(6Li, 6He) AT INTERMEDIATE ENERGIES

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(6Li, 6He) AT INTERMEDIATE ENERGIES

J. Winfield, N. Anantaraman, S. Austin, C. Chang, Ziping Chen, G.

Ciangaru, S. Gales, A. Galonsky, J. van der Plicht, H.-L. Wu

To cite this version:

J. Winfield, N. Anantaraman, S. Austin, C. Chang, Ziping Chen, et al.. (6Li, 6He) AT IN- TERMEDIATE ENERGIES. Journal de Physique Colloques, 1986, 47 (C4), pp.C4-171-C4-174.

�10.1051/jphyscol:1986419�. �jpa-00225785�

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( Li, He ) AT INTERMEDIATE ENERGIES

J.S. WINFIELD, N. ANANTARAMAN, S.M. AUSTIN, C.C. CHANG*, ZIPING CHEN, G. CIANGARU' , S

.

^GALES*^'^,A. GALONSKY, J. VAN DER PLICHT and H.-L. WU

National Superconducting Cyclotron Laboratory and Physics Department, Michigan State University, East Lansing, MI 48824.

U.S.A.

'physics Department, University of Maryland, College Park.

MD 20742, U.S.A.

Schlumberger Well Services, Houston, TX 77210, U. S. A.

"'~nstitut de Physique NuclBaire, F-91406 Orsay Cedex, France

R6sum6

-

Les noyaux de 'Li,

'

^*C."Mg e t = O Z r ont 6 t 6 etudie's en r e a c t i o n dt6change de charge ( 6 L i ,6He), aux e n e r g i e s i n c i d e n t e s de 35, 25 e t 14 MeV/A.

Les r a p p o r t s de s e c t i o n s eff i c a c e s observe's pour c e r t a i n s e'tats dans

'

^*N^{e t}

'Be semblent indiquer que l e me'canisme de r e a c t i o n e s t doming par l e proces- sus h une Btape a 35 MeV/A, avec une contribut,ion c r o i s s a n t e du t r a n s f e r t s e q u e n t i e l aux Bnergies i n c i d e n t e s plus f a i b l e s

.

Les c a l c u l s DWBA 1 une Btape 2 35 e t 25 MeV/A sont en bon accord avec l e s donnges experimentales

.

Une bonne p r o p o r t i o n n a l i t e e n t r e l e s s e c t i o n s e f f i c a c e s GT mesurBes e t l e s 616ments de matrice 0 correspondants e s t observe'e

.

Abstract

-

Angular d i s t r i b u t i o n s have been measured f o r t h e ($Li ,6He) reac- t i o n on t a r g e t s of 'Li. " ' l * C , 26Mg and "Zr a t bombarding e n e r g i e s of 35, 25 and 14 MeV/A. The r a t i o s of observed c r o s s s e c t i o n s f o r s t a t e s i n '*N and 'Be suggest t h a t t h e r e a c t i o n mechanism i s p r e d ~ m i n a n t l y one-step a t 35 MeV/A with an i n c r e a s i n g c o n t r i b u t i o n from s e q u e n t i a l t r a n s f e r a t lower e n e r g i e s . One-step DWBA c a l c u l a t i o n s a t 35 and 25 MeV/A a r e c o n s i s t e n t with t h e d a t a . Good p r o p o r t i o n a l i t y i s observed between the measured c r o s s s e c t i o n s f o r Gamow-Teller t r a n s i t i o n s and t h e corresponding 0-decay m a t r i x elements.

The heavy-ion charge-exchange r e a c t i o n ('Li ,'He) i s p o t e n t i a l l y a very u s e f u l probe of s p i n - f l l p s t r e n g t h i n n u c l e i because, providing t h a t t h e r e a c t i o n mechanism is one-step, t h e s p i n s and p a r i t i e s of 'Li and 6He impose a s p i n t r a n s f e r of AS-1.

Then a t forward a n g l e s , where AL=O t r a n s f e r s a r e s t r o n g , Gamow-Teller (GT) s t r e n g t h may be measured, w h i l s t a t more backward a n g l e s , higher m u l t i p o l a r i t y s p i n - f l i p e x c i t a t i o n s may be observed. The main reason why (6Li,6He) h a s not been e x p l o i t e d t o d a t e i s the u n c e r t a i n t y a t previously a v a i l a b l e bombarding e n e r g i e s (E/A<15 MeV) over whether t h e one-step (r-exchange ) mechanism dominates t h e competing s e q u e n t i a l nucleon t r a n s f e r processes, involving e .g

.

6Li+7Li+6He. Such second-order processes a r e expected / I / t o become l e s s important a s t h e bombarding energy i s i n c r e a s e d , which was t h e motivation f o r t h e present e x p l o r a t o r y survey of GT t r a n s i t i o n s induced by t h e ( 6 L i , 'He) r e a c t i o n a t E/A = 14, 25 and 35 MeV. A r e l a t e d i s s u e i s whether t h e measured GT c r o s s s e c t i o n s a r e proportional t o known B-decay m a t r i x elements, a s has been found /2/ i n ( p , n ) r e a c t i o n s above 100 MeV.

Measurements were performed w i t h t h e NSCL S-320 spectrograph and i t s f o c a l plane d e t e c t o r . The t a r g e t s s t u d i e d were 7 ~ i , 12c, 1 4 c ( f o r a11 bombarding e n e r g i e s ) , 2 6 ~ g ( E / A = 35 MeV o n l y ) , and 9 0 ~ r ( E / A = 25 and 35 MeV). Figure 1 shows a

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986419

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JOURNAL DE PHYSIQUE

BY = 5 deg.

E/A = 14 MeV

POSITION (channel no.) U .

\ E 4 o L

5 s

^20,

0 '

spectt-um f o r 1*C(6Li ,6He) '*N a t E/A = 1 4 MeV. T y p i c a l l y , a r e s o l u t i o n of AE/E = 1/600 was o b t a i n e d . For t h e lower bombarding e n e r g i e s , t h e a b s o l u t e r e s o l u t i o n i s t h u s s i g n i f i c a n t l y b e t t e r t h a n i n s t a t e - o f - t h e - a r t i n t e r m e d i a t e e n e r g y ( p , n ) experiments.

A simple model-independent t e s t of t h e n a t u r e of t h e r e a c t i o n mechanism i s t o compare t h e r a t i o of one-step allowed and one-step suppressed t r a n s i t i o n s t o s t a t e s

i p t h e r e s i d u a l n u c l e i . Three s t a t e s i n '*N a r e of i n t e r e s t - 5 f o r t h i s purpose. The 1 ground s t a t e 3 s a B(GT) v a l u e from 8-decay only about 10 of t h a t f o r t h e s t r o n g 3.95 MeV 1 l e v e l , and i n t h e ( p , n ) r e a c t i o n a t 160 MeV t h e r a t i o of c r o s s s e c t i o n s is observed t o be 0.01. For ( 6 L i , 6 H e ) . t h e r a t i o s (taken a t q = 100 MeV/c) a r e 0.11. 0.15 and 0.21 a t E/A

-

35, 25 and 14 MeV, r e s p e c t i v e l y ; o t h e r work / 3 / shows t h e r a t i o c o n t i n u e s t o i n c r e a s e t o 0.30 a t E/A

-

^{10 MeV.} However, t e n s o r and exchange terms i n t h e i n t e r a c t i o n may account f o r t h e d i f f e r e n c e s between t h e v a r i o u s r e a c t i o n s and t h e dependence of t h e r a t i o s f o r (6Li.6He) on energy.+ A b e t t e r t e s t of t h e c o n t r i b u t i o n of two-step p r o c e s s e s i s t h e r a t i o of t h e 0+

i s o b a r i c a n a l o g s t a t e (IAS) a t 2.31 MeV t o t h e 3.95 MeV s t a t e , because a 0 t o 0' t r a n s i t i o n can o n l y be mediated by t h e non-local p a r t of t h e exchange i n t e r a c t i o n i n a o n e - s t e p process. T h i s r a t i o is 0.05 a t E/A

-

35 and 25 MeV, 0.08 a t E/A

-

^{14 MeV}

and 0.1 f o r t h e E/A = 1 0 MeV d a t a / 3 / . The f a c t t h a t t h e IAS is suppressed by a f a c t o r of two a t E/A = 35 and 25 MeV .compared w i t h t h e lower bombarding e n e r g i e s , s u g g e s t s t h a t m u l t i - s t e p processes a r e much l e s s s i g n i f i c a n t a t t h e higher e n e r g i e s .

A s i m i l a r t e s t t o t h e above i s t o compare t h e r a t i o of ALSO c r o s s s e c t i o n s f o r t h e ground and 0.43 MeV e x c i t e d s t a t e s of 'Be w i t h t h e r a t i o . 1.18, of B(GT) values known from @-decay. The r a t i o s f o r t h e s m a l l e s t a n g l e s s t u d i e d (= 2.5O) a r e 1.08k0.06. 1.34k0.07 and 1.7850.05 f o r E/A

-

35. 25 and 14 YeV. r e s p e c t i v e l y . Thus o n l y a t E/A = 35 MeV i s t h e ( 6 ~ i , 6 ~ e ) r a t i o i n agreement with t h e B(GT) r a t i o . There is a t r e n d away from t h i s value a s t h e bombarding energy is reduced, which s u g g e s t s a s i m i l a r c o n c l u s i o n a s f o r t h e "N IAS comparisons above.

A second c l a s s of t e s t s of t h e r e a c t i o n mechanism i s t o compare t h e d a t a with one-step d i s t o r t e d wave Born approximation (DWBA) c a l c u l a t i o n s . I n Fig. 2, a n g u l a r d i s t r i b u t i o n s a r e shown f o r t h e c a l c u l a t i o n s completed s o f a r ; o t h e r s a r e i n pro- g r e s s . The DWBA code used i s a modified v e r s i o n /4/ o f DWUCK which a l l o w s f o r t h e f i n i t e s i z e o f t h e p r o j e c t i l e system and i n c l u d e s d i r e c t (V,,) + exchan e + t e n s o r terms i n t h e i n t e r a c t i o n . D p t i c a l p o t e n t i a l s o b t a i n e d /5/ from 150 MeV

ki

e l a s t i c s c a t t e r i n g d a t a were used. The E/A = 3 5 MeV c a l c u l a t i o n s f o r and o v e r p r e d i c t t h e d a t a a t s m a l l a n g l e s , p o s s i b l y because o f t h e l a c k o f a p p r o p r i a t e o p t i c a l model p o t e n t i a l s o r t h e n e g l e c t of t h e exchange p a r t o f t h e t e n s o r i n t e r a c t i o n . The normal- i z a t i o n s o f t h e c u r v e s r e q u i r e d t o f i t t h e d a t a ( e x c l u d i n g p o i n t s f o r B<lro), w i t h a Yukawa i n t e r a c t i o n of 1 fm range and with t h e s i n g l e - p a r t i c l e wave f u n c t i o n s r e n o r - malized t o g i v e t h e experimental B(GT) v a l u e , correspond t o VoT v a l u e s r a n g i n g from 7 t o 14 MeV. These a r e t o be compared with t h e average v a l u e o f 11.7f1.7 MeV ob- t a i n e d / 6 / from ( p , n ) s t u d i e s a t s i m i l a r e n e r g i e s / n u c l e o n . A t e n s o r f o r c e o f s t r e n g t h h 1.5 MeV g i v e s t h e b e s t r e p r o d u c t i o n of t h e d a t a .

. 2-.5.11

I

^-.a.^4B F i g . 1

-

Spectrum of 1*C(6Li .6He)1*N

;

^: f o r E(6Li)/A

-

^{14 MeV.} S t a t e s i n

1*.0.0 '*N a r e i n d i c a t e d w i t h t h e e x c i t a - t i o n energy given iri MeV. The

' -' : observed r e s o l u t i o n i s 140 keV.

200 250 300 350

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t h e f i n a l s t a t e nucleus and e x c i t a t i o n energy i n d i c a t e d f o r each case. The curves a r e DWBA c a l c u l a t i o n s normalized t o t h e data.

We have i n v e s t i g a t e d how c l o s e l y t h e observed forward-angle AL-0 c r o s s s e c t i o n s measure Gamow-Teller s t r e n g t h . Wharton and Debevec /7/ have shown t h a t even a t E ( 6 L i ) = 34 MeV, where m u l t i s t e p processes a r e known t o give s i z e a b l e c o n t r i b u t i o n s , t h e r e i s s t i l l some c o r r e l a t i o n between (6Li.6He) ALSO c r o s s s e c t i o n s and t h e corresponding 0-decay m a t r i x elements w i t h i n a given nucleus. Their d a t a do show a marked mass dependence, however. Our r e s u l t s . which cover a mass range from 7 t o 90. a r e shown i n Fig. 3 a s a plot of c r o s s s e c t i o n a t ~R/R("c) = 100 MeV/c, where q i s t h e l i n e a r momentum t r a n s f e r and R/R(14C) i s t h e sum of t h e p r o j e c t i l e and t a r g e t r a d i i divided by t h a t of 6Li+L*C. A t E/A ⁼14 MeV t h e o v e r l a p of equivalent qR values is too l i m i t e d f o r comparison, but a t 25 and 35 MeV/nucleon t h e r e i s a high degree of p r o p o r t i o n a l i t y f o r a l l Pinal s t a t e s measured.

B(GT) f r o m p o r (p,n)

Fig. 3

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P l o t of c r o s s s e c t i o n s a t qR/R(14C) = 100 MeV/c f o r various GT t r a n s i t i o n s induced by ( 6 L i , 'He) a t 25 and 35 MeV/nucleon. The f i n a l s t a t e s , w i t h e x c i t a t i o n e n e r g i e s i n MeV, a r e : l4N(3.95). 90Nb(2.3),

12N(0.0), ' ~ e ( O . 0 and 0.43), and 2 6 ~ 1 ( 1 . 0 6 and 1.85).

I n summary, t h e r a t i o s of observed c r o s s s e c t i o n s f o r c e r t a i n s t a t e s i n '*N and 'Be i n d i c a t e t h a t t h e c o n t r i b u t i o n t o t h e c6Li,'He) r e a c t i o n from higher-order processes i s i n c r e a s i n g l y n o t i c e a b l e as t h e bombarding energy i s reduced. One-step

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JOURNAL DE PHYSIQUE

DWBA c a l c u l a t i o n s give a reasonable account of the d a t a a t E/A = 35 and 25 MeV.

The most pleasing r e s u l t , however, i s the observed p r o p o r t i o n a l i t y between t h e c r o s s s e c t i o n s f o r GT t r a n s i t i o n s and 8-decay m a t r i x elements a t E/A = 35 and 25 MeV. The r e a c t i o n may s t i l l be a useful probe of s p i n s t r e n g t h i n n u c l e i a t even lower

ener g i as

.

This work was supported i n p a r t by t h e National Science Foundation under Grants No. PHY83-12245 and PHY83-17437.

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