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The single aliquot regenerative dose protocol: potential for improvements in reliability
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Radiation Measurements 37 (2003) 377 – 381 www.elsevier.com/locate/radmeas
The single aliquot regenerative dose protocol: potential for improvements in reliability A.S. Murraya , A.G. Wintleb;∗ a Nordic
Laboratory for Luminescence Dating, Department of Earth Sciences, Arhus University, Ris National Laboratory, DK-4000 Roskilde, Denmark b Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB, UK Received 21 August 2002; received in revised form 8 January 2003; accepted 14 January 2003
Abstract This paper examines the e/ects of optically stimulated luminescence (OSL) components, other than that usually termed the fast component, on dose determination by the single-aliquot regenerative-dose (SAR) protocol. Results are presented for “dose recovery tests”, in which a known laboratory dose, delivered after optical bleaching at room temperature, is measured using the SAR protocol. Data obtained using either the initial OSL signal or the fast component, derived by curve 7tting, are compared. Dose recovery tests are also carried out when an additional step is added to the SAR protocol, aimed at reducing recuperation, i.e. the residual signal observed in a SAR cycle when no regenerative dose is applied. The results for quartz from various sources indicate much improved dose recovery when only a well-separated fast component is analysed. c 2003 Elsevier Science Ltd. All rights reserved. Keywords: Optically stimulated luminescence; Quartz; Fast component; Single-aliquot regeneration; Dose recovery
1. Introduction The single aliquot regenerative dose (SAR) protocol was introduced as a method of obtaining a reliable value for an unknown dose received by quartz grains being used as dosimeters for dating or accident dosimetry (Murray and Wintle, 2000a; Banerjee et al., 2000). The SAR experimental sequence makes use of both the optically stimulated luminescence (OSL) signal arising from the unknown dose to be measured, and a number of OSL signals resulting from laboratory doses. The OSL sensitivity is monitored by the OSL response to a non-varying test dose given following completion of the main OSL measurement for each dose. Both the regenerated OSL growth curve, and the natural OSL from the unknown dose, are then sensitivity-normalised, by dividing by the appropriate test dose OSL response, before the unknown dose is calculated. The need to allow for sensitivity change when using a regenerative dose procedure
∗ Corresponding author. Tel.: +44-1970-622-658; fax: +441970-622-659. E-mail address: [email protected] (A.G. Wintle).
with sedimentary or heated quartz is widely recognised, with sensitivity changes of up to a factor of two occurring when the grains were heated (Wintle and Murray, 2000; Turney et al., 2001). These changes result mainly from the heat treatments applied prior to OSL measurement in order to isolate a thermally stable signal, e.g. 240◦ C for 10 s. The OSL signals used for both the main OSL measurements and the test dose measurements are usually the initial part of the continuous wave (CW) OSL decay curve. For blue (470 nm) diode stimulation giving ∼50 mW=cm2 at the sample, the signal is typically that obtained in less than the 7rst second of stimulation. The grains are usually held at 125◦ C during stimulation in order to prevent the 110◦ C TL trap from accumulating charge and subsequently contributing to the OSL signal (Murray and Wintle, 1998). The initial signal, being the most light-sensitive, is most likely to have been removed by daylight during grain transport prior to incorporation in a sedimentary deposit. The SAR protocol has been applied to a large number of both sedimentary and heated quartz grains in both our laboratories and others around the world. For sedimentary quartz, the reliability has been demonstrated by comparisons for about 50 samples with independent age information
c 2003 Elsevier Science Ltd. All rights reserved. 1350-4487/03/$ - see front matter doi:10.1016/S1350-4487(03)00053-2
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A.S. Murray, A.G. Wintle / Radiation Measurements 37 (2003) 377 – 381
spanning the last 500,000 years (Murray and Olley, 2002). These samples were from deposits with di/erent depositional histories (aeolian, Euvial, marine and glacial), and it was concluded that there was no evidence for systematic uncertainties in the results, and that in almost all cases total uncertainties had been correctly assessed. However, there have been a few reports of SAR OSL ages that were not in agreement with the expected age (e.g. Stokes et al., 2000). This paper investigates some samples that had been thought to be problematic and suggests small modi7cations to methods of analysis or measurement procedure.
2. The SAR protocol In our view, the SAR protocol is distinguished from other measurement protocols by the use of the response to a 7xed test dose (measured after the signal resulting from the natural dose and every regenerative dose) to monitor sensitivity changes. The basic SAR protocol applied to quartz OSL signals (it can, for instance, also be applied to TL signals; Murray and Wintle, 2000b) is given in Table 1, where it can be seen that although the sequence is simple, the exact measurement conditions can be varied considerably. For example, a range of preheat temperatures may be employed in order to test for thermal transfer, isolate the most stable component, and/or accentuate potential sensitivity changes and thus provide a more strenuous test of the SAR technique. The fundamental assumption is that the response (T ) to the test dose provides an appropriate measure of the sensitivity that pertained during measurement of the main OSL signal (L). Besides its widespread application to sand-sized quartz grains, the measurement sequence given in Table 1 has been used in a modi7ed form by Banerjee et al. (2001) and by Roberts and Wintle (2001) with the OSL from samples of mixed mineralogy, containing both quartz and feldspar grains, and for the infrared stimulated luminescence (IRSL) from potassium-rich feldspar grains
Table 1 Generalised single-aliquot regenerative-dose protocol incorporating an extra step to reduce recuperation
1 2 3 4 5 6 7 8
Treatment
Observe
Give dose, Di Preheat (160 –300◦ C for 10 s) Optically stimulate for 40 s at 125◦ C Give test dose, Dt Heat to 160◦ C (to ¡preheat in step 2) Optically stimulate for 40 s at 125◦ C Optically stimulate for 40 s at ¿preheat Return to 1
— — Li — — Ti — —
Note: 1. The stimulation time is dependent on the stimulation light (blue diodes) intensity. 2. Step 7 is justi7ed in this paper. 3. Changes to original protocol given in bold text in parentheses.
(Wallinga et al., 2000), and even for blue stimulated luminescence from various arti7cial phosphors of application to accident dosimetry (Thomsen et al., 2002). For the response (T ) to the test dose to monitor the luminescence sensitivity, the assumption is made that the electron traps that are being probed are the same as those responsible for the main signal (L). However, it has long been known that there is more than one OSL signal from quartz, each with a di/erent optical stability. Bailey et al. (1997) termed these the fast, medium and slow components and they have been further investigated by Singarayer and Bailey (2003) using linearly modulated OSL (LM-OSL). The SAR protocol was developed using a mixture of these quartz signals. In the samples used to validate the protocol, this mixed signal was dominated by the fast component, as is found for the OSL of many quartz samples. However, if the medium and slow components are relatively large compared to the fast component and if they sensitise at di/erent rates as the result of a given thermal treatment, then a simple analysis using the initial (mixed) signal may not be appropriate. Where the medium and slow components make up a large proportion of the signal, as identi7ed by LM-OSL curves or from log-linear plots of CW-OSL, then it may be necessary to use a curve 7tting procedure to isolate the fast component of the OSL signals. Furthermore, for a small number (¡10%) of samples an ultra-fast component has been found following laboratory irradiation (Jain et al., 2003). Since it is not present in the natural OSL signal, it presumably represents a less thermally stable, but more light sensitive, component. If this ultra-fast component is not detected, e.g. by using LM-OSL or log-linear plots of CW-OSL, signi7cant age underestimation can result (Choi et al., 2003). To obtain the correct age for such samples, it is necessary to employ a preheat temperature range (e.g. 160 –280◦ C), 7rst to identify the problem, and second, to select an appropriate thermal treatment in order to erase the ultra-fast component resulting from laboratory irradiation (e.g. above 220◦ C). In addition, it may be necessary to employ a more stringent thermal treatment (cut heat) to remove the ultra-fast component from the test dose signal (e.g. if the component uses di/erent luminescence centres); such a cut heat may be as high as 200◦ C or 220◦ C, or be selected to be about 20◦ C below the preheat temperature. A further problem causing inaccurate dose determination for some samples could be high levels of OSL recuperation, e.g. as reported by Stokes et al. (2000). This is also likely to be due to the measured OSL signal not being derived only from the fast component. We have attempted to minimise the e/ect of recuperation by modifying the SAR procedure in Table 1 by the addition of a high temperature (e.g. 280◦ C) optical stimulation following each measurement of the test dose signal. In this paper we examine the e/ects of these three changes to analysis and measurement procedures using a range of samples that have shown atypical behaviour.
Recycling ratio
1.4
379
(a)
1.2 1.0 0.8 0.6 0.0 1.4
(b)
1.2 1.0 0.8 0.6 0.0 0.3
(c)
0.2
0.1
0.0 00 03 0 01 1 18 02 00 25 3 00 0 25 6 98 8 32 0 99 5 10 26 00 09 0 99 5 07 0 99 5 49 01
For most samples there is no independent age control, and thus it is important to be able to detect atypical behaviour in samples of unknown age. For this reason, an experimental procedure is needed to test that any SAR protocol (modi7ed or otherwise) results in an accurate determination of the unknown dose for an arbitrary sample. We suggest using a “dose recovery test”, in which SAR is applied to an aliquot that has been given a laboratory dose following optical bleaching of the aliquot. Such an approach has been reported for the SAR protocol applied to single grains of quartz (Roberts et al., 1999) and also to multiple grain aliquots of feldspar (Wallinga et al., 2000). A similar experimental design was employed by Folz and Mercier (1999), though they used the 110◦ C TL peak for sensitivity correction (Murray and Roberts, 1998). In such a test, the ratio of the measured to given dose should be unity. Using large doses in their study of aliquots of quartz grains from Eemian samples, Murray et al. (2002) obtained an average ratio of 0:952 ± 0:010. This ratio provides a more stringent test than that of the recycling ratio that is part of the SAR protocol (Murray and Wintle, 2000a), since the unknown dose is applied prior to any thermal treatment (e.g. the preheat); it thus mimics the natural dose (measured as the equivalent dose), except that it is delivered at a dose rate ∼109 times higher. Any successful measurement protocol must be able to recover such a known dose, but although this is a necessary test, it is not a complete test. The most complete test available is that a sample should meet all laboratory criteria, and yield good agreement with the known age. For the current dose recovery tests, two room temperature optical stimulations using the blue diodes for 40 s were used to bleach the aliquots, with a 10 ks room temperature storage between the two stimulations so that any charge transferred into the 110◦ C TL trap has time to thermally decay. It is possible that 40 s stimulation at 125◦ C may achieve the same e/ect, without causing any additional change. For each sample, the dose given was chosen to be close to the expected natural dose. The SAR protocol was then applied to three aliquots of each sample, employing a 10 s preheat at 260◦ C and a cut heat at 160◦ C. The results for nine samples from RisH are shown in Fig. 1. For all samples, the ratio of the measured to given dose (Fig. 1a) is within 26% of unity when using the initial signal (7lled circles) in what would be considered a standard analysis; the mean ratio was 0:96 ± 0:16. When the fast component was separated by curve 7tting (open circles), the ratios were slightly closer to unity, giving a mean of 1:00 ± 0:14. For those samples giving values close to 1.00, the value obtained was the same for both methods of analysis. However, there are still some samples (e.g. 000301 and 994901) for which selection of the fast component by curve 7tting resulted in the ratio only being consistent with unity at the 2 level. For the nine samples of this study, only a
Recuperation
3. Test for suggested changes
Measured/Given ratio
A.S. Murray, A.G. Wintle / Radiation Measurements 37 (2003) 377 – 381
Sample Code Fig. 1. Data obtained from dose recovery experiments on nine samples using a 10 s preheat at 260◦ C and a cut heat at 160◦ C in the basic SAR sequence: (a) measured to given dose ratio; (b) recycling ratio; and (c) recuperation expressed as a fraction of the OSL response to a given dose. Data obtained using the initial part of the OSL decay curve are given by 7lled circles; those obtained using the fast component derived from curve 7tting are given by open circles, with error bars being 1 .
slight reduction was found for the scatter in the measured to given dose ratio when a higher preheat (10 s at 280◦ C) and cut heat (240◦ C) were used (data not shown). The recycling ratios are also shown (Fig. 1b), and for some samples they are outside the range of 0.90 –1.10 suggested as a possible criterion for acceptance by Murray and Wintle (2000a). If accurate remeasurement of a point on the laboratory-generated dose-response curve is not possible, then some systematic error in the interpolation of Ln =Tn onto the dose-response curve is inevitable. Nevertheless, from the data presented in Fig. 1a and b, it can be seen that acceptable dose recovery can be achieved, even when a poor recycling ratio is obtained (e.g. sample 002530) and vice versa (e.g. sample 991026), indicating that the recycling ratio is not a particularly sensitive measure for the suitability of the measurement protocol.
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4. Recuperation
0.15
0.10 With blue light
Stimulation time, s 1
10 OSL (0.16 s)-1
Recuperation
No blue light
0.05
0.00 150
10 4 5x10 3
200 250 Temperature,oC
300
Fig. 2. Fractional recuperation obtained after holding an aliquot at the stated temperature for 40 s whilst exposing to light from blue diodes. Data shown for sample 994801 (preheat 260◦ C for 10 s). Open circles obtained when no stimulation is applied. The inset shows the recuperation decay curves after a 260◦ C preheat and when 40 s blue light stimulation is applied at 280◦ C prior to the 260◦ C preheat.
1.4
(a)
1.2 1.0
Measured/Given ratio
0.8 0.6 0.0 1.4
o
Preheat 260, cut-heat 160 C (b)
1.2 1.0 0.8 o
0.6
Preheat 260, cut-heat 220 C o 40 s blue stimulation at 280 C
0.0 00 03 0 01 1 18 0 00 2 25 3 00 0 25 6 98 8 32 0 99 5 10 26 00 09 0 99 5 07 0 99 5 49 01
The recuperated OSL signal is that measured following a zero Gy regenerative dose in a SAR measurement cycle. Of the 7ve samples showing high recuperation signals (when expressed as a fraction of the response to the given dose, 0.07– 0.25 in Fig. 1c, RisH sample numbers 000301, 011802, 000905, 990705 and 994901) only one (000905) had recuperation reduced to zero when component stripping was applied (when the higher preheat (10 s at 280◦ C) and cut heat (240◦ C) were used, these samples still showed similar amounts of recuperation (data not shown)). When the initial signal was used, four of the samples gave ratios of measured to given dose that were signi7cantly less than unity for both sets of thermal treatment. This possible dependence of recuperation on OSL components, and its e/ect on dose measurement, is now considered in more detail. From the timing of the recuperation measurement in the SAR cycle, it can be deduced that recuperation results from charge remaining in a light insensitive trap at the end of step 6 of the protocol (Table 1). This charge has only experienced relatively low temperature (test dose cut-heat, step 5) since storage during beta irradiation. The preheat (step 2) prior to measurement and after the zero regenerative dose, thermally stimulates this charge and transfers some of it to an optically sensitive trap, thus resulting in a measurable OSL signal. The fact that, for four of the 7ve samples, use of component 7tting reduces the amount of recuperation (Fig. 1c) (in 2 cases to zero) suggests that the trap responsible for the recuperated OSL signal is probably not the main OSL trap. A similar conclusion has been reached by Jain et al. (2003). This recuperation may thus be interpreted as an artefact of not using only the fast component of the OSL signal in subsequent data analysis. Rather than having to employ curve stripping, it may be possible to minimise this component experimentally. Optical stimulation at an elevated temperature should result in optical removal of charge as soon as it arrives at the OSL trap as the result of thermal transfer from the optically insensitive trap. This can be achieved by following the test dose measurements (step 6) with an additional optical stimulation at a temperature close to (or higher than) the preheat temperature (step 7). The e/ect of such an elevated temperature stimulation step on the measured recuperation can be seen for a sample with relatively high fractional recuperation (Fig. 2); the subsequent preheat (step 2) has been kept constant at 260◦ C, and recuperation measured as a function of the temperature of the preceding optical stimulation (step 7). The amount of recuperation decreases signi7cantly as the stimulation temperature is increased. The inset in Fig. 2 compares the decay curves after zero dose, measured using (i) the standard SAR protocol and (ii) a modi7ed protocol using the additional 40 s stimulation at 280◦ C. For measurements made without the extra optical stimulation step, increasing treatment temperature increased the amount of recuperation. When the
Sample Code Fig. 3. Measured to given dose ratios (a) same data as in Fig. 1a and (b) obtained using an additional step (7) in the SAR protocol given in Table 1.
additional step (step 7, including blue light stimulation) is applied to the samples that previously showed high fractional recuperation values (0.07– 0.25), a fractional recuperation of below 0.03 was achieved when using the initial signal and the measured to given dose ratios became much closer to unity and less scattered (Fig. 3b). Thus applying the modi7ed SAR protocol certainly improves the recovery of a known dose administered in the laboratory. We anticipate that it should also result in more accurate determination
A.S. Murray, A.G. Wintle / Radiation Measurements 37 (2003) 377 – 381
of unknown natural doses, and may also result in more precise equivalent dose evaluations. The former is diMcult to demonstrate, although the latter is testable in the laboratory. It should be pointed out that the experimental results presented in this paper were obtained using aliquots made up of several hundred grains. The dose recovery test applied assumes homogeneous OSL behaviour. However, studies of single quartz grains have shown a wide variation in various OSL characteristics. For this test to be useful with single grains or small aliquots, it would need to be applied to a suMcient number to demonstrate that both the average behaviour and the variability about this average were acceptable. 5. Conclusions Several years of experience of using the simple SAR protocol of Murray and Wintle (2000a) has demonstrated the usefulness of this procedure, as judged by its application to independently dated samples. For these samples, the OSL signal was almost certainly dominated by the fast component. Following the work of others on samples for which the fast component was not dominant, use of curve stripping and application of higher preheats suggested that samples for which SAR appeared to be unsatisfactory were those for which the fast component was not dominant. For some of the samples in this study, recuperation was found to be relatively large compared with the natural OSL, and to be linked to underestimation of a known dose given in the laboratory prior to any thermal treatment. This recuperation could be minimised by carrying out optical bleaching at an elevated temperature between each cycle of the SAR protocol. The e/ects of not using a well-separated fast component can be detected by using dose recovery tests. It is important that such tests are carried out routinely on samples being dated, and when procedures are being modi7ed. The dose recovery test is more diagnostic than the recycling ratio criterion previously used, although it must be remembered that it does not test whether the natural dose can be measured accurately. Acknowledgements The authors wish to thank Dr. M. Jain, J.H. Choi and T. Watanuki for discussions and access to their unpublished work, and Dr. R.G. Roberts for critical comments on the manuscript. References Bailey, R.M., Smith, B.W., Rhodes, E.J., 1997. Partial bleaching and the decay form characteristics of quartz OSL. Radiat. Meas. 27, 123–136. Banerjee, D., BHtter-Jensen, L., Murray, A.S., 2000. Retrospective dosimetry: estimation of the dose to quartz using the
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single-aliquot regenerative-dose protocol. Appl. Radiat. Isot. 52, 831–844. Banerjee, D., Murray, A.S., BHtter-Jensen, L., Lang, A., 2001. Equivalent dose estimation using a single aliquot of polymineral 7ne grains. Radiat. Meas. 33, 73–94. Choi, J.H., Murray, A.S., Jain, M., Cheong, C.S., Chang, H.W., 2003. Luminescence dating of well-sorted marine terrace sediments on the southeastern coast of Korea. Quat. Sci. Rev. 22, 407–421. Folz, E., Mercier, N., 1999. A single-aliquot OSL protocol using bracketing regenerative doses to accurately determine equivalent doses in quartz. Radiat. Meas. 30, 477–485. Jain, M., Murray, A.S., BHtter-Jensen, L., 2003. Characterisation of blue-light stimulated luminescence components in di/erent quartz samples: implications for dose measurement. Radiat. Meas. this issue. Murray, A.S., Olley, J.M., 2002. Precision and accuracy in the optically stimulated luminescence dating of sedimentary quartz: a status review. Geochronometria 21, 1–16. Murray, A.S., Roberts, R.G., 1998. Measurement of the equivalent dose in quartz using a regenerative-dose single-aliquot protocol. Radiat. Meas. 29, 503–515. Murray, A.S., Wintle, A.G., 1998. Factors controlling the shape of the OSL decay curve in quartz. Radiat. Meas. 29, 65–79. Murray, A.S., Wintle, A.G., 2000a. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiat. Meas. 32, 57–73. Murray, A.S., Wintle, A.G., 2000b. Application of the single-aliquot regenerative-dose protocol to the 375◦ C quartz TL signal. Radiat. Meas. 32, 579–583. Murray, A.S., Wintle, A.G., Wallinga, J., 2002. Dose estimation using quartz OSL in the non-linear region of the growth curve. Radiat. Prot. Dosim. 101, 371–374. Roberts, H.M., Wintle, A.G., 2001. Equivalent dose determinations for polymineralic 7ne-grains using the SAR protocol: application to a Holocene sequence of the Chinese Loess Plateau. Quat. Sci. Rev. 20, 859–863. Roberts, R.G., Galbraith, R.F., Olley, J.M., Yoshida, H., Laslett, G.M., 1999. Optical dating of single and multiple grains of quartz from Jinmium rock shelter, northern Australia: Part II, Results and implications. Archaeometry 41, 365–395. Singarayer, J.S., Bailey, R.M., 2003. Component-resolved bleaching spectra of quartz optically stimulated luminescence: preliminary results and implications for dating. Radiat. Meas., submitted for publication. Stokes, S., Colls, A.E.L., Fattahi, M., Rich, J., 2000. Investigations of the performance of quartz single aliquot DE determination procedures. Radiat. Meas. 32, 585–594. Thomsen, K.J., BHtter-Jensen, L., Murray, A.S., 2002. Household and workplace chemicals as retrospective luminescence dosemeters. Radiat. Prot. Dosim. 101, 515–518. Turney, C.S.M., Bird, M.I., Fi7eld, L.K., Roberts, R.G., Smith, M., Dortch, C.E., GrPun, R., Lawson, E., Ayli/e, L.K., Miller, G.H., Dortch, J., Cresswell, R.G., 2001. Early human occupation at Devil’s Lair, southwestern Australia 50,000 years ago. Quat. Res. 55, 3–13. Wallinga, J., Murray, A.S., Duller, G.A.T., 2000. Underestimation of equivalent dose in single-aliquot optical dating of feldspars caused by preheating. Radiat. Meas. 32, 691–695. Wintle, A.G., Murray, A.S., 2000. Quartz OSL: e/ects of thermal treatment and their relevance to laboratory dating procedures. Radiat. Meas. 32, 387–400.
Radiation Measurements 37 (2003) 377 – 381 www.elsevier.com/locate/radmeas
The single aliquot regenerative dose protocol: potential for improvements in reliability A.S. Murraya , A.G. Wintleb;∗ a Nordic
Laboratory for Luminescence Dating, Department of Earth Sciences, Arhus University, Ris National Laboratory, DK-4000 Roskilde, Denmark b Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB, UK Received 21 August 2002; received in revised form 8 January 2003; accepted 14 January 2003
Abstract This paper examines the e/ects of optically stimulated luminescence (OSL) components, other than that usually termed the fast component, on dose determination by the single-aliquot regenerative-dose (SAR) protocol. Results are presented for “dose recovery tests”, in which a known laboratory dose, delivered after optical bleaching at room temperature, is measured using the SAR protocol. Data obtained using either the initial OSL signal or the fast component, derived by curve 7tting, are compared. Dose recovery tests are also carried out when an additional step is added to the SAR protocol, aimed at reducing recuperation, i.e. the residual signal observed in a SAR cycle when no regenerative dose is applied. The results for quartz from various sources indicate much improved dose recovery when only a well-separated fast component is analysed. c 2003 Elsevier Science Ltd. All rights reserved. Keywords: Optically stimulated luminescence; Quartz; Fast component; Single-aliquot regeneration; Dose recovery
1. Introduction The single aliquot regenerative dose (SAR) protocol was introduced as a method of obtaining a reliable value for an unknown dose received by quartz grains being used as dosimeters for dating or accident dosimetry (Murray and Wintle, 2000a; Banerjee et al., 2000). The SAR experimental sequence makes use of both the optically stimulated luminescence (OSL) signal arising from the unknown dose to be measured, and a number of OSL signals resulting from laboratory doses. The OSL sensitivity is monitored by the OSL response to a non-varying test dose given following completion of the main OSL measurement for each dose. Both the regenerated OSL growth curve, and the natural OSL from the unknown dose, are then sensitivity-normalised, by dividing by the appropriate test dose OSL response, before the unknown dose is calculated. The need to allow for sensitivity change when using a regenerative dose procedure
∗ Corresponding author. Tel.: +44-1970-622-658; fax: +441970-622-659. E-mail address: [email protected] (A.G. Wintle).
with sedimentary or heated quartz is widely recognised, with sensitivity changes of up to a factor of two occurring when the grains were heated (Wintle and Murray, 2000; Turney et al., 2001). These changes result mainly from the heat treatments applied prior to OSL measurement in order to isolate a thermally stable signal, e.g. 240◦ C for 10 s. The OSL signals used for both the main OSL measurements and the test dose measurements are usually the initial part of the continuous wave (CW) OSL decay curve. For blue (470 nm) diode stimulation giving ∼50 mW=cm2 at the sample, the signal is typically that obtained in less than the 7rst second of stimulation. The grains are usually held at 125◦ C during stimulation in order to prevent the 110◦ C TL trap from accumulating charge and subsequently contributing to the OSL signal (Murray and Wintle, 1998). The initial signal, being the most light-sensitive, is most likely to have been removed by daylight during grain transport prior to incorporation in a sedimentary deposit. The SAR protocol has been applied to a large number of both sedimentary and heated quartz grains in both our laboratories and others around the world. For sedimentary quartz, the reliability has been demonstrated by comparisons for about 50 samples with independent age information
c 2003 Elsevier Science Ltd. All rights reserved. 1350-4487/03/$ - see front matter doi:10.1016/S1350-4487(03)00053-2
378
A.S. Murray, A.G. Wintle / Radiation Measurements 37 (2003) 377 – 381
spanning the last 500,000 years (Murray and Olley, 2002). These samples were from deposits with di/erent depositional histories (aeolian, Euvial, marine and glacial), and it was concluded that there was no evidence for systematic uncertainties in the results, and that in almost all cases total uncertainties had been correctly assessed. However, there have been a few reports of SAR OSL ages that were not in agreement with the expected age (e.g. Stokes et al., 2000). This paper investigates some samples that had been thought to be problematic and suggests small modi7cations to methods of analysis or measurement procedure.
2. The SAR protocol In our view, the SAR protocol is distinguished from other measurement protocols by the use of the response to a 7xed test dose (measured after the signal resulting from the natural dose and every regenerative dose) to monitor sensitivity changes. The basic SAR protocol applied to quartz OSL signals (it can, for instance, also be applied to TL signals; Murray and Wintle, 2000b) is given in Table 1, where it can be seen that although the sequence is simple, the exact measurement conditions can be varied considerably. For example, a range of preheat temperatures may be employed in order to test for thermal transfer, isolate the most stable component, and/or accentuate potential sensitivity changes and thus provide a more strenuous test of the SAR technique. The fundamental assumption is that the response (T ) to the test dose provides an appropriate measure of the sensitivity that pertained during measurement of the main OSL signal (L). Besides its widespread application to sand-sized quartz grains, the measurement sequence given in Table 1 has been used in a modi7ed form by Banerjee et al. (2001) and by Roberts and Wintle (2001) with the OSL from samples of mixed mineralogy, containing both quartz and feldspar grains, and for the infrared stimulated luminescence (IRSL) from potassium-rich feldspar grains
Table 1 Generalised single-aliquot regenerative-dose protocol incorporating an extra step to reduce recuperation
1 2 3 4 5 6 7 8
Treatment
Observe
Give dose, Di Preheat (160 –300◦ C for 10 s) Optically stimulate for 40 s at 125◦ C Give test dose, Dt Heat to 160◦ C (to ¡preheat in step 2) Optically stimulate for 40 s at 125◦ C Optically stimulate for 40 s at ¿preheat Return to 1
— — Li — — Ti — —
Note: 1. The stimulation time is dependent on the stimulation light (blue diodes) intensity. 2. Step 7 is justi7ed in this paper. 3. Changes to original protocol given in bold text in parentheses.
(Wallinga et al., 2000), and even for blue stimulated luminescence from various arti7cial phosphors of application to accident dosimetry (Thomsen et al., 2002). For the response (T ) to the test dose to monitor the luminescence sensitivity, the assumption is made that the electron traps that are being probed are the same as those responsible for the main signal (L). However, it has long been known that there is more than one OSL signal from quartz, each with a di/erent optical stability. Bailey et al. (1997) termed these the fast, medium and slow components and they have been further investigated by Singarayer and Bailey (2003) using linearly modulated OSL (LM-OSL). The SAR protocol was developed using a mixture of these quartz signals. In the samples used to validate the protocol, this mixed signal was dominated by the fast component, as is found for the OSL of many quartz samples. However, if the medium and slow components are relatively large compared to the fast component and if they sensitise at di/erent rates as the result of a given thermal treatment, then a simple analysis using the initial (mixed) signal may not be appropriate. Where the medium and slow components make up a large proportion of the signal, as identi7ed by LM-OSL curves or from log-linear plots of CW-OSL, then it may be necessary to use a curve 7tting procedure to isolate the fast component of the OSL signals. Furthermore, for a small number (¡10%) of samples an ultra-fast component has been found following laboratory irradiation (Jain et al., 2003). Since it is not present in the natural OSL signal, it presumably represents a less thermally stable, but more light sensitive, component. If this ultra-fast component is not detected, e.g. by using LM-OSL or log-linear plots of CW-OSL, signi7cant age underestimation can result (Choi et al., 2003). To obtain the correct age for such samples, it is necessary to employ a preheat temperature range (e.g. 160 –280◦ C), 7rst to identify the problem, and second, to select an appropriate thermal treatment in order to erase the ultra-fast component resulting from laboratory irradiation (e.g. above 220◦ C). In addition, it may be necessary to employ a more stringent thermal treatment (cut heat) to remove the ultra-fast component from the test dose signal (e.g. if the component uses di/erent luminescence centres); such a cut heat may be as high as 200◦ C or 220◦ C, or be selected to be about 20◦ C below the preheat temperature. A further problem causing inaccurate dose determination for some samples could be high levels of OSL recuperation, e.g. as reported by Stokes et al. (2000). This is also likely to be due to the measured OSL signal not being derived only from the fast component. We have attempted to minimise the e/ect of recuperation by modifying the SAR procedure in Table 1 by the addition of a high temperature (e.g. 280◦ C) optical stimulation following each measurement of the test dose signal. In this paper we examine the e/ects of these three changes to analysis and measurement procedures using a range of samples that have shown atypical behaviour.
Recycling ratio
1.4
379
(a)
1.2 1.0 0.8 0.6 0.0 1.4
(b)
1.2 1.0 0.8 0.6 0.0 0.3
(c)
0.2
0.1
0.0 00 03 0 01 1 18 02 00 25 3 00 0 25 6 98 8 32 0 99 5 10 26 00 09 0 99 5 07 0 99 5 49 01
For most samples there is no independent age control, and thus it is important to be able to detect atypical behaviour in samples of unknown age. For this reason, an experimental procedure is needed to test that any SAR protocol (modi7ed or otherwise) results in an accurate determination of the unknown dose for an arbitrary sample. We suggest using a “dose recovery test”, in which SAR is applied to an aliquot that has been given a laboratory dose following optical bleaching of the aliquot. Such an approach has been reported for the SAR protocol applied to single grains of quartz (Roberts et al., 1999) and also to multiple grain aliquots of feldspar (Wallinga et al., 2000). A similar experimental design was employed by Folz and Mercier (1999), though they used the 110◦ C TL peak for sensitivity correction (Murray and Roberts, 1998). In such a test, the ratio of the measured to given dose should be unity. Using large doses in their study of aliquots of quartz grains from Eemian samples, Murray et al. (2002) obtained an average ratio of 0:952 ± 0:010. This ratio provides a more stringent test than that of the recycling ratio that is part of the SAR protocol (Murray and Wintle, 2000a), since the unknown dose is applied prior to any thermal treatment (e.g. the preheat); it thus mimics the natural dose (measured as the equivalent dose), except that it is delivered at a dose rate ∼109 times higher. Any successful measurement protocol must be able to recover such a known dose, but although this is a necessary test, it is not a complete test. The most complete test available is that a sample should meet all laboratory criteria, and yield good agreement with the known age. For the current dose recovery tests, two room temperature optical stimulations using the blue diodes for 40 s were used to bleach the aliquots, with a 10 ks room temperature storage between the two stimulations so that any charge transferred into the 110◦ C TL trap has time to thermally decay. It is possible that 40 s stimulation at 125◦ C may achieve the same e/ect, without causing any additional change. For each sample, the dose given was chosen to be close to the expected natural dose. The SAR protocol was then applied to three aliquots of each sample, employing a 10 s preheat at 260◦ C and a cut heat at 160◦ C. The results for nine samples from RisH are shown in Fig. 1. For all samples, the ratio of the measured to given dose (Fig. 1a) is within 26% of unity when using the initial signal (7lled circles) in what would be considered a standard analysis; the mean ratio was 0:96 ± 0:16. When the fast component was separated by curve 7tting (open circles), the ratios were slightly closer to unity, giving a mean of 1:00 ± 0:14. For those samples giving values close to 1.00, the value obtained was the same for both methods of analysis. However, there are still some samples (e.g. 000301 and 994901) for which selection of the fast component by curve 7tting resulted in the ratio only being consistent with unity at the 2 level. For the nine samples of this study, only a
Recuperation
3. Test for suggested changes
Measured/Given ratio
A.S. Murray, A.G. Wintle / Radiation Measurements 37 (2003) 377 – 381
Sample Code Fig. 1. Data obtained from dose recovery experiments on nine samples using a 10 s preheat at 260◦ C and a cut heat at 160◦ C in the basic SAR sequence: (a) measured to given dose ratio; (b) recycling ratio; and (c) recuperation expressed as a fraction of the OSL response to a given dose. Data obtained using the initial part of the OSL decay curve are given by 7lled circles; those obtained using the fast component derived from curve 7tting are given by open circles, with error bars being 1 .
slight reduction was found for the scatter in the measured to given dose ratio when a higher preheat (10 s at 280◦ C) and cut heat (240◦ C) were used (data not shown). The recycling ratios are also shown (Fig. 1b), and for some samples they are outside the range of 0.90 –1.10 suggested as a possible criterion for acceptance by Murray and Wintle (2000a). If accurate remeasurement of a point on the laboratory-generated dose-response curve is not possible, then some systematic error in the interpolation of Ln =Tn onto the dose-response curve is inevitable. Nevertheless, from the data presented in Fig. 1a and b, it can be seen that acceptable dose recovery can be achieved, even when a poor recycling ratio is obtained (e.g. sample 002530) and vice versa (e.g. sample 991026), indicating that the recycling ratio is not a particularly sensitive measure for the suitability of the measurement protocol.
380
A.S. Murray, A.G. Wintle / Radiation Measurements 37 (2003) 377 – 381
4. Recuperation
0.15
0.10 With blue light
Stimulation time, s 1
10 OSL (0.16 s)-1
Recuperation
No blue light
0.05
0.00 150
10 4 5x10 3
200 250 Temperature,oC
300
Fig. 2. Fractional recuperation obtained after holding an aliquot at the stated temperature for 40 s whilst exposing to light from blue diodes. Data shown for sample 994801 (preheat 260◦ C for 10 s). Open circles obtained when no stimulation is applied. The inset shows the recuperation decay curves after a 260◦ C preheat and when 40 s blue light stimulation is applied at 280◦ C prior to the 260◦ C preheat.
1.4
(a)
1.2 1.0
Measured/Given ratio
0.8 0.6 0.0 1.4
o
Preheat 260, cut-heat 160 C (b)
1.2 1.0 0.8 o
0.6
Preheat 260, cut-heat 220 C o 40 s blue stimulation at 280 C
0.0 00 03 0 01 1 18 0 00 2 25 3 00 0 25 6 98 8 32 0 99 5 10 26 00 09 0 99 5 07 0 99 5 49 01
The recuperated OSL signal is that measured following a zero Gy regenerative dose in a SAR measurement cycle. Of the 7ve samples showing high recuperation signals (when expressed as a fraction of the response to the given dose, 0.07– 0.25 in Fig. 1c, RisH sample numbers 000301, 011802, 000905, 990705 and 994901) only one (000905) had recuperation reduced to zero when component stripping was applied (when the higher preheat (10 s at 280◦ C) and cut heat (240◦ C) were used, these samples still showed similar amounts of recuperation (data not shown)). When the initial signal was used, four of the samples gave ratios of measured to given dose that were signi7cantly less than unity for both sets of thermal treatment. This possible dependence of recuperation on OSL components, and its e/ect on dose measurement, is now considered in more detail. From the timing of the recuperation measurement in the SAR cycle, it can be deduced that recuperation results from charge remaining in a light insensitive trap at the end of step 6 of the protocol (Table 1). This charge has only experienced relatively low temperature (test dose cut-heat, step 5) since storage during beta irradiation. The preheat (step 2) prior to measurement and after the zero regenerative dose, thermally stimulates this charge and transfers some of it to an optically sensitive trap, thus resulting in a measurable OSL signal. The fact that, for four of the 7ve samples, use of component 7tting reduces the amount of recuperation (Fig. 1c) (in 2 cases to zero) suggests that the trap responsible for the recuperated OSL signal is probably not the main OSL trap. A similar conclusion has been reached by Jain et al. (2003). This recuperation may thus be interpreted as an artefact of not using only the fast component of the OSL signal in subsequent data analysis. Rather than having to employ curve stripping, it may be possible to minimise this component experimentally. Optical stimulation at an elevated temperature should result in optical removal of charge as soon as it arrives at the OSL trap as the result of thermal transfer from the optically insensitive trap. This can be achieved by following the test dose measurements (step 6) with an additional optical stimulation at a temperature close to (or higher than) the preheat temperature (step 7). The e/ect of such an elevated temperature stimulation step on the measured recuperation can be seen for a sample with relatively high fractional recuperation (Fig. 2); the subsequent preheat (step 2) has been kept constant at 260◦ C, and recuperation measured as a function of the temperature of the preceding optical stimulation (step 7). The amount of recuperation decreases signi7cantly as the stimulation temperature is increased. The inset in Fig. 2 compares the decay curves after zero dose, measured using (i) the standard SAR protocol and (ii) a modi7ed protocol using the additional 40 s stimulation at 280◦ C. For measurements made without the extra optical stimulation step, increasing treatment temperature increased the amount of recuperation. When the
Sample Code Fig. 3. Measured to given dose ratios (a) same data as in Fig. 1a and (b) obtained using an additional step (7) in the SAR protocol given in Table 1.
additional step (step 7, including blue light stimulation) is applied to the samples that previously showed high fractional recuperation values (0.07– 0.25), a fractional recuperation of below 0.03 was achieved when using the initial signal and the measured to given dose ratios became much closer to unity and less scattered (Fig. 3b). Thus applying the modi7ed SAR protocol certainly improves the recovery of a known dose administered in the laboratory. We anticipate that it should also result in more accurate determination
A.S. Murray, A.G. Wintle / Radiation Measurements 37 (2003) 377 – 381
of unknown natural doses, and may also result in more precise equivalent dose evaluations. The former is diMcult to demonstrate, although the latter is testable in the laboratory. It should be pointed out that the experimental results presented in this paper were obtained using aliquots made up of several hundred grains. The dose recovery test applied assumes homogeneous OSL behaviour. However, studies of single quartz grains have shown a wide variation in various OSL characteristics. For this test to be useful with single grains or small aliquots, it would need to be applied to a suMcient number to demonstrate that both the average behaviour and the variability about this average were acceptable. 5. Conclusions Several years of experience of using the simple SAR protocol of Murray and Wintle (2000a) has demonstrated the usefulness of this procedure, as judged by its application to independently dated samples. For these samples, the OSL signal was almost certainly dominated by the fast component. Following the work of others on samples for which the fast component was not dominant, use of curve stripping and application of higher preheats suggested that samples for which SAR appeared to be unsatisfactory were those for which the fast component was not dominant. For some of the samples in this study, recuperation was found to be relatively large compared with the natural OSL, and to be linked to underestimation of a known dose given in the laboratory prior to any thermal treatment. This recuperation could be minimised by carrying out optical bleaching at an elevated temperature between each cycle of the SAR protocol. The e/ects of not using a well-separated fast component can be detected by using dose recovery tests. It is important that such tests are carried out routinely on samples being dated, and when procedures are being modi7ed. The dose recovery test is more diagnostic than the recycling ratio criterion previously used, although it must be remembered that it does not test whether the natural dose can be measured accurately. Acknowledgements The authors wish to thank Dr. M. Jain, J.H. Choi and T. Watanuki for discussions and access to their unpublished work, and Dr. R.G. Roberts for critical comments on the manuscript. References Bailey, R.M., Smith, B.W., Rhodes, E.J., 1997. Partial bleaching and the decay form characteristics of quartz OSL. Radiat. Meas. 27, 123–136. Banerjee, D., BHtter-Jensen, L., Murray, A.S., 2000. Retrospective dosimetry: estimation of the dose to quartz using the
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