GAMMA RAIS FROM THE S i2 9 (p, lf)p 30 REACTION DISSERTATION
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1 GAMMA RAIS FROM THE S i2 9 (p, lf)p 30 REACTION DISSERTATION P re s e n te d in P a r t i a l F u lf illm e n t o f th e R equirem ents f o r th e D egree D octor o f P h ilo so p h y i n th e G raduate S chool o f The Ohio S ta te U n iv e rs ity By SALVATORE MILANI, B. S. The Ohio S ta te U n iv e rs ity 1956 Approved b y : j y A d v iser 0 J y D epartm ent o f t ' P h vy sic s and Astronomv Astronotny
2 ACKNOWLEDGMENTS I w is h to e x p r e s s ny s in c e re a p p re c ia tio n t o D r..j ohn N. C ooper f o r s u g g e s tin g th e problem and f o r h is c o n s ta n t i n t e r e s t, and encouragem ent th ro u g h o u t my a s s o c i a t i o n w ith th e V an de G raaff g e n e ra to r p r o j e c t. O r a titu d e i s e x p r e s s e d fo r th e a s s is ta n c e and coopera- tio n g iv e n by o t h e r s connected w ith th e p r o je c t - D r 3, Jam es C* H a r r i s a n d Donald W. Q reen, G raduate A s s i s t a n t s D onald L. S h ir e r and James H. B lackburn, a n d te c h n ic ia n s O tis E. Campbell and Ig n as S ald u k as. X wish t o th a n k my w i f e and ny p a re n ts f o r t h e i r p a tie n c e and u n d e rs ta n d in g d u rin g my p e rio d of stu d y. X also w is h t o th a n k t h e Development Fund and th e R esearch F o u n d a tio n o f t h e O hio S ta te U n iv e rs ity f o r t h e i r a id. i i
3 TABLE OF CONTENTS PAGE I n t r o d u c t io n... 1 PART I. GAMMA RAYS FROM THS S i2 9 ( p t if JP^0 REACTION.. 2 C h a p te r I. B ackground 3 A. S ta te m e n t o f th e p r o b l e m... 3 B. I s o t o p i c s p i n... * C. R eview o f l i t e r a t u r e C h a p te r I I. M a te r ia ls and A p p a r a t u s A. P r e p a r a t io n o f t a r g e t s B. A p p a ra tu s f a r th e d e te r m in a tio n o f re s o n a n c e s 19 C. Gamma r a y s p e c t r o m e t e r D. H a l f - l i f e a p p a r a tu s C h a p te r I I I. E x p e rim e n ta l P ro c e d u re and R e s u lts. 32 A. R eso n ances T h ic k t a r g e t y i e l d T h in t a r g e t y i e l d B. M easurem ent o f gamma r a y e n e r g ie s.. 1. The 4li+ kev re s o n a n c e The 698 kev re s o n a n c e The 731 kev re s o n a n c e R. The 918 kev re s o n a n o e The 957 kev re s o n a n c e i i i
4 iv PAGE C. H a lf - lif e m easu re m e n ts... 7^ C hapter IV I n te r p r e ta tio n of R e su lts and C onclusions 79 PART I I. STQPPIKG CROSS SECTIONS OF CASES C hapter I. Background 93 C hapter I I. M a te ria ls and A pparatus A. A pparatus and p r o c e d u r e B. P re p a ra tio n of f o i l s C hapter I I I. R e su lts and C o n c l u s i o n s APPENDIX BIBLIOGRAPHY
5 LIST OP TABLES TABLE PAGE I. R eported V alues o f th e H a l f - l i f e o f and Maximum Energy of th e P o s itro n I I. T hick T arg e t Y ie ld from N a tu ra l and E n rich ed S ilic o n...35 I I I. O bserved W idths o f th e Resonances...49 IV. Gamma Ray E n erg ies of th e 414 kev Resonance... 6 l V. Gamma Ray E n e rg ie s o f th e 698 kev Resonance V I. Gamma Ray E n erg ies o f th e 731 kev Resonance V II. Gamma Ray E n erg ies o f th e 918 kev R esonance V H I. Gamma Ray E n e rg ie s o f th e 957 kev R esonance IX. H a l f - l i f e D ata X. R esonances and t h e i r C orresponding Energy L evels. 81 X I. D ata f o r N i t r o g e n X II. D ata fo r Xenon... I l l X I II. D ata f o r A ir 114 XIV. D ata f o r Carbon D ioxide XV. D ata fo r W ater V a p o r XVI. D ata f o r Benzene v
6 LIST OF FIGURES FIGURE PAGE 1. Ia o to p ic S pin T r ip le t Iso to p ic Spin T r ip le t w ith o u t Coulonto C o rre c tio n 9 3* A p p lic a tio n o f S e le c tio n R u l e s P revious Work T hick T arget Assembly T hin T arg et E vaporation A s s e m b l y T arg e t Assembly Gamma-Ray S p e c t r o m e t e r S p e c tra o f Cs^-^ and C o ^ P h o to m u ltip lie r and Cathode F ollow er C ir c u it C a lib ra tio n Curve 2? 12. Photograph of Cs^37 and C o ^ D ensitom eter T race of Csl37 and C o ^ T hick T arget Y ie ld s T hin T arget Y ield o f S i 2 9 ( p, y ) p T hin T arget Y ield o f S i2 9 (p, r ) P ^ Gamma Rays from T arget w ith Beam On Ganna Rays from T arg et w ith Beam O f f Gamma Rays from th e Mev R e s o n a n c e Decay o f th e 414 and 698 kev R e s o n a n c e Decay o f th e 732* 918 and 960 kev R esonances T hin T arget Y ield s fo r the 698 kev R esonances v i
7 v i l 2 3. A p p a ra tu s f o r th e D e te rm in a tio n o f th e T h ic k n e s s o f th e L if T a r g e t T h ic k n e s s o f L if T a r g e t T h in T a rg e t Y ie ld f o r th e R e g io n from 800 t o 1000 kev Gamma Rays from t h e 4 l 4 kev R esonance P o s s ib le Decay Scheme o f th e 414 kev R esonance l 2 0. Gamma Rays fro m th e 693 kev R esonance P o s s ib le Decay Scheme o f th e 698 kev R esonance Gamma Rays f r a n th e 731 k v R esonance P o s s ib le Decay Scheme o f th e 731 k v R esonance Gamma R ays fro m th e 918 kev R esonance (show ing th e lo w e r e n e rg y gamma r a y s ) * Gamma Rays from th e 918 kev R esonance ( T o ta l S p e c tru m ) P o s s ib le D ecay Scheme o f th e 918 kev R esonance Gamma Rays from th e 957 kev R esonance P o s s ib le D ecay Scheme o f th e 957 kev R esonance H a l f - l i f e o f P E nergy L e v e ls o f P ^ and S i ^ * A p p a ra tu s f o r M e asu rin g Gas S to p p in g C ross S e c tio n E f f e c t o f Beam on P r e s s u re A tom ic S to p p in g Pow er o f N itro g e n A tom ic S to p p in g Pow er o f X e n o n A tom ic S to p p in g Pow er o f A ir M o le c u la r S to p p in g Power o f C arbon D io x id e
8 v i i i 4 5. M olecular S to p p in g Power o f W ater Vapor M o lecu lar S topping Power o f B e n z e n e I n te g r a to r C ir c u it I n te g r a to r Response
9 INTRODUCTION The work p re s e n te d h e re c o n s is ts o f two u n re la te d p ro b lem s. The f i r s t i s a stu d y o f th e S i^ 9 (p t Jf)p30 r e a c tio n c o n s is tin g o f a d e te rm in a tio n o f th e re s o n a n c e s, m easurement o f th e gamma-ray e n e rg ie s from th e e x c ite d p30 n u cleu s and a measurement o f th e h a l f - l i f e o f P -0. The second problem i s concerned w ith th e s to p p in g c ro s s s e c tio n s o f g a s e s. The sto p p in g c ro s s s e c tio n s o f n itro g e n, xenon, a i r, carbon d io x id e, w a te r vapor and benzene w ere d e term in ed
10 PART I GAMMA RAYS FROM THE S I29(p, 1T)P30 REACTION - 2 -
11 CHAPTER I BACKGROUND A. S tatem en t o f th e problem. Much re c e n t i n t e r e s t in Is o to p ic S pin has been fo cu sed on th e N * Z, A * **n 2 n u c le i. In o rd e r to t e s t th e h y p o th e sis o f charge independence o f n u c le a r f o rc e s upon which th e th e o ry o f I s o to p ic S p in i s b a se d t i t i s n e c e ssa ry to cons id e r th e s e n u c le i s in c e th e y a lo n e have more th a n one I s o to p io Spin quantum number among t h e i r low ly in g sta te s^ -. Some assignm ents have been made to th e low er s t a t e s o f Li^» N ^, Na^2 and C l^ 4, b u t th e is o to p ic s p in p r o p e r tie s o f F^, A l2 6, p30 and K38 a re l a r g e ly unknown^*3. The work re p o rte d h e re i s concerned w ith th e stu d y o f th e p30 n u c le u s. I t was produced on th e Ohio S ta te U n iv e rs ity Van de Q raaff a c c e le r a to r by means o f th e S i ^ ( p ( y* )p30 r e a c tio n. The ex p erim e n tal work i s d iv id e d in to th r e e p a r t s : 1. The d e te rm in a tio n o f reso n an ces in th e above r e a c tio n which g iv es in fo rm a tio n on th e h ig h e r energy le v e ls in p3<>. 2. The measurement o f th e gamma ra y e n e rg ie s from each o f th e reso n an ces which g iv e s in fo rm a tio n on th e low ly in g energy le v e ls. 3. p3 i s u n s ta b le, as a re a l l o f th e odd-odd n u c le i above la**, decaying by p o s itr o n e m issio n. Measurement o f th e h a l f - l i f e o f th e p o s itr o n decay le a d s to in fo rm a tio n on th e ty p e o f b e ta decay t h a t e x i s t s
12 B. Is o to o lc S p in * The h y p o th e s is o f charge sym netry and charg e Independence o f n u c le a r f o rc e s a re im p o rta n t i n n u c le a r p h y s ic s. Charge symmetry assum es t h a t th e n u c le a r f o r c e s betw een two p ro to n s a re e q u al t o th e n u c le a r f o r c e s betw een two n e u tro n s ; t h a t i s, pp * nn. T h at c h arg e symmetry i s a v a lid concept i s shown m ost r e a d ily by th e m irro r n u c le i w ith t h e i r co rresp o n d in g energy l e v e l s. The h y p o th e s is o f c h arg e independence i s much s tr o n g e r th a n th e h y p o th e sis o f c h arg e symmetry f o r i t assum es t h a t th e n u c le a r fo rc e s betw een any p a i r o f n u c le o n s a re e q u a l; t h a t i s, pp * nn * np. T here i s v e ry l i t t l e e x p e rim e n ta l ev id en ce to su p p o rt t h i s h y p o th e s is, e x cept f o r th e r e s u l t s o f some s c a t t e r i n g e x p e rim e n ts. H eisenberg in 1932** in tro d u c e d th e concept o f is o to p ic s p in to make u se o f th e h y p o th e s is o f ch arg e independence and s im p lify th e th e o ry o f n u c le a r s t r u c t u r e. The n e u tro n and th e p ro to n a re n o t t r e a t e d a s d i f f e r e n t p a r t i c l e s b u t as d i f f e r e n t s t a t e s o f th e same p a r t i c l e - th e n u c le o n. A new quantum number i s in tro d u c e d, mr, w hich has a v a lu e l / 2 i f th e n u c le o n i s a p ro to n, and - 1 /2 i f th e n u cleo n i s a neutron* The c h arg e on th e n u c le o n i s th e n q «mt + 1 / 2 in u n i ts o f e. Of c o u rse th e n e u tro n and p ro to n a re n o t i d e n t i c a l i n e i t h e r t h e i r m asses o r c h a rg e. The c h arg e on th e p ro to n makes th e t o t a l pp fo rc e d i f f e r e n t from th e nn o r pn f o r c e s. However, th e s e f a c to r s a re n e g le c te d a t t h i s p o in t and a re t r e a t e d l a t e r as c o rre c t i o n s. An is o to p ic s p in o p e ra to r t i s a ls o in tro d u c e d w hich has comp o n ents t x, t y, t z in is o to p ic s p in s p a c e. T hese o p e ra to rs have th e same form as th e o rd in a ry P a u li s p in m a tric e s and a re a s fo llo w s :
13 In f u r t h e r analo g y to o rd in a ry s p in, th e wave fu n c tio n o f th e nucleon i s w r itte n as a two component fu n c tio n depending on w hether th e nuc leo n i s a p ro to n o r n e u tro n. The o p e ra to r t % o p e ra tin g on e i t h e r o f th e wave fu n c tio n s g iv e s 2mT and i s r e l a te d t o th e charge s in c e * q - l / 2. The is o to p ic s p in o p e ra to rs have a l l o f th e p r o p e r tie s o f o rd in a ry s p in o p e ra to rs which e x p la in s th e o r ig in o f th e term " is o to p ic s p in 11. I f th e system c o n ta in s A nucleons* th e o p e ra to rs a re g iv en as A A A t X t y t i The e ig e n v alu e o f t ^ i s Tz * ^ 'rvv w ith o u t th e l / 2. S in ce q = m^. + l / 2, tn e t o t a l charge on a n u cleus o f A p a r t i c l e s i s Z and we have The t o t a l ls o to p ic s p in o p e ra to r f o r th e system i s t where t^ t ^ + + t ^. Because o f th e analogy w ith o rd in a ry s p in, th e x y z e ig e n v alu es o f t ^ a re giv en as T (T +1) where T i s c a lle d th e t o t a l is o to p ic sp in quantum number. S in ce Tz i s an e ig e n v alu e o f t z and T i s th e t o t a l quantum number* th e r e e x is ts * f o r a ry T, s t a t e s w ith
14 -6 - The wave fu n c tio n o f th e sy a te n b e fo re th e in tr o d u c tio n of is o to p ic s p in had th e form ^ (r, s,» rt r* s > r s VM... raak ) w here r sf f o r i» l to N re p re s e n ts th e space and s p in c o o rd in a te s o f th e n e u tro n s, and r- S; f o r i=n+l to A re p re s e n ts th e sp ace and sp in c o o rd in a te s o f th e p ro to n s. The P a u li p r in c ip le r e q u ir e s t h a t th e wave fu n c tio n change s ig n i f any two n e u tro n s o r any two p ro to n s a re in te rc h a n g e d. No req u irem ent i s made i f a n eutro n and a p ro to n a re in te rc h a n g e d. A new c o o rd in a te I i s in tro d u c e d to r e p re s e n t th e i s o to p ic s p in c o o rd in a te s. The wave fu n c tio n becomes y (rt s, I,, rt s^ I %... s^ I ). The nucleons a re tr e a te d as b ein g i d e n t i c a l and th e wave fu n c tio n must change i t s s ig n i f a l l o f th e c o o rd in a te s ( r t sv- I c ) of any two p a r t i c l e s a re in te rc h a n g e d. In th e case o f two n u c le o n s, th e wave fu n c tio n can be se p a ra te d in th e fo llo w in g form : p.p n - n V ' n - p * i a, S. I., A, S, U) I *. V " W l... iila.v.a.iol'*,1''v ' 1- V 0 V* J "I1 The is o to p ic s p in wave fu n c tio n f o r th e f i r s t th r e e c a se s i s sym m etric a l w ith r e s p e c t to th e in te rc h a n g e o f th e two p a r t i c l e s and sin c e th e t o t a l wave fu n c tio n must be antisy m m etric, th e sp a c e -sp in component must be a n tisy m m etric. In th e l a s t case th e is o to p ic sp in wave fu n c tio n i s antisy m m etric and th e r e f o re th e sp a c e -sp in component m ust be sym m etric. There th e n e x i s t th r e e sym m etric ls o to p ic s p in wave fu n c tio n s denoted by ; th e y a re :
15 -7 - x i, i * f o r p - p X i'o - ^ [V1 f" L^ ^ ^pix)j fop n - p *,r \ * 'fn10 f o r n - n Each o f th e s e th r e e fu n c tio n s I s a s s o c ia te d w ith th e same sp a c e -sp in s t a t e. The p o s tu la te o f charge Independence o f n u c le a r fo rc e s s t a te s t h a t th e fo rc e s do n o t depend upon th e charge I f th e sp a c e -sp in s t a t e s o f th e system a re th e same. T h e re fo re, th e energy le v e ls re p re s e n te d by th e s e fu n c tio n s a l l have th e same energy s in c e th e y a l l have th e same sp a c e -sp in component of th e wave f u n c tio n. The th r e e X( l form an ls o to p lc sp in t r i p l e t a s s o c ia te d w ith an an tisy m m etric sp a c e -sp in wave f u n c tio n. Each has T=1 w ith T^ * 1 f o r th e f i r s t,» 0 f o r th e second, and T * -1 f o r th e t h i r d. The l a s t is o to p ic s p in wave fu n c tio n i s antisy m m etric and Is a s s o c ia te d w ith a sym m etric sp a c e -sp in wave f u n c tio n. I t i s r e p r e se n te d by X* # * ^. I t has T = 0, Te = 0 and i s an is o to p ic sp in s in g le t s t a t e. S ince th e sp a c e -sp in p a r t i s d i f f e r e n t from th e c a se s w ith T = 1, t h i s s t a t e w i l l behave q u ite d i f f e r e n t l y. I t w i l l occur only in th e Ta * 0 is o b a r and w i l l have no c o u n te rp a rt in th e T# a 1 o r T# = -1 is o b a r s. The n u c le i w ith A * hn + 2 o f f e r a means o f t e s t i n g th e charge independence h y p o th e s is. They c o n s is t o f a c e n te r core o f hn nucleons and have two nucleons in o u te r s h e l l s. In th e case of th e th re e i s o b a rs S i30, P3 f and S3, S i30 has * -1, P3 has T# - 0, and S3 has T * 1. The T = 0 is o b a r, P3, can have s t a t e s re p re s e n te d by X, _ o r I I <9 X99 and th e r e f o re can have s t a te s w ith e i t h e r T * 0 o r 1. The is o b a rs S i30 and 330 can have s t a t e s re p re se n te d by X and X r e s p e c tiv e ly, I. *1 4'
16 -8 - and th e r e f o re can have only T * 1 s t a t e s. T his i s i l l u s t r a t e d in th e F ig u re T- i T * o T* o T I VS T * I G.S T»o S S J* T». I P >0 T,. o 5 i Tt.-I F ig u re 1. Is o to p ic S pin T r ip l e t In th e is o b a rs S -^ and S i ^ th e two e x tr a nucleons a re i d e n t i c a l in each c a s e. In th e ground s t a t e s, th e s e two nucleons can only e x is t w ith t h e i r m echanical s p in s a n t i p a r a l l e l ; t h a t i s. in th e s i n g l e t s t a t e. These s t a t e s a re th e n is o to p ic s p in t r i p l e t s and m echanical s p in s i n g l e t s. I f F ig u re 1 i s c o r r e c t, f i r s t e x c ite d s t a t e o f i s I d e n tic a l to th e s e two s t a t e s. However, in th e ground s t a t e o f P-^ th e two nucleons a re d i f f e r e n t and can e x is t in th e m echanical sp in p a r a l l e l o r t r i p l e t s t a t e. T h e re fo re, th e d if f e r e n c e betw een th e ground s t a t e o f P30 and i t s f i r s t e x c ite d s t a t e l i e s in th e f a c t t h a t one of th e nucleons has changed i t s s p in. I f, in a d d itio n to th e charge independence assum ption, an assum ption o f sp in independent fo rc e s had a ls o been made th e ground s t a t e and f i r s t e x c ite d s t a t e o f P-^ would o c c u r a t th e same en erg y. T h ere fo re such charge and s p in in d ep en d en t n u c le a r fo rc e s would make th e is o to p ic s p in t r i p l e t and
17 -9 - is o to p ic s p in s i n g l e t merge i n to a " s u p e rm u ltip le t." In r e a l i t y, th e s t a t e s do n o t l i e on th e same le v e l as shown b u t a re d is p la c e d due to th e charge on th e p ro to n s and th e d iffe re n c e in m asses o f th e n eu tro n and p ro to n. The ty p e o f scheme t h a t i s obta in e d i s shown in F ig u re 2. T. T«o T»o T»» J e F ig u re 2. Is o to p ic Spin T r ip le t w ith o u t Coulomb C o rre c tio n The c o rre c tio n f o r t h i s d isp lacem en t i s ^ t AEc * ^ L (A -2 ) 2 - (M^c2 - Mf c 2 ) * A ( a. 2 ) w here ae i s th e downward d isp lacem en t o f th e ground s t a t e energy o f c th e T# * -1 n u cleu s r e l a t i v e to th e corresp o n d in g le v e l in th e = 0 n u c le u s. The is o to p ic sp in s e le c tio n r u le s f o r t r a n s i t i o n s betw een s t a t e s a re l i s t e d as fo llo w s:
18 n, p, H^, He^, have T * l / 2 A T * ± 1/2 a*, d have T = 0 A T * 0 0 AT 3 0, ±1 t A T * 0, +1 No T a 0-*0 t r a n s itio n s f o r E( r a d ia tio n ^ The t r a n s itio n s o f co u rse a re a ls o s u b je c t to th e s e le c tio n r u le s f o r th e o th e r quantum num bers. A pplying th e s e s e le c tio n r u le s f o r is o to p ic s p in to th e S i ^ (p» r e a c tio n g iv e s th e scheme in F ig u re 3» T«o, i i t j o T" I F ig u re 3* A p p lic a tio n o f S e le c tio n R ules *7 S i i s known to have a T * l / 2 ground s t a t e and S i^ can have only a T=1 s t a t e. The a d d itio n of a p ro to n to S i ^ can give s t a t e s w ith T*0 o r 1 as expected* The gamma t r a n s itio n s o ccur w ith AT = 0* ±1 and th e re fo re can o ccu r betw een any le v e l in P-& s u b je c t to th e o rd in a ry s e le c tio n r u l e s. However, i f T = 0 e x is ts f o r th e i n i t i a l s t a t e, no e l e c t r i c d ip o le r a d ia tio n can o ccu r to a n o th e r T = 0 s t a t e. The t r a n s i t i o n can occur by m agnetic r a d ia tio n o r e l e c t r i c r a d ia tio n o f
19 o rd e r h ig h e r th a n one. The p o s itro n decay can o ccu r s in c e th e ground s t a t e o f P^ w i l l be e i t h e r T 8 0 o r 1, The stu d y o f P^ th e n o f f e r s a means o f te s tin g th e charge and s p in dependences o f n u c le a r f o r c e s. I f energy le v e ls a re found in P-^ which correspond to th e known le v e ls in Si^, th e charge in dependence h y p o th e sis i s s tre n g th e n e d. In a d d itio n i f e l e c t r i c d ip o le t r a n s i t i o n s a re a b se n t betw een two T 3 0 s t a t e s w ith allow ed a n g u la r momentum and p a r ity values* th e charge independence h y p o th e sis can be p ro v en. The absence of a s u p e rm u ltip le t in P ^ w i l l su g g e st t h a t th e n u c le a r fo rc e s a re s p in dependent. C. Review o f T A te ra tu re. Phosphorus-30 h as th e d i s t in c t io n o f b ein g th e f i r s t a r t i f i c i a l l y produced r a d io a c tiv e is o to p e. I. g C urie and F. J o l i o t in 193** re p o rte d t h a t a t a r g e t o f aluminum t h a t had been bombarded w ith alp h a p a r t i c l e s rem ained r a d io a c tiv e when th e a lp h a so u rc e was rem oved. They m easured th e h a l f - l i f e and found i t to be 3 *25 m in u te s. They i d e n t i f i e d th e e m itte d p a r t i c l e as a p o s itro n w ith a th r e e Mev maximum en erg y. By chem ical means* th e i d e n t i f i e d th e r a d io a c tiv e elem ent as phosphorus and a tt r ib u t e d i t to th e r e a c t i o n : A l27 + He^ P3 + n ; P30- * S i^ + /3* + >\ T his d isc o v e ry o f induced r a d io a c tiv ity i n i t i a t e d a flo o d o f experim ents e s p e c ia lly on th e above r e a o tio n. The energy o f th e p o s itro n and th e h a l f - l i f e have been re p o rte d many tim e s. T able I l i s t s th e s e re p o rte d v a lu e s along w ith th e d a te o f p u b lic a tio n. As can be seen* th e r e e x is ts a v e ry la rg e sp re a d in th e re p o rte d v a lu e s.
20 - 12- TABLE I HALF-LIFE OF P3 and MAXIMUM ENERGY OF POSITRON H a lf-l ife Energy Year R eferen ce m in. 3 Mev 193* **±.5 193* * o ± ".i ± ** *4 27 The h a l f - l i f e v a lu e s re p o rte d s in c e 1940 seem to be c o n s is te n t w ith th e v a lu e o f m inutes re p o rte d by R idenour and H enderson in 1937* A v a lu e o f 3-27 Mev h a s been a c c e p te d as th e b e s t v a lu e f o r th e maximum p o s itr o n e n e rg y. An a c c u ra te knowledge o f th e h a l f - l i f e and en erg y i s n e c e ssa ry f o r th e c a lc u la tio n o f th e L o g (ft) v a lu e w hich i s e s p e c ia lly im p o rta n t i n th e c a se o f P^O \ S e v e ra l in v e s tig a to r s have re p o rte d e x p erim ents perform ed to d e te rm in e th e energy le v e ls o f P - ^. F ig u re 4 i l l u s t r a t e s th e s e 23 re p o rte d e x p e rim e n ts. R.A. Peck in 1948 m easured th e n e u tro n
21 414 ^ >t«s i#v * ) p10 m m m t.o l.4«.0 0 FIGURE 4 * PREVIOUS WORK 13
22 e n e rg ie s by n u c le a r em ulsion te c h n iq u e s f o r th e r e a c tio n A l2? (»c.n)p3<> and re p o rte d a le v e l a t Mev. He a ls o stu d ie d th e S i29(d,n )P ^ r e a c tio n u sin g n a tu r a l s i l i c o n and re p o rte d a le v e l a t 1.2? + *48 Mev. The S i^ 9 (d,n) r e a c tio n was a ls o s tu d ie d by Kande- 29 v i l l e e t a l. who re p o rte d le v e ls a t , and Mev. The S i 29(p. r e a c tio n was f i r s t s tu d ie d by T an g e n ^ who re p o rte d reso n an ce le v e ls a t p ro to n e n e rg ie s o f 326 and 414 kev. The gamma e n e rg ie s from th e s e two reso n an ces w ere m easured by E ndt, KLuyver and Van d e r Leun^0. They re p o rte d t h a t th e 326 kev reso n an ce le v e l decays d i r e c t l y to th e ground s t a t e e m ittin g a 5*86 Mev gamma; th e 414 kev reso n an ce le v e l decays by th e em issio n o f two gammas of e n e rg ie s.688 and 5*27 Mev corresp o n d in g to a decay through th e f i r s t e x c ite d s t a t e o f P^ a t.688 Mev.
23 CHAPTER XI MATERIALS AND APPARATUS Phosphorus-30 was s tu d ie d by bom barding S ilic o n -2 9 w ith p ro to n s in th e energy range from 250 kev to 1.3 Mev. These p ro to n s were a c c e le ra te d by th e Ohio S ta te U n iv e rs ity Van de G raaff a c c e le r a t o r. E xcept f o r th e a d d itio n o f a new i n te g r a t o r, th e a c c e le r a to r i s as d e sc rib e d by Qrove^1, and T a y lo r^ 1. I t was found n e c e ssa ry to d e sig n a new in te g r a to r in J u ly 195^ when th e p ro to n beam in te n s i t y dropped to such a low v a lu e t h a t th e e x is tin g in te g r a to r ceased to fu n c tio n. The new in te g r a to r i s d e s c rib e d in th e appendix. A l though th e i n te n s i t y of th e beam re tu rn e d to norm al, th e o ld i n t e g r a to r was d isc a rd e d and a l l o f th e work re p o rte d h e re has been p e r formed w ith th e new i n te g r a t o r. A. P re p a ra tio n o f t a r g e t s. The s i l i c o n used in th e s e e x p e r im ents was o b ta in e d from th e Oak R idge N a tio n a l L ab o rato ry in th e form o f SiOg e n ric h e d in S i 2^ by e le c tro m a g n e tic s e p a r a tio n. S p e c tro - g ra p h ic and is o to p ic a n a ly se s w ere perform ed a t th e Oak R idge N a tio n a l L ab o rato ry and a re as fo llo w s: S p e c tro g ra p h ic A n aly sis S i mg Na.3* Mg.0 l Cu.02* Sn.05/
24 Is o to p ic A n aly sis s i Atomic $> P re c is io n ± ± e V-h CD +.01 B efore any a tte m p ts were made to p re p a re t a r g e ts o f S i- 2 9» experim ents were conducted w ith n a tu r a l s i l i c o n to d eterm in e th e b e s t p ro ced u re f o r th e p re p a ra tio n of th e th ic k and th in ta rg e ts *, o C T his was n e c e ssa ry s in c e th e m e ltin g p o in t o f S102 i s 1*1-70. n ear th e upper l i m i t o f th e e v a p o ra tin g system. In a d d itio n to th is * c a re was re q u ire d in h a n d lin g th e S i-2 9 to in s u re as l i t t l e w aste as p oss i b l e. The m a te r ia l was o n ly on lo a n to t h i s la b o ra to ry because of th e g r e a t expense o f th e m agnetic s e p a ra tio n p ro c e s s, and as much o f i t as p o s s ib le had to be re tu rn e d to th e Oak Ridge L ab o rato ry. N a tu ra l s ilic o n - d io x id e powder was p rep a red by g rin d in g a q u a rtz ro d u n t i l th e te x tu r e o f th e powder was approxim ately th e same as th e S i2^02. A fte r some e x p e rim e n ta tio n, th e fo llo w in g p ro cedures w ere adopted : F or th e p r e p a ra tio n o f th e th ic k t a r g e t s, only two req u irem ents had to be m et. The t a r g e t had to be th ic k enough to s to p com pletely th e m ost e n e rg e tic proton* and th e t a r g e t assem bly had to be designed in such a way t h a t th e m a te ria l c o u ld be reclaim ed a f t e r u s e. These re q u ire -m e n ts w ere met by com pressing th e SiO^ powder in to a h o ld e r w ith a p re s s u re of 5*000 pounds p e r sq u are in c h. A h y d ra u lic p re s s was used to o b ta in t h i s p re s s u re. The t a r g e t h o ld e r and p lu n g er a re
25 shown in F ig u re 5* I t was found t h a t th e t a r g e ts p rep a red in t h i s manner would n o t c ra ck under bombardment i f th e f i n a l th ic k n e s s o f th e m a te ria l was a t l e a s t l / l 6 ". T his th ic k n e s s s a t i s f i e s th e f i r s t req u irem ent v ery e a s i l y. The t h in t a r g e ts were p rep a red by vacuum e v a p o ra tio n of th e S i(>2 powder upon l / 2 in ch tan ta lu m d is c s. The e v ap o ratin g u n it was th e same as re p o rte d by G reen^^, and Seiler^** except f o r th e ty p e o f b a s k e t u se d. No a ttem p t was made to u se th e c o ile d w ire b a s k e ts sin c e th e m a te r ia l would f a l l through th e lo o p s. The b e s t r e s u l t s w ere o b ta in e d by u sin g a b o a t made from a l / h in ch x 2 in c h s t r i p of.005 in ch ta n ta lu m. The edges of th e s t r i p w ere b en t to g iv e a l / l 6 in c h w a ll on b o th s id e s to p re v e n t th e m a te ria l from s p illin g * Upon h e a tin g * how ever, th e ta n ta lu m expanded and tw is te d throw ing th e m a te r ia l o u t o f th e b o a t. To p re v e n t th is * th e s t r i p was c o rru g a te d by sq u eezin g i t betw een th e k n u rle d jaws of a v is e. The e f f e c t of expansion was d e crease d f u r th e r by lo o s e ly clamping th e b o a t in th e e v a p o ra tin g u n i t which allow ed th e b o at to expand w ith o u t tw is tin g. The m a te r ia l was evap o rated onto th e t a r g e t d is k s as shown in F ig u re 6. The s i x d is k s w ere arranged in t h i s manner so t h a t ta r g e ts of v a ry in g th ic k n e s s e s could be o b ta in e d from one e v a p o ra tio n. th ic k n e s s of th e evap o rated la y e r was judged by th e c o lo r; The a t a r g e t t h a t appeared g reen was found to weigh 120/*g which i s e q u iv a le n t to ab o u t 15 k ev. I t was found t h a t th e t a r g e ts had to be s to r e d i n a CaClg d e s ic c a to r to p re v e n t d e te r io r a tio n due to th e m o istu re i n th e air.
26 FIGURE 5 -THICK TARGET ASSEMBLY n SiOt aft 0 >- FIGURE 6 THIN TARGET EVAPORATION ASSEMBLY
27 B. A pparatus f o r th e d e te rm in a tio n of re so n a n c e s: Resonances In any (p» Jf ) r e a c tio n may be determ ined by p lo ttin g th e number of gamma ra y s e m itte d p e r In c id e n t p ro to n as a fu n c tio n o f th e p ro to n en erg y. The gamma-ray co u n tin g r a t e a t a d e f i n i t e p ro to n energy w i l l depend upon th e c ro s s s e c tio n o f th e r e a c tio n, th e p ro to n beam in te n s i t y, th e number o f atoms of m a te ria l p re s e n t p e r u n it th ic k n e s s o f th e t a r g e t, and th e e ffic ie n c y of th e co u n tin g a p p a ra tu s. S in ce th e c ro s s s e c tio n of many r e a c t i o n s, and e s p e c ia lly of th e S i29(p, f ) p 3 r e a c tio n, i s q u ite low, one must t r y to In c re a s e th e rem aining f a c to r s to o b ta in a co u n tin g r a t e t h a t i s as la r g e as p o s s ib le compared to th e background r a t e. The p ro to n beam i n t e n s i t y w i l l be c h a r a c t e r i s t i c o f th e p a r t i c u l a r a c c e le r a to r and l i t t l e can be done to in c re a s e i t w ith o u t m ajor changes. The number o f atoms p re s e n t p e r u n i t t a r g e t th lo k n e ss can be in c re a s e d by u sin g s e p a ra te d is o to p e s. For in s ta n c e, in c re a s in g th e is o to p ic p e rc e n t o f S i29 from 4.6? to in c re a s e s th e counting r a t e by about 17. The l a s t f a c to r has been g r e a tly i n c re a se d by th e in tro d u c tio n o f th e s c i n t i l l a t i o n c o u n te r to re p la c e th e Q elger c o u n te r f o r gamma r a y s. A s ta n d a rd Nal s c i n t i l l a t i o n c o u n te r h as been used fo r s e v e ra l y e a rs in t h i s la b o ra to ry. P re lim in a ry experim ents to d eterm in e th e resonances o f th e S i29(p, r e a c tio n using th e sta n d a rd f l a t - f a c e d Nal c r y s t a l were v ery d isa p p o in tin g because o f th e low y i e l d o f th e r e a c tio n. I t was f e l t t h a t th e co u n tin g e ff ic ie n c y o f th e s c i n t i l l a t i o n c o u n te r could be in c re a s e d by u sin g a w e ll-ty p e Nal c r y s t a l in p la c e o f th e f l a t - f a c e c r y s t a l. T h is would se rv e to in c re a s e th e geom etry from 2 TT to alm ost
28 b TT. A w e ll-ty p e c r y s t a l was p urchased from th e Harsh aw Chemical Company and assem bled In th e u n it shown in F ig u re?. The c r y s t a l was o p tic a lly coupled to a p h o to m u ltip lie r tu b e u sin g im m ersion o il* The t a r g e ts which were 1 /2 in c h in d iam ete r f i t t e d in to a b ra s s h o ld e r w ith an o u ts id e d ia m e te r of in c h e s. The h o ld e r s l i d i n to th e w e ll c r y s t a l to p o s itio n th e t a r g e t i n th e c e n te r o f th e c r y s t a l. The t a r g e t h o ld e r had to be in s u la te d s in c e i t se rv ed as th e F araday Cage. T his was done by w rapping e l e c t r i c a l ta p e around th e h o ld e r b e fo re in s e r tio n in to th e w e ll c r y s t a l. G lass and L u c ite se rv ed to i n s u la te th e h o ld e r from th e grounded d r i f t tu b e o f th e a c c e le r a to r. The p u ls e s from th e p h o to m u ltip lie r fe d th rough a cathode fo llo w e r in to an Atomic 20h-B l i n e a r a m p lifie r and d is c rim in a to r. The p u lse s from th e d is c rim in a to r w ere counted w ith a N a tio n a l C ounter L ab o rato ry s c a l e r Model 10A. The d is c rim in a to r was a d ju s te d so t h a t o n ly p u ls e s o f energy g r e a te r th a n Mev w ere co u n ted. T his was d e s ir a b le s in c e th e p ro d u c t n u c le u s, P -30, i s a p o s itr o n e m itte r. The two.51 Mev gammas produced when th e p o s itr o n i s a n n ih ila te d can be absorbed g iv in g a p u lse o f Mev h e ig h t. I f t h i s p u ls e is counted, th e d elayed em issio n of th e p o s itro n s aau ses a "sm earing" o f th e resonance c u rv e s. With t h i s arran g em en t, l i t t l e d i f f i c u l t y was enc o u n tered in lo c a tin g th e reso n an ces of t h i s r e a c tio n. I t was a ls o used by S e l le r - ^ f o r th e d e te rm in a tio n o f th e reso n an ces in n a tu r a l s i l i c o n.
29 PM 5 Sit MAI CRYSTAL TARGET B BRASS B RUBBER BLASS LUCITE TO INTEGRATOR FIGURE T - TARGET ASSEMBLY
30 C. Gamma Ray S p e c tro m e te r. A s in g le c r y s t a l N a l(t l) s c i n t i l l a t i o n sp e c tro m e te r was used to m easure th e e n e rg ie s of th e gaam a-rays from th e e x c ite d Phosphorous-30 n u c le i, A diagram of th e in stru m e n t i s shown in F ig u re 8. A 1,5 in c h d iam ete r x 1 in ch N al(t l) c r y s t a l was coupled to a DuMont 6292 p h o to m u ltip lie r tu b e usin g im m ersion o i l, A m u-m etal s h ie ld surrounded th e p h o to m u ltip lie r tu b e and a 3 ^ in ch le a d s h ie ld surrounded th e c r y s t a l. The le a d s h ie ld was used p rim a rily to s h ie ld th e c r y s t a l from th e x -ra y s produced a t th e h ig h v o lta g e end o f th e Van de O ra a ff a c c e le r a to r. The p h o to m u ltip lie r was powered by a R a d ia tio n In stru m en ts Company High V oltage Supply Model 515 At * v o lta g e o f minus 900 v o lts* The supply was v e ry s ta b le w ith a d r i f t of l e s s th a n.02 p e rc e n t p e r d ay, and a r e g u la tio n of.0002 p e rc e n t f o r a one p e rc e n t change in l i n e v o lta g e. The p u ls e s from th e p h o to m u ltip lie r tu b e fe d through a cathode fo llo w e r i n to an Atomic 204-B l i n e a r a m p lifie r. The a m p lifie d p u ls e s could be scanned by two m ethods; i. e., w ith an Atomic model 510 s in g le ch an n el a n a ly z e r, o r by photographing th e spectrum which was d isp la y e d on an o s c illo s c o p e. r a t e s w ere a v a ila b le. The f i r s t method was used only when h ig h co u n tin g In th e second method which was e q u iv a le n t to a m u ltic h a n n e l a n a ly z e r, th e p u ls e s were fe d in to th e v e r t i c a l d e f le c tio n p la te s o f a T ex tro n!c Model 511 o s c illo s c o p e u sin g a h o r iz o n ta l sweep o f about 8 m icroseconds. A DuMont Model 296 O scillo sco p e-r ecord in g camera was vised to photograph th e p u ls e s ; sta n d a rd 35 mm Plux-X film was used in th e cam era. A fte r d evelo p in g th e f ilm, i t was scanned w ith e ith e r a m ic ro d e n sito m e te r, o r a low power tr a v e lin g m icroscope to m easure th e h e ig h ts o f th e p eak s.
31 PM CATHODE HV ADD 6AMEDA TIMED FIGURE 8 GAMMA RAY SPECTROM ETER
32 - 2U- A s in g le c r y s t a l sp e c tro m e te r has one d isa d v an tag e t h a t i s n o t en co u n tered in a m u ltic r y s ta l sp e c tro m e te r; t h a t i s t photons oan produce as many as fo u r peaks in th e observed spectrum * These peaks a re th e p h o to e le c tr ic peak* th e Compton edge, and i f th e gamma energy i s g r e a te r th a n 1*02 Mev, th e f i r s t and second escape peaks o f p a ir p ro d u c tio n. The f u l l energy peak in p a ir p ro d u c tio n w i l l c o in c id e w ith th e p h o to e le c tr ic p eak. (These escape peaks a r i s e in th e escape from th e c r y s t a l of one o r b o th o f th e *51 Mev quanta produced when th e p a ir-p ro d u c tio n p o s itro n i s a n n ih i l a t e d.) I f th e spectrum to be an aly zed i s com plex, th e peaks from gamma ra y s o f d i f f e r e n t energy w i l l o v e rla p and i t becomes v e ry d i f f i c u l t to re s o lv e and i d e n t i f y them* For t h i s re a s o n, a tte m p ts w ere made to in c re a s e th e r e s o lu tio n o f th e sp e c tro m e te r as much as p o ssib le * As a m easure o f th e r e s o lu tio n o f a s c i n t i l l a t i o n sp e c tro m e te r, th e f u l l w id th a t h a lf maximum of th e Cs Mev p h o to e le c tr ic peak has fre q u e n tly been u s e d. ^ A r e s o lu tio n o f was o b ta in e d when th e sp e c tro m e te r was f i r s t assem bled* The resp o n se o f t h i s sp e c tro m e te r i s shown by th e dashed cu rv es o f F ig u re 9 which were o b ta in e d u sin g th e s in g le channel a n a ly z e r. As can be se e n, th e two Co-60 peaks a re n o t to o w e ll re s o lv e d. The s o lid curves in th e fig u r e show th e r e s u l t s o b tain ed a f t e r th e r e s o lu tio n was in c re a s e d t o 1 0 *5$. A lthough th e appearance o f th e new cesium peak i s n o t much d i f f e r e n t from th e o ld, th e advantage of in c re a s e d r e s o lu tio n can be seen in th e much g r e a te r s e p a ra tio n of th e two c o b a lt p eak s. The g r e a te s t c o n trib u tio n to th e in c re a s e in r e s o lu tio n was made by changing th e v a lu e s o f th e dynode r e s i s t o r s o f th e
33 - c o u n ts pen channel,117 c s AO CO MEV FIGURE 9 - SPE C T R A OF Cs'*T AND C<? Z S
34 6A65 150V VWi ** IM IM IM IM M 5M 200 OUTPUT V 2 K FIGURE 10 PHOTOMULTIPLIER AND CATHODE FOLLOWER CIRCUIT
35 ENERGY* MEV PULSE HEIGHT * MM FIGURE II - CALIBRATION CURVE 27
36 -2 8 -
37 0EN8ITY FIGURE 13 -DENSITOMETER TRACE OF Gs'*7 AND
38 p h o to m u ltip lie r tu b e from th e u s u a l r a t i o s o f to T his I s shown i n F ig u re 10 w ith th e c i r c u i t diagram o f th e cath o d e fo llo w e r* The r a t i o t h a t g iv e s th e b e s t r e s u l t s w i l l depend upon th e p a r t i c u l a r tu b e t h a t i s b e in g u se d. R a tio s as h ig h a s have been u a e d ^. In c re a s in g t h i s r a t i o in c r e a s e s th e v o lta g e fro st th e p h o to s u rfa c e t o th e f i r s t dynode w hich in c re a s e s th e e f f ic ie n c y o f e le c tr o n c o lle c t io n from th e p h o to su rfa c e * O ther w o rk e rs, u sin g s e le c te d c r y s t a l s and p h o to m u ltip lie r tu b e s, have re p o rte d r e s o lu t io n s a s h ig h a s f o r th e cesium p e a k ^. To check th e l i n e a r i t y of th e s p e c tro m e te r, th e p h o to g ra p h ic method was used f o r gammas o f known e n e rg y. The r e s u l t s a re shown in F ig u re 1 1. The h e ig h ts o f th e peaks on th e f ilm w ere m easured w ith th e tr a v e lin g m icroscope* The peaks o b serv ed w ere a s fo llo w s : *51 Mev from p o s itr o n a n n ih i l a t io n,.661 Mev from a Cs-137 s o u rc e, 1*17 and 1*33 from Co-60; 1*38, 1*76, and 2*76 Mev from Na-2*i-, and 5»11 and 5*62 which w ere th e f i r s t and second escape peaks o f th e 6*13 Mev gamma from th e F l ^ ( p, «. ir4) ^ r e a c tio n. As an exam ple o f th e p h o to g ra p h ic m ethod, th e superim posed spectrum s o f Cs-137 and Co-60 a re shown in F ig u re 12* A d e n sito m e te r t r a c e o f th e p hotograph i s shown in F ig u re 1 3. D. H a lf-l ife A p p a ra tu s. As can be seen from T able I, th e r e p o rte d v a lu e s o f th e h a l f - l i f e o f p30 a re n o t in v e ry good agreem ent. The re a so n s f o r t h i s v a ria n c e a re n o t known, b u t c o n ta m in a tio n in th e t a r g e t s, low y i e l d and p o o r c o u n tin g a p p a ra tu s p ro b ab ly w ere th e main f a c t o r s. In th e s e re p o rte d e x p e rim e n ts, th ic k t a r g e ts m ainly w ere used which made i t p o s s ib le t o e x c ite th e reso n an c es of any
39 p o s s ib le c o n ta m in a tio n. In o rd e r to d e c re a se th e p r o b a b ility o f e r r o r s o c c u rrin g due to c o n ta m in a tio n, th e f o i l owing a p p a ra tu s was assem bled to m easure th e h a l f - l i f e o f P ^ : A t h i n t a r g e t (15 kev) o f e n ric h e d S i-2 9 was used in s te a d o f a th ic k t a r g e t, and i t was bombarded a t a known reso n an ce o f P T h e re fo re, i f a s i g n i f i c a n t amount o f c o n ta m in a tio n w ere p r e s e n t, i t was u n lik e ly t h a t i t would have a reso n an c e w ith in 15 kev o f th e P-30 re s o n a n c e. The u se o f a th in t a r g e t in s te a d o f a th ic k one h as one d is a d v a n ta g e ; i. e., th e y i e l d w i l l be somewhat lo w er f o r th e t h i n t a r g e t making i t n e c e ssa ry to have a more e f f i c i e n t co u n tin g a p p a ra tu s. The p o s itr o n s from th e P-30 w ere n o t d i r e c t l y co u n te d, b u t th e.51 Mev gamma ra y s produced d u rin g a n n ih ila tio n w ere d e te c te d in s te a d - To do t h i s, th e gamma s p e c tro m e te r was u sed in c o n ju n c tio n w ith th e s in g le channel a n a ly z e r w ith th e w idth o f th e window s e t a t 15 p e rc e n t o f th e b a se l i n e v a lu e. The 3in g le channel a n a ly z e r se rv e d n o t only t o s e l e c t th e.51 Mev p h o to n s, b u t a ls o t o low er th e background r a t e o f th e s c i n t i l l a t i o n c o u n te r t o com pensate f o r th e low er y ie ld o f th e t h i n t a r g e t. The p u ls e s from th e s in g le chan n el a n a ly z e r w ere fe d in to a B erk ley Model 2001 s c a l e r and tim e r. As a check on th e s c a le r and tim e r, ru n s a ls o w ere made u sin g a R a d ia tio n C ounter L a b o ra to rie s S c a le r Model 10A w ith a L le b e l F la rsh e im tim e r Model U2.
40 CHAPTER I I I EXPERIMENTAL PROCEDURE AND RESULTS A. R esonances. The reso n an ces in th e S i^ 9 (p f )p30 r e a c tio n were s tu d ie d by bombarding th ic k and t h i n ta r g e ts i n th e p ro to n energy ran g e from 400 kev to 1.3 Mev. The p ro to n s w ere analy sed w ith a m agnetic a n a ly s e r. A c a l ib r a t io n curve f o r th e a n a ly z e r was d e te r mined by u sin g th e w ell-know n reso n an ces o f L if ; i. e. * 340, 441, 669, 873*5 and 935 kev. C h ilto n has found t h a t i f th e lo g a rith m o f th e c u rre n t through th e a n a ly z e r i s p lo tte d a g a in s t th e lo g a rith m of 37 th e p ro to n en erg y, th e curve w i l l be a s t r a i g h t l i n e. The p ro ced u re f o r d eterm in in g th e reso n an ces was to determ ine f i r s t t h e i r approxim ate lo c a tio n s w ith a th ic k t a r g e t, and th e n to survey th e s e a re a s w ith a t h in t a r g e t to d e term in e a c c u ra te ly th e e n e rg ie s o f th e re so n a n c e s. The th ic k t a r g e t o f f e r s a means o f r a p id l y d e te rm in in g th e resonanoes s in c e i t s y i e l d i s a s te p fu n c tio n. A reso n an ce cannot be overlooked even when la r g e in te r v a ls a re used between m easurem ents. The d e t a i l s of th e p ro ced u res and th e r e s u l t s o f b o th th e th ic k and t h in t a r g e ts a re d isc u sse d s e p a r a te ly in th e fo llo w in g s e c tio n s. 1. T hick T a rg e t Y ie ld. The gamma-ray y ie ld s from th ic k t a r g e ts o f b o th n a tu r a l and e n ric h e d s i l io o n were d eterm in ed. The u se o f th e two m a te ria ls o f f e r s a means o f id e n tif y in g any s te p t h a t m ight appear in th e y i e l d. S in ce th e n a tu r a l s i l ic o n has a high p e rc e n ta g e o f S l-2 8, w h ile th e e n ric h e d has a h ig h p e rc e n ta g e o f S i-2 9,
41 one can i d e n t i f y th e reso n an c e by i t s r e l a t i v e y i e l d i n th e two m a te r ia ls. The is o to p ic p e rc e n t o f th e two m a te r ia ls i s i Is o to p e N a tu ra l S i E n ric h e d S i The t a r g e t s w ere p re p a re d a s d e s c rib e d i n th e p re v io u s c h a p te r. A p ro to n beam o f about 1 /u* i n t e n s i t y was u sed i n co n ju n c t i o n w ith a 1*5 mm d e fin in g s l i t * The c u rre n t th ro u g h th e a n a ly z e r was v a rie d in s te p s o f 4 m illia m p e re s w hich co rre sp o n d s to 8 kev p ro to n e n erg y. The w e ll-ty p e N a l(t l) s c i n t i l l a t i o n c o u n te r was vised t o m easure th e y i e l d. The v o lta g e on th e p h o to m u ltip lie r was s e t a t 900 v o l ts and th e d is c r im in a to r was a d ju s te d a t about 1.2 Mev by u sin g a Co-60 so u rc e a s a s ta n d a rd. The background o f th e c o u n te r was 120 co u n ts p e r m in u te. F ig u re 14 shows th e r e s u l t s o b ta in e d f o r th e two m a te r ia ls a f t e r s u b tr a c tio n o f th e background. The dashed curv e shows th e n a tu r a l s i l i c o n y i e l d which was s tu d ie d o n ly as f a r as 800 kev s in c e th e e f f e c t o f f lu o r in e c o n ta m in a tio n a t g r e a te r e n e rg ie s was v e ry h ig h. t a r g e t. The s o l i d curv e shows th e y i e l d f o r th e e n ric h e d s i l i c o n The f i r s t reso n an ce i s th e known 414 kev re so n a n c e o f th e S i2? (p. / ) p 3 r e a c tio n. I t a p p ears t h a t th e s te p s a t 700 and 725 kev a re a ls o due t o t h i s r e a c tio n w h ile th e one a t 622 kev i s due to th e S i^ (p, fc*)p2? r e a c tio n. To o b ta in a more q u a n tita t iv e a n a ly s is o f th e c u rv e s, th e h e ig h t o f each s te p f o r th e e n ric h e d th ic k t a r g e t
42 6 RELATIVE GAMMA RAY YIELO IT / J T - ~ - 'i i i i J 45o Too" 600 ENRICHED 81 NATURAL Si» i» > PROTON ENERGY (KEV) FIGURE 14 -THICK TARGET YIELDS
43 curve was compared to th e s te p s f o r th e n a tu r a l th ic k t a r g e t curve a s shown in T able 2. As can be se en, th e r a t i o s o f th e two y ie ld s f o r th e 700 and 725 kev reso n an ces compare v e ry fa v o ra b ly w ith th e r a t i o f o r th e known 414 kev reso n an ce and th e r e f o r e th e s e two re s o nances a re m ost p ro b ab ly due to th e S i^ ^ (p, r e a c tio n. The sm a ll s te p a t 5 0 kev in th e n a tu r a l curve does n o t appear in th e e n ric h e d curve and i s p ro b ab ly due to th e Si^ (p, f ) o r S i ^ ( p, y*) r e a c tio n s. The s te p a t 669 kev in th e n a tu r a l curve i s due to f lu o r in e co n ta m in a tio n. The rem ainder of th e e n ric h ed curve shews reso n an ces a t.9 2 0, 960, 1.0 1, and Mev. The sm a ll s te p a t 873 kev i s due to a s l i g h t f lu o r in e contam in atio n in th e t a r g e t. T able I I T hick T a rg e t Y ield s from N a tu ra l and E nriched 3 ilic o n Resonance Kev N a tu ra l S i Y ield E nriched S i Y ield N at. Y E n r. Y Thin T a rg e t Y ie ld. The w e ll-ty p e N al(t l) c r y s t a l was used to d e te c t th e gamma-rays from th in t a r g e t s. The d is c rim in a to r was a d ju s te d t o g iv e a background r a t e o f one count p e r second. T his was more th a n enough to exclude th e.51 Mev gamma ra y s from th e p o s itr o n a n n ih ila tio n.
44 A lthough th e th ic k t a r g e t d a ta gave in fo rm a tio n on th e lo c a tio n s of th e re so n a n c e s, th e e n t i r e re g io n from 300 kev to 1*3 Mev was surveyed to be c e r ta in t h a t no reso n an ces w ere overlooked* The m agnet was v a rie d in s te p s o f 2 m illia m p ere s which corresponded to a f o u r kev sp acin g a t 700 kev p ro to n en erg y. In th e v i c i n i t y o f a reso n an c e, th e magnet was v a rie d in s te p s o f one m illiam pere* The beam s tre n g th was appro x im ately one microampere* The in te g r a to r was s e t f o r 4,096 counts f o r d a ta ta k e n a t p o in ts o f f re so n a n c e, and 8192 f o r d a ta ta k e n in th e v i c i n i t y o f a re so n a n c e. T his co rresp o n d ed t o re c o rd in g tim es o f 20 and 40 seconds r e s p e c tiv e ly. The th ic k e s t o f th e s ix evap o rated t a r g e ts was ru n f i r s t. The r e s u l t s o f t h i s t a r g e t f o r th e re g io n from J00 kev to 1 Mev a re shewn in F ig u re 13* to 1.3 Mev. F ig u re 16 g iv e s th e r e s u l t s f o r th e re g io n from 1 Mev The peaks a t 340, 669, 873, and 935 kev a re due to f lu o r in e contam in atio n in th e t a r g e t. The peaks a t 326 and 414 a re th e two reso n an ces in th e S i2 9 (p, )f)p30 t h a t have been re p o rte d by Tangen22. a com parison of th e s e d a ta w ith t h a t re p o rte d by S e ile r who s tu d ie d th e y ie ld from n a tu r a l s i l i c o n, shows t h a t th e 622 and 989 kev peaks a re due to th e S i28 (p, / )p29 r e a c tio n. The peaks a t 698, 732, 918, and 960 kev a re due to th e S i29 (p, )p30 r e a c tio n. T his w i l l be shewn in th e fo llo w in g p a ra g ra p h s. The d a ta ta k e n in th e re g io n above one Mev i s e r r a t i c s in c e th e a c c e le r a to r sparked c o n sid e ra b ly. The y ie ld d id n o t drop to z ero a t p o s itio n s o f f re s o nance because o f th e in c re a s e in th e x -ra y i n te n s i t y from th e high v o lta g e end o f th e a c c e le r a to r. However, th e d a ta does show
45 FIGURE 15 THIN TARGET YIELD i KEV
46 a 5- ui >- Z z U i > St _> U i (E 1.3 MEV FIGURE 16 -THIN TARGET YIELD OF Si*f(p
47 reso n an c es a t 1.1 6, 1,2 1, and 1.2? Mav. C l a s s if ic a tio n o f th e r e a c tio n s t h a t a re re s p o n s ib le f o r th e peaks i s a id e d by th e stu d y o f th e p ro d u c t nucleus* Phosphorus- 30 i s a p o s itr o n e m itte r w ith a h a l f - l i f e o f ap p ro x im ate ly 2*5 m inu t e s. P hosphorus-29 i s a ls o a p o s itr o n e m itte r b u t h as a h a l f - l i f e o f o n ly h *6 seconds* T herefore* i f a reso n an c e p ro d u ces a p ro d u c t t h a t does n o t em it p o s itr o n s, we can be s u re t h a t i t i s n o t due to e i t h e r of th e two r e a c tio n s. The s c i n t i l l a t i o n sp e c tro m e te r was used t o d e te rm in e i f th e p ro d u c t n u c le i w ere p o s itr o n e m itte r s. The t a r g e t was f i r s t bombarded f o r a t l e a s t te n m inutes (u s u a lly th e bom barding tim e was o f th e o rd e r o f two h o u rs s in c e gamma-ray measurem ents w ere b e in g made a t t h i s tim e ). The beam was th e n removed from th e t a r g e t and th e t a r g e t was allow ed to decay f o r t h i r t y se c o n d s. S in c e P29 has a h a l f - l i f e o f o n ly ***6 seconds* i f i t w ere p re s e n t i t w ould decay to 11$ o f i t s i n i t i a l v a lu e in t h i s p e rio d * The p u lse from th e sp e c tro m e te r was th e n photographed on th e o s c illo s c o p e f o r a p e rio d o f th r e e m in u te s. The f ilm was developed and any l i n e s p r e s e n t w ere m easured w ith a tr a v e lin g m icro scope. F ig u re 1? shows a photograph ta k e n f o r th e 732 kev reso n an c e w ith th e beam on th e t a r g e t. The beam was tu rn e d o f f and F ig u re 18 shews th e photograph ta k e n a f t e r th e t h i r t y second w a itin g p e rio d * The.51 Mev gamma-ray was observ ed in b o th p ic tu r e s f o r th e 698, 732, 918, and 9&0 kev reso n an c es in d ic a tin g t h a t th e s e reso n an c es w ere due to th e S i29 (P. / )P -^ r e a c tio n. In a d d itio n to t h i s, none o f th e photo g rap h s ta k e n w ith th e beam o f f showed th e p re sen c e o f any gamma o th e r th a n
48 -1*0- F ig u re 17 Gam a Rays from T a rg e t w ith Bean On
49 F lg u re 1 8. Qanna Rays from T arg e t w ith Beam Off
50 th e.51 Mev a n n ih ila tio n gamma. F ig u re 19 shows th e photograph f o r th e 1,1 6 Mev resonance w ith th e beam on th e t a r g e t. No lin e s w ere p re s e n t a f t e r th e beam was tu rn e d o f f in d ic a tin g t h a t th e p ro d u c t nucleus i s n o t phosphorus-30. The absence o f any.51 Mev l i n e in th e photograph in d ic a te s t h a t th e p ro d u c t n u cleu s i s n o t a p o s itro n e m itte r and c e r ta in ly th e reso n an ce i s n o t th e r e s u l t o f th e two r e a c tio n s in q u e s tio n. The p h o to g rap h, how ever, shows a gamma o f 1.6 Mev. The photographs ta k e n f o r th e and Mev reso n an ces a re i d e n t i c a l to th e ones o b tain ed f o r th e Mev reso n an c e. These th r e e reso n an ces have been a tt r ib u t e d t o th e N a ^ (p,«c )N e ^ * r e a c tio n f o r th e fo llo w in g re a so n s: 1. T his r e a c tio n has reso n an ces a t , and Mev38 which a re in good agreem ent w ith th e observed v a lu e s of 1.1 6, 1.2 1, and 1.2? Mev. 2. Neon-20 i s a s t a b le is o to p e which i s c o n s is te n t w ith th e f a c t t h a t no p o s itro n s w ere o b serv ed. 3 * Only a Mev gamma has been re p o rte d f o r t h i s r e a c t i o n 3 9 which i s due to th e r e le a s e o f an alp h a p a r t i c l e from th e M g^ compound n u cleu s to th e f i r s t e x c ite d s t a t e in Ne. The Ne th e n decays t o i t s ground s t a t e r e le a s in g th e Mev gaama. T his i s c o n s is te n t w ith our o b s e rv a tio n o f o n ly one gamma ra y o f energy 1.6 Mev. 4. The sp e c tro g ra p h ic a n a ly s is shows t h a t sodium i s p re s e n t in th e t a r g e t w ith a c o n c e n tra tio n o f. 3 < As a l a s t check on th e i d e n t i ty o f th e 698, 732, 918. and 960 kev re so n a n c e s, m easurements w ere made to d eterm in e th e h a l f - l i f e
51 F ig u re 1 9. G am a Rays from th e H er Resonance
52 o f th e p o s itro n em issio n. The p ro ced u re was s im ila r to th e one d i s cussed in th e l a s t s e c tio n of t h i s c h a p te r w ith one e x cep tio n ; i. e., no a tte o p t was made to d eterm in e a c c u ra te ly th e h a l f - l i f e, b u t only to e s ta b lis h t h a t i t was of th e o rd e r of 2.5 m in u tes. The h a l f - l i f e f o r th e h lh key reso n an ce was a ls o m easured as a check. F ig u re s 20 and 21 g iv e th e r e s u l t s t h a t w ere o b ta in e d. As can be se e n, th e f iv e reso n an ces produce p ro d u ct n u c le i whose h a l f - l i v e s a re c o n s is te n t w ith th e re p o rte d v a lu e f o r phosp h o ru s-30. The resonance a t 698 kev was s tu d ie d more c lo s e ly by o b serv in g th e y ie ld from a l l s ix o f th e t a r g e t s. F ig u re 22 g iv e s th e r e s u l t s f o r t a r g e ts 2, 5, and 6. T a rg e ts 1 and 3 were found to g iv e th e same r e s u l t s as t a r g e t 2. As was ex p ected from th e geom etry o f th e evapora tio n, th e h ig h e r numbered t a r g e ts gave a low er y ie ld and a more narrow w id th. The w id th a t half-maximum which v a rie d from kev f o r t a r g e t 2 to 3*8 kev f o r t a r g e t 6, i s th e r e s u l t o f th r e e f a c to r s ; nam ely, th e th ic k n e s s o f th e t a r g e t, th e energy sp re a d in th e beam, and th e n a tu r a l w idth of th e reso n an c e. The n a tu r a l w idth o f th e resonance could be c a lc u la te d from th e s e d a ta i f th e o th e r two f a c to r s a re known. An e stim a te o f th e w id th o f th e s l i t can be o b ta in e d from th e geom etry o f th e an aly zin g m agnet, b u t th e th ic k n e s s of th e t a r g e t was n o t found s in c e th e r e a re no d a ta a v a ila b le on th e sto p p in g power o f t h i s m ateri a l and th e w eight o f th e m a te r ia l was n o t known. However, th e n a tu r a l w idth o f th e reso n an ce can be found from t a r g e t 2 i f i t i s a m odified" th ic k t a r g e t. A m odified th ic k t a r g e t i s one whose th ic k n e s s i s s e v e ra l tim es th e n a tu r a l w id th o f th e reso n an ce so t h a t th e maximum
53 4 0 1 COUNTS PEN SECOND KEV S9S KEV T. 2.3 MIN S MIN. FIGURE DECAY OF THE 414 AND KEV R E S O N A N C E S 4 5 *
54 COUNTS PEN SECONO KEV KEV KEV.2 - T 2.2 MIN «9 MIN. FIGURE 21 - DECAY OF THE 7 3 2,9 1 8 AND KEV RESO N A N C ES 6
55 YIELD TOO Z b I M too* too- T-2 T "5 T-S TOO t io T to TOO TIO TOO KEV FIGURE 2 2 THIN TARGET YIELDS FOR THE KEV RESONANCE
56 y i e l d o b ta in e d i s th e same as w ould be o b ta in e d w ith th e o rd in a ry t h ic k t a r g e t s. t h i c k t a r g e t. A lso th e i n i t i a l s lo p e w i l l be th e same a s f o r a I n a t h ic k t a r g e t w here Y i s th e maximum y i e l d obt a i n e d, th e en erg y s p re a d from 1 /4 Y t o 3 /4 Y i s due o n ly t o th e n a tu r a l w id th o f th e re so n a n c e p lu s th e h a lf - w id th o f th e d e fin in g s l i t. A m o d ifie d th ic k t a r g e t i s more u s e f u l f o r t h i s p u rp o se th a n a th ic k t a r g e t s in c e th e l a t t e r w i l l have a h ig h background due to t h e low er en erg y re s o n a n c e s. To show t h a t t a r g e t 2 i s a m o d ifie d th ic k t a r g e t, th e d a ta from t a r g e t 6 i s f i r s t u se d. The h a lf - w id th o f th e d e fin in g s l i t i s x 10 3 e kev/mm w hich, f o r a mm s l i t and E * 700 k e v, i s e q u a l t o 1.6 k e v. Then (3» 8 )^ * ( 1.6 ) 2 + P' w here P i s th e w id th o f th e re so n a n c e p lu s th e c o n tr ib u tio n due to th e t a r g e t th ic k n e s s. # S o lv in g t h i s e q u a tio n g iv e s P * 3*^ k e v. T h e re fo re, th e n a tu r a l w id th i s c e r t a i n l y l e s s th a n k e v, and t a r g e t 2 i s a t l e a s t 3*7 tim e s t h i c k e r th a n th e n a tu r a l w id th and can be c o n s id e re d as a "m odif i e d " th ic k t a r g e t. A lso th e to p o f th e peak o f t a r g e t 2 i s se en t o be f l a t w hich i s f u r t h e r p ro o f t h a t i t i s a m o d ified t h ic k t a r g e t. T ab le I I I l i s t s th e r e s u l t s o b ta in e d w ith th e m o d ifie d th ic k t a r g e t s. The r e s u l t s f o r th e f u l l w id th a t half-m axim um w i l l be u se d l a t e r i n c o n n e c tio n w ith th e d e te rm in a tio n o f th e e x a c t reso n an c e e n e rg y. The t h i r d column l i s t s th e v a lu e s o b ta in e d f o r th e en erg y s p re a d from 1 /4 Y t o 3 /4 Y. From th e av erag e o f th e s e v a lu e s, th e n a tu r a l w id th s o f th e re so n a n c e s w ere c a lc u la te d and a r e g iv e n i n th e f o u r th colum n. As an exang)le, th e c a l c u l a t i o n f o r th e 414 kev
57 TABLE I I I O bserved W idths o f th e R esonances F u ll W idth a t W idth o f S l i t R esonance H a lf M axim * and Resonance 4 l kev 1.7 kev * A verage 13*6 1* * Q 6 A verage 11*5 1* Average Average 9* r
58 reso n an ce i s as fo llo w st 1.55 kev 3 W idth of resonance p lu s s l i t.7 ^ kev * x 10"^ E kev f a r 1mm s l i t 3 w idth o f s l i t r * N a tu ra l w idth of resonance (1.5 5 )2 - (.7 * 0 2 * T * kev Prom th e s e re s u lts * i t can be seen t h a t a l l of th e resonances a re narrow having a n a tu r a l w idth of l e s s th a n two kev. Up to t h i s p o in t, th e e x a c t p ro to n energy a t which th e re s o nances occur h as n o t been d isc u sse d s in c e th e p reced in g in fo rm a tio n on th e th ic k n e s s of th e ta r g e ts i s n e c e ssa ry. The v a lu e s o f 698, 732, 918, and 960 w ere used f o r i d e n t i f i c a t i o n purposes o n ly. The e x a c t energy was determ ined by two m ethods. In th e f i r s t, which was used o n ly fo r th e 698 and 732 kev re so n a n c e s, a s e p a ra te LiF t a r g e t was used f o r c a lib r a tio n o f th e m agnet. In th e second m ethod, th e f lu o r in e co n tam in atio n in th e SiC^ t a r g e t was u t i l i z e d. These methods a re d isc u sse d in th e fo llo w in g p a ra g ra p h s. I f a s e p a ra te LiF c a l ib r a t io n t a r g e t i s u se d, c o rre c tio n s have to be made to th e m easured p ro to n energy to com pensate f o r th e f i n i t e th ic k n e s s o f b o th th e SiO^ and LiF t a r g e t s. I f i s th e m easured p ro to n energy, W^ th e th ic k n e s s o f th e SiC^ t a r g e t, and W^ th e th ic k n e s s of th e LiF t a r g e t, th e n th e energy o f th e resonance i s : Er * HJp + I / 2CW2 - W-^). The v a lu e s f o r W-^ a re g iv en in th e second column o f T able I I I. The th ic k n e s s of th e LiF t a r g e t was determ ined in th e fo llo w in g m anner: The LiF was evap o rated
59 a i m u l t a n e o u s l y upon a ta n ta lu m b la n k and a s i l v e r f o i l. The s i l v e r f o i l was p re p a re d by th e e v a p o ra tio n of s i l v e r upon a Form var backin g. (D e ta ils a re g iv en in th e s e c tio n on gas sto p p in g p o w er.) The two t a r g e t s w ere th e n bombarded w ith p ro to n s a s shown i n F ig u re 23. The L if s id e o f th e f o i l fa c e d th e L if on th e ta n ta lu m b la n k. In t h i s m anner, th e p ro to n s p a sse d th ro u g h th e Form var and s i l v e r b e fo r e s t r i k i n g th e f i r s t t a r g e t. Two N a l(t l) s o i n t i l l a t i o n c o u n te rs w ere used t o d e te c t th e gammas from th e s e two t a r g e t s. The t a r g e t s w ere p la c e d 17 cm a p a r t to m inim ize i n t e r a c t i o n o f th e two t a r g e t s on th e c o u n te rs. As th e p ro to n energy i s in c re a s e d in th e v i c i n i t y o f a L if re s o n a n c e, b o th c o u n te rs w i l l d e te c t a p eak, b u t th e two peaks w i l l n o t o c cu r a t th e same m agnet s e t t i n g. T h is i s shown i n F ig u re 2k- f o r th e f i r s t L if re s o n a n c e. The maximum o f each peak o c c u rs when th e p ro to n energy a t th e c e n te r o f th e t a r g e t i s e q u a l t o th e reso n an ce e n erg y. S in c e th e p ro to n s m ust p a ss th ro u g h two h a lv e s o f th e t a r g e t s, and b o th a re assum ed to have th e same t h ic k n e s s, th e energy d if f e r e n c e i n th e two peaks i s th e n th e th ic k n e s s o f e i t h e r L if t a r g e t. The d if f e r e n c e s i n th e m agnet s e t t i n g s f o r th e reso n an c es w ere found to be as fo llo w s : Ep M (am ps) y* l*k ** I n te r p o la tin g th e s e r e s u l t s g iv e s a AM a t 700 kev which co rre sp o n d s to a t a r g e t th ic k n e s s o f kev, and a AM = a t
60 LiF LIF SILVER FORMVAR 17 CM. PROTON BEAM NAl - 1 FIGURE 23 "APPARATUS FOR THE DETERMINATION OF THE THICKNESS OF THE LiF TARGET
61 AMP. 10- RELATIVE 6AMMA RAY YIELO X FRONT RACK AMPERES MAONET CURRENT FIGURE 2 4 ' THICKNESS OF LiF T A R G E T * 3
62 kev which co rresp o n d s to a t a r g e t th ic k n e s s o f 2*76 k ev. The v a lu e f o r th e th ic k n e s s o f th e S K ^ t a r g e t as g iv en in T able I I I i s 11*5 kev f o r th e 698 resonance* and 10*7 kev f o r th e 732 reso n an ce. T a rg e ts numbers 1, 2, and 3 w ere used to f in d Ep. The r e s u l t s were as fo llo w s: ( ) * 699*3 kev Average Average ( ) * kev. 2 The second method o f c a lib r a tio n which u t i l i s e s th e f lu o r in e co n tam in atio n t h a t i s d i s t r ib u t e d u n ifo rm ly th ro u g h o u t th e SiC^ t a r g e t has th e advantage t h a t no c o rre c tio n s have to be made to th e observed reso n an ce e n e rg ie s. In t h i s case i s c e r ta in ly eq u al to Vfg s in c e o n ly one t a r g e t i s u sed which makes E^, * Ep. F ig u re 15 shored th e spectrum o b ta in e d in c lu d in g th e f lu o r in e reso n an ces which w ere used f o r c a l ib r a t io n. F ig u re 25 shows th e r e s u l t s u sin g a 4 th in n e r t a r g e t (#5) f o r th e re g io n from 850 kev to 1 Mev. The r e s u l t s t h a t w ere o b ta in e d a re g iv en in th e fo llo w in g ta b le :
63 YIELO ISO TS I I 100 I U TARGET-5 Mo ITI *00 lift KEV FIGURE 2 6 ~ THIN TARGET YIELD FOR THE REGION FROM TO KEV
64 E, P i p2 *P3 V ** 956 « ** Av Av. 918 Av. 957 A veraging th e r e s u l t s f o r th e f i r s t two reso n an c es by th e f i r s t method g iv e s th e v a lu e s 698* , and 957 kev a s th e e n e rg ie s o f th e re so n a n c e s. B. M easurement o f Qamma-Ray E n e rg ie s. The gamma-ray e n e rg ie s from th e **1**, 698, 731* 918, and 957 kev reso n an c es w ere m easured w ith th e s in g le c r y s t a l s p e c tro m e te r. In a l l c a s e s, th e p ro ced u re was th e same e x c e p t f o r th e le n g th o f th e cam era exposure whioh depended upon th e y i e l d from th e re s o n a n c e. The cam era was s e t a t f5*6 w ith th e exposu res ra n g in g from 1.5 h o u rs f o r th e **1*+ and 698 kev reso n a n c es t o fo u r h o u rs f o r th e 957 kev re s o n a n c e. Two s e t s o f p ic tu r e s were ta k e n f o r each re so n a n c e ; one covered th e ra n g e up to th r e e Mev u sin g C s-137. Co-60, and Na-2*t f o r c a l i b r a t i o n, and th e o th e r covered th e ran g e up t o s i x Mev u sin g th e gamma ra y s from th e r e a c tio n p lu s th e above th r e e Is o to p e s f o r c a l i b r a t i o n. One o f th e main problem s in t h i s p h o to g ra p h ic m ethod i s to d eterm in e a c c u ra te ly th e b a se l i n e s of th e p h o to g ra p h s. I f th e b ase l i n e s o f th e c a l ib r a t io n p h o to g rap h s do n o t c o in c id e w ith th e b ase l i n e s o f th e photo g rap h s o f th e reso n an c e gammas, i t i s obvious t h a t a la r g e e r r o r w i l l r e s u l t. The b a se l i n e o f th e o s c illo s c o p e was n o t u se d f o r t h i s purp o se b ecau se i t was to o b ro ad and n o t w e ll d e fin e d.
65 To e s ta b lis h a w e ll d e fin e d b ase l i n e, a g r id was p la c e d over th e fa c e o f th e o s c illo s c o p e tu b e. The g rid was a sta n d a rd o s c illo s c o p e g rid composed of h o r iz o n ta l lin e s * In th e photographs* i t appeared as v e ry sh arp h o r iz o n ta l l in e s w ith a sp a cin g o f one m illim e te r. I f a l l of th e c o n tro ls rem ained th e same f o r th e c a lib r a tio n and unknown p i c t u r e s, th e n th e s e g r id l in e s se rv ed as a v ery a c c u ra te re fe re n c e p o i n t. I t was n o tic e d t h a t th e m agnetic f i e l d o f th e a n a ly z e r had a la r g e e f f e c t on th e c a l ib r a t io n. Por exam ple, when th e c u rre n t through th e a n a ly z e r changed from.9000 amperes to.9300 amperes* th e c a lib r a tio n was seen to change as much as te n p e r cent* In o rd e r to d e c re a se t h i s e f f e c t, a m u-m etal s h ie ld was used around th e photom u ltip lie r. In a d d itio n to th is * th e magnet c u rre n t was n o t changed d u rin g th e p e rio d from c a lib r a tio n to p hotographing th e unknown gammas* Thus th e LIP t a r g e t was bombarded a t th e energy o f a S i^ C p,* * ) re so n an c e. However* th e y ie ld from th e LiF t a r g e t i s so h ig h, t h a t a good photograph could be o b ta in e d, a t t h i s o ff-re so n a n o e p o s itio n, in j u s t a few m in u tes. The p ro cedure th a t was used f o r a l l o f th e reso n an ces was as fo llo w s : 1. The reso n an ce was lo c a te d by v a ry in g th e magnet c u rre n t u n t i l th e y ie ld was a maximum. Prom t h i s p o in t on, th e magnet c u rre n t was n o t changed. 2. A photograph o f th e superim posed spectrum o f Cs-137 and Co-60 was ta k e n. A nother o f Na-2h a ls o was ta k e n. (These w ere ta k e n w ith th e beam o f f o f th e S i ^ t a r g e t ).
66 -5 8-3* The S i29 p h o to g rap h was ta k e n. I*. A nother Cs-137 and Co-60 p h o to g rap h was ta k e n. 5. The S i t a r g e t was re p la c e d w ith a L if t a r g e t w hich was bom barded a t th e same p ro to n e n erg y a s th e S i t a r g e t. A p h o to g rap h was ta k e n o f th e s e gamma r a y s. The film s w ere d ev elo p ed and re a d w ith th e t r a v e l i n g m icro sc o p e. The c a l i b r a t i o n p o in ts w ere p l o t t e d on la r g e g rap h p a p e r and th e unknown e n e r g ie s w ere re a d from th e r e s u l t i n g c a l i b r a t i o n c u rv e. The r e s u l t s f o r each re so n a n c e w i l l be d is c u s s e d i n th e fo llo w in g p a ra g ra p h s. I n th e f i g u r e s and t a b l e s t o fo llo w, th e fo llo w in g n o ta tio n h a s b een u s e d : E0 - E nergy in Mev o f t h e o b se rv ed l i n e. P - O bserved l i n e i s a p h o t o e l e c t r ic p e a k, o r a f u l l en erg y peak o f p a i r p ro d u c tio n. - F i r s t e sc a p e p e a k. p - Second e sc a p e p e a k. w - The o b serv ed l i n e i s weak com pared t o th e o th e r s. (No a c c u ra te d e te rm in a tio n o f th e i n t e n s i t i e s c an be made w ith th e p h o to g ra p h ic m eth o d ). 1. Qamma-Ray E n e rg ie s o f th e 414 kev R eso n an ce: The 41^ kev re so n a n c e i s e x p e c te d to c o rre sp o n d t o a l e v e l i n P-30 a t an en erg y e q u a l t o th e Q v a lu e p lu s t h a t f r a c t i o n o f th e p ro to n k i n e t i c en erg y t h a t i s im p a rte d t o th e compound n u c le u s. From C h a p te r 4 th e e x p e c te d Q v a lu e i s 5*53 Mev. The f r a c t i o n o f th e p ro to n en erg y i s 29/30 o f
67 *, o r M ev. An e n e rg y l e v e l I s t h e n e x p e c te d a t 5*93 Mev. T he gamma s p e c tru m o f t h e hi** k e v r e s o n a n c e w as b y f a r t h e s i m p l e s t o b s e r v e d. A p h o to g ra p h o f t h e s p e c tr u m i s show n I n F ig u r e 2 6 ; t h e r e s u l t s o b s e rv e d a r e g iv e n i n T a b le IV. I n a d d i t i o n t o t h e.5 1 Mev gamma r a y fro m p o s i t r o n a n n i h i l a t i o n, tw o s t r o n g ganma r a y s o f e n e r g i e s 5 * 2 7 ± *05 a n d Mev w e re o b s e rv e d i n e x c e l l e n t a g re e m e n t w ith E n d t, e t &1, who r e p o r t e d gamma r a y s o f >07 an d M ev. T he sura o f t h e tw o e n e r g i e s o b s e r v e d h e r e i s 5*95 Mev i n good a g re e m e n t w i t h t h e e x p e c te d 5*93 Mev. T h is I n d i c a t e s t h a t t h e d e c a y i s th r o u g h t h e l e v e l a t.6 8 M ev. F i g u r e 27 show s t h e d e c a y sc h e m e. I n a d d i t i o n t o t h e tw o s t r o n g gammas* a w eak l.h O Mev gamma w as o b s e r v e d. ( I t c a n b e s e e n i n t h e p h o to g r a p h o f F i g u r e 2 6 ;much o f t h e r e s o l u t i o n w as l o s t i n e n l a r g i n g a n d p r i n t i n g t h e p h o to g r a p h ). The o r i g i n o f t h i s w eak gamma i s n o t know n. 2. Oamma-Ray E n e r g ie s fro m t h e 698 k e v R e s o n a n c e : W ith t h e e x p e c te d Q v a lu e o f 5*53 Mev p l u s 2 9 /3 0 o f 6 9 8, a n e x c i t e d l e v e l a t Mev s h o u ld c o r r e s p o n d t o t h e 698 k e v r e s o n a n c e. T he p h o to g ra p h s t a k e n o f t h i s r e s o n a n c e w e re v e r y c o m p lex ; a p r i n t o f a t y p i c a l p h o to g r a p h i s shown i n F ig u r e 2 8. T a b le V l i s t s t h e e n e r g i e s o f th e l i n e s t h a t w e re o b s e r v e d. Column tw o o f t h i s t a b l e g iv e s t h e d e s ig n a t i o n o f t h e T y p e" o f l i n e o b s e r v e d. T h is d e s i g n a t i o n w as q u i t e d i f f i c u l t t o m ake b e c a u s e o f t h e many l i n e s, e s p e c i a l l y i n t h e r e g i o n fro m 1.^ 7 t o M ev. I n a d d i t i o n t o t h e.5 1 Mev l i n e d u e t o p o s i t r o n a n n i h i l a t i o n, t h e.6 9 a n d Mev l i n e s a r e d e f i n i t e l y
68 -6 o - F ig u re 26. Gamma Rays from th e kev Resonance
69 - 61- TABLE IT Garnma-Ray E n erg ies o f th e 414 kev Resonance s (Mev) Type 51 t.01 P ±.01 P v i.02 P w t.05? * ±.05 P i +.51 * P i.05 S \1 7 1 f F igu re 27. P o s s ib le Decay Scheme o f th e 414 kev Resonance
70
71 TAJBIE V Ganma-Ray E n e rg ie s o f th e 698 kev Resonance E0 (Mev) Type E.5 1 ±.0 1 P ±.01 P.69 i P ? ±.05 P & p t P A Pz P i w i P i P P w P2 T P i P t.05 27/»* 2.*<8 <.*/7 1.7/ t.*8» 2.*<8 r 1.16 e* a F igu re 29* P o ssib le Decay Scheme o f th e 698 kev Resonance
72 photopeaks s in c e th e sp a cin g betw een them was la r g e enough to obse rv e a c le a r Compton ed g e. The and 3-74 Mev l in e s a re b e lie v e d to be th e escape peaks o f a Mev gamma; th e y g iv e : = *74.51 * * Mev Average The sum o f th e 4.2 4, 1.2 6, and.69 Mev l i n e s g iv e s Mev in e x c e lle n t agreem ent w ith th e expected v a lu e o f Mev. These th r e e gammas th e n form one mode o f decay o f th e e x c ite d s t a t e. Compton edges w ere observed f o r th e 1.47 and Mev lin e s in d ic a tin g t h a t th e s e a re p h o to p eak s. However, th e y a ls o appear a t th e p ro p er energy t o be second escape peaks o f two gammas a t and A lso th e I.96 and Mev l in e s a re a t th e p ro p er energy to be th e f i r s t escape peaks o f gammas a t and Two s e r i e s th e n e x is t; « * = * = Compton edges w ere a ls o observed f o r th e l i n e s a t and 2*70 Mev in d ic a tin g t h a t th e s e a re p h o to p eaks. However, i n com paring th e r e l a t i v e i n t e n s i t i e s o f th e escape peaks w ith th o se o b ta in e d f o r th e escape peaks o f th e Mev l i n e o f Na-24, i t was n o tic e d t h a t th e i n t e n s i t i e s o f th e 1.4 7, 1.6 8, and Mev l i n e s w ere to o h ig h w h ile th e weak 2.23 Mev l i n e was o f th e p ro p e r i n t e n s i t y. T h is f a c t, p lu s th e o b se rv a tio n o f th e Compton ed g e, le a d to th e b e l i e f t h a t gammas o f energy 1.47 and Mev w ere a ls o b ein g e m itte d from th e t a r g e t.
73 The I.96 Mev l i n e showed no Compton edge and th e in c re a se d in te n s i t y i s b e lie v e d t o be due to th e c o in c id e n ce o f th e.69 and Mev l in e s whose sum i s 1*95* Two o th e r inodes o f decay a re b e lie v e d to e x is t; th e y a re : These v a lu e s, 6*13 and 6*11, compare fa v o ra b ly w ith th e expected v a lu e of Mev. The o r ig in o f th e weak l i n e a t 3**0 i* not known. I t may have been due to c o in c id e n ces o f low er energy gammas* The p o s s ib le decay scheme f o r th e e x c ite d s t a t e a t Mev i s shown in F ig u re 29* 3. Gamma-Ray E n erg ies of th e 731 kev R esonance; The photographs ta k e n of th e gamma ra y s from t h i s reso n an ce show o n ly s ix l i n e s in c lu d in g th e expected *51 Mev l i n e. F ig u re 30 shews one of th e photographs ta k e n. (In t h i s p h o to g rap h, th e.51 Mev l i n e i s p a r t i a l l y h id d en by a h ig h s e t t i n g o f th e tr ig g e r in g a m p lifie r o f o s c illo s c o p e ). T able VI l i s t s th e e n e rg ie s t h a t w ere found f o r th e se l i n e s. The.5 1,.6 9, and 1.U6 Mev l in e s a l l have c le a r Compton edges and can d e f i n i t e l y be a ssig n e d as th e e n e rg ie s o f gamma ra y s from th e t a r g e t. The 2.25 Mev l i n e i s a t th e e x a c t energy to be th e second escape peak o f th e observed l in e a t 3*27* However, i t a ls o e x h ib its a Compton edge. ( I t can be seen in th e photograph o f F ig u re 3 0.) The 2.25 Mev l i n e i s a ls o b e lie v e d to be due to a gamma ra y o f t h is en erg y. The l i n e a t Mev i s due to th e Compton edge and th e
74 F ig u re 3 0. Gamma Rays from th e 731 kev Resonance
75 67- TABLE VI Gaama-Ray E n erg ies o f th e 731 kev Resonance fio Type E.51 i.0 1 P.5 1 ± P.6 9 ± P f> & * ± P i P ^ <* 3 Figure 31. P o ssib le Decay Scheme o f th e 731 kev Resonance
76 f i r s t escape peak o f th e Mev gamma. The energy o f th e e x c ite d s t a t e f o r t h i s reso n an ce was c a l c u la te d as b e fo re and found to be Mev. The sum o f th e.69* and Mev gamma ra y s i s 6.21 Mev which i s w e ll w ith in th e lim i ts of th e e x p erim e n tal e r r o r s. These th r e e gamma ra y s form a mode o f decay th rough th e e s ta b lis h e d le v e l a t.69 Mev, and th rough a new le v e l a t e i t h e r o r Mev. The p o s s ib le decay scheme i s shown in F ig u re 31. The gamma ra y a t Mev does n o t f i t in to any decay scheme. 4. Qamma Ray E n erg ies o f th e 918 kev R esonancei A photograph o f th e t o t a l gamma-ray spectrum from th e 918 kev reso n an ce i s shown in F ig u re 33. The low energy p o rtio n of th e spectrum i s shown in F ig u re 32. The m easured e n e rg ie s a re given in T able V II. The.5 1, and Mev l in e s a l l show Compton edges and a re ty p ed as p h o to e le c tr ic p e a k s. The 3*45 and 3*95 Mev l in e s a re b e lie v e d to be th e escape peaks of a gamma o f Mev en erg y. A l i n e was obse rv ed a t t h i s energy which was v e ry weak. The f lu o r in e contam inatio n on th e t a r g e t made i t im p o ssib le to m easure any lin e s o f energy above 5 Mev. The weak l i n e a t Mev i s b e lie v e d to be th e second escape peak o f a 5*74 Mev gaana ra y, b u t th e f lu o r in e co n tam in atio n p rev en ted th e d e te c tio n o f th e rem aining two l i n e s. F ig u re 34 g iv e s th e p o s s ib le decay schem e. The c a lc u la te d v a lu e o f th e energy o f t h i s le v e l i s Mev. The observed gammas form ed two modes o f decay which added as fo llo w s: ^
77 F ig u re 32. Oansna Bays from th e 918 kev Resonance (show ing th e lo w er energy gamma ra y s ) I F ig u re 33. Gamma Rays from th e 918 kev Resonance (T o ta l Spectrum )
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