As a test of soft-pion theorems in nuclei, we measured near threshold the ratio of positive-pion photoproduction cross sections on ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$ and on protons. If the total cross section of the reaction $\gamma +p\to {\pi }^{+}+n$ is expressed as ${\sigma }_p=a_p\frac{q}{k}$, our result for the reaction $\gamma +{}_{}{}^6{}_{}{}^{}\mathrm{Li}\to {\pi }^{+}+{}_{}{}^6{}_{}{}^{}\mathrm{He}_{\mathrm{g}.\mathrm{s}.\mathrm{}}^{}{}_{}{}^{}$ is ${\sigma }_{\mathrm{Li}}=(0.073\mathrm{}\pm 0.002)a_p\frac{q}{k}\frac{\frac{12.5}{q}}{\exp \left(\frac{12.5}{q}\right)-1\mathrm{}},$ where $q$ is the pion and $k$ the photon c.m. momenta expressed in units of MeV/c. This result is considerably lower than the existing theoretical predictions.

• Received 21 May 1974

DOI:https://doi.org/10.1103/PhysRevLett.33.316