Radiation Protection Procedures

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1 S A F E T Y S E R IE S N o. 38 Radiation Protection Procedures IN T E R N A T IO N A L A T O M IC E N E R G Y A G E N C Y, V IEN N A, 1973

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3 R A D I A T I O N P R O T E C T I O N P R O C E D U R E S

4 The following States are Members of the International Atomic Energy Agency: AFGHANISTAN GUATEMALA PANAMA ALBANIA HAITI PARAGUAY ALGERIA HOLY SEE PERU ARGENTINA HUNGARY PHILIPPINES AUSTRALIA ICELAND POLAND AUSTRIA INDIA PORTUGAL BANGLADESH INDONESIA ROMANIA BELGIUM IRAN SAUDI ARABIA BOLIVIA IRAQ SENEGAL BRAZIL IRELAND SIERRA LEONE BULGARIA ISRAEL SINGAPORE BURMA ITALY SOUTH AFRICA BYELORUSSIAN SOVIET IVORY COAST SPAIN SOCIALIST REPUBLIC JAMAICA SRI LANKA CAMEROON JAPAN SUDAN CANADA JORDAN SWEDEN CHILE KENYA SWITZERLAND CHINA KHMER REPUBLIC SYRIAN ARAB REPUBLIC COLOMBIA KOREA, REPUBLIC OF THAILAND COSTA RICA KUWAIT TUNISIA CUBA LEBANON TURKEY CYPRUS LIBERIA UGANDA CZECHOSLOVAK SOCIALIST LIBYAN ARAB REPUBLIC UKRAINIAN SOVIET SOCIALIST REPUBLIC LIECHTENSTEIN REPUBLIC DENMARK LUXEMBOURG UNION OF SOVIET SOCIALIST DOMINICAN REPUBLIC MADAGASCAR REPUBLICS ECUADOR MALAYSIA UNITED KINGDOM OF GREAT EGYPT, ARAB REPUBLIC OF MALI BRITAIN AND NORTHERN EL SALVADOR MEXICO IRELAND ETHIOPIA MONACO UNITED STATES OF AMERICA FINLAND MOROCCO URUGUAY FRANCE NETHERLANDS VENEZUELA GABON NEW ZEALAND VIET-NAM GERMANY, FEDERAL REPUBLIC OF NIGER YUGOSLAVIA GHANA NIGERIA ZAIRE, REPUBLIC OF GREECE NORWAY ZAMBIA PAKISTAN T he A g e n c y 's S ta tu te was approved on 23 O ctober by the C onference on the S tatu te o f the IAEA h eld a t U nited N ations H eadquarters, New York; i t en tered in to force on 29 July T he H eadquarters of the A gency a re situ ated in V ienna. Its prin cip al o b jectiv e is "to a c c e le ra te and e n la rg e the contrib u tio n of a to m ic energy to p e a c e, h e a lth and prosperity throughout th e w orld". IAEA, 1973 Perm ission to reproduce or tran slate the in fo rm atio n contain ed in this p u b lic atio n m ay be obtain ed by writing to the International Atomic Energy Agency, Kamtner Ring 11, P.O. Box 590, A -1Q11 Vienna, Austria. Printed by the IAEA in A ustria M ay 1973

5 SAFETY SERIES N o.38 RADIATION PROTECTION PROCEDURES I N T E R N A T I O N A L A T O M I C E N E R G Y A G E N C Y V I E N N A,

6 T H I S S A F E T Y S E R I E S W I L L A L S O B E P U B L I S H E D IN F R E N C H, R U S S I A N A N D S P A N IS H R A D I A T I O N P R O T E C T I O N P R O C E D U R E S I A E A, V I E N N A, S T I / P U B / 2 5 7

7 F O R E W O R D T h e I n t e r n a t i o n a l A t o m i c E n e r g y A g e n c y p u b li s h e d in a m a n u a l, S a f e t y S e r i e s N o.2, e n t i t l e d " S a f e H a n d lin g o f R a d i o i s o t o p e s : H e a lt h P h y s i c s A d d e n d u m ", w h ic h w a s p r e p a r e d b y tw o a u t h o r s, th e l a t e G. J. A p p le t o n o f th e U n ite d K in g d o m A t o m i c E n e r g y A u t h o r i t y a n d P. N. K r i s h n a m o o r t h y o f th e D i r e c t o r a t e o f R a d ia t io n P r o t e c t i o n, D e p a r t m e n t o f A t o m i c E n e r g y, T r o m b a y, In d ia. T h e A d d e n d u m c o n t a i n e d t e c h n i c a l i n f o r m a t i o n n e c e s s a r y f o r th e i m p l e m e n t a t i o n o f th e c o n t r o l s g i v e n in th e c o d e o f p r a c t i c e e n t i t le d " S a f e H a n d lin g o f R a d i o i s o t o p e s ", I A E A S a f e t y S e r i e s N o. l. In a d d it io n, it w a s in te n d e d to s e r v e a s a b r i e f i n t r o d u c t i o n to th e t e c h n i c a l p r o b l e m s e n c o u n t e r e d in r a d i o l o g i c a l p r o t e c t i o n w o r k an d to th e m e t h o d s o f r e s o l v i n g t h e m. D u r i n g th e p a s t t w e l v e y e a r s c o n s i d e r a b l e d e v e l o p m e n t s h a v e t a k e n p l a c e o n v a r i o u s a s p e c t s o f r a d i a t i o n p r o t e c t i o n, e s p e c i a l l y o n m e a s u r i n g t e c h n i q u e s a n d i n s t r u m e n t s. T h e m a n u a l h a s t h e r e f o r e b e e n t h o r o u g h l y r e v i s e d a n d b r o u g h t up to d a t e, j o i n t l y b y P.N. K r i s h n a m o o r t h y, w h o w a s a p p o in te d a s th e c o n s u l t a n t, an d J. U. A h m e d o f th e I A E A. T h e r e v i s e d m a n u a l h a s b e e n e x p a n d e d b y th e i n c l u s i o n o f n e w c h a p t e r s, t a b l e s a n d f i g u r e s. It h a s th u s b e c o m e a c o m p l e t e g u id e in i t s e l f an d i s n o w p u b li s h e d u n d e r th e t i t l e " R a d i a t i o n P r o t e c t i o n P r o c e d u r e s ".

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9 CONTENTS 1. I n t r o d u c t i o n F u n d a m e n t a l s o f n u c l e a r p h y s i c s I n t e r a c t i o n o f r a d i a t i o n w ith m a t t e r I n t e r a c t i o n o f r a d i a t i o n w ith l i v i n g c e l l s R a d i a t i o n u n it s a n d a s s o c i a t e d c o n c e p t s M a x i m u m p e r m i s s i b l e l e v e l s o f r a d i a t i o n M e t h o d s o f r a d i a t i o n m e a s u r e m e n t R a d i a t i o n m o n i t o r i n g i n s t r u m e n t s C a l i b r a t i o n a n d m a i n t e n a n c e o f r a d i a t i o n m o n i t o r i n g i n s t r u m e n t s R a d i a t io n c o n t r o l m e a s u r e s R a d i a t io n s h i e l d i n g B a s i c d e s i g n f e a t u r e s o f r a d i a t i o n i n s t a l l a t i o n s H a n d lin g e q u i p m e n t f o r r a d i a t i o n s o u r c e s C o n t a i n e r s f o r r a d i o a c t i v e m a t e r i a l s P r o t e c t i v e c l o t h i n g A r e a a n d e n v i r o n m e n t a l m o n i t o r i n g D e c o n t a m i n a t i o n M a n a g e m e n t o f r a d i o a c t i v e w a s t e s T r a n s p o r t o f r a d i o a c t i v e m a t e r i a l s R a d i a t i o n a c c i d e n t s a n d e m e r g e n c y p r o c e d u r e s A d m i n i s t r a t i o n o f r a d i a t i o n p r o t e c t i o n A n n e x : A c o l l e c t i o n o f u s e f u l h e a l t h p h y s i c s d a t a T a b l e s F i g u r e s I l l u s t r a t i o n s B i b l i o g r a p h y

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11 1. INTRODUCTION A l l r a d i o a c t i v e m a t e r i a l s an d r a d i a t i o n a r e p o t e n t i a l l y h a z a r d o u s. T h e y c a n g i v e r i s e, w h e n o u t s i d e th e b o d y, to e x t e r n a l r a d i a t i o n e x p o s u r e o f p e r s o n n e l, an d w h e n i n s i d e th e b o d y, to th e i r r a d i a t i o n o f c e r t a i n o r g a n s. T h e h e a l t h p h y s i c i s t m u s t t h e r e f o r e b e c a p a b l e o f a d v is i n g th e u s e r o f th e r a d i o a c t i v e m a t e r i a l o r o t h e r r a d i a t i o n s o u r c e s o n th e m e t h o d s n e c e s s a r y f o r th e p r e v e n t i o n an d c o n t r o l o f t h e s e h a z a r d s. E v i d e n c e an d e x p e r i e n c e i n d i c a t e t h a t l i m i t e d e x p o s u r e to e x t e r n a l r a d i a t i o n, o r th e i n t a k e o f s m a l l a m o u n t s o f r a d i o a c t i v e m a t e r i a l in to th e b o d y, a r e a s s o c i a t e d w ith a n e g l i g i b l e p r o b a b i l i t y o f s e v e r e s o m a t i c o r g e n e t i c i n ju r y. T h i s h a s le d to th e c o n c e p t o f th e m a x i m u m p e r m i s s i b l e l e v e l s o f e x p o s u r e f o r p e r s o n n e l w o r k i n g w ith r a d i o a c t i v e m a t e r i a l s o r r a d i a t i o n s o u r c e s. In a d d itio n to th e e x p o s u r e o f r a d i a t i o n w o r k e r s, t h e r e i s a l s o th e p o s s i b i l i t y o f e x p o s u r e o f th e g e n e r a l p u b l i c. T h e h e a l t h p h y s i c i s t m u s t t h e r e f o r e b e a r in m in d tw o m a i n a s p e c t s o f r a d i o l o g i c a l p r o t e c t i o n : (a) p r o t e c t i o n o f r a d i a t i o n w o r k e r s ; an d (b) p r o t e c t i o n o f th e g e n e r a l p u b l i c. T h e l a r g e n u m b e r in v o lv e d an d t h e i m p r a c t i c a b i l i t y o f l a r g e - s c a l e m e d i c a l s u p e r v i s i o n n e c e s s i t a t e s e v e n m o r e s t r i n g e n t p r e c a u t i o n s f o r th e p r o t e c t i o n o f th e g e n e r a l p u b l ic t h a n f o r th e p r o t e c t i o n o f r a d i a t i o n w o r k e r s. T h e r e a r e t h r e e p r i n c i p l e s w h ic h c a n b e a p p lie d to p r e v e n t o r c o n t r o l th e e x p o s u r e o f p e r s o n n e l to r a d i a t i o n h a z a r d s : (a) R e m o v e t h e h a z a r d (b) G u a r d t h e h a z a r d (c ) G u a r d t h e w o r k e r. T h e s e p r i n c i p l e s s h o u ld b e a p p lie d in th e a b o v e o r d e r f o r p e r s o n n e l p r o t e c t i o n. T h e f i r s t i s a n o b v i o u s o n e, th e s e c o n d i m p l i e s th e p r o p e r d e s i g n o f w o r k p l a c e s an d th e p r o v i s i o n o f a p p r o p r i a t e e q u i p m e n t an d s h i e l d i n g to e n s u r e th e m a x i m u m a m o u n t o f p r o t e c t i o n, an d th e t h i r d r e f e r s to t h e m e a s u r e s r e q u i r e d to m a k e a p e r i o d i c c h e c k o n th e c o n t i n u in g a d e q u a c y o f th e c o n t r o l s, th e p e r s o n a l p r o t e c t i o n m e a s u r e s an d th e e q u ip m e n t. T h e p r o c e d u r e s n e c e s s a r y f o r th e i m p l e m e n t a t i o n o f t h e s e t h r e e p r i n c i p l e s f o r r a d i o l o g i c a l p r o t e c t i o n in a n y s p e c i f i c s i t u a t i o n c a n b e a s s e s s e d o n l y a f t e r a p r o p e r e v a l u a t i o n o f th e t e c h n i c a l and o t h e r a s p e c t s in v o l v e d. S u c h a r e v i e w, w h ic h t a k e s in to a c c o u n t th e r e q u i r e m e n t s o f r a d i a t i o n an d n o n - r a d i a t i o n w o r k e r s, h a s b e e n p r e s e n t e d in t a b u l a r f o r m in th e a n n e x at t h e e n d o f t h i s b o o k. 9

12 2. FUNDAMENTALS OF NUCLEAR PHYSICS T h e a t o m S t r u c t u r e T h e a t o m i s th e s m a l l e s t p a r t o f an e l e m e n t w h ic h c a n p a r t i c i p a t e in a n y c h e m i c a l r e a c t i o n. It i s s o s m a l l (a b o u t 1 0 " 8 c m in d i a m.) t h a t in a g r a m a t o m i c w e ig h t o f a n y s u b s t a n c e ( e.g. 1 g o f h y d r o g e n o r 12 g o f c a r b o n ) t h e r e a r e X a t o m s o f th e e l e m e n t. T h e a t o m c o n s i s t s o f a s m a l l p o s i t i v e l y c h a r g e d c e n t r a l n u c l e u s (a b o u t 1 0 ' 1 2 c m in d i a m.), in w h ic h m o s t o f i t s m a s s is c o n c e n t r a t e d. T h e v o l u m e o f th e n u c l e u s, h o w e v e r, i s o n l y a s m a l l f r a c t i o n o f th e v o lu m e o f th e a t o m. T h e n u c l e u s, in t u r n, c o n s i s t s o f p o s i t i v e l y c h a r g e d p a r t i c l e s c a l l e d p r o t o n s an d u n c h a r g e d p a r t i c l e s c a l l e d n e u t r o n s. P a r t i c l e s i n s i d e a n u c l e u s a r e a l s o r e f e r r e d to a s n u c l e o n s. T h e p o s i t i v e c h a r g e o f th e n u c l e u s r e s u l t s f r o m th e p r e s e n c e o f p r o t o n s in i t. N e g a t i v e l y c h a r g e d p a r t i c l e s c a l l e d e l e c t r o n s w h i r l a r o u n d th e n u c l e u s in w e l l - d e f in e d o r b i t s o r s h e l l s at d i f f e r e n t r a d i a l d i s t a n c e s f r o m th e n u c l e u s ( s e e F i g. 2.1 ). T h e s i m p l e s t a t o m i s t h a t o f h y d r o g e n w h ic h c o n s i s t s o f a p r o t o n a s i t s n u c l e u s an d a n e l e c t r o n in th e o r b i t. T h e h e l i u m a t o m h a s 2 p r o t o n s an d 2 n e u t r o n s in th e n u c l e u s an d 2 o r b i t a l e l e c t r o n s. S i m i l a r l y 238U h a s 9 2 p r o t o n s an d n e u t r o n s in th e n u c l e u s an d 9 2 o r b i t a l e l e c t r o n s. ELECTRONS N U C LEU S ATOM NUCLEUS FIG Schematic visualization of an atom and a nucleus. 10

13 T h e p r o t o n i s t i m e s h e a v i e r th a n th e e l e c t r o n w h ile th e m a s s o f th e n e u t r o n i s s l i g h t l y g r e a t e r t h a n t h a t o f a p r o t o n. T h e c h a r g e o f an e l e c t r o n i s 4. 8 X 1 0 " 10 e l e c t r o s t a t i c u n i t s ( e s u ) and i s th e s m a l l e s t c h a r g e k n o w n to e x i s t. T h e p r o t o n c a r r i e s a c h a r g e e q u a l to t h a t o f a n e l e c t r o n b u t o f o p p o s i t e s i g n A t o m i c n u m b e r a n d m a s s n u m b e r T h e a t o m a s a w h o le i s e l e c t r i c a l l y n e u t r a l a n d h e n c e th e n u m b e r o f p r o t o n s in th e n u c l e u s i s e q u a l to t h e n u m b e r o f o r b i t a l e l e c t r o n s. T h u s th e t o t a l n u m b e r o f p r o t o n s in th e n u c l e u s o r th e t o t a l n u m b e r o f o r b i t a l e l e c t r o n s c h a r a c t e r i z e s a n e l e m e n t an d is k n o w n a s th e " a t o m i c n u m b e r " - d e n o te d b y th e s y m b o l Z. T h e s u m o f th e n u m b e r s o f p r o t o n s an d n e u t r o n s i s c a l l e d t h e " m a s s n u m b e r " - d e n o te d b y th e s y m b o l A an d i s an in d e x o f th e m a s s o f th e a t o m. T h e n u m b e r o f n e u t r o n s in th e n u c l e u s i s th u s g iv e n b y A - Z. A n y i n d iv id u a l a t o m, w ith i t s c h a r a c t e r i s t i c n u m b e r o f p r o t o n s an d n e u tr o n s., i s c a l l e d a n u c l i d e I s o t o p e s T h e a t o m i c n u m b e r Z i s th e in d e x o f a n e l e m e n t. T h e r e f o r e, n u c l i d e s h a v i n g th e s a m e Z b u t d i f f e r e n t m a s s n u m b e r A a r e c a l l e d t h e i s o t o p e s o f th e e l e m e n t o f a t o m i c n u m b e r Z. T h e i s o t o p e s o f th e s a m e e l e m e n t h a v e i d e n t i c a l c h e m i c a l p r o p e r t i e s, b u t g e n e r a l l y h i g h l y d i s s i m i l a r n u c l e a r p r o p e r t i e s. T h e n u c l id e o f a n e l e m e n t X o f a t o m i c n u m b e r Z an d m a s s n u m b e r A i s r e p r e s e n t e d a s ( e. g., ^ H e d e n o t e s th e n u c l e u s o f h e l i u m a t o m c o n s i s t i n g o f 2 p r o t o n s an d 2 n e u t r o n s ), b u t in p r a c t i c e Z i s u s u a l ly o m i t t e d, s i n c e th e u s e o f th e c h e m i c a l s y m b o l an d m a s s n u m b e r is s u f f i c i e n t i d e n t i f i c a t i o n. In T a b l e 2. 1, e x a m p l e s o f n u c l i d e s and i s o t o p e s a r e s h o w n. T A B L E E X A M P L E S O F N U C L I D E S A N D I S O T O P E S Term Characterized by Examples Remarks Nuclide Z, A ih, 126c, 23s u More than 700 nuclides known Isotope Constant Z \h, i H, 5h 3 to 19 isotopes known per element 11

14 TA BLE 2.2. SOME COMMON TYPES OF RADIATION Type of radiation Symbol Charge Rest mass (amu) Alpha particle a Beta particle (a) electron B, e" (b) positron B+, e Proton P Neutron n Electromagnetic radiations (a) X-rays X 0 (b) Gamma-rays y N u c l e a r r a d i a t i o n T y p e s o f n u c l e a r r a d i a t i o n T h e t e r m n u c l e a r r a d i a t i o n c o m m o n l y r e f e r s to t h e w id e v a r i e t y o f e m a n a t i o n s a s s o c i a t e d w ith s y s t e m s u n d e r g o i n g n u c l e a r t r a n s f o r m a t i o n..in t h i s g r o u p a r e a l s o in c lu d e d s u b - a t o m i c and a t o m i c p a r t i c l e s a s w e l l a s X - an d g a m m a r a d i a t i o n. S i n c e a d i s c u s s i o n o f a l l t y p e s o f n u c l e a r r a d i a t i o n i s b e y o n d th e s c o p e o f t h i s m a n u a l, o n ly a fe w c o m m o n t y p e s p r e s e n t e d in T a b l e 2.2 w i l l b e d i s c u s s e d A lp h a p a r t i c l e s : A lp h a p a r t i c l e s a r e th e h e l i u m n u c l e i ( H e) e m i t t e d b y r a d i o n u c l i d e s, m a i n l y b y h e a v y n u c l e i s u c h a s p o l o n i u m, r a d i u m, t h o r i u m, u r a n i u m, e t c. In a lp h a d e c a y th e m a s s o f th e p a r e n t n u c l e u s i s g r e a t e r th a n th e s u m o f t h e m a s s e s o f t h e p r o d u c t s, an d t h i s m a s s d i f f e r e n c e i s r e l e a s e d a s t h e k i n e t i c e n e r g y o f th e a lp h a p a r t i c l e. T h e a lp h a p a r t i c l e s e m i t t e d b y a n y r a d i o n u c l id e h a v e g e n e r a l l y o n e o r tw o, an d r a r e l y m o r e, d i s c r e t e e n e r g i e s, w h ic h a r e c h a r a c t e r i s t i c o f th e r a d i o n u c l i d e B e t a p a r t i c l e s : B e t a p a r t i c l e s a r e h i g h - e n e r g y e l e c t r o n s o r p o s i t r o n s c r e a t e d a n d e m i t t e d b y c e r t a i n r a d i o n u c l i d e s. 12

15 U n lik e a lp h a p a r t i c l e s, b e t a p a r t i c l e s a r e n o t m o n o e n e r g e t i c b u t a r e e m i t t e d w ith a c o n t in u o u s s p e c t r u m o f e n e r g y. B e t a p a r t i c l e s e m i t t e d b y r a d i o n u c l i d e s a r e o f tw o k in d s - th e n e g a t i v e e l e c t r o n an d th e p o s i t i v e e l e c t r o n r e s u l t i n g f r o m n e u t r o n o r p r o t o n e x c e s s r e s p e c t i v e l y in th e p a r e n t r a d i o n u c l i d e. T h e e m i s s i o n o f th e p o s i t i v e e l e c t r o n is c a l l e d p o s i t r o n d e c a y. (A p r o c e s s e q u i v a l e n t in e f f e c t to p o s i t r o n d e c a y i s e l e c t r o n c a p t u r e, in w h i c h t h e n u c l e u s c a p t u r e s an i n n e r o r b i t a l e l e c t r o n. ) O w in g to th e e m i s s i o n o f a b e t a p a r t i c l e, th e m a s s o f th e n u c l e u s r e m a i n s p r a c t i c a l l y u n c h a n g e d b u t t h e a t o m i c n u m b e r i s c h a n g e d b y o n e u n i t. A s m e n t i o n e d e a r l i e r, b e t a p a r t i c l e s f r o m r a d i o n u c l i d e s a r e e m i t t e d in a c o n t i n u o u s e n e r g y s p e c t r u m, an d h e n c e t a b l e s o f b e t a e n e r g i e s a l w a y s l i s t th e m a x i m u m e n e r g y o f e m i s s i o n w h i c h i s c h a r a c t e r i s t i c o f e a c h r a d i o n u c l i d e. F o r m a n y p u r p o s e s, h o w e v e r, a m e a n e n e r g y e q u a l to o n e - t h i r d o f th e m a x i m u m e n e r g y i s t a k e n P r o t o n s : P r o t o n s a r e h y d r o g e n n u c l e i an d a r e t h e r e f o r e p o s i t i v e l y c h a r g e d. P r o t o n b e a m s a r e p r o d u c e d in a c c e l e r a t o r s o f d i f f e r e n t t y p e s an d m a y h a v e e n e r g i e s o f s e v e r a l h u n d r e d s o f m e g a e l e c t r o n v o l t s o r m o r e. P r o t o n s a r e a l s o e m i t t e d in th e i n t e r a c t i o n s o f f a s t n e u t r o n s w ith h y d r o g e n a t o m s N e u t r o n s : T h e n e u t r o n i s a n u n c h a r g e d p a r t i c l e h a v in g a m a s s s l i g h t l y g r e a t e r th a n t h a t o f a p r o t o n. It s u f f e r s n o C o u lo m b i n t e r a c t i o n w ith e i t h e r th e o r b i t a l e l e c t r o n s o r th e n u c l e u s o f th e a t o m. N e u t r o n s a r e g e n e r a l l y c l a s s i f i e d a c c o r d i n g to t h e i r e n e r g i e s u n d e r f o u r b r o a d c a t e g o r i e s. (a) T h e r m a l n e u t r o n s a r e t h o s e w h ic h a r e in t h e r m a l e q u i l i b r i u m w ith th e s u r r o u n d i n g m a t t e r, s o t h a t o n th e a v e r a g e t h e r e i s no n e t e x c h a n g e o f k i n e t i c e n e r g y b e t w e e n th e n e u t r o n s an d th e t h e r m a l l y a g i t a t e d a t o m s o f th e s u r r o u n d i n g m a t t e r '. T h e n e u t r o n s in t h i s c a s e w i l l h a v e a M a x w e l l i a n d i s t r i b u t i o n o f v e l o c i t i e s \Vith a m o s t p r o b a b l e v e l o c i t y o f 2.2 X c m / s w h ic h c o r r e s p o n d s to a k i n e t i c e n e r g y o f e V. (b) I n t e r m e d i a t e n e u t r o n s a r e t h o s e f a l l i n g in th e e n e r g y r a n g e o f 0.5 e V to 10 k e V. N e u t r o n s h a v in g e n e r g i e s l e s s th a n e V a r e a l s o r e f e r r e d to a s s lo w n e u t r o n s. (c ) F a s t n e u t r o n s h a v e e n e r g i e s b e t w e e n 10 k e V an d 10 M e V. (d) R e l a t i v i s t i c n e u t r o n s h a v e e n e r g i e s g r e a t e r th a n 1 0 M e V. In t h i s r a n g e t h e k i n e t i c e n e r g y b e c o m e s a s i g n i f i c a n t f r a c t i o n o f th e t o t a l e n e r g y o f th e n e u t r o n s, s o t h a t r e l a t i v i s t i c c o r r e c t i o n s s h o u ld b e a p p lie d in a n a l y s e s o f n e u t r o n i n t e r a c t i o n s. 13

16 X - a n d g a m m a - r a y s : X - a n d g a m m a - r a y s a r e e l e c t r o m a g n e t i c r a d i a t i o n o f v e r y s h o r t w a v e l e n g t h. T h e r e i s no d i f f e r e n c e b e t w e e n X - an d g a m m a - r a y s e x c e p t in t h e i r o r i g i n. W h i l e g a m m a r a y s a r e e m i t t e d w ith d i s c r e t e e n e r g i e s c h a r a c t e r i s t i c o f th e n u c l i d e f o r m e d, X - r a y e m i s s i o n i s o f tw o t y p e s, c h a r a c t e r i s t i c r a d i a t i o n ( d i s c r e t e e n e r g i e s ) an d b r e m s s t r a h l u n g ( c o n tin u o u s s p e c t r u m o f e n e r g i e s ). C h a r a c t e r i s t i c X - r a y s a r e p r o d u c e d f r o m t r a n s i t i o n s b e t w e e n e n e r g y l e v e l s o f i n n e r e l e c t r o n s in a n a t o m, w h i le g a m m a r a y s a r e e m i t t e d b e c a u s e o f t r a n s i t i o n s o f th e n u c l e u s f r o m h i g h e r to l o w e r e n e r g y s t a t e s. T h e n u c l e u s i s l e f t u n c h a n g e d b y g a m m a e m i s s i o n. P u r e g a m m a e m i s s i o n is u n k n o w n in n a t u r a l r a d i o a c t i v i t y ; i t f r e q u e n t l y f o ll o w s a lp h a o r b e t a d e c a y R a d i o a c t i v i t y It h a s b e e n m e n t i o n e d e a r l i e r t h a t, e x c e p t f o r th e s i m p l e s t n u c l e u s, t h a t o f h y d r o g e n, a l l o t h e r n u c l e i c o n s i s t o f n e u t r o n s and p r o t o n s. T h e r a t i o o f n e u t r o n s to p r o t o n s i s u n ity f o r l i g h t e r i s o t o p e s an d i n c r e a s e s g r a d u a l l y a s o n e a p p r o a c h e s th e h e a v i e r e l e m e n t s at th e e n d o f th e p e r i o d i c t a b l e. A s t h i s r a t i o i n c r e a s e s, a s t a g e i s r e a c h e d w h e r e th e n u c li d e i s n o l o n g e r s t a b l e. T h e h e a v i e s t s t a b l e n u c l id e i s 2g g B i. N u c l id e s h e a v i e r th a n t h i s a r e u n s t a b l e b e c a u s e t h e y h a v e e x c e s s e n e r g y to d i s s i p a t e. S u c h u n s t a b l e n u c l i d e s a r e c a l l e d r a d i o n u c l i d e s and t h e y d i s s i p a t e t h e i r s u r p l u s e n e r g y b y th e e m i s s i o n o f r a d i a t i o n. T h i s p r o c e s s is c a l l e d r a d i o a c t i v i t y o r r a d i o a c t i v e d e c a y. T h e m o r e f r e q u e n t m o d e s o f d e c a y o f r a d i o n u c l i d e s a r e a lp h a, b e t a and g a m m a d e c a y s. R a d i o a c t i v i t y c a n b e o f tw o t y p e s : (1 ) n a t u r a l r a d i o a c t i v i t y e x h i b it e d b y m o r e th a n 5 0 n a t u r a l l y o c c u r r i n g i s o t o p e s ( e. g., 238U, 226R a, 40K, e t c. ), an d (2) a r t i f i c i a l r a d i o a c t i v i t y w h ic h i s th e r a d i o a c t i v i t y i n d u c e d in s o m e e l e m e n t s b y b o m b a r d i n g t h e m w ith n e u t r o n s, c h a r g e d p a r t i c l e s o r p h o t o n s. T h e r e s u l t a n t n u c l e i ( e. g., is ^ C s, 60C o, 32P, e t c. ) c o u ld b e in e x c i t e d s t a t e s an d w i ll t h e r e f o r e d e c a y b y o n e o f th e m o d e s d e s c r i b e d a b o v e. M o s t r a d i o n u c l i d e s c u r r e n t l y in u s e a r e a r t i f i c i a l l y p r o d u c e d. T h e d e c a y o f a r a d i o n u c l id e i s a s t a t i s t i c a l p r o c e s s in th e s e n s e t h a t it i s n o t p o s s i b l e to p r e d i c t e x a c t l y w h e n a p a r t i c u l a r n u c l e u s w i l l d i s i n t e g r a t e. O n e m a y, h o w e v e r, a s c r i b e a p r o b a b i l i t y th a t a n u c l e u s w i l l d e c a y in u n it t i m e. T h i s p r o b a b i l i t y is k n o w n a s t h e r a d i o a c t i v e d e c a y c o n s t a n t ( t r a n s f o r m a t i o n c o n s t a n t ), X, o f th e r a d i o n u c l i d e. T h e n u m b e r o f a t o m s o f a r a d i o a c t i v e s u b s t a n c e d i s i n t e g r a t i n g p e r u n it t i m e, d N / d t, w h ic h i s r e f e r r e d to a s th e 14

17 a c t i v i t y o f th e s u b s t a n c e, i s p r o p o r t i o n a l to th e t o t a l n u m b e r, N, o f r a d i o a c t i v e a t o m s p r e s e n t a t t i m e t ; th e c o n s t a n t o f p r o p o r t i o n a l i t y b e i n g X. T h u s, I n t e g r a t i n g t h i s e q u a t i o n, o n e h a s N = N 0 e ' M (2) w h e r e No i s th e i n i t i a l n u m b e r o f r a d i o a c t i v e a t o m s p r e s e n t, and N, a s a l r e a d y s t a t e d, t h e n u m b e r o f r a d i o a c t i v e a t o m s a t t i m e t. R e w r i t i n g E q. ( l ), o n e h a s d N -x t - = X N = X N 0 e (3) E q u a t i o n (3) i n d i c a t e s t h a t th e n u m b e r o f r a d i o a c t i v e a t o m s p r e s e n t a s w e l l a s th e d i s i n t e g r a t i o n r a t e ( a c t i v i t y ) d e c r e a s e e x p o n e n t i a l l y w ith t i m e. T h e t i m e t a k e n f o r h a l f th e r a d i o a c t i v e a t o m s o r i g i n a l l y p r e s e n t to d e c a y i s c a l l e d th e h a l f - l i f e o f th e r a d i o n u c l i d e. S u b s t it u t i n g N = N 0 /2 an d t = t± in E q. ( 2 ), o n e h a s N 0 / 2 = N 0 e ' Xti o r X t i = In 2 = o r t i = / X (4) T h e n u m b e r o f r a d i o a c t i v e a t o m s p r e s e n t an d h e n c e t h e r a t e o f d i s i n t e g r a t i o n d e c r e a s e s to o n e - h a l f in o n e h a l f - l i f e, to o n e - q u a r t e r in tw o h a l f - l i v e s, to o n e - e i g h t h in t h r e e h a l f - l i v e s, an d s o o n. T h e h a l f - l i f e is c h a r a c t e r i s t i c o f a n y p a r t i c u l a r r a d i o i s o t o p e. A n o t h e r u s e f u l q u a n t it y i s th e m e a n l i f e o r th e a v e r a g e l i f e o f a r a d i o n u c l i d e w h ic h i s th e r e c i p r o c a l o f th e d e c a y c o n s t a n t, ( t m = 1/ X ). 15

18 R adioactive equilibrium A r a d i o n u c l i d e u p o n u n d e r g o i n g d i s i n t e g r a t i o n o f a p a r t i c u l a r t y p e y i e l d s a s p e c i f i e d n u c l i d e. T h e o r i g i n a l r a d i o n u c l id e i s c a l l e d th e p a r e n t an d th e d e c a y p r o d u c t i s c a l l e d th e d a u g h t e r. T h e d a u g h t e r m a y a l s o b e a r a d i o n u c l i d e. A s u c c e s s i o n o f n u c l i d e s, e a c h o f w h i c h t r a n s f o r m s b y r a d i o a c t i v e d i s i n t e g r a t i o n in to th e n e x t u n t i l a s t a b l e n u c l i d e r e s u l t s, i s c a l l e d a r a d i o a c t i v e s e r i e s. E x a m p l e s o f s u c h s e r i e s a r e th e u r a n i u m s e r i e s an d th e t h o r i u m s e r i e s. R a d i o a c t i v e e q u i l i b r i u m r e f e r s to t h a t s t a t e in w h ic h th e r a t i o s b e t w e e n th e a m o u n t s o f s u c c e s s i v e m e m b e r s o f th e s e r i e s r e m a i n c o n s t a n t. U n d e r t h e s e c o n d i t i o n s th e d i s i n t e g r a t i o n r a t e s o f th e p a r e n t an d a l l th e s u b s e q u e n t r a d i o a c t i v e d a u g h t e r s w i l l b e th e s a m e U n it o f r a d i a t i o n e n e r g y T h e e n e r g y o f a t o m i c r a d i a t i o n i s e x p r e s s e d in u n i t s o f e l e c t r o n v o l t s ( e V ). T h e e l e c t r o n v o lt i s d e fin e d a s th e k i n e t i c e n e r g y a c q u ir e d b y a n e l e c t r o n w h e n it f a l l s t h r o u g h a p o t e n t i a l d i f f e r e n c e o f o n e v o l t. E x p r e s s e d in t e r m s o f e r g s, o n e h a s O n e e l e c t r o n v o lt, 1 e V = 1.6 X 1 0 "12 e r g O n e m i l l i o n e l e c t r o n v o l t s, 1 M e V = 1.6 X 1 0 " 6 e r g A s a l r e a d y s t a t e d, th e e n e r g y r e l e a s e d in a d e c a y p r o c e s s o c c u r s a s a r e s u l t o f th e d i f f e r e n c e in m a s s - o f th e p a r e n t n u c l e u s an d th e s u m o f th e m a s s e s o f th e p r o d u c t s. T h e m a g n it u d e o f t h i s e n e r g y is g i v e n b y E i n s t e i n ' s m a s s ( m ) - e n e r g y (E ) r e l a t i o n E = m e 2 w h e r e c is th e v e l o c i t y o f l ig h t, w h ic h i s c o n s t a n t and e q u a l to 3 X c m / s. F r o m t h i s r e l a t i o n s h i p o n e c a n e a s i l y s e e th a t 1 a t o m i c m a s s u n it = M e V. A l s o, th e r e s t m a s s o f an e l e c t r o n c a n b e s e e n to b e M e V. 16

19 3. INTERACTION OF RADIATION WITH MATTER 3.1. D i r e c t l y i o n i z i n g r a d i a t i o n s D i r e c t l y io n iz i n g r a d i a t i o n s i n c l u d e a l l c h a r g e d p a r t i c l e s s u c h a s a lp h a p a r t i c l e s a n d h e a v i e r i o n s a n d b e t a p a r t i c l e s. A l l c h a r g e d p a r t i c l e r a d i a t i o n s l o s e e n e r g y b y i n t e r a c t i o n w ith t h e o r b i t a l e l e c t r o n s o r n u c l e i o f a t o m s i n t h e m a t e r i a l s t h e y t r a v e r s e. T h e r e a r e tw o m a i n p r o c e s s e s in v o lv in g t h e o r b i t a l e l e c t r o n s : (a) A t o m i c o r m o l e c u l a r e x c i t a t i o n, w ith t h e e m i s 's i o n o f lig h t r e s u l t i n g f r o m s u b s e q u e n t d e - e x c i t a t i o n (b) I o n i z a t i o n, w h ic h i n v o l v e s t h e e j e c t i o n o f a n o r b i t a l e l e c t r o n, r e s u l t i n g in th e c r e a t i o n o f a n io n p a i r. O n t h e a v e r a g e, a b o u t 3 4 e V i s e x p e n d e d in t h e c r e a t i o n o f e a c h i o n p a i r i n a i r. T h e i o n i z a t i o n o f a n a t o m w h ic h f o r m s p a r t o f a m o l e c u l e c o u ld r e s u l t in t h e d i s s o c i a t i o n o f t h e m o l e c u l e A l p h a p a r t i c l e s A l p h a p a r t i c l e s e m i t t e d f r o m r a d i o n u c l i d e s h a v e w e l l - d e f i n e d a n d c h a r a c t e r i s t i c e n e r g i e s. A s t h e y a r e d o u b ly c h a r g e d a n d m o v e r e l a t i v e l y s lo w ly, t h e y a r e d e n s e l y io n iz i n g a n d h e n c e, in s p i t e o f t h e i r h ig h e n e r g i e s, t h e i r p e n e t r a t i n g p o w e r o r r a n g e i s e x t r e m e l y l i m i t e d. I n f a c t, a lp h a p a r t i c l e s o f e n e r g i e s u p to 7. 5 M e V a r e i n c a p a b l e o f p e n e t r a t i n g t h e p r o t e c t i v e l a y e r o f t h e s k i n o n m o s t p a r t s o f t h e b o d y ( e.g., th e h a n d ). T h e i r r a f i g e in a i r i s o n ly a f e w c e n t i m e t r e s. T h u s, s h i e l d i n g a g a i n s t t h i s t y p e o f r a d i a t i o n p r e s e n t s n o p r o b l e m B e t a p a r t i c l e s B e t a p a r t i c l e s l o s e e n e r g y m a i n l y t h r o u g h i o n i z a t i o n. A n o t h e r p r o c e s s b y w h ic h b e t a p a r t i c l e s l o s e e n e r g y i s b y t h e p r o d u c t i o n o f b r e m s s t r a h l u n g ( b r a k i n g r a d i a t i o n ). T h e p r o d u c t i o n o f X - r a y s b y b o m b a r d i n g h e a v y m e t a l t a r g e t s w ith h i g h - e n e r g y e l e c t r o n s i s a l s o a b r e m s s t r a h l u n g p r o c e s s. S i n c e b e t a p a r t i c l e s a r e m u c h l i g h t e r t h a n o t h e r c h a r g e d p a r t i c l e s, t h e i r v e l o c i t y f o r a g i v e n e n e r g y i s m u c h h i g h e r a n d t h e i r s p e c i f i c i o n i z a t i o n ( n u m b e r o f io n p a i r s p e r u n it l e n g t h ) m u c h s m a l l e r. W h e r e a s t h e s p e c i f i c i o n i z a t i o n i s a b o u t io n p a i r s p e r m i c r o n i n w a t e r f o r a l p h a p a r t i c l e s o f 4 M e V it i s o n ly a b o u t 5 f o r b e t a p a r t i c l e s o f 1 M e V. 17

20 T h u s, f o r a g i v e n e n e r g y, b e t a p a r t i c l e s h a v e a m u c h g r e a t e r r a n g e t h a n a l p h a p a r t i c l e s. I n a d d it io n, b e c a u s e o f t h e i r s m a l l m a s s, b e t a p a r t i c l e s u n d e r g o f r e q u e n t s c a t t e r i n g w ith l i t t l e l o s s o f e n e r g y, a n d th u s f o ll o w t o r t u o u s p a t h s. T h i s c a n c a u s e a p r o c e s s a n a l o g o u s to r e f l e c t i o n f r o m s u r f a c e s. T h i s p r o c e s s i s r e f e r r e d to a s b a c k - s c a t t e r i n g a n d t h e e x t e n t o f b a c k s c a t t e r i n g i n c r e a s e s w ith t h e a t o m i c n u m b e r o f t h e s u r f a c e m a t e r i a l. B e t a p a r t i c l e s a r e a t t e n u a t e d e x p o n e n t i a l l y f o r t h e g r e a t e r p a r t o f t h e i r m a x i m u m r a n g e. I t h a s b e e n o b s e r v e d t h a t f o r li g h t e l e m e n t s t h e r a n g e f o r b e t a p a r t i c l e s ( m e a s u r e d in g / c m 2 ) i s a l m o s t in d e p e n d e n t o f t h e n a t u r e o f t h e a b s o r b e r. T h e r e l a t i o n s h i p b e t w e e n r a n g e ( g / c m 2 ) a n d e n e r g y (M e V ) c a n b e e x p r e s s e d a s a n d R = E max f o r E max > 0. 8 M e V R = ( E ^ J 1-38 f o r E max b e t w e e n a n d 0. 8 M e V I n o t h e r w o r d s, f o r p r o t e c t i o n p u r p o s e s, t h e r a n g e e x p r e s s e d in g / c m 2 c a n b e t a k e n to b e a p p r o x i m a t e l y h a l f o f t h e b e t a p a r t i c l e e n e r g y e x p r e s s e d in M e V. T h e e n e r g y l o s s r e s u l t i n g f r o m t h e b r e m s s t r a h l u n g p r o c e s s d e p e n d s o n t h e e n e r g y o f th e b e t a p a r t i c l e a n d o n t h e a t o m i c n u m b e r o f t h e a b s o r b e r m a t e r i a l. T h e r e l a t i v e m a g n it u d e s o f t h e e n e r g y l o s s b y b r e m s s t r a h l u n g a n d th e t o t a l e n e r g y l o s s a r e g i v e n b y th e f o ll o w in g r e l a t i o n : E n e r g y l o s s b y b r e m s s t r a h l u n g _ Z E T o t a l e n e r g y l o s s Z E w h e r e E i s t h e a v e r a g e e n e r g y, w h ic h i s E max/3 f o r th e b e t a e m i t t e r. I t w i l l b e s e e n t h a t f o r a n E max o f a b o u t 2 M e V, t h e b r e m s s t r a h l u n g e n e r g y l o s s i s o n ly 0.7% in l u c i t e c o m p a r e d w ith a v a l u e o f 8% in l e a d. T h u s, i n t h e d e s i g n o f s h i e l d i n g f o r p u r e b e t a e m i t t e r s p a r t i c u l a r a t t e n t i o n s h o u ld b e p a id t o t h e p o s s i b l e p r o d u c t i o n o f b r e m s s t r a h l u n g I n d i r e c t l y io n iz i n g r a d i a t i o n s I n d i r e c t l y io n i z i n g r a d i a t i o n s i n c lu d e s o m e t y p e s o f e l e c t r o m a g n e t i c r a d i a t i o n s a n d n e u t r o n s. T h e s e r a d i a t i o n s i n t e r a c t w ith m a t t e r b y g iv in g r i s e t o s e c o n d a r y r a d i a t i o n w h ic h i s i o n iz i n g. I n d i r e c t l y i o n i z in g r a d i a t i o n s l o s e e n e r g y b y c o l l i s i o n s w ith e l e c t r o n s, o r a t o m i c n u c l e i, a n d t h e c h a r g e d p a r t i c l e s th u s s e t in m o t i o n i n t e r a c t i n t u r n w ith t h e o r b i t a l e l e c t r o n s o r n u c l e i. 1 8

21 T A B L E 3.1. E F F E C T S I N T E R A C T I O N M O D E S A N D T H E I R P O S S I B L E Kinds of interaction Results of interaction 1. In tera ctio n w ith a to m ic electro n s (a) C o m p lete absorption 2. Interaction with nucleons (b) Elastic scattering 3. Interaction with e lectric field (c) Inelastic scattering surrounding the nuclei or electrons 4. Interaction with the nuclear field X - a n d g a m m a - r a y s E l e c t r o m a g n e t i c r a d i a t i o n i s c l a s s i f i e d a c c o r d i n g to i t s o r i g i n, i n d e p e n d e n t l y o f i t s e n e r g y. T h e p r o d u c t i o n o f c o n t in u o u s X - r a y s o r b r e m s s t r a h l u n g h a s b e e n d e s c r i b e d e a r l i e r. C h a r a c t e r i s t i c X - r a y s a r e e m i t t e d in a t o m i c t r a n s i t i o n s o f b o u n d e l e c t r o n s b e t w e e n t h e v a r i o u s e l e c t r o n i c s h e l l s i n t h e a t o m. A n n i h i l a t i o n r a d i a t i o n i s p r o d u c e d b y t h e i n t e r a c t i o n o f p o s i t r o n s a n d e l e c t r o n s, w h e r e b y t h e m a s s e s o f t h e tw o p a r t i c l e s a r e c o m p l e t e l y c o n v e r t e d in to e n e r g y i n a c c o r d a n c e w ith E i n s t e i n ' s m a s s - e n e r g y r e l a t i o n s h i p. H o w e v e r, t h e m e c h a n i s m o f i n t e r a c t i o n o f r a d i a t i o n s o f t h e a b o v e - m e n t i o n e d t y p e s w ith m a t t e r a r e d e p e n d e n t o n ly o n t h e i r e n e r g y a n d n o t o n t h e i r o r i g i n. T h i s i s a n i m p o r t a n t f a c t o r t o b e b o r n e i n m in d w h i l e p l a n n in g r a d i a t i o n p r o t e c t i o n. T h e r e a r e a n u m b e r o f w a y s in w h i c h e l e c t r o m a g n e t i c r a d i a t i o n m a y i n t e r a c t w ith m a t t e r. T h e m o d e s o f i n t e r a c t i o n a n d t h e i r p o s s i b l e e f f e c t s a r e l i s t e d in T a b l e 3.1. T h e tw o c o l u m n s in T a b l e 3.1. c a n b e c o m b i n e d in 1 2 d i f f e r e n t w a y s. H o w e v e r, o n ly t h r e e o f t h e s e p r o c e s s e s a r e o f i m p o r t a n c e i n t h e i n t e r a c t i o n o f X - a n d g a m m a - r a y s w ith m a t t e r. T h e s e a r e t h e p h o t o e l e c t r i c e f f e c t (1 (a )), t h e C o m p t o n e f f e c t (1 (b )) a n d p a i r p r o d u c t i o n (3 (a )) P h o t o e l e c t r i c e f f e c t : T h e m o s t i m p o r t a n t e n e r g y l o s s m e c h a n i s m f o r l o w - e n e r g y p h o t o n s in t h e r a n g e o f h u n d r e d s o f e l e c t r o n v o l t s i s t h e p h o t o e l e c t r i c e f f e c t. I n t h i s p r o c e s s t h e lo w - e n e r g y p h o t o n i n t e r a c t s w it h a b o u n d e l e c t r o n i n o n e o f t h e v a r i o u s s h e l l s o f t h e a t o m a n d d i s a p p e a r s b y t h e t r a n s f e r o f i t s e n t i r e e n e r g y 19

22 to t h e e l e c t r o n, w h ic h i s t h e n e j e c t e d f r o m th e a t o m a s a p h o t o e l e c t r o n. T h e k i n e t i c e n e r g y T o f t h e e l e c t r o n i s g i v e n b y T = h v - $ w h e r e h v i s t h e p h o t o n e n e r g y a n d $ t h e b in d in g e n e r g y o f t h e e l e c t r o n. T h u s, f o r t h e p h o t o e l e c t r i c e f f e c t to o c c u r, t h e p h o t o n e n e r g y m u s t b e g r e a t e r t h a n t h e b in d in g e n e r g y. T h e i m p o r t a n t f e a t u r e s o f th e p h o t o e l e c t r i c e f f e c t a r e th e f o llo w in g : (a) (b) T h e c r o s s - s e c t i o n f o r t h i s p r o c e s s d e c r e a s e s w ith i n c r e a s i n g p h o t o n e n e r g y a n d a t h i g h e r e n e r g i e s t h i s p r o c e s s p l a y s a n i n s i g n i f i c a n t r o l e, o t h e r p r o c e s s e s l i k e t h e C o m p t o n e f f e c t b e i n g m o r e i m p o r t a n t. F u r t h e r, t h e c r o s s - s e c t i o n f o r t h i s p r o c e s s i n c r e a s e s w ith i n c r e a s i n g a t o m i c n u m b e r o f t h e a b s o r b e r. F o r l e a d, th e p h o t o e l e c t r i c e f f e c t i s s i g n i f i c a n t u p to a b o u t 1 M e V. T h e p r o c e s s i s m o s t l i k e l y to o c c u r w h e n t h e p h o t o n e n e r g y i s s l i g h t l y h i g h e r t h a n th e b in d in g e n e r g y. T h e m o s t l i k e l y e l e c t r o n t o b e d i s lo d g e d i s t h a t w h i c h i s m o s t t i g h t l y b o u n d in t h e a t o m, i.e. t h e K e l e c t r o n, p r o v i d e d th e p h o t o n e n e r g y i s s u f f i c i e n t t o r e m o v e i t f r o m i t s o r b i t C o m p t o n e f f e c t : W h e r e a s t h e p h o t o e l e c t r i c e f f e c t o c c u r s in t h e c a s e o f a b o u n d e l e c t r o n, t h e C o m p t o n e f f e c t c a n o c c u r w ith a f r e e o r l o o s e l y b o u n d e l e c t r o n. I n t h i s p r o c e s s, t h e i n c i d e n t p h o t o n u n d e r g o e s a n e l a s t i c c o l l i s i o n w ith a f r e e o r l o o s e l y b o u n d e l e c t r o n a n d s h a r e s i t s e n e r g y a n d m o m e n t u m w ith t h e e l e c t r o n, w h i c h i s t h e n a c c e l e r a t e d a n d t h e p h o t o n i s d e f l e c t e d w ith l o w e r e n e r g y. C o m p t o n s c a t t e r i n g c a n n o t b e c h a r a c t e r i z e d e x c l u s i v e l y a s a n a b s o r p t i o n p r o c e s s s i n c e t h e s c a t t e r e d p h o t o n s m a y n o t b e a p p r e c i a b l y d e f l e c t e d o r d e g r a d e d i n e n e r g y. T h e C o m p t o n e f f e c t d e p e n d s o n t h e n u m b e r o f e l e c t r o n s p r e s e n t in t h e m a t e r i a l w h i c h t h e p h o t o n s t r a v e r s e. T h i s p r o c e s s i s th e d o m in a n t a b s o r p t i o n p r o c e s s f o r i n t e r m e d i a t e e n e r g y g a m m a r a y s. I n t h e c a s e o f l e a d, t h i s p r o c e s s p r e d o m i n a t e s in th e e n e r g y r a n g e 1-5 M e V ; t h e c o r r e s p o n d i n g e n e r g y r a n g e f o r a l u m i n i u m i s M e V. T h e c r o s s - s e c t i o n f o r C o m p t o n i n t e r a c t i o n d e c r e a s e s m o n o t o n i c a l l y w ith i n c r e a s i n g p h o t o n e n e r g y P a i r p r o d u c t i o n : A t p h o t o n e n e r g i e s e x c e e d i n g 1.02 M e V, t h e p h o t o n m a y i n t e r a c t e i t h e r w ith t h e C o u l o m b f i e l d o f t h e n u c l e u s o r, l e s s f r e q u e n t l y, w ith t h a t o f a n e l e c t r o n to p r o d u c e a p o s i t r o n - e l e c t r o n p a i r. T h i s p r o c e s s c a n b e r e g a r d e d a s th e i n v e r s e o f t h e 20

23 a n n i h i l a t i o n p r o c e s s d e s c r i b e d e a r l i e r. A n y e n e r g y o f t h e p h o t o n i n e x c e s s o f 1.02 M e V a p p e a r s a s th e k i n e t i c e n e r g y o f t h e tw o p a r t i c l e s c r e a t e d. T h e p o s i t r o n c r e a t e d in t h i s p r o c e s s w i l l, a f t e r s l o w in g d o w n, b e a n n i h i l a t e d w ith t h e e m i s s i o n o f tw o p h o t o n s. T h e s e p h o t o n s, e a c h o f M e V, a r e e j e c t e d n e a r l y in o p p o s i t e d i r e c t i o n s. I t m a y f u r t h e r b e n o te d t h a t t h e s e p h o t o n s a r e e m i t t e d i s o t r o p i c a l l y. T h e c r o s s - s e c t i o n f o r t h e p a i r p r o d u c t i o n p r o c e s s v a r i e s f r o m e l e m e n t to e l e m e n t, a p p r o x i m a t e l y, a s Z 2. I t i n c r e a s e s w ith i n c r e a s i n g p h o t o n e n e r g y a n d b e c o m e s a p r e d o m i n a n t m o d e o f i n t e r a c t i o n a t a b o u t 1 0 M e V f o r e l e m e n t s o f h ig h a t o m i c n u m b e r (e.g., l e a d ) a n d a t m u c h h i g h e r e n e r g i e s f o r e l e m e n t s o f lo w a t o m i c n u m b e r (e.g., a l u m i n i u m ) N e u t r o n s T h e i n t e r a c t i o n o f n e u t r o n s w ith m a t t e r i s q u it e d i f f e r e n t f r o m t h a t o f e i t h e r c h a r g e d p a r t i c l e s o r g a m m a r a y s. D e p e n d in g o n t h e i r e n e r g y, n e u t r o n s i n t e r a c t w ith m a t t e r b y v a r i o u s p r o c e s s e s. (a) E l a s t i c s c a t t e r i n g : T h e n e u t r o n s h a r e s i t s i n i t i a l k i n e t i c e n e r g y w ith t h e n u c l e u s, w h i c h s u f f e r s a r e c o i l o n ly a n d i s n o t l e f t in a n e x c i t e d s t a t e. T h e s m a l l e r t h e m a s s o f t h e n u c l e u s, th e g r e a t e r t h e f r a c t i o n o f t h e k i n e t i c e n e r g y t a k e n b y i t. T h e a v e r a g e f r a c t i o n o f t h e n e u t r o n e n e r g y t r a n s f e r r e d p e r c o l l i s i o n to a m e d i u m o f a t o m i c w e ig h t A i s g i v e n b y 2 A / ( 1 + A ) 2. A 2 - M e V n e u t r o n g e t s t h e r m a l i z e d in a b o u t 1 8 c o l l i s i o n s i n w a t e r a n d in a b o u t c o l l i s i o n s i n l e a d. (b) I n e l a s t i c s c a t t e r i n g : I n e l a s t i c s c a t t e r i n g i s p o s s i b l e o n ly w ith f a s t n e u t r o n s : t h e s c a t t e r e d n e u t r o n c a r r i e s l e s s e n e r g y t h a n t h e i n c i d e n t n e u t r o n a n d th e n u c l e u s g o e s in to a n e x c i t e d s t a t e. T h e e x c i t e d n u c l e u s e i t h e r e m i t s a g a m m a r a y o r r e m a i n s in a m e t a s t a b l e s t a t e. (c ) C a p t u r e : T h e i n c i d e n t n e u t r o n i s c a p t u r e d b y th e t a r g e t n u c l e u s f o r m i n g a c o m p o u n d n u c l e u s w h ic h m a y b e e x c i t e d a n d e m i t g a m m a r a d i a t i o n. T h i s r e a c t i o n i s p r o b a b l y t h e m o s t c o m m o n s i n c e t h e r m a l n e u t r o n s c a n i n d u c e t h i s r e a c t i o n in n e a r l y a l l n u c l i d e s. T h e e x c i t a t i o n e n e r g y o f t h e t a r g e t n u c l e u s m a y b e e m i t t e d i n a s i n g l e p h o t o n o r in s e v e r a l. E v e r y s u c h c a p t u r e r e s u l t s i n e n e r g y e m i s s i o n a m o u n t in g to a b o u t 6 t o 1 0 M e V. H e n c e, m a t e r i a l s i n w h ic h n e u t r o n c a p t u r e i s a l l o w e d to t a k e p l a c e f o r p u r p o s e s o f a t t e n u a t i o n a r e s o c h o s e n t h a t, a s a r e s u l t o f t h e c a p t u r e, c h a r g e d p a r t i c l e s o r p h o t o n s a r e e m i t t e d t h a t 21

24 c a n b e e a s i l y a b s o r b e d. C a d m i u m a n d b o r o n a r e c o m m o n l y u s e d f o r c a p t u r i n g t h e r m a l n e u t r o n s. (d) P a r t i c l e e m i s s i o n : I n t h i s t y p e o f r e a c t i o n t h e i n t e r a c t i o n o f t h e i n c i d e n t n e u t r o n w ith t h e t a r g e t n u c l e u s m a y l e a d t o t h e e m i s s i o n o f p a r t i c l e s s u c h a s p r o t o n s, n e u t r o n s a n d a l p h a s. S i n c e th e c h a r g e d p a r t i c l e s w i l l h a v e to o v e r c o m e t h e C o u l o m b b a r r i e r b e f o r e e s c a p i n g th e n u c l e u s, t h i s t y p e o f r e a c t i o n i s m o s t p r o b a b l e f o r lig h t n u c l i d e s a n d f a s t n e u t r o n s. (e ) F i s s i o n : I n t h i s p r o c e s s t h e c o m p o u n d n u c l e u s s p l i t s in to tw o f i s s i o n f r a g m e n t s w ith t h e e m i s s i o n o f o n e o r m o r e n e u t r o n s. F i s s i o n r e a c t i o n s t a k e p l a c e w ith t h e r m a l n e u t r o n s i n 235U, 239P u a n d 233U a n d w ith f a s t n e u t r o n s i n m a n y h e a v y n u c l i d e s. E s s e n t i a l l y, t h e a b s o r p t i o n o f n e u t r o n s o c c u r s in tw o d i s t i n c t s t a g e s. F a s t n e u t r o n s a r e s lo w e d d o w n b y e l a s t i c a n d i n e l a s t i c s c a t t e r i n g p r o c e s s e s w ith n u c l e i, p a r t i c u l a r l y l ig h t n u c l e i l i k e c a r b o n a n d h y d r o g e n. T h e s l o w e d - d o w n n e u t r o n s a r e t h e n c a p t u r e d, a s t h e c a p t u r e c r o s s - s e c t i o n f o r l o w - e n e r g y n e u t r o n s i s h ig h f o r m o s t e l e m e n t s A t t e n u a t i o n, q u a l i t y a n d c r o s s - s e c t i o n A t t e n u a t i o n W h e n r a d i a t i o n p a s s e s t h r o u g h m a t t e r i t s u f f e r s a r e d u c t i o n i n i n t e n s i t y a s a r e s u l t o f c o m p l e x i n t e r a c t i o n s b e t w e e n t h e r a d i a t i o n a n d t h e m a t e r i a l c o n c e r n e d. T h i s r e d u c t i o n in i n t e n s i t y i s c a l l e d a t t e n u a t i o n. T h e i n t e n s i t y o f r a d i a t i o n ( o r r a d i a n t e n e r g y f lu x d e n s i t y ) a t a g i v e n p o i n t i s d e f in e d a s t h e e n e r g y p e r u n i t t i m e e n t e r i n g a s m a l l s p h e r e o f u n it c r o s s - s e c t i o n a l a r e a c e n t r e d o n t h a t p o in t. I t i s m e a s u r e d in t e r m s o f e r g s p e r s q u a r e c e n t i m e t r e p e r s e c o n d o r w a t t s p e r s q u a r e c e n t i m e t r e. T h e d e g r e e o f a t t e n u a t i o n d e p e n d s u p o n t h e t y p e o f r a d i a t i o n a n d t h e m a t e r i a l u s e d. M a t e r i a l s u s e d f o r p u r p o s e s o f r a d i a t i o n a t t e n u a t i o n a r e c a l l e d s h i e l d i n g m a t e r i a l s a n d i n t h i s c o n n e c t i o n th e t e r m s " h a l f - v a l u e t h i c k n e s s ( H V T ) " a n d " t e n t h - v a l u e t h i c k n e s s " a r e o f t e n u s e d. T h e h a l f - v a l u e t h i c k n e s s o f a m a t e r i a l f o r a c e r t a i n q u a l i t y o f r a d i a t i o n i s t h e t h i c k n e s s r e q u i r e d t o r e d u c e t h e i n t e n s i t y o f t h a t q u a li t y o f r a d i a t i o n b y o n e h a l f s i m i l a r l y, t h e t e n t h - v a l u e t h i c k n e s s i m p l i e s a n i n t e n s i t y r e d u c t i o n b y a f a c t o r o f 1 0. I n th e c a s e o f a l p h a a n d b e t a p a r t i c l e s, d e f i n it e t h i c k n e s s e s o f s h i e l d i n g m a t e r i a l s a r e s u f f i c i e n t t o s to p t h e s e p a r t i c l e s c o m p l e t e l y. H o w e v e r, 22

25 f o r g a m m a r a y s, t h e r a d i a t i o n i s a b s o r b e d e x p o n e n t i a l l y a n d th e i n t e n s i t y I a t a n y p o in t i s g i v e n b y t h e r e l a t i o n I = I 0 e-f w h e r e Io i s t h e i n i t i a l i n t e n s i t y o f t h e b e a m, I th e i n t e n s i t y o f t h e b e a m e m e r g i n g t h r o u g h a m a t e r i a l o f t h i c k n e s s x, a n d p t h e l i n e a r a b s o r p t i o n c o e f f i c i e n t f o r t h a t m a t e r i a l. T h e H V T c a n b e s e e n to b e e q u a l to /ju. T h e a t t e n u a t i o n f o r a g i v e n r a d i a t i o n i n c r e a s e s w ith i n c r e a s i n g a t o m i c n u m b e r a n d d e n s i t y o f t h e s h i e l d i n g m a t e r i a l. T h u s, f o r g a m m a r a y s, l e a d i s a s u i t a b l e s h i e l d i n g m a t e r i a l Q u a l i t y o f r a d i a t i o n T h e e x a c t s p e c i f i c a t i o n o f t h e " q u a l i t y " o f r a d i a t i o n i s p o s s i b l e in t h e c a s e o f a m o n o e n e r g e t i c b e a m, o r in t h e c a s e o f h e t e r o g e n e o u s b e a m s w h e r e t h e s p e c t r a l d i s t r i b u t i o n s o f t h e v a r i o u s e n e r g i e s p r e s e n t a r e k n o w n. F o r t u n a t e l y, h o w e v e r, in t h e c a s e o f h e t e r o g e n e o u s b e a m s i t i s u n n e c e s s a r y t o s p e c i f y t h e " q u a l i t y " f o r m o s t a p p l i c a t i o n s. I n s u c h c a s e s, a n " e f f e c t i v e e n e r g y " o f t h e h e t e r o g e n e o u s b e a m i s s p e c i f i e d, w h i c h i s t h a t e n e r g y o f a m o n o e n e r g e t i c b e a m w h i c h h a s t h e s a m e h a l f - v a l u e t h i c k n e s s a s t h e h e t e r o g e n e o u s b e a m i n q u e s t i o n. H o w e v e r, t h e e f f e c t i v e e n e r g y o f a h e t e r o g e n o u s b e a m d e r i v e d f r o m t h e h a l f - v a l u e t h i c k n e s s d o e s n o t c o i n c i d e w ith a n d i s i n v a r i a b l y l o w e r t h a n t h e m e a n e n e r g y m o r e c o r r e c t l y d e d u c e d f r o m t h e d e t a i l e d s p e c t r u m. A n e v e n b e t t e r r e p r e s e n t a t i o n o f th e q u a li t y o f a h e t e r o g e n e o u s b e a m ( e. g., X - r a y ) c o u ld b e g i v e n b y s p e c i f y i n g t h e r a t i o o f t h e f i r s t a n d t h e s e c o n d H V T s f o r t h e h e t e r o g e n e o u s b e a m in q u e s t i o n. I t m u s t b e n o te d h o w e v e r t h a t t h e s e c o n d H V T r e f e r r e d to h e r e i s t h a t H V T w h i c h i s d e t e r m i n e d f o r t h e s p e c t r a l q u a li t y o f t h e r a d i a t i o n w h ic h e m e r g e s a f t e r p a s s a g e th r o u g h t h e f i r s t H V T C r o s s - s e c t i o n I n a l l t h e p r o c e s s e s d e s c r i b e d a b o v e, t h e c o n c e p t o f " c r o s s - s e c t i o n " i s a n i m p o r t a n t o n e. T h e c r o s s - s e c t i o n i s t h e e f f e c t i v e a r e a o r c r o s s - s e c t i o n w h i c h a t a r g e t e n t it y ( s u c h a s a n a t o m ) p r e s e n t s t o a b o m b a r d i n g p a r t i c l e o r p h o t o n f o r t h e o c c u r r e n c e o f t h e p r o c e s s i n q u e s t i o n, a n d e x p r e s s e s t h e p r o b a b i l i t y o f t h e p r o c e s s. T h i s a r e a i s n o t n e c e s s a r i l y e q u a l to t h e g e o m e t r i c a l a r e a o f t h e t a r g e t. C r o s s - s e c t i o n i s e x p r e s s e d i n b a r n s, 1 b a r n b e i n g 1CT24 c m. 23

26 4. INTERACTION OF RADIATION WITH LIVING CELLS 4.1. T h e c e l l E a c h o r g a n o f t h e liv in g b o d y i s c o m p o s e d o f t i s s u e s w h ic h a r e m a d e u p o f c e l l s o f v a r i o u s t y p e s. T h e l i v i n g c e l l, w h i c h i s th e u n it o f o u r c o m p l e x b i o l o g i c a l s y s t e m, i s in i t s e l f a h ig h ly c o m p l e x e n t i t y. I t i s c o m p o s e d o f p r o t o p l a s m, w h i c h i s p r i m a r i l y w a t e r, c o n t a in in g a v a r i e t y o f c o m p o u n d s i n c lu d in g i n o r g a n i c s a l t s, c a r b o h y d r a t e s, f a t s, a m i n o a c i d s a n d p r o t e i n s. W i t h i n e a c h c e l l i s a n u c l e u s w h i c h c o n t a i n s a c h a r a c t e r i s t i c n u m b e r o f c h r o m o s o m e s. T h e s e c h r o m o s o m e s c a r r y th e g e n e t i c m a t e r i a l w h ic h i s d e o x y r i b o n u c l e i c a c i d (D N A ). T h e n u c l e u s i s e s s e n t i a l f o r s u s t a i n e d l i f e o f t h e c e l l a n d f o r i t s r e p r o d u c t i o n. T h e p r o t o p l a s m o u t s i d e t h e n u c l e u s i s c a l l e d c y t o p l a s m ; i t c o n t a i n s m a n y p a r t i c l e s, d i s s o l v e d fo o d m a t e r i a l s, a n d e n z y m e s w h ic h a r e u t il i z e d i n m e t a b o l i s m. T h e r e i s o n e m e m b r a n e a r o u n d th e n u c l e u s a n d a n o t h e r a r o u n d t h e e n t i r e c e l l, b o t h o f w h i c h h a v e s e l e c t i v e p e r m e a b i l i t y p r o p e r t i e s A c t i o n o f i o n i z i n g r a d i a t i o n o n c e l l s T h e a b s o r p t i o n o f io n i z in g r a d i a t i o n b y c e l l s i s fo ll o w e d b y t h e p r o d u c t i o n o f i o n i z a t i o n a n d e x c i t a t i o n. T h e i o n i z e d a n d e x c i t e d a t o m s a n d m o l e c u l e s r e a r r a n g e t h e m s e l v e s to f o r m s t a b l e o r u n s t a b l e m o l e c u l e s o r f r e e r a d i c a l s, th u s c a u s i n g n e w c h e m i c a l r e a c t i o n s to t a k e p l a c e w ith a d ja c e n t m o l e c u l e s. S u c h c h a n g e s in a n y p a r t o f t h e c o m p l e x s t r u c t u r e o f t h e c e l l c o u ld r e s u l t i n a n y o f a s e r i e s o f h a r m f u l e f f e c t s, s u c h a s in h i b i t i o n o f c e l l d i v i s i o n, i m p a i r e d f u n c t i o n o f t h e c e l l, c e l l d e a t h o r a l t e r a t i o n i n t h e g e n e s t r u c t u r e o f r e p r o d u c t i v e c e l l s w h i c h c o u ld u l t i m a t e l y p r o d u c e g e n e t i c c h a n g e s. T h e e x t e n t o f t h e d a m a g e d e p e n d s o n t h e a m o u n t, r a t e a n d m e c h a n i s m o f e n e r g y a b s o r p t i o n, an d i s c u m u l a t i v e o v e r e x t e n d e d i n t e r v a l s o f t i m e M e c h a n i s m o f r a d i a t i o n i n ju r y a n d r e p a i r T h e m e c h a n i s m o f r a d i a t i o n i n ju r y i s a c o m p l e x p r o c e s s. T h e r e l a t i v e i m p o r t a n c e o f t h e p a r t o f t h e c e l l i m p a i r e d b y r a d i a t i o n d e t e r m i n e s w h e t h e r o r n o t th e c e l l i s a f f e c t e d. I f, f o r e x a m p l e, o n e o f t h e s e v e r a l p r o t e i n m o l e c u l e s in t h e c e l l i s i n a c t i v a t e d b y r a d i a t i o n, t h e c e l l m a y s u r v i v e t h e d a m a g e. O n th e o t h e r h a n d, i f t h e m o l e c u l e a f f e c t e d i s t h e D N A, w h i c h i s v e r y e s s e n t i a l f o r 24

27 t h e c e l l f u n c t io n i n g, t h e e f f e c t c o u ld b e l e t h a l. T h i s f a c t i s c le a r ly - b o r n e o u t b y e x p e r i m e n t a l f i n d i n g s t h a t a h a p lo id c e l l c o n t a i n in g o n ly o n e s e t o f c h r o m o s o m e s ( g e n e s ) i s m o r e s e n s i t i v e to r a d i a t i o n t h a n a d ip lo id c e l l o f th e s a m e s p e c i e s w h i c h c o n t a i n s tw o s e t s o f c h r o m o s o m e s. T h e e x a c t d e t e r m i n a t i o n o f th e d a m a g e i s c o m p l i c a t e d b y s u c h f a c t o r s a s t h e i n t e r r e l a t i o n s o f t h e c e l l s in t h e t i s s u e s, t i s s u e r e p a i r p r o c e s s e s an d o t h e r r e a c t i o n s w h ic h a r e s e c o n d a r y in n a t u r e. T i s s u e r e p a i r c a n o c c u r in tw o w a y s. O n e i s t h e r e c o v e r y o f p a r t i a l l y d a m a g e d c e l l s a n d t h e o t h e r i s t h e r e p l a c e m e n t o f t h e f u ll y o r p a r t i a l l y d a m a g e d c e l l s. H o w e v e r, t h e s e r e p a i r p r o c e s s e s c o u ld t h e m s e l v e s b e a f f e c t e d b y r a d i a t i o n - i n d u c e d d a m a g e t o t h e r e c o v e r y o r r e p a i r m e c h a n i s m i t s e l f D i r e c t a n d i n d i r e c t a c t i o n o f r a d i a t i o n D i r e c t a c t i o n o f r a d i a t i o n o n c e l l s o r o t h e r m o l e c u l e s o f i n t e r e s t r e f e r s to t h e p r i m a r y i o n i z a t i o n o r e x c i t a t i o n e v e n t s p r o d u c i n g d i r e c t d a m a g e in t h e s e c e l l s o r m o l e c u l e s i n d e p e n d e n t o f t h e n a t u r e o f th e s u r r o u n d i n g m e d i u m. T h i s d i r e c t e f f e c t i s r e a d i l y o b s e r v e d in p r o c e s s e s s u c h a s i n a c t i v a t i o n o f (a) e n z y m e s i n t h e d r y s t a t e, (b) b a c t e r i a l s p o r e s, a n d (c ) w e t c e l l s i n t h e f r o z e n s t a t e. S u c h i n a c t i v a t i o n b y d i r e c t a c t i o n c a n b e e x p l a i n e d b y t h e t a r g e t t h e o r y w h ic h a s s u m e s t h a t b i o l o g i c a l e f f e c t s r e s u l t o n ly w h e n o n e o r m o r e i o n i z a t i o n s ( h its ) o c c u r in, o r in t h e i m m e d i a t e v i c i n i t y o f, t h e s e n s i t i v e s i t e. T h i s t h e o r y h a s b e e n s u c c e s s f u l l y u s e d to e x p l a i n t h e r a d i a t i o n - i n d u c e d c h e m i c a l c h a n g e s in o r g a n i c s u b s t a n c e s, a n d t h e i n a c t i v a t i o n o f m a c r o m o l e c u l e s, v i r u s e s a n d b a c t e r i a. T h e i n d i r e c t e f f e c t i s d u e t o t h e i n t e r a c t i o n o f i n t e r m e d i a t e c h e m i c a l p r o d u c t s c r e a t e d in t h e a q u e o u s m e d i u m i n t h e c e l l. T h i s i n v o l v e s t h e p r o d u c t i o n o f t h e h y d r a t e d e l e c t r o n ( e q), H, O H, a n d H a n d o t h e r r e a c t i v e s p e c i e s. T h e s e d i f f u s e t h r o u g h t h e w a t e r m e d i u m a n d r e a c t w ith t h e c r i t i c a l m o l e c u l e s i n t h e c e l l. C o n t r i b u tio n t o t h e b i o l o g i c a l e f f e c t f r o m i n d i r e c t a c t i o n c a n b e c l e a r l y d e m o n s t r a t e d b y o b s e r v i n g t h e r e d u c t i o n i n r a d i o s e n s i t i v i t y w h e n s c a v e n g e r s o f t h e s e r a d i c a l s a r e in c l u d e d i n t h e m e d i u m. S e n s i t i z a t i o n b y a f a c t o r o f a b o u t 2. 5, w h ic h i s c o m m o n l y o b s e r v e d w h e n d i s s o l v e d o x y g e n i s p r e s e n t i n t h e m e d i u m a s o p p o s e d t o a n o x i c c o n d i t io n s, i s a t t r i b u t e d t o t h e e n h a n c e d p r o d u c t i o n o f h y d r o g e n p e r o x i d e a n d o t h e r o r g a n i c p e r o x i d e s i n t h e p r e s e n c e o f o x y g e n. T h e tw o m e c h a n i s m s d e s c r i b e d a b o v e a r e n o t m u t u a l l y e x c l u s i v e r a t h e r, t h e y m a y b e c o m p l e m e n t a r y. I n b o t h d i r e c t a n d i n d i r e c t e f f e c t s, a c h a i n o f c h e m i c a l r e a c t i o n s i s i n d u c e d w h ic h 25

28 may result in a significant biological effect. O n the basis of these mechanisms, the observations described in the following paragraphs may be expected Dose-effect relationship (a) The effect of radiation on some specified biological function, e.g. survival, is usually studied to determine the relationship of response to increasing dose, and the interrelationship plotted as a survival curve. When loss of biological activity results from the passage of a charged particle through (or near) the biological target, or when the inactivation of a molecule is caused by a radical, the survival curve will be exponential. Inactivation of macromolecules, viruses and bacteria follows this pattern. (b) (c) W hen the passage of several ionizing particles through the sensitive targets is required (multi-hits), or several targets are required to be inactivated before a specified biological function can be impaired, a shouldered exponential survival curve (sigmoid) is expected. Such survival curves are common with cells of higher organisms. Impairment of the reproductive capacity of mamm alian cells, chromosome aberration frequencies and survival of organisms as a whole follow this pattern. Quantitation of survival curves for these systems is complicated because of the existence of repair mechanisms and other factors involved. Identical doses of different qualities and types of radiation would produce different biological effects because of the different values of the linear energy transfer ( L E T ) along the paths of the different qualities and types of radiation (L E T being defined as the linear rate of energy loss by an ionizing particle traversing a medium) and the space distribution of the effective primary events. In the case of molecular, viral and bacterial inactivation, the biological effectiveness stays constant at low L E T s and continuously decreases in the high L E T region. For m am m alian cells, the biological effectiveness gradually increases with increasing L E T, reaches a m axim um for very heavily ionizing radiations and then decreases with further increase in L E T. F r o m consideration of the above mechanisms it is obvious that certain physical factors, such as the type, quality and quantity of radiation, its distribution in time, its distribution over the body and whether the radiation source is external or internal to the body, 26

29 influence the biological effects. These are also dependent upon biological factors, such as the degree of oxygenation and water content of a tissue, and its metabolic state. In the case of an internally deposited radionuclide, the emission of radiation is often accompanied by recoil effects or transmutation into an atom having new chemical properties. For example, incorporated 32P is transmuted to 32S and this chemical change in an important place in a macromolecule may have serious consequences for the vitality of the cell. The dose-effect relationship is different for different types of radiation effects in higher organisms and m an. Genetic effects are linearly related to the radiation dose. Other effects, such as the production of chromosome breakage and subsequent adhesion or lumping, are highly dependent on the dose-rate the effect is greater the higher the intensity Somatic and genetic effects Somatic effects Somatic effects relate to injuries to cells which are concerned with the maintenance of the body functions, such as cells in the blood and bone m arrow. Some of the somatic effects arising from radiation exposures are listed below: (a) Local acute effects (1) Skin damage, erythema, epilation, and necrosis of the skin and deep-seated tissues (2) Temporary or permanent sterility, caused by irradiation of the gonads (3) Reduced or abnormal reproduction of proliferative tissues, such as the epithelia of the gastro-intestinal tract and the blood-forming tissue (4) Impaired function of the nervous system and other differentiated systems. (b) Generalized acute effects Acute radiation sickness for details please see Table IV of the Annex. 27

30 (c) Delayed effects after either a single high exposure or chronic exposure (1) Chronic skin damage, which may be ulcerous or cancerous in form (2) Local atropic scars or dystrophic processes in heavily irradiated organs and tissues (3) Cataract of the lens of the eye (4) Bone sarcoma due to irradiation of the bone tissue (5) Cancer of the lung caused by inhalation and deposition of radioactive material in the lung (6) Aplastic anaemias caused by radiation damage in the bone marrow (7) Leukemia, which is a malignant disease with increased numbers of white blood cells of various kinds. (d) Possible delayed effects (established in animal experiments but not in man) (1) Shortening of the life span and premature aging (2) Increased incidence of tumours other than those already mentioned above. The dose-effect relationship in respect of acute effects im mediately following exposure is reasonably well understood in most cases; in general, it is possible to specify a m inimum dose and a minimum dose-rate which will bring about an observable effect. For delayed effects, however, this relationship is very poorly understood at present. For certain effects, such as cancer of the skin, bone or lungs, or the cataract of the eye, high doses are necessary. In the case of effects like aplastic anaemia and leukemia, it is not known whether a threshold dose exists at all, Genetic effects Genetic effects relate to injuries to cells in the gonads which are responsible for the propagation of genetic characteristics to subsequent generations. As mentioned earlier, the tissues of the gonads are more radiosensitive. Irradiation of the germ cells may cause mutations which manifest themselves in later generations. Mutations, having once occurred, are permanent. The great majority of observed mutations are deleterious. No conclusive answer is available to the question whether a threshold dose exists for mutations. Small doses may be cumulative and the end result may not appear until many generations later. 28

31 S. R A D I A T I O N U N I T S A N D A S S O C I A T E D C O N C E P T S 5.1. Introduction to radiation units W hen ionizing radiation passes through matter it interacts with the atoms and molecules in the medium it traverses, producing ionizations and excitations. Depending on the medium, the absorbed energy may give rise to observable effects, for example, ionization, photographic effects, biological effects and heating. The energy absorbed may be expressed in ergs per gram, which has led to the concept of radiation dose. In recent years the basic concepts governing the formulation of units for measurement of radiation have undergone extensive examination and considerable modification. Currently three radiation units are used the roentgen is used for the measurement of exposure, the rad for absorbed dose and the rem for biological dose. Earlier the term "dose" was rather loosely used to specify both exposure dose and absorbed dose. However, according to the present usage, the term "exposure" relates to what was previously understood as the exposure dose, and the term "dose" relates to the concept of absorbed dose. Thus the unit for exposure is the roentgen and that for dose is the rad. The amount of energy absorbed in a layer of material depends on both the quality and quantity of radiation falling on it. In living tissues, in particular, the same amount of absorbed energy from different types of radiation can lead to different biological effects. To take this factor into account it was therefore necessary to introduce a term "relative biological effectiveness (R B E )" and consequently a new unit, rem ( = rad X R B E ), to give the measure of radiation dose received by irradiated living tissues. While the individual values of R B E would undoubtedly relate to specified narrow biological end points (although experimental results in this regard are sparse), the same could not apply in respect of assessing broad effectiveness of various types of radiations on biological systems. Therefore, for the purposes of radiation protection a term quality factor (Q) has been introduced to give a measure of the dose equivalent, thus defining rem =.r a d X Q The roentgen (R) The roentgen (R), as first defined in 1928, was applicable to X-rays only. In 1937, however, it was redefined as follows, so as to include gam m a rays: 29

32 "The roentgen shall be the quantity of X- or gam m a radiation such that the associated corpuscular emission per g of air produces, in air, ions carrying 1 esu of quantity of electricity of either sign. " The roentgen, which is the unit of exposure, is not a radiation unit in the strict sense of the term. It describes neither the number of photons in the beam, nor their energy; it merely gives the effect of that radiation in 1 cm 3 of air at N. T. P. W hen the charge carried by all ions of either sign produced by the charged particles arising in each cm 3 of air is found to be 1 esu, then the original volume of air is said to have been exposed to one roentgen of X- or gam m a radiation. It can easily be shown that one roentgen of X- or gam m a radiation results in the absorption of erg/g of air, but in substances of different atomic number and different density the amount of energy per unit mass for the same quantity of radiation will be different for instance, in soft tissue the energy absorbed per gram is about 98 erg The rad, D The term "roentgen" applies only to measurements in air of X-rays and gam m a energies up to 3 M e V. However, ionization in tissue can also be produced by radiations other than photons, such as alpha particles, beta radiation, neutrons and protons. As radiation has to be measured in terms of the energy absorbed in the medium of interest, it was considered necessary to introduce a unit which would depend only on the amount of energy absorbed per unit mass of irradiated material and not on the energy or type of radiation, or on the nature of the absorber. Hence the International Commission on Radiological Units and Measurements introduced the concept of "absorbed dose" (now called "dose") in 1953 and this is defined as the amount of energy imparted to matter by ionizing particles per unit mass of irradiated material at the place of interest. The rad is the unit of dose and is 100 erg/g. Thus the term radiation dose means one quantity only, i.e. the amount of energy absorbed. It may be noted that in soft tissue one rad is approximately equal to the absorbed dose delivered when such tissue is exposed to one roentgen of medium-energy X-rays The rem, H (dose equivalent) From the biological point of view, evidence has accumulated that the effects of the various types of ionizing radiations are not 30

33 the sam e. One can assume that radiation can bring about a change in a system only by virtue of the energy actually absorbed. A biological effect, however, may also depend upon the spatial distribution of the energy released along the track of the ionizing particle. It will depend therefore on the type and quality of the radiation, and equal energy absorptions of different types of radiations may not produce the same biological effects. Generally, the effect of radiation on cell structures increases with increasing L E T, although certain "all-or-none" effects, like the inactivation of bacteria and viruses, become less efficient per unit energy absorption as the L E T of the radiation increases. The term R B E (relative biological effectiveness) is used to specify the variation in the degrees of effectiveness of different types of radiation and is defined as follows: _ Dose of 250-kV X-rays to produce a certain biological effect Dose of a given radiation to produce the same biological effect The biological effect of a particular type of radiation depends therefore not only on the absorbed dose but also on the R B E of the radiation. To take these factors into account, a new unit, the "rem " (roentgen equivalent man) was defined as that dose of any ionizing radiation which, when delivered to m an or m am m al, is biologically equivalent to the dose of one rad of X- or gam m a radiation. R e m is here taken as the unit of R B E dose. One thus has the relationship: R B E dose in rems = Dose in rads X R B E The use of the term R B E both in radiobiology and in radiation protection presents certain problems. Hence the International C o m mission on Radiological Units and Measurements (ICRU) recommended that the term R B E be used only in radiobiology and that for protection purposes a separate name, quality factor, Q, be used for the linear-energy-transfer-dependent factor. Therefore, the dose equivalent, H, as defined by IC R U in its Report No. 19 is given by: 1 H = D Q N where D is the absorbed dose, Q is the quality factor and N is the product of any modifying factors such as distribution factors, etc. 31

34 The special unit of the dose equivalent is rem. It should be noted that the quantity H may be used when its value is in the region of the m axim um permissible dose equivalent and not for high-level accidental exposures. In the case of radiation dose delivered by particles having the range of L values (L is the linear energy transfer), the dose equivalent may be taken as H = D Q N where oo Q = 5 - / Q D L d L 0 The relation between Q and L E T recommended for radiation protection is given in Table The curie Simultaneously with the roentgen (R), the concept of curie (Ci), which is the unit of radioactivity, was being developed. The curie was originally defined as the disintegration rate from one gram of radium. Later on it was defined as the amount of radon in equilibrium with one gram of radium. Still later, in 1930, it was defined as the amount of any decay product of radium that is in equilibrium with one gram of radium. The term curie is now used for all radionuclides and is defined as the activity of that amount of a substance T A B L E 5.1. Q-L» R E L A T IO N S H I P 3 Lgoin water kev/pm Q 3. 5 or less a From ICRU Rep. 19 (1971). 32

35 which undergoes 3. 7 X 1010 disintegrations per second. There are few pure g am m a emitters, and the emission of one or more photons usually follows charged-particle emission or equivalent processes. Therefore, if photon emission follows a charged-particle emission for the purposes of this definition, the whole process (i.e. a chargedparticle emission plus photon emission) is treated as a single disintegration Specific activity: The "specific activity" of a radioactive material is the concentration of activity measured in C i/g in the material concerned. It is defined as the disintegration rate per unit mass of the source. A radionuclide in a source (whether present only as the element in question or as a mixture, alloy or compound) can be either in a carrier-free state or accompanied by a carrier. In the carrier-free state, all the atoms of the element are those of the radionuclide only. The radionuclide is said to be accompanied by a carrier when only part of the total number of atoms are those of the radionuclide in question and the rest are of one or more of the other isotopes of the same nuclide. These other nuclides are normally inactive, but could in special cases be radioactive. The specific activity of a source either in a carrier-free state or otherwise is thus the activity per unit mass of the source and is expressed in C i/g. Thus, in the case of a carrier-free radionuclide, the specific activity depends only on the half-life of the radionuclide and its atomic weight, and is given by:,, * X l O 8 Specific activity (in Ci/g) = -- rrr=where A is the atomic weight and Tx the half-life of the radionuclide in days. However, in special cases where more than one radionuclide is present (e.g. fission products), the calculation of the specific activity may become very complex, and it is usually determined empirically IC RU recommendations To summarize, the IC RU recommendations specify the use of units to be restricted as follows: 33

36 Rad solely for absorbed dose Roentgen solely for exposure Curie solely for activity R e m for dose equivalent 5.2. Radiation exposure and dose rates Exposure rate A fundamental requirement of radiological protection when one is working with radiation sources is a knowledge of the radiation exposure rate associated with the source being used. For certain radiation sources, the exposure rate at a specified distance is known, but in other cases it is necessary to calculate or measure it. The exposure rates associated with some commonly used radionuclides are given in Table Reduction of radiation intensity with distance Once the exposure rate from a radiation source has been determined at a certain distance, the exposure rate at any other distance in vacuum or in air varies inversely as the square of the distances according to the formula where Dj^ is the exposure rate at a distance d x from the source D2 is the exposure rate at a distance d2 from the source. This formula is accurate only when the dimensions of the source are small compared with the distances involved. For large sources, the calculation of the exposure rate (D) at a distance is more com plicated, and is a function of the solid angle subtended by the source at the point of interest, as given by the formula where D s is the exposure rate at the surface of the source, and & the solid angle subtended by the source at the point of interest. The latter formula assumes that the source is a segment of a spherical shell with the centre at the point at which the exposure rate is required, but it may be used without serious error even 34

37 T A B L E 5.2. G A M M A - R A Y E X P O S U R E R A T E A T O N E M E T R E F R O M A O N E- C U R IE S O U R C E O F V A R IO U S IS O T O P E S Isotope Half-life Gamma-ray energy (MeV) Exposure rate a at 1 metre (R h"1 Ci"1) 22Na 2. 6 yr Na 15 h 1.38, k 12.4 h lCr 27 d Mn 5.7 d 0.73, % n 300 d Fe 45 d 1.1, sCo 72 d 0.50, Co 5.3 yr 1.17, Zn 245 d As 17.5 d A As 27 h Br 36 h ' j 12.6 h I31I 8 d (mainly 0.36) I 2.3 h Cs 30 yr Tm 129 d I82Ta 111 d I92lr 74 d rt Au 2.7 d (mainly 0.41) 0.24 Ra (B+C) Filtered through 0.5 mm Pb 0.83 Exposure rate in mr*h-1-mci-1at 1 ftmay be obtained by multiplying the values in column 4 by

38 when the source presents a convex surface to the point of interest, provided that the height of convexity does not exceed about onefifth of the distance from the source to the point Calculation of dose rate and relation between flux and dose rate The dose rate in rads at a point resulting from any radioactive source is a function of (a) the flux of radiation, measured in particles or photons per unit area in unit time, and (b) the absorption of energy in tissue, measured in M e V per unit distance. The energy absorbed per second in 1 cm3 of tissue in a flux of N particles or photons/cm2 is N A E M e V, where A is the fraction of energy absorbed per cm of path in water (which corresponds to wet tissue), and E is the energy of the particle or photon in M e V (Table 5.3). Dose rate = N A E MeV/cm3-s N A E = --- MeV/g- s, where p is the density of the tissue. Since 1 M e V = 1. 6 X 10"6 erg, 1 h = 3600 s and 1 rad = 100 erg/g, N A E v 1.6X 10"6X 3600,,, Dose rate = --- X rad/h p = N A E X X 10'5 rad/h = N A E X X 10 5X Q F re m /h. T A B L E 5.3. E N E R G Y A B S O R B E D P E R cm O F P A T H IN W A T E R (AE) F O R G A M M A R A Y S O F E N E R G Y E E (MeV) AE (MeV/cm)

39 C alculation of flux For point sources, the calculation of flux is easy; but for sources of complicated shape, or for a collection of sources in any configuration, the calculations require specialized treatment and will not be considered here. To calculate the flux from any source it is necessary to know the decay scheme, i. e. the number of photons or particles emitted per disintegration. For instance, in 60Co there are two gam m a photons per disintegration and both of these must be taken into account in the calculation of the flux Flux from a point source: Most sources used in radiography can be described as point sources. The same description will apply where the distance from the source to the point of interest is large compared with the dimensions of the source (at least five times the m axim um dimension of the source). The flux at a distance from a point source emitting N photons or particles per second in air = ^ T(d) particles or photons/cm2 s where N is the number of particles or photons emitted by the source per second d is the distance in cm from the source to the point of interest T(d) is the transmission through air, which for gam m a rays may be taken as 100% but for beta particles should be obtained from a suitable graph Flux from a line source: The flux at any point at a distance d from a line source emitting N particles or photons per second per unit length of the source = r, *an particles or photons/cm2 s 37

40 where N is the number of particles or photons emitted per second per unit length in cm of the source d is the distance in cm on the normal from the middle of the source L is half the source length in cm. In this formula, the absorption in air has been neglected Flux from a plane disc source: The flux at a point distant d cm from a plane disc source emitting N particles or photons per second per unit area of the source N i "4 lqg + 1 particles or photons/cm2 s where N is the number of particles or photons emitted per second per unit area of the source R is the radius of the source in cm d is the distance of the point from the source along the central axis normal to the plane of the source Some useful formulae for the determination of dose rates Point sources: (a) G a m m a emitter where Dose rate at 1 metre = C E rad/h for gam m a radiation of energy M e V C is the strength of the source in curies E is the total energy of gam m a radiation per disintegration in M e V. (b) Beta emitter Dose rate at 10 cm = 2700 C rad/h (varies slowly with beta energy) 38

41 Here the self-absorption of the source and also absorption in air has been neglected. This dose rate varies slowly with beta energy. The inverse square law applies, but absorption in air introduces complications at distances of the order of 1 metre Thick sources (sources of large dimensions): G a m m a or beta emitters Dose rate at the surface of a source in the shape of a solid bar of large dimensions = 1.07 SE rad/h where S is the specific activity of the source inpci/g E is the mean energy in M e V per disintegration. In the case of beta rays, E is the m ean beta energy and is given by one-third of the m axim um energy of the beta spectrum. 39

42 6. MAXIMUM PERMISSIBLE LEVELS OF RADIATION 6.1. General M an has been continuously exposed to natural sources of radiation, namely cosmic rays and naturally occurring radionuclides. The average dose due to this background radiation is estimated to be about 100 m rem /y r, although there are places where the background dose may be times higher than this average value. There have been varying estimates which indicate that 2-10% of the natural mutations in m an could be caused by background radiation. Any increase in these environmental radiation levels due to man-made sources may entail a risk of deleterious effects. However, in the interest of progress, the use of ionizing radiations cannot be entirely dispensed with. The solution to this problem is therefore to limit radiation doses to those which would involve risks which are not entirely unacceptable to the individual radiation worker and to the population at large. With this consideration in view the International Commission on Radiological Protection (ICRP) has defined the m axim um permissible dose for an individual as that dose accumulated over a long period of time or resulting from a single exposure which in the light of present knowledge carries a negligible probability of occurrence of severe somatic or genetic injuries Development of the concept of m axim um permissible dose Soon after the discovery of X-rays and radioactivity, it was recognized that exposure to intense beams of radiation could result in a variety of injuries to the human body, such as skin erythema, smarting of the eyes, epilation of hair and induction of tumours. It was not until 1925 however, that the concept of "tolerance dose" emerged in quantitative terms. The tolerance dose was then defined as that dose which a person could occupationally receive continuously or at repeated intervals without suffering from changes in the blood or damage to the skin or the reproductive organs. It was also estimated that if the total dose per month did not exceed onehundredth of the dose required to cause skin erythema, no injury would result in the long run. The unit roentgen had come into use at about that time, and an exposure rate of 10 5 R / s was recognized 40

43 as a tolerance level. In terms of the roentgen, the skin erythema dose was estimated to be 600 R, and the corresponding daily tolerance dose was 200 m R, In 1936, the value of the tolerance dose was reduced to 100 m R /d on the basis of two important factors. One was that a dose of m R with backscatter corresponded to a free air dose of 100 m R for the quality of radiation then encountered. The other was that more and more energetic X-rays were being produced and used and it was recognized that a greater percentage of the surface dose was reaching the deep-seated critical organs. The advent of the atomic age in 1942 resulted in the exposure of large numbers of occupational workers to various types of radiation. In addition, it became recognized that it was imperative to extend the concept of the m axim um permissible dose to the general public as well. With (1) the recognition of the concept of R B E, (2) the coming into vogue of the new radiation units, especially the rem and (3) the increasing knowledge of the biological effects of radiation, it was decided: (a) (b) (c) (d) (e) (f) (g) (h) To reduce the earlier accepted tolerance dose by a factor of two, and make it 50 m r e m /d To express the m axim um permissible dose over a period of one week, i. e. as 300 m rem per week (for administrative and technical reasons connected with the enforcement of these tolerance limits, it was decided that dose integration over a period of one week was most convenient) To regard the skin as a critical organ and to set the permissible dose for it at'600 m rem per week (at a depth of 7 m g /c m 2) To take the blood-forming organs as the most critical tissue and to apply the permissible limit of 300 m rem per week to these organs To make the permissible dose for persons over 45 years of age twice that permissible for young adults To allow larger weekly doses for the hands and feet (1. 5 rem per week) To make a suitable recommendation about accidental exposure involving a single dose that might be as large as 25 rem To recommend R B E values of 1 for X-rays, gam m a rays and beta rays, 5 for thermal neutrons, 10 for fast neutrons and 10 for alpha particles. O n the basis of subsequent studies on low-level irradiation over extended periods of time, the IC R P in 1958 reduced the m axim um permissible weekly dose to an average of 100 m rem per week. 41

44 Another factor necessitating such a decision was the increasing evidence that irreversible genetic damage could result from the exposure of the reproductive organs to any amount of radiation, however small. For practical reasons, the Commission allowed a certain flexibility whereby a m axim um quarterly cumulative dose (i.e. over a period of 13 consecutive weeks) of 3 rem could be permitted with the restriction however, that the annual dose must not exceed 5 rem. This annual limit of 5 rem was further subject to the flexibility that it could be exceeded provided the accumulated total dose at any age N did not exceed the value of 5 (N-18) rem. It is customary that no person below the age of 18 should be engaged in radiation work. However, should a person be engaged in radiation work from the age of 16 yr (which is the minimum age specified by the International Labour Organisation), care must be taken to ensure that his accumulated dose at the age of 30 does not exceed 60 rem Current dose limits The IC R P in its 1965 report gave recommendations on the radiation dose limits for the following two categories of individuals: (1) Adults exposed in the course of their work (2) M em bers of the public. The dose limits for the above two categories of individuals are given in Table 6.1. It is important that in all radiation work care be taken to avoid any unnecessary exposure. It is also important that the doses received by individuals or the population as a whole should be kept as low as is readily achievable. In the modified recommendations, the Commission has suggested that a period of one year is the most reasonable length of time over which to assess accumulated exposure but that it was also necessary to limit the magnitude of a single dose. The C o m mission has further recommended that, although the annual m axim um permissible dose is 5 rem, in a period of a quarter of a year up to half the annual m axim um permissible dose could be accumulated, except in the case of women of reproductive capacity whose abdomen should not receive more than 1. 3 rem of penetrating radiation per quarter. In the case of a woman whose pregnancy has been diagnosed, necessary arrangements should be made to ensure that the dose to her foetus, accumulated during the remaining period of the pregnancy, does not exceed 1 rem. 42

45 T A B L E 6.1. SUMMARY OF DOSE LIMITS FO R INDIVIDUALS Organ or tissue Maximum permissible doses for adults exposed in the course of their work Dose limits for members of the public Gonads, red 5 rem in 1 yr 0.5 rem in 1 yr bone-marrow Skin, bone, 30 rem in 1 yr 3 rem in 1 yra thyroid Hands and forearms; 75 rem in 1 yr 7.5 rem in 1 yr feet and ankles Other single organs 15 rem in 1 yr 1. 5 rem in 1 yr For the thyroid of children the dose is 1.5 rem. In certain circumstances it may be necessary to permit a few workers to receive exposures in excess of the quarterly limits. This may be recommended only in exceptional circumstances when alternative techniques to avoid exposure of workers are either unavailable or impractical. In such a situation a person may be allowed exposure or intake of radioactive materials provided the dose commitment does not exceed twice the annual dose limit in any single event, and in a lifetime, five times this limit. Such planned special exposures should not be permitted if the addition of the intended dose to the worker's accumulated dose exceeds 5(N-18)rem. In other cases such as pregnant women, there are other limitations, the details of which may be found in IC R P Report No. 9, As shown in Table 6. 1, the dose limits for m em bers of the public are set at one tenth the values for occupational workers with the additional stipulation that the dose limit for the thyroid of children up to 16 years of age be 1. 5 rem. The genetic dose to a population, as defined by the IC R P, is that dose which, if received by each person from conception to the mean age of childbearing (taken to be 30 yr), would result in the same genetic burden to the whole population as do the actual doses received by the individuals. The m axim um permissible genetic 43

46 dose to the whole population should not exceed 5 rem from all sources additional to the dose from natural background radiation and from medical procedures Internal contamination Internal contamination may result from the inhalation or ingestion of radioactive materials or in some cases from direct absorption through skin. Increased attention was therefore paid to limit the internal contamination so that the m axim um permissible doses are not exceeded. From the yearly dose limits the m axim um permissible annual intakes (M PAI) for various radionuclides were calculated. The values of M P A I are given in Tables IIA and IIB of the Basic Safety Standards for Radiation Protection, IA E A Safety Series No. 9 (revised edition, 1967). Other derived values such as the derived air concentration (DAC) and derived ingestion concentration (DIC) Water computed by dividing M P A I by the yearly intakes of air or water, are presented in the Appendix of IA E A Safety Series No. 1 (Revised edition, 1973). These values of D A C and DIC water are the same as the m axim um permissible concentration in air (M P C )a and m axim um permissible concentration in water (M P C )W previously given by the IC R P. The above-mentioned figures were derived on the basis of the following general considerations: (a) (b) (c) (d) Only general m axim um permissible concentration values for transportable and non-transportable compounds were considered Only ingestion and inhalation processes were taken into account All calculations were based on a "reference m an" in whom all characteristics were assumed to be those of an average m an and under average conditions of physical work Under conditions of continuous exposure to the concentrations prescribed, the dose to the whole body, or to any critical organ, would not exceed the maximum permissible doses. The most recent reinterpretation of the basic standards is the concept of dose commitment. The m aximum permissible annual dose commitment corresponds to the exposure at the M P C of the radionuclide, and in such cases the integrated dose to 50 yr should not exceed the m axim um permissible annual dose. The following important factors should be borne in mind while considering internal hazards. Radionuclides gaining entry into the body distribute themselves non-uniformly. Further, depending upon their chemical characteristics and the chemical interactions which they undergo in the body, they concentrate in particular organs, 44

47 in many cases in a non-uniform manner. It is difficult, if not im possible, to estimate the amount of the substance present and its distribution in the body, and it is equally difficult to measure its rate of elimination, if any. The biological half-life (defined as the time taken by the body to eliminate half the quantity of stable isotope of the element gaining entry into the body) is very long for some radionuclides, and attempts to accelerate their elimination from the body have not been entirely successful. They remain in intimate contact with the tissues and irradiate the body continuously until they are eliminated. Thus, alpha and low-energy beta emitters, which are not hazardous as external sources of radiation, assume considerable importance as radiation hazards once they gain entry into the body Mixed exposures external and internal Although the m axim um permissible doses, both internal and external, are individually based on the basic average criterion of 5 rem /yr, special consideration must be given to those cases where both types of exposure are possible. It should be stressed that the IC R P recommendations for m axim um permissible levels of radiation are also applicable to doses resulting from a combination of both external and internal exposures. In particular, the case of a person who has been subjected to internal contamination as a result of a radiation incident will be considered. The extent of contamination, and hence the extent and degree of irradiation of the critical organs, can be determined in such a case. In planning all future radiation work for this person, the exposure to which he is continuously subject as a result of the internal contamination arising from the incident must be fully taken into account Excessive exposure In cases of excessive exposures to radiation, careful consideration must be given to the detailed provisions made in this regard in the latest recommendations of the IC R P, and future radiation work must be planned accordingly. 45

48 7. METHODS OF RADIATION MEASUREMENT 7.1. General All radiations, whether directly or indirectly ionizing, interact with the atoms or molecules of-the medium which they traverse. They produce ionization in gaseous media, scintillations in certain phosphors, blackening in photographic emulsions and decomposition in chemical media. These properties have been utilized in the design and development of various devices for the detection as well as measurement of radiation. The ionization chamber, the proportional counter, the Geiger-Miiller counter and some solid state detectors are based on the principle of collection of ions formed by radiation in gases or solids. Scintillation counters have been developed on the principle that the passage of ionizing radiation through certain phosphors produces visible light which can be detected by a suitable device such as a photomultiplier. The blackening of photographic films by exposure to radiation and subsequent development of the films has been utilized for the detection and measurement of radiation Gas-filled detectors The ionization chamber Since the roentgen is the unit of exposure and is defined in terms of the release of total electric charge resulting from the irradiation of a unitvolume of air at N.T.P., the ion-chamber method is the most direct method of measuring exposure. Since, further, the definition of the roentgen requires that the total ionization produced by the secondary electrons formed in this volume of air be measured, and as some of the secondary electrons may have ranges of several metres, large and cumbersome apparatus would be needed for this purpose. To avoid the use of such large and unwieldy apparatus, air-equivalent wall chambers or thimble chambers have been developed. Their use is based on the principle that when a tiny cavity, such as a small ionization chamber, is placed in a large homogeneous absorbing medium which is uniformly irradiated, the flux of secondary electrons in the cavity is identical to the electron flux which existed in the medium before the cavity was introduced. If the chamber gas is air and if the walls are composed of materials having an atomic number near that of air, then the energy loss per gram of air in the chamber will be 46

49 substantially the same as the energy loss per gram of air at the point where the chamber is located. Ideally, therefore, an ionization chamber consists of a chamber of known volume and with air-equivalent walls, the inside surfaces of which are made electrically conducting. Inside the chamber, and insulated from the walls, is a central conductor to which a positive potential is applied. Ions produced in the chamber as a result of the passage of radiation cause a voltage change on the central electrode, and this change is measured on a charge- or current-measuring system which gives a reading directly in roentgens per unit time. An ideal chamber can cover only a limited range of energies, for many factors enter into the construction and use of such an instrument. Some of these factors are the use of only approximately air-equivalent walls, the use of metallic electrodes, the absorption of low-energy radiation in the chamber walls and the lack of sufficient wall thickness for electron equilibrium at high energies. One example of such a device is a pocket dosimeter which has a 2 -cm3 chamber, a quartz fibre electrometer which is also part of the collecting electrode and a compound microscope for viewing the fibre. A separate battery charger is used to charge the instrument prior to use. These instruments are available in ranges of m R and 1, 5, 10, and 50 R. It will be readily seen that as the exposure levels to be covered are higher, the volume of the chamber used is smaller. A non-self-reading dosimeter or pocket chamber consists of an ion chamber which is charged and read on a separate chargereader unit containing an electrometer and a voltage source. Although less convenient to use, this instrument is sturdier than the directreading pocket dosimeter described above. In portable survey meters, and in installed types of meters, the ion chamber is often used with an electrometer valve circuit with a pen recorder or meter display. The design of a comprehensive monitoring apparatus involves a number of real problems because of the range of dose rate to be covered a few mr/h to a few thousand R/h. The most commonly used instrument is an ion chamber operated with a logarithmictype DC amplifier. Such logarithmic D C amplifiers covering 3 to 6 decades of exposure rate have been described in the literature. One such instrument has been described in which the exposure rate is indicated on a single meter or pen recorder, without range switching, from 1 m R /h to 103 R/h. Suitable outputs can be provided in this instrument, at any desired level, to operate interlocks and alarm circuits as required. 47

50 The prop ortional counter The proportional counter has the same basic construction features as an ionization chamber, except that the electric field strength near the central electrode is much higher. This high field strength results in the electrons being accelerated towards the central electrode, and they gain sufficient energy to ionize further gas atoms by collision. Thus, the charge collected, and hence the initial voltage developed, is amplified considerably. An important factor which makes this type of detector particularly valuable is the fact that the magnitude of the voltage pulse is proportional to the amount of the primary ionization. Therefore, such a detector can be used for differentiating between sparsely and heavily ionizing radiations. This detector is particularly suitable for detecting alpha particles because of its great degree of discrimination against beta and gam m a radiation The Geiger-Muller counter In this type of detector the amplification of the voltage pulse described in the case of a proportional counter is carried one step further by increasing to an even higher level the potential of the central electrode. This process of amplification results in the entire length of the central electrode (which is a wire) being used in the discharge and thus makes the size of the ultimate pulse completely independent of the extent of the primary ionization. The process of the initial avalanche of electrons approaching the central wire will be accompanied by the positive ions produced in the counter gas moving towards the counter wall. W hen these ions hit the wall, electrons are ejected, resulting in a second avalanche and similarly in successive avalanches. This process is undesirable and is inhibited by "quenching" the discharge by filling the tube with a suitable mixture of a gas and organic vapour such as argon and ethyl alcohol. These measures, however, result in restrictions being imposed on the life of the tube and on the counting rates that can be attained. Recently, special halogen-quenched Geiger-Muller counters have been developed which are useful for measuring exposure rates as high as 1000 R/h. These counters have almost indefinite life and can, without damage, be operated at much higher pulse rates than is possible with an organic quenching gas. These counters give larger pulses and operate at considerably lower voltages than G M counters containing organic vapours. 48

51 7.3. The scin tillation counter The interaction of radiation with certain materials, called scintillation phosphors, results in the energy of the radiation being dissipated both by ionization and by excitation. Excitation occurs in cases where the amount of energy transferred to the electron is not sufficient to dislodge it from the atom and thus causes the em ission of visible or near visible light which can be converted into a voltage pulse by means of a photomultiplier. The scintillator can be solid, liquid, or gas and hence the versatility of this type of detector is almost unlimited. For the detection of alpha particles, a thin layer of activated powdered zinc sulphide is used. As the specific ionization of alpha particles is large, they produce much larger pulses than beta or gam m a rays and thus it is possible to choose only alpha pulses and eliminate the rest. This type of detector is frequently used to measure alpha contamination of surfaces. For the detection of extremely low-level gam m a radiation, large transparent crystals (dimensions of the order of a few inches) for example thallium-activated sodium iodide are used. Such crystals are highly sensitive and are capable of detecting photons with high efficiency. The characteristics of scintillators which make them particularly valuable for gam m a radiation monitoring include an energy response for scintillators (of low atomic number) which is reasonably close to that of an air-wall ion chamber, high sensitivity per unit volume and ability to cover a large range of intensities. One of their disadvantages is that the spectrum of the radiation must be known. Liquid scintillators have been described in the literature which have specially desirable characteristics, although they would have obvious drawbacks for use as portable instruments for area monitoring purposes Semiconductor detectors Semiconductor detectors have, during the past few years, proved to be a very important class of radiation detectors. There are many types of these detectors available, such as diffused junction detectors, surface-barrier silicon detectors, lithium-ion-drifted junction detectors and totally depleted junction detectors. The passage of charged particles through a semiconducting medium results in the production of electron-hole pairs. The electron in the pair exists in the conduction band and is free to 49

52 move under the influence of an electric field. The hole refers to the property of the medium to carry charge by virtue of the fact that the net positive charge existing at a lattice point can be passed along from neighbour to neighbour under the influence of an electric field. An applied electric field causes these charge carriers to move toward the appropriate electrodes and in so doing induces a charge in the circuit external to the detector. A semiconductor detector has a sensitive volume, usually at the middle, and an insensitive region, referred to as the dead layer. A nuclear particle entering the detector from outside must pass through this dead layer before reaching the sensitive region. This dead layer is called the window of the detector. Semiconductor detectors possess very good energy-resolution characteristics. They display excellent linearity relationships in respect of pulse height versus energy, have rapid response time, convenient dimensions and thin windows, are insensitive to ambient magnetic fields, are easy to fabricate even in special configurations and have variable sensitivity with respect to particle energy. However, there are some drawbacks to semiconductor detectors, such as the short operating lifetime of some semiconductors, relatively small output signals, variation of operational charactersitcs with ambient conditions, inability to stop particles of relatively long range and nuclear radiation damage Luminescence detectors Radiophotoluminescence Radiophotoluminescence is the phenomenon whereby a material, which is originally nonluminscent under ultraviolet excitation, is made responsive to such excitation by exposure to X-rays or gam m a rays. Thus, when silver-activated phosphate glass is exposed to X- or gam m a rays, new stable photoluminescent centres, termed F-centres, are created. Such F-centres are formed when the electrons released by radiation from negative ions are trapped by the interstitial silver ions (Ag+). The Ag atoms formed by the capture of the liberated electrons fluoresce with a peak at about 6400 A when exposed to ultraviolet radiation of wavelength 3650 A. The intensity of the fluorescent radiation emitted by a phosphate glass which has been exposed to X- or gam m a radiation is related to the radiation dose. The fluorescence emission of such glass is independent of the radiation dose rate and bears a linear relation to the total radiation dose. 50

53 Silver-activated phosphate glasses can be used for thermal neutron dosimetry as both i<»ag and 109Ag, with cross-sections of 30 and 84 barns respectively, capture neutrons and are transmuted to short-lived 108Ag and 110Ag isotopes, both of which are beta and gam m a emitters. Beta rays emitted by these two isotopes contribute to most of the response. In mixed gam m a and neutron fields, the contribution from thermal neutrons alone can be obtained by covering one of two glass detectors with a neutron absorber. Glass dosimeters, however, are insensitive to fast neutrons. Glass dosimeters are noted for accuracy, negligibly small fading, little directional dependence, remeasurability, and stability under varying temperature and humidity conditions. The glass dosimeter is also suitable for accident dosimetry Thermoluminescence Some materials, such as LiF and C a F 2, when exposed to radiation can store energy which by heating to a certain predetermined temperature is re-emitted in the form of visible light. This phenomenon is known as thermoluminescence and has been utilized in the design of devices for the measurement of radiation doses. As in the case of radiophotoluminescence, when a thermoluminescent material is exposed to ionizing radiation, electron-hole pairs are formed. The liberated electrons are trapped at impurity centres or crystal deformations, resulting in the formation of F-traps. W hen the material is heated to a temperature of the order of a few hundred degrees centigrade, the trapped electrons are released and while combining with the holes emit visible radiation which can be detected and collected using a photomultiplier. Pure LiF or C a F 2 is not suitable for thermoluminescent dosimetry (T L D ), but when suitably activated with materials such as manganese or dysprosium, exhibit very good thermoluminescent properties. Thermoluminescent dosimeters are more sensitive than radiophoto luminescent dosimeters and exposures of the order of 1 m R have been detected with C a F 2 detectors. With L if this system of dosimetry has less energy dependence than the radiophotoluminescent system; but with C af2, which consists of high-z material, dependence of response on photon energy has been observed. The lithium fluoride detector is sensitive to neutrons and this can prove to be a disadvantage in the case of mixed radiation fields. For personnel monitoring, thermoluminescent dosimeters are being used in certain centres in place of film badge dosimeters, although the film badge continues to be used as the principal per 51

54 sonnel monitoring device in most centres. In finger and hand dosimetry, LiF-teflon dosimeters have proved to be particularly satisfactory Neutron detectors Neutrons cannot be detected directly by any of the abovementioned detection techniques, as they are uncharged and do not produce any direct ionization. However, by introducing into these detecting systems suitable materials, in which the cross-section for reactions like (n,^), (n, (3), (n, p), or (n, 7 ) is high, or from which the recoil of nuclei of light elements takes place, the secondary ionizing radiations resulting from interactions of slow or fast neutrons can be detected. For example, thermal neutrons are detected by means of the 2-MeV alpha particle which results from the absorption of these neutrons by 10 B. The boron is coated on the inner walls of the ionization chamber, or introduced into the chamber as boron-trifluoride gas. Fast neutrons are detected by observing recoil protons which are ejected by elastic collisions of fast neutrons with hydrogenous material such as polyethylene. A proportional counter is used to detect the protons so that extraneous electrons which would otherwise give rise to spurious counts may be excluded. In some cases, the isotope resulting from neutron interactions could itself be radioactive, in which case a measurement of the amount of such induced activity provides a measure of the neutron flux. This method is often used for the absolute determination of neutron flux. Some of the foils used for this purpose are indium, gold and tantalum. Similar foils are also used in accident dosimetry. Non-photographic nuclear track detectors have recently been developed which are based on the principle that the passage of a heavy charged particle through an insulating solid leaves a track of damaged material which has a cross-section of the order of 50 A. This damage can, in some cases, be observed under an electron microscope. W hen treated with a chemical solution, the damaged zone has a preferential etching, thus making the particle track visible under an ordinary microscope Photographic detectors It is well known that the action of ionizing radiation on photographic emulsion is similar to that of visible light. The degree of blackening is a measure of the dose, although this relationship 52

55 is vitiated by energy dependence the photographic film shows high sensitivity below 150 kev. To overcome this energy dependence and also to be able to differentiate between the blackenings caused by different types of radiation, metallic filters are used between the radiation and the film. Photographic films can also be used to monitor slow neutrons, by covering the film with a cadmium filter. This filter, by absorbing slow neutrons, gives rise to capture gam m a rays which blacken the film. The degree of blackening is a measure of the slow-neutron dose. Fast-neutron doses can also be measured by counting the number of tracks produced by recoil protons from a hydrogenous radiator in a suitable nuclear emulsion. The greatest single advantage of the photographic film is that it provides permanent records. It has other advantages too, such as its small size and weight, sturdiness, low individual cost, simultaneous recording of more than one type of radiation and measurement over a wide range of total exposures, which make it an appropriate personnel monitoring device in most situations. The disadvantages include a large energy dependence within a certain energy range, latent-image fading with some types of emulsion, problems of development and densitometry and inconvenience of storage. Present-day procedures, however, have overcome many of these shortcomings Chemical dosimeters The changes induced in chemical systems by ionization resulting from the passage of radiation is another method employed for measuring radiation doses. In this method, chemical systems are used in conjunction with indicators in which the ions produced by radiation combine to form new compounds. A typical chemical decomposition system is the chloroformwater mixture which, when exposed to radiation, produces hydrochloric acid in proportion to the radiation absorbed. This acid depresses the ph and by the use of a suitable indicator it is possible to determine when a predetermined dose has been received by a chemical system. An indicator frequently used in this system is bromocresol purple. Other systems used include the ferrous-ferric and ceric-cerous systems and the reduction of methylene-blue dye. An inherent drawback of chemical decompositon systems is the fact that the sensitivity to radiation is quite low. Doses of the order of 25 R are required before any detectable chemical change is induced in any of the above systems. Hence, these systems are 53

56 best suited for measuring the dose from large sources of radiation. Recently, however, liquid-chemical dosimeters prepared in small containers have been successfully used for dose measurements within body cavities, in tumour areas or at positions close to highintensity sources Calorimetric methods Another method adopted is calorimetry, where the heat resulting from energy dissipation by radiation is measured. Thus calorimetry can be employed to measure the total energy emitted by a specific source of radiation, or the total energy in a defined beam of radiation. To determine the total energy directly it is necessary (1 ) that the absorbing system converts all the dissipated energy into heat, and (2 ) that the geometrical dimensions and arrangement of the absorber are such as to absorb all the radiation. W hen either or both of these conditions are not satisfied, owing to chemical changes or geometrical limitations, total energy m easurements can still be achieved by comparison with a known radiation source or beam. In the latter case, the calorimeter serves as a means of comparison rather than as a fundamental and independent method of energy determination. Calorimetry can also be employed to measure the absorbed dose in a finite amount of material due to the partial absorption of radiation incident on the material. In this case, if there is no chemical change in the absorber and all the dissipated energy appears as heat, the absorbed dose may be determined directly. The advantage of calorimetry is that it gives a measurement of radiation energy in terms of fundamental energy units. This provides an absolute basis for the evaluation and comparison of experiments, treatments, and any procedure employing radiation Special radiation detectors Radiation elements for dosimetry Radiation elements are devices which can convert radiation energy directly into electrical energy. Such detectors consist of two electrodes contained in a vacuum tube. Radiation elements meant for the detection of gam m a rays have the two electrodes made of materials of different atomic num bers. W hen exposed to radiation the two electrodes emit different numbers of electrons resulting in a potential difference which is a measure 54

57 of radiation dose rate. The potential difference is measured using an electrometer. For purposes of detection of neutrons, radiation elements have been designed in which one electrode is made of material having a cross-section sufficient for inducing beta activity. The beta-active isotope must however have a short half-life. Radiation elements have two advantages: (a) they are selfpowered and can function with unlimited storing time, no batteries or charging units being necessary, and (b) they give quantitative measurement of the dose rate with sufficient accuracy, as no recombination processes are involved Exoelectron emission Low-energy electrons (maximum energy ~ 5 ev, average energy ~ 0. 2 ev) are emitted from thin layers of ionic crystals during several physical and chemical processes such as mechanical deformation, solidification of melts, changes in the crystalline structure and chemical reactions. Also, after exposure to ionizing radiation, exoelectron emission can take place spontaneously or by thermal or optical stimulation. Of these, only the radiationinduced exoelectron emission is of dosimetric interest. Thermally stimulated exoelectron emission (TSEE) is similar to the thermoluminescence process. However, unlike the thermoluminescence process, which is a volume effect, T S E E is purely a surface effect. Thermoluminescence peaks in certain materials can be observed without the corresponding T S E E peaks. Two substances, namely L if and B e O, have been found to have good T S E E characteristics, the latter having higher sensitivity. The emission peaks of LiF and B e O are at approximately 150 C and 2 80 C respectively. Compared with T L D systems, T S E E detectors have certain advantages, i.e. relatively high sensitivty; an energy response which can be adjusted easily by low-z additives to a completely flat or other desired energy dependence; and relative ease of preparation of detectors in almost any laboratory, using inexpensive, unactivated L i F. 55

58 8. RADIATION MONITORING INSTRUMENTS General W ork with radiation sources or radioactive materials may lead to external exposures or internal contamination of the working personnel. External exposure may result from work with radiation sources such as X-ray machines, accelerators, sealed or unsealed radioactive materials, etc. Internal contamination on the other hand may arise from the handling of damaged sealed sources or open sources of radioactive materials. It should be borne in mind that in the case of internal contamination the radionuclides tend to remain in the body for varied periods of time and could lead to radiation exposures to critical organs in the body. The control of external radiation exposures is accomplished principally by two general methods: (1 ) personnel monitoring and (2) area and environmental monitoring. In area and environmental monitoring the adequacy of built-in safety features and working procedures is periodically checked, whereas personnel monitoring instruments assess in retrospect whether the personnel in question have received doses which are well within the envisaged limits.. The most frequently measured external exposure is the whole-body exposure whereas, for special operations, doses to the extremities of the body are also measured. The control of internal radiation exposure is accomplished by two types of measurements: (1 ) the measurement of environmental airborne and surface contamination and (2 ) the measurement of internal contamination by bioassay and whole-body burden measurement techniques. Some of the instruments which are principally used for such measurements will now be described briefly Monitoring of external radiation Personnel monitoring Film badges: The most versatile device now available for the assessment of whole-body exposure is still the personnel monitoring film. The film is partially covered by filters of cadmium, lead or other suitable material, so as to make its response energy independent, and also to permit the assessment of doses due to different types and qualities of radiation when the exposures involve mixed radiations; the whole assembly is called a film badge. The blackening of the uncovered portion of the film would be more than that of the covered portions if the film badge were exposed 56

59 to low-energy electromagnetic radiation or beta rays, as these radiations would be partially or totally absorbed by the filters. The measurement of the degree of blackening in both the covered and uncovered regions m ay be used to differentiate and assess doses from different components of mixed radiation. Also, in the case of exposure to slow neutrons, the increased blackening beneath a suitable filter, such as cadmium, arising from the (n, 7 ) reaction in the filter may be used for assessment of slow-neutron doses. For the assessment of fast-neutron doses, a special film-pack which uses a nuclear emulsion to record recoil proton tracks from the hydrogenous cellulose acetate base of the emulsion is used. The whole assembly is made light-tight and part of it is covered with a cadmium filter to absorb slow neutrons which would otherwise give rise to confusing proton tracks resulting from the (n, p) reaction with the nitrogen of the cellulose base. Thus there are two types of film badge - one for fast neutrons and the other for other types of radiation. Either or both of these badges are used as the occasion demands, and they are normally worn on the chest to enable a whole-body dose assessment to be made Thermoluminescent dosimeters: Thermoluminescent dosimeters have shown considerable promise for applications in personnel radiation dosimetry. They are already in use in certain institutions for personnel monitoring purposes. The phosphors commonly used are suitably activated L if, C a F 2, and C a S 0 4. Lithium fluoride is nearly tissue-equivalent and its response is nearly energy independent. Calcium fluoride on the other hand has a higher sensitivity but because of its higher effective atomic number its response is highly energy dependent Radiophotoluminescent dosimeters: Radiophotoluminescent dosimeters have also been used for personnel radiation dosimetry purposes. They have a number of advantages such as good accuracy, stability, negligible fading, and remeasurability. The most widely used detector of this type is the silver-activated phosphate glass. Using such glass, it has been determined that precision of the order of ± 10% for 10 m R, ± 5% for 2 5 m R, and ± 2% for 50 m R or more of gam m a rays can be obtained under normal laboratory conditions provided, of course, appropriate glass cleaning methods are employed. For the detection of thermal neutrons, one method employs two phosphate glasses, of which one glass contains natural Li and the other only 7 Li. The 7Li glass has only one-fourteerith the thermal neutron response of the natural L i glass. Another method uses a boron-containing spherical glass capsule and the response is rem 57

60 equivalent. This method, however, cannot differentiate gam m a ray dose from that of thermal neutrons. Glass dosimeters have negligible response for fast neutrons Pocket electroscope ionization chambers: These instruments are used to provide an immediate assessment of radiation exposures. Wherever the radiation exposure levels are known to be high but cannot be exactly mapped as in the case of emergencies, the pocket ionization chamber issued to personnel will give an on-the-spot indication of radiation exposure. The normal type of pocket ionization chamber can be used for assessing'gamma radiation exposure only. Special pocket ionization chambers are available for dealing with X-ray and neutron fields. Most of these pocket dosimeters are fitted with a scale on which the radiation exposure may be read directly in roentgens. They are not as useful as film badges for providing an integrated radiationexposure assessment over a period of a number of days. Furthermore, such factors as mechanical shocks and variations in atmospheric conditions could result in their being discharged. A s far as possible, duplicate chambers should be used and the lower of the two readings should be taken. To avoid any wrong assessment of exposures resulting from such discharges, these instruments should always be used in conjunction with other personnel monitoring devices such as film badges Finger badges: Where only partial exposure of the body is involved, either of the instruments described above may be employed. For the fingers, however, smaller devices such as finger film badges have been used to give an assessment of the exposure in the area of interest without hindering operations. The films used in such badges are essentially a smaller version of the films previously described. They have usually been fitted into rings which may be slipped on the fingers. Small-size glass dosimeters have also been successfully used as finger dosimeters. L if T L D phosphors uniformly imbedded in teflon ribbons and discs are also in use as finger dosimeters at some centres. Generally, glass and T L D finger badges are found to be entirely satisfactory for determining finger tip doses Area monitoring systems Area monitoring instruments using any one of the principles described earlier usually consist of a probe and the associated electronic circuitry. The probe contains the sensitive element of the 58

61 instrument (for example, the ionization chamber, Geiger-Muller tube, or the scintillation phosphor). The ionization occurring in the sensitive volume as a result of exposure of the probe to radiation provides a measure of the quantity of radiation. The probe is generally fitted with a removable shield of suitable thickness. Thus, where mixed radiation fields are involved, when the shield is in place, one type of radiation is absorbed and the ionization produced by the radiation not absorbed by the shield is measured by the instrument. When the shield is removed, the ionization produced by both types of radiation can be measured. For example, in a mixed field of beta and gamma radiation, a thin metal shield would permit differential measurement of the individual fields. A shield of cadmium could be used similarly for differentiation between fast and slow neutrons Ionization chamber instruments: As already described, the ionization chamber instrument can operate at relatively low voltages, which makes it particularly useful in places where atmospheric conditions are subject to large variations. This, coupled with the robustness of the instrument and the fact that it can be powered by batteries, makes it particularly versatile as a portable instrument. However the availability, in recent years, of halogen-quenched G M counters with relatively low working voltages and DC- DC converters have made instruments with G M probes equally suitable as portable battery-operated devices Instruments using G M counters: The instrument.using a G M tube as a probe is extremely sensitive. It can detect alpha and beta particles when fitted with a very thin "window". Such counters are efficient for beta-particle counting but are less efficient for gam m a radiation. The G M counter essentially measures individual particles; its readings are therefore usually given in counts per second. By suitable calibration procedures, it is possible to relate the number of counts per second to dose rate and the G M counter can therefore be used to give an indication of dose rate. The relationship between counts per second observed and the corresponding dose rate in m r e m /h will depend upon the instrument, its construction, and the energy and type of the radiation being measured Scintillation counters: The scintillation counter is the most expensive of all radiation detectors. However, a number of factors have led to its being developed as a versatile measuring 59

62 device and have further resulted in its replacing many of the more conventional types of ionization chambers and G M instruments. These factors are: (a) The detector volume can easily be made tissue-equivalent (b) The detection efficiency is extremely high, and this makes the instrument particularly valuable for low-level counting (c) The detector volume can be made as large or as small as necessary (e. g., extremely small crystals are used for measurements near radium sources in the body and very large crystals for body-burden measurements) (d) A high degree of discrimination between radiations of different energies is possible and this makes the instrument particularly valuable for use in mixed radiation fields Monitoring against internal contamination Surface contamination measurement instruments Where open sources of radioactive materials are handled, suitable instruments for measuring surface contamination are necessary. The Geiger-Miiller counter is particularly suitable for this purpose and, as described earlier, the observed counting rates can, by suitable calibration, be converted into radiation levels. By similar calibration, the observed counting rates can be converted into the degree of contamination in terms of activity per unit area. The scintillation counter is also extremely useful as a contamination measuring device particularly where alpha contamination, which cannot easily be measured by conventional methods, is present. Furthermore, when the contamination levels are extremely low, the large sensitive area presented by a scintillation probe is of particular advantage. In the laboratory a very useful instrument consists of one versatile electronic unit which can be coupled to any one of the types of probes described above. Such instruments are in use in many laboratories Air monitoring devices In addition to area monitoring and the detection and measurement of contamination on surfaces, the determination of the degree of air contamination is another important factor in working areas where open sources are handled. 60

63 A simple method m ay be used for detecting the presence and level of radioactive material in air. Air is drawn through a fine filter paper, any active material present in the air is deposited on the paper and the assessment of the activity on the filter paper gives a measure of the activity of the air sampled. A typical instrument for this purpose is a modified vacuum pump which has at its intake a suitable housing into which a filter paper may be securely fixed. This instrument is also provided with means for measuring the amount of air passing through the filter paper. Thus, the quantity of activity found on the filter paper may be related to the amount of air that has passed through, to give a measurement of the concentration of the radioactivity in the air in terms of activity per unit volume. Other instruments, using different principles for the collection of airborne contamination, such as electrostatic precipitators, have also been developed Biological indicators A s a supplement to the physical methods described above, biological indicators can also be used to detect, and in some cases assess, radiation exposure to individuals. However, none of the biological indicators yet proposed can achieve the convenience, sensitivity and accuracy of the chemical and physical systems now in use. Nevertheless, in special circumstances in which the physical or chemical personnel dosimeter readings could be in doubt or in accident situations in which auxiliary confirmatory dose estimates could prove to be of value, biological indicators can be employed. In addition, biochemical estimates of the extent of radiation injury may also, under ideal circumstances, serve as a guide for the physician to schedule appropriate clinical steps in respect of persons involved in radiation accidents. The biological indicators used in such studies include breakdown products of cell and tissue components such as deoxycytidine, thymidine and amino acids which are excreted in increased amounts in urine. Radiohaematological methods are also employed to evaluate changes in the peripheral blood, as well as quantitative and qualitative changes in the bone marrow cells. Of all the indicators studied to date, the most promising one is the cytogenetic analysis of peripheral blood lymphocytes following radiation exposure. The chromosome aberration frequency is a measure of the dose and the technique has been perfected to the extent that doses as low as 10 rad can be measured by this method. The study of the lymphocyte system as a potential biological dosimeter has many features that are of particular interest to the health physicist. 61

64 Body burden m easu rem ents The body burden of many radionuclides can be estimated in certain cases by analysing excreta (urine or faeces) for concentration of the radioisotope, using suitable radiochemical methods. The maxim um permissible body burdens and hence the excretion rates of high-toxicity radionuclides are low and hence extremely sensitive methods have to be used in such cases. This method is valuable for the routine monitoring of personnel handling materials such as uranium] and plutonium and also as a follow-up procedure in cases of accidental intake of radionuclides. Another method of measuring body burden is by using a wholebody monitor. One version of this instrument has several large sodium iodide crystals located at suitable positions inside a heavily shielded enclosure where the background radiation is extremely low. The person under examination must remain in this enclosure while whole-body counting is done. The pulses so obtained are fed into a pulse-height analyser for a determination of the energy spectrum of the radiations emitted by the body. These results are compared with measurements on a phantom containing standard sources and thus the concentrations and locations of radioisotopes in the body can be estimated. This instrument, which is elaborate and expensive, is not used for routine measurements, but is of great value for following up cases of persons who suffer from accidental intake of radioisotopes. The radium body burden of an individual can be estimated by measuring the concentration of radon exhaled in that person's breath. Radioiodine concentrates selectively in the thyroid gland, and hence its concentration can be estimated by scanning the thyroid externally. A scintillation counter is usually used for this purpose. 62

65 9. C A L I B R A T I O N A N D M A I N T E N A N C E O F R A D I A T I O N M O N I T O R I N G I N S T R U M E N T S 9.1. Introduction All radiation monitoring instruments should be carefully calibrated as soon as they are received for use. In addition they should be subjected to both mechanical and electrical inspection, and further, all environmental factors that could lead to any malfunction of the instruments should be carefully considered. The instruments should be stored in dust-free atmospheres and they should not be subjected to extreme changes of environmental conditions such as temperature and humidity. In the case of battery-operated instruments where the batteries are not leak-proof, it is important to ensure that the batteries are removed. These steps will generally ensure troublefree performance of such instruments. Another important factor involved in the use of radiation monitoring instruments is the need for regular calibration of these instruments at carefully predetermined intervals Objectives of calibration The main objectives of calibration procedures are: (a) (b) (c) (d) To ensure sound functioning of the instrument T o ensure that the measurements of the instrument are reliable and accurate in all ranges and for all energies in which the instrument is expected to provide energy-independent response To ensure further that the instrument functions reliably in the dose-rate range specified and that its directional-dependent characteristics are fully understood To confirm the performance of the instrument under extremes of environmental conditions in which it is certified to perform satisfactorily. A well-developed instrument calibration facility would enable the user to intercompare the relative performances of various types and makes of instruments in order to choose those designs of instruments which would meet the user's requirements with the m axim um reliability and accuracy. 63

66 9. 3. P rim a ry and secon d ary standards The proper calibration of instruments can be accomplished by one of two alternative means. One would be to use absolute standard radiation measuring devices such as free air chambers, calorimeters and extrapolation chambers for intercomparison with the instrument in question. The other approach, which would be more appropriate for most laboratories, wouldbethe use of a secondary standard instrument The secondary standard instrument, which in stability, reliability, accuracy and performance is a mean between primary standards and field instruments, can be periodically calibrated against a primary standard (say, once a year) and used in the laboratory for regular calibration of the other field instruments. Instead of using primary or secondary standard dosimetric devices, one can use standard radionuclide sources which have been previously calibrated against primary standard devices. Thus, for most laboratories, the use of standard sources and secondary standard instruments would be the most convenient procedure, since the setting up and maintenance of primary standard instruments are sophisticated and complicated operations. It is obvious that factors such as energy dependence, direction dependence, source-to-detector distance in relation to the sizes of the source and the detector and the possible contribution of scatter are crucial factors that should be taken into account in any calibration procedure Calibration techniques There are basically two approaches involved. In the first, standard sources whose radiation fields are well defined are used and the calibration procedures are conducted in scatter-free conditions In the second, approach, which is termed the substitution technique, the response of the instrument being calibrated is compared with that of a secondary standard instrument which has been previously calibrated against a primary standard. In this approach, as long as geometry and other factors are taken into account, it is not imperative to ensure scatter-free conditions Calibration procedure for personnel dosimeters Beta and gam m a personnel dosimeters: The various types of instruments under this category are film badges, pocket chambers and dosimeters, thermoluminescent dosimeters and radiophotoluminescent dosimeters. 64

67 Photographic film badges can most conveniently be calibrated by positioning the badges on a series of spider arms arranged radially with respect to a standard source. The film badges-are positioned on the spider arms at different distances from the source. With this setup, it is possible to calibrate several dosimeters to different predetermined dose levels at the same time. The same device can also be used to calibrate other personnel dosimeters. Care should be taken to ensure that the jig used for calibration is made of low atomic number material and that the geometry is reproducible. Together with the setup, an accurate timing device should be used. For beta calibration, a 90Sr - 90Y point source can be used. First the dose as a function of distance can be determined by using an extrapolation chamber. Then a suitable precalibrated field instrument can be used to verify the radiation field at any point prior to the calibration of personnel monitoring devices. A surface beta radiation calibration can also be done using a polished uranium disc Neutron personnel dosimeter: The Q-values (i. e. the number of neutrons emitted by the source per second) of laboratory neutron sources are determined by using primary standardization techniques such as the manganese activation method or the use of the long counter. Such sources are extensively used for fast-neutron calibration in the laboratory. For thermal neutron calibration purposes a sigma pile or a paraffin- or polyethylene-moderated system is normally used. The gold foil activation method is employed to determine the absolute values of flux and fluence at any point Calibration procedures for area monitoring instruments Area monitoring instruments can be classified both by the types of radiation they detect (e. g. beta, gam m a or neutron) and by the type of measurements they are intended for (e. g. dose rate, exposure rate or contamination). As stated earlier these instruments can be calibrated either by using a standard source under scatter-free conditions or in some cases by using a pre-calibrated secondary standard dosimeter. In the latter case, the provision of scatter-free conditions is not absolutely essential. It will be seen that the secondary standard dosimeter will be particularly useful in laboratories where space is at a premium and where, therefore, it would be difficult to ensure scatter-free conditions. Many survey meters currently in use are provided with built-in check sources which mainly serve as spot-check devices in the field, 65

68 to provide assurance that the instrument maintains its calibration. In a sense it is similar to a battery check device. Calibration of instruments in beta fields presents some unusual problems. Uncertainties, arising from factors such as (a) backscatter from the source and (b) absorption and scatter in the intervening medium with the consequent changes in beta energy characteristics, should be carefully weighed in calibration procedures. Neutron field instruments are normally calibrated using neutron sources whose Q-values or flux factors are known. Other neutron sources such as neutron generators can also be used for calibration purposes provided they have been standardized by primary techniques. In neutron calibration, the possible interference of gam m a radiation from the source should always be considered. 6 6

69 10. RADIATION CONTROL MEASURES General Where suitable precautions are taken, the handling of radiation sources is a safe operation. The hazards arising from the use of radiation sources can, as already described, be divided into two broad classes: (a) external hazards, and (b) internal hazards External hazards External hazards can be minimized by reducing all externa], radiation levels to values which are as low as practicable. This can be accomplished by a number of methods, including the following: (a) (b) (c) (d) (e) Ensuring that the minimum radiation output in the case of a radiation source such as an X-ray machine or the minimum quantity of radionuclide is used for any specified operation Maintaining the m axim um possible distance compatible with effective working methods between the radiation source and the worker Limiting the time spent in the vicinity of the source to the minimum necessary Using proper shielding between the source and the worker and, where necessary, using additional shielding to ensure that other persons in the vicinity or in adjoining areas are appropriately protected from radiation exposure Conducting regular area and personnel monitoring checks with a view to ensuring minimal radiation exposures Internal hazards Precautions against internal hazards include the following additional measures: (a) (b) (c) Conducting all operations with open sources in enclosures such as fume cupboards and glove boxes Ensuring that good housekeeping habits are maintained in all areas where open sources are handled Taking extreme care to ensure that any radioactive spill is confined to well-defined areas by using special appliances such as trays and, in the event of an uncontrolled spill, taking immediate 67

70 (d) steps to prevent further spread of any air, surface, effluent, personnel or other contamination Conducting periodic surveys aimed at ensuring that the degree of air, surface, effluent and personnel contamination is well within acceptable limits Radiation control measures Some typical radiation control measures which must be adopted to conform to the conditions laid down above will now be briefly described Sealed sources In a hospital which has facilities for brachytherapy (cancer therapy with sealed sources) it is frequently necessary to sterilize and thread a number of radium or cobalt needles, each of a few millicuries strength. The safe execution of such an operation involves: (a) (b) (c) (d) (e) Ensuring that the needles are safely stored with sufficient shielding when not in use and that appropriately shielded transport containers are provided for transporting them to their place of use Removing one needle at a time for threading while the remaining needles are held in their secondary shielded storage container Shielding the needle (except the eyelet) with an appropriate lead container during the threading operation to minimize radiation exposures during threading Using an automatic threader to minimize the time involved in the threading operation and hence the exposure to the technician Conducting all sterilizing operations in adequately shielded enclosures. Needless to say, the actual implantation operation should also be conducted in the m inim um time possible with maximal shielding facilities and suitable remote handling devices. Patients who have radiation source implants in them should themselves be treated as radiation sources and hence should be suitably segregated. The sources in their bodies should be provided with adequate external shielding to ensure that other patients or medical or paramedical personnel are not unduly exposed to radiation. Similar problems arise in the handling of industrial radiography sources. Where a camera with a suitably collimated source is used 6 8

71 in industrial radiography, adequate steps should be taken to segregate the operation and to provide appropriate shielding for all persons involved in the operation and for others in the vicinity. Similar steps of a more stringent nature will have to be taken in the case of panoramic exposures. Area monitoring instruments and, where necessary, radiation alarms should be provided so that any accidental situation that could arise could be immediately detected and adequate remedial measures taken Open sources Where open sources of radionuclides are handled, even more stringent precautions have to be taken since, once a radionuclide is released in an uncontrolled manner to the environment, it is very difficult to decontaminate or to contain the spread of contamination. This is particularly true in respect of ingestion and inhalation hazards, since radionuclides, once they gain entry into the body, cannot easily be eliminated from the body. Thus it is important to (a) (b) (c) (d) (e) Effectively contain radionuclides at all stages of handling Undertake all measures to minimize accidental spills, etc. Follow good housekeeping procedures Employ well-qualified and adequately trained personnel Provide, in advance, for well-thought-out measures to deal with any emergency. Some special aspects of control measures involved in dealing with open sources will now be briefly described. On completion of any operation in which radioactive materials are handled, even when gloves are worn, the hands of the personnel should be thoroughly washed and subsequently monitored. Whilst it is possible to install special equipment for monitoring hands, the small user can effectively use the conventional contamination monitor. The probe of this instrument should be moved slowly and carefully over the entire surface of the hands, with particular attention being paid to the edges of the hand and finger nails, and also to the ridges between the fingers. During normal operations it is unlikely that any part of the body, except possibly the hands and wrist, will become contaminated. However, in the event of a spill or other accident involving radioactive materials it is necessary to monitor the skin of the face and other exposed portions of the body. Again, the conventional contamination monitor can be used for this purpose. 69

72 Where open sources of radioactive material are used (1) the white coat or overall which should always be worn during operations and (2) when necessary, the clothing worn beneath the white coat or overall, should be carefully monitored for contamination by the method described above. 70

73 11. RADIATION SHIELDING G eneral One method of controlling external radiation exposure is to provide adequate shielding. Suitable radiation absorbing material is placed between the source of radiation and the personnel exposed to reduce the intensity of the radiation to acceptable levels. This reduction in intensity is known as attenuation and is a result of complex interactions between the radiations and the absorbing materials. These interactions have been described previously but are summarized in this chapter. The absorbing material used and the thickness required to attenuate the radiations to acceptable levels depend upon the type of radiation, its energy, the flux and the dimensions of the source. The amount of shielding material required may be calculated with reasonable accuracy in most instances, but only very approximately in others, and it is essential that experimental methods should be used wherever possible to evaluate the accuracy of the theoretical estimates. It is stressed that the final proof of the adequacy of shielding must be obtained by measurement of the intensity of the radiation with suitable instruments. The terms "tenth-value thickness" (T V T ) and "halfvalue thickness" (H V T ), which are frequently used in discussing radiation absorption, have been described in Chapter 3. The absorbing material should be installed as close to the source as possible to obtain m axim um economy; for, although the thickness required is the same, the area and hence the total volume of the absorber will be greatly reduced. Details of some suitable absorbing materials for the various radiations and some of the methods of assessing the amount of shielding required are given in the following sections. The important role played by distance in the control of radiation exposure should not be overlooked. Each case involving the exposure of personnel should be examined with a view to increasing the distance between the source and personnel as much as possible, com patible with operational efficiency Shielding for particle radiation Alpha particles Alpha particles lose energy rapidly in passage through matter and hence do not penetrate very far. For the energy range of alpha particles usually encountered, a fraction of a millimetre of any 71

74 ordinary material is sufficient to absorb them. Thin rubber, perspex, stout paper or cardboard will absorb them. This radiation is normally not an external radiation hazard since only at high energies can the alpha particles penetrate even to the basal layer of the skin Beta particles Beta particles do not lose energy so rapidly and are more penetrating than alpha particles. Their range in air varies from about 1 m at 0. 5 M ev to about 10 m at 3 M e V, but in comparatively dense material their range is very much reduced. For reasons explained in Chapter 3, materials composed mostly of elements of low atomic number such as perspex, aluminium and thick rubber are most appropriate for the absorption of beta particles. For example, \ inch of perspex will absorb all beta particles up to 1 M e V and 1 inch of perspex will absorb all beta particles up to an energy of 4 M ev. Hence it is advisable to use materials of low atomic number for beta shielding since, generally speaking, only a very small percentage of the beta radiation energy will give rise to bremsstrahlung from these materials. With high-energy beta particles from large sources, the bremsstrahlung contribution may become significant and it may be necessary to provide additional shielding of high atomic weight material (such as lead) to attenuate the bremsstrahlung radiation Neutrons Neutrons are uncharged particles and are therefore capable of considerable penetration in matter. The use of shielding to attenuate a neutron beam should be directed towards reducing the energy of the neutrons to levels at which they can be easily absorbed. A reduction of the energy of neutrons is best accomplished by collisions with atoms of light elements, such as hydrogen. For neutrons of energies above 1 M ev the use of heavier elements is also effective. Neutron interaction in these materials produces inelastic collisions from which the neutrons are ejected with reduced energy and gam m a photons are also emitted. Additional light elements must be used, however, to reduce the neutron energy to below that at which neutron capture becomes possible. The absorption of neutrons is most easily accomplished at thermal energy levels in suitable materials. The absorption process gives rise to secondary-particle and/or gamma-ray emission. 72

75 Water and paraffin wax are easily handled hydrogenous com pounds which are effective for reducing the energy of fast neutrons. For example, 10 inches of paraffin wax will attenuate 1 M e V fast neutrons by about a factor of 10. A thin sheet of cadmium (about 1 m m in thickness) is adequate for the absorption of neutrons of thermal energy. It must be remembered that the absorption process is accompanied by gamma-ray emission, and in places where this absorption is considerable it may be necessary to provide additional shielding of lead or other similar material to attenuate the gam m a radiation Shielding for X- and gamma rays General principles The attenuation of X- and gam m a rays in an absorbing material is the result of a combination of the photoelectric effect, the Compton effect and pair production, and is a complex process. These three types of interaction have been described in Chapter 3. The photoelectric effect is the predominant type of interaction at low energies, the Compton effect at medium energies and pair production at very high energies. Because of the increasing crosssections for interaction in materials of high atomic number, the most suitable materials for this type of shielding are lead and iron. In the medium energy range ( M ev ), the density of the material is more important than the atomic number, but in the COLLIMATOR ABSORBER FIG Narrow beam condition, 73

76 higher and lower energy ranges, materials of higher atomic number are more effective. The scattering of X- and gam m a rays in passing through the absorbing material involves two geometrical conditions which must be considered in shielding calculations: narrow beam and broad beam Narrow beam conditions: Here one considers a narrow collimated beam of gamma radiation which might be obtained from a small source such as is used in gamma radiography, but is usually produced only for experimental purposes. Photons scattered in the shielding material placed in the collimated beam- are removed from the emergent beam (Fig. 11.1). The total attenuation is then an exponential process expressed by where i = i0«i is the intensity (flux) of the radiation beam emerging from the shielding material, in photons / cm 2 s I0 is the intensity (flux) of the beam incident on the shielding material /u is the linear absorption coefficient for the shielding material referred to a thickness of 1 cm x is the thickness of the shielding material, in cm. It can be shown that the half-value thickness of a material is related to its linear absorption coefficient by the relation H V T = Similarly T V T = The reciprocal of the linear absorption coefficient has dimensions of length and is known as the mean free path (mfp). A thickness of 1 mfp will produce an attenuation to l/e, 2 mfp an attenuation to l/e2, and so on. The linear absorption coefficient varies with (a) the energy of the photon, (b) the atomic number of the material in question, and 74

77 (c) the density of the material. The term mass absorption coefficient, which is equ'al to the linear absorption coefficient divided by the density, is also often used. Mass absorption coefficient values reflect the relative efficiencies of different shielding materials for the same weight of the various materials Broad beam conditions: This is the more frequently encountered condition in radiation shielding problems. When broad parallel beams or divergent beams of radiation pass through attenuating material, some scattered radiation re-enters the emergent beam (Fig ). Thus the attenuation no longer follows an exponential process, as it is reduced by an amount known as the "build-up factor" for any particular source-shield arrangement. ABSORBER FIG Broad beam conditions. The calculations involved in the assessment of the thickness of shielding required for broad beam conditions are complicated and will be dealt with only briefly in this chapter. Although the basic requirements for large-source teletherapy and gamma-radiography installations have been described in the IA E A Manual, Safe Handling of Radionuclides, Safety Series No.l, 1973 Edition, it is stressed that in planning such installations expert opinion should be obtained at the design stage itself. There are, 75

78 however, a number of useful points which can be discussed in this chapter with regard to the basic shielding requirements in such cases. Protection of occupied areas adjacent to X- and gamma-ray installations may be achieved by shields or walls which absorb radiation. A s the cost of such shielding can prove to be an important consideration, judicious siting of these installations at a distance from occupied areas will be advantageous. For example, an isolated basement which is surrounded on all sides by the walls of its excavation and which therefore only needs appropriate shielding material on the roof may provide an economical site. The protective barriers can be of two types the primary protective barrier which is a barrier sufficient to attenuate the useful beam to the permissible levels and the secondary protective barrier which is a barrier sufficient to attenuate the scattered and/or leakage radiation to the permissible levels (Fig ). FIG Protective barriers for a radiotherapy installation. While calculating the primary shielding requirements for the useful beam of a radiation source, it should be assumed, in the interests of safety, that there is no irradiated object in the beam. The primary radiation in these installations is generally accompanied by scattered radiation, not only from the walls, floor and ceiling, but also from the object being irradiated. Therefore, while calculating secondary shielding requirements it should be assumed that scattered radiation, both from the object being irradiated and from all other areas such as the walls, floor and ceiling, is present. 76

79 The overall requirements of shielding can be reduced to a minimum if the useful beam is so controlled that it is used only in certain specified directions. The apparatus should be provided with suitable electrical or mechanical interlocks to ensure that the useful beam can only be used in the chosen directions. Where these directions may be chosen so as to avoid the walls of adjacent occupied areas, further economies can be effected in shielding requirements. The protection afforded by shielding must be spatially continuous and hence special attention should be given to joints, bolts or openings to ensure that suitable overlaps of the absorbing material are provided. W here a number of radiation units are contiguously present in the same general area, it is necessary to assume, in determining the appropriate shielding, that all the units will be simultaneously in use. As has been said previously, it is essential to know the exposure rate of the radiation which would be received at the point of interest T A B L E X- R A Y E X P O S U R E R A T E S Excitation voltage (constant potential) (kv) Total filtration -1 mm glass (or 0.07 mm Cu) X_ray exposure rate in R^mA'^min 1 at 1 metre Total filtration = 0.1 mm Cu Total filtration = 0.5 mm Cu a Exposure rates for pulsating potential generators are about one-half to two-thirds those for constant potential generators. 77

80 without any protection, and also the transmission values of the radiation in question through various thicknesses of absorbers. The exposure rate in the useful beam from any particular X-ray tube must be measured or obtained from the manufacturer; however, an approximate guide is given in Table Calculation of shielding requirements for X- and gamma-ray beams To ensure that adequate protection is afforded, the shielding thickness for an X-ray machine should be calculated for the m axim u m rated voltage and current of that particular machine. Similarly, for gamma-ray installations, shielding calculations should be based on the m axim um strength of the gam m a source proposed to be employed. For both X- and gamma-ray installations, the shortest distances between the source and occupied areas should be considered. Other factors to be taken into account are: (a) The work load (W ). This factor is related to the m axim um expected extent of use of an X-ray or gamma-ray source in a specified period of time. For X-ray equipment operating below 4 M V, the work load is usually expressed in milliampere minutes per week. For gamma beam therapy sources, and for X-ray equipment operating at 4 M V or above, the work load is usually stated in terms of the weekly exposure of the useful beam at one metre from the source and is expressed in roentgens per week at one metre. (b) The use factor (U). This factor indicates the fraction of the work load during which the radiation beam under consideration is directed at a particular barrier. (c) The occupancy factor (T). This is the factor by which the work load should be multiplied to correct for the degree of occupancy of the area in question while the source is "on". The broad basis for all shielding calculations is as follows. For any occupied area in the vicinity of an X- or gamma-ray source, the shielding provided is such that the integral dose which takes into account the work load, the use factor and the occupancy factor, (i) will not exceed 100 m R per week if occupational radiation workers are involved, and (ii) will not exceed 10 m R per week if persons who are not occupationally exposed to radiation are involved. The weekly m axim a of 100 m R per week and 10 m R per week have been arrived at from the annual maxima of 5 R and 0. 5 R respectively for occupational workers and members of the public. 78

81 Protection against the useful beam : The thickness of absorber required to reduce the exposure rate from the useful beam to that which would be the permissible weekly exposure rate at the point of interest may be calculated from the following expressions. The exposure per week E u at any point of interest is given by where E E t : d* E is the exposure rate in R /m in at 1 metre from the source in the useful beam t is the m axim um duration of operation of the unit in minutes per week d is the distance in metres between the source and the point of interest. If E u is greater than the permissible weekly exposure, P, a primary barrier of sufficient thickness to give a transmission factor of B ux (for X-rays) must be inserted into the beam between the source and the point of interest. It can be shown that E _ P d 2 B ux i w Taking into account the use factor and the occupancy factor, this equation can be modified as follows R - K - F d UX I UX W U T where I is the tube current in milliamperes. number of roentgens in a week for the useful beam normalized at one metre. P, the permissible weekly exposure, is taken as 0. 1 R for controlled areas and R for uncontrolled areas open to the public. The value of K ux can be used to obtain the barrier thickness from appropriate curves. For gamma- and high-energy X-ray equipment the work load is expressed as roentgens/week at 1 metre and 'u g Bux P d 2 W U T where B ug is the transmission factor for the useful gamma-ray beam and B ux the transmission factor for the useful X-ray beam. Using 79

82 appropriate curves of transmission factor versus barrier thickness, estimates of the required barrier thickness can be made Protection against leakage radiation: All X- and gammaray machines have in their design features a limit on the permitted amount of leakage radiation. The shielding requirements in respect of leakage radiation can be computed using the m axim um leakage radiation levels permitted for such machines. Here the use factor is considered as unity Protection against scattered radiation: The amount of scattered radiation will depend on the field size, the effective atomic number of the scatterer and the angle between the direction of scatter and the direction of the initial useful beam. Available experimental evidence on scattering from materials composed of elements of low atomic number (e.g. human tissue, wood, brick or concrete) indicates that at 1 metre to the side of an irradiated object (i. e. one metre away from the object in a direction at right angles to the initial useful beam) the exposure rate is about % of that at the surface of the irradiated body for each 100 c m 2 of irradiated field on the object. To calculate the shielding required against scattered radiation, the exposure rate of the radiation at the point of interest must first be estimated, from which the percentage transmission required to reduce this exposure rate to appropriate permissible weekly exposures can be evaluated. For radiations of high energy, softening of the beam due to scatter should be taken into account in computing barrier thicknesses. The transmission factor, B sg, needed to calculate the thickness of the secondary protective barriers for gamma beam sources is given by where a is the ratio of the scattered to the incident exposure. (The scattered radiation is measured at one metre from a phantom, which is typical of the object that is usually exposed to the beam, when the field area is 400 cm 2 at the surface of the phantom. The incident exposure is measured at the centre of the'field but without the phantom) dj is the distance from the radiation source to the person to be protected d2 is the distance from the radiation source to the phantom. 8 0

83 12. B A S I C D E S I G N F E A T U R E S O F R A D I A T I O N I N S T A L L A T I O N S General The hazards peculiar to the handling of radiation sources necessitate special features of design and construction which are not required for conventional laboratories or working areas. These special design features are so incorporated that the radiation worker is not unduly hampered in his operations while at the same time ensuring that he is not exposed to undue external or internal radiation hazards. Access to all areas in which exposure to radiation and to radionuclides could occur must be controlled not only in respect of the personnel who m ay be permitted to enter such areas but also in respect of the type of clothing they should wear and the precautions they should take. In the administration of such control measures, the classification of radiation areas, based either on the presence of ionizing radiations or on the presence of radioactive contamination or on both, is of great assistance. The introduction of such classification concepts at the planning stage itself will result in the installation being provided with all the built-in features that would render operations with radiation sources relatively less hazardous Classification of working areas and laboratory types A suggested form of such a classification of working areas is given in Table In addition to the broad classifications outlined above, it would be useful to classify, in a suitable manner, laboratories in which open sources of radionuclides are handled. The basis for such a classification is the grouping of radionuclides according to their relative radiotoxicities per unit activity. A classification of radionuclides is given in Table Group I could be classified as high toxicity radionuclides, Group II as medium-high toxicity radionuclides. Group III as medium toxicity radionuclides and Group IV as low toxicity radionuclides. O n the basis of toxicity considerations, the amounts of radionuclides which will be handled in a laboratory and the types of operation involved, three broad types of laboratories are envisaged. Table gives the types of laboratories along with the toxicity classifications, the m axim um amounts of radionuclides that can be handled and the modifying factors that apply in respect of the type 81

84 T A B L E Area type CLASSIFICATION O F WORKING AREAS Definition Control of access Typical examples 4 Areas within the confines of a radiation facility where the external radiation levels are negligible and where radioactive contamination is also not present. 3 Areas in which the average external radiation level is not greater than O.IR/week. Contamination is negligible and no special operating instructions are required. 2 Areas in which external radiation levels could exist and in which the possibility of contamination necessitatesspecial operating instructions. 1 Areas in which the external radiation exposure levels and/or radioactive contamination levels could be high. Unrestricted Access limited to radiation workers, no special clothing necessary. Access limited to radiation workers in appropriate clothing and foot-wear. Controlled access toradiation workers only, under strictly controlled working conditions and with appropriate clothing. Administration block Working areas in the immediate vicinity of radiography operations, such as control rooms, etc. Luminizing factories and other equivalent installations. Hot laboratories and similar facilities. Highly contaminated areas. of operations. The numbers in Table should be used only as broad guidelines. However, the requirements in respect of specified laboratories should be arrived at on the basis of detailed consideration of the various factors involved for each individual laboratory Considerations governing the siting/location of radiation installations The site for any radiation installation has to be chosen carefully and based on a variety of considerations. Some of the more important criteria are presented below. 82

85 T A B L E CLASSIFICATION O F ISOTOPES ACCORDING TO R E L A T IV E RADIOTOXICITY a Group 1 210pb 2ioPo 223Ra 225Ra 228Ra 221 Ac 227Th 228Th 230Th 231Pa 230JJ 232u 233U 234U 237Np 238Pu 239Pu 240pu 24ipu 212pu 241Am Am 242Cm 243Cm 244Cm 243Cm 246Cm 249Cf 250Cf 252Cf Group 2 22N a 36C1 45Ca 46Sc 54Mn 56Co 60Co 89Sr 90Sr 91Y 95Zr 106Ru no^gm 115Cdm iuinm 124Sb 125Sb 127Tem i29jem 124j 126j 131j 133j I34Cs 137Cs 140Ba 144Ce 152Eu (13 yr) 154Eu 160Tb 1,0Tm l8lhf 182Ta 192It 204-pj 207Bi 210Bi 211At 212Pb 224Ra 228Ac 230Pa 234Th 236JJ 249Bk Group 3 7Be 14C isf 24Na 38C1 31Si 32p 35g 41a 42k 43K 47Ca 47Sc 48Sc 48V 51Cr 52Mn 56Mn 52Fe 55Fe 59Fe 57Co 58Co 63Ni 65Ni 64Cu 65Zn 69Znm 72Ga 73As 74As 76As,7As 75Se 82Br 85 S7Kr 86Rb 85Sr 91Sr 90y 92y 93y 97Zr 93Nbm 95Nb 99Mo 96Tc 9ITcm 97Tc 99Tc 97Ru 103Ru IOSRu losrh 103pd 109Pd :05Ag Ag 109Cd 1I5Cd 115Inm I13Sn 125Sn 122Sb i25tem 127Te 129Te 131 xem 132Te 130j 132j 134j 135j 135Xe 131Cs 136Cs 131Ba 140La 141Ce 143Ce 142Pr 143Pr 147Nd 149Nd 147Pm 149Pm l51sm 153Sm i52eum(9.2h) 155Eu 153Gd 159Gd 165Dy 166Dy 166Ho 169Er 171Er nltm HSyb 177Lu 181w «SW 187W 183Re 186Re 188Re 1850s 191Os 193Os 190Ir 194Ir 191pt 193Pt 197pt 196Au 198Au I99Au 197Hgm 197Hg 203Hg 200-p^ pj 2»3Pb 206Bi 212Bi 220Rn 222Rn 23iTh 233Pa 239Np Group 4 3H is0 37A a C om 39Ni 69Zn 71Ge 85Kr 85Srm 87Rb 9iy m 93Zr 97Nb 96T c m 99X cm i»3rhm I13Inm X em 133Xe 134Csm 135Cs,47Sm 187Re i9i 0 sm i93pt m 197ptm 232T h T h -N a t 235u 238U U -N a t. a From ICRP Publication 5(1964). 83

86 T A B L E CLASSIFICATION OF LABORATORIES FOR HANDLING RADIONUCLIDES a Group of radionuclide Type of laboratory required for levels of activity specified below Type 1 Type 2 Type 3 1 < 10 jjci 10 /ici to 1 mci > 1 mci 2 < 1 mci 1 mci to 100 mci > 100 mci 3 < 100 mci 100 mci to 10 Ci > 10 Ci 4 < 10 Ci 10 Ci to 1000 Ci > 1000 Ci Modifying conditions Multiplication factors for activity levels Simple storage x 100 Very simple wet operations (e.g. preparation of aliquots of stock solutions) x 10 Normal chemical operations (e.g. analysis, simple chemical preparations) x 1 Complex wet operations (e.g. multiple operations, or operations with complex glass apparatus) x 0.1 b Simple dry operations e.g. manipulation of powders) and work with volatile radioactive compounds Dry and dusty operations (e.g. grinding) X 0.01 b X o a From ICRP Publication 5(1964). b These figures could be increased by one or more oiders of magnitude if operations are carried out in closed boxes General considerations Facilities in which highly toxic materials are handled or stored in quantity, or in which large sources are present, should be separated from other buildings to mitigate the effect of any radiation incident on nearby buildings and their occupants. Facilities of this type must not be sited close to potential fire hazards such as petrol, oil, or paint stores. It will also be necessary to consider these facilities in relation to the general public in the neighbourhood, and for large nuclear facilities, appropriate separation distances from centres of population will be necessary. Careful consideration should also be given to the potential release of low-level gaseous and liquid effluents from the installation, and to its impact on the environment. 84

87 For facilities in which less toxic materials are handled or in which smaller sources are installed, the conditions of siting need not be so stringent, but similar criteria should be applied. A facility m ay necessarily have to be served by operations in other buildings, e. g. the supply of radioactive materials from a production unit or the transfer of irradiated fuel from a reactor to a fuel reprocessing facility. The location of the various buildings in such a complex should be so planned as to minimize potential hazards from the movement of radioactive materials. Similarly, radioisotope laboratories in a building may have to receive radioactive materials from a central storage area in the building. Here again the same considerations would apply Location of a radiation installation in a building W hen a radiation installation is part of a large building, the following points should be borne in mind when deciding on the location of such an installation. (a) (b) (c) (d) The installation should be located in a relatively unfrequented part of the building so that access to the area can be easily controlled. Fire hazard potential should be minimal in the area chosen. The location of the installation and the ventilation facilities provided should be such that possibilities for spread of both surface and airborne contamination are minimal. The location should be judiciously chosen so that, with minimum expenditure on shielding, radiation levels can be effectively maintained within permissible limits in the immediate vicinity Planning of radiation installations General Where a gradation of levels of activity is envisaged, the laboratory should be so located that access to high radiation areas is gradual, i.e. one first enters from a non-radiation area into a low activity area, and then on to a medium activity area and so on. This kind of layout has been found to be most useful in limiting spread of contamination and also in controlling radiation levels. In laboratories where tracer work is being done, the limits of contamination in the counting 85

88 CORRIDOR DETAIL OF BAFFLE SECTION AT XX' V.W.-VIEWING WINDOW C.R - CONTROL PANEL C.-CONDUIT A.CB.-AIR CONDITIONER WITH BAFFLE BRICK WALL CONCRETE WALL FIG Layout of a cobalt teletherapy installation.

89 areas m ay be much more stringent than those permitted from the point of view of personnel contamination. This is an important factor to be borne in mind. In the design of radiation installations, heavy shielding requirements can be minimized by the adoption of certain simple measures. For example, in a typical cobalt beam therapy room the provision of a maze obviates the need for a heavy lead-lined door. The absence of such a heavy door also makes operational procedures more simple. Further reduction in the amount of shielding required for the beam therapy room can be effected by suitably tapering the primary protective barrier in areas which are not directly in the path of the useful beam. It is also necessary that careful attention be paid to the proper positioning of auxiliary fittings such as the lead glass viewing window, the underground conduit cables and the baffles for ventilation systems. In placing these fittings in position, care should be taken to ensure that there is no leakage of radiation. This can be done by providing sufficient overlap of shielding. Similarly, in the design of a maze, it is necessary that sufficient geometric shadow-shielding is provided so that the radiation field, for example, in the control room, is well within permissible limits. In the layout of the cobalt room as shown in Fig , it will be seen that the viewing window intercepts only the scattered radiation and hence its shielding requirements are minimized. In radiation installations such as accelerators, betatrons and high-intensity cobalt units, the use of closed circuit television in place of viewing windows results in improvement in shielding specifications. The provision of closed circuit television further improves the operational efficiency of the unit involved. The same considerations would apply in respect of hot cells as well. In small laboratories the need for elaborate control of ventilation can be avoided by the use of fumehoods or glove boxes for handling open sources of radionuclides. In the simplest case an ordinary chemical fumehood with an appropriate air flow-rate would be adequate. Where larger quantities of more toxic radioactive materials are handled a total enclosure type glove box may be required. Depending upon the operation carried out, fumehoods and glove boxes can be supplied with water, gas and vacuum lines. Where low-level discrete sources such as radium and cobalt needles and tubes are handled, as in a hospital, the need for elaborate shielded cells can be obviated by employing a simple L-bench. It is important that operations (1) in glove boxes, (2) with remote-handling devices and (3) behind the L-benches should be practised with dummies and confidence gained before starting actual operations with radiation sources. 87

90 Sto rag e fa c ilitie s for rad iatio n so u rces The sto rage a re a fo r radiation so u rce s and fo r stock solutions of radionuclides should p referab ly be sep arated from the a re a s w here d isp en sin g/ad m in istration /actu al handling of radionuclides is c a rrie d out. In the choice of the a re a for so u rce sto rag e, con sid eration m ust be given to p ossible damage to undeveloped photographic film s that m ay be p resen t in the vicinity. Depending on the type and quantity of radionuclides to be stored, sto rag e units such as radium safes or b uilt-in sto rag e sp aces such as co n crete pits can be provided. S pecial attention should be paid to the ventilation of sto rag e a re a s. F o r exam ple, where radium needles a re sto red, the p ossib ility of radon leakage from defective needles m ust be taken into account. In certain situation s, sm all au xiliary sto rag e fa cilitie s, as d istinct from the m ain sto rag e facility, m ay also be useful for exam ple, au xiliary sto rag e facilities in the operation th eatre of a ca n ce r hospital. W here only sm all quantities of rad ioactive m a te ria ls are handled, they can be conveniently stored in one co rn e r of the room, p referab ly inside a fumehood, provided they a re adequately shielded. In the provision of adequate shielding, carefu l con sid eration should be given to all occupied a re a s around, above and below the a re a of sto rag e W aste sto rag e facilities In m ost ca se s rad ioactive w astes could also be sto red in a sep arate a re a earm ark ed fo r the purpose in the so u rce sto rag e a re a itself. W here sm all volum es of relativ ely h igh-activity w astes are anticipated, sp ecial sinks with carb oys to contain the effluent should be provided to ensure that these w astes do not find th eir way into the sew age. Obviously in such operations m inim al amount of w ater should be used in o rd er to effectively r e s t r ic t the volum e of w astes co llected. How ever, in ca se s where larg e volum es of w astes of re la tiv e ly high activ ity a re envisaged, sep arate effluent lines leading to sp ecially built sto ra g e /d e la y tanks of adequate cap acity should be provided. In c a s e s w here only e x trem ely sm all-activ ity liquid effluents a re involved, these effluents can be allowed to go into the sew age d ire ctly. How ever, carefu l con sid eration should be given to the dilution fa cto rs involved and also to any p ossib ilities of reco n cen tratio n. 88

91 V entilation The aim of any ventilating sy stem should be to ensure (a) C om fortable working conditions (b) Continuous a ir change to ensure the dilution and rem oval of undesirable a ir contam inants. The ventilation req u irem en ts in a radioisotope lab o rato ry a re fundam entally the sam e as those appropriate to n orm al ch em ical lab o rato ries and working sp a ces. T h ere a re, how ever, featu res of the fo rm e r which req u ire som e carefu l attention. The ventilating a ir should be p assed only once through the a re a con cern ed. The a ir intake and exhaust positions should be well sep arated to avoid any re circu la tio n. The inlet side of the ventilation sy stem within the working sp ace should be as diffuse as p ossible to en sure good m ixing of a ir. The extractio n m ay be through fume cupboards o r cells and this will n orm ally be adequate. C onsideration m u st be given to ensuring that the sy stem is balanced to avoid failure of the extractio n sy stem in any fume cupboard o r ce ll. The position in which the exhaust a ir is released to the atm osphere should be at a suitable height above the ground and away from open windows. W here it is d esired to ventilate a num ber of working sp aces within one building, the flow of a ir should be from the n on-active into the active a re a s. P ro v isio n s should be made fo r shutdown of the ventilating system, eith er in individual working sp aces o r in the building as a whole, in the event of an accid en t. The con trols for this shutdown should be in easily a cce ssib le positions Ventilation req u irem en ts The quantitative asse ssm e n t of ventilation req u irem en ts in any working sp ace will depend on the operating equipment in that a re a, i. e. whether the work is c a rrie d out in totally enclosed fa cilitie s, such a s glove boxes, or in p a rtially enclosed fa cilitie s, such as fume cupboards. The ventilation req u irem ents for these a re a s a re v e ry different and a re d escrib ed below Ventilation in a re a s equipped with total en clo su res: The ventilation req u irem en ts in an a re a of this kind a re not d issim ila r to those in n orm al ch em ical la b o ra to rie s. In a n orm al working a re a th ree a ir changes p e r hour would probably be adequate; if n e cessary, this could be in cre a se d to five changes. H igher ventilation flow- 89

92 ra te s in a working sp ace of this kind a re not norm ally n e c e s s a ry and m ay have the disadvantage of leading to e x ce ssiv e and w idespread d isp ersal of rad io active m a te ria l in the event of an accid en t. In facilities of this kind, and p a rticu la rly w here highly toxic rad ioactiv e m a te ria ls a re involved, con sid eration should be given to the in stallation of filte rs on the exhaust side to lim it the d ischarge of these m a te ria ls to the atm osp h ere Ventilation in a re a s equipped with p artial en closu res: The crite rio n in a re a s of this kind is that ventilation flo w -rates a c ro s s the working ap ertu res of the equipment a re adequate to ensure that th ere is no backflow of contam inated a ir into the working sp ace. F o r fumehoods and fo r operating ap ertu res leading into p artially contained ce lls, lin ear flo w -rates of m /s should be provided. The total ventilation req u irem en t of the working a re a will th e re fore be determ ined by the num ber of openings fo r which such flowra te s a re n e ce ssa ry. However, in a working a re a where the num ber of fumehoods, cupboards o r cells is such that only half or le ss will be in use at any one tim e, the ventilation req u irem en ts m ay be a s s e s s e d accord in gly. In such c a s e s, provision m ust be made for reducing the a p ertu res of fume cupboards or cells when they a re not in u se. Individual filte rs a re not norm ally req u ired on fumehoods and ce lls, but w here it is thought d esirab le to in stall them they should be fitted on the exhaust side in positions w here they m ay be easily changed Surface finishes within a working a re a The carefu l design of equipment and an ap propriate ventilating sy stem will a s s is t in preventing the d isp ersal of rad ioactive con tam ination in any working a re a. N everth eless, w henever su rfaces a re potentially exposed to contam ination they should be such that they will r e s is t contam ination and facilitate decontam ination. It is not intended in this ch ap ter to deal with the equipment installed in the working a re a but ra th e r with the in ternal su rfaces of the building itself. How ever, the p rin cip les applied in this context a re equally applicable to equipment. The in ternal su rfa ce s of the building should be finished as sm oothly as p ossib le. This m eans that unplastered brickw ork should be avoided, and that e le c tric a l conduits, sw itches and sim ila r fittings should be so installed that the p ossib ility of th eir becom ing con tam inated is reduced to a minimum and that cleaning will be easy. 90

93 L edges of all kinds, such as open shelves or the extern al upper su rfaces of ventilation sy ste m s, should be avoided, as th ese will provide su rfa ce s on which contam ination m ay co lle ct. W here cupboards a re provided they should be fitted with sliding doors, and open shelves should be avoided. T hese standards apply p a rticu la rly to lab o rato ries of type 3 and a re d esirab le, but not n e ce ssa ry, for lab o ra to rie s of types 2 and 1. A ll raw su rfa ce s, such as p la ste r, co n cre te, wood, e t c., should be perm anently sealed off with a suitable m aterial, which m ay be paint, tiles, linoleum o r som e other suitable covering. The choice should be made with the following con sid erations in mind: (a) (b) (c) (d) The p rovision of a sm ooth, ch em ically in ert su rface The environm ental conditions of tem p eratu re, humidity, and m ech an ical w ear and te a r to which the su rfa ce s m ay be exposed C om patibility with radiation fields and ch em icals involved in operations in the a re a The need fo r re p a ir in the event of dam age. In addition to perm anent finishes on all su rfa ce s, th ere m ay be som e c a se s w here a tem p o rary finish is also useful. F o r in stan ce, in cells w here levels of contam ination m ay be consid erab le, the application of a su rfacing m a te ria l which can be rem oved easily m ay ease the problem of decontam ination la te r on. Strippable m a te ria ls should be applied only to good perm anent su rfa ce s. T here a re a num ber of strippable paints and lacq u ers available. However, the rem oval of such m a te ria ls m ay som etim es be difficult and m essy Change room s E n try to all a re a s in which open so u rce s of radionuclides a re handled should be arran g ed through a change room of appropriate stan d ard.. The classificatio n of a re a s given e a r lie r provides a b asis fo r deciding on the stan d ard of the change room req u ired, since the operating in stru ction s and req u irem en ts of any a re a will be based on the potential exposure of person nel to radiation and contam ination. E n try from a re a type 4 into an a re a type 3 would not n orm ally be through a change room, but th ere should be p rovision for person nel to keep th eir lab o rato ry coats or co v e ra lls in the a re a and to wash th eir hands. In a re a type 3 the contam ination would be kept to insignificant levels and th ere would be no req u irem en t for foot-w ear change. S im ilarly, the p rovision of hand and clothing m onitoring apparatus would be optional. 91

94 E n try into a re a type 2 would req u ire facilities in the change room fo r la b o ra to ry co ats o r co v e ra lls, hand-w ashing, foot-w ear change and com p u lsory hand and clothing m on itors. In a re a type 2 the conditions of contam ination would n ece ssita te s tr ic t com pliance with operating in stru ction s since significant levels of contam ination might be p resen t. E n tran ce into a re a type 1 would req u ire a carefu lly operated con trol point backed up by a fully equipped change room, in which th ere is provision for putting on p ro tectiv e clothing and foot-w ear and also fo r using show ers as well as wash b asin s. Hand and clothing m onitors would also be com pulsory. In a change room of this stan d ard it would be essen tial to have a suitable sto rag e facility for p ro tectiv e clothing. A ll change room s should be provided with suitable bins for the collection of contam inated clothing, such as co ats, gloves and fo o t-w ear. W here a single operating a re a contains sections ap propriate to a re a type 3 and a re a type 2, a fo ot-w ear change b a r r ie r between the two might be provided. This b a r r ie r is usually designed in the form of a bench beneath which a re ra ck s on each side fo r the appropriate fo o t-w ear. The su rface finish of all change room s should be the sam e as that fo r la b o ra to rie s, i. e. sm ooth and nonperm eable. 92

95 1 3. H A N D L I N G E Q U I P M E N T F O R R A D I A T I O N S O U R C E S G eneral In handling many types of radiation s o u rc e s, sp ecial equipment o r sp ecially modified conventional ch em ical equipment is n e ce ssa ry to provide adequate p rotection against the h azard s involved. Such d evices should be chosen carefully so that any re s tric tio n on the operations involved is reduced to a m inim um. The b asic aim s of such d evices would be: (a) Reduction of extern al radiation exposure (b) P revention of contam ination (c) P revention of intake of rad io active m a te ria l The shielded cell The shielded c e ll is used only in work p laces w here la rg e, highly active so u rce s a re used. The m ain featu res of such a ce ll are the following: (a) (b) (c) The m o st econ om ical site for a shielded cell is in the b asem ent of a building where the n atu ral shielding provided by the surrounding earth would red u ce costly shielding s tru ctu re s to a m inim um. In all c a s e s where such a ce ll is surrounded by oth er occupied a r e a s, the final th ickness of the w alls should re fle ct con sid eration of the m axim um stren gth of so u rce s to be used. This should include any future expansion p ro g ram s involving the possible use of la rg e so u rc e s, so as to obviate the need for exten sive m odifications la te r on. It would, of c o u rs e, be n e c e s s a ry to provide shielding above an d /o r below the cell to red u ce the s c a tte re d rad iation and to shield any working a re a s that m ay be p resen t. To prohibit unauthorized entry into the ce ll, adequate administra tiv e con trol m e a su re s should be set up, as recom m ended in the IAEA Manual, Safe Handling of R adionuclides, Safety S eries No. 1, 1973 Edition. All op erations within the cell should be cle a rly visible from the outside. F o r this purpose, a s e rie s of m i r r o r s, le a d -g la ss windows, windows filled with zinc brom ide solution o r suitable p erisco p es m ay be used. It is also essen tial that the lighting in the c e ll should be so arran ged and of such intensity that the o p erato r will not be ham pered by shadows o r dark a re a s. 93

96 (d) (e) Where the c e ll is to be used to handle solutions o r radionuclides in friable form, a suitable a ir extractor of sufficient capacity should be provided to ensure the prevention of leakage of radioactive m aterial. In all c a s e s, as emphasized e a r lie r, it is im portant to provide smooth, nonperm eable, chem ically in ert su rfaces in such ce lls so that any contamination can be easily rem oved. ( f ) The controls for all feeds into the c e ll, i. e. ele c tricity, w ater, a ir or g ases, e t c., should be situated outside the ce ll in a lo cation where they are easily a cc e ssib le. A tem porary version of a radiation c e ll, which can be easily made up, modified or dism antled, consists of lead or m ild -steel b rick s of dimensions approximating the conventional building brick. Furth er refinem ents, which become essen tial where large sources are to be handled, include interlocking faces on the b rick s to prevent radiation leakage between adjoining units and arrangem ents to perm it rem ote-handling equipment to be operated through the shield without decreasing the efficiency of the shield its e lf (Fig ). W here it is n ecessary to construct large shields,the sm aller units may be replaced by large m etal-loaded concrete blocks of a size equivalent to perhaps twelve or m ore of the sm a ller b rick s. FIG Interlocking lead bricks. 94

97 These loaded concrete blocks must be provided with rem ovable lifting arrangem ents, of which the sim plest is a pair of eye-bolts screw ing into appropriately tapped holes in the top of the block. Here again, ca re must be taken to ensure that such holes do not affect continuity of protection. In all ca ses it is essen tial that the b rick s o r blocks have smooth nonperm eable su rfa ces, and a suitable paint must be used to ensure th is, particularly in the case of the m ild -steel b rick s and concrete blocks, the su rfaces of which tend to be rough and porous Rem ote-handling apparatus In handling sources of beta- and/or gam m a-em itting m aterials, the use of distance as a means of reducing radiation exposure is very im portant. To provide this protection and at the sam e tim e perm it easy manipulation of the active m aterial, long-handled tongs have been devised for working at distances up to about 1 m from such so u rces; long-handled forcep s may also be used up to a d istance of about 15 cm from the source. The long-handled tongs of the sim plest pattern have a pistol grip together with a trig g er operating a pincers grip which can be used to seize the radiation sou rce. An additional elaboration is the FIG Remote-hand ling tongs. 95

98 provision of a threaded screw which can be screw ed up so that the pincers rem ain securely fastened to the radioactive source. In this way one can secure a source with one pair of rem ote handling tongs and use another pair to carry out some operation on it. Where the rem ote handling tongs must pass through the lead or m ild -steel brick w all, sp ecial brick s are n ecessary which, while ensuring ease of operation, do not give rise to radiation leakage. The longhandled forceps are sim ilar to those which are extensively used in m edicine and surgery (F ig, 13. 2) Shielded flask s and test-tu be holders F o r te st tubes containing radioactive solutions o r m aterial, sm all sections of conventional lead piping long enough to enclose the te st tube com pletely can be used to advantage. On account of the FIG Temporary perspex shielding for handling beta-active material. 96

99

100 frag ile n ature of the te s t tube and the fact that it holds rad ioactive m a te ria ls, a cushion should be placed at the bottom of such a contain er so that when the te st tube is placed in it th ere will be no chance of breakage and resu ltan t re le a s e of contam ination. A s e rie s of these sections of lead piping can be arran ged in a ra ck to hold as many te st tubes as req u ired. F o r flask s, p articu larly sm all ones, conventional lead piping m ay be suitable, o r sheet lead may be used to co n stru ct a suitable cylin d rical section into which the flask containing the rad ioactive m a te ria l m ight be placed. Cushioning m a te ria l should again be placed at the bottom of the shielded en clo su re T em p orary shields for b eta-activ e radionuclides P e rsp e x in sheet o r moulded form o r m a te ria l such as aluminium o r copper may be kept at hand and used as tem p orary shielding when b eta -a ctiv e nuclides are handled (see F ig ) Liquid d isp en sers M outh-operated apparatus should not be used in any rad ioactive w ork. V arious designs of m echanically op erated pipettes and other sim ila r apparatus have been produced; som e of them work by m eans of ru b ber bulbs and oth ers use a syringe action. F in g er-o p erated valves a re used for the re le a se of liquids from the pipette. It is also possible to co n stru ct a m echanically operated pipette from lab o rato ry m a te ria ls (F ig. 13.4). Shallow tra y s made of m etal o r of suitable p lastic m ust be used as a precaution agains sp illage. F ra g ile con tain ers of rad io active solutions should alw ays be placed in th ese tr a y s, which should be lined with absorbent m a te ria l. The tray should be of sufficient cap acity to hold the en tire contents of the con tain er. In addition, the tray should be without rough su rfa ce s, edges o r co rn e rs so as to facilitate decontam ination R adioactive w aste bins F o r the collection of w astes in the la b o rato ry, a modified version of the ord in ary foot-op erated dustbin is the m ost suitable. The m odification co n sists in the dustbin being so made and furnished as to facilitate easy decontam ination. Inside the dustbin, suitable lining bags of p lastic o r waxed paper should be provided to contain the w astes. 9 8

101 1 4. C O N T A I N E R S F O R R A D I O A C T I V E M A T E R I A L S G eneral The b asic prin cip les governing the design and con stru ction of co n tain ers for rad io active m a te ria ls a re the sam e whether the conta in e rs a re used for sto rag e o r fo r tran sp o rt. H ow ever, in the actu al designing of these co n tain ers, an a sse ssm e n t of the risk s to which they m ay be exposed under conditions of actu al use is an essen tial and im portant step. The m ain fa cto rs which need to be con sid ered in the design of co n tain ers include the following: (a) The extern al radiation hazard (b) The p resen ce of rad io active m a te ria l in friab le o r powder form (c) The p resen ce of rad ioactive m a te ria l in solution form (d) The degree of containm ent req u ired and the sev erity of the operational s tr e s s e s R equirem ents Alpha, beta and gam m a so u rces The essen tial feature of con tain ers for rad io active m a te ria ls is that they m ust be provided with a thickness of shielding m a te ria l adequate to reduce the extern al rad iation s to accep tab le lev els at which they m ay be safely handled, sto red o r tran sp o rted. F o r beta e m itte rs, con tain ers are m ade of low -Z m a te ria ls, such as p ersp ex, having sufficient wall th ickness to absorb the beta rad iation. Additional shielding for b rem sstrah lu n g should be con sid ered where n e c e s s a ry. As for alpha e m itte rs, the n e cessa ry shielding can even be provided by sm all th ick n esses of these m a te ria ls. F o r gam m a e m itte rs, the con tain er m ust include a sufficient thickness of highdensity m a te ria l to reduce the gam m a radiation on the outside to accep tab le le v e ls. T h ere are two possible approaches to this problem. In one, the con tain er m ight be designed for continuous handling by p ersonnel during working h ours, in which case the thickness of m a te ria l req u ired could be con sid erab le. In the o th er, a co n serv a tive estim ate could be made of the av erage period of tim e for which a person would be expected to handle the con tain er o r be in its v icin ity, in which case the shielding req u irem en ts would be considerably red u ced. L ead, iron o r co n cre te can be used for the 99

102 co n tain ers, 'but it is recom m ended that where lead is used som e form of m etal containm ent should be provided so that in the event of the con tain er being involved in a fire, although the lead would m e lt, the con tain er would still m aintain its distribution and, th e re fo re, its shielding p ro p erties. While fab ricatin g such co n tain ers, c a re should be taken to prevent voids. It is also recom m ended that the extern al and in tern al su rfaces of th ese contain ers should be suitably painted to provide a su rface which will r e s is t contam ination and can be easily cleaned. The lids of these contain ers should fit reason ab ly well (though not so tightly as to ren d er operations difficult) and should be provided with sufficient shielding overlap so as to avoid any leakage of rad iation. As an added p recaution, a suitably long ca rry in g handle o r providing the con tain er with wheels would ensure minimum exposure to the person handling the con tain er. E ach con tain er should be provided with adequate m arkings and app rop riate radiation sym bols which cannot be easily rem oved o r era se d Neutron so u rces F o r reaso n s outlined in C hapter 11 on shielding, fast-n eu tron so u rce s should be firs t surrounded by som e slowing-down medium such as paraffin wax which should be contained in a hollow v essel of suitable fire -re s is tin g m a te ria l. The th erm alized neutrons em erging from this p rim ary shield can be absorbed by using suitable m a te ria ls such as cadm ium. The n e cessary thickness of lead or oth er sim ila r m a te ria l should, how ever, be provided as additional shielding against any radiation that m ight resu lt from such absorption Solid rad ioactive m a te ria ls It is recom m ended that solid rad ioactive m a te ria ls should be placed within sm all contain ers of m etal, p lastic o r other suitable m a te ria l b efore they are placed in the outer shielded con tain ers re fe rre d to above. This will facilitate the rem o val of active m a te ria l from the larg e container and will further prevent the inside of the m ain con tain er from being contam inated R adioactive m a te ria ls in friab le o r powder form F o r rad ioactiv e m a te ria ls in friab le o r powder form a container of the type d escrib ed in Section should be used. Such a container should be firm ly sealed to prevent any escap e of rad ioactive m a te ria l, but it should also be easy to open sin ce, in rem oving the stopper, 1 0 0

103 the m inim um amount of exertion is d esirab le le st any vigorous action should sp ill the m a te ria l Liquid rad ioactiv e m a te ria ls It is recom m ended that rad io active m a te ria ls in liquid form should be placed in a suitable sealed con tain er which in turn m ust be packed inside a second sealed con tain er. The in tersp ace between th ese two con tain ers m ust be packed with an absorbent m a te ria l sufficient to ab sorb the en tire liquid content and of such a nature that its efficiency will not be im paired by ch em ical reactio n with the contents T estin g of sealed so u rces All sealed so u rce s of rad ioactive m a te ria ls m ust be regu larly tested for possible leak age. D iscrete so u rce s such as radium or cobalt needles and tu b es, which a re extensively used in h ospitals and which a re subjected to consid erab le handling and m ech an ical s tr e s s e s, should be tested v ery frequently for leakage as well as total content of activ ity, p referab ly once every th ree m onths. Other lab o rato ry sealed so u rce s and larg e in stalled s o u rc e s, such as the so u rce in the head of a teleth erap y unit, a re not d irectly a cce ssib le for leak -testin g p urposes. In such c a s e s (i) the inner su rfa ce s of lab o rato ry so u rce con tain ers and (ii) sh u tter openings o r inner su rfa ce s of co llim ato rs in the c a se of la rg e so u rce h old ers should be tested for any deposition of activity by taking swipe sam p les. T e sts of this type should be conducted p referably annually o r at le a st once every two y e a rs T estin g of radium needles and tubes A sim ple method of testin g radium needles and tubes is to put the so u rce in a te st tube, co v e r it with a cotton wad and leave the assem b ly fo r about 24 h. During such sto rag e it may be advisable to apply m od erate heat to the te st tube, e. g. with steam, to d isco v er any pin-hole leak. It is, h ow ever, v ery im portant to avoid e x ce ssiv e tem p eratu res during this p ro c e s s. The cotton wad is then a sse sse d for activity. Another method of conducting this te st is to p lace the sou rce in a glass tube with a length of polythene tubing (about 4 cm long x 3 m m diam. ) attached to it. The tubing is closed with a rubber plug and left fo r 48 h. Any beta activity in the tube is then m easu red with an end-window GM counter to indicate leak age. 101

104 G en eral m ethods of te stin g seale d so u rce s fo r leakage Wipe te s t: All exposed extern al su rfaces of the capsule to be tested are wiped thoroughly with a piece of filte r paper of high wet strength and absorption cap acity, m oistened with a solution which will not attack the m a te ria l of which the container o r so u rce holder is m ade and which, under the conditions of this te s t, has been dem on strated to be effective in rem oving the radioisotope involved. The paper is allowed to dry. Then rad ioactivity on the paper is m easu red C ellu lose-tap e te st: This te s t is applicable to a device containing a sealed so u rce in a so u rce holder which has an opening through which the radiation em e rg e s. This opening is kept covered for a m inim um of 7 d with a p iece of thin adhesive cellu lose tape. The tape is rem oved carefully and the rad ioactivity which may have been deposited on its adhesive side as a resu lt of any leakage from the sealed so u rce is m easu red. Wipe te sts can also be c a rrie d out at the openings, sh u tters o r co llim ato rs of such so u rces Scrub te s t: The capsule to be tested is im m ersed in a solution which will not attack the m a te ria l of which the container o r so u rce holder is m ade and which has been dem onstrated to be effective in rem oving the radioisotope involved when the ob ject is scrubbed with a brush thoroughly under these conditions. The total rad io activ ity is m easu red in the resid u e obtained by evaporation of the solution Im m ersion te st I: The ob ject to be tested is im m ersed in a solution which will not attack the m a te ria l of which the container o r so u rce holder is made and which, under the conditions of this te st, has been dem onstrated to be effective in rem oving the radioisotope involved. The solution is heated to about 50 C and held at this tem p eratu re fo r 8 h. The total rad ioactivity is m easu red in the resid u e obtained by evaporation of the solution Im m ersion te st II: The so u rce is im m ersed in w ater which is heated to 100 C and held at that tem p eratu re for 5 min. The w ater is then rem oved, the so u rce cooled and the p roced ure rep eated tw ice. S ources are p assed if the activity e xtracted in the final p roced u re does not exceed nci. In conducting any of the above-m entioned te s ts, all n e ce ssa ry sp ecial d evices m u st be used and p recau tion s taken to avoid any e x ce ssiv e exp osu res. 10 2

105 1 5. P R O T E C T I V E C L O T H I N G G eneral P ro te ctiv e clothing for rad ioactive work m ay be con sid ered under two headings routine and em ergen cy. The la tte r includes p ro tectiv e clothing fo r p urposes such as sp ecial m aintenance, decontam ination and em erg en cy operations Routine p ro tectiv e clothing L a b o ra to ry coats Conventional white cotton d rill o r nylon co ats of p rop er size which should extend below the knees a re suitable fo r clean a re a s O veralls (b o ile rsu it) T hese a re on e-p iece cotton d rill garm ents so designed as to co v er the body com p letely excep t fo r the head and neck, w rists and hands, and feet and ankles. The fastenings for these garm ents a re u sually at the front. They a re e x trem ely useful as they p ro te ct all the clothing worn below Aprons W here the p ro c e s s e s involve working at benches with liquids, an apron of suitable im pervious m a te ria l such as PV C, polyethylene or neoprene will be found useful in preventing the clothing below from becom ing contam inated by co rro siv e liquids o r dust Rubber gloves F o r gen eral lab o ra to ry w ork, su rg ica l gloves a re adequate for m ost op erations. W here it is n e c e s s a ry to handle b eta -a ctiv e m a te ria l d ire ctly with the hands, rubber gloves of a h eav ier type o r le a th e r gloves m ay be used to reduce the b eta-rad iatio n dose to the hands Footw ear Shoes: T hese should p referab ly be ru b b er-so led to p reven t the uptake of contam ination and to facilitate cleaning. It is 103

106 recom m ended that the p attern of the rubber sole should not be too deeply indented. The upper p art of the shoes should be well waxed to r e s is t the absorption of contam inated solutions, etc O versh oes: These a re worn over the n orm al walking shoe and a re suitable fo r use by v isito rs to active a re a s or fo r general use in la b o ra to rie s. The conventional rubber oversh oes a re suitable but the soles should not be too deeply indented. A ch eap er form of overshoe made of rubber, p lastic o r canvas is also available Rubber boots: T hese a re p a rticu la rly useful for w ear in a re a s in which the p ro ce sse s involve contam inated solutions o r wet conditions, such as in a re a s being decontam inated. The half-length ru b ber boot is usually adequate fo r this purpose. The soles of these boots should not be too deeply indented B reath in g apparatus F o r work in a re a s of low or medium level of airb o rn e activity, a fu ll-face re s p ira to r with an efficient filte r provides adequate p rotectio n. The filte r used m ust be reliab le; suggested types are the resin wool and ch arco al o r the highly efficient paper filte rs which a re co m m ercially available. C are m ust be taken to ensure that these re s p ira to rs fit p rop erly and do not allow a ir to be taken in from the sides of the fa ce-p ie ce. F o r work in a re a s of v e ry low a ctiv ity a h alf-face re s p ira to r m ay be used E m erg en cy p ro tectiv e clothing (including sp ecial clothing for purposes such as m aintenance work and decontamination) P ro v isio n of this clothing is usually indicated in a re a s where the operations con cern ed will expose the personnel involved to a high risk of contam ination and of breathing contam inated a ir. B efo re w earing em ergen cy p rotective clothing, including fully im perviou s clothing, one has to have a full change, which im plies rem o val of all p erson al clothing and the w earing of sim ple clothing supplied by the lab o rato ry. A typical change would be d rill tro u se rs, u nderw ear, sh irt, sock s and shoes F u lly im pervious clothing This co n sists of garm ents so designed that they co v er the individual com pletely, excep t fo r the head and neck and the hands and feet, 104

107 with a la y e r of im pervious m a te ria l. As a p rotection for the head, and to allow a re s p ir a to r o r breathing apparatus to be worn at the sam e tim e, hoods of sim ila r m a te ria l a re used. Fin ally, com plete exclusion from contam ination is obtained by w earing rubber gauntlet gloves pulled well over the cuffs of the p ro tectiv e suit and rubber boots with the bottom of the im pervious suit tro u sers brought over them P re s s u riz e d clothing This is a suit made of im pervious m ate ria l which com pletely en closes the individual. Such a suit effectively iso lates the individual inside from any contam ination on the su rfaces o r in the a ir. C om p re sse d a ir supplied to the suit enables n orm al breathing during op erations. The co m p re sse d -a ir line d elivers its a ir im m ed iately in front of the face, as this arran g em en t provides plenty of a ir fo r breathing and at the sam e tim e helps to reduce the m isting of the tran sp aren t h ead -p iece. Com plete p rotection of the hands and w rists is afforded by w earing rubber gloves, which a re se cu re ly taped to the suit to preven t the in g ress of any contam ination. Rubber boots a re usually worn with the su it. A ssistan ce is alw ays n e c e s s a ry to d re ss an individual in any im pervious clothing. Such a ssista n ce is p a rticu la rly essen tial when the im pervious clothing is being rem oved. If the individual concerned u n d resses h im self it is m ore than likely that he will b ecom e contam inated from the activ e m a te ria l p resen t on the suit B reath in g apparatus W here a c o m p re sse d -a ir supply is not available for use in the p re ssu riz e d su its d escrib ed above, breathing se ts m ay be obtained. T hese com p rise a w ell-fitting fa ce -p ie ce in which suitable goggles a re in serted. This fa ce-p ie ce is attached to a cylind er o r cylinders of co m p ressed a ir c a rrie d in a h arn ess on the w e a re r's back. Regulating valves attached to the cylinders enable the w e a re r to con trol his a ir supply. This b reath in g -set m ay be used in conjunction with the fully p ro tectiv e im pervious clothing and, so d ressed, the individual con cern ed can en ter a contam inated atm osphere under em erg en cy o r m aintenance conditions D econtam ination of p ro tectiv e clothing Routine p ro tectiv e clothing should be cleaned reg u larly to avoid the build-up and fixation of contam ination. F o r sim ila r reaso n s, the 105

108 clothing should not be left in sto rag e fo r long'periods before cleaning, a s exp erien ce in d icates that such sto rag e re su lts in the contam ination being fixed and ren d ers decontam ination in creasin gly difficult. Another m easu re taken in such clothing to avoid build-up and fixation of contam ination is to provide no pockets o r belts and to m inim ize folds White coats and co v eralls In sm all estab lishm ents it will not usually be n e c e s s a ry to s e g r e gate this clothing, before washing, accord in g to the different con tam inants and different levels of contam ination. How ever, in la rg e r establishm ents w here a v a rie ty of rad ioactive m ate ria l is used, it is often n e c e s s a ry to seg reg ate the clothing to p reven t c r o s s contam ination during the cleaning p ro c e s s. Contam inated clothing should be handled as little as p ossible, sin ce it can give ris e to airborne contam ination. The p ro ce ss used in cleaning this type of clothing is not d issim i la r to the p ro c e s s e s adopted in conventional lau n d ries. It is useful to rem em b er that the m ore strin gent washing solutions used n e c e s s i tate the use of suitably re sista n t m etals in the con stru ction of washing m ach in es. The actu al routine of washing will depend on exp erien ce, but as a guide it is usual to give clothing two full w ashes (of 10 min each) with a rin se (of 5 min) in c le a r w ater a fte r each wash. The reagen ts or soaps used will also depend upon exp erien ce, but a washing solution consisting of unbuilt d etergent, sodium m etasilica te, sodium acid phosphate and c itric acid has given good re su lts with this type of clothing. The clothing should be dried thoroughly before being m onitored Rubber gloves It is essen tial that the p ersonnel w earing these gloves, and p a r ticu la rly the heavy ru b ber gloves, should wash them on com pletion of th eir w ork. Bulk collection of contam inated gloves is m ost u n satisfacto ry, sin ce th ere is no efficient way in which they m ay be washed in larg e num bers without tra n sfe rrin g contam ination to the inside. G ood-quality soap o r d etergents and scrubbing brushes should be provided at ap propriate p laces w here person nel m ay wash th eir gloves. The gloves should be well scrubbed and rin sed and then dried with p aper tow els o r p referab ly with sm all p ieces (say 2 x 1 ft) of towelling, which have proved econ om ical and m o re sa tisfa cto ry 106

109 than p ap er tow els. The sm all tow els should be used once only and then p laced in a suitable collectin g bin R e sp ira to rs and dust m asks The only sa tisfa cto ry way in which these item s m ay be cleaned is by individual swabbing with suitable d etergen ts. The d etergent used should be a m ild one and unlikely to cau se skin com plaints if it com es into con tact with the faces of individuals. Sm all cloth or cotton swabs should be used all over and inside the fa c e -p ie c e s. T hese swabs should be changed frequently. C are should be taken to clean the outside fir s t and then the inside using clean sw abs. The fa c e -p ie c e s should be well swabbed with clean w ater, following the detergen t treatm en t, and then dried. The filte rs m ust be handled carefu lly and the solutions should not be allowed to get into the filte r m a te ria l, otherw ise the efficien cy of the filte r m ay be im paired Im pervious p ro tectiv e clothing On com pletion of an operation involving the use of this kind of clothing and w here e x tre m e contam ination is expected, it is essen tial that the o p erato r, s till w earing the suit, should p ass through som e fo rm of washing. This m ay co n sist of an installed show er or sim ply of a rin se with buckets of w ater. T h ere is obviously a need to choose the c o r r e c t position for this so a s to preven t d isp ersal of con tam ination, and it should be followed by washing with detergen t and soap solutions, a ssiste d by swabs and soft b ristled b ru sh es. O perations in th ese su its usually involve at le a s t two individuals and it is a convenient p ra c tic e fo r them to wash each oth er. A fterw ard s the su its should be rin sed in clean w ater and quickly dried. When dry, the suit is rem oved and m onitored. Any resid u al contam ination can be treated se p a ra te ly with mild ab rasiv e p astes o r sim ila r m a te ria l. Suitable detergen ts should be used fo r washing these suits and it is also advantageous to use solutions containing weak c itric acid and com plexing agents such as e th y le n e -d ia m in e -te tra -a ce tic acid (EDTA) Footw ear R u b b er-soled shoes m ay req u ire decontam ination from tim e to tim e. The so les should be scrubbed with d etergent and com plexing solutions, and for re sista n t contam ination it m ay be n e c e s s a ry to rem ove the su rface of the rubber by the application of acetone or by 107

110 m ech an ical buffing. W here m ech an ical buffing is used, it will be n e c e s s a ry to provide fo r lo cal a ir extractio n on the m achine. The upper p a rt of the sh oes, if kept p rop erly waxed, can be easily decontam inated. Rubber boots should be cleaned a fter each operation. Scrubbing in d etergent and com plexing solutions should be followed by the use of ab rasiv e p astes where n e ce ssa ry. R esistan t contam ination will n e ce ssita te the rem oval of the rubber su rface by acetone or m echanica l buffing. P recau tio n s should be taken to p reven t contam inated liquids from entering the boots

111 1 6. A R E A A N D E N V I R O N M E N T A L M O N I T O R I N G G eneral R adiation m onitoring of the working environm ent and the surrounding a re a s is an essen tial p a rt of any effective radiation p rotection p ro g ram to ensure that neith er the operating p ersonnel n or the gen eral population re ce iv e s radiation doses in e x ce ss of p erm issib le lim its. The type and exten t of the environm ental m onitoring p ro g ram for a p a rticu la r radiation in stallation will la rg e ly depend on the individual circu m sta n ce s. Some typical guiding fa cto rs a re the types of facilities and th eir in ven tories, the n ature of work with rad iations and rad io active m a te ria ls, the types and quantities of radionuclides handled, and the possib le rou tes and ra te s of re le a s e of rad io activ ity into the environm ent. A continuing a s s e s s m ent of the radiation situation in the working environm ent is n e ce ssa ry to ensure safe working conditions. This p roced ure is an essen tial adjunct to the p erson nel m onitoring sy stem which m easu res the ra d ia tion doses receiv ed by operating p ersonnel A rea and environm ental su rveys including p re-o ccu p atio n al su rveys In ca se s w here open so u rce s a re to be handled, p relim in ary su rv ey s should be conducted w here ap propriate, to determ ine background radionuclide con centrations in the environm ent. Such studies on environm ental radionuclide con centrations should be continued to ensure that during operations radionuclides a re being released to the environm ent in a con trolled m an n er. F o r exam ple, in a hospital, this could m ean p eriod ic m onitoring of the effluents disch arged from the hospital. Depending upon the individual circu m sta n ce s, the working environm ent m ay be m onitored by the use of in strum ents such as installed radiation m on itors, p ortab le su rvey m e te rs, a ir m onitors and contam ination m o n ito rs. O ff-site m onitoring is n o rm ally c a rrie d out by methods of routine a ir and effluent sam pling Installed radiation m onitors Installed radiation m onitors a re in variably fitted with visu al and au ral in d icators so that when a p red eterm in ed lev el of radiation is reach ed, this fa ct is im m ed iately brought to the attention of the o p erato r. In a re a s of potential critic a lity accid en ts, a re a m onitors 10 9

112 con sisting of TLD or g lass d o sim eters, ch em ical d o sim eters, tra ck d etecto rs and neutron activation d etecto rs a re in stalled. In case of an accid en t th ese m onitors a re im m ed iately retriev ed and dose e s tim ates m ade. In la rg e in stallation s, rem ote-m on itorin g devices for m easu ring rad iation levels and airborne and su rface contam ination levels a re in stalled with devices fo r continuously relayin g in form a tion to a ce n tra l record in g facility P o rtab le su rv ey m eters Routine a re a m onitoring both inside any radiation working a re a and also in its im m ediate environm ent is an im portant a sp e ct of a com prehensive radiation safety p ro g ram. P o rtab le su rvey m eters which m ainly use ion ch am b ers, GM cou nters and scin tillato rs a re used in th ese op erations. W here th ere is potential for high radiation in ten sities eith er as p a rt of n orm al planned operations o r as a resu lt of off-n orm al or accid en t situations, fish -p ole probe type su rvey in strum ents with high ran ges should be used. Such instrum ents should p referab ly be b attery -o p erated and should be p ortab le Surface contam ination m easu rem en ts In m onitoring for the contam ination of su rfa ce s, one of the following methods m ay be adopted. In the d ire ct method, the probe of the conventional contam ination m onitor is used to scan the a re a which is su sp ected of contam ination and the reading so obtained provides a d irect m easu re of the degree of contam ination. W here the su rface a re a s involved a re very larg e, it m ay not be p racticab le to m onitor the en tire su rface and in such c a s e s it will be sufficient to m onitor rep resen tativ e a re a s. W here such d ire ct m easu rem en ts a r e im possible, a swab technique should be used. This involves rubbing a filte r paper lightly over the contam inated su rfa ce, usually covering an a re a of up to 100 c m 2. This filte r p aper is then a sse sse d fo r activ ity, using the contam ination m on itor A ir m onitoring Routine m onitoring of the a ir is a valuable m eans of ensuring the effectiven ess of safety p recau tion s taken to p reven t undue re le a se of rad io activ ity eith er in the working a re a o r to the environm ent. This p roced u re is p a rticu la rly essen tial in those lab o rato ries where significant quantities of unsealed rad io active su bstan ces a re handled

113 A irb orn e rad ioactiv e contam ination m ay co n sist of fine p a rticle s of radionuclides o r th eir compounds in th eir pure form o r in a s s o c ia tion with dust p a rtic le s. When contam inated a ir is drawn through a sp ecial filte r p ap er, m ost of the p articu late m atter is retained on the filte r p ap er. The rad io activ ity co llected on the filter p aper provides a m easu re of the contam ination in the a ir. The sam pling devices m ay be of installed o r p ortable typ es. Installed a ir sam p lers m ay be useful in la b o ra to rie s handling sign ificant quantities of rad ioactive su bstan ces and also in stack s which exh aust a ir from fume cupboards and other sim ila r en clo su res used for handling open s o u rce s. P o rtab le a ir sam p lers a re used fo r spot sam pling in a re a s w here airb orn e contam ination is suspected Effluent m onitoring The re le a s e of rad ioactive w astes to the environm ent is based on two concepts: (a) con cen trate and contain and (b) dilute and d isp erse. In the ca se of the la tte r it is im portant that the cap acity of the environm ent to absorb the amount of rad io activ ity released should be carefu lly p re -e stim a te d and the re le a s e should be based on such e stim a te s. To ensure that the re le a se s a re indeed taking p lace as planned, effluents m ay have to be m onitored eith er p eriod i ca lly o r on a continuing b asis depending on the volum e and com plexity of the operation involving rad ioactive m a te ria ls O ff-site m onitoring O ff-site m onitoring sy stem s m ay include d evices such as continuous a ir sam p lers or w ater sam p lers which a re rem otely installed and from which the readings a re continuously relayed to a con trol ce n tre. How ever, the n ece ssity and the extent of sop h istication of such sy stem s would depend upon the type of installation and on the distribution of population in the surrounding a re a s. While in som e c a se s installed m onitors m ay be essen tial, in other c a se s p ortab le o r mobile sam pling units would su ffice. In addition to a ir and w ater sam pling, p eriod ic sam pling of vegetation and so il and oth er biological in d icato rs in the vicinity m ay be useful fo r m on itoring the re le a s e of rad io activ ity to the environm ent. Ill

114 1 7. D E C O N T A M I N A T I O N G eneral D econtam ination is the p ro cess of rem oval of rad ioactive con tam ination from the skin or from su rfaces such as the wall o r floor of working a r e a s. D econtam ination can prove to be an expensive operation, in te rm s of both tim e and m oney, and hence the m ain aim in the design and operation of any working place for rad ioactive su bstan ces should be to red u ce the p ossib ilities of any contam ination to the absolute m inim um. N ev erth eless, w henever work with rad io active m a te ria l is ca rrie d out, a certain amount of contam ination will inevitably a ris e and one of the principal objectives in the design of a radioisotope lab o rato ry m ust be to prevent the attachm ent of contam ination to su rfaces and to facilitate the r e m oval of contam ination with the least possible dam age to the a f fected su rfa ce s. This m ay be accom plished by (a) A ppropriate segregation, both in te rm s of a re a and ventilation, of operations involving open so u rce s. (b) T raining operating personnel in m ethods of (i) Minimizing contam ination (ii) Controlling the spread of contam ination (iii) D econtam ination. (c) The use of the c o r r e c t p rotective clothing, such as gloves, footw ear, and im pervious su its. (d) The provision of smooth nonperm eable su rfaces in all working places and on all equipment. (e) The use of ap propriate m onitoring instrum ents to detect and effectively deal with contam ination incidents. R adioactive contam ination m ay e x ist in loose form o r m ay be m o re o r le ss fixed as a re su lt of physical and ch em ical fa cto rs. A ll loose contam ination should be rem oved for obvious re a so n s, but the effort involved in rem oving any fixed contam ination m ay be con sid erab le. Ideally, all contam ination should be rem oved; how ever th ere a re ce rta in con sid erations which should be taken into account in determ ining the degree of decontam ination r e quired. Some of these considerations will now be outlined in b rief Skin and su rface contam ination In decontam inating the skin, while it would be ideal to rem ove the en tire contam ination, this m ay not alw ays be possible, because 11 2

115 the d ra stic m e a su re s which m ay be n e c e s s a ry in certain ca se s could re su lt in such dam age to the skin that the rad io activ e m a te ria l could gain en try into the body and so give r is e to an in ternal h azard. In such c a s e s, it should be considered sa tisfa cto ry to red u ce the levels of contam ination to within p erm issib le lim its. S im ilar con sid erations would apply to the decontam ination of su rfaces such as w alls, floo rs, and table tops, and of contam inated equipm en t. T here could, how ever, be situations in which exp erim ental req u irem en ts ren d er it essen tial that the decontam ination be ab solu te. On the other hand, in dealing with contam ination of ce rta in a rtic le s and types of equipm ent, it might turn out to be m o re econ om ical to sto re the contam inated object tem p o rarily, with a view to letting the activ ity die down natu rally to within p erm issib le le v e ls, o r to dispose of it as w aste. The above con sid erations im ply the setting up of m axim um p erm issib le levels of contam ination for the skin and for su rfa ce s in controlled and uncontrolled a r e a s. Some rep resen tativ e values fo r th ese levels have been provided in the IAEA Manual, Safe Handling of R adionuclides, Safety S eries No.l, 1973 Edition. The fundam ental p rin cip les which a re applicable to all d e contam ination p ro ced u res a re : (a) W et decontam ination m ethods should alw ays be used in p referen ce to dry. (b) Mild decontam ination m ethods should be tried b efore reso rtin g to treatm en t which can dam age the su rfaces involved. (c) P recau tio n s m ust alw ays be taken to prevent the fu rth er spread of contam ination during decontam ination op eration s. (d) W here p o ssib le, contam ination involving sh ort-liv ed activ ities should be isolated and segregated to allow natu ral decay to take its co u rse D econtam ination of p ersonnel Soap and w ater is the first req u irem ent for rem oving co n tam ination from the hands and o th er exposed a re a s of the skin. The soap chosen should be mild so that it will not produce skin dam age after frequent u se. F o r hands, a so ft-b ristle nail brush should be provided for u se in conjunction with soap and w ater o v er the en tire su rface of the hands and the w ris ts. P a rtic u la r attention should be given to the n ails, to the ridges between the fingers and to the edges of the hand. Freq u en t rinsing is essen tial during the en tire operation. 113

116 F o r the face, copious amounts of w ater and soap should be u sed, the hands alone being used to c re a te the lath er. Isolated a re a s of high contam ination should be carefu lly scrubbed. All p erson nel should be in structed to keep the eyes and the mouth closed during treatm en t and to rin se the face frequently with copious amounts of w ater. While using tow els, o r oth er m a te ria ls suitable for drying, rubbing should be avoided. All c a se s of face contam ination should be re fe rre d to the m ed ical o fficer. Contaminated h air should be washed sev e ra l tim es with an efficient shampoo and copious amounts of w ater should be used for rinsing. The la tte r is p articu larly im portant to ensure that contam ination rem oved from the h air does not rem ain in the e a rs o r on the face. In the event of contam ination which p e rs is ts although the above-m entioned p ro ced u res have been followed a num ber of tim e s, the individual concerned should be re fe rre d to the m ed ical departm ent w here m o re effective decontam ination can be c a rrie d out under m ed ical supervision. It is essen tial that skin d econtam ination should not be taken to the point of damaging the skin. In ca se of contam inated sm all open wounds, cu ts, p unctures, etc., the wound should be im m ediately washed, bleeding should be encouraged if n e ce ssa ry, and the m ed ical o fficer should be consulted. W henever in tern al contam ination o c c u rs, it essen tially becom es a m ed ical p rob lem, p arallel in som e ways to the absorption of ch em ical toxins. All c o rre c tiv e m e asu res should be c a rrie d out under m ed ical supervision D econtam ination of working a re a s A p relim in ary contam ination survey w ill indicate those a re a s which req u ire decontam ination and such a re a s should be c le a rly m arked. The decontam ination m easu res should be re s tric te d to those a re a s and ev ery endeavour should be made to prevent the spread of contam ination. The decontam ination m easu res taken will depend upon the n atu re of the contam ination, i.e. whether it is in loose form o r is relativ ely fixed, and details of recom m ended p roced u res a re given in this o rd e r in the following sectio n s R em oval of loose contam ination S pecial decontam ination ap p aratu s, such as vacuum cle a n e rs fitted with sp ecial filte rs, m ay be used to rem ove loose con tam ina- 114

117 tion. No attem pt should be made to brush o r dust it off, though in the ca se of slight contam ination on the floor a wet medium such as dampened sawdust sprinkled o v er the contam inated a re a before brushing is accep tab le. F o r all oth er su rfa ce s, wet methods such as swabbing a re e ssen tial. The rem oval of contam ination should be done with the minimum of rubbing and the swabs should be frequently d iscard ed as rad io active w aste. Decontam ination solu tions which contain com plexing agents a re p articu larly useful in such c a s e s. W here th ere is copious loose contam ination, a suitable strippable lacq u er m ay be carefu lly applied to the contam inated s u rfa ce s. This lacq u er is allowed to dry and in so doing will take up the contam ination. A fterw ard s the strippable lacq u er can be rem oved together with the contam ination. During this op eration, c a re should be taken in using the spraying d evice to avoid disturbing the loose contam ination and thus giving ris e to an airb orn e h azard. As a p recau tion, p ersonnel should be in fully p ro tectiv e clothing. A fter stripping, the affected a re a s should be washed as d escrib ed above R em oval of relativ ely fixed contam ination Only wet m ethods should be used. The firs t wash should be with suitable d etergent solution which will rem ove loose contam ination and all g rease-h eld m a te ria l. Only light rubbing should be used at this stage and the swabs should be d iscard ed frequently as rad ioactive w aste. Contam ination rem aining after this tr e a t m ent should be rem oved by fu rth er washing with suitable decontam inating solutions. The decontam inating solution should contain com plexing agents to prevent redeposition of the contam ination. It is advisable to allow decontam inating solutions to rem ain in con tact with the contam inated su rfaces as long as possible so that ch em ical reactio n at the su rface m ay a s s is t the decontam ination. Contam ination rem aining after sev e ra l attem pts at rem oval with the above treatm en t will n orm ally be confined to sm all a re a s (unless a m ajo r spill has o ccu rre d ) and fu rth er treatm en t should be undertaken with ab rasive m a te ria l. The p ro p rietary ab rasive cle a n e rs a re useful for gen eral application in this "spotting" treatm en t. M etal polish can be used to advantage on m etal s u r fa ce s; ab rasiv e c re a m s, containing a com plexing agent, m ay be rubbed into the affected a re a s and left in con tact fo r a period before being washed off. M ore stringent treatm en t would involve the use of steel wool o r a sim ila r scouring agent. 115

118 If contam ination p e rs is ts, it will be n e c e s s a ry to rem ove the su rface on which the contam ination is fixed, u nless it is so fixed and in such sm all amount that it can be left in p lace. In the la tte r c a s e, p recau tion s should be taken to seal in the contam ination with co n cre te, paint o r other ap propriate m a te ria l. Such sealed - in contam ination m ust be record ed so that in any future m od ifications to the building suitable precautions can be taken against d isp ersin g the contam ination and creatin g a h azard D econtam ination of equipment It is im possible to d escrib e h ere the m easu res to be used in the decontam ination of the individual p ieces of equipment encountered in radiation work. H ow ever, such item s of equipment m ay be conveniently classified into groups accord in g to the m a te ria ls of which they a re m ade, and the decontam ination m ethods d escrib ed accord in gly. D econtam ination methods for equipment a re of two kinds: (a) (b) R em oval of contam ination without dam age to the su rface below R em oval of the su rface of the equipment togeth er with the adhering contam ination. In all c a s e s the firs t method should be used initially and only if se v e ra l attem pts fail should the second method be trie d, since damaged su rfa ce s m ay be unsuitable for re -u s e b ecause of th eir tendency to co llect contam ination easily. D econtam ination of equipment should be c a rrie d out as soon as p ossib le a fte r its rem oval from the active a re a. Contamination left in situ o v er periods of tim e b ecom es fixed and b ecom es increasin g ly difficult to rem ove. All decontam ination should be c a rrie d out using wet m ethods. The routine to be followed is the sam e for all equipment, the only d ifferen ce being in the reagen ts used for various m a te ria ls. The routine p ro ced u res a re : 116 (a) W ash in d etergen t solution at raised tem p eratu res. This will rem ove all loose and g rease-h eld contam ination. This m ay be followed by swabbing and light scrubbing with the sam e solution. (b) D econtam inated equipment should be washed in clean w ater and dried before m onitoring.

119 (c) F u rth e r scrubbing and also steeping techniques m ay be used, w here contam ination rem ain s after the above treatm en t. In the la tte r the equipment is placed in solu tions of suitable decontam inating reag en ts, p referab ly at raised te m p e ra tu re s, and is left th ere for suitable periods of tim e. The inclusion of com plexing agents in the decontam inating solution is recom m ended to prevent redeposition of the contam ination. (d) Equipment is washed in clean w ater on rem oval from the decontam ination solution and is then dried before being m onitored. (e) F u rth e r m ethods will depend upon the extent and nature of resid u al contam ination, when contam ination still rem ain s after the above treatm en t. If the contam ination is p resen t in sp ots, a treatm en t known as "spottin g" m ay be c a rrie d out using ab rasiv es o r strong acid s on the sm all a re a s involved. W here acids a re u sed, c a re should be taken to ensure that the su rface of the equipment is not unduly etched. If the contam ination is g en eral, it m ay be possible to apply a b rasiv es o v er the whole su rfa ce, but if this fails, steepage in acid solution will be n e c e s s a ry. P r e cautions should be taken to prevent the acid s from damaging the su rface of the equipment m o re than is absolutely n e c e s s a ry to rem ove the contam ination. Special apparatus in the form of fume hoods o r glove boxes will be n e c e s s a ry fo r the acid tre a tm e n t, because of noxious fum es. (f) Equipm ent should be well washed and dried before m onitoring. The decontam ination of clothing, including p rotective clothing, has alread y been dealt with in C hapter 15 dealing with p ro tectiv e clothing. 117

120 1 8. M A N A G E M E N T O F R A D I O A C T I V E W A S T E S G eneral R adioactive w astes can origin ate eith er from the production of n u clear power o r from the use of radionuclides in m edicine, industry, ag ricu ltu re and re s e a r c h. The w astes that a ris e can be classified on the b asis of th eir physical sta te, the type and lev el of rad ioactivity and th eir com position (see T echnical R eports S eries No.101, Standardization of R adioactive W aste C a te g o rie s, IAEA, Vienna (1970)) W aste m anagem ent The b asic objective of rad ioactive w aste m anagem ent is to keep the radiation dose to man as low as is p racticab le and under any circu m stan ce within the annual dose lim its recom m ended by the IC RP. The m anagem ent of rad ioactive w aste is broadly governed by the application of th ree w idely-accep ted p rin cip les: (a) Dilute and d isp erse for low -level solid, liquid and gaseous w astes (b) Delay and decay for solid, liquid and gaseous w astes that contain sh o rt-liv ed radionuclides (c) C oncentrate and contain for in term ed iate- and h igh-level solid, liquid and gaseous w astes. The w aste m anagem ent system often co n sists of a combination of the p rin cip les outlined above Dilution and disp ersion The p rinciple of dilution and disp ersion is based on the a s sumption that the environm ent has a finite cap acity for dilution of radionuclides to an innocuous level. The application of this principle req u ires an understanding of the behaviour of rad ioactive m a te ria ls in the environm ent and of the ways in which the r e leased rad ion u clid es, p articu larly those that a re considered to be c r itic a l, m ay lead.la te r to the exposure of m an. It is e sp e cially im portant to take into consideration environm ental p ro ce sse s which m ay cau se recon cen tration of radionuclides. A larg e body of knowledge is available for use in the application of. this p rin

121 cip le, esp ecially in m eteorology, geology, geography, hydrology, hydrography, oceanography, ecology, soil scien ce and environm en tal engineering. A pplications of this p rinciple m ust be made cau tiou sly, ensuring that the re le a s e s a re m inim al and in any ca se a re w ell within the cap acity of the total environm ent to re ce iv e them. The varied n ature of solid w astes and the degree and type of contam ination a re such that this principle is not read ily applicable in a s tr ic t sen se. However, low -level and certain in term ed iate-lev el solid w astes, including m assiv e p ieces of contam inated equipm ent, m ay be buried at suitable depths. Any site chosen for such b urial op erations should be exam ined c a r e fully with reg ard to its geologic and hydrologic p ro p erties and an a sse ssm e n t should be m ade of the possible contam ination of w ater supplies and of eco lo g ical system s that might lead to human e x p o su res. The depth chosen for b urial m ust be sufficient to prevent leakage of any harm ful level of rad ioactivity into usable su rface w aters o r ground w a te rs. The whole of the burial a re a and its im m ediate vicinity should be isolated suitably and fenced to prevent use of the a re a by m em b ers of the g en eral public. It is also im portant that a ce n tra l re co rd should be m aintained of all b u rials in o rd e r to a ssu re that the a re a is kept under continuing s u r veillan ce. Under such conditions som e leaching of the ra d io activ ity into the ground w ater can take p lace, and th is m ay th e re fore be con sid ered under the principle of dilute and d isp erse to a sm all extent (see Safety S eries No.15, R adioactive W aste D isposal into the Ground, IA EA, Vienna (1965).) With liquid w astes this p rinciple is re s tric te d in its application to w aste stre a m s that have origin ally v ery low levels of rad ioactiv ity o r which have been treated for rem oval of m ost of the rad ioactivity (see Safety S eries N o.5, R adioactive W aste D isposal into the Sea, IA EA,Vienna (1961) and Safety S eries N o.36, D isposal of R adioactive W astes into R iv e rs, L a k e s, and E s tu a rie s, IAEA, Vienna (1971). In the "sm a ll u s e r" estab lish m en ts, gaseous w astes a ris e in the handling of rad ioactive m a te ria ls in hoods o r ce lls and are c a rrie d away in the exhaust g ases from th ese fa cilitie s. In installation s w here larg e am ounts of airb orn e activity a re involved it m ay be n e c e s s a ry to use suitable a ir filtration sy stem s and to d isch arg e the filtered effluent through a tall stack. The height of the stack can be chosen, on the b asis of lo cal m eteo ro lo g ical cond itions, to a ssu re that the rad ioactivity is sufficiently diluted before it re a ch e s ground level. Dilution of low -lev el liquid w astes can be achieved by (a) Addition of uncontam inated liquid to reduce the co n ce n tra tions p rio r to d isch arge. 119

122 (b) (c) R elease of the liquid w astes at sm all ra te s o ver long p eriods of tim e. R elease of w astes into larg e bodies of w ater Delay and decay The second of the th ree p rin cip les is based on the fact that radionuclides lose th eir rad ioactiv ity through d ecay, and this fact m ay be utilized in the treatm en t not only of in term ed iate- and h igh -level solid, liquid and gaseous w astes but in som e c a s e s also in that of low -lev el w astes. The aim is to ease problem s in subsequent handling o r to lessen risk s of re le a s e s to the environm ent, taking advantage of the d ecay of som e radionuclides - p a rticu larly those having short h alf-liv es - with the p assage of tim e. If highlev el w aste is held in sto rag e in a liquid fo rm, the risk of accid en tal re le a se m ight in som e circu m sta n ce s dictate e a rly con version to solid form. T his p rinciple is esp ecially useful for those in stallations w here a substantial reduction in the activ ity level of a w aste stre a m can be achieved by delaying d isch arge of effluents for a few days C oncentrate and contain The p rinciple of con centration and containm ent d eriv es from the concept that the m ajo rity of the rad ioactivity generated in n u clear p ro g ram s m ust be kept in isolation from the human environm ent. Since som e radionuclides take a long tim e to decay to innocuous le v e ls, som e w astes m ust be contained for extended p eriod s of tim e. T his p rinciple is invoked in techniques for a ir and gas cleaning; the treatm en t of liquid w astes by scavenging and p re cipitation; ion exchange and evaporation; the treatm en t of lowlev el, solid w astes by in cin eration, baling and packaging; the treatm en t of in term ed iate-lev el solid and liquid w astes by insolubilization in asphalt; con version of h igh -level liquid w astes to insoluble solids by h igh -tem p eratu re calcination o r incorporation in g la ss; tank sto rag e of in term ed iate- and h igh-level liquid w astes; sto rag e of solid w astes in vaults o r ca v e rn s; and disposal of solid and liquid w astes in deep geological form ation s. When liquid and gaseous rad ioactive w astes a re subjected to treatm en t th ere re su lts a con centrated resid u e and a low -level effluent from which m ost of the rad ioactiv ity has been rem oved. The lo w -level effluent can be disch arged to the environm ent a fter 1 2 0

123 adequate treatm en t for rem ov al of rad io activity but the con cen trated resid u es m ust be kept in containm ent. The d egree of containm ent required for solid w astes depends on th e ir physical and ch em ical p ro p erties as w ell as on the type and am ounts of radionuclide p resen t. It is gen erally advisab le to reduce the volume of bulky solid w astes, such as con tam inated tra s h, b efore d isp osal. Com bustible low -level rad io active w astes m ay be in cin erated to reduce th eir bulk, provided adequate p recau tion s a re taken to prevent the d isp ersal of the rad ioactiv e m a te ria ls in the sm oke, and the residue collected for b u rial. W here in cin eration is not feasib le, baling m ay be used for volum e reduction. When the activity lev el of the solid w aste is too high for d ire ct burial n ear the land su rface it is n e c e s s a ry to provide additional b a r r ie r s again st the m igration of the rad ion u clid es. One method to accom p lish this is to in co rp orate these w astes into a solid with low leach ab ility. F u rth e r reduction in the p rob ability of m ovem ent of the radionuclides contained can be achieved by using sp ecially p rep ared sto rag e vaults o r by placing the solids w ell below zones of circu latin g ground w a te rs. Rock salt fo rm a tions a re a ttra ctiv e for sto rag e of such w astes because th eir p resen ce in d icates that the region has not been subjected to the leaching action of ground w ater and salt is p lastic so that any fra c tu re s a re se lf-se a lin g. All m ethods of rad ioactive w aste m anagem ent should be planned and conducted in a m anner that en su res com pliance with national regulations and international recom m endations on rad iation exp osure. 121

124 1 9. T R A N S P O R T O F R A D I O A C T I V E M A T E R I A L S G eneral R adioactive m a te ria ls are being re g u la rly tran sp orted by land, w ater or a ir in com pliance with the cu rre n t national or international regulations. Even during n orm al conditions of tran sp o rt and handling, th ere e x ist p ossib ilities of radiation exposure of (a) person nel involved in such tran sp o rt (crew, p a sse n g e rs, e tc. ) and (b) undeveloped photographic film s that m ay be contiguously p resen t in the m eans of tran sp o rt. F u rth e r, the re le a se of rad ioactive m ate ria ls to the environm ent under accid en tal conditions cannot be ruled out. To m inim ize these h azard s and to ensure safe tran sp o rt of rad ioactive m a te ria ls sev e ra l steps a re taken, which include (i) lim iting the amount of rad io activ e m a te ria l in a package, depending on the ability of the package to withstand both n orm al and off-n orm al conditions encountered during tran sp o rt; (ii) lim iting the radiation level on the su rface of the package and at a distance of 1 m etre from the su rface of the package; and (iii) segregatin g such packages from p assen ger a re a s and undeveloped photographic film s. B ased on th ese co n sid erations national and international regulations governing the tran sp o rt of rad ioactive m a te ria ls have been form ulated. The broad req u irem en ts to be m et for the tran sp o rt of rad ioactive m aterials a re outlined below P ackage and packaging req u irem ents By package is m eant the packaging together with its rad ioactive contents as p resen ted fo r tran sp o rt. P ackaging is the assem b ly of components n e c e s s a ry to ensure com pliance with the packaging r e quirem ents of the regu lation s. It m ay, in p articu lar, con sist of one or m ore re ce p ta cle s, absorbent m a te ria ls, spacing stru ctu re s, radiation shielding, and devices for cooling, for absorbing m echanical shocks and fo r th erm al insulation. T hese devices m ay include the vehicle with tie-down sy stem when these a re intended to form an in teg ral p a rt of the packaging. Two types of packaging, Type A and T ypeb, have been recom m ended and they a re required to m eet certain sp ecification s including recom m ended te s ts. A Type A packaging is designed to withstand the n orm al conditions of tran sp o rt and is r e quired to dem on strate its suitability by the retention of in teg rity of shielding and containm ent under n orm al conditions incident to tra n s p o rt. A Type B packaging, how ever, is designed to withstand the 1 2 2

125 damaging effects of a tran sp o rt accid en t as dem onstrated by the retention of in tegrity of shielding and containm ent. It follows that the lim its of activ ity p erm itted fo r Type B packagings would be higher than fo r Type A packagings. Type A package is a Type A packaging togeth er with its lim ited rad ioactive contents and does not req u ire com petent auth ority approval. Type B(U) package is a Type B packaging togeth er with its rad ioactive contents, which req u ires unila te ra l approval only of the package design and of any stowage p ro v i sions that m ay be n e c e s s a ry fo r heat dissipation. Type B(M) package is a Type B packaging together with its rad ioactive contents as p resen ted fo r tran sp o rt and which req u ires m u ltilateral approval of the package design and of the conditions of shipm ent. P ack ag es a re fu rth er classified as C ategory I-W hite, C ategory II-Y ellow and C ateg o ry Ill-Y ellow packages based on the radiation levels on th eir su rface and at 1 m etre from any point on the su rfa ce. While m ost of the pure alpha and beta em itte rs could be tran sp orted as White p ack ages, it would be econom ical to tra n sp o rt gam m a em itte rs as Yellow p ack ages. O therw ise con sid erab le am ounts of shielding would be req u ired to bring the radiation levels to those correspon d in g to White p ack ages. E ach package has to be ap prop ria te ly labelled and the activ ity lim its as well as labelling re q u ire m ents have to follow the relevan t tran sp o rt regu lation s. P ack ag es containing rad ioactive m a te ria ls which a re also fissile m a te ria ls are designed, excep t for ce rta in sp ecified c a s e s, to com ply with the gen eral p rovisions fo r n u clear safety. All fissile m a te ria ls are packed and shipped in such a m anner that critic a lity cannot be reach ed under any fo reseeab le circu m sta n ce s of tran sp o rt. P ack ag es of fissile m ate ria ls a re divided into: (a) (b) (c) F is s ile C lass I: packages which a re n uclearly safe in any num ber and in any arran g em en t under all foreseeab le circ u m stan ces of tran sp o rt F is s ile C lass II: p ackages which in lim ited num ber a re n u clearly safe in any arran g em en t under all fo reseeab le circu m stan ces of tra n sp o rt F is s ile C lass III: p ack ag es which a re n u clearly safe under all fo reseeab le circu m sta n ce s of tran sp o rt by reaso n of sp ecial p recau tio n s, o r sp ecial ad m in istrative o r operational con trols im posed upon the tran sp o rt of the consignm ent. If the package fo r rad io active m a te ria ls is returned empty, the relevan t lab els fo r rad io active m a te ria ls have to be rem oved or ob literated and it has to be ensured that the package is fre e from contam ination. 12 3

126 Som e design fe a tu re s of tra n sp o rt p ackages Sealed so u rce s o r solid rad ioactiv e m a te ria ls: The provision of adequate shielding fo r alpha and soft beta em itters to ensure that radiation level req u irem en ts a re fully m et p resen ts no sp ecial p rob lem s. F o r en ergetic beta em itters and gam m a em itte rs shielding con sid erations outlined in the e a rlie r ch ap ters should be kept in mind. F o r neutron so u rces, it m ay be n e ce ssa ry to provide suitable m o d erato rs and neutron a b so rb ers and, w here relevant, additional gam m a shielding S ources in liquid form : S ources in liquid form a re n o r m ally contained in glass or p lastic bottles or v ials. Hence, additional p recau tion s have to be taken to ensure that th ere is no b reak age and consequent leakage of the so u rces during tra n sit. This is done by providing an extern al leak -p roof con tain er in which the p rim a ry so u rce con tain er is cen trally lo cated. F u rth e r, the sp ace between the p rim a ry con tain er and the inner w alls of the secon d ary container is filled with suitable absorbing m a te ria l which will not in any way re a c t with the rad ioactive liquid but is capable of absorbing it com pletely B ird -cag e type of packaging: In m any c a s e s, the judicious use of two concepts (1) distance from the so u rce and (2) shielding would re su lt in optim ization of both the size and the weight of the p ackage. In such c a s e s, in addition to m inim al shielding, the distance approach is made use of by providing b ird -cag e or other sim ila r con stru ctional featu res so that the distance of the su rface of the package from the so u rce is in creased. Thus, the dose lev els on the su rface of the package and at a distance of 1 m etre from the su rface a re brought down to values within p erm issib le lim its. In such c a s e s the in tegrity of the stru ctu re around the shielding should also be capable of withstanding the rig o u rs of tran sp o rt. T h ere have been c a s e s w here, during tran sp o rt, the b ird -cag e stru ctu re s have been damaged, resu ltin g in the inner shielded container being shifted. This has given r is e to higher radiation levels on the su rface of the package than, those p erm itted. It is obvious that to provide m axim al shielding fo r minimum package weight the shielding should be as clo se to the so u rce as p ossib le E xp o su re lim itations T ran sp o rt regu lation s excluding packaging req u irem en ts a re so form ulated that no p erson should re ce iv e, under any condition 124

127 incidental to tran sp ortation (including the o ff-n orm al situation s), a radiation dose in e x c e s s of p erm issib le lev els. T ran sp o rt and sto rag e w orkers shall not be so exposed as to re ceiv e rad iation doses in e x c e s s of th ree-ten th s of the m axim um p erm issib le doses specified fo r radiation w orkers (in the B a sic Safety Standards fo r R adiation P ro tectio n ) unless arran g em en ts have been m ade to provide the sp ecial health su pervision and p erson nel m onitoring req u ired fo r such w o rk ers. In the co u rse of tran sp o rt, p ackages containing rad ioactiv e m a te ria ls, excep t the White p ackages, have to be kept sep arated from (a) (b) (c) (d) Livin g accom m odations R eg u larly occupied working sp aces Spaces in the m eans of tran sp o rt that m ay be continually occupied by p assen g ers Undeveloped photographic film s or p lates so that they do not re ce iv e a cum ulative exp osure equivalent to m ore than 10 m rem. A ll tra n sp o rt and sto rag e w orkers shall re ce iv e all n e c e s s a ry in stru ction s concerning the h azard s involved and the p recau tio n s to be observed in the handling of packages containing rad ioactive m a te ria ls O ther fo rm s of h azard s A s a gen eral ru le rad ioactive m a te ria ls p o ssessin g additional hazardous p ro p erties such as exp losiven ess, inflam m ability, p y ro - p h oricity, ch em ical to xicity and co rro siv e n e ss have to be tran sp orted in com pliance with relev an t tra n sp o rt regu lation s both fo r dangerous goods and fo r rad io activ e m a te ria ls G eneral accid en t provisions In the event of a package of rad ioactive m a te ria ls breaking or leaking, o r becom ing involved in a cra sh, w reck, o r fire, the affected a re a should be suitably seg reg ated and no p erson should be allowed to en ter o r to rem ain within the seg reg ated a re a until qualified p erso n s a re available to check radiation h azard s and su p ervise subsequent operations including salvage op erations. How ever, the p re se n ce of rad io activ e m a te ria ls should not be con sid ered to be a b ar to re scu e operations o r fighting of fire s by qualified p erso n s. All p erson s who m ay have becom e contam inated with rad ioactive 125

128 m a te ria ls should be subjected to im m ediate exam ination and ap prop riate decontam ination m e a su re s. Any conveyance, building, location, equipment o r p a rt th ereof which has becom e contam inated as a re su lt of an accid en t in the co u rse of tran sp o rt of rad ioactive m ate ria ls should be decontam inated by qualified p erson s as soon as p ossib le Custom s exam inations C ustom s operations involving exam ination of the contents of a package containing rad ioactive m a te ria ls should be c a rrie d out in a p lace w here adequate m eans of radiation exposure con trol a re provided and in the p resen ce of p erson s qualified to deal with ra d io active m a te ria ls. Any package opened on custom s in struction s should, before being forw arded to the consignee to its final destination be re sto re d to its original packaging sp ecification s so that all radiatior p rotection req u irem en ts are re sto re d. 12 6

129 2 0. R A D I A T I O N A C C I D E N T S A N D E M E R G E N C Y P R O C E D U R E S G eneral A radiation accid en t is an unusual o ccu rre n ce resu ltin g from the lo ss of con trol over a rad iation so u rce which could d irectly or in d irectly involve h azard s to life, health and p ro p erty. A radiation accid en t could o ccu r at any stage of an operation involving radiation so u rce s. F o r exam ple, a neutron so u rce which m ay be used on the f ir s t flo or of a building and which m ay be provided with adequate shielding all round excep t tow ards the floor could irra d ia te p erson s in the room which is below the so u rce. S im ilarly the beam of an X -r a y m achine could be inadvertently turned tow ards a wall to which it should not n orm ally be d irected, as a re su lt of which p erson s in the adjoining room who m ay not even be routine radiation w orkers could be exposed to relativ ely high levels of radiation. R adiation accid en ts would norm ally conform to one of the following broad gen eral p attern s: (a) (b) (c) A ccid ental extern al exposure to e x ce ssiv e amounts of radiation, a s, for exam ple, when a p erson inadvertently rem ain s clo se to a stron g so u rce or accid en tally gets exposed to a beam of radiation. A ccid ental spill o r explosion in a working place (accom panied by a fire o r otherw ise) resu ltin g in su rface and a ir contam ination of the surroundings and contam ination of p erson nel. In such c a se s the intake of rad io active su bstan ces into the body could be by inhalation, through open wounds resu ltin g from the a c c i dent, o r by d ire ct absorption through the skin. D isp ersal of rad ioactive m a te ria l to the environm ent as a resu lt of an explosion, fire, m ech an ical shock or other incident o c c u r ring in a public p lace, as fo r in stan ce when rad io active substances a re being tran sp orted Types of rad iation accid en ts O n -site accid en ts High levels of exp osure to radiation o ccu r as a re su lt of a p erson inadvertently entering a high radiation field such a s, for exam ple, a p erson walking through an X-ray beam when the machine is on, or when a p erso n gets dangerously clo se to an industrial so u rce while 1 2 7

130 it is being used fo r p anoram ic exp osu res. Such accid en tal exp osures can also o ccu r when shielding m easu res a re inadequate. In p laces w here a potential for such accid en ts e x ists, appropriate con trol m easu res should be taken well in advance to ensure that the chances of any p erson being accid en tally exposed to high levels of radiation a re m inim ized. Such m easu res include (1) the provision of rad iation - actu ated and other sim ila r in terlock s which would ensure that no p erson can en ter a radiation a re a when an exposure is in p ro g re ss; (2) p rovision of visu al o r au ral indication to identify high radiation lev el a re a s ; (3) p rovision of adequate radiation a la rm s which can be eith er located at s tra te g ic points or c a rrie d by individuals whenever they a r e n ear high radiation level a re a s ; (4) the adoption of detailed ad m in istrative p ro ced u res such as p rovision of suitable cordoned-off a re a s and prohibition of en try into such a re a s during radiation op erations. A second categ o ry of accid en tal radiation exp osure, which could a ris e in a radiation work a re a, could re su lt from one of the following contingencies; (a) (b) (c) The inability to get a rem o tely controlled so u rce back into its shielded con tain er b ecause of m ech an ical o r pneum atic failure A ccid ental breakage of a sealed so u rce o r the con tain er of an open so u rce, resu ltin gin h igh contam ination of both su rface and a ir in the vicinity Breakdown of c ru c ia l ventilation sy stem s in a re a s w here open so u rces a re being handled (d ) A c c i d e n t s w h i c h c o u l d i n v o l v e f i r e o r e x p l o s i o n a n d w h i c h c o u l d r e s u l t i n t h e b r e a k d o w n o f t h e i n t e g r i t y o f s h i e l d i n g, o f r a d i o n u c l i d e s i n t h e e n v i r o n m e n t o f t h e l a b o r a t o r y, o r d i s p e r s a l e. g. i n t h e c a s e o f a m a j o r n u c l e a r f a c i l i t y, a c r i t i c a l i t y a c c i d e n t O ff-site accid en ts Such an accid en t could o ccu r in a re a s to which public have a c c e s s and m ay re su lt from one of the following contingencies: (a) (b) Unplanned re le a s e of airb orn e activ ity to the environm ent of a rad iation fa cility owing to unusual conditions such as fire, explosion, breakdown of the ventilation sy stem o r breakdown of the filte r system A ccid ent to consignm ents of radionuclides when such consignm ents a re in a c a r r i e r such as a truck, tra in o r a ir c ra ft, or when such consignm ents a re held in sto rag e during tra n sit

131 E m e rg en cy p ro ced u res The b asic p rin cip les underlying p ro tectiv e m easu res to be adopted in dealing with radiation accid en ts have alread y been e la borated in e a r lie r ch ap ters. It m ust, how ever, be borne in mind that the firs t essen tial step in dealing with a rad iation accid en t is to identify, seg reg ate and tr e a t all p erso n s who m ay have been subjected to radiation exp osures both extern al and in tern al. Im m ediate steps should be taken to a s s e s s the extent of exposure by sending the person nel m onitoring devices used by the exposed p erso n s for im m ediate p ro ce ssin g. B iological m onitoring and body burden m easu rem en ts m ust a lso be im m ed iately conducted w here n e ce ssa ry. In radiation em erg en cies, radiation fields m ay be encountered which a re m uch higher than n orm al, and sp ecial radiation m easu ring in strum ents will be n e c e s s a ry for m easu ring such field s. F u rth e r m ore, these m easu rem en ts m ay have to be made under p a rticu la rly difficult conditions such as high radiation fields, high levels of s u r face and airborne contam ination, e tc. The in stru m en ts should be capable of m easu ring much higher dose ran ges and dose ra te s than usual. W here n e ce ssa ry, they should be provided with d etecto rs which a re telesco p ically coupled to the m e te rs so that while a d etecto r would be in clo se p roxim ity of a high rad iation field, the p erson reading the m e te r can be at a reason ab ly safe distance from the field. Such in strum ents should be p erio d ically calib rated and kept in working shape if they a re to be useful in dealing with e m e r gen cies. A ir and su rface contam ination sam p les m ay have to be drawn and analysed urgen tly to enable fu rth er n e c e s s a ry action to be taken. The instrum entation n e c e s s a ry fo r this purpose should also be on hand. It would be extre m e ly difficult to stipulate hard and fa st ru les fo r m eeting the wide v a rie ty of situations that accom pany radiation accid en ts, but it would not be out of place to outline broadly the b asic con sid erations that should be borne in mind while tackling em erg en cy situ ation s. Among oth ers, the following step s m ay be con sid ered for dealing with an em ergency: (a) (b) E vacu ate the im m ediate a re a while sim ultaneously ensuring that the rad iation field and the extent of sp read of contam ination a re kept to the absolute m inim um. Identify and im m ed iately isolate all p erso n s who m ight have receiv ed high exp osu res or who could have been contam inated. In such c a se s arran g e for the im m ediate evaluation of the p erson n el m onitoring devices worn by th ese p erson s and also co lle ct sam p les of body fluids such as blood, urine, e tc. for fu rth er an aly sis. 129

132 (c) W here person nel contam ination is involved, a rran g e fo r im m ediate de contam inati on. (d) R egulate en try to the scen e of the accid en t so as to m inim ize all subsequent exp osu res and incidents of contam ination. (e) Notify prom ptly the ap propriate au th orities through suitable m edia such as telephone, telegraph, etc., and seek im m ediate advice on fu rth er steps to be taken. A rran ge fo r the im m ediate a v a ilability of exp erts who a re train ed to deal with such accid en tal conditions. (P erso n s responsible for radiation p rotection in ev ery institution should have p rio r inform ation regarding exp erts and organizations to be contacted to deal with radiation e m e r g e n c ie s.) (f) Contain the contam ination at the site of the accid en t. In ca se of a sm all spill involving a rad ioactive liquid, it m ay be d esirab le to contain and clean up the contam ination im m ediately. During such a clean-up operation, routine p ro tectiv e m easu res such as w earing gloves and segregatin g the mop as rad ioactive w aste, e tc. should be adopted. In the ca se of a relativ ely larg e re le a se of rad ioactive powder or a e ro so l in a room, such a room m ust be im m ed iately isolated from its surroundings by shutting off m ech an ical ventilation and by closing windows and d oors. E n try into the room excep t by exp erts with all n e c e s s a ry p rotectiv e equipment and devices should be forbidden. A room with heavy a ir contam ination can be decontam inated from within by drawing the a ir of the room through an ap propriate filte r. (g) In all em erg en cy p roced u res taken in the ca se of a radiation accid en t, p rio rity should be given to human safety. It is in recogn ition of this fact that the ICRP has provided fo r a 10-rem dose lim it for planned sp ecial exp o su res. It is highly d esirab le that the staff a re in stru cted in b asic em ergen cy p ro ced u res and fu rth er that a list of these p roced u res including p erso n s to be con tacted in ca se of an accid en t be displayed at appropriate location s in rad iation in stallation s. Mock-up operations for dealing with com p licated situations a sso ciated with high radiation fields and contam ination a re a s should alw ays be p art of a ra d ia tion em erg en cy p roced u re. (h) M aintain com plete re co rd s of the accid en t and follow-up p ro c e d u res. This sim ple in struction is often not followed, resu ltin g in enorm ous com plications in investigating such incidents and in the adoption of subsequent rem ed ial m e a su re s. (i) In re sp e ct of accid en ts in a public a re a, the a re a involved should be cordoned off and the ap propriate au th orities contacted im m ed iately fo r fu rth er c o rre c tiv e action. 130

133 R e sp o n sib ility in the co n tro l of rad iatio n a ccid e n ts F i r s t of all, the resp o n sib ility for controlling the use of radiation so u rce s within a cou ntry should r e s t on the public au th orities and the op erato rs of estab lishm ents in which the so u rce s a re p resen t. The Governm ent of the country should designate and define the functions of those public au th orities which a re resp on sib le fo r the con trol of radiation so u rce s and dealing with the radiation accid en ts. T hese public au th orities will (a) (b) (c) (d) (e) (f) (g) A rran g e fo r con trol of the u ses of radiation so u rces through licen sin g and regulations P r e s c r ib e p rotectio n standards and guidelines E stab lish lines of au th ority among national bodies Define the authority to be notified in the event of a radiation accid en t E stab lish n e c e s s a ry liaison with national au th orities in neighbouring cou ntries D eterm ine the need fo r train ed p erson nel in the state and arran g e fo r train in g, if n e c e s s a ry D eterm ine and p erio d ically review the availability and location of train ed p erson nel, and all other n e c e s s a ry equipment and s e rv ic e s. R espon sib ility fo r im m ediate action following an accid en t origin ating within the establishm ent will r e s t on the o p erato r. The o p erator, in acco rd an ce with the req u irem en ts of his work and regulations applied by the public au th orities, should estab lish an in tern al o rg an ization. This in tern al organization will (a) (b) (c) (d) (e) (f) E n su re that he is p rep ared, within the lim its im posed by his r e s o u rc e s, to deal with any accid en t that m ay o ccu r within his p re m ise s A rran g e fo r a ssista n ce from public au th orities and other o ff-site organizations if n e c e s s a ry P ro vid e im m ediate notification of the designated public au th oritie s of accid en ts whose consequences m ay extend off-site P rovid e a ssista n ce to public au th orities as req u ired P ro vid e notification of designated public au th orities of all rad iation accid en ts Keep adequate re c o rd s, and make an an alysis of any radiation accid en ts that o ccu r. F o r d etails, the International A tom ic E n erg y A gency Safety S eries No. 32, Planning fo r the Handling of Radiation A ccid ents, IAEA, Vienna (1969), m ay be consulted. 131

134 2 1. A D M I N I S T R A T I O N O F R A D I A T I O N P R O T E C T I O N G eneral The concluding ch ap ter of this book will be devoted to summing up the vario u s ad m in istrative and tech n ical m easu res which could form the b asis for establishing a su ccessfu l radiation protection p ro g ram. It should be borne in mind that the su cce ss of any radiation safety p ro g ram depends on the efficien cy of the safety organization in the individual installation and on the efficien cy of the individual radiation w ork er. Only by continuing in te re st and carefu l followup at all levels of m anagem ent can this efficien cy be m aintained M easu res H azards evaluation An im portant step in the form ulation of any radiation safety p ro g ram is the determ ination of the n ature and extent of the health h azard s involved in any operation with radiation so u rces. On the b asis of such an a sse ssm e n t the installation has to be ap propriately planned and operating p ro ced u res have to be form ulated so that the levels both fo r radiation dose and fo r con centrations of radionuclides in a ir and w ater will be kept as low as p racticab le and in any ca se will not exceed the m axim um p erm issib le v alu es. F o r m any y e a rs, the International C om m ission on R adiological P ro tectio n (ICRP) has been activ e in establishing and keeping under review such m axim um p erm issib le.values. T hese valu es have been identified both for occupational w orkers and fo r m em b ers of the public, and have been a rriv e d at on the b asis that the risk of dam age to the health of the p erso n s exposed to rad iation at levels not g re a te r than th ese m axim um p erm issib le valu es is con sid ered to be negligible and hence accep tab le. The valu es a re widely accep ted as determ ining the b asis fo r the design of in stallatio n s, the developm ent of suitable operational p ro c e dures and the form ulation of ap propriate ad m in istrativ e co n strain ts. H ow ever, the IC RP has also recom m ended that all radiation exp osures should be kept at the low est p ra ctica b le lim its. Thus, while m axim um p erm issib le levels can se rv e as a guide fo r planning purposes they should not be tre a te d as levels to which rad iation w ork ers could be 132

135 routinely exposed. Fortu n ately, worldwide exp erien ce in this regard has gen erally been a happy one, sin ce the average dose to radiation w ork ers has proved to be only a sm all fractio n of the m axim um p erm issib le lev els. On the other hand, the use of the term "m axim u m p erm issib le le v e ls" is open to ce rta in leg al and p sychological objections. While the aim is to keep all levels of radiation to the low est level p r a c ticable, it would not be tru e to sa y that if under ce rta in off-n orm al conditions a p erson re ce iv e s exp osu res in e x c e s s of the m axim um p erm issib le lev els, this n e c e s s a rily leads to a hazardous situation. The leg al im plication of this statem en t is obvious. Thus in the form ulation of reg u lato ry p ra c tic e s with legal overtones the use of the te rm "m axim u m p erm issib le lev els" should p referab ly be avoided and te rm s such as "op eratio n al lim its" or "accep tab le dose" should be used Design of in stallation s o r devices C areful design of the installation or d evices in which radiation so u rce s a re to be used is another im portant step in a radiation p rotection p ro g ram. In the planning and actu al con stru ction of an installation o r d evice, a con sid erab le degree of safety can be built in. The p rovision of such built-in safety featu res red u ces the influence of human e r r o r on the occupational exp osures ultim ately reco rd ed. The amount of built-in safety which can be provided will depend on the type of installation and on the nature of the work to be undertaken. In som e c a s e s this can be v e ry high, as in the c a se of la rg e so u rces used in m ed ical o r in d ustrial applications; in other c a s e s it m ay be m uch low er, thus req u iring m o re carefu lly form ulated operational p ro ced u res and ap propriate ad m in istrative co n tro ls. The design of a radiation installation includes the location, layout and shielding of the working a re a. In arriv in g at a suitable design, con sid eration m ust be given to many fa c to rs, which include the type and intensity of the radiation so u rce to be handled both in itially and in the fo reseeab le future, environm ental conditions which could have a bearin g on the d isp ersal of rad io active m aterial and the natu re of occupancy in the a re a s in the im m ediate neighbourhood of the in stallation. The working a re a should be so designed and co n stru cted as to m inim ize the possib le sp read of contam ination and to facilitate decontam ination p ro ced u res. Inherent safety m ay appear to be expensive initially, but w here it can be provided it will eventually prove to be the m ost reliab le and econ om ical method of providing adequate p rotection. 133

136 O perational p ro ced u res The inherent p rotection provided by the design and construction of the installation m ust alw ays be supplemented by action taken by the o p erato r to p ro te ct h im self. Safe operation will then depend to a la rg e exten t on his knowledge of the h azard s involved and his skill and con scien tiou sn ess in the im plem entation of recom m ended op erational p ro ced u res and in the use of p ro tectiv e d evices. F o r this reason the radiation w orker should be train ed by p articip ation in ap propriate radiation safety training p ro g ra m s. In addition, he m ust alw ays work under the close supervision of a qualified and exp erien ced p erson. W ell-defined operational p ro ced u res should be established for all radiation in stallation s; the p erson nel involved should be furnished with w ritten copies of th ese p roced u res and should be provided with all in strum ents and equipment n e c e s s a ry to im plem ent such p ro c e d u res. Special p ro ced u res, including the p rovision of m edical fa cilitie s, should be drawn up to m eet fo reseeab le accid en ts and em ergen cy situ ation s. When dealing with any em ergen cy situation, a p ro p er balance m u st be stru ck between controlling the em ergen cy and exposing the operational p erson nel involved to e x ce ssiv e radiation h azard s. F in ally it would be useful to re ite ra te that all work should be so planned that the likelihood of o ccu rre n ce of an em ergen cy is kept to a m inim um M aintenance of discipline and re co rd s The m aintenance of good operational discipline in a radiation in stallation is of the g re a te st im p ortan ce. The lines of resp on sib ility in all m a tte rs connected with radiation safety should be v e ry c le a rly drawn and each individual should be aw are of his duties and resp o n sib ilities. N e ce ssa ry liaison should be established and m aintained with all extern al agen cies which could be involved, such as the fire au th orities, the p olice, and the public health au th orities. A routine p erson nel m onitoring p rogram m ust be established to en sure that radiation doses receiv ed by the person nel during the co u rse of th eir norm al occupation do not exceed the m axim um p e rm issible lim its set by the national or international au th orities. Depending upon the size and type of the facility this could be eith er an "in -h ou se" p ro g ra m or p a rt of a cen tralized se rv ic e. A ccu ra te and com plete re co rd s of each individual's exposure to all fo rm s of ionizing radiation should be m aintained and p reserv ed during his lifetim e. T hese re co rd s should co v e r e x tern al and in ternal exp osu res and should include doses both to whole body and to lim ited region s of the body such as the hands and feet o r specified o rgan s. 134

137 D oses receiv ed as a re su lt of any planned em ergen cy o r accid en tal exposure should be reco rd ed. D etails of lay -o ff p erio d s, if any, resu ltin g from e x ce ssiv e exp osures should a lso be included Inspection A sy stem of inspection and follow-up should be established by those resp on sib le for the rad iation safety p ro g ram as a whole to en sure that the p ro g ram is being im plem ented efficiently and that exposure levels a re held at the low est p ra ctica b le lev els..inspection p ro ced u res should aim at verifyin g that: (a) Individual w ork ers p o sse ss the exp erien ce and training n e ce ssa ry to c a r r y out th eir work safely and efficiently (b) A ppropriate safety featu res have been built into the installation, o r device, and suitable op erational p ro ced u res a re p ractised (c) P e rso n s who have resp on sib ilities for radiation safety in the installation in question a re fam iliar with the approved operating p ro ced u res and with lines of resp on sib ility in the organization and a re d ischarging th eir duties sa tisfa cto rily (d) Adequate re c o rd s of p ersonnel exp osu res and of m onitoring o p e ra tions a re m aintained so that th ere is no likelihood of any p erson in the organization receivin g radiation doses in e x ce ss of the m axim um p erm issib le lev els Summing up of the resp on sib ilities of an o v erall radiation safety organization To sum up, it can be said that the organization in ch arge of an o v erall rad iation safety p ro g ram should be resp on sib le for: (a) (b) (c) The form ulation and im plem entation of ap propriate radiation p rotection regulations. The siting, location and design of radiation in stallations with p a rticu la r re fe re n ce to (i) the types of radiation so u rce s to be used, (ii) environm ental fa cto rs related to the disposal of rad io activ e m a te ria l and to its d isp ersal both under norm al and em erg en cy conditions and (iii) the p resen ce of occupied a re a s in the v icin ity of the in stallation. S tru ctu ral design featu res which would have a bearin g on (i) the p ossib le sp read of contam ination throughout the a re a and (ii) ease of decontam ination. 135

138 (d) The setting up of house ru les and w ell-defined operational p ro ced u res. (e) The p rop er in struction of person nel in th ese ru les and p ro c e d u res. (f) The p rovision of all n e ce ssa ry facilities for (i) personnel m onitoring, (ii) a re a m onitoring, (iii) m ed ical supervision and (iv) the m aintenance of all the relevan t re c o rd s. (g) The drawing up of p ro ced u res fo r m eeting em erg en cies and the provision of all the facilities n e c e s s a ry for carry in g out these p ro ced u res. (h) The m aintenance of p rop er liaison with extern al ag en cies, such as the fire, p olice, tran sp o rt, and public health au th orities. (i) The m aintenance of cum ulative whole-body radiation exposure re c o rd s covering both internal and extern al exp osu res. (j) The initiation of ap propriate action in c a se s of e x ce ssiv e e x p o su res or radiation em erg en cies Additional featu res In any radiation installation, re p a irs and m odifications to plant and equipment will be n e ce ssa ry from tim e to tim e. Such operations m ay involve p erson s not norm ally employed in radiation work, and som e exposure to radiation both in ternal and extern al m ay be inevitab le. In all such ca ses it is essen tial that the operations be subject to adequate reg u lato ry con trol. An im portant fa cto r to be rem em b ered in planning radiation p rotection facilities for individual in stallation s is that the sm all u se r of radiation so u rces could take advantage of the se rv ice s of centralized] agen cies to m eet m any of his m onitoring req u irem en ts. P o ck et d osim eters for p ersonnel m onitoring and other sim ple instrum ents fo r a re a m onitoring are relativ ely e a sy to operate and m aintain in the in stallation itself, but m ore sp ecialized p ro ced u res such as a film - badge se rv ice and the m easu rem ent of the body burden of rad ioactive m a te ria ls m ay req u ire exp ert knowledge and m ore elab orate equipm ent, T hese con sid erations also apply to decontam ination operations, disposal of rad ioactive w aste m aterial, standardization and calibration of radiation m easu ring instrum ents and m easu res to deal with radiatioil em e rg e n cie s. Much can be done by the sm all u se r with the aid of re la tiv e ly sim ple equipment, but fo r the m ore difficult o r sp ecialized p ro ced u res he should seek the a ssista n ce of a cen tralized organization offering the ap propriate s e rv ic e s

139 A N N E X A COLLECTION OF USEFUL HEALTH PHYSICS DATA T A B L ES FIG URES ILLUSTRATIONS BIBLIO G RAPH Y

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141 T A B L E S T A B L E I. PR IN C IPLES O F RADIATION PRO TECTIO N A P E R S P E C T IV E RADIATION WORKER NON-RADIATION WORKER MINIMIZE THE HAZARD (1) Keep the amount of radioactive material required to a minimum (2) Choose radioactive material presenting the least possible hazard (3) Choose the safest and most practicable procedures (4) Dispose of radioactive waste safely (5) Restrict the movement of radioactive materials to a minimum GUARD THE HAZARD (CONTAINMENT) General Prevent hazardous release of radioactive material to the environment Local Local (1) Partial (fume cupboards) (1) Sealed transport containers (2) Total (glove boxes) (3) Temporary (sealed containers) General (1) Reduce radiation level outside the controlled area to well within permissible levels Local (1) Permanent (hot cell) (2) Temporary (lead bricks, lead coffins) SHIELDING General (1) Reduce radiation level in public access position to well below non-occupational levels Local (1) Shielded transport containers GUARD THE WORKER (PLANNING AND INSTRUCTION) (1) Choice of material, instru- (1) Transport regulations ments and facilities (2) Emergency procedures (2) House rules (3) Operating instructions (4) Emergency instructions MONITORING (RADIATION AND CONTAMINATION) (1) Personnel (1) Transport containers (2) Equipment (3) Area and site (4) Biological PROTECTIVE CLOTHING (1) General types for routine operations (2) Special types for emergency use Note: in all processes involving radioactive materials (storage, experiment or process, handling of materials, transport and the disposal of active waste) consideration must be given to the appropriate methods of control.

142 T A B L E II. S P E C IFIC ACTIVITIES AND H A L F-L IV E S O F SOME T Y P IC A L RADIOISOTOPES Radioisotope Radiations Specific activity (Ci/g) assuming the isotope is carrier free Half-life 3H yr 24Na 0 and y 8.3 x h 3Zp x 10s d 42 K 0 and y 5. 9 x h 45 Ca x d 56 Mn 0 and y 2.2 x h 55 Fe 0 and y 5.0 x d 60 Co 0 and y 1.1 x yr 89 St 0 and y 2.9 x d 90 Sr x 10z yr m Ag 0 and y 1.6 x d 131 j 0 and y 1.25 x 10s 8.09 d 137 Cs 0 and y yr 92 Ir 0 and y d 197 Pt 0 and y 9.0 x h 198 Au 0 and y 2.4 x d 204 & 5.8 x 10z 3. 0 yr 210 po aand y 4.5 x d 226 Ra a and y yr 232 Th a and y 1.1 x x 1010 yr 238,j a and y 3.3 x 10' x 109 yr 239 pu a and y 6.2 x 10' yr 140

143 T A B L E III. IM PO RTA N T NEUTRON SO URCES Source Type Half-life Maximum neutron energy (MeV) Average neutron energy (MeV) Neutron output of source (n s-1 * Ci-1) Remarks 226Ra-Be (a,n) reaction 1622 yr x 106 Large gamma background 210Po-Be (a,n) reaction d x 106 Short half-life, very low gamma background 239Pu-Be (a,n) reaction yr x 106 Low gamma background 241 Am-Be (a, n) reaction 462 yr x 106 Low gamma background 24Na-Be (y, n) reaction h x 106 Very large gamma background, monoenergetic neutron source 124Sb-Be (y, n) reaction 60 d x 106 Very large gamma background, monoenergetic neutron source 252 Cf Spontaneous fission yi (effective half-life) x 1012 (per gram) Fission neutron spectrum. There will be fission and fission-product gamma background

144 T A B L E IV. TH E BIO LO G ICA L E F F E C T S O F RADIATION (X - or gam m a ra y s) Acute doses Probable effect 0-25 rad No obvious injury rad Possible blood changes but no serious injury rad Blood-cell changes, some injury, no disability rad Injury, possible disability rad Injury and disability certain, death possible rad 50% fatal within 30 d 600 or more rad Probably fatal SUMMARY OF E F F E C T S L IK E L Y TO R ESU LT FROM W H O LE-BO D Y EXPO SU R E TO RADIATION Mild dose Moderate dose Semi-lethal dose Lethal dose 0-25 rad 50 rad 100 rad 200 rad 400 rad 600 rad No detectable clinical effects. Probably no delayed effects. Slight transient blood changes. No other clinically detectable effects. Delayed effects possible, but serious effect on average individual very improbable. Nausea and fatigue with possible vomiting above 125 rad. Marked changes in blood picture with delayed recovery. Shortening of life expectancy. Nausea and vomiting within 24 h. Following latent period of about one week, epilation, loss of appetite, general weakness anu other symptoms such as sore throat and diarrhoea. Possible death in 2 to 6 weeks in a small fraction of the individuals exposed. Recovery likely unless complicated by poor previous health, superimposed injuries or infections. Nausea and vomiting in 1-2 h. After a latent period of about one week, beginning of epilation, loss of appetite, and general weakness accompanied by fever. Severe inflammation of mouth and throat in the third week. Symptoms such as pallor, diarrhoea, nosebleeds and rapid emaciation in about the fourth week. Some deaths in 2 to 6 weeks. Eventual death to probably 50% of the exposed individuals. Nausea and vomiting in 1-2 h. Short latent period following initial nausea. Diarrhoea, vomiting, inflammation of mouth and throat toward end of first week. Fever, rapid emaciation and death as early as the second week with eventual death of probably all exposed individuals.

145 T A B L E V. TH ERM A L NEUTRON ACTIVA TIO N D E T E C T O R S Element Reaction Isotopic abundance (%) Crosssection (barns) Halflife Gamma ray energy (MeV) In 113In(n, 7) 1I4In d In(n, y) 116In min 1.29 Au 197Au(n, 7) 198Au d Co f Co(n, 7) 60Co yr \l.33 Mn 55Mn(n, 7)56Mn h 0.85 Cu 63Cu(n, 7)64Cu h Cu(n, 7) e6cu min 1.04 W 184W(n, 7) I85w d W(n, 7) 187W h Na 23Na(n, 7)24Na h 2.75 Ni "Ni(n, 7) 65Ni h 1.49 Dy 164Dy(n, 7)165Dy min T A B L E VI. FAST NEUTRON THRESHOLD DETECTORS Element Reaction Half-life Approximate threshold energy (MeV) 32 s ^(n.p)32? 14.3 d p 31P(n, p)31si 2.6 h Si 28Si(n, p) 28A min Fe 56Fe(n, p)56ivin 2.6 h C I2C(n, 2n)n C 20.5 min Np 237Np(n, f) y 238U(n, f) Th 232Th(n,f)

146 T A B L E V II. NEUTRON F L U X -D O SE EQ U IVA LEN TS Neutron energy (MeV) QF Average flux to deliver 100 mrem in 40 h (a/cm2 s) Fluence to deliver 100 mrem (n/cm2) Thermal x x x x x x x x x 10s x x 106 T A B L E VIII. RATIO, a, O F THE SCA TTERED TO THE INCIDENT EXPO SU R E (C hapter 11) Scattering angle (from central ray) Gamma rays from X-rays 137Cs 60 Co 4 MV 6 MV CO o 15 9 x 10"3 6.5 x x 10"3 7 x x 10"3 3.6 x 10"3 2.7 X 10"3 1.8 x 10" x 10"3 2.3 x 10"3 1.1 x 10" x 10"3 0.9 x 10"3 0.6 x 10' x x x 10'3 144

147 T A B L E IX. Instrument and detector This publication is no longer valid P ER SO N N EL MONITORING D EV IC ES Radiation detected Range Usefulness Remarks Film dosimeter Photographic emulsion Low-kV X-rays. High-kV X-rays. Gamma rays. Slow neutrons. Fast neutrons. 1 mrem - 35 rem 1 mrem - 50 rem 5 mrem rem 5 mrem rem 3 mrem - 5 rem Inexpensive and easy to use. Has advantage of indicating radiation qualities in mixed fields when used with suitable array of filters. Provides permanent records. The ranges for X- and gammarays and slow neutrons are stated for Kodak Type-2 films, whereas for fast neutrons the values are for Kodak NTA films. In fast-neutron monitoring, the upper limit is restricted by the gamma background. Pocket dosimeter Ionization chamber X- and gammarays. 2.5 mr mr 100 mr - 5 R 200 mr - 10 R Particularly useful for measurement of exposure during any planned operation. Both self-reading and separate readout types of pocket chambers are available. In the X-ray region the chambers are energy dependent. Boron-lined ionization chamber Slow neutrons. 2.5 mrem mrem Chemical dosimeter Tetrachloroethylene system Gamma rays. 5 rem - 2 x 10 rem Useful for dosimetry in radiation accidents. Sensitive to neutrons also. Glass dosimeter Silver metaphosphate Gamma rays. 10 mrem rem Particularly useful for dosimetry in radiation accidents. The dosimeter requires a separate reader. Energy dependent in the X-ray region. Thermoluminescent dosimeter LiF (Eu, Mg) Li2B407(Mn) CaF2 Gamma rays. 10 mrem - 10 rem Suitable for a wide range of doses. LiF and Li2B407 are tissue equivalent and can be used for dosimetry experiments in phantom. CaF2 has an energy-dependent response. Requires a separate reader. Has good potential as a personnel dosimeter.

148 T A B L E X. P O R T A B L E SU R V E Y IN STRU M EN TS Instrument Detector Radiation detected Range Usefulness Remarks Gun-type survey meter GM-type survey meter Ionization chamber (air) GMtube (either end window type or thin-wall type) Gammas, X-rays, beta radiation Gamma and beta radiation 0-5 R/h For survey around X-ray and gamma ray installations and storage areas of radioactive materials mr/h counts/s For checking general contamination levels. The most widely used instrument for these measurements. J It has a shield for beta radiation. For lowenergy betas, thinwalled GMcounter detector is used. Alpha scintillation counter Scintillator and photomultiplier Alphas counts/s Assay of alpha emitters, estimation of alpha contamination on surfaces, filter paper counts, etc. Neutron flux meter Moderated BF3 counter Neutrons n/cm2 s General survey around reactors, accelerators, neutron sources, etc. for fast and thermal neutrons.

149 T A B L E X I. Type of laboratory This publication is no longer valid F A C IL IT IE S AND IN STRU M EN TS FO R WORKING P L A C E S Facilities required Instruments required (numbers given refer to illustrations in Annex) a Essential Optional Remarks Type 1 Those of a good chemical laboratory. Possibly fume hoods. 2 3, 8 4, 5 Type 2 All facilities to restrict radiation levels and avoid contamination. Fume hoods and glove boxes. Decontamination facilities. Facilities for waste disposal. 2 1, 3 4, 5, 8 (1) Detailed operating instructions may be essential. (2) Supplementary instruments for monitoring clothing, air monitoring, etc. Type 3 All those for type 2 plus special structural features related to working surfaces, ventilation, etc. Change room. Special decontamination facilities. 1, 2, 3 6, 7, 9 4, 5, 8 Special hand Perma- &clothing nently monitors. installed Air monito- area moniring equip- tors, ment. (1) Siting and ultimate location to be carefully done. (2) Strict compliance with detailed operating instructions essential. (3) Special instrumentation for measuring high-level radiation and for following up all operational phases from the radiation protection point of view. Sealed sources and radiation machines. Adequate shielding and clear demarcation of controlled areas. Special facilities for mobile equipment. 2 1, 3, 11 5, 8, 10 (1) Restricted access to controlled areas. a See Illustrations following Tables and Figures.

150 T A B L E XII. LIN EAR ABSORPTION C O EFFIC IEN TS n FOR GAMMA RADIATION IN VARIOUS M ATERIALS Energy (E) (MeV) (cm"1) Pb Fe Al h2o T A B L E XIII. RELATIO N B ET W E E N THICKNESS OF ORDINARY CONCRETE (DENSITY 2.2 g/cm3) AND LEAD EQUIVALENCE AT VARIOUS X -R A Y QUALITIES (BROAD-BEAM CONDITIONS) Thickness of Lead equivalent (in mm) for X-rays excited at the following voltages (kv) (cm) '

151 T A B L E XIV. M ATERIALS S E L E C T E D THERM AL NEUTRON SHIELDING Element Reaction Isotopic abundance i l ) Crosssection (barns) Remarks n3cd 113Cd(n, y) 1I4Cd Capture gamma rays have a wide energy spectrum Natural cadmium 2450 Cadmium is used in the form of natural cadmium sheets. Hence the averaged cross-section is given. Isotopes of cadmium other than 113Cd do not participate in the reaction. 10B I0B(n, cl)1li In 95% of the reactions a MeV gamma ray is emitted. Natural boron 759 Only 10Bparticipates in the reaction. The averaged cross-section for boron is given. 6li 6Li(n, a)3 H Natural lithium 70.7 Only 6 Li participates in the reaction. The averaged cross-section for lithium is given. 149

152 T A B L E XV. This publication is no longer valid THE TREA TM EN T OF SU R FA C ES IN LA BO RA TO RIES AND WORK P LA C E S Conditions of exposure Normal Chem ical attack M echanical wear and tear Humidity Irradiation Surface Ceiling Good-quality distemper Chlorinatedrubber-based paint - Chlorinatedrubber-based paint Epoxide resin-based paint Walls Good-quality gloss paint (i) Chlorinatedrubber-based paint (ii) Glazed tiles a (i) Epoxide resin- based paint (ii) Glazed tiles a (i) Chlorinated- rubber-based paint (ii) Glazed tiles a (i) Epoxide resin- based paint (ii) Tiles Floors (i) Concrete sealed off by (i) Concrete sealed off with (i) Concrete sealed off with (i) Concrete sealed off with (i) Concrete sealed off with painting (ii) Good-quality cork line (iii) Compositions, e.g. organic chlorinated-rubber- based paint (ii) Neoprene rubber floorcovering epoxide resin-based paint (ii) Compositions (iii) Good-quality tiles or bricks a chlorinated- rubber-based paint (ii) Compositions (iii) T iles epoxide resin-based paint (ii) T iles a (iii) Acid resistant bricks a Metalwork Good-quality gloss paint Chlorinated-rubber- based paint Epoxide resin-based paint Chlorinatedrubber-based paint Epoxide resin-based paint Woodwork Same as for metalwork. Where fixed benches are concerned the working tops should be covered permanently with lam inated plastics such as form ica or should be well waxed and covered with temporary coverings such as polythene or stout good-quality paper. Concrete Must always be sealed off with appropriate material such as paint, compositions or tiles. a Attention must be given to the correct choice of bedding and pointing m aterial for tiles and bricks. considerable contamination. Porous m aterials may take up

153 TA B LE XV I. This publication is no longer valid RECOMMENDED REAGENTS FOR DECONTAMINATION OF VARIOUS MATERIALS D econtam i- nating agent M aterial being decontam - inated (1) FOR REM OVAL OF CONTAM I NATION ONLY Detergents Decontaminating reagents including chelating or sequestering agents Glass Stainless steel Copper Aluminium Lead Rubber Unbuilt detergents, i.e. obtainable as by-products of the oil industry. Solutions made up of the following m aterial: Sodium carbonate or built detergent: 5-10% Ethylene diamine tetra -acetic acid: 1-2% Citric acid: z-1% Strengths of solutions to be determined by experience. (2) FOR REM OVAL OF SURFACE AND CONTAMINANT Acids and other vigorous reagents Chromic acid Sulphuric acid 1%. Inhibited phosphoric acid 20% Sodium hydroxide, C itric acid Aqua regia Acetone General abrasives Proprietary abrasive powders and pastes. Wire brushes and steel wool. Special abrasives Vapour blast Cutting tools Buffing

154 TABLE XVII. PACKAGING AND RADIATION L E V E L REQUIREMENTS FOR TRANSPORT OF RADIOACTIVE M ATERIALS Packaging type Category I (White) (a) Radiation lev el at any tim e during transport at any point on the external surface of the package Category II (Yellow ) (b) Category III (Yellow) a (c) Type A Designed to withstand normal conditions of transport Type B Designed to withstand the damaging effects of a transport accident Does not exceed 0. 5 mrem/h, and the package does not belong to Fissile Class II or Class III Is between mrem/h and the transport index b does not exceed 1.0. The package also belongs to this category when the package belongs to Fissile Class II (including the case when radiation lev el is within the lim its specified in column (a)) Exceeds one of the lim its specified in column (b) but does not exceed 100 mrem/h and the transport index does not exceed 10 or when the package belongs to Fissile Class 111 Sp ecial requirements for all packages containing liquid radioactive m aterials: Receptacle surrounded by adequate quantity of absorbent m aterial; receptacle and outer container leak-proof. a These lim its may be exceeded provided that the package is transported as "fu ll-load " in accordance with the requirements relevant to the mode of transport used. b The number expressing the maximum radiation level in mrem per hour at one metre from the external surface of the package.

155 T A B L E XVII (continued) Limitations on number of packages Labels and markings Vehicle, aircraft or inland water craft: Sum of transport indices of Category-II and C ategory-ill Yellow packages should not exceed 50. Where this control is not effected by reference to transport indices, there shall not be more than 50 Category-II or 5 Category-Ill packages in any one group of packages. Where packages of both categories are present, one Category-Ill package shall be taken as equivalent to 10 C ate- gory-ii packages. Seagoing vessel: Sum of the transport indices of Category-II and C ategory-ill Yellow packages should not exceed 200 provided that the sum of transport indices of no individual cluster or group of such packages exceeds 50. Where this control is not effected by reference to transport indices, there shall not be more than 50 Category-II or 5 Category-Ill packages in any one group. Where packages of both categories are present, one C ate- gory-iii shall be taken as equivalent to 10 Category-II packages. Com pliance with the requirements for labelling and marking should be the responsibility of the consignor. Type-A and Type-B packaging should be plainly and durably marked on the outside of the package with inscriptions ' Type A* and Type B1 respectively. Each package should bear two labels, according to the category, on two opposite sides of the outside of the package. In case of Low specific activity m aterials, the principal radioactive contents shall be described on the package labels as "LSA... " Marking of v eh icles: While carrying Category-II and -III Yellow packages, placards should be displayed: in rail-freight vehicles on each side of the two external lateral walls in road vehicles on each side of the two external lateral walls and on the external rear wall.

156 T A B L E XV II (continued) Exemptions from packaging requirements Transport documents ITEMS CONDITIONS Should include information on I. Low specific activity radioactive materials such as (1) Uranium or thorium ores and physical or chem ical concentrates of these ores. (2) Un-irradiated natural or depleted uranium or un-irradiated natural thorium. When packed in strong industrial packages which will prevent loss of contents under normal conditions of transport can be transported as individual packages or as "fu ll-load " in a veh icle, inland water craft or seagoing vessel. Radioactive m aterials: Package: Name of the radioactive m aterial, physical or ch em ical form. Activity of the radioactive material (Ci). Category of package I (W hite), II (Yellow ), 111 (.Yellow). (3) Tritium oxide in aqueous solutions provided the concentration does not exceed 15 Ci/litre. External radiation: Dose rate at the outer surface of the package and at I metre from the package. Transport index when II. Radioactive materials of less than a specified activity limit. III. Manufactured instruments and articles having radioactive m aterials of less than specified activity lim its. When securely packed in strong packages in such a manner that during transport there is no leakage of radioactive materials. When the radiation level at 10 cm from any point on the external surface of any unpacked instrument or article does not belonging to categories II and HI Yellow. Any other special instructions where necessary and ce rtificate of consigner declaring com pliance with relevant transport regu exceed 10 mrem/h. lations.

157 FIGURES EXPOSURE RATE (R/h) GAMMA ENERGY (MeV) FIG. 1. Exposure rate from a 1 -C i point source of gamma radiation. 155

158 IN VACUUM i i IN AIR N r ± T I DOSE RATE (rad/h) I i I * / i / J- / /\ f l IN VACUUM -I L 1in AIR BETA ENERGY, Emax (MeV) FIG. 2. Dose rate from a 1 -C i point source of beta radiation. 156

159 RANGE OF ELECTRON IN ALUMINIUM ( m g /c m 2) LET. { MeV cm g ) PAR TIC LE ENERG Y (MeV) FIG. 3. Range of electrons, protons and alphas In aluminium. FIG. 4. Energy dissipated by electrons, protons and alphas in water.

160 TRANSMISSION ABSORPTION FACTOR ABSORBER THICKNESS (mg/cm2) FIG. 5a. Transmission of 6 particles of various energies in aluminium. Note: between 0 and mg/cm2 the curves are extrapolated. The absorber is assumed to be near the counter. 158

161 TRANSMISSION a b s o r p t io n f a c t o r ABSORBER THICKNESS (mg/cm2) FIG. 5b. Transmission of 0 particles of various energies in aluminium. Note: between 0 and mg/cm the curves are extrapolated. The absorber is assumed to be near the counter. 159

162 ATTENUATION COEFFICIENT flip (cmvg) GAMMA ENERGY (MeV) FIG. 6. Mass attenuation coefficients for various elements. 160

163 ATTENUATION COEFFICIENT ju / p c m 2/ g ) G AM M A ENERGY (MeV) FIG. 7. Mass attenuation coefficients for various materials. 161

164 1 l.f 2.: 2.: THICKNESS TO ATTENUATE BY A FACTOR OF 10 (cm) RAY ENERGY (MeV) FIG. 8. Thickness to attenuate narrow beam of gamma rays by a factor of

165 k (R/mA min at 1 m) FIG. 9. Attenuation in lead of X-rays produced by potentials of kv. 163

166 10 \W k (R/mA min at 1 m) L \ v \ K ^ F A \ 250 k V p \ kvcp kvcp: kvp \ IS LEAD (mm) FIG. 10. Attenuation in lead of X-rays produced by potentials of kv. 164

167 k (R/mA min at 1 m) CONCRETE (in.) (DENSITY = 2.35 g/cm3) FIG. 11. Attenuation in concrete (density 147 lb/ft3, g/cm3) o f X-rays produced by potentials of kv. 165

168 k {R/mA min at 1 m) LEAD (mm) FIG. 12. Attenuation in lead of X-rays produced by potentials of MV constant potential. 166

169 k (R/mA min at 1 m) FIG. 13. Attenuation in concrete (density 147 lb/ft3, g/cm3) of X-rays produced by potentials of MV constant potential. 167

170 FIG. 14. Attenuation in lead of X-rays produced at 4-10 MV. CONCRETE THICKNESS (in.) FIG. 15. Attenuation in concrete (density 147 lb/ft3, g/cm3) of X-rays produced at 6 and 10 MV. 168

171 TRANSMISSION, B FIG. 16. Transmission through lead of gamma rays from selected radionuclides. 169

172 CONCRETE (in.) FIG. 17. Transmission through concrete (density 147 lb/ft3, g/cm3) of gamma rays from selected radionuclides. r 170

173 TRANSMISSION, B IRON (in.) FIG. 18. Transmission through iron of gamma rays from selected radionuclides. 171

174 z o < cc V PRESOWOOD LEAD STEEL THICKNESS (in.) FIG. 19. Transmission through lead, Presdwood and steel o f 6-MW scattered radiation (90 scatter only). CONCRETE THICKNESS (in.) FIG. 20. Broad beam attenuation in concrete (density 147 lb/ft3, g/cm3) for the 6-M V X -iays scattered at six different angles from a Presdwood phantom. 172

175 1.0 FIG. 21. Transmission through lead of ^Co scattered radiation from a Masonite phantom, 20-cm -diam eter field at 1 m from source. 173

176 FIG. 22. Transmission through concrete (density 147 lb/ft3, g/cm3) of 60Co scattered radiation from Masonite phantom, 20-cm -diam eter field at 1 m from source. 174

177 FIG. 23. LEAD (cm) Transmission through lead of 137 Ce scattered radiation. FIG, 24. radiation. Transmission through concrete (density 147 lb/ft3, g/cm3) of 137Ce scattered 175

178 TRANSMISSION FIG. 25. Fast neutron transmission in paraffin wax and water for a Po-Be neutron source. 176

179 FIG. 26. Approximate broad beam absorption of neutrons in water and concrete (concrete density 148 lb/ft3, g/cm3). Note that the curve shows the reduction of neutron flux density, not of dose rate. 177

180 FIG. 27. L-bench designed for a work load of 600 mg* h o f radium or equivalent in one week. Construction m aterial is lead. The L-bench is always located 30 cm from the edge of a sturdy work table so that the person will be provided protection both by distance and shielding. 178

181 FIG. 28. Radiochem ical fume hood. (1) Hood; (2) strippable paint, if desired; (3) paper or tray on hood floor; (4) safety lin e, 8 in. from front; (5) cup sink; (6) exhaust duct damper to adjust air velocity between 50 and 80 linear ft/min with open sash; (7) by-pass for air when sash is nearly closed; (8) sash; (9) adjustable baffle; (10) trough to catch sash condensate; (11) air foil; (12) air foil showing air space; (13) service outlets; (14) controls for service outlets; (15) electrical outlets; (16) base to support 2000 lb. 179

182 FIG. 29. Glove box. (1) Flange for mounting tubes for housing columns; (2) ele ctric power panel; (3) fluorescent lamp; (4) service inlets; (5) equipment frame; (6) glass window; (7) door; (8) long rubber gloves; (9) plywood box heavily enam elled; (10) airlock; (11) electric outlets; (12) filter; (13) exhaust outlet; (14) to hood or duct; (15) exhaust blower. 180

183 ILLUSTRATIONS ILLUSTRATION 1 P ocket dosim eter Rem arks: Typical ranges: 100 m R, 200 m R, 1 R, 2 R and 5 R. Special m odels available for slow neutron monitoring (wall lined with suitable m aterial such as 10B). 181

184 IL L U S T R A T IO N 2 2: 0 Z * S 2 * a. >< UJ * «0 x -* g. 3 y u > -i~-r-j Film badge Rem arks: Suitable for assessm en t of a wide range of radiation exposures for all types of radiation (except fast neutrons). Location and thicknesses of filters are shown. Generally worn on the chest to a sse ss whole body dose. Can be worn on other parts of the body (e.g. wrist) to a sse ss radiation exposures to that part of the body. Cadmium filter enables assessm en t of therm al neutron dose equivalent. 182

185 IL L U S T R A T IO N 3 Wrist badge in use Rem arks: Two pocket dosim eters and a chest film badge are also shown in use. 183

186 IL L U S T R A T IO N 4 Contamination monitor (N orm ally fitted with a number of probes such as a thin-wall GM counter with beta shield, end-window counter, and scintillation detector) Rem arks: Mains-operated instrument with count-rate m eter and scaler. Provided with aural indicator. It can be used routinely for alpha, beta and gam ma contamination monitoring with appropriate probes. When fitted with special background reduction devices, this instrument can be used for extrem ely lowlevel counting such as in urine and effluent sampling and in air sampling. This instrument can also be used for neutron surveys by using suitable scintillation detectors for fast and slow neutrons. 184

187 IL L U S T R A T IO N 5 Portable contamination monitor (Fitted with an end-window GM counter) Rem arks: Battery-operated instrument for routine m easurem ents of beta-gam m a contamination in laboratories. Provided with visual and aural indication. Count-rate m eter covers a span of Hz corresponding to m R /h in three ranges. 185

188 IL L U S T R A T IO N 6 Portable spectrometer with scintillation counter for field use Rem arks: Battery-operated gamma ray spectrom eter. Can be used to analyse filter paper and effluent sam ples. Composed of m odules-decade sca le r, linear am plifier, single channel analyser, tim er and EHT. W ell-type scintillation probe is shown on the right. 186

189 IL L U S T R A T IO N 7 Ionization-chamber-type survey meter with dosimeter Rem arks; Battery-operated ionization-cham ber-type instrument for monitoring X - and gamma rays. It can be fitted with a irequivalent ionization chambers of different volum es to cover a wide range of exposure rates of 5 m R /h to 5000 R /h. When fitted with adapter which has an integrating capacitor, it can m easure exposures in the range 50 m R to 500 R. Facility available for fitting long cable for making remote m easurements in high fields. 187

190 IL L U S T R A T IO N 8 Gun-type ion chamber survey instrument R em arks: Portable, extrem ely light, operates on one flash light cell. Fitted with air-equivalent ionization chamber. Used for X -, gamma and beta radiation. Has special window for betas. Useful in the range of 2 m R /h to 2 R /h. Uses solid state electronics. 188

191 IL L U S T R A T IO N 9 Portable wide-range survey meter Rem arks: Battery-operated radiation survey m eter for monitoring X - and gam m a-ray exposure rates from 10 m fi/h to 100 R /h. U ses sm all halogen-filled GM tube and telescopic extension rod 1 metre in length for remote monitoring of high radiation fields. 189

192 IL L U S T R A T IO N 10 Mains-operated X- and gamma-ray dosimeter (With four ionization chambers of different sensitive volumes) Rem arks: Can cover exposure rates up to 500 R/ min and integrated exposures up to R. A long cable is provided to facilitate m easurem ents to be carried out while the ion chamber is in the useful beam. 190

193 IL L U S T R A T IO N 11 TLD reader Rem arks: TLD reader and analyser suitable for TLD research and routine dosim etry. Has provision for (i) variable linear heating rates from 0.5 C /s to 2 0 C /s ; (ii) simultaneous measurement of integrated TL em ission and peak height of glow curve. 191

194

195 BIBLIOGRAPHY The Bibliography presented below is meant to be sufficiently representative but not n ecessarily exhaustive. General ( INTERNATIONAL ATOMIC ENERGY AGENCY, Safe Handling of Radioisotopes (1st Edn with Revised Appendix 1), Safety Series No. 1, IAEA, Vienna (1962). INTERNATIONAL ATOMIC ENERGY AGENCY, M edical Supervision of Radiation Workers, Safety Series No. 25, IAEA, Vienna (1968). ALEXANDER, P., Atomic Radiation and Life, Revised Edn, Penguin Books, In c., Baltimore (1965). ATTIX, F. H., ROESCH, W.C. (Eds), Radiation Dosimetry l t 2nd Edn, Academic Press, New York (1968). ATTIX, F. H., ROESCH, W.C. (Eds), Radiation Dosimetry 2, 2nd Edn, Academic Press, New York (1966). A TTIX, F. H., TOCHILIN, E. (Eds), Radiation Dosimetry ^3, 2nd Edn, Academ ic Press, New York (1969). BLATZ, H. (Ed.), Radiation Hygiene Handbook, McGraw Hill Book C o., Oxford (1959). BRAESTRUP, C, B., WYCKOFF, H. O,, Radiation Protection, Charles C. Thomas, Publisher, Springfield, 111. (1958). EISENBUD, M., Environmental Radioactivity, McGraw Hill Book C o., In c., New York (1963). FAIRES, R.A., PARKS, B.H., Radioisotope Laboratory Techniques, George Newnes Ltd., London (1960). GLASSTONE, S., Source Book on Atomic Energy, D. Van Nostrand C o., In c., London (1967). JOHNS, H. E., CUNNINGHAM, J. R,, The Physics of Radiology, 3rd Edn, Charles C, Thomas, Publisher, Springfield, 111. (1969). KINSMAN, S., Radiological Health Handbook, Division of Radiological Health, US Department o f Health, Education and Welfare (1960). LAPP, R. E., ANDREWS, H. L., Nuclear Radiation Physics, 3rd Edn, Prentice Hall In c., New Jersey (1963). MORGAN, K. Z., TURNER, J. E. (Eds), Principles of Radiation Protection, John Wiley & Sons, In c., New York (1967). OVERMAN, R. T., CLARK, H. M., Radioisotope Techniques, McGraw Hill Book C o., In c., New York (1960). PRICE, W.J., Nuclear Radiation D etection, 2nd Edn, McGraw H ill Book C o., In c., New York (1964). SEGRE, E. (Ed.), Experimental Nuclear Physics, John Wiley & Sons, In c., New York (1960). STEPHENSON, R., Introduction to Nuclear Engineering, McGraw Hill Book C o., In c., New York (1958). Chapter 2 HALLIDAY, D., Introductory Nuclear Physics, John Wiley & Sons, In c., New York (1955). 193

196 Chapter 3 EVANS, R.D., The Atomic Nucleus, McGraw Hill Book C o., In c., New York (1955). C hapter 4 BACQ, Z. H., ALEXANDER, P., Fundamentals of Radiobiology, Pergamon Press, London (1961). CLAUS, W.D,, Radiation Biology and M edicine, Addison Wesley Publishing C o., In c., Reading, Mass. (1958). C h apter 5 INTERNATIONAL COMMISSION ON RADIOLOGICAL UNITS AND MEASUREMENTS, Rep. 9, Natn Bur. Stand. Handbk 78, Washington, D.C. (1959). INTERNATIONAL COMMISSION ON RADIOLOGICAL UNITS AND MEASUREMENTS, Radiation Quantities and Units, Rep. 10a, Natn Bur. Stand. Handbk 84, Washington, D.C. (1962). INTERNATIONAL COMMISSION ON RADIOLOGICAL UNITS AND MEASUREMENTS, Radioactivity, Rep, 10c, Natn Bur. Stand. Handbk 86, Washington, D.C. (1962). INTERNATIONAL COMMISSION ON RADIOLOGICAL UNITS AND MEASUREMENTS, Radiation Quantities and Units, Rep. 11, Washington, D.C. (1968). C h apter 6 INTERNATIONAL ATOMIC ENERGY AGENCY, Basic Safety Standards for Radiation Protection, Safety Series No. 9, IAEA, Vienna (1967). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Recommendations, Pergamon Press, London (1958). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Report of Com m ittee IV on Protection Against Electrom agnetic Radiation Above 3 MeV and Electrons, Neutrons and Protons, Publication 4, Pergamon Press, London (1964), INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Recommendations, Publication 6, Pergamon Press, London (1964). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Recommendations, Publication 9, Pergamon Press, London (1969). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Report of Com m ittee IV on Evaluation of Radiation Doses to Body Tissues for Internal Contamination due to Occupational Exposure, Publication 10, Pergamon Press, London (1968). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, General Principles of Monitoring for Radiation Protection of Workers, Publication 12, Pergamon Press, London (1969). 194

197 Chapters 7 and 8 US NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, Radiological Monitoring Methods and Instruments, Rep. 10, Natn Bur. Stand. Handbk 51, Washington, D. C. (1952). HANDLOSER, J. S., Health Physics Instrumentation, Pergamon Press, London (1959). SNELL, A. H. (Ed.), Nuclear Instruments and Their Uses.1, JohnWiley & Sons, In c., New York (1962). INTERNATIONAL ATOMIC ENERGY AGENCY, The Use of Film Badges for Personnel Monitoring, Safety Series No. 8, IAEA, Vienna (1962). INTERNATIONAL ATOMIC ENERGY AGENCY, The Basic Requirements for Personnel Monitoring, Safety Series No. 14, IAEA, Vienna (1965). CAMERON, J. R., SUNTHARAL1NGAM, N., KENNEY, G.N., Thermoluminescent Dosimetry, University of Wisconsin Press, Madison (1968). Chapter 9 INTERNATIONAL ATOMIC ENERGY AGENCY, Handbook on Calibration of Radiation Protection Monitoring Instruments, Techn. Reps Series No. 133, IAEA, Vienna (1971). Chapter 10 US NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, Control and Removal of Radioactive Contamination in Laboratories, Rep. 8, Natn Bur. Stand. Handbk 48, Washington, D.C. (1951). US NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, Safe Handling of Radioactive Materials, Rep. 30, Natn Bur. Stand. Handbk 92, Washington, D.C. (1964). US NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, Radiation Protection in Educational Institutions, Rep. 32, Washington, D.C. (1966). INTERNATIONAL LABOUR OFFICE, Manual o f Industrial Radiation Protection, Part 1 - Convention and Recommendation, ILO, Geneva (1963). Chapters 11 and 14 US NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, Protection Against Neutron Radiation up to 30 M illion Electron Volts, Rep. 20, Natn Bur. Stand. Handbk 63, Washington, D.C. (1957). US NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, M edical X-Ray and Gamma-Ray Protection for Energies up to 10 MeV (Equipment Design and Use), Rep. 33, Washington, D. C. (1968). US NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, M edical X-Ray and Gamma-Ray Protection for Energies up to 10 MeV (Structural Shielding Design and Evaluation), Rep. 34, Washington, D. C. (1970). 195

198 US NATIONAL BUREAU OF STANDARDS, Safe Design and Use o f Industrial Beta-Ray Sources, Handbk 66, Washington, D.C. (1958). INTERNATIONAL LABOUR OFFICE, Manual of Industrial Radiation Protection, Part II - Model Code of Safety Regulations (Ionising Radiations), ILO, Geneva (1959). JAEGER, R. G. (Ed.), Engineering Compendium on Radiation Shielding 1, 2, 3, Springer Verlag In c., New York (1968). McMASTER, R. C. (E d.), Nondestructive Testing Handbook 1, The Ronald Press C o., New York (1963). Chapter 12 INTERNATIONAL ATOMIC ENERGY AGENCY, Techniques for Controlling Air Pollution for the Operation of Nuclear Facilities, Safety Series No. 17, IAEA, Vienna (1966). INTERNATIONAL ATOMIC ENERGY AGENCY, Respirators and Protective Clothing, Safety Series No. 22, IAEA, Vienna (1967). INTERNATIONAL ATOMIC ENERGY AGENCY, Manual on Safety Aspects of the Design and Equipment of Hot Laboratories, Safely Series No. 30, IAEA, Vienna (1969). WORLD HEALTH ORGANIZATION, Planning of Radiotherapy Facilities, Techn. Reps Series No. 328, WHO, Geneva (1966). SCOTT, W. G., Planning Guide for Radiologic Installations, Williams and Wilkins, Publishers, Baltimore (1966). Chapter 13 INTERNATIONAL ATOMIC ENERGY AGENCY, Manual on Safety Aspects of the Design and Equipment o f Hot Laboratories, Safety Series No. 30, IAEA, Vienna (1969). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Handling and Disposal o f Radioactive Materials in Hospitals and M edical Research Establishments, Publication 5, Pergamon Press, London (1965). US NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, A Manual of Radioactivity Procedures, Rep. 28, Natn Bur. Stand. Handbk 80, Washington, D.C. (1961). Chapter 15 INTERNATIONAL ATOMIC ENERGY AGENCY, Respirators and Protective Clothing, Safety Series No. 22, IAEA, Vienna (1967). Chapter 16 INTERNATIONAL ATOMIC ENERGY AGENCY, Manual on Environmental Monitoring in Normal Operation, Safety Series No. 16, IAEA, Vienna (1966). INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Monitoring in Emergency Situations, Safety Series No. 18, IAEA, Vienna (1966). 196

199 Chapter 17 INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Handling and Disposal of Radioactive Materials in Hospitals and M edical Research Establishments, Publication 5, Pergamon Press, London (1964). US NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, Control and Removal of Radioactive Contamination in Laboratories, Rep. 8, Natn Bur. Stand. Handbk 48, Washington, D.C. (1951). Chapter 18 INTERNATIONAL ATOMIC ENERGY AGENCY, Radioactive Waste Disposal into the Sea, Safety Series No. 5, IAEA, Vienna (1961). INTERNATIONAL ATOMIC ENERGY AGENCY, Radioactive Waste Disposal into the Ground, Safety Series No. 15, IAEA, Vienna (1965). INTERNATIONAL ATOMIC ENERGY AGENCY, Basic Factors for the Treatm ent and Disposal of Radioactive Waste, Safety Series No. 24, IAEA, Vienna (1967). INTERNATIONAL ATOMIC ENERGY AGENCY, Management of Radioactive Wastes a t Nuclear Power Plants, Safety Series No. 28, IAEA, Vienna (1968). INTERNATIONAL ATOMIC ENERGY AGENCY, Disposal of Radioactive Wastes into Rivers, Lakes and Estuaries, Safety Series No. 36, IAEA, Vienna (1971). US NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, Recommendations for Waste Disposal of P-32 and for M edical Users, Rep. 9, Natn Bur. Stand. Handbk 49, Washington, D.C. (1951). US NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS. Recommendations for the Disposal of C -14 Wastes, Rep. 12, Natn Bur. Stand. Handbk 53, Washington, D.C. (1953). US NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, Radioactive Waste Disposal in the Ocean, Rep. 16, Natn Bur. Stand. Handbk 58, Washington, D.C. (1954). Chapter 19 INTERNATIONAL ATOMIC ENERGY AGENCY, Regulations for the Safe Transport of Radioactive Materials, Safety Series No. 6, IAEA, Vienna (1961). INTERNATIONAL ATOMIC ENERGY AGENCY, Regulations for the Safe Transport of Radioactive M aterials, Notes on Certain Aspects of the Regulations, Safety Series No. 7, IAEA, Vienna (1961). INTERNATIONAL AIR TRANSPORT ASSOCIATION, IATA Restricted Articles Regulations, 14th Edn, Montreal (1971). Chapter 20 INTERNATIONAL ATOMIC ENERGY AGENCY, Environmental Monitoring in Emergency Situations, Safety Series No. 18, IAEA, Vienna (1966). 197

200 INTERNATIONAL ATOMIC ENERGY AGENCY, Planning for the Handling of Radiation Accidents, Safety Series No. 32, IAEA, Vienna (1969). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Report of Com m ittee V on the Handling and Disposal of Radioactive M aterials in Hospitals and M edical Research Establishments, Publication 5, Pergamon Press, London (1965). US NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, Exposure to Radiation in an Emergency, Rep. 29, Natn Bur. Stand, Handbk 91, Washington, D.C. (1962). Chapter 21 INTERNATIONAL ATOMIC ENERGY AGENCY, The Provision of Radiological Protection, Safety Series No. 13, IAEA, Vienna (1965). INTERNATIONAL ATOMIC ENERGY AGENCY, H ie Basic Requirements for Personnel Monitoring, Safety Series No. 14, IAEA, Vienna (1965). INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, General Principles of Monitoring for Radiation Protection of Workers, Publication 12, Pergamon Press, London (1969). US NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, Safe Handling of Radioactive Materials, Rep. 29, Natn Bur. Stand. Handbk 92, Washington, D.C. (1964). US NATIONAL COUNCIL ON RADIATION PROTECTION AND MEASUREMENTS, Protection in the Management of Patients Who Have Received Therapeutic Amounts o f Radionuclides, Rep. 37, Washington, D.C. (1970). 198

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