Radioactivity refers to the ability to spontaneously emit. Radioactivity as evidence of the complex structure of atoms
Radioactivity is the ability of atoms of certain isotopes to spontaneously decay by emitting radiation. For the first time, such radiation emitted by uranium was discovered by Becquerel, therefore, at first, radioactive radiation was called Becquerel rays. The main type of radioactive decay is the ejection of alpha particles from the nucleus of an atom - alpha decay (see Alpha radiation) or beta particles - beta decay (see Beta radiation).
During radioactive decay, the original element turns into an atom of another element. As a result of the ejection of an alpha particle from the nucleus of an atom, which is a combination of two protons and two neutrons, the mass number of the resulting atom (see) decreases by four units, and it turns out to be shifted in the Mendeleev table by two cells to the left, since the serial number of the element in the table is equal to the number of protons in the nucleus of the atom. When a beta particle (electron) is ejected, one neutron is converted into a proton in the nucleus, as a result of which the resulting atom is shifted in the Mendeleev table by one cell to the right. Its mass remains almost unchanged. The ejection of a beta particle is usually associated with (see).
The decay of any radioactive isotope occurs according to the following law: the number of atoms decaying per unit time (n) is proportional to the number of atoms (N) available at a given time, i.e. n=λN; the coefficient λ is called the radioactive decay constant and is related to the half-life of the isotope (T) by the ratio λ= 0.693/T. The indicated decay law leads to the fact that for each time interval equal to the half-life T, the amount of the isotope is halved. If the atoms formed as a result of radioactive decay are also radioactive, then their gradual accumulation occurs until a radioactive equilibrium is established between the parent and daughter isotopes; in this case, the number of atoms of the daughter isotope formed per unit time is equal to the number of atoms decaying in the same time.
More than 40 natural radioactive isotopes are known. Most of them are located in three radioactive rows (families): uranium-radium, and actinium. All of these radioactive isotopes are widely distributed in nature. Their presence in rocks, waters, atmosphere, plant and living organisms causes natural or natural radioactivity.
In addition to natural radioactive isotopes, about a thousand artificially radioactive isotopes are now known. They are obtained by nuclear reactions, mainly in nuclear reactors (see). Many natural and artificially radioactive isotopes are widely used in medicine for treatment (see Radiation therapy) and especially for the diagnosis of diseases (see). See also Ionizing radiation.
Radioactivity (from Latin radius - beam and activus - effective) - the ability of unstable atomic nuclei to spontaneously transform into other, more stable or stable nuclei. Such transformations of nuclei are called radioactive, and the nuclei themselves or the corresponding atoms are called radioactive nuclei (atoms). During radioactive transformations, the nuclei emit energy either in the form of charged particles, or in the form of gamma quanta of electromagnetic radiation or gamma radiation.
Transformations in which the nucleus of one chemical element turns into the nucleus of another element with a different atomic number is called radioactive decay. Radioactive isotopes (see), formed and existing in natural conditions, call naturally radioactive; the same isotopes obtained artificially through nuclear reactions are artificially radioactive. There is no fundamental difference between naturally and artificially radioactive isotopes, since the properties of the nuclei of atoms and the atoms themselves are determined only by the composition and structure of the nucleus and do not depend on the method of their formation.
Radioactivity was discovered in 1896 by A. N. Becquerel, who discovered the radiation of uranium (see), capable of causing blackening of the photographic emulsion and ionizing the air. Curie-Sklodowska (M. Curie-Sklodowska) was the first to measure the radiation intensity of uranium and simultaneously with the German scientist Schmidt (G. S. Schmidt) discovered radioactivity in thorium (see). The property of isotopes to spontaneously emit invisible radiation was called radioactivity by the Curies. In July 1898, they reported their discovery of a new radioactive element, polonium, in uranium resin ore (see). In December 1898, together with G. Bemont, they discovered radium (see).
After the discovery of radioactive elements, a number of authors (Becquerel, the Curies, Rutherford, and others) found that these elements can emit three types of rays that behave differently in a magnetic field. At the suggestion of Rutherford (E. Rutherford, 1902), these rays were called alpha (see Alpha radiation), beta (see Beta radiation) and gamma rays (see Gamma radiation). Alpha rays consist of positively charged alpha particles (doubly ionized helium atoms He4); beta rays - from negatively charged particles of small mass - electrons; Gamma rays are similar in nature to X-rays and are quanta of electromagnetic radiation.
In 1902, Rutherford and F. Soddy explained the phenomenon of radioactivity by the spontaneous transformation of atoms of one element into atoms of another element, occurring according to the laws of chance and accompanied by the release of energy in the form of alpha, beta and gamma rays.
In 1910, M. Curie-Sklodowska, together with A. Debierne, obtained pure metallic radium and investigated its radioactive properties, in particular, measured the decay constant of radium. A number of other radioactive elements were soon discovered. Debjorn and F. Giesel discovered sea anemones. Gan (O. Halm) discovered radiothorium and mesothorium, Boltwood (VV Boltwood) discovered ionium, Gan and L. Meitner discovered protactinium. All isotopes of these elements are radioactive. In 1903, Pierre Curie and C. A. Laborde showed that a preparation of radium always has an elevated temperature and that 1 g of radium with its decay products releases about 140 kcal in 1 hour. In the same year, W. Ramsay and Soddy found that a sealed radium ampule contained gaseous helium. The works of Rutherford, Dorn (F. Dorn), Debierne and Gisel showed that among the decay products of uranium and thorium there are rapidly decaying radioactive gases, called the emanations of radium, thorium and actinium (radon, thoron, actinon). Thus, it was proved that during decay, radium atoms turn into helium and radon atoms. The laws of radioactive transformations of some elements into others during alpha and beta decays (displacement laws) were first formulated by Soddy, Fajans (K. Fajans) and Russell (W. J. Russell).
These laws are as follows. In alpha decay, another element is always obtained from the original element, which is located in the periodic system of D. I. Mendeleev two cells to the left of the original element (the serial or atomic number is 2 less than the original); in beta decay, another element is always obtained from the original element, which is located in the periodic system one cell to the right of the original element (the atomic number is one more than that of the original element).
The study of the transformations of radioactive elements led to the discovery of isotopes, that is, atoms that have the same chemical properties and atomic numbers, but differ from each other in mass and physical properties, in particular, in radioactive properties (type of radiation, decay rate). Of the large number of discovered radioactive substances, only radium (Ra), radon (Rn), polonium (Po) and protactinium (Ra) turned out to be new elements, and the rest were isotopes of previously known uranium (U), thorium (Th), lead (Pb) , thallium (Tl) and bismuth (Bi).
After the discovery by Rutherford of the nuclear structure of atoms and the proof that it is the nucleus that determines all the properties of the atom, in particular the structure of its electron shells and its chemical properties (see Atom, atomic nucleus), it became clear that radioactive transformations are associated with the transformation of atomic nuclei. Further study of the structure of atomic nuclei made it possible to fully decipher the mechanism of radioactive transformations.
The first artificial transformation of nuclei - a nuclear reaction (see) - was carried out by Rutherford in 1919 by bombarding the nuclei of nitrogen atoms with polonium alpha particles. At the same time, the nitrogen nuclei emitted protons (see) and turned into O17 oxygen nuclei. In 1934, F. Joliot-Curie and I. Joliot-Curie (F. Joliot-Curie, I. Joliot-Curie) were the first to artificially obtain a radioactive isotope of phosphorus by bombarding Al atoms with alpha particles. P30 nuclei, in contrast to the nuclei of naturally radioactive isotopes, during decay emitted not electrons, but positrons (see Cosmic radiation) and turned into stable Si30 silicon nuclei. Thus, in 1934, artificial radioactivity and a new type of radioactive decay, positron decay, or β + decay, were simultaneously discovered.
The Joliot-Curies suggested that all fast particles (protons, deuterons, neutrons) cause nuclear reactions and can be used to produce naturally radioactive isotopes. Fermi (E. Fermi) et al., By bombarding various elements with neutrons, received radioactive isotopes of almost all chemical elements. At present, with the help of accelerated charged particles (see Accelerators of charged particles) and neutrons, a wide variety of nuclear reactions have been carried out, as a result of which it has become possible to obtain any radioactive isotopes.
In 1937, Alvarez (L. Alvarez) discovered a new type of radioactive transformation - electronic capture. In electron capture, the nucleus of an atom captures an electron from the shell of the atom and turns into the nucleus of another element. In 1939, Hahn and F. Strassmann discovered the fission of the uranium nucleus into lighter nuclei (fission fragments) when bombarded with neutrons. In the same year, Flerov and Petrzhak showed that the process of fission of uranium nuclei is carried out spontaneously without external influence. Thus, they discovered a new type of radioactive transformation - the spontaneous fission of heavy nuclei.
At present, the following types of radioactive transformations are known that occur without external influences, spontaneously, due to only internal causes due to the structure of atomic nuclei.
1. Alpha decay. A nucleus with atomic number Z and mass number A emits an alpha particle - a helium nucleus He4 - and turns into another nucleus with Z less than 2 units and A less than 4 units than the original nucleus. In general, alpha decay is written as follows:
Where X is the original nucleus, Y is the nucleus of the decay product.
2. beta decay there are two types: electronic and positron, or β - - and β + -decay (see Beta radiation). During electronic decay, an electron and a neutrino fly out of the nucleus and a new nucleus is formed with the same mass number A, but with an atomic number Z one greater than that of the original nucleus:
During positron decay, the nucleus emits a positron and a neutrino and a new nucleus is formed with the same mass number, but with Z one less than that of the original nucleus:
During beta decay, on average, 2/3 of the energy of the nucleus is carried away by neutrino particles (neutrino particles of very small mass, interacting very weakly with matter).
3. Electronic capture(former name K-capture). The nucleus captures an electron from one of the shells of the atom, most often from the K-shell, emits a neutrino and turns into a new nucleus with the same mass number A, but with an atomic number Z less than 1 than that of the original nucleus.
The transformation of nuclei during electron capture and positron decay is the same; therefore, these two types of decay are observed simultaneously for the same nuclei, i.e., they are competing. Since after the capture of an electron from the inner shell of an atom, an electron from one of the orbits more distant from the nucleus passes to its place, the electron capture is always accompanied by the emission of characteristic x-ray radiation.
4. Isomeric transition. After emitting an alpha or beta particle, some types of nuclei are in an excited state (a state with excess energy) and emit excitation energy in the form of gamma rays (see Gamma radiation). In this case, during radioactive decay, the nucleus, in addition to alpha or beta particles, also emits gamma quanta. Thus, the nuclei of the isotope Sr90 emit only β-particles, the nuclei of Na24 emit, in addition to β-particles, also gamma quanta. Most of the nuclei are in an excited state for very short periods of time that cannot be measured (less than 10 -9 sec.). However, only a relatively small number of nuclei can be in an excited state for relatively long periods of time - up to several months. Such nuclei are called isomers, and their corresponding transitions from an excited state to a normal one, accompanied by the emission of only gamma rays, are isomeric. During isomeric transitions A and Z, the nuclei do not change. Radioactive nuclei emitting only alpha or beta particles are called pure alpha or beta emitters. Nuclei in which alpha or beta decay is accompanied by the emission of gamma rays are called gamma emitters. Pure gamma emitters are only nuclei that are in an excited state for a long time, i.e., undergoing isomeric transitions.
5. Spontaneous nuclear fission. As a result of fission, two lighter nuclei are formed from one nucleus - fission fragments. Since identical nuclei can be divided in different ways into two nuclei, in the process of fission many different pairs of lighter nuclei with different Z and A are formed. During fission, neutrons are released, on average 2-3 neutrons per nuclear fission event, and gamma quanta . All fragments formed during fission are unstable and undergo β - decay. The probability of fission is very small for uranium, but increases with increasing Z. This explains the absence of nuclei heavier than uranium on Earth. In stable nuclei, there is a certain ratio between the number of protons and neutrons, in which the nucleus has the greatest stability, i.e. the highest binding energy of particles in the nucleus. For light and medium nuclei, their greatest stability corresponds to an approximately equal content of protons and neutrons. For heavier nuclei, a relative increase in the number of neutrons in stable nuclei is observed. With an excess of protons or neutrons in the nucleus, nuclei with an average value of A are unstable and undergo β - - or β + -decays, during which the mutual transformation of a neutron and a proton occurs. With an excess of neutrons (heavy isotopes), one of the neutrons is converted into a proton with the emission of an electron and a neutrino: 
With an excess of protons (light isotopes), one of the protons is converted into a neutron with the emission of either a positron and a neutrino (β + decay), or only a neutrino (electron capture): 
All heavy nuclei with an atomic number greater than Pb82 are unstable due to the significant number of protons repelling each other. Chains of successive alpha and beta decays in these nuclei occur until stable nuclei of lead isotopes are formed. With the improvement of experimental technique, more and more nuclei, previously considered stable, are found to have very slow radioactive decay. There are currently 20 known radioactive isotopes with Z less than 82.
As a result of any radioactive transformations, the number of atoms of a given isotope continuously decreases. The law of decrease over time of the number of active atoms (the law of radioactive decay) is common to all types of transformations and all isotopes. It is statistical in nature (applicable only to a large number of radioactive atoms) and is as follows. The number of active atoms of a given isotope decaying per unit time ΔN/Δt is proportional to the number of active atoms N, i.e., the same fraction k of active atoms of a given isotope decays per unit time, regardless of their number. The value of k is called the radioactive decay constant and represents the fraction of active atoms decaying per unit time, or the relative decay rate. k is measured in reciprocal units of time, i.e. in sec.-1 (1 / sec.), day-1, year-1, etc., for each radioactive isotope has its own specific value, which changes over a very wide range for different isotopes. The value characterizing the absolute decay rate is called the activity of a given isotope or drug. The activity of 1 g of a substance is called the specific activity of the substance.
From the law of radioactive decay it follows that the decrease in the number of active atoms N first occurs rapidly, and then more and more slowly. The time during which the number of active atoms or the activity of a given isotope is halved is called the half-life (T) of the given isotope. The law of decrease of N from time t is exponential and has the following analytical expression: N=N0e-λt, where N0 is the number of active atoms at the moment of the beginning of the time reference (r=0), N is the number of active atoms after time t, e is the base of natural logarithms (a number equal to 2.718...). Between the decay constant k and the half-life λ there is the following relationship: λT-0.693. From here 
Half-lives are measured in seconds, min. etc., and for various isotopes vary over a very wide range from small fractions of a second to 10 + 21 years. Isotopes with large λ and small T are called short-lived, isotopes with small λ and large T are called long-lived. If the active substance consists of several radioactive isotopes with different half-lives, genetically unrelated, then over time the activity of the substance will also continuously decrease and the isotopic composition of the drug will change all the time: the proportion of short-lived isotopes will decrease and the proportion of long-lived isotopes will increase. After a sufficiently long period of time, practically only the longest-lived isotope will remain in the preparation. From the decay curves of radioactive substances consisting of one or a mixture of isotopes, one can determine the half-lives of individual isotopes and their relative activities for any moment in time.
The laws of change in the activity of genetically related isotopes are qualitatively different; they depend on the ratio of their half-lives. For two genetically related isotopes with period T1 for the original isotope and T2 for the decay product, these laws have the simplest form. At T1>T2, the activity of the initial isotope Q1 decreases all the time according to an exponential law with a half-life of T1. Due to the decay of the nuclei of the initial isotope, the nuclei of the final isotope will be formed and its activity Q2 will increase. After a certain time, the rate of decay of the nuclei of the second isotope (will become close to the rate of formation of nuclei of this isotope from the original one (the rate of decay of the initial isotope Q1) and these rates will be in a certain and constant ratio for the rest of the time - radioactive equilibrium sets in.
The activity of the initial isotope continuously decreases with the period T1, therefore, after reaching radioactive equilibrium, the activity of the final isotope Q2 and the total activity of the two isotopes Q1 + Q2 will also decrease with the half-life of the initial isotope T1. At T1>T2 Q2=Q1. If several short-lived isotopes are formed successively from the original long-lived isotope, as is the case in the radioactive series of uranium and radium, then after equilibrium is reached, the activities of each short-lived isotope become practically equal to the activity of the original isotope. In this case, the total activity is equal to the sum of the activities of all short-lived decay products and decreases with the half-life period of the initial long-lived isotope, as well as the activity of all isotopes in equilibrium.
Radioactive equilibrium is reached practically in a time equal to 5-10 half-lives of the isotope of the decay products that has the longest half-life. If T1 Naturally radioactive isotopes include about 40 isotopes of the periodic table of elements with Z greater than 82, which form three consecutive series of radioactive transformations: the uranium series (Fig. 1), the thorium series (Fig. 2) and the actinium series (Fig. 3). Through successive alpha and beta decays, the final stable isotopes of lead are obtained from the initial isotopes of the series. The arrows in the figures indicate successive radioactive transformations, indicating the type of decay and the percentage of atoms undergoing decay of this type. Horizontal arrows indicate transformations that occur in almost 100% of cases, and slanted arrows - in a small part of cases. When designating isotopes, their half-lives are indicated. In parentheses are the former names of the members of the series, indicating a genetic relationship, without parentheses - the currently accepted designations of isotopes, corresponding to their chemical and physical nature. Long-lived isotopes are enclosed in frames, and final stable isotopes are enclosed in double frames. Alpha decay is usually accompanied by very low intensity gamma radiation, some beta emitters emit intense gamma radiation. The natural background is due to natural radioactivity-radiation and exposure to naturally radioactive isotopes contained on the surface of the Earth, in the biosphere and air, and cosmic radiation (see). In addition to these isotopes, various substances also contain the K40 isotope and about 20 other radioactive isotopes with very long half-lives (from 109 to 1021 years), as a result of which their relative activity is very low compared to the activity of other isotopes. The radioactive isotopes contained in the Earth's shell have played and are playing an exceptional role in the development of our planet, in particular in the development and preservation of life, since they compensated for the heat losses occurring on the Earth and ensured the temperature on the planet was practically constant for many millions of years. Radioactive isotopes, like all other isotopes, are found in nature mainly in a diffuse state and are present in all substances, plant and animal organisms. Due to the difference in the physicochemical properties of isotopes, their relative content in soils and waters is not the same. The gaseous decay products of uranium, thorium and actinium - thoron, radon and actinon - from soil water continuously enter the air. In addition to these gaseous products, the air also contains alpha and beta active decay products of radium, thorium and actinium (in the form of aerosols). From the soil, radioactive elements, as well as stable ones, enter plants together with soil water, so the stems and leaves of plants always contain uranium, radium, thorium with their decay products, potassium and a number of other isotopes, although in relatively low concentrations. Plants and animals also contain isotopes C14, H3, Be7 and others, which are formed in the air under the influence of cosmic radiation neutrons. Due to the fact that there is a continuous exchange between the human body and the environment, all radioactive isotopes contained in food, water and air are contained in the body. Isotopes are found in the body in the following doses: in soft tissues - 31 mrem / year, in bones - 44 mrem / year. The dose from cosmic radiation is 80-90 mrem/year, the dose from external gamma radiation is 60-80 mrem/year. The total dose is 140-200 mrem/year. The dose falling on the lungs is 600-800 mrem/year. Artificially radioactive isotopes are obtained by bombarding stable isotopes with neutrons or charged particles as a result of various nuclear reactions, various types of accelerators are used as sources of charged particles. For measurements of fluxes and doses of various types of ionizing radiation, see Dosimetry, Doses of ionizing radiation, Neutron. Due to the fact that large doses of radiation adversely affect human health, special protective measures are applied when working with radiation sources and radioactive isotopes (see). In medicine and biology, isotopes are used to study metabolism, for diagnostic and therapeutic purposes (see). The content of radioactive isotopes in the body and the dynamics of their metabolism is determined using counters of external radiation from a person. The assumption that all bodies are made up of tiny particles was made by the ancient Greek philosophers Leucippus and Democritus about 2500 years ago. These particles were called atoms, which means "indivisible." An atom is the smallest, simplest, non-component and therefore indivisible particle. But from about the middle of the XIX century. experimental facts began to appear that cast doubt on the idea of the indivisibility of atoms. The results of these experiments suggested that atoms have a complex structure and that they contain electrically charged particles. The most striking evidence of the complex structure of the atom was the discovery of the phenomenon of radioactivity, made by the French physicist Henri Becquerel in 1896. Henri Becquerel (1852-1908) Becquerel discovered that the chemical element uranium spontaneously (that is, without external influences) emits previously unknown invisible rays, which were later called radioactive radiation. Since radioactive radiation had unusual properties, many scientists began to study it. It turned out that not only uranium, but also some other chemical elements (for example, radium) also spontaneously emit radioactive rays. The ability of atoms of some chemical elements to spontaneous radiation began to be called radioactivity (from Latin radio - I radiate and activus - effective). Ernest Rutherford (1871-1935) In 1899, as a result of an experiment conducted under the guidance of the English physicist Ernest Rutherford, it was discovered that the radioactive radiation of radium is inhomogeneous, that is, it has a complex composition. Let's see how this experiment was carried out. Figure 156a shows a thick-walled lead vessel with a grain of radium at the bottom. A beam of radioactive radiation from radium exits through a narrow hole and hits a photographic plate (radium radiation occurs in all directions, but it cannot pass through a thick layer of lead). After developing the photographic plate, one dark spot was found on it - just in the place where the beam hit. Rice. 156. Scheme of Rutherford's experiment to determine the composition of radioactive radiation Then the experiment was changed (Fig. 156, b): a strong magnetic field was created, which acted on the beam. In this case, three spots appeared on the developed plate: one, the central one, was in the same place as before, and the other two were on opposite sides of the central one. If two streams deviated from the previous direction in a magnetic field, then they are streams of charged particles. The deviation in different directions indicated different signs of the electric charges of the particles. In one stream, only positively charged particles were present, in the other, negatively charged ones. And the central flow was radiation that did not have an electric charge. Positively charged particles are called alpha particles, negatively charged particles are called beta particles, and neutral particles are called gamma particles or gamma quanta. Joseph John Thomson(1856-1940) Some time later, as a result of studying various physical characteristics and properties of these particles (electric charge, mass, etc.), it was possible to establish that the β-particle is an electron, and the α-particle is a fully ionized atom of the chemical element helium (i.e., an atom helium, which has lost both electrons). It also turned out that γ-radiation is one of the types, or rather ranges, of electromagnetic radiation (see Fig. 136). The phenomenon of radioactivity, i.e., spontaneous emission of α-, β- and α-particles by matter, along with other experimental facts, served as the basis for the assumption that the atoms of matter have a complex composition. Since it was known that the atom as a whole is neutral, this phenomenon led to the assumption that the composition of the atom includes negatively and positively charged particles. Based on these and some other facts, the English physicist Joseph John Thomson proposed in 1903 one of the first models of the structure of the atom. According to Thomson, an atom is a sphere, throughout the volume of which a positive charge is evenly distributed. Inside this sphere are electrons. Each electron can oscillate around its equilibrium position. The positive charge of the ball is equal in absolute value to the total negative charge of the electrons, so the electric charge of the atom as a whole is equal to zero. The model of the structure of the atom proposed by Thomson needed experimental verification. In particular, it was important to check whether the positive charge is indeed distributed over the entire volume of the atom with a constant density. Therefore, in 1911, Rutherford, together with his colleagues, conducted a series of experiments to study the composition and structure of atoms. To understand how these experiments were carried out, consider Figure 157. In the experiments, a lead vessel C was used with a radioactive substance P that emits α-particles. From this vessel, α-particles fly out through a narrow channel at a speed of the order of 15,000 km/s. Rice. 157. Scheme of the installation of Rutherford's experiment on the study of the structure of the atom Since α-particles cannot be directly seen, a glass screen E is used to detect them. The screen is covered with a thin layer of a special substance, due to which flashes occur at the points where α-particles hit the screen, which are observed using a microscope M. This method of registering particles is called the method , scintillations (i.e. flashes). This entire setup is placed in a vessel from which the air has been evacuated (in order to eliminate the scattering of α-particles due to their collisions with air molecules). If there are no obstacles in the way of α-particles, then they fall on the screen in a narrow, slightly expanding beam (Fig. 157, a). In this case, all flashes appearing on the screen merge into one small spot of light. If, on the way of α-particles, a thin foil F of the metal under study is placed (Fig. 157, b), then when interacting with the substance, α-particles scatter in all directions at different angles φ (only three angles are shown in the figure: φ1, φ2 and φ3). When the screen is at position 1, the most flashes are located in the center of the screen. This means that the main part of all α-particles passed through the foil, almost without changing the original direction (scattered at small angles). As you move away from the center of the screen, the number of flashes becomes smaller. Consequently, with an increase in the scattering angle φ, the number of particles scattered at these angles sharply decreases. By moving the screen along with the microscope around the foil, one can find that a certain (very small) number of particles are scattered at angles close to 90° (this position of the screen is indicated by the number 2), and some single particles are scattered at angles of the order of 180°, i.e. as a result of interaction with the foil were thrown back (position 3). It was these cases of large-angle scattering of α-particles that gave Rutherford the most important information for understanding how the atoms of matter are arranged. After analyzing the results of the experiments, Rutherford came to the conclusion that such a strong deflection of α-particles is possible only if there is an extremely strong electric field inside the atom. Such a field could be created by a charge concentrated in a very small volume (compared to the volume of an atom). One of the examples of a schematic representation of the nuclear model of the atom proposed by E. Rutherford Rice. 158. Flight trajectories of α-particles when passing through the atoms of matter Since the mass of an electron is approximately 8000 times less than the mass of an α-particle, the electrons that make up the atom could not significantly change the direction of motion of the α-particles. Therefore, in this case, we can only talk about the forces of electrical repulsion between α-particles and the positively charged part of the atom, the mass of which is much greater than the mass of the α-particle. These considerations led Rutherford to create a nuclear (planetary) model of the atom (which you already know about from the 8th grade physics course). Recall that, according to this model, a positively charged nucleus is located in the center of the atom, occupying a very small volume of the atom. Electrons move around the nucleus, the mass of which is much less than the mass of the nucleus. An atom is electrically neutral because the charge of the nucleus is equal to the modulus of the total charge of the electrons. Rutherford was able to estimate the size of atomic nuclei. It turned out that, depending on the mass of an atom, its nucleus has a diameter of the order of 10 -14 - 10 -15 m, i.e., it is tens and even hundreds of thousands of times smaller than an atom (an atom has a diameter of about 10 -10 m). Figure 158 illustrates the passage of α-particles through the atoms of matter from the point of view of the nuclear model. This figure shows how the trajectory of the flight of α-particles changes depending on how far from the nucleus they fly. The strength of the electric field created by the nucleus, and hence the force of action on the α-particle, decreases rather quickly with increasing distance from the nucleus. Therefore, the direction of the particle's flight changes greatly only if it passes very close to the nucleus. Since the diameter of the nucleus is much smaller than the diameter of the atom, most of all α-particles pass through the atom at such distances from the nucleus, where the repulsive force of the field created by it is too small to significantly change the direction of the α-particles. And only very few particles fly near the nucleus, i.e., in the region of a strong field, and are deflected at large angles. It is these results that were obtained in Rutherford's experiment. Thus, as a result of experiments on the scattering of α-particles, the inconsistency of the Thomson model of the atom was proved, the nuclear model of the structure of the atom was put forward, and the diameters of atomic nuclei were estimated. The purpose of the lesson: to give students an idea of radioactivity During the classes The hypothesis that all bodies are made up of tiny particles was put forward by the ancient Greek philosophers Leucippus and Democritus over two millennia ago. These particles were called "atoms", which means indivisible. But from the middle of the 9th century, the idea of the indivisibility of the atom was called into question. Experimental work has shown that their structure includes electrically charged particles. Becquerel Antoine Henri French physicist (for the discovery of the radioactivity of uranium he was awarded the Nobel Prize in 1903, holder of all distinctions of the Paris Academy of Sciences, Member of the Royal Society). The discovery of natural radioactivity, a phenomenon that proves the complex composition of the atomic nucleus, happened due to a happy accident. In 1896, French physicist Antoine Becquerel discovered that uranium salt lying next to a packaged photographic plate caused it to turn black. The study of this penetrating uranium radiation, together with Pierre and Marie Curie, led to the discovery of radioactivity. Thus began the atomic era in human history. Becquerel discovered that the chemical element uranium spontaneously, that is, without any external influences, emits previously invisible rays. Intensive research began. It was found that the radiation of uranium salts ionizes the air and rarefies the electroscope. These rays were later called radioactive radiation. This ability of the atoms of some chemical elements to spontaneous radiation began to be called radioactivity. RADIOACTIVITY (from Latin radio - I emit rays and activus - effective), the spontaneous transformation of unstable atomic nuclei into the nuclei of other elements, accompanied by the emission of particles or a g-quantum. 4 types of radioactivity are known: alpha decay, beta decay, spontaneous fission of atomic nuclei, proton radioactivity (two-proton and two-neutron radioactivity have been predicted, but have not yet been observed). Radioactivity is characterized by an exponential decrease in the average number of nuclei over time. In 1899, Ernest Rutherford experimentally discovered that the radioactive radiation of radium is inhomogeneous and has a complex composition. In a thick-walled lead vessel, he placed a grain of radium. A beam of radioactive radiation from radium passed through a narrow hole and hit a photographic plate. After developing the photographic plate, one spot was found on it. Then the experiment was modified, now the radiation beam passed through the region of the magnetic field before hitting the photographic plate. As a result, the magnetic field divided this beam into three, and after development, three spots were found on the photographic plate - one in the center, two - to the side of it. This suggests that the radiation beam consisted of positively charged α alpha particles, negatively charged β beta particles and neutral γ gamma particles. These three types of radiation are very different from each other in penetrating power. α alpha rays have the least penetrating power. A layer of paper about 0.1 mm thick is already opaque for them. For β beta rays, an aluminum plate with a thickness of several millimeters is opaque, γ gamma rays are the most penetrating, a lead layer 1 cm thick is not an insurmountable obstacle for them. In terms of their properties, γ gamma rays resemble X-rays. These are electromagnetic waves with a length of 10 -8 to 10 -11 cm. It was easier to experiment with β beta rays, as they were strongly deflected in both magnetic and electric fields. In the study, it was found that they are electrons moving at speeds very close to the speed of light. It turned out to be more difficult to reveal the nature of α alpha particles. Rutherford finally solved this riddle. The alpha particles turned out to be the nuclei of the helium atom, i.e. it is a fully ionized atom of the chemical element helium. What happens to matter when exposed to radiation? First, the amazing constancy with which radioactive elements emit radiation. During the day, months, years, the radiation intensity does not noticeably change. It is not affected by heating or an increase in pressure, the chemical reactions in which the radioactive element entered also did not affect the intensity of the radiation. Secondly, radioactivity is accompanied by the release of energy, and it is released continuously over a number of years. Where does this energy come from? When radioactive, the substance undergoes some changes. It was suggested that the atoms themselves undergo transformations. Later it was discovered that as a result of atomic transformation, a new substance is formed, of a completely new type, completely different in its physical and chemical properties from the original. This new substance itself is also unstable and undergoes transformations with the emission of characteristic radioactive radiation. So, the phenomenon of radioactivity indicates that the atoms of substances have a complex composition. III. Consolidation of the studied IV. Homework 1. § 55, answer questions. ELECTRON (e, e -), stable negatively charged elementary particle with spin 1/2, mass approx. 9·10 -28 g and a magnetic moment equal to the Bohr magneton; refers to leptons and participates in electromagnetic, weak and gravitational interactions. An electron is one of the main structural elements of matter; The electron shells of atoms determine the optical, electrical, magnetic, and chemical properties of atoms and molecules, as well as most of the properties of solids. ALPHA DECAY (a-decay), a type of radioactive decay of atomic nuclei, when an alpha particle is emitted, the charge of the nucleus decreases by 2 units, the mass number - by 4. St. 3000 a-active nuclei, most of which are obtained artificially. ALPHA PARTICLE (a-particle), the nucleus of a helium atom containing 2 protons and 2 neutrons. Questions. 1. What was the discovery made by Becquerel in 1896? Becquerel in 1896 discovered that the chemical element uranium U spontaneously emits invisible rays. 2. How did they begin to call the ability of atoms of some chemical elements to spontaneous radiation? This ability came to be called radioactivity. 3. Tell us how the experiment was carried out, the scheme of which is shown in Figures 167, a, b. What emerged from this experience? In the experiment in Fig. 167 a grain of radium Ra was placed in a thick-walled vessel. From it, through the slit, a beam of radioactive radiation comes out, which illuminates the photographic plate. Then the beam was affected by a magnetic field, as a result of which the beam splits into three streams: positively charged, negatively charged and neutral, which was recorded by the formation of three spots on the photographic plate. 4. What were the names of the particles that make up the radioactive emission? What are these particles? It was found that radioactive radiation consists of three types of particles: α-particles - ionized helium atoms He, β-particles - electrons and γ-particles - photons. THEME OF THE LESSON “Discovery of Radioactivity.
Alpha, beta and gamma radiation.
Lesson goals.
Educational
- expansion of students' ideas about the physical picture of the world on the example of the phenomenon of radioactivity; study patterns Educational
– to continue the formation of skills: the theoretical method of studying physical processes; compare, generalize; to establish connections between the studied facts; put forward hypotheses and justify them. educators
– on the example of the life and work of Marie and Pierre Curie to show the role of scientists in the development of science; show the non-randomness of random discoveries; (thought: the responsibility of a scientist, a discoverer for the fruits of his discoveries), continue the formation of cognitive interests, collective skills, combined with independent work. The course and content of the lesson
.Organizing time
Message about the topic and purpose of the lesson
2. The stage of preparation for the study of a new topic
Actualization of the available knowledge of students in the form of checking homework and a cursory frontal survey of students. 3. Stage of assimilation of new knowledge (25 min)
Radioactivity appeared on the earth from the time of its formation, and man in the entire history of the development of his civilization was under the influence of natural sources of radiation. The Earth is exposed to the radiation background, the sources of which are solar radiation, cosmic radiation, radiation from radioactive elements lying in the Earth.
What is radiation? How does it arise? What types of radiation exist? And how to protect yourself from it? The word "radiation" comes from the Latin radius and stands for beam. In principle, radiation is all types of radiation existing in nature - radio waves, visible light, ultraviolet, and so on. But radiations are different, some of them are useful, some are harmful. In ordinary life, we are accustomed to the word radiation to call harmful radiation arising from the radioactivity of certain types of matter. Let's analyze how the phenomenon of radioactivity is explained in physics lessons
The discovery of radioactivity was due to a happy accident. Becquerel studied the luminescence of substances previously irradiated with sunlight for a long time. He wrapped the photographic plate in thick black paper, placed grains of uranium salt on top, and exposed it to bright sunlight. After developing, the photographic plate turned black in those areas where the salt lay. Becquerel thought that the radiation of uranium arises under the influence of sunlight. But one day, in February 1896, he failed to conduct another experiment due to cloudy weather. Becquerel put the record back in a drawer, placing on top of it a copper cross covered with uranium salt. Having developed the plate, just in case, two days later, he found blackening on it in the form of a distinct shadow of a cross. This meant that uranium salts spontaneously, without any external influences, create some kind of radiation. Intensive research began. Soon, Becquerel established an important fact: the intensity of radiation is determined only by the amount of uranium in the preparation, and does not depend on which compounds it is included in. Therefore, radiation is inherent not in compounds, but in the chemical element uranium. Then a similar quality was discovered in thorium. Becquerel Antoine Henri French physicist. He graduated from the Polytechnic School in Paris. The main works are devoted to radioactivity and optics. In 1896 he discovered the phenomenon of radioactivity. In 1901, he discovered the physiological effect of radioactive radiation. Becquerel was awarded the Nobel Prize in 1903 for his discovery of the natural radioactivity of uranium. (1903, together with P. Curie and M. Sklodowska-Curie). Discovery of radium and polonium.
In 1898, other French scientists Marie Skłodowska-Curie and Pierre Curie isolated two new substances from the uranium mineral, much more radioactive than uranium and thorium. So two previously unknown radioactive elements were discovered - polonium and radium. It was exhausting work, for four long years the couple almost did not leave their damp and cold barn. Polonium (Po-84) was named after Mary's homeland, Poland. Radium (Ra -88) - radiant, the term radioactivity was proposed by Maria Sklodowska. All elements with serial numbers greater than 83 are radioactive, i.e. located in the periodic table after bismuth. For 10 years of joint work, they have done a lot to study the phenomenon of radioactivity. It was a selfless work in the name of science - in a poorly equipped laboratory and in the absence of the necessary funds, the researchers received the preparation of radium in 1902 in the amount of 0.1 g. To do this, they took 45 months of hard work there and more than 10,000 chemical liberation and crystallization operations. Nobel Prize in Physics. RADIOACTIVITY is the ability of some atomic nuclei to spontaneously transform into other nuclei, while emitting various particles: any spontaneous radioactive decay is exothermic, that is, it occurs with the release of heat. The body of Marie Sklodowska-Curie, enclosed in a lead coffin, still emits radioactivity with an intensity of 360 becquerel/M3 at a rate of about 13 bq/M3... She was buried with her husband... The complex composition of radioactive radiation
In 1899, under the guidance of the English scientist E. Rutherford, an experiment was carried out that made it possible to detect the complex composition of radioactive radiation. As a result of an experiment conducted under the guidance of an English physicist ,
it was found that the radioactive radiation of radium is inhomogeneous, i.e. it has a complex structure. Rutherford Ernst (1871-1937), English physicist, one of the creators of the theory of radioactivity and the structure of the atom, founder of a scientific school, foreign corresponding member of the Russian Academy of Sciences (1922) and honorary member of the USSR Academy of Sciences (1925). Director of the Cavendish Laboratory (since 1919). Opened (1899) alpha and beta rays and established their nature. Created (1903, together with F. Soddy) the theory of radioactivity. He proposed (1911) a planetary model of the atom. Carried out (1919) the first artificial nuclear reaction. Predicted (1921) the existence of the neutron. Nobel Prize (1908). A classic experiment that made it possible to detect the complex composition of radioactive radiation. The radium preparation was placed in a lead container with a hole. A photographic plate was placed opposite the hole. A strong magnetic field acted on the radiation. Almost 90% of known nuclei are unstable. Radioactive nuclei can emit particles of three types: positively charged (α-particles - helium nuclei), negatively charged (β-particles - electrons) and neutral (γ-particles - quanta of short-wave electromagnetic radiation). The magnetic field allows these particles to be separated. 4) Penetration α .β. γ radiation
α-rays have the least penetrating power. A layer of paper 0.1 mm thick is already opaque for them. . β-rays are completely blocked by an aluminum plate several mm thick. γ-rays, when passing through a 1 cm layer of lead, reduce the intensity by 2 times. 5) Physical nature of α .β. γ radiation
γ-radiation electromagnetic waves 10 -10 -10 -13 m Gamma radiation is photons, i.e. electromagnetic wave that carries energy. In the air, it can travel long distances, gradually losing energy as a result of collisions with the atoms of the medium. Intense gamma radiation, if not protected from it, can damage not only the skin, but also internal tissues. Dense and heavy materials such as iron and lead are excellent barriers to gamma radiation.
β-rays - a stream of electrons moving at speeds close to the speed of light. α -rays - the nucleus of the helium atom Stage of consolidation of new knowledge.
1. What was the discovery made by Becquerel in 1896? 2. How did they begin to call the ability of atoms of some chemical elements to spontaneous radiation? 3. Tell us how the experiment was carried out, the scheme of which is shown in the figure. What emerged from this experience? 4. What were the names of the particles that make up the radioactive emission? 5. What are these particles? 6. What did the phenomenon of radioactivity testify to? 5. Stage of debriefing, information about homework.
Homework §§
99,100
Rice. 1. Series of uranium. 
Rice. 2. Thorium series. 
Rice. 3. A series of sea anemones.
French physicist. One of the discoverers of radioactivity
English physicist. He discovered the complex composition of the radioactive radiation of radium, proposed a nuclear model of the structure of the atom. Discovered the proton
English physicist. Opened electron. He proposed one of the first models of the structure of the atom
Questions
Lesson 51
Radioactivity as evidence of the complex structure of atoms
Analysis of control work
Learning new material
What was the discovery made by Becquerel in 1896?
How did the ability of atoms of some chemical elements to spontaneous radiation come to be called?
What is the name of the particles that make up radioactive radiation?
What does the phenomenon of radioactivity indicate?
Loading...