Im looking for an old novelty toy, that lets you watch atoms decay?

Its supposed to be a small container with a lens on top and a miniscule amount of some isotope that emits a photon as the element decays. when in a dark room you can watch one at a time. trouble is i cant remember what its called or where to find one, i think it was in popular science where i read about it. Looked like a really cool novelty