The design of a bismuth-based auxiliary filter for the removal of spurious background scattering associated with filter-analyzer neutron spectrometers

The ultimate sensitivity of filter-analyzer-type neutron spectrometers, which are invaluable for measuring the vibrational spectra of materials, is typically limited by spurious background scattering features ubiquitous in all measured spectra. These undesirable features arise from neutrons that are elastically scattered by the sample and then are inelastically scattered by phonon excitations in the beryllium filter material. Such features can be significantly reduced by adding an auxiliary polycrystalline bismuth filter in front of the main filter analyzer. An optimal Bi filter requires a high-purity, low-hydrogen-content material with a sufficiently small crystallite grain size distribution to ensure a sharp Bragg cutoff in the energy dependence of the neutron total cross-section. We were able to produce such a material by the room-temperature compression of dense, high-purity, finely polycrystalline, Bi needles (with an average crystallite grain size of ≈230 μm) synthesized by the rapid water-quenching of melted Bi dripped into a spinning centrifuge. This material resulted in an improved filter performance compared to that using more coarsely polycrystalline Bi synthesized via melt-casting.