Neutron-capture 36Cl, 41Ca, 36Ar, and 150Sm in large chondrites: evidence for high fluences of thermalized neutrons

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dc.contributor.authorBogard, DDen_AU
dc.contributor.authorNyquist, LEen_AU
dc.contributor.authorBansal, BMen_AU
dc.contributor.authorGarrison, DHen_AU
dc.contributor.authorWiesmann, Hen_AU
dc.contributor.authorHerzog, GFen_AU
dc.contributor.authorAlbrecht, Aen_AU
dc.contributor.authorVogt, Sen_AU
dc.contributor.authorKlein, Jen_AU
dc.date.accessioned2021-10-28T00:27:18Zen_AU
dc.date.available2021-10-28T00:27:18Zen_AU
dc.date.issued1995-05-25en_AU
dc.date.statistics2021-10-19en_AU
dc.description.abstractWe have measured significant concentrations of 36Cl, 41Ca, 36Ar from decay of 36Cl, and 150Sm produced from the capture of thermalized neutrons in the large Chico L6 chondrite. Activities of 36Cl and 41Ca, corrected for a high-energy spallogenic component and a terrestrial age of ∼50 ka, give average neutron-capture production rates of 208 atoms/min/g-Cl and 1525 atoms/min/kg-Ca, which correspond to thermal neutron (n) fluxes of 6.2 n/cm2/s and 4.3 n/cm2/s, respectively. If sustained for the ∼65 Ma single-stage, cosmic ray exposure age of Chico, these values correspond to thermal neutron fluences of ∼1.3×1016 and 0.8 × 1016 n/cm2 for 36Cl and 41Ca, respectively. Stepwise temperature extraction of Ar in Chico impact melt shows 36Ar/38Ar ratios as large as ∼9. The correlation of high 36Ar/38Ar with high Cl/Ca phases in neutron-irradiated Chico indicates that the excess 36Ar above that expected from spallation is due to decay of neutron-produced 36Cl. Excess 36Ar in Chico requires a thermal neutron fluence of 0.9–1.7×1016 n/cm2. Decreases in 149Sm/152Sm due to neutron-capture by 149Sm correlate with increases in 150Sm/152Sm for three samples of Chico, and one of the Torino H-chondrite. The 0.08% decrease in 149Sm/152Sm shown by Chico corresponds to a neutron fluence of 1.23×1016 n/cm2. This fluence derived from Sm considers capture of epithermal neutrons and effects of chemical composition on the neutron energy distribution. Excess 36Ar identified in the Arapahoe, Bruderheim, and Torino chondrites and the Shallowater aubrite suggest exposure to neutron fluences of ∼0.2–0.6×1016 n/cm2. Depletion of 149Sm in Torino and the LEW86010 angrite suggest neutron fluences of 0.8×1016 n/cm2 and 0.25×1016 n/cm2, respectively. Neutron fluences of ∼1016 n/cm2 in Chico are almost as large as those previously observed for some lunar soils. Consideration of exposure ages suggests that the neutron flux in Chico may have been greater than that in many lunar soils. Neutron-capture effects, although seldom reported, may be common for large meteorites and could affect calculation of exposure ages based on cosmogenic Ar. Combining measurements of radioactive and stable species produced from neutron-capture has the potential for identifying large meteorites with complex exposure histories. © 2021 American Geophysical Unionen_AU
dc.identifier.citationBogard, D. D., Nyquist, L. E., Bansal, B. M., Garrison, D. H., Wiesmann, H., Herzog, G. F., Albrecht, A. A., Vogt, S., & Klein, J. (1995). Neutron-capture 36Cl, 41Ca, 36Ar, and 150Sm in large chondrites: evidence for high fluences of thermalized neutrons. Journal of Geophysical Research: Planets, 100(E5), 9401-9416. doi:10.1029/95JE00663en_AU
dc.identifier.issn2169-9100en_AU
dc.identifier.issueE5en_AU
dc.identifier.journaltitleJournal of Geophysical Research: Planetsen_AU
dc.identifier.pagination9401-9416en_AU
dc.identifier.urihttps://doi.org/10.1029/95JE00663en_AU
dc.identifier.urihttps://apo.ansto.gov.au/dspace/handle/10238/12158en_AU
dc.identifier.volume100en_AU
dc.language.isoenen_AU
dc.publisherWileyen_AU
dc.subjectDecayen_AU
dc.subjectThermal neutronsen_AU
dc.subjectNeutron reactionsen_AU
dc.subjectEpithermal neutronsen_AU
dc.subjectChemical compositionen_AU
dc.subjectEnergy spectraen_AU
dc.subjectNeutron fluxen_AU
dc.subjectChondritesen_AU
dc.subjectMeteoritesen_AU
dc.titleNeutron-capture 36Cl, 41Ca, 36Ar, and 150Sm in large chondrites: evidence for high fluences of thermalized neutronsen_AU
dc.typeJournal Articleen_AU
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