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.