Nuclear isomer

Isomers (from Greek ἴσος isos, equal to ' and μέρος meros, part '; singular: the isomer ) ( ie, the same atomic number and same mass number ), but are in nuclear physics are atomic nuclei with equal numbers of protons and neutrons in different internal states. To distinguish them from the isomerism in chemistry also the designations nuclear isomerism, Kernisomer be used.

In contrast to the chemical isomer is considered, however, generally not the core in the ground state, but only one in an excited state called, and also, if this condition is particularly durable. The isomer is considered as a separate nuclide and by an "m" ( for " metastable" ) in addition to the mass number called. To distinguish between several isomers of the same nuclide can the " m" is a number to be adjusted, for example 152m1Eu. In Nuklidkarten to Kernisomere can be represented by the field in question is divided into columns.

Explanation and examples

For each nucleus (of mass number 4 and up), there are excited states other than the ground state. As isomers particularly durable states are denoted (see metastable state). The boundary between short-and long-lived is not exactly defined, but is at least 1 nanosecond, that is orders of magnitude over the lifetimes of most excited nuclear states. The transition from the metastable state to the ground state, for example due to a necessary major change in angular momentum unlikely ( " forbidden ").

The longest-lived Kernisomer is 180mTa with a half-life of at least 1.2 x 1015 years. So it has a longer than the core in the initial state, which has only a half-life of 8 hours. Another isomer, 99mTc, is described in the article on radioactivity.

Isomers can go into a state of lower energy or decay radioactively otherwise, by emission of gamma radiation ( Isomerieübergang ) or by interior conversion.

Like all excited nuclear states of isomeric nuclei may also differ in spin and parity of the ground state and from each other. The different states describe different charge distributions in the core. This influences the energy of the electrons attached to the core and can lead to a shift of the spectral lines of the atom that is referred to as isomer and indication of the core structure and the electron density can be in the core.

History

The first isomeric cores were discovered in 1921 by Otto Hahn in the study of decay series of uranium. In addition to the 234mPa already known ( "Uranium X2 ", " Brevium " ) with a half-life of 1.17 minutes, he took a second beta radiating nuclide 234Pa ( "Uranium Z") with the same mass number, which differs from 234mPa only by its longer half-life differs from 6.7 hours. In 1936 the Kernisomere were declared by Carl Friedrich von Weizsäcker as metastable states.