Actually, the government (NNSA, https://nnsa.energy.gov/ https://nnsa.energy.gov/) trains for that on a pretty regular basis. They scan cities for known source

Actually, the government (NNSA, https://nnsa.energy.gov/ https://nnsa.energy.gov/) trains for that on a pretty regular basis. They scan cities for known sources of radiation (hospitals, certain types of industries, natural sources) and map it all out. They'll also go back and conduct drills using those maps to see if they can locate planted radiation sources. A real-life thriller...