Mossbauer spectroscopy radiation sources

Nuclear y-ray resonance spectroscopy. This technique is based on the resonance absorption of y radiation and is more conventionally known as Mossbauer spectroscopy. The source of the radiation is a nuclide fixed in a solid crystal lattice held below the Debye temperature. In this condition, y radiation of energies less than 150 keV are emitted with no loss of energy. Such quantized y photons can undergo resonance absorption by the appropriate identical stable nuclide in a solid sample matrix. If the chemical environment of the absorbing nuclide is different from the emitter, energy must be added or subtracted from the radiation to establish resonance. This can be achieved by introducing net motion to the source or absorber to establish a Doppler motion energy term. 

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... 

The spectroscopic techniques that have been most frequently used to investigate biomolecular dynamics are those that are commonly available in laboratories, such as nuclear magnetic resonance (NMR), fluorescence, and Mossbauer spectroscopy. In a later chapter the use of NMR, a powerful probe of local motions in macromolecules, is described. Here we examine scattering of X-ray and neutron radiation. Neutrons and X-rays share the property of being found in expensive sources not commonly available in the laboratory. Neutrons are produced by a nuclear reactor or spallation source. X-ray experiments are routinely performed using intense synclirotron radiation, although in favorable cases laboratory sources may also be used. 

Fig. 3.19 Schematic illustration of the measurement geometry for Mossbauer spectrometers. In transmission geometry, the absorber (sample) is between the nuclear source of 14.4 keV y-rays (normally Co/Rh) and the detector. The peaks are negative features and the absorber should be thin with respect to absorption of the y-rays to minimize nonlinear effects. In emission (backscatter) Mossbauer spectroscopy, the radiation source and detector are on the same side of the sample. The peaks are positive features, corresponding to recoilless emission of 14.4 keV y-rays and conversion X-rays and electrons. For both measurement geometries Mossbauer spectra are counts per channel as a function of the Doppler velocity (normally in units of mm s relative to the mid-point of the spectrum of a-Fe in the case of Fe Mossbauer spectroscopy). MIMOS II operates in backscattering geometry circle), but the internal reference channel works in transmission mode...

Mossbauer spectroscopy The Mossbauer effect is resonance absorption of 7 radiation of a precisely defined energy, by specific nuclei. It is the basis of a form of spectroscopy used for studying coordinated metal ions. The principal application in bioinorganic chemistry is Fe. The source for the 7 rays is Co, and the frequency is shifted by the Doppler effect, moving it at defined velocities (in mm/s) relative to the sample. The parameters derived from the Mossbauer spectrum (isomer shift, quadrupole splitting, and the hyperfine coupling) provide information about the oxidation, spin and coordination state of the iron. 

Most often the transmission mode is found to be the most convenient in Mossbauer spectroscopy, i.e., the y radiation passes from the source through the absorber, and the attenuation of the primary beam is measured at the various Doppler velocities. However, there are a number of cases where a "scattering geometry may be advantageous (SO). The basis for this geometry lies in those processes that take place after resonant absorption of y radiation by the Mossbauer isotope. Specifically, after excitation the Mossbauer isotope may reemit the y ray, or it may decay by emission of internal conversion electrons and X rays [with the probability of internal conversion equal to a/(l + a)]. 

Fig. 10.19. Mossbauer spectroscopy in 57Fe the 14.4 keV source radiation is generated in a nuclear...

In conventional Mossbauer spectroscopy, X-rays with energies corresponding to nuclear transitions (5-150keV) can be produced only by use of radioactive sources containing a parent isotope of the absorbing nucleus in an appropriate excited state from which it decays into the ground state with emission of a y-quantum. For spectroscopic applications, the y-radiation must be variable. The chemical perturbations... 

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... 

The samples were characterized by means of X-ray diffraction (XRD) analysis, Fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy (TEM), electron diffraction (ED), and Mossbauer spectroscopy. XRD analysis was carried out on a HZG-4A diffractometer by using Ni-filtered Co Ka radiation. IR-spectra were recorded on an AVATAR FTIR-330 spectrometer. TEM/ED examinations were performed with a LEO 906E and a JEOL 4000 EX transmission electron microscopes. The resonance spectra were recorded in air at 298 K and processed by using a commercial SM2201 MSssbauer spectrometer equipped with a 15 mCi Co (Rh) source. 

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. " ... 

Table 2.4 Properties of selected nuclei observed by Mossbauer spectroscopy. The radioisotope source provides the y-radiation required for the Mossbauer effect.

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). 

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. 

The effect is exploited in Mossbauer spectroscopy in which a gamma-ray source is mounted on a moving platform and a similar sample is mounted nearby. A detector measures gamma rays scattered by the sample. The source is moved slowly towards the sample at a varying speed, so as to continuously change the frequency of the emitted gamma radiation by the Doppler effect. A sharp decrease in the signal from the detector at a particular speed (i.e. frequency) indl-... 

A substantial advantage of emission Mossbauer spectroscopy in comparison with the transmission technique is that if the material to be investigated contains heavy elements, then the required dopant concentration (e.g., Co) may be 1-2 orders of magnitude lower in the emission experiment than the Fe concentration in an analogous transmission experiment. This is in connection with the intensity loss of the Mossbauer radiation due to electronic absorption, which is always self absorption in the source and regular absorption in the absorber (Vertes and Homonnay 1997). Low dopant concentration is very important in impurity Mossbauer spectroscopy, where the investigated material does not contain a Mbssbauer element thus, a conveniently measurable Mbssbauer nuclide is introduced artificially as an impurity with a potential risk of perturbing the physicochemical properties of the host phase. 

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