C 1818 (OTTO) Cesium iodide, 99.995% Cas 7789-17-5 - used in medicine as the scintillating material in flat panel x-ray detectors. C 1818 (OTTO) Cesium iodide, 99.995% Cas 7789-17-5 - used as the input phosphor of an X-ray image intensifier tube found in fluoroscopy equipment. Caesium iodide photocathodes are highly efficient at extreme ultraviolet wavelengths.