Which of the following materials requires the smallest half-value layer (HVL) to effectively attenuate the x-ray beam?

• A. Glass
• B. Aluminum
• C. Lead
• D. Concrete

Answer: (C)

Attenuation efficiency in X-ray shielding is measured by how thick a material has to be to cut the beam’s intensity in half, described by the half-value layer. The HVL is inversely related to the material’s linear attenuation coefficient, μ. Materials with higher effective atomic number and density have larger μ for X-rays, so they attenuate more strongly per millimeter and require less thickness to reach the same half-value reduction.

Lead has a very high atomic number and high density, so it absorbs diagnostic X-rays far more effectively per unit thickness than glass, aluminum, or concrete. That means the smallest HVL comes from lead.

Glass and aluminum are lighter with lower Z, so they attenuate less efficiently and need more thickness to achieve a 50% reduction. Concrete, while denser than glass and aluminum, still has a lower μ for X-rays than lead, so its HVL per unit thickness is larger than lead’s as well.

In short, because lead provides the greatest attenuation per unit thickness at diagnostic X-ray energies, it requires the smallest HVL to effectively attenuate the beam.