Bremsstrahlung in the nuclear medium: search for coherence phenomena

Photon angular distributions and energy spectra up to the kinematical limit have been measured in proton reactions with light and heavy targets. For the first time the dependence of photon spectra on scattered protons has been measured to investigate the influence of multiple-scattering processes on the photon production. A strong suppression of bremsstrahlung relative to a quasi-free production model using the free pn bremsstrahlung cross section is observed in the low-energy regime of the photon spectrum in p + nucleus reactions. We attribute this effect to modifications of the bremsstrahlung amplitude in the nuclear medium and to interference of photon amplitudes due to multiple scattering of nucleons. (author)