Figure 1. A schematic diagram explaining the principle of the model introduced by Petrantonaki et al [3]. If a material Y, e.g. a concrete wall, of thickness y₁ is already present, then the thickness x₁ of another shielding material X, e.g. lead, that should be placed in front of y₁ to bring the exposure down to the limit, is that attenuating the X-ray beam by as much as thickness y₂ of material Y, where B(y₂+y₁) is the required relative attenuation to comply with the dose limit.