Part 1 of 5The Three Types
Radioactive substances emit radiation from the nuclei of their atoms without any external cause.
Radioactive decay
- An unstable nucleus emits radiation to become more stable; the process is random,
- so we cannot predict when one particular nucleus will decay.
Alpha (α)
- A particle made of two protons and two neutrons, identical to a helium nucleus, so it is relatively heavy and carries a positive charge.
Beta (β)
- A fast-moving electron that comes from the nucleus when a neutron changes into a proton, carrying a negative charge.
Gamma (γ)
- Electromagnetic radiation of very high frequency, with no mass and no charge, emitted by the nucleus after other decays.
Detection
- A Geiger–Müller tube and counter, or photographic film, detects all three radiations by the ionisation they cause.
Part 2 of 5Penetrating Power
Penetrating Power
Alpha
- Stopped by a sheet of paper, a few centimetres of air or the dead outer layer of skin, so it has the lowest penetrating power.
Beta
- Passes through paper but is stopped by a few millimetres of aluminium or a thin sheet of metal.
Gamma
- Passes through paper and aluminium, and is reduced but never completely stopped by thick lead or very thick concrete.
Range in air
- Alpha travels only a few centimetres, beta travels about a metre or so, and gamma travels very much further.
Absorbers
- The denser and thicker the absorber, the more radiation it stops, which is why lead shielding is used.
| Radiation | Nature | Stopped by |
|---|---|---|
| Alpha | Helium nucleus (2 protons, 2 neutrons) | Paper |
| Beta | Fast electron | Few mm of aluminium |
| Gamma | Electromagnetic wave | Thick lead or concrete (reduced, not stopped) |
Part 3 of 5Identifying Radiation Experimentally
1
Measure the background count rate with the source removed and write it down, so it can be subtracted from later readings.
2
Place the source at a fixed distance from the Geiger–Müller tube and record the count rate with no absorber in the way.
3
Insert a sheet of paper and record the count rate again; if it drops sharply the source emits alpha.
4
Replace the paper with a few millimetres of aluminium; a further drop shows beta radiation is present.
5
Finally place a thick lead sheet in the path; any count still above background after the aluminium indicates gamma radiation.
Part 4 of 5Worked Example
Question
- Three unknown sources are tested.
- Source A is completely stopped by paper.
- Source B passes through paper but is completely stopped by 3 mm of aluminium.
- Source C is still detected after 3 mm of aluminium but is greatly reduced by a thick sheet of lead.
- Identify each source.
- Source A is stopped by paper, the least penetrating radiation, so it emits alpha.
- Source B passes through paper but is stopped by thin aluminium, so it emits beta.
- Source C passes through aluminium and needs thick lead to reduce it, so it emits gamma.
- State the order of penetrating poweralpha is lowest, then beta, then gamma highest.
Answer A is alpha, B is beta and C is gamma.
Part 5 of 5Exam Checklist & Tips
6.1.1 – Key Points for the Exam
- Radioactive decay The random emission of radiation from an unstable nucleus.
- Alpha particle Two protons and two neutrons, the same as a helium nucleus.
- Beta particle A high-speed electron emitted from the nucleus.
- Gamma ray High-frequency electromagnetic radiation emitted from a nucleus.
- Geiger–Müller tube A detector that produces an electrical pulse for each radiation particle or ray it absorbs.
Common Mistakes
- Saying gamma radiation is completely stopped by lead; it is only reduced to a lower level.
- Thinking beta particles come from the electron shells instead of the nucleus.
- Forgetting to subtract background radiation before comparing readings.
- Describing gamma as a particle with mass and charge.