As a result of a fission chain reaction, uranium is formed. Nuclear fission reactions and fission chain reactions

Physics lesson in grade 9

"Fission of uranium nuclei. Chain reaction"

The purpose of the lesson: to acquaint students with the process of fission of atomic nuclei of uranium, the mechanism of the chain reaction.

Tasks:

educational:

to study the mechanism of nuclear fission of uranium-235; introduce the concept of critical mass; determine the factors that determine the course of a chain reaction.

educational:

to bring students to an understanding of the significance of scientific discoveries and that the danger that can come from scientific achievements with a thoughtless, illiterate or immoral attitude towards them.

developing:

development of logical thinking; development of monologue and dialogic speech; development of mental operations in students: analysis, comparison, learning. Formation of the idea of the integrity of the picture of the world

Lesson type: learning lesson.

Competences, the formation of which the lesson is aimed at:

value-semantic - the ability to see and understand the world around,

general cultural - mastering the scientific picture of the world by the student,

educational and cognitive - the ability to distinguish facts from conjectures,

Communicative - skills of working in a group, possession of various social roles in a team,

competencies of personal self-improvement - culture of thinking and behavior

Course of the lesson: 1. Organizational moment.

A new lesson has come. I will smile at you and you will smile at each other. And think: how good it is that we are all here together today. We are modest and kind, friendly and affectionate. We are all healthy. - Inhale deeply and exhale. Exhale yesterday's resentment, anger and anxiety. I wish us all a good lesson .

2. Checking homework.

Test.

1. What is the charge on the nucleus?

1) positive 2) negative 3) the nucleus has no charge

2. What is an alpha particle?

1) electron 2) nucleus helium atom

3) electromagnetic radiation

3. How many protons and neutrons does the nucleus of a beryllium atom contain?

1) Z=9, N=4 2) Z=5, N=4 3) Z=4, N=5

4. The nucleus of what chemical element is formed during the α - decay of radium?

Ra → ? +He.

1) radon 2) uranium 3) fermium

5. The mass of the nucleus is always ... the sum of the masses of the nucleons of which it consists.

1) greater than 2) equal to 3) less

6. The neutron is a particle

1) having a charge of +1, an atomic mass of 1;

2) having a charge – 1, atomic mass 0;

3) having a charge of 0, an atomic mass of 1.

7. Specify the second product of the nuclear reaction

Answers: Option 1. 1)1; 2)2; 3)3; 4)1; 5)3; 6)3; 7)3.

8. How do protons electrically interact with each other in the nucleus?

9. What is a mass defect? Write down the formula.

10. What is bond energy? Write down the formula.

Learning new material.

We recently learned that some chemical elements are converted into other chemical elements during radioactive decay. And what do you think will happen if some particle is directed into the nucleus of an atom of a certain chemical element, well, for example, a neutron into the nucleus of uranium?

In 1939, German scientists Otto Hahn and Fritz Strassmann discovered the fission of uranium nuclei. They found that when uranium is bombarded with neutrons, elements of the middle part of the periodic system appear - radioactive isotopes of barium (Z = 56), krypton (Z = 36), etc.

Let us consider in more detail the process of fission of a uranium nucleus during bombardment by a neutron according to the figure. A neutron entering the uranium nucleus is absorbed by it. The nucleus is excited and begins to deform like a liquid drop.

The nucleus enters a state of excitation and begins to deform. Why does the core break into 2 parts? What forces cause the break?

What forces act inside the nucleus?

– Electrostatic and nuclear.

Okay, so how do electrostatic forces manifest themselves?

– Electrostatic forces act between charged particles. The charged particle in the nucleus is the proton. Since the proton is positively charged, it means that repulsive forces act between them.

Right, but how do nuclear forces manifest themselves?

– Nuclear forces are forces of attraction between all nucleons.

So, under the action of what forces does the nucleus break?

– (If there are difficulties, I ask leading questions and lead students to the correct conclusion) Under the action of electrostatic repulsive forces, the nucleus is torn into two parts, which scatter in different directions and emit 2-3 neutrons.

It stretches until the electrical repulsive forces begin to prevail over the nuclear ones. The nucleus breaks into two fragments, throwing out two or three neutrons. This is the technology of fission of the uranium nucleus.

The fragments scatter at a very high speed. It turns out that part of the internal energy of the nucleus is converted into the kinetic energy of flying fragments and particles. The fragments are released into the environment. What do you think is happening to them?

– Fragments are decelerated in the environment.

In order not to violate the law of conservation of energy, we must say what will happen to the kinetic energy?

– The kinetic energy of the fragments is converted into the internal energy of the medium.

Is it possible to notice that the internal energy of the medium has changed?

Yes, the environment is warming up.

But will the change in internal energy be influenced by the factor that a different number of uranium nuclei will participate in fission?

- Of course, with the simultaneous fission of a large number of uranium nuclei, the internal energy of the environment surrounding uranium increases.

From the course of chemistry, you know that reactions can occur both with the absorption of energy and with the release. What can we say about the course of the uranium fission reaction?

- The reaction of fission of uranium nuclei goes with the release of energy into the environment.

(Slide 13)

Uranium occurs in nature in the form of two isotopes: U (99.3%) and U (0.7%). In this case, the U fission reaction proceeds most intensively on slow neutrons, while U nuclei simply absorb a neutron, and fission does not occur. Therefore, the main interest is the fission reaction of the U nucleus. At present, about 100 different isotopes with mass numbers from about 90 to 145 are known, arising from the fission of this nucleus. Two typical fission reactions of this nucleus have the form:

Note that the energy released during the fission of uranium nuclei is enormous. For example, with the complete fission of all the nuclei contained in 1 kg of uranium, the same energy is released as with the combustion of 3000 tons of coal. Moreover, this energy can be released instantly.

(Slide 14)

Figured out what's going to happen to the shards How will neutrons behave?

In the fission of a uranium-235 nucleus, which is caused by a collision with a neutron, 2 or 3 neutrons are released. Under favorable conditions, these neutrons can hit other uranium nuclei and cause them to fission. At this stage, from 4 to 9 neutrons will already appear, capable of causing new decays of uranium nuclei, etc. Such an avalanche-like process is called chain reaction. (Notebook entry: Chain nuclear reaction- a sequence of nuclear reactions, each of which is caused by a particle that appeared as a reaction product at the previous step of the sequence). The scheme of development of the chain reaction of fission of uranium nuclei will be considered in more detail in the video clip in slow motion for a more detailed consideration

We see that the total number of free neutrons in a piece of uranium increases like an avalanche with time. What can this lead to?

- To the explosion.

Why?

- The number of nuclear fission increases and, accordingly, the energy released per unit of time.

But after all, another option is also possible, in which the number of free neutrons decreases with time, the nucleus did not meet the neutron on its way. In this case what happens to the chain reaction?

- It will stop.

Can the energy of such reactions be used for peaceful purposes?

How should the reaction proceed?

The reaction must proceed in such a way that the number of neutrons remains constant over time.

How is it possible to ensure that the number of neutrons remains constant all the time?

– (guys suggestions)

To solve this problem, it is necessary to know what factors influence the increase and decrease in the total number of free neutrons in a piece of uranium in which a chain reaction takes place.

(Slide 15)

One of these factors is mass of uranium . The fact is that not every neutron emitted during nuclear fission causes the fission of other nuclei. If the mass (and, accordingly, the size) of a piece of uranium is too small, then many neutrons will fly out of it, not having time to meet the nucleus on their way, cause its fission and thus generate a new generation of neutrons necessary to continue the reaction. In this case, the chain reaction will stop. In order for the reaction to continue, it is necessary to increase the mass of uranium to a certain value, called critical.

Why does a chain reaction become possible with an increase in mass?

For a chain reaction to occur, it is necessary that the so-called multiplication factor neutrons was greater than one. In other words, there should be more neutrons in each subsequent generation than in the previous one. The multiplication factor is determined not only by the number of neutrons produced in each elementary event, but also by the conditions under which the reaction proceeds - some of the neutrons can be absorbed by other nuclei or leave the reaction zone. Neutrons released during the fission of uranium-235 nuclei can only cause fission of the nuclei of the same uranium, which accounts for only 0.7% of natural uranium. This concentration is insufficient to start a chain reaction. The U isotope can also absorb neutrons, but no chain reaction occurs.

( Notebook entry: Neutron multiplication factork - the ratio of the number of neutrons of the next generation to the number in the previous generation in the entire volume of the medium multiplying neutrons)

A chain reaction in uranium with a high content of uranium-235 can only develop when the mass of uranium exceeds the so-called critical mass. In small pieces of uranium, most of the neutrons, without hitting any nucleus, fly out. For pure uranium-235, the critical mass is about 50 kg.

( Notebook entry: Critical mass- the minimum amount of fissile material required to start a self-sustaining fission chain reaction).

(Slide 16)

The critical mass of uranium can be reduced many times over by using so-called neutron moderators. The fact is that neutrons produced during the decay of uranium nuclei have too high speeds, and the probability of capture of slow neutrons by uranium-235 nuclei is hundreds of times greater than that of fast ones. The best neutron moderator is heavy water H 2 O. When interacting with neutrons, ordinary water itself turns into heavy water.

A good moderator is also graphite, whose nuclei do not absorb neutrons. During elastic interaction with deuterium or carbon nuclei, neutrons slow down their movement.

The use of neutron moderators and a special beryllium shell that reflects neutrons makes it possible to reduce the critical mass to 250 g (0.25 kg).

Notebook entry:

The critical mass can be reduced if:

Use retarders (graphite, ordinary and heavy water)

Reflective shell (beryllium)).

And in atomic bombs, just, a chain uncontrolled nuclear reaction occurs when two pieces of uranium-235 are quickly combined, each of which has a mass slightly lower than the critical one.

The atomic bomb is a terrible weapon. The damaging factors of which are: 1) Light radiation (including X-ray and thermal radiation here); 2) shock wave; 3) radiation contamination of the area. But the fission of uranium nuclei is also used for peaceful purposes - this is in nuclear reactors at nuclear power plants. We will consider the processes occurring in these cases in the next lesson.

The middle of the 20th century is defined by the acceleration of science: a fantastic acceleration, the introduction of scientific achievements into production and into our lives. All this makes us think - what will science give us tomorrow?
To alleviate all the hardships of human existence - this is the main goal of a truly progressive science. To make humanity happier - not one, not two, but humanity. And this is very important, because, as you know, science can also act against a person. The atomic explosion in Japanese cities - Hiroshima and Nagasaki is a tragic example of this.

So, 1945, August. World War II is coming to an end.

(slide 2)

On August 6, at 1:45 a.m., an American B-29 bomber, commanded by Colonel Paul Tibbets, took off from an island about 6 hours from Hiroshima.

(Slide 3)

Hiroshima after the atomic explosion.

Whose shadow wanders there invisibly,
Are you blind from misfortune?
This is Hiroshima crying
Ash clouds.
Whose voice is there in the hot darkness
Heard frenzied?
This is Nagasaki crying
On the burnt land
In this weeping and sobbing
There is no falsehood
The whole world is frozen in anticipation -
Who will cry next?

(Slide 4)

The number of deaths from the direct impact of the explosion ranged from 70 to 80 thousand people. By the end of 1945, due to the effects of radioactive contamination and other post-effects of the explosion, the total number of deaths ranged from 90 to 166 thousand people. After 5 years, the total death toll reached 200,000 people.

(Slide 5)

On August 6, after receiving news of the successful atomic bombing of Hiroshima, US President Truman announced that

“We are now ready to destroy, even faster and more completely than before, all Japanese land-based production facilities in any city. We will destroy their docks, their factories, and their communications. Let there be no misunderstanding - we will completely destroy Japan's ability to wage war."

(Slide 6)

At 2:47 on August 9, an American B-29 bomber under the command of a major, carrying an atomic bomb on board, took off from the island. At 10:56 B-29 arrived at Nagasaki. The explosion occurred at 11:02 local time.

(Slide 7)

The death toll by the end of 1945 ranged from 60 to 80 thousand people. After 5 years, the total death toll, including deaths from cancer and other long-term effects of the explosion, could reach or even exceed 140,000 people.

Such is the story, sad and warning

Every person is not an island,

each person is part of a large continent.
And never ask for whom the bell tolls.
He calls for you...

Consolidation.

What did we learn in class today? (with the mechanism of fission of uranium nuclei, with a chain reaction)

What are the conditions for a chain reaction to take place?

What is critical mass?

What is the multiplication factor?

What serves as a neutron moderator?

Reflection.

In what mood do you leave the lesson?

Evaluation.

Homework: p. 74.75, questions pp. 252-253

Chain nuclear reaction. As a result of experiments on neutron irradiation of uranium, it was found that under the action of neutrons, uranium nuclei are divided into two nuclei (fragments) of approximately half the mass and charge; this process is accompanied by the emission of several (two or three) neutrons (Fig. 402). In addition to uranium, some more elements from among the last elements of the periodic system of Mendeleev are capable of fission. These elements, like uranium, fission not only under the influence of neutrons, but also without external influences (spontaneously). Spontaneous fission was established experimentally by Soviet physicists K. A. Petrzhak and Georgy Nikolaevich Flerov (b. 1913) in 1940. It is a very rare process. So, in 1 g of uranium, only about 20 spontaneous fissions occur per hour.

Rice. 402. Fission of a uranium nucleus under the influence of neutrons: a) the nucleus captures a neutron; b) the impact of a neutron on the nucleus causes the latter to oscillate; c) the nucleus is divided into two fragments; more neutrons are emitted.

Due to mutual electrostatic repulsion, fission fragments scatter in opposite directions, acquiring huge kinetic energy (about ). The fission reaction thus occurs with a significant release of energy. Fast-moving fragments intensely ionize the atoms of the medium. This property of fragments is used to detect fission processes using an ionization chamber or cloud chamber. A photograph of traces of fission fragments in a cloud chamber is shown in fig. 403. It is extremely significant that the neutrons emitted during the fission of a uranium nucleus (the so-called secondary fission neutrons) are capable of causing the fission of new uranium nuclei. Thanks to this, it is possible to carry out a fission chain reaction: once having arisen, the reaction, in principle, can continue by itself, covering an increasing number of nuclei. The development scheme of such a growing cellon reaction is shown in Fig. 404.

Rice. 403. Photo of traces of uranium fission fragments in a cloud chamber: fragments () scatter in opposite directions from a thin layer of uranium deposited on a plate blocking the chamber. The image also shows many thinner traces belonging to protons knocked out by neutrons from water car molecules contained in the chamber.

Carrying out a fission chain reaction is not easy in practice; Experience shows that in the mass of natural uranium a chain reaction does not occur. The reason for this lies in the loss of secondary neutrons; in natural uranium most of the neutrons are out of the game without causing fission. As studies have revealed, the loss of neutrons occurs in the most common isotope of uranium - uranium - 238 (). This isotope easily absorbs neutrons in a reaction similar to the reaction of silver with neutrons (see § 222); this produces an artificially radioactive isotope. It divides with difficulty and only under the action of fast neutrons.

An isotope that is contained in natural uranium in an amount has more successful properties for a chain reaction. It is divided under the action of neutrons of any energy - fast and slow, and the better, the lower the neutron energy. The process competing with fission - the simple absorption of neutrons - is unlikely in contrast to. Therefore, in pure uranium-235, a fission chain reaction is possible, provided, however, that the mass of uranium-235 is large enough. In low-mass uranium, the fission reaction is terminated due to the emission of secondary neutrons outside its matter.

Rice. 404. Development of a Valuable Fission Reaction: It is conditionally accepted that two neutrons are emitted during nuclear fission and there are no neutron losses, i.e. each neutron causes a new fission; circles - fission fragments, arrows - fission neutrons

Indeed, due to the tiny size of atomic nuclei, a neutron travels a long distance in matter (measured in centimeters) before accidentally hitting a nucleus. If the dimensions of the body are small, then the probability of a collision on the way to the exit is small. Almost all secondary fission neutrons fly out through the surface of the body without causing new fissions, i.e., without continuing the reaction.

From a body of large dimensions, it is mainly neutrons that are formed in the surface layer that fly out. The neutrons formed inside the body have a sufficient thickness of uranium in front of them and for the most part cause new fissions, continuing the reaction (Fig. 405). The greater the mass of uranium, the smaller the fraction of the volume is the surface layer, from which many neutrons are lost, and the more favorable the conditions for the development of a chain reaction.

Rice. 405. Development of a fission chain reaction in . a) In a small mass, most fission neutrons fly out. b) In a large mass of uranium, many fission neutrons cause the fission of new nuclei; the number of divisions increases from generation to generation. Circles - fission fragments, arrows - fission neutrons

By gradually increasing the amount, we will reach the critical mass, i.e., the smallest mass, starting from which a sustained fission chain reaction in is possible. With a further increase in mass, the reaction will begin to develop rapidly (it will be initiated by spontaneous fission). When the mass decreases below the critical value, the reaction decays.

So, you can carry out a chain reaction of fission. If you have enough pure , separated from .

As we saw in §202, isotope separation is a complex and expensive operation, but it is still possible. Indeed, extraction from natural uranium was one of the ways in which the fission chain reaction was put into practice.

Along with this, the chain reaction was achieved in another way, which did not require the separation of uranium isotopes. This method is somewhat more complicated in principle, but easier to implement. It uses the slowing down of fast secondary fission neutrons to the speeds of thermal motion. We have seen that in natural uranium the immediate secondary neutrons are mainly absorbed by the isotope . Since absorption in does not lead to fission, the reaction terminates. Measurements show that when neutrons are slowed down to thermal velocities, the absorbing power increases more than the absorbing power . Absorption of neutrons by the isotope , leading to fission, gets the upper hand. Therefore, if fission neutrons are slowed down, preventing them from being absorbed into , a chain reaction will become possible with natural uranium.

Rice. 406. A system of natural uranium and a moderator in which a fission chain reaction can develop

In practice, this result is achieved by placing flue rods of natural uranium in the form of a rare lattice in the moderator (Fig. 406). Substances having a low atomic mass and weakly absorbing neutrons are used as moderators. Good moderators are graphite, heavy water, beryllium.

Let the fission of the uranium nucleus take place in one of the rods. Since the rod is relatively thin, the fast secondary neutrons will fly almost all into the moderator. The rods are located in the lattice quite rarely. Before hitting the new rod, the emitted neutron experiences many collisions with the nuclei of the moderator and slows down to the speed of thermal motion (Fig. 407). Having then hit the uranium rod, the neutron will most likely be absorbed and cause a new fission, thereby continuing the reaction. The fission chain reaction was first carried out in the United States in 1942. a group of scientists led by the Italian physicist Enrico Fermi (1901-1954) in a system with natural uranium. This process was independently implemented in the USSR in 1946. Academician Igor Vasilievich Kurchatov (1903-1960) with employees.

Rice. 407. Development of a valuable fission reaction in a system of natural uranium and a moderator. A fast neutron, flying out of a thin rod, hits the moderator and slows down. Once again in uranium, the slowed down neutron is likely to be absorbed into , causing fission (symbol: two white circles). Some neutrons are absorbed at without causing fission (symbol: black circle)

Purpose: to form students' understanding of the fission of uranium nuclei.

check previously studied material;
consider the mechanism of fission of the uranium nucleus;
consider the condition for the occurrence of a chain reaction;
find out the factors influencing the course of a chain reaction;
develop the speech and thinking of students;
develop the ability to analyze, control and adjust their own activities within a given time.

Equipment: computer, projection system, didactic material (test “Composition of the core”), disks “Interactive course. Physics 7-11kl ”(Fizikon) and“ 1C-repeater. Physics” (1C).

Lesson progress

I. Organizational moment (2 ').

Greetings, lesson plan announcement.

II. Repetition of previously studied material (8’).

Independent work of students - performing a test ( Appendix 1 ). In the test, you must indicate one correct answer.

III. Learning new material (25’). Making notes during the lesson(application 2 ).

Let's check your assumptions (work with the interactive model “Nuclear Fission”“Interactive course. Physics 7-11kl” ).

What was the result?

- When a neutron hits the uranium nucleus, we see that as a result 2 fragments and 2-3 neutrons are formed.

The same effect was obtained in 1939 by the German scientists Otto Hahn and Fritz Strassmann. They found that as a result of the interaction of neutrons with uranium nuclei, radioactive fragment nuclei appear, the masses and charges of which are approximately half the corresponding characteristics of uranium nuclei. Nuclear fission occurring in this way is called forced fission, in contrast to spontaneous fission, which occurs during natural radioactive transformations.

The nucleus enters a state of excitation and begins to deform. Why does the core break into 2 parts? What forces cause the break?

What forces act inside the nucleus?

– Electrostatic and nuclear.

Okay, so how do electrostatic forces manifest themselves?

– Electrostatic forces act between charged particles. The charged particle in the nucleus is the proton. Since the proton is positively charged, it means that repulsive forces act between them.

Right, but how do nuclear forces manifest themselves?

– Nuclear forces are forces of attraction between all nucleons.

So, under the action of what forces does the nucleus break?

- (If there are any difficulties, I ask leading questions and lead students to the correct conclusion) Under the influence of electrostatic repulsive forces, the nucleus is torn into two parts, which scatter in different directions and emit 2-3 neutrons.

– Fragments are decelerated in the environment.

In order not to violate the law of conservation of energy, we must say what will happen to the kinetic energy?

– The kinetic energy of the fragments is converted into the internal energy of the medium.

Is it possible to notice that the internal energy of the medium has changed?

Yes, the environment is warming up.

But will the change in internal energy be influenced by the factor that a different number of uranium nuclei will participate in fission?

- Of course, with the simultaneous fission of a large number of uranium nuclei, the internal energy of the environment surrounding uranium increases.

From the course of chemistry, you know that reactions can occur both with the absorption of energy and with the release. What can we say about the course of the uranium fission reaction?

- The reaction of fission of uranium nuclei goes with the release of energy into the environment.

The energy contained in the nuclei of atoms is colossal. For example, with the complete fission of all the nuclei present in 1 g of uranium, the same amount of energy would be released as is released during the combustion of 2.5 tons of oil. Figured out what's going to happen to the shards How will neutrons behave?

– (I listen to the assumptions of students, check the assumptions, working with the interactive model “Chain Reaction”“1C repeater. Physics" ).

True, neutrons on their way can meet uranium nuclei and cause fission. Such a reaction is called a chain reaction.

So, what is the condition for a chain reaction to occur?

- A chain reaction is possible due to the fact that during the fission of each nucleus, 2-3 neutrons are formed, which can take part in the fission of other nuclei.

We see that the total number of free neutrons in a piece of uranium increases like an avalanche with time. What can this lead to?

- To the explosion.

- The number of nuclear fission increases and, accordingly, the energy released per unit of time.

- It will stop.

Can the energy of such reactions be used for peaceful purposes?

How should the reaction proceed?

The reaction must proceed in such a way that the number of neutrons remains constant over time.

How is it possible to ensure that the number of neutrons remains constant all the time?

- (children's suggestions)

To solve this problem, it is necessary to know what factors influence the increase and decrease in the total number of free neutrons in a piece of uranium in which a chain reaction takes place.

Why does a chain reaction become possible with an increase in mass?

– The larger the mass of the piece, the greater the probability of neutrons meeting with nuclei. Accordingly, the number of nuclear fissions and the number of emitted neutrons increase.

At a certain so-called critical mass of uranium, the number of neutrons that appeared during the fission of nuclei becomes equal to the number of lost neutrons (ie, captured by nuclei without fission and flying out of the piece).

Therefore, their total number remains unchanged. In this case, the chain reaction can go on for a long time, without stopping and without acquiring an explosive character.

The smallest mass of uranium at which a chain reaction is possible is called the critical mass.

How will the reaction proceed if the mass of uranium is greater than the critical mass?

– As a result of a sharp increase in the number of free neutrons, a chain reaction leads to an explosion.

What if it's less critical?

The reaction does not proceed due to the lack of free neutrons.

It is possible to reduce the loss of neutrons (which fly out of uranium without reacting with nuclei) not only by increasing the mass of uranium, but also by using a special reflective shell . To do this, a piece of uranium is placed in a shell made of a substance that reflects neutrons well (for example, beryllium). Reflected from this shell, neutrons return to uranium and can take part in nuclear fission.

In addition to the mass and the presence of a reflective shell, there are several other factors on which the possibility of a chain reaction depends. For example, if a piece of uranium contains too much impurities other chemical elements, they absorb most of the neutrons and the reaction stops.

Another factor that influences the course of the reaction is Availability in the so-called uranium neutron moderator . The fact is that the nuclei of uranium-235 are most likely to fission under the action of slow neutrons. Nuclear fission produces fast neutrons. If fast neutrons are slowed down, then most of them will be captured by uranium-235 nuclei with subsequent fission of these nuclei; substances such as graphite, hearth, heavy water and some others are used as moderators. These substances only slow down neutrons, almost without absorbing them.

So, what are the main factors that can influence the course of a chain reaction?

- The possibility of a chain reaction is determined by the mass of uranium, the amount of impurities in it, the presence of a shell and a moderator.

The critical mass of a spherical piece of uranium-235 is approximately 50 kg. At the same time, its radius is only 9 cm, since uranium has a very high density.

By using a moderator and a reflective shell, and by reducing the amount of impurities, it is possible to reduce the critical mass of uranium to 0.8 kg.

Nuclear fission is the splitting of a heavy atom into two fragments of approximately equal mass, accompanied by the release of a large amount of energy.

The discovery of nuclear fission began a new era - the "atomic age". The potential of its possible use and the ratio of risk to benefit from its use have not only generated many sociological, political, economic and scientific achievements, but also serious problems. Even from a purely scientific point of view, the process of nuclear fission has created a large number of puzzles and complications, and its full theoretical explanation is a matter of the future.

Sharing is profitable

The binding energies (per nucleon) differ for different nuclei. Heavier ones have lower binding energies than those located in the middle of the periodic table.

This means that for heavy nuclei with an atomic number greater than 100, it is advantageous to divide into two smaller fragments, thereby releasing energy, which is converted into the kinetic energy of the fragments. This process is called splitting

According to the stability curve, which shows the dependence of the number of protons on the number of neutrons for stable nuclides, heavier nuclei prefer more neutrons (compared to the number of protons) than lighter ones. This suggests that along with the splitting process, some "spare" neutrons will be emitted. In addition, they will also take on some of the released energy. The study of nuclear fission of the uranium atom showed that 3-4 neutrons are released: 238 U → 145 La + 90 Br + 3n.

The atomic number (and atomic mass) of the fragment is not equal to half the atomic mass of the parent. The difference between the masses of atoms formed as a result of splitting is usually about 50. However, the reason for this is not yet entirely clear.

The binding energies of 238 U, 145 La, and 90 Br are 1803, 1198, and 763 MeV, respectively. This means that as a result of this reaction, the fission energy of the uranium nucleus is released, equal to 1198 + 763-1803 = 158 MeV.

Spontaneous division

The processes of spontaneous splitting are known in nature, but they are very rare. The average lifetime of this process is about 10 17 years, and, for example, the average lifetime of alpha decay of the same radionuclide is about 10 11 years.

The reason for this is that in order to split into two parts, the nucleus must first be deformed (stretched) into an ellipsoidal shape, and then, before finally splitting into two fragments, form a “neck” in the middle.

Potential Barrier

In the deformed state, two forces act on the core. One is the increased surface energy (the surface tension of a liquid drop explains its spherical shape), and the other is the Coulomb repulsion between fission fragments. Together they produce a potential barrier.

As in the case of alpha decay, in order for the spontaneous fission of the uranium atom nucleus to occur, the fragments must overcome this barrier using quantum tunneling. The barrier is about 6 MeV, as in the case of alpha decay, but the probability of tunneling an alpha particle is much greater than that of a much heavier atom fission product.

forced splitting

Much more likely is the induced fission of the uranium nucleus. In this case, the parent nucleus is irradiated with neutrons. If the parent absorbs it, they bind, releasing binding energy in the form of vibrational energy that can exceed the 6 MeV required to overcome the potential barrier.

Where the energy of the additional neutron is insufficient to overcome the potential barrier, the incident neutron must have a minimum kinetic energy in order to be able to induce the splitting of an atom. In the case of 238 U, the binding energy of additional neutrons is about 1 MeV short. This means that fission of the uranium nucleus is induced only by a neutron with a kinetic energy greater than 1 MeV. On the other hand, the 235 U isotope has one unpaired neutron. When the nucleus absorbs an additional one, it forms a pair with it, and as a result of this pairing, additional binding energy appears. This is enough to release the amount of energy necessary for the nucleus to overcome the potential barrier and the isotope fission occurs upon collision with any neutron.

beta decay

Even though the fission reaction emits three or four neutrons, the fragments still contain more neutrons than their stable isobars. This means that cleavage fragments are generally unstable against beta decay.

For example, when uranium 238U fission occurs, the stable isobar with A = 145 is neodymium 145Nd, which means that the lanthanum 145La fragment decays in three steps, each time emitting an electron and an antineutrino, until a stable nuclide is formed. The stable isobar with A = 90 is zirconium 90 Zr; therefore, the bromine 90 Br splitting fragment decomposes in five stages of the β-decay chain.

These β-decay chains release additional energy, which is almost all carried away by electrons and antineutrinos.

Nuclear reactions: fission of uranium nuclei

Direct emission of a neutron from a nuclide with too many of them to ensure the stability of the nucleus is unlikely. The point here is that there is no Coulomb repulsion and so the surface energy tends to keep the neutron in bond with the parent. However, this sometimes happens. For example, a 90 Br fission fragment in the first beta decay stage produces krypton-90, which can be in an excited state with enough energy to overcome the surface energy. In this case, the emission of neutrons can occur directly with the formation of krypton-89. still unstable with respect to β decay until converted to stable yttrium-89, so that krypton-89 decays in three steps.

Fission of uranium nuclei: a chain reaction

The neutrons emitted in the fission reaction can be absorbed by another parent nucleus, which then itself undergoes induced fission. In the case of uranium-238, the three neutrons that are produced come out with energies less than 1 MeV (the energy released during the fission of the uranium nucleus - 158 MeV - is mainly converted into the kinetic energy of the fission fragments), so they cannot cause further fission of this nuclide. Nevertheless, at a significant concentration of the rare isotope 235 U, these free neutrons can be captured by 235 U nuclei, which can indeed cause fission, since in this case there is no energy threshold below which fission is not induced.

This is the principle of a chain reaction.

Types of nuclear reactions

Let k be the number of neutrons produced in a sample of fissile material in stage n of this chain, divided by the number of neutrons produced in stage n - 1. This number will depend on how many neutrons produced in stage n - 1 are absorbed by the nucleus, which may be forced to divide.

If k< 1, то цепная реакция просто выдохнется и процесс остановится очень быстро. Именно это и происходит в природной в которой концентрация 235 U настолько мала, что вероятность поглощения одного из нейтронов этим изотопом крайне ничтожна.

If k > 1, then the chain reaction will grow until all the fissile material has been used. This is achieved by enriching natural ore to obtain a sufficiently large concentration of uranium-235. For a spherical sample, the value of k increases with an increase in the neutron absorption probability, which depends on the radius of the sphere. Therefore, the mass U must exceed a certain amount in order for the fission of uranium nuclei (chain reaction) to occur.

If k = 1, then a controlled reaction takes place. This is used in nuclear reactors. The process is controlled by distributing cadmium or boron rods among the uranium, which absorb most of the neutrons (these elements have the ability to capture neutrons). The fission of the uranium nucleus is automatically controlled by moving the rods in such a way that the value of k remains equal to one.

Class

Lesson #42-43

Chain reaction of fission of uranium nuclei. Nuclear energy and ecology. Radioactivity. Half life.

Nuclear reactions

A nuclear reaction is the process of interaction of an atomic nucleus with another nucleus or elementary particle, accompanied by a change in the composition and structure of the nucleus and the release of secondary particles or γ-quanta.

As a result of nuclear reactions, new radioactive isotopes can be formed that are not found on Earth in natural conditions.

The first nuclear reaction was carried out by E. Rutherford in 1919 in experiments to detect protons in nuclear decay products (see § 9.5). Rutherford bombarded nitrogen atoms with alpha particles. When the particles collided, a nuclear reaction occurred, which proceeded according to the following scheme:

During nuclear reactions, several conservation laws: momentum, energy, angular momentum, charge. In addition to these classical conservation laws, the so-called conservation law holds true in nuclear reactions. baryon charge(that is, the number of nucleons - protons and neutrons). A number of other conservation laws specific to nuclear physics and elementary particle physics also hold.

Nuclear reactions can proceed when atoms are bombarded by fast charged particles (protons, neutrons, α-particles, ions). The first reaction of this kind was carried out using high-energy protons obtained at the accelerator in 1932:

where M A and M B are the masses of the initial products, M C and M D are the masses of the final reaction products. The value ΔM is called mass defect. Nuclear reactions can proceed with the release (Q > 0) or with the absorption of energy (Q< 0). Во втором случае первоначальная кинетическая энергия исходных продуктов должна превышать величину |Q|, которая называется порогом реакции.

For a nuclear reaction to have a positive energy yield, specific binding energy nucleons in the nuclei of the initial products must be less than the specific binding energy of nucleons in the nuclei of the final products. This means that ΔM must be positive.

There are two fundamentally different ways of releasing nuclear energy.

1. Fission of heavy nuclei. In contrast to the radioactive decay of nuclei, accompanied by the emission of α- or β-particles, fission reactions are a process in which an unstable nucleus is divided into two large fragments of comparable masses.

In 1939, German scientists O. Hahn and F. Strassmann discovered the fission of uranium nuclei. Continuing the research begun by Fermi, they found that when uranium is bombarded with neutrons, elements of the middle part of the periodic system arise - radioactive isotopes of barium (Z = 56), krypton (Z = 36), etc.

Uranium occurs in nature in the form of two isotopes: (99.3%) and (0.7%). When bombarded by neutrons, the nuclei of both isotopes can split into two fragments. In this case, the fission reaction proceeds most intensively with slow (thermal) neutrons, while nuclei enter into a fission reaction only with fast neutrons with an energy of the order of 1 MeV.

Nuclear fission is of primary interest to nuclear power engineering. Currently, about 100 different isotopes with mass numbers from about 90 to 145 are known to occur during the fission of this nucleus. Two typical fission reactions of this nucleus have the form:

Note that as a result of nuclear fission initiated by a neutron, new neutrons are produced that can cause fission reactions in other nuclei. Other isotopes of barium, xenon, strontium, rubidium, etc. can also be fission products of uranium-235 nuclei.

The kinetic energy released during the fission of one uranium nucleus is enormous - about 200 MeV. The energy released during nuclear fission can be estimated using specific binding energy nucleons in the nucleus. The specific binding energy of nucleons in nuclei with mass number A ≈ 240 is about 7.6 MeV/nucleon, while in nuclei with mass numbers A = 90–145 the specific energy is approximately equal to 8.5 MeV/nucleon. Therefore, the fission of a uranium nucleus releases an energy of the order of 0.9 MeV/nucleon, or approximately 210 MeV per uranium atom. With the complete fission of all the nuclei contained in 1 g of uranium, the same energy is released as during the combustion of 3 tons of coal or 2.5 tons of oil.

The fission products of the uranium nucleus are unstable, since they contain a significant excess number of neutrons. Indeed, the ratio N / Z for the heaviest nuclei is about 1.6 (Fig. 9.6.2), for nuclei with mass numbers from 90 to 145 this ratio is about 1.3–1.4. Therefore, fragment nuclei experience a series of successive β - decays, as a result of which the number of protons in the nucleus increases, and the number of neutrons decreases until a stable nucleus is formed.

Figure 9.8.1. Scheme of development of a chain reaction.

For a chain reaction to occur, it is necessary that the so-called neutron multiplication factor was greater than one. In other words, there should be more neutrons in each subsequent generation than in the previous one. The multiplication factor is determined not only by the number of neutrons produced in each elementary event, but also by the conditions under which the reaction proceeds - some of the neutrons can be absorbed by other nuclei or leave the reaction zone. Neutrons released during the fission of uranium-235 nuclei can only cause fission of the nuclei of the same uranium, which accounts for only 0.7% of natural uranium. This concentration is insufficient to start a chain reaction. An isotope can also absorb neutrons, but no chain reaction occurs.

A chain reaction in uranium with a high content of uranium-235 can only develop when the mass of uranium exceeds the so-called critical mass. In small pieces of uranium, most of the neutrons, without hitting any nucleus, fly out. For pure uranium-235, the critical mass is about 50 kg. The critical mass of uranium can be reduced many times over by using the so-called moderators neutrons. The fact is that neutrons produced during the decay of uranium nuclei have too high speeds, and the probability of capture of slow neutrons by uranium-235 nuclei is hundreds of times greater than that of fast ones. The best neutron moderator is heavy water D 2 O. When interacting with neutrons, ordinary water itself turns into heavy water.

A good moderator is also graphite, whose nuclei do not absorb neutrons. Upon elastic interaction with deuterium or carbon nuclei, neutrons are slowed down to thermal velocities.

The use of neutron moderators and a special beryllium shell that reflects neutrons makes it possible to reduce the critical mass to 250 g.

In atomic bombs, an uncontrolled nuclear chain reaction occurs when two pieces of uranium-235, each of which has a mass slightly below the critical one, quickly join.

A device that maintains a controlled nuclear fission reaction is called nuclear(or atomic) reactor. The scheme of a nuclear reactor on slow neutrons is shown in fig. 9.8.2.

Figure 9.8.2. Scheme of the device of a nuclear reactor.

The nuclear reaction takes place in the reactor core, which is filled with a moderator and pierced with rods containing an enriched mixture of uranium isotopes with a high content of uranium-235 (up to 3%). Control rods containing cadmium or boron are introduced into the core, which intensively absorb neutrons. The introduction of rods into the core allows you to control the speed of the chain reaction.

The core is cooled by a pumped coolant, which can be water or a metal with a low melting point (for example, sodium, which has a melting point of 98 °C). In a steam generator, the heat transfer medium transfers heat energy to water, converting it into high-pressure steam. The steam is sent to a turbine connected to an electric generator. From the turbine, steam enters the condenser. To avoid leakage of radiation, the circuits of coolant I and steam generator II operate in closed cycles.

The turbine of a nuclear power plant is a heat engine that determines the overall efficiency of the plant in accordance with the second law of thermodynamics. At modern nuclear power plants, the efficiency is approximately equal. Therefore, to produce 1000 MW of electrical power, the thermal power of the reactor must reach 3000 MW. 2000 MW must be carried away by the water cooling the condenser. This leads to local overheating of natural water bodies and the subsequent emergence of environmental problems.

However, the main problem is to ensure the complete radiation safety of people working at nuclear power plants and to prevent accidental releases of radioactive substances that accumulate in large quantities in the reactor core. Much attention is paid to this problem in the development of nuclear reactors. Nevertheless, after the accidents at some nuclear power plants, in particular at the nuclear power plant in Pennsylvania (USA, 1979) and at the Chernobyl nuclear power plant (1986), the problem of the safety of nuclear energy has become especially acute.

Along with the above-described nuclear reactor operating on slow neutrons, reactors operating without a moderator on fast neutrons are of great practical interest. In such reactors, the nuclear fuel is an enriched mixture containing at least 15% of the isotope. The advantage of fast neutron reactors is that during their operation, uranium-238 nuclei, absorbing neutrons, through two successive β - decays are converted into plutonium nuclei, which are then can be used as nuclear fuel:

The breeding ratio of such reactors reaches 1.5, that is, for 1 kg of uranium-235, up to 1.5 kg of plutonium is obtained. Conventional reactors also produce plutonium, but in much smaller quantities.

The first nuclear reactor was built in 1942 in the USA under the leadership of E. Fermi. In our country, the first reactor was built in 1946 under the leadership of IV Kurchatov.

2. thermonuclear reactions. The second way to release nuclear energy is associated with fusion reactions. During the fusion of light nuclei and the formation of a new nucleus, a large amount of energy should be released. This can be seen from the dependence of the specific binding energy on the mass number A (Fig. 9.6.1). Up to nuclei with a mass number of about 60, the specific binding energy of nucleons increases with increasing A. Therefore, the fusion of any nucleus with A< 60 из более легких ядер должен сопровождаться выделением энергии. Общая масса продуктов реакции синтеза будет в этом случае меньше массы первоначальных частиц.

Fusion reactions of light nuclei are called thermonuclear reactions, as they can only flow at very high temperatures. In order for two nuclei to enter into a fusion reaction, they must approach at a distance of action of nuclear forces of the order of 2·10 -15 m, overcoming the electrical repulsion of their positive charges. For this, the average kinetic energy of the thermal motion of molecules must exceed the potential energy of the Coulomb interaction. The calculation of the required temperature T for this leads to a value of the order of 10 8 –10 9 K. This is an extremely high temperature. At this temperature, the substance is in a fully ionized state, which is called plasma.

The energy released in thermonuclear reactions per nucleon is several times higher than the specific energy released in chain reactions of nuclear fission. So, for example, in the fusion reaction of deuterium and tritium nuclei

3.5 MeV/nucleon is released. In total, 17.6 MeV is released in this reaction. This is one of the most promising thermonuclear reactions.

Implementation controlled thermonuclear reactions will give humanity a new environmentally friendly and practically inexhaustible source of energy. However, obtaining ultra-high temperatures and keeping plasma heated to a billion degrees is the most difficult scientific and technical task on the way to the implementation of controlled thermonuclear fusion.

At this stage in the development of science and technology, only uncontrolled fusion reaction in a hydrogen bomb. The high temperature required for nuclear fusion is achieved here by detonating a conventional uranium or plutonium bomb.

Thermonuclear reactions play an extremely important role in the evolution of the universe. The radiation energy of the Sun and stars is of thermonuclear origin.

Radioactivity

Almost 90% of the known 2500 atomic nuclei are unstable. An unstable nucleus spontaneously transforms into other nuclei with the emission of particles. This property of nuclei is called radioactivity. For large nuclei, instability arises due to the competition between the attraction of nucleons by nuclear forces and the Coulomb repulsion of protons. There are no stable nuclei with charge number Z > 83 and mass number A > 209. But atomic nuclei with significantly lower Z and A numbers can also turn out to be radioactive. If the nucleus contains significantly more protons than neutrons, then instability is caused by an excess of the Coulomb interaction energy . Nuclei, which would contain a large excess of neutrons over the number of protons, are unstable due to the fact that the mass of the neutron exceeds the mass of the proton. An increase in the mass of the nucleus leads to an increase in its energy.

The phenomenon of radioactivity was discovered in 1896 by the French physicist A. Becquerel, who discovered that uranium salts emit unknown radiation that can penetrate through barriers that are opaque to light and cause blackening of the photographic emulsion. Two years later, French physicists M. and P. Curie discovered the radioactivity of thorium and discovered two new radioactive elements - polonium and radium

In subsequent years, many physicists, including E. Rutherford and his students, were engaged in the study of the nature of radioactive radiation. It was found that radioactive nuclei can emit particles of three types: positively and negatively charged and neutral. These three types of radiation were called α-, β- and γ-radiation. On fig. 9.7.1 shows the scheme of the experiment, which makes it possible to detect the complex composition of radioactive radiation. In a magnetic field, α- and β-rays deviate in opposite directions, and β-rays deviate much more. γ-rays in a magnetic field do not deviate at all.

These three types of radioactive radiation differ greatly from each other in their ability to ionize the atoms of matter and, consequently, in their penetrating power. α-radiation has the least penetrating power. In air, under normal conditions, α-rays travel a distance of several centimeters. β-rays are much less absorbed by matter. They are able to pass through a layer of aluminum several millimeters thick. γ-rays have the highest penetrating power, being able to pass through a layer of lead 5–10 cm thick.

In the second decade of the 20th century, after the discovery by E. Rutherford of the nuclear structure of atoms, it was firmly established that radioactivity is property of atomic nuclei. Studies have shown that α-rays represent a stream of α-particles - helium nuclei, β-rays are a stream of electrons, γ-rays are short-wave electromagnetic radiation with an extremely short wavelength λ< 10 –10 м и вследствие этого – ярко выраженными корпускулярными свойствами, то есть является потоком частиц – γ-квантов.

Alpha decay. Alpha decay is the spontaneous transformation of an atomic nucleus with the number of protons Z and neutrons N into another (daughter) nucleus containing the number of protons Z - 2 and neutrons N - 2. In this case, an α-particle is emitted - the nucleus of a helium atom. An example of such a process is the α-decay of radium:

Alpha particles emitted by the nuclei of radium atoms were used by Rutherford in experiments on scattering by the nuclei of heavy elements. The speed of α-particles emitted during the α-decay of radium nuclei, measured along the curvature of the trajectory in a magnetic field, is approximately equal to 1.5 10 7 m/s, and the corresponding kinetic energy is about 7.5 10 -13 J (approximately 4. 8 MeV). This value can be easily determined from the known values of the masses of the parent and daughter nuclei and the helium nucleus. Although the speed of the ejected α-particle is enormous, it is still only 5% of the speed of light, so the calculation can use a non-relativistic expression for the kinetic energy.

Studies have shown that a radioactive substance can emit α-particles with several discrete energy values. This is explained by the fact that nuclei can be, like atoms, in different excited states. A daughter nucleus can be in one of these excited states during α-decay. During the subsequent transition of this nucleus to the ground state, a γ-quantum is emitted. The scheme of α-decay of radium with the emission of α-particles with two values of kinetic energies is shown in fig. 9.7.2.

Thus, the α-decay of nuclei is in many cases accompanied by γ-radiation.

In the theory of α-decay, it is assumed that groups consisting of two protons and two neutrons, that is, an α-particle, can form inside nuclei. The parent nucleus is for α-particles potential well, which is limited potential barrier. The energy of the α-particle in the nucleus is insufficient to overcome this barrier (Fig. 9.7.3). The escape of an α-particle from the nucleus is possible only due to a quantum mechanical phenomenon called tunnel effect. According to quantum mechanics, there is a non-zero probability of a particle passing under a potential barrier. The phenomenon of tunneling has a probabilistic character.

Beta decay. In beta decay, an electron is emitted from the nucleus. Inside nuclei, electrons cannot exist (see § 9.5), they arise during β-decay as a result of the transformation of a neutron into a proton. This process can occur not only inside the nucleus, but also with free neutrons. The average lifetime of a free neutron is about 15 minutes. When a neutron decays into a proton and an electron

The measurements showed that in this process there is an apparent violation of the law of conservation of energy, since the total energy of the proton and electron arising from the decay of the neutron is less than the energy of the neutron. In 1931, W. Pauli suggested that during the decay of a neutron, another particle is released with zero mass and charge, which takes away part of the energy. The new particle is named neutrino(small neutron). Due to the absence of a charge and mass in a neutrino, this particle interacts very weakly with the atoms of matter, so it is extremely difficult to detect it in an experiment. The ionizing ability of neutrinos is so small that one act of ionization in air falls on approximately 500 km of the path. This particle was discovered only in 1953. Currently, it is known that there are several varieties of neutrinos. In the process of neutron decay, a particle is produced, which is called electronic antineutrino. It is denoted by the symbol Therefore, the neutron decay reaction is written as

A similar process also occurs inside nuclei during β-decay. An electron formed as a result of the decay of one of the nuclear neutrons is immediately ejected from the “parent house” (nucleus) at a tremendous speed, which can differ from the speed of light by only a fraction of a percent. Since the distribution of the energy released during β-decay between an electron, a neutrino and a daughter nucleus is random, β-electrons can have different velocities over a wide range.

During β-decay, the charge number Z increases by one, while the mass number A remains unchanged. The daughter nucleus turns out to be the nucleus of one of the isotopes of the element, the serial number of which in the periodic table is one higher than the serial number of the original nucleus. A typical example of β-decay is the transformation of the thorium isotone arising from the α-decay of uranium into palladium

Gamma decay. Unlike α- and β-radioactivity, γ-radioactivity of nuclei is not associated with a change in the internal structure of the nucleus and is not accompanied by a change in the charge or mass numbers. In both α- and β-decay, the daughter nucleus can be in some excited state and have an excess of energy. The transition of the nucleus from the excited state to the ground state is accompanied by the emission of one or several γ-quanta, the energy of which can reach several MeV.

Law of radioactive decay. Any sample of radioactive material contains a huge number of radioactive atoms. Since radioactive decay is random and does not depend on external conditions, the law of decrease in the number N(t) of nuclei that have not decayed by a given time t can serve as an important statistical characteristic of the radioactive decay process.

Let the number of undecayed nuclei N(t) change by ΔN over a short period of time Δt< 0. Так как вероятность распада каждого ядра неизменна во времени, что число распадов будет пропорционально количеству ядер N(t) и промежутку времени Δt:

The coefficient of proportionality λ is the probability of the decay of the nucleus in the time Δt = 1 s. This formula means that the rate of change of the function N(t) is directly proportional to the function itself.

where N 0 is the initial number of radioactive nuclei at t = 0. During the time τ = 1 / λ, the number of undecayed nuclei will decrease by e ≈ 2.7 times. The value τ is called average life time radioactive nucleus.

For practical use, it is convenient to write the law of radioactive decay in a different form, using the number 2 as the base, and not e:

The value of T is called half-life. During time T, half of the initial number of radioactive nuclei decays. The values of T and τ are related by the relation

The half-life is the main quantity that characterizes the rate of radioactive decay. The shorter the half-life, the more intense the decay. Thus, for uranium T ≈ 4.5 billion years, and for radium T ≈ 1600 years. Therefore, the activity of radium is much higher than that of uranium. There are radioactive elements with a half-life of a fraction of a second.

Not found in natural conditions, and ends in bismuth This series of radioactive decays occurs in nuclear reactors.

An interesting application of radioactivity is the method of dating archaeological and geological finds by the concentration of radioactive isotopes. The most commonly used method is radiocarbon dating. An unstable carbon isotope occurs in the atmosphere due to nuclear reactions caused by cosmic rays. A small percentage of this isotope is found in air along with the usual stable isotope. Plants and other organisms consume carbon from the air and accumulate both isotopes in the same proportion as they do in air. After the death of plants, they cease to consume carbon and the unstable isotope gradually turns into nitrogen as a result of β-decay with a half-life of 5730 years. By accurately measuring the relative concentration of radioactive carbon in the remains of ancient organisms, it is possible to determine the time of their death.

Radioactive radiation of all types (alpha, beta, gamma, neutrons), as well as electromagnetic radiation (X-ray radiation) have a very strong biological effect on living organisms, which consists in the processes of excitation and ionization of atoms and molecules that make up living cells. Under the action of ionizing radiation, complex molecules and cellular structures are destroyed, which leads to radiation damage to the body. Therefore, when working with any source of radiation, it is necessary to take all measures for the radiation protection of people who can fall into the zone of radiation.

However, a person can be exposed to ionizing radiation in domestic conditions. Radon, an inert, colorless, radioactive gas, can pose a serious danger to human health. As can be seen from the diagram shown in Fig. 9.7.5, radon is a product of the α-decay of radium and has a half-life T = 3.82 days. Radium is found in small amounts in soil, in stones, and in various building structures. Despite the relatively short lifetime, the concentration of radon is continuously replenished due to new decays of radium nuclei, so radon can accumulate in enclosed spaces. Getting into the lungs, radon emits α-particles and turns into polonium, which is not a chemically inert substance. This is followed by a chain of radioactive transformations of the uranium series (Fig. 9.7.5). According to the American Commission on Radiation Safety and Control, the average person receives 55% of their ionizing radiation from radon and only 11% from medical care. The contribution of cosmic rays is about 8%. The total dose of radiation that a person receives in a lifetime is many times less maximum allowable dose(SDA), which is established for people of certain professions exposed to additional exposure to ionizing radiation.