Fusion hindrance and Pauli blocking in Ni 58 + Ni 64

Background: 58Ni + 64Ni is the first case where the influence of positive Q-value transfer channels on subbarrier fusion was evinced, in a very well known experiment by Beckerman et al. [Phys. Rev. Lett. 45, 1472 (1980)], by comparing with the two systems 58Ni + 58Ni and 64Ni + 64Ni. Subsequent measurements on 64Ni + 64Ni showed that fusion hindrance is clearly present in this case. On the other hand, no indication of hindrance can be observed for 58Ni + 64Ni down to the measured level of 0.1 mb. Purpose: Measuring deep subbarrier fusion cross sections for 58Ni + 64Ni, where the influence of positive Qvalue transfer channels on near-barrier fusion was evidenced previously, in order to investigate whether hindrance shows up. Methods: 167–200 MeV 58Ni beams from the XTU Tandem Accelerator of INFN-Laboratori Nazionali di Legnaro were used, bombarding thin metallic 64Ni (50 μg/cm2) enriched to 99.6% in mass 64. An electrostatic beam deflector allowed fusion evaporation residues to be detected at very forward angles in a detector telescope. Results: The excitation function has been extended by two orders of magnitude downward. The cross sections for 58Ni + 64Ni continue decreasing very smoothly below the barrier, down to about 1 μb. 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-channel (CC) calculations using a Woods-Saxon potential and including inelastic excitations only underestimate the subbarrier cross sections by a large amount. Good agreement is found by adding two-nucleon transfer couplings to a schematical level. This behavior is quite different from what already observed for 64Ni + 64Ni (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. Conclusions: No hindrance effect is observed in 58Ni + 64Ni in the measured energy range. This trend at deep subbarrier 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.