Introduction to Radioactivity

The hidden energy locked in the nucleus

At the centre of every atom sits a tiny, dense nucleus made of protons and neutrons. For most atoms, this nucleus is perfectly stable and will sit unchanged forever. But for some atoms, the nucleus holds the "wrong" balance of protons and neutrons and is unstable. An unstable nucleus cannot stay as it is — sooner or later it will spit out energy and particles to rearrange itself into a more stable form. This process is called radioactive decay, and the particles and energy it releases are radiation.

Radioactivity was discovered by accident in 1896 when Henri Becquerel found that uranium fogged a photographic plate kept in the dark. Marie and Pierre Curie went on to identify new radioactive elements and gave the phenomenon its name. Crucially, radioactivity is a nuclear process, not a chemical one — it comes from deep inside the nucleus, and nothing you do chemically (heating, cooling, dissolving) can speed it up or slow it down. To understand it, you first need a picture of the atom itself, covered in introduction to atoms.

Isotopes: same element, different nucleus

An element is defined by its number of protons (the atomic number). But atoms of the same element can have different numbers of neutrons — these are called isotopes. For example, carbon always has 6 protons, but carbon-12 has 6 neutrons (stable), while carbon-14 has 8 neutrons (unstable and radioactive).

It is the unstable isotopes — called radioisotopes — that are radioactive. They decay to reach a more stable configuration.

The three types of radiation

There are three main types of nuclear radiation, named after the first three Greek letters. They differ enormously in their penetrating power (how far they travel and what stops them) and their ionising power (how strongly they knock electrons off atoms they pass).

Alpha radiation (α) is a helium nucleus — 2 protons and 2 neutrons — ejected from the nucleus. It is relatively large, slow and carries a +2 charge.

• Penetration: very low. Stopped by a sheet of paper, a few centimetres of air, or the outer layer of your skin.
• Ionising power: very high. Because it is big and highly charged, it collides with and ionises many atoms.
• Danger: mostly harmless outside the body, but extremely dangerous if an alpha-emitter is swallowed or inhaled, because it deposits all its energy in nearby tissue.

Beta radiation (β) is a fast-moving electron emitted when a neutron in the nucleus changes into a proton. It is tiny and carries a −1 charge.

• Penetration: medium. Stopped by a few millimetres of aluminium.
• Ionising power: moderate — less than alpha, more than gamma.

Gamma radiation (γ) is not a particle at all but a high-energy electromagnetic wave, released when a nucleus has excess energy after other decays. It has no charge and no mass.

• Penetration: very high. Needs thick lead or concrete to reduce it significantly; it is never fully stopped, only weakened.
• Ionising power: low per interaction, but it travels far and is hard to shield against.

Gamma rays sit at the high-energy end of the electromagnetic spectrum, beyond X-rays.

Type	What it is	Charge	Stopped by	Ionising power
Alpha (α)	Helium nucleus (2p + 2n)	+2	Paper / skin	Very high
Beta (β)	Fast electron	−1	~3 mm aluminium	Medium
Gamma (γ)	EM wave (photon)	0	Thick lead / concrete	Low (but penetrating)

What happens to the nucleus during decay

Decay actually changes one element into another — a process the alchemists dreamed of. Two key examples:

Alpha decay: the nucleus loses 2 protons and 2 neutrons. The atomic number drops by 2 and the mass number drops by 4. For example, uranium-238 (92 protons) alpha-decays into thorium-234 (90 protons).

Beta-minus decay: a neutron turns into a proton, emitting a fast electron (the beta particle). The atomic number increases by 1 while the mass number stays the same. For example, carbon-14 (6 protons) beta-decays into nitrogen-14 (7 protons).

In both cases the total electric charge and the total number of nucleons are conserved — the bookkeeping always balances.

Half-life: the clock of the nucleus

Radioactive decay is random. You can never predict when a single particular nucleus will decay — it might be a microsecond or a million years. Yet with billions of nuclei, the average behaviour is wonderfully predictable. We describe it using the half-life.

Half-life is the average time taken for half the radioactive nuclei in a sample to decay (equivalently, the time for the activity to halve).

Each isotope has its own fixed half-life, ranging from fractions of a second to billions of years. Carbon-14 has a half-life of about 5730 years; uranium-238 about 4.5 billion years; some medical isotopes only a few hours.

Worked example 1. A sample contains 8000 undecayed nuclei of an isotope with a half-life of 2 hours. How many remain after 6 hours?

6 hours = 3 half-lives (6 ÷ 2 = 3). Each half-life halves the count: 8000 → 4000 (after 2 h) → 2000 (after 4 h) → 1000 (after 6 h).

Worked example 2. A radioactive source has an activity of 480 counts per second. After 12 days the activity has fallen to 60 counts per second. What is the half-life?

The activity has dropped from 480 → 240 → 120 → 60, which is three halvings. So 3 half-lives = 12 days, meaning one half-life = 4 days.

A useful shortcut: after n half-lives, the fraction remaining is (1/2)ⁿ. After 4 half-lives, (1/2)⁴ = 1/16 of the original remains.

Ionising radiation: danger and protection

What makes nuclear radiation hazardous is that it is ionising — it carries enough energy to knock electrons off atoms in its path. In living tissue, this can damage molecules and, critically, the DNA inside cells. Low doses may be repaired by the body; high doses can kill cells outright (radiation sickness) or cause mutations that lead to cancer.

Protection relies on three principles, often summarised as time, distance and shielding:

• Time — minimise how long you are exposed.
• Distance — radiation intensity falls rapidly with distance (gamma intensity follows an inverse-square law).
• Shielding — put appropriate material between you and the source (paper or skin for alpha, aluminium for beta, thick lead or concrete for gamma).

In a school lab, sources are weak, handled with tongs, kept pointing away from people, and locked away when not in use.

Why radioactivity is useful

Despite the dangers, controlled radioactivity is enormously valuable:

• Medicine: gamma rays sterilise equipment and treat cancer (radiotherapy targets and kills tumour cells); tracer isotopes with short half-lives let doctors image organs.
• Carbon dating: because carbon-14 decays at a known rate, measuring how much remains in ancient wood, bone or charcoal reveals its age — a cornerstone of archaeology.
• Power generation: nuclear power stations release vast energy by splitting heavy nuclei (fission), providing low-carbon electricity. The energy released here is enormous compared with chemical reactions, a theme explored in energy, work and power.
• Smoke detectors: many contain a tiny alpha source that ionises the air; smoke disrupts the current and triggers the alarm.

Try it yourself! 🧪

You cannot — and must not — handle real radioactive sources at home, but you can model the two key ideas safely.

Experiment 1 — model random decay with dice. Take 100 small dice (or coins). Each die represents one undecayed nucleus.

1. Roll all 100. Remove every die showing a six — these have "decayed" this round. Record how many remain.
2. Roll the survivors again, removing the sixes each time, and record the count after each round.
3. Plot remaining dice against round number. You will get a smooth decay curve, and roughly 1/6 decay each round. The "half-life" (rounds to halve the count) is about 3.8 rounds — and crucially, you cannot predict which individual die will decay, only the average. This is exactly how real nuclei behave.

Experiment 2 — model penetration and shielding. Use a torch as a stand-in for radiation and different materials as shields. Shine it through tissue paper (lets most through, like gamma through thin material), then card, then a book. While light is not nuclear radiation, the principle is identical: thicker, denser shielding stops more radiation. It reinforces why lead stops gamma but paper only stops alpha.

These models capture the heart of radioactivity — randomness governed by a fixed half-life, and the role of shielding — without any real hazard.