Unresolved Range

For the heavier nuclei, there is a region above the resolved range where the resonances get so close together that they can no longer be separated. However, there can still be important resonance effects, such as self shielding, active in this "unresolved" range. In ENDF-format evaluations, the unresolved range is represented by giving average resonance widths and spacings and the statistical rules obeyed by these widths and spacings. RECONR converts these statistical representations into actual "infinitely-dilute" cross sections. These are the values appropriate for thin samples or dilute mixtures where resonances don't interfere with each other due to their effects on the neutron flux. The effects of each resonance simply add together in computing the net cross section, and the cross section can be computed by doing weighted averages using the specified probability distributions.

There are some complicated rules used for determining the energy grid in the unresolved range. Modern evaluations normally give the parameters as functions of energy, and the rule is to first compute the cross sections at these energy points, and then to interpolate between the cross sections to get intermediate values. However, some of the older evaluations contain rather large jumps in energy in between the grid values given, e.g., decade steps. The evaluators did not expect that their work would be linearly interpolated across such large steps, because earlier ENDF rules did not require that. Therefore, RECONR watches for large steps, and it automatically subdivides them using points from a standard grid with roughly logarithmic steps. This gives a better approximation of the roughly 1/v shapes expected. Some evaluations for minor isotopes that were made without benefit of detailed experimental data just assume that the average resonance parameters are constant across the unresolved range. RECONR automatically subdivides the range using its standard grid, thus yielding a cross section with a reasonable shape resulting from the natural energy dependences in the resonance formulas.

RECONR includes the unresolved contributions in the normal elastic, fission, capture, and total tabulations in File 3 of the PENDF tape. However, it also generates a new section of File 2 with the special value MT=152 that also contains the unresolved cross sections. This section will be overwritten with self-shielded cross sections by the UNRESR module, if it is run. Otherwise, the MT=152 section from RECONR will be available for GROUPR to use while generating multigroup constants.


23 January 2013 T-2 Nuclear Information Service