Fusion of 32S+94Zr: Further exploration of the effect of the positive Qxn value neutron transfer channels

The effect of the positive Q-value neutron transfer (PQNT) channels on the subbarrier fusion process is still far from a clear understanding and hence is worth further investigation. To this end, we have measured the fusion excitation function for S-32 + Zr-94 at energies near and below the Coulomb barrier with rather good accuracy, and deduced the experimental fusion barrier distribution. The reaction system S-32 + Zr-94 is a good candidate for the evaluation of PQNT effect because it has multineutron transfer channels with positive ground-state Q(xn) values as well as the relatively weak low-lying 3(-) vibrational state in Zr-94. Our results show that the fusion cross sections of S-32 + Zr-94 are strongly enhanced at the lower-energy region. By means of a cross comparison of the S-32, Ca-40 + Zr-90,Zr-94,Zr-96 systems, further evidence is given that the positive Qxn-value neutron transfer channels, rather than the strong low-lying 3(-) vibrational state, bring about the additional subbarrier fusion enhancement. Relative to the coupled-channels results with couplings to the low-lying inelastic collective excitations, an unexpectedly large enhancement for the subbarrier fusion cross sections is observed in S-32 + Zr-94 compared to S-32 + Zr-94. Some existing theoretical models have been discussed, but a good explanation cannot be achieved. Exact coupled-channels theory and more experiments on transfer reactions themselves are strongly desired to comprehensively understand the effect of transfer on fusion.