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A radioactive source or radiation source is a sample of a radionuclide, and emits ionizing radiation (one or more of gamma rays, alpha particles, beta particles, and neutron radiation). Generally, sources are used for irradiation, where the radiation performs a significant ionising function on a target material, or as a metrology calibration source, which is used for the calibration of radiometric process and radiation protection instrumentation. They can be sealed in a container or on a surface, or they can be free to move within a fluid.
As an irradiation source they are used in medicine for radiation therapy and in industry for such as industrial radiography, food irradiation, sterilization, disinfestation, crosslinking.
Radionuclides are chosen according to the type and character of the radiation they emit, intensity of emission, and the half-life of their decay. Common source radionuclides include cobalt-60, iridium-192, and strontium-90. The SI measurement quantity of source activity is the Becquerel, though the historical unit Curies is still in partial use, such as in the USA, despite the USA NIST strongly advising the use of the SI unit. The SI unit for health purposes is mandatory in the EU.
An irradiation source typically lasts for between 5 and 15 years before its activity drops below useful levels. However sources with long half-life radionuclides when utilised as calibration sources can be used for much longer.
Disposal
Disposal of expired radioactive sources presents similar challenges to the disposal of other nuclear waste, although to a lesser degree. Spent low level sources will sometimes be sufficiently inactive that they are suitable for disposal via normal waste disposal methods — usually landfill. Other disposal methods are similar to those for higher-level radioactive waste, using various depths of borehole depending on the activity of the waste.
A notorious incident of neglect in disposing of a high level source was the Goiânia accident, which resulted in several fatalities.