Polonium 210 is a radioactive element which emits alpha radiation.
Regular People data:
The disk is 100% legal and technically produces 222,000 particle emissions each minute! Most Geiger counters cannot actually detect all of the emissions, and I only see one side of the disk at a time, so I only detect about 26,000 clicks.
I purchased a 0.1 uCi source disk, which is NRC exempt and legal to own. The specific activity of the disk is 3700 Bq. My GM detects up to 26,000 CPM @ 2-4 mm distance.
Click the like button if you think Polonium 210 is cool! =)