Here's an IAEA report from an incident where a worker walked into an irradiation room of a commercial irradiator with the radioactive source still up - he was only there for about one minute and received a dose of "only" about 10-20Gy total, but realized something was wrong because of intense pain that developed shortly after entering the room, followed by intense neusea and diarrhea 5 minutes later. Despite receiving urgent care immediately he still died a month later.

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.