About X-Ray

X-rays, or Roentgen rays, are electromagnetic waves; they are the same as visible light, except that they have shorter wavelengths (higher photon energies). Thus x-rays, visible light, ultraviolet, infrared, microwaves, and radio waves are all electromagnetic radiation in different wavelength (energy) spectral regions. X-rays are generated when fast-moving electrons slow down and stop in matter, when an innershell vacancy in an atom is filled by another electron, and when electrons moving at relativistic speeds (speeds near the speed of light) change their direction of motion in space.

Roentgen's findings.

X-rays were discovered by W.C. Roentgen in 1895. This discovery came about by accident. Roentgen was studying gas discharges in his laboratory when he noticed that unknown radiation from a gas discharge could induce fluorescence in certain materials. In his first communication, Roentgen described the properties ofthese rays as follows: They were invisible; moved in straight lines; were unaffected by electric or magnetic fields, and hence not electrically charged; passed through matter opaque to ordinary light (since they penetrated through the black cardboard around his cathode-ray tube); were differentially absorbed by matter of different densities or of different atomic weights; affected photographic plates; produced fluorescence in certain chemicals, such as in the barium platinocyanide screen with which the initial discovery was made and in the wall of his glass tube opposite the cathode; produced ionization in gases; and were evidently produced at the anode by the beam of rays (identified by J. J. Thomson in 1897 as electrons) issuing from the cathode in his vacuum tube.
Along with all these definitive characteristics of the rays, however, other crucial experiments designed to establish similarity or differences from visible light were clearly called for. The fundamental optical properties of visible light were well established in 1895: reflection from mirrors; refraction in prisms (change in direction in passing from air into glass, for example), by means of which a beam of white light could be spread out into a rainbow or spectrum of colors; diffraction by narrow slits or ruled gratings, also a method of producing spectra; and polarization, or constraint of the electric field of the light wave to a single direction. In spite of the best efforts
of Roentgen, no evidence of any of these four optical phenomena could be found. Hence the designation "x"—unknown—was assigned by Roentgen. Many theories were proposed to account for the apparently unique quality of x-rays, which seemed to be so closely similar and yet so greatly different from visible light.

Later discoveries.

Other scientists studying x-rays found the essential experimental conditions to prove that x-rays can be polarized (C. Barkla, 1905, by scattering from carbon); diffracted by crystals (M. von Laue, W Friedrich, and P. Knipping, 1912); refracted in prisms and in crystals; reflected by mirrors; and diffracted by ruled gratings (A. Compton, 1921-1922). Instead of being refracted in passing from a less dense medium (air) to a more dense medium (a glass prism or a crystal) in the same direction as light (the index of refraction for visible light is always greater than 1), x-rays are deviated in the opposite direction by a very small amount: the index of refraction is less than 1 by an amount as small as 10-6. Total reflection from mirrors is observed only when the beam impinges at a very small angle (grazing angle), a necessary condition understandably missed by Roentgen. Similarly, the beam must be incident at a very small angle on a ruled diffraction grating if diffraction (a spectrum, or spatial dispersion by wavelength) is to be observed.
From 1895 to 1912 there seemed to be no analyzer capable of dispersing an x-ray beam into a spectrum. The spectacular Laue diffraction pattern of a zinc sulfide crystal in 1912 proved the electromagnetic wave nature of x-rays and the ordered structure of crystals, with atoms lying on families of planes to constitute three-dimensional diffraction gratings, all governed by the simple Bragg law nk = 2d sin 6 (which must be corrected for refraction in extremely accurate work). Here n is an integer indicating the order of the spectrum, k the wavelength, d the crystal lattice spacing of one set of planes, and 6 the angle between the incident ray and this set of planes.
The wavelength range of x-rays in the electromagnetic spectrum, as excited in x-ray tubes by the bombardment of a target by electrons under a high accelerating potential, overlaps the ultraviolet range on the order of 100 nanometers on the long-wavelength side, and the shortest-wavelength limit moves downward as voltages increase. An accelerating potential of 109 V, now readily generated, produces a k of 10-6 nm. An average wavelength used in research is 0.1 nm, or about 1/6000 the wavelength of yellow light.

Quantum mechanics.

X-rays (and visible light) can be considered as an electromagnetic wave. X-rays can also be considered as discontinuous bundles of energy, or quanta, in accordance with the laws first enunciated by M. Planck and extended by A. Einstein early in the twentieth century. In diffraction, refraction, polarization, and interference phenomena, x-rays, together with all other electromagnetic radiation, appear to act as waves and k has a real significance. There is duality, meaning that light and x-rays have both wave and particle properties, although these are generally not observed at the same time in a given experiment. Beams of electrons and neutrons also have wave properties, and are diffracted in appropriate media. In other phenomena—such as the appearance of sharp spectral lines, a definite short-wavelength limit k0 of the continuous "white" spectrum [defined by k0 = hc/eV, where h is Planck's constant, c the velocity of electromagnetic radiation (including light and x-rays), e the charge of electron, and V the accelerating voltage], the shift in wavelength of x-rays scattered by electrons in atoms (Compton effect), and the photoelectric effect— the energy is propagated and transferred in quanta (called photons) defined by values of hv, where the frequency v is c/x.


X-rays are a valuable probe of matter, as they interact selectively with electrons; electrons in matter account for most of the significant properties of matter (excepting nuclear properties). Medical uses abound, as x-rays can penetrate matter, and are selectively absorbed by atoms containing many electrons—elements with a high atomic number. Thus dental x-rays show fillings as dark, teeth and bone in grey, and are only lightly absorbed in the cheek. X-rays in high intensities are also used for therapy, to treat certain cancers. X-rays are also used in many industrial processes, as in checking the integrity of welds. Tomographic techniques allow three-dimensional images to be obtained, which are invaluable in medical diagnosis.
X-rays are used in research for the study of both the electronic structure of matter and its spatial structure. X-ray microscopes and microprobes have been developed which allow imaging with high spatial resolution while obtaining contrast with different elemental discrimination, distinguishing chemical bonds, and, by use of circularly polarized x-rays, the orientation of magnetization. X-rays are used in protein crystallography to determine the spatial structure of proteins, viruses, and other objects that can be made into crystals but that otherwise cannot be seen.


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