Radioactive decay: The half-life of cobalt-60 is 5.3 years, while that of strontium-90 is 28 years. Suppose that samples of cobalt-60 and strontium-90 are such that they initially have the same activity (number of decays per second). What is true about the initial numbers of cobalt-60 and strontium-90 nuclei in these samples?

Question

Quizplus · Accepted Answer

Given that both samples have the same activity initially, and knowing that the activity (number of decays per second) of a radioactive sample is directly proportional to the number of radioactive nuclei present and inversely proportional to the half-life, a shorter half-life (like that of cobalt-60) means its nuclei decay faster than those of strontium-90. Therefore, to have the same activity, there must be more strontium-90 nuclei to compensate for its longer half-life.

Radioactive Decay: the Half-Life of Cobalt-60 Is 5