Sr - Strontium 90 - periodic table of elements

Strontium-90 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.