A smoke detector contains a small amount of a radioactive substance that emits alpha particles. The alpha particles ionise the air inside the detector, allowing a small electric current to flow. When smoke enters the detector, the current drops and the alarm sounds. Explain why alpha particles are suitable for use inside the smoke detector, and why this type of detector does not pose a significant radiation risk to people in the building.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Smoke detectors used in homes and offices contain a radioactive source. The source has a long half-life and emits alpha particles continuously.
Model answer (5 marks)
Alpha particles are highly ionising, producing many ion pairs in the air of the detector. This ionisation allows a small electric current to flow between the detector plates. The alpha particles have a very short range in air – only a few centimetres – so they are stopped by the plastic casing of the detector. Consequently they cannot reach people in the building. Even if a few escaped, alpha particles cannot penetrate the skin; dead skin cells absorb them, so they would not reach living tissue.
Examiner tips
- Mention strong ionisation and short range to justify current and safety
- Show that the detector’s plastic casing stops the particles
- Explain skin absorption to cover safety
- Use exam‑specific terminology such as ‘short range’ and ‘ionisation’
Common mistakes
- Confusing alpha particles with beta or gamma radiation
- Failing to state that the particles are stopped by the casing
- Overlooking the skin absorption point
Mark scheme (5 marks)
- Alpha particles are strongly ionising / cause strong ionisation of air
- This allows a current to flow through the ionised air between the detector plates
- Alpha particles have a very short range in air (a few centimetres)
- Alpha particles cannot penetrate the plastic casing of the detector / are stopped before reaching people
- Alpha particles cannot penetrate the skin / dead skin cells absorb alpha radiation, so even if some escaped they would not reach living tissue
Key terms in this question
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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