Explain why alpha radiation is more ionising than gamma radiation.

WJEC GCSE Physics (Wales) — 2.7 Types of radiation · Explain · 4 marks · View as Markdown

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

Radioactive sources can emit different types of radiation. These types of radiation differ in their ability to ionise atoms in the materials they pass through.

Model answer (4 marks)

Alpha particles are relatively large and have a mass of two protons and two neutrons, giving them a charge of +2. This strong positive charge means they interact strongly with the electrons of the atoms they pass through, causing many ionising collisions as they travel. Because they move relatively slowly, they lose energy quickly and produce a high density of ionisations over a short distance. In contrast, gamma rays are massless, uncharged photons; they interact only weakly with matter and therefore produce far fewer ionisations per unit distance.

Examiner tips

Use the terms ‘mass’, ‘charge’ and ‘ionising collisions’ to show understanding of the mechanism.
Show that alpha particles lose energy quickly and give a short, clear comparison with gamma rays.
Remember to mention the +2 charge and the lack of charge in gamma rays.

Common mistakes

Confusing alpha particles with beta particles or gamma rays in terms of charge or mass.
Failing to explain that alpha particles lose energy quickly, leading to many ionisations.
Using vague phrases like ‘more energetic’ without linking to ionisation density.

Mark scheme (4 marks)

Alpha particles are relatively large / have a large mass compared to gamma radiation
Alpha particles carry a charge of +2 / are strongly charged, so they interact strongly with electrons in surrounding atoms
Alpha particles move relatively slowly / lose energy quickly as they travel through matter, causing many ionising collisions per unit distance
Gamma radiation has no charge and no mass, so it interacts very little with atoms / passes through matter without causing much ionisation

Key terms in this question

alpha radiation · gamma radiation · ionising