Hard protons from heavy-ion collisions

Abstract

The production of high energy γ-rays in intermediate energy nucleus-nucleus collisions is studied microscopically within two different approaches. First we calculate energetic photons as emerging from collective bremsstrahlung within TDHF as well as Extended TDHF. It is found, however, that the double differential spectra disagree with experimental data, both in magnitude and angular distribution. In a second step we assume high energy γ-rays to be produced by incoherent nucleon-nucleon collisions. For this purpose we evaluate covariantly the photon production from proton-neutron collisions in a vector (ω) and scalar meson (σ) exchange model with coupling constants given by the M2Y G-matrix in the nonrelativistic limit. We furthermore follow the proton-neutron collisional history by means of a phase-space simulation based on the Vlasov-Uehling-Uhlenbeck approach for proton-nucleus and nucleus-nucleus collisions adding up incoherently the yields from each individual collision. The satisfactory agreement we obtain in comparison with experimental data allows to conclude that energetic photons dominantly arise from proton-neutron bremsstrahlung during the early stage of the collision. Similar statements are found to hold also for subthreshold pion production by independent nucleon-nucleon collisions.