LOW-ENERGY SURFACE VIBRATIONS AND NEUTRON TRANSFER INITIATING HEAVY-ION SUBBARRIER FUSION

This paper reviews recent experimental results on heavy-ion sub-barrie r fusion, which have been obtained with the beams of the XTU Tandem ac celerator of LNL and a simple set-up using an electrostatic beam defle ctor. Various systems have been studied, i.e. Ni-58 + Ni-60, S-32,S-36 + Pd-110 and more recently Ca-40 + Zr-90,Zr-96. In most cases, the su b-barrier fusion cross section enhancements can be attributed to inela stic channel couplings which also produce discrete structures in the r epresentations 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 distribu tions,, important effects of low-energy surface vibrations up to the t wo-and possibly three-phonon level have been put in evidence. In two c ases (Ca-40 + Zr-96, and S-32 + Pd-110) the effect of transfer couplin gs on cross sections and barrier distributions is rather clear, althou gh the two barrier distributions have very different shapes. Further c omparisons are made with the results of older experiments on heavy-ion systems with various nuclear structure situations, and preliminary da ta are shown on the sub-barrier fusion of Ca-40 + Sn-116,Sn-124, where interesting connections may be deduced with the multi-nucleon transfe r reactions already measured for the heavier target.