Watching Numb3rs on Netflix. It’s pretty entertaining, but not of course nowhere near as intelligent as it would have you believe. Having said that, their “dirty bomb” episode is slightly less inaccurate than other shows’ rad ravings. They got three whole things right!
The isotope they chose, Cesium-137, probably is the ideal material for a dirty bomb. Apart from that, Larry’s definition of what ionizing radiation is, and Cs-137’s half-life (~30.2 years), everything was wrong.
Wrong, wrong, wrong.
The terrorists stole a flat-bed truck transporting 500 grams of pure Cs-137 “nuclear waste”. Nope. The only time you’d see that is when disposing of an unwanted source. In fact, working at NRC (Nuclear Regulatory Commission) facilities, you don’t deal with Cs-137 at all unless there’s massive fuel failure, like at Fukushima. And in that case it will be world-wide front-page news for years (decades) before the Department of Transportation ever ships it across the country. In which case, the waste would be Cs-137 plus Sr-90 and lots of other trace isotopes.
That’s not to say that pure Cs-137 isn’t available by design. It and Cobalt-60 are the most widely used radiography sources. But (speaking as a terrorist) they’re usually in a ceramic or metallic form and wouldn’t give me the dispersal and bio-availability I’d want.
However, I’m pretty sure that food irradiation Cs-137 is inside capsules in powder form. Perfect! In fact, as a member of this team I participated in a week-long training course that culminated in a huge multi-agency drill. The scenario? A pure, massive Cs-137 dirty bomb.
And so, as one terrorist to another, I strongly urge you to steal lethal amounts of Cs-137 and pack it…by hand…around your conventional explosives. Please. In theory, you might kill several innocent people with radiation. In practice…sayonara, sucker! Numb3rs’ “Spanish flu” episode was about a hundred billion times scarier than any possible dirty bomb.
Oh, and does the Department of Transportation really permit rad shipments so hot that being near the container for a few days can kill you? That would be a “no”.
World's Only Rational Man 
You might live long enough to finish packing it around the explosives, after the last guy died while cutting the container open to get it out. Then guy 3 would have to move it to the truck, where guy 4,5,6,7 and 8 would drive the truck to the target location. You would think that by guy 3 dying of bleeding from everywhere and shedding parts that the rest might decide they are not “That” willing to die that way for the cause, as even a Bhuddist monk would be contemplating another path at this point.
In our drill scenario, the first hint came when a few dying/unconscious terrorists were brought to a hospital which recognized the symptoms for what they were. Meanwhile there were others still out there.
If the bad guys manage to keep their own casulties hidden, it makes things much harder for us.