IGCSE Physics Radioactivity Notes
Unstable nuclei decay by emitting radiation. Learn the three types of radiation, decay equations and half-life to master this topic.
Key concepts
- Alpha (α)
- A helium nucleus (2 protons, 2 neutrons); highly ionising but stopped by paper.
- Beta (β)
- A fast electron from the nucleus; moderately ionising, stopped by a few mm of aluminium.
- Gamma (γ)
- A high-energy electromagnetic wave; weakly ionising but very penetrating, reduced by thick lead.
- Radioactive decay is random and cannot be predicted for a single nucleus.
- Alpha decay reduces mass number by 4 and atomic number by 2.
- Beta decay increases atomic number by 1 (a neutron becomes a proton).
Key formulas
Alpha decay
A → (A−4) + alpha
mass −4, atomic number −2.
Beta decay
n → p + electron
atomic number +1.
Required practicals
- Investigate the penetrating power of alpha, beta and gamma radiation with absorbers.
- Measure and correct for background radiation.
Worked example
A nucleus emits an alpha particle. How do its mass number and atomic number change?
- 1Alpha is a helium nucleus with mass 4 and charge 2.
- 2Emitting it removes 4 from the mass number.
- 3It removes 2 from the atomic number.
- 4State the change clearly.
Answer: Mass number decreases by 4; atomic number decreases by 2.
Common mistakes
✗ Confusing ionising power with penetrating power.
✓ Alpha is most ionising but least penetrating; gamma is the opposite.
✗ Forgetting to subtract background radiation.
✓ Always subtract the background count rate from measured readings.
✗ Getting alpha and beta nuclear changes mixed up.
✓ Alpha: mass −4, number −2. Beta: number +1, mass unchanged.
Quick check
Try these, then reveal the answers.
Which radiation is stopped by a sheet of paper?Reveal answer
Alpha.
Which radiation is the most penetrating?Reveal answer
Gamma.
What happens to the atomic number in beta decay?Reveal answer
It increases by 1.
Exam tips
- Learn the penetration table: paper (α), aluminium (β), lead (γ).
- Always mention subtracting background radiation in count-rate questions.
- Balance mass and atomic numbers on both sides of a decay equation.
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Frequently asked questions
Why is radioactive decay described as random?+
You cannot predict when a particular nucleus will decay, only the probability across many nuclei.
What is background radiation?+
The low-level radiation always present from rocks, cosmic rays, food and medical sources.
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