The concept of radioactivity

Law of radioactive decay

Quantification of radioactivity and its units

Ionizing radiation, their characteristics.

Sources of AI

1. The concept of radioactivity

Radioactivity is the spontaneous process of transformation (decay) of atomic nuclei, accompanied by the emission of a special type of radiation called radioactive.

In this case, the transformation of atoms of one element into atoms of others occurs.

Radioactive transformations are characteristic only of individual substances.

A substance is considered radioactive if it contains radionuclides and undergoes a process of radioactive decay.

Radionuclides (isotopes) - the nuclei of atoms capable of spontaneous decay are called radionuclides.

As a characteristic of a nuclide, the symbol of a chemical element is used, the atomic number (number of protons) and mass number of the nucleus (number of nucleons, i.e. total number protons and neutrons).

For example, 239 94 Pu means that the nucleus of a plutonium atom contains 94 protons and 145 neutrons, for a total of 239 nucleons.

There are the following types of radioactive decay:

beta decay;

Alpha decay;

Spontaneous fission of atomic nuclei (neutron decay);

Proton radioactivity (proton fusion);

Two-proton and cluster radioactivity.

beta decay - this is the process of transformation in the nucleus of an atom of a proton into a neutron or a neutron into a proton with the release of a beta particle (positron or electron)

Alpha decay - characteristic of heavy elements, the nuclei of which, starting from the number 82 of the table of D.I. Mendeleev, are unstable, despite the excess of neutrons and spontaneously decay. The nuclei of these elements predominantly eject the nuclei of helium atoms.

Spontaneous fission of atomic nuclei (neutron decay) - this is the spontaneous fission of some nuclei of heavy elements (uranium-238, californium 240.248, 249, 250, curium 244, 248, etc.). The probability of spontaneous nuclear fission is negligible compared to alpha decay. In this case, the nucleus is divided into two fragments (nuclei) that are close in mass.

1. Law of radioactive decay

The stability of nuclei decreases as the total number of nucleons increases. It also depends on the ratio of the number of neutrons and protons.

The process of successive nuclear transformations, as a rule, ends with the formation of stable nuclei.

Radioactive transformations obey the law of radioactive decay:

N = N 0 e λ t ,

where N, N 0 is the number of atoms that have not decayed at times t and t 0 ;

λ is the radioactive decay constant.

The value λ has its own individual value for each type of radionuclide. It characterizes the decay rate, i.e. shows how many nuclei decay per unit time.

According to the equation of the law of radioactive decay, its curve is an exponential.

1. Quantification of radioactivity and its units

The time during which, due to spontaneous nuclear transformations, half of the nuclei decay, is called half-life T 1/2 . The half-life T 1/2 is associated with the decay constant λ dependence:

T 1/2 \u003d ln2 / λ \u003d 0.693 / λ.

The half-life T 1/2 for different radionuclides is different and varies widely - from fractions of a second to hundreds and even thousands of years.

Half-lives of some radionuclides:

Iodine-131 - 8.04 days

Cesium-134 - 2.06 years

Strontium-90 - 29.12 years

Cesium-137 - 30 years

Plutonium-239 - 24065 years

Uranium-235 - 7.038. 10 8 years

Potassium-40 - 1.4 10 9 years.

The reciprocal of the decay constant, calledaverage lifetime of a radioactive atom t :

The decay rate is determined by the activity of substance A:

A \u003d dN / dt \u003d A 0 e λ t \u003d λ N,

where A and A 0 are the activities of the substance at times t and t 0 .

Activity is a measure of radioactivity. It is characterized by the number of decays of radioactive nuclei per unit of time.

The activity of a radionuclide is directly proportional to the total number of radioactive atomic nuclei at time t and inversely proportional to the half-life:

A \u003d 0.693 N / T 1/2.

In the SI system, the becquerel (Bq) is taken as the unit of activity. One becquerel is equal to one disintegration per second. The off-system unit of activity is the curie (Ku).

1 Ku \u003d 3.7 10 10 Bq

1Bq = 2.7 10 -11 Ku.

The unit of curie activity corresponds to the activity of 1 g of radium. In the practice of measurements, the concepts of volume A v (Bq / m 3, Ku / m 3), surface A s (Bq / m 2, Ku / m 2), specific A m (Bq / m, Ku / m) activity are also used.

1. Radioactivity. Basic law of radioactive decay. Activity.

2. The main types of radioactive decay.

3. Quantitative characteristics of the interaction of ionizing radiation with matter.

4. Natural and artificial radioactivity. radioactive rows.

5. Use of radionuclides in medicine.

6. Charged particle accelerators and their use in medicine.

7. Biophysical foundations of the action of ionizing radiation.

8. Basic concepts and formulas.

9. Tasks.

The interest of physicians in natural and artificial radioactivity is due to the following.

Firstly, all living things are constantly exposed to the natural radiation background, which is cosmic radiation, the radiation of radioactive elements that occur in the surface layers of the earth's crust, and the radiation of elements that enter the body of animals along with air and food.

Secondly, radioactive radiation is used in medicine itself for diagnostic and therapeutic purposes.

33.1. Radioactivity. Basic law of radioactive decay. Activity

The phenomenon of radioactivity was discovered in 1896 by A. Becquerel, who observed the spontaneous emission of unknown radiation from uranium salts. Soon, E. Rutherford and the Curies found that during radioactive decay, He nuclei (α-particles), electrons (β-particles) and hard electromagnetic radiation (γ-rays) are emitted.

In 1934, decay with the emission of positrons (β + -decay) was discovered, and in 1940 new type radioactivity - spontaneous fission of nuclei: a fissile nucleus falls apart into two fragments of comparable mass with the simultaneous emission of neutrons and γ -quanta. Proton radioactivity of nuclei was observed in 1982.

Radioactivity - the ability of some atomic nuclei to spontaneously (spontaneously) transform into other nuclei with the emission of particles.

Atomic nuclei are composed of protons and neutrons, which have a general name - nucleons. The number of protons in the nucleus determines Chemical properties atom and is denoted by Z (this serial number chemical element). The number of nucleons in a nucleus is called mass number and denote A. Nuclei with the same serial number and different mass numbers are called isotopes. All isotopes of one chemical element have the same Chemical properties. Physical Properties isotopes can vary greatly. To designate isotopes, the symbol of a chemical element is used with two indices: A Z X. The lower index is the serial number, the upper one is the mass number. Often the subscript is omitted because the element symbol itself points to it. For example, they write 14 C instead of 14 6 C.

The ability of a nucleus to decay depends on its composition. The same element can have both stable and radioactive isotopes. For example, the 12C carbon isotope is stable, while the 14C isotope is radioactive.

Radioactive decay is a statistical phenomenon. The ability of an isotope to decay characterizes decay constantλ.

decay constant is the probability that the nucleus of a given isotope will decay per unit time.

The probability of nuclear decay in a short time dt is found by the formula

Taking into account formula (33.1), we obtain an expression that determines the number of decayed nuclei:

Formula (33.3) is called the main the law of radioactive decay.

The number of radioactive nuclei decreases with time according to an exponential law.

In practice, instead of decay constantλ often use another value called half-life.

Half life(T) - the time during which it decays half radioactive nuclei.

The law of radioactive decay using the half-life is written as follows:

Dependence graph (33.4) is shown in fig. 33.1.

The half-life can be either very long or very short (from fractions of a second to many billions of years). In table. 33.1 shows the half-lives for some elements.

Rice. 33.1. The decrease in the number of nuclei of the original substance during radioactive decay

Table 33.1. Half-lives for some elements

For rate degree of radioactivity isotopes use a special quantity called activity.

Activity - the number of nuclei of a radioactive preparation decaying per unit of time:

Unit of measure of activity in SI - becquerel(Bq), 1 Bq corresponds to one decay event per second. In practice, more

resourceful off-system unit of activity - curie(Ci) equal to the activity of 1 g of 226 Ra: 1 Ci = 3.7x10 10 Bq.

Over time, activity decreases in the same way as the number of undecayed nuclei decreases:

33.2. Main types of radioactive decay

In the process of studying the phenomenon of radioactivity, 3 types of rays emitted by radioactive nuclei were discovered, which were called α-, β- and γ-rays. Later it was found that α- and β-particles are products of two various kinds radioactive decay, and γ-rays are a by-product of these processes. In addition, γ-rays also accompany more complex nuclear transformations, which are not considered here.

Alpha decay consists in the spontaneous transformation of nuclei with emissionα -particles (helium nuclei).

The α-decay scheme is written as

where X, Y are the symbols of the parent and child nuclei, respectively. When writing α-decay, instead of "α" you can write "Not".

In this decay, the atomic number Z of the element decreases by 2, and the mass number A - by 4.

During α-decay, the daughter nucleus, as a rule, is formed in an excited state and, upon transition to the ground state, emits a γ-quantum. A common property of complex micro-objects is that they have discrete set of energy states. This also applies to cores. Therefore, the γ-radiation of excited nuclei has a discrete spectrum. Consequently, the energy spectrum of α-particles is also discrete.

The energy of emitted α-particles for almost all α-active isotopes lies within 4-9 MeV.

beta decay consists in the spontaneous transformation of nuclei with the emission of electrons (or positrons).

It has been established that β-decay is always accompanied by the emission of a neutral particle - a neutrino (or antineutrino). This particle practically does not interact with matter, and will not be considered further. The energy released during β-decay is distributed between the β-particle and the neutrino randomly. Therefore, the energy spectrum of β-radiation is continuous (Fig. 33.2).

Rice. 33.2. Energy spectrum of β-decay

There are two types of β-decay.

1. Electronicβ - -decay consists in the transformation of one nuclear neutron into a proton and an electron. In this case, another particle ν" appears - an antineutrino:

An electron and an antineutrino fly out of the nucleus. The scheme of electronic β - decay is written as

During electronic β-decay, the serial number of the Z-element increases by 1, the mass number A does not change.

The energy of β-particles lies in the range of 0.002-2.3 MeV.

2. Positronβ + -decay consists in the transformation of one nuclear proton into a neutron and a positron. In this case, another particle ν appears - a neutrino:

Electron capture itself does not generate ionizing particles, but it does accompanied by x-rays. This radiation occurs when the space vacated by the absorption of an inner electron is filled by an electron from an outer orbit.

Gamma radiation has an electromagnetic nature and is a photon with a wavelengthλ ≤ 10 -10 m.

Gamma radiation is not independent view radioactive decay. Radiation of this type almost always accompanies not only α-decay and β-decay, but also more complex nuclear reactions. It is not deflected by electric and magnetic fields, has a relatively weak ionizing and very high penetrating power.

33.3. Quantitative characteristics of the interaction of ionizing radiation with matter

The impact of radioactive radiation on living organisms is associated with ionization, which it induces in the tissues. The ability of a particle to ionize depends both on its type and on its energy. As the particle moves deeper into the substance, it loses its energy. This process is called ionization braking.

To quantitatively characterize the interaction of a charged particle with matter, several quantities are used:

After the energy of the particle falls below the ionization energy, its ionizing effect ceases.

Average linear mileage(R) of a charged ionizing particle - the path traveled by it in a substance before losing its ionizing ability.

Let us consider some characteristic features of the interaction of various types of radiation with matter.

alpha radiation

The alpha particle practically does not deviate from the initial direction of its movement, since its mass is many times greater

Rice. 33.3. Dependence of the linear ionization density on the path traveled by an α-particle in a medium

the mass of the electron with which it interacts. As it penetrates deep into the substance, the ionization density first increases, and when end of run (x = R) drops sharply to zero (Fig. 33.3). This is explained by the fact that with a decrease in the speed of movement, the time that it spends near the molecule (atom) of the medium increases. In this case, the probability of ionization increases. After the energy of the α-particle becomes comparable with the energy of molecular thermal motion, it captures two electrons in the substance and turns into a helium atom.

The electrons generated during the ionization process, as a rule, move away from the track of the α-particle and cause secondary ionization.

Characteristics of the interaction of α-particles with water and soft tissues are presented in Table. 33.2.

Table 33.2. Dependence of the characteristics of interaction with matter on the energy of α-particles

beta radiation

For movement β -particles in matter are characterized by a curvilinear unpredictable trajectory. This is due to the equality of the masses of the interacting particles.

Characteristics of interaction β -particles with water and soft tissues are presented in Table. 33.3.

Table 33.3. Dependence of the characteristics of interaction with matter on the energy of β-particles

As with α particles, the ionization power of β particles increases with decreasing energy.

Gamma radiation

Absorption γ -radiation by a substance obeys an exponential law similar to the law of absorption of x-rays:

The main processes responsible for absorption γ -radiation are the photoelectric effect and Compton scattering. This produces a relatively small amount of free electrons (primary ionization), which have a very high energy. It is they who cause the processes of secondary ionization, which is incomparably higher than the primary one.

33.4. natural and artificial

radioactivity. radioactive ranks

Terms natural and artificial radioactivity are conditional.

Natural call the radioactivity of isotopes that exist in nature, or the radioactivity of isotopes formed as a result of natural processes.

For example, the radioactivity of uranium is natural. The radioactivity of carbon 14 C, which is formed in the upper layers of the atmosphere under the influence of solar radiation, is also natural.

Artificial called the radioactivity of isotopes that arise as a result of human activities.

This is the radioactivity of all isotopes produced in particle accelerators. This also includes the radioactivity of soil, water and air, which occurs during an atomic explosion.

natural radioactivity

AT initial period In studying radioactivity, researchers could only use natural radionuclides (radioactive isotopes) contained in terrestrial rocks in a sufficiently large amount: 232 Th, 235 U, 238 U. Three radioactive series begin with these radionuclides, ending with stable Pb isotopes. Subsequently, a series starting from 237 Np was discovered, with a final stable nucleus 209 Bi. On fig. 33.4 shows a row starting with 238 U.

Rice. 33.4. Uranium-radium series

Elements of this series are the main source of internal human exposure. For example, 210 Pb and 210 Po enter the body with food - they are concentrated in fish and shellfish. Both of these isotopes accumulate in lichens and are therefore present in reindeer meat. The most significant of all natural sources of radiation is 222 Rn - a heavy inert gas resulting from the decay of 226 Ra. It accounts for about half of the dose of natural radiation received by humans. Formed in earth's crust, this gas seeps into the atmosphere and enters the water (it is highly soluble).

The radioactive isotope of potassium 40 K is constantly present in the earth's crust, which is part of natural potassium (0.0119%). From the soil, this element comes through root system plants and with plant foods (cereals, fresh vegetables and fruits, mushrooms) - into the body.

Another source of natural radiation is cosmic radiation (15%). Its intensity increases in mountainous areas due to a decrease in the protective effect of the atmosphere. Sources of natural background radiation are listed in Table. 33.4.

Table 33.4. Component of the natural radioactive background

33.5. The use of radionuclides in medicine

radionuclides called radioactive isotopes of chemical elements with a short half-life. Such isotopes do not exist in nature, so they are obtained artificially. AT modern medicine radionuclides are widely used for diagnostic and therapeutic purposes.

Diagnostic Application is based on the selective accumulation of certain chemical elements by individual organs. Iodine, for example, is concentrated in the thyroid gland, while calcium is concentrated in the bones.

The introduction of radioisotopes of these elements into the body makes it possible to detect areas of their concentration by radioactive radiation and thus obtain important diagnostic information. This diagnostic method is called by the labeled atom method.

Therapeutic use radionuclides is based on the destructive effect of ionizing radiation on tumor cells.

1. Gamma Therapy- the use of high-energy γ-radiation (source 60 Co) for the destruction of deeply located tumors. So that superficially located tissues and organs are not subjected to a destructive effect, the effect of ionizing radiation is carried out in different sessions in different directions.

2. alpha therapy- therapeutic use of α-particles. These particles have a significant linear ionization density and are absorbed even by a small layer of air. Therefore, therapeutic

the use of alpha rays is possible with direct contact with the surface of the organ or with the introduction inside (with a needle). For superficial exposure, radon therapy (222 Rn) is used: exposure to the skin (baths), digestive organs (drinking), respiratory organs (inhalations).

In some cases, medicinal use α -particles is associated with the use of neutron flux. With this method, elements are first introduced into the tissue (tumor), the nuclei of which, under the action of neutrons, emit α -particles. After that, the diseased organ is irradiated with a neutron flux. In this manner α -particles are formed directly inside the organ, on which they should have a destructive effect.

Table 33.5 lists the characteristics of some radionuclides used in medicine.

Table 33.5. Isotope characterization

33.6. Particle accelerators and their use in medicine

Accelerator- an installation in which, under the action of electric and magnetic fields, directed beams of charged particles with high energy (from hundreds of keV to hundreds of GeV) are obtained.

Accelerators create narrow beams of particles with a given energy and a small cross section. This allows you to provide directed impact on irradiated objects.

The use of accelerators in medicine

Electron and proton accelerators are used in medicine for radiation therapy and diagnostics. In this case, both the accelerated particles themselves and the accompanying X-ray radiation are used.

Bremsstrahlung X-ray obtained by directing a particle beam to a special target, which is the source x-rays. This radiation differs from the X-ray tube by a much higher photon energy.

Synchrotron X-rays occurs in the process of accelerating electrons in ring accelerators - synchrotrons. Such radiation has a high degree orientation.

The direct action of fast particles is associated with their high penetrating power. Such particles pass through surface tissues without causing serious damage, and have an ionizing effect at the end of their journey. By selecting the appropriate particle energy, it is possible to achieve the destruction of tumors at a given depth.

The areas of application of accelerators in medicine are shown in Table. 33.6.

Table 33.6. Application of accelerators in therapy and diagnostics

33.7. Biophysical foundations of the action of ionizing radiation

As noted above, the impact of radioactive radiation on biological systems is associated with ionization of molecules. The process of interaction of radiation with cells can be divided into three successive stages (stages).

1. physical stage consists of energy transfer radiation to the molecules of a biological system, resulting in their ionization and excitation. The duration of this stage is 10 -16 -10 -13 s.

2. Physico-chemical the stage consists of various kinds of reactions leading to a redistribution of the excess energy of excited molecules and ions. As a result, highly active

products: radicals and new ions with a wide range of chemical properties.

The duration of this stage is 10 -13 -10 -10 s.

3. Chemical stage - this is the interaction of radicals and ions with each other and with surrounding molecules. At this stage, structural damage of various types is formed, leading to a change in biological properties: the structure and functions of membranes are disturbed; lesions occur in DNA and RNA molecules.

The duration of the chemical stage is 10 -6 -10 -3 s.

4. biological stage. At this stage, damage to molecules and subcellular structures leads to a variety of functional disorders, to premature cell death as a result of the action of apoptosis mechanisms or due to necrosis. Damage received at the biological stage can be inherited.

The duration of the biological stage is from several minutes to tens of years.

We note the general patterns of the biological stage:

Large violations with low absorbed energy (a lethal dose of radiation for a person causes heating of the body by only 0.001 ° C);

Action on subsequent generations through the hereditary apparatus of the cell;

A latent, latent period is characteristic;

Different parts of cells have different sensitivity to radiation;

First of all, dividing cells are affected, which is especially dangerous for a child's body;

The destructive effect on the tissues of an adult organism, in which there is a division;

The similarity of radiation changes with the pathology of early aging.

33.8. Basic concepts and formulas

Table continuation

33.9. Tasks

1. What is the activity of the drug if 10,000 nuclei of this substance decay within 10 minutes?

4. The age of ancient wood samples can be approximately determined by the specific mass activity of the 14 6 C isotope in them. How many years ago was a tree cut down that was used to make an object, if the specific mass activity of carbon in it is 75% of the specific mass activity of a growing tree? The half-life of radon is T = 5570 years.

9. After Chernobyl accident in some places, soil contamination with radioactive caesium-137 was at the level of 45 Ci/km 2 .

After how many years the activity in these places will decrease to a relatively safe level of 5 Ci/km 2 . The half-life of cesium-137 is T = 30 years.

10. The permissible activity of iodine-131 in the human thyroid gland should be no more than 5 nCi. In some people who were in the area of ​​the Chernobyl disaster, the activity of iodine-131 reached 800 nCi. After how many days did activity decrease to normal? The half-life of iodine-131 is 8 days.

11. The following method is used to determine the volume of blood in an animal. A small volume of blood is taken from the animal, the erythrocytes are separated from the plasma and placed in a solution with radioactive phosphorus, which is assimilated by the erythrocytes. Labeled erythrocytes are reintroduced into the circulatory system of the animal, and after some time the activity of the blood sample is determined.

ΔV = 1 ml of this solution was injected into the blood of some animal. The initial activity of this volume was A 0 = 7000 Bq. The activity of 1 ml of blood taken from the vein of the animal a day later was equal to 38 pulses per minute. Determine the volume of the animal's blood if the half-life of radioactive phosphorus is T = 14.3 days.

Lecture 16

Elements of physics atomic nucleus

Questions

1. Law of radioactive decay.

Nuclear reactions and their main types.

patterns ,  and decays.

Doses of radiation.

Fission chain reaction.

6. Fusion reactions (thermonuclear reactions).

1. Law of radioactive decay

Under radioactive decay understand the natural radioactive transformation of nuclei, which occurs spontaneously.

An atomic nucleus undergoing decay is called maternal, the emerging core is child.

The theory of radioactive decay obeys the laws of statistics. Number of cores d N, disintegrated over a period of time from t before t+ d t, proportional to time interval d t and number N undecayed nuclei by the time t:

d N = – λ N d t , (1)

λ  constant radioactive decay, s  1 ; the minus sign indicates that the total number of radioactive nuclei decreases during the decay process.

(2)

where N 0 - initial number undecayed nuclei at a time t = 0;N number undecayed nuclei at a time t.

Law of radioactive decay: the number of undecayed nuclei decreases with time according to an exponential law.

The intensity of the decay process is characterized by two quantities:

half lifeT 1/2  the time during which the initial number of radioactive nuclei is halved;

average lifetime τ of a radioactive nucleus.

. (3)

half-life, T 1 /2
				4.510 9 years

Total lifespan d N cores is t|dN| = λ Nt d t. By integrating this expression over t(i.e. from 0 to ∞) and divide by the initial number of cores N 0 , we obtain the average lifetime τ of a radioactive nucleus:

. (4)

Table integral:

Thus, the average lifetime τ of a radioactive nucleus is the reciprocal of the radioactive decay constant λ.

ActivityBUT nuclide in a radioactive source is the number of decays that occur with the nuclei of matter in 1 s:

Bq - becquerel, (5)

1Bq is the activity of the nuclide, at which one act of decay occurs in 1 s.

The off-system unit is curie [Ci]: 1[Ci] = 3.710 10 [Bq].

Radioactive decay occurs in accordance with the so-called displacement rules (they are a consequence of the laws of conservation of charge and mass number), which make it possible to establish which nucleus arises as a result of the decay of a given parent nucleus.

Displacement rule for α-decay:
. (6)

Shift rule for β-decay:
, (7)

where
- maternal nucleus; Y child nucleus symbol;
 helium nucleus (α-particle);  symbolic designation of an electron (its charge is  e, and the mass number is zero).

The nuclei resulting from radioactive decay can be, in turn, radioactive. This leads to a chain or series of radioactive transformations. , ending with a stable element. The final nuclides are:
,
,
,
.

1. Nuclear reactions and their main types

nuclear reaction – this 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– quantums .

, , (8)

X, Y initial and final nuclei; FROM intermediate compound core; a, b bombarding and emitted particles.

The first nuclear reaction was carried out by E. Rutherford in 1919

(9)

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 (i.e., the number of nucleons - protons and neutrons).

Classification of nuclear reactions

according to the type of particles involved :

under the influence of neutrons ;

under the action of charged particles (protons,  particles, etc.);

under the influence of quanta.

2. by the energy of the particles that cause them :

low energies  1 eV (with neutrons);

average energies  1 MeV (with quanta, particles);

high energies  10 3 MeV (birth of new elementary particles);

3. According to the type of nuclei involved in them:

on light nuclei (A<50);

on medium cores (50<А<100);

on heavy nuclei (A>100);

4. by the nature of nuclear transformations :

with the emission of neutrons;

with the emission of charged particles;

capture reactions (radiated quantum).

3. Regularities of ,  and decays

decay: active are the nuclei of mainly heavy elements ( BUT> 200, Z > 82), for example:

(10)

 particle is formed when two protons and two neutrons meet, has a speed of 1.410 7 …210 7 m/s, which corresponds to energies of 4.0…8.8 MeV.

Geiger-Nattall law:
, (11)

R  run, the distance traveled by a particle in a substance to a complete stop;
.The shorter the half-life of a radioactive element, the greater the range, and hence the energy particles.

 particle with an energy of 4.2 MeV is surrounded by a potential barrier of Coulomb forces of 8.8 MeV. Its departure is explained in quantum mechanics by the tunnel effect.

 decay: electron is born as a result of processes occurring inside the nucleus. Because the number of nucleons does not change, but Z increases by 1, then one of the neutrons turns into a proton with the formation of an electron and emission antineutrino:

(12)

The theory of  decay with the emission of neutrinos was proposed by Pauli in 1931 and experimentally confirmed in 1956. It has a high penetrating power: a neutrino with an energy of 1 MeV in lead runs a path of 10 18 m!

decay: is not independent, but accompanies  and  decays.  the spectrum is discrete, it is characterized not by wave, but by corpuscular properties.  quanta, having zero rest mass, not possessing a charge, cannot slow down in the medium, but can either be absorbed, or dissipate. The large penetrating power of  radiation is used in  flaw detection.

N=N 0 e - λt is the law of radioactive decay, where N is the number of undecayed nuclei, N 0 is the number of initial nuclei.

physical meaning decay constant - the probability of nuclear decay per unit time. The characteristic lifetimes for radioactive nuclei are τ> 10 -14 s. The lifetimes of nuclei due to the emission of nucleons 10 -23 s< <10 -20 c. T 1/2 – период полураспада – время, за которое распадается половина начального количества ядер. Активность радиоактивного источника – число распадов в единицу времени: A=λN.

Types of radioactive decay. α - decay, decay scheme, decay patterns.

Radioactive decay is the process of transformation of unstable atomic nuclei into the nuclei of other elements, which is accompanied by the emission of particles.

Types of radioactive decay:

1)α - decay - is accompanied by the emission of helium atoms.

2)β - decay - emission of electrons and positrons.

3)γ - decay - the emission of photons during transitions between states of nuclei.

4) Spontaneous nuclear fission.

5) Nucleon radioactivity.

α - decay: A 2 X→ A-Y Z-2 Y+ 4 2 He. Α-decay is observed in heavy nuclei. The spectrum of α - decay is discrete. Run length α - particles in the air: 3-7 cm; for dense substances: 10 -5 m. T 1/2 10 -7 s ÷ 10 10 years.

β - decay. Schemes β + , β - and K-capture. Regularities of β - decay.

β - decay is due to weak interaction. It is weak in relation to strong nuclei. All particles except photons participate in weak interactions. The point is the degeneration of new particles. T 1/2 10 -2 s ÷ 10 20 years. The free path of the neutron is 10 19 km.

β - decay includes 3 types of decay:

1) β - or electronic. The nucleus emits electrons. In general:

A 2 X→ A Z -1 Y+ 0 -1 e+υ e .

2)β + or positron. Electron antiparticles are emitted – positrons: 1 1 p→ 1 0 n+ 0 1 e+υ e – reaction of transformation of a proton into a neutron. The reaction does not go away on its own. General view of the reaction: A Z X→ A Z -1 Y+ 0 1 e+υ e . Observed in artificial radioactive nuclei.

3) Electronic capture. There is a transformation of the nucleus, captures the K-shell and turns into a neutron: 1 1 p+ 0 -1 e→ 1 0 n+υ e . General view: A Z X+ 0 1 e→ A Z -1 Y+υ e . As a result of electrical capture, only one particle flies out of the nuclei. Accompanied by characteristic x-ray radiation.

Activity BUT nuclide(general name for atomic nuclei that differ in the number of protons Z and neutrons N) in a radioactive source is the number of decays that occur with the nuclei of the sample in 1 s:

SI unit of activity - becquerel(Bq): 1 Bq is the activity of the nuclide, at which one act of decay occurs in 1 s. Until now, in nuclear physics, an off-system unit of nuclide activity in a radioactive source is also used - curie(Ci): 1 Ci = 3.710 10 Bq.

Radioactive decay occurs according to the so-called displacement rules, allowing to establish which nucleus arises as a result of the decay of a given parent nucleus. Offset Rules:

where X is the parent nucleus, Y is the symbol of the daughter nucleus, He is the helium nucleus (  -particle), e- symbolic designation of an electron (its charge is -1, and its mass number is zero). The displacement rules are nothing more than a consequence of two laws that hold during radioactive decays - conservation electric charge and mass number conservation: the sum of the charges (mass numbers) of emerging nuclei and particles is equal to the charge (mass number) of the original nucleus.

28. Main regularities of a-decay. tunnel effect. Properties of a-radiation.

α-decay called the spontaneous decay of the atomic nucleus into a daughter nucleus and an α-particle (the nucleus of the 4 He atom).

α-decay, as a rule, occurs in heavy nuclei with a mass number BUT≥140 (although there are a few exceptions). Inside heavy nuclei, due to the property of saturation of nuclear forces, separate α-particles are formed, consisting of two protons and two neutrons. The resulting α-particle is subject to a greater action of the Coulomb repulsive forces from the protons of the nucleus than individual protons. At the same time, the α-particle experiences less nuclear attraction to the nucleons of the nucleus than the rest of the nucleons. The resulting alpha particle at the boundary of the nucleus is reflected inward from the potential barrier, but with some probability it can overcome it (see tunnel effect) and fly out. As the energy of the alpha particle decreases, the permeability of the potential barrier decreases exponentially, so the lifetime of nuclei with a lower available energy of alpha decay, other things being equal, is longer.

Soddy's shift rule for α-decay:

As a result of α-decay, the element is shifted by 2 cells to the beginning of the periodic table, the mass number of the daughter nucleus decreases by 4.

tunnel effect- overcoming a potential barrier by a microparticle in the case when its total energy (remaining unchanged during tunneling) is less than the barrier height. The tunnel effect is a phenomenon of exclusively quantum nature, impossible and even completely contrary to classical mechanics. An analogue of the tunnel effect in wave optics can be the penetration of a light wave into a reflecting medium (over distances of the order of the wavelength of light) under conditions when, from the point of view of geometric optics, total internal reflection occurs. The phenomenon of tunneling underlies many important processes in atomic and molecular physics, in the physics of the atomic nucleus, solid state, etc.

The tunnel effect can be explained by the uncertainty relation. Written as:

it shows that when a quantum particle is limited along the coordinate, that is, its certainty along x, its momentum p becomes less certain. Randomly, the momentum uncertainty can add energy to the particle to overcome the barrier. Thus, with some probability, a quantum particle can penetrate the barrier, while the average energy of the particle remains unchanged.

Alpha radiation has the lowest penetrating power (to absorb alpha particles, a sheet of thick paper is enough) in human tissue to a depth of less than a millimeter.

29. Basic regularities of b-decay and its properties. Neutrino. Electronic capture. (see 27)

Becquerel proved that β-rays are a stream of electrons. β-decay is a manifestation of the weak interaction.

β-decay(more precisely, beta minus decay, -decay) is a radioactive decay, accompanied by the emission of an electron and an antineutrino from the nucleus.

β decay is an intranucleon process. It occurs as a result of the transformation of one of d-quarks in one of the neutrons of the nucleus in u-quark; in this case, the neutron is converted into a proton with the emission of an electron and an antineutrino:

Soddy's shift rule for -decay:

After -decay, the element is shifted by 1 cell to the end of the periodic table (the nuclear charge increases by one), while the mass number of the nucleus does not change.

There are also other types of beta decay. In positron decay (beta plus decay), the nucleus emits a positron and a neutrino. In this case, the charge of the nucleus decreases by one (the nucleus is shifted one cell to the beginning of the periodic table). Positron decay always accompanied by a competing process - electron capture (when the nucleus captures an electron from the atomic shell and emits a neutrino, while the charge of the nucleus also decreases by one). However, the converse is not true: many nuclides, for which positron decay is forbidden, experience electron capture. The rarest known type of radioactive decay is double beta decay, which has been detected to date for only ten nuclides, with half-lives exceeding 10 19 years. All types of beta decay conserve the mass number of the nucleus.

Neutrino- a neutral fundamental particle with a half-integer spin, participating only in weak and gravitational interactions, and belonging to the class of leptons.

Electronic grip, e capture - one of the types of beta decay of atomic nuclei. In electron capture, one of the protons of the nucleus captures an orbiting electron and turns into a neutron, emitting an electron neutrino. The charge of the nucleus is then reduced by one. The mass number of the nucleus, as in all other types of beta decay, does not change. This process is characteristic of proton-rich nuclei. If the energy difference between parent and child atom (the available energy of beta decay) exceeds 1.022 MeV (twice the mass of an electron), electron capture always competes with another type of beta decay, positron decay. For example, rubidium-83 is converted to krypton-83 only via electron capture (available energy is about 0.9 MeV), while sodium-22 decays to neon-22 via both electron capture and positron decay (available energy is about 2.8 MeV).

Since the number of protons in the nucleus (i.e., the nuclear charge) decreases during electron capture, this process turns the nucleus of one chemical element into the nucleus of another element located closer to the beginning of the periodic table.

General formula for electron capture

30. γ-radiation of nuclei and its properties. Interaction of γ-radiation with matter. The emergence and destruction of electron-positron pairs.

It has been experimentally established that  -radiation is not an independent type of radioactivity, but only accompanies  - and  -decays and also occurs during nuclear reactions, during the deceleration of charged particles, their decay, etc.  - The spectrum is a line.  -Spectrum is the distribution of a number  -quanta in energy. discreteness  -spectrum is of fundamental importance, since it is proof of the discreteness of the energy states of atomic nuclei.

It is now firmly established that  -radiation is emitted by the child (and not the parent) nucleus. The daughter nucleus at the moment of its formation, being excited, passes into the ground state with emission  -radiation. Returning to the ground state, the excited nucleus can go through a series of intermediate states, so  -radiation of the same radioactive isotope may contain several groups  -quanta differing from one another in their energy.

At  - radiation BUT and Z kernels do not change, so it is not described by any displacement rules.  - The radiation of most nuclei is so short-wavelength that its wave properties are very weakly manifested. Here, corpuscular properties come to the fore, therefore  -radiation is considered as a stream of particles -  -quanta. During radioactive decays of various nuclei  -quanta have energies from 10 keV to 5 MeV.

A nucleus in an excited state can go over to the ground state not only by emitting  -quantum, but also with direct transfer of excitation energy (without prior emission  -quantum) to one of the electrons of the same atom. This produces the so-called conversion electron. The phenomenon itself is called internal conversion. Internal conversion is a process that competes with  -radiation.

Conversion electrons correspond to discrete energy values, which depend on the work function of the electron from the shell from which the electron escapes, and on the energy E, given by the nucleus during the transition from the excited state to the ground state. If all energy E stands out in the form  -quantum, then the radiation frequency  is determined from the known relation E=h. If internal conversion electrons are emitted, then their energies are equal to E-A K , E-A L , .... where A K , A L , ... - work function of an electron TO- and L-shells. The monoenergetic nature of conversion electrons makes it possible to distinguish them from  -electrons, the spectrum of which is continuous. The vacancy on the inner shell of the atom that has arisen as a result of the emission of an electron will be filled with electrons from the overlying shells. Therefore, internal conversion is always accompanied by characteristic X-ray emission.

 -Quantums, having zero rest mass, cannot slow down in the medium, therefore, when passing through  - radiation through the substance, they are either absorbed or scattered by it.  -Quanta do not carry an electric charge and thus do not experience the influence of Coulomb forces. When passing the beam  -quanta through matter, their energy does not change, but as a result of collisions, the intensity is weakened, the change of which is described by the exponential law I=I 0e- x (I 0 and I- intensity  -radiation at the entrance and exit of the layer of absorbing material with a thickness x,  - absorption coefficient). Because  radiation is the most penetrating radiation, then  for many substances - a very small value;  depends on the properties of matter and on energy  -quanta.

 -Quanta, passing through matter, can interact both with the electron shell of the atoms of matter, and with their nuclei. In quantum electrodynamics, it is proved that the main processes accompanying the passage  -radiation through matter are the photoelectric effect, the Compton effect (Compton scattering) and the formation of electron-positron pairs.

photoelectric effect, or photoelectric absorption  - radiation, is the process by which an atom absorbs  -quantum and emits an electron. Since the electron is knocked out of one of the inner shells of the atom, the vacated space is filled with electrons from the overlying shells, and the photoelectric effect is accompanied by characteristic X-ray radiation. The photoelectric effect is the predominant absorption mechanism in the low-energy region  -quanta ( E 100 keV). The photoelectric effect can only occur on bound electrons, since a free electron cannot absorb  -quantum, while the laws of conservation of energy and momentum are not simultaneously satisfied.

As energy increases  -quanta ( E0.5 MeV) the probability of the photoelectric effect is very small and the main mechanism of interaction  -quanta with matter is Compton scattering.

At E>l,02 MeV=2 m e c 2 (t e - rest mass of an electron) the process of formation of electron-positron pairs in electric fields nuclei. The probability of this process is proportional to Z 2 and increases with growth E. Therefore, when E10 MeV main interaction process  -radiation in any substance is formed electron-positron pairs.

If the energy  -quantum exceeds the binding energy of nucleons in the nucleus (7-8 MeV), then as a result of absorption  - quantum can be observed nuclear photoelectric effect- ejection from the nucleus of one of the nucleons, most often a neutron.

Great penetrating power  - radiation is used in gamma flaw detection - a flaw detection method based on different absorption  -radiation when it propagates over the same distance in different environments. The location and size of defects (cavities, cracks, etc.) are determined by the difference in the intensities of the radiation that has passed through different parts of the translucent product.

Impact  - radiation (as well as other types of ionizing radiation) on a substance characterize dose of ionizing radiation. Differ:

Absorbed radiation dose - physical quantity, equal to the ratio of the radiation energy to the mass of the irradiated substance.

Unit of absorbed radiation dose - gray(Gy)*: 1 Gy= 1 J/kg - dose of radiation at which the energy of any ionizing radiation of 1 J is transferred to an irradiated substance weighing 1 kg.

31. Obtaining transuranium elements. Basic laws of nuclear fission reactions.

TRANSURANE ELEMENTS, chemical elements located in periodic system after uranium, that is, with atomic number Z >92.

All transuranium elements have been synthesized by nuclear reactions (only trace amounts of Np and Pu have been found in nature). Transuranium elements are radioactive; with increasing Z half life T 1/2 transuranium elements is sharply reduced.

In 1932, after the discovery of the neutron, it was suggested that when uranium was irradiated with neutrons, isotopes of the first transuranium elements should be formed. And in 1940, E. Macmillan and F. Ableson synthesized neptunium (serial number 93) using a nuclear reaction and studied its most important chemical and radioactive properties. At the same time, the discovery of the next transuranium element, plutonium, took place. Both new elements were named after planets in the solar system.

All transuranium elements up to and including 101 were synthesized through the use of light bombarding particles: neutrons, deuterons and alpha particles. The synthesis process consisted in irradiating the target with fluxes of neutrons or charged particles. If U is used as a target, then with the help of powerful neutron fluxes generated in nuclear reactors or during the explosion of nuclear devices, it is possible to obtain all transuranium elements, up to Fm ( Z= 100) inclusive. Elements with Z 1 or 2 less than the synthesized element. Between 1940 and 1955 American scientists led by G. Seaborg synthesized nine new elements that do not exist in nature: Np (neptunium), Pu (plutonium), Am (americium), Cm (curium), Bk (berkelium), Cf (californium), Es ( einsteinium), Fm (fermium), Md (mendelevium). In 1951, G. Seaborg and E. M. Macmillan were awarded Nobel Prize"for their discoveries in the chemistry of the transuranium elements."

Possibilities of the method for the synthesis of heavy radioactive elements, in which irradiation with light particles is used, are limited, it does not allow obtaining nuclei with Z> 100. The element with Z = 101 (mendelevium) was discovered in 1955 by irradiating 253 99Es (einsteinium) with accelerated a-particles. The synthesis of new transuranium elements became more and more difficult as we moved to higher values Z. The values ​​of the half-lives of their isotopes turned out to be smaller and smaller.

Nuclear reaction - the process of transformation of atomic nuclei, which occurs when they interact with elementary particles, gamma rays and with each other, often resulting in the release of enormous amounts of energy. During the course of nuclear reactions, the following laws are fulfilled: conservation of electric charge and the number of nucleons, conservation of energy and

momentum conservation, angular momentum conservation, parity conservation, and

isotopic spin.

Fission reaction - the division of an atomic nucleus into several lighter nuclei. Divisions are forced and spontaneous.

The fusion reaction is the fusion of light nuclei into one. This reaction occurs only at high temperatures, on the order of 10 8 K, and is called a thermonuclear reaction.

The energy yield of the reaction Q is the difference between the total rest energies of all particles before and after the nuclear reaction. If Q > 0, then the total rest energy decreases in the course of a nuclear reaction. Such nuclear reactions are called exoenergetic. They can proceed at an arbitrarily small initial kinetic energy of the particles. Conversely, for Q<0 часть исходной кинетической энергии частиц превращается в энергию покоя. Такие ядерные реакции называются эндоэнергетическими. Для их протекания необходимо, чтобы кинетическая энергия частиц превышала некоторую величину.

32. Fission chain reaction. Controlled chain reaction. Nuclear reactor.

Secondary neutrons emitted during nuclear fission can cause new fission events, which makes it possible to carry out fission chain reaction- a nuclear reaction in which the particles causing the reaction are formed as products of this reaction. The fission chain reaction is characterized by multiplication factor k neutrons, which is equal to the ratio of the number of neutrons in a given generation to their number in the previous generation. Necessary condition for the development of a fission chain reaction is requirement k 1.

It turns out that not all of the resulting secondary neutrons cause subsequent nuclear fission, which leads to a decrease in the multiplication factor. First, due to the finite dimensions core(the space where the chain reaction takes place) and the high penetrating power of neutrons, some of them will leave the core before they are captured by any nucleus. Secondly, part of the neutrons is captured by the nuclei of non-fissile impurities, which are always present in the core. In addition, along with fission, competing processes of radiative capture and inelastic scattering can take place.

The multiplication factor depends on the nature of the fissile material, and for a given isotope, on its quantity, as well as the size and shape of the active zone. The minimum dimensions of the active zone at which a chain reaction is possible are called critical dimensions. The minimum mass of fissile material located in a system of critical sizes, necessary for the implementation chain reaction, called critical mass.

The rate of development of chain reactions is different. Let T - the average lifetime of one generation, and N- the number of neutrons in a given generation. In the next generation, their number is kN, t. e. increase in the number of neutrons per generation dN=kN-N=N(k- one). The increase in the number of neutrons per unit of time, i.e. the rate of growth of the chain reaction,

Integrating (266.1), we obtain

where N 0 is the number of neutrons at the initial moment of time, and N- their number at a time t. N is defined by the sign ( k- one). At k> 1 is coming developing reaction, the number of divisions grows continuously and the reaction can become explosive. At k=1 goes self-sustaining reaction, at which the number of neutrons does not change with time. At k<1 идет затухающая реакция.

Chain reactions are divided into managed and unmanaged. The explosion of an atomic bomb, for example, is an uncontrolled reaction. To prevent an atomic bomb from exploding during storage, U (or Pu) in it is divided into two parts remote from each other with masses below critical. Then, with the help of an ordinary explosion, these masses approach each other, the total mass of the fissile material becomes more critical, and an explosive chain reaction occurs, accompanied by an instantaneous release of a huge amount of energy and great destruction. Explosive reaction starts due to available spontaneous fission neutrons or cosmic radiation neutrons. Controlled chain reactions are carried out in nuclear reactors.

There are three isotopes in nature that can serve as nuclear fuel (U: natural uranium contains approximately 0.7%) or raw materials for its production (Th and U: natural uranium contains approximately 99.3%). Th serves as the initial product for obtaining artificial nuclear fuel U (see reaction (265.2)), and U, absorbing neutrons, through two successive  – -decays - for transformation into a Pu nucleus:

Reactions (266.2) and (265.2), thus, open up a real possibility of reproduction of nuclear fuel in the process of a fission chain reaction.

Nuclear reactor- This is a device in which a controlled nuclear chain reaction is carried out, accompanied by the release of energy. The first nuclear reactor was built and launched in December 1942 in the USA under the leadership of E. Fermi. The first reactor built outside the United States was ZEEP, launched in Canada in September 1945. In Europe, the first nuclear reactor was the F-1 installation, which was launched on December 25, 1946 in Moscow under the leadership of I. V. Kurchatov.

By 1978, about a hundred nuclear reactors of various types were already operating in the world. The components of any nuclear reactor are: a core with nuclear fuel, usually surrounded by a neutron reflector, a coolant, a chain reaction control system, radiation protection, a remote control system. The main characteristic of a nuclear reactor is its power. A power of 1 MW corresponds to a chain reaction in which 3·10 16 fission events occur in 1 sec.

33. Thermonuclear fusion. Star energy. Controlled thermonuclear fusion.

thermonuclear reaction is a reaction of fusion of light nuclei into heavier ones.

For its implementation, it is necessary that the initial nucleons or light nuclei approach each other to distances equal to or less than the radius of the sphere of action of the nuclear forces of attraction (ie, up to distances of 10 -15 m). Such mutual approach of the nuclei is prevented by the Coulomb repulsive forces acting between the positively charged nuclei. For a fusion reaction to occur, it is necessary to heat a substance of high density to ultrahigh temperatures (on the order of hundreds of millions of Kelvin) so that the kinetic energy of the thermal motion of the nuclei is sufficient to overcome the Coulomb repulsive forces. At such temperatures, matter exists in the form of a plasma. Since fusion can only occur at very high temperatures, nuclear fusion reactions are called thermonuclear reactions (from the Greek. therme"warmth, heat").

Thermonuclear reactions release enormous energy. For example, in the reaction of deuterium fusion with the formation of helium

3.2 MeV of energy is released. In the reaction of deuterium synthesis with the formation of tritium

4.0 MeV of energy is released, and in the reaction

17.6 MeV of energy is released.

Controlled thermonuclear fusion (TCB) - the synthesis of heavier atomic nuclei from lighter ones in order to obtain energy, which, unlike explosive thermonuclear fusion (used in thermonuclear explosive devices), is controlled. Controlled thermonuclear fusion differs from traditional nuclear energy in that the latter uses a fission reaction, during which lighter nuclei are obtained from heavy nuclei. The main nuclear reactions planned to be used for controlled fusion will use deuterium (2 H) and tritium (3 H), and in the longer term helium-3 (3 He) and boron-11 (11 B).

34. Sources and methods of registration of elementary particles. Types of interactions and classes of elementary particles. Antiparticles.

Geiger counter
- serves to count the number of radioactive particles (mainly electrons).

It is a glass tube filled with gas (argon) with two electrodes inside (cathode and anode).
During the passage of a particle, impact ionization of the gas occurs and an electric current pulse occurs.

Advantages:
- compactness
- efficiency
- performance
- high accuracy (10000 particles/s).
Where is used:
- registration of radioactive contamination on the ground, in premises, clothing, products, etc.
- at radioactive materials storage facilities or with operating nuclear reactors
- when searching for deposits of radioactive ore (U, Th)

cloud chamber
- serves to observe and photograph traces from the passage of particles (tracks).
The internal volume of the chamber is filled with vapors of alcohol or water in a supersaturated state:
when the piston is lowered, the pressure inside the chamber decreases and the temperature decreases, as a result of the adiabatic process, supersaturated steam is formed.
Moisture droplets condense along the path of the passage of the particle and a track is formed - a visible trace.
When a camera is placed in a magnetic field, the track can be used to determine the energy, velocity, mass, and charge of a particle.

The characteristics of a flying radioactive particle are determined by the length and thickness of the track, by its curvature in a magnetic field.
For example, an alpha particle gives a continuous thick track,
proton - thin track,
electron - dotted track.

bubble chamber

Cloud chamber variant

With a sharp decrease in the piston, the fluid under high pressure goes into a superheated state. With the rapid movement of the particle along the trail, vapor bubbles are formed, i.e. the liquid boils, the track is visible.
Advantages over cloud chamber:
- high density of the medium, hence short tracks
- particles get stuck in the chamber and further observation of the particles can be carried out
- more speed.
Method of thick-layer photographic emulsions
- serves to register particles
- allows you to register rare phenomena due to the long exposure time.
The photographic emulsion contains a large amount of microcrystals of silver bromide.
Incoming particles ionize the surface of photographic emulsions. AgBr crystals disintegrate under the action of charged particles, and upon development, a trace from the passage of a particle, a track, is revealed.
The energy and mass of the particles can be determined from the length and thickness of the track.

Particle classes and types of interactions

At present, there is a firm belief that everything in nature is built from elementary particles, and all natural processes are due to the interaction of these particles. Today, elementary particles are understood as quarks, leptons, gauge bosons and Higgs scalar particles. Under fundamental interactions - strong, electro-weak and gravitational. Thus, it is conditionally possible to single out four classes of elementary particles and three types of fundamental interactions.

Neutrinos are electrically neutral; the electron, muon and tau lepton have electric charges. Leptons participate in the electroweak and gravitational interactions.

Third class are quarks. Today, six quarks are known - each of which can be "colored" in one of three colors. Like leptons, it is convenient to arrange them in the form of three families

Free quarks are not observed. Together with gluons, they are the components of hadrons, of which there are several hundred. Hadrons, like the quarks that make them up, participate in all types of interactions.

fourth grade- Higgs particles, experimentally not yet detected. In the minimal scheme, one Higgs scalar is sufficient. Their role in nature today is mostly "theoretical" and is to make the electro-weak interaction renormalizable. In particular, the masses of all elementary particles are the "handiwork" of the Higgs condensate. Perhaps the introduction of Higgs fields is necessary to solve fundamental problems of cosmology, such as the homogeneity and causality of the universe.

Subsequent lectures on the theory of the quark structure of hadrons are devoted to hadrons and quarks. The focus will be on particle classification, symmetries and conservation laws.

35. Laws of conservation in transformations of elementary particles. The concept of quarks.

A quark is a fundamental particle in the Standard Model that has an electric charge that is a multiple of e/3, and is not observed in the free state. Quarks are point particles up to a scale of about 0.5·10 −19 m, which is about 20 thousand times smaller than the size of a proton. Quarks make up hadrons, specifically the proton and neutron. Currently, 6 different "sorts" (more often they say - "flavors") of quarks are known, the properties of which are given in the table. In addition, for the gauge description of the strong interaction, it is postulated that quarks also have an additional internal characteristic called "color". Each quark corresponds to an antiquark with opposite quantum numbers.

The hypothesis that hadrons are built from specific subunits was first put forward by M. Gell-Mann and, independently of him, J. Zweig in 1964.

The word "quark" was borrowed by Gell-Mann from the novel Finnegans Wake by J. Joyce, where in one of the episodes the phrase "Three quarks for Muster Mark!" (usually translated as "Three quarks for Master/Muster Mark!"). The very word "quark" in this phrase is supposedly an onomatopoeia of the cry of seabirds.

Radioactivity. Basic law of radioactive decay.

Radioactivity is the spontaneous decay of unstable nuclei with the emission of other nuclei and elementary particles.

Types of radioactivity:

1. Natural

2. Artificial.

Ernest Rutherford - the structure of the atom.

Types of radioactive decay:

α-decay: à + ; β-decay: à +

Basic law of radioactive decay. N \u003d N o e -lt

The number of undecayed radioactive nuclei decreases exponentially. L(lambda) is the decay constant.

decay constant. Half life. Activity. Types of radioactive decay and their spectra.

L (lambda) - decay constant, proportional to the probability of decay of a radioactive nucleus and different for different radioactive substances.

Half life ( T )- is the time it takes for half of the radioactive nuclei to decay. T=ln2/l=0.69/l.

Activity is characterized by the decay rate. A=-dN/dT=lN=lN o e -lt =(N/T)*ln2

[A]-becquerel (Bq)= 1 disintegration/second.

[A]-curie (Ci) . 1 Ci=3.7*10 10 Bq=3.7*10 10 s -1

[A]-rutherford(Rd). 1Rd=10 6

Types of radioactive decay. displacement rule.

Alpha decay (weakest): A Z X> 4 2 He + A-4 Z-2 Y

Beta decay: A Z X> 0 -1 e + A Z+1 Y

The energy spectra of particles of many radioactive elements consist of several lines. The reason for the appearance of such a spectrum structure is the decay of the initial nucleus (A, Z) into an excited state of the nucleus (A-4, Z-2. For alpha decay, for example). By measuring the spectra of particles, one can obtain information about the nature of the excited states of the nucleus.

Characteristics of the interaction of charged particles with matter: linear ionization density, linear stopping power, average linear range. Penetrating and ionizing abilities of alpha, beta and gamma radiation.

Charged particles, propagating in matter, interact with electrons and nuclei, as a result of which the state of both matter and particles changes.

Linear ionization density is the ratio of ions of sign dn, formed by charged ionized particles on the elementary path dL, to the length of this path. I=dn/dL.

Linear stopping power - this is the ratio of the energy dE lost by a charged ionizing particle during the passage of an elementary path dL to the length of this path. S=dE/dL.

Average linear run- is the distance that an ionizing particle travels in a substance without colliding. R is the average linear mileage.

It is necessary to take into account the penetrating power of radiation. For example, heavy nuclei of atoms and alpha particles have an extremely short path in matter, so radioactive alpha sources are dangerous if they enter the body. On the contrary, gamma rays have a significant penetrating power, since they consist of high-energy photons that do not have a charge.

The penetrating power of all types of ionizing radiation depends on the energy.