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

1. Radiation enters through thin mica window
2. Ionises argon gas inside tube
3. Ions accelerated by high voltage
4. Movement of ions = current pulse
5. 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