Mossbauer source experiments

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. 

Most Mossbauer experiments are currently performed with commercially available radioactive sources. For some applications, however, a so-called source experiment may be useful, in which the sample is labeled with the radioactive parent-isotope of the Mossbauer nucleus such as Co. The y-radiation of the radioactive sample is then analyzed by moving a single-line absorber for Doppler modulation in front of the detector. 

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 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. 

Parent nuclides produced by the processes mentioned above can all be used for several half-lives. In contrast, one can also populate the Mossbauer excited state directly via Coulomb excitation (84). In this technique, a beam of high-energy ( 10 MeV) charged particles (e.g., O4+, Cl7 +) is directed onto the Mossbauer isotope and the electromagnetic field generated by these particles induces nuclear transitions, which can include transitions to the Mossbauer excited state. Subsequent decay to the nuclear ground state then provides the appropriate y radiation. The half-life of a source created in this manner is the half-life of the Mossbauer excited state (e.g., several nanoseconds), and thus Coulomb excitation is necessarily an in situ technique, i.e., the Mossbauer effect experiment must be performed at the location of the charged particle beam. 

Due to effects caused by the nuclear decay in the sample, these so-called source experiments may be difficult to perform and interpret. Several papers dealing with these effects can be found (23). In principle, however, the applicability of Mossbauer spectroscopy to catalytic studies can be extended to include both the Mossbauer isotopes and the corresponding parent nuclides. We therefore list below the Mossbauer isotopes and corresponding parent nuclides that may be of greatest use in catalytic studies, as deduced from their nuclear properties. 

In a standard Mossbauer transmission experiment the absorber is placed between the source and the detector. In order to observe the effect. 

In I960 Ruderfer [10] proposed a uni-directional photon experiment to verify the existence or otherwise of an aether drift. Mounting the Mossbauer source at the centre of a revolving turntable with the absorber and detector fixed on its circumference would cause a line-shift of a diurnal nature as the orientation of the laboratory frame in the aether alters with the earth s revolution. Champeney et a/. [11] observed count-rates through absorbers at opposite ends of a rotor diameter with time of day, and found no evidence of a steady drift past the earth resolved parallel to the equatorial plane down to an upper limit of 1-9 m s" L... 

In-beam Mossbauer spectroscopy is a source experiment and, as such, has three distinct advantages over conventional absorption experiments. First, the concentration of Mossbauer probe atoms in a sample is typically several orders of magnitude lower than that in absorption experiments. This makes source experiments more sensitive than absorption experiments and leads to a preponderance of radioactive sources for dilute systems. A concentration in the range 10 10 atoms cm is generally sufficient to observe the Mossbauer effect. It has recendy become possible to measure a Fe Mossbauer spectrum using an implantation dose rate of I O particles s of Mn nuclei by using a... 

Second, the ability of source experiments to use Mossbauer parent atoms permits many more elements to be used than in absorption experiments by employing the excited Mossbauer ievei. The nature of an experiment will determine how Mossbauer 7-emitters are introduced into a sample. Using an energetic Rl beam, it is possible to implant Mossbauer probes to depths of several hundred micrometers and with straggling widths as wide as several tenths of a micrometer. The measurement duration can be controlled by using a probe nuclide with an appropriate half-life for example, for Mn, the Mossbauer effect can be observed for a few minutes after implantation, whereas an in-beam experiment using Coulomb excitation and recoil implantation has a duration of only several hundred nanoseconds after implantation. 

Fe Mossbauer spectra of Mn implanted in an n-type floating-zone Si wafer between 330 and 1200 K [54]. The isomer shift is given relative to a-Fe at room temperature and the sign of the velocity is given in the emission source experiment. (Reproduced from Ref. 54 with permission of Elsevier.)... 

Similar experiments were conducted in a last time [ 14,15] on the basis of Mossbauers sources ( "Sn02 oxide or "Sn metal) and nearest (not distant) and distributed in space resonance screen, made of Sn02 oxide. Activity of all samples was Q = 1.5 x lO Ci. Powder source (" " Sn02 oxide) and powder absorber (" Sn02 oxide) have been mixed and placed in small plastic scintillator. Analysis of low of gamma decay was conducted by delayed (e — y) coincidence method. 

Therefore, the measured isomer shift is always given with respect to a standard material this can be the Mossbauer source used in the particular experiment or any conventional absorber material. 

In conventional Mossbauer spectroscopy one uses a single-line source, e.g. Co embedded in a rhodium matrix in the case of Fe spectroscopy, and the iron containing material under study as absorber. This technique is termed Mossbauer Absorption Spectroscopy (MAS) in order to distinguish it from the so-called source experiment, also known as Mossbauer Emission Spectroscopy (MES). In a MES... 

Similar experiments were carried out with systems whose corresponding Fe compounds possess intermediate ligand field strengths and show thermal spin crossover. [Fe(phen)2(NCS)2] undergoes thermal ST as already discussed above (Sect. 2.3.2.1). The temperature dependent MAS spectra are shown on the left of Fig. 2.34. The analogous Co compound doped with Co and used as Mossbauer source (or the corresponding iron compound doped with Co as source which... 

SEDMS experiment is a transmission experiment with the scatterer being the Mossbauer source. 

Most Mossbauer spectroscopy experiments are conducted either in the transmission mode, in which a source of well-defined characteristics is used to examine the spectral properties of an unknown absorber, or in the emission mode, in which the source of radiation becomes the sample under investigation and a known or standard absorber is employed to determine the transition energy differences. In both cases, any of a number of y-ray detectors is utilized to record the amount of radiation transmitted through the absorber. 

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. 

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). 

The arguments seen in section 2.3 suggest that resonant y-absorption should decrease at very low temperatures because the Doppler broadening of the y-lines decreases and may even drop below the value of the recoil energy. In his experiments with solid sources and absorbers, however, R.L. Mossbauer ([1] in Chap. 1) observed on the... 

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]. 

The emission spectmm of Co, as recorded with an ideal detector with energy-independent efficiency and constant resolution (line width), is shown in Fig. 3.6b. In addition to the expected three y-lines of Fe at 14.4, 122, and 136 keV, there is also a strong X-ray line at 6.4 keV. This is due to an after-effect of K-capture, arising from electron-hole recombination in the K-shell of the atom. The spontaneous transition of an L-electron filling up the hole in the K-shell yields Fe-X X-radiation. However, in a practical Mossbauer experiment, this and other soft X-rays rarely reach the y-detector because of the strong mass absorption in the Mossbauer sample. On the other hand, the sample itself may also emit substantial X-ray fluorescence (XRF) radiation, resulting from photo absorption of y-rays (not shown here). Another X-ray line is expected to appear in the y-spectrum due to XRF of the carrier material of the source. For rhodium metal, which is commonly used as the source matrix for Co, the corresponding line is found at 22 keV. 

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