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The range of nuclear forces. Nuclear forces. See what "nuclear forces" are in other dictionaries

The atomic nucleus, consisting of a certain number of protons and neutrons, is a single entity due to the specific forces that act between the nucleons of the nucleus and are called nuclear. It has been experimentally proven that the nuclear forces are very large, far exceeding 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 of 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. The 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 rapidly 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 character of nuclear forces manifests itself in the approximate constancy of the specific binding energy of nucleons at a charge number A>40. Indeed, if there were no saturation, then the specific binding energy would increase with an increase in 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 nucleons, so the nuclear interactions between protons and neutrons are the same. The charge independence of nuclear forces is seen from a comparison of the binding energies mirror nuclei . So called 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 nuclei and heavy hydrogen - tritium 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 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 the interacting nucleons. This is confirmed by the different nature of neutron scattering by ortho- and para-hydrogen molecules. In the orthohydrogen molecule, the spins of both protons are parallel to each other, while in the parahydrogen molecule they are antiparallel. Experiments have shown that the scattering of neutrons by parahydrogen is 30 times greater than the scattering by 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 due to the exchange - mesons, i.e. elementary particles, the mass of which is approximately 7 times less than the mass of nucleons. According to this model, a nucleon in time m- the mass of the meson) emits a meson, which, moving at a speed close to the speed of light, travels 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 coincides in order of magnitude 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 charge modulus of - or - mesons is numerically equal to the elementary charge e. The mass of charged - mesons is the same and equal to (140 MeV), the mass of the meson 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 the neutron, which consequently turns into a proton, then the same process proceeds 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 the corresponding number of practically resting neutrons is found 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 run into serious difficulties. Here are some of them:

1. Insufficiency 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 bodies).

These difficulties compel us to follow the path of creating nuclear models that make it possible to describe a certain set of properties of the nucleus with the help of relatively simple mathematical means. None of these models can give an absolutely accurate description of the nucleus. Therefore, several models have to be used.

Under kernel model in nuclear physics understand the totality of physical and mathematical assumptions with which you can 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 (drop) model of the core was developed in 1939. N. Bor 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 in the case of an ordinary liquid drop, the surface of the nucleus can oscillate. If the oscillation amplitude 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 a nucleus and explained the mechanism of some nuclear reactions. However, this model does not allow one to 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 nucleus.

Shell model of the kernel developed in 1940-1950 by the American physicist M. Goeppert - Mayer and the German physicist H. Jensen. It assumes that each nucleon moves independently of the others in a certain average potential field (potential well created by the remaining nucleons of the nucleus. In the framework of the shell model, the function is not calculated, but is selected so that the best agreement with the 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 motion of nucleons in the average potential field is in conflict with the main 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 nucleons of filled shells, and external nucleons moving in the field created by the core nucleons. In this regard, the motion of the core is described by the hydrodynamic model, while the motion of external nucleons is described by the shell model. Due to the interaction with external nucleons, the core can be deformed, and the nucleus 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 for some of them.

We have considered the main phenomenological, i.e. descriptive, core models. However, for a complete understanding of 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 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 rapidly 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 due to the exchange - mesons, i.e. elementary particles, the mass of which is approximately 7 times less than the mass of nucleons. According to this model, a nucleon in time m- the mass of the meson) emits a meson, which, moving at a speed close to the speed of light, travels 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 coincides in order of magnitude 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 charge modulus of - or - mesons is numerically equal to the elementary charge e . The mass of charged - mesons is the same and equal to (140 MeV), the mass of the meson 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 understand the totality of physical and mathematical assumptions with which you can calculate the characteristics of a nuclear system consisting of A nucleons.

Hydrodynamic (drop) 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, independent motion of individual nucleons is impossible and the nucleus is a drop of charged liquid with a 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 nucleons of filled shells, and external nucleons moving in the field created by the core nucleons. In this regard, the motion of the core is described by the hydrodynamic model, while the motion of external nucleons is described by the shell model. Due to the interaction with external nucleons, the core can be deformed, and the nucleus can rotate around an axis perpendicular to the deformation axis.

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

Nuclear reactions 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 at the same time protons are emitted. Rutherford came to the conclusion that nitrogen nuclei were converted into oxygen nuclei as a result of a nuclear reaction of the form:

, (22.11)

where − - particle; − proton (hydrogen).

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

(22.12)

Here and are the sums of the rest masses of the particles before and after the reaction. When nuclear reactions proceed with the absorption of energy, therefore they are called endothermic, and at - with the release of energy. In this case they are called exothermic.

In any nuclear reaction, there are always conservation laws :

− electric charge;

− number of nucleons;

− energy;

− impulse.

The first two laws make it possible to correctly write down nuclear reactions even when one of the particles participating in the reaction or one of its products is unknown. Using the laws of conservation of energy and momentum, one can determine the kinetic energies of the particles that are formed during the reaction, as well as the direction of their subsequent movement.

To characterize endothermic reactions, the concept is introduced threshold kinetic energy , or nuclear reaction threshold , those. the smallest kinetic energy of an incident particle (in the reference frame where the target nucleus is at rest) at which a nuclear reaction becomes possible. It follows from the law of conservation of energy and momentum 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 immobile nucleus - 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 has been called nuclear fission.

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

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

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

+neutrons . (22.15)

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. 22.2).

The excess energy that the nucleus receives as a result of the absorption of a neutron causes a 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 some energy called activation energy , then under the influence of the electrostatic repulsion of protons, the nucleus splits into two parts, with the emission 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 main

Between the nucleons that make up the nucleus act nuclear forces , significantly exceeding the Coulomb repulsive forces between protons. From the point of view of the field theory of elementary particles, nuclear forces are mainly forces of interaction of magnetic fields of nucleons in the near zone. At large distances, the potential energy of such interaction decreases according to the law 1/r 3 - this explains their short-range character. At a distance (3 ∙10 -13 cm) nuclear forces become dominant, and at distances less than (9.1 ∙10 -14 cm) they turn into even more powerful repulsive forces.

nuclear forces are short-range forces. They appear only at very small distances between nucleons in the nucleus of the order of 10–15 m. The length (1.5–2.2) 10–15 m is called range of nuclear forces.

Nuclear forces discover charge independence : the attraction between two nucleons is the same regardless of the charge state of the nucleons - proton or neutron. The charge independence of nuclear forces is seen from a comparison of the binding energies mirror nuclei . What are the nuclei 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, nuclei of helium and heavy hydrogen - tritium. The binding energies of these nuclei are 7.72 MeV and 8.49 MeV.

The difference in the binding energies of the nuclei, equal to 0.77 MeV, corresponds to the energy of the Coulomb repulsion of two protons in the nucleus.

Nuclear forces have saturation property , which manifests itself in, that a nucleon in a nucleus interacts only with a limited number of neighboring nucleons closest to it. That is why there is a linear dependence of the binding energies of nuclei on their mass numbers A. Almost complete saturation of nuclear forces is achieved in the α-particle, which is a very stable formation.

Nuclear forces depend on spin orientations interacting nucleons. This is confirmed by the different character of neutron scattering by ortho- and para-hydrogen molecules. In the orthohydrogen molecule, the spins of both protons are parallel to each other, while in the parahydrogen molecule they are antiparallel. Experiments have shown that the scattering of neutrons by parahydrogen is 30 times greater than the scattering by orthohydrogen. nuclear forces are not central.

The interaction between nucleons arises as a result of the emission and absorption of quanta of the nuclear field – π- mesons . They define the nuclear field by analogy with the electromagnetic field, which arises as a result of the exchange of photons.

Bond energy

The strength of the nuclei is characterized by the binding energy. The magnitude of the binding energy is equal to the work that must be expended to destroy the nucleus into its constituent nucleons without imparting kinetic energy to them . The same amount of energy is released during the formation of a nucleus from nucleons. The nuclear binding energy is the difference between the energy of all free nucleons that make up the nucleus and their energy in the nucleus.

When a nucleus is formed, its mass decreases: the mass of the nucleus is less than the sum of the masses of its constituent nucleons. The decrease in the mass of the nucleus during its formation is explained by the release of binding energy. The amount of energy contained in matter is directly related to its mass by the Einstein relation

E=mc2 .

V According to this relation, mass and energy are different forms of the same phenomenon. Neither mass nor energy disappears, but under appropriate conditions they pass from one species to another, i.e. any change in mass m system corresponds to the equivalent change in its energy E.

The difference between the sum of masses of free nucleons and the mass of the nucleus is called mass defect atomic nucleus. If the nucleus with mass m formed from Z protons with mass m p and from (A - Z) neutrons with mass m n , then the mass defect Δ m is determined by the ratio

When a nucleus is formed from particles, the latter, due to the action of nuclear forces at small distances, rush with great acceleration towards each other. The gamma rays emitted in this case just have energy E St. and weight m .

By the mass defect, using the Einstein equation ( E \u003d mc 2 ) it is possible to determine the energy released as a result of the formation of the nucleus, i.e. bond energy (E cv ):

E cv = Δ m c 2

The binding energy per nucleon (i.e., the total binding energy divided by the number of nucleons in the nucleus) is called specific binding energy :

The greater the absolute value of the specific binding energy, the stronger the interaction between nucleons and the stronger the nucleus. The highest binding energy per nucleon, about 8.75 MeV, is inherent in the elements of the middle part of the periodic table.

Nuclear spectra

The atomic nucleus, like other objects of the microworld, is a quantum system. This means that the theoretical description of its characteristics requires the involvement of quantum theory. In quantum theory, the description of the states of physical systems is based on wave functions, or probability amplitudesψ(α,t). The square of the modulus of this function determines the probability density of detecting the system under study in a state with characteristic α – ρ(α,t) = |ψ(α,t)| 2. The argument of the wave function can be, for example, the coordinates of the particle.

The quantum nature of atomic nuclei manifests itself in the patterns of their excitation spectra. Nuclei have discrete spectra of possible energy states. Thus, the quantization of energy and a number of other parameters is a property not only of atoms, but also of atomic nuclei. The state of the atomic nucleus with the minimum amount of energy is called main, or normal, states with excess energy (compared to the ground state) are called excited .

Spectrum of kernel states 12 WITH

Atoms are usually in excited states for about 10 -8 seconds, and excited atomic nuclei get rid of excess energy in a much shorter time - about 10 -15 - 10 -16 seconds. Like atoms, excited nuclei are released from excess energy by emitting quanta of electromagnetic radiation. These quanta are called gamma quanta (or gamma rays). A discrete set of energy states of the atomic nucleus corresponds to a discrete spectrum of frequencies emitted by them gamma rays.

Many patterns in nuclear spectra can be explained using the so-called shell model of the structure of the atomic nucleus. According to this model, nucleons in the nucleus are not mixed in disorder, but, like electrons in an atom, they are arranged in bound groups, filling the allowed nuclear shells. In this case, the proton and neutron shells are filled independently of each other. The maximum numbers of neutrons: 2, 8, 20, 28, 40, 50, 82, 126 and protons: 2, 8, 20, 28, 50, 82 in filled shells are called magic. Nuclei with magic numbers of protons and neutrons have many remarkable properties: an increased value of the specific binding energy, a lower probability of entering into a nuclear interaction, resistance to radioactive decay, etc. "Doubly magic" are, for example, nuclei 4 He, 16 O, 28 Si. It is precisely because of their particularly high stability that these nuclei are the most common in nature.

The transition of the nucleus from the ground state to the excited state and its return to the ground state, from the point of view of the shell model, is explained by the transition of the nucleon from one shell to another and back.

Spontaneous transitions of nuclei from higher excited states discrete the spectrum of the nucleus to lower (including the ground state) are realized, as a rule, by radiation of γ-quanta, i.e. at the expense electromagnetic interactions. In the region of high excitation energies, when E > E ot, the level widths of the excited nucleus increase sharply. The fact is that in the separation of the nucleon from the nucleus, the main role is played by nuclear forces - i.e. strong interactions. The probability of strong interactions is orders of magnitude higher than the probability of electromagnetic ones, so the decay widths for strong interactions are large and the levels of nuclear spectra in the region E > E sep overlap - the spectrum of the nucleus becomes continuous. The main mechanism for the decay of highly excited states from this energy range is the emission of nucleons and clusters (α-particles and deuterons). The emission of γ-quanta in this region of high excitation energies E > E resp occurs with a lower probability than the emission of nucleons. An excited nucleus has, as a rule, several paths, or channels, decay.

The huge binding energy of nucleons in the nucleus indicates that there is a very intense interaction between nucleons. This interaction is in the nature of attraction. It keeps the nucleons at distances cm from each other, despite the strong Coulomb repulsion between protons. The nuclear interaction between nucleons is called the strong interaction. It can be described using the field of nuclear forces. Let us list the distinctive features of these forces.

1. Nuclear forces are short-range. Their range is of the order of . At distances much smaller than , the attraction of nucleons is replaced by repulsion.

2. Strong interaction does not depend on the charge of nucleons. The nuclear forces acting between two protons, a proton and a neutron and two neutrons, are of the same magnitude. This property is called the charge independence of nuclear forces.

3. Nuclear forces depend on the mutual orientation of the nucleon spins. So, for example, a neutron and a proton are held together, forming a heavy hydrogen nucleus deuteron (or deuteron) only in that. if their spins are parallel to each other.

4. Nuclear forces are not central. They cannot be represented as directed along a straight line connecting the centers of interacting nucleons. The non-centrality of nuclear forces follows, in particular, from the fact that they depend on the orientation of the nucleon spins.

5. Nuclear forces have the property of saturation (this means that each nucleon in the nucleus interacts with a limited number of nucleons). Saturation manifests itself in the fact that the specific binding energy of nucleons in the nucleus does not increase with an increase in the number of nucleons, but remains approximately constant. In addition, the saturation of nuclear forces is also indicated by the proportionality of the volume of the nucleus to the number of nucleons forming it (see formula (66.8)).

According to modern concepts, the strong interaction is due to the fact that nucleons virtually exchange particles, called mesons. In order to understand the essence of this process, let us first consider what the electromagnetic interaction looks like from the point of view of quantum electrodynamics.

The interaction between charged particles is carried out through an electromagnetic field. We know that this field can be represented as a collection of photons.

According to the concepts of quantum electrodynamics, the process of interaction between two charged particles, such as electrons, consists in the exchange of photons. Each particle creates a field around itself by continuously emitting and absorbing photons. The action of the field on another particle is manifested as a result of its absorption of one of the photons emitted by the first particle. Such a description of interaction cannot be taken literally. The photons through which the interaction is carried out are not ordinary real photons, but virtual ones. In quantum mechanics, particles are called virtual if they cannot be detected during their lifetime. In this sense, virtual particles can be called imaginary.

To better understand the meaning of the term "virtual", consider an electron at rest. The process of creating a field in the surrounding space can be represented by the equation

The total energy of a photon and an electron is greater than the energy of an electron at rest. Consequently, the transformation described by equation (69.1) is accompanied by a violation of the energy conservation law. However, for a virtual photon this violation is apparent. According to quantum mechanics, the energy of a state that exists time is determined only with an accuracy that satisfies the uncertainty relation:

(see formula (20.3)). It follows from this relationship that the energy of the system can undergo deviations AE, the duration of which should not exceed the value determined by condition (69.2). Therefore, if a virtual photon emitted by an electron is absorbed by the same or another electron before the expiration of time (where ), then the violation of the energy conservation vacon cannot be detected.

When an electron is given additional energy (this can happen, for example, when it collides with another electron), a real photon can be emitted instead of a virtual one, which can exist indefinitely.

For the time determined by the condition (69.2), a virtual photon can transfer the interaction between points separated by a distance

The photon energy can be arbitrarily small (the frequency varies from 0 to ). Therefore, the range of the electrode magnetic forces is unlimited.

If the particles exchanged by the interacting electrons had a mass other than zero, then the radius of action of the corresponding forces would be limited by the value

where is the Compton wavelength of the given particle (see (11.6)). We assumed that the particle - the carrier of the interaction - moves with a speed c.

In 1934, I. E. Tamm suggested that the interaction between nucleons is also transmitted through some kind of virtual particles. At that time, apart from nucleons, only the photon, electron, positron and neutrino were known. The heaviest of these particles, the electron, has a Comptonian wavelength (see (11.7)), which is two orders of magnitude greater than the radius of action of nuclear forces. In addition, the magnitude of the forces that could be due to virtual electrons, as shown by calculations, turned out to be extremely small. Thus, the first attempt to explain nuclear forces with the help of the exchange of virtual particles turned out to be unsuccessful.

In 1935, the Japanese physicist H. Yukawa expressed a bold hypothesis that in nature there are still undiscovered particles with a mass 200-300 times greater than the mass of an electron, and that these particles act as carriers of nuclear interaction, just as photons are carriers of electromagnetic interaction. Yukawa called these hypothetical particles heavy photons. Due to the fact that in terms of mass these particles occupy an intermediate position between electrons and nucleons, they were subsequently called mesons (Greek "mesos" means medium),

In 1936, Anderson and Neddermeyer discovered in cosmic rays particles with a mass equal to . Initially, it was believed that these particles, called mesons, or muons, are the carriers of the interaction predicted by Yukawa. However, later it turned out that muons interact very weakly with nucleons, so that they cannot be responsible for nuclear interactions. Only in 1947 did Okchialini and Powell discover another type of mesons in cosmic radiation - the so-called -mesons, or pions, which turned out to be carriers of nuclear forces predicted 12 years earlier by Yukawa.

There are positive negative and neutral mesons. The charge of u-mesons is equal to the elementary charge. The mass of charged pions is the same and equal to , the mass of -meson is equal to .

The spin of both charged and neutral -mesons is equal to zero. All three particles are unstable. The lifetime of and -mesons is , -mesons - .

The vast majority of charged mesons decay according to the scheme

( - positive and negative muons, v - neutrino, - antineutrino). On average, 2.5 decays out of a million proceed according to other schemes (for example, etc., and in the case, i.e., a positron is formed, and in the case, i.e., an electron is formed).

On average, -mesons decay into two -quanta:

The remaining decays are carried out according to the schemes:

The particles called -mesons or muons belong to the class of leptons (see § 74) and not of mesons. Therefore, in what follows we will call them muons. Muons have a positive or negative charge equal to the elementary charge (there is no neutral muon). The mass of the muon is , spin - half . Muoys, like -mesons, are unstable, they decay according to the scheme:

The lifetime of both muons is the same and equal.

Let us turn to the consideration of the exchange interaction between nucleons. As a result of virtual processes

The nucleon turns out to be surrounded by a cloud of virtual -mesons, which form the field of nuclear forces. The absorption of these mesons by another nucleon leads to a strong interaction between nucleons, which is carried out according to one of the following schemes:

The corresponding number of practically resting neutrons is found in the target. It is absolutely unbelievable that such a large number of neutrons would completely transfer their momentum to previously resting protons as a result of frontal impacts. Therefore, one has to admit that a part of the neutrons flying near the protons captures one of the virtual mesons. As a result, the neutron turns into a proton, and the proton that has lost its charge turns into a neutron (Fig. 69.2).

If the nucleon is given an energy equivalent to the mass of the -meson, then the virtual -meson can become real. The necessary energy can be imparted by the collision of sufficiently accelerated nucleons (or nuclei) or by the absorption of a quantum by a nucleon. At very high energies of colliding plants, several real

1. Nuclear forces are short-range forces of attraction . They appear only at very small distances between nucleons in the nucleus of the order of 10–15 m. The length (1.5–2.2) 10–15 m is called range of nuclear forces they rapidly decrease 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, so the nuclear interactions between protons and neutrons are the same. The charge independence of nuclear forces can be seen from a comparison of the binding energies mirror nuclei.What are the nuclei called?, in which the total number of nucleons is the same, night the number of protons in one is equal to the number of neutrons in the other. For example, the binding energies of helium nuclei and heavy hydrogen - tritium are respectively 7.72 MeV and 8.49 MeV The difference between 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 increase to be equal, it can be found that the average distance r between protons in the nucleus is 1.9·10 -15 m, which is consistent with the value of the radius of action of nuclear forces.

4. Nuclear forces are not central and depend on the mutual orientation of the spins of the interacting nucleons. This is confirmed by the different character of neutron scattering by ortho- and para-hydrogen molecules. In the orthohydrogen molecule, the spins of both protons are parallel to each other, while in the parahydrogen molecule they are antiparallel. Experiments have shown that the scattering of neutrons by parahydrogen is 30 times greater than the scattering by 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 due to the exchange - mesons, i.e. elementary particles, the mass of which is approximately 7 times less than the mass of nucleons. According to this model, a nucleon over time m- the mass of the meson) emits a meson, which, moving at a speed close to the speed of light, travels 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 coincides in order of magnitude 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 has been called nuclear fission.

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

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

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

Neutrons (7.15)

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

condition.

An important feature of the nuclear fission reaction is the ability to implement on its basis a self-sustaining nuclear chain reaction . This is due to the fact that more than one neutron is released on average during each fission event. Mass, charge and kinetic energy of fragments X and U, formed in the course of a fission reaction of the type (7.15) are different. These fragments are quickly decelerated 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 medium 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 into the ground state by emitting β - particles and -quanta.

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

Question 27. 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 in the process of 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) an energy of approximately 17.5 MeV is released, i.e. 3.5 MeV per nucleon (17.5:5=3.5 MeV). In this way, the fusion process is about 4 times more efficient than the uranium fission process (calculated per one nucleon of the nucleus participating in the fission reaction).

The high rate of these reactions and the relatively high energy release make an equal-component 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, are limited on Earth. But the deuterium contained in the water of the oceans 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 looks like:), does not occur in nature. Therefore, to ensure the work fusion reactor that uses tritium as a nuclear fuel, the possibility of its reproduction should be provided.

For this purpose, the working zone 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 based on 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 fusion products, the biological hazard of the reactor can be reduced by four to five orders of magnitude, both in comparison with nuclear fission reactors and with thermonuclear reactors operating on deuterium and tritium fuel, there is no need for industrial processing radioactive materials and their transportation, qualitatively simplifies the disposal of radioactive waste. However, the prospects for the creation in the future of an environmentally friendly thermonuclear reactor based on 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 influence of om deuterium in the future of environmentally friendly thermonuclear

On the way to the implementation of fusion reactions under terrestrial conditions, the problem of electrostatic repulsion of light nuclei arises when they approach distances at which nuclear forces of attraction 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 the temperature T ≈10 8 K and higher. Therefore, thermonuclear reactions in natural conditions occur only in the interiors of stars. For their implementation under terrestrial conditions, a strong heating of the substance is necessary either by a nuclear explosion, or by a powerful gas discharge, or by a giant pulse of laser radiation, or by bombardment with an intense particle beam. Thermonuclear reactions have been carried out so far only in test explosions of thermonuclear (hydrogen) bombs.

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

First, reliable hot plasma confinement (≈10 8 K) in the reaction zone. The fundamental idea, which determined for many years the way to solve this problem, was expressed in the middle of the 20th century in the USSR, the 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 an isotope of hydrogen with high radioactivity), 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 the need for periodic complete dismantling of the installation and its subsequent reassembly with the help of remotely operating 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 will more than compensate for the energy expended from external sources to maintain the reaction itself. Of great interest are "pure" thermonuclear reactions,

that do not produce neutrons (see (7.20) and the reaction below:

Question 28 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 the radioactivity observed in naturally occurring unstable isotopes.

artificial radioactivity called the radioactivity of isotopes obtained as a result of nuclear reactions carried out on accelerators and nuclear reactors.

Radioactive transformations occur with a change in the structure, composition and energy state of the nuclei of atoms, 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 the nuclear (short-range) attractive forces and the electromagnetic (long-range) repulsive forces of positively charged protons.

ionizing radiation– a flow 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; radioactive; by 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 made it possible to divide it into three different components: α-radiation is a stream of positively charged particles - helium nuclei (), β-radiation is the flow of electrons or positrons, γ radiation – flux of short-wave electromagnetic radiation.

Usually, 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 child 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 . The Coulomb energy and the energy of equivalent masses must be conserved in all reactions and decays.

4.momentum and angular momentum . The 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 the emission from an atomic nucleus α− particles. At α− decay, as always, the law of conservation of energy must be satisfied. 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 parent 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 parent nucleus was at rest before the decay). Thus, in the case α− decay must satisfy the condition

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

Energy α− particles produced during the decay of nuclei lies in a relatively narrow range from 2 to 11 MeV. In this case, there is a tendency for the half-life to decrease with increasing energy α− particles. This tendency is especially manifested in successive radioactive transformations within the same radioactive family (the Geiger-Nattall law). For example, energy α− particles during the decay of uranium (T \u003d 7.1. 10 8 years) is 4.58 mev, with the decay of protactinium (T \u003d 3.4. 10 4 years) - 5.04 Mevy during the decay of polonium (T \u003d 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 resulting decay nucleus, 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 rest of the states are called excited . The nucleus can stay in them for a very short time (10 -8 - 10 -12 sec), and then goes into a state with a lower energy (not necessarily immediately into the main one) with emission γ− quantum.

In progress α− There are two stages of decay: the formation α− particles from nucleons of the nucleus and emission α− core particles.

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

There are much more beta-radioactive isotopes than alpha-active ones. They are present in the entire region of variation in the mass numbers of nuclei (from light nuclei to the heaviest ones).

The beta decay of atomic nuclei is due to weak interaction elementary particles and, like decay, obeys certain laws. During the decay, one of the neutrons of the nucleus turns into a proton, while emitting an electron and an electron antineutrino. This process occurs according to the scheme: . (8.8)

During -decay, one of the protons of the nucleus is converted into a neutron with the emission of a positron and an electron neutrino:

A free neutron that is 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 a.m.u. greater than the proton mass a.m.u. by the a.m.u. value, which exceeds the electron rest mass a.m.u. (the rest mass of the neutrino is zero). The decay of a free proton is forbidden by the law of conservation of energy, since the sum of the rest masses of the resulting particles - the neutron and the positron - is greater than the mass of the proton. The decay (8.9) of a proton, therefore, is possible only in the nucleus, if the mass of the daughter nucleus is less than the mass of the parent nucleus by a value exceeding the rest mass of the positron (the rest masses of the positron and electron are equal). On the other hand, a similar condition must also be satisfied in the case of the decay of a neutron that is part of the nucleus.

In addition to the process occurring according to reaction (8.9), the transformation of a proton into a neutron can also occur by capturing 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 the 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 parent nucleus and the electron is greater than the mass of the daughter nucleus. The very possibility of a meeting of protons 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 along strictly defined orbits, as is accepted in Bohr's theory, but there is some probability of meeting an electron in any region of space inside the atom, in particular, and in the region occupied by the nucleus.

The transformation of a nucleus 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 (K-capture) occurs. The capture of an electron that is part of the next L-shell (L-capture) occurs approximately 100 times less frequently.

Gamma radiation. Gamma radiation is short-wavelength electromagnetic radiation, which has an extremely short wavelength and, as a result, pronounced corpuscular properties, i.e. is a flux 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 the medium, during the decay of mesons, is present in cosmic radiation, in nuclear reactions, etc. 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 as their energy decreases and as the difference between the spins of the nucleus in the initial and final states increases.

The emission of a quantum also occurs during the radiative transition of the atomic nucleus from an excited state with energy E i into the ground or less excited state with energy E k (Ei >Ek). According to the law of conservation of energy (up to the recoil energy of the nucleus), the quantum energy 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 fact, the energy spectrum of the nucleus is divided into discrete and continuous regions. In the region of the discrete spectrum, the distances between the energy levels of the nucleus are much larger than the energy width G level determined by the lifetime of the nucleus in this state:

Time determines the decay rate of an 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, in the resulting (daughter) nucleus, in comparison with the initial (parent) nucleus, the mass number is four less, and the serial number is two less.

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

During decay, one of the neutrons of 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, there is a transformation of one isobar into another. However, the charge of the daughter nucleus and its ordinal number change. During -decay, when a neutron turns into a proton, the serial number increases by one, i.e. in this case, an element appears that is shifted in the periodic table compared to the original one by one cell to the right:

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

In expressions (8.14) − (8.16) X- symbol of the mother nucleus, Y is the symbol of the daughter nucleus; is the helium nucleus; 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 family of actinia (). They got their names for the long-lived isotopes with the longest half-lives. All families after the chain of α- and β-decays end at stable nuclei of lead isotopes - , and. The family of neptunium, starting from the transuranium element neptunium, was obtained artificially and ends with the bismuth isotope.