Relative measurements of ${\pi }^0$ photoproduction yields have been performed on hydrogen, deuterium, ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$, in the region of 1 to 10 MeV above threshold. A simplified distorted-wave impulse approximation model of the four reactions is described; it leads to an overall understanding of the results. Large rescattering effects are brought to evidence in deuterium and ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, making the extraction of precise values for the dipole photoproduction amplitudes $E_{0^{+}}$ on nucleons strongly dependent on the theoretical description of the processes.

NUCLEAR REACTIONS ($\gamma$,${\pi }^0$), ${}_{}{}^1{}_{}{}^{}\mathrm{H}$, ${}_{}{}^2{}_{}{}^{}\mathrm{H}$, ${}_{}{}^3{}_{}{}^{}\mathrm{He}$, and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ targets; measured reaction yields, $E_{{\pi }^0}=1-10\mathrm{}$ MeV.

• Received 9 October 1979

DOI:https://doi.org/10.1103/PhysRevC.21.1416