Physics and Engineering of Radiation Detection

12.1. Spectroscopy of Photons 685

used are the so calledfluorescencespectroscopy andAuger electronspectroscopy. To
understand these methods, the reader is referred to Fig.12.1.11. The sketches drawn
here depict the possible relaxation modes of an atom after absorbing an x-ray photon.
Note that absorption of an x-ray photon, resulting in the emission of a photoelectron,
makes the atom unstable. To regain the stability one or more electronic transitions
take place quickly after the photoelectric absorption. Let us first concentrate on
Fig.12.1.11(a). Here the vacancy left behind by the photoelectron is shown to have
been filled by an electron from the L-shell. Since this electron was originally at
a higher energy level therefore, while making the transition to the lower level, it
releases the excess energy in the form of a fluorescence photon. Obviously the energy
of this photon is less than the energy of the original x-ray photon. The fluorescence
spectroscopy involves detecting these photons and deriving the absorption coefficient
from the measured fluorescence intensity. However now the absorption coefficient
for x-rays does not follow the relation 12.1.2. In fact the actual dependence is
fairly complicated due to the self absorption of these photons in the material. The
reader should note that now we are dealing with two attenuation coefficients, one
for the x-rays and the other for the fluorescence photons. The fluorescence photons,
having longer wavelengths as compared to x-ray photons, generally get absorbed
more quickly in the material. Nevertheless a very simple relationship between the
absorption coefficient for x-rays and the intensity of fluorescence photons exists and
is given by

μ(E)∝

If

I 0

. (12.1.10)

Note that hereIfrefers to the intensity of the fluorescence photons whileI 0 is the
intensity of incident x-rays. This relationship assumes little or no self absorption of
fluorescence photons and therefore should be used carefully while interpreting the
data. The most damaging effect of self absorption is the damping of the fine struc-
tures making the technique useless as far as spectroscopy is concerned. Fig.??shows
a basic setup designed for fluorescence and transmission spectroscopy of x-rays. Note
that, in terms of detection setup, fluorescence spectroscopy is more demanding than
its transmission counterpart. The main reason is that the fluorescence photons are
essentially emitted in all directions.

Ix

If

I 0

Collimator

Monochromator

X−rays

Sample

Figure 12.1.12: A basic setup for XAFS spectroscopy using trans-

mission and fluorescence techniques.

As mentioned earlier, another type of absorption spectroscopy is the Auger spec-
troscopy. It involves detection of the Auger electrons that are produced following