Neutron-Excess Dependence of Fusion—Ni+Sn

Excitation functions ($E_{\mathrm{lab}}=235-320\mathrm{}$ MeV) for producing nuclei close to the compound system (evaporation residues) were measured for ${}_{}{}^{58,64}{}_{}{}^{}\mathrm{Ni}$+ ${}_{}{}^{112-124}{}_{}{}^{}\mathrm{Sn}$, 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.