emissions are sometimes known as 'white X-rays'. Superimposed on the continuum will be emission
peaks characteristic of transitions involving the K and L shells of the elements within the target (Figure
8.32). The wavelengths of the peaks are related to the atomic numbers of the emitting atoms by the
Moseley equation (8.6) and thus may be used for the qualitative analysis of a target material,
where a is a constant and b has a specific value for all lines in one series, i.e. b(Kα lines) = 1.0, and b(Lα
lines) = 7.4.
Figure 8.32
Spectra of X-rays emitted from a molybdenum
target as a result of electron (25 keV) and X-ray
(35 keV) bombardment.Furthermore, under controlled bombardment conditions, peak intensity measurements may be used for a
quantitative determination of the appropriate element. Measurements of the characteristics and intensity
of primary X-rays produced by electron bombardment constitute the basis of electron probe
microanalysis. Figure 8.33 illustrates the complex nature of the reactions initiated by the impact of an
electron beam on a target. As a consequence of this complexity it has proved extraordinarily difficult to
make fully quantitative measurements, and it is only recently with the widespread application of
dedicated computers and sophisticated software that this has become possible.
In electron probe microanalysis the surface characteristics of the specimen are also of importance.
Figure 8.34 illustrates how an uneven surface can lead to variable attenuation of the emitted X-rays and
the importance of the angle between detector and specimen surface being as near 90° as possible. Such
a high take off angle will minimize the surface effects.