X-ray optics can be used to collimate the isotropic output into a beam or to focus the x-rays on the target. We have tested different kinds of optics with the result that each of them serve a specific task best. Depending on the experiment, relevant parameters are the total photon flux, the flux per unit angle or the flux density in the focus.
a) A 100 µm thick Germanium crystal cut along the (111) plane is bonded to a toroidally bent quartz substrate. Here the (444) reflex is used and the curvature of the crystal is chosen in such a way that the entire surface area (12 x 40 mm2) is reflecting. The mirror is made for 1:1 imaging with a working distance of 468 mm, and the deflection angle is given by the angle 2Q = 70° of the Ge (444) reflex. The focus is rather close to the source (200 mm) which imposes some geometric constraints on the experimental design of the setup.
b) The second imaging device consists of two perpendicular elliptical multilayer mirrors (MLM) reflecting at a Bragg angle of approx. 3°, determined by the multilayer periodicity. There are two possible optical paths to focus in two dimensions by the Kirkpatrick-Baez scheme. The reflectivity of the optic over the entire solid angle W0 is very large (approx. 0.3 for a 15 µm small source size), because the thickness of the multilayers is graded along the ellipse. This makes the design of the MLM very flexible, e.g., regarding the magnification ratio. We have chosen a magnification M = 2 : 1 with a source-focus distance of 300 mm. The multilayer periodicity is designed to suppress Cu Kb radiation efficiently. The white Bremsstrahlung background is strongly suppressed with an average reflectivity of approximately 10-4.
c) Capillary x-ray optics are glass capillaries or poly-capillaries, which guide x-rays by total reflection. The elliptical capillary optic is made from a thin lead glass tube which is pulled in such a way as to approximate the shape of an ellipsoid. The resulting focusing is cylindrically symmetric and occurs by total reflection of the x-rays on the Helium/Glass interface. (The optic is filled with He.) We selected a 150 mm long section of the ellipsoid which images the x-rays with a magnification of M = 1 : 7. The small deflection angle is determined by the angle of total reflection (0:3° for lead glass), and the calculated average reflectivity of the capillary is 0.8. The high energetic x-rays are somewhat suppressed, as the angle of total reflection decreases with increasing photon energy. The source to optic-housing distance is 50 mm.
d) The principle of operation of typical poly-capillary elements consisting of bundles of glass or quartz capillaries is also based on the effect of total internal reflection of radiation, however, here several reflections are needed. This reduces the transmission through the optic and leads to an increase of the x-ray pulse duration. A more substantial broadening stems from the different single capillary lengths within the array of 59000 capillaries. The spectral response is similarly unselective as in the single-capillary case, and the convergence angle is very large (3.4°).