[cowsandmilk]

> Fixed lifetime suggests ionization rather than optical sensor?

Not at all. The 10 year lifetime is something that was set by people concerned by the lifetime of the electronics. The half life of the isotope in smoke detectors allow them to last well beyond 10 years. (The half life is 432.2 years)

[tialaramex]

I was aware that the half-life was much longer but I'd assumed the 10 years was based on the fact that nevertheless we're talking a degraded sensor performance over time, however it does seem like it's actually 10 years because they're worried the electronics will crap out, so you're right.