X-ray optics is a branch of applied optics that studies the processes of X-ray propagation in media, as well as developing elements for X-ray devices. X-ray optics, unlike conventional optics, consider electromagnetic waves in the X-ray wavelength range of 10 −4 to 100 Å (from 10 −14 to 10 −8 m ) and gamma radiation <10 −4 Å.
Content
General Information
One of the reasons for the development of X-ray optics is the possibility of obtaining on X-ray microscopes images of objects with incredibly small sizes due to the increased resolution of optical systems using shorter wavelengths. X-ray optics are also used in X-ray lasers and X-ray telescopes .
Materials used in conventional optics are not applicable to x-ray optics due to their proximity to a unit of the X-ray refractive index . In other words, X-rays pass through the substance, practically without changing their trajectory. In addition, X-rays are strongly absorbed in the substance due to the photoelectric effect . So a layer of air 1 cm thick is almost completely opaque to soft X-rays. Therefore, a vacuum is necessary for the operation of X-ray optical systems, and X-ray telescopes are placed outside the atmosphere .
History
Working Principles
The main task of X-ray optics is focusing X-rays. Therefore, the most important characteristics of optical systems is the focal length and width of the output beam. There are several types of optical systems, depending on the principle of operation.
Reflective X-ray Optics
X-ray mirror
The reflection of electromagnetic waves from the interface between two media is described in optics by the Fresnel formulas . With a normal incidence on the mirror, the reflection coefficient is too small, that is, X-rays are practically not reflected, but only absorbed by the mirror or pass through it. Therefore, normal incidence mirrors are not used in X-ray optics. As the angle of incidence increases, the reflection coefficient increases, making it possible to use oblique incidence mirrors (the beam slides along the surface of the mirror) used in X-ray astronomy (see the Voltaire telescope ).
Conical capillary
This device is a hollow conical tube. X-ray vacuum is an optically more dense medium, so if the beam falls on the smooth surface of the capillary at an angle less than the critical, then it is fully reflected [1] This principle is realized in Kumakhov optics .
Diffractive optics
Zone records
Fresnel zone plate can also be used to focus X-rays. The principle of its operation is based on dividing the wave front into zones in such a way that the neighboring zones are in antiphase. For example, if you close (darken) all even zones, the odd zones that remain open will be all in one phase. As a result of interference, the signal will be amplified many times. For the first time, X-ray zone plates were obtained in 1988 at the Lawrence Livermore National Laboratory [1] .
Bragg Fresnel Optics
The width of the zones in the Fresnel plate depends on the wavelength of the radiation, so the more monochromatic it is , the better the plate works. Therefore, the zone plate is sprayed onto a single crystal and the monochromaticity of the radiation is provided by Bragg diffraction [1] .
X-ray refractive optics
In the X-ray range, almost all materials have a refractive index close to unity. Therefore, a separate lens would have an extremely large focal length, which cannot be used in an x-ray experiment. This problem is solved by creating voids of a certain size and shape in a certain material that behave like a sequence of lenses; and also, by creating isolated parabolic refractive lenses, sets of which can focus X-rays at a small focal length. Such devices in the English literature are called Compound refractive lens ( composite refractive lenses ) [2] .
X-ray Waveguides
Such devices are analogous to devices used in conventional optics. Radiation is transported along curved waveguides and collected at point [1] .
Other ways to build an image
- Collimators
- Coding aperture
See also
- X-ray microscope
- X-ray telescope
- X-ray laser
Notes
Literature
- Pinsker Z. G. X-ray crystal optics. M .: Science, 1982.
- Vysotsky, V.I., Vorontsov, V.I., Kuzmin, R.N., et al. The Sagnac experiment on X-rays, Uspekhi nat. sciences. 1994. T. 164, No. 3. P. 309-324.
- Bushuev V.A., Kuzmin R.N. Secondary processes in X-ray optics. M .: Publishing House of Moscow State University, 1990.
- Ingal VN, Beliaevskaya EA // J. Phys. D .: Appl. Phys. 1995. Vol. 28. P. 2314.
- Duax WL Holograhy with X-rays // Intern. Union Crystallography // Newsletter. 1996. Vol. 4, No. 2. P. 3.
- Elton R. X-ray lasers / Trans. from English by ed. A.V. Vinogradov. M .: Mir, 1994.
- Schmal G., Rudolf D. X-ray optics and microscopy: Trans. from English M .: Mir, 1987. 463 p.