Physics for Geologists, Second edition

68 Electromagnetic radiation
Source
Figure 6.1 X-ray diffraction. The lower path is longer than the upper by
2d sin 8.
Figure 6.2 The Debye-Scherrer powder camera. The powder embedded in
a non-crystalline medium is oriented at random to the X-ray path.
(Courtesy Philips Nederland B.V.)
X-ray diffraction analysis is based on the assumption that no two crystals
of different composition have identical atomic spacing, and that mea-
surement through all possible values of 0 will give a unique pattern of
wavelengths. The Debye-Scherrer powder camera (Figure 6.2) can be used
for simple solids that are fairly pure. The powder is embedded in a non-
crystalline medium and it is assumed that random orientation of the particles
will cover the full range of 0. Photographic film is in a circular holder. After
exposure and development, the film has lines that are spaced according to 0,
symmetrically arrayed around the point 0 = 0".
X-ray fluorescence (XRF) or X-ray emission spectroscopy
When metals and other massive samples with atomic numbers less than about
20 are irradiated with high-energy X-rays, characteristic X-rays of lesser
energy are excited. These wavelengths are analysed using XRD methods and
a crystal grating of known spacing (an X-ray spectrometer, which is strictly
Copyright 2002 by Richard E. Chapman