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Input phosphor and photocathode

Cross-section of an image intensifier input assembly. The input screen sits on a curved surface (not flat) to match the electron optics, which is a source of pincushion distortion. A magnified layer stack shows the x-ray beam passing through a thin aluminium or titanium input window, then a caesium iodide (CsI) input phosphor made of columnar needle crystals that channel light and limit lateral spread, and finally the Sb-Cs photocathode on the inner surface. The sequence is x-ray to light to electron: about 60 percent of incident x-rays are absorbed in the phosphor, each absorbed x-ray gives about 3000 blue-green light photons channelled along one crystal, and the photocathode emits photoelectrons in response to that light, not to x-rays directly.

Image intensifier input screen Cross-section of an image intensifier input assembly. The input screen sits on a curved surface (not flat) to match the electron optics, which is a source of pincushion distortion. A magnified layer stack shows the x-ray beam passing through a thin aluminium or titanium input window, then a caesium iodide (CsI) input phosphor made of columnar needle crystals that channel light and limit lateral spread, and finally the Sb-Cs photocathode on the inner surface. The sequence is x-ray to light to electron: about 60 percent of incident x-rays are absorbed in the phosphor, each absorbed x-ray gives about 3000 blue-green light photons channelled along one crystal, and the photocathode emits photoelectrons in response to that light, not to x-rays directly. Image intensifier input screen (cross-section, magnified) sequence: x-ray → light (in caesium iodide) → electron (from photocathode) x-rays curved input surface a flat screen (worse, not sufficient) pincushion distortion Curved to match the electron optics, keeping electron path lengths more uniform across the field. The curvature (with the electrostatic focusing field) is a source of pincushion distortion; a perfectly flat screen is neither required nor sufficient for an undistorted image. input window Al or Ti, low-Z envelope input phosphor — caesium iodide (CsI), x-ray-facing outer layer ~0.2–0.5 mm; columnar needle crystals, not one flat slab photoelectrons → to electron optics incident x-rays ~60% absorbed (QDE ≈ 0.6); ~40% pass through ~3000 light photons blue/blue-green, matched to the photocathode; kept within one crystal channel → good resolution photocathode (Sb–Cs) on the inner surface: light, not x-rays, frees the electrons
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