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VIOLENT COLLISIONS BETWEEN IONS AS HEAVY
AS KRYPTON AT GANIL AND GSI
G. Rudolf
To cite this version:
JOURNAL DE PHYSIQUE
Colloque C4, supplément au n° 8, Tome 47, août 1986 C4-351
VIOLENT COLLISIONS BETWEEN IONS ftS HEAVY AS KRYPTON AT GANIL AND GSI
G. RUDOLF
Centre de
Recherches Nucléaires, BP 20 CR, F-67037 Strasbourg
Cedex, France
Résumé - Des résultats récents obtenus pour des collisions violentes entre un faisceau de masse élevée (A ^ 100) et des cibles de masse comparable sont exposés. Jusqu'à 20 MeV/u, le mécanisme est essentiellement celui des colli-sions profondément inélastiques, bien que certaines déviations apparaissent. Au-dessus de 20 MeV/u, certaines données sont interprétées dans le cadre d'un modèle participant-spectateur, mais en tenant compte des effets de champ moyen qui restent très importants. A 35 et 44 MeV/u, ces mécanismes permet-tent d'atteindre des températures telles que la multifragmentation devient possible.
Abstract - Data from violent collisions between heavy (A ^ 100) beams and targets of comparable mass are reviewed. The deep-inelastic mechanism domi-nates up to 20 MeV/u, although some departures are already noticed. Above 20 MeV/u, some data have been interpreted in the frame of a participant-spectator model, but taking in account mean field effects which are still important. At 35 and 44 MeV/u, such mechanisms allow to reach temperatures where the multifragmentation becomes possible.
I - INTRODUCTION
Since heavy ion beams have become available, two bombarding energy regions have mainly been explored:below E/A = 10 MeV/u where one observes collective phenomena, and above E/A = 100 MeV/u where nucleon-nucleon interactions dominate. The inter-mediate energy region has only recently begun to be investigated. Projectiles with mass A > 100 can be accelerated at GSI (E/A < 20 MeV/u) and at Ganil (E/A < 50 MeV/u for such heavy beams).
The very first results obtained at Ganil with a Kr beam showed events indicati-ve of a tremendous relaxation (1) : indeed only fragments with small Z values and low velocities had been observed in the Kr + Au reaction at 35.5 MeV/u (Figs. 1 and 2). These data had been measured at angles larger than the grazing angle.
They appeared completely different from the first results which had been obtai-ned previously with an Ar beam (2) : in the Ar + Ni and Au reactions at 44 MeV/u, fragments with Z values equal to or smaller than the beam one and velocities close to that of the projectile dominated the energy spectra measured at 3°.
Since than, data measured over a broad angular range showed that the difference is much less dramatic. For example, Fig.3 illustrates that, in the Kr + Au reaction at 44 MeV/u, fragmentation-type events as well as fragments relaxed in Z and velo-city are emitted (3).
If no difference between Kr and Au induced reactions existed, the Ar beam would be, at Ganil, more attractive than the Kr one : indeed, Ar can be accelerated up to 60 MeV/u, while Kr only up to 44 MeV/u. Moreover, the Kr beam was not, until recen-tly, of the "push button" type, although of good quality.
The aim of this paper is to explain why experimentalists prefer occasionally Kr or other mass 100 beams : the best way to do this is to present some results obtained with these beams. The aim of my introduction - emphasizing the difference between the very first data gained with Ar and Kr projectiles - was to indicate you which is the main philosophy of the paper: mass 100 ions induced reactions are
JOURNAL
DE-PHYSIQUE
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r e a c t i o n s : f u s i o n and deep i n e l a s t i c c o l l i s i o n s . Fusion i s n o t s e e n f o r systems of t o t a l mass l a r g e r t h a n 300 a.m.u. ( 4 ) .On t h e o t h e r hand, mass 100 i o n s a r e very s t a b l e a g a i n s t f i s s i o n when compared t o much h e a v i e r ones. They a r e t h e r e f o r e good p r o j e c t i l e s t o s t u d y deep i n e l a s t i c reac- t i o n s , and c e r t a i n l y a l s o t o f o l l o w t h e e v o l u t i o n of t h i s mechanism i n t h e t r a n s i - t i o n r e g i o n opened by Ganil.
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DEEP INELASTIC COLLISIONS AT LOW BOMBARDING ENERGIESD e e p - i n e l a s t i c c o l l i s i o n s a r e q u i t e f a s c i n a t i n g . We have l e a r n e d through them t h a t two heavy n u c l e i can s t i c k t o g e t h e r f o r a r a t h e r long time : s e c o r so. During t h i s time, they exchange many nucleons, t r a n s f o r m a s much k i n e t i c energy a s p o s s i b l e i n t o e x c i t a t i o n energy, e q u i l i b r a t e t h e i r t e m p e r a t u r e s and deform conside- r a b l y b e f o r e r e s e p a r a t i n g ; t h e s e a r e t h e f e a t u r e s , among a l l t h o s e which have been s t u d i e d , which w i l l r e t a i n o u r a t t e n t i o n i n t h e following.
I1 b
-
THE NUCLEON EXCHANGE PROCESS UP TO 20 MeV/uThe e v o l u t i o n of t h e nucleon exchange p r o c e s s h a s been s t u d i e d by G r a l l a e t a l . (5). These a u t h o r s have measured t h e 9 2 ~ 0 + 9 2 ~ 0 and ~ ~ O M O
+
~ O ~ M O r e a c t i o n s between 12 and 18.8 MeV/u. The f a c t t h a t t h e two ions a r e i d e n t i c a l a l l o w s t o t e s t i f t h e r e a c t i o n i s s t i l l of two-body n a t u r e ( s e e s e c t i o n I1 e ) . It i s t h e n p o s s i b l e , by measuring t h e time of f l i g h t of both r e a c t i o n p a r t n e r s , t o o b t a i n primary charge and mass d i s t r i b u t i o n s , and hence moments f o r them which a r e l i t t l e d i s t u r b e d by t h e l i g h t p a r t i c l e s e v a p o r a t i o n .Charge and mass v a r i a n c e s i n c r e a s e w i t h i n c r e a s i n g energy l o s s : t h i s proves t h a t , when t h e two n u c l e i come t o c o n t a c t , t h e p o t e n t i a l b a r r i e r between them va- n i s h e s , a window opens and t h e nucleons, through t h e Fermi motion, p a s s t h i s window i n b o t h d i r e c t i o n s ( F i g . 4 ) . I t has be shown (6) t h a t
02
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t h e t o t a l number of nucleons which have passed t h e window.
C4-354
JOURNAL DE PHYSIQUE
The i n c r e a s e of maximum
ax
w i t h t h e bombarding e n e r g y may i n d i c a t e a n i n c r e a s e of t h e i n t e r a c t i o n t i m e . I n Ref.5) i t i s .suggested t h a t i t i s d u e , r a t h e r , t o t h e i n c r e a s e o f t h e t e m p e r a t u r e of t h e d i n u c l e a r system. T h i s assumption i s based on a u n i v e r s a l l a w which h a s been shown t o be f o l l o w e d by a l l systems a t low bombarding e n e r g y ( 7 ) . When e x t r a p o l a t e d t o 18.8 ~ e V / u , i t i s s t i l l f o l l o w e d by t h e Mo + Mo systems ( F i g . 5 ) : t h i s p r o v e s t h a t t h e mechanism h a s n o t changed i n i t s n a t u r e .The consequence of t h i s l a r g e number o f n u c l e o n exchanges i s shown i n F i g . 6 :
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[am1v e r y broad mass d i s t r i b u t i o n s a r e o b s e r v e d i n t h e f u l l y damped r e g i o n . They e x t e n d t o v a n i s h i n g v a l u e s o f t h e mass : t h i s can be i n t e r p r e t e d a s a k i n d of "slow f u s i o n " which i s a c h i e v e d t h r o u g h t h e s e q u e n t i a l t r a n s f e r o f a l l n u c l e o n s from one n u c l e u s t o t h e o t h e r . T h i s mechanism a p p e a r s , however a t t e m p e r a t u r e s where t h e system may n o more b e s t a b l e a g a i n s t m u l t i f r a g m e n t a t i o n ( s e e s e c t i o n I1 e ) . But f i r s t , we s h a l l examin some d e v i a t i o n s from t h e c l a s s i c a l deep i n e l a s t i c p r o c e s s which a r e a l r e a d y s e e n a t E / A < 20 MeV/u.
I1 c
- THE ONSET OF THREE-BODY MECHANISMS
Three f r a g m e n t s - e v e n t s can be observed i n a heavy i o n r e a c t i o n e i t h e r a f t e r a s e q u e n t i a l f i s s i o n o r a f t e r t h e f a s t break-up o f one o f t h e p a r t n e r s . I n t h e f i r s t c a s e , t h e mechanism i s s t i l l o f two body n a t u r e , l i k e a deep i n e l a s t i c c o l l i s i o n ( F i g . 7 ) . I n t h e second c a s e , t h e p r o c e s s i s a f a s t three'body mechanism, l i k e t h o s e which a r e o b s e r v e d a t h i g h bombarding e n e r g i e s .
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Second,deformations modify t h e p o t e n t i a l energy and can induce thereby s t r o n g mass d r i f t s (9) s o t h a t f i n a l masses do n o t a l l o w t o i d e n t i f y unambiguously t h e p a r t n e r s of t h e f i r s t o r second s c i s s i o n . L a s t , s t r o n g f i n a l - s t a t e coulomb i n t e r a c t i o n s modi- f y t h e energy and a n g u l a r c o r r e l a t i o n s between t h e fragments.
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The f i r s t i n d i c a t i o n f o r a three-body break-up was observed i n t h e 8 6 ~ r
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1 6 6 ~ r r e a c t i o n a t 12.1 MeV/u (10 This phenomenon was i n v e s t i g a t e d i n g r e a t d e t a i l i n t h es4Kr
+
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liiSn r e a c t i o n s a t n e a r l y t h e same energy ( 1 1 ) . A s t r o n g y i e l d of three-body e v e n t s was observed. It was shown t h a t t h e y a r e produced i n a mechanism i n t e r m e d i a t e between t h e s e q u e n t i a l f i s s i o n and t h e f a s t break-up : i n d e e 4 a time s c a l e of 1. s e c between t h e c o n s e c u t i v e s c i s s i o n s was e s t a b l i s h e d . This time i s l a r g e enough t o c o n s i d e r t h e mechanism t o b e a s e q u e n t i a l one, and s m a l l enough t o a l l o w a Coulomb f i n a l - s t a t e i n t e r a c t i o n t o show up.The e f f e c t s of t h i s i n t e r a c t i o n were s t u d i e d i n Ref.11) a s a f u n c t i o n of t h e azimuthal a n g l e
Gf
: t h e r e l a t i v e v e l o c i t y , t h e y i e l d and t h e mean v a l u e of t h e f i s s i o n fragment-mass d i s t r i b u t i o n f l u c t u a t e .These f e a t u r e s show t h a t we a r e no more d e a l i n g w i t h an e q u i l i b r a t e d , s e q u e n t i a l f i s s i o n f o l l o w i n g a damped c o l l i s i o n . It i s a l s o n o t a f a s t break-up of t h e p a r t i - " c i p a n t - s p e c t a t o r type seen a t much h i g h e r e n e r g i e s . Indeed, Fig.9 shows t h a t t h r e e -
2-BODY TOTAL KINETIC ENERGY (MeV)
0 200 LOO 600 0 200 LOO 600 800
lo7
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C4-356
JOURNAL
DE
PHYSIQUE
body e v e n t s a r e a s s o c i a t e d w i t h t h e l a r g e s t energy l o s s e s , i . e . w i t h t h e l a r g e s t deformations. From t h i s , we may conclude t h a t d e f o r m a b i l i t y can " a c c e l e r a t e " t h e second s c i s s i o n .
Recently, a long time s c a l e between t h e two s c i s s i o n s has been found i n t h e 3 5 ~ 1
+
Ta r e a c t i o n a t 20 MeV/u ( 1 2 ) . The low d e f o r m a b i l i t y of 3 3 ~ 1 , when compared t o 129xe, may e x p l a i n i t .I1 d - PARTICIPANTS AND SPECTATORS
It i s well-known, a t low bombarding energy, t h a t two p a r t n e r s of a damping c o l l i s i o n tend t o s h a r e t h e i r e x c i t a t i o n energy so a s t o e q u i l i b r a t e t h e i r tempera- t u r e s . A t high energy, on t h e c o n t r a r y , zones of low temperature ( t h e s p e c t a t o r s ) c o e x i s t w i t h zones of high one ( t h e p a r t i c i p a n t s ) .
Ternary r e a c t i o n s , l i k e t h o s e we have seen i n t h e preceeding s e c t i o n , may be a good t o o l t o observe d i f f e r e n c e s of t e m p e r a t u r e s between t h e t h r e e fragments : indeed, d u r i n g t h e f i r s t d i s s i p a t i v e s t e p of t h e r e a c t i o n , one f i s s i o n fragment i s c l o s e r t o t h e c o n t a c t zone t h a n t h e o t h e r . The t h r e e c a s e s of Fig.8 correspond t h e n t o t h e c r e a t i o n of an independant f i r e - b a l l ( a ) , p a r t i a l f u s i o n of t h e h o t zone on t h e t a r g e t (b) o r on t h e p r o j e c t i l e ( c ) . Because mean f i e l d e f f e c t s a r e s t i l l impor- t a n t a t Ganil e n e r g i e s , t h e two l a s t p i c t u r e s a r e q u i t e l i k e l y t o be observed.
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Various d a t a confirm such i d e a s . However, t h e models which have been used t o ana- l y z e them p r e s e n t some d e c i n i t e d i f f e r e n c e s w i t h t h o s e of Fig.8. F i r s t , complete energy damping i s no more reached (12) ( s e e a l s o s e c t i o n I1 e )
.
Second, deforma- tdons a r e n o t c o n s i d e r e d , but t h e e x i s t e n c e of a l o c a l i z e d zone of p a r t i c i p a n t nucleons i s a c r u c i a l i n g r e d i e n t of t h e models.Model 1 i s found t o b e t t e r d e s c r i b e t h e 8 4 ~ r + 9 8 ~ 0 and 8 4 ~ r
+
lg78u d a t a (15) (Fig.12). To e x p l a i n t h i s o b s e r v a t i o n , i t was s p e c u l a t e d t h a t c a l e f a c t i o n , a con- c e p t w e l l known i n condensed m a t t e r p h y s i c s , can b e a p p l i e d a l s o t o n u c l e a r p h y s i c s(15). 2.0
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TEMPERATURES AND MULTIFRAGMENTATIONC o l l i s i o n s between i d e n t i c a l i o n s a l l o w t o t e s t i f t h e mechanism i s of two-body n a t u r e : i n t h i s c a s e , indeed, primary charge o r mass d i s t r i b u t i o n s must remain cent- e r e d a t p r o j e c t i l e v a l u e . Fig.6 shows t h a t , even f o r t h e l a r g e s t energy l o s s e s , t h i s i s f a i r l y checked i n t h e 9 2 ~ 0 + 9 2 ~ 0 a t 18.2 MeV/u when c o i n c i d e n c e s a r e s e l e c t e d by c o n d i t i o n s of c o p l a n a r i t y and center-of-mass c o l i n e a r i t y . This e n s u r e s t h a t , dur- i n g t h e s e l e c t e d c o l l i s i o n s , t h e observed k i n e t i c energy l o s s of o v e r 650 MeV i s transformed i n t o e x c i t c t i o n energy. The d i n u c l e a r system r e a c h e s thereby tempera- t u r e s of more than 5 MeV. This is confirmed i n Fig.13 by t h e good agreement between temperatures c a l c u l a t e d from t h e k i n e t i c energy l o s s and t h o s e measured through t h e s l o p e of evaporated p r o t o n energy s p e c t r a ( 8 ) .
Mass 100 n u c l e i d e e x c i t e p r e f e r e n t i a l l y by l i g h t p a r t i c l e e v a p o r a t i o n . When t h e bombarding energy a t t a i n s 20 MeV/u, one can e s t i m a t e t h a t up t o 30% of t h e t o t a l mass is evaporated i n f u l l y damped r e a c t i o n s . This r a c e would of c o u r s e i n c r e a s e w i t h t h e bombarding energy i f f u l l damping can s t i l l b e reached.
JOURNAL
DE
PHYSIQUE
Another mechanism may p u t a n end t o t h e i n c r e a s e o f e n e r g y d e p o s i t d u r i n g a heavy i o n r e a c t i o n . I n d e e d , m u l t i f r a g m e n t , a t i o n h a s r e c e n t l y been p r e d i c t e d t o o c c u r when t h e t e m p e r a t u r e o f a n u c l e u s a t t a i n s a b o u t 6 MeV ( 1 6 ) .
E x p e r i m e n t a l h i n t s f o r t h i s mechanism a r e based on t h e o b s e r v a t i o n o f intermed- i a t e m H s s f r a g m e n t s i n v a r i o u s e x p e r i m e n t s . Almost a l l d a t a i n t h i s f i e l d a r e i n c l - u s i v e : q u i t e few c o i n c i d e n c e measurements h a v e been r e p o r t e d and o n l y a t
E / A C 20 MeV/u ( 1 2 ) . At G a n i l , i n c l u s i v e fragment-measurements have been performed f o r s e v e r a l 35 MeV/u Kr-induced r e a c t i o n s : on 2 7 ~ 1 (17)
,
9 3 ~ b (18) and Au ( 3 , 19).
While, i n t h e f i r s t r e a c t i o n , a n e q u i l i b r a t e d cornposit system seems t o have been formed ( 1 7 ) , t h e two o t h e r s r e v e a l t h e c o e x i s t e n c e of d i f f e r e n t mechanisms. T h i s i s i l l u s t r a t e d i n F i g . 1 4 where two g r o u p s o f f r a g m e n t s a r e observed a t a n g l e s s m a l l e r o r l a r g e r t h a n t h e g r a z i n g a n g l e ( s e e a l s o F i g . 3 ) .100 200 300 LOO 500 600 700 800 0 . 7 ~ ~ t ~ ~ ~ t ~ o ~ t ~ ~ t * ~
Totai Kinetic Energy:
TKE [MeV1
12 16 20 2~ 28 32 36z
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-
36 f r a g m e n t s e m i t t e d i n forward d i r e c t i o n ( 9 = 3' t o 30") have been measured f o r t h e K r+
Au r e a c t i o n a t 35 and 44 MeV/u ( 3 ) . I n F i g . 15 i s shown t h e ECep spectrum. E : ~ ~ is t h e d i f f e r e n c e between t h e r e l a t i v e e n e r g y of t h e two f r a g m e n t s and t h e i r Coulomb r e p u l s i o n . The a b s e n c e of c o u n t s below Egel = 0 shows t h a t t h e f r a g m e n t s a r e n o t e m i t t e d s e q u e n t i a l l y by a s i n g l e n u c l e u s . R a t h e r , i t a p p e a r s t h a t t h e y a r e e i t h e r e m i t t e d i n c l o s e c o n t a c t (E:,1'
0) by t h e same s o u r c e , o r t h a t t h e y o r i g i n a t e from two s o u r c e s (E:,~ >> 01, t h e r e l a t i v e v e l o c i t y of which i n c r e a s e s w i t h t h e bombarding e n e r g y .The fragment v e l o c i t y s p e c t r a c o r r e s p o n d i n g t o t h e s e two k i n d s o f c o i n c i d e n c e s a r e shown i n Fig.16. Three components a p p e a r : f a s t f r a g m e n t s (V/Vp % 0.9) a r e i n
c o i n c i d e n c e w i t h slow o n e s (V/Vp < 0 . 7 ) , w h i l e t h e f r a g m e n t s w i t h
"
0 have a broad v e l o c i t y s p e c t r u m w i t h a maximum around V/Vp = 0.6.The f a s t - s l o w c o i n c i d e n c e s (Fig.16a) can b e i n t e r p r e t e d i n t h e framework o f t h e a b l a t i o n - a b r a s i o n model (20) : p r o j e c t i l e s p e c t a t o r s , whose v e l o c i t y h a s been slowed down t o 0 . 9 V by t h e s e p a r a t i o n of t h e p a r t i c i p a n t n u c l e o n s , a r e d e t e c t e d i n c o i n c i d e n c e w i t h f r a g m e n t s e m i t t e d by t h e p a r t i c i p a n t zone. The v e l o c i t y of t h i s l a t e r s o u r c e i s p r e d i c t e d t o b e e q u a l t o 0.35 Vp, due t o t h e f a c t t h a t p r o j e c t i l e and t a r g e t do n o t c o n t r i b u t e e q u a l l y t o i t .
shared over a l l t h e n u c l e u s (model 2 o r 3), i t s temperature a t t a i n s 8.5 MeV. I f t h i s h o t n u c l e u s undergoes m u l t i f r a g m e n t a t i o n , i t w i l l emit fragments which may be d e t e c - t e d i n t h e
" 0
coincidences. I n v e r s e l y , one can imagine t h a t t h e nucleons a r e t r a n s f e r e d on t h e t a r g e t . More than 30 nucleons have t o be t r a n s f e r e d t h i s way t o a c c e l e r a t e t h e target-composite s u f f i c i e n t l y t o a l l o w fragments emitted by i t t o be d e t e c t e d i n t h e set-up. It happens t h a t , a t t h e same t i m e , i t s temperature becomes l a r g e r than 6 MeV : i t may t h e r e f o r e emit fragments which we d e t e c t a s slow o n e s , i n coincidence w i t h t h e remainder of t h e p r o j e c t i l e (Fig.16a).r . t t 1 . 1 1 P
Kr
+Au
400
-
-
E,,
=
44MeVIu
----
EKr=
35
MeVIu
-
2
300-
- Z 3 0"
200
-100-
0-
0
50
100
150
200
E;,
,
(MeV1
Fig.
75
-
Codom6
cohtleoted ndcLtive
enmgy
Eid
npecaXm
a4 0.6 0.8 1.0 '47.2 /"P
Fig.
16-
V e l o c i t yn p e d 0 4
4tragmevLtb
detect-
ed
in
n
elected coincidencu
.
Linear momentum t r a n s f e r , a s we s e e , i s a c r u c i a l q u a n t i t y t o e x p l a i n t h e s e d a t a . The u n d e r l y i n g mechanism may be t h e i n t e r a c t i o n between t h e p a r t i c i p a n t zone and one of t h e two s p e c t a t o r s zones l i k e i n Fig.11, but a l s o multinucleon t r a n s f e r o r perhaps even a d i s s i p a t i v e c o l l i s i o n l i k e i n Fig.8. While kinematics a r e i n s e n s i t i v e on t h e choice of t h e model, temperatures and t h e r e f o r e t h e r a t e of evaporated par-t i c l e s depend s t r o a g l y on i t . This f a c t was used i n Refs.13 and 15 t o d i s t i n g u i s h between t h e models i n Fig.11. A t bombarding e n e r g i e s a s h i g h a s 44 MeV/u, t h i s i s no more p o s s i b l e . Coincidences between heavy fragments have t h e g r e a t m e r i t t o
JOURNAL
DE
PHYSIQUE111
-
CONCLUSIONDeep i n e l a s t i c r e a c t i o n s have been o b s e r v e d between mass 100 i o n s up t o 20 MeV/u. Deformations, however, i n c r e a s e t h e f i s s i o n p r o b a b i l i t y o f t h e p a r t n e r s and s h o r t e n t h e i r l i f e - t i m e , g i v i n g r i s e t o a c r o s s s e c t i o n f o r a n o n - e q u i l i b r a t e d t h r e e body mechanism. T h i s mechanism, which h a s a 10-21 s e c c h a r a c t e r i s t i c t i m e s c a l e a t 12 MeV/u, i s n e t h e r t h e l e s s q u i t e d i f f e r e n t from f a s t break-up.
Above 20 MeV/u, m u l t i f r a g m e n t a t i o n s e t s p r o b a b l y i n b e c a u s e t h e t e m p e r a t u r e of t h e p a r t n e r s can exceed 6 MeV. D i r e c t measurement of t e m p e r a t u r e s c l o s e t o 6 MeV i n a deep i n e l a s t i c r e a c t i o n h a s been r e a l i s e d s l i g h t l y below 20 MeV/u by s e l e c t i n g two body e v e n t s i n c o l l i s i o n s between i d e n t i c a l i o n s . Such a n experiment is S f i l l p o s s i - b l e a t a somewhat h i g h e r e n e r g y , and may a l l o w a measure o f t h e l i m i t i n g t e m p e r a t u r e f o r m u l t i f r a g m e n t a t i o n .
Around 25 MeV/u, p a r t i c i p a n t - s p e c t a t o r models i n c l u d i n g c o l l e c t i v e n u c l e a r mean- f i e l d e f f e c t s have been used t o i n t e r p r e t e s i n g l e s d a t a . They showed t h a t c o n c e p t s , borrowed from condensed m a t t e r p h y s i c s , may b e a p p l i e d t o n u c l e i . E x c l u s i v e e x p e r i - ments a r e now needed t o c o n f i r m t h e s e i d e a s .
Below 35 M e ~ / u , Coulomb f i n a l s t a t e i n t e r a c t i o n s may jumble somewhat t h e c o r r e - l a t i o n s measured i n f r a g m e n t - c o i n c i d e n c e s . A t 44 MeV/u, on t h e c o n t r a r y , i t h a s been p o s s i b l e t o s e p a r a t e components c o r r e s p o n d i n g t o d i f f e r e n t s o u r c e s . Only two of them can p o s s i b l y b e u n d e r s t o o d i n t h e frame of a p a r t i c i p a n t - s p e c t a t o r model. A l l t h r e e can b e e x p l a i n e d by mechanisms i n v o l v i n g a l a r g e energy damping. More i n v e s t i g a t i o n s a r e needed t o e l u c i d a t e t h e n a t u r e of t h i s mechanism.
ACKNOWLEDGEMENTS
Thanks a r e due t o a l l my c o l l e a g u e s a t S t r a s b o u r g , Caen, S a c l a y and Darmstadt f o r h e l p f u l l d i s c u s s i o n s . I n p a r t i c u l a r , A. Gobbi, K.D. H i l d e n b r a n d t and Ch. Ngo have p r o v i d e d m e w i t h d a t a p r i o r t o p u b l i c a t i o n .
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