Krypton-85

Krypton-85

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

It decays into rubidium-85, with a half-life of 10.756 years and a maximum decay energy of 0.687 MeV. Its most common decay (99.57%) is by beta particle emission with maximum energy of 687 keV and an average energy of 251 keV. The second most common (0.43%) is by beta particle emission (maximum energy of 173 keV) followed by gamma ray emission (energy of 514 keV). Other decay modes have very small probabilities and emit less energetic gammas. The only other long-lived radioisotope of krypton is krypton-81 with a 210,000 year half-life; others have half-lives of less than two days.

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 cubic meter). However, since the mid-1940s, much larger quantities have been artificially produced as a product of nuclear fission. When uranium-235, or another fissile nucleus fissions, it usually splits into two large fragments (fission products) 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%). This is only about 20% of the total fission product of mass 85, as most decay from a short-lived excited state of 85Kr directly to 85Rb without passing through the longer-lived nuclear isomer.

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. The 1979 accident at the Three Mile Island nuclear power plant released about 50,000 curies of Kr-85 into the atmosphere

and the Chernobyl accident released about 5 million curies. 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, according to the ANL factsheet.

For wide-area atmospheric monitoring, krypton-85 is the best indicator for clandestine plutonium separations.

A large nuclear power plant produces about 300,000 curies of the isotope per year, most or all retained in the spent nuclear fuel rods. Nuclear reprocessing currently releases Kr-85 to the atmosphere when the spent fuel is dissolved. It would also be possible to capture and store it as nuclear waste or for use.

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.

The sealed spark gap assemblies contained in ignition excitors used in some older Turbine/Jet engines contain a very small amount of Krypton 85 in order to obtain consistent ionization levels and uniform operation. The amount of radiation from the average gap is approximatley the same as that of a radium-dialed wrist watch but should be handled carefully.

Kr 85 is used in indicator lights in appliances such as clothes washers and dryers, stereos, and coffee makers; used to gauge the thickness of thin plastics and sheet metal, rubber, textiles and paper, and to measure dust and pollutant levels.

See also

Footnotes

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