Model for photonuclear reactions at intermediate energies

In the present paper we discuss photonuclear reactions for photon energies $E_{\gamma }>50\mathrm{}$ MeV. We show in detail how the different processes concerning the nucleon-nucleon interactions enter the transition matrix. Explanations are given for the similar behavior of ($\gamma , n$) and ($\gamma , p$) and for a strong relative suppression of the reactions ($\gamma , \mathrm{pp}$) and ($\gamma , \mathrm{nn}$) compared to ($\gamma , \mathrm{np}$). This solves the standing problems of photonuclear reactions at intermediate photon energies. We present numerical calculations for the reactions ($\gamma , p$) and ($\gamma , n$) on ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ from 50 to 140 MeV photon energy and show in detail how the different physical processes numerically enter the total cross sections and angular distributions. Over-all good agreement with the experiments is achieved.