Mdssbauer nuclides

Application of Mdssbauer spectrometry depends on the availability of suitable sources with half-lives of excited states between about 10 and 10 s. The photon energy must not exceed lOOkeV and conversion must not be too high to ensure recoilless emission and absorption. As already mentioned, Fe, the daughter of Co, is the most frequently used Mdssbauer nuclide. Co is used as Mdssbauer source and iron of natural isotopic composition (2.17% Fe) or enriched Fe as absorber. 

Mdssbauer spectrometry gives information about the chemical environment of the Mdssbauer nuclide in the excited state at the instant of emission of the photon. It does not necessarily reflect the normal chemical state of the daughter nuclide, because of the after-effects that follow the decay of the mother nuclide (recoil and excitation effects, including emission of Auger electrons). At very short lifetimes of the excited state, ionization and excitation effects may not have attained relaxation at the instant of emission of the y-ray photon this results in a time-dependent pattern of the Mdssbauer spectrum. 

Often, the observed iron-57 Mossbauer spectral hyperfme parameters, when combined with a knowledge of the magnetic properties of the complexes, can easily lead to an unambiguous assignment of the iron oxidation state, coordination number, and yield the extent of the distortion of its coordination environment. The same is also true for many of the other Mdssbauer nuclides mentioned above. 

When the Mdssbauer nuclides are in different microenvironments, the determination of the relative values of the Mdssbauer—Lamb factors belonging to each microenvironment is necessary in order to obtain the correct relative molar quantities. 

However, the source, which is supposed to be a material doped with a Mdssbauer nuclide in its excited state caimot be prepared because the half-life is so short that the source would completely decay before one would start the experiment (for Fe, T1/2 10 s). Thus, one has to find a parent nuclide with a reasonably long half-life, which produces the excited Mossbauer level by nuclear decay. This has the consequence that in an emission experiment the dopant element may be different from the Mossbauer active one. In Fe Mdssbauer spectroscopy, this parent nuclide is Co, which decays by electron capture, Co(EC) Fe, with a half-life of 9 months. 

In Table 7.1 at the end of the book), nuclear data are collected for those Mdssbauer transitions of transition metal nuclides that are used in Mossbauer spectroscopy. The symbols used in this table have the following meaning ... 

The Mossbauer effect is the emission and resonant absorption of nuclear y-rays studied under conditions such that the nuclei have negligible recoil velocities when y-rays are emitted or absorbed. This is only achieved by working with solid samples in which the nuclei are held rigidly in a crystal lattice. The energy, and thus the frequency of the y-radiation involved, corresponds to the transition between the ground state and the short-lived excited state of the nuclide concerned. Table 2.4 lists properties of several nuclei which can be observed using Mdssbauer spectroscopy. 

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