In direct transmission, where is the radiation source positioned relative to the material?

• A. At the surface
• B. Lowered to depth
• C. Inside the gauge
• D. Behind the material

Answer: (B)

Direct transmission relies on a straight-line beam that travels completely through the material from source to detector. To make that path well-defined and minimize scattering or edge effects, the radiation source is placed at depth within the material—lowered to depth—so the beam passes through the material in a predictable, through-thickness path. If the source were at the surface, behind the material, or inside the gauge, the geometry wouldn't produce the clean direct-through signal needed for accurate transmission measurements, so lowering the source to depth best fits how direct transmission is designed to work.