Preparation of Mossbauer Sources and Absorbers

Deviating from the setup discussed earlier, the y-ray beam can also be consistently collimated by structures other than the absorber holder. If this is the entrance window of the detector, the counter should have a lead shield, and the absorber must be sufficiently large to prevent radiation from passing by. For Mossbauer scattering experiments, the same arguments have to be considered. 

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. 

Most Mossbauer experiments are currently performed with commercially available radioactive sources. For some applications, however, a so-called source experiment may be useful, in which the sample is labeled with the radioactive parent-isotope of the Mossbauer nucleus such as Co. The y-radiation of the radioactive sample is then analyzed by moving a single-line absorber for Doppler modulation in front of the detector. 

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In this section, some general features concerning the preparation of Mossbauer sources and absorbers will be discussed details which are specific to individual nuclides are deferred until later chapters, in which each element is considered in turn. 

Most of the Zn Mossbauer experiments so far have been carried out with ZnO as absorber. De Waard and Perlow [54] used polycrystaUine ZnO enriched to 90% in Zn with various pretreatments. They intended to determine (1) the quadrupole splitting in ZnO, (2) the influence of source and absorber preparation on the width and depth of a resonance, (3) the SOD shift, and (4) the influence of pressure on the source. 

In this chapter we shall consider the various techniques which have been used for observation of the Mbssbauer effect, together with methods of source and absorber preparation and computer techniques for data analysis. Some of the advantages and limitations of Mossbauer spectroscopy will become apparent during the discussion of these problems. References to more recent development will be found in the review by J. R. De Voe and J. J. Spijkerman in Analytical Chemistry, 1970, 42, 366R, and in Spectroscopic Properties of Inorganic and Organometallic Compounds published annually by the Chemical Society (London). 

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]. 

In Mossbauer emission spectroscopy one prepares the catalyst with the radioactive source element (e.g. Co) and uses a suitable moving single-line absorber of Fe to record the spectra. In this way one can also study Co-containing catalysts, although strictly speaking the information concerns the iron in the catalyst that forms by the Co —> Fe decay process. 

Unfortunately, the requirements of recoil-free emission and resonant absorption and transmission through the absorber limit the useable energy range of the Mossbauer effect 7-ray to approximately 10-100 keV. Further, in order to obtain rather sharp absorption lines and a reasonable spectral resolution, the mean lifetime of the Mossbauer 7-ray precursor state should be between 1 ns and 100 ns. Further, the Mossbauer nuclide must have a sufficiently high isotopic abundance in the element to yield a usable signal-to-noise ratio over a reasonable acquisition time. Finally, the radioactive source containing the Mossbauer 7-ray precursor state must be easily prepared and have a mean lifetime of several weeks to be practical. These various requirements limit the number of nuclides available for typical Mossbauer spectral studies. 

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