DYNAMICS AND THERMALIZATION IN VIOLENT COLLISIONS BETWEEN AR-40 AND AG AT 27 MEV NUCLEON

For the violent collisions of 27 MeV/nucleon Ar-40 with Ag, coincidence measurements have been made between heavy residues and intermediate mass fragments (3 less-than-or-equal-to Z less-than-or-equal-to 14) or light charged particles. From the analysis of the correlation between heavy residues (mass and velocity) and intermediate mass fragments, the main characteristics of the dominant mechanisms, fusion and partially damped collisions preceded or accompanied by a preequilibrium emission, are presented. Balances concerning mean values of parallel linear momentum, mass and atomic number, are established and confirm that a complete description of violent collisions was obtained. Then thermalization is discussed, first in terms of excitation energies derived from kinematics between heavy residues and intermediate mass fragments, and secondly in terms of initial temperature estimates derived from light charged particle spectra. Very hot nuclei (T congruent-to 5.7-6.6 MeV) are produced over a large impact parameter range from very central collisions to medium peripheral ones. Various experimental results are compared to predictions obtained with semi-classical calculations (Landau-Vlasov equation). From their good agreement one may conclude that, depending on the impact parameter, thermal equilibrium is achieved within 4-10 x 10(-22) s.