Half-life measurement of the 199-keV isomeric state in Ga-76

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 = 28-50. 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 Ga-76. Purpose: The purpose of this experiment was to observe short-lived isomeric states in the vicinity of Ni-78. Methods: In-beam fragmentation of a Kr-86 primary beam at the National Superconducting Cyclotron Laboratory produced radioactive ions which were delivered to and deposited in a CeBr3 scintillator coupled to a position-sensitive photomultiplier tube. Beta-delayed gamma rays were measured by ancillary HPGe clover and LaBr3 detectors which surrounded the implantation detector. Results: The previously observed J(pi) =1(+), 199-keV level in Ga-76, populated following the beta decay of Zn-76, was identified as isomeric with a half-life of 34(1)(stat)(8)(sys.) 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.