Smoke detectors in homes use a radioactive source that emits alpha particles. Explain why alpha particles are suitable for use inside a smoke detector in a home, and why the detector does not pose a significant radiation hazard to the people living there.

Eduqas GCSE Physics — 9.3 Hazards and uses of radioactive emissions · Explain · 4 marks · View as Markdown

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

A smoke detector contains a small amount of a radioactive source. Under normal conditions, alpha particles ionise the air inside the detector, allowing a small electric current to flow. When smoke enters the detector, the current is interrupted and the alarm sounds.

Model answer (4 marks)

Alpha particles are highly ionising, so they can ionise the air inside the detector effectively to maintain the current.
Alpha particles have a very short range in air (a few centimetres), so they cannot travel far enough to reach people in the room.
Alpha particles are also stopped by the casing of the detector, so they cannot escape to irradiate people nearby.
If a more penetrating radiation (beta or gamma) were used instead, it would pass through the casing and through the body, posing a greater hazard to the occupants.

Examiner tips

  • Use the word ‘highly ionising’ to show understanding of ionisation. Mention the short range (few centimetres) and the protective casing. Contrast with beta/gamma to justify low hazard.
  • Use concise, exam‑style sentences and avoid unnecessary words.

Common mistakes

  • Confusing alpha with beta or gamma radiation. Claiming alpha particles can travel metres. Failing to mention the detector casing as a barrier.

Mark scheme (4 marks)

  1. Alpha particles are highly ionising, so they can ionise the air inside the detector effectively to maintain the current.
  2. Alpha particles have a very short range in air (a few centimetres), so they cannot travel far enough to reach people in the room.
  3. Alpha particles are also stopped by the casing of the detector, so they cannot escape to irradiate people nearby.
  4. If a more penetrating radiation (beta or gamma) were used instead, it would pass through the casing and through the body, posing a greater hazard to the occupants.

Key terms in this question

alpha particles

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