Definition of strontium 90
: a heavy radioactive isotope of strontium of mass number 90 that has a half-life of 29 years, that is present in nuclear waste and fallout, and that is hazardous because like calcium it can be assimilated in biological processes and deposited in the bones of human beings and animals
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Merriam-Webster unabridged
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Time Traveler for strontium 90
The first known use of strontium 90 was in 1952
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