of Radiation for Industrial Radiography
Industrial radiography uses two sources of radiation: X-radiation
and Gamma radiation. X-rays and Gamma rays differ only in their source of origin. X-rays
are produced by an X-ray generator, and Gamma radiation is the
product of radioactive atoms. An in depth discussion on radiation
production can be found in other areas of this site, but will
be reviewed briefly in the following sections.
are two different atomic processes that can produce X-ray photons. One process produces Bremsstrahlung radiation
and the other produces K-shell or
characteristic emission. Both processes involve a change in the
energy state of electrons. X-rays are generated when an electron
is accelerated and then made to rapidly decelerate, usually due
to interaction with other atomic particles.
In an X-ray system, a large amount of electric current is passed
through a tungsten filament, which heats the filament to several
thousand degrees centigrade to create a source of free electrons.
A large electrical potential is established between the filament
(the cathode) and a target (the anode). The cathode and anode
are enclosed in a vacuum tube to prevent the filament from burning
up and to prevent arcing between the cathode and anode. The electrical
potential between the cathode and the anode pulls electrons from
the cathode and accelerates them as they are attracted towards
the anode or target, which is usually made of tungsten. X-rays
are generated when free electrons give up some of their energy
when they interact with the electrons or nucleus of an
atom. The interaction of the electrons in the target results in
the emission of a continuous Bremsstrahlung spectrum and also
characteristic X-rays from the target material.
Production of Gamma Rays
Gamma radiation is the product of radioactive atoms. Depending
upon the ratio of neutrons to protons within its nucleus, an isotope
of a particular element may be stable or unstable. Over time, the
nuclei of unstable isotopes spontaneously disintegrate, or transform,
in a process known as radioactive decay. Various types of radiation
may be emitted from the nucleus and/or its surrounding electrons
when an atom experiences radioactive decay. Nuclides which undergo
radioactive decay are called radionuclides. Any material which
contains measurable amounts of one or more radionuclides is a
There are many naturally occurring radioactive materials, but
manmade radioactive isotopes or radioisotopes are used for industrial
radiography. Man-made sources are produced by introducing an extra
neutron to atoms of the source material. For example, Cobalt-60
is produced by bombarding a sample of Cobalt-59 with an excess
of neutrons in a nuclear reactor. The Cobalt-59 atoms absorb some
of the neutrons and increase their atomic weight by one to produce
the radioisotope Cobalt-60. This process is known as activation.
As a material rids itself of atomic particles to return to a balance
state, energy is released in the form of Gamma rays and sometimes
alpha or beta particles.
Physical size of isotope materials will very slightly between
manufacturer, but generally an isotope is a pellet that measures 1.5 mm x 1.5
mm. Depending on the activity (curies) desired, a pellet or pellets
are loaded into a stainless steel capsule and sealed. Unlike X-ray
tubes, radioactive sources provide a continual source of radiation
that cannot be turned off. Once radioactive decay starts, it
continues until all of the atoms have reached a stable state.
The radioisotope can only be shielded to prevent exposure to the
radiation. In industrial radiography, the instruments that are
used to shield the radioisotope so that they can be safely handled
and used are commonly called cameras or exposure devices. Exposure
devices will be discussed later in more detail.