Which of the following properties is NOT among the three key properties of lead that contribute to shielding effectiveness?

• A. High atomic number
• B. High mass density
• C. High electron density
• D. High attenuation

Answer: (C)

Shielding effectiveness for X- and gamma-rays comes from how strongly photons interact with matter as they pass through. The factors that matter most are the material’s atomic number, how dense it is, and how much it attenuates the beam per unit thickness. A high atomic number increases the likelihood of photon interactions, especially at the energies where the photoelectric effect dominates, while a high mass density means more atoms packed into a given volume, giving photons more chances to interact in a short distance. The resulting attenuation per thickness combines these effects into a measure that reflects how good the shielding is.

High electron density is not listed as one of the main descriptors of shielding effectiveness. Although having more electrons per volume can influence some interaction probabilities, it is not the primary criterion used to characterize shielding performance. The three key properties are therefore high atomic number, high density, and high attenuation.