measurement of target thickness by a nuclear track technique

Nucl. Tracks Radiat. Meas., Vol. 19, Nos 1--4, pp. 597-598, 1991 Int. J. Radiat. AppL lnstvum., Part D Printed in Great Britain

0735-245X/91 $3.00 + .00 Pe~mon Press plc

NEASURKHENT OF TARGET TNICENKSS BY A NUCLEAR TRACK TECHNIQUE

K . K . D w i v e d i , S .Ghosh and J . R a j u

D e p a r t m e n t o f C h e m i s t r y , N o r t h - E a s t e r n H i l l U n i v e r s i t y , S h i l l o n g - 793 003, I n d i a .

ABSTRACT

A new a p p l i c a t i o n o f t h e n u c l e a r t r a c k t e c h n i q u e f o r t h e m e a s u r e m e n t o f t a r g e t t h i c k n e s s i s p r e s e n t e d h e r e . A w e l l c o l l i m a t e d b e a m o f h e a v y i o n s i s a l l o w e d t o p e n e t r a t e t h r o u g h t h i n m e t a l t a r g e t s and t h e e n e r g i e s o f t h e h e a v y i o n s e m e r g i n g o u t of t h e s e t a r g e t s a r e o b t a i n e d f rom t h e maximum e t c h a b l e t r a c k l e n g t h s i n a s u i t a b l e t r a c k d e t e c t o r u s i n g e n e r g y - t r a c k l e n g t h c a l i b r a t i o n c u r v e . T a r g e t t h i c k n e s s i s t h e n d e t e r m i n e d f rom r a n g e - e n e r g y c a l i b r a t i o n p l o t s . I n t h i s p a p e r we d i s c u s s t h i s t r a c k t e c h n i q u e and h i g h l i g h t i t s s c o p e .

KETWORDS

T a r g e t t h i c k n e s s , r a n g e - e n e r g y c a l i b r a t i o n , h e a v y i o n s , t r a c k t e c h n i q u e .

INTRODUCTION

T a r g e t t h i c k n e s s m e a s u r e m e n t s a r e o f p a r a m o u n t i m p o r t a n c e i n n u c l e a r p h y s i c s t o d e t e r m i n e t h e c r o s s - s e c t i o n o f n u c l e a r r e a c t i o n s , t h e t h i c k n e s s o f b a c k i n g s a n d windows o r t h e e n e r g y l o s t by i o n s w h i l e p a s s i n g t h r o u g h d e g r a d e r f o i l s . I n o r d e r t o e v a l u a t e t h e s e p a r a m e t e r s c o r r e c t l y i t i s n e c e s s a r y t o d e t e r m i n e t h e t a r g e t t h i c k n e s s as a c c u r a t e l y a s p o s s i b l e . D i f f e r e n t m e t h o d s ( K a t o , 1969) which h a v e been u s e d t o d e t e r m i n e t h e t a r g e t t h i c k n e s s s u f f e r f rom one d r awback o r t h e o t h e r - l i k e r e p r o d u c i b i l i t y of t h e known c r o s s - s e c t i o n and d e s t r u c t i o n of t a r g e t s i n t h i c k n e s s m e a s u r e m e n t s . A l s o a h i g h f l u x o f a - p a r t i c l e s c an c h a n g e t h e t a r g e t p r o p e r t i e s by r a d i a t i o n damage and t h e r m a l h e a t i n g . I t i s t h e r e f o r e n e c e s s a r y t o e v o l v e a t e c h n i q u e w h i c h i s a c c u r a t e and i s a l s o n o n - d e s t r u c t i v e ( X u G u o j i e t a l . , 1 9 8 5 ) . B e a r i n g t h e s e i n m i n d , we p r e s e n t h e r e , a new a p p l i c a t i o n o f t h e n u c l e a r t r a c k t e c h n i q u e t o m e a s u r e t a r g e t t h i c k n e s s u s i n g a h e a v y i on beam.

THE TRACK TECHNIQUE

N e a s u r e m e n t of t a r g e t t h i c k n e s s by t h e p r o p o s e d n u c l e a r t r a c k t e c h n i q u e i n v o l v e s two s i m p l e s t e p s .

S t e p 1 - The E n e r g y - T r a c k L e n g t h C a l i b r a t i o n of t h e D e t e c t o r

F i r s t of a l l a s e n s i t i v e t r a c k d e t e c t o r i s c a l i b r a t e d i n t e r m s of maximum e t c h a b l e t r a c k l e n g t h (L) and e n e r g y (E) o f t h e h e a v y i o n s . k t h i r d o r d e r p o l y n o m i a l may be u s e d t o f i t t h e t r a c k l e n g t h d a t a and i s e x p r e s s e d by

n~3 E(MeV/u) ~ ~ anLn(~m) (I)

S t e p 2- The R a n q e - E n e r a y C a l i b r a t i o n of T a r g e t E lement

T h i s c a l i b r a t i o n can be o b t a i n e d f rom t h e a v a i l a b l e e x p e r i m e n t a l d a t a on r a n g e and e n e r g y o f a h e a v y i o n i n t h e t a r g e t m a t e r i a l . A g a i n a t h i r d o r d e r p o l y n o m i a l p r o d u c e s t h e b e s t f i t f o r t h e r a n g e - e n e r g y d a t a . I t may be e x p r e s s e d as

597

598 K . K . DWlVEDI et al.

m=3 R(gm) = ~ = bmEm (MeV/u) (2)

where b m r e p r e s e n t s a s e t of c o e f f i c i e n t s .

I f t h e i n i t i a l e n e r g y of a h e a v y i o n be E i t h e n t h e t r a n s m i t t e d e n e r g y , E x, of t h e h e a v y i o n a f t e r p a s s i n g t h r o u g h a t a r g e t t h i c k n e s s ' x ' may be o b t a i n e d u s i n g e q . ( 1 ) . From e q . ( 2 ) one g e t s t h e v a l u e s of r a n g e R f o r a c o r r e s p o n d i n g v a l u e of e n e r g y E of t h e h e a v y i o n s . The t a r g e t t h i c k n e s s may t h e n be e a s i l y o b t a i n e d f rom

x = R(E i) - R(E X) (3)

where R(E i) and R(E x) are the ranges of heavy ions corresponding to the initial E i and transmitted energy E x.

EXPERIMENTAL REQUIREMENTS

T a r g e t s of v a r y i n g t h i c k n e s s may be s e l e c t e d and t h e n mounted on a s u i t a b l e t r a c k d e t e c t o r . The t a r g e t - d e t e c t o r a s s e m b l y may t h e n he i r r a d i a t e d by a w e l l c o l l i m a t e d low f l u x beam of h e a v y i o n s a t a s u i t a b l e a n g l e ( e g . 4 5 ° ) . Two s e t s of i r r a d i a t i o n s may t h e n be p e r f o r m e d - o n e w i t h and t h e o t h e r w i t h o u t t a r g e t s . The two s e t s o f d e t e c t o r s may t h e n b e c h e m i c a l l y e t c h e d t i l l t h e t r a c k t i p s a r e r o u n d e d . M e a s u r e m e n t s may be done u s i n g an o p t i c a l m i c r o s c o p e and t h e maximum e t c h a b l e t r a c k l e n g t h s may be o b t a i n e d u s i n g e q u a t i o n g i v e n by Dwived i and M u k h e r j i ( 1 9 7 9 ) .

DISCUSSION

The t r a c k t e c h n i q u e f o r m e a s u r e m e n t o f t a r g e t t h i c k n e s s r e q u i r e s two t y p e s o f c a l i b r a t i o n s (a) Between h e a v y i o n e n e r g y and t h e maximum e t c h a b l e t r a c k l e n g t h i n a s e n s i t i v e t a r c k d e t e c t o r and (b) b e t w e e n r a n g e e n e r g y o f s ame h e a v y i o n i n a r e q u i r e d t a r g e t m a t e r i a l . F r o m t h e s e c a l i b r a t i o n s t h e b e s t s e t of c o e f f i c i e n t s a r e e v a l u a t e d by u s i n g s u i t a b l e p o l y n o m i a l f u n c t i o n s . The t h i c k n e s s o f a t a r g e t may s i m p l y be o b t a i n e d by ( i ) m e a s u r i n g maximum e t c h a b l e t r a c k l e n g t h of t h e t r a n s m i t t e d i o n s i n t h e t r a c k d e t e c t o r , ( i i ) d e t e r m i n i n g i o n e n e r g y f rom eq . ( 1 ) , ( i i i ) f i n d i n g r a n g e s of h e a v y i o n s i n t a r g e t m a t e r i a l f rom eq . (2) and f i n a l l y ( i v ) f rom e q ( 3 ) , t h e t a r g e t t h i c k n e s s may he d e r i v e d . Thus i n t h e p r o p o s e d t r a c k t e c h n i q u e w i t h t h e h e l p of r eadymade c a l i b r a t i o n c u r v e s one can d e t e r m i n e t h e t h i c k n e s s of t h e t a r g e t s by s i m p l y m e a s u r i n g t h e maximum e t c h a b l e t r a c k l e n g t h s o f h e a v y i o n s a f t e r p a s s i n g t h r o u g h t h e t a r g e t s . The p r o p o s e d t r a c k t e c h n i q u e h a s b e e n e m p l o y e d t o m e a s u r e t h i c k n e s s s e v e r a l A l - t a r g e t s u s i n g 238U i o n s and Z n P - g l a s s d e t e c t o r s . The d e t a i l s of e x p e r i m e n t and r e s u l t s a r e n o t p r e s e n t e d h e r e due to page l i m i t a t i o n s and a r e b e i n g p u b l i s h e d s e p e r a t e l y .

CONCLUSION

I t may be c o n c l u d e d t h a t t h i s t r a c k t e c h n i q u e i s s i m p l e , n o n - d e s t r u c t i v e and c a p a b l e o f y i e l d i n g f a i r l y a c c u r a t e r e s u l t s . A low d o s e of h e a v y i o n s i s r e q u i r e d compared t o a h i g h f l u x of = - p a r t i c l e s w i t h t h e i r a t t e n d a n t d r a w b a c k s l i k e r a d i a t i o n damage, t h e r m a l h e a t i n g t o name a f e w . M o r e o v e r , h e a v y i o n s b e i n g s u p e r i o r p r o j e c t i l e s p r o d u c e v e r y l i t t l e s t r a g g l i n g and n e g l i g i b l e l a r g e a n g l e s c a t t e r i n g . I t i s e n v i s a g e d t h a t t h o s e c e n t r e s w h e r e h e a v y i o n b e a m s a r e r o u t i n e l y a v a i l a b l e , n u c l e a r t r a c k t e c h n i q u e may p r o v e t o be q u i t e c o n v e n i e n t and p r e f e r a b l e o v e r ~ - p a r t i c l e s f o r m e a s u r i n g t a r g e t t h i c k n e s s .

REFERENCES

Dwlvedi. K.K and S. Mukherji (1979). Track lengths of heavy ions in solid dielectric track detector. Nucl.Instrum. Meth., 161, 317-326.

Kato. S (1969). Methods of Target thicness measurements. Nucl. Instrum. Meth., 75, 293-296.

Xu Guoj i , Zhao Yuhua, Zheng Shengnan and Xu Yongcbeng (1985). Determinat ion of thicnesses and impurities of targets by measuring back-scattered particles from 2 MeV 4He +- bombardment. Nucl. Instrum. Meth., A236, 627-629.