Fusion hindrance and Pauli blocking in ⁵⁸Ni

The early experiments on fusion of Ni + Ni systems are well-known and indicated for the first time the possible influence of transfer reactions on subbarrier cross sections. The influence of the positive Q-value transfer channels on sub-barrier fusion cross sections of the system Ni-58 + Ni-64 was evidenced in an experiment by Beckerman et al. Subsequent experiments for the two systems Ni-58 + Ni-58 and Ni-64 + Ni-64 showed that fusion hindrance is clearly present in both cases. The lowest measured cross section for Ni-58 + Ni-64 was relatively large (0.1 mb), so that no hindrance was observed. In the present measurement the excitation function has been extended by two orders of magnitude downward. The present experiment indicates that the flat trend of the sub-barrier cross sections for Ni-58 + Ni-64 continues down to the level of mu b and the logarithmic slope of the excitation function increases slowly, showing a tendency to saturate at the lowest energies. No maximum of the astrophysical S-factor is observed, so fusion hindrance is not observed. This trend at far sub-barrier energies suggests that the presence of the transfer channel with Q > 0, effectively counterbalances the effect of Pauli repulsion that is predicted to reduce tunneling probability.