Fun fact: The same group that figured out how to make the atom bomb also paved the way for the invention of the modern smoke detector.
It's true. While the physicists of the highly-secretive Manhattan Project were trying to figure out what Plutonium was all about, they created an element new to science called Americium-241 by bombarding the Plutonium with neutrons. They kept it secret for years, but eventually it found its way into our homes.
The most common type of smoke detector around today—an ionization chamber smoke detector—uses this human-created radioactive element to sense smoke and save your life in the event of a fire.
This radiation strips off electrons and gives some of the air molecules a charge, which completes a circuit that smoke is really good at breaking.
Don't fucking panic, though! Alpha radiation is pathetic at penetrating anything, including your skin.