HICOFED OUTLOOK

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HICOFED OUTLOOK

M. Lefort

To cite this version:

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JOURNAL DE PHYSIQUE

C o l l o q u e C4, s u p p l 6 m e n t a u n o 8 , Tome 47, a o Q t 1 9 8 6

HICOFED OUTLOOK

M. LEFORT

I n s t i t u t d e Physique N u c l B a i r e , BP 1 , F - 9 1 4 0 6 O r s a y , F r a n c e

Since the subject of HICOFED i s r a t h e r new, i t would be c e r t a i n l y a mistake t o draw any kind of conclusion from t h i s l i v e l y meeting. Neither could we make a summary. Although I am not sure t o we1 l understand what i t means, l e t us c a l l "outlook" the few considerations which follow.

I should l i k e t o t e l l you about some aspects which appeared t o me q u i t e typical and r a t h e r exciting f o r the future. Probably these remarks will not be accepted by every- body, but t h i s is a t l e a s t the privilege of the l a s t t a l k of the Conference, and probably the l a s t t a l k in my s c i e n t i f i c l i f e , t o be a b l e t o a s s e r t various considera- t i o n s without being contradicted.

I. GENERAL REMARKS

The main topics of t h e Conference were selected one year ago and i t seems t o me t h a t t h e choice was not so bad. Therefore, i t may be worthwhile t o follow i t . Before, l e t us consider some general aspects. When t h e f i r s t experiments i n t h e Fermi energy domain s t a r t e d with the CERN 85 MeV.A Carbon beam, we had nearly no idea about what we should look f o r . I t was a l i t t l e l i k e t h e s a i l o r s who were landing 500 years ago on t h e shore of t h e new world and who were Tooking a t t h e country w i t h t h e i r european eyes and habits. So, we had t o use r a t h e r simple concepts and methods t h a t we borrowed from other domains of nuclear reaction s t u d i e s , i .e. from t h e heavy ion c o l l i s i o n s e i t h e r a t lower energies or a t very high energies. Then, i t was not surprising t h a t the German group and the Scandinavian-Grenoble group were inspired by t h e abrasion model taken from the high energy s i d e whereas we were, a t Orsay, more i n t e r e s t e d in the l i n e a r momentum t r a n s f e r and i t s connection with fusion process. Nowadays, i t is real l y s t r i k i n g t o observe t h a t t h e r e a r e enough experimental data and theoretical approaches so t h a t s p e c i f i c attempts can be devoted t o the intermediate energy domain. The second remark is t h a t , a f t e r a period of three o r four years during which inclusive measurements were c a r r i e d on, equipments and ideas a r e ready f o r developing coincidence experiments and mu1 t i p1 i c i t y measurements.

Comparing with t h e evolution of t h e studies i n t h e low energy region during t h e se- venties i t i s i n t e r e s t i n g t o notice how the s i t u a t i o n i s d i f f e r e n t . The low energy *experiments s t a r t e d nearly without any theoretical model and the f i r s t r e s u l t s on

what was c a l l e d l a t e r on Deep I n e l a s t i c Collisions were t o t a l l y unexpected, since on- l y two c l a s s e s of c o l l i s i o n s were considered, compound nucleus formation and quasi- e l a s t i c t r a n s f e r reactions. We were indeed delighted when my old f r i e d Dieter ~ r o s s !

_B

around 1970, produced a model which was able t o describe our results by introducing f r i c t i o n forces along t h e deflection function of t h e c o l l i s i o n . A t t h a t time, i t was hard t o follow a leading thread and t o f i n d a frame i n order t o make prospects f o r new experiments.

Nowadays, in t h e Fermi Energy Domain, t h e s i t u a t i o n i s very d i f f e r e n t , a s i t appears p a r t i c u l a r l y well in t h i s Conference. The abundance of theoretical approaches is real- l y impressive, and t h e d i f f i c u l t y i s perhaps t o decide which of them is the most useful f o r designing more s p e c i f i c experiments. I t i s one of t h e thoughtsof Einstein t h a t theory should be elaborated in the purpose of designing which observable should be searched.

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CC428 JOURNAL DE PHYSIQUE

My l a s t remark concerns t h e so c a l l e d "exclusive" experiments. Nearly every t a l k was ended by the c a l l f o r exclusive experiments. I am not sure t h a t we a r e always prepared f o r them. S t r i c t l y speaking, do we know c l e a r l y what should be triggered and what should be excluded ? There a r e a few cases where coincidence experiments have been c a r r i e d on with success, because they were very s p e c i f i c a l l y prepared by precise hypothesis made in t h e frame of inclusive r e s u l t s . Typically, i t i s the case of f a s t l i g h t p a r t i c l e s emitted in coincidence with recoiling quasi-compound nuclei. Those p a r t i c l e s travel with t h e beam velocity and t h e i r observation brings an evidence t h a t the incomplete l i n e a r momentum t r a n s f e r and incomplete fusion can be treated l i k e an energy deposit from a p a r t i a l mass of t h e p r o j e c t i l e (see S. Leray ' S

contribution^.

Another example i s t h e study394 of l i g h t charged p a r t i c l e s emitted i s o t r o p i c a l l y in t h e c e n t e r of mass of t h e recoiling heavy nucleus source. I t shows t h a t a very excited heavy system evaporates protons, alpha p a r t i c l e s , deuterons and probably neutrons prior t o f i s s i o n ,even when the excitation enervy per nucleon reaches more than 3#eV/n. I am sure t h a t the fragment m u l t i p l i c i t y will be measured soon i n various conditions, i.e., in peripheral and in central c o l l i s i o n s , with the lowest possible energy thres- hold. Preliminary r e s u l t s were already given. My personal r e g r e t i s t h a t the delay i s now so long between data acquisitions and t h e f i n a l r e s u l t , a t l e a s t f o r people l i k e me who a r e eager t o r e a l l y understand what happens t o these nuclei heated a t 1 GeV !

Let us now review b r i e f l y the various topics.

11. NUCLEUS-NUCLEUS POTENTIAL

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ELASTIC SCATTERING

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TOTAL REACTION CROSS SECTION The heavy ion optical potential model i s always t h e s t a r t i n g point f o r describing the s c a t t e r i n g and the reactions. The extension of the Briickner method t o heavy ions l i k e i t was described by A. Faessler seems t o me q u i t e an important progress. I t i s indeed a l s o i n t e r e s t i n g t o see how t h e sudden and the a d i a b a t i c optical p o t e n t i a l s a r e near- l y i d e n t i c a l , both f o r t h e real part and the imaginary p a r t , a t the surface of the nucleus. Therefore, i t j u s t i f i e s t o t r e a t t h e f i r s t s t e p of t h e c o l l i s i o n i n terms of the sudden approximation (see f i g . 6 i n A. Faessler's contribution).

The knowledge of t o t a l reaction cross sections w i t h a g r e a t accuracy i s indeed q u i t e important, p a r t i c u l a r l y i n the Fermi Energy Domain where the nucleon mean f r e e path i n varying strongly, where the N-P, N-N and P-P cross sections a r e decreasing. Also we would l i k e t o know more about t h e neutron skin especially f o r l i g h t nuclei. The experimental methods, as they were described by Bruandet have been improved and i t will be q u i t e i n t e r e s t i n g t o apply them f o r UR measurements on reactions l i k e

1 2 ~ + I ~ c , 1% + 11C, where t h e neutron skin changes so much, when secundary "exotic" beams a r e available. We shall come back t o t h i s point l a t e r .

111. DISCRETE STATE EXCITATION

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QUASI-ELASTIC REACTIONS

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GIANT RESONANCES The big s p e c t r m e t e r SPEG i s now operating a t GANIL. I t i s too early t o know about i t s usefulness. I t i s certainly the best tool f o r studying quasi-elastic t r a n s f e r reactions which s t i l l occur a t 50 MeV.A, but w i t h very low cross sections and in an angular range i n t h e v i c i n i t y of zero degree. The t r a n s f e r of one proton from 208pb t o 2 0 9 ~ i iqduced by 160 a t $0 MeV.A was observed with t h e e f f e c t of matching by popu- l a t i o n inversion. I t i s too early t o say i f t h e very high s e l e c t i v i t y will be of such i n t e r e s t t h a t i t compensates the low cross section. Spin f l i p studies a r e a l s o a ra- t h e r new aspect of the high ve1,ocity of the p r o j e c t i l e .

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phonon excitation i s compatible with t h e Paul i principle i n t h e frame of t h e sudden approximation. A t lower energies, such a dissipation i s followed by t h e deep inelas- t i c process. For higher v e l o c i t i e s , such an energy dissipation may be a t the origine of the observed slowing down e f f e c t s on p r o j e c t i l e l i k e fragments which have t o be taken i n account in peripheral collisions below 100 MeV.A.

IV. PERIPHERAL COLLISIONS

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PROJECTILE-LIKE FRAGMENTS

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EMISSION OF NUCLEONS

P r o j e c t i l e - l i k e fragments emitted with a velocity not very f a r from t h e i n i t i a l pro- j e c t i l e velocity have been studied f o r quite a few years. But a l s o l i q h t e r fragme- n t s a r e emitted in a wide spectrum of q u i t e lower energies. Moreover, there a r e trag- ments issued from a source t r a v e l l i n g a t a velocity close t o half t h e beam velocity. That work s t a r t e d naturally with high energy considerations i n mind, and t h e elegant ~ o l d h a b e r s prescription was followed since i t was so successful a t high energies. I t i s of course always possible t o e x t r a c t a width from a bell shape spectrum, even i f t h e d i s t r i b u t i o n i s not symmetric. Therefore, a values were extracted, and compared with ao, t h e Fermi momentum width, according the pppular expression :

A p (Ap-AF) <P;>

a2 =a; A - 1 and a; =

P 5

where AF and Ap a r e the masses of the observed fragment and of the p r o j e c t i l e respec- t i v e l y (see f o r example R. Dayras' contribution).

I t i s now r a t h e r c l e a r t h a t such a procedure may lead t o wrong conclusions when ap- plied t o t h e Fermi Energy Domain. What we observe t h e r e i s not the "fragmentation" described a t 1. GeV.A. F i r s t , there i s a contribution of c l a s s i c a l t r a n s f e r reactions which should be subtracted even f o r fragments w i t h Z values equal t o Zp-2. Second,as was a1 ready mentioned ,sl,owi ng down 'processes due t o the/mean f i e l d a r e s t i l l occuring

,

even i f the clutching i n t o a two center intermediate+'is not produced. Third t h e separation i n t o spectators and a p a r t i c i p a n t which i s the basis of t h e abrasion-fragmen- t a t i o n model, i s indeed too schematic. Particularly i t i s d i f f i c u l t t o imagine a clean-cut of the p r o j e c t i l e spectator and a participant zone f l y i n g away l i k e t h e

" f i r e b a l l

"

and a t a r g e t spectator.

A big e f f o r t has been made in several laboratories t o build-up l a r g e multidetectors and coincidence experiments have s t a r t e d f o r measuring both fragments and f a s t l i g h t charged p a r t i c l e s . A t GANIL t h e huge chamber "Nautilus" has been used i n t h a t purpose and we have heard the f i r s t r e s u l t s on t h e system 40Ar + Ag a t 35 MeV.A. I t i s r a t h e r surprising, f o r example, t o see t h a t 50 % of t h e fragments Z = 16 a r e emitted without being escorted by a l i g h t fragment. B u t we don't know i f the primary fragment was Z = 18 and because of i t s excitation energy evaporated an alpha p a r t i c l e (not observed by t h e p l a s t i c wall of Nautilus), o r i f two protons o r an 4 ~ e were d i r e c t l y t r a n s f e r - red t o t h e target. So, we may repeat t h a t , before making very exclusive experiments, one should accumulate more information about m u l t i p l i c i t i e s and various s e t s of coincidence experiments.

P a r t i c l e - p a r t i c l e c o r r e l a t i o n s have been made. Their purpose i s t o t r y t o d e t e c t a hot zone from which t h e f a s t emission would originate. There a r e many objections against t h i s interpretation and f o r t h e moment t h e s i t u a t i o n i s r a t h e r confused. Amongst the coincidence experiments between t h e quasi-projectile and l i g h t charged p a r t i c l e s , i t is worthwhile t o notice an interesting attempt t o observe the deflec- t i o n of t h e p a r t i c l e s by the nuclear mean f i e l d , using f o r t h a t t h e amount of circu- l a r polarisation of y rays i n coincidence. The r e s u l t indicates t h a t they a r e emitted predominantly t o negative angles, which i s consistent with t h e deflection by an a t - t r a c t i v e nuclear mean f i e l d . If t h e interaction was purely of t h e type nucleon-nucleon c o l l i s i o n , t h e polarisation should be smaller. Perhaps we a r e here in presence of a new method, t y p i c a l l y adapted t o t h e Fermi energy domain, which i s able t o estimate the interplay between nucl e a r mean-field and nucleon-nucleon col l isions (see comnu- nication 27 by M.B. Tsang).

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C4-430 JOURNAL DE PHYSIQUE

i n t h e i n t e r m e d i a t e energy range by t r e a t i n g b o t h mean-field and nucleon-nucleon i n t e r a c t i o n s . There a r e several approaches, a l l r e f e r i n g more o r l e s s t o t h e Bol tzmann- Uehling-Uhlenbeck equation. I n t h a t respect,, t h e movy p i c t u r e presented by Gregoire was r e a l l y very suggestive. On t h e o t h e r hand such treatments l o o k a l i t t l e l i k e b l a c k magic boxes f o r e x p e r i m e n t a l i s t s and i t i s sometimesnice t o f o l l o w r a t h e r simple+ approaches l i k e t h e p r o d u c t i o n o f fragments as a r e s u l t o f c o l l i s i o n s o f pro- j e c t i l e nucleons w i t h t a r g e t nucleons (see B. Harvey). The neutron enrichment o f t h e fragments which i s observed when t h e t a r g e t i s neutron r i c h would then be t h e r e s u l t o f t h e known f a c t t h a t n-p s c a t t e r i n g cross s e c t i o n i s about t h r e e t i m e l a r g e r than n-n o r p-p cross sections, i n t h e range o f k i n e t i c energy which i s o f i n t e r e s t f o r us. A l a s t t r i v i a l remark : we a r e s t i l l unable t o answer p r e c i s e l y t o t h e question :

What i s t h e energy, o r t h e v e l o c i t y , where Deep i n e l a s t i c c o l l i s i o n s a r e n o t seen any more ? T h i s v e r y simple q u e s t i o n i s r e l a t e d t o t h e i m p o r t a n t problem t o know when t h e c l u t c h i n g becomes so d i f f i c u l t t h a t i t f a i l s . It i s a l s o r e l a t e d t o t h e p o s s i b i l i t y o f f u s i o n f o r a g i v e n p a r t i a l wave. A v e r y n a i v e idea i s t o compare t h e average r e l a - t i v e v e l o c i t y o f p r o j e c t i l e nucleons, vp, w i t h t h e Fgrmi v e l o c i t y VF. The average k i n e t i c energy o f i n c i d e n t nucleons i s <Ek> = mo/2 (vp + 3/5 V F ) ~ and t h e c l u t c h i n g becomes d i f f i c u l t i f <Ek> l EF + S, where S i s t h e average separation energy of a nucleon. T h i s would correspond t o v around 0.2 c, i.e. around 20 MeV/n. Rudolf has shown us t h e v e r y t y p i c a l deep i n e l g s t i c r e s u l t i n t h e r e a c t i o n 9 2 ~ 0 + 9 2 ~ 0 a t

18.2 MeV.A, w i t h a huge energy d i s s i p a t i o n , whereas f o r 86Kr + 9 8 ~ 0 and 86Kr + 9 3 ~ b a t 35 MeV.A, o t h e r processes, q u i t e new, a r e r e p l a c i n g D.I.C.

V. CENTRAL COLLISIONS

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HIGHLY EXCITED NUCLEI

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EQUATION OF STATE AND PHASE TRANSITION

I t i s n o t so c l e a r t o d e f i n e a c e n t r a l c o l l i s i o n when, f o r example one r e a l i s e s t h a t a t 30 MeV.A, a Krypton p r o j e c t i l e c a r r i e s a l i n e a r momentum o f 19 GeV/c, b u t a l s o can induce an a n g u l a r momentum o f 100 fi f o r an impact parameter as small as 1 Fermi. I n Central C o l l i s i o n s Studies t h e r e a r e t h r e e main aspects : l i n e a r momentum t r a n s f e r (LMT) r e l a t e d t o t h e e x t e n t of f u s i o n , h i g h l y e x c i t e d n u c l e i , l i m i t i n temperature o r e x c i t a t i o n energy f o r a n u c l e a r system.

I n o r d e r t o know t o which e x t e n t t h e f u s i o n process between two n u c l e i i s complete o r not,the e s t i m a t e o f t h e l i n e a r momentum t r a n s f e r i s an obvious method. The measurements a r e made by determining v e l o c i t i e s and p o s s i b l y masses o f r e c o i l i n g n u c l e i . The r e s u l t t h a t f u l l momentum t r a n s f e r c o u l d n o t be observed6 any more w i t h 12C, a t 30 MeV.A was t h e beginning o f a s e r i e s o f studies. The data a r e o f t e n presented i n terms o f

E,

which i s t h e r a t i o o f t h e average LMT t o t h e i n i t i a l l i n e a r momentum (so c a l l e d V i o l a systematics7). S y l v i e Leray has shown us a simple r e l a t i o n s h i p between and t h e r e l a t i v e v e l o c i t y ( ( E-v,)/A)~/~. There a r e t h e o r e t i c a l treatments, u s i n g Boltzmann equation which reproduce more o r l e s s t h i s behaviour, assuming t h a t nucleon-nucleon i n t e r a c t i o n s i n h i b i t more and more t h e mean-field f u s i o n process when t h e v e l o c i t y increases.

But another p r e s e n t a t i o n o f t h e r e s u l t s works as w e l l . It was suggested by S a i n t - Laurent e t a1 on t h e b a s i s o f a systematic study o f helium induced r e a c t i o n s 8 and then was g e n e r a l i z e d t o heavy ionsg. The e s s e n t i a l remark i s t h a t t h e l i m i t increases w i t h t h e mass o f t h e p r o j e c t i l e and f i n a l l y corresponds t o a v a l u e o f t h e o r d e r o f 180 MeV/c p e r nucleon. Such a remark d i r e c t s towards o t h e r p o s s i b i l i t i e s f o r e x p l a i - n i n g t h e r e s u l t . t o r example one may n o t i c e t h a t t h e corresponding v e l o c i t y , 0.2 c, approaches t h e e s t i m a t e o f t h e sound v e l o c i t y i n n u c l e a r m a t t e r .

Fusion, Incomplete f u s i o n , Energy d e p o s i t

The r e l a t i o n between l i n e a r momentum and energy d e p o s i t i o n i s s t r a i g h t forward i f t h e l a c k o f momentum i s due t o a l a c k o f t r a n s f e r r e d mass, as t h i s has been demons- t r a t e d . Therefore nowadays, we know t h a t e x c i t a t i o n energies c l o s e t o 1 GeV can r e a l l y b e deposited i n n u c l e a r composite systems, as I made t h e hypothesis9 a l r e a d y a t t h e German Physical S o c i e t y i n Bad Honeff (1981) a f t e r t h e CERN experiments. The simple r e l a t i o n i s t h e f o l l o w i n g :

E* = <P>' MT

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where <p> i s t h e mean t r a n s f e r of l inear momentum,

M;

t h e mass incorporated i n t o the fusion t o the t a r g e t mass MT, assuming <p>/pi = <M1>/Mp

.

Jacob Bondorf t o l d me t h a t we a r e discovering what Niels Bohr was t e l l i n g in 8936, exactly 50 years ago, in one of h i s three papers where the compound nucleus was invented : " I t i s conceivable t h a t a compound nucleus may sustain an excitation energy of t h e order of 1 GeV".

B u t a s e t of experiments have shown t h a t t h e s t a t i s t i c a l treatment i s not e n t i r e l y c o r r e c t when i t predicts f o r very heavy systems the predominance of f i s s i o n over any p a r t i c l e evaporation. We heard a t t h i s meeting t h a t a more elaborated analysis ap- plying the Fokker-Planck equation t o the very short-lived composite system predicts indeed t o a large extent l i g h t charged p a r t i c l e and neutron emission p r i o r t o f i s s i o n . H. Delagrange has shown t h a t when f i s s i o n does occur, nuclei have l o s t indeed so many neutrons t h a t they a r e not so hot. Also, i f the composite system has been so much depleted in mass, the f i s s i o n process may not occur a t a l l . This was experimentally shown in t h e case of Ar + 1 6 5 ~ 0 by Rivet e t a l . (communication 39).

A careful analysis of the k i n e t i c energy spectra of l i g h t charged p a r t i c l e s emitted in backward angles ( t h e evaporation origin should be checked) i s probably t h e only way t o estimate t h e temperature of such excited nuclei. There a r e several cares t h a t should be taken. J . Alexander has explained how t h e angular momentum and t h e deforma- tion could modify both t h e slope and the maximum of t h e spectra. Also nuclei a r e coo- led down through a long cascade of p a r t i c l e evaporation and a s Natowitz has pointed out, t h e experimental r e s u l t i s an averaging of a long s e t of decreasing temperatures from the very hot i n i t i a l system down t o a nearly cold nucleus. Fortunately,for very heavy systems, a small number of l i g h t charged p a r t i c l e s (alpha and protons) a r e evaporated in the very f i r s t stage, followed by neutron emission and f i n a l l y f i s s i o n . Therefore a natural selection i s made of very hot s p e c i e d . A l a s t word about the important improvement due t o the measurements of neutron m u l t i p l i c i t i e s which a r e now possible with the help of the "old" neutron s c i n t i l l a t o r b a l l . I t provides a very di- r e c t tool t o estimate t h e excitation energy and a t t h e same time t h e reaction cross section (see Jahnke's contribution).

Now we a r e overcoming t h e l i m i t s of E* above which t h e known processes of de-excitatim a r e not observed any more. I am more and more convinced of t h a t experimental f a c t , but we s t i l l don't know what is i t s signification. In t h a t respect the recent systematic study a t various incident energies f o r t h e angular correlation of f i s s i o n fragments emitted in the reaction 5 8 ~ i + 2 3 2 ~ h shows very t y p i c a l l y what was already observed f o r 40Ar + 238U, i . e . t h e decrease in heiqht f o r the bump of l a r g e momentum t r a n s f e r a t 9.5 GeV/c ( C . Volant, communication 40). The corresponding excitation energy f o r t h e highest bombarding energy of 30 MeV.A, reaches 920 MeV, about t h e same as f o r Ar + U a t 35 MeV.A. This i s j u s t on t h e shore of t h e l i m i t around

E* = E*/A = 3.5 MeV per nucleon. Then t h e f i g u r e which was given10 l a s t year i s s t i l l

valid. I t has been confirmed by some new r e s u l t s which I have added.

l 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

60 100 l60 ZOO 260

A eomposita

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What happens ? What should we l o o k f o r ? Here i s a r e a l challenge. A v e r y a t t r a c t i v e p o s s i b i l i t y i s t h e m u l t i f r a g m e n t a t i o n which was proposed by J. Bondorf i n t h e frame o f thermal s t a t i s t i c a l theory, and by X. Campi using a p e r c o l a t i o n model. Both approaches l e a d t o s i m i l a r conclusions and a r e p h y s i c a l l y e q u i v a l e n t . Furthermore t h e y c o u l d concern c e n t r a l c o l l i s i o n s as w e l l as those c o l l i s i o n s w i t h l a r g e r impact para- m e t e r s ' f o r which one observes l i g h t fragments o r i g i n a t i n g from a source a t roughly h a l f t h e beam v e l o c i t y (Borderie). There a r e a l s o t h e i n t e r e s t i n g r e s u l t s given by Rudolf on two fragment c o r r e l a t i o n s f o r c o l l i s i o n s between Krypton + Molybenum and Xenon + S i l v e r . But t h e r e a l answer w i l l be g i v e n by c a r e f u l l m u l t i p l i c i t y measurements o f l i g h t and medium mass fragments, and f o r t h e moment we have i n hands o n l y t h e r e s u l t s o f n u c l e a r emulsions, which a r e indeed good C IT detectors. The experimen- t a l e f f o r t t o be,made f o r e l e c t r o n i c counters i s huge and we have u n f o r t u n a t e l y t o w a i t . Now, o f course, m u l t i f r a g m e n t a t i o n i s n o t t h e o n l y t h e o r e t i c a l p o s s i b i l i t y which has been proposed, and we heard how a h o t nucleus c o u l d undergo v a p o r i s a t i o n a t a tempera- t u r e h i g h e r than some 8 MeV, i n t h e frame o f a s t a t i c + dynamic treatment o f t h e nu- c l e a r i n t e r a c t i o n w i t h t h e mean-field.

F i n a l l y , t h e proposal o f equation o f s t a t e and phase t r a n s i t i o n i n n u c l e a r matter, which was made several years ago, has l e a d t o many t h e o r e t i c a l discussions more than t o a r e a l p r e d i c t i o n f o r a c r u c i a l experiment.

V I . COOPERATIVE PROCESSES

Only a very few words on t h e subject. The i n t r o d u c t i o n o f h i g h l y e n e r g e t i c gamma r a y s as a p o s s i b l e e f f e c t o f cooperative processes i n t h e n u c l e a r c o l l i s i o n s a t energies between 30 MeV.A and 100 MeV.A has been v e r y f r u i t f u l . The idea o f proton-neutron bremsstrahlungll goes back t o 1966, and t h e r e s u l t s shown by Pinston a r e q u i t e con- vincing. The review on p i o n p r o d u c t i o n made by K n o l l and Gross i s i n f a v o r o f a coo- p e r a t i v e model w i t h composite p a r t i c l e s as f i n a l s t a t e s .

V I I . PRODUCTION OF NUCLEI FAR OFF THE BETA STABILITY LINE

The l a s t b u t n o t t h e l e a s t : I should confess t h a t I was n o t very o p t i m i s t i c about t h e i n t e r e s t o f heavy i o n p r o j e c t i l e s as compared t o e n e r g e t i c protons f o r t h e produc- t i o n o f new isotopes, p a r t i c u l a r l y along t h e d r i p l i n e . The reason was t h a t I was t h i n k i n g o n l y about t h e evaporation residues. As D. Guerreau has shown us, t h e p e r i - pheral r e a c t i o n s a r e indeed a v e r y e f f i c i e n t t o o l which s p r i n k l e s b o t h sides o f t h e s t a b i l i t y v a l l e y . The d r i p l i n e has been reached f o r neutron r i c h n u c l e i up t o Z = 20. On t h e o t h e r side, i n t e r e s t i n g n u c l e i have been discovered l i k e 3 1 ~ r which i s a good c a n d i d a t e f o r 2 p r o t o n r a d i o a c t i v i t y . Moreover spectroscopic s t u d i e s have been c a r r i e d on w i t h v e r y ingenious technics.

My main remark on t h i s s u b j e c t i s f i n a l l y on t h e experimental aspect. Most o f t h e r e s u l t s i n d i c a t e d above were o b t a i n e d because t h e r e was ready a t GANft s i n c e t h e beginning an apparatus c a l l e d L i s e , p a r t i c u l a r l y w e l l adapted f o r t h e i d e n t i f i c a t i o n of p e r i p h e r a l c o l l i s i o n products. The c o n s t r u c t i o n o f L i s e was a d i f f i c u l t d e c i s i o n and I am proud t h a t i t was taken i n time.

I should l i k e t o f i n i s h t h i s paragraph w i t h a remark on t h e secondary beamwhich were described by Ren6 Bimbot. Again, L i s e appears l i k e a wonderful instrument, s i n c e i t i s p o s s i b l e t o s e l e c t u n s t a b l e r e a c t i o n products, l i k e I ~ N , 1 6 ~ o r 3 8 ~ , t o s t e e r t h e beam o f these n u c l e i and t o reach a h i g h degree o f p u r i f i c a t i o n w i t h an i n t e n s i t y which i s a l r e a d y o f i n t e r e s t , although we a r e o n l y a t t h e beginning o f t h e t e c h n i c a l

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Our HICOFED Conference comes t o

i t s end, three years only a f t e r the f i r s t beams

a t Vicksi, GANIL, SARA and MSU. You will forgive me t o show again a f i g u r e where t h e main c h a r a c t e r i s t i c s of our Energy Domain a r e indicated (Fig. 2 ) .

Fig. 2

-

Energies in MeV.A, center of mass energies on 23% f o r various p r o j e c t i l e s Ap. Also indicated nucleon wave length, v e l o c i t i e s in c units.

There were, along these f i v e days, so many i n t e r e s t i n g r e s u l t s presented, and t h e discussion was so a c t i v e t h a t we can already claim t h a t the choice of theses acce- l e r a t o r s was indeed a good one. Also, both t h e theoretical ideas and t h e experimental equipment, which has been b u i l t and which i s in preparation, give us t h e g r e a t e s t hope f o r t h e future. I wish t h a t t h e beautiful work which i s going on will overcome the pessimistic propency which f l o a t s over nuclear physics p a r t i c u l a r l y i n our coun- t r y . My l a s t wishes will be f o r our colleagues who a r e expecting the success of t h e i r mew accelerators in Catania, Chalk River, Texas, Tokyo and Lanchow. I hope t h a t you will come back t o your laboratories with in mind t h i s E p i c t e t e l s thought : "Those thinqs belong t o you t h a t you have stored i n your memory and t h a t you keep i n your heart".

References

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2. Galin,J., Gatty, B., Lefort, M., Peter, J 7 a r r a g 0 , X. and Basile, R., Phys. Rev. 182 (1969) 1267.

3. %g, S., Rivet, M.F., Bimbot, R., Borderie, B., Forest, I., Galin, J . , Gardes, D., Gatty, B., Lefort, M., Oeschler, H., Tamain, B., Tarrago, X . , Phys.Lett.130B (1983) 14.

4. Jacquet, D., Duek, E., Alexander, J.M., Borderie, B., Galin, J., GardBs, D.,

Guerreau, D., Lefort, M., Yonnet, F., R.ivet, M.F., Tarrago, X., Phys.Rev.Lett.

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5. Goldhaber, A.S., Phys.Rev. C17 (1978) 2243.

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Galin, J., Oeschler, H., Song, S., ~ o r * i e , B., Rivet, M.F., Forest, I.,

Bimbot, R., Gardes, D., Gatty, B., Guillemot, H., L e f o r t , M., Tamain, B.,

Tarrago, X., Phys.Rev.Lett. 48 (1982) 1787.

7. Back, B.B., Wolf, K.L., M i g n z y , A.C., Gel bke, C.K., Awes, T.C., Breuer, H., V i o l a Jr., V.E. and Dyer, P., Phys.Rev. C22 (1980) 1927.

8. S a i n t Laurent, F., Conjeaud, H., ~ a y r a s , X , Harar, S., Oeschler, H. and Volant, C., Phys.Lett'. 110B (1982) 372.

9. L e f o r t , M., Nucl.Phys. A587 (1982) 3c-24c.

lO.Lefort, M,, Proc. I t a l i f i y s . Soc., R.A. R i c c i and C. V i l l i , 467 (1986).

Borderie, B., IPNO-DRE-85-23 (1985).