Americium-241 is a radioactive isotope used in smoke alarms. It is an alpha emitter with a half-life of 432 years. Explain why Americium-241 is suitable for use in a smoke alarm, and explain why alpha emission rather than gamma emission makes it safe for household use.

OCR A-Level Physics B: Advancing Physics (H557) — 6.3 Nuclear and particle physics · Explain · 5 marks · View as Markdown

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

Smoke alarms are found in most homes. They contain a small amount of a radioactive isotope that continuously emits radiation into the air inside the alarm. When smoke enters the alarm, the count rate of radiation detected inside the alarm drops, triggering the alarm to sound.

Model answer (5 marks)

Americium‑241 has a very long half‑life (432 yr), so a small amount remains active for the life of the alarm and does not need frequent replacement.

The isotope emits alpha particles. Inside the alarm the alphas ionise the air, producing a small current that can be measured as a count rate.

When smoke enters, the smoke particles absorb or scatter the alpha particles, reducing the ionisation and the count rate. The detector senses this drop and triggers the alarm.

Alpha radiation has very low penetrating power; a few centimetres of air or the alarm housing stops it, so it cannot reach people in the room. In contrast, gamma rays are highly penetrating and would pass through the housing, exposing occupants to radiation.

Examiner tips

  • Mention the long half‑life to show longevity of the source. Explain the ionisation mechanism and the drop in count rate when smoke blocks alphas. State the low penetration of alpha particles and the danger of gamma rays.
  • common_mistakes
  • :
  • Confusing alpha with beta or gamma emission. Forgetting to mention the long half‑life. Over‑emphasising the alarm mechanism without linking it to alpha absorption.

Mark scheme (5 marks)

  1. Americium-241 has a long half-life (432 years), so it does not need to be replaced / remains active for the lifetime of the alarm
  2. Alpha particles ionise the air inside the alarm, allowing a current to flow / creating a detectable signal
  3. When smoke enters, the alpha particles are absorbed / blocked by smoke particles, reducing ionisation / count rate, so the alarm sounds
  4. Alpha radiation has very low penetrating power / is stopped by a few centimetres of air, so it cannot penetrate the casing of the alarm or reach people in the room
  5. Gamma radiation would be dangerous in a household as it is highly penetrating and would pass through the casing and irradiate people nearby

Key terms in this question

half-life

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