6.2.2Radioactivity
Properties of Radiation — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions
5 min read 7 parts Lesson 10 of 20
Section 1 of 7The three types compared
| Property | Alpha (α) | Beta (β) | Gamma (γ) |
|---|---|---|---|
| What it is | 2 protons + 2 neutrons (helium nucleus) | A fast electron (β−) or positron (β+) from the nucleus | Electromagnetic wave |
| Relative charge | +2 | −1 (β+: +1) | 0 |
| Ionising power | Very strong | Moderate | Weak |
| Range in air | A few centimetres | Tens of centimetres to a metre | Very long; intensity falls with distance |
| Stopped by | Paper or a few cm of air | A few millimetres of aluminium | Thick lead or several metres of concrete |
Section 2 of 7Trends
1Ionisation decreases
- Ionisation decreases Alpha ionises the most, gamma the least.
2Penetration increases
- Penetration increases Alpha is stopped easily; gamma passes through most materials.
3Range increases
- Range increases Strongly ionising radiation loses its energy quickly, so it has a short range.
Section 3 of 7Why ionisation and range are linked
Ionising
- Ionising Ionisation means knocking electrons from atoms to make ions, which transfers energy from the radiation each time.
Alpha
- Alpha Large, slow, highly charged, so it interacts with many atoms and soon runs out of energy.
Gamma
- Gamma No charge, so it interacts rarely and travels far.
Section 4 of 7Identifying an unknown source
1
Place a Geiger–Müller tube a fixed distance from the source and record the count rate.
2
Subtract the background count rate, measured with no source present.
3
Put a sheet of paper in the way; a big drop in count rate means alpha is present.
4
Replace it with a few millimetres of aluminium; a big drop here means beta.
5
If the count rate falls only with thick lead, the source emits gamma.
6
Remember that thin aluminium does not stop beta, so check the thickness used.
Section 5 of 7Example interpretation
| Observation | Conclusion |
|---|---|
| Count unchanged by paper, large drop with 3 mm aluminium | Beta emitter |
| Count reduced by paper to background | Alpha emitter |
| Count only reduced by thick lead | Gamma emitter |
Section 6 of 7Key terms
| Term | Meaning |
|---|---|
| Ionisation | The process of an atom losing or gaining electrons to become an ion. |
| Penetrating power | How far radiation can pass through a material. |
| Count rate | The number of radiation detections per second or minute. |
| Geiger–Müller tube | A detector that clicks or counts each time radiation enters it. |
| Background radiation | Low-level radiation always present from natural and artificial sources. |
Section 7 of 7Exam technique
1
Saying alpha is the most dangerous outside the body because it penetrates the furthest: alpha is stopped by skin; it is most hazardous when inside the body.
2
Forgetting to subtract background radiation before drawing conclusions.
3
Stating that aluminium of any thickness stops beta: only a few millimetres do.
4
Mixing up the trendsmore ionising means less penetrating.
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