5.2.1 Detection of Radioactivity
Background Radiation
Background radiation refers to the ionising radiations or radioactive emissions that are always present in the surroundings.
- Comes from both natural and artificial sources.
- Varies from place to place depending on soil, altitude, and building materials.
- Measured before experiments to allow correction of readings from radioactive samples.
Sources of Background Radiation
- Radon gas in the air.
- Rocks and building materials that contain radioactive isotopes.
- Food and drink containing trace radioactive substances (e.g., potassium-40).
- Cosmic rays from space and the Sun.
- Nuclear waste and fallout from past nuclear testing.
Detection of Radiation
Ionising nuclear radiation can be detected using a radiation detector connected to a counter.
- The count rate is measured in counts per second (counts/s) or counts per minute (counts/min).
- Before measurement, the detector reading should be corrected by subtracting the background count rate.
- Corrected count rate = measured rate − background rate.
Common Detectors
| Detector | Working Principle | Typical Use |
|---|---|---|
| Geiger–Müller Tube | Detects ionising particles that produce pulses in a gas-filled tube. | Measuring count rate of α, β, or γ radiation. |
| Photographic Film | Darkens when exposed to radiation; intensity of darkening shows exposure level. | Used in radiation badges for safety monitoring. |
| Scintillation Detector | Converts ionising events into flashes of light detected by sensors. | Used in hospitals and nuclear laboratories. |
Safety Precautions During Detection
- Always measure and record the background count before starting experiments.
- Keep sources at a safe distance and use lead shielding when appropriate.
- Do not touch radioactive sources with bare hands — use tongs or forceps.
- Minimise exposure time and never point the source at yourself or others.
Exam Pointers
- Define background radiation as naturally occurring ionising radiation present everywhere.
- Always mention correction for background count in practical or calculation questions.
- Identify Geiger–Müller tube as the most common radiation detector in IGCSE contexts.
- Be able to list sources: rocks, radon, food, cosmic rays, nuclear waste.
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