A passive environmental neutron spectrometer using SSNTD

Nuclear Tracks a n d Radiation Measurements, Vol. 8, NOS. 1-4, pp. 267-270, 1984. Printed in Great Britain.

A PASSIVE

P.F.

0191-278X/84 $3.00 + .00 .° 1984 Pergamon Press Ltd.

ENVIRONMENTAL NEUTRON USING SSNTD

Green*,

S.A. D u r r a n l

SPECTROMETER

and J. W a l k e r

Department of Physics, University of Birmingham, B i r m i n g h a m BI5 2TT, UK. N o w at the Department of Geology, University of M e l b o u r n e , P a r k v l l l e 3052, V i c t o r i a , A u s t r a l i a

ABSTRACT A d e s i g n is p r o p o s e d for a P a s s i v e E n v i r o n m e n t a l N e u t r o n S p e c t r o m e t e r u s i n g SSNTD. Fast n e u t r o n s p r o d u c e (n,~ ) r e a c t i o n s In a 10B c o n v e r t e r , and the a-partlcles d e t e c t e d in L R I I 5 . B y i n t e r p o s i n g an a b s o r b e r of a c e r t a i n t h i c k n e s s b e t w e e n c o n v e r t e r and d e t e c t o r , t h e d e t e c t o r Is e f f e c t i v e l y s h i e l d e d f r o m n e u t r o n s b e l o w a c e r t a i n threshold e n e r g y . B y u s i n g a set of a b s o r b e r s , a set of t h r e s h o l d d e t e c t o r s c a n be c r e a t e d . T h i s d e v i c e is a s s e s s e d by c o m p u t e r m o d e l l i n g , and a l s o e x p e r i m e n t a l l y . P r e l i m i n a r y r e s u l t s s u g g e s t s u c c e s s m a y be p o s s l b l e , a l t h o u g h several problems still remain.

KEYWORDS Fast lOB.

neutron

spectrometry;

LRII5;

(n,~)

reactions;

computer modelling;

absorbers;

INTRODUCTION S o l i d S t a t e N u c l e a r T r a c k D e t e c t o r s (SSNTD) o f f e r c o n s i d e r a b l e a d v a n t a g e s for use in fast n e u t r o n s p e c t r o m e t r y o w i n g to t h e i r s m a l l slze~ l o n g - t e r m s t a b i l i t y and I n s e n s l t l v l t y to y - r a y s . U n t i l now, h o w e v e r , few a t t e m p t s h a v e been m a d e to d e v e l o p a w o r k a b l e s p e c t r o m e t e r b a s e d on S S N T D . M o s t i n v e s t i g a t i o n s of fast n e u t r o n d e t e c t i o n u s i n g S S N T D h a v e been m a d e w l t h d o s l m e t r y in mlnd~ and h a v e r e l i e d on t h e r e g i s t r a t i o n of p a r t i c l e s p r o d u c e d by n e u t r o n - l n d u c e d r e a c t i o n s w i t h i n t h e S S N T D i t s e l f . T h l s w i l l be t e r m e d the " i n t r i n s i c r e s p o n s e " of the d e t e c t o r . T h e p o s s i b i l i t y a l s o e x i s t s to use an S S N T D to d e t e c t p a r t i c l e s p r o d u c e d by r e a c t i o n s in an e x t e r n a l c o n v e r t e r , as an " e x t r i n s i c " t r a c k d e n s i t y . L i t t l e a t t e n t i o n has been paid, u n t i l r e c e n t l y , to t h e l a t t e r , in fast n e u t r o n d e t e c t i o n . In t h i s study, v a r i o u s o p t i o n s w e r e c o n s i d e r e d w h i c h m i g h t l e a d to a s u c c e s s f u l S S N T D - b a s e d n e u t r o n s p e c t r o m e t e r for the e n e r g y r a n g e 0 . I - i 0 MeV. T h e s e are: a) use of C R 3 9 as a p r o t o n r e c o i l s p e c t r o m e t e r ; b) u s e of v a r i o u s e l e m e n t s w i t h (n,~) reactions having d i f f e r e n t t h r e s h o l d e n e r g i e s , t o g e t h e r w l t h an S S N T D s e n s i t i v e to Q - p a r t l c l e s but not p r o t o n s , to g i v e a set of t h r e s h o l d d e t e c t o r s ; c) u s e of a s i n g l e (n, ~ ) r e a c t i o n w i t h p o s i t i v e Q v a l u e , w i t h t h e d e p e n d e n c e of 5p a r t i c l e e n e r g y on n e u t r o n e n e r g y g i v i n g n e u t r o n s p e c t r o m e t r i c i n f o r m a t i o n ; d) use of a s i n g l e (n, ~ ) r e a c t i o n w i t h p o s i t i v e Q v a l u e , t o g e t h e r w i t h a set of a b s o r b e r s of d i f f e r e n t t h i c k n e s s , to p r o v i d e a set of t h r e s h o l d d e t e c t o r s . T h e ~p a r t i c l e e n e r g y i n c r e a s e s w i t h n e u t r o n e n e r g y . B y i n t e r p o s i n g an a b s o r b e r b e t w e e n t h e c o n v e r t e r (In w h i c h t h e (n, ~ ) r e a c t i o n s o c c u r ) and the d e t e c t o r , the l a t t e r c a n be s h i e l d e d f r o m Q - p a r t l c l e s below a c e r t a i n e n e r g y , w h i c h do not emerge from the a b s o r b e r . T h u s t h e d e t e c t o r is r e n d e r e d i n s e n s i t i v e to n e u t r o n s b e l o w a c e r t a i n t h r e s h o l d e n e r g y . B y u s i n g a set of a b s o r b e r s of v a r i o u s t h i c k n e s s e s , a set of t h r e s h o l d d e t e c t o r s m a y t h u s be c o n s t r u c t e d w h i c h w o u l d t h e n p r o v i d e n e u t r o n - s p e c t r u m i n f o r m a t i o n by u n f o l d i n g the r e s p o n s e of e a c h e l e m e n t (i.e. e a c h 267

268

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a b s o r b e r t h i c k n e s s ) in turn. O p t i o n a) a b o v e r e l i e s on t h e I n t r i n s i c S S N T D r e s p o n s e , w h i l e b)-d) r e l y on e x t r i n s i c s o u r c e s of t r a c k s . For v a r i o u s r e a s o n s , of w h i c h l l m l t a t l o n s of s p a c e do not a l l o w d l s c u s s l o n , w o r k has been c o n c e n t r a t e d on d) above. B e c a u s e of the low t r a c k d e n s i t i e s to be e x p e c t e d in p r a c t i c e , and also b e c a u s e of its i n s e n s i t i v i t y to p r o t o n s , L R I I 5 (Kodak, F r a n c e ) w a s s e l e c t e d as the d e t e c t o r . To o b t a i n a t h o r o u g h a s s e s s m e n t of the v a r i o u s o p t i o n s d e t a i l e d above, a c o m p u t e r m o d e l was c o n s t r u c t e d to s i m u l a t e the r e g i s t r a t i o n of ~-partlcles r e s u l t i n g f r o m (n, a ) r e a c t i o n s in an e x t e r n a l c o n v e r t e r , as t h r o u g h - e t c h e d h o l e s in L R I I 5 , w l t h or w i t h o u t a c e r t a i n t h i c k n e s s of a b s o r b e r . V a r i o u s e x p e r i m e n t a l m e a s u r e m e n t s h a v e been m a d e to v e r i f y t h e p r e d i c t i o n s of t h e m o d e l . T h e s e w e r e p e r f o r m e d w i t h n e u t r o n s b e t w e e n 0.57 MeV and 6.1 MeV on t h e D y n a m l t r o n a c c e l e r a t o r at B i r m i n g h a m U n i v e r s i t y R a d i a t i o n C e n t r e , u s i n g p r o t o n s or deuterons on l i t h i u m , t r i t i u m or d e u t e r i u m t a r g e t s .

THE MODEL S i m p l e t h e o r y p r e d i c t s that the t r a c k density in a t h i c k c o n v e r t e r Is g i v e n by: p

=

NO~

~RcosZ @

O(cm -2) obtained from (n,a) reactions

c

,

(I)

w h e r e N z n u m b e r of r e a c t i n g a t o m s per cm3; o = (n,a) cross section (cm2); ~ = number of n e u t r o n s per cm2; ~ is a g e o m e t r i c a l term; R = r a n g e of ~ - p a r t l c l e s in c o n v e r t e r ; @c = c r i t i c a l a n g l e for a - p a r t l c l e s in the d e t e c t o r . T h i s s i m p l e e q u a t i o n r e l i e s on i s o t r o p l c a-emlsslon; a - e n e r g y I s o t r o p l c and i n d e p e n d e n t of n e u t r o n energy; @c c o n s t a n t . In t h e c a s e of fast neutron-induced (n,a ) r e a c t i o n s , n o n e of t h e s e p r o v i s i o n s Is v a l i d . T h e ~ - p a r t l c l e e n e r g y d e p e n d s on n e u t r o n e n e r g y a n d o n t h e a n g l e of e m i s s i o n r e l a t i v e to t h e n e u t r o n i n c i d e n c e d i r e c t i o n . In a d d l t l o n , L R l l 5 does not g i v e t h r o u g h - e t c h e d h o l e s for all ~-particles but for only those within a certain energy window, in which @c will vary. F u r t h e r , the e q u a t i o n d o e s not a l l o w the i n c o r p o r a t i o n of an a b s o r b e r (this c a n e a s i l y be a c c o m m o d a t e d , but o n l y for an a b s o r b e r of s i m i l a r c o m p o s i t i o n to t h e converter). Thus the term ~Rcos2@c has been r e p l a c e d by ~@y, w h e r e e a c h p a r a m e t e r r e p l a c e s its r e s p e c t i v e p r e d e c e s s o r . T h e e x p r e s s i o n for the t r a c k yield, Y = 0/~ , t h e n b e c o m e s : Y

=

No~6y

.

(2)

F o r a c o n v e r t e r in w h i c h a n u m b e r of (n~ ~) r e a c t i o n s occur, t h e t o t a l t r a c k y i e l d is g i v e n by s u m m i n g t h e c o n t r i b u t i o n of e a c h r e a c t i o n . R a n g e - e n e r g y r e l a t i o n s h i p s for a - p a r t l c l e s in c o n v e r t e r , a b s o r b e r and d e t e c t o r are t a b u l a t e d u s i n g the c o m p u t e r p r o g r a m of H e n k e and B e n t o n (1967). F o r a g i v e n n e u t r o n e n e r g y , the m a x i m u m e n e r g y of a ~ a r t l c l e is c a l c u l a t e d , and t h e r a n g e of this a - p a r t l c l e in the converter becomes 6 in e q u a t i o n (2). T h i s d e p t h is t h e n d i v i d e d into d i s c r e t e depth increments A x at d e p t h x. In e a c h Ax, t h e e n e r g y of an a-partlcle p r o d u c e d at an a n g l e ~ to t h e n e u t r o n d i r e c t i o n is c a l c u l a t e d , and its r a n g e found. If t h i s r a n g e is s u f f i c i e n t to get through ~ e c o n v e r t e r , t h e e n e r g y of the p a r t i c l e e m i t t e d f r o m t h e c o n v e r t e r is c a l c u l a t e d . If an a b s o r b e r is p r e s e n t , t h i s p r o c e s s is r e p e a t e d to g l v e t h e e n e r g y of a-partlcle emitted from the absorber. T h e e n e r g y of t h e a - p a r t l c l e r e a c h i n g t h e d e t e c t o r is t h e n t e s t e d as to w h e t h e r it w i l l f o r m a t h r o u g h - e t c h e d h o l e in L R I I 5 , for a s p e c i f i c e t c h i n g c o n d i t i o n , u s i n g i n f o r m a t i o n o b t a i n e d in a p a r a l l e l s t u d y ( D u r r a n l and G r e e n , 1983). T h e a n g l e e is t h e n i n c r e m e n t e d in s t e p s A~ u n t i l d e t e c t i o n as a t r a c k is no l o n g e r p o s s i b l e . W h e n t h i s o c c u r s , t h e p r o b a b i l i t y of t r a c k d e t e c t i o n is i n c r e m e n t e d by the a p p r o p r i a t e s o l i d a n g l e f a c t o r , t h e d e p t h i n c r e m e n t e d and t h e p r o c e s s r e p e a t e d . In t h i s way, t h e t o t a l p r o b a b i l l t ~ , o v e r all d e p t h s , is s u m m e d to g i v e a factor ~ w h i c h is a n a l o g o u s to the ~cosZ0 c t e r m In e q u a t l o n ( 1 ) . T h e y i e l d m a y t h e n be c a l c u l a t e d , u s i n g k n o w n e n e r g y d e p e n d e n c e of ( n , a ) c r o s s s e c t i o n s . In t h i s study, t h i s has been t a k e n f r o m the E N D F - I V n e u t r o n d a t a file. T h e d i s c u s s i o n so far is v a l i d o n l y for n o r m a l l y - l n c l d e n t n e u t r o n s . V a r y i n g d e g r e e s of n e u t r o n t n c l d e n c e c a n be a c c o m m o d a t e d u s i n g m o d i f i c a t i o n s to t h e s i m p l e g e o m e t r y to a l l o w s o l i d a n g l e c a l c u l a t i o n s . B e s i d e s s i m p l e y i e l d s f r o m e q u a t i o n (2), the m o d e l also

A PASSIVE ENVIRONMENTAL NEUTRON SPECTROMETER

predicts

spectra

of

t e s t e d by i n s e r t i n g angle llmltatlons. c a s e s v a l u e s of ~ range spectra were of c o n s i s t e n c y and r e m o v a l h=6 ~m.

TOWARDS

A PASSIVE

~-partlcle

energy

and/or

range

in

the

detector.

269

The

model

was

d a t a for t h e r m a l n e u t r o n s on 1 0 B a n d 6 L I , w i t h no c r i t i c a l H e r e , t h e s i m p l e t h e o r y of e q u a t i o n i) is a p p r o p r i a t e . In b o t h e q u a l t o 0 . 2 5 w e r e f o u n d , as e x p e c t e d . The emitted ~-partlcle rectangular, which also agrees wlth simple theory. For the sake simplicity, all calculations have been made for a LRII5 surface

ENVIRONMENTAL

NEUTRON

SPECTROMETER

T h e Tnodel w a s f i r s t a p p l i e d t o t h e i n t r i n s i c response of L R I I 5 , on t h e a s s u m p t i o n that t h i s is e n t i r e l y d u e to (n, ~ ) r e a c t i o n s in 1 2 C , 1 4 N a n d 160. T h e r e s u l t s are s h o w n in F i g . i as t h e t r a c k y i e l d ( t r a c k s p e r n e u t r o n ) at v a r i o u s n e u t r o n energies. Also shown are experimental measurements of t h e s a m e q u a n t i t y . It s h o u l d be s t r e s s e d t h a t no n o r m a l l s a t l o n has b e e n m a d e . T h e a g r e e m e n t between theory and e~perlment g i v e s us f u r t h e r c o n f i d e n c e in p r e d i c t i o n s of t h e m o d e l . A p p l i c a t i o n of t h e m o d e l to o p t i o n b) a b o v e , u s i n g C, N, O , Be, F a n d A I as r e a c t i n g elements, showed very clearly t h a t t h i s is not a p r a c t i c a l proposition, as t h e I n t r i n s i c contribution in L R I I 5 is of e q u a l or g r e a t e r m a g n i t u d e than the extrinsic yields, in a l l c a s e s . T h l s p o i n t h a s o f t e n b e e n i g n o r e d In t h e p a s t . A s e a r c h for a material wlth sufficiently high cross section and abundance for o p t i o n d) led t o the selection of l O B . A p r e l l n t n a r y d e s i g n , on w h i c h w e h a v e p e r f o r m e d extensive t e s t s , t a k e s t h e f o r m of a s t a c k of t e n 1 - 1 n c h s q u a r e s of a l u m l n f u m , each coated w l t h a i00 ~m l a y e r o~ 1 0 B in a r u b b e r i z e d b i n d e r . T h e b o r o n Is e n r i c h e d to ~ 97%10B,and boron comprises ~ 7 0 % of t h e l a y e r , w h i c h is o t h e r w i s e dominantly carbon. The upper and lower converters (numbered I a n d I0) h a v e no a b s o r b e r (orlg[nally designed to t e s t for n e u t r o n a t t e n u a t i o n and consistency of r e s p o n s e down the stack), while the remaining converter sheets (numbered 2 - 9 ) h a v e 2, 4, 5, 6, 8, I0, 15 a n d 20 ~ m of p o l y c a r b o n a t e plastic absorber between converter and LRII5 detector. For normally-lncldent neutrons, t h e " f r o n t " of t h e d e t e c t o r is converter n u m b e r i, w i t h a b s o r b e r thickness i n c r e a s i n g down the stack. Direct neutrons traverse the c o n v e r t e r b e f o r e the detector, r i g . 2 s h o w s t h e f u l l set of predicted response functions for t h e p r o p o s e d spectrometer assembly, for t h e c a s e of h = 6 ~ m in t h e L R I I 5 d e t e c t o r s , for normally-lncldent neutrons. A l s o s h o w n In F l g . 2 Is t h e i n t r i n s i c y i e l d of L R I I 5 f r o m V l g . 1, w h i c h c a n be s e e n to be m u c h r e d u c e d in c o m p a r i s o n to the extrinsic yields. Each spectrometer package has a b a r e s h e e t of L R I I 5 at e a c h end, t o a l l o w a s s e s s m e n t of t h e i n t r i n s i c contribution. Flg. 3 shows the predicted response to n o r m a l l y - l n c i d e n t neutrons of the converter p l u s 2 ~m a b s o r b e r , together wlth experimental measurements of t h e response. The agreement h e r e is q u i t e g o o d , a n d is s i m i l a r t o t h a t f o u n d for t h e other elements of t h e s p e c t r o m e t e r . Fig. 4 shows predicted yields for the converter p l u s I0 ~ m a b s o r b e r , f o r a n g l e s of n e u t r o n Incidence of 90 ° , 60 ° , 30 ° a n d 0 °. T h e s e a n d s i m i l a r c u r v e s for o t h e r a b s o r b e r t h i c k n e s s e s indicate that for a n g l e s of i n c i d e n c e a b o v e ~ 3 0 ° , t h e f o r m of t h e r e s p o n s e Is not t o o s e n s i t i v e to angle. Thls suggests t h a t t h e d e v i c e m a y be s a t i s f a c t o r y for a situation in w h i c h neutrons are dominantly produced by a c e n t r a l s o u r c e , w i t h o n l y a s m a l l contribution from scattered secondaries, as is i n d e e d t h e c a s e in t h e a p p l i c a t i o n for w h i c h t h l s s p e c t r o m e t e r was designed. As a f i n a l t e s t , a s p e c t r o m e t e r package was exposed to a Pu-Be neutron s o u r c e , at a d i s t a n c e of ~ 2 m to p r o v i d e n o r m a l incidence. The response of e a c h s p e c t r o m e t e r element was predicted using the known energy spectrum of n e u t r o n s from this source. The predictions a r e in b r o a d agreement with the observed yields, although some disagreement is f o u n d In d e t a i l .

PROBLEMS Although these early results are p r o m i s i n g , the spectrometer Is by no m e a n s a practical proposition as yet, o w l n g t o various problems encountered b o t h in t h e above development w o r k a n d In v a r i o u s exposures in t h e d e s i g n e n v i r o n m e n t . A poor d e g r e e of c o n s i s t e n c y was found between measured y i e l d s In e l e m e n t s i a n d i0 ( b a r e converter) at a l l e n e r g i e s s t u d i e d . T h l s w a s not t o o s u r p r i s i n g , b e c a u s e of inconsistencies in t h e b e h a v l o u r of L R I I 5 a l r e a d y n o t i c e d . More surprising, h o w e v e r , w a s a l a r g e e x c e s s of n o n - t h r o u g h - e t c h e d tracks, due to low-energy particles In t h e L R I I 5 a d j a c e n t to e l e m e n t i0, at t h e b o t t o m of t h e s t a c k . T h l s

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P . F . G R E E N , S. A. D U R R A N I and J. W A L K E R

excess amounts to a factor of -'5 in some cases. No e x p l a n a t i o n has been found yet for this. Although the model predictions are generally validated by e x p e r t : a e ; ~ t , lq some cases quite serious departures are evident. Above 4MeV, the dependeuce of

yield upon a b s o r b e r t h i c k n e s s is predicted, by the model, to be c o n s t a n t ; w h e r e a s e x p e r i m e n t a l l y m e a s u r e d yzelds fall r a p i d l y with a b s o r b e r t h i c k n e s s . In addition, the p r e d i c t e d d e p e n d e n c e of yield on n e u t r o n angle of ~ n c t d e n c e is also In m a r k e d d~agreement with e x p e r i m e n t a l m e a s u r e m e n t s above 4MeV~ w h i l e below 4~leV, agreement is quite good. The most likely e x p l a n a t i o n Fur these features is a!iisotropy in the p r o b a b i l i t y of ~ - p a r t i c l e e m i s s i o n from the IOB (n, .~ ) reaction (the above t r e a t m e n t assumes i s o t r o p y ) . L i t t l e data a r e a v a l l a b l e on this, however, to allow r i g o r o u s assess:neut. Such data could, whert available, be easily i n c o r p o r a t e d into the model. $o at present there r e m a i n s e v e r a l problems to be overcome. However, the r e s u l t s of the w o r k so far suggest that success should be p o s s i b l e tn tlme, and result in a fully o p e r a t i o n a l S S N T D - b a s e d P a s s i v e Environmental Neutron Spectrometer.

ACKNOWLE/)CEMENTS

We w o u l d llke to thank the sponsoring authorities for financial support; M. LongdenT h u r g o o d and c o l l e a g u e s for many useful discussions; and the staff of the Birmingham Radiation Centre for neutron irradiations.

REF;]RENCES

Durranl, S.A. and P.F. G r e e n (1983). The effect of e t c h i n g c o n d i t z o n s on the r e s p o n s e of LRII5. (This c o n f e r e n c e . ) Henke, R.P. and E.V. B e n t o n (1967). U S N R D L - T R - 6 7 - 1 2 2 ~ San Francisco, California. Fig. 1 (far left) Intrinsic response of LRII5 (h = 6 Nm). (removed-layer thickness h : 6 ~m ).

1 0 -5

io -e g •

c

~

i- I ° - I

10 -e

i]~i. 5 ~= ~t

1 0 -7

~o B ,

~

i

)

, 3

4

5 6 7 8 9 Neutron Energy (MeV)

, 0.5

I 1.0

Neutron

i 5.0

Fig. 2 (]eft) Modelled response functions of ]OB converter plus various thicknesses of absorber (shown in ~m).

I 10.0

10 -s

Energy (MeV) Z

10-5 --

,,r

.=

Fig. 3 (left) Modelled response of lOB converter plus 2 ~m absorber, with experimental measurements (thickness removed by etching, h = 6 ~m).

~- 10 -e

l .90

°

lO

16

0.5

1.0 ' 2.0 5 i0 1 o .0 Neutron Energy (MeV)

-7

.......2:

Fig. 4 (right) Modelled response of 10B converter plus i0 Nm absorber for four angles of neutron incidence.

..... o °

I 10 Neutron

Energy

(MeV)