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On the separation of quartz OSL signal components using different stimulation modes
Radiation Measurements 43 (2008) 742 – 747 www.elsevier.com/locate/radmeas
On the separation of quartz OSL signal components using different stimulation modes Jakob Wallinga a,∗ , Adrie J.J. Bos b , Geoff A.T. Duller c a Netherlands Centre for Luminescence Dating, Delft University of Technology, Mekelweg 15, NL-2629 JB Delft, The Netherlands b Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, NL-2629 JB Delft, The Netherlands c Institute of Geography and Earth Sciences, Aberystwyth University, SY23 3DB Aberystwyth, Wales, UK
Abstract We investigate both theoretically and experimentally the effect of stimulation mode on the separation of quartz optically stimulated luminescence (OSL) components. We find that, when assuming first-order kinetics with the detrapping probability proportional to stimulation intensity, the OSL signal is a function of the cumulative stimulation energy and not affected by the stimulation mode. This is confirmed by close correspondence between continuous wave (CW), linearly modulated (LM) and hyperbolically modulated (HM) OSL data for some of the samples studied. For other samples the data obtained using LM stimulation differ from that obtained using the other stimulation modes. This may be due to a contribution to the OSL signal from feldspars, or it may indicate that the behaviour of these samples is not adequately described by first-order kinetics. We suggest that CW stimulation is the method of choice for dating purposes as it allows the fastest readout with the greatest signal-to-noise ratio, and because it has a constant background. HM stimulation provides a good alternative when higher resolution is needed for the initial part of the shine-down curve. © 2008 Elsevier Ltd. All rights reserved. Keywords: Quartz; OSL components; Stimulation mode; First-order kinetics
1. Introduction The quartz optically stimulated luminescence (OSL) signal is composed of a number of components with different photoionization cross sections. Based on the relative rapidity of resetting upon light exposure, these are referred to as fast, medium and slow components (e.g. Bailey et al., 1997). Including the ultrafast component, up to seven components have now been identified (Jain et al., 2003). At constant stimulation intensity (continuous wave, CW), the OSL signal is a sum of exponential decays (Smith and Rhodes, 1994). It is also possible to ramp the stimulation intensity during readout. Using linearly modulated (LM) stimulation, an OSL signal is obtained where the components are represented by peaks (Bulur, 1996; BZtter-Jensen et al., 1999). For dating using the OSL signal from quartz, the fast component is most suitable as it is: (1) rapidly reset by light exposure ∗ Corresponding author.
E-mail address: [email protected] (J. Wallinga). 1350-4487/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2008.01.013
prior to deposition and burial; (2) it is stable on geological timescales (e.g. Singarayer and Bailey, 2003) and (3) the single aliquot regenerative (SAR) dose procedure was developed to use this component (Wintle and Murray, 2006). For dating of samples where the fast OSL component is at or near saturation, one of the slow components may potentially be used for dating (Singarayer and Bailey, 2003). A number of recent studies have reported the use of LM stimulation to improve component separation for dating purposes (e.g. Singarayer and Bailey, 2003; Li and Li, 2006; Choi et al., 2006b). Some of these studies suggest that the use of LM stimulation allows better separation of components. In a recent letter to Ancient TL, Huntley (2006) points out that the separation of OSL components is independent of the stimulation mode used. In this study we investigate theoretically and experimentally whether the overlap of OSL signals from the components is independent of the distribution of the stimulation energy in time. In addition to the widely used CW and LM stimulation modes, we introduce hyperbolically modulated (HM) stimulation. We discuss the advantages and disadvantages of different
J. Wallinga et al. / Radiation Measurements 43 (2008) 742 – 747
stimulation modes and conclude that CW stimulation is the method of choice, provided that the resolution of the data acquisition is adequate. 2. Considerations on the separation of quartz OSL signal components In this section we discuss whether it is theoretically possible to improve the separation of quartz OSL signal components using different stimulation modes. To do so, we assume a firstorder kinetic system where the detrapping probability is proportional to stimulation intensity. This assumption is based on evidence in the literature, which we briefly summarize here. The quartz photo-ionization cross sections with blue stimulation have been shown to be independent of the stimulation power up to 50 mW cm−2 (Bulur et al., 2001) and quartz LMOSL curves and linearly transformed CW-OSL are in close correspondence (Jain et al., 2003; Poolton et al., 2003). Moreover, Singarayer and Bailey (2003) demonstrated that the componentspecific dose response curves could be approximated well by single saturating exponentials. We note that Kuhns et al. (2000) provided experimental evidence that the quartz OSL signal may not follow first-order kinetics, but it has been suggested that their data may have been affected by an isothermal TL signal (Jain et al., 2007). For a system obeying the assumptions outlined above, the OSL signal from a single component is described by IOSL (t) = −
dn = n(t)()(t), dt
(1)
where IOSL (t) is the OSL intensity at time t (s), n(t) is the charge carrier (electron or hole) trapped charge population (m−3 ) at time t, () is the photo-ionization cross section (m2 ) for a given wavelength of stimulation () and (t) is the stimulation intensity (directly proportional to the number of stimulation photons, m−2 s−1 ) at time t. In Table 1 we provide the functions for the stimulation mode and the mathematical form that would be expected for the OSL signal from a single trap for CW, LM and HM stimulation. The information is also provided graphically in Fig. 1A. HM stimulation is here introduced; the stimulation intensity increases rapidly at a rate (s−1 ) at the start of the measurement after which it stabilizes at a maximum P after a stimulation period P. In Fig. 1B we show calculated OSL signals for a hypothetical sample with fast, medium and slow(3) components
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with equal trapped charge populations at t = 0. Based on Jain et al. (2003), we chose the photo-ionization cross sections of the medium and slow(3) components to be 0.24 and 0.0013 relative to that of the fast component. Through integration of Eq. (1) we can calculate the charge carrier concentration at time t: n t dn (t ) dt , (2) = − n0 n 0
t n(t) = n0 exp − (3) (t ) dt . 0
The OSL intensity ratio of two components (a and b) with photo-ionization cross a and b and initial trapped charge concentrations n0,a and n0,b follows from Eq. (1): IOSL,a (t) na (t) a . = IOSL,b (t) nb (t) b
(4)
Substitution of Eq. (3) gives
t IOSL,a (t) n0,a a exp (b − a ) (t ) dt . = IOSL,b (t) n0,b b 0
(5)
From Eq. (5) we see that the OSL signal ratio of two components depends on: (i) the ratio of the initial trapped charge concentrations, (ii) the relative photo-ionization cross sections (at ), (iii) the time integrated stimulation intensity, which is proportional to the cumulative stimulation energy. The OSL signal ratio is thus independent of the distribution of stimulation photons in time. This is demonstrated in Fig. 1C where we show that the calculated CW-, LM- and HM-OSL signals given in Fig. 1B are identical when plotted as a function of cumulative stimulation energy. We conclude that under the assumptions outlined above, the mode of stimulation has no influence on the separation of components. An alternative approach that gives the same results is to consider the change in trapped electrons per incident photon instead of unit time (Jain and Lindvold, 2007). We wish to point out that owing to the dependence of the photo-ionization cross sections on the wavelength of stimulation, the separation can be improved by stimulating at a longer wavelength (Singarayer and Bailey, 2003).
Table 1 OSL signals under different stimulation modes derived from Eq. (1) Stimulation intensity (s−1 )
Decay rate parameter (s−1 )
OSL signal intensity (counts s−1 )
CW (t) = CW LM (t) = P t
= CW
ICW (t) = n0 exp(− t)
= P
ILM (t) = n0
P
HM (t) =
t c 1 + t P
= cP
t exp −1/2 t 2 P P IHM (t) = n0 t exp − 1 ln(1 + t) − t
P is the stimulation time (s), P the stimulation intensity at time P (s−1 ) and c a dimensionless constant given by [(1 + P )/P ].
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3. Experimental findings 3.1. Equipment and samples We selected five samples to compare the OSL responses to different stimulation modes experimentally. Selected samples are BR-4 (brick from Denmark), ECS0 (fluvial sediment from Germany), NCL-2205126 (colluvial sediment from South Africa), NCL-3505025 (fluvial sediment from The Netherlands) and NCL-2105095 (colluvial sediment from Israel). For each sample, a single stainless-steel disc was prepared containing approximately 10 mg of purified quartz in the grain size of 150 or 180–212 m. A RisZ TL-DA-15 TL/OSL reader was used for the experiments (BZtter-Jensen et al., 2003). Optical stimulation sources are three clusters of seven blue diodes filtered with a long pass GG-420 filter (470 ± 30 nm, maximum stimulation power (at 90%) ∼ 30 mW cm−2 at the sample position) and IR diodes (emission 875 nm, maximum stimulation power ∼135 mW cm−2 at the sample position). Photon detection was achieved by an EMI9235QA photomultiplier tube. For OSL measurements a single 7.5 mm Hoya U-340 filter was used. For monitoring the stimulation intensity, we used an aluminium diaphragm combined with three Schott neutral density filters (2 mm NG-4, 1 mm NG-9 and 2 mm NG-10).
3.2. Checking the stimulation intensity
Fig. 1. (A) CW, HM and LM stimulation intensities as a function of time, both theoretical (lines) and measured (symbols). The right-hand axis shows the ratio of measured and theoretical stimulation intensities. For the sake of clarity the LM data are plotted on the same timescale as the CW and HM data. Actual LM readout times are twice the values indicated on the x-axis (also for B). (B) OSL curves for a hypothetical sample containing fast, medium and slow(3) components with equal initial trapped charge concentrations and photo-ionization cross sections of the fast, medium and slow(3) components in the ratio 1:0.24:0.0013. (C) Calculated OSL signals per unit of stimulation energy plotted as a function of the cumulative stimulation energy, which is proportional to the time integrated stimulation intensity. Because the unit of both axes is stimulation energy rather than time, this plotting method allows direct comparison of CW, HM and LM measurements. For CW-OSL the representation is identical to the usual plot of OSL signal intensity versus readout time as the cumulative stimulation energy is directly proportional to the time, and the stimulation intensity is constant. The values given on both axes are based on a stimulation intensity of 30 mW cm−2 at the sample position at 90% power of the blue LEDs.
We conducted experiments for CW, LM and HM stimulation and tested the stimulation intensity by monitoring the reflection from an empty sample disc with the PM tube shielded by a diaphragm and neutral density filters. The RisZ control software was modified to allow HM stimulation. For the CW experiment we stimulated for 3600 s at 90% stimulation power. For the LM experiment we stimulated for 7200 s while ramping the stimulation power from 0% to 90%. For the HM experiment we stimulated for 3600 s, ramping the stimulation power from 0% to 90% with set to 1 s−1 . These stimulation times were chosen to obtain similar cumulative stimulation energies at the end of the experiment. We repeated the initial parts of the measurements with a higher sampling rate to improve the resolution for these intervals. All experiments used 3600 data channels. For analysis we combined the data obtained with different sampling rates; for CW and HM stimulation the first 10 s was taken from the data set with 100 data points s−1 , the interval from 10 to 100 s from the data set with 10 data points s−1 and the remainder from the data set with 1 data point s−1 . For the LM experiment the first 200 s was taken from the data set with 5 data points s−1 and the remainder from the data set with 0.5 data point s−1 . Instrumental background (50 counts s−1 ) was subtracted for all experiments. We found that for LM stimulation the diodes did not respond to power settings below ∼ 0.14%, producing a time off-set. To eliminate this off-set we ignored the first 11.5 s of the LM-OSL measurements. For the HM-OSL measurements the first 0.0016 s was ignored for the same reason.
J. Wallinga et al. / Radiation Measurements 43 (2008) 742 – 747
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In Fig. 1A results are shown together with the theoretical stimulation curves. The LM and HM data could be very well fitted with the expected equations (Table 1) giving an r 2 of 0.9998 and 0.994 for LM and HM, respectively. However, detailed analysis showed that for stimulation intensities below the maximum the observed stimulation intensity is lower than the desired intensity (Fig. 1A). Although the absolute difference is small (< 1.7% of the maximum intensity), it results in a stimulation intensity that is about 15% below the level that it should be for the first 100 s of the LM measurement (Fig. 1A). This unwanted behaviour is observed for the first ∼ 10 s of the HM measurement and for the complete LM measurement. These findings seem to contradict the linearity of LM stimulation on a RisZ reader reported by Choi et al. (2006a) (Fig. 1B). However, careful re-analysis of their data reveals a similar, though less distinct, pattern. In their case the absolute difference is < 0.8% of the maximum stimulation intensity, which results in a stimulation intensity that is about 8% below the level that it should be for the initial part of the LM measurement. We suggest that the stimulation intensity should be investigated in detail when using ramped stimulation as the observed behaviour will affect the measurements and may lead to erroneous conclusions with regard to photo-ionization cross sections. 3.3. OSL measurements To prevent sensitivity changes during our experiments, we repeatedly dosed and measured the samples prior to the experiment. A regenerative dose (∼ 25 Gy) procedure was used with a preheat at 250 ◦ C for 10 s. After preheating, the aliquots were exposed to IR light (500 s at 30 ◦ C) to minimize contributions to the OSL signal from any contaminating feldspar grains or inclusions without affecting the quartz OSL signal (Wallinga et al., 2002). IR signals ranged from 750 to 2400 counts in the first second of stimulation. For samples BR-4, NCL-2205126 and NCL-2105095, the IR signals were small (< 0.4%) compared to the post-IR blue stimulated OSL signals, indicating that the feldspar contribution to the post-IR blue signal can be safely neglected. For samples ECS0 and NCL-3505025 the relative IR signals were 6.3% and 2.3%, respectively. These greater values are mainly caused by the dimness of the post-IR blue OSL signal and indicate that feldspars may contribute to the post-IR blue OSL signal. OSL measurements were made at 125 ◦ C to prevent retrapping in the 110 ◦ C TL trap (Murray and Wintle, 1998). After each measurement the OSL signal was reset through a 500 s exposure to the blue LEDs (90% power) while holding the sample at 275 ◦ C. CW, LM and HM measurements were made using the experimental parameters outlined in the previous section. Following Choi et al. (2006a), instrumental backgrounds were subtracted based on measurements of empty sample discs. 3.4. OSL as a function of stimulation energy The measurements of the stimulation intensity and the OSL signal allow us to plot the OSL signal as a function of
Fig. 2. CW-, LM- and HM-OSL signals for five quartz samples as a function of cumulative stimulation energy. Set-ups of the plots are explained in the caption of Fig. 1C. Samples (A) BR-4; (B) ECS0; (C) NCL-2205126; (D) NCL-3505025; (E) NCL-2105095.
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cumulative stimulation energy for the different stimulation modes (Fig. 2). This analysis circumvents the differences seen between the theoretical and measured patterns of increase in stimulation intensity as discussed in Section 3.2. For samples NCL-2205126 and NCL-2105095, the results obtained using different stimulation modes are nearly identical, as we expected based on the theory. For the other samples the CW and HM agree but the LM data are different, especially in the region from 60 to 6000 mJ cm−2 cumulative stimulation energy (∼2–200 s of CW stimulation). These observations cannot be explained through a shift in stimulation wavelength due to heating of the diodes (Huntley, 2006; Jain and Lindvold, 2007), as this would result in an off-set for all samples in the same direction. For samples ECS0 and NCL-3505025, the behaviour may be related to the presence of feldspar contamination. Alternatively, the medium and slow components that are dominant in this region may not obey first-order kinetics. Additional research will be needed to draw firm conclusions. To quantify our findings we attempted fitting components to the resulting OSL data shown in Fig. 2 using Origin 7.5 software. However, we found that the fitting results depend strongly on the choice of initial parameters, both the number of components as well as their photo-ionization cross section. This is likely to be a consequence of the ill-posed nature of the fitting problem (Bulur et al., 2002), which means that the solution ‘may not be unique, may not exist and may not depend continuously on the data’ (Istratov and Vyvenko, 1999). Because fitting of the data of Fig. 2 proved unreliable, we were not able to provide a quantitative comparison of the parameters of OSL components underlying the CW-, HM- and LM-OSL data.
4. Implications and conclusions; selecting an appropriate stimulation mode For quartz OSL dating one would like to use a signal that: (a) (b) (c) (d)
is dominated by the fast OSL component, can be collected in a short measurement time, is little affected by instrumental noise, has a constant background.
For samples where the OSL components show a first-order kinetic behaviour, the overlap of the OSL components at a given wavelength of stimulation is identical regardless of the stimulation mode chosen. Hence, the stimulation mode does not affect the separation of components and the mode of choice can be selected based on the considerations (b)–(d). We propose that CW stimulation is the method of choice as it allows the fastest readout of the signal and thus the best signal-to-noise ratio. In addition, the CW-OSL signal has a constant background that allows straightforward processing of the data. For visualization a peak shaped representation may be desired; this may be achieved by ramped stimulation or by mathematical transformation of the CW-OSL signal (Bulur, 2000; Poolton et al., 2003). We note that for some of the investigated samples we
observed differences between OSL signals measured using different stimulation modes. This may be an artefact of a contribution to the OSL signals from feldspar minerals or it may suggest that assuming first-order kinetics is not justified for all quartz OSL components or for all samples. The disadvantage of CW-OSL is that a very high resolution is required to observe details in the initial part of the shine-down curve; this is illustrated by the ultrafast component in sample NCL-3505025, which is visible from the LM and HM data but indistinguishable from the CW data due to a lack of resolution (Fig. 2D). If measuring at very high resolution is problematic, HM stimulation provides a good alternative, combining fast readout of the signal with high resolution for the (ultra)fast component(s). Acknowledgements Samples for this study were kindly provided by Mayank Jain (RisZ National Laboratory, Denmark, sample BR-4), Nicole Klasen (Cologne University, Germany, sample ECS0), Arnaud Temme and Eke Buis (Wageningen University, The Netherlands, samples NCL-2205126 and 2105095, respectively) and Denise Maljers (TNO, The Netherlands, sample NCL-3505025). The paper was improved following comments by an anonymous reviewer. J.W. acknowledges financial support from the Technology Foundation (STW Grant DSF.7553).
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. BZtter-Jensen, L., Duller, G.A., Murray, A.S., Banerjee, D., 1999. Blue light emitting diodes for optical stimulation of quartz in retrospective dosimetry and dating. Radiat. Prot. Dosimetry 84, 225–340. BZtter-Jensen, L., Andersen, C.E., Duller, G.A.T., Murray, A.S., 2003. Developments in radiation, stimulation and observation facilities in luminescence measurements. Radiat. Meas. 37, 535–541. Bulur, E., 1996. An alternative technique for optically stimulated luminescence (OSL) experiment. Radiat. Meas. 26, 701–709. Bulur, E., 2000. A simple transformation for converting CW-OSL curves to LM-OSL curves. Radiat. Meas. 32, 141–145. Bulur, E., BZtter-Jensen, L., Murray, A.S., 2001. LM-OSL signals from some insulators: an analysis of the dependency of the detrapping probability on stimulation light intensity. Radiat. Meas. 33, 715–719. Bulur, E., Duller, G.A.T., Solongo, S., BZtter-Jensen, L., Murray, A.S., 2002. LM-OSL from single grains of quartz: a preliminary study. Radiat. Meas. 35, 79–85. Choi, J.H., Duller, G.A.T., Wintle, A.G., 2006a. Analysis of quartz LM-OSL curves. Ancient TL 24, 9–20. Choi, J.H., Duller, G.A.T., Wintle, A.G., Cheong, C.S., 2006b. Luminescence characteristics of quartz from the Southern Kenyan Rift Valley: dose estimation using LM-OSL SAR. Radiat. Meas. 41, 847–854. Huntley, D.J., 2006. Thoughts arising from “Choi, Duller and Wintle: analysis of quartz LM-OSL curves. Ancient TL 24, 9–20 (2006)”. Ancient TL 24, 69–70. Istratov, A.A., Vyvenko, O.F., 1999. Exponential analysis in physical phenomena. Rev. Sci. Instrum. 70, 1233–1257. Jain, M., Lindvold, L.R., 2007. Blue light stimulation and linearly modulated optically stimulated luminescence. Ancient TL 25, 69–80.
J. Wallinga et al. / Radiation Measurements 43 (2008) 742 – 747 Jain, M., Murray, A.S., BZtter-Jensen, L., 2003. Characterisation of bluelight stimulated luminescence components in different quartz samples: implications for dose measurement. Radiat. Meas. 37, 441–449. Jain, M., BZtter-Jensen, L., Murray, A.S., Essery, R., 2007. A peak structure in isothermal luminescence signals in quartz: origin and implications. J Lumin. 127, 678–688. Kuhns, C.K., AgersnapLarsen, N., McKeever, S.W.S., 2000. Characteristics of LM-OSL from several different types of quartz. Radiat. Meas. 32, 413–418. Li, S.H., Li, B., 2006. Dose measurement using the fast component of LMOSL signals from quartz. Radiat. Meas. 41, 534–541. Murray, A.S., Wintle, A.G., 1998. Factors controlling the shape of the OSL decay curve in quartz. Radiat. Meas. 29, 65–79. Poolton, N.R.J., BZtter-Jensen, L., Andersen, C.E., Jain, M., Murray, A.S., Malins, A.E.R., Quinn, F.M., 2003. Measuring modulated luminescence
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using non-modulated stimulation: ramping the sample period. Radiat. Meas. 37, 639–645. Singarayer, J.S., Bailey, R.M., 2003. Further investigations of the quartz optically stimulated luminescence components using linear modulation. Radiat. Meas. 37, 451–458. Smith, B.W., Rhodes, E.J., 1994. Charge movements in quartz and their relevance to optical dating. Radiat. Meas. 23, 329–333. Wallinga, J., Murray, A.S., BZtter-Jensen, L., 2002. Measurement of the dose in quartz in the presence of feldspar contamination. Radiat. Prot. Dosimetry 101, 367–370. Wintle, A.G., Murray, A.S., 2006. A review of quartz optically stimulated luminescence characteristics and their relevance in single-aliquot regeneration dating protocols. Radiat. Meas. 41, 369–391.
On the separation of quartz OSL signal components using different stimulation modes Jakob Wallinga a,∗ , Adrie J.J. Bos b , Geoff A.T. Duller c a Netherlands Centre for Luminescence Dating, Delft University of Technology, Mekelweg 15, NL-2629 JB Delft, The Netherlands b Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, NL-2629 JB Delft, The Netherlands c Institute of Geography and Earth Sciences, Aberystwyth University, SY23 3DB Aberystwyth, Wales, UK
Abstract We investigate both theoretically and experimentally the effect of stimulation mode on the separation of quartz optically stimulated luminescence (OSL) components. We find that, when assuming first-order kinetics with the detrapping probability proportional to stimulation intensity, the OSL signal is a function of the cumulative stimulation energy and not affected by the stimulation mode. This is confirmed by close correspondence between continuous wave (CW), linearly modulated (LM) and hyperbolically modulated (HM) OSL data for some of the samples studied. For other samples the data obtained using LM stimulation differ from that obtained using the other stimulation modes. This may be due to a contribution to the OSL signal from feldspars, or it may indicate that the behaviour of these samples is not adequately described by first-order kinetics. We suggest that CW stimulation is the method of choice for dating purposes as it allows the fastest readout with the greatest signal-to-noise ratio, and because it has a constant background. HM stimulation provides a good alternative when higher resolution is needed for the initial part of the shine-down curve. © 2008 Elsevier Ltd. All rights reserved. Keywords: Quartz; OSL components; Stimulation mode; First-order kinetics
1. Introduction The quartz optically stimulated luminescence (OSL) signal is composed of a number of components with different photoionization cross sections. Based on the relative rapidity of resetting upon light exposure, these are referred to as fast, medium and slow components (e.g. Bailey et al., 1997). Including the ultrafast component, up to seven components have now been identified (Jain et al., 2003). At constant stimulation intensity (continuous wave, CW), the OSL signal is a sum of exponential decays (Smith and Rhodes, 1994). It is also possible to ramp the stimulation intensity during readout. Using linearly modulated (LM) stimulation, an OSL signal is obtained where the components are represented by peaks (Bulur, 1996; BZtter-Jensen et al., 1999). For dating using the OSL signal from quartz, the fast component is most suitable as it is: (1) rapidly reset by light exposure ∗ Corresponding author.
E-mail address: [email protected] (J. Wallinga). 1350-4487/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2008.01.013
prior to deposition and burial; (2) it is stable on geological timescales (e.g. Singarayer and Bailey, 2003) and (3) the single aliquot regenerative (SAR) dose procedure was developed to use this component (Wintle and Murray, 2006). For dating of samples where the fast OSL component is at or near saturation, one of the slow components may potentially be used for dating (Singarayer and Bailey, 2003). A number of recent studies have reported the use of LM stimulation to improve component separation for dating purposes (e.g. Singarayer and Bailey, 2003; Li and Li, 2006; Choi et al., 2006b). Some of these studies suggest that the use of LM stimulation allows better separation of components. In a recent letter to Ancient TL, Huntley (2006) points out that the separation of OSL components is independent of the stimulation mode used. In this study we investigate theoretically and experimentally whether the overlap of OSL signals from the components is independent of the distribution of the stimulation energy in time. In addition to the widely used CW and LM stimulation modes, we introduce hyperbolically modulated (HM) stimulation. We discuss the advantages and disadvantages of different
J. Wallinga et al. / Radiation Measurements 43 (2008) 742 – 747
stimulation modes and conclude that CW stimulation is the method of choice, provided that the resolution of the data acquisition is adequate. 2. Considerations on the separation of quartz OSL signal components In this section we discuss whether it is theoretically possible to improve the separation of quartz OSL signal components using different stimulation modes. To do so, we assume a firstorder kinetic system where the detrapping probability is proportional to stimulation intensity. This assumption is based on evidence in the literature, which we briefly summarize here. The quartz photo-ionization cross sections with blue stimulation have been shown to be independent of the stimulation power up to 50 mW cm−2 (Bulur et al., 2001) and quartz LMOSL curves and linearly transformed CW-OSL are in close correspondence (Jain et al., 2003; Poolton et al., 2003). Moreover, Singarayer and Bailey (2003) demonstrated that the componentspecific dose response curves could be approximated well by single saturating exponentials. We note that Kuhns et al. (2000) provided experimental evidence that the quartz OSL signal may not follow first-order kinetics, but it has been suggested that their data may have been affected by an isothermal TL signal (Jain et al., 2007). For a system obeying the assumptions outlined above, the OSL signal from a single component is described by IOSL (t) = −
dn = n(t)()(t), dt
(1)
where IOSL (t) is the OSL intensity at time t (s), n(t) is the charge carrier (electron or hole) trapped charge population (m−3 ) at time t, () is the photo-ionization cross section (m2 ) for a given wavelength of stimulation () and (t) is the stimulation intensity (directly proportional to the number of stimulation photons, m−2 s−1 ) at time t. In Table 1 we provide the functions for the stimulation mode and the mathematical form that would be expected for the OSL signal from a single trap for CW, LM and HM stimulation. The information is also provided graphically in Fig. 1A. HM stimulation is here introduced; the stimulation intensity increases rapidly at a rate (s−1 ) at the start of the measurement after which it stabilizes at a maximum P after a stimulation period P. In Fig. 1B we show calculated OSL signals for a hypothetical sample with fast, medium and slow(3) components
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with equal trapped charge populations at t = 0. Based on Jain et al. (2003), we chose the photo-ionization cross sections of the medium and slow(3) components to be 0.24 and 0.0013 relative to that of the fast component. Through integration of Eq. (1) we can calculate the charge carrier concentration at time t: n t dn (t ) dt , (2) = − n0 n 0
t n(t) = n0 exp − (3) (t ) dt . 0
The OSL intensity ratio of two components (a and b) with photo-ionization cross a and b and initial trapped charge concentrations n0,a and n0,b follows from Eq. (1): IOSL,a (t) na (t) a . = IOSL,b (t) nb (t) b
(4)
Substitution of Eq. (3) gives
t IOSL,a (t) n0,a a exp (b − a ) (t ) dt . = IOSL,b (t) n0,b b 0
(5)
From Eq. (5) we see that the OSL signal ratio of two components depends on: (i) the ratio of the initial trapped charge concentrations, (ii) the relative photo-ionization cross sections (at ), (iii) the time integrated stimulation intensity, which is proportional to the cumulative stimulation energy. The OSL signal ratio is thus independent of the distribution of stimulation photons in time. This is demonstrated in Fig. 1C where we show that the calculated CW-, LM- and HM-OSL signals given in Fig. 1B are identical when plotted as a function of cumulative stimulation energy. We conclude that under the assumptions outlined above, the mode of stimulation has no influence on the separation of components. An alternative approach that gives the same results is to consider the change in trapped electrons per incident photon instead of unit time (Jain and Lindvold, 2007). We wish to point out that owing to the dependence of the photo-ionization cross sections on the wavelength of stimulation, the separation can be improved by stimulating at a longer wavelength (Singarayer and Bailey, 2003).
Table 1 OSL signals under different stimulation modes derived from Eq. (1) Stimulation intensity (s−1 )
Decay rate parameter (s−1 )
OSL signal intensity (counts s−1 )
CW (t) = CW LM (t) = P t
= CW
ICW (t) = n0 exp(− t)
= P
ILM (t) = n0
P
HM (t) =
t c 1 + t P
= cP
t exp −1/2 t 2 P P IHM (t) = n0 t exp − 1 ln(1 + t) − t
P is the stimulation time (s), P the stimulation intensity at time P (s−1 ) and c a dimensionless constant given by [(1 + P )/P ].
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3. Experimental findings 3.1. Equipment and samples We selected five samples to compare the OSL responses to different stimulation modes experimentally. Selected samples are BR-4 (brick from Denmark), ECS0 (fluvial sediment from Germany), NCL-2205126 (colluvial sediment from South Africa), NCL-3505025 (fluvial sediment from The Netherlands) and NCL-2105095 (colluvial sediment from Israel). For each sample, a single stainless-steel disc was prepared containing approximately 10 mg of purified quartz in the grain size of 150 or 180–212 m. A RisZ TL-DA-15 TL/OSL reader was used for the experiments (BZtter-Jensen et al., 2003). Optical stimulation sources are three clusters of seven blue diodes filtered with a long pass GG-420 filter (470 ± 30 nm, maximum stimulation power (at 90%) ∼ 30 mW cm−2 at the sample position) and IR diodes (emission 875 nm, maximum stimulation power ∼135 mW cm−2 at the sample position). Photon detection was achieved by an EMI9235QA photomultiplier tube. For OSL measurements a single 7.5 mm Hoya U-340 filter was used. For monitoring the stimulation intensity, we used an aluminium diaphragm combined with three Schott neutral density filters (2 mm NG-4, 1 mm NG-9 and 2 mm NG-10).
3.2. Checking the stimulation intensity
Fig. 1. (A) CW, HM and LM stimulation intensities as a function of time, both theoretical (lines) and measured (symbols). The right-hand axis shows the ratio of measured and theoretical stimulation intensities. For the sake of clarity the LM data are plotted on the same timescale as the CW and HM data. Actual LM readout times are twice the values indicated on the x-axis (also for B). (B) OSL curves for a hypothetical sample containing fast, medium and slow(3) components with equal initial trapped charge concentrations and photo-ionization cross sections of the fast, medium and slow(3) components in the ratio 1:0.24:0.0013. (C) Calculated OSL signals per unit of stimulation energy plotted as a function of the cumulative stimulation energy, which is proportional to the time integrated stimulation intensity. Because the unit of both axes is stimulation energy rather than time, this plotting method allows direct comparison of CW, HM and LM measurements. For CW-OSL the representation is identical to the usual plot of OSL signal intensity versus readout time as the cumulative stimulation energy is directly proportional to the time, and the stimulation intensity is constant. The values given on both axes are based on a stimulation intensity of 30 mW cm−2 at the sample position at 90% power of the blue LEDs.
We conducted experiments for CW, LM and HM stimulation and tested the stimulation intensity by monitoring the reflection from an empty sample disc with the PM tube shielded by a diaphragm and neutral density filters. The RisZ control software was modified to allow HM stimulation. For the CW experiment we stimulated for 3600 s at 90% stimulation power. For the LM experiment we stimulated for 7200 s while ramping the stimulation power from 0% to 90%. For the HM experiment we stimulated for 3600 s, ramping the stimulation power from 0% to 90% with set to 1 s−1 . These stimulation times were chosen to obtain similar cumulative stimulation energies at the end of the experiment. We repeated the initial parts of the measurements with a higher sampling rate to improve the resolution for these intervals. All experiments used 3600 data channels. For analysis we combined the data obtained with different sampling rates; for CW and HM stimulation the first 10 s was taken from the data set with 100 data points s−1 , the interval from 10 to 100 s from the data set with 10 data points s−1 and the remainder from the data set with 1 data point s−1 . For the LM experiment the first 200 s was taken from the data set with 5 data points s−1 and the remainder from the data set with 0.5 data point s−1 . Instrumental background (50 counts s−1 ) was subtracted for all experiments. We found that for LM stimulation the diodes did not respond to power settings below ∼ 0.14%, producing a time off-set. To eliminate this off-set we ignored the first 11.5 s of the LM-OSL measurements. For the HM-OSL measurements the first 0.0016 s was ignored for the same reason.
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In Fig. 1A results are shown together with the theoretical stimulation curves. The LM and HM data could be very well fitted with the expected equations (Table 1) giving an r 2 of 0.9998 and 0.994 for LM and HM, respectively. However, detailed analysis showed that for stimulation intensities below the maximum the observed stimulation intensity is lower than the desired intensity (Fig. 1A). Although the absolute difference is small (< 1.7% of the maximum intensity), it results in a stimulation intensity that is about 15% below the level that it should be for the first 100 s of the LM measurement (Fig. 1A). This unwanted behaviour is observed for the first ∼ 10 s of the HM measurement and for the complete LM measurement. These findings seem to contradict the linearity of LM stimulation on a RisZ reader reported by Choi et al. (2006a) (Fig. 1B). However, careful re-analysis of their data reveals a similar, though less distinct, pattern. In their case the absolute difference is < 0.8% of the maximum stimulation intensity, which results in a stimulation intensity that is about 8% below the level that it should be for the initial part of the LM measurement. We suggest that the stimulation intensity should be investigated in detail when using ramped stimulation as the observed behaviour will affect the measurements and may lead to erroneous conclusions with regard to photo-ionization cross sections. 3.3. OSL measurements To prevent sensitivity changes during our experiments, we repeatedly dosed and measured the samples prior to the experiment. A regenerative dose (∼ 25 Gy) procedure was used with a preheat at 250 ◦ C for 10 s. After preheating, the aliquots were exposed to IR light (500 s at 30 ◦ C) to minimize contributions to the OSL signal from any contaminating feldspar grains or inclusions without affecting the quartz OSL signal (Wallinga et al., 2002). IR signals ranged from 750 to 2400 counts in the first second of stimulation. For samples BR-4, NCL-2205126 and NCL-2105095, the IR signals were small (< 0.4%) compared to the post-IR blue stimulated OSL signals, indicating that the feldspar contribution to the post-IR blue signal can be safely neglected. For samples ECS0 and NCL-3505025 the relative IR signals were 6.3% and 2.3%, respectively. These greater values are mainly caused by the dimness of the post-IR blue OSL signal and indicate that feldspars may contribute to the post-IR blue OSL signal. OSL measurements were made at 125 ◦ C to prevent retrapping in the 110 ◦ C TL trap (Murray and Wintle, 1998). After each measurement the OSL signal was reset through a 500 s exposure to the blue LEDs (90% power) while holding the sample at 275 ◦ C. CW, LM and HM measurements were made using the experimental parameters outlined in the previous section. Following Choi et al. (2006a), instrumental backgrounds were subtracted based on measurements of empty sample discs. 3.4. OSL as a function of stimulation energy The measurements of the stimulation intensity and the OSL signal allow us to plot the OSL signal as a function of
Fig. 2. CW-, LM- and HM-OSL signals for five quartz samples as a function of cumulative stimulation energy. Set-ups of the plots are explained in the caption of Fig. 1C. Samples (A) BR-4; (B) ECS0; (C) NCL-2205126; (D) NCL-3505025; (E) NCL-2105095.
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cumulative stimulation energy for the different stimulation modes (Fig. 2). This analysis circumvents the differences seen between the theoretical and measured patterns of increase in stimulation intensity as discussed in Section 3.2. For samples NCL-2205126 and NCL-2105095, the results obtained using different stimulation modes are nearly identical, as we expected based on the theory. For the other samples the CW and HM agree but the LM data are different, especially in the region from 60 to 6000 mJ cm−2 cumulative stimulation energy (∼2–200 s of CW stimulation). These observations cannot be explained through a shift in stimulation wavelength due to heating of the diodes (Huntley, 2006; Jain and Lindvold, 2007), as this would result in an off-set for all samples in the same direction. For samples ECS0 and NCL-3505025, the behaviour may be related to the presence of feldspar contamination. Alternatively, the medium and slow components that are dominant in this region may not obey first-order kinetics. Additional research will be needed to draw firm conclusions. To quantify our findings we attempted fitting components to the resulting OSL data shown in Fig. 2 using Origin 7.5 software. However, we found that the fitting results depend strongly on the choice of initial parameters, both the number of components as well as their photo-ionization cross section. This is likely to be a consequence of the ill-posed nature of the fitting problem (Bulur et al., 2002), which means that the solution ‘may not be unique, may not exist and may not depend continuously on the data’ (Istratov and Vyvenko, 1999). Because fitting of the data of Fig. 2 proved unreliable, we were not able to provide a quantitative comparison of the parameters of OSL components underlying the CW-, HM- and LM-OSL data.
4. Implications and conclusions; selecting an appropriate stimulation mode For quartz OSL dating one would like to use a signal that: (a) (b) (c) (d)
is dominated by the fast OSL component, can be collected in a short measurement time, is little affected by instrumental noise, has a constant background.
For samples where the OSL components show a first-order kinetic behaviour, the overlap of the OSL components at a given wavelength of stimulation is identical regardless of the stimulation mode chosen. Hence, the stimulation mode does not affect the separation of components and the mode of choice can be selected based on the considerations (b)–(d). We propose that CW stimulation is the method of choice as it allows the fastest readout of the signal and thus the best signal-to-noise ratio. In addition, the CW-OSL signal has a constant background that allows straightforward processing of the data. For visualization a peak shaped representation may be desired; this may be achieved by ramped stimulation or by mathematical transformation of the CW-OSL signal (Bulur, 2000; Poolton et al., 2003). We note that for some of the investigated samples we
observed differences between OSL signals measured using different stimulation modes. This may be an artefact of a contribution to the OSL signals from feldspar minerals or it may suggest that assuming first-order kinetics is not justified for all quartz OSL components or for all samples. The disadvantage of CW-OSL is that a very high resolution is required to observe details in the initial part of the shine-down curve; this is illustrated by the ultrafast component in sample NCL-3505025, which is visible from the LM and HM data but indistinguishable from the CW data due to a lack of resolution (Fig. 2D). If measuring at very high resolution is problematic, HM stimulation provides a good alternative, combining fast readout of the signal with high resolution for the (ultra)fast component(s). Acknowledgements Samples for this study were kindly provided by Mayank Jain (RisZ National Laboratory, Denmark, sample BR-4), Nicole Klasen (Cologne University, Germany, sample ECS0), Arnaud Temme and Eke Buis (Wageningen University, The Netherlands, samples NCL-2205126 and 2105095, respectively) and Denise Maljers (TNO, The Netherlands, sample NCL-3505025). The paper was improved following comments by an anonymous reviewer. J.W. acknowledges financial support from the Technology Foundation (STW Grant DSF.7553).
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