[en] Between the exit window of a radiation source and the material to be irradiated radiation absorbing plates are located for closure of the window. The plates are fixed to two opposite walls of a box by means of tilting levers on two turnable shafts. The axis of the shafts are parallel to the long side of the window. A drive on one axis can be used to swivel the plates from the radiation area of the source to allow the irradiation of the material. By means of connecting pieces located on the opposite side of the drive, the plates can be cooled using a circulating cooling liquid. The device described above facilitates access to the window if the window foil has to be repaired