$0^{+}\to 0^{-}{\beta }^{+}$ Decay of ${}_{}{}^{18}{}_{}{}^{}\mathrm{Ne}$ and the Determination of $F_{\pi }$

The ${\beta }^{+}$ branch of ${}_{}{}^{18}{}_{}{}^{}\mathrm{Ne}$ to the 1081-keV $J^{\pi }=0^{-}$, $I=0\mathrm{}$ level of ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$ is observed to have an intensity of (1.71 ± 0.41)×${10}^{-4\mathrm{}}$ times that of the superallowed Fermi transition to the 1042-keV $J^{\pi }=0^{+}$, $I=1\mathrm{}$ level. This result plus the previously measured upper limit on parity mixing in the $0^{+}$, $0^{-}$ doublet of ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$ yields an upper limit of ± (0.5 ± 1.3) × ${10}^{-6\mathrm{}}$ for $F_{\pi }$. This limit is largely independent of details of nuclear structure.