Optimizing a microwave gas ion source for continuous-flow accelerator mass spectrometry

Author Posting. © The Author(s), 2011. This is the author's version of the work. It is posted here by permission of American Institute of Physics for personal use, not for redistribution. The definitive version was published in Review of Scientific Instruments 83 (2012): 02B304, doi:10.1063/1.3656408.A 2.45 GHz microwave ion source coupled with a magnesium charge exchange canal (CxC) has been successfully adapted to a large acceptance radiocarbon accelerator mass spectrometry system at the National Ocean Sciences AMS Facility (NOSAMS), Woods Hole Oceanographic Institution. CO2 samples from various preparation sources are injected into the source through a glass capillary at 370 µl/min. Routine system parameters are about 120 - 140 µA of negative 12C current after the CxC, leading to about 400 14C counts per second for a modern sample and implying a system efficiency of 0.2%. While these parameters already allow us to perform high quality AMS analyses on large samples, we are working on ways to improve the output of the ion source regarding emittance and efficiency. Modeling calculations suggest modifications in the extraction triode geometry, shape and size of the plasma chamber could improve emittance and hence ion transport efficiency. Results of experimental tests of these modifications are presented.This work has been supported by the US National Science Foundation through Cooperative Agreement OCE-0753487

The Keck Carbon Cycle AMS Laboratory, University of California, Irvine : status report

Author Posting. © The Authors. This work is licensed under a Creative Commons Attribution 3.0 License The definitive version was published in Radiocarbon 52 (2010): 301-309.We present a status report of the accelerator mass spectrometry (AMS) facility at the University of California, Irvine, USA. Recent spectrometer upgrades and repairs are discussed. Modifications to preparation laboratory procedures designed to improve sample throughput efficiency while maintaining precision of 2–3‰ for 1-mg samples (Santos et al. 2007c) are presented

Mean-field calculations of quasi-elastic responses in 4He

We present calculations of the quasi-elastic responses functions in 4He based upon a mean-field model used to perform analogous calculations in heavier nuclei. The meson exchange current contribution is small if compared with the results of calculations where short-range correlations are explicitly considered. It is argued that the presence of these correlations in the description of the nuclear wave functions is crucial to make meson exchange current effects appreciable.Comment: uuencoded file containing 7 LaTex peges plus 3 ps figures. To be published in Physical Review

High-precision Studies of the 3^{\bf{3}}He(e,e′^{\bf{\prime}}p) Reaction at the Quasielastic Peak

Precision studies of the reaction $^{3}$He(e,e$^\prime$p) using the three-spectrometer facility at the Mainz microtron MAMI are presented. All data are for quasielastic kinematics at $|\vec{q} | =685$ MeV/c. Absolute cross sections were measured at three electron kinematics. For the measured missing momenta range from 10 to 165 MeV/c, no strength is observed for missing energies higher than 20 MeV. Distorted momentum distributions were extracted for the two-body breakup and the continuum. The longitudinal and transverse behavior was studied by measuring the cross section for three photon polarizations. The longitudinal and transverse nature of the cross sections is well described by a currently accepted and widely used prescription of the off-shell electron-nucleon cross-section. The results are compared to modern three-body calculations and to previous data.Comment: 4 pages, 3 figures. Submitted for publication in Phys. Rev. Let

Quasielastic 12C(e,e'p) Reaction at High Momentum Transfer

We measured the 12C(e,e'p) cross section as a function of missing energy in parallel kinematics for (q,w) = (970 MeV/c, 330 MeV) and (990 MeV/c, 475 MeV). At w=475 MeV, at the maximum of the quasielastic peak, there is a large continuum (E_m > 50 MeV) cross section extending out to the deepest missing energy measured, amounting to almost 50% of the measured cross section. The ratio of data to DWIA calculation is 0.4 for both the p- and s-shells. At w=330 MeV, well below the maximum of the quasielastic peak, the continuum cross section is much smaller and the ratio of data to DWIA calculation is 0.85 for the p-shell and 1.0 for the s-shell. We infer that one or more mechanisms that increase with $\omega$ transform some of the single-nucleon-knockout into multinucleon knockout, decreasing the valence knockout cross section and increasing the continuum cross section.Comment: 14 pages, 7 figures, Revtex (multicol, prc and aps styles), to appear in Phys Rev

Natural-abundance radiocarbon as a tracer of assimilation of petroleum carbon by bacteria in salt marsh sediments

Author Posting. © The Authors, 2005. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 70 (2006): 1761-1771, doi:10.1016/j.gca.2005.12.020.The natural abundance of radiocarbon (14C) provides unique insight into the source and cycling of sedimentary organic matter. Radiocarbon analysis of bacterial phospholipid lipid fatty acids (PLFAs) in salt-marsh sediments of southeast Georgia (USA) – one heavily contaminated by petroleum residues – was used to assess the fate of petroleum-derived carbon in sediments and incorporation of fossil carbon into microbial biomass. PLFAs that are common components of eubacterial cell membranes (e.g., branched C15 and C17, 10-methyl-C16) were depleted in 14C in the contaminated sediment (mean Δ14C value of +25 ± 19 ‰ for bacterial PLFAs) relative to PLFAs in uncontaminated “control” sediment (Δ14C = +101 ± 12‰). We suggest that the 14C-depletion in bacterial PLFAs at the contaminated site results from microbial metabolism of petroleum and subsequent incorporation of petroleum-derived carbon into bacterial membrane lipids. A mass balance calculation indicates that 6-10% of the carbon in bacterial PLFAs at the oiled site could derive from petroleum residues. These results demonstrate that even weathered petroleum may contain components of sufficient lability to be a carbon source for biomass production by marsh sediment microorganisms. Furthermore, a small but significant fraction of fossil carbon is assimilated even in the presence of a much larger pool of presumably more-labile and faster-cycling carbon substrates.This study was supported by Georgia Sea Grant (RR100-221/926784), the National Science Foundation (OCE-9911678) and NOSAMS (thanks to J. M. Hayes)

Direct application of compound-specific radiocarbon analysis of leaf waxes to establish lacustrine sediment chronology

Author Posting. © Springer, 2007. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Journal of Paleolimnology 39 (2008): 43-60, doi:10.1007/s10933-007-9094-1.This study demonstrates use of compound-specific radiocarbon analysis (CSRA) for dating Holocene lacustrine sediments from carbonate-hosted Ordy Pond, Oahu, Hawaii. Long-chain odd-numbered normal alkanes (n-alkanes), biomarkers characteristic of terrestrial higher plants, were ubiquitous in Ordy Pond sediments. The δ13C of individual n-alkanes ranged from −29.9 to −25.5‰, within the expected range for n-alkanes synthesized by land plants using the C3 or C4 carbon fixation pathway. The 14C ages of n-alkanes determined by CSRA showed remarkably good agreement with 14C dates of rare plant macrofossils obtained from nearby sedimentary horizons. In general, CSRA of n-alkanes successfully refined the age-control of the sediments. The sum of n-alkanes in each sample produced 70–170 μg of carbon (C), however, greater age errors were confirmed for samples containing less than 80 μg of C. The 14C age of n-alkanes from one particular sedimentary horizon was 4,155 years older than the value expected from the refined age-control, resulting in an apparent and arguable age discrepancy. Several lines of evidence suggest that this particular sample was contaminated by introduction of 14C-free C during preparative capillary gas chromatography. This study simultaneously highlighted the promising potential of CSRA for paleo-applications and the risks of contamination associated with micro-scale 14C measurement of individual organic compounds.This project was funded by Petroleum Research Fund (PRF #40088-ACS) and in part by Sigma Xi, The Scientific Research Society (Grants in aid of research, 2003)

14C AMS Measurements of <100 Microgram Samples with a High-Current System

From the 16th International Radiocarbon Conference held in Gronigen, Netherlands, June 16-20, 1997.The NOSAMS facility at Woods Hole Oceanographic Institution has started to develop and apply techniques for measuring very small samples on a standard Tandetron accelerator mass spectrometry (AMS) system with high-current hemispherical Cs sputter ion sources. Over the past year, results on samples ranging from 7 to 160 micrograms C showed both the feasibility of such analyses and the present limitations on reducing the size of solid carbon samples. One of the main factors affecting the AMS results is the dependence of a number of the beam optics parameters on the extracted ion beam current. The extracted currents range from 0.5 to 10 micro-A of 12Cfor the sample sizes given above. We here discuss the setup of the AMS system and methods for reliable small-sample measurements and give the AMS-related limits to sample size and the measurement uncertainties.This material was digitized as part of a cooperative project between Radiocarbon and the University of Arizona Libraries.The Radiocarbon archives are made available by Radiocarbon and the University of Arizona Libraries. Contact [email protected] for further information.Migrated from OJS platform February 202