Most modern smoke detectors use a radioactive element called Americium-241 to sense fire.

Ionization smoke detectors rely on Americium-241, a synthetic radioactive isotope discovered in 1944 by Glenn T. Seaborg and his team at the University of Chicago. This element is produced in nuclear reactors by bombarding Plutonium-239 with neutrons. Inside the detector, about 0.29 micrograms of Americium-241 dioxide are placed between two metal plates to create an ionization chamber.The isotope emits alpha particles that collide with air molecules, stripping away electrons and creating a constant flow of ions between the plates. This flow is measured as a steady electrical current. When smoke enters the chamber, the heavy particles attach to the ions and neutralize them, causing the electrical current to drop significantly.The integrated circuit detects this drop in current and activates the horn. While Americium-241 has a half-life of 432 years, the amount used is extremely small, typically around 0.9 microcuries. The alpha radiation it emits is easily blocked by a single sheet of paper or a few centimeters of air, ensuring it poses no health risk while contained within the plastic housing of the detector.