| Popis: |
The advent of high-brightness x-ray sources in the 10–40 keV region opens new possibilities of experiments with microbeams. Techniques to form these focused beams may be based on glancing mirrors, phase elements, or diffractive optics, in particular Fresnel zone plates (FZPs). Because of the long focal length and large acceptance, FZPs designed to work in the hard x-ray region tend to have quite large diameters and large numbers of zones. For instance, the zone plate described in this article has a 1860 μm diam, a focal length of f=3 m (for 8 keV), and 1860 zones. On a standard pattern generator, circular shapes are always approximated as simpler structures. The tolerance requirement for shape and positions of zones depends on the number of zones, and it is necessary to guarantee that the circular structures are approximated to the required degree of accuracy while keeping the size of the data structure to a reasonable size for processing by the exposure system. For instance, if polygons are used to appro... |
