The total cross sections relative to photoproduction off the proton have been measured for the reactions ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$($\gamma$,${\pi }^{+}$)${}_{}{}^{16}{}_{}{}^{}\mathrm{N}$ and ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$($\gamma$,${\pi }^{+}$)${}_{}{}^9{}_{}{}^{}\mathrm{Li}$ over the energy region from the production threshold up to 12 MeV over threshold. A distorted wave impulse approximation calculation has been performed for the ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$($\gamma$,${\pi }^{+}$)${}_{}{}^{16}{}_{}{}^{}\mathrm{N}$ reaction and is seen to be ≃ 30% higher than the observed cross section from threshold to 10 MeV. The ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$($\gamma$,${\pi }^{+}$)${}_{}{}^9{}_{}{}^{}\mathrm{Li}$ reaction represents the first ($\gamma$,${\pi }^{+}$) measurement near threshold in which the cross section is not dominated by the $\overrightarrow{\sigma }·\overrightarrow{\epsilon }$ term in the production Hamiltonian.

NUCLEAR REACTIONS ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$($\gamma$,${\pi }^{+}$)${}_{}{}^{16}{}_{}{}^{}\mathrm{N}$, ${}_{}{}^9{}_{}{}^{}\mathrm{Be}$($\gamma$,${\pi }^{+}$)${}_{}{}^9{}_{}{}^{}\mathrm{Li}$, bremsstrahlung end point energies to 175 MeV, measured $\sigma \mathrm{}\left(E\right)\mathrm{}$; calculated $\sigma \mathrm{}\left(E\right)\mathrm{}$, DWIA.

• Received 19 June 1979

DOI:https://doi.org/10.1103/PhysRevC.20.1603