Isomer (nuclear physics)

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Isomers (from ancient Greek ἴσος ísos "equal" and μέρος méros "part"; singular: the isomer) in nuclear physics are atomic nuclei that do not differ in the number of protons or neutrons, but differ in different internal (energy) states are located. To differentiate from isomerism in chemistry, the terms nuclear isomer or nuclear isomer are also used.

The isomer does not refer to the nucleus in the ground state , but only to the one in an excited state , and only if this state is particularly long-lived. The isomer is viewed as a separate nuclide and is designated by an “m” (for “ metastable ”) next to the mass number. To distinguish between several isomers of a nucleus, the “m” can be followed by a number, e.g. B. 152m1 Eu . In table of nuclides is Kernisomere may be made by the field in question is divided into columns.

Which conditions are viewed as "particularly durable" is subject to a certain arbitrariness. In addition, more and more core isomers are being discovered. Therefore, you can only specify a lower limit to the number of core isomers, which should be a four-digit number.

Explanation and examples

All atomic nuclei with at least four nucleons can exist in excited states as well as in the ground state . Normally, these have  very short lifetimes of 10 −22 to 10 −14 seconds , which are measured over the line width of the emitted radiation (e.g. gamma radiation ). Isomers are longer-lived (metastable) excited states with lifetimes of around 10 −9  seconds. These extended lifetimes are due to the fact that transitions into deeper states are not impossible, but are orders of magnitude less likely than usual. The cause is usually a particularly large difference in nuclear spins , so that the radiation has to carry away a correspondingly large angular momentum. This is called the angular momentum barrier , based on the conditions when a particle is emitted through a potential barrier by virtue of the tunnel effect . The potential barrier increases by the term when the particle of mass with angular momentum quantum number has to be emitted, and hampers the emission accordingly. In the case of the core isomer 180m Ta , the opposite occurs, namely that it is practically stable in contrast to the ground state.

When nuclear isomers, like other excited nuclei, change into low-excited states or the ground state, they emit the released energy mostly through emission of gamma radiation ( isomerism transition ) or through internal conversion . In the case of heavy isomers of nuclei with an unstable ground state, in particular, one finds its decay channels even in the isomer; the isomer thus continues to decompose, bypassing the ground state. The easiest example of this is the beta decay of 24m Na. In the case of the isomer 184m Hf, there is the extreme case that no transition to the ground state 184 Hf can be observed at all . Due to the low spin difference to 184 Ta it is energetically favorable for this isomer, like the beta decay to 184 to make Ta.

Different core states always have different charge distributions in the core. These influence the energy of the electrons bound to the nucleus. In the case of spectral lines, in addition to the frequent hyperfine structure splitting, this also leads to a shift, the isomer shift . Both provide information about the core structure.

The core isomer 99m Tc is used medically and diagnostically for scintigraphy . There the core isomer is incorporated into a complex .

Because of the equivalence of mass and energy, a core isomer is always heavier than the same core in the ground state.


Core isomers were predicted by Frederick Soddy in 1917 . The first isomeric nuclei were discovered by Otto Hahn in 1921 while studying the decay series of uranium . In addition to the already known 234m Pa (“Uranium X 2 ”, “Brevium”) with a half-life of 1.16 minutes, he found a second beta-emitting nuclide of the same element, 234 Pa (“Uranium Z”), with the same mass number differed from 234m Pa only by its longer half-life of 6.7 hours. The discovery, which Hahn later considered to be one of his most important, was ahead of its time and only received greater attention from 1935 with the discovery of further examples. In 1936, Carl Friedrich von Weizsäcker declared core isomers to be states whose decay is delayed because they have to emit radiation with a particularly large angular momentum. Weizsäcker worked temporarily at the Hahn Institute at that time. Since isomers were initially only discovered in nuclei with an unstable ground state, namely on the basis of the different half-lives of the ground state and isomer, it was not until 1939 that isomers of stable (or at that time considered stable) ground states were identified, first at 115 In.


  • Klaus Bethge , Gertrud Walter, Bernhard Wiedemann, Kernphysik, 3rd edition, Springer 2008, p. 271
  • Theo Mayer-Kuckuk : Introduction to Nuclear Physics, 7th edition, Teubner, 2002, p. 97

Web links

Wiktionary: isomer  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. A. D. McNaught, A. Wilkinson: nuclide . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . 2nd Edition. Blackwell Scientific Publications, Oxford 1997, ISBN 0-9678550-9-8 , doi : 10.1351 / goldbook.No4257 (English, corrected version (XML; 2006–) by M. Nic, J. Jirat, B. Kosata; with updates by A. Jenkins -).
  2. Frederick Soddy, Nature, Volume 99, 1917, p. 433
  3. ^ Hahn: About a new radioactive decay product in uranium , Die Naturwissenschaften, Volume 9, 1921, Issue 5, p. 84
  4. ^ Klaus Hoffmann: Guilt and Responsibility. Otto Hahn, Conflict of a Scientist , Springer 1993, p. 94
  5. ^ Carl Friedrich von Weizsäcker: Metastable states of the atomic nuclei. In: Natural Sciences . Vol. 24, No. 51, 1936, pp. 813-814, doi : 10.1007 / BF01497732 .
  6. ^ M. Goldhaber, RD Hill: Radioactivity Induced by Nuclear Excitation . In: The Physical Review . tape 55 , no. 1 , 1939, p. 47 , doi : 10.1103 / PhysRev.55.47 .
  7. J. Mattauch: About the occurrence of isomeric atomic nuclei . In: Journal of Physics . tape 117 , 1941, pp. 246-255 , doi : 10.1007 / BF01342313 .