[lucas]

You must have a physical barrier at 7.5 ìSv/hr. How do you achieve this? Let’s say you use 250 kV and 4 mA in your example. If no object is in the way for the radiation this gives 6 Sv/hr at 1 metre. If you then use the inverse square law the safety distance is 895 m. For steel and 250 kV the half value layer is 12 mm. So if the incoming radiation is 6 Sv/hr the radiation out is approx. 3 Sv/hr, then you need a safety distance of… 632 m. –Wow. This is how to calculate the safety distance for the central beam of radiation, but in the real life you have other objects around you that absorb the radiation (pipes, valves, etc.), so if don’t want to set up a barrier around half the world you need to measure the radiation level with a dose rate meter.