Radioactive what though?
Hi Jacob
The Moon has lots of short lived isotopes made from cosmics reacting with heavy elements, but such are also rather diffusely spread in the crust. The rest of the elements in the Moon are light and liable to form stable products through spallation, not radioisotopes. And there's no carbon 14 because there's no nitrogen to make it from - many expected a lot of organics and volatiles on the Moon, perhaps that's why the "puzzle" arose.
Alot of cosmic radiation loses energy through making collision tracks in crystals on the Moon rather than making radioisotopes - I think the puzzle was more about ignorance than real mystery.
Adam