[en] A detector is provided which responds to a two-dimensional pattern of x-radiation emergent from an object under examination to generate discrete electrical signals indicative of the amounts of radiation received at each of many locations, distributed over the pattern. The detector includes a substantially flat chamber, containing a noble gas, with an x-ray permeable window disposed to receive the pattern of radiation, a target at the rear of the chamber comprising high resistance material capable of storing charge and means for reading the charge pattern from the target. To improve resolution for such a flat chamber, as opposed to one which is spherical about the x-ray origin, a potential pattern is provided at least on the input surface of the detector to provide lines of force in the chamber more closely radial from the x-ray source than would otherwise be the case. The potential pattern may be applied by conductive rings. By the use of a suitable resistor chain, to which a variable potential can be applied, the detector can be adapted to x-ray sources at different distances from the detector. (author)