What is radioactivity, and how do alpha, beta, and gamma emissions differ?
Describe alpha, beta, and gamma radiation and compare their nature, penetration, and ionising power
A focused answer to the O-Level Physics outcome on radioactivity. Radioactive decay from unstable nuclei, the nature of alpha, beta, and gamma radiation, their penetrating and ionising power, and how each is stopped.
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What this dot point is asking
SEAB wants you to explain that radioactivity comes from unstable nuclei, to describe the three types of emission (alpha, beta, gamma), and to compare their nature, penetrating power, and ionising power. The big idea is that an unstable nucleus becomes more stable by emitting radiation, and the three types behave very differently.
The answer
What radioactivity is
Some atomic nuclei are unstable. They become more stable by emitting radiation from the nucleus, a process called radioactive decay. The decay is random (you cannot predict when any one nucleus will decay) and spontaneous (it is not affected by temperature, pressure, or chemical state).
The three types of emission
- Alpha (): a helium nucleus, two protons and two neutrons, with a charge of . It is relatively large and slow.
- Beta (): a fast-moving electron emitted from the nucleus, with a charge of . It is light and fast.
- Gamma (): a high-frequency electromagnetic wave, with no charge and no mass. It travels at the speed of light.
Penetrating power
The three types penetrate matter very differently:
| Radiation | Stopped by |
|---|---|
| Alpha | A sheet of paper (or a few cm of air) |
| Beta | A few mm of aluminium |
| Gamma | Greatly reduced only by thick lead or concrete |
Ionising power
Ionising power is the ability to knock electrons off atoms, creating ions. It runs in the opposite order to penetration:
- Alpha is the most strongly ionising (large, slow, highly charged), but the least penetrating.
- Gamma is the least ionising, but the most penetrating.
- Beta is in between.
A radiation that ionises strongly loses its energy quickly, which is exactly why alpha does not penetrate far.
Examples in context
Example 1. Smoke detectors. A household smoke alarm uses a weak alpha source. The alpha particles ionise the air in a small chamber, allowing a tiny current to flow. Smoke particles absorb the alpha radiation and reduce this current, triggering the alarm. Alpha is chosen because it ionises strongly but is safely stopped by the alarm's casing and the air.
Example 2. Thickness control in factories. A beta source and a detector are placed on opposite sides of a moving sheet of metal or paper. If the sheet gets thicker, more beta is absorbed and the count falls, so an automatic system adjusts the rollers. Beta is used because its penetration is sensitive to small thickness changes, unlike gamma (too penetrating) or alpha (too easily stopped).
Try this
Q1. State what alpha, beta, and gamma radiation each are. [3 marks]
- Cue. Alpha: a helium nucleus. Beta: a fast-moving electron. Gamma: a high-frequency electromagnetic wave.
Q2. State the material that stops each of alpha, beta, and gamma radiation. [3 marks]
- Cue. Alpha: paper. Beta: a few mm of aluminium. Gamma: thick lead (or concrete).
Q3. Explain why alpha radiation is the most ionising but the least penetrating. [2 marks]
- Cue. Alpha is large, slow, and highly charged, so it interacts strongly with atoms (ionising heavily) and loses its energy quickly, so it does not penetrate far.
Exam-style practice questions
Practice questions written in the style of SEAB exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Original5 marksCompare alpha, beta, and gamma radiation in terms of (a) what each one is, and (b) what material is needed to stop each.Show worked answer →
(a) Alpha () is a helium nucleus (two protons and two neutrons), positively charged. Beta () is a fast-moving electron from the nucleus, negatively charged. Gamma () is a high-frequency electromagnetic wave, with no charge and no mass.
(b) Alpha is stopped by a sheet of paper (or a few centimetres of air). Beta is stopped by a few millimetres of aluminium. Gamma is only greatly reduced by thick lead or concrete.
Markers reward the correct nature of each radiation (helium nucleus, fast electron, EM wave) and the correct stopping material for each (paper, aluminium, lead/concrete).
Original4 marksA radioactive source is tested. Its radiation passes through paper but is stopped by a few millimetres of aluminium. (a) Identify the type of radiation, with reasoning. (b) State whether it is more or less ionising than alpha radiation, and why.Show worked answer →
(a) The radiation is beta. It passes through paper (so it is not alpha, which paper stops) but is stopped by a few millimetres of aluminium (which beta cannot pass but gamma would), so it must be beta.
(b) Beta is less ionising than alpha. Alpha is the most strongly ionising because it is large, slow, and highly charged, so it interacts strongly with atoms; beta is lighter, faster, and less charged, so it ionises less.
Markers reward identifying beta from the penetration test, and that beta is less ionising than the strongly ionising alpha, with a sensible reason.
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