Mossbauer source

Mossbauer spectra are usually recorded in transmission geometry, whereby the sample, representing the absorber, contains the stable Mossbauer isotope, i.e., it is not radioactive. A scheme of a typical spectrometer setup is depicted in Fig. 3.1. The radioactive Mossbauer source is attached to the electro-mechanical velocity transducer, or Mossbauer drive, which is moved in a controlled manner for the modulation of the emitted y-radiation by the Doppler effect. The Mossbauer drive is powered by the electronic drive control unit according to a reference voltage (Fr), provided by the digital function generator. Most Mossbauer spectrometers are operated in constant-acceleration mode, in which the drive velocity is linearly swept up and down, either in a saw-tooth or in a triangular mode. In either case. 

The motion of a Mossbauer source must be controlled with an accuracy of 10 mm s or better to resolve sharp Mossbauer spectral lines, such as those of... 

The relative absorption depth of the Mossbauer line is determined by the product of the recoU-free fraction/s of the Mossbauer source and the fractional absorption z t) of the sample, abs = fs-e f), where c(t) is a zeroth-order Bessel function ((2.32) and Fig. 2.8). Since c(t) increases Unearly for small values of t, the thin absorber approximation, c(t) t/2, holds up to t 1. On the other hand, values as small as t = 0.2 may cause already appreciable thickness broadening of the Mossbauer lines, according to (2.31), Fexp + 0.135t). In practice, therefore the sample... 

Fig. 3.18 Pulse-height analysis (PHA) spectrum (or energy spectrum) for Co/Rh Mossbauer source radiation backscattered nonresonantly and/or resonantly from aluminum and stainless steel plates. Data were obtained with Si-PIN diodes with sensitive area of 1 cm per diode and a thickness of 400 pm (from [36, 46])...

Amersham International Limited, Technical Catalog RS16-8, Mossbauer Sources (1981)... 

The nuclear y-resonance effect in ° Ni was first observed in 1960 by Obenshain and Wegener [2]. However, the practical application to the study of nickel compounds was hampered for several years by (1) the lack of a suitable single-line source, (2) the poor resolution of the overlapping broad hyperfine lines due to the short excited state lifetime, and (3) the difficulties in producing and handling the short-lived Mossbauer sources containing the Co and Cu parent nuclides, respectively. 

Single-line sources are now available which cut down the number of resonance lines in a spectrum and thereby reduce the resolution problems considerably. Since many laboratories have access to electron and ion accelerators to produce the parent nuclides Co and Cu, the major experimental obstacles to Ni spectroscopy have been overcome and a good deal of successful work has been performed in recent years. Moreover, the development of synchrotron radiation instead of conventional Mossbauer sources is of additional advantage for future Mossbauer applications (see below). 

Quasielastic (Rayleigh) scattering of the 46.5 keV Mossbauer radiation was used to examine the liquid dynamics of glycerol [245, 246] and the harmonic vibrations of the nonhydrogen atoms in polycrystalline myoglobin [247] as a function of temperature. The y-quanta emitted by the Mossbauer source are... 

Salomon and Shirley [296] have performed Mossbauer-source experiments on Fe( Os) alloys with less than 0.1 at% osmium against a metallic Ir absorber. Their spectra were well-resolved magnetic eight-line patterns, which the authors could satisfactorily analyze using the Hamiltonian... 

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. 

The use of synchrotron radiation overcomes some of the limitations of the conventional technique. The high brilliance of up to 10 ° photons s mm mrad /0.1% bandwidth of energy, and the extremely collimated synchrotron beam lead to a large flux of photons through a very small cross section (0.1-1 mm ). This allows measurements with samples of small volume if isotopi-cally enriched (with the relevant Mossbauer isotope, e.g., Fe). Measurements that were described earlier [4] and that require a polarized Mossbauer source now become experimentally more feasible by making use of the polarization of the synchrotron radiation. Additionally, the energy can be tuned over a wide range. This facilitates measurements with those Mossbauer nuclei for which conventional sources are available but with life times that are too short for most experimental purposes, e.g., 99 min for Co —> Ni and 78 h for Ga —> Zn. 

Asymmetry in the ligand environment, either geometric or in charge distribution (or both), affect the asymmetry parameter, tp An r = 0 value corresponds to complete axial symmetry, whereas r = 1 corresponds to pure rhombic symmetry. Electric monopole interactions between the nuclear charge distributions and the electrons at the nucleus cause a shift of the nuclear ground and excited states. These interactions are known as the isomer shift, 8. Both the Mossbauer source and the absorber (the sample of interest) experience an isomer shift, and it is customary to quote 8 relative to a standard, usually Fe metal or Na2[Fe(CN)5NO] 2H2O at... 

The Mossbauer source nuclide Co-57 was obtained commercially Sb-119 was produced by the 160 cm RIKEN cyclotron at our institute. 

Figure 1. Simplified decay schemes of the Mossbauer source nuclides.

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. 

Mossbauer sources with convenient lifetimes can be made from Sb, or the 58d isomer of Te. This is evident from the level diagram shown in Figure 1. Single line sources can be made with Sb and 251 if these isotopes are incorporated into materials of cubic symmetry, thereby avoiding quadrupole eflFects. This has been done by electroplating Sb or on a copper foil and annealing (13, 16). Sources have also been made by neutron irradiation of pure Te enriched with (5). The 58-day Te isomer is then produced by neutron... 

FIGURE 1. Radioactive processes for Mossbauer sources (a) 127I, (b) 129I... 

The most frequently used Mossbauer nuclide is Fe, originating from the Mossbauer source Co by electron capture (Fig. 10.1). Source and Mossbauer nuclide form a Mossbauer pair. The half-life of the first excited state of Fe at E = 14.4 keV is 98 ns (r = 1.4 x 10 s) and the natural linewidth is T = 4.6 10 eV. 

Figure 10.1. Transmutation of Co (Mossbauer source) into Fe (Mossbauer nuclide) Mossbauer level at 0.0144 MeV (excitation energy).

More information on this system came from non-destructive Mossbauer measure-ments In cis- and trans-(Co(en)2Cl2)N03 about 10 Co cm were implanted and the targets used as Mossbauer sources. The spectra were compared with emission spectra of the same compounds labelled with Co via chemical synthesis. One must keep in mind that the Mossbauer spectrum contains no direct information on the Co in the matrix but only on the actual Mossbauer nuclide pe arising from the Co -> Fe 3-decay. Because the corresponding compounds cis- and trans--(Fe(en)2Cl2)N03 were not accessible by synthesis, a comparison with the absorption spectra was not possible. The interpretation followed the line ... 

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