americium

Why is americium used in a smoke detector instead of a more available and cheaper radioactive isotope?
Answered by Silk Road, Quora
Americium is a radioactive metal that spits out alpha particles, which are like tiny bullets made of helium nuclei.
Alpha particles can ionize the air, which means they knock off electrons from the air molecules and make them charged.
Smoke detectors have a chamber with two metal plates that have opposite charges. The ionized air allows a small electric current to flow between the plates.
When smoke gets in the way, it blocks the alpha particles and reduces the ionization. The current drops and the alarm rings.
Americium is preferred because it has a long half-life of about 432 years, which means it stays radioactive for a long time and doesn't need to be replaced often.
It emits alpha particles with a low energy of about 5.5 MeV, which means they can be easily stopped by a thin layer of foil or ceramic.
This makes americium safe to handle and prevents any radiation from escaping the detector.
Americium is relatively cheap and easy to make from plutonium-241, which is a waste product of nuclear reactors.
(Americium is a synthetic chemical element; it has symbol Am and atomic number 95, and was named for the USA, where it was first made in 1944.)