This parameter is mainly generated by alpha and beta rays emitted from elements within the sample. In enamel and dentine of teeth, the U-decay chains are in disequilibrium. This affects the average dose rates and has to be taken into account mathematically. The alpha efficiency of tooth enamel is difficult to determine because of size requirements for ESR measurements. The best value is about 0.13 ± 0.02.
The dose rate calculations in tooth enamel are further complicated by the fact that bones and teeth tend to accumulate uranium over time. Modern bones are virtually uranium free (with concentrations around 1-10 parts per billion, ppb), whereas archaeological samples may contain many orders of magnitude higher concentrations (up to several hundreds ppm). The explicit history of the uranium uptake may have significant implications for the calculation of the average dose rate. Conventionally, two uranium uptake models have been calculated: early U-uptake (EU) and linear U-uptake (LU). For the EU model it is assumed that all the uranium that is measured today was accumulated by the teeth within a short time after its burial, whereas the LU model predicts that the uranium has been accumulated continuously over time in a linear fashion. The differences in EU and LU age calculations are small as long as the U-concentra-tions in the teeth are moderate (less than about 2 ppm in dentine). However, with increasing U-concentra-tions in the teeth, the EU/LU age difference increases until the LU age is about twice the EU age. In the interpretation of the ESR dating results, it is generally assumed that the correct age of the sample lies somewhere between the bracketing EU and LU age calculations. It has been recognized, however, that teeth from a considerable number of sampling sites (perhaps 10-20%) may have U-uptake histories that lie outside the age range defined by the EU and LU models. This may be due to U-leaching or strongly delayed U-uptake. The reason may lie in changes in the hydrological environment of the samples. If ESR is used as an independent dating technique and unless U-series analyses are carried out on the teeth (see below), there is no first-principles argument that could be used to prefer one model over another.
To overcome the problem of the unknown U-uptake history in teeth, Grtin et al. suggested analyzing the enamel and dentine samples for U-series isotopes. Uranium series dating results on bones and teeth are similarly influenced by U-uptake, but to a different extent than ESR (the LU U-series age is always nearly twice the EU age). Combining ESR and U-series allows the establishment of an equation system that can be solved for a one-parameter diffusion equation and age.
World History
