Abstract: A photosensitive matrix sensor include a faceplate of caesium iodide CsI mounted on a graphite base so as to transform high-frequency radiation, X-ray radiation, into low-frequency radiation, in the visible spectrum. It is shown that if the CsI is grown on such a graphite base, a sensor with much better resolution and much better sensitivity is obtained than if a gadolinium oxysulphide scintillator were used. Precautions in preparing the graphite may furthermore be taken rendering the surface of the graphite denser. It can thus be covered with a layer of amorphous carbon and or be made to undergo impregnation. This densification contributes to the homogeneity of the sensor. Protection of the CsI is then effected by a synthetic resin layer allied with a layer of liquid resin for optical coupling with a detector.