K. Marti1, B. Lavielle2
California at San Diego, Dept. of Chemistry, 9500 Gilman Drive, La Jolla, CA
2CNRS; Laboratoire de Chimie Nucléaire Analytique et Bio-environnementale (CNAB); Universités Bordeaux 1 & 2 - UMR 5084 CNRS; Domaine Le Haut Vigneau - BP 120; 33175 Gradignan Cedex, France
Galactic cosmic rays (GCR) provide information on the solar neighborhood during the suns motion in the galaxy. There is now considerable evidence supporting a model of GCR acceleration by shock waves of supernova in active star-forming regions (OB associations) in the galactic spiral arms. During times of passage into star-forming regions an increase in the GCR-flux is expected. Recent data from the Spitzer Space Telescope are shedding light on the structure of the Milky Way and of its star-forming spiral arms. Flux changes should have left records in solar system metallic detectors. Iron meteorites with GCR-exposure times of several hundred million years have long been considered to represent potential detectors of GCR-flux variations (e.g. Voshage, 1962). Variable concentration ratios of GCR-produced stable and radioactive nuclides, with the latter depending on the integration times of the studied nuclides, were reported by Lavielle et al. (1999). These data indicate a recent 38% GCR-flux increase. Potential flux recorders consisting of different pairs of nuclides which measure average fluxes on different time scales are being investigated (Lavielle et al., 2007; Mathew and Marti, 2008). Important characteristics of two pairs of recorders (81Kr-Kr and 129I-129Xe) are properties which make them self-correcting for GCR-shielding (flux variability within the iron meteorites of varying sizes). The 81Kr-Kr method (Marti, 1967) uses Kr isotope ratios for these corrections, while 129Xe accumulates the total isobaric yield from decay of 129I, since this nuclide is produced by secondary neutron reactions on Te in troilites of iron meteorites. The two chronometers provide records of average GCR fluxes over the 1 and 100 million year time scales, respectively. We discuss briefly improved sensitivities due to recent advances which lower the detection limit of 81Kr to a few thousand atoms.
Lavielle B., Marti K., Jeannot J.-P., Nishiizumi K., and Caffee M. W. (1999) Earth Planet. Sci. Lett. 170, 93-104.
Lavielle B., Gilabert E., and Thomas B. (2007) MAPS 42-S, A92.
Mathew K. J. and Marti K. (2008) Galactic cosmic-ray-produced 129Xe and 131Xe excesses in troilites of the Cape York iron meteorite (submitted to MAPS).
Marti K. (1967) Phys. Rev. Lett. 18(7), 264-266.
Voshage H. (1962) Z. Naturforsch 17a, 422-432.