On a measurement approach to support evaluation of thermal scattering law data

• Published in: Annals of nuclear energy 2020-01, Vol.135 (C), p.106940, Article 106940
• Main Author: Hawari, Ayman I.
• Format: Article
• Language: English
• Subjects: Cross section; ENDF; Measurement; Neutron; Thermal scattering law
• ISSN: 0306-4549; 1873-2100
• DOI: 10.1016/j.anucene.2019.106940

•Experiments are reviewed for use in the evaluation of the thermal scattering law.•The utility of this approach is illustrated for the evaluation of “nuclear” graphite.•The results showed that “nuclear” graphite can be treated as a porous material.•For TSL evaluation, measurements and experiments should begin at the atomistic level.•This approach can identify deficiencies at each stage of the evaluation process. Inelastic thermal neutron scattering in materials that act as neutron moderators, reflectors, and filters results in shaping the neutron spectrum at low energies. This phenomenon is described using differential scattering cross sections calculated from three components including the bound atom (i.e., nuclear) scattering cross section of the neutron, the ratio of the outgoing and incoming neutron energy, and the thermal scattering law (TSL), i.e., S(α,β), where α and β represent dimensionless momentum and energy exchange variables, respectively. To date, no TSL libraries are generated using measured data. However, valuable information may be derived from measurements and “targeted” experiments that can validate TSL data and the related inelastic scattering cross sections. As a demonstration, a suite of coordinated measurements and experiments is described that was designed and used to support the evaluation of the TSL for “nuclear” graphite. This experimental suite includes neutron powder diffraction (for structure analysis), positron annihilation (for nano porosity assessment), inelastic neutron scattering measurements using a chopper spectrometer, transmission experiments using neutrons with energy below the Bragg cutoff thereby accessing the total (inelastic) cross section, and a slowing-down-time experiment to observe the developing neutron spectrum in the material. This experimental suite was key to understanding the difference in TSL between “nuclear” and “ideal” graphite and for the inclusion of “nuclear” graphite in the ENDF/B-VIII.0 nuclear data library release.