The electron exit window assembly contains a metal cover plate, a titanium insert piece and a titanium exit window. The cover plate covers the interior of the last cavity of the accelerator. It contains an insert opening into which is inserted the insert piece in a vacuum tight manner. The insert piece in turn contains a hole which is covered by the exit window. The cover plate preferably is made of stainless steel, and the insert piece is preferably brazed into the insert opening of the cover plate.