|MadSci Network: Physics|
From your description, and without seeing the actual device, it sounds like an instrument called a spinthariscope. The spinthariscope was very popular during the first half of the century. It contains a small screen coated with zinc sulfide. A tiny bit of uranium or thorium (often in the form of ore or oxide) is mounted on a pinhead above the screen. In a darkened room, you can observe the interaction of the emitted alpha particles with the zinc sulfide screen. As an alpha particle traverses the screen, it leaves a greenish-white flash. The use of a zinc sulfide screen was common in early experiments in nuclear science. One was used in the famous Rutherford Scattering Experiments (which led to the discovery of the nucleus) to count the scattered alpha particles. I do not know of any way that you would be able to "rejeuvenate" this device. The screen may have aged (part of the zinc sulfide having come off of the substrate it was coated on), or the surface of the source oxidized a bit more and decreased the number of alphas that reach the screen.
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