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At higher energies in the heavier isotopes, the resonances get so
close together that they can no longer be resolved into separate
lines. However, the fluctuations in the cross section in this
energy range do lead to important effects, such as self shielding,
in some applications (for example, fast breeder reactors). In
ENDF-format evaluations, this "unresolved range" is handled by
giving average values for the resonance spacing and the various
partial widths, together with probability distributions for the
spacing and partial widths. These unresolved resonance
parameters are used three ways in practice:
- Infinitely-dilute cross sections: the cross sections
that would be measured for a thin sample (which are equivalent
to the cross sections that would act in a very dilute mixture)
can be calculated using direct integrals over the probability
distributions. These calculations are made with codes like
RECENT and the RECONR module of NJOY.
- Self-shielded effective cross sections: effective
cross sections for thicker targets or less dilute mixtures
show self-shielding effects that can be computed vs
temperature and background cross section. These calculations
are made with the UNRESR module of NJOY.
- Probability tables: probability tables for the
total cross section and the dependent elastic, fission, and
capture cross sections can be used to sample for cross sections
in continueous-energy Monte Carlo codes like MCNP. The
probability table can be generated using the PURR module of NJOY.
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