The importance of $n$-$p$ radiative capture, utilizing polarized cold neutrons, as a means of measuring the weak pion coupling constant is reviewed. Parity conserving processes of the form ${\mathbf{k}}_{\gamma }\cdot ({\mathbf{s}}_n\times {\mathbf{k}}_n)$ can contribute to the ${\mathbf{s}}_n\cdot {\mathbf{k}}_{\gamma }$ photon asymmetry in any such experiment, if the apparatus is not perfectly symmetric. For an incident laboratory neutron energy of 0.003 eV a value of $A_{\gamma }^{\mathrm{PC}}=0.67\mathrm{}\times {10}^{-8}$ is obtained for two different potential models (Argonne AV14 and Nijmegen Reid93). Serving as an extreme test case, the Reid soft core potential yields $0.61\times {10}^{-8}$, close to the result of the contemporary forces. Implications for extracting the weak pion coupling constant and for monitoring the beam polarization are discussed.

• Received 29 April 1997

DOI:https://doi.org/10.1103/PhysRevC.56.631