Charged-particle evaporation from hot composite nuclei: Evidence over a broadZrange for distortions from cold nuclear profiles

Charged-particle emission has been studied in ten heavy-ion reactions spanning composite systems of 16≤Z≤64. By means of reversed kinematics, high-quality energy spectra were obtained for proton and alpha evaporation from compound nuclei with temperatures ∼1.5–3.5 MeV and spins up to ∼70ħ. In some cases, deuteron and triton spectra were recorded as well. Statistical model calculations for spherical nuclei require systematically low emission barriers to reproduce the data. For the lighter systems, the effective spins (or moments of inertia) must be modified as well, whereas for the heavier systems this effect is of less importance. The roles of multistep particle emission and competition in the model calculations are assessed and found to be inadequate to explain the observed discrepancies. Calculations that model statically deformed nuclear emitters suggest that large deformations may be present, but they do not give a satisfactory overall picture. The very low proton energies in particular seem to indicate an extended density profile. Such an effect, together with the requirements for cluster preformation, may explain the observed low barriers for ${}_{}{}^1{}_{}{}^{}\mathrm{H}$ ${/}^4$He and the increasing mean energies found for the series ${}_{}{}^1{}_{}{}^{}\mathrm{H}$, ${}_{}{}^2{}_{}{}^{}\mathrm{H}$, ${}_{}{}^3{}_{}{}^{}\mathrm{H}$, respectively.