Low-energy surface vibrations and neutron transfer initiating heavy-ion sub-barrier fusion

Abstract

This paper reviews recent experimental results on heavy-ion sub-barrier fusion, which have been obtained with the beams of the XTU Tandem accelerator of LNL and a simple set-up using an electrostatic beam deflector. Various systems have been studied, i.e. , and more recently . In most cases, the sub-barrier fusion cross section enhancements can be attributed to inelastic channel couplings which also produce discrete structures in the representations of fusion barrier distributions obtained as the second derivatives of the excitation functions. From combined analyses of the evaporation residue cross sections and of the fusion barrier distributions, important effects of low-energy surface vibrations up to the two- and possibly three-phonon level have been put in evidence. In two cases (40Ca + 96Zr and 32S + 110Pd) the effect of transfer couplings on cross sections and barrier distributions is rather clear, although the two barrier distributions have very different shapes. Further comparisons are made with the results of older experiments on heavy-ion systems with various nuclear structure situations, and preliminary data are shown on the sub-barrier fusion of 40Ca +116,124Sn, where interesting connections may be deduced with the multi-nucleon transfer reactions already measured for the heavier target.