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Nearby normal cells, also exposed to the ionizing X-rays, are typically more capable of repair. In a related application, in agricultural industries the irradiation of some foods with X-rays and gamma rays is used to inhibit selectively the growth of bacteria see food preservation: Food irradiation. X-rays are a powerful diagnostic tool for revealing the structure and composition of materials.

The great utility of X-ray images derives from the differential absorption of X-rays by materials of different density, composition, and homogeneity. In a common application, X-rays are used for quick examination of the contents of airline baggage. In industry, X-ray images are used to detect flaws nondestructively in castings that are inaccessible to direct observation.

X-ray microscopes are capable of magnifying X-ray absorption images so as to resolve features on scales as small as about 40 nanometres nm; billionths of a metre , or roughly atomic diameters. This resolution, about five times greater than that achieved by the best visible light microscopes, is possible because of the small diffraction effects associated with the very short wavelengths of X-rays.

Because water is relatively transparent in the soft X-ray region, these microscopes are ideal for studying biological materials in an aqueous environment. The detailed atomic structure of the double-helix polymer deoxyribonucleic acid DNA was famously revealed by James Watson and Francis Crick via the X-ray crystallography studies of Maurice Wilkins. X-ray fluorescence is a complementary method for the quantitative analysis of the composition of materials. In this technique, a sample is exposed to either an electron beam or a beam of primary X-rays; the resulting atomic excitations lead to X-ray emissions with wavelengths characteristic of the elements in the sample.

The electron microprobe uses this process to identify the constituents of sample regions as small as a few micrometres millionths of a metre. X-ray fluorescence and diffraction techniques are valuable methods for the nondestructive analysis of art objects. Brushstroke techniques and the arrangements of painted-over pigments in oil paintings, the presence of coatings and varnishes, and the compositions of glasses, porcelain, and enamels are revealed through X-ray analysis.

Many of the above techniques are enhanced by the exceptionally high X-ray intensities produced in modern synchrotron light facilities. Extremely bright, short X-ray pulses, tuned to selected wavelength regions, are used to probe chemical reactions on surfaces, the electronic structures of semiconductors and magnetic materials, and the structure and function of proteins and biological macromolecules.

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Another promising source of high-intensity X-rays is the X-ray laser. Coherent X-rays a signature of lasing at the longer-wavelength end of the spectral region have been produced in the laboratory. There are three common mechanisms for the production of X-rays: the acceleration of a charged particle, atomic transitions between discrete energy levels, and the radioactive decay of some atomic nuclei. Each mechanism leads to a characteristic spectrum of X-ray radiation.

Fundamental characteristics

In the theory of classical electromagnetism, accelerating electric charges emit electromagnetic waves. In the most common terrestrial source of X-rays, the X-ray tube , a beam of high-energy electrons impinges on a solid target. As the fast-moving electrons in the beam interact with the electrons and nuclei of the target atoms, they are repeatedly deflected and slowed. Most of the energy radiated in an X-ray tube is contained in this continuous spectrum.

Far more powerful and far larger sources of a continuum of X-rays are synchrotron particle accelerators and storage rings. In a synchrotron, charged particles usually electrons or positrons are accelerated to very high energies typically billions of electron volts and then confined to a closed orbit by strong magnets. When the charged particles are deflected by the magnetic fields and hence accelerated via the change in their direction of motion , they emit so-called synchrotron radiation —a continuum whose intensity and frequency distribution are determined by the strength of the magnetic fields and the energy of the circulating particles.

Specially designed synchrotron light sources are used worldwide for X-ray studies of materials. In an X-ray tube, in addition to the continuous spectrum of radiation emitted by the decelerating electrons, there is also a spectrum of discrete X-ray emission lines that is characteristic of the target material.

Most commonly, a collision first causes a tightly bound inner-shell electron to be ejected from the atom; a loosely bound outer-shell electron then falls into the inner shell to fill the vacancy. In the process, a single photon is emitted by the atom with an energy equal to the difference between the inner-shell and outer-shell vacancy states. This energy difference usually corresponds to photon wavelengths in the X-ray region of the spectrum.

Characteristic X-ray radiation can also be produced from a target material when it is exposed to a primary X-ray beam. In this case, the primary X-ray photons initiate the sequence of electron transitions that result in the emission of secondary X-ray photons. In the English physicist Henry Moseley discovered a simple relationship between the wavelengths of the X-ray emission lines from a target and the atomic number of the target element—the wavelengths are inversely proportional to the square of the atomic number.

X-ray fluoresence techniques, in which the wavelengths of characteristic X-rays are recorded following the excitation of a target, are now commonly used to identify the elemental constituents of materials. X-ray emission is sometimes a by-product of a nuclear transformation. In the process of electron capture , an inner-shell atomic electron is captured by the atomic nucleus, initiating the transformation of a nuclear proton into a neutron and lowering the atomic number by one unit see radioactivity: Types of radioactivity.

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The vacant inner-shell orbit is then quickly filled by an outer-shell electron, producing a characteristic X-ray photon. The relaxation of an excited nucleus to a lower-energy state also sometimes results in the emission of an X-ray photon. However, the photons emitted in most nuclear transitions of this type are of even higher energy than X-rays—they fall into the gamma-ray region of the electromagnetic spectrum. Many astronomical sources of X-rays have been discovered over the past 50 years; collectively they are a rich resource of information about the universe see X-ray sources.

Many binary star systems emit copious X-rays; the strongest such sources produce, in the X-ray region alone, more than 1, times the entire energy output of the Sun. Supernova remnants are also strong sources of X-rays, which are sometimes associated with synchrotron radiation produced by high-energy charged particles circulating in intense magnetic fields and sometimes with atomic emissions from extremely hot gases in the range of 10 million kelvins.

Powerful extragalactic sources of X-rays, including active galaxies, quasars, and galactic clusters, are currently under intense scientific scrutiny; in some cases the exact mechanisms of X-ray production are still uncertain or unknown. The launch of the Chandra X-Ray Observatory in greatly advanced the observational capabilities of X-ray astronomy see telescope: X-ray telescopes. The process of exposure is initiated by X-ray photons ionizing radiation-sensitive silver halide crystals in an emulsion on the film surface; the resulting photochemical change of the affected crystals darkens the exposed area see radiation measurement: Photographic emulsions.

A number of more effective detection methods have been developed. An applied high voltage induces further ionizations from collisions between liberated electrons and neutral atoms, creating an avalanche of charged particles and a large electrical pulse that is easily detected. More sophisticated detection schemes based on the ionization of gas atoms can discriminate between X-rays of different energies see radiation measurement: Proportional counters. Other common detection schemes rely on the ability of X-rays to produce visible fluorescence in crystals see scintillation counter and charge separation in semiconductors see radiation measurement: Semiconductor detectors.

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