Chemistry Reference
In-Depth Information
to a high background in the diffraction pattern. The antiscatter slit prevents most air-
scattered X-rays from reaching the receiving slit.
The functions of the Soller slits can be better explained with cross-section view of
the diffractometer plane as shown in Figure 3.4(b). The Soller slits aremade of equally
spaced tantalum or molybdenum foils. The Soller slits 1 on the primary beam side
control the beam divergence in the goniometer axial direction. Due to the narrow
spacing between the metal foils, the X-rays from the line focus source with high
divergence in axial direction are blocked. The primary line beam sliced by the Soller
slits 1 can also be considered an array of point beams in parallel diffractometer
planes. Each of these point beams will produce a diffraction cone from the sample.
The diffraction cone extends out from the diffractometer plane. The aperture of
the receiving slit in the goniometer axial direction is typically the same size as the
line focus source. If the diffraction cone in the range of the receiving slit is measured,
the overlap of all the diffraction cones will create a smeared diffraction peak. The
diffracted beam along the diffraction cone makes an angle with the diffractometer
plane except the part of the cone near the diffractometer plane. The Soller slits 2 in the
receiving side allow only those diffracted beams nearly parallel to the diffractometer
plane pass through, so the smearing effect is eliminated.
The radiation spectrum from the X-ray tube contains white radiation and several
characteristic radiations. If there is no monochromator in the primary side, all
wavelengths of the spectrum will diffract from the sample when the Bragg law is
satisfied. To collect a diffraction pattern from a single wavelength, for instance, from
K
a
line, a crystal monochromator is installed before the detector. This monochroma-
tor is also called as diffracted-beam monochromator. The receiving slit, monochro-
mator crystal, and detector are so positioned that only the K
a
line or the K
a1
line
satisfy the Bragg law. The diffracted X-rays from other wavelengths are blocked by
the monochromator. Another important function of the diffracted-beam monochro-
mator is to block the fluorescence radiation from the sample since the wavelength of
the fluorescence is different from the K
a
line.
Figure 3.5 shows a diffractometer with a point detector in the parallel beam
geometry. A single parabolic bent G
obel mirror transforms the divergent primary
beam from the source into a parallel beam. The G
€
obel mirror is a multilayer mirror
that serves also as a monochromator, so that the incident parallel beam is a single
wavelength X-ray beam. The incident X-rays spread over the sample surface with the
same incident angle u. The surface of the irradiated region can either be flat or uneven,
but only the diffracted X-rays in the direction of 2u defined by the Soller slits can reach
the point detector. It is noticeable that the Soller slits have an orientation different
from the Soller slits in the Bragg-Brentano geometry. In the Bragg-Brentano
geometry, the foils in the Soller slits are parallel to the diffractometer plane, while
in the parallel beam geometry the foils are perpendicular to the diffractometer plane
and aligned in the direction between the instrument center and detector. The Soller
slits here also cover the entire active area of the detector so that only the X-rays of the
same direction can reach the detector. Therefore, the diffractometer with parallel
beam geometry is not sensitive to the surface roughness of the sample. This is
especially beneficial when collecting diffraction data from samples with a rough,
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