# A hospital uses americium-241 in smoke alarms fitted throughout its wards. Americium-241 is an alpha emitter with a half-life of 432 years. A physicist suggests that, despite being radioactive, americium-241 in smoke alarms poses very little health risk to patients. Explain why the physicist is correct, using your knowledge of the properties of alpha radiation and the half-life of americium-241.

> OCR A-Level Physics A (H556) — 6.3 Nuclear and particle physics · Explain · 5 marks

> Americium-241 is sealed inside the smoke alarm casing and continuously emits alpha particles into a small internal chamber. The alarm sounds when smoke disrupts the flow of alpha particles inside the chamber.

## Mark scheme (5 marks)

1. Alpha particles are highly ionising but have very low penetrating power / can only travel a few centimetres in air
2. Alpha radiation cannot penetrate the casing of the smoke alarm / cannot reach patients from inside the device
3. The long half-life of 432 years means the activity / count rate of the source remains low and roughly constant over time
4. The long half-life means the source will remain weakly radioactive for a very long time / does not need replacing
5. There is no risk of contamination to patients because the americium is sealed / contained within the device and irradiation from outside the casing is negligible

## Key terms

- [half-life](https://www.gradenine.co.uk/glossary/half-life)

## Related

- [Revision notes for OCR A-Level Physics A (H556)](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)

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Source: [GradeNine](https://www.gradenine.co.uk/q/a-hospital-uses-americium-241-in-smoke-2b33cd9e) · Published by Druglandscape Ltd.