Entrance effects in medium mass nuclear systems at 25 MeV/nucleon

We discuss experimental data concerning 48Ca+48Ca and 42Ca+54Fe reactions at 25 MeV/nucleon obtained by using the 4pi multi-detector Chimera. Effects due due to the different neutron to proton ratios (N/Z) of the two systems have been investigated. Charge distributions of forwardly emitted fragments show even-odd staggering whose amplitude is larger for the neutron poor system. Moreover, the neutron excess of the total system influence the evolution of fusion-like sources formed in semi-central collisions. In particular, for the neutron rich system, the relative emission yield of heavy residues is larger, while multi-fragmentation and binary-like events prevail in the neutron poor system