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

Abstract

A high-precision branching ratio measurement for the superallowed Fermi β+ emitter Ga62 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 γ-ray transitions, with intensities down to ≈1 part per million (ppm) per Ga62β+ decay, to be placed in the level scheme of the daughter nucleus Zn62, establishing the superallowed β branching ratio for Ga62 decay to be 99.8577-0.0029+0.0023%, a factor of 4 more precise than the previous world average. For several cascades, γ-γ 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 Zn62 was definitively assigned as J=0. This assignment resolves a discrepancy between previous measurements and has important implications for the isospin symmetry breaking correction, δC1, in Ga62 superallowed Fermi β decay.