How do X-ray tubes work?
X-ray radiation is created by converting the energy of electrons into photons, which occurs in an X-ray tube. The quantity( exposure) and quality( spectrum) of radiation can be regulated by changing the current, voltage and operating time of the instrument.
Principle of operation
X-ray tubes( photo is given in the article) are energy converters. They get it from the network and turn it into other forms - penetrating radiation and heat, while the latter is an undesirable by-product. The X-ray tube device is such that it maximizes the production of photons and dissipates heat as quickly as possible.
The tube is a relatively simple device, usually containing two fundamental elements - the cathode and the anode. When the current flows from the cathode to the anode, the electrons lose energy, which leads to the generation of X-rays.
Anode is a component in which high-energy photons are emitted. This is a relatively massive element of metal that connects to the positive pole of the electrical circuit. It performs two main functions:
- converts the energy of electrons into X-rays,
- dissipates heat.
The material for the anode is chosen to enhance these functions.
Ideally, most electrons should form high-energy photons, rather than heat. The proportion of their total energy, which is converted into X-ray radiation,( EFFICIENCY) depends on two factors:
- atomic number( Z) anode material,
- electron energy.
In most X-ray tubes, tungsten is used as an anode material, the atomic number of which is 74. In addition to the large Z, this metal has some other characteristics that make it suitable for this purpose. Tungsten is unique in its ability to maintain strength when heated, has a high melting point and a low evaporation rate.
For many years the anode was made from pure tungsten. In recent years, we have started using an alloy of this metal with rhenium, but only on the surface. The anode itself under the tungsten-rhenium coating is made of a light material that accumulates heat well. Two such substances are molybdenum and graphite.
X-ray tubes used for mammography are made with anode coated with molybdenum. This material has an intermediate atomic number( Z = 42), which generates characteristic photons with energies that are convenient for shooting a breast. Some mammography instruments also have a second anode made of rhodium( Z = 45).This allows you to increase energy and achieve greater penetration for a dense chest.
The use of rhenium-tungsten alloy improves long-term radiation yield - over time, the efficiency of devices with an anode of pure tungsten decreases due to thermal damage to the surface.
Most anodes have the shape of beveled discs and are attached to the shaft of the electric motor, which rotates them at relatively high speeds during the emission of X-rays. The purpose of rotation is to remove heat.
The whole anode does not participate in the generation of X-rays. It occurs on a small area of its surface - a focal spot. The dimensions of the latter are determined by the dimensions of the electron beam coming from the cathode. In most devices, it has a rectangular shape and varies within the range of 0.1-2 mm.
X-ray tubes project with a certain focal spot size. The smaller it is, the less the blur and the sharper the image, and the more it is, the better the heat is removed.
The size of the focal spot is one of the factors that must be considered when choosing X-ray tubes. Manufacturers produce devices with small focal spots when it is necessary to achieve high resolution and sufficiently small radiation. For example, it is required in the study of small and thin parts of the body, as in mammography.
X-ray tubes are mainly produced with focal spots of two sizes - large and small, which can be selected by the operator in accordance with the procedure for image formation.
The main function of the cathode is to generate electrons and collect them in a beam directed at the anode. As a rule, it consists of a small wire spiral( filament), immersed in a cup-shaped depression.
Electrons passing through a circuit usually can not leave the conductor and go into free space. However, they can do this if they get enough energy. In the process known as thermionic emission, heat is used to expel electrons from the cathode. This becomes possible when the pressure in the pumped X-ray tube reaches 10-6-10-7 mm Hg. Art. The filament is heated in the same way as the filament of the incandescent lamp when the current flows through it. The work of the X-ray tube is accompanied by heating of the cathode to the luminescence temperature with the displacement of a part of the electrons by the thermal energy from it.
The anode and the cathode are contained in a sealed cylinder casing. The balloon and its contents are often called an insert that has a limited lifespan and can be replaced. X-ray tubes mainly have glass bulbs, although metal and ceramic cylinders are used for some applications.
The main function of the cylinder is to provide support and insulation of the anode and cathode, and maintain a vacuum. The pressure in the evacuated X-ray tube at 15 ° C is 1.2 · 10 -3 Pa. The presence of gases in the cylinder would allow electricity to flow through the device freely, and not only in the form of an electron beam.
The X-ray tube device is such that, in addition to the fence and support of other components, its housing serves as a shield and absorbs radiation, except for a useful beam passing through the window. Its relatively large outer surface dissipates much of the heat generated inside the device. The space between the body and the insert is filled with oil, which provides insulation and its cooling.
An electrical circuit connects the tube to an energy source, which is called a generator. The source receives power from the network and converts the alternating current into a constant one. The generator also allows you to adjust some parameters of the circuit:
- KV - voltage or electric potential;
- MA - current that flows through the tube;
- S - the duration or time of exposure, in fractions of a second.
The chain provides the movement of electrons. They are charged with energy, passing through the generator, and give it to the anode. As they move, two transformations occur:
- , the potential electrical energy turns into kinetic;
- kinetic, in turn, is converted to X-ray radiation and heat.
When electrons enter a flask, they have potential electrical energy, the amount of which is determined by the voltage KV between the anode and the cathode. The X-ray tube works under voltage, to create 1 KV of which each particle should have 1 keV.By adjusting the KV, the operator assigns each electron an amount of energy.
The low pressure in the evacuated X-ray tube( at 15 ° C it is 10-6-10-7 mm Hg) allows the particles to escape from the cathode to the anode under the action of thermionic emission and electric force. This force accelerates them, which leads to an increase in velocity and kinetic energy and a decrease in the potential energy. When a particle hits the anode, its potential is lost, and all its energy goes to the kinetic energy. A 100-keV electron reaches a velocity exceeding half the speed of light. Hitting the surface, the particles very quickly slow down and lose their kinetic energy. It turns into X-rays or heat.
Electrons come into contact with individual atoms of the anode material. Radiation is generated when they interact with orbitals( X-ray photons) and with the nucleus( bremsstrahlung).
Each electron within an atom has a certain binding energy, which depends on the size of the latter and the level at which the particle is located. The binding energy plays an important role in the generation of characteristic X-ray radiation and is necessary for removing the electron from the atom.
Brake radiation produces the largest number of photons. Electrons penetrating the anode material and passing near the nucleus are deflected and slowed down by the force of attraction of the atom. Their energy, lost during this meeting, appears in the form of an X-ray photon.
Only a few photons have an energy close to the energy of electrons. Most of them are lower. Suppose that there is a space, or a field, surrounding the nucleus, in which the electrons experience the force of "inhibition".This field can be divided into zones. This gives the field of the nucleus the form of a target with an atom in the center. An electron that hits any point of the target undergoes deceleration and generates an X-ray photon. Particles that come closest to the center are exposed to the greatest impact and, therefore, lose the most energy, producing the most high-energy photons. Electrons entering the outer zones experience weaker interactions and generate quanta with a lower energy. Although the zones have the same width, they have different area, depending on the distance to the core. Since the number of particles falling on a given zone depends on its total area, it is obvious that the outer zones capture more electrons and create more photons. According to this model, it is possible to predict the energy spectrum of X-ray radiation.
Emax photons of the fundamental spectrum of bremsstrahlung correspond to Emax electrons. Below this point, with a decrease in the energy of quanta, their number increases.
A significant number of photons with low energies are absorbed or filtered, as they try to pass through the anode surface, the tube window or the filter. Filtration, as a rule, depends on the composition and thickness of the material through which the beam passes, which determines the final form of the low-energy curve of the spectrum.
Effect of KV
The high-energy part of the spectrum determines the voltage in the X-ray tubes kV( kilovolt).This is because it determines the energy of the electrons reaching the anode, and photons can not have a potential greater than this. Under what voltage does the X-ray tube work? The maximum photon energy corresponds to the maximum applied potential. This voltage can vary during exposure due to the AC mains current. In this case, the Emax of the photon is determined by the peak voltage of the oscillation period KVp.
In addition to the potential of quanta, KVp determines the amount of radiation produced by a given number of electrons entering the anode. Since the total efficiency of bremsstrahlung increases due to the growth of the energy of bombarding electrons, which is determined by KVp, it follows that KVp influences the efficiency of the device.
Changing KVp, as a rule, changes the spectrum. The total area under the energy curve is the number of photons. Without a filter, the spectrum is a triangle, and the amount of radiation is proportional to the square of KV.In the presence of a filter, an increase in KV also increases the penetration of photons, which reduces the percentage of filtered radiation. This leads to an increase in the radiation yield.
The interaction type, which produces characteristic radiation, involves the collision of high-speed electrons with orbiting. Interaction can occur only when the incoming particle has Ek greater than the binding energy in the atom. When this condition is met, and a collision occurs, the electron is knocked out. This leaves a vacancy filled with a particle of a higher energy level. As the motion moves, the electron gives off the energy radiated in the form of an X-ray quantum. This is called characteristic radiation, since the E photon is a characteristic of the chemical element from which the anode is made. For example, when an electron of the tungsten K-level is knocked out with a link = 69.5 keV, the vacancy is filled by an electron from the L-level with E = 10.2 keV.The characteristic X-ray photon has an energy equal to the difference between these two levels, or 59.3 keV.
In fact, this anode material leads to the appearance of a number of characteristic X-ray energy. This is because electrons at different energy levels( K, L, etc.) can be knocked out by bombarding particles, and vacancies can be filled from different energy levels. Although the filling of L-level vacancies generates photons, their energies are too small to be used in diagnostic imaging. Each characteristic energy is given a designation that indicates the orbital in which the vacancy was formed, with an index that indicates the source of the electron filling. The alpha index( α) indicates the filling of the electron from the L-level, and beta( β) indicates the filling from the M or N level.
- Spectrum of tungsten. The characteristic radiation of this metal produces a linear spectrum consisting of several discrete energies, and the bremsstrahlung creates a continuous distribution. The number of photons created by each characteristic energy differs in that the probability of filling a K-level vacancy depends on the orbital.
- Spectrum of molybdenum. Anodes from this metal used for mammography produce two quite intense characteristic X-ray energies: K-alpha at 17.9 keV, and K-beta at 19.5 keV.The optimal X-ray tube spectrum, which achieves the best balance between contrast and radiation dose for a medium-sized breast, is achieved at Ef = 20 keV.However, bremsstrahlung is produced by high energies. In the mammography equipment, a molybdenum filter is used to remove the undesirable part of the spectrum. The filter operates according to the K-edge principle. It absorbs radiation that exceeds the binding energy of electrons at the K-level of the molybdenum atom.
- Spectrum of rhodium. Rhodium has an atomic number of 45, and molybdenum has 42. Therefore, the characteristic X-ray radiation of the rhodium anode will have a slightly higher energy than that of molybdenum, and more penetrating. This is used to obtain images of a dense breast.
Anodes with double surface areas, molybdenum-rhodium, enable the operator to select a distribution optimized for mammary glands of different sizes and densities.
Effect of KV on the
spectrum The KV value strongly affects the characteristic radiation, since it will not be produced if KV is less than the K-level electron energy. When the KV exceeds this threshold, the amount of radiation is usually proportional to the difference between the KV tube and the threshold KV.
The energy spectrum of the X-ray photons emitted from the instrument is determined by several factors. As a rule, it consists of quanta of the bremsstrahlung and the characteristic interaction.
The relative composition of the spectrum depends on the material of the anode, KV and the filter. In a tube with a tungsten anode, characteristic radiation is not formed at KV & lt;69.5 keV.At higher values of CV used in diagnostic studies, the characteristic radiation increases the total radiation to 25%.In molybdenum devices, it can account for most of the total generation.
Only a small fraction of the energy delivered by electrons is converted to radiation. The main part is absorbed and converted into heat. The radiation efficiency is defined as the fraction of total radiated energy from the total electrical, reported to the anode. The factors that determine the efficiency of the X-ray tube are the applied voltage KV and the atomic number Z. The approximate ratio is as follows:
- Efficiency = KV x Z x 10-6.
The relationship between efficiency and KV has a specific effect on the practical use of X-ray equipment. Due to heat generation, the tubes have a certain limit in terms of the amount of electrical energy they can dissipate. This imposes a limitation on the power of the device. With the increase in KV, however, the amount of radiation produced per unit heat is significantly increased.
The dependence of the efficiency of X-ray generation on the anode composition is of only academic interest, since most devices use tungsten. Exceptions are molybdenum and rhodium, used in mammography. The efficiency of these devices is much lower than that of tungsten because of their lower atomic number.
The efficiency of an X-ray tube is defined as the amount of irradiation in milli-radents delivered to a point in the center of the useful beam at a distance of 1 m from the focal spot for every 1 mA of electrons passing through the instrument. Its value expresses the ability of the device to convert the energy of charged particles into X-rays. Allows you to determine the exposure of the patient and the picture. Like efficiency, the efficiency of the device depends on a number of factors, including KV, voltage waveform, anode material and the degree of damage to its surface, filter and the time of use of the device.
Voltage KV effectively controls the output radiation of the X-ray tube. As a rule, it is assumed that the output is proportional to the square of KV.Doubling KV increases the exposure by 4 times.
The waveform describes the way in which KV varies with time during the generation of radiation due to the cyclic nature of the power supply. Several different waveforms are used. The general principle is this: the smaller the shape of KV, the more efficient X-ray radiation is produced. In modern equipment, generators with a relatively constant KV are used.
X-ray tubes: manufacturers
Oxford Instruments manufactures various devices, including glass ones with a power of up to 250 W, a potential of 4-80 kV, a focal spot up to 10 microns and a wide range of anode materials, including Ag, Au, Co, Cr, Cu, Fe, Mo, Pd, Rh, Ti, W.
Varian offers more than 400 different types of medical and industrial X-ray tubes. Other well-known manufacturers are Dunlee, GE, Philips, Shimadzu, Siemens, Toshiba, IAE, Hangzhou Wandong, Kailong, etc.
Russia produces X-ray tubes "Svetlana-Roentgen".In addition to traditional devices with a rotating and stationary anode, the company manufactures devices with a cold cathode, controlled by a light flux. The advantages of the device are as follows:
- work in continuous and pulsed modes;
- regulation of the current intensity of the LED;
- spectrum purity;
- the possibility of obtaining X-rays of different intensity.