Conditions for the decay of radioactive elements. General properties of beta decay

Radioactive decay is a process in which elementary particles are lost from the nucleus of an isotope, causing the isotope to become more stable element. These subatomic substances leave the atom at tremendous speed. As the isotope decays, it emits radioactive gamma radiation, as well as alpha and beta particles. Explanation this process is that most nuclei are unstable. Isotopes are varieties of the same chemical element with the same number of protons, but with different amounts neutrons.

Types of radioactive alpha and beta decay. Read more about them below. During alpha decay, helium is released, which is also called an alpha particle; during beta decay, the nucleus of an atom loses an electron, moving forward along the periodic table one position, and gamma radiation is the decay of nuclei with the simultaneous emission of photons, or gamma rays. IN the latter case the process occurs with a loss of energy, but without modification of the chemical element.

The radioactive decay reaction proceeds in such a way that, over a certain period of time, a number of nucleons emanate from the nucleus of the elements, proportional to the number of nucleons that still remain in the nucleus. That is, the more of them still remain in the atom, the more of them will come out of it. The rate of decay of an atom is determined by the so-called radioactivity constant, which is also known as the radioactive decay constant. However, this is not what is usually measured in physics. Instead, a value such as the half-life is used - the time during which the nucleus will lose half of its nucleons. It depends on the type of substance and can last from insignificant shares seconds to billions of years. In other words, some atomic nuclei can exist forever, while others can exist for a very short time before decaying.

The isotope that was the original one during the decay process is called the parent isotope, and the resulting result is called the daughter isotope.

Radioactive elements are created in the vast majority of cases as a result of a chain of atomic fission reactions. For example: the “mother” (primary) nucleus breaks up into several “daughter” ones, which, in turn, also divide. And this chain is not interrupted until stable isotopes are formed. For example: the half-life of uranium is more than four and a half billion years. During this time, this element first forms thorium, which in turn becomes palladium, and at the end of this whole long chain there will be lead. Or rather, its stable isotope.

Radioactive decay has a number of its own characteristics. One cannot remain silent about his “ side effects" For example, if we take a sample of any radioactive isotope, as a result of its decay we will obtain a series with different weights kernels. Many fission chains can be cited as examples. Radioactivity is by and large, natural phenomenon. After all, nuclear decay of substances occurred long before man discovered these mechanisms. However, the activity of this decay led to an increase in the radioactive background of the entire planet. In particular, due to the artificial acceleration of such natural processes.

Radioactive decay for humanity results in both new opportunities and dangers. It is worth remembering at least the process. It, in particular, leads to the formation of radon-222. This gas is large quantities found on the planet. In itself, it does not pose any danger, but only until the nuclei of its atoms begin to disintegrate into other elements. Its fission products, especially in an unventilated area, are harmful to human health.

Radioactive decay as a process can be beneficial. But only if you use its products correctly. For example, radioactive phosphorus, injected into the body, helps obtain information about the condition of the patient's bones. The rays emitted by it are recorded by light-sensitive equipment, which makes it possible to obtain accurate images with recorded fracture sites. The degree of its radioactivity is very small and cannot cause any harm to humans.

Let us consider the second case using the example of the decay of the Chlorine-17 isotope, the diagram of which is shown in Figure 7.

It is clear from the diagram that the actual b-decay of Chlorine-17 can occur in three ways (blue lines).

In the first case, the atom of the daughter nuclide Argon-18 is formed in the ground state. This completes the act of single decay.

In the second case, the daughter nuclide atom is formed in an excited state (excitation energy is 2.170 MeV). An atom is in an excited state limited time, after which it goes to the ground state, emitting a g-quantum. The energy of this quantum is exactly equal to the excitation energy.

In the third case, the daughter nuclide atom is also formed in an excited state (excitation energy is 3.77 MeV). However, unlike the second case, here the atom of the daughter nuclide can go to the ground state in two ways.

Firstly, the atom can immediately go to the ground state by emitting a gamma quantum with an energy of 3.77 MeV.

The probability of such a transition is low and only 0.06% of atoms “go” along this path. Secondly, (for this the way goes

the vast majority of atoms - 99.94%) an atom can first emit a g-quantum with an energy of 1.60 MeV and go into a state with a lower excitation energy, and then, after some time, go to the ground state, emitting a g-quantum with the energy 2.17 MeV. Such sequential emission of g-quanta is called a g-cascade. It is obvious that the energy spectrum of g-quanta in in this case will ruled

. The spectrum will have three lines with energies of 1.60 MeV, 2.17 MeV and 3.77 MeV. If the atoms of the daughter nuclide are formed only in the ground state, then in this case the parent nuclide will be clean

an a - or b -emitter, but there will be no g -radiation.

An example is the decay of Polonium-210 (pure a-emitter), the diagram of which is shown in Fig. 8. When gamma quanta are emitted, the energy of the quanta can be in the range from 5 KeV to 7 MeV, with the lower limit being in the region of the characteristic.

x-ray radiation Due to the fact that g-quanta have neither electric charge

, nor the rest mass, the emission of g-quanta does not lead to a change in the number of nucleons A and the charge of the nucleus Z. Quantum with energy D E, equal difference

energies of the daughter nuclide nucleus in the initial (excited) E 2 and E 1 final (main or excited with a lower excitation energy):

D E = E 2 - E 1 = E g

It is not always possible to leave the atom. It often interacts with one of the shell electrons of the atom. If the energy D E is greater than the binding energy of the electron E St, then the electron has a chance to leave the atom. Such electrons are called conversion electrons . Obviously, the energy of such electrons will be the same as the energy of g-quanta:

discrete

where E is the recoil energy of the daughter nuclide (see Fig. 9).

In most cases, the conversion electrons are the electrons of the K-shell closest to the nucleus. If the energy donated by the nucleus is less than Est for the K-shell electrons, then the conversion electrons are split off from the outer shells (L, M), where the binding energy is lower.

After the removal of a conversion electron, a vacancy is formed, which is filled with electrons from the outer shells. In this case, the corresponding X-ray radiation is formed, called characteristic K a, K b, La, ...

The characteristic X-ray radiation can in turn be converted. The electrons emitted in this case are called Auger electrons after the scientist who discovered them.

Figure 10 shows a diagram that explains everything that has been said.

Most atomic nuclei are unstable. Sooner or later they spontaneously (or, as physicists say, spontaneously) decay into smaller nuclei and elementary particles, which are commonly called decomposition products or child elements. Decaying particles are usually called starting materials or parents. All of the chemicals we are familiar with (iron, oxygen, calcium, etc.) have at least one stable isotope. (Isotopes varieties of a chemical element with the same number of protons in the nucleus are called - this number of protons corresponds to the atomic number of the element - but different numbers neutrons.) The fact that these substances are well known to us indicates their stability - which means they live long enough to significant quantities accumulate in natural conditions, without breaking up into components. But each of them natural elements There are also unstable isotopes - their nuclei can be obtained in the process of nuclear reactions, but they do not live long because they quickly decay.

Nuclear decay radioactive elements or isotopes can occur in three main ways, and the corresponding reactions nuclear fission named with the first three letters Greek alphabet. At alpha decay A helium atom consisting of two protons and two neutrons is released - it is usually called an alpha particle. Since alpha decay entails a decrease in the number of positively charged protons in an atom by two, the nucleus that emitted the alpha particle turns into the nucleus of an element two positions lower from it in the periodic table. At beta decay the nucleus emits an electron and the element moves one position forward according to the periodic table (in this case, essentially, a neutron turns into a proton with the radiation of this very electron). Finally, gamma decay - This decay of nuclei with the emission of high-energy photons, which are commonly called gamma rays. In this case, the nucleus loses energy, but the chemical element does not change.

However, the mere fact of instability of one or another isotope of a chemical element does not mean that by collecting together a certain number of nuclei of this isotope, you will get a picture of their instantaneous decay. In reality, the decay of the nucleus of a radioactive element is somewhat reminiscent of the process of frying corn when making popcorn: the grains (nucleons) fall off the “cob” (kernel) one at a time, in a completely unpredictable order, until all of them fall off. The law describing the reaction of radioactive decay, in fact, only states this fact: over a fixed period of time, a radioactive nucleus emits a number of nucleons proportional to the number of nucleons remaining in its composition. That is, the more grains-nucleons still remain in the “undercooked” cob-kernel, the more of them will be released during a fixed “frying” time interval. When translating this metaphor into language mathematical formulas we get an equation describing radioactive decay:

d N = λN d t

where d N— number of nucleons emitted by a nucleus with total number nucleons N in time d t, A λ - experimentally determined radioactivity constant test substance. The above empirical formula represents a linear differential equation, whose solution is next function, describing the number of nucleons remaining in the nucleus at a time t:

N = N 0 e - λt

Where N 0 is the number of nucleons in the nucleus per starting moment observations.

The radioactivity constant thus determines how quickly the nucleus decays. However, experimental physicists usually measure not it, but the so-called half-life nucleus (that is, the period during which the nucleus under study emits half of the nucleons it contains). For different isotopes of different radioactive substances, half-lives vary (in full accordance with theoretical predictions) from billionths of a second to billions of years. That is, some nuclei live almost forever, and some decay literally instantly (here it is important to remember that after the half-life time, half of the total mass remains starting material, after two half-lives - a quarter of its mass, after three half-lives - one-eighth, etc.).

As for the emergence of radioactive elements, they are born in different ways. In particular, the ionosphere (the upper rarefied layer of the atmosphere) of the Earth is constantly bombarded cosmic rays, consisting of particles with high energies (cm. Elementary particles). Under their influence, long-lived atoms are split into unstable isotopes: in particular, from stable nitrogen-14 to earth's atmosphere the unstable isotope carbon-14 with 6 protons and 8 neutrons in the nucleus is constantly formed ( cm. Radiometric dating).

But the above case is rather exotic. Much more often, radioactive elements are formed in reaction chains nuclear fission . This is the name given to a series of events during which the original (“mother”) nucleus decays into two “daughter” (also radioactive), which, in turn, decay into four “granddaughter” nuclei, etc. The process continues until until stable isotopes are obtained. As an example, let's take the isotope uranium-238 (92 protons + 146 neutrons) with a half-life of about 4.5 billion years. This period, by the way, is approximately equal to the age of our planet, which means that approximately half of the uranium-238 from the composition of the primordial matter of the formation of the Earth is still found in the totality of elements earthly nature. Uranium-238 turns into thorium-234 (90 protons + 144 neutrons), which has a half-life of 24 days. Thorium-234 turns into palladium-234 (91 protons + 143 neutrons) with a half-life of 6 hours - etc. After more than ten decay stages, the stable isotope of lead-206 is finally obtained.

ABOUT radioactive decay Much can be said, but a few points need to be especially noted. Firstly, even if we take a pure sample of any one radioactive isotope as a starting material, it will decay into different components, and soon we will inevitably get a whole “bouquet” of different radioactive substances with different nuclear masses. Secondly, natural chains of reactions atomic decay reassure us in the sense that radioactivity is a natural phenomenon, it existed long before man, and there is no need to take sin on our souls and blame only human civilization in what is available on Earth background radiation. Uranium-238 existed on Earth from its very inception, decayed, is decaying - and will decay, and nuclear power plants speed up this process, in fact, by a fraction of a percent; so nothing special harmful influence in addition to what is provided by nature, they do not affect you and me.

Finally, the inevitability of radioactive atomic decay poses both potential problems and potential opportunities for humanity. In particular, in the chain of reactions of the decay of uranium-238 nuclei, radon-222 is formed - a noble gas without color, smell and taste, which does not enter into any form. chemical reactions because it is not capable of forming chemical bonds. This inert gas, and it literally oozes from the depths of our planet. Usually it has no effect on us - it simply dissolves in the air and remains there in a slight concentration until it breaks down into even lighter elements. However, if this harmless radon remains in an unventilated room for a long time, then over time its decay products will begin to accumulate there - and they are harmful to human health (if inhaled). This is how we get the so-called “radon problem”.

On the other side, radioactive properties chemical elements bring significant benefits to people if you approach them wisely. Radioactive phosphorus, in particular, is now injected to produce a radiographic picture of bone fractures. The degree of its radioactivity is minimal and does not cause harm to the patient’s health. Entering bone tissue body along with ordinary phosphorus, it emits enough rays to record them on light-sensitive equipment and get pictures of a broken bone literally from the inside. Surgeons, accordingly, have the opportunity to operate on a complex fracture not blindly and at random, but by studying the structure of the fracture in advance using such images. In general, applications radiography there are countless numbers in science, technology and medicine. And they all work on the same principle: Chemical properties atom (essentially, the properties of the external electron shell) make it possible to assign a substance to a certain chemical group; then, using the chemical properties of this substance, the atom is delivered “to the right place”, after which, using the property of the nuclei of this element to decay in strict accordance with established by laws physicists “graphically”, decay products are recorded.

All atomic nuclei can be divided into two groups - stable and radioactive (unstable) nuclei. The number of stable isotopes and isotopes with a half-life comparable to the lifetime of the Earth is ~ 350. Most nuclei are unstable isotopes. In order for a radioactive substance to be discovered in nature, the half-life should not be much less age Earth or it must be formed as a result of the decay of another radioactive substance or in a nuclear reaction. Along with α-, β-, γ-radioactivity, and fission of atomic nuclei, new types of radioactive decay were discovered.
To more rare types radioactive decay include

• double β decay,
• proton and two-proton radioactivity,
• neutron radioactivity,
• cluster radioactivity.

In all types of radioactivity (except gamma radioactivity), the composition of the nucleus changes - the number of protons Z, mass number A or both.
The characteristics of radioactive decay are significantly influenced by the interactions that cause the decay. α decay is caused by strong interactions. β-decay is caused by weak interaction, and gamma decay is caused by electromagnetic interaction.
Exist various reasons, due to which the lifetimes of unstable nuclei can vary by several orders of magnitude.
a) The emission of heavy positively charged particles is strongly suppressed by the potential (Coulomb) barrier.
b) The reason for the long lifetimes of radioactive nuclei may be the low intensity of interaction due to which decay occurs.
c) The lifetime of a radioactive nucleus strongly depends on the energy released during decay. If this energy is low, then the lifetime increases sharply. The weak interaction is characterized by a particularly sharp dependence on the decay energy Q: τ ~ 1/Q 5 .
d) The lifetime of a radioactive nucleus also strongly depends on the difference between the spins of the initial and final nuclei.

Alpha decay. The phenomenon of α decay is that heavy nuclei spontaneously emit α particles. In this case, the mass number of the nucleus decreases by four units, and the atomic number by two:

(A,Z) → (A-4,Z-2) + 4 He.

Let us list the characteristic empirical features of α-decay:
a) α decay occurs on heavy nuclei with Z > 60.
b) The half-lives of known α-radioactive nuclei vary widely. Thus, the tungsten isotope 182 W has T 1/2 > 8.3·10 18 years, and the protactinium isotope 219 Pa has T 1/2 = 5.3·10 -8 s.

For even-even isotopes, the dependence of the half-life on the α-decay energy Q α well described by the empirical Geiger-Nettol law

log T 1/2 = A + B/√Q α ,

where A and B are constants that weakly depend on Z. Taking into account the charge of the final nucleus Z the relationship between the half-life T 1/2 and the α-decay energy can be represented as

log T 1/2 = 9.54·Z 0.6 /√Q α − 51.37,

where the half-life T 1/2 is expressed in seconds, and Q α in MeV. The picture shows experimental values half-lives for a radioactive even-even nuclei (Z varies from 74 to 106) and their description using the Geiger-Nettol relation.
For odd-even, even-odd and odd-odd kernels The general trend is preserved, but the half-lives are 2-1000 times longer than for even-even nuclei with the same Z and Q α.

E. Rutherford, 1936“In 1919 I showed that during the bombingα -particles of light elements can be destroyed by emitting a proton, i.e., a hydrogen nucleus. Therefore, we assumed that the proton should be one of structural units, of which the nuclei of other atoms are made, and theorists tried to explain the properties of the nucleus by combinations of protons and negative electrons. However, it is very difficult to combine a slow and heavy proton with a light and mobile electron in such a limited space as a nucleus, and when Chadwick discovered the existence of an uncharged particle - the neutron, this question apparently found its theoretical solution. It then became possible to assume that the nuclei of all atoms consist of a combination of protons and neutrons, so that, for example, oxygen with charge 8 and mass 16 has 8 protons and 8 neutrons. It was very simple idea, the meaning of which was that the particles composing the nucleus had the same mass. However, the question arose: how to explain the fact that a negative electron often flies out of the nucleus during radioactive transformations and that a positive electron appears during some artificial transformations? In response, theorists suggested that in the confined space of the nucleus, where the forces of interaction between particles are enormous, protons turn into neutrons, and vice versa. For example, if a neutron loses a negative electron, it becomes a proton, and if a proton loses a positive electron, it becomes a neutron, so in the first case a negative particle can be emitted, and in the second a positive one. Electrons and positrons do not exist in a free state in the nucleus, they are associated with a neutron or proton, depending on the circumstances, and can only be released under certain conditions when large changes in energy occur within the nucleus."

N-Z diagram of atomic nuclei. Dark color stable isotopes are shown.

G. Gamov, 1930: “The phenomenon of radioactivity, already discovered at the end of the last century, indicated that the nucleus of an atom is not a simple unit, but has a very complex structure. The α and β particles observed during the radioactive decay of elements were interpreted by Rutherford as constituent parts of the nucleus ejected from the unstable nuclei of heavy atoms, and the very hard radiation observed during the decay, γ-rays, were interpreted as electromagnetic disturbances caused by the restructuring of nuclei after decay. Further experiments by Rutherford also showed the possibility of artificial splitting of the nuclei of usually stable elements under the influence of external energetic influences.
The discovery of isotopes and Aston's research, which showed that their atomic weights are expressed in numbers very close to whole numbers, made it more than probable that the nuclei of all elements are built from protons and electrons, and formations consisting of four protons play a very important role in the structure of the nucleus and two electrons (α-particles) and have very high stability.
Very precise measurements of the atomic weights of isotopes revealed small deviations from whole numbers (mass defect), which led to the possibility of determining the total energy binding individual structural elements kernels into one whole.
Detailed studies of the spectra of γ-rays, which showed their line structure - studies for which we owe mainly to Ellis and Meitner - led to the conclusion that in the nucleus of the atom we are dealing with the existence of certain quantum energy levels, quite similar to those that we encounter in electronic system atom.
Most amazing fact, which we encounter in the theory of spontaneous nuclear decay, are those often incredibly long periods of time during which an unstable nucleus remains in statu quo before ejecting an α or β particle. The average lifespan of radioactive elements varies from an insignificant fraction of a second to extraordinarily long periods of many millions of years and, for each of this element, is a well-defined quantity.
It seemed very difficult to find the reasons that delay the departure of a particle for such long periods of time, if the particle has enough energy to leave the nucleus, and meanwhile the α- and β-particles ejected from the nucleus carry very, very significant reserves of energy.
It has long been known that there is a well-defined relationship between the energy of an ejected particle and the average period of its stay in the nucleus in an unstable state (the period of nuclear decay). In 1912, Geiger and Nattall noticed that if for elements that decay, we plot the energy of alpha particles on the abscissa axis, and the logarithm of the corresponding decay constant on the ordinate axis, then for a given radioactive family the points will lie approximately on a straight line. The three known radioactive families of uranium, thorium and actinium are represented by three parallel straight lines.

The experiments of Rutherford and Chadwick showed that in the case of very close collisions of α-particles with the nuclei of light elements, deviations of the number of scattered particles from the formula derived under the assumption are observed Coulomb interaction. The observed deviations can be explained by the assumption of the existence of these attractive forces - in this way we can form an idea of ​​​​the scope and laws of these forces. Unfortunately, at present there is no sufficiently detailed study of the anomalous scattering of α-particles, and theoretical conclusions boil down to approximately the following. For light elements (Mg, A1), anomalous attractive forces begin to affect distances of the order of 10 -12 cm, varying approximately in inverse proportion to the fourth or fifth power of the distance and overpower Coulomb repulsions at a distance of about 3∙10 -13 cm from the center of the nucleus - at smaller distances the α-particle is obviously already under the influence of the total attractive forces. For the nuclei of heavy radioactive elements that interest us, due to their large charge, the α particles at our disposal cannot approach such close distances and reach the region of anomalous forces. Rutherford and Chadwick, in experiments with the scattering of α particles in uranium, could reach (using the fastest α particles) only a distance of 3∙10 -12 cm and no deviations from normal scattering were noticed - the region of attractive forces obviously lies here much closer to the nucleus than 3∙10 -12 cm.
It would seem that the results of these experiments with uranium can help us very little - since the region of attractive forces could not be reached; These experiments contained the key to unraveling the phenomenon of α-decay.
When compared with data on the decay of uranium nuclei themselves, these experiments lead to a paradox that is completely inexplicable from the point of view classical mechanics. Indeed: the nuclei of uranium atoms are unstable and emit α-particles with an energy of about 6.8.10 -6 erg. According to our assumption about the existence of attractive forces near the nucleus, an alpha particle sitting in the nucleus of a radioactive element is surrounded by a kind of potential barrier, as shown in the figure. The fact that even at distances of 3∙10 -12 see we only have Coulomb forces, indicates that the maximum height of the barrier is in any case greater than

How can a uranium alpha particle with an energy of only 6.8.10 -6 erg "roll" over such a barrier? In other words: if α-particles "RaG", used in scattering experiments in uranium, "rolling" along the outer slope of the barrier, could not yet reach its top, how can α-particles of uranium, which have significantly with less energy, roll over the barrier and fly out? From the point of view of classical mechanics, an alpha particle, passing through such a barrier higher than its total energy, should have a “negative” inside the barrier kinetic energy" and therefore "imaginary speed".
However, the possibility of such a phenomenon, which is in sharp contradiction with classical mechanics, is a direct consequence of modern wave mechanics. Just as in wave optics light, incident on the interface between two media at an angle greater than the angle of full internal reflection, partly penetrates into the second medium - just as in wave mechanics, de Broglie-Schrödinger waves can partly penetrate into the region of “imaginary speed”, allowing particles to “roll” over the barrier.
Moving on to the question of the emission of an alpha particle from a nucleus surrounded by some potential barrier, we must first of all know the shape of this barrier. We have already seen that the course of the potential of anomalous attractive forces near and inside the core (internal ramp) is not precisely known; on the other hand, it is easy to see that the exact course of the potential on the internal steep slope of the barrier has relatively little effect on its permeability. In this case. it is most rational to make the simplest assumptions about its form; for subsequent calculations we will accept the barrier model given by the formulas

This model is characterized by two unknown quantities: core radius r 0 And internal potential U. The question of the departure of an alpha particle from space surrounded by a potential barrier comes down to solving the wave equation, which gives a receding spherical wave outside the nucleus. This problem leads to a number of discrete (quantum) energies of the α particle sitting inside the barrier, and to a number of corresponding escape probabilities.
In this essay, however, we will not dwell on the exact solution of the problem and will be satisfied with an approximate conclusion, which, however, is quite sufficient for comparison with experimental data. In view of high altitude barrier, we can, as a first approximation, consider the movement of a particle inside the nucleus as enclosed between infinitely high walls, forgetting that in two million years the particle will still fly out. We will be interested only in the state of lowest energy (the main orbit), since now it can be considered more than probable that all α particles in the nucleus have quantum number- one.
The probability of escape λ can be calculated approximately as the product of the number of collisions of an α-particle with a barrier and its permeability

.

It would seem that the phenomenon of β-decay should be easily explained using the same general principles, as well as α decay.
Indeed, the phenomenon of ejection of a nuclear electron is in many respects similar to the ejection of an α particle. We encounter here the same very long periods and the same quantitatively the same relationship between energy and decay period: slower β particles correspond to longer periods of nuclear life.
A significant difference, however, is the fact that the spectrum of β-particles is blurred.
Ellis's research has quite reliably established that β particles leave nuclei with velocities varying within very wide limits; on the other hand, there is absolutely no process that can compensate for this blurring of energies and bring balance total energy kernels. According to the law of conservation of energy, nuclei resulting from β-decay should have a very diverse supply of energy, and yet the discreteness of the velocities of the particles and the linearity of the γ-spectra indicate a well-defined discrete energy of the nuclei.
We thus come to the conclusion that for electrons inside the nucleus and escaping from it, the law of conservation of energy turns out to be inapplicable.
This and a number of other difficulties associated with the question of the movement of electrons inside the nucleus indicate that here we have come across something completely new, which cannot be explained on the basis of modern theoretical concepts. There is no doubt that all these difficulties in quantizing particles moving at a speed very close to the speed of light are in direct connection with the fundamental contradictions that modern theoretical physics in attempts to generalize wave mechanics to cases of relativistic motion. The study of the properties of electrons in the nucleus is currently the only area that can provide experimental material for the further development of the basic principles of theoretical physics.”

β-decay. The mentioned problem of non-conservation of energy during β-decay was solved by Pauli, who suggested that β-decay produces a neutrino simultaneously with an electron. The total energy of β-decay is distributed between the electron and the neutrino. Therefore, recording the energy of only the electron leads to an apparent non-conservation of the β-decay energy. The missing energy is carried away by neutrinos, the detection of which is an extremely difficult problem.
The study of β-decay has played an extremely important role in understanding the processes occurring in atomic nuclei. The phenomenon of β-decay is that the nucleus (A,Z) spontaneously emits leptons of the 1st generation - an electron (positron) and an electron neutrino (electron antineutrino), passing into a nucleus with the same mass number A, but with atomic number Z , one more or less. At e- capture, the nucleus absorbs one of the electrons atomic shell(usually from the K-shell closest to it), emitting neutrinos. In the literature, the term EC (Electron Capture) is often used for e-capture.
There are three types of β-decay β - -decay, β + -decay and e-capture.
β - : (A, Z) → (A, Z+1) + e - + e ,
β + : (A, Z) → (A,Z-1) + e + + ν e ,
e: (A, Z) + e - → (A,Z-1) + ν e .
Main featureβ-decay is that it is caused by weak interaction. Beta decay is not an intranuclear process, but an intranucleon one. A single nucleon decays in a nucleus. The nucleon transformations occurring inside the nucleus and the energy conditions for β-decay have the form (we assume zero neutrino mass):
β - (n → p + e - + e), M(A, Z) > M(A, Z+1) + m e ,
β + (p → n + e + + ν e), M(A, Z) > M(A, Z-1) + m e ,
e-capture (p + e - → n + ν e), M(A, Z) + m e > M(A, Z-1).

β-decay, like α-decay, occurs between discrete states of the initial (A,Z) and final (A,Z±1) nuclei. That's why for a long time After the discovery of the phenomenon of β-decay, it was not clear why the spectra of electrons and positrons emitted from the nucleus during β-decay were continuous, and not discrete, like the spectra of α-particles.
Experimental facts seemed incompatible with the laws of conservation of energy, momentum, and angular momentum. Thus, the total energy of the electron and the nucleus formed as a result of decay was less than the energy of the initial nucleus. In order to save the conservation laws, W. Pauli in 1930, in a letter to participants in a physics conference in Tübingen, suggested that in the process of β - decay, along with the electron, another very light (elusive) particle with zero electric charge and spin J = 1/2.

W. Pauli, 1930: “Dear radioactive ladies and gentlemen. With... the continuous β spectrum in mind, I made a desperate attempt to save exchange statistics and the law of conservation of energy. Namely, there is the possibility that in nuclei there are electrically neutral particles, which I will call “neutrons” and which have a spin of 1/2. The mass of the “neutron” should be comparable in order of magnitude to the mass of the electron and in any case not more than 0.01 of the mass of the proton. The continuous β-spectrum would then become understandable if we assume that during decay, along with the electron, a “neutron” is also emitted in such a way that the sum of the energies of the “neutron” and the electron remains constant.”

After the discovery of the neutron in 1932, E. Fermi proposed calling W. Pauli’s particle “neutrino.” In 1933, at the Solvay Congress, W. Pauli made a report on the mechanism of β-decay involving a neutral particle with spin J = l/2. Pauli's hypothesis saved not only the law of conservation of energy, but also the laws of conservation of angular momentum and momentum. The last doubts that the conservation laws reliably proven in classical physics are violated in quantum processes were rejected. In 1934, E. Fermi developed the theory of β-decay, based on the law of conservation of energy and the assumption that an electron and a neutrino are simultaneously emitted from the nucleus. Fermi explained the observed energy spectrum of electrons and related the rate of β decay to the maximum energy of electrons emitted during β decay. Most important element Fermi's theory of beta decay was the statement that there are no electrons in the nucleus.

Electron and neutrino are created at the moment of beta decay atomic nucleus.

This decay is analogous to the emission of light by an atom. A light quantum does not exist in an atom, but arises as a result of a change in the state of the atom. Neutrinos were experimentally discovered in 1956 in experiments by F. Reines and C. Cohen.

Basic characteristics of the electron

Main characteristics of the electron neutrino

+ ?.

After?-breakup atomic number element changes and it shifts one cell in the periodic table.
?-decay
?-decay called the spontaneous disintegration of an atomic nucleus into a product nucleus and a?-particle (nucleus of an atom).
?-decay is a property of heavy nuclei with mass number A >= 200. Inside such nuclei due to the saturation property nuclear forces Isolated?-particles are formed, consisting of two protons and two neutrons. A particle formed in this way feels Coulomb repulsion from other protons of the nucleus more strongly than individual protons. At the same time, the β-particle is less affected by the nuclear inclination of attraction due to strong interaction than for the remaining nucleons.
Soddy's displacement rule for?-decay:

As a result of?-decay, the element shifts 2 cells to the beginning of the periodic table. The daughter nucleus formed as a result of?-decay usually also turns out to be radioactive and after some time also decays. The process of radioactive decay will occur until a stable, that is, non-radioactive nucleus appears, which is most often a lead or bismuth nucleus.
?-decay
Gamma rays are electromagnetic waves with wavelength smaller sizes atom. They are usually formed during the transition of the atomic nucleus from an excited state to the ground state. In this case, the number of neutrons or protons in the nucleus does not change, which means the nucleus remains the same element. However, the emission of gamma rays can accompany other nuclear reactions.
During radioactive decay, transformations of atomic nuclei occur. The energy of the particles that are formed in this case is much greater than the energy released in typical chemical reactions. Therefore, these processes are practically independent of the chemical environment of the atom and the compounds in which this atom is included. Radioactive decay occurs spontaneously. This means that it is impossible to determine the moment when a particular nucleus will decay. However, for each type of decay there is a characteristic time during which half of all radioactive nuclei decay. This time is called the half-life. For different radioactive isotopes, the half-life can vary over a very wide range - from nanoseconds to millions of years. Isotopes with short half-lives are very radioactive but disappear quickly. Isotopes with long period half-lives are weakly radioactive, but this radioactivity persists for a very long time.

The detection of radioactive radiation is based on its action on a substance, in particular its ionization. Historically, radiation was first recorded due to the blackening of an irradiated photographic plate. Photographic emulsions, in which chemical reactions occur under the influence of radiation, still remain one of the detection methods. Another detection principle is used in Geiger counters - the occurrence of non-self electrical discharge in irradiated gas. Dosimeters that do not record individual passages of a fast charged particle often use changes in properties, such as conductivity, of the irradiated material
Radioactivity depends on the number of unstable isotopes and their lifetime. The SI system defines the unit of measurement for activity as the Becquerel - the amount of radioactive substance in which one decay event occurs per second. In practice, this value is not very convenient, so non-systemic units - Curie - are more often used. The Rutherford unit is sometimes used.
Regarding the effects of radioactive radiation on irradiated substances, the same units are used as for x-ray radiation. The unit of measurement for the dose of absorbed ionizing radiation in the C system is Gray - a dose at which one Joule of energy is released per kilogram of a substance. Unit biological action irradiance in the SI system is sievert. An extra-system unit of energy released during irradiation - tips.
A unit such as the x-ray is a measure not of the released energy, but of the ionization of a substance during radioactive irradiation. To simulate the biological effects of radiation, the biological equivalent of an X-ray, the rem, is used.
To characterize the intensity of radiation, units are used that describe the rate of dose accumulation, for example, roentgens per hour.
Radiation exposure causes significant tissue damage. Ionization of chemicals in biological tissue creates the possibility of chemical reactions that are unusual for biological processes, and to education harmful substances. Radiation damage to DNA causes mutations. Working with radioactive substances requires careful adherence to safety regulations. Radioactive substances are marked with a special symbol at the top of the page.
Radioactive substances are stored in special containers designed to absorb radioactive radiation. Big problem is the disposal of radioactive waste nuclear energy.
Radioactive substances can be used to produce energy in conditions where other energy sources are not available, e.g. spacecraft designed for flights to distant planets solar system. The energy released during radioactive decay in such devices can be converted into electricity using thermoelements.
In medicine radiation exposure used in the treatment of some forms of cancer, relying on the fact that cancer cells, which divide quickly, are sensitive to radiation and therefore attack more quickly.
The tagged atom method makes it possible to analyze metabolism in the body and helps in diagnosing diseases.
Dating for radioactive isotopes helps establish the age of objects and rocks and is used in geology, archeology, and paleontology.
Radioactivity and radioactive substances also widely used in various areas scientific research.
All types of radioactive radiation accompanying radioactivity are called ionizing radiation. Ionizing radiation is the process of excitation and ionization of atoms of matter during the passage of gamma quanta and particles formed as a result of?-and?-decay. When, for example, gamma rays pass through matter, the quanta turn into an electron-positron pair, provided that the energy of the gamma ray exceeds the energy of these two particles (> 1 MeV). ?-particles quickly lose all their energy because they excite all the atoms that they encounter in their path (1-10 cm in air, 0.01-0.2 mm in liquids). ?-particles interact less effectively with substances (2-3 m in air, 1-10 mm in liquids). ?-quanta have the greatest penetrating ability. Neutrons, which have no electrical charge, do not directly ionize atoms. However, as a result of the interaction of neutrons with nuclei, fast charged particles and gamma quanta appear, which are ionizing particles. When a person stays for a long time in the zone of radioactive radiation, ionization and excitation of its cells occurs. As a result, cells enter into new chemical reactions and form new ones. chemical substances that disrupt the normal functioning of the body. A measure of the effect of ionizing radiation is the absorbed dose of radiation (Gray), equal to ratio energy transferred by ionizing radiation to the mass of matter (D = E / m). Radiation dose rate is measured by the ratio of absorbed radiation dose to time (Pв = D/t). Radioactive radiation used for x-ray examination.