Energy equilibration in composite nuclei at high energy and spin: Correlations between evaporativeH1,He4, and fission

Correlations have been studied between evaporative H/He emission and fissionlike products for the reactions 333 MeV ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$ + ${}_{}{}^{116}{}_{}{}^{}\mathrm{Sn}$, ${}_{}{}^{154}{}_{}{}^{}\mathrm{Sm}$, and ${}_{}{}^{197}{}_{}{}^{}\mathrm{Au}$. Fission products were detected at 60° and 320° in coincidence with H and He at 120° in plane and 27° and 60° out of plane. The coincidence energy spectra of H and He are not consistent with evaporation from the moving fragments. An analysis has been made with the assumption of complete energy equilibration in the particle emission (i.e., evaporation) prior to scission. The rather large data set is essentially consistent with this assumption. From the spectral shapes one can deduce level density parameters and barriers to evaporation. Values inferred for the Fermi gas level density parameters are similar to those inferred from experiments at much lower energies. Values deduced for the emission barriers are much smaller than those from systematics of fusion barriers. The observed cross sections for fission and ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ evaporation are not easily reconciled with statistical model calculations. In particular, the proton evaporation seems to compete more favorably with fission and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ evaporation than implied by phase space considerations alone.