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

TL;DR

This study measures the charge radii of $^{55,56}$Ni to explore shell closure effects, revealing unexpected similarities with calcium isotopes and providing insights into nuclear structure near $N=28$.

Contribution

It provides new experimental charge radii data for nickel isotopes and compares them with theoretical models, highlighting the softness of the $^{56}$Ni core and the impact of M1 excitations.

Findings

Charge radii of $^{55,56}$Ni complete the understanding at $^{56}$Ni shell closure.

Charge radii trends in Ca and Ni are surprisingly similar despite different core softness.

Good agreement between experimental results and theoretical calculations.

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 $\mu(^{55}\mathrm{Ni})=-1.108(20)\,\mu_N$ 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.