Radioactive sources are used in industry to measure the thickness of metal sheets during manufacturing. A beta source is placed on one side of the metal sheet and a detector is placed on the other side. Explain why a beta source is chosen for this process, rather than an alpha source or a gamma source.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
In a metal sheet factory, the thickness of steel sheets must be carefully controlled. A radioactive source and detector are used to monitor the thickness automatically.
Model answer (4 marks)
Alpha particles are stopped by the metal sheet, so none reach the detector.
Beta particles can penetrate the sheet but are partially absorbed; the amount reaching the detector decreases with increasing thickness.
Gamma rays pass straight through the sheet with little absorption, so the detector reading would not vary with thickness.
Therefore a beta source is chosen because its transmission through the sheet depends on thickness, giving a measurable change in detector signal.
Beta particles can penetrate the sheet but are partially absorbed; the amount reaching the detector decreases with increasing thickness.
Gamma rays pass straight through the sheet with little absorption, so the detector reading would not vary with thickness.
Therefore a beta source is chosen because its transmission through the sheet depends on thickness, giving a measurable change in detector signal.
Examiner tips
- State that alpha particles are stopped by the sheet. Explain that beta particles are partially absorbed, giving a thickness‑dependent signal. Mention that gamma rays would give almost no change in signal. Use the exact terminology: ‘penetrate’, ‘absorbed’, ‘detector reading’.
Common mistakes
- Confusing alpha with beta particles. Claiming gamma rays are absorbed by the metal. Failing to explain that the detector signal changes with thickness.
Mark scheme (4 marks)
- Alpha particles would be stopped/absorbed by the metal sheet, so none would reach the detector
- Beta particles can penetrate the metal sheet but are partially absorbed by it
- The amount of beta radiation reaching the detector depends on the thickness of the sheet
- Gamma radiation would pass straight through the metal with very little absorption, so the detector reading would not change significantly with thickness
Key terms in this question
detector
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