Prediction of fission mass yield distributions based on cross-section evaluations

The statistical model for fission cross-section evaluations has been extended by including the concept of multi-modality of the fission process. The three most dominant fission modes, i.e. the two asymmetric standard I (S1) and standard II (S2) modes and the symmetric super long mode, are taken into account. Based on calculations of the nuclear surface within the multi-modal random neck rupture model, separate outer fission barriers are considered for each mode, while the inner barriers and isomeric wells are assumed to be the same. Deconvoluted fission cross-sections for the S1, S2 and super long modes of 235,238U(n,f) and 237Np(n,f), based on experimental branching ratios, were calculated for the first time in the incident neutron energy range from 0.01 to 5.5 MeV, providing good agreement with the experimental fission cross-section data. In this way, branching ratios can also be deduced for incident neutron energies when no experimental data exist, and used for the prediction of the corresponding fragment mass yield distributions. (author)