Figure excerpt from NCRP Report No. 147, Structural Shielding Design for Medical X-Ray Imaging Facilities
Fig. 4.5c. The lead thickness requirements for secondary barriers in the representative radiographic room as a function of NT/Pd2. P is in milligray per week, N is the weekly total number of patients examined in the radiographic
room, and d (in meters) is chosen as the distance from the most intense radiation source to the occupied area. If the Wnorm values given in Table 4.5 do not match the per patient workload for the facility under consideration, then the original value of NT/Pd2 can be multiplied by Wsite / Wnorm, and the modified value can be used to obtain the required shielding
from Figure 4.5c.