Re: Principle behind Lone Ranger 'Atom Bomb' Ring of 1947?

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.