Americium-241 is an alpha emitter with a half-life of 432 years. It is used in household smoke alarms. Explain why Americium-241 is a suitable isotope for use in a smoke alarm, and explain why alpha emission, rather than gamma emission, makes it safer for people living in the home.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Americium-241 is placed inside a smoke alarm. The alpha particles it emits ionise the air inside the alarm. When smoke enters the alarm, the count rate of alpha particles detected drops and the alarm sounds.
Model answer (5 marks)
A long half‑life (432 y) means the source remains radioactive for many years, so the alarm does not need frequent replacement.
The activity therefore stays roughly constant, giving a stable count rate and reliable operation.
Alpha particles are highly ionising; they efficiently ionise the air inside the alarm, producing the current that is measured.
Alpha particles are poorly penetrating – a few centimetres of air or the alarm casing stops them – so they do not reach people in the room.
If the source emitted gamma rays instead, the radiation would be far more penetrating, passing through the casing and walls and exposing occupants to a health risk.
The activity therefore stays roughly constant, giving a stable count rate and reliable operation.
Alpha particles are highly ionising; they efficiently ionise the air inside the alarm, producing the current that is measured.
Alpha particles are poorly penetrating – a few centimetres of air or the alarm casing stops them – so they do not reach people in the room.
If the source emitted gamma rays instead, the radiation would be far more penetrating, passing through the casing and walls and exposing occupants to a health risk.
Examiner tips
- Use the exact phrases from the mark scheme – e.g. "long half‑life", "stable count rate", "highly ionising", "poorly penetrating".
- Show the logical chain: half‑life → stable activity → reliable alarm; alpha → ionisation → detection; alpha → low penetration → safety.
- Keep each point brief and separate with line breaks or bullet numbers.
Common mistakes
- Confusing alpha with beta radiation; forgetting to mention the ionising property.
- Claiming alpha particles are harmless without noting their limited penetration.
- Using vague terms like "safe" without explaining why alpha is less penetrating than gamma.
Mark scheme (5 marks)
- A long half-life means the source remains radioactive for a long period of time / does not need to be replenished frequently
- The long half-life means the activity / count rate remains roughly constant / weakly radioactive over time, so the alarm functions reliably
- Alpha particles are the most ionising type of radiation, so they effectively ionise the air inside the alarm to create a detectable current / allow the alarm to function
- Alpha particles are the least penetrating type of radiation and are stopped by a few centimetres of air / the casing of the alarm, so they do not reach the people in the room
- Gamma radiation would be more penetrating / would pass through the alarm casing and through walls, irradiating people in the home and posing a greater health risk
Key terms in this question
Related
- All OCR A-Level Physics B: Advancing Physics (H557) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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