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

• Received 25 October 1993

DOI:https://doi.org/10.1103/PhysRevLett.72.1608