Fusion of124,132Sn with40,48Ca

Fusion excitation functions for the reactions of ${}^{124,132}$Sn with ${}^{40,48}$Ca were measured at energies near and below the Coulomb barrier. Weak sub-barrier fusion enhancement accounted for by coupling to the first 2${}^{+}$ and 3${}^{\ensuremath{-}}$ states in the target and projectile was observed for ${}^{124,132}\text{Sn}+{}^{48}\text{Ca}$. However, the ${}^{124,132}\text{Sn}+{}^{40}\text{Ca}$ fusion cross sections are very strongly enhanced below the barrier and this difference survives even after correcting for trivial size effects. Although the enhancement appears to be related to the existence of large positive $Q$ values for neutron transfer reactions, it is not proportional to the magnitudes of those $Q$ values, which are much larger for ${}^{132}\mathrm{Sn}+{}^{40}\mathrm{Ca}$ than for ${}^{124}\mathrm{Sn}+{}^{40}\mathrm{Ca}$.