Physics and Engineering of Radiation Detection

12.1. Spectroscopy of Photons 683

χ

(k)

0.6

k

−0.2

0.0

0.4

0.2

Figure 12.1.9: Typical XAFS oscillation curve normalized to the

edge step.

I 0 Ix

Collimator

Monochromator

Sample

X−rays d

Figure 12.1.10: A basic setup for XAFS spectroscopy using trans-

mission technique.

beams of x-rays of continuous spectra. Hence most XAFS experiments are

performed using synchrotron radiation. In fact, modern synchrotron facilities

are equipped with dedicated XAFS beam lines, which are designed to provide

highly stable beams of x-rays.

Monochromator: The job of a monochromator is to tune the synchrotron

beam to well defined frequencies. Most monochromators in use today are made

of silicon. They exploit the process of Bragg diffraction to tune the x-rays. It is

important that the monochromator provides good energy resolution in order to

reveal all the fine structures. In general one strives to have an energy resolution

of better than 10−^4 , which is achievable through silicon devices.

Collimator:A collimator is used to align the beam to minimize dispersion.

Detectors:In general one needs two detectors to correctly identify the fine

structures. The detector before the sample ensures that any structure in the

beam itself does not get wrongly identified as a fine structure. However there

are stringent requirements on the working of this detector. For example, it

should not attenuate the beam too much, say more than 10%. At the same

time its resolution should be high enough to identify fine details in the spec-