Knowledge Organiser: Radiation Detection
Key Terms
- GM tube — ionisation-based counter for count rate
- Count rate — detections per minute/second
- Film badge — cumulative dose monitor for workers
- Cloud chamber — makes particle tracks visible
- Ionisation — removing electrons from atoms
- Activity — decays per second (becquerels)
GM Tube Mechanism
- Radiation enters through thin mica window
- Ionises argon gas inside tube
- Ions accelerated by high voltage
- Movement of ions = current pulse
- Each pulse counted by electronic counter
Cloud Chamber Tracks
- Alpha: thick, straight, short tracks
- Beta: thin, wiggly, longer tracks
- Gamma: no tracks (no charge)
- Tracks = vapour condensing on ions
Film Badge Layers
- Open window: all radiation
- Plastic: blocks alpha, lets beta+gamma through
- Aluminium: blocks alpha+beta, only gamma
- Darker film = higher dose received
- Different windows identify radiation types
Key Equations
- Count rate (counts/s or counts/min) ∝ activity of source
- Corrected count rate = measured − background count rate
- Alpha: stopped by paper/few cm air; Beta: stopped by ~3mm aluminium; Gamma: reduced by thick lead/concrete
- GM tube detects all three types; film badge distinguishes by comparing windows
Common Mistakes
- Confusing count rate and activity: Activity (Bq) is decays per second at the source; count rate is what the detector registers — count rate is always lower than activity due to distance and detector efficiency
- Saying GM tubes only detect gamma: Geiger-Muller tubes detect alpha, beta, and gamma radiation — but alpha is easily absorbed before reaching the tube unless very close
- Misidentifying cloud chamber tracks: Alpha: short, thick, straight tracks (high ionisation); beta: longer, thinner, curved tracks (deflected by magnetic field); gamma: very faint or no visible track
- Forgetting to subtract background in experiments: Always subtract background count rate from all readings — this is a standard required practical step that examiners specifically ask about
- Thinking film badges measure activity: Film badges measure the dose received by the worker (how much radiation the body has absorbed) — not the activity of any source
Memory Aid
- GM tube — Goes "click" — counts radiation events in real time
- Film badge — Film gets darker — measures dose over weeks or months
- Cloud chamber — Clouds form tracks — shows paths of individual particles