Effects of the final-state interaction in (γ, pn) and (γ, pp) processes

A model is presented to describe electromagnetically induced two-nucleon emission processes in a shell-model picture. Distortions in the outgoing nucleon waves are accounted for by performing a partial-wave expansion in a real mean-field potential. The antisymmetry condition for the A-body wave functions is shown to be naturally preserved. The model is used to calculate (γ, pn) and (γ, pp) cross sections off the target nuclei ¹⁶O and ¹²C for photon energies ranging from 50 MeV up to the Δ(1232) isobar threshold. Effects due to the pionic currents and intermediate Δ creation are implemented. The impact of the distortions due to the interaction of the outgoing nucleon waves with the A — 2 core is examined. Hadronic form factors are introduced to regularize the πNN vertices and the sensitivity of the cross section to the pion cutoff mass is examined. The relative contribution of the (γ, pp) and the (γ, pn) channel to the total photoabsorption strength is discussed. Further, the photon energy dependence of the (γ, pp)/(γ, pn) ratio is investigated.