Hindrance of heavy-ion fusion at extreme sub-barrier energies in open-shell colliding systems

Abstract

The excitation function for the fusion-evaporation reaction $^{64}\mathrm{Ni}+^{100}\mathrm{Mo}$ has been measured down to a cross section of \ensuremath{\sim}5 nb. Extensive coupled-channels calculations have been performed, which cannot reproduce the steep falloff of the excitation function at extreme sub-barrier energies. Thus, this system exhibits a hindrance for fusion, a phenomenon that has been discovered only recently. In the S-factor representation introduced to quantify the hindrance, a maximum is observed at ${E}_{s}=120.6\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$, which corresponds to 90% of the reference energy ${E}_{s}^{\mathrm{ref}}$, a value expected from systematics of closed-shell systems. A systematic analysis of Ni-induced fusion reactions leading to compound nuclei with mass $A=100\text{\ensuremath{-}}200$ is presented in order to explore a possible dependence of fusion hindrance on nuclear structure.