The major components of an X-ray generator are the tube, the high voltage generator, the control console, and the cooling system. As discussed earlier in this material, X-rays are generated by directing a stream of high speed electrons at a target material such as tungsten, which has a high atomic number. When the electrons are slowed or stopped by the interaction with the atomic particles of the target, X-radiation is produced. This is accomplished in an X-ray tube such as the one shown here. The X-ray tube is one of the components of an X-ray generator and tubes come a variety of shapes and sizes. The image below shows a portion of the Roentgen tube collection of Grzegorz Jezierski, a professor at Opole University of Technology. For more information on X-ray tubes visit Dr. Jezierski's website at www.xraylamp.webd.pl
The tube cathode (filament) is heated with a low-voltage current of a few amps. The filament heats up and the electrons in the wire become loosely held. A large electrical potential is created between the cathode and the anode by the high-voltage generator. Electrons that break free of the cathode are strongly attracted to the anode target. The stream of electrons between the cathode and the anode is the tube current. The tube current is measured in milliamps and is controlled by regulating the low-voltage, heating current applied to the cathode. The higher the temperature of the filament, the larger the number of electrons that leave the cathode and travel to the anode. The milliamp or current setting on the control console regulates the filament temperature, which relates to the intensity of the X-ray output.
The high-voltage between the cathode and the anode affects the speed at which the electrons travel and strike the anode. The higher the kilovoltage, the more speed and, therefore, energy the electrons have when they strike the anode. Electrons striking with more energy results in X-rays with more penetrating power. The high-voltage potential is measured in kilovolts, and this is controlled with the voltage or kilovoltage control on the control console. An increase in the kilovoltage will also result in an increase in the intensity of the radiation.
A focusing cup is used to concentrate the stream of electrons to a small area of the target called the focal spot. The focal spot size is an important factor in the system's ability to produce a sharp image. See the information on image resolution and geometric unsharpness for more information on the effect of the focal spot size. Much of the energy applied to the tube is transformed into heat at the focal spot of the anode. As mentioned above, the anode target is commonly made from tungsten, which has a high melting point in addition to a high atomic number. However, cooling of the anode by active or passive means is necessary. Water or oil recirculating systems are often used to cool tubes. Some low power tubes are cooled simply with the use of thermally conductive materials and heat radiating fins.
It should also be noted that in order to prevent the cathode from burning up and to prevent arcing between the anode and the cathode, all of the oxygen is removed from the tube by pulling a vacuum. Some systems have external vacuum pumps to remove any oxygen that may have leaked into the tube. However, most industrial X-ray tubes simply require a warm-up procedure to be followed. This warm-up procedure carefully raises the tube current and voltage to slowly burn any of the available oxygen before the tube is operated at high power.
The other important component of an X-ray generating system is the control console. Consoles typically have a keyed lock to prevent unauthorized use of the system. They will have a button to start the generation of X-rays and a button to manually stop the generation of X-rays. The three main adjustable controls regulate the tube voltage in kilovolts, the tube amperage in milliamps, and the exposure time in minutes and seconds. Some systems also have a switch to change the focal spot size of the tube.
X-ray Generator Options
Kilovoltage - X-ray generators come in a large variety of sizes and configurations. There are stationary units that are intended for use in lab or production environments and portable systems that can be easily moved to the job site. Systems are available in a wide range of energy levels. When inspecting large steel or heavy metal components, systems capable of producing millions of electron volts may be necessary to penetrate the full thickness of the material. Alternately, small, lightweight components may only require a system capable of producing only a few tens of kilovolts.
Focal Spot Size - Another important consideration is the focal spot size of the tube since this factors into the geometric unsharpness of the image produced. Generally, the smaller the spot size the better. But as the electron stream is focused to a smaller area, the power of the tube must be reduced to prevent overheating at the tube anode. Therefore, the focal spot size becomes a tradeoff of resolving capability and power. Generators can be classified as a conventional, minifocus, and microfocus system. Conventional units have focal-spots larger than about 0.5 mm, minifocus units have focal-spots ranging from 50 microns to 500 microns (.050 mm to .5 mm), and microfocus systems have focal-spots smaller than 50 microns. Smaller spot sizes are especially advantageous in instances where the magnification of an object or region of an object is necessary. The cost of a system typically increases as the spot size decreases and some microfocus tubes exceed $100,000. Some manufacturers combine two filaments of different sizes to make a dual-focus tube. This usually involves a conventional and a minifocus spot-size and adds flexibility to the system.
AC and Constant Potential Systems - AC X-ray systems supply the tube with sinusoidal varying alternating current. They produce X-rays only during one half of the 1/60th second cycle. This produces bursts of radiation rather than a constant stream. Additionally, the voltage changes over the cycle and the X-ray energy varies as the voltage ramps up and then back down. Only a portion of the radiation is useable and low energy radiation must usually be filtered out. Constant potential generators rectify the AC wall current and supply the tube with DC current. This results in a constant stream of relatively consistent radiation. Most newer systems now use constant potential generators.
Flash X-Ray Generators
Flash X-ray generators produce short, intense bursts of radiation. These systems are useful when examining objects in rapid motion or when studying transient events such as the tripping of an electrical breaker. In these type of situations, high-speed video is used to rapidly capture images from an image intensifier or other real-time detector. Since the exposure time for each image is very short, a high level of radiation intensity is needed in order to get a usable output from the detector. To prevent the imaging system from becoming saturated from a continuous exposure high intensity radiation, the generator supplies microsecond bursts of radiation. The tubes of these X-ray generators do not have a heated filament but instead electrons are pulled from the cathode by the strong electrical potential between the cathode and the anode. This process is known as field emission or cold emission and it is capable of producing electron currents in the thousands of amperes.