Thats it, sometimes called a gamma scan. I had one of these done, it supposedly leaves some trace radioactivity for a week or so, I had no idea they could detect it though! This is interesting!
Okay .. then I guess July 4th wasn't joking after all .. now I don't know whether to be worried or not. I guess it's a good thing they are checking for the stuff.
But .. the ones who are carrying such stuff would probably not come through the checkpoint .. they would be sneaking across the porous borders. Very scary to find out they are looking for this stuff.