Isomerism of atomic nuclei. Nuclear isomerism "nuclear isomerism" in books

Historical information

The concept of isomerism of atomic nuclei arose in 1921, when the German physicist O. Hahn discovered a new radioactive substance uranium-Z (UZ), which did not differ either in chemical properties or in mass number from the already known uranium-X2 (UX 2), however had a different half-life. In modern notation, UZ and UX 2 correspond to the ground and isomeric states of the isotope. In 1935, B.V. Kurchatov, I.V. Kurchatov, L.V. Mysovsky and L.I. Rusinov discovered an isomer of the artificial bromine isotope 80 Br, formed along with the ground state of the nucleus during the capture of neutrons by stable 79 Br. This laid the basis for a systematic study of this phenomenon.

Theoretical information

Isomeric states differ from ordinary excited states of nuclei in that the probability of transition to all underlying states for them is strongly suppressed by the spin and parity exclusion rules. In particular, transitions with high multipolarity (that is, a large spin change required for a transition to the underlying state) and low transition energy are suppressed.

Sometimes the appearance of isomers is associated with a significant difference in the shape of the nucleus in different energy states (as in 180 Hf).

Of greatest interest are relatively stable isomers with half-lives from 10 −6 sec to many years. Isomers are designated by the letter m(from English metastable) in the mass number index (for example, 80 m Br) or in the upper right index (for example, 80 Br m). If a nuclide has more than one metastable excited state, they are designated in order of increasing energy by the letters m, n, p, q and further in alphabetical order, or by letter m with the number added: m 1, m 2, etc.

Some examples

Notes

Literature

1. L. I. Rusinov // Isomerism of atomic nuclei. UFN. 1961. T. 73. No. 4. P. 615-630.
2. E. V. Tkalya. // Induced decay of the nuclear isomer 178m2 Hf and the “isomer bomb”. UFN. 2005. T. 175. No. 5. P. 555-561.

See also

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See what “Isomerism of atomic nuclei” is in other dictionaries:

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A phenomenon consisting in the existence of long-lived excited (metastable) states of atomic nuclei. The transition to an unexcited state occurs due to γ ​​radiation or internal conversion. * * * ISOMERISM OF ATOMIC NUCLEI ISOMERISM OF ATOMIC NUCLEI,... ... Encyclopedic Dictionary

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The existence of nuclei of certain nuclides in metastable excited energy. states. Nuclides with metastable nuclei are designated by the Latin letter tv top. index to the left of the mass number. Thus, the metastable isomer 236Np is designated 236mNp. AND … Chemical encyclopedia

The phenomenon of artificial radioactive isotopes, an outstanding world discovery (1935) of the Russian scientist I.V. Kurchatov.

ISOMERIA NUCLEAR- the existence of certain nuclei, along with the ground state, of fairly long-lived (metastable) excited states, called. isomeric. Phenomenon I. I. was discovered in 1921 by O. Hahn, who discovered a radioact. a substance he called uranium Z (UZ), which had the same atomic number Z and mass number A, like another radioact, the substance UX 2, but differed from it in its half-life. Both substances were products of the p-decay of the same element UX 1 (234 90 Th). Later it turned out that UZ and UX 2 are the ground and isomeric states of the 234 91 Pa nucleus (the isomeric state is denoted by the index T, eg. 234m 91 Ra). In 1935, I.V. Kurchatov, B.V. Kurchatov, L.V. Mysovsky and L.I. Rusinov discovered that when the stable isotope 79 35 Br is irradiated with neutrons, a radioact is formed. isotope 80 35 Br, having two, which corresponded to decays from the ground and isomeric states. Further studies revealed a large number of isomeric states of nuclei with decomposition. half-lives from 3. 10 6 years (210m Bi) to several. mks and not even. Mn. the nuclei have 2, and, for example, 160 But it has 4 isomeric states. The reason I. I. is a weakening of the probability of gamma ray emission from an excited state (see. Gamma radiation This usually happens when a small transition energy is combined with a large difference in the values ​​of the moments of the number of motions I (angular moments) of the beginning. and final states. The higher the multipolarity and the lower the hw transition energy, the lower the probability of a y-transition. In some cases, the weakening of the probability of emission of g-quanta is explained by more complex structural features of the states of the nucleus, between which a transition occurs (different structures of the nucleus in the isomeric and underlying states). In Fig. Figures 1 and 2 show fragments of the decomposition schemes for the 234m 91 Pa and 80m 35 Br isomers. In the case of protactinium, the reason for I. i. is low energy and high multipolarity EZ g-transition. It is so difficult that in the overwhelming majority of cases the isomer undergoes b-decay (see. Beta decay nuclei). For certain isomers, the isomeric transition often becomes completely unobservable. In the case of 80m 35 Vr I. I. is obliged to the g-transition of the multipolarity of the MS. The nucleus goes from the isomeric state (I p = 5 -) to a lower energy state (2 -), which in a short time goes into the main state. nuclear state 80 35 Br. In the case of the 242 Am nucleus (Fig. 3) I. i. associated with the g-transition of multipolarity E4.

Rice. 1. Scheme of the decay of the 234m 91 Ra isomer. The ground (0) and isomeric states are highlighted with thick lines; on the left are the values ​​of spins and parities (I p), to the right are multipolarity, level energies (in keV) and half-lives; The probabilities of various channels of nuclear decay from the isomeric state are given in %.

The isomeric state mainly decays through the g-transition, but in 5 out of 1000 cases it is observed alpha decay In the examples given, isomeric transitions are accompanied in most cases by the emission of conversion electrons rather than g-quanta (see Fig. Conversion internal).

Rice. 2. Scheme of the decomposition of the 80m 35 Br isomer; E.Z - electronic capture.

Rice. 3. Scheme of the decay of 242m 95 Am.

A large number of isomeric transitions of multipolarity M4 are observed during the “discharge” of excited states of odd nuclei, when the number of protons or neutrons approaches the magic number. numbers (isomerism islands). This is explained shell model of the nucleus, as a consequence of the filling by nucleons of neighboring, similar in energy, but very different in spin states g 9/2 and p 1/2, as well as h 11/2 and d 3/2 (g, p, h, d- designations of the orbital moments of nucleons, the indices for them are the spin values).

Rice. 4. Scheme of the decay of 180m 72 Hf.

In contrast to the examples given, the isomeric state 180m 72 Hf (Fig. 4) belongs to a stable nucleus and has a relatively high excitation energy. The reason for the isomerism is the strongly weakened g-transition E1 with an energy of 57.6 keV, which is inhibited 10 16 times due to the structural differences between the 8 - and 8 + states. In 1962, a new type of i.e. fission isomerism was discovered at JINR. It turned out that certain isotopes of transuranium elements U, Pu, Am, Cm and Bk have excited states with an energy of ~2-3 MeV, which decay by

It was discovered that there are nuclei with the same number values ​​but with different half-lives. Such nuclei are called isomers.

A study of the phenomenon of nuclear isomerism in artificially radioactive nuclei was carried out by a group of Soviet physicists led by Kurchatov and Rusinov. Artificial

radioactivity resulting from the irradiation of a natural mixture of stable isotopes by slow neutrons. In this case, two radioactive isotopes of bromine are formed, chemically inseparable from each other:

The surprising result of these experiments was the discovery of not two, but three different half-lives:

It is obvious that one of the isotopes decays in two ways. The experiment was modified and was irradiated not with neutrons, but with -rays, which caused the so-called nuclear photoelectric effect

The resulting bromine isotopes are also -active and decay according to the following scheme:

Research has shown that in this case, not two, but three half-lives are also observed:

From a comparison of the processes, it became clear that it is with the isotope Brzb, formed in both cases, that two half-lives are associated: min and hour, which are also found in both series of experiments. It was necessary to explain the existence of two different half-lives for the same isotope.

Further experiments showed that isomerism is explained by the presence of a metastable state in this nucleus, that is, an excited state from which the probability of transition to the ground state is low. To understand this, consider

in more detail the diagram of nuclear decay. As a result of the preceding nuclear reaction, the nucleus appears in a highly excited state.

Rice. 45. Decay scheme

Removal of excitation occurs in two ways: the nucleus is transferred to the ground state within a second by a transition, from which the emission of c particles already occurs, or the nucleus moves to a metastable level, a further transition from which to the ground state is prohibited by selection rules. As a result, the nucleus is “stuck” at a metastable level with a lifespan of 4.4 hours; The transition from the metastable to the ground state is accompanied by both -radiation and internal electron conversion. Subsequently, the transition from the ground level again occurs with the help of -decay with the formation of .

Thus, we observe, in fact, the same spectrum of particles formed during the transition from the main level to the main level with a single half-life of mines, but due to the delay of the transitions inside the bromine nucleus, an effect arises that leads to two half-lives.

Nuclear isomerism is not a rare phenomenon among nuclear transformations. Currently, more than 100 isomers are known.

In connection with the phenomenon of isomerism described above, the question arises: what time is necessary for the nucleus to move from the excited state to the ground state? What does the emission time depend on? To estimate it, we use the fact that the energy width of a level is a measure of the uncertainty of the energy of the system located at this level. The time the system remains in this state can be estimated from the uncertainty relation:

In the case under consideration, the value and will be the average lifetime of the nucleus in the excited state, and the energy width of this excited level. It is known from experience that the width of the spectral line is usually of the order of , therefore,

(it is impossible to measure this time with existing instruments, whereas the value can be measured quite accurately).

Thus, usually Let us now consider how we can explain the presence of isomers and the existence of forbidden transitions for -radiation.

At different levels, the core, as already mentioned, has different angular momenta. Since the law of conservation of angular momentum must be fulfilled, during the transition the difference between the moments of the initial and final levels carries away the -quantum. This determines the selection rules.

The radiation associated with the restructuring of the system is called dipole radiation; -quadrupole radiation; on octupole radiation; in general by the radiation of a multipole of order.

According to the theory of such transitions, developed by Weidsäcker, -quanta of different multipolarity arise as a result of different oscillations inside the nucleus. Some of these processes are associated with the redistribution of electric charges inside the nucleus (electric dipole, quadrupole, etc. radiation), others with the redistribution of currents or magnetic moments of nucleons (magnetic dipole, quadrupole, etc. radiation). moments of the initial state of the nucleus and the final state of the nucleus and the moment carried away by the -quantum, there must be a relation

However, from classical electrodynamics it is known that if the dimensions of the system are small compared to X, then the intensities of radiation of different multipolarities differ to the extent of the factor (thus the radius of the nucleus, K is the wavelength of the radiation).

ISOMERISTY OF ATOMIC NUCLEI, the existence of some atomic nuclei, along with the ground state, of long-lived (metastable) excited states, called isomeric. Historically, states with lifetimes that can be measured directly (more than 0.01 μs) are considered isomeric. The phenomenon of isomerism arises due to a sharp difference in the structure of neighboring states (excited and ground), which leads to a significant decrease in the probability of decay of the excited state (sometimes by many orders of magnitude).

The first indication of the existence of nuclear isomers was obtained in 1921 by O. Hahn, who discovered among the decay products of uranium a radioactive substance that, with the same atomic number Z and mass number A, had two completely different radioactive decay paths. However, the date of discovery of isomerism of atomic nuclei is considered to be 1935, when a group of Soviet scientists led by I.V. Kurchatov discovered the formation of three radioactive isotopes with different half-lives when irradiating bromine with slow neutrons.

Subsequently, it turned out that this phenomenon is quite widespread; several hundred isomeric states are already known, and some nuclei may have several such states. For example, the hafnium nucleus with A = 175 has 5 states with lifetimes greater than 0.1 μs.

An indispensable condition for the existence of an isomeric state of the nucleus is the presence of some kind of prohibition for radiative transitions from isomeric to states with lower energy. There are a number of known features of the nuclear structure that cause such a prohibition: the difference in angular momenta (spins) of the isomeric and ground states, leading to radiative transitions of high multipoleity, different orientations of spins relative to a preferred axis in the nucleus, different shapes of nuclei in both states.

The decay of isomeric states is usually accompanied by the emission of electrons or γ quanta, resulting in the formation of the same nucleus, but in a state with lower energy. Sometimes beta decay is more likely. Isomers of heavy elements can decay through spontaneous fission. Isomeric states of nuclei with a high probability of spontaneous fission are called fissile isomers. About 30 nuclei are known (isotopes U, Pu, Am, Cm, Bk), for which the probability of spontaneous fission in the isomeric state is approximately 10 26 times greater than in the main state.

Isomerism of atomic nuclei is an important source of information about the structure of atomic nuclei; the study of isomers helped to establish the order of filling of nuclear shells. Based on the lifetimes of isomers, one can judge the values ​​of the prohibitions for radiative transitions and their connection with the nuclear structure.

Nuclear isomers also find practical applications. For example, in activation analysis, their formation in some cases makes it possible to achieve greater sensitivity of the method. Long-lived nuclear isomers are considered as possible energy storage devices in the future.

Lit.: Korsunsky M.I. Isomerism of atomic nuclei. M., 1954; Polikanov S. M. Isomerism of the shape of atomic nuclei. M., 1977.

In all underlying states, they are strongly suppressed by the rules of the ban on spin and parity. In particular, transitions with high multipolarity (that is, a large spin change required for a transition to the underlying state) and low transition energy are suppressed. Sometimes the appearance of isomers is associated with a significant difference in the shape of the nucleus in different energy states (as in 180 Hf).

Isomers are designated by the letter m(from the English metastable) in the mass number index (for example, 80 m Br) or in the upper right index (for example, 80 Br m). If a nuclide has more than one metastable excited state, they are designated in order of increasing energy by the letters m, n, p, q and further in alphabetical order, or by letter m with the number added: m 1, m 2, etc.

Of greatest interest are relatively stable isomers with half-lives from 10 −6 sec to many years.

Story

The concept of isomerism of atomic nuclei arose in 1921, when the German physicist O. Hahn, studying the beta decay of thorium-234, known at that time as “uranium-X1” (UX 1), discovered a new radioactive substance “uranium-Z” (UZ ), which did not differ either in chemical properties or in mass number from the already known “uranium-X2” (UX 2), but had a different half-life. In modern notations, UZ and UX 2 correspond to the isomeric and ground states of the isotope 234 Pa. In 1935, B.V. Kurchatov, I.V. Kurchatov, L.V. Mysovsky and L.I. Rusinov discovered an isomer of the artificial bromine isotope 80 Br, formed along with the ground state of the nucleus during the capture of neutrons by stable 79 Br. Three years later, under the leadership of I.V. Kurchatov, it was established that the isomeric transition of bromine-80 occurs mainly through internal conversion, and not through the emission of gamma rays. All this laid the basis for a systematic study of this phenomenon. Theoretically, nuclear isomerism was described by Karl Weizsäcker in 1936.

Physical properties

The decomposition of isomeric states can be carried out by:

• isomeric transition to the ground state (by emission of a gamma quantum or through internal conversion);
• beta decay and electron capture;
• spontaneous fission (for heavy nuclei);
• proton radiation (for highly excited isomers).

The probability of a particular decay option is determined by the internal structure of the nucleus and its energy levels (as well as the levels of nuclei - possible decay products).

In some areas of mass numbers there are so-called. islands of isomerism (in these areas isomers are especially common). This phenomenon is explained by the nuclear shell model, which predicts the existence in odd nuclei of energetically close nuclear levels with large spin differences when the number of protons or neutrons is close to magic numbers.

Some examples

See also

Notes

1. Otto Hahn.Über eine neue radioaktive Substanz im Uran (German) // Berichte der Deutschen Chemischen Gesellschaft (English) Russian: magazine. - 1921. - Bd. 54, Nr. 6. - S. 1131-1142. - DOI:10.1002/cber.19210540602.
2. D. E. Alburger. Nuclear isomerism// Handbuch der physik / S. Flügge. - Springer-Verlag, 1957. - T. 42: Kernreaktionen III / Nuclear Reactions III. - P. 1.
3. J. V. Kourtchatov, B. V. Kourtchatov, L. V. Misowski, L. I. Roussinov. Sur un cas de radioactivité artificielle provoquée par un bombardement de neutrons, sans capture du neutron (French) // Comptes rendus hebdomadaires des séances de l "Académie des sciences (English) Russian: magazine. - 1935. - Vol. 200. - P. 1201-1203.
4. , With. 617.
5. C. von Weizsäcker. Metastabile Zustände der Atomkerne (English) // Naturwissenschaften (English) Russian: journal. - 1936. - Vol. 24, no. 51. - P. 813-814.
6. Konstantin Mukhin. Exotic nuclear physics for the curious (Russian) // Science and life. - 2017. - No. 4. - pp. 96-100.
7. G.Audi et al. The NUBASE evaluation of nuclear and decay properties. Nuclear Physics A, 1997, vol. 624, page 1-124. Archived copy (undefined) (unavailable link). Retrieved March 17, 2008.