Explain why alpha radiation is more ionising than gamma radiation, and why alpha radiation is stopped by a thin sheet of paper whilst gamma radiation can penetrate several centimetres of lead.

AQA A-Level Physics (7408) — 3.8.1 Radioactivity · Explain · 5 marks · View as Markdown

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

Model answer (5 marks)

Alpha particles are large, massive and carry a +2 charge, so they interact strongly with surrounding atoms and electrons. They transfer a large amount of energy to many atoms over a very short distance, producing dense ionisation.

Gamma rays are electromagnetic radiation with no mass and no charge. They interact only weakly with matter, so they can pass through most materials. Only very dense materials such as lead can absorb them effectively.

Examiner tips

  • Use the word "carry a +2 charge" to show charge effect. Explain dense ionisation for alpha. Mention no mass/charge for gamma. State that gamma needs dense material to be absorbed.

Common mistakes

  • Confusing alpha with beta radiation. Saying gamma has a charge. Not explaining why lead is needed for gamma.

Mark scheme (5 marks)

  1. Alpha particles are large / have a large mass (compared to gamma)
  2. Alpha particles carry a charge (2+) so interact strongly with surrounding atoms / electrons
  3. Alpha particles transfer energy to many atoms in a short distance, causing dense ionisation
  4. Gamma radiation is electromagnetic radiation / has no mass and no charge
  5. Gamma radiation interacts weakly with matter so passes through most materials / requires dense materials such as lead to be absorbed

Key terms in this question

alpha radiation · gamma radiation · ionising

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