An X-ray tube is a vacuum tube that converts electrical input power into X-rays. 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 CT scanners, 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.
History X-ray tubes evolved from experimental Crookes tubes with which X-rays were first discovered on November 8, 1895, by the German physicist Wilhelm Conrad Röntgen. The first-generation cold cathode or Crookes X-ray tubes were used until the 1920s. These tubes work by ionisation of residual gas within the tube. The positive ions bombard the cathode of the tube to release electrons, which are accelerated toward the anode and produce X-rays when they strike it. The Crookes tube was improved by William Coolidge in 1913. The Coolidge tube, also called a hot cathode tube, uses thermionic emission, where a tungsten cathode is heated to a sufficiently high temperature to emit electrons, which are then accelerated toward the anode in a near perfect vacuum. 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.
Physics
As with any vacuum tube, there is a cathode, which emits electrons into the vacuum and an anode to collect the electrons, thus establishing a flow of electrical current, known as the beam, through the tube. A high voltage power source, for example 30 to 150 kilovolts (kV), called the tube voltage, is connected across cathode and anode to accelerate the electrons. The X-ray spectrum depends on the anode material and the accelerating voltage. 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 energies 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 released Heat is produced in the focal spot of the anode. Since a small fraction (less than or equal to 1%) of electron energy is converted to X-rays, it can be ignored in heat calculations. The quantity of heat produced (in joules) in the focal spot is given by :
E h e a t = w V p I t {\displaystyle E_{\mathrm {heat} }=w\mathrm {V_{p}} \mathrm {I} \mathrm {t} }
w {\displaystyle w} being the waveform factor
V p {\displaystyle \mathrm {V_{p}} } = peak AC voltage (in kilo Volts)
I {\displaystyle \mathrm {I} } = tube current (in milli Amperes)
t {\displaystyle \mathrm {t} } = exposure time (in seconds) Heat Unit (HU) was used in the past as an alternative to joules. 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, w {\displaystyle w} = 1 2 ≈ 0.707 {\displaystyle {\frac {1}{\sqrt {2}}}\approx 0.707} , thus the heat unit:
1 HU = 0.707 J 1.4 HU = 1 J
Types
Crookes tube (cold cathode tube)
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