More than 15 years ago, the study of nanodiamond (ND) powders as a material for designing reflectors of very cold neutrons (VCNs) and cold neutrons (CNs) began. Such reflectors can significantly increase the efficiency of using such neutrons and expand the scope of their application for solving applied and fundamental problems. This review considers the principle of operation of VCN and CN reflectors based on ND powders and their advantages. Information is presented on the performed experimental and theoretical studies of the effect of the size, structure, and composition of NDs on the efficiency of reflectors. Methods of chemical and mechanical treatments of powders in order to modify their chemical composition and structure are discussed. The aim is to avoid, or at least to decrease, the neutron inelastic scatterers and absorbers (mainly hydrogen atoms but also metallic impurities and nitrogen) as well as to enhance coherent elastic scattering (to destroy ND clusters and sp2 carbon shells on the ND surface that result from the preparation of NDs). Issues requiring further study are identified. They include deeper purification of NDs from impurities that can be activated in high radiation fluxes, the stability of NDs in high radiation fluxes, and upscaling methods for producing larger quantities of ND powders. Possible ways of solving these problems are proposed.
Keywords: cold neutrons; fluorination; nanodiamonds; neutron reflectors; very cold neutrons.