A2 Daily A Level Physics question
In a school lab, a sealed radioactive source is placed next to a Geiger–Müller tube. With the source in place, the total count rate is 430 counts per minute. A separate background measurement (same setup, no source) is 30 counts per minute. The isotope’s half-life is 20 minutes. If the source is left in place and nothing else is changed, what total count rate is expected after 1.0 hour?
Answer
The correct answer is A.
Correct: A — About 80 counts per minute. A is correct because the net source count is 430 − 30 = 400 cpm, and after three half-lives (1 hour) it becomes 400/8 = 50 cpm; adding the unchanged 30 cpm background gives 80 cpm total. B is wrong because it divides the total (including background) by 8, ignoring that background does not decay. C is wrong because it finds the decayed net source (50 cpm) but forgets to add the background back in. D is wrong because it both divides the total by 8 and then adds background, effectively counting some background twice.