Evolution of fragmentation modes in central reactions at Fermi energies: theory and experiment

Recently, the INDRA collaboration has investigated links between entrance channel, collective expansion and fragmentation features, in the context of heavy ion collisions at Fermi energies . To go further in this direction, a full comparison between transport theory results (the Stochastic-Mean-Field (SMF) approach is employed) and multifragmentation data [3] will be discussed. In a first part, the question of centrality selection is addressed. Starting from simulation events in the range of impact parameter 0-4 fm, we apply the same dynamical selection as that used experimentally to sort out the so-called "Quasi-Fusion" sources. To probe the link between the fragmentation regime and the system collective expansion, we analyse reactions in the beam-energy range between 25 and 50 MeV/A. Then we extract, looking at the evolution of the spatial density profile in the simulations, the corresponding evolution of the collective expansion energy, which turns out to be in very good agreement with the experimental results reported. The simulations also indicate a change in the fragmentation mechanism, from sequential decay of a composite source to prompt multifragmentation. We investigate the connection between the evolution of the fragmentation regime and the trend experimentally observed for the amount of stopping reached in heavy ion reactions, which is also determined by the interplay between one-body (mean-field) and two-body (nucleon-nucleon collisions) effects