Alpha decay is the type of radioactive decay used in smoke detectors. Smoke detectors contain a very small amount of the radioactive element americium-241. The americium-241 undergoes alpha decay, which ionizes air molecules and allows a small electric current to flow. When smoke particles enter the chamber, they attach to the ionized air molecules, disrupting the flow of current. This drop in current triggers the alarm to go off, alerting people to the presence of smoke. The use of alpha radiation from americium-241's decay makes smoke detectors very sensitive at detecting even small amounts of smoke.