as gamma rays are difficult to produce in large numbers, difficult to focus, and interact too strongly with matter, producing particle–antiparticle pairs. Similar diffraction patterns can be produced by scattering electrons or neutrons. X-rays are usually not diffracted from atomic nuclei, but only from the electrons surrounding them. Interference X-ray interference is the addition (superposition) of two or more X-ray waves that results in a new wave pattern. X-ray interference usually refers to the interaction of waves that are correlated or coherent with each other, either because they come from the same source or because they have the same or nearly the same frequency. Two non-monochromatic X-ray waves are only fully coherent with each other if they both have exactly the same range of wavelengths and the same phase differences at each of the constituent wavelengths. The total phase difference is derived from the sum of both the path difference and the initial phase difference (if the X-ray waves are generated from two or more different sources). It can then be concluded whether the X-ray waves reaching a point are in phase (constructive interference) or out of phase (destructive interference). Technologies There are a variety of techniques used to funnel X-ray photons to the appropriate location on an X-ray detector: Grazing incidence mirrors in a Wolter telescope, or a Kirkpatrick–Baez X-ray reflection microscope. Zone plates. Bent crystals. Normal-incidence mirrors making use of multilayer coatings. A normal-incidence lens much like an optical lens, such as a compound refractive lens. Microstructured optical arrays, namely, capillary/polycapillary optical systems. Coded aperture imaging. Modulation collimators. X-ray waveguides. Most X-ray optical elements (with the exception of grazing-incidence mirrors) are very small and must be designed for a particular incident angle and energy, thus limiting their applications in divergent radiation. Although the technology has advanced rapidly, its practical uses outside research are still limited. Efforts are ongoing, however, to introduce X-ray optics in medical X-ray imaging. 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. Another application is to optimize the energy distribution of the X-ray beam to improve contrast-to-noise ratio compared to conventional energy filtering. Mirrors for X-ray optics The mirrors can be made of glass, ceramic, or metal foil, coated by a reflective layer. The most commonly used reflective materials for X-ray mirrors are gold and iridium. Even with