Neutron filter efficiency of beryllium and magnesium fluorides

The neutron filter efficiency of MgF₂ and BeF₂ has been investigated as a function of neutron incident energy at different temperatures, starting from the phonon density of states (PDOS) calculated using first-principles techniques, and the results are compared with those of MgO and BeO. Recently, MgF₂ has been suggested as a neutron filter and neutron transmission through it has been experimentally studied. For MgF₂, excellent agreement between calculated and available experimental data has been achieved for the phonon dispersion relations, constant-volume specific heat, inelastic scattering cross sections and neutron transmission. The PDOSs of MgF₂ and BeF₂ are found to differ significantly owing to the crystal structure and the cations' mass difference. The inelastic scattering cross sections and filter efficiencies of MgF₂ and BeF₂ show different behaviours, which can be understood to originate from the above PDOS incongruity and the large difference in absorption cross sections of Be and Mg nuclei. BeF₂ is predicted to be a better neutron filter than MgF₂ and MgO, over the temperature range of interest, while it has less ability than BeO to transmit low-energy neutrons.