Fusion Hindrance and Quasi-Fission in48Ca Induced Reactions

Fusion-fission and fusion-evaporation cross sections have been measured in a large energy range for the 4 8 Ca+ 1 6 8 Er, 1 5 4 Sm reactions. The comparison of the reduced evaporation data for such reactions and for collisions induced by light projectiles leading to the same compound nuclei 2 1 6 Ra* and 2 0 2 Pb* puts in evidence a fusion hindrance, which is particularly large for the first composite system. This fusion suppression is consistent with a noticeable contribution coming from quasi-fission events observed in the mass-energy distribution of fission fragments. The asymmetric shape of the quasi-fission component has been explained in terms of shell effects in the exit channel, favouring the population of closed shell fission fragments. The comparison of 4 8 Ca+ 1 5 4 Sm with preliminary data on 4 8 Ca+ 1 4 4 Sm seems to suggest that the target deformation plays a role in the onset of the quasi-fission mechanism.