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
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.