From the fact of the existence of nuclei it follows that specific nuclear forces act between the nucleons of the nucleus, irreducible to electromagnetic forces. Nuclear forces have the following properties.
1. Nuclear forces are short-range. They decrease exponentially with distance The interaction radius of nucleons is less cm and is related to the mass of the interaction carrier particle (pi-meson).
2. Nuclear forces are forces of attraction and at distances of 1 fermi times greater than the Coulomb repulsive forces of protons in the nucleus. This follows from the positive value of the binding energy of the nucleus and the existence of the deuteron. Coulomb repulsion energy of two protons
The specific binding energy of a nucleon in a helium nucleus is approximately 7 mev.
3. Nuclear forces have a non-central (tensor) character, i.e. depend on the mutual arrangement of nucleons. This follows from the presence of an electric quadrupole moment in the deuteron.
4. The potential of nuclear forces depends on the mutual orientation of the spins of the interacting particles and their spins. This is indicated by experiments on the scattering of slow neutrons by molecular hydrogen.
5. Nuclear forces have the property of saturation. Each nucleon interacts only with a limited number of nucleons closest to it. This follows from the fact that the binding energy is proportional to the number of nucleons BUT. If each nucleon interacted with all the others, then it would be E st ~ BUT 2 .
6. Nuclear forces have the property charge independence(isotopic invariance). The interaction of two protons, two neutrons, a neutron with a proton in the same quantum spatial and spin states is the same, if the Coulomb interaction is excluded. This is evidenced by scattering experiments ( n,p) and ( p,p), as well as reactions with the formation of two neutrons in the final states. in mirror nuclei (when all protons are replaced by neutrons), all properties are almost the same.
7. Nuclear forces have an exchange character. Nucleons interacting exchange coordinates, spins. and charges. The π meson is a quantum of nuclear interaction at low energies.
8. The high intensity and repulsive nature of nuclear forces at very small distances () follows from the presence of massive charged particles (quarks) inside nucleons.
9. Spin-orbit dependence of nuclear forces is observed experimentally.
10. A significant dependence of nuclear forces on the value of the isotopic spin is observed T(1 or 0) at nucleon energies less than 1 gav, and independence from isospin at energies greater than 10 gav.
11. General character ( n,p) and ( p,p) - scattering at high energies greater than 100 mev leads to the conclusion that there is a very strong repulsion of nucleons at distances less than 0.5 10 -13 cm, the exchange character of the nuclear forces, and the spin-orbital dependence of the nuclear forces (the non-central tensor character of the nuclear forces follows from the phase analysis ( p,p) - scattering).
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 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.
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 BUT>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 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)
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 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
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 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 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 BUT 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 BUT. 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 the process 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 the 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 Ei 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, compared to the original (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, is obtained artificially and ends with the bismuth isotope.
Nuclear interaction testifies that in nuclei there are special nuclear forces , not reducible to any of the types of forces known in classical physics (gravitational and electromagnetic).
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. 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 value of the radius of nuclear forces.
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.
So let's list general properties of nuclear forces :
short range of nuclear forces ( R~ 1 fm);
large nuclear potential U~ 50 MeV;
· dependence of nuclear forces on spins of interacting particles;
· tensor character of interaction of nucleons;
· nuclear forces depend on the mutual orientation of the spin and orbital moments of the nucleon (spin-orbit forces);
nuclear interaction has the property of saturation;
charge independence of nuclear forces;
exchange character of nuclear interaction;
attraction between nucleons at large distances ( r> 1 fm), is replaced by repulsion at small ( r < 0,5 Фм).
in 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. Interaction between nucleons resulting from the exchange of mass quanta m, leads to the appearance of the potential U I ( r):
.
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nuclear forces(eng. Nuclear forces) are the forces of interaction of nucleons in the atomic nucleus. They rapidly decrease with increasing distance between nucleons and become almost imperceptible at distances above 10 -12 cm.
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. A graph of the potential energy of the interaction of the electric and magnetic fields of two protons, demonstrating the presence of nuclear forces, is shown in the figure.
Proton - proton, proton - neutron and neutron - neutron interactions will be somewhat different because the structure of the magnetic fields of the proton and neutron is different.
There are several basic properties of nuclear forces.
1. Nuclear forces are forces of attraction.
2. Nuclear forces are short acting. Their action is manifested only at distances of about 10-15 m.
With an increase in the distance between nucleons i, the nuclear forces rapidly decrease to zero, and at distances smaller than their radius of action ((1.5 2.2) 1 0 ~15 m), they turn out to be approximately 100 times greater than the Coulomb forces acting between protons at the same distance.
3. Nuclear forces exhibit charge independence: the attraction between two nucleons is constant and does not depend on the charge state of the nucleons (proton or neutron). This means that nuclear forces are of a non-electronic nature.
The charge independence of nuclear forces is seen from a comparison of the binding energies in mirror nuclei. So called nuclei, in which the total number of nucleons is the same, this number of protons in one is equal to the number of neutrons in the other.
4. Nuclear forces have the property of saturation, that is, each nucleon in the nucleus interacts only with a limited number of nucleons closest to it. Saturation manifests itself in the fact that the specific binding energy of nucleons in the nucleus remains constant with an increase in the number of nucleons. Almost complete saturation of the nuclear forces is achieved with the a-particle, which is very stable.
5. Nuclear forces depend on the mutual orientation of the spins of the interacting nucleons.
6. Nuclear forces are not central, that is, they do not act along the line connecting the centers of interacting nucleons.
The complexity and ambiguous nature of nuclear forces, as well as the difficulty of accurately solving the equations of motion of all nucleons of the nucleus (a nucleus with a mass number A is a system of A bodies, has not allowed us to develop a unified coherent theory of the atomic nucleus to this day.
35. Radioactive decay. Law of radioactive transformation.
radioactive decay(from lat. radius"beam" and activus"effective") - a spontaneous change in the composition of unstable atomic nuclei (charge Z, mass number A) by emitting elementary particles or nuclear fragments. The process of radioactive decay is also called radioactivity, and the corresponding elements are radioactive. Substances containing radioactive nuclei are also called radioactive.
It has been established that all chemical elements with an atomic number greater than 82 (that is, starting with bismuth), and many lighter elements (promethium and technetium do not have stable isotopes, and some elements, such as indium, potassium or calcium, have part of the natural isotopes are stable, while others are radioactive).
natural radioactivity- spontaneous decay of the nuclei of elements found in nature.
artificial radioactivity- spontaneous decay of the nuclei of elements obtained artificially through the corresponding nuclear reactions.
acon of radioactive decay- a physical law describing the dependence of the intensity of radioactive decay on time and the number of radioactive atoms in the sample. Discovered by Frederick Soddy and Ernest Rutherford
The law was first formulated as :
In all cases when one of the radioactive products was separated and its activity was studied, regardless of the radioactivity of the substance from which it was formed, it was found that the activity in all studies decreases with time according to the law of geometric progression.
from what with Bernoulli's theorems scientists concluded [ source unspecified 321 days ] :
The rate of transformation is always proportional to the number of systems that have not yet undergone transformation.
There are several formulations of the law, for example, in the form of a differential equation:
which means that the number of decays that occurred in a short time interval is proportional to the number of atoms in the sample.
