Introduction
Electromagnetic waves like gamma rays and X-rays when pass through material medium, loses
energy. The energy loss can be through photo-electric effect, through Compton scattering,
through pair production or through Rayleigh scattering. All these channels of energy loss are
dominant at certain energies. Gamma rays are the most energetic in nature because they arise
from nuclear transitions. They can pass through very large thicknesses and can damage the living
tissues. Thus protection is required against them because these rays can be very harmful if the
body is exposed to a high dose of them in research labs and nuclear reactors. The protection
could be provided by material in which the energy loss of these rays is maximum. The loss is
determined by the attenuation coefficient of the material. The attenuation coefficient has
contribution coming from all of the above discussed processes. Lets discuss all of them briefly.
Simple Scattering (Rayleigh Scattering).
Rayleigh scattering (named after Lord Rayleigh) is the elastic scattering of light or other
electromagnetic radiation by particles much smaller than the wavelength of the light. It can occur
when light travels in transparent solids and liquids, but is most prominently seen in gases.
The incident photon energy is much less than the binding energy of the electron in an atom. The
photon is scattered without change of energy. But Raleigh scattering is important for low energy
photons and high Z material. It is the common practice to ignore the Rayleigh scattering in
shielding calculation.
Photoelectric Effect
The photoelectric effect, in which the photon disappears, is an interaction between a photon and
a tightly bound electron whose binding energy is equal to or less than the energy of the photon.
The primary ionizing particle resulting from this interaction is the photoelectron, whose energy is
given by as
EPhotoelectron = h f − φ.
The photoelectron dissipates its energy in the absorbing medium mainly by excitation and
ionization. The binding energy φ is transferred to the absorber by means of the fluorescent
radiation that follows the initial interaction. These low-energy photons are absorbed by outer
electrons or in other photoelectric interactions not far from their points of origin. The
photoelectric effect is favored by low-energy photons and high-atomic-numbered absorbers. The
cross section for this reaction varies approximately as Z4λ3 (Z4/E 3). It is this very strong
dependence of photoelectric absorption on the atomic number Z that makes lead such a good
material for shielding against X-rays. For very low-atomic-numbered absorbers, the photoelectric
effect is relatively unimportant. The following fig. depicts the process
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