High-precision branching ratio measurement and spin assignment implications for Ga-62 superallowed beta decay

A high-precision branching ratio measurement for the superallowed Fermi beta(+) emitter Ga-62 was performed with the Gamma-Ray Infrastructure for Fundamental Investigations of Nuclei (GRIFFIN) spectrometer at the Isotope Separator and Accelerator (ISAC) radioactive ion beam facility at TRIUMF. The high efficiency of the GRIFFIN spectrometer allowed 63 gamma -ray transitions, with intensities down to approximate to 1 part per million (ppm) per Ga-62 beta(+) decay, to be placed in the level scheme of the daughter nucleus Zn-62, establishing the superallowed beta branching ratio for Ga-62 decay to be 99.8577(-0.0029)(+0.0023)%, a factor of 4 more precise than the previous world average. For several cascades, gamma-gamma angular correlation measurements were performed to assign spins and/or determine the mixing ratios of transitions. In particular, the spin of the 2.342 MeV excited state in the daughter nucleus Zn-62 was definitively assigned as J = 0. This assignment resolves a discrepancy between previous measurements and has important implications for the isospin symmetry breaking correction, delta(C1), in Ga-62 superallowed Fermi beta decay.