What you'll learn
This revision guide covers all testable content on radioactivity from the Edexcel GCSE Physics specification. You'll understand the structure of atoms, the nature of ionising radiation, nuclear equations, and how to calculate half-life. These notes prepare you for questions worth approximately 8-12% of your final exam.
Key terms and definitions
Isotope — atoms of the same element with the same number of protons but different numbers of neutrons, resulting in different mass numbers.
Radioactive decay — the spontaneous and random process by which unstable atomic nuclei emit radiation to become more stable.
Activity — the rate at which unstable nuclei decay, measured in becquerels (Bq), where 1 Bq = 1 decay per second.
Half-life — the time taken for the number of radioactive nuclei in a sample to halve, or for the activity or count rate to fall to half its initial value.
Ionisation — the process by which an atom gains or loses electrons to form charged particles (ions), caused when radiation removes electrons from atoms.
Background radiation — low-level ionising radiation present in the environment at all times from natural and artificial sources.
Contamination — radioactive material getting onto or into objects, including living tissue, where it continues to emit radiation.
Irradiation — exposure to radiation from a source outside the body; the object exposed does not become radioactive.
Core concepts
Atomic structure and isotopes
All matter consists of atoms. Each atom has a small central nucleus containing protons and neutrons, surrounded by electrons in shells.
Atomic number (Z) — the number of protons in the nucleus, which defines the element.
Mass number (A) — the total number of protons and neutrons in the nucleus.
Notation: An element is written with its mass number A at the top left and its atomic number Z at the bottom left of the chemical symbol X, as in ²²⁶₈₈Ra.
Isotopes have identical chemical properties because they have the same electron configuration. However, isotopes have different physical properties, particularly nuclear stability. For example:
- Carbon-12 (¹²₆C) is stable
- Carbon-14 (¹⁴₆C) is radioactive
Unstable isotopes undergo radioactive decay to reach a more stable configuration. This process is:
- Random — impossible to predict which nucleus will decay next
- Spontaneous — not affected by external conditions like temperature or pressure
Types of ionising radiation
Three main types of radiation are emitted from radioactive nuclei:
Alpha (α) particles
- Composition: 2 protons + 2 neutrons (helium nucleus)
- Notation: ⁴₂He or ⁴₂ α
- Charge: +2
- Range in air: a few centimetres (typically 3-5 cm)
- Stopped by: paper, skin, a few centimetmetres of air
- Ionising power: strongly ionising
- Penetrating power: weakly penetrating
Beta (β) particles
- Composition: high-speed electron emitted from the nucleus
- Notation: ⁰₋₁e or ⁰₋₁ β
- Charge: -1
- Range in air: up to 1 metre
- Stopped by: thin aluminium (few millimetres), Perspex
- Ionising power: moderately ionising
- Penetrating power: moderately penetrating
Gamma (γ) rays
- Composition: electromagnetic radiation (high-frequency wave)
- Notation: γ
- Charge: 0
- Range in air: effectively unlimited (follows inverse square law)
- Stopped by: thick lead, several metres of concrete (never completely absorbed)
- Ionising power: weakly ionising
- Penetrating power: highly penetrating
Nuclear decay equations
Nuclear equations must balance for both mass number (top) and atomic number (bottom).
Alpha decay: When a nucleus emits an alpha particle, the mass number decreases by 4 and the atomic number decreases by 2.
²²⁶₈₈Ra → ²²²₈₆Rn + ⁴₂ α
Beta-minus decay: A neutron in the nucleus converts to a proton and an electron. The electron is emitted as a beta particle. Mass number stays the same, atomic number increases by 1.
¹⁴₆C → ¹⁴₇N + ⁰₋₁ β
Gamma emission: Often follows alpha or beta decay. The nucleus releases excess energy as a gamma ray. No change to mass number or atomic number.
⁶⁰₂₇Co → ⁶⁰₂₇Co + γ
You must be able to complete nuclear equations by calculating missing mass numbers or atomic numbers using conservation principles.
Half-life calculations
Half-life is a characteristic property of each radioactive isotope, ranging from fractions of a second to billions of years.
Method 1: Using the half-life formula (counting halvings)
Number of half-lives = total time ÷ half-life
Final activity = initial activity ÷ 2ⁿ where n = number of half-lives
Method 2: Reading from a graph
Plot activity (or count rate) against time. The half-life is the time taken for the activity to halve from any starting point on the curve.
The graph shows an exponential decay curve. To find half-life:
- Choose a point on the curve
- Halve the y-value
- Read the time difference on the x-axis
Key points:
- After 1 half-life: 50% remains
- After 2 half-lives: 25% remains
- After 3 half-lives: 12.5% remains
- After n half-lives: (½)ⁿ × 100% remains
Background radiation sources
Background radiation exists everywhere at low levels. You must subtract background count when measuring the activity of a source.
Natural sources (≈85% of total):
- Radon gas from rocks (mainly granite) — 50%
- Cosmic rays from space — 10%
- Rocks and soil containing uranium, thorium — 14%
- Food and drink containing carbon-14, potassium-40 — 11%
Artificial sources (≈15% of total):
- Medical procedures (X-rays, radiotherapy) — 14%
- Nuclear weapons testing (legacy) — <1%
- Nuclear power stations — <1%
Background count rate varies by:
- Location (higher at altitude, higher in granite areas like Cornwall)
- Occupation (airline crew, medical staff)
Uses and hazards of radiation
Medical applications:
- Tracers — beta or gamma emitters with short half-lives (hours to days) injected or swallowed to diagnose conditions
- Radiotherapy — gamma rays targeted at cancer cells to destroy them
- Sterilisation — gamma rays kill bacteria on surgical instruments
Industrial applications:
- Thickness monitoring — beta sources detect paper/foil thickness in manufacturing
- Smoke detectors — alpha sources ionise air; smoke disrupts the current
- Carbon dating — measuring carbon-14 content determines age of organic materials
Safety precautions:
Contamination hazards:
- Radioactive material enters the body through ingestion, inhalation or broken skin
- Continues to emit radiation from inside the body
- Particularly dangerous for alpha emitters (highly ionising at close range)
Prevention:
- Wear protective clothing, gloves, masks
- Sealed containers
- Good ventilation
- Monitor contamination with detectors
Irradiation hazards:
- Exposure to external radiation
- Can damage or kill cells
- Risk of cancer from damaged DNA
Prevention:
- Keep distance from sources
- Keep exposure time short
- Use shielding (appropriate to radiation type)
- Store in lead-lined containers
- Use tongs/robot arms for handling
- Wear film badges to monitor exposure
Worked examples
Example 1: Nuclear equation completion
Complete the nuclear equation for the alpha decay of polonium-210:
²¹⁰₈₄Po → Pb + ⁴₂α, where the lead nucleus has mass number A and atomic number Z to be found
Solution:
Mass number: 210 = A + 4, so A = 206 [1 mark]
Atomic number: 84 = Z + 2, so Z = 82 [1 mark]
²¹⁰₈₄Po → ²⁰⁶₈₂Pb + ⁴₂ α [1 mark for correct equation]
Example 2: Half-life calculation
A radioactive sample has an initial activity of 800 Bq. The half-life is 6 hours. Calculate the activity after 18 hours. [3 marks]
Solution:
Number of half-lives = 18 hours ÷ 6 hours = 3 half-lives [1 mark]
After 1 half-life: 800 ÷ 2 = 400 Bq After 2 half-lives: 400 ÷ 2 = 200 Bq After 3 half-lives: 200 ÷ 2 = 100 Bq [1 mark for method]
OR: Activity = 800 ÷ 2³ = 800 ÷ 8 = 100 Bq
Final activity = 100 Bq [1 mark]
Example 3: Choosing appropriate radiation
A hospital needs to sterilise surgical equipment without damaging heat-sensitive plastics. Explain which type of radiation would be most suitable and why. [3 marks]
Solution:
Gamma radiation [1 mark]
Gamma rays are highly penetrating so will pass through the entire equipment and packaging, killing all bacteria throughout [1 mark]
The equipment does not become radioactive (only irradiated, not contaminated) so is safe to use immediately [1 mark]
Common mistakes and how to avoid them
Confusing isotopes with ions. Isotopes have different numbers of neutrons (different mass number, same atomic number). Ions have different numbers of electrons (charged particles). Isotopes are identified by their mass number.
Incorrect nuclear equations. Always check both mass numbers (top) and atomic numbers (bottom) balance on both sides. For beta decay, remember the atomic number increases by 1 even though mass number stays constant.
Forgetting background radiation. When calculating the activity of a source from count rate measurements, you must subtract the background count. Net count rate = measured count rate - background count rate.
Half-life arithmetic errors. Keep dividing by 2 for each half-life, or use 2ⁿ as the denominator. After 4 half-lives, 1/16 remains, not ¼.
Mixing up contamination and irradiation. Contamination means radioactive material is present on/in an object (can be transferred). Irradiation means exposure to radiation from an external source (object doesn't become radioactive).
Wrong radiation for the application. Alpha is blocked by paper/skin, so useless outside the body but very dangerous inside. Gamma penetrates everything, so good for sterilisation and cancer treatment. Beta is in between, ideal for thickness detection.
Exam technique for "Radioactivity"
Command word "Explain" requires you to give reasons using scientific knowledge. State your point then justify it using physics principles. For 3 marks, typically need two developed points or three simple points.
Calculation questions always show your working. Even if the final answer is wrong, method marks are available. Include units in your final answer. For half-life problems, clearly state the number of half-lives elapsed.
Comparing radiation types — use comparative language: "Alpha is more ionising than gamma" or "Beta has greater range in air than alpha." Create a clear comparison, don't just list properties.
6-mark extended response questions require logical structure. Use the writing lines available as a guide to how much detail is expected. Include: identification of correct radiation/process, scientific explanation of properties, and application to the specific context given.
Quick revision summary
Radioactive decay is spontaneous and random, emitting alpha (helium nuclei), beta (electrons) or gamma (electromagnetic waves). Alpha is highly ionising but easily stopped; gamma is weakly ionising but highly penetrating. Nuclear equations must balance for mass and atomic number. Half-life is the time for activity to halve. Background radiation comes mainly from natural sources. Contamination involves radioactive material on/in objects; irradiation is exposure to external radiation. Medical and industrial uses exploit different radiation properties. Safety requires shielding, distance, limiting time, and protective equipment.