Fusion of 28Si + 28Si: oscillations above the barrier and the behavior down to 1μb

Fusion excitation functions of light heavy-ion systems show oscillatory struc- tures above the Coulomb barrier, caused by resonances or due to the penetration of suc- cessive centrifugal barriers well separated in energy. In heavier systems, the amplitude of oscillations decreases and the peaks get nearer to each other. This makes the measure- ments very challenging. We have performed a first experiment for 28 Si + 28 Si, by measuring fusion cross sections ( ) in an energy range of'15 MeV above the barrier, with a small Elab = 0.5 MeV step. Three regular oscillations are clearly observed, which are best revealed by plotting the energy-weighted derivative of the excitation function. The excitation function has been recently measured down to cross sections 1 b with larger energy steps. Coupled-channel (CC) calculations based on a shallow potential in the entrance channel are able to reproduce the oscillations. A further analysis will provide a stringent test for the calculations, in particular for the choice of the ion-ion potential, because the sub- barrier excitation function has to be reproduced as well.