High-precision Half-life And Branching Ratio Measurements For Superallowed β+ Emitters At TRIUMF-ISAC

Abstract

High precision measurements of the $ft$ values for superallowed Fermi $\beta$ transitions between $J^\pi~=~0^+$ isobaric analogue states allow for stringent tests of the electroweak interaction described by the Standard Model. These transitions provide an experimental probe of the Conserved-Vector-Current hypothesis, enable the most precise determination of the up-down ($V_{ud}$) element of the Cabibbo-Kobayashi-Maskawa quark-mixing matrix, and allow one to set stringent limits on the existence of scalar currents in the weak interaction. An extensive program of superallowed branching-ratio and half-life measurements at TRIUMF's Isotope Separator and Accelerator (ISAC) facility has covered the full range of superallowed emitters, from the lightest case, $^{10}$C, to the heaviest case for which precision data are currently available, $^{74}$Rb. These experiments have been performed using a 4$\pi$ continuous-flow gas proportional $\beta$ counter, the 8$\pi$ $\gamma$-ray spectrometer, and, most recently, the new high-efficiency GRIFFIN $\gamma$-ray spectrometer. In this paper, recent highlights from the superallowed Fermi $\beta$ decay program at TRIUMF will be summarized, including high-precision half-life measurements for all three of the lightest superallowed emitters, $^{10}$C, $^{14}$O, and $^{18}$Ne, with the greatest sensitivity to a potential weak scalar current contribution, as well as high-precision branching-ratio measurements for the heavy superallowed emitters $^{62}$Ga and $^{74}$Rb.