STRESS MEASUREMENT - Two-Dimensional X-Ray Diffraction - page 296

condition of the data frame. The larger the segment size (Dg) is, the better the
integrated diffraction profile due to more counts being integrated. g-integration also
produces a smearing effect on the diffraction ring distortion because the counts
collected within the segment size (Dg) are considered a single g value at the segment
center. The 2u shift in the segment is averaged. The segment size (Dg) should be
sufficient to produce a smooth diffraction profile, but not so large as to introduce
significant smearing effect. For data frames containing high pixel counts, the
integration segment can be sufficiently small, Dg 2 for instance, and still have
a smooth profile for each segment. For data frames having low pixel counts—for
instance, the frames collected from a microarea, with a small sample, or with a short
data collection time—it is critical to choose an appropriate segment size. The segment
size can be determined by observing the smoothness of the integrated profile or by
comparing the stress results and standard error at various segment sizes.
Peak evaluation on each segment can be done by the same algorithm that was used
in the conventional method. The corrections on the integrated profiles are performed
before or during the peak evaluation. Absorption correction eliminates the influence
of the irradiated area and the diffraction geometry on the measured intensity
distribution. The absorption for a given material and radiation level depends on the
incident angle to the sample and the reflected angle from the sample. For XRD 2 , the
reflected angle is a function of g on each frame. The polarization effect is also
a function of g. Therefore, the correction for polarization and absorption should be
applied to the frame before integration. The details on these corrections are discussed
in Chapter 6 on data reduction. The absorption correction is not always necessary if
the error caused by absorption can be tolerated for the application, or if the data
collection strategy involves only c and f scans.
In most cases, the combined K a1 and K a2 radiation is used for stress measurement,
in which case the weighted average wavelength is used in calculations. For samples
with a broad peak width, the diffractions from the K a1 and the K a2 radiation are
merged together as a single peak profile, and the profile can be evaluated as if there is
a single K a line without introducing much error to the measured d-spacing. For
samples with a relatively narrow peak width, the diffraction profile shows strong
asymmetry or may even reveal two peaks corresponding to the K a1 and K a2 lines,
especially at high 2u angles. This phenomenon has been referred to as K a1 -K a2
doublet or K a doublet [37]. In this case, the profile fitting should include contributions
from both K a1 and K a2 lines. It is common practice to use the peak position from
the K a1 line and the K a1 wavelength to calculate the d-spacing after the contribution
of the K a2 line is eliminated. Therefore, the correction is also referred to as K a2
correction or K a2 stripping. The line profiles of K a1 and K a2 are typically assumed to
be identical, but with different intensities. The intensity ratio of the K a2 line to the K a1
line is approximately 0.5. K a2 stripping can be done before the peak evaluation
or during the profile fitting, depending on the algorithms used for peak evalua-
tion [38,39]. K a2 stripping is not necessary if the K a2 line has been removed from the
incident X-ray beam—for instance, when a channel-cut monochromator is used.
Background correction is necessary if there is a strong background or the peak
evaluation algorithms are sensitive to the background, such as in K a2 stripping, peak