Distribution of compound-nucleus shapes and its influence on evaporation -art. no. 054614

The tail of the equilibrium distribution of compound-nucleus shapes is show n to extend out to very deformed shapes even for a moderate excitation ener gy of 100 MeV. The standard Hauser-Feshbach formalism is extended to predic t the decay of a deformed compound nucleus as a function of spin, spin proj ection,and excitation energy. The inclusion of the equilibrium distribution s of shapes is found to have little effect on the predicted neutron and pro ton kinetic energy spectra, but for ct particles, the low-energy ''sub-barr ier'' region of the spectrum is enhanced, in agreement with experimental da ta. Langevin simulations, using dissipation given by the wall formula, pred ict that compound nuclei start evaporating before the shape distribution eq uilibrates for excitation energies above similar to 100 MeV.