Isotopes, Mdssbauer

In a Mdssbauer transmission experiment, the absorber containing the stable Mdssbauer isotope is placed between the source and the detector (cf. Fig. 2.6). For the absorber, we assume the same mean energy q between nuclear excited and ground states as for the source, but with an additional intrinsic shift A due to chemical influence. The absorption Une, or resonant absorption cross-section cr( ), has the same Lorentzian shape as the emission line and if we assume also the same half width , cr( ) can be expressed as ([1] in Chap. 1)... 

Fig. 7.52 Simplified nuclear decay schemes for the four osmium Mdssbauer isotopes...

Doppler velocity may provide a Mossbauer spectrum with a large increase in the signal to noise ratio compared to that obtained in the transmission mode. The type of radiation used to generate the scattered Mossbauer spectrum depends on the internal conversion coefficient a a large value of a, which favors the emission of X rays by the Mdssbauer isotope, makes X-ray detection appropriate, while a small value of a favors y-ray detection. 

Although GEMS is a more difficult technique experimentally than normal Mdssbauer spectroscopy it offers many exciting possibilities for the study of Mdssbauer isotopes on or near the surface. It could be a useful technique to study the properties of bifunctional zeolite catalysts and metal-impregnated zeolite catalysts. 

The properties of the two common Mdssbauer isotopes, and Sn, are compared in Table I. The parent isotope for Sn is the 87-keV " Sn, which has a half-life of 245 days and decays directly to the 23.8-keV MOssbauer level. The 25.2-keV x-rays which are also emitted from such sources can be removed by a palladium filter or through the use of a high-resolution detector. The Sn is obtained by neutron irradiation of separated Sn, but the cross section is low and high fluxes are necessary. The natural half-width of the 23.8-keV y-ray is 0.31 mm sec that is, the lines of a good spectrum will be 0.7 to 0.8 mm sec wide, and lines closer than about 0.4 mm sec will not be resolved. The spectra of Sn are therefore much less well-resolved than those of Fe (cf. Fig. 1). 

Fig. 1.1. A periodic table of elements with Mossbauer isotopes. The elements with easier to use and more extensively studied isotopes are shown shaded, while those with more difficult to use and less extensively studied isotopes are shown hatched. These two categories were distinguished by whether more or less than one hundred papers were published on work with these Mdssbauer isotopes between 1953 and 1982 (Long, 1984). It should be noted that in this period 10000 papers were published on studies involving the most extensively used isotope Fe.

The ground- and excited-state magnetic moments ji are tabulated as /ig = 0.09062(3) n.m. (nuclear magnetons, /i ) and jig, = 0.1549 n.m., respectively (see Table Properties of Isotopes Relevant to Mdssbauer Spectroscopy provided by courtesy of Professor J. G. Stevens, Mdssbauer Effect Data Center, cf. CD-ROM). Considering that nuclear magnetic moments are given by the relation p, = giPj I, the nuclear g factors for Ee with /g = 1/2 and 1 = 3/2 are gg = 0.09062 X 2 and ge = 0.1549 x 2/3. With these values and taken from... 

Fig. 5. Basic equipment for measuring an emission Mdssbauer spectrum. The sample is prepared with a radioactive isotope and used as the source.

First, the sample must be isotopically enriched with Fe or other Mdssbauer nucleus of interest. Small molecule... 

Shallow escape depth of electron provides information on the nature (oxidation state, spin state, coordination, etc.) of the Mdssbauer-active isotope in the surface layer... 

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. 

A Mdssbauer resonance is known in at least one isotope of fourteen tran sition metals in addition to iron. However, none has been extensively used up to the present time. Several of them present extreme difficulties in measurement, but as this chapter will show, sufficient background information has been collected to assess the feasibility of chemical application. 

The nucleus of an isotope suitable for Mdssbauer spectroscopy must have an excited state of moderately low energy (less than about 200 keV) to permit the occurrence of recoilless emission and absorption, and the excited state must be accessible, preferably by the spontaneous decay of a parent isotope with a... 

The term microenvironment is of fundamental importance in M5ssbauer spectroscopy. It is directly related to the hyperfine interactions sensed by the nucleus. In a solid studied by Mdssbauer spectroscopy, there are a large number of nuclei of the M5ssbauer isotope. They all sense local hyperfine interactions, which are determined by the product of a nuclear parameter (charge equivalent nuclear radius, Ry nuclear quadrupole moment, Q, and nuclear magnetic moment, /r) and a solid-state parameter (electronic density, p, electric field gradient represented by V22 and rj, and magnetic flux density, B) (see OEqs. (25.60), (0 25.68), and (025.74)). 

The existence of a proper y-ray transition is an outcome of nuclear structure. Therefore, a Mdssbauer transition is coupled to a certain isotope of the atom of interest. A low isotopic abundance of the resonant isotope may be a serious restriction. Next, if the excited state is rather short lived then F becomes wide and the resolution for hyperfine spectroscopy (see section 2.3) becomes too limited. The same effect can be caused by unfortunate values of nuclear moments of the two states involved in the Mdssbauer transition. Finally, difficulties to produce the source activity may hinder a wider application of measurements with that particular isotope. Difficulties in source preparation and poor resolution are, for example, present in and have prevented Mdssbauer spectroscopic studies of compounds of this most interesting actinide. 

Typical required absorber thickness (in mg/cm of natural isotopic admixture of the resonant atoms) for selected Mdssbauer resonances in the lanthanides and actinides for different Debye temperatures 0 and measuring temperatures T. 

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