Radioactive fallout One of the problems of in-air testing of nuclear weapons (or

Question

Radioactive fallout One of the problems of in-air testing of nuclear weapons (or, even worse, the use of such weapons!) is the danger of radioactive fallout. One of the most problematic nuclides in such fallout is strontium - 90 (90Sr), which breaks down by beta- decay with a half-life of 28 years. It is chemically similar to calcium and therefore can be incorporated into bones and teeth, where, due to its rather long half-life, it remains for years as an internal source of radiation. What is the daughter nucleus of the 90Sr decay? What percent of the original level of 90Sr is left after 56 years? How long would you have to wait for the original level to be reduced to 0.195% of its original value?

Explanation / Answer

A) 39Y is daughter (mass no=90) B) after 56 yrs i.e 2 half lives 25% will be left C) k= 0.693/t1/2 = 0.693/28 =0.02475 t= 1/k x ln(a/a-x) = 1/0.02475 x ln(100/0.195) =252.118 yrs time equired = 252.118 yrs

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