A message on the topic of nuclear forces. Nuclear forces and their properties

NUCLEAR FORCES

Physical encyclopedic dictionary. - M.: Soviet Encyclopedia. . 1983 .

NUCLEAR FORCES

Interaction forces between nucleons; provide a greater amount of nuclear binding energy compared to other systems. I'm with. are the most important and common example strong interaction(SV). Once upon a time, these concepts were synonymous and the term “” itself was introduced to emphasize the enormous magnitude of Ya. in comparison with other forces known in nature: electric-magnetic, weak, gravitational. After opening p -, r - and etc. mesons, hyperons, etc. hadrons the term “strong” began to be used in a broader sense - as the interaction of hadrons. In the 1970s quantum chromodynamics(QCD) has established itself as a generally recognized microscope. SV theory. According to this theory, are composite particles consisting of quarks And gluons, and by SV they began to understand the interaction of these funds. particles.

On the other hand, Ya. s. how the forces of interaction between nucleons include not only SW, but also el.-magn., weak and gravitational. interactions of nucleons. From the point of view of modern theories, el.-magn. and weak interactions are manifestations of one, more fundamental, electroweak interaction. However, at those space-time scales (~10 -13 cm, ~10 -23 s), which are usually dealt with in atomic nuclei, the single nature of the electric magnet. and weak forces practically do not appear and they can be considered as independent. These interactions, being much weaker than SW, are insignificant in most nuclear processes, but situations are possible when their role becomes decisive. So, el.-magn. interaction (the most significant part of which is the Coulomb repulsion between protons), in contrast to SV, is long-range. Therefore, conditioned by him will put. the Coulomb nucleus increases with increasing number of particles A in the kernel is faster than negative. part of nuclear energy due to SW. As a result, heavy nuclei become A unstable - first with respect to fission (see. nuclear fission), and then completely unstable. Co weak interaction nucleons are associated with such a phenomenon as parity nonconservation in nucleon-nucleon scattering and in other nuclear phenomena (see. Non-conservation of parity in kernels). Gravity the forces acting between nucleons are negligible in all nuclear phenomena and are significant only in astrophysics. conditions (see Neutron).

The basis of Ya.s. is the strong interaction of nucleons. The strong interaction of nucleons in nuclei differs from the interaction of free nucleons, but the latter is the foundation on which the entire theory of nuclear energy is built. This interaction has isotopic invariance. Its essence is that the interaction between 2 neutrons, 2 protons, or between a proton and a neutron in the same quantum states is the same. Therefore, we can talk about the interaction between nucleons without specifying which nucleons we are talking about (see also Isotopic invariance nuclear forces). I'm with. are short-range (their radius of action is ~10 -13 cm) and have a saturation property, which means that with an increase in the number of nucleons in the nucleus, the beat. nucleons remains approximately constant (Fig. 1). This leads to the possibility of existence nuclear matter.

Since nucleons in a nucleus move, as a rule, at relatively low speeds (3-4 times less than the speed of light), then to construct a model of SW nucleons in nuclei, one can use a non-relativistic theory and approximately describe its potential, which is a distance function r between nucleons. Unlike Coulomb and gravitation. potentials inversely proportional to distance, Ya.s. depends on r much more difficult. In addition, the potential of Ya. depends on nucleon spins and orbital momentum L relative motion of nucleons.

Nonrelativistic potential of Ya.s. contains several components: central V C , tensor V T, spin-orbit VLS and quadratic spin-orbit potential VLL. Naib. an important one - the central one - is a combination of strong repulsion at short distances (i.e. nuclear matter). There are models of SW nucleons with an infinite (“hard”) core (for example, the phenomenological Hamada-Johnston potential), as well as more realistic ones. models with a finite (“soft”) core (for example, the Reid potential, Fig. 2). From the end 1950s Attempts were made to build the potential of the Ya. based on the field theory of meson-nucleon interaction. The obvious difficulties of such a theory are associated with the high strength of interaction and the inapplicability of perturbation theory and methods based on it. Semi-phenomenological is very popular. potential of “one-boson exchange”, based on the concepts of meson-nucleon field theory, but using the simplest model of one-meson exchange. It turned out that to describe attraction at intermediate distances it is necessary, in addition to the known mesons p, p, w,... introduce also the exchange of the non-existent s-meson, which is interpreted as eff. taking into account the exchange of two p-mesons. The meson-nucleon interaction constants were considered as phenomenological. parameters, which were selected so that the potential described the experiment. phases of nucleon-nucleon scattering. The w- and r-mesons turned out to be responsible for short-range repulsion, and for long-range attraction - pi meson. The one-pion exchange term contributes to the central and tensor potentials:

Where f p NN- pion-nucleon interaction constant, T p - pion mass, l= With/m p =1.4 fm - Compton wavelength peony, a s 1 , s 2-spin Pauli matrices. As can be seen from expressions (1), (2), the one-pion exchange potential decreases exponentially over a distance on the order of the Compton length of the pion. Dr. terms of the one-boson exchange potential have the same type of exponential. factors, but with Compton lengths of the corresponding bosons, which are several. times less than peony. At such distances there are several exchanges. pions can be as significant as the exchange of one heavy meson. This explains why the terms corresponding to the exchange of heavy mesons are perceived as semi-phenomenological. At the same time, the type of potential Ya.c, at large distances, is undoubtedly described by expressions (1), (2). So asymptotic. all, without exception, have a phenomenological appearance. potentials. Currently max. the so-called is considered accurate. Parisian and Bonn potentials, which combine phenomenological features. soft-core potentials and single-boson exchange potentials.

Modern ideas about the nature of SW based on QCD have posed the problem of calculating the potential of SW nucleons within the framework of QCD, but it has not yet been solved, since the simpler problem of constructing a theory of one nucleon has not been solved. There are several quark models hadrons, of which the most the model of bags in different types is known. options. It allows us to qualitatively understand the nature of the repulsive core, estimate its radius and height, but does not allow us to calculate the type of potential at large distances. From the QCD point of view, the status of mesons (with the exception of the p-meson) in the formation of the SW potential of nucleons is a big question: the exchange of heavy mesons between nucleons occurs at such small distances that their quark-gluon nature becomes significant. A special place in the QCD theory of SW belongs to the p-meson. According to modern ideas, it is interpreted as a collective vacuum, consisting of a large number of quark-antiquark ( gold stone, associated with spontaneous breakdown in QCD chiral symmetry). Therefore, in most modern models, all other hadrons are considered to consist of a small number of quarks (antiquarks, gluons), and the r-meson is additionally introduced as an independent particle. From this point of view, the status of potentials (1), (2) as describing the “tail” of the nucleon interaction potential is understandable.

Since Wed. the distance between the nucleons in the nucleus (1.8 fm) does not greatly exceed the radius of action of the nuclear system, then in the nuclei there are multiparticle (primarily 3-particle) forces arising due to the exchange of quarks and gluons between several. nucleons almost simultaneously. In terms of hadrons, this corresponds to processes of meson exchange between, for example, three nucleons, which cannot be reduced to a set of successive pair exchanges. Ch. A role in the formation of 3-particle forces is played by the exchange of p-mesons, and beings. the contribution is also made by the virtual excitation of the D-isobar - the first excited nucleon. Thus, D-isobars are the main non-nucleon degrees of freedom, which are important in nuclear processes. Many-particle forces in nuclei are relatively small: their contribution to the binding energy does not exceed 10-15%. However, there are phenomena where they play a major role. role.

Ch. part el.-magn. The interaction of nucleons is the Coulomb repulsion between protons. At large distances it is determined only by the charges of protons. SV leads to the fact that the electric proton is not point-like, but distributed at distances of 1 fm (the root-mean-square radius of a proton is 0.8 fm; see "Size" of an elementary particle). Electric interaction at short distances also depends on the charge distribution inside the proton. This is modern. SW theory cannot reliably calculate, but it is quite well known from experiments. data on electron-proton scattering. Neutrons are generally electrically neutral, but due to the SW charge inside the neutron also exists, which leads to electric. interaction between two neutrons and between a neutron and a proton. Magn. the interaction between neutrons is of the same order as between protons due to the large magnitude anomalous magnetic moment, caused by SV. The situation with the weak interaction of nucleons is less clear. Although the weak interaction is well known, the SW leads to a renormalization of the corresponding interaction constants (an analogue of the anomalous magnetic moment) and the appearance form factors. As in the case of el.-magn. interactions, the effects of weak interaction cannot be reliably calculated, but in this case they are not known experimentally. The available data on the magnitude of parity nonconservation effects in a 2-nucleon system make it possible to establish the intensity of this interaction, but not its structure. There are several alternative models of weak interaction of nucleons, which describe 2-nucleon experiments equally well, but lead to differences. consequences for atomic nuclei.

Lit.: Bohr O., Mottelson B., Structure of the atomic nucleus, trans. from English, vol. 1-2, M., 1971-77; Calogero F., Simonov Yu. A., Nuclear forces, saturation and structure of nuclei, in: The Future of Science, v. 9, M., 1976. E. E. Saperstein.

Physical encyclopedia. In 5 volumes. - M.: Soviet Encyclopedia. Editor-in-chief A. M. Prokhorov. 1988 .

See what “NUCLEAR FORCES” is in other dictionaries:

Modern encyclopedia

Forces that hold nucleons (protons and neutrons) in the nucleus. Nuclear forces act only at distances of no more than 10-13 cm and reach a value 100-1000 times greater than the force of interaction of electric charges. Nuclear forces do not depend on charge... ... Big Encyclopedic Dictionary

Nuclear forces- NUCLEAR FORCES, forces that hold nucleons (protons and neutrons) in the nucleus. Nuclear forces act only at distances of no more than 10-13 cm, are 100-1000 times greater than the force of interaction of electric charges and do not depend on the charge of nucleons. Nuclear forces... Illustrated Encyclopedic Dictionary

The collective name of units, formations and associations intended to carry out military tasks using nuclear weapons. The concept of “Nuclear Forces” includes: military formations armed with various carriers... ... Naval Dictionary

NUCLEAR FORCES- cm … Big Polytechnic Encyclopedia

Forces that hold nucleons (protons and neutrons) in the nucleus. They cause the most intense interactions known in physics (see Strong interactions). I'm with. are short-range (their radius of action Nuclear forces is 10 13 cm, ... ... Great Soviet Encyclopedia

nuclear forces- Short-range forces that bind protons and neutrons in atomic nuclei; have the property of charge independence. [A.S. Goldberg. English-Russian energy dictionary. 2006] Topics: energy in general EN nuclear forces ... Technical Translator's Guide

An atomic nucleus, consisting of a certain number of protons and neutrons, is a single whole due to specific forces that act between the nucleons of the nucleus and are called nuclear. It has been experimentally proven that nuclear forces have very large values, much greater than the forces of electrostatic repulsion between protons. This is manifested in the fact that the specific binding energy of nucleons in the nucleus is much greater than the work done by the Coulomb repulsion forces. Let us consider the main features of nuclear forces.

1. Nuclear forces are short-range attractive forces . They appear only at very small distances between nucleons in the nucleus of the order of 10–15 m. A distance of the order of (1.5 – 2.2)·10–15 m is called the radius of action of nuclear forces; with its increase, nuclear forces quickly decrease. At a distance of the order of (2-3) m, nuclear interaction between nucleons is practically absent.

2. Nuclear forces have the property saturation, those. each nucleon interacts only with a certain number of nearest neighbors. This nature of nuclear forces is manifested in the approximate constancy of the specific binding energy of nucleons at charge number A>40. Indeed, if there were no saturation, then the specific binding energy would increase with the number of nucleons in the nucleus.

3. A feature of nuclear forces is also their charge independence , i.e. they do not depend on the charge of the nucleons, so the nuclear interactions between protons and neutrons are the same. The charge independence of nuclear forces is visible from a comparison of binding energies mirror cores . This is the name given to nuclei in which the total number of nucleons is the same, but the number of protons in one is equal to the number of neutrons in the other. For example, the binding energies of helium and heavy hydrogen – tritium nuclei are respectively 7.72 MeV and 8.49 MeV. The difference in binding energies of these nuclei, equal to 0.77 MeV, corresponds to the energy of the Coulomb repulsion of two protons in the nucleus. Assuming this value to be equal to , we can find that the average distance r between protons in the nucleus is 1.9·10 –15 m, which is consistent with the radius of action of nuclear forces.

4. Nuclear forces are not central and depend on the mutual orientation of the spins of interacting nucleons. This is confirmed by the different nature of neutron scattering by ortho- and parahydrogen molecules. In an orthohydrogen molecule, the spins of both protons are parallel to each other, while in a parahydrogen molecule they are antiparallel. Experiments have shown that neutron scattering on parahydrogen is 30 times greater than scattering on orthohydrogen.

The complex nature of nuclear forces does not allow the development of a single, consistent theory of nuclear interaction, although many different approaches have been proposed. According to the hypothesis of the Japanese physicist H. Yukawa, which he proposed in 1935, nuclear forces are caused by exchange - mesons, i.e. elementary particles whose mass is approximately 7 times less than the mass of nucleons. According to this model, a nucleon in time m- meson mass) emits a meson, which, moving at a speed close to the speed of light, covers a distance , after which it is absorbed by the second nucleon. In turn, the second nucleon also emits a meson, which is absorbed by the first. In H. Yukawa’s model, therefore, the distance at which nucleons interact is determined by the meson path length, which corresponds to a distance of about m and in order of magnitude coincides with the radius of action of nuclear forces.

Let us turn to the consideration of the exchange interaction between nucleons. There are positive, negative and neutral mesons. The modulus of charge - or - mesons is numerically equal to the elementary charge e. The mass of charged mesons is the same and equal to (140 MeV), meson mass is 264 (135 MeV). The spin of both charged and neutral mesons is 0. All three particles are unstable. The lifetime of - and - mesons is 2.6 With, - meson – 0.8·10 -16 With. The interaction between nucleons is carried out according to one of the following schemes:

(22.7)
1. Nucleons exchange mesons:

In this case, the proton emits a meson, turning into a neutron. The meson is absorbed by a neutron, which consequently turns into a proton, then the same process occurs in the opposite direction. Thus, each of the interacting nucleons spends part of the time in a charged state and part in a neutral state.

2. Nucleons exchange - mesons:

3. Nucleons exchange - mesons:

. (22.10)

All these processes have been proven experimentally. In particular, the first process is confirmed when a neutron beam passes through hydrogen. Moving protons appear in the beam, and a corresponding number of practically resting neutrons are detected in the target.

Kernel models. The absence of a mathematical law for nuclear forces does not allow the creation of a unified theory of the nucleus. Attempts to create such a theory encounter serious difficulties. Here are some of them:

1. Lack of knowledge about the forces acting between nucleons.

2. The extreme cumbersomeness of the quantum many-body problem (a nucleus with a mass number A is a system of A tel).

These difficulties force us to take the path of creating nuclear models that make it possible to describe a certain set of nuclear properties using relatively simple mathematical means. None of these models can give an absolutely accurate description of the nucleus. Therefore, you have to use several models.

Under kernel model in nuclear physics they understand a set of physical and mathematical assumptions with the help of which it is possible to calculate the characteristics of a nuclear system consisting of A nucleons. Many models of varying degrees of complexity have been proposed and developed. We will consider only the most famous of them.

Hydrodynamic (drip) model of the core was developed in 1939. N. Bohr and Soviet scientist J. Frenkel. It is based on the assumption that, due to the high density of nucleons in the nucleus and the extremely strong interaction between them, the independent movement of individual nucleons is impossible and the nucleus is a drop of charged liquid with density . As with a normal drop of liquid, the surface of the core can oscillate. If the amplitude of vibrations becomes large enough, the process of nuclear fission occurs. The droplet model made it possible to obtain a formula for the binding energy of nucleons in the nucleus and explained the mechanism of some nuclear reactions. However, this model does not explain most of the excitation spectra of atomic nuclei and the special stability of some of them. This is due to the fact that the hydrodynamic model very approximately reflects the essence of the internal structure of the core.

Shell model of the kernel developed in 1940-1950 by the American physicist M. Geppert - Mayer and the German physicist H. Jensen. It assumes that each nucleon moves independently of the others in some average potential field (potential well created by the remaining nucleons of the nucleus. Within the framework of the shell model, the function is not calculated, but is selected so that the best agreement with experimental data can be achieved.

The depth of the potential well is usually ~ (40-50) MeV and does not depend on the number of nucleons in the nucleus. According to quantum theory, nucleons in a field are at certain discrete energy levels. The main assumption of the creators of the shell model about the independent movement of nucleons in an average potential field is in conflict with the basic provisions of the developers of the hydrodynamic model. Therefore, the characteristics of the core, which are well described by the hydrodynamic model (for example, the value of the binding energy), cannot be explained within the framework of the shell model, and vice versa.

Generalized kernel model , developed in 1950-1953, combines the main provisions of the creators of the hydrodynamic and shell models. In the generalized model, it is assumed that the nucleus consists of an internal stable part - the core, which is formed by the nucleons of filled shells, and external nucleons moving in the field created by the nucleons of the core. In this regard, the motion of the core is described by a hydrodynamic model, and the motion of external nucleons by a shell model. Due to interaction with external nucleons, the core can be deformed, and the core can rotate around an axis perpendicular to the deformation axis. The generalized model made it possible to explain the main features of the rotational and vibrational spectra of atomic nuclei, as well as the high values of the quadrupole electric moment of some of them.

We have considered the main phenomenological ones, i.e. descriptive,kernel models. However, to fully understand the nature of nuclear interactions that determine the properties and structure of the nucleus, it is necessary to create a theory in which the nucleus would be considered as a system of interacting nucleons.

1. Nuclear forces are short-range forces of attraction . They appear only at very small distances between nucleons in a nucleus of the order of 10 –15 m. The length (1.5 – 2.2) 10 –15 m is called range of nuclear forces they decrease rapidly with increasing distance between nucleons. At a distance of (2-3) m, nuclear interaction is practically absent.

3. A feature of nuclear forces is also their charge independence , i.e. they do not depend on the charge of nucleons, therefore the nuclear interactions between protons and neutrons are the same. The charge independence of nuclear forces is visible from a comparison of binding energies mirror cores.This is what the kernels are called, in which the total number of nucleons is the same, but the number of protons in one is equal to the number of neutrons in the other. For example, the binding energies of helium and heavy hydrogen – tritium nuclei are respectively 7.72 MeV and 8.49 MeV The difference in the binding energies of these nuclei, equal to 0.77 MeV, corresponds to the energy of the Coulomb repulsion of two protons in the nucleus. Assuming this value to be equal, we can find that the average distance r between protons in the nucleus is 1.9·10 –15 m, which is consistent with the radius of action of nuclear forces.

4. Nuclear forces are not central and depend on the mutual orientation of the spins of interacting nucleons. This is confirmed by the different nature of neutron scattering by ortho- and parahydrogen molecules. In an orthohydrogen molecule, the spins of both protons are parallel to each other, while in a parahydrogen molecule they are antiparallel. Experiments have shown that neutron scattering from parahydrogen is 30 times greater than scattering from orthohydrogen.

The complex nature of nuclear forces does not allow the development of a single, consistent theory of nuclear interaction, although many different approaches have been proposed. According to the hypothesis of the Japanese physicist H. Yukawa (1907-1981), which he proposed in 1935, nuclear forces are caused by exchange - mesons, i.e. elementary particles whose mass is approximately 7 times less than the mass of nucleons. According to this model, nucleon time m- meson mass) emits a meson, which, moving at a speed close to the speed of light, covers a distance, after which it is absorbed by a second nucleon. In turn, the second nucleon also emits a meson, which is absorbed by the first. In H. Yukawa’s model, therefore, the distance at which nucleons interact is determined by the meson path length, which corresponds to a distance of about m and in order of magnitude coincides with the radius of action of nuclear forces.

Question 26. Fission reactions. In 1938, German scientists O. Hahn (1879-1968) and F. Strassmann (1902-1980) discovered that when uranium is bombarded with neutrons, nuclei sometimes appear that are approximately half the size of the original uranium nucleus. This phenomenon was called nuclear fission.

It represents the first experimentally observed nuclear transformation reaction. An example is one of the possible fission reactions of the uranium-235 nucleus:

The process of nuclear fission proceeds very quickly (within ~10 -12 s). The energy released during a reaction of type (7.14) is approximately 200 MeV per fission event of the uranium-235 nucleus.

In general, the fission reaction of the uranium-235 nucleus can be written as:

Neutrons (7.15)

The mechanism of the fission reaction can be explained within the framework of the hydrodynamic model of the nucleus. According to this model, when a neutron is absorbed by a uranium nucleus, it goes into an excited state (Fig. 7.2).

The excess energy that the nucleus receives due to the absorption of a neutron causes more intense movement of nucleons. As a result, the nucleus is deformed, which leads to a weakening of the short-range nuclear interaction. If the excitation energy of the nucleus is greater than a certain energy called activation energy , then under the influence of the electrostatic repulsion of protons the nucleus splits into two parts, emitting fission neutrons . If the excitation energy upon absorption of a neutron is less than the activation energy, then the nucleus does not reach

critical stage of fission and, having emitted a quantum, returns to the ground

state.

An important feature of the nuclear fission reaction is the ability to implement a self-sustaining nuclear chain reaction on its basis. . This is due to the fact that each fission event produces, on average, more than one neutron. Mass, charge and kinetic energy of fragments X And Uh, formed during a fission reaction of type (7.15) are different. These fragments are quickly inhibited by the medium, causing ionization, heating and disruption of its structure. The use of the kinetic energy of fission fragments due to their heating of the environment is the basis for the conversion of nuclear energy into thermal energy. The fragments of nuclear fission are in an excited state after the reaction and pass to the ground state by emitting β - particles and -quanta.

Controlled nuclear reaction carried out in nuclear reactor and is accompanied by the release of energy. The first nuclear reactor was built in 1942 in the USA (Chicago) under the leadership of physicist E. Fermi (1901 - 1954). In the USSR, the first nuclear reactor was created in 1946 under the leadership of I.V. Kurchatov. Then, after gaining experience in controlling nuclear reactions, they began to build nuclear power plants.

Question 27. Synthesis reaction. Nuclear fusion called the fusion reaction of protons and neutrons or individual light nuclei, as a result of which a heavier nucleus is formed. The simplest nuclear fusion reactions are:

, ΔQ = 17.59 MeV; (7.17)

Calculations show that the energy released during nuclear fusion reactions per unit mass significantly exceeds the energy released in nuclear fission reactions. During the fission reaction of the uranium-235 nucleus, approximately 200 MeV is released, i.e. 200:235=0.85 MeV per nucleon, and during the fusion reaction (7.17) the energy released is approximately 17.5 MeV, i.e. 3.5 MeV per nucleon (17.5:5=3.5 MeV). Thus, the fusion process is approximately 4 times more efficient than the uranium fission process (per one nucleon of the nucleus participating in the fission reaction).

The high speed of these reactions and the relatively high energy release make an equal mixture of deuterium and tritium the most promising for solving the problem controlled thermonuclear fusion. Mankind's hopes for solving its energy problems are connected with controlled thermonuclear fusion. The situation is that the reserves of uranium, as a raw material for nuclear power plants, on Earth are limited. But deuterium contained in ocean water is an almost inexhaustible source of cheap nuclear fuel. The situation with tritium is somewhat more complicated. Tritium is radioactive (its half-life is 12.5 years, the decay reaction is:), and does not occur in nature. Therefore, to ensure work fusion reactor using tritium as a nuclear fuel, the possibility of its reproduction must be provided.

For this purpose, the working area of the reactor must be surrounded by a layer of light lithium isotope, in which the reaction will take place

As a result of this reaction, the hydrogen isotope tritium () is formed.

In the future, the possibility of creating a low-radioactive thermonuclear reactor using a mixture of deuterium and helium isotope is being considered; the fusion reaction has the form:

MeV.(7.20)

As a result of this reaction, due to the absence of neutrons in the synthesis products, the biological hazard of the reactor can be reduced by four to five orders of magnitude compared to both nuclear fission reactors and thermonuclear reactors operating on deuterium and tritium fuel, and there is no need for industrial processing radioactive materials and their transportation, the disposal of radioactive waste is qualitatively simplified. However, the prospects for creating in the future an environmentally friendly thermonuclear reactor using a mixture of deuterium () with a helium isotope () are complicated by the problem of raw materials: the natural reserves of the helium isotope on Earth are insignificant. The impact of deuterium in the future of environmentally friendly thermonuclear

On the path to implementing fusion reactions under terrestrial conditions, the problem of electrostatic repulsion of light nuclei arises when they approach distances at which nuclear attractive forces begin to act, i.e. about 10 -15 m, after which the process of their merging occurs due to tunnel effect. To overcome the potential barrier, the colliding light nuclei must be given an energy of ≈10 keV, which corresponds to temperature T ≈10 8 K and higher. Therefore, thermonuclear reactions under natural conditions occur only in the interior of stars. To implement them under terrestrial conditions, a strong heating of the substance is required, either by a nuclear explosion, or a powerful gas discharge, or a giant pulse of laser radiation, or bombardment with an intense beam of particles. Thermonuclear reactions have so far only been carried out in test explosions of thermonuclear (hydrogen) bombs.

The basic requirements that a thermonuclear reactor must satisfy as a device for implementing controlled thermonuclear fusion are as follows.

Firstly, reliable confinement of hot plasma is necessary (≈10 8 K) in the reaction zone. The fundamental idea, which determined the ways to solve this problem for many years, was expressed in the mid-20th century in the USSR, USA and Great Britain almost simultaneously. This idea is use of magnetic fields for containment and thermal insulation of high-temperature plasma.

Secondly, when operating on fuel containing tritium (which is a highly radioactive isotope of hydrogen), radiation damage to the walls of the fusion reactor chamber will occur. According to experts, the mechanical resistance of the first wall of the chamber is unlikely to exceed 5-6 years. This means that the installation must be periodically completely dismantled and then reassembled using remote robots due to the exceptionally high residual radioactivity.

Thirdly, the main requirement that thermonuclear fusion must satisfy is that the energy release as a result of thermonuclear reactions more than compensates for the energy consumed from external sources to maintain the reaction itself. Of great interest are “pure” thermonuclear reactions,

not producing neutrons (see (7.20) and the reaction below:

Question 28. Radioactive decay α−, β−, γ− radiation.

Under radioactivity understand the ability of some unstable atomic nuclei to spontaneously transform into other atomic nuclei with the emission of radioactive radiation.

Natural radioactivity called radioactivity observed in naturally occurring unstable isotopes.

Artificial radioactivity is the radioactivity of isotopes obtained as a result of nuclear reactions carried out in accelerators and nuclear reactors.

Radioactive transformations occur with a change in the structure, composition and energy state of atomic nuclei, and are accompanied by the emission or capture of charged or neutral particles, and the release of short-wave radiation of an electromagnetic nature (gamma radiation quanta). These emitted particles and quanta are collectively called radioactive (or ionizing ) radiation, and elements whose nuclei can spontaneously decay for one reason or another (natural or artificial) are called radioactive or radionuclides . The causes of radioactive decay are imbalances between nuclear (short-range) attractive forces and electromagnetic (long-range) repulsive forces of positively charged protons.

Ionizing radiation– a stream of charged or neutral particles and quanta of electromagnetic radiation, the passage of which through a substance leads to ionization and excitation of atoms or molecules of the medium. By its nature, it is divided into photon (gamma radiation, bremsstrahlung, X-ray radiation) and corpuscular (alpha radiation, electron, proton, neutron, meson).

Of the 2500 nuclides currently known, only 271 are stable. The rest (90%!) are unstable, i.e. radioactive; through one or more successive decays, accompanied by the emission of particles or γ-quanta, they turn into stable nuclides.

The study of the composition of radioactive radiation has allowed it to be divided into three different components: α-radiation is a stream of positively charged particles - helium nuclei (), β radiation – flow of electrons or positrons, γ radiation – flux of short-wave electromagnetic radiation.

Typically, all types of radioactivity are accompanied by the emission of gamma rays - hard, short-wave electromagnetic radiation. Gamma rays are the main form of reducing the energy of excited products of radioactive transformations. A nucleus undergoing radioactive decay is called maternal; emerging subsidiary the nucleus, as a rule, turns out to be excited, and its transition to the ground state is accompanied by the emission of a quantum.

Conservation laws. During radioactive decay, the following parameters are preserved:

1. Charge . Electric charge cannot be created or destroyed. The total charge before and after the reaction must be conserved, although it may be distributed differently among different nuclei and particles.

2. Mass number or the number of nucleons after the reaction must be equal to the number of nucleons before the reaction.

3. Total Energy . Coulomb energy and the energy of equivalent masses must be conserved in all reactions and decays.

4.Momentum and angular momentum . Conservation of linear momentum is responsible for the distribution of Coulomb energy among nuclei, particles, and/or electromagnetic radiation. Angular momentum refers to the spin of particles.

α-decay called emission from an atomic nucleus α− particles. At α− decay, as always, the law of conservation of energy must be fulfilled. At the same time, any changes in the energy of the system correspond to proportional changes in its mass. Therefore, during radioactive decay, the mass of the mother nucleus must exceed the mass of the decay products by an amount corresponding to the kinetic energy of the system after the decay (if the mother nucleus was at rest before the decay). Thus, in case α− decay condition must be satisfied

where is the mass of the mother nucleus with mass number A and serial number Z, is the mass of the daughter nucleus and is the mass α− particles. Each of these masses, in turn, can be represented as the sum of the mass number and the mass defect:

Substituting these expressions for the masses into inequality (8.2), we obtain the following condition for α− decay:, (8.3)

those. the difference in the mass defects of the mother and daughter nuclei must be greater than the mass defect α− particles. Thus, when α− decay, the mass numbers of the mother and daughter nuclei must differ from each other by four. If the difference in mass numbers is four, then when the mass defects of natural isotopes always decrease with increasing A. Thus, when inequality (8.3) is not satisfied, since the mass defect of the heavier nucleus, which should be the mother nucleus, is less than the mass defect of the lighter nucleus. Therefore, when α− nuclear decay does not occur. The same applies to most artificial isotopes. The exceptions are several light artificial isotopes, for which the jumps in binding energy, and therefore in mass defects, compared with neighboring isotopes are especially large (for example, the beryllium isotope, which decays into two α− particles).

Energy α− particles resulting from the decay of nuclei is contained within a relatively narrow range from 2 to 11 MeV. At the same time, there is a tendency for the half-life to decrease with increasing energy α− particles. This tendency is especially evident during successive radioactive transformations within the same radioactive family (Geiger-Nattall law). For example, energy α− particles during the decay of uranium (T = 7.1 . 10 8 years) is 4.58 Mev, during the decay of protactinium (T = 3.4 . 10 4 years) - 5.04 Maev during the decay of polonium (T = 1.83 . 10 -3 With)- 7,36Mev.

Generally speaking, nuclei of the same isotope can emit α− particles with several strictly defined energy values (in the previous example, the highest energy is indicated). In other words, α− particles have a discrete energy spectrum. This is explained as follows. The daughter nucleus resulting from decay, according to the laws of quantum mechanics, can be in several different states, in each of which it has a certain energy. The state with the lowest possible energy is stable and is called main . The remaining states are called excited . The nucleus can remain in them for a very short time (10 -8 - 10 -12 sec), and then passes into a state with lower energy (not necessarily immediately into the main one) with emission γ− quantum.

In progress α− There are two stages of decay: formation α− particles from nuclear nucleons and emission α− particles with a nucleus.

Beta decay (radiation). The concept of decay combines three types of spontaneous intranuclear transformations: electron decay, positron decay and electron capture ( E- capture).

There are significantly more beta radioactive isotopes than alpha radioactive isotopes. They are present in the entire range of changes in the mass numbers of nuclei (from light nuclei to the heaviest).

Beta decay of atomic nuclei is caused by weak interaction elementary particles and, just like -decay, is subject to certain laws. During decay, one of the neutrons in the nucleus turns into a proton, emitting an electron and an electron antineutrino. This process occurs according to the following scheme: . (8.8)

During − decay, one of the protons of the nucleus transforms into a neutron with the emission of a positron and an electron neutrino:

A free neutron, not part of the nucleus, decays spontaneously according to reaction (8.8) with a half-life of about 12 minutes. This is possible because the mass of the neutron is amu. greater than the mass of a proton a.m.u. by the value of the amu, which exceeds the rest mass of the electron amu. (neutrino rest mass is zero). The decay of a free proton is prohibited by the law of conservation of energy, since the sum of the rest masses of the resulting particles - the neutron and positron - is greater than the mass of the proton. Decay (8.9) of a proton is thus possible only in a nucleus if the mass of the daughter nucleus is less than the mass of the mother nucleus by an amount greater than the rest mass of the positron (the rest masses of the positron and electron are equal). On the other hand, a similar condition must be satisfied in the case of the decay of a neutron included in the nucleus.

In addition to the process occurring according to reaction (8.9), the transformation of a proton into a neutron can also occur through the capture of an electron by a proton with the simultaneous emission of an electron neutrino

Just like process (8.9), process (8.10) does not occur with a free proton. However, if a proton is inside the nucleus, then it can capture one of the orbital electrons of its atom, provided that the sum of the masses of the mother nucleus and the electron is greater than the mass of the daughter nucleus. The very possibility of meeting protons located inside the nucleus with the orbital electrons of an atom is due to the fact that, according to quantum mechanics, the movement of electrons in an atom does not occur in strictly defined orbits, as is accepted in Bohr’s theory, but there is a certain probability of meeting an electron in any region of space inside the atom, in particular, and in the region occupied by the nucleus.

The nuclear transformation caused by the capture of an orbital electron is called E-capture. Most often, the capture of an electron belonging to the K-shell closest to the nucleus occurs (K-capture). Capture of an electron included in the next L-shell (L-capture) occurs approximately 100 times less frequently.

Gamma radiation. Gamma radiation is short-wave electromagnetic radiation, which has an extremely short wavelength and, as a result, pronounced corpuscular properties, i.e. is a stream of quanta with energy ( ν − radiation frequency), momentum and spin J(in units ħ ).

Gamma radiation accompanies the decay of nuclei, occurs during the annihilation of particles and antiparticles, during the deceleration of fast charged particles in a medium, during the decay of mesons, is present in cosmic radiation, in nuclear reactions, etc. It has been experimentally established that an excited nucleus formed as a result of decay can go through a series of intermediate, less excited states. Therefore, the radiation of the same radioactive isotope may contain several types of quanta, differing from each other in energy values. The lifetime of excited states of nuclei usually increases sharply with a decrease in their energy and with an increase in the difference between the nuclear spins in the initial and final states.

Quantum emission also occurs during the radiative transition of an atomic nucleus from an excited state with energy E i to the ground or less excited state with energy Ek (E i >E k). According to the law of conservation of energy (up to the recoil energy of the nucleus), the energy of a quantum is determined by the expression: . (8.11)

During radiation, the laws of conservation of momentum and angular momentum are also satisfied.

Due to the discreteness of the energy levels of the nucleus, the radiation has a line spectrum of energy and frequencies. In reality, the energy spectrum of the nucleus is divided into discrete and continuous regions. In the discrete spectrum region, the distances between the energy levels of the nucleus are significantly greater than the energy width G level determined by the lifetime of the kernel in this state:

Time determines the decay rate of the excited nucleus:

where is the number of cores at the initial time (); number of undecayed nuclei at a time t.

question 29. Laws of displacement. When emitting a particle, the nucleus loses two protons and two neutrons. Therefore, the resulting (daughter) nucleus, compared to the original (mother) nucleus, has a mass number less by four and an ordinal number by two.

Thus, upon decay, an element is obtained, which in the periodic table occupies a place two cells to the left compared to the original:. (8.14)

During decay, one of the neutrons in the nucleus turns into a proton with the emission of an electron and an antineutrino (–decay). As a result of decay, the number of nucleons in the nucleus remains unchanged. Therefore, the mass number does not change, in other words, the transformation of one isobar into another occurs. However, the charge of the daughter nucleus and its atomic number change. During –decay, when a neutron turns into a proton, the atomic number increases by one, i.e. in this case, an element appears that is shifted in the periodic table by one cell to the right compared to the original one:

During decay, when a proton turns into a neutron, the atomic number decreases by one, and the newly resulting element is shifted one cell to the left in the periodic table:

In expressions (8.14) − (8.16) X- symbol of the maternal core, Y– symbol of the daughter nucleus; – helium nucleus, and – symbolic designations, respectively, of the electron for which A= 0 and Z= –1, and a positron, for which A= 0 and Z=+1.

Naturally radioactive nuclei form three radioactive families , called uranium family (), thorium family ()And sea anemone family (). They got their names from long-lived isotopes with the longest half-lives. All families after a chain of α− and β− decays end on stable nuclei of lead isotopes – , and. The neptunium family, starting with the transuranium element neptunium, is produced artificially and ends at the isotope bismuth.

Our task: introduce the basic properties of nuclear forces arising from the available experimental data.

Let's start by listing the known properties of nuclear forces, so that we can then move on to their justification:

These are the forces of attraction.
They are short-acting.
These are forces of great magnitude (compared to electromagnetic, weak and gravitational).
They have the property of saturation.
Nuclear forces depend on the mutual orientation of interacting nucleons.
They are not central.
Nuclear forces do not depend on the charge of interacting particles.
Depend on the relative orientation of the spin and orbital momentum.
Nuclear forces are of an exchange nature.
At short distances ( r m) are repulsive forces.

There is no doubt that nuclear forces are forces of attraction. Otherwise, the Coulomb forces of repulsion of protons would make the existence of nuclei impossible.

The property of saturation of nuclear forces follows from the behavior of the dependence of the specific binding energy on the mass number (see lecture).

Dependence of binding energy per nucleon on mass number

If nucleons in a nucleus interacted with all other nucleons, the interaction energy would be proportional to the number of combinations of A 2 each, i.e. A(A-1)/2 ~ A 2. Then the binding energy per nucleon was proportional to A. In fact, as can be seen from the figure, it is approximately constant ~8 MeV. This indicates a limited number of nucleon bonds in the nucleus.

Properties resulting from the study of the bound state - the deuteron

The deuteron 2 1 H is the only bound state of two nucleons - a proton and a neutron. There are no bound states proton - proton and neutron - neutron. Let us list the experimentally known properties of the deuteron.

Binding energy of nucleons in a deuteron G d = 2.22 MeV.
Has no excited states.
Deuteron spin J=1, parity is positive.
Magnetic moment of the deuteron μ d = 0.86 μ i, Here μ i = 5.051·10 -27 J/T - nuclear magneton.
The quadrupole electric moment is positive and equal to Q = 2.86·10 -31 m 2.

To a first approximation, the interaction of nucleons in a deuteron can be described by a rectangular potential well

Here μ - reduced mass equal to μ = m p m n /(m p +m n).

This equation can be simplified by introducing the function χ = r*Ψ(r). We get

We solve separately for regions r and r > a(take into account that E for the bound state we are looking for)

Coefficient B must be set equal to zero, otherwise when r → 0 wave function Ψ = χ/r turns to infinity; and coefficient B 1 = 0, otherwise the solution diverges at r → ∞.

Solutions must be stitched together r = a, i.e. equate the values of functions and their first derivatives. This gives

Fig.1 Graphic solution of equation (1)

Substituting the values into the last equation k, k 1 and believing E = -Gd we obtain an equation relating the binding energy Gd, pit depth U 0 and its width a

The right side, given the low binding energy, is a small negative number. Therefore, the cotangent argument is close to π/2 and slightly exceeds it.

If we take the experimental value of the deuteron binding energy G d = 2.23 MeV, then for the product a 2 ·U 0 we get ~2.1·10 -41 m 2 J (unfortunately, individual values U 0 And a cannot be obtained). Wondering reasonable a = 2·10 -15 m (follows from experiments on neutron scattering, more on this later), for the depth of the potential well we obtain approximately 33 MeV.

Let's multiply the left and right sides of equation (1) by a and introduce auxiliary variables x = ka And y = k 1 a. Equation (1) takes the form

1. Nuclear forces are short-range attractive forces . They appear only at very small distances between nucleons in the nucleus of the order of 10 –15 m. A distance of the order of (1.5 – 2.2) 10 –15 m is called range of nuclear forces, with its increase, nuclear forces quickly decrease. At a distance of the order of (2-3) m, nuclear interaction between nucleons is practically absent.

The complex nature of nuclear forces does not allow the development of a single, consistent theory of nuclear interaction, although many different approaches have been proposed. According to the hypothesis of the Japanese physicist H. Yukawa (1907-1981), which he proposed in 1935, nuclear forces are caused by exchange - mesons, i.e. elementary particles whose mass is approximately 7 times less than the mass of nucleons. According to this model, a nucleon in time m- meson mass) emits a meson, which, moving at a speed close to the speed of light, covers a distance , after which it is absorbed by the second nucleon. In turn, the second nucleon also emits a meson, which is absorbed by the first. In H. Yukawa’s model, therefore, the distance at which nucleons interact is determined by the meson path length, which corresponds to a distance of about m and in order of magnitude coincides with the radius of action of nuclear forces.

Let us turn to the consideration of the exchange interaction between nucleons. There are positive, negative and neutral mesons. The modulus of charge - or - mesons is numerically equal to the elementary charge e . The mass of charged mesons is the same and equal to (140 MeV), meson mass is 264 (135 MeV). The spin of both charged and neutral mesons is 0. All three particles are unstable. The lifetime of - and - mesons is 2.6 With, - meson – 0.8·10 -16 With. The interaction between nucleons is carried out according to one of the following schemes:

1. Nucleons exchange mesons: . (22.8)

2. Nucleons exchange - mesons:

3. Nucleons exchange - mesons:

, (22.10)

Kernel models. Under kernel model in nuclear physics they understand a set of physical and mathematical assumptions with the help of which it is possible to calculate the characteristics of a nuclear system consisting of A nucleons.

Hydrodynamic (drip) model of the core It is based on the assumption that, due to the high density of nucleons in the nucleus and the extremely strong interaction between them, the independent movement of individual nucleons is impossible and the nucleus is a drop of charged liquid with the density .

Shell model of the kernel It assumes that each nucleon moves independently of the others in some average potential field (potential well created by the remaining nucleons of the nucleus.

Generalized kernel model, combines the main provisions of the creators of the hydrodynamic and shell models. In the generalized model, it is assumed that the nucleus consists of an internal stable part - the core, which is formed by the nucleons of filled shells, and external nucleons moving in the field created by the nucleons of the core. In this regard, the motion of the core is described by a hydrodynamic model, and the motion of external nucleons by a shell model. Due to interaction with external nucleons, the core can be deformed, and the core can rotate around an axis perpendicular to the deformation axis.

26. Reactions of fission of atomic nuclei. Nuclear energy.

Nuclear reactions are called transformations of atomic nuclei caused by their interaction with each other or with other nuclei or elementary particles. The first message about a nuclear reaction belongs to E. Rutherford. In 1919, he discovered that when particles pass through nitrogen gas, some of them are absorbed, and protons are simultaneously emitted. Rutherford concluded that nitrogen nuclei were converted into oxygen nuclei as a result of a nuclear reaction of the form:

, (22.11)

where − is a particle; − proton (hydrogen).

An important parameter of a nuclear reaction is its energy yield, which is determined by the formula:

(22.12)

Here and are the sums of the rest masses of particles before and after the reaction. When nuclear reactions occur with the absorption of energy, that’s why they are called endothermic, and when - with the release of energy. In this case they are called exothermic.

In any nuclear reaction, the following are always fulfilled: conservation laws :

− electric charge;

– number of nucleons;

− energy;

− impulse.

The first two laws allow nuclear reactions to be written correctly even in cases where one of the particles participating in the reaction or one of its products is unknown. Using the laws of conservation of energy and momentum, it is possible to determine the kinetic energies of particles that are formed during the reaction process, as well as the directions of their subsequent movement.

To characterize endothermic reactions, the concept is introduced threshold kinetic energy , or nuclear reaction threshold , those. the lowest kinetic energy of an incident particle (in the frame of reference where the target nucleus is at rest) at which a nuclear reaction becomes possible. From the law of conservation of energy and momentum it follows that the threshold energy of a nuclear reaction is calculated by the formula:

. (22.13)

Here is the energy of the nuclear reaction (7.12); -mass of the stationary core – target; is the mass of the particle incident on the nucleus.

Fission reactions. In 1938, German scientists O. Hahn and F. Strassmann discovered that when uranium is bombarded with neutrons, nuclei sometimes appear that are approximately half the size of the original uranium nucleus. This phenomenon was called nuclear fission.

It represents the first experimentally observed nuclear transformation reaction. An example is one of the possible fission reactions of the uranium-235 nucleus:

The process of nuclear fission proceeds very quickly in a time of ~10 -12 s. The energy released during a reaction like (22.14) is approximately 200 MeV per fission event of the uranium-235 nucleus.

In general, the fission reaction of the uranium-235 nucleus can be written as:

+neutrons . (22.15)

critical stage of fission and, having emitted a quantum, returns to the main

A message on the topic of nuclear forces. Nuclear forces and their properties

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