# A scientist is investigating the half-life of a radioactive isotope in a laboratory. Explain why the scientist must measure the background radiation before recording the count rate of the sample, and describe how the half-life of the isotope can be determined from the corrected count rate readings taken over time.

> Pearson Edexcel International GCSE Physics (4PH1) — 7.2 Radioactivity · Explain · 4 marks

> A Geiger-Muller tube connected to a counter is used to measure the count rate of a radioactive sample at regular time intervals over several hours.

## Mark scheme (4 marks)

1. Background radiation must be subtracted from the measured count rate to give an accurate/corrected count rate for the sample alone.
2. Background radiation is present from natural and/or man-made sources (e.g. rocks, cosmic rays, nuclear accidents) and would cause the count rate to appear artificially high.
3. Record the corrected count rate at regular time intervals and identify the initial corrected count rate; note the time at which the count rate has fallen to half the initial value.
4. The half-life can be confirmed/is constant by checking that the count rate halves again in the same time interval (i.e. the half-life is the same regardless of when it is measured).

## Key terms

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

## Related

- [Revision notes for Pearson Edexcel International GCSE Physics (4PH1)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-scientist-is-investigating-the-half-life-d1a6b510) · Published by Druglandscape Ltd.