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Putting radiation to use - smoke alarms

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How do they work?

  • smoke detectors have an ionization section and a source of ionizing radiation. The source of ionizing radiation is a small quantity of americium-241 ,which is a source of alpha particles . The ionization section consists of two plates separated by about a centimetre. The battery applies a voltage to the plates, charging one plate positive and the other plate negative. Alpha particles constantly released by the americium knock electrons off of the atoms in the air, ionizing the oxygen and nitrogen atoms in the chamber. The positively-charged oxygen and nitrogen atoms are attracted to the negative plate and the electrons are attracted to the positive plate, generating a small, continuous electric current. When smoke enters the ionization chamber, the smoke particles attach to the ions and neutralize them, so they do not reach the plate. The drop in current between the plates triggers the alarm.  

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 Many detectors that use a small amount of radioactive material to ionize air in the detector will alarm when there is a sudden change in room air opening doors to the outside or cooking food in a kitchen and an over-ride button will silence the false alarm until the air clears. Using a magazine, etc. to fan and clear a false alarm will help save the battery.

A general concern is that only one person will hear the alarm and must alert all others in the house who must follow a prior plan to escape (out windows using rope ladders, etc.) to avoid the flames. Everyone should meet at an assigned location so that no one races back inside to rescue someone who has already escaped.

Importance of having a smoke detector?

Fire safety organizations

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The amount of radioactive material in each smoke alarm is extremely small and, from environmental and public health perspectives, the disposal of individual smoke alarms with domestic rubbish does not represent any hazard;

The radioactive material is securely bound in a metal foil within the smoke alarm; and

The amount of naturally-occurring alpha-emitting radioactivity in normal soils is equivalent to a dozen or more smoke alarms in every cubic metre. The dispersal of smoke alarms, even in large numbers, through refuse land-fill sites is therefore not significant in comparison.


Individual (up to 10) smoke alarms can be safely disposed of in domestic rubbish.

When more than ten smoke alarms are collected together for bulk disposal, they must be treated as radioactive waste and the requirements of the National Health and Medical Research Council's Code of Practice for the Near-Surface Disposal of Radioactive Waste in Australia (1992) must be met.

  • Websites used
  • http://www.nswfb.nsw.gov.au/page.php?id=704

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