Charge radii of ${}^{55,56}$Ni reveal a surprisingly similar behavior at $N=28$ in Ca and Ni isotopes

Abstract

Nuclear charge radii of ${}^{55,56}$Ni were measured by collinear laser spectroscopy. The obtained information completes the behavior of the charge radii at the shell closure of the doubly magic nucleus ${}^{56}$Ni. The trend of charge radii across the shell closures in calcium and nickel is surprisingly similar despite the fact that the ${}^{56}$Ni core is supposed to be much softer than the ${}^{48}$Ca core. The very low magnetic moment $\hat{I}¼{(}^{55}\mathrm{Ni})=-1.108(20)\hat{I}{¼}_N$ indicates the impact of M1 excitations between spin-orbit partners across the $N,Z=28$ shell gaps. Our charge-radii results are compared to \textit{ab initio} and nuclear density functional theory calculations, showing good agreement within theoretical uncertainties.