Isomerism of atomic nuclei

The isomerism of atomic nuclei is the phenomenon of the existence of metastable (isomeric) excited states with sufficiently long lifetimes in atomic nuclei .

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 rules of spin and parity banning . In particular, transitions with a high multipolarity (i.e., a large change in the spin necessary for the transition to the underlying state) and a 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 denoted 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 denoted in the order of energy growth by the letters m , n , p , q and then alphabetically, or by the letter m with the addition of a number: m 1, m 2, etc.

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

Content

History

The concept of isomerism of atomic nuclei arose in 1921 ^[1] when the German physicist O. Hahn , studying the beta decay of thorium-234 , known at the time as “uranium-X1” (UX ₁ ), discovered a new radioactive substance “uranium-Z "(UZ), which did not differ in chemical properties or mass number from the already known" uranium-X2 "(UX ₂ ), but had a different half-life. In modern notation, UZ and UX ₂ correspond to the isomeric and ground states of the ²³⁴ Pa isotope ^[2] . In 1935 ^[3] B.V. Kurchatov , I.V. Kurchatov , L.V. Mysovsky and L.I. Rusinov discovered an isomer of the artificial bromine isotope ⁸⁰ Br, which is formed along with the ground state of the nucleus upon neutron capture by a stable ⁷⁹ Br . Three years later, under the leadership of IV Kurchatov, it was found that the isomeric transition of bromine-80 occurs mainly through internal conversion , and not by emission of gamma rays ^[4] . All this laid the foundation for a systematic study of this phenomenon. Theoretically, nuclear isomerism was described by Karl Weizsacker in 1936 ^[5] ^[6] .

Physical Properties

The lifetime of isomeric states exceeds fractions of a microsecond (and can be measured in years), while the typical lifetime of nonisomeric excited states is of the order of picoseconds or less. There is no natural difference, apart from the lifetime, between the two: the boundary between the isomeric and non-isomeric excited states of the nucleus is a matter of agreement. So, in the reference book on the properties of the Nubase'1997 isotopes ^[7] , isomers are assigned to isomers with a half-life of more than 1 ms, while in newer versions of this reference book Nubase'2003 ^[8] and Nubase'2016 ^[9] , states are added to them with a half-life of about 100 ns or more. In 2016, a total of 3437 nuclides are known, of which 1318 nuclides have one or more isomeric states with a half-life exceeding 100 ns ^[9] .

The decay of isomeric states can be carried out by:

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

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

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

Some examples

The tantalum isomer -180 ( ^180m Ta) is the only stable (within the sensitivity of modern methods) isomer. Unlike radio- or cosmogenic short-lived radionuclides , it exists in the earth's crust from the moment of its formation, occurring in natural tantalum in a ratio of 1 to 8300. Although ^180m Ta can theoretically decay in at least three ways ( isomeric transition , beta-minus decay , electronic capture ), none of them were experimentally detected; the lower limit on its half-life is 7.1⋅10 ¹⁵ years ^[9] . At the same time, the ground state of ¹⁸⁰ Ta is beta-active with a half-life of 8.154 (6) hours ^[9] . The spin and parity of the ground state are 1 ⁺ , the isomer - 9 ^- ^[8] . Due to the high difference between the spins of states and the proximity of their energies (the isomeric level lies 75.3 (14) keV higher than the ground state ^[9] ), the isomeric transition is extremely strongly suppressed. It is expected that ^180m Ta, like any other nuclear isomer, can be artificially transferred to the ground state by stimulated emission when irradiated with gamma rays with an energy exactly equal to the difference between the energies of the excited and ground states.

In the natural radioactive series of uranium ²³⁸ U there is a protactinium isomer -234 ^234m Pa (half-life 1,159 (11) minutes ^[9] ).

A very low lying metastable level of ^235m U (half-life of 25.7 (1) minutes ^[9] ) was found at the uranium-235 nucleus, which is only 76.0 (4) electron-volts away from the ground level ^[9] .

The hafnium isomer is -178 ^178m2 Hf with a half-life of 31 (1) years ^[9] (index 2 means that there is also a lower-lying isomer ^178m1 Hf). It has the highest excitation energy among isomers with a half-life of more than a year. Three kilograms of pure ^178m2 Hf contains approximately 4 TJ of energy, which is equivalent to a kiloton of TNT . All this energy is released in the form of cascade gamma rays and conversion electrons with an energy of 2446 keV per nucleus. As with ^180m Ta, there is a discussion of the possibility of artificially transferring ^178m2 Hf to the ground state. The results obtained (but not confirmed in other experiments) indicate a very rapid release of energy (power on the order of exavatts). Theoretically, hafnium isomers can be used both to create gamma lasers , energy storage devices, and to develop a fairly powerful nuclear weapon that does not create radioactive contamination of the area. Nevertheless, the prospects here remain generally rather vague, since neither experimental nor theoretical works on this issue give unambiguous answers, and the production of macroscopic quantities of ^178m2 Hf, with the modern development of technology, is practically unavailable ^[10]

The iridium isomer -192 ^192m2 Ir has a half-life of 241 (9) years and an excitation energy of 168.14 (12) keV ^[9] . Sometimes it is proposed to use it for the same purposes as the hafnium-178 ^178m2 Hf isomer.

The largest number of isomers (six each, not counting the ground state) was found for the tantalum isotopes -179 ( ¹⁷⁹ Ta) and radium -214 ( ²¹⁴ Ra) ^[9] .

Notes

↑ Otto Hahn. Über eine neue radioaktive Substanz im Uran (German) // Berichte der Deutschen Chemischen Gesellschaft : magazin. - 1921. - Bd. 54 , Nr. 6 - S. 1131-1142 . - DOI : 10.1002 / cber.19210540602 .
↑ DE Alburger. Nuclear isomerism // Handbuch der physik / S. Flügge. - Springer-Verlag, 1957. - T. 42: Kernreaktionen III / Nuclear Reactions III. - P. 1.
↑ JV Kourtchatov, BV Kourtchatov, LV Misowski, LI Roussinov. Sur un cas de radioactivité artificielle provoquée par un bombardement de neutrons, sans capture du neutron (fr.) // Comptes rendus hebdomadaires des séances de l'Académie des sciences : magazine. - 1935. - Vol. 200 . - P. 1201-1203 .
↑ Rusinov, 1961 , p. 617.
↑ C. von Weizsäcker. Metastabile Zustände der Atomkerne (Eng.) // Naturwissenschaften : journal. - 1936. - Vol. 24 , no. 51 . - P. 813-814 .
↑ Konstantin Mukhin. Exotic nuclear physics for the curious (Rus.) // Science and Life . - 2017. - No. 4 . - S. 96-100 .
↑ G. Audi et al. The NUBASE evaluation of nuclear and decay properties. Nuclear Physics A, 1997, vol. 624, page 1-124. Archived copy (unopened) (inaccessible link) . Date of treatment March 17, 2008. Archived May 4, 2006.
↑ ¹ ² Audi G. , Bersillon O. , Blachot J. , Wapstra AH The NUBASE evaluation of nuclear and decay properties // Nuclear Physics A. - 2003. - T. 729 . - p . 3-128 . - DOI : 10.1016 / j.nuclphysa.2003.11.001 . - .
↑ ¹ ² ³ ⁴ ⁵ ⁶ ⁷ ⁸ ⁹ ¹⁰ ¹¹ Audi G. , Kondev FG , Wang M. , Huang WJ , Naimi S. The Nubase2016 evaluation of nuclear properties (English) // Chinese Physics C. - 2017 .-- Vol. 41 , iss. 3 - P. 030001-1-030001-138 . - DOI : 10.1088 / 1674-1137 / 41/3/030001 . - .
↑ Tkalya E.V. Induced decay of the nuclear isomer ^178m2 Hf and the "isomeric bomb" // Uspekhi Fizicheskikh Nauk : Journal. - 2005. - T. 175, No. 5. - S. 555-561.

Literature

Rusinov L.I. Isomerism of atomic nuclei // Uspekhi Fizicheskikh Nauk : Journal. - 1961. - T. 73, No. 4. - S. 615-630.

Links

The boundary of the proton stability of nuclei can be quite blurred // Elements.ru
The theoretical inconsistency of the hafnium bomb is proved // Elements.ru

[1] Otto Hahn. Über eine neue radioaktive Substanz im Uran (German) // Berichte der Deutschen Chemischen Gesellschaft : magazin. - 1921. - Bd. 54 , Nr. 6 - S. 1131-1142 . - DOI : 10.1002 / cber.19210540602 .

[2] DE Alburger. Nuclear isomerism // Handbuch der physik / S. Flügge. - Springer-Verlag, 1957. - T. 42: Kernreaktionen III / Nuclear Reactions III. - P. 1.

[3] JV Kourtchatov, BV Kourtchatov, LV Misowski, LI Roussinov. Sur un cas de radioactivité artificielle provoquée par un bombardement de neutrons, sans capture du neutron (fr.) // Comptes rendus hebdomadaires des séances de l'Académie des sciences : magazine. - 1935. - Vol. 200 . - P. 1201-1203 .

[_3436ab87e871c599-4] Rusinov, 1961 , p. 617.

[5] C. von Weizsäcker. Metastabile Zustände der Atomkerne (Eng.) // Naturwissenschaften : journal. - 1936. - Vol. 24 , no. 51 . - P. 813-814 .

[6] Konstantin Mukhin. Exotic nuclear physics for the curious (Rus.) // Science and Life . - 2017. - No. 4 . - S. 96-100 .

[Nubase1997-7] G. Audi et al. The NUBASE evaluation of nuclear and decay properties. Nuclear Physics A, 1997, vol. 624, page 1-124. Archived copy (unopened) (inaccessible link) . Date of treatment March 17, 2008. Archived May 4, 2006.

[Nubase2003-8] ¹ ² Audi G. , Bersillon O. , Blachot J. , Wapstra AH The NUBASE evaluation of nuclear and decay properties // Nuclear Physics A. - 2003. - T. 729 . - p . 3-128 . - DOI : 10.1016 / j.nuclphysa.2003.11.001 . - .

[Nubase2016-9] ¹ ² ³ ⁴ ⁵ ⁶ ⁷ ⁸ ⁹ ¹⁰ ¹¹ Audi G. , Kondev FG , Wang M. , Huang WJ , Naimi S. The Nubase2016 evaluation of nuclear properties (English) // Chinese Physics C. - 2017 .-- Vol. 41 , iss. 3 - P. 030001-1-030001-138 . - DOI : 10.1088 / 1674-1137 / 41/3/030001 . - .

[10] Tkalya E.V. Induced decay of the nuclear isomer ^178m2 Hf and the "isomeric bomb" // Uspekhi Fizicheskikh Nauk : Journal. - 2005. - T. 175, No. 5. - S. 555-561.