Explain why alpha radiation is used in smoke alarms rather than beta or gamma radiation.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Smoke alarms contain a small amount of the radioactive isotope americium-241, which has a half-life of 432 years. Under normal conditions the alarm remains silent, but it sounds when smoke enters the device.
Model answer (4 marks)
Alpha particles are highly ionising; they ionise air molecules in the alarm’s ionisation chamber, producing a measurable current.
When smoke enters, the particles are absorbed or scattered, reducing the ionisation and the current falls below the alarm threshold, so the alarm sounds.
Alpha radiation is weakly penetrating – a few centimetres of air or a sheet of paper stops it – so the device is safe and the radiation is contained.
Beta or gamma rays would penetrate the smoke and the alarm’s housing, so they could not be blocked by the smoke and would pose a greater external radiation hazard.
When smoke enters, the particles are absorbed or scattered, reducing the ionisation and the current falls below the alarm threshold, so the alarm sounds.
Alpha radiation is weakly penetrating – a few centimetres of air or a sheet of paper stops it – so the device is safe and the radiation is contained.
Beta or gamma rays would penetrate the smoke and the alarm’s housing, so they could not be blocked by the smoke and would pose a greater external radiation hazard.
Examiner tips
- Use the term ‘highly ionising’ and explain the current drop; mention the alarm’s threshold.
- State that alpha rays are stopped by a few centimetres of air – this shows safety and containment.
Common mistakes
- Confusing ionisation with heating; forgetting to mention the current drop.
- Claiming beta/gamma are harmless without explaining why they would not be blocked by smoke.
Mark scheme (4 marks)
- Alpha particles are highly ionising / ionise air particles to create a current
- When smoke enters, it absorbs/blocks the alpha particles, reducing the ionisation and causing the current to drop, triggering the alarm
- Alpha radiation is weakly penetrating / stopped by a few centimetres of air, so it is contained safely within the device
- Beta or gamma radiation would be too penetrating to be blocked by smoke / would pass straight through, so the alarm would not work / would present a greater radiation hazard outside the device
Related
- All Edexcel GCSE Physics (1PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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