PNNL

Abstract

The shape of the atomic nucleus is a key property which underpins our understanding of nuclear systems, impacts the limits of nuclear existence, and enables more sensitive probes of physics beyond the Standard Model. Nuclei can adopt a variety of shapes such as spheres, axially deformed spheroids (frisbees/football shapes), and even pear shapes. In some regions of the nuclear chart where a spherical nucleus would naively be expected, a deformed nuclear state 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, giving rise to a phenomenon known as shape coexistence. Here we present conclusive spectroscopic evidence for the coexistence of spherical and deformed states in 70Co, separated by less than 275 keV. The result confirms previous indications and unambiguously identifies the Co isotopes as the transition between deformed ground states observed in nearby Cr isotopes and spherical ground states seen in closed-shell Ni isotopes. The identification of two states with similar energies but drastically different shapes demonstrates an extreme example of the complex interplay between independent-particle motion and collective behavior in nuclear systems and resultant effects on nuclear structure far from stability.