Commonly-used gamma emitting isotopes

Fission products
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.

Activation products
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.

Minor actinides
Americium-241 has been used as a source of low energy gamma photons, it has been used in some applications such as portable X-ray fluorescence equipment (XRF).

Natural radioisotopes
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. Due to the fact that 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.

Cyclotron generated
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.