• Background Components of a Liquid Scintillation Counter in the 14C Window

      Jonsson, G.; Theodórsson, P. (Department of Geosciences, The University of Arizona, 2007-01-01)
      We present a broad and detailed study of the background components of a liquid scintillation (LS) detector, using a simple laboratory-built system, ICELS. It was specifically designed for radiocarbon dating and is compact and easily transportable (total weight 35 kg). Its flexible LS detector unit has a dome-shaped vial with 3 mL benzene to which 45 mg butyl-PBD is added. The vial sits on the top of a vertical 28-mm-diameter phototube. The gamma radiation, to which the benzene is exposed under varying conditions, was measured by replacing the vial with a 38-mm-diameter NaI crystal. The pulse-height spectra of the 14C LS background and the NaI gamma background were measured in a surface laboratory and in a deep underground counting room with: 1) a lead shield of varying thickness; 2) lead of normal and low 210Pb concentration; 3) phototubes of 2 different types; and 4) varying benzene volume. The beta emission from the face of the tubes was measured with a low-level Geiger counter.
    • Background Concentration of 14C in Aquatic Samples from Brackish Lake Obuchi, Rokkasho, Japan, Adjacent to Nuclear Fuel Reprocessing Facilities

      Ueda, Shinji; Kondo, Kunio; Inaba, Jiro (Department of Geosciences, The University of Arizona, 2007-01-01)
      The brackish Lake Obuchi in Rokkasho, Japan, is adjacent to the first Japanese commercial nuclear fuel reprocessing facilities, which are now undergoing performance testing, with commercial operation scheduled to start in 2007. Preparatory surveys were made by measuring the background levels of radiocarbon for water, aquatic biota, and sediment samples using accelerator mass spectrometry (AMS) in order to study the potential effects of 14C released by the plant to the 14C concentration in aquatic samples. Concentrations of 14C in Futamata River in 2004 ranged from 102 +/- 0.5 to 109 +/- 0.6 pMC (average 106 +/- 0.6 pMC), while 14C concentrations in brackish water from Lake Obuchi and in seawater were 89 +/- 0.5 to 104 +/- 0.4 pMC (average 98 +/- 0.5 pMC) and 82 +/- 0.6 to 102 +/- 0.4 pMC (average 93 +/- 0.5 pMC), respectively. The relationship between 14C concentration and salinity showed a negative correlation (r = 0.68, P < 0.01, n = 20). 14C concentration in selected aquatic biota (i.e. fish, benthos, and seagrass) from 2003 to 2004 ranged from 105 0.7 to 107 0.6 pMC and in zooplankton and phytoplankton was 103 +/- 2.4 to 105 +/- 1.7 pMC. The depth profile of 14C in 3 core sediment samples from Lake Obuchi showed maximum concentrations from 103 +/- 0.5 to 106 +/- 0.5 pMC at 520 cm depth. The vertical profile of 14C concentration in the sediment did not follow global atmospheric 14C fallout. We confirmed that the background level of 14C concentration in aquatic samples in brackish Lake Obuchi before operation of the reprocessing plant was similar to the concentration (~106 pMC) in the recent atmosphere.
    • Background Measurements with Different Shielding and Anticoincidence Systems

      Loosli, H. H.; Forster, Markus; Otlet, R. L. (American Journal of Science, 1986-01-01)
      Extremely low background count rates are a necessary condition for both the measurement of 39Ar concentrations in ground and ocean water and of 14C activities in small samples using gas proportional counting techniques. A systematic comparison of the performance of three different designs of shielding systems in four different installations has been made. Background values of selected gas proportional counters were measured, compared and separated into their various components. Acceptably low backgrounds were obtained in all the systems tried. The performance of a Nail shield in a surface laboratory was found to be at least equal to the best obtained with a gas ariticoincidence detector in a deep underground laboratory.
    • Balance and Behavior of Carbon Dioxide at an Urban Forest Inferred from the Isotopic and Meteorological Approaches

      Takahashi, Hiroshi Aoki; Hiyama, Tetsuya; Konohira, Eiichi; Takahashi, Atsuhiro; Yoshida, Naohiro; Nakamura, Toshio (Department of Geosciences, The University of Arizona, 2001-01-01)
      Diurnal variations in δ14C, delta-13C and the concentration of atmospheric carbon dioxide in an urban forest were measured on 9 February 1999 to discriminate and quantify contributions from different CO2 sources. The biogenic CO2 concentration remained relatively constant throughout the day. However, anthropogenic CO2 concentration fluctuated with the atmospheric CO2 concentration, and seemed to be controlled by wind velocity and the amount of exhaust gases from fossil fuel burning. The vertical profiles of anthropogenic, biogenic, and total CO2 showed a constant concentration within forest during daytime because of the large vertical CO2 influx, strong winds, and neutral atmospheric condition. The biogenic contribution at night decreased from the forest floor upwards with a smooth gradient, while the anthropogenic contribution showed a direct mirror because of the location of respective CO2 sources—the vertical gradient of wind velocity and the horizontal CO2 supply
    • Balanced Window Method in 14C Liquid Scintillation Counting

      Theodórsson, P.; Ingvarsdottir, S.; Gudjonsson, G. I. (Department of Geosciences, The University of Arizona, 2003-01-01)
      The authors present a detailed theoretical and experimental study of the liquid scintillation balanced counting method, widely used in radiocarbon dating, using a simple, laboratory-made system. A fixed counting window becomes a balanced window when the high voltage is set where the 14C count rate rises to a maximum. Using a measured 14C pulse height spectrum, we have calculated the lower and upper limits for 11 balanced windows of varying width and their respective counting efficiencies. Furthermore, we have studied: (1) theoretically and experimentally, the counting efficiency for up to a +/15% shift in pulse height from the balanced setting, (2), the change in pulse height due to temperature variations, (3), the long-time stability of the system, and (4), a method that allows a quick determination of the balance voltage for individual samples, using the Compton spectrum of 133Ba. The standard deviation for thirty 24-hr measuring periods for a 14C standard (190 Bq) was within the expected statistical error (0.03%).
    • Balanced-Energy Counting Window for Stable Liquid Scintillation Radiocarbon Dating

      Theodórsson, Pall (Department of Geosciences, The University of Arizona, 2011-01-01)
      This paper describes an optimal radiocarbon counting window for liquid scintillation (LS) 14C dating that secures for unquenched as well as for heavily quenched dating samples maximal stability of 14C counting efficiency and theoretically minimal quench correction. In high-precision dating, a balanced counting window with fixed channel limits is frequently used, where about 3% of the highest part of the 14C spectrum is sacrificed for high 14C counting stability. The stability is, however, diminished for quenched samples. Therefore, this window is here replaced by a balanced fixed-energy 14C counting window where the channel limits depend on the quench level. The LS system used must have a linear amplifier and a multichannel analyzer. All samples are measured at a fixed high voltage. For energy calibration and determination of the quench level, the channel number of the middle of the 59.5-keV peak from an external 241Am gamma source is determined before and after measuring each sample. This counting mode is valuable in high-precision dating. It could be widely applied if adapted to systems with a logarithmic amplifier, generally used in LS dating.
    • Bayesian Analysis of High-Precision AMS 14C Dates from a Prehistoric Mexican Shellmound

      Kennett, Douglas J.; Culleton, Brendan J.; Voorhies, Barbara; Southon, John R. (Department of Geosciences, The University of Arizona, 2011-01-01)
      We establish a precision accelerator mass spectrometry (AMS) radiocarbon chronology for the Archaic period Tlacuachero shellmound (Chiapas, Mexico) within a Bayesian statistical framework. Carbonized twig samples were sequentially selected from well-defined stratigraphic contexts based on iterative improvements to a probabilistic chronological model. Analytical error for these measurements is 15 to 20 14C yr. This greater precision and the absence of stratigraphic reversals eclipses previous 14C work at the site. Based on this, we establish a chronological framework for a sequence of 3 clay floors dating to between 4930 and 4270 cal BP and determine that the bedded shell deposits that formed the mound accumulated rapidly during 2 episodes: a lower 2-m section below the floors that accumulated over a 0-150 cal yr period at 5050-4875 cal BP and, an upper 3.5-m section above the floors that accumulated over a 0-80 cal yr period at 4380-4230 cal BP.
    • Bayesian Analysis of Radiocarbon Dates

      Ramsey, Christopher Bronk (Department of Geosciences, The University of Arizona, 2009-01-01)
      If radiocarbon measurements are to be used at all for chronological purposes, we have to use statistical methods for calibration. The most widely used method of calibration can be seen as a simple application of Bayesian statistics, which uses both the information from the new measurement and information from the 14C calibration curve. In most dating applications, however, we have larger numbers of 14C measurements and we wish to relate those to events in the past. Bayesian statistics provides a coherent framework in which such analysis can be performed and is becoming a core element in many 14C dating projects. This article gives an overview of the main model components used in chronological analysis, their mathematical formulation, and examples of how such analyses can be performed using the latest version of the OxCal software (v4). Many such models can be put together, in a modular fashion, from simple elements, with defined constraints and groupings. In other cases, the commonly used "uniform phase" models might not be appropriate, and ramped, exponential, or normal distributions of events might be more useful. When considering analyses of these kinds, it is useful to be able run simulations on synthetic data. Methods for performing such tests are discussed here along with other methods of diagnosing possible problems with statistical models of this kind.
    • Bayesian Evaluation of the Southern Hemisphere Radiocarbon Offset during the Holocene

      Hogg, Alan; Bronk Ramsey, Christopher; Turney, Chris; Palmer, Jonathan (Department of Geosciences, The University of Arizona, 2009-01-01)
      While an interhemispheric offset in atmospheric radiocarbon levels from AD 1950-950 is now well established, its existence earlier in the Holocene is less clear, with some studies reporting globally uniform 14C levels while others finding Southern Hemisphere samples older by a few decades. In this paper, we present a method for wiggle-matching Southern Hemisphere data sets against Northern Hemisphere curves, using the Bayesian calibration program OxCal 4.1 with the Reservoir Offset function accommodating a potential interhemispheric offset. The accuracy and robustness of this approach is confirmed by wiggle-matching known-calendar age sequences of the Southern Hemisphere calibration curve SHCal04 against the Northern Hemisphere curve IntCal04. We also show that 5 of 9 Holocene Southern Hemisphere data sets are capable of yielding reliable offset information. Those data sets that are accurate and precise show that interhemispheric offset levels in the Early Holocene are similar to modern levels, confirming SHCal04 as the curve of choice for calibrating Southern Hemisphere samples.
    • Bayesian Methods: What Can We Gain and at What Cost?

      Scott, Marian (Department of Geosciences, The University of Arizona, 2000-01-01)
    • Bayesian Periodic Signal Detection Applied to INTCAL98 Data

      Palonen, V.; Tikkanen, P. (Department of Geosciences, The University of Arizona, 2004-01-01)
      A Bayesian multiple-frequency model has been developed for spectral analysis of data with unknown correlated noise. A description of the model is given and the method is applied to decadal atmospheric INTCAL98 Delta-14C data. The noise of the INTCAL98 data is found to be red, and there seems to be no support for continuous harmonic frequencies in the data.
    • Bayesian Refinement of a Stratified Sequence of Radiometric Dates from Punta de Chimino, Guatemala

      Bachand, Bruce R. (Department of Geosciences, The University of Arizona, 2008-01-01)
      Bayesian analysis of 6 radiocarbon and 2 luminescence determinations from Punta de Chiminos acropolis provides subcentury chronometric accuracy for a Protoclassic hiatus and a more decisive, incipient Early Classic abandonment. For the latter event, sensitivity tests and a redundant modal value pattern reduce the period of historical interest from a few centuries to several decades. The findings aid in selecting between 2 historical scenarios and demonstrate that improved chronological accuracy is attainable for sites and contexts lacking calendrical dates.
    • Bayesian Spatiotemporal Analysis of Radiocarbon Dates from Eastern Fennoscandia

      Onkamo, Päivi; Kammonen, Juhana; Pesonen, Petro; Sundell, Tarja; Moltchanova, Elena; Oinonen, Markku; Haimila, Miikka; Arjas, Elja (Department of Geosciences, The University of Arizona, 2012-10-15)
      Archaeological phenomena, especially those that have been radiocarbon dated, can be utilized as indications of human activity and occupancy in space and time. 14C dates from archaeological contexts have been used as proxies for population history events in several recent studies (e.g. Gamble et al. 2005; Shennan and Edinborough 2007; Oinonen et al. 2010; Tallavaara et al. 2010; Pesonen et al. 2011). As a step towards a larger spatiotemporal modeling effort, we present examples of spatial distributions obtained using Bayesian methodology, analyzing all available archaeological 14C dates from the Stone Age (9000–1500 cal BC) in eastern Fennoscandia. The resulting maps follow the patterns of pioneer settlement in Finland beginning at ~9000 cal BC and provide supporting evidence for the postulated population peak around 4000–3500 cal BC in Finland and the subsequent population decline.
    • Belfast Radiocarbon Dates I

      Smith, A. G.; Pearson, G. W.; Pilcher, J. R. (American Journal of Science, 1969-12-31)
    • Belfast Radiocarbon Dates II

      Smith, A. G.; Pearson, G. W.; Pilcher, J. R. (American Journal of Science, 1969-12-31)
    • Belfast Radiocarbon Dates III

      Smith, A. G.; Pearson, G. W.; Pilcher, J. R. (American Journal of Science, 1970-12-31)
    • Belfast Radiocarbon Dates IV

      Smith, A. G.; Pearson, G. W.; Pilcher, J. R. (American Journal of Science, 1971-12-31)
    • Belfast Radiocarbon Dates IX

      Pearson, G. W. (American Journal of Science, 1979-01-01)
    • Belfast Radiocarbon Dates V

      Smith, A. G.; Pearson, G. W.; Pilcher, J. R. (American Journal of Science, 1973-01-01)
    • Belfast Radiocarbon Dates VI

      Smith, A. G.; Pearson, G. W.; Pilcher, J. R. (American Journal of Science, 1973-01-01)