Physics and Engineering of Radiation Detection

682 Chapter 12. Radiation Spectroscopy

(k)

μ

k 0

μ 0 (k)

∆μ

0

k

Figure 12.1.8: Typical variation of x-ray absorption coefficient with

respect to photoelectron wavenumber as obtained by XAFS spec-

troscopy.

The variation of the absorption coefficient with respect to energy or wavenumber
are generally known asoscillations. These oscillations are believed to be due to
interference effect. This implies that in the absence of any interference, that is when
there is only an isolated atom, one should have a nearly flat response. Note that this
does not mean thatμshould not vary with respect tok. In Fig. reffig:spectxray6
such a case is depicted by the functionμ 0 (k). This function is very useful in XAFS
since it can be used to quantify the oscillations according to the following equation.

χ(k)=

μ(k)−μ 0 (k)

μ 0 (k 0 )

(12.1.9)

Here μ 0 (k 0 ) is simply a normalization factor, called theedge step. A plot ofchi(k)
with respect tokdepicts the oscillations. The structure of these oscillations gives
insight into the structure of the material and its constituents. A typical XAFS
oscillation curve is shown in Fig.12.1.9.
Let us now discuss how XAFS spectra can be obtained using the transmis-
sion technique. The basic building blocks of transmission experiment are shown
in Fig.12.1.12 and described below.

X-ray Source:One of the basic requirements of an XAFS measurement setup

is the intensity of x-rays, which should be as high as possible. The reason is

that the number of photons surviving the absorption should be large enough

to obtain a well defined spectrum showing the fine structure details. Another

important requirement is that the x-rays should be tunable. That is, one

should be able to tune the laser to particular frequencies. Synchrotron radiation

sources fulfill both of these requirements since they produce highly intense