Americium

Americium (AM) is a synthetic chemical element, atomic number 95, named for the USA where it was first made in 1944.

Q. Why is americium used in a smoke detector instead of a more available and cheaper radioactive isotope?

A. 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.