Page 128 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Minerals
Soil Minerals 123

Replacement of orbiting electrons from outer shells in atoms involves a stepwise
energy change, so X-rays have particular wavelengths. Other ‘‘white radiation’’
wavelengths also are produced and are used for medical X-rays, but for
diffraction measurements only a single wavelength can be used, so the rest are
filtered out.

The strongest monochromatic or single-wavelength X-rays are produced when a
position in the innermost or ‘‘K’’ atomic shell is replaced by one falling in from
the next outer shell, which generates ‘‘K radiation.’’ The most useful X-rays for
mineral identification are from a copper target and are therefore referred to as
‘‘copper K radiation.’’

6.2.3 Geometry of Diffraction

Figure 6.1 shows a diffractometer that is designed to measure diffraction angles
from a sample placed in an X-ray beam. A beam is obtained by simply looking at
the target through some narrow slits, and is directed toward the sample that is at
the center of a circular goniometer. A counter then is moved around the outside
of the circle to detect X-rays, and is synchronized with a chart recorder so that
when diffraction occurs and the count increases, the angle can be measured.
One of the most famous uses of X-ray diffraction was to determine the structure
of DNA.

Unlike reflections in a mirror the diffraction angle is the same on both sides of the
crystal and is designated by . As shown in Fig. 6.1, the angle measured around
the circumference of a diffractometer is 2 .

Figure 6.1
Schematic
diagram of
an X-ray
diffractometer used
for identifying
minerals in soils
and measuring
clay mineral
expansion.

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