1) An Intercomparison of Some AMS and Small Gas Counter Laboratories
Richard Burleigh, Morven Lese and Michael Tite
Radiocarbon, Vol 28, No 2A, 1986, pp571-577
Abstract: The performance of six laboratories with the capacity to date small samples (4 AMS and 2 small gas-counter laboratories) has been compared using 100mg samples of textiles from Ancient Egypt and Peru, with the British Museum laboratory acting as independent cordinator. This intercomparison was one of normal practices and has demonstrated that a coherent series of results can be obtained when several laboratories undertake blindfold measurements, although the ocurence of outliers emphasizes the continuing need for the dating of unusually important or controversial samples to be undertaken by a group of laboratories.
The AMS labs were Arizona, Bern (using the Zurich AMS Facility), Oxford and Rochester, the small gas-counter ones Brookhaven and Harwell. They were all given samples from one Peruvian and one Egyptian textile, apparently of known age. However all the labs found that the Peruvian sample was much more recent than previously thought, so a second was included. The results were:
The pink areas define the ‘known’ ages of the samples. All the results are consistent, with a spread of about 500 years, with the exception of two clear outliers. Unfortunately, but in accordance with protocol, the laboratories responsible for them were not identified. The authors stated that: “There do not appear to be differences between the AMS and small counter techniques.”
[Following an investigation of the circumstances of this Intercomparison among the Shroud data at the British Museum, it is clear that both the outliers came from the Bern (Zurich) laboratory.]
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2) Archaeologic Sherd Dating: Comparison of Thermoluminescence dates
with Radiocarbon dates by Beta Counting and Accelerator Techniques
R.A. Johnson, J.J. Stip and M.A. Tamers (University of Miami)
Georges Bonani, Martin Suter, Willy Wölfli (ETH, Zurich)
Radiocarbon, Vol 28, No 2A, 1986, pp719-725
Abstract: Sherds can be dated by four independent methods: 14C beta counting on associated material, accelerator mass spectrometry on carbon traces on and within the sherd, thermoluminescence studies on minerals within the sherd, and stylistic form. Age analyses of materials and sherds from several sites are shown in this work. Each technique has its own frequently encountered non-laboratory sources of error. A combination of at least two independent techniques is indispensable for the highest level of confidence.
Sherds are notoriously awkward to radiocarbon date, and there were a few outliers for both beta counters and AMS procedures. The thermoluminescence results were assumed to be the more accurate, but there is at least one thermoluminescent outlier.
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3) Report on Stage 3 of the International Collaborative Program
T.C. Aitchison, E.M. Scott, D.D. Harkness, M.S. Baxter, G.T. Cook
Radiocarbon, Vol 32, No 3, 1990, pp271-278
Abstract: 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.
This was the huge, four-year, interlaboratory comparison project of which Murdoch Baxter, whose Liquid Scintillation laboratory was one of those which took part, was particularly scathing of the AMS method, which was in fact demonstrably better than his.
Baxter’s remarks have been seized upon with enthusiasm by Shroud authenticists, so let’s see how accurate they are.
“Baxter says that accelerator mass spectrometry, used last year by a laboratory at the University of Oxford to date the Turin shroud, allegedly the burial shroud of Jesus Christ, came out of the survey badly. Five of the 38 participating laboratories used this technique, for which samples weighing a few milligrams are acceptable. The other techniques require grams of the sample. Baxter says that some of the accelerator laboratories were way out when dating samples as little as 200 years old.
Because so little material is used in accelerator mass spectrometry, the effects of chemical pre-treatment are likely to be more serious, says Baxter. ‘The samples are probably more prone to atmospheric dust or dandruff,’ he said.” (New Scientist, 30 September 199)
A mere glance at the results data above is enough to show that the AMS labs produced results that were better grouped, and closer to the known ages of the samples, in all cases. By contrast, the Liquid Scintilators were undeniably the worst. Baxter was only correct that “some of the accelerator laboratories were way out when dating samples as little as 200 years old,” if it be admitted that both the other methods of dating were considerably more so.
Furthermore, this survey summed up the findings thus: “It seems reasonable to consider that a laboratory performs adequately if it has no significant systematic bias and assesses its internal and external variability adequately (i.e. with error multipliers not significantly different from one). Accordingly, Table 3 presents the number of laboratories failing to meet each of these requirements.”
This shows that of the 5 AMS labs that took part, 3 (60%) did not fail any of the designated criteria. Of the 13 Liquid Scintillation Counter labs, between 7 and 13 failed, giving a success rate of between 0% and 46%, and of the 20 Gas Counter labs, between 6 and 15 failed, giving a success rate of between 25% and 70%.
The Conclusion, however, goes on: “In total only 15 of the 38 laboratories meet all 3 of these adequacy requirements, whereas eight have two or more problems. ” We can see that of the 15 successful labs, 3 were AMS (60%), leaving only 12 of the other 33 labs (36%). None of the AMS labs had two or more problems; all eight such labs were from the other two groups (24%).