Equivalent dose measurement using a single aliquot of quartz

Radiation Measurements, Vol. 27, No. 2, pp. 171-184, 1997

Pergamon

© 1997 ElsevierScience Ltd. All rights reserved Printed in Great Britain PII: S1350..4487(96)00130-8 1350-4487/97 $17.00 + 0.00

EQUIVALENT DOSE MEASUREMENT USING A SINGLE ALIQUOT OF QUARTZ A. S. MURRAY'*, R. G. ROBERTS:t and A. G. WINTLE$ *CSIRO Division of Water Resources, P.O. Box 1666, Canberra, ACT 2601, Australia, tDivision of Archaeology and Natural History, Research School of Pacific and Asian Studies, The Australian National University, Canberra, ACT 0200, Australia and :~Institute of Earth Studies, University of Wales, Aberystwyth, SY23 3DB, U.K. Abstract--The principles behind an additive-dose single-aliquot protocol and the need for such a protocol are outlined. It is shown for two Australian sedimentary quartz samples that the decay of OSL during a repeated measurement cycle, consisting of a 10 s preheat at a given temperature followed by a 0.1 s exposure to green light at 110°C, can be well represented by exponential decay. The decay constant of about 8% per cycle is insensitive to preheat temperatures below about 250°C, but increases to about 35% per cycle at 280°C. This increase is attributed to increasing thermal erosion of the main OSL trap. The decay constant is also shown to be insensitive to dose. An additive-dose protocol is presented which requires only one aliquot for the estimation of the equivalent dose (Dr). This protocol has been applied to quartz from 11 Australian sites. To illustrate the value of the single-aliquot approach, the apparent values of Dr for 14 samples, containing doses of between 0.01 and 100 Gy, have been measured in triplicate at preheat temperatures of between 150 and 300°C, using a single aliquot for each De measurement. It is shown that low temperature preheats ( ~ 200°C for 10 s) are appropriate for the younger samples containing the smaller doses, but a much higher temperature preheat (>280°C for 10 s) is required for the older samples containing the larger doses. Excellent agreement is found between these single-aliquot estimates of D° and those from additive-dose multiple-aliquot protocols, over the entire dose range. © 1997 Elsevier Science Ltd

1. I N T R O D U C T I O N Optical dating of quartz was first proposed by Huntley et al. (1985) as the natural extension of thermoluminescence (TL) procedures which had been developed for dating sedimentary materials (Wintle and Huntley, 1980). These TL procedures involved the measurement of the thermoluminescence from individual aliquots of sediment which had been subjected to different laboratory irradiation and optical bleaching regimes prior to measurement. The measurement of the TL signal of the mineral grains involved heating the grains to 500°C, thereby removing all information concerning the trapped electron population giving rise to the TL signal. Hence, TL dating procedures involved the preparation and measurement of several tens of identical sub-samples (aliquots). This multiple-aliquot approach has been extended to optical dating methods, although Huntley et al. (1985) pointed out that the equivalent dose (Do) should be able to be determined on a single aliquot of material, since it is possible to obtain an optically stimulated luminescence (OSL) signal using a brief light exposure. For quartz, several procedures have been used for the estimation of D0, all requiring a theoretical

minimum of at least two aliquots (see review by Wintle, 1993). In practice, usually tens of sub-samples with identical characteristics are required for a single, precise estimate of the Dr. This can be a practical limitation, either because of a shortage of sample or instrument time, or it can be more fundamental if "representative" aliquots are not available, i.e. identical sub-samples cannot be taken because of inherent heterogeneity in the material to be dated. Such heterogeneity in both feldspars and quartz has been reported by several authors (e.g. Li, 1994; Lamothe et al., 1994; Rhodes and Pownall, 1994; Murray et al., 1995), and it is likely to be c o m m o n in fluvially deposited material (Perkins and Rhodes, 1994). Li (1994) has pointed out the value of using single-aliquot measurements on feldspars to identify such heterogeneous materials. Thus there is both a practical and a theoretical need for a single-aliquot protocol, where all measurements needed to provide an estimate of De are made on only one sub-sample. OSL methods measure some proportion of the trapped electron population and can be expected to produce less alteration in the sample as a result of measurement than TL. It is likely that, before final burial, wind-blown or water-lain quartz grains have

'Present address: The Nordic Laboratory for Luminescence Dating, Riso National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark. 2Present address: School of Earth Sciences, La Trobe University, Melbourne, VIC 3083, Australia. 171

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been illuminated (by sunlight) many times since given a laboratory dose prior to the determination of crystallisation, whereas such sediments will not have the decay curve shape. Although this proposal was been heated significantly since the crystal formed. not tested, a related procedure was put forward by OSL is thus likely to be better suited than TL to Galloway (1996), in a study of the IRSL of feldspars. repeated measurement on the same sample. He was able to represent the decay of IRSL with the The principle behind an additive-dose single-ali- number of measurement cycles (n) by the function quot OSL protocol is to sample the trapped electron f(n) = I - a In(n), where a is a constant. He used population using a brief light exposure, give a repeat measurements on the same feldspar aliquot, laboratory dose, and sample the population again. after the completion of the additive-dose growth Repeating this process defines an additive-dose curve, to obtain the correction curve appropriate to growth curve; the intercept on the dose axis is D0. If that aliquot. the light exposure used to sample the trapped electron There have been a few attempts to apply the Duller population is sufficiently small, no correction for (1991) protocols to quartz. Galloway (1994) used a population depletion will be necessary. Smith et al. l0 s green light-emitting diode (565 nm, (1986) were the first to suggest that such a small light 0.2 mW cm 2) exposure and a preheat of l min at exposure can be repeated after additional doses to 200~C. He tested the additive-dose feldspar protocol generate an additive-dose growth curve. However, on a heated quartz, by comparing the shape of a they did not discuss the effect of heating the sample single-aliquot additive-dose growth curve with that between successive laboratory doses (needed to from a multiple-aliquot protocol. He considered the redistribute unstable charge), nor did they attempt agreement acceptable, but did not attempt to measure any Dc estimates. A practical additive-dose single-ali- any unknown doses. Stokes (1994) also applied the quot procedure is likely to produce a significant single-aliquot additive-dose feldspar protocol to reduction in OSL with each illumination/dose/beat quartz, using a 514.5 mn laser exposure of cycle for two reasons. First, it may be necessary to 0.25 mJ cm 2 which reduced the overall OSL signal stimulate a significant fraction of the trapped electron by less than 1%, and a preheat of 16 h at 160°C. The population to obtain light levels large enough to be quartz sample came from near Derby, in northern reproducible. Second, the heating of samples after Western Australia, and he obtained De estimates addition of any dose will also reduce the OSL signal; consistent with those from multiple-aliquot additivethis so-called "preheating" is required to ensure that dose procedures. Liritzis et al. (1994) employed the any charge redistribution that has occurred during additive-dose single-aliquot protocol to estimate the the period of burial is reproduced in the samples D,. of quartz extracted from pottery, using the given a laboratory dose. These preheats may have to Galloway (1994) green light-emitting diode array for be at a sufficiently high temperature to partly stimulation. Duller (1995) has remarked that this thermally erode the main OSL trap, which is emptied represents one of the few descriptions in the literature at about 32Y'C at a heating rate of 5 K s ' (as stated that takes advantage of one of the principal benefits by Spooner et al., 1988) or 20 K s ~ (as stated by of a single-aliquot protocol, i.e. the requirement for Kaylor et al., 1995). small sample mass. Significant loss of charge is not necessarily a Apart from Galloway's study on feldspar (Galproblem provided the loss as a function of preheat loway, 1996), all these applications of the existing and/or stimulation is known and can be corrected for. "single-aliquot" protocol to either quartz or feldspar In the simplest case, if the loss is a constant fraction have in fact used at least two aliquots, a point which of any population present immediately before is not always emphasised in the published accounts. stimulation, the decay is exponential; any OSL Lamothe et al. (1994) used the single-aliquot protocol measurement can then be corrected for the depletion on single 500-1000 ktm diameter feldspar grains. resulting from an arbitrary number of preheat/stimu- They required an estimate of the shape of the IRSL lation cycles. Duller (1991) was the first to propose a depletion curve which they obtained from a second workable "single-aliquot" protocol, which he devel- sample, despite their conclusion that individual grains oped for feldspars. Unfortunately, he found that the had different bleaching and anomalous fading signal loss was not exponential in his feldspar histories. They did not comment on the reproducibilsamples; the infrared stimulated luminescence (IRSL) ity of the shape of the depletion curve from grain to response to each dose required its own correction. He grain. showed that this could be obtained either from a Duller (1991) also investigated a regenerationsecond natural aliquot, or from an aliquot which had based single-aliquot protocol for feldspar, and Stokes been optically bleached and then given a laboratory (1994) and Mejdahi and Botter-Jensen (1994) have dose before the preheat and measurement cycles. attempted to apply this to quartz. Both concluded These decay curves could then be applied to the that sensitivity changes precluded such an approach; additive-dose curve for another aliquot. In addition, the same conclusion was reached for feldspars by the similarity of the two data sets led him to suggest Duller (1991). Because of this, Mejdahl and a true single-aliquot approach in which, following the Botter-Jensen (1994) developed a new Single-Aliquot/ additive-dose sequence, the sample is bleached and Regeneration and Added dose (SARA) protocol for

EQUIVALENT DOSE M E A S U R E M E N T heated materials which took account of these sensitivity changes, and Murray (1996) extended this to sedimentary materials. The SARA protocol requires a minimum of two aliquots. Tso and Li (1994) have also shown how a regenerative single-aliquot protocol can be used, if three aliquots are available, to bracket the true Do in fine grains extracted from Chinese pottery. This paper examines the suitability of Australian sedimentary quartz for estimation of the equivalent dose using an additive-dose single-aliquot protocol, where all measurements are made on the same sub-sample. The dependence of the shape of the pulse stimulation curve (i.e. an OSL decay curve generated by repeated stimulation using a brief light exposure) on preheat temperature and added dose is examined. A single-aliquot protocol is applied to several samples and the results compared with multiple-aliquot De estimates.

2. EXPERIMENTAL PROCEDURES All the single-aliquot experiments reported here, and the multiple-aliquot measurements on samples D S I - 4 and WIDG8 (see Section 3), were carried out using two automated Riso TL/OSL readers (BotterJensen and Duller, 1992). The stimulating light source is a tungsten-halogen lamp filtered from 420 to approx. 550 nm using a GG-420 filter in combination with an interference filter to reduce the scattered light reaching the Thorn-EMI 9235QA photomultiplier tube; this delivers about 13 m W c m -2 to the sample (H. Christiansen, personal communication, 1995). The OSL is detected through an HA-3 and two 3 mm thick U-340 filters. Experiments were run using Ris~ software, version 4.65. This is particularly important for the work described here, because software prior to version 4.64 added an optical stimulation of about 0.4 s to that programmed by the user (H. Christiansen, personal communication, 1995); e.g. if an exposure of 0.1 s was entered, an actual exposure of 0.5 s was given. This additional exposure has been removed in software versions 4.64 and 4.65. The readers are also equipped with 9°Srfl°Y beta sources delivering 0.022 and 0.027 Gy s -~ to quartz in nickel cups. The dose rates to stainless-steel discs are about 7% higher. The multiple-aliquot analyses performed on samples ABH54, NEST6/3, NEST6/7, NEST4/2, TSS4, NC12/13 and C U D D I E i employed light from a tungsten-halogen lamp source filtered through a 500 4-40 nm interference filter, delivering about 5 mW cm-2 to the sample, for optical stimulation. Luminescence was detected through 2.5 mm thick U-340 and 3 mm thick U G - l l filters, using a Thorn-EMI 9235 QA photomultiplier tube, and Elsec 9010 unit and software. The multiple-aliquot analysis performed on sample

173

ACIS0 employed the 514.5nm line from an argon-ion laser for optical stimulation (Smith et al., 1986), delivering ~ 12.5 mW cm -2 to the sample. Luminescence was detected through Coming 7-51 and Schott BG-39 filters using a Thorn-EMI 9635Q photomultiplier tube, and modified Elsec 9010 unit and software (Roberts et al., 1994a). All sample processing was undertaken in subdued red ( > 590 nm) light. Samples were treated with hydrogen peroxide (where necessary, to remove organic material) and hydrochloric acid to remove carbonates. All samples other than the DS series were then treated with fluorosilicic acid and fluoboric acid to remove feldspars and micas. For all samples, heavy minerals were removed by density separation using solutions of sodium polytungstate. The residual quartz grains of 90-125 ~tm diameter were isolated by dry sieving and finally etched in 40% hydrofluoric acid for 45 min. Samples were either mounted on 10 mm diameter stainless-steel discs with a silicon oil spray, or placed loose in shallow nickel cups of 10 mm diameter.

3. S A M P L E DESCRIPTIONS Sample BR94001 was collected from a modem beach deposit at Tathra, New South Wales. It is believed to be < 20 years old, and Murray et al. (1995) measured a De of 0.011 + 0.004 Gy. (Note that the laboratory dose rate used in that paper has since been revised, and so the De reported there must be multiplied by 0.62.) Samples DS1, 2, 3 and 4 were collected from a coastal dune system at Cooloola, on the southern Queensland coast. DSI is known by direct observation to be < 5 years old. Total-bleach TL dates are available (Tejan-Kella et al., 1990), although not at identical sites. Sample AC150 was obtained from the Allen's Cave archaeological excavation on the semi-arid Nullarbor Plain, South Australia. Stratigraphic details of the site and the close correspondence between calibrated T4C ages and multiple-aliquot OSL and TL age estimates ( ~ 10 ka) are presented elsewhere (Roberts et al., 1994a, 1996a). Sample NC12/13 was collected from Ngarrabullgan Cave in Cape York, northern Queensland. Stratigraphic details of this rock shelter deposit are given by David (1993) and the multiple-aliquot OSL age estimate ( ~ 34 ka) is confirmed by 20 AMS ~4C determinations on individual pieces of charcoal from the same stratigraphic layer (David et al., 1997). Four samples were obtained from the Kimberley region of northern Western Australia: WIDG8 was collected from the base of the Widgingarri 1 archaeological site, and was stratigraphically overlain by a charcoal sample dated by ~4C to ~ 28 ka B.P. (Veth, 1995); and NEST6/3, NEST6/7 and NEST 4/2 are the inner portions of mud-wasp nests removed

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from the ceiling of a painted rock shelter, as discussed in detail by Roberts et al. (1996b). Sample ABH54 was collected from a sand dune in the northern Simpson Desert (25'30' S, 136~25' E), from a depth of about 75 cm. Details of this and other sand dune sites and grain surface features are given in Pell and Chivas (1995). Samples TSS4 and CUDDIEI were obtained from the archaeological and megafaunal sites of Tambar Springs (Wright, 1986), and Cuddie Springs (Dodson et al., 1993) in northern New South Wales. At Tambar Springs, the multiple-aliquot OSL age ( ~ 5 ka) is confirmed by '4C age determinations on charcoal, while the multiple-aliquot optical date of 36 ka for the Cuddie Springs sample is consistent with eight '4C ages of 29-34 ka B.P. on charcoal from the same excavation unit. To illustrate the TL characteristics of two of these materials, the TL curves before and after optical stimulation have been measured for samples DS2 and ACI50. Similar curves for sample WIDG8 are given in Wintle and Murray (1997), and some OSL characteristics of sample ACI50 have also been published previously (Roberts et al., 1994a). Figure 1 a presents the TL curve for (i) a natural sample of o

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Fig. I. (a) Dashed/dotted line--TL glow curve obtained for a natural aliquot of sample DS2 (De = 2.7 Gy) (right-hand axis). Dashed line--TL glow curve observed after adding a beta dose of 20 Gy to a natural aliquot, and then heating at II0°C for 10 s (left-hand axis). Solid line~ifference between the latter TL glow curve, and one which was subsequently given 25 s of green-light exposure at room temperature (22°C) (right-hand axis). (b) Sample AC150, presented as in (a), except that the difference TL curve should be referred to the left-hand axis. The Dr is 23 Gy, and the added dose was 43 Gy.

DS2 (De = 2.7 Gy) (right-hand axis), and (ii) a natural sample after a laboratory added dose of 20 Gy and a preheat of I10°C for 10 s (left-hand axis). Each curve is the average of three aliquots. Various peaks have been regenerated in the added-dose glow curve, especially at about 160 and 220°C, in addition to those seen in the natural glow curve, between 280 and 375°C. The lower temperature TL peaks are thermally unstable over time-scales of a few hours to a few millennia. As they decay, some of the electrons entering the conduction band will be retrapped by the main OSL trap at about 325°C. Any laboratory dose must therefore be followed by a preheat to eject charge from shallow traps and redistribute the appropriate fraction to the deep OSL trap, thus simulating the redistribution of charge that occurs during the period of sample burial. The TL curve for the light-sensitive traps only is also shown in Fig. l(a). This was obtained by measuring the TL from a third group of three aliquots which had been treated similarly to the second, except that they were finally also given a 25 s green-light exposure at 22"C. This third curve was then subtracted from the second to give the data shown (right-hand axis). The loss centred on 325"C is usually associated with the main OSL trap, but other decreases (e.g. at 240°C) and increases (e.g. at I I0°C) resulting from optical stimulation are also seen. These result from charge transfer between different traps (Spooner et al., 1988). Similar curves are shown for AC150 in Fig. l(b). (Measurement conditions were identical for AC150 and DS2.) For this sample, the high temperature peak appears less complex in the added dose (N + 43 Gy) TL curve (le;t axis), presumably because the 280°C TL peak is less important. The overall luminescence in the difference curve (i.e. the curve of TL lost and gained as a result of optical stimulation) is a larger fraction of the initial TL in the older sample AC 150 than in sample DS2, even allowing for the difference in added beta dose. (Note that the natural TL (right axis) and the TL lost during 25 s OSL (left axis) are on different vertical scales in Fig. l(b), unlike in Fig. l(a). The 110°C peak visible in both natural glow curves is phototransferred TL from a 0.1 s light exposure for normalisation.) This fractional difference between the two samples may reflect more complete bleaching of the less light-sensitive TL traps (e.g. at 280 and 375°C) in sample AC150. It is interesting that the high temperature peak in the difference curve is significantly different in shape between the two samples (and also different from sample WIDG8; see Wintle and Murray, 1997). Unlike sample DS2, there is no significant decrease in TL at about 240°C. The 190°C region is relatively more affected by light in sample AC150, and the relative importance of the 110, 140 and 160°C peaks is significantly different. It is noteworthy that there is no significant transfer of charge into the 110°C peak

EQUIVALENT DOSE M E A S U R E M E N T in sample ACI50, in contrast to both samples DS2 and WIDG8 (Wintle and Murray, 1997).

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8 4. PULSED STIMULATION CURVES AND THE EFFECT OF PREHEAT TEMPERATURE OSL signals can be obtained under continuous optical stimulation to produce continuous decay curves, and stimulation may take place while the sample is held at room temperature, or at an elevated temperature. In an earlier study, we have shown that, under continuous stimulation, charge is transferred into and out of the trap responsible for the TL peak at I10°C (Wintle and Murray, 1997). To reduce the effect of this mechanism on the OSL, it was suggested that OSL measurement should be made at elevated temperatures, thus preventing charge entering the l l0°C trap. In a subsequent study, Murray and Wintle (1997) showed that decay curves can also be constructed by measuring the OSL for 0.1 s and then heating to a fixed temperature in the range 80-300°C (and holding there for l0 s). They repeated this measurement cycle seven times to give OSL decay curves which could be closely approximated by exponential functions; these were described as pulsed stimulation curves. They are conceptually identical to the correction curves for feldspars used by Duller (1991) and are what would be obtained in the additive-dose single-aliquot protocol if no laboratory dose was given. We decided first to examine the shape of a pulsed stimulation curve, and to repeat this experiment at various preheat temperatures. Three aliquots of sample DS2 were given a dose of 6.7 Gy in addition to the natural dose of 2.7 Gy, to ensure that shallow traps contained a significant dose. They were then preheated at 160°C for l0 s, and given a 0.1 s exposure to green light at an elevated temperature of ! 10°C; this preheat/stimulation cycle was repeated 14 times in total. The experimental cycle was repeated for preheats between 160 and 300°C, in 20°C intervals, and the OSL signals observed during the 0.1 s stimulations are shown in Fig. 2(a). All data points are the average of three identical treatments on separate aliquots and the results are normalised to the same initial count. For preheats between 160 and 260°C, the data are well represented by exponential decay (the best fits are shown as solid lines). Lip to 240°C, about 8% of the signal is lost during each preheat/stimulation cycle. The 280°C preheat curve also decays exponentially (about 35% loss per cycle) until the l lth stimulation. This corresponds to an absolute light level of 529 counts/0.1 s exposure (6% of the initial intensity, cf. instrument background of 1-2 counts/0.1 s stimulation). The data included in the decay constant analyses are represented in Fig. 2(a) as solid symbols; those excluded are shown as open symbols. The 300°C curve decays exponentially for the first few stimulations, i.e. until an

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Fig. 2. (a) Pulsed decay curves for sample DS2 (N + 6.7 Gy, D~ = 2.7 Gy) for various preheat temperatures (held at the given temperature for 10 s between optical stimulations). Stimulation was for 0.1 s at 110°C. The solid lines are single exponential decay curves fitted to the solid data points. (b) Similar pulsed decay curves for sample WIDG8 (N + 43 Gy, De = 58 Gy).

absolute light level of between 542 and 290 counts is reached (between 10% and 5% of initial intensity). It is deduced that preheats of 260°C and above involve detectable thermal erosion of the deep OSL trap [related to the TL trap at about 325°C, see Fig. l(a)]. It seems that the decay of 90-95% of the OSL signal intensity is exponential, whether the loss is caused primarily by optical stimulation (in Fig. 2(a), < 240°C) or by a combination of thermal erosion and optical stimulation ( _> 240°C). There was a marked loss of TL in the 240°C region of the glow curve resulting from a 25 s optical stimulation [see Fig. l(a)]. Despite this, the rate of OSL decay is not significantly increased by those preheats which erode or empty this region of the glow curve. It is concluded that these TL traps do not contribute significantly to the OSL signal. The deviation from exponential decay at low light levels (5-10% of initial intensifies) is unlikely to result from interactions with shallow traps, because these are emptied by preheating between each 0.1 s exposure. Moreover, optical

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stimulation is performed at I I0'~C, which prevents charge accumulating in the II0°C TL trap. Figure 2(b) shows the corresponding data for sample WIDG8 (Do = 58 Gy). The decay rates of the curves are very similar to those of sample DS2, and the curves resulting from 280 and 300°C preheats also deviate from exponential decay at low light levels. Murray and Wintle (1997) show that part of this non-exponential component is associated with the 330-375'~C TL glow curve interval, and part with the TL glow curve interval between 375 and 500'C. Samples heated to above 500°C have an OSL signal within a factor of ten of background [1-2 counts on the vertical scale of Fig. 2(b)]. It is concluded that the deviation from exponential behaviour arises, at least in part, from a longer-term decay component originating from other deep traps which would empty when the sample is heated to above 325°C.

6. AN ADDITIVE-DOSE SINGLE-ALIQUOT PROTOCOL From the data shown in Figs 2 and 3 it is concluded that: (a) pulsed stimulation curves can be closely approximated by a single exponential decay term for the majority of the decay curve at all likely preheat temperatures; and (b) the dose dependency of the exponential decay constant is small. These observations allow the formulation of a simple model to describe the decay correction necessary in an additive-dose single-aliquot protocol. If the OSL signal observed during the first stimulation cycle is f~, then the contribution to the second cycle will be f,e ~:, where 2' is the decay constant appropriate to a particular preheat temperature. Then the total OSL signal observed during the second stimulation will be L2 = E2 + [,e ~"

5. DOSE DEPENDENCY OF PULSE STIMULATION DECAY CONSTANTS

The data in Fig. 2 offer the possibility that a true single-aliquot protocol can be developed for these quartz samples, because a correction factor for exponential decay can be applied across a wide range of light levels. However, it is important to first determine whether or not the pulse stimulation decay constants are independent of dose. Three natural aliquots of sample DS2 were repeatedly pulse stimulated as before, at l l0C'C, using a preheat of 200'~C for 10 s. This experiment was repeated on a total of 24 aliquots, except that various doses (of between 1.7 and 215 Gy) were given to the remaining 21 aliquots before the first preheat. The process was then repeated for another 24 aliquots, but using a preheat of 280"C for 10 s. Data for sample DS2 are presented in Fig. 3(a), and similar data for sample WlDG8, using only a 280°C preheat, are shown in Fig. 3(b). For ease of comparison, light levels have again been normalised to the initial count. Based on the results in Fig. 2, only absolute light levels above 700 counts/0.1 s exposure have been considered when evaluating the decay constants. The data included in these analyses are shown as solid symbols; those excluded are shown as open symbols. For both the 200 and 280°C preheats, there is a weak dependence of decay constant on dose: for the 200°C preheat, the slope decreases from 0.090 to 0.073 with increasing dose; for the 280°C preheat, the slope tends to increase with dose, from 0.24 to 0.29 (DS2) and from 0.31 to 0.34 (WlDG8). Nonetheless, over the two orders-of-magnitude dose range examined, the change in decay constant is small; over any practical dose range (i.e. zero added dose to maximum added dose) used in the construction of an additive-dose growth curve, the effect may be safely ignored.

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Stimulation Cycle Fig. 3. (a) Pulsed decay curves for sample DS2 (preheats of 200°C for l0 s and 280°C for l0 s) for various added doses (0, 1.7, 3.4, 6.8, 13.5, 27, 54, 108, 215 Gy), The solid lines are single exponential decay curves fitted to the solid data points. Optical stimulation was for 0.1 s at ll0°C. (b) Similar pulsed decay curves for sample WIDG8 (preheat of 280°C for 10 s, for the same added doses as in (a) above).

EQUIVALENT DOSE M E A S U R E M E N T dose given at the beginning of the second cycle. Similarly, that during the third stimulation will be L3 = Y~ + [2e - ;" + [~e - 2;.. w h e r e L 3 is

the total OSL signal observed during the third stimulation, ¢3 is the signal resulting from the dose added in the third cycle, and 2" is the decay constant for the second added dose. From Fig. 3, 2'~2"=2, and so L3 -- [3 + ( [ : +

[,e-~) e-~-

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L2e- ~

Had there been no decay with each cycle, then the OSL signal that would have been observed during the second cycle is given by [~ + [:, and that during the third cycle becomes [~ + [2 + [3. Thus [i + [2 = It + L2 - [ l e - ~ [ , + [2 + [3 = L3 - -

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(3)

i=l

where L0, d0 = 0. Thus the observed OSL signals (Li) can be corrected to give the signals that would have been observed (EEl) had there been no depletion of the trap population by either stimulation or preheating. Galloway (1996) also used a data set obtained after additional preheats (of feldspars) with no added dose. The major difference between his procedure and ours is that in our case the decay per cycle (2) is constant. For feldspars, Duller (1991) found that the decay rate decreased with each measurement cycle, thus requiring a separate correction for each part of the IRSL signal (due to each added dose). Galloway (1996) found an empirical relationship between the amount of decay and the number of measurement cycles. He then used this relationship to correct previous measurements; no physical mechanism for this behaviour was proposed. Our correction procedure is analogous to the "luminescence correction" procedure of Duller (1994a), and he has cautioned that his procedure will be significantly in error if the growth curve is markedly non-linear. That this applies here can be appreciated most easily by considering the result of applying equation (3) to a fully saturated sample. If each preheat/stimulation cycle depletes the trapped charge measurably, but each added dose returns the sample to saturation, the use of equation (3) will give rise to an apparent increase in luminescence with

177

added dose; the growth curve will appear to increase linearly for equal added doses. For unequal doses, an increasing response will still be generated, but the smaller added-dose points will lie systematically above the average increase, and the larger added-dose points will lie below. Thus the applicability of equation (3) to older samples can be tested by using unequal added doses when generating a growth curve. In any case, equation (3) should be applied with caution to samples whose OSL sensitivity varies significantly with dose, because it will tend to make non-linear growth curves appear more linear. The estimated Dr is not necessarily sensitive to this effect, however, as will be seen later. Because the decay of the OSL signal with repeated stimulation is exponential, with a decay constant that is effectively independent of dose, this constant can be evaluated at the end of an additive-dose sequence; repeating the preheat and pulse stimulation cycle without further addition of dose will give a decay curve of the type shown in Fig. 2. Thus the entire measurement procedure can be carried out on a single aliquot, or even on a single grain; there is no need for a second aliquot with this protocol. It should also be noted that this approach can only be regarded as a true additive-dose protocol for those measurement cycles that negligibly decrease the population of the deep OSL trap. For high temperature preheats, where > 30% of the OSL signal is lost with each measurement cycle, a significant fraction of the OSL comes from regenerative (rather than additive) dose, even after only one added-dose cycle. Thus the protocol will be applicable only to those samples that require a low temperature preheat, or to those that do not show pronounced luminescence efficiency changes with regeneration. The observation of exponential decay with repeated measurement cycle (with no added dose) is believed to be a reliable indicator of suitable characteristics.

7. APPLICATION OF THE SINGLE-ALIQUOT PROTOCOL To test the application of equation (3), a 5 mg aliquot of sample DS2 (De = 2.7 Gy) was preheated at 200°C for 10 s, and then stimulated by green light for 0.1 s at 110°C. The same aliquot was then given a dose of 2.7 Gy, preheated and stimulated again. This dose/preheat/stimulation cycle was repeated five times in total. The aliquot was then preheated and stimulated a further eight times without the addition of dose. The data are presented in Fig. 4(a) (solid squares). An exponential decay curve has been fitted to the last nine data points, and the decay constant is 0.082 + 0.002 cycle-~ (R 2 = 0.996). The data were then corrected as described by equation (3), and the revised data are shown in Fig. 4(a) as solid circles. The (average) decay correction has satisfactorily

178

A . S . MURRAY et al.

2"0x104

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Fig. 4. (a) Application of the single-aliquot protocol to sample DS2, using a preheat of 200'~Cfor 10 s, and a 0.1 s stimulation at 110r~C.Solid squares show the raw data, and solid circles show the data corrected using equation (3). The last eight data points received no added dose between preheat/stimulation cycles. The corrected data, up to a cumulative added dose of 13.5 Gy, have been fitted with a saturating exponential to extrapolate to the dose axis; the estimated Dcis 2.77 + 0.10 Gy. The open triangle data point is discussed in the text. (b) Multiple-aliquot additive-dose growth curve for sample DS2 (preheated at 20ff~Cfor 10 s, with 100 s optical stimulation at 110°C). The solid line is a best-fit saturating exponential, and the estimated De is 2.4 + 0.3 Gy.

accounted for the loss of signal in the last nine points; the data do not deviate systematically from a horizontal straight line. The first six data points have been fitted with a weighted saturating exponential to extrapolate to the dose axis. The De estimated by this procedure is 2.77 + 0.10 Gy. To confirm that the curvature in the dose response curve is not an artefact of the repeated measurement cycle, the first and last dose points (i.e. points 1 and 6) were re-measured on three further aliquots, but all intermediate points were omitted, i.e. after the first measurement cycle (preheated/stimulated naturals), the aliquots were given a dose of 13.5 Gy, preheated and stimulated again. The pulse stimulation decay curve shape was then obtained by multiple preheat/ stimulation cycles as before, and the data corrected accordingly. The average of the three first cycle measurements was normalised to the corresponding

OSL intensity (at 0 Gy) from the earlier data set, and the resulting average OSL intensity at 13.5 Gy is shown in Fig. 4(a) as an open triangle. The agreement with the earlier data point at a cumulative dose of 13.5 Gy is excellent, confirming that the non-linearity is not an artefact of the multiple measurement cycle; no significant sensitivity changes appear to have occurred as a result of the first six pulse stimulation cycles. A full multiple-aliquot additive-dose protocol (using 48 aliquots) has also been applied to this sample, using initial 0.1 s OSL "short shine" normalisation (first proposed by Smith et al., 1986) and the 0-100 s OSL integral after subtraction of the background signal (derived from the stimulation interval 90-100 s; see Murray, 1996). The multiplealiquot growth curve is shown in Fig. 4(b). The curve is much less well defined than in Fig. 4(a), illustrating another advantage of any single-aliquot protocol; if all measurements are made on a single sub-sample, uncertainties arising from heterogeneity between sub-samples are avoided. Rhodes (1990) suggested that examination of the reproducibility of luminescence signals as a function of dose, following normalisation using the OSL from brief stimulation of the natural aliquots, gives information about the completeness of bleaching at deposition. He proposed that if the relative scatter increases with dose, the sample is probably made up of grains that have been bleached to different degrees before deposition. If the relative scatter is independent of added dose, then the sample has been homogeneously bleached. This approach was subsequently discussed and employed by Duller (1994b). For sample DS2, there is no significant correlation with dose of the relative standard deviation (i.e. standard deviation divided by the mean) of the normalised OSL data shown in Fig. 4(b) (R 2 = 0.06), confirming that the degree of bleaching of this sample has been uniform, rather than heterogeneous. Despite the poor reproducibility (average relative standard deviation of 11%), there is a slight dose-response curvature similar to that in Fig. 4(a), confirming that the additive-dose growth curve is sub-linear; the De is 2.4 + 0.3 Gy. Figure 5(a) shows the result of repeating these single-aliquot experiments on the older sample WIDG8 (D~, = 58 Gy; the mean of three multiple-aliquot analyses, see Table 1). Wintle and Murray (1997) suggest that a low temperature preheat would be inappropriate for this sample, because there has been some transfer of the contents of the 280°C TL trap to the deep OSL trap in the natural sample. Accordingly, the experimental procedure was the same as for sample DS2, except that a preheat of 280°C for 10 s was used. In this test, the doses were not given in equal increments as for DS2 [Fig. 4(a)]. this was in order to produce a non-monotonic growth curve for the raw data (solid squares). The decay-corrected data (solid circles) produce a smooth curve, with no systematic tendency for the smaller

EQUIVALENT DOSE M E A S U R E M E N T added-dose points to lie above the average growth curve, or the larger added-dose points to lie below. This observation confirms that the correction procedure of equation (3) is not detectably compromised by the dose-response curvature observed in the multiple-aliquot additive-dose growth curve [see below, and Fig. 5(b)]. This possibility was raised in the discussion following equation (3). The exponential decay correction for the 280°C preheat is considerable (the decay rate of the raw data shown in Fig. 5(a) is 0.398_+ 0.003 per cycle; R 2 = 0.9996). This is similar to the mean decay rate derived from the 24 aliquots shown in Fig. 3(b), of 0.322 _+ 0.005 per cycle, over an added-dose range from 0 to 215 Gy; the two data sets were collected on different OSL readers, and the difference must, in part, be attributed to different stimulation rates, and to heterogeneity between aliquots. Since each preheat/stimulation cycle decreases the OSL signal by about 40%, and the subsequent added-dose refills the OSL trap (at least in part), the raw data signal in Fig. 5(a) never decreases to less than 75% of the initial count rate. The decay is known to be 1.5x105

i

(a)

I

179

exponential in form over the first 90% of the decrease of any OSL signal [Fig. 3(b)]; in this case, the luminescence never drops close to the level where non-exponential decay behaviour becomes important. This is why, despite the significant deviation from exponential decay at low light levels (Figs 2 and 3), equation (3) successfully corrects for the decay of the final growth curve point at 120 Gy. The D, estimate for this aliquot, of 54.1 _+ 1.3 Gy, is consistent with that obtained from a 24 aliquot additive-dose growth curve [De = 61 + 9 Gy, Fig. 5(b)], both for a preheat of 280~C for 10 s. The latter analysis used the 0-100 s OSL integral after subtraction of the background signal (derived from the stimulation interval 90-100 s), and the natural aliquots were normalised using the OSL signal from a 0.1 s stimulation. The multiple-aliquot growth curve is distinctly non-linear when using added-doses of up to 215 Gy [Fig. 5(b)], and contrasts with the single-aliquot growth curve [Fig. 5(a); after correction] which appears to be linear. However, the multiple-aliquot data set might have been construed as almost linear, if the data for 120-215 Gy had not been taken. In addition, the apparent linearity of the corrected single-aliquot data when unequal addeddoses were given, gives no indication of differential sensitivity changes.

r-

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Fig. 5. (a) Application of the single-aliquot protocol to sample WIDG8, using a preheat of 280°C for l0 s, and a 0.1 s optical stimulation at 110°C. The data are presented in a similar manner to those in Fig. 4(a). The estimated De is 54.1 _+ 1.3 Gy. (b) Multiple-aliquot additive-dose growth curve for sample WIDG8 (preheated at 280°C for 10 s, with 100 s optical stimulation at l l0°C). The solid line is a best-fit saturating exponential, and the estimated D, is 61 _+9Gy.

To illustrate the saving in time and sample mass achieved using the single-aliquot protocol, a preheat plateau has been prepared for sample DS2 using 48 aliquots. A full single-aliquot protocol was applied in triplicate at preheat temperatures between 150 and 300°C (at 100°C intervals), to give 16 data points, each with an external estimate of uncertainty. The results of this analysis are shown in Fig. 6(c) [these include the result from the aliquot used in Fig. 4(a)]. These data were obtained within 36 h; to obtain a similar data set by conventional multiple-aliquot analysis would have taken several weeks. For sample DS2, there is a plateau in the De estimates beginning by at least 160°C, and continuing to 300~C. However, while the growth curves for preheats below 280'~C were of the same form as in Fig. 4(a), those at 280°C and above were supra-linear, and the higher temperature D, intercepts (shown as open squares) are accordingly less reliable. Because of this, the average D0 (2.69 + 0.04 Gy) is based only on the temperature interval 160--270°C. Four other preheat plateaus are shown for samples of various ages. Figure 6(a) presents similar data for a sample of modern coastal dune sand (DSI). This sample is known to have been collected within five years of deposition. There is a clear plateau in D~ values (D~ = 0.026 + 0.006 Gy) between 160 and 200°C, but above this range the apparent Do rises steadily. However, this pattern is not repeated

180

A . S . M U R R A Y et al. Table 1. Comparison between estimates of De from single-aliquot and multiple-aliquot additive-dose protocols Single-aliquot

Sample code BR94001 DS I ABH54 NEST6/3 NEST6/7 DS2 NEST4/2 DS3 BR94001 TSS4 AC 150 DS4 NC 12/13 WIDG8 CUDDLE1

Preheat ('C, s) 200, 160-200, 280-300, 220-300, 280, 160-270 280-300 180-240 260-300 260-300 260-300 270-280 280-300 280-290 290-300

10 10 10 10 10 10 10 10 10 10 10 10 10 10 10

Multiple-aliquot'

Dr (Gy)

(n)

0.013 _ 0.001 0.026 +_ 0.006 0.103 + 0.012 0.41 _ 0.03 0.86 _ 0.06 2.69 4- 0.04 3.1 4- 0.4 5.17 4- 0.06 9.8 + 0.6 8.6 + 0.4 22.4 4- 0.7 24.0 + 1.2 48 + 2 52.1 + 1.0 100 4- 6

10 9 6 15 3 36 6 12 9 9 9 18 6 6 6

Preheat (C, s) 200, 200, 220, 220, 220, 200, 220, 200, 280, 220, 220, 280, 220, 26(~290, 220,

10 10 300 300 300 10 300 10 10 300 300 10 300 10 300

Dr (Gy) 0.011 -0.04 0.076 0.36 0.84 2.4 3.7 5.3

_ 0.004 _ 0.07 + 0.008 + 0.04 4- 0.17 4- 0.3 4- 0.4 4- 1.1 10 9.0 4- 0.9 23.5 4- 0.6 25 4- 3 47 4- 4 58 4- 6 91 4- 8

(n)

(m)

1 24 1 24 1 46 1 20 1 52 1 48 1 52 1 48 Note 5 1 52 1 52 1 48 1 52 3 72 1 52

Lab. CSIRO CSIRO ANU ANU ANU CSIRO ANU CSIRO CSIRO ANU Oxford CSIRO ANU CSIRO ANU

Note: (1) all multiple-aliquot Do estimates were obtained using the additive-dose protocol, except sample BR94001, for which the modified SARA protocol was used (Murray et al., 1995). (2) More than one preheat temperature (e.g. 160-240) indicates the range (in r'C) over which the De, has been calculated. (3) The uncertainties are standard errors on the mean. (4) 'n' is the number of independent estimates of De; for the single-aliquot protocol this corresponds to the number of single aliquots employed. 'm" is the number of aliquots used in the multiple-aliquot protocol. (5) This sample contained a natural dose of ~ 0.01 Gy (see first entry); a laboratory beta dose of 10 Gy was then added to all aliquots, and these were preheated at 28ffC for l0 s prior to analysis using the single-aliquot protocol.

in Fig. 6(b), which shows data obtained from sample NEST6/3, a mud-wasp nest associated with Aboriginal rock paintings. Here the plateau begins above 200°C, and continues to 300°C (D~ = 0.41 + 0.03 Gy). Values derived at temperatures of 200"C and below are greater and more scattered than those from higher temperatures. Figure 6(d) shows the De plateau for sample ACI50, a wind-blown quartz deposit in a limestone cave. There is a pronounced drop in apparent D~ as the preheat temperature increases to about 260°C. F r o m 260 to 300°C, the values are indistinguishable and the average Dr is 22.4 + 0.7 Gy. Finally, Fig. 6(e) shows the "plateau" for sample W l D G 8 [these data include the result for the aliquot shown in Fig. 5(a)]. The apparent values of De decrease rapidly from 220 to 270°C. If there is a plateau in this sample, it begins at 280°C; the average of the 280 and 290°C estimates o f De is 52.1 ± 1.0Gy.

9. COMPARISON OF S I N G L E - A L I Q U O T A N D M U L T I P L E - A L I Q U O T De E S T I M A T E S The single-aliquot protocol has now been applied to samples from eleven sites from around Australia, and the results compared with those from multiplealiquot additive-dose procedures (Table 1). The multiple-aliquot analyses were undertaken at three different laboratories (Research Laboratory for Archaeology and the History of Art, Oxford University; Research School of Pacific and Asian Studies, The Australian National University, Canberra; and C S I R O Division of Water Resources, Canberra). The beta source calibrations in each

laboratory are also derived from different national standards (British, German, and Danish, respectively). Any systematic uncertainties in the three different calibrations must be kept in mind when comparing the analyses presented below. Furthermore, eight of the comparisons are with multiple-aliquot De values obtained using a 220°C preheat for 300 s (Smith et al., 1986; Rhodes, 1988; Roberts et al., 1994a). Despite these potential additional sources of uncertainty, the comparison (presented graphically in Fig. 7) shows that the agreement between single-aliquot and multiple-aliquot additivedose estimates of De is very satisfactory. As a further test of the single-aliquot protocol, 24 aliquots of the beach sand sample BR94001 (Dr ~ 0.01 Gy) were each given a beta dose of 10 Gy, and heated at 280°C for 10 s. These aliquots were then individually analysed as a function of preheat temperature using the single-aliquot protocol; a plateau was found between 260 and 300°C, and the average Dr was 9.8 + 0.6 Gy. This result is also included in Table 1 and Fig. 7 (shown as an open square). The single-aliquot data in Table 1 were obtained at stimulation temperatures of either 25 or I I0°C. Various authors (e.g. Wintle and Murray, 1997) have suggested that the initial OSL signal should arise from detrapping of electrons from the deep OSL trap only, provided that the shallow traps are initially empty. Second-order kinetic effects should only become significant as stimulation progresses. In the single-aliquot protocol used in this study, shallow traps are kept empty by preheating between each optical stimulation cycle. Thus, there is no a priori

EQUIVALENT DOSE M E A S U R E M E N T reason to expect that the apparent De should depend on optical stimulation temperature. This has been tested on samples DS4 and WIDG8, by repeating the single-aliquot analysis at stimulation temperatures of 25, 110, 160 and 200°C. Each analysis was done in triplicate, using a 270°C preheat for 10 s. The results are summarised in Table 2, from which it can be seen that there is no observable effect of stimulation temperature on the average value of D , although the scatter in the results may be greater for low and high temperature stimulations. The natural OSL counts from 0.1 s stimulation are also shown (average of three aliquots); there is no reason to believe that any variation in reproducibility depends on photon counting statistics. Finally, it is important to add that we have examined samples from one site, Deaf Adder Gorge in the Northern Territory (samples K166 and KI72,

0.2

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Fig. 7. Summary of all single-aliquot estimates of De plotted against estimates derived from multiple-aliquot additivedose procedures (see Table 1). The data point marked with an open square is a single-aliquot estimate of a given (i.e. known) dose. The solid straight line passes through the origin, and has unit slope. Axes are logarithmic, to best display the data at low doses. The linear plot is inset. Roberts et al., 1994b), which did not show exponential decay of the OSL signal with continued preheat/0.1 s OSL measurement after the last addition of dose. This is attributed to pronounced luminescence efficiency changes with pulsed stimulation, and these samples are now under further investigation.

0.4

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are given in the figure. The difference between the solid circles and open squares in (c) is discussed in the text.

The De values compared in Table 1 and Fig. 7 extend over four orders-of-magnitude, and there is no significant disagreement between the single-aliquot and multiple-aliquot De determinations in that range. In particular, the agreement at higher values of De is consistent with uncertainties, indicating that the growth curve non-linearity [see Fig. 5(b)] is not giving rise to significant systematic errors. This comparison also gives confidence in the validity of short-duration, high-temperature preheats; these are of particular value in an automated procedure, where instrument time is at a premium. There seems little doubt that the single-aliquot protocol gives results at least as reliable as the conventional multiple-aliquot additive-dose protocol. Given this, the procedural implications of Fig. 6 can now be considered. Godfrey-Smith (1994) demonstrated the need for a preheat using a sedimentary quartz which had been given a laboratory dose of 100 or 200 Gy. The OSL signals decayed with storage time at room temperature, with most loss occurring within five days of being dosed. Thus, for older samples, preheating of the laboratory dose will be required to simulate the effects of burial time at ambient temperature. One way of estimating the appropriate preheat conditions is to apply a Do preheat plateau test (Aitken, 1994). Despite their acknowledged importance, there have

182

A.S. MURRAY et al.

been only a few OSL preheat plateaus published (e.g. Rhodes, 1988; Stokes, 1992; Roberts et al., 1993; Murray, 1996), presumably because of the labour and equipment time involved using multiple-aliquot techniques. Wintle and Murray (1997) have suggested that low temperature preheats should be appropriate for samples younger than about 20 ka, because the 280°C TL peak should be stable over these time-scales. Godfrey-Smith et al. (1988) found that preheating quartz samples, which had been optically bleached by 18 h of sunlight, increased their OSL signals for preheat temperatures above 175°C, reaching a maximum value between 300 and 400'~C. These experiments used modern and bleached quartz extracts for which sensitivity increases were found for preheats above 225°C. These authors and others (Huntley et al., 1985; Godfrey-Smith and Haskell, 1993) suggested that this effect is the result of charge transfer from optically insensitive traps to optically sensitive traps. Murray (1996) presented data for young sedimentary quartz (<_ 2000 years old) that show a preheat plateau commencing at 180°C, and continuing to at least 240°C. The appropriateness of low temperature preheats is confirmed by the data for three of the youngest samples examined here (DSI, NEST6/3 and DS2). The apparent De for DS1 [Fig. 6(a)] increases systematically at temperatures above 200°C, consistent with the observations of Godfrey-Smith et al. (1988). The De plateau for DS2 [2.7 Gy, Fig. 6(c); age ~ 3 ka] extends from 160°C to at least 270~C (the growth curves cannot be interpreted reliably above this temperature). However, it should be noted that the plateau for sample NEST6/3 does not commence until 220°C, although this sample is thought to be less than 500 years old (Roberts et al., 1996b). Nevertheless, the minimum treatment needed to reach the plateau value of De for all three samples is substantially less than that needed for the much older ( > 30 ka) samples WIDG8 [Fig. 6(e)], NC12/13 and CUDDIEI (Table 1). Sample ACI50 [Fig. 6(d)] is of intermediate age ( ~ 10 ka). Wintle and Murray (1997) have argued that low temperature preheats should still be useful in this range, if the 280°C TL trap is the dominant source of thermally unstable charge. From our data, a minimum preheat of 260°C for 10 s is required to reach the De plateau, suggesting either that the 280°C peak is much less stable than thought, or that there are other lower temperature traps present in this

sample [see Fig. l(b)] which provide charge to the deep OSL trap. These will have been at least partly emptied during the period of burial, and so both the natural doses and the laboratory added doses must be preheated to completely empty these traps. Wintle and Murray (1997) also concluded that the main effect of preheating sample WlDG8 (age > 30 ka) is the transfer of charge from lower temperature traps (especially that at 280°C) to the deep OSL trap. For such older samples, the 280°C trap will have lost some charge relative to the deeper trap during the burial period, and hence a preheat is necessary to complete the transfer in the natural sub-sample and totally transfer the charge from this trap for all laboratory-dosed sub-samples. Based on the change in absolute OSL intensifies with preheat temperature (rather than changes in De estimates), they deduced that a preheat of at least 260°C for 10 s, or equivalent, would be necessary for sample WlDG8. The results in Fig. 6(e) and Table 1 support this deduction, suggesting a minimum preheat of about 270°C for 10 s. 11. CONCLUSIONS Samples from 11 sites from around Australia have been examined in this study. Except for material from one site (Deaf Adder Gorge), all samples responded to repeated pulse stimulation with a decrease in the OSL signal that could be approximated by exponential decay; the decay constant was independent of dose, and included a contribution from both optical stimulation and thermal excitation which depended on preheat temperature. Such exponential decay implies that the luminescence efficiency was unaffected by the dose/preheat/pulse stimulation cycle. Samples from the Deaf Adder Gorge site did not show exponential decay of the OSL, presumably because the luminescence efficiency was sensitive to the preheat. For the first time, using the protocol described here, it is possible to determine the De from only one aliquot of quartz. Related work (to be described elsewhere; Murray and Roberts, 1997) has shown that this protocol can be applied to single grains of quartz, allowing a detailed examination of variations in De, from grain to grain. This will be especially valuable in studies of young, incompletely bleached materials such as fluvial sediments, and in the

Table 2. Effect of varying stimulation temperature on D~ Sample DS4 Stimulation temperature (°C)

Do (Gy)

OSL (counts/0.1 s)

25 21 + 5 5880 110 19.4 + 1.4 6990 160 24 + 7 6330 200 21 +_6 2060 Uncertainties are standard errors based on three analyses.

Sample WIDG8 De

(Gy) 57 +_6 51.3 _ 0.3 51.7 -I- 1.8 52 -I- 7

OSL (counts/0.1 s) 38650 35640 39520 13040

EQUIVALENT DOSE MEASUREMENT identification o f sediment d i s t u r b a n c e in n a t u r a l a n d a n t h r o p o g e n i c deposits. It has also been s h o w n here t h a t it is n o w practical to routinely examine such f u n d a m e n t a l concepts as the preheat plateau. We conclude t h a t the additive-dose single-aliquot protocol represents a powerful new a p p r o a c h to estimating the D, in sedimentary quartz. Acknowledgements--We thank Geoff Duller and Martin Aitken for their comments on drafts of this manuscript, and Jon Olley for providing prepared material from the DS sequence of sites. A.G.W. gratefully acknowledges financial assistance from the British Council and CS1RO Division of Water Resources, which assisted a study visit to Canberra. R.G.R is in receipt of a Queen Elizabeth II Fellowship from the Australian Research Council. Some of the equipment used in this study was purchased with the assistance of the Co-operative Research Centre for Catchment Hydrology. Fundamental luminescence research in Aberystwyth is currently supported by NERC grant GST/02/0762.

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