X-Ray Tube

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X-ray tube

An electronic device used for the generation of x-rays. X-rays are produced in the x-ray tube by accelerating electrons to a high velocity by an electrostatic field and then suddenly stopping them by collision with a solid body, the so-called target, interposed in their path. The x-rays radiate in all directions from the spot on the target where the collisions take place. The x-rays are due to the mutual interaction of the fast-moving electrons with the electrons and positively charged nuclei which constitute the atoms of the target. Depending upon the method used in generating the electrons, x-ray tubes may all be classified in two general groups, gas tubes and high-vacuum tubes. See X-rays

In gas tubes electrons are freed from a cold cathode by positive ion bombardment. For the existence of the positive ions a certain gas pressure is required without which the tube will allow no current to pass. Metals, such as platinum and tungsten, are placed in the path of the electron beam to serve as the target. Concave metal cathodes are used to focus the electrons on a small area of the metal target and increase the sharpness of the resulting shadows on the fluorescent screen or the photographic film. Many designs of gas tubes have been built for useful application, particularly in the medical field.

The operational difficulties and erratic behavior of gas x-ray tubes are inherently associated with the gas itself and the positive ion bombardment that takes place during operation. The high-vacuum x-ray tube eliminates these difficulties by using other means of emitting electrons from the cathode. The original type of high-vacuum x-ray tube had a hot tungsten-filament cathode and a solid tungsten target. This tube permitted stable and reproducible operation with relatively high voltages and large masses of metals. A modern commercial hot-cathode high-vacuum x-ray tube is built with a liquid-cooled, copper-backed tungsten target.

X-Ray Tube

 

an electron tube used to generate X-radiation. The X rays are produced when the electrons emitted by the cathode are retarded or stopped by the anode, or anticathode. When electrons accelerated by the strong electric field in the space between the anode and the cathode strike the anode, their energy is partially converted into the energy of X-radiation. The radiation of an X-ray tube consists of bremsstrahlung and the characteristic radiation of the substance composing the anode.

X-ray tubes can be classified in various ways. With respect to the method used to produce the electron stream, there are distinguished hot-cathode tubes, which are based on thermionic emission; point-cathode tubes, which make use of field emission; tubes whose cathodes are bombarded with positive ions; and tubes containing a radioactive (β) source of electrons. Depending on the method of evacuation, X-ray tubes are classed as sealed-off or continuously evacuated. With respect to radiation time, X-ray tubes can be divided into continuous-action and pulse types. X-ray tubes can be classified according to the type of cooling used for the anode: water cooling, oil cooling, air cooling, or radiant cooling. Large-focus, small-focus, and microfocus X-ray tubes are distinguished according to the size of the focal spot, that is, the area of the anode that is bombarded by the electron beam. With respect to shape, X-ray tubes are classed as toroidal, circular, or linear. X-ray tubes with electrostatic, magnetic, and electromagnetic focusing are distinguished on the basis of the method used to focus the electrons on the anode.

Figure 1. Schematic of an X-ray tube for X-ray diffraction analysis: (1) metallic anode block (usually grounded); (2) beryllium window for exit of X-radiation; (3) hot cathode; (4) glass envelope insulating the anode part of the tube from the cathode part; (5) cathode leads, to which the filament voltage and high (with respect to the anode) voltage are supplied; (6) electrostatic electron-focusing system; (7) anode (anticathode); (8) inlet and outlet pipes for the flowing water that cools the anode block

X-ray tubes are used in X-ray diffraction analysis, X-ray spectral analysis, flaw detection, roentgen diagnosis, roentgenotherapy, X-ray microscopy, and microradiography. The type of X-ray tube used most widely in all fields is the sealed-off hot-cathode tube with a water-cooled anode and an electrostatic electron-focusing system (Figure 1). The hot cathode of an X-ray tube is usually a spiral or straight filament of tungsten wire that is heated to a high temperature by an electric current. The target surface of the anode is a mirrorlike metallic surface positioned perpendicular or at an angle to the electron stream. In order to obtain a continuous spectrum of high-energy and high-intensity X-radiation, anodes made of Au and W are used. X-ray tubes with anodes made of Ti, Cr, Fe, Co, Ni, Cu, Mo, and Ag are used in X-ray diffraction analysis.

The maximum permissible accelerating voltage of X-ray tubes varies from 1 to 500 kilovolts. The electron current ranges from 0.01 milliampere to 1 ampere. The specific power dissipation of the anode varies from 10 to 104 watts /mm2, and the total power consumption varies from 0.002 watt to 60 kilowatts. The focal spot can have dimensions ranging from 1 micrometer to 10 mm. The efficiency of X-ray tubes is between 0.1 and 3 percent.

REFERENCES

Taylor, A. Rentgenovskaia metallografiia. Moscow, 1965. (Translated from English.)
Umanskii, Ia. S. Rentgenografiia metallov i poluprovodnikov. Moscow, 1969.
Shmelev, V. K. Rentgenovskie apparaty. Moscow, 1973.

V.G. LIUTTSAU

x-ray tube

[′eks ‚rā ‚tüb]
(electronics)
A vacuum tube designed to produce x-rays by accelerating electrons to a high velocity by means of an electrostatic field, then suddenly stopping them by collision with a target.