Abstract: Image-detector for depicting differences in intensity in high energy photon beams with the aid of a photon-sensitive element, the photon-sensitive element is an ionization chamber, consisting in the main of two mainly equivalent plates of an electrically insulating material, which are attached to each other by a ring-shaped electrically insulating part as a divider, while the outer walls of both plates are covered with electrically conductive material, whereby one of the plates is equipped with a number of parallel high voltage electrodes over a central part of its inner wall and the other plate is equipped over a central part of its inner wall with a number of parallel ionization current electrodes which extend perpendicularly towards the high voltage electrodes, while the inner walls of both plates around the central parts are covered with an electrically conductive material and a liquid dieectric is situated in the space between the plates.