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The existence of excited states of atomic nuclei with unusually long lifetimes. If the lifetime of a specific excited state is unusually long, compared with the lifetimes of other excited states in the same nucleus, the state is said to be isomeric. The definition of the boundary between isomeric and normal decays is arbitrary, and the term is therefore used loosely. See Excited state, Parity (quantum mechanics), Spin (quantum mechanics)
The predominant decay mode of excited nuclear states is by γ-ray emission. The rate at which this process occurs is determined largely by the spins, parities, and excitation energies of the decaying state and of those to which it is decaying. In particular, the rate is extremely sensitive to the difference in the spins of initial and final states and to the difference in excitation energies. Both extremely large spin differences and extremely small energy differences can result in a slowing of the γ-ray emission by many orders of magnitude, resulting in some excited states having unusually long lifetimes and therefore being termed isomeric.
In addition to spin isomers, two other types of isomers have been identified. The first of these arises from the fact that some excited nuclear states represent a drastic change in shape of the nucleus from the shape of the ground state. In many cases this extremely deformed shape displays unusual stability, and states with this shape are therefore isomeric. A particularly important class of these shape isomers is observed in the decay of heavy nuclei by fission, and the study of such fission isomers has been the subject of intensive effort. See Nuclear fission
A more esoteric form of isomer has also been observed, the so-called pairing isomer which results from differences in the microscopic motions of the constituent nucleons in the nucleus. A state of this type has a quite different character from the ground state of the nucleus, and is therefore also termed isomeric. See Nuclear structure
the existence in some atomic nuclei of metastable states—excited states with relatively long lifetimes. Some atomic nuclei have several isomeric states with different lifetimes.
The concept of nuclear isomerism originated in 1921, when the German physicist O. Hahn discovered the radioactive substance uranium Z (UZ), which did not differ from the then known uranium VX2 either in chemical properties or in mass number. Later it was established that UZ and VX2 are two states of the same isotope, 234Pa, having a different energy and half-life. By analogy with isomeric organic compounds, UZ and UX2w were called nuclear isomers. In 1935, B.V. Kurchatov, I.V. Kurchatov, L.V. Mysovskii, and L.I. Rusinov detected an isomeric state in an artificial radioactive isotope of bromine, 80Br, which was the beginning of the systematic study of nuclear isomerism. A large number of isomeric states, with half-lives of 10 6 sec to many years, are known. One of the longest lived isomers is 236Np, with a half-life of 5,500 years.
The decay of isomers is generally accompanied by the emission of conversion electrons or y-quanta; as a result a nucleus of the same isotope is formed, but in a lower energy state. Sometimes beta decay, which leads to the appearance of an isotope of a different element (see Figure 1), is more probable. The isomers of heavy elements may decay through spontaneous fission.
Nuclear isomerism is due to the structural peculiarities of atomic nuclei. Isomeric states are formed in cases when the transition of the nucleus from a state with higher energy to a lower-energy state through the emission of a γ-quantum is difficult. This is most often associated with a great difference in the values of the spins S of the nuclei in these states. If the difference in the energy in the two states is not great, then the probability of the emission of a γ-quantum becomes small, and consequently the half-life of the excited state proves to be great. Isomers are encountered especially often in nuclei in certain ranges of mass-number values (isomeric islands). This fact is explained by the shell model of the nucleus, which predicts the existence of nuclear levels that are close in energy and have a great difference in spin for certain numbers of protons and neutrons in the nucleus. In some cases (for example, for 180Hf) the appearance of isomers is related to the significant difference in the shape of the nucleus in two nearby energy states. This also leads to a decline in the probability of γ-radiation.
REFERENCESMukhin, K.N. Vvedenie v iadernuiu fiziku. Moscow, 4963. Moszkowsky, S. “Teoriia mul’tipol’nogo izlucheniia.” In Al’fa-, beta- i gamma-spektroskopiia, no. 3. Edited by K. Siegbahn. Moscow, 1969. Page 5. (Translated from English.)
N. N. DELIAGIN