Alpha, beta and gamma radiation explained

TutorAura guide · Updated 9 November 2026

Some atoms have unstable nuclei. They become more stable by giving out radiation, a random process called radioactive decay. The three main types of nuclear radiation are alpha (α), beta (β) and gamma (γ). They differ in what they are, how far they travel, what stops them and how much damage they do.

Key takeaways

Alpha: a helium nucleus. Most ionising, stopped by paper.
Beta: a fast electron from the nucleus. Stopped by a few millimetres of aluminium.
Gamma: an electromagnetic wave. Least ionising, reduced by thick lead or concrete.
Alpha decay: mass number −4, atomic number −2. Beta decay: atomic number +1.

The three types

	Alpha (α)	Beta (β)	Gamma (γ)
What it is	2 protons + 2 neutrons (a helium nucleus)	A high-speed electron from the nucleus	An electromagnetic wave
Charge	+2	−1	0
Ionising power	Strong	Moderate	Weak
Range in air	A few centimetres	Further than alpha	Very far
Stopped by	A sheet of paper or skin	A few millimetres of aluminium	Several centimetres of lead or metres of concrete (reduced, not fully stopped)

The more ionising a radiation is, the more quickly it loses energy, so the less penetrating it is.

Alpha decay

The nucleus loses 2 protons and 2 neutrons. Its mass number falls by 4 and its atomic number falls by 2, so it becomes a different element.

{}^{238}_{92}\text{U} \rightarrow {}^{234}_{90}\text{Th} + {}^{4}_{2}\text{He}

Check: the top numbers balance ( $238 = 234 + 4$ ) and the bottom numbers balance ( $92 = 90 + 2$ ).

Beta decay

A neutron in the nucleus turns into a proton and an electron, and the electron is emitted. The mass number stays the same and the atomic number increases by 1.

{}^{14}_{6}\text{C} \rightarrow {}^{14}_{7}\text{N} + {}^{0}_{-1}\text{e}

Gamma emission doesn't change the mass number or atomic number: the nucleus just loses energy, often straight after alpha or beta decay. Practise with our nuclear equation questions.

Uses

Use	Radiation	Why
Smoke detectors	Alpha	Ionises air; smoke absorbs alpha, breaking the circuit and setting off the alarm
Paper and foil thickness control	Beta	Partly absorbed: the amount getting through depends on thickness
Sterilising medical equipment	Gamma	Penetrates packaging and kills bacteria
Medical tracers	Gamma (sometimes beta)	Detected outside the body; a short half-life limits the dose
Radiotherapy	Gamma (or beta)	Kills cancer cells

How long a source stays active depends on its half-life.

Contamination and irradiation

Irradiation is exposure to radiation from a source outside the body. The object doesn't become radioactive.
Contamination is when radioactive atoms get onto or into something. It stays exposed until the atoms are removed or decay.

Outside the body, alpha is the least dangerous because it can't get through skin. Inside the body (if swallowed or breathed in), alpha is the most dangerous because it's so strongly ionising. People working with sources use gloves and tongs, keep their distance, limit their time, and use shielding.

Background radiation

We're exposed to low levels of radiation all the time from natural sources (rocks such as granite, radon gas, cosmic rays from space) and from human activity (medical X-rays, fallout from past nuclear tests and accidents). When measuring a source's activity, subtract the background count.

Common mistakes

Saying beta particles come from the electron shells. They come from the nucleus.
Saying gamma is "stopped" by lead. Thick lead greatly reduces it.
Getting the beta equation wrong: the electron is written with mass 0 and atomic number −1.
Saying an irradiated object becomes radioactive.

Practice questions

Which type of radiation is stopped by paper?
Polonium-210 (atomic number 84) emits an alpha particle. What are the mass number and atomic number of the new nucleus?
Why is beta radiation used to monitor the thickness of paper, rather than alpha or gamma?

Answers: 1. Alpha. 2. Mass number 206, atomic number 82 (lead). 3. Alpha would be completely absorbed by the paper and gamma would pass straight through whatever the thickness; the amount of beta getting through changes with thickness.

Frequently asked questions

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

Alpha is a helium nucleus, strongly ionising and stopped by paper. Beta is a fast electron from the nucleus, stopped by a few millimetres of aluminium. Gamma is an electromagnetic wave, weakly ionising and only reduced by thick lead or concrete.

Which radiation is the most dangerous?

Outside the body, gamma and beta are more dangerous because they penetrate skin. Inside the body, alpha is the most dangerous because it's the most ionising.

What happens in beta decay?

A neutron turns into a proton and an electron, and the electron is emitted. The atomic number goes up by 1 and the mass number stays the same.

Sources

GCSE Physics (8463) specification: subject content. AQA.
Cambridge IGCSE Physics 0625 syllabus for 2026, 2027 and 2028. Cambridge International.

Half-life and radioactive decay explained

Half-life calculations and graphs, alpha, beta and gamma, nuclear equations and safety.

Half-life calculator for radioactive decay

Amount remaining, starting amount, time or half-life for radioactive decay, with working.

Atomic structure: protons, neutrons and electrons

Subatomic particles, atomic and mass numbers, isotopes, electron shells and atomic models.

Nuclear equations questions and answers

8 GCSE and IGCSE questions with worked answers, checked as you type.