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. 2023 Apr 25;56(Pt 3):633-642.
doi: 10.1107/S1600576723002819. eCollection 2023 Jun 1.

POWTEX visits POWGEN

Andreas Houben  1 , Yannick Meinerzhagen  1 , Noah Nachtigall  1 , Philipp Jacobs  2 , Richard Dronskowski  1
Affiliations

POWTEX visits POWGEN

Andreas Houben et al. J Appl Crystallogr. .

Abstract

The high-intensity time-of-flight (TOF) neutron diffractometer POWTEX for powder and texture analysis is currently being built prior to operation in the eastern guide hall of the research reactor FRM II at Garching close to Munich, Germany. Because of the world-wide 3He crisis in 2009, the authors promptly initiated the development of 3He-free detector alternatives that are tailor-made for the requirements of large-area diffractometers. Herein is reported the 2017 enterprise to operate one mounting unit of the final POWTEX detector on the neutron powder diffractometer POWGEN at the Spallation Neutron Source located at Oak Ridge National Laboratory, USA. As a result, presented here are the first angular- and wavelength-dependent data from the POWTEX detector, unfortunately damaged by a 50g shock but still operating, as well as the efforts made both to characterize the transport damage and to successfully recalibrate the voxel positions in order to yield nonetheless reliable measurements. Also described is the current data reduction process using the PowderReduceP2D algorithm implemented in Mantid [Arnold et al. (2014). Nucl. Instrum. Methods Phys. Res. A, 764, 156-166]. The final part of the data treatment chain, namely a novel multi-dimensional refinement using a modified version of the GSAS-II software suite [Toby & Von Dreele (2013). J. Appl. Cryst.46, 544-549], is compared with a standard data treatment of the same event data conventionally reduced as TOF diffraction patterns and refined with the unmodified version of GSAS-II. This involves both determining the instrumental resolution parameters using POWGEN's powdered diamond standard sample and the refinement of a friendly-user sample, BaZn(NCN)2. Although each structural parameter on its own looks similar upon comparing the conventional (1D) and multi-dimensional (2D) treatments, also in terms of precision, a closer view shows small but possibly significant differences. For example, the somewhat suspicious proximity of the a and b lattice parameters of BaZn(NCN)2 crystallizing in Pbca as resulting from the 1D refinement (0.008 Å) is five times less pronounced in the 2D refinement (0.038 Å). Similar features are found when comparing bond lengths and bond angles, e.g. the two N-C-N units are less differently bent in the 1D results (173 and 175°) than in the 2D results (167 and 173°). The results are of importance not only for POWTEX but also for other neutron TOF diffractometers with large-area detectors, like POWGEN at the SNS or the future DREAM beamline at the European Spallation Source.

Keywords: DREAM beamline; POWGEN beamline; POWTEX detector; Rietveld refinement; angular-dispersive refinement; multi-dimensional refinement; neutron detectors; powder diffraction; time-of-flight diffraction; wavelength-dispersive refinement.

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Figures

Figure 1
Figure 1
(Left) A schematic drawing showing one of the POWTEX cylinder surface detector mounting units (left-hand part) mounted as a pack of eight detector modules in the POWTEX detector geometry. It is aligned orthogonal to 2θ = 90° in the horizontal scattering plane (paper plane) at a distance of 81 cm (instead of the ideal 80 cm) from the sample at the center of the coordinate system. The maximum angular coverage of this type of POWTEX detector is Δ2θ = 90°. Each mounting unit covers 9° in the φ direction (almost perpendicular to the paper plane). The drawing results from the instrument definition file (IDF) as used with Mantid. The much larger POWGEN detector system is shown on the opposite hemisphere. (Right) A photograph of the detector (plus mockup and shielding) touching the POWGEN detector vessel, i.e. the neutron window, as closely as possible to give the best match to the ideal 80 cm sample-to-detector distance.
Figure 2
Figure 2
An example diffractogram of the TOF values measured similarly for each voxel. The experimental data points are shown as blue dots and the fitted function as a red line.
Figure 3
Figure 3
A one-dimensional diffractogram for the diamond sample measured on POWTEX at POWGEN, in red for the uncorrected and in blue for the corrected detector alignment accounting for the detector damage.
Figure 4
Figure 4
A two-dimensional diffractogram for the diamond POWTEX at POWGEN data sets, with an uncorrected (red) and corrected (blue) detector alignment also handling the effects of the detector damage. By the definition of formula image , the corrected reflections (blue) need to be exactly vertical for each d hkl value. The widths of the blue and red reflections in this plot are proportional to the FWHM for each reflection and are meant just as a guide to the eye.
Figure 5
Figure 5
The three basic steps of data reduction applied by the workflow algorithm PowderReduceP2D to convert raw event data to refineable .p2d data.
Figure 6
Figure 6
A diagram showing the offsets assigned to each voxel as identified by their detector ID. Red voxels were selected for further data treatment while blue voxels, all having an offset of exactly zero, were masked for different reasons. Each of the eight detector modules per mounting unit consists of two subunits. Therefore, the four subunits of modules 7 and 8 were masked because their anode wires were broken. Similarly, in the third module, half of the detector volume (in depth) had to be shut down.
Figure 7
Figure 7
Angle- and wavelength-dispersive diffraction pattern (2θ–λ plot) of the reduced observed data for the diamond sample
Figure 8
Figure 8
Results of the multi-dimensional Rietveld refinement for a neutron TOF measurement of diamond. (a) The observed pattern, (b) the calculated pattern, (c) the difference pattern, (d) a 1D plot of multi-dimensional refinement and (e) the conventional one-dimensional refinement. Blue lines indicate peak positions. The color scale shows normalized intensity in plots (a)–(c).
Figure 9
Figure 9
Results of the multi-dimensional Rietveld refinement for a neutron TOF measurement of BaZn(NCN)2. (a) The observed pattern, (b) the calculated pattern, (c) the difference pattern, (d) a 1D plot of multi-dimensional refinement and (e) the conventional one-dimensional refinement result. Blue lines indicate peak positions. The color scale shows normalized intensity in plots (a)–(c).
See this image and copyright information in PMC

References

    1. Arnold, O., Bilheux, J. C., Borreguero, J. M., Buts, A., Campbell, S. I., Chapon, L., Doucet, M., Draper, N., Ferraz Leal, R., Gigg, M. A., Lynch, V. E., Markvardsen, A., Mikkelson, D. J., Mikkelson, R. L., Miller, R., Palmen, K., Parker, P., Passos, G., Perring, T. G., Peterson, P. F., Ren, S., Reuter, M. A., Savici, A. T., Taylor, J. W., Taylor, R. J., Tolchenov, R., Zhou, W. & Zikovsky, J. (2014). Nucl. Instrum. Methods Phys. Res. A, 764, 156–166.
    1. Chebyshev, P. L. (1858). J. Math. Pure Appl. 3, 289–323.
    1. Conrad, H., Brückel, T., Schäfer, W. & Voigt, J. (2008). J. Appl. Cryst. 41, 836–845.
    1. Corkett, A. J., Konze, P. M. & Dronskowski, R. (2018). Inorganics, 6, 1.
    1. Houben, A., Schweika, W., Brückel, T. & Dronskowski, R. (2012). Nucl. Instrum. Methods Phys. Res. A, 680, 124–133.