Diamond powders as neutron filters

Low-energy band-pass filters are essential components for many neutron-scattering measurements, particularly cold-neutron inelastic scattering. There has been widespread use of polycrystalline Be filters for typical cutoff energies near 5 meV. The need to cool the filters to minimize thermal diffuse scattering is inconvenient for many experiments and for this reason we have investigated alternatives. Polycrystalline diamond appears to be an excellent candidate, with the large Debye temperature reducing the need for cooling, an abundant supply of inexpensive material, and a large scattering length density. Using the ORELA pulsed neutron source and the HFIR at ORNL, we have characterized the energy dependence of the room temperature neutron transmission for several commercially available powders of both natural and artificial diamond with homogeneous particle sizes ranging from single digits to hundreds of microns. Sharp cutoffs are observed near neutron energies of 5 meV. The low-energy transmission is reduced by small-angle scattering from voids, and we have begun to investigate methods of overcoming this limitation. Nevertheless, for some applications room temperature diamond powders are viable neutron filters.