Instruct: Given a question, retrieve documents that answer the question.
Query: What is the wavelength of X-rays?
X-rays make up X-radiation, a form of electromagnetic radiation. Most X-rays have a wavelength ranging from 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz (3×1016 Hz to 3×1019 Hz) and energies in the range 100 eV to 100 keV. X-ray wavelengths are shorter than those of UV rays and typically longer than those of gamma rays. In many languages, X-radiation is referred to with terms meaning Röntgen radiation, after the German scientist Wilhelm Röntgen who discovered these on November 8, 1895,[1] who usually is credited as its discoverer, and who named it X-radiation to signify an unknown type of radiation.[2] Spelling of X-ray(s) in the English language includes the variants x-ray(s), xray(s), and X ray(s).[3]
The composition of X-rays was unknown, his father argued that X-rays are streams of particles, others argued that they are waves. Max von Laue directed an X-ray beam at a crystal in front of a photographic plate; alongside of the spot where the beam struck there were additional spots from deflected rays hence X-rays are waves. In 1912, as a first-year research student at Cambridge, W L Bragg, while strolling by the river, had the insight that crystals made from parallel sheets of atoms would not diffract X-ray beams that struck their surface at most angles because X-rays deflected by collisions with atoms would be out of phase, cancelling one another out. However, when the X-ray beam stuck at an angle at which the distances it passed between atomic sheets in the crystal equaled the X-ray's wavelength then those deflected would be in phase and produce a spot on a nearby film. From this insight he wrote the simple Bragg equation that relates the wavelength of the X-ray and the distance between atomic sheets in a simple crystal to the angles at which an impinging X-ray beam would be reflected.
X-ray lithography originated as a candidate for next-generation lithography for the semiconductor industry, with batches of microprocessors successfully produced. Having short wavelengths (below 1 nm), X-rays overcome the diffraction limits of optical lithography, allowing smaller feature sizes. If the X-ray source isn't collimated, as with a synchrotron radiation, elementary collimating mirrors or diffractive lenses are used in the place of the refractive lenses used in optics. The X-rays illuminate a mask placed in proximity of a resist-coated wafer. The X-rays are broadband, typically from a compact synchrotron radiation source, allowing rapid exposure. Deep X-ray lithography (DXRL) uses yet shorter wavelengths on the order of and modified procedures such as the LIGA process, to fabricate deep and even three-dimensional structures.
X-rays with high photon energies (above 5–10 keV, below 0.2–0.1nm wavelength) are called hard X-rays, while those with lower energy (and longer wavelength) are called soft X-rays.[53] Due to their penetrating ability, hard X-rays are widely used to image the inside of objects, e.g., in medical radiography and airport security. The term X-ray is metonymically used to refer to a radiographic image produced using this method, in addition to the method itself. Since the wavelengths of hard X-rays are similar to the size of atoms, they are also useful for determining crystal structures by X-ray crystallography. By contrast, soft X-rays are easily absorbed in air; the attenuation length of 600 eV (~2nm) X-rays in water is less than 1 micrometer.[54]
From the observed X-ray spectrum, combined with spectral emission results for other wavelength ranges, an astronomical model addressing the likely source of X-ray emission can be constructed. For example, with Scorpius X-1 the X-ray spectrum steeply drops off as X-ray energy increases up to 20 keV, which is likely for a thermal-plasma mechanism. In addition, there is no radio emission, and the visible continuum is roughly what would be expected from a hot plasma fitting the observed X-ray flux. The plasma could be a coronal cloud of a central object or a transient plasma, where the energy source is unknown, but could be related to the idea of a close binary.
The distinction between X-rays and gamma rays has changed in recent decades. Originally, the electromagnetic radiation emitted by X-ray tubes had a longer wavelength than the radiation emitted by radioactive nuclei (gamma rays). So older literature distinguished between X- and gamma radiation on the basis of wavelength, with radiation shorter than some arbitrary wavelength, such as 10 m, defined as gamma rays. However, as shorter wavelength continuous spectrum "X-ray" sources such as linear accelerators and longer wavelength "gamma ray" emitters were discovered, the wavelength bands largely overlapped. The two types of radiation are now usually distinguished by their origin: X-rays are emitted by electrons outside the nucleus, while gamma rays are emitted by the nucleus.
In X-ray crystallography, anomalous scattering refers to a change in a diffracting X-ray’s phase that is unique from the rest of the atoms in a crystal due to strong X-ray absorbance. The amount of energy that individual atoms absorb depends on their atomic number. The relatively light atoms found in proteins such as carbon, nitrogen, and oxygen do not contribute to anomalous scattering at normal X-ray wavelengths used for X-ray crystallography. Thus, in order to observe anomalous scattering, a heavy atom must be native to the protein or a heavy atom derivative should be made. In addition, the X-ray’s wavelength should be close to the heavy atom’s absorption edge.
The water window is a region of the electromagnetic spectrum in which water is transparent to soft x-rays. The window extends from the K-absorption edge of carbon at 282 eV (68 PHz, 4.40 nm wavelength) to the K-edge of oxygen at 533 eV (129 PHz, 2.33 nm wavelength). Water is transparent to these X-rays, but carbon (and thus, organic molecules) is absorbing. These wavelengths could be used in an x-ray microscope for viewing living specimens.
There is no consensus for a definition distinguishing between X-rays and gamma rays. One common practice is to distinguish between the two types of radiation based on their source: X-rays are emitted by electrons, while gamma rays are emitted by the atomic nucleus.[55][56][57][58] This definition has several problems: other processes also can generate these high-energy photons, or sometimes the method of generation is not known. One common alternative is to distinguish X- and gamma radiation on the basis of wavelength (or, equivalently, frequency or photon energy), with radiation shorter than some arbitrary wavelength, such as 10−11 m (0.1 Å), defined as gamma radiation.[59] This criterion assigns a photon to an unambiguous category, but is only possible if wavelength is known. (Some measurement techniques do not distinguish between detected wavelengths.) However, these two definitions often coincide since the electromagnetic radiation emitted by X-ray tubes generally has a longer wavelength and lower photon energy than the radiation emitted by radioactive nuclei.[55] Occasionally, one term or the other is used in specific contexts due to historical precedent, based on measurement (detection) technique, or based on their intended use rather than their wavelength or source. Thus, gamma-rays generated for medical and industrial uses, for example radiotherapy, in the ranges of 6–20 MeV, can in this context also be referred to as X-rays.
These scattering methods generally use "monochromatic" X-rays, which are restricted to a single wavelength with minor deviations. A broad spectrum of X-rays (that is, a blend of X-rays with different wavelengths) can also be used to carry out X-ray diffraction, a technique known as the Laue method. This is the method used in the original discovery of X-ray diffraction. Laue scattering provides much structural information with only a short exposure to the X-ray beam, and is therefore used in structural studies of very rapid events (Time resolved crystallography). However, it is not as well-suited as monochromatic scattering for determining the full atomic structure of a crystal and therefore works better with crystals with relatively simple atomic arrangements.
X-rays range in wavelength from 10 to 0.01 nanometers; a typical wavelength used for crystallography is 1 Å (0.1 nm), which is on the scale of covalent chemical bonds and the radius of a single atom. Longer-wavelength photons (such as ultraviolet radiation) would not have sufficient resolution to determine the atomic positions. At the other extreme, shorter-wavelength photons such as gamma rays are difficult to produce in large numbers, difficult to focus, and interact too strongly with matter, producing particle-antiparticle pairs. Therefore, X-rays are the "sweetspot" for wavelength when determining atomic-resolution structures from the scattering of electromagnetic radiation.
When an electron from the inner shell of an atom is excited by the energy of a photon, it moves to a higher energy level. When it returns to the low energy level, the energy which it previously gained by the excitation is emitted as a photon which has a wavelength that is characteristic for the element (there could be several characteristic wavelengths per element). Analysis of the X-ray emission spectrum produces qualitative results about the elemental composition of the specimen. Comparison of the specimen's spectrum with the spectra of samples of known composition produces quantitative results (after some mathematical corrections for absorption, fluorescence and atomic number). Atoms can be excited by a high-energy beam of charged particles such as electrons (in an electron microscope for example), protons (see PIXE) or a beam of X-rays (see X-ray fluorescence, or XRF or also recently in transmission XRT). These methods enable elements from the entire periodic table to be analysed, with the exception of H, He and Li.
In electron microscopy an electron beam excites X-rays; there are two main techniques for analysis of spectra of characteristic X-ray radiation: energy-dispersive X-ray spectroscopy (EDS) and wavelength dispersive X-ray spectroscopy (WDS). In X-Ray Transmission (XRT), the equivalent atomic composition (Zeff) is captured based on photo electric and Compton effects.
X-ray diffraction is a form of elastic scattering; the outgoing X-rays have the same energy, and thus same wavelength, as the incoming X-rays, only with altered direction. By contrast, inelastic scattering occurs when energy is transferred from the incoming X-ray to an inner-shell electron exciting it to a higher energy level. Such inelastic scattering reduces the energy (or increases the wavelength) of the outgoing beam. Inelastic scattering is useful for probing such electron excitation, but not in determining the distribution of atoms within the crystal.
Monochromatic wavelength dispersive x-ray fluorescence ("MWD XRF") is an enhanced version of conventional wavelength-dispersive X-ray spectroscopy ("WDXRF") elemental analysis. The key difference is that MWD XRF uses a doubly curved crystal X-ray optic between the X-ray source and the sample resulting in monochromatic excitation. This additional optic creates a high-intensity X-ray beam on a small spot size without increasing the power of the X-ray source. An MWD XRF instrument is constructed from a low-power X-ray tube, a point-to-point focusing optic for excitation, a sample cell, a focusing optic that collects the fluorescence from the sample, and an X-ray detector. By using an optic between the X-ray source and the sample, a monochromatic beam free of bremsstrahlung, excites the sample, eliciting the secondary fluorescence X-rays needed for elemental analysis. By restricting the band of wavelengths used for excitation, a much higher signal to background ratio is achieved. This type of excitation allows much lower limits of detection and faster reading times.
By the early 20th century, research into the interaction of X-rays with matter was well under way. It was observed that when X-rays of a known wavelength interact with atoms, the X-rays are scattered through an angle formula_1 and emerge at a different wavelength related to formula_1. Although classical electromagnetism predicted that the wavelength of scattered rays should be equal to the initial wavelength, multiple experiments had found that the wavelength of the scattered rays was longer (corresponding to lower energy) than the initial wavelength.
In X-ray crystallography, there are several ways to recover the lost phases. A powerful solution is the "Multi-wavelength Anomalous Diffraction" (MAD) method. In this technique, atoms' inner electrons absorb X-rays of particular wavelengths, and reemit the X-rays after a delay, inducing a phase shift in all of the reflections, known as the "anomalous dispersion effect". Analysis of this phase shift (which may be different for individual reflections) results in a solution for the phases. Since X-ray fluorescence techniques (like this one) require excitation at very specific wavelengths, it is necessary to use synchrotron radiation when using the MAD method.
Wavelength-dispersive X-ray spectroscopy (WDXS or WDS) is a method used to count the number of X-rays of a specific wavelength diffracted by a crystal. The wavelength of the impinging X-ray and the crystal's lattice spacings are related by Bragg's law and produce constructive interference if they fit the criteria of Bragg's law. Unlike the related technique of energy-dispersive X-ray spectroscopy (EDS), WDS reads or counts only the X-rays of a single wavelength at a time, not producing a broad spectrum of wavelengths or energies simultaneously. WDS is primarily used in chemical analysis, in an X-ray fluorescence spectrometer, in an electron microprobe, and may also be used in a scanning electron microscope.
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 gets appreciable. Since the refractive index is less than 1 for x-rays these lenses must be concave to achieve focusing, contrary to visible light lenses. Radii of curvature are typically less than a millimeter making the usable x-ray beam width at most about 1 mm. To reduce the absorption of x-rays in these stacks, materials with very low atomic number such as beryllium or lithium are typically used. Since the refractive index depends strongly on X-ray wavelength these lenses are highly chromatic and thus only suitable for monochromatic X-rays.
X-ray telescopes, using shorter wavelengths than ultraviolet light Ultraviolet telescopes, using shorter wavelengths than visible light Optical telescopes, using visible light Infrared telescopes, using longer wavelengths than visible light Submillimetre telescopes, using longer wavelengths than infrared light Fresnel Imager, an optical lens technology X-ray optics, optics for certain X-ray wavelengths
X-ray crystallography is a form of elastic scattering; the outgoing X-rays have the same energy, and thus same wavelength, as the incoming X-rays, only with altered direction. By contrast, "inelastic scattering" occurs when energy is transferred from the incoming X-ray to the crystal, e.g., by exciting an inner-shell electron to a higher energy level. Such inelastic scattering reduces the energy (or increases the wavelength) of the outgoing beam. Inelastic scattering is useful for probing such excitations of matter, but not in determining the distribution of scatterers within the matter, which is the goal of X-ray crystallography.
The characteristic X-rays are used for chemical analysis. Specific X-ray wavelengths or energies are selected and counted, either by wavelength dispersive X-ray spectroscopy (WDS) or energy dispersive X-ray spectroscopy (EDS). WDS utilizes Bragg diffraction from crystals to select X-ray wavelengths of interest and direct them to gas-flow or sealed proportional detectors. In contrast, EDS uses a solid state semiconductor detector to accumulate X-rays of all wavelengths produced from the sample. While EDS yields more information and typically requires a much shorter counting time, WDS is generally a more precise technique with lower limits of detection because its superior X-ray peak resolution and greater peak to background ratio.
An X-ray filter is a material placed in front of an X-ray source in order to reduce the intensity of particular wavelengths from its spectrum and selectively alter the distribution of X-ray wavelengths within a given beam.
The anomalous aspects of X-ray scattering have become the focus of considerable interest in the scientific community because of the availability of synchrotron radiation. In contrast to desktop X-ray sources that work at a limited set of fixed wavelengths, synchrotron radiation is generated by accelerating electrons and using an undulator(device of periodic placed dipole magnets) to "wiggle" the electrons in their path, to generate the wanted wavelength of X-rays. This allows scientists to vary the wavelength, which in turn makes it possible to vary the scattering factor for one particular element in the sample under investigation. Thus a particular element can be highlighted. This is known as "contrast variation". In addition to this effect the anomalous scatter is more sensitive to any deviation from sphericity of the electron cloud around the atom. This can lead to resonant effects involving transitions in the outer shell of the atom: resonant anomalous X-ray scattering.
In a wavelength dispersive X-ray spectrometer the single crystal diffracts the photons (Bragg's law) which are collected by a detector. Without any motion there will be just one wavelength detected. By moving crystal and detector, a wide region of spectrum is observed (to collect all parts of spectrum three of four different single crystals may be needed). In contrast to EDS, WDS method is a method of sequential spectrum acquisition. While WDS is slower than EDS and more sensitive to positioning specimen in the spectrometer, it has superior spectral resolution and sensitivity. WDS is widely used in microprobes (where X-ray microanalysis is the main task) and in XRF; 
it is widely used in the field of x ray diffraction to calculate various data such as interplanar spacing and wavelength of the incident x ray using Bragg's law.
With the initial detection of an extrasolar X-ray source, the first question usually asked is "What is the source?" An extensive search is often made in other wavelengths such as visible or radio for possible coincident objects. Many of the verified X-ray locations still do not have readily discernible sources. X-ray astrometry becomes a serious concern that results in ever greater demands for finer angular resolution and spectral radiance.