I wonder if the 10 year life of a detector really was linked to the depletion of the radioactive source? With a half life of 432 years, this would be quite some margin of safety
I am grateful to wiki for the following information on americium 241.
Americium can be produced in kilogram amounts and has some uses, mostly involving 241Am since it is easiest to produce relatively pure samples of this isotope. Americium is the only synthetic element to have found its way into the household, where one common type of smoke detector contains a tiny amount (about 0.2 microgram) of 241Am as a source of ionizing radiation.
This amount emits about 1 microcurie of nuclear radiation when new, with the amount declining slowly as the americium decays into neptunium, a different transuranic element, with a much longer half-life (about 2.14 million years). With its half-life of 432 years, the americium-241 in a smoke detector includes about 5% neptunium after 22 years, and about 10% after 43 years.
After the 432-year americium-241 half-life, a smoke detector's original americium would, by definition, be more than half neptunium.
