Thermally Transferred Optically Stimulated Luminescence Signal Research Articles

Multi dating approach of long marine core sediments from the south-eastern continental shelf of Korea: Comparison of SAR OSL, TT-OSL and pIRIR dates

Marine sediments constitute important archives of past oceanic, climate, and sea-level changes. However, it is difficult to derive accurate chronological models due to the absence of long sediment cores and limitations of dating methods. To investigate the feasibility of applying optically stimulated luminescence (OSL) ages to establish high-resolution chronologies in the East Sea of Korea, we obtained three cores with lengths up to 120 m from the eastern continental shelf of Korea. Most of the core sediments were homogeneous or crudely laminated mud, with some sand and gravel bed. Multiple dating methods, including fine-grained quartz single-aliquot regenerative-dose (SAR) OSL, thermally transferred OSL (TT-OSL) from fine-grained quartz, and single-grain K-feldspar post-infrared (IR) infrared stimulated luminescence (IRSL) (pIRIR), following the above description, were applied to overcome the age limitation and create an accurate age model through cross-checking among the different dating methods. We also investigated the applicability of the standardised growth curve (SGC) to TT-OSL signals in continental shelf environments. The equivalent dose (D e ) values from the SGC were consistent with those from the SAR TT-OSL procedure, which suggests that the SGC is valid for long core sediments on the continental shelf. A detailed chronology derived from the radiocarbon, U/Th, and multiple OSL age dating methods was compared. Especially, in core 19ESDP-101, a comparison between TT-OSL ages from fine-grained quartz and pIRIR ages from coarse K-feldspar showed agreement to within 10% at most depths for which ages were measured. We demonstrated that OSL dating from long marine core sediments in a continental shelf margin environment can provide an accurate and precise chronology in undatable regions.

Thermally Transferred Optically Stimulated Luminescence, TT-OSL: a study on the responsible traps

Abstract Aim of this paper is to better investigate the Thermally Transferred Optically Stimulated Luminescence (TT-OSL) signal generated in quartz samples, both mineral quartz and quartz extracted from sediment, when they are heated after an optical bleaching. In recent years, TT-OSL has become an interesting dating technique because it can extend by an order of magnitude the age range over which OSL dating can be applied. We focused in particular on the source traps that generate this signal, studying the changes in TT-OSL due to the preheat temperature that induces the thermal transfer, and analysing the Thermoluminescence (TL) signals caused by OSL and TT-OSL measurements. By varying the preheating temperature, it was possible to identify the presence of a peak at ∼280-300°C, likely related to a trap responsible for the TT-OSL signal. A similar result was obtained by analysing the glow curves after an optical bleaching, which evidenced a peak at ∼270°C, comparable with a peak previously identified. It was also possible to highlight the fact that the OSL and TT-OSL signals are originated by different mechanisms due to the differences between the source traps responsible for them.

Insights into the relationship between luminescence and ESR dating signals from Spanish sedimentary quartz samples of different geologic origins

Trapped charge dating of optically bleached quartz deposits that lie beyond the dose-saturation limits of conventional optically stimulated luminescence (OSL) has typically focused on two main techniques over the last decade: electron spin resonance (ESR) and thermally transferred optically stimulated luminescence (TT-OSL). In spite of a clear association between these two dating techniques through their shared dose rate terms, the exact relationships between the various ESR centre (i.e., Al and Ti) and TT-OSL dating signals remain largely unexplored. In this study we undertake ESR and luminescence signal comparisons on paired sedimentary quartz samples from two Middle Pleistocene Spanish archaeological sites (Porto Maior in northwest Spain and Cuesta de la Bajada in northeast Spain) to examine the empirical relationships between quartz dating signals across different geological provinces. For this purpose we undertake 3-dimensional (3D) thermoluminescence (TL) spectral measurements, single-grain TT-OSL dating and signal analyses, normalised multiple centre (MC) ESR signal characterisation (ESR intensities, signal composition, dose-response curve properties), MC ESR De evaluation, and ICP-OES analysis of quartz extracts to evaluate potential correlations between elemental impurity concentrations and Al, Li–Ti and Li–H centre signals. The ESR samples from Cuesta de la Bajada and Porto Maior display similar Al centre signal intensities and dose-response behaviour, but they exhibit major differences in their Ti centre properties. Geochemical analysis reveals that the Cuesta de la Bajada quartz extracts contain higher concentrations of Al and Ti impurities (15 and 21% higher, respectively) when compared with the Porto Maior quartz extracts. These compositional differences appear correlated with the relative Ti–H signal intensities for the two sample datasets but not with the relative Al and Ti–Li signal intensities. The luminescence characterisation comparisons reveal a higher proportion of TT-OSL-producing grains in the Cuesta de la Bajada samples (6.2% versus ~1.6% for Porto Maior), and single-grain TT-OSL signal intensities that are almost an order of magnitude higher than for the Porto Maior samples. The 3D TL spectra for the Cuesta de la Bajada quartz are dominated by two major TL peaks in the red emission band (620 nm) at 265–310 °C and 365 °C (when using a heating rate of 1 °C/s), which are up to seven times more intense than the corresponding signals observed for the Porto Maior quartz. However, no major inter-sample differences were observed in the blue and UV-blue bands of the 3D TL spectra. These contrasting luminescence characteristics may be indicative of variable signal sensitisation histories related to dissimilar sedimentary recycling or geological origins between the two sites. Our results provide a useful baseline dataset for future comparative ESR and luminescence characterisation studies, and indicate a potentially close relationship between the quartz ESR Ti centre and single-grain TT-OSL signals. There is significant potential to refine understanding of these signal relationships in the future by expanding the number, and geographic coverage, of paired ESR–luminescence comparison studies, and by incorporating comparative 3D TL spectral investigations that focus exclusively on TT-OSL-producing grains (as opposed to bulk grain populations) to better elucidate luminescence trapping and recombination characteristics.

Refining the chronology of Acheulean deposits at Porto Maior in the River Miño basin (Galicia, Spain) using a comparative luminescence and ESR dating approach

Trapped charge dating of optically bleached quartz, which involves techniques such as electron spin resonance (ESR) and thermally transferred optically stimulated luminescence (TT-OSL), is increasingly being used to establish Middle and Early Pleistocene chronologies for archaeological, palaeontological and palaeoenvironmental sedimentary sequences. Comparative dating studies that make use of several independent or semi-independent radiometric techniques in tandem are an invaluable means of assessing the reliability of optically bleached quartz dating methods and ascertaining empirical relationships between different quartz dating signals. In this study, we undertake a comparative dating evaluation of single-grain TT-OSL and various ESR signals that exhibit different optical resetting properties (Al, Ti–Li and, where measurable, Ti–H centres) at the Middle Pleistocene archaeological site of Porto Maior, northwest Spain, which contains a distinctive, regionally significant Acheulean stone tool assemblage. We expand the existing chronological framework for Porto Maior using single-grain TT-OSL dating, derive combined numerical ages for individual depositional units using Bayesian modelling, and further examine the relationship between ESR and TT-OSL signals by reviewing a range of published comparative dating studies. The new single-grain TT-OSL ages obtained for unit PM3 at Porto Maior (225 ± 31 ka and 242 ± 32 ka) are in close agreement with previously published ESR Ti–Li ages for this unit (266 ± 23 ka and 279 ± 26 ka) and significantly younger than those reported for the Al centre (441 ± 75 ka and 452 ± 66 ka). Our broader comparison of published TT-OSL – ESR dating results also shows statistically significant agreement between single-grain TT-OSL and Ti–Li centre ages in other sedimentary settings, suggesting a potentially closer relationship between the TT-OSL-producing traps and the Ti–Li paramagnetic centres in quartz, rather than between the TT-OSL and Al centre signals. The Bayesian modelling results for Porto Maior enable us to refine the existing bracketing depositional age for the archaeological accumulation at Porto Maior and provide a stronger statistical foundation for chronostratigraphic correlations with other Acheulean archaeological deposits found in the Miño River basin. The Bayesian model indicates that the entire Porto Maior sedimentary sequence was deposited between 312.6 ± 32.6 ka and 18.8 ± 7.7 ka, and that the in situ large cutting tool assemblage from level PM4 most likely accumulated during Marine Isotope Stage 7 (mean depositional age = 210.7 ± 24.7 ka). The results of this study reinforce the usefulness of applying the multiple centre (MC) ESR dating approach in many dating contexts, and demonstrate the value of undertaking comparisons between semi-independent quartz dating signals in order to maximise the reliability of ESR and single-grain TT-OSL quartz chronologies.

Characterization of the thermally transferred optically stimulated luminescence (TT-OSL) of BeO

The objective of this work was to characterize more in depth the newly discovered thermally transferred optically stimulated luminescence (TT-OSL) of BeO. The studies include a more detailed investigation of the pre-heating temperature and time required to induce the TT-OSL signal; the reproducibility, additivity, and stability of the TT-OSL signal at room temperature and under ambient light exposure; the TT-OSL response to high doses (up to 25 Gy); and the possible recuperation of the OSL signal at room temperature. The results indicate that the TT-OSL is maximum for a pre-heating to 260 °C, if a heating rate of 5 °C/s is used. The TT-OSL decreases with readouts (pre-heating plus OSL readout), indicating depletion of the source of trapped charges. Sensitivity changes are observed in both OSL and TT-OSL signals to the same extend. Furthermore, it was demonstrated that the TT-OSL is additive: the TT-OSL is proportional to the total dose delivered to the detector, whereas the OSL signal is proportional to the dose since the last bleaching. This relationship holds up to ∼5 Gy; above this dose, the TT-OSL starts to saturate. The kinetics of the TT-OSL signal with pre-heating temperature and duration is consistent with a simplified model in which charges from an optically insensitive (or “difficult to bleach”) trap are transferred to an optically sensitive trap, both having similar thermal stabilities with a difference in activation energy of the order of 0.1 eV. The TT-OSL signal is stable in the dark up to the maximum duration investigated here (50 days), decaying slowly under exposure to ambient light. Furthermore, we report evidence of recuperation of the OSL signal at room temperature, which may be caused by a similar process as the one responsible for the TT-OSL signal. The results have obvious implications for practical dosimetry using BeO, including the possibility of re-estimating the doses if the detectors are accidently exposed to light, the possibility to check if the detectors have been exposed to light, the potential recuperation of OSL signal at room temperature and its influence on dosimetry if the detectors have been exposed to high doses, or the potential to use the shape of the OSL curve or the ratio between the OSL and TT-OSL signals for linear energy transfer estimation in ion beam dosimetry.

Investigating the thermal stability of TT-OSL main source trap

Thermally-Transferred Optically Stimulated Luminescence (TT-OSL) from quartz is an extended-range luminescence dating technique, with an assumed potential to date sediments as old as early Pleistocene (0.8–2.6 Ma). However, one of the main drawbacks of the TT-OSL signal is its relatively low thermal stability. The few and scattered estimates in the literature of the relatively low thermal stability highlight the possibility that some reported TT-OSL ages are thermal artefacts (i.e. minimum ages only). In this study, we rigorously investigate the thermal stability of the main TT-OSL source trap, using a combination of laboratory and analytical techniques (varying heating rates, isothermal decay, alongside several models) on multiple aliquots of a modern sand sample from the eastern Mediterranean coastal plain. The varying heating rates method constrains the Arrhenius parameters of the TT-OSL main trap to E = 1.50 ± 0.06 eV and s = 1012.8±0.6 s−1; these values translate into a trap lifetime of 3.2−1.9+4.8 Ma at 10 °C. Isothermal decay data further exhibits significant departures from first-order kinetic behavior, which can be well captured by either the general order kinetics model, or a Gaussian distribution of first-order systems. However, extrapolations of these models to geological timescales are at odds with a large volume of observations, thus suggesting that the deviation from first-order kinetics may be a laboratory artefact. Overall, our study reinforces the concern, that thermal loss progressively affects the TT-OSL signal in the Ma-scale age range, and that some previously reported results may be “minimum ages” only.

Natural saturation of OSL and TT-OSL signals of quartz grains from Nilotic origin

Abstract The upper limit of optically stimulated luminescence (OSL) dating technique is usually defined by signal saturation. In nature the OSL signal saturates even faster than in the laboratory. Here we present a study on natural saturation of OSL and thermally transferred OSL (TT-OSL) signals of quartz grains from Nilotic origin. Quartz samples from two representative sections from the coastal plain of Israel, and which span approximately similar time frames, were used to construct semi-natural dose response curves (DRC) for OSL and TT-OSL by plotting the natural signal of samples against their depth and equivalent dose (De). Samples from these two sections, as well as from other sites, define typical OSL and TT-OSL DRC's for Nilotic quartz that was transported to the eastern shores of the Mediterranean Sea. The constructed semi-natural OSL DRC is unique to the quartz of Nilotic origin and indicates the upper limit for reliable OSL dating of this quartz at 140 Gy due to signal saturation. The natural TT-OSL luminescence grows linearly up to 400 Gy, and is constant for 400–600 Gy. Steady-state between TT-OSL trap-filling, due to ionizing radiation, and concurrent thermal emptying of these traps is achieved at 600 Gy. TT-OSL dating of the Nilotic quartz is expected to underestimate ages over ∼120 ka due to the low thermal stability of the signal.

Single-grain OSL dating of the Middle Palaeolithic site of Galería de las Estatuas, Atapuerca (Burgos, Spain)

This study presents single-grain optically stimulated luminescence (OSL) chronologies for the archaeological site of Galeria de las Estatuas – the first systematically excavated Middle Palaeolithic site within the karst system of the Sierra de Atapuerca archaeological complex, northern Spain. The single-grain OSL ages are compared with paired single-grain thermally transferred OSL (TT-OSL) dating results for a selection of samples in order to better assess quartz signal bleaching characteristics of endokarstic deposits preserved at Atapuerca. In total, seven luminescence dating samples were collected from four lithostratigraphic units exposed in two excavation pits (GE-I and GE-II). The single-grain OSL equivalent dose (De) distributions are characterised by generally low overdispersion (20–30%), suggesting appropriate bleaching at deposition. The resultant single-grain OSL ages reveal that the sediment sequence and archaeological remains excavated in pit GE-I accumulated 80–112 ka, while the upper layers of excavation area GE-II were deposited 70–79 ka. The replicate single-grain TT-OSL ages are in agreement with the OSL chronologies at 2σ for three of the four samples investigated; although in all cases the TT-OSL ages were systematically older than their single-grain counterparts. Apparent TT-OSL residual doses (i.e., TT-OSL De values in excess of their corresponding OSL De values) of 9–65 Gy were observed for all samples. These excess TT-OSL De values are generally low in comparison to the natural dose ranges of TT-OSL dating applications undertaken elsewhere in the Atapuerca karst system. The single-grain TT-OSL and OSL dating comparisons build on daylight bleaching experiments and modern analogue studies performed on other Atapuerca exogeneous infill deposits and suggest reasonable potential for TT-OSL signal resetting down to relatively low levels for at least some sediments preserved in the Atapuerca karstic cavities. The quartz single-grain OSL chronologies obtained in this study place the Middle Palaeolithic sequence of Galeria de las Estatuas within marine isotope stage (MIS) 5 and the beginning of MIS 4, and provide firm evidence for human occupation of the Sierra de Atapuerca during a previously unreported time period.

Single-grain TT-OSL bleaching characteristics: Insights from modern analogues and OSL dating comparisons

Previous assessments of thermally transferred optically stimulated luminescence (TT-OSL) signal resetting in natural sedimentary settings have been based on relatively limited numbers of observations, and have been conducted primarily at the multi-grain scale of equivalent dose (De) analysis. In this study, we undertake a series of single-grain TT-OSL bleaching assessments on nineteen modern and geological dating samples from different sedimentary environments. Daylight bleaching experiments performed over several weeks confirm that single-grain TT-OSL signals are optically reset at relatively slow, and potentially variable, rates. Single-grain TT-OSL residual doses range between 0 and 24 Gy for thirteen modern samples, with > 50% of these samples yielding weighted mean De values of 0 Gy at 2σ. Single-grain OSL and TT-OSL dating comparisons performed on well-bleached and heterogeneously bleached late Pleistocene samples from Kangaroo Island, South Australia, yield consistent replicate age estimates. Our results reveal that (i) single-grain TT-OSL residuals can potentially be reduced down to insignificant levels when compared with the natural dose range of interest for most TT-OSL dating applications; (ii) the slow bleaching properties of TT-OSL signals may not necessarily limit their dating applicability to certain depositional environments; and (iii) non-trivial differences may be observed between single-grain and multi-grain TT-OSL bleaching residuals in some modern samples. Collectively, these findings suggest that single-grain TT-OSL dating may offer advantages over multi-grain TT-OSL dating in certain complex depositional environments.

Evaluating isothermal thermoluminescence and thermally transferred optically stimulated luminescence for dating of Pleistocene sediments in Amazonia

Abstract The paleogeography of Amazonia lowlands during the Pleistocene remains hampered by the lack of reliable absolute ages to constrain sediment deposition in the hundred thousand to few million years timescales. Optically stimulated luminescence (OSL) dating applied to quartz has provided important chronological control for late Quaternary sediments, but the method is limited to the last ∼150 ka. In order to extend the age range of luminescence dating, new signals from quartz have been investigated. This study tested the application of isothermal thermoluminescence (ITL) and thermally transferred optically stimulated luminescence (TT-OSL) signals of quartz for dating of fluvial terraces from eastern Amazonia. ITL and TT-OSL signals measured in a modern fluvial sediment sample have shown small residual doses (4 and 16 Gy), suggesting adequate bleached sediments for the target dose range (>150 Gy). This sample responded well to dose recovery test, which showed that the ITL and TT-OSL signals grow to higher doses compared to the doses estimated by the conventional OSL signal. The ITL signal saturated for doses significantly lower than doses reported in the literature. Most dating samples were beyond the ITL saturation doses and only TT-OSL signals were suitable to estimate equivalent doses. Burial ages ranging from 107 to 340 ka were estimated for the fluvial terraces in the lower Xingu River. The main ages uncertainties are related to dose rate changes through time. Despite the uncertainties, these ages should indicate a higher channel base level during the Middle Pleistocene followed by channel incision, possibly due to episodes of increased precipitation in the Xingu watershed.

Natural and laboratory TT-OSL dose response curves: Testing the lifetime of the TT-OSL signal in nature

This study compares natural and laboratory generated thermally transferred optically stimulated luminescence (TT-OSL) dose response curves (DRCs) for fine-grain quartz extracts from the Luochuan loess section in central China. Both DRCs saturate at high doses relative to the quartz OSL signal; the natural TT-OSL DRC saturates at about 2200 Gy and laboratory DRCs saturate at about 2700 Gy. However, the natural and laboratory TT-OSL DRCs deviate from one another at circa 150 Gy resulting in TT-OSL equivalent dose underestimation relative to palaeodoses expected from dose rates and independent age control. The lifetime of the TT-OSL signal at 10 °C, calculated from values of trap parameters E and s, is compared against the value for lifetime of the TT-OSL signal in nature at average burial temperature as determined from the age underestimation caused by deviation of the natural and laboratory generated DRCs. These two independent assessments of TT-OSL signal lifetime at Luochuan give similar values, suggesting that laboratory measurements of thermal stability reflect natural burial lifetimes and can potentially be used to correct TT-OSL ages for the difference between natural and laboratory dose response curves.

Insights into TT-OSL signal stability from single-grain analyses of known-age deposits at Atapuerca, Spain

Single-grain thermally transferred optically stimulated luminescence (TT-OSL) dating has recently been applied to a number of the Atapuerca palaeoanthropological sites. As yet, however, there have not been any direct assessments of TT-OSL signal stabilities for specific grains used for dating. In this study, we undertake a series of TT-OSL suitability assessments on known-age samples from Gran Dolina and Sima del Elefante. Our results suggest that the Atapuerca samples contain populations or sub-populations of grains with suitably stable TT-OSL signals for dating over late-Early and Middle Pleistocene timescales. Equivalent dose (De) distribution analysis in combination with pulse-annealing assessments provides a useful means of identifying inter-grain and inter-sample differences in TT-OSL signal stabilities. We also show that obtaining De values using different preheat conditions may help to identify potentially problematic TT-OSL behaviours. Analyses of multi-grain aliquot TL curves for these samples reveal that 'bulk' TL signal loss experiments may provide limited insights into TT-OSL source trap lifetimes due to averaging effects, the dominance of grain populations that do not produce TT-OSL, and interference from slowly bleaching OSL components. Our results improve the robustness and precision of existing TT-OSL chronologies for units TD6-3 and TE16-TE17 at Gran Dolina and Sima del Elefante, and support the broader suitability of the single-grain TT-OSL approach at the Atapuerca sites.

Assessing the impact of pulsed-irradiation procedures on the thermally transferred OSL signal in quartz

Thermally transferred optically stimulated luminescence (TT-OSL) dating protocols have been suggested as a means of extending the age range of luminescence dating. Several studies demonstrate that TT-OSL signals increase with large radiation doses (>2000 Gy) and yet, few studies report older TT-OSL ages (>400 Gy) in agreement with independent absolute age control. In one such study, agreement with independent chronology was only achieved for the old samples by implementing a pulsed-irradiation procedure. Pulsed-irradiation is suggested to compensate for dose rate dependent competition effects by dividing the laboratory irradiation into discrete irradiation steps interspersed with heat treatments. However, every inter-step heat treatment has the potential to anneal part of the TT-OSL dating signal. This study compares constant- and pulsed-irradiation TT-OSL protocols and investigates the degree of partial thermal annealing. The results suggest that almost all of the difference in outcome between constant- and pulsed-irradiation protocols can be explained by partial annealing of the TT-OSL signal rather than by competition effects. Partial annealing distorts the laboratory dose response curve but has no impact on the natural signal, resulting in unreliable equivalent dose estimates. This means that pulsed-irradiation procedures may not be viable for TT-OSL dating measurements. Future studies implementing pulsed-irradiation procedures should carefully consider the extent to which inter-step thermal treatments partially anneal the signal.

Evaluating a SAR TT-OSL protocol for dating fine-grained quartz within Late Pleistocene loess deposits in the Missouri and Mississippi river valleys, United States

The Mississippi and Missouri river valleys in the midcontinental United States contain extensive loess-paleosol sequences that are used to constrain the timing of expansion and retreat of the Laurentide Ice Sheet. Previous studies have been unsuccessful in producing finite ages for sediments older than ∼150 ka due to saturation of luminescence emissions. The thermally transferred optically stimulated luminescence (TT-OSL) dating technique is tested on the fine-grained (4–11 μm) quartz fraction of these loess deposits because the TT-OSL signal has been shown to grow with high (kGy) radiation doses. The TT-OSL signal continued to increase with radiation dose above 900 Gy. The optical and thermal stabilities of this TT-OSL signal are evaluated. Equivalent dose values are highly sensitive to preheat temperatures. Recycling ratios, zero-dose response values, and dose recovery tests all yield acceptable values for samples with burial doses >∼200 Gy. The apparent TT-OSL ages for the Roxana Silt (∼52–63 ka), Teneriffe Silt (∼66 ka), and Loveland Silt (∼133–192 ka) agree at 1σ level with previously published TL and IRSL ages derived from the same samples. For the oldest unit, the Crowley's Ridge Silt, TT-OSL ages (∼167–200 ka) are younger than IRSL or TL ages by ∼20%. This is interpreted as underestimation related to TT-OSL signal contamination, which can be avoided by isolating the fast component of the TT-OSL. Preliminary fast component TT-OSL ages for the Crowley's Ridge Silt (∼197–241 ka) favor deposition during marine oxygen-isotope stage (MIS) 7 or 8, contrary to a previous inference of a MIS 12 deposition.

Dosimetry in the multi kilo-Gray range using optically-stimulated luminescence (OSL) and thermally-transferred OSL from quartz

Abstract This study explores the potential for using photon-stimulated luminescence of quartz grains to retrospectively evaluate multi-kGy gamma radiation doses. Subsamples from two ceramic tiles were given 60 Co gamma doses of 1, 3, 5, 15 and 30 kGy (nominal), accompanied by PMMA dosimeters and quartz grains from a geological sample known to exhibit thermally-transferred optically-stimulated luminescence (TTOSL). Following gamma irradiation, quartz grains were prepared from each subsample. OSL and TTOSL signals were measured for different preheat temperatures, and following re-irradiation with beta doses designed to equal the nominal gamma doses. OSL responses to 10 Gy beta test doses were measured following each cycle of high dose irradiation. Gamma doses were predicted from the beta responses, to evaluate the effectiveness of retrospective dose evaluation for different signal integrals, preheat combinations, and dose intervals. The use of linear modulation limited maximum OSL signal levels to 1.5 × 10 6 cps without reducing detector sensitivity, for the measurement of smaller TTOSL signals. The dosimetric behaviour of the three samples differed significantly: the best results were obtained from the pre-prepared geological quartz. OSL signals evident at short stimulation times, which are conventionally used for dosimetry and dating, did not in general appear appropriate for dosimetry in the range of doses examined. They exhibited dose response characteristics that were highly preheat dependant and variable in form between samples, which contained rapidly saturating and/or non-monotonic components. Higher preheats, up to 300 °C, preferentially removed OSL and TTOSL signals evident at longer stimulation times: those that remained yielded growth in signal with dose to approximately 10 kGy, but the accuracy of retrospective dose determinations was variable. TTOSL signals evident at short stimulation times increased proportionally with dose when 300 °C preheats were used, up to 30 kGy of beta irradiation for two of the samples, but the given gamma doses were underestimated by 15–23%. Refinement of the present measurement protocol may improve assessment of the gamma doses, and the short term stability of the signal needs to be characterised, but the observed level of accuracy is already sufficient to provide approximate estimates for severity of radiation exposure. The present study has identified certain elements of OSL and TTOSL signals that can be used, in combination with a relatively severe preheating regime, to retrospectively evaluate doses of tens of kGy: beyond the range traditionally possible using quartz.

Improving the TT-OSL SAR protocol through source trap characterisation

Abstract Thermally-transferred optically stimulated luminescence (TT-OSL) extends the age range of OSL dating using quartz. A set of experiments have been undertaken to determine the kinetic parameters of the TT-OSL source traps, and this information has been used to propose an improved TT-OSL single aliquot regenerative (SAR) dose protocol. By combining together OSL and thermoluminescence (TL) measurements on fine-grained quartz, a correspondence between TL peaks and the TT-OSL signal is found. The thermal stability of the main TT-OSL trap was estimated by applying Hoogenstraaten’s method and allowing for thermal quenching; this predicts a lifetime of 4.5 Ma at 10 °C. A set of experiments were undertaken to refine the treatment needed at the end of each SAR cycle to erase the previously acquired TT-OSL signal. An improved TT-OSL SAR protocol using this treatment is proposed, and it is tested on quartz from a young Holocene sample. These tests yielded excellent recycling ratios and excellent dose recovery.

Re-evaluation of the chronology of the palaeolithic site at Jeongokri, Korea, using OSL and TT-OSL signals from quartz

Seven samples were collected from sediments at the Palaeolithic site of Jeongokri, Korea, known for the first discovery of Acheulian-like handaxes in East Asia. Optically stimulated luminescence (OSL) ages obtained from chemically purified quartz from the upper five samples are in stratigraphic order, but the ages of the lower samples are not. The OSL signal of the oldest samples occurs well above the saturation level of the exponential part of the dose response curve; however, the OSL signal has an additional component that grows linearly at high doses, making it possible to calculate a D e value and thus an age. However the reliability of calculating D e using this linear component is not clear. In contrast, the thermally transferred OSL (TT-OSL) signal grows linearly with dose up to at least 1600 Gy, thus permitting its use for dating. The TT-OSL ages are in agreement with those from the OSL for the four uppermost samples, but give older ages for the lower three samples. Ages obtained using TT-OSL are in stratigraphic order and indicate that the oldest artefacts have an age of ∼195 ka.

The mechanism of thermally transferred optically stimulated luminescence in quartz

In this paper we assess two mechanisms for the production of the thermally transferred optically stimulated luminescence (TT-OSL) signal in quartz when all stimulation is carried out at 125 °C. One mechanism is based on the double transfer process previously put forward for OSL recuperation following storage at room temperature. In this mechanism, electrons from the trap giving rise to the fast OSL component are released by optical stimulation and some are transferred into a refuge trap via the conduction band; these electrons are then released from the refuge trap by a thermal treatment and some are retrapped in the trap responsible for the fast OSL component. The other mechanism is based on a single transfer process in which electrons are transferred by thermal treatment from a light-sensitive trap via the conduction band to the trap that gives rise to the fast component of the OSL signal, this trap having been emptied by the initial optical stimulation. The analysis of the measured OSL and TT-OSL decay curves suggest that the two signals are derived from the same traps and are dominated by the fast OSL component. The experimental data presented support the hypothesis that the original source of the electrons that are thermally transferred is a trap that is less light sensitive than that giving rise to the fast OSL, but one that has similar thermal stability. Thus, a single transfer mechanism explains the production of the TT-OSL signal observed for stimulation above 120 °C when the 110 °C TL trap is kept empty.

Characteristics of thermally transferred optically stimulated luminescence (TT-OSL) in quartz and its potential for dating sediments

The temperature dependence, dose response and bleaching characteristics of the thermally transferred optically stimulated luminescence (TT-OSL) of sedimentary quartz were studied, in order to assess the potential of the TT-OSL signal for dating. The TT-OSL was separated into two components; recuperated OSL (ReOSL) and basic transfer (BT-OSL) by annealing samples at 300 °C for 10 s as suggested in an earlier study. Four quartz extracts were studied, two from loess from China and two from coastal sands from South Africa. The equivalent doses of the two recent samples (one sand and one loess) were ∼ 15 Gy and this suggests that the signal can be bleached by sunlight but may not be totally zeroed. The sensitivity-corrected ReOSL from the older samples did not reach zero and gave doses of 14 and 52 Gy, respectively, after 7 days bleaching with a solar simulator. A single aliquot regenerative dose (SAR) protocol using ReOSL was proposed and tested. In this protocol, a blue light stimulation at 280 °C for 100 s at the end of each cycle resulted in the recovery of identical sensitivity-corrected ReOSL values, in spite of ∼ 20 – 30 % loss in sensitivity for the four samples that were tested. Two dose response curves were constructed using the sensitivity-corrected ReOSL, one for the initial 2 s signal and the other for the fast component obtained by curve fitting. Using the additional high temperature bleach and the separated fast component of the ReOSL, it was possible to recover given doses within 10%, up to ∼ 1000 Gy for the loess and ∼ 2000 Gy for the coarse grained quartz. However, the natural dose obtained for the older sand was twice that obtained using the conventional SAR OSL method.

Thermally transferred luminescence in fine-grained quartz from Chinese loess: Basic observations

Results of a series of experiments designed to investigate the properties of thermally transferred optically stimulated luminescence (TT-OSL) in fine-grained quartz from Chinese loess are reported. This signal is confirmed as having two sources, recuperation and basic transfer, as proposed by earlier workers. The combined TT-OSL signal is shown to be due to electrons being ejected from electron traps that are very similar to the main OSL traps, and may indeed be identical to them. However, the dose dependences of the main OSL signal and the recuperated TT-OSL signal behave in a different way, with the TT-OSL signal showing slower onset of saturation. This behavior is interpreted in terms of the OSL production being limited by the number of available luminescence centres, rather than electron traps. This opens up the possibility of a new OSL dating procedure capable of extending the age range by almost an order of magnitude. The thermal and optical stabilities of the two sources of electrons that give rise to the TT-OSL signal are investigated. The basic TT-OSL signal is the result of the direct thermal transfer of electrons from a trap that is thermally deeper than the main OSL trap and which is optically insensitive. The recuperated TT-OSL is directly derived from an optically insensitive refuge trap which trapped electrons during the initial optical stimulation following irradiation. These two signals can be separated easily by a sequence of repeated thermal and optical treatments (or a single isothermal treatment). It is found that the recuperated TT-OSL signal is the major contributor to the TT-OSL signal when these procedures are carried out at 260 ∘ C for the fine-grained quartz studied here.