The branching ratio for the superallowed ${\beta }^{+}$ decay of ${}^{38}\mathrm{K}^m$ was measured at TRIUMF’s ISAC radioactive ion beam facility. The $M3$ internal transition between the isomer and the ground state of ${}^{38}\mathrm{K}^m$ was observed with a branching ratio of 330(43) ppm. A search for the nonanalogue $\beta$-decay branch to the first excited $0^{+}$ state in ${}^{38}\mathrm{Ar}$ was also performed and yielded an upper limit of $\le 12\mathrm{ppm}$ at 90% C.L. These measurements lead to a revised superallowed branching ratio for ${}^{38}\mathrm{K}^m$ of 99.967(4)%, and increase the ${}^{38}\mathrm{K}^m$ $ft$ value by its entire quoted uncertainty to $ft=3052.1(10)\mathrm{s}$. Implications for tests of the nuclear-structure dependent corrections in superallowed $\beta$ decays and the extraction of the Cabibbo-Kobayashi-Maskawa matrix element $V_{\mathrm{ud}}$ are discussed.

• Received 20 November 2007

DOI:https://doi.org/10.1103/PhysRevLett.100.192504