Multinucleon mechanisms in (γ,N) and (γ,NN) reactions

The similarities in the experimental indications for multinucleon mechanisms in (γ,p) and (e,e’p) processes are pointed out. For both types of reactions, the substantial role of two-nucleon emission processes for transitions to high excitation energies in the residual nucleus is stressed. A microscopic model for the calculation of the two-body knockout contributions to the inclusive (γ,N) reaction is presented. It is based on an unfactorized formalism for the calculation of electromagnetically induced two-nucleon emission cross sections. The model is shown to yield a reasonable description of the overall behavior of the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(γ,p) and ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$(γ,n) data at high excitation energies in the residual nucleus. In the calculations, effects from nonresonant and resonant pion exchange currents are included. Photoabsorption on these currents is predicted to produce the major contributions to the exclusive ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$(γ,${\mathit{n}}_0$ ${)}^{15}$O process at photoenergies above the pion threshold. Double differential cross sections for photon induced pp and pn emission from ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$ are calculated and compared with the data.