Crowbar
Could this be made only using mechanical (and simple electronic - no computation) components?
Probably, although I'm saying that based on my general understanding of this stuff; might need a different approach in specific cases.
Portable radiation meters, despite containing a bunch of electronics with quite probably a microprocessor at its centre, are meant to be autonomous (and usually portable, but that's not really relevant here). So they would make a decent base for such a crowbar system, provided they're sensitive to the radiation used in the CT or treatment device.
Biting the hand that feeds IT
