The dirty bomb is a real and serious threat, but it's been exaggerated compared to other kinds of attacks. Dirty bombs aren't the indiscriminate weapons of mass destruction many make them out to be. They're more akin to really dirty conventional bombs.

A dirty bomb attack would have two steps:

1) Terrorist detonates an explosive bomb.

2) Radioactive contamination sprays out from the point of detonation.

Here are the pros and cons of using gamma radiography devices for dirty bombs.

Pros

* Gamma radiography devices are widely available and used in many industries, so obtaining one without raising suspicion would be relatively easy for a terrorist group.

* They can be quite powerful.

Cons

* The radioactivity they produce is limited. While a dirty bomb from one of these devices could contaminate an area, the actual health effects would be limited to the immediate vicinity of the blast. The dirty bomb would still do harm, but its destructive potential is often overstated.

* The source itself would be immediately identifiable, making it harder to set off without authorities narrowing in on the source.

Overall, dirty bombs are a relatively crude and ineffective weapon. They're unlikely to cause mass destruction, and they would be difficult for a terrorist group to build without being detected. While gamma radiography devices could potentially be used in a dirty bomb, their limited radioactive output and ease of identification would make them a less attractive option for terrorists than other materials.

In conclusion, the threat of a dirty bomb from a gamma radiography device is real but exaggerated, and there are other materials and methods terrorists would likely prioritize in constructing a dirty bomb.