Fusion Hindrance and Pauli Blocking in $^{58}$Ni + $^{64}$Ni
Résumé
58 Ni + 64 Ni 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 58 Ni + 58 Ni and 64 Ni + 64 Ni. Subsequent measurements on 64 Ni + 64 Ni showed that fusion hindrance is clearly present in this case. On the other hand, no indication of hindrance can be observed for 58 Ni + 64 Ni 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 58 Ni + 64 Ni continue decreasing very smoothly below the barrier, down to 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 Sfactor 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 64 Ni+ 64 Ni (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 58 Ni+ 64 Ni 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.
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