FIT2D for Powder Diffraction with Area Detectors

The POWDER DIFFRACTION (2-D) interface allows Debye-Scherrer rings which have been recorded of a 2-D detector to be integrated to the equivalent of a 2-theta scan with a zero-dimensional detector (Hammersley et al, 1996). An example of an input image and the main menu are shown in Figure 14. To aid with this the MASK sub-menu allows interactive definition of masked areas i.e. pixels which will be ignored during further operations. The TILT button allows the beam centre and any non-orthogonality of the detector to the main beam to be refined, and this tilt is used by the INTEGRATE command. Other types of output are also available. The CAKE sub-menu allows different regions of the image to be defined with graphical coordinate input for the starting azimuth, end azimuth and inner and outer radii of the integrated region. The number of output bins in both the azimuthal and the 2-theta or radial direction are user specified. This allows a variety of different output possibilities e.g. A number of different 2-theta scans, for different azimuth ranges; a 1-D profile of intensity of a ring as a function of azimuth (e.g. for texture studies); or a polar transform of the data. The output formats include one of the GSAS formats (Larson & Von Dreele, 1994), one of the Cerius formats, and a general purpose ASCII two column format, which may easily be edited.

Figure 14: The Powder Diffraction Menu

The CALIBRANT button allows the fitting of the powder pattern from a standard calibrant sample. The known D-spacings of the diffracting planes allow the beam centre and tilt to be defined with greater accuracy than with an unknown sample, and if reasonably high angle data is present allow the sample to detector distance, and the wavelength to be refined independently. This has been verified to agree to within the accuracy that may be obtained from scanning an absorption edge using a Ge detector on ID-30 at the ESRF.