Radioactive Decay: Alpha, Beta and Gamma Radiation — AQA GCSE Physics
Unstable nuclei decay randomly, emitting different types of nuclear radiation.
Radioactive decay
Some atomic nuclei are unstable and decay randomly, giving out radiation. This is a random process — we cannot predict when a particular nucleus will decay. Activity (rate of decay) is measured in becquerels (Bq); the count-rate is measured with a Geiger–Müller tube.
The three main types
| Radiation | What it is | Penetration | Ionising power |
|---|---|---|---|
| Alpha (α) | a helium nucleus (2 protons + 2 neutrons) | stopped by paper/skin; range a few cm in air | most ionising |
| Beta (β) | a high-speed electron from the nucleus | stopped by a few mm of aluminium | moderate |
| Gamma (γ) | a high-energy electromagnetic wave | stopped by thick lead/concrete | least ionising |
(Neutron emission can also occur.)
Ionising power vs penetration
There is a trade-off: alpha is the most ionising but least penetrating, while gamma is the least ionising but most penetrating.
Exam tips
- Radioactive decay is a random process; activity is in becquerels.
- Learn the three radiations with nature, penetration and ionising power.
- Alpha = most ionising, least penetrating; gamma = the opposite.
- Match each to what stops it (paper / aluminium / lead).