Andrew S. Murray

Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol

Single aliquot protocols are now widely used as a means of measuring the equivalent dose (De) in quartz and feldspar optical stimulated luminescence (OSL) dating of both heated and sedimentary materials. The most recent of these is the single-aliquot regenerative-dose (SAR) protocol, first suggested by Murray and Roberts (Radiation Measurements 29, 503–515, 1998). In this approach, each natural or regenerated dose OSL measurement is corrected for changes in sensitivity using the OSL response to a subsequent test dose (10–20% of De). If the sensitivity correction is adequate, then the corrected OSL response should be independent of prior dose and thermal/optical treatment, i.e. there should be no change in the sensitivity-corrected dose–response curve on remeasurement. Here we examine the interpretation of the sensitivity corrected growth curve as a function of dose, and the effect of changing measurement conditions (e.g. preheat temperature, size of test dose, stimulation temperature) on the estimation of De. The dependence of the dose response on prior treatment is tested explicitly, and the significance of thermal transfer discussed. It is concluded that a robust SAR protocol is now available for quartz, and that it is applicable to a wide range of heated and unheated materials.

The single aliquot regenerative dose protocol: potential for improvements in reliability

Abstract This paper examines the effects of optically stimulated luminescence (OSL) components, other than that usually termed the fast component, on dose determination by the single-aliquot regenerative-dose (SAR) protocol. Results are presented for “dose recovery tests”, in which a known laboratory dose, delivered after optical bleaching at room temperature, is measured using the SAR protocol. Data obtained using either the initial OSL signal or the fast component, derived by curve fitting, are compared. Dose recovery tests are also carried out when an additional step is added to the SAR protocol, aimed at reducing recuperation, i.e. the residual signal observed in a SAR cycle when no regenerative dose is applied. The results for quartz from various sources indicate much improved dose recovery when only a well-separated fast component is analysed.

Developments in radiation, stimulation and observation facilities in luminescence measurements

Abstract This paper summarises recent developments in the Riso TL/OSL reader. These new developments include an elevated temperature irradiation facility that has been expanded to measure variable temperature radioluminescence (RL) during beta irradiation. This RL facility allows the measurement of TL, OSL and RL in the same software-controlled automatic sequence. To provide an alternative to the 90 Sr beta source, the performance of a software-controlled mini X-ray generator has been investigated. This has the advantage of providing a range of dose rates spanning more than three orders of magnitude up to more than 2 Gy / s . A separate bleaching facility has also been added to the reader, new powerful infrared (IR) light-emitting diodes have replaced the IR laser diode in the general OSL unit, and an IR laser has been included in the single-grain OSL attachment. This paper describes these new facilities, and illustrates their applications with representative measurements.

The effects of disequilibria in the uranium and thorium decay chains on burial dose rates in fluvial sediments

Abstract Buried sediments receive about 53% of their annual dose of ionising radiation from radionuclides in the uranium and thorium decay chains. In luminescence dating of sediment samples, it is usually assumed that the dose rate does not change over the period of burial, implying that the uranium and thorium decay series are in secular equilibrium. For the 232 Th decay chain there is little literature available on the equilibrium conditions in sediments, but given the short half-lives of the longer-lived daughters in the series, 228 Ra (5.75 years) and 228 Th (1.91 years), the decay chain is expected to be in secular equilibrium in most natural materials. However, for the 238 U decay chain, disequilibrium is commonplace in the surficial environment and the half-lives of several members of this decay chain ( 234 U, 230 Th, 226 Ra) are sufficiently long that any disequilibrium, once established, may persist for millennia. In these circumstances, the dose rate will vary with time unless the decay rate is matched by the transport and deposition of the relevant (unsupported) nuclide. We present data from a variety of fluvial and lacustrine depositional environments, and demonstrate that disequilibria is common in these Australian surficial sediments. The origins of the disequilibria and their likely evolution in time are discussed. The effect on the dose rate is assessed and, in the majority of cases, is found to be comparable with other luminescence dating uncertainties of typically 5–10%.

Equivalent dose estimation using a single aliquot of polymineral fine grains

We have tested the suitability of a new single-aliquot regenerative-dose protocol for estimating the equivalent dose (De) in polymineral fine grains extracted from colluvia from various sites in Germany. First, we report the behaviour of three OSL signals: (i) blue-stimulated, (ii) infrared-stimulated luminescence, and (iii) blue-stimulated luminescence following infrared (IR) stimulation, using a near-UV (290–380 nm) detection window in each case. For these three signals, there is a significant change in sensitivity with regeneration cycle; this change can be compensated for using the response to a fixed test dose after each natural or regenerated measurement. The source of the three luminescence signals is then investigated using pulse-anneal and elevated-temperature experiments. Fading tests on laboratory-induced signals show that although the IR signals fade by up to 23% in 15 days at 100°C, the post-IR blue signals are stable. The preheat dependence of estimates of De obtained using fine grains is presented for the first time, for both blue- and IR-derived signals. Our results are compared with De estimates derived from multiple-aliquot additive-dose IR luminescence data, obtained using a blue detection window, and also with expected values of De based on independent age estimates and measured dose rates. We conclude that post-IR blue-stimulated luminescence provides reliable estimates of De, and that these are probably superior to the IRSL estimates obtained using both near-UV and blue detection windows.

Characterisation of blue-light stimulated luminescence components in different quartz samples: implications for dose measurement

Abstract Over the last few years, we have become increasingly confident that quartz is a reliable natural dosimeter for sediment dating. Nevertheless, there is only a limited understanding of the behaviour of the different components of optically stimulated luminescence (OSL) from quartz. Recent single-aliquot dose-evaluation protocols seem to be relatively free of complications when applied to quartz dominated by the fast OSL component coming from 325°C TL region, but this may not be true for quartz in which other components are more significant. An adequate understanding of how different OSL components behave during various measurement cycles is critical to ensuring that our dose evaluation protocols are robust and it is also important to our interpretation of the variation of apparent dose with optical stimulation time for identification of partial bleaching. We report here the principal results from measurement of (a) sensitisation, (b) thermal stability, (c) recuperation, and (d) infrared response as a function of stimulation temperature from 3 different samples of sedimentary quartz selected on the basis of relative OSL contribution from different blue light stimulated linearly modulated (LM-OSL) components. We then discuss the implications of these characteristics for dose assessment using the single aliquot regeneration dose protocol.

The human colonisation of Australia: optical dates of 53,000 and 60,000 years bracket human arrival at Deaf Adder Gorge, Northern Territory

The date at which people entered Australia has important implications for the debate on modern human origins. Thermoluminescence dates of 50–60 ka, reported for initial occupation of the Malakunanja II site in northern Australia, have been used as a means of calibrating the rate of mitochondrial and nuclear DNA evolution in modern humans. Optical dating of unburnt quartzose sediments from a rock shelter site (Nauwalabila I, Lindner Site) in Deaf Adder Gorge, 70 km south of Malakunanja II, provides new evidence for the timing of the colonisation of the continent. Optical dates were determined for several stratigraphic levels within a 3 m deep excavation, in which flaked stone artefacts and ground pigments were found in primary depositional setting. The lowest human occupation levels are bracketed by dates of 53.4 ± 5.4 ka and 60.3 ± 6.7 ka, while the upper levels show good agreement between optical and calibrated 14C age estimates. High-quality haematite with ground facets and striations is associated directly with the 53 ka level and indicates the use of pigments by these early Australians. The optical dates independently confirm evidence for the colonisation of northern Australia shortly after 60 ka and should be seen in the context of this region as having been a likely entry route for the first human movements into Sahul.

The single-aliquot regenerative-dose (SAR) protocol applied to coarse-grain feldspar

The application of the single-aliquot regenerative-dose (SAR) protocol to equivalent dose determination in coarse-grain feldspar is investigated. An outline of the protocol is presented, and basic tests of its performance are discussed. It is shown that the method is robust and yields results similar to those obtained with a single-aliquot additive-dose (SAAD) procedure. The use of the SAR procedure for fading tests is also discussed. We conclude that the SAR protocol for coarse-grain feldspar yields several advantages over previously used multiple-aliquot and single-aliquot methods. The method is fast and precise, the equivalent dose is obtained by interpolation, and the method corrects for sensitivity changes. The luminescence age obtained by SAR and additive-dose dating of the coarse-grain feldspar separates used in this study do, however, underestimate the independently estimated geological age of the samples.

Determining the burial time of single grains of quartz using optically stimulated luminescence

Abstract In the optical dating of sediments it is usually assumed that the optically stimulated luminescence (OSL) signal has been completely reset by light exposure prior to burial; this assumption is often not valid. One approach to testing, and perhaps circumventing, this assumption is to examine the apparent date of last exposure to daylight of individual sediment grains. This paper reports on the application, for the first time, of 2 new measurement protocols to the estimation of the radiation dose received during burial for individual quartz grains from an aeolian deposit of known age (10,000 year old), which is considered likely to have been completely reset by sunlight at deposition. Additive-dose (laboratory doses added to the burial dose before OSL measurement) and regenerative-dose (doses added after measurement of OSL from burial dose) single-aliquot protocols are applied to 28 and 25 individual grains, respectively; each grain provides an independent estimate (D e ) of the burial dose. The average D e from the additive-dose protocol (21.8±1.1 Gy) is in good agreement with the average from the regenerative-dose protocol (23.8±1.0 Gy). Both agree well with: (1) 13 multiple-grain regenerative-dose single-aliquot measurements, each on 1 mg sub-samples, of 23.9±0.3 Gy; (2) 9 multiple-grain additive-dose single-aliquot measurements, also on 1 mg sub-samples, of 22.4±0.7 Gy; and (3) one previously published multiple-aliquot additive-dose estimate of 23.5±0.6 Gy using 52 sub-samples, each of 5 mg. The resulting optical ages are in good accord with 14 C and thermoluminescence age determinations. The distribution of equivalent doses in the single grains is, however, unexpectedly large ( σ ≈23% of mean D e ), given the very likely complete resetting of the OSL signal at deposition. Possible reasons are discussed, and it is concluded that heterogeneity in beta dosimetry is the most likely explanation. The single-grain optical dating protocols reported here allow a detailed examination of the dose distribution in very small samples. Thus, they should enable accurate dates to be obtained for sediments and soils that contain poorly bleached or mixed-age components, as well as deposits in which quartz grains are present in extremely low abundance.

Optical dating of single sand-sized grains of quartz: sources of variability

Optically stimulated luminescence (OSL) measurements have been made of over 3000 sand-sized grains of quartz. Analysis at this scale highlights the variability in the luminescence sensitivity and the dose saturation characteristics of individual quartz grains. Using a new instrument capable of measuring single grains it is feasible to routinely measure the equivalent dose from many hundreds of grains from each sample. Analysis of such datasets requires assessment of the uncertainties on each equivalent dose since these may vary significantly. This paper assesses the significance of signal intensity, dose saturation characteristics and instrument uncertainty in equivalent dose calculation.