The US actually does have a radiation detector network that covers most major cities. If you have enough radioactive material to pose a threat to people's healt

The US actually does have a radiation detector network that covers most major cities. If you have enough radioactive material to pose a threat to people's health from brief exposure over a non-trivial area, it would be detected. You'd probably also wind up on a lot of intelligence agencies' radars trying to acquire that much radioactive material and the equipment for handling it.