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This lesson covers the three main types of nuclear radiation — alpha (α), beta (β) and gamma (γ) — as required by the Edexcel GCSE Physics specification (1PH0), Topic 6: Radioactivity. You need to know the nature, charge, mass, penetrating power, ionising ability and range of each type of radiation.
Some atomic nuclei are unstable. An unstable nucleus will emit radiation in order to become more stable — this process is called radioactive decay. It is a random process: you cannot predict exactly when a particular nucleus will decay, but you can predict the behaviour of a large number of nuclei.
The three main types of nuclear radiation are:
Exam Tip: Radioactive decay is a nuclear process — it involves changes in the nucleus of the atom. It is NOT a chemical reaction. Chemical reactions involve electrons; radioactive decay involves protons and neutrons in the nucleus.
An alpha particle consists of 2 protons and 2 neutrons — it is identical to a helium-4 nucleus.
| Property | Detail |
|---|---|
| Composition | 2 protons + 2 neutrons (helium nucleus) |
| Charge | +2 (two protons, each with +1 charge) |
| Relative mass | 4 (2 protons + 2 neutrons, each with relative mass 1) |
| Ionising ability | Strongly ionising — knocks electrons off atoms very easily |
| Penetrating power | Low — stopped by a few centimetres of air, a sheet of paper, or skin |
| Range in air | Approximately 3–5 cm |
| Speed | Relatively slow (compared to beta and gamma) |
| Deflection in electric/magnetic fields | Deflected — towards the negative plate (because it is positive) |
Exam Tip: There is an inverse relationship between ionising ability and penetrating power. Alpha is the MOST ionising but the LEAST penetrating. Gamma is the LEAST ionising but the MOST penetrating. This pattern is tested frequently.
A beta particle is a high-speed electron emitted from the nucleus of an atom.
The nucleus does not contain electrons. Instead, a neutron in the nucleus transforms into a proton and an electron. The electron is immediately ejected at high speed — this is the beta particle.
neutron → proton + electron (beta particle)
| Property | Detail |
|---|---|
| Composition | A high-speed electron emitted from the nucleus |
| Charge | −1 |
| Relative mass | Negligible (~1/1836 of a proton) |
| Ionising ability | Moderately ionising |
| Penetrating power | Moderate — passes through paper and air, stopped by a few millimetres of aluminium |
| Range in air | Approximately 1 metre |
| Speed | Fast (up to ~90% the speed of light) |
| Deflection in electric/magnetic fields | Deflected — towards the positive plate (because it is negative); deflected more than alpha due to smaller mass |
Gamma radiation is a form of electromagnetic radiation (electromagnetic wave) — it has the highest frequency and shortest wavelength on the electromagnetic spectrum.
| Property | Detail |
|---|---|
| Nature | Electromagnetic wave (not a particle) |
| Charge | 0 (no charge) |
| Mass | 0 (no mass) |
| Ionising ability | Weakly ionising |
| Penetrating power | Very high — passes through paper, aluminium, and most materials; only significantly reduced by thick lead or several metres of concrete |
| Range in air | Effectively unlimited (inverse square law applies) |
| Speed | Speed of light (3 × 10⁸ m/s) |
| Deflection in electric/magnetic fields | Not deflected (no charge) |
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| What is it? | 2 protons + 2 neutrons (He nucleus) | High-speed electron from nucleus | Electromagnetic wave |
| Charge | +2 | −1 | 0 |
| Relative mass | 4 | ~0 (negligible) | 0 |
| Ionising ability | Strong | Moderate | Weak |
| Penetrating power | Low | Moderate | High |
| Stopped by | Paper, skin, few cm of air | Aluminium (few mm) | Thick lead or concrete |
| Range in air | ~3–5 cm | ~1 m | Effectively unlimited |
| Deflected by electric/magnetic fields? | Yes (towards negative) | Yes (towards positive) | No |
| Speed | Slow (relative) | Fast | Speed of light |
Exam Tip: Learn this comparison table thoroughly. Questions often ask you to compare two or more types of radiation, or to identify a type of radiation from its properties. A 6-mark question might ask you to describe and compare all three types.
Ionisation is the process by which radiation removes electrons from atoms, turning them into ions (charged particles).
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