Caesium-137 is a radioactive isotope which is formed mainly by nuclear fission. It has a half-life of 30 years, and decays by pure beta decay to a metastable state of barium-137 (Ba-137m). 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 photon energy of Ba-137m is about 662 keV. These photons can be used in food irradation, or in cancer treatment. Cs-137 is not widely used for industrial radiography as other isotopes offer higher gamma activities for a given volume. For instance, cobalt-60 and iridium-192 can be made by the neutron irradiation of normal non-radioactive cobalt and iridium metal.
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 (brachytherapy). The iridium wires for brachythereapy 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 photon 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 expensive), these natural radioisotopes have fallen out of use. For industrial radiography, iridium-192 and cobalt-60 can be used as photon sources as both of these are much shorter lived than radium-226 and are less radiotoxic.
Iodine-125 is used in a variety of biochemical assays. Often, 125I is conjugated to the tyrosine of a target protein. This protein can then be followed by monitoring the gamma radiation. With the advent of non-radioactive fluorogenic labels, the use of Iodine-125 has fallen off.