Abstract: Radiometric gage for contactless measurement of the surface density or the thickness of a flat product, includes:an X-radiation emitter, whose tube, is directed toward the flat productan X-radiation receiver, whose detector is disposed in order to receive X-radiation scattered or transmitted in attenuated form through said flat product, said receiver delivering a measurement signala member for processing the measurement signal, in order to obtain information representing the surface density or the thickness of the flat productThe anticathode of the tube is made of a material with atomic number as high as possible, preferably greater than 70, which is a good conductor of heat and has a high melting point.