Stepped-irradiation SAR: A viable approach to circumvent OSL equivalent dose underestimation in last glacial loess of northwestern China

The equivalent dose (De) obtained with the continuous irradiation SAR (CI-SAR) protocol for fine-grained quartz from loess of northwestern China is found to be lower than the expected value for samples older than 70 ka based on the regional stratigraphy. This is attributed to the difference in the response of the quartz to natural radiation and laboratory beta irradiation whose rates vary by ∼108 times. A stepped irradiation SAR protocol was employed to evaluate the influence of such a ‘dose rate effect’ on the equivalent dose determination. After investigating the effects of thermal treatment and ‘unit-dose’ on OSL signal and De, we refined the stepped irradiation strategy with a ‘unit-dose’ of ∼25 Gy and successive thermal treatments at 250 °C for 10 s, and applied it to the SAR protocol. This stepped irradiation SAR (SI-SAR) protocol led to a 20%–70% increase in De value for loess deposited during the early last glacial period.