# A radioactive sample has a half-life of 20 years. Explain what is meant by the term 'half-life' and describe how the count rate of the sample changes over a period of 60 years.

> WJEC GCSE Physics (Wales) — 2.8 Half-life · Explain · 4 marks

## Mark scheme (4 marks)

1. Half-life is the time taken for the number of radioactive nuclei (or activity/count rate) in a sample to halve
2. After 20 years (one half-life) the count rate falls to half its original value
3. After 40 years (two half-lives) the count rate falls to one quarter of its original value
4. After 60 years (three half-lives) the count rate falls to one eighth of its original value

## Key terms

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

## Related

- [Revision notes for WJEC GCSE Physics (Wales)](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/a-radioactive-sample-has-a-half-life-6b0252da) · Published by Druglandscape Ltd.