Modern smoke detectors seem to have done away with ionization in favour of optical technology, and so no longer contain radioactive materials.
Bad news for mad scientists!
Bad news for mad scientists!
Radioactive ones are usually cheaper but come with plenty of warnings about not disassembling or ingesting the source. Best to have a mix spread around the home. It's also well worth having a CO alarm if you have non-electric heating/cooking.
Edit: Seems I'm not right regarding sensitivity, wiki says:
"Ionization detectors are more sensitive to the flaming stage of fires than optical detectors, while optical detectors are more sensitive to fires in the early smouldering stage"
Wow it also says:
"Where an ionizing smoke detector has been the only detector, fires in the early stages have not always been effectively detected."
That's worrying.
You don't really want the thing going off every time someone burns a bit of toast or it just becomes an annoyance, and therefore at risk or being removed or disabled.