HIGH EXCITATION ENERGY STRUCTURES IN HEAVY ION COLLISIONS ON A208Pb TARGET

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HIGH EXCITATION ENERGY STRUCTURES IN

HEAVY ION COLLISIONS ON A208Pb TARGET

Y. Blumenfeld, Ph. Chomaz, N. Frascaria, J. Garron, J. Jacmart, J.

Roynette, W. Bohne, W. von Oertzen, M. Buenerd, A. Gamp, et al.

To cite this version:

J O U R N A L

DE

PHYSIQUE

Colloque C4, supplirnent au

n03, Tome

45,

mars

1984 page C4-201

H I G H E X C I T A T I O N ENERGY STRUCTURES I N HEAVY I O N C O L L I S I O N S ON A

2 0 8 ~ b

TARGET

Y. Blumenfeld, Ph. Chornaz, N. F r a s c a r i a , J . P . Garron, J.C. Jacmart, J.C. Roynette, W. ~ o h n e * , W. Von ~ e r t z e n * , M. ~uenerd**, A. Gamp**,

D . Lebrun** and Ph.

art in**

I n s t i t u t de Physique NucZdaire, Orsay, France

*Hahn Meitner I n s t i t u t , Berlin,

F . R. G.

* * l n s t i t u t des Sciences NucZ&aires, GrenobZe, France

R6sum6 : Des s t r u c t u r e s o n t 6 t Q observees dans l e s s p e c t r e s en e n e r g i e des f r a g - ments i s s u s des r e a c t i o n s 3 6 ~ r

+

208pb h 1 1 ~ e V / n e t 2 0 ~ e + 208pb 3

30 MeV/n jusqu'h une e n e r g i e d ' e x c i t a t i o n d ' e n v i r o n 130 MeV. L ' i d e n t i f i c a - t i o n complBte en masse e t en charge, a

ete

obtenue

B

l ' a i d e d'un systeme de temps de v o l a s s o c i 6 h un t g l e s c o p e AE.E. I1 e s t montre que c e s s t r u c - t u r e s s o n t dues 2 l ' e x c i t a t i o n du noyau c i b l e .

A b s t r a c t : Energy s p e c t r a of fragments from t h e 3 6 ~ r

+

2 0 8 ~ b r e a c t i o n a t 11 MeV/n and 2 0 ~ e 4 208pb r e a c t i o n a t 30 MeV/n were measured u s i n g a time of f l i g h t spectrometer. S t r u c t u r e s ranging up t o 130 MeV e x c i t a t i o n energy a r e obser- ved i n t h e i n e l a s t i c s p e c t r a . These s t r u c t u r e s a r e shown t o be due t o an e x c i t a t i o n of t h e 208pb t a r g e t n u c l e u s .

I - I n t r o d u c t i o n

The g r o s s f e a t u r e s of t h e energy s p e c t r a of e j e c t i l e s from heavy ion c o l l i - s i o n s a t low bombarding e n e r g i e s ( 2 10 MeV/n) can be i n t e r p r e t e d i n terms of two main r e a c t i o n mechanisms : p e r i p h e r i c a l c o l l i s i o n s ( i n e l a s t i c o r t r a n s f e r r e a c t i o n s ) and more c e n t r a l c o l l i s i o n s c a r a c t e r i s e d by a complete energy r e l a x a t i o n (deep ine- l a s t i c r e a c t i o n s ) . A d e t a i l e d s t u d y o f t h e s e energy s p e c t r a i n s e v e r a l symmetric o r n e a r l y symmetric heavy i o n r e a c t i o n s revealed t h e a d d i t i o n a l presence of broad s t r u c - t u r e s ranging up t o an e x c i t a t i o n energy of approximately 130 MeV /I-31. More d e t a i - l e d information on t h e s e bumps was subsquently o b t a i n e d by t h e s t u d y of t h e

4 0 ~ a

+

4 0 ~ a c o l l i s i o n a t t h r e e d i f f e r e n t i n c i d e n t e n e r g i e s 4 MeV/n

141,

7. 5 MeV/n / I / and 10 MeV/n / 5 / . The main f e a t u r e s of t h e high e x c i t a t i o n energy (H.E.E.) s t r u c t u r e s can be summarized a s f o l l o w : i ) They a r e only observed i n t h e i n e l a s t i c and few nu- c l e o n t r a n s f e r channels. i i ) Their e x c i t a t i o n e n e r g i e s a r e independent of a n g l e . i i i ) Within t h e allowed e x c i t a t i o n energy ranges, t h e i r p o s i t i o n s a r e independent of beam energy. i i i i ) The s t r u c t u r e s a r e observed more c l e a r l y a t higher e x c i t a t i o n e- nergy. i i i i i ) T h e i r widths i n c r e a s e with i n c r e a s i n g e x c i t a t i o n energy E ~ [ T 'L 5 MeV f o r Ex '7. 25 MeV and

r

25 MeV f o r Ex 'L 100 M ~ V ]

.

S e v e r a l i n t e r p r e t a t i o n s have been given / 5 / up t o now b u t have f a i l e d i n reproducing a l l t h e f e a t u r e s of t h e d a t a . A p o s s i b l e i n t e r p r e t a t i o n c o n s i s t e n t with t h e s e o b s e r v a t i o n s i s t h e e x c i t a t i o n of high energy c o l l e c t i v e modes such a s g i a n t resonances.

U n t i l r e c e n t l y t h e s e s t u d i e s were l i m i t e d t o symmetric systems. Thus a cer- t a i n number of open q u e s t i o n s r e m a i q t h e f i r s t of which being whether o r n o t such s t r u c t u r e s a r e a l s o observed i n very asymmetric c o l l i s i o n s . Furthermore, i n such r e a c t i o n s where two heavy n u c l e i c o l l i d e many degrees of freedom a r e involved and it i s l e g i t i m a t e t o ask i f t h e observed bumps a r e due t o an e x c i t a t i o n of t h e t a r g e t nu- c l e u s , t h e p r o j e c t i l e nucleus, a mutual e x c i t a t i o n o r even t o t h e e x i s t e n c e of a s h o r t l i v e d composite system.

To check t h e above mentionned i n t e r p r e t a t i o n and t o answer t h e preceding q u e s t i o n s , two new experiments have been performed by bombarding a 2 0 8 ~ b t a r g e t w i t h

C4-202 JOURNAL

DE PHYSIQUE

11 MeV/n 3 6 ~ r and 30 MeV/n 2 0 ~ e beams. The experimental procedures w i l l be d e s c r i b e d below and subsequently t h e r e s u l t s w i l l be p r e s e n t e d and d i s c u s s e d .

I1 - Experimental s e t up

11.1 2 2 ~ r ~ - _ ' 3 " ~ b s e x ~ s r i m e n t

The 392 MeV 36Ar beam from t h e VICKSX a c c e l e r a t o r of B e r l i n was used t o bombard a 500 pg/cm2 i s o t o p i c a l l y pure Pb t a r g e t d e p o s i t e d on a t h i n carbon backing. I n o r d e r t o o b t a i n an unambiguous mass and charge i d e n t i f i c a t i o n of t h e heavy e j e c - t i l e s t o g e t h e r with a g o o d e n e r g y r e s o l u t i o n a time of f l i g h t s e t up was used. The s t a r t s i g n a l was o b t a i n e d by a c c e l e r a t i n g t h e e l e c t r o n s e m i t t e d by a 40 pg/cm2 c a r - bon f o i l s e t a t 45O t o t h e i n c i d e n t p a r t i c l e s onto two microchannel p l a t e s . The ac- c e l e r a t i n g system used a s e t of 5 e l e c t r o d e s i n s t e a d of t h e u s u a l g r i d i n o r d e r t o avoid c r e a t i n g p a r a s i t e s t r u c t u r e s i n t h e energy s p e c t r a of t h e d e t e c t e d fragments. The s t o p s i g n a l was given by two s o l i d s t a t e E d e t e c t o r s having an a n g u l a r separa- t i o n of I " a s s o c i a t e d w i t h a AE gaz i o n i s a t i o n chamber f o r charge i d e n t i f i c a t i o n . The f l i g h t p a t h was 160 cm long and t h e time r e s o l u t i o n % 130 p s y i e l d i n g a mass r e - s o l u t i o n

&

% I %. The o v e r a l l energy r e s o l u t i o n was AE % 2.5 MeV. The measured an-

m

g l e s were Olab = 15, 20, 25 and 30°, t h e c a l c u l a t e d g r a z i n g angle f o r t h e r e a c t i o n being Ograz % 28".

I n t h i s c a s e , a 1 mg/cm2 2 0 8 ~ b t a r g e t was bombarded w i t h t h e 600 MeV 2 0 ~ e beam of t h e SARA f a c i l i t y i n Grenoble. The experimental s e t up was s i m i l a r t o t h e one d e s c r i b e d above, t h e i o n i s a t i o n chamber being r e p l a c e d by two AE s i l i c o n s u r f a c e b a r r i e r d e t e c t o r s from which t h e s t o p s i g n a l s were d e r i v e d . Because of t h e l i g h t e r p r o j e c t i l e and h i g h e r i n c i d e n t energy a conventionnal E.AE s o l i d s t a t e t e - lescope a l s o y i e l d e d an e x c e l l e n t mass i d e n t i f i c a t i o n i n a d d i t i o n t o t h e usual charge s e p a r a t i o n . The measurement was performed a t only one a n g l e Olab = 10'. t h e g r a z i n g a n g l e f o r t h e r e a c t i o n . The t o t a l energy r e s o l u t i o n was AE 2. 3.5 MeV. 111 - R e s u l t s and d i s c u s s i o n

This paper w i l l be focused on t h e i n e l a s t i c s p e c t r a measured i n both s t u d i e d r e a c t i o n s . The t r a n s f e r channel s p e c t r a , w h e r e s t r u c t u r e s a r e a l s o observed w i l l be p r e s e n t e d elsewhere / 6 / . Because of t h e very high e l a s t i c peaks, a l l t h e i n e l a s t i c s p e c t r a w i l l be p r e s e n t e d u s i n g a l o g a r i t h m i c s c a l e . The i n d i c a t e d e x c i t a t i o n ener- g i e s a r e c a l c u l a t e d assuming two body r e a c t i o n s .

111.1 r i c k u g ~ b r e a k

;p

c o n t r i b u t i o n

It i s w e l l known t h a t t h e c o n t r i b u t i o n t o t h e s p e c t r a of t h e so-called pick up-break up mechanism can be important. This p r o c e s s c o n s i s t s of p a r t i c l e t r a n s f e r s from t h e t a r g e t t o t h e p r o j e c t i l e nucleus followed by t h e subsequent de-excita- t i o n of t h i s q u a s i p r o j e c t i l e by emission of nucleons of t h e same type a s t h e t r a n s - f e r e d nucleons ( f o r example 36Ar

+

2 0 8 ~ b + 37Ar + 2 0 7 ~ b + 3 6 ~ r

+

207pb

+

n ) . The do- minant c o n t r i b u t i o n i s due t o t h e one nucleon t r a n s f e r . It can g e n e r a t e bumps i n t h e energy spectrum of t h e r e s i d u a l nucleus c e n t e r e d a t an a p p a r e n t e x c i t a t i o n energy E* which can b e c a l c u l a t e d bv

A , L

- - .

- ,

where Ap and AT a r e r e s p e c t i v e l y t h e p r o j e c t i l e and t a r g e t mass numbers, Ela?, t h e i n c i d e n t l a b o r a t o r y energy, E l p t h e average k i n e t i c energy of t h e e m i t t e d nucleon and Q t h e Q v a l u e f o r t h e p i c k up-break up r e a c t i o n . Table 1 2 0 ~ e + 2 0 8 ~ b , Elab=600 M:e E* 2r 39'MeV E* 2, 41 MeV , Neutron transfer Proton transfer 4.

36Ar+208pb, Elab=400 MeV E* 'Ir 2 3 M e V

This d e t e r m i n a t i o n i s s t r a i g h t forward b u t a c o r r e c t c a l c u l a t i o n of t h e width and c r o s s s e c t i o n s of t h e s e c o n t r i b u t i o n s i s much more d e l i c a t e 131. Thus, i n t h e follow- i n g paragraphs we w i l l n o t i n c l u d e i n our d i s c u s s i o n t h e r e g i o n of t h e i n e l a s t i c s p e c t r a where t h i s c o n t r i b u t i o n i s p r e s e n t .

111.2 Experimental r e s u l t s

111 2a. 3 6 ~ r + 2 0 8 ~ b experiment

Figure 1 p r e s e n t s t h e low e x c i t a t i o n energy p a r t (Ex

<

40 MeV) of t h e i n e l a s t i c s p e c t r a from t h e A r + Pb r e a c t i o n a t t h e f o u r measured a n g l e s . Bumps su- perimposed on t h e high t a i l of t h e e l a s t i c peak, c l e a r l y show up a t a l l a n g l e s a t e x c i t a t i o n e n e r g i e s Ex = 11.5, 13.5, 17.5, 21.5, 25.5 and 35 MeV

'

1 MeV. The posi- t i o n of t h e f i r s t one i s i n good agreement with t h e w e l l known g i a n t quadrupole r e - sonance (G.Q.R.) i n 2 0 8 ~ b . The t h r e e f o l l o w i n g bumps have a l r e a d y been observed i n o t h e r l i g h t ion and l i g h t heavy ion experiments

/

7-81. The peak a t 13.5 MeV was a s - sumed t o a r i s e from both t h e g i a n t monopole (G.M.R.) and t h e g i a n t d i p o l e (G.D.R.) resonances 181 while t h e peak a t 17.5 MeV was r e l a t e d t o t h e high e x c i t a t i o n octu- p o l e resonance (H.E.O.R.) 171. The 21.5 MeV bump was found t o be a mixture of t h e g i a n t i s o s c a l a r d i p o l e and high m u l t i p o l a r i t y g i a n t resonances 171.

Since t h e s t r u c t u r e s a t much higher e x c i t a t i o n energy a r e expected t o be very wide a compression of t h e s p e c t r a (2 MeVIchannel i n s t e a d of 0.5 MeVIchannel) p e r m i t s t h e i r c l e a r o b s e r v a t i o n up t o e x c i t a t i o n e n e r g i e s above 100 MeV ( f i g . 2 ) . The e s t i m a t e d e r r o r on t h e p o s i t i o n s of t h e s e h i g h e x c i t a t i o n energy s t r u c t u r e s i s about ? 3 MeV. The width of t h e observed s t r u c t u r e s a r e seen t o i n c r e a s e with i n c r e a - s i n g e x c i t a t i o n energy.

Angular d i s t r i b u t i o n s f o r f o u r of t h e observed s t r u c t u r e s a r e presen- t e d on f i g . 3 . They r e s u l t from t h e i n t e g r a t i o n of t h e bumps a f t e r s u b t r a c t i o n of an e s t i m a t e d background. The e r r o r b a r s i n c l u d e both t h e s t a t i s c a l e r r o r and an estima- t e d e r r o r on t h e background. While a t Ex = 25 MeV t h e background i s very important due t o t h e t a i l of t h e e l a s t i c peak a t t h e h i g h e r e x c i t a t i o n e n e r g i e s (Ex = 96 and 138 MeV) it has almost e n t i r e l y d i s s a p e a r e d l e a v i n g only t h e s t a t i s t i c a l e r r o r s . The angular d i s t r i b u t i o n of t h e Ex = 25 MeV s t r u c t u r e shows a s t r o n g maximum a t t h e gra- zing a n g l e (Ograz = 28'). A t Ex = 52 MeV t h e maximum i s l e s s pronounced and h a s mo- ved towards a s m a l l e r a n g l e while t h e s t r u c t u r e s a t h i g h e s t e x c i t a t i o n energy have a monotically d e c r e a s i n g a n g u l a r d i s t r i b u t i o n and t h e i r c r o s s s e c t i o n v a n i s h e s almost completely a t @ l a b = 30".

-

Fig. 5 d i s p l a y s t h e i n e l a s t i c s p e c t r a of 36Ar and 2 0 ~ e measured r e s - p e c t i v e l y a t Olab = 20" and 10'. The g e n e r a l t r e n d s of t h e s e two s p e c t r a a r e s t r i - k i n g l y s i m i l a r , except i n t h e r e g i o n of 2. 35 MeV e x c i t a t i o n where t h e c o n t r i b u t i o n of t h e p i c k up-break up p r o c e s s i s expected i n t h e 20Ne

+

208pb experiment. I n addi- t i o n of t h e low l y i n g g i a n t resonances a t 11.5, 17.5, 21 MeV which a r e c l e a r l y obser- ved i n b o t h r e a c t i o n s ( s e e f i g . 2 ) , a t e x c i t a t i o n e n e r g i e s above 40 MeV t h e H.E.E. s t r u c t u r e s a r e p r e s e n t a t t h e same e x c i t a t i o n e n e r g i e s i n b o t h s p e c t r a up t o appro- ximately 100 MeV e x c i t a t i o n energy (Ex = 44, 52, 61, 73

+

3 MeV). This independence of t h e p o s i t i o n of t h e s t r u c t u r e s on bombarding energy and p r o j e c t i l e mass unambi- gously demonstrates t h a t an e x c i t a t i o n of t h e 208pb t a r g e t i s r e s p o n s i b l e f o r t h e s e s t r u c t u r e s .

C4-204 J O U R N A L

DE PHYSIQUE

Fig.l.2

-

Energy spectra at different angles of 'nelastically scattered 3 6 ~ r from the

46Ar + 208pb reaction at Elab=ll MeV/n

(see text).

Fig.3

-

Angular distribution for 4 of the obser- ved structures (see text)

Fig.4

-

Comparison of the inelastic channels for both studied reac- tions.

t

%A, + 2 0 8 ~ b I

IV

-

Summary and conclusion

The inelastic spectra produced in two very asymmetric heavy ion collisions 3 6 ~ r + 2 0 8 ~ b and

ON^

+

208pb have been measured at bombarding energies of respec- tively

1 1

MeV/n and 30 MeV/n. The giant quadrupole resonance is clearly excited in these reactions. At high excitation energies, structures of the same type as those previously observed in symmetric systems are strongly populated in these asymmetric collisions but are more numerous and narrower than the ones observed in the

4 0 ~ a + 4 0 ~ a experiment. Furthermore, these structures lie at the same excitation energies for the two studied reactions. It is now clearly established that their positions are independent of angle, incident energy and the nature of the projec- tile. This clearly proves that they aredueto an excitation of the target nucleus. In the light of these new results the hypothesis of an excitation of collective modes such as giant resonances in these heavy ion reactions is strengthened.

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T - 81-02