High-precision branching-ratio measurement for the superallowedβ+emitter74Rb

Abstract

A high-precision branching-ratio measurement for the superallowed ${\ensuremath{\beta}}^{+}$ decay of ${}^{74}$Rb was performed at the TRIUMF Isotope Separator and Accelerator (ISAC) radioactive ion-beam facility. The scintillating electron-positron tagging array (SCEPTAR), composed of 10 thin plastic scintillators, was used to detect the emitted $\ensuremath{\beta}$ particles; the 8$\ensuremath{\pi}$ spectrometer, an array of 20 Compton-suppressed HPGe detectors, was used for detecting $\ensuremath{\gamma}$ rays that were emitted following Gamow-Teller and nonanalog Fermi ${\ensuremath{\beta}}^{+}$ decays of ${}^{74}$Rb; and the Pentagonal Array of Conversion Electron Spectrometers (PACES), an array of 5 Si(Li) detectors, was employed for measuring $\ensuremath{\beta}$-delayed conversion electrons. Twenty-three excited states were identified in ${}^{74}$Kr following $8.241(4)\ifmmode\times\else\texttimes\fi{}{10}^{8}$ detected ${}^{74}$Rb $\ensuremath{\beta}$ decays. A total of 58 $\ensuremath{\gamma}$-ray and electron transitions were placed in the decay scheme, allowing the superallowed branching ratio to be determined as ${B}_{0}=99.545(31)%$. Combined with previous half-life and $Q$-value measurements, the superallowed branching ratio measured in this work leads to a superallowed $ft$ value of 3082.8(65) s. Comparisons between this superallowed $ft$ value and the world-average-corrected $\overline{\mathcal{F}t}$ value, as well as the nonanalog Fermi branching ratios determined in this work, provide guidance for theoretical models of the isospin-symmetry-breaking corrections in this mass region.