Americium. Close-up view of the active part of a domestic smoke detector. At the centre of this is a piece of americium dioxide (AmO2). Americium is a radioactive transuranic metal of the actinide group. Americium-241 is used in smoke detectors as is decays with alpha radiation and has a half-life of over 400 years. The radiation it produces passes through an ionisation chamber where it enables a constant current to pass. If any smoke particles enter the chamber, the current is interrupted and the alarm triggered.