Apparently Toe-Knee Was Right About that Tracking Nuclear Isotopes Thing...

Crooks and Liars has always been so good about supplying me with my Olbermann fix.  It's a vice I know, but I enjoys my little indulgences...

When stories about national security and nuclear threats hit the networks, isn't it curious how they never bring in a nuclear scientist to talk about such security?  How hard is it to track a nuke?  What actually is done other than tapping our phones and torturing suspected terrorists?  
Okay, I'm doing a bit of research which is thus-far incomplete, but I work in the oil industry, and one of the things we in the business do to scan and understand rock formations 2 to 3 km below the surface is that we shoot nuclear isotopes down there into the rock formations, and those isotopes send readings back to our instruments allowing geologists to literally map that formation based on where the isotopes travel to.  That means they send particles through MILES of rock that we can sense.  
So if we have instruments that can read particle readings through miles of rock, it maybe makes sense that we might have instruments that can say pick them up through miles of say... air?  Obviously there are probably ways to shield these signatures, but how hard is it?  How heavy would such a container be, and how would you ship it over?  The logistics of such an endeavor make it not feasible.