Abstract: The invention relates to an arrangement for generating an X-ray or gamma beam with small cross-section and variable direction, having an X-ray or gamma emitter, from the focus of which a bundle of rays emerges, and a diaphragm arrangement, which cuts out a beam from the bundle of rays and comprises a hollow-cylindrical first diaphragm body which is rotatable about its axis of symmetry and has two mutually offset helical slits on the circumference. In this arrangement, an X-ray beam with at least approximately square cross-section is cut out on a relatively long hollow-cylindrical body with small diameter by the slits winding around the diaphragm body in at least one turn each and being shaped in such a way that at least one straight line runs through the slits towards the focus, the position of which line can be varied by turning the diaphragm body.