FAST FISSION PHENOMENA IN HEAVY ION REACTIONS

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FAST FISSION PHENOMENA IN HEAVY ION REACTIONS

C. Grégoire, R. Lucas, C. Ngô, H. Ngô, B. Schürmann

To cite this version:

C. Grégoire, R. Lucas, C. Ngô, H. Ngô, B. Schürmann. FAST FISSION PHENOMENA IN HEAVY ION REACTIONS. Journal de Physique Colloques, 1980, 41 (C10), pp.C10-247-C10-248.

�10.1051/jphyscol:19801027�. �jpa-00220646�

JOURNAL DE PHYSIQUE CoZZoque CIO, suppZ6ment au n012, Tome 41, d6eembre 1980, page C10-247

F A S T F I S S I O N PHENOMENA I N HEAVY I O N REACTIONS

C. GrGgoire, R. Lucas, C. Ng8, H. ~ ~and 8 *B. Schiirmann.

**

2Ph-N/MF, CEN SacZay, B.P. N02, 91290 Gif-sup-Yvette, France.

.%I. P. N., Laboratoire de Physique Thgorique, B. P. N o l , 91406 Orsay, France.

l'U Wnchen, 0-8046 Garching, R.F.A.

Until recently the classification, at low bombar- ding energy, of heavy ion nuclear reactions into 3 categories : quasi elastic, deep inelastic (DI) and compound nucleus (CN) reactions was quite satisfac- tory. However some experimental data which are as- sociated to the de-excitation of the CN cannot be fully understood within this framework. A CN has completely forgotten about its formation and will de-excite by emitting light particles and y rays

(leading to evaporation residues) or by fissioning

ments. It turns out that one property seems to be different : the FWHM of the mass distribution seems to be larger for FFP than for ordinary fission [3,

4 1 . This is illustrated in figure 1 where the FWHM

of the symmetric fragmentation component of a sys- tem 2::~t increases quite a lot for II crit

'

^{'B~ due}

to the appearance of FFP. We have developedasimple static model to try to understand such an evolu- tion

m.

The collision is assumed to be pictured as follows :

into two fragments (symmetric fragmentation compo- 1. The entrance channel is assumed to be domina- nent). The effective barrier against fission de- ted by a sudden interaction potential. For a given creases with increasing angular momentum and for II value, the trajectory can either be scattered af- some value kgf vanishes [I]. Consequently, according ter a loss of kinetic energy (DI) or it can be trap- to this picture a real CN cannot be formed. What is ped in the pocket of the potential. In this latter usually called fusion cross section is the sum of case we will have either CN formation or FFP. Due the evaporation residues and of the symmetric frag- to the Coulomb field it should be noted that the po- mentation cross sections. This latter is usually cket only exists if Z1Z2 < 2500-3000.

associated to the fission of the CN. From the fa- 2. When the system is trapped in the pocket it sion cross section one can deduce, within the sharp will drift along the mass asymmetry coordinate.

cut off approximation, a critical angular momentum This drift will lead to a symmetric quasimolecular tcrit. If the fusion cross section is the same as system if it is not too asymmetric. Indeed it is the CN cross section formation Rcrit shouldbebound only if the system is initially not too asymmetric by RBf. This seems not to be the case for medium that FFP will be observed. .

systems [2]. The symmetric fragmentation cross 3. In the last step it is assumed that a reorga- section is too large to be entirely associated to nization of the densities takes place (transition the fission of a CN. This is likely to indicatethat from a sudden to an adiabatic potential). If g c Itg f a new mechanism intermehiate between DI and CN for- a real CN is formed which will forget about its mation exists. We can call it fast fission phenome- formation : the FWHM of the fission mass distribu- non (FFP) because the fragments have properties tion will be given by the statistical fluctuations which are similar to those of the CN fission frag- in the adiabatic potential. On the contrary for

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19801027

c10-248 JOURNAL DE PHYSIQUE

R > IIB no r e a l compound n u c l e u s i s formed and t h e f

memory of t h e sudden p o t e n t i a l w i l l b e k e p t a s f a r a s mass asymmetry i s concerned. The FWHM w i l l a r i s e from t h e s t a t i s t i c a l f l u c t u a t i o n s i n t h e sudden po- t e n t i a l . I n f i g u r e 1 t h e f u l l curve shows t h e re- s u l t o f a s i m p l e c a l c u l a t i o n based on t h e p r e c e d i n g assumptions. For Rcrit < RBf a FWHM of 29 was t a k e n a s a n i n p u t and f o r acrit > RBf t h e c a l c u l a t i o n was performed u s i n g a sudden p o t e n t i a l . T h i s c a l c u l a t i o n p r e d i c t s t h a t Rcrit s a t u r a t e s around 130 which seems to-'be e x p e r i m e n t a l l y observed.

F i g . 1

-

The FWHM of t h e mass d i s t r i b u t i o n of t h e symmetric p r o d u c t s i s shown a s a f u n c t i o n of c r i - t i c a l a n g u l a r momentum f o r t h e A r + Ho (o), Ne + Re

( A ) , p + B i , a + Pb, C + P t (I) systems. The f u l l c u r v e c o r r e s p o n d s t o t h e f a s t f i s s i o n model.

REFERENCES

[I] S. Cohen, F. P l a s i l and W . J . S w i a t e c k i , Ann. of [4] C. GrBgoire, M. Berlanger, B. Borderie,

Phys.

82

(1974) 557. F. Hanappe, D. GardBs, J. G i r a r d , J. Matuszek,

[2] For a c o m p i l a t i o n of r e s u l t s s e e M. L e f o r t , C. Nga and B. Tamain, p r e p r i n t 1980.

Symposium on deep i n e l a s t i c and f u s i o n r e a c t i o n s

E]

C. GrCgoire, R. Lucas, C. NgB, B. Schiirmann w i t h heavy i o n s , B e r l i n (1979). and H. Ng6, p r e p r i n t DPh-N/MF 79/33 (1979).

[3] C. Lebrun, F. Hanappe, J.F. L e c o l l e y ,

F. L e f e b v r e s , C. Ng6, J. P e t e r and B. Tamain, Nucl. Phys.

A321

(1979) 207.