Explain why alpha radiation is more ionising than gamma radiation.

OCR GCSE Physics A: Gateway Science (J249) — P4.3 Radioactive emissions · Explain · 4 marks · View as Markdown

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

Scientists studying radioactive sources measure how effectively different types of radiation can ionise the air around them. They find that alpha particles cause far more ionisation per centimetre of path than gamma rays do.

Model answer (4 marks)

Alpha particles are relatively large and have a mass of 4 u, so they interact strongly with matter. They carry a +2 charge, which allows them to attract and transfer energy to electrons in atoms. This results in the ejection of many electrons, producing a large amount of ionisation over a short distance. In contrast, gamma rays have no charge and no mass; they interact only weakly with electrons and atoms, so they produce far less ionisation per centimetre of path.

Examiner tips

Mention size/mass and charge of alpha particles
Explain energy transfer to electrons
Contrast with gamma’s lack of charge and mass
Use terms: ionisation, ejection of electrons

Common mistakes

Confusing gamma rays with alpha particles; describing gamma as having charge
Failing to mention the mass of alpha particles
Using vague terms like ‘more energetic’ instead of ‘more ionising’

Mark scheme (4 marks)

Alpha particles are relatively large / have a large mass (compared to gamma)
Alpha particles carry a charge (of +2) / interact strongly with electrons in atoms
Alpha particles transfer energy to / knock electrons from atoms more readily, causing ionisation
Gamma radiation has no charge / no mass, so it interacts weakly with matter and causes much less ionisation per unit distance

Key terms in this question

alpha radiation · gamma radiation