Low-lying excited states in ${}^{72}\mathrm{Ni}$ have been investigated in an in-flight fission experiment at the RIBF facility of the RIKEN Nishina Center. The combination of the state-of-the-art BigRIPS and EURICA setups has allowed for a very accurate study of the $\beta$ decay from ${}^{72}\mathrm{Co}$ to ${}^{72}\mathrm{Ni}$, and has provided first experimental information on the decay sequence ${}^{72}\mathrm{Fe}\to {}^{72}\mathrm{Co}\to {}^{72}\mathrm{Ni}$ and on the delayed neutron-emission branch ${}^{73}\mathrm{Co}\to {}^{72}\mathrm{Ni}$. Accordingly, we report nearly 60 previously unobserved $\gamma$ transitions which deexcite 21 new levels in ${}^{72}\mathrm{Ni}$. Evidence for the location of the so-sought-after $\left(4_2^{+}\right),\left(6_2^{+}\right)$, and $\left(8_1^{+}\right)$ seniority states is provided. As well, the existence of a low-spin $\beta$-decaying isomer in odd-odd neutron-rich Co isotopes is confirmed for mass $A=72$. The new experimental information is compared to simple shell-model calculations including only neutron excitations across the $fpg$ shells. It is shown that, in general, the calculations reproduce well the observed states.

• Received 11 November 2015
• Revised 26 January 2016

DOI:https://doi.org/10.1103/PhysRevC.93.034328