X-ray radiation. Mysterious rays that changed the world

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X-ray radiation. Mysterious rays that changed the world
X-ray radiation. Mysterious rays that changed the world
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In the 19th century, radiation invisible to the human eye, capable of passing through flesh and other materials, seemed like something completely fantastic. Now, X-rays are widely used to create medical images, conduct radiation therapy, analyze works of art and solve nuclear energy problems. How X-ray radiation was discovered and how it helps people - we find out together with physicist Alexander Nikolaevich Dolgov.

The discovery of X-rays

From the end of the 19th century, science began to play a fundamentally new role in shaping the picture of the world. A century ago, the activities of scientists were of an amateur and private nature. However, by the end of the 18th century, as a result of the scientific and technological revolution, science turned into a systematic activity in which every discovery became possible thanks to the contribution of many specialists. Research institutes, periodical scientific journals began to appear, competition and struggle arose for the recognition of copyright for scientific achievements and technical innovations. All these processes took place in the German Empire, where by the end of the 19th century, the Kaiser encouraged scientific achievements that increased the country's prestige on the world stage.

One of the scientists who worked with enthusiasm during this period was the professor of physics, rector of the University of Würzburg Wilhelm Konrad Roentgen. On November 8, 1895, he stayed late in the laboratory, as often happened, and decided to conduct an experimental study of the electric discharge in glass vacuum tubes. He darkened the room and wrapped one of the tubes in opaque black paper to make it easier to observe the optical phenomena that accompany the discharge. To his surprise, Roentgen saw a fluorescence band on a nearby screen covered with barium cyanoplatinite crystals. It is unlikely that a scientist could then imagine that he was on the verge of one of the most important scientific discoveries of his time. Next year, over a thousand publications will be written about X-rays, doctors will immediately take the invention into service, thanks to it, radioactivity will be discovered in the future and new directions of science will appear.

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Crookes tube - a device with which for the first time unknowingly produced

Crookes tube - a device with the help of which X-rays were unconsciously produced for the first time // wikipedia.org

Roentgen devoted the next few weeks to investigating the nature of the incomprehensible glow and found that fluorescence appeared whenever he applied current to the tube. The tube was the source of the radiation, and not some other part of the electrical circuit. Not knowing what he was facing, Roentgen decided to designate this phenomenon as X-rays, or X-rays. Further Roentgen discovered that this radiation can penetrate almost all objects to different depths, depending on the thickness of the object and the density of the substance. Thus, a small lead disk between the discharge tube and the screen turned out to be impervious to X-rays, and the bones of the hand cast a darker shadow on the screen, surrounded by a lighter shadow from soft tissues. Soon, the scientist found out that X-rays cause not only the glow of the screen covered with barium cyanoplatinite, but also darkening of photographic plates (after development) in those places where X-rays fell on the photographic emulsion.

In the course of his experiments, Roentgen was convinced that he had discovered radiation unknown to science. On December 28, 1895, he reported on the research results in the article "On a new type of radiation" in the journal "Annals of Physics and Chemistry". At the same time, he sent scientists the pictures of the hand of his wife, Anna Bertha Ludwig, which later became famous. Thanks to Roentgen's old friend, Austrian physicist Franz Exner, the inhabitants of Vienna were the first to see these photos on January 5, 1896 in the pages of the newspaper Die Presse. The very next day, information about the opening was transmitted to the London Chronicle newspaper. So the discovery of Roentgen gradually began to enter the daily life of people. Practical application was found almost immediately: on January 20, 1896, in New Hampshire, doctors helped a man with a broken arm using a new diagnostic method - an X-ray.

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X-ray of the hand of Anna Berta Ludwig // wikipedia.org

Early use of X-rays

Over the course of several years, X-ray images have begun to be actively used for more accurate operations. Friedrich Otto Valkhoff took the first dental X-ray just 14 days after their discovery. And then, together with Fritz Giesel, they founded the world's first dental X-ray laboratory.

By 1900, 5 years after its discovery, the use of X-rays in diagnosis was considered an integral part of medical practice.

The statistics compiled by the oldest hospital in Pennsylvania can be considered indicative of the spread of technologies based on X-ray radiation. According to her, in 1900, only about 1–2% of patients received help with X-rays, while by 1925 there were already 25%.

X-rays were used in a very unusual way at the time. For example, they were used to provide hair removal services. For a long time, this method was considered preferable in comparison with the more painful ones - forceps or wax. In addition, X-rays have been used in shoe fitting apparatuses - try-on fluoroscopes (pedoscopes). These were X-ray machines with a special notch for the feet, as well as windows through which the client and the sellers could evaluate how the shoes sat down.

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Fluoroscope for shoes // wikipedia.org

The early use of X-ray imaging from a modern safety perspective raises many questions. The problem was that at the time of the discovery of X-rays, practically nothing was known about radiation and its consequences, which is why the pioneers who used the new invention faced its harmful effects in their own experience. The negative consequences of increased exposure became a mass phenomenon at the turn of the 19th century. XX centuries, and people began to gradually come to the realization of the dangers of mindless use of X-rays.

The nature of the x-rays

X-ray radiation is electromagnetic radiation with photon energies from ~ 100 eV to 250 keV, which lies on the scale of electromagnetic waves between ultraviolet radiation and gamma radiation. It is part of the natural radiation that occurs in radioisotopes when atoms of elements are excited by a stream of electrons, alpha particles or gamma quanta, in which electrons are ejected from the electron shells of the atom. X-ray radiation occurs when charged particles move with acceleration, in particular, when electrons are decelerated, in the electric field of atoms of a substance.

Soft and hard X-rays are distinguished, the conditional boundary between which on the wavelength scale is about 0.2 nm, which corresponds to a photon energy of about 6 keV. X-ray radiation is both penetrating, due to its short wavelength, and ionizing, since when passing through a substance, it interacts with electrons, knocking them out of atoms, thereby breaking them into ions and electrons and changing the structure of the substance on which it acts.

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Characteristics of radioisotopes

X-rays cause a chemical compound called fluorescence to glow. Irradiating the atoms of the sample with high-energy photons causes the emission of electrons - they leave the atom. In one or more electron orbitals, "holes" - vacancies are formed, due to which the atoms go into an excited state, that is, they become unstable. Millionths of a second later, the atoms return to a stable state when the vacancies in the inner orbitals are filled with electrons from the outer orbitals. This transition is accompanied by the emission of energy in the form of a secondary photon, hence fluorescence arises.

X-ray astronomy

On Earth, we rarely encounter X-ray radiation, but it is quite often found in space. There it occurs naturally due to the activity of many space objects. This made X-ray astronomy possible. The energy of X-ray photons is much higher than that of optical ones; therefore, in the X-ray range it emits a substance heated to extremely high temperatures. X-ray sources are black holes, neutron stars, quasars. Thanks to X-ray astronomy, it became possible to distinguish black holes from neutron stars, Fermi bubbles were discovered, and it was possible to capture the process of destruction of an ordinary star that approached a black hole.

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One of the first X-ray sources in the sky - Cygnus X-1 - was discovered in 1964, and today most scientists are sure that this is a black hole with a mass of about 15 solar masses // NASA

These cosmic sources of X-ray radiation are not a noticeable part of the natural background radiation for us and therefore do not threaten people in any way. The only exception can be such a source of hard electromagnetic radiation as a supernova explosion, which occurred close enough to the solar system.

How to create X-rays artificially?

X-ray devices are still widely used for non-destructive introscopy (X-ray images in medicine, flaw detection in technology). Their main component is an X-ray tube, which consists of a cathode and an anode. The tube electrodes are connected to a high voltage source, usually tens or even hundreds of thousands of volts. When heated, the cathode emits electrons, which are accelerated by the generated electric field between the cathode and the anode. When the electrons collide with the anode, they are decelerated and lose most of their energy. In this case, X-ray bremsstrahlung radiation arises, but the predominant part of the electron energy is converted into heat, so the anode is cooled.

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Ekaterina Zolotoryova for PostNauki

The X-ray tube of constant or pulsed action is still the most widespread source of X-ray radiation, but it is far from the only one. To obtain high-intensity radiation pulses, high-current discharges are used, in which the plasma channel of the flowing current is compressed by its own magnetic field of the current - the so-called pinching. If the discharge takes place in a medium of light elements, for example, in a hydrogen medium, then it plays the role of an effective accelerator of electrons by the electric field arising in the discharge itself. This discharge can significantly exceed the field generated by an external current source. In this way, pulses of hard X-ray radiation with high energy of generated quanta (hundreds of kiloelectronvolts), which have a high penetrating power, are obtained.

To obtain X-ray radiation in a wide spectral range, electron accelerators - synchrotrons are used. In them, radiation is formed inside an annular vacuum chamber, in which a narrowly directed beam of high-energy electrons, accelerated almost to the speed of light, moves in a circular orbit. During rotation under the influence of a magnetic field, flying electrons emit beams of photons tangentially to the orbit in a wide spectrum, the maximum of which falls on the X-ray range.

How X-rays are detected

For a long time, a thin layer of phosphor or photographic emulsion applied to the surface of a glass plate or transparent polymer film was used to detect and measure X-ray radiation. The first, under the action of X-ray radiation, shone in the optical range of the spectrum, while the optical transparency of the coating changed in the film under the action of a chemical reaction.

At present, electronic detectors are most often used to register X-ray radiation - devices that generate an electric pulse when a quantum of radiation is absorbed in the sensitive volume of the detector. They differ in the principle of converting the energy of the absorbed radiation into electrical signals. X-ray detectors with electronic registration can be divided into ionization, the action of which is based on the ionization of a substance, and radioluminescent, including scintillation, using the luminescence of a substance under the action of ionizing radiation. Ionization detectors, in turn, are divided into gas-filled and semiconductor, depending on the detection medium.

The main types of gas-filled detectors are ionization chambers, Geiger counters (Geiger-Muller counters) and proportional gas discharge counters. Radiation quanta entering the working environment of the counter cause ionization of the gas and the flow of current, which is recorded. In a semiconductor detector, electron-hole pairs are formed under the action of radiation quanta, which also make it possible for an electric current to flow through the body of the detector.

The main component of scintillation counters in a vacuum device is a photomultiplier tube (PMT), which uses the photoelectric effect to convert radiation into a stream of charged particles and the phenomenon of secondary electron emission to enhance the current of the generated charged particles. The photomultiplier has a photocathode and a system of sequential accelerating electrodes - dynodes, upon impact on which accelerated electrons multiply.

Secondary electron multiplier is an open vacuum device (operates only under vacuum conditions), in which X-ray radiation at the input is converted into a stream of primary electrons and then amplified due to the secondary emission of electrons as they propagate in the multiplier channel. Microchannel plates, which are a huge number of separate microscopic channels that penetrate the plate detector, work according to the same principle. They can additionally provide spatial resolution and the formation of an optical image of the cross-section of the X-ray flux incident on the detector by bombarding a semitransparent screen with a phosphor deposited on it with an outgoing electron flow.

X-rays in medicine

The ability of X-rays to shine through material objects not only gives people the ability to create simple X-rays, but also opens up possibilities for more advanced diagnostic tools. For example, it is at the heart of computed tomography (CT). The X-ray source and receiver rotate inside the ring in which the patient lies. The obtained data on how the tissues of the body absorb X-rays are reconstructed by a computer into a 3D image. CT is especially important for diagnosing stroke, and although it is less accurate than magnetic resonance imaging of the brain, it takes much less time.

A relatively new direction, which is now developing in microbiology and medicine, is the use of soft X-ray radiation. When a living organism is translucent, it allows one to obtain an image of blood vessels, to study in detail the structure of soft tissues, and even to conduct microbiological studies at the cellular level. An X-ray microscope using radiation from a pinch-type discharge in the plasma of heavy elements makes it possible to see such details of the structure of a living cell, which an electron microscope cannot see even in a specially prepared cellular structure.

One of the types of radiation therapy used to treat malignant tumors uses hard X-rays, which becomes possible due to its ionizing effect, which destroys the tissue of a biological object. In this case, an electron accelerator is used as a radiation source.

Radiography in technology

Soft X-rays are used in research aimed at solving the problem of controlled thermonuclear fusion. To start the process, it is necessary to create a recoil shock wave by irradiating a small deuterium and tritium target with soft X-rays from an electric discharge and instantly heating the shell of this target to a plasma state. This wave compresses the target material to a density thousands of times higher than the density of a solid, and heats it up to a thermonuclear temperature. The release of thermonuclear fusion energy occurs in a short time, while the hot plasma scatters by inertia.

The ability to translucent makes possible radiography - an imaging technique that allows you to display the internal structure of an opaque object made of metal, for example. It is impossible to determine by eye whether the bridge structures have been firmly welded, whether the seam at the gas pipeline is tight, and whether the rails fit tightly to each other. Therefore, in the industry, X-ray is used for flaw detection - monitoring the reliability of the main working properties and parameters of an object or its individual elements, which does not require taking the object out of service or dismantling it.

X-ray fluorescence spectrometry is based on the effect of fluorescence - an analysis method used to determine the concentrations of elements from beryllium to uranium in the range from 0,0001 to 100% in substances of various origins. When a sample is irradiated with a powerful flux of radiation from an X-ray tube, characteristic fluorescent radiation of atoms appears, which is proportional to their concentration in the sample. At present, practically every electron microscope makes it possible to determine, without any difficulty, the detailed elemental composition of the microobjects under study by the method of X-ray fluorescence analysis.

X-rays in art history

The ability of X-rays to shine through and create a fluorescence effect is also used to study paintings. What is hidden under the top coat of paint can tell a lot about the history of the creation of the canvas. For example, it is in the skillful work with several layers of paint that the unique properties of the artist's work may lie. It is also important to consider the structure of the layers of the painting when selecting the most suitable storage conditions for the canvas. For all this, X-ray radiation is indispensable, which allows you to look under the upper layers of the image without harm to it.

Important developments in this direction are new methods specialized for working with works of art. The macroscopic fluorescence method is a variant of X-ray fluorescence analysis that is well suited for visualizing the distribution structure of key elements, mainly metals, present in areas of about 0.5-1 square meter or more. On the other hand, X-ray laminography, a variant of computed X-ray tomography, which is more suitable for studying flat surfaces, seems promising for obtaining images of individual layers of a picture. These methods can also be used to study the chemical composition of the paint layer. This allows the canvas to be dated, including in order to identify a forgery.

X-rays allow you to find out the structure of a substance

X-ray crystallography is a scientific direction associated with the identification of the structure of matter at the atomic and molecular levels. A distinctive feature of crystalline bodies is a multiple ordered repetition in the spatial structure of the same elements (cells), consisting of a certain set of atoms, molecules or ions.

The main research method consists in exposing a crystalline sample to a narrow beam of X-rays using an X-ray camera. The resulting photograph shows a picture of diffracted X-rays passing through the crystal, from which scientists can then visually display its spatial structure, called the crystal lattice. Various ways of implementing this method are called X-ray structural analysis.

X-ray structural analysis of crystalline substances consists of two stages:

  1. Determination of the size of the unit cell of the crystal, the number of particles (atoms, molecules) in the unit cell and the symmetry of the arrangement of particles. These data are obtained by analyzing the geometry of the location of the diffraction maxima.
  2. Calculation of the electron density inside the unit cell and determination of the coordinates of the atoms, which are identified with the position of the electron density maxima. These data are obtained by analyzing the intensity of the diffraction maxima.
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A photograph of the diffraction pattern of DNA in its so-called B-configuration

Some molecular biologists predict that in imaging the largest and most complex molecules, X-ray crystallography may be replaced by a new technique called cryogenic electron microscopy.

One of the newest tools in chemical analysis was Henderson's film scanner, which he used in his pioneering work in cryogenic electron microscopy. However, this method is still quite expensive and therefore is unlikely to completely replace X-ray crystallography in the near future.

A relatively new area of research and technical applications associated with the use of X-rays is X-ray microscopy. It is designed to obtain an enlarged image of the object under study in real space in two or three dimensions using focusing optics.

The diffraction limit of spatial resolution in X-ray microscopy due to the small wavelength of the radiation used is about 1000 times better than the corresponding value for an optical microscope. In addition, the penetrating power of X-ray radiation makes it possible to study the internal structure of samples that are completely opaque to visible light. And although electron microscopy has the advantage of a slightly higher spatial resolution, it is not a non-destructive method of investigation, since it requires a vacuum and samples with metal or metallized surfaces, which is completely destructive, for example, for biological objects.

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