# A student investigates the count rate from a radioactive source over several weeks. The student notices that the count rate decreases over time and never reaches zero. Explain why the count rate decreases over time and why it never reaches zero.

> AQA A-Level Physics (7408) — 3.8.1 Radioactivity · Explain · 5 marks

## Mark scheme (5 marks)

1. Radioactive decay is random / the nucleus decays at random
2. Each decay reduces the number of undecayed nuclei remaining
3. Fewer undecayed nuclei means fewer decays per second, so count rate falls
4. There is always background radiation present (which is always detected)
5. Background radiation comes from natural sources in the environment (e.g. cosmic rays / rocks / radon gas)

## Key terms

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

## Related

- [Revision notes for AQA A-Level Physics (7408)](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-student-investigates-the-count-rate-65336993) · Published by Druglandscape Ltd.