Hi-
This is a question that has been discussed a few times, if you check out the Safecast website you can find a number of articles involving radon:
An important note is that the bGiegie is a geiger counter, not a scintillator - so it will measure radiation but will not speculate on the isotope or source. It’s not a gas measurement device and is not looking for Radon specifically. It can detect the radiation given off by Radon when it’s close to the sensor, but as gas dilutes and flows that would be unpredictable, and an environment outside with a strong breeze would read differently than an enclosed room. So the short answer is that yes it can detect Radon if it’s present, but no it’s not designed for measuring Radon and would not give you any information about it.
Hope that helps!
-sean