Who this is for: Cambridge IGCSE Physics (0625) students who can name radiation types but lose marks on half-life calculations, penetration comparisons, or balancing nuclear equations.
What query it owns: how to understand and revise radioactivity in Cambridge IGCSE Physics.
Why this is safe: this page owns the radioactivity revision-guide angle, while Tutopiya’s Radioactivity subtopic page owns the learning resource and the free Radioactivity quiz owns the practice.

Radioactivity is the spontaneous emission of radiation from unstable nuclei. Cambridge IGCSE Physics (0625) tests whether you can compare alpha, beta and gamma properties, use half-life in calculations, and explain detection and safety precautions. This guide covers the syllabus pathway and the command words that appear every year.

Key takeaways

• Alpha (α): helium nucleus (²⁴He), +2 charge, most ionising, least penetrating — stopped by paper/skin.
• Beta (β): high-speed electron from nucleus, −1 charge, stopped by thin aluminium.
• Gamma (γ): electromagnetic wave, no charge, least ionising, most penetrating — reduced by thick lead/concrete.
• Half-life: time for activity (or count rate) to halve; random for individual atoms, predictable for large samples.
• Background radiation comes from rocks, cosmic rays, medical sources and nuclear tests.

What is radioactivity in Cambridge IGCSE Physics?

Radioactivity is the random, spontaneous decay of unstable atomic nuclei, emitting alpha, beta or gamma radiation. Decay changes the nucleus (new element after alpha/beta) and reduces the number of undecayed nuclei over time. Half-life quantifies how quickly activity falls. Read the full notes on Tutopiya’s Radioactivity subtopic page before attempting questions.

The core ideas you must master

Radiation	Nature	Charge	Ionising power	Penetration	Stopped by
Alpha (α)	2 protons + 2 neutrons	+2	Highest	Lowest	Paper, few cm air, skin
Beta (β)	Electron from nucleus	−1	Medium	Medium	Thin aluminium (~5 mm)
Gamma (γ)	EM wave	0	Lowest	Highest	Thick lead or concrete

Alpha and beta decay — nuclear equations

Decay type	What is emitted	Mass number change	Atomic number change	Example
Alpha	²⁴He nucleus	−4	−2	²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He
Beta	Electron (neutron → proton)	0	+1	¹⁴₆C → ¹⁴₇N + ⁰₋₁e
Gamma	γ photon	0	0	Nucleus loses energy only

Half-life — interpretation table

Half-lives elapsed	Fraction remaining	Fraction decayed
0	1 (100%)	0
1	½ (50%)	½
2	¼ (25%)	¾
3	⅛ (12.5%)	⅞
n	(½)ⁿ	1 − (½)ⁿ

Radioactivity in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical radioactivity stem
Define	Precise syllabus definition	”Define half-life.”
Compare	Similarities and differences	”Compare alpha and beta radiation.”
Calculate	Use half-life or decay data	”Calculate the activity after 3 half-lives.”
Explain	Cause and effect	”Explain why gamma needs thick shielding.”
Suggest	Safety or application	”Suggest precautions when handling radioactive sources.”

Worked exam-style stems (how to answer the wording)

1. “Define half-life.” The time taken for half the radioactive nuclei in a sample to decay, or the time for the activity to halve. Reward: both nuclei and activity wording accepted.
2. “A source has a count rate of 800 counts/min. After two half-lives, what is the count rate?” After 1 half-life: 400. After 2: 200 counts/min. Reward: correct halving twice.
3. “State which radiation type is stopped by a sheet of paper.” Alpha. Reward: alpha named; optional brief reason (large, +2 charge, strong interaction).

When you can recognise the wording instantly, work the Radioactivity quiz and mixed stems in the Nuclear Physics topical past paper questions.

How radioactivity connects to the rest of the syllabus

Radioactivity builds on the Nuclear Model of an Atom — you need proton and neutron numbers to balance decay equations. The Cambridge IGCSE Physics resource hub links every Nuclear Physics subtopic.

Common mistakes students make

• Stating beta particles come from electron shells (they come from the nucleus).
• Reversing penetration and ionising power (alpha is most ionising, least penetrating).
• Subtracting half-lives arithmetically instead of halving repeatedly.
• Confusing activity with total number of nuclei in half-life definition.
• Forgetting background radiation when interpreting count-rate graphs.

When you need more support

If half-life and radiation-comparison questions keep costing marks, work through the Radioactivity quiz, then get focused help from a Cambridge IGCSE Physics tutor.

Frequently asked questions

Is radioactivity hard in Cambridge IGCSE Physics? The radiation table is learnable; marks are lost on half-life arithmetic and incomplete decay equations.

What is the difference between irradiation and contamination? Irradiation = exposed to radiation; contamination = radioactive material on or inside the object.

Do I need to balance nuclear equations? Yes — mass number and atomic number must balance on both sides for alpha and beta decay.

How do I revise radioactivity effectively? Learn the properties table, practise half-life halving, balance two decay equations, then take the Radioactivity quiz.

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