Coherence effects in nuclear bremsstrahlung

The production of nuclear bremsstrahlung (Egamma > 30 MeV) has been studied in heavy-ion collisions, as well as proton and alpha-particle collisions with nuclei. In heavy-ion reactions the measured photon spectra show an exponential shape dominated by the incoherent sum of photons produced in first-chance collisions. Photon spectra, angular distributions and multiplicities at 60A MeV indicate that a significant fraction of photons is emitted in secondary nucleon-nucleon collisions from a thermally equilibrated system. In 200 MeV alpha + p collisions the incoherent contribution to the photon spectrum is observed as well at low photon energies, while coherent bremsstrahlung is observed at the highest photon energies from radiative capture into unbound states of ILL In 190 MeV proton reactions with light and heavy targets photon spectra have been measured up to the kinematic limit. At high photon energies the spectra show the expected behaviour from first-chance collisions. Below ca 80 MeV a significant suppression of the photon yield is observed. We attribute this effect to the interference of photon amplitudes due to multiple scattering of nucleons in the nuclear medium