INTERACTION OF RADIATION WITH MATTER
Atomic and Nuclear Physics
Gamma radiation is electromagnetic radiation. It is commonly referred to as a gamma ray and
is very similar to an x-ray. The difference is that gamma rays are emitted from the nucleus of
an atom, and x-rays are produced by orbiting electrons. The x-ray is produced when orbiting
electrons move to a lower energy orbit or when fast-moving electrons approaching an atom are
deflected and decelerated as they react with the atom's electrical field (called Bremsstrahlung).
The gamma ray is produced by the decay of excited nuclei and by nuclear reactions. Because
the gamma ray has no mass and no charge, it is difficult to stop and has a very high penetrating
power. A small fraction of the original gamma stream will pass through several feet of concrete
or several meters of water.
There are three methods of attenuating gamma rays. The first method is referred to as the
photo-electric effect. When a low energy gamma strikes an atom, the total energy of the gamma
is expended in ejecting an electron from orbit. The result is ionization of the atom and expulsion
of a high energy electron. This reaction is most predominant with low energy gammas
interacting in materials with high atomic weight and rarely occurs with gammas having an energy
above 1 MeV. Annihilation of the gamma results. Any gamma energy in excess of the binding
energy of the electron is carried off by the electron in the form of kinetic energy.
The second method of attenuation of gammas is called Compton scattering. The gamma interacts
with an orbital or free electron; however, in this case, the photon loses only a fraction of its
energy. The actual energy loss depending on the scattering angle of the gamma. The gamma
continues on at lower energy, and the energy difference is absorbed by the electron. This
reaction becomes important for gamma energies of about 0.1 MeV and higher.
At higher energy levels, a third method of attenuation is predominant. This method is
pair-production. When a high energy gamma passes close enough to a heavy nucleus, the gamma
completely disappears, and an electron and a positron are formed. For this reaction to take place,
the original gamma must have at least 1.02 MeV energy. Any energy greater than 1.02 MeV
becomes kinetic energy shared between the electron and positron. The probability of pair-
production increases significantly for higher energy gammas.