The primary beam from a 250-kVp diagnostic X-ray machine in a doctor’s ofﬁce is directed perpendicularly at a wall that separates the ofﬁce from a public sidewalk just outside, 12 ft from the X-ray machine. The wall consists of 1.0 in. of hard brick covered by 2.67 in. of sand plaster. The machine is
operated 45 minutes per week with a beam current of 200 mA, and the beam is always directed at this wall. Calculate the thickness of lead that should be added to the wall to provide a proper primary protective barrier.