Explain why alpha radiation is more ionising than gamma radiation but is also stopped more easily by materials.

Eduqas GCSE Physics — 9.2 Absorption and emission of ionising radiations · Explain · 4 marks · View as Markdown

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

Model answer (4 marks)

Alpha particles are massive and carry a charge of +2, so they interact strongly with electrons in matter and ionise many atoms over a very short distance. Because they lose energy rapidly through frequent ionisation, they are stopped quickly and have a short range. In contrast, gamma rays are uncharged photons with no mass; they interact only weakly with matter, so they penetrate far and are less ionising.

Examiner tips

  • Mention the +2 charge and large mass of alpha particles
  • Explain rapid energy loss leading to short range
  • Contrast with uncharged, massless gamma rays
  • Use terms: ionising, penetrating, range

Common mistakes

  • Confusing alpha with beta radiation
  • Saying alpha is more penetrating instead of less
  • Omitting the charge or mass explanation

Mark scheme (4 marks)

  1. Alpha particles have a large mass and/or charge (doubly charged / charge of +2)
  2. The large charge / size of alpha causes it to interact strongly with matter, ionising many atoms in a short distance
  3. Because alpha loses energy rapidly through frequent ionisation events, it is stopped quickly / has a short range
  4. Gamma radiation has no charge / no mass, so it rarely interacts with atoms and is therefore more penetrating but less ionising

Key terms in this question

alpha radiation · gamma radiation · ionising

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