Light-charged-particle emission inAr40-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 ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$+${}_{}{}^{116}{}_{}{}^{}\mathrm{Sn}$, ${}_{}{}^{154}{}_{}{}^{}\mathrm{Sm}$, ${}_{}{}^{164}{}_{}{}^{}\mathrm{Dy}$, and ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$. 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 lowtemperature evaporative component and a high-temperature forward peaked component. For Li through O the forward-peaked emission was studied for 340 MeV ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$ only. The evaporation residue cross sections decrease strongly with increasing $Z$ reflecting the increasing fissionability. However, the cross sections for ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ in coincidence with fission are essentially constant with $Z$. Coincidence measurements, energy spectra, and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$/${}_{}{}^1{}_{}{}^{}\mathrm{H}$ 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.