The RD200M uses a custom ion chamber.
Geiger tubes are very insensitive to alpha particles, even with special windows. The pancake type tube is perhaps 15% efficient (1 out of 6 particles detected), so if you want to measure low levels of radon alpha emission, a Geiger tube is not the way to go.
An ion chamber does not have to have a window, so samples can be introduced directly into the chamber.