# A scientist is monitoring a radioactive isotope used in medical imaging. After 6 hours the count rate has fallen from 800 counts per minute to 100 counts per minute. Explain what this tells us about the half-life of this isotope and how the number of radioactive nuclei in the sample changes over time.

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

> A radioactive isotope used in medical imaging has a count rate of 800 counts per minute at the start of monitoring. After 6 hours, the count rate has fallen to 100 counts per minute.

## Mark scheme (4 marks)

1. The count rate has fallen to half (400) after the first half-life, then halved again to 200, then halved again to 100 — so three half-lives have passed
2. The half-life of the isotope is 2 hours (6 hours ÷ 3)
3. The number of radioactive nuclei in the sample decreases over time as nuclei decay
4. The rate of decrease slows over time / the number of radioactive nuclei never reaches zero

## Key terms

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

## 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-scientist-is-monitoring-a-radioactive-04ec707d) · Published by Druglandscape Ltd.