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: alpha, beta, gamma and neutron
All four radiation types are compared across five properties: symbol, what it is, range in air, what stops it, ionising power, and a decay equation example.
- Alpha (α) - a helium nucleus (2 protons + 2 neutrons), symbol ⁴₂He. Range: a few centimetres in air. Stopped by paper or skin. Ionising power: strong. In a decay equation, atomic number decreases by 2 and mass number decreases by 4.
- Beta (β⁻) - a fast-moving electron, symbol ⁰₋₁e. Range: a few metres in air. Stopped by a few millimetres of aluminium. Ionising power: moderate. Atomic number increases by 1; mass number is unchanged.
- Gamma (γ) - an electromagnetic wave, symbol γ. Range: hundreds of metres. Stopped by thick lead or metres of concrete. Ionising power: weak. Atomic number and mass number are both unchanged.
- Neutron (n) - a neutron, symbol n. Range: varies (wide range). Stopped by thick concrete or water. Ionising power: moderate. Neutrons may be emitted from some unstable nuclei.
Total mass number and total atomic number must balance on both sides of every nuclear equation.
Randomness, activity and count rate
- Radioactive decay is random - you cannot predict which nucleus decays 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, measured in counts per second.
- Subtract background radiation from the count rate to get a corrected count rate.
A Geiger-Müller tube is used to detect radiation.
Half-life
- Half-life is the time for the number of unstable nuclei (or the activity, or the count rate) to halve.
- Read it off the graph by finding the time for activity to drop from 800 Bq to 400 Bq.
- After n half-lives, the remaining fraction of the original activity is (1/2)ⁿ.
- After 3 half-lives, only 1/8 of the original activity is left.
Example: Initial activity = 640 Bq. After 2 half-lives: 640 ÷ 1 = 160 Bq. As a fraction of the initial activity: 160/640 = 1/4 = 25%.