FORMATION AND DECAY OF HOT NUCLEI

HAL Id: jpa-00225805

https://hal.archives-ouvertes.fr/jpa-00225805

Submitted on 1 Jan 1986

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

FORMATION AND DECAY OF HOT NUCLEI

M. Mostefaï, Y. Cassagnou, M. Conjeaud, R. Dayras, S. Harar, R. Legrain, E.

Pollacco, C. Volant, G. Klotz-Engmann, H. Oeschler

To cite this version:

M. Mostefaï, Y. Cassagnou, M. Conjeaud, R. Dayras, S. Harar, et al.. FORMATION AND DECAY OF HOT NUCLEI. Journal de Physique Colloques, 1986, 47 (C4), pp.C4-361-C4-364.

�10.1051/jphyscol:1986439�. �jpa-00225805�

Colloque C4, supplément au n" 8, Tome 47, août 1986 Cf-361

FORMATION AND DECRY OF HOT N U C L E I

M. MOSTEFAI, Y . CASSAGNOU, M. CONJEAUD, R . DAYRAS, S . HARAR, R . LEGRAIN, E . C . POLLACCO, C . VOLANT, G. KLOTZ-ENGMANN* a n d H . OESCHLER*

Service de Physique Nucleaire, Basse Energie, CEN-Saclay, F-91191 Gif-sur-Yvette Cedex, France

* I n s t i t u t fur Kernphysik, Technische Hochschule, D-6100 Darmstadt, F.R.G.

Résumé - Les corrélations angulaires entre fragments de fission pour le sys- tëme

Ni +

Th à 20, 25 et 30 MeV/u montrent une persistance de la bosse correspondant aux grands transferts qui suggère que sa disparition pour le système

Ar +

Th à 44 MeV/u n'est pas due à une l i m i t a t i o n en énergie d'excitation et que la t a i l l e et la vitesse du p r o j e c t i l e ont aussi un rôle important. De grandes vitesses relatives sont observées pour les fragments de fission pour les grands transferts (~ 9,5 GeV/c) qui pourraient provenir d'un mécanisme de fission plus exotique des noyaux chauds.

Abstract - Fission fragment angular correlations for the system

Ni +

Th at 20, 25 and 30 MeV/u show s t i l l a bump for large momentum transfers which suggest that i t s disappearance for the **°Ar +

Th system at 44 MeV/u is not due to a l i m i t a t i o n in the excitation energy and that the size and velocity of projectiles are also important. High relative velocities of fission frag- ments are measured for large transfers (~ 9.5 GeV/c), this could be due to more exotic fission of hot nuclei.

The aim of the present investigation is to determine the entrance channel conditions to form nuclei as hot as possible and also to study t h e i r decay properties. The technics used is the angular correlation measurement.

A report of the results on the '••'Ar induced fission on

Th target at energies ranging from 31 to 44 MeV/u i s given elsewhere / l / and the main features of the an- gular correlations shown in the l e f t part of Fig. 1 can be summarized : i ) A pro- minent peak i s observed a t r e l a t i v e angle 8 f f = 170° corresponding to sequential f i s s i o n induced by quasi-elastic collisions (p

.= 0.8 GeV/c). i i ) A bump located at high momentum t r a n s f e r ( 8

f = 110° corresponding to p

= 7 GeV/c) which can be attributed to central c o l l i s i o n s . These events result from incomplete fusion process since t h e i r recoil velocities are lower than the one corresponding to f u l l momentum transfer (arrows in F i g . l ) Two aspects are very s t r i k i n g : the disappearance of the bump at 44 Mev/u and the fixed average momentum transfer with incident energies.

The different nature of the two main processes have been also i l l u s t r a t e d i n / l / ; namely the constant behavior with the increase of the energy of the measured masses and cross sections for the peripheral collisions and a decrease of the total masses for the fusion-fission processes evidencing an increase of the violence of the c o l - l i s i o n s associated with a decrease of cross sections.

In order to check i f the washing out of the bump at 44 MeV/u i s due to the maximum excitation energy acceptable by nuclei or, to entrance channel conditions, we stud-

'^•'Experiment performed at the GANIL facility

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986439

C4-362 JOURNAL DE PHYSIQUE

i e d t h e Ni + Th system which a l l o w s t h e i n t r o d u c t i o n o f more momentum and e x c i t a t i o n energies i n t o t h e t a r g e t n u c l e i a t t h e same r e l a t i v e v e l o c i t i e s than Ar p r o j e c t i l e s . Experimental r e s u l t s a r e presented i n t h e r i g h t o f f i g . 1. The main d i f f e r e n c e i s t h a t now t h e bump corresponding t o c e n t r a l c o l l i s i o n s i s l o c a t e d a t PI, = 9.5 GeV/c which imp1 i e s i n t h e massive t r a n s f e r framework, f i s s i o n i n g n u c l e i w i t h A - ^{270 and}

about 900 MeV e x c i t a t i o n energy. T h i s r e s u l t suggests t h a t t h e disappearance o f t h e bump observed a t 44 MeV/u Ar i s n o t due t o a l i m i t i n g e x c i t a t i o n energy i n t h e com- posi t e l i k e system s i n c e i n b o t h Ni and Ar induced incomplete f u s i o n a temperature of about 5 MeV i s reached ; hence entrance channel p r o p e r t i e s p l a y an e s s e n t i a l r o l e i n f i x i n g t h e c e n t r a l c o l l i s i o n y i e l d s .

Fig. 1

A t t h i s stage, several remarks can be p o i n t e d o u t : i ) The washing o u t o f t h e bump a t 44 MeV/u Ar does n o t mean t h a t f i s s i o n events corresponding t o h i g h momentum t r a n s f e r s (p > 7 GeV/c) cannot be produced by such entrance channels. I n f a c t , f o r t h e Ar induced f i s s i o n on 165Ho f o r which p e r i p h e r a l c o l l i s i o n s a r e s t r o n g l y sup- pressed because o f t h e f i s s i o n b a r r i e r height, one observes t h a t h i g h momentum t r a n s f e r s a r e imparted t o t a r g e t /2/. Several r e a c t i o n mechanisms a r e probably i n - v o l ved i n c e n t r a l c o l 1 i sions induced by b o t h nucl eus-nucl eus and nucl eon-nucl eon i n t e r a c t i o n s . The present data c o u l d i n d i c a t e t h a t t h e one body d i s s i p a t i o n i s v a n i s h i n g f o r such entrance channel w h i l e two body d i s s i p a t i o n s t i l l c o n t r i b u t e s t o t h e p o p u l a t i o n o f h o t n u c l e i . i i ) The r e l a t i v e v e l o c i t y o f 44 MeV/u Ar p r o j e c t i l e s i s n o t s u f f i c i e n t t o e x p l a i n such t r a n s i t i o n s i n c e h i g h e r v e l o c i t y p r o j e c t i l e s such as 1 2 C (ref./3/) and

(ref./4/) a t 60 MeV/u on Th can induce a shoulder i n t h e c e n t r a l c o l l i s i o n region. It i s probably both t h e v e l o c i t y and s i z e o f t h e p r o j e c t i - l e s which determine t h e nature o f mechanisms i n v o l v e d i n these c e n t r a l c o l l i s i o n s . iii ) Average momentum t r a n s e r s a r e s a t u r a t e d a t 2.6 GeV/c, 7 GeV/c and 9.5 GeV/c f o r 14N, 4 0 ~ r and 5 8 ~ i r e s p e c t i v e l y . These values a r e n o t s i g n i f i c a n t l y dependent o f t h e t a r g e t n u c l e i f o r masses , ^{165 a.m.u.} Then one observes a remarkable s c a l i n g o f momentum i m p a r t e d t o t a r g e t n u c l e i w i t h t h e p r o j e c t i l e masses s i n c e PI, = (180+_20) MeV/c. T h i s r e s u l t i s very i m p o r t a n t since i t i n d i c a t e s t h a t t h e whole p r o j e c t i l e i n t e r a c t s w i t h t h e t a r g e t n u c l e i and t h a t each escaping nucleon c a r r i e s away more than 180 MeV/c i n average.

I n order t o have more i n s i g h t i n t h e o r i g i n o f t h e l i m i t a t i o n we performed same c a l - c u l a t i o n s u s i n g t h e e x c i t o n model /5/ which a r e presented i n t a b l e l. The t o t a l mo- mentum t r a n s f e r s f i t q u i t e w e l l t h e experimental r e s u l t s and e x c i t a t i o n energies a r e compatible w i t h those deduced from masses o f t h e f i s s i o n fragments /I/.

I n t h e framework o f a n a i v e massive t r a n s f e r p i c t u r e , u s i n g as i n p u t t h e experiment-

a! $,, v a l u e s , one deduces q u i t e s i m i l a r e x c i t a t i o n energies (Table I ) as o b t a i n e d

w ~ t h t h e p r e e q u i l i b r i u m model, w h i l e t h e basic assumptions a r e very d i f f e r e n t . T h i s

i s t r a c e d t o t h e f a c t t h a t i n t h e e x c i t o n model t h e escaping nucleons c a r r y away

t o r nucleon in the massive transfer picture. However, the determination of the ori- gin of the momentum transfer saturation needs further studies.

Another fascinating aspect concerns the recoil properties of fission fragments. Two dimensional plots of the coincident fission fragment masses are presented in Fig. 2 for both low and high momentum transfers. In these two conditions, the fission re- gion i s well defined, b u t for central collisions the mass asymmetry becomes rather large. The relative velocities (Vff) vs the total detected masses presented in Fig.2 are well f i t t e d by the Viola's systematics showing that we observed normal fission process for peripheral collisions. In contrast for biolent collisions, the velocity

distribution i s shifted towards higher

values and shows a t a i l up to 2 cm/ns Table I

which represents quite abnormal total For t h e massive t r a n s f e r , P,, i s deduced kinetic energies for fission processes. from the angular correlation, while i t i s This effect might be due to a compact predicted in the precompound model.

configuration a t he saddle point and

partly to angular momentum effect.

+

In summary, important amount of momentum

transfers and excitation energies have been imparted to composite-1 ike-nuclei with A = 270. A scaling for momentum transfer with the projectile mass i s evidenced up to the Ni ions for central collisions. Velocity and size of projec- t i l e s play an important role as well as excitation energies deposited in f i s- sioning nuclei, t o f i x the central col- lision cross sections. A t l a s t , high relative velocities of fission fragments have been measured and open an inter- esting problem on the fission properties

of nuclei a t high excitation energies and angular momenta.

-

Mtot

Fig. 2

50 100 150 200 0.5 1.0 1.5 2.0

C4-364 JOURNAL DE PHYSIQUE

REFERENCES

111 Pollacco, E.C. e t al., Phys. L e t t . 146 (1984) 29 ; Conjeaud, M. e t al., Phys. L e t t . 1 5 9 1 9 8 5 ) 244.

/2/ C ~ S S ~ ~ ~ O U , Y. e t al., Winter W o x o p on nuclear dynamics- I V , Cooper Mountain (Colorado), Feb. 24-28, 1986.

/3/ Galin, J. e t al., Phys. Rev. Lett. 48 (1982) 1787.

141 Volant, C., Proc. o f the X X I I I I n c Winter Meeting on Nuclear Physics, Bormio ( I t a l y ) , Jan. 21-26, 1985 (ed. Universita Degli Studi d i Milano - 47 (1985) 370).

151 Blann, M., Phys. Rev,. C31 (1985) 1245.