Onset of midvelocity emissions in symmetric heavy ion reactions

Experimental data obtained with the 4 pi multidetector system INDRA are used to study midvelocity emissions of light charged particles (LCP) and intermediate mass fragments for peripheral and semicentral collisions of Xe and Sn at energies between 25 and 50 MeV/nucleon. The analysis is performed as a function of incident energy and of impact parameter, defined through the total transverse energy of LCP. The onset of midvelocity emissions is found to be close to 25 MeV/nucleon. Evaporative processes are also identified and are found to be sensitive to the impact parameter but show, for a given impact parameter, little dependence on the incident energy. A chemical analysis of the midvelocity component is performed. Compared to the evaporative process, midvelocity matter is found to be more neutron rich. Results are compared with the predictions of a dynamical model (CHIMERA). The general trends are well reproduced but some interesting differences are observed, notably in the amount of relative energy dissipation, showing that midvelocity emissions could be sensitive to the finer details of the interaction.