Chemistry Reference
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convert each pixel position (in a and z) into a point in diffraction space (g and 2u).
The two-dimensional diffraction image can then be displayed and analyzed using
two-dimensional diffraction theory. Other equations may be used to calculate the
diffraction space parameters (g and 2u) for the same geometry or different geometry.
All the previous algorithms for phase identification, stress analysis, texture analysis,
and other diffraction applications are applicable to the diffraction frame collected by
scanning line detector, except those equations specifically developed for the geometry
of flat 2D detectors.
13.2.2 Advantages of Scanning Line Detector
There are many advantages of two-dimensional diffraction with a line detector. In
addition to most functions of a conventional two-dimensional diffraction with a 2D
detector, there are other advantages that include, but are not limited to, the following:
(a) Low Cost: With current technology, the cost of a line detector is typically
much less than an area detector. The low cost makes two-dimensional
diffraction more affordable to many users.
(b) Higher Resolution: There are at least three factors resulting in higher resolu-
tion with line scan diffraction. Firstly, a line detector can be built with smaller
pixel size, which translates to higher resolution. Secondly, the line scan step
with a typical goniometer can be much smaller than the pixel size in a typical
2D detector. Thirdly, it is possible to add slits along the line direction to control
the detection line width.
(c) No Defocusing Effect: The defocusing effect is observed with the data
collected by a 2D detector when a diffraction peak is collected with the
low-incident angle (
u) over a flat sample surface.With line scan diffraction,
the incident angle u
1
can change simultaneously with the scanning angle a of
detector scan so as to keep u
1
¼u
2
in the diffractometer plane. In this case the
defocusing effect is eliminated with a constant defocusing factor of 1.
(d) Reduces Air Scattering from the Diffracted Beam: Air scatter with a 2D
detector is a significant contribution to the intensity background due to the
open space between the sample and the 2D detector. The intensity of the air
scatter from the incident beam is proportional to the length of the open incident
beam path, which is the distance between the sample and the beam collimation
exit. With a line detector, air scatter may be blocked by an air scatter shield as
shown in Figure 13.4. The air scatter from the primary beam between the
collimator exit and the sample is blocked by the scatter shield. The air scatter
from the diffracted X-rays is also blocked by the scatter shield. Only the
diffracted X-rays from the sample and in the correct direction can reach the
line detector through the narrow channel of the air scatter shield.
(e) Secondary Monochromator on the Detector Side: The spectrum impurity of
the incident beam and/or radiation fluorescence from the sample are the sources
of intensity background with a 2D detector—for example, when Cu-K
a
<
