Speaker
Description
In recent years there has been resurgence in the field of thermal scattering law (TSL) evaluations and experiments. This comes after years that this field was dormant with little updates to ENDF evaluations. Evaluated TSL is based on atomistic calculations that in most cases results in a phonon spectrum that can be processed to create the scattering kernel for use in applications. The scattering kernel accounts for the thermal motion of the atoms in the target material (usually a moderator) which results in scattered neutrons both losing and gaining energy. As a result of this process the scattering cross section generated from a TSL evaluation is very different from the free-gas cross section at energies below a few eV, particularly for the elements in a moderator for example C and H in CH2 (polyethylene). Similarly the angular distribution of the scattered neutrons can be calculated from the TSL and thus the neutron slowing down rate in a moderator is function of the TSL. Thermal scattering evaluations will affect the calculated results of criticality benchmarks, but these systems usually involve other materials that contribute to the uncertainty of the calculated multiplication factor, thus experiments that are only sensitive to the TSL should be used for validation. These experiments must have the sensitivity to resolve differences between evaluations when they occur. The experiments discussed include: accurate total thermal cross section measurements, neutron die-away experiments, and neutron leakage experiments. The experimental setups and results will be reviewed for some moderators including polyethylene, and YHx. For the measurements preformed thus far, there is generally good agreement between the measured total cross section and the current TSL evaluation, however in certain energy ranges differences were observed and can help resolve differences between evaluations.