StrategyPage has some excellent posts on dirty bombs, two comforting and one downright chilling:
Irradiation devices for food-processing are so potent that they can inflict lethal doses in seconds on persons who touch them, and incapacitate in a few hours. But one suicide terrorist could hack off a smaller chunk, drop it in a not-very shielded portable container, toss that to a better-shielded friend, and they leave. The first dies quickly. The second drives to a target (ballpark, theater, shopping mall, etc.), hauls the violently radioactive chunk out and sits there until he starts vomiting. Hundreds or even thousands of people could receive lethal doses without knowing it until days later. That is really scary.
Enjoy the movie.