# Explain why background radiation must be subtracted from measurements before the half-life of a radioactive source is determined.

> Cambridge International IGCSE Physics (0625) — 5.2 Radioactivity · Explain · 4 marks

> A student uses a Geiger-Müller tube connected to a counter to measure the count rate of a radioactive source at regular time intervals in order to find its half-life.

## Mark scheme (4 marks)

1. Background radiation is always present / exists independently of the source being investigated
2. Background radiation contributes to / adds to the measured count rate
3. Subtracting gives the corrected count rate / count rate due only to the source
4. Without subtraction the half-life calculated would be incorrect / too long, because the count rate would not fall to zero / the background count prevents the count rate from halving correctly

## Key terms

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

## Related

- [Revision notes for Cambridge International IGCSE Physics (0625)](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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