# Explain why a radioactive isotope with a long half-life is chosen for use in a smoke alarm, and explain why alpha radiation is the most suitable type of radiation for this application rather than gamma radiation.

> Edexcel GCSE Physics (1PH0) — 6.1 Radioactivity · Explain · 4 marks

> Smoke alarms contain a small amount of the radioactive isotope americium-241, which has a half-life of 432 years. The radiation ionises air particles inside the alarm to create a small electrical current. When smoke enters the alarm, the current drops and the alarm sounds.

## Mark scheme (4 marks)

1. A long half-life means the activity of the source remains roughly constant / does not significantly decrease over the lifetime of the alarm
2. Alpha radiation is highly ionising, so it effectively ionises the air particles to produce the current needed to operate the alarm
3. Alpha radiation has very low penetrating power / is stopped by a few centimetres of air or the casing of the alarm
4. Gamma radiation is highly penetrating and would pass straight through the alarm casing, posing a greater radiation risk to people nearby / would not be safely contained

## Key terms

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

## Related

- [Revision notes for Edexcel GCSE Physics (1PH0)](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/explain-why-a-radioactive-isotope-with-66980028) · Published by Druglandscape Ltd.