In backscatter geometry, gamma photons are reflected onto the material and must have at least one reach the gauge. This describes which geometry?

• A. The gauge
• B. The detector
• C. The shielding
• D. The source

Answer: (A)

Backscatter readings rely on the instrument's arrangement where the source and the detector are on the same side of the material. Gamma photons are sent into the surface, scatter back, and, crucially, at least some of those backscattered photons must reach the instrument on that same side to be counted. That means the geometry is defined by the gauge’s configuration—the way the gauge is placed and operates to collect backscattered photons. The other components aren’t defining the layout of the measurement: shielding changes attenuation, while the source or detector alone don’t describe how the measurement path is arranged.