Quantum efficiency of cesium iodide photocathodes at soft x-ray and extreme ultraviolet wavelengths.

We have measured the quantum efficiency for normal incidence radiation of microchannel plate detectors which use opaque or semitransparent photocathodes made of fluffy CsI, solid CsI, or both. At wavelengths below ~44 A, detectors with fluffy CsI semitransparent photocathodes are more efficient than those with opaque photocathodes of solid CsI, but the opposite is true at longer wavelengths. Fluffy CsI semitransparent photocathodes with surface densities between 150 and 400 μg/cm2 are optimum at soft x-ray wavelengths, and we have obtained efficiencies of 35 and 41% at 8 and 44 A, respectively, for a single-layer photocathode. The measured peak efficiency for an opaque layer of solid CsI, deposited at 0° coating angle and 5000 A (226 μg/cm2) in thickness, is 56% at 110 A.

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