A2 Daily A Level Physics question
In a school lab, a GM tube at fixed distance records a background of 20 counts per minute (cpm). With a sealed iodine-131 source in place at the same geometry, the total count rate at t = 0 is 340 cpm. The isotope’s half-life is 8.0 days. Assuming no change in setup or absorption, what total count rate is expected 24 days later?
Answer
The correct answer is D.
Correct: D — 60 counts per minute. Over 24 days there are three half-lives (24/8 = 3), so the source’s corrected count rate falls by a factor of 8: (340 − 20) cpm → 320/8 = 40 cpm; adding background back gives 40 + 20 = 60 cpm total. A 40 cpm is the corrected rate only; it omits adding the background to get the total. B 20 cpm treats the activity as having dropped to background or subtracts background again, which is incorrect. C 100 cpm results from halving only twice (treating 24 days as two half-lives) and then adding background. D matches three half-lives (divide by 8) and then adds the background to give the total rate.