# A radiographer is deciding which radioactive isotope to use in a smoke alarm for a hospital ward. One isotope has a half-life of 432 years and emits alpha radiation. A second isotope has a half-life of 6 hours and emits gamma radiation. Explain why the isotope with a half-life of 432 years and alpha emission is more suitable for use in a smoke alarm than the second isotope.

> OCR A-Level Physics A (H556) — 6.1 Capacitors · Explain · 5 marks

> Smoke alarms work by detecting a drop in the number of radiation particles reaching a sensor inside the alarm. When smoke enters the alarm, it absorbs the radiation and the count rate at the sensor falls, triggering the alarm.

## Mark scheme (5 marks)

1. A long half-life means the source remains active / does not need replacing for a very long time (accept: will not decay away quickly / will last hundreds of years)
2. The 6-hour half-life isotope would decay too quickly / lose its activity too fast to be useful in a smoke alarm
3. Alpha radiation is the least penetrating / is stopped by a few centimetres of air
4. Alpha radiation is highly ionising so it ionises the air inside the alarm effectively / produces a detectable ionisation current
5. Gamma radiation is the most penetrating so it would pass through the alarm casing / smoke without being absorbed / would not be blocked by smoke, making it unsuitable for detecting smoke

## Key terms

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

## Related

- [Revision notes for OCR A-Level Physics A (H556)](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-radiographer-is-deciding-which-radioactive-fe22842b) · Published by Druglandscape Ltd.