Physics and Engineering of Radiation Detection

2.3. Interaction of Photons with Matter 101

Fig.2.3.9 shows the mass attenuation coefficients of potassium and lead. As one
would expect, the attenuation coefficient decreases with increase in energy. However
at certain energies there are abrupt jumps in attenuation coefficients. These jumps
occur when the energy of the photon matches one of the binding energies of the atom.
At that energy the absorption of the photons increases abruptly and the attenuation
coefficient jumps to a higher value. These abrupt changes in attenuation coefficients
has been exploited in radiation imaging techniques to enhance image contrast.

Energy(MeV)

10 -3 10 -2 10 -1 1 10

2 /g)

(cmm

μ

10 -1

1

10

102

103

Pb

K

Figure 2.3.9: Depen-

dence of mass attenua-

tion coefficients of potas-

siumandleadonenergy.

B.1 MeasuringAttenuationCoefficient

To see how one can determine the attenuation coefficient experimentally, we note
that according to equation 2.3.33 the total linear attenuation coefficient can be
written as

μt=

1

d

ln

(

I 0

I

)

. (2.3.43)

Hence, to determine the linear attenuation coefficient we just need to measure the
incoming and the outgoing intensities of a photon beam that passes through a slab
of thicknessdof the material. Such an experimental setup is shown in Fig.2.3.10.
The intensitiesI 0 andIgoing in and coming out of the material can be measured
by two detectors installed before and after the material. Due to the energy de-
pendence of the attenuation coefficient, either the source should be able to provide
monochromatic beam or a monochromator setup should be used. The choice of the
source depends on the application. For composite materials, such as body organs,
radioactive sources are generally preferred due to the ease in their deployment and
use. For more accurate measurements, high intensity x-ray beams, such as the ones
provided by synchrotron sources are used. However, since the x-ray spectra of such
sources is quite broad, therefore the setup must also include a monochromator to
selectively use photon beam of the desired wavelength.