Fission and neutron emission in the actinides at excitation energies of 10-35 MeV

Abstract

The statistical model description of fission through a double humped barrier has been extended from 12 to \ensuremath{\cong}35 MeV. Calculations have been compared with measured cross sections for $^{237}\mathrm{Np}(^{4}\mathrm{He},xn)$ reactions giving evaporation residues and fission isomers. A generally good fit to the data has been obtained with level densities from theory and fission barriers that give good fits to the direct reaction fission probabilities at low energies (\ensuremath{\le} 12 MeV). In detail there are some significant differences between calculation and experiment. We have explored the sensitivity to the various parameters but have not suggested major redefinitions of level densities, second barriers, and/or isomer masses. The effect of preequilibrium mechanisms has been estimated via the hybrid model of Blann et al. These effects are small for energies at or below the excitation function peaks, but they dominate at high energies. The hybrid model seems to predict too many very high energy neutrons.