A radiographer is deciding which radioactive isotope to use in a smoke alarm for a hospital ward. One isotope has a half-life of 432 years and emits alpha radiation. A second isotope has a half-life of 6 hours and emits gamma radiation. Explain why the isotope with a half-life of 432 years and alpha emission is more suitable for use in a smoke alarm than the second isotope.

OCR A-Level Physics A (H556) — 6.1 Capacitors · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Smoke alarms work by detecting a drop in the number of radiation particles reaching a sensor inside the alarm. When smoke enters the alarm, it absorbs the radiation and the count rate at the sensor falls, triggering the alarm.

Model answer (5 marks)

A long half‑life means the source remains active for a very long time, so it will not need to be replaced for many years. The 6‑hour isotope would decay too quickly and lose its activity too fast to be useful in a smoke alarm.

Alpha particles are the least penetrating radiation; they are stopped by a few centimetres of air. This means that the alpha source will be contained within the alarm and will not pose a radiation hazard to the surrounding area.

Alpha radiation is highly ionising, so it ionises the air inside the alarm effectively, producing a detectable ionisation current that can be used to trigger the alarm when the count rate falls.

Gamma radiation is the most penetrating; it would pass through the alarm casing and through smoke without being absorbed. It would not be blocked by smoke, so it would not provide a reliable signal for detecting smoke.

Examiner tips

  • Use the word "long" or "very long" to describe the half‑life advantage. Mention that a 6‑hour half‑life would mean rapid loss of activity. Explain that alpha particles are stopped by air and are highly ionising. State that gamma rays would penetrate the casing and smoke, making detection unreliable.

Common mistakes

  • Confusing alpha with beta radiation. Forgetting to mention the ionising effect of alpha particles. Saying gamma rays are useful for smoke detection instead of noting their high penetration.

Mark scheme (5 marks)

  1. A long half-life means the source remains active / does not need replacing for a very long time (accept: will not decay away quickly / will last hundreds of years)
  2. The 6-hour half-life isotope would decay too quickly / lose its activity too fast to be useful in a smoke alarm
  3. Alpha radiation is the least penetrating / is stopped by a few centimetres of air
  4. Alpha radiation is highly ionising so it ionises the air inside the alarm effectively / produces a detectable ionisation current
  5. Gamma radiation is the most penetrating so it would pass through the alarm casing / smoke without being absorbed / would not be blocked by smoke, making it unsuitable for detecting smoke

Key terms in this question

half-life

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