For shielding gamma emitters, which material property is most relevant to attenuation?

• A. Color
• B. Thermal conductivity
• C. Material density/atomic number
• D. Mechanical strength

Answer: (C)

Gamma attenuation depends on how often gamma photons interact with matter as they travel through it. The chance of interaction per unit path length is described by the linear attenuation coefficient, which for a given energy equals the mass attenuation coefficient times the density. Materials with higher density and higher average atomic number have more atoms and electrons per unit volume, increasing the probability of interactions like the photoelectric effect, Compton scattering, and pair production. That’s why dense, high‑Z materials (like lead) are so effective at reducing gamma intensity per unit thickness.

Other properties such as color, thermal conductivity, or mechanical strength don’t directly affect the probability of gamma interactions in the shielding material, though they matter for fabrication and engineering considerations.