Lineshape analysis and filter difference method for a high intensity time-of-flight inelastic neutron scattering spectrometer

The use of polycrystalline filter analysers in inelastic neutron scattering allows relatively high count rates to be achieved. Its application to time-of-flight (TOF) techniques is important because the new pulsed neutron sources are expected to provide higher neutron fluxes than currently available. We present therefore a detailed lineshape analysis for inelastic spectra taken by a TOF filter analyser. By means of a convolution of the instrinsic line shape for the vibrational mode with that for instrumental parameters and the filter transmission function, the asymmetry caused by the latter can be modelled out by a fitting procedure. Most of the drawbacks of this approach are eliminated by taking a difference between two filter analysers with different cutoffs, which results in symmetrized, narrow experimental lineshapes. The analysis is extended to this case and the design along with some experimental results of a successfully operating prototype filter difference spectrometer at the Los Alamos pulsed spallation neutron source (WNR/PSR Facility) are presented.