The HVL is the thickness of material required to reduce beam intensity by half. Which of the following best describes this definition?

• A. The thickness of material required to reduce beam intensity by half
• B. The thickness of material required to reduce beam intensity to zero
• C. The energy of the incoming photons
• D. The distance between the source and detector

Answer: (A)

HVL measures how much material is needed to cut the beam’s intensity in half. Attenuation of X-ray or gamma-ray photons through a material follows I = I0 e^{-μx}, where μ is the linear attenuation coefficient and x is thickness. To make the beam half as intense, set I = I0/2 and solve for x: x = ln(2)/μ. That thickness is the half-value layer. It depends on the material and the photon energy (μ varies with both), so different materials or energies have different HVLs. It is not the photon energy itself, nor the distance from source to detector, nor a complete stop to zero intensity.