90Sr

90Sr, or strontium-90, is a radioactive isotope of strontium with mass number 90. It is produced primarily as a fission product in nuclear reactors and in nuclear weapons tests.

The nucleus decays by beta emission to 90Y, with a half-life of about 28.8 years. 90Y then decays to stable 90Zr with a half-life of roughly 64 hours. The beta decay of 90Sr has an endpoint energy of about 0.546 MeV, while 90Y emits higher-energy beta radiation with an endpoint near 2.28 MeV. 90Sr itself is a pure beta emitter with little gamma radiation.

Chemically, strontium behaves similarly to calcium, so when 90Sr is ingested it tends to be incorporated into bone and teeth. Biological retention in bone is relatively long, which can lead to a prolonged internal radiation dose, especially to the bone marrow. External exposure from 90Sr is less penetrating but still requires shielding and controls to limit intake and contamination.

Occurrence and risk: 90Sr is a notable fission product present in nuclear fallout and contaminated environments. It is monitored in air, water, soil, and food after nuclear events, and in routine waste streams from reactors.

Applications and regulation: 90Sr has been used as a radiation source in medical and industrial devices, including ophthalmic brachytherapy sources that employ Sr-90/Y-90. Regulation treats 90Sr as a radioactive material requiring appropriate shielding, handling, and waste management. Health risks from internal exposure include elevated risks of bone cancer and leukemia due to irradiation of bone marrow.