Are you sure a Geiger counter will pick up alpha radiation? I thought that angeoger counter had a window in front of the detector which would stop alpha which is why we used scintillation cocktail fluid and read alphas indirectly as light flashes in a specialized counter. I could be wrong it's been a long time. Also, there are many isotopes of U. I doubt the highly energeticly emitting ones are sold for photography. In other words, they might be alpha emitters not beta or gamma and if so, the Geiger counter might not find them.