Neutron Ray-Tracing Simulations of a New Supermirror Guide for the Osiris Spectrometer
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eutron Ray-Tracing Simulations of a New Supermirror Guide for the Osiris Spectrometer A. Perrichona, b, *, F. Fernandez-Alonsob, c, M. Wolffa, M. Karlssond, and F. Demmelb aDepartment
of Physics and Astronomy, Uppsala University, Uppsala, 752 37 Sweden Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX United Kingdom c Department of Physics and Astronomy, University College London, London, WC1E 6BT United Kingdom dDepartment of Chemistry and Chemical Engineering, Chalmers University of Technology, Göteborg, 412 96 Sweden *e-mail: [email protected] bISIS
Received June 29, 2019; revised July 31, 2019; accepted August 5, 2019
Abstract—A new supermirror guide has been proposed to replace the current neutron guide of the indirect time-of-flight near-backscattering spectrometer OSIRIS at the ISIS facility. Here we present an extensive Monte Carlo simulation study for the design and optimisation of a new guide system. Among the several guide geometry assessed, a curved guide with elliptical defocusing and focusing sections is shown to perform best. The estimated gain in intensity is a factor of 5–6 at the sample position with a homogeneous distribution of the divergence. The elliptic geometry results in a smaller beam spot and smaller samples will particularly benefit from this upgrade. The proposed guide replacement will ensure that the OSIRIS spectrometer will remain competitive in the years to come. Keywords: neutron spectrometer, Monte Carlo simulations, OSIRIS, quasielastic neutron scattering DOI: 10.1134/S1027451020070381
INTRODUCTION Quasielastic neutron scattering studies are traditionally performed on time-of-flight (TOF) and backscattering spectrometers. At pulsed sources, the idea of backscattering from an analyzer crystal has been combined with the analysis of the incoming energy by measuring the time-of-flight to the detector. One of the first instruments of this type was IRIS at the ISIS Facility (Rutherford Appleton Laboratory, Didcot, UK) [1, 2]. The OSIRIS spectrometer, based on a similar design as IRIS, achieved a large gain in intensity by using a supermirror guide and an analyzer unit with increased angular coverage [3–5]. An extensive Monte Carlo simulation study, using the McStas program package [6–8], was performed to describe in detail the resolution and line shape of the spectrometer [9]. OSIRIS is mainly used as a cold TOF-spectrometer in quasielastic neutron scattering experiments to reveal stochastic particle dynamics and for high-resolution investigations of low-energy excitations (for example, [10, 11]). On the quasielastic side of applications, a strong increase in ion-mobility studies for battery materials has recently been observed. These are quite challenging experiments, since, compared to hydrogen, the studied ions are weak incoherent scatterers [12], which leads to low count-rates of detected neutrons. Here we propose a substantial
increase in flux with a new supermirror guide, which will benefit these studies in particular. The current OSIRIS
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