A scientist is investigating the properties of alpha, beta and gamma radiation by placing different materials between a radioactive source and a detector. Describe the properties of each type of radiation, including their penetrating power and what materials are needed to stop each one.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
A Geiger-Müller tube is used to detect radiation from three separate radioactive sources. Each source emits only one type of radiation. When a sheet of paper is placed between the first source and the tube, the count rate drops to background level. When a 3 mm sheet of aluminium is placed between the second source and the tube, the count rate drops to background level. A thick slab of lead is needed to significantly reduce the count rate from the third source.
Model answer (5 marks)
Alpha radiation consists of helium nuclei (2 p + 2 n) and is positively charged. It has very low penetrating power and is stopped by a sheet of paper or a few centimetres of air.
Beta radiation is a fast‑moving electron (negatively charged). It is more penetrating than alpha particles but can be stopped by a few millimetres of aluminium or an equivalent thin metal sheet.
Gamma radiation is high‑energy electromagnetic radiation. It has the greatest penetrating power and requires a thick slab of lead or several centimetres of concrete to significantly reduce its intensity; it cannot be completely stopped.
Beta radiation is a fast‑moving electron (negatively charged). It is more penetrating than alpha particles but can be stopped by a few millimetres of aluminium or an equivalent thin metal sheet.
Gamma radiation is high‑energy electromagnetic radiation. It has the greatest penetrating power and requires a thick slab of lead or several centimetres of concrete to significantly reduce its intensity; it cannot be completely stopped.
Examiner tips
- Use the exact terminology (helium nucleus, fast‑moving electron, high‑energy electromagnetic radiation).
- Show the order of penetration: alpha < beta < gamma.
- Mention the specific materials and thicknesses that stop each type.
Common mistakes
- Confusing the charge of alpha particles (positive) with beta particles (negative).
- Saying gamma rays can be completely stopped or using the wrong material (e.g., paper).
Mark scheme (5 marks)
- Alpha radiation is stopped by paper (or a few centimetres of air)
- Beta radiation is stopped by a few millimetres of aluminium (or equivalent thin metal sheet)
- Gamma radiation requires thick lead (or several centimetres of concrete) to significantly reduce its intensity, and cannot be completely stopped
- Alpha particles are positively charged (and are helium nuclei / consist of 2 protons and 2 neutrons)
- Beta particles are fast-moving electrons (negatively charged) and gamma rays are high-energy electromagnetic waves / radiation
Key terms in this question
gamma radiation · penetrating power
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Describe" question →
- Decode the mark scheme abbreviations →
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