Evaluating dose-rate distributions in natural sediments using α-Al2O3: C grains

Radioactive nuclides in natural sediments are not always distributed homogeneously, and this can lead to variations in dose rate within the sediment on both a macroscopic and microscopic scale. Single-grain dating protocols are expected to allow a detailed examination of the dose distribution in a sample; although the equivalent doses are measured for individual grains, the dose rate is usually assumed to be homogeneous and is obtained from bulk measurements. Then, if an asymmetric dose distribution is observed, it is usually attributed to incomplete bleaching or physical intrusion of older or younger gains. As part of our initial studies on the importance of spatial dose-rate variations, we report here on the first direct measurements to test the dose rate homogeneity in a natural sediment. alpha-Al2O3: C gains with diameters of 200-300 mum were mixed with a loess sediment and the doses to the single grains were measured. Additionally, the homogeneity of the gamma response of the grains was investigated. The results from 26 grains buried in the loess show a distribution with a standard deviation of 27%; this can be compared with a standard deviation of 20% measured using a gamma-irradiated sample, for which the dose should be homogeneous. We conclude that 18% of the dose variation is caused by dose-rate inhomogencities in the loess sediment. (C) 2003 Elsevier Science Ltd. All rights reserved.