Cobalt Mossbauer source

The nuclear decay of radioactive atoms embedded in a host is known to lead to various chemical and physical after effects such as redox processes, bond rupture, and the formation of metastable states [46], A very successful way of investigating such after effects in solid material exploits the Mossbauer effect and has been termed Mossbauer Emission Spectroscopy (MES) or Mossbauer source experiments [47, 48]. For instance, the electron capture (EC) decay of Co to Fe, denoted Co(EC) Fe, in cobalt- or iron-containing compormds has been widely explored. In such MES experiments, the compormd tmder study is usually labeled with Co and then used as the Mossbauer source versus a single-line absorber material such as K4[Fe(CN)6]. The recorded spectrum yields information on the chemical state of the nucleogenic Fe at ca. 10 s, which is approximately the lifetime of the 14.4 keV metastable nuclear state of Fe after nuclear decay. 

Figure 3.26 Cobalt-57 source of 14.41-keV y radiation used in Mossbauer experiments. Isomer shift and quadrupole splitting characteristics are shown at right. (Adapted from Figure 2.26 of reference 3 and Figure 1 of reference 24.)...

The present method is still in its early stage of application. Both ex situ and in situ type measurements are applicable to a variety of mineral/aqueous solution interfaces. For example, the mechanism of selective adsorption of cobaltous ions on manganese minerals can be studied by this method. In addition to the two Mossbauer source nuclides described in the present article, there are a number of other nuclides which can be studied. We have recently started a series of experiments using Gd-151 which is a source nuclide of Eu-151 Mossbauer spectroscopy. Development of theory on surface magnetism, especially one including relaxation is desirable. Such a theory would facilitate the interpretation of the experimental results. 

Corrosion of Metals. Corrosion phenomena of iron, cobalt, tin, and their alloys have been investigated by a variety of Mossbauer techniques by the transmission and scattering methods and by the emission technique in which the specimen under study is doped with a Mossbauer source isotope (e.g., Co) and measured against a single-line resonant... 

In order to dissipate the recoil energy Mossbauer was the first to use atoms in solid crystal lattices as emitters and also to cool both emitter and absorber. In this way it could be shown that the 7-ray emission from radioactive cobalt metal was absorbed by metallic iron. However, it was also found that if the iron sample were in any other chemical state, the different chemical surroundings of the iron nucleus produce a sufficient effect on the nuclear energy levels for absorption no longer to occur. To enable a search for the precisely required absorption frequency, a scan based on the Doppler effect was developed. It was noted that a velocity of 102 ms-1 produced an enormous Doppler shift and using the same equation (7) it follows that a readily attainable displacement of the source at a velocity of 1 cms-1 produces a shift of 108 Hz. This shift corresponds to about 100 line-widths and provides a reasonable scan width. 

The most direct information on the state of cobalt has come from Mossbauer spectroscopy, applied in the emission mode. As explained in Chapter 5, such experiments are done with catalysts that contain the radioactive isotope 57Co as the source and a moving single-line absorber. Great advantages of this method are that the Co-Mo catalyst can be investigated under in situ conditions and the spectrum of cobalt can be correlated to the activity of the catalyst. One needs to be careful, however, because the Mossbauer spectrum one obtains is strictly speaking not that of cobalt, but that of its decay product, iron. The safest way to go is therefore to compare the spectra of the Co-Mo catalysts with those of model compounds for which the state of cobalt is known. This was the approach taken... 

The AAS method has several limitations. For the trace elements, particularly the colorants cobalt and nickel, the dilution factor required for analyses of 12 elements by continuous nebulization places these elements close to the detection limits for flame AAS. More accurate data on these and other trace elements are necessary before conclusions can be drawn on the source minerals used to impart color. Phosphorus, a ubiquitous minor component of medieval stained glass, has not been determined by AAS in the course of this work, but has the potential to provide key information on sources of plant ash. A full understanding of the colorant role of the transition metal elements is not possible on the basis of analysis alone UV-visible spectroscopy, electron spin resonance spectrometry, and Mossbauer spectroscopy, for example, are necessary adjuncts to achieve this aim. The results of the application of these techniques and the extension of the AAS method to trace element determination by pulse nebulization and furnace atomization will be addressed in future reports. 

The source most commonly employed with Fe Mossbauer spectroscopy is elemental Co, which is incorporated into rhodium or copper metal. During the radioactive decay of the cobalt isotope into Fe, the needed gamma radiation is emitted. For measurements with tin ( Sn), sources of CaSnOs or BaSnOs enriched with Sn are used, which again release the proper radiation during their radioactive decay. The source is moved at constant positive and subsequently negative accelerations (i.e. linearly varying speed) to probe the resonant absorption. 

Iron-57 is the most important Mossbauer nuclide even though it has a natural abundance of only 2.2%. The half-life of its 14.41 keV excited state is 98.1ns and the half-life of its precursor source, cobalt-57, is 270 days. As a result, the resolution of iron-57 Mossbauer spectroscopy, 6.5 X 10 —the ratio of the linewidth to the 7-ray energy—is excellent. Because the typical iron-57 hyperfme parameters are of the order of a few millimeters per second and range up to many times the natural linewidth of 0.194mm s they are easily measured with excellent resolution. 

Table 1.38 summarises the main features of Mossbauer spectroscopy. The great advantage of Mossbauer spectroscopy for in-polymer additive analysis is that it provides in situ information. An economic advantage is that the technique is relatively inexpensive in comparison to electron microscopy or XPS. The technique is limited to those isotopes that exhibit the Mossbauer effect. The detection limit is 10 atoms of the nuclear isotope studied. Through the Mossbauer effect in iron, it is possible to obtain information on the state of cobalt. Whereas in Mossbauer absorption spectroscopy (MAS) a single-line source is moved and... 

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