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.