What are X-rays - properties and applications of radiation. Lecture X-rays The essence of X-rays

X-rays were discovered by accident in 1895 by the famous German physicist Wilhelm Roentgen. He studied cathode rays in a low-pressure gas-discharge tube at high voltage between its electrodes. Despite the fact that the tube was in a black box, Roentgen noticed that a fluorescent screen, which happened to be nearby, glowed every time the tube was in use. The tube turned out to be a source of radiation that could penetrate paper, wood, glass and even a one and a half centimeter thick aluminum plate.

X-ray determined that the gas-discharge tube was a source of a new type of invisible radiation with great penetrating power. The scientist could not determine whether this radiation was a stream of particles or waves, and he decided to give it the name X-rays. They were later called X-rays

It is now known that X-rays are a type electromagnetic radiation, having a shorter wavelength than ultraviolet electromagnetic waves. The wavelength of X rays ranges from 70 nm up to 10 -5 nm. The shorter the wavelength of X-rays, the greater the energy of their photons and the greater their penetrating power. X-rays with a relatively long wavelength (more than 10 nm), are called soft. Wavelength 1 - 10 nm characterizes hard X-rays. They have enormous penetrating power.

Receiving X-rays

X-rays are produced when fast electrons, or cathode rays, collide with the walls or anode of a low-pressure gas discharge tube. A modern X-ray tube is a evacuated glass cylinder with a cathode and anode located in it. The potential difference between the cathode and anode (anti-cathode) reaches several hundred kilovolts. The cathode is a tungsten filament heated by electric current. This causes the cathode to emit electrons as a result of thermionic emission. The electrons are accelerated by the electric field in the X-ray tube. Since there is a very small number of gas molecules in the tube, the electrons practically do not lose their energy on the way to the anode. They reach the anode at a very high speed.

X-rays are produced whenever electrons moving at high speed are slowed down by the anode material. Most of the electrons' energy is dissipated as heat. Therefore, the anode must be artificially cooled. The anode in the X-ray tube must be made of a metal that has a high melting point, such as tungsten.

The part of the energy that is not dissipated in the form of heat is converted into the energy of electromagnetic waves (X-rays). Thus, X-rays are the result of electron bombardment of the anode substance. There are two types x-ray radiation: inhibitory and characteristic.

Bremsstrahlung X-rays

Bremsstrahlung X-ray radiation occurs when electrons moving at high speed are slowed down by the electric fields of the anode atoms. The conditions for stopping individual electrons are not the same. As a result, various parts of their kinetic energy are converted into X-ray energy.

The spectrum of X-ray bremsstrahlung does not depend on the nature of the anode substance. As is known, the energy of X-ray photons determines their frequency and wavelength. Therefore, X-ray bremsstrahlung is not monochromatic. It is characterized by a variety of wavelengths that can be represented continuous (continuous) spectrum.

X-rays cannot have an energy greater than the kinetic energy of the electrons that form them. The shortest wavelength of X-ray radiation corresponds to the maximum kinetic energy of decelerating electrons. The greater the potential difference in the X-ray tube, the shorter the wavelengths of X-ray radiation can be obtained.

Characteristic X-ray radiation

The characteristic X-ray radiation is not continuous, but line spectrum. This type of radiation occurs when a fast electron, reaching the anode, penetrates the inner orbitals of atoms and knocks out one of their electrons. As a result, a free space appears that can be filled by another electron descending from one of the upper atomic orbitals. This transition of an electron from a higher to a lower energy level produces x-rays of a specific discrete wavelength. Therefore, the characteristic X-ray radiation has line spectrum. The frequency of the characteristic radiation lines completely depends on the structure of the electron orbitals of the anode atoms.

Spectral lines of characteristic radiation of different chemical elements have the same appearance, since the structure of their internal electron orbitals is identical. But their wavelength and frequency are due to energy differences between the internal orbitals of heavy and light atoms.

The frequency of the lines in the spectrum of characteristic X-ray radiation changes in accordance with the atomic number of the metal and is determined by the Moseley equation: v 1/2 = A(Z-B), Where Z- atomic number of a chemical element, A And B- constants.

Primary physical mechanisms of interaction of X-ray radiation with matter

The primary interaction between X-rays and matter is characterized by three mechanisms:

1. Coherent scattering. This form of interaction occurs when the X-ray photons have less energy than the binding energy of the electrons to the atomic nucleus. In this case, the photon energy is not sufficient to release electrons from the atoms of the substance. The photon is not absorbed by the atom, but changes the direction of propagation. In this case, the wavelength of X-ray radiation remains unchanged.

2. Photoelectric effect (photoelectric effect). When an X-ray photon reaches an atom of a substance, it can knock out one of the electrons. This occurs if the photon energy exceeds the binding energy of the electron with the nucleus. In this case, the photon is absorbed and the electron is released from the atom. If a photon carries more energy than is needed to release an electron, it will transfer the remaining energy to the released electron in the form of kinetic energy. This phenomenon, called the photoelectric effect, occurs when relatively low-energy X-rays are absorbed.

An atom that loses one of its electrons becomes a positive ion. The lifetime of free electrons is very short. They are absorbed by neutral atoms, which turn into negative ions. The result of the photoelectric effect is intense ionization of the substance.

If the energy of the X-ray photon is less than the ionization energy of the atoms, then the atoms go into an excited state, but are not ionized.

3. Incoherent scattering (Compton effect). This effect was discovered by the American physicist Compton. It occurs when a substance absorbs X-rays of short wavelength. The photon energy of such X-rays is always greater than the ionization energy of the atoms of the substance. The Compton effect results from the interaction of a high-energy X-ray photon with one of the electrons in the outer shell of an atom, which has a relatively weak connection with the atomic nucleus.

A high-energy photon transfers some of its energy to the electron. The excited electron is released from the atom. The remaining energy from the original photon is emitted as an x-ray photon of longer wavelength at some angle to the direction of motion of the original photon. The secondary photon can ionize another atom, etc. These changes in the direction and wavelength of X-rays are known as the Compton effect.

Some effects of interaction of X-rays with matter

As mentioned above, X-rays are capable of exciting atoms and molecules of matter. This may cause certain substances (such as zinc sulfate) to fluoresce. If a parallel beam of X-rays is directed at opaque objects, you can observe how the rays pass through the object by placing a screen covered with a fluorescent substance.

The fluorescent screen can be replaced with photographic film. X-rays have the same effect on photographic emulsion as light. Both methods are used in practical medicine.

Another important effect of X-rays is their ionizing ability. This depends on their wavelength and energy. This effect provides a method for measuring the intensity of x-rays. When X-rays pass through the ionization chamber, an electric current is generated, the magnitude of which is proportional to the intensity of the X-ray radiation.

Absorption of X-rays by matter

As X-rays pass through matter, their energy decreases due to absorption and scattering. The attenuation of the intensity of a parallel beam of X-rays passing through a substance is determined by Bouguer’s law: I = I0 e -μd, Where I 0- initial intensity of X-ray radiation; I- intensity of X-rays passing through the layer of matter, d- absorbent layer thickness , μ - linear attenuation coefficient. He equal to the sum two quantities: t- linear absorption coefficient and σ - linear dissipation coefficient: μ = τ+ σ

Experiments have revealed that the linear absorption coefficient depends on the atomic number of the substance and the wavelength of the X-rays:

τ = kρZ 3 λ 3, Where k- coefficient of direct proportionality, ρ - density of the substance, Z- atomic number of the element, λ - wavelength of x-rays.

The dependence on Z is very important from a practical point of view. For example, the absorption coefficient of bones, which are composed of calcium phosphate, is almost 150 times higher than that of soft tissue ( Z=20 for calcium and Z=15 for phosphorus). When X-rays pass through the human body, bones stand out clearly against the background of muscles, connective tissue and so on.

It is known that the digestive organs have the same absorption coefficient as other soft tissues. But the shadow of the esophagus, stomach and intestines can be distinguished if the patient takes a contrast agent - barium sulfate ( Z= 56 for barium). Barium sulfate is very opaque to x-rays and is often used for x-ray examination of the gastrointestinal tract. Certain opaque mixtures are injected into the bloodstream in order to examine the condition of blood vessels, kidneys, etc. In this case, iodine, whose atomic number is 53, is used as a contrast agent.

Dependence of X-ray absorption on Z also used to protect against the possible harmful effects of x-rays. Lead is used for this purpose, the amount Z for which it is equal to 82.

Application of X-rays in medicine

The reason for the use of x-rays in diagnostics was their high penetrating ability, one of the main properties of x-ray radiation. In the early days after its discovery, X-rays were used mostly to examine bone fractures and determine the location of foreign bodies (such as bullets) in the human body. Currently, several diagnostic methods using x-rays (x-ray diagnostics) are used.

X-ray . An X-ray device consists of an X-ray source (X-ray tube) and a fluorescent screen. After X-rays pass through the patient's body, the doctor observes a shadow image of him. A lead window should be installed between the screen and the physician's eyes to protect the physician from the harmful effects of X-rays. This method makes it possible to study the functional state of certain organs. For example, the doctor can directly observe the movements of the lungs and the passage of the contrast agent through the gastrointestinal tract. The disadvantages of this method are insufficient contrast images and relatively large doses of radiation received by the patient during the procedure.

Fluorography . This method consists of taking a photograph of a part of the patient's body. Typically used for preliminary research state internal organs patients using low doses of X-ray radiation.

Radiography. (X-ray radiography). This is a research method using x-rays in which an image is recorded on photographic film. Photographs are usually taken in two perpendicular planes. This method has some advantages. X-ray photographs contain more detail than a fluorescent screen and are therefore more informative. They can be saved for further analysis. The total radiation dose is less than that used in fluoroscopy.

Computed X-ray tomography . Equipped with computer technology, an axial tomography scanner is the most modern X-ray diagnostic device that allows you to obtain a clear image of any part of the human body, including soft tissues of organs.

The first generation of computed tomography (CT) scanners include a special X-ray tube that is attached to a cylindrical frame. A thin beam of X-rays is directed at the patient. Two X-ray detectors are attached to the opposite side of the frame. The patient is in the center of the frame, which can rotate 180° around his body.

An X-ray beam passes through a stationary object. The detectors obtain and record the absorption values of various tissues. Recordings are made 160 times while the X-ray tube moves linearly along the scanned plane. Then the frame is rotated 1 0 and the procedure is repeated. Recording continues until the frame rotates 180 0 . Each detector records 28,800 frames (180x160) during the study. The information is processed by a computer, and an image of the selected layer is formed using a special computer program.

The second generation of CT uses several X-ray beams and up to 30 X-ray detectors. This makes it possible to speed up the research process up to 18 seconds.

The third generation of CT uses a new principle. A wide fan-shaped beam of X-rays covers the object under study, and the X-ray radiation passing through the body is recorded by several hundred detectors. The time required for research is reduced to 5-6 seconds.

CT has many advantages over earlier x-ray diagnostic methods. It is characterized by high resolution, which makes it possible to distinguish subtle changes in soft tissues. CT allows you to detect pathological processes that cannot be detected by other methods. In addition, the use of CT makes it possible to reduce the dose of X-ray radiation received by patients during the diagnostic process.

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 electromagnetic wave scale 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 located in the electron shells of atoms). At the same time, it is impossible to determine from the photon itself during what 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 a wavelength of 2 angstroms and 6 keV of energy.

An X-ray generator is a tube in which a vacuum is created. There are electrodes located there - 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 surface of the anode 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. Some of the electrons in the atoms of the anode substance are excited, that is, they move to higher orbits, and then return to their normal state, emitting photons of a certain wavelength. In a standard generator, both types of X-ray radiation are produced.

History of discovery

On November 8, 1895, the German scientist Wilhelm Conrad Roentgen discovered that certain substances began to glow when exposed to “cathode rays,” that is, a stream of electrons generated by a cathode ray tube. He explained this phenomenon by the influence of certain X-rays - this is how this radiation is now called in many languages. Later V.K. Roentgen studied the phenomenon he discovered. On December 22, 1895, he gave a report on this topic at the University of Würzburg.

Later it turned out that X-ray radiation had been observed earlier, but then the phenomena associated with it were not given of great importance. The cathode ray tube was invented a long time ago, but before V.K. Nobody paid much attention to the X-rays about the blackening of photographic plates near it, etc. phenomena. The danger posed by penetrating radiation was also unknown.

Types and their effects on the body

“X-ray” is the mildest type of penetrating radiation. Excessive exposure to soft x-rays resembles the effects of ultraviolet radiation, but in a more severe form. A burn forms on the skin, but the damage 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 apart the protein molecules that make up the tissues of the human body, as well as the DNA molecules of the genome. But even if the X-ray quantum breaks up a water molecule, it makes no difference: in this case, chemically active free radicals H and OH are formed, which themselves are capable of affecting proteins and DNA. Radiation sickness occurs in a more severe form, the more the hematopoietic organs are affected.

X-rays have mutagenic and carcinogenic activity. This means that the likelihood of spontaneous mutations in cells during irradiation increases, and sometimes healthy cells can degenerate into cancerous ones. An increased likelihood of malignant tumors is a standard consequence of any radiation 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 use of X-rays is fluoroscopy. “X-raying” of the human body allows you to obtain a detailed image of both bones (they are visible most clearly) and images of internal organs.

The different transparency of body tissues in X-rays is associated with their chemical composition. The structural features of bones are that they contain a lot of calcium and phosphorus. Other tissues consist mainly of carbon, hydrogen, oxygen and nitrogen. A phosphorus atom weighs almost twice as much as an oxygen atom, and a calcium atom by 2.5 times (carbon, nitrogen and hydrogen are even lighter than oxygen). In this regard, the absorption of X-ray photons in 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 radiation received from one image is small: it is approximately equal to the radiation received during a 2-hour flight in an airplane at an altitude of 10 km.

X-ray flaw detection allows you to detect minor internal defects in products. Hard x-rays are used for it, since many materials (metal, for example) are poorly “transparent” due to high atomic mass 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 diffraction scattering of X-rays on 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 fluxes of alpha particles (nuclei of helium atoms), beta particles (electrons), neutrons, protons, and heavy ions. Wave uses rays of the electromagnetic spectrum - x-rays and gamma.

Radiotherapy methods are used primarily for the treatment of cancer. The fact is that radiation primarily affects actively dividing cells, which is why the hematopoietic organs suffer so much (their cells are constantly dividing, producing more and more new red blood cells). Cancer cells also constantly divide and are more vulnerable to radiation than healthy tissue.

A level of radiation is used that suppresses the activity of cancer cells while having a moderate effect on healthy cells. Under the influence of radiation, it is not the destruction of cells as such that occurs, but the damage to their genome - DNA molecules. A cell with a destroyed genome can exist for some time, but can no longer divide, that is, tumor growth stops.

X-ray therapy is the mildest form of radiotherapy. Wave radiation is softer than corpuscular radiation, and x-rays are softer than gamma radiation.

During pregnancy

Using ionizing radiation during pregnancy is dangerous. X-rays are mutagenic and can cause problems 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. The restrictions on fluoroscopy are milder, but in the first months it is also strictly prohibited.

When emergency X-ray examination is replaced by magnetic resonance imaging. But in the first trimester they try to avoid it too (this method appeared recently, and we can say with absolute certainty that there are no harmful consequences).

A clear 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 approximately 50 times less. In order to receive such a dose at one time, you need to undergo a detailed computed tomography.

That is, the fact of a 1-2 x “X-ray” in itself at an early stage of pregnancy does not threaten serious consequences (but it is 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 in that healthy tissues fare little better and there are numerous side effects. The hematopoietic organs are in particular danger.

In practice, various methods are used to reduce the impact of x-rays on healthy tissue. The rays are directed at an angle so that the tumor is in the area of their intersection (due to this, the main absorption of energy occurs right 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. In this case, healthy tissues are in the irradiation zone only occasionally, and sick tissues are constantly exposed.

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 used is softer, side effects similar to those that occur in the treatment of tumors are not observed.

Modern medical diagnosis and treatment of certain diseases cannot be imagined without devices that use the properties of x-ray radiation. The discovery of X-rays occurred more than 100 years ago, but even now work continues on the creation of new techniques and devices to minimize the negative effects of radiation on the human body.

Who discovered X-rays and how?

Under natural conditions, X-ray fluxes are rare and are emitted only by certain radioactive isotopes. X-rays or X-rays were only discovered in 1895 by the German scientist Wilhelm Röntgen. This discovery occurred by chance, during an experiment to study the behavior of light rays in conditions approaching a vacuum. The experiment involved a cathode gas-discharge tube with reduced pressure and a fluorescent screen, which each time began to glow the moment the tube began to operate.

Interested in the strange effect, Roentgen conducted a series of studies showing that the resulting radiation, invisible to the eye, is capable of penetrating through various obstacles: paper, wood, glass, some metals, and even through the human body. Despite the lack of understanding of the very nature of what is happening, whether such a phenomenon is caused by the generation of a stream of unknown particles or waves, the following pattern was noted - radiation easily passes through the soft tissues of the body, and much harder through hard living tissues and non-living substances.

Roentgen was not the first to study this phenomenon. In the middle XIX century, similar possibilities were studied by the Frenchman Antoine Mason and the Englishman William Crookes. However, it was Roentgen who first invented a cathode tube and an indicator that could be used in medicine. He was the first to publish treatise, which brought him the title of first Nobel laureate among physicists.

In 1901, a fruitful collaboration between three scientists began, who became the founding fathers of radiology and radiology.

Properties of X-rays

X-rays are component general spectrum of electromagnetic radiation. The wavelength lies between gamma and ultraviolet rays. X-rays have all the usual wave properties:

diffraction;
refraction;
interference;
speed of propagation (it is equal to light).

To artificially generate a flux of X-rays, special devices are used - X-ray tubes. X-ray radiation occurs due to the contact of fast electrons from tungsten with substances evaporating from the hot anode. Against the background of interaction, electromagnetic waves of short length appear, located in the spectrum from 100 to 0.01 nm and in the energy range of 100-0.1 MeV. If the wavelength of the rays is less than 0.2 nm, this is hard radiation; if the wavelength is greater than this value, they are called soft X-rays.

It is significant that the kinetic energy arising from the contact of electrons and the anode substance is 99% converted into heat energy and only 1% is X-rays.

X-ray radiation – bremsstrahlung and characteristic

X-radiation is a superposition of two types of rays - bremsstrahlung and characteristic. They are generated in the tube simultaneously. Therefore, X-ray irradiation and the characteristics of each specific X-ray tube - its radiation spectrum - depend on these indicators and represent their overlap.

Bremsstrahlung or continuous X-rays are the result of the deceleration of electrons evaporated from a tungsten filament.

Characteristic or line X-ray rays are formed at the moment of restructuring of the atoms of the substance of the anode of the X-ray tube. The wavelength of the characteristic rays directly depends on the atomic number of the chemical element used to make the anode of the tube.

The listed properties of X-rays allow them to be used in practice:

invisibility to ordinary eyes;
high penetrating ability through living tissues and non-living materials that do not transmit rays of the visible spectrum;
ionization effect on molecular structures.

Principles of X-ray imaging

The properties of X-rays on which imaging is based is the ability to either decompose or cause the glow of certain substances.

X-ray irradiation causes a fluorescent glow in cadmium and zinc sulfides - green, and in calcium tungstate - blue. This property is used in medical x-ray imaging techniques and also increases the functionality of x-ray screens.

The photochemical effect of X-rays on photosensitive silver halide materials (exposure) allows for diagnostics - taking X-ray photographs. This property is also used when measuring the total dose received by laboratory assistants in X-ray rooms. Body dosimeters contain special sensitive tapes and indicators. The ionizing effect of X-ray radiation makes it possible to determine the qualitative characteristics of the resulting X-rays.

A single exposure to radiation from conventional X-rays increases the risk of cancer by only 0.001%.

Areas where X-rays are used

The use of X-rays is permissible in the following industries:

Safety. Stationary and portable devices for detecting dangerous and prohibited items at airports, customs or in crowded places.
Chemical industry, metallurgy, archeology, architecture, construction, restoration work - to detect defects and conduct chemical analysis of substances.
Astronomy. Helps monitor cosmic bodies and phenomena using X-ray telescopes.
Military industry. To develop laser weapons.

The main application of X-ray radiation is in the medical field. Today, the section of medical radiology includes: radiodiagnosis, radiotherapy (x-ray therapy), radiosurgery. Medical universities graduate highly specialized specialists – radiologists.

X-Radiation - harm and benefits, effects on the body

The high penetrating power and ionizing effect of X-rays can cause changes in the structure of cell DNA, and therefore pose a danger to humans. The harm from x-rays is directly proportional to the radiation dose received. Different organs respond to radiation to varying degrees. The most susceptible include:

bone marrow and bone tissue;
lens of the eye;
thyroid;
mammary and reproductive glands;
lung tissue.

Uncontrolled use of X-ray irradiation can cause reversible and irreversible pathologies.

Consequences of X-ray irradiation:

damage to the bone marrow and the occurrence of pathologies of the hematopoietic system - erythrocytopenia, thrombocytopenia, leukemia;
damage to the lens, with subsequent development of cataracts;
cellular mutations that are inherited;
development of cancer;
receiving radiation burns;
development of radiation sickness.

Important! Unlike radioactive substances, X-rays do not accumulate in body tissues, which means that X-rays do not need to be removed from the body. The harmful effect of X-ray radiation ends when the medical device is turned off.

The use of X-ray radiation in medicine is permissible not only for diagnostic (traumatology, dentistry), but also for therapeutic purposes:

X-rays in small doses stimulate metabolism in living cells and tissues;
certain limiting doses are used for the treatment of oncological and benign neoplasms.

Methods for diagnosing pathologies using X-rays

Radiodiagnostics includes the following techniques:

Fluoroscopy is a study during which an image is obtained on a fluorescent screen in real time. Along with the classic acquisition of an image of a body part in real time, today there are X-ray television transillumination technologies - the image is transferred from a fluorescent screen to a television monitor located in another room. Several digital methods have been developed for processing the resulting image, followed by transferring it from the screen to paper.
Fluorography is the cheapest method of examining the chest organs, which consists of taking a reduced-scale image of 7x7 cm. Despite the likelihood of error, it is the only way to conduct a mass annual examination of the population. The method is not dangerous and does not require removal of the received radiation dose from the body.
Radiography is the production of a summary image on film or paper to clarify the shape of an organ, its position or tone. Can be used to assess peristalsis and the condition of mucous membranes. If there is a choice, then among modern X-ray devices, preference should be given neither to digital devices, where the x-ray flux can be higher than that of old devices, but to low-dose X-ray devices with direct flat semiconductor detectors. They allow you to reduce the load on the body by 4 times.
Computed X-ray tomography is a technique that uses X-rays to obtain the required number of images of sections of a selected organ. Among the many varieties of modern CT devices, low-dose high-resolution computed tomographs are used for a series of repeated studies.

Radiotherapy

X-ray therapy is a local treatment method. Most often, the method is used to destroy cancer cells. Since the effect is comparable to surgical removal, this treatment method is often called radiosurgery.

Today, x-ray treatment is carried out in the following ways:

External (proton therapy) – a radiation beam enters the patient’s body from the outside.
Internal (brachytherapy) - the use of radioactive capsules by implanting them into the body, placing them closer to the cancerous tumor. The disadvantage of this method of treatment is that until the capsule is removed from the body, the patient needs to be isolated.

These methods are gentle, and their use is preferable to chemotherapy in some cases. This popularity is due to the fact that the rays do not accumulate and do not require removal from the body; they have a selective effect, without affecting other cells and tissues.

Safe exposure limit to X-rays

This indicator of the norm of permissible annual exposure has its own name - genetically significant equivalent dose (GSD). This indicator does not have clear quantitative values.

This indicator depends on the patient’s age and desire to have children in the future.
Depends on which organs were examined or treated.
The GZD is influenced by the level of natural radioactive background in the region where a person lives.

Today the following average GZD standards are in effect:

the level of exposure from all sources, with the exception of medical ones, and without taking into account the natural background radiation - 167 mrem per year;
the norm for an annual medical examination is not higher than 100 mrem per year;
the total safe value is 392 mrem per year.

X-ray radiation does not require removal from the body, and is dangerous only in case of intense and prolonged exposure. Modern medical equipment uses low-energy irradiation of short duration, so its use is considered relatively harmless.

Modern medicine uses many doctors for diagnosis and therapy. Some of them have been used relatively recently, while others have been practiced for dozens or even hundreds of years. Also, one hundred and ten years ago, William Conrad Roentgen discovered amazing X-rays, which caused significant resonance in the scientific and medical world. And now doctors all over the world use them in their practice. The topic of our conversation today will be X-rays in medicine; we will discuss their use in a little more detail.

X-rays are a type of electromagnetic radiation. They are characterized by significant penetrating qualities, which depend on the wavelength of the radiation, as well as on the density and thickness of the irradiated materials. In addition, X-rays can cause a number of substances to glow, influence living organisms, ionize atoms, and also catalyze some photochemical reactions.

Application of X-rays in medicine

Today, 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 are used when carrying out:

X-ray (radioscopy);
- radiography (image);
- fluorography;
- X-ray and computed tomography.

X-ray

To conduct such a study, the patient must position himself between the X-ray tube and a special fluorescent screen. A specialist radiologist selects the required rigidity of the X-rays, obtaining on the screen an image of the internal organs, as well as the ribs.

Radiography

To conduct this study, the patient is placed on a cassette containing a special photographic 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 small details, more detailed than during a fluoroscopic examination.

Fluorography

This study is carried out during mass medical examinations of the population, including to detect tuberculosis. In this case, a picture from a large screen is projected onto a special film.

Tomography

When performing tomography, computer beams help to obtain images of organs in several places at once: in specially selected cross sections of tissue. This series of x-rays is called a tomogram.

Computer tomogram

This study allows you to record sections of the human body using an X-ray scanner. Afterwards, the data is entered into a computer, resulting in one cross-sectional image.

Each of the listed diagnostic methods is based on the properties of an X-ray beam to illuminate photographic film, as well as on the fact that human tissues and bones differ in different permeability to their effects.

X-ray therapy

The ability of X-rays to influence tissue in a special way is used to treat tumor formations. Moreover, the ionizing qualities of this radiation are especially noticeable when affecting cells that are capable of rapid division. It is precisely 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. This effect has an aggressive effect on 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

While studying X-rays, doctors found that they can lead to changes in the skin that resemble a sunburn, but are accompanied by deeper damage to the skin. Such ulcerations take an extremely long time to heal. Scientists have found that such injuries can be avoided by reducing the time and dose of radiation, as well as using special shielding and techniques. remote control.

The aggressive effects of X-rays can also manifest themselves 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: which organ is irradiated and for how long. Irradiation of the hematopoietic organs can lead to blood diseases, and exposure to the genitals 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

When conducting an examination, doctors use the minimum possible number of x-rays. All radiation doses meet certain acceptable standards and cannot harm a person. X-ray diagnostics pose a significant danger only to the doctors who perform them. And then modern methods protections help reduce the aggression of rays to a minimum.

The safest methods of X-ray diagnostics include radiography of the extremities, as well as dental X-rays. The next place in this ranking is mammography, followed by computed tomography, and then radiography.

In order for the use of X-rays in medicine to bring only benefits to humans, it is necessary to conduct research with their help only when indicated.

In the study and practical use of atomic phenomena, X-rays play one of the most important roles. Thanks to their research, many discoveries were made and methods for analyzing substances were developed, used in a variety of fields. Here we will look at one type 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 electromagnetic 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 region 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 quite arbitrary 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 powers, 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 photoelectric effect (photoabsorption) and incoherent (Compton) scattering. In photoabsorption, an X-ray photon, 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 produce beams, glass vacuum cylinders with electrodes located inside are used. The potential difference across the electrodes needs to be very high - up to hundreds of kilovolts. Thermionic emission occurs on the tungsten cathode, heated by current, 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 creation of a photon, types of X-ray radiation are distinguished: bremsstrahlung and characteristic.

Electrons can, when meeting the anode, be slowed down, that is, lose energy in electric fields its atoms. This energy is emitted in the form of X-ray photons. This type of 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 a bremsstrahlung photon cannot exceed the kinetic energy of the electron generating it, so the maximum frequency (and shortest wavelength) of bremsstrahlung radiation corresponds to the highest value of the 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 radiation, the source of which is a different process. This radiation is called characteristic radiation, and we will dwell on it in more detail.

How does characteristic X-ray radiation arise?

Having reached the anti-cathode, a fast electron can penetrate inside the atom and knock out an 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 in the atom occupied by electrons, the vacated space will not remain empty.

It must be remembered that the electronic structure of the atom, like any energy system, tends to minimize energy. The vacancy formed as a result of knocking out is filled with an electron from one of the higher levels. Its energy is higher, and, occupying a lower level, it emits the excess 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 only have strictly defined energy values, reflecting the difference in levels. As a result, the characteristic X-ray radiation has a spectrum that is not continuous, but line-shaped. This spectrum makes it possible to characterize the substance of the anode - hence the name of these rays. It is thanks to the spectral differences that it is clear what is meant by bremsstrahlung and characteristic X-ray radiation.

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, therefore the spectra of such electrons are also discrete in nature.

General view of the characteristic spectrum

Narrow characteristic lines are present in the X-ray spectral picture along with a continuous bremsstrahlung spectrum. If we imagine the spectrum as a graph 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. As the voltage on the tube electrodes increases, 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 appearance regardless of the material of the anticathode irradiated by electrons, but for different materials they are located at different frequencies, uniting in series based on the proximity of the frequency values. Between the series themselves, the difference in frequencies is much more significant. The type of maxima does not depend in any way on whether the anode material is a pure chemical element or a complex substance. In the latter case, the characteristic X-ray spectra of its constituent elements are simply superimposed on each other.

As the atomic number of a chemical element increases, all lines of its X-ray spectrum shift toward higher frequencies. The spectrum retains its appearance.

Moseley's Law

The phenomenon of spectral shift of characteristic lines was experimentally discovered by the English physicist Henry Moseley in 1913. This allowed him to connect the frequencies of the spectrum maxima with the serial numbers of chemical elements. Thus, the wavelength of characteristic X-ray radiation, as it turned out, can be clearly correlated with a specific 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 serial number of the element, S n is the screening constant, n is the principal quantum number and R is the constant Rydberg. This dependence is linear and on the Moseley diagram looks like a series of straight lines for each value of n.

The n values correspond to individual series of characteristic X-ray emission peaks. Moseley's law makes it possible to determine the serial number of a chemical element irradiated by hard electrons based on the measured wavelengths (they are uniquely related to the frequencies) of the maxima of the X-ray spectrum.

The structure of the electronic 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 to each element.

The role of Moseley's law in atomic physics

There are slight deviations from the strict linear relationship expressed by Moseley's law. They are associated, firstly, with the peculiarities of the order of filling the electron shells of some elements, and, secondly, with the relativistic effects of the movement of electrons of heavy atoms. In addition, when the number of neutrons in the nucleus changes (the so-called isotopic shift), the position of the lines may change slightly. This effect made it possible to study the atomic structure in detail.

The significance of Moseley's law is extremely great. Applying it sequentially to elements periodic table Mendeleev established a pattern of increasing the ordinal number corresponding to each small shift in the characteristic maxima. This helped to clarify the question of the physical meaning of the ordinal number of elements. The Z value is not just a number: it is the positive electric charge of the nucleus, which is the sum of the unit positive charges of the particles that make up its composition. The correct placement of elements in the table and the presence of empty positions in it (they still existed then) received powerful confirmation. The validity of the periodic law was proven.

Moseley's law, in addition, became the basis on which a whole direction of experimental research arose - X-ray spectrometry.

The structure of the electron shells of an atom

Let us briefly recall how the electron structure is structured. It consists of shells designated 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 principal quantum number n, which determines the possible energy values. In the outer shells, the electron energy is higher, and the ionization potential for the 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 designated by the shell to which they belong, for example, 2s, 4d, and so on.

The sublevel contains which are specified, in addition to the main and orbital ones, by another quantum number - magnetic, which determines the projection of the orbital momentum of the electron 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 electronic configurations.

Mechanism for generating characteristic X-ray radiation

So, the cause of this radiation is the formation of electron vacancies in the inner shells, caused by 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 to occur within tightly 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 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 a characteristic series is formed, in this example the K-series. An indication of where the electron that fills the vacancy comes from is given in the form of a Greek index in the series designation. "Alpha" means it comes from the L shell, "beta" means it comes from the M shell. Currently, there is a tendency to replace the Greek letter indices with the Latin ones adopted for designating shells.

The intensity of the alpha line in the series is always the highest - this 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 a quantum of characteristic X-ray radiation. It is determined by the difference in the energy values of the levels between which the electron transition occurs, according to the formula E = 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 (the height of the peaks) is the lowest, since they are the least probable.

If, due to insufficient voltage at 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 as a result of an electronic transition, a new vacancy appears in the overlying shell. This creates the conditions for generating the next series. Electron 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, as in optical spectra, is expressed in line splitting.

Fine structure is due to the fact that the energy level is electron shell- is a set of closely spaced components - subshells. To characterize the subshells, another internal quantum number j is introduced, reflecting the interaction of the electron’s own and orbital magnetic moments.

Due to the influence of spin-orbit interaction, the energy structure of the atom becomes more complex, and as a result, the characteristic X-ray radiation has a spectrum characterized by split lines with very closely spaced elements.

Elements of fine structure are usually designated by additional digital indices.

Characteristic X-ray radiation has a feature reflected only in the fine structure of the spectrum. The transition of an electron to a lower energy level does not occur from the lower subshell of the higher level. Such an event has a negligible probability.

Use of X-rays in spectrometry

This radiation, due to its characteristics described by Moseley’s law, underlies various X-ray spectral methods for analyzing substances. When analyzing the X-ray spectrum, either diffraction of radiation on 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 kind of X-ray spectrometry attachments.

Wave-dispersive spectrometry is particularly accurate. Using special filters, the most intense peaks in the spectrum are highlighted, making it possible to obtain almost monochromatic radiation with a precisely known frequency. The anode material is selected very carefully to ensure that a monochromatic beam of the desired frequency is obtained. Its diffraction by crystal lattice of the substance being studied allows one to study the lattice structure with great accuracy. This method is also used in the study of DNA and other complex molecules.

One of the features of characteristic X-ray radiation is also taken into account in gamma spectrometry. This is a high intensity characteristic peak. Gamma spectrometers use lead shielding against external background radiation that interferes with measurements. But lead, absorbing gamma rays, experiences internal ionization, as a result of which it actively emits in the X-ray range. To absorb the intense peaks of the characteristic X-ray radiation of lead, additional cadmium shielding is used. 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, when analyzing substances, the absorption spectra of continuous X-rays by various substances are studied.