Multinucleon Mechanisms in ($γ$,N) and ($γ$,NN) Reactions

Abstract

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 behaviour of the $^{12}$C$(γ,p)$ and $^{12}$C$(γ,n)$ data at high excitation energies in the residual nucleus. In the calculations effects from non-resonant and resonant pion exchange currents are included. Photoabsorption on these currents are predicted to produce the major contributions to the exclusive $^{16}$O$(γ,n_0)^{15}$O process at photonenergies above the pion threshold. Double differential cross sections for photon induced $pp$ and $pn$ emission from $^{16}$O are calculated and compared with the data.