LECTURE
X-RAY RADIATION
The nature of X-rays
Bremsstrahlung X-ray, its spectral properties.
Characteristic x-ray radiation (for review).
Interaction of X-ray radiation with matter.
Physical basis for the use of X-rays in medicine.
X-rays (X - rays) were discovered by K. Roentgen, who in 1895 became the first Nobel Laureate in physics.
The nature of X-rays
x-ray radiation - electromagnetic waves with a length of 80 to 10 -5 nm. Long-wave X-ray radiation is covered by short-wave UV radiation, and short-wave radiation by long-wave radiation.
X-rays are produced in x-ray tubes. fig.1.
K - cathode
1 - electron beam
2 - X-ray radiation
Rice. 1. X-ray tube device.
The tube is a glass flask (with a possibly high vacuum: the pressure in it is about 10–6 mm Hg) with two electrodes: anode A and cathode K, to which a high voltage U (several thousand volts) is applied. The cathode is a source of electrons (due to the phenomenon of thermionic emission). The anode is a metal rod that has an inclined surface in order to direct the resulting X-ray radiation at an angle to the axis of the tube. It is made of a highly heat-conducting material to remove the heat generated during electron bombardment. On the beveled end there is a plate made of refractory metal (for example, tungsten).
The strong heating of the anode is due to the fact that the main number of electrons in the cathode beam, having hit the anode, experience numerous collisions with the atoms of the substance and transfer a large amount of energy to them.
Under the action of high voltage, the electrons emitted by the hot cathode filament are accelerated to high energies. The kinetic energy of an electron is equal to mv 2 /2. It is equal to the energy that it acquires by moving in the electrostatic field of the tube:
mv 2 /2 = eU(1)
where m, e are the electron mass and charge, U is the accelerating voltage.
The processes leading to the appearance of bremsstrahlung X-rays are due to the intense deceleration of electrons in the anode material by the electrostatic field of the atomic nucleus and atomic electrons.
The origin mechanism can be represented as follows. Moving electrons are some kind of current that forms its own magnetic field. Electron deceleration - a decrease in current strength and, accordingly, a change in induction magnetic field, which will cause the occurrence of an alternating electric field, i.e. appearance of an electromagnetic wave.
Thus, when a charged particle flies into matter, it slows down, loses its energy and speed, and emits electromagnetic waves.
Spectral properties of X-ray bremsstrahlung .
So, in the case of electron deceleration in the anode material, bremsstrahlung radiation.
The bremsstrahlung spectrum is continuous. The reason for this is as follows.
When electrons decelerate, each of them has part of the energy used to heat the anode (E 1 \u003d Q), the other part to create an X-ray photon (E 2 \u003d hv), otherwise, eU \u003d hv + Q. The ratio between these parts is random.
Thus, the continuous spectrum of X-ray bremsstrahlung is formed due to the deceleration of many electrons, each of which emits one X-ray quantum hv (h) of a strictly defined value. The value of this quantum different for different electrons. Dependence of the X-ray energy flux on the wavelength , i.e. the X-ray spectrum is shown in Fig.2.
Fig.2. Bremsstrahlung spectrum: a) at different voltages U in the tube; b) at different temperatures T of the cathode.
Short-wave (hard) radiation has a greater penetrating power than long-wave (soft) radiation. Soft radiation is more strongly absorbed by matter.
From the side of short wavelengths, the spectrum ends abruptly at a certain wavelength m i n . Such short-wavelength bremsstrahlung occurs when the energy acquired by an electron in an accelerating field is completely converted into photon energy (Q = 0):
eU = hv max = hc/ min , min = hc/(eU), (2)
min (nm) = 1.23/UkV
The spectral composition of the radiation depends on the voltage on the X-ray tube; with increasing voltage, the value of m i n shifts towards short wavelengths (Fig. 2a).
When the temperature T of the cathode incandescence changes, the electron emission increases. Consequently, the current I in the tube increases, but the spectral composition of the radiation does not change (Fig. 2b).
The energy flux Ф of bremsstrahlung is directly proportional to the square of the voltage U between the anode and the cathode, the current strength I in the tube and the atomic number Z of the anode substance:
Ф = kZU 2 I. (3)
where k \u003d 10 -9 W / (V 2 A).
Characteristic X-rays (for familiarization).
Increasing the voltage on the X-ray tube leads to the fact that against the background of a continuous spectrum, a line appears, which corresponds to the characteristic X-ray radiation. This radiation is specific to the anode material.
The mechanism of its occurrence is as follows. At a high voltage, accelerated electrons (with high energy) penetrate deep into the atom and knock electrons out of its inner layers. Electrons from upper levels pass to free places, as a result of which photons of characteristic radiation are emitted.
The spectra of characteristic X-ray radiation differ from optical spectra.
- Uniformity.
The uniformity of the characteristic spectra is due to the fact that the internal electron layers of different atoms are the same and differ only energetically due to the force action from the nuclei, which increases with increasing elemental number. Therefore, the characteristic spectra shift towards higher frequencies with increasing nuclear charge. This was experimentally confirmed by an employee of Roentgen - Moseley, who measured X-ray transition frequencies for 33 elements. They made the law.
MOSELY'S LAW – the square root of the frequency of the characteristic radiation is a linear function of the ordinal number of the element:
= A (Z - B), (4)
where v is the frequency of the spectral line, Z is the atomic number of the emitting element. A, B are constants.
The importance of Moseley's law lies in the fact that this dependence can be used to accurately determine the atomic number of the element under study from the measured frequency of the X-ray line. This played a big role in the placement of the elements in the periodic table.
Independence from a chemical compound.
The characteristic X-ray spectra of an atom do not depend on the chemical compound in which the atom of the element enters. For example, the X-ray spectrum of an oxygen atom is the same for O 2, H 2 O, while the optical spectra of these compounds differ. This feature of the x-ray spectrum of the atom was the basis for the name " characteristic radiation".
Interaction of X-ray radiation with matter
The impact of X-ray radiation on objects is determined by the primary processes of X-ray interaction. photon with electrons atoms and molecules of matter.
X-ray radiation in matter absorbed or dissipates. In this case, various processes can occur, which are determined by the ratio of the X-ray photon energy hv and the ionization energy Аu (the ionization energy Аu is the energy required to remove internal electrons from the atom or molecule).
a) Coherent scattering(scattering of long-wave radiation) occurs when the relation
For photons, due to interaction with electrons, only the direction of movement changes (Fig. 3a), but the energy hv and the wavelength do not change (therefore, this scattering is called coherent). Since the energies of a photon and an atom do not change, coherent scattering does not affect biological objects, but when creating protection against X-ray radiation, one should take into account the possibility of changing the primary direction of the beam.
b) photoelectric effect happens when
In this case, two cases can be realized.
The photon is absorbed, the electron is detached from the atom (Fig. 3b). Ionization occurs. The detached electron acquires kinetic energy: E k \u003d hv - A and. If the kinetic energy is large, then the electron can ionize neighboring atoms by collision, forming new ones. secondary electrons.
The photon is absorbed, but its energy is not enough to detach the electron, and excitation of an atom or molecule(Fig. 3c). This often leads to the subsequent emission of a photon in the visible radiation region (X-ray luminescence), and in tissues - to the activation of molecules and photochemical reactions. The photoelectric effect occurs mainly on the electrons of the inner shells of atoms with high Z.
in) Incoherent scattering(Compton effect, 1922) occurs when the photon energy is much greater than the ionization energy
In this case, the electron is detached from the atom (such electrons are called recoil electrons), acquires some kinetic energy E k, the energy of the photon itself decreases (Fig. 4d):
hv=hv" + A and + E k. (5)
The resulting radiation with a changed frequency (length) is called secondary, it scatters in all directions.
Recoil electrons, if they have sufficient kinetic energy, can ionize neighboring atoms by collision. Thus, as a result of incoherent scattering, secondary scattered X-ray radiation is formed and the atoms of the substance are ionized.
These (a, b, c) processes can cause a number of subsequent ones. For example (Fig. 3d), if during the photoelectric effect electrons are detached from the atom on the inner shells, then electrons from higher levels can pass to their place, which is accompanied by secondary characteristic X-ray radiation of this substance. Photons of secondary radiation, interacting with electrons of neighboring atoms, can, in turn, cause secondary phenomena.
coherent scattering
uh 
energy and wavelength remain unchanged
photoelectric effect
photon is absorbed, e - detached from the atom - ionization
hv \u003d A and + E to
atom A is excited upon absorption of a photon, R is X-ray luminescence
incoherent scattering
hv \u003d hv "+ A and + E to
secondary processes in the photoelectric effect
Rice. 3 Mechanisms of X-ray interaction with matter
Physical basis for the use of X-rays in medicine
When X-rays fall on a body, it is slightly reflected from its surface, but mainly passes deep into, while it is partially absorbed and scattered, and partially passes through.
The law of weakening.
The X-ray flux is attenuated in matter according to the law:
F \u003d F 0 e - x (6)
where is linear attenuation factor, which essentially depends on the density of the substance. It is equal to the sum of three terms corresponding to coherent scattering 1, incoherent 2 and photoelectric effect 3:
= 1 + 2 + 3 . (7)
The contribution of each term is determined by the photon energy. Below are the ratios of these processes for soft tissues (water).
|
Energy, keV |
photoelectric effect |
Compton - effect |
enjoy mass attenuation coefficient, which does not depend on the density of the substance :
m = /. (eight)
The mass attenuation coefficient depends on the energy of the photon and on the atomic number of the absorbing substance:
m = k 3 Z 3 . (9)
The mass attenuation coefficients of bone and soft tissue (water) are different: m bone / m water = 68.
If an inhomogeneous body is placed in the path of X-rays and a fluorescent screen is placed in front of it, then this body, absorbing and attenuating the radiation, forms a shadow on the screen. By the nature of this shadow, one can judge the shape, density, structure, and in many cases the nature of bodies. Those. a significant difference in the absorption of x-ray radiation by different tissues allows you to see the image of the internal organs in the shadow projection.
If the organ under study and the surrounding tissues equally attenuate x-rays, then contrast agents are used. So, for example, filling the stomach and intestines with a mushy mass of barium sulfate (BaSO 4 ), one can see their shadow image (the ratio of the attenuation coefficients is 354).
Use in medicine.
In medicine, X-ray radiation with photon energy from 60 to 100-120 keV is used for diagnostics and 150-200 keV for therapy.
X-ray diagnostics – Recognition of diseases by transilluminating the body with X-rays.
X-ray diagnostics is used in various options, which are given below.
With fluoroscopy the x-ray tube is located behind the patient. In front of it is a fluorescent screen. There is a shadow (positive) image on the screen. In each individual case, the appropriate hardness of the radiation is selected so that it passes through soft tissues, but is sufficiently absorbed by dense ones. Otherwise, a uniform shadow is obtained. On the screen, the heart, the ribs are visible dark, the lungs are light.
When radiography the object is placed on a cassette, which contains a film with a special photographic emulsion. The X-ray tube is placed over the object. The resulting radiograph gives a negative image, i.e. the opposite in contrast to the picture observed during transillumination. In this method, there is a greater clarity of the image than in (1), therefore, details are observed that are difficult to see when transilluminated.
A promising variant of this method is X-ray tomography and "machine version" - computer tomography.
3. With fluoroscopy, On a sensitive small-format film, the image from the large screen is fixed. When viewed, the pictures are examined on a special magnifier.
X-ray therapy- the use of X-rays to destroy malignant tumors.
The biological effect of radiation is to disrupt vital activity, especially rapidly multiplying cells.
COMPUTED TOMOGRAPHY (CT)
The method of X-ray computed tomography is based on the reconstruction of an image of a certain section of the patient's body by registering a large number of X-ray projections of this section, made at different angles. Information from the sensors that register these projections enters the computer, which, according to a special program calculates distribution tightlysample size in the investigated section and displays it on the display screen. The image of the section of the patient's body obtained in this way is characterized by excellent clarity and high information content. The program allows you to increase image contrast in dozens and even hundreds of times. This expands the diagnostic capabilities of the method.
Videographers (devices with digital X-ray image processing) in modern dentistry.
In dentistry, X-ray examination is the main diagnostic method. However, a number of traditional organizational and technical features of X-ray diagnostics make it not quite comfortable for both the patient and dental clinics. This is, first of all, the need for the patient to come into contact with ionizing radiation, which often creates a significant radiation load on the body, it is also the need for a photoprocess, and, consequently, the need for photoreagents, including toxic ones. This is, finally, a bulky archive, heavy folders and envelopes with x-ray films.
In addition, the current level of development of dentistry makes the subjective assessment of radiographs by the human eye insufficient. As it turned out, of the variety of shades of gray contained in the x-ray image, the eye perceives only 64.
Obviously, to obtain a clear and detailed image of the hard tissues of the dentoalveolar system with minimal radiation exposure, other solutions are needed. The search led to the creation of so-called radiographic systems, videographers - digital radiography systems.
Without technical details, the principle of operation of such systems is as follows. X-ray radiation enters through the object not on a photosensitive film, but on a special intraoral sensor (special electronic matrix). The corresponding signal from the matrix is transmitted to a digitizing device (analog-to-digital converter, ADC) that converts it into digital form and is connected to the computer. Special software builds an x-ray image on the computer screen and allows you to process it, save it on a hard or flexible storage medium (hard drive, floppy disks), print it as a picture as a file.
In a digital system, an x-ray image is a collection of dots having different digital grayscale values. The information display optimization provided by the program makes it possible to obtain an optimal frame in terms of brightness and contrast at a relatively low radiation dose.
In modern systems, created, for example, by Trophy (France) or Schick (USA), 4096 shades of gray are used when forming a frame, the exposure time depends on the object of study and, on average, is hundredths - tenths of a second, a decrease in radiation exposure in relation to film - up to 90% for intraoral systems, up to 70% for panoramic videographers.
When processing images, videographers allow:
Get positive and negative images, false color images, embossed images.
Increase contrast and magnify the area of interest in the image.
Assess changes in the density of dental tissues and bone structures, control the uniformity of canal filling.
In endodontics, determine the length of the canal of any curvature, and in surgery, select the size of the implant with an accuracy of 0.1 mm.
The unique Caries detector system with elements of artificial intelligence during the analysis of the image allows you to detect caries in the stain stage, root caries and hidden caries.
"F" in formula (3) refers to the entire range of radiated wavelengths and is often referred to as "Integral Energy Flux".
Radiology is a branch of radiology that studies the effects of X-ray radiation on the body of animals and humans arising from this disease, their treatment and prevention, as well as methods for diagnosing various pathologies using X-rays (X-ray diagnostics). A typical X-ray diagnostic apparatus includes a power supply (transformers), a high-voltage rectifier that converts the alternating current of the electrical network into direct current, a control panel, a tripod and an X-ray tube.
X-rays are a type of electromagnetic oscillations that are formed in an X-ray tube during a sharp deceleration of accelerated electrons at the moment of their collision with the atoms of the anode substance. At present, the point of view is generally accepted that X-rays, in their physical nature are one of the types of radiant energy, the spectrum of which also includes radio waves, infrared rays, visible light, ultraviolet rays and gamma rays of radioactive elements. X-ray radiation can be characterized as a collection of its smallest particles - quanta or photons.
Rice. 1 - mobile x-ray machine:
A - x-ray tube;
B - power supply;
B - adjustable tripod.
Rice. 2 - X-ray machine control panel (mechanical - on the left and electronic - on the right): A - panel for adjusting exposure and hardness;
B - high voltage supply button.
Rice. 3 is a block diagram of a typical x-ray machine 1 - network;
2 - autotransformer;
3 - step-up transformer;
4 - x-ray tube;
5 - anode;
6 - cathode;
7 - step-down transformer.
Mechanism of X-ray generation
X-rays are formed at the moment of collision of a stream of accelerated electrons with the anode material. When electrons interact with a target, 99% of their kinetic energy is converted into thermal energy and only 1% into X-rays.
An X-ray tube consists of a glass container in which 2 electrodes are soldered: a cathode and an anode. Air is pumped out of the glass cylinder: the movement of electrons from the cathode to the anode is possible only under conditions of relative vacuum (10 -7 -10 -8 mm Hg). On the cathode there is a filament, which is a tightly twisted tungsten filament. When an electric current is applied to the filament, electron emission occurs, in which electrons are separated from the spiral and form an electron cloud near the cathode. This cloud is concentrated at the focusing cup of the cathode, which sets the direction of electron movement. Cup - a small depression in the cathode. The anode, in turn, contains a tungsten metal plate on which the electrons are focused - this is the site of the formation of x-rays.
Rice. 4 - X-ray tube device: A - cathode; 
B - anode;
B - tungsten filament;
G - focusing cup of the cathode;
D - stream of accelerated electrons;
E - tungsten target;
G - glass flask;
З - a window from beryllium;
And - formed x-rays;
K - aluminum filter.
2 transformers are connected to the electron tube: step-down and step-up. A step-down transformer heats the tungsten filament with a low voltage (5-15 volts), resulting in electron emission. A step-up, or high-voltage, transformer goes directly to the cathode and anode, which are supplied with a voltage of 20–140 kilovolts. Both transformers are placed in the high-voltage block of the X-ray machine, which is filled with transformer oil, which provides cooling of the transformers and their reliable insulation.
After an electron cloud has formed with the help of a step-down transformer, the step-up transformer is turned on, and high-voltage voltage is applied to both poles of the electrical circuit: a positive pulse to the anode, and a negative pulse to the cathode. Negatively charged electrons are repelled from a negatively charged cathode and tend to a positively charged anode - due to such a potential difference, a high speed of movement is achieved - 100 thousand km / s. At this speed, electrons bombard the tungsten anode plate, completing an electrical circuit, resulting in X-rays and thermal energy.
X-ray radiation is subdivided into bremsstrahlung and characteristic. Bremsstrahlung occurs due to a sharp deceleration of the speed of electrons emitted by a tungsten filament. Characteristic radiation occurs at the moment of rearrangement of the electron shells of atoms. Both of these types are formed in an X-ray tube at the moment of collision of accelerated electrons with atoms of the anode material. The emission spectrum of an X-ray tube is a superposition of bremsstrahlung and characteristic X-rays.
Rice. 5 - the principle of the formation of bremsstrahlung X-rays.
Rice. 6 - the principle of formation of the characteristic x-ray radiation.
Basic properties of X-rays
- X-rays are invisible to visual perception.
- X-ray radiation has a high penetrating power through the organs and tissues of a living organism, as well as dense structures. inanimate nature that do not transmit visible light.
- X-rays cause certain chemical compounds to glow, called fluorescence.
- Zinc and cadmium sulfides fluoresce yellow-green,
- Crystals of calcium tungstate - violet-blue.
Scale of electromagnetic oscillations
X-rays have a specific wavelength and frequency of oscillation. Wavelength (λ) and oscillation frequency (ν) are related by the relationship: λ ν = c, where c is the speed of light, rounded to 300,000 km per second. The energy of X-rays is determined by the formula E = h ν, where h is Planck's constant, a universal constant equal to 6.626 10 -34 J⋅s. The wavelength of the rays (λ) is related to their energy (E) by the relation: λ = 12.4 / E.
X-ray radiation differs from other types of electromagnetic oscillations in wavelength (see table) and quantum energy. The shorter the wavelength, the higher its frequency, energy and penetrating power. The X-ray wavelength is in the range
. By changing the wavelength of X-ray radiation, it is possible to control its penetrating power. X-rays have a very short wavelength, but a high frequency of oscillation, so they are invisible to the human eye. Due to their enormous energy, quanta have a high penetrating power, which is one of the main properties that ensure the use of X-rays in medicine and other sciences.X-ray characteristics
Intensity- quantitative characteristic of x-ray radiation, which is expressed by the number of rays emitted by the tube per unit time. The intensity of X-rays is measured in milliamps. Comparing it with the intensity of visible light from a conventional incandescent lamp, we can draw an analogy: for example, a 20-watt lamp will shine with one intensity, or power, and a 200-watt lamp will shine with another, while the quality of the light itself (its spectrum) is the same . The intensity of X-ray radiation is, in fact, its quantity. Each electron creates one or more radiation quanta on the anode, therefore, the number of X-rays during exposure of the object is regulated by changing the number of electrons tending to the anode and the number of interactions of electrons with atoms of the tungsten target, which can be done in two ways:
- By changing the degree of incandescence of the cathode spiral using a step-down transformer (the number of electrons produced during emission will depend on how hot the tungsten spiral is, and the number of radiation quanta will depend on the number of electrons);
- By changing the value of the high voltage supplied by the step-up transformer to the poles of the tube - the cathode and the anode (the higher the voltage is applied to the poles of the tube, the more kinetic energy the electrons receive, which, due to their energy, can interact with several atoms of the anode substance in turn - see Fig. rice. 5; electrons with low energy will be able to enter into a smaller number of interactions).
The X-ray intensity (anode current) multiplied by the exposure (tube time) corresponds to the X-ray exposure, which is measured in mAs (milliamps per second). Exposure is a parameter that, like intensity, characterizes the amount of rays emitted by an x-ray tube. The only difference is that the exposure also takes into account the operating time of the tube (for example, if the tube works for 0.01 sec, then the number of rays will be one, and if 0.02 sec, then the number of rays will be different - twice more). The radiation exposure is set by the radiologist on the control panel of the X-ray machine, depending on the type of examination, the size of the object under study and the diagnostic task.
Rigidity- qualitative characteristic of x-ray radiation. It is measured by the high voltage on the tube - in kilovolts. Determines the penetrating power of x-rays. It is regulated by the high voltage supplied to the X-ray tube by a step-up transformer. The higher the potential difference is created on the electrodes of the tube, the more force the electrons repel from the cathode and rush to the anode, and the stronger their collision with the anode. The stronger their collision, the shorter the wavelength of the resulting X-ray radiation and the higher the penetrating power of this wave (or the hardness of the radiation, which, like the intensity, is regulated on the control panel by the voltage parameter on the tube - kilovoltage).
λ - wavelength; 
Rice. 7 - Dependence of the wavelength on the energy of the wave:
E - wave energy 
Rice. 8 - The ratio of the voltage on the X-ray tube and the wavelength of the resulting X-ray radiation:
Classification of x-ray tubes
- By appointment
- Diagnostic
- Therapeutic
- For structural analysis
- For transillumination
- By design
- By focus
- Single-focus (one spiral on the cathode, and one focal spot on the anode)
- Bifocal (two spirals of different sizes on the cathode, and two focal spots on the anode)
- By type of anode
- Stationary (fixed)
- Rotating
X-rays are used not only for radiodiagnostic purposes, but also for therapeutic purposes. As noted above, the ability of X-ray radiation to suppress the growth of tumor cells makes it possible to use it in radiation therapy of oncological diseases. In addition to the medical field of application, X-ray radiation has found wide application in the engineering and technical field, materials science, crystallography, chemistry and biochemistry: for example, it is possible to identify structural defects in various products (rails, welds, etc.) using X-ray radiation. The type of such research is called defectoscopy. And at airports, railway stations and other crowded places, X-ray television introscopes are actively used to scan hand luggage and luggage for security purposes.
Depending on the type of anode, X-ray tubes differ in design. Due to the fact that 99% of the kinetic energy of the electrons is converted into thermal energy, during the operation of the tube, the anode is significantly heated - the sensitive tungsten target often burns out. The anode is cooled in modern x-ray tubes by rotating it. The rotating anode has the shape of a disk, which distributes heat evenly over its entire surface, preventing local overheating of the tungsten target.
The design of X-ray tubes also differs in focus. Focal spot - the section of the anode on which the working X-ray beam is generated. It is subdivided into the real focal spot and the effective focal spot ( rice. 12). Due to the angle of the anode, the effective focal spot is smaller than the real one. Different focal spot sizes are used depending on the size of the image area. The larger the image area, the wider the focal spot must be to cover the entire image area. However, a smaller focal spot produces better image clarity. Therefore, when producing small images, a short filament is used and the electrons are directed to a small area of the anode target, creating a smaller focal spot.
Rice. 9 - x-ray tube with a stationary anode.
Rice. 10 - X-ray tube with a rotating anode.
Rice. 11 - X-ray tube device with a rotating anode.
Rice. 12 is a diagram of the formation of a real and effective focal spot.
Basic properties of X-rays
1. Great penetrating and ionizing ability.
2. Not deflected by electric and magnetic fields.
3. They have a photochemical effect.
4. Cause the glow of substances.
5. Reflection, refraction and diffraction as in visible radiation.
6. Have a biological effect on living cells.
1. Interaction with matter
The wavelength of X-rays is comparable to the size of atoms, so there is no material that could be used to make an X-ray lens. In addition, when X-rays are incident perpendicular to the surface, they are almost not reflected. Despite this, in X-ray optics, methods have been found for constructing optical elements for X-rays. In particular, it turned out that diamond reflects them well.
X-rays can penetrate matter, and different substances absorb them differently. The absorption of x-rays is their most important property in x-ray photography. The intensity of X-rays decreases exponentially depending on the path traveled in the absorbing layer (I = I0e-kd, where d is the layer thickness, the coefficient k is proportional to Z³λ³, Z is the atomic number of the element, λ is the wavelength).
Absorption occurs as a result of photoabsorption (photoelectric effect) and Compton scattering:
Photoabsorption is understood as the process of knocking out an electron from the shell of an atom by a photon, which requires that the photon energy be greater than a certain minimum value. If we consider the probability of the act of absorption depending on the energy of the photon, then when a certain energy is reached, it (probability) increases sharply to its maximum value. For higher energies, the probability continuously decreases. Because of this dependence, it is said that there is an absorption limit. The place of the electron knocked out during the act of absorption is occupied by another electron, while radiation with a lower photon energy is emitted, the so-called. fluorescence process.
An X-ray photon can interact not only with bound electrons, but also with free and weakly bound electrons. There is a scattering of photons on electrons - the so-called. Compton scattering. Depending on the scattering angle, the wavelength of a photon increases by a certain amount and, accordingly, the energy decreases. Compton scattering, compared to photoabsorption, becomes predominant at higher photon energies.
In addition to these processes, there is one more fundamental possibility of absorption - due to the appearance of electron-positron pairs. However, this requires energies greater than 1.022 MeV, which lie outside the above X-ray emission boundary (<250 кэВ). Однако при другом подходе, когда "ренгеновским" называется излучение, возникшее при взаимодействии электрона и ядра или только электронов, такой процесс имеет место быть. Кроме того, очень жесткое рентгеновское излучение с энергией кванта более 1 МэВ, способно вызвать Ядерный фотоэффект.
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2. Biological impact
X-rays are ionizing. It affects the tissues of living organisms and can cause radiation sickness, radiation burns, and malignant tumors. For this reason, protective measures must be taken when working with X-rays. It is believed that the damage is directly proportional to the absorbed dose of radiation. X-ray radiation is a mutagenic factor.
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3. Registration
Luminescence effect. X-rays can cause some substances to glow (fluorescence). This effect is used in medical diagnostics during fluoroscopy (observation of an image on a fluorescent screen) and X-ray photography (radiography). Medical photographic films are usually used in combination with intensifying screens, which include X-ray phosphors, which glow under the action of X-rays and illuminate the light-sensitive photographic emulsion. The method of obtaining a life-size image is called radiography. With fluorography, the image is obtained on a reduced scale. The luminescent substance (scintillator) can be optically connected to an electronic light detector (photomultiplier tube, photodiode, etc.), the resulting device is called a scintillation detector. It allows you to register individual photons and measure their energy, since the energy of a scintillation flash is proportional to the energy of an absorbed photon.
photographic effect. X-rays, as well as ordinary light, are able to directly illuminate the photographic emulsion. However, without the fluorescent layer, this requires 30-100 times the exposure (i.e. dose). This method (known as screenless radiography) has the advantage of sharper images.
In semiconductor detectors, X-rays produce electron-hole pairs in the p-n junction of a diode connected in the blocking direction. In this case, a small current flows, the amplitude of which is proportional to the energy and intensity of the incident X-ray radiation. In the pulsed mode, it is possible to register individual X-ray photons and measure their energy.
Individual X-ray photons can also be registered using gas-filled detectors of ionizing radiation (Geiger counter, proportional chamber, etc.).
Application
With the help of X-rays, it is possible to “enlighten” the human body, as a result of which it is possible to obtain an image of the bones, and in modern instruments, of internal organs (see also X-ray). This uses the fact that the element calcium (Z=20) contained mainly in the bones has an atomic number much larger than the atomic numbers of the elements that make up soft tissues, namely hydrogen (Z=1), carbon (Z=6) , nitrogen (Z=7), oxygen (Z=8). In addition to conventional devices that give a two-dimensional projection of the object under study, there are computed tomographs that allow you to obtain a three-dimensional image of the internal organs.
The detection of defects in products (rails, welds, etc.) using X-rays is called X-ray flaw detection.
In materials science, crystallography, chemistry and biochemistry, X-rays are used to elucidate the structure of substances at the atomic level using X-ray diffraction scattering (X-ray diffraction analysis). A famous example is the determination of the structure of DNA.
In addition, X-rays can be used to determine the chemical composition of a substance. In an electron beam microprobe (or in an electron microscope), the analyzed substance is irradiated with electrons, while the atoms are ionized and emit characteristic x-ray radiation. X-rays can be used instead of electrons. This analytical method is called X-ray fluorescence analysis.
At airports, X-ray television introscopes are actively used, which allow viewing the contents of hand luggage and baggage in order to visually detect dangerous objects on the monitor screen.
X-ray therapy is a section of radiation therapy that covers the theory and practice of the therapeutic use of X-rays generated at an X-ray tube voltage of 20-60 kV and a skin-focal distance of 3-7 cm (short-range radiotherapy) or at a voltage of 180-400 kV and a skin-focal distance 30-150 cm (remote radiotherapy).
X-ray therapy is carried out mainly with superficially located tumors and with some other diseases, including skin diseases (ultrasoft X-rays of Bucca).
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natural x-rays
On Earth, electromagnetic radiation in the X-ray range is formed as a result of ionization of atoms by radiation that occurs during radioactive decay, as a result of the Compton effect of gamma radiation that occurs during nuclear reactions, and also by cosmic radiation. Radioactive decay also leads to direct emission of X-ray quanta if it causes a rearrangement of the electron shell of the decaying atom (for example, during electron capture). X-ray radiation that occurs on other celestial bodies does not reach the Earth's surface, as it is completely absorbed by the atmosphere. It is being explored by satellite X-ray telescopes such as Chandra and XMM-Newton.
X-ray radiation (synonymous with X-rays) is with a wide range of wavelengths (from 8·10 -6 to 10 -12 cm). X-ray radiation occurs when charged particles, most often electrons, decelerate in the electric field of the atoms of a substance. The resulting quanta have different energies and form a continuous spectrum. The maximum photon energy in such a spectrum is equal to the energy of incident electrons. In (see) the maximum energy of X-ray quanta, expressed in kiloelectron-volts, is numerically equal to the magnitude of the voltage applied to the tube, expressed in kilovolts. When passing through a substance, X-rays interact with the electrons of its atoms. For X-ray quanta with energies up to 100 keV, the most characteristic type of interaction is the photoelectric effect. As a result of such an interaction, the quantum energy is completely spent on pulling out an electron from the atomic shell and imparting kinetic energy to it. With an increase in the energy of an X-ray quantum, the probability of the photoelectric effect decreases and the process of scattering of quanta on free electrons becomes predominant - the so-called Compton effect. As a result of such an interaction, a secondary electron is also formed and, in addition, a quantum flies out with an energy lower than the energy of the primary quantum. If the energy of an X-ray quantum exceeds one megaelectron-volt, a so-called pairing effect can occur, in which an electron and a positron are formed (see). Consequently, when passing through a substance, the energy of X-ray radiation decreases, i.e., its intensity decreases. Since low-energy quanta are more likely to be absorbed in this case, X-ray radiation is enriched with higher-energy quanta. This property of X-ray radiation is used to increase the average energy of quanta, i.e., to increase its rigidity. An increase in the hardness of X-ray radiation is achieved using special filters (see). X-ray radiation is used for X-ray diagnostics (see) and (see). See also Ionizing radiation.
X-ray radiation (synonym: x-rays, x-rays) - quantum electromagnetic radiation with a wavelength of 250 to 0.025 A (or energy quanta from 5 10 -2 to 5 10 2 keV). In 1895, it was discovered by V.K. Roentgen. The spectral region of electromagnetic radiation adjacent to x-rays, whose energy quanta exceed 500 keV, is called gamma radiation (see); radiation, whose energy quanta are below 0.05 keV, is ultraviolet radiation (see).
Thus, representing a relatively small part of the vast spectrum of electromagnetic radiation, which includes both radio waves and visible light, X-ray radiation, like any electromagnetic radiation, propagates at the speed of light (about 300 thousand km / s in a vacuum) and is characterized by a wavelength λ ( the distance over which the radiation propagates in one period of oscillation). X-ray radiation also has a number of other wave properties (refraction, interference, diffraction), but it is much more difficult to observe them than for longer-wavelength radiation: visible light, radio waves.
X-ray spectra: a1 - continuous bremsstrahlung spectrum at 310 kV; a - continuous bremsstrahlung spectrum at 250 kV, a1 - spectrum filtered by 1 mm Cu, a2 - spectrum filtered by 2 mm Cu, b - K-series of the tungsten line.
To generate x-rays, x-ray tubes are used (see), in which radiation occurs when fast electrons interact with atoms of the anode substance. There are two types of x-rays: bremsstrahlung and characteristic. Bremsstrahlung X-ray radiation, which has a continuous spectrum, is similar to ordinary white light. The distribution of intensity depending on the wavelength (Fig.) is represented by a curve with a maximum; in the direction of long waves, the curve falls gently, and in the direction of short waves, it steeply and breaks off at a certain wavelength (λ0), called the short-wavelength boundary of the continuous spectrum. The value of λ0 is inversely proportional to the voltage on the tube. Bremsstrahlung arises from the interaction of fast electrons with atomic nuclei. The bremsstrahlung intensity is directly proportional to the strength of the anode current, the square of the tube voltage, and the atomic number (Z) of the anode material.
If the energy of electrons accelerated in the X-ray tube exceeds the critical value for the anode substance (this energy is determined by the tube voltage Vcr, which is critical for this substance), then characteristic radiation occurs. The characteristic spectrum is line, its spectral lines form a series, denoted by the letters K, L, M, N.
The K series is the shortest wavelength, the L series is longer wavelength, the M and N series are observed only in heavy elements (Vcr of tungsten for the K-series is 69.3 kv, for the L-series - 12.1 kv). Characteristic radiation arises as follows. Fast electrons knock atomic electrons out of the inner shells. The atom is excited and then returns to the ground state. In this case, electrons from the outer, less bound shells fill the spaces vacated in the inner shells, and photons of characteristic radiation with an energy equal to the difference between the energies of the atom in the excited and ground states are emitted. This difference (and hence the energy of the photon) has a certain value, characteristic of each element. This phenomenon underlies the X-ray spectral analysis of elements. The figure shows the line spectrum of tungsten against the background of a continuous spectrum of bremsstrahlung.
The energy of electrons accelerated in the X-ray tube is converted almost entirely into thermal energy (the anode is strongly heated in this case), only an insignificant part (about 1% at a voltage close to 100 kV) is converted into bremsstrahlung energy.
The use of x-rays in medicine is based on the laws of absorption of x-rays by matter. The absorption of x-rays is completely independent of the optical properties of the absorber material. The colorless and transparent lead glass used to protect personnel in x-ray rooms absorbs x-rays almost completely. In contrast, a sheet of paper that is not transparent to light does not attenuate X-rays.
The intensity of a homogeneous (i.e., a certain wavelength) X-ray beam, when passing through an absorber layer, decreases according to an exponential law (e-x), where e is the base of natural logarithms (2.718), and the exponent x is equal to the product of the mass attenuation coefficient (μ / p) cm 2 /g per absorber thickness in g / cm 2 (here p is the density of the substance in g / cm 3). X-rays are attenuated by both scattering and absorption. Accordingly, the mass attenuation coefficient is the sum of the mass absorption and scattering coefficients. The mass absorption coefficient increases sharply with increasing atomic number (Z) of the absorber (proportional to Z3 or Z5) and with increasing wavelength (proportional to λ3). This dependence on the wavelength is observed within the absorption bands, at the boundaries of which the coefficient exhibits jumps.
The mass scattering coefficient increases with increasing atomic number of the substance. For λ≥0,3Å the scattering coefficient does not depend on the wavelength, for λ<0,ЗÅ он уменьшается с уменьшением λ.
The decrease in the absorption and scattering coefficients with decreasing wavelength causes an increase in the penetrating power of X-rays. The mass absorption coefficient for bones [absorption is mainly due to Ca 3 (PO 4) 2 ] is almost 70 times greater than for soft tissues, where absorption is mainly due to water. This explains why the shadow of the bones stands out so sharply on the radiographs against the background of soft tissues.
The propagation of an inhomogeneous X-ray beam through any medium, along with a decrease in intensity, is accompanied by a change in the spectral composition, a change in the quality of the radiation: the long-wave part of the spectrum is absorbed to a greater extent than the short-wave part, the radiation becomes more uniform. Filtering out the long-wavelength part of the spectrum makes it possible to improve the ratio between deep and surface doses during X-ray therapy of foci located deep in the human body (see X-ray filters). To characterize the quality of an inhomogeneous X-ray beam, the concept of "half attenuation layer (L)" is used - a layer of a substance that attenuates the radiation by half. The thickness of this layer depends on the voltage on the tube, the thickness and material of the filter. Cellophane (up to an energy of 12 keV), aluminum (20–100 keV), copper (60–300 keV), lead, and copper (>300 keV) are used to measure half attenuation layers. For X-rays generated at voltages of 80-120 kV, 1 mm of copper is equivalent in filtering capacity to 26 mm of aluminum, 1 mm of lead is equivalent to 50.9 mm of aluminum.
Absorption and scattering of X-rays is due to its corpuscular properties; X-rays interact with atoms as a stream of corpuscles (particles) - photons, each of which has a certain energy (inversely proportional to the wavelength of X-rays). The energy range of X-ray photons is 0.05-500 keV.
The absorption of X-ray radiation is due to the photoelectric effect: the absorption of a photon by the electron shell is accompanied by the ejection of an electron. The atom is excited and, returning to the ground state, emits characteristic radiation. The emitted photoelectron carries away all the energy of the photon (minus the binding energy of the electron in the atom).
Scattering of X-ray radiation is due to the electrons of the scattering medium. There are classical scattering (the wavelength of the radiation does not change, but the direction of propagation changes) and scattering with a change in wavelength - the Compton effect (the wavelength of the scattered radiation is greater than the incident one). In the latter case, the photon behaves like a moving ball, and the scattering of photons occurs, according to the figurative expression of Comnton, like a game of billiards with photons and electrons: colliding with an electron, the photon transfers part of its energy to it and scatters, having already less energy (respectively, the wavelength of the scattered radiation increases), the electron flies out of the atom with a recoil energy (these electrons are called Compton electrons, or recoil electrons). The absorption of X-ray energy occurs during the formation of secondary electrons (Compton and photoelectrons) and the transfer of energy to them. The energy of X-rays transferred to a unit mass of a substance determines the absorbed dose of X-rays. The unit of this dose 1 rad corresponds to 100 erg/g. Due to the absorbed energy in the substance of the absorber, a number of secondary processes occur that are important for X-ray dosimetry, since it is on them that X-ray measurement methods are based. (see Dosimetry).
All gases and many liquids, semiconductors and dielectrics, under the action of X-rays, increase electrical conductivity. Conductivity is found by the best insulating materials: paraffin, mica, rubber, amber. The change in conductivity is due to the ionization of the medium, i.e., the separation of neutral molecules into positive and negative ions (ionization is produced by secondary electrons). Ionization in air is used to determine the exposure dose of X-ray radiation (dose in air), which is measured in roentgens (see Ionizing Radiation Doses). At a dose of 1 r, the absorbed dose in air is 0.88 rad.
Under the action of X-rays, as a result of the excitation of the molecules of a substance (and during the recombination of ions), in many cases a visible glow of the substance is excited. At high intensities of X-ray radiation, a visible glow of air, paper, paraffin, etc. is observed (metals are an exception). The highest yield of visible light is given by such crystalline phosphors as Zn·CdS·Ag-phosphorus and others used for screens in fluoroscopy.
Under the action of X-rays, various chemical processes can also take place in a substance: the decomposition of silver halides (a photographic effect used in X-rays), the decomposition of water and aqueous solutions of hydrogen peroxide, a change in the properties of celluloid (clouding and release of camphor), paraffin (clouding and bleaching) .
As a result of complete conversion, all the X-ray energy absorbed by the chemically inert substance is converted into heat. The measurement of very small amounts of heat requires highly sensitive methods, but is the main method for absolute measurements of X-rays.
Secondary biological effects from exposure to x-rays are the basis of medical radiotherapy (see). X-rays, the quanta of which are 6-16 keV (effective wavelengths from 2 to 5 Å), are almost completely absorbed by the skin integument of the tissue of the human body; they are called boundary rays, or sometimes Bucca rays (see Bucca rays). For deep X-ray therapy, hard filtered radiation with effective energy quanta from 100 to 300 keV is used.
The biological effect of x-ray radiation should be taken into account not only in x-ray therapy, but also in x-ray diagnostics, as well as in all other cases of contact with x-rays that require the use of radiation protection (see).
X-ray radiation occurs when electrons moving at high speeds interact with matter. When electrons collide with atoms of any substance, they quickly lose their kinetic energy. In this case, most of it is converted into heat, and a small fraction, usually less than 1%, is converted into X-ray energy. This energy is released in the form of quanta - particles called photons that have energy but have zero rest mass. X-ray photons differ in their energy, which is inversely proportional to their wavelength. With the conventional method of obtaining x-rays, a wide range of wavelengths is obtained, which is called the x-ray spectrum. The spectrum contains pronounced components, as shown in Fig. one.
Rice. one. A CONVENTIONAL X-RAY SPECTRUM consists of a continuous spectrum (continuum) and characteristic lines (sharp peaks). The Kia and Kib lines arise due to the interactions of accelerated electrons with the electrons of the inner K-shell.
The wide "continuum" is called the continuous spectrum or white radiation. The sharp peaks superimposed on it are called characteristic x-ray emission lines. Although the entire spectrum is the result of collisions of electrons with matter, the mechanisms for the appearance of its wide part and lines are different. A substance consists of a large number of atoms, each of which has a nucleus surrounded by electron shells, and each electron in the shell of an atom of a given element occupies a certain discrete energy level. Usually these shells, or energy levels, are denoted by the symbols K, L, M, etc., starting from the shell closest to the nucleus. When an incident electron of sufficiently high energy collides with one of the electrons bound to the atom, it knocks that electron out of its shell. The empty space is occupied by another electron from the shell, which corresponds to a higher energy. This latter gives off excess energy by emitting an X-ray photon. Since the shell electrons have discrete energy values, the resulting X-ray photons also have a discrete spectrum. This corresponds to sharp peaks for certain wavelengths, the specific values of which depend on the target element. The characteristic lines form K-, L- and M-series, depending on which shell (K, L or M) the electron was removed from. The relationship between the wavelength of X-rays and the atomic number is called Moseley's law (Fig. 2).
Rice. 2. The wavelength of the CHARACTERISTIC X-RAY RADIATION emitted by chemical elements depends on the atomic number of the element. The curve corresponds to Moseley's law: the larger the atomic number of the element, the shorter the wavelength of the characteristic line.
If an electron collides with a relatively heavy nucleus, then it slows down, and its kinetic energy is released in the form of an X-ray photon of approximately the same energy. If he flies past the nucleus, he will lose only part of his energy, and the rest will be transferred to other atoms that fall in his way. Each act of energy loss leads to the emission of a photon with some energy. A continuous X-ray spectrum appears, the upper limit of which corresponds to the energy of the fastest electron. This is the mechanism for the formation of a continuous spectrum, and the maximum energy (or minimum wavelength) that fixes the boundary of the continuous spectrum is proportional to the accelerating voltage, which determines the speed of the incident electrons. The spectral lines characterize the material of the bombarded target, while the continuous spectrum is determined by the energy of the electron beam and practically does not depend on the target material.
X-rays can be obtained not only by electron bombardment, but also by irradiating the target with X-rays from another source. In this case, however, most of the energy of the incident beam goes into the characteristic X-ray spectrum, and a very small fraction of it falls into the continuous spectrum. Obviously, the incident X-ray beam must contain photons whose energy is sufficient to excite the characteristic lines of the bombarded element. The high percentage of energy per characteristic spectrum makes this method of X-ray excitation convenient for scientific research.
X-ray tubes. In order to obtain X-ray radiation due to the interaction of electrons with matter, it is necessary to have a source of electrons, means of accelerating them to high speeds, and a target capable of withstanding electron bombardment and producing X-ray radiation of the required intensity. The device that has all this is called an x-ray tube. Early explorers used "deep vacuum" tubes such as today's discharge tubes. The vacuum in them was not very high.
Discharge tubes contain a small amount of gas, and when a large potential difference is applied to the electrodes of the tube, the gas atoms turn into positive and negative ions. The positive ones move towards the negative electrode (cathode) and, falling on it, knock electrons out of it, and they, in turn, move towards the positive electrode (anode) and, bombarding it, create a stream of X-ray photons.
In the modern X-ray tube developed by Coolidge (Fig. 3), the source of electrons is a tungsten cathode heated to a high temperature. The electrons are accelerated to high speeds by the high potential difference between the anode (or anticathode) and the cathode. Since the electrons must reach the anode without colliding with atoms, a very high vacuum is required, for which the tube must be well evacuated. This also reduces the probability of ionization of the remaining gas atoms and the associated side currents.
Rice. 3. X-RAY TUBE COOLIDGE. When bombarded with electrons, the tungsten anticathode emits characteristic x-rays. The cross section of the X-ray beam is less than the actual irradiated area. 1 - electron beam; 2 - cathode with a focusing electrode; 3 - glass shell (tube); 4 - tungsten target (anticathode); 5 - cathode filament; 6 - actually irradiated area; 7 - effective focal spot; 8 - copper anode; 9 - window; 10 - scattered x-rays.
The electrons are focused on the anode by a specially shaped electrode surrounding the cathode. This electrode is called the focusing electrode and, together with the cathode, forms the "electronic spotlight" of the tube. The anode subjected to electron bombardment must be made of a refractory material, since most of the kinetic energy of the bombarding electrons is converted into heat. In addition, it is desirable that the anode be made of a material with a high atomic number, since the x-ray yield increases with increasing atomic number. The most commonly chosen anode material is tungsten, whose atomic number is 74.
The design of X-ray tubes may vary depending on the application and requirements.