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State autonomous educational institution

middle vocational education -

Novokuibyshev State College of Humanities and Technology

abstract

by discipline:"Chemistry"

topic: "The use of radioactive isotopes in technology"

Grazhdankina Daria Igorevna

1st year students of group 16

specialty 230115

2013

1. What are isotopes and how to get them

Bibliography

radioactive isotope atom flaw detection

1. What are isotopes?

Isotopes are varieties of any chemical element in D.I. Mendeleev, with different atomic weights. Different isotopes of any chemical element have the same number of protons in the nucleus and the same number of electrons on the shells of the atom, have the same atomic number and occupy certain places in the table D.I. Mendeleev. The difference in atomic weight between isotopes is explained by the fact that the nuclei of their atoms contain a different number of neutrons.

Radioactive isotopes - isotopes of any element of the periodic system of D. I. Mendeleev, whose atoms have unstable nuclei and go into a stable state by radioactive decay accompanied by radiation. For elements with an atomic number greater than 82, all isotopes are radioactive and decay by alpha or beta decay. These are the so-called natural radioactive isotopes, which are usually found in nature. The atoms formed during the decay of these elements, if their atomic number is higher than 82, in turn undergo radioactive decay, the products of which can also be radioactive. It turns out, as it were, a sequential chain, or the so-called family of radioactive isotopes. Three naturally occurring radioactive families are known, named after the first element of the series as families of uranium, thorium, and actinouranium (or actinium). The uranium family includes radium and radon. The last element of each series is converted as a result of decay into one of the stable isotopes of lead with serial number 82. In addition to these families, individual natural radioactive isotopes of elements with serial numbers less than 82 are known. These are potassium-40 and some others. Of these, potassium-40 is important, since it is found in any living organism.

All radioactive isotopes chemical elements can be obtained artificially.

There are several ways to get them. Radioactive isotopes of elements such as strontium, iodine, bromine and others, occupying middle places in the periodic system, are fission products of the uranium nucleus. From a mixture of such products obtained in a nuclear reactor, they are isolated using radiochemical and other methods. Radioactive isotopes of almost all elements can be produced in a particle accelerator by bombarding certain stable atoms with protons or deuterons. A common method is to obtain radioactive isotopes from stable isotopes of the same element by irradiating them with neutrons in a nuclear reactor. The method is based on the so-called radiative capture reaction. If a substance is irradiated with neutrons, the latter, having no charge, can freely approach the nucleus of an atom and, as it were, “stick” to it, forming a new nucleus of the same element, but with one extra neutron. In this case, a certain amount of energy is released in the form gamma radiation, which is why the process is called radiative capture. Nuclei with an excess of neutrons are unstable, so the resulting isotope is radioactive. With rare exceptions, radioactive isotopes of any element can be obtained in this way.

The decay of an isotope can form an isotope, also radioactive. For example, strontium-90 turns into yttrium-90, barium-140 into lanthanum-140, etc.

Transuranium elements unknown in nature with an atomic number greater than 92 (neptunium, plutonium, americium, curium, etc.) were obtained artificially, all of whose isotopes are radioactive. One of them gives rise to another radioactive family, the neptunium family.

During the operation of reactors and accelerators, radioactive isotopes are formed in the materials and parts of these installations and surrounding equipment. This "induced activity", which persists for a more or less long time after the operation of the installations has ceased, represents an undesirable source of radiation. Induced activity also occurs in a living organism exposed to neutrons, for example, during an accident or an atomic explosion.

The activity of radioactive isotopes is measured in units of curie or its derivatives - millicurie and microcurie.

For chemical and physical chemical properties radioactive isotopes practically do not differ from natural elements; their admixture to any substance does not change its behavior in a living organism.

It is possible to replace stable isotopes with such labeled atoms in various chemical compounds. The properties of the latter will not change from this, and if introduced into the body, they will behave like ordinary, unlabeled substances. However, due to radiation, it is easy to detect their presence in the blood, tissues, cells, etc. Radioactive isotopes in these substances thus serve as indicators, or indicators, of the distribution and fate of substances introduced into the body. Therefore, they are called "radioactive tracers". Many inorganic and organic compounds labeled with various radioactive isotopes have been synthesized for radioisotope diagnostics and various experimental studies.

2. Application of radioactive isotopes in engineering

One of the most outstanding studies carried out with the help of "tagged atoms" was the study of metabolism in organisms. It has been proven that in a relatively short time the body undergoes an almost complete renewal. Its constituent atoms are replaced by new ones. Only iron, as experiments on the isotopic study of blood have shown, is an exception to this rule. Iron is part of the hemoglobin in red blood cells. When radioactive iron atoms were introduced into food, it was found that the free oxygen released during photosynthesis was originally part of water, and not carbon dioxide. The field of application of radioactive isotopes in industry is extensive. One example of this is the following method for monitoring piston ring wear in engines internal combustion. By irradiating the piston ring with neutrons, they cause nuclear reactions in it and make it radioactive. When the engine is running, particles of the ring material enter the lubricating oil. By examining the level of radioactivity of the oil after a certain time of engine operation, the wear of the ring is determined. Radioactive isotopes make it possible to judge the diffusion of metals, processes in blast furnaces, etc.

Powerful gamma radiation of radioactive preparations is used to study the internal structure of metal castings in order to detect defects in them.

Radioactive isotopes emitting gamma rays can be used instead of bulky X-ray units for transillumination of products, since the properties of gamma rays are similar to those of x-rays. A source of gamma rays is placed on one side of the product to be tested, and a photographic film on the other. This testing method is called gamma flaw detection. In this way, black and non-ferrous castings, finished products (steel products up to 300 mm thick) and the quality of welds are currently checked. With the help of radioactive isotopes, it is easy to measure the thickness of a metal strip or rolled metal sheets on the go and without contact and automatically maintain the thickness constant. A source of beta particles is placed under the moving belt running out from under the rollers of the machine. A change in the thickness of the tape leads, therefore, to a change in the current in the meter. This current is amplified and directed either to measuring device, or into an automatic machine that instantly brings together or, conversely, pushes the rollers apart. Devices of this type are also used in the paper, rubber and leather industries. Radioisotope sources created electrical energy. They use the heat generated in the sample that absorbs the radiation. Thermocouples convert this heat into electricity. A source weighing several kilograms provides a power of several tens of watts for 10 years of uninterrupted operation. Such sources are used to power automatic beacons and automatic weather stations operating in hard-to-reach areas. More powerful sources were installed on Soviet lunar rovers launched to the Moon. They worked reliably at temperatures from -140 to +120.

One of the most outstanding studies carried out with the help of "tagged atoms" was the study of metabolism in organisms. It has been proven that in a relatively short time the body undergoes an almost complete renewal. Its constituent atoms are replaced by new ones. Only iron, as experiments on the isotopic study of blood have shown, is an exception to this rule. Iron is part of the hemoglobin in red blood cells. When radioactive iron atoms were introduced into food, it was found that the free oxygen released during photosynthesis was originally part of water, and not carbon dioxide. Radioactive isotopes are used in medicine for both diagnosis and therapeutic purposes. Radioactive sodium, introduced in small quantities into the blood, is used to study blood circulation, iodine is intensively deposited in the thyroid gland, especially in Graves' disease. By monitoring the deposition of radioactive iodine with a counter, a diagnosis can be made quickly. Large doses of radioactive iodine cause partial destruction of abnormally developing tissues, and therefore radioactive iodine is used to treat Graves' disease. Intense cobalt gamma radiation is used in the treatment of cancer (cobalt gun).

List of used literature

1. Gaisinsky M.N., Nuclear chemistry and its applications, transl. from French, Moscow, 1961

2. Experimental nuclear physics, ed. E. Segre, trans. from English, vol. 3, M., 1961; INTERNET Network Tools

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Course work

Presentation on theme: "Radioactivity.

The use of radioactive isotopes in technology"

Introduction

1. Types of radioactive radiation

2. Other types of radioactivity

3. Alpha decay

4.Beta decay

5. Gamma decay

6. Law of radioactive decay

7. Radioactive rows

8. The effect of radioactive radiation on humans

9. Application of radioactive isotopes

List of used literature

Introduction

Radioactivity- the transformation of atomic nuclei into other nuclei, accompanied by the emission of various particles and electromagnetic radiation. Hence the name of the phenomenon: in Latin radio - I radiate, activus - effective. This word was introduced by Marie Curie. During the decay of an unstable nucleus - a radionuclide, they fly out of it with high speed one or more high energy particles. The flow of these particles is called radioactive radiation or simply radiation.

X-rays. The discovery of radioactivity was directly related to the discovery of Roentgen. Moreover, for some time it was thought that this is one and the same type of radiation. Late 19th century in general, he was rich in the discovery of various kinds of previously unknown "radiations". In the 1880s, the English physicist Joseph John Thomson began to study elementary carriers. negative charge, in 1891 the Irish physicist George Johnston Stoney (1826–1911) called these particles electrons. Finally, in December, Wilhelm Konrad Roentgen announced the discovery of a new kind of rays, which he called X-rays. Until now, in most countries they are called so, but in Germany and Russia, the proposal of the German biologist Rudolf Albert von Kölliker (1817–1905) to call X-rays is accepted. These rays are produced when electrons (cathode rays) traveling rapidly in a vacuum collide with an obstacle. It was known that when cathode rays hit glass, it emits visible light- green luminescence. Roentgen discovered that at the same time some other invisible rays emanate from the green spot on the glass. This happened by chance: in a dark room, a nearby screen was glowing, covered with barium tetracyanoplatinate Ba (earlier it was called barium platinum cyanide). This substance gives a bright yellow-green luminescence under the action of ultraviolet, as well as cathodic rays. But the cathode rays did not hit the screen, and moreover, when the device was covered with black paper, the screen continued to glow. Roentgen soon discovered that radiation passes through many opaque substances, causing a blackening of a photographic plate wrapped in black paper or even placed in a metal case. The rays passed through a very thick book, through a spruce board 3 cm thick, through an aluminum plate 1.5 cm thick ... X-ray realized the possibilities of his discovery: “If you hold your hand between the discharge tube and the screen,” he wrote, “then dark shadows are visible bones against the background of lighter outlines of the hand. It was the first X-ray examination in history.

Roentgen's discovery instantly spread all over the world and amazed not only specialists. On the eve of 1896, a photograph of a hand was exhibited in a bookstore in a German city. On it were visible the bones of a living person, and on one of the fingers - a wedding ring. It was an x-ray photograph of Roentgen's wife's hand. Roentgen's first message About a new kind of rays” was published in the "Reports of the Würzburg Physico-Medical Society" on December 28, it was immediately translated and published in different countries, published in London, the most famous scientific journal "Nature" ("Nature") published an article by Roentgen on January 23, 1896.

New rays began to be investigated all over the world, in just one year over a thousand papers were published on this topic. Simple in design, X-ray machines also appeared in hospitals: the medical application of the new rays was obvious.

Now X-rays are widely used (and not only for medical purposes) throughout the world.

Rays of Becquerel. Roentgen's discovery soon led to an equally remarkable discovery. It was made in 1896 by the French physicist Antoine Henri Becquerel. He was on January 20, 1896 at a meeting of the Academy, at which the physicist and philosopher Henri Poincaré spoke about the discovery of Roentgen and demonstrated x-rays of a human hand already made in France. Poincaré did not confine himself to a story about new rays. He suggested that these rays are associated with luminescence and, perhaps, always occur simultaneously with this type of luminescence, so that cathode rays can probably be dispensed with. The glow of substances under the action of ultraviolet - fluorescence or phosphorescence (in the 19th century there was no strict distinction between these concepts) was familiar to Becquerel: his father Alexander Edmond Becquerel (1820–1891) and grandfather Antoine Cesar Becquerel (1788–1878) were engaged in it - both physicists; the son of Antoine Henri Becquerel, Jacques, became a physicist, who “by inheritance” accepted the chair of physics at the Paris Museum natural history, Becquerelis headed this department for 110 years, from 1838 to 1948.

Becquerel decided to check whether the X-rays were associated with fluorescence. Some uranium salts, for example, uranyl nitrate UO 2 (NO 3) 2, have bright yellow-green fluorescence. Such substances were in Becquerel's laboratory, where he worked. His father also worked with uranium preparations, who showed that after the cessation of sunlight, their glow disappears very quickly - in less than a hundredth of a second. However, no one has checked whether this glow is accompanied by the emission of some other rays capable of passing through opaque materials, as was the case with Roentgen. It was this that, after Poincaré's report, Becquerel decided to test. On February 24, 1896, at the weekly meeting of the Academy, he said that taking a photographic plate wrapped in two layers of thick black paper, placing crystals of double potassium uranyl sulfate K 2 UO 2 (SO 4) 2 2H2O on it and exposing all this for several hours on sunlight, then after the development of the photographic plate on it you can see a somewhat blurred contour of the crystals. If a coin or a figure cut out of tin is placed between the plate and the crystals, then after development, a clear image of these objects appears on the plate.

All this could indicate a relationship between fluorescence and X-rays. The recently discovered X-rays can be obtained much more easily - without cathode rays and the necessary vacuum tube and high voltage for this, but it was necessary to check whether it turns out that the uranium salt, heated in the sun, releases some kind of gas that penetrates under the black paper and acts on photographic emulsion To eliminate this possibility, Becquerel laid a sheet of glass between the uranium salt and the photographic plate - it still lit up. “From here,” concluded his short message Becquerel, - we can conclude that the luminous salt emits rays that penetrate through black paper that is not transparent to light and restore the silver salts in the photographic plate. As if Poincaré was right and Roentgen's X-rays can be obtained in a completely different way.

Becquerel began to set up many experiments in order to better understand the conditions under which rays appear that illuminate a photographic plate, and to investigate the properties of these rays. He placed various substances between the crystals and the photographic plate - paper, glass, plates of aluminum, copper, lead of different thicknesses. The results were the same as those obtained by Roentgen, which could also serve as an argument in favor of the similarity of both radiations. In addition to direct sunlight, Becquerel illuminated uranium salt with light reflected by a mirror or refracted by a prism. He found that the results of all previous experiments had nothing to do with the sun; what mattered was how long the uranium salt was near the photographic plate. The next day, Becquerel reported this at a meeting of the Academy, but, as it turned out later, he made the wrong conclusion: he decided that uranium salt, at least once "charged" in the light, was then itself capable of emitting invisible penetrating rays for a long time.

Becquerel until the end of the year, he published nine articles on this topic, in one of them he wrote: “Different uranium salts were placed in a thick-walled lead box ... Protected from the action of any known radiation, these substances continued to emit rays passing through glass and black paper..., in eight months."

These rays came from any uranium compounds, even those that do not glow in the sun. Even stronger (about 3.5 times) was the radiation of metallic uranium. It became obvious that the radiation, although similar in some manifestations to X-rays, has a greater penetrating power and is somehow connected with uranium, so Becquerel began to call it "uranium rays."

Becquerel also discovered that "uranium rays" ionize the air, making it a conductor of electricity. Almost simultaneously, in November 1896, the English physicists J. J. Thomson and Ernest Rutherford (discovered the ionization of air under the action of X-rays. To measure the radiation intensity, Becquerel used an electroscope in which the lightest golden leaves, suspended by the ends and electrostatically charged, repel and their free ends diverge.If the air conducts current, the charge drains from the leaves and they fall off - the faster, the higher the electrical conductivity of the air and, consequently, the greater the radiation intensity.

The question remained how the substance emits continuous and unabated radiation for many months without energy supply from an external source. Becquerel himself wrote that he was not able to understand where uranium receives the energy that it continuously emits. A variety of hypotheses, sometimes quite fantastic, have been put forward on this occasion. For example, the English chemist and physicist William Ramsay wrote: “... physicists wondered where the inexhaustible supply of energy in uranium salts could come from. Lord Kelvin was inclined to suggest that uranium is a kind of trap that catches otherwise undetectable radiant energy reaching us through space and converts it into a form in which it is made capable of producing chemical effects.

Becquerel could neither accept this hypothesis, nor come up with something more plausible, nor abandon the principle of conservation of energy. It ended up that he generally quit working with uranium for a while and took up fission spectral lines in a magnetic field. This effect was discovered almost simultaneously with the discovery of Becquerel by the young Dutch physicist Peter Zeeman and explained by another Dutchman, Hendrik Anton Lorentz.

This completed the first stage of the study of radioactivity. Albert Einstein compared the discovery of radioactivity with the discovery of fire, because he believed that both fire and radioactivity are equally important milestones in the history of civilization.

1. Types of radioactive radiation

When powerful sources of radiation appeared in the hands of researchers, millions of times stronger than uranium (these were preparations of radium, polonium, actinium), it was possible to become more familiar with the properties of radioactive radiation. Ernest Rutherford, spouses Maria and Pierre Curie, A. Becquerel, and many others took an active part in the first studies on this topic. First of all, the penetrating power of the rays was studied, as well as the effect of the magnetic field on the radiation. It turned out that the radiation is inhomogeneous, but is a mixture of "rays". Pierre Curie discovered that when a magnetic field acts on radium radiation, some rays are deflected while others are not. It was known that the magnetic field deflects only charged flying particles, both positive and negative in different directions. By the direction of the deflection, we made sure that the deflected β-rays were negatively charged. Further experiments showed that there is no fundamental difference between cathode and β-rays, from which it followed that they represent a stream of electrons.

The deflecting rays had a stronger ability to penetrate various materials, while the non-deflecting ones were easily absorbed even by thin aluminum foil - this is how, for example, the radiation of the new element polonium behaved - its radiation did not penetrate even through the cardboard walls of the box in which the drug was stored.

When using stronger magnets, it turned out that α-rays also deviate, only much weaker than β-rays, and in the other direction. From this it followed that they are positively charged and have a much larger mass (as it was later found out, the mass of α-particles is 7740 times greater than the mass of an electron). This phenomenon was first discovered in 1899 by A. Becquerel and F. Gisel. Later it turned out that α-particles are the nuclei of helium atoms (nuclide 4 He) with a charge of +2 and a mass of 4 cu. β-rays, he discovered in the radiation of radium a third type of rays that do not deviate in the strongest magnetic fields, this discovery was soon confirmed by Becquerel. This type of radiation, by analogy with alpha and beta rays, was called gamma rays, the designation of different radiations by the first letters of the Greek alphabet was proposed by Rutherford. Gamma rays turned out to be similar to X-rays, i.e. they represent electromagnetic radiation, but with shorter wavelengths and correspondingly higher energy. All these types of radiation were described by M. Curie in her monograph "Radium and Radioactivity". Instead of a magnetic field, an electric field can be used to “split” radiation, only charged particles in it will deviate not perpendicularly lines of force, and along them - towards the deflecting plates.

For a long time it was not clear where all these rays come from. For several decades, the nature of radioactive radiation and its properties were elucidated by the works of many physicists, new types of radioactivity were discovered.γ

Alpha rays emit mainly the nuclei of the heaviest and therefore less stable atoms (in the periodic table they are located after lead). These are high energy particles. Usually there are several groups of α-particles, each of which has a strictly defined energy. So, almost all α-particles emitted from 226 Ra nuclei have an energy of 4.78 MeV (megaelectron-volt) and a small fraction of α-particles with an energy of 4.60 MeV. Another radium isotope, 221 Ra, emits four groups of α-particles with energies of 6.76, 6.67, 6.61 and 6.59 MeV. This indicates the presence of several energy levels in the nuclei, their difference corresponds to the energy of α-quanta emitted by the nucleus. "Pure" alpha emitters are also known (for example, 222 Rn).

According to the formula E = mu 2 /2 one can calculate the speed of α-particles with a certain energy. For example, 1 mol of α-particles with E= 4.78 MeV has energy (in SI units) E\u003d 4.78 10 6 eV  96500 J / (eV mol) \u003d 4.61 10 11 J / mol and mass m= 0.004 kg/mol, whence uα 15200 km/s, which is tens of thousands of times greater than the speed of a pistol bullet. Alpha particles have the strongest ionizing effect: colliding with any other atoms in a gas, liquid, or solid, they “rip off” electrons from them, creating charged particles. In this case, α-particles lose energy very quickly: they are retained even by a sheet of paper. In air, α-radiation of radium passes only 3.3 cm, α-radiation of thorium - 2.6 cm, etc. Ultimately, the alpha particle, which has lost kinetic energy, captures two electrons and turns into a helium atom. The first ionization potential of the helium atom (He - e → He +) is 24.6 eV, the second (He + - e → He +2) is 54.4 eV, which is much more than that of any other atoms. When electrons are captured by α-particles, huge energy is released (more than 7600 kJ / mol), therefore, not a single atom, except for the atoms of helium itself, is able to retain its electrons if an α-particle is in the neighborhood.

The very high kinetic energy of α-particles makes it possible to "see" them with the naked eye (or with an ordinary magnifying glass), this was first demonstrated in 1903 by the English physicist and chemist William Crookes (1832 - 1919. He barely glued visible to the eye a grain of radium salt and fixed the needle in a wide glass tube. At one end of this tube, not far from the tip of the needle, was placed a plate coated with a layer of phosphor (zinc sulfide served as a layer), and at the other end there was a magnifying glass. If you look at the phosphor in the dark, you can see: the entire field of view is dotted with flashing and immediately dying sparks. Each spark is the result of the impact of one α-particle. Crookes called this device a spinthariscope (from the Greek spintharis - a spark and skopeo - I look, I observe). With the help of this simple method counting α-particles, a number of studies were carried out, for example, in this way it was possible to determine the Avogadro constant quite accurately.

In the nucleus, protons and neutrons are held together. nuclear forces, Therefore, it was not clear how an alpha particle, consisting of two protons and two neutrons, could leave the nucleus. Answered in 1928 American physicist(who emigrated in 1933 from the USSR) George (Georgy Antonovich) Gamov). According to the laws of quantum mechanics, α-particles, like any particles of small mass, have a wave nature and therefore they have some small probability of being outside the nucleus, on a small (about 6 · 10–12 cm) distance from it. As soon as this happens, the Coulomb repulsion from a very nearby positively charged nucleus begins to act on the particle.

Alpha decay is mainly affected by heavy nuclei - more than 200 of them are known, α-particles are emitted by most of the isotopes of elements following bismuth. Lighter alpha emitters are known, mainly rare earth atoms. But why are alpha particles emitted from the nucleus, and not individual protons? Qualitatively, this is explained by the energy gain in α-decay (α-particles - helium nuclei are stable). The quantitative theory of α-decay was created only in the 1980s, and domestic physicists also took part in its development, including Lev Davidovich Landau, Arkady Beinusovich Migdal (1911–1991), Stanislav Georgievich Kadmensky, head of the Department of Nuclear Physics at Voronezh University, and colleagues .

The departure of an α-particle from the nucleus leads to the nucleus of another chemical element, which is shifted in the periodic table by two cells to the left. An example is the transformation of seven isotopes of polonium (nucleus charge 84) into different lead isotopes (nucleus charge 82): 218 Po → 214 Pb, 214 Po → 210 Pb, 210 Po → 206 Pb, 211 Po → 207 Pb, 215 Po → 211Pb, 212Po → 208Pb, 216Po → 212Pb. Lead isotopes 206 Pb 207 Pb and 208 Pb are stable, the rest are radioactive.

Beta decay is observed in both heavy and light nuclei, such as tritium. These light particles (fast electrons) have a higher penetrating power. So, in air, β-particles can fly several tens of centimeters, in liquid and solids- from fractions of a millimeter to about 1 cm. Unlike α-particles, the energy spectrum of β-rays is not discrete. The energy of electrons escaping from the nucleus can vary almost from zero to some maximum value characteristic of a given radionuclide. Usually, the average energy of β particles is much less than that of α particles; for example, the energy of β-radiation 228 Ra is 0.04 MeV. But there are exceptions; so the β-radiation of the short-lived nuclide 11 Be carries an energy of 11.5 MeV. For a long time it was not clear how particles with different speeds fly out of identical atoms of the same element. When it became known the structure of the atom and atomic nucleus, a new mystery has appeared: where do the β-particles emitted from the nucleus come from - after all, there are no electrons in the nucleus. After the English physicist James Chadwick discovered the neutron in 1932, Russian physicists Dmitry Dmitrievich Ivanenko (1904–1994) and Igor Evgenievich Tamm and, independently, the German physicist Werner Heisenberg suggested that atomic nuclei consist of protons and neutrons. In this case, β-particles should be formed as a result of the intranuclear process of the transformation of a neutron into a proton and an electron: n → p + e. The mass of the neutron slightly exceeds the total mass of the proton and electron, the excess mass, in accordance with Einstein's formula E = mc 2 gives the kinetic energy of an electron escaping from the nucleus; therefore, β-decay is observed mainly in nuclei with an excess number of neutrons. For example, the nuclide 226 Ra is an α-emitter, and all the heavier isotopes of radium (227 Ra, 228 Ra, 229 Ra and 230 Ra) are β-emitters.

It remained to find out why β-particles, in contrast to α-particles, have continuous spectrum energy, which meant that some of them had very little energy, while others had very much energy (and at the same time they were moving at a speed close to the speed of light). Moreover, the total energy of all these electrons (it was measured with a calorimeter) turned out to be less than the energy difference between the original nucleus and its decay product. Again, physicists were faced with a "violation" of the law of conservation of energy: part of the energy of the original nucleus disappeared in an unknown direction. The unshakable physical law was “saved” in 1931 by the Swiss physicist Wolfgang Pauli, who suggested that during β-decay two particles fly out of the nucleus: an electron and a hypothetical neutral particle - a neutrino with almost zero mass, which carries away excess energy. The continuous spectrum of β-radiation is explained by the distribution of energy between electrons and this particle. Neutrino (as it turned out later, the so-called electron antineutrino is formed during β-decay) interacts very weakly with matter (for example, it easily pierces the globe and even a huge star in diameter) and therefore was not detected for a long time - experimentally free neutrinos were registered only in 1956 Thus, the refined scheme of beta decay is as follows: n → p + . The quantitative theory of β-decay based on Pauli's ideas about the neutrino was developed in 1933 by the Italian physicist Enrico Fermi, who also proposed the name neutrino (in Italian, "neutron").

The transformation of a neutron into a proton during β-decay practically does not change the mass of the nuclide, but increases the nuclear charge by one. Consequently, a new element is formed, shifted in the periodic table by one cell to the right, for example: →, →, →, etc. (simultaneously, an electron and an antineutrino fly out of the nucleus).

2. Other types of radioactivity

In addition to alpha and beta decays, other types of spontaneous radioactive transformations are also known. In 1938, the American physicist Luis Walter Alvarez discovered a third type of radioactive transformation, electron capture (K-capture). In this case, the nucleus captures an electron from the energy shell closest to it (K-shell). When an electron interacts with a proton, a neutron is formed, and a neutrino flies out of the nucleus, carrying away excess energy. The transformation of a proton into a neutron does not change the mass of the nuclide, but reduces the nuclear charge by one. Consequently, a new element is formed, which is one cell to the left in the periodic table, for example, a stable nuclide is obtained from it (it was on this example that Alvarez discovered this type of radioactivity).

With K-capture in the electron shell of an atom, an electron from a higher energy level, the excess energy is either released in the form of X-rays, or is spent on the escape of one or more weakly bound electrons from the atom - the so-called Auger electrons, named after the French physicist Pierre Auger (1899–1993), who discovered this effect in 1923 (to knock out internal electrons, he used ionizing radiation).

In 1940, Georgy Nikolaevich Flerov (1913–1990) and Konstantin Antonovich Petrzhak (1907–1998) discovered spontaneous fission using the example of uranium, in which an unstable nucleus decays into two lighter nuclei, the masses of which do not differ very much, for example: → + + 2n. This type of decay is observed only in uranium and heavier elements - more than 50 nuclides in total. In the case of uranium, spontaneous fission occurs very slowly: the average lifetime of a 238U atom is 6.5 billion years. In 1938, the German physicist and chemist Otto Hahn, the Austrian radiochemist and physicist Lise Meitner (the element Mt - meitnerium is named after her) and the German physicochemist Fritz Strassmann (1902–1980) discovered that when bombarded by neutrons, uranium nuclei are divided into fragments, moreover, flying out of neutrons are capable of causing the fission of neighboring uranium nuclei, which leads to chain reaction). This process is accompanied by the release of huge (compared to chemical reactions) energy, which led to the creation nuclear weapons and construction of nuclear power plants.

In 1934 Marie Curie's daughter Irene Joliot-Curie and her husband Frédéric Joliot-Curie discovered positron decay. In this process, one of the protons of the nucleus turns into a neutron and an antielectron (positron) - a particle with the same mass, but positively charged; at the same time, a neutrino flies out of the nucleus: p → n + e + + 238. The mass of the nucleus does not change, but the displacement occurs, unlike β - decay, to the left, β + decay is characteristic of nuclei with an excess of protons (the so-called neutron-deficient nuclei ). So, heavy isotopes of oxygen 19 O, 20 O and 21 O β - are active, and its light isotopes 14 O and 15 O β + are active, for example: 14 O → 14 N + e + + 238. As antiparticles, positrons immediately they are destroyed (annihilated) when they meet electrons with the formation of two γ-quanta. Positron decay often competes with K-capture.

In 1982, proton radioactivity was discovered: the emission of a proton from a nucleus (this is possible only for some artificially obtained nuclei that have excess energy). In 1960, physical chemist Vitaly Iosifovich Gol'danskii (1923–2001) theoretically predicted two-proton radioactivity: the ejection of two protons with paired spins by the nucleus. It was first observed in 1970. Two-neutron radioactivity is also very rarely observed (discovered in 1979).

In 1984, cluster radioactivity was discovered (from the English cluster - bunch, swarm). In this case, in contrast to spontaneous fission, the nucleus decays into fragments with very different masses, for example, nuclei with masses from 14 to 34 fly out of a heavy nucleus. Cluster decay is also observed very rarely, and this made it difficult to detect for a long time.

Some nuclei are able to decay in different directions. For example, 221 Rn decays by 80% with the emission of α-particles and 20% by β-particles, many isotopes of rare earth elements (137 Pr, 141 Nd, 141 Pm, 142 Sm, etc.) decay either by electron capture or with emission of a positron. Different kinds radioactive emissions are often (but not always) accompanied by γ-radiation. This happens because the resulting nucleus may have excess energy, from which it is released by emitting gamma rays. The energy of γ-radiation lies within a wide range, so, during the decay of 226 Ra, it is equal to 0.186 MeV, and during the decay of 11 Be it reaches 8 MeV.

Almost 90% of the known 2500 atomic nuclei are unstable. An unstable nucleus spontaneously transforms into other nuclei with the emission of particles. This property of nuclei is called radioactivity. For large nuclei, instability arises due to the competition between the attraction of nucleons by nuclear forces and the Coulomb repulsion of protons. There are no stable nuclei with charge number Z > 83 and mass number A > 209. But atomic nuclei with significantly lower Z and A numbers can also turn out to be radioactive. If the nucleus contains significantly more protons than neutrons, then the instability is caused by an excess of the Coulomb interaction energy . Nuclei, which would contain a large excess of neutrons over the number of protons, are unstable due to the fact that the mass of the neutron exceeds the mass of the proton. An increase in the mass of the nucleus leads to an increase in its energy.

The phenomenon of radioactivity was discovered in 1896 by the French physicist A. Becquerel, who discovered that uranium salts emit unknown radiation that can penetrate through barriers that are opaque to light and cause blackening of the photographic emulsion. Two years later, French physicists M. and P. Curie discovered the radioactivity of thorium and discovered two new radioactive elements - polonium and radium

In subsequent years, many physicists, including E. Rutherford and his students, were engaged in the study of the nature of radioactive radiation. It was found that radioactive nuclei can emit particles of three types: positively and negatively charged and neutral. These three types of radiation were called α-, β- and γ-radiation. These three types of radioactive radiation differ greatly from each other in their ability to ionize the atoms of matter and, consequently, in their penetrating power. α-radiation has the least penetrating power. In air, under normal conditions, α-rays travel a distance of several centimeters. β-rays are much less absorbed by matter. They are able to pass through a layer of aluminum several millimeters thick. γ-rays have the highest penetrating power, being able to pass through a layer of lead 5–10 cm thick.

In the second decade of the 20th century, after the discovery by E. Rutherford of the nuclear structure of atoms, it was firmly established that radioactivity is a property of atomic nuclei. Studies have shown that α-rays represent a stream of α-particles - helium nuclei, β-rays are a stream of electrons, γ-rays are short-wave electromagnetic radiation with an extremely short wavelength λ< 10 –10 м и вследствие этого – ярко выраженными corpuscular properties, i.e. is a stream of particles - γ-quanta.

3. Alpha decay

Alpha decay is the spontaneous transformation of an atomic nucleus with the number of protons Z and neutrons N into another (daughter) nucleus containing the number of protons Z - 2 and neutrons N - 2. In this case, an α-particle is emitted - the nucleus of a helium atom. An example of such a process is the α-decay of radium: Alpha particles emitted by the nuclei of radium atoms were used by Rutherford in experiments on scattering by the nuclei of heavy elements. The speed of α-particles emitted during the α-decay of radium nuclei, measured along the curvature of the trajectory in a magnetic field, is approximately equal to 1.5 10 7 m/s, and the corresponding kinetic energy is about 7.5 10 -13 J (approximately 4. 8 MeV). This value can be easily determined from the known values ​​of the masses of the parent and daughter nuclei and the helium nucleus. Although the speed of the ejected α-particle is enormous, it is still only 5% of the speed of light, so the non-relativistic expression for the kinetic energy can be used in the calculation. Studies have shown that a radioactive substance can emit α-particles with several discrete energy values. This is explained by the fact that nuclei can be, like atoms, in different excited states. A daughter nucleus can be in one of these excited states during α-decay.

During the subsequent transition of this nucleus to the ground state, a γ-quantum is emitted. Scheme of α-decay of radium with the emission of α-particles with two values kinetic energies shown in Fig.2. Thus, the α-decay of nuclei is in many cases accompanied by γ-radiation.

In the theory of α-decay, it is assumed that groups consisting of two protons and two neutrons can be formed inside nuclei, i.e. α-particle. The parent nucleus is a potential well for α-particles, which is limited by a potential barrier. The energy of the α-particle in the nucleus is insufficient to overcome this barrier (Fig. 3). The escape of an α-particle from the nucleus is only possible due to a quantum-mechanical phenomenon called the tunnel effect. According to quantum mechanics, there is a non-zero probability of the particle passing under the potential barrier. The phenomenon of tunneling has a probabilistic character.

4. Beta decay

In beta decay, an electron is emitted from the nucleus. Electrons cannot exist inside nuclei, they arise during β-decay as a result of the transformation of a neutron into a proton. This process can occur not only inside the nucleus, but also with free neutrons. The average lifetime of a free neutron is about 15 minutes. When a neutron decays into a proton and an electron

The measurements showed that in this process there is an apparent violation of the law of conservation of energy, since the total energy of the proton and electron arising from the decay of the neutron is less than the energy of the neutron. In 1931, W. Pauli suggested that during the decay of a neutron, another particle with zero mass and charge is released, which takes away part of the energy with it. The new particle was named neutrino (small neutron). Due to the absence of a charge and mass in a neutrino, this particle interacts very weakly with the atoms of matter, so it is extremely difficult to detect it in an experiment. The ionizing ability of neutrinos is so small that one act of ionization in air falls on approximately 500 km of the path. This particle was discovered only in 1953. Currently, it is known that there are several varieties of neutrinos. In the process of neutron decay, a particle is formed, which is called an electron antineutrino. It is marked with a symbol. Therefore, the neutron decay reaction is written as

A similar process occurs inside nuclei during β-decay. An electron formed as a result of the decay of one of the nuclear neutrons is immediately ejected from the “parent house” (nucleus) at a tremendous speed, which can differ from the speed of light by only a fraction of a percent. Since the distribution of the energy released during β-decay between an electron, a neutrino and a daughter nucleus is random, β-electrons can have different velocities over a wide range.

In β-decay charge number Z increases by one, while the mass number A remains unchanged. The daughter nucleus turns out to be the nucleus of one of the isotopes of the element, the serial number of which in the periodic table is one higher than the serial number of the original nucleus. A typical example of β-decay is the transformation of the thorium isotone arising from the α-decay of uranium into palladium

5. Gamma decay

Unlike α- and β-radioactivity, γ-radioactivity of nuclei is not associated with a change in the internal structure of the nucleus and is not accompanied by a change in charge or mass numbers. In both α- and β-decay, the daughter nucleus can be in some excited state and have an excess of energy. The transition of the nucleus from the excited state to the ground state is accompanied by the emission of one or several γ-quanta, the energy of which can reach several MeV.

6. Law of radioactive decay

Any sample of radioactive material contains a huge number of radioactive atoms. Since radioactive decay is random and does not depend on external conditions, the law of decrease in the number N(t) of undecayed k present moment time t nuclei can serve as an important statistical characteristic radioactive decay process.

Let the number of undecayed nuclei N(t) change by ΔN over a short period of time Δt< 0. Так как вероятность распада каждого ядра неизменна во времени, что число распадов будет пропорционально количеству ядер N(t) и промежутку времени Δt:

The coefficient of proportionality λ is the probability of the decay of the nucleus in the time Δt = 1 s. This formula means that the rate of change of the function N(t) is directly proportional to the function itself.

where N 0 is the initial number of radioactive nuclei at t = 0. During the time τ = 1 / λ, the number of undecayed nuclei will decrease by e ≈ 2.7 times. The value τ is called the average lifetime of a radioactive nucleus.

For practical use, it is convenient to write the law of radioactive decay in a different form, using the number 2 as the base, and not e:

The value of T is called the half-life. During time T, half of the initial number of radioactive nuclei decays. The values ​​of T and τ are related by the relation

The half-life is the main quantity that characterizes the rate of radioactive decay. The shorter the half-life, the more intense the decay. Thus, for uranium T ≈ 4.5 billion years, and for radium T ≈ 1600 years. Therefore, the activity of radium is much higher than that of uranium. There are radioactive elements with a half-life of a fraction of a second.

During α- and β-radioactive decay, the daughter nucleus may also be unstable. Therefore, a series of successive radioactive decays are possible, which end in the formation of stable nuclei. In nature, there are several such series. The longest is a series consisting of 14 consecutive decays (8 - alpha decays and 6 beta decays). This series ends with a stable lead isotope (Fig. 5).

In nature, there are several more radioactive series, similar to the series. There is also a series that begins with neptunium not found in natural conditions, and ends with bismuth. This series of radioactive decays occurs in nuclear reactors.

displacement rule. The displacement rule specifies exactly what kind of transformations a chemical element undergoes when emitting radioactive radiation.

7. Radioactive rows

The displacement rule made it possible to trace the transformations of natural radioactive elements and build three genealogical trees from them, the ancestors of which are uranium-238, uranium-235 and thorium-232. Each family starts with an extremely long-lived radioactive element. The uranium family, for example, is headed by uranium with a mass number of 238 and a half-life of 4.5·10 9 years (in Table 1, in accordance with the original name, it is designated as uranium I).

uranium family. Most of the properties of radioactive transformations discussed above can be traced to the elements of the uranium family. So, for example, the third member of the family has nuclear isomerism. Uranium X 2, emitting beta particles, turns into uranium II (T = 1.14 min). This corresponds to the beta decay of the excited state of protactinium-234. However, in 0.12% of cases, excited protactinium-234 (uranium X 2) emits a gamma quantum and goes into the ground state (uranium Z). The beta decay of uranium Z, which also leads to the formation of uranium II, occurs in 6.7 hours.

Radium C is interesting because it can decay in two ways: by emitting either an alpha or a beta particle. These processes compete with each other, but in 99.96% of cases beta decay occurs with the formation of radium C. In 0.04% of cases, radium C emits an alpha particle and turns into radium C (RaC). In turn, RaC and RaC are converted into radium D by the emission of alpha and beta particles, respectively.

Isotopes. Among the members of the uranium family, there are those whose atoms have the same atomic number ( same charge nuclei) and different mass numbers. They are identical in chemical properties, but differ in the nature of radioactivity. For example, radium B, radium D, and radium G, which have the same atomic number of 82 as lead, are similar in chemical behavior to lead. Obviously, the chemical properties do not depend on the mass number; they are determined by the structure of the electron shells of the atom (hence, and Z). On the other hand, the mass number is critical to the nuclear stability of the radioactive properties of the atom. Atoms with the same atomic number and different mass numbers are called isotopes. Isotopes of radioactive elements were discovered by F. Soddy in 1913, but soon F. Aston proved with the help of mass spectroscopy that many stable elements also have isotopes.

8. The effect of radioactive radiation on humans

Radioactive radiation of all types (alpha, beta, gamma, neutrons), as well as electromagnetic radiation (X-ray radiation) have a very strong biological effect on living organisms, which consists in the processes of excitation and ionization of atoms and molecules that make up living cells. Under the action of ionizing radiation, complex molecules and cellular structures are destroyed, which leads to radiation damage to the body. Therefore, when working with any source of radiation, it is necessary to take all measures for the radiation protection of people who can fall into the zone of radiation.

However, a person can be exposed to ionizing radiation in domestic conditions. An inert, colorless, radioactive gas radon can pose a serious danger to human health. As can be seen from the diagram shown in Fig. 5, radon is a product of the α-decay of radium and has a half-life T = 3.82 days. Radium is found in small amounts in soil, in stones, and in various building structures. Despite the relatively short lifetime, the concentration of radon is continuously replenished due to new decays of radium nuclei, so radon can accumulate in enclosed spaces. Getting into the lungs, radon emits α-particles and turns into polonium, which is not a chemically inert substance. This is followed by a chain of radioactive transformations of the uranium series (Fig. 5). According to the American Commission on Radiation Safety and Control, the average person receives 55% of ionizing radiation from radon and only 11% from medical care. The contribution of cosmic rays is approximately 8%. The total dose of radiation that a person receives in a lifetime is many times less than the maximum allowable dose (MAD), which is set for people of certain professions who are exposed to additional exposure to ionizing radiation.

9. Use of radioactive isotopes

One of the most outstanding studies carried out with the help of "tagged atoms" was the study of metabolism in organisms. It has been proven that in a relatively short time the body undergoes an almost complete renewal. Its constituent atoms are replaced by new ones. Only iron, as experiments on the isotopic study of blood have shown, is an exception to this rule. Iron is part of the hemoglobin in red blood cells. When radioactive iron atoms were introduced into food, it was found that the free oxygen released during photosynthesis was originally part of water, and not carbon dioxide. Radioactive isotopes are used in medicine for both diagnosis and therapeutic purposes. Radioactive sodium, introduced in small quantities into the blood, is used to study blood circulation, iodine is intensively deposited in the thyroid gland, especially in Graves' disease. By monitoring the deposition of radioactive iodine with a counter, a diagnosis can be made quickly. Large doses of radioactive iodine cause partial destruction of abnormally developing tissues, and therefore radioactive iodine is used to treat Graves' disease. Intense cobalt gamma radiation is used in the treatment of cancer (cobalt gun).

No less extensive are the applications of radioactive isotopes in industry. One example of this is the following method for monitoring piston ring wear in internal combustion engines. By irradiating the piston ring with neutrons, they cause nuclear reactions in it and make it radioactive. When the engine is running, particles of the ring material enter the lubricating oil. By examining the level of radioactivity of the oil after a certain time of engine operation, the wear of the ring is determined. Radioactive isotopes make it possible to judge the diffusion of metals, processes in blast furnaces, etc.

Powerful gamma radiation of radioactive preparations is used to study the internal structure of metal castings in order to detect defects in them.

Increasingly, radioactive isotopes are being used in agriculture. Irradiation of plant seeds (cotton, cabbage, radish, etc.) with small doses of gamma rays from radioactive preparations leads to a noticeable increase in yield. Large doses of "radiation cause mutations in plants and microorganisms, which in some cases leads to the emergence of mutants with new valuable properties (radioselection). Thus, valuable varieties of wheat, beans and other crops have been bred, and highly productive microorganisms used in the production of antibiotics have been obtained. Gamma radiation from radioactive isotopes is also used to control harmful insects and for conservation food products. "Tagged atoms" are widely used in agricultural technology. For example, to find out which of the phosphate fertilizers is better absorbed by the plant, various fertilizers are labeled with radioactive phosphorus 15 32P. By examining the plants for radioactivity, one can determine the amount of phosphorus absorbed by them from different varieties of fertilizer.

An interesting application of radioactivity is the method of dating archaeological and geological finds by the concentration of radioactive isotopes. The most commonly used method is radiocarbon dating. An unstable carbon isotope occurs in the atmosphere due to nuclear reactions caused by cosmic rays. A small percentage of this isotope is found in the air along with the usual stable isotope . Plants and other organisms consume carbon from the air, and they accumulate both isotopes in the same proportion as in the air. After the plants die, they stop consuming carbon, and as a result of β-decay, the unstable isotope gradually turns into nitrogen with a half-life of 5730 years. By accurately measuring the relative concentration of radioactive carbon in the remains of ancient organisms, it is possible to determine the time of their death.

List of used literature

1. The doctrine of radioactivity. History and modernity. M. Nauka, 1973 2. Nuclear radiation in science and technology. M. Nauka, 1984 Furman VI 3. Alpha decay and related nuclear reactions. M. Science, 1985

4. Landsberg G.S. Elementary textbook of physics. Volume III. - M.: Nauka, 19865. Seleznev Yu. A. Fundamentals of elementary physics. –M.: Nauka, 1964.6. CD-ROM Big Encyclopedia Cyril and Methodius, 1997.

7. M. Curie, Radioactivity, trans. from French, 2nd ed., M. - L., 1960

8. A. N. Murin, Introduction to radioactivity, L., 1955

9. A. S. Davydov, Theory of the atomic nucleus, Moscow, 1958

10. Gaisinsky M.N., Nuclear chemistry and its applications, transl. from French, Moscow, 1961

11. Experimental Nuclear Physics, ed. E. Segre, trans. from English, vol. 3, M., 1961; INTERNET Network Tools

ISOTOPS-varieties of the same chemical element, similar in their physical and chemical properties, but having different atomic masses. The name "isotopes" was proposed in 1912 by the English radiochemist Frederick Soddy, who formed it from two Greek words: isos - the same and topos - place. Isotopes occupy the same place in the cell of Mendeleev's periodic system of elements.

An atom of any chemical element consists of a positively charged nucleus and a cloud of negatively charged electrons surrounding it. The position of a chemical element in the periodic system of Mendeleev (its serial number) is determined by the charge of the nucleus of its atoms. isotopes are called therefore varieties of the same chemical element whose atoms have the same nuclear charge (and therefore almost the same electron shells), but differ in the values ​​of the mass of the nucleus. According to the figurative expression of F. Soddy, the atoms of isotopes are the same "outside", but different "inside".

The neutron was discovered in 1932 - a particle that has no charge, with a mass close to the mass of the nucleus of a hydrogen atom - a proton , and created proton-neutron model of the nucleus. As a result in science, the final modern definition of the concept of isotopes has been established: isotopes are substances whose atomic nuclei consist of the same number of protons and differ only in the number of neutrons in the nucleus . Each isotope is usually denoted by a set of symbols, where X is the symbol of a chemical element, Z is the charge of the atomic nucleus (the number of protons), A is the mass number of the isotope ( total number nucleons - protons and neutrons in the nucleus, A = Z + N). Since the charge of the nucleus is unambiguously associated with the symbol of the chemical element, often the notation A X is simply used for abbreviation.

Of all the isotopes known to us, only the isotopes of hydrogen have their own names. Thus, the 2 H and 3 H isotopes are called deuterium and tritium and are designated D and T, respectively (the 1 H isotope is sometimes called protium).

They occur naturally as stable isotopes. , and unstable - radioactive, the nuclei of atoms of which are subject to spontaneous transformation into other nuclei with the emission of various particles (or processes of the so-called radioactive decay). Now about 270 stable isotopes are known, and stable isotopes are found only in elements with atomic number Z Ј 83. The number of unstable isotopes exceeds 2000, the vast majority of them were obtained artificially as a result of various nuclear reactions. The number of radioactive isotopes in many elements is very large and can exceed two dozen. The number of stable isotopes is much less. Some chemical elements consist of only one stable isotope (beryllium, fluorine, sodium, aluminum, phosphorus, manganese, gold and a number of other elements). Largest number stable isotopes - 10 found in tin, in iron, for example, they are 4, in mercury - 7.

Discovery of isotopes, historical background.

In 1808, the English naturalist John Dalton first introduced the definition of a chemical element as a substance consisting of atoms of one kind. In 1869, the chemist DIMendeleev discovered the periodic law of chemical elements. One of the difficulties in substantiating the concept of an element as a substance that occupies a certain place in the cell of the periodic system was the experimentally observed non-integer atomic weights of elements. In 1866, the English physicist and chemist - Sir William Crookes put forward the hypothesis that each natural chemical element is a mixture of substances that are identical in their properties, but have different atomic masses, but at that time this assumption did not yet have experimental confirmation and therefore little passed. noticed.

An important step towards the discovery of isotopes was the discovery of the phenomenon of radioactivity and the hypothesis of radioactive decay formulated by Ernst Rutherford and Frederick Soddy: radioactivity is nothing more than the decay of an atom into a charged particle and an atom of another element, which differs in its chemical properties from the original one. As a result, the concept of radioactive series or radioactive families arose. , at the beginning of which there is the first parent element, which is radioactive, and at the end - the last stable element. An analysis of the chains of transformations showed that in their course one and the same radioactive elements, differing only in atomic masses, can appear in one cell of the periodic system. In fact, this meant the introduction of the concept of isotopes.

Independent confirmation of the existence of stable isotopes of chemical elements was then obtained in the experiments of J. J. Thomson and Aston in 1912-1920 with beams of positively charged particles (or so-called canal rays ) emerging from the discharge tube.

In 1919 Aston designed an instrument called the mass spectrograph. (or mass spectrometer) . The discharge tube was still used as the ion source, but Aston found a way in which the successive deflection of the particle beam in the electrical and magnetic fields led to the focusing of particles with the same charge-to-mass ratio (regardless of their speed) at the same point on the screen. Along with Aston, a mass spectrometer of a slightly different design was created in the same years by the American Dempster. As a result of the subsequent use and improvement of mass spectrometers by the efforts of many researchers, by 1935 an almost complete table of the isotopic compositions of all chemical elements known by that time was compiled.

Isotope separation methods.

To study the properties of isotopes, and especially to use them for scientific and applied purposes, it is necessary to obtain them in more or less noticeable quantities. In conventional mass spectrometers, almost complete separation of isotopes is achieved, but their number is negligible. Therefore, the efforts of scientists and engineers were directed to the search for other possible methods of isotope separation. First of all, physical and chemical separation methods were mastered, based on differences in such properties of isotopes of the same element as evaporation rates, equilibrium constants, rates chemical reactions etc. The most effective among them were methods of rectification and isotopic exchange, which are widely used in the industrial production of isotopes of light elements: hydrogen, lithium, boron, carbon, oxygen and nitrogen.

Another group of methods is formed by the so-called molecular-kinetic methods: gaseous diffusion, thermal diffusion, mass diffusion (diffusion in a vapor stream), and centrifugation. Methods gas diffusion, based on different diffusion rates of isotopic components in highly dispersed porous media, were used during the Second World War to organize industrial production separation of uranium isotopes in the United States in the framework of the so-called Manhattan project to create atomic bomb. To obtain the required quantities of uranium, enriched up to 90% with the light isotope 235 U - the main "combustible" component of the atomic bomb, plants were built that occupied an area of ​​about four thousand hectares. More than 2 billion dollars were allocated for the creation of an atomic center with plants for the production of enriched uranium. After the war, plants for the production of enriched uranium for military purposes, also based on the diffusion separation method, were developed and built in the USSR. AT last years this method has given way to a more efficient and less costly centrifugation method. In this method, the effect of separation of the isotope mixture is achieved due to the different action of centrifugal forces on the components of the isotope mixture that fills the centrifuge rotor, which is a thin-walled cylinder limited from above and below, rotating at a very high speed in a vacuum chamber. Hundreds of thousands of centrifuges connected in cascades, the rotor of each of which makes more than a thousand revolutions per second, are currently used in modern separation plants both in Russia and in other developed countries of the world. Centrifuges are used for more than just getting the enriched uranium needed to run nuclear reactors nuclear power plants, but also for the production of isotopes of about thirty chemical elements of the middle part of the periodic table. For the separation of various isotopes, electromagnetic separation installations with powerful ion sources are also used; in recent years, laser separation methods have also become widespread.

The effect of radioactive radiation on humans

Radioactive radiation of all types (alpha, beta, gamma, neutrons), as well as electromagnetic radiation (X-ray radiation) have a very strong biological effect on living organisms, which consists in the processes of excitation and ionization of atoms and molecules that make up living cells. Under the influence of ionizing radiation, complex molecules and cellular structures are destroyed, which leads to radiation damage to the body. Therefore, when working with any source of radiation, it is necessary to take all measures for the radiation protection of people who can fall into the zone of radiation.

However, a person can be exposed to ionizing radiation in domestic conditions. An inert, colorless, radioactive gas radon can pose a serious danger to human health. It is a decay product of radium and has a half-life T = 3.82 days. Radium is found in small amounts in soil, in stones, and in various building structures. Despite the relatively short lifetime, the concentration of radon is continuously replenished due to new decays of radium nuclei, so radon can accumulate in enclosed spaces. Getting into the lungs, radon emits -particles and turns into polonium, which is not a chemically inert substance. This is followed by a chain of radioactive transformations of the uranium series. According to the American Commission on Radiation Safety and Control, the average person receives 55% of ionizing radiation from radon and only 11% from medical care. The contribution of cosmic rays is approximately 8%. The total dose of radiation that a person receives in a lifetime is many times less maximum allowable dose(SDA), which is established for people of certain professions exposed to additional exposure to ionizing radiation.

The use of radioactive isotopes

One of the most outstanding studies carried out with the help of "tagged atoms" was the study of metabolism in organisms. It has been proven that in a relatively short time the body undergoes an almost complete renewal. Its constituent atoms are replaced by new ones. Only iron, as experiments on the isotopic study of blood have shown, is an exception to this rule. Iron is part of the hemoglobin in red blood cells. When radioactive iron atoms were introduced into food, it was found that the free oxygen released during photosynthesis was originally part of water, and not carbon dioxide. Radioactive isotopes are used in medicine for both diagnosis and therapeutic purposes. Radioactive sodium, introduced in small quantities into the blood, is used to study blood circulation, iodine is intensively deposited in the thyroid gland, especially in Graves' disease. By monitoring the deposition of radioactive iodine with a counter, a diagnosis can be made quickly. Large doses of radioactive iodine cause partial destruction of abnormally developing tissues, and therefore radioactive iodine is used to treat Graves' disease. Intense cobalt gamma radiation is used in the treatment of cancer (cobalt gun).

No less extensive are the applications of radioactive isotopes in industry. One example of this is the following method for monitoring piston ring wear in internal combustion engines. By irradiating the piston ring with neutrons, they cause nuclear reactions in it and make it radioactive. When the engine is running, particles of the ring material enter the lubricating oil. By examining the level of radioactivity of the oil after a certain time of engine operation, the wear of the ring is determined. Radioactive isotopes make it possible to judge the diffusion of metals, processes in blast furnaces, etc.

Powerful gamma radiation of radioactive preparations is used to study the internal structure of metal castings in order to detect defects in them.

Radioactive isotopes are being used more and more widely in agriculture. Irradiation of plant seeds (cotton, cabbage, radish, etc.) with small doses of gamma rays from radioactive preparations leads to a noticeable increase in yield. Large doses of "radiation cause mutations in plants and microorganisms, which in some cases leads to the emergence of mutants with new valuable properties (radioselection). Thus, valuable varieties of wheat, beans and other crops have been bred, and highly productive microorganisms used in the production of antibiotics have been obtained. Gamma radiation from radioactive isotopes is also used to control harmful insects and to preserve food. "Tagged atoms" are widely used in agricultural technology. For example, to find out which of the phosphorus fertilizers is better absorbed by the plant, various fertilizers are labeled with radioactive phosphorus 15 32P. then plants for radioactivity, you can determine the amount of phosphorus absorbed by them from different varieties of fertilizer.An interesting application of radioactivity is the method of dating archaeological and geological finds by the concentration of radioactive isotopes.The most commonly used method of radiocarbon dating.Unstable and an isotope of carbon occurs in the atmosphere due to nuclear reactions caused by cosmic rays. A small percentage of this isotope is found in air along with the usual stable isotope. Plants and other organisms consume carbon from the air and accumulate both isotopes in the same proportion as they do in air. After the death of plants, they cease to consume carbon and the unstable isotope, as a result of decay, gradually turns into nitrogen with a half-life of 5730 years. By accurately measuring the relative concentration of radioactive carbon in the remains of ancient organisms, it is possible to determine the time of their death.

The use of radioactivity.

1. Biological actions. Radioactive radiation has a disastrous effect on living cells. The mechanism of this action is associated with the ionization of atoms and the decomposition of molecules inside cells during the passage of fast charged particles. Cells that are in a state of rapid growth and reproduction are especially sensitive to the effects of radiation. This circumstance is used for the treatment of cancerous tumors.

For the purposes of therapy, radioactive preparations emitting g-radiation are used, since the latter penetrate the body without noticeable weakening. At not too high doses of radiation, cancer cells die, while the patient's body does not suffer significant damage. It should be noted that cancer radiotherapy, like X-ray therapy, is by no means a universal remedy that always leads to a cure.

Excessively high doses of radioactive radiation cause severe diseases in animals and humans (the so-called radiation sickness) and can lead to death. In very small doses, radioactive radiation, mainly a-radiation, on the contrary, has a stimulating effect on the body. Related to this is the healing effect of radioactive mineral waters containing small amounts of radium or radon.

2. Luminous compounds. Luminescent substances glow under the action of radioactive radiation (cf. § 213). By adding a very small amount of radium salt to a luminescent substance (for example, zinc sulfide), permanently luminous paints are prepared. These paints, when applied to the dials and hands of watches, sights, etc., make them visible in the dark.

3. Determining the age of the Earth. Atomic mass of ordinary lead, mined from ores that do not contain radioactive elements, is 207.2; these minerals at the time of formation (crystallization from a melt or solution) did not contain lead; all lead available in such minerals has accumulated as a result of the decay of uranium. Using the law of radioactive decay, it is possible to determine its age by the ratio of the amounts of lead and uranium in a mineral.

The age of minerals of various origins containing uranium, determined by this method, is measured in hundreds of millions of years. The oldest minerals are over 1.5 billion years old.

Isotopes, especially radioactive ones, have numerous applications. In table. 1.13 shows selected examples of some of the industrial applications of isotopes. Each technique mentioned in this table is also used in other industries. For example, the technique for determining the leakage of a substance using radioisotopes is used: in the beverage industry to determine the leakage from storage tanks and pipelines; in the construction of engineering structures for

Table 1.13. Some applications of radioisotopes

determination of leakage from underground conduits; in the energy industry to detect leakage from heat exchangers in power plants; in the oil industry to determine the leakage from underground pipelines; in the service of control of waste and sewer water to determine the leakage from the main collectors.

Isotopes are also widely used in scientific research. In particular, they are used to determine the mechanisms of chemical reactions. As an example, consider the use of water labeled with stable oxygen isotope 180 to study the hydrolysis of esters like ethyl acetate (see also Section 19.3). Using mass spectrometry to detect the isotope 180, it was found that during hydrolysis, the oxygen atom from the water molecule goes into acetic acid, and not into ethanol.

Radioisotopes are widely used as labeled atoms in biological research. In order to trace metabolic pathways in living systems, the radioisotopes carbon-14, tritium, phosphorus-32, and sulfur-35 are used. For example, the absorption of phosphorus by plants from fertilized soil can be monitored using fertilizers that contain an admixture of phosphorus-32.

radiation therapy.

Ionizing radiation can destroy living tissue. Tissues of malignant tumors are more sensitive to radiation than healthy tissues. This makes it possible to treat cancers with γ-rays emitted from a source, which is the radioactive isotope cobalt-60. The radiation is directed to the area of ​​the patient's body affected by the tumor; The treatment session lasts a few minutes and is repeated daily for 2-6 weeks. During the session, all other parts of the patient's body must be carefully covered with radiation-impervious material to prevent the destruction of healthy tissues.

Determination of the age of samples using radiocarbon.

A small part of the carbon dioxide that is in the atmosphere contains a radioactive isotope. Plants absorb this isotope during photosynthesis. Therefore, the tissues of all

plants and animals also contain this isotope. Living tissues have a constant level of radioactivity, because its decrease due to radioactive decay is compensated by the constant supply of radiocarbon from the atmosphere. However, as soon as the death of a plant or animal occurs, the flow of radiocarbon into its tissues stops. This leads to a gradual decrease in the level of radioactivity of dead tissues.

The radioactivity of the isotope is due to -decay

The radiocarbon method of geochronology was developed in 1946 by W.F. Libby, who received for him Nobel Prize in Chemistry in 1960. This method is now widely used by archaeologists, anthropologists and geologists to date specimens up to 35,000 years old. The accuracy of this method is approximately 300 years. The best results are obtained when determining the age of wool, seeds, shells and bones. To determine the age of a sample, the activity of p-radiation (decays per minute) is measured per 1 g of carbon contained in it. This allows the age of the sample to be determined using the radioactive decay curve for the isotope.

The half-life for is 5700 years. Living tissue in active contact with the atmosphere has an activity of 15.3 dispersal/min per 1 g of carbon. These data require:

a) determine the decay constant for

b) build a decay curve for

c) calculate the age of Lake Oregon Crater in the USA), which is of volcanic origin. It has been established that a tree turned upside down during

The eruption that resulted in the formation of the lake has an activity of 6.5 dispersal/min per 1 g of carbon.

a) The decay constant can be found from the equation

b) The decay curve is a plot of activity versus time. The data needed to plot this curve can be calculated from the half-life and initial activity of the sample (living tissue activity); these data are given in table. 1.14. The decay curve is shown in fig. 1.32.

c) The age of the lake can be determined using the decay curve (see dashed lines in Fig. 1.32). This age is 7000 years.

Table 1.14. Data for constructing a curve of radioactive decay of carbon used in determining the age of samples

Rice. 1.32. Radioactive isotope decay curve

Many rocks on Earth and the Moon contain radioisotopes with half-lives on the order of years. By measuring and comparing the relative content of these radioisotopes with the relative content of their decay products in samples of such rock formations, one can determine their age. The three most important methods of geochronology are based on the determination of the relative abundance of isotopes (half-life years). (half-life years) and (half-life years).

Method of dating by potassium and argon.

Minerals such as mica and some varieties of feldspar contain small amounts of the radioisotope potassium-40. It decays, undergoing electron capture and turning into argon-40:

The age of the sample is determined based on calculations that use data on the relative content of potassium-40 in the sample compared to argon-40.

Rubidium and strontium dating method.

Some of the oldest rocks on earth, such as the granites off the west coast of Greenland, contain rubidium. Approximately one third of all rubidium atoms are radioactive rubidium-87. This radioisotope decays into the stable isotope strontium-87. Calculations based on the use of data on the relative content of rubidium and strontium isotopes in samples make it possible to determine the age of such rocks.

Dating method for uranium and lead.

Uranium isotopes decay into lead isotopes. The age of minerals such as apatite, which contain impurities of uranium, can be determined by comparing the content in their samples of certain isotopes of uranium and lead.

All three methods described have been used to date terrestrial rocks. The resulting data indicate that the age of the Earth is years. These methods were also used to determine the age of lunar rocks brought to Earth from space missions. The age of these breeds is from 3.2 to years.