Mossbauer transmission experiment

Fig. 2.6 Schematic illustration of a Mossbauer transmission experiment in five steps. The Absorption bars indicate the strength of recoilless nuclear resonant absorption as determined by the overlap of emission and absorption lines when the emission line is shifted by Doppler modulation (velocities Uj,. .., 1)5). The transmission spectrum T v) is usually normalized to the transmission T oo) observed for v oo by dividing T(v)IT(oo). Experimental details are found in Chap. 3...

In a standard Mossbauer transmission experiment the absorber is placed between the source and the detector. In order to observe the effect. 

Fig. 2. Apparatus (a) Mossbauer transmission experiment, (b) Mossbauer scattering experiment...

Scattered radiation. In a transmission experiment, the Mossbauer sample emits a substantial amount of scattered radiation, originating from XRF and Compton scattering, but also y-radiation emitted by the Mossbauer nuclei upon de-excitation of the excited state after resonant absorption. Since scattering occurs in 4ti solid angle, the y-detector should not be positioned too close to the absorber so as not to collect too much of this unwanted scattered radiation. The corresponding pulses may not only uimecessarily overload the detector and increase the counting dead time, but they may also affect the y-discrimination in the SCA and increase the nonresonant background noise. 

A Mossbauer transmission cell similar to that prepared by Delgass and coworkers (20) was used for all Mossbauer experiments. Zeolite pellets between 200 mg and 300 mg were used as samples. 

The Mossbauer-effect experiment can also be applied to the study of surfaces in the variation known as conversion electron Mossbauer spectroscopy (CEMS). Here, what is monitored as a function of incident y-ray energy is not absorption, but the emission of electrons through a process of internal conversion (i.e., as a byproduct of the absorption of Mossbauer y rays). Since the conversion electrons can only escape from the surface layers of the solid, data are selectively acquired for the surface region, arising from the Mossbauer effect in the (most commonly iron) atoms of the surface layers. The monitoring of emitted electrons results in a mirror image of the usual absorption spectrum. Transmission and CEM spectra of vivianite [Ee3(P04)2-8H20] are illustrated in Fig. 2.49 (after Tricker et al., 1979]. 

Figure 1. Typical geometries for Mossbauer spectroscopy experiments (a) transmission (b) scattering...

Even the nuclear heat capacity of Tm has presented problems. Holmstrom et al. (1%9) in measurements between 0.02 and 0.4, obtain a good fit with the parameters in table 5.2, provided that only 95.0 0.8% (by weight) of the specimen is assumed to contribute to Cn- As these parameters, although smaller than the theoretical values (Bleaney 1%3), agree well with those found from Mossbauer (Kalvius et al. 1%3) and neutron transmission experiments (Al-Kital et al. 1967), they may be taken to represent the situation in thulium at low temperatures. 

A substantial advantage of emission Mossbauer spectroscopy in comparison with the transmission technique is that if the material to be investigated contains heavy elements, then the required dopant concentration (e.g., Co) may be 1-2 orders of magnitude lower in the emission experiment than the Fe concentration in an analogous transmission experiment. This is in connection with the intensity loss of the Mossbauer radiation due to electronic absorption, which is always self absorption in the source and regular absorption in the absorber (Vertes and Homonnay 1997). Low dopant concentration is very important in impurity Mossbauer spectroscopy, where the investigated material does not contain a Mbssbauer element thus, a conveniently measurable Mbssbauer nuclide is introduced artificially as an impurity with a potential risk of perturbing the physicochemical properties of the host phase. 

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. 

The Gonversion Electron Mossbauer Spectroscopy experiments are very useful, particularly in nondestructive testing and study of surfaces and thin layers because low-energy conversion electrons in matter have a limited range (typically 100 nm for the nucleus Fe), the method is especially useful for the study of thin layers. Apart from the transmission spectroscopy, the... 

Figure 5.1 Schematic illustration of recording the resonance absorption line in a Mossbauer spectroscopy experiment and relative transmission of gamma quanta as a function of Doppler velocity. 2000 lOP Publishing Ltd.

A typical device for accumulating the Mossbauer spectrum is the multichannel analyser, where the count rate is a function of a definite value of the Doppler velocity. The count rate is normalized relative to the off-resonance count rate. Hence, for transmission-mode Mossbauer spectroscopy relative intensities are always less than unity (or 100%). In Mossbauer scattering experiments relative intensities always exceed 100% and can reach several hundred percent in the case of electron detection from samples with a high abundance of the resonant isotope. It is most often that the -y ,ax value corresponds to the first channel and the +y ,ax value to the last channel. The quality of a Mossbauer spectrometer is determined by how accurately the modulation of the y-quanta energy follows the chosen mode of movement. 

SEDMS experiment is a transmission experiment with the scatterer being the Mossbauer source. 

Most Mossbauer spectroscopy experiments are conducted either in the transmission mode, in which a source of well-defined characteristics is used to examine the spectral properties of an unknown absorber, or in the emission mode, in which the source of radiation becomes the sample under investigation and a known or standard absorber is employed to determine the transition energy differences. In both cases, any of a number of y-ray detectors is utilized to record the amount of radiation transmitted through the absorber. 

Four different material probes were used to characterize the shock-treated and shock-synthesized products. Of these, magnetization provided the most sensitive measure of yield, while x-ray diffraction provided the most explicit structural data. Mossbauer spectroscopy provided direct critical atomic level data, whereas transmission electron microscopy provided key information on shock-modified, but unreacted reactant mixtures. The results of determinations of product yield and identification of product are summarized in Fig. 8.2. What is shown in the figure is the location of pressure, mean-bulk temperature locations at which synthesis experiments were carried out. Beside each point are the measures of product yield as determined from the three probes. The yields vary from 1% to 75 % depending on the shock conditions. From a structural point of view a surprising result is that the product composition is apparently not changed with various shock conditions. The same product is apparently obtained under all conditions only the yield is changed. 

The lr Mossbauer experiments are usually carried out in transmission geometry with both source and absorber kept at liquid helium temperature and a Ge(Li) diode or a 3 mm Nal(Tl) crystal used to detect the 73 keV y-rays. The absorbers typically contain 50-500 mg cm of natural iridium, which contains 62.7% of the Mossbauer isotope lr. The isomer shifts are generally given with respect to iridium metal (the isomer shift between Os/Os and Ir metal is (0.540 0.004) mm s at 4.2 K ([268]). 

Figure 5.11 A constant velocity Mossbauer experiment reveals the kinetics of the denitridation of an iron nitride in different gases at 525 K. The negative part of the time scale gives the transmission of the most intense peak of the nitride at time zero the gas atmosphere is changed to the desired gas. Denitridation occurs relatively fast in H2, but is retarded by CO, whereas the nitride is stable in an inert gas such as helium (from Hummel etal. [33]).

Figure 5.4 provides a schematic diagram of a Mossbauer experiment in transmission mode with a moving single-line source and the absorbing sample in fixed position. A Mossbauer spectrum is a plot of the y-ray intensity transmitted by the sample, against the velocity v of the source. The latter is related to the... 

In a basic Mossbauer experiment, the reduction in transmission (9) (Figure 2) or the increase in scattered intensity of radiation (2) (Figure 3) is observed as a function of the relative velocity between a source and an absorber. The full width at half maximum of the resonance curve r is related to the mean life of the radiating state by the uncertainty relation r 2h/r. The depth of the curve, c, is related to /, the magnitude of the recoilless fraction of gamma rays emitted, and hence to the crystalline properties of the solid. Finally, the displacement of the curve from zero relative velocity indicates the energy difference between emitted and absorbed radiation and is proportional to the s-electron... 

Mossbauer experiments were performed in transmission mode using a calcium stannate source. Spectra were deconvoluted using standard methods to separate contributions from Tin (II) and Tin (IV) peaks. The Mfissbauer spectrum of the cured rubber (see Figure 1) shows the Tin (II) and Tin (IV) oxidation states, with the Tin (IV) species representing approximately 67% of the total tin signal. Overall, the IS/QS values suggest that the Tin (IV) species formed in the rubber is most probably Tin (IV) oxide, Sn02. The presence of some residual unreacted Tin (II) catalyst within the cured rubber is clearly evident. 

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