A hospital uses americium-241 in smoke alarms and a gamma-emitting isotope in cancer treatment. Explain why americium-241 is suitable for use in a smoke alarm, and explain why a gamma emitter, rather than an alpha or beta emitter, is used to treat cancerous cells inside the body.

OCR A-Level Physics B: Advancing Physics (H557) — 6.1 Capacitors · Explain · 5 marks · View as Markdown

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

Americium-241 is an alpha emitter with a half-life of 432 years. It is used in smoke alarms found throughout a hospital building. Separately, doctors use a gamma-emitting radioactive isotope to treat cancerous tumours located deep inside a patient's body.

Model answer (5 marks)

1. Americium‑241 has a long half‑life of 432 yr, so a smoke alarm can operate for many years without replacement.
2. It emits alpha particles, which are highly ionising but have very low penetration – a few centimetres of air stops them.
3. The alarm contains a small chamber of air; when smoke particles enter, they absorb the alpha particles, reducing the ionisation of the air.
4. The reduced ionisation causes a change in the electric current between two electrodes, triggering the alarm.
5. For cancer treatment a gamma emitter is used because gamma rays are the most penetrating radiation and can reach tumours deep inside the body.
6. Alpha or beta particles would be stopped by healthy tissue before reaching the tumour and would therefore be ineffective.

Examiner tips

  • Use the exact terms ‘long half‑life’, ‘alpha particles’, ‘penetration’, ‘ionisation’, ‘gamma rays’.
  • Explain the chain: long half‑life → long‑term activity; alpha → low penetration → safety; smoke → reduces ionisation → alarm; gamma → high penetration → deep tumour.
  • Keep each point concise and directly linked to the mark scheme points.

Common mistakes

  • Confusing alpha with beta radiation; writing that alpha particles are highly penetrating.
  • Failing to mention the ionisation change that triggers the alarm.
  • Using vague phrases like ‘good for long use’ instead of ‘long half‑life’.

Mark scheme (5 marks)

  1. Americium-241 has a long half-life (432 years), so it does not need to be replenished / remains active for a very long time
  2. Alpha radiation is the least penetrating / is stopped by a few centimetres of air, so it does not travel far and is not harmful to people nearby
  3. When smoke enters the alarm, it reduces / absorbs the alpha particles in the surrounding air, triggering the alarm to sound
  4. Gamma radiation is the most penetrating type of nuclear radiation, so it can pass through body tissue to reach tumours deep inside the body
  5. Alpha or beta radiation would be absorbed by surrounding healthy tissue before reaching the tumour / would not penetrate deep enough into the body

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