<Tr> <Th> Decay mode </Th> <Th> Decay energy </Th> </Tr> <Tr> <Td> Beta decay </Td> <Td> 0.546 MeV </Td> </Tr> <P> Strontium - 90 (90 Sr) is a radioactive isotope of strontium produced by nuclear fission, with a half - life of 28.8 years . It undergoes β decay into yttrium - 90, with a decay energy of 0.546 MeV . Strontium - 90 has applications in medicine and industry and is an isotope of concern in fallout from nuclear weapons and nuclear accidents . </P> <P> Naturally occurring strontium is nonradioactive and nontoxic at levels normally found in the environment, but Sr is a radiation hazard . Sr undergoes β decay with a half - life of 28.79 years and a decay energy of 0.546 MeV distributed to an electron, an anti-neutrino, and the yttrium isotope Y, which in turn undergoes β decay with half - life of 64 hours and decay energy 2.28 MeV distributed to an electron, an anti-neutrino, and Zr (zirconium), which is stable . Note that Sr / Y is almost a pure beta particle source; the gamma photon emission from the decay of Y is so infrequent that it can normally be ignored . </P>

The element 90sr undergoes beta decay to form which isotope