We have measured significant concentrations of 3c'c1, 4•Ca, 36Ar from decay of 3c'c1, and •5øSm produced from the capture of thermalized neutrons in the large Chico L6 chondrite. Activities of 3r121 and 4•Ca, 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 ~l.3x1016 and 0.8 x1016 n/cm 2 for 3r'CI and n•Ca, respectively. Stepwise temperature extraction of Ar in Chico impact melt shows 3•Ar/3SAr ratios as large as ~9. The correlation of high 3aAr/3SAr with high C1/Ca phases in neutronirradiated Chico indicates that the excess 3•Ar above that expected from spallation is due to decay of neutron-produced 3c'c1. Excess 3•Ar in Chico requires a thermal neutron fluence of 0.9-1.7x10 • n/cm 2. Decreases in a49Sm/aS2Sm due to neutron-capture by •49Sm correlate with increases in 15øSm/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 1.23x10 la n/cm 2. This fluence derived from Sm considers capture of epithermal neutrons and effects of chemical composition on the neutron energy distribution. Excess 3•Ar identified in the Arapahoe, Bmderheim, and Torino chondrites and the Shallowater aubrite suggest exposure to neutron fluences of --0.2-0.6x10 la n/cm 2. Depletion of 149Sm in Torino and the LEW86010 angrite suggest neutron fluences of 0.8x1016 n/cm 2 and 0.25x10 la n/cm 2, respectively. Neutron fluences of ~10 • n/cm 2 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.Copyfight 1995 by the American Geophysical Union.Paper number 95JE00663. 0148-0227/95/95JE-00663505.00The rates at which these high-energy or spallation reactions proceed vary with depth in the meteorold, but all of them reach maxima at shielding depths less than 150 g/cm 2, some much less. The exact depth of maximum production depends on the size of the body and the specific product [e.g., Reedy, 1985; Graf et al., 1990; Bhandari et al., 1993]. Some of the neutrons produced by cosmic rays do not interact, however, until they are slowed to thermal energies. In large meteoroids (radius >200 g/cm 2) these neutron-capture products are predicted to reach maxima in their production rams at shielding depths of 100-300 g/cm 2 [e.g., Eberhardt et al.,...
Migration of radionuclides in soils and their transfer to edible plants are usually estimated using volume-averaged bulk concentrations. However, radionuclides might not be homogeneously distributed in soils due to heterogeneous water flow and solute transport. One important cause of heterogeneous transport is preferential flow. The aim of this study was to investigate the spatial distribution of radionuclides in the soil in relation to preferential flow paths and to assess the possible consequences for their transfer from soil to plants. We identified the preferential flow paths in a forest soil by staining them with a blue dye, and we compared radionuclide activity in samples from the stained preferential flow paths with those from the unstained soil matrix. The activities of the atmospherically deposited radionuclides 137 Cs, 210 Pb, 239,240 Pu, 238 Pu, and 241 Am were enriched in the preferential flow paths by a factor of up to 3.5. Despite their different depositional histories, the distribution of the radionuclides between preferential flow paths and matrix was similar. Our findings indicate increased transport of radionuclides through the preferential flow paths, representing a possible risk of groundwater contamination. Furthermore, enrichment of radionuclides in the preferential flow paths might influence the uptake by plants. The heterogeneous radionuclide distribution in the soil and the more intense rooting in the preferential flow paths can be incorporated into soil-to-plant transfer models. Taking the correlated radionuclide and root distribution between the two flow regions into account provides a more physical and biological basis for the calculation of plant activities with transfer models than using the homogeneously mixed bulk soil activities as input parameters.
In this letter, we report the profiles (-160 samples) of løBe, 9Be, Zn in a Mediterranean sediment core spanning the last 60 kyr. We show the existence of a løBe peak, whose absolute age is estimated to be 34+3 kyr B P, based on its stratigraphic position between two tephra layers originating from two volcanic eruptions (Campanian Ignimbrite and Citara), dated by K-Ar, '*øAr-39Ar and methods. This peak is similar to the increase in the løBe concentration observed in Vostok and Dome C ice cores at the same age. The increase with approximately similar amplitude in both ice and sediment cores at different latitudes and hemispheres cannot be explained by changes in the archives. The present result supports the view of a well defined global enhancement of the løBe flux, related to an increase of the løBe production rate and not to a redistribution of the løBe fallout between different latitudes.
After closure of a waste disposal cell in a repository for radioactive waste, resaturation is likely to cause the release of soluble species contained in cement and bituminous matrices, such as ionic species (nitrates, sulfates, calcium and alkaline ions, etc.), organic matter (mainly organic acids), or gases (from steel containers and reinforced concrete structures as well as from radiolysis within the waste packages). However, in the presence of nitrates in the near-field of waste, the waste cell can initiate oxidative conditions leading to enhanced mobility of redox-sensitive radionuclides (RN). In biotic conditions and in the presence of organic matter and/or hydrogen as electron donors, nitrates may be microbiologically reduced, allowing a return to reducing conditions that promote the safety of storage. Our work aims to analyze the possible microbial reactivity of nitrates at the bitumen - concrete interface in conditions as close as possible to radioactive waste storage conditions in order (i) to evaluate the nitrate reaction kinetics; (ii) to identify the by-products (NO2(-), NH4(+), N2, N2O, etc.); and (iii) to discriminate between the roles of planktonic bacteria and those adhering as a biofilm structure in the denitrifying activity. Leaching experiments on solid matrices (bitumen and cement pastes) were first implemented to define the physicochemical conditions that microorganisms are likely to meet at the bitumen-concrete interface, e.g. highly alkaline pH conditions (10 < pH < 11) imposed by the cement matrix. The screening of a range of anaerobic denitrifying bacterial strains led us to select Halomonas desiderata as a model bacterium capable of catalyzing the reaction of nitrate reduction in these particular conditions of pH. The denitrifying activity of H. desiderata was quantified in a batch bioreactor in the presence of solid matrices and/or leachate from bitumen and cement matrices. Denitrification was relatively fast in the presence of cement matrix (<100 h) and 2-3 times slower in the presence of bituminous matrix (pH 9.7). The maximal rate of denitrification was approximately 0.063 mM h(-1) and some traces of nitrite were detected for a few hours (<2%). Overall, the presence of solid cement promoted the kinetics of denitrification. The inspection of the solid surfaces at the end of the experiment revealed the presence of a biofilm of H. desiderata on the cement paste surface. These attached bacteria showed a comparable denitrifying activity to planktonic bacterial culture. However, no colonization of bitumen was observed either by SEM or by epifluorescence microscopy.
Abstract-We report measurements of 26Al and loge 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 x 10-8 cm3 STP/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 x 10-8 cm3 STP/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 3HeP1Ne 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.
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