Equivalent dose estimation of calcite using isothermal thermoluminescence signals

Thermoluminescence (TL) signals of calcite can be used to potentially date geological and archaeological events back to several million years. However, several issues, such as spurious TL signals appearing at temperatures above 300 °C, have hindered its application to a wide range of samples. A single-aliquot regenerative-dose (SAR) protocol for calcite with low-temperature measurements is proposed to measure the equivalent dose (D_e). It uses the isothermal TL (ITL) signals measured at around 225-240 °C, where a D_e vs. ITL temperature (D_e-T) plateau can be observed. The width of the temperature range of such a plateau can be sample dependent, as it relates to the proportional contributions of the signals from corresponding TL peaks. The signal at the ITL temperature plateau range largely corresponds to the TL signals of the 280 °C TL peak. D_e values obtained by the SAR-ITL protocol are in agreement with those of the multiple-aliquot additive-dose (MAA)-TL and MAA-ITL protocols. The absence of detectable anomalous fading of ITL signals at 235 °C in this and previous studies indicates that the signal is free of fading. Dose recovery tests confirm the suitability of the SAR-ITL protocol for D_e estimation. The SAR-ITL protocol measured with temperatures below 300 °C avoids the effects of spurious luminescence signals induced by high-temperature heating. The dose-response curves for ITL signals at 230-235 °C have large characteristic saturation doses (D₀) of ∼2000-2400 Gy. The SAR-ITL protocol for calcite thus has the potential to date geological and archaeological samples spanning the entire Quaternary period.