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 pairproduction increases significantly for higher energy gammas.

Summarv

The important information in this chapter is summarized below.

Interaction of Radiation with Matter Summary

An alpha particle deposits a large amount of energy in a short distance of travel due to its large mass and charge.

Beta-minus particles interact with the electrons orbiting the nucleus of atoms, causing ionization by displacing the electrons. The beta particle loses energy with each interaction. After the beta particle loses enough energy, it is captured in the orbital shells of an atom.

Positrons interact with matter much the same way as beta minus particles. After the positron has lost most of its energy by ionizing atoms, it is annihilated by interaction with an electron. The electron-positron pair disappear and are replaced by two gammas, each with the energy equivalent of the mass of an electron (0.51 MeV).

Neutrons interact with matter by elastic scattering, inelastic scattering, or absorption.

Photoelectric effect is where a gamma interacts with an electron orbiting an atom. The entire energy of the gamma is transferred to the electron, and the electron is ejected from its orbit.

In Compton scattering a gamma interacts with an orbital electron, but only part of the gamma energy is transferred to the electron. The electron is ejected from its orbit, and the gamma is scattered off at a lower energy.

In pair-production, a gamma interacts with the electric field of a nucleus and is converted into an electron-positron pair. The gamma must have an energy greater than 1.02 MeV for this to occur.