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Smoke Detectors : Uses of radioisotopes

Key Concepts

An ionization smoke detector contains:
  • a positive electrode

  • a negative electrode

  • a radiation source (eg, Americium-241)

  • a current detector

  • Americium-241 decays to produce Neptunium-237 and an alpha particle (42He).

    241     4     237  
      Am --->   He +   Np
    95     2     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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