Physics and Engineering of Radiation Detection

680 Chapter 12. Radiation Spectroscopy

Note that the absorption coefficientμis energy dependent. If the energy of the
photons is increased slowly, there comes a point when the absorption coefficient
suddenly increases (see Fig.12.1.6). This happens when the photon energy equals
the binding energy of the core electron. The sudden increase in the absorption
coefficient results in a corresponding decrease in the measured intensity. This change
in absorption coefficient is known as anedge. A material can have a number of edges
corresponding to electronic transitions from different shells. For x-ray energies, one
generally deals with K-edges. The important point to note here is that an edge
is a unique signature of an element and can therefore be used to identify it from
a mixture. This technique is generally known astransmissionspectroscopy since
here one simply measures the intensity of transmitted x-rays at different energies.
The simplicity of this technique should not undermine its usefulness, though. As a
matter of fact, as we will shortly see, besides being simple it is also the most efficient
technique available for absorption spectroscopy.

Energy(MeV)

10 -3 10 -2 10 -1 1 10

2 /g)

(cmm

μ

10 -1

1

10

102

103 Fe

Cu

Figure 12.1.6: Variation of mass absorp-

tion coefficients of iron and copper for

photons. The edges shown correspond

to K-shell transitions.

Fig.12.1.7 shows a typical absorption spectrum obtained through transmission
spectroscopy. As shown in the figure, such spectra are generally divided into two
regions. The one near the absorption edge contains the near edge x-ray fine structure
or NEXAFS. The spectroscopy done in this region is then referred to as NEXAFS
or XANES (x-ray absorption near edge structure) spectroscopy. The second region
on the side in Fig.12.1.7 contains the extended x-ray absorption fine structure or
EXAFS. These structures are explored through the EXAFS spectroscopy.
The attenuation coefficient shown in Fig.12.1.7 has been plotted against the x-ray
photon energy. However the general practice is to plot it with respect to the photo-
electron wavenumber instead. The rationale behind this is that the fine structures
are generated due to the interference effect and are therefore more closely related
to the wave nature of the photoelectron. The transition from the photon energy
to the photoelectron wavenumber is fairly straightforward. We start with the basic