Light-charged-particle emission in /sup 40/Ar-induced reactions: A probe of the very early evolution of the collisions

Charged-particle emission was studied for the reactions 222, 274, and 340 MeV /sup 40/Ar+/sup 116/Sn, /sup 154/Sm, /sup 164/Dy, and /sup 197/Au. Energy and angular distributions were measured for H and He and angular distributions for Li, Be, B, C, N, and O, fissionlike fragments and evaporation residues. For H and He the data are consistent with a low-temperature evaporative component and a high-temperature forward peaked component. For Li through O the forward-peaked emission was studied for 340 MeV /sup 40/Ar only. The evaporation residue cross sections decrease strongly with increasing Z reflecting the increasing fissionability. However, the cross sections for /sup 1/H and /sup 4/He in coincidence with fission are essentially constant with Z. Coincidence measurements, energy spectra, and /sup 4/He//sup 1/H ratios indicate that most H/He evaporation precedes fission. This implies that the energy thermalization is very rapid indeed and that the light particle spectra can probe the very early history of the composite systems.