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TILT/BEAM CENTRE allows any non-orthogonality of the detector with respect to the beam to be fitted from the shape of one or more powder rings. Similarly the beam centre on the detector can be refined.
From the tilt and beam centre found by TILT/BEAM CENTRE or input by the user, the ADR may be re-binned to a 1-D equal angle or equal radial distance bin scan using the POWDER DIFFRACTION command. This allows the option or simultaneously correcting spatial distortion.
Users of these commands are kindly asked to acknowledge and cite:
A P Hammersley, S O Svensson, M Hanfland, A N Fitch, and D Häusermann, ``Two-Dimensional Detector Software: From Real Detector to Idealised Image or Two-Theta Scan'', High Pressure Research, 14, pp235-248, (1996)
(Note: Other options to allow more flexible integration as a function of azimuth are under development. At present R/THETA RE-BINNING performs a polar transformation to the ADR, with user control of the number and size of radial and azimuthal pixels.)
In general the effect of polarisation is dependent on both the
2 angle and the azimuthal angle of diffracted radiation. Thus,
it is important to correct for polarisation when the data is being
converted from a 2-D image to one or more 1-D spectra. If the
polarisation correction is to be applied the user will be prompted for
the polarisation degree of the X-ray beam and the geometry of the
experiment. The beam polarisation is defined as:
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(5) |
where: Ih is the horizontal component of the intensity, and Iv is the vertical component.
At a synchrotron the X-rays are usually highly horizontally polarised, so
, but the beam-line scientists should be able to supply
an appropriate number for their beam-lines.
The correction applied is based on the formula of Kahn [16].
Having corrected for polarisation it is important that subsequent software does not also apply a polarisation correction !
Andy Hammersley