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An X-ray tube is a vacuum tube that converts electrical input power into . The availability of this controllable source of X-rays created the field of radiography, the imaging of partly opaque objects with penetrating radiation. In contrast to other sources of ionizing radiation, X-rays are only produced as long as the X-ray tube is energized. X-ray tubes are also used in , airport luggage scanners, X-ray crystallography, material and structure analysis, and for industrial inspection.
Increasing demand for high-performance computed tomography (CT) scanning and angiography systems has driven development of very high-performance medical X-ray tubes.
Until the late 1980s, X-ray generators were merely high-voltage, AC to DC variable power supplies. In the late 1980s a different method of control was emerging, called high-speed switching. This followed the electronics technology of switching power supplies (aka switch mode power supply), and allowed for more accurate control of the X-ray unit, higher quality results and reduced X-ray exposures.
Electrons from the cathode collide with the anode material, usually tungsten, molybdenum or copper, and accelerate other electrons, ions and nuclei within the anode material. About 1% of the energy generated is emitted/radiated, usually perpendicular to the path of the electron beam, as X-rays. The rest of the energy is released as heat. Over time, tungsten will be deposited from the target onto the interior surface of the tube, including the glass surface. This will slowly darken the tube and was thought to degrade the quality of the X-ray beam. Vaporized tungsten condenses on the inside of the envelope over the "window" and thus acts as an additional filter and decreases the tube's ability to radiate heat. Eventually, the tungsten deposit may become sufficiently conductive that at high enough voltages, arcing occurs. The arc will jump from the cathode to the tungsten deposit, and then to the anode. This arcing causes an effect called "crazing" on the interior glass of the X-ray window. With time, the tube becomes unstable even at lower voltages and must be replaced. At this point, the tube assembly (also called the "tube head") is removed from the X-ray system, and replaced with a new tube assembly. The old tube assembly is shipped to a company that reloads it with a new X-ray tube.
The two X-ray photon-generating effects are generally called the 'Characteristic effect' and the bremsstrahlung effect, a compound of the German bremsen meaning to brake, and Strahlung meaning radiation.
The range of photonic energy emitted by the system can be adjusted by changing the applied voltage, and installing aluminum filters of varying thicknesses. Aluminum filters are installed in the path of the X-ray beam to remove "soft" (non-penetrating) radiation. The number of emitted X-ray photons, or dose, are adjusted by controlling the current flow and exposure time.
Heat Unit (HU) was used in the past as an alternative to Joule. It is a convenient unit when a single-phase power source is connected to the X-ray tube. With a full-wave rectification of a sine wave, =, thus the heat unit:
To operate, a Direct current voltage of a few to as much as 100 kV was applied between the anodes and the cathode, usually generated by an induction coil, or for larger tubes, an electrostatic machine.
Crookes tubes were unreliable. As time passed, the residual air would be absorbed by the walls of the tube, reducing the pressure. This increased the voltage across the tube, generating 'harder' X-rays, until eventually the tube stopped working. To prevent this, 'softener' devices were used (see picture). A small tube attached to the side of the main tube contained a mica sleeve or chemical that released a small amount of gas when heated, restoring the correct pressure.
The glass envelope of the tube would blacken with usage due to the X-rays affecting its structure.
There are two designs: end-window tubes and side-window tubes. End window tubes usually have "transmission target" which is thin enough to allow X-rays to pass through the target (X-rays are emitted in the same direction as the electrons are moving.) In one common type of end-window tube, the filament is around the anode ("annular" or ring-shaped), the electrons have a curved path (half of a toroid).
What is special about side-window tubes is an electrostatic lens is used to focus the beam onto a very small spot on the anode. The anode is specially designed to dissipate the heat and wear resulting from this intense focused barrage of electrons. The anode is precisely angled at 1-20 degrees off perpendicular to the electron current to allow the escape of some of the X-ray photons which are emitted perpendicular to the direction of the electron current. The anode is usually made of tungsten or molybdenum. The tube has a window designed for escape of the generated X-ray photons.
The power of a Coolidge tube usually ranges from 0.1 to 18 kilowatt.
The focal spot temperature can reach during an exposure, and the anode assembly can reach following a series of large exposures. Typical anodes are a tungsten-rhenium target on a molybdenum core, backed with graphite. The rhenium makes the tungsten more ductile and resistant to wear from the impact of the electron beams. The molybdenum conducts heat from the target. The graphite provides thermal storage for the anode, and minimizes the rotating mass of the anode.
There are two basic types of microfocus X-ray tubes: solid-anode tubes and metal-jet-anode tubes.
Solid-anode microfocus X-ray tubes are in principle very similar to the Coolidge tube, but with the important distinction that care has been taken to be able to focus the electron beam into a very small spot on the anode. Many microfocus X-ray sources operate with focus spots in the range 5-20 μm, but in the extreme cases spots smaller than 1 μm may be produced.
The major drawback of solid-anode microfocus X-ray tubes is their very low operating power. To avoid melting the anode, the electron-beam power density must be below a maximum value. This value is somewhere in the range 0.4-0.8 W/μm depending on the anode material.D. E. Grider, A Wright, and P. K. Ausburn (1986), "Electron beam melting in microfocus x-ray tubes", J. Phys. D: Appl. Phys. 19: 2281-2292 This means that a solid-anode microfocus source with a 10 μm electron-beam focus can operate at a power in the range 4-8 W.
In metal-jet-anode microfocus X-ray tubes the solid metal anode is replaced with a jet of liquid metal, which acts as the electron-beam target. The advantage of the metal-jet anode is that the maximum electron-beam power density is significantly increased. Values in the range 3-6 W/μm have been reported for different anode materials (gallium and tin).M. Otendal, T. Tuohimaa, U. Vogt, and H. M. Hertz (2008), "A 9 keV electron-impact liquid-gallium-jet x-ray source", Rev. Sci. Instrum. 79: 016102T. Tuohimaa, M. Otendal, and H. M. Hertz (2007), "Phase-contrast x-ray imaging with a liquid-metal-jet-anode microfocus source", Appl. Phys. Lett. 91: 074104 In the case with a 10 μm electron-beam focus a metal-jet-anode microfocus X-ray source may operate at 30-60 W.
The major benefit of the increased power density level for the metal-jet X-ray tube is the possibility to operate with a smaller focal spot, say 5 μm, to increase image resolution and at the same time acquire the image faster, since the power is higher (15-30 W) than for solid-anode tubes with 10 μm focal spots.
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