Explain why a radioactive isotope with a long half-life is chosen for use in a smoke alarm, and explain why alpha radiation is the most suitable type of radiation for this application rather than 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. The radiation ionises air particles inside the alarm to create a small electrical current. When smoke enters the alarm, the current drops and the alarm sounds.
Model answer (4 marks)
A long half‑life means the activity of the source remains roughly constant over the life of the alarm, so the current produced does not fall significantly.
Alpha radiation is highly ionising, so it efficiently ionises the air particles to produce the current needed to operate the alarm.
Alpha particles have very low penetrating power; they are stopped by a few centimetres of air or the alarm casing, keeping the radiation confined.
Gamma radiation is highly penetrating and would pass through the casing, posing a greater radiation risk to people nearby and would not be safely contained.
Alpha radiation is highly ionising, so it efficiently ionises the air particles to produce the current needed to operate the alarm.
Alpha particles have very low penetrating power; they are stopped by a few centimetres of air or the alarm casing, keeping the radiation confined.
Gamma radiation is highly penetrating and would pass through the casing, posing a greater radiation risk to people nearby and would not be safely contained.
Examiner tips
- Use the exact terms "half‑life", "activity", "alpha radiation", "highly ionising", "low penetrating power", "gamma radiation".
- Show the logical link: long half‑life → stable activity; alpha → ionising & confined; gamma → penetrating & unsafe.
Common mistakes
- Confusing alpha with beta radiation; not recognising alpha’s low penetration.
- Failing to mention that the long half‑life keeps the activity constant over the alarm’s lifetime.
Mark scheme (4 marks)
- A long half-life means the activity of the source remains roughly constant / does not significantly decrease over the lifetime of the alarm
- Alpha radiation is highly ionising, so it effectively ionises the air particles to produce the current needed to operate the alarm
- Alpha radiation has very low penetrating power / is stopped by a few centimetres of air or the casing of the alarm
- Gamma radiation is highly penetrating and would pass straight through the alarm casing, posing a greater radiation risk to people nearby / would not be safely contained
Key terms in this question
Related
- All Edexcel GCSE Physics (1PH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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