Photon energy (8–250 keV) response of optically stimulated luminescence: Implications for luminescence geochronology

Understanding the energy response of dosimeters and scintillators for measurements of ionising radiation is an important area in the field of solid state dosimetry. Such an understanding is particularly critical for source calibration and for quantifying effective dose (rates) in wide-energy mixed-radiation fields. While there have been several investigations on characterising the ionising- energy response of artificial dosimeters (e.g. LiF, Al2O3:C, etc.), there exist no studies on natural dosimeters such as quartz and feldspar which are extensively used in luminescence geochronology and retrospective dosimetry. Currently, luminescence geochronology assumes that all beta and gamma irradiations in nature (from the decay of K, the U and Th radioactive chains) are equivalent in terms of luminescence production per unit dose to each other, and to the laboratory 137Cs, 60Co (gamma) and 90Sr/90Y (beta) irradiations used for calibration and/or equivalent dose determination. In this study, we experimentally determine the quartz Optically Stimulated Luminescence (OSL) and feldspar Infra-Red Stimulated Luminescence (IRSL: IRSL at 50 °C and post-IR IRSL at 290 °C) response to irradiations with photons of in the energy range 8–250 keV. We show that microdosimetric effects lead to a steady, significant decrease in quartz OSL production efficiency for<80 keV photons. Similar to quartz, the local saturation of the electron traps also leads to a systematic decrease in luminescence efficiency for<120 keV photons in feldspar; however, this effect competes with an increase in recombination efficiency due an increased number density of hole centres in the vicinity of the trap, thus giving a peak-shaped IRSL efficiency response in the range of 250–8 keV. Our results suggest that dose-rate conversion factors, specific to radiation type and radioisotope, should be used for age calculation, and significant care should be taken for luminescence based calibration of the dose rate of the laboratory irradiation source.