Agriculture Reference
In-Depth Information
Reflected
X-rays
Incident
X-rays
q
Lattice
planes
Figure 8.2.
The diffraction of X rays from crystal planes.
incident angles, a soil clay sample. When the incident angle
, shown in Figure
8.2, results in constructive interference of the reflected X rays the distance
between the layers can be calculated using Bragg's law (see the text by Atkins
and de Paula [4] for an explanation of Bragg's law). In addition to the distance
between lattice planes, the swelling characteristics of various clays are differ-
ent when they are exposed to different solvents. This provides additional infor-
mation about the type of clay present and its characteristics.
In practice, clay from a soil sample is prepared on a microscope slide and
dried and its X-ray diffraction measured; subsequently the same clay is placed
in atmospheres saturated with, for example, glycerol, with subsequent X-ray
diffraction and again the distance between layers determined. Changes in the
diffraction pattern, or the lack thereof, will help identify the type of clay
present [5].
q
8.4.
X-RAY FLUORESCENCE
Exposure of elements to a broad spectrum of X rays results in the ejection of
electrons from their inner shells. Electrons from outer shells falling into
these vacancies emit radiation, an X-ray photon, of specific wavelength (see
Figure 8.3). Analysis of this radiation, X-ray fluorescence (XRF), allows for
the identification of the element from which the photon is emitted. Instru-
ments for carrying out this analysis can be either laboratory-size or a hand-
held unit that can be taken to the field. The excitation radiation for the X-ray
fluorescence instrument must pass through a window. The window material,
