Fusion hindrance and Pauli blocking in Ni58+Ni64

Background: Ni58+Ni64 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)PRLTAO0031-900710.1103/PhysRevLett.45.1472], by comparing with the two systems Ni58+Ni58 and Ni64+Ni64. Subsequent measurements on Ni64+Ni64 showed that fusion hindrance is clearly present in this case. On the other hand, no indication of hindrance can be observed for Ni58+Ni64 down to the measured level of 0.1 mb. Purpose: Measuring deep subbarrier fusion cross sections for Ni58+Ni64, where the influence of positive Q-value transfer channels on near-barrier fusion was evidenced previously, in order to investigate whether hindrance shows up. Methods: 167–200 MeV Ni58 beams from the XTU Tandem Accelerator of INFN-Laboratori Nazionali di Legnaro were used, bombarding thin metallic Ni64(50μg/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 Ni58+Ni64 continue decreasing very smoothly below the barrier, down to ≃1μb. 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 Ni64+Ni64 (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 Ni58+Ni64 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.