Dipole and quadrupole moments of ${}^{73-78}$Cu as a test of the robustness of the $Z=28$ shell closure near ${}^{78}$Ni

Nuclear spins and precise values of the magnetic dipole and electric quadrupole moments of the ground-states of neutron-rich ${}^{76-78}$Cu isotopes were measured using the Collinear Resonance Ionization Spectroscopy (CRIS) experiment at ISOLDE, CERN. The nuclear moments of the less exotic ${}^{73,75}$Cu isotopes were re-measured with similar precision, yielding values that are consistent with earlier measurements. The moments of the odd-odd isotopes, and ${}_{29}^{78}$Cu ($N=49$) in particular, are used to investigate excitations of the assumed doubly-magic ${}^{78}$Ni core through comparisons with large-scale shell-model calculations. Despite the narrowing of the $Z=28$ shell gap between $N\sim 45$ and $N=50$, the magicity of $Z=28$ and $N=50$ is restored towards ${}^{78}$Ni. This is due to weakened dynamical correlations, as clearly probed by the present moment measurements.