Isomeric shift

The isomer shift is a physical effect, which manifests itself in the fact that the positions of the spectral lines in atomic spectra and gamma radiation of different isomers of a chemical element are not identical. In the case of gamma radiation is called the effect also Mössbauer isomer shift. , The spectra due to the magnetic moments of the nuclei and the hyperfine structure, the displacement of the center of gravity of the spectral relates. The isomer shift provides important information on nuclear structure and on the physical, chemical and biological environment of atoms. It has recently been proposed to use the effect for the examination of the time variation of physical constants.

The isomer shift of atomic spectra

The isomer of the atomic spectra of the energy or frequency shift in the atomic spectra as a result of substitution of one Kernisomers by another. The effect was predicted in 1956 by Weiner in, ( qv). The isomer shift was observed in 1959 for the first time experimentally. Developed by Weiner theory is reflected in the Declaration of Mössbauer isomer shift to bear.

Terminology

The first work on isomer shift related to atomic spectra and used the term Kernisomerieverschiebung of spectral lines (nuclear isomeric shift on spectral lines). After the discovery of the Mössbauer effect, the isomer shift was also detected in gamma spectra and there is also Mössbauer isomer shift called. S For more details on the history of the isomer shift and the terminology used

Isomerism versus isotope shift in atomic spectra

Atomic spectral lines occur at transitions of electrons between different energy levels of atomic E, which will be accompanied by photon emission. Atomic levels are a manifestation of the electromagnetic interaction between electrons and nuclei. The energy levels of two atoms whose nuclei are different isotopes of the same element that differ, although the electric charges Z of the isotopes are the same. This is a consequence of the fact that two isotopes differ in the number of neutrons, and therefore by their masses and volumes. These differences are responsible for the isotope of atomic spectral lines. At two Kernisomeren are the same in both the number of protons, as well as that of the neutrons, but the quantum state of the core is different from what δφ a difference in the electric charge distribution in the core and thus a difference of the corresponding electrostatic core potentials φ has the consequence that finally a difference AE in the atomic energy levels leads. The isomer shift of atomic spectral lines is given by

Here ψ is the wave function of the electron involved in the transition, with its electric charge -e (negative elementary charge ) and the integration is performed over the electron coordinates. The isotope and isomer shift are both a consequence of the finite dimensions of the nucleus. The isotope was discovered experimentally and theoretically explained. The isomer shift, however, was predicted and later experimentally demonstrated ( see also ). While in the case of the isotope is the calculation of the interaction energy between electrons and nuclei a relatively simple electromagnetic problem in the case of isomers, the problem is more difficult, because the Isomerieanregung and hence the difference in charge between the two Isomeriezuständen of the strong interaction is caused. This partly explains why the isomer was not detected earlier: an adequate theory and in particular the shell model was developed only in the late 1940s and early 1950s. Also, the experimental evidence of this effect was made ​​possible by a new technique - the spectroscopy of metastable isomeric nuclear states - which was also developed only in the 1950s. The difference of the isotope, the ( first approximation ) is independent of the structure of the cores, the isomer depends on that structure. Therefore, the information that you can get from the isomer shift, largely as resulting from the isotope shift. The measurement, eg with the help of the isomer shift, the difference between the core radii of the excited and the ground state, is one of the most sensitive tests of nuclear models. In addition, the isomer shift in combination with the Mössbauer effect a detail like instrument that has been found in many other areas outside of physics applications.

The isomer shift and the shell model

As part of the shell model, there is a class of isomers in which a single (as "optical ", designated ) nucleon is responsible for the difference in charge distributions of two isomeric states. This is especially true for nuclei with odd proton and even neutron numbers near closed shells, for example, In -115, for which the effect was calculated and predicted that he would be far greater than the natural line width, and thus measurable. The value of the three years later in measured displacement in Hg -197 was quite close to the calculated with In115, although in Hg -197, In -115, in contrast to the optical nucleon a neutron and a proton is not, and the interaction electron - free neutron is much smaller than the interaction electron - free proton and thus by a factor of 100 smaller effect was expected. This discrepancy can be explained by the fact that optical nucleons are not free but bound particles. The measurement results could be explained in the framework of the theory developed in by the optical neutron ascribed to an effective electric charge.

Mössbauer isomer shift

The Mössbauer isomer shift is observed in the gamma spectroscopy shift of spectral lines, when comparing two different Kernisomeriezuständen in two different physical, chemical or biological environments. It is a consequence of the combined effect of the recoil -free Mößbauerübergangs between two Kernisomeriezuständen and the transition between two atomic states in the given medium. The isomer shift of atomic spectral lines depends on the electron wave function ψ and the difference δφ the electrostatic potentials φ from the two Isomeriezuständen. For a given Kernisomer in two different environments (eg, different physical phases or different chemical combinations ) to the corresponding electron wave functions differ. For this reason, there is in addition to the isomer of the spectral lines, which is the difference of the cores due Isomeriezustände, a shift resulting from the two different environments. For experimental reasons, the latter source or absorber are called. This combined shift is the Mössbauer isomer shift and it is mathematically described using the same formalism as the Kernisomerieverschiebung of atomic spectra, except that one has to consider now instead of a two- electron wave functions (from source ψQuelle and absorber ψ_Absorber ) and the difference between the respective shifts:

The first measurement of the isomer shift in the gamma spectroscopy using the Mössbauer effect was 1960. This effect provides important and highly accurate information about both the Isomeriezustände, as well as on the physical, chemical and biological environments of the atoms. Thus, the isomer shift found important applications in fields as diverse as nuclear physics, solid state physics, nuclear physics, chemistry, biology, metallurgy, mineralogy, geology, Moon and Mars research (see also ). The Kerniosmerieverschiebung was also detected in muonic atoms. These atoms a muon is captured from the excited core and then a transition from the excited state to the ground state of atomic place in a time interval which is smaller than the lifetime of the excited Kernisomeriezustandes.