From MariachiWiki

We had a question: "I am still confused about how the scintillator works and I'd like to know more about that."

Scintillators are materials whose molecules that emit light when charged particles pass near the atoms and transfer some of their energy by attracting or repelling the electrons around the atom. When these atoms come back to their normal state they release energy in the form of light. Usually this light is in the ultraviolet and can't travel very far before getting reabsorbed. So the material is "doped" with another material that absorbs the UV very quickly and re-emits light in the visible spectrum. This light then bounces around the plastic and some of it gets to the end where the photumultiplier sits. It is helped by this bouncing by the phenomenon of "total internal reflection" where if light hits a surface between a denser material (ie the plastic) and less dense (the air), it is completely reflected. You have witnessed this effect if you ever looked up under water in a pool or fish tank. I will show you an example this afternoon. Anyway, some fraction of the original light that is emitted gets to the phototube which then converts this tiny flash into an electrical signal that you can work with. CC