Browsing by Author "Masarik, J"

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    Cosmic-ray exposure history of the Norton County enstatite achondrite
    (Wiley, 2011-01-28) Herzog, GF; Albrecht, A; Ma, P; Fink, D; Klein, J; Middleton, R; Bogard, DD; Nyquist, LE; Shih, CY; Garrison, DH; Reese, Y; Masarik, J; Reedy, RC; Rugel, G; Faestermann, T; Korschinek, G
    We report measurements of cosmogenic nuclides in up to 11 bulk samples from various depths in Norton County. The activities of 36Cl, 41Ca, 26Al, and 10Be were measured by accelerator mass spectrometry; the concentrations of the stable isotopes of He, Ne, Ar, and Sm were measured by electron and thermal ionization mass spectrometry, respectively. Production rates for the nuclides were modeled using the LAHET and the Monte Carlo N-Particle codes. Assuming a one-stage irradiation of a meteoroid with a pre-atmospheric radius of approximately 50 cm, the model satisfactorily reproduces the depth profiles of 10Be, 26Al, and 53Mn (<6%) but overestimates the 41Ca concentrations by about 20%. 3He, 21Ne, and 26Al data give a one-stage cosmic-ray exposure (CRE) age of 115 Ma. Argon-36 released at intermediate temperatures, 36Arn, is attributed to production by thermal neutrons. From the values of 36Arn, an assumed average Cl concentration of 4 ppm, and a CRE age of 115 Ma, we estimate thermal neutron fluences of 1–4 × 1016 neutrons cm−2. We infer comparable values from ε149Sm and ε150Sm. Values calculated from 41Ca and a CRE age of 115 Ma, 0.2–1.4 × 1016 neutrons cm−2, are lower by a factor of approximately 2.5, indicating that nearly half of the 149Sm captures occurred earlier. One possible irradiation history places the center of proto-Norton County at a depth of 88 cm in a large body for 140 Ma prior to its liberation as a meteoroid with a radius of 50 cm and further CRE for 100 Ma. © The Meteoritical Society, 2011
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    Depth profile of 41Ca in an Apollo 15 drill core and the low energy neutron flux in the Moon
    (Elsevier, 1997-05-01) Nishiizumi, K; Fink, D; Klein, J; Middleton, R; Masarik, J; Reedy, RC; Arnold, JR
    Systematic measurements of the concentrations of cosmogenic41Ca (half-life = 1.04 × 105 yr) in the Apollo 15 long core 15001–15006 were performed by accelerator mass spectroscopy. Earlier measurements of cosmogenic10Be,14C,26Al,36Cl, and53Mn in the same core have provided confirmation and improvement of theoretical models for predicting production profiles of nuclides by cosmic ray induced spallation in the Moon and large meteorites. Unlike these nuclides,41Ca in the lunar surface is produced mainly by thermal neutron capture reactions on40Ca. The maximum productions of41Ca, about 1 dpm/g Ca, was observed at a depth in the Moon of about 150 g/cm2. For depths below about 300 g/cm2,41Ca production falls off exponentially with an e-folding length of 175 g/cm2. Neutron production in the Moon was modeled with the Los Alamos High Energy Transport Code System, and yields of nuclei produced by low-energy thermal and epithermal neutrons were calculated with the Monte Carlo N-Particle code. The new theoretical calculations using these codes are in good agreement with our measured41Ca concentrations as well as with60Co and direct neutron fluence measurements in the Moon. © 1997 Elsevier B.V.
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    Exposure history of the Torino meteorite
    (Wiley, 1996-03) Wieler, R; Graf, T; Signer, P; Vogt, S; Herzog, GF; Tuniz, C; Fink, D; Fifield, LK; Klein, J; Middleton, R; Jull, AJT; Pellas, P; Masarik, J; Dreibus, G
    We determined He, Ne, Ar, 10Be, 26Al, 36Cl, and 14C concentrations, as well as cosmic-ray track densities and halogen concentrations in different specimens of the H6 chondrite Torino, in order to constrain its exposure history to cosmic radiation. The Torino meteoroid had a radius of ∼20 cm and travelled in interplanetary space for 2.5–10 Ma. Earlier, Torino was part of a larger body. The smallest possible precursor had a radius of 55 cm and a journey through space longer than ∼65 Ma. If the first-stage exposure took place in a body with a radius of >3 m or in the parent asteroid, then it lasted nearly 300 Ma. The example of Torino shows that it is easy to underestimate first-stage exposure ages when constructing two-stage histories. © 1999-2021 John Wiley & Sons, Inc.