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The Collaborative International Dictionary
radiostrontium

Strontium-90 \Strontium-90\ A radioactive isotope of strontium produced by certain nuclear reactions, and constituting one of the prominent harmful components of radioactive fallout from nuclear explosions; also called radiostrontium. It has a half-life of 28 years.

Wiktionary
radiostrontium

n. any radioactive isotope of strontium, especially (nuclide 90 38 Sr)