Explain why alpha particles are used as the radiation source in smoke alarms rather than beta particles or gamma rays.
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, alpha particles emitted from the source ionise air particles between two electrodes, allowing a small current to flow. When smoke enters the alarm, the current drops and the alarm sounds.
Model answer (5 marks)
Alpha particles are highly ionising, so they efficiently ionise the air between the electrodes and allow a current to flow.
Alpha particles are weakly penetrating; a few centimetres of air stop them, so the current is reduced when smoke particles capture the ions.
Beta particles and gamma rays are far more penetrating; they would pass through the alarm casing and would not be stopped by smoke, so the current would not change appreciably.
Because the alpha particles are stopped by the smoke, the alarm can be triggered reliably.
The long half‑life (432 yr) of Am‑241 keeps the activity nearly constant during the alarm’s lifetime, so the source does not need frequent replacement.
Alpha particles are weakly penetrating; a few centimetres of air stop them, so the current is reduced when smoke particles capture the ions.
Beta particles and gamma rays are far more penetrating; they would pass through the alarm casing and would not be stopped by smoke, so the current would not change appreciably.
Because the alpha particles are stopped by the smoke, the alarm can be triggered reliably.
The long half‑life (432 yr) of Am‑241 keeps the activity nearly constant during the alarm’s lifetime, so the source does not need frequent replacement.
Examiner tips
- Use the term ‘highly ionising’ and ‘weakly penetrating’ – these are key phrases. Explain how ionisation leads to current and how smoke reduces it. Mention the long half‑life to justify the source’s longevity. Keep the answer concise, matching the 5 marks.
Mark scheme (5 marks)
- Alpha particles are highly ionising
- Alpha particles ionise the air particles between the electrodes, allowing a current to flow
- Alpha particles are weakly penetrating / stopped by a few centimetres of air
- Gamma rays or beta particles are too penetrating and would pass through the alarm casing / would not be stopped by smoke, so could not trigger the alarm effectively
- The long half-life of americium-241 (432 years) means the activity remains approximately constant over the lifetime of the alarm, so it does not need replacing
Key terms in this question
Related
- All Edexcel A-Level Physics (9PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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