Curriculum fit: GCSE Physics and Combined Science (atomic structure and radioactivity). US: high school physical science (nuclear processes).
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What Is It?
Some atoms have unstable nuclei. They become more stable by emitting radiation, a process called radioactive decay. This is random: you cannot predict when a particular nucleus will decay.
The three main types are alpha, beta and gamma radiation. They differ in what they are, how strongly they ionise and how far they penetrate matter.
Key Formulas and Ideas
- Alpha (α): a helium nucleus (2 protons and 2 neutrons). Stopped by paper. Strongly ionising.
- Beta (β): a fast electron from the nucleus. Stopped by a few millimetres of aluminium.
- Gamma (γ): electromagnetic radiation. Reduced by thick lead or concrete. Weakly ionising.
- Half-life: the time taken for the activity (or the number of unstable nuclei) to halve.
Worked Examples
Half-Life Calculation
A sample has an activity of 800 counts per minute and a half-life of 3 days. What is the activity after 9 days?
- 9 days = 3 half-lives.
- 800 → 400 → 200 → 100.
✓ Answer 100 counts per minute
Fraction Remaining
What fraction of a radioactive sample remains after 2 half-lives?
- After 1 half-life: 1/2 remains.
- After 2: 1/4 remains.
✓ Answer 1/4 (25%)
Alpha Decay
Uranium-238 (atomic number 92) emits an alpha particle. What nucleus results?
- Alpha particle: mass number 4, atomic number 2.
- New mass number: 238 − 4 = 234. New atomic number: 92 − 2 = 90.
- Atomic number 90 is thorium.
✓ Answer Thorium-234
Common Mistakes to Avoid
- Saying half-life is half the time until the sample is gone.
- Mixing up the penetrating power of alpha and gamma.
- Forgetting to change both mass and atomic numbers in decay equations.
- Saying radioactivity can be switched off.
Practice Questions
Frequently Asked Questions
What is background radiation?
Low-level radiation from natural sources such as rocks, cosmic rays and food, as well as some human-made sources.
What are medical uses of radiation?
Gamma sources are used as tracers and in radiotherapy to destroy cancer cells.
Keep Going With These Guides
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Use v = fλ, tell transverse from longitudinal waves and put the types of electromagnetic radiation in order, with worked examples.
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Use the kinetic and gravitational potential energy formulas, apply conservation of energy and calculate efficiency, with worked examples.
GCSEHigh SchoolSources and Review
Reviewed by StudyMorrow editorial review on 5 Nov 2026.