Charged particle emission fromHg194compound nuclei: Energy and spin dependence of fission-evaporation competition

Twelve reactions have been studied that produce the compound system ${}_{}{}^{194}{}_{}{}^{}\mathrm{Hg}$* at excitation energies of 57 - 195 MeV and with $l_{\mathrm{crit}}$ values of $25-142\hslash$. Beams of ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$, ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$, ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}$, and ${}_{}{}^{40}{}_{}{}^{}\mathrm{Ar}$ ions in conjunction with appropriate targets have been used to measure cross sections for evaporative H/He, fission, and evaporation residues. These results confirm that most ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ and ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ is evaporated prior to fission or instead of fission and very little if any from the fission fragments. The probability of H/He evaporation increases dramatically with excitation energy. The evaporation residue cross sections ($\frac{{\sigma }_{\mathrm{ER}}}{\pi {\lambda }^2}$) indicate fission survival for entrance channel $l$ up to $27-39\hslash$. Fission survival becomes stronger and corresponding fission competition becomes weaker for excitation ≥ 100 MeV; a connection with charged particle emission is suggested. The dimensionless cross section for evaporation residue ($\frac{{\sigma }_{\mathrm{ER}}}{\pi {\lambda }^2}$) depends on both the entrance channel and on energy, indicating that nonequilibrium mechanisms must play an important role, even for $l\lesssim 40$. Heretofore evaporation residue production has been usually thought to arise from lower partial waves while direct reactions have been thought to dominate only for the higher partial waves.