High-energy photons in neutron-proton and proton-nucleus collisions

In an effort to better understand the dynamics of nucleon-nucleon bremsstrahlung in producing energetic photons, we have investigated the contribution of each part of the current associated with the different terms in the nuclear Hamiltonian to the neutron-proton bremsstrahlung cross sections. In the range of photon energy investigated here, we find meson-exchange currents to be the dominant source of high-energy photons (ω≳50 MeV). Although the one-body current contribution is small for such energetic photons, its interference with the two-body current is important and enhances the cross section. At low photon energy the convection current dominates. We find that the bremsstrahlung cross section is very sensitive to the ${}_{}{}^3{}_{}{}^{}$(SD${)}_1$ states and offers a potential means of investigating short-range correlation effects. Off-energy-shell effects are estimated to have only a small influence on the neutron-proton bremsstrahlung inclusive cross section. In connection with the neutron-proton bremsstrahlung process, the nucleon-nucleon collisional mechanism in producing high-energy photons in proton-induced reactions has been also investigated within the nuclear matter approximation. Nuclear medium effects in intermediate states are shown to have a minor influence on the photon cross section; therefore, the use of a free nucleon-nucleon T matrix instead of a G matrix seems to be well justified. However, the results are found to be very sensitive to medium effects in the initial and final states.