Californium is the heaviest element which currently has practical applications, but it is for niche uses such as nuclear reactor start-up, to detect underground water and other applications which require a portable fast neutron source.
It’s also only certain kinds of smoke detectors that work by ionization. There’s still the mundane ones that use a laser to check how clear the air is.
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u/davidarblack 13h ago
That's americium, Californium is only made in labs and is highly unstable