On Sun, 1 Nov 2009 20:02:40 +0000 (UTC), (David
Combs) wrote Re Smoke detectors for the elderly:

2: the kind that works via some wee radioactive thing, that
senses, I think, the FIRE. It has some OTHER single-letter
printed on the package.

Most of the above type of alarm uses Americium-241 (an Alfa particle
emitter) in it's detector chamber to ionize air and detect combustion
products.
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