Distribution of compound-nucleus shapes and its influence on evaporation

The tail of the equilibrium distribution of compound-nucleus shapes is shown to extend out to very deformed shapes even for a moderate excitation energy of 100 MeV. The standard Hauser-Feshbach formalism is extended to predict the decay of a deformed compound nucleus as a function of spin, spin projection, and excitation energy. The inclusion of the equilibrium distributions of shapes is found to have little effect on the predicted neutron and proton kinetic energy spectra, but for α particles, the low-energy "sub-barrier" region of the spectrum is enhanced, in agreement with experimental data. Langevin simulations, using dissipation given by the wall formula, predict that compound nuclei start evaporating before the shape distribution equilibrates for excitation energies above ∼100 MeV.