I have taught a chemistry class where we use those disks. They have a stamp on them that say something like "NRC exempt quantity."

So, apparently, the disks have enough radioactive material to detect, but not enough to cause anyone to worry.

Smoke detectors have radioactive material

Not all though. Photoelectric detectors also exist and seem to be more common in homes these days.

TIL that you can buy antimatter on the internet.