Fusion of 40 Ca + 40 Ca and other Ca + Ca systems near and below the barrier

The fusion excitation function of Ca-40 + Ca-40 has been measured from well above the Coulomb barrier, down to low energies where the cross section is as small as similar or equal to 20 mu b, and the astrophysical S factor possibly reaches a maximum vs energy. The results of coupled-channels calculations using the M3Y + repulsion potential are presented. A detailed comparison is made with recently published data on the fusion of Ca-40 + Ca-48 and of Ca-48 + Ca-48, including the excitation functions, their low-energy slopes and the barrier distributions. The presence of the fusion hindrance phenomenon in all cases is pointed out, as well as the influence of the strong octupole excitation in Ca-40 and of nucleon transfer channels with positive Q values in Ca-40 + Ca-48.