A new measurement of the circular polarization $P_{\gamma }$ of the 1081-keV $\gamma$ line in ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$ has been performed. The result is $P_{\gamma }\left(1081\mathrm{} \mathrm{keV}\right)=(2.7\mathrm{}\pm 5.7)\times {10}^{-4\mathrm{}}$. This result, when averaged with those of recent measurements in ${}_{}{}^{18}{}_{}{}^{}\mathrm{F}$, sets an upper constraint for the absolute value of the coupling constant $f_{\pi }$ of the weak $\pi \mathrm{NN}$ interaction of $\left|f_{\pi }\right|<~1.5\times {10}^{-7\mathrm{}}$.

• Received 25 July 1985

DOI:https://doi.org/10.1103/PhysRevLett.55.795