Radioactive Decay and Nuclear Radiation

Not every atomic nucleus is stable. Some nuclei have a combination of protons and neutrons that makes them unstable, and to become more stable they give out radiation. This is called radioactive decay.

Radioactive decay is the process by which an unstable nucleus gives out radiation as it changes to become more stable.

A crucial feature of decay is that it is random. You cannot predict which nucleus in a sample will decay next, or exactly when any particular nucleus will decay. There is no trigger and no build-up: it simply happens by chance.

Being random does not mean disorganised. With enormous numbers of nuclei in even a tiny sample, the overall behaviour is very predictable on average, which is what makes half-life a useful measurement. But at the level of a single nucleus, decay is purely a matter of probability.

To describe how radioactive a source is, we measure how quickly it decays.

Activity is the rate at which a source of unstable nuclei decays. It is measured in becquerel (Bq), where 1 Bq is one decay per second.

Activity is a property of the source itself. A source with a high activity has many nuclei decaying every second.

In practice we measure activity using a detector, most commonly a Geiger-Müller tube. The detector does not catch every single decay, so we talk about the count-rate.

Count-rate is the number of decays per second recorded by a detector such as a Geiger-Müller tube.

The count-rate is always a little lower than the true activity, because radiation is emitted in all directions and the detector only samples part of it. Even so, changes in count-rate mirror changes in activity, so a falling count-rate tells you the activity is falling.

There are four types of nuclear radiation you must know, along with what each one actually is.

• Alpha (α) — a helium nucleus: 2 protons and 2 neutrons.
• Beta (β) — a high-speed (fast) electron ejected from the nucleus when a neutron turns into a proton.
• Gamma (γ) — electromagnetic radiation emitted from the nucleus.
• Neutron (n) — a neutron emitted from the nucleus.

Take care with the beta particle. It is an electron, but it does not come from the electron shells around the atom. It is created and ejected from the nucleus at the moment a neutron changes into a proton.

Gamma radiation is different in kind from the other three. Alpha, beta and neutron radiation are all fast-moving particles, whereas gamma is a wave of electromagnetic radiation, the same family as X-rays and visible light, but emitted from the nucleus itself.

The three main types differ hugely in how far they travel and what stops them. This is one of the most heavily tested tables in the whole topic.

Radiation	Stopped by (penetration)	Range in air
Alpha (α)	A sheet of paper or skin	A few cm
Beta (β)	A few mm of aluminium	About 1 m
Gamma (γ)	Thick lead or concrete (only reduced)	Very large (km)

Alpha particles are the largest and slowest, so they collide with air molecules quickly and are absorbed within a few centimetres. Beta particles are much smaller and faster, so they travel further and need aluminium to stop them. Gamma radiation is the most penetrating of all: thick lead or concrete reduces it, but it is never fully absorbed, only weakened.

A quick memory hook: alpha stopped by a sheet of paper, beta by a few mm of aluminium, gamma only reduced by lead. The penetrating power increases in the order alpha, beta, gamma.

Ionising means knocking electrons off atoms, turning them into charged ions. Ionising power runs in the opposite order to penetrating power.

Ionising power: alpha is the most ionising, beta is in the middle, and gamma is the least ionising.

The reason follows from what each radiation is. An alpha particle is large and highly charged (+2), so it interacts strongly with the atoms it passes, ripping off electrons and quickly losing its energy. That same strong interaction is why it does not penetrate far.

Gamma radiation carries no charge and interacts only weakly with matter, so it ionises very little but travels a long way. Beta particles sit in between on both counts.

Radiation	Ionising power	Penetrating power
Alpha (α)	Highest	Lowest
Beta (β)	Medium	Medium
Gamma (γ)	Lowest	Highest

Notice the inverse relationship: the most ionising radiation is the least penetrating, because ionising strongly means losing energy quickly.

Exam questions often ask you to pick the best type of radiation for a task, using the properties above. The trick is to match penetration and ionising power to what the job needs.

Worked example — a source is used to check the thickness of aluminium sheet on a production line. A detector measures how much radiation passes through the sheet. Which type is best?

Alpha would be completely stopped by even thin aluminium, so no radiation would get through to compare. Gamma would pass through almost unchanged whatever the thickness, so it could not detect small changes. Beta is ideal: enough gets through to be detected, but the amount passing through changes measurably as the thickness changes.

Worked example — why is an alpha source dangerous if swallowed, but relatively safe outside the body?

Outside the body, alpha cannot penetrate the dead outer layer of skin, so it does little harm. Once inside, it is close to living tissue and, being the most ionising, it causes the most damage. This is why the hazard depends on both penetration and ionising power together.

1. Saying a beta particle comes from the electron shells

A beta particle is an electron, but it is created in and ejected from the nucleus when a neutron becomes a proton. It does not come from the electrons orbiting the atom.

2. Confusing penetrating power with ionising power

They run in opposite orders. Alpha is the most ionising but least penetrating; gamma is the most penetrating but least ionising. Do not assume the most dangerous radiation is the one that travels furthest.

3. Muddling activity and count-rate

Activity is the true rate of decay of the source in becquerel. Count-rate is what the detector actually records, which is lower because it only samples part of the radiation.

4. Forgetting that gamma is only reduced, not stopped

Gamma is never fully absorbed by a shield. Thick lead or concrete reduces its intensity, but some always gets through. Alpha and beta, by contrast, can be completely stopped.

5. Treating decay as predictable for a single nucleus

Radioactive decay is random. You cannot say when a particular nucleus will decay, only describe the average behaviour of a large sample.