Dominance of nucleon-nucleon bremsstrahlung in mass-asymmetric heavy-ion collisions at 15 MeV/nucleon

Photons from the reaction {sup 16}O+{sup nat}W have been measured using the Darmstadt-Heidelberg crystal ball. The source velocity for photons above 25 MeV deduced from the angular distribution identifies the nucleon-nucleon system as the emitting system consistent with proton-neutron bremsstrahlung. This observation is supported by a comparison of the experimental angle-integrated spectra with statistical model calculations, which suggest that photons below 20 MeV arise from thermal emission, whereas hard photons above 25 MeV cannot be explained by this mechanism.