What is alpha decay and beta decay? Beta decay, alpha decay: formulas and reactions. Path of an α particle in air under normal conditions

Lecture: Radioactivity. Alpha decay. Beta decay. Electronic β-decay. Positron β-decay. Gamma radiation

Radioactivity

Radioactivity was discovered completely by accident as a result of experiments carried out by A. Becquerel in 1896. The newly discovered X-rays led scientists to want to find out whether they were produced by illumination. sunlight some elements. For his experiment, Becquerel chose a uranium salt.

The salt was placed on a photographic plate and wrapped in black paper to ensure the quality of the experiment. As a result of the fact that the salt lay for several hours in direct sunlight, the developed photographic plate contained a photograph that completely corresponded to the outlines of the salt crystals. This experience allowed Becquerel to speak at a conference where he spoke about new manifestations of X-rays. In a few weeks he was expected to announce new results from similar studies.

However, the weather prevented the scientist. Since it was cloudy all the time, the salt lay wrapped together with the photographic plate in black paper in the desk drawer. In desperation, the scientist developed a photographic plate, as a result of which he noticed that the salt left its mark even without sunlight.

It turned out that uranium emits some kind of rays, which are also capable of penetrating paper and leaving a mark on the plate.

This phenomenon is called radioactivity.

It later turned out that not only uranium is radioactive. The Curie family discovered similar properties in thorium, polonium, and radium.

Types of radioactive radiation

In the course of numerous experiments in which uranium was placed in a magnetic field, it was found that any radioactive element has three main types of radiation - alpha, beta and gamma.

As a result of placing a radioactive element in a lead plate exposed to a magnetic field, three spots were observed on the screen, located at some distance from each other.

1. Alpha rays (alpha particles) is a positive particle that has 4 nucleons and two positive charges. This radiation is the weakest. You can change the direction of motion of an alpha particle even with a piece of paper.

Equation and examples of such decay:

2 . Beta radiation or beta particle . This radiation occurs as a result of knocking out one negative or positive electron (positron).

3. Gamma radiation is radiation that produces an electromagnetic wave similar to x-rays.

The nuclei of most atoms are fairly stable formations. However, the nuclei of atoms of radioactive substances during the process of radioactive decay spontaneously transform into the nuclei of atoms of other substances. So in 1903, Rutherford discovered that radium placed in a vessel after some time turned into radon. And additional helium appeared in the vessel: (88^226)Ra→(86^222)Rn+(2^4)He. To understand the meaning of the written expression, study the topic of mass and charge number of the nucleus of an atom.

It was possible to establish that the main types of radioactive decay: alpha and beta decay occur according to the following displacement rule:

Alpha decay

During alpha decay an alpha particle (the nucleus of a helium atom) is emitted. From a substance with the number of protons Z and neutrons N in the atomic nucleus, it turns into a substance with the number of protons Z-2 and the number of neutrons N-2 and, accordingly, atomic mass A-4: (Z^A)X→(Z-2^ (A-4))Y +(2^4)He. That is, the resulting element is shifted two cells back in the periodic table.

Example of α decay:(92^238)U→(90^234)Th+(2^4)He.

Alpha decay is intranuclear process. As part of a heavy nucleus, due to a complex combination of nuclear and electrostatic forces, an independent α-particle is formed, which is pushed out by Coulomb forces much more actively than other nucleons. Under certain conditions, it can overcome the forces of nuclear interaction and fly out of the nucleus.

Beta decay

During beta decay an electron (β particle) is emitted. As a result of the decay of one neutron into a proton, electron and antineutrino, the composition of the nucleus increases by one proton, and the electron and antineutrino are emitted outward: (Z^A)X→(Z+1^A)Y+(-1^0)e+(0 ^0)v. Accordingly, the resulting element is shifted one cell forward in the periodic table.

Example of β decay:(19^40)K→(20^40)Ca+(-1^0)e+(0^0)v.

Beta decay is intranucleon process. The neutron undergoes the transformation. There is also beta plus decay or positron beta decay. In positron decay, the nucleus emits a positron and a neutrino, and the element moves back one cell on the periodic table. Positron beta decay is usually accompanied by electron capture.

Gamma decay

In addition to alpha and beta decay, there is also gamma decay. Gamma decay is the emission of gamma quanta by nuclei in an excited state, in which they have high energy compared to the unexcited state. Nuclei can come to an excited state during nuclear reactions or during radioactive decays of other nuclei. Most excited states of nuclei have a very short lifetime - less than a nanosecond.

There are also decays with the emission of a neutron, proton, cluster radioactivity and some other, very rare types of decays. But prevailing

Alpha and beta radiation are generally called radioactive decays. This is a process that is emission from the nucleus, occurring at enormous speed. As a result, an atom or its isotope can change from one chemical element to another. Alpha and beta decays of nuclei are characteristic of unstable elements. These include all atoms with a charge number greater than 83 and a mass number greater than 209.

Conditions for the reaction to occur

Decay, like other radioactive transformations, can be natural or artificial. The latter occurs due to the entry of some foreign particle into the nucleus. How much alpha and beta decay an atom can undergo depends only on how quickly a stable state is achieved.

Under natural circumstances, alpha and beta minus decays occur.

Under artificial conditions, neutron, positron, proton and other, rarer types of decays and transformations of nuclei are present.

These names were given by someone who studied radioactive radiation.

Difference between stable and unstable kernel

The ability to decay directly depends on the state of the atom. The so-called “stable” or non-radioactive nucleus is characteristic of non-decaying atoms. In theory, such elements can be observed indefinitely to finally verify their stability. This is required in order to separate such nuclei from unstable ones, which have an extremely long half-life.

By mistake, such a “slowed down” atom can be taken for a stable one. However, a striking example can be tellurium, and more specifically, its isotope with number 128, which has a lifespan of 2.2·10 24 years. This case is not isolated. Lanthanum-138 has a half-life of 10-11 years. This period is thirty times the age of the existing universe.

The essence of radioactive decay

This process occurs randomly. Each decaying radionuclide acquires a speed that is constant for each case. The rate of decay cannot change under the influence of external factors. It doesn’t matter whether the reaction will occur under the influence of enormous gravitational force, at absolute zero, in an electric and magnetic field, during any chemical reaction, etc. The process can only be influenced by direct influence on the inside of the atomic nucleus, which is practically impossible. The reaction is spontaneous and depends only on the atom in which it occurs and its internal state.

When talking about radioactive decay, the term “radionuclide” is often used. For those unfamiliar with it, the word refers to a group of atoms that have radioactive properties, their own mass number, atomic number, and energy status.

Various radionuclides are used in technical, scientific and other areas of human activity. For example, in medicine, these elements are used in diagnosing diseases, processing medications, instruments and other items. There are even a number of therapeutic and prognostic radiotherapy drugs.

Equally important is the determination of the isotope. This word refers to a special type of atom. They have the same atomic number as a regular element, but a different mass number. This difference is caused by the number of neutrons, which do not affect the charge, like protons and electrons, but change the mass. For example, simple hydrogen has as many as 3 of them. This is the only element whose isotopes have been given names: deuterium, tritium (the only radioactive one) and protium. In other cases, names are given according to atomic masses and the main element.

Alpha decay

This is a type of radioactive reaction. Characteristic of natural elements from the sixth and seventh periods of the periodic table of chemical elements. Particularly for artificial or transuranium elements.

Elements subject to alpha decay

The metals that are characterized by this decay include thorium, uranium and other elements of the sixth and seventh periods from the periodic table of chemical elements, starting from bismuth. Isotopes of heavy elements are also subjected to the process.

What happens during the reaction?

During alpha decay, particles consisting of 2 protons and a pair of neutrons begin to be emitted from the nucleus. The emitted particle itself is the nucleus of a helium atom, with a mass of 4 units and a charge of +2.

As a result, a new element appears, which is located two cells to the left of the original one in the periodic table. This arrangement is determined by the fact that the original atom has lost 2 protons and, at the same time, the initial charge. As a result, the mass of the resulting isotope decreases by 4 mass units compared to the initial state.

Examples

During this decay, thorium is formed from uranium. From thorium comes radium, from it comes radon, which ultimately gives polonium, and finally lead. In this process, isotopes of these elements arise, and not themselves. So, we get uranium-238, thorium-234, radium-230, radon-236 and so on, until a stable element appears. The formula for such a reaction is as follows:

Th-234 -> Ra-230 -> Rn-226 -> Po-222 -> Pb-218

The speed of the isolated alpha particle at the moment of emission ranges from 12 to 20 thousand km/sec. Being in a vacuum, such a particle would circle the globe in 2 seconds, moving along the equator.

Beta decay

The difference between this particle and an electron is in the place of its appearance. Beta decay occurs in the nucleus of an atom, not the electron shell surrounding it. The most common of all existing radioactive transformations. It can be observed in almost all currently existing chemical elements. It follows from this that each element has at least one isotope susceptible to decay. In most cases, beta decay results in beta minus decomposition.

Reaction progress

At this process An electron is ejected from the nucleus, resulting from the spontaneous transformation of a neutron into an electron and a proton. In this case, protons, due to their greater mass, remain in the nucleus, and the electron, called a beta minus particle, leaves the atom. And since there are more protons by one, the nucleus of the element itself changes upward and is located to the right of the original one in the periodic table.

Examples

The decay of beta with potassium-40 turns it into the isotope calcium, which is located on the right. Radioactive calcium-47 becomes scandium-47, which can become stable titanium-47. What does this beta decay look like? Formula:

Ca-47 -> Sc-47 -> Ti-47

The emission speed of a beta particle is 0.9 times the speed of light, which is 270 thousand km/sec.

There are not too many beta-active nuclides in nature. There are quite a few significant ones. An example is potassium-40, which naturally contains only 119/10,000. Also significant natural beta-minus active radionuclides are the products of alpha and beta decay of uranium and thorium.

Beta decay has a typical example: thorium-234, which, by alpha decay, turns into protactinium-234, and then in the same way becomes uranium, but with a different isotope numbered 234. This uranium-234 again becomes thorium due to alpha decay , but of a different variety. This thorium-230 then becomes radium-226, which turns into radon. And in the same sequence, up to thallium, only with different beta transitions back. This radioactive beta decay ends with the formation of stable lead-206. This transformation has the following formula:

Th-234 -> Pa-234 -> U-234 -> Th-230 -> Ra-226 -> Rn-222 -> At-218 -> Po-214 -> Bi-210 -> Pb-206

Natural and significant beta-active radionuclides are K-40 and the elements thallium to uranium.

Beta plus decay

There is also beta plus transformation. It is also called positron beta decay. In it, a particle called a positron is emitted from the nucleus. The result is the transformation of the original element into the one on the left, which has a lower number.

Example

When electron beta decay occurs, magnesium-23 becomes a stable isotope of sodium. Radioactive europium-150 becomes samarium-150.

The resulting beta decay reaction can create beta+ and beta- emissions. The particle emission speed in both cases is 0.9 times the speed of light.

Other radioactive decays

Apart from reactions such as alpha decay and beta decay, the formula of which is widely known, there are other, rarer processes that are characteristic of artificial radionuclides.

Neutron decay. A neutral particle of 1 mass unit is emitted. During it, one isotope transforms into another with a lower mass number. An example would be the conversion of lithium-9 to lithium-8, helium-5 to helium-4.

When the stable isotope iodine-127 is irradiated with gamma rays, it becomes isotope number 126 and acquires radioactivity.

Proton decay. It is extremely rare. During it, a proton is emitted, having a charge of +1 and 1 unit of mass. The atomic weight decreases by one value.

Any radioactive transformation, in particular radioactive decays, is accompanied by the release of energy in the form of gamma radiation. They are called gamma quanta. In some cases, X-rays of lower energy are observed.

It is a stream of gamma quanta. It is electromagnetic radiation, harder than x-rays, which is used in medicine. As a result, gamma quanta, or energy flows from the atomic nucleus, appear. X-ray radiation is also electromagnetic, but arises from the electron shells of the atom.

Alpha particle range

Alpha particles with a mass of 4 atomic units and a charge of +2 move in a straight line. Because of this, we can talk about the range of alpha particles.

The range value depends on the initial energy and ranges from 3 to 7 (sometimes 13) cm in the air. In a dense environment it is a hundredth of a millimeter. Such radiation cannot penetrate a sheet of paper or human skin.

Due to its own mass and charge number, the alpha particle has the greatest ionizing ability and destroys everything in its path. In this regard, alpha radionuclides are the most dangerous for people and animals when exposed to the body.

Penetrating power of beta particles

Due to its small mass number, which is 1836 times less than a proton, negative charge and size, beta radiation has a weak effect on the substance through which it flies, but the flight is longer. Also, the path of the particle is not linear. In this regard, they talk about penetrating ability, which depends on the energy received.

The penetrating ability of beta particles produced during radioactive decay reaches 2.3 m in air; in liquids the calculation is carried out in centimeters, and in solids - in fractions of a centimeter. The tissues of the human body transmit radiation to a depth of 1.2 cm. To protect against beta radiation, a simple layer of water up to 10 cm can serve. The flow of particles with a fairly high decay energy of 10 MeV is almost entirely absorbed by the following layers: air - 4 m; aluminum - 2.2 cm; iron - 7.55 mm; lead - 5.2 mm.

Given their small size, beta radiation particles have low ionizing ability compared to alpha particles. However, when ingested, they are much more dangerous than during external exposure.

Neutron and gamma radiation currently have the highest penetrating rates among all types of radiation. The range of these radiations in the air sometimes reaches tens and hundreds of meters, but with lower ionizing characteristics.

Most isotopes of gamma rays do not exceed 1.3 MeV in energy. Rarely values ​​of 6.7 MeV are reached. In this regard, to protect against such radiation, layers of steel, concrete and lead are used for the attenuation factor.

For example, to attenuate cobalt gamma radiation tenfold, lead protection with a thickness of about 5 cm is required; for 100-fold attenuation, 9.5 cm will be required. Concrete protection will be 33 and 55 cm, and water protection - 70 and 115 cm.

The ionizing properties of neutrons depend on their energy parameters.

In any situation, the best protective method against radiation will be to stay as far away from the source as possible and spend as little time as possible in an area of ​​high radiation.

Fission of atomic nuclei

By atoms we mean spontaneous, or under the influence of neutrons, into two parts, approximately equal in size.

These two parts become radioactive isotopes of elements from the main part of the table of elements. They start from copper to lanthanides.

During the release, a couple of extra neutrons escape and an excess of energy appears in the form of gamma rays, which is much greater than during radioactive decay. Thus, during one act of radioactive decay one gamma quanta appears, and during a fission act 8.10 gamma quanta appear. Also, scattered fragments have high kinetic energy, which turns into thermal indicators.

The released neutrons can provoke the separation of a pair of similar nuclei if they are located nearby and the neutrons hit them.

In this regard, there is a possibility of a branching, accelerating chain reaction of the separation of atomic nuclei and the creation large quantity energy.

When such a chain reaction is under control, it can be used for certain purposes. For example, for heating or electricity. Such processes are carried out in nuclear power plants and reactors.

If you lose control of the reaction, an atomic explosion will occur. This is what is used in nuclear weapons.

Under natural conditions, there is only one element - uranium, which has only one fissile isotope with the number 235. It is weapons-grade.

In an ordinary uranium nuclear reactor, a new isotope number 239 is formed from uranium-238 under the influence of neutrons, and from it plutonium, which is artificial and does not occur naturally. In this case, the resulting plutonium-239 is used for weapons purposes. This process of fission of atomic nuclei is the essence of everything atomic weapons and energy.

Phenomena such as alpha decay and beta decay, the formula of which is studied in school, are widespread in our time. Thanks to these reactions, there are nuclear power plants and many other industries based on nuclear physics. However, we should not forget about the radioactivity of many of these elements. When working with them, special protection and compliance with all precautions are required. Otherwise, it could lead to an irreparable disaster.

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Subtitles

Everything we've discussed so far in chemistry has been based on the stability of electrons, and where they are most likely to reside in stable shells. They leave the core. Electrons in neutrons and all that. Let's sign. a certain amount protons and neutrons. Here is an alpha particle. The helium nucleus has an atomic mass of four and an atomic number of two.

Theory

Alpha decay from main state is observed only in fairly heavy nuclei, for example, in radium-226 or uranium-238. Alpha radioactive nuclei in the table of nuclides appear starting with atomic number 52 (tellurium) and a mass number of about 106-110, and with an atomic number greater than 82 and a mass number greater than 200, almost all nuclides are alpha radioactive, although they may have alpha decay and a non-dominant decay mode. Among natural isotopes alpha radioactivity is observed in several nuclides of rare earth elements (neodymium-144, samarium-147, samarium-148, europium-151, gadolinium-152), as well as in several nuclides of heavy metals (hafnium-174, tungsten-180, osmium- 186, platinum-190, bismuth-209, thorium-232, uranium-235, uranium-238) and in short-lived decay products of uranium and thorium.

Alpha decay from highly excited nuclear states are also observed in a number of light nuclides, for example, lithium-7.

An alpha particle undergoes a tunnel transition through a potential barrier, caused by nuclear forces, so alpha decay is an essentially quantum process. Since the probability of the tunneling effect depends exponentially on the barrier height, the half-life of alpha-active nuclei increases exponentially with decreasing alpha particle energy (this fact constitutes the content of the Geiger-Nattall law). When the alpha particle energy is less than 2 MeV, the lifetime of alpha-active nuclei significantly exceeds the lifetime of the Universe. Therefore, although most natural isotopes heavier than cerium are in principle capable of decaying through this channel, only a few of them have actually recorded such decay. Danger to living organisms

Being quite heavy and positively charged, alpha particles from radioactive decay have a very short range in matter and, when moving through a medium, quickly lose energy at a short distance from the source. This results in all the radiation energy being released in a small volume of the substance, which increases the chances of cell damage when the radiation source enters the body. However external Radiation from radioactive sources is harmless, since alpha particles can be effectively retained by a few centimeters of air or tens of micrometers of dense matter - for example, a sheet of paper and even the stratum corneum of the epidermis, without reaching living cells. Even touching a source of pure alpha radiation is not dangerous, although it should be remembered that many sources of alpha radiation also emit much more penetrating types of radiation (beta particles, gamma rays, sometimes neutrons). However, if an alpha source enters the body, it results in significant radiation exposure. The quality factor of alpha radiation is 20 (more than all other types of ionizing radiation, with the exception of heavy nuclei and fission fragments). This means that in living tissue, an alpha particle creates an estimated 20 times more damage than a gamma ray or beta particle of equal energy.

All of the above applies to radioactive sources of alpha particles, the energies of which do not exceed 15 MeV. Alpha particles produced at an accelerator can have significantly higher energies and create a significant dose even with external irradiation of the body.

2.3 Patternsα - Andβ -decay

ActivityAnuclidein a radioactive source, the number of decays occurring with the nuclei of a sample in 1 s is called:

Activity unit — becquerel (Bq): 1Bq - activity of a nuclide, at which one decay event occurs in 1 s.Non-system unit of activitynuclide in a radioactive source -curie (Ku): 1 Ku=3.7 1010 Bk.

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

Alpha particles emitted by the nuclei of radium atoms were used by Rutherford in experiments on scattering by the nuclei of heavy elements. The speed of α-particles emitted during the α-decay of radium nuclei, measured from the curvature of the trajectory in a magnetic field, is approximately equal to 1.5 107 m/s, and the corresponding kinetic energy is about 7.5 10–13 J (approximately 4.8 MeV). This value can be easily determined from the known values ​​of the masses of the mother and daughter nuclei and the helium nucleus. Although the speed of the escaping α-particle is enormous, it is still only 5% of the speed of light, so when calculating, you can use a non-relativistic expression for kinetic energy.

Research has shown that a radioactive substance can emit alpha particles with several discrete energies. This is explained by the fact that nuclei can be, like atoms, in different excited states. The daughter nucleus may end up in one of these excited states during α decay. During the subsequent transition of this nucleus to the ground state, a γ-quantum is emitted. A diagram of the α-decay of radium with the emission of α-particles with two values ​​of kinetic energies is shown in Figure 2.4.

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

In the theory of α-decay, it is assumed that groups consisting of two protons and two neutrons, i.e., an α particle, can be formed inside nuclei. The mother nucleus is a potential well for α particles, which is limited by a potential barrier. The energy of the α particle in the nucleus is not sufficient to overcome this barrier (Figure 2.5). The escape of an alpha particle from the nucleus is possible only due to a quantum mechanical phenomenon called the tunneling effect. According to quantum mechanics, there is a non-zero probability of a particle passing under a potential barrier. The phenomenon of tunneling is probabilistic in nature.

Beta decay. During beta decay, an electron is ejected from the nucleus. Electrons cannot exist inside nuclei (see § 1.2); they arise during beta decay as a result of the transformation of a neutron into a proton. This process can occur not only inside the nucleus, but also with free neutrons. The average lifetime of a free neutron is about 15 minutes. When a neutron decaysturns into a protonand electron

Measurements have shown that in this process there is an apparent violation of the law of conservation of energy, since the total energy of the proton and electron resulting from the decay of a neutron is less than the energy of the neutron. In 1931, W. Pauli suggested that during the decay of a neutron, another particle with zero mass and charge is released, which takes away part of the energy. The new particle is namedneutrino(small neutron). Due to the lack of charge and mass of a neutrino, this particle interacts very weakly with the atoms of matter, so it is extremely difficult to detect in experiment. The ionizing ability of neutrinos is so small that one ionization event in the air occurs approximately 500 km of the way. This particle was discovered only in 1953. It is now known that there are several types of neutrinos. During the decay of a neutron, a particle is created, which is called an electronantineutrino. It is indicated by the symbolTherefore, the neutron decay reaction is written in the form

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

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

Along with electronic β decay, the so-called positron β decay was discovered+ -decay in which a positron is emitted from the nucleusand neutrinos. A positron is a particle twin of an electron, differing from it only in the sign of its charge. The existence of the positron was predicted by the outstanding physicist P. Dirac in 1928. A few years later, the positron was discovered in cosmic rays. Positrons arise as a result of the reaction of converting a proton into a neutron according to the following scheme:

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