Which statement best describes backscatter geometry?

Which statement best describes backscatter geometry?

• A. Photons are reflected onto the material and at least one reaches the gauge
• B. Photons pass through the material to the gauge
• C. The source is placed above the material
• D. The gauge is used without a shield

Answer: (A)

Backscatter geometry means the detector reads photons that have interacted with the material on the same side as the source. The photons are emitted into the material, scatter back toward the surface, and at least some of them return to the gauge to be detected. This is exactly what the statement describes: photons go into the material and, through scattering, some reach the gauge. If photons had to pass all the way through the material to reach a detector on the opposite side, that would be transmission geometry, not backscatter. Saying only that the source is above the material misses the crucial backscatter path. And using the gauge without shielding relates to safety and design, not the geometry of how the photons travel and are detected.

Backscatter geometry means the detector reads photons that have interacted with the material on the same side as the source. The photons are emitted into the material, scatter back toward the surface, and at least some of them return to the gauge to be detected. This is exactly what the statement describes: photons go into the material and, through scattering, some reach the gauge.

If photons had to pass all the way through the material to reach a detector on the opposite side, that would be transmission geometry, not backscatter. Saying only that the source is above the material misses the crucial backscatter path. And using the gauge without shielding relates to safety and design, not the geometry of how the photons travel and are detected.