A hospital uses americium-241 in smoke alarms fitted throughout its wards. Americium-241 is an alpha emitter with a half-life of 432 years. A physicist suggests that, despite being radioactive, americium-241 in smoke alarms poses very little health risk to patients. Explain why the physicist is correct, using your knowledge of the properties of alpha radiation and the half-life of americium-241.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Americium-241 is sealed inside the smoke alarm casing and continuously emits alpha particles into a small internal chamber. The alarm sounds when smoke disrupts the flow of alpha particles inside the chamber.
Model answer (5 marks)
Alpha particles are highly ionising but have very low penetrating power – they can only travel a few centimetres in air.
Alpha radiation cannot penetrate the metal casing of the smoke alarm, so it cannot reach patients from inside the device.
The long half‑life of 432 years means the activity of the source is very low and remains roughly constant over time.
Because of this long half‑life the source will stay weakly radioactive for a very long time and does not need frequent replacement.
The americium is sealed inside the device, so there is no risk of contamination and irradiation from outside the casing is negligible.
Alpha radiation cannot penetrate the metal casing of the smoke alarm, so it cannot reach patients from inside the device.
The long half‑life of 432 years means the activity of the source is very low and remains roughly constant over time.
Because of this long half‑life the source will stay weakly radioactive for a very long time and does not need frequent replacement.
The americium is sealed inside the device, so there is no risk of contamination and irradiation from outside the casing is negligible.
Examiner tips
- Use the exact terms ‘highly ionising’, ‘low penetrating power’, ‘sealed’, ‘long half‑life’.
- Show the five points clearly, each matching a mark‑scheme element.
- Keep sentences short and to the point.
Common mistakes
- Confusing alpha particles with beta or gamma radiation.
- Claiming alpha particles can penetrate the casing.
Mark scheme (5 marks)
- Alpha particles are highly ionising but have very low penetrating power / can only travel a few centimetres in air
- Alpha radiation cannot penetrate the casing of the smoke alarm / cannot reach patients from inside the device
- The long half-life of 432 years means the activity / count rate of the source remains low and roughly constant over time
- The long half-life means the source will remain weakly radioactive for a very long time / does not need replacing
- There is no risk of contamination to patients because the americium is sealed / contained within the device and irradiation from outside the casing is negligible
Key terms in this question
Related
- All OCR A-Level Physics A (H556) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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