3
BASIC DEFINITIONS
ATTENUATION COEFFICIENT
Since, it is known that electromagnetic radiations interact with the atoms of the absorbing
material mainly by three phenomena given as;
Photoelectric Effect Compton Scattering Pair Production
Here we define a parameter as; the probability of interaction per unit length in a given
material is called the linear attenuation coefficient µ of that material.
Thus theoretically it can be defined as follows:
Where τ, δc and k are the photoelectric, Compton scattering and pair production
probabilities respectively.
The intensity of gamma rays is attenuated by a shielding material according as:
( )
Where I0 is the intensity in the absence of shielding material and I(x) is the intensity
after the insertion of a shield of thickness x in a fixed source detector geometry.
The above equation can also be written as follows:
( ) ( )
This is the equation of a straight line with slope equal to -µ. It should be noted that the
last two equations are based on the assumption that the scattered photons are
completely removed from the gamma rays beam. The quantity I(x) then gives what is
called the uncollided intensity.
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