Near-Barrier Fusion Of S-32+Zr-90,Zr-96: The Effect Of Multi-Neutron Transfers In Sub-Barrier Fusion Reactions

Fusion excitation functions have been measured for the first time with rather good accuracy for S-32+Zr-90 and S-32+Zr-96 near and below the Coulomb barrier. The sub-barrier cross sections for S-32+Zr-96 are much larger than for S-32+Zr-90. A coupled-channels calculation considering the inelastic excitations is capable of describing sub-barrier enhancement only for S-32+Zr-90. The unexplained part for S-32+Zr-96 is found to be correlated with the positive-Q-value intermediate neutron transfers in this system. The comparison with Ca-40+Zr-96 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 S-32+Zr-90.