What types of beta decay exist? Radioactive transformations. Alpha and beta decay. Characteristics of radioactive transformations

Beta decay

β-decay, radioactive decay of an atomic nucleus, accompanied by the emission of an electron or positron from the nucleus. This process is caused by the spontaneous transformation of one of the nucleons of the nucleus into a nucleon of a different kind, namely: the transformation of either a neutron (n) into a proton (p), or a proton into a neutron. In the first case, an electron (e -) flies out of the nucleus - the so-called β - decay occurs. In the second case, a positron (e +) flies out of the nucleus - β + decay occurs. Departing under B.-r. electrons and positrons are collectively called beta particles. The mutual transformations of nucleons are accompanied by the appearance of another particle - the neutrino ( ν ) in the case of β+ decay or antineutrino A, equal to the total number of nucleons in the nucleus, does not change, and the nuclear product is an isobar of the original nucleus, standing next to it to the right in the periodic system of elements. On the contrary, during β + -decay, the number of protons decreases by one, and the number of neutrons increases by one, and an isobar is formed, which is adjacent to the left of the original nucleus. Symbolically, both processes of B.-r. are written in the following form:

where -Z neutrons.

The simplest example of β - decay is the transformation of a free neutron into a proton with the emission of an electron and an antineutrino (neutron half-life ≈ 13 min):

A more complex example (β - decay - the decay of a heavy isotope of hydrogen - tritium, consisting of two neutrons (n) and one proton (p):

Obviously, this process comes down to the β - decay of a bound (nuclear) neutron. In this case, the β-radioactive tritium nucleus turns into the nucleus of the next element in the periodic table - the nucleus of the light isotope of helium 3 2 He.

An example of β + decay is the decay of the carbon isotope 11 C according to the following scheme:

The transformation of a proton into a neutron inside a nucleus can also occur as a result of the proton capturing one of the electrons from the electron shell of the atom. Most often, electron capture occurs

B.-r. observed in both naturally radioactive and artificially radioactive isotopes. In order for a nucleus to be unstable with respect to one of the types of β-transformation (that is, it could experience a transformation), the sum of the masses of the particles on the left side of the reaction equation must be greater than the sum of the masses of the transformation products. Therefore, with B.-r. energy is released. Energy B.-r. Eβ can be calculated from this mass difference using the relation E = mc2, Where With - speed of light in vacuum. In the case of β decay

Where M - masses of neutral atoms. In the case of β+ decay, a neutral atom loses one of the electrons in its shell, the energy of the b.-r. is equal to:

Where me - electron mass.

Energy B.-r. distributed between three particles: electron (or positron), antineutrino (or neutrino) and nucleus; each of the light particles can carry away almost any energy from 0 to E β i.e. their energy spectra are continuous. Only during K-capture does a neutrino always carry away the same energy.

So, with β - decay, the mass of the initial atom exceeds the mass of the final atom, and with β + decay this excess is at least two electron masses.

Study of B.-r. Nuclei have repeatedly presented scientists with unexpected mysteries. After the discovery of radioactivity, the phenomenon of B.-r. has long been considered as an argument in favor of the presence of electrons in atomic nuclei; this assumption turned out to be in obvious contradiction with quantum mechanics (see Atomic nucleus). Then, the inconstancy of the energy of electrons emitted during B.-R. even gave rise to some physicists’ disbelief in the law of conservation of energy, because It was known that nuclei that are in states with a very definite energy participate in this transformation. The maximum energy of electrons escaping from the nucleus is exactly equal to the difference between the energies of the initial and final nuclei. But in this case, it was not clear where the energy disappears if the emitted electrons carry less energy. The assumption of the German scientist W. Pauli about the existence of a new particle - the neutrino - saved not only the law of conservation of energy, but also another important law of physics - the law of conservation of angular momentum. Since the Spins (i.e., the intrinsic moments) of the neutron and proton are equal to 1/2, then to preserve the spin on the right side of the B.-r. equations. There can only be an odd number of particles with spin 1/2. In particular, during the β - decay of a free neutron n → p + e - + ν, only the appearance of an antineutrino eliminates the violation of the law of conservation of angular momentum.

B.-r. occurs in elements of all parts of the periodic table. The tendency towards β-transformation arises due to the presence of an excess of neutrons or protons in a number of isotopes compared to the amount that corresponds to maximum stability. Thus, the tendency to β + -decay or K-capture is characteristic of neutron-deficient isotopes, and the tendency to β - -decay is characteristic of neutron-rich isotopes. About 1500 β-radioactive isotopes of all elements of the periodic table are known, except for the heaviest ones (Z ≥ 102).

Energy B.-r. currently known isotopes range from

half-lives are in a wide range from 1.3 10 -2 sec(12 N) to Beta decay 2 10 13 years (natural radioactive isotope 180 W).

Subsequent study of B.-r. has repeatedly led physicists to the collapse of old ideas. It was found that B.-r. governed by forces of a completely new nature. Despite the long period that has passed since the discovery of B.-r., the nature of the interaction that determines B.-r. has not been fully studied. This interaction was called “weak” because it is 10 12 times weaker than nuclear and 10 9 times weaker than electromagnetic (it exceeds only the gravitational interaction; see Weak interactions). Weak interaction is inherent in all elementary particles (See Elementary particles) (except for the photon). Almost half a century passed before physicists discovered that in B.-r. the symmetry between “right” and “left” may be broken. This nonconservation of spatial parity has been attributed to the properties of weak interactions.

Study of B.-r. had another important side. The lifetime of the nucleus relative to the B.-r. and the shape of the spectrum of β-particles depend on the states in which the original nucleon and the product nucleon are located inside the nucleus. Therefore, the study of magnetic resonance, in addition to information about the nature and properties of weak interactions, has significantly expanded the understanding of the structure of atomic nuclei.

Probability of B.-r. depends significantly on how close the states of the nucleons in the initial and final nuclei are to each other. If the state of the nucleon does not change (the nucleon seems to remain in the same place), then the probability is maximum and the corresponding transition of the initial state to the final state is called allowed. Such transitions are characteristic of B.-r. light nuclei. Light nuclei contain almost the same number of neutrons and protons. Heavier nuclei have more neutrons than protons. The states of nucleons of different types are significantly different from each other. This makes it difficult for B.-r.; transitions appear in which B.-r. occurs with low probability. The transition is also complicated by the need to change the spin of the nucleus. Such transitions are called forbidden. The nature of the transition also affects the shape of the energy spectrum of β-particles.

An experimental study of the energy distribution of electrons emitted by β-radioactive nuclei (beta spectrum) is carried out using a Beta spectrometer. Examples of β spectra are shown in rice. 1 And rice. 2 .

Lit.: Alpha, beta and gamma spectroscopy, ed. K. Siegbana, trans. from English, V. 4, M., 1969, ch. 22-24; Experimental Nuclear Physics, ed. E. Segre, trans. from English, vol. 3, M., 1961.

E. M. Leikin.

Neutron beta spectrum. The abscissa axis shows kinetic. electron energy E in kev, on the ordinate - the number of electrons N (E) in relative units (vertical bars indicate the limits of measurement errors for electrons with a given energy).

Great Soviet Encyclopedia. - M.: Soviet Encyclopedia. 1969-1978 .

Synonyms:

See what “Beta decay” is in other dictionaries:

Beta decay, radioactive transformations of atomic nuclei; in the process, nuclei emit electrons and antineutrinos (beta decay) or positrons and neutrinos (beta+ decay). Departing during B. r. electrons and positrons are collectively called. beta particles. At… … Big Encyclopedic Polytechnic Dictionary

Modern encyclopedia

Beta decay- (b decay), a type of radioactivity in which a decaying nucleus emits electrons or positrons. In electron beta decay (b), a neutron (intranuclear or free) turns into a proton with the emission of an electron and an antineutrino (see ... ... Illustrated Encyclopedic Dictionary

Beta decay- (β decay) radioactive transformations of atomic nuclei, during which the nuclei emit electrons and antineutrinos (β decay) or positrons and neutrinos (β+ decay). Departing during B. r. electrons and positrons are collectively called beta particles (β particles)... Russian encyclopedia of labor protection

- (b decay). spontaneous (spontaneous) transformations of a neutron n into a proton p and a proton into a neutron inside the at. nuclei (as well as the transformation of a free neutron into a proton), accompanied by the emission of electron e or positron e+ and electron antineutrinos... ... Physical encyclopedia

Spontaneous transformations of a neutron into a proton and a proton into a neutron inside an atomic nucleus, as well as the transformation of a free neutron into a proton, accompanied by the emission of an electron or positron and a neutrino or antineutrino. double beta decay... Nuclear energy terms

- (see beta) radioactive transformation of an atomic nucleus, in which an electron and an antineutrino or a positron and a neutrino are emitted; During beta decay, the electric charge of the atomic nucleus changes by one, but the mass number does not change. New dictionary... ... Dictionary of foreign words of the Russian language

beta decay- beta rays, beta decay, beta particles. The first part is pronounced [beta]... Dictionary of difficulties of pronunciation and stress in modern Russian language

Noun, number of synonyms: 1 decay (28) ASIS Dictionary of Synonyms. V.N. Trishin. 2013… Synonym dictionary

Beta decay, beta decay... Spelling dictionary-reference book

BETA DECAY- (ß decay) radioactive transformation of an atomic nucleus (weak interaction), in which an electron and an antineutrino or a positron and a neutrino are emitted; with B. r. the electric charge of the atomic nucleus changes by one, the mass (see) does not change... Big Polytechnic Encyclopedia

E. Resenford, together with the English radiochemist F. Soddy, proved that radioactivity is accompanied by the spontaneous transformation of one chemical element into another.
Moreover, as a result of radioactive radiation, the nuclei of atoms of chemical elements undergo changes.

DESIGNATION OF THE ATOMIC NUCLEUS

ISOTOPES

Among the radioactive elements, elements were discovered that were chemically indistinguishable, but different in mass. These groups of elements were called "isotopes" ("occupying one place in the periodic table"). The nuclei of atoms of isotopes of the same chemical element differ in the number of neutrons.

It has now been established that all chemical elements have isotopes.
In nature, all chemical elements, without exception, consist of a mixture of several isotopes, therefore, in the periodic table, atomic masses are expressed in fractional numbers.
Isotopes of even non-radioactive elements can be radioactive.

ALPHA - DECAY

Alpha particle (nucleus of a helium atom)
- characteristic of radioactive elements with a serial number greater than 83
.- the law of conservation of mass and charge number is necessarily satisfied.
- often accompanied by gamma radiation.

Alpha decay reaction:

During the alpha decay of one chemical element, another chemical element is formed, which in the periodic table is located 2 cells closer to its beginning than the original one

Physical meaning of the reaction:

As a result of the emission of an alpha particle, the charge of the nucleus decreases by 2 elementary charges and a new chemical element is formed.

Offset rule:

During the beta decay of one chemical element, another element is formed, which is located in the periodic table in the next cell after the original one (one cell closer to the end of the table).

BETA - DECAY

Beta particle (electron).
- often accompanied by gamma radiation.
- may be accompanied by the formation of antineutrinos (light electrically neutral particles with high penetrating power).
- the law of conservation of mass and charge number must be fulfilled.

Beta decay reaction:

Physical meaning of the reaction:

A neutron in the nucleus of an atom can turn into a proton, electron and antineutrino, as a result the nucleus emits an electron.

Offset rule:

FOR THOSE WHO ARE NOT TIRED YET

I suggest writing the decay reactions and handing in the work.
(make a chain of transformations)

1. The nucleus of which chemical element is the product of one alpha decay
and two beta decays of the nucleus of a given element?

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

Beta decay is the process of spontaneous transformation of a neutral nucleus into a nucleus - an isobar with a different charge at Z = ±1. The speed of particles emitted during beta decay is close to the speed of light.

Like -radiation, -radiation is deflected in magnetic and electric fields, but in the opposite direction and over a greater distance. This indicates that beta radiation is a stream of negatively charged particles of low mass. Based on the e/m ratio, Rutherford identified beta particles as ordinary electrons.

According to the Fajans-Soddy displacement rule, decay results in the appearance of an isotope of an element shifted one cell to the right of the original element without changing the mass number.

In order to distinguish electrons arising during nuclear transformations, they began to be called beta particles. Despite what is usually said about nuclei emitting electrons, atomic nuclei in their pure form do not contain electrons. A beta particle is formed in the very act of nuclear transformation.

Three types of decay are known: electronic-decay, positron + -decay and electronic K-capture electron by the nucleus with one of the shells closest to the nucleus.

During beta decay, the mass numbers of nuclei do not change, but only the charge changes, one more in the case of - decay and one less in the case of + decay and K-capture. According to Fajans-Soddy shift rule, for these types of decay we can write:

All three types of -decay are reduced to the following types of mutual transformation of nucleons in the nucleus.

Decay - n o r + + e - + ; Р S + e - + ; (-decay);

Decay - p n o + e + + ; C B + e + + (+ -decay);

K-capture - p + + e - n + ; Cs + e - Xe + (K- capture)

Thus, electrons and positrons are not located in the nucleus, but appear at the moment of transition of one nucleon to another. As can be seen from the transformation diagrams, a characteristic feature of all types of transformations is the emission neutrino or antineutrino.

For the first time the concept of neutrino introduced by W. Pauli into 1930 to explain the “lost” part of the energy during radioactive decay with the emission of an electron. Total particle energy and gamma quanta turned out to be several lower energy of particles entering into interaction. Pauli suggested that the missing part of the energy escapes with a particle, which he called "neutrino". Neutrino is an uncharged elementary particle with a rest mass close to zero. Neutrinos have exceptional penetrating power. It is extremely difficult to detect, since the passage of neutrinos through a material medium is practically not accompanied by any effect. Antineutrinos have the same properties.

As can be seen from the transformation schemes during electronic beta decay, one of the neutrons turns into a proton, and the mother nucleus emits an electron and an antineutrino. Schematically, this process is represented as follows:

Electronic beta decay can also be accompanied by gamma radiation. This occurs when, during the decay process, a nucleus is formed that is not in the ground state, but in an excited state. An example of such a decay is the transformation of strontium into yttrium:

The reverse process of converting a proton into a neutron in a free state is impossible, since the mass of the neutron is greater than the mass of the proton. However, nuclei located in N and Z coordinates below the stability line, as a result of nucleon rearrangement, can move from a less stable state to a more stable state by replacing one proton with a neutron. In this case, the proton loses its charge, turning into a neutron and positron (e +), a particle carrying a positive charge, but having the mass of an electron. Since when a positron is emitted, an electron is captured from the electron shell, ensuring the preservation of the electrical neutrality of the atom, positron decay can occur if the energy difference in the final and initial states exceeds 1.02 MeV, that is, more than the rest mass of two electrons. During positron decay, the positron immediately leaves the nucleus, and after slowing down, its mass annihilates along with the mass of the electron. The presence of positron decay is evidenced by the registration of two gamma quanta with energies of 0.51 MeV. This process occurs with the absorption of energy, since the mass of the neutron is greater than the mass of the proton.

When a positron and an electron annihilate, their mass is completely converted into the energy of two quanta. This energy is generated due to the restructuring of the rest of the nucleus:

e_+e+2+1.02 MeV

Positron emission is very rare in natural radionuclides and occurs mainly in artificially produced radionuclides using particle accelerators:

O N + e + ;Fe Mn + e + +

If the transformation energy value is less than 1.02 MeV, then positron emission is not possible. In this case, the parent nuclide passes into the daughter by capturing an electron, the so-called K-grab.

For nuclei of heavy elements with a lack of neutrons (neutron-deficient nucleus), the transformation of protons into neutrons occurs only through the electron K-capture mechanism. Since in an atom K-electrons are on average closest to the nucleus, there is some probability of the nucleus capturing an electron from the K-shell.

Since the mass of a neutron is greater than the total mass of a proton and an electron, additional energy is needed to carry out this reaction. This energy is taken by increasing the binding energy of the newly formed nucleus. For atoms of heavy elements, K-capture is more likely than positron emission.

The capture of an electron by a nucleus is always accompanied by x-ray radiation, since orbital electrons from shells located above immediately move to the vacant space at the lower energy level.

In addition, K-capture is accompanied by emission of Auger electrons from the excited electron shells of the atom.

For the nuclei of light elements, all three options are common - decay.

Beta decay is energetically possible if the rest mass of the system in the initial state is greater than its rest mass in the final state.

Since the rest mass of a neutrino (antineutrino) is equal to 0, the energy conditions for transformations have the form:

М(Z,A) М(Z + 1), A + m e- () - decay

M(Z,A) M(Z - 1), A + m e+ (+) decay

M(Z,A) + m e M(Z - 1), A -K capture

From these conditions it is clear that K-capture is energetically more favorable than positron decay.

Since the excitation energy that is carried away from nuclei during decay is redistributed between an electron and an antineutrino or between a positron and a neutrino and obeys the law of randomness, decay has a continuous energy spectrum. The sum of energies - particles and neutrinos (antineutrinos) is always equal to a constant value characteristic of a given isotope and is called maximum energy - spectrum.

E. Fermi derived an empirical equation relating the maximum energy of radiation to the decay constant, l:

The maximum energy of beta particles lies in the range of 0.015 - 15 MeV, and half-lives vary from 0.3 s to 6.10 14 years

Beta decay nucleus is the process of spontaneous transformation of an unstable nucleus into an isobar nucleus as a result of the emission of an electron (positron) or the capture of an electron. About 900 beta radioactive nuclei are known. Of these, only 20 are natural, the rest are obtained artificially.

There are three types of β-decay: electron β - decay, positron β + decay and electron capture (e-capture). The main type is the first.

At electronic β - decay one of the neutrons of the nucleus turns into a proton with the emission of an electron and an electron antineutrino.

Examples: decay of a free neutron

T 1/2 =10.7 min;

tritium decay

T 1/2 = 12 years.

At positron β+ decay one of the protons of the nucleus turns into a neutron with the emission of a positively charged electron (positron) and an electron neutrino

When electronic e-capture the nucleus captures an electron from the electron shell (usually the K-shell) of its own atom.

Beta decay is possible. when the difference between the masses of the initial and final nuclei exceeds the sum of the masses of the electron and neutrino. Whenever β+ decay is energetically possible, it is also possible e-capture. Beta decay is observed in nuclei with any mass number. The observable characteristics of beta decays are the half-life T 1/2, shapes of energy β-spectra and other characteristics.

The β - -decay energy lies in the range

()0,02 Mev < Е β < 13,4 Mev ().

The energy released during beta decay is distributed between the electron, neutrino and daughter nucleus. Spectrum of emitted β particles continuous from zero to maximum value. Calculation formulas maximum energy of beta decays:

where is the mass of the mother nucleus, is the mass of the daughter nucleus. m e–electron mass.

Half life T 1/2 associated with probability beta decay relation

The probability of beta decay strongly depends on the beta decay energy ( ~ Eβ 5 at Eβ >> m e c 2) therefore the half-life T 1/2 varies widely