Fusion of $^{124}$Sn with $^{40,48}$Ca*

Fusion of $^{124}$Sn with $^{40,48}$Ca near and below the barrier has been measured at ORNL. The ultimate goal is to compare with fusion of $^{132}$Sn on the same targets to study the effects of neutron excess and neutron-transfer Q-values on fusion. The Ca isotopes are ideal for this purpose since an identical $^{172}$Yb composite system is produced using two essentially spherical targets, thus reducing deformation effects on fusion. The $^{124}$Sn+$^{48}$Ca system displays weak sub-barrier fusion enhancement which is completely accounted for by coupling to the first 2$^+$ and 3$^-$ states in the target and projectile, but $^{124}$Sn+$^{40}$Ca fusion is strongly enhanced below the barrier. This effect appears to be related to the large positive Q-values for $^{124}$Sn+$^{40}$Ca neutron transfer reactions. The implications for $^{132}$Sn+$^{40,48}$Ca fusion will be discussed.