N2868i40: Magicity versus Superfluidity

The neutron-rich ${}^{66,68}\mathrm{Ni}$ have been produced at GANIL via interactions of a $65.9A\mathrm{MeV}$ ${}^{70}\mathrm{Zn}$ beam with a ${}^{58}\mathrm{Ni}$ target. Their reduced transition probability $B({E2;0\mathrm{}}_1^{+}\to 2^{+})$ has been measured for the first time by Coulomb excitation in a ${}^{208}\mathrm{Pb}$ target at intermediate energy. The $B\left(E2\mathrm{}\right)$ value for ${}^{68}{\mathrm{Ni}}_{40}$ is unexpectedly small. An analysis in terms of large scale shell model calculations stresses the importance of proton core excitations to reproduce the $B\left(E2\mathrm{}\right)$ values and indicates the erosion of the $N=40\mathrm{}$ harmonic-oscillator subshell by neutron-pair scattering.