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Learn5054Paper 2 · TheoryNuclear physicsRadioactive emissions and decay
Nuclear physics · Subtopic 2 of 4

Radioactive Emissions and Decay

Unstable nuclei decay randomly and spontaneously, giving out alpha particles (helium nuclei), beta particles (fast electrons) or gamma rays (electromagnetic waves). Alpha is the most ionising but is stopped by paper; gamma is the least ionising and most penetrating. Decay equations balance both the nucleon numbers and the proton numbers.

Cambridge 5054 Paper 1Cambridge 5054 Paper 2Cambridge 0625 Papers 2, 4Cambridge 0625 Papers 1, 3Edexcel 4PH1 Paper 1PEdexcel 4PH1 Paper 2P≈ 30 minRevision 5 · updated 2026-10-27
Section 1 of 14

Learning objectives

  • Describe the nature, ionising effect and penetration of α, β and γ.
  • Describe their deflection in electric and magnetic fields.
  • Detect radiation with a GM tube and correct for background radiation.
  • Write and complete decay equations.
Pause and think

Why is an alpha source harmless held in your hand behind a sheet of paper, but extremely dangerous if swallowed?

Section 2 of 14

Core idea

Start herePaper stops it, lead barely does

Some radiation is stopped by a sheet of paper; some passes through centimetres of lead. Three types of nuclear radiation behave very differently, and that decides which is useful, and which is dangerous, where.

The big idea

Unstable nuclei decay randomly: α (24^4_2He), β (fast electron), γ (EM wave). α most ionising, least penetrating; γ the reverse. Balance AA and ZZ in equations.

1. The three radiations

Penetrating power
αβγpaperaluminiumthick lead
Alpha travels only a few cm in air. Gamma is reduced by lead but never completely stopped.
  • α: stopped by paper; strongly ionising; charge +2.
  • β: stopped by a few mm of aluminium; charge −1.
  • γ: reduced by thick lead; weakly ionising; no charge.

2. Decay equations

Worked exampleAlpha and beta decay

Complete:  92238^{238}_{\ 92}U emits α;  614^{14}_{\ 6}C emits β.

Goal
Daughter nuclei.
Strategy
Top numbers and bottom numbers each balance.
  1.  92238U→ 90234Th+24α^{238}_{\ 92}\text{U}\to{}^{234}_{\ 90}\text{Th}+{}^4_2\alpha
    A−4A-4, Z−2Z-2.
  2.  614C→ 714N+−1 0β^{14}_{\ 6}\text{C}\to{}^{14}_{\ 7}\text{N}+{}^{\ 0}_{-1}\beta
    A neutron becomes a proton: AA same, Z+1Z+1.
AnswerThorium-234; nitrogen-14.
✓ Check238=234+4238=234+4, 92=90+292=90+2; 6=7−16=7-1 ✓.
Key takeawaySafety Background radiation is always present. Use tongs, minimise time, maximise distance, use shielding.
Pause and think

Which radiation for a smoke detector?

Summary

  • α β γ properties.
  • Balance AA and ZZ.
Key words
Ionising
Removing electrons from atoms.
HistoryBecquerel's cloudy day. In 18961896 Henri Becquerel planned to test whether sunlight made uranium salts emit X-rays. It was cloudy, so he left them in a drawer on a photographic plate. Developing it anyway, he found the plate fogged: uranium emitted radiation all by itself. Radioactivity had been discovered.
Fun factBananas are (slightly) radioactive. Bananas contain potassium-40, a natural radioactive isotope. Eating one gives a dose so tiny that scientists use the "banana equivalent dose" as a joke unit. You'd need to eat about 1010 million bananas at once for a dangerous dose.
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