We report a measurement of the β asymmetry of the first-forbidden transition from the 1/$2^{+}$ ground state of ${}_{}{}^{19}{}_{}{}^{}\mathrm{Ne}$ to the 1/$2^{\mathrm{-}}$ level at 110 keV in ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$. Using atomic beam methods to prepare a 100% polarized source of ${}_{}{}^{19}{}_{}{}^{}\mathrm{Ne}$, we identify the 1/$2^{+}$→1/$2^{\mathrm{-}}$ decays by the detection of the subsequent 110 keV gamma emitted in coincidence with a beta. The average β asymmetry of the first-forbidden transition is measured to be 〈A〉=17±12 %. We simultaneously make an improved measurement of the branching ratio of the same transition and find Γ=(1.13±0.09)×${10}^{\mathrm{-}4}$ in good agreement with previous results. Our measurement is used to obtain an improved calibration of the isovector parity-mixing matrix element between the 1/$2^{+}$ ground state of ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$ and the 1/$2^{\mathrm{-}}$ level at 110 keV.

• Received 9 July 1993

DOI:https://doi.org/10.1103/PhysRevC.48.3069