The quantity of heat produced (in Joule) in the focal spot is given by :Į h e a t = w V p I t, thus the heat unit:ġ HU = 0.707 J 1.4 HU = 1 J Types Crookes tube (cold cathode tube) Ĭrookes X-ray tube from early 1900s. 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. Heat is produced in the focal spot of the anode. The number of emitted X-ray photons, or dose, are adjusted by controlling the current flow and exposure time.
Aluminum filters are installed in the path of the X-ray beam to remove "soft" (non-penetrating) 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. The X-ray photon-generating effect is generally called the bremsstrahlung effect, a compound of the German bremsen meaning to brake, and Strahlung meaning radiation. The old tube assembly is shipped to a company that reloads it with a new X-ray tube. 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. As time goes on, the tube becomes unstable even at lower voltages, and must be replaced. This arcing causes an effect called " crazing" on the interior glass of the X-ray window. The arc will jump from the cathode to the tungsten deposit, and then to the anode. Eventually, the tungsten deposit may become sufficiently conductive that at high enough voltages, arcing occurs. 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.
This will slowly darken the tube and was thought to degrade the quality of the X-ray beam. Over time, tungsten will be deposited from the target onto the interior surface of the tube, including the glass surface. The rest of the energy is released as heat. About 1% of the energy generated is emitted/radiated, usually perpendicular to the path of the electron beam, as X-rays. Įlectrons from the cathode collide with the anode material, usually tungsten, molybdenum or copper, and accelerate other electrons, ions and nuclei within the anode material. The X-ray spectrum depends on the anode material and the accelerating voltage. 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. Note that the emission starts around wavelength of 20pm corresponding to E=hc/λ.Īs 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. The smooth, continuous curve is due to bremsstrahlung, and the spikes are characteristic K lines for rhodium atoms. Spectrum of the X-rays emitted by an X-ray tube with a rhodium target, operated at 60 kV. 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. In the late 1980s a different method of control was emerging, called high speed switching. Until the late 1980s, X-ray generators were merely high-voltage, AC to DC variable power supplies. The Coolidge tube, also called a hot cathode tube, uses thermionic emission, where a tungsten cathode is heated to a high enough temperature to emit electrons, which are then accelerated toward the anode in a near perfect vacuum. The Crookes tube was improved by William Coolidge in 1913. 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. These tubes work by ionisation of residual gas within the tube. The first-generation cold cathode or Crookes X-ray tubes were used until the 1920s. 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. Increasing demand for high-performance Computed tomography (CT) scanning and angiography systems has driven development of very high performance medical X-ray tubes. X-ray tubes are also used in CT scanners, airport luggage scanners, X-ray crystallography, material and structure analysis, and for industrial inspection. In contrast to other sources of ionizing radiation, X-rays are only produced as long as the X-ray tube is energized. The availability of this controllable source of X-rays created the field of radiography, the imaging of partly opaque objects with penetrating radiation. William Coolidge explains medical imaging and X-rays.Īn X-ray tube is a vacuum tube that converts electrical input power into X-rays.
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