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

> OCR A-Level Physics B: Advancing Physics (H557) — 6.2 Electric and magnetic fields · Explain · 5 marks

> Americium-241 is placed inside a smoke alarm. The alpha particles it emits ionise the air inside the alarm. When smoke enters the alarm, the count rate of alpha particles detected drops and the alarm sounds.

## Mark scheme (5 marks)

1. A long half-life means the source remains radioactive for a long period of time / does not need to be replenished frequently
2. The long half-life means the activity / count rate remains roughly constant / weakly radioactive over time, so the alarm functions reliably
3. Alpha particles are the most ionising type of radiation, so they effectively ionise the air inside the alarm to create a detectable current / allow the alarm to function
4. Alpha particles are the least penetrating type of radiation and are stopped by a few centimetres of air / the casing of the alarm, so they do not reach the people in the room
5. Gamma radiation would be more penetrating / would pass through the alarm casing and through walls, irradiating people in the home and posing a greater health risk

## Key terms

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

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