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Optimizing a neutron-beam focusing device for the direct geometry time-of-flight spectrometer TOFTOF at the FRM II reactor source

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Authors:
  • Rasmussen, N. G. ;
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    Condensed Matter Physics, Niels Bohr Institute, Faculty of Science, Københavns Universitet
  • Simeoni, G. G. ;
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    Technical University of Munich
  • Lefmann, K.
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    Orcid logo0000-0003-4282-756X
    Xray and Neutron Science, Niels Bohr Institute, Faculty of Science, Københavns Universitet
DOI:
10.1016/j.nima.2016.01.082
Abstract:
A dedicated beam-focusing device has been designed for the direct geometry thermal-cold neutron time-of-flight spectrometer TOFTOF at the neutron facility FRM II (Garching, Germany). The prototype, based on the compressed Archimedes' mirror concept, benefits from the adaptive-optics technology (adjustable supermirror curvature) and the compact size (only 0.5 m long). We have simulated the neutron transport across the entire guide system. We present a detailed computer characterization of the existing device, along with the study of the factors mostly influencing the future improvement. We have optimized the simulated prototype as a function of the neutron wavelength, accounting also for all relevant features of a real instrument like the non-reflecting side edges. The results confirm the "chromatic" displacement of the focal point (flux density maximum) at fixed supermirror curvature, and the ability of a variable curvature to keep the focal point at the sample position. Our simulations are in excellent agreement with theoretical predictions and the experimentally measured beam profile. With respect to the possibility of a further upgrade, we find that supermirror coatings with m-values higher than 3.5 would have only marginal influence on the optimal behaviour, whereas comparable spectrometers could take advantage of longer focusing segments, with particular impact for the thermal region of the neutron spectrum.
Type:
Journal article
Language:
English
Published in:
Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 2016, Vol 816, p. 106-112
Keywords:
Adaptive optics; McStas; Neutron optics; Neutron spectroscopy; Ray-tracing simulations
Main Research Area:
Science/technology
Publication Status:
Published
Review type:
Peer Review
Submission year:
2016
Scientific Level:
Scientific
ID:
2291831123

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