Chemistry Reference
In-Depth Information
frames collected with the Bruker Hi-Star MWPC with the fluorescence radiation of
glassy iron foil irradiated by a 0.5mm point beam at a 15 cm sample-to-detector
distance. Figure 6.1(a) is a raw image used for flood-field correction. The nonuniform
response can be seen on this frame. Since this frame is froman isotropic source, a flood-
field correction projection file can be generated from this frame. Figure 6.1(b) is an
image collected under the same conditions with flood-field correction, that is, with the
projection file loaded during the data collection. This image shows a homogeneous
intensity distribution. The flood-field calibrationmust be donewith the same sample-to-
detector distance for the diffraction data collection. However, it is possible to collect
several flood-field calibration files at different distances and then use them as needed.
6.3 SPATIAL CORRECTION
In an ideal flat area detector, each pixel not only has the same intensity response but
also has an accurate position. The pixels are aligned in the x and y-direction with
equal spacing. In most cases, we assume the detective area is completely filled by
pixels, so the distance between two neighboring pixels in the x-ory-direction is
equivalent to the pixel size. The deviation from this perfect pixel array is called
spatial distortion. In reality, all area detectors exhibit spatial distortions to some
extent due to the imperfection in the components, design or manufacturing. The
extent of spatial distortion is dependent on the nature and limitation of the detector
technology. A CCD detector with 1:1 demagnification may have a negligible spatial
distortion, but the barrel distortion in the coupling fiber optic taper can introduce
substantial spatial distortion [11]. An image plate system may have spatial distortion
caused by imperfections in the scanning system [15]. Gaseous detectors typically
exhibit more severe spatial distortion due to the window curvature and the imperfect
wire anode [16]. Spatial correction is necessary to get a diffraction image with
accurate pixel positions so as to get the correct 2u and g values for data analysis and
integration.
6.3.1 Fiducial Plate and Detector Plane
Spatial correction computation requires knowledge of the spatial distortion. The
spatial distortion is measured from the X-ray images collected with a reasonably
uniformly radiating point source positioned at the instrument center and a fiducial
plate fastened to the front surface of the detector. The calibration sources for flood-
field and spatial corrections have different requirements. The point source for a flood
field should have isotropic brightness, but the size and location are not very critical.
The source for spatial correction should have a very accurate position, point-like
shape, and small size. In practice, the same point source is used for both flood-field and
spatial corrections. Figure 6.2 illustrates the spatial correction geometry for a
MikroGap detector (Bruker VA
NTEC-2000). A fiducial plate is mounted to the
front surface of the detector. The fiducial plate is made of brass with accurately
