A MICROSCOPIC CALCULATION OF PHOTOABSORPTION CROSS-SECTIONS ON PROTONS AND NUCLEI

A recently developed model for p-meson propagation in dense hadronic m atter is applied to total photoabsorption cross sections in gamma-prot on and gamma-nucleus reactions. Within the vector dominance model the photon coupling to the virtual pion cloud of the nucleon, two-body mes on-exchange currents, as well as gamma-nucleon resonances are included . Whereas the gamma p reaction is determined by the low-density limit of the model, higher orders in the nuclear density are important to co rrectly account for the experimental spectra observed on both light an d heavy nuclei over a wide range of photon energies, including the reg ion below the pion threshold. in connection with soft dilepton spectra in high-energy heavy-ion collisions we emphasize the importance of ph otoabsorption to further constrain the parameters of the model. (C) 19 98 Elsevier Science B.V.