On the periodic table, americium is used in some smoke detectors as americium-241.
Americium has atomic number 95 and belongs to the actinide series. Americium-241 emits alpha particles, which ionize air inside an ionization smoke detector. Smoke entering the chamber disrupts the electrical current, triggering the alarm.
The isotope’s half-life is about 432 years, long enough for devices to operate for many years. The source is sealed inside the detector, and the quantity used is very small. Americium itself is a radioactive synthetic element that was first produced during the Manhattan Project.
Americium is sometimes confused with radium because both have a history of association with radioactivity. Modern household ionization detectors generally use americium-241, not radium.