Abstract
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° ≤ 2θ ≤ 168°, with only minimal gaps between each bank. These detectors have a substantially increased solid angle coverage (Ω ∼ 5.67 sr) compared to the previous instrument (Ω ∼ 0.82 sr), resulting in increases in count rate of between 2× and 10×, depending on 2θ 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.
Highlights
The neutron diffraction technique is well established as an essential tool for the characterization of both crystalline and disordered materials, especially in those areas which focus on the intrinsic strengths of the neutron
The instrumental resolution has been improved with Δd/d decreasing from 0.45% to 0.31% at backscattering angles; and instrumental backgrounds have been minimized through an innovative advanced sample tank and radial collimator design
The upgrade to the Polaris powder diffractometer at ISIS has had a major impact in the area of “routine” structural studies, allowing more rapid characterization of a diverse range of materials as a function of, e.g., temperature or gas environment, and during chemical reactions using Rietveld refinement methods
Summary
The neutron diffraction technique is well established as an essential tool for the characterization of both crystalline and disordered materials, especially in those areas which focus on the intrinsic strengths of the neutron (its use as a bulk probe, sensitivity to magnetic order, sensitivity to light elements, Q-independent form factor, etc.). Measurements performed on Polaris demonstrated the importance of epithermal neutrons to collect data at high scattering vectors Q (or, equivalently, low d-spacings) and highlighted how the fixed scattering geometry at a pulsed neutron source could be used to develop specialized sample environment (such as the Paris-Edinburgh pressure cell7) and to measure residual stress distributions within engineering components.8 These advances inevitably led to a rapid increase in the breadth of the scientific programme and a high over-demand for beamtime on Polaris, culminating in the construction of new instruments dedicated to diffraction at high pressures (PEARL) and engineering applications (ENGIN, itself later upgraded to ENGINX).
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