The story is still very new. I have tried to find some clear descriptions of the device, but am inclined to believe that the specifications are not available as this is now a pending federal criminal courts case. The federal attorney also states the weapon was feasible, although he stated covert agents posing as supporters/buyers made sure that no working radiological supply was ever part of the device the conspirators were working on. I don't think the FBI or US attorneys are misleading about the device' potential to harm. They have access to a wide array of qualified people for assessing the device and it's feasibility, though. I checked the FBI's press release on fbi.gov, and the statements are the same as in the several news articles appearing on the web.
The way I can see this working would be to focus the x-ray emissions in a very tight cone, even a straight line as is done with other electromagnetic spectrum rays, as in lasers. Although the amount of radiation would require a large power capacity to generate 8 Grays, as claimed by one of the terrorists, it would be enough to cause serious harm in only a brief exposure at near range, perhaps a split second.
Think of; park the van next to a mosque pedestrian entrance or car park driveway where the device is pointing across where people or vehicles are slowly passing on their way to worship services. Remotely toggle the emitter on for a split second x-ray burst as a victim passes in front of the beam or cone of emitted x-rays. For a power source, super capacitors are quite small and efficient for this need, and a van could hold a lot of them, or if the van is disguised as a repair company vehicle, run a heavy gauge electric cable to the nearest contractor's service hookup to recharge an array of deep cycle batteries or combination with large capacitors. Not sure just how much charge capacity/how quick the drain would be to produce a split second burst.
There are industrial inspection and research x-ray machines with focusable emitters, and x-ray lasers used to inspect things such as ship propellers and jet engine turbine vanes for structural cracks, or to produce micrographs of very small objects and sub atomic particles. They are extremely large and expensive, but obviously these are orders of magnitude larger and more powerful than what the terrorists' came up with. I am also guessing that as to radiological source material, the connection with GE or with some other potential medical scan device supplier was arranged by the terrorists, albeit they unknowingly were thwarted by the undercover agents. These materials are regulated, but if a purchase were attempted by the GE employee using the equivalent of "company letterhead", it would not likely have raised suspicions on the part of the regulated supplier.
