Multistep Coulomb excitation of Ni64 : Shape coexistence and nature of low-spin excitations

Abstract

The structure of $^{64}\mathrm{Ni}$, the heaviest stable Ni isotope, has been investigated via high-statistics, multistep safe Coulomb excitation to search for shape coexistence, a phenomenon recently observed in neutron-rich $^{66}\mathrm{Ni}$ and $^{70}\mathrm{Ni}$ as well as in doubly magic, $N=40, ^{68}\mathrm{Ni}$. The study was motivated by recent, state-of-the-art Monte Carlo shell-model calculations (MCSM), where a Hamiltonian with effective interactions incorporating the monopole tensor force predicts the existence of shape coexistence, also in the lower-mass $^{62,64}\mathrm{Ni}$ isotopes. A set of transition and static $E2$ matrix elements for both yrast and near-yrast structures was extracted from the differential Coulomb excitation cross sections. From comparisons between the new results and MCSM as well as other shell-model calculations, a clearer picture of the structure of $^{64}\mathrm{Ni}$ emerges. Specifically, the low-spin states are shown to be dominated by proton and neutron excitations mainly within the $fp$ shell, with minimal contribution from the ${g}_{9/2}$ shape-driving neutron orbital. The agreement between experimental data and MCSM results indicates a small oblate deformation for the ${0}_{2}^{+}$ level and a spherical shape for the ${0}_{3}^{+}$ state. In addition, the small upper limit determined for the $B(E2)$ probability of a transition associated with the decay of the recently observed 3463-keV, ${0}_{4}^{+}$ state agrees with its proposed assignment to a prolate shape, herewith providing first evidence for triple shape coexistence in a stable Ni isotope.