Mossbauer spectroscopy measurement geometries

Fig. 3.19 Schematic illustration of the measurement geometry for Mossbauer spectrometers. In transmission geometry, the absorber (sample) is between the nuclear source of 14.4 keV y-rays (normally Co/Rh) and the detector. The peaks are negative features and the absorber should be thin with respect to absorption of the y-rays to minimize nonlinear effects. In emission (backscatter) Mossbauer spectroscopy, the radiation source and detector are on the same side of the sample. The peaks are positive features, corresponding to recoilless emission of 14.4 keV y-rays and conversion X-rays and electrons. For both measurement geometries Mossbauer spectra are counts per channel as a function of the Doppler velocity (normally in units of mm s relative to the mid-point of the spectrum of a-Fe in the case of Fe Mossbauer spectroscopy). MIMOS II operates in backscattering geometry circle), but the internal reference channel works in transmission mode...

Conversion electron Mossbauer spectroscopy (CEMS) measurements with back scattering geometry have the merit that spectra can be obtained from a sample with much less isotope content compared with transmission measurements. Another merit is that a sample, deposited on a thick substrate, could be measured, and that because of the limited escape depth of the conversion electrons, depth-selective surface studies are possible. The CEMS technique was found to be best applicable to specimens of 10-100 pg Au cm, i.e., about two orders of magnitudes thinner than required for measurements in transmission mode [443]. This way (1) very thin films of gold alloys, as well as laser- and in beam-modified surfaces in the submicrometers range of depth [443], and (2) metallic gold precipitates in implanted MgO crystals [444] were investigated. 

Most often the transmission mode is found to be the most convenient in Mossbauer spectroscopy, i.e., the y radiation passes from the source through the absorber, and the attenuation of the primary beam is measured at the various Doppler velocities. However, there are a number of cases where a "scattering geometry may be advantageous (SO). The basis for this geometry lies in those processes that take place after resonant absorption of y radiation by the Mossbauer isotope. Specifically, after excitation the Mossbauer isotope may reemit the y ray, or it may decay by emission of internal conversion electrons and X rays [with the probability of internal conversion equal to a/(l + a)]. 

These molecular adducts were investigated by IR (351, 353-365), and electronic spectroscopy (353-365), magnetic measurements (353-365), mass spectrometry (in some cases) (357, 358, 361, 364), Mossbauer spectroscopy [for iron(III) complexes] (353, 354, 365). These physicochemical studies led to the proposal of plausible geometries. 

Schematic representations of (a) parallel and (b) perpendicular measuring geometry, most frequently used In in-field Mossbauer spectroscopy.

Figure 8.47 Principle of Mossbauer spectroscopy. An electromagnetic drive system (EDS) moves the energy source (Sj towards and awayficm the absorber (A) with a constant velocity 8. Transmitted radiation is measured by the proportional counter (Z). lead collimators (C) dejme beam geometry.

Fig. 5. Principle of Mossbauer spectroscopy. Top Set-up for measurement in transmission geometry., Bottom Overlap of Doppler-sKifted zero-phonon emission line with the zero-phonon absorption line. The shift A is given by eq. (10)l [Taken from Kalvius (1987).]...

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