DIRECT EXCITATION OF HIGHER MULTIPOLE RESONANCES IN HEAVY NUCLEI

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DIRECT EXCITATION OF HIGHER MULTIPOLE RESONANCES IN HEAVY NUCLEI

H.-P. Morsch

To cite this version:

H.-P. Morsch. DIRECT EXCITATION OF HIGHER MULTIPOLE RESONANCES IN HEAVY NU- CLEI. Journal de Physique Colloques, 1984, 45 (C4), pp.C4-185-C4-200. �10.1051/jphyscol:1984414�.

�jpa-00224079�

J O U R N A L DE PHYSIQUE

Colloque C4, supplCment au n03, Tome 45, mars 1984 page C4-185

DIRECT EXCITATION OF HIGHER MULTIPOLE RESONANCES I N HEAVY NUCLEI

H.-P. Morsch

I n s t i t u t fiir Kemphysik, Kernfo~schungsan Zage JuZich, 0-5170 JiiZich, F.R.G.

Resume - Nous revoyons l e s 6tudes rgcentes de rgsonances g6antes de haute m u l t i p o l a r i t 6 dans l e s noyaux lourds. Nous discutons l a rgsonance octupo- l a i r e g6ante S haute gnergie, puis l e s composantes de hautes m u l t i p o l a r i - t@s dans l a r6gion d ' e x c i t a t i o n de 2 % ~ . Enfin nous prPsentons l e s premiers r g s u l t a t s d ' exp6riences s u r l e fond continu dont on peut d6duire des ren- seignements s u r l e mecanisme d ' e x c i t a t i o n e t l a mu1 t i p o l a r i t 6 moyenne.

Abstract - A review i s given on recent g i a n t resonance s t u d i e s i n heavy nuclei i n v e s t i g a t i n g higher multipole e x c i t a t i o n s . Three t o p i c s a r e d i s - cussed: t h e high 1 ing g i a n t octupole resonance, higher mu1 t i p o l e components in t h e region of 2Kw e x c i t a t i o n s , and f i r s t i n v e s t i g a t i o n s of t h e continuum background which can y i e l d information about e x c i t a t i o n mechanism and average mu1 t i p o l a r i t y .

I - INTRODUCTION

During the l a s t decade a rapid development of t h e f i e l d of g i a n t resonances took place. Whereas i n the s e v e n t i e s mainly t h e i s o s c a l a r g i a n t quadrupole resonance(GQR) was s t u d i e d i n hadron s c a t t e r i n g experiments, a t present the main i n t e r e s t l i e s in two p o i n t s : f i r s t the study of compressional modes of e x c i t a t i o n with mu1 t i p o l a r i ty L=O and 1 ( t h i s i s discussed i n d e t a i l i n the t a l k by M . Buenerd), and second higher multipole e x c i t a t i o n s which a r e discussed i n t h i s t a l k . Nuclear shape o s z i l l a t i o n s of h i g h e r mu1 t i p o l a r i ty a r e expected t o be important a l s o i n heavy ion c o l l i s i o n s . Of s p e c i a l i n t e r e s t i s the damping mechanism of t h e s e s u r f a c e v i b r a t i o n s r e l a t e d t o the question whether i t i s due t o one-body o r two-body d i s s i p a t i o n .

To a l a r g e e x t e n t t h e r e c e n t progress i n s t u d i e s of g i a n t resonances was due t o l a r g e improvements of experimental techniques mainly through the p o s s i b i l i t y t o study i n e l a s t i c a l l y s c a t t e r e d p a r t i c l e s a t extremely small angles including OO. This i s f e a s i b l e only by extremely good beam qua1 i ty and magnetic spectrometer systems i n combination with r a t h e r s o p h i s t i c a t e d d e t e c t o r devices

.

In t h i s t a l k I want t o discuss mainly multipole e x c i t a t i o n s with L ? 3 although in t h e case of Pb (where some confusion has e x i s t e d ) a l s o the L=2 s t r e n g t h i s b r i e f l y discussed. (Ex a 30 A d ] 3MeV) has been i n v e s t i g a t e d / 3 / . Recently, by improved energy r e s o l u t i o n A ready f o r a number of y e a r s the low lying

I

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1984414

C4-186 J O U R N A L D E PHYSIQUE

a + 208pb Ea=172 MeV

(see c o n t r i b u t i o n s t o t h i s conference) T h i s s h o u l d n o t be d i s c u s s e d here.

I w i l l c o n c e n t r a t e on t h r e e p o i n t s : - E x c i t a t i o n o f t h e g i a n t o c t u p o l e r e -

sonance (3kw e x c i t a t i o n )

- H i g h e r m u l t i p o l a r i t y e x c i t a t i o n i n t h e 2hw resonance r e g i o n

- S t u d i e s o f t h e i n e l a s t i c continuum background u n d e r l y i n g the g i a n t r e - sonances. Here experiments a r e d i s - cussed w h i c h can g i v e i n f o r m a t i o n on t h e r e a c t i o n mechanism and t h e mul- t i p o l a r i ty d i s t r i b u t i o n o f t h e con- tinuum.

F i g u r e 1 : A n g u l a r d i s t r i b u t i o n s f o r g i a n t resonance e x c i t a t i o n s o f d i f f e r e n t m u l t i p o l a r i t i e s c a l c u l a t e d w i t h i n a m i c r o s c o p i c DWBA approach u s i n g f o l d i n g t y p e f o r m f a c t o r s .

I1 - EXCITATION OF THE GIANT OCTUPOLE RESONANCE fGOR)

P.round 1980 from r a t h e r d i f f e r e n t s c a t t e r i n g experiments, 130 MeV 3 ~ e s c a t t e - r i n g /4/ 172 MeV a s c a t t e r i n g /5/ and 800 MeV p s c a t t e r i n g / 6 / evidence has been p r e - sented f o r t h e GOR. S p e c t r a f r o m these experiments f o r t h e case o f 20*Pb a r e shown i n f i g . 2. A r a t h e r b r o a d s t r u c t u r e i s observed a t a h i g h e x c i t a t i o n energy o f about 20 MeV w e l l above t h e g i a n t quadrupole and monopole resonances. A summary of d i f f e r e n t experiments i s g i v e n i n t a b l e 1. I n a l l these experiments /4-10/ t h e GOR i s c l e a r l y observed. Whereas m o s t l y one resonance i s observed o u r J u l i c h r e s u l t s i n d i - c a t e a t l e a s t two resonances, t h e GOR and t h e i s o s c a l a r d i p o l e resonance / 5 / , a compressional mode o f e x c i t a t i o n .

I n t h e a n a l y s i s o f t h e d i f f e r e n t d a t a ( f i g . 2) r a t h e r d i f f e r e n t background shapes have been s u b t r a c t e d . I n t h e 3He and a work a t t e m p t s were made t o e s t i m a t e t h e continuous background below t h e g i a n t resonance whereas i n t h e a n a l y s i s o f t h e (p,pl) s p e c t r a o n l y a f l a t t a i l from t h e quadrupole and monopole resonances was s u b t r a c -

"t- ^{h !\}

1

140 MeV ( 3 ~ e , 3 ~ e 1 ) Osaka, October 1980 Phys.Rev. C23, 937 (

172 MeV ( a . a l ) dUlich, A p r i l 1980

Phys.Rev.Lett. 45, 337 (1980)

Figure 2 : Comparison of spectra from the s c a t t e r i n g of d i f f e r e n t p r o j e c t i l e s .

C4-188 JOURNAL DE PHYSIQUE

Pro-

j e c - Einc Exp. Mu1 ti -

t i l e p o l a r i t y T a r g e t s Refs.

3 ~ e 130 Osaka 1979,80 L=3 90zr,116,118,120Sn,144Sm,208pb

172 J u l i c h 1980 L=3,1 208pb ,23ZTh ,238" 5

p 800 LAMPF 1980 L=3 4 0 ~ a ,'16sn , 2 0 8 ~ b 6

p 200 TRIUMF 1981 L=3 9 0 ~ r , 1 2 0 ~ n , 2 0 8 ~ b 7

p 200 Orsay 1 9 8 1 L=1(3) '08pb 8

a 340,480 S a c l a y 1981,82 L=3 '16sn, '08pb 9

a 172 J u l i c h 1982 L=3 12OSn-238" 10

T a b l e 1: Evidence f o r new i s o s c a l a r g i a n t resonances a t h i g h e r Ex i n hadron s c a t t e r i n g .

t e d g i v i n g r i s e t o r e l a t i v e l y s m a l l e r c r o s s s e c t i o n s . A t t h i s p o i n t a few comments on' t h e background problem i n h i g h e r energy hadron s c a t t e r i n g s h o u l d be made. D i f f e r e n t f r o m l o w e r i n c i d e n t e n e r g i e s where more c o m p l i c a t e d processes c o n t r i b u t e t o t h e background, i n i n e l a s t i c s c a t t e r i n g a t h i g h e r e n e r g i e s t h e i n e l a s t i c continuum i s expected t o be dominated by d i r e c t e x c i t a t i o n /11/ which may be i n t e r p r e t e d by b r o a d o v e r l a p p i n g d i s t r i b u t i o n s o f many d i f f e r e n t ( m o s t l y h i g h e r ) m u l t i p o l a r i t i e s / 1 2 / . So, we have a s i t u a t i o n i n which b o t h resonance e x c i t a t i o n and p a r t o f t h e continuum a r e o f s i m i l a r d i r e c t c h a r a c t e r . I n t h i s case, r a t h e r t h a n t o e x t r a c t c r o s s s e c t i o n s o f resonances i n an a b s o l u t e manner i n which t h e f u l l u n c e r t a i n t i e s o f t h e back- ground e n t e r , one can determine resonance y i e l d s f o r g i v e n background shapes. Such cross s e c t i o n s depend, o f course, on t h e background h e i g h t . N e v e r t h e l e s s , i f t h e background l i n e i s drawn s u f f i c i e n t l y low, t h a n f o r d i f f e r e n t background choices a m u l t i p o l e decomposition s h o u l d y i e l d t h e same amount o f l o w e r mu1 t i p o l a r i t y r e - sonant s t r e n g t h . An example i s g i v e n i n r e f . 5 where t h e d i f f e r e n t i a l c r o s s sec- t i o n of t h e 13.8 MeV resonance i s e x t r a c t e d f o r two r a t h e r d i f f e r e n t background shapes. B o t h s e t s o f d a t a /2,5/ w i t h d i f f e r e n t a b s o l u t e c r o s s s e c t i o n y i e l d t h e same amount o f L=O ( a n d I ) s t r e n g t h , t h e data s e t d e r i v e d f r o m t h e l o w e r back- ground y i e l d s a d d i t i o n a l c o n t r i b u t i o n s o f h i g h e r mu1 t i p o l a r i ty ( d e s c r i b e d b y L=4 and 6). T h e r e f o r e i n t h i s approach i t i s o n l y i m p o r t a n t t o s u b t r a c t t h e continuum i n a c o n s i s t e n t way f o r a l l angles and energies. I n o u r g i a n t resonance analyses we used always t h e f o l l o w i n g procedure: The background i s f i t t e d above t h e r e s o - nances t o t h e h i g h e r energy continuum. T h i s i s connected t o t h e low energy d i s - c r e t e spectrum by a p o l y n o m i a l f i t o f l e a s t c u r v a t u r e .

I n t h e f o l l o w i n g t h e mass s y s t e m a t i c s o f t h e GOR / l o / i s discussed, p a r t i c u l a r l y i n view o f d e f o r m a t i o n e f f e c t s . J t i s w e l l known t h a t t h e g i a n t d i p o l e resonance i n deformed n u c l e i s p l i t s i n t o t h e K=O- and 1- components. F o r resonances o f h i g h e r m u l t i p o l a r i t y one e x p e c t s a b r o a d e n i n g o f t h e resonance r a t h e r t h a n a s p l i t t i n g be-

cause o f a l a r g e r number o f K components, t h e energy o f which a r e more c l o s e l y spaced. F o r t h e GQR i n deformed n u c l e i o n l y a r a t h e r s m a l l b r o a d e n i n g o f t h e o r d e r o f 0 . 4 - 1 MeV has been observed. T h i s q u a n t i t y has l a r g e u n c e r t a i n t i e s aue t o s e p a r a t i o n of t h e n e i g h b o u r i n g g i a n t monopole resonance (GMR) which a l s o shows d e f o r m a t i o n e f f e c t s .

The GOR can be s t u d i e d v e r y n i c e l y i n 172 MeV a s c a t t e r i n g i n t h e a n g u l a r range 3-5'. A t these angles t h e GOR i s t h e o n l y s t r o n g e x c i t a t i o n i n t h e 3 % ~ e x c i t a t i o n r e g i o n ( f i g . 3). S p e c t r a taken a t 4' f r o m o u r J u l i c h work a r e g i v e n i n f i g . 4 f o r d i f f e r e n t t a r g e t n u c l e i between 120Sn and 238U. They show r a t h e r c l e a r l y t h e GOR peak i n a d d i t i o n t o t h e GQR and GMR resonances. E x t r a c t e d resonance parame- t e r s and sum r u l e s t r e n g t h s a r e summarized i n f i g . 5 and compared w i t h o t h e r i n - v e s t i g a t i o n s /4,6,7/. A c l e a r d e v i a t i o n i n w i d t h and e x c i t a t i o n energy f r o m a smooth A dependence i s o b t a i n e d f o r deformed n u c l e i . We observe an i n c r e a s e i n

F i g u r e 3: D i f f e r e n t i a l cross sections f o r the GOR e x c i t a t i o n i n 208Pb i n comparison w i t h DWBA c a l c u l a t i o n s f o r d i f f e r e n t mul- t i p o l a r i t i e s i n t h e 3 % ~ e x c i t a t i o n region.

w i d t h up t o about 2 MeV. Somewhat s u r p r i - sing, a l s o the e x c i t a t i o n energy shows de- formation e f f e c t s which are opposite i n r a r e e a r t h and a c t i n i d e n u c l e i . I n general a good agreement o f o u r r e s u l t s w i t h o t h e r data i s obtained. A s t r e n g t h i n t h e o r d e r o f 40-70 % o f the EWSR s t r e n g t h has been e x t r a c t e d . Adding the lhw octupole e x c i t a - t i o n s /3/ p r a c t i c a l l y the whole i s o s c a l a r L=3 s t r e n g t h i s observed. From 800 MeV (p,pl) a much smaller sum r u l e s t r e n g t h has been r e p o r t e d /6/. This i s due t o a too h i g h background s u b t r a c t i o n , f u r t h e r an i n - e l a s t i c form f a c t o r was used which does n o t g i v e r e l i a b l e sum r u l e strengths.

F i g u r e 4: Small angle s p e c t r a a t 4' o f 172 MeV ^u s c a t t e r i n q from d i f f e - r e n t n u c l e i .

From the q u i t e l a r g e increase i n resonance w i d t h from L=2 t o L=3 g i a n t resonances ( f o r 208Pb from 2.6t0.2 MeV t o 5 . 0 i - 0 . 9 MeV) i t i s obvious t h a t h i g h e r L resonances a t h i g h e x c i t a t i o n s have q u i t e l a r g e widths and are t h e r e f o r e extremely d i f f i c u l t t o d e t e c t experimentally. Using a macroscopic damping model /13/ t h e w i d t h o f t h e g i a n t hexadecapole resonance i s estimated t o be a t l e a s t 9-10 PeV. Experimental attempts t o f i n d h i g h e r n u l t i p o l e resonancesat l a r g e e x c i t a t i o n energies have been made so f a r o n l y by t h e Saclay group using h i g h e r energy p a r t i c l e s 191.

JOURNAL DE PHYSIQUE

F i g u r e 5: Mass dependence o f e x c i t a t i o n energy, w i d t h and energy weighted sum r u l e s t r e n g t h of the GOR e x c i t a t i o n . The s o l i d li es i n d i c a t e

EX = 112 A-l73 MeV and r ⁼ 73 A - l / 2 MeV.

The s o l i d p o i n t s are from r e f . 10, the open p o i n t s are from r e f s . 4 , 6 , 7 .

W i t h i n a s h e l l model p i c t u r e h i g h e r m u l t i p o l e e x c i t a t i o n s are n o t expected o n l y a t h i g h energies b u t a l s o a t lower e x c i t a t i o n . For a closed s h e l l nucleus l i k e 208Pb c o l l e c t i v e L=4 e x c i t a t i o n s are ex e c t e d i n t h e r e g i o n of 2hw energies and a t 4hw, L=6 e x c i t a t i o n s a t 2kw,L4hw and 6#w. S i m i l a r l y , the odd p a r i t y L=3 e x c i t a t i o n has con- t r i b u t i o n s a t 160~ and 3hw (discussed i n t h e l a s t s e c t i o n ) . O f l a r g e i n t e r e s t i n the moment i s t h e question whether h i g h e r m u l t i p o l e com onents c o n t r i b u t e t o t h e cross s e c t i o n i n t h e & w r e g i o n (GQR and GMR). As i n the 2

!

so h i g h e r energy (e,e1) /15/. Therefore I w i l l mainly concentrate on t h e case o f 2Q8Pb.

I n d i c a t i o n s f o r L c o n t r i b u t i o n s d i f f e r e n t from L=2 i n the GQR r e g i o n were f i r s t ob- t a i n e d from angular d i s t r i b u t i o n s . I n several d i f f e r e n t s c a t t e r i n g systems d i f f e r e n - t i a l cross s e c t i o n s f o r t h e GQR were measured which d i d n o t show a pronounced d i f f r a c - t i o n p a t t e r n c h a r a c t e r i s t i c f o r pure m u l t i p o l a r i t y . P a r t i c u l a r l y , i n a and d s c a t t e - r i n g /2/ r a t h e r f l a t angular d i s t r i b u t i o n s were observed which i n d i c a t e c l e a r l y a m i x t u r e o f d i f f e r e n t L values ( f i g . 6 ) . Attempts t o f i t t h e data w i t h an a d d i t i o n a l L=4 component o n l y were n o t s a t i s f a c t o r y . For d s c a t t e r i n g L=4 c o n t r i b u t i o n s g i v e r i s e t o a s i g n i f i c a n t smearing o u t o f t h e d i f f r a c t i o n p a t t e r n as observed experimen- t a l l y /2/ whereas i n 172 MeV a s c a t t e r i n g L=4 angular d i s t r i b u t i o n s a r e n o t much o u t o f phase w i t h t h a t o f L=2 and thus c r e a t e t o o much s t r u c t u r e . Therefore, a c o n s i s t e n t d e s c r i p t i o n o f the data i n f i g . 6 a r e obtained o n l y by assuming L=2, 4 and 6 e x c i t a - t i o n /2/. A l t e r n a t i v e l y , t h e data c o u l d be described a l s o by a L=2, 4 a x a s t r o n g

F i g u r e 6: D i f f e r e n t i a l cross s e c t i o n s f o r t h e e x c i t a t i o n o f t h e 10.9 MeV resonance i n a and d s c a t t e r i n g i n comparison w i t h m i - croscopic DWBA c a l c u l a t i o n s (see r e f . 2).

L=3 e x c i t a t i o n . However, t h i s i s n o t c o n s i s t e n t w i t h t h e r e s u l t o f a concentrated GOR a t h i g h e r e x c i t a t i o n (previous s e c t i o n ) . Our i n d i c a t i o n s . f o r a s t r o n g L=6 com- ponent i n t h e GQR r e g i o n are supported by r e s u l t s o f a 208Pb(a,a'n) c o r r e l a t i o n ex- periment /16/ i n which a s t r o n g decay i n t o t h e 13/2+ s t a t e o f 207Pb has been observed.

This decay can o n l y be favourably populated from a L=6 e x c i t a t i o n .

Now I w i l l discuss r e c e n t small angle 208Pb(a,a') experiments from J u l i c h which y i e l d more d e t a i l e d i n f o r m a t i o n . The angular r e g i o n s t u d i e d was 1.5 - 8O. I n f i g . 7 small angle spectra a r e given. The background s u b t r a c t i o n was done i n t h e f o l l o w i n g way: the shape was f i x e d a t 4' as discussed i n sect. 2. This background shape was used a l s o f o r t h e o t h e r s m a l l e r angle spectra. I n a d d i t i o n a h i g h energy background was assumed (dashed l i n e ) due t o t h e t a i l o f t h e e l a s t i c peak. This was adjusted t o t h e 2' spectrum and scaled f o r the o t h e r angles w i t h t h e e l a s t i c cross section. To- wards s m a l l e r angles a decrease o f t h e 10.9 MeV resonance (GQR) i s observed w i t h a cross s e c t i o n minimum o f about 70 mb/sr. As f o r these angles h i g h e r L components are small t h i s determines q u i t e we1 1 t h e L=2 s t r e n g t h . I t corresponds t o about 60 % o f the i s o s c a l a r EWSR c o n s i s t e n t w i t h o l d e r r e s u l t s /2/. This i s a f a c t o r o f two more than e x t r a c t e d from lower energy e l e c t r o n s c a t t e r i n g /14/.

As discussed i n f i g . 4 the spectra a t 3-4O are w e l l described by t h r e e Gaussian peaks which represent GQR, GMR and GOR. I n f i j . 8 a comparison i s made between 4O spectrum and those taken a t l a r g e r angles o f 6-8 . I n t h i s r e g i o n we expect l a r g e r c o n t r i b u - t i o n s of h i g h e r m u l t i p o l a r i t y ( f i g . 1 ) . Indeed, by comparing t h e 4O spectrum w i t h t h e

C4-192 JOURNAL DE PHYSIQUE

s p e c t r a a t 6 and 8' we observe s i g n i f i - c a n t d i f f e r e n c e s . T h i s i s seen on t h e l e f t s i d e o f f i g . 8 i n which t h e same Three-Gaussian f i t which d e s c r i b e s w e l l t h e 40 spectrum i s a p p l i e d t o t h e 6 and 80 s p e c t r a . D e f i c i e n c i e s o f t h e s e f i t s a r e observed a t t h r e e e x c i t a t i o n s i n d i c a t e d by t h e dashed arrows. A t t h e e x c i t a t i o n energy above t h e GOR t h e e x t r a y i e l d i s i n t e r p r e t e d t o be due t o t h e h i g h l y i n g

300 L = l e x c i t a t i o n /5/ which i s weakly e x c i -

t e d a t 4O. A t t h e o t h e r two p o s i t i o n s ,

200 a t Ex = 12.5 MeV and 16 MeV,new s t r u c -

100 t u r e s were i n t r o d u c e d i n o r d e r t o o b t a i n

a good f i t t o t h e s p e c t r a ( r i g h t s i d e ,

v, f i g . 8 ) . F o r l a r g e r angles t h e y i e l d s

I- show a decrease o f t h e 12.5 MeV

Z 300 s t r u c t u r e and a r i s e a t 16 MeV. The d i f -

=) 200 f e r e n t i a l c r o s s s e c t i o n s g i v e n i n f i g . 9

g ¹⁰⁰ a r e d e s c r i b e d by L=4 (Ex = 12.5 MeV)

and L=6 ( E x = 16 MeV) w i t h EWSR s t r e n g t h o f 14 and 15 %, r e s p e c t i v e l y . The h i g h l y i n g s t r u c t u r e s a r e w e l l d e s c r i b e d by

400 L=3 and 1, r e s p e c t i v e l y . The 16 MeV

200 s t r u c t u r e , which has an a n g u l a r d i s t r i b u -

t i o n q u i t e s i m i l a r t o t h a t o f L=3 a t l a r g e r angles, i s r e s p o n s i b l e f o r t h e f a c t t h a t i n t h e e a r l i e r (a,al) s t u d y

600 ( r e f . 5 ) t h e GOR was e x t r a c t e d a t t o o

400 low an e x c i t a t i o n energy. The f a c t t h a t

200 i n t h e r e g i o n o v e r l a p p i n g w i t h t h e GMR

a r a t h e r s t r o n g L=4 c o n t r i b u t i o n was 30 20 10 found e x p l a i n s why i n a s c a t t e r i n g t o o

Ex(MeVl l a r g e monopole cross s e c t i o n s have been e x t r a c t e d /2/.

Summaring up t h e d i f f e r e n t L components

Figure 7: a spectra 2 0 8 p b . from peaks assumed a t 10.9, 12.5, 13.8

C o n s i s t e n t background f i t s a r e shown (see and 16 MeV we obtain the multipole

t e x t ) . s t r e n g t h d i s t r i b u t i o n s g i v e n i n f i g . 10.

These d i s t r i b u t i o n s a r e q u i t e d i f f e r e n t f r o m Gaussian o r L o r e n t z i a n shapes w i t h a r a t h e r narrow w i d t h f o r L=2 e x c i t a t i o n ( r 2.7 MeV), a l a r g e r w i d t h f o r L=4

( r

-

There a r e v e r y r e c e n t l y p u b l i s h e d d a t a /17/ f o r GQR and GMR e x c i t a t i o n i n 3 ~ e s c a t t e - r i n g s t r e s s i n g t h e importance o f L=4 c o n t r i b u t i o n s . However, p o s s i b l e L=6 s t r e n g t h which was c r u c i a l t o d e s c r i b e a s c a t t e r i n g d a t a ( f i g . 6 ) was n o t mentioned a t a l l . Based on o u r a s c a t t e r i n g r e s u l t s d i s c u s s e d above we c a l c u l a t e d t h e c o r r e s p o n d i n g 3He s c a t t e r i n g c r o s s s e c t i o n s which a r e g i v e n t o g e t h e r w i t h t h e d a t a /17/ i n f i g . 11.

F o r a check we c a l c u l a t e d a l s o a b s o l u t e c r o s s s e c t i o n s f o r t h e low l y i n g 3- e x c i t a t i - on which a r e i n e x c e l l e n t agreement w i t h t h e d a t a i n f i g . 11. Using o u r m u l t i p o l e assignments we o b t a i n a p e r f e c t d e s c r i p t i o n f o r t h e GMR r e g i o n (13.7 MeV), f o r t h e GQR r e g i o n t h e e s t i m a t e d c r o s s s e c t i o n (dashed l i n e ) i s s l i g h t l y below t h e data. Using a d d i t i o n a l s t r e n g t h o f L-4 and 6 ( 6 % EWSR s t r e n g t h each) y i e l d s a l s o i n t h i s case a

F i g u r e 8: Comparison o f 4' spectrum w i t h t h e measured spectra a t l a r g e r angles 6-8'. L e f t hand side: t h r e e Gaussian f i t s t o t h e g i a n t resonances w i t h t h e energies i n d i c a t e d . E x c i t a t i o n regions w i t h l a r g e d e f i c i e n c i e s o f the f i t s a r e marked by dashed arrows.

R i g h t hand side: f i t s t o t h e resonances i n c l u d i n g a d d i t i o n a l s t r u c t u r e s a t 12.5, 16 and 21.3 MeV.

C4-194 JOURNAL DE PHYSIQUE

- a+za8Pb Ea=172 MeV

-

4 Ex 4 1 8 MeV

'1 ^*p

L ul

k E ^k\,L=4 &=12.5M.V

-

F i g u r e 9: D i f f e r e n t i a l cross sec- t i o n s f o r d i f f e r e n t resonances i n t h e angular r e g i o n 5-8O i n compa- r i s o n w i t h DWBA c a l c u l a t i o n s . The upper l i n e s a r e due t o e x c i t a t i o n o f t h e g i a n t monopole resonance w i t h some a d d i t i o n a l c o n t r i b u t i o n s o f h i g h e r mu1 t i p o l a r i t y .

600 m

I-

Z 3 0 0

LOO

200

F i g u r e 10: R e l a t i - ve m u l t i p o l e s t r e n g t h d i s t r i b u - t i o n s f o r even m u l t i p o l a r i t i e s obtained from t h e a n a l y s i s o f t h e resonances a t 10.9, 12.5, 13.8 and 16 MeV.

F i g u r e 11: D i f f e r e n t i a l cross sections f o r e x c i t a t i o n o f the low l y i n g 3- s t a t e and t h e g i a n t resonances a t 10.9 and 13.7 MeV i n 140 MeV 3He s c a t t e r i n g (data a r e from r e f . 17) i n comparison w i t h microscopic DWBA c a l c u l a- t i o n s w i t h m u l t i p o l e s t r e n g t h s c o n s i s t e n t w i t h a s c a t t e r i n g r e - s u l t s .

good account o f t h e data ( s o l i d l i n e ) . The f a c t t h a t somewhat s m a l l e r cross sections a r e p r e d i c t e d f o r t h e GQR from our a s c a t t e r i n g data i s presumably due t o a somewhat d i f f e r e n t background s u b t r a c t i o n i n the two experiments analysed completely indepen- dent. The f i t s t o t h e data i n f i g . 11 based on o u r a s c a t t e r i n g r e s u l t s a r e much b e t t e r than those given i n r e f . 17. D i f f e r e n t from t h e conclusions o f t h e authors they support our r e s u l t s o f a s t r o n g L=2 e x c i t a t i o n and the importance o f L=6 con- t r i b u t i o n s . There are a l s o new ( p , p S ) experiments which support o u r r e s u l t s o f con- c e n t r a t e d L=4 s t r e n g t h around 12 MeV /18/. As compared t o a s c a t t e r i n g ( f i g . 1) the s i t u a t i o n i n 200 MeV p r o t o n s c a t t e r f r r g i s somewhat d i f f e r e n t i n so f a r t h a t h i g h e r m u l t i p o l e e x c i t a t i o n s have l a r g e cross sections o n l y a t much l a r g e r s c a t t e r i n g angles a t which low mu1 t i p o l e e x c i t a t i o n s have a l r e a d y dropped o f f considerably

(see f i g . 12). Indeed, t h e l a r g e angle proton spectra i n f i g . 13 c o u l d be f i t t e d o n l y assuming a L=4 resonance peak a t 12 MeV.

A few attempts have been made a l s o t o study t h e d e t a i l s o f t h e 2kw e x c i t a t i o n s i n l i g h t e r n u c l e i . I n h i g h e r energy a s c a t t e r i n g / 9 / L=4 s t r e n g t h has been found i n t h e Sn r e g i o n q u i t e c o n s i s t e n t t o t h a t i n 2Q8Pb. I n c o n t r a s t , i n t h e same mass r e g i o n p r o t o n s c a t t e r i n g angular d i s t r i b u t i o n s /7/ gave i n d i c a t i o n f o r o n l y very small L=4 y i e l d .

C4-196 JOURNAL DE PHYSIQUE

4 4 0

6 8 10 12 14 16 18 2 0 2 2 2 4 EXCITATION ENERGY (MeV)

F i g u r e 12: Angular d i s t r i b u t i o n s f o r g i a n t F i g u r e 13: Spectra o f 200 MeV p s c a t t e - resonance e x c i t a t i o n s o f d i f f e r e n t m u l t i - r i n g form r e f . 18. The l a r g e r angle p o l a r i t i e s i n 200 MeV p r o t o n s c a t t e r i n g spectra (14 and 16O) c o u l d be f i t t e d

/18/. o n l y assuming a L=4 resonance a t 12 MeV.

I V - STUDY OF THE INELASTIC CONTINUUM UNDERLYING THE GIANT RESONANCES

I n t h e g i a n t resonance peaks discussed i n t h e previous sections l a r g e sum r u l e s t r e n g t h s a r e found o n l y f o r low m u l t i p o l a r i t i e s up t o L=3. This i s o n l y a small f r a c - t i o n o f t h e t o t a l i n e l a s t i c y i e l d which i n v o l v e s e x c i t a t i o n s up t o l a r g e m u l t i p o l a r i - t i e s . The l a r g e r p a r t o f the i n e l a s t i c s t r e n g t h i s found i n t h e continuous background below t h e sharp g i a n t resonances. To g e t i n f o r m a t i o n about t h e features o f h i g h mul- t i p o l e v i b r a t i o n s one has t o study t h e p r o p e r t i e s o f t h e background, t h i s i s a l s o o f general i n t e r e s t .

A t t h e beam energies used i n t h e experiments reviewed we expect t h a t t h e i n e l a s t i c continuum i s t o a l a r g e e x t e n t due t o d i r e c t e x c i t a t i o n . Under t h i s assumption we can c a l c u l a t e t h e background continuum using microscopic o r macroscopic models/12,13/.

Using t h e macroscopic model o f r e f . 13 a continuum i s obtained as shown by t h e dashed l i n e i n f i g . 14. I n t h i s c a l c u l a t i o n o n l y surface v i b r a t i o n s ( m u l t i p o l a r i t y L=2 - 10) were assumed. The c e n t r o i d energies increase w i t h i n c r e a s i n g mu1 t i p o . l a r i t y , a1 so t h e widths a r e s t r o n g l y L dependent. The resonance widths were c a l c u l a t e d u s i n g two-body d i s s i p a t i o n , t h i s gives a q u i t e remarkable agreement w i t h o u r experimental f i n d i n g s f o r t h e widths o f L=2 and 3 resonances. I n f i g . 12 these r e s u l t s (dashed l i n e ) a r e compared w i t h an a ' spectrum taken a t 15' which extends up t o h i g h e r e x c i t a t i o n ener- gies. Although t h e absolute magnitude i s o f f by a l a r g e amount a q u a l i t a t i v e agree- ment w i t h t h e experimental s p e c t r a l form i s obtained, i n p a r t i c u l a r i n t h e h i g h e r e x c i t a t i o n region. A t lower e x c i t a t i o n s the y i e l d i s r a t h e r s m a l l . This can be s i g - n i f i c a n t l y improved by adding t o t h e macroscopic model I 1 3 1 features o f t h e s h e l l mo- d e l as discussed i n s e c t i o n 111. These a r e L c o n t r i b u t i o n s a t lower energies, as e.g.

F i g u r e 14: Higher mu1 t i p o l e continuum f o r a s c a t t e r i n g

(lower dashed and s o l i d l i n e s ) c a l c u l a t e d by use o f macrosco- p i c models (see r e f . 13) i n comparison w i t h experimental data from r e f . 5.

L=4 s t r e n g t h a t 2 h ~ e x c i t a t i o n and n o t o n l y a t 4%~. The r e s u l t s o f such a schematic model c a l c u l a t i o n are shown by t h e lower s o l i d l i n e i n f i g . 12 which shows a s p e c t r a l form q u i t e s i m i l a r t o those used experimentally. The widths o f t h e h i g h l y i n g broad resonances were assumed t o be t h e same as i n t h e macroscopic model, those a t lower e x c i t a t i o n were taken from RPA c a l c u l a t i o n s /12/ o r d i r e c t l y from our experiment.

S t i l l the problem o f the absolute background h e i g h t i s n o t solved. I n t h e c a l c u l a - t i o n s presented o n l y e x c i t a t i o n s up t o L=10 were included, f u r t h e r compression modes were n o t assumed. Both e f f e c t s can g i v e r i s e t o a s i g n i f i c a n t increase o f t h e d i r e c t y i e l d .

A t t h e end I l i k e t o discuss two types o f experiments from which i n f o r m a t i o n on t h e n a t u r e o f t h e background can be obtained. The f i r s t i s r e l a t e d t o t h e question o f d i r e c t e x c i t a t i o n , t h e second type o f experiments probes the average m u l t i p o l a r i t y o f t h e continuum.

The f i r s t question may be answered i n c o r r e l a t i o n experiments. As an example I want t o discuss a f i s s i o n decay coincidence experiment s t u d i e d a t J u l i c h . We measured angu- l a r c o r r e l a t i o n s of f i s s i o n fragments i n t h e 23*~(a,a'flf2) r e a c t i o n . The idea o f t h i s experiment was t o measure t h e l i n e a r momentum t r a n s f e r r e d t o t h e nucleus, t h i s gives r i s e t o an angle between t h e f i s s i o n products o f l e s s than 180'. The f i s s i o n products were measured by two p o s i t i o n s e n s i t i v e p a r a l l e l p l a t e detectors c o i n c i d e n t w i t h i n - e l a s t i c a l l y s c a t t e r e d a p a r t i c l e s detected a t 11'. F i s s i o n angle spectra a r e shown i n f i g . 15. For d i f f e r e n t outgoing a ' energies they peak r a t h e r c l o s e t o 180'. This i s very d i f f e r e n t from t h e i n c l u s i v e 238U(a,flf2) r e a c t i o n . I n t h e lower p a r t t h e average f i s s i o n angle and t h e t r a n s f o r m a t i o n t o t h e momentum p a r a l l e l t o the beam d i r e c t i o n i s p l o t t e d versus t h e outgoing a ' energy. The p a r a l l e l momentum t r a n s f e r c a l c u l a t e d i n two-body kinematics i s shown by t h e upper l i n e . Deviations from t h i s l i n e are observed which are due t o f a s t nucleon emission d u r i n g t h e s c a t t e r i n g pro- cess. I n t h e data t h e r e i s l i t t l e d e v i a t i o n from t h e two-body kinematical l i n e up t o

C4-198 JOURNAL DE PHYSIQUE

Figure 15: Fission angle s p e c t r a from t h e r e a c t i o n 2 3 8 ~ ( a , n ' f l f 2 ) and momen- t u m t r a n s f e r p a r a l l e l t o t h e beam a x i s ( P I / ) and corresponding average f i s s i o n ang e a s a f u n c t i o n o f outgoing I X ' energy. Two-body and three-body kine- matical 1 in e s a r e i n d i c a t e d .

Figure 16: Spectral form and angular d i s t r i b u t i o n of t h e continuum c a l c u l a t e d i n a quasl'free s c a t t e r i n g model ( s o l i d l i n e s ) i n comparison with 800 MeV pro- ton s c a t t e r i n g /19/.

30-40 MeV o f e x c i t a t i o n , t h i s supports t h e p i c t u r e o f a l a r g e d i r e c t e x c i t a t i o n i n the background y i e l d i n the r e g i o n o f t h e observed g i a n t resonances. Other type o f c o r r e l a t i o n experiments d e t e c t i n g d i f f e r e n t secondary p a r t i c l e s may g i v e s i m i l a r o r complementary i n f o r m a t i o n .

A second type o f experiment i s t h e i n v e s t i g a t i o n of t h e background behaviour a t ex- treme small angles. A t l a r g e r angles t h e background behaviour shows an exponential slope /ill. This i s expected t o change a t small angles due t o t h e f a c t t h a t f o r the dominant l a r g e angular momenta small momentum t r a n s f e r i s s t r o n g l y reduced i n d i r e c t s c a t t e r i n g . A f i r s t experiment was performed a t Los Alamos u t i l i z i n g 800 MeV proton s c a t t e r i n g /19/ which i n d i c a t e s a r e d u c t i o n o f t h e background y i e l d a t small angles (see f i g . 16). Both, s p e c t r a l form o f t h e background and t h e angular d i s t r i b u t i o n ( f i g . 16) are w e l l described using a q u a s i f r e e s c a t t e r i n g model /11,19/. By these c a l c u l a t i o n s and those presented i n f i g . 14 t h e usual background shape s u b t r a c t e d (discussed i n sect. 11) appears t o be j u s t i f i e d a p o s t e r i o r i . P r e l i m i n a r y r e s u l t s o f g i a n t resonance s t u d i e s i n ( a y e ' ) and a l s o ( 3 ~ e , t ) a t J u l i c h show t h e same t r e n d b u t a r a t h e r f l a t small angle behaviour. From these angular d i s t r i b u t i o n s t h e average m u l t i p o l a r i t i e s i n the e x c i t a t i o n o f t h e background continuum can be deduced.

SUMMARY

Giant resonance e x c i t a t i o n s i n hadron s c a t t e r i n g experiments have been reviewed which g i v e i n f o r m a t i o n on h i g h e r m u l t i p o l e e x c i t a t i o n s . Apart from t h e g i a n t quadrupole r e - sonance s t u d i e d f o r almost a l l s t a b l e n u c l e i t h e g i a n t octupole resonance i s t h e o n l y r a t h e r concentrated resonance o f h i g h e r m u l t i p o l a r i t y f o r which a systematic study i s possible. Resonances w i t h L 2 4 a r e expected t o have widths o f t h e o r d e r o f 10 MeV o r more, which appear t o be extremely d i f f i c u l t t o d e t e c t experimentally. Such h i g h L e x c i t a t i o n s have components i n t h e lower e x c i t a t i o n region; these could be s t u d i e d i n t h e case o f 208Pb f o r L=4 and L=6 e x c i t a t i o n s . I t i s c e r t a i n l y o f l a r g e i n t e r e s t t o study these type o f e x c i t a t i o n s a l s o f o r o t h e r n u c l e i . To l e a r n more about e x c i - t a t i o n s o f h i g h m u l t i p o l e s t r u c t u r e s i t i s important t o study t h e i n e l a s t i c continuum be1 ow t h e narrow resonances. Two examples of such experiments were discussed, c o r r e - l a t i o n experiments and small angle s t u d i e s which can g i v e i n f o r m a t i o n on mechanism and average mu1 t i p o l a r i t y . The comparison o f experimental background shapes w i t h t h e o r e t i c a l estimates confirms t h e importance o f h i g h L c o n t r i b u t i o n s a t lower ex- c i t a t i o n energies. F u r t h e r i t may g i v e i n s i g h t i n t o t h e nuclear damping mechanism i n t h e regime o f h i g h mu1 t i p ~ l a r i t i e s .

I n t h i s t a l k experimental r e s u l t s o n l y from proton and l i g h t i o n s c a t t e r i n g were co- vered which g i v e very u s e f u l complementary r e s u l t s . S p e c i f i c aspects of heavy i o n s c a t t e r i n g were n o t discussed; t h i s i s the s u b j e c t of several c o n t r i b u t i o n s t o t h i s meeting.

1 thank my colleagues P. Decowski, M. Rogge, P. Turek, L. Zem40, C. Mayer-Boricke, S.A. Martin, G.P.A. Berg, J. MeiBburger and J.G.M. Romer w i t h whom I have worked on t h e d i f f e r e n t a s c a t t e r i n g experiments discussed. Further, a very f r u i t f u l c o l l a b o r a - t i o n i n t h e c o r r e l a t i o n experiments w i t h G. Gaul, R. Glasow, 6. Ludewigt, R. Santo and W. Schumcher from the I n s t i t u t Fur Kernphysik, U n i v e r s i t y o f M i n s t e r , i s acknow- 1 edged.

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