Photonuclear Reactions up to Pion Threshold

Dr. Tolio Rodrigues
Institute of Physics
University of Sau Paulo

Photonuclear reactions at intermediate energies provide a convenient tool for the understanding of the dynamics of an excited nuclear ensemble.  In this colloquium, I will present a sophisticated monte Carlo model suitable to describe low and intermediate energy nuclear reactions.  The basic idea of the model is the interpretation of the reaction mechanism in terms of a two-step process.  The first stage incorporates the initial photoabsorption mechanism followed by a time dependent intranuclear cascade framework to account for the direct and pre-equilibrium emissions.  The second step addresses the compound nucleus decay taking into account the competition between paricle evaporation and nuclear fission.  The quasideuteron channel is included using the Levinger model to sort neutron-proton pairs embedded in the nuclei plus a non-stochastic Pauli-blocking mechanism to account for the short range correlations.  Results for photo and electro induced reactions within a wide target mass range are presented and compared with the available experimental data.  A detailed analysis of the electrodisintegration yields of protons on 90 Zr indicates a strong evidence of a collective isovector giant quadrupole resonance with a strength parameter compatible with the E2 energy-weighted sum rule.  The calculation provides a consistent interpretation for the exotic steady-increase behavior of the (e, p) yield in terms of a multiple proton emission scenario.  The available data of the inclusive photoneutron cross sections for  90 Zr  and  nat Zr from Livermore and Saclay are reproduced with reasonable accuracy up to 140 MeV.