X-rays are created by converting electron energy into photons, which occurs in an x-ray tube. The amount (exposure) and quality (spectrum) of radiation can be adjusted by changing the current, voltage and operating time of the device.
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 design of the X-ray tube is such that it maximizes photon production and dissipates heat as quickly as possible.
The tube is a relatively simple device, as a rule, containing two fundamental elements - the cathode and the anode. When current flows from the cathode to the anode, the electrons lose energy, which leads to the generation of x-rays.
Anode
An anode is a component in which high-energy photons are emitted. This is a relatively massive metal element that connects to the positive pole of an electrical circuit. It performs two main functions:
- converts electron energy into x-rays,
- dissipates heat.
The material for the anode is selected to enhance these functions.
Ideally, most electrons should form high-energy photons, not heat. The fraction of their total energy, which is converted into x-ray radiation, (efficiency) depends on two factors:
- atomic number (Z) of the anode material,
- energy of electrons.
Most x-ray tubes use tungsten with an atomic number of 74 as the anode material. 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 low evaporation rate.
For many years, the anode was made of pure tungsten. In recent years, they began to use 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.
The x-ray tubes used for mammography are made with a molybdenum-coated anode. This material has an intermediate atomic number (Z = 42), which generates characteristic photons with energies convenient for shooting breasts. Some mammography devices also have a second anode made of rhodium (Z = 45). This allows you to increase energy and achieve greater penetration for a tight chest.
The use of rhenium-tungsten alloy improves the long-term radiation yield - over time, the efficiency of devices with an anode made of pure tungsten decreases due to thermal damage to the surface.
Most of the anodes are in the form of beveled disks and attached to the shaft of an electric motor, which rotates them at relatively high speeds during the emission of X-rays. The purpose of rotation is to remove heat.
Focal spot
Not the entire anode is involved in the generation of x-ray radiation. 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 between 0.1–2 mm.
X-ray tubes are designed with a specific focal spot size. The smaller it is, the lower the blurriness and higher clarity of the image, and the larger 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 fairly small radiation. For example, this is required when examining 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 imaging procedure.
Cathode
The main function of the cathode is to generate electrons and collect them into a beam directed at the anode. As a rule, it consists of a small wire spiral (thread) immersed in a bowl-shaped recess.
Electrons passing through the circuit usually cannot leave the conductor and go into free space. However, they can do this if they get enough energy. In a process known as thermal emission, heat is used to expel electrons from the cathode. This becomes possible when the pressure in the evacuated X-ray tube reaches 10 -6 –10 -7 mm Hg. Art. The thread is heated in the same way as an incandescent lamp spiral when current is passed through it. The operation of the X-ray tube is accompanied by heating of the cathode to a glow temperature with the displacement of some electrons by thermal energy from it.
Balloon
The anode and cathode are contained in a sealed housing - a cylinder. The balloon and its contents are often referred to as inserts, which have a limited life and can be replaced. X-ray tubes generally have glass flasks, although metal and ceramic cylinders are used for some applications.
The main function of the cylinder is to provide support and isolation 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 just in the form of an electron beam.
Housing
The arrangement of the X-ray tube is such that, in addition to enclosing and supporting other components, its body serves as a shield and absorbs radiation, with the exception of the useful beam passing through the window. Its relatively large outer surface dissipates most 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.
Chain
An electrical circuit connects the tube to an energy source called a generator. The source receives power from the network and converts alternating current into direct current. The generator also allows you to adjust some parameters of the circuit:
- KV - voltage or electric potential;
- MA is the current that flows through the tube;
- S - duration or exposure time, in fractions of a second.
The circuit 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:
- potential electrical energy is converted into kinetic;
- kinetic, in turn, is converted to x-ray radiation and heat.
Potential
When electrons enter the flask, they have potential electric energy, the amount of which is determined by the voltage KV between the anode and cathode. The X-ray tube operates under voltage, to create 1 KV which each particle must have 1 keV. By adjusting KV, the operator gives each electron a certain amount of energy.
Kinetics
The low pressure in the evacuated X-ray tube (at 15 ° C it is 10 -6 –10 -7 mm Hg) allows particles to escape from the cathode to the anode under the influence of thermionic emission and electric force. This force accelerates them, which leads to an increase in speed and kinetic energy and a decrease in potential. When a particle hits the anode, its potential is lost, and all its energy goes into kinetic. A 100-keV electron reaches a speed exceeding half the speed of light. Hitting the surface, the particles slow down very quickly and lose their kinetic energy. It turns into x-ray radiation or heat.
Electrons come into contact with individual atoms of the anode material. Radiation is generated by their interaction with orbitals (x-ray photons) and with the nucleus (bremsstrahlung).
Communication energy
Each electron inside 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 an electron from an atom.
Brake light
Bremsstrahlung produces the largest number of photons. Electrons penetrating the anode material and passing near the nucleus are deflected and slowed by the force of attraction of the atom. Their energy, lost during this meeting, appears in the form of an x-ray photon.
Range
Only a few photons have energy close to the energy of electrons. Most of them have it lower. Suppose that there is a space, or field, surrounding the nucleus, in which the electrons experience the force of "braking". This field can be divided into zones. This gives the field of the nucleus the appearance of a target with an atom in the center. An electron falling at any point of the target is inhibited and generates an x-ray photon. Particles closest to the center are most affected and therefore lose the most energy, producing the highest energy photons. Electrons entering the outer zones experience weaker interactions and generate lower energy quanta. Although the zones have the same width, they have a different area, depending on the distance to the core. Since the number of particles falling into a given zone depends on its total area, it is obvious that the outer zones capture more electrons and create more photons. Using this model, the energy spectrum of x-rays can be predicted.
E max of photons of the main bremsstrahlung spectrum corresponds to E max of electrons. Below this point, with a decrease in the energy of quanta, their number grows.
A significant number of low-energy photons are absorbed or filtered as they try to pass through the surface of the anode, 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.
KV influence
The high-energy part of the spectrum is determined by the voltage in the x-ray tubes kV (kilovolt). This is because it determines the energy of the electrons reaching the anode, and photons cannot have a potential greater than this. What voltage does the x-ray tube work under? The maximum photon energy corresponds to the maximum applied potential. This voltage may change during exposure due to AC mains. In this case, the E max of the photon is determined by the peak voltage of the oscillation period KV p .
In addition to the quantum potential, KV p determines the amount of radiation created by a given number of electrons entering the anode. Since the total efficiency of bremsstrahlung is increased due to the increase in the energy of the bombarding electrons, which is determined by KV p , it follows that KV p affects the efficiency of the device.
A change in KV p , 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 KV. With 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 radiation yield.
Characteristic radiation
The type of interaction that produces characteristic radiation involves the collision of high-speed electrons with orbital ones. The interaction can occur only when the incoming particle has E to greater than the binding energy in the atom. When this condition is met, and a collision occurs, the electron is knocked out. In this case, a vacancy remains, filled with a particle of a higher energy level. As it moves, the electron gives off energy emitted 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 a tungsten K-level electron with an E bond = 69.5 keV is knocked out, a vacancy is filled with an electron from the L level with an E bond = 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 energies. 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. Despite the fact that the filling of L-level vacancies generates photons, their energies are too small for use in diagnostic imaging. Each characteristic energy is given a designation that indicates the orbital in which the vacancy formed, with an index that indicates the source of electron filling. Index alpha (α) indicates the filling of an electron from the L level, and beta (β) indicates the filling from level M or N.
- Tungsten spectrum. 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 is different in that the probability of filling a K-level vacancy depends on the orbital.
- Molybdenum spectrum. The anodes of this metal used for mammography produce two fairly intense characteristic energies of X-rays: K-alpha at 17.9 keV, and K-beta at 19.5 keV. The optimal spectrum of x-ray tubes, allowing to achieve the best balance between contrast and radiation dose for a medium-sized breast, is achieved at E f = 20 keV. However, bremsstrahlung is produced by high energies. Mammography equipment uses a molybdenum filter to remove the unwanted part of the spectrum. The filter works according to the “K-edge” principle. It absorbs radiation exceeding the electron binding energy at the K level of the molybdenum atom.
- The spectrum of rhodium. Rhodium has atomic number 45, and molybdenum - 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 capture tight breasts.
Anodes with double surface sections, molybdenum-rhodium, enable the operator to select a distribution optimized for mammary glands of different sizes and densities.
The influence of KV on the spectrum
The value of KV strongly affects the characteristic radiation, because it will not be produced if KV is less than the energy of the K-level electrons. When KV exceeds this threshold value, the amount of radiation is usually proportional to the difference between the tube KV and the threshold KV.
The energy spectrum of the photons of an x-ray coming out of the device is determined by several factors. As a rule, it consists of quanta of inhibitory and characteristic interactions.
The relative composition of the spectrum depends on the anode material, KV and filter. In a tube with a tungsten anode, characteristic radiation is not formed at KV <69.5 keV. At higher HF values used in diagnostic studies, the characteristic radiation increases the total radiation up to 25%. In molybdenum devices, it can make up most of the total generation volume.
Efficiency
Only a small fraction of the energy delivered by the electrons is converted to radiation. The bulk is absorbed and converted into heat. The radiation efficiency is defined as the fraction of the total radiated energy from the total electrical energy communicated to the anode. Factors that determine the efficiency of an X-ray tube are the applied voltage KV and atomic number Z. An approximate ratio is as follows:
- Efficiency = KV x Z x 10 -6 .
The relationship between efficiency and KV has a specific impact on the practical use of X-ray equipment. Due to heat generation, the tubes have a certain limit on the amount of electrical energy that they can dissipate. This imposes a limitation on the power of the device. With an increase in KV, however, the amount of radiation produced per unit of heat increases significantly.
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X-ray tubes Svetlana-X-ray are produced in Russia. In addition to traditional devices with a rotating and stationary anode, the company manufactures devices with a cold cathode controlled by the light flux. The advantages of the device are as follows:
- continuous and pulsed operation;
- inertia;
- intensity regulation by LED current;
- spectrum clarity;
- the possibility of obtaining x-rays of various intensities.