Performance of the new pyrolytic graphite analyser bank on the near-backscattering spectrometer IRIS on the ISIS pulsed source

The pyrolytic graphite (PG) analyser bank on the IRIS high-resolution quasi-elastic/low-energy inelastic neutron scattering spectrometer at ISIS has been upgraded. While the original analyser utilised a 6-row by 225-column array of graphite crystals cooled to 30 K, the new design is comprised of 4212 crystal pieces (18 rows by 234 columns). In addition, the graphite is now cooled close to liquid-helium temperature to further improve the sensitivity of the spectrometer. In this paper, the performance of the newly upgraded instrument is determined by comparing resolution measurements collected from the complete IRIS spectrometer before and after modification.