The best way to prevent a dirty bomb explosion is if terrorists know that in retaliation we will nuke either their country or their general headquarters.
This is a behavioral phenomenon, just as the provocation of aggression by a display of weakness is a behavioral phenomenon.
Yes, dirty bombs can come from outside our borders.
You are operating under the assumption that your enemy fears death.
Unfortunately this is not the case. They would simply say, “Bring it on. I want to be with Allah.”
Threats like that work us up a lot more than it scares them.