Mossbauer absorbance

In this chapter, we present the principles of conventional Mossbauer spectrometers with radioactive isotopes as the light source Mossbauer experiments with synchrotron radiation are discussed in Chap. 9 including technical principles. Since complete spectrometers, suitable for virtually all the common isotopes, have been commercially available for many years, we refrain from presenting technical details like electronic circuits. We are concerned here with the functional components of a spectrometer, their interaction and synchronization, the different operation modes and proper tuning of the instrument. We discuss the properties of radioactive y-sources to understand the requirements of an efficient y-counting system, and finally we deal with sample preparation and the optimization of Mossbauer absorbers. For further reading on spectrometers and their technical details, we refer to the review articles [1-3]. 

So far, we have discussed only the detection of y-rays transmitted through the Mossbauer absorber. However, the Mossbauer effect can also be established by recording scattered radiation that is emitted by the absorber nuclei upon de-excitation after resonant y-absorption. The decay of the excited nuclear state proceeds for Fe predominantly by internal conversion and emission of a conversion electron from the K-shell ( 90%). This event is followed by the emission of an additional (mostly Ka) X-ray or an Auger electron when the vacancy in the K shell is filled again. Alternatively, the direct transition of the resonantly excited nucleus causes re-emission of a y-photon (14.4 keV). 

Particularly for thin Mossbauer absorbers with a low concentration of the resonance nuclide and high mass absorption, it may be problematic to apply the recommendation for sample preparation (f 0.2), because the resulting electronic absorption may be prohibitively high. In such a case, it may pay well to optimize the absorber thickness, i.e., the area density f. To this end, following the approach of Long et al. [33], we adopt the general expression ... 

The ideal thickness of a Mossbauer absorber is therefore given by [33] ... 

From the discussion in Sect. 4.2 in Chap. 4, it became evident that the isomer shift is linearly related to the electron density at the Mossbauer absorbing nucleus. Thus, one is tempted to write ... 

In molecular DFT calculations, it is natural to include all electrons in the calculations and hence no further subtleties than the ones described arise in the calculation of the isomer shift. However, there are situations where other approaches are advantageous. The most prominent situation is met in the case of solids. Here, it is difficult to capture the effects of an infinite system with a finite size cluster model and one should resort to dedicated solid state techniques. It appears that very efficient solid state DFT implementations are possible on the basis of plane wave basis sets. However, it is difficult to describe the core region with plane wave basis sets. Hence, the core electrons need to be replaced by pseudopotentials, which precludes a direct calculation of the electron density at the Mossbauer absorber atom. However, there are workarounds and the subtleties involved in this subject are discussed in a complementary chapter by Blaha (see CD-ROM, Part HI). 

As a check on the standard technique that is often used for other types of measurement [54,33], a Mossbauer absorber was also prepared by dissolving Au in CHjClj and allowing it to evaporate in the absorber dish. Subsequent MES measurements [45] were carried out at temperatures up to 60 K. A spectrum identical (within statistics) to that of Au plus a singlet corresponding to about 5% metallic gold was obtained at 4.2 K. The fact that a measurement... 

Rancourt DG, Klingelhofer G (1994) Possibility of a Mossbauer resonant-electron microscope. Fourth Seeheim Workshop on Mossbauer Spectroscopy, p 129 Rancourt DG, Mcdondd AM, Lalonde AE, Ping JY (1993) Mossbauer absorber thicknesses for accurate site populations in Fe-bearing minerals. Am Mineral 78 1-7 Riedel E, Karl R (1980) Mossbauer studies of thiospinels. 1. The system FeCr2S4-FeRh2S4. J Sol State Chem 35 77-82... 

Rancourt DG, Mcdonald AM, Lalonde AE, Ping JY (1993) Mossbauer absorber thicknesses for accurate site populations in Fe-bearing minerals. Am Mineral 78 1-7... 

Normally, the studied sample is used either as an absorber or a scatterer. The absorber has to contain the ground-state nuclei of the nuclide responsible for the Mossbauer radiation of the source. Mossbauer absorbers used for transmission experiments can be sheets or powders of solids. Frozen solutions can also be measured. 

Weyer [50] developed a conversion electron detector based on a PPAC for Mossbauer spectroscopy. Conversion electrons are emitted from the surbce of a Mossbauer absorber after resonance absorption with a conversion coefficient a (a = 8.2 for 14.4 keV Fe). In principle, a conversion electron detector is very sensitive only for resonant Mossbauer 7-rays without interference of nonresonant 7-radiations. The PPAC is importantfor in-beam Mossbauer spectroscopy of implanted excited atoms and Rl nuclei in environments with high backgrounds. 

It was stated in the preceding section that 2 is a suitable choice of effective absorber thickness. Table 4 presents the needed area density of the lanthanide (in natural isotopic abundance) or the actinide for different measuring temperatures and various Debye temperatures. For intermetallic compounds 0 200 K is a good guess. A typical Mossbauer absorber covers an area of 2-4 cm. For most cases about 0.5 g of material suffices. As said before, powder samples are fine, single crystals are not necessary. 

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