Browsing by Author "Vogt, S"

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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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    Accelerator mass spectrometry analyses of environmental radionuclides: sensitivity, precision and standardisation
    (Elsevier, 2000-10-01) Hotchkis, MAC; Fink, D; Tuniz, C; Vogt, S
    Accelerator Mass Spectrometry (AMS) is the analytical technique of choice for the detection of long-lived radionuclides which cannot be practically analysed with decay counting or conventional mass spectrometry. AMS allows an isotopic sensitivity as low as one part in 1015 for 14C (5.73 ka), 10Be (1.6 Ma), 26Al (720 ka), 36Cl (301 ka), 41Ca (104 ka), 129I (16 Ma) and other long-lived radionuclides occurring in nature at ultra-trace levels. These radionuclides can be used as tracers and chronometers in many disciplines: geology, archaeology, astrophysics, biomedicine and materials science. Low-level decay counting techniques have been developed in the last 40–50 years to detect the concentration of cosmogenic, radiogenic and anthropogenic radionuclides in a variety of specimens. Radioactivity measurements for long-lived radionuclides are made difficult by low counting rates and in some cases the need for complicated radiochemistry procedures and efficient detectors of soft β-particles and low energy x-rays. The sensitivity of AMS is unaffected by the half-life of the isotope being measured, since the atoms not the radiations that result from their decay, are counted directly. Hence, the efficiency of AMS in the detection of long-lived radionuclides is 106–109 times higher than decay counting and the size of the sample required for analysis is reduced accordingly. For example, 14C is being analysed in samples containing as little as 20 μg carbon. There is also a world-wide effort to use AMS for the analysis of rare nuclides of heavy mass, such as actinides, with important applications in safeguards and nuclear waste disposal. Finally, AMS microprobes are being developed for the in-situ analysis of stable isotopes in geological samples, semiconductors and other materials. Unfortunately, the use of AMS is limited by the expensive accelerator technology required, but there are several attempts to develop compact AMS spectrometers at low (⩽0.5 MV) terminal voltages. Recent advances in AMS will be reviewed with highlights from the scientific programs at Lucas Heights and other AMS centres. © 2000 Elsevier Science Ltd.
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    The application of synchrotron radiation induced X-ray emission in the measurement of zinc and lead in Wistar rat ameloblasts
    (Elsevier, 2007-10) Arora, M; Kennedy, BJ; Ryan, CG; Boadle, RA; Walker, DM; Harland, CL; Lai, B; Cai, ZH; Vogt, S; Zoellner, H; Chan, SWY
    The development of analytical techniques for the measurement of trace elements in cellular compartments of developing teeth remains an important methodological issue in dental research. Recent advances in third generation synchrotron facilities have provided high brilliance X-ray sources that can be effectively used to study trace element distributions in small spatial regions with low detection limits. The present study describes for the first time the application of synchrotron radiation induced X-ray emission (SRIXE) in measuring the distribution of zinc and lead in the ameloblasts of developing Wistar rat teeth. Wistar rats were fed a standard rat diet, containing the normal dietary requirements of zinc, ad libitum and exposed to 100ppm of lead in drinking water. Resin embedded sections of first mandibular molars were analysed using a 13.3keV incident monochromatic X-ray beam focussed to a 0.2μm spot. Characteristic X-rays arising from the entire thickness of the sample were measured using an energy dispersive detector for quantitative analysis of elemental concentrations. The results showed that intranuclear concentrations of zinc were greater than levels in the cytoplasm. Furthermore, nuclear and cytoplasmic concentrations of zinc in the maturation stage (742±27 and 424±25ppm, respectively) were significantly higher than the zinc levels observed in the nucleus and cytoplasm of presecretory stage ameloblasts (132±10 and 109±10ppm, respectively) (p<0.05). A clear lead signal above the background was not detected in the ameloblasts and lead concentrations could only be reliably measured in the developing enamel. Overall, SRIXE was an effective method of studying the spatial distribution of zinc in the cells of developing teeth and offered a unique combination of sub-micron spatial resolution and parts-per-million detection limits (0.8–1 and 0.6–1ppm for zinc and lead, respectively). © 2007, Elsevier Ltd.
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    Complex exposure histories for meteorites with “short” exposure ages
    (Wiley, 1997-05) Herzog, GF; Vogt, S; Albrecht, A; Xue, S; Fink, D; Klein, J; Middleton, R; Weber, H; Schultz, L
    We report measurements of 26Al and 10Be activities in nine ordinary chondrites and of the light noble gas concentrations and 36Cl and 41Ca activities in subsets of those meteorites. All but Murray have low 21Ne concentrations (<1.0 × 10−8cm3STP/g) and have previously been used to estimate 21Ne production rates. Ladder Creek, Murchison, Sena, and Timochin have inventories of cosmogenic radionuclides that are compatible with a single stage of irradiation and give 21Ne production rates that are consistent with the standard L-chondrite value of 0.33 × 10−8cm3STP/g/Ma. In contrast, Cullison, Guenie, Shaw, and Tsarev experienced complex irradiation histories. They and several other meteorites with low nominal exposure ages also have lower 3He/21Ne ratios than expected based on their 22Ne/21Ne ratios. A general association between low 21Ne contents and 3He losses suggests that meteorites with short lifetimes often occupy orbits with small perihelia. However, meteorites with low 21Ne contents, one-stage exposure histories, and losses of cosmogenic 3He are rare. Possible explanations for the scarcity are (1) statistical, (2) that it is harder for more deeply buried protometeoroids to lose gas in a liberating collision, and (3) that it is harder to insert more deeply buried protometeoroids directly into orbits with small perihelia. © 1999-2021 John Wiley & Sons, Inc.
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    Cross sections for 36Cl from Ti at EP = 35-150 MeV: applications to in-situ exposure dating
    (Elsevier, 2000-10) Fink, D; Vogt, S; Hotchkis, MAC
    We have measured the low-energy yield of 36Cl from Ti for proton energies from 35 to 150 MeV. Thin Ti foil irradiations were performed at the Harvard University Cyclotron Laboratory and 36Cl concentrations were determined using the ANTARES AMS facility at ANSTO. Cross-sections ranged smoothly with energy from 0.32±0.05 mb at 35 MeV to 5.3±0.4 mb at 150 MeV. Results for E<110 MeV are new, while the upper region from 110 to 150 MeV agrees well with overlapping data from other studies. The in-situ production rate for 36Cl from Ti at the earth’s surface and high latitude based on this excitation function and calculations of Masarik and Reedy (normalised to the mean measured yield of 36Cl from Ca) is estimated at ∼(13±3) atoms 36Cl (g Ti yr)−1. We thus conclude that in Ti-rich, Ca-poor rocks or in typical basalts, 36Cl yield from Ti can amount to ∼5–10% of total. This is similar to the contribution from slow muon capture on 40Ca and in some cases, from thermal neutron capture on native Cl. © 2000 Elsevier Science 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.
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    Light noble gases and cosmogenic radionuclides in Estherville, Budulan and other mesosiderites: Implications for exposure histories and production rates
    (Wiley, 2000-08) Albrecht, A; Schnabel, C; Vogt, S; Xue, S; Herzog, GF; Begemann, F; Weber, H; Middleton, R; Fink, D; Klein, J
    We report measurements of 26AI, 10Be, 41Ca, and 36Cl in the silicate and metal phases of 11 mesosiderites, including several specimens each of Budulan and Estherville, of the brecciated meteorite Bencubbin, and of the iron meteorite Udei Station. Average production rate ratios (atom/atom) for metal phase samples from Estherville and Budulan are 26Al/10Be = 0.77 ± 0.02; 36Cl/10Be = 5.3 ± 0.2. For a larger set of meteorites that includes iron meteorites and other mesosiderites, we find 26Al/10Be = 0.72 ± 0.01 and 36Cl/10Be = 4.5 ± 0.2. The average 41Ca/36Cl production rate ratio is 1.10 ± 0.04 for metal separates from Estherville and four small iron falls. The 41Ca activities in dpm/(kg Ca) of various silicate separates from Budulan and Estherville span nearly a factor of 4, from <400 to >1600, indicating preatmospheric radii of >30 cm. After allowance for composition, the activities of 26Al and 10Be (dpm/kg silicate) are similar to values measured in most ordinary chondrites and appear to depend only weakly on bulk Fe content. Unless shielding effects are larger than suggested by the 36Cl and 41Ca activities of the metal phases, matrix effects are unimportant for 10Be and minor for 26Al. Noble gas concentrations and isotopic abundances are reported for samples of Barea, Emery, Mincy, Morristown, and Marjalahti. New estimates of 36Cl/36Ar exposure ages for the metal phases agree well with published values. Neon-21 production rates for mesosiderite silicates calculated from these ages and from measured 21Ne contents are consistently higher than predicted for L chondrites despite the fact that the mesosiderite silicates have lower Mg contents than L chondrites. We suggest that the elevation of the 21Ne production rate in mesosiderite silicates reflects a “matrix effect,” that is, the influence of the higher Fe content of mesosiderites, which acts to enhance the flux of low-energy secondary particles and hence the 21Ne production from Mg. As 10Be production is relatively insensitive to this matrix effect, 10Be/21Ne ages give erroneously low production rates and high exposure ages. By coincidence, standard 22Ne/21Ne based “shielding” corrections give fairly reliable 21Ne production rates in the mesosiderite silicates. © 1999-2021 John Wiley & Sons, Inc.
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    Mechanisms of murine cerebral malaria: multimodal imaging of altered cerebral metabolism and protein oxidation at hemorrhage sites
    (American Association for the Advancement of Science, 2015-12-18) Hackett, MJ; Aitken, JB; El-Assaad, F; McQuillan, JA; Carter, EA; Ball, HJ; Tobin, MJ; Paterson, DJ; de Jonge, MD; Siegele, R; Cohen, DD; Vogt, S; Grau, GE; Hunt, NH; Lay, PA
    Using a multimodal biospectroscopic approach, we settle several long-standing controversies over the molecular mechanisms that lead to brain damage in cerebral malaria, which is a major health concern in developing countries because of high levels of mortality and permanent brain damage. Our results provide the first conclusive evidence that important components of the pathology of cerebral malaria include peroxidative stress and protein oxidation within cerebellar gray matter, which are colocalized with elevated nonheme iron at the site of microhemorrhage. Such information could not be obtained previously from routine imaging methods, such as electron microscopy, fluorescence, and optical microscopy in combination with immunocytochemistry, or from bulk assays, where the level of spatial information is restricted to the minimum size of tissue that can be dissected. We describe the novel combination of chemical probe–free, multimodal imaging to quantify molecular markers of disturbed energy metabolism and peroxidative stress, which were used to provide new insights into understanding the pathogenesis of cerebral malaria. In addition to these mechanistic insights, the approach described acts as a template for the future use of multimodal biospectroscopy for understanding the molecular processes involved in a range of clinically important acute and chronic (neurodegenerative) brain diseases to improve treatment strategies. 2015 © The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. Distributed under a Creative Commons Attribution Non Commercial Licence 4.0 (CC BY-NC).
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    Neutron-capture 36Cl, 41Ca, 36Ar, and 150Sm in large chondrites: evidence for high fluences of thermalized neutrons
    (Wiley, 1995-05-25) Bogard, DD; Nyquist, LE; Bansal, BM; Garrison, DH; Wiesmann, H; Herzog, GF; Albrecht, A; Vogt, S; Klein, J
    We 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 Union