A student investigates the count rate from a radioactive source over several weeks. The student notices that the count rate decreases over time and never reaches zero. Explain why the count rate decreases over time and why it never reaches zero.

AQA A-Level Physics (7408) — 3.8.1 Radioactivity · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

Radioactive decay is a random process – each nucleus decays at a random time.

Because the number of undecayed nuclei decreases, the probability of a decay occurring in a given second falls, so the count rate measured by the detector decreases.

However, the detector always records some background radiation, which comes from natural sources such as cosmic rays, terrestrial radioisotopes in the rocks and radon gas. This background count rate is constant, so the total count rate never falls to zero.

Examiner tips

  • Use the term ‘random’ to describe decay; link it to the decreasing number of undecayed nuclei. Include the constant background radiation as the reason the rate never reaches zero. Show the logical sequence: random decay → fewer nuclei → lower rate, but background remains.

Common mistakes

  • Failing to mention that decay is random. Confusing the decrease with the source being exhausted. Omitting the background radiation explanation.

Mark scheme (5 marks)

  1. Radioactive decay is random / the nucleus decays at random
  2. Each decay reduces the number of undecayed nuclei remaining
  3. Fewer undecayed nuclei means fewer decays per second, so count rate falls
  4. There is always background radiation present (which is always detected)
  5. Background radiation comes from natural sources in the environment (e.g. cosmic rays / rocks / radon gas)

Key terms in this question

count rate

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