# Americium-241 is an alpha emitter used in smoke alarms. Explain why americium-241 is a suitable radioactive source for use in a smoke alarm, referring to its type of emission, its half-life, and the difference between irradiation and contamination in this context.

> OCR A-Level Physics A (H556) — 6.2 Electric and magnetic fields · Explain · 5 marks

> Smoke alarms contain a small amount of americium-241, which has a half-life of 432 years. The americium-241 is sealed inside the alarm and continuously emits radiation into a small chamber of air within the device. If smoke enters the chamber, the alarm sounds.

## Mark scheme (5 marks)

1. Alpha radiation is weakly penetrating / cannot travel far in air, so it does not present a significant radiation risk to people near the alarm
2. The long half-life of 432 years means the source remains weakly radioactive / active for a very long time, so it does not need to be replaced
3. The long half-life means the activity at any given time is low / the source is only weakly radioactive, so it is not harmful to people nearby
4. The americium-241 is sealed inside the alarm, so users are only irradiated (exposed to radiation from the source) rather than contaminated (radioactive atoms being deposited on them)
5. Alpha radiation is highly ionising, so it ionises the air in the chamber, allowing a current to flow; when smoke enters, fewer alpha particles reach the air in the chamber, the current drops and the alarm sounds

## Key terms

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

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