Abstract: A detector for X-radiation or other ionizing radiation comprises a pair of curved metal bars lying in spaced apart parallel planes. Ceramic members are bonded to the respective bars in facing relationship. Each ceramic member has a plurality of radially extending circumferentially spaced grooves facing similar grooves in the other member. A plurality of electrode plates are held in juxtaposed and circumferentially spaced apart relationship by disposing their respective upper and lower edges in the grooves of opposed ceramic members. The faces of adjacent electrode plates define gas-filled gaps in which photoelectron-ion pairs are produced when radiation enters from the front edges of the plates. The above described assembly is disposed within a pressurized gas-filled chamber having an X-ray transmissive window adjacent the front edges of the plates. A cover encloses the chamber. It has insulator feed-throughs for connecting the electrode plates with external electric circuits.