Isotopic Processes

Elements beyond zinc. Despite extensive investigations carried out in parallel with the FUN inclusions, clear-cut mass-independent isotopic heterogeneities (e.g., more than 5 cr of a single measurement) have not been identified yet in normal inclusions for the rest of the elements besides extinct radioactivities. Nevertheless, there are consistent hints for excesses of about 2 e in Zr (Harper et al. 1991 Schonbachler et al. 2003) which is an r-process isotope for which some coupling may exist with neutron-rich statistical equilibrium (Meyer 1994). 

Figure 4. Non-linear effects for Sr, Ba, Ndand Sm in the FUN inclusions Cl andEKldl (McCulloch and Wasserburg 1978a,b Papanastassiou and Wasserburg 1978). Relative deviations from terrestrial standard ratios are plotted after normalization with the isotope pair represented with large open squares. Each isotope is labeled with its primary nucleosynthetic source. In using s-process isotopes for normalization, clear excesses in r-process nuclei are seen for Ba and Sm in EK141. Sr is normal in both inclusions except for a deficit in the p-process Sr. AsNdhas only one pure s-process isotope at mass 142, the data in EK141 have been further corrected to yield an excess in Nd identical to that of Sm as these two isotope are pure r-process nuclei expected to be produced in comparable abundances.

Figure 5. Relative abundances of the Kr, Xe isotopes (Huss and Lewis 1994) in presolar diamonds have been measured in bulk samples (= many grains) and are plotted relative to solar wind abundances. The terrestrial atmosphere is shown for comparison and displays a pattern close to mass dependent fractionation relative to the solar wind. The primary nucleosynthetic processes at the origin of the different nuclei are also listed. Both Kr and Xe are elevated in the r-process isotopes, whereas only Xe is also enriched in the p-isotopes. These patterns are a strong argument in favor of a supernova origin for the diamonds. Ne isotopes in presolar diamond is within the field of bulk meteorite data.

There are also indications that Ru displays systematic deficits in Ru in iron meteorites and in Allende relative to terrestrial composition (Chen et al. 2003). A deficit in s-process isotopes of Ru is the favored interpretation. 

Fig. 5.5. Decomposition of Solar System abundances into r and s processes. Once an isotopic abundance table has been established for the Solar System, the nuclei are then very carefully separated into two groups those produced by the r process and those produced by the s process. Isotope by isotope, the nuclei are sorted into their respective categories. In order to determine the relative contributions of the two processes to solar abundances, the s component is first extracted, being the more easily identified. Indeed, the product of the neutron capture cross-section with the abundance is approximately constant for aU the elements in this class. The figure shows that europium, iridium and thorium come essentially from the r process, unlike strontium, zirconium, lanthanum and cerium, which originate mainly from the s process. Other elements have more mixed origins. (From Sneden 2001.)... 

Along with the abundances of the isotopes, Table 4.2 also provides information on the processes that synthesized the isotopes. The processes are listed in order of importance, with minor processes shown in lower case. Note that most isotopes are produced by more than one process, and that isotopes of elements heavier than iron are produced predominantly by the s- and r-processes. / -process isotopes invariably have quite low abundances. 

Precise and accurate isotope analyses by mass spectrometry have attained growing importance in the last few years due to instrumental improvements with respect to sensitivity, detection limits, precision and accuracy.1 As mentioned before, because the isotope abundances of several elements are not constant and vary as a result of nuclear, biological, chemical, geochemical and physical processes, isotope ratio measurements are required for different research and application fields. Isotope ratio measurements are therefore necessary for elements with two or more isotopes for inves-... 

In the last few years, we have seen the application of isotope dilution methodologies to some new analytical fields. One of these is elemental speciation , where the aim is to determine individual chemical species in which an element is distributed in a given sample. IDMS has also proved its usefulness in element speciation, in which either species-specific or species-unspecific spikes can be used. For example, species-specific IDMS is nowadays used in several laboratories as an effective tool to validate analytical procedures for speciation and to investigate and document eventual interconversion between species. In addition, the study of induced variations in the isotopic composition of a target element can also provide insight into various (bio)chemical and physical processes isotopic analysis is, therefore, also of increasing importance in biological studies. 

Isotopic enrichment has also been found by monoisotopic photosensitization for mixtures of natural mercury and alkyl chlorides and vinyl chloride by similar processes. Isotopic enrichment is dependent on such factors as lamp temperatures, flow rates, and substrate pressures. Enrichment increases with decreasing lamp temperature and increasing flow rate, since process (VIII-1) is more ellicient at low temperatures and Cl atoms react with natural mercury containing higher fractions of 202Hg in (VIII-3) at higher flow rates of HC1 or under intermittent illumination. The intermittent illumination results in higher enrichment than the steady illumination. 

Like the s isotopes, the r isotopes are produced by neutron irradiation, but the flux must be sufficiently intense that the neutron captures would be rapid compared to the beta-decay lifetimes of the radioactive isotopes. For the s-process path through stable isotopes these beta-decay times are typically minutes to hours but on very neutron-rich paths encountered in the r-process, the corresponding times may be but tiny fractions of a second. In s-process circumstances (10 to loo years between captures), decay of radioactive isotopes will occur first and the r-process isotopes are... 

Isbell, Horace S., and Pigman, Ward, Mutarotation of Sugars in Solution Part II, Catalytic Processes, Isotope Effects, Reaction Mechanisms, and Biochemical Aspects, 24, 13-65 Isbell, Horace S. See also, Pigman, Ward. 

The environment provided by thermal pulses in the helium shells of intermediate-mass stars on the AGB provides conditions consistent with the synthesis of the bulk of the heavy s-process isotopes through bismuth. Neutron captures in AGB stars are driven by a combination of neutron sources the C(a, n) 0 reaction provides... 

It is generally accepted that the r-process synthesis of the heavy neutron capture elements in the mass regime A S 130-140 occurs in an environment associated with massive stars. This results from two factors (i) the two most promising mechanisms for r-process synthesis—a neutrino heated hot bubble and neutron star mergers— are both tied to environments associated with core collapse supernovae and (ii) observations of old stars (discussed in Section 1.01.6) confirm the early entry of r-process isotopes into galactic matter. 

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