Browsing by Author "Reedy, RC"

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    41Ca, 26Al, and 10Be in lunar basalt 74275 and 10Be in the double drive tube 74002/74001
    (Elsevier, 1998-07) Fink, D; Klein, J; Middleton, R; Vogt, S; Herzog, GF; Reedy, RC
    We report depth profiles of the cosmogenic radionuclides 10Be, 26Al, and 41Ca in the titanium-rich lunar basalt 74275. The 10Be profile is flat: 10Be activities are confined to a narrow range between 9.6 and 11.2 dpm/kg but are nonetheless consistent with a small contribution of about 1–2 dpm/kg from solar cosmic rays. The 26Al profile shows the steep decrease with increasing depth that is characteristic of nuclides whose production is dominated by solar cosmic rays. 41Ca activities decrease from about 22 dpm/kg at the surface to a minimum of ∼9 dpm/kg at a depth of 4.7 g/cm2 and then increase to ∼11 dpm/kg at a depth of 15.8 g/cm2. The sharp decrease near the surface identifies for the first time production of 41Ca by solar cosmic rays. We also report 10Be measurements for six samples from lunar core 74002/1. The 10Be activities range from approximately 8 to 14 dpm/kg. We model the production of 10Be, 26Al, and 41Ca in lunar rock 74275 by including published data that indicate a long exposure to galactic cosmic rays at a depth of 140 g/cm2 followed by one at the surface lasting 2.8 Ma. Cosmogenic radionuclide production by galactic cosmic rays, and, in the case of 41Ca, by thermal neutrons is estimated from published measurements and semi-empirical calculations. Our model includes a new calculation of production rates due to solar cosmic rays and incorporates recently published cross section measurements. Although many parameterizations of the flux of solar energetic particles give acceptable fits to the experimental data for 74275, we prefer a best fit obtained for 10Be and 26Al alone, which incorporates an erosion rate of ∼2 mm/Ma, a rigidity of 100 MV, and a 4π flux of protons with energies greater than 10 MeV of 89 cm−2 s−1. For 41Ca alone, the corresponding values are 2 mm/Ma, 80 MV, and 198 cm−2 s−1. The differences between the two sets of parameters may reflect uncertainties in the calculations of 41Ca production or a secular change in the solar cosmic ray flux. Calculations for a slab and for a hemispherical knob with a radius of 23 cm yield similar results. © 1998 Elsevier Science Ltd.
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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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    Report on workshop on production rates of terrestrial produced in-situ cosmogenic radionuclides
    (Elsevier, 1994-06-03) Reedy, RC; Tuniz, C; Fink, D
    Production rates of cosmogenic nuclides made in-situ in terrestrial samples and how they are applied to the interpretation of measured radionuclide concentrations were discussed at a one-day Workshop held 2 October 1993 in Sydney, Australia. The status of terrestrial in-situ studies using the long-lived radionuclides 14C, 10Be, 26Al, 36Cl, and 41Ca and of various models were presented. The uncertainties in and magnitudes of various factors that go into data interpretation were discussed; no single factor was found to dominate the final uncertainty.© 1994 Elsevier B.V.