Study of bound hydrogen in powders of diamond nanoparticles

In order to access feasibility of increasing albedo of very cold neutrons from powder of diamond nanoparticles, we studied hydrogen bound to surface of diamond nanoparticles, which causes unwanted losses of neutrons. We showed that one could decrease a fraction of hydrogen atoms from a ratio C7.4 ± 0.15H to a ratio C12.4 ± 0.2H by means of thermal treatment and outgasing of powder. Measurements of atomic excitation spectra of these samples, using a method of inelastic incoherent neutron scattering, indicate that residual hydrogen is chemically bound to carbon, while a removed fraction was composed of adsorbed water. The total cross section of scattering of neutrons with a wavelength of 4.4 Å on residual hydrogen atoms equals 108 ± 2 b; it weakly changes with temperature. Thus preliminary cleaning of powder from hydrogen and its moderate cooling do not improve considerably neutron albedo from powder of nano-diamonds. An alternative approach is isotopic replacement of hydrogen by deuterium.