the differentially absorbed x radiation impinges upon an exterior surface 14a of a first conductive electrode 14 spaced a selected distance d from and substantially parallel to a second conductive electrode 15 advantageously at least first electrode 14 is formed of a light material such as aluminum and the like having substantially no absorption of x ray photons having energies commonly encountered in medical diagnostic apparatus e g on the order of 60 kev a source 16 of electrical potential of magnitude v is coupled between first and second electrodes 14 and 15 with polarity selected to form an electric field in the direction of arrow e from second electrode 15 to first electrode 14 a removable sheet 17 of an insulative material such as a plastic and the like is initially positioned essentially parallel to and substantially in abutment with the interior surface 15a of the second electrode the gap 18 defined between first and second electrodes 14 and 15 respectively is advantageously filled with an 34 avalanche amplifying 34 gas such as air argon and the like for charge multiplication a structured photocathode 20 preferably formed of a photocathodic material such as cesium iodide csi and the like is fabricated upon the inward surface 14b of first electrode 14 and extends a distance h h lt d therefrom into gap 18 toward second electrode 15 photocathode 20 is structured to have a geometry increasing the photoemitting surface area.