# 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.

> OCR GCSE Physics A: Gateway Science (J249) — P4.4 Uses and hazards of radiation · Explain · 4 marks

> 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.

## Mark scheme (4 marks)

1. Alpha radiation has a very short range in air (a few centimetres) and cannot penetrate the casing of the detector
2. Alpha radiation is stopped by a thin layer of material / skin and so cannot penetrate into the body from an external source
3. The source is sealed / enclosed inside the detector so the americium cannot be inhaled or ingested
4. 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

- [alpha radiation](https://www.gradenine.co.uk/glossary/alpha-radiation)
- [ionising](https://www.gradenine.co.uk/glossary/ionising)

## Related

- [Revision notes for OCR GCSE Physics A: Gateway Science (J249)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-smoke-detectors-that-use-d07c7ba4) · Published by Druglandscape Ltd.