Threshold π0 photoproduction from complex nuclear targets

The photoproduction of neutral pions near threshold from complex nuclear targets in investigated. Momentum dependent terms in the photoproduction operator and a two-nucleon charge exchange production mechanism are included. Various selection rules related to the quantum numbers of the nuclear states involved are discussed. Differential cross sections for pion energies 7.5 MeV from $^{4}\mathrm{He}$ and $^{6}\mathrm{Li}$ are calculated using realistic shell model wave functions. The effects of distortion of the pion wave function are found to be small, but the two-nucleon charge exchange mechanism is found to dominate the coherent production from $J\ensuremath{\ne}0$ targets near threshold.[NUCLEAR REACTIONS ($\ensuremath{\gamma}$, ${\ensuremath{\pi}}^{0}$) near threshold; realistic photoproduction operator; shell model wave functions; calculated differential cross section on $^{4}\mathrm{He}$ and $^{6}\mathrm{Li}$.]