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

Fusion excitation functions of light heavy-ion systems show oscillatory structures above the Coulomb barrier, caused by resonances or due to the penetration of successive 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 measurements very challenging. We have performed a first experiment for Si-28 + Si-28, by measuring fusion cross sections (sigma) in an energy range of similar or equal to 15 MeV above the barrier, with a small Delta E-lab = 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 mu 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.