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Strontium-90 (90Sr) is a radioactive isotope of strontium, with a half life of 28.8 years. 90Sr undergoes beta decay with decay energy of 0.546 MeV to the yttrium isotope 90Y, which in turn undergoes beta decay with half life of 64 hours and decay energy 2.28 MeV for beta particles to 90Zr (zirconium), which is stable. Note that 90Sr/Y is almost a perfectly pure beta source, the gamma photon emission from the decay of 90Y is so weak that it can normally be ignored.
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90Sr finds extensive use in medicine and industry, as a radioactive source for thickness gauges and for superficial radiotherapy of some cancers. Controlled amounts of 90Sr and 89Sr can be used in treatment of bone cancer. As the radioactive decay of strontium-90 generates significant amount of heat, and is cheaper than the alternative 238Pu, it is used as a heat source in many Russian/Soviet radioisotope thermoelectric generators, usually in the form of strontium fluoride. It is also used as a radioactive tracer in medicine and agriculture. It is obtained during nuclear reprocessing of spent nuclear fuel.
90Sr is a product of nuclear fission. It is present in significant amount in spent nuclear fuel and in radioactive waste from nuclear reactors and in nuclear fallout from nuclear tests. For thermal neutron fission as in today's nuclear power plants, the fission product yield from U-235 is 5.8%, from U-233 6.8%, but from Pu-239 only 2.1%.
Together with cesium isotopes 134Cs, 137Cs, and iodine isotope 131I it was between the most important isotopes regarding health impacts after the Chernobyl disaster. Slightly elevated levels of 90Sr may be present in the vicinity of nuclear power plants.
Strontium has biochemical behavior similar to calcium. After entering the organism, most often by ingestion with contaminated food or water, about 70-80% of the dose gets excreted. Virtually all remaining strontium is deposited in bones and bone marrow, with the remaining 1% remaining in blood and soft tissues. Its presence in bones can cause bone cancer, cancer of nearby tissues, and leukemia. Exposition to 90Sr can be tested by a bioassay, most commonly by urinalysis.
In the vicinity of nuclear waste and nuclear test sites, strontium also enters the metabolism of plants in lieu of calcium. For example, specimens of chamisa growing in Bayo Canyon, near Los Alamos, New Mexico, exhibit a concentration of radioactive strontium 300,000 times higher than normal plants. Their roots reach into a nuclear waste treatment area that has been closed since 1963; the radioactive shrubs are "indistinguishable from other shrubs without a Geiger counter" . The same happens with the tumbleweed plants at the Hanford Site; "crews armed with pitchforks" are employed to prevent the contaminated plants from spreading .
Accidental mixing of radioactive sources containing strontium with metal scrap can result in production of radioactive steel. Discarded radioisotope thermoelectric generators are a major source of 90Sr contamination in the area of the former Soviet Union.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Strontium-90". A list of authors is available in Wikipedia.|