A2 Daily A Level Physics question

2026-06-19 OCR A Nuclear physics (A2): activity, decay law (qual.), half-life OCR A Module 6.4 Nuclear physics: Activity and decay law; half-life (qualitative)

In a school lab, a GM tube measures background radiation at 40 s⁻¹. With a sealed source at a fixed distance, the initial total count rate is 1040 s⁻¹. Several hours later, under identical setup, the total count rate is 140 s⁻¹. Assuming the detector response and geometry are unchanged, which statement must be true about the time elapsed?

A About three half-lives, since the total count fell by roughly a factor of 8.
B Exactly four half-lives, with the activity now at about one‑sixteenth of the start.
C Two half-lives, because 140 s⁻¹ is close to a quarter of 1040 s⁻¹.
D Between three and four half-lives, since the net source count fell from 1000 s⁻¹ to 100 s⁻¹ (one‑tenth). (correct)

Answer

The correct answer is D.

Correct: D — Between three and four half-lives, since the net source count fell from 1000 s⁻¹ to 100 s⁻¹ (one‑tenth). Subtracting background gives a factor of 1/10; three half-lives give 1/8 and four give 1/16, so the time lies between these. A ignores the background and uses total counts, giving a misleading factor; after correction the fall is 1/10, not about 1/8. B assumes a one‑sixteenth reduction, but the net source rate is 1/10 of the initial, so fewer than four half-lives have passed. C both misjudges the factor and ignores background; 100 is not a quarter of 1000, so it cannot be two half-lives. D correctly uses background subtraction and half‑life ratios to bound the time between three and four half‑lives.

Try today's guest Daily MCQ A Level Physics MCQ practice Problem-solving previews