Threshold region photoproduction of neutral π mesons off complex nuclei

Photoproduction of π⁰ mesons off targets of ⁶Li, ¹²C, ²⁸Si, ⁴⁰Ca, 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 ⁶Li(γ, π⁰)⁶Li and also in disagreement with a simple schematic model which assumed only coherent contributions from the M₁₊ multipole. The schematic model, however, did approximately predict the relative magnitudes of the yield curves for the energy range 14-20 MeV over threshold. NUCLEAR REACTIONS ⁶Li, ¹²C, ²⁸Si, ⁴⁰Ca, Cd, Pb, (γ, π⁰) E_γ=140-155 MeV; measured σ test of reaction model.