Good article from the Swiss Federal Institute of Technology in Zurich:
What began with a particle accelerator in the Laboratory of Nuclear Physics at ETH Zurich nearly half a century ago is now one of the world’s leading labs for ion beam physics. Having been converted into an accelerator mass spectrometer, today the particle accelerator is used to determine the age of historically significant objects.
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This was the technique used at ETH Zurich to date renowned objects like the Turin Shroud or Ötzi, the frozen mummified corpse found in the Alps in 1991. Up until now, they have been the most famous objects to be dated at the Laboratory for Ion Beam Physics. To determine how old something is, a few milligrams of the object are burned and a graphite sample is produced from the resulting carbon dioxide. This is then analyzed in the AMS. The ratio of 14C atoms to 12C atoms compared to the initial concentration of 14C in the atmosphere defines the radiocarbon age which after calibration by the tree ring curves the true age of the sample. Traditional methods measure radioactive degradation. However, as the half-life increases, the decay becomes seldom and much more material is needed to obtain a good signal, explains Synal. The AMS method is three to four orders of magnitude more efficient. Here, the natural isotope ratios have a concentration of between 10-15 and 10-12 atoms in the material being examined. There are 1015 atoms of 12C for one 14C atom. Such a low proportion of 14C in the ratio therefore has to be measured with extreme precision, which is only possible with the AMS.
But we need to look at the full story:
There were a number of people after 1988, including several scientists, who were not convinced that the carbon dating results were right. In part, this was because there was a mountain of other evidence that suggested a much earlier provenance for the shroud and there were some very puzzling mysteries about the nature of the image. Some speculated on why the carbon dating might be wrong but none of the proposals seemed very scientific. It was mostly hypotheses that could not be falsified (ala Popper).
Two researchers, Sue Benford and Joe Marino, who were not scientists, proposed that the cloth had been mended in the seventeenth century in a corner from which the carbon dating samples were taken and thus what had been dated was probably a mixture of original cloth (presumably first century) and newer thread.
Raymond Rogers, a Fellow of the Los Alamos Laboratory was perplexed by this proposal that seemed to him very unscientific. As a chemist, he had personally examined the shroud in 1978, warning church official that he would report whatever he found. As it turns out, he did offer an opinion on the cloth’s authenticity because there were too many unanswered questions. However, in 1988, he accepted the carbon dating results and withdrew from further shroud study. When he read about what Benford and Marino were suggesting, he was certain that they were wrong. They were, as he put it, part of the lunatic fringe of shroud research. He was certain that he could prove they were wrong. He had some material from the sample corner and set out to do so.
Much to Rogers’ surprise, Benford and Marino were right. Rogers not only found substantial evidence of mending, he found stark chemical differences between the corner from which the carbon dating sample had been taken and the rest of the cloth. If there were chemical differences then the sample could not be reliably considered to be representative of the whole cloth. This invalidated the carbon dating.
Before publishing his findings in the peer-reviewed journal, Thermochimica Acta (vol 425  pp 189–194) in 2005, Rogers, with Anna Arnoldi of the University of Milan, published an informal paper in 2002. Though it was widely distributed, it received no comment from those who had been involved in the carbon dating. It wasn’t until 2004 when the Journal of Research of the National Institute of Standards and Technology (U.S. Department of Commerce, NIST, U.S.
Government Printing Office) published an important paper by Lloyd A. Currie. Currie, a highly regarded specialist in the field of radiocarbon dating and an NIST Fellow Emeritus, wrote a seminal retrospective on carbon 14 dating. Because the Shroud of Turin was such a famous test, Currie devoted much of his paper to it.
Like Rogers, Currie dismissed any argument that radiocarbon labs had done anything wrong in dating the Shroud of Turin. Currie also rejected, as Rogers also had done, other very unscientific proposal. But Currie did acknowledge that disguised mending was a viable explanation. He cited the work of Rogers and Arnoldi. He found it credible.
Rogers also asked John Brown, a materials forensic expert from Georgia Tech to confirm his finding using different methods. Brown did so. He also concluded that the shroud had been mended with newer material.
Since then, a team of nine scientists at Los Alamos has also confirmed Rogers work, also with different methods and procedures. Much of this new information has been recently published in Chemistry Today.
Article: The custodians of time