A rational approximation to Reich–Moore collision matrix of non-fissile nuclides

The cross sections of many important nuclides are represented in Reich±Moore (RM) formalism in the recent American Evaluated Nuclear Data file, ENDF/B-VI. Processing of cross sections with RM resonance parameters is much more dicult than the other multilevel formalisms such as MLBW and Adler±Adler. In this paper, we derive a rational approxima- tion to the RM collision matrix in the vicinity of a resonance. This simplifies the cross section processing. The energy range of the validity of this approximation in the vicinity of a reso- nance is also derived. Choosing Ni58 as an example, results of our approximation for a non- fissile nuclide are given for two typical s-wave resonances. Our rational approximation method is found to work with good accuracies in the vicinity of resonances