Angled anode
(click photo to enlarge)
(click photo to enlarge)
The modern X-ray tube has evolved from the early studies of electrical discharges through gases. These studies found that as the gas pressure in the discharge tube was reduced, the gases started to conduct. This conduction gave rise to a glow over the walls of the tube. By replacing the cold cathode of the discharge tube with an electrically heated filament, it was found that the filament provided a source of electrons that were drawn to the anode. Wilhelm Roentgen discovered in 1895 that a type of radiation was emitted from the anode in these discharge tubes, and that this radiation could pass through solid objects and darken photographic plates.
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The cathode is the source of electrons. The cathode is a coiled tungsten filament mounted in a shaped electrode, called the ‘focusing cup’. The focusing cup acts to focus the electrons into a beam by electrostatic forces.
In early X-ray tubes the anode was made of solid tungsten which became white hot because most of the energy of the impinging electrons was converted to heat. The copper anode with the embedded tungsten target was developed to enable excess heat to be conducted away.
In many X-ray tubes the anode is surrounded by a hood. This hood serves several purposes:
Note that apertures in the hood are needed to allow electrons to reach the target, and allow X-rays to be emitted.
The face of the anode is set at an angle of about 70° to the axis joining the anode and cathode. The purpose of this is to provide a relatively large surface area to dissipate the energy of the electron beam, and a relatively small effective area as seen from the X-ray beam. A small focal spot size maximises the sharpness of the image in the radiograph.
The image below is of an X-ray tube that has been cut in half. The cathode (left) and anode (right) have been positioned so their internal surfaces are facing the viewer. The angled surface of the anode is visible through the hood.
The radiation emitted from the anode has maximum intensity along a line at about 10° forward of the centre-line of the beam. The intensity diminishes as you move away from this plane.
Furthermore, at the anode side of the beam, some X-rays are absorbed by the target itself. This results in a greater fall-off in intensity at this end of the beam and is known as the heel effect.
The practical effect of this is small and will generally not be noticed in normal industrial radiographic exposures.
In the traditional X-ray tube, the anode and cathode are mounted in a borosilicate glass envelope that hermetically seals the tube to maintain the high level of internal vacuum.
In some X-ray tubes, the glass envelope contains a ‘window’ made from the metal beryllium. The purpose of this window is to allow a greater proportion of low energy X-rays to pass through the tube. This is particularly important for tubes that operate from a low voltage, and so produce a higher proportion of low-energy X-rays.
The glass envelope contains silicon and will absorb some low energy X-rays. Silicon has the atomic number 14 in the periodic table of elements. Beryllium has an atomic number 4 and is the lightest solid stable element in the periodic table. Beryllium absorbs much less of the low energy X-rays than silicon. A photograph of an X-ray tube is shown below.
A more recent development in X-ray tube construction is the metal-ceramic tube, which is made from a steel cylinder brazed to alumina ceramic insulators at each end. These insulators carry the anode and cathode assemblies. The metal-ceramic tubes are smaller and more robust than their glass equivalents. They have another advantage, in that they enable more flexibility in the electrical circuitry associated with the tube.
The three basic methods of wiring an X-ray tube are:
Many industrial X-ray tubes are centre grounded, meaning that for a tube operating at 200 kV, the cathode is at -100 kV and the anode at +100 kV, giving a total potential difference of 200 kV between cathode and anode. This arrangement allows for lighter electricals and insulation to be used, resulting in greater portability. It has been used satisfactorily up to about 400 kV equipment, but provides problems for equipment operating above this voltage. This arrangement is also known as a bi-polar tube.
Negative grounding is rarely used as it provides no particular advantage. In a negative grounded 200 kV set, the cathode is at earth (zero) potential and the anode operates at +200 kV.
Positive grounding does provide an advantage. A positive grounded 200 kV set has the cathode operating at -200 kV and the anode is at earth (zero) potential. With the anode at ground potential, it can be water cooled because the pipes carrying the water will be at earth potential. This improves the efficiency of heat extraction, enabling higher currents and smaller focal spot sizes to be used. Negative and positive grounding arrangements are known as unipolar tubes.
Because all X-ray tubes operate from an alternating current (AC) supply, the voltage on the tube is continually varying from positive, through zero, to negative 50 times each second due to Australia’s 50 Hertz supply.
This gives rise to two effects:
Where:
For example, 200 kV the minimum wavelength (λ0) is 1.23 × 10-6 / 200,000 = 6.2 × 10-12 m.
There are several different circuits used in X-ray equipment to provide this rectification of the voltage.
In this circuit, no specific external rectification is provided. The X-ray tube itself provides rectification because normally electrons cannot escape from the cathode during the negative half cycle. If the anode is overheated however, electrons can escape, and strike-back will occur. Its advantage is lightness due to the absence of heavy rectification components. This is used only in smaller portable equipment as it has significant voltage and current limitations due to the thermal restraints at the anode.
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The Kearsley circuit has been used in X-ray sets operating up to 250 kV. A diode suppressor circuit, it reduces the high voltage applied to the tube during the negative half-cycle. The diode, typically a gas rectifier, is designed to carry a heavy current. It has a low resistance in the forward direction and consequently there is little or no drop in primary voltage across the diode on the conducting cycle, and the high-tension transformer delivers its normal load voltage. On the inverse half-cycle, the primary current is carried by the resistance, across which there is a large voltage drop. There is less primary voltage applied to the transformer and a lower voltage is applied to the X-ray tube on the inverse or no-load half-cycle.
This is a simple circuit in which the rectification is in the secondary (high voltage) circuit. This completely eliminates the inverse voltage applied to the tube. However, the rectifier must be capable of handling voltages in the megavolt range.
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In all of the above circuits, the X-ray tube is effectively turned off for 50% of its operating time during the negative half cycle of the power supply.
The Graetz circuit, reverses the negative half cycle so that both halves of the alternating wave are used. Thus the idle time of the above circuits is removed and the X-ray output is effectively doubled. Its disadvantage is the requirement of four rectifiers connected in a form of Wheatstone bridge circuit. This makes for a bulky and heavy instrument that is suitable for a permanent installation only.
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The rectifying valve, the high-tension capacitor, the high-tension transformer, and the tube filament transformer are all housed in a single oil-filled tank. The peak voltage between the tube cathode and anode is twice the peak voltage generated by the transformer. Two of these units connected to a shielded, centre-grounded X-ray tube form the basis of many 200-300 kV X-ray sets.
Consider the polarity such that the rectifier conducts. During this initial half-cycle the capacitor charges and the small voltage drop across it appears as inverse voltage on the X-ray tube. As the transformer voltage falls the capacitor supplies current to the tube and its voltage falls by a small amount, dependent on its capacity and the tube current; at the end of this half-cycle the transformer reverses polarity; its voltage is added to the capacitor voltage, and current is maintained through the tube while the voltage across it builds up to a maximum at the crest of this half-wave; it falls again at the next reversal of polarity of the transformer.
In subsequent action the capacitor voltage is alternately supplemented and reduced by the transformer voltage, and hence the voltage applied to the tube is sinusoidal, varying from 0 to 2 × V, where V is the peak voltage of the transformer. The rectifier valve must be capable of handling twice the tube voltage, which makes for a very heavy voltage requirement.
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One disadvantage of all of the above circuits, including the Villard circuit, is that the applied voltage has a sinusoidal waveform. In these circuits, the spectrum of the emitted X-rays contains a high proportion of low energy photons and the maximum intensity occurs at photon energies corresponding to about 0.6 of the peak voltage applied to the tube.
A constant potential applied to the X-ray tube results in no idle-time and a lower proportion of low energy photons in the emitted X-ray beam and a maximum emission corresponding to about 0.76 of the peak applied voltage. As a result, the same applied AC voltage gives both a higher output, and X-rays of slightly greater penetration, reducing the exposure times. Constant potential operation imposes greater stresses on the X-ray tube. This circuit is now used extensively in industrial X-ray sets operating up to 400 kV.
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The most recent development in X-ray generator circuits is the high frequency circuit, with radically improved performance, stability and reproducibility. These circuits depart from the 50 Hz frequencies of mains supply current to operate at high 12 kHz frequencies, using electronic circuitry and thyristor power inverters with metal-ceramic X-ray tubes. The output is an accurately monitored constant potential in kilovolts and a current in milliampere with less than 1.5% ripple in the tube voltage.
X-ray radiography can be performed on thick steel sections if a high-energy source of X-rays is available. The most common is the linear accelerator, although other sources such as betatrons and Van de Graaff generators have also been used.
Linear accelerators, or linacs, operate at energies ranging from 1 million electron volts (MeV) to 25 MeV. They incorporate an electron gun (source of electrons) and a target. The electrons are accelerated towards the target by ‘surfing’ a high frequency electromagnetic wave (microwave) travelling in a straight line through a tube known as a wave guide. The electrons ‘ride on the front of the wave’ just as a surfer rides a wave. As the microwave is travelling at the speed of light, and as the electron is a particle, it will slip over one crest and be picked up by the next wave. The electrons are accelerated to an energy of perhaps 5 million electron volts (5 MeV). X-rays are emitted from the far side of the target, the side opposite that struck by the electrons, in a narrow beam with very high intensity. Because of the high energies, these X-rays can deeply penetrate and may be used to radiograph steel of 300 mm and greater thickness.
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The betatron is a donut shaped (toroidal) machine incorporating a number of electromagnets around an evacuated chamber. Electrons are injected from an electron gun and accelerated using an alternating magnetic field. This acceleration continues as the electrons pass many orbits around the chamber until they reach their required velocity, at which stage, they are released to strike the target with energies typically ranging from 15 MeV to 30 MeV. The output of X-rays from the betatron is much lower than that from linacs. As such, they have not achieved success as a commercial sources of high energy X-rays.
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The Van de Graaff generator accumulates large amounts of static charge. The principle is the same as that used to ‘charge’ a plastic ruler by rubbing it vigorously over a woolen fabric such as a pullover. The charge on the belt is discharged through a conducting comb. These devices usually operate at around 1 MeV to 3 MeV with very low currents. Their advantages include a true constant potential and very small (0.1 mm) focal spot size. Their limitations include a low intensity of X-rays and a very bulky installation required to generate the electric energy.
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Answer: b - A source of electrons, a means of accelerating the electrons and a target.
Answer: c - At about 70° to the direction of the electron flow.
Answer: a - Heel effect
Answer: d - To allow softer X-rays to be emitted
Answer: a - It allows for direct cooling of the anode
Answer: a - Enables lighter electrical transforms to be used.
Answer: Refer to X-ray Circuits for a description of X-ray tube circuits.
Answer: c - microwaves
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