Nuclear reactions are classified. Classification and mechanisms of nuclear reactions

Turchica N.V. Physics in tasks for entering universities - M.: Onyx, 2008. - 768 c.
ISBN 978-5-94666-452-3.
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20.5.7. With the resonant grip of the neutron, the uranium isotope 292U is formed by the radioactive isotope of uranium 239U. It is experiencing a p-decay and turns into an isotope of the transuranone element of neptune 2 ^ Np. Neptune is a P-radioactive and turning

in Plutonium 94PU, which plays an essential role in obtaining nuclear energy. Write down the described nuclear reactions.

20.5.8. Most nuclear reactions can go in several ways that received the name "reaction channels". For example, when irradiation of lithium isotope, 7LL protons can register

398
: a) two identical kernels; b) The beryllium beryllium isotope core and neutron. Write the reactions of the specified "reaction channels".

20.5.9. Write the missing designations in the following reactions:

h 27 .., 1 A, 4TT ... 56--, and "56", 1

a) 13ai + 0 n ^ z x + 2 he; b) 25mm + z x ^ 26fe + 0 n;

A 1 22 4 27 26 A

c) ZX + IH ^ NNA + 2He; d) 13Al + Y ^ 12mg + zx *

20.5.10. Runfords element got, irradiating plutonium

94pu neon 10ne nuclei. Write the reaction if you know that more than four neutrons are formed besides it.

20.6. Nuclear reaction energy

20.6.1. Determine the energy of the 3li + 1H ^ 24he nuclear reaction.

20.6.2. Determine thermal effects The following reactions:

a) 3li + 1p ^ 4BE + 0N; b) 4Be + 0N ^ 4BE + Y;

7 4 10 1 16 2 14 4

c) 3li + 2a ^ 5 b + 0n; d) 8o + 1 d ^ 7n + 2a.

20.6.3. What minimal energy should have a particle

to implement a 3li + 2He ° 5b + 0n nuclear reaction?

20.6.4. Find the Y-quanta energy emitted with nuclear

23 reactions 1H + N ^ 1H + Y.

20.6.5. When exploding hydrogen bombs The thermalide reaction of the formation of 4He helium atoms from Datery 1H and tritium 1H is flowing.

Write a nuclear reaction and determine its energy output.

20.6.6. Determine the energy of the nuclear reaction 4BE + 1H ^

^ 14Be + ^ h. What energy is released when full reaction Beryllium mass M \u003d 1 g?

20.6.7. Thermonuclear reaction 1H + 2He ^ 4He + ^ P comes with the emission of energy E1 \u003d 18.4 MeV. What energy will be extended in

3He + 2He ^! HE + 2 ^, if the 2He core mass defect is

AM \u003d 0.006 A.E.M. More than the kernel 1h?

399
20.6.8. Using the definition of communication energy, show that the energy required for the separation of the c key on the kernel A and B can be represented as: Eab \u003d Ec - (EA + EB), where EA, EB, EC - the communication energy of the corresponding cores. Determine the energy necessary to separate the oxygen 16o kernel on the A-particle and the carbon kernel 12c. Communication Energy: E16 ^ \u003d 127.62 MeV, EA \u003d 28.30 MeV, E12 ^ \u003d

92.16 MeV.

20.6.9. With a 3li + 1H ^ 3Li + 1R reaction, the energy Q \u003d 5.028 MeV is released. Lithium core binding energy E1 \u003d 39.2 MeV, deuterium E2 \u003d 1.72 MeV. Determine the mass of lithium core.

20.6.10. When dividing nuclei with a specific bonding energy є \u003d 8.5 MeV / NUKL, two fragments are formed - one with a mass number Ai \u003d 140 and the specific energy of communication єїї \u003d 8.3 MeV / Nucl, the other - with a mass number A2 \u003d 94 and the specific Energy of communication є2 \u003d \u003d 8.6 MeV. Evaluate the amount of heat that is released when dividing the mass M \u003d 1 g of the initial nuclei. Count Tr \u003d Mn \u003d

1,6724 10-27 kg.

20.6.11. Considering that in one act of dividing the kernel of uranium 235U, the energy of EO \u003d 200 MeV is released, determine the energy released during combustion M \u003d 1 kg of uranium, and a mass of stone coal Mi equivalent to 1 kg of uranium.

20.6.12. When dividing the uranium core 235U, the energy Q \u003d 200 MeV is released. What share of uranium peace energy is the excreted energy?

20.6.13. Determine the mass consumption of nuclear fuel 235U in the nuclear reactor of the nuclear power plant. Thermal power of power plant P \u003d 10 MW; Its efficiency n \u003d 20%. Energy released at one act of division, Q \u003d 200 MeV.

20.6.14. Find the power of the nuclear power plant consuming per day M \u003d 220 g of uranium isotope 235U and having efficiency n \u003d 25%. It is possible that in one act of division 235U, the energy Q \u003d 200 MeV is released.

20.6.15. For aluminum melting, the energy released during the positron p-decay of carbon isotopes 11c is used, each of the carbon kernel eats one positron. Disintegration products are not radioactive. How much carbon 1i1c required for

performing melting M \u003d 100 T aluminum per І \u003d 30 min, if the initial temperature of aluminum 0o \u003d 20 ° С?

20.6.16. Sodium and Na weighing m \u003d 10 g, experiencing electronic Р-decay, placed in an ampoule to a tank containing

400
M \u003d 1000 tons of water. Disintegration products are not radioactive. The period of

luraspad sodium t \u003d ^ day. How many degrees will increase the water temperature in the first day from the beginning of sodium decay?

20.6.17. Polonium 84p0 decays with emitting a-particle

and the formation of lead cores. Disintegration products are not radioactive. Half-life polonium t \u003d 140 days. What kind of ice, taken at a temperature of 0 \u003d 0 0c, can melt, using the energy that is released during the decay M \u003d 10 g of polonium during T \u003d 35 days?

20.7. Nuclear reactions and conservation laws

20.7.1. Rowing the core of polonium 84p0 threw a particle with kinetic energy EK \u003d 5.3 MeV. Determine the kinetic energy of the nucleus of recoil and the total energy distinguished with the decay.

Nuclear reactions are the conversion of atomic nuclei when interacting with elementary particles (including with y-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 original and finite nuclei, but and B - bombarding and emitted (or emitted) in the nuclear particle reaction.

In any nuclear reaction, the laws of preservation and mass numbers are carried out: amount charge (mass) numbers of nuclei and particles entering the nuclear reaction is equal to the amount of charge (mass) numbers of finite products (cores and particles) of the reaction. Performed also energy conservation laws, impulse and moment of momentum.

In contrast to the radioactive decay, which always proceeds with the release of energy, nuclear reactions can be both exothermic (with energy release) and endothermic (with energy absorption).

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

The first stage is the seizure of the kernel X particle A, approaching it to the range nuclear power (approximately 2 10 15 m), and the formation of an intermediate kernel with a composite (or compound nucleus). The energy of the particles flew in the core is quickly distributed between the nucleons of the composite nucleus, as a result of which it turns out to be in the excited state. In the collision of the nucleons of the compound kernel, one of the nucleons (or their combination, for example, a deuteron - a hydrogen heroic core - deuterium containing one proton and one neutron) or an CX particle can get energy sufficient for departure from the kernel. As a result, the second stage of the nuclear reaction is possible - the decay of the composite core on the kernel of the y and particle B

Classification of nuclear reactions

By the nature of parties participating in the reactions:

• reactions under the action of neutrons;
• Reactions under the action of charged particles (for example, protons, (X-particles).

By the energy of causing particles:

• reactions at low energies (order of eV), which occur mainly with the participation of neutrons;
• reactions at average energies (somewhat MEV), taking place with the participation of bodies and charged particles;
• High Energy Reactions (hundreds and Thousands MeV), resulting in the birth of missing in free state elementary particles and having great importance For their study.

By the nature of the cores participating in reactions:

• Reactions on light nuclei (And 50);
• reactions on medium nuclei (50 A
• Reactions on heavy nuclei (A... 150).

By the nature of the nuclear transformations occurring:

• reactions with neutron emission;
• Reactions with emission of charged particles. First in history nuclear reaction (Rangeford; 1919)

11.1. Definition and classification of nuclear reactions. There are various interpretations of the term nuclear reactions. In a broad sense, a nuclear reaction is called any process beginning with a collision of two, rarely several, particles (simple or complex) and coming, as a rule, with the participation of strong interactions. This definition satisfy and nuclear reactions In the narrow sense of the word, under which processes beginning with a collision of a simple or complex particle (Nucleon, α- particle, γ-quantum) with the kernel. Note that the definition of the reaction satisfies as a special case, and the scattering of particles. Two examples of nuclear reactions are shown below.

Historically, the first nuclear reaction (Rutherford, 1919 - the opening of the proton):

α + 14 N → 17 O + r.

Opening of Neutron (Chadwick, 1932):

α + 9 ve → 12 s + n.

The study of nuclear reactions is necessary to obtain information on the properties of new nuclei and elementary particles excited by the states of the nuclei, etc. We should not forget that in the micrometer due to the presence of quantum patterns on a particle or nucleus you cannot "see". Therefore, the main method of studying microjects is the study of their collisions, i.e. nuclear reactions. In applied relations, nuclear reactions are needed to use nuclear energy, as well as to obtain artificial radionuclides.

Nuclear reactions can occur in natural conditions (for example, in bowels of stars or in space rays). But their study is usually carried out in laboratory conditions, on experimental installations. To carry out nuclear reactions, it is necessary to bring together particles or kernels with nuclei to the distance of the nuclear power radius. An approach of charged particles with nuclei is hampered by the Coulomb barrier. Therefore, for the implementation of nuclear reactions on charged particles use acceleratorsIn which the particles that accelerate in the electric field acquire the energy necessary to overcome the barrier. Sometimes this energy is comparable to a particle rest energy or even exceeds it: in this case, the movement is described by the laws of relativistic mechanics. In conventional accelerators ( linear accelerator, cyclotron etc.) the heavier of two acupunged particles, as a rule, rests, and easier on it flies. People's particle called target (english - Target). Flying, or bombarding, particles in Russian special name did not receive (in english language The term Projectile is used - projectile). In accelerators on counter beams ( colliider.) Both accessed particles move, so that the separation of the target and the bundle of the flutter particles loses its meaning.

The energy of the charged particle in the reaction can be less than the height of the Coulomb barrier, as it was in the classical experiments of J. Kokkroft and E. Walton, which in 1932 carried out artificial cleavage of lithium cores by bombing by their accelerated protons. In their experiments, the proton penetration in the target core occurred by tuning through the Coulomb potential barrier (see lecture 7). The probability of such a process, of course, is very small due to the small transparency of the barrier.

For symbolic recording of nuclear reactions, there are several ways, two of which are shown below:

A combination of colliding particles in a certain quantum state (for example, r and 7 Li) called inlet canal nuclear reaction. In collisions of the same particles (fixed input channel), various reaction products can appear in the general case. So, in collisions of protons with 7 Li, reactions 7 Li ( p., 2α), 7 Li ( p., n.) 7 be, 7 Li ( p., d.) 6 bes and others. In this case, they talk about competing processes, or about the set output channels.

Often nuclear reactions are recorded in an even shorter form: ( a., b.) - i.e. Indicating only light particles and without pointing the kernels involved in the reaction. For example, recording ( p., n.) means knocking out a protone neutron from any nucleus, ( n., γ ) - neutron absorption core with emission γ -Kvanta, etc.

Classification of nuclear reactions Can be carried out on the following signs:

I. By type of proceeding process

1) Radiation capture: ( n., γ ), (p., γ )

2) Nuclear photo effect: ( γ , n.), (γ , p.)

3) nucleon-nucleon reactions:

a) knocking out nucleon or group of nucleons ( n., P.), (p., α), etc.

b) "evaporation" of nucleons ( p., 2n.), (p., 2p.) etc.

c) disruption ( d., P.), (d., N.) and pickup ( p., D.), (n., D.)

4) division: ( n., f.), (p., f.), (γ , f.)

5) Synthesis (merge)

6) inelastic scattering: ( n., n ')

7) Elastic scattering: ( n., N.)

II. On the basis of excretion or absorption of energy

1) exothermic reactions

2) endothermic reactions

III. On the energy of bombarding particles

1) small energies (< 1 кэВ)

2) average energies (1 keV-10MEV)

3) High Energy (\u003e 10 MeV)

IV. By weight of bombarded nuclei

1) on light nuclei ( BUT < 50)

2) on the medium cores (50< BUT < 100)

3) on heavy nuclei ( BUT > 100)

V. By type of bombarding particles

1) on charged particles ( p., d., α and heavier ions)

2) on neutron

3) on photons (photonuclear reactions)

11.2. Law of energy conservation. For the nuclear reaction of the general form

A. + B. → C.+ D + E + ...

we write the law of conservation of energy through the energy of peace and kinetic energies:

Value Q., Defined as the difference in the energy of rest:

called reaction energy. It's obvious that

If a Q. \u003e 0, then such a reaction is called exothermic. In this case Q. - This is the difference in the kinetic energies of all participants in the reaction before and after the expansion defined in the coordinate system associated with the center of inertia (SCI, or c-system). The exothermic reaction can go with any value of the kinetic energy of the colliding particles, including, and at zero.

If a Q. < 0, то реакцию называют endothermal. The reaction of the reverse exothermic reaction is always endothermic, and vice versa. Value - Q.in c-system- this is the minimum kinetic energy of colliding particles at which the reaction is still possible, or, threshold reactions.

When switching to the laboratory coordinate system (Fig. 11.1), LSK, or simply l-Systemin which one of the reacting particles is resting - the target value of the reaction threshold E POR. increases, because Part of the kinetic energy goes on the movement of the center of inertia for the reaction. Indeed, the kinetic energy of the Motion of the Center inertia may be arbitrarily large, but if the particles rest relative to each other, the reaction will not go.

To determine the reaction threshold in l-system We use the fact that the mass, and therefore the energy of rest is invariant. The value is independent of the choice of coordinate system. As , for any number of particles

If the target in the reaction under consideration is a particle IN, then B. L-system

IN c-system

As mentioned above, the threshold in c-system Corresponds to the birth of particles FROM, D. etc. with zero kinetic energies, i.e. etc. and . Invariant Mass B. l-system

Mass-responding threshold mass in c-system

If you now equate the two invariants, then

. (11.3)

Thus, the threshold of the endothermic reaction is always greater than the energy of the reverse exothermic reaction Q.. As can be seen from the resulting expression, the threshold of the endothermic reaction is lower than more Massa target.

11.3. The role of the orbital moment.The moment of the pulse of the particle with momentum rpowered by a fixed kernel is equal pB.where b. - Aimal parameter. According to classical ideas, the reaction can occur only in cases where this target parameter is less than the radius of the nuclear forces, i.e. b. < R.. In quantum mechanics, the value of the orbital moment

(- De Brogly wavelength). Then there should be inequality

. (11.4)

For neutron with energy T. \u003d 1 MeV, i.e. Compare with the sizes of the kernel. For neutrons and protons with less energy, it is much larger. So., for particles of small and medium energies, inequality (11.4) is performed, strictly speaking, only under the condition l. \u003d 0 (less often l. = 1).

Taking into account the quantum properties of the system, the reaction is possible in principle for any l.But the probability of the reaction drops sharply if the ratio (11.4) is not performed. The reason is that neutrons in this case need to overcome the centrifugal barrier. But, as was shown in the consideration of the emission of γ-quanta nuclei (lecture 9), the transparency coefficient of the centrifugal barrier

,

those. sharply decreases with increasing l.. If the long-wave approximation ceases to be performed (i.e., bombarding particles have very high energy), interaction is possible with l.other than zero.

11.4. Section and yield of nuclear reaction.Quantitative description of nuclear reactions in terms of quantum mechanics Maybe only statistical. In which it is fundamentally, it is possible to speak only about the likelihood of an act of the reaction itself. The most probabilistic characteristics of nuclear reactions are section and outputwhose definition is given below. Suppose when falling a flow of particles BUT On a thin (but macroscopic) target containing kernels IN, it is formed in it dN S. kernels FROM (Fig. 11.2). This amount is proportional to the number of particles BUT, target particle density n B. (M -3) and target thickness dX (m):

.

Section Reactions BUT + IN → FROM + ··· is determined while the coefficient of proportionality, i.e.

, (11.5)

From the definition (11.5) it follows that the section has dimension of the area (m 2). In nuclear physics, 1 is used as a unit of section barne: 1 b \u003d 10 -28 m 2.

A clear section can be viewed as an effective target area, falling into which the particle causes the required reaction. But due to the wave properties of particles, such an interpretation has a limited area of \u200b\u200bapplicability. After all, from the point of view of quantum mechanics for a particle, there is a non-zero probability to go without deviation through the area in which the forces act on it. Then the actual cross section of the reaction will be less than the cross section of the area in which the interaction occurs. In this case, by analogy with optics, the target core is called partially transparent, or gray.

In real physical experiments, it is not always possible to measure the reaction cross section. Directly measured magnitude is output Reactions, defined as the proportion of the beam particles that have entered the reaction with the target nuclei. Express the yield of the reaction through its cross section provided that the latter remains constant when the incident particles pass through the target. Number of kernels FROMformed in a thin layer of target as a result of reaction with particles BUTwell

,

where N. 0 - Total Particle BUTin a layer thick dX, N A. - The number of particles under the layer without reaction. From here . Then, in accordance with (11.5),

Number of particles BUTwho passed the target layer of the ultimate thickness h., find the integration of this equation:

,

Using the definition of reaction output as a fraction of particles that experienced the transformation, we find that

Thin target Corresponds to small compared to the unit exponential indicator. In this case, decomposition (11.6) in a series of Taylor gives

11.5. Mechanisms of nuclear reactions.In addition to the classification given in paragraph 11.1., Nuclear reactions differ in time and in connection with this by the mechanism of their flow. As a temporary scale, it is convenient to use a nuclear time - the time of the span of particles through the kernel: τ i. = 2R./v.≈ 10 -22 C (p. 2.2). It's obvious that τ poison - The minimum time required to complete the elementary act of the rapid response itself.

We will use the following classification of reactions by the flow mechanism. If the time of the elementary act t R. ≈ τ poison, such reactions are called straight. In case of direct particle reactions a. transmits energy to one or more nucleon nucleons A., after which they immediately leave the kernel, did not have time to exchange energy with the rest:

a. + A. → b. + B..

If a t R. >> τ poison, then the reaction goes through the stage of education compound kernel:

a. + A. → FROM* → b. + B..

The idea of \u200b\u200bthe composite core was introduced into the physics of N. Borok in 1936. The composite core FROM* - excited core state FROM, and the excitation energy

(11.7)

where T A.- kinetic particle energy but, W A. - energy of separation from the kernel FROM. Excitation energy is divided between BUT+ but nucleons of the composite core, and on average, one nucleon has to

. (11.8)

Thus, each of the nucleons separately the energy is insufficient for departure. As a result of a plurality of particle collisions but "Confused" in the kernel and loses its individuality. Only through time t R.>> τ poison As a result of randomly redistribution of energy, its sufficient amount can concentrate on one of the nucleons (or the nucleon group). In this case, the nucleon (group of nucleons) leaves the composite core - it takes place.

Approximately evaluate the average life of the compound kernel FROM*can be as follows. We will take that immediately after the collision of the particles takes place the distribution n. excitation energy quanta f. single-duccant degrees of freedom. The total number of possible distributions is equal

. (11.9)

The output of formula (11.9) can be illustrated by the following visual scheme: - distribution n. quantum cross f. Cells separated from each other f.minus by the temper. The total number of permutations (i.e. the total number of states of the system) of all crosses and all the thrust is equal ( n.+ f -one)! However, the permutations of only crosses and only the thoughts whose numbers are equal n.! and ( f -one)! Accordingly, do not lead to new states. As a result, the true number of states turns out to be in n.!(f -one)! Once smaller.

We will further form for the simplicity of reasoning that the reaction of the nucleon departure occurs under the action of low-energy particles, so E * ≈ W A.. Then to flow the reaction to focus all N. quanta for one degree of freedom, the number of states in this case is simply equal f.. Attitude w. = f./g. and will determine the likelihood of nucleon departure from the composite nucleus, i.e. reactions.

Nucleon binding energy with a kernel is an average of about 8 meV. The magnitude of the excitation quantum is about 0.5 MeV. Then n.\u003d 8 MeV / 0.5 MeV \u003d 16. Considering that as a result of the reaction, the nucleon branch is most likely only from the outer shell, you can put f. ≈ n.. Substituting it in (11.9), we will find that

For n.\u003d 16 have w. \u003d 5 ∙ 10 -8. Changes to the state of the kernel occur with a frequency of 1 / τ poisontherefore constant decay of the composite core λ s * = w. /τ poison, and average lifetime τ s * \u003d1/ λ s * - About 10 -14 p. So really τ s *>> τ poison.

It can be noted that the composite core is not fundamentally different from the radioactive kernel. It also strives to lose energy due to any possible process conditions. One of these processes (the gap of nucleon) was already considered above. For composite kernel, there may be simultaneously several decay channels. In addition, the transition to the ground state can occur as a result of the emission of γ-quantum (such a reaction is called radiation capture). The flashing of the core of γ-quanta occurs under the action of electromagnetic forces, i.e. In a nuclear scale, it is also quite slow (after 10 -11 -10 -7 s - see clause 9.3). Thus, the reactions of radiation capture also go through the composite core.

The cross section of the reaction going through the composite core can be written as

, (11.11)

where w B. - probability of decay of the composite core via B., and

The dependence of the cross section of the nuclear reaction from the kinetic energy of the flutter particles is called function excitation.

Similar information.

IN general Nuclear interaction can be written in the form:

The most common type of nuclear reaction is the interaction of light particles a. with kernel X., as a result of which a particle is formed b. and kernel Y.. It is written symbolically like this:

The role of particles a. and b. Most often neutron n., Proton p., deuteron d., α-particle and γ-quantum.

The process (4.2) is usually ambiguous, since the reaction can go several competing methods, i.e. Particles born as a result of a nuclear reaction (4.2) may be different:

.

Different possibilities of nuclear reaction in the second stage are sometimes called reaction channels. The initial stage of the reaction is called the input channel.

The two recent reaction channels refer to cases of inelastic ( A 1. + a.) and elastic ( A. + a.) Nuclear scattering. These particular cases of nuclear interaction differ from other facts that the reaction products coincide with the reaction particles, and with elastic scattering, not only the type of kernel is maintained, but also its interior, When inelastic scattering, the inner state of the kernel changes (the kernel goes into an excited state).

Figure 4.1. Qualitative addiction The probability of the decay of the nucleus of energy.

When studying a nuclear reaction, the identification of the reaction channels is of interest, the comparative probability of it in different channels at various energies of incident particles. Kernels may be in various energy states . The state of a stable or radioactive nucleus that corresponds to minimal energy (mass) E 0 called basic. From quantum mechanics it is known that between the state of the state and its time of life takes place gaisenberg's ratio: ΔE \u003d ћ / Δt,

Excited kernels experience different kinds Energy transitions. The excitation energy can be discharged along various channels (translating the kernel to the ground state): the emission of γ-quanta, the division of the nucleus, etc. For this reason, the concept of partial level width is introduced Γ I. . Partial width of the resonance level is the probability of decay by i.- Channel. Then the probability of decay per unit time ω May be presented in the form:

.

It is also of great interest is the energy and angular distribution of the resulting particles, and their inner state (excitation energy, spin, parity, isotopic spin).

Many of nuclear reactions can be obtained as a result of the application of conservation laws.

For more information on this section, you can see.

A large role in the development of ideas about the structure of the nuclei was played by the study of nuclear reactions, which gave extensive information about the backs and parces of the initiated states of the nuclei, contributed to the development of the model of the shells. The study of reactions with the exchange of several nucleons between the encoding nuclei allowed us to investigate the nuclear dynamics in a state with large angular moments. As a result, long rotary stripes were opened, which served as one of the foundations of creating a generalized kernel model. In the collision of heavy nuclei, the kernels are formed, which are not in nature. The synthesis of transuran elements is largely based on the physics of the interaction of heavy nuclei. In the reactions with severe ions, the kernels removed from the β-stability band are formed. The kernels removed from the β-stability band differ from the stable nuclei by another relationship between the Coulomb and nuclear interactions, the ratio between the number of protons and the number of neutrons, significant differences in proton and neutron communication energies, which manifests itself in new types of radioactive decay - proton and neutron radioactivity - proton and neutron radioactivity and a number of other specific features of atomic nuclei.
When analyzing nuclear reactions, it is necessary to take into account the wave nature of particles interacting with the nuclei. The wave nature of the process of interaction of particles with nuclei is clearly manifested with elastic scattering. So for the nucleons with an energy of 10 MeV, the core wavelength is less than the kernel radius and during nucleon scattering there is a characteristic picture of diffraction maxima and minima. For nucleons with an energy of 0.1 MeV, the wavelength is greater than the radius of the kernel and the diffraction is absent. 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 of studying nuclear dynamics. Nuclear reactions occur in the interaction of two particles. With a nuclear reaction, an active exchange of energy and pulse between particles occurs, as a result of which one or more particles are formed, flying out of the interaction area. As a result of a nuclear reaction, a complex process of restructuring atomic nucleus occurs. As with the description of the structure of the kernel, when describing nuclear reactions, it is almost impossible to obtain an accurate solution of the problem. And just as the structure of the nucleus is described by various nuclear models, the nuclear reaction is described by various reaction mechanisms. The mechanism of the flow of a nuclear reaction depends on several factors - on the type of inclusive particle, the type of target nucleus, the energy of the flushing particle and from a number of other factors. One of the limiting cases of a nuclear reaction is Direct nuclear reaction. In this case, the flutter particle transmits the energy to one or two nucleon nucleons, and they leave the kernel without interacting with other nucleons of the nucleus. The characteristic time of the flow of direct nuclear reaction is 10 -23 s. The direct nuclear reactions go on all nuclei at any energy of the inclusive particle. Direct nuclear reactions are used to study single-particle states of atomic nuclei, because Reaction products carry information about the position position from which the nucleon is knocked out. With the help of direct nuclear reactions, detailed information was obtained about the energies and filling the single-particle states of the nuclei, which was formed the basis of the kernel's shell model. Another limit case is reactions going through education of composite core.

The description of the mechanism of nuclear reactions was given in the works of V.Quiskopfa.

V.Viscopp: "What happens when the particle enters the kernel and faces one of the nuclear components? Figure illustrates some of these features.
1) The falling particle loses part of its energy, lifting the nuclear particle into a higher state. This will be the result of inelastic scattering if the incident particle remains with an energy sufficient to leave the kernel again. This process is called direct inelastic scattering, since it involves scattering only on one component of the kernel.
2) The incident particle transmits energy to collective movement, as it is symbolically shown in the second pattern of the pattern, it is also direct interaction.
3) On the third diagram of the figure, the transmitted energy is large enough to snatch the nucleon from the target. This process also contributes to a straight nuclear reaction. In principle, it does not differ from 1), it corresponds to the "exchange reaction".
4) The falling particle may lose so much energy that remains connected inside the kernel, transmitted energy can be accepted by low-laying nucleon in such a way that it will not be able to leave the kernel. We then get an excited core that cannot be chopped by a nucleon. This state with necessity leads to further excitation of nucleons internal collisions, in which the energy on an excited particle decreases on average, so in most cases the nucleon cannot leave the kernel. Therefore, a state will be achieved with a very long life time, which can only disappear in the case when one particle in collisions inside the nucleus will accidentally acquire sufficient energy in order to leave the kernel. This situation we call the formation of the compound nucleus. Energy can also be lost by radiation, after which the car departure becomes energetically impossible: the incident nucleon will test the radiation capture.
5) The formation of the compound nucleus can be carried out in two or more steps, if after process type 1) or 2) the incident nucleon hit another nucleon and excites it in such a way that the departure of the kernel is impossible for any nucleon. "

For the first time, the idea of \u200b\u200bthe nuclear reaction flow through the stage of the component was expressed by N. Bloom. According to the component kernel, the incident particle after interaction with one or two nucleons of the core transmits the kernel most of its energy and turns out to be a captured kernel. The time of life of the composite kernel is much more than the time of the span of the flutter particle through the core. The energy made by the flushing particle in the core is redistributed between the nucleons of the nucleus until its large part focuses on one particle and then it flies out of the kernel. The formation of a long-lived excited state may result in its division as a result of deformation.

N. Bor: "The phenomenon of the capture of neutrons makes us assume that the collision between the rapid neutron and a heavy core 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 departure of the material particle or the transition to the final state with the emission of the rays energy quantum should be considered as independent processes that are not directly related to the first phase of collision. We meet here with a significant difference, previously unrecognized, between real nuclear reactions - conventional collisions of rapid particles and atomic systems - collisions that have so far for us were the main source of information relative to the structure of the atom. Indeed, the ability to account through such collisions of individual atomic particles and the study of their properties are obliged, first of all, the "openness" of the systems under consideration, which makes very unlikely energy exchange between the individual components into continuation of the strike. However, due to the close packing of particles in the core, we must be prepared for the fact that it is this exchange of energy that plays a major 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 flutter particle is larger than the sizes of the nucleus, then in such experiments it turns out information about the kernel as a whole. If less than the sizes of the core, then information on the distribution of the density of nuclear matter, the structure of the kernel surface, correlation between nucleons in the kernel, the distribution of nucleons through nuclear shells is extracted from the reaction sections.

• Coulomb excitation of nuclei under the action of charged particles relative to large mass (protons, α-particles and heavy carbon, nitrogen ions) are used to study low-laying rotational levels of heavy nuclei.
• Reactions with heavy ions on heavy nuclei, leading to the merger of encountered nuclei, are the main method of obtaining superheavy nuclear cores.
• The merger reactions of light nuclei at a relatively low collision energies (so-called thermonuclear reactions). These reactions occur due to quantum mechanical tunneling through the Coulomb barrier. Thermonuclear reactions proceed inside the stars at temperatures of 10,7 -10 10 K and are the main source of stars.
• Photonuclear and electrical reactions occur in a collision with nuclei of γ-quanta and electrons with Energy E\u003e 10 MeV.
• The reaction of the division of heavy nuclei, accompanied by deep perestroika kernel.
• Reactions on the beams of radioactive cores open the possibilities of obtaining and studying nuclei with an unusual ratio of the number of protons and neutrons, distant from the stability line.

The classification of nuclear reactions is usually carried out by type and energy of the flushing particle, the type of target nuclear and the energy of the flutter particle.

Reactions on slow neutrons

"1934 Once in the morning Bruno Pontecorvo and Eduardo Amaldi were tested on radioactivity some metals. These samples were granted a form of small hollow cylinders. equal valueIn addition to which the source of neutrons could be placed. To irradiate such a cylinder, the source of neutrons was inserted into it, and then everything was placed in a lead box. At this momentary Morning Amaldi and Pontecorvo conducted experiments with silver. And suddenly Pontecorvo noticed that something strange was happening with a silver cylinder: his activity is not always the same, it changes depending on where it is placed in the middle or in the angle of the lead box. In the complete bewilderment of Amaldi and Pontecorvo went to report on this miracle of Fermi and delighted. Franke was inclined to attribute these oddities to some statistical error or inaccurate measurements. And Enrico, who believed that every phenomenon requires verification, offered them to try to irradiate this silver cylinder outside the lead drawer and see what it would work out. And here they went very incredible wonders. It turned out that items located nearby the cylinder are able to influence its activity. If the cylinder irradiated when he stood on a wooden table, his activity was higher than when it was put on a metal plate. Now the whole group is interested in this and everyone took part in the experiments. They placed the source of neutrons outside the cylinder and between it and the cylinder put different items. The lead plate slightly increased activity. Lead− The substance is heavy. "Well, let's try easy!− suggested Fermi.− Say, paraffin. " On the morning of October 22 and experience with paraffin was made.
They took a big piece of paraffin, woven the pocket in it, and the source of neutrons was placed inside, the silver cylindrik was irradiated and brought it to the Geiger counter. The counter, as if the chain was broken, and snapped. The whole building was rapped by the exclamations: "unthinkable! Unimaginable! Black magic!" Paraffin increased the artificial radioactivity of silver a hundred times.
At noon, the group of physicists reluctantly dismissed the break set for breakfast, which usually continued with their two hours ... Enrico took advantage of her loneliness, and when he returned to the laboratory, he was already ready the theory that explained the strange paraffin action. "