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Data reduction

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Data Reduction

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In order to reduce raw data collected as images (or frames) on an area +detector as a function of sample rotation angle and transform the +results into reciprocal space maps, i.e., S(Q), NXRefine +performs the following steps:

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  • combining the frames into a single three-dimensional array.

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  • harvesting metadata collected during the sample rotations.

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  • summing detector frames to facilitate absorption corrections.

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  • searching for Bragg peaks embedded within the raw data.

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  • defining an orientation matrix.

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  • transforming the raw data into reciprocal space coordinates.

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When multiple sample rotations are performed to collect a single data +set, these steps have to be applied to each rotation scan and the +results merged to produce a single three-dimensional array representing +S(Q). Optionally, NXRefine also transforms the data after applying +masks that eliminate spurious signals caused by the scattering of Bragg +peaks within the detector sensor layer.

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Once the data has been transformed into S(Q), it is possible to +generate 3D-ΔPDF maps, which transform the data back into real space, +producing difference Patterson maps, i.e., maps of interatomic vector +probabilities, which differ from the average crystalline structure. In +this way, continuous distributions of diffuse scattering intensity are +typically reduced to discrete peaks, with positive and negative +intensities, representing these probability differences. NXRefine +implements the “punch-and-fill” method, described by Weber and Simonov.

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Nearly all of the steps in the NXRefine data reduction workflow can +either be performed from the command line or launched from a NeXpy GUI. +The exception is determining the crystal orientation, which must first +be performed using the Refine Lattice dialog in NeXpy. Once the sample +orientation has been determined from one of the measurements, e.g., at +room temperature, it can be copied and refined automatically when +reducing the data from other measurements, provided the space group has +not changed or is still compatible with the observed Bragg peaks.

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