Abstract
58Ni +64Ni 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 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. In the present experiment 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 '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-channels (CC) calculations using a Woods-Saxon potential and includinginelastic 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 64Ni+ 64Ni (no positive Q-value transfer channels available), where a clear low-energy maximum of the S -factorappears, and whose excitation function is overestimated by a standard Woods-Saxon CC calculation. No hindrance effect is observed in 58Ni+ 64Ni 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.
Highlights
The sequence of stable nickel isotopes from 58Ni to 64Ni offers several opportunities of studying fusion dynamics near and below the Coulomb barrier
Fusion-evaporation cross sections have been measured for the system 58Ni + 64Ni at several energies near and below the Coulomb barrier, using the 58Ni beam provided by the XTU Tandem accelerator of the Laboratori Nazionali di Legnaro (LNL) of INFN in the energy range 167-201 MeV
We observe that the logarithmic slope of the excitation function has a slow increase, and tends to saturate at the lowest energies
Summary
The sequence of stable nickel isotopes from 58Ni to 64Ni offers several opportunities of studying fusion dynamics near and below the Coulomb barrier. The early experiments on fusion of Ni + Ni systems [1] are well-known and indicated for the first time the possible influence of transfer reactions on near- and sub-barrier cross sections. In more recent years it was found for many systems [4] that, at deep sub-barrier energies, the cross section decreases very rapidly [5], so that the excitation function is much steeper than the prediction of standard coupledchannels (CC) calculations.
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