Krypton-85

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Krypton 85 (⁸⁵Kr) is a radioisotope of krypton.

It decays by beta decay into rubidium-85, with a half-life of 10.756 years and a maximum decay energy of 0.687 MeV. (Actually, a 687 keV electron - beta particle - and a 514 keV photon - gamma ray.) It is one of two unstable krypton isotopes which have relatively long half-lives – all of its other radiosotopes have half-lives of less than two days. The other isotope is krypton-81 with a 210,000 year half-life.

Krypton-85 is produced in small quantities by the interaction of cosmic rays with the stable krypton-84 (which is present in concentrations of about 1 cm³ per metre³). However, since the mid-1940s, much larger quantites have been artificially produced as a product of nuclear fission. When uranium, or another fissile nucleus fissions, it usually splits into two large fragments with mass numbers around 90-140, and two or three neutrons. About three atoms of krypton-85 are produced for every 1000 fissions (i.e. it has a fission yield of 0.3%).^[1]

About 5 million curies of the isotope was released into the atmosphere as a result of nuclear weapons tests between 1945 and the end of atmospheric testing in 1962. A large nuclear power plant produces about 300,000 curies of the isotope per year, usually retained in the fuel assemblies and released into the atmosphere if the spent elements are reprocessed. The 1979 accident at the Three Mile Island nuclear power plant released about 50,000 curies into the atmosphere. ^[1]

The atmospheric concentration of krypton-85 peaked in around 1970, when it reached around 10 picocuries per metre³. Since then the cessation of atmospheric weapons tests and the reduced production of plutonium has, because of the short half life of the isotope, led to a sharp reduction in the atmospheric concentration.

[edit] Uses in Industry

It is used for indicator lights in appliances such as stereos, and in arc discharge lamps commonly used in the entertainment industry for large HMI film lights as well as most moving lights. It is also used to detect leaks in piping.