A radioactive isotope used in smoke detectors undergoes alpha decay. Explain why an alpha emitter with a long half-life is suitable for use in a smoke detector, and why a gamma emitter would not be suitable for this application.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Smoke detectors contain a small radioactive source that produces a continuous current inside the detector. When smoke enters the device, the alarm is triggered.
Model answer (4 marks)
Alpha particles are highly ionising, so they create a continuous ionisation current inside the detector.
A long half‑life keeps the source active for many years, so the detector does not need frequent replacement.
Gamma rays are lowly ionising, so they would not produce enough ionisation to sustain the current.
Gamma rays are also highly penetrating, so they would pass through the detector’s casing and expose people to radiation.
A long half‑life keeps the source active for many years, so the detector does not need frequent replacement.
Gamma rays are lowly ionising, so they would not produce enough ionisation to sustain the current.
Gamma rays are also highly penetrating, so they would pass through the detector’s casing and expose people to radiation.
Examiner tips
- Use the word "highly ionising" for alpha particles and "lowly ionising" for gamma rays. Mention the long half‑life in terms of detector lifetime. Explain the penetration issue for gamma radiation.
- common_mistakes
- :
- Confusing alpha with beta radiation. Forgetting that a long half‑life is required. Claiming gamma radiation is harmless without noting its penetration.
Mark scheme (4 marks)
- Alpha particles are highly ionising, so they cause a current to flow inside the detector.
- A long half-life is needed so that the source remains active / does not decay significantly over the lifetime of the detector.
- Gamma radiation is lowly ionising, so it would not produce enough ionisation to maintain the current inside the detector.
- Gamma radiation is highly penetrating, so it would pass through the casing of the detector and pose a radiation hazard to people nearby.
Key terms in this question
Related
- All Pearson Edexcel International GCSE Physics (4PH1) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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