Uran-Thorium datingOver the past 60 years, the uranium series – dating methodology of natural secondary carbonates such as travertine, calcareous tufa, cave deposits, and corals has evolved dramatically. In the beginning, the measurement of radioactive disequilibrium between 230Th and its mother nuclide 234U was performed via nuclear physics methods. In the early 90, the use of mass spectrometers revolutionized the age determination leading to precision levels of a few ‰ compared to a few %. At the same time the required sample sizes diminished and the associated processing methods were strongly improved – hence the performance. The scientific research fields and applications consequently expanded considerably since then. Sea level reconstructions, the development of cave sinters as the best dated climate archives, a multitude of new archaeological discoveries (including the oldest cave paintings) and the calibration of atmospheric radiocarbon concentrations are just a few outstanding examples. Only a decade later, a second revolution has begun, with the development of multi-collector inductively coupled plasma source mass spectrometry. Today, analyses in the sub-‰ range are possible, comparable and sometimes better than the radiocarbon method. Results from cave deposits allow dating from a few years up to more than 600'000 years and the limit of technological development has not yet been reached. What began more than 40 years ago with the Research Unit of the Academy of Sciences for Radiometric Dating of Water and Sediment in Heidelberg has now developed into a unique laboratory for ultra-high precision dating based on natural uranium disequilibria at the Institute for Environmental Physics at the Heidelberg University. By means of some examples we show the possibilities and still existing methodical limitations of existing and future technologies.Uranium/thorium-helium dating During the radioactive decay of uranium and thorium, alpha radiation produces the noble gas helium. At the beginning of the 20th century, this process was used for the first application of radioactivity to determine the age of rocks and minerals. Assuming that these materials store helium, an age can in principle be calculated from the ratio of uranium to helium. However, it soon turned out that this assumption is not justified. Despite many experiments on various materials, this method has not provided reliable age values. Gold, on the other hand, has the unusual property of actually being able to store helium in its crystal lattice. This helium can be extracted and measured by heating gold to near its melting point. This has proved successful for the geological dating of gold deposits, for example. In principle, the uranium/thorium-helium method can also be applied to archaeological gold objects by determining the time of the last melting of the gold (resetting of the helium concentration to zero), but the difficulties involved are considerable. Firstly, the amount of helium to be measured is much less than in geological gold because of the short time span. In addition, even a small sample of gold objects is not easy to obtain and, in addition, extremely low concentrations of uranium and thorium must be measured in the same sample from which helium has been extracted. In this unit the basics of the method are presented in detail and possible complications are discussed.The ²¹⁰Pb testThe so-called ²¹⁰Pb test is a method adapted from the geosciences for the authenticity examination of metal objects. There, for example, the age of younger sediments or peat can be determined or sedimentation rates reconstructed using this method. Within the scope of authentication, objects made of various copper alloys (copper, bronze, brass), silver, tin and lead objects or parts of objects made of these metals can be tested to determine whether the object used was produced from ores within the last 150 years. However, the exact age of a metal object cannot be determined. The method is based on the fact that the radioactive isotope of lead, namely ²¹⁰Pb, which is found almost everywhere in nature, is absorbed by the metal during the production of non-ferrous and precious metals, making it radioactive. ²¹⁰Pb is a daughter product in the decay chain of 238U, which is present in almost all rocks and is usually in radiochemical equilibrium with it. This equilibrium is disturbed during metal extraction, so that ²¹⁰Pb decays with its relatively short half-life of 22.4 years. For this reason, this radioactivity can no longer be detected after about five half-lives. Although this does not allow an exact dating of older objects, it does make it possible to identify metals that have only been produced within the last 100 to 150 years.

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