They are called nuclear reactions. Nuclear reaction

And the ability to use nuclear energy, as in creative ( nuclear power), and destructive (atomic bomb) purposes became, perhaps, one of the most significant inventions of the last twentieth century. Well, at the heart of all that formidable power that lurks in the depths of a tiny atom are nuclear reactions.

What are nuclear reactions

Nuclear reactions in physics mean the process of interaction atomic nucleus with another similar nucleus or different elementary particles, resulting in changes in the composition and structure of the nucleus.

A little history of nuclear reactions

The first nuclear reaction in history was made by the great scientist Rutherford back in 1919 during experiments to detect protons in nuclear decay products. The scientist bombarded nitrogen atoms with alpha particles, and when the particles collided, a nuclear reaction occurred.

And this is what the equation for this nuclear reaction looked like. It was Rutherford who was credited with the discovery of nuclear reactions.

This was followed by numerous experiments by scientists in implementing various types nuclear reactions, for example, a very interesting and significant for science was the nuclear reaction caused by the bombardment of atomic nuclei with neutrons, which was carried out by the outstanding Italian physicist E. Fermi. In particular, Fermi discovered that nuclear transformations can be caused not only fast neutrons, but also slow, which move at thermal speeds. By the way, nuclear reactions caused by exposure to temperature are called thermonuclear reactions. As for nuclear reactions under the influence of neutrons, they very quickly gained their development in science, and what kind of reactions, read about it further.

Typical formula for a nuclear reaction.

What nuclear reactions are there in physics?

In general, nuclear reactions known today can be divided into:

fission of atomic nuclei
thermonuclear reactions

Below we will write in detail about each of them.

Nuclear fission

The fission reaction of atomic nuclei involves the disintegration of the actual nucleus of an atom into two parts. In 1939, German scientists O. Hahn and F. Strassmann discovered the fission of atomic nuclei, continuing the research of their scientific predecessors, they established that when uranium is bombarded with neutrons, elements of the middle part arise periodic table Mendeleev, namely radioactive isotopes barium, krypton and some other elements. Unfortunately, this knowledge was initially used for horrifying, destructive purposes, as the second World War and German, and on the other hand, American and Soviet scientists were racing to develop nuclear weapons(which was based on the nuclear reaction of uranium), which ended in the infamous “ nuclear mushrooms" above Japanese cities Hiroshima and Nagasaki.

But back to physics, the nuclear reaction of uranium during the splitting of its nucleus simply has colossal energy, which science has been able to put to its service. How does such a nuclear reaction occur? As we wrote above, it occurs as a result of the bombardment of the nucleus of a uranium atom by neutrons, which causes the nucleus to split, creating a huge kinetic energy of the order of 200 MeV. But what is most interesting is that as a product of the nuclear fission reaction of a uranium nucleus from a collision with a neutron, several free new neutrons appear, which, in turn, collide with new nuclei, split them, and so on. As a result, there are even more neutrons and even more uranium nuclei are split from collisions with them - a real nuclear chain reaction occurs.

This is how it looks on the diagram.

In this case, the neutron multiplication factor must be greater than unity, this is necessary condition nuclear reaction of this type. In other words, in each subsequent generation of neutrons formed after the decay of nuclei, there should be more of them than in the previous one.

It is worth noting that, according to a similar principle, nuclear reactions during bombardment can also take place during the fission of the nuclei of atoms of some other elements, with the nuances that the nuclei can be bombarded by a variety of elementary particles, and the products of such nuclear reactions will vary, so we can describe them in more detail , we need a whole scientific monograph

Thermonuclear reactions

Thermonuclear reactions are based on fusion reactions, that is, in fact, the process opposite to fission occurs, the nuclei of atoms do not split into parts, but rather merge with each other. This also releases a large amount of energy.

Thermonuclear reactions, as the name suggests (thermo-temperature), can occur exclusively at very high temperatures Oh. After all, for two atomic nuclei to merge, they must come very close close quarters to each other, while overcoming their electrical repulsion positive charges, this is possible in the presence of high kinetic energy, which, in turn, is possible at high temperatures. It should be noted that thermonuclear reactions of hydrogen do not occur, however, not only on it, but also on other stars; one can even say that it lies at the very basis of their nature of any star.

Nuclear reactions, video

And finally, an educational video on the topic of our article, nuclear reactions.

For a long time, people have been haunted by dreams of the interconversion of elements - more precisely, of the transformation of different metals into one. After realizing the futility of these attempts, the point of view about the inviolability of chemical elements was established. And only the discovery of the structure of the nucleus at the beginning of the 20th century showed that the transformation of elements into one another is possible - but not chemical methods, that is, the impact on external electronic shells atoms, but by interfering with the structure of the atomic nucleus. This kind of phenomenon (and some others) relate to nuclear reactions, examples of which will be discussed below. But first we need to remember some basic concepts that will be required during this discussion.

General concept of nuclear reactions

There are phenomena in which the nucleus of an atom of one or another element interacts with another nucleus or some elementary particle, that is, it exchanges energy and momentum with them. Such processes are called nuclear reactions. Their result may be a change in the composition of the nucleus or the formation of new nuclei with the emission of certain particles. In this case, the following options are possible:

turning one chemical element to another;
synthesis, that is, the fusion of nuclei in which the nucleus of a heavier element is formed.

The initial phase of the reaction, determined by the type and state of the particles entering it, is called the entrance channel. Output channels are possible ways along which the reaction will proceed.

Rules for recording nuclear reactions

The examples below demonstrate the ways in which reactions involving nuclei and elementary particles.

The first method is the same as that used in chemistry: the initial particles are placed on the left side, and the reaction products are placed on the right side. For example, the interaction of a beryllium-9 nucleus with an incident alpha particle (the so-called neutron discovery reaction) is written as follows:

9 4 Be + 4 2 He → 12 6 C + 1 0 n.

The upper indices indicate the number of nucleons, that is, the mass numbers of the nuclei, the lower indices indicate the number of protons, that is, the atomic numbers. The sums of both on the left and right sides must coincide.

A shorthand way of writing nuclear reaction equations, often used in physics, looks like this:

9 4 Be (α, n) 12 6 C.

The general form of such a record is: A (a, b 1 b 2 ...) B. Here A is the target nucleus; a - incident particle or nucleus; b 1, b 2 and so on are light reaction products; B is the final core.

Energy of nuclear reactions

In nuclear transformations, the law of conservation of energy is fulfilled (along with other conservation laws). In this case, the kinetic energy of particles in the input and output channels of the reaction can differ due to changes in the rest energy. Since the latter is equivalent to the mass of the particles, the masses before and after the reaction will also be different. But the total energy of the system is always conserved.

The difference in rest energy between particles entering a reaction and those leaving it is called energy output and is expressed in a change in their kinetic energy.

In processes involving nuclei, three types are involved fundamental interactions- electromagnetic, weak and strong. Thanks to the latter, the nucleus has such an important feature as high binding energy between its constituent particles. It is significantly higher than, for example, between the core and atomic electrons or between atoms in molecules. This is evidenced by a noticeable mass defect - the difference between the sum of the nucleon masses and the nuclear mass, which is always less by the amount proportional to energy connections: Δm = E St / c 2. The mass defect is calculated using the simple formula Δm = Zm p + Am n - M i, where Z is the nuclear charge, A is the mass number, m p is the proton mass (1.00728 amu), m n is the neutron mass ( 1.00866 amu), M i - core mass.

When describing nuclear reactions, the concept is used specific energy bonds (that is, per nucleon: Δmc 2 /A).

Binding energy and nuclear stability

The greatest stability, that is, the highest specific binding energy, is distinguished by nuclei with a mass number from 50 to 90, for example, iron. This “peak stability” is due to the non-central nature of nuclear forces. Since each nucleon interacts only with its neighbors, it is bound weaker on the surface of the nucleus than inside. The fewer interacting nucleons in a nucleus, the lower the binding energy, so light nuclei are less stable. In turn, as the number of particles in the nucleus increases, the Coulomb repulsive forces between protons increase, so that the binding energy of heavy nuclei also decreases.

Thus, for light nuclei the most probable, that is, energetically favorable, are fusion reactions with the formation of a stable nucleus average weight, for heavy ones, on the contrary, processes of decay and fission (often multi-stage), as a result of which more stable products are also formed. These reactions are characterized by a positive and often very high energy yield, which accompanies an increase in binding energy.

Below we will look at some examples of nuclear reactions.

Decay reactions

Nuclei can undergo spontaneous changes in composition and structure, during which some elementary particles or fragments of the nucleus are emitted, such as alpha particles or heavier clusters.

Thus, during alpha decay, possible due to quantum tunneling, the alpha particle overcomes the potential barrier of nuclear forces and leaves the mother nucleus, which, accordingly, reduces atomic number by 2, and the mass number by 4. For example, the nucleus of radium-226, emitting an alpha particle, turns into radon-222:

226 88 Ra → 222 86 Rn + α (4 2 He).

The decay energy of the radium-226 nucleus is about 4.87 MeV.

Beta decay occurs without a change in the number of nucleons (mass number), but with an increase or decrease in the charge of the nucleus by 1, with the emission of an antineutrino or neutrino, as well as an electron or positron. An example of a nuclear reaction of this type is the beta-plus decay of fluorine-18. Here, one of the protons of the nucleus turns into a neutron, a positron and a neutrino are emitted, and fluorine turns into oxygen-18:

18 9 K → 18 8 Ar + e + + ν e .

The beta decay energy of fluorine-18 is about 0.63 MeV.

Nuclear fission

Fission reactions have much greater energy output. This is the name of the process in which the nucleus spontaneously or forcibly disintegrates into fragments of similar mass (usually two, rarely three) and some lighter products. A nucleus divides if its potential energy exceeds the initial value by a certain amount, called the fission barrier. However, the probability of a spontaneous process even for heavy nuclei is low.

It increases significantly when the nucleus receives the corresponding energy from the outside (when a particle hits it). The neutron penetrates the nucleus most easily, since it is not subject to electrostatic repulsion forces. The impact of a neutron leads to an increase internal energy nucleus, it is deformed with the formation of a constriction and divides. The fragments fly apart under the influence Coulomb forces. An example of a nuclear fission reaction is demonstrated by uranium-235 absorbing a neutron:

235 92 U + 1 0 n → 144 56 Ba + 89 36 Kr + 3 1 0 n.

Splitting into barium-144 and krypton-89 is just one of possible options fission of uranium-235. This reaction can be written as 235 92 U + 1 0 n → 236 92 U* → 144 56 Ba + 89 36 Kr + 3 1 0 n, where 236 92 U* is a highly excited compound nucleus with high potential energy. Its excess, along with the difference in binding energies of the mother and daughter nuclei, is released mainly (about 80%) in the form of the kinetic energy of the reaction products, and also partially in the form potential energy fission fragments. Total energy fission of a massive nucleus - approximately 200 MeV. In terms of 1 gram of uranium-235 (assuming that all nuclei have reacted), this amounts to 8.2 ∙ 10 4 megajoules.

Chain reactions

The fission of uranium-235, as well as nuclei such as uranium-233 and plutonium-239, is characterized by one important feature- presence among the reaction products free neutrons. These particles, penetrating into other nuclei, in turn, are able to initiate their fission, again with the release of new neutrons, and so on. This process is called a nuclear chain reaction.

The course of the chain reaction depends on how the number of neutrons emitted from the next generation compares with their number in the previous generation. This ratio k = N i /N i -1 (here N is the number of particles, i is serial number generation) is called the neutron multiplication factor. At k< 1 chain reaction doesn't work. When k > 1, the number of neutrons, and therefore fissile nuclei, increases like an avalanche. An example of a nuclear chain reaction of this type is an explosion atomic bomb. At k = 1, the process occurs in a steady state, as exemplified by the reaction controlled by neutron-absorbing rods in nuclear reactors.

Nuclear fusion

The greatest energy release (per nucleon) occurs during the fusion of light nuclei - the so-called fusion reactions. To react, positively charged nuclei must overcome the Coulomb barrier and approach a distance strong interaction, not exceeding the size of the core itself. Therefore, they must have extremely high kinetic energy, which means high temperatures (tens of millions of degrees and above). For this reason, fusion reactions are also called thermonuclear reactions.

An example of a nuclear fusion reaction is the formation of helium-4 with the release of a neutron during the fusion of deuterium and tritium nuclei:

2 1 H + 3 1 H → 4 2 He + 1 0 n.

Here, an energy of 17.6 MeV is released, which per nucleon is more than 3 times higher than the fission energy of uranium. Of these, 14.1 MeV occurs in kinetic energy neutron and 3.5 MeV - helium-4 nuclei. Such a significant value is created due to the huge difference in the binding energies of the nuclei of deuterium (2.2246 MeV) and tritium (8.4819 MeV) on the one hand, and helium-4 (28.2956 MeV) on the other.

In nuclear fission reactions, the energy of electrical repulsion is released, while in fusion, energy is released due to the strong interaction - the most powerful in nature. This determines such a significant energy yield of this type of nuclear reactions.

Examples of problem solving

Consider the fission reaction 235 92 U + 1 0 n → 140 54 Xe + 94 38 Sr + 2 1 0 n. What is its energy output? IN general view the formula for its calculation, reflecting the difference in the rest energies of particles before and after the reaction, is as follows:

Q = Δmc 2 = (m A + m B - m X - m Y + ...) ∙ c 2.

Instead of multiplying by the square of the speed of light, you can multiply the mass difference by a factor of 931.5 to get the energy value in megaelectronvolts. Substituting the corresponding values into the formula atomic masses, we get:

Q = (235.04393 + 1.00866 - 139.92164 - 93.91536 - 2∙1.00866) ∙ 931.5 ≈ 184.7 MeV.

Another example is the synthesis reaction. This is one of the stages of the proton-proton cycle - the main source of solar energy.

3 2 He + 3 2 He → 4 2 He + 2 1 1 H + γ.

Let's apply the same formula:

Q = (2 ∙ 3.01603 - 4.00260 - 2 ∙ 1.00728) ∙ 931.5 ≈ 13.9 MeV.

The main share of this energy - 12.8 MeV - occurs in in this case per gamma photon.

We have considered only the simplest examples of nuclear reactions. The physics of these processes is extremely complex; they are extremely diverse. The study and application of nuclear reactions has great importance how in practical area(energy) and in fundamental science.

– the process of interaction of a nucleus with an elementary particle or another nucleus, during which a change in the structure and properties of the nucleus occurs. For example, the emission of elementary particles by the nucleus, its fission, the emission of high-energy photons ( gamma rays). One of the results of nuclear reactions is the formation of isotopes that do not exist naturally on Earth.

Nuclear reactions can occur when atoms are bombarded by fast particles ( protons , neutrons , ions , alpha particles ).

More useful information By different topics- in our teleram.

Nuclear reactions

One of the first nuclear reactions carried out by humans was carried out Rutherford V 1919 year in order to detect the proton. At that time it was not yet known that the nucleus consisted of nucleons (protons And neutrons). During the splitting of many elements, a particle was discovered that was the nucleus of a hydrogen atom. Based on experiments, Rutherford made the assumption that this particle is part of all nuclei.

This reaction exactly describes one of the scientist’s experiments. In the experiment, the gas is higher ( nitrogen) is bombarded alpha particles (helium nuclei), which, knocking out nitrogen nuclei proton , convert it into an isotope of oxygen. The recording of this reaction looks like this:

When solving problems involving nuclear reactions, it should be remembered that during their occurrence the following conditions are fulfilled: classical laws saving: charge , angular momentum , impulse And energy .

There is also baryon charge conservation law . This means that the number of nucleons participating in the reaction remains unchanged. If we look at the reaction, we see that the amounts mass numbers (number above) and atomic numbers l (bottom) on the right and left sides of the equation are the same.

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Specific binding energy of nuclei

As is known, one of the fundamental forces operates inside the nucleus at distances of the order of its size. physical interactions – strong interaction . To overcome it and “destroy” the core, it is necessary a large number of energy.

Nuclear binding energy - the minimum energy required to split the nucleus of an atom into its constituent elementary particles.

The mass of any atomic nucleus is less than the mass of its constituent particles. The difference between the masses of a nucleus and its constituent nucleons is called mass defect:

Numbers Z And N are easily determined using periodic tables, and you can read about how this is done. The binding energy is calculated using the formula:

Energy of nuclear reactions

Nuclear reactions accompanied by energy transformations. There is a quantity called the energy yield of the reaction and is determined by the formula

Delta M – mass defect, but in this case it is the difference in mass between the initial and final products of a nuclear reaction.

Reactions can occur both with the release of energy and with its absorption. Such reactions are called respectively exothermic And endothermic .
To leak exothermic reaction , execution required next condition: the kinetic energy of the initial products must be greater than the kinetic energy of the products formed during the reaction.

Endothermic reaction possible when specific binding energy nucleons in original products less specific binding energy of the nuclei of the final products.

Examples of solving nuclear reaction problems

And now a couple practical examples with solution:

Even if you come across a problem with an asterisk, it is worth remembering that there are no unsolvable problems. Student service will help you complete any task.

Isomeric transition

Mechanisms of nuclear reaction

Compound nucleus

The theory of the reaction mechanism to form a compound nucleus was developed by Niels Bohr in 1936 together with the theory of the droplet model of the nucleus and forms the basis modern ideas about most nuclear reactions.

According to this theory, the nuclear reaction occurs in two stages. At the beginning, the initial particles form an intermediate (composite) nucleus after nuclear time, that is, the time required for a particle to cross the nucleus, approximately equal to 10 −23 - 10 −21. In this case, a compound nucleus is always formed in an excited state, since it has excess energy brought by the particle into the nucleus in the form of the binding energy of the nucleon in the compound nucleus and part of its kinetic energy, which is equal to the sum of the kinetic energy of the target nucleus with mass number and the particle in the system center of inertia.

Excitation energy

The excitation energy of a compound nucleus formed upon absorption of a free nucleon is equal to the sum of the binding energy of the nucleon and part of its kinetic energy:

Most often due to big difference in the masses of the nucleus and nucleon is approximately equal to the kinetic energy of the nucleon bombarding the nucleus.

On average, the binding energy is 8 MeV, varying depending on the characteristics of the resulting compound nucleus, but for the given target nucleus and nucleon this value is a constant. The kinetic energy of the bombarding particle can be anything, for example, when excitation of nuclear reactions by neutrons, the potential of which does not have a Coulomb barrier, the value can be close to zero. Thus, the binding energy is the minimum excitation energy of a compound nucleus.

Reaction channels

The transition to a non-excited state can be carried out in various ways, called reaction channels. The types and quantum state of incident particles and nuclei before the start of the reaction are determined by input channel reactions. After completion of the reaction, the totality of the resulting reaction products and their quantum states determines output channel reactions. The reaction is completely characterized by input and output channels.

The reaction channels do not depend on the method of formation of the compound nucleus, which can be explained by the long lifetime of the compound nucleus; it seems to “forget” how it was formed, therefore, the formation and decay of the compound nucleus can be considered as independent events. For example, it can be formed as a compound nucleus in an excited state in one of the following reactions:

Subsequently, provided that the excitation energy is the same, this compound nucleus can decay in the opposite way to any of these reactions, with a certain probability that does not depend on the history of the appearance of this nucleus. The probability of the formation of a compound nucleus depends on the energy and on the type of target nucleus.

Direct nuclear reactions

The course of nuclear reactions is also possible through the mechanism direct interaction Basically, this mechanism manifests itself at very high energies of bombarding particles, when the nucleons of the nucleus can be considered as free. Direct reactions differ from the compound nucleus mechanism primarily in the distribution of the momentum vectors of the product particles relative to the momentum of the bombarding particles. In contrast to the spherical symmetry of the compound nucleus mechanism, direct interaction is characterized by the predominant direction of flight of the reaction products forward relative to the direction of movement of the incident particles. The energy distributions of product particles in these cases are also different. Direct interaction is characterized by an excess of high-energy particles. In collisions with nuclei complex particles(that is, other nuclei), processes of nucleon transfer from nucleus to nucleus or nucleon exchange are possible. Such reactions occur without the formation of a compound nucleus and they have all the features of direct interaction.

Nuclear reaction cross section

The probability of a reaction is determined by the so-called nuclear cross section reactions. In a laboratory frame of reference (where the target nucleus is at rest), the probability of interaction per unit time is equal to the product of the cross section (expressed in units of area) and the flux of incident particles (expressed in the number of particles crossing a unit area per unit time). If several output channels can be implemented for one input channel, then the ratio of the probabilities of the output reaction channels is equal to the ratio of their cross sections. In nuclear physics, reaction cross sections are usually expressed in special units - barns, equal to 10 −24 cm².

Reaction output

The number of reaction cases divided by the number of particles bombarding the target is called the output of a nuclear reaction. This value is determined experimentally at quantitative measurements. Since the yield is directly related to the reaction cross section, measuring the yield is essentially a measurement of the reaction cross section.

Conservation laws in nuclear reactions

In nuclear reactions, all conservation laws of classical physics are satisfied. These laws place restrictions on the possibility of a nuclear reaction. Even an energetically favorable process always turns out to be impossible if it is accompanied by a violation of any conservation law. In addition, there are conservation laws specific to the microworld; some of them are always fulfilled, as far as is known (law of conservation of baryon number, lepton number); other conservation laws (isospin, parity, strangeness) only suppress certain reactions, since they are not satisfied for some of the fundamental interactions. The consequences of conservation laws are the so-called selection rules, indicating the possibility or prohibition of certain reactions.

Law of energy conservation

If , , , are the total energies of two particles before and after the reaction, then based on the law of conservation of energy:

When more than two particles are formed, the number of terms on the right side of this expression should accordingly be greater. Total Energy particle is equal to its rest energy Mc 2 and kinetic energy E, That's why:

The difference between the total kinetic energies of particles at the “output” and “input” of the reaction Q = (E 3 + E 4) − (E 1 + E 2) called reaction energy(or energy yield of the reaction). It satisfies the condition:

Multiplier 1/ c 2 is usually omitted when calculating the energy balance, expressing particle masses in energy units (or sometimes energy in mass units).

If Q> 0, then the reaction is accompanied by the release free energy and is called exoenergetic , If Q < 0, то реакция сопровождается поглощением свободной энергии и называется endoenergetic .

It's easy to see that Q> 0 when the sum of the masses of the product particles is less than the sum of the masses of the initial particles, that is, the release of free energy is possible only by reducing the masses of the reacting particles. And vice versa, if the sum of the masses of secondary particles exceeds the sum of the masses of the initial ones, then such a reaction is possible only if a certain amount of kinetic energy is spent to increase the rest energy, that is, the masses of new particles. Minimum value kinetic energy of an incident particle at which an endoenergetic reaction is possible is called threshold reaction energy. Endoenergetic reactions are also called threshold reactions, since they do not occur at particle energies below the threshold.

Law of conservation of momentum

The total momentum of the particles before the reaction is equal to the total momentum of the reaction product particles. If , , , are the momentum vectors of two particles before and after the reaction, then

Each of the vectors can be independently measured experimentally, for example, with a magnetic spectrometer. Experimental data indicate that the law of conservation of momentum is valid both in nuclear reactions and in the processes of scattering of microparticles.

Law of conservation of angular momentum

Nuclear fusion reaction

Nuclear fusion reaction- the process of fusion of two atomic nuclei to form a new, heavier nucleus.

In addition to the new nucleus, during the fusion reaction, as a rule, various elementary particles and (or) quanta of electromagnetic radiation are also formed.

Without supply external energy fusion of nuclei is impossible, since positively charged nuclei experience electrostatic repulsion forces - this is the so-called “Coulomb barrier”. To synthesize nuclei, it is necessary to bring them closer to a distance of about 10–15 m, at which the action of strong interaction will exceed the forces of electrostatic repulsion. This is possible if the kinetic energy of approaching nuclei exceeds the Coulomb barrier.

Such conditions can arise in two cases:

If matter is heated to extremely high temperatures in a star or fusion reactor. According to kinetic theory, the kinetic energy of moving microparticles of a substance (atoms, molecules or ions) can be represented as temperature, and, therefore, by heating a substance, a nuclear fusion reaction can be achieved. In this case, they talk about thermonuclear fusion or thermonuclear reaction.

Thermonuclear reaction

Thermonuclear reaction- the fusion of two atomic nuclei to form a new, heavier nucleus, due to the kinetic energy of their thermal motion.

For a nuclear fusion reaction, the initial nuclei must have relatively high kinetic energy, since they experience electrostatic repulsion, since they are positively charged.

First of all, among them it should be noted the reaction between two isotopes (deuterium and tritium) of hydrogen, which is very common on Earth, as a result of which helium is formed and a neutron is released. The reaction can be written as:

+ energy (17.6 MeV).

The energy released (arising from the fact that helium-4 has very strong nuclear ties) turns into kinetic energy, most of which, 14.1 MeV, carries away the neutron as a lighter particle. The resulting nucleus is tightly bound, which is why the reaction is so highly exoenergetic. This reaction is characterized by the lowest Coulomb barrier and big yield, so she represents special interest for managed thermonuclear fusion.

Photonuclear reaction

When a gamma quantum is absorbed, the nucleus receives excess energy without changing its nucleon composition, and a nucleus with excess energy is a compound nucleus. Like other nuclear reactions, absorption of a gamma quantum by a nucleus is possible only if the necessary energy and spin relationships are met. If the energy transferred to the nucleus exceeds the binding energy of a nucleon in the nucleus, then the decay of the resulting compound nucleus occurs most often with the emission of nucleons, mainly neutrons. Such decay leads to nuclear reactions and, which are called photonuclear, and the phenomenon of nucleon emission in these reactions is nuclear photoelectric effect.

Other

Recording nuclear reactions

Nuclear reactions are written in the form of special formulas in which the designations of atomic nuclei and elementary particles are found.

First way writing formulas for nuclear reactions is similar to writing formulas for chemical reactions, that is, the sum of the original particles is written on the left, the sum of the resulting particles (reaction products) is written on the right, and an arrow is placed between them.

Thus, the reaction of radiative capture of a neutron by a cadmium-113 nucleus is written as follows:

We see that the number of protons and neutrons on the right and left remains the same (the baryon number is conserved). The same applies to electric charges, lepton numbers and other quantities (energy, momentum, angular momentum, ...). In some reactions where the weak interaction is involved, protons can turn into neutrons and vice versa, but their total number does not change.

Second way notation, more convenient for nuclear physics, has the form A (a, bcd...) B, Where A- target core, A- bombarding particle (including the nucleus), b, c, d, …- emitted particles (including nuclei), IN- residual core. Lighter reaction products are written in brackets, heavier ones are written outside. Thus, the above neutron capture reaction can be written in this form.

At low (< 1 МэВ), средних (1-100 МэВ) и высоких (>100 MeV) energies. Distinctions are made on light nuclei (target nuclei A< 50), ядрах ср. массы (50 < А < 100) и тяжелых ядрах (А > 100).
I nuclear can occur if the two particles involved in it approach at a distance less than the diameter of the nucleus (approx. 10 -13 cm), i.e. at a distance at which the forces of intranuclear interaction act. between the constituent nucleons of the nucleus. If both participants nuclear particles Since both the bombardment and the target core are positively charged, the approach of the particles is prevented by the repulsive force of the two positive particles. charges, and the bombarding particle must overcome the so-called. Coulomb potential barrier. The height of this barrier depends on the charge of the bombarding particle and the charge of the target nucleus. For kernels responding with avg. values of , and bombarding particles with charge +1, the barrier height is approx. 10 MeV. If particles that do not have a charge () participate in the nuclear process, there is no Coulomb potential barrier, and nuclear reactions can proceed with the participation of particles that have thermal energy(i.e. energy corresponding to thermal vibrations).
The possibility of nuclear nuclei occurring not as a result of bombardment of target nuclei by incident particles, but due to ultra-strong convergence of nuclei (i.e., approaching at distances comparable to the diameter of the nucleus) located in a solid or on a surface (for example, with the participation of nuclei, dissolved in); So far (1995) there is no reliable data on the implementation of such nuclear ("cold thermonuclear fusion").
I nuclear are subject to the same general laws of nature as ordinary chem. r-tion (and energy, conservation of charge, momentum). In addition, during the course of nuclear reactions, certain specific effects also occur. laws that do not appear in chemistry. p-tions, for example, the law of conservation of baryon charge (baryons are heavy).
Nuclear nuclei can be written as shown in the example of the transformation of Pu nuclei into Ku nuclei when a plutonium target is irradiated with nuclei:

From this record it is clear that the sums of charges on the left and right (94 + 10 = 104) and the sums (242 + 22 = 259 + 5) are equal. Because the chemical symbol element clearly indicates its at. number (nuclear charge), then when writing nuclear values of the charge of particles, they are usually not indicated. More often nuclear ones are written shorter. Thus, the nuclear formation of 14 C during irradiation of 14 N nuclei is recorded as follows. way: 14 N(n, p) 14 C.
In brackets indicate first the bombarding particle or quantum, then, separated by commas, the resulting light particles or quantum. In accordance with this recording method, (n, p), (d, p), (n, 2n) and other nuclear ones are distinguished.
When the same particles collide, nuclear particles can separate. ways. For example, when an aluminum target is irradiated, a trace may occur. nuclear: 27 А1(n,) 28 А1, 27 А1(n, n) 27 А1, 27 А1(n, 2n) 26 А1, 27 А1(n, p) 27 Mg, 27 Al(n,) 24 Na and etc. The collection of colliding particles is called. the nuclear input channel, and the particles born as a result of the nuclear one form the output channel.
I Nuclear nuclear reactions can occur with the release and absorption of energy Q. If in general terms we write nuclear energy as A(a, b)B, then for such nuclear energy is equal to: Q = [(M A + M a) - (M b + M b)] x c 2, where M is the mass of the nuclear particles involved; c is the speed of light. In practice, it is more convenient to use deltaM values (see), then the expression for calculating Q has the form: and for reasons of convenience, it is usually expressed in kiloelectronvolts (keV, 1 amu = 931501.59 keV = 1.492443 x 10 -7 kJ).
The change in energy that is accompanied by nuclear energy can be 10 6 times or more greater than the energy released or absorbed during chemical reactions. r-tions. Therefore, during a nuclear one, a change in the masses of interacting nuclei becomes noticeable: the energy released or absorbed is equal to the difference in the sums of the masses of particles before and after the nuclear one. The possibility of releasing huge amounts of energy during the implementation of nuclear lies at the basis of nuclear (see). The study of the relationships between the energies of particles participating in nuclear reactions, as well as the relationships between the angles at which the resulting particles fly apart, constitutes the section nuclear physics- kinematics of nuclear systems.

Nuclear outputs, i.e., the ratio of the number of nuclear particles to the number of particles falling per unit area (1 cm 2) of the target usually does not exceed 10 -6 -10 -3. For thin targets (simplistically, a thin target can be called a target, when passing through it the flow of bombarding particles does not noticeably weaken), the nuclear yield is proportional to the number of particles falling on 1 cm 2 of the target surface, the number of nuclei contained in 1 cm 2 of the target, and also the value of the effective nuclear cross section. Even when using such a powerful source of incident particles as nuclear reactor, within 1 hour it is possible, as a rule, to obtain when carrying out nuclear under the influence of no more than a few. mg containing new nuclei. Usually, the mass of a substance obtained in one or another nuclear facility is significantly less.

Bombarding particles. To carry out nuclear reactions, n, p, deuterons d, tritons t, particles, heavy (12 C, 22 Ne, 40 Ar, etc.), e quanta are used. Sources (see) when carrying out nuclear are: mixtures of metallic. Be and a suitable emitter, e.g. 226 Ra (so-called ampoule sources), neutron generators, nuclear reactors. Since in most cases, nuclear ones are higher for low energies (thermal), then before directing the flow to the target, they are usually slowed down using, and other materials. In the case of slow fundamentals. the process for almost all nuclei is radiation capture - nuclear type, since the Coulomb barrier of the nucleus prevents the escape of particles. Under the influence, chain flows occur.
If used as bombarding particles, deuterons, etc., carrying positive. charge, the bombarding particle is accelerated to high energies(from tens of MeV to hundreds of GeV), using decomp. accelerators. This is necessary so that a charged particle can overcome the Coulomb potential barrier and enter the irradiated nucleus. When irradiating targets with positively charged particles, max. Nuclear yields are achieved using deuterons. This is due to the fact that the binding energy in the deuteron is relatively small, and accordingly, the distance between and is large.
When deuterons are used as bombarding particles, only one nucleon often penetrates into the irradiated nucleus - or, the second nucleon of the deuteron nucleus flies further, usually in the same direction as the incident deuteron. High effective cross sections can be achieved by conducting nuclear tests between deuterons and light nuclei at relatively low energies of incident particles (1-10 MeV). Therefore, nuclear nuclei with the participation of deuterons can be carried out not only by using deuterons accelerated at an accelerator, but also by heating a mixture of interacting nuclei to a temperature of approx. 10 7 K. Such nuclear ones are called thermonuclear. IN natural conditions they occur only in the depths of stars. On Earth, thermonuclear r-tions involving,