The decay daughter of strontium 90, yttrium 90 (Y), is also a beta decay nuclide. The maximum energy of beta rays is 2.288 megaelectron volts and the half-life is 64.1 hours. Due to the short half-life of yttrium-90 and the high energy of beta rays, the content of strontium-90 is often calculated by measuring the radioactivity of yttrium-90 that is in radioactive equilibrium with strontium-90. The time for strontium-90 and yttrium-90 to reach radioactive equilibrium is about 25 days. . Strontium 90 weighs 3.7×10 becquerels 7.19×10 mg. Strontium-90 is a highly toxic nuclide. It mainly accumulates in bones and is difficult to excrete. Its effective half-life for the human body is about 16 years. The maximum allowable accumulation amount in the human body is 7.4×10 Bq. The maximum allowable concentrations of strontium-90 in the air of radioactive workplaces and in open water sources are 3.7×10 and 2.6 Bq/L respectively.
Strontium-90 is one of the fission products of uranium-235, with a yield of 5.90%. Large amounts of strontium-90 are separated from highly radioactive waste streams from the recovery of uranium and plutonium as a by-product of nuclear fuel reprocessing plants.