Chemistry Reference
In-Depth Information
TABLE 3.6 X-Ray Beam Divergence Angle (b), Convergence Angle (a), and
Beam Spot Size on Sample (
S
) for a 0.8mm Point Focus Tube with
Graphite Monochromator or Cross-Coupled Gobel Mirrors
Graphite Monochromator
Gobel Mirrors
Pinhole
b (
)
a (
)
b (
)
a (
)
d (mm)
S (mm)
f (mm)
S (mm)
0.05
0.041
0.017
0.07
0.15
0.041
0.017
0.07
0.10
0.082
0.034
0.14
0.30
0.060
0.034
0.13
0.20
0.164
0.067
0.29
0.60
0.060
0.060
0.23
0.30
0.246
0.101
0.42
0.80
0.060
0.060
0.33
0.50
0.266
0.148
0.64
0.80
0.060
0.060
0.53
0.80
0.327
0.148
0.97
0.80
0.060
0.060
0.83
are H¼280 mm, h¼140 mm, g¼30mm, and F¼0.8mm for the 0.4mm0.8mm
fine focus tube. A collimator may determine the maximum divergence of an X-ray
beam passing through it or reduce the divergence by cutting off the high divergent X-
rays, but it can never increase the divergence from the optics coupled preceding it. The
graphite monochromator has a rocking curve of 0.4
, and cross-coupled Gobel
mirrors of 0.06
. The beam divergence and convergence angles should not be above
these values even if the collimator may allow a more divergent beam to pass. For the
same pinhole size, if the focal spot is larger than the pinhole size, the beam divergence
is inversely proportional to the distance between two pinholes. If the focal spot size is
smaller than the pinhole size, the beam divergence is inversely proportional to the
distance between the source and the second pinhole. In every case, the lower
divergence is typically associated with a long beam path. At the same time the beam
flux is inversely proportional to the square of the distance between the source and
sample. There are mainly two factors determining the length of primary beam path:
the first is the required distance for collimating the beam into the required divergence,
the second is the space required for primary X-ray optics, sample stage, and detector.
On the condition that
the above two factors are satisfied,
the primary
X-ray beam path should be kept as short as possible.
Table 3.6 also shows that the beam divergence decreases continuously with
decreasing pinhole size for the combination of double-pinhole collimator and
monochromator. In cases where the application requires a small beam size but not
necessarily a small divergence, it is advisable to remove the pinhole 1 from the
collimator tube to achieve higher beam intensity. Table 3.7 gives the comparison
between double-pinhole collimators and single-pinhole collimators in terms of
intensity gain (the approximate ratio of single to double pinhole), beam divergence,
and beam spot size on the sample.
The 50 and 100 mm collimators are preferred for microdiffraction and 0.5 or
0.8mm collimators are typically used for quantitative analysis, texture, or percent
crystallinity measurements. In the case of quantitative analysis and texture measure-
ments, using too small a collimator can actually be a detriment, causing poor
statistical grain sampling. In such cases, the statistics can be improved by oscillating
