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(Updated January 2009)
Smoke detectors/alarms are important safety devices, because of their obvious potential to save lives and property. There are two types of smoke detector commonly available in many countries.
One type uses the radiation from a small amount of radioactive material to detect the presence of smoke or heat sources. These 'ionisation chamber' smoke detectors are the most popular, because they are inexpensive and are sensitive to a wider range of fire conditions than the other type.
The other type of detector does not contain radioactive material; it uses a photoelectric sensor to detect the change in light level caused by smoke. This type is more expensive to purchase and install, and is less effective.
Americium
The element americium (atomic number 95) was discovered in 1945 during the Manhattan Project in USA. The first sample of americium was produced by bombarding plutonium with neutrons in a nuclear reactor at the University of Chicago.
Americium is a silvery metal, which tarnishes slowly in air and is soluble in acid. Its most stable isotope, Am-243, has a half-life of over 7500 years, although Am-241, with a half-life of 432 years, was the first isotope to be isolated. Am-241 decays by emitting alpha particles and intense gamma radiation to become neptunium-237.
Americium oxide, AmO2, was first offered for sale by the US Atomic Energy Commission in 1962 and the price of US$1500 per gram has remained virtually unchanged since. One gram of americium oxide provides enough active material for more than three million household smoke detectorsa.
Americium (in combination with beryllium) is also used as a neutron source in non-destructive testing of machinery and equipment, and as a thickness gauge in the glass industry. However, its most common application is as an ionisation source in smoke detectors, and most of the several kilograms of americium made each year is used in this way.
Formation of americium
Typical used fuel from a power reactor contains about 1% plutonium. Of this, about 12% is the plutonium-241 isotope, which has a half life of only 14 years, decaying to Am-241 through emission of beta particles. Am-241 has a half life of 432 years, emitting alpha particles and intense gamma radiation to become neptunium-237.
The plutonium-241 is formed in any nuclear reactor by neutron capture from uranium-238. The detailed steps are:
This will decay both in the reactor and subsequently to form Am-241.
It is of interest (and some significance in recycling spent fuel) that if too much Am-241 builds up in plutonium separated from spent fuel, it cannot readily be used in the manufacture of mixed oxide (MOX) fuel because it is too radioactive for handling in a normal MOX fuel fabrication plant. For instance, the Sellafield MOX Plant in the UK can handle plutonium containing up to 3% Am-241, hence up to six years old (any more would need special shielding).
Operation of smoke detectors
The vital ingredient of household smoke detectors is a very small quantityb of Am-241 as americium dioxide (AmO2).
Americium-241 emits alpha particles and low energy gamma raysc. The alpha particles emitted by the Am-241 collide with the oxygen and nitrogen in air in the detector's ionisation chamber to produce charged particles (ions). A low-level electric voltage applied across the chamber is used to collect these ions, causing a steady small electric current to flow between two electrodes. When smoke enters the space between the electrodes, the smoke particles attach to the charged ions, neutralizing them. This causes the number of ions present - and therefore the electric current - to fall, which sets off an alarm.
The radiation dose to the occupants of a house from a domestic smoke detector is essentially zero, and in any case very much less than that from natural background radiation. The alpha particles are absorbed within the detector, while most of the gamma rays escape harmlessly. The small amount of radioactive material that is used in these detectors is not a health hazard and individual units can be disposed of in normal household waste.
Even swallowing the radioactive material from a smoke detector would not lead to significant internal absorption of Am-241. Americium dioxide is insoluble, so will pass through the digestive tract without delivering a significant radiation dose. (Americium-241 is however a potentially dangerous isotope if it is taken into the body in soluble form. It decays by both alpha activity and gamma emissions and it would concentrate in the skeleton).
Further Information
Notes
a. The activity of Am-241 is 127 GBq/g (3.43 Ci/g). [Back]
b. An average smoke detector for domestic use contains about about 0.29 micrograms of Am-241 (in the form of americium dioxide), so its activity is around 37,000 Bq (or about 1 µCi). [Back]
c. Am-241 emits low energy gamma rays of 60 keV. The Am-241 gamma dose constant of 3.14 x 10-9 Sv m2 h-1 Ci-1 gives an annual dose at one metre of 27 µSv/yr for an average household smoke detector - around 100 times lower than the dose from natural background radiation. [Back]
General sources
Ionization Technology webpage on the U.S. Environmental Protection Agency website (www.epa.gov)Radioactivity in Domestic Smoke Alarms Fact Sheet on Australian Radiation Protection and Nuclear Safety Agency (ARPANSA) website (www.arpansa.gov.au)How Smoke Detectors Work on HowStuffWorks website (www.howstuffworks.com)Smoke Detectors and Americium-241, Canadian Nuclear Society (CNS) Educational Brochure on CNS website (www.cns-snc.ca)
Related information pages
Processing of Used Nuclear FuelPlutonium