– 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 ).
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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 when they occur, the classical conservation laws are satisfied: 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 physical interactions operates inside the nucleus at distances of the order of its size - strong interaction . To overcome it and “destroy” the core, a large amount of energy is needed.
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 are 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 in this case is the difference in mass between the initial and final products 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
, the following condition must be met: kinetic energy 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 the initial products are less than the 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.
>> Nuclear reactions
§ 106 NUCLEAR REACTIONS
Atomic nuclei undergo transformations during interactions. These transformations are accompanied by an increase or decrease in the kinetic energy of the particles involved in them.
Nuclear reactions call changes atomic nuclei when they interact with elementary particles or with each other. You have already seen examples of nuclear reactions in § 103. Nuclear reactions occur when particles come close to the nucleus and fall within the sphere of action of nuclear forces. Likely charged particles repel each other. Therefore, the approach of positively charged particles to nuclei (or nuclei to each other) is possible if these particles (or nuclei) are given a sufficiently large kinetic energy. This energy is imparted to protons, deuterium nuclei - deuterons, -particles and other heavier nuclei using accelerators.
For carrying out nuclear reactions, this method is much more effective than the use of helium nuclei emitted by radioactive elements. Firstly , with the help of accelerators, particles can be given an energy of the order of 10 5 MeV, i.e., much greater than that which alpha particles have (maximum 9 MeV). Secondly , you can use protons that are in the process radioactive decay do not appear (this is expedient because the charge of protons is half the charge of -particles, and therefore the repulsive force acting on them from the nuclei is also 2 times less). Third , it is possible to accelerate nuclei heavier than helium nuclei.
The first nuclear reaction using fast protons was carried out in 1932. It was possible to split lithium into two particles:
In chemistry lessons you learned about chemical reactions that lead to the transformation of molecules. However, atoms do not change during chemical reactions. Let us now consider the so-called nuclear reactions, which lead to transformations of atoms. Let's introduce some conventions:
Here X is the symbol of the chemical element (as in the periodic table), Z is the charge number of the isotope nucleus, A is the mass number of the isotope nucleus.
Nuclear charge number is the number of protons in the nucleus, equal to the number of the element in the periodic table. Nucleus mass number is the number of nucleons (protons and neutrons) entering the nucleus. Charge and mass numbers are physical quantities that do not coincide with the charge and mass of the nucleus.
For example, the symbol means that the nucleus of this carbon atom has a charge number of 6 and a mass number of 12. There are others isotopes carbon, for example. The nucleus of such an isotope contains one more neutron for the same number of protons (compare the figures).
Rutherford's first laboratory nuclear reaction proceeded as follows:
The nucleus of a nitrogen atom interacted with an a-particle (the nucleus of a helium atom). This produced a fluorine nucleus, an unstable intermediate product of the reaction. And then oxygen and hydrogen nuclei formed from it, that is, it happened transformation of one chemical element into another.
Based on the results of this nuclear reaction Let's make the following table.
From a comparison of the cells in the table, it can be seen that the sums of mass numbers, as well as the sums of charge numbers before and after the nuclear reaction are pairwise equal. Experiments show that for all nuclear reactions it is true law of conservation of charge and mass numbers: the sums of charge and mass numbers of particles before and after a nuclear reaction are equal in pairs.
Most nuclear reactions end after the formation of new nuclei. However, there are reactions whose products cause new nuclear reactions, called nuclear chain reactions. An example is the fission reaction of uranium-235 nuclei (see figure). When a neutron hits a uranium nucleus, it decays into two other nuclei and 2-3 new neutrons. These neutrons hit other uranium nuclei, and the chain reaction continues. This situation is ideal. In fact, many of the neutrons produced fly out of the substance and therefore cannot be absorbed by uranium.
However, with a high degree of purity of uranium, that is, with a large mass fraction of it, as well as with its compact placement, the probability of neutron capture by a neighboring nucleus increases. The minimum mass of a radioactive substance at which a chain reaction occurs is called critical mass. For pure uranium-235, this is several tens of kilograms. An uncontrolled chain reaction occurs very quickly, resulting in an explosion. For its use in for peaceful purposes it is necessary to make the reaction controllable, which is achieved in a special device - nuclear reactor(see § 15).
Nuclear reactions are very common in nature. For example, more than half of the elements of the periodic table have radioactive isotopes.
Let us briefly recall what we already know about the atom:
- the nucleus of an atom has an extremely high density with a very small size (relative to the atom itself);
- the nucleus contains protons and neutrons;
- electrons are found outside the nucleus at energy levels;
- protons have a positive charge, electrons have a negative charge, and neutrons have no charge. In general, the atom is neutral, because has an equal number of protons and electrons;
- the number of neutrons found in each atom of the same element may vary. Atoms that have the same nuclear charge but a different number of neutrons are called isotopes.
In the periodic table, the chemical element oxygen is designated as follows:
- 16 - mass number (sum of protons and neutrons);
- 8 - serial (atomic) number of the element (the number of protons in the nucleus of an atom);
- ABOUT- element designation.
1. Radioactivity
The spontaneous transformation of an unstable isotope of one chemical element into an isotope of another element, during which elementary particles are emitted, is called radioactivity.
If we know one of the particles resulting from the decay, then we can calculate the other particle, since during a nuclear reaction the so-called mass balance of the nuclear reaction is observed.
The essence of a nuclear reaction can be schematically expressed as follows:
Reactants that react → Products resulting from the reaction
Nuclear reaction is considered balanced, if the sum of the atomic numbers of the elements on the left side of the expression is equal to the sum of the atomic numbers of the elements obtained after the reaction. The same condition must be met for sums of mass numbers. Suppose a nuclear reaction occurs: an isotope of chlorine (chlorine-35) is bombarded by a neutron to form an isotope of hydrogen (hydrogen-1):
35 17 Cl + 1 0 n → 35 16 X + 1 1 H
What X-element will be on the right side of the reaction equation?
Based on the mass balance of the nuclear reaction, the atomic number of the unknown element will be equal to 16. V periodic table Under this number is the element sulfur (S). Thus, we can say that as a result of our nuclear reaction, bombarding the chlorine isotope (chlorine-35) with a neutron produces a hydrogen isotope (hydrogen-1) and a sulfur isotope (sulfur-35). This process is also called nuclear transformation.
35 17 Cl + 1 0 n → 35 16 S + 1 1 H
With the help of such nuclear transformations, scientists have learned to produce artificial isotopes that are not found in nature.
2. Why do isotopes decay?
The nucleus of an atom contains protons (positively charged particles) that are concentrated in a very small space. Earlier we said that in the nucleus of an atom there are certain holding forces (the so-called “nuclear glue”) that prevent similarly charged neutrons from tearing apart the nucleus of the atom. But sometimes the energy of repulsion of particles exceeds the energy of gluing, and the nucleus splits into pieces - radioactive decay occurs.
Scientists have found that everything chemical elements, in the nucleus of which there are more than 84 protons (under this serial number in the table is polonium - Po), are unstable and from time to time undergo radioactive decay. However, there are isotopes that have fewer than 84 protons in their nucleus, but they are also radioactive. The fact is that the stability of an isotope can be judged by the ratio of the number of protons and neutrons of an atom. An isotope will be unstable if the difference between the number of protons and neutrons is large (many protons and few neutrons, or few protons and many neutrons). An isotope of an element will be stable if the number of neutrons and protons in its atom is approximately equal.
Therefore, unstable isotopes, undergoing radioactive decay, turn into other elements. The transformation process will continue until a stable isotope is formed.
3. Half-life
When does radioactive decay of an atom of an unstable element occur? This can happen at any moment: in a couple of moments, or in 100 years. But, if the sample of atoms for a certain element is large enough, then a certain pattern can be derived.
The table below shows half-life data for some radioactive isotopes
The half-life must be known in order to determine the time when a radioactive element will become safe - this will happen when its radioactivity has dropped so much that it can no longer be detected, i.e., after 10 half-lives.
4. Nuclear chain reaction
In the 1930s, scientists began trying to control nuclear reactions. As a result of bombardment (usually by a neutron), the nucleus of an atom heavy element splits into two lighter nuclei. For example:
235 92 U + 1 0 n → 142 56 Ba + 91 36 Kr + 3 1 0 n
This process is called splitting (fission) of the nucleus. As a result, a colossal amount of energy is released. Where does it come from? If you very accurately measure the masses of particles before and after the reaction, it turns out that as a result of the nuclear reaction, part of the mass disappeared without a trace. This loss of mass is usually called a mass defect. Disappearing matter is converted into energy.
The great Albert Einstein came up with his famous formula: E = mc 2, Where
E- amount of energy;
m- mass defect (disappearing mass of a substance);
With- speed of light = 300,000 km/s
Since the speed of light is a very large quantity in itself, and in the formula it is squared, even an insignificantly small “disappearance of mass” leads to the release of quite large quantity energy.
From the above equation for the fission of uranium-235, it can be seen that in the process of nuclear fission one electron is consumed, but three are obtained at once. In turn, these three newly received electrons, having met three uranium-235 nuclei on their “path”, will produce another splitting, resulting in 9 neutrons, etc.... Such a continuously increasing cascade of splittings is called chain reaction.
A chain reaction is possible only with those isotopes whose splitting creates an excess of neutrons. So a chain reaction with an isotope of uranium (uranium-238) is impossible, because only one neutron will be released:
238 92 U + 1 0 n → 142 56 Ba + 91 36 Kr + 1 0 n
For nuclear reactions, isotopes of uranium (uranium-235) and pluton (pluton-239) are used. For a nuclear reaction to proceed independently, a certain amount of fissionable substance is required, called critical mass. Otherwise, the number of excess neutrons will be insufficient to carry out a nuclear reaction. The mass of the fissionable substance less than critical is called subcritical.
Nuclear reaction – this is the transformation of atomic nuclei when interacting with elementary particles(including with γ-quanta) or with each other. The most common type of nuclear reaction is the reaction written symbolically as follows:
Where X And Y– initial and final kernels, A And b– a particle bombarding and emitted (or emitted) in a nuclear reaction.
In nuclear physics, the efficiency of interaction is characterized by effective cross section σ. Each type of particle-nucleus interaction is associated with its own effective cross section: effective scattering cross section ;effective absorption cross section .
The effective cross section of the nuclear reaction σ is found by the formula:
 ,
|
(9.5.1) |
Where N– number of particles falling per unit time per unit area cross section substance having per unit volume n cores; d N is the number of these particles reacting in a layer of thickness d x. The effective cross section σ has the dimension of area and characterizes the probability that a reaction will occur when a beam of particles falls on a substance.
Unit of measurement of the effective cross section of nuclear processes – barn (1 barn = 10–28 m2).
In any nuclear reaction are being carried out conservation laws electric charges And mass numbers : sum of charges(and sum of mass numbers) nuclei and particles, reacting is equal to the sum of the charges(and sum of mass numbers) final products(nuclei and particles) reactions. In progress Also laws of conservation of energy , impulse And angular momentum .
Unlike radioactive decay, which always releases energy, nuclear reactions can be either exothermic (with the release of energy), and endothermic (with energy absorption).
The most important role in explaining the mechanism of many nuclear reactions was played by N. Bohr’s assumption (1936) that nuclear reactions proceed in two stages according to the following scheme:
| . | (9.5.2) |
First stage – this is capture by the core X particles a, approaching it at the distance of action of nuclear forces (approximately), and the formation of an intermediate nucleus WITH, called a composite (or compound core). The energy of a particle flying into the nucleus is quickly distributed among the nucleons of the compound nucleus, as a result of which it finds itself in an excited state. When nucleons collide in a compound nucleus, one of the nucleons (or a combination of them, such as a deuteron) or α - the particle can receive energy sufficient to escape from the nucleus. As a result comes second stage of nuclear reaction – decay of a compound nucleus into a nucleus Y and a particle b.
In nuclear physics it is introduced characteristic nuclear time – time,required for a particle to travel a distance of the order of magnitude equal to the diameter of the nucleus(). So for a particle with an energy of 1 MeV (which corresponds to its speed of 10 7 m/s), the characteristic nuclear time is . On the other hand, it has been proven that the lifetime of a compound nucleus is 10 –16 – 10 –12 s, i.e. is (10 6 – 10 10)τ. This means that during the lifetime of a compound nucleus a lot of collisions of nucleons with each other can occur, i.e. redistribution of energy between nucleons is indeed 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 particles emitted by it b) – the second stage of a nuclear reaction – does not depend on the method of formation of the compound nucleus, the first stage.
If the emitted particle is identical to the captured one (), then scheme (4.5.2) describes the scattering of the particle: elastic – at ; inelastic – at . If the emitted particle is not identical to the captured one (), then we have similarities with a nuclear reaction in the literal sense of the word.
Some reactions take place without formation of a compound nucleus, they're called direct nuclear interactions(for example, reactions caused by fast nucleons and deuterons).
Nuclear reactions are classified according to the following criteria:
· by the type of particles involved in them - reactions under the influence of neutrons; reactions under the influence of charged particles (for example, protons, deuterons, α-particles); reactions under the influence of γ-quanta;
· according to the energy of the particles causing them - reactions at low energies (on the order of electron volts), occurring mainly with the participation of neutrons; reactions at medium energies (up to several MeV), occurring with the participation of γ-quanta and charged particles (protons, α-particles); reactions occurring at high energies (hundreds and thousands of MeV), leading to the appearance of elementary particles that are absent in the free state and have great importance to study them;
· by the type of nuclei involved in them - reactions on light nuclei (A< 50); реакции на средних ядрах (50 < A < 100); реакции на тяжёлых ядрах (A > 100);
· according to the nature of the nuclear transformations occurring - reactions with the emission of neutrons; reactions with the emission of charged particles; capture reactions (in these reactions the compound nucleus does not emit any particles, but transitions to the ground state, emitting one or more γ-quanta).
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