ANALYZING POWER OF NEUTRON CONTINUUM SPECTRA FOR THE 93Nb([MATH], nX) REACTION AT 65 MeV

(1)

HAL Id: jpa-00224069

https://hal.archives-ouvertes.fr/jpa-00224069

Submitted on 1 Jan 1984

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

ANALYZING POWER OF NEUTRON CONTINUUM SPECTRA FOR THE 93Nb([MATH], nX) REACTION

AT 65 MeV

H. Sakai, K. Hatanaka, N. Matsuoka, T. Motobayashi, T . Saito, T. Shibata, A. Shimizu

To cite this version:

H. Sakai, K. Hatanaka, N. Matsuoka, T. Motobayashi, T . Saito, et al.. ANALYZING POWER OF NEUTRON CONTINUUM SPECTRA FOR THE 93Nb([MATH], nX) REACTION AT 65 MeV.

Journal de Physique Colloques, 1984, 45 (C4), pp.C4-31-C4-35. �10.1051/jphyscol:1984404�. �jpa- 00224069�

(2)

JOURNAL DE PHYSIQUE

CoJioque C4, supplément au n°3, Tome 45, mars 1984 page C4-31

ANALYZING POWER OF NEUTRON CONTINUUM SPECTRA FOR THE Nb(p,nX) REACTION A T 65 MeV

H. Sakai, K. Hatanaka, N. Matsuoka, T. Motobayashi* , T. Saito, T. Shibata*and A. Shimizu

Research Center for Nuelear Physics, Osaka University, 10-1, Mihogaoka, Ibaraki, Osaka 567, Japan

*Department of Physics, Osaka University, Toyonaka, Osaka 560, Japan RESUME - Le pouvoir d'analyse des spectres continus de neutrons dans la réaction

Nb(p*,nX) à 65 MeV a été mesuré dans une grande plage angulaire 20° < 8n <g 130°.

Les pouvoirs d'analyse sont grands et positifs aux grands angles (8n > 40°). Les ca- ractéristiques des pouvoirs d'analyse des réactions 93fjb(p, nX) et ^"Nb(p*, p'x) sont très similaires, ce qui indique l'importance d'un mécanisme de réaction commun à ces deux réactions. Les résultats d'un calcul DWBA simple et à une étape sont com- parés aux données.

ABSTRACT_Analyzinq power of neutron continuum spectra for the 9%b(p,nX) reaction at 65 MeV has been measured for the wide angular range 6n = 20° -130°. The analyzing powers are positive and large at the large angles 9n> 4 0 ° . The characteristic features in the analyzing power for the 93Nb(p,nX) reaction are very similar to those for the 93fft>(l3,p'x) reaction indicating the importance of a common reaction mechanism for both reactions. A simple single step DWBA calculation was performed

to compare with the data.

1. INTRODUCTION

A typical emitted particle spectrum induced by medium energy protons contains structureless continuum region in between the low energy evaporation peak and the higher energy discrete peaks which has a strong forward peaking angular distribution. A lot of theoretical as well as experimental studies have been devoted to reveal the origin of this continuum spectrum. However because of the structureless nature of the continuum spectra our understanding is still in a preliminary stage.

Recently we have shown that the analyzing power (Ay) data of the continuum spectrum are very useful to investigate the reaction mechanism/1-3/. From the theoretical view point the nucleon emission process such as in the (p,p'X) or

(p,nx) reactions is rather simple to describe yet up to now no neutron analyzing power data for the !p,nX) reaction has been published. Here we report, for the first time, the analyzing power of the continuum for the 93fjb(prnx) reaction at 65 Mev. It is very interesting to compare the neutron analyzing power data with the proton analyzing power data for the (P*,p'X) reaction, since the difference in analyzing power is solely due to the difference in the reaction mechanism of the

(p,nX) and (p.p'X) reactions if we neglect a small difference in the exit channel optical potential.

2. EXPERIMENTS

The 65 MeV polarized proton beam was provided by the RCNP AVF cycltoron.

Neutrons were detected by using two 5"*x 3" liquid scintillation (NE 213) counters.

Neutron energy was determined by the Time of Flight (TOF) method. The flight path was aobut 4m. The 9-%b target was a self-supporting metalic foil with thickness

43.6 mg/cm2. The use of a thick target compensates for the inefficiency of the neutron detector by yielding reasonable counting rates with a little sacrifice of an energy resolution. The background neutron spectra were obtained by inserting Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984404

(3)

C4-32 JOURNAL DE PHYSIQUE

t h e 60 cm l o n g i r o n shadow bar i n between t h e t a r g e t and t h e d e t e c t o r f o r each measured angle. The time r e s o l u t i o n ( F h m ) of t h e beam b u r s t was t y p i c a l l y 1.0 -

1 . 5 ns. For t h i s time r e s o l u t i o n t h e energy r e s o l u t i o n s of t h e n e u t r o n s were a b u t 4 and 2 NeV f o r 50 and 30 MeV n e u t r o n s , r e s p e c t i v e l y . The c a l c u l a t e d e f f i - c i e n c y f o r t h e neutron d e t e c t o r was used. The neutron a t t e n u a t i o n due t o t h e m a t e r i a l s between t h e t a r g e t and t h e d e t e c t o r such a s t h e w a l l of t h e s c a t t e r i n g chamber o r t h e a i r volume was c o r r e c t e d f o r . The e r r o r s i n t h e a b s o l u t e niagnitucies of t h e c r o s s s e c t i o n s due t o u n c e r t a i n t i e s i n t h e neutron d e t e c t i o n e f f i c i e n c y c a i - c u l a t i o n a r e estimated t o be a b u t 10%. The bean, p o l a r i z a t i o n was measured c o n t i - nuously with a 12c-polarimeter placed down stream of t h e main s c a t t e r i n g chamber.

3 . RESULTS

F i g u r e 1 shows t y p i c a l energy s p e c t r a and t h e i r anaiyzlng powers. F i g u r e s 2 show t h e angular distribution of t h e c r o s s s e c t i o n s and t h e analyzing powers f o r t h e e x c i t a t i o n energy b i n s of 12-16 MeV. Open symbols a r e f o r t h e 93~b($,nx) r e a c t i o n and c l o s e d symbols a r e f o r t h e 9 3 N b [ 3 , p q ~ ) r e a c t i o n taken from r e f e r e n c e 2. The observed (p,nX) and (p,plX) energy s p e c t r a show t h e same shapes and angular dependence i n a wide continuum r e g i o n above t h e e v a p o r a t i o n peaks. A s f a r a s t h e analyzing power i s concerned t h e g r o s s f e a t u r e s such a s p o s i t i v e analyzing power v a l u e s , s m a l l magnitude i n t h e forward a n g l e s and r a t h e r l a r g e magnitude i n t h e backward a n g l e s a r e , rouqhly speaking, s i m i l a r t o t h o s e of t h e proton r e s u l t s . This c l e a r l y i n d i c a t e s t h a t t h e same r e a c t i o n mechanism c o n t r i b u t e s i n l a r g e p a r t t o t h e continuun~ s p e c t r a of both t h e (p,nX) and (p,plX) r e a c t i o n s . There a r e d i f - f e r e n c e s i n d e t a i l however a s f o r example t h e analyzing power f o r t h e Ex = 14 KeV bin which has d i f f e r e n t angular dependence f o r t h e two r e a c t i o n s . E s p e c i a l l y a t around 60 t h e a n a l y z i n g power of t h e neutron spectrum is a b u t 25%, while t h a t of t h e protcjn spectrum is about 10%.

En(lab) (MeV) Fig. 1. Neutron energy s p e c t r a and analyzing powers f o r some a n g l e s from t h e r e a c t i o n of 9 3 ~ b (p,nX) a t 65 MeV.

(4)

4. A SIMPLE SINGLE STEP DWBA ANALYSIS AND DISCUSSIONS

Until now only two theories are capable of taking into account the spin- dependent interaction in the calculation of the continuum spectrum. They are Multi Step Direct Reaction Theory of Tamura, Udagawa, and Lenske/4/ and of Feshbach, Kerman ana Koonin/S/. Therefore, in principle, we can calculate the continuum spectra and their analyzing powers. However, in practice, it is almost impossible to carry out the calculation without various simplifications and assumptions.

Therefore we performed a simple single-step DWBA calculation according to the prescription of ref. 2 an6 6 as a first attempt. The analyzing power Ay is given by

do(e E)

where <A ( 8,E) >L and are the DWBA predictions af ter averaging over all

Y d n

possible particle-hole pairs which have the same angular momentum transfer L. E is the excitation energy. The one particle-one hole level density (which corresponds to two exciton states) P(E) and the spin distribution function R(L) of the residual nucleus levels are given as

P ( E ) = g E' and

where g and P are the level density parameter for equally spaced single particle states and the spin cutoff parameter, respectively.

Fig. 2. Angular distributions of the cross sections and analyzing powers for the Ex = 12-16 MeV bin. Open and closed symbols are for the 93~b(p,nX) reaction and for the 9 3 ~ b (p,pl~) reaction, respectively. The indicated errors are statistical.

The solid line and the dashed line are the present calculations for the 93~b(p,n~) and 93~b(p,p'~) reactions, respectively.

(5)

C4-34 J O U R N A L DE PHYSIQUE

S i n c e our aim was n o t t o f i t t h e experimental d a t a , but r a t h e r t o understand t h e o v e r a l l behaviour and t o s e e t h e d i f f e r e n c e s between t h e ( p , p t x ) and t h e (p,nX) r e a c t i o n , we have f i x e d a l l t h e parameters "a p r i o r i " . The DWBA c a l c u l a t i o n s were made with t h e code ~ ~ ~ 7 4 7 ) which i n c l u d e s knock-on exchange amplitudes. A micro- s c o p i c form f a c t o r was employed. The e f f e c t i v e two-body i n t e r a c t i o n was taken from r q f . 8 ( h e r e a f t e r r e f e r r e d t o a s M3Y) s i n c e f o r t h e p r e s e n t energy r e g i o n t h e M3Y e f f e c t i v e i n t e r a c t i o n reproduces n o t only t h e c r o s s s e c t i o n s but a l s o t h e a n a l y z i n g powers f o r t h e low-lying d i s c r e t e s t a t e s r a t h e r nicely/9/. We n e g l e c t e d t h e t a r g e t s p i n , t h e r e f o r e o n l y t h e t r a n s i t i o n s t o n a t u r a l p a r i t y s t a t e s were taken i n t o account. The t r a n s f e r r e d angular momentum considered h e r e was L=2-7. P o s s i b l e neutron l p - l h t r a n s i t i o n s w i t h i n four major s h e l l s a r e c a l c u l a t e d assuming t h e bound s t a t e s h e l l model wave f u n c t i o n w i t h a harmonic o s c i l l a t o r p o t e n t i a l . The e m p i r i c a l l y determined values/lO/ 7.0 M~V-1 and 2.0 were used f o r g and P, respec- t i v e l y . The o p t i c a l p o t e n t i a l parameters f o r t h e i n c i d e n t proton were t h o s e of Sakaguchi/ll/, and t h o s e f o r t h e e x i t proton and neutron were t h o s e of B e c c h e t t i and Greenless/lZ/. A l l t h e c a l c u l a t i o n s were performed f o r t h e 12-16 MeV bin assuming Ex = 14 MeV and r e s u l t s a r e shown i n f i g u r e 2. S o l i d and dashed l i n e s a r e f o r t h e (if,nx) and ( $ , p ' ~ ) r e a c t i o n , r e s p e c t i v e l y . C a l c u l a t e d (3,nx) c r o s s

s e c t i o n s a r e normalized t o t h e experimental d a t a a t 40' and t h e same n o r m a l i z a t i o n f a c t o r was used a l s o f o r t h e ( $ , P I X ) c r o s s s e c t i o n s . I t is remarkable t h a t both proton and neutron a n g u l a r d i s t r i b u t i o n s of t h e c r o s s s e c t i o n s a r e reproduced almost p e r f e c t l y from 0' t o 150'. However DWBA c a l c u l a t i o n s f a i l t o reproduce t h e analyzing power d a t a a s can be s e e n from f i g . 2. The c a l c u l a t i o n o v e r e s t i m a t e s l a r g e l y t h e large-angle analyzing power and u n d e r e s t i m a t e s t h e analyzing power a t around 45"-90". The analyzing power d i f f e r e n c e i n t h e (6,nx) and ( $ , p ' ~ ) r e a c t i o n s a t around 60' is n o t reproduced by t h e p r e s e n t simple DWBA c a l c u l a t i o n which p r e d i c t s r a t h e r s i m i l a r analyzing power v a l u e s f o r both r e a c t i o n s .

I n o r d e r t o s e e t h e s e n s i t i v i t i e s of c a l c u l a t e d a n a l y z i n g powers and c r o s s s e c t i o n s of t h e continuum t o t h e v a r i o u s parameters, we performed DWBA c a l c u l a t i o n s by switching o f f t h e non-central component e i t h e r VLST o r VTT i n M3Y o r by switching o f f t h e s p i n o r b i t p o t e n t i a l e i t h e r i n t h e e n t r a n c e channel @! o r i n t h e e x i t channel fig. The r e s u l t s a r e shown i n t h e Fig. 3 . The c a l c u l a t e d

- _a~ _-_full_M3Y

-

L ⁵^. ^-'-ULS~^off

w 1 '..\ _'.,

""' un off

z

_;_0.1 _,.

w _'0 _{- -}_-VF ^off .%. -...\ . ..<>

0.01 _{Vls off}ex ...>. ...-- .' .

Fig. 3 . S e n s i t i v i t y of t h e DWBA c a l c u l a t i o n s f o r t h e v a r i o u s parameters. See t e x t f o r d e t a i l .

(6)

a n g u l a r d i s t r i b u t i o n o f t h e c r o s s s e c t i o n is n o t s e n s i t i v e t o t h e change o f t h e s t r e n g t h o f t h e n o n - c e n t r a l components o f M3Y and/or o p t i c a l p o t e n t i a l p a r a m e t e r s . T h i s f a c t i n d i c a t e s t h a t t h e c r o s s s e c t i o n d a t a a l o n e a r e n o t s u f f i c i e n t t o have a n i n s i g h t i n t o t h e r e a c t i o n mechanism. However t h e c a l c u l a t e d a n a l y z i n g power f o r t h e ( 6 , n x ) r e a c t i o n d o e s depend s t r o n g l y o n t h e t e n s o r component VTT of M3Y and a l s o o n t h e ve&, component o f t h e i n c i d e n t o p t i c a l p o t e n t i a l p a r a m e t e r s which g e n e r a t e t h e d i s t o r t e d waves. Thus t h e o v e r e s t i m a t i o n o f a n a l y z i n g powers a t backward a n g l e s c o u l d be e x p l a i n e d by c h a n g i n g t h e p a r a m e t e r s o r by i n c l u d i n g t h e m u l t i s t e p p r o c e s s e s which a r e i m p o r t a n t i n e x p l a i n i n g t h e backward a n a l y z i n g powers /13/. On t h e o t h e r hand t h e r e seems t o be v e r y l i t t l e p o s s i b i l i t y t o g e t b e t t e r a g r e e m e n t a t around 60" s i n c e none o f t h e c a l c u l a t i o n s made above w i t h d r a s t i c p a r a m e t e r v a r i a t i o n s i s a b l e t o improve t h e f i t and moreover t h e c o n t r i b u t i o n s of t h e m u l t i s t e p p r o c e s s e s a r e l e s s i m p o r t a n t a t 60°/13/. Same s q r t o f d i s a g r e e m e n t o f t h e a n a l y z i n g pow5r a t a r o u n d 60" is a l s o s e e n i n t h e S 8 ~ i ( p , p ' X ) r e a c t i o n /13/and i n t h e 93iYb(d,drx) r e a c t i o n a t 56 MeV /3/. Thus it m i g h t be v e r y i n t e r e s t - i n g t o u n d e r s t a n d t h e o r i g i n o f t h e d i s c r e p a n c y a t a r o u n d 6 0 " t o l o o k i n t o t h e r e a c t i o n mechanism o f t h e continuum s p e c t r a .

I t is hoped t h a t a more r e f i n e d c a l c u l a t i o n s u c h a s by MSDR t h e o r y w i l l f o l l o w i n t h e n e a r f u t u r e , which s h o u l d improve o n t h e p r e l i m i n a r y a n a l y s i s of t h e p r e s e n t r e s u l t s .

REFERENCES

H. S a k a i , K. Hosono, N. Matsuoka, S. Naganachi, K. Okada, K. Maeda, and H. S h i m i z u , Phys. Rev. L e t t . 44 (1980) 1193.

I I . S a k a i , K. Hosono, N. Matsuoka, S. Magamachi, K. Okada, K. Maeda and

H. S h i m i z u , Nucl. Phys. A 344 (1980) 40.

H. S a k a i , M. Matsuoka, K. Hatanaka, K. Okada and H . Shimizu, Phys. Rev. C24

(1981) 2766.

T. Tamura, T. Udagawa and H. Lenske, Phys. Rev. C26 (1982) 379.

H. Feshbach, A.K. Kerman, and W.E. Koonin, Ann. Phys. (N.Y.) 125 (1980) 429.

R. B o n e t t i , M. Camnasio, L. C o l l i M i l a z z o and P.E. Hodgson, Phys. Rev. C24

(1981) 71.

R. S c h a e f f e r and J. Raynal ( u n p u b l i s h e d ) .

G. B e r t s c h , J. Borysowicz, H. McManus and W.G. Love, Nucl. Phys. A 284 (1977) 399.

9. H. F u j i w a r a , p r i v a t e communication.

10. A. Bohr and B.K. N o t t e l s o n , N u c l e a r S t r u c t u r e , Vol I , Benjamin, New York, 1965.

11. H. S a k a g u c h i e t a l . , ~ h y s . Rev. C26 (1982) 944.

1 2 . F.D. B e c c h e t t i and G.W. G r e e n l e s s , Phys. Rev. 182 (1969) 1190.

1 3 . R. B o n e t t i , L. C o l l i M i l a z z o , I. Doda and P.E. Hodgson, Phys. Rev. 26C (1982) 2417.