Decays, Mossbauer spectroscopy

Apart from XeF, which is the light-emitting species in certain Xe/F2 lasers, there is no evidence for the existence of any odd-valent fluorides. Reports of XeFg have not been confirmed. Of the other halides, XeCl2, XeBr2 and XeCl4 have been detected by Mossbauer spectroscopy as products of the -decay of their... 

The precursor of ° Ru is ° Rh (tip, = 3 years). It is prepared by irradiating natural ruthenium metal with 20 MeV deuterons, " Ru (d, n) Rh. The target is then allowed to decay for several months to diminish the accompanying Rh activity. In a report on ° Ru Mossbauer spectroscopy [111], the authors reported on spectra of Ru metal, RuOa, and [Ru(NH3)4(HS03)2] at liquid helium temperature in standard transmission geometry using a Ge(Li) diode to detect the 127 keV y-rays. The absorber samples contained 1 g of ruthenium per cm. ... 

There are two iridium isotopes, ir and Ir, suitable for Mossbauer spectroscopy. Each of them possesses two nuclear transitions with which nuclear resonance absorption has been observed. Figure 7.58 (from [266]) shows the (simplified) nuclear decay schemes for both iridium Mossbauer isotopes the Mossbauer transitions are marked therein with bold arrows. The relevant nuclear data known to date for the four Mossbauer transitions are collected in Table 7.1 at the end of the book. 

Effect of Nuclear Decay Mossbauer Emission Spectroscopy... 

Figure 5.3 The decay of 57Co to 57Fe. The encircled part is the transition commonly used for Mossbauer spectroscopy of iron-containing samples.

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... 

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. 

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)]. 

Figure 27. Distribution of cobalt among the various species of an activated HDS catalyst, as determined by emission Mossbauer spectroscopy. The catalyst, prepared by impregnation of ammonium heptamolybdate and 57Co nitrate, was as similar as possible to the commercial one used for obtaining the results presented in the previous figures (3wt% as CoO, 13 wt% as M0O3 on -/-alumina). All precautions were taken to avoid possible errors due to noninstantaneous charge compensation in the sequence of nuclear events in 57Co decay (this has to be taken into account in the nonconducting M0S2 matrix) [155, 156].

Mossbauer spectroscopy was bom in the 1957 Ph.D. dissertation of Mossbauer,189 who measured and explained the resonant emission and absorption of a y ray from the radioactive decay of 77lr191 [87,88]. 

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... 

Xenon tetrachloride and xenon dibromide have been detected by (see Mossbauer Spectroscopy) as products of the -decay of their relatives (equation 13). 

Figure 1 depicts a typical sequence of events started by absorption of an incident photon with an energy near the nuclear excited state energy Eq. The Fe nucleus has an excited state lifetime of 141ns, and excited nuclei have two decay channels. About 10% of them reemit a 14.4kev photon. For recoilless absorption, where no vibrational levels are excited, time-resolved measurements of 14.4kev photons scattered in the forward direction reveal information on hyperfine interactions comparable to conventional Mossbauer spectroscopy (see Mossbauer Spectroscopy). The remaining nuclei expel electrons from the atomic K shell, followed by... 

Co(bpy)3] are particularly amenable to study by Mossbauer spectroscopy and related techniques and enable the study of the chemical effects of the decay (and exchange) of Co 5,6,152,322,467-469,520,... 

Hawthorne et al. (1991) used a wide variety of analytical techniques (EMPA, TGA, powder XRD and IR, single crystal XRD, Mossbauer spectroscopy, Si MAS-NMR, EXAFS/XANES spectroscopies and HRTEM) to determine the stmcture of a-decay damaged titanite. The metamictization process was proposed to begin with the formation of isolated defects caused by a-particles and amorphous domains caused by a-recoil tracks which, with increasing dose, overlapped to produce the metamict state. Fe" was reduced to Fe with increasing a-decay dose. In a set of annealing experiments of... 

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