• A Quality Assurance Protocol for Radiocarbon Laboratories

      Long, Austin (Department of Geosciences, The University of Arizona, 1990-01-01)
    • An Overview of All Three Stages of the International Radiocarbon Intercomparison

      Scott, E. Marian; Aitchison, T. C.; Harkness, D. D.; Cook, G. T.; Baxter, M. S. (Department of Geosciences, The University of Arizona, 1990-01-01)
      The International Collaborative Study involved a wide range of sample materials and ages and, on completion, assessed each stage independently (Scott et al 1989; Aitchison et al 1990). We combine here the three stages of the study and provide an overview of the uncertainties in the dating procedure as a whole and in its component processes. Three key optimal performance indices, related to internal and external precision and to bias, have been defined to allow quantitative assessment of Internal Consistency and External Consistency (Aitchison et al 1990). We believe that these measures provide quantitative descriptions of a laboratory's reproducibility, accuracy and precision. For the internal consistency, we have defined the Internal Error Multiplier of the quoted error and, for the external consistency of any laboratory relative to an appropriate baseline, we have defined two indices, the Systematic Bias and External Error Multiplier of the quoted error. We have evaluated the three indices over the three stages and have assessed the relative performances of gas counting, accelerator and liquid scintillation laboratories. The quoted errors describe adequately the variability in duplicate results, but there is evidence of systematic biases and underestimation of interlaboratory variability. We have considered the contribution of pretreatment, synthesis counting to the overall variability for each laboratory type. We found that, for liquid scintillation (LS) and gas counting (GC) laboratories, ca 66% of the total variation is due to counting and sample synthesis whereas, for accelerator mass spectrometry (AMS) laboratories, the major sources of variability are the sampling and pretreatment processes.
    • An Overview of Some Interlaboratory Studies

      Scott, E. Marian; Baxter, M. S.; Aitchison, T. C.; Harkness, D. D.; Cook, G. T. (Department of Geosciences, The University of Arizona, 1990-01-01)
      Many interlaboratory studies have been made in the 14C community at irregular intervals over the past ten years. At times, the results from these studies have been contentious, mostly because of the lack of consistency in their findings. The importance of regular exercises has become particularly acute due to the large number of operating laboratories and the diversity of their methodologies. Hence, we briefly review the studies that have been made in the 1980s, focusing on those in which our laboratories participated. These include the 14C Interlaboratory Comparison in the UK (Otlet et al 1980), the International Comparison (ISG 1982, 1983) and the first two parts of the current International Collaborative Program (Scott et al 1989a, b). The development of each study, its findings and shortcomings, are highlighted in order to assess the concordance of the conclusions.
    • Checking Back on an Assemblage of Published Radiocarbon Dates

      Baillie, M. G. L. (Department of Geosciences, The University of Arizona, 1990-01-01)
      It is clear that radiocarbon researchers take a forward view towards the improvement of accuracy and precision in dating. Unfortunately, archaeologists base much of their research on the published dates produced in the past. Archaeologists and other users of radiocarbon dates should understand the limitations associated with past dates. This article addresses these limitations by looking at a large number of routine radiocarbon dates associated with a block of English tree-ring chronologies, the true ages of which are now known within close limits. My conclusion supports the idea of global multiplication factors as proposed by the International Study Group (1982).
    • High-Precision Intercomparison at IsoTrace

      Beukens, Roelf P. (Department of Geosciences, The University of Arizona, 1990-01-01)
      I conducted a high-precision comparison at the 0.2% to 0.3% level with samples supplied by the radiocarbon laboratory of the Quaternary Research Center at the University of Washington (QRC). Four samples with ages ranging from modern to > 50,000 BP were dated in a blind test. The absence of cosmic-radiation background in AMS dating is a major advantage for dating samples > 35,000 BP. The reliability of AMS dates > 35,000 BP depends entirely on understanding the contamination processes. By comparing results with laboratories capable of sample enrichment, such as QRC, it is possible to identify and estimate the intrinsic 14C in the background samples as well as the contamination introduced by sample preparation.
    • International Collaborative Study: Structuring and Sample Preparation

      Cook, G. T.; Harkness, D. D.; Miller, B. F.; Scott, E. Marian; Baxter, M. S.; Aitchison, T. C. (Department of Geosciences, The University of Arizona, 1990-01-01)
      The success of any intercomparison exercise depends largely on participation and cooperation of a sufficient number of laboratories and the selection of a suitable suite of samples. Unless the latter is satisfactorily devised, the former cannot be guaranteed. The hierarchical nature of this study has necessarily resulted in a far more comprehensive set of sample types than has previously been employed. The exercise was structured to satisfy the following criteria: 1) to enable the participating laboratories to assess the experimental precision and accuracy of the component stages of the dating process; 2) samples should be typical of those routinely dated by the laboratories. This takes on a particular significance in Stage 1 where they should resemble as closely as possible the counting medium; 3) an objective statistical analysis of the results at each component stage of the study.
    • Radiocarbon Dating Reproducibility at the Museo de la Plata Radiocarbon Laboratory

      Figini, Anibal J.; Huarte, Roberto; Carbonari, Jorge (Department of Geosciences, The University of Arizona, 1990-01-01)
      We discuss here the variability, for our laboratory, in counting for radiocarbon dating of replicate measurements of background and secondary modern standard, duplicate measurements of samples provided by the International Collaborative Study, and replicate measurements of the dilution of the 14C-labeled benzene standard. The variability in the measurements of the International Collaborative Study samples suggest the existence of systematic bias.
    • Report on Stage 3 of the International Collaborative Program

      Aitchison, T. C.; Scott, E. Marian; Harkness, D. D.; Baxter, M. S.; Cook, G. T. (Department of Geosciences, The University of Arizona, 1990-01-01)
      This report on the third and final stage of the International Collaborative Program concentrates on the analysis of internal and external variability of 14C dates obtained from samples involved in the full 14C dating process. Thirty-eight laboratories took part in this stage with most producing 8 14C dates from 3 sets of duplicate material (eg, wood, shell and peat) and 2 single samples of wood of known ages 190 yr BP apart. From the 3 sets of duplicates for each laboratory, the internal precision of most laboratories was adequate; 6 labs grossly underestimated their internal reproducibility. From the 14C determinations from the 5 distinct samples for each laboratory, we discovered significant systematic biases, often greater than 100 years, in 15 laboratories and even accounting for bias, 12 laboratories had significantly greater external variability than explained by their quoted errors. In total, 23 out of the 38 laboratories in this stage of the study, FAILED to meet these 3 basic criteria for an adequate performance in the production of 14C dates.
    • Routine Checks in the Uppsala Conventional 14C Laboratory to Achieve Reliable Results

      Olsson, Ingrid U. (Department of Geosciences, The University of Arizona, 1990-01-01)
      I describe here a series of routine self-checks that the Uppsala 14C laboratory performs with all measurements. We estimate all uncertainties in the physical measurement of a sample. We study long-term stability, calculate mean values for oxalic acid and background and compare expected and real statistical distributions of uncertainties. To reduce the risk of bias, the samples from each series are almost exclusively run on the same counter. Some samples are, however, run on two or more counters to check the possible bias to achieve reliable activity comparisons with other laboratories. It is always possible to trace which counter is used, since different number series are used for different counters.
    • Sources of Random Error in the Debrecen Radiocarbon Laboratory

      Hertelendi, Ede (Department of Geosciences, The University of Arizona, 1990-01-01)
      A new high-pressure methane-filled counter system for 14C dating was installed in 1986 when the first stage of the International Collaborative Study (ICS) started. Random errors in the new measuring system and in the process of chemical pretreatment and preparation were checked during the three years of intercomparison. Results show that the most important source of error in our laboratory is gas contamination. This causes variation of the count rate to exceed the statistically expected variability. Other sources of error are also discussed and limits of their contributions are given.
    • Systematic Biases in Results of the International Collaborative Study and Their Probable Sources

      Pazdur, Mieczysław F.; Awsiuk, Romuald; Goslar, Tomasz; Pazdur, Anna (Department of Geosciences, The University of Arizona, 1990-01-01)
      Results of the International Collaborative Study show an unexpectedly large scatter of individual dates as well as systematic biases. Very high values of linear correlation coefficients are observed for all results of Stage 2 and for benzene samples of Stage 1. We observed moderate correlations for carbonate samples and the lowest for natural samples of wood and peat of Stage 3. The correlation is practically negligible among results obtained in different stages. The probable reasons for such effects are seen in medium-term changes in the calibration of the counting systems.