A modified SAR protocol for optical dating of individual grains from young quartz samples

Radiation Measurements 42 (2007) 360 – 369 www.elsevier.com/locate/radmeas

A modified SAR protocol for optical dating of individual grains from young quartz samples M. Ballarini a , J. Wallinga a,∗ , A.G. Wintle b , A.J.J. Bos c a Netherlands Centre for Luminescence Dating, Delft University of Technology, Faculty of Applied Sciences, Mekelweg 15, NL-2629 JB Delft, The Netherlands b Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Ceredigion, SY23 3DB, UK c Delft University of Technology, Faculty of Applied Sciences, Mekelweg 15, NL-2629 JB Delft, The Netherlands

Received 6 July 2006; received in revised form 6 December 2006; accepted 22 December 2006

Abstract We investigate the feasibility of a modified single-aliquot regenerative-dose (SAR) protocol for OSL dating of individual grains from young samples (< 300 years) using dose-recovery tests. Parameters such as pre-heat temperature, test- and regenerative-dose size, an additional bleaching step at high-temperature and optical stimulation time are optimised to give the highest percentage of accepted grains. The optimised protocol makes use of a 50 Gy test dose, a single regeneration dose of 5 Gy, an additional bleaching step and an optical stimulation of 10 s. This protocol is applied to two coastal-dune samples. Equivalent doses close to those obtained for multiple-grain single-aliquot measurements can be retrieved if: (1) short integration intervals (0.034 s) are used for the natural and regenerated OSL signals as well as the test-dose responses and (2) the background is estimated from the subsequent 0.034 s, and the net signals are used to determine the equivalent dose. Standard background subtraction methods lead to overestimation of the equivalent dose for these samples due to incorporation of a slow OSL component that was not completely reset at the time of burial. © 2007 Published by Elsevier Ltd. Keywords: Quartz; OSL; Single grain; SAR; Young deposits

1. Introduction The optically stimulated luminescence (OSL) signals from naturally irradiated quartz grains are used for dating sedimentary deposits. The recent expansion of the use of OSL as a dating method has been the result of the development of a measurement procedure that enables the equivalent dose (De ) to be determined for a single aliquot made up of several thousand grains. The single-aliquot regenerative-dose (SAR) protocol of Murray and Wintle (2000) has its basis in numerous studies on the OSL signals from natural quartz (Wintle and Murray, 2006) and relies upon the measured OSL signal being dominated by the fast OSL component, i.e., that which is most rapidly removed by exposure to light. Several tests have been established as checks on the reliability of the protocol applied (Murray and

∗ Corresponding author.

E-mail address: [email protected] (J. Wallinga). 1350-4487/$ - see front matter © 2007 Published by Elsevier Ltd. doi:10.1016/j.radmeas.2006.12.016

Wintle, 2000). When changes are made to a particular protocol, e.g., change in the size of the test dose or the use of an additional high-temperature optical bleach between cycles (Murray and Wintle, 2003), these tests must be applied to ensure that any such changes result in reliable values of De . The SAR protocol has also been applied to single grains of quartz. Most of these studies have been carried out on grains that are from aeolian deposits that are over 500 years old (e.g., Bush and Feathers, 2003; Feathers, 2003; Jacobs et al., 2003; Olley et al., 2004a, b). The SAR protocol has also been applied to single grains extracted from construction materials, such as concrete (e.g., Thomsen et al., 2002, 2003) and the mortar between building bricks (e.g., Jain et al., 2004); these would be expected to have very small equivalent doses. The same tests that were developed for single aliquots have also been applied to single grains. However, for single grains there are the additional complications that the signal levels are considerably lower and only a small percentage of grains give rise to a De value (e.g., Duller et al., 2000; Jacobs et al., 2003).

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The SAR protocol has also been applied to single grains that have been optically bleached, subsequently given a laboratory dose and then used in a dose-recovery experiment of the type that was first applied by Roberts et al. (1999). For example, Jacobs et al. (2006) used a known dose of 108 Gy and recovered a value of 108 ± 1 Gy when they applied the SAR protocol in a dose-recovery test to a sample of coastal sand. It is interesting to note that in their SAR protocol, they used a test dose of 54 Gy to maximise the number of grains for which it was possible to calculate the dose. They also used an additional high-temperature bleach as advocated by Murray and Wintle (2003). Both these modifications are investigated in the current study on much younger sediments. In this paper we aim to develop an appropriate SAR protocol for optical dating of single grains of quartz younger than about 300 years. Firstly, previously suggested modifications (such as the use of the additional high-temperature bleach) are tested and other parameters used in the SAR protocol are varied with the aim of adjusting the experimental procedure so that the percentage of grains accepted is maximised. Then we select appropriate intervals for signal and background integration to obtain a net signal that is dominated by the fast OSL component. Finally, we test a SAR protocol that uses the selected parameters by comparing single grain De values with those obtained in previous single aliquot studies on very young samples with good independent age control (Ballarini et al., 2003). In a separate paper we discuss the application of the proposed single-grain SAR protocol to modern samples (Ballarini et al., 2007).

2. Instrumentation and samples A RisZ TL/OSL reader with a single-grain attachment (BZtter-Jensen et al., 2000) was used to investigate both naturally irradiated quartz grains and grains that had been optically bleached and then given a laboratory dose. A Nd:YVO4 diodepumped laser ( = 532 nm) delivering a maximum power of 50 W/cm2 to a grain was used for optical stimulation. In this study we chose only 2.5 mm thickness of Hoya U340 filter for OSL detection in order to increase the light collection efficiency (Ballarini et al., 2005), instead of the standard 7.5 mm supplied by RisZ. One hundred grains were mounted on each aluminium disc that had a ten-by-ten grid of 300 m holes drilled in its surface. During OSL measurements, the disc was held at 125 ◦ C in order to prevent the 110 ◦ C TL trap in quartz accumulating charge and subsequently contributing to the OSL signal (Murray and Wintle, 1998). The samples (TX02-29 and TX02-31) selected for grainby-grain investigations were from a single coastal-dune ridge formed about 300 years ago on the island of Texel, the Netherlands. Chemically purified quartz grains were sieved to obtain grains of 180.212 m diameter and these were used for all OSL measurements. The De values measured using multiplegrain aliquots were 0.236 ± 0.010 and 0.244 ± 0.009 Gy, for sample TX02-29 and TX02-31, respectively (Ballarini et al., 2003). These doses combined with the environmental dose rate

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(∼ 1 Gy/ka) gave ages in agreement within errors with independent age control. 3. The SAR protocol for single grains 3.1. The basic protocol First the natural OSL signal of the grain is recorded (Ln ). Subsequent laboratory (regenerative) doses are administered and the OSL signals (Li ) are used to characterise the OSL response of the grains. By administering a test dose (Dt ) after each measurement of Li and recording the OSL response to this dose (Ti ) it is possible to correct for sensitivity changes caused by pre-heating ahead of the main OSL measurements. The sensitivity-corrected regenerative OSL signals are given by the ratio (Ri = Li /Ti ) and are used to construct a sensitivitycorrected dose–response curve. The equivalent dose (De ) is obtained by projecting the ratio (Rn = Ln /Tn ) onto this curve and finding the dose that gives rise to this value. 3.2. Signals from single grains In the experiments reported in this paper, the OSL signals were measured for either 1 or 10 s, with data collected in 0.017 s intervals. The beginning and end of the natural OSL decay curve for a bright grain from sample TX02-31 are shown in Fig. 1. The inset presents the complete record collected over 10 s; the main figure shows the data collected up to the first 0.68 s and for the last 0.68 s of the 10 s stimulation time. In channels 0–5 and 596–600 the laser has not been switched on; the photomultiplier noise is represented by the single count in channel 2 in Fig. 1. The signal in the first channel of stimulation is 47 counts. Rapid decay occurs over the first 0.2 s of laser stimulation time,

Fig. 1. Beginning and end of a natural OSL decay curve for a bright quartz grain from sample TX02-31. Total laser stimulation (shown in inset) was for 10 s, with data collected in channels of 0.017 s length. Laser stimulation starts at channel 5 and ends at channel 595; before and after these channels, the noise from the photomultiplier tube was recorded. Dotted lines relate to areas of integration discussed in the text.

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Table 1 Details of the experiments described in Section 4 Step 1 2 3 4 5 6 7 8 9 10 11 12

(a) Additional step

(b) Pre-heat temperature 210 ◦ C

Bleach 10 s OSL at (×3) Dose = 0, 20 Gy 10 s PH at 180 ◦ C 1 s OSL at 125 ◦ C Test dose = 10 Gy Cut-heat 160 ◦ C 1 s OSL at 125 ◦ C 0, 1 or 10 s OSL at 210 ◦ C Return to 2

210 ◦ C

Bleach 10 s OSL at (×3) Dose = 50 Gy 1 s OSL at room temperature Dose = 10 Gy 10 s PH at 120, 150, . . . , 300 ◦ C 1 s OSL at 125 ◦ C Test dose = 5 Gy Cut-heat 160 ◦ C 1 s OSL at 125 ◦ C 10 s OSL at 210 ◦ C Repeat 4–10 (1 cycle) Return to 1

but there is still a measurable signal out to 0.6 s. In the experiments reported in the following sections, and summarised in Table 1, both 1 and 10 s stimulation times are used. The parts of the OSL signals that are used as the data set are either the photon counts collected in the first 0.034 s (for the 1 and 10 s stimulation) or 0.17 s (for the 10 s stimulation). The limits of these two integration times are shown in Fig. 1; also shown are the signal regions used for subtraction of the background, termed early and late background (EBG and LBG), in order to obtain the appropriate net signal. The different background subtraction regions are discussed in Section 5.1. 3.3. Acceptance criterion based on signal statistics To determine whether a value for De (or the value obtained for a known dose in the case of dose-recovery experiments) is meaningful for the weak signals from single grains, we have used a criterion based on the relative standard error (RSE) for the OSL signal resulting from the test dose, as suggested by Banerjee et al. (2000). In this paper we have used the definition for RSE given in Galbraith (2002), Eq. (3):  RSE() =

Y0 + Y¯ /k , Y0 − Y¯

where  is the net signal calculated by subtracting the background from the initial signal; Y0 is the initial signal measured in the first n channels and Y¯ is the background measured in m channels divided by the number of channels used for the initial signal (n); k is defined as m/n. It can be seen from this formula that the RSE is inversely proportional to the brightness of a grain (i.e., number of counts). Selection of grains based on the RSE has been used in studies on optically bleached grains extracted from construction materials that also had low natural doses and low OSL sensitivity (e.g., Thomsen et al., 2002, 2003; Jain et al., 2004) and sedimentary grains from archaeological sites (e.g., Bush and Feathers, 2003). Following their studies, we have chosen to apply a RSE threshold of 30%; grains are only accepted for analysis if the RSE on all test-dose responses is calculated to be less than 30%.

(c) Test-dose size

(d) Dose–response curves 210 ◦ C

Bleach 10 s OSL at (×3) Dose Dn = 10, 0 Gy 10 s PH at 180 ◦ C 1 or 10 s OSL at 125 ◦ C Test dose = 0.15–1000 Gy Cut-heat 160 ◦ C 1 or 10 s OSL at 125 ◦ C 0, 1 or 10 s OSL at 210 ◦ C Repeat 2–8 (3 cycles) Return to 1

Bleach 10 s OSL at 210 ◦ C (×3) Dose = 10, 40, 70, 100, 130, 40 Gy 10 s PH at 180 ◦ C 10 s OSL at 125 ◦ C Test dose = 50 Gy Cut-heat 160 ◦ C 10 s OSL at 125 ◦ C 10 s OSL at 210 ◦ C Return to 2

3.4. Tests within the SAR protocol Murray and Wintle (2000) suggested two tests that should be carried out within the SAR measurement sequence. In the first, the ratio between two sensitivity-corrected OSL responses regenerated from the same regenerative dose is determined. Murray and Wintle (2000) proposed the criterion that this recycling ratio should be within 10% of unity. Because of the large uncertainties on the sensitivity corrected OSL responses for most individual grains, application of the Murray and Wintle (2000) recycling criterion would result in rejection of a vast amount of data. We adopted an alternative approach where we accepted grains for which the two responses used for the recycling test were within ±1. Because of the large error terms associated with each value of R, no grains were rejected on this criterion for any of the experiments. In the second test of Murray and Wintle (2000), the OSL response is measured when no regenerative dose is given. When expressed as a percentage of the natural OSL response, this value should be zero. However, transfer of charge from deeper traps into the OSL traps may occur during previous heating, resulting in this signal being greater than zero. Murray and Wintle (2000) proposed the criterion that this recuperated response is less than 5% of the natural response. Because our samples have very low natural OSL responses, this rejection criterion was not applied in this study. In addition to the two internal SAR tests discussed above, Duller (2003) proposed to screen for feldspar grains by monitoring the reduction of the test-dose OSL response due to a 40 s IR exposure at room temperature applied after an extra irradiation at the end of the SAR sequence. Following Duller (2003) we rejected grains which showed a reduction of 70% or more; application of this criterion resulted in discarding less than 1% of the grains. 3.5. Dose-recovery test In a dose-recovery test, the natural OSL is removed by bleaching and a laboratory dose is applied. Instead of measuring the natural OSL signal derived from irradiation in the

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sedimentary environment, the OSL signal due to this laboratory dose is used (e.g., Roberts et al., 1999; Jacobs et al., 2006). For the experiments in this paper, the grains were optically bleached three times using the laser for 10 s at 210 ◦ C, with a 1000 s pause at room temperature between each bleach, and then given a dose. When the SAR protocol is then applied, this laboratory dose is treated as an unknown quantity. The ability to accurately recover this dose tests the appropriateness of the SAR protocol for this material. 4. Testing parameter changes in the SAR protocol When the SAR procedure was applied to aliquots of these samples made up of several thousand grains (Ballarini et al., 2003), the original SAR protocol of Murray and Wintle (2000) was employed. In that study, a pre-heat of 10 s at 180 ◦ C was employed for the OSL response to the natural and regenerative doses, together with a cut-heat at 160 ◦ C for the test-dose response. In the following subsections, several changes are made to this protocol, with the aim of optimising the procedure for single grains. These tests are applied to grains from sample TX02-29. 4.1. Testing use of an additional bleaching step Murray and Wintle (2003) proposed the inclusion of an optical bleach at the end of each SAR cycle (i.e., each made up of a paired L and T measurement), holding the disc at a temperature higher than the one used for pre-heating. The purpose of this extra step is to remove charge that is thermally transferred from light-insensitive traps into the main OSL traps during the preheat. For their single grain dose-recovery test using a known dose of 108 Gy, Jacobs et al. (2006) used laser stimulation for 2 s at 280 ◦ C after each test dose OSL measurement as the additional bleaching step. In our study, we have used stimulation at 210 ◦ C with the laser at 90% of full power for each of the 800 grains investigated. Our choice of 210 ◦ C follows the suggestion of Murray and Wintle (2003) of using a temperature which is slightly above the pre-heat temperature. The same pre-heat (10 s at 180 ◦ C) and cut-heat (160 ◦ C) conditions were applied as for the single aliquot measurements. Instead of constructing a dose–response curve, our experiment consisted of repeatedly administering doses of either 20 or 0 Gy with a test dose of 10 Gy being used. Optical stimulation for 1 or 10 s at 210 ◦ C (Step 8 in Table 1a) was introduced after each SAR cycle (Steps 2–7 in Table 1a) to investigate whether the longer exposure time increased the bleaching efficiency, and the results were compared with those obtained when no additional bleaching step was used. In Fig. 2, the sensitivity-corrected OSL (Li /Ti ) for the 20 Gy doses obtained when no additional high-temperature bleaching step is applied and when using a 1 or 10 s stimulation at 210 ◦ C are shown (left axis) as a function of cycle. The measured recuperation, observed as the signal for zero dose (right axis), is presented as a percentage of the first 20 Gy measurement (L1 /T1 ). The recuperated signal is reduced from ∼ 10 to

Fig. 2. Effect of the additional bleaching step at the end of each SAR cycle (Table 1a). Six repeated cycles, in which doses of 20, 20, 0, 20, 20 and 0 Gy are given in turn, are shown. For the 20 Gy dose, values of Li /Ti are shown with respect to the left axis. For the two cycles for which the dose was 0 Gy, recuperation is expressed on the right axis as a percentage of the sensitivity-corrected OSL signal for the 20 Gy dose measured in the first cycle. Each data point is the average of the grains (about 50 for each measurement) that showed RSE of less than 30% for the test dose (10 Gy) OSL measurements.

∼ 5% by introducing the additional bleaching step after each measurement of the signal due to the test dose. Each data point is the average for about 50 grains (i.e., those that gave a RSE of less than 30%). A similar spread in corrected OSL signals is observed through the bleaching cycles, irrespective of the use of the bleaching step. In the light of these results, and keeping in mind our need to use large test doses, we chose to use an additional bleaching step after each measurement of the test dose; this step consists of a 10 s optical stimulation at 90% of full power whilst the aliquot is held at 210 ◦ C in order to keep recuperation as low as possible. 4.2. Dependence of recovered dose on the pre-heat temperature The effect of the pre-heat temperature on the value of De obtained using a SAR procedure has been discussed for aliquots made up of many grains (Duller, 1991; Stokes, 1994; Jain et al., 2004). Each aliquot is assumed to be representative of the whole sample. However, single grains vary greatly in terms of their luminescence properties (e.g., Adamiec, 2000). To study the effect of different pre-heat temperatures on the OSL signals of single grains for our sample, the set of 800 grains used in the previous experiment was subjected to the measurement sequence in Table 1b. First, the grains were given a triple optical bleach at 210 ◦ C (Step 1 in Table 1b). A dose of 50 Gy was then given as a surrogate for the geological dose, followed by laser stimulation for 1 s at room temperature (Steps 2 and 3 in Table 1b). The thermally unstable traps filled by the laboratory irradiation were not depleted by this optical stimulation, but the fast OSL due to the 50 Gy dose will have been completely zeroed (Fig. 1); thus, this experiment will allow the influence of thermal transfer during the first OSL measurement to be investigated.

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Fig. 3. Dose-recovery ratios for a 10 Gy dose administered after a 1 s green laser bleach at room temperature of a 50 Gy surrogate natural dose (Table 1b). The dose-recovery ratio obtained using three different net signals discussed in the text is plotted as a function of pre-heat temperature. Insets (a) and (b) show the distributions for pre-heat temperatures of 180 and 270 ◦ C, respectively, with both being obtained using the longer signal integration time (0.17 s) and late background subtraction, as discussed in the text.

A SAR protocol was applied to these grains using repeated measurement of a known dose of 10 Gy and a 5 Gy test dose (Steps 4–11 in Table 1b). First, a 10 s pre-heat at 120 ◦ C was used both for the surrogate natural and for the regenerative dose. Subsequently, the complete experiment was repeated (including Steps 1–3, Table 1b) using 30 ◦ C higher temperatures up to 300 ◦ C. The value of dose obtained was then compared with the known dose (10 Gy given in Step 4) and the ratio of measured/given dose was calculated. These ratios are shown in Fig. 3 as a function of the pre-heat temperature, with values being calculated using three combinations of signal integration region and background subtraction, as shown in Fig. 1. For higher pre-heat temperatures (above 180 ◦ C), the value of the recovered dose is progressively overestimated. The insets in Fig. 3 show as histograms the ratios obtained for pre-heat temperatures of 180 ◦ C and 270 ◦ C for grains for which the RSE was less than 30%. For the lower pre-heat temperature shown in inset (a), 67 out of 800 grains passed the 30% RSE criterion and a dose-recovery ratio of 1.08 ± 0.05 was calculated when the signal was integrated over 0.17 s. For the higher pre-heat temperature shown in inset (b), only 400 grains were measured and 34 of them passed the criterion leading to a dose-recovery ratio of 1.31 ± 0.10. For the other pre-heat temperatures, the average ratios given in the main figure were each obtained using about 50 grains. The results indicate that the dose can be recovered for pre-heat temperatures of 120, 150 and 180 ◦ C irrespective of the integration intervals used. 4.3. Effect of changing size of test dose In earlier versions of the SAR protocol (Murray and Roberts, 1998; Murray and Mejdahl, 1999) it was suggested that the test dose should be kept small compared with the natural dose to avoid recuperation effects. Murray and Wintle (2000) suggest that for very young and/or dim samples, it is advantageous to use larger test doses to increase the strength of the test dose

Fig. 4. Dependence on test-dose (Dt ) of: (a) the dose-recovery ratio (R1 /N) for a dose of 10 Gy; (b) the recycling ratio (R3 /R1 ), and (c) percentage of accepted grains. Data are presented for a set of 100 grains and the experiment was carried out under two sets of conditions, as shown and discussed in the text and in Table 1c.

signal. These authors and Galbraith et al. (2005) examined the effect of varying the test dose size up to and above the value of the equivalent dose; their results suggested that larger test doses would probably not have an adverse effect of De determination, provided that there was no build-up of signal during the implementation of the SAR protocol. In our study of the coastal sands from Texel, equivalent doses are not expected to be above about 0.3 Gy, but we would like to have as large a test dose as possible in order that as many grains as possible pass the RSE acceptance criterion of 30%. To investigate the effects of using a test dose in the range from 0.15 to 1000 Gy, 100 grains were subjected to the experimental sequence given in Table 1c. This sequence was used as a simple dose-recovery test with a dose of 10 Gy being given three times—once to act as the unknown dose and provide its data point (N ), once to provide the first regeneration point (R1 ) and the third time to act as a recycling point (R3 ). Between R1 and R3 , a measurement was made with no dose being given (R2 ) in order to measure recuperation. The sequence in Table 1c was run both using 1 and 10 s stimulation times, and with and without the additional bleaching step discussed in Section 4.1. For both stimulation times the OSL signal used was from the first 0.085 s (from channels 6–10 in Fig. 1) and the background subtracted was half that for the last 0.17 s of the stimulation time. In Fig. 4c, it can be seen that the number of grains that are accepted using the criterion of RSE being less than 30% increases from 0 to ∼ 5% for test dose sizes of 0.15–150 Gy, respectively. Although only 100 grains were studied, it seems that for this sample a test dose between 10 and 100 Gy would lead to a similar percentage (∼ 5%) of grains being accepted. The OSL signals from these grains were then used to calculate the dose-recovery ratio (R1 /N , Fig. 4a) and the recycling ratio

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Table 2 Modified SAR protocol used for De estimation of single quartz grains

Fig. 5. Six dose–response curves for single grains of quartz from sample TX02-29 corrected for any sensitivity change through the SAR sequence using a test dose of 50 Gy (Table 1d). Results from exponential fitting of the curves are shown in the legend and discussed in the text.

(R3 /R1 , Fig. 4b); neither ratio shows a dependence on the size of test dose. Recycling ratios obtained with a 10 s stimulation time in combination with the additional optical stimulation step at 210 ◦ C were found to be closest to unity (Fig. 4b). Moreover we note that, although it appears possible to recover a dose of 10 Gy when using a test dose of 1000 Gy, we observed recuperation of a few percent (not shown) when the additional optical stimulation at 210 ◦ C was not used. 4.4. Dose–response curves Since the equivalent doses for the Texel samples were expected to be ∼ 0.3 Gy, we considered using a SAR protocol with only one regeneration dose and use of linear interpolation between the origin and this point in order to determine De . A single regeneration point had been used by Murray and Roberts (1997) in their single-grain quartz study. In order to select an appropriate value for the regeneration dose, we first constructed dose–response curves for six grains from sample TX02-29, using the sequence in Table 1d. Suitable parameters for the SAR protocol were selected based on the experiments described above and are given in Table 1d. In Fig. 5 the sensitivity-corrected OSL is shown as a function of laboratory doses up to 130 Gy. The sensitivity-corrected OSL data were fitted with a single exponential function of the form I = Isat (1 − e−D/D0 ), where Isat is the sensitivity-corrected OSL intensity at saturation, I is the sensitivity-corrected OSL intensity produced by D, the laboratory regenerative dose, and D0 is a dose parameter indicative of the onset of saturation. The shapes of the curves in Fig. 5 are similar to those presented by others for single grains (e.g., Adamiec, 2000; Jacobs et al., 2003; Yoshida et al., 2000). Based on the range of values of D0 shown in the table inset in Fig. 5, we selected the regeneration dose for the SAR protocol for samples from Texel to be 5 Gy, i.e., half the lowest dose used in these measurements of the dose–response curve. This

Step

Treatment

Observed

1 2 3 4 5 6 7 8 9 10 11 12

Dose, Di (i = 1, . . . , 4) 10 s PH at 180 ◦ C 10 s OSL at 125 ◦ C Test dose, Dt = 50 Gy Cut-heat 160 ◦ C 10 s OSL at 125 ◦ C 10 s OSL at 210 ◦ C Return to 1 Test dose, Dt = 50 Gy Cut-heat 160 ◦ C 40 s IR at room temp. 10 s OSL at 125 ◦ C

– – Li – – Ti – – – – – TIR

a

the first cycle (i = 1) no dose was given and the natural OSL signal recorded. The administered regenerative doses were 5, 0 and 5 Gy.

a In

value was unlikely to cause more than a few percent error in the evaluation of De when using a linear interpolation. 4.5. Parameters selected for single-grain dating studies on Texel From the results in this section, based on sample TX02-29, it was concluded that a test dose of 50 Gy would be appropriate when dating the individual grains from the young (< 300 year old), coastal sands on the island of Texel. Also, the SAR protocol should employ a high-temperature optical stimulation (210 ◦ C for 10 s) after each test dose measurement in order to prevent recuperation of the OSL signal through the SAR protocol. In addition, a pre-heat of 10 s at 180 ◦ C was chosen for use before the main OSL measurements related to the natural and regenerative doses in order to avoid any effects due to thermal transfer. Based on the dose–response curve shape, a single regenerative dose of 5 Gy was selected. Because of the large scatter (both positive and negative values) obtained when the zero dose point was measured, it was decided to use the origin (0,0) for calculation of De . The modified SAR protocol is given in Table 2. 5. Selection of the integration regions for OSL signal used for dating Texel sands 5.1. Selecting the fast component OSL signal As indicated in Fig. 1, it is necessary to select part of the OSL decay curve as the signal and also a later part as the background to be subtracted. The net signal thus obtained should be dominated by the fast component of the OSL signal. It is important to select the fast component because the slower components may be less well reset in nature and cause overestimation of the equivalent dose (e.g., Bailey, 2003) and because the SAR protocol has been developed for the fast component of the OSL signal (Wintle and Murray, 2006). Hence, we decided to choose integration regions that preferentially selected the fast component relative to the medium and slow components.

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where i is indicative of the component (i = fast, medium and slow), Ai are the maximum intensities and i are the decay constants. The instrumental background is accounted for by adding a constant to the sum of the fast, medium and slow OSL component signals shown in Fig. 6 as 265 cts/channel. The percentage contribution of each component to the net OSL signal after EBG subtraction is compared to that after LBG subtraction. The equation we used is

Fig. 6. Synthetic aliquot OSL decay curve obtained for 400 grains of sample TX02-29 in the first 0.5 s of laser stimulation for a 50 Gy test dose. Resolved contributions for the fast, medium and slow components and for the background are shown; fitting parameters are given in the legend. The fitting procedure is discussed in the main text.

In order to apply this philosophy to the samples from Texel, we first compared the OSL decay curve obtained over 0.5 s with published information on OSL signals to investigate what components were present in our grains. Since the signals from individual grains were weak, with that for one of the brightest grains being shown in Fig. 1, the OSL signals for 400 grains were summed, i.e., by adding the signals obtained for all the grains mounted on 4 of the single-grain discs without applying any rejection criteria. Fig. 6 shows the OSL signal obtained at 125 ◦ C in response to a test dose of 50 Gy. The data were fitted using three decay constants of 45, 5.52 and 0.33 s−1 and a constant background. The value for the first decay constant was chosen as that for the fast component reported in another single-grain study that used the same wavelength and power of stimulation (Bulur et al., 2002). The second value was based on the relative photoionisation cross-section for the fast and medium components at 525 nm stimulation given by Singarayer and Bailey (2004). The decay constant of the slow component was obtained by fitting the decay curve of Fig. 6 for the region from 1 to 10 s. The amplitudes of the three components and the constant background were entered as free parameters in a fitting procedure using Microcal Origin 7.5 software while the decay constants were fixed. The results are shown in the legend in Fig. 6. Using the values of the amplitude and decay constants from this simple approach, we investigated what integration regions could be selected to minimise the relative contributions to the net signal from medium and slow components. The initial signal was calculated for integration over the first 0.017, 0.034 and 0.17 s of stimulation (channels 6, 6–7 and 6–15, respectively). The background signals were calculated both for the region immediately following the initial signal integration region (0.017–0.034, 0.034–0.068 and 0.170–0.234 s) and for the final part of stimulation (9.93–10.00 s); the background regions are termed early background (EBG) and late background (LBG). Each OSL component was represented by an exponential decay of the form OSLi (t) = Ai exp(−i t),

OSLEBGi OSLLBGi   t 1  tEBG exp(− t) dt Ai 0s exp(−i t) dt − i kEBG ts  , =  ts 1  t595 Ai 0 exp(−i t) dt − exp(−i t) dt kLBG t591 where ts is the time over which the initial signal was integrated in the first n channels (ts = nQ0.017 s; n = 1, 2, 10); tEBG is the time used for integrating the EBG in the subsequent m channels (tEBG =mQ0.017 s; m=1, 2); t591 −t595 is the fixed time interval used for the LBG and corresponds to the last five channels of the decay curve (9.85–9.93 s; m = 5); kEBG and kLBG are defined as m/n for the EBG and LBG regions, respectively. The results (Table 3) indicate that EBG subtraction methods are highly effective in reducing the contribution of the slow component. The contribution of the medium component depends on the integration region used for the initial signal; short integration intervals are most effective in this respect. However, the drawback of using short initial signal integration intervals is that only part of the fast OSL component is contained; this affects the signal intensity and thus the precision of the measurements. Based on our investigation we suggest that a combination of the first two channels (0.034 s) for the initial signal and the subsequent two channels for the EBG subtraction provides a good compromise. This approach reduces the contribution of the slow and medium components by 99 and 83%, respectively, compared to LBG subtraction, while the fast component OSL signal is only reduced by 22% (Table 3, second column). The data obtained in the next section are used to test the suitability of these integration regions. 5.2. Checking the suitability of integration intervals using synthetic aliquots We investigated the dependence of De on the integration interval by using the collective OSL output from individual grains in the way that the OSL decay curve shown in Fig. 6 was obtained. The grains can be seen as making up a synthetic aliquot, an approach previously applied by Henshilwood et al. (2002) and Jacobs et al. (2003) in their study of coastal-dune sand. For this experiment we measured the OSL signals from 2000 grains; only grains that passed the rejection criteria outlined in Sections 3.3 and 3.4 were included in the synthetic aliquot. It should be pointed out that this method can only be used provided the dose rate from the laboratory source is uniform across the single-grain disc, a condition that is not always fulfilled (Ballarini et al., 2006). Use of the summed OSL output can

M. Ballarini et al. / Radiation Measurements 42 (2007) 360 – 369

367

Table 3 Reduction in net OSL signal due to EBG subtraction Component

Fast Medium Slow a For

Reduction in net OSL: (1 −

OSLEBGi a OSLLBGi )×100%

Signal = 0–0.017 s EBG = 0.017–0.034 s (%)

Signal = 0–0.034 s EBG = 0.034–0.068 s (%)

Signal = 0–0.170 s EBG = 0.170–0.204 s (%)

47 91 99

22 83 99

0 11 97

LBG the initial signal indicated in the column header is used in combination with the background signal integrated over 9.85–9.93 s.

Fig. 7. Natural and regenerative dose OSL responses of 2000 grains from sample TX02-29 were measured. The percentage of grains accepted for inclusion in the synthetic aliquot and the synthetic aliquot De is given as a function of the RSE criterion used (expected palaeodose 0.236 ± 0.010 Gy, indicated by the shaded bar). Results for four different integration regions for obtaining the net signal are shown; only those obtained using EBG subtraction are independent of the RSE threshold used.

be applied in this study as the radiation source that we used (source 6088) was uniform (Ballarini et al., 2006). The synthetic aliquot De was obtained by projecting the ratio Ln /Tn onto the line drawn between the origin (0,0) and R1 for the first regeneration dose of 5 Gy; this approach was preferred to that using the measurement for a zero dose (R2 ) as the latter are scattered as discussed at the end of Section 4. In Fig. 7 we show De values for synthetic aliquots obtained using OSL signals from grains from sample TX02-29 as a function of the RSE threshold applied. Results are shown for two integration intervals for the initial signal (0.034 and 0.17 s) combined with EBG and LBG subtraction. Similar results were obtained for sample TX02-31 (not shown). The data obtained using the late background subtraction method (as used in standard single aliquot dating) show an increase in the value of the equivalent dose when dim grains are included in the synthetic aliquot (Fig. 7; greater RSE threshold). We interpret this trend to be the result of the incorporation of an OSL signal from slow OSL components that were not completely reset prior to deposition. This contribution may strongly affect the results for our samples because the natural OSL signal is very small, especially for dim grains. The effect is enhanced by the strong power of the stimulation source and the long stimulation time

(10 s) adopted for this study. When the signal observed after 1 s stimulation is used for the background calculation, the dependency of the equivalent dose on the RSE threshold used is reduced but not eliminated (data not shown). When the EBG subtraction methods are used, the contribution of the slow component to the net signal is minimised (Table 3) and no dependency of the equivalent dose on the RSE threshold is found. We conclude that, especially for young samples and for dim grains, it is important to avoid contribution of the slow OSL components to the natural signal. This may be achieved by obtaining a net signal through subtraction of an EBG signal immediately following the integration interval used for the initial signal. Based on the calculations presented in Section 5.1 and our synthetic aliquot results, we adopt a signal integration interval of 0.034 s combined with a background measured over the subsequent 0.034 s for application in our dating study.

6. Application of the SAR protocol to two samples from Texel In a previous study, a sample of modern sand (TX02-8) was taken from an embryo dune on Texel. From a study of recent maps, this dune was known to be no more than 10 years old. For this sample, it was found that using the combination of the initial 0.034 s OSL signal and the subsequent 0.034 s for EBG subtraction (as used in this study) resulted in a narrow and symmetric De distribution (Ballarini et al., 2007). For this modern sample, the single aliquot De was 6 ± 1 mGy, in agreement with the expected value of 8 mGy, or less. The value obtained from the Gaussian fit to the De values obtained for 31 grains (using the RSE criterion set to 10%) was 22 ± 5 mGy, equivalent to an age of 27 years. Having established this apparent limit, it was then appropriate to apply the same procedure to the two samples listed in Section 2. The SAR protocol given in Table 2 was applied to 2000 grains from each of the samples in order to determine De values. The rejection criteria outlined in Section 3, the signal integration interval presented at the end of the previous section and a RSE threshold of 30% were used. The values of De for samples TX02-29 and TX02-31 are shown in the histograms in Fig. 8. The data sets are for 90 and 94 grains, respectively, together with a Gaussian fit for each data set

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M. Ballarini et al. / Radiation Measurements 42 (2007) 360 – 369

calculation. It should be noted that the De estimates based on single-grain measurements are less precise than the estimates obtained using single-aliquot methods. 7. Conclusion

Fig. 8. Histograms and Gaussian fits for single grain De values obtained after application of the rejection criteria discussed in the text for sample TX02-29 and TX02-31. The mean value was calculated with Gaussian fitting.

Table 4 Results from single-grain analysis for two natural samples Sample (n = 2000)

TX02-29

TX02-31

Expected dose, SA (Gy)a

0.236 ± 0.010

0.244 ± 0.010

Single grainb

De (Gy)

Average Weighted mean Median Min Max

0.457 ± 0.281 (62%) 0.136 ± 0.032 (16%) 0.289 −16 12

0.310 ± 0.193 (62%) 0.197 ± 0.016 (8%) 0.219 −9 5

Accepted grainsc Rejected as feldspar grainsc

90 (4.5%) 8 (0.4%)

94 (4.7%) 11 (0.5%)

Synthetic aliquot

0.245 ± 0.031 (13%)

0.240 ± 0.020 (9%)

Gaussian fit (Fig. 8)

0.298 ± 0.020 (7%)

0.199 ± 0.028 (14%)

a Multiple-grain

single-aliquot equivalent-dose giving age in agreement with independent age control (Ballarini et al., 2003). b The net signal was obtained from the initial signal in the first 0.034 s of the decay curve and the EBG over the subsequent 0.034 s. cAcceptance criteria used are RSE < 30% and (post-IR) OSL to OSL ratio < 0.70.

and a dashed line representing the De value obtained in an earlier study using multiple-grain single-aliquots (Ballarini et al., 2003). The mean values listed in Fig. 8 are from the Gaussian fit and can be compared with the average values, weighted mean and median values given in Table 4 for each sample. The synthetic aliquot results are also presented in Table 4. The De values obtained using the Gaussian fit and those obtained from the synthetic aliquots most closely resemble the expected dose based on the single-aliquot study (Ballarini et al., 2003). The median values are also similar to these values. However, the inclusion of grains that gave exceptionally high or low De values (shown in Table 4 as Max or Min values) biased the values obtained when average or weighted mean De values were calculated. It is concluded that these are inappropriate ways of calculating a De value for use in the age

In an investigation of laboratory-irradiated quartz grains from sand samples from the island of Texel in the Netherlands, we have tested a SAR protocol that uses a single regenerative dose (5 Gy), a large test dose (50 Gy) and an additional high temperature bleach (210 ◦ C for 10 s) after measurement of the testdose response. This protocol has been designed for measuring the equivalent doses of grains from young sands with expected doses of less than 1 Gy. A low temperature pre-heat (180 ◦ C for 10 s) before measurement of the natural and regenerative doses was selected on the basis of a dose recovery test designed to detect thermal transfer effects. Measurements of De were made using this protocol on two 300-year-old sands from Texel. Following application of a rejection criterion that requires the RSE on the OSL signal from all test doses used in the SAR protocol to be less than 30%, about 5% of the initially measured 2000 grains were accepted for calculation of De . From investigation of different regions of integration used to obtain the net signal, it was concluded that the most appropriate signal, i.e., that containing the fast OSL component which is most likely to have been completely bleached and for which the SAR protocol was designed, is that obtained from the first 0.034 s of laser stimulation, with the background for subtraction coming from the next 0.034 s. This net signal was shown to contain a relatively small contribution from the medium and slow OSL signal components. When standard background subtraction methods were used, equivalent doses were greatly overestimated for dim grains due to the incorporation in the net signal of a slow OSL component that was apparently not completely reset at the time of burial. Values of the equivalent dose close to those previously obtained using multiple-grain single-aliquots were obtained using Gaussian fitting of the single grain equivalent dose distributions and using a synthetic aliquot approach. Although we have demonstrated the feasibility of a SAR protocol for two young sands from one site on the coast of the Netherlands, further testing using a larger number of well-dated young samples from other locations is required. Acknowledgements The manuscript was greatly improved following suggestions by the editor, Prof. Ian Bailiff, and two anonymous reviewers. We thank the Netherlands Organisation for Scientific Research for supporting this research through NWO grants 863.03.006 and 834.03.003. References Adamiec, G., 2000. Variations in luminescence properties of single quartz grains and their consequences for equivalent dose estimation. Radiat. Meas. 32, 427–432.

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