Strontium-90 (90Sr) is a radioactive isotope of strontium, with a half life of 28.8 years. Natural strontium is nonradioactive and nontoxic, but 90Sr is a radioactivity hazard. 90Sr undergoes β− decay with decay energy of 0.546 MeV to an electron and the yttrium isotope 90Y, which in turn undergoes β− decay with half life of 64 hours and decay energy 2.28 MeV for beta particles to an electron and 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.