Parity Mixing of0+and0−Levels inF18

The circular polarization of the $\gamma$ rays emitted in the transition from the 1.081-MeV state to the ground state in ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$ has been measured to be (1.6±5.6)×${10}^{-4\mathrm{}}$, corresponding to a parity-nonconserving (PNC) matrix element $|〈0^{+}, I=1|V^{\mathrm{PNC}}|0^{-}, I=0\mathrm{}〉|=0.03\mathrm{}\pm 0.10$ eV. The weak pion-nucleon coupling constant deduced from the weighted average of all recent ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$ measurements is (${0.28}_{-0.28\mathrm{}}^{+0.89\mathrm{}}$)×${10}^{-7\mathrm{}}$. This result, together with PNC matrix elements in other experiments, suggests that the isovector weak $\mathrm{NN}$ interaction may be strongly suppressed compared with the isoscalar weak $\mathrm{NN}$ interaction.