[Vinnie]

Not only that, but they would show up like a lighted flare on the radiation monitors now installed nearly everywhere

I'm not so sure. If plutonium was used ( if it could be used in this device) doesn't it only give off alpha particles? They can be stopped by a sheet of paper. Shielding would have to be minimal.

[js1138]

No matter how you shield there are always secondary particles and radiation that can be detected. Plutonium is the most potent ingredient for a small nuke, but it is also almost impossible to detonate without an elaborate technical support system. Any terrorist bomb would most likely be a dirty fizzle.