An improved constant-Q spectrometer for pulsed neutron sources

We describe a constant-Q spectrometer which has been installed recently at the Los Alamos Neutron Scattering Center. Although the instrument is based on the concept proposed originally by Windsor, it incorporates a number of important improvements. Features which facilitate spectrometer alignment include a 90° line-up detector flight path which can be collimated both vertically and horizontally and an analyser crystal which is mounted in a way which allows it to be translated accurately in the scattering plane. The latter feature permits the analyser to be moved to the sample position so that its rocking curve can be measured in the line-up detector. A new multi-element analyser crystal is proposed and tested. It exploits the anisotropic mosaic distribution of plastically deformed germanium single crystals in a new way. As test experiments, we have measured incoherent scattering from ZrH₂ and the scattering by phonons in an aluminium single crystal. The data from the latter experiment are presented as an intensity map over a plane in energy-wavevector space and a number of constant-Q scans have been constructed from this map. The results are in agreement with previous work on aluminium and serve to demonstrate the power of the constant-Q spectrometer as a survey instrument. Both dispersive and non-dispersive phonon branches have been recorded. Their line-widths are in agreement with a calculation of the instrumental resolution which is presented in appendix A. A further appendix presents a prescription for data normalisation.