βdecay of32Cl: Precisionγ-ray spectroscopy and a measurement of isospin-symmetry breaking

Abstract

Background: Models to calculate small isospin-symmetry-breaking effects in superallowed Fermi decays have been placed under scrutiny in recent years. A stringent test of these models is to measure transitions for which the correction is predicted to be large. The decay of ${}^{32}$Cl decay provides such a test case.Purpose: The purpose of this paper is to improve the $\ensuremath{\gamma}$ yields following the $\ensuremath{\beta}$ decay of ${}^{32}$Cl and to determine the $ft$ values of the the $\ensuremath{\beta}$ branches, particularly the one to the isobaric-analog state in ${}^{32}$S.Method: Reaction-produced and recoil-spectrometer-separated ${}^{32}$Cl is collected in a tape and transported to a counting location where $\ensuremath{\beta}\ensuremath{-}\ensuremath{\gamma}$ coincidences are measured with a precisely calibrated high-purity germanium detector.Results: The precision on the $\ensuremath{\gamma}$ yields for most of the known $\ensuremath{\beta}$ branches has been improved by about an order of magnitude, and many new transitions have been observed. We have determined ${}^{32}$Cl-decay transition strengths extending up to ${E}_{x}\ensuremath{\sim}11$ MeV. The $ft$ value for the decay to the isobaric-analog state in ${}^{32}$S has been measured. A comparison to a shell-model calculation shows good agreement.Conclusions: We have experimentally determined the isospin-symmetry-breaking correction to the superallowed transition of this decay to be ${({\ensuremath{\delta}}_{C}\ensuremath{-}{\ensuremath{\delta}}_{\mathrm{NS}})}_{\mathrm{exp}}=5.4(9)%$, significantly larger than for any other known superallowed Fermi transition. This correction agrees with a shell-model calculation, which yields ${\ensuremath{\delta}}_{C}\ensuremath{-}{\ensuremath{\delta}}_{\mathrm{NS}}=4.8(5)%$. Our results also provide a way to improve the measured $ft$ values for the $\ensuremath{\beta}$ decay of ${}^{32}$Ar.