Physics and Engineering of Radiation Detection

684 Chapter 12. Radiation Spectroscopy

trum. A resolution of better than 10−^4 is generally required but is difficult to

achieve. The main reason is the conflicting requirements of the resolution and

the attenuation. For higher resolution the detector should absorb large number

of photons. Recall that the physical limit on resolution is set by the statistical

noise of photon absorption. The larger the number of photons absorbed, the

lower the relative uncertainty ornoise. Since the detector after the sample does

not have the attenuation constraint, it can absorb as many photons as required

to achieve good signal to noise ratio. In most XAFS applications, use to ion-

ization chambers as transmission detectors is fairly common. Such a detector

is based on parallel plate geometry and generally has several chambers.

νi

νf<νi

νi νf<νi

(a)

(b)

X−ray Photon

M

L

K

Fluorescence Photon

Continuum Photoelectron Auger Electron

Hole

X−ray Photon Fluorescence Photon

Continuum

M

L

K

Hole

Photoelectron

Figure 12.1.11: Generation of

(a) fluorescence photon and (b)

Auger electron following x-ray

photon absorption.

Even though it is the most widely used one, transmission measurement is not
the only technique available for absorption spectroscopy. The other two techniques