# A smoke detector contains a small amount of americium-241, which is a radioactive isotope that emits alpha radiation. Explain why alpha radiation is used inside the smoke detector rather than beta or gamma radiation, and describe what is meant by the term 'isotopes'.

> Eduqas A-Level Physics — 3.5 Nuclear decay · Explain · 5 marks

> Smoke detectors use radioactive sources to ionise air between two metal plates. When smoke enters the detector, the ionisation is reduced and the alarm sounds.

## Mark scheme (5 marks)

1. Alpha radiation has a high ionising ability / is strongly ionising
2. Alpha radiation has a very short range in air (a few centimetres), so it is contained safely within the device / does not penetrate the casing
3. Beta or gamma radiation would penetrate the casing and could be harmful to people nearby / would escape the detector
4. Isotopes are atoms of the same element (same number of protons / same atomic number)
5. Isotopes have different numbers of neutrons / different mass numbers

## Key terms

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

## Related

- [Revision notes for Eduqas A-Level Physics](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)

---
Source: [GradeNine](https://www.gradenine.co.uk/q/a-smoke-detector-contains-a-small-a9534f49) · Published by Druglandscape Ltd.