In an electron accelerator having a target which is subjected to an electron beam for the production of deceleration radiation, a collimator is provided behind the target which has a passageway adapted to receive a beam cone of maximum dimensions. Adjustable x-ray aperture plates are provided behind the collimator and an electron absorber is provided behind the target. An additional electron absorber is positioned in a widened portion at one end of the passageway in the collimator. Both electron absorbers are made of a material having a relatively low atomic number as compared with electron absorbers of the prior art.