As we know that detection efficiency for gas filled detector is low, then how we use them to detect radiation levels inside X-ray or nuclear medicine department rooms ?
Won't they be giving false readings due to the low detection efficiency?
If the detector's efficiency is known, then it can simply be taken into account using a calibration. If the goal is to get doses in units of Sieverts, then there is a complicated calibration in any case, because doses depend on things like quality factors.