What is the wavelength of X-rays?
X-ray
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 (3×10 Hz to 3×10 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, who usually is credited as its discoverer, and who named it "X-radiation" to signify an unknown type of radiation. Spelling of "X-ray(s)" in the English language includes the variants "x-ray(s)", "xray(s)", and "X ray(s)".
Wavelength
In the case of electromagnetic radiation—such as light—in free space, the phase speed is the speed of light, about 3×10 m/s. Thus the wavelength of a 100 MHz electromagnetic (radio) wave is about: 3×10 m/s divided by 10 Hz = 3 metres. The wavelength of visible light ranges from deep red, roughly 700 nm, to violet, roughly 400 nm (for other examples, see electromagnetic spectrum).
X-ray
In August 1896 Dr. HD. Hawks, a graduate of Columbia College, suffered severe hand and chest burns from an x-ray demonstration. It was reported in "Electrical Review" and led to many other reports of problems associated with x-rays being sent in to the publication. Many experimenters including Elihu Thomson at Edison's lab, William J. Morton, and Nikola Tesla also reported burns. Elihu Thomson deliberately exposed a finger to an x-ray tube over a period of time and suffered pain, swelling, and blistering. Other effects were sometimes blamed for the damage including ultraviolet rays and (according to Tesla) ozone. Many physicians claimed there were no effects from X-ray exposure at all.
On August 3, 1905 at San Francisco, California, Elizabeth Fleischman, American X-ray pioneer, died from complications as a result of her work with X-rays.
X-ray
In about 1906, the physicist Charles Barkla discovered that X-rays could be scattered by gases, and that each element had a characteristic X-ray spectrum. He won the 1917 Nobel Prize in Physics for this discovery.
Terahertz radiation
Terahertz radiation – also known as submillimeter radiation, terahertz waves, tremendously high frequency (THF), T-rays, T-waves, T-light, T-lux or THz – consists of electromagnetic waves within the ITU-designated band of frequencies from 0.3 to 3 terahertz (THz). One terahertz is 10 Hz or 1000 GHz. Wavelengths of radiation in the terahertz band correspondingly range from 1 mm to 0.1 mm (or 100 μm). Because terahertz radiation begins at a wavelength of one millimeter and proceeds into shorter wavelengths, it is sometimes known as the "submillimeter band", and its radiation as "submillimeter waves", especially in astronomy.
Compton scattering
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.
X-ray photoelectron spectroscopy
Because the energy of an X-ray with particular wavelength is known (for Al K X-rays, "E" = 1486.7 eV), and because the emitted electrons' kinetic energies are measured, the electron binding energy of each of the emitted electrons can be determined by using an equation that is based on the work of Ernest Rutherford (1914):
X-ray astronomy
X-rays from the Sun were detected by the U.S. Naval Research Laboratory Blossom experiment on board. An Aerobee 150 rocket was launched on June 12, 1962 and it detected the first X-rays from other celestial sources (Scorpius X-1). It is now known that such X-ray sources as Sco X-1 are compact stars, such as neutron stars or black holes. Material falling into a black hole may emit X-rays, but the black hole itself does not. The energy source for the X-ray emission is gravity. Infalling gas and dust is heated by the strong gravitational fields of these and other celestial objects. Based on discoveries in this new field of X-ray astronomy, starting with Scorpius X-1, Riccardo Giacconi received the Nobel Prize in Physics in 2002.
X-ray
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. 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 m (0.1 Å), defined as gamma radiation.
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.
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.
Thermosphere
The solar X-ray and extreme ultraviolet radiation (XUV) at wavelengths < 170 nm is almost completely absorbed within the thermosphere. This radiation causes the various ionospheric layers as well as a temperature increase at these heights (Figure 1).
While the solar visible light (380 to 780 nm) is nearly constant with a variability of not more than about 0.1% of the solar constant, the solar XUV radiation is highly variable in time and space. For instance, X-ray bursts associated with solar flares can dramatically increase their intensity over preflare levels by many orders of magnitude over a time span of tens of minutes. In the extreme ultraviolet, the Lyman α line at 121.6 nm represents an important source of ionization and dissociation at ionospheric D layer heights. During quiet periods of solar activity, it alone contains more energy than the rest of the XUV spectrum. Quasi-periodic changes of the order of 100% or greater, with periods of 27 days and 11 years, belong to the prominent variations of solar XUV radiation. However, irregular fluctuations over all time scales are present all the time. During low solar activity, about half of the total energy input into the thermosphere is thought to be solar XUV radiation. Evidently, that solar XUV energy input occurs only during daytime conditions, maximizing at the equator during equinox.
X-ray transient
A gamma-ray burst (GRB) is a highly luminous flash of gamma rays — the most energetic form of electromagnetic radiation. GRB 970228 was a GRB detected on Feb 28 1997 at 02:58 UTC. Prior to this event, GRBs had only been observed at gamma wavelengths. For several years physicists had expected these bursts to be followed by a longer-lived afterglow at longer wavelengths, such as radio waves, x-rays, and even visible light. This was the first burst for which such an afterglow was observed.
X unit
The x unit (symbol xu) is a unit of length approximately equal to 0.1 pm (10 m). It is used to quote the wavelength of X-rays and gamma rays.
X-ray astronomy
Stellar X-ray astronomy is said to have started on April 5, 1974, with the detection of X-rays from Capella. A rocket flight on that date briefly calibrated its attitude control system when a star sensor pointed the payload axis at Capella (α Aur). During this period, X-rays in the range 0.2–1.6 keV were detected by an X-ray reflector system co-aligned with the star sensor. The X-ray luminosity of "L" = 10 erg·s (10 W) is four orders of magnitude above the Sun's X-ray luminosity.
X-ray astronomy
X-ray detectors collect individual X-rays (photons of X-ray electromagnetic radiation) and count the number of photons collected (intensity), the energy (0.12 to 120 keV) of the photons collected, wavelength (~0.008 to 8 nm), or how fast the photons are detected (counts per hour), to tell us about the object that is emitting them.
Crab Pulsar
The Crab Nebula is often used as a calibration source in X-ray astronomy. It is very bright in X-rays and the flux density and spectrum are known to be constant, with the exception of the pulsar itself. The pulsar provides a strong periodic signal that is used to check the timing of the X-ray detectors. In X-ray astronomy, 'crab' and 'millicrab' are sometimes used as units of flux density. A millicrab corresponds to a flux density of about () in the 2–10 keV X-ray band, for a "crab-like" X-ray spectrum, which is roughly a powerlaw in photon energy, "I" = 9.5 "E". 
Very few X-ray sources ever exceed one crab in brightness.
Resonant inelastic X-ray scattering
In RIXS one measures both the "energy and momentum" change of the scattered photon. Comparing the energy of a neutron, electron or photon with a wavelength of the order of the relevant length scale in a solid— as given by the de Broglie equation considering the interatomic lattice spacing is in the order of Ångströms—it derives from the "relativistic energy–momentum relation" that an X-ray photon has less energy than a neutron or electron. The scattering phase space (the range of energies and momenta that can be transferred in a scattering event) of X-rays is therefore without equal. In particular, high-energy X-rays carry a momentum that is comparable to the inverse lattice spacing of typical condensed matter systems so that, unlike Raman scattering experiments with visible or infrared light, RIXS can probe the full dispersion of low energy excitations in solids.
Electromagnetic radiation
The last portion of the EM spectrum to be discovered was associated with radioactivity. Henri Becquerel found that uranium salts caused fogging of an unexposed photographic plate through a covering paper in a manner similar to X-rays, and Marie Curie discovered that only certain elements gave off these rays of energy, soon discovering the intense radiation of radium. The radiation from pitchblende was differentiated into alpha rays (alpha particles) and beta rays (beta particles) by Ernest Rutherford through simple experimentation in 1899, but these proved to be charged particulate types of radiation. However, in 1900 the French scientist Paul Villard discovered a third neutrally charged and especially penetrating type of radiation from radium, and after he described it, Rutherford realized it must be yet a third type of radiation, which in 1903 Rutherford named gamma rays. In 1910 British physicist William Henry Bragg demonstrated that gamma rays are electromagnetic radiation, not particles, and in 1914 Rutherford and Edward Andrade measured their wavelengths, finding that they were similar to X-rays but with shorter wavelengths and higher frequency, although a 'cross-over' between X and gamma rays makes it possible to have X-rays with a higher energy (and hence shorter wavelength) than gamma rays and vice versa. The origin of the ray differentiates them, gamma rays tend to be a natural phenomena originating from the unstable nucleus of an atom and X-rays are electrically generated (and hence man-made) unless they are as a result of bremsstrahlung X-radiation caused by the interaction of fast moving particles (such as beta particles) colliding with certain materials, usually of higher atomic numbers.
European XFEL
Electrons are accelerated to an energy of up to 17.5 GeV by a long linear accelerator with superconducting RF-cavities. The use of superconducting acceleration elements developed at DESY allows up to 27,000 repetitions per second, significantly more than other X-ray lasers in the U.S. and Japan can achieve. The electrons are then introduced into the magnetic fields of special arrays of magnets called undulators, where they follow curved trajectories resulting in the emission of X-rays whose wavelength is in the range of 0.05 to 4.7 nm.
Industrial radiography
Industrial radiography is a method of non-destructive testing where many types of manufactured components can be examined to verify the internal structure and integrity of the specimen. Industrial Radiography can be performed utilizing either X-rays or gamma rays. Both are forms of electromagnetic radiation. The difference between various forms of electromagnetic energy is related to the wavelength. X and gamma rays have the shortest wavelength and this property leads to the ability to penetrate, travel through, and exit various materials such as carbon steel and other metals.
Superficial X-rays
Superficial X-rays are low-energy X-rays that do not penetrate very far before they are absorbed. They are produced by X-ray tubes operating at voltages in the 10–100 kV range, and therefore have peak energies in the 10–100 keV range. Precise naming and definitions of energy ranges may vary, and X-rays at the lower end of this range may also be known as Grenz rays. They are useful in radiation therapy for the treatment of various benign or malignant skin problems, including skin cancer and severe eczema. They have a useful depth of up to 5 mm. In some locations, orthovoltage treatment is being replaced by electron therapy or brachytherapy.
Radio
Radio frequencies occupy the range from a 30 Hz to 300 GHz, although commercially important uses of radio use only a small part of this spectrum. Other types of electromagnetic radiation, with frequencies above the RF range, are infrared, visible light, ultraviolet, X-rays and gamma rays. Since the energy of an individual photon of radio frequency is too low to remove an electron from an atom, radio waves are classified as non-ionizing radiation.
X-ray fluorescence
In order to excite the atoms, a source of radiation is required, with sufficient energy to expel tightly held inner electrons. Conventional X-ray generators are most commonly used, because their output can readily be "tuned" for the application, and because higher power can be deployed relative to other techniques. However, gamma ray sources can be used without the need for an elaborate power supply, allowing an easier use in small portable instruments. When the energy source is a synchrotron or the X-rays are focused by an optic like a polycapillary, the X-ray beam can be very small and very intense. As a result, atomic information on the sub-micrometre scale can be obtained. X-ray generators in the range 20–60 kV are used, which allow excitation of a broad range of atoms. The continuous spectrum consists of "bremsstrahlung" radiation: radiation produced when high-energy electrons passing through the tube are progressively decelerated by the material of the tube anode (the "target"). A typical tube output spectrum is shown in Figure 3.
Scattering amplitude
The scattering length for X-rays is the Thomson scattering length or classical electron radius, .
Phase-contrast X-ray imaging
The first to discover X-rays was Wilhelm Conrad Röntgen in 1895, which is the reason why they are even today sometimes referred to as "Röntgen rays". He found out that the "new kind of rays" had the ability to penetrate materials opaque for visible light, and he thus recorded the first X-ray image, displaying the hand of his wife. He was awarded the first Nobel Prize in Physics in 1901 "in recognition of the extraordinary services he has rendered by the discovery of the remarkable rays subsequently named after him". Since then, X-rays were used as an invaluable tool to non-destructively determine the inner structure of different objects, although the information was for a long time obtained by measuring the transmitted intensity of the waves only, and the phase information was not accessible.
X-ray fluorescence
In wavelength dispersive analysis, the fluorescent X-rays emitted by the material sample are directed into a diffraction grating monochromator. The diffraction grating used is usually a single crystal. By varying the angle of incidence and take-off on the crystal, a single X-ray wavelength can be selected. The wavelength obtained is given by Bragg's law:
Electromagnetic spectrum
Next in frequency comes ultraviolet (UV). The wavelength of UV rays is shorter than the violet end of the visible spectrum but longer than the X-ray.
Astrophysical X-ray source
Ultraluminous X-ray sources (ULXs) are pointlike, nonnuclear X-ray sources with luminosities above the Eddington limit of 3 × 10 W for a black hole. Many ULXs show strong variability and may be black hole binaries. To fall into the class of intermediate-mass black holes (IMBHs), their luminosities, thermal disk emissions, variation timescales, and surrounding emission-line nebulae must suggest this. However, when the emission is beamed or exceeds the Eddington limit, the ULX may be a stellar-mass black hole. The nearby spiral galaxy NGC 1313 has two compact ULXs, X-1 and X-2. For X-1 the X-ray luminosity increases to a maximum of 3 × 10 W, exceeding the Eddington limit, and enters a steep power-law state at high luminosities more indicative of a stellar-mass black hole, whereas X-2 has the opposite behavior and appears to be in the hard X-ray state of an IMBH.
Radiation
Light, or visible light, is a very narrow range of electromagnetic radiation of a wavelength that is visible to the human eye, or 380–750 nm which equates to a frequency range of 790 to 400 THz respectively. More broadly, physicists use the term "light" to mean electromagnetic radiation of all wavelengths, whether visible or not.
High-energy X-rays
High-energy X-rays (HEX-rays) between 100 and 300 keV bear unique advantage over conventional hard X-rays, which lie in the range of 5–20 keV They can be listed as follows:With these advantages, HEX-rays can be applied for a wide range of investigations. An overview, which is far from complete:
Gamma ray
The conventional distinction between X-rays and gamma rays has changed over time. Originally, the electromagnetic radiation emitted by X-ray tubes almost invariably had a longer wavelength than the radiation (gamma rays) emitted by radioactive nuclei. 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. Since the energy of photons is proportional to their frequency and inversely proportional to wavelength, this past distinction between X-rays and gamma rays can also be thought of in terms of its energy, with gamma rays considered to be higher energy electromagnetic radiation than are X-rays.
X-ray astronomy satellite
Wavelength dispersive X-ray spectroscopy (WDS) is a method used to count the number of X-rays of a specific wavelength diffracted by a crystal. WDS only counts X-rays of a single wavelength or wavelength band. In order to interpret the data, the expected elemental wavelength peak locations need to be known. For the ESRO-2B WDS X-ray instruments, calculations of the expected solar spectrum had to be performed and were compared to peaks detected by rocket measurements.
X-ray
X-rays are also produced in lightning accompanying terrestrial gamma-ray flashes. The underlying mechanism is the acceleration of electrons in lightning related electric fields and the subsequent production of photons through Bremsstrahlung. This produces photons with energies of some few keV and several tens of MeV. In laboratory discharges with a gap size of approximately 1 meter length and a peak voltage of 1 MV, X-rays with a characteristic energy of 160 keV are observed. A possible explanation is the encounter of two streamers and the production of high-energy run-away electrons; however, microscopic simulations have shown that the duration of electric field enhancement between two streamers is too short to produce a significantly number of run-away electrons. Recently, it has been proposed that air perturbations in the vicinity of streamers can facilitate the production of run-away electrons and hence of X-rays from discharges.
Industrial radiography
Radiography started in 1895 with the discovery of X-rays (later also called Röntgen rays after the man who first described their properties in detail), a type of electromagnetic radiation. Soon after the discovery of X-rays, radioactivity was discovered. By using radioactive sources such as radium, far higher photon energies could be obtained than those from "normal" X-ray generators. Soon these found various applications, with one of the earliest users being Loughborough College. X-rays and gamma rays were put to use very early, before the dangers of ionizing radiation were discovered. After World War II new isotopes such as caesium-137, iridium-192 and cobalt-60 became available for industrial radiography, and the use of radium and radon decreased.
Radio wave
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies as high as 300 gigahertz (GHz) to as low as 30 hertz (Hz). At 300 GHz, the corresponding wavelength is 1 mm, and at 30 Hz is 10,000 km. Like all other electromagnetic waves, radio waves travel at the speed of light. They are generated by electric charges undergoing acceleration, such as time varying electric currents. Naturally occurring radio waves are emitted by lightning and astronomical objects.
Manne Siegbahn
Manne Siegbahn began his studies of X-ray spectroscopy in 1914. Initially he used the same type of spectrometer as Henry Moseley had done for finding the relationship between the wavelength of some elements and their place at the periodic system. Shortly thereafter he developed improved experimental apparatus which allowed him to make very accurate measurements of the X-ray wavelengths produced by atoms of different elements. Also, he found that several of the spectral lines that Moseley had discovered consisted of more components. By studying these components and improving the spectrometer, Siegbahn got an almost complete understanding of the electron shell. He developed a convention for naming the different spectral lines that are characteristic to elements in X-ray spectroscopy, the Siegbahn notation. Siegbahn's precision measurements drove many developments in quantum theory and atomic physics.
Duane–Hunt law
In an X-ray tube, electrons are accelerated in a vacuum by an electric field and shot into a piece of metal called the "target". X-rays are emitted as the electrons slow down (decelerate) in the metal. The output spectrum consists of a continuous spectrum of X-rays, with additional sharp peaks at certain energies (see graph on right). The continuous spectrum is due to "bremsstrahlung", while the sharp peaks are characteristic X-rays associated with the atoms in the target.
Electromagnetic spectrum
After hard X-rays come gamma rays, which were discovered by Paul Ulrich Villard in 1900. These are the most energetic photons, having no defined lower limit to their wavelength. In astronomy they are valuable for studying high-energy objects or regions, however as with X-rays this can only be done with telescopes outside the Earth's atmosphere. Gamma rays are used experimentally by physicists for their penetrating ability and are produced by a number of radioisotopes. They are used for irradiation of foods and seeds for sterilization, and in medicine they are occasionally used in radiation cancer therapy. More commonly, gamma rays are used for diagnostic imaging in nuclear medicine, an example being PET scans. The wavelength of gamma rays can be measured with high accuracy through the effects of Compton scattering.
Coherent diffraction imaging
Coherent x-ray diffraction imaging (CXDI or CXD) uses x-rays (typically .5-4keV) to form a diffraction pattern which may be more attractive for 3D applications than electron diffraction since x-rays typically have better penetration. For imaging surfaces, the penetration of X-rays may be undesirable, in which case a glancing angle geometry may be used such as GISAXS. A typical x-ray CCD is used to record the diffraction pattern. If the sample is rotated about an axis perpendicular to the beam a 3-Dimensional image may be reconstructed.
Due to radiation damage, resolution is limited (for continuous illumination set-ups) to about 10 nm for frozen-hydrated biological samples but resolutions of as high as 1 to 2 nm should be possible for inorganic materials less sensitive to damage (using modern synchrotron sources). It has been proposed that radiation damage may be avoided by using ultra short pulses of x-rays where the time scale of the destruction mechanism is longer than the pulse duration. This may enable higher energy and therefore higher resolution CXDI of organic materials such as proteins. However, without the loss of information “the linear number of detector pixels fixes the energy spread needed in the beam” which becomes increasingly difficult to control at higher energies.
Astrophysical X-ray source
RX J0648.0-4418 is an X-ray pulsator in the Crab nebula. HD 49798 is a subdwarf star that forms a binary system with RX J0648.0-4418. The subdwarf star is a bright object in the optical and UV bands. The orbital period of the system is accurately known. Recent XMM-Newton observations timed to coincide with the expected eclipse of the X-ray source allowed an accurate determination of the mass of the X-ray source (at least 1.2 solar masses), establishing the X-ray source as a rare, ultra-massive white dwarf.
X-ray microscope
The Advanced Light Source (ALS) in Berkeley, California, is home to XM-1, a full-field soft X-ray microscope operated by the Center for X-ray Optics and dedicated to various applications in modern nanoscience, such as nanomagnetic materials, environmental and materials sciences and biology. XM-1 uses an X-ray lens to focus X-rays on a CCD, in a manner similar to an optical microscope. XM-1 held the world record in spatial resolution with Fresnel zone plates down to 15 nm and is able to combine high spatial resolution with a sub-100ps time resolution to study e.g. ultrafast spin dynamics. In July 2012, a group at DESY claimed a record spatial resolution of 10 nm, by using the hard X-ray scanning microscope at PETRA III.
Far infrared
The infrared radiation (IR) band covers the wavelength range of 700 nm – 1 mm, frequency range of 430 THz – 300 GHz, and photon energy range of 1.24 meV – 1.7 eV. Far-infrared radiation (FIR) is found on the wavelength spectrum at 15–1000 µm with a frequency range of 0.3–20 THz, and photon energy range of 1.2–83 meV. In these IR radiation bands, researchers have noted that the far-infrared radiation band "transfers energy purely in the form of heat which can be perceived by the thermoreceptors in human skin as radiant heat." They report that this radiant heat can penetrate up to 1.5 inches (almost 4 cm) beneath the skin. Biomedical researchers have experimented with the use of FIR emitting ceramics which are embedded into various fibers and woven into the fabric of garments. These researchers noted in subjects a "delay" in the "onset of fatigue induced by muscle contractions." They propose that this ceramic-emitted FIR (cFIR) has the potential to promote cell repair.
Henry Moseley
X-ray spectrometers are the foundation-stones of X-ray crystallography. The X-ray spectrometers as Moseley knew them worked as follows. A glass-bulb electron tube was used, similar to that held by Moseley in the photo at the top of this article. Inside the evacuated tube, electrons were fired at a metallic substance (i.e. a sample of pure element in Moseley's work), causing the ionization of electrons from the inner electron shells of the element. The rebound of electrons into these holes in the inner shells next causes the emission of X-ray photons that were led out of the tube in a semi-beam, through an opening in the external X-ray shielding. These are next diffracted by a standardized salt crystal, with angular results read out as photographic lines by the exposure of an X-ray film fixed at the outside the vacuum tube at a known distance. Application of Bragg's law (after some initial guesswork of the mean distances between atoms in the metallic crystal, based on its density) next allowed the wavelength of the emitted -rays to be calculated.
History of X-ray astronomy
The study of X-ray astronomy continued to be carried out using data from a host of satellites that were active from the 1980s to the early 2000s: the HEAO Program, EXOSAT, Ginga, RXTE, ROSAT, ASCA, as well as BeppoSAX, which detected the first afterglow of a gamma-ray burst (GRB). Data from these satellites continues to aid our further understanding of the nature of these sources and the mechanisms by which the X-rays and gamma rays are emitted. Understanding these mechanisms can in turn shed light on the fundamental physics of our universe. By looking at the sky with X-ray and gamma-ray instruments, we collect important information in our attempt to address questions such as how the universe began and how it evolves, and gain some insight into its eventual fate.
History of X-ray astronomy
Our large area PCA (Proportional Counter Array) on the current RXTE (Rossi X-ray Timing Explorer) mission genuinely reflects the heritage of our sounding rocket program. RXTE continues to provide very valuable data as it enters the second decade of successful operation. 
Goddard's ASM (All-Sky Monitor) pin-hole X-ray camera on Ariel-5 (1974-1980) was the first X-ray astronomy experiment to use imaging proportional counters (albeit one-dimensional); it provided information on transient sources and the long-term behavior of several bright objects. Jean Swank joined the program in time for the beginning of our OSO-8 experiment (1975-1978), the first broadband (2-40 keV) orbiting observatory based on multi-anode multi-layer proportional chambers, one that showed the power of X-ray spectroscopy; for example, it established that iron K-line emission is a ubiquitous feature of clusters of galaxies.
X-ray crystallography
The idea that crystals could be used as a diffraction grating for X-rays arose in 1912 in a conversation between Paul Peter Ewald and Max von Laue in the English Garden in Munich. Ewald had proposed a resonator model of crystals for his thesis, but this model could not be validated using visible light, since the wavelength was much larger than the spacing between the resonators. Von Laue realized that electromagnetic radiation of a shorter wavelength was needed to observe such small spacings, and suggested that X-rays might have a wavelength comparable to the unit-cell spacing in crystals. Von Laue worked with two technicians, Walter Friedrich and his assistant Paul Knipping, to shine a beam of X-rays through a copper sulfate crystal and record its diffraction on a photographic plate. After being developed, the plate showed a large number of well-defined spots arranged in a pattern of intersecting circles around the spot produced by the central beam. Von Laue developed a law that connects the scattering angles and the size and orientation of the unit-cell spacings in the crystal, for which he was awarded the Nobel Prize in Physics in 1914.
Spectral line
For each element, the following table shows the spectral lines which show up in the visible spectrum, from about 400nm-700nm.
Without qualification, "spectral lines" generally implies that one is talking about lines with wavelengths which fall into the range of the visible spectrum. However, there are also many spectral lines which show up at wavelengths outside this range. At the much shorter wavelengths of x-rays, these are known as characteristic X-rays. Other frequencies have atomic spectral lines as well, such as the Lyman series, which falls in the ultraviolet range.
Ethics of technology
Since the introduction of full body X-ray scanners to airports in 2007, many concerns over traveler privacy have arisen. Individuals are asked to step inside a rectangular machine that takes an alternate wavelength image of the person's naked body for the purpose of detecting metal and non-metal objects being carried under the clothes of the traveler. This screening technology comes in two forms, millimeter wave technology (MM-wave technology) or backscatter X-rays (similar to x-rays used by dentists). Full-body scanners were introduced into airports to increase security and improve the quality of screening for objects such as weapons or explosives due to an increase of terrorist attacks involving airplanes occurring in the early 2000s.
X-ray telescope
The Einstein Observatory (1978–1981), also known as HEAO-2, was the first orbiting X-ray observatory with a Wolter Type I telescope (R. Giacconi et al., ApJ 230,540 (1979)). It obtained high-resolution X-ray images in the energy range from 0.1 to 4 keV of stars of all types, supernova remnants, galaxies, and clusters of galaxies. HEAO-1 (1977-1979) and
HEAO-3 (1979-1981) were others in that series. Another large project was ROSAT (active from 1990-1999), which was a heavy X-ray space observatory with focusing X-ray optics.
History of X-ray astronomy
X-ray wavelengths reveal information about the bodies (sources) that emit them.
History of X-ray astronomy
The study of astronomical objects at the highest energies of X-rays and gamma rays began in the early 1960s. Before then, scientists knew only that the Sun was an intense source in these wavebands. Earth's atmosphere absorbs most X-rays and gamma rays, so rocket flights that could lift scientific payloads above Earth's atmosphere were needed. The first rocket flight to successfully detect a cosmic source of X-ray emission was launched in 1962 by a group at American Science and Engineering (AS&E). The team of scientists on this project included Riccardo Giacconi, Herbert Gursky, Frank Paolini, and Bruno Rossi. This rocket flight used a small X-ray detector, which found a very bright source they named Scorpius X-1, because it was the first X-ray source found in the constellation Scorpius.
X-ray astronomy satellite
ISRO launched the multi-wavelength space observatory Astrosat in 2015. One of the unique features of ASTROSAT mission is that it enables the simultaneous multi-wavelength observations of various astronomical objects with a single satellite. ASTROSAT observes universe in the optical, Ultraviolet, low and high energy X-ray regions of the electromagnetic spectrum, whereas most other scientific satellites are capable of observing a narrow range of wavelength band.
Röntgen Memorial Site
On the late Friday evening of 8. November 1895 Röntgen discovered for the first time the rays which penetrate through solid materials and gave them the name "X-rays". He presented this in a lecture and publication "On a new type of rays - Über eine neue Art von Strahlen" on 23 January 1896 at the Physical Medical Society of Würzburg.
Wavelength-dispersive X-ray spectroscopy
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.
X-ray
On November 8, 1895, German physics professor Wilhelm Röntgen stumbled on X-rays while experimenting with Lenard tubes and Crookes tubes and began studying them. He wrote an initial report "On a new kind of ray: A preliminary communication" and on December 28, 1895 submitted it to Würzburg's Physical-Medical Society journal. This was the first paper written on X-rays. Röntgen referred to the radiation as "X", to indicate that it was an unknown type of radiation. The name stuck, although (over Röntgen's great objections) many of his colleagues suggested calling them Röntgen rays. They are still referred to as such in many languages, including German, Hungarian, Danish, Polish, Swedish, Finnish, Estonian, Russian, Japanese, Dutch, Georgian, Hebrew and Norwegian. Röntgen received the first Nobel Prize in Physics for his discovery.
X-ray astronomy
Several types of astrophysical objects emit, fluoresce, or reflect X-rays, from galaxy clusters, through black holes in active galactic nuclei (AGN) to galactic objects such as supernova remnants, stars, and binary stars containing a white dwarf (cataclysmic variable stars and super soft X-ray sources), neutron star or black hole (X-ray binaries). Some solar system bodies emit X-rays, the most notable being the Moon, although most of the X-ray brightness of the Moon arises from reflected solar X-rays. A combination of many unresolved X-ray sources is thought to produce the observed X-ray background. The X-ray continuum can arise from bremsstrahlung, black-body radiation, synchrotron radiation, or what is called inverse Compton scattering of lower-energy photons by relativistic electrons, knock-on collisions of fast protons with atomic electrons, and atomic recombination, with or without additional electron transitions.
X-ray
X-rays have much shorter wavelengths than visible light, which makes it possible to probe structures much smaller than can be seen using a normal microscope. This property is used in X-ray microscopy to acquire high resolution images, and also in X-ray crystallography to determine the positions of atoms in crystals.
Radio spectrum
Radar applications use relatively high power pulse transmitters and sensitive receivers, so radar is operated on bands not used for other purposes. Most radar bands are in the microwave part of the spectrum, although certain important applications for meteorology make use of powerful transmitters in the UHF band.
Radio waves are a type of electromagnetic radiation with wavelengths in the electromagnetic spectrum longer than infrared light. Radio waves have frequencies as high as 300 GHz to as low as 3 kHz, though some definitions describe waves above 1 or 3 GHz as microwaves, or include waves of any lower frequency. At 300 GHz, the corresponding wavelength is 1 mm (0.039 in), and at 3 kHz is 100 km (62 mi). Like all other electromagnetic waves, they travel at the speed of light. Naturally occurring radio waves are generated by lightning, or by astronomical objects.
Radiation
Wilhelm Röntgen discovered and named X-rays. While experimenting with high voltages applied to an evacuated tube on 8 November 1895, he noticed a fluorescence on a nearby plate of coated glass. Within a month, he discovered the main properties of X-rays that we understand to this day.
Gamma-ray astronomy
Observation of gamma rays first became possible in the 1960s. Their observation is much more problematic than that of X-rays or of visible light, because gamma-rays are comparatively rare, even a "bright" source needing an observation time of several minutes before it is even detected, and because gamma rays are difficult to focus, resulting in a very low resolution. The most recent generation of gamma-ray telescopes (2000s) have a resolution of the order of 6 arc minutes in the GeV range (seeing the Crab Nebula as a single "pixel"), compared to 0.5 arc seconds seen in the low energy X-ray (1 keV) range by the Chandra X-ray Observatory (1999), and about 1.5 arc minutes in the high energy X-ray (100 keV) range seen by High-Energy Focusing Telescope (2005).
Standing wave
Interference between X-ray beams can form an X-ray standing wave (XSW) field. Because of the short wavelength of X-rays (less than 1 nanometer), this phenomenon can be exploited for measuring atomic-scale events at material surfaces. The XSW is generated in the region where an X-ray beam interferes with a diffracted beam from a nearly perfect single crystal surface or a reflection from an X-ray mirror. By tuning the crystal geometry or X-ray wavelength, the XSW can be translated in space, causing a shift in the X-ray fluorescence or photoelectron yield from the atoms near the surface. This shift can be analyzed to pinpoint the location of a particular atomic species relative to the underlying crystal structure or mirror surface. The XSW method has been used to clarify the atomic-scale details of dopants in semiconductors, atomic and molecular adsorption on surfaces, and chemical transformations involved in catalysis.
Anomalous X-ray scattering
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.
Bremsstrahlung
The spectrum has a sharp cutoff at formula_72, which is due to the limited energy of the incoming electrons. For example, if an electron in the tube is accelerated through 60 kV, then it will acquire a kinetic energy of 60 keV, and when it strikes the target it can create X-rays with energy of at most 60 keV, by conservation of energy. (This upper limit corresponds to the electron coming to a stop by emitting just one X-ray photon. Usually the electron emits many photons, and each has an energy less than 60 keV.) A photon with energy of at most 60 keV has wavelength of at least 21 pm, so the continuous X-ray spectrum has exactly that cutoff, as seen in the graph. More generally the formula for the low-wavelength cutoff, the Duane-Hunt law, is:
where "h" is Planck's constant, "c" is the speed of light, "V" is the voltage that the electrons are accelerated through, "e" is the elementary charge, and "pm" is picometres.
Soft x-ray microscopy
Unlike visible light, X-rays do not reflect or refract easily, and they are invisible to the human eye. Therefore, the basic process of an X-ray microscope is to expose film or use a charge-coupled device (CCD) detector to detect X-rays that pass through the specimen. It is a contrast imaging technology using the difference in absorption of soft x-ray in the water window region (wavelength region: 2.34 — 4.4 nm, photon energy region: 280 – 530 eV) by the carbon atom (main element composing the living cell) and the oxygen atom (main element for water).
X-ray binary
X-ray binaries are a class of binary stars that are luminous in X-rays. 
The X-rays are produced by matter falling from one component, called the "donor" (usually a relatively normal star), to the other component, called the "accretor", which is very compact: a neutron star or black hole. 
The infalling matter releases gravitational potential energy, up to several tenths of its rest mass, as X-rays. (Hydrogen fusion releases only about 0.7 percent of rest mass.) The lifetime and the mass-transfer rate in an X-ray binary depends on the evolutionary status of the donor star, the mass ratio between the stellar components, and their orbital separation.
Wilhelm Röntgen
In Würzburg, where he discovered the X-rays, a non-profit organization maintains his laboratory and provides guided tours to the Röntgen Memorial Site.World Radiology Day:
The International Day of Radiology is an annual event promoting the role of medical imaging in modern healthcare. It is celebrated on November 8 each year, coincides with the anniversary of the discovery of X-rays by Wilhelm Roentgen in 1895. It was first introduced in 2012, as a joint initiative, by the European Society of Radiology (ESR), the Radiological Society of North America (RSNA), and the American College of Radiology (ACR).
X-ray astronomy
X-ray emission is expected from astronomical objects that contain extremely hot gases at temperatures from about a million kelvin (K) to hundreds of millions of kelvin (MK). Moreover, the maintenance of the E-layer of ionized gas high in the Earth's Thermosphere also suggested a strong extraterrestrial source of X-rays. Although theory predicted that the Sun and the stars would be prominent X-ray sources, there was no way to verify this because Earth's atmosphere blocks most extraterrestrial X-rays. It was not until ways of sending instrument packages to high altitude were developed that these X-ray sources could be studied.
X-ray optics
Since X-rays and visible light are both electromagnetic waves they propagate in space in the same way, but because of the much higher frequency and photon energy of X-rays they interact with matter very differently. Visible light is easily redirected using lenses and mirrors, but because the refractive index of all materials is very close to 1 for X-rays, they instead tend to initially penetrate and eventually get absorbed in most materials without changing direction much.
X-ray vision
Although called X-ray vision, this power has little to do with the actual effect of X-rays. Instead, it is usually presented as the ability to selectively see through certain objects as though they are invisible or translucent in order to see objects or surfaces beyond or deep to the affected object or material. Thus, Superman can see through walls to see the criminals beyond, or see through Lois Lane's dress to determine the color of her underwear (in "Superman: The Movie", Warner Brothers, 1978). In such cases, the visions seen are generally in full color and three-dimensional. How such an effect might be created via x-rays is unexplained (the x-rays from the viewer's eyes would need to bounce back to his eyes the same way as normal light reflects off objects and into the viewer's eyes: x-rays simply pass through an object and continue on their way. X-ray films are made as x-rays pass through an object and then through the x-ray film. The images seen on x-ray film are "shadows" of the objects the x-rays passed through on their way to the film). As depicted, x-ray vision is actually more of a form of the supposed psychic ability of remote viewing.
X: The Man with the X-ray Eyes
Dr. Xavier develops eyedrops intended to increase the range of human vision, allowing one to see beyond the "visible" spectrum into the ultraviolet and x-ray wavelengths and beyond. Believing that testing on animals and volunteers will produce uselessly subjective observations, he tests the drops on himself.
Roentgen (unit)
The roentgen or röntgen () (symbol R) is a legacy unit of measurement for the exposure of X-rays and gamma rays. It is defined as the electric charge freed by such radiation in a specified volume of air divided by the mass of that air. 
In 1928 it was the first international measurement quantity for ionising radiation to be defined for radiation protection, and was an easily replicated method of measuring air ionization directly by using an ion chamber. It is named after the German physicist Wilhelm Röntgen, who discovered X-rays.
X-ray astronomy detector
X-rays span 3 decades in wavelength (~8 nm - 8 pm), frequency (~50 PHz - 50 EHz) and energy (~0.12 - 120 keV). In terms of temperature, 1 eV = 11,604 K. Thus X-rays (0.12 to 120 keV) correspond to 1.39 × 10 to 1.39 × 10 K. From 10 to 0.1 nanometers (nm) (about 0.12 to 12 keV) they are classified as soft X-rays, and from 0.1 nm to 0.01 nm (about 12 to 120 keV) as hard X-rays.
X-ray optics
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.
Ionizing radiation
X-rays normally have a lower energy than gamma rays, and an older convention was to define the boundary as a wavelength of 10 m or a photon energy of 100 keV. That threshold was driven by limitations of older X-ray tubes and low awareness of isomeric transitions. Modern technologies and discoveries have resulted in an overlap between X-ray and gamma energies. In many fields they are functionally identical, differing for terrestrial studies only in origin of the radiation. In astronomy, however, where radiation origin often cannot be reliably determined, the old energy division has been preserved, with X-rays defined as being between about 120 eV and 120 keV, and gamma rays as being of any energy above 100 to 120 keV, regardless of source. Most astronomical "gamma-ray astronomy" are known "not" to originate in nuclear radioactive processes but, rather, result from processes like those that produce astronomical X-rays, except driven by much more energetic electrons.
Non-line-of-sight propagation
By definition, line of sight is the "visual" line of sight, that is determined by the ability of the average human eye to resolve a distant object. Our eyes are sensitive to light but optical wavelengths are very short compared to radio wavelengths. Optical wavelengths range from about 400 nanometer (nm) to 700 nm but radio wavelengths range from approximately 1 millimetre (mm) at 300 GHz to 30 kilometres (km) at 10 kHz. Even the shortest radio wavelength is therefore about 2000 times longer than the longest optical wavelength. For typical communications frequencies up to about 10 GHz the difference is more like 60,000 times so it is not always reliable to compare visual obstructions, such as might suggest a NLOS path, with the same obstructions as they might affect a radio propagation path.
X-ray crystallography
Crystals are regular arrays of atoms, and X-rays can be considered waves of electromagnetic radiation. Atoms scatter X-ray waves, primarily through the atoms' electrons. Just as an ocean wave striking a lighthouse produces secondary circular waves emanating from the lighthouse, so an X-ray striking an electron produces secondary spherical waves emanating from the electron. This phenomenon is known as elastic scattering, and the electron (or lighthouse) is known as the "scatterer". A regular array of scatterers produces a regular array of spherical waves. Although these waves cancel one another out in most directions through destructive interference, they add constructively in a few specific directions, determined by Bragg's law:
X-ray astronomy detector
Although the more energetic X-rays, photons with an energy greater than 30 keV (4,800 aJ), can penetrate the air at least for distances of a few meters, the Earth's atmosphere is thick enough that virtually none are able to penetrate from outer space all the way to the Earth's surface (they would have been detected and medical X-ray machines would not work if this was not the case). X-rays in the 0.5 to 5 keV (80 to 800 aJ) range, where most celestial sources give off the bulk of their energy, can be stopped by a few sheets of paper; 90% of the photons in a beam of 3 keV (480 aJ) X-rays are absorbed by traveling through just 10 cm of air.
X-ray lithography
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-ray crystallography
Wilhelm Röntgen discovered X-rays in 1895, just as the studies of crystal symmetry were being concluded. Physicists were initially uncertain of the nature of X-rays, but soon suspected (correctly) that they were waves of electromagnetic radiation—in other words, another form of light. At that time, the wave model of light—specifically, the Maxwell theory of electromagnetic radiation—was well accepted among scientists, and experiments by Charles Glover Barkla showed that X-rays exhibited phenomena associated with electromagnetic waves, including transverse polarization and spectral lines akin to those observed in the visible wavelengths. Single-slit experiments in the laboratory of Arnold Sommerfeld suggested that X-rays had a wavelength of about 1 angstrom. However, X-rays are composed of photons, and thus are not only waves of electromagnetic radiation but also exhibit particle-like properties. Albert Einstein introduced the photon concept in 1905, but it was not broadly accepted until 1922, when Arthur Compton confirmed it by the scattering of X-rays from electrons. Therefore, these particle-like properties of X-rays, such as their ionization of gases, caused William Henry Bragg to argue in 1907 that X-rays were "not" electromagnetic radiation. Nevertheless, Bragg's view was not broadly accepted and the observation of X-ray diffraction by Max von Laue in 1912 confirmed for most scientists that X-rays were a form of electromagnetic radiation.
Lawrence Bragg
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.
Waves (Juno)
Another issue that came up in 2002, was when Chandra determined with its high angular resolution that X-rays were coming from Jupiter's poles. Einstein Observatory and Germany's ROSAT previously observed X-rays from Jupiter. The new results by Chandra, which took the observations during December 2000, showed X-rays coming from the magnetic north pole not the aurora. Roughly every 45 minutes Jupiter sends out a multi-gigawatt X-ray pulse, and this is synchronized with an emission in radio at 1 to 200 kHz. "Galileo" orbiter and "Ulysses" solar orbiter picked up the radio emissions every 45 minutes. The radio emissions were discovered before the X-rays, they have been detected since the 1950s, and there is even Citizen astronomer project orchestrated by NASA called Radio Jove for anyone to listen to Jupiter's radio signals. Kilometric radio radiation was not detected until the Voyager flybys of Jupiter in the late 1970s. Two candidates for the source of the X-rays are particles of Solar wind or from Io.
X-ray astronomy
Balloon flights can carry instruments to altitudes of up to 40 km above sea level, where they are above as much as 99.997% of the Earth's atmosphere. Unlike a rocket where data are collected during a brief few minutes, balloons are able to stay aloft for much longer. However, even at such altitudes, much of the X-ray spectrum is still absorbed. X-rays with energies less than 35 keV (5,600 aJ) cannot reach balloons. On July 21, 1964, the Crab Nebula supernova remnant was discovered to be a hard X-ray (15 – 60 keV) source by a scintillation counter flown on a balloon launched from Palestine, Texas, USA. This was likely the first balloon-based detection of X-rays from a discrete cosmic X-ray source.
High-energy X-rays
High-energy X-rays or HEX-rays are very hard X-rays, with typical energies of 80–1000 keV (1 MeV), about one order of magnitude higher than conventional X-rays (and well into gamma-ray energies over 120 keV). They are produced at modern synchrotron radiation sources such as the beamline ID15 at the European Synchrotron Radiation Facility (ESRF). The main benefit is the deep penetration into matter which makes them a probe for thick samples in physics and materials science and permits an in-air sample environment and operation. Scattering angles are small and diffraction directed forward allows for simple detector setups.
Astrophysical X-ray source
NASA's Swift Gamma-Ray Burst Mission satellite was monitoring Comet Lulin as it closed to 63 Gm of Earth. For the first time, astronomers can see simultaneous UV and X-ray images of a comet. "The solar wind -- a fast-moving stream of particles from the sun -- interacts with the comet's broader cloud of atoms. This causes the solar wind to light up with X-rays, and that's what Swift's XRT sees", said Stefan Immler, of the Goddard Space Flight Center. This interaction, called charge exchange, results in X-rays from most comets when they pass within about three times Earth's distance from the sun. Because Lulin is so active, its atomic cloud is especially dense. As a result, the X-ray-emitting region extends far sunward of the comet.
X-ray vision
X-rays have many practical uses for scientific and medical imaging. Security agencies are experimenting with applications of imaging devices which can "see" through clothing (using terahertz waves). Such devices are being deployed in some airports as a way of detecting contraband, such as guns, knives, and any other weapons in particular which may be carried beneath a person's clothing, bag, etc. The devices have created some degree of controversy from personal privacy advocates who worry about screeners being able to see people "naked." There also exist certain night-vision equipped video cameras that can be modified to see through clothing at a frequency just below visible light. Such imaging is not true x-ray vision, but rather shows variations in heat radiation rising from the skin beneath the clothing which can provide some detail of the body beneath.
Cargo scanning
X-ray radiography is similar to gamma-ray radiography but instead of using a radioactive source, it uses a high-energy bremsstrahlung spectrum with energy in the 5-10 MeV range created by a linear particle accelerator (LINAC). Such X-ray systems can penetrate up to 30–40 cm of steel in vehicles moving with velocities up to 13 km/h. They provide higher penetration but also cost more to buy and operate. They are more suitable for the detection of special nuclear materials than gamma-ray systems. They also deliver about 1000 times higher dose of radiation to potential stowaways.
Astronomy
X-ray astronomy uses X-ray wavelengths. Typically, X-ray radiation is produced by synchrotron emission (the result of electrons orbiting magnetic field lines), thermal emission from thin gases above 10 (10 million) kelvins, and thermal emission from thick gases above 10 Kelvin. Since X-rays are absorbed by the Earth's atmosphere, all X-ray observations must be performed from high-altitude balloons, rockets, or X-ray astronomy satellites. Notable X-ray sources include X-ray binaries, pulsars, supernova remnants, elliptical galaxies, clusters of galaxies, and active galactic nuclei.
X-ray Raman scattering
X-ray Raman scattering (XRS) is non-resonant inelastic scattering of x-rays from core electrons. 
It is analogous to vibrational Raman scattering, which is a widely used tool in 
optical spectroscopy, with the difference being that the wavelengths of the
exciting photons fall in the x-ray regime and the corresponding excitations
are from deep core electrons.
Radiation
X-rays are electromagnetic waves with a wavelength less than about 10 m (greater than 3x10 Hz and 1,240 eV). A smaller wavelength corresponds to a higher energy according to the equation E=hc/λ. ("E" is Energy; "h" is Planck's constant; "c" is the speed of light; "λ" is wavelength.) When an X-ray photon collides with an atom, the atom may absorb the energy of the photon and boost an electron to a higher orbital level or if the photon is very energetic, it may knock an electron from the atom altogether, causing the atom to ionize. Generally, larger atoms are more likely to absorb an X-ray photon since they have greater energy differences between orbital electrons. Soft tissue in the human body is composed of smaller atoms than the calcium atoms that make up bone, hence there is a contrast in the absorption of X-rays. X-ray machines are specifically designed to take advantage of the absorption difference between bone and soft tissue, allowing physicians to examine structure in the human body.
Frequency
Visible light is an electromagnetic wave, consisting of oscillating electric and magnetic fields traveling through space. The frequency of the wave determines its color: is red light, is violet light, and between these (in the range 4-) are all the other colors of the visible spectrum. An electromagnetic wave can have a frequency less than , but it will be invisible to the human eye; such waves are called infrared (IR) radiation. At even lower frequency, the wave is called a microwave, and at still lower frequencies it is called a radio wave. Likewise, an electromagnetic wave can have a frequency higher than , but it will be invisible to the human eye; such waves are called ultraviolet (UV) radiation. Even higher-frequency waves are called X-rays, and higher still are gamma rays.
Electromagnetic spectrum
The last portion of the electromagnetic spectrum was filled in with the discovery of gamma rays. In 1900 Paul Villard was studying the radioactive emissions of radium when he identified a new type of radiation that he first thought consisted of particles similar to known alpha and beta particles, but with the power of being far more penetrating than either. However, in 1910, British physicist William Henry Bragg demonstrated that gamma rays are electromagnetic radiation, not particles, and in 1914, Ernest Rutherford (who had named them gamma rays in 1903 when he realized that they were fundamentally different from charged alpha and beta particles) and Edward Andrade measured their wavelengths, and found that gamma rays were similar to X-rays, but with shorter wavelengths and higher frequencies.
Chandra X-ray Observatory
The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space observatory launched on STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources 100 times fainter than any previous X-ray telescope, enabled by the high angular resolution of its mirrors. Since the Earth's atmosphere absorbs the vast majority of X-rays, they are not detectable from Earth-based telescopes; therefore space-based telescopes are required to make these observations. Chandra is an Earth satellite in a 64-hour orbit, and its mission is ongoing .
X-ray astronomy
During the solar cycle, as shown in the sequence of images at right, at times the Sun is almost X-ray dark, almost an X-ray variable. Betelgeuse, on the other hand, appears to be always X-ray dark. Hardly any X-rays are emitted by red giants. There is a rather abrupt onset of X-ray emission around spectral type A7-F0, with a large range of luminosities developing across spectral class F. Altair is spectral type A7V and Vega is A0V. Altair's total X-ray luminosity is at least an order of magnitude larger than the X-ray luminosity for Vega. The outer convection zone of early F stars is expected to be very shallow and absent in A-type dwarfs, yet the acoustic flux from the interior reaches a maximum for late A and early F stars provoking investigations of magnetic activity in A-type stars along three principal lines. Chemically peculiar stars of spectral type Bp or Ap are appreciable magnetic radio sources, most Bp/Ap stars remain undetected, and of those reported early on as producing X-rays only few of them can be identified as probably single stars. X-ray observations offer the possibility to detect (X-ray dark) planets as they eclipse part of the corona of their parent star while in transit. "Such methods are particularly promising for low-mass stars as a Jupiter-like planet could eclipse a rather significant coronal area."
Electromagnetic spectrum
In 1895 Wilhelm Röntgen noticed a new type of radiation emitted during an experiment with an evacuated tube subjected to a high voltage. He called these radiations x-rays and found that they were able to travel through parts of the human body but were reflected or stopped by denser matter such as bones. Before long, many uses were found for them in the field of medicine.
X-ray transient
X-ray emission occurs from many celestial objects. These emissions can have a pattern, occur intermittently, or as a transient astronomical event. In X-ray astronomy many sources have been discovered by placing an X-ray detector above the Earth's atmosphere. Often, the first X-ray source discovered in many constellations is an X-ray transient. These objects show changing levels of X-ray emission. NRL astronomer Dr. Joseph Lazio stated: " ... the sky is known to be full of transient objects emitting at X- and gamma-ray wavelengths, ...". There are a growing number of recurrent X-ray transients. In the sense of traveling as a transient, the only stellar X-ray source that does not belong to a constellation is the Sun. As seen from Earth, the Sun moves from west to east along the ecliptic, passing over the course of one year through the twelve constellations of the Zodiac, and Ophiuchus.
X-ray optics
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.
Gamma spectroscopy
Radioactive nuclei (radionuclides) commonly emit gamma rays in the energy range from a few keV to ~10 MeV, corresponding to the typical energy levels in nuclei with reasonably long lifetimes. Such sources typically produce gamma-ray "line spectra" (i.e., many photons emitted at discrete energies), whereas much higher energies (upwards of 1 TeV) may occur in the continuum spectra observed in astrophysics and elementary particle physics. The boundary between gamma rays and X rays is somewhat blurred, as X rays typically refer to the high energy electronic emission of atoms, which may extend to over 100 keV, whereas the lowest energy emissions of nuclei are typically termed gamma rays, even though their energies may be below 20
Arthur Compton
The quantity is known as the Compton wavelength of the electron; it is equal to . The wavelength shift lies between zero (for ) and twice the Compton wavelength of the electron (for ). He found that some X-rays experienced no wavelength shift despite being scattered through large angles; in each of these cases the photon failed to eject an electron. Thus the magnitude of the shift is related not to the Compton wavelength of the electron, but to the Compton wavelength of the entire atom, which can be upwards of 10,000 times smaller.
X-ray transient
X-ray bursters are one class of X-ray binary stars exhibiting periodic and rapid increases in luminosity (typically a factor of 10 or greater) peaked in the X-ray regime of the electromagnetic spectrum. These astrophysical systems are composed of an accreting compact object, typically a neutron star or occasionally a black hole, and a companion 'donor' star; the mass of the donor star is used to categorize the system as either a high mass (above 10 solar masses) or low mass (less than 1 solar mass) X-ray binary, abbreviated as LMXB and HMXB, respectively. X-ray bursters differ observationally from other X-ray transient sources (such as X-ray pulsars and soft X-ray transients), showing a sharp rise time (1 – 10 seconds) followed by spectral softening (a property of cooling black bodies). Individual bursts are characterized by an integrated flux of 10 ergs.
X-ray laser
An X-ray laser is a device that uses stimulated emission to generate or amplify electromagnetic radiation in the near X-ray or extreme ultraviolet region of the spectrum, that is, usually on the order of several of tens of nanometers (nm) wavelength.
X-ray fluorescence
In wavelength dispersive spectrometers (WDX or WDS), the photons are separated by diffraction on a single crystal before being detected. Although wavelength dispersive spectrometers are occasionally used to scan a wide range of wavelengths, producing a spectrum plot as in EDS, they are usually set up to make measurements only at the wavelength of the emission lines of the elements of interest. This is achieved in two different ways:In order to keep the geometry of the tube-sample-detector assembly constant, the sample is normally prepared as a flat disc, typically of diameter 20–50 mm. This is located at a standardized, small distance from the tube window. Because the X-ray intensity follows an inverse-square law, the tolerances for this placement and for the flatness of the surface must be very tight in order to maintain a repeatable X-ray flux. Ways of obtaining sample discs vary: metals may be machined to shape, minerals may be finely ground and pressed into a tablet, and glasses may be cast to the required shape. A further reason for obtaining a flat and representative sample surface is that the secondary X-rays from lighter elements often only emit from the top few micrometres of the sample. In order to further reduce the effect of surface irregularities, the sample is usually spun at 5–20 rpm. It is necessary to ensure that the sample is sufficiently thick to absorb the entire primary beam. For higher-Z materials, a few millimetres thickness is adequate, but for a light-element matrix such as coal, a thickness of 30–40 mm is needed.
UVS (Juno)
UVS, known as the Ultraviolet Spectrograph or Ultraviolet Imaging Spectrometer is the name of an instrument on the "Juno" orbiter for Jupiter. The instrument is an imaging spectrometer that observes the ultraviolet range of light wavelengths, which is shorter wavelengths than visible light but longer than X-rays. Specifically, it is focused on making remote observations of the aurora, detecting the emissions of gases such as hydrogen in the far-ultraviolet. UVS will observes light from as short a wavelength as 70 nm up to 200 nm, which is in the extreme and far ultraviolet range of light. The source of aurora emissions of Jupiter is one of the goals of the instrument. UVS is one of many instruments on "Juno", but it is in particular designed to operate in conjunction with JADE, which observes high-energy particles. With both instruments operating together, both the UV emissions and high-energy particles at the same place and time can be synthesized. This supports the Goal of determining the source of the Jovian magnetic field. There has been a problem understanding the Jovian aurora, ever since Chandra determined X-rays were coming not from, as it was thought Io's orbit but from the polar regions. Every 45 minutes an X-ray hot-spot pulsates, corroborated by a similar previous detection in radio emissions by Galileo and Cassini spacecraft. One theory is that its related to the solar wind. The mystery is not that there are X-rays coming Jupiter, which has been known for decades, as detected by previous X-ray observatories, but rather why with the Chandra observation, that pulse was coming from the north polar region.