Nowadays there's noise about putting radiation detectors in various places to intercept nuclear terrorism. Hmm. Here's a little bit of background info.
There's only so much potential for radiation in a given specimen. What is significant is the rate at which it is released. Release it rapidly and it's more dangerous while it lasts, but it won't last long. Release it slowly and exposure rates aren't so bad, but it will hang around longer. If you think about it you'll realize that isotopes with rapid rates of decay don't exist naturally in appreciable quantities, because they've all pretty much burned out by now. The slow ones have been here since evolution day one so we're inherently acclimated to consistent low levels of radiation.
Unfortunately there are some isotopes which lie between the extremes, giving dangerous levels of radiation while they last, while lasting long enough to force us to deal with them. That stuff is what the Yucca Mountain depository is designed for. It's not any good for nuclear weapons for reasons outside the scope of this blog.
But you can use it for a "dirty bomb". This is a bomb designed to cause great amounts of radiation exposure. Hmm - I have to ask why?
Small doses aren't worth the trouble - people might not even know they're getting them, and any results take years to show up, so what's the point?
Large doses? Unless they're truly astronomical, they don't kill people right away. I'm not a terrorist, but I don't think I'd be interested in merely injuring a bunch of people, who'll have plenty of time left to hunt me down and know they're going to die anyway.
About anything that might cause large doses causes huge logistical problems. It's not like nerve gases where the agent can be created at delivery with binary mixtures - it's dangerous all along. The radiation is easily detected even from a distance with simple instruments. Unless of course you're talking about a lot of shielding, which is incredibly heavy and otherwise causes problems of its own.
Then what? If they just scatter highly radioactive substances, it can be cleaned up. It's expensive to do, but I don't see why Al Qaeda or other creeps would be interested in doing what amounts to little more than making a mess.
Order your own geiger counter here. I think you can beat that price.