When gammas of this energy pass through materials, either of two
interactions may occur. At the original energy of 0.662 MeV, the primary
effect is collision with the loosely bound electrons of the material with
a scattering (change in direction) and transfer of energy. As scattering
continues and the energy decreases, photoelectric absorption occurs in
which the gamma transfers all of its energy to a more tightly bound electron
and the electron leaves the atom which may result in some X radiation.
As evident from the above, the interaction is with the electrons in a material
and not the nucleus which contains most of the mass. Consequently the
Gauge actually measures the electron density of the material which is only
approximately related to the mass density. The relationship is the ratio of the
Z (atomic number or number of electrons per atom) and A (atomic mass of
the atom). The term Z/A is used frequently.
The process is further complicated by the probability that the interaction will
or will not occur. Atoms are mostly voids so many gammas will simply pass
through with no interaction.
The probability is a function of both the atomic number and the energy
of the gamma and is different for scattering and photoelectric absorption.
We will combine the two and call the resultant probability as the "mass
attenuation coefficient" or u/p.
The classic equation for the attenuation of gamma passing
through material is:
-L*p*u/p
I = Io * e
Where:
I = resulting intensity
Io = Initial intensity
L = path length
p = density of material
u/p = attenuation coefficient
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