Influence of nuclear structure on sub-barrier hindrance in Ni+Ni fusion

Fusion-evaporation cross sections for ${}^{64}$Ni+${}^{64}$Ni have been measured down to the 10 nb level. For fusion between two open-shell nuclei, this is the first observation of a maximum in the $S$-factor, which signals a strong sub-barrier hindrance. A comparison with the ${}^{58}$Ni+${}^{58}$Ni, ${}^{58}$Ni+${}^{60}$Ni, and ${}^{58}$Ni+${}^{64}$Ni systems indicates a strong dependence of the energy where the hindrance occurs on the stiffness of the interacting nuclei.