Which of the following can be used to determine barrier thickness (shielding) needed to attenuate radiation to the required degree?

• A. Pair production
• B. Newton's 2nd law
• C. Planck's constant
• D. Half-value layer

Answer: (D)

Shielding calculations rely on the half-value layer. The HVL is the thickness of material needed to reduce the radiation intensity by half. Because attenuation is exponential, each HVL halves the beam again. To size a barrier, determine how many HVLs are needed to reach the required final intensity and multiply by the HVL for that material and energy. In formula form, I = I0 × (1/2)^{n}, where n is the number of HVLs, and barrier thickness = n × HVL. The other options aren’t used to determine shielding thickness: they relate to different physics and don’t provide a practical measure for sizing barriers.