Strontium-90 is a radioactive isotope of strontium, with a half life of 29.1 years. Strontium-90 decays by beta decay with decay energy of 0.546 MeV to the isotope yttrium-90, which in turn undergoes beta decay with half time of 2.67 days and decay energy 1.54 MeV for beta particles to zirconium-90, 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.}

Strontium-90 finds extensive use in medicine and industry, as a radioactive source for thickness gauges and for superficial radiotherapy of some cancers. Controlled amounts of strontium-90 and strontium-89 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 plutonium-238, 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.

Strontium-90 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. Together with caesium-134, caesium-137, and iodine-131 it was between the most important isotopes regarding health impacts after the Chernobyl disaster. Slightly elevated levels of strontium-90 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 strontium-90 can be tested by a bioassay, most commonly by urinalysis.

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 strontium-90 contamination in the area of the former Soviet Union.

Isotopes