Which property contributes to lead's shielding effectiveness?

• A. High atomic number
• B. Low density
• C. High thermal conductivity
• D. Magnetic susceptibility

Answer: (A)

Shielding effectiveness comes from how likely photons are to be absorbed or scattered as they pass through a material, quantified by the attenuation coefficient. For diagnostic X-rays and gamma rays, the photoelectric interaction dominates at many energies, and its probability increases dramatically with the atomic number. Lead has a very high atomic number (82), so it massively increases the chance that incoming photons will be absorbed per unit thickness, yielding strong attenuation. While density also helps (more material means more opportunities for interaction), the primary reason lead is so effective is its high atomic number, which boosts photon absorption. The other properties listed don’t directly affect photon attenuation in the same way: low density would reduce shielding, high thermal conductivity isn’t about photon shielding, and magnetic susceptibility isn’t relevant to X-ray attenuation.