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

Abstract

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 wavefunctions is shown to be naturally preserved. The model is used to calculate ($γ$,pn) and ($γ$,pp) cross sections off the target nuclei $^{16}$O and $^{12}$C for photon energies ranging from 50 MeV up to the $\bigtriangleup$(1232) isobar threshold. Effects due to the pionic currents and intermediate $\bigtriangleup$ 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 $π$NN vertices and the sensitivity of the cross section to the pion cut-off 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.