The upgraded Polaris powder diffractometer at the ISIS neutron source

This paper describes the design and operation of the Polaris time-of-flight powder neutron diffractometer at the ISIS pulsed spallation neutron source, Rutherford Appleton Laboratory, UK. Following a major upgrade to the diffractometer in 2010-2011, its detector provision now comprises five large ZnS scintillator-based banks, covering an angular range of 6 degrees <= 2 theta <= 168 degrees, with only minimal gaps between each bank. These detectors have a substantially increased solid angle coverage (Omega similar to 5.67 sr) compared to the previous instrument (Omega similar to 0.82 sr), resulting in increases in count rate of between 2x and 10x, depending on 2 theta angle. The benefits arising from the high count rates achieved are illustrated using selected examples of experiments studying small sample volumes and performing rapid, time-resolved investigations. In addition, the enhanced capabilities of the diffractometer in the areas of in situ studies (which are facilitated by the installation of a novel design of radial collimator around the sample position and by a complementary programme of advanced sample environment developments) and in total scattering studies (to probe the nature of short-range atomic correlations within disordered crystalline solids) are demonstrated. (C) 2019 Author(s).