A non-uniform ''Picket-fence'' model for truncated levels contribution estimation in thermal and resolved resonance energy range for even-even actinides

A non-uniform ''picket-fence''-model to compute the contributions of the truncated levels to elastic scattering, capture and fission cross sections for even-even actinides in thermal and resolved resonance range is proposed. The recurrent algorithm developed, takes into account the exponential energy dependence of the level density and accepte variation of other resonance average parameters as well. BWSL and ''s'' wave resonances are considered. The proposed model gives a better representation of neutron data in thermal and resolved resonance range for END F/B evaluations, as well as the possibility to obtain effective radii really constant. To obtain better results, a number of marginal resonances are recommended to be included into evaluated files, and the present parameters for negative resonances as well as the effective radii are to be reconsidered. The new model has been developed and applied to 15 even-even actinide nuclei and can be considered an improved version of the uniform ''picket-fence'' model previously developed at ORNL for U-238 exclusively. (author)