IGCSE §7: Radioactivity and Particles · IGCSE
Alpha, Beta and Gamma Radiation
Revision notes on Alpha, Beta and Gamma Radiation for the OCR A-level Physics specification (H556). Free to read, with 4 practice questions in the app.
Three kinds of radiation are emitted by unstable nuclei, and almost every question turns on the differences between them.
Alpha (α) — a helium nucleus: 2 protons and 2 neutrons. Relative charge +2, relative mass 4.
Beta (β) — a fast-moving electron, emitted from the nucleus when a neutron changes into a proton. Relative charge −1, mass effectively 0. Note it does not come from the electron shells.
Gamma (γ) — a high-frequency electromagnetic wave. No charge and no mass. Emitted when a nucleus rearranges itself into a lower-energy state, often after an alpha or beta decay.
Penetrating power and what stops each:
| Stopped by | Range in air | |
|---|---|---|
| Alpha | A sheet of paper, or skin | A few cm |
| Beta | A few mm of aluminium | About a metre |
| Gamma | Several cm of lead, or thick concrete | Effectively unlimited, only reduced |
Gamma is never fully stopped — it is reduced by shielding. Saying "lead stops gamma" is imprecise and loses marks; thick lead greatly reduces it.
Ionising power runs the other way. Alpha is by far the most ionising, beta moderate, gamma the least.
The two properties are connected, and the connection is worth understanding rather than memorising: a heavy, doubly charged alpha particle interacts strongly with the atoms it passes, so it ionises heavily — and in doing so it loses its energy very quickly and does not travel far. Gamma interacts weakly, so it ionises little and travels a long way. Strongly ionising means poorly penetrating, necessarily.
Behaviour in fields:
- In a magnetic field, alpha and beta are deflected in opposite directions, because their charges are opposite. Beta deflects far more because it has a much smaller mass. Gamma is not deflected at all, having no charge.
- In an electric field, alpha is attracted towards the negative plate and beta towards the positive plate. Gamma passes straight through.
Example: to identify an unknown source, place paper in front of it. If the count drops to background, it was alpha. If not, try a few millimetres of aluminium; a drop there indicates beta. If it takes lead to reduce the reading, gamma is present.
4 questions on Alpha, Beta and Gamma Radiation
Reading is not revising. This topic has 4 questions in the app — multiple choice, calculations, and written answers marked against the mark scheme, point by point. Miss one and it comes back later with its options reshuffled, so you have to know it rather than remember where the answer sat.
- Every topic of Edexcel International GCSE Physics (4PH1)
- Flashcards and every equation, linked to the topic it comes from
- Full mock papers in the real paper structure
20 minutes a day is free. No card, nothing to cancel — and the paid version never renews itself either.
Start Alpha, Beta and Gamma Radiation