Commonly used gamma-emitting isotopes
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Radionuclides which emit gamma radiation are valuable in a range of different industrial, scientific and medical technologies. This article lists some common gamma-emitting radionuclides of technological importance, and their properties.
Many artificial radionuclides of technological importance are produced as fission products within nuclear reactors. A fission product is a nucleus with approximately half the mass of a uranium or plutonium nucleus which is left over after such a nucleus has been "split" in a nuclear fission reaction.
Caesium-137 is one such radionuclide. It has a half-life of 30 years, and decays by pure beta decay to a metastable state of barium-137 (137mBa). Barium-137m has a half-life of minutes and is responsible for all of the gamma ray emission. The ground state of barium-137 is stable.
The gamma ray (photon) energy of Ba-137m is about 662 keV. These gamma rays can be used, for example, in radiotherapy such as for the treatment of cancer, in food irradation, or in industrial gauges or sensors. Cs-137 is not widely used for industrial radiography as other nuclides, such as cobalt-60 or iridium-192 for example, offer higher radiation output for a given volume.
Iodine-131 is another important gamma-emitting radionuclide produced as a fission product. With a short half-life of 8 days, this radioisotope is not of practical use in radioactive sources in industrial radiography or sensing. However, since iodine is a component of biological molecules such as thyroid hormones, iodine-131 is of great importance in nuclear medicine, and in medical and biological research as a radioactive tracer.
Some radionuclides, for example cobalt-60 and iridium-192, are made by the neutron irradiation of normal non-radioactive cobalt and iridium metal in a nuclear reactor, creating radioactive nuclides of these elements which contain extra neutrons, compared to the original stable nuclides.
In addition to their uses in radiography, both cobalt-60 (Co-60) and iridium-192 (Ir-192) are used in the radiotherapy of cancer. Cobalt-60 tends to be used in teletherapy units as a higher photon energy alternative to Cs-137, while iridium-192 tends to be used in a different mode of therapy, internal radiotherapy or brachytherapy. The iridium wires for brachytherapy are a palladium-coated iridium/palladium alloy wire made radioactive by neutron activation. This wire is then inserted into a tumor such as a breast tumor, and the tumor is irradiated by gamma ray photons from the wire. At the end of the treatment the wire is removed.
A rare but notable gamma source is sodium-24, this has a very short half-life but it emits photons with very high energies (>2 MeV). It could be used for radiography of thick steel objects if the radiography occurred close to the point of production. In common with Co-60 and Ir-192 it is formed by the neutron activation of the commonly found stable isotope.
Many years ago radium-226 and radon-222 sources were used as gamma-ray sources for industrial radiography: for instance a radon-222 source was used to examine the mechanisms inside an unexploded V-1 flying bomb, while some of the early Bathyspheres could be examined using radium-226 to check for cracks. Because both radium and radon are very radiotoxic and very expensive due to their natural rarity, these natural radioisotopes have fallen out of use over the last half-century, replaced by artificially created radioisotopes.