Thermal neutron standards

Over the 50-year history of Section III (Neutron Measurements) of the Consultative Committee for Ionizing Radiation of the International Committee for Weights and Measures, only a single interlaboratory comparison of thermal neutron measurements has been completed, although a new comparison of this kind is in progress at the time of this writing. The initial comparison was perhaps considered to have demonstrated a sufficient degree of agreement that its repetition was not of particularly urgent priority. Thermal neutron measurements are indeed an old and well-developed field, as is seen in the many references cited in this paper. This paper describes briefly the production of thermal neutron fields from fast neutron sources, the moderation of neutron spectra towards thermal equilibrium, various conventions for describing thermal neutron fields, corrections for self-absorption and scattering in detector materials and the selection of detectors based on established thermal neutron cross-section standards.