X-ray tube

An X-ray tube is a special electron tube for generating X- rays. It consists in its simplest form consists of a cathode and an anode, which was formerly referred to as anti-cathode, which were melted in a vacuum inside a sealed glass body. The naming took place after the German physicist Wilhelm Conrad Röntgen.

  • 6.1 Fixed or standing anode
  • 6.2 rotating anode

Function

From the cathode, electrons are emitted ( emitted ) by a high- voltage accelerated ( 25-150 kV) to the anode and penetrating into the anode material. They are decelerated and produce three different types of radiation: the so-called characteristic X-rays, bremsstrahlung and Lilienfeld radiation (a form of transition radiation ).

By high-performance tubes, as used in computer tomography (CT) and angiography, the vacuum container is made ​​of metal, which can withstand substantially greater thermal influence. Over time, technical improvements have been made also in X-ray tubes, however, at the very principle of the generation of X-rays do not change.

Discrete and characteristic X-rays

While visible light sources only the outer shell electrons of the atoms are involved, beat the accelerated in the X-ray tube high-energy electrons in the anode and electrons out of the inner shells of the atoms of the anode material. In these gaps "jump" either electrons from higher energy levels or free electrons. Since the binding energy of the innermost electron levels is very large and causes no visible light, but the characteristic X-ray radiation with materials typical discrete quantum energies or wavelengths. This energy corresponds to the difference between the binding energy of, for example, the K-shell and the energy-rich N- shell. Of course all sorts of other discrete quantum energies are possible, so for example, between K and L shell, between M- and K-shell, M - and L-shell or, as mentioned, also of "free" electrons to the K or L shell.

This discrete and characteristic X-rays with the respective quantum energies and thus wavelengths is used, however, with the exception of mammography and the crystal analysis, or only to a small extent for imaging with fluoroscopy. In mammography, an anode plate made ​​of molybdenum is used with appropriate filters so that in this case the K radiation of molybdenum for receiving the mammary gland is used. Also, the crystal structure analysis of discrete wavelengths are needed. Only the X-ray bremsstrahlung is these exceptions for imaging in medicine and material testing used.

It should be noted that the electrons in the inner shells to be ejected not only by external impacts, such as in the X-ray tube, but also through the process of internal conversion from the atom.

X-ray bremsstrahlung

The bremsstrahlung caused by the deceleration of electrons as it passes through the metal of the anode means any accelerating electric charge generated electromagnetic radiation. The wavelength of the radiation depends on the value of the acceleration ( or deceleration ) so that at a higher acceleration voltage or anode voltage harder X-ray radiation ( higher-energy quanta) is formed. The bremsstrahlung spectrum has a minimum wavelength at which the total kinetic energy of the electron is given to a single photon. This lower cut-off wavelength is dependent only on the anode voltage, and not from the anode material.

Lilienfeld radiation

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". Its origin could be explained only in later years.

Cathode types

Cathode can be characterized by the type of electron generation.

Thermal emission

The cathode consists of a filament ( filament), which usually consists of a tungsten wire. These thermionic cathode is heated by passage of current at about 2000 ° C so that thermal emission of electrons from the metal occurs. The electrons form a negatively charged electron cloud which opposes the exit of further electrons. Only on the application of a positive voltage to the anode, the electrons are accelerated to it. If the tube only from the cathode and anode, it is called a diode. The anode current is determined by the field and from a saturation value of the heating current of the filament. With the addition of so-called Wehnelt front of the cathode, the anode current can be independently regulate it. The Wehnelt cylinder and acts as a control gate is negative relative to the cathode. He thus counteracting the acceleration field of the anode. In this case one speaks of a triode.

Field emission

The filament is heated only to moderate temperatures depending on the material. By heating alone, nor standeth in no emission. However, many electrons are thus on a higher energy level above the Fermi level. If you create a so-called extraction grid over the filament, which is positive with respect to this, it produces very high field strengths of several volts per micrometer in the space between the two. This causes electrons to be removed from the filament. The potential of the so-called vacuum levels - the potential of which has to reach an electron, to be truly free of the original solids - is reduced by the strong external field with distance from the surface of the metal / filament. The electrons can now this potential to the vacuum level to tunnel out and leave the solid. Behind the extraction grid again follows the negatively charged grid control - the Wehnelt.

Field emission cathodes have a very small emission area, so that with appropriate electron lenses and a small incidence can be achieved on the anode. Characterized the origin of the X-ray radiation is approximately a point source, which enables a more detailed analysis of very small objects.

Anode types

Fixed or standing anode

In a stationary anode, the electrons hit a typically 1 x 10 mm ² large area. In the region of the focal point, the wear of the anode material can be very high. We used, for example embedded in copper tungsten plates. Tungsten has a very high rate of conversion from electric energy into X-ray energy with a high melting point.

The stationary anode of equipment for the crystal structure analysis are mostly water-cooled because of the long measurement times, with more and more re-cooling is used to save water.

Rotating anode

The rotating anode is usually made of a composite plate made of a tungsten outer layer and an underlying, high heat-resistant molybdenum alloy, which is attached via a shaft to a rotor ( squirrel cage ). Outside the X-ray tube is the core of the stator coils to drive the rotor according to the principle of an induction motor. The electrons strike the edge of the plate. Due to the rotation of the plate heat from the focal spot is spread on the bigger picture. This leads to a longer lifetime of the anode and makes it possible a greater beam intensity than would be achievable with a fixed anode and anode to melt the material.

The revolution number of such anodes is different: during anode target rotating at about 8 to 12 cm in diameter with 8,000 to 9,000 revolutions / minute, and usually not in continuous operation ( the service life of ball bearings is in vacuum only a few hundred hours, the plate is therefore accelerated by recording stops it again ), turn high-performance anodes with 20 cm diameter at 3500-6000 revolutions / minute in continuous operation and preferably mounted on wear-free hydrodynamic bearings. Due to intense heat (99% of the energy used is to heat) to the anode plate to be cooled. This is in addition into only by radiation and in tubes with liquid metal bearings by direct heat dissipation inside the camp and then into the cooling water or oil in tubes with ball bearings. Another advantage of the hydrodynamic bearings, the wear-free and almost noiseless running, so that also for this reason the acceleration and deceleration of the anode can be eliminated.

A more recent development is the rotary tube ( rotating envelope tube ). In this technology, the anode is designed as a part of the wall of the tube and to rotate the whole tube. The cathode is placed centrally in the axis of rotation of the tube and the electron beam is magnetically deflected to the circular path of the anode. By this construction, it is possible to cool the anode directly with oil, since it is a part of the housing of the tube. This allows very powerful tubes.

Equipment

For the safe operation of an x-ray tube a suitable shielding in a housing is required. This shielding results in:

  • You can protect the tube against external mechanical stress.
  • Electrical isolation for the required high voltage.
  • Shielding of X-rays in undesired directions. For this purpose, lead is used. In the desired radiation direction, an exit window is ( usually made of glass or beryllium ).

Often, the tube is cooled by oil and also isolated.

Applications

  • Electronics ( X-ray lithography )
  • Fluoroscopy in medicine in baggage checks and for non-destructive material testing (eg quality control of welds )
  • Crystal structure analysis by X-ray diffraction
  • Quantitative analysis by X-ray fluorescence analysis

Specific methods and designs

  • High power X-ray tubes;
  • Soft X-ray beam, for example important for mammographic examinations, where by "soft" X-rays an increased level of detail of the image is reached;
  • X-ray lenses;
  • Phase-contrast X-ray;
  • Microfocus X-ray tubes.

Miscellaneous

And the electron tube used as an amplifier or switch elements in various fields of the electronic type with high voltages from undesirably X-rays. This fact led to some serious damage to health by radar technicians who worked from the 1950s to the 1980s for radar devices, the high-voltage switch tubes were insufficiently protected.

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