Presently, the world data for superallowed $\beta$ decay leads to a result in disagreement (at the $98\%$ confidence level) with the predictions of the minimal standard model for the unitarity of the Cabibbo-Kobayashi-Maskawa matrix. Precise data for the superallowed $0^{+}\to 0^{+}$ $\beta$ decay of ${}^{74}\mathrm{Rb}$ would provide a critical test of the nucleus-dependent isospin symmetry-breaking corrections that must be calculated for these superallowed Fermi $\beta$ decays. The present work reports the first precise measurement of the half-life for ${}^{74}\mathrm{Rb}$ ( $t_{1/2}=64.761\mathrm{}\pm 0.031\mathrm{ms}$). The data were obtained at the radioactive beam facility (ISAC) at TRIUMF using a beam of $\sim 4000$ ${}^{74}\mathrm{Rb}$ ions ${\mathrm{s}}^{-1\mathrm{}}$.

• Received 16 August 2000

DOI:https://doi.org/10.1103/PhysRevLett.86.1454