Threshold region photoproduction of neutralπmesons off complex nuclei

Photoproduction of ${\pi }^0$ mesons off targets of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}$, ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{28}{}_{}{}^{}\mathrm{Si}$, ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ca}$, natural Cd, and natural Pb was studied using a bremsstrahlung beam with endpoint energies of 140, 145, 150, and 155 MeV. Photoproduction from a liquid hydrogen target was employed as a normalization. The measured yields were found to be in disagreement with published theoretical cross sections for ${}_{}{}^6{}_{}{}^{}\mathrm{Li}(\gamma , {\pi }^0){}_{}{}^6{}_{}{}^{}\mathrm{Li}$ and also in disagreement with a simple schematic model which assumed only coherent contributions from the $M_{1+}$ multipole. The schematic model, however, did approximately predict the relative magnitudes of the yield curves for the energy range 14-20 MeV over threshold.