Experiment. The Pinhole Neutron. Pinex. c. c. sartain UCRL-ID November 21,1958

Transcription

1 UCRLD Pinex = The Pinhole Neutron Experiment c c sartain November 21,1958 This is an informal report intended primarily for internal or limited external distribution The opinions and conclusions stated are those of the author and may or may not be those of the Laboratory Work performed under the auspices of the US Department of Energy by the Lawrence Livermore National Laboratory under Contract W7405ENG48 BE

2 DSCLAMER This report was prepared as an account of work sponsored by an agency of the United States Government Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, mommendation, or favoring by the United States Government or any agency thereof The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof

3 DSCLAMER Portions of this document may be illegible in electronic image products mages are produced from the best available original document

4 , 2 \' COpZl~337, 21, 1958 ;>&mber _ PNEX, the Pnhole Neutron Experiment SUBJ: The pinhole neutron experiment is sometimes called "Pinex", a name which has a l s o been used t o describe the pin method of measuring t h e time required f o r imploding metals t o t r a v e l t o c e r t a i n locations i n space The two experiments a r e not related and should not be confused with each other The pinhole neutron experiment is very similar t o the optical pinhole camera i n which light on passing through a pinhole i n an opaque screen produces an inverted image of t h e source n the pinhole neutron experiment14 Mev neutrons f r o m a thermo _, % +;~~ nuclear device t r a v e l i n straight'lines from t h e i r respective points of origin outward i n a l l directions Those which pass through a pin hole in an opaque neutron shield make an inverted neutron image of the source Some of the neutrons which form the image a r e captured by_ threshold detector plates which have been suitably located behind t h e pinhole Neutrons which have sufficient energy r e a c t w i t h t h e nuclei of t h e detector p l a t e t o form radioactive nuclei which by t h e i r decay locate the position of the image on the plate The image may be made v i s i b l e by au&oradiography or by counting techniques n the autoradiograph, an xray film i s placed i n contact w i t h the image plate As the radioactive nuclei decay, they expose t h e film, 7 %is document consists of pages No / of / copies: Series /:&

5 c i 2 _, The image i s v i s i b l e when the f i l m has been adequateljj exposed and developed n the counting method, the image p l a t e i s cut i n t o small pieces; each the s i z e of a resolution element d 9 %;ch piece i s separately counted A The number of neutrons causing i t s radioadtivity is determined and plotted on a drawing of the plate These numbers indicate the shape of t h e DT plasma a t t h e time of "burn" t is the object of t h i s paper t o discuss the fzctors affecting the various parameters i n the experiment and what information i s required t o optimize these parameters f o r a given set of conditions Formulae are written i n many alternative ways t o emphasize the effect t o be expected' from a change i n any one of t h e many parameters Consider an extended source which e m i t s a quantity No of 14 Mev neutrons L e t the source be a distance neutron shgeld $ from a pinhole i n an opaque L e t a threshold detector p l a t e be placed s o t h a t a neutron image of t h e source i s formed a t a distance q from t h e pinhole Then L = p + q is the distance from t h e source t o the image As i n optics, t h e magnification i s given by 4 = /O = 4/p 0 ' L e t d equal the diameter of the circulax pinhole 2;

6 c&r 357 p Consider two points A and B on the object P o u t A transforms i n t o t h e c i r c l e A1 and point B transforms i n t o the c i r c l e B1 resolved if the c i r c l e s are tangent and 3, A and B a r e j u s t Let o be the distance between A two points which a r e j u s t resolved Let i be the distance between the centers of t h e c i r c l e s A1 and B1 the c i r c l e s A1 and B1 have equal radii, s o i equals the diameter of Now either circle m = i / o = g/p o = Pi i / m 9 Consider the similar triangles having a common vertex A and bases which are the diameter of the pinhole and t h e c i r c l e A i/dq+pl= P m + l 3 i = (m+1) D Since o = i/m, o=m + l or m m D=m + l then D = L D 9 9 o = c o = i m + l Hepce, the diameter of the pinhole is fixed by the s i z e of the resolution F exement and by the magnification of t h e system

7 ' _ *e J > :c cm357 uz P A C 5 The fraction of the total neutroas which pass through t h e pinhole i s determined by the solid angle subtended by the pinhole The neutrons a r e attenuated by the device, by the air or other medium, and by the blast shield placed i n front of the image p l a t e (detector plate) Let t h e fraction of the neutrons transmitted by these materials be respecitvely td, h, and ts Then the number Nn of neutrons which pass through t h e pinhole and impinge on the detector plate i s given by Nn = No t d t a t s S Let t h e product td ta ts = t Nn = No t 52 = Not (D)2 4 P Before a device has been tested, the exact s i z e and shape of t h e,, : "source of 14 Mev at the time of burn is not known Hawever, a, the, average number of neutrons per u n i t area at t h e image can be obtained by dividing Nn by Ai the area of the image Ai i s the product of R t h e number of resolution elements times ;the area of one resolution element The number of neutrons per u n i t area i s inversely proportional t o t h e square of the distance between t h e device and t h e image p l a t e and inversely proportional t o the number of resolution elements into which

8 1 ', c*357' \ \ the device has been divided The number of neutrons per resolution element may be found by noting t h a t t h e s o l i d angle subtended by one resolution element i s R = x/bl 2 /hl 2 = ( i/4l) 2 = (D/4p) 2 Hence t h e number of neutrons per resolution element i s Alternatively, N n= R no of neutronscmc! 2 cm resolution element = CJ 2 Not 1 16 L ~ R,,, ' 'NOW i = mo = (m + i) D 1/4= 2 as before ' Therefore, Nn/R 4 4 x N o t (m+l) D 64 x Not (mo) 64 ALL2 4 2 Not (m+l)?d4 n choosing values f o r the above quantities f o r a given s e t of t e s t conditions, it appears t h a t L should be chosen first L should have t h e minimum value consistant with a good probability of recovering t h e image p l a t e intact Fixing L also f i x e s t, since td i s a function of t h e device, ta i s a function of L, and i n d i r e c t l y ts i s a function of L After L has been fixed, one needs t o consider the number of neutrons per u n i t area

9 The number of resolution elements i s inversely proportional t o t h e number of neutrons per u n i t area available for autoradiography N ~ T determines t h e "exposure" R = area of' image/area of resolution element at t h e image = area of object/area of resolution element a t object, s o R f i x e s the s i z e of the resolution element Therefore, the 0 resolution element can be made as small as i s consistant w i t h obtaining a s u f f i c i e n t number of activated atoms per unit are8 t o give the photographic density required f o r the photograph The counting r a t e over background per resolution element f i x e s the l i m i t of detection of the system f o r obtaining an i m g e by counting and plotting The counting rate per resolution element i s determined u l t i m t e l y by the number of neutrons per resolution element NJR = After L has been selected f o r safe recovery and R f o r b e s t photography, " then i t h e s i z e of the resolution element a t t h e image should be selected,, large enough t o give an adequate counting r a t e enough t o go i n t o the counter, O f course i must be small N a 7 o has been fixed by t h e requirement f o r photographjr, so choosing i f i x e s the magnification m, and the pinhole diameter D Since L has been fixed, m fixes p and q, t h e object and image distances from the pinhole SUmmam 1 The requirement f o r the recovery of t h e pinex image plate, i n t a c t f i x e s L the distance between the device and t h e image plate 2 The requirement f o r adequate photographic density f i x e s number of nectrons per unit area at the image the This f i x e s R the t o t a l number of resolution elements seen by t h e photograph and o t h e l i n e a r : dimension of t h e resolution element at the object,

10 cob35'7 7 1 L 71 3 The counting r a t e over background per resolution element fixes i t h e s i z e o f the resolution element a t t h e image, m the magnification, D t h e pinhole diameter, p and q t h e object and image distances from t h e pinhole, and R t h e s o l i d angle subtended at t h e device by the pinhole CCS/do DSTRBUTON: B 3A S: 4A S la Boyrie 2A A Clark 3A 6A 7A 9A DA 8A UA l2a l3a ka 15A Freden Fultz R Grader G Johnson M Knapp W McMaster C Sartain S White Woodward L Wouters file file file *