Types of Nuclear Radiation

When an unstable nucleus decays, it emits nuclear radiation — and there is more than one kind. The three you must know are alpha, beta and gamma radiation, named after the first three letters of the Greek alphabet ($\alpha$α, $\beta$β, $\gamma$γ); a fourth, neutron emission, also occurs. These types differ enormously in what they are made of, how far they travel, what stops them, and how strongly they ionise the materials they pass through. Knowing these differences is the key to understanding the uses and the hazards of radiation later in the topic. This lesson, part of Topic P6 (Radioactivity) of OCR Gateway Science A, describes each type of radiation, compares their penetration and ionising power, and (for Higher tier) shows how they behave in electric and magnetic fields.

By the end of this lesson you should be able to describe the nature, charge and mass of alpha, beta and gamma radiation (and neutron emission), compare their penetrating ability and ionising power, state what stops each one, and explain how the charged radiations are deflected in electric and magnetic fields.

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Alpha Radiation ($\alpha$α)

An alpha particle is a helium nucleus — two protons and two neutrons bound together — emitted at high speed from an unstable nucleus. It is written as $^{4}_{2}\text{He}$24​He or $^{4}_{2}\alpha$24​α.

• What it is: 2 protons + 2 neutrons (a helium nucleus).
• Relative charge: $+2$+2 (it has two protons and no electrons).
• Relative mass: $4$4 (the heaviest of the three radiations).
• Penetration: very low — alpha is stopped by a sheet of paper, by a few centimetres of air, or by the skin.
• Ionising power: very strong — because it is large, slow and highly charged, it interacts strongly with atoms it passes, knocking off many electrons.
• Range in air: short, only a few centimetres.

Because alpha particles are big and highly charged, they "bump into" the atoms of any material very readily, transferring their energy over a short distance. This makes them strongly ionising but weakly penetrating — the two properties go hand in hand.

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Beta Radiation ($\beta$β, beta-minus)

A beta-minus particle is a fast-moving electron emitted from the nucleus. It is written $^{\ \ 0}_{-1}\text{e}$−1  0​e or $^{\ \ 0}_{-1}\beta$−1  0​β. It may seem strange that an electron comes out of the nucleus, but it is created at the moment of decay when a neutron turns into a proton, emitting an electron in the process (covered fully in the next lesson).

• What it is: a high-speed electron from the nucleus.
• Relative charge: $-1$−1.
• Relative mass: $\dfrac{1}{1836}$18361​ — effectively $0$0 (very small).
• Penetration: moderate — beta is stopped by a few millimetres of aluminium (about 3 mm), and travels a few metres in air.
• Ionising power: moderate — less than alpha but more than gamma.
• Range in air: up to about a metre or more.

Beta particles are far smaller, faster and less charged than alpha particles, so they interact less strongly with the atoms they pass — making them more penetrating but less ionising than alpha.

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Gamma Radiation ($\gamma$γ)

Gamma radiation is not a particle at all but a high-frequency electromagnetic wave (a burst of high-energy EM radiation) released from the nucleus, often straight after an alpha or beta decay leaves the nucleus with surplus energy. It is written simply as $\gamma$γ.

• What it is: a high-frequency electromagnetic wave (high-energy photon).
• Relative charge: $0$0 (it is not charged).
• Relative mass: $0$0 (it has no mass).
• Penetration: very high — gamma is only reduced (never fully stopped) by thick lead or thick concrete; it is never completely absorbed.
• Ionising power: weak — because it has no charge and no mass, it interacts only weakly with matter.
• Range in air: very long (effectively unlimited; it spreads out and weakens with distance).

Note the careful wording: thick lead and concrete reduce gamma radiation, but a thickness of lead can only cut it down, not remove it entirely. This is the opposite extreme from alpha: gamma is highly penetrating but only weakly ionising.

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Neutron Emission

Some unstable nuclei — particularly very large or neutron-rich ones, and nuclei undergoing fission — emit neutrons. A neutron, written $^{1}_{0}\text{n}$01​n, has:

• relative charge $0$0 (it is uncharged), and
• relative mass $1$1.

Because they are uncharged, neutrons do not ionise directly by attracting or repelling electrons, but they are very penetrating and can be absorbed by other nuclei (making them unstable in turn). Neutrons are best absorbed by materials rich in light atoms, such as water or concrete. Neutron emission is important in nuclear reactors, which you will meet in the lesson on fission and fusion.

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What "Ionising" Means and Why It Matters

The single most important property of nuclear radiation is its ionising power — its ability to knock electrons off atoms, turning neutral atoms into charged ions. When a fast-moving alpha or beta particle, or a gamma ray, passes through a material, it transfers energy to the atoms it meets and can pull electrons away from them. Each "knock" creates an ion and uses up a little of the radiation's energy.

This explains the inverse link between ionising power and penetration in a physical way:

• A strongly ionising radiation (like alpha) creates a great many ions in a short distance, so it loses its energy quickly and is stopped after only a short distance — it penetrates poorly.
• A weakly ionising radiation (like gamma) creates very few ions as it passes, so it loses its energy slowly and can travel a long way before being absorbed — it penetrates deeply.

Ionising power also explains why radiation is hazardous to living things: ionising the atoms in living cells can damage or kill them, or alter the DNA, which is the basis of both the dangers and several of the medical uses of radiation covered later in the topic.

Exam Tip: "Ionising" means knocking electrons off atoms to form ions. The more ionising a radiation, the more energy it loses per centimetre, so the less it penetrates — which is exactly why alpha (most ionising) is stopped most easily and gamma (least ionising) penetrates furthest.

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Comparing Penetration and Ionising Power

The defining feature of the three radiations is the trade-off between penetration and ionising power: the more strongly a radiation ionises, the less far it penetrates, and vice versa.