Neutron-Excess Dependence of Fusion—Ni+Sn

Abstract

Excitation functions (${E}_{\mathrm{lab}}=235\ensuremath{-}320$ MeV) for producing nuclei close to the compound system (evaporation residues) were measured for $^{58,64}\mathrm{Ni}$+ $^{112\ensuremath{-}124}\mathrm{Sn}$, corresponding to a 20% variation in the compound-nucleus neutron number. A dramatic, order-of-magnitude increase was observed in the maximum cross section as a function of the compound-nucleus neutron neutron excess. The subbarrier energy dependence of the cross sections is well described by an optical-model calculation, but not by the frequently used parabolic approximation.