# A geologist discovers a rock sample that contains a radioactive isotope. She uses a Geiger-Muller tube to measure the activity of the sample and records a value of 2400 Bq. State what is meant by the unit becquerel and explain why the geologist must account for background radiation before using this reading to determine the half-life of the isotope.

> Pearson Edexcel International GCSE Physics (4PH1) — 7.1 Units · State and Explain · 4 marks

## Mark scheme (4 marks)

1. One becquerel is equal to one decay (disintegration) per second
2. Background radiation is present all around us / is always being detected by the Geiger-Muller tube
3. The background count rate must be subtracted from the measured activity to give the true activity of the sample alone
4. If background radiation is not subtracted, the activity reading will be too high / the half-life calculated will be incorrect

## Key terms

- [becquerel](https://www.gradenine.co.uk/glossary/becquerel)
- [activity](https://www.gradenine.co.uk/glossary/activity)
- [background radiation](https://www.gradenine.co.uk/glossary/background-radiation)
- [half-life](https://www.gradenine.co.uk/glossary/half-life)
- [Geiger-Muller tube](https://www.gradenine.co.uk/glossary/geiger-muller-tube)

## Related

- [Revision notes for Pearson Edexcel International GCSE Physics (4PH1)](https://www.gradenine.co.uk/learn)
- [How to answer "State and 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/a-geologist-discovers-a-rock-sample-950d6f75) · Published by Druglandscape Ltd.