The Detection and Analysis of Emitted X-rays
The characteristic X-radiations emitted by a sample may be analysed in a number of different ways. In a
procedure akin to filter photometry (p. 277) the absorption edge phenomenon is exploited to construct
narrow bandpass filters which can be employed to present the radiation of a single characteristic line to
the detector (Figure 8.42). Dispersive analysis of X-rays is achieved using a crystal to effect diffraction.
The angle, θ, of diffraction is related to the wavelength, λ, of the radiation and the spacing, d, of the
crystal planes by the Bragg equation in which n has integral values.
To provide for a comprehensive range of wavelengths a variety of crystals with different spacings are
required (Table 8.8).
Figure 8.42
Primary X-ray spectrum for Cu with the
absorption edge of a nickel filter superimposed
showing how it may be used to isolate the long
wavelength emission peak.The analysing crystal is rotated to direct radiations of varying wavelengths on to the detector in turn
(Figure 8.40). It follows that the detector must also move and it will be seen from the Bragg equation
(8.11) that it must do so through twice the angle of rotation of the crystal. As a result, X-ray spectra are
often plotted with the intensity as a function of 2θ (Figure 8.43). Any detector sensitive to ionizing
electromagnetic radiations will serve for monochromatic radiation presented by a filter system or
dispersing crystal. Such detectors and their characteristics are discussed in detail in Chapter 10. Geiger-
Müller, scintillation and proportional counters have all been employed, with the latter pair proving the
most useful. In these two devices the voltage pulses generated have sizes proportional to the energies of
the incident photons, whence a measure of pulse height analysis may be applied to reduce the
background and improve the resolution of the spectrum.