Competition between fusion-evaporation and multifragmentation in central collisions in Ni-58+Ca-48 at 25A MeV

The experimental data concerning the Ni-58+Ca-48 reaction at E-lab(Ni)=25A MeV, collected by using the CHIMERA 4 pi device, have been analyzed in order to investigate the competition among different reaction mechanisms for central collisions in the Fermi energy domain. As a main criterion for centrality selection we have chosen the flow angle (9(flow)) method, making an event-by-event analysis that considers the shape of events, as it is determined by the eigenvectors of the experimental kinetic-energy tensor. For the selected central events (9(flow)>60 degrees) some global variables, good to characterize the pattern of central collisions have been constructed. The main features of the reaction products were explored by using different constraints on some of the relevant observables, like mass and velocity distributions and their correlations. Much emphasis was devoted, for central collisions, to the competition between fusion-evaporation processes with subsequent identification of a heavy residue and a possible multifragmentation mechanism of a well defined (if any) transient nuclear system. Dynamical evolution of the system and pre-equilibrium emission were taken into account by simulating the reactions in the framework of transport theories. Different approaches have been envisaged (dynamical stochastic BNV calculations + sequential SIMON code, QMD, CoMD, etc.). Preliminary comparison of the experimental data with BNV calculations shows reasonable agreement with the assumption of sequential multifragmentation emission in the mass region of IMFs close to the heavy residues. Possible deviations from sequential processes were found for those IMFs in the region of masses intermediate between the mass of heavy residues and the mass of light IMF's. Further simulations are in progress. The experimental analysis will be enriched also by information obtained inspecting the IMF-IMF correlation function, in order to elucidated the nature of space-time decay property of the emitting source associated with events having the largest IMF multiplicity.