Abstract: A device for forming an X-ray beam or gamma beam (11) having a small cross-section and a variable direction, includes an X-ray source or gamma source (1) which supplies an X-ray beam and a diaphragm device which forms the X-ray beam from the radiation beam. The diaphragm device has a stationary diaphragm section (7) provided with a rectilinear slit (8) and a cylindrical first diaphragm body (3) which rotates about an axis of rotation (5) and which is provided with a helical slit (9) on its outer surface. In order to reduce the expenditure for manufacturing a device which is also suitable for different distances between the radiation source and the axis of rotation, the diaphragm body (3) has an at least approximately semi-circular cross-section over at least a part of its length.