Nuclear charge radii of $^{62−80}$Zn and their dependence on cross-shell proton excitations

Nuclear charge radii of $^{62−80}$Zn have been determined using collinear laser spectroscopy of bunched ion beams at CERN-ISOLDE. The subtle variations of observed charge radii, both within one isotope and along the full range of neutron numbers, are found to be well described in terms of the proton excitations across the $Z=28$ shell gap, as predicted by large-scale shell model calculations. It comprehensively explains the changes in isomer-to-ground state mean square charge radii of $^{69−79}$Zn, the inversion of the odd-even staggering around $N=40$ and the odd-even staggering systematics of the Zn charge radii. With two protons above $Z=28$ , the observed charge radii of the Zn isotopic chain show a cumulative effect of different aspects of nuclear structure including single particle structure, shell closure, correlations and deformations near the proposed doubly magic nuclei, $^{68}$Ni and $^{78}$Ni.