A2 Daily A Level Physics question
In a lab, a sealed radioactive source is kept at a fixed distance from a Geiger–Müller tube. After subtracting background, the count rate drops from 800 counts min⁻¹ to 200 counts min⁻¹ in 12 hours. What is the source’s half-life?
Answer
The correct answer is A.
Correct: A — 6 hours. A two halvings occur: 800 → 400 → 200 in 12 hours, so each half-life is 12/2 = 6 hours. B 3 hours treats 25% as a quarter of the time, ignoring that decay proceeds in equal halving intervals, not proportional time. C 4 hours assumes a linear drop (splitting the 600 decrease into three equal 200 steps), which is not how radioactive decay behaves. D 12 hours wrongly equates the time to reach 25% with one half-life, but 25% corresponds to two half-lives.