A radiographic imaging device includes a gas avalanche detector detecting and locating X-ray or gamma ray ionizing radiation. The detector has a gas enclosure with an admission window for admitting incident X-ray photons; an intermediate plane electrode in the enclosure between and parallel to two end plane electrodes thereby forming an amplification space constituting a conversion space where the photons are convertible into electrical charges made up of primary electrons and corresponding ions; the intermediate electrode being operable at an electrical potential relative to the electrical potentials of the end electrodes suitable for generating an electric field causing multiplication of the electrons by the avalanche phenomenon in the amplification space near the intermediate electrode; one of the end electrodes being a collector electrode for picking up the electrical signals induced by the ions; and said window being level with the amplification space and between the intermediate plane and collector electrodes.