Entrance channel limitations in the high energy fusion of light-heavy systems leading to 40Ca and 42Ca

The fusion cross section of light-heavy ions drops below the total reaction cross section with increasing energy and then decreases rapidly when the bombarding energy is further increased. This decrease in the fusion cross section (σ/sub fus/) at high energy can be understood in terms of a limit on the angular momentum the fusing system is able to support. It is not clear however what this limit is due to. It may be due to the paucity of high spin states in the compound nucleus, or to its inability to sustain the high angular momentum brought in by the entrance channel. A limitation to σ/sub fus/ can be also brought about at high angular momentum when the repulsive forces of the combined Coulomb and centrifugal potentials overcome the attractive nuclear forces and prevent the fusion of the colliding nuclei. The question of compound nucleus or entrance channel-type limitation to the fusion cross section has been approached effectively in experiments where the same compound nucleus is populated by different projectile-target combinations. Fusion data are available for several systems leading to the compound nuclei 40Ca and 42Ca. In this study we report on measurements of fusion cross sections done on 28Si + 12C and 30Si + 12C over a wide range of bombarding energies. These data are combined with available data to show the presence of a strong entrance channel limit for fusion in these two systems