Radioisotopes in Smoke Detectors Chemistry Tutorial

Key Concepts

• An ionization smoke detector contains:

  ⚛ a positive electrode

  ⚛ a negative electrode

  ⚛ a radiation source

  ⚛ a current detector

• The radiation source is an unstable isotope such as americium-241
• An unstable isotope undergoes nuclear decay (radioactive decay) by emitting radiation.
• Normally, charged particles in the air allow a current to flow through the detector.
  Smoke particles in a fire prevent the current flowing causing an alarm to sound.

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How a Smoke Detector Works

Inside the smoke detector:

• The unstable isotope of americium, americium-241, undergoes nuclear decay to produce neptunium-237 and an alpha particle (a helium nucleus, α).

  241 Am 95

  →

  4 He 2

  +

  237 Np 93

• Alpha particles cause the oxygen and nitrogen molecules in air to ionize:

  O₂   →   O₂⁺ + e^-

• Positively charged particles, alpha particles and O₂⁺, move to the negative electrode.

  Negatively charged particles, electrons, move to the positive electrode.

• The flow of charged particles between the electrodes causes a current to flow.
• When smoke is present, alpha particles attach to the smoke particles so air molecules are no longer ionized, the current drops, causing an alarm to sound.

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