• Iron-Manganese System for Preparation of Radiocarbon AMS Targets: Characterization of Procedural Chemical-Isotopic Blanks and Fractionation

      Verkouteren, R. Michael; Klinedinst, Donna B.; Currie, Lloyd A. (Department of Geosciences, The University of Arizona, 1997-01-01)
      We report a practical system to mass-produce accelerator mass spectrometry (AMS) targets with 10-100 micrograms carbon samples. Carbon dioxide is reduced quantitatively to graphite on iron fibers via manganese metal, and the Fe-C fibers are melted into a bead suitable for AMS. Pretreatment, reduction and melting processes occur in sealed quartz tubes, allowing parallel processing for otherwise time-intensive procedures. Chemical and isotopic (13C, 14C) blanks, target yields and isotopic fractionation were investigated with respect to levels of sample size, amounts of Fe and Mn, pretreatment and reduction time, and hydrogen pressure. With 7-day pretreatments, carbon blanks exhibited a lognormal mass distribution of 1.44 micrograms (central mean) with a dispersion of 0.50 micrograms (standard deviation). Reductions of 10 micrograms carbon onto targets were complete in 3-6 h with all targets, after correction for the blank, reflecting the 13C signature of the starting material. The 100 micrograms carbon samples required at least 15 h for reduction; shorter durations resulted in isotopic fractionation as a function of chemical yield. The trend in the 13C data suggested the presence of kinetic isotope effects during the reduction. The observed CO2-graphite 13C fractionation factor was 3-4% smaller than the equilibrium value in the simple Rayleigh model. The presence of hydrogen promoted methane formation in yields up to 25%. Fe-C beaded targets were made from NIST Standard Reference Materials and compared with graphitic standards. Although the 12C ion currents from the beads were one to two orders of magnitude lower than currents from the graphite, measurements of the beaded standards were reproducible and internally consistent. Measurement reproducibility was limited mainly by Poisson counting statistics and blank variability, translating to 14C uncertainties of 5-1% for 10-100 micrograms carbon samples, respectively. A bias of 5-7% (relative) was observed between the beaded and graphitic targets, possibly due to variations in sputtering fractionation dependent on sample size, chemical form and beam geometry.
    • On the Isotopic Composition of Dissolved Inorganic Carbon in Rivers and Shallow Groundwater: A Diagrammatic Approach to Process Identification and a More Realistic Model of the Open System

      Taylor, C. B. (Department of Geosciences, The University of Arizona, 1997-01-01)
      Rivers and shallow groundwater are deep groundwater precursors. Their dissolved inorganic carbon content (DIC) and its isotopic composition are end members in the evolution of these properties in confined situations, and are therefore essential information when applying carbon isotopes as tracers of groundwater processes and determining aquifer residence times using 14C. During studies of regional aquifer systems in New Zealand, a simple model has been developed to explain the isotopic compositions of DIC encountered in rivers and shallow groundwater. The model format incorporates a diagrammatic approach, providing a framework for tracing the subsequent evolution of DIC in both precipitation-and river-recharged aquifers under closed conditions. DIC concentration of rivers continuously adjusts toward chemical and isotopic equilibrium between direct addition of CO2 to the water (via plant respiration and decay of dead organic material) and exchange of CO2 across the river-air interface. In the shallow groundwater situation, the gaseous reservoir is soil CO2, generally at significantly higher partial pressure. In both cases, calcite dissolution or other processes may be an additional source of DIC directly added to the bicarbonate and dissolved CO2 components; while these may add or remove DIC, steady-state isotopic concentrations are considered to be determined only by the dynamic balance between directly added CO2 and gas exchange. This model allows the calculation of steady states, using selectable parameters in river or groundwater situations. These appear as straight lines in 13C or 14C vs. 1/DIC, or total 14C vs. DIC plots, into which the experimental data can be inserted for interpretation. In the case of 14C, the steady-state balance is very often complicated by the presence of an old component in the directly added DIC; the understanding achieved via the 13C patterns is helpful in recognizing this. Data from four contrasting aquifer systems in New Zealand. The success of the approach has depended crucially on DIC concentrations measured very accurately on the isotope samples, rather than separate chemical analyses.
    • Radiocarbon Laboratories

      Department of Geosciences, The University of Arizona, 1997-01-01
    • Radiocarbon Releases at the Krško Nuclear Power Plant

      Vokal, Barbara; Kobal, Ivan (Department of Geosciences, The University of Arizona, 1997-01-01)
      Since 1991, radiocarbon analyses of exhaust air have been part of the regular radioactivity monitoring program at the Krško Nuclear Power Plant (NPP), a Westinghouse 632 MWe pressurized water reactor (PWR). Activity of CO2 and hydrocarbons has been identified; the former contributes ca. 43%. A normalized release of total 14C of 0.219 TBgGWe-1a-1 was obtained. Indoor air 14C concentrations in selected rooms inside the plant have generally been <5 Bq m-3, although rare peaks of >1000 Bq m-3 may be reached. Tree rings have shown slight enhanced 14C activity due to the operation of the plant.
    • Radiocarbon Updates

      Department of Geosciences, The University of Arizona, 1997-01-01
    • Radiocarbon, Volume 39, Number 3 (1997)

      Department of Geosciences, The University of Arizona, 1997-01-01
    • Study of the Parameters Affecting the Correlation of Background Versus Cosmic Radiation in CO2 Counters: Reliability of Dating Results

      Facorellis, Yorgos; Maniatis, Yannis; Kromer, Bernd (Department of Geosciences, The University of Arizona, 1997-01-01)
      Systematic treatment of the data recorded by our guard counters and corrections introduced for meteorological factors has allowed observations on solar events clearly manifested in the readings. Examples are the solar flares of March 1989 and especially of June 1991, which caused a ca. 10% decrease in the cosmic radiation flux reaching the counters. A sinusoidal variation in the cosmic-ray flux with a period of one year is also clearly manifested in the data. The observation that the background in the 14C measurements depends on the intensity of the cosmic radiation has led to the use of monthly correlations for the determination of the best background value to be used in the age calculations. This reduces the error significantly. However, various factors such as random statistical fluctuations of the background measurements may affect the slope of the correlations and consequently the calculated age of the samples. Long-term observations of the relation between background values and coincidence counts have led to constraints in the slope of the correlation. A simple extension of the fitting procedure is explored, which maintains the physically meaningful range of the slopes, but is flexible to adjust for the seasonally varying contributions to the variations of the cosmic-ray flux.
    • Subject Index Volume 39, 1997

      Department of Geosciences, The University of Arizona, 1997-01-01
    • The 14C Content of Lithium Metal Used in Benzene Synthesis

      Eastoe, C. J.; Long, A.; Tucek, C. S.; Jull, A. J. T. (Department of Geosciences, The University of Arizona, 1997-01-01)
      The lithium reagent used in the synthesis of benzene for 14C dating contributes insignificant 14C in high-precision measurements on 7 g benzene samples. Blank experiments on three different batches of lithium yielded small amounts of carbon, most of which probably originated as "memory" in the reaction vessel.
    • University of Granada Radiocarbon Dates VIII

      González-Gómez, Cecilio; Villafranca-Sánchez, Elena (Department of Geosciences, The University of Arizona, 1997-01-01)