# Americium-241 is a radioactive isotope used in smoke alarms. It is an alpha emitter with a half-life of 432 years. Explain why Americium-241 is suitable for use in a smoke alarm, and explain why alpha emission rather than gamma emission makes it safe for household use.

> OCR A-Level Physics B: Advancing Physics (H557) — 6.3 Nuclear and particle physics · Explain · 5 marks

> Smoke alarms are found in most homes. They contain a small amount of a radioactive isotope that continuously emits radiation into the air inside the alarm. When smoke enters the alarm, the count rate of radiation detected inside the alarm drops, triggering the alarm to sound.

## Mark scheme (5 marks)

1. Americium-241 has a long half-life (432 years), so it does not need to be replaced / remains active for the lifetime of the alarm
2. Alpha particles ionise the air inside the alarm, allowing a current to flow / creating a detectable signal
3. When smoke enters, the alpha particles are absorbed / blocked by smoke particles, reducing ionisation / count rate, so the alarm sounds
4. Alpha radiation has very low penetrating power / is stopped by a few centimetres of air, so it cannot penetrate the casing of the alarm or reach people in the room
5. Gamma radiation would be dangerous in a household as it is highly penetrating and would pass through the casing and irradiate people nearby

## Key terms

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

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Source: [GradeNine](https://www.gradenine.co.uk/q/americium-241-is-a-radioactive-isotope-used-70199c9c) · Published by Druglandscape Ltd.