Symmetry Energy Effects on Low Energy Dissipative Heavy Ion Collisions

Abstract

We investigate the reaction path followed by Heavy Ion Collisions with exotic nuclear beams at low energies. We focus on the interplay between reaction mechanisms, fusion vs. break-up (fast-fission, deep-inelastic), that in exotic systems is expected to be influenced by the symmetry energy term at densities around the normal value. The method described here, based on the event by event evolution of phase space quadrupole collective modes, will nicely allow to extract the fusion probability at relatively early times, when the transport results are reliable. Fusion probabilities for reactions induced by 132Sn on 64,58Ni targets at 10 AMeV are evaluated. We obtain larger fusion cross sections for the more n-rich composite system, and, for a given reaction, with a soft symmetry term above saturation. A collective charge equilibration mechanism (the Dynamical Dipole Resonance, DDR) is revealed in both fusion and break-up events, depending on the stiffness of the symmetry term just below saturation. Finally we investigate the effect of the mass asymmetry in the entrance channel for systems with the same overall isospin content and similar initial charge asymmetry. As expected we find reduced fusion probabilities for the more mass symmetric case, while the DDR strength appears not much affected. This is a nice confirmation of the prompt nature of such collective isovector mode.