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.
Take the following
links to learn more about the three principle mechanisms of
ionization:
