EQUILIBRIUM FEATURES AND DYNAMIC INSTABILITIES IN NUCLEAR FRAGMENTATION

We study equilibrium and non-equilibrium features of complex fragment production in medium-energy heavy-ion collisions. We consider first statistical sequential emission from equilibrated primary sources formed in the entrance-channel dynamics. The inputs of cascade calculations are derived from a suitable clustering procedure at some carefully fixed equilibration time. Large pre-equilibrium dissipation is revealed. Applications are shown to the reactions Kr-84+Al-27 at 34.4 MeV/u, and La-139+Al-27 and La-139+Cu-63 at 55 MeV/u beam energies. Absolute comparisons with experiments show a quite good overall agreement. Some discrepancies revealed at higher energies, for high multiplicity events and when a larger excitation energy is available, appear as a signature of the onset of a direct multifragmentation mechanism. A microscopic study of the nuclear-matter phase diagram in the participant zone seems to relate this multifragment emission to dynamical instabilities.