Really sensitive nuclear detectors are based upon a photo-multiplier tube and a phosphorescent film of some sort. PM tubes can pick up light in a fiber optic cable merely by wrapping the fiber around the tube. This is caused by photon leakage through the sides of the fiber and its jacket. Those tubes can detect a single photon of light.
When I used them they were inside an aluminum casing ... and we still had to keep the room light off.
The phosphorescent material was also cooled with liquid nitrogen. (About 1974)
Thank you for replying with your personal experience working with radiation detectors.
I did not know some of the specific things you mentioned.
We seem to have a wealth of very knowledgeable people with a broad range of experiences in this forum.