Hello all, I have posted on Physics Forums a few times in the past, but mostly for help with my old physics classes and not anything in the real world. Part of my work involves radiography, but it is generally done in a field environment where we just shut down large sections of land to safely use our generator. We also use shielding material, but this is generally very thin sheets of lead or copper directly on the X-ray generator or imager. My co-workers and I are thinking of making a pitch for a new building where we can exercise our X-ray machines, keeping everything outside to less than 2 mR/hr. With the energy (up to 7.5 MeV) and intensity (420 R/hr @ 1 m) of our biggest generator, I came up with needing almost 12' concrete walls in front of the beam (assuming it was placed 1 m in front of the wall) to knock it down below 2 mR/hr. Since it is going almost 12' through the concrete the inverse square law would also knock out a sizeable amount of that radiation, so we'd be left with well below the 2 mR/hr outside the cell. My question is, then, is there an equation that combines both the inverse square law and the HVL calculations? I know it can't hurt to be more safe, but it just might be a more feasible plan with the walls being a bit narrower. Thanks in advance for any help.