X-ray (English. X-ray) are electromagnetic waves with a photon energy, which is higher than from ultraviolet light.

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classification in the electromagnetic spectrum

the energy regions to that gamma - and X-ray overlap within a wide range. Both kinds of radiation are electromagnetic radiation and with same energytherefore equivalent. The distinction criterion is the origin: X-ray result contrary to the gamma-rays not with processes in the atomic nucleus separate from high-energy electron processes. Roentgen photons have an energy of approximately 100 eV to 250 keV. That corresponds to a frequencyof approximately 3·10 16 cycles per second to 6·10 19 cycles per second and a wavelength of approximately 5 pm - 10 Nm (there is no uniform definition of the lower cutoff-wavelength), whereby yield X-ray the smallest energy and lowest frequency and the largest wavelengthhave, hard X-ray accordingly the largest energy, highest frequency and the smallest wavelength.


schematic design of an x-ray tube (s.h. Putting one)
Feynman-Graph der Bremsstrahlungserzeugung (Zeit von links nach rechts)
Feynman graph of the bremsstrahlung production (time from left to right)

X-ray to develop by strong acceleration of charged particles (mostlyElectrons) or by high-energy transitions in the electron sheaths of atoms or molecules. Both effects are used in the x-ray tube, in which electrons are accelerated first by a Glühwendel (cathode) out (to set it no x-ray freely, because accelerationenough is not large) and afterwards on the anode meet, in which they are strongly braked (here x-ray develops:Bremsstrahlung) and electrons from the bowls of the metal atoms drive out. The holes in the bowls are filled up by other electrons, howX-ray with a element-specific energy develops (characteristic x-ray). Anodes are nowadays usually of ceramic(s), whereby the places, on which the electrons hit, consist of molybdenum. With the process of accelerating and the deceleration develops only to 1%X-ray and to 99% warmth.

A further possibility of producing X-ray are particle accelerators. In them, if the particle beam in a strong magnetic field is diverted and accelerated thus transverse to its direction of propagation, synchrotron radiation develops. Up to a maximum energythe synchrotron radiation contains the entire electromagnetic spectrum, with suitably selected parameters (strength of the magnetic field and particle energy) is represented thereby also of x-ray.

reciprocal effect with subject

the refractive index of subject for X-ray deviates only little from 1. Thishas the consequence that there is no material, from which one lenses for X-ray manufacture can. The moreover X-ray are hardly reflected with perpendicular idea. Nevertheless one found in the Roentgen optics ways to develop optical elements for X-ray.

X-raycan subject penetrate. They are differently strongly weakened thereby depending upon kind of fabrics. The attenuation of the X-ray is the most important factor during the radiological imaging. The intensity of the X-ray takes D with the distance put back in the material exponentially off (I =I 0 e - kd), the coefficient k is about proportional to Z 3 λ 3 (Z… Ordinal number, λ… Wavelength).

The absorption results from the photo absorption and the Compton dispersion:

  • During the photo absorption the photon strikesan electron from the electron sheath of an atom. But a certain minimum energy is necessary. If one regards the probability of absorption as a function of the photon energy, she rises with reaching the minimum energy abruptly to a maximum value. To higher photon energies the probability takes thenagain continuously off. Because of this dependence one speaks also of an absorption edge. The hole in the electron sheath is filled up again by other electrons. Low-energy fluorescence radiation develops.
  • Except at strongly bound electrons as with the photo absorption can Roentgen photon also at free or weakly bound electrons to be strewn. One calls this process Compton dispersion. The photons experience an extension of the wavelength by a firm amount and thus a loss of energy, dependent on the scattering angle, by the dispersion. In the relationshipfor photo absorption the Compton dispersion steps only with high photon energy and particularly with light atoms into the foreground.

During the photo absorption and the Compton dispersion it concerns inelastic processes, with which the photon loses finally energy and one absorbs.Besides also flexible dispersion ( Rayleigh scattering) is possible. The strewn photon remains coherent to the incident and keeps its energy.

Additionally to the processes mentioned also in principle the generation of pairs is possible for photons. However energies are beyond ofapprox. 1 MeV necessarily, those not into the range for Roentgen photons, indicated above, (< 250 keV) falls.

biological effect

x-ray is ionizing, it can thereby changes in the living organism up to cancer cause. This usuallyunwanted effects justify the necessity for the radiation protection. The sensitive structure for the emergence of cancer is the hereditary substance (DNA), from a linear rise of the damage with the dose one proceeds. This means that also a very small dosea risk saves to induce cancer although evenly a very small risk.


  • luminescence effect. X-ray move certain materials on to the light delivery (“fluorescence”). This effect is used also during the radiological imaging. Medical radiographic films mostly containa fluorescent foil, which sends light with impact of a Roentgen photon and which surrounding photo-sensitive photoemulsion exposes.
  • Photographic effect. X-ray can blacken photographic films directly just like light. Without a fluorescent foil however an about 10-twentyfoldly higher intensity is needed.The advantage is in the larger sharpness of the taken up picture.
  • Individual Roentgen photons are proven in the Geiger-Mueller counter by the ionization of a counting gas.
  • In semiconductor radiation detectors produce the Roentgen photons of electron pairs of holes in the intrinsischen zone of a diode operated in check direction. Thus becomesa small river caused, whose strength is proportional to the energy and intensity of the incident x-ray.


radiograph of a male hip

with X-ray the human body, whereby above all bone, but with modern devices can be analyzed alsointernal organs become visible (see also Roentgen). The fact is used that the element calcium with Z=20 a clearly higher ordinal number, occurring in the bones, has than the elements, of which mainly consists the soft fabric, i.e. hydrogen (Z=1), carbon (Z=6), nitrogen (Z=7) and oxygen (Z=8). Beside conventional devices, which produce a two-dimensional projection, also Computertomographen are used, which make a spatial reconstruction possible of the inner body.

In material physics, chemistry and biochemistry dispersion of X-ray becomesfor the structure clearing-up uses. A well-known example is the structure clearing-up of the DNA.

Beyond that also the element composition of a material can be determined with X-ray. In an electron beam micro probe (and/or equivalent in the electron microscope) the substance which can be analyzed is illuminated with electrons,on which the atoms to be ionized and characteristic x-ray delivered. Instead of with electrons can be illuminated also with X-ray. Then one speaks of the x-ray analysis (RFA).

natural x-ray

on earth X-ray in small intensity in the course developthe absorption of other kinds of radiation, which come from radioactive decay and the cosmic radiation. X-ray, which develop on other heavenly bodies, do not reach the earth's surface, because they become shielded by the atmosphere. They are examined with Roentgen satellites such as Chandra and XMM Newton.

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Discovery history

to 28. December 1895 published William Conrad Roentgen a work over x-ray with the title over a new kind of jets. Roentgen is considered as its discoverer, although it stands firmly that already different produced X-ray before him.In the cathode ray tubes, which used also Roentgen for its experiments, developed by Johann hit peat and William Crookes, result to x-ray, those in experiments of Crookes and starting from 1892 of Heinrich Hertz and its pupil Philipp Lenard from density of photographicPlates one proved, without realizing itself however obvious the meaning of the discovery. Also Nikola Tesla experimented starting from 1887 with cathode ray tubes and produced thereby X-ray, published its results however not.

There the scientists mentioned their knowledge not, also Roentgen anything did not know publications of it. He discovered the X-ray independently, when he observed fluorescent light with the enterprise of the cathode ray tube. To X-raying celebrity surely also the radiograph of a hand of its wife contributed, who it in its first publicationfor x-ray illustrated. This celebrity registered 1901 the first Nobelpreis for physics to him , whereby the Nobelpreiskomitee emphasized the practical meaning of the discovery. 1896 were introduced for the first time the today's name. In Germany the designation X-ray was in-patriated, during inmost linguistic areas (for example English. x-rays) the old name remained.


  • CH. R. Friedrich: 100 years X-ray. First Nobelpreis for physics. Material science and materials engineering 26 (11-12), S. 598 - 607 (1995), ISSN 0933-5137
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See also

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