Origin and removal of spurious background peaks in vibrational spectra measured by filter-analyzer neutron spectrometers

Inelastic neutron scattering is an invaluable technique for measuring the vibrational spectra of materials. One of the standard methods for analyzing the energies of neutrons scattered by vibrational modes involves the use of polycrystalline beryllium filters. Here, we demonstrate that the spurious background features between 50 and 85 meV accompanying vibrational spectra measured with filter-analyzer neutron spectrometers are due to phonon excitations of the beryllium filter. These features are significantly reduced by an auxiliary polycrystalline bismuth filter placed in front of the main filter. Such a bismuth filter can result in only a minor attenuation in sample scattering intensity concomitant with a reduction in the thermal- and fast-neutron background from the sample.