But it is well possible that some people could still try. The Four Lions, only in the real world.
The other stuff sure, but it takes a truly mind boggling amount of radiation to incapacitate someone (near) instantly. A lot more than that.
100-150 Gy? The only concrete number I found looking around for ‘instant’ was 1000 Gy to likely kill someone within an hour.
I think incapacitation is what matters. The human body will fight death for some time, but it won't be able to running around with a heavy backpack of fission material. Various tissues will scream bloody hell almost immediately from being torn apart by the radiation.
Definitely a death sentence either way.
It's a fascinating(if very morbid) read if you want to have a look:
https://www-pub.iaea.org/MTCD/publications/PDF/Pub925_web.pd...
The point is, that maybe the terrorists wouldn't be inacapacitated, but they certainly wouldn't be able to ignore the effects for very long if at all.
These people are not stupid and would likely know what they are getting in to. I'd be more worried about a single unhinged person, but that's why we have security guards, and the problem would likely solve itself in short order in that case.