I don't know if something like that would help, but in Switzerland there are annual measurement flights (with a helicopter): https://blog.alertswiss.ch/en/categories/focp-news/annual-ne...

Part of this seems to be (sometimes?) to find radioactive sources: "The goal of this part of the exercise was to find two radioactive sources which, for training purposes, had been placed somewhere in an area of about 2,500 km2"