An important role in the development of ideas about the structure of nuclei was played by the study of nuclear reactions, which provided extensive information on the spins and parities of the excited states of nuclei and contributed to the development of the shell model. The study of reactions involving the exchange of several nucleons between colliding nuclei made it possible to study nuclear dynamics in a state with large angular momenta. As a result, long rotational bands were discovered, which served as one of the foundations for creating a generalized model of the nucleus. When heavy nuclei collide, nuclei are formed that do not exist in nature. The synthesis of transuranium elements is largely based on the physics of the interaction of heavy nuclei. In reactions with heavy ions, nuclei are formed that are far from the β-stability band. Nuclei far from the β-stability band differ from stable nuclei in a different ratio between the Coulomb and nuclear interactions, the ratio between the number of protons and the number of neutrons, a significant difference in the binding energies of protons and neutrons, which manifests itself in new types of radioactive decay - proton and neutron radioactivity and a number of other specific features atomic nuclei.
When analyzing nuclear reactions, it is necessary to take into account the wave nature of particles interacting with nuclei. The wave nature of the process of interaction of particles with nuclei is clearly manifested in elastic scattering. Thus, for nucleons with an energy of 10 MeV, the reduced de Broglie wavelength is less than the radius of the nucleus, and a characteristic pattern of diffraction maxima and minima arises during the scattering of a nucleon. For nucleons with an energy of 0.1 MeV, the wavelength is greater than the radius of the nucleus and there is no diffraction. For neutrons with energy<< 0.1 МэВ сечение реакции ~π 2 гораздо больше, чем характерный размер площади ядра πR.
Nuclear reactions are an effective method for studying nuclear dynamics. Nuclear reactions occur when two particles interact. During a nuclear reaction, there is an active exchange of energy and momentum between the particles, resulting in the formation of one or more particles that fly away from the interaction region. As a result of a nuclear reaction, a complex process of rearrangement of the atomic nucleus occurs. As in the description of the structure of the nucleus, in the description of nuclear reactions it is practically impossible to obtain an exact solution of the problem. And just as the structure of the nucleus is described by various nuclear models, the course of a nuclear reaction is described by various reaction mechanisms. The mechanism of a nuclear reaction depends on several factors: the type of the incident particle, the type of the target nucleus, the energy of the incident particle, and a number of other factors. One of the limiting cases of a nuclear reaction is direct nuclear reaction. In this case, the incident particle transfers energy to one or two nucleons of the nucleus, and they leave the nucleus without interacting with other nucleons of the nucleus. The characteristic time of a direct nuclear reaction is 10 -23 s. Direct nuclear reactions take place on all nuclei at any energy of the incident particle. Direct nuclear reactions are used to study single-particle states of atomic nuclei, because the reaction products carry information about the position of the levels from which the nucleon is knocked out. Using direct nuclear reactions, detailed information was obtained about the energies and occupation of single-particle states of nuclei, which formed the basis of the shell model of the nucleus. The other limiting case is reactions proceeding through compound nucleus formation.

The description of the mechanism of nuclear reactions was given in the works of W. Weisskopf.

W.Weiskopf: “What happens when a particle enters a nucleus and collides with one of the nuclear constituents? The figure illustrates some of these possibilities.
1) The falling particle loses some of its energy, raising the nuclear particle to a higher state. This will be the result of inelastic scattering if the incident particle is left with enough energy to leave the nucleus again. This process is called direct inelastic scattering because it involves scattering from only one constituent part of the nucleus.
2) The falling particle transfers energy to the collective motion, as it is symbolically shown in the second diagram of the figure, this is also a direct interaction.
3) In the third diagram of the figure, the transferred energy is large enough to pull the nucleon out of the target. This process also contributes to the direct nuclear reaction. In principle, it does not differ from 1), it corresponds to the "exchange reaction".
4) An incoming particle can lose so much energy that it remains bound inside the nucleus, the transferred energy can be taken up by a low-lying nucleon in such a way that it cannot leave the nucleus. We then get an excited nucleus that cannot emit a nucleon. This state necessarily leads to further excitations of nucleons by internal collisions, in which the energy per excited particle decreases on average, so that in most cases the nucleon cannot leave the nucleus. Consequently, a state with a very long lifetime will be reached, which can decay only if one particle, in collisions inside the nucleus, accidentally acquires sufficient energy to leave the nucleus. We call this situation the formation of a compound nucleus. Energy can also be lost by radiation, after which the escape of a particle becomes energetically impossible: the incident nucleon experiences radiative capture.
5) The formation of a compound nucleus can be carried out in two or more steps, if after a process of type 1) or 2) the incident nucleon on its way hits another nucleon and excites it in such a way that it is impossible for any nucleon to leave the nucleus.

For the first time, the idea of ​​a nuclear reaction proceeding through the stage of a compound nucleus was expressed by N. Bohr. According to the compound nucleus model, an incident particle, after interacting with one or two nucleons of the nucleus, transfers most of its energy to the nucleus and is captured by the nucleus. The lifetime of a compound nucleus is much longer than the time of flight of an incident particle through the nucleus. The energy introduced by the incident particle into the nucleus is redistributed between the nucleons of the nucleus until a significant part of it is concentrated on one particle, and then it flies out of the nucleus. The formation of a long-lived excited state can lead to its fission as a result of deformation.

N. Bor: “The phenomenon of neutron capture leads us to assume that a collision between a fast neutron and a heavy nucleus should lead, first of all, to the formation of a complex system characterized by remarkable stability. The possible subsequent decay of this intermediate system with the ejection of a material particle or the transition to the final state with the emission of a quantum of radiant energy should be considered as independent processes that have no direct connection with the first phase of the collision. We meet here with an essential difference, hitherto unrecognized, between real nuclear reactions—the ordinary collisions of fast particles and atomic systems—collisions which have hitherto been our main source of information about the structure of the atom. Indeed, the possibility of counting individual atomic particles through such collisions and studying their properties is due, first of all, to the "openness" of the systems under consideration, which makes the exchange of energy between individual constituent particles during the impact very unlikely. However, due to the close packing of particles in the nucleus, we must be prepared for the fact that it is this exchange of energy that plays the main role in typical nuclear reactions.

Classification of nuclear reactions. Nuclear reactions are an effective means of studying the structure of atomic nuclei. If the wavelength of the incident particle is greater than the size of the nucleus, then in such experiments information is obtained about the nucleus as a whole. If the size of the nucleus is smaller, then information about the distribution of the density of nuclear matter, the structure of the surface of the nucleus, the correlation between nucleons in the nucleus, and the distribution of nucleons over nuclear shells is extracted from the reaction cross sections.

• Coulomb excitation of nuclei under the action of charged particles of relatively large mass (protons, α-particles and heavy ions of carbon, nitrogen) is used to study the low-lying rotational levels of heavy nuclei.
• Reactions with heavy ions on heavy nuclei, leading to the fusion of colliding nuclei, are the main method for obtaining superheavy atomic nuclei.
• Fusion reactions of light nuclei at relatively low collision energies (the so-called thermonuclear reactions). These reactions occur due to quantum mechanical tunneling through the Coulomb barrier. Thermonuclear reactions take place inside stars at temperatures of 10 7 –10 10 K and are the main source of stellar energy.
• Photonuclear and electronuclear reactions occur when γ-quanta and electrons with energy E > 10 MeV collide with nuclei.
• Fission reactions of heavy nuclei, accompanied by a deep rearrangement of the nucleus.
• Reactions in beams of radioactive nuclei open up possibilities for obtaining and studying nuclei with an unusual ratio of the number of protons and neutrons that are far from the line of stability.

The classification of nuclear reactions is usually carried out according to the type and energy of the incident particle, the type of target nuclei and the energy of the incident particle.

Reactions on slow neutrons

“1934 One morning Bruno Pontecorvo and Eduardo Amaldi were testing certain metals for radioactivity. These samples were shaped into small hollow cylinders of the same size, inside which a neutron source could be placed. To irradiate such a cylinder, a neutron source was inserted into it, and then everything was placed in a lead box. On this momentous morning, Amaldi and Pontecorvo were experimenting with silver. And suddenly Pontecorvo noticed that something strange was happening with the silver cylinder: its activity is not always the same, it changes depending on where it is placed, in the middle or in the corner of the lead box. Completely bewildered, Amaldi and Pontecorvo went to report this miracle to Fermi and Razetti. Franke was inclined to attribute these oddities to some statistical error or inaccurate measurements. And Enrico, who believed that every phenomenon required verification, suggested that they try to irradiate this silver cylinder outside the lead box and see what happens. And then they went absolutely incredible miracles. It turned out that objects in the vicinity of the cylinder can influence its activity. If the cylinder was irradiated while standing on a wooden table, its activity was higher than when it was placed on a metal plate. Now the whole group became interested in this and everyone took part in the experiments. They placed the neutron source outside the cylinder and placed various objects between it and the cylinder. The lead plate slightly increased the activity. Lead−heavy substance. "Come on, let's try the easy one now!−suggested by Fermi.−Let's say paraffin. On the morning of October 22, an experiment was made with paraffin.
They took a large piece of paraffin, hollowed out a hole in it, and placed a neutron source inside, irradiated a silver cylinder and brought it to a Geiger counter. The counter, as if off the chain, snapped. The whole building thundered with exclamations: “Unthinkable! Unimaginable! Black magic!" Paraffin increased the artificial radioactivity of silver a hundred times.
At noon, a group of physicists reluctantly dispersed for a break set for breakfast, which usually lasted two hours for them ... Enrico took advantage of his loneliness, and when he returned to the laboratory, he already had a theory ready that explained the strange effect of paraffin.

emission elementary particles and thermal energy. Nuclear rii can be accompanied by both the release of energy and its absorption. The amount of energy is called the energy of rii, this is the difference between the masses of the initial and final nuclei. Classifications according to trace features: L by energy, element particles participate in nuclear fractions: at low energies 1 eV, fractions on slow neutrons: fractions on electron particles of medium energy with the charge of particles of electrons, protons, ions, deuterons = 1 MeV; on particles of high energy 103 MeV, particles receive cosmic rays in accelerators ...

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45. Nuclear reactions and their classification

Nuclear reactions are a process of intense interaction of an atomic nucleus with an elementary particle or with another nucleus, leading to the transformation of nuclei. Emission of elementary particles and thermal energy. The interaction of reacting particles arises when they approach each other up to a distance of the order of 10~ 13 see due to the action of nuclear forces. The most common nuclear reaction is that light particles interact, and with the nucleus X , in which the resulting image is an el particle b and the nucleus X. Nuclear radiation can be accompanied by both the release of energy and its absorption. The amount of energy is called the energy of p-ii - this is the difference between the masses of the initial and final nuclei. Classifications according to the following features: L in terms of energy, element particles participate in nuclear p-tions: at low energies 1eV - p-tions on slow neutrons: p-tions on e-particles of medium energy with a charge of particles - electrons, protons, ions, deuterons> = 1 MeV; on high-energy particles (~10 3 MeV - cosmic rays, particles are obtained in accelerators) by nature, the particle element neutrons are involved; on charged particles; caused by y - quanta, by nature (mass) of the nuclei participate in the district: on the lungs (A<50);средних (50<А<100);тяжелых(А>100). P o the nature of the transformations: p-radioactivity; fission of heavy nuclei, chain division fission; synthesis of light nuclei into heavy, thermonuclear districts.

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Nuclear reaction is a complex process of rearrangement of the atomic nucleus. As in the description of the structure of the nucleus, it is practically impossible to obtain an exact solution of the problem here. And just as the structure of the nucleus is described by various nuclear models, the course of nuclear reactions is described by various reaction mechanisms.

There are many different reaction mechanisms. We will consider only the main ones. First, a classification of reaction mechanisms will be given, and then the most important of them will be considered in more detail.

We will classify the reactions according to the time of occurrence. It is convenient to use as a time scale nuclear time - the time of flight of a particle through the nucleus: t i = 2R/v≈10 -22 s. (9.11)

We will use the following classification of nuclear reactions according to the time of occurrence:

1. If the reaction time is t p ≈ t i, then this is a direct reaction (reaction time is minimal).

2. If t p >>t i, then the reaction goes through the compound nucleus.

In the first case (direct reaction) particle a transfers energy to one or two nucleons of the nucleus without affecting the rest, and they immediately leave the nucleus without having time to exchange energy with the rest of the nucleons. For example, the reaction (p, n) can occur as a result of the collision of a proton with one neutron of the nucleus. The direct processes include the breakdown reactions (d,p), (d, n) and the reverse reactions of pickup (p,d), (n,d), fragmentation reactions, in which a high-energy nucleon, colliding with the nucleus, knocks out it is a fragment consisting of several nucleons.

In the second case (compound kernel) the particle a and the nucleon to which it has transferred energy "get entangled" in the nucleus. The energy is distributed among many nucleons, and for each nucleon it is insufficient to escape from the nucleus. Only after a relatively long time, as a result of random redistributions, does it concentrate in sufficient quantity on one of the nucleons (or an object of several bound nucleons) and it leaves the nucleus. The compound nucleus mechanism was introduced by Niels Bohr in 1936.

An intermediate position between the reaction mechanism through the compound nucleus and the direct reaction mechanism is occupied by mechanism of preequilibrium nuclear reactions.

The duration of nuclear reactions can be determined by analyzing the widths of excited nuclear states.

To describe elastic scattering averaged over nuclear resonances, we use optical model, in which the core is treated as a continuous medium capable of refracting and absorbing de Broglie waves of particles incident on it.

The nature of the nuclear reaction depends on a number of factors: the type of a projectile particle, the type of a target nucleus, the energy of their collision and some others, which makes any classification of nuclear reactions rather conditional. The simplest one is projectile particle type classification. Within the framework of such a classification, the following main types of nuclear reactions can be distinguished:

Reactions under the action of protons, deuterons, α-particles and other light nuclei. It was these reactions that gave the first information about the structure of atomic nuclei and the spectra of their excited states.

Reactions with heavy ions on heavy nuclei leading to the fusion of colliding nuclei. These reactions are the main method for obtaining superheavy atomic nuclei.

Fusion reactions of light nuclei at relatively low collision energies ( so-called thermonuclear reactions). These reactions occur due to quantum mechanical tunneling through the Coulomb barrier. Thermonuclear reactions take place inside stars at temperatures of 10 7 -10 10 K and are the main source of stellar energy.

Coulomb excitation of nuclei under the action of protons, α-particles, and especially multiply ionized heavy ions of such elements as carbon, nitrogen, argon, etc. These reactions are used to study the low-lying rotational levels of heavy nuclei.

Reactions under the action of neutrons, primarily (n, n), (n, γ) and nuclear fission reactions (n, f).

Many specific properties are possessed by photonuclear and electronuclear reactions that occur in collisions with nuclei of γ-quanta and electrons with energies E > 10 MeV.

Reactions on beams of radioactive nuclei. Modern technical means make it possible to generate sufficiently intense beams of such nuclei, which opens up the possibility of obtaining and studying nuclei with an unusual ratio of the number of protons and neutrons that are far from the stability line.

Nuclear reactions are the transformations of atomic nuclei when interacting with elementary particles (including g-quanta) or with each other. The most common type of nuclear reaction is the reaction, written symbolically as follows:

where X and Y are the source and destination kernels, a and b- bombarding and emitted (or emitted) in a nuclear reaction particles.

In nuclear physics, the efficiency of interaction is characterized by the effective cross section a. Each type of interaction between a particle and a nucleus is associated with its effective cross section: the effective scattering cross section determines the scattering processes, while the effective absorption cross section determines the absorption processes. Effective cross section of a nuclear reaction

where N- the number of particles falling per unit time per unit area of ​​the cross-section of a substance having n nuclei per unit volume, dN - the number of these particles entering into a nuclear reaction in a layer of thickness dx . Effective cross section a has the dimension of area and characterizes the probability that a reaction will occur when a particle beam falls on a substance.

Unit of effective cross section of nuclear processes - barn(1 barn \u003d 10 -28 m 2).

In any nuclear reaction, laws of conservation of electric charges and mass numbers: the sum of charges (and the sum of mass numbers) of nuclei and particles entering into a nuclear reaction is equal to the sum of charges (and the sum of mass numbers) of the final products (nuclei and particles) of the reaction. Also performed laws of conservation of energy, momentum and angular momentum.

An important role in explaining the mechanism of many nuclear reactions was played by the assumption of N. Bohr (1936) that nuclear reactions proceed in two stages according to the following scheme:

The first stage is the capture of the X particle by the nucleus a, approaching it at a distance of action of nuclear forces (approximately 2 × 10 -15 m), and the formation of an intermediate nucleus C, called a compound (or compound-nucleus). The energy of a particle that has flown into the nucleus is quickly distributed among the nucleons of the compound nucleus, as a result of which it is in an excited state. In the collision of nucleons of a compound nucleus, one of the nucleons (or a combination of them, for example, a deuteron - the nucleus of a heavy isotope of hydrogen - deuterium, containing one proton and one neutron) or an a-particle can receive energy sufficient to escape from the nucleus. As a result, the second stage of the nuclear reaction is possible - the decay of the compound nucleus into the nucleus Y and the particle b .

In nuclear physics, a characteristic nuclear time is introduced - the time required for a particle to fly a distance of the order of magnitude equal to the diameter of the nucleus (d» 10 -15 m). Thus, for a particle with an energy of 1 MeV (which corresponds to its velocity v » 10 7 m/s), the characteristic nuclear time is t = 10 -15 m/10 7 m/s = 10 -22 s. On the other hand, it has been proved that the lifetime of the compound nucleus is 10 - 16 -10 - 12 s, i.e. is (10 6 -10 10) t. This also means that during the lifetime of a compound nucleus a lot of collisions of nucleons can occur, i.e., the redistribution of energy between nucleons is really possible. Consequently, the compound nucleus lives so long that it completely "forgets" how it was formed. Therefore, the nature of the decay of the compound nucleus (the emission of particles b) - the second stage of the nuclear reaction - does not depend on the method of formation of the compound nucleus - the first stage.

Nuclear reactions are classified according to the following criteria:

1) by the kind of particles involved- reactions under the action of neutrons; reactions under the action of charged particles (for example, protons, deuterons, a-particles); reactions under the action of g-quanta;

2) by the energy of the particles that cause them - reactions at low energies (of the order of electron volts), occurring mainly with the participation of neutrons; reactions at medium energies (up to several megaelectronvolts) involving g-quanta and charged particles (protons, a-particles); reactions at high energies (hundreds and thousands of megaelectronvolts), leading to the birth of elementary particles absent in the free state and having great importance to study them;

3) according to the type of nuclei involved- reactions on light nuclei (A<50); реакции на средних ядрах (50 < A < 100); реакции на тяжелых ядрах (А > 100);

4) by the nature of the ongoing nuclear transformations- reactions with neutron emission; reactions with the emission of charged particles; capture reactions (in these reactions, the compound nucleus does not emit any particles, but goes into the ground state, emitting one or more g-quanta).

The first nuclear reaction in history was carried out by E. Rutherford (1919) by bombarding a nitrogen nucleus with a-particles emitted by a radioactive source.