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CAPTURE RADIATIVE DE NEUTRONS RAPIDES SUR LES NOYAUX Si, Rb, Sr ET Y
F. Rigaud, J. Roturier, J. Irigaray, G. Petit, G. Longo, F. Saporetti
To cite this version:
F. Rigaud, J. Roturier, J. Irigaray, G. Petit, G. Longo, et al.. CAPTURE RADIATIVE DE NEU- TRONS RAPIDES SUR LES NOYAUX Si, Rb, Sr ET Y. Journal de Physique Colloques, 1970, 31 (C2), pp.C2-154-C2-155. �10.1051/jphyscol:1970247�. �jpa-00213805�
C2-154 Session 2B
CAPTURE RADIATIVE DE NEUTRONS RAPIDES SUR LES NOYAUX Si, Rb, Sr ET Y
"* A A A O O
F . R i g a u d , J. R o t u r i e r , J . L . I r i g a r a y , G . Y , P e t i t , G . L o n g o e t F, S a p p r e t t i
C . E . N . B . G . , L e Haut-^Vigneau, 3 3 - G r a d i g n a n ( F r a n c e )
C o m i t a t o N a z i o n a l e E n e r g i a N u c l e a r e , C e n t r o d i C a l c o l o , B o l o g n a ( I t a l i e ^
Les spectres en énergie des raies y issues de la capture radiative de outrons de 14,Ûb MeV par des noyaux de Si, Rb, Sr et Y, ont été mesurés avec un spectromètre à temps de vol. Les résultats ont été interprétés par les mécanismes d'interaction di recte et collective en tenant compte du terme d'interférence. L'accord entre la th¥o
rie et l'expérience semble satisfaisant. —
G a m m a - r a y s p e c t r a f r o m t h e r a d i a t i v e c a p t u r e o f 1 4 . 0 6 M e V n e u t r o n s i n n a t u r a l S i , R b , Sr a n d Y h a v e b e e n i n v e s t i g a t e d b y m e a n s o f a t i m e of f l i g h t s p e c t r o m e t e r . T h e r e s u l t s h a v e b e e n i n t e r p r e t e d u s i n g d i r e c t a n d c o l l e c t i v e c a p t u r e m e c h a n i s m s i n c l u d i n g t h e c o n
t r i b u t i o n o f t h e i n t e r f e r e n c e b e t w e e n t h e m . T h e a g r e e m e n t b e t w e e n t h e o r y a n d e x p e r i m e n t s e e m s s a t i s f a c t o r y .
G a m m a - r a y s p e c t r a f r o m c a p t u r e of f a s t n e u t r o n s w e r e m e a s u r e d u p to 8.3 M e V n e u t r o n e n e r g y [l] f o r F e , N i , P b , B i a n d at 1 4 . 1 M e V e n e r g y [2]
f o r s o m e l i g h t a n d m e d i u m w e i g h t n u c l e i . T h e p r e s e n t w o r k e x t e n d s t h i s k i n d o f m e a s u r e m e n t s t o o t h er n u c l e i a n d c o m p a r e s t h e g a m m a r a y s p e c t r a o b t a i n e d w i t h t h e p r e d i c t i o n s o f d i r e c t a n d c o l l e c t i v e t h e o r i e s . T h e t a r g e t n u c l e i c h o s e n a r e s i H c o n , r u b i d i u m , s t r o n t i u m a n d y t t r i u m .
I n c i d e n t n e u t r o n s a r e p r o d u c e d a t 1 4 . 0 6 M e V b y t h e r e a c t i o n T ( d , n ) H e . G a m m a - r a y s a r e d e t e c t e d w i t h a c y l i n d r i c a l I N a ( T l ) s c i n t i l l a t o r of 5 " x 5 " s i z e , c o u p l e d to a f a s t p h o t o m u l t i p l i e r X P 1 0 4 0 . T h e t i m e o f f l i g h t m e t h o d i s u s e d t o a c h i e v e h i g h p r e c i s i o n in t h e d e t e r m i n a t i o n o f the e n e r g y of i n c i d e n t n e u t r o n s a n d to d i s c r i m i n a t e p h o t o n s f r o m n e u t r o n s s c a t t e r e d b y t h e s a m p i e . P u l s e s f r o m t h e g a m m a - r a y d e t e c t o r a r e r e g i s t e r e d in a m u l t i c h a n n e l a n a l y s e r , w h i c h i s g a t e d b y t h e c o i n c i d e n c e p u l s e s b e t w e e n t h e t i m e - t o - p u l s e - h e i g h t c o n v e r t e r s i g n a l a n d a l i n e a r a l p h a - p a r t i c l e d e t e c t o r s i g n a l . T h e e n e r g y r e s o l u t i o n is -2 M e V in t h e r e g i o n of i n t e r e s t .
T h e m e a s u r e d i n t e g r a t e d c r o s s - s e c t i o n s f o r S i , R b , S r a n d Y a r e 6 4 0 + 1 6 5 u b , 7 7 0 ± 2 3 0 u b , 1 9 7 0 ± 5 8 0 u b a n d 1 0 2 0 ± 2 8 0 y b r e s p e c t i v e l y . T h e e x p e r i m e n t a l g a m m a - r a y s p e c t r u m f r o m t h e r a d i a t i v e c a p t u r e of 1 4 . 0 6 M e V n e u t r o n s i n s i l i c o n i s s h o w n i n t h e f i g u r e . A s c a n b e s e e n , t h e a g r e e m e n t b e t w e e n t h e e x p e r i m e n t a l p o i n t s o f t h e p r e s e n t w o r k ( J ) a n d t h o s e (jjj) o f r e f . [2] i s s a t i s f a c t o r y o v e r t h e w h o l e e n e r g y r a n g e o f i n t e r e s t e x c e p t
n e a r E = 1 5 M e V , w h e r e t h e " b a c k g r o u n d " o f g a m m a - r a y s f r o m i n e l a s t i c n e u t r o n scattering, i n t h e s a m p i e i s p r o b a b l y g r e a t .
C a l c u l a t i o n s h a v e b e e n c a r r i e d o u t i n o r d e r t o v e r i f y w h e t h e r d i r e c t a n d c o l l e c t i v e c a p t u r e m o d e l s c a n r e p r o d u c e b o t h s h a p e a n d m a g n i t u d e of m e a s u r e d g a m m a s p e c t r a . T h e f o r m u l a d e r i v e d i n ref, [3], which includes both direct and semidi rect processes and the interference between them is used. The calculated curve for the Si(n,y) reaction is obtained by using the optical poten tial of ref. [4]; the bound state wave functions are calculated with the parameters of ref. [5] ad justing the potential depth for each bound state to give the single particle binding energies. The excitation energy and the width of the dipole state are taken equal to 20 MeV and to 4.5 MeV respec tively. The strength of the isospin term of the
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1970247
Session 2B
optical potential is put equal to 130 MeV. The level structure of "Si, the angular momentum as signment and the corresponding spactroscopic fa2 tors are taken from ref. [6]. The single particle binding energies are obtained as the centre of gravity of the real nuclear states. For these s i g gle particle states, indicated by arrows in the figure, the cross sections are calculated and dik tributed among real nuclear states by using the spectroscopic factors mentioned.
As can be seen, the agreement between cai culated curve and experimental points is good in the (Ot4) MeV excitation energy range. Here, the positions, spin assignments and spectroscopic fa2 tors of the levels are known from experiment and are taken into account in the calculations. For (418) MeV excitation energy the information on si;
gle particle levels is incomplete and therefore most of the levels listed in ref. [6] are omitted
in the calculations. The capture to these levels makes some contribution to the cross section. How ever, the strength of the transitions to single particle levels is greater in the (04) MeV exci -
tation energy range than for higher energies. It seems, therefore, that even a cross section calcu lation performed with all the real nuclear states, would not be able to reproduce the experimental
spectrum of gamma rays emitted in the 28~i(n,y)
,
action in the (14I18) MeV energy range, taking i; to account only the direct and collective or semi- direct mechanisms. On the other hand, for the highest excitation energies of light nuclei,cap
7
ture through a compound nucleus may be relevant.
For the y-yay spectrum emitted at 14.06 MeV nel tron energy by a light nucleus such as 2 8 ~ i , these considerations lead to the same conclusions given for the spectra emitted at 7.4 MeV by medium or heavy nuclei [7]: the measured spectra seem to be explained by a superposition of the compound nucle us, direct and collective mechanisms.
Work is in progress to investigate the spectra from the radiative capture of 14.06 MeV neutrons in Rb, Sr and Y.
References
[I] I. Bergqvist et al., Phys. Let. 2 (1966) 670 and Nucl. Phys. A (1968) 161.
[2] F. Cvelbar et al., Nucl. Phys. A (1969) 401 and Nucl. Phys. A (1969) 412.
[3] G . Longo and F. Sa~oretti, Nuovo Cimento
(1968) 264.
[4] J. Htrhn et al., Nucl. Phys. A (1969) 289.
[5] K. Bleuler et al., Nuovo Cimento B (1967)
45.
[6] P .M. Endt and C. Van Der Leun, Nucl. Phys. 105
(1967) 157.
[7] G. Longo and F. Sa~oretti, Nucl. Phys.
(1969) 503.
