Radioactive atomic transformations. Transformations of atomic nuclei

Lesson type
Lesson objectives:

Continue studying the phenomenon of radioactivity;

Study radioactive transformations (displacement rules and the law of conservation of charge and mass numbers).

To study fundamental experimental data in order to explain in an elementary way the basic principles of the use of nuclear energy.
Tasks:
educational
developing
educational

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Lesson on the topic "Radioactive transformations of atomic nuclei."

Physics teacher of the 1st category Medvedeva Galina Lvovna

Lesson type : a lesson in learning new material
Lesson objectives:

Continue studying the phenomenon of radioactivity;

Study radioactive transformations (displacement rules and the law of conservation of charge and mass numbers).

To study fundamental experimental data in order to explain in an elementary way the basic principles of the use of nuclear energy.
Tasks :
educational - familiarizing students with the bias rule; expansion of students' ideas about the physical picture of the world;
developing - to work out the skills of the physical nature of radioactivity, radioactive transformations, the rules of displacement by periodic system chemical elements; continue developing skills in working with tables and diagrams; continue the development of work skills: highlighting the main thing, presenting the material, developing attentiveness, the ability to compare, analyze and generalize facts, promote the development of critical thinking.
educational - to promote the development of curiosity, to form the ability to express their point of view and defend their innocence.

Lesson summary:

Text for the lesson.

Good afternoon, everyone present at our today's lesson.

Teacher: So we are at the second stage research work on the topic "Radioactivity". What is it? That is, today we will study radioactive transformations and displacement rules. ----This is the subject of our research and, accordingly, the topic of the lesson

Research equipment: periodic table, working map, collection of problems, crossword puzzle (one for two).

Teacher, Epigraph: "At one time, when the phenomenon of radioactivity was discovered, Einstein compared it with the production of fire in ancient times, since he believed that fire and radioactivity are equally major milestones in the history of civilization."

Why did he think so?

The students of our class did some theoretical research and here is the result:

Student message:

1. Pierre Curie placed an ampoule of radium chloride in the calorimeter. It absorbed α-, β-, γ-rays, and due to their energy the calorimeter was heated. Curie determined that 1 g of radium releases about 582 J of energy in 1 hour. And this energy has been released over the years.
2. The formation of 4 grams of helium is accompanied by the release of the same energy as in the combustion of 1.5-2 tons of coal.
3. The energy contained in 1 g of uranium is equal to the energy released during the combustion of 2.5 tons of oil.

During the day, months and years, the radiation intensity did not change noticeably. It was not affected in any way by such usual influences as heating or increasing pressure. The chemical reactions that the radioactive substances entered did not affect the radiation intensity either.

Each of us is not only "under the supervision" of a radiation vigilant "nanny", each of us is a little radioactive in and of itself. Radiation sources are not only outside of us. When we drink, with every sip we introduce into the body a certain number of atoms of radioactive substances, the same happens when we eat. Moreover, when we breathe, our body again receives something from the air that is capable of radioactive decay - maybe the radioactive isotope of carbon C-14, maybe potassium K-40 or some other isotope.

Teacher: Where does the amount of radioactivity that is constantly present around and inside us come from?

Student communication:

According to nuclear geophysics, there are many sources of natural radioactivity in nature. In the rocks crustOn average, one ton of rocks accounts for 2.5 - 3 grams of uranium, 10 - 13 g of thorium, 15 - 25 g of potassium. True, radioactive K-40 is only up to 3 milligrams per ton. All this abundance of radioactive, unstable nuclei is continuously, spontaneously decaying. Every minute, an average of 60,000 K-40 nuclei, 15,000 Rb-87 isotope nuclei, 2,400 Th-232 nuclei, 2,200 U-238 nuclei disintegrate in 1 kg of terrestrial material. The total value of natural radioactivity is about 200 thousand decays per minute. Did you know that natural radioactivity is different for men and women? The explanation for this fact is obvious - soft and dense tissues have different structures, absorb and accumulate radioactive substances in different ways..

PROBLEM: What equations, rules, laws describe these reactions of decomposition of substances?

Teacher: What problem will we solve with you? What solutions do you suggest?

Students work and make their assumptions.

Students' answers:

Solution ways:

Student 1: Recall the basic definitions and properties of radioactive radiation.

Student 2: Using the proposed reaction equations (according to the map), get general equations for radioactive conversion reactions using the periodic table, formulate general displacement rules for alpha and beta decays.

Apprentice 3 : To consolidate the knowledge gained in order to apply it for further research (problem solving).

Teacher.

Okay. Let's get down to the solution.

Stage 1: working with maps. You have been given questions to which you must give writtenanswers.

Five questions - five correct answers. We estimate on a five-point system.

(Give time to work, then verbally voice the answers, check with the slides, give ourselves a grade according to the criteria).

1. Radioactivity is ...
2. α-rays are ...
3. β-rays are….
4. γ-radiation -….
5. Formulate the law of conservation of charge and mass numbers.

ANSWERS AND POINTS:

STEP 2. Teacher.

We work independently and at the blackboard (3 students).

A) We write down the reaction equations, which are accompanied by the release of alpha particles.

2. Write the reaction of uranium α-decay235 92 U.

3. .Write the alpha decay of a polonium nucleus

Teacher:

CONCLUSION # 1:

As a result of alpha decay, the mass number of the resulting substance decreases by 4 amu, and the charge number by 2 elementary charges.

B) We write down the reaction equations, which are accompanied by the release of beta particles (3 studying at the blackboard).

one. . Write the reaction of β-decay of plutonium239 94 Pu.

2. Write beta decay of thorium isotope

3.Write the reaction β-decay of curium247 96 Cm

Teacher: What general expression can we write down with you and draw an appropriate conclusion?

CONCLUSION # 2:

As a result of beta decay, the mass number of the resulting substance does not change, and the charge number increases by 1 elementary charge.

STEP 3.

Teacher: At one time, after these expressions were obtained, Rutherford's student Frederick Soddy,proposed bias rules for radioactive decays, with the help of which the formed substances can be found in the periodic table. Let's look at the equations we got.

QUESTION:

one). WHAT REGULARITY IS OBSERVED IN THE ALPHA DECAY?

ANSWER: With alpha - decay, the resulting substance is shifted by two cells to the beginning of the periodic table.

2). WHAT REGULARITY IS OBSERVED DURING THE BETA DECAY?

ANSWER: During beta decay, the resulting substance is displaced by one cell towards the end of the periodic table.

STEP 4.

Teacher. : And the last stage of our activity for today:

Independent work (based on the collection of problems by Lukashik):

Option 1.

Option 2.

CHECK: on the board, by yourself.

CRITERIA FOR EVALUATION:

"5" - completed tasks

"4" - completed 2 tasks

"3" - completed 1 task.

SELF-ASSESSMENT PER LESSON:

IF TIME REMAINS:

Question to the class:

What topic did you study in class today? Having guessed the crossword puzzle, you will learn the name of the radiation release process.

1. Which scientist discovered the phenomenon of radioactivity?

2. A particle of matter.

3. The surname of the scientist who determined the composition of the radioactive radiation.

4. Nuclei with the same number of protons, but with a different number of neutrons are ...

5. A radioactive element discovered by the Curies.

6. The isotope of polonium is alpha-radioactive. What element is formed in this case?

7. The name of a woman - a scientist who became Nobel laureate twice.

8. What is in the center of the atom?

In the previous lesson, we discussed a question related to Rutherford's experiment, as a result of which we now know that the atom is a planetary model. that's what it's called - the planetary model of the atom. At the center of the nucleus is a massive, positively charged nucleus. And electrons revolve around the nucleus in their orbits.

Figure: 1. Rutherford's planetary model of the atom

Frederick Soddy took part in the experiments with Rutherford. Soddy is a chemist, so he carried out his work precisely in terms of identifying the obtained elements by their chemical properties. It was Soddy who was able to figure out what exactly are a-particles, the flux of which fell on the gold plate in Rutherford's experiments. When the measurements were made, it turned out that the mass of the a-particle is 4 atomic mass units, and the charge of the a-particle is 2 elementary charges. Comparing these things, having accumulated a certain number of a-particles, the scientists found out that these particles turned into a chemical element - helium gas.

The chemical properties of helium were known, thanks to this Soddy argued that the nuclei, which are a-particles, captured electrons from outside and turned into neutral helium atoms.

Subsequently, the main efforts of scientists were directed to the study of the atomic nucleus. It became clear that all the processes that occur with radioactive radiation do not occur with the electron shell, not with the electrons that surround the nuclei, but with the nuclei themselves. It is in the nuclei that some transformations take place, as a result of which new chemical elements are formed.

The first such chain was obtained for the transformation of the element radium, which was used in experiments on radioactivity, into the inert gas radon with the emission of an a-particle; the reaction in this case is written as follows:

First, an a-particle is 4 atomic mass units and a double, twice the elementary charge, and the charge is positive. Radium has a serial number 88, its mass number is 226, and radon has a serial number already 86, mass number 222, and an a-particle appears. This is the nucleus of a helium atom. In this case, we just write helium. Ordinal number 2, mass number 4.

Reactions resulting in the formation of new chemical elements and at the same time the formation of new radiation and other chemical elements are called nuclear reactions.

When it became clear that radioactive processes are taking place inside the nucleus, they turned to other elements, not only to radium. Studying various chemical elements, scientists realized that there are not only reactions with emission, emission of a-particle of the nucleus of a helium atom, but also other nuclear reactions. For example, reactions with the emission of a b-particle. We now know that these are electrons. In this case, a new chemical element is also formed, respectively, new particle, this is a b-particle, it is also an electron. Of particular interest in this case are all chemical elements with a serial number greater than 83.

So, we can formulate the so-called. Soddy's rules, or bias rules for radioactive transformations:

... With alpha decay, the ordinal number of the element decreases by 2 and the atomic weight decreases by 4.

Figure: 2. Alpha decay

During beta decay, the serial number increases by 1, while the atomic weight does not change.

Figure: 3. Beta decay

List of additional literature

1. Bronstein M.P. Atoms and electrons. "Library" Quant "". Issue 1.M .: Nauka, 1980
2. Kikoin I.K., Kikoin A.K. Physics: Textbook for Grade 9 high school... M .: "Education"
3. Kitaygorodsky A.I. Physics for everyone. Photons and nuclei. Book 4.M .: Science
4. Myakishev G.Ya., Sinyakova A.Z. Physics. Optics Quantum Physics. Grade 11: textbook for in-depth study physics. M .: Bustard
5. Rutherford E. Selected scientific works... Radioactivity. M .: Science
6. Rutherford E. Selected scientific works. The structure of the atom and the artificial transformation of elements. M .: Science

Discovery history

Already in 1903, physicists Rutherford and Soddy discovered that during radioactive alpha decay the element radium is converted into another chemical element - radon. These two chemical elements have completely different properties. Radium is a solid, metal, and radon is an inert gas. The radium and radon atoms differ in mass, the number of electrons in the electron shell, and the charge of the nucleus. Further studies showed that beta decay converts some chemical elements into others. In 1911, Rutherford proposed a nuclear model of the atom. The essence of the model was as follows: an atom consists of a positively charged nucleus and negatively charged electrons that move around the nucleus. It was logical to assume that in such a model of the atom with radioactive alpha or beta decay, it is in the nucleus of the atom that a change occurs, since if only the number of electrons changed, then a new chemical element would not be obtained, but an ion of the same chemical element would be obtained ...

Formula decay

The alpha decay of radium is written as follows:

(226.88) Ra -\u003e (222.86) Rn + (4.2) He.

Picture

In the formula above, (226.88) Ra denotes the nucleus of a radium atom, (222.86) Rn is the nucleus of a radon atom and (4.2) He is an alpha particle, or the nucleus of a helium atom.

Please note that the same notation is used to denote the nucleus of an atom as for the atom itself. Let's deal with indices. The number on top is called the mass number. The mass number of the nucleus of an atom shows how many atomic mass units are contained in the mass of the nucleus of a given atom. The number written down below is called the charge number. The charge number of the nucleus of an atom shows how many elementary electric charges are contained in the charge of the nucleus of a given atom. The mass and charge numbers are always whole and positive values. They do not have a separate unit of designation, since they express how many times the mass and charge of the nucleus of a given atom is greater than single indicators.

The essence of the phenomenon

Let us analyze the reaction equation that we wrote down for the alpha decay of the radium atom nucleus.

(226.88) Ra -\u003e (222.86) Rn + (4.2) He.

We have that the nucleus of a radium atom during the emission of an alpha particle has lost 4 units of mass and two elementary charges, and thus turned into the nucleus of a radon atom. It can be seen that the laws of conservation of mass number and charge are satisfied. Let's add separately the mass numbers and charge numbers of the resulting two elements:

As you can see, they add up to the same values \u200b\u200bas the radium nucleus. From all of the above, it follows that the nucleus of an atom also consists of some particles, that is, in other words, it has a complex composition. And we can now refine the definition of radioactivity. Radioactivity - the ability of the nuclei of some atoms to spontaneously transform into other nuclei, while emitting particles.

S.G.Kadmensky
Voronezh State University

Radioactivity of atomic nuclei: history, results, latest achievements

In 1996, the physical community celebrated the centenary of the discovery of the radioactivity of atomic nuclei. This discovery led to the birth of a new physics, which made it possible to understand the structure of the atom and atomic nucleus, and served as a gateway to the strange and harmonious quantum world of elementary particles. As with many outstanding discoveries, the discovery of radioactivity happened by accident. At the beginning of 1896, immediately after the discovery of V.K. Using X-rays, the French physicist Henri Becquerel, in the process of testing the hypothesis of the fluorescent nature of X-ray radiation, discovered that the uranium-potassium salt spontaneously, spontaneously, without external influences emits hard radiation. Later Becquerel found that this phenomenon, which he called radioactivity, that is, radiation activity, is entirely related to the presence of uranium, which became the first radioactive chemical element. A few years later, similar properties were found in thorium, then in polonium and radium, discovered by Marie and Pierre Curie, and later in all chemical elements, whose numbers are more than 82.With the advent of accelerators and nuclear reactors, radioactive isotopes were found in all chemical elements, most of which are practically not found in natural conditions.

TYPES OF RADIOACTIVE CONVERSIONS OF ATOMIC NUCLEI

Analyzing the penetrating ability of radioactive radiation from uranium, E. Rutherford discovered two components of this radiation: less penetrating, called α-radiation, and more penetrating, called γ-radiation. The third component of uranium radiation, the most penetrating of all, was discovered later, in 1900, by Paul Willard and named by analogy with the Rutherford series of γ-radiation. Rutherford and his collaborators showed that radioactivity is associated with the decay of atoms (much later it became clear that we are talking about the decay of atomic nuclei), accompanied by the release of a certain type of radiation from them. This conclusion dealt a crushing blow to the concept of the indivisibility of atoms that prevailed in physics and chemistry.
In subsequent studies by Rutherford, it was shown that α-radiation is a flux of α-particles, which are nothing more than the nuclei of the helium isotope 4 He, and β-radiation consists of electrons. Finally, γ-radiation turns out to be a relative of light and X-ray radiation and is a flux of high-frequency electromagnetic quanta emitted by atomic nuclei during the transition from excited to lower-lying states.
The nature of β-decay of nuclei turned out to be very interesting. The theory of this phenomenon was created only in 1933 by Enrico Fermi, who used the hypothesis of Wolfgang Pauli about the birth in β-decay of a neutral particle with a rest mass close to zero and called a neutrino. Fermi discovered that β-decay is due to a new type of interaction of particles in nature - "weak" interaction and is associated with the processes of transformation in the parent nucleus of a neutron into a proton with the emission of an electron e - and antineutrino (β - decay), a proton into a neutron with the emission of a positron е + and neutrinos ν (β + -decay), as well as with the capture of an atomic electron by a proton and the emission of a neutrino ν (electron capture).
The fourth type of radioactivity discovered in Russia in 1940 by young physicists G.N. Flerov and K.A. Petrzhak, is associated with spontaneous nuclear fission, in the process of which some rather heavy nuclei decay into two fragments with approximately equal masses.
But fission has not exhausted all types of radioactive transformations of atomic nuclei. Beginning in the 1950s, physicists have methodically approached the discovery of the proton radioactivity of nuclei. For a nucleus in the ground state to be able to spontaneously emit a proton, it is necessary that the energy of separation of a proton from the nucleus is positive. But such nuclei do not exist under terrestrial conditions, and they had to be created artificially. Russian physicists in Dubna were very close to obtaining such nuclei, but proton radioactivity was discovered in 1982 by German physicists in Darmstadt using the world's most powerful accelerator of multiply charged ions.
Finally, in 1984, independent groups of scientists in England and Russia discovered the cluster radioactivity of some heavy nuclei that spontaneously emit clusters - atomic nuclei with atomic weights from 14 to 34.
Table 1 presents the history of the discovery of various types of radioactivity. Time will tell whether they have exhausted all possible types of radioactive transformations of nuclei. In the meantime, the search continues intensively for nuclei that would emit from the ground states a neutron (neutron radioactivity) or two protons (two-proton radioactivity).

Table 1. History of the discovery of various types of radioactivity

Nuclei radioactivity type	Type of radiation detected	Opening year	Authors of the discovery
Radioactivity of atomic nuclei	Radiation	1896	A. Becquerel
Alpha decay	4 Not	1898	E. Rutherford
Beta decay	e -	1898	E. Rutherford
Gamma decay	γ -Quantum	1900	P. Willard
Spontaneous nuclear fission	Two shards	1940	G.N. Flerov, K.A. Petrzhak
Proton decay	p	1982	3. Hoffman et al.
Cluster decay	14 C	1984	X. Rose, G. Jones; D.V. Alexandrov and others.

MODERN CONCEPTS OF ALPHA DECAY

All types of radioactive transformations of nuclei satisfy an exponential law:

N (t) \u003d N (0) exp (-λt),

where N (t) is the number of radioactive nuclei that survived by the time t > 0 if at the moment t \u003d 0 their number was N (0). The value of λ, coincides with the probability of decay of a radioactive nucleus per unit time. Then the time T 1/2, called the half-life, during which the number of radioactive nuclei decreases by half, is defined as

T 1/2 \u003d (ln2) / λ ,.

The T 1/2 values \u200b\u200bfor α-emitters vary in a wide range from 10 -10 seconds to 10 20 years, depending on the value of the energy Q of the relative motion of the α-particle and the daughter nucleus, which, when using the laws of conservation of energy and momentum during α-decay, is determined as

Q \u003d B (A-4, Z-2) + B (4,2) - B (A, Z),

where B (A, Z) is the binding energy of the parent nucleus. For all investigated α-transitions, the value Q\u003e 0 and does not exceed 10 MeV. In 1910, Hans Geiger and George Nettall experimentally discovered a law relating the half-life T 1/2 to energy Q:

logT 1/2 \u003d B + CQ -1/2

(1)

where the values \u200b\u200bof B and C do not depend on Q. Figure 1 illustrates this law for the even-even isotopes of polonium, radon and radium. But then a very serious problem arises. The interaction potential V (R) of the α-particle and the daughter nucleus, depending on the distance R between their centers of gravity, can be qualitatively represented as follows (Fig. 2). At large distances R they interact in a Coulomb manner and the potential

At small distances R, short-range nuclear forces come into play, and the potential V (R) becomes attractive. Therefore, a barrier appears in the potential V (R), the position of the R B maximum of which V B \u003d V (R B) lies for heavy nuclei with Z ≈ 82 in the region of 10 -12 cm, and the value V B \u003d 25 MeV. But then the question arises as to how an a-particle with energy Q < V B can escape from the radioactive nucleus if in the sub-barrier region its value kinetic energy К \u003d Q - V (R) becomes negative and from the point of view of classical mechanics the motion of a particle in this area is impossible. The solution to this problem was found in 1928 by the Russian physicist G.A. Gamow. Relying on the quantum mechanics created not long before that time, Gamow showed that the wave properties of the α-particle allow it to penetrate through the potential barrier with a certain probability P. Then, if we accept that the α-particle exists in a fully formed form inside the nucleus, for the probability of its α-decay per unit time A, the formula appears

where 2 ν is the number of impacts of an α-particle on the inner wall of the barrier, determined by the frequency ν vibrations of an α-particle inside the parent nucleus. Then, having calculated the quantum-mechanical value of P and estimating v in the simplest approximations, Gamow obtained the Geiger-Nettol law for logT 1/2 (1). Gamow's result had a tremendous resonance among physicists, as he demonstrated that the atomic nucleus is described by the laws of quantum mechanics. But the main problem of α-decay remained unresolved: where do α-particles come from in heavy nuclei consisting of neutrons and protons?

MULTIPARTICLE ALPHA-DECAY THEORY

The many-particle theory of α-decay, in which the problem of the formation of an α-particle from neutrons and protons of the parent nucleus is consistently solved, arose in the early 1950s and in last years received conceptual completion in the works of several physicists, including the author and his collaborators. This theory is based on the shell model of the nucleus, which was substantiated within the framework of the Fermi liquid theory by L.D. Landau and A.B. Migdal, in which it is assumed that the proton and neutron in the nucleus move in an independent way in the self-consistent field created by the rest of the nucleons. Using the shell wave functions of two protons and two neutrons, one can find the probability with which these nucleons will be in the -particle state. Then Gamow's formula (2) can be generalized as

where W if is the probability of the formation of an alpha particle from the nucleons of the parent nucleus i with the formation of a specific state f of the daughter nucleus. Calculations of the W if values \u200b\u200bhave demonstrated the fundamental importance of taking into account the superfluid properties of atomic nuclei for understanding the nature of alpha decay.
A bit of history. In 1911, Heike Kamerling-Onnes discovered the phenomenon of superconductivity of some metals, for which at temperatures below a certain critical value, the resistance drops abruptly to zero. In 1938 P.L. Kapitsa discovered the phenomenon of superfluidity of liquid helium 4 He, which consists in the fact that at temperatures below a certain critical value, liquid helium flows through thin capillary tubes without friction. Both of these phenomena have long been regarded as independent, although many physicists intuitively felt their relationship. The superfluidity of liquid helium was explained in the works of N.N. Bogolyubov and S.T. Belyaev in that Bose condensation occurs in it at low temperatures, in which most helium atoms are accumulated in a state with zero momentum. This is possible because helium atoms have a spin of zero, and therefore are Bose particles, which can be in any quantity in a certain quantum state, for example, in a state with a zero momentum. Unlike helium atoms, electrons, protons and neutrons have half-integer spin and are Fermi particles for which the Pauli principle is valid, which allows only one particle to be in a certain quantum state. The explanation of the superconductivity of metals is based on the phenomenon predicted by L. Cooper, when two electrons in a superconductor form a bound system, called a Cooper pair. The total spin of this pair is zero, and it can be considered as a Bose particle. Then a Bose condensation of Cooper pairs with momenta equal to zero occurs in the superconductor, and the phenomenon of superfluidity of these pairs arises in them, akin to the phenomenon of superfluidity of liquid helium. It is the superfluidity of Cooper pairs that forms the superconducting properties of metals. Thus, two phenomena that formally belong to different branches of physics - superconductivity and superfluidity - turned out to be physically related. Nature does not like to lose her beautiful finds. She uses them in various physical objects. This forms the unity of physics.
In 1958, Oge Bohr hypothesized the existence of superfluid properties in atomic nuclei. In almost one year, this hypothesis was fully confirmed and implemented in the creation of a superfluid model of the atomic nucleus, in which it is assumed that pairs of protons or neutrons combine into Cooper pairs with a spin equal to zero, and the Bose condensation of these pairs forms the superfluid properties of nuclei.
Since the α-particle consists of two protons and two neutrons with total spins equal to zero, its internal symmetry coincides with the symmetry of the Cooper pairs of protons and neutrons in atomic nuclei. Therefore, the probability of the formation of an α-particle W if is maximum if it is formed from two Cooper pairs of protons and neutrons. α-Transitions of this type are called facilitated and occur between the ground states of even-even nuclei, where all nucleons are paired. For such transitions in the case of heavy nuclei with Z\u003e 82, the value W if \u003d 10 -2. If the α-particle contains only one Cooper pair (proton or neutron), then such α-transitions, characteristic of odd nuclei, are called semi-lightweight and for them W if \u003d 5 * 10 -4. Finally, if a -particle is formed from unpaired protons and neutrons, then the α-transition is called non-facilitated and for it the value W if \u003d 10 -5. Based on the superfluid model of the nucleus, the author and his collaborators by 1985 succeeded in describing, on the basis of formulas like (3), not only the relative, but also the absolute probabilities of the alpha decay of atomic nuclei.

MULTIPARTICLE THEORY OF PROTON RADIOACTIVITY

To reliably observe the proton decay of atomic nuclei from the ground and low-lying excited states, it is necessary that the energy of the relative motion of the proton and the daughter nucleus Q be positive and at the same time noticeably less than the height of the proton potential barrier VB, so that the lifetime of the proton decay nucleus is not too short for its experimental research. Such conditions, as a rule, are satisfied only for strongly neutron-deficient nuclei, the production of which has become possible only in recent years. At present, more than 25 proton decayers have been discovered from the ground and isomeric (rather long-lived) excited states of nuclei. From a theoretical point of view, proton decay looks much simpler than alpha decay, since the proton is part of the nucleus, and therefore it seemed that it was possible to use formulas like formula (2). However, very soon it became clear that almost all proton transitions are sensitive to the structure of the parent and daughter nuclei and it is necessary to use formula (3), and to calculate the probabilities W if the author and his co-workers had to develop a many-particle theory of proton radioactivity taking into account superfluid effects. On the basis of this theory, it was possible to successfully describe all the observed cases of proton decay, including the particularly incomprehensible case of the decay of the long-lived isomeric state of the 53Co nucleus, and to make predictions about the most likely new candidates for observing proton radioactivity. At the same time, it was demonstrated that the majority of proton-decaying nuclei are nonspherical, in contrast to the initial ideas.

CLUSTER DECAY OF ATOMIC NUCLEI

At present, 25 nuclei from 221 Fr to 241 Аm have been experimentally discovered, emitting from the ground states clusters of the 14 С, 20 О, 24 Ne, 26 Ne, 28 Mg, 30 Mg, 32 Si, and 34 Si types. The energies of relative motion of the ejected cluster and daughter nucleus Q vary from 28 to 94 MeV and in all cases turn out to be noticeably lower than the height of the potential barrier V B. At the same time, all investigated cluster radioactive nuclei are also α-decay nuclei, and the ratios of the probability k of their cluster decay per unit time to the similar probability λ α for α-decay decrease with increasing mass of the ejected cluster and lie in the range from 10 -9 to 10 -16. Such small values \u200b\u200bof such ratios have never before been analyzed for other types of radioactivity and demonstrate record achievements of experimenters in observing cluster decay.
Currently, two theoretical approaches are being developed to describe the dynamics of cluster decay of atomic nuclei, which are actually two possible limiting cases. The first approach considers cluster decay as deep subbarrier spontaneous fission, strongly asymmetric in the masses of the resulting fragments. In this case, the parent core, which is in the state and until the moment of rupture in it smoothly rebuilds, noticeably changing its shape and passing through an intermediate configuration b, which is illustrated in Fig. 3. The description of such a rearrangement is carried out on the basis of collective nuclear models, which are a generalization of the hydrodynamic model. This approach currently encounters significant difficulties in describing the fine characteristics of cluster decay.

The second approach is constructed by analogy with the α-decay theory. In this case, the description of the transition to the final configuration in is carried out without introducing an intermediate configuration b directly from the configuration a in the language of a formula like (3) using the concept of the probability of cluster formation W if. A good argument in favor of the second approach is the fact that for cluster decay, as in the case of α decay, the Geiger - Nettol law (1) is fulfilled, which relates the cluster half-life T 1/2 and energy Q. This fact is illustrated in Fig. 4. Within the framework of the second approach, the author and his collaborators succeeded, by analogy with α-decay, to classify cluster transitions according to the degree of lightness, using the ideology of the superfluid model of the nucleus, and to predict the fine structure in the spectra of outgoing clusters. Later, this structure was discovered in the experiments of the French group at Saclay. This approach also made it possible to reasonably describe the scale of the relative and absolute probabilities of known cluster decays and make predictions by observing cluster radioactivity in new cluster-decay nuclei.

CONCLUSION

Studies of various types of radioactivity of atomic nuclei continue at the present time. Of particular interest is the study of proton decay of nuclei, since in this case it is possible to obtain unique information about the structure of nuclei lying beyond the boundaries of nucleon stability of nuclei. Quite recently, a team of physicists led by Professor K. Davids at the Argonne National Laboratory (USA) synthesized the strongly neutron-deficient nucleus 131 Eu and discovered not only proton decay, but also for the first time the fine structure of its proton spectrum. An analysis of these phenomena on the basis of the theory developed by the author made it possible to convincingly confirm the idea of \u200b\u200bthe strong nonsphericity of this core.
An illustration of the interest in such research is an article by journalist M. Brownie entitled "A Look at Unusual Nuclei Changes the View of Atomic Structure", which appeared in the March 1998 issue of the New York Times, which describes the results in a popular form, obtained by the Argonne group, and methods of their interpretation.
The above review, which illustrates the development of ideas about the nature of radioactivity of atomic nuclei over a whole century, demonstrates a clear acceleration in the rate of obtaining new knowledge in this area, especially in the last 25 years. And although nuclear physics is a fairly developed science in the experimental and theoretical sense, there is no doubt that ongoing research within its framework, as well as at the interface with other sciences, can in the near future give mankind new very beautiful and amazing results.

Radioactive transformations of nuclei

Structure of matter

Everything in nature consists of simple and complex substances. Simple substances include chemical elements, complex - chemical compounds... It is known that substances in the world around us consist of atoms, which are the smallest part of a chemical element. An atom is the smallest particle of matter that defines it chemical properties, it has a complex internal structure... In nature, only inert gases are found in the form of atoms, since their outer shells are closed, all other substances exist in the form of molecules.

In 1911, E. Rutherford proposed a planetary model of the atom, which was developed by N Bohr (1913). According to the generally accepted model of the structure of the atom, two regions are distinguished in it: a heavy, positively charged nucleus located in the center, in which almost all the mass of the atom is concentrated, and a light electron shell, consisting of negatively charged particles - electrons, rotating around the nucleus with tremendous speed.

Electron (e -)- a stable elementary particle with a rest mass equal to 9.1 · 10 -31 kg or 0.000548 amu. (atomic mass unit is a dimensionless value of atomic mass, which shows how many times an atom of a given element or particle is heavier than 1/12 of an atom of the isotope carbon-12; the energy equivalent of 1 amu is 931 MeV). An electron carries one elementary negative charge of electricity (q \u003d 1.6 · 10 -19 C), that is, the smallest amount of electricity found in nature. Proceeding from this, the electron charge is taken as one elementary unit of electric charge.

Depending on the energy that holds the electrons as they rotate around the nucleus, they are grouped in different orbits (levels or layers). The number of layers for different atoms is not the same. In atoms with a large mass, the number of orbits reaches seven. They are designated by numbers, or letters of the Latin alphabet, starting from the nucleus: K, L, M, N, O, P, Q. The number of electrons in each layer is strictly defined. So, the K-layer has no more than 2 electrons, the L-layer - up to 8, the M-layer - up to 18, the N-layer - 32 electrons, etc.

The size of an atom is determined by the size of its electron shell, which has no strictly defined boundaries. The approximate linear dimensions of the atom are 10 -10 m.

Core - the central massive part of the atom, consisting of protons and neutrons, which is positively charged. Almost all the mass of an atom (more than 99.95%) is concentrated in the nucleus. The total number of electrons in orbits is always equal to the sum of protons in the nucleus. For example, an oxygen atom contains 8 protons in its nucleus and has 8 electrons in its orbits, a lead atom has 82 protons in its nucleus and 82 electrons in its orbits. Due to the equality of the sum of positive and negative charges, an atom is an electrically neutral system. Two equal, oppositely directed forces act on each of the electrons moving around the nucleus: the Coulomb force attracts the electrons to the nucleus, and the equal centrifugal force of inertia tends to "snatch" the electron from the atom. In addition, electrons, moving (rotating) around the nucleus in an orbit, simultaneously have their own moment of motion, which is called spin, which is simplified as rotation like a top around its own axis. The spins of individual electrons can be oriented parallel (rotation in the same direction) and antiparallel (rotation in different directions). In a simplified form, all this ensures the stable movement of electrons in the atom.

It is known that the bond of an electron with a nucleus is affected not only by the Coulomb force of attraction and centrifugal force of inertia, but also by the repulsive force of other electrons. This effect is called screening. The further the electron orbit is from the nucleus, the stronger the screening of the electrons on it and the weaker the energy bond between the nucleus and the electron. In outer orbits, the binding energy of electrons does not exceed 1–2 eV, while for electrons of the K-layer it is many times greater and increases with an increase in the atomic number of an element. For example, for carbon, the binding energy of electrons in the K-layer is 0.28 keV, for strontium - 16 keV, for cesium - 36 keV, for uranium - 280 keV. Therefore, the electrons of the outer orbit are more susceptible to external factors, in particular, low-energy radiation. When imparting additional energy to electrons from outside, they can move from one energy level to another, or even leave the limits of a given atom. If the energy of the external influence is weaker than the binding energy of the electron with the nucleus, then the electron can only go from one energy level to another. Such an atom remains neutral, but it differs from the rest of the atoms of this chemical element by an excess of energy. Atoms with an excess of energy are called excited, and the transition of electrons from one energy level to another, more distant from the nucleus, is an excitation process. Since in nature any system tends to go to a stable state in which its energy will be the smallest, then the atom after a while passes from the excited state to the ground (initial) state. The return of the atom to the ground state is accompanied by the release of excess energy. The transition of electrons from external to internal orbits is accompanied by radiation with a wavelength characteristic only for a given transition from one energy level to another. Transitions of electrons within the orbits farthest from the core produce radiation consisting of ultraviolet, light and infrared rays. Under strong external influences, when the energy exceeds the binding energy of the electrons with the nucleus, the electrons are torn out of the atom and removed from it. An atom that has lost one or more electrons turns into a positive ion, and one that has “attached” one or more electrons to itself turns into a negative ion. Consequently, for each positive ion, one negative ion is formed, i.e., a pair of ions arises. The formation of ions from neutral atoms is called ionization... An atom in the state of an ion exists under normal conditions for an extremely short period of time. The free space in the orbit of a positive ion is filled with a free electron (an electron not bound to the atom), and the atom again becomes a neutral system. This process is called ion recombination (deionization) and is accompanied by the release of excess energy in the form of radiation. The energy released during ion recombination is numerically approximately equal to the energy spent on ionization.

Proton(r) Is a stable elementary particle with a mass equal to 1.6725 · 10 -27 kg or 1.00758 amu, which is approximately 1840 times the mass of an electron. The charge of a proton is positive and equal in magnitude to the charge of an electron. A hydrogen atom is a nucleus containing one proton, around which one electron revolves. If you “rip off” this electron, then the rest of the atom will be the proton, so the proton is often defined as the nucleus of hydrogen.

Each atom of any element contains a certain number of protons in the nucleus, which is constant and determines the physical and chemical properties of the element. For example, there are 47 of them in the nucleus of the silver atom, and 92 in the uranium nucleus. The number of protons in the nucleus (Z) is called the atomic number or charge number, it corresponds to the ordinal number of the element in the periodic system of DI Mendeleev.

Neutron(n) Is an electrically neutral elementary particle with a mass slightly exceeding the mass of a proton and equal to 1.6749 10 -27 kg or 1.00898 amu. Neutrons are stable only as part of stable atomic nuclei. Free neutrons decay into protons and electrons.

The neutron, due to its electrical neutrality, is not deflected under the influence of a magnetic field, is not repelled by the atomic nucleus and, therefore, has a high penetrating ability, which poses a serious danger as a factor in the biological effect of radiation. The number of neutrons in the nucleus gives only basically the physical characteristics of the element, since in different nuclei of the same chemical element there can be a different number of neutrons (from 1 to 10). In the nuclei of light stable elements, the number of protons is related to the number of neutrons as 1: 1. With an increase in the atomic number of an element (starting from the 21st element - scandium), the number of neutrons in its atoms exceeds the number of protons. In the heaviest nuclei, the number of neutrons is 1.6 times the number of protons.

Protons and neutrons are the constituent parts of the nucleus, therefore, for convenience, they are called nucleons. Nucleon(from Latin nucleus - nucleus) - a common name for protons and neutrons of the nucleus. Also, when talking about a specific atomic nucleus, the term nuclide is used. Nuclide - any atomic nucleus with a given number of protons and neutrons.

When denoting nuclides or atoms, they use the symbol of the element to which the nucleus belongs, and indicate the mass number above - A, below - the atomic (ordinal) number - Z in the form of indices, where E is the symbol of a chemical element. A shows the number of nucleons that make up the nucleus of an atom (A \u003d Z + N). Z shows not only the charge of the nucleus and the ordinal number, but also the number of protons in the nucleus and, accordingly, the number of electrons in the atom, since the atom is generally neutral. N is the number of neutrons in the nucleus, which is most often not indicated. For example, - a radioactive isotope of cesium, A \u003d 137, hence the nucleus consists of 137 nucleons; Z \u003d 55, which means there are 55 protons in the nucleus and, accordingly, 55 electrons in the atom; N \u003d 137 - 55 \u003d 82 is the number of neutrons in the nucleus. The ordinal number is sometimes omitted, since the symbol of the element completely determines its place in the periodic system (for example, Cs-137, He-4). The linear dimensions of the atomic nucleus are 10 -15 -10 -14 m, which is 0.0001 of the diameter of the entire atom.

Protons and neutrons are held inside the nucleus by forces called nuclear... In terms of their intensity, they are much more powerful than electrical, gravitational and magnetic forces. Nuclear forces are short-range with a range of 10 -14 -10 -15 m. They manifest themselves in the same way between proton and neutron, proton and proton, neutron and neutron. With an increase in the distance between nucleons, nuclear forces decrease very quickly and become practically equal to zero. Nuclear forces have the property of saturation, that is, each nucleon interacts only with a limited number of neighboring nucleons. Therefore, with an increase in the number of nucleons in the nucleus, nuclear forces significantly weaken. This explains the lower stability of the nuclei of heavy elements, which contain a significant number of protons and neutrons.

To divide the nucleus into its constituent protons and neutrons and remove them from the field of action of nuclear forces, it is necessary to perform work, i.e. expend energy. This energy is called the binding energy of the nucleus... On the contrary, when a nucleus is formed from nucleons, binding energy is released.

m i \u003d m p N p + m n N n,

where m i is the mass of the nucleus; m p is the proton mass; N p is the number of protons; m n is the neutron mass; N n is the number of neutrons, then it will be equal to 1.0076 · 2 + 1.0089 · 2 \u003d 4.033 amu.

At the same time, the actual mass of the helium nucleus is 4.003 amu. Thus, the actual mass of the helium nucleus turns out to be less than the calculated one by 0.03 amu. and in this case the nucleus is said to have a mass defect (lack of mass). The difference between the calculated and actual mass of the nucleus is called the mass defect (Dm). The mass defect shows how strongly the particles are bound in the nucleus, as well as how much energy was released during the formation of the nucleus from individual nucleons. You can connect mass with energy using the equation derived by A. Einstein:

where DE - energy change; Dm - mass defect; c is the speed of light.

Considering that 1 amu. \u003d 1,661 10 -27 kg, and in nuclear physics the electron-volt (eV) is taken as a unit of energy, and 1 amu. is equivalent to 931 MeV, then the energy released during the formation of a helium nucleus will be equal to 28 MeV. If there were a way to divide the nucleus of a helium atom into two protons and two neutrons, then this would require spending at least 28 MeV of energy.

The binding energy of nuclei increases commensurately with an increase in the number of nucleons, but not strictly proportional to their number. For example, the binding energy of the nitrogen nucleus is 104.56 MeV, and that of uranium is 1800 MeV.

The average binding energy per nucleon is called specific binding energy... For helium, it will be 28: 4 \u003d 7 MeV. Except for the lightest nuclei (deuterium, tritium), the binding energy per nucleon is about 8 MeV for all nuclei.

Most of the chemical elements in nature are certain mixtures of atoms with nuclei of different masses. The difference in masses is due to the presence in the nuclei different numbers neutrons.

Isotopes (from the Greek isos - the same and topos - place) - varieties of an atom of the same chemical element, which have the same number of protons (Z) and a different number of neutrons (N). They have practically the same physical and chemical properties; it is very difficult to separate them in a natural mixture. The number of isotopes of the elements varies from 3 for hydrogen to 27 for polonium. Isotopes are stable and unstable. Stable isotopes do not undergo any changes over time, if there is no external influence. Unstable or radioactive isotopes, due to the processes taking place inside the nucleus, over time are converted into isotopes of other chemical elements. Stable isotopes are found only in elements with a serial number Z≤83. At present, about 300 stable and more than 2000 radioactive isotopes are known. For all elements of the periodic system of D.I.Mendeleev, radioactive isotopes, called artificial, were synthesized.

The phenomenon of radioactivity

All chemical elements are stable only in a narrow range of the ratio of the number of protons to the number of neutrons in the nucleus. In light nuclei, there should be approximately equal proportions of protons and neutrons, i.e., the value of the ratio n: p is close to 1, for heavy nuclei this ratio decreases to 0.7. If there are too many neutrons or protons in the nucleus, then such nuclei become unstable (unstable) and undergo spontaneous radioactive transformations, as a result of which the composition of the nucleus changes and at the same time charged or neutral particles are emitted. The phenomenon of spontaneous radiation was called radioactivity, and substances emitting radiation were called radioactive.

Radioactivity (from Latin radio - radiate, radius - ray, aktivus - effective) - these are spontaneous transformations (decays) of atomic nuclei of some chemical elements into atomic nuclei of other elements with the emission of a special kind of radiation. Radioactivity leads to a change in the atomic number and mass number of the original chemical element.

The discovery of the phenomenon of radioactivity was facilitated by two major discoveries of the 19th century. In 1895, W. Roentgen discovered rays that arose when a high voltage current was passed between electrodes placed in a sealed glass tube from which air was evacuated. The beams were called X-rays. And in 1896 A. Becquerel discovered that uranium salts spontaneously emit invisible rays with great penetrating power, causing blackening of the photographic plate and the glow of certain substances. He called this radiation radioactive. In 1898 Pierre Curie and Maria Sklodowska-Curie discovered two new radioactive elements - polonium and radium, which emitted similar radiation, but their intensity was many times higher than that of uranium. In addition, radioactive substances have been found to continuously release energy in the form of heat.

Radioactive radiation is also called ionizing radiation, as it can ionize the environment, or nuclear, emphasizing that radiation is emitted by a nucleus, not an atom.

Radioactive decay is associated with changes in atomic nuclei and the release of energy, the value of which, as a rule, is several orders of magnitude higher than the energy chemical reactions... Thus, with the complete radioactive decay of 1 g-atom of 14 C, 3 is released. 10 9 calories, while the combustion of the same amount of 14 C to carbon dioxide is released only 9.4. 10 4 calories.

As a unit of energy of radioactive decay, 1 electron-volt (eV) and its derivatives 1 keV \u003d 10 3 eV and 1 MeV \u003d 10 6 eV are taken. 1 eV \u003d 1.6. 10 -19 J. 1 eV corresponds to the energy acquired by an electron in an electric field when passing a path on which the potential difference is 1 Volt. In the decay of most radioactive nuclei, the released energy ranges from several keV to several MeV.

Radioactive phenomena occurring in nature are called natural radioactivity; similar processes occurring in artificially obtained substances (through the corresponding nuclear reactions) - artificial radioactivity. However, both types of radioactivity obey the same laws.

Types of radioactive decay

Atomic nuclei are stable, but change their state when a certain ratio of protons and neutrons is violated. Light nuclei should contain approximately equal proportion of protons and neutrons. If there are too many protons or neutrons in the nucleus, then such nuclei are unstable and undergo spontaneous radioactive transformations, as a result of which the composition of the nucleus changes and, therefore, the nucleus of an atom of one element turns into the nucleus of an atom of another element. This process emits nuclear radiation.

There are the following main types of nuclear transformations or types of radioactive decay: alpha decay and beta decay (electronic, positron and K-capture), internal conversion.

Alpha decay -it is the emission of alpha particles by the nucleus of a radioactive isotope. Due to the loss of two protons and two neutrons with an alpha particle, the decaying nucleus turns into another nucleus, in which the number of protons (nuclear charge) decreases by 2, and the number of particles (mass number) by 4. Therefore, for a given radioactive decay, in accordance with the rule displacement (shift), formulated by Faience and Soddy (1913), the resulting (daughter) element is displaced to the left relative to the original (parent) by two cells to the left in the periodic system of D.I.Mendeleev. The alpha decay process is generally written as follows:

,

where X is the symbol of the original kernel; Y is the symbol of the nucleus of the decay product; 4 2 He - alpha particle, Q - released excess energy.

For example, the decay of radium-226 nuclei is accompanied by the emission of alpha particles, while radium-226 nuclei are converted into radon-222 nuclei:

The energy released during alpha decay is divided between the alpha particle and the nucleus in inverse proportion to their masses. The energy of alpha particles is strictly related to the half-life of a given radionuclide (Geiger-Nettol law) . This suggests that, knowing the energy of alpha particles, it is possible to establish the half-life, and by the half-life to identify the radionuclide. For example, the nucleus of polonium-214 is characterized by the values \u200b\u200bof the energy of alpha particles E \u003d 7.687 MeV and T 1/2 \u003d 4.5 × 10 -4 s, while for the uranium-238 nucleus E \u003d 4.196 MeV and T 1/2 \u003d 4, 5 × 10 9 years. In addition, it was found that the greater the alpha decay energy, the faster it proceeds.

Alpha decay is a fairly widespread nuclear transformation of heavy nuclei (uranium, thorium, polonium, plutonium, etc. with Z\u003e 82); more than 160 alpha emitting nuclei are currently known.

Beta decay -spontaneous transformations of a neutron into a proton or a proton into a neutron inside the nucleus, accompanied by the emission of electrons or positrons and antineutrino or neutrino ne.

If there is a surplus of neutrons in the nucleus (“neutron overload” of the nucleus), then electronic beta decay occurs, in which one of the neutrons turns into a proton, emitting an electron and an antineutrino:

With this decay, the charge of the nucleus and, accordingly, the atomic number of the daughter nucleus increases by 1, and the mass number does not change, that is, the daughter element is shifted in the periodic system of D.I.Mendeleev by one cell to the right of the initial one. The beta decay process is generally written as follows:

.

In this way, nuclei with an excess of neutrons decay. For example, the decay of strontium-90 nuclei is accompanied by the emission of electrons and their conversion into yttrium-90:

Often, the nuclei of the elements formed during beta decay have excess energy, which is released by the emission of one or more gamma quanta. For instance:

Electronic beta decay is characteristic of many naturally occurring and artificially obtained radioactive elements.

If the unfavorable ratio of neutrons and protons in the nucleus is due to an excess of protons, then positron beta decay occurs, in which the nucleus emits a positron and neutrino as a result of the transformation of a proton into a neutron inside the nucleus:

The charge of the nucleus and, accordingly, the atomic number of the child element decreases by 1, the mass number does not change. The child element will occupy a place in the periodic system of D.I.Mendeleev one cell to the left of the parent:

Positron decay is observed in some artificially obtained isotopes. For example, the decay of the isotope phosphorus-30 with the formation of silicon-30:

A positron, having escaped from the nucleus, strips off the “extra” electron (weakly bound to the nucleus) from the shell of the atom or interacts with a free electron, forming a “positron-electron” pair. Due to the fact that a particle and an antiparticle are instantly annihilated with the release of energy, the formed pair turns into two gamma quanta with an energy equivalent to the mass of the particles (e + and e -). The process of transformation of a pair "positron-electron" into two gamma quanta is called annihilation (destruction), and the resulting electromagnetic radiation called annihilation. In this case, there is a transformation of one form of matter (particles of matter) into another (radiation). This is confirmed by the existence of the reverse reaction - the reaction of pair formation, in which electromagnetic radiation of sufficiently high energy, passing near the nucleus under the action of the strong electric field of the atom, turns into an “electron-positron” pair.

Thus, during positron beta decay, in the final result, not particles fly out of the mother nucleus, but two gamma quanta, each having an energy of 0.511 MeV, equal to the energy equivalent of the rest masses of particles - a positron and an electron E \u003d 2m ec 2 \u003d 1.022 MeV ...

The transformation of the nucleus can be carried out by electron capture, when one of the protons of the nucleus spontaneously captures an electron from one of the inner shells of the atom (K, L, etc.), most often from the K-shell, and turns into a neutron. This process is also called K-capture. A proton turns into a neutron according to the following reaction:

In this case, the nuclear charge decreases by 1, and the mass number does not change:

For instance,

In this case, the place vacated by the electron is taken by the electron from the outer shells of the atom. As a result of the rearrangement of the electron shells, an X-ray quantum is emitted. The atom still retains electrical neutrality, since the number of protons in a nucleus during electron capture decreases by one. Thus, this type of decay leads to the same results as positron beta decay. It is typical, as a rule, for artificial radionuclides.

The energy released by the nucleus during the beta decay of a particular radionuclide is always constant, but since this type of decay produces not two, but three particles: a recoil nucleus (daughter), an electron (or positron) and a neutrino, then the energy is different in each decay act, it is redistributed between an electron (positron) and a neutrino, since the daughter nucleus always carries away the same portion of energy. Depending on the angle of expansion, a neutrino can carry away more or less energy, as a result of which an electron can receive any energy from zero to a certain maximum value. Consequently, in beta decay, beta particles of the same radionuclide have different energies, from zero to a certain maximum value characteristic of the decay of a given radionuclide. By the energy of beta radiation, it is almost impossible to identify the radionuclide.

Some radionuclides can decay simultaneously in two or three ways: through alpha and beta decays and through K-capture, a combination of three types of decays. In this case, the transformations are carried out in a strictly defined ratio. So, for example, the natural long-lived radioisotope potassium-40 (T 1/2 \u003d 1.49 × 10 9 years), the content of which in natural potassium is 0.0119%, undergoes electronic beta decay and K-capture:

(88% - electronic decay),

(12% - K-capture).

From the types of decays described above, we can conclude that gamma decay does not exist in “pure form”. Gamma radiation can only accompany various types of decays. When gamma radiation is emitted in the nucleus, neither the mass number nor its charge change. Consequently, the nature of the radionuclide does not change, but only the energy contained in the nucleus changes. Gamma radiation is emitted when nuclei pass from excited levels to more than low levels, including the main one. For example, when cesium-137 decays, an excited nucleus of barium-137 is formed. The transition from an excited to a stable state is accompanied by the emission of gamma quanta:

Since the lifetime of nuclei in excited states is very short (usually t<10 -19 с), то при альфа- и бета-распадах гамма-квант вылетает практически одновременно с заряженной частицей. Исходя из этого, процесс гамма-излучения не выделяют в самостоятельный вид распада. By the energy of gamma radiation, as well as by the energy of alpha radiation, it is possible to identify the radionuclide.

Internal conversion.Excited (as a result of one or another nuclear transformation) state of the atomic nucleus indicates the presence of an excess of energy in it. An excited nucleus can go to a state with a lower energy (normal state) not only by emitting a gamma quantum or ejecting a particle, but also by internal conversion, or conversion with the formation of electron-positron pairs.

The phenomenon of internal conversion consists in the fact that the nucleus transfers the excitation energy to one of the electrons of the inner layers (K-, L- or M-layer), which, as a result, is pulled out of the atom. Such electrons are called conversion electrons. Consequently, the emission of conversion electrons is due to the direct electromagnetic interaction of the nucleus with the shell electrons. Conversion electrons have a linear energy spectrum, in contrast to beta decay electrons, which give a continuous spectrum.

If the excitation energy exceeds 1.022 MeV, then the transition of the nucleus to the normal state can be accompanied by the emission of a pair "electron-positron" with their subsequent annihilation. After the internal conversion has taken place, a “vacant” place of the torn out conversion electron appears in the electron shell of the atom. One of the electrons from more distant layers (from higher energy levels) performs a quantum transition to a "vacant" place with the emission of characteristic X-rays.

Properties of nuclear radiation

Nuclear (radioactive) radiation is radiation that is generated as a result of radioactive decay. The radiation of all natural and artificial radionuclides is divided into two types - corpuscular and electromagnetic. Corpuscular radiation is a stream of particles (corpuscles), which are characterized by a certain mass, charge and speed. These are electrons, positrons, nuclei of helium atoms, deuterons (nuclei of the hydrogen isotope of deuterium), neutrons, protons and other particles. As a rule, corpuscular radiation directly ionizes the medium.

Electromagnetic radiation is a flux of quanta or photons. This radiation has neither mass nor charge and produces indirect ionization of the medium.

The formation of 1 pair of ions in air requires an average of 34 eV. Therefore, ionizing radiation includes radiation with an energy of 100 and above eV (does not include visible light and UV radiation).

To characterize ionizing radiation, the concepts of mileage and specific ionization are used. Range - the minimum thickness of the absorber (some substance) required for the complete absorption of ionizing radiation. Specific ionization is the number of ion pairs formed per unit path length in a substance under the influence of ionizing radiation. Note that the concept of mileage and distance traveled are not identical concepts. If the particles move in a straight line, then these values \u200b\u200bcoincide, if the trajectory of the particles is a broken winding line, then the range is always less than the length of the path traveled.

Alpha radiation is a stream of a-particles, which are the nuclei of helium atoms sometimes called doubly ionized helium atoms). An alpha particle consists of 2 protons and 2 neutrons, is positively charged and carries with it two elementary positive charges. Particle mass m a \u003d 4.003 amu. Is the largest of the particles. The speed of movement is (14.1-24.9) × 10 6 m / s. In matter, alpha particles move in a straight line, which is associated with a relatively large mass and significant energy. Deflection occurs only in a head-on collision with the nuclei.

The range of alpha particles in a substance depends on the energy of the alpha particle and on the nature of the substance in which it moves. On average, the range of an alpha particle in air is 2.5–9 cm, the maximum is up to 11 cm, in biological tissues - 5–100 microns, in glass - 4. 10 -3 cm. The energy of the alpha particle is in the range of 4-9 MeV. You can completely stop the alpha radiation with a sheet of paper. Over its entire path, an alpha particle can create 116,000 to 254,000 ion pairs.

Specific ionization is approximately 40,000 ion pairs / cm in air, the same specific ionization in the body is on the way of 1-2 microns.

After the energy is expended, the alpha particle is decelerated, the ionization process stops. The laws governing the process of formation of atoms come into force. The nuclei of helium atoms attach 2 electrons and a full-fledged helium atom is formed. This explains the fact of the obligatory presence of helium in rocks containing radioactive substances.

Of all types of radioactive radiation, alpha radiation is the most strongly fluorescent (glows).

Beta radiation Is a stream of beta particles, which are electrons or positrons. One elementary electric charge is carried, m b \u003d 0.000548 amu. They move at speeds close to the speed of light, i.e. (0.87-2.994) × 10 8 m / s.

Unlike a-particles, b-particles of one and the same radioactive element have different energy reserves (from zero to a certain maximum value). This is due to the fact that with each beta decay, two particles are simultaneously emitted from the atomic nucleus: a b-particle and a neutrino (n e). The energy released in each decay act is distributed between the b-particle and the neutrino in different proportions. Therefore, the energy of beta particles ranges from tenths and hundredths of MeV (soft b-radiation) to 2-3 MeV (hard radiation).

Due to the fact that beta particles emitted by one and the same beta emitter have different energy reserves (from minimum to maximum), both the path length and the number of ion pairs are not the same for beta particles of a given radionuclide. Typically, the range in air is tens of cm, sometimes several meters (up to 34 m), in biological tissues - up to 1 cm (up to 4 cm with an energy of beta particles of 8 MeV).

Beta radiation has significantly less ionizing effect than alpha radiation. Thus, in the air, beta particles form from 1000 to 25500 ion pairs along their entire path. On average, for the entire path in air, or 50-100 pairs of ions per 1 cm of path. The degree of ionization depends on the speed of the particle, the lower the speed, the more ionization. The reason for this is that high-energy beta particles fly past atoms too quickly and do not have time to cause the same strong effect as slow beta particles.

Since beta particles have a very low mass, they easily deviate from their original direction when they collide with atoms and molecules. This deflection phenomenon is called scattering. Therefore, it is very difficult to determine the length of the path of beta particles, and not the range, since it is too tortuous.

When energy is lost, an electron is captured either by a positive ion to form a neutral atom, or by an atom to form a negative ion.

Gamma radiation Is a flux of photons (quanta) of electromagnetic radiation. Their speed of propagation in a vacuum is equal to the speed of light - 3 × 10 8 m / s. Since gamma radiation is wavelength, it is characterized by wavelength, vibration frequency and energy. The energy of the g-quantum is proportional to the frequency of vibrations, and the frequency of vibrations is related to their wavelength. The longer the wavelength, the lower the vibration frequency, and vice versa, i.e., the vibration frequency is inversely proportional to the wavelength. The shorter the wavelength and the greater the vibration frequency of the radiation, the greater its energy and, consequently, the penetrating ability. The energy of gamma radiation of natural radioactive elements ranges from several keV to 2-3 MeV and rarely reaches 5-6 MeV.

Gamma quanta, having no charge and rest mass, cause a weak ionizing effect, but they have a great penetrating ability. In the air, they can travel up to 100-150 m. This radiation passes through the human body without attenuation.

Measurements

Dose concept

The result of exposure to ionizing radiation on irradiated objects is physicochemical or biological changes in these objects. Examples of such changes are body heating, photochemical reaction of X-ray films, changes in biological parameters of a living organism, etc. The radiation effect depends on physical quantities X icharacterizing the radiation field or the interaction of radiation with matter:

The quantities X ifunctionally related to the radiation effect η , are called dosimetric. The purpose of dosimetry is to measure, study and theoretically calculate dosimetric quantities to predict or assess the radiation effect, in particular, the radiobiological effect.

The system of dosimetric quantities is being formed as a result of the development of radiobiology, dosimetry and radiation safety. The safety criteria are largely determined by society, therefore, different systems of dosimetric quantities have been formed in different countries. An important role in the unification of these systems is played by the International Commission on Radiological Protection (ICRP) - an independent organization uniting experts in the field of biological effects of radiation, dosimetry and