Near-barrier fusion ofS32+Zr90,96: The effect of multi-neutron transfers in sub-barrier fusion reactions

Abstract

Fusion excitation functions have been measured for the first time with rather good accuracy for $^{32}\mathrm{S}+^{90}\mathrm{Zr}$ and $^{32}\mathrm{S}+^{96}\mathrm{Zr}$ near and below the Coulomb barrier. The sub-barrier cross sections for $^{32}\mathrm{S}+^{96}\mathrm{Zr}$ are much larger than for $^{32}\mathrm{S}+^{90}\mathrm{Zr}$. A coupled-channels calculation considering the inelastic excitations is capable of describing sub-barrier enhancement only for $^{32}\mathrm{S}+^{90}\mathrm{Zr}$. The unexplained part for $^{32}\mathrm{S}+^{96}\mathrm{Zr}$ is found to be correlated with the positive-$Q$-value intermediate neutron transfers in this system. The comparison with $^{40}\mathrm{Ca}+^{96}\mathrm{Zr}$ suggests that couplings to the positive-$Q$-value neutron transfer channels may play a role in the sub-barrier fusion enhancement. Multi-neutron transfers are taken into account in Zagrebaev's semiclassical model to explain the discrepancies of the sub-barrier fusion cross sections for $^{32}\mathrm{S}+^{96}\mathrm{Zr}$.