Comparison of X-Ray Sources and Applications Conventional Generators Synch Rot Ron Others

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Comparison of X-ray sources and applications: conventional generators/synchrotron/others Introduction Initially the identity of x-rays was mysterious, their connotations including morbidity and the otherworldly. Gradually they became routine, finding a place in the high-energy end of the electromagnetic spectrum. X-Ray photons are electromagnetic radiation with wavelengths in the range 0.1 - 100 Å that can be produced by conventional generators, by synchrotrons, and by plasma sources. Modern la
  1 Comparison of X-ray sources and applications: conventionalgenerators/synchrotron/others Introduction Initially the identity of x-rays was mysterious, their connotations including morbidity andthe otherworldly. Gradually they became routine, finding a place in the high-energy endof the electromagnetic spectrum. X-Ray photons are electromagnetic radiation withwavelengths in the range 0.1 - 100 Å that can be produced by conventional generators,by synchrotrons, and by plasma sources. Modern laboratory X-rays sources are basedon the same principle that was used at the beginning when the X-rays were discovered:energetic electrons impact a metal target generating Brehmsstrahlung andcharacteristic X-rays. While the method used to generate X-rays is still the same,source technology has advanced significantly. Because of the rapid improvements, thelaboratory sources can now rival the intensity of second generation synchrotron beamlines. The user has more options than ever before, but this can also lead to confusionabout how different sources perform compared to one another and which is the mostappropriate for different applications. This note compares various sources and theirapplications. 1. What are X-Ray sources? X-ray sources are vacuum tubes that use an electrostatic field to produce X-rays. Theyaccelerate electrons to a high velocity and then suddenly stop them. X-ray tubes, as X-ray sources are sometimes called, are devices in which energy conversion takes place,the kinetic energy of fast moving electrons is converted into heat and X-ray energy.To produce X-radiation, large amounts of electrical energy must be transferred to the X-ray tube. Typically less than 1% of the energy deposited in the tube is converted into X-rays the other 99% appears in the form of heat. Consequently, this limits the use of X-ray apparatus. If excessive heat is produced in the X-ray tube, the temperature will riseabove critical values, and the tube can be damaged.There are various sources of X-ray radiation, but X-rays can be generated by an X-raytube when a charged particle can be accelerated or decelerated. Usually, in laboratoriesX-rays are produced from sealed tubes or rotating anodes. In medical X-ray tubes thetarget is usually tungsten or a more crack-resistant alloy of rhenium (5%) and tungsten(95%). Sometimes molybdenum is used for more specialized applications, such as  2 when soft X-rays are needed as in mammography. In crystallography, a copper target ismost common. But cobalt is often used when fluorescence from iron content in thesample might otherwise present a problem. 2. Conventional generators2.1. Crookes tube X-rays tubes evolved from the experimental Crookes tubes which were used for thediscovery of the X-rays when the medical and other uses of X-rays became apparent.They are used in radiography, CAT scanners, airport luggage scanners, X-raycrystallography and for industrial inspections.The earliest x-ray tubes were of the cold cathode variety which means that they didn’t contain a heated filament in them like the later electronic vacuum tubes do. Thesetubes, referred to as Crookes tube after the inventor’s name, the English physicist William Crookes and other around 1869  – 1875 are a development of the Geisslertubes. These first generation cold cathode or Crookes X-ray tubes were used until the1920s. In the Crookes tube the electrons are generated by the ionization of the residualair in the tube by a DC voltage applied between the electrodes, usually by an inductioncoil. Crookes tubes were of the general class of gas tubes since the pressure had to be in the ‘soft’ vacuum range (about 10 -3 to 10 -4 Torr) to permit the passage of electronsfrom the cathode to the x-ray producing target, the anode, in a so- called ‘dark’ discharge. If high voltage is applied to the tube, the small number of the electricallycharged ions present in the gas is accelerated by the electric field and collides withother gas molecules knocking electrons off them and creating positive ions in a chainreaction. The positive ions are attracted to the cathode which is the negative electrodeand they produce a large number of electrons when they strike the cathode. Theseelectrons are attracted by the anode which is the positive electrode. This seemed to bean invisible ray, but when it hit the end of the tube, for some reason it made the glassglow green. These rays were called cathode rays by Eugen Goldstein, because at that time they didn’t know if the rays came from the cathode or from the anode , but if theanode was moved ‘round the corner’ then it was clear that the green glow was oppositethe negative electrode. The high speed electrons which struck the atoms of the anodecreate X-rays by one of the two processes: Bremsstrahlung or X-ray fluorescence.In order to make the glow more visible, later on, researchers painted the inside backwall of the tube with phosphor which is a fluorescent chemical. After striking the wall,the electrons eventually make their way to the anode, flow through the anode wire, thepower supply, and back to the cathode.  3 Crookes tubes were unreliable and temperamental. Both the energy and the quantity ofcathode rays produced depended on the pressure of residual gas in the tube. As timepassed the gas was absorbed by the walls of the tube, reducing the pressure. Thisreduced the amount of cathode rays produced and caused the voltage across the tubeto increase, creating 'harder' more energetic cathode rays. Soon the pressure got solow the tube stopped working. To prevent this, 'softener' devices were used (Figure 1).A small tube attached to the side of the main tube contained a mica sleeve or chemicalthat released a small amount of gas when heated, restoring the correct pressure. Theglass envelope of the tube would blacken in use due to the X-rays affecting its structure,but the glow could be seen.Figure 1 Crookes tube 2.2. Coolidge tube The cold cathode tube went out of use shortly after 1913 when W. D. Coolidgeintroduced a tube with a hot cathode in which the electrons are produced by thermioniceffect from a tungsten filament heated by an electric current. This is a more reliablesource of electrons. The Coolidge tube, which uses high vacuum (typically below 10 -6  Torr), has a number of advantages over the gas tube, but despite its superiority it didnot immediately replaced the cold cathode tubes. The cold cathode tubes were beingmanufactured into the 1920s and were employed for instances in radiology as late asthe 1960s.The Coolidge Tube (figure 2) is the most popular X-ray source because of the highvacuum and its use of a heated filament as the source of the electrons. The fact that theCoolidge tube contains very little gas inside which is not involved in the production of X-rays differentiates it from previous X-rays sources. In order to operate the device, the  4 cathode filament is heated and the Coolidge tube emits electrons. The hotter thefilament gets, the greater the emission of electrons. The electrons are acceleratedtowards the positively charged anode and upon their collision they change direction andemit X-rays with a continuous range of energies. Some undesirable X-rays called strayradiation are produced by electrons striking other tube components, in addition to the x-rays produced at the focal spot of the anode. The fact that the production of x-rays wascontrollable by heating the cathode instead of varying the vacuum rendered the x-rayquality much more stable.The advantages of the Coolidge tube are its stability, and the fact that the intensity andenergy of the X-rays can be controlled independently. If the current to the cathodeincreases, then its temperature also increases. This will lead to an increased number ofelectrons emitted by the cathode, and as a result, the intensity of the x-rays. Increasingthe high voltage potential difference between the anode and the cathode increases thevelocity of the electrons striking the anode, and this increases the energy of the emittedx-rays. The opposite effects would be given by the decreasing of the current or the highvoltage. The high degree of control over the tube output meant that the earlyradiologists could do with one Coolidge tube what before had required a stable of finickycold cathode tubes and the Coolidge tube could function almost indefinitely unlessbroken or badly abused.Figure 2 Coolidge tubeIn certain Coolidge tubes there is actually a third electrode in front of the heater andmuch like a triode, different voltages applied to this grid will allow different amounts ofcurrent through. However for hobbyist purposes this grid can be left unconnected andthe tube can be controlled via heater voltage, which is much easier to adjust.Coolidge tubes are formed as either end-window tubes or side-window tubes.
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