Source and absorber preparation

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. 

1-86-hour half-life. Br gives a decay to 2-41-hour half-life. 

The likelihood of success in observing a Mossbauer resonance will depend on numerous factors as follows  

The absorption cross-section Oq is proportional to Ey and also decreases rapidly as E increases. These are the main reasons why the Mbssbauer effect 

1 and 100 ns. As we saw in Section 1.3 it is this time which controls the Heisenberg linewidth of the y-ray energy. If the relative linewidth F/Ey is too narrowly defined (i.e. a long half-life) there are considerable problems in damping out mechanical vibrations in the spectrometer. Conversely, a short lifetime gives a broad line which is difficult to observe and which usually also obscures any chemical hyperline effects. 

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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). 

De Waard and Perlow [54] observed a marked influence of the method of preparation of source and absorber on both the line width and the depth of resonance. For instance, the line width was found to be 2.7 pm s for a powdered ZnO absorber as compared to 0.8 pm s for a sintered one. 

The 64-keV level of Gd has a lifetime of 460 ns, giving it a very narrow natural linewidth of only 0-0093 mm s the resonance was first reported in 1966 [84]. The Eu parent has a short lifetime of 15-4 h and the i Gd(y,p) Eu preparation requires separation of the required source activity from other contaminants. The combined quadrupole splitting of the GdaOj source and absorber from the f, f El transition gave line separations an order of magnitude greater than the linewidth. 

We close with a plea to investigators to report shift data with respect to a standard absorber, either clean iron foil or sodium nitroprusside or both. The logic behind this request is obvious since anyone who has tried to intercompare results between different papers, knows how frustrating it can be when no common substances were studied. We also suggest that inherently narrow line sources be used—i.e., Cu, Pt, Pd, or Cr. Stainless steel sources are usually so broad that they can often mask important features of the spectrum. Of the host matrices mentioned we prefer Cu since it doesn t give rise to any interfering x-rays and when prepared properly doesn t show any broadening at low temperatures. 

For photostability studies, accelerated tests for liquid preparations may involve the use of some controlled, high intensity photon sources, and should take into account maintenance of a uniform temperature and adequate mixing of the preparation. If the protocol of the photostability study is properly designed, the extent of degradation should be directly proportional to the number of photons absorbed, as reported in the study by Shin et al. (97), and presented in Figure 12. 

The digestibility of starch depends on source, physical condition, and food preparation. With the exception of lignin, dietary fibers may be partially or fully hydrolyzed by enzymes of the microflora present In the colon, iJome products arc absorbed into the bloodstream, but most serve as energy sources for the colono-cy tes. Specific dietary fibers can influence fecal bulk, the rate of passage of material through the intestines, and certain maladies such as constipation. 

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