Direct transmission uses which device, and how is it positioned?

• A. A sealed gamma source placed on the surface
• B. A rod with Cesium-137 lowered to depth
• C. A broad beam X-ray source mounted above the material
• D. A laser-based scanning device at the surface

Answer: (B)

Direct transmission measures density by sending radiation straight through the material from a source to a detector on the opposite side. To create that clear path, the radioactive source is placed inside the material—typically in a rigid rod that is lowered into a borehole to a set depth—so the gamma rays travel directly through the material to the detector. Cesium-137 is used because its gamma rays penetrate soil and asphalt well, and lowering the rod to depth minimizes scattering, giving a more direct, density-related signal. This approach differs from methods that keep the source on the surface (backscatter) or use broad-beam X-ray setups or laser scanners, which do not provide the same direct transmission measurement.