X-rays are produced when electrons, traveling at high speed, collide with matter or change direction. In the usual type of x-ray tube, an incandescent filament supplies the electrons and thus forms the cathode, or negative electrode, of the tube. A high voltage applied to the tube drives the electrons to the anode, or target. The sudden stopping of these rapidly moving electrons in the surface of the target results in the generation of x-radiation.
The design and spacing of the electrodes and the degree of vacuum are such that no flow of electrical charge between cathode and anode is possible until the filament is heated.
The X-Ray Tube
Figure 4 is a schematic diagram of the essential parts of an x-ray tube. The filament is heated by a current of several amperes from a low-voltage source, generally a small transformer. The focusing cup serves to concentrate the stream of electrons on a small area of the target, called the focal spot. This stream of electrons constitutes the tube current and is measured in milliamperes.
Figure 4: Schematic diagram of an x-ray tube.
The higher the temperature of the filament, the greater is its emission of electrons and the larger the resulting tube current. The tube current is controlled, therefore, by some device that regulates the heating current supplied to the filament. This is usually accomplished by a variable-voltage transformer, which energizes the primary of the filament transformer. Other conditions remaining the same, the x-ray output is proportional to the tube current.
Most of the energy applied to the tube is transformed into heat at the focal spot, only a small portion being transformed into x-rays. The high concentration of heat in a small area imposes a severe burden on the materials and design of the anode. The high melting point of tungsten makes it a very suitable material for the target of an x-ray tube. In addition, the efficiency of the target material in the production of x-rays is proportional to its atomic number.1 Since tungsten has a high atomic number, it has a double advantage. The targets of practically all industrial x-ray machines are made of tungsten.
Cooling
Circulation of oil in the interior of the anode is an effective method of carrying away the heat. Where this method is not employed, the use of copper for the main body of the anode provides high heat conductivity, and radiating fins on the end of the anode outside the tube transfer the heat to the surrounding medium. The focal spot should be as small as conditions permit, in order to secure the sharpest possible definition in the radiographic image. However, the smaller the focal spot, the less energy it will withstand without damage. Manufacturers of x-ray tubes furnish data in the form of charts indicating the kilovoltages and milliamperages that may be safely applied at various exposure times. The life of any tube will be shortened considerably if it is not always operated within the rated capacity.
Focal-Spot Size
The principle of the line focus is used to provide a focal spot of small effective size, though the actual focal area on the anode face may be fairly large, as illustrated figure 5. By making the angle between the anode face and the central ray small, usually 20 degrees, the effective area of the spot is only a fraction of its actual area. With the focal area in the form of a long rectangle, the projected area in the direction of the central ray is square.
Figure 5: Diagram of a line-focus tube depicting the relation between actual focal-spot area (area of bombardment) and effective focal spot, as projected from a 20° anode.
Effects of Kilovoltage
As will be seen later, different voltages are applied to the x-ray tube to meet the demands of various classes of radiographic work. The higher the voltage, the greater the speed of the electrons striking the focal spot. The result is a decrease in the wavelength of the x-rays emitted and an increase in their penetrating power and intensity. It is to be noted that x-rays produced, for example, at 200 kilovolts contain all the wavelengths that would be produced at 100 kilovolts, and with greater intensity. In addition, the 200-kilovolt x-rays include some shorter wavelengths that do not exist in the 100-kilovoIt spectrum at all. The higher voltage x-rays are used for the penetration of thicker and heavier materials.
Most x-ray generating apparatus consists of a filament supply and a high-voltage supply.
The power supply for the x-ray tube filament consists of an insulating step-down transformer. A variable-voltage transformer or a choke coil may serve for adjustment of the current supplied to the filament.
The high-voltage supply consists of a transformer, an autotransformer, and, quite frequently, a rectifier.
A transformer makes it possible to change the voltage of an alternating current. In the simplest form, it consists of two coils of insulated wire wound on an iron core. The coil connected to the source of alternating current is called the primary winding, the other the secondary winding. The voltages in the two coils are directly proportional to the number of turns, assuming 100 percent efficiency. If, for example, the primary has 100 turns, and the secondary has 100,000, the voltage in the secondary is 1,000 times as high as that in the primary. At the same time, the current in the coils is decreased in the same proportion as the voltage is increased. In the example given, therefore, the current in the secondary is only 1/1,000 that in the primary. A step-up transformer is used to supply the high voltage to the x-ray tube.
An autotransformer is a special type of transformer in which the output voltage is easily varied over a limited range. In an x-ray generator, the autotransformer permits adjustment of the primary voltage applied to the step-up transformer and, hence, of the high voltage applied to the x-ray tube.
The type of voltage waveform supplied by a high-voltage transformer is shown in part A of the figure 6 and consists of alternating pulses, first in one direction and then in the other. Some industrial x-ray tubes are designed for the direct application of the high-voltage waveform of part A of the figure below, the x-ray tube then acting as its own rectifier. Usually, however, the high voltage is supplied to a unit called a rectifier, which converts the pulses into the unidirectional form illustrated in part B of the figure below. Another type of rectifier may convert the waveform to that shown in part C of the figure below, but the general idea is the same in both cases--that is, unidirectional voltage is supplied to the x-ray tube. Sometimes a filter circuit is also provided that "smooths out" the voltage waves shown in parts Band C of the figure below, so that essentially constant potential is applied to the x-ray tube, part D of the figure below. Many different high- voltage waveforms are possible, depending on the design of the x-ray machine and its installation. Figure 6 shows idealized waveforms difficult to achieve in practical high-voltage equipment. Departures from these terms may vary in different x-ray installations. Since x-ray output depends on the entire waveform, this accounts for the variation in radiographic results obtainable from two different x-ray machines operating at the same value of peak kilovoltage.
Figure 6: Typical voltage waveforms of x-ray machines.
Tubes with the anodes at the end of a long extension cylinder are known as "rod-anode" tubes. The anodes of these tubes can be thrust through small openings (See Figure 7, top) to facilitate certain types of inspection. If the target is perpendicular to the electron stream in the tube, the x- radiation through 360 degrees can be utilized (See Figure 7, bottom), and an entire circumferential weld can be radiographed in a single exposure.
With tubes of this type, one special precaution is necessary. The long path of the electron stream down the anode cylinder makes the focusing of the electrons on the target very susceptible to magnetic influences. If the object being inspected is magnetized--for example, if it has undergone a magnetic inspection and has not been properly demagnetized--a large part of the electron stream can be wasted on other than the focal-spot area, and the resulting exposures will be erratic.
The foregoing describes the operation of the most commonly used types of x-ray equipment. However, certain high-voltage generators operate on principles different from those discussed.
Figure 7: Top: Rod-anode tube used in the examination of a plug weld. Bottom: Rod- anode tube with a 360° beam used to examine a circumferential weld in a single exposure.
Flash X-Ray Machines
Flash x-ray machines are designed to give extremely short (microsecond), extremely intense bursts of x-radiation. They are intended for the radiography of objects in rapid motion or the study of transient events (See "High Speed Radiography"). The high-voltage generators of these units give a very short pulse of high voltage, commonly obtained by discharging a condenser across the primary of the high-voltage transformer. The x-ray tubes themselves usually do not have a filament. Rather, the cathode is so designed that a high electrical field "pulls" electrons from the metal of the cathode by a process known as field emission, or cold emission. Momentary electron currents of hundreds or even thousands of amperes--far beyond the capacity of a heated filament--can be obtained by this process.
High-Voltage Equipment
The betatron may be considered as a high-voltage transformer, in which the secondary consists of electrons circulating in a doughnut-shaped vacuum tube placed between the poles of an alternating current electromagnet that forms the primary. The circulating electrons, accelerated to high speed by the changing magnetic field of the primary, are caused to impinge on a target within the accelerating tube.
In the linear accelerator, the electrons are accelerated to high velocities by means of a high- frequency electrical wave that travels along the tube through which the electrons travel.
Both the betatron and the linear accelerator are used for the generation of x-radiation in the multimillion-volt range.
Table I - Typical X-ray Machines and Their Applications
Maximum voltage (kV)
|
Screens
|
Applications and Approximate Thickness Limits
|
50
|
None
|
Thin sections
of most metals; moderate thickness of
graphite and beryllium; small electronic components; wood, plastics, etc.
|
150
|
None or lead foil
|
5-inch aluminum or equivalent. (See Table IV)
1-inch steel
or equivalent.
|
Fluorescent
|
11/ -inch steel or equivalent. (See Table IV)
2
|
|
300
|
Lead foil
|
3-inch steel
or equivalent.
|
Fluorescent
|
4-inch steel
or equivalent.
|
|
400
|
Lead foil
|
31/ -inch steel or equivalent.
2
|
Fluorescent
|
41/ -inch steel or equivalent.
2
|
|
1000
|
Lead foil
|
5-inch steel
or equivalent.
|
Fluorescent
|
8-inch steel
or equivalent.
|
|
2000
|
Lead foil
|
8-inch steel
or equivalent.
|
8 to 25 MeV
|
Lead foil
|
16-inch steel
or equivalent.
|
Fluorescent
|
20-inch steel
or equivalent.
|
In the high-voltage electrostatic generator, the high voltage is supplied by static negative charges mechanically conveyed to an insulating electrode by a moving belt. Electrostatic generators are used for machines in the 1- and 2-million-volt range.
No attempt is made here to discuss in detail the various forms of electrical generating equipment. The essential fact is that electrons must be accelerated to very great velocities in order that their deceleration, when they strike the target, may produce x-radiation.
In developing suitable exposure techniques, it is important to know the voltage applied to the x- ray tube. It is common practice for manufacturers of x-ray equipment to calibrate their machines at the factory. Thus, the operator may know the voltage across the x-ray tube from the readings of the volt-meter connected to the primary winding of the high-voltage transformer.