Americium-241 is used as the radioactive source in smoke alarms. It is an alpha emitter with a half-life of 432 years. Explain why Americium-241 is well suited for use in a smoke alarm, and explain how a smoke alarm using this source detects smoke.

OCR A-Level Physics A (H556) — 6.1 Capacitors · Explain · 5 marks · View as Markdown

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

Americium-241 is an alpha-emitting radioactive isotope with a half-life of 432 years. It is used as the radioactive source inside domestic smoke alarms.

Model answer (5 marks)

Alpha particles are highly ionising but have very low penetration – a few centimetres of air or a thin sheet of paper stops them. This means the source does not expose people to radiation outside the alarm.

The isotope has a very long half‑life (432 yr). Over the life of a typical alarm the activity changes only slightly, giving a stable, reliable count rate and eliminating the need for frequent replacement.

In the alarm the alpha particles ionise the air between two electrodes, producing a small electric current.

When smoke enters the chamber the particles are absorbed or scattered, reducing the ionisation and the current. The drop in current triggers the alarm.

Examiner tips

  • Use the command word ‘explain’ – give reasons and describe the process.
  • Mention both the low penetration and the long half‑life as safety and longevity factors.
  • Show the ionisation–current–alarm sequence clearly.
  • Keep the answer concise – 5 marks, not a paragraph of explanation.

Common mistakes

  • Confusing alpha with beta or gamma radiation; forgetting the low penetration point.
  • Failing to link the long half‑life to a stable activity and no need for replacement.
  • Not describing the ionisation of air and the current drop when smoke is present.

Mark scheme (5 marks)

  1. Alpha radiation is the least penetrating type of radiation / is stopped by a few centimetres of air, so it does not travel far from the source and does not pose a significant radiation hazard to people nearby.
  2. The long half-life of 432 years means the source remains active for a very long time and does not need to be replaced.
  3. The long half-life means the activity / count rate changes very little over time, so the source gives a steady, reliable output of alpha particles.
  4. In normal conditions, alpha particles emitted from the source ionise the air between the detector plates, allowing a small electric current to flow.
  5. When smoke enters the alarm, smoke particles absorb / block the alpha particles, reducing ionisation of the air, so the current drops / falls and the alarm sounds.

Key terms in this question

half-life

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