Improved end blocks for the electrode array of a multicell radiation detector support the array and isolate it from the interior walls of an enclosed ionization chamber. A plunger extends from the rear wall of each end block and is biased by a spring within the end block to bear against the rear wall of the detector chamber, biasing the electrode array forwardly against front spacers to position the front edges of the electrodes in equidistantly spaced relation to a x-ray transmissive window formed in the front wall of the detector housing. The bias exerted by the springs is adjustable. A tool is inserted into each end block during assembly of the detector to retract the plungers while the array is being positioned within the opened ionization chamber.The invention greatly reduces noise in the detector caused by microphonics, thus improving the integrity of the detected signal and permitting shorter scanning times and other advantages.