Extreme Shape Coexistence Observed in $^{70}$Co

TL;DR

This paper reports the discovery of an extreme case of shape coexistence in $^{70}$Co, where spherical and deformed nuclear states with the same spin and parity are nearly degenerate, revealing complex nuclear structure dynamics.

Contribution

It provides the first spectroscopic evidence of shape coexistence in $^{70}$Co, highlighting a transition point between different nuclear shape regimes.

Findings

Spherical and deformed states in $^{70}$Co are separated by less than 275 keV.

The coexistence demonstrates interplay of single-particle and collective nuclear behaviors.

$^{70}$Co is a transitional nucleus between Cr and Ni isotopes.

Abstract

The shape of the atomic nucleus is a property which underpins our understanding of nuclear systems, impacts the limits of nuclear existence, and enables probes of physics beyond the Standard Model. Nuclei can adopt a variety of shapes, including spheres, axially deformed spheroids, and pear shapes. In some regions of the nuclear chart where a spherical nucleus would naively be expected, deformed nuclear states can result from collective action of constituent protons and neutrons. In a small subset of nuclei both spherical and deformed nuclear states have been experimentally observed, a phenomenon termed shape coexistence. We present spectroscopic evidence for the coexistence of $J^{\pi}=1+$ spherical and deformed states in $^{70}$Co, separated by less than 275~keV. This close degeneracy of levels with the same $J^{\pi}$ and different shapes demonstrates an extreme example of shape coexistence resulting from the interplay of independent particle motion and collective behavior in highly unstable nuclear systems and identifies the Co isotopes as a transition point between deformed ground states observed in the Cr isotopes and spherical configurations observed in the closed-shell Ni isotopes.