This research was sponsored by the National Science Foundation Grant NSF PHY-931478

Friedman, W.A.

Kwiatkowski, K.

Pollacco, E.C.

Viola, V.E.

Volant, C.

Woo, L.W.

Yennello, S.J.

English

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DECAY OF HIGHLY EXCITED SYSTEMS 
ENERGY DISSIPATION AND MULTIFRAGMENT DECAY 
IN LIGHT-ION-INDUCED REACTIONS 
K. Kwiatkowski, L.W. Woo, and V.E. Viola 
D e p a r t m e n t  o f  C h e m i s t r y  and  Ind iana  Un ive r s i t y  Cyc lo t ron  Facili ty,  
B looming ton ,  I n d i a n a  4 7 4 0 8  
W.A. Friedman 
Univers i ty  of Wi scons in ,  Mad i son ,  W i s c o n s i n  53706  
E.C. Pollacco and C. Volant 
Cen t re  d ' ~ t u d e s  Nucle'aires, D A P N I A / S P h N ,  Saclay,  France 
S . J . Yennello 
Texas  A & M Univers i ty ,  College S ta t i on ,  Texas  7 7 8 4 3  
Multifragment emission data from the 3He + Ag reaction at 3.6 GeV (Ref. 1) have 
been examined in the framework of a hybrid model that treats the reaction dynamics 
in terms of an intranuclear cascade cal~ulation,~ followed by decay from an expanding, 
emitting source. 
The INC calculations demonstrate that the average excitation of the excited residues 
formed in central collisions increases rapidly with bombarding energy; for more peripheral 
collisions this increase is much more gradual. The important role of the A resonance in 
producing highly excited residues is illustrated by the calculations. This mechanism pro- 
vides a rapid, efficient means of energy dissipation that would appear to be essential in 
forming the highly excited species required for multifragmentation in light-ion-induced re- 
actions. The results suggest that mult ifragment ation studies, when complement ed by pion 
emission probabilities, may provide valuable insight into the question of pion reabsorption 
in the nuclear m e d i ~ m . ~  The INC calculations also show that pll and p l  are comparable for 
excitation energies above about 500 MeV; these calculated values are in good agreement 
with source velocities derived from a rapidity analysis of the spectra for high multiplicity 
events in the 3 ~ e  + Ag system. 
Fits to the 3.6-GeV 3He + Ag multiplicity data with the combined INC/EES model 
require a relatively soft equation of state (K = 144). This value also described the charge 
distribution data best. Calculations with stiffer equations of state or without A excitations 
in the cascade severely underpredict the high mulitplicity data; in fact, the calculations 
are quite sensitive to all parameters. In order to describe the data with a stiffer equation 
of state, the probability for A excitation and/or pion absorption in the INC code would 
need to be increased significantly in order to enhance the probability for high excitation 
energy residues. The importance of employing a realistic excitation energy distribution 
in multifragmentation calculations is also stressed by this work. If the EES calculations 
are performed with the average INC excitation energy instead of the full distribution, 
the data are significantly underpredicted. Comparison of the INCJEES calculations with 
experimental kinetic energy spectra accounts for the major features of the data as shown in 
Fig. 1. The calculations successfully predict changes in IMF spectral shapes for bombarding 
energies below and above about 1 GeV, i.e., broadening of the Coulomb peaks toward 
lower energies and flattening of the spectral tails at high projectile energies, as observed 
in the data.' In the context of the EES model, this behavior is explained in terms of 
energetic IMFs being emitted early in the expansion, where the expansion velocity is 
large, and low energy IMFs being emitted from nuclear matter at low density. While the 
quantitative agreement with the spectra is not perfect, the qualitative features of the data 
are successfully described. 
0 25 50 75 100 125 0 25 50 75 100 125 150 
Energy (MeV) 
Figure 1. Left frame: inclusive spectra for C fragments at backward angles for 
3He+natAg reaction (Ref. 1) as several bombarding energies. Right frame: Spectral shapes 
predicted by EES model for emission of Z=6 fragments at 117" for several residue excitation 
energies, as indicated on figure. 
It is also of interest to compare the charge distributions predicted by the INC/EES 
model with the experimental data. I n  Fig. 2, we present such a comparison with a cal- 
culation which employs the standard (with A) INC code with the EES model for various 
values of the compressibility parameter K. The calculation is quite sensitive to the value 
of K; a value of K = 144 gives the best fit to the data. Thus, the results for the charge 
distributions are self-consistent with other observables. 
Figure 2. Experimental inclusive charge distribution for 3.6-GeV 3He+nat Ag reaction 
(points) compared with predictions of INC/EES model for K=144, I<=2OO and K=m. 
These INC-EES calculations provide a physically transparent framework for under- 
st anding the basic features of light-ion-induced multifragment at ion reactions. The funda- 
mental roles of inelastic N-N scattering and expansion of the nuclear medium are essential 
in confrontation of this model with the data. 
1. S.J. Yennello, e t  al., Phys. Rev. Lett. 67, 671 (1991); Phys. Lett. B 246, 26 (1990). 
2. Y. Yariv and 2. Fraenkel, Phys. Rev. C 20, 2227 (1979); ibid, 26, 2138 (1982). 
3. W.A. Friedman, Phys. Rev. 42, 667 (1990). 
4. J. Cugnon and M.C. Lemaire, Nucl. Phys. A 489, 781 (1988). 


Energy Dissipation and Multifragment Decay in Light-Ion-Induced Reactions

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Energy Dissipation and Multifragment Decay in Light-Ion-Induced Reactions

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