Your thriller plot is dumb. ISIS did not almost kill with radioactivity.
Some of us root for the bad guys in the movies. They get to be much more ingenious than the good guys, most of whom are either corrupt or boring. With this mindset, I read the Washington Post article today that claimed ISIS almost had a dirty bomb. The upshot: two decommissioned cobalt-60 radiation therapy devices were in storage in Mosul, which was controlled by ISIS up until recently. The danger, however, was minimal.
A dirty bomb is not a cobalt bomb. A cobalt bomb is a nuclear weapon that contains nonradioactive cobalt metal (cobalt-59, usually as a wire). When exploded, fast neutrons hit the metal, and in a nuclear reaction, the cobalt-59 absorbs a neutron to become cobalt-60. The heat vaporizes the cobalt-60 and the blast spreads it over a large area.
A cobalt bomb would indeed be dangerous, but there is no proof one has ever been developed as a weapon. The concept was first proposed by Leo Szilard, a Hungarian-born physicist working in the US, in a radio show in 1950. In 1957, the British military exploded a cobalt-containing weapon, although the point of the cobalt was as a tracer to understand the blast. The test failed. It wasn't repeated. In 2015, the design of a Russian nuclear-armed torpedo was leaked that may have been intended as a cobalt bomb. It does not appear to have been produced.
A dirty bomb is not a nuclear bomb. It is a dispersal device (perhaps as simple as fireworks) that is duped with radioactive material, so that the explosion spreads the material. These exist. However, the material doesn't spread far. Cobalt-60 comes as either a wire or a sheet, so it's not that easy to disperse. Cesium-137, on the other hand, another isotope used in radiation therapy machines, is a chloride salt that can be dispersed as easily as table salt. That's a good image to have, some table salt in a salt-shaker. Let's say you're a budding terrorist. How far can you disperse table salt? Not very. Also, once you've dispersed the table salt, how much is at any particular place? Not much.
The only way these medical devices could be dangerous is if the relatively small amount of radioactive material in them stays in one place. And that has happened. In 2013, a Mexican driver trucking decommissioned devices all the way across Mexico went to sleep in his truck and got robbed. The robbers removed the shielding from the devices. The robbers undoubtedly died; however, nobody else did. One can also slip radioactive material into a meal, as Russians have done with diplomats they no longer needed. These are horrible incidents but they are hardly at the scale the Washington Post wanted you to believe.
The prime danger of these weapons is their use to create terror. Scare the population (most of whom not being conversant with nuclear chemistry), they panic, and bad things happen. Medical care shuts down as entire cities think they've been dosed and flood the system. Traffic accidents, loss of productivity as city centers are shut down, expensive clean-ups, sure, those will happen.
Still scared of these things? Don't be. One crazed yahoo with a gun and a vendetta can do a lot more damage than a terrorist group with a medical device they don't understand. The upshot of the event in Mosul was that upon recapturing the city, the military found the medical devices in the same place they were before ISIS invaded. The Post speculates that ISIS fighters may have lacked the no-how to open up the devices safely, or may have been distracted. These guys aren't dummies. Do recall that Osama bin laden, trained as an engineer, did his own mathematical calculations of how much heat would be needed to melt the metal holding up the Twin Towers before he authorized the attack. I think it much more likely that the ISIS leadership knew exactly what they had and made the rational decision not to bother. How much more terrorized could the population of Iraq have become?
That doesn't mean they were disinterested in the technology. Both ISIS and al-Qaeda have looked into it. But if you can't explode something in a public space in a major city (one not accustomed to bombings) so as to generate terror, there is little reason to bother.
For those of us who write hard science fiction (plausible and based on actual science), there's nothing here to write about. For those writing the squishier science fantasy, there's far better stuff to make heads explode: holy green electric fire anti-Nazi stuff (Indiana Jones), dementors and death eaters (Harry Potter), cosmic bulldozers that destroy Earth to make way for a highway (Hitchhiker's Guide to the Galaxy).
Cue the mad scientist laugh.