An isotope of krypton has a half-life of 3 minutes. A sample of this isotope produces 1000 counts per minute in a Geiger counter. Determine the number of counts per minute produced after 15 minutes.
A)zero
B)15
C)30
D)60
E)1000

Question

Quizplus · Accepted Answer

The half-life of the isotope is 3 minutes, so after 15 minutes (which is 5 half-lives), the count rate will be reduced by a factor of 2^5 (32). Starting from 1000 counts per minute, the count rate after 15 minutes will be 1000/32 = 31.25, which rounds to 30 counts per minute.

An Isotope of Krypton Has a Half-Life of 3 Minutes