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K F K | - < o 3 - 5 J f

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2 , 6 MeV NEUTRON INDUCED REACTIONS IN C s £ I . B o r b é l y , T . D o l i n s z k y , G . H r e h u s s , S . Z s o l d o s

C e n t r a l R e s e a r c h I n s t i t u t e f o r P h y s i c s B u d a p e s t

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mi

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Government or any agency thereof.

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DISCLAIMER

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о

Издательская группа

Центрального института физических исследований Венгерской Академии Наук

Будапешт Н А, п/я А9

Техн. редактор: Э. йестер Тираж: 200 э к з . I6/IX 1963

P u b l i s h e d by t h e

P u D l i s n i n g Group o f t h e C e n t r a l R e s e a r c h I n s t i t u t e f o r P h y s i c s o f t h e

H u n g a ria n Academy o f S c i e n c e s Вu dap e s t 1 1 4 , РОВ 4 9 •

T e c h n . E d i t o r : E . M e s s t e r ' N o . o f c o p i e s : 2 0 0 .

l ü » űOG ü • I 9 Оp .

f N

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2 , 6 MeV NEUTRON INDUCED REACTIONS IN C s I I . B o r b é l y , T . D o l i n s z k y , G . H r e h u s s , S . Z s o l d o s

C e n t r e d R e s e a r c h I n s t i t u t e f o r P h y s i c s B u d a p e s t

Summary

The c r o s s s e c t i o n s f o r 2 , 6 MeV n e u t r o n i n d u c e d r e a c t i o n s i n C s J h a v e b e e n i n v e s t i g a t e d . U p p e r l i m i t s o f 0 , 1 mb w e re f o u n d f o r t h e / п , р / r e a c t i o n s i n b o t h o f t h e c o m p o n e n t n u c l e i a n d 6 , 3 mb a n d 2 8 , 3 mb f o r t h e C s - 1 3 3 / n , r /C s - 1 3 4 a n d _ I - 1 2 7 / n, r / 1 - 1 2 8 r e a c t i o n s , r e s p e c t i v e l y . The h a l f l i f e o f t h e .137 keV / 8“ / i s o m e r i c s t a t e o f C s - 1 3 4 a n d t h a t f o r t h e g r o u n d s t a t e /З- a c t i v i t y o f I —128 w e r e m e a s u r e d t o b e 2 , 7 5 h o u r s a n d 2 4 , 6 m i n u t e s , r e s p e c t i v e l y .

I n t h e c a s e o f n u c l e i b e i n g i n t h e n e i g h b o u r h o o d o f C s - 1 3 3 and 1 -1 2 7 r e g a r d i n g t h e i r m ass n u m b e r , t h e l e v e l d e n s i t y a t e x c i t a t i o n s t o s e v e r a l MeV-s above t h e n u c l e o n b i n d i n g e n e r g y i s s o h i g h , t h a t t h e a p p l i c a b i l i t y o f t h e s t a t i s t i c a l t h e o r y f o r t h e n u c l e a r r e a c t i o n s may b e e x p e c t e d . A c c o r d i n g l y , i t i s p o s s i b l e t o c a l c u l a t e t h e c r o s s s e c t i o n s o f t h e d i f f e r e n t r e a c t i o n s e . g . / п , р / , / п , с ( . / , / n , 2n / e t c . a n d t h e c o m p a r i s o n o f t h e c a l c u l a t e d a n d m e a s u r e d v a l u e s g i v e s i m m e d i a t e i n f o r m a t i o n a b o u t t h e v a l i d i t y o f t h e a s s u m p t i o n s m ade.

We know s e v e r a l m e a s u r e m e n t s , p u b l i s h e d i n t h e p a s t f i v e y e a r s w h ic h s u g g e s t , t h a t t h e v a l i d i t y o f t h e s t a t i s t i c a l t h e o r y b r e a k s down f o r n e u c r o n i n d u c e d r e a c t i o n s a t 1 4 MeV b o m b a r d i n g e n e r g y f o r t h e l i g h t e r n u c l e i o f A « 6 0 [ 1] , ( 2 ] . I t h a s b e e n f o u n d t h a t t h e r e a c t ..on c r o s s s e c t i o n s a r e much g r e a t e r t h a n e x p e c t e d a n d t h a t t h e d e v i a t i o n s a r e d u e b o t h t o t h e lo w and t h e h i g h e n e r g y p a r t o f t h e e n e r g y s p e c t r u m . I n some c a s e s t h e a n g u l a r d i s t r i b u t i o n o f t h e e m i s s i o n c o u l d be m e a s u r e d , a n d t h e h i g h e n e r g y p a r t p r o v e d t o be s t r o n g l y d i r e c t e d f o r w a r d s , c o n s e q u e n t l y a t t r i b u t e d t o d i r e c t p r o c e s s e s . A d i r e c t p r o c e s s f a v o u r s i n e m i s s i o n t h e h i g h e r e n e r g y t h e r e f o r e e v e n i n c a s e s when no a n g u l a r d i s t r i b u t i o n was m e a s u r e d , t h e p e a k s a t t h e t o p o f t h e e v a p o r a t i o n s p e c t r u m u s u a l l y w e r e b e l i e v e d t o r e p r e s e n t d i r e c t c o n t r i b u t i o n s . I n some p a p e r s [ 8 ] , [9 ] c e r t a i n h i g h e n e r g y p e a k s w e r e a l s o a t t r i b u t e d t o p r o t o n s p r o d u c e d b y d i r e c t p r o c e s s . I n a c a r e f u l r e p e t i t i o n o f t h e e x p e r i m e n t t h e p e a k s i n q u e s t i o n a c t u a l l y d i s a p p e a r e d : D e t a i l s a r e t o b e f o u n d i n [ 4 ] . On t h e o t h e r h a n d t h e lo w e n e r g y d i s c r e p a n c i e s s e e m e d t o b e s u c c e s s f u l l y e x p l a i n e d b y A l l a n [ 3 ] a s c o n t r i b u t i o n s from t h e / n , n p /

KFKI 1 4 6 7 / f

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p r o c e s s . However t h e r e a r e s e v e r a l c o n t r a d i c t i o n s r e g a r d i n g t h i s p o i n t among w h ic h t h e s t r o n g e s t i s t h e m e a s u r e m e n t o f Z n - 6 4 / п , р / C u - 6 4 w here t h e b o m b a r d in g e n e r g y was c h o s e n j u s t b e lo w t h e t h r e s h o l d o f t h e / n , n p / p r o c e s s [ 5 ] . The num b er o f s i m i l a r m e a s u r e m e n ts i s , h o w e v e r , p o o r a n d v e r y much i s s t i l l t o be done i n t h i s f i e l d . One may c o n c l u d e t h a t t h e p e n e t r a b i l i t y o f t h e Coulomb b a r r i e r i s somewhat h i g h e r [ 6 ] , t h e n i t was t h o u g h t b e f o r e . At a n y r a t e t h e s u c c e s s o f t h e s t a t i s t i c a l model i n e x p l a i n i n g t h e m ain

f e a t u r e s o f t h e / n , n ’ / a n d / n , 2 n / r e a c t i o n s w h ere no Coulomb b a r r i e r

p r o b le m s a r i s e seems t o g i v e s t r o n g s u p p o r t t o t h e h y p o t h e s i s o f t h e l o w e r e d Coulomb b a r r i e r .

We were t o i n v e s t i g a t e t h i s p r o b le m . F o r i t we u s e d b o m b a r d in g n e u t r o n e n e r g y 2 , 6 MeV t o a v o i d any t w o - p a r t i c l e e m i s s i o n a n d t a r g e t n u c l e i o f a b o u t 1 3 О m ass number i n o r d e r t o s a t i s f y t h e a s s u m p t i o n s o f t h e

s t a t i s t i c a l t h e o r y . S i n c e we e x p e c t e d v e r y low v a l u e s f o r t h e c r o s s s e c t i o n s , g r e a t number o f t h e t a r g e t n u c l e i h a d t o b e u s e d . A 1 1 / 2 " x 2 "

C s J / T l / s c i n t i l l a t i o n c r y s t a l was i r r a d i a t e d w i t h 2 , 6 MeV n e u t r o n s o f t h e d - d r e a c t i o n a t 400 keV b o m b a r d in g d e u t e r o n e n e r g y . A f t e r t h r e e d a y s o f c o n t i n u o u s a c t i v a t i o n w h i l e t h e v a r i a t i o n s i n t h e n e u t r o n f l u x was c a r e f u l l y c o n t r o l l e d , t h e a c t i v i t y o f t h e c r y s t a l l was m e a s u r e d b y i t s e l f i n a m e r c u r y s h i e l d o f 5 cm a n d a n i r o n b o x o f 20 cm w a l l t h i c k n e s s . The p u l s e s o f t h e s c i n t i l l a t i o n c o u n t e r were r e c o r d e d by a m u l t i c h a n n e l a n a l y z e r , so t h a t t h e p u l s e s due t o a s p e c i f i c d e c a y c o u l d e a s i l y be c o l l e c t e d . The

m eth o d a p p e a r e d t o be v e r y u s e f u l e . g . i n m e a s u r i n g t h e h a l f l i f e o f C s - 1 3 4 m i n t h e p r e s e n c e o f h i g h 1 - 1 2 8 a c t i v i t y ; t h e p r e v i o u s one d e c a y s w i t h a n 0 ,1 3 7 i n t e r n a l c o n v e r s i o n w n i l e t h e l a t t e r i s ß e m i t t i n g w i t h a s p e c t r u m e x t e n d e d t o a b o u t 2 MeV. The n e u t r o n f l u x m e a s u r e m e n t was made w i t h p l a s t i c s c i n t i l l a t o r a n d we c o u l d n o t a c h i e v e a n e r r o r i n f l u x m e a s u r e m en t l e s s t h a n 20 %. T h i s i s t h e d e t e r m i n i n g e r r o r i n o u r c r o s s s e c t i o n v a l u e s . The b a c k g r o u n d i n t h e c r y s t a l l was m e a s u r e d a l s o i n t h a t b o x b e f o r e t h e a c t i v a t i o n . I n o u r p r e v i o u s m e a s u r e m e n t c a r r i e d o u t t h r o u g h a p u l s e s h a p e d i s c r i m i n a t i o n m e t h o d t h e c r o s s s e c t i o n ⁶ / n , o t / was f o u n d t o b e c o m p l e t e l y n e g l i g i b l e a s c o m p e r e d t o S' / n , p / + . T h e r e f o r e t h e p r o c e s s e s w h ic h c a n p l a y a r o l e i n t h i s m e a s u r e m e n t a r e a s f o l l o w s :

+ 'u n p u b lis h e d KFKI 1 4 6 7 / f

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- 3 ^-

КЖ1 1 4 6 7 / f

(8)

KFKI 3A67/Í

C s - 1 3 5 /n , ? / l e^{- 13331} C s -1 3 3 /n ,p /I d -1 3 3 I - 1 2 7 / n ,p /T e - 1 2 7 m I - 1 2 7 / n , p /Т е - 1 2 7 C s - 1 3 3 /n ,y /C s - 1 3 4 m I - 1 2 7 / n , - j 1/ ! - 1 2 8

T h r e s h o l d e n e r g y

- 0 , 3 5 - 0 ,3 5 - 0 , 0 9 - 0 , 0 9 - 6 , 7 3 - 6 , 5 9

A c t i v i t y o f r e s i d u a l

n u c l e u s 0 , 2 3 3 MeV

0,345

0 ,0 8 3 0 , 6 9 5 0 , 1 3 7 2 , 1 2

Ts o f r e s i d u a l n u c le u s C ro ss s e c t i o n

l i t e r a r y p r e s e n t l i t e r a r y p r e s e n t c a l c u - d a t a m e a s u r e m e n t d a t e m ea s u re m e n t l a t e d

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+ 0 , 0 0 5 h 2 , 7 5 + 0 , 1 - 6 , 3 + 1 , 2

mb 2 4 , 9 9 min 2 4 , 6 5 + 0 , 1 2 27 mb 2 8 ,3 + 6 mb

min

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- 5 ^-

I n Gaia t a b l e we g i v e a l l t h e d e c a y c o n s t a n t s a n d c r o s s s e c t i o n s g i v e n i n t h e l i t e r a t u r e a s co m p a red t o t h e d a t a we m e a s u r e d . I t seem s t h a t w n i l e i n t h e c a s e o f 1 - 1 2 7 / n , ^g- / 1 - 1 2 8 t h e r e i s a good a g r e e m e n t w i t h t h e c r o s s s e c t i o n m e a s u r e d by M e s c h e r y a k o v [ 7 j u n d a l s o t h e h a l f l i v e s we o b t a i n e d a r e g o o d , t h e c r o s s s e c t i o n f o r t h e / п , р / p r o c e s s e s a r e b e lo w t h e s e n s i t i v i t y o f o u r m e a s u r e m e n t . Anyway t h e r e s u l t s o b t a i n e d do n o t seem t o be i n c o n t r a d i c t i o n t o t h e s t a t i s t i c a l t h e o r y a n d i t i s p r o b a b l e t h a t t h e r e i s no d i r e c t p r o c e s s o f c o n s i d e r a b l e a m o u n t.

R e f e r e n c e s

[1 ] C o l l i , F a c c h i n i e t a l . Nuovo C im e n to 1 3 , 7 3 0 / 1 9 5 9 / [ 2 ] R .A .P e e k , P h y s .- ^ e v . 1 2 3 , 1738 / 1 9 6 1 /

[ 3 ] I ) . I . . A l l a n : N u c l . P h y s . 2 4 , 274 / 1 9 6 1 /

[ 4 ] B'.a e l d i n M .S a y e d , Hi-e h u s s G y u l a : KFKI K ö z i . 1 0 , 204 / 1 9 6 2 / [ 5 ] A .A rm s tro n g and L . R o s e n : N u c l . P h y s . 1 3 , 40 / I 9 6 0 /

[ó ] K.Kikuchy, P r o g r . T h e o r . P h y s . ! £ , 6 4 3 / 1 9 5 7 / [7 ] Hughes, H a r v e y : N e u t r o n C r o s s e l e c t i o n s / 1 9 5 5 / [8] W .R.D ixon, N u c l e a r P h y s i c s 42, 27 / 1 9 6 3 /

[9 ] M a r c a z z a n - M e n i c h e l l a S a e t t a - T o n o l i n i , Nuovo C im e n to 2 0 , 9 0 4 /1 9 6 1 / [10] i U a t t - W e i s s k o p f : T h e o r e t i c a l N u c l e a r P h y s i c s

[11] Dzelepow—Peker: D ecay Schemes o f R a d i o a c t i v N u c l e i /1998/

K FK I 1 4 6 7 / f

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