ESR dating of quartz grains: evaluating the performance of various cryogenic systems for dosimetric purpose

Abstract

We present the results of the first detailed comparative and quantitative study of various cryogenic systems that can be used for ESR measurements of quartz grains. Three experimental setups were tested: (i) a standard liquid nitrogen Variable Temperature Unit (VTU), operating at 90–100 K and used in most ESR dating studies; (ii) a helium-based VTU that can reach measurement temperatures as low as 15–20 K; and a (iii) finger dewar filled with liquid nitrogen operating at 77 K. As expected, our results show significant gains in signal intensity and resolution when working at temperatures below 90 K, which is extremely useful when dealing with samples with weak intensities or poorly-resolved spectra. The improved signal resolution at 40 K allows the differentiation of the Ti–Li and Ti–H absorption lines around g = 1.913 that are typically merged at 90 K or above. It is therefore possible to extract the ESR intensity of a resolved Ti–Li signal for dating. The results obtained with each experimental configuration are highly consistent for both the Al and Ti centres. In particular, equivalent dose values obtained from eleven of the twelve samples analysed agree at a 1σ confidence level. The various cryogenic systems do no induce any significant systematic bias for ESR dose evaluation.