Radioactive decay and nuclear radiation — AQA GCSE Physics

SPEC 8463 • MODULE 4 • PAPER 1

Radioactive Decay & Nuclear Equations

Four types of radiation, their properties, and how to balance nuclear equations. Recurring marks every series — and the foundation for half-life, contamination, and uses of radiation in the lessons that follow.

Why This Lesson Matters

Two skill clusters every Paper 1 tests: knowing the properties of alpha, beta, and gamma (penetration, range, ionising power) and balancing nuclear equations. Both are usually straightforward marks — if you've memorised the values and the balancing rules.

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High Priority

4.4.2

Spec Reference

4

Emissions to Know

By the end of this lesson you can…

Define radioactive decay, activity (in becquerels), and count rate

Name the four nuclear emissions and what each one is

State the penetration, range in air, and ionising power of α, β, and γ

Balance a nuclear equation for single alpha or beta decay

Explain why gamma emission doesn't change A or Z

What Is Radioactive Decay?

Radioactive decay

Some atomic nuclei are unstable. The nucleus gives out radiation as it changes to become more stable. This is a random process called radioactive decay.

Three things to know

It's random. You can't predict when any individual nucleus will decay — only the average behaviour of a large sample

The driver is stability. Decay happens because the nucleus is unstable; emitting radiation makes it more stable

The element can change. Some types of decay change the proton number, so the daughter nucleus is a different element

Bridge from Lesson 4.01

In Lesson 4.01 you saw isotopes — same proton number, different neutron count. Some isotopes are stable; others have unstable nuclei that decay. This lesson explains how those unstable nuclei emit radiation.

Activity & Count Rate

Two related but different quantities. Both measure how fast decay is happening — but in different ways.

Activity

The rate at which a source of unstable nuclei decays.

Unit: the becquerel (Bq)

1 Bq = 1 decay per second.

Count rate

The number of decays recorded each second by a detector (e.g. a Geiger-Müller tube).

Count rate is always lower than activity because some decays miss the detector.

Quick distinction

Activity = how many decays happen per second (in the source itself).

Count rate = how many decays we actually detect per second (depends on detector position, efficiency, etc.). Both are measured in counts per second or per minute.

The Four Nuclear Emissions

An unstable nucleus can emit one of four types of radiation. Memorise all four — and their symbols.

Alpha

A helium nucleus

(2 protons + 2 neutrons)

Beta

A high-speed electron ejected from the nucleus as a neutron turns into a proton

Gamma

Electromagnetic radiation from the nucleus

Neutron

A neutron emitted from the nucleus

Spec note

The spec lists all four as types of nuclear emission. However, the spec also says required knowledge of properties (penetration, range, ionising) is limited to α, β, and γ — neutron properties aren't examinable at this level.

Symbols for Nuclear Equations

Each particle has an A/Z notation you'll need for balancing equations.

ParticleSymbolMass (rel.)Charge (rel.)What the numbers mean

Alpha (α)4 2 He4+2A helium nucleus (2p + 2n)

Beta (β)0 −1 e0−1Electron has negligible mass

Gamma (γ)γ00Pure EM energy — no particle

Neutron (n)1 0 n10Same mass as a proton, no charge

Memorise exactly

The single biggest error in nuclear equations is mixing up the values. Alpha has mass 4 and charge +2. Beta has mass 0 and charge −1. Writing "⁴₂e" for beta (instead of ⁰₋₁e) is a recurring examiner-flagged mistake — it confuses beta with alpha.

Properties: Penetration, Range & Ionising Power

The spec limits required knowledge to these three properties for alpha, beta, and gamma. Memorise all three columns.

RadiationPenetration (stopped by)Range in airIonising power

Alpha (α)A sheet of paper , a few cm of air, or human skinA few centimetresVery high (highly ionising)

Beta (β)A few millimetres of aluminiumAbout 1 m (up to a few metres)Medium

Gamma (γ)Reduced (never fully stopped) by thick lead or many cm of concreteVery long (effectively no upper limit)Low (weakly ionising)

The pattern

Penetration and ionising power are opposites. Alpha = most ionising, least penetrating. Gamma = least ionising, most penetrating. Beta is in the middle on both. This pattern alone unlocks most "best source for X" questions.

Penetration — Visualised

Three sources fire α, β and γ at three materials in series. Each is stopped (or reduced) by a different barrier.

Mark-scheme phrasing

Gamma is "reduced" (or "absorbed by"), not "stopped", because some gamma always penetrates even thick shielding. Saying gamma is "stopped by lead" loses marks — say "reduced by lead" instead.

Choosing the Right Source

"Suggest the best source for…" questions use the properties table to match a radiation to a use. The logic depends on whether radiation needs to get out, stop quickly, or be absorbed.

UseBest sourceWhy

Smoke detector (sources inside, alarm if blocked)AlphaStops at smoke particles or in a few cm of air — safe inside the device, sensitive to blockage

Medical tracer (inside the body, detected outside)GammaPenetrates body tissue, low ionisation = low cell damage at small doses

Thickness control on a factory production line (e.g. paper, foil)Beta (for paper or thin metal)Penetrates the material but is partly absorbed — count rate changes with thickness

Sterilising surgical equipment (radiation must reach inside packaging)GammaPenetrates packaging; long range; kills bacteria

How to argue these in the exam

Always state: the property you're using (penetration / range / ionising) AND why that property fits the use. "Gamma is most penetrating" alone isn't enough — say "gamma is most penetrating, so it can pass through body tissue and be detected outside the patient".

Nuclear Equations — The Two Rules

Nuclear equations represent radioactive decay using the AZX notation from Lesson 4.01. Two rules govern balancing — and the spec is strict on both.

Rule 1: Mass numbers balance

The sum of mass numbers on the left equals the sum of mass numbers on the right.

Rule 2: Atomic numbers balance

The sum of atomic numbers on the left equals the sum of atomic numbers on the right.

Spec note

The spec is explicit: "This is limited to balancing the atomic numbers and mass numbers. The identification of daughter elements from such decays is not required." So you only need to find the missing A and Z values — you don't need to name the resulting element. (Though mark schemes often credit the element name if you do.)

Alpha Decay — Worked Example

Polonium-210 undergoes alpha decay. Complete the nuclear equation. 2 marks

21084Po → ??Pb + 42He

1

Apply Rule 1 (mass numbers): 210 = A + 4, so A = 206

2

Apply Rule 2 (atomic numbers): 84 = Z + 2, so Z = 82

3

Final equation:

21084Po → 20682Pb + 42He

Pattern: in alpha decay, mass number drops by 4 and atomic number drops by 2.

Beta Decay — Worked Example

Tritium (hydrogen-3) undergoes beta decay. Complete the nuclear equation. 2 marks

31H → ??He + 0−1e

1

Apply Rule 1 (mass numbers): 3 = A + 0, so A = 3

2

Apply Rule 2 (atomic numbers): 1 = Z + (−1), so Z = 2

3

Final equation:

31H → 32He + 0−1e

Pattern: in beta decay, mass number is unchanged and atomic number increases by 1 (a neutron has turned into a proton + electron).

Gamma Emission — A Special Case

Gamma is electromagnetic radiation, not a particle. It has no mass and no charge, so gamma emission doesn't change the nucleus's mass number or atomic number.

Spec wording

"The emission of a gamma ray does not cause the mass or the charge of the nucleus to change."

The nucleus simply loses energy — going from a higher-energy state to a lower-energy state — without changing what it's made of. Same protons, same neutrons, just less energy.

For example, an excited technetium-99 nucleus (used in medical imaging) emits a gamma ray and becomes its ground-state form:

9943Tc* → 9943Tc + γ

Mass number 99 → 99 (unchanged) · Atomic number 43 → 43 (unchanged)

Why is this useful?

Gamma is often emitted alongside alpha or beta decay — the alpha or beta carries away most of the change, and the leftover excess energy is released as a gamma ray. So a single nucleus can emit multiple types of radiation.

Balance the Equation

A radium-226 nucleus undergoes alpha decay.

22688Ra → ??Rn + ???

(a) Find the mass number and atomic number of the daughter nucleus. 2 marks

(b) Write the full balanced equation. 2 marks

a1

Alpha decay → mass number drops by 4, atomic number drops by 2.

a2

A = 226 − 4 = 222 · Z = 88 − 2 = 86

b1

The alpha particle on the right is ⁴₂He.

b2

Full equation:

22688Ra → 22286Rn + 42He

Check: mass numbers 226 = 222 + 4 ✓ | atomic numbers 88 = 86 + 2 ✓

Examiner Alerts

Four recurring errors that lose easy marks every series.

Alert 1 — Beta particle mass

The beta particle is ⁰₋₁e — mass number 0, charge −1. Writing "⁴₋₁e" (with mass 4) confuses beta with alpha. Examiner reports flag this recurringly.

Alert 2 — Alpha is a helium NUCLEUS, not atom

"Alpha is a helium nucleus" ✓ or "Alpha consists of 2 protons and 2 neutrons" ✓ — both creditworthy. But writing "Alpha is a helium atom" is wrong (an atom would include electrons). And under the AQA "list principle", writing both ("a helium atom consisting of 2 protons and 2 neutrons") gets zero because one phrase contradicts the other.

Alert 3 — Specific shielding material

"Beta is stopped by paper" is wrong (paper stops alpha only). Beta needs a few millimetres of aluminium. Gamma is reduced by thick lead or concrete (not "stopped"). State both the material and the approximate thickness.

Alert 4 — Equations must balance

The mark scheme is strict: if mass numbers OR atomic numbers don't balance on both sides, the equation marks are not awarded. Always check: sum on left = sum on right, for both top and bottom numbers.

Common Errors — and the Fixes

Students often write…Mark-scheme versionWhy it matters

"Alpha decay decreases atomic number by 4"Alpha decay decreases atomic number by 2 , mass number by 4Magnitudes swapped

"Beta decay decreases the atomic number"Beta decay increases the atomic number by 1 (neutron → proton)Wrong direction

"Gamma has charge −1"Gamma is EM radiation — no mass, no chargeFundamental property error

"Beta is stopped by paper"Alpha is stopped by paper; beta needs ~5 mm aluminiumWrong material

"Gamma is stopped by lead"Gamma is reduced (never fully stopped) by thick lead or concrete"Stopped" is incorrect — gamma always partly penetrates

"Activity is measured in counts per second"Activity is in becquerels (Bq) ; count rate is in counts per secondConfuses the two quantities

Stretch Question

Two radioactive sources, X and Y, both emit a single type of radiation:

X emits alpha radiation

Y emits gamma radiation

A student says: "X is more dangerous than Y because alpha is more ionising than gamma."

Evaluate this statement, considering whether the source is held outside the body or inside the body. 4 marks

1

The student is partly correct but oversimplifies — the danger depends on whether the source is outside or inside the body.

2

Outside the body: Y (gamma) is more dangerous, because gamma can penetrate skin and reach internal tissue. Alpha is stopped by skin or even air, so X is much less of a hazard externally.

3

Inside the body: X (alpha) is more dangerous, because alpha is highly ionising and deposits all its energy in a small volume of nearby tissue. Gamma passes through with relatively little ionisation.

4

Conclusion: the student's statement is correct for sources inside the body but wrong for sources outside. Ionising power alone doesn't determine risk — you must also consider penetration.

Checkpoint — What You Can Now Do

Tick each one off before moving on

✓ Define radioactive decay as a random process where unstable nuclei emit radiation to become more stable

✓ State that activity is measured in becquerels (Bq), and count rate is decays per second on a detector

✓ Name the four emissions: α, β, γ, n — and what each one consists of

✓ State the penetration, range in air, and ionising power of α, β and γ

✓ Use the properties to suggest the best source for a given use

✓ Write balanced nuclear equations for single α and β decay

✓ Explain why gamma emission doesn't change the mass or atomic number

Next Lesson

Lesson 4.04 — Half-Lives and the Random Nature of Radioactive Decay. Activity drops over time as nuclei decay. Half-life is the time taken for that drop to halve — and it's the most-tested concept in this whole module.