Synchrotron Mossbauer sources

Mossbauer spectroscopy with synchrotron radiation can be used in a straightforward manner with single-line Fe Mossbauer sources obtained using iron borate (FeBOs) single crystals set under diffraction conditions at the Neel temperature (75.3°C) (Smirnov et al. 1969, 1997 Smirnov 1999 Mitsui et al. 2005, 2007). In this method, the spectrum obtained using synchrotron Mossbauer sources is basically identical to that obtained using conventional radioactive sources. This approach, therefore, is easily applicable and beneficial for a wide range of Mossbauer research. However, Fe is the only available nuclide so far. 

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). 

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. 

Potential Mossbauer isotopes for nuclear resonance scattering, which are within the spectral reach of synchrotron radiation sources, are summarized in Table 9.5 [118-120], and the synchrotron radiation sources which provide dedicated beam lines for specific Mossbauer isotopes are listed in Table 9.6 (adopted from [85]). 

Table 9.5 Potential Mossbauer isotopes for nuclear resonance scattering, which are within the spectral reach of currently available synchrotron radiation sources...

Recent developments in Mossbauer spectroscopy may also lead to interesting high-pressure applications. Many years ago it was proposed that the special properties of synchrotron radiation could be used to provide nuclear excitation without the use of radioactive sources, and recently progress with modern synchrotron-radiation sources could mean that such experiments could be feasible for Fe. Due to the natural high collimation of the most favourable undulator radiation from synchrotron insertion devices, one can expect that high-pressure measurements will be one of the first applications of this technique, which will eventually be applied to isotopes for which no suitable radioactive sources exist. " ... 

Seto et al. 2000a, 2002). However, when synchrotron radiation is used as an excitation source (excluding one method discussed later), the energy width is much broader (>0.1 meV) than that of conventional Mossbauer sources. Therefore, special measuring techniques are needed to obtain information on the hyperfine interactions when using synchrotron radiation. 

Resonant y-ray absorption is directly connected with nuclear resonance fluorescence. This is the re-emission of a (second) y-ray from the excited state of the absorber nucleus after resonance absorption. The transition back to the ground state occurs with the same mean lifetime t by the emission of a y-ray in an arbitrary direction, or by energy transfer from the nucleus to the K-shell via internal conversion and the ejection of conversion electrons (see footnote 1). Nuclear resonance fluorescence was the basis for the experiments that finally led to R. L. Mossbauer s discovery of nuclear y-resonance in ir ([1-3] in Chap. 1) and is the basis of Mossbauer experiments with synchrotron radiation which can be used instead of y-radiation from classical sources (see Chap. 9). 

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]. 

As was stated in Section II.A, the energy resolution of the radioactive sources used in conventional Mossbauer spectroscopy is typically 10 eV. This resolution is determined by the natural line width and the maximum energy range obtained by Doppler-shifting techniques. In the case of synchrotron radiation, the energy resolution, which is related to the time period following the excitation of the isotope, is superior to that in conventional Mossbauer spectroscopy. This period can be as short as 2.8 ps, which leads to an energy resolution of about 10 ° eV. However, the... 

Tsun-Kong) Sham received his PhD in Chemistry from the University of Western Ontario for the studies of Mossbauer spectroscopy. He was on the staff of the Chemistry Department at Brookhaven National Laboratory for ten years before returning to the University of Western Ontario in 1988 and is presently a Professor in Chemistry and the Scientific Director of the Canadian Synchrotron Radiation Facility at the Synchrotron Radiation Center, University of Wisconsin-Madison. He has been involved in synchrotron research since 1975, is a scientific member of the SRI-CAT at the Advanced Photon Source and a Senior Scientific Consultant for the Canadian Light Source (University of Saskatchewan, Saskatoon, Canada). 

Synchrotron-based nuclear resonance methods have revealed the vibrational dynamics of the iron atom in numerous systems, including alloys, amorphous materials, nanomaterials, and materials under high pressure. The above-mentioned selectivity for the probe nucleus is particularly valuable for biological macromolecules, which may contain many thousands of atoms, but a localized active site is often the true center of interest. Since its availability, NRVS has been applied to study the vibrational dynamics of Fe in proteins, porphyrin model compounds, " and iron-sulfur clusters. It is shown that NRVS can provide frequencies, amplitudes, and directions for Fe vibrations in the samples. It helps to clarify mode assignments in vibrational spectra and reveals many important vibrational modes of Fe that cannot be seen by other methods. In particular, NRVS reveals low-frequency motions of the Fe down to below 100 cm that control biological reactions. The applications presented here use Fe as the probe nucleus, but the principle applies to other Mossbauer isotopes such as " Sn, Kr, Ni, and Zn if appropriate sources are available. 

Although hyperfine interaction studies with both radioactive sources and synchrotron radiation have their respective advantages, new techniques based on synchrotron radiation widen the possibilities for studying hyperfine interactions and open up new prospects to perform studies that are out of the reach of conventional Mossbauer spectroscopy. 

A schematic of the principles and experimental setups for conventional Mossbauer spectroscopy (left) and SR-based NFS (right). In conventional Mossbauer spectroscopy, a 7-ray is generated by a radioactive source f Co for Fe Mossbauer). A driver system is attached to the source to provide a Doppler shift to the energy to the emitted 7-ray. This 7-ray can be resonantly absorbed in the sample. The transmitted 7-ray intensity is registered in the detector as a function of Doppler velocity. In SR-based NFS, millielectron volt bandwidth 7 radiation is provided by synchrotron radiation and subsequent monochromators. This pulse coherently excites different nuclear transitions in the sample. The forward-scattered signal generated from the nuclear excited states is registered in the detector placed in the forward direction as a function of time. 

Nuclear excitation and nuclear resonant scattering with synchrotron radiation have opened new fields in Mossbauer spectroscopy and have quite different aspects with the spectroscopy using a radioactive source. For example, as shown in Fig. 1.10, when the high brilliant radiation pulse passed through the resonant material and excite collectively the assemblies of the resonance nuclei in time shorter than the lifetime of the nuclear excited state, the nuclear excitons are formed and their coherent radiation decay occurs within much shorter period compared with an usual spontaneous emission with natural lifetime. This is called as speed-up of the nuclear de-excitation. The other de-excitations of the nuclei through the incoherent channels like electron emission by internal conversion process are suppressed. Synchrotron radiation is linearly polarized and the excitation and the de-excitation of the nuclear levels obey to the selection rule of magnetic dipole (Ml) transition for the Fe resonance. As shown in Fig. 1.10, the coherent de-excitation of nuclear levels creates a quantum beat Q given by... 

Nasu Sabmo High-Pressure Mossbauer spectroscopy using synchrotron radiation and radioactive sources. Hyperfine Int., 128, 101-113 (2000)... 

New methodological developments in Mossbauer spectroscopy are the use of monochromatic synchrotron radiation and Coulomb excitation instead of radioisotope sources, the simultaneous detection of Mossbauer )/-rays, internal conversion electrons and x-rays from different depths of one specimen [844], A competitor technique yielding similar information on chemical order is EXAFS. 

Adsorption Atomic Spectrometry Auger Electron Spectroscopy Microwave Molecular Spectroscopy Mossbauer Spectroscopy Multiphoton Spectroscopy Radiation Sources Sureace Chemistry Vacuum TBchnology X-Ray Analysis X-Ray, Synchrotron Radiation, and Neutron Diffraction... 

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