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Comparison of De estimates using the fast component and the medium component of quartz OSL
Radiation Measurements 41 (2006) 125 – 136 www.elsevier.com/locate/radmeas
Comparison of De estimates using the fast component and the medium component of quartz OSL Bo Li, Sheng-Hua Li∗ Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China Received 9 March 2005; received in revised form 1 June 2005; accepted 30 June 2005
Abstract In equivalent dose (De ) determination of quartz using the single aliquot regenerative dose (SAR) technique, it is commonly observed that the De values are strongly dependent on the stimulation time for some well bleached samples. Such dependence is not controlled by the degree of recuperation of the optically stimulated luminescence (OSL) signals. Instead, this behavior is interpreted as being due to the medium component being thermally unstable for these samples and giving a lower De value than the fast component. An alternative method is described for the De determination of samples that exhibit such a phenomenon. © 2005 Elsevier Ltd. All rights reserved. Keywords: Equivalent dose; Quartz; OSL; Fast and medium components
1. Introduction Optically stimulated luminescence (OSL) from quartz consists of different components having different detrapping probabilities or photoionization cross-sections (Smith and Rhodes, 1994; Huntley et al., 1996; Bailey et al., 1997; Bulur et al., 2000; Jain et al., 2003). The different components in quartz OSL have been studied from many points of view, such as thermal stability, photoionization crosssection, recuperation, dose response, sensitization (Jain et al., 2003; Singarayer and Bailey, 2003, 2004). These studies have indicated that different characteristics of the different components may lead to significantly different results in the determination of the equivalent dose (De ) using a single aliquot regenerative dose (SAR) protocol (Murray and Wintle, 2000). In optical dating, the fast component is primarily of interest, because it can be bleached quickly and completely. Hence, it can be used even for sediments containing grains that only received a brief bleaching prior to ∗ Corresponding author. Tel.: +852 22 415 486; fax: +852 2517 6912. E-mail address: [email protected] (S.-H. Li).
1350-4487/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2005.06.037
deposition. It also has a high signal-to-noise ratio due to its high detrapping probability. In the single aliquot regenerative dose (SAR) procedure with stimulation using constant power to obtain continuous wave OSL (CW-OSL) signals, the OSL signal used for De determination is usually the initial part of the OSL curve, with subtraction of both the background and the slow component. This method of signal selection is successful for samples that OSL signals are dominated by the fast component (Murray and Wintle, 2000; Murray and Olley, 2002). However, this signal selection cannot efficiently separate the fast component from the medium component because of their relatively close detrapping rates and the negligible amount of the medium component in the end signals for subtraction. The appearance of the medium component in the initial part of OSL signals may interfere with the fast component and may yield problematic results in De determination. Choi et al. (2003) reported a significant underestimation of De values using the simple integration of initial OSL signals where a large proportion of medium component contributed to the initial signal. Similar results were also found in other studies (e.g. Tsukamoto et al., 2003). Thomas et al. (2005) also reported a significant
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underestimation of De in the single-aliquot method as compared to those using the single-grain method. They interpreted it as there being more medium components present in the initial light in single aliquot measurements because the wavelength used for stimulation is different from that in the single grain luminescence system. A simple and useful method for checking the influence of the medium component is to use a De (t) plot. The form of the De (t) plot is affected by the composition of the OSL signals; thus the shape of the De (t) curve could be an effective tool for assessing the bleaching history of sediment samples (e.g. Aitken, 1998; Bailey, 2000a; Bailey et al., 2003). For well-bleached samples, the De should be independent of the illumination time. If the samples were partially bleached before burial and more relatively slower components remained, the De value will increase with illumination time. This method has been successfully applied for assessing the bleaching history of modern samples and young samples (Bailey et al., 2003; Singarayer et al., in press). For older samples, other factors, i.e. recuperation and thermal instability, may contribute potentially to the shape of De (t) plot. Tsukamoto et al. (2003) observed a decrease of De with illumination time. This was interpreted as a result of recuperation in the medium component. Singarayer et al. (in press) reported a falling De (t) plot for an old sample, which was attributed to the thermal instability of one of the slow components (S2). Several methods for separating the fast component from other components have been proposed. One is curve fitting of the OSL curves by deconvolution (Bailey et al., 1997; Bulur et al., 2000; Murray and Wintle, 2003; Jain et al., 2003; Choi et al., 2003; Li and Li, in press). This approach can have uncertainties in curve fitting procedure and the number of components in the OSL curves. Another method of separating components is to use different photon energies for stimulation. It was reported that the detrapping rate of the quartz OSL is strongly dependent on the stimulation wavelength or photon energy and the stimulation temperature (Godfrey-Smith et al., 1988; Spooner, 1994; Duller and BZtter-Jensen, 1996; Huntley et al., 1996). The fast component can be depleted by infrared (IR) stimulation at elevated temperature (100–200 ◦ C), with negligible depletion of the medium component and slow components (Bailey, 1998; Singarayer and Bailey, 2004; Jain et al., 2003, 2005). Based on such observations, a differential-OSL SAR procedure, using the difference between two short OSL measurements separated by elevated temperature IR stimulation, for De evaluation was proposed (Jain et al., 2005; Singarayer and Bailey, 2004). This approach requires long time IR stimulation and may not be appropriate for young samples. In this paper, we compared the De estimates using the fast component and the medium component, and present an alternative method for De determination of samples having significant dependence of De on illumination time. The possible reasons for problems with the De estimates using the medium component were investigated.
2. Samples and analytical facilities Three aeolian samples, RKZ, DGF-1 and 4155, from China were used in this study. These samples were well bleached prior to burial. All samples were treated first with 20% H2 O2 and 10% HCl to remove organic materials and carbonates. Grains between 90–150 m were selected by wet and dry sieving methods in subdued red safe-light conditions. Grains with densities between 2.62 and 2.75 g/cm3 were separated using sodium polytungstate heavy liquid. The separated grains were treated with 40% HF acid for 2 h to remove feldspar grains. For measurement, the mineral grains were mounted on the 10-mm-diameter aluminum discs with Silkospray silicone oil. The purity of the quartz was tested by monitoring the presence of feldspar through measuring the IRSL signals at room temperature and using the approach of the OSL IR depletion ratio in the SAR procedure proposed by Duller (2003). OSL measurements were performed using an automated RisZ TL/OSL DA-15 reader equipped with excitation units containing blue light-emitting diodes (LEDs, 470 ± 30 nm) (BZtter-Jensen et al., 1999). The OSL signal was detected through two 3 mm U-340 filters. Irradiation was carried out using a 90 Sr/90Y beta source built into the reader with the dose rate of 0.0899 Gy/s to quartz on aluminum discs. All OSL measurements were carried out at 125 ◦ C in order to prevent retrapping of charges from the shallow trap corresponding to the 110 ◦ C TL peak (Murray and Wintle, 1998). 3. Experiments and results 3.1. The CW-OSL signals The representative CW-OSL curve from sample DGF-1 is shown in Fig. 1(a). In order to examine how different components contribute to the OSL signals, the CW-OSL curve was deconvoluted into three components (fast, medium and slow) using first-order equations, respectively. The relative contributions as a function of illumination time of different components to the total signal are shown in Fig. 1(b). The fast component contributes above 80% of the total signal in the initial time (0–0.4 s). However, the proportion of the fast component decays quickly after 1 s and became less than 1% of the total signal after 5 s. In contrast, the proportion of the medium component increased from 15% at the beginning to about 50% at 3 s. The contribution from the slow component was negligible in the first 1 s; it started to increase quickly at 2 s and reached to 50% at 4 s. Therefore, the fast component and medium component contribute most to the De evaluation using the CW-OSL technique when the initial signal (0–2 s) is used. After 4 s, the slow component might contribute considerably to the total signal. 3.2. The dependence of De values on illumination time A SAR protocol (Murray and Wintle, 2000) was used for obtaining De for these samples. Each disc was treated with
B. Li, S.-H. Li / Radiation Measurements 41 (2006) 125 – 136
120
1.E+04 Experiment Medium Sum
Fast Slow
100
1.E+03
Apparent De, Gy
CW-OSL signal, counts
127
1.E+02
1.E+01
80 60 40 Initial test OSL 20
1.E+00 0
(a)
10
20 30 Stimulation time, s
Successive integrals
0
40
0
0.8
1.6 2.4 Stimulation time, s
3.2
Relative contribution
1.2
0.8 Fast
0.6
Medium 0.4
F+M Slow
0.2 0 0.1
(b)
Fig. 2. Dependence of De value on stimulation time for sample 4155 using different parts of the test dose OSL for correcting for sensitivity change, namely the initial signals (full diamonds) and successive integrals (empty squares).
1
1 10 Stimulation time, s
100
Fig. 1. Representative CW-OSL curve from sample DGF-1. Three exponential components (fast, medium and slow) are shown. (b) Relative contributions from different components to the CW-OSL curve (a) plotted against the stimulation time.
measurement cycles of preheating at 260 ◦ C for 10 s, OSL measurement, test dose, cut-heat to 220 ◦ C, OSL measurement and irradiation with different regenerative dose in each cycle. Five regenerative doses including one zero dose and one repeat dose were applied to each disc. The De values obtained were plotted as a function of illumination time. The time range examined is 0–3.6 s where the fast component and medium component dominate (Fig. 1(b)). Any dependence of De on illumination time could thus be considered to be the result of the interaction between the fast component and the medium component. In order to investigate whether the sensitivity changes of the signals at different integration intervals are the same or not, initial signal and successive integrals through the stimulation time for the test dose OSL were used for correcting for the sensitivity change. The De (t) plots obtained from the same data set from one aliquot of sample 4155 using both correction methods are shown in Fig. 2. A similar pattern was obtained for both methods, suggesting that sensitivity changes of the signals at different integration intervals are the same. It is thus indicated that the fast component and the medium component are sensitized in the same way. Based on this observation, in this study, the sensitivity change of different integration intervals was corrected using the initial
signal of test dose OSL to minimize the effect of significant reduction of precision in the test dose OSL as illumination time increases, which may lead to large errors in constructing De (t) plots. Typical De (t) plots for a single aliquot of samples DGF-1, 4155 and RKZ are shown in Fig. 3(a)–(c). The De values are dependent on the illumination time to different extents. The oldest sample, DGF-1, yields the largest dependence. The De value decreases from 143 Gy at 0–0.4 s to 65 Gy at 3.2–3.6 s. The De value of the aliquot from sample 4155 (Fig. 3(b)) also decreases from 110 to 56 Gy for the same period. The youngest sample RKZ shows the smallest dependence, from 48 to 30 Gy (Fig. 3(c)). Fig. 3(d) shows the dose response curves and the natural values at t =0–0.4 s and 3.2–3.6 s for the sensitivity-corrected data used for Fig. 3(a). The response curves were fitted with a saturating exponential function of the form I = I0 (1 − exp[−D/D0 ]) with D0 = 119 and 293 Gy for t = 0–0.4 s and 3.2–3.6 s, respectively. Both natural signals are not saturated, suggesting that the dependence of De on illumination time is not related to the saturation of one signal relative to the other (e.g. the fast component compared with the medium component) which may also affect the form of De (t) (Bailey et al., 2003). The good recycling ratios obtained for both intervals also suggest that the sensitivity correction using the initial signal from test dose OSL was successful. Fig. 4 shows four De (t) plots for different aliquots from the sample DGF-1 in which one of them (Fig. 4(d)) was measured using a higher cut-heat temperature (260 ◦ C) after the test dose, equal to that used for the preheat in order to check the influence of cut-heat on this temperature dependence. Bailey (2000b) suggested that for some samples a low cut-heat temperature might result in incorrect De estimation. The decreasing trend of De with illumination time appears for both cut-heat temperatures, suggesting that such dependence is not the result of inappropriate use of cut-heat temperature. The extent of the decrease in De with
128
B. Li, S.-H. Li / Radiation Measurements 41 (2006) 125 – 136
DGF-1
150
Apparent De, Gy
Apparent De, Gy
180
120 90 60 Experimental Best fitted
30 0
0
0.8
(a)
1.6 2.4 Stimulation time, s
4155
120 90 60
Experimental Best fitted
30 0
3.2
0.8
0
(b)
1.6 2.4 Stimulation time, s
3.2
Apparent De,Gy
60 RK Z
50 40 30 20
Experimental Best fitted
10 0 0
(c)
0.6
1.2 1.8 2.4 3 Stimulation time, s
0-0.4s 3.2-3.6s
3 2.5 2
0.2
1.5 1
143 Gy
0
4.2
0.1
65 Gy
0.5 3.6
0.3
0
50
(d)
100 Dose, Gy
150
Corrected OSL,3.2-3.6s
Corrected OSL,0-0.4s
3.5
0 200
200
250
160
200
Apparent De, Gy
Apparent De, Gy
Fig. 3. Typical De (t) plots for samples DGF-1 (a), 4155 (b) and RKZ (c) using a 260 ◦ C preheat temperature. Every experimental data point is calculated using an integral of 0.4 s of the OSL signals at each different illumination time. The trend lines are the best fitted data using Eq. (3) (see text). (d) The dose response curves and natural values at t = 0–0.4 s and 3.2–3.6 s for the sensitivity-corrected data used for (a).
120 80 40 0
0
0.8
(a)
1.6 2.4 Stimulation time, s
50
0
0.8
1.6 2.4 Stimulation time, s
3.2
0
0.8
1.6 2.4 Stimulation time, s
3.2
(b) 200
200
Apparent De, Gy
Apparent De, Gy
100
0
3.2
250
150 100 50 0 0
(c)
150
0.8
1.6 2.4 Stimulation time, s
150 100 50 0
3.2
(d)
Fig. 4. De (t) plots for different aliquots of DGF-1 measured using the standard SAR protocol. The cut-heat temperature for the test dose used for (a), (b) and (c) was 220 ◦ C. The cut-heat temperature for the test dose used for (d) was 260 ◦ C. Every experimental data point was calculated using an integral of 0.4 s of the OSL signals at each different illumination time. The trend lines are the best fitted data using Eq. (3).
250
250
200
200
Apparent De, Gy
Apparent De, Gy
B. Li, S.-H. Li / Radiation Measurements 41 (2006) 125 – 136
150 100 50 0
0
100 50 0
3.2
250
200
200
150 100 50 0
0.8
1.6 2.4 3.2 Stimulation time, s
0
0.8
0
0.8
(b)
250
0 (c)
150
Apparent De, Gy
Apparent De, Gy
(a)
0.8 1.6 2.4 Stimulation time, s
129
1.6 2.4 Stimulation time, s
3.2
150 100 50 0
4 (d)
2.4 1.6 Stimulation time, s
3.2
Fig. 5. De (t) plots for “dose recovery” aliquots of DGF-1 measured using the standard SAR protocol. The lines are the laboratory dose of 174 Gy.
3.3. The role of recuperation in De (t) dependence Tsukamoto et al. (2003) reported that there was an exponential relationship between De and the degree of recuperation at different illumination time; this might be the reason for the underestimation of De . In order to check the role of recuperation in the time dependence of De , six aliquots of sample DGF-1 were measured using the similar SAR protocol in which a high temperature (280 ◦ C) OSL measurement for 40 s was added at the end of each measurement of test dose OSL. This was initially proposed by Murray
4 No 280˚C OSL Recuperation, %
illumination time for these aliquots is not the same. This might be expected as different aliquots have different proportions of fast component and medium component, analogous to the observations that different aliquots may have different proportions of well bleached grains (Li, 2001). Fig. 5 shows four De (t) plots of aliquots for a dose recovery test for sample DGF-1. Quartz aliquots were bleached for 100 s with the blue LED light. Then, a laboratory dose of 174 Gy was given to each aliquot. The laboratory irradiated aliquots were treated as the natural to determine the De using the same SAR protocol as described above. The average De value is 184 ± 16 Gy for a laboratory dose of 174 Gy. It is noting that in contrast to the natural aliquots of the same sample DGF-1 (Fig. 4), the De (t) plots for these dose recovery aliquots were flat over the same illumination time range. It is suggested that De decreasing with illumination time only happens for the natural aliquots.
280˚C OSL
3
2
1
0
0
1
2
3
4
5
6 7 8 Aliquot No.
9 10 11 12 13
Fig. 6. Recuperation of the sample DGF-1 observed during SAR measurements; (a) the filled squares are the recuperation of aliquots measured using the standard SAR protocol. (b) The empty squares are the recuperation of aliquots measured using a SAR protocol in which a high temperature (280 ◦ C) OSL measurement for 40 s was added at the end of each OSL measurement of test dose. The recuperation was expressed as the percentage of the natural signal.
and Wintle (2003) for reducing recuperation from the residual signals after each SAR cycle; it has since proved to be useful for improving the precision and the accuracy of dose recovery. After this step was added, the recuperation for all aliquots was reduced significantly to less than 0.5% of the initial level of natural signals (Fig. 6) and can be
130
B. Li, S.-H. Li / Radiation Measurements 41 (2006) 125 – 136 200 Apparent De, Gy
Apparent De, Gy
150
100
50
0
0
0.8
(a)
1.6 2.4 Stimulation time, s
150 100 50 0
3.2
(c)
0.8
1.6 2.4 Stimulation time, s
0
0.8
2.4 1.6 Stimulation time, s
3.2
200
200
Apparent De, Gy
Apparent De, Gy
250
150 100 50 0 0
0
(b)
0.8
2.4 1.6 Stimulation time, s
150 100 50 0
3.2
(d)
3.2
Fig. 7. De (t) plots for four different aliquots of DGF-1 measured using SAR protocol in which a high temperature (280 ◦ C) OSL measurement for 40 s was applied at the end of each measurement of the test dose OSL for which data points in Fig. 6(b) were obtained. Every point of experimental data was calculated using an integral of 0.4 s of the OSL signals at each different stimulation time. The trend lines are the best fitted data using Eq. (3).
considered to be negligible. The relationship between De and illumination time of these aliquots was then examined (Fig. 7). Although negligible recuperation was achieved in this experiment, no significant diminution in the dependence of De on illumination time was achieved and there was still a strongly decreasing trend of De with illumination time. Similar results were observed for samples RKZ and 4155. It is thus concluded that recuperation of the medium component is not the main factor controlling the time dependence of De for these samples. 3.4. De determination using the medium component In order to check the validity of De estimation using the medium component, a SAR protocol for the medium component is used for De estimation (Singarayer and Bailey, 2004; Jain et al., 2003). The outline of the post-IR OSL SAR protocol is given in Table 1. In this protocol, IR stimulation for 7000 s at 160 ◦ C is applied before each OSL measurement. It was demonstrated that there is no significant decay of the medium component by 6000–8000 s IR stimulation at 160 ◦ C while the fast component can be depleted to a negligible level (Singarayer and Bailey, 2004). Therefore, the OSL recorded after such high temperature IR stimulation is only from medium and slow components. At the end of
Table 1 The post-IR OSL single-aliquot regenerative-dose protocol for measuring the medium OSL component Step
Treatment
Observed
1 2 3 4 5 6 7 8 9 10
Give regenerative dose, Di a Preheat at 260 ◦ C for 10 s IR stimulation at 160 ◦ C for 7000 s OSL measurement at 125 ◦ C for 40 s
Li (medium)
Give test dose, Dt Cut-heat to 260 ◦ C IR stimulation at 160 ◦ C for 7000 s OSL measurement at 125 ◦ C for 40 s OSL measurement at 280 ◦ C for 40 s Return to 1
Ti (medium)
a For the natural sample, i = 0 and D = 0. The whole sequence 0 is repeated for several regenerative doses including a zero dose and a repeat dose.
each SAR cycle an additional OSL measurement is made at 280 ◦ C for 40 s to minimize the effect of recuperation. A preheat of 260 ◦ C for 10 s and a cut-heat of 260 ◦ C was used for regenerative doses and test doses, respectively, so that any sensitivity change due to the 160 ◦ C IR stimulation can be ignored (Jain et al., 2005).
B. Li, S.-H. Li / Radiation Measurements 41 (2006) 125 – 136
280
4
200 6
De, Gy
Corrected OSL
5
No post-IR
240
8
4
160
3
120
2
80 2
1
40 0
0 0
50
(a)
100 150 200 Regenerative dose, Gy
250
300
0
2
4
8 6 Aliquot No.
10
12
0
Fig. 9. De values and recycling ratios obtained using the post-IR OSL SAR protocol (Table 1) and the standard SAR protocol (no IR) for sample DGF-1. The filled diamonds with error bars are individual De values. The recycling ratios are represented as open squares.
120 100 De(medium), Gy
Post-IR OSL
Recycling ratio
10
131
80 60
component for De evaluation gives lower values than using the fast component.
40 20
3.5. The thermal stability of the medium component
0 0
(b)
0.6
1.2 1.8 2.4 Stimulation time, s
3
3.6
Fig. 8. (a) Dose response curve of the medium component of the sample DGF-1 obtained using the post-IR OSL SAR protocol described in Table 1. The experimental data were fitted with a single saturating exponential function of the form I =I0 (1−exp[−D/D0 ]) with D0 = 147.8 Gy. (b) De (t) plot taken from the same aliquot used in (a). The dashed line represents the value obtained in (a).
Sample DGF-1 was measured in this experiment. The OSL recorded after IR stimulation was used for De estimation. The net OSL signals Li (medium) and Ti (medium) are derived from the initial integral (0–2 s) of the OSL signals minus an equivalent part of the residual signal from the end part of the curves. The dose response curves were fitted with single saturating exponential functions (Fig. 8(a)). It is interesting to note that there was little dependence of De value on illumination time (Fig. 8(b)) in such a postIR OSL dating procedure when comparing to those without IR bleaching, indicating that the OSL is dominated by the signals from a single type of traps corresponding to the medium component. The results obtained from this protocol and those from the standard SAR procedure are compared in Fig. 9. The average De value of the medium component is 106.3 ± 9.2 Gy (n = 8), significantly lower than the value of 167.6 ± 13.6 Gy (n = 9) given by using the standard SAR protocol which utilizes the initial part of the OSL signal. The average recycling ratios of both procedures are 0.99 ± 0.03 and 1.00 ± 0.05, respectively, suggesting that the sensitivity change during regeneration cycles was well corrected. Thus it can be concluded that using the medium
The thermal stabilities of different components in OSL have been suggested as a potential factor contributing to the shape of De (t) plot for old samples (Bailey et al., 2003; Singarayer et al., in press). Bailey et al. (2003) have modeled the influence of the unstable slow component (S2 in their study) on De (t) plot. However, according to their numerical simulation, the effect is much less significant than the results observed for our samples. Since the dependence of De on illumination time examined in our study occurs over 0–3.6 s where the fast component and medium component dominate (Fig. 1(b)), only when the medium component yields a significantly lower De value than the fast component will the remarkable time dependence of De be observed. In order to check whether the decrease in the De (t) plots is the result of thermal instability of the medium component, a pulsed annealing study using CW-OSL was carried out using a single aliquot of sample DGF-1. The aliquot was first optically bleached at 125 ◦ C for 100 s to remove the natural signal. The aliquot was then optically stimulated at a high temperature (280 ◦ C) for 40 s to remove the recuperation as suggested by Murray and Wintle (2003). The aliquot was then given a dose of 90 Gy prior to heating to T (◦ C), followed a CW-OSL measurement at 125 ◦ C for 100 s to measure the remaining signal. The aliquot was then given a test dose of 9 Gy and heated to 220 ◦ C. The test dose OSL was then measured for monitoring sensitivity change. Such measurement cycles were repeated by increasing T from 220 to 380 ◦ C in steps of 20 ◦ C. At the end of each measurement of test dose OSL, an OSL measurement at 280 ◦ C for 40 s was applied to remove the recuperation.
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et al., 2003). This result also implies more complex characteristics of different components of quartz OSL, because different quartz may have different thermal stabilities for different components.
Normalized signals
1.2 1 0.8 0.6
3.6. Analyzing the De (t) plots
0.4 0.2 0 200
(a)
0s 1.2 s 2.4 s 3.6 s
0.4 s 1.6 s 2.8 s
0.8 s 2.0 s 3.2 s
240 280 320 360 Preheating temperature, °C
400
1.2
Remnant OSL
1 0.8 0.6 0.4
In De determination using CW-OSL, if the medium component yields a different De value from the fast component, the De will be dependent upon the relative proportions of the fast and medium components, and thus depend on stimulation time. Only when the fast component and medium component give the same De values, will such dependence disappear and a single De value will be obtained. Assuming that the contribution of slow components to the net OSL is negligible after subtracting the later part of OSL curves, only the fast component and medium component contribute to the De estimation. For the fast component, assuming that the OSL decaying exponentially with stimulation time, the OSL from stimulating time t to t + t is given by
Fast
0.2 0 200 (b)
Medium
250 300 350 Preheating temperature, °C
Lf (t) = 400
Fig. 10. (a) Pulse annealing curves for different integral intervals of CW-OSL signal at different stimulation times for sample DGF-1. (b) The pulse annealing curves for the fast and medium components separated from the CW-OSL signal for sample DGF-1.
In order to check the influence of the thermal stability of the medium component on different time intervals of CW-OSL signal, the signals at different time intervals (0–3.6 s) observed after preheating to different temperatures are shown in Fig. 10(a). The signals at later intervals decay faster with increasing preheating temperature than the signals at earlier intervals, indicating that most unstable signal is mainly from the medium component that has an increased concentration in the later time intervals. The fast and medium components in each CW-OSL curve were also separated by deconvoluting the CW-OSL curves. Fig. 10(b) shows the remnant fast and medium components after preheating to different temperatures. The fast component begins to decay beyond 300 ◦ C. However, the medium component shows earlier decay above 240 ◦ C, suggesting that the medium component is not stable when compared with the fast component. Thus it can be concluded that the instability of the medium component contributes the falling of De (t) plots for our samples. This result contrasts with those reported previously where the medium component is more stable than the fast component (Singarayer et al., 2003; Jain
t+t t
If exp(−f I0 t) dt
=
If {exp(−f I0 t) − exp[−f I0 (t + t)]} f I0
=
If exp(−f I0 t)[1 − exp(−f I0 t)] f I0
= Cf exp(−f I0 t),
(1)
where Cf = [1 − exp(−f I0 t)]If /(f I0 ) and is independent of t, Lf is the integrated signal of the fast component from t to t + t, If is the initial intensity of the fast component at t = 0, f refers to the photoionization cross-section of the fast component, I0 is the intensity of the stimulation light (in this experiment I0 is ∼ 45 mW/cm2 ). As for the fast component, we have for the medium component, Lm (t) =
t+t t
Im exp(−m I0 t) dt
= Cm exp(−m I0 t),
(2)
where Cm = [1 − exp(−m I0 t)]Im /(m I0 ), Lm is the integrated signal of the medium component from t to t + t, Im is the initial OSL intensity of the medium component, m is the photoionization cross-section of the medium component. If the De values of the fast component and medium component are expressed as Df and Dm respectively, and the contributions of the fast component and medium component to the apparent De value are proportional to their relative
B. Li, S.-H. Li / Radiation Measurements 41 (2006) 125 – 136
proportions in the OSL signals, the apparent De observed at time t will be: Lf (t) Lm (t) + Dm Lf (t) + Lm (t) Lf (t) + Lm (t)
Df Cf exp(−f I0 t) + Dm Cm exp(−m I0 t) Cf exp(−f I0 t) + Cm exp(−m I0 t)
Df k exp(−f I0 t) + Dm exp(−m I0 t) , = k exp(−f I0 t) + exp(−m I0 t)
D0(f)=D0(m)=150 Gy 250 Apparent De, Gy
=
300
(3)
where k is a factor relating to the ratio of initial OSL intensity of fast component to that of medium component (k = Cf /Cm ). The values of f and m are 2.3 × 10−17 and 5.6 × 10−18 cm2 , respectively (Jain et al., 2003; Li and Li, in press). If the integration interval t used for the calculation is kept the same (0.4 s is chosen for ensuring good precision and enough data points for fitting in this study), k is a constant. Therefore, the apparent De can be expressed as a function of time. The values of Df , Dm and k can be given as parameters by fitting the De (t) plot with Eq. (3). Thus, the true De , i.e. Df , can be obtained. For linear growth in the low dose range, it is true that the contributions of the fast component and medium component to the apparent De value are proportional to their relative proportions in the OSL signals. However, this may not be the case when either one or both of these signals grows nonlinearly. In order to test the feasibility of applying Eq. (3) for analyzing the De (t) close to the region of dose saturation (up to 2D0 ), the De (t) plot was simulated using different parameters and analyzed using Eq. (3). Assuming that the true values of Df and Dm are 300 and 150 Gy, respectively, and the values of the detrapping rates for the fast component and the medium component, bf and bm , are 2.5 and 0.62 s−1 (after Jain et al., 2003), respectively, the De (t) plot can be constructed (Fig. 11) using different ratios of Imax (the maximum signal after saturation) and D0 for the fast component and the medium component (Table 2). The simulated De (t) plots were fitted using Eq. (3) to obtain Df and Dm . Fig. 11(a) shows two De (t) curves where the fast component and the medium component have same D0 (D0 (f ) = D0 (m) = 150 Gy) but their relative Imax is different. Fig. 11(b) shows three De (t) curves where the medium component has different D0 value but same relative Imax . The shape of the De (t) curves are very similar to those observed in experiment (e.g. Figs. 3, 4 and 7). The fitting results were also shown in Table 2. Significant underestimation (up to 20%) of the apparent De values (De (a) in Table 2) obtained using the first 0.4 s of the OSL signal was obtained. However, the deviation of all values of Df given by fitting are within 5% of the true value of 300 Gy. The Dm can also be obtained correctly without the influence of saturation. This result suggests that Eq. (3) can be safely applied to the samples with De less than 2D0 where the SAR protocol can be applied.
200 150 100 Imax(f)/Imax(m)=4 Imax(f)/Imax(m)=2
50 0 0
0.8
(a)
1.6 2.4 Stimulation time, s
3.2
300 Imax(f)/Imax(m)=3.3 250 Apparent De, Gy
De (t) = Df
133
200 150 100 D0(f)/D0(m)=2 D0(f)/D0(m)=1 D0(f)/D0(m)=0.5
50 0 0 (b)
0.8
1.6 2.4 Stimulation time, s
3.2
Fig. 11. De (t) plots constructed using different parameters for the fast component and the medium component as shown in Table 2. (a) De (t) curves where the fast component and the medium component have the same D0 (D0 (f ) = D0 (m) = 150 Gy) but different relative Imax . (b) De (t) curves where the medium component has a different D0 value but the same relative Imax . All De (t) curves were fitted using Eq. (3).
3.7. De estimation using De (t) plots The De (t) plots taken from all aliquots of the sample DGF-1 were fitted with Eq. (3) using nonlinear least squares method with software Origin 7.0. It is found that all the De (t) plots are well fitted, as already shown in Figs. 3, 4 and 7. The Df and Dm values obtained by fitting the De (t) plots and the corresponding apparent De obtained by using the initial 1.2 s of the OSL signals are summarized in Fig. 12(a) and (b). For the nine aliquots examined, the De values of the fast component are significantly higher than those of the medium component, except for one aliquot giving a coincident value for both components and which has a flat De (t) curve. The mean Dm value obtained by fitting the De (t) plots is 98.4 ± 5.2 Gy (n = 9), which agrees well with the value of 106.3 ± 9.3 Gy given by the post-IR OSL method described in Section 3.2, indicating that the medium
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Table 2 Numerical parameters for constructing the De (t) plots and the fitting results obtained using Eq. (3). The true values for Df and Dm are 300 and 150 Gy, respectively. The values of the detrapping rates for the fast component and the medium component, bf and bm , are 2.5 and 0.62 s−1 , respectively (after Jain et al., 2003). De (a) is the value obtained for the first 0.4 s OSL signal Numerical parameters
Results Relative Imax (m)
D0 (f ) (Gy)
D0 (m) (Gy)
Df (Gy)
Dm (Gy)
De (a) (Gy)
1 1 1 1 1
0.25 0.5 0.3 0.3 0.3
150 150 150 150 150
150 150 75 150 300
288.2 ± 2.8 298.5 ± 6.5 309.3 ± 1.1 296.7 ± 3.5 293.2 ± 3.7
149.4 ± 0.5 149.9 ± 0.5 149.2 ± 0.2 149.4 ± 0.4 149.9 ± 0.6
255 235 270 250 253
300
200
250
150 100 De (fast) De (apparent) De (medium)
50 0
0
1
2
De (apparent), Gy
(a)
3
4 5 6 Aliquot No.
7
8
9 10
300
300
250
250
200
200
150
150
100
100
0
0
50
100 150 200 De (fast), Gy
250
200 150 100 50 0
50
50
(b)
De from fast component, Gy
250
De (medium), Gy
De, Gy
Relative Imax (f )
0 300
Fig. 12. Comparison of De obtained with different methods. (a) The De values of the fast component and medium component of sample DGF-1 given by fitting the De (t) plots with Eq. (3) and the apparent De values obtained using the initial 1.2 s of the OSL signals. (b) The De values of the fast component plotted against the De values of the medium component (open squares at the right scale) and the apparent De values (filled squares at the left scale).
component was separated successfully. The mean Df value is 194.8 ± 13.9 Gy (n = 9), systematically higher than the apparent De value 167.6 ± 13.6 Gy obtained by using the first 1.2 s of OSL signals. In order to confirm that the fast
0
50
100 150 200 250 Df from the De(t) plot, Gy
300
Fig. 13. De values of fast component obtained by curve fitting individual OSL curves plotted against the values obtained by fitting the De (t) plots.
component was successfully separated in this method, the individual OSL curves of these aliquots were also analyzed using a curve fitting method to separate the fast component for De estimation (Li and Li, in press). The De value given by separated fast component is 187.7 ± 13.3 Gy. It is in good agreement with the result 194.8 ± 13.9 Gy for Df given by fitting the De (t) plot. The individual values of the aliquots obtained from both methods are shown in Fig. 13. The De values for fast component obtained by curve fitting individual OSL curves are coincident with the values obtained by fitting the De (t) plots, suggesting that the fast component was successfully separated by fitting the De (t) plots.
4. Discussion A strong dependence of De on illumination time was observed. The De values decrease with illumination time for
B. Li, S.-H. Li / Radiation Measurements 41 (2006) 125 – 136
a large portion of aliquots measured, although the samples were well bleached before burial. It is shown that the dependence of De on stimulation time was independent of the degree of recuperation, indicating that recuperation is not the main reason controlling the time dependence of De , although recuperation might be a potential factor contributing to such a dependence when considering that the recuperation in the medium component is larger than the fast component (Jain et al., 2003). The thermal instability of the medium component in these samples has been demonstrated by annealing tests. Such an unstable medium component occurs in the initial OSL signal in the observed stimulation range (0–3.6 s) and it contributes to the significant decrease of De with illumination time when these results are compared with those reported by Bailey et al. (2003) and Singarayer et al., (in press). In their studies, a significant decrease in De occurred at a later illumination time, and which they attributed to the thermal instability of one of the slow components (S2). These results suggest that different quartz may have different characteristics in regard to the thermal stabilities of different components. Therefore, checking the thermal stabilities of different components used for dating is necessary. The plots of De versus stimulation time can provide information about the interaction between the fast component and the medium component. The apparent De values can be expressed as a function (Eq. (3)) of stimulation time t, the De value from the fast component Df , and the value from the medium component Dm . Df and Dm can thus be obtained by fitting the De (t) plots using Eq. (3). For sample DGF-1, the De value from the fast component obtained by this method agrees with the result of fitting individual OSL curves but systematically higher than the apparent De value obtained by using the initial OSL signals (Fig. 13). The De value from the medium component obtained by this method is also consistent with the result obtained by using the postIR OSL for the experimentally separated medium component. This result indicates that, in the De evaluation using the standard SAR protocol, underestimation of De values might happen when using the initial part of the OSL signals if the medium component is unstable. It thus suggested that, besides using criteria of “recycling ratio”, “recuperation”, “dose recovery” and “preheat plateau” for testing the reliability of the De estimation, it is also important to check the De (t) plot to assess the validity of De results. These results also indicate that analyzing the De (t) plots is an effective way for De estimation. Because no analysis of individual OSL curves is required, this procedure is less time consuming than curve fitting of each OSL curve and only fitting of the De (t) curve is needed for each aliquot. It should be also noted that this method of De (t) analysis is insensitive to the relationship between De values of the fast component and medium component. It can distinguish the De values of both components regardless of whether the De of the medium component is higher or lower than that of the fast component. For partially bleached samples,
135
the De may increase with illumination time because of the incomplete bleaching of the medium component and slower components. Therefore, this method of analysis can also be used to evaluate the De value of the fast component for partially bleached samples where the medium component may yield larger De values. As mentioned previously, it is assumed in Eq. (3) that the contribution of the fast component and medium component to the apparent De value is proportional to their proportions in the OSL signals. However, erroneous results may be obtained if the signals are close to saturation. It is suggested that the De values of the fast component and the medium component should not exceed their 2D0 values that are obtained from growth curve fitting with a saturating exponential function. Another limitation of De estimation using a De (t) plot is the assumption that there are negligible signals of slow components after subtraction of late part of the OSL curve. In fact, the thermally unstable component (S2) also contributes to the decrease of De with illumination time (Bailey et al., 2003), although relatively small in the earlier stage of illumination. If there is a significant portion of such a slow component in the OSL signal region of interest, and if it has a different De value to Df and Dm , the fitted results might be erroneous. Therefore, for the samples where the fast component is relatively small and the slow components are relatively important, the best method is to separate the fast component by using curve fitting (Singarayer et al., in press; Li and Li, in press) or by an experimental method as that suggested by Jain et al. (2005).
5. Conclusions Equivalent dose values decreasing with illumination time were observed in several samples. Recuperation in the medium component is not the main factor controlling the underestimation of De values when the initial part of the OSL signals was used. The medium component yields systematically lower De values than those from the fast component due to its thermal instability; this causes the underestimation of De values using the initial part of the OSL signals and the significant dependence of De on illumination time. True De values for the fast component and medium component can be evaluated by analyzing the De (t) plots.
Acknowledgements The authors thank Ann Wintle for her inspirational and critical comments and Richard Bailey for suggestions on the manuscript. This work was supported by grants to S.-H. Li from the Research Grant Council of the HKSAR, China (Project No. 7106/02P) and from CRCG of HKU.
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References Aitken, M.J., 1998. An Introduction to Optical Dating. Oxford University Press, Oxford. Bailey, R.M., 1998. Depletion of quartz OSL signal using low photon energy stimulation. Ancient TL 16, 33–36. Bailey, R.M., 2000a. The interpretation of quartz optically stimulated luminescence equivalent dose versus time plots. Radiat. Meas. 32, 129–140. Bailey, R.M., 2000b. Circumventing possible inaccuracies of the single aliquot regeneration method for the optical dating of quartz. Radiat. Meas. 32, 833–840. 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. Bailey, R.M., Singarayer, J.S., Ward, S., Stokes, S., 2003. Identification of partial resetting using De as a function of illumination time. Radiat. Meas. 37, 511–518. BZtter-Jensen, L., Duller, G.A.T., Murray, A.S., Banerjee, D, 1999. Blue light emitting diodes for optical stimulation of quartz in retrospective dosimetry and dating. Radiat. Prot. Dosim. 84, 335–340. Bulur, E., BZtter-Jensen, L., Murray, A.S., 2000. Optically stimulated luminescence from quartz measured using the linear modulation technique. Radiat. Meas. 32, 407–411. Choi, J.H., Murray, A.S., Cheong, C.S., Hong, D.G., Chang, H.W., 2003. The resolution of stratigraphic inconsistency in the luminescence ages of marine terrace sediments from Korea. Quat. Sci. Rev. 22, 1201–1206. Duller, G.A.T., 2003. Distinguishing quartz and feldspar in single grain luminescence measurements. Radiat. Meas. 37, 161–165. Duller, G.A.T., BZtter-Jensen, L., 1996. Comparison of optically stimulated luminescence signals from quartz using different stimulation wavelengths. Radiat. Meas. 26, 603–609. Godfrey-Smith, D.I., Huntley, D.J., Chen, W.H., 1988. Optical dating studies of quartz and feldspar sediment extracts. Quat. Sci. Rev. 7, 373–380. Huntley, D.J., Short, M.A., Dunphy, K., 1996. Deep traps in quartz and their use for optical dating. Can. J. Phys. 74, 81–91. Jain, M., Murray, A.S., BZtter-Jensen, L., 2003. Characterisation of blue-light stimulated luminescence components in different quartz samples: implications for dose measurement. Radiat. Meas. 37, 441–449.
Jain, M., Murray, A.S., BZtter-Jensen, L., Wintle, A.G., 2005. A single-aliquot regenerative-dose method based on IR(1.49 eV) bleaching of the fast OSL component in quartz. Radiat. Meas. 39, 309–318. Li, S.-H., 2001. Identification of well-bleached grains in the optical dating of quartz. Quat. Sci. Rev. 20, 1365–1370. Li, S.H., Li, B., Dose measurement using fast component of LMOSL signals from quartz. Radiat. Meas., in press. 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., 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., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiat. Meas. 32, 57–73. Murray, A.S., Wintle, A.G., 2003. The single aliquot regenerative dose protocol: potential for improvements in reliability. Radiat. Meas. 37, 377–381. Singarayer, J.S., Bailey, R.M., 2003. Further investigations of the quartz optically stimulated luminescence components using linear modulation. Radiat. Meas. 37, 451–458. Singarayer, J.S., Bailey, R.M., 2004. Component-resolved bleaching spectra of quartz optically stimulated luminescence: preliminary results and implications for dating. Radiat. Meas. 38, 111–118. Singarayer, J.S., Bailey, R.M., Ward, S., Stokes, S., in press. Assessing the completeness of optical resetting of quartz OSL in the natural environment. Radiat. Meas.,doi:10.1016/j.radmeas.2005.02.005. Smith, B.W., Rhodes, E.J., 1994. Charge movements in quartz and their relevance to optical dating. Radiat. Meas. 23, 329–333. Spooner, N.A., 1994. On the optical dating signal from quartz. Radiat. Meas. 23, 593–600. Thomas, P.J., Jain, M., Juyal, N., Singhvi, A.K., 2005. Comparison of single-grain and small-aliquot OSL dose estimates in < 3000 years old river sediments from South India. Radiat. Meas. 39, 457–469. Tsukamoto, S., Rink, W.J., Watanuki, T., 2003. OSL of tephric loess and volcanic quartz in Japan and an alternative procedure for estimation De from a fast OSL component. Radiat. Meas. 37, 459–465.
Comparison of De estimates using the fast component and the medium component of quartz OSL Bo Li, Sheng-Hua Li∗ Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China Received 9 March 2005; received in revised form 1 June 2005; accepted 30 June 2005
Abstract In equivalent dose (De ) determination of quartz using the single aliquot regenerative dose (SAR) technique, it is commonly observed that the De values are strongly dependent on the stimulation time for some well bleached samples. Such dependence is not controlled by the degree of recuperation of the optically stimulated luminescence (OSL) signals. Instead, this behavior is interpreted as being due to the medium component being thermally unstable for these samples and giving a lower De value than the fast component. An alternative method is described for the De determination of samples that exhibit such a phenomenon. © 2005 Elsevier Ltd. All rights reserved. Keywords: Equivalent dose; Quartz; OSL; Fast and medium components
1. Introduction Optically stimulated luminescence (OSL) from quartz consists of different components having different detrapping probabilities or photoionization cross-sections (Smith and Rhodes, 1994; Huntley et al., 1996; Bailey et al., 1997; Bulur et al., 2000; Jain et al., 2003). The different components in quartz OSL have been studied from many points of view, such as thermal stability, photoionization crosssection, recuperation, dose response, sensitization (Jain et al., 2003; Singarayer and Bailey, 2003, 2004). These studies have indicated that different characteristics of the different components may lead to significantly different results in the determination of the equivalent dose (De ) using a single aliquot regenerative dose (SAR) protocol (Murray and Wintle, 2000). In optical dating, the fast component is primarily of interest, because it can be bleached quickly and completely. Hence, it can be used even for sediments containing grains that only received a brief bleaching prior to ∗ Corresponding author. Tel.: +852 22 415 486; fax: +852 2517 6912. E-mail address: [email protected] (S.-H. Li).
1350-4487/$ - see front matter © 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2005.06.037
deposition. It also has a high signal-to-noise ratio due to its high detrapping probability. In the single aliquot regenerative dose (SAR) procedure with stimulation using constant power to obtain continuous wave OSL (CW-OSL) signals, the OSL signal used for De determination is usually the initial part of the OSL curve, with subtraction of both the background and the slow component. This method of signal selection is successful for samples that OSL signals are dominated by the fast component (Murray and Wintle, 2000; Murray and Olley, 2002). However, this signal selection cannot efficiently separate the fast component from the medium component because of their relatively close detrapping rates and the negligible amount of the medium component in the end signals for subtraction. The appearance of the medium component in the initial part of OSL signals may interfere with the fast component and may yield problematic results in De determination. Choi et al. (2003) reported a significant underestimation of De values using the simple integration of initial OSL signals where a large proportion of medium component contributed to the initial signal. Similar results were also found in other studies (e.g. Tsukamoto et al., 2003). Thomas et al. (2005) also reported a significant
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underestimation of De in the single-aliquot method as compared to those using the single-grain method. They interpreted it as there being more medium components present in the initial light in single aliquot measurements because the wavelength used for stimulation is different from that in the single grain luminescence system. A simple and useful method for checking the influence of the medium component is to use a De (t) plot. The form of the De (t) plot is affected by the composition of the OSL signals; thus the shape of the De (t) curve could be an effective tool for assessing the bleaching history of sediment samples (e.g. Aitken, 1998; Bailey, 2000a; Bailey et al., 2003). For well-bleached samples, the De should be independent of the illumination time. If the samples were partially bleached before burial and more relatively slower components remained, the De value will increase with illumination time. This method has been successfully applied for assessing the bleaching history of modern samples and young samples (Bailey et al., 2003; Singarayer et al., in press). For older samples, other factors, i.e. recuperation and thermal instability, may contribute potentially to the shape of De (t) plot. Tsukamoto et al. (2003) observed a decrease of De with illumination time. This was interpreted as a result of recuperation in the medium component. Singarayer et al. (in press) reported a falling De (t) plot for an old sample, which was attributed to the thermal instability of one of the slow components (S2). Several methods for separating the fast component from other components have been proposed. One is curve fitting of the OSL curves by deconvolution (Bailey et al., 1997; Bulur et al., 2000; Murray and Wintle, 2003; Jain et al., 2003; Choi et al., 2003; Li and Li, in press). This approach can have uncertainties in curve fitting procedure and the number of components in the OSL curves. Another method of separating components is to use different photon energies for stimulation. It was reported that the detrapping rate of the quartz OSL is strongly dependent on the stimulation wavelength or photon energy and the stimulation temperature (Godfrey-Smith et al., 1988; Spooner, 1994; Duller and BZtter-Jensen, 1996; Huntley et al., 1996). The fast component can be depleted by infrared (IR) stimulation at elevated temperature (100–200 ◦ C), with negligible depletion of the medium component and slow components (Bailey, 1998; Singarayer and Bailey, 2004; Jain et al., 2003, 2005). Based on such observations, a differential-OSL SAR procedure, using the difference between two short OSL measurements separated by elevated temperature IR stimulation, for De evaluation was proposed (Jain et al., 2005; Singarayer and Bailey, 2004). This approach requires long time IR stimulation and may not be appropriate for young samples. In this paper, we compared the De estimates using the fast component and the medium component, and present an alternative method for De determination of samples having significant dependence of De on illumination time. The possible reasons for problems with the De estimates using the medium component were investigated.
2. Samples and analytical facilities Three aeolian samples, RKZ, DGF-1 and 4155, from China were used in this study. These samples were well bleached prior to burial. All samples were treated first with 20% H2 O2 and 10% HCl to remove organic materials and carbonates. Grains between 90–150 m were selected by wet and dry sieving methods in subdued red safe-light conditions. Grains with densities between 2.62 and 2.75 g/cm3 were separated using sodium polytungstate heavy liquid. The separated grains were treated with 40% HF acid for 2 h to remove feldspar grains. For measurement, the mineral grains were mounted on the 10-mm-diameter aluminum discs with Silkospray silicone oil. The purity of the quartz was tested by monitoring the presence of feldspar through measuring the IRSL signals at room temperature and using the approach of the OSL IR depletion ratio in the SAR procedure proposed by Duller (2003). OSL measurements were performed using an automated RisZ TL/OSL DA-15 reader equipped with excitation units containing blue light-emitting diodes (LEDs, 470 ± 30 nm) (BZtter-Jensen et al., 1999). The OSL signal was detected through two 3 mm U-340 filters. Irradiation was carried out using a 90 Sr/90Y beta source built into the reader with the dose rate of 0.0899 Gy/s to quartz on aluminum discs. All OSL measurements were carried out at 125 ◦ C in order to prevent retrapping of charges from the shallow trap corresponding to the 110 ◦ C TL peak (Murray and Wintle, 1998). 3. Experiments and results 3.1. The CW-OSL signals The representative CW-OSL curve from sample DGF-1 is shown in Fig. 1(a). In order to examine how different components contribute to the OSL signals, the CW-OSL curve was deconvoluted into three components (fast, medium and slow) using first-order equations, respectively. The relative contributions as a function of illumination time of different components to the total signal are shown in Fig. 1(b). The fast component contributes above 80% of the total signal in the initial time (0–0.4 s). However, the proportion of the fast component decays quickly after 1 s and became less than 1% of the total signal after 5 s. In contrast, the proportion of the medium component increased from 15% at the beginning to about 50% at 3 s. The contribution from the slow component was negligible in the first 1 s; it started to increase quickly at 2 s and reached to 50% at 4 s. Therefore, the fast component and medium component contribute most to the De evaluation using the CW-OSL technique when the initial signal (0–2 s) is used. After 4 s, the slow component might contribute considerably to the total signal. 3.2. The dependence of De values on illumination time A SAR protocol (Murray and Wintle, 2000) was used for obtaining De for these samples. Each disc was treated with
B. Li, S.-H. Li / Radiation Measurements 41 (2006) 125 – 136
120
1.E+04 Experiment Medium Sum
Fast Slow
100
1.E+03
Apparent De, Gy
CW-OSL signal, counts
127
1.E+02
1.E+01
80 60 40 Initial test OSL 20
1.E+00 0
(a)
10
20 30 Stimulation time, s
Successive integrals
0
40
0
0.8
1.6 2.4 Stimulation time, s
3.2
Relative contribution
1.2
0.8 Fast
0.6
Medium 0.4
F+M Slow
0.2 0 0.1
(b)
Fig. 2. Dependence of De value on stimulation time for sample 4155 using different parts of the test dose OSL for correcting for sensitivity change, namely the initial signals (full diamonds) and successive integrals (empty squares).
1
1 10 Stimulation time, s
100
Fig. 1. Representative CW-OSL curve from sample DGF-1. Three exponential components (fast, medium and slow) are shown. (b) Relative contributions from different components to the CW-OSL curve (a) plotted against the stimulation time.
measurement cycles of preheating at 260 ◦ C for 10 s, OSL measurement, test dose, cut-heat to 220 ◦ C, OSL measurement and irradiation with different regenerative dose in each cycle. Five regenerative doses including one zero dose and one repeat dose were applied to each disc. The De values obtained were plotted as a function of illumination time. The time range examined is 0–3.6 s where the fast component and medium component dominate (Fig. 1(b)). Any dependence of De on illumination time could thus be considered to be the result of the interaction between the fast component and the medium component. In order to investigate whether the sensitivity changes of the signals at different integration intervals are the same or not, initial signal and successive integrals through the stimulation time for the test dose OSL were used for correcting for the sensitivity change. The De (t) plots obtained from the same data set from one aliquot of sample 4155 using both correction methods are shown in Fig. 2. A similar pattern was obtained for both methods, suggesting that sensitivity changes of the signals at different integration intervals are the same. It is thus indicated that the fast component and the medium component are sensitized in the same way. Based on this observation, in this study, the sensitivity change of different integration intervals was corrected using the initial
signal of test dose OSL to minimize the effect of significant reduction of precision in the test dose OSL as illumination time increases, which may lead to large errors in constructing De (t) plots. Typical De (t) plots for a single aliquot of samples DGF-1, 4155 and RKZ are shown in Fig. 3(a)–(c). The De values are dependent on the illumination time to different extents. The oldest sample, DGF-1, yields the largest dependence. The De value decreases from 143 Gy at 0–0.4 s to 65 Gy at 3.2–3.6 s. The De value of the aliquot from sample 4155 (Fig. 3(b)) also decreases from 110 to 56 Gy for the same period. The youngest sample RKZ shows the smallest dependence, from 48 to 30 Gy (Fig. 3(c)). Fig. 3(d) shows the dose response curves and the natural values at t =0–0.4 s and 3.2–3.6 s for the sensitivity-corrected data used for Fig. 3(a). The response curves were fitted with a saturating exponential function of the form I = I0 (1 − exp[−D/D0 ]) with D0 = 119 and 293 Gy for t = 0–0.4 s and 3.2–3.6 s, respectively. Both natural signals are not saturated, suggesting that the dependence of De on illumination time is not related to the saturation of one signal relative to the other (e.g. the fast component compared with the medium component) which may also affect the form of De (t) (Bailey et al., 2003). The good recycling ratios obtained for both intervals also suggest that the sensitivity correction using the initial signal from test dose OSL was successful. Fig. 4 shows four De (t) plots for different aliquots from the sample DGF-1 in which one of them (Fig. 4(d)) was measured using a higher cut-heat temperature (260 ◦ C) after the test dose, equal to that used for the preheat in order to check the influence of cut-heat on this temperature dependence. Bailey (2000b) suggested that for some samples a low cut-heat temperature might result in incorrect De estimation. The decreasing trend of De with illumination time appears for both cut-heat temperatures, suggesting that such dependence is not the result of inappropriate use of cut-heat temperature. The extent of the decrease in De with
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B. Li, S.-H. Li / Radiation Measurements 41 (2006) 125 – 136
DGF-1
150
Apparent De, Gy
Apparent De, Gy
180
120 90 60 Experimental Best fitted
30 0
0
0.8
(a)
1.6 2.4 Stimulation time, s
4155
120 90 60
Experimental Best fitted
30 0
3.2
0.8
0
(b)
1.6 2.4 Stimulation time, s
3.2
Apparent De,Gy
60 RK Z
50 40 30 20
Experimental Best fitted
10 0 0
(c)
0.6
1.2 1.8 2.4 3 Stimulation time, s
0-0.4s 3.2-3.6s
3 2.5 2
0.2
1.5 1
143 Gy
0
4.2
0.1
65 Gy
0.5 3.6
0.3
0
50
(d)
100 Dose, Gy
150
Corrected OSL,3.2-3.6s
Corrected OSL,0-0.4s
3.5
0 200
200
250
160
200
Apparent De, Gy
Apparent De, Gy
Fig. 3. Typical De (t) plots for samples DGF-1 (a), 4155 (b) and RKZ (c) using a 260 ◦ C preheat temperature. Every experimental data point is calculated using an integral of 0.4 s of the OSL signals at each different illumination time. The trend lines are the best fitted data using Eq. (3) (see text). (d) The dose response curves and natural values at t = 0–0.4 s and 3.2–3.6 s for the sensitivity-corrected data used for (a).
120 80 40 0
0
0.8
(a)
1.6 2.4 Stimulation time, s
50
0
0.8
1.6 2.4 Stimulation time, s
3.2
0
0.8
1.6 2.4 Stimulation time, s
3.2
(b) 200
200
Apparent De, Gy
Apparent De, Gy
100
0
3.2
250
150 100 50 0 0
(c)
150
0.8
1.6 2.4 Stimulation time, s
150 100 50 0
3.2
(d)
Fig. 4. De (t) plots for different aliquots of DGF-1 measured using the standard SAR protocol. The cut-heat temperature for the test dose used for (a), (b) and (c) was 220 ◦ C. The cut-heat temperature for the test dose used for (d) was 260 ◦ C. Every experimental data point was calculated using an integral of 0.4 s of the OSL signals at each different illumination time. The trend lines are the best fitted data using Eq. (3).
250
250
200
200
Apparent De, Gy
Apparent De, Gy
B. Li, S.-H. Li / Radiation Measurements 41 (2006) 125 – 136
150 100 50 0
0
100 50 0
3.2
250
200
200
150 100 50 0
0.8
1.6 2.4 3.2 Stimulation time, s
0
0.8
0
0.8
(b)
250
0 (c)
150
Apparent De, Gy
Apparent De, Gy
(a)
0.8 1.6 2.4 Stimulation time, s
129
1.6 2.4 Stimulation time, s
3.2
150 100 50 0
4 (d)
2.4 1.6 Stimulation time, s
3.2
Fig. 5. De (t) plots for “dose recovery” aliquots of DGF-1 measured using the standard SAR protocol. The lines are the laboratory dose of 174 Gy.
3.3. The role of recuperation in De (t) dependence Tsukamoto et al. (2003) reported that there was an exponential relationship between De and the degree of recuperation at different illumination time; this might be the reason for the underestimation of De . In order to check the role of recuperation in the time dependence of De , six aliquots of sample DGF-1 were measured using the similar SAR protocol in which a high temperature (280 ◦ C) OSL measurement for 40 s was added at the end of each measurement of test dose OSL. This was initially proposed by Murray
4 No 280˚C OSL Recuperation, %
illumination time for these aliquots is not the same. This might be expected as different aliquots have different proportions of fast component and medium component, analogous to the observations that different aliquots may have different proportions of well bleached grains (Li, 2001). Fig. 5 shows four De (t) plots of aliquots for a dose recovery test for sample DGF-1. Quartz aliquots were bleached for 100 s with the blue LED light. Then, a laboratory dose of 174 Gy was given to each aliquot. The laboratory irradiated aliquots were treated as the natural to determine the De using the same SAR protocol as described above. The average De value is 184 ± 16 Gy for a laboratory dose of 174 Gy. It is noting that in contrast to the natural aliquots of the same sample DGF-1 (Fig. 4), the De (t) plots for these dose recovery aliquots were flat over the same illumination time range. It is suggested that De decreasing with illumination time only happens for the natural aliquots.
280˚C OSL
3
2
1
0
0
1
2
3
4
5
6 7 8 Aliquot No.
9 10 11 12 13
Fig. 6. Recuperation of the sample DGF-1 observed during SAR measurements; (a) the filled squares are the recuperation of aliquots measured using the standard SAR protocol. (b) The empty squares are the recuperation of aliquots measured using a SAR protocol in which a high temperature (280 ◦ C) OSL measurement for 40 s was added at the end of each OSL measurement of test dose. The recuperation was expressed as the percentage of the natural signal.
and Wintle (2003) for reducing recuperation from the residual signals after each SAR cycle; it has since proved to be useful for improving the precision and the accuracy of dose recovery. After this step was added, the recuperation for all aliquots was reduced significantly to less than 0.5% of the initial level of natural signals (Fig. 6) and can be
130
B. Li, S.-H. Li / Radiation Measurements 41 (2006) 125 – 136 200 Apparent De, Gy
Apparent De, Gy
150
100
50
0
0
0.8
(a)
1.6 2.4 Stimulation time, s
150 100 50 0
3.2
(c)
0.8
1.6 2.4 Stimulation time, s
0
0.8
2.4 1.6 Stimulation time, s
3.2
200
200
Apparent De, Gy
Apparent De, Gy
250
150 100 50 0 0
0
(b)
0.8
2.4 1.6 Stimulation time, s
150 100 50 0
3.2
(d)
3.2
Fig. 7. De (t) plots for four different aliquots of DGF-1 measured using SAR protocol in which a high temperature (280 ◦ C) OSL measurement for 40 s was applied at the end of each measurement of the test dose OSL for which data points in Fig. 6(b) were obtained. Every point of experimental data was calculated using an integral of 0.4 s of the OSL signals at each different stimulation time. The trend lines are the best fitted data using Eq. (3).
considered to be negligible. The relationship between De and illumination time of these aliquots was then examined (Fig. 7). Although negligible recuperation was achieved in this experiment, no significant diminution in the dependence of De on illumination time was achieved and there was still a strongly decreasing trend of De with illumination time. Similar results were observed for samples RKZ and 4155. It is thus concluded that recuperation of the medium component is not the main factor controlling the time dependence of De for these samples. 3.4. De determination using the medium component In order to check the validity of De estimation using the medium component, a SAR protocol for the medium component is used for De estimation (Singarayer and Bailey, 2004; Jain et al., 2003). The outline of the post-IR OSL SAR protocol is given in Table 1. In this protocol, IR stimulation for 7000 s at 160 ◦ C is applied before each OSL measurement. It was demonstrated that there is no significant decay of the medium component by 6000–8000 s IR stimulation at 160 ◦ C while the fast component can be depleted to a negligible level (Singarayer and Bailey, 2004). Therefore, the OSL recorded after such high temperature IR stimulation is only from medium and slow components. At the end of
Table 1 The post-IR OSL single-aliquot regenerative-dose protocol for measuring the medium OSL component Step
Treatment
Observed
1 2 3 4 5 6 7 8 9 10
Give regenerative dose, Di a Preheat at 260 ◦ C for 10 s IR stimulation at 160 ◦ C for 7000 s OSL measurement at 125 ◦ C for 40 s
Li (medium)
Give test dose, Dt Cut-heat to 260 ◦ C IR stimulation at 160 ◦ C for 7000 s OSL measurement at 125 ◦ C for 40 s OSL measurement at 280 ◦ C for 40 s Return to 1
Ti (medium)
a For the natural sample, i = 0 and D = 0. The whole sequence 0 is repeated for several regenerative doses including a zero dose and a repeat dose.
each SAR cycle an additional OSL measurement is made at 280 ◦ C for 40 s to minimize the effect of recuperation. A preheat of 260 ◦ C for 10 s and a cut-heat of 260 ◦ C was used for regenerative doses and test doses, respectively, so that any sensitivity change due to the 160 ◦ C IR stimulation can be ignored (Jain et al., 2005).
B. Li, S.-H. Li / Radiation Measurements 41 (2006) 125 – 136
280
4
200 6
De, Gy
Corrected OSL
5
No post-IR
240
8
4
160
3
120
2
80 2
1
40 0
0 0
50
(a)
100 150 200 Regenerative dose, Gy
250
300
0
2
4
8 6 Aliquot No.
10
12
0
Fig. 9. De values and recycling ratios obtained using the post-IR OSL SAR protocol (Table 1) and the standard SAR protocol (no IR) for sample DGF-1. The filled diamonds with error bars are individual De values. The recycling ratios are represented as open squares.
120 100 De(medium), Gy
Post-IR OSL
Recycling ratio
10
131
80 60
component for De evaluation gives lower values than using the fast component.
40 20
3.5. The thermal stability of the medium component
0 0
(b)
0.6
1.2 1.8 2.4 Stimulation time, s
3
3.6
Fig. 8. (a) Dose response curve of the medium component of the sample DGF-1 obtained using the post-IR OSL SAR protocol described in Table 1. The experimental data were fitted with a single saturating exponential function of the form I =I0 (1−exp[−D/D0 ]) with D0 = 147.8 Gy. (b) De (t) plot taken from the same aliquot used in (a). The dashed line represents the value obtained in (a).
Sample DGF-1 was measured in this experiment. The OSL recorded after IR stimulation was used for De estimation. The net OSL signals Li (medium) and Ti (medium) are derived from the initial integral (0–2 s) of the OSL signals minus an equivalent part of the residual signal from the end part of the curves. The dose response curves were fitted with single saturating exponential functions (Fig. 8(a)). It is interesting to note that there was little dependence of De value on illumination time (Fig. 8(b)) in such a postIR OSL dating procedure when comparing to those without IR bleaching, indicating that the OSL is dominated by the signals from a single type of traps corresponding to the medium component. The results obtained from this protocol and those from the standard SAR procedure are compared in Fig. 9. The average De value of the medium component is 106.3 ± 9.2 Gy (n = 8), significantly lower than the value of 167.6 ± 13.6 Gy (n = 9) given by using the standard SAR protocol which utilizes the initial part of the OSL signal. The average recycling ratios of both procedures are 0.99 ± 0.03 and 1.00 ± 0.05, respectively, suggesting that the sensitivity change during regeneration cycles was well corrected. Thus it can be concluded that using the medium
The thermal stabilities of different components in OSL have been suggested as a potential factor contributing to the shape of De (t) plot for old samples (Bailey et al., 2003; Singarayer et al., in press). Bailey et al. (2003) have modeled the influence of the unstable slow component (S2 in their study) on De (t) plot. However, according to their numerical simulation, the effect is much less significant than the results observed for our samples. Since the dependence of De on illumination time examined in our study occurs over 0–3.6 s where the fast component and medium component dominate (Fig. 1(b)), only when the medium component yields a significantly lower De value than the fast component will the remarkable time dependence of De be observed. In order to check whether the decrease in the De (t) plots is the result of thermal instability of the medium component, a pulsed annealing study using CW-OSL was carried out using a single aliquot of sample DGF-1. The aliquot was first optically bleached at 125 ◦ C for 100 s to remove the natural signal. The aliquot was then optically stimulated at a high temperature (280 ◦ C) for 40 s to remove the recuperation as suggested by Murray and Wintle (2003). The aliquot was then given a dose of 90 Gy prior to heating to T (◦ C), followed a CW-OSL measurement at 125 ◦ C for 100 s to measure the remaining signal. The aliquot was then given a test dose of 9 Gy and heated to 220 ◦ C. The test dose OSL was then measured for monitoring sensitivity change. Such measurement cycles were repeated by increasing T from 220 to 380 ◦ C in steps of 20 ◦ C. At the end of each measurement of test dose OSL, an OSL measurement at 280 ◦ C for 40 s was applied to remove the recuperation.
132
B. Li, S.-H. Li / Radiation Measurements 41 (2006) 125 – 136
et al., 2003). This result also implies more complex characteristics of different components of quartz OSL, because different quartz may have different thermal stabilities for different components.
Normalized signals
1.2 1 0.8 0.6
3.6. Analyzing the De (t) plots
0.4 0.2 0 200
(a)
0s 1.2 s 2.4 s 3.6 s
0.4 s 1.6 s 2.8 s
0.8 s 2.0 s 3.2 s
240 280 320 360 Preheating temperature, °C
400
1.2
Remnant OSL
1 0.8 0.6 0.4
In De determination using CW-OSL, if the medium component yields a different De value from the fast component, the De will be dependent upon the relative proportions of the fast and medium components, and thus depend on stimulation time. Only when the fast component and medium component give the same De values, will such dependence disappear and a single De value will be obtained. Assuming that the contribution of slow components to the net OSL is negligible after subtracting the later part of OSL curves, only the fast component and medium component contribute to the De estimation. For the fast component, assuming that the OSL decaying exponentially with stimulation time, the OSL from stimulating time t to t + t is given by
Fast
0.2 0 200 (b)
Medium
250 300 350 Preheating temperature, °C
Lf (t) = 400
Fig. 10. (a) Pulse annealing curves for different integral intervals of CW-OSL signal at different stimulation times for sample DGF-1. (b) The pulse annealing curves for the fast and medium components separated from the CW-OSL signal for sample DGF-1.
In order to check the influence of the thermal stability of the medium component on different time intervals of CW-OSL signal, the signals at different time intervals (0–3.6 s) observed after preheating to different temperatures are shown in Fig. 10(a). The signals at later intervals decay faster with increasing preheating temperature than the signals at earlier intervals, indicating that most unstable signal is mainly from the medium component that has an increased concentration in the later time intervals. The fast and medium components in each CW-OSL curve were also separated by deconvoluting the CW-OSL curves. Fig. 10(b) shows the remnant fast and medium components after preheating to different temperatures. The fast component begins to decay beyond 300 ◦ C. However, the medium component shows earlier decay above 240 ◦ C, suggesting that the medium component is not stable when compared with the fast component. Thus it can be concluded that the instability of the medium component contributes the falling of De (t) plots for our samples. This result contrasts with those reported previously where the medium component is more stable than the fast component (Singarayer et al., 2003; Jain
t+t t
If exp(−f I0 t) dt
=
If {exp(−f I0 t) − exp[−f I0 (t + t)]} f I0
=
If exp(−f I0 t)[1 − exp(−f I0 t)] f I0
= Cf exp(−f I0 t),
(1)
where Cf = [1 − exp(−f I0 t)]If /(f I0 ) and is independent of t, Lf is the integrated signal of the fast component from t to t + t, If is the initial intensity of the fast component at t = 0, f refers to the photoionization cross-section of the fast component, I0 is the intensity of the stimulation light (in this experiment I0 is ∼ 45 mW/cm2 ). As for the fast component, we have for the medium component, Lm (t) =
t+t t
Im exp(−m I0 t) dt
= Cm exp(−m I0 t),
(2)
where Cm = [1 − exp(−m I0 t)]Im /(m I0 ), Lm is the integrated signal of the medium component from t to t + t, Im is the initial OSL intensity of the medium component, m is the photoionization cross-section of the medium component. If the De values of the fast component and medium component are expressed as Df and Dm respectively, and the contributions of the fast component and medium component to the apparent De value are proportional to their relative
B. Li, S.-H. Li / Radiation Measurements 41 (2006) 125 – 136
proportions in the OSL signals, the apparent De observed at time t will be: Lf (t) Lm (t) + Dm Lf (t) + Lm (t) Lf (t) + Lm (t)
Df Cf exp(−f I0 t) + Dm Cm exp(−m I0 t) Cf exp(−f I0 t) + Cm exp(−m I0 t)
Df k exp(−f I0 t) + Dm exp(−m I0 t) , = k exp(−f I0 t) + exp(−m I0 t)
D0(f)=D0(m)=150 Gy 250 Apparent De, Gy
=
300
(3)
where k is a factor relating to the ratio of initial OSL intensity of fast component to that of medium component (k = Cf /Cm ). The values of f and m are 2.3 × 10−17 and 5.6 × 10−18 cm2 , respectively (Jain et al., 2003; Li and Li, in press). If the integration interval t used for the calculation is kept the same (0.4 s is chosen for ensuring good precision and enough data points for fitting in this study), k is a constant. Therefore, the apparent De can be expressed as a function of time. The values of Df , Dm and k can be given as parameters by fitting the De (t) plot with Eq. (3). Thus, the true De , i.e. Df , can be obtained. For linear growth in the low dose range, it is true that the contributions of the fast component and medium component to the apparent De value are proportional to their relative proportions in the OSL signals. However, this may not be the case when either one or both of these signals grows nonlinearly. In order to test the feasibility of applying Eq. (3) for analyzing the De (t) close to the region of dose saturation (up to 2D0 ), the De (t) plot was simulated using different parameters and analyzed using Eq. (3). Assuming that the true values of Df and Dm are 300 and 150 Gy, respectively, and the values of the detrapping rates for the fast component and the medium component, bf and bm , are 2.5 and 0.62 s−1 (after Jain et al., 2003), respectively, the De (t) plot can be constructed (Fig. 11) using different ratios of Imax (the maximum signal after saturation) and D0 for the fast component and the medium component (Table 2). The simulated De (t) plots were fitted using Eq. (3) to obtain Df and Dm . Fig. 11(a) shows two De (t) curves where the fast component and the medium component have same D0 (D0 (f ) = D0 (m) = 150 Gy) but their relative Imax is different. Fig. 11(b) shows three De (t) curves where the medium component has different D0 value but same relative Imax . The shape of the De (t) curves are very similar to those observed in experiment (e.g. Figs. 3, 4 and 7). The fitting results were also shown in Table 2. Significant underestimation (up to 20%) of the apparent De values (De (a) in Table 2) obtained using the first 0.4 s of the OSL signal was obtained. However, the deviation of all values of Df given by fitting are within 5% of the true value of 300 Gy. The Dm can also be obtained correctly without the influence of saturation. This result suggests that Eq. (3) can be safely applied to the samples with De less than 2D0 where the SAR protocol can be applied.
200 150 100 Imax(f)/Imax(m)=4 Imax(f)/Imax(m)=2
50 0 0
0.8
(a)
1.6 2.4 Stimulation time, s
3.2
300 Imax(f)/Imax(m)=3.3 250 Apparent De, Gy
De (t) = Df
133
200 150 100 D0(f)/D0(m)=2 D0(f)/D0(m)=1 D0(f)/D0(m)=0.5
50 0 0 (b)
0.8
1.6 2.4 Stimulation time, s
3.2
Fig. 11. De (t) plots constructed using different parameters for the fast component and the medium component as shown in Table 2. (a) De (t) curves where the fast component and the medium component have the same D0 (D0 (f ) = D0 (m) = 150 Gy) but different relative Imax . (b) De (t) curves where the medium component has a different D0 value but the same relative Imax . All De (t) curves were fitted using Eq. (3).
3.7. De estimation using De (t) plots The De (t) plots taken from all aliquots of the sample DGF-1 were fitted with Eq. (3) using nonlinear least squares method with software Origin 7.0. It is found that all the De (t) plots are well fitted, as already shown in Figs. 3, 4 and 7. The Df and Dm values obtained by fitting the De (t) plots and the corresponding apparent De obtained by using the initial 1.2 s of the OSL signals are summarized in Fig. 12(a) and (b). For the nine aliquots examined, the De values of the fast component are significantly higher than those of the medium component, except for one aliquot giving a coincident value for both components and which has a flat De (t) curve. The mean Dm value obtained by fitting the De (t) plots is 98.4 ± 5.2 Gy (n = 9), which agrees well with the value of 106.3 ± 9.3 Gy given by the post-IR OSL method described in Section 3.2, indicating that the medium
134
B. Li, S.-H. Li / Radiation Measurements 41 (2006) 125 – 136
Table 2 Numerical parameters for constructing the De (t) plots and the fitting results obtained using Eq. (3). The true values for Df and Dm are 300 and 150 Gy, respectively. The values of the detrapping rates for the fast component and the medium component, bf and bm , are 2.5 and 0.62 s−1 , respectively (after Jain et al., 2003). De (a) is the value obtained for the first 0.4 s OSL signal Numerical parameters
Results Relative Imax (m)
D0 (f ) (Gy)
D0 (m) (Gy)
Df (Gy)
Dm (Gy)
De (a) (Gy)
1 1 1 1 1
0.25 0.5 0.3 0.3 0.3
150 150 150 150 150
150 150 75 150 300
288.2 ± 2.8 298.5 ± 6.5 309.3 ± 1.1 296.7 ± 3.5 293.2 ± 3.7
149.4 ± 0.5 149.9 ± 0.5 149.2 ± 0.2 149.4 ± 0.4 149.9 ± 0.6
255 235 270 250 253
300
200
250
150 100 De (fast) De (apparent) De (medium)
50 0
0
1
2
De (apparent), Gy
(a)
3
4 5 6 Aliquot No.
7
8
9 10
300
300
250
250
200
200
150
150
100
100
0
0
50
100 150 200 De (fast), Gy
250
200 150 100 50 0
50
50
(b)
De from fast component, Gy
250
De (medium), Gy
De, Gy
Relative Imax (f )
0 300
Fig. 12. Comparison of De obtained with different methods. (a) The De values of the fast component and medium component of sample DGF-1 given by fitting the De (t) plots with Eq. (3) and the apparent De values obtained using the initial 1.2 s of the OSL signals. (b) The De values of the fast component plotted against the De values of the medium component (open squares at the right scale) and the apparent De values (filled squares at the left scale).
component was separated successfully. The mean Df value is 194.8 ± 13.9 Gy (n = 9), systematically higher than the apparent De value 167.6 ± 13.6 Gy obtained by using the first 1.2 s of OSL signals. In order to confirm that the fast
0
50
100 150 200 250 Df from the De(t) plot, Gy
300
Fig. 13. De values of fast component obtained by curve fitting individual OSL curves plotted against the values obtained by fitting the De (t) plots.
component was successfully separated in this method, the individual OSL curves of these aliquots were also analyzed using a curve fitting method to separate the fast component for De estimation (Li and Li, in press). The De value given by separated fast component is 187.7 ± 13.3 Gy. It is in good agreement with the result 194.8 ± 13.9 Gy for Df given by fitting the De (t) plot. The individual values of the aliquots obtained from both methods are shown in Fig. 13. The De values for fast component obtained by curve fitting individual OSL curves are coincident with the values obtained by fitting the De (t) plots, suggesting that the fast component was successfully separated by fitting the De (t) plots.
4. Discussion A strong dependence of De on illumination time was observed. The De values decrease with illumination time for
B. Li, S.-H. Li / Radiation Measurements 41 (2006) 125 – 136
a large portion of aliquots measured, although the samples were well bleached before burial. It is shown that the dependence of De on stimulation time was independent of the degree of recuperation, indicating that recuperation is not the main reason controlling the time dependence of De , although recuperation might be a potential factor contributing to such a dependence when considering that the recuperation in the medium component is larger than the fast component (Jain et al., 2003). The thermal instability of the medium component in these samples has been demonstrated by annealing tests. Such an unstable medium component occurs in the initial OSL signal in the observed stimulation range (0–3.6 s) and it contributes to the significant decrease of De with illumination time when these results are compared with those reported by Bailey et al. (2003) and Singarayer et al., (in press). In their studies, a significant decrease in De occurred at a later illumination time, and which they attributed to the thermal instability of one of the slow components (S2). These results suggest that different quartz may have different characteristics in regard to the thermal stabilities of different components. Therefore, checking the thermal stabilities of different components used for dating is necessary. The plots of De versus stimulation time can provide information about the interaction between the fast component and the medium component. The apparent De values can be expressed as a function (Eq. (3)) of stimulation time t, the De value from the fast component Df , and the value from the medium component Dm . Df and Dm can thus be obtained by fitting the De (t) plots using Eq. (3). For sample DGF-1, the De value from the fast component obtained by this method agrees with the result of fitting individual OSL curves but systematically higher than the apparent De value obtained by using the initial OSL signals (Fig. 13). The De value from the medium component obtained by this method is also consistent with the result obtained by using the postIR OSL for the experimentally separated medium component. This result indicates that, in the De evaluation using the standard SAR protocol, underestimation of De values might happen when using the initial part of the OSL signals if the medium component is unstable. It thus suggested that, besides using criteria of “recycling ratio”, “recuperation”, “dose recovery” and “preheat plateau” for testing the reliability of the De estimation, it is also important to check the De (t) plot to assess the validity of De results. These results also indicate that analyzing the De (t) plots is an effective way for De estimation. Because no analysis of individual OSL curves is required, this procedure is less time consuming than curve fitting of each OSL curve and only fitting of the De (t) curve is needed for each aliquot. It should be also noted that this method of De (t) analysis is insensitive to the relationship between De values of the fast component and medium component. It can distinguish the De values of both components regardless of whether the De of the medium component is higher or lower than that of the fast component. For partially bleached samples,
135
the De may increase with illumination time because of the incomplete bleaching of the medium component and slower components. Therefore, this method of analysis can also be used to evaluate the De value of the fast component for partially bleached samples where the medium component may yield larger De values. As mentioned previously, it is assumed in Eq. (3) that the contribution of the fast component and medium component to the apparent De value is proportional to their proportions in the OSL signals. However, erroneous results may be obtained if the signals are close to saturation. It is suggested that the De values of the fast component and the medium component should not exceed their 2D0 values that are obtained from growth curve fitting with a saturating exponential function. Another limitation of De estimation using a De (t) plot is the assumption that there are negligible signals of slow components after subtraction of late part of the OSL curve. In fact, the thermally unstable component (S2) also contributes to the decrease of De with illumination time (Bailey et al., 2003), although relatively small in the earlier stage of illumination. If there is a significant portion of such a slow component in the OSL signal region of interest, and if it has a different De value to Df and Dm , the fitted results might be erroneous. Therefore, for the samples where the fast component is relatively small and the slow components are relatively important, the best method is to separate the fast component by using curve fitting (Singarayer et al., in press; Li and Li, in press) or by an experimental method as that suggested by Jain et al. (2005).
5. Conclusions Equivalent dose values decreasing with illumination time were observed in several samples. Recuperation in the medium component is not the main factor controlling the underestimation of De values when the initial part of the OSL signals was used. The medium component yields systematically lower De values than those from the fast component due to its thermal instability; this causes the underestimation of De values using the initial part of the OSL signals and the significant dependence of De on illumination time. True De values for the fast component and medium component can be evaluated by analyzing the De (t) plots.
Acknowledgements The authors thank Ann Wintle for her inspirational and critical comments and Richard Bailey for suggestions on the manuscript. This work was supported by grants to S.-H. Li from the Research Grant Council of the HKSAR, China (Project No. 7106/02P) and from CRCG of HKU.
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