Which factor does NOT govern shielding design for gamma emitters?

• A. Photon energy
• B. Source activity
• C. Geometry
• D. Distance to the source

Answer: (D)

The key idea is that shielding thickness is determined by how penetrating the photons are, how many photons are being emitted, and how the photons can reach the barrier, not by how far the receptor sits from the source.

Photon energy tells you how easily a material attenuates the photons (through the attenuation coefficient μ). Source activity tells you how many photons are produced per unit time. Geometry tells you where photons can travel and whether there are direct paths or openings that photons can stream through. Together, these factors set the required barrier thickness to achieve a safe dose.

Distance to the source affects the dose you’d measure at a point after the shield (via inverse-square behavior), but it does not change how much the shield must attenuate the photons to meet the safety limit. The shield’s attenuation property is governed by energy, activity, and geometry, not by how far the receptor is from the source.