Lecture 16

Elements of physics of the atomic nucleus

Questions

1. The law of radioactive decay.

Nuclear reactions and their main types.

Patterns ,  And decays.

Radiation doses.

Fission chain reaction.

6. Fusion reactions (thermonuclear reactions).

1. The law of radioactive decay

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

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

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

d N = – λ N d t , (1)

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

(2)

Where N 0  starting 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  time during which the initial number of radioactive nuclei is halved;

average lifetime τ of a radioactive nucleus.

. (3)

Half-lives T 1 /2
				4.510 9 years

Total life expectancy d N cores is equal to 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.

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

Bk - becquerel, (5)

1Bq is the activity of a nuclide, in which one decay event occurs in 1 s.

Extra-system unit – curie [Ci]: 1[Ci] = 3.710 10 [Bq].

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

Displacement rule for α-decay:
. (6)

Bias rule for beta decay:
, (7)

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

Nuclei resulting from radioactive decay can, in turn, be radioactive. This leads to the occurrence of 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 – is the process of interaction of an atomic nucleus with another nucleus or elementary particle, accompanied by a change in the composition and structure of the nucleus and the release of secondary particles or γ– quanta .

, , (8)

X, Y- initial and final kernels; WITH 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: impulse, energy, angular momentum, charge. In addition to these classical conservation laws in nuclear reactions, the conservation law of the so-called baryon charge (i.e., the number of nucleons - protons and neutrons).

Classification of nuclear reactions

by the type of particles involved :

under the influence of neutrons ;

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

under the influence of quanta.

2. according to the energy of the particles causing 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. By 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 (a quantum is emitted).

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

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

(10)

 particle is formed by the meeting of two protons and two neutrons, 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  mileage, the distance traveled by a particle in a substance until it comes to a complete stop;
.The shorter the half-life of a radioactive element, the greater the range, and therefore the energy particles.

 a 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 neutrino emission was proposed by Pauli in 1931 and experimentally confirmed in 1956. It has a high penetrating ability: a neutrino with an energy of 1 MeV in lead travels 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 and no charge, cannot slow down in the medium, but can either be absorbed, or dissipate. The high penetrating power of  radiation is used in  flaw detection.

Radioactivity. The 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 +

The basic law of radioactive decay. N= N o e -lt

The number of undecayed radioactive nuclei decreases according to an exponential law. L(lambda) is the decay constant.

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

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

Half life ( T )- This is the time during which half of the radioactive nuclei decay. T=ln2/l=0.69/l.

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

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

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

[A]-rutherford(Rd). 1Рд=10 6

Types of radioactive decay. Offset 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, spreading 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 a charged ionized particle on an elementary path dL to the length of this path. I=dn/dL.

Linear braking capacity - this is the ratio of the energy dE lost by a charged ionizing particle when passing through an elementary path dL to the length of this path. S=dE/dL.

Average linear mileage - This 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 ability of radiation. For example, heavy atomic nuclei and alpha particles have an extremely short range in matter, so radioactive alpha sources are dangerous if they enter the body. On the contrary, gamma rays have significant penetrating power because they consist of high-energy photons that have no charge.

The penetrating ability of all types of ionizing radiation depends on energy.

1. Radioactivity. The basic law of radioactive decay. Activity.

2. Main types of radioactive decay.

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

4. Natural and artificial radioactivity. Radioactive series.

5. Use of radionuclides in medicine.

6. Accelerators of charged particles and their use in medicine.

7. Biophysical basis of the action of ionizing radiation.

8. Basic concepts and formulas.

9. Tasks.

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

Firstly, all living things are constantly exposed to natural background radiation, which consists of cosmic radiation, radiation from radioactive elements located in the surface layers of the earth’s crust, and radiation from elements entering 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. The 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 established 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, a new type of radioactivity was discovered - spontaneous fission of nuclei: a fissioning 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 consist of protons and neutrons, which have a general name - nucleons. The number of protons in the nucleus determines the chemical properties of the atom and is denoted by Z (this is serial number chemical element). The number of nucleons in a nucleus is called mass number and denote A. Nuclei with the same atomic number and different mass numbers are called isotopes. All isotopes of one chemical element have the same Chemical properties. The physical properties of isotopes can vary greatly. To designate isotopes, use the symbol of a chemical element with two indices: A Z X. The lower index is the serial number, the upper index is the mass number. Often the subscript is omitted because it is indicated by the element's symbol itself. 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 carbon isotope 12 C is stable, but the isotope 14 C is radioactive.

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

Decay constant- 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 law of radioactive decay.

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

In practice, instead decay constantλ another quantity is often used, called half-life.

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

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

The graph of dependence (33.4) is shown in Fig. 33.1.

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

Rice. 33.1. 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 isotope use a special quantity called activity.

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

The SI unit of activity is becquerel(Bq), 1 Bq corresponds to one decay event per second. In practice, more

childish non-systemic unit of activity - curie(Ci), equal to the activity of 1 g 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. It was later discovered that α- and β-particles are products of two different types of radioactive decay, and γ-rays are a byproduct of these processes. In addition, γ-rays accompany more complex nuclear transformations, which are not considered here.

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

The α-decay scheme is written as

where X, Y are the symbols of the mother and daughter nuclei, respectively. When writing α-decay, you can write “He” instead of “α”.

During 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. The general property of complex microobjects is that they have discrete a set of energy states. This also applies to kernels. Therefore, γ-radiation from excited nuclei has a discrete spectrum. Consequently, the energy spectrum of α-particles is discrete.

The energy of emitted α-particles for almost all α-active isotopes lies in the range of 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 beta decay is distributed randomly between the beta particle and the neutrino. 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 of 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 from the nucleus. The electron β - decay scheme is written in the form

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

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

2. Positronicβ + -decay involves 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 produce ionizing particles, but it does accompanied by X-ray radiation. This radiation occurs when the space vacated by the absorption of an internal electron is filled by an electron from the outer orbit.

Gamma radiation has an electromagnetic nature and represents photons with a wavelengthλ ≤ 10 -10 m.

Gamma radiation is not an independent type of 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 ability.

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 causes in tissues. The ability of a particle to ionize depends on both its type and its energy. As a particle moves deeper into matter, it loses its energy. This process is called ionization inhibition.

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

Once the particle's energy drops 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 linear ionization density on the path traveled by an α-particle in the medium

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

Electrons formed during the ionization process, as a rule, move away from the α-particle track 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 interacting particles.

Interaction Characteristics 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

Like α particles, the ionization ability of β particles increases with decreasing energy.

Gamma radiation

Absorption γ -radiation by matter obeys an exponential law similar to the law of absorption of X-ray radiation:

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

33.4. Natural and artificial

radioactivity. Radioactive series

Terms natural And artificial radioactivity are conditional.

Natural called the radioactivity of isotopes existing 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 radioactivity of isotopes that arise as a result of human activity.

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

Natural radioactivity

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

Rice. 33.4. Uranium-radium series

Elements of this series are the main source of internal human radiation. 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 the dose of natural radiation received by humans. Formed in the earth's crust, this gas seeps into the atmosphere and enters 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 enters through the root system of 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 natural radioactive background

33.5. Use of radionuclides in medicine

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

Diagnostic Application based on the selective accumulation of certain chemical elements by individual organs. Iodine, for example, is concentrated in the thyroid gland, and calcium 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- use of high-energy γ-radiation (60 Co source) to destroy deep-lying tumors. To prevent superficial tissues and organs from being subjected to harmful effects, exposure to 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 by even a small layer of air. Therefore therapeutic

The use of alpha rays is possible through direct contact with the surface of the organ or when administered internally (using a needle). For surface exposure, radon therapy (222 Rn) is used: exposure to the skin (baths), digestive organs (drinking), and respiratory organs (inhalation).

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 influence of neutrons, emit α -particles. After this, the diseased organ is irradiated with a stream of neutrons. In this manner α -particles are formed directly inside the organ on which they should have a destructive effect.

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

Table 33.5. Characteristics of isotopes

33.6. Charged particle accelerators and their use in medicine

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

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

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-rays are obtained by directing a beam of particles to a special target, which is the source of X-rays. This radiation differs from the X-ray tube by significantly higher quantum energy.

Synchrotron X-rays occurs during the acceleration of electrons in ring accelerators - synchrotrons. Such radiation has a high degree of directionality.

The direct effect of fast particles is associated with their high penetrating ability. Such particles pass through superficial 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 destroy 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 basis 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 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 types of reactions leading to the redistribution of 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 changes in biological properties: the structure and functions of membranes are disrupted; 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 various functional disorders, to premature cell death as a result of the action of apoptotic 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.

Let us note the general patterns of the biological stage:

Large disturbances with low absorbed energy (a lethal dose of radiation for humans causes the body to heat up by only 0.001°C);

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

Characterized by a hidden, latent period;

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;

Detrimental effect on tissues of an adult organism in which there is division;

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 the 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 the growing tree? The half-life of radon is T = 5570 years.

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

After how many years will activity in these places 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 Chernobyl disaster zone, 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. To determine the blood volume of an animal, the following method is used. A small volume of blood is taken from the animal, red blood cells are separated from the plasma and placed in a solution with radioactive phosphorus, which is assimilated by the red blood cells. The labeled red blood cells are reintroduced into the animal's circulatory system, and after some time the activity of the blood sample is determined.

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

Radioactive radiation and its types

In 1896, the French physicist A. Becquerel, while studying the luminescence of uranium salts, accidentally discovered their spontaneous emission of radiation of an unknown nature, which acted on a photographic plate, ionized the air, penetrated through thin metal plates, and caused luminescence of a number of substances. Continuing the study of this phenomenon, the Curie spouses - Marie and Pierre - discovered that Becquerel radiation is characteristic not only of uranium, but also of many other heavy elements, such as thorium and actinium. They also showed that uranium pitchblende (the ore from which uranium metal is mined) emits radiation whose intensity is many times greater than that of uranium. Thus, it was possible to isolate two new elements - carriers of Becquerel radiation: polonium and radium.

The detected radiation was named radioactive radiation , and the phenomenon itself is the emission of radioactive radiation - radioactivity.

Types of radioactive radiation:

1) - radiation

It is deflected by electric and magnetic fields, has a high ionizing ability and low penetrating ability. Represents a stream of helium nuclei; the charge of the -particle is +2e, and the mass coincides with the mass of the helium isotope nucleus. Based on the deviation of particles in electric and magnetic fields, their specific charge was determined, the value of which confirmed the correctness of ideas about their nature.

2) -radiation

Deflected by electric and magnetic fields; its ionizing ability is much lower (by about two orders of magnitude), and its penetrating ability is much greater than that of particles. It is a flow of fast electrons (this follows from the definition of their specific charge).

3) -radiation

It is not deflected by electric and magnetic fields, has a relatively weak ionizing ability and very high penetrating ability, and exhibits diffraction when passing through crystals. It is short-wave electromagnetic radiation with an extremely short wavelength m and, as a result, pronounced corpuscular properties, i.e. is a flow of particles – quanta (photons).

Radioactivity– the ability of some atomic nuclei to spontaneously (spontaneously) transform into other nuclei with the emission of various particles:

1)Natural - observed in unstable isotopes existing in nature;

2) Artificial - observed in isotopes synthesized through nuclear reactions in the laboratory.

Law of Radioactive Decay

Radioactive decay- natural transformation of nuclei that occurs spontaneously.

This phenomenon is statistical, therefore the conclusions following from the laws of radioactive decay are probabilistic in nature.

Radioactive decay constant- probability of nuclear decay per unit time, equal to the fraction of nuclei decaying in 1 s.

Law of Radioactive Decay: Due to the spontaneity of radioactive decay, we can assume that the number of nuclei dN that decayed on average during the time interval from t to t+dt is proportional to the time interval dt and the number N of nuclei that did not decay by time t:

[ N is the number of undecayed nuclei at time t; - the initial number of undecayed nuclei at time t=0; -radioactive decay constant]

Half life ()- the period of time during which, on average, the number of undecayed nuclei decreases by half.

Average lifetime of a radioactive nucleus:

Nuclide activity- the number of decays occurring with the sample nuclei in 1 s:

Unit of activity - 1 Bq: 1 becquerel - the activity of a nuclide in a radioactive source, in which one decay event occurs in 1 s. 1Bq= 2.703 curies.

5. Offset rules for - And -decays

Mother core- an atomic nucleus undergoing radioactive decay.

Child kernel- an atomic nucleus resulting from radioactive decay.

Offset Rules– rules that allow one to determine which nucleus arises as a result of the decay of a given parent nucleus. These rules are a consequence of the laws that apply during radioactive decays - the law of conservation of charge numbers and the law of conservation of mass numbers.

Laws of conservation of charge and mass numbers

1) The sum of the charge numbers of the emerging nuclei and particles is equal to the charge number of the original nucleus.

2) the sum of the mass numbers of the emerging nuclei and particles is equal to the mass number of the initial nucleus.

The displacement rules are a consequence of the laws of conservation of charge and mass numbers.

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

Alpha decay usually occurs in heavy nuclei with mass number

A≥ 140 (although there are a few exceptions).

Displacement rule for α-decay: , where is the helium nucleus (a-particle),

Example (alpha decay uranium-238 to thorium-234):

As a result of α-decay, the atom moves 2 cells to the beginning periodic tables(that is, the nuclear charge Z decreases by 2), the mass number of the daughter nucleus decreases by 4.

Beta decay

Becquerel proved that β-rays are a flux electrons. Beta decay is a manifestation weak interaction.

Radioactivity concept

Law of Radioactive Decay

Quantification of radioactivity and its units

Ionizing radiation, their characteristics.

AI Sources

1. Radioactivity concept

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 some elements into atoms of others occurs.

Radioactive transformations are characteristic only of individual substances.

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

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

To characterize a nuclide, use the symbol of a chemical element, indicate the atomic number (number of protons) and the mass number of the nucleus (number of nucleons, i.e. the total number of 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.

The following types of radioactive decay exist:

Beta decay;

Alpha decay;

Spontaneous fission of atomic nuclei (neutron decay);

Proton radioactivity (proton fusion);

Two-proton and cluster radioactivity.

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

Alpha decay – characteristic of heavy elements, the nuclei of which, starting from number 82 of D.I. Mendeleev’s table, are unstable, despite the excess of neutrons and spontaneously decay. The nuclei of these elements predominantly emit 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 insignificant compared to alpha decay. In this case, the nucleus divides into two fragments (nuclei) of similar 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 rate of decay, i.e. shows how many nuclei decay per unit time.

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

1. Quantification of radioactivity and its units

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

T 1/2 = ln2/λ = 0.693/λ.

The half-life T 1/2 of 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 is calledaverage lifetime of a radioactive atom t :

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

A = dN/dt = A 0 e λ t = λ N,

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

Activity– a measure of radioactivity. It is characterized by the number of decays of radioactive nuclei per unit 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 = 0.693 N/T 1/2.

The SI unit of activity is the becquerel (Bq). One becquerel equals one decay per second. The extrasystemic unit of activity is the curie (Ku).

1 Ku = 3.7 10 10 Bq

1Bq = 2.7 10 -11 Ku.

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