Nuclear reactions. The interaction of a particle with an atomic nucleus, leading to the transformation of this nucleus into a new nucleus with the release of secondary particles or gamma rays, is called a nuclear reaction.
First nuclear reaction was carried out by Rutherford in 1919. He discovered that collisions of alpha particles with the nuclei of nitrogen atoms produce rapidly moving protons. This meant that the nucleus of the nitrogen isotope, as a result of a collision with an alpha particle, was transformed into the nucleus of the oxygen isotope:
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Nuclear reactions can occur with the release or absorption of energy. Using the law of the relationship between mass and energy, the energy output of a nuclear reaction can be determined by finding the difference in the masses of the particles entering the reaction and the reaction products:
Chain reaction of fission of uranium nuclei. Among the various nuclear reactions, especially important in modern life human society have chain reactions of fission of some heavy nuclei.
The fission reaction of uranium nuclei when bombarded with neutrons was discovered in 1939. As a result of experimental and theoretical research carried out by E. Fermi, I. Joliot-Curie, O. Hahn, F. Strassmann, L. Meitner, O. Frisch, F. Joliot-Curie, it was found that when one neutron hits a uranium nucleus, the nucleus is divided into two three parts.
The fission of one uranium nucleus releases about 200 MeV of energy. The kinetic energy of the movement of fragment nuclei accounts for approximately 165 MeV, the rest of the energy is carried away by gamma quanta.
Knowing the energy released during the fission of one uranium nucleus, it can be calculated that the energy output from the fission of all nuclei of 1 kg of uranium is 80 thousand billion joules. This is several million times more than what is released when burning 1 kg coal or oil. Therefore, a search was made for ways to liberate nuclear energy in significant quantities to use it for practical purposes.
F. Joliot-Curie was the first to suggest the possibility of chain nuclear reactions in 1934. In 1939, he, together with H. Halban and L. Kowarski, experimentally discovered that during the fission of a uranium nucleus, in addition to nuclear fragments, 2 -3 free neutrons. At favorable conditions these neutrons can hit other uranium nuclei and cause them to fission. When three uranium nuclei fission, 6-9 new neutrons should be released, they will fall into new uranium nuclei, etc. A diagram of the development of a chain reaction of fission of uranium nuclei is presented in Figure 316.
Rice. 316
The practical implementation of chain reactions is not like that simple task how it looks on the diagram. Neutrons released during the fission of uranium nuclei are capable of causing the fission of only nuclei of the uranium isotope with a mass number of 235, but their energy is insufficient to destroy the nuclei of a uranium isotope with a mass number of 238. In natural uranium, the share of uranium with mass number 238 is 99.8%, and the share of uranium with mass number 235 is only 0.7%. Therefore the first possible way The implementation of a fission chain reaction is associated with the separation of uranium isotopes and the production of the isotope in its pure form in sufficiently large quantities. A necessary condition for a chain reaction to occur is the presence of sufficient large quantity uranium, since in a small sample most of the neutrons fly through the sample without hitting any nucleus. The minimum mass of uranium in which a chain reaction can occur is called the critical mass. The critical mass for uranium-235 is several tens of kilograms.
The simplest way to carry out a chain reaction in uranium-235 is the following: two pieces of uranium metal are made, each with a mass slightly less than the critical one. A chain reaction cannot occur in each of them separately. When these pieces are quickly connected, a chain reaction develops and colossal energy is released. The temperature of uranium reaches millions of degrees, the uranium itself and any other substances nearby turn into steam. The hot gaseous ball expands rapidly, burning and destroying everything in its path. This is how a nuclear explosion occurs.
It is very difficult to use the energy of a nuclear explosion for peaceful purposes, since the release of energy is uncontrollable. Controlled chain reactions of fission of uranium nuclei are carried out in nuclear reactors.
Nuclear reactor. The first nuclear reactors were slow neutron reactors (Fig. 317). Most of the neutrons released during the fission of uranium nuclei have an energy of 1-2 MeV. Their speeds are approximately 107 m/s, which is why they are called fast neutrons. At such energies, neutrons interact with uranium and uranium nuclei with approximately the same efficiency. And since there are 140 times more uranium nuclei in natural uranium than uranium nuclei, most of these neutrons are absorbed by uranium nuclei and a chain reaction does not develop. Neutrons moving at speeds close to the speed thermal movement(about 2·10 3 m/s), are called slow or thermal. Slow neutrons interact well with uranium-235 nuclei and are absorbed by them 500 times more efficiently than fast neutrons. Therefore, when natural uranium is irradiated with slow neutrons, most of them are absorbed not in the nuclei of uranium-238, but in the nuclei of uranium-235 and cause their fission. Consequently, for a chain reaction to develop in natural uranium, neutron velocities must be reduced to thermal ones.
Rice. 317
Neutrons slow down as a result of collisions with atomic nuclei of the medium in which they move. To slow down neutrons in a reactor, a special substance called a moderator is used. The nuclei of atoms of the moderator substance must have a relatively small mass, since when colliding with a light nucleus, a neutron loses more energy than when colliding with a heavy one. The most common moderators are ordinary water and graphite.
The space in which the chain reaction occurs is called the reactor core. To reduce neutron leakage, the reactor core is surrounded by a neutron reflector, which rejects a significant portion of the escaping neutrons into the core. The same substance that serves as a moderator is usually used as a reflector.
The energy released during reactor operation is removed using a coolant. Only liquids and gases that do not have the ability to absorb neutrons can be used as a coolant. Ordinary water is widely used as a coolant; sometimes carbon dioxide and even liquid metallic sodium.
The reactor is controlled using special control (or control) rods inserted into the reactor core. Control rods are made of boron or cadmium compounds, which absorb thermal neutrons with very high efficiency. Before the reactor starts operating, they are completely introduced into its core. By absorbing a significant portion of neutrons, they make it impossible for a chain reaction to develop. To start the reactor, the control rods are gradually removed from the core until the energy release reaches a predetermined level. When increasing power above established level automatic machines are switched on, plunging the control rods deep into the core.
Nuclear energy. Nuclear energy was put to the service of peace for the first time in our country. The first organizer and leader of work on atomic science and technology in the USSR was Academician Igor Vasilyevich Kurchatov (1903-1960).
Currently, the largest in the USSR and Europe is the Leningrad NPP named after. IN AND. Lenin has a capacity of 4000 MW, i.e. 800 times the power of the first nuclear power plant.
The cost of electricity generated at large nuclear power plants, lower than the cost of electricity generated at thermal power plants. That's why nuclear power is developing at an accelerated pace.
Nuclear reactors are used as power plants in sea ships. The world's first peaceful ship with a nuclear power plant, the nuclear-powered icebreaker Lenin, was built in the Soviet Union in 1959.
The Soviet nuclear-powered icebreaker Arktika, built in 1975, became the world's first surface ship to reach the North Pole.
Thermonuclear reaction. Nuclear energy is released not only in nuclear reactions of fission of heavy nuclei, but also in reactions of combination of light atomic nuclei.
To connect like-charged protons, it is necessary to overcome the Coulomb repulsive forces, which is possible at sufficiently high velocities of colliding particles. The necessary conditions for the synthesis of helium nuclei from protons are available in the interior of stars. On Earth, thermonuclear fusion reaction was carried out during experimental thermonuclear explosions.
The synthesis of helium from the light isotope of hydrogen occurs at a temperature of about 108 K, and for the synthesis of helium from the heavy isotopes of hydrogen - deuterium and tritium - according to the scheme
requires heating to approximately 5 10 7 K.
When 1 g of helium is synthesized from deuterium and tritium, the energy released is 4.2·10 11 J. This energy is released when 10 tons of diesel fuel are burned.
The reserves of hydrogen on Earth are practically inexhaustible, so the use of thermal energy nuclear fusion for peaceful purposes is one of most important tasks modern science and technology.
The controlled thermonuclear reaction of helium synthesis from heavy isotopes of hydrogen by heating is supposed to be carried out by passing an electric current through the plasma. A magnetic field is used to keep the heated plasma from contacting the chamber walls. On experimental setup"Tokamak-10" Soviet physicists managed to heat the plasma to a temperature of 13 million degrees. Hydrogen can be heated to higher temperatures using laser radiation. To do this, light beams from several lasers must be focused on a glass ball containing a mixture of heavy isotopes of deuterium and tritium. In experiments on laser installations, plasma with a temperature of several tens of millions of degrees has already been obtained.
Class
Lesson No. 42-43
Chain reaction of fission of uranium nuclei. Nuclear energy and ecology. Radioactivity. Half life.
Nuclear reactions
A nuclear reaction is the process of interaction of an atomic nucleus with another nucleus or elementary particle, accompanied by a change in the composition and structure of the nucleus and the release of secondary particles or γ quanta.
As a result of nuclear reactions, new radioactive isotopes can be formed that are not found on Earth under natural conditions.
The first nuclear reaction was carried out by E. Rutherford in 1919 in experiments to detect protons in nuclear decay products (see § 9.5). Rutherford bombarded nitrogen atoms with alpha particles. When the particles collided, a nuclear reaction occurred, proceeding according to the following scheme:
During nuclear reactions several conservation laws: impulse, energy, angular momentum, charge. In addition to these classical laws conservation in nuclear reactions the law of conservation of the so-called baryon charge(that is, the number of nucleons - protons and neutrons). A number of other conservation laws specific to nuclear and particle physics also hold.
Nuclear reactions can occur when atoms are bombarded with fast charged particles (protons, neutrons, α-particles, ions). The first reaction of this kind was carried out using high-energy protons produced at an accelerator in 1932:
where M A and M B are the masses of the initial products, M C and M D are the masses of the final reaction products. The quantity ΔM is called mass defect. Nuclear reactions can occur with the release (Q > 0) or with the absorption of energy (Q< 0). Во втором случае первоначальная кинетическая энергия исходных продуктов должна превышать величину |Q|, которая называется порогом реакции.
In order for a nuclear reaction to have a positive energy output, specific binding energy nucleons in the nuclei of the initial products must be less than the specific binding energy of nucleons in the nuclei of the final products. This means that the ΔM value must be positive.
There are basically two possible different ways liberation of nuclear energy.
1. Fission of heavy nuclei. Unlike the radioactive decay of nuclei, which is accompanied by the emission of α- or β-particles, fission reactions are a process in which an unstable nucleus is divided into two large fragments of comparable masses.
In 1939, German scientists O. Hahn and F. Strassmann discovered the fission of uranium nuclei. Continuing the research begun by Fermi, they found that when uranium is bombarded with neutrons, elements of the middle part appear periodic table– radioactive isotopes of barium (Z = 56), krypton (Z = 36), etc.
Uranium occurs in nature in the form of two isotopes: (99.3%) and (0.7%). When bombarded by neutrons, the nuclei of both isotopes can split into two fragments. In this case, the fission reaction occurs most intensely with slow (thermal) neutrons, while nuclei enter into a fission reaction only with fast neutrons with an energy of the order of 1 MeV.
The main interest for nuclear energy is the fission reaction of a nucleus. Currently, about 100 different isotopes with mass numbers from approximately 90 to 145 are known, resulting from the fission of this nucleus. Two typical fission reactions of this nucleus are:
Note that nuclear fission initiated by a neutron produces new neutrons that can cause fission reactions in other nuclei. The fission products of uranium-235 nuclei can also be other isotopes of barium, xenon, strontium, rubidium, etc.
The kinetic energy released during the fission of one uranium nucleus is enormous - about 200 MeV. An estimate of the energy released during nuclear fission can be made using specific binding energy nucleons in the nucleus. The specific binding energy of nucleons in nuclei with mass number A ≈ 240 is about 7.6 MeV/nucleon, while in nuclei with mass numbers A = 90–145 the specific energy is approximately 8.5 MeV/nucleon. Consequently, the fission of a uranium nucleus releases energy of the order of 0.9 MeV/nucleon, or approximately 210 MeV per uranium atom. The complete fission of all nuclei contained in 1 g of uranium releases the same energy as the combustion of 3 tons of coal or 2.5 tons of oil.
The fission products of the uranium nucleus are unstable because they contain a significant excess number of neutrons. Indeed, the N / Z ratio for the heaviest nuclei is of the order of 1.6 (Fig. 9.6.2), for nuclei with mass numbers from 90 to 145 this ratio is of the order of 1.3–1.4. Therefore, fragment nuclei undergo a series of successive β – -decays, as a result of which the number of protons in the nucleus increases and the number of neutrons decreases until a stable nucleus is formed.
When a uranium-235 nucleus fissions, which is caused by a collision with a neutron, 2 or 3 neutrons are released. Under favorable conditions, these neutrons can hit other uranium nuclei and cause them to fission. At this stage, from 4 to 9 neutrons will appear, capable of causing new decays of uranium nuclei, etc. Such an avalanche-like process is called a chain reaction. Development scheme chain reaction fission of uranium nuclei is shown in Fig. 9.8.1.
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| Figure 9.8.1. Diagram of the development of a chain reaction. |
For a chain reaction to occur, it is necessary that the so-called neutron multiplication factor was more than one. In other words, in each subsequent generation there should be more neutrons than in the previous one. The multiplication coefficient is determined not only by the number of neutrons produced in each elementary act, but also by the conditions under which the reaction occurs - some of the neutrons can be absorbed by other nuclei or leave the reaction zone. Neutrons released during the fission of uranium-235 nuclei are capable of causing the fission of only the nuclei of the same uranium, which accounts for only 0.7% of natural uranium. This concentration is insufficient to start a chain reaction. The isotope can also absorb neutrons, but this does not cause a chain reaction.
A chain reaction in uranium with an increased content of uranium-235 can develop only when the mass of uranium exceeds the so-called critical mass. In small pieces of uranium, most neutrons fly out without hitting any nucleus. For pure uranium-235, the critical mass is about 50 kg. The critical mass of uranium can be reduced many times by using so-called retarders neutrons. The fact is that neutrons produced during the decay of uranium nuclei have too high speeds, and the probability of capturing slow neutrons by uranium-235 nuclei is hundreds of times greater than fast ones. The best neutron moderator is heavy water D 2 O. When interacting with neutrons, ordinary water itself turns into heavy water.
Graphite, whose nuclei do not absorb neutrons, is also a good moderator. During elastic interaction with deuterium or carbon nuclei, neutrons are slowed down to thermal speeds.
The use of neutron moderators and a special beryllium shell, which reflects neutrons, makes it possible to reduce the critical mass to 250 g.
IN atomic bombs An uncontrolled nuclear chain reaction occurs when two pieces of uranium-235, each of which has a mass slightly below critical, quickly combine.
A device that supports a controlled nuclear fission reaction is called nuclear(or atomic) reactor. The diagram of a nuclear reactor using slow neutrons is shown in Fig. 9.8.2.
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| Figure 9.8.2. Diagram of a nuclear reactor. |
The nuclear reaction takes place in the reactor core, which is filled with a moderator and penetrated by rods containing an enriched mixture of uranium isotopes with a high content of uranium-235 (up to 3%). Control rods containing cadmium or boron are introduced into the core, which intensively absorb neutrons. Inserting rods into the core allows you to control the speed of the chain reaction.
The core is cooled using a pumped coolant, which can be water or a metal with a low melting point (for example, sodium, which has a melting point of 98 °C). In a steam generator, the coolant transfers thermal energy to water, turning it into steam high pressure. The steam is sent to a turbine connected to an electric generator. From the turbine, steam enters the condenser. To avoid radiation leakage, the coolant I and steam generator II circuits operate in closed cycles.
The turbine of a nuclear power plant is a heat engine that determines the overall efficiency of the plant in accordance with the second law of thermodynamics. Modern nuclear power plants have a coefficient useful action approximately equal Therefore, to produce 1000 MW of electrical power, the thermal power of the reactor must reach 3000 MW. 2000 MW must be carried away by the water cooling the condenser. This leads to local overheating of natural reservoirs and the subsequent emergence of environmental problems.
However, the main problem is to ensure complete radiation safety people working at nuclear power plants and preventing accidental releases of radioactive substances that accumulate in large quantities in the reactor core. When developing nuclear reactors, much attention is paid to this problem. However, after accidents at some nuclear power plants, in particular at the Pennsylvania nuclear power plant (USA, 1979) and at Chernobyl nuclear power plant(1986), the problem of nuclear energy safety has become particularly acute.
Along with the nuclear reactor operating on slow neutrons described above, reactors operating without a moderator on fast neutrons are of great practical interest. In such reactors, the nuclear fuel is an enriched mixture containing at least 15% of the isotope. The advantage of fast neutron reactors is that during their operation, uranium-238 nuclei, absorbing neutrons, are transformed into plutonium nuclei through two successive β - decays, which then can be used as nuclear fuel:
The breeding factor of such reactors reaches 1.5, that is, for 1 kg of uranium-235 up to 1.5 kg of plutonium is obtained. Conventional reactors also produce plutonium, but in much smaller quantities.
The first nuclear reactor was built in 1942 in the USA under the leadership of E. Fermi. In our country, the first reactor was built in 1946 under the leadership of I.V. Kurchatov.
2. Thermonuclear reactions . The second way to release nuclear energy is associated with fusion reactions. When light nuclei fuse and form a new nucleus, a large amount of energy must be released. This can be seen from the curve of specific binding energy versus mass number A (Fig. 9.6.1). Up to nuclei with a mass number of about 60, the specific binding energy of nucleons increases with increasing A. Therefore, the synthesis of any nucleus with A< 60 из более легких ядер должен сопровождаться выделением энергии. Общая масса продуктов реакции синтеза будет в этом случае меньше массы первоначальных частиц.
Fusion reactions of light nuclei are called thermonuclear reactions, since they can only occur at very high temperatures. In order for two nuclei to enter into a fusion reaction, they must approach to a distance of nuclear forces of the order of 2·10 –15 m, overcoming their electrical repulsion positive charges. For this, the average kinetic energy of thermal motion of molecules must exceed potential energy Coulomb interaction. Calculation of the temperature T required for this leads to a value of the order of 10 8 –10 9 K. This is an extremely high temperature. At this temperature, the substance is in a completely ionized state, which is called plasma.
The energy released during thermonuclear reactions per nucleon is several times higher than the specific energy released in chain reactions of nuclear fission. For example, in the fusion reaction of deuterium and tritium nuclei
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3.5 MeV/nucleon is released. Overall, this reaction releases 17.6 MeV. This is one of the most promising thermonuclear reactions.
Implementation controlled thermonuclear reactions will give humanity a new environmentally friendly and practically inexhaustible source of energy. However, obtaining ultra-high temperatures and confining plasma heated to a billion degrees represents the most difficult scientific and technical task on the path to implementing controlled thermonuclear fusion.
At this stage of development of science and technology, it was possible to implement only uncontrolled fusion reaction V hydrogen bomb. The high temperature required for nuclear fusion is achieved here by the explosion of a conventional uranium or plutonium bomb.
Thermonuclear reactions play extremely important role in the evolution of the Universe. The radiation energy of the Sun and stars is of thermonuclear origin.
Radioactivity
Almost 90% of the known 2500 atomic nuclei are unstable. An unstable nucleus spontaneously transforms into other nuclei, emitting particles. This property of nuclei is called radioactivity. U large kernels instability arises due to competition between the attraction of nucleons by nuclear forces and the Coulomb repulsion of protons. There are no stable nuclei with a charge number Z > 83 and a mass number A > 209. But atomic nuclei with significantly lower values of the Z and A numbers can also be radioactive. If the nucleus contains significantly more protons than neutrons, then the instability is caused by an excess of Coulomb interaction energy . Nuclei that would contain a large excess of neutrons over the number of protons turn out to be 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 barriers 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, studied 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. In Fig. 9.7.1 shows an experimental diagram that allows you to detect complex composition radioactive radiation. In a magnetic field, α- and β-rays experience deviations of opposite sides, and β-rays are deviated much more. γ-rays in a magnetic field are not deflected at all.
These three types of radioactive radiation differ greatly from each other in their ability to ionize atoms of matter and, therefore, in their penetrating ability. α-radiation has the least penetrating ability. In the air at normal conditionsα-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 greatest penetrating ability, capable of passing through a layer of lead 5–10 cm thick.
In the second decade of the 20th century, after E. Rutherford’s discovery of the nuclear structure of atoms, it was firmly established that radioactivity is property of atomic nuclei. Studies have shown that α-rays represent a flow of α-particles - helium nuclei, β-rays are a flow of electrons, γ-rays represent short-wavelength electromagnetic radiation with extremely short wavelength λ< 10 –10 м и вследствие этого – ярко выраженными corpuscular properties, that is, it is a flow of particles - γ-quanta.
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:
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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 from the curvature of the trajectory in a magnetic field, is approximately 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 by known values masses of the mother and daughter nuclei and the helium nucleus. Although the speed of the escaping α-particle is enormous, it is still only 5% of the speed of light, so when calculating, you can use a non-relativistic expression for kinetic energy.
Research has shown that a radioactive substance can emit alpha particles with several discrete energies. This is explained by the fact that nuclei can be, like atoms, in different excited states. The daughter nucleus may end up 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. A diagram of the α-decay of radium with the emission of α-particles with two values of kinetic energies is shown in Fig. 9.7.2.
Thus, 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, that is, an α-particle, can be formed inside nuclei. The mother nucleus is for α-particles potential hole, which is limited potential barrier. The energy of the α particle in the nucleus is not sufficient to overcome this barrier (Fig. 9.7.3). The departure of an alpha particle from the nucleus is possible only due to a quantum mechanical phenomenon called tunnel effect. According to quantum mechanics, there is a non-zero probability of a particle passing under a potential barrier. The phenomenon of tunneling is probabilistic in nature.
Beta decay. During beta decay, an electron is ejected from the nucleus. Electrons cannot exist inside nuclei (see § 9.5); they arise during beta 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. During decay, a neutron turns into a proton and an electron
Measurements have shown 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 resulting from the decay of a 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. The new particle is named neutrino(small neutron). Due to the lack of charge and mass of a neutrino, this particle interacts very weakly with the atoms of matter, so it is extremely difficult to detect in experiment. The ionizing ability of neutrinos is so small that one ionization event in the air occurs approximately 500 km of the way. This particle was discovered only in 1953. It is now known that there are several types of neutrinos. During the decay of a neutron, a particle is produced, which is called electron antineutrino. It is denoted by the symbol Therefore, the neutron decay reaction is written as
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A similar process occurs inside nuclei during β-decay. An electron produced by the decay of one of the nuclear neutrons, is immediately thrown out of the “parental home” (core) with enormous speed, which can differ from the speed of light by only a fraction of a percent. Since the distribution of energy released during β-decay between the electron, neutrino and daughter nucleus is random, β-electrons can have different velocities over a wide range.
During β-decay, the charge number Z increases by one, but the mass number A remains unchanged. The daughter nucleus turns out to be the nucleus of one of the isotopes of the element, serial number which in the periodic table is one higher than the ordinal number of the original nucleus. A typical example of β-decay is the transformation of thorium isotone resulting from the α-decay of uranium into palladium
Gamma decay. Unlike α- and β-radioactivity, γ-radioactivity of nuclei is not associated with a change internal structure nucleus and is not accompanied by a change in charge or mass numbers. Both during α- and β-decay, the daughter nucleus may find itself in some excited state and have an excess of energy. The transition of a nucleus from an excited state to a ground state is accompanied by the emission of one or more γ quanta, the energy of which can reach several MeV.
Law of Radioactive Decay. In any sample radioactive substance contains a huge number of radioactive atoms. Since radioactive decay is random in nature and does not depend on external conditions, the law of decrease in the number N(t) of undecayed at this moment time t nuclei can serve as an important statistical characteristic process of radioactive decay.
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 proportionality coefficient λ is the probability of nuclear decay in 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 quantity τ is called average life time radioactive nucleus.
For practical use It is convenient to write the law of radioactive decay in a different form, using the number 2 rather than e as the base:
The quantity T is called half-life. During time T, half of the original number of radioactive nuclei decays. The quantities T and τ are related by the relation
Half-life is the main quantity characterizing 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 half-lives of a fraction of a second.
Not found naturally, and ends in bismuth. This series of radioactive decays occurs in nuclear reactors.
Interesting application radioactivity is a method of dating archaeological and geological finds by concentration radioactive isotopes. The most commonly used method of dating is radiocarbon dating. An unstable isotope of carbon appears 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 take up carbon from the air and accumulate both isotopes in the same proportions as in the air. After the plants die, they stop consuming carbon and the unstable isotope gradually turns into nitrogen as a result of β-decay with a half-life of 5730 years. By precise measurement The relative concentration of radioactive carbon in the remains of ancient organisms can determine the time of their death.
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 ionizing radiation are destroyed complex molecules And cellular structures, that leads to radiation injury body. Therefore, when working with any source of radiation, it is necessary to take all measures to radiation protection people who may be exposed to radiation.
However, a person can be exposed to ionizing radiation at home. Inert, colorless, radioactive gas radon As can be seen from the diagram shown in Fig. 9.7.5, radon is a product of the α-decay of radium and has a half-life T = 3.82 days. Radium is found in small quantities in soil, stones, and various building structures. Despite the relatively little time life, the radon concentration is continuously replenished due to new decays of radium nuclei, so radon can accumulate in enclosed spaces. Once in the lungs, radon emits α-particles and turns into polonium, which is not a chemically inert substance. What follows is a chain of radioactive transformations of the uranium series (Fig. 9.7.5). According to the American Radiation Safety and Control Commission, the average person receives 55% of ionizing radiation from radon and only 11% from medical services. Contribution cosmic rays is approximately 8%. The total radiation dose a person receives during his life is many times less maximum permissible dose(SDA), which is established for people in certain professions who are subject to additional exposure to ionizing radiation.
Nuclear fission is a process in which 2 (sometimes 3) fragment nuclei are formed from one atomic nucleus, which are similar in mass.
This process is beneficial for everyone β -stable nuclei with mass number A > 100.
Uranium nuclear fission was discovered in 1939 by Hahn and Strassman, who unequivocally proved that when neutrons bombard uranium nuclei U Radioactive nuclei are formed with masses and charges approximately 2 times less than the mass and charge of the uranium nucleus. In the same year, L. Meitner and O. Frischer introduced the term “ nuclear fission"and it was noted that this process releases enormous energy, and F. Joliot-Curie and E. Fermi simultaneously found out that several neutrons are emitted during fission (fission neutrons). This became the basis for putting forward the idea self-sustaining fission chain reaction and the use of nuclear fission as a source of energy. The basis of modern nuclear energy is nuclear fission 235 U And 239 Pu under the influence of neutrons.
Nuclear fission can occur due to the fact that the rest mass of the heavy nucleus turns out to be larger amount rest masses of fragments that arise during fission.
The graph shows that this process turns out to be beneficial with energy point vision.
The mechanism of nuclear fission can be explained on the basis of the droplet model, according to which a bunch of nucleons resembles a droplet of a charged liquid. The nucleus is kept from decay by nuclear attractive forces, greater than the Coulomb repulsion forces that act between protons and tend to tear the nucleus apart.
Core 235 U has the shape of a ball. After absorbing a neutron, it is excited and deformed, acquiring an elongated shape (in the figure b), and stretches until the repulsive forces between the halves of the elongated core become greater than the attractive forces acting in the isthmus (in the figure V). After this, the nucleus breaks into two parts (in the figure G). The fragments, under the influence of Coulomb repulsive forces, fly away at a speed equal to 1/30 of the speed of light.
Emission of neutrons during fission, which we talked about above, is explained by the fact that the relative number of neutrons (relative to the number of protons) in the nucleus increases with increasing atomic number, and for the fragments formed during fission, the number of neutrons becomes greater than is possible for the nuclei of atoms with smaller numbers.
Division often occurs into fragments not equal mass. These fragments are radioactive. After the series β -decays ultimately produce stable ions.
Except forced, it happens spontaneous fission of uranium nuclei, which was opened in 1940 Soviet physicists G. N. Flerov and K. A. Petrzhak. The half-life for spontaneous fission corresponds to 10 16 years, which is 2 million times greater than the half-life for α -decay of uranium.
The synthesis of nuclei occurs in thermonuclear reactions. Thermonuclear reactions is a reaction of fusion of light nuclei at very high temperature. The energy that is released during fusion (synthesis) will be maximum during the synthesis of light elements that have the lowest binding energy. When two light nuclei, such as deuterium and tritium, combine, a heavier helium nucleus with higher binding energy is formed:
With this process of nuclear fusion, significant energy is released (17.6 MeV), equal to the difference in the binding energies of a heavy nucleus and two light nuclei 
. The neutron produced during reactions acquires 70% of this energy. A comparison of the energy per nucleon in the reactions of nuclear fission (0.9 MeV) and fusion (17.6 MeV) shows that the fusion reaction of light nuclei is energetically more favorable than the fission reaction of heavy nuclei.
The fusion of nuclei occurs under the influence of nuclear attraction forces, so they must approach to distances less than 10 -14 at which nuclear forces act. This approach is prevented by the Coulomb repulsion of positively charged nuclei. It can be overcome only due to the high kinetic energy of the nuclei, which exceeds the energy of their Coulomb repulsion. From the corresponding calculations it is clear that the kinetic energy of nuclei, which is needed for the fusion reaction, can be achieved at temperatures of the order of hundreds of millions of degrees, therefore these reactions are called thermonuclear.
Thermonuclear fusion- a reaction in which, at high temperatures above 10 7 K, heavier nuclei are synthesized from light nuclei.
Thermonuclear fusion is the source of energy for all stars, including the Sun.
The main process by which thermonuclear energy is released in stars is the conversion of hydrogen into helium. Due to the mass defect in this reaction, the mass of the Sun decreases by 4 million tons every second.
The large kinetic energy that is needed for thermonuclear fusion is obtained by hydrogen nuclei as a result of strong gravitational attraction to the center of the star. After this, the fusion of helium nuclei produces heavier elements.
Thermonuclear reactions play a major role in evolution chemical composition substances in the Universe. All these reactions occur with the release of energy, which is emitted by stars in the form of light over billions of years.
The implementation of controlled thermonuclear fusion would provide humanity with a new, practically inexhaustible source of energy. Both deuterium and tritium needed for its implementation are quite accessible. The first is contained in the water of the seas and oceans (in quantities sufficient for use for a million years), the second can be obtained in a nuclear reactor by irradiating liquid lithium (the reserves of which are huge) with neutrons:
One of the most important advantages of controlled thermonuclear fusion is the absence radioactive waste during its implementation (in contrast to fission reactions of heavy uranium nuclei).
The main obstacle to the implementation of controlled thermonuclear fusion is the impossibility of confining high-temperature plasma using strong magnetic fields for 0.1-1. However, there is confidence that sooner or later thermonuclear reactors will be created.
So far it has only been possible to produce uncontrollable reaction explosive type synthesis in a hydrogen bomb.
Nuclear fission is the splitting of a heavy atom into two fragments of approximately equal mass, accompanied by the release of a large amount of energy.
The discovery of nuclear fission began new era- "atomic age". The potential of its possible use and the risk-to-benefit ratio of its use not only gave rise to many sociological, political, economic and scientific achievements, but also serious problems. Even with clean scientific point view the process of nuclear fission created big number puzzles and complications, and its full theoretical explanation is a matter for the future.
Sharing is profitable
Binding energies (per nucleon) differ for different nuclei. Heavier ones have lower binding energy than those located in the middle of the periodic table.
This means that heavy nuclei that have atomic number more than 100, it is advantageous to divide into two smaller fragments, thereby releasing energy, which turns into kinetic energy of the fragments. This process is called splitting
According to the stability curve, which shows the dependence of the number of protons on the number of neutrons for stable nuclides, heavier nuclei are preferred larger number neutrons (compared to the number of protons) than lighter ones. This suggests that some "spare" neutrons will be emitted along with the fission process. In addition, they will also absorb part of the released energy. A study of the fission of the nucleus of a uranium atom showed that 3-4 neutrons are released: 238 U → 145 La + 90 Br + 3n.
The atomic number (and atomic mass) of the fragment is not equal to half atomic mass parent. The difference between the masses of atoms formed as a result of splitting is usually about 50. However, the reason for this is not yet entirely clear.
The binding energies of 238 U, 145 La and 90 Br are 1803, 1198 and 763 MeV, respectively. This means that as a result of this reaction, the fission energy of the uranium nucleus is released, equal to 1198 + 763-1803 = 158 MeV.
Spontaneous fission
Spontaneous fission processes are known in nature, but they are very rare. Average life time the specified process is about 10 17 years, and, for example, the average lifetime of alpha decay of the same radionuclide is about 10 11 years.
The reason for this is that in order to split into two parts, the core must first undergo deformation (stretch) into an ellipsoidal shape, and then, before finally splitting into two fragments, form a “neck” in the middle.
Potential barrier
In a deformed state, two forces act on the core. One is increased surface energy (the surface tension of a liquid drop explains its spherical shape), and the other is Coulomb repulsion between fission fragments. Together they produce potential barrier.
As in the case of alpha decay, for spontaneous fission of the nucleus of a uranium atom to occur, the fragments must overcome this barrier with the help of quantum tunneling. The barrier value is about 6 MeV, as in the case of alpha decay, but the probability of an alpha particle tunneling is much greater than that of the much heavier atomic fission product.
Forced splitting
Much more likely is the induced fission of the uranium nucleus. In this case, the mother nucleus is irradiated with neutrons. If the parent absorbs it, they bond, releasing binding energy in the form of vibrational energy that can exceed the 6 MeV required to overcome the potential barrier.
Where the energy of the additional neutron is not sufficient to overcome the potential barrier, the incident neutron must have a minimum kinetic energy in order to be able to induce atomic fission. In the case of 238 U, the binding energy of additional neutrons is missing by about 1 MeV. This means that the fission of a uranium nucleus is induced only by a neutron with a kinetic energy greater than 1 MeV. On the other hand, the 235 U isotope has one unpaired neutron. When a nucleus absorbs an additional one, it pairs with it, and this pairing results in additional binding energy. This is enough to release the amount of energy necessary for the nucleus to overcome the potential barrier and the isotope fission occurs upon collision with any neutron.
Beta decay
Even though the fission reaction produces three or four neutrons, the fragments still contain more neutrons than their stable isobars. This means that cleavage fragments tend to be unstable to beta decay.
For example, when the fission of the uranium nucleus 238 U occurs, the stable isobar with A = 145 is neodymium 145 Nd, which means that the lanthanum 145 La fragment decays in three stages, each time emitting an electron and an antineutrino, until a stable nuclide is formed. A stable isobar with A = 90 is zirconium 90 Zr, so the cleavage fragment of bromine 90 Br decays in five stages of the β-decay chain.
These β-decay chains release additional energy, almost all of which is carried away by electrons and antineutrinos.
Nuclear reactions: fission of uranium nuclei
Direct neutron emission from a nuclide with too many neutrons to ensure nuclear stability is unlikely. The point here is that there is no Coulomb repulsion and so the surface energy tends to keep the neutron bound to the parent. However, this happens sometimes. For example, the fission fragment of 90 Br in the first stage of beta decay produces krypton-90, which can be in an excited state with enough energy to overcome the surface energy. In this case, neutron emission can occur directly with the formation of krypton-89. is still unstable to β decay until it becomes stable yttrium-89, so krypton-89 decays in three steps.
Fission of uranium nuclei: chain reaction
Neutrons emitted in the fission reaction can be absorbed by another parent nucleus, which then itself undergoes induced fission. In the case of uranium-238, the three neutrons that are produced come out with an energy of less than 1 MeV (the energy released during the fission of a uranium nucleus - 158 MeV - is mainly converted into the kinetic energy of the fission fragments), so they cannot cause further fission of this nuclide. However, with a significant concentration of the rare isotope 235 U, these free neutrons can be captured by 235 U nuclei, which can indeed cause fission, since in this case there is no energy threshold below which fission is not induced.
This is the principle of a chain reaction.
Types of nuclear reactions
Let k be the number of neutrons produced in a sample of fissile material at stage n of this chain, divided by the number of neutrons produced at stage n - 1. This number will depend on how many neutrons produced at stage n - 1 are absorbed by the nucleus that may undergo forced division.
If k< 1, то цепная реакция просто выдохнется и процесс остановится очень быстро. Именно это и происходит в природной в которой концентрация 235 U настолько мала, что вероятность поглощения одного из нейтронов этим изотопом крайне ничтожна.
If k > 1, then the chain reaction will grow until all the fissile material has been used up. This is achieved by enriching natural ore until enough high concentration uranium-235. For a spherical sample, the value of k increases with increasing probability of neutron absorption, which depends on the radius of the sphere. Therefore, the mass U must exceed a certain amount so that the fission of uranium nuclei (chain reaction) can occur.
If k = 1, then a controlled reaction takes place. This is used in nuclear reactors. The process is controlled by the distribution of cadmium or boron rods among the uranium, which absorb most neutrons (these elements have the ability to capture neutrons). The fission of the uranium nucleus is controlled automatically by moving the rods so that the value of k remains equal to unity.
The fission of uranium nuclei was discovered in 1938 by German scientists O. Hahn and F. Strassmann. They managed to establish that when uranium nuclei are bombarded with neutrons, elements of the middle part of the periodic table are formed: barium, krypton, etc. The correct interpretation of this fact was given by the Austrian physicist L. Meitner and English physicist O. Frisch. They explained the appearance of these elements by the decay of uranium nuclei that captured a neutron into two approximately equal parts. This phenomenon is called nuclear fission, and the resulting nuclei are called fission fragments.
see also
- Vasiliev A. Uranium fission: from Klaproth to Hahn // Quantum. - 2001. - No. 4. - P. 20-21,30.
Droplet model of the nucleus
This fission reaction can be explained based on the droplet model of the nucleus. In this model, the core is considered as a drop of electrically charged incompressible fluid. In addition to the nuclear forces acting between all nucleons of the nucleus, protons experience additional electrostatic repulsion, as a result of which they are located at the periphery of the nucleus. In an unexcited state, the forces of electrostatic repulsion are compensated, so the nucleus has a spherical shape (Fig. 1, a).
After a \(~^(235)_(92)U\) neutron is captured by the nucleus, intermediate nucleus\(~(^(236)_(92)U)^*\), which is in an excited state. In this case, the neutron energy is evenly distributed among all nucleons, and the intermediate nucleus itself is deformed and begins to vibrate. If the excitation is small, then the nucleus (Fig. 1, b), freeing itself from excess energy by emitting γ -quantum or neutron, returns to steady state. If the excitation energy is sufficiently high, then the deformation of the core during vibrations can be so great that a waist is formed in it (Fig. 1, c), similar to the waist between two parts of a bifurcating drop of liquid. Nuclear forces, operating in a narrow constriction, can no longer withstand significant Coulomb force repulsion of parts of the nucleus. The waist breaks, and the core breaks up into two “fragments” (Fig. 1, d), which fly off in opposite directions.
Currently, about 100 different isotopes with mass numbers from about 90 to 145 are known, resulting from the fission of this nucleus. Two typical fission reactions of this nucleus are:
\(~^(235)_(92)U + \ ^1_0n \ ^(\nearrow)_(\searrow) \ \begin(matrix) ^(144)_(56)Ba + \ ^(89)_( 36)Kr + \ 3^1_0n \\ ^(140)_(54)Xe + \ ^(94)_(38)Sr + \ 2^1_0n \end(matrix)\) .
Note that nuclear fission initiated by a neutron produces new neutrons that can cause fission reactions in other nuclei. The fission products of uranium-235 nuclei can also be other isotopes of barium, xenon, strontium, rubidium, etc.
When the nuclei of heavy atoms fission (\(~^(235)_(92)U\)), very large energy is released - about 200 MeV during the fission of each nucleus. About 80% of this energy is released as kinetic energy of fragments; the remaining 20% comes from the energy of radioactive radiation from fragments and the kinetic energy of prompt neutrons.
An estimate of the energy released during nuclear fission can be made using the specific binding energy of nucleons in the nucleus. Specific binding energy of nucleons in nuclei with mass number A≈ 240 of the order of 7.6 MeV/nucleon, while in nuclei with mass numbers A= 90 – 145 specific energy is approximately 8.5 MeV/nucleon. Consequently, the fission of a uranium nucleus releases energy of the order of 0.9 MeV/nucleon, or approximately 210 MeV per uranium atom. The complete fission of all nuclei contained in 1 g of uranium releases the same energy as the combustion of 3 tons of coal or 2.5 tons of oil.
see also
- Varlamov A.A. Droplet model of the nucleus //Quantum. - 1986. - No. 5. - P. 23-24
Chain reaction
Chain reaction- a nuclear reaction in which the particles causing the reaction are formed as products of this reaction.
When a uranium-235 nucleus fissions, which is caused by a collision with a neutron, 2 or 3 neutrons are released. Under favorable conditions, these neutrons can hit other uranium nuclei and cause them to fission. At this stage, from 4 to 9 neutrons will appear, capable of causing new decays of uranium nuclei, etc. Such an avalanche-like process is called a chain reaction. A diagram of the development of a chain reaction of fission of uranium nuclei is shown in Fig. 3.
Uranium occurs in nature in the form of two isotopes \[~^(238)_(92)U\] (99.3%) and \(~^(235)_(92)U\) (0.7%). When bombarded by neutrons, the nuclei of both isotopes can split into two fragments. In this case, the fission reaction \(~^(235)_(92)U\) occurs most intensively with slow (thermal) neutrons, while the nuclei \(~^(238)_(92)U\) react fission only with fast neutrons with energies of the order of 1 MeV. Otherwise, the excitation energy of the resulting nuclei \(~^(239)_(92)U\) turns out to be insufficient for fission, and then nuclear reactions occur instead of fission:
\(~^(238)_(92)U + \ ^1_0n \to \ ^(239)_(92)U \to \ ^(239)_(93)Np + \ ^0_(-1)e\ ) .
Uranium isotope \(~^(238)_(92)U\) β -radioactive, half-life 23 minutes. The neptunium isotope \(~^(239)_(93)Np\) is also radioactive, with a half-life of about 2 days.
\(~^(239)_(93)Np \to \ ^(239)_(94)Pu + \ ^0_(-1)e\) .
The plutonium isotope \(~^(239)_(94)Np\) is relatively stable, with a half-life of 24,000 years. The most important property plutonium is that it fissions under the influence of neutrons in the same way as \(~^(235)_(92)U\). Therefore, with the help of \(~^(239)_(94)Np\) a chain reaction can be carried out.
The chain reaction diagram discussed above is perfect case. IN real conditions Not all neutrons produced during fission participate in the fission of other nuclei. Some of them are captured by the non-fissile nuclei of foreign atoms, others fly out of the uranium (neutron leakage).
Therefore, a chain reaction of fission of heavy nuclei does not always occur and not for any mass of uranium.
Neutron multiplication factor
The development of a chain reaction is characterized by the so-called neutron multiplication factor TO, which is measured by the ratio of the number N i neutrons causing fission of the nuclei of a substance at one of the stages of the reaction, to the number N i-1 neutrons that caused fission at the previous stage of the reaction:
\(~K = \dfrac(N_i)(N_(i - 1))\) .
The reproduction coefficient depends on a number of factors, in particular on the nature and amount of fissile material, on geometric shape the volume it occupies. Same quantity of this substance It has different meaning TO. TO maximum if the substance has a spherical shape, since in this case the loss of prompt neutrons through the surface will be minimal.
The mass of fissile material in which the chain reaction occurs with a multiplication factor TO= 1 is called critical mass. In small pieces of uranium, most neutrons fly out without hitting any nucleus.
Meaning critical mass determined by the geometry of the physical system, its structure and external environment. Thus, for a ball of pure uranium \(~^(235)_(92)U\) the critical mass is 47 kg (a ball with a diameter of 17 cm). The critical mass of uranium can be reduced many times by using so-called neutron moderators. The fact is that neutrons produced during the decay of uranium nuclei have too high speeds, and the probability of capturing slow neutrons by uranium-235 nuclei is hundreds of times greater than fast ones. The best neutron moderator is heavy water D 2 O. When interacting with neutrons, ordinary water itself turns into heavy water.
Graphite, whose nuclei do not absorb neutrons, is also a good moderator. During elastic interaction with deuterium or carbon nuclei, neutrons are slowed down to thermal speeds.
The use of neutron moderators and a special beryllium shell, which reflects neutrons, makes it possible to reduce the critical mass to 250 g.
At the multiplication rate TO= 1 the number of fissioning nuclei is maintained at a constant level. This mode is provided in nuclear reactors.
If the mass of nuclear fuel is less than the critical mass, then the multiplication factor TO < 1; каждое новое поколение вызывает все меньшее и smaller number divisions, and the reaction without external source neutrons decay quickly.
If the mass of nuclear fuel is greater than the critical mass, then the multiplication factor TO> 1 and each new generation of neutrons causes an increasing number of fissions. The chain reaction grows like an avalanche and has the character of an explosion, accompanied by a huge release of energy and an increase in temperature environment up to several million degrees. This kind of chain reaction occurs when an atomic bomb explodes.
Nuclear bomb
In its normal state, a nuclear bomb does not explode because the nuclear charge in it is divided into several small parts by partitions that absorb the decay products of uranium - neutrons. The nuclear chain reaction that causes a nuclear explosion cannot be sustained under such conditions. However, if fragments of a nuclear charge are combined together, their total mass will become sufficient for a chain reaction of uranium fission to begin to develop. The result is a nuclear explosion. In this case, the explosion power developed nuclear bomb relatively small sizes, is equivalent to the power released during the explosion of millions and billions of tons of TNT.
Rice. 5. Atomic bomb