Welcome to the forum @Bertin !
What sort of device will you have in the field for data collection?
IE: What/how are you recording the radiation readings?
If you are building a rig, the PostCard might be a good choice to incorporate.
If you already have your kit sorted out, then the new TX2 might be a good fit ?
There really isn’t a “bad” answer in the RTK lineup, but some serve different purposes a little better.
Do you have a RTK Correction service already picked out (many States in the USA have free RTK Networks, for example). Otherwise, PointPerfect Flex is a great service.
If you wont have internet/cellular service at the project sites for RTK corrections, you can look at a Base/Rover combo, or satellite internet service such as StarLink Mini.