X-ray

X-ray radiation refers to electromagnetic waves with photon energies between 100 eV and several MeV, corresponding to wavelengths between 10-8 m (10 nanometers) and about 10-12 m (1 picometer ). X-rays lie in the electromagnetic spectrum between the ultraviolet light and gamma radiation with which they overlap partially. The X-ray radiation was discovered X-ray on November 8, 1895 by Wilhelm Conrad and bears her name in German- and almost the entire Central and Eastern Europe in his honor. In other language areas it is often the term X - rays originally used for their own X-ray called (English X -rays ). X-rays are ionizing radiation.

Alignment of the electromagnetic spectrum

The spectrum of the X-rays begins below the extreme ultraviolet radiation at a wavelength of 50 nm ( soft X-rays ) and extends to less than 1 pm down (hard or high energy X-rays ). The energy ranges of gamma and X-rays overlap in a wide range. Both types of radiation are electromagnetic radiation and therefore at the same energy equivalent. The distinguishing criterion is the origin: X- rays are produced in contrast to gamma radiation is not in processes in the nucleus, but by high-energy electron processes. In the X-ray tube (see below) the radiation spectrum generated is a superposition of a continuous a discrete spectrum. The position of the intensity maximum is dependent on the operating voltage of the tube. The minimum wavelength can be calculated with the Duane -Hunt law. Photons from x-ray tubes have an energy of about 1 keV to 250 keV, corresponding to a frequency of about 2.5 x 1017 Hz to 6.1019 Hz in the short wavelength region is no uniform definition of the cutoff wavelength exists. However, the generation of ever shorter wavelength X-rays are technical limits.

Generation

X-rays may be caused by two different processes:

  • By rapid acceleration of charged particles (usually braking or deflection of electrons ) - this is the bremsstrahlung, the spectrum is continuous,
  • And by high-energy transitions in the electron shells of atoms or molecules. Which is the characteristic X-rays. Always has a line spectrum.

Both effects are used in the X-ray tube in which electrons initially of a filament (cathode) from accelerated (where they put no X-ray radiation -free, because the acceleration is not large enough ) and then hit the anode, where they are strongly braked. This X-ray radiation is generated (braking radiation, with a total of around 1% of the incident energy ) and heat ( around 99 %). Also, are knocked out of the shells of the metal atoms by electron collisions electrons. The holes in the shells are filled up with other electrons, characteristic X-ray radiation is generated.

The anodes are made today usually made ​​of ceramics, wherein the locations to which the electrons impinge, are made of molybdenum, copper or tungsten.

Another possibility to generate x-ray radiation, particle accelerators are, in particular during acceleration of electrons. Here arises is when the particle beam deflected in a strong magnetic field, thereby accelerating transverse to its direction of propagation, synchrotron radiation, a form of bremsstrahlung. Up to a maximum energy of synchrotron radiation includes the entire electromagnetic spectrum. With suitably chosen parameters ( strength of the magnetic field and particle energy ) while also X-ray radiation is represented.

Interaction with matter

The refractive index of matter for X-rays deviates only slightly from 1. This has the consequence that a single X-ray lens only focuses weak or out of focus and you need for a stronger effect lens stack. Furthermore, hardly reflected X-rays at normal incidence. Nevertheless, it has been found in the X-ray optics ways to develop optical components for X-rays.

X-rays can penetrate matter. She will be greatly weakened differently depending on the type of fabric. The attenuation of X- rays is the most important factor in radiological imaging. The intensity of the X-ray beam, following the Lambert -Beer law with the distance in the material path length exponentially (), the coefficient is dependent on the material and approximately proportional to (: order number: wavelength).

The absorption takes place by photoabsorption, Compton scattering and, at high photon energies, pair production.

  • In the photoabsorption the photon knocks an electron from the electron shell of an atom. Therefore, depending on the electron shell a certain minimum energy is required. The likelihood of this process as a function of photon energy increases abruptly when reaching the minimum energy at a high value ( absorption edge ) and decreases at higher photon energies then continuously until the next absorption edge. The "hole " in the electron shell is filled by an electron from a higher shell again. This creates low-energy fluorescence radiation.
  • Apart from strongly bound electrons as in the photoabsorption can an X-ray photon are also scattered to unbound or weakly bound electrons. This process is called Compton scattering. The photon scattering experienced by the one dependent on the scattering angle extension of the wavelength by a fixed amount and thus a loss of energy. In relation to the photoabsorption Compton scattering occurs only at high photon energies and especially for light atoms in the foreground.

Photo absorption and Compton scattering are inelastic processes in which the photon loses energy, and is finally absorbed. In addition, elastic scattering (Thomson scattering, Rayleigh scattering ) is also possible. Here, the scattered photon remains coherent with the incident and retains its energy.

  • At energies above 2 mec2 ≈ 1.03 MeV also occurs on electron-positron pair production. She is from about 5 MeV the dominant absorption process.

Biological effect

X-rays are ionizing. It can therefore cause changes in living organisms and cause damage and even cancer. It must therefore be observed when working with the radiation of radiation protection. The disregard of this fact led, for example, in military personnel up to the 1980s to insufficiently shielded radars did in the 1950 service to health caused by the by-produced X-rays of the devices.

The delicate structure for the development of cancer is the genetic material ( DNA). In this case, it is assumed that a linear increase of the damage to the dosage, that is, even a very small radiation dose holds a non-zero risk of causing cancer. This risk shall be weighed against the benefits of medical diagnosis or therapy with X-rays.

Proof

  • Luminescent effect. X-rays stimulate certain substances to emit light on ( " fluorescence" ). This effect is also used in the radiological imaging. Medical X-ray films usually contain a fluorescent film, which emits light when struck by a X-ray photon light and the surrounding light-sensitive photographic emulsion exposed.
  • A photographic effect. X-rays as well as light blacken photographic films directly. Without a fluorescent film is about a 10 - to 20 -fold higher required intensity. The advantage lies in the greater sharpness of the captured image.
  • Individual X-ray photons are detected by scintillation counters or Geiger counters.
  • In semiconductor diodes ( semiconductor detectors ) the X-ray photons generate electron -hole pairs within the semiconductor, which are separated in the space charge zone. Thus, a small current is caused, whose strength is proportional to the energy and intensity of incident X- radiation.

Visibility to the human eye

Contrary to widespread belief to the contrary, the human eye X-ray perceive partially. Shortly after Roentgen's discovery in 1895 reported fire from a weak, blue - gray bill, which was apparently in the eye itself, when he was in a darkened room next to an X-ray tube. Then X-ray noted that he also had this effect observed. At first he had thought it for imagination, because the effect was produced only from the strongest X-ray tube and he therefore had seen him only once.

The knowledge that X-rays can be seen with the naked adapted to the dark eye, is now largely forgotten. The reason is probably that the attempt is now regarded as unnecessarily dangerous and harmful. The exact mechanism of perception is not released. Possible is the normal way via the excitation of the retina, a direct excitation of the optic nerve or, for example, that the X-rays cause phosphorescence in the eyeball, and then a "normal" light is perceived.

Julius Edgar Lilienfeld in 1919 described the first time visible to the human eye gray - white radiation at the anode of X-ray tubes, which are named after him, " Lilienfeld radiation". Their origin could only be explained in later years as a form of transition radiation.

Applications

With X-rays of the human body can be illuminated, with particular bones, but with modern appliances are also visible internal organs (see also X ). This makes use of that which occurs in the bone element calcium with Z = 20 a much higher atomic number than the elements from which the soft tissues are mainly composed, namely hydrogen (Z = 1 ), carbon ( Z = 6 ), nitrogen ( Z = 7) and of oxygen (Z = 8). In addition to conventional devices, which produce a two-dimensional projection, and computed tomography are used which allow the spatial reconstruction of the interior of the body. Can with X-rays but also fight cancer by, as part of a radiation damage the cancer cells A., are radiation sensitive than the surrounding tissue by selective irradiation.

Until the development of the first antifungal and skin fungal infections were treated by X-rays ( see also ringworm affair).

In material physics, chemistry, biochemistry, crystallography and in other sciences diffraction of X-rays used for structure determination in the broadest sense, for example, to study the texture or the actual crystal structure analysis. A well-known example is the structural elucidation of the DNA. By X-ray photoelectron spectroscopy ( XPS ), the elemental composition of a sample to be investigated. In addition, XPS has the ability to examine chemical bonds.

In addition, it can be determined with X-ray radiation, the elemental composition of a material. In an electron micro- probe (or equivalent in an electron microscope ), the substance to be analyzed is irradiated with electrons, and then the atoms are ionized and emit characteristic x rays. Instead of electrons can also be irradiated with X-rays. This is known as the X-ray fluorescence analysis ( XRF).

Natural X-ray radiation

On Earth, X-rays produced in low intensity during the absorption of other types of radiation that come from radioactive decay and cosmic radiation. X-ray radiation, which is created on the other celestial bodies, reaches the earth's surface, not because it is shielded by the atmosphere. They will be examined with X-ray satellites such as Chandra and XMM- Newton.

Note: X-ray radiation in a narrow sense means radiation that the same is artificially generated at a target by accelerating electrons and striking. In another way, resulting electromagnetic waves in the frequency range of the X-rays are usually rather referred to as gamma rays.

Discovery history

The first observation of X-rays by Wilhelm Conrad Röntgen was performed at the Physics Institute of the Julius -Maximilians -Universität Würzburg late on Friday evening of November 8, 1895, when - as he described it himself - " no more helping hands were in the home." Just seven weeks later, on December 28, 1895, he submitted a work for publication under the title A: About a new kind of rays.

X-ray is considered the discoverer of today rays named after him, although it is clear that others have generated X-rays before him. In developed by Johann Hittorf and William Crookes cathode ray tubes, which also used X-ray for his experiments, but generates X- rays, which has been demonstrated in experiments by Crookes and from 1892 by Heinrich Hertz and his student Philipp Lenard by blackening of photographic plates, without apparently about the importance of the discovery to be aware of. Nikola Tesla also experimented from 1887 with cathode ray tubes and X-ray radiation produced here, but did not publish his results. It is also claimed that the X - rays were discovered by an Austrian scientist of Ukrainian origin Johann Puluj already in 1881.

Because the scientists mentioned not announced their skills, including those X knew nothing about it. He has the X-rays independently discovered when he observed fluorescence- able objects close to the tube during operation of the cathode ray tube ( black cardboard) began to glow brightly, despite a cover of the tube. Roentgen's merit is to have the importance of the newly discovered rays detected early and the first to study this scientifically. To Roentgen's celebrity certainly has the X-ray image of a hand of his wife helped that he abbildete in its first release for X-rays. This fame brought him in 1901 the first Nobel Prize in Physics, the Nobel Prize Committee highlighted the practical importance of the discovery. X-ray called his discovery of X - rays.

The designation of X-rays is due to the anatomist Albert von Kölliker, who suggested that on 23 January 1896. The occasion was the first public lecture about his Roentgen's discovery, which took place at the invitation of the Physico- Medical Society at Wuerzburg, whose founder was Kölliker. In some language areas it remained established by X-ray self- named X - rays ( for example, English X -rays ).

Related Topics

  • The discovered shortly after the X-rays N- rays proved to be scientific aberration.
  • The Z- machine in New Mexico is currently the strongest X-ray source in the world.
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