FRAGMENTATION IN MEDIUM ENERGY HEAVY-ION COLLISIONS

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 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 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 clearly shows the relation of this multifragment emission to dynamical instabilities. The dynamical informations on the systems which enter such instability regions are used as inputs for a statistical multifragmentation approach where all possible final partitions are considered with probabilities given by the corresponding entropies. Secondary evaporations are also accounted for. In this way a very good agreement with final Intermediate Mass Fragment (IMF) yields is found.