Interplay between nuclear shell evolution and shape deformation revealed by magnetic moment of 75Cu

TL;DR

This study investigates how shell evolution and shape deformation interact in neutron-rich nuclei, using magnetic moment measurements of 75Cu to reveal the dominance of single-particle states and implications for nuclear magicity.

Contribution

The paper provides experimental evidence and numerical analysis showing shell evolution's role in nuclear structure, especially in the context of 75Cu and 78Ni.

Findings

Low-lying states in 75Cu are mainly single-particle in nature.

Shell evolution significantly influences nuclear structure despite collective deformation.

Potential restoration of double magicity in 78Ni is discussed.

Abstract

Exotic nuclei are characterized by a number of neutrons (or protons) in excess relative to stable nuclei. Their shell structure, which represents single-particle motion in a nucleus, may vary due to nuclear force and excess neutrons, in a phenomenon called shell evolution. This effect could be counterbalanced by collective modes causing deformations of the nuclear surface. Here, we study the interplay between shell evolution and shape deformation by focusing on the magnetic moment of an isomeric state of the neutron-rich nucleus 75Cu. We measure the magnetic moment using highly spin-controlled rare-isotope beams and achieving large spin alignment via a two-step reaction scheme that incorporates an angular-momentum-selecting nucleon removal. By combining our experiments with numerical simulations of many-fermion correlations, we find that the low-lying states in 75Cu are, to a large extent, of single-particle nature on top of a correlated 74Ni core. We elucidate the crucial role of shell evolution even in the presence of the collective mode, and within the same framework, we consider whether and how the double magicity of the 78Ni nucleus is restored, which is also of keen interest from the perspective of nucleosynthesis in explosive stellar processes.