Unresolved Resonance Self Shielding

At higher energies in heavier nuclei, the resonances get so close together that they cannot be given separately. Instead of giving individual resonances with their energies and characteristic widths, ENDF-format evaluations give average values for the resonance spacing and the various characteristic widths, together with the probability distributions needed to describe the quantities.

In this unresolved range, it is no longer possible to compute simple cross section vs energy tables; instead, you can calculate the effective cross section in the region of a given energy, which depends on the environment through self-shielding effects, or you can calculate probability distributions for the total cross section and the related elastic, fission, and capture distributions. The self-shielded cross sections are computed by UNRESR, and the probability tables are computed by PURR.

The effective self-shielded cross sections from UNRESR are normally used by the GROUPR module for generating self-shielded multigroup constants. The probability tables from PURR are usually processed by the ACER module and made available to the MCNP Monte Carlo code.


23 January 2013 T-2 Nuclear Information Service ryxm@lanl.gov