Evidence for stopping in heavy-ion collisions from a study of hard-photon source velocities

We present data on hard-photon production in [sup 36]Ar+[sup 197]Au and [sup 36]Ar+[sup 12]C collisions at 95 MeV/nucleon. The photon source velocity [beta][sub [ital S]] is investigated as a function of system size, photon energy, and impact parameter. In the heavy system, [beta][sub [ital S]] is found to be sizably smaller than [beta][sub beam]/2, except for very peripheral reactions. This unambiguously demonstrates that at intermediate bombarding energies photon emission is not solely dominated by first-chance [ital p]-[ital n] collisions, but carries also information on the later, stopping phase of the reaction.