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This lesson covers the properties, penetrating power, and ionising ability of the three main types of nuclear radiation — alpha, beta, and gamma — as required by the AQA GCSE Physics specification (4.4.2). You need to know the nature of each type, how they interact with matter, and how they can be identified.
An alpha particle consists of 2 protons and 2 neutrons — it is identical to a helium-4 nucleus. Alpha particles are emitted from the nuclei of very heavy, unstable atoms (such as uranium, radium, and polonium).
| Property | Detail |
|---|---|
| Composition | 2 protons + 2 neutrons (helium nucleus) |
| Charge | +2 |
| Relative mass | 4 |
| Speed | Relatively slow (about 5-10% of the speed of light) |
| Ionising ability | Strongly ionising |
| Penetrating power | Very low — stopped by a few centimetres of air or a sheet of paper |
| Deflection in electric/magnetic fields | Deflected (towards negative plate, due to positive charge) |
Alpha particles are large and heavily charged (+2). Because of this, they interact strongly with atoms in the material they pass through. They knock electrons off atoms, creating ions. Each alpha particle causes many ionisation events over a short distance, which is why they lose their energy quickly and are stopped after travelling only a few centimetres.
Exam Tip: There is an inverse relationship between ionising ability and penetrating power. Alpha is the most ionising, so it loses energy the fastest and therefore has the least penetrating power. Gamma is the least ionising, so it retains energy for longer and has the greatest penetrating power.
A beta particle is a high-speed electron emitted from the nucleus when a neutron transforms into a proton. Despite being an electron, the beta particle comes from the nucleus, not from the electron shells. The transformation inside the nucleus is:
neutron --> proton + electron (beta particle)
| Property | Detail |
|---|---|
| Composition | High-speed electron |
| Charge | -1 |
| Relative mass | Negligible (1/1836 of a proton) |
| Speed | Fast (up to 90% of the speed of light) |
| Ionising ability | Moderately ionising |
| Penetrating power | Moderate — stopped by a few millimetres of aluminium |
| Deflection in electric/magnetic fields | Deflected (towards positive plate, due to negative charge); deflected more than alpha due to smaller mass |
Beta particles are smaller and less charged than alpha particles, so they interact less strongly with atoms as they pass through a material. They cause fewer ionisation events per unit distance, which means they travel further before losing all their energy.
Gamma rays are high-frequency electromagnetic waves emitted from the nucleus. They are part of the electromagnetic spectrum and travel at the speed of light. Gamma emission often accompanies alpha or beta decay — after emitting an alpha or beta particle, the nucleus may still have excess energy, which it releases as a gamma ray.
| Property | Detail |
|---|---|
| Composition | Electromagnetic wave (photon) |
| Charge | 0 |
| Mass | 0 |
| Speed | Speed of light (3 x 10^8 m/s) |
| Ionising ability | Weakly ionising |
| Penetrating power | Very high — only significantly reduced by several centimetres of lead or several metres of concrete |
| Deflection in electric/magnetic fields | Not deflected (no charge) |
Gamma rays have no charge and no mass, so they interact very weakly with atoms. They pass through most materials without causing ionisation, which is why they are so penetrating. However, when they do interact, they can still knock electrons off atoms.
Exam Tip: A common question asks you to compare the three types of radiation. Use a table with rows for: nature/composition, charge, mass, ionising ability, penetrating power, and what stops them. This structured approach guarantees full marks.
| Property | Alpha (a) | Beta (b) | Gamma (g) |
|---|---|---|---|
| What is it? | Helium nucleus (2p + 2n) | High-speed electron | Electromagnetic wave |
| Charge | +2 | -1 | 0 |
| Mass | 4 | Very small | 0 |
| Ionising ability | Strong | Moderate | Weak |
| Penetrating power | Low | Moderate | High |
| Stopped by | Paper / few cm of air | Few mm of aluminium | Several cm of lead / metres of concrete |
| Deflected by fields? | Yes (towards negative) | Yes (towards positive) | No |
graph LR
A["Radioactive Source"] --> B["Alpha particles"]
A --> C["Beta particles"]
A --> D["Gamma rays"]
B --> E["Stopped by<br>PAPER"]
C --> F["Stopped by<br>ALUMINIUM"]
D --> G["Reduced by<br>LEAD / CONCRETE"]
style A fill:#e74c3c,color:#fff
style B fill:#e67e22,color:#fff
style C fill:#2980b9,color:#fff
style D fill:#9b59b6,color:#fff
style E fill:#f1c40f,color:#000
style F fill:#95a5a6,color:#fff
style G fill:#2c3e50,color:#fff
In experiments, you can identify which type of radiation a source emits by placing different materials between the source and a detector (such as a Geiger-Muller tube):
Important: Always subtract the background count rate from all measurements before comparing them.
Exam Tip: When describing an experiment to identify the type of radiation, always mention subtracting the background count. Also mention repeating measurements to improve reliability. These are easy marks that students often miss.
When radiation passes through an electric field (between charged plates) or a magnetic field:
Ionisation is the process of knocking electrons off atoms, turning them into ions. This is significant because:
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