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At thermal energies, e.g., up to about 0.5 eV for temperatures
around room temperature and maybe up to as high as 4 eV for hotter
materials, the energy transferred by the scattering of a neutron is
similar to the kinetic energies of motion of the atoms in liquids and
to the energies of excitations in molecules and crystalline lattices.
Therefore, you cannot picture the target atoms as being initially
stationary and recoiling freely as is normally done for higher neutron
energies. The motion of the target atoms and their binding in liquids
and solids affects both cross sections and the distribution in energy
and angle of the scattered neutrons. The THERMR module of NJOY is
used to compute these effects and add them to a PENDF tape for use
by other modules.
For free-gas scattering, where only the thermal motion of the targets
is taken into account, not internal modes of excitation, THERMR can
generate the cross sections and scattering distributions using
analytic formulas. For real bound scattering, it uses an input
scattering function and other parameters from an ENDF-format
thermal evaluation in File 7 format. A number of such evaluations
for common moderator materials have been available for years in
various ENDF-format libraries, and new ones have been produced
recently using the LEAPR module of NJOY. The results of THERMR's
work are stored into the new PENDF tape in Files 3 and 6 using a
special set of MT numbers:
| MT | Moderator |
| 221 | free gas |
| 222 | H in H2O |
| 223,224 | H in polyethylene |
| 225,226 | H in ZrHn |
| 227 | benzine |
| 228 | D in D2O |
| 229,230 | C in graphite |
| 231,232 | Be |
| 233,234 | BeO |
| 235,236 | Zr in ZrHn |
As will be discussed below, the lines with two MT values refer
to the inelastic and elastic components of scattering, respectively.
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