I've just been learning some things about smoke detectors. Were you aware that most actually have a small amount of a radioactive element in them? Any that use ionization will have some kind of radioactive source. (Now there are also optical detectors which use light to detect the smoke, but these are more expensive so you're not going to find them as often.) Now checking out Wikipedia the most common radioactive element used is americium-241 (which denotes the isotope of americium with an atomic weight of 241). This element doesn't occur in nature and was first created in 1944.