A factory uses a radioactive source to monitor the thickness of aluminium sheets as they are produced on a production line. The source is placed on one side of the sheet and a detector is placed on the other side. Describe how this thickness monitoring system works, including why a beta emitter with a long half-life is used rather than an alpha or gamma emitter.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In industrial thickness monitoring, the count rate recorded by the detector changes if the thickness of the sheet changes. The source chosen must have a suitable type of radiation and a long half-life.
Model answer (4 marks)
Beta particles are used because they are stopped by the aluminium sheet – the thicker the sheet the more beta particles are absorbed, so the detector records a lower count rate. If the sheet is thinner, fewer beta particles are absorbed and the count rate rises.
Alpha particles would be stopped completely by even a thin sheet of aluminium, giving no useful change in count rate. Gamma rays would pass through the sheet with little attenuation, so the detector would not register a significant change.
A long half‑life is chosen so that the source activity remains essentially constant over the life of the equipment; this keeps the count rate stable and avoids the need to correct for decay of the source.
Alpha particles would be stopped completely by even a thin sheet of aluminium, giving no useful change in count rate. Gamma rays would pass through the sheet with little attenuation, so the detector would not register a significant change.
A long half‑life is chosen so that the source activity remains essentially constant over the life of the equipment; this keeps the count rate stable and avoids the need to correct for decay of the source.
Examiner tips
- Use the word ‘beta’ and explain attenuation by thickness; mention alpha stopped, gamma passes. Show the two possible changes in count rate. Explain long half‑life as keeping activity constant. Use correct terminology: ‘count rate’, ‘attenuation’, ‘half‑life’.
Common mistakes
- Confusing gamma with beta or saying gamma is used. Forgetting to mention the effect of sheet thickness on count rate. Not explaining why a long half‑life is required.
Mark scheme (4 marks)
- Beta particles are used because they are stopped by / their count rate is reduced by changes in the thickness of aluminium, whereas alpha particles would be stopped entirely by the sheet and gamma rays would pass straight through with little change in count rate.
- If the sheet becomes too thick, fewer beta particles reach the detector, so the count rate drops.
- If the sheet becomes too thin, more beta particles reach the detector, so the count rate rises.
- A long half-life is required so that the activity / count rate does not decrease significantly over time, ensuring readings remain reliable and do not need to be attributed to decay of the source.
Key terms in this question
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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