Fusion Hindrance and Pauli Blocking in Ni-58 + Ni-64

Ni-58 +Ni- 64 is the first case where the influence of positive Q-value transfer channels on sub-barrier fusion was evidenced, in a very well known experiment by Beckerman et al., by comparing with the two systems Ni-58 + Ni-58 and Ni-64 + Ni-64. Subsequent measurements on Ni-64 + Ni-64 showed that fusion hindrance is clearly present in this case. On the other hand, no indication of hindrance can be observed for Ni-58 + Ni-64 down to the measured level of 0.1 mb. In the present experiment the excitation function has been extended by two orders of magnitude downward. The cross sections for Ni-58 + Ni-64 continue decreasing very smoothly below the barrier, down to similar or equal to 1 mu b. 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. Coupled-channels (CC) calculations using a Woods-Saxon potential and including inelastic excitations only, underestimate the sub-barrier cross sections by a large amount. Good agreement is found by adding two-neutron transfer couplings to a schematical level. This behaviour is quite different from what already observed for Ni-64+Ni-64 (no positive Q-value transfer channels available), where a clear low-energy maximum of the S-factor appears, and whose excitation function is overestimated by a standard Woods-Saxon CC calculation. No hindrance effect is observed in Ni-58+ Ni-64 in the measured energy range. This trend at deep sub-barrier energies reinforces the recent suggestion that the availability of several states following transfer with Q >0, effectively counterbalances the Pauli repulsion that, in general, is predicted to reduce tunneling probability inside the Coulomb barrier.