Americium-241 is an alpha emitter used in smoke alarms. Explain why americium-241 is a suitable radioactive source for use in a smoke alarm, referring to its type of emission, its half-life, and the difference between irradiation and contamination in this context.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Smoke alarms contain a small amount of americium-241, which has a half-life of 432 years. The americium-241 is sealed inside the alarm and continuously emits radiation into a small chamber of air within the device. If smoke enters the chamber, the alarm sounds.
Model answer (5 marks)
Alpha particles are weakly penetrating; they cannot travel far in air, so the alarm does not expose people to significant radiation.
The half‑life of 432 yr is very long, so the source remains weakly radioactive for many years and does not need frequent replacement.
Because of this long half‑life the activity at any time is low, making the alarm safe for users.
The americium is sealed inside the device, so users are only irradiated (exposed to the radiation field) and not contaminated with radioactive material.
Alpha particles are highly ionising; they ionise the air in the chamber, creating a current. When smoke enters, fewer alpha particles reach the air, the current falls and the alarm sounds.
The half‑life of 432 yr is very long, so the source remains weakly radioactive for many years and does not need frequent replacement.
Because of this long half‑life the activity at any time is low, making the alarm safe for users.
The americium is sealed inside the device, so users are only irradiated (exposed to the radiation field) and not contaminated with radioactive material.
Alpha particles are highly ionising; they ionise the air in the chamber, creating a current. When smoke enters, fewer alpha particles reach the air, the current falls and the alarm sounds.
Examiner tips
- Use the exact phrases "weakly penetrating" and "highly ionising"; mention the 432‑yr half‑life twice – once for longevity, once for low activity.
- Explain both irradiation and contamination to show understanding of safety.
- Show the chain: alpha → ionisation → current → alarm trigger.
Common mistakes
- Confusing alpha with beta or gamma radiation; not noting low penetration.
- Forgetting to mention the sealed source and the difference between irradiation and contamination.
- Using vague terms like "safe" without linking to half‑life or penetration.
Mark scheme (5 marks)
- Alpha radiation is weakly penetrating / cannot travel far in air, so it does not present a significant radiation risk to people near the alarm
- The long half-life of 432 years means the source remains weakly radioactive / active for a very long time, so it does not need to be replaced
- The long half-life means the activity at any given time is low / the source is only weakly radioactive, so it is not harmful to people nearby
- The americium-241 is sealed inside the alarm, so users are only irradiated (exposed to radiation from the source) rather than contaminated (radioactive atoms being deposited on them)
- Alpha radiation is highly ionising, so it ionises the air in the chamber, allowing a current to flow; when smoke enters, fewer alpha particles reach the air in the chamber, the current drops and the alarm sounds
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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