Precision Mass Measurements of Neutron-Rich Co Isotopes Beyond N=40

TL;DR

This paper reports precise mass measurements of neutron-rich cobalt isotopes beyond N=40 using Penning trap techniques, complemented by ab initio calculations, to better understand nuclear structure evolution near $^{68}$Ni.

Contribution

It provides the first Penning trap mass measurements of $^{68,69}$Co and combines them with advanced theoretical calculations to explore nuclear shell evolution.

Findings

Masses of $^{68,69}$Co measured with high precision.

Ab initio calculations support experimental data.

Insights into shell closure at N=40 near $^{68}$Ni.

Abstract

The region near Z=28, N=40 is a subject of great interest for nuclear structure studies due to spectroscopic signatures in $^{68}$Ni suggesting a subshell closure at N=40. Trends in nuclear masses and their derivatives provide a complementary approach to shell structure investigations via separation energies. Penning trap mass spectrometry has provided precise measurements for a number of nuclei in this region, however a complete picture of the mass surfaces has so far been limited by the large uncertainty remaining for nuclei with N > 40 along the iron and cobalt chains. Here we present the first Penning trap measurements of $^{68,69}$Co, performed at the Low-Energy Beam and Ion Trap facility at the National Superconducting Cyclotron Laboratory. In addition, we perform ab initio calculations of ground state and two-neutron separation energies of cobalt isotopes with the valence-space in-medium similarity renormalization group approach based on a particular set of two- and three-nucleon forces which predict saturation in infinite matter. We discuss the importance of these measurements and calculations for understanding the evolution of nuclear structure near $^{68}$Ni.