X-ray optics
X-ray beams are often collimated (reduced in size) using pinholes or movable slits typically made of tungsten or some other high-Z material.
Narrow parts of an X-ray spectrum can be selected with monochromators based on one or multiple Bragg reflections by crystals.
Polycapillary optics are achromatic and thus suitable for scanning fluorescence imaging and other applications where a broad X-ray spectrum is useful.
The zone widths are designed so that a transmitted wave gets constructive interference in a single point giving a focus.
[4] Zone plates can be used as condensers to collect light, but also for direct full-field imaging in e.g. an X-ray microscope.
Zone plates are highly chromatic and usually designed only for a narrow energy span, making it necessary to have monochromatic X-rays for efficient collection and high-resolution imaging.
Since refractive indices at X-ray wavelengths are so close to 1, the focal lengths of normal lenses get impractically long.
To overcome this, lenses with very small radii of curvature are used, and they are stacked in long rows, so that the combined focusing power becomes appreciable.
Radii of curvature are typically less than one millimeter, making the usable X-ray beam width at most about 1 mm.
X-rays produce a diffraction pattern because their wavelength typically has the same order of magnitude (0.1–10.0 nm) as the spacing between the atomic planes in the crystal.
If the atoms are arranged symmetrically (as is found in a crystal) with a separation d, these spherical waves will be in phase (add constructively) only in directions where their path-length difference 2d sin θ is equal to an integer multiple of the wavelength λ.
For instance, one of the applications showing greater promise is in enhancing both the contrast and resolution of mammographic images, compared to conventional anti-scatter grids.
[17] X-ray mirrors can be made of glass, ceramic, or metal foil, coated by a reflective layer.
The materials for multilayers are selected to give the highest possible reflection at each boundary and the smallest absorption or the propagation through the structure.
The absorption in the heavier material can be reduced by positioning it close to the nodes of the standing-wave field inside the structure.
An X-ray mirror optic for the NuSTAR space telescope working at 79 keV (hard, i.e. high-energy X-radiation) was made using multilayered coatings, computer-aided manufacturing, and other techniques.
