The World of Nuclear Science

Household Uses of Radiation

Apart from household electricity, radiation is also used elsewhere within our homes.

Smoke detectors are important safety devices that are in many modern homes. Two types of smoke detectors exist. One type uses a photoelectric sensor to detect changes in light caused by smoke, and the other uses radioisotopes - usually americium-241. (American spelling: americum) The photoelectric type is more expensive and is less effective, whereas the other type is much cheaper and more sensitive to a wider range of fire conditions. These radioisotope-based detectors are the most common.

Inside a radioisotope-based smoke detector, the americium-241 is placed inside an air chamber. The ionising alpha radiation emitted collides with air moleculrs in the air chamber, ionising them (making the air particles charged). The ionised air molecules are then able to conduct an electrical current between two electrodes on either side of the air chamber.

When smoke enters the air chamber, the ionised air molecules attract the smoke particles, causing a decrease in the current conducted. (The ionised air molecules are now "carrying" extra smoke particles, decreasing the flow of current.) This current decrease is then detected by electrical circuitry, activating an alarm.

This is best illustrated by the smoke detector simulation. You will need a JavaScript enabled browser to view it.

Smoke Detectors and Americium
The americium-241 used in smoke detectors exists in the form of americium oxide AmO₂. This chemical is expensive - the US Atomic Energy Commission sells it for about US$1500 per gram. However, 1 gram of americium-241 is enough for over 5000 household smoke detectors. The americium-241 radioisotope was first discovered 50 years ago as part of the Manhattan project - the US's attempt to build the first nuclear bomb. Americium-241 is formed in nuclear reactors and has a half life of 432 years.

Typical smoke detectors use less than 35 kilobecquerels of americium-241, which is a very small amount. The radiation emitted by a smoke detector containing this amount of americium poses less danger to human health than background radiation in the atmosphere, so smoke detectors being "radioactive" are not a health hazard. Even still, Australian standards require that detectors be labelled as containing radioactive material, and must not be disposed through general garbage collection, although in some states smoke detectors are compulsory in all homes.

the internationally accepted nuclear trefoil symbol is required on all Australian smoke detectors. In many other countries smoke detectors must also carry this symbol.