The half-life of cobalt-60 is $5.3$ years, while that of strontium-90 is about 29 years. Suppose you have samples of both isotopes, and that they initially have the same activity (number of decays per second). What must be true of the numbers of cobalt-60 and strontium-90 nuclei in these samples?
A) There are more strontium-90 than cobalt-60 nuclei.
B) There are more cobalt-60 than strontium-90 nuclei.
C) There are equal numbers of cobalt-60 and strontium-90 nuclei.
D) It is not possible to compare numbers of nuclei without knowing the masses of the samples.

Question

Quizplus · Accepted Answer

Since both samples have the same activity but cobalt-60 has a shorter half-life, there must be more strontium-90 nuclei to compensate for its slower decay rate.

The Half-Life of Cobalt-60 Is 5.35.35.3 Years, While That of Strontium-90 Is About 29 Years

The Half-Life of Cobalt-60 Is $5.3$ Years, While That of Strontium-90 Is About 29 Years