Effect of Particle Sizes on the Efficiency of Fluorinated Nanodiamond Neutron Reflectors

Abstract

Over a decade ago, it was confirmed that detonation nanodiamond (DND) powders reflect very cold neutrons (VCNs) diffusively at any incidence angle and that they reflect cold neutrons quasi-specularly at small incidence angles. In the present publication, we report the results of a study on the effect of particle sizes on the overall efficiency of neutron reflectors made of DNDs. To perform this study, we separated, by centrifugation, the fraction of finer DND nanoparticles (which are referred to as S-DNDs here) from a broad initial size distribution and experimentally and theoretically compared the performance of such a neutron reflector with that from deagglomerated fluorinated DNDs (DF-DNDs). Typical commercially available DNDs with the size of ~4.3 nm are close to the optimum for VCNs with a typical velocity of ~50 m/s, while smaller and larger DNDs are more efficient for faster and slower VCN velocities, respectively. Simulations show that, for a realistic reflector geometry, the replacement of DF-DNDs (a reflector with the best achieved performance) by S-DNDs (with smaller size DNDs) increases the neutron albedo in the velocity range above ~60 m/s. This increase in the albedo results in an increase in the density of faster VCNs in such a reflector cavity of up to ~25% as well as an increase in the upper boundary of the velocities of efficient VCN reflection.

Effect of Nanodiamond Sizes on the Efficiency of the Quasi-Specular Reflection of Cold Neutrons

“…The results for DND and F-DND samples were the same as reported in ref. [ 35 ] and other previous publications.…”

“…Powders of this type were also used in refs. [ 11 , 12 , 14 , 15 , 23 , 27 , 28 , 34 , 35 ]. The sample density is 0.29 g/cm 3 .…”

“…We developed a method for simulating the transport of slow neutrons in ND powders, as described in refs. [ 28 , 34 , 35 ]. NDs are described by an optical potential.…”

“…In addition, DND reflectors are being integrated into standard neutron simulation packages [ 28 , 29 , 30 , 31 ]. The efficiency of reflectors has been improved by removing hydrogen (H) from the DND surface using fluorination [ 32 , 33 ], accompanied by destroying clusters in the DND powder [ 34 ] and, in the case of the diffuse reflection of VCNs, by choosing the optimal DND size [ 35 ].…”

“…In reference to ref. [ 35 ], and on the basis of our model ideas about the interaction of CNs with nanodiamonds (NDs), we assumed that it is possible to optimize the sizes of NDs to increase the efficiency of the QSR of CNs. In particular, an even narrower angular distribution of quasi-specularly reflected CN, as well as an even larger total probability of neutron reflection at relatively long wavelengths, can be obtained by increasing the characteristic size of NDs.…”