Thermal Cross Sections and Scattering

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:

221free gas
222H in H2O
223,224H in polyethylene
225,226H in ZrHn
228D in D2O
229,230C in graphite
235,236Zr in ZrHn

As will be discussed below, the lines with two MT values refer to the inelastic and elastic components of scattering, respectively.


23 January 2013 T-2 Nuclear Information Service