Home - Education Resources - Science of NDT - Radiography


After reading this section you will be able to:

  • Explain the process of ionization.

What does the process of ionization entail?

Radiation is absorbed by the material it penetrates by a process known as ionization. Radiation creates ions in the material that it passes through, and some or all of the radiation energy is lost during this process. An ion is an atom, group of atoms, or a particle with a positive or negative charge. Ionization is any process that changes the electrical balance within an atom. If we remove an electron from a stable atom, the atom becomes electrically incomplete. That is, there are more protons in the nucleus (positive charges) than there are electrons (negative charges). With an electron removed, the atom possesses a plus one charge, therefore it is a positive ion. Consequently, the liberated electron is a negative ion, as long as it exists by itself and does not combine with another atom.

Remember from our discussion on atomic structure, that atoms are held together by a binding energy. This means that the electrons are held in their orbital shells by a quantity of binding energy. In order to liberate an electron from its parent atom, it will take energy that is at least equal to the binding energy that holds the electron.

When X and gamma ray photons penetrate matter, they interact with the atomic particles in the material and are said to be absorbed by the material. This absorption can result due to ionization or bremsstruhlung processes. The Bremsstruhlung mechanism was covered in the material on X-ray generation and, therefore, will not be re-addressed here.

The three mechanism of ionization

The amount of ionization that occurs is dependent upon two principle factors, (1) the radiation energy, and (2) the type of material for which the radiation is interacting. For a given material, the level of ionization will vary with varying levels of radiation energy. There are three principle mechanisms of ionization that are of interest in radiography. These include the Photoelectric effect, Compton effect, and Pair Production.



  1. The three principle level of ionization are the Photoelectric effect, the Compton Effect, and Pair-Production.
  2. This process of radiation absorption is called ionization.