Crystallography
2. experimental, 3. simulations, 4. results and comments, 5. conclusion.
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JOURNAL OF APPLIED CRYSTALLOGRAPHY |
a Jülich Centre for Neutron Science at Heinz Maier-Leibnitz Zentrum (MLZ), Forschungszentrum Jülich, Lichtenbergstraße 1, Garching, 85747, Germany * Correspondence e-mail: [email protected]
For a reliable characterization of materials and systems featuring multiple structural levels, a broad length scale from a few ångström to hundreds of nanometres must be analyzed and an extended Q range must be covered in X-ray and neutron scattering experiments. For certain samples or effects, it is advantageous to perform such characterization with a single instrument. Neutrons offer the unique advantage of contrast variation and matching by D-labeling, which is of great value in the characterization of natural or synthetic polymers. Some time-of-flight small-angle neutron scattering (TOF-SANS) instruments at neutron spallation sources can cover an extended Q range by using a broad wavelength band and a multitude of detectors. The detectors are arranged to cover a wide range of scattering angles with a resolution that allows both large-scale morphology and crystalline structure to be resolved simultaneously. However, for such analyses, the SANS instruments at steady-state sources operating in conventional monochromatic pinhole mode rely on additional wide-angle neutron scattering (WANS) detectors. The resolution must be tuned via a system of choppers and a TOF data acquisition option to reliably measure the atomic to mesoscale structures. The KWS-2 SANS diffractometer at Jülich Centre for Neutron Science allows the exploration of a wide Q range using conventional pinhole and lens focusing modes and an adjustable resolution Δ λ / λ between 2 and 20%. This is achieved through the use of a versatile mechanical velocity selector combined with a variable slit opening and rotation frequency chopper. The installation of WANS detectors planned on the instrument required a detailed analysis of the quality of the data measured over a wide angular range with variable resolution. This article presents an assessment of the WANS performance by comparison with a McStas [Willendrup, Farhi & Lefmann (2004). Physica B , 350 , E735–E737] simulation of ideal experimental conditions at the instrument.
Keywords: small-angle neutron scattering ; SANS ; wide-angle neutron scattering ; WANS ; McStas simulations ; semi-crystalline materials .
Scattering patterns from silver behenate (AgBeh) and C60-fullerene obtained by XRD. |
Scattering patterns from the fullerene-C60 powder sample obtained by TOF-SANS at KWS-2 for the following experimental conditions: λ = 2.8 Å and Δλ/λ = 14%, as delivered by the tilted velocity selector, = 1.25 m, = 98.24 Hz, Δφ = 25.72°, τ = 0.00072 s, Δλ/λ = 4.7% (central TOF channel), number of channels = 7. |
Measured ( , ) and simulated ( , ) two-dimensional scattering patterns from the fullerene-C60 powder sample for λ = 3.0 Å and Δλ/λ = 22.7% ( , ) and λ = 2.8 Å and Δλ/λ = 4.7% ( , ). |
Measured with an HRD ( ) and simulated ( ) two-dimensional scattering patterns from the fullerene-C60 powder sample for λ = 5.0 Å and Δλ/λ = 6.2%. The HRD was used in the TOF-WANS mode (see text). Panel ( ) shows the simulated data as a function of the scattering angle using the component. |
Measured (symbols) and simulated (yellow curve) one-dimensional scattering patterns from the fullerene-C60 powder sample under different experimental conditions in SANS and WANS geometries, as explained in the legends, in parallel with the XRD pattern (green curve). For better comparison, the XRD data were normalized to the SANS peak intensity and elevated so that the baseline corresponds to the flat SANS background. |
The resolution at KWS-2 for the SANS and WANS detectors under different experimental conditions ( , , λ and Δλ). |
Help from Dr Ralf Engels and Dr Georg Brandl, both from Forschungszentrum Jülich, during the WANS test with the HRD at KWS-2 is gratefully acknowledged. Open access funding enabled and organized by Projekt DEAL.
This is an open-access article distributed under the terms of the Creative Commons Attribution (CC-BY) Licence , which permits unrestricted use, distribution, and reproduction in any medium, provided the original authors and source are cited.
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