In recent models, the neutron-rich Ni isotopes around $N=40$ are predicted to exhibit multiple low-energy excited $0^{+}$ states attributed to neutron and proton excitations across both the $N=40$ and $Z=28$ shell gaps. In ${}^{68}\mathrm{Ni}$, the three observed $0^{+}$ states have been interpreted in terms of triple shape coexistence between spherical, oblate, and prolate deformed shapes. In the present work a new $\left(0_2^{+}\right)$ state at an energy of 1567 keV has been discovered in ${}^{70}\mathrm{Ni}$ by using $\beta$-delayed, $\gamma$-ray spectroscopy following the decay of ${}^{70}\mathrm{Co}$. The precipitous drop in the energy of the prolate-deformed $0^{+}$ level between ${}^{68}\mathrm{Ni}$ and ${}^{70}\mathrm{Ni}$ with the addition of two neutrons compares favorably with results of Monte Carlo shell-model calculations carried out in the large $fp{g}_{9/2}{d}_{5/2}$ model space, which predict a $0_2^{+}$ state at 1525 keV in ${}^{70}\mathrm{Ni}$. The result extends the shape-coexistence picture in the region to ${}^{70}\mathrm{Ni}$ and confirms the importance of the role of the tensor component of the monopole interaction in describing the structure of neutron-rich nuclei.

• Received 15 June 2015
• Revised 9 September 2015
• Corrected 28 December 2015

DOI:https://doi.org/10.1103/PhysRevC.92.061302