Production 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.
Production of X-Rays
There 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 radioactive material.
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.