Explain why smoke detectors that use americium-241, an alpha emitter, are considered safe for household use, despite alpha radiation being the most ionising type of radiation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Americium-241 is a radioactive isotope used inside domestic smoke detectors. It emits alpha radiation, which ionises air inside the detector chamber. When smoke enters the chamber, it disrupts the ionisation and triggers the alarm. Americium-241 has a half-life of around 430 years.
Model answer (4 marks)
Alpha radiation has a very short range in air (a few centimetres) and cannot penetrate the casing of the detector.
Alpha radiation is stopped by a thin layer of material or by the skin, so it cannot enter the body from an external source.
The americium source is sealed inside the detector, preventing inhalation or ingestion.
Alpha particles would only be hazardous if the source were inside the body, because they are highly ionising only at very close range to tissue.
Alpha radiation is stopped by a thin layer of material or by the skin, so it cannot enter the body from an external source.
The americium source is sealed inside the detector, preventing inhalation or ingestion.
Alpha particles would only be hazardous if the source were inside the body, because they are highly ionising only at very close range to tissue.
Examiner tips
- Use the exact wording from the mark scheme (e.g. "short range in air", "sealed inside the detector").
- Show the logical sequence: range → shielding → containment → risk only if internal.
- Keep each point concise and separate them clearly.
Common mistakes
- Claiming alpha radiation is harmless in all situations; ignore the internal hazard condition.
- Forgetting to mention the detector’s sealed design or the short range of alpha particles.
Mark scheme (4 marks)
- Alpha radiation has a very short range in air (a few centimetres) and cannot penetrate the casing of the detector
- Alpha radiation is stopped by a thin layer of material / skin and so cannot penetrate into the body from an external source
- The source is sealed / enclosed inside the detector so the americium cannot be inhaled or ingested
- Alpha radiation would only be hazardous if the source were inside the body (e.g. inhaled or swallowed), because alpha particles are highly ionising at close range to tissue
Key terms in this question
Related
- All OCR GCSE Physics A: Gateway Science (J249) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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