EVIDENCE FOR STOPPING IN HEAVY-ION COLLISIONS FROM A STUDY OF HARD-PHOTON SOURCE VELOCITIES

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