Radon sensors might be dedicated and expensive, or they might just count clicks in the gamma spectrum.
If you have a basement | enclosed space in a granite rich area then you'll probably get radon gas pooling in that space unless it's sealed and|or vented.
You're unlikely to have an unspecified gamma source (lump of uranium, limp of potassium, lump of thorium) in your basement so ...
A generic geiger counter (click counter) should give a reading (low) outside your space in the open air, and another reading (higher) inside that space .. that can vary with time (as gas is expressed, pools, vented).
That generic click difference is almost entirely radon .. you'll be taking readings within 20 feet, one inside, anothe outside.
So you can easily get away with a cheap geiger counter to indicate if there's an issue .. if there is, you'll need to either:
* minise time spent inside space, and|or
* vent the space (fan blower, + pipe to outside)
* accept a cancer risk ~ a pack a day smoking if you use space as an office.
Source: a couple of decades geophysical surveying making radiometric maps of entire countries.
it's a heavy gas that "oozes" out of granites and settles in low areas .. valleys until a breeze comes through or basements until pumped out ( floor level extractor fan | gas sump ).
