A technician is testing three different radioactive sources in a laboratory. Source A emits alpha particles, Source B emits beta particles, and Source C emits gamma rays. The technician places each source near a Geiger-Müller tube and then places different materials between the source and the tube. Explain how the technician could use paper, a few millimetres of aluminium, and thick lead to identify which type of radiation each source emits.

WJEC A-Level Physics (Wales) — 6.1 Radioactivity · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Radioactive sources can emit three types of ionising radiation: alpha particles, beta particles, and gamma rays. Each type has a different penetrating power and is stopped by different materials.

Model answer (5 marks)

The technician can use the different stopping powers of the materials to identify the radiation.
1. Place a sheet of paper between Source A and the Geiger‑Müller tube. If the count rate falls to background, the source is emitting alpha particles (alpha particles are stopped by paper). If the count rate remains unchanged, the source is not alpha.
2. Place a few millimetres of aluminium between the remaining source and the tube. If the count rate drops to background, the source is emitting beta particles (beta particles are stopped by a few millimetres of aluminium). If the count rate is unchanged, the source is not beta.
3. Finally, place thick lead between the remaining source and the tube. If the count rate is still reduced or stopped, the source is emitting gamma rays (gamma rays are reduced or stopped by thick lead). The order of penetrating power is therefore alpha < beta < gamma.

Examiner tips

  • Use the order of materials to match the known stopping powers; state the result of each test clearly.
  • Show the reasoning that a drop to background confirms the radiation type.
  • Mention the penetrating‑power order to demonstrate understanding.

Common mistakes

  • Confusing the order of materials (e.g. using lead before aluminium).
  • Failing to state that a drop to background confirms the type of radiation.

Mark scheme (5 marks)

  1. Alpha particles are stopped by paper (or a few centimetres of air)
  2. Beta particles pass through paper but are stopped by a few millimetres of aluminium
  3. Gamma rays pass through both paper and aluminium but are reduced/stopped by thick lead
  4. The source whose count rate drops to background when paper is placed in front is alpha (Source A)
  5. Gamma rays are the most penetrating / alpha particles are the least penetrating, so the order of penetrating power is alpha < beta < gamma

Key terms in this question

alpha particles · beta particles · gamma rays · Geiger-Müller tube

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