COHERENCE EFFECTS IN NUCLEAR BREMSSTRAHLUNG

At the AGOR cyclotron of the KVI a series of bremsstrahlung measurements has been carried out with proton and α beams. The elementary bremsstrahlung process for real and virtual photons has been studied in the pp system. Coherent bremsstrahlung has been found in the α+p system. Photon angular distributions and energy spectra up to the kinematic limit have been measured in 190 MeV proton reactions with light and heavy targets. Based on predictions for the free pn bremsstrahlung amplitude a strong suppression of bremsstrahlung relative to a quasi-free production model is observed in the low-energy regime of the photon spectrum in p+nucleus reactions. We attribute this effect partly to modifications of the bremsstrahlung amplitude in the nuclear medium and partly to interference of photon amplitudes due to multiple scattering of nucleons.