Nuclear Radiation: Alpha, Beta, and Gamma Rays — KS3 Physics Guide

Nuclear radiation is energy or particles emitted from an unstable atomic nucleus as it decays to a more stable state. There are three main types — alpha (α), beta (β), and gamma (γ) — each with different masses, charges, speeds, ionising power, and penetrating ability.

What makes a nucleus unstable?

Before we can understand nuclear radiation, we need to think carefully about what is happening inside an atomic nucleus. What do you predict happens when a nucleus has too many protons crammed together?

Every atom has a nucleus containing protons and neutrons, held together by the strong nuclear force. In stable atoms, this force comfortably balances the electrostatic repulsion between the positively charged protons. In some atoms — particularly those with a large number of protons, or with an unusual ratio of protons to neutrons — the nucleus is in an unstable configuration. To reach a more stable arrangement, it spontaneously emits radiation.

This process is called radioactive decay. It is entirely random at the level of any individual nucleus: it is impossible to predict exactly when a particular atom will decay. Crucially, radioactive decay cannot be controlled, speeded up, or slowed down by heating, cooling, or any chemical process — it depends only on the composition of the nucleus itself. The original atom (the parent nucleus) is transformed into a different atom (the daughter nucleus) after each decay event.

What is alpha radiation?

Imagine a helium atom stripped of its two electrons. What remains is an alpha particle: 2 protons and 2 neutrons bound together, written as ⁴₂He or simply α. It carries a charge of +2 and has a relative mass of 4, making it the heaviest and most massive of the three types of nuclear radiation.

Because of its double positive charge and relatively slow speed (roughly 5% of the speed of light), an alpha particle interacts very strongly with matter as it passes through. It continuously attracts electrons away from the atoms it encounters, causing approximately 10,000 ionisations per centimetre of air. This makes alpha radiation the most ionising of the three types — but this also means it loses energy rapidly and is stopped after just a few centimetres of air or a single sheet of paper.

Alpha radiation from outside the body presents little hazard because even skin stops it entirely. However, if an alpha-emitting substance is inhaled or ingested, it becomes acutely dangerous: the radiation is then deposited directly in sensitive lung or gut tissue, causing intense localised damage. Common alpha emitters include radon-222 (a gas that seeps from certain rock types) and polonium-210.

What is beta radiation?

What happens when a neutron inside a nucleus transforms into a proton? The answer is beta radiation. A beta particle is a high-speed electron ejected from the nucleus at the moment a neutron decays into a proton. It is written as ⁰₋₁e or β. Its charge is −1, and its mass is approximately 1/2000 of a proton's mass, making it far lighter than an alpha particle.

Beta particles travel at speeds up to around 90% of the speed of light. Their single negative charge means they cause fewer ionisations per centimetre than alpha particles — beta radiation has moderate ionising power. They penetrate further than alpha particles, passing through several metres of air or several millimetres of paper, but are stopped by approximately 3 mm of aluminium or other similarly dense materials.

Beta particles can penetrate human skin and deposit energy in underlying tissue, so protective clothing or shielding is needed when working with beta sources. Common beta emitters include carbon-14 (used in radiocarbon dating) and strontium-90.

What is gamma radiation?

Gamma radiation is quite different from alpha and beta: it is not a particle at all. A gamma ray is a photon of high-energy electromagnetic radiation — the same family as visible light and X-rays, but with far shorter wavelength and far higher frequency. It has no charge and no mass. It travels at the speed of light (3 × 10⁸ m/s).

Because gamma rays carry no charge, they interact only weakly with matter compared with alpha and beta particles. This makes them the least ionising of the three types per unit distance — but also the most penetrating. A significant fraction of gamma radiation passes through the entire human body. Reducing gamma radiation to safe levels requires several centimetres of dense lead or metres of concrete.

Gamma emission often accompanies alpha or beta decay as the daughter nucleus releases excess energy. Common gamma emitters include cobalt-60 (used in radiotherapy) and technetium-99m (a medical tracer used in gamma cameras).

How do the three types of radiation compare?

Before reading the table, predict: which type do you think is most penetrating? Which is most ionising? Notice how the two properties trade off against each other.

Property | Alpha (α) | Beta (β) | Gamma (γ)
What is it? | 2 protons + 2 neutrons | Fast electron | Electromagnetic wave (photon)
Charge | +2 | −1 | 0
Relative mass | 4 | ~1/2000 | 0
Speed | ~5% c | up to ~90% c | c (3 × 10⁸ m/s)
Ionising power | Very high | Moderate | Low
Penetrating power | Very low | Moderate | Very high
Stopped by | Paper / a few cm air | ~3 mm aluminium | Several cm lead or metres of concrete
Example source | Radon-222 | Carbon-14 | Cobalt-60

The pattern to observe is that ionising power and penetrating power are inversely related: the more strongly a type of radiation ionises matter, the more rapidly it loses energy and the less far it travels.

How is nuclear radiation detected?

How would you know radiation was passing through a room if you could not see, hear, smell, or feel it at low doses? Several instruments make the invisible detectable.

The Geiger-Müller (GM) tube is the most common detector in school laboratories. Radiation entering the tube ionises the gas inside, creating a brief electrical pulse that is amplified and counted or made to produce a clicking sound. The count rate (counts per second or per minute) indicates the intensity of the radiation.

Photographic film badges are worn by radiation workers. Radiation exposes the film in proportion to the cumulative dose received; the badge is developed and analysed regularly to ensure workers stay within safe dose limits.

A cloud chamber reveals the tracks left by radiation in supersaturated vapour: alpha particles leave short, thick, straight tracks; beta particles leave longer, thinner, more curved tracks; gamma rays produce faint, wispy trails.

An essential concept for all experiments is background radiation — the low-level ionising radiation always present in the environment from natural sources such as radon gas in the air, cosmic rays from space, radioactive minerals in soil and rocks, and tiny amounts in food. Before taking readings from a source, students must measure the background count rate and subtract it from their results.

What are the uses and dangers of nuclear radiation?

Here is a striking prediction question: how can something that damages living cells also be used to save lives? The answer lies in controlling which cells are exposed and to what dose.

Uses in medicine: Gamma-emitting tracers (such as technetium-99m) injected into the bloodstream concentrate in specific organs; a gamma camera detects them and produces an image of blood flow or organ function. Radiotherapy uses focused gamma rays or beta emitters to destroy cancerous tumours. Medical instruments are sterilised using gamma radiation, killing bacteria without the heat damage of autoclaving.

Uses in industry: Smoke detectors contain a tiny americium-241 alpha source that ionises the air between two electrodes; smoke particles disrupt the ion current and trigger the alarm. Beta sources are used as thickness gauges in paper and metal manufacturing: a detector on the far side of the material measures how much beta radiation passes through, and the production line automatically adjusts if the reading changes.

Dangers: All ionising radiation — whether alpha, beta, or gamma — can damage DNA in living cells. At low doses, the body's repair mechanisms cope, but high doses or repeated exposure increase the risk of cancer and, at very high acute doses, cause radiation burns or radiation sickness. Precautions include maintaining distance from sources, using appropriate shielding, and minimising exposure time.

Frequently asked questions

What is the difference between alpha, beta, and gamma radiation?

Alpha radiation consists of particles (2 protons + 2 neutrons) that are heavy and slow, making them highly ionising but easily blocked — even paper or skin stops them. Beta radiation consists of fast electrons that penetrate further and are stopped by a few millimetres of aluminium. Gamma radiation is high-energy electromagnetic waves with no mass or charge; it penetrates the most and requires lead or concrete shielding. As ionising power goes up, penetrating power goes down.

Why is alpha radiation the most ionising?

An alpha particle carries a double positive charge (+2) and is relatively slow and massive. As it passes through matter, its strong charge strips electrons from atoms it encounters, causing about 10,000 ionisations per centimetre of air. This very high ionising ability means it loses energy rapidly and is stopped after just a few centimetres. Beta particles carry only a single charge (−1) and travel faster, causing fewer ionisations per centimetre. Gamma rays carry no charge, so they cause far fewer ionisations per unit distance.

Why is background radiation important in nuclear experiments?

Background radiation is the low-level ionising radiation always present in the environment from natural sources: radon gas seeping from rocks (the largest contributor in the UK), cosmic rays from space, naturally radioactive materials in soil and building materials, and tiny amounts in food and water. Before measuring radiation from a source, students must measure the background count rate separately and subtract it from their readings, otherwise results will be misleadingly high.

What is radioactive decay?

Radioactive decay is the spontaneous, random emission of radiation from an unstable atomic nucleus. It is random because it is impossible to predict exactly WHEN a particular nucleus will decay, although for a large sample of identical atoms we can predict the average rate accurately using the concept of half-life (the time for half the nuclei to decay). Decay changes the nucleus: alpha decay reduces proton number by 2 and mass number by 4; beta decay increases proton number by 1; gamma emission changes neither.

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