You'll want an alpha-detecting Geiger counter for the best results Paolo, not all of them are.

It's a very minimal risk - the amount of radioactive material in the lume is tiny and it has a lot of dense metals to get through before it reaches your skin, energy dissipating all the while. The dangers were more for the people manufacturing the dials in the first place. Still, I doubt its use would get approved in this Health and Safety conscious age. Maybe I'll start marketing a range a line of lead straps for the concerned... hmmm, but what to line the lead with, you don't want that next to the skin either really... :)