A device made to detect the presence of smoke, and, by association, fire inside a building.

Two types of smoke detectors are currently in widespread use: ionization detectors and photoelectric detectors. Their principles of operation are entirely different, and each has its own particular benefits and drawbacks.

Ionization detector

The ionization smoke detector is an extremely ironic byproduct of the Manhattan Project. The most immediately destructive scientific research program in human history produced, as a mere afterthought, a mildly radioactive isotope. Americium-241, a product of the decay of plutonium-241, has made possible an inexpensive safety device which has likely saved thousands of lives since the Project's intended output seared, poisoned, and vaporized hundreds of thousands of Japanese. I'm not sure what I think of that.

Americium, discovered by Glenn Seaborg at the University of Chicago in 1944 or 1945, has several isotopes. Americium-241 is not the most stable; its half-life is 432 years. Decaying, it emits gamma rays of relatively low energy, 59.5 keV (kilo-electron-volts), as well as alpha particles.

An ionization-type smoke detector contains about 0.9 microcurie of Am-241, though it's bound in oxide form (Americium oxide, AmO2). The total mass of the radioactive material is about 1/5000 of a gram. The source is positioned near an ionization chamber, essentially two electrodes with a potential across them supplied by a battery. The emitted alpha particles ionize the air molecules in the chamber, causing a minute current to flow between the plates. If significant numbers of smoke molecules are present, they'll neutralize enough of the ions to reduce the net current. The detector's electronics are sensitive enough to respond to very tiny changes (as anyone who has tried to fry fish near one can tell you!), and will set off the alarm immediately upon a current drop.

The gamma radiation emitted by the americium is incidental to its function within the smoke detector; however, it is able to escape the detector's confines, unlike the alpha particles. In practice, what this means is that an ionization-chamber smoke detector gives a tangible, but insignificant, dose of radiation to the inhabitants of the home or office in which it is installed. The average yearly dose received by a smoke-detector user is several orders of magnitude smaller than that imparted by a single chest X-ray scan. The only real danger from the isotope comes if the radioactive material is somehow freed, then inhaled or ingested.

Ionization detectors tend to be more effective in detecting fires with open flames, rather than smoldering ones.

Photoelectric detector

Photoelectric detectors are considerably more expensive to purchase and maintain than those of the ionization variety, and are less commonly used. For those who are paranoid about radiation in any form or quantity, though, they present a viable alternative to installing a decaying isotope on one's ceiling.

Two types of photoelectric smoke detectors exist. The first uses an LED to send a beam of light down a chamber. A photocell lies at the end of a branch of the chamber perpendicular to the light's path. Under normal conditions, the sensor remains unilluminated. But when large smoke particles enter the chamber, light scatters off them and some makes it to the photocell at the end of the perpendicular chamber, setting off the alarm.

A second variety of photoelectric smoke detector, somewhat less sensitive than the first and less popular, directs the light beam toward the photocell, and registers an alarm when the light reaching the detector is reduced.

Photoelectric smoke detectors are better at detecting smoldering fires, which tend to release larger smoke particles into the air.