on this cheat sheet
Everything on the GCSE Physics Radioactivity poster is written out below, section by section. Use it to search the sheet, copy parts into your own notes, or check a fact quickly.
Types of Radiation
| Type | Symbol | What it is | Range in air | Stopped by | Ionising power | Nuclear equation |
|---|---|---|---|---|---|---|
| Alpha | Helium nucleus (2 protons + 2 neutrons) | A few cm | Paper or skin | Strong | – atomic number decreases by 2, mass number decreases by 4 | |
| Beta | Fast-moving electron | A few metres | ~5 mm of aluminium | Moderate | – atomic number increases by 1, mass number unchanged | |
| Gamma | Electromagnetic wave | Hundreds of metres | Thick lead or metres of concrete | Weak | – atomic and mass numbers unchanged | |
| Neutron | n | Neutron | Varies | Thick concrete or water | Moderate | Neutrons may be emitted from some unstable nuclei |
Total mass number and total atomic number must balance on both sides of every equation.
Randomness, Activity and Count Rate
- Radioactive decay is random – you cannot predict which nucleus will decay next.
- Activity is the rate of decay, measured in becquerels (Bq), where 1 Bq = 1 decay per second.
- Count rate is the radiation detected by a Geiger–Müller tube per second.
- Subtract the background count to get a corrected count rate.
Half-Life
Half-life is the time for the number of unstable nuclei (or the activity, or the count rate) to halve.
- Read half-life from a graph by finding the time for activity to drop from one value to half that value (for example from 800 Bq to 400 Bq).
- After half-lives, the remaining fraction = .
- After 3 half-lives, only of the original activity remains.
Example: Initial activity = 640 Bq. After 2 half-lives: activity = 640 ÷ 2 ÷ 2 = 160 Bq. As a fraction of the initial value: .