Ionising radiation is detected by its effect on matter: it knocks electrons out of atoms, creating ions. This ionisation can be detected and measured.
Detection methods
| Detector | How it works |
|---|---|
| Geiger-Muller (GM) tube and counter | Gas inside the tube is ionised by radiation. Each ionisation event produces a pulse of current, which is counted. Gives a count rate (counts per second or per minute) |
| Photographic film | Radiation fogs the film. Used in film badges worn by workers to monitor exposure |
Background radiation
Radiation is always present from natural and artificial sources. This is called background radiation.
| Source | Type |
|---|---|
| Rocks and soil (e.g. radon gas) | Natural |
| Cosmic rays from space | Natural |
| Food and drink (e.g. potassium-40) | Natural |
| Medical X-rays and scans | Artificial |
| Nuclear power and weapons testing | Artificial |
When measuring the activity of a source, the background count rate must be subtracted to find the corrected count rate from the source alone.
Hazards of ionising radiation
Radiation can damage or kill living cells by ionising molecules in DNA. This can cause:
- Cell death (radiation burns, radiation sickness at high doses).
- Mutations in DNA (which may cause cancer).
Alpha radiation is the most dangerous inside the body (strongest ioniser). Gamma is the most dangerous outside the body (most penetrating).
Safety precautions
| Precaution | Purpose |
|---|---|
| Minimise exposure time | Reduce total dose received |
| Maximise distance from source | Radiation intensity decreases with distance |
| Use shielding (lead, concrete) | Absorbs radiation before it reaches people |
| Use tongs to handle sources | Keeps the source away from the body |
| Point sources away from people | Reduces exposure |
| Store sources in lead-lined containers | Contains radiation when not in use |
Common errors and how to correct them
Forgetting to subtract background radiation. The GM tube detects background radiation as well as the source. Always subtract the background count to find the source’s activity.
Saying radiation makes objects radioactive. Exposure to alpha, beta or gamma radiation does not normally make other objects radioactive (neutron radiation can, but this is not required at IGCSE level).
How to apply this in an exam
When calculating corrected count rate, show: measured count rate minus background count rate. For safety, give at least two precautions with reasons. For hazards, distinguish between internal and external exposure.
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