# Explain why alpha particles are used as the radiation source in smoke alarms rather than beta particles or gamma rays.

> Edexcel A-Level Physics (9PH0) — 6.1 Circular motion and momentum · Explain · 5 marks

> Smoke alarms contain a small amount of the radioactive isotope americium-241, which has a half-life of 432 years. Under normal conditions, alpha particles emitted from the source ionise air particles between two electrodes, allowing a small current to flow. When smoke enters the alarm, the current drops and the alarm sounds.

## Mark scheme (5 marks)

1. Alpha particles are highly ionising
2. Alpha particles ionise the air particles between the electrodes, allowing a current to flow
3. Alpha particles are weakly penetrating / stopped by a few centimetres of air
4. Gamma rays or beta particles are too penetrating and would pass through the alarm casing / would not be stopped by smoke, so could not trigger the alarm effectively
5. The long half-life of americium-241 (432 years) means the activity remains approximately constant over the lifetime of the alarm, so it does not need replacing

## Key terms

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

## Related

- [Revision notes for Edexcel A-Level Physics (9PH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/explain-why-alpha-particles-are-used-c1c9b3fa) · Published by Druglandscape Ltd.