Transporting that crap around is complicated and just chocked full of uncertainty the least of which is extensive radiation monitoring in likely targets.
Well, discounting the logistical impossibility of say... a
lead box, I do agree that the detectors are the biggest issue for the enemy. They are a force multiplier for us. Since they don't need to be manned, and can just passively wait and detect any neutrons at the proper MeV, and actually even have a good idea of what nasty it was that passed by it.
However, unless it's an actual physical barrier like a toll booth, or border crossing, I'm not sure if just driving a van into the target region hell-bent-for leather wouldn't suffice.
Also, there's the whole psy-ops factor of striking a target that's in "Podunk, Nowhereville", like we've discussed wondering about Mumbai-style attacks. That makes security and detectors non-existent.
One of them figures out to skip NYC and D.C. and various federal parks and monuments, and just strike a big mall or stadium or school in the Midwest etc. the media and FUD-factor would be sky-high.