
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 ENDFformat 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 "infinitelydilute" 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 selfshielded 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.
