No sub-Coulomb-barrier fusion enhancement due to positive Q -value two-neutron stripping: The O 18 + Ni 58 case

Coupling to transfer reactions is a long-standing problem in heavy-ion fusion at near-barrier energies. The 18O + 58Ni system, that was claimed to show a large sub-barrier fusion enhancement, is a typical case to demonstrate the coupled-channels effects of positive Q-value two-neutron (2n) transfer based on the existing experimental data. We remeasured the near-barrier fusion excitation function of 18O + 58Ni extending to lower energies by detecting the fusion evaporation residues. In order to understand the effects of couplings to neutron transfers, coupled-reaction-channels calculations by the code fresco were performed in a nonadjustable mode. Results show that couplings to low-lying collective excitations and one-neutron transfer are enough to explain the experimental fusion excitation function and barrier distribution. No evidence shows a sub-barrier fusion enhancement attributed to positive Q-value 2n stripping.