Other
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.588t&link_type=abstract
Meteoritics, vol. 30, no. 5, page 588
Other
Grains, Interstellar, Instrumentation, Isotopes, Kr, Trace, Xe
Scientific paper
Since the first reliable indication of the existence of non-terrestrial composition of Xe isotopes in meteorites by Reynolds [1,2], anomalies have been found in the isotopic composition of many elements in meteorites, implying a plurality of nucleosynthetic processes, and indicating that the initial conditions leading to the formation of the solar system were quite diverse [3]. Noble gases provide a unique window into the composition of the progenitor material to the solar system [4]. Stellar evolution may contribute by the s-process, r-process, p-process, etc., each with its own isotopic distribution. Kr and Xe are especially useful as the large number of isotopes (7 in the 78Kr to 86Kr range, and 9 in the 124Xe to 136Xe range) provide many clues to help unravel the multiple sources to the measured abundances. Although Xe has been measured and analyzed considerably more frequently than Kr in meteorite studies, this has only partially been due to the wealth of information derivable from Xe studies, but also, due to experimental difficulties encountered in Kr measurements. Early in the development of stellar nucleosynthesis, 86Kr was proposed as a chronometer of the s-process [5], but until recently has provided unreliable results [6]. Other interesting properties derivable from Kr isotope measurements include 78Kr as an indication of spallation history, 80Kr as a stellar thermometer, and 81Kr and 83Kr to determine cosmic ray exposure ages. Studies of highly refractory microscopic grains (diamond, graphite and SiC) separated from the much more abundant carbonaceous matrix in primitive meteorites have shown a remarkable isotopic composition diversity in a small fraction of single grains from the same meteorite, implying multiple stellar sources [4]. Ion microprobe measurements have been possible of the major constituents of single interstellar grains [7,8] and of He and Ne using a state-of-the-art static noble gas mass spectrometer [9-11]. But, a recent attempt to measure noble gases from fourteen individual "X" SiC grains, previously identified by ion microprobe analysis, was unsuccessful with the 2,000 132Xe atom detection limit of the mass spectrometer [12,13]. From the Kr concentration measurements of SiC particles KJF by Lewis et al. [6], a 2 micrometer diameter particle will on average contain 134 Kr atoms. If only 4% of the SiC grains contain the majority of the noble gas atoms, then a single gas rich grain will contain 3,350 Kr atoms, or 12, 75, 385, 398, 1910, and 580 atoms for 78Kr through 86Kr, respectively. The Xe single-grain abundances would be similar. Resonance ionization, an emerging laser-based element analysis technique, is being harnessed to a wide variety of problems in which minute quantities of a particular element need to be measured efficiently in the presence of an overwhelmingly larger background of other materials [14]. By utilizing lasers tuned to specific atomic energy levels of the analyte element, ions are produced selectively in a mass spectrometer with much higher efficiency than possible using conventional methods, such as electron bombardment, thermal ionization, or ion sputtering. In a static resonance ionization system for noble gases, the combination of high ionization efficiency and sample concentrator results in an extremely fast (~3 min. detection half-life vs. ~60 min. for conventional systems) analyzer with a detection limit of ~100 85Kr atoms [15]. In addition to the almost complete absence of interferences, the short analysis time significantly reduces the background contribution of outgassing in the mass spectrometer. Although using a less efficient laser scheme resulting in slightly slower analyses, a similar system has recently been completed and dedicated to extraterrestrial Xe measurements [16]. At the newly formed Institute for Rare Isotope Measurements [17], the noble gas equipment that had previously been at Atom Sciences [14,15] is being re-installed and upgraded to provide routine noble gas measurements from terrestrial and extraterrestrial samples. The present system, data illustrating current capabilities, and improvements that should reduce the detection limit significantly below 100 atoms for both Kr and Xe will be described. An early application will be simultaneous Kr and Xe isotopic measurements from single microscopic interstellar SiC grains. References: [1]Reynolds J. H. (1960) Phys. Rev. Lett., 4, 351-354. [2] Reynolds J. H. and Turner G. (1964) J. Geo. Phys. Res., 69, 3263-3281. [3] Lee T. (1988) (J. F. Kerridge and M. S. Matthews, eds.), 1063-1089, Univ. of Arizona, Tucson. [4] Anders A. and Zinner E. (1993) Meteoritics, 28, 490-514. [5] Burbidge E. M. et al. (1957) Rev. Mod. Phys., 29, 547-650. [6] Lewis R. S. et al. (1994) GCA, 58, 471-494. [7] Zinner E. et al. (1989) GCA, 53, 730-732. [8] Amari S. et al. (1992) Astrophys. J. Lett., 394, L43-L46. [9] Hohenberg C. M. et al. (1990) GCA, 54, 2133-2140. [10] Nichols R. H. Jr. et al. (1991) Meteoritics, 26. [11] Nichols R. H. Jr. et al. (1993) Meteoritics, 28, 410-411. [12] Nittler L. et al. (1995) LPS XXVI, 1057-1058. [13]Hohenberg C. M. (1994) personal communication. [14]Payne M. G. et al. (1994) Rev. Sci. Instrum., 65, 2433-2459. [15] Thonnard N. et al. (1992) Inst. Phys. Conf. Ser. 128, 27-30. [16] Gilmour J. D. et al. (1994) Rev. Sci. Instrum., 65, 617-625. [17] Thonnard N. and Lehmann B. L. (1995) AIP Conf. Proc., 329, 335-338.
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