A smoke detector contains a small amount of americium-241, which is a radioactive isotope that emits alpha radiation. Explain why alpha radiation is used inside the smoke detector rather than beta or gamma radiation, and describe what is meant by the term 'isotopes'.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Smoke detectors use radioactive sources to ionise air between two metal plates. When smoke enters the detector, the ionisation is reduced and the alarm sounds.
Model answer (5 marks)
Alpha radiation is highly ionising, so it efficiently ionises the air between the plates.
Alpha particles have a very short range in air (a few centimetres) and cannot penetrate the detector’s casing, keeping the radiation safely contained.
Beta or gamma radiation would penetrate the casing and could escape, posing a health risk to people nearby.
Isotopes are atoms of the same element – they have the same number of protons (same atomic number) but different numbers of neutrons, giving them different mass numbers.
Alpha particles have a very short range in air (a few centimetres) and cannot penetrate the detector’s casing, keeping the radiation safely contained.
Beta or gamma radiation would penetrate the casing and could escape, posing a health risk to people nearby.
Isotopes are atoms of the same element – they have the same number of protons (same atomic number) but different numbers of neutrons, giving them different mass numbers.
Examiner tips
- Mention ionising ability and short range to justify use of alpha; note safety of containment; define isotopes clearly with proton and neutron differences
Common mistakes
- Confusing alpha with beta or gamma as the ionising agent; forgetting to state the short range; describing isotopes as having different numbers of protons instead of neutrons
Mark scheme (5 marks)
- Alpha radiation has a high ionising ability / is strongly ionising
- Alpha radiation has a very short range in air (a few centimetres), so it is contained safely within the device / does not penetrate the casing
- Beta or gamma radiation would penetrate the casing and could be harmful to people nearby / would escape the detector
- Isotopes are atoms of the same element (same number of protons / same atomic number)
- Isotopes have different numbers of neutrons / different mass numbers
Key terms in this question
Related
- All Eduqas A-Level Physics revision notes →
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