Glossary of terms used on this site
Alle
- Tube output
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Product of tube voltage* and tube current*. Only a small part of the actual tube output is transformed into X-radiation (see X-ray yield*).
- Tube voltage
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The difference in potential between filament and anode in an X-ray tube. The speed at which the electrons travel from cathode to anode is dependant on the tube voltage. At a tube voltage of 100 kV, the electrons travel to the anode with appr. one third of the speed of light. The tube voltage determines the maximum energy of the X-ray spectrum*, since the higher the voltage, the faster, and therefore more energetic, the electrons when they strike the anode, resulting in X-rays with more penetrating power. The photon intensity is approximately proportional to the square tube voltage.
- Tungsten
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Element No. 74. Because of its high atomic number, which makes for a relatively high X-ray yield, and melting point of 3410 ?C, tungsten is commonly used as target and filament material.
- Vacuum
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A vacuum is a volume of space that is substantively empty of matter so that gaseous pressure is much less than standard atmospheric pressure. It is necessary that the X-ray tube is evacuated because otherwise the electrons would be stopped by air or any other gas. Unlike in closed (or sealed) X-ray tubes, in which the vacuum is permanent, open tube systems have to be outfitted with a vacuum pump for the purpose of creating vacuum during tube warm-up.
- Wehnelt electrode
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Wehnelt electrode or grid. Cylindrical electrode enclosing the cathode of an X-ray tube. The Wehnelt electrode has a negative potential with respect to the cathode. Varying the voltage applied to the grid regulates the tube current, which, in turn, affects the intensity of the X-ray photons.
- X-ray film
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A photographic film used to generate a visual X-ray image. X-ray films provide very good spatial resolution and contrast, but need long exposures times and need to be chemically processed. Moreover, they cannot be digitally processed.
- X-ray generator
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Supplies heating current and both acceleration and grid voltage for the X-ray tube. It is crucial that grid voltage is smooth and stable to prevent defocussing and chromatic aberrations (unsharpness).
- X-ray spectrum
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The distribution of the energy (wavelength, frequency) of the X-ray photon emerging from an X-ray source. Typically the spectrum of an X-ray tube consists of the continuous bremsspectrum (s. bremsstrahlung), which is superimposed by the lines of the characteristic spectrum.
- X-ray tube
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Device for the production of X-rays. An evacuated tube in which a swift electron beam is generated by an electron gun setup and then stopped by an anode. The deceleration of electrons leads to the emission of electromagnetic radiation, namely X-radiation. As opposed to conventional X-ray tubes, where the anode of the electron gun is also the target, in nanofocus and microfocus* X-ray tubes, the electron beam is transmitted trough a hole in the anode where it is then focussed onto a small spot on the target. This way, a very small but bright X-ray source is produced. Depending on the type of tube housing, we differentiate between two types of X-ray tubes: open and sealed tubes. Either one can be outfitted with either a directional* or transmission* type target.
- X-ray yield
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The X-ray yield is the percentage of tube power* transformed into X-ray radiation. The main part of the tube power is used for warming up the target. An increase in tube voltage results in a linear increase in X-ray yield. Also important in this context is the atomic number of the target material: The higher the atomic number, the better the X-ray yield. Hence, target materials with high atomic numbers, such as tungsten, should be used. Under a tube voltage of 100kv, tungsten provides an X-ray yield of 0.7%.
- X-rays
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X-rays are a form of electromagnetic radiation with a wavelength in the range of about 10-9m (1 nm) to 6 x 10-12m (6 pm), or frequencies in the range of 3 x 1017Hz to 5 x 1019Hz and photon energy between 1.2 keV and 240 keV. The most common way of producing X-rays is by bremsstrahlung* (German for braking radiation). Another type of X-rays is produced by the inner, more tightly bound electrons in atoms (characteristic radiation). For more information on the generation of X-rays, please see X-ray tube.