Which geometry uses gamma photons reflected back toward the gauge?

• A. Backscatter geometry
• B. Direct transmission
• C. Transmission geometry
• D. Absorption geometry

Answer: (A)

Backscatter geometry relies on the detector and the gamma source being on the same side of the material. A portion of the photons collide with the material and scatter back toward the gauge, so the detector reads those backscattered photons. The intensity of this backscatter depends on the material’s bulk properties, like density and moisture, which is why this setup is used for quick, practical readings without needing access to the far side of the sample. In contrast, direct transmission or transmission geometry require photons to pass straight through to a detector on the opposite side, so you’re measuring transmitted photons rather than those returning to the gauge. Absorption-focused setups deal with attenuation rather than backscattering, so they don’t produce the backscattered signal described here.