Question:
I saw a circuit that uses an ordinary neon lamp as a radiation (gamma) detector. Has anyone ever tried using a neon-filled 0A3?

I have a circuit built up that adjusts the voltage to an NE-2 just below its firing point. It seems to respond to visible light (flashlight).

Next step is to canabalize a smoke detector for the radiation source (weak gamma radiation) and see if that will trigger it.

Answer: I doubt that the wimpy alphas or gammas from a smoke detector can penetrate the glass of either. Even then, a neon is a terrible radiation detector.

The key feature of a Geiger tube is the extremely high electric field close to the hair-thin central wire. This gives a nice secondary ionization gain factor that provides a comfortable margin between radiation-induced breakdown and spontaneous breakdown from the HV itself. A neon doesn't have this margin.

A simple electroscope dramatically detects the alphas from a smoke detector source.

See Knoll's classic book, Radiation Detection and Measurement; Powell's sometimes has used copies.