Smoke Detectors

Smoke detectors are important safety devices that are in many homes, businesses and factories.

Smoke detectors trigger an alarm well before a smoldering fire bursts into flame.

There are two different types of smoke detectors; one type uses a photoelectric sensor to detect changes in light caused by smoke, and the other is
ionization detectors which uses radioisotopes - usually americium-241.

1.     Photo-electric smoke detectors

The photo-electric Smoke detector contains a small light source that shines light into a dark sensing chamber. The chamber also contains a photo-electric cell, which converts light into a small electrical current. The light source and the cell are arranged so that the light does not normally strike the cell. When smoke particles enter the sensing chamber the light is reflected off the surface of the smoke particles, striking the cell, and increasing the voltage generated by the cell. The increase in voltage triggers the alarm.

2.     Ionization smoke detectors

The ionization Smoke detector uses a small amount of radioactive material ((americium-241)).The americium-241 is placed inside an air chamber. The ionizing alpha radiation emitted collides with air molecules in the air chamber, ionizing them. The ionized 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 ionized air molecules attract the smoke particles, causing a decrease in the current conducted. (The ionized 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.

Americium-241 is formed in nuclear reactors and has a half life of 432 years.

Typical ionization smoke detectors use less than 35 kilobecquerels of americium-241, which is a very small amount. The radiation emitted by ionization smoke detector containing this amount of americium poses less danger to human health than background radiation in the atmosphere, so ionization smoke detectors being "radioactive" are not a health hazard.