Pre-equilibriumαEmission in Reactions of 724-MeVKr86with Au: A Coincidence Study of Direct and Evaporation Mechanisms

The production of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ in coincidence with a heavy fragment has been studied with a solid-state telescope and a gas telescope. For heavy fragments detected at 36° and 42°, we have mapped coincident emission of ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ from 80° to 320° in the reaction plane. We find evidence for $\alpha$ evaporation from both fragments and for a significant number of pre-equilibrium ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ particles at c.m. angles near to 90° from the detected heavy fragment with average c.m. energy of ≈ 30 MeV.