Are published ESR dose assessments on fossil tooth enamel reliable?

Abstract

We performed dose recovery tests on proto-historic enamel samples that were dosed to approximately 200 Gy to present equivalents of Middle to Late Pleistocene fossil teeth. The experimental results confirmed that the ESR dose response of fossil tooth enamel can be best described by a double saturating exponential (DSE) function. When using a DSE for dose evaluation, it is necessary to irradiate samples to very high doses (at least 20 kGy, ideally up to 50 kGy) and use at least 15 additive dose steps.In past studies, dose estimations were mostly carried out using a single saturating exponential (SSE) function and maximum doses (Dmax) in the range of around five to ten times the equivalent dose, usually less than 5 kGy. To address the question whether older dose estimations have to be disregarded as unreliable, we carried out computer simulations using DSE dose response curves which were then fitted with SSEs. Artificial data sets were generated for a wide range of preset dose values between 10 and 5000 Gy. The results indicate a strong correlation between the magnitude of the DE value with systematic and random errors. Reliable doses can be obtained with SSEs for preset dose values of <1000 Gy using 10 exponentially distributed dose points with maximum dose values in the range of up to ten times the DE value. Higher preset doses (>1 kGy) require modified Dmax values. Although here the application of DSEs is preferable, the use of SSEs can nevertheless provide reliable results with reasonably small systematic errors for doses of up to 2000 Gy, provided Dmax is adjusted to about 1*Dset and measurement precision is better than 0.3%. The overall uncertainty remains acceptable (∼10%) using DSEs for DE values of up to 5000 Gy, provided the measurement precision is better than 0.5% and up to 20 dose steps are used. 5000 Gy is apparently the maximum dose than can be reliably assessed with ESR.Finally, we provide some detailed recommendations to ensure reliable dose evaluations with either the SSE or DSE functions. These recommendations can also be used to evaluate the reliability of previously published ESR dose assessments.