Threshold region photoproduction of neutral π mesons off complex nuclei

Photoproduction of ..pi../sup 0/ mesons off targets of /sup 6/Li, /sup 12/C, /sup 28/Si, /sup 40/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 /sup 6/Li(..gamma..,..pi../sup 0/)/sup 6/Li and also in disagreement with a simple schematic model which assumed only coherent contributions from the M/sub 1//sub +/ multipole. The schematic model, however, did approximately predict the relative magnitudes of the yield curves for the energy range 14--20 MeV over threshold.