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

TL;DR

This study measures nuclear magnetic and quadrupole moments of neutron-rich copper isotopes near $^{78}$Ni to test the robustness of the $Z=28$ shell closure, revealing that magicity is restored despite shell gap narrowing.

Contribution

It provides new high-precision measurements of nuclear moments for isotopes near $^{78}$Ni, and compares them with shell-model calculations to assess shell closure robustness.

Findings

Nuclear moments are consistent with shell-model predictions.

Magicity of $Z=28$ and $N=50$ is restored towards $^{78}$Ni.

Weakened dynamical correlations are probed by moment measurements.

Abstract

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 $^{78}_{29}$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 $N\sim45$ 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.