Fusion Hindrance and Pauli Blocking in 58Ni +64Ni

The argument of this thesis is the measurement of deep sub-barrier fusion cross sections for 58Ni +64Ni. In this system the influence of positive Q-value transfer channels on sub-barrier fusion was evidenced in a famous experiment by Beckerman et al.. Subsequent experiments for the two symmetric systems 58Ni +58Ni and 64Ni +64Ni showed that fusion hindrance is clearly present in both cases. The lowest measured cross section for 58Ni +64Ni, however, was relatively large (0.1 mb), so that no hindrance was observed. The present measurements have been recently performed at the XTU Tandem accelerator of LNL and the excitation function has been extended by two orders of magnitude downward. The case of 58Ni +64Ni is very similar to 40Ca+96Zr because of the flat shape of the two sub-barrier fusion excitation functions, originating from the couplings to several Q>0 neutron pick-up channels. 40Ca+96Zr was studied to very small cross sections (2b) and fusion hindrance does not show up, suggesting that this unusual behavior is due to the Q>0 transfer couplings, since the valence nucleons can flow freely from one nucleus to the other without being hindered by Pauli blocking. The present experiment indicates that the flat trend of the sub-barrier cross sections for 58Ni +64Ni continues down to the level of b and fusion hindrance is not observed. This trend at far sub-barrier energies reinforces the suggestion that the availability of several states following transfer with Q>0, effectively counterbalances the effect of Pauli repulsion that, in general, is predicted to reduce tunneling probability inside the Coulomb barrier.