Now we have finally come to the answer to the question of the origin of the mysterious beta particles. The source of their appearance is the process reverse to the transformation of a proton into a neutron, namely: the transformation of a neutron into a proton. From logical considerations, such a process is analogously associated with the emission of an electron (the same beta particle). After all, the loss of a negative charge is equivalent to the acquisition of a positive one. But where in a completely uncharged neutron can one find a negative charge and release it?
In fact, if everything was limited only to the emission of a negatively charged particle, this would simply be impossible. Centuries of experience have accustomed physicists to the idea that neither negative nor positive charge can arise from nothing. Just like none of these charges can disappear without any trace. This is the law of conservation of electric charge.
In reality, the neutron does not simply release a beta particle; at the same time, it also forms a proton, which completely balances the negative charge of the latter and maintains overall neutrality. Thus, in total no additional charge is formed. Likewise, when an electron meets a positron and annihilates, the net change in charge is also zero.
When a proton emits a positron to become a neutron, the original particle (proton) has a unit positive charge, and the two resulting particles (neutron and positron) also have a total charge of +1.
The nucleus is also capable of absorbing an electron, then the proton inside the nucleus turns into a neutron. An electron and a proton (their total charge is zero) form a chargeless neutron. Typically, the nucleus captures an electron from the K-shell closest to it, so this process is called K-capture. Immediately, the vacant place is occupied by an electron from a more distant L-shell, which is accompanied by the release of energy in the form of X-rays. This effect was first described in 1938 by the American physicist L. Alvarez. As a rule, chemical transformations that involve the movement of electrons do not affect nuclear reactions. But since K-capture involves not only nuclei, but also electrons, this process is to some extent associated with chemical changes.
Heavy ion storage devices open up fundamentally new opportunities in studying the properties of exotic nuclei. In particular, they allow the accumulation and long-term use of fully ionized atoms - “naked” nuclei. As a result, it becomes possible to study the properties of atomic nuclei that do not have an electronic environment and in which there is no Coulomb effect of the outer electron shell with the atomic nucleus.
Rice. 3.2 Scheme of e-capture in an isotope (left) and fully ionized atoms and (right)
Decay into a bound state of an atom was first discovered in 1992. The β-decay of a fully ionized atom into bound atomic states was observed. The 163 Dy nucleus is marked in black on the N-Z diagram of atomic nuclei. This means that it is a stable core. Indeed, being part of a neutral atom, the 163 Dy nucleus is stable. Its ground state (5/2 +) can be populated as a result of e-capture from the ground state (7/2 +) of the 163 Ho nucleus. The 163 Ho nucleus, surrounded by an electron shell, is β - radioactive and its half-life is ~10 4 years. However, this is only true if we consider the nucleus surrounded by an electron shell. For fully ionized atoms the picture is fundamentally different. Now the ground state of the 163 Dy nucleus is higher in energy than the ground state of the 163 Ho nucleus and the possibility opens up for the decay of 163 Dy (Fig. 3.2)
| → + e - + e . | (3.8) |
The electron resulting from the decay can be captured into the vacant K or L shell of the ion. As a result, decay (3.8) has the form
→ + e - + e (in a bound state).
The energies of β-decays into the K and L shells are equal to (50.3±1) keV and (1.7±1) keV, respectively. To observe the decay into bound states of the K- and L-shell, 10 8 fully ionized nuclei were accumulated in the ESR storage ring at GSI. During the accumulation time, nuclei were formed as a result of β + decay (Fig. 3.3).
Rice. 3.3. Dynamics of ion accumulation: a - current of Dy 66+ ions accumulated in the ESR storage ring during different stages of the experiment, β- intensities of Dy 66+ and Ho 67+ ions, measured by external and internal position-sensitive detectors, respectively
Since the Ho 66+ ions have practically the same M/q ratio as the ions of the primary Dy 66+ beam, they accumulate in the same orbit. The accumulation time was ~30 min. In order to measure the half-life of the Dy 66+ nucleus, the beam accumulated in orbit had to be purified from the admixture of Ho 66+ ions. To clean the beam from ions, an argon gas jet with a density of 6·10 12 atom/cm 2 and a diameter of 3 mm was injected into the chamber, which crossed the accumulated ion beam in the vertical direction. Due to the fact that Ho 66+ ions captured electrons, they left the equilibrium orbit. The beam was cleaned for approximately 500 s. After which the gas stream was blocked and Dy 66+ ions and Ho 66+ ions, newly formed (after turning off the gas stream) as a result of decay, continued to circulate in the ring. The duration of this stage varied from 10 to 85 minutes. The detection and identification of Ho 66+ was based on the fact that Ho 66+ can be further ionized. To do this, at the last stage, a gas jet was again injected into the storage ring. The last electron was stripped from the 163 Ho 66+ ion, resulting in the 163 Ho 67+ ion. A position-sensitive detector was located next to the gas jet, which recorded the 163 Ho 67+ ions leaving the beam. In Fig. Figure 3.4 shows the dependence of the number of 163 Ho nuclei formed as a result of β-decay on the accumulation time. The inset shows the spatial resolution of the position-sensitive detector.
Thus, the accumulation of 163 Ho nuclei in the 163 Dy beam was evidence of the possibility of decay
→ + e - + e (in a bound state).
Rice. 3.4. The ratio of daughter ions 163 Ho 66+ to primary 163 Dy 66+ depending on the accumulation time. Inset: peak 163 Ho 67+, recorded by the internal detector
By varying the time interval between cleaning the beam from the Ho 66+ impurity and the time of recording the Ho 66+ ions newly formed in the beam, it is possible to measure the half-life of the fully ionized Dy 66+ isotope. It turned out to be equal to ~0.1 year.
A similar decay was discovered for 187 Re 75+. The result obtained is extremely important for astrophysics. The fact is that neutral 187 Re atoms have a half-life of 4·10 10 years and are used as radioactive clocks. The half-life of 187 Re 75+ is only 33±2 years. Therefore, it is necessary to make appropriate corrections to astrophysical measurements, because In stars, 187 Re is most often found in an ionized state.
The study of the properties of fully ionized atoms opens up a new direction of research into the exotic properties of nuclei devoid of the Coulomb influence of the outer electron shell.
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 table 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
.
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
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The nuclei of atoms are stable, but change their state when a certain ratio of protons and neutrons is violated. Light nuclei should have approximately equal numbers of protons and neutrons. If there are too many protons or neutrons in the nucleus, then such nuclei are unstable and undergo spontaneous radioactive transformations, as a result of which the composition of the nucleus changes and, consequently, the nucleus of an atom of one element turns into the nucleus of an atom of another element. During this process, nuclear radiation is emitted.
There are the following main types of nuclear transformations or types of radioactive decay: alpha decay and beta decay (electron, positron and K-capture), internal conversion.
Alpha decay – This is the emission of alpha particles by a nucleus of a radioactive isotope. Due to the loss of two protons and two neutrons with an alpha particle, the decaying nucleus turns into another nucleus, in which the number of protons (nuclear charge) decreases by 2, and the number of particles (mass number) by 4. Therefore, for a given radioactive decay, in accordance with the rule displacement (shift), formulated by Fajans and Soddy (1913), the resulting (daughter) element is shifted to the left relative to the original (mother) by two cells to the left in the periodic table of D. I. Mendeleev. The alpha decay process is generally written as follows:
where X is the symbol of the original nucleus; Y is the symbol of the decay product nucleus; 4 2 He – alpha particle, Q – released excess energy.
For example, the decay of radium-226 nuclei is accompanied by the emission of alpha particles, while radium-226 nuclei turn into radon-222 nuclei:
The energy released during alpha decay is divided between the alpha particle and the nucleus in inverse proportion to their masses. The energy of alpha particles is strictly related to the half-life of a given radionuclide (Geiger-Nettol law) . This suggests that, knowing the energy of alpha particles, it is possible to establish the half-life, and by the half-life to identify the radionuclide. For example, the polonium-214 nucleus is characterized by alpha particle energy values E = 7.687 MeV and T 1/2 = 4.510 -4 s, while for the uranium-238 nucleus E = 4.196 MeV and T 1/2 = 4, 510 9 years. In addition, it has been established that the higher the energy of alpha decay, the faster it proceeds.
Alpha decay is a fairly common nuclear transformation of heavy nuclei (uranium, thorium, polonium, plutonium, etc. with Z > 82); Currently, more than 160 alpha-emitting nuclei are known.
Beta decay – spontaneous transformations of a neutron into a proton or a proton into a neutron inside a nucleus, accompanied by the emission of electrons, positrons and antineutrinos 
or neutrino e.
If there is an excess of neutrons in the nucleus (“neutron overload” of the nucleus), then electron beta decay occurs, in which one of the neutrons turns into a proton, emitting an electron and an antineutrino:
.
During this decay, the charge of the nucleus and, accordingly, the atomic number of the daughter nucleus increases by 1, but the mass number does not change, i.e., the daughter element is shifted in the periodic system of D.I. Mendeleev by one cell to the right of the original one. The beta decay process is generally written as follows:
.
In this way, nuclei with an excess of neutrons decay. For example, the decay of strontium-90 nuclei is accompanied by the emission of electrons and their transformation into yttrium-90:
Often the nuclei of elements produced by beta decay have excess energy, which is released by the emission of one or more gamma rays. For example:
Electronic beta decay is characteristic of many natural and artificially produced radioactive elements.
If the unfavorable ratio of neutrons to protons in the nucleus is due to an excess of protons, then positron beta decay occurs, in which the nucleus emits a positron and a neutrino as a result of the conversion of a proton to a neutron within the nucleus:
The charge of the nucleus and, accordingly, the atomic number of the daughter element decreases by 1, the mass number does not change. The daughter element will occupy a place in D.I. Mendeleev’s periodic table one cell to the left of the parent:
Positron decay is observed in some artificially obtained isotopes. For example, the decay of the isotope phosphorus-30 to form silicon-30:
A positron, escaping from the nucleus, rips off an “extra” electron (weakly bound to the nucleus) from the shell of the atom or interacts with a free electron, forming a “positron-electron” pair. Due to the fact that the particle and antiparticle instantly annihilate each other with the release of energy, the formed pair turns into two gamma quanta with energy equivalent to the mass of the particles (e + and e -). The process of transformation of a positron-electron pair into two gamma quanta is called annihilation (destruction), and the resulting electromagnetic radiation is called annihilation. In this case, there is a transformation of one form of matter (particles of matter) into another (radiation). This is confirmed by the existence of a reverse reaction - a pair formation reaction, in which electromagnetic radiation of sufficiently high energy, passing near the nucleus under the influence of a strong electric field of the atom, turns into an electron-positron pair.
Thus, during positron beta decay, the final result is not particles, but two gamma rays, each with an energy of 0.511 MeV, equal to the energy equivalent of the rest mass of particles - a positron and an electron E = 2m e c 2 = 1.022 MeV .
The transformation of a nucleus can be carried out by electron capture, when one of the protons of the nucleus spontaneously captures an electron from one of the inner shells of the atom (K, L, etc.), most often from the K-shell, and turns into a neutron. This process is also called K-capture. A proton turns into a neutron according to the following reaction:
In this case, the nuclear charge decreases by 1, but the mass number does not change:
For example,
In this case, the space vacated by the electron is occupied by an electron from the outer shells of the atom. As a result of the restructuring of electron shells, an X-ray quantum is emitted. The atom still remains electrically neutral, since the number of protons in the nucleus decreases by one during electron capture. Thus, this type of decay produces the same results as positron beta decay. It is typical, as a rule, for artificial radionuclides.
The energy released by the nucleus during the beta decay of a particular radionuclide is always constant, but due to the fact that this type of decay produces not two, but three particles: a recoil nucleus (daughter), an electron (or positron) and a neutrino, the energy varies in each decay event it is redistributed between the electron (positron) and the neutrino, since the daughter nucleus always carries away the same portion of energy. Depending on the angle of expansion, a neutrino can carry away more or less energy, as a result of which an electron can receive any energy from zero to a certain maximum value. Hence, during beta decay, beta particles of the same radionuclide have different energies, from zero to a certain maximum value characteristic of the decay of a given radionuclide. It is almost impossible to identify a radionuclide based on beta radiation energy.
Some radionuclides can decay simultaneously in two or three ways: by alpha and beta decay and through K-capture, a combination of the three types of decay. In this case, transformations are carried out in a strictly defined ratio. For example, the natural long-lived radioisotope potassium-40 (T 1/2 = 1.4910 9 years), the content of which in natural potassium is 0.0119%, undergoes electronic beta decay and K-capture:
(88% – electronic decay),
(12% – K-grab).
From the types of decays described above, we can conclude that gamma decay does not exist in its “pure form.” Gamma radiation can only accompany various types of decays. When gamma radiation is emitted in the nucleus, neither the mass number nor its charge changes. Consequently, the nature of the radionuclide does not change, but only the energy contained in the nucleus changes. Gamma radiation is emitted when nuclei pass from excited levels to lower levels, including the ground level. For example, the decay of cesium-137 produces an excited barium-137 nucleus. The transition from an excited to a stable state is accompanied by the emission of gamma quanta:
Since the lifetime of nuclei in excited states is very short (usually t10 -19 s), during alpha and beta decays a gamma quantum is emitted almost simultaneously with the charged particle. Based on this, the process of gamma radiation is not distinguished as an independent type of decay. By the energy of gamma radiation, as well as by the energy of alpha radiation, it is possible to identify a radionuclide.
Internal conversion. The excited (as a result of one or another nuclear transformation) state of the nucleus of an atom indicates the presence of excess energy in it. An excited nucleus can transition to a state with lower energy (normal state) not only through the emission of a gamma quantum or the ejection of a particle, but also through internal conversion, or conversion with the formation of electron-positron pairs.
The phenomenon of internal conversion is that the nucleus transfers excitation energy to one of the electrons of the inner layers (K-, L- or M-layer), which as a result escapes outside the atom. Such electrons are called conversion electrons. Consequently, the emission of conversion electrons is due to the direct electromagnetic interaction of the nucleus with shell electrons. Conversion electrons have a line energy spectrum, unlike beta decay electrons, which give a continuous spectrum.
If the excitation energy exceeds 1.022 MeV, then the transition of the nucleus to the normal state can be accompanied by the emission of an electron-positron pair, followed by their annihilation. After internal conversion has occurred, a “vacant” place for the ejected conversion electron appears in the electron shell of the atom. One of the electrons in more distant layers (from higher energy levels) carries out a quantum transition to a “vacant” place with the emission of characteristic X-ray radiation.