Gamma and X-ray shielding: which material best blocks them?

• A. Water
• B. Plastic
• C. Lead, steel, concrete
• D. Vacuum

Answer: (C)

The main idea here is how materials stop gamma and X-ray photons. Photons are attenuated when they interact with matter, and the amount of attenuation depends on the material’s density and its atomic number. Dense, high‑Z materials have more electrons and stronger interaction probabilities, so photons are absorbed or scattered more readily. This is captured by the attenuation relationship I = I0 e^(−μx), where μ = (μ/ρ) ρ; a higher density or a higher mass attenuation coefficient (which increases with atomic number) means greater shielding effectiveness for the same thickness.

Water and plastic are relatively low in density and have lower effective atomic numbers, so they don’t slow down gamma and X-rays as effectively per unit thickness. Vacuum provides no material to interact with the photons, so it offers essentially no shielding at all.

Lead (and materials like steel and concrete) are much denser and have higher effective atomic numbers, giving them the strongest attenuation per thickness. Lead, in particular, is a standard shielding material because it combines high density with a high atomic number, making it the best option among the choices for blocking gamma and X-ray photons. Steel and concrete also shield well but typically require greater thickness to achieve the same reduction as a thinner layer of lead.