Cesium iodide is the preferred material for the scintillation layer because it can be formed into:

• A. Columns which focus light photons
• B. Squares which focus x-ray photons
• C. Columns which focus x-ray photons
• D. Squares which focus light photons

Answer: (A)

Cesium iodide is used because it can be grown as columnar, needle-like crystals. This columnar structure acts as light guides, channeling the light produced by each X-ray interaction along the length of the column and toward the detector with minimal spreading. That improves light collection efficiency and spatial resolution, since the signal stays tied to the original interaction site. It doesn’t focus X-ray photons, and a square arrangement wouldn’t provide the same light-guiding advantage. So the best description is columns which focus light photons.