Internal [gamma] decay and the superallowed branching ratio for the [beta][superscript +] emitter [superscript 38]K[superscript m]

Abstract

The branching ratio for the superallowed [beta][superscript +] decay of [superscript 38]K[superscript m] was measured at TRIUMF’s ISAC radioactive ion beam facility. The M3 internal transition between the isomer and the ground state of [superscript 38]K[superscript m] was observed with a branching ratio of 330(43) ppm. A search for the nonanalogue [beta]-decay branch to the first excited 0[superscript +] state in [superscript 38]Ar was also performed and yielded an upper limit of [less than or equal to] 12 ppm at 90% C.L. These measurements lead to a revised superallowed branching ratio for [superscript 38]K[superscript m] of 99.967(4)%, and increase the [superscript 38]K[superscript m] ft value by its entire quoted uncertainty to ft = 3052.1(10) 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[subscript ud] are discussed.