Background: Isomeric states in atomic nuclei are a sensitive probe of their underlying microscopic structure and can be used to study the evolution of shell structure far from stability. Recent studies have identified and provided detailed spectroscopy of isomers in neutron-rich nuclei with $Z=28\text{−}50$. Isomeric states in the odd-odd gallium isotopes have been reported for all gallium isotopes from $A=72$ to $A=80$ with the exception of ${}^{76}\mathrm{Ga}$.

Purpose: The purpose of this experiment was to observe short-lived isomeric states in the vicinity of ${}^{78}\mathrm{Ni}$.

Methods: In-beam fragmentation of a ${}^{86}\mathrm{Kr}$ primary beam at the National Superconducting Cyclotron Laboratory produced radioactive ions which were delivered to and deposited in a $\mathrm{Ce}{\mathrm{Br}}_3$ scintillator coupled to a position-sensitive photomultiplier tube. Beta-delayed $\gamma$ rays were measured by ancillary HPGe clover and $\mathrm{La}{\mathrm{Br}}_3$ detectors which surrounded the implantation detector.

Results: The previously observed $J^{\pi }=1^{+}$, 199-keV level in ${}^{76}\mathrm{Ga}$, populated following the $\beta$ decay of ${}^{76}\mathrm{Zn}$, was identified as isomeric with a half-life of $34{\left(1\right)}_{\text{stat.}}{\left(8\right)}_{\text{sys.}}\mathrm{ns}$. Shell-model calculations suggest this state is formed by the coupling of protons in negative-parity configurations to $1/2^{-}$ neutron configurations. Transition strengths assuming a ground-state spin of $J=2$ and $J=3$ were determined from the experimental data.

• Received 9 November 2021
• Accepted 2 February 2022

DOI:https://doi.org/10.1103/PhysRevC.105.024319