X-rays are electromagnetic waves with a wavelength of approximately 80 to 10 -5 nm. The longest-wavelength X-ray radiation is covered by short-wavelength ultraviolet, the short-wavelength - by long-wavelength γ-radiation. According to the method of excitation, X-ray radiation is divided into bremsstrahlung and characteristic.
31.1. DEVICE OF X-RAY TUBE. Bremsstrahlung X-RAY
The most common source of x-rays is the x-ray tube, which is a two-electrode vacuum device (Fig. 31.1). Heated cathode 1 emits electrons 4. Anode 2, often referred to as the anticathode, has an inclined surface in order to direct the resulting X-rays 3 at an angle to the axis of the tube. The anode is made of a highly heat-conducting material to remove the heat generated by the impact of electrons. The anode surface is made of refractory materials having a large atomic number in the periodic table, such as tungsten. In some cases, the anode is specially cooled with water or oil.
For diagnostic tubes, the pinpointness of the X-ray source is important, which can be achieved by focusing electrons in one place of the anticathode. Therefore, constructively, two opposite tasks have to be taken into account: on the one hand, electrons must fall on one place of the anode, on the other hand, in order to prevent overheating, it is desirable to distribute electrons over different parts of the anode. As one of the interesting technical solutions is an X-ray tube with a rotating anode (Fig. 31.2).
As a result of deceleration of an electron (or other charged particle) by the electrostatic field of the atomic nucleus and atomic electrons of the substance of the anticathode, a bremsstrahlung radiation.
Its mechanism can be explained as follows. A moving electric charge is associated with a magnetic field, the induction of which depends on the speed of the electron. When braking, the magnetic
induction and, in accordance with Maxwell's theory, an electromagnetic wave appears.
When electrons decelerate, only part of the energy goes to create an X-ray photon, the other part is spent on heating the anode. Since the ratio between these parts is random, when a large number of electrons decelerate, a continuous spectrum of X-ray radiation is formed. In this regard, bremsstrahlung is also called continuous. On fig. 31.3 shows the dependence of the X-ray flux on the wavelength λ (spectra) at different voltages in the X-ray tube: U 1< U 2 < U 3 .
In each of the spectra, the shortest wavelength bremsstrahlung λ ηίη arises when the energy acquired by an electron in an accelerating field is completely converted into the energy of a photon:
Note that on the basis of (31.2) one of the most accurate methods for the experimental determination of Planck's constant has been developed.
Short-wavelength X-rays usually have a greater penetrating power than long-wavelength ones and are called hard, and longwave soft.
By increasing the voltage on the X-ray tube, the spectral composition of the radiation is changed, as can be seen from Fig. 31.3 and formulas (31.3), and increase the rigidity.
If the cathode filament temperature is increased, then the electron emission and the current in the tube will increase. This will increase the number of X-ray photons emitted every second. Its spectral composition will not change. On fig. 31.4 shows the spectra of bremsstrahlung x-rays at one voltage, but at different strength cathode filament current: / n1< / н2 .
The X-ray flux is calculated by the formula:
Where U And I- voltage and current in the x-ray tube; Z- serial number of an atom of the anode substance; k- coefficient of proportionality. Spectra obtained from different anticathodes at the same U and I H are shown in fig. 31.5.
31.2. CHARACTERISTIC X-RAY RADIATION. ATOMIC X-RAY SPECTRA
By increasing the voltage on the X-ray tube, one can notice the appearance of a line, which corresponds to
characteristic x-rays(Fig. 31.6). It arises due to the fact that accelerated electrons penetrate deep into the atom and knock out electrons from the inner layers. Electrons from upper levels move to free places (Fig. 31.7), as a result, photons of characteristic radiation are emitted. As can be seen from the figure, the characteristic X-ray radiation consists of series K, L, M etc., the name of which served to designate the electronic layers. Since the emission of the K-series frees up space in the higher layers, the lines of other series are simultaneously emitted.
In contrast to optical spectra, the characteristic x-ray spectra of different atoms are of the same type. On fig. 31.8 shows the spectra of various elements. The uniformity of these spectra is due to the fact that the inner layers of different atoms are the same and differ only energetically, since the force effect from the nucleus increases as the element's atomic number increases. This circumstance leads to the fact that the characteristic spectra shift towards higher frequencies with increasing nuclear charge. This pattern is visible from Fig. 31.8 and known as Moseley's law:
Where v- spectral line frequency; Z- atomic number of the emitting element; A And IN- permanent.
There is another difference between optical and x-ray spectra.
The characteristic X-ray spectrum of an atom does not depend on the chemical compound in which this atom is included. For example, the X-ray spectrum of the oxygen atom is the same for O, O 2 and H 2 O, while the optical spectra of these compounds are significantly different. This feature of the x-ray spectrum of the atom was the basis for the name characteristic.
Characteristic radiation always occurs when there is free space in the inner layers of an atom, regardless of the reason that caused it. So, for example, characteristic radiation accompanies one of the types of radioactive decay (see 32.1), which consists in the capture of an electron from the inner layer by the nucleus.
31.3. INTERACTION OF X-RAY RADIATION WITH SUBSTANCE
The registration and use of X-ray radiation, as well as its impact on biological objects, are determined by the primary processes of interaction of an X-ray photon with electrons of atoms and molecules of a substance.
Depending on the ratio of energy hv photon and ionization energy 1 A and there are three main processes.
Coherent (classical) scattering
Scattering of long-wavelength X-rays occurs mainly without a change in wavelength, and is called coherent. It occurs if the photon energy is less than the ionization energy: hv< A and.
Since in this case the energy of the X-ray photon and the atom does not change, coherent scattering in itself does not cause a biological effect. However, when creating protection against X-ray radiation, one should take into account the possibility of changing the direction of the primary beam. This kind of interaction is important for X-ray diffraction analysis (see 24.7).
Incoherent scattering (Compton effect)
In 1922 A.Kh. Compton, observing the scattering of hard X-rays, discovered a decrease in the penetrating power of the scattered beam compared to the incident beam. This meant that the wavelength of the scattered X-rays was greater than that of the incident X-rays. The scattering of X-rays with a change in wavelength is called incoherent nym, and the phenomenon itself - the Compton effect. It occurs if the energy of the X-ray photon is greater than the ionization energy: hv > A and.
This phenomenon is due to the fact that when interacting with an atom, the energy hv photon is spent on the production of a new scattered X-ray photon with energy hv", to the detachment of an electron from an atom (ionization energy A u) and communication to the electron kinetic energy E to:
hv \u003d hv " + A and + E k.(31.6)
1 Here, ionization energy is understood as the energy required to remove internal electrons from an atom or molecule.
Since in many cases hv>> A and and the Compton effect occurs on free electrons, then we can write approximately:
hv = hv"+ E K .(31.7)
It is significant that in this phenomenon (Fig. 31.9), along with secondary X-ray radiation (energy hv" photon) recoil electrons appear (kinetic energy E to electron). Atoms or molecules then become ions.
photoelectric effect
In the photoelectric effect, X-ray radiation is absorbed by an atom, as a result of which an electron flies out, and the atom is ionized (photoionization).
The three main interaction processes discussed above are primary, they lead to subsequent secondary, tertiary, etc. phenomena. For example, ionized atoms can emit a characteristic spectrum, excited atoms can become sources of visible light (X-ray luminescence), etc.
On fig. 31.10 is a diagram of the possible processes that occur when X-ray radiation enters a substance. Several tens of processes similar to the one shown may occur before the energy of the X-ray photon is converted into the energy of molecular thermal motion. As a result, there will be changes in the molecular composition of the substance.
The processes represented by the diagram in fig. 31.10, underlie the phenomena observed under the action of X-rays on matter. Let's list some of them.
X-ray luminescence- the glow of a number of substances under x-ray irradiation. Such a glow of platinum-cyanogen barium allowed Roentgen to discover the rays. This phenomenon is used to create special luminous screens for the purpose of visual observation of x-rays, sometimes to enhance the action of x-rays on a photographic plate.
The chemical action of X-ray radiation is known, for example, the formation of hydrogen peroxide in water. A practically important example is the effect on a photographic plate, which makes it possible to detect such rays.
The ionizing effect is manifested in an increase in electrical conductivity under the influence of X-rays. This property is used
in dosimetry to quantify the effect of this type of radiation.
As a result of many processes, the primary X-ray beam is weakened in accordance with the law (29.3). Let's write it in the form:
I = I0 e-/", (31.8)
Where μ - linear attenuation coefficient. It can be represented as consisting of three terms corresponding to coherent scattering μ κ , incoherent μ ΗΚ and photoeffect μ f:
μ = μ k + μ hk + μ f. (31.9)
The intensity of X-ray radiation is attenuated in proportion to the number of atoms of the substance through which this flow passes. If we compress matter along the axis x, for example, in b times by increasing b times its density, then
31.4. PHYSICAL FOUNDATIONS OF THE APPLICATION OF X-RAY RADIATION IN MEDICINE
One of the most important medical applications of X-rays is the transillumination of internal organs for diagnostic purposes. (X-ray diagnostics).
For diagnostics, photons with an energy of about 60-120 keV are used. At this energy, the mass extinction coefficient is mainly determined by the photoelectric effect. Its value is inversely proportional to the third power of the photon energy (proportional to λ 3), which manifests a large penetrating power of hard radiation, and proportional to the third power of the atomic number of the absorbing substance:
A significant difference in the absorption of x-ray radiation by different tissues allows you to see images of the internal organs of the human body in a shadow projection.
X-ray diagnostics is used in two versions: fluoroscopy the image is viewed on an X-ray luminescent screen, radiography - the image is fixed on the film.
If the organ under study and the surrounding tissues attenuate x-rays approximately equally, then special contrast agents are used. So, for example, filling the stomach and intestines with a mushy mass of barium sulfate, one can see their shadow image.
The brightness of the image on the screen and the exposure time on the film depend on the intensity of the x-rays. If it is used for diagnosis, then the intensity cannot be large so as not to cause unwanted biological consequences. Therefore, there are a number of technical devices that improve the image at low X-ray intensities. An example of such a device is intensifier tubes (see 27.8). In a mass examination of the population, a variant of radiography is widely used - fluorography, in which an image from a large X-ray luminescent screen is recorded on a sensitive small-format film. When shooting, a lens of large aperture is used, the finished pictures are examined on a special magnifier.
An interesting and promising option for radiography is a method called x-ray tomography, and its "machine version" - CT scan.
Let's consider this question.
A plain radiograph covers a large area of the body, with various organs and tissues shading each other. You can avoid this if you periodically move the X-ray tube together (Fig. 31.11) in antiphase RT and film Fp relative to the object About research. The body contains a number of inclusions that are opaque to X-rays; they are shown by circles in the figure. As you can see, x-rays at any position of the x-ray tube (1, 2 etc.) pass through
cutting the same point of the object, which is the center, relative to which the periodic movement is performed RT And Fp. This point, more precisely a small opaque inclusion, is shown by a dark circle. His shadow image moves with fp, occupying successively positions 1, 2 etc. The remaining inclusions in the body (bones, seals, etc.) create on Fp some general background, since x-rays are not permanently obscured by them. By changing the position of the swing center, it is possible to obtain a layer-by-layer X-ray image of the body. Hence the name - tomography(layered recording).
It is possible, using a thin X-ray beam, to screen (instead of Fp), consisting of semiconductor detectors of ionizing radiation (see 32.5), and a computer, to process the shadow x-ray image in tomography. This modern version of tomography (computed or computed x-ray tomography) allows you to get layered images of the body on the screen of a cathode ray tube or on paper with details of less than 2 mm with a difference in x-ray absorption of up to 0.1%. This allows, for example, to distinguish between the gray and white matter of the brain and to see very small tumor formations.
Wilhelm Conrad Roentgen, a scientist from Germany, can rightfully be considered the founder of radiography and the discoverer of key features x-rays.
Then back in 1895, he did not even suspect the breadth of application and popularity of X-radiation discovered by him, although even then they raised a wide resonance in the world of science.
It is unlikely that the inventor could have guessed what benefit or harm the fruit of his activity would bring. But today we will try to find out what effect this kind of radiation has on the human body.
- X-radiation is endowed with a huge penetrating power, but it depends on the wavelength and density of the material that is irradiated;
- under the influence of radiation, some objects begin to glow;
- the x-ray affects living beings;
- thanks to X-rays, some biochemical reactions begin to occur;
- An x-ray beam can take electrons from some atoms and thereby ionize them.
Even the inventor himself was primarily concerned with the question of what exactly the rays he discovered were.
After a series of experimental studies, the scientist found that X-rays are intermediate waves between ultraviolet and gamma radiation, the length of which is 10 -8 cm.
The properties of the X-ray beam, which are listed above, have destructive properties, but this does not prevent them from being used for useful purposes.
So where in modern world can you use x-rays?
- They can be used to study the properties of many molecules and crystalline formations.
- For flaw detection, that is, to check industrial parts and devices for defects.
- In the medical industry and therapeutic research.
Due to the short lengths of the entire range of these waves and their unique properties, the most important application of the radiation discovered by Wilhelm Roentgen became possible.
Since the topic of our article is limited to the impact of X-rays on the human body, which encounters them only when going to the hospital, then we will consider only this branch of application.
The scientist who invented X-rays made them an invaluable gift for the entire population of the Earth, because he did not patent his offspring for further use.
Since World War I, portable X-ray machines have saved hundreds of wounded lives. Today, X-rays have two main applications:
- Diagnosis with it.
X-ray diagnostics is used in various options:
- X-ray or transillumination;
- x-ray or photograph;
- fluorographic study;
- tomography using x-rays.
Now we need to understand how these methods differ from each other:
- The first method assumes that the subject is located between a special screen with a fluorescent property and an X-ray tube. Doctor Based individual characteristics selects the required strength of the rays and receives an image of the bones and internal organs on the screen.
- In the second method, the patient is placed on a special x-ray film in a cassette. In this case, the equipment is placed above the person. This technique allows you to get an image in the negative, but with finer details than with fluoroscopy.
- Mass examinations of the population for lung disease allows for fluorography. At the time of the procedure, the image is transferred from a large monitor to a special film.
- Tomography allows you to get images of internal organs in several sections. A whole series of images are taken, which are hereinafter referred to as a tomogram.
- If you connect the help of a computer to the previous method, then specialized programs will create a complete image made using an x-ray scanner.
All these methods of diagnosing health problems are based on the unique property of X-rays to light up photographic film. At the same time, the penetrating ability of inert and other tissues of our body is different, which is displayed in the picture.
After another property of X-rays to influence tissues from a biological point of view was discovered, this feature began to be actively used in tumor therapy.
Cells, especially malignant ones, divide very quickly, and the ionizing property of radiation has a positive effect on therapeutic therapy and slows down tumor growth.
But the other side of the coin is Negative influence x-rays on cells of the hematopoietic, endocrine and immune systems, which also divide rapidly. As a result of the negative influence of the X-ray, radiation sickness manifests itself.
The effect of x-rays on the human body
Literally immediately after such a loud discovery in the scientific world, it became known that X-rays can affect the human body:
- In the course of research on the properties of X-rays, it turned out that they are capable of causing burns on the skin. Very similar to thermal. However, the depth of the lesion was much greater than domestic injuries, and they healed worse. Many scientists dealing with these insidious radiations have lost their fingers.
- By trial and error, it was found that if you reduce the time and vine of endowment, then burns can be avoided. Later, lead screens and the remote method of irradiating patients began to be used.
- The long-term perspective of the harmfulness of rays shows that changes in the composition of the blood after irradiation leads to leukemia and early aging.
- The degree of severity of the impact of X-rays on the human body directly depends on the irradiated organ. So, with X-rays of the small pelvis, infertility can occur, and with the diagnosis of hematopoietic organs - blood diseases.
- Even the most insignificant exposures, but over a long period of time, can lead to changes at the genetic level.
Of course, all studies were conducted on animals, but scientists have proven that pathological changes will also apply to humans.
IMPORTANT! Based on the obtained data, X-ray exposure standards were developed, which are uniform throughout the world.
Doses of x-rays for diagnosis
Probably, everyone who leaves the doctor's office after an x-ray is wondering how this procedure will affect their future health?
Radiation exposure in nature also exists and we encounter it daily. To make it easier to understand how x-rays affect our body, we compare this procedure with the natural radiation received:
- on a chest x-ray, a person receives a dose of radiation equivalent to 10 days of background exposure, and the stomach or intestines - 3 years;
- tomogram on the computer of the abdominal cavity or the whole body - the equivalent of 3 years of radiation;
- examination on chest x-ray - 3 months;
- limbs are irradiated, practically without harming health;
- dental x-ray due to the precise direction of the beam beam and the minimum exposure time is also not dangerous.
IMPORTANT! Despite the fact that the given data, no matter how frightening they may sound, meet international requirements. However, the patient has every right to ask for additional means of protection in case of strong fear for his well-being.
All of us are faced with x-ray examination, and more than once. However, one category of people outside of the prescribed procedures are pregnant women.
The fact is that X-rays extremely affect the health of the unborn child. These waves can cause intrauterine malformations as a result of the effect on the chromosomes.
IMPORTANT! The most dangerous period for x-rays is pregnancy before 16 weeks. During this period, the most vulnerable are the pelvic, abdominal and vertebral regions of the baby.
Knowing about this negative property of x-rays, doctors all over the world are trying to avoid prescribing it for pregnant women.
But there are other sources of radiation that a pregnant woman may encounter:
- microscopes powered by electricity;
- color TV monitors.
Those who are preparing to become a mother must be aware of the danger that awaits them. During lactation, X-rays do not pose a threat to the body of the nursing and the baby.
What about after the x-ray?
Even the most minor effects of X-ray exposure can be minimized by following a few simple recommendations:
- drink milk immediately after the procedure. As you know, it is able to remove radiation;
- dry white wine or grape juice has the same properties;
- it is desirable at first to eat more foods containing iodine.
IMPORTANT! You should not resort to any medical procedures or use medical methods after visiting the x-ray room.
No matter how negative the properties of the once discovered X-rays, the benefits of their use far outweigh the harm. In medical institutions, the transillumination procedure is carried out quickly and with minimal doses.
Modern medicine uses many physicians for diagnosis and therapy. Some of them have been used relatively recently, while others have been practiced for more than a dozen or even hundreds of years. Also, a hundred and ten years ago, William Conrad Roentgen discovered amazing X-rays, which caused a significant resonance in scientific and medical world. And now doctors all over the planet use them in their practice. The topic of our today's conversation will be X-rays in medicine, we will discuss their application in a little more detail.
X-rays are one of the varieties electromagnetic radiation. They are characterized by significant penetrating qualities, which depend on the wavelength of radiation, as well as on the density and thickness of the irradiated materials. In addition, X-rays can cause the glow of a number of substances, affect living organisms, ionize atoms, and also catalyze some photochemical reactions.
The use of X-rays in medicine
To date, the properties of x-rays allow them to be widely used in x-ray diagnostics and x-ray therapy.
X-ray diagnostics
X-ray diagnostics is used when carrying out:
X-ray (transmission);
- radiography (picture);
- fluorography;
- X-ray and computed tomography.
Fluoroscopy
To conduct such a study, the patient needs to position himself between the X-ray tube and a special fluorescent screen. A specialist radiologist selects the required hardness of the X-rays, receiving on the screen a picture of the internal organs, as well as the ribs.
Radiography
For this study The patient is placed on a cassette containing a special film. The X-ray machine is placed directly above the object. As a result, a negative image of the internal organs appears on the film, which contains a number of fine details, more detailed than during a fluoroscopic examination.
Fluorography
This study is carried out during mass medical examinations of the population, including for the detection of tuberculosis. At the same time, a picture from a large screen is projected onto a special film.
Tomography
When conducting tomography, computer beams help to obtain images of organs in several places at once: in specially selected transverse sections of tissue. This series of x-rays is called a tomogram.
Computed tomogram
Such a study allows you to register sections of the human body by using an X-ray scanner. After the data is entered into the computer, getting one picture in cross section.
Each of the listed diagnostic methods is based on the properties of the X-ray beam to illuminate the film, as well as on the fact that human tissues and bone skeleton differ in different permeability to their effects.
X-ray therapy
The ability of X-rays to influence tissues in a special way is used to treat tumor formations. At the same time, the ionizing qualities of this radiation are especially actively noticeable when exposed to cells that are capable of rapid division. It is these qualities that distinguish the cells of malignant oncological formations.
However, it is worth noting that X-ray therapy can cause a lot of serious side effects. Such an impact aggressively affects the state of the hematopoietic, endocrine and immune systems, the cells of which also divide very quickly. Aggressive influence on them can cause signs of radiation sickness.
The effect of X-ray radiation on humans
During the study of x-rays, doctors found that they can lead to changes in the skin that resemble sunburn, however, are accompanied by deeper skin lesions. Such ulcers heal for a very long time. Scientists have found that such lesions can be avoided by reducing the time and dose of radiation, as well as using special shielding and methods. remote control.
The aggressive influence of X-rays can also manifest itself in the long term: temporary or permanent changes in the composition of the blood, susceptibility to leukemia and early aging.
The effect of x-rays on a person depends on many factors: on which organ is irradiated, and for how long. Irradiation of the hematopoietic organs can lead to blood ailments, and exposure to the genital organs can lead to infertility.
Carrying out systematic irradiation is fraught with the development of genetic changes in the body.
The real harm of x-rays in x-ray diagnostics
During the examination, doctors use the minimum possible amount of x-rays. All radiation doses meet certain acceptable standards and cannot harm a person. X-ray diagnostics poses a significant danger only for the doctors who carry it out. And then modern methods protections help to reduce the aggression of the rays to a minimum.
The safest methods of radiodiagnosis include radiography of the extremities, as well as dental x-rays. In the next place of this rating is mammography, followed by computed tomography, and after it is radiography.
In order for the use of X-rays in medicine to bring only benefit to a person, it is necessary to conduct research with their help only according to indications.
X-rays play one of the most important roles in the study and practical use of atomic phenomena. Thanks to their research, many discoveries were made and methods for analyzing substances were developed, which are used in various fields. Here we will consider one of the types of X-rays - characteristic X-rays.
Nature and properties of X-rays
x-ray radiation is a high-frequency change in the state of the electrical magnetic field propagating in space at a speed of about 300,000 km / s, that is, electromagnetic waves. On the scale of the range of electromagnetic radiation, X-rays are located in the wavelength range from approximately 10 -8 to 5∙10 -12 meters, which is several orders of magnitude shorter than optical waves. This corresponds to frequencies from 3∙10 16 to 6∙10 19 Hz and energies from 10 eV to 250 keV, or 1.6∙10 -18 to 4∙10 -14 J. It should be noted that the boundaries of the frequency ranges of electromagnetic radiation are rather conventional due to their overlap.
Is the interaction of accelerated charged particles (high-energy electrons) with electric and magnetic fields and with atoms of matter.
X-ray photons are characterized by high energies and high penetrating and ionizing power, especially for hard X-rays with wavelengths less than 1 nanometer (10 -9 m).
X-rays interact with matter, ionizing its atoms, in the processes of the photoelectric effect (photoabsorption) and incoherent (Compton) scattering. In photoabsorption, an X-ray photon, being absorbed by an electron of an atom, transfers energy to it. If its value exceeds the binding energy of an electron in an atom, then it leaves the atom. Compton scattering is characteristic of harder (energetic) X-ray photons. Part of the energy of the absorbed photon is spent on ionization; in this case, at a certain angle to the direction of the primary photon, a secondary one is emitted, with a lower frequency.
Types of X-ray radiation. Bremsstrahlung
To obtain rays, glass vacuum bottles with electrodes located inside are used. The potential difference across the electrodes needs to be very high - up to hundreds of kilovolts. On a tungsten cathode heated by current, thermionic emission occurs, that is, electrons are emitted from it, which, accelerated by the potential difference, bombard the anode. As a result of their interaction with the atoms of the anode (sometimes called the anticathode), X-ray photons are born.
Depending on what process leads to the birth of a photon, there are such types of X-ray radiation as bremsstrahlung and characteristic.
Electrons can, meeting with the anode, slow down, that is, lose energy in electric fields its atoms. This energy is emitted in the form of X-ray photons. Such radiation is called bremsstrahlung.
It is clear that the braking conditions will differ for individual electrons. This means that different amounts of their kinetic energy are converted into X-rays. As a result, bremsstrahlung includes photons of different frequencies and, accordingly, wavelengths. Therefore, its spectrum is continuous (continuous). Sometimes for this reason it is also called "white" X-rays.
The energy of the bremsstrahlung photon cannot exceed the kinetic energy of the electron that generates it, so that the maximum frequency (and smallest wavelength) of bremsstrahlung corresponds to highest value kinetic energy of electrons incident on the anode. The latter depends on the potential difference applied to the electrodes.
There is another type of X-ray that comes from a different process. This radiation is called characteristic, and we will dwell on it in more detail.
How characteristic X-rays are produced
Having reached the anticathode, a fast electron can penetrate inside the atom and knock out any electron from one of the lower orbitals, that is, transfer to it energy sufficient to overcome the potential barrier. However, if there are higher energy levels occupied by electrons in the atom, the vacated place will not remain empty.
It must be remembered that the electronic structure of the atom, like any energy system, seeks to minimize energy. The vacancy formed as a result of the knockout is filled with an electron from one of the higher levels. Its energy is higher, and, occupying a lower level, it radiates a surplus in the form of a quantum of characteristic X-ray radiation.
The electronic structure of an atom is a discrete set of possible energy states of electrons. Therefore, X-ray photons emitted during the replacement of electron vacancies can also have only strictly defined energy values, reflecting the level difference. As a result, the characteristic X-ray radiation has a spectrum not of a continuous, but of a line type. Such a spectrum makes it possible to characterize the substance of the anode - hence the name of these rays. It is precisely because of the spectral differences that it is clear what is meant by bremsstrahlung and characteristic X-rays.
Sometimes the excess energy is not emitted by the atom, but is spent on knocking out the third electron. This process - the so-called Auger effect - is more likely to occur when the electron binding energy does not exceed 1 keV. The energy of the released Auger electron depends on the structure of the energy levels of the atom, so the spectra of such electrons are also discrete.
General view of the characteristic spectrum
Narrow characteristic lines are present in the X-ray spectral pattern along with a continuous bremsstrahlung spectrum. If we represent the spectrum as a plot of intensity versus wavelength (frequency), we will see sharp peaks at the locations of the lines. Their position depends on the anode material. These maxima are present at any potential difference - if there are X-rays, there are always peaks too. With increasing voltage at the electrodes of the tube, the intensity of both continuous and characteristic X-ray radiation increases, but the location of the peaks and the ratio of their intensities does not change.
The peaks in the X-ray spectra have the same shape regardless of the material of the anti-cathode irradiated by electrons, but for different materials they are located at different frequencies, uniting in series according to the proximity of the frequency values. Between the series themselves, the difference in frequencies is much more significant. The shape of the maxima does not depend in any way on whether the anode material represents a pure chemical element or whether it is a complex substance. In the latter case, the characteristic X-ray spectra of its constituent elements are simply superimposed on each other.
With an increase in the atomic number of a chemical element, all lines of its X-ray spectrum are shifted towards increasing frequency. The spectrum retains its form.
Moseley's law
The phenomenon of spectral shift of characteristic lines was experimentally discovered English physicist Henry Moseley in 1913 This allowed him to associate the frequencies of the spectrum maxima with serial numbers chemical elements. Thus, the wavelength of the characteristic X-ray radiation, as it turned out, can be clearly correlated with a particular element. In general, Moseley's law can be written as follows: √f = (Z - S n)/n√R, where f is the frequency, Z is the element's ordinal number, S n is the screening constant, n is the principal quantum number, and R is the constant Rydberg. This relationship is linear and appears on the Moseley diagram as a series of straight lines for each value of n.
The values of n correspond to individual series of characteristic X-ray peaks. Moseley's law allows one to determine the serial number of a chemical element irradiated by hard electrons from the measured wavelengths (they are uniquely related to the frequencies) of the X-ray spectrum maxima.
The structure of the electron shells of chemical elements is identical. This is indicated by the monotonicity of the shift change in the characteristic spectrum of X-ray radiation. The frequency shift reflects not structural, but energy differences between electron shells, unique for each element.
The role of Moseley's law in atomic physics
There are small deviations from the strict linear relationship expressed by Moseley's law. They are connected, firstly, with the peculiarities of the filling order of the electron shells in some elements, and, secondly, with the relativistic effects of the motion of electrons in heavy atoms. In addition, when the number of neutrons in the nucleus changes (the so-called isotopic shift), the position of the lines can change slightly. This effect made it possible to study the atomic structure in detail.
The significance of Moseley's law is extremely great. Sequentially applying it to elements periodic system Mendeleev established a pattern of increasing the serial number according to each small shift in the characteristic maxima. This helped clarify the issue of physical sense the ordinal number of the elements. The Z value is not just a number: it is positive electric charge nucleus, which is the sum of unit positive charges of the particles included in its composition. The correct placement of elements in the table and the presence of empty positions in it (then they still existed) received powerful confirmation. The validity of the periodic law was proved.
Moseley's law, in addition, became the basis on which a whole area of experimental research arose - X-ray spectrometry.
The structure of the electron shells of the atom
Let us briefly recall how the electronic structure is arranged. It consists of shells, denoted by the letters K, L, M, N, O, P, Q, or numbers from 1 to 7. Electrons within the shell are characterized by the same main quantum number n, which determines the possible energy values. In outer shells, the energy of electrons is higher, and the ionization potential for outer electrons is correspondingly lower.
The shell includes one or more sublevels: s, p, d, f, g, h, i. In each shell, the number of sublevels increases by one compared to the previous one. The number of electrons in each sublevel and in each shell cannot exceed a certain value. They are characterized, in addition to the main quantum number, by the same value of the orbital electron cloud that determines the shape. Sublevels are labeled with the shell they belong to, such as 2s, 4d, and so on.
The sublevel contains which are set, in addition to the main and orbital, by one more quantum number - magnetic, which determines the projection of the electron's orbital momentum onto the direction of the magnetic field. One orbital can have no more than two electrons, differing in the value of the fourth quantum number - spin.
Let us consider in more detail how characteristic X-ray radiation arises. Since the origin of this type of electromagnetic emission is associated with phenomena occurring inside the atom, it is most convenient to describe it precisely in the approximation of electronic configurations.
The mechanism of generation of characteristic X-rays
So, the cause of this radiation is the formation of electron vacancies in the inner shells, due to the penetration of high-energy electrons deep into the atom. The probability that a hard electron will interact increases with the density of the electron clouds. Therefore, collisions are most likely within densely packed inner shells, such as the lowest K-shell. Here the atom is ionized, and a vacancy is formed in the 1s shell.
This vacancy is filled by an electron from the shell with a higher energy, the excess of which is carried away by the X-ray photon. This electron can "fall" from the second shell L, from the third shell M and so on. This is how the characteristic series is formed, in this example, the K-series. An indication of where the electron filling the vacancy comes from is given in the form of a Greek index when designating the series. "Alpha" means that it comes from the L-shell, "beta" - from the M-shell. At present, there is a tendency to replace the Greek letter indices with the Latin ones adopted to designate shells.
The intensity of the alpha line in the series is always the highest, which means that the probability of filling a vacancy from a neighboring shell is the highest.
Now we can answer the question, what is the maximum energy of the characteristic x-ray quantum. It is determined by the difference in the energy values of the levels between which the electron transition occurs, according to the formula E \u003d E n 2 - E n 1, where E n 2 and E n 1 are the energies of the electronic states between which the transition occurred. The highest value of this parameter is given by K-series transitions with maximum high levels atoms of heavy elements. But the intensity of these lines (peak heights) is the smallest, since they are the least likely.
If, due to insufficient voltage on the electrodes, a hard electron cannot reach the K-level, it forms a vacancy at the L-level, and a less energetic L-series with longer wavelengths is formed. Subsequent series are born in a similar way.
In addition, when a vacancy is filled, a new vacancy appears in the overlying shell as a result of an electronic transition. This creates the conditions for generating the next series. Electronic vacancies move higher from level to level, and the atom emits a cascade of characteristic spectral series, while remaining ionized.
Fine structure of characteristic spectra
Atomic X-ray spectra of characteristic X-ray radiation are characterized by a fine structure, which is expressed, as in optical spectra, in line splitting.
The fine structure is due to the fact that the energy level - the electron shell - is a set of closely spaced components - subshells. To characterize the subshells, one more, internal quantum number j is introduced, which reflects the interaction of the intrinsic and orbital magnetic moments of the electron.
Due to the influence of the spin-orbit interaction, the energy structure of the atom becomes more complicated, and as a result, the characteristic X-ray radiation has a spectrum that is characterized by split lines with very closely spaced elements.
Fine structure elements are usually denoted by additional digital indices.
The characteristic X-ray radiation has a feature that is reflected only in the fine structure of the spectrum. The transition of an electron to the lowest energy level does not occur from the lower subshell of the overlying level. Such an event has a negligible probability.
The use of X-rays in spectrometry
This radiation, due to its features described by Moseley's law, underlies various X-ray spectral methods for the analysis of substances. When analyzing the X-ray spectrum, either diffraction of radiation by crystals (wave-dispersive method) or detectors sensitive to the energy of absorbed X-ray photons (energy-dispersive method) are used. Most electron microscopes are equipped with some form of X-ray spectrometry attachment.
Wave-dispersive spectrometry is characterized by especially high accuracy. With the help of special filters, the most intense peaks in the spectrum are selected, thanks to which it is possible to obtain almost monochromatic radiation with a precisely known frequency. The anode material is chosen very carefully to ensure that a monochromatic beam of the desired frequency is obtained. Its diffraction to crystal lattice of the studied substance makes it possible to study the structure of the lattice with great accuracy. This method is also used in the study of DNA and other complex molecules.
One of the features of the characteristic X-ray radiation is also taken into account in gamma spectrometry. This is the high intensity of the characteristic peaks. Gamma spectrometers use lead shielding against external background radiation that interferes with measurements. But lead, absorbing gamma quanta, experiences internal ionization, as a result of which it actively emits in the X-ray range. Additional cadmium shielding is used to absorb the intense peaks of the characteristic x-ray radiation from lead. It, in turn, is ionized and also emits X-rays. To neutralize the characteristic peaks of cadmium, a third shielding layer is used - copper, the X-ray maxima of which lie outside the operating frequency range of the gamma spectrometer.
Spectrometry uses both bremsstrahlung and characteristic X-rays. Thus, in the analysis of substances, the absorption spectra of continuous X-rays by various substances are studied.
X-rays are a type of high-energy electromagnetic radiation. It is actively used in various branches of medicine.
X-rays are electromagnetic waves whose photon energy on the scale of electromagnetic waves is between ultraviolet radiation and gamma radiation (from ~10 eV to ~1 MeV), which corresponds to wavelengths from ~10^3 to ~10^−2 angstroms ( from ~10^−7 to ~10^−12 m). That is, it is incomparably harder radiation than visible light, which is on this scale between ultraviolet and infrared (“thermal”) rays.
The boundary between X-rays and gamma radiation is distinguished conditionally: their ranges intersect, gamma rays can have an energy of 1 keV. They differ in origin: gamma rays are emitted during processes occurring in atomic nuclei, while X-rays are emitted during processes involving electrons (both free and those in the electron shells of atoms). At the same time, it is impossible to determine from the photon itself during which process it arose, that is, the division into the X-ray and gamma ranges is largely arbitrary.
The X-ray range is divided into “soft X-ray” and “hard”. The boundary between them lies at the wavelength level of 2 angstroms and 6 keV of energy.
The X-ray generator is a tube in which a vacuum is created. There are electrodes - a cathode, to which a negative charge is applied, and a positively charged anode. The voltage between them is tens to hundreds of kilovolts. The generation of X-ray photons occurs when electrons “break off” from the cathode and crash into the anode surface at high speed. The resulting X-ray radiation is called “bremsstrahlung”, its photons have different wavelengths.
At the same time, photons of the characteristic spectrum are generated. Part of the electrons in the atoms of the anode substance is excited, that is, it goes to higher orbits, and then returns to its normal state, emitting photons of a certain wavelength. Both types of X-rays are produced in a standard generator.
Discovery history
On November 8, 1895, the German scientist Wilhelm Konrad Roentgen discovered that some substances, under the influence of "cathode rays", that is, the flow of electrons generated by a cathode ray tube, begin to glow. He explained this phenomenon by the influence of certain X-rays - so (“X-rays”) this radiation is now called in many languages. Later V.K. Roentgen studied the phenomenon he had discovered. On December 22, 1895, he gave a lecture on this topic at the University of Würzburg.
Later it turned out that X-ray radiation had been observed before, but then the phenomena associated with it were not given much importance. The cathode ray tube was invented a long time ago, but before V.K. X-ray, no one paid much attention to the blackening of photographic plates near it, etc. phenomena. The danger posed by penetrating radiation was also unknown.
Types and their effect on the body
“X-ray” is the mildest type of penetrating radiation. Overexposure to soft x-rays is similar to ultraviolet exposure, but in a more severe form. A burn forms on the skin, but the lesion is deeper, and it heals much more slowly.
Hard X-ray is a full-fledged ionizing radiation that can lead to radiation sickness. X-ray quanta can break the protein molecules that make up the tissues of the human body, as well as the DNA molecules of the genome. But even if an X-ray quantum breaks a water molecule, it doesn't matter: in this case, chemically active free radicals H and OH are formed, which themselves are able to act on proteins and DNA. Radiation sickness proceeds in a more severe form, the more the hematopoietic organs are affected.
X-rays have mutagenic and carcinogenic activity. This means that the probability of spontaneous mutations in cells during irradiation increases, and sometimes healthy cells can degenerate into cancerous ones. Increasing the likelihood of malignant tumors is a standard consequence of any exposure, including x-rays. X-rays are the least dangerous type of penetrating radiation, but they can still be dangerous.
X-ray radiation: application and how it works
X-ray radiation is used in medicine, as well as in other areas of human activity.
Fluoroscopy and computed tomography
The most common application of X-rays is fluoroscopy. "Transillumination" of the human body allows you to get a detailed image of both the bones (they are most clearly visible) and images of the internal organs.
Different transparency of body tissues in x-rays is associated with their chemical composition. Features of the structure of bones is that they contain a lot of calcium and phosphorus. Other tissues are composed mainly of carbon, hydrogen, oxygen and nitrogen. The phosphorus atom exceeds the weight of the oxygen atom almost twice, and the calcium atom - 2.5 times (carbon, nitrogen and hydrogen are even lighter than oxygen). In this regard, the absorption of X-ray photons in the bones is much higher.
In addition to two-dimensional “pictures”, radiography makes it possible to create a three-dimensional image of an organ: this type of radiography is called computed tomography. For these purposes, soft x-rays are used. The amount of exposure received in a single image is small: it is approximately equal to the exposure received during a 2-hour flight in an airplane at an altitude of 10 km.
X-ray flaw detection allows you to detect small internal defects in products. Hard x-rays are used for it, since many materials (metal, for example) are poorly “translucent” due to the high atomic mass of their constituent substance.
X-ray diffraction and X-ray fluorescence analysis
X-rays have properties that allow them to examine individual atoms in detail. X-ray diffraction analysis is actively used in chemistry (including biochemistry) and crystallography. The principle of its operation is the diffraction scattering of X-rays by atoms of crystals or complex molecules. Using X-ray diffraction analysis, the structure of the DNA molecule was determined.
X-ray fluorescence analysis allows you to quickly determine chemical composition substances.
There are many forms of radiotherapy, but they all involve the use of ionizing radiation. Radiotherapy is divided into 2 types: corpuscular and wave. Corpuscular uses flows of alpha particles (nuclei of helium atoms), beta particles (electrons), neutrons, protons, heavy ions. Wave uses rays of the electromagnetic spectrum - x-rays and gamma.
Radiotherapy methods are used primarily for the treatment of oncological diseases. The fact is that radiation primarily affects actively dividing cells, which is why the hematopoietic organs suffer this way (their cells are constantly dividing, producing more and more new red blood cells). Cancer cells are also constantly dividing and are more vulnerable to radiation than healthy tissue.
A level of radiation is used that suppresses the activity of cancer cells, while moderately affecting healthy ones. Under the influence of radiation, it is not the destruction of cells as such, but the damage to their genome - DNA molecules. A cell with a destroyed genome may exist for some time, but can no longer divide, that is, tumor growth stops.
Radiation therapy is the mildest form of radiotherapy. Wave radiation is softer than corpuscular radiation, and X-rays are softer than gamma radiation.
During pregnancy
It is dangerous to use ionizing radiation during pregnancy. X-rays are mutagenic and can cause abnormalities in the fetus. X-ray therapy is incompatible with pregnancy: it can only be used if it has already been decided to have an abortion. Restrictions on fluoroscopy are softer, but in the first months it is also strictly prohibited.
In case of emergency, X-ray examination is replaced by magnetic resonance imaging. But in the first trimester they try to avoid it too (this method has appeared recently, and with absolute certainty to speak about the absence of harmful consequences).
An unequivocal danger arises when exposed to a total dose of at least 1 mSv (in old units - 100 mR). With a simple x-ray (for example, when undergoing fluorography), the patient receives about 50 times less. In order to receive such a dose at a time, you need to undergo a detailed computed tomography.
That is, the mere fact of a 1-2-fold “X-ray” at an early stage of pregnancy does not threaten with serious consequences (but it’s better not to risk it).
Treatment with it
X-rays are used primarily in the fight against malignant tumors. This method is good because it is highly effective: it kills the tumor. It is bad because healthy tissues are not much better, there are numerous side effects. The organs of hematopoiesis are at particular risk.
In practice, apply various methods to reduce the impact of x-rays on healthy tissue. The beams are directed at an angle in such a way that a tumor appears in the zone of their intersection (due to this, the main absorption of energy occurs just there). Sometimes the procedure is performed in motion: the patient's body rotates relative to the radiation source around an axis passing through the tumor. At the same time, healthy tissues are in the irradiation zone only sometimes, and the sick - all the time.
X-rays are used in the treatment of certain arthrosis and similar diseases, as well as skin diseases. In this case, the pain syndrome is reduced by 50-90%. Since the radiation is used in this case is softer, side effects similar to those that occur in the treatment of tumors are not observed.
