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Use of the LM-OSL technique for the detection of partial bleaching in quartz
Radiation Measurements 32 (2000) 419±425
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Use of the LM-OSL technique for the detection of partial bleaching in quartz N. Agersnap Larsen a, E. Bulur b, L. Bùtter-Jensen b, S.W.S. McKeever a,* a
Department of Physics, Oklahoma State University, Stillwater, OK 74078-3072, USA b Risù National Laboratory, DK-4000 Roskilde, Denmark Received 30 October 1999; accepted 1 March 2000
Abstract We present a study of the sensitivity to light (ease-of-bleaching) of the trapped charge in sedimentary quartz grains using an optically stimulated luminescence (OSL) technique in which the intensity of the stimulation light is linearly increased during the measurement period. The technique is known as linear modulation OSL (LM-OSL). In controlled laboratory conditions, this technique has been employed to study the ease-of-bleaching of the trapped charge in quartz by comparing the OSL curves of quartz aliquots which have been either: (1) fully bleached, followed by a laboratory dose of b-irradiation, or (2) partially bleached, followed by the laboratory b-dose. The ratio of the OSL signals due to the b-dose from the partly and fully bleached aliquots is illustrated to be a potential indicator of the degree of optical resetting of the OSL signal in dating material. The key parameter governing the ease-of-bleaching is the photoionization cross-section of the trap involved. The concept is also demonstrated in a model study from which very good agreement with the experimental observations has been found. Potential applications of the technique to dating are discussed. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction One of the necessary criteria for successful dating using optically stimulated luminescence (OSL) is a complete resetting to zero, or to a known level, of the latent OSL signal at the time of deposition through exposure of the sediment to sunlight. Incomplete resetting (partial bleaching) at deposition time leaves the material with an unknown residual signal leading to an apparent higher accumulated paleodose during the burial period and this in turn results in an overestimation of the age. This is particularly a danger for ¯uvial
* Corresponding author. Tel.: +1-405-744-5802; fax: +1405-744-6811. E-mail address: [email protected] (S.W.S. McKeever).
and colluvial sediments since these materials may have been transported under conditions of short transport distances and reduced light intensities owing to the sediment load in the water during transport. A multigrain aliquot of these sediments may therefore consist of grains that have received dierent light exposure energies, leaving grains with dierent residuals at deposition time. This consequently causes a grain-to-grain variation of the apparent paleodose, or age. These circumstances cause not only the OSL dating technique to overestimate the age, but also to be sensitive to the number of grains used in an aliquot, which in turn leads to poor reproducibility (Huntley and Berger, 1995). The demand for a tool to recognize and characterize incomplete resetting at the time of deposition is therefore high in OSL dating of sediments. Sev-
1350-4487/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 4 4 8 7 ( 0 0 ) 0 0 0 7 1 - 8
420
N. Agersnap Larsen et al. / Radiation Measurements 32 (2000) 419±425
eral methods have been suggested in the literature to extract information on incomplete resetting. Olley et al. (1998) suggested an approach utilizing the grain-to-grain variation of the paleodose in quartz sediments. They represented the paleodose variation in a histogram constructed from, typically, 100 small aliquots of 60±100 grains and concluded that the lowest value observed gave the optimum estimate of the true paleodose. Huntley et al. (1985), in their pioneering work on OSL dating, proposed a test for incomplete resetting in quartz by plotting paleodose against OSL illumination time. A constant illumination intensity was employed to stimulate the OSL signal (using a 514 nm Ar2+ laser) and the technique is hereafter termed continuous wave OSL (CW-OSL). In the case of partial bleaching, the paleodose is expected to increase with longer illumination time since the OSL signal at longer times originates from traps which are more dicult to empty optically. In contrast, in a sample which has been fully bleached, the paleodose is constant with stimulation time. (This procedure is sometimes called the ``shine-plateau'' test.) A more detailed study of CW-OSL curves from quartz stimulated at the same wavelength as above was reported by Smith and Rhodes (1994), wherein they concluded that the luminescence was associated with three traps characterized by having dierent decay constants Ð the so-called ``fast'', ``medium'' and ``slow'' components. Bailey et al. (1997) studied CW-OSL curves from quartz when stimulated with a broadband light source (420±550 nm) and reached the same conclusions as those of previous authors. The presence of dierential light sensitivity of the traps led the latter authors to propose a method of detecting partial bleaching in an aliquot by comparing certain parts of the natural CW-OSL decay curve to that from a matching regeneration dose delivered in the laboratory. Recently, Bulur (1996) introduced an OSL technique in which the intensity of the stimulation light is linearly increased during the measurement period. The technique is known as the linear modulation OSL (LM-OSL). The temporal order for which the peaks in an LM-OSL curve occur is inversely proportional to the photoionization cross-section of the involved traps. Consequently, the ®rst LM-OSL peak corresponds to the easy-to-bleach traps, i.e. having a high cross-section. Correspondingly, the signals for the harder-tobleach traps, having lower cross-sections, follow later. This distinct property of the LM-OSL technique has been applied in this work to study partial bleaching in quartz. Following Bulur (1996), the time dependence of LM-OSL from a single, ®rst-order, optically sensitive trap is expressed by:
1 I t I0 sgt exp ÿ sgt 2 2
1
where s is the photoionization cross-section of the trap. Thermal assistance from the trap has been speci®cally excluded. The ¯ux of the stimulation light F is linearly ramped in according to F t gt, where g is the ramp rate. I0 is the area under the curve and t is the time elapsed since the start of the OSL measurement. For further details of the expected shape of LMOSL curves and their relationship to conventional CW-OSL decay curves, see the accompanying papers in these proceedings by Kuhns et al. (2000) and Bulur et al. (2000). The purpose of this paper is to present an investigation of partial bleaching in quartz and to develop a method to characterize incomplete resetting of the preburial OSL signal using the LM-OSL technique. The work describes bleaching experiments of a previously heated quartz aliquot and its comparison with model simulations based on parameters extracted from the obtained data. The implications for detecting incomplete resetting using this technique are discussed.
2. Experimental details OSL measurements were performed using an array of blue light emitting diodes of type Nichia NSPB500S using a Risù automated TL/OSL apparatus (Bùtter-Jensen et al., 1999). The diodes had an emission wavelength at 470 D 20 nm and delivered an optical power output of about 20 mW cmÿ2 at the sample position. The luminescence was recorded by a 9168AQ PMT ®tted with two 3 mm Hoya U340 ®lters. Schott GG420 long-pass ®lters placed in front of each blue diode reduced the detection of the most energetic light from these diodes. The sample was exposed to a 90 Sr/90Y b-source delivering a dose rate of 25 mGy ÿ1 s . All the experiments were carried out on the same material, a glacio¯uvial quartz sediment from a marine core in Jutland, Denmark (Risù laboratory code Q914805) which was kindly provided by A.S. Murray at The Nordic Laboratory for Luminescence Dating, Risù National Laboratory, Denmark. The work was performed with 10 mg of approximately 125 mm grains, extracted from the sediments. Prior to the study, the aliquot was irradiated, illuminated and heated to 5508C repeatedly in the laboratory until the OSL sensitivity and the shape of the OSL curve were unchanged. This procedure inevitably results in some dierences between the behavior of these samples and those of as-found sedimentary materials, especially in terms of the sensitivity. Nevertheless, our concern in this work was to enable comparison between the dier-
N. Agersnap Larsen et al. / Radiation Measurements 32 (2000) 419±425
ent measurements conducted with the same aliquot in order to establish the principle of the method. Additional studies will be required to determine the utility of the procedures to natural sedimentary quartz. The blue light emitting diodes were used to both bleach and readout the OSL signal. 3. Results 3.1. Partial bleaching and ratio plots The eect of partial bleaching of the OSL signal in one aliquot of quartz was studied by exposing an irradiated aliquot to light using dierent illumination durations prior to the OSL measurements. The details of the measuring sequence are: Laboratory dose
partial bleaching
z}|{ z}|{ annealed 4 irradiation 4 preheat 4 bleaching 25 Gy at RT 5508C=10 s 2208C=10 s Dt at 1608C OSL curve
z}|{ 4 preheat 4 OSL readout 4000s at 1608C 2208C=10 s
421
The preheat and the elevated temperatures during the bleaching and OSL measurement were applied to eliminate accumulation of charge in the shallow traps in order to prevent thermal release of charge from these traps during the OSL readout. The illumination duration of the partial bleach Dt was varied from 0 to 10,000 s (at maximum light power), and the LM-OSL signal was stimulated by ramping the light power linearly from 0 to 100% of maximum light power in 4000 s at the elevated temperature of 1608C. Fig. 1(a) shows the LM-OSL curves measured after these various partial bleaches. The LM-OSL curves contain up to three separate peaks. The position (in time) of each peak is seen to be the same in each of the OSL curves. By assuming that the peaks are caused by the optically stimulated release of charge from three dierent traps obeying ®rst-order kinetics, the LMOSL curve for no bleaching Dt 0 s) was ®tted to the sum of three ®rst-order curves given by Eq. (1). The traps were found to be characterized by photoionization cross-sections of s1 9:0 10 ÿ17 cmÿ2 (easy-tobleach), s2 6:0 10 ÿ19 cmÿ2 (medium bleachability) and s3 4:2 10 ÿ20 cmÿ2 (hard-to-bleach). (These values should be compared with similar estimates by Kuhns et al. (2000) for LM-OSL emission from quartz from Oklahoma eolian and ¯uvial sediments using
Fig. 1. Partially bleached LM-OSL curves (a), and CW-OSL curves (c) recorded at an elevated temperature of 1608C. The ratio between the partially bleached and the non-bleached LM-OSL curves (b) and CW-OSL curves (d) for various bleaching durations.
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longer wavelength, green light stimulation.) Consistency between the photoionization cross-section and the degree of partial bleaching is clearly observed as the easy-to-bleach peak reduces faster than the harder-tobleach peaks. To describe the bleaching eect more qualitatively, the ratio between the partially bleached and the unbleached OSL curves has been plotted in Fig. 1(b). These ratio plots show distinct plateaux, of dierent durations, apparently corresponding to the presence of each of the three traps. In addition, the plot contains direct information of the degree of depletion of each of the dierent traps. Also plotted in Fig. 1 are CW-OSL curves obtained for the same sample after similar irradiation and partial bleaching sequences (Fig. 1(c)), and the corresponding ratio plots of the OSL from the partially bleached samples to that from the unbleached sample (Fig. 1(d)). The regions corresponding to the dierent
traps are much less distinct. No true plateaux are obtained but instead upwardly sloping curves are observed over narrow time regions. This arises because of the greater overlap of the easy-to-bleach and hardto-bleach signals in the CW-OSL curves compared to the LM-OSL curves. 3.2. The dating scenario Sediments examined in dating studies are expected to have undergone several irradiation and bleaching stages corresponding to pre-depositional (i.e. pre-burial) irradiation, solar resetting (partial or complete), and depositional irradiation (i.e. during burial) before the dating procedure is applied in the laboratory. The dating technique adopted in this work is the single aliquot regeneration (SAR) procedure described initially by Murray and Roberts (1998) and recently improved by Murray and Wintle (2000). The following measuring sequence has been conducted to imitate the dating scenario: pre-deposition dose z}|{ annealed 4 irradiation 4 preheat 25Gy at RT 5508C=10 s 2208C=10 s resetting event
deposition dose
z}|{ z}|{ 4 bleaching 4 preheat 4 irradiation 25 Gy at RT Dt at1608C 2208C=10 s natural OSL signal
z}|{ 4 4 preheat 4 OSL readout 4000 s at 1608C 2208C=10 s regeneration dose
z}|{ 4 4 bleaching 4 irradiation 25 Gy at RT 1000 s at 1608C dose-regenerated OSL signal z}|{ 4 4 preheat 4 OSL readout 4000 s at 1608C 2208C=10 s
Fig. 2. (a) Natural and dose-regenerated LM-OSL curves from a single aliquot of quartz which we have subjected to the imitated dating scenario, as described in the text. (b) Ratio between the natural and dose-regenerated LM-OSL curves for various bleaching durations.
The stages included consist of a ``pre-depositional'' irradiation, an optical resetting event using various illumination durations Dt from 0 s to 10000 s, a ``depositional'' irradiation followed by a single regenerative-dose step to imitate part of the SAR procedure. To simplify the study, the pre-depositional, depositional and the regeneration doses were selected to be identical (25 Gy). The preheat stages (2208C for 10 s) introduced after the ``pre-depositional'' dose and the ``resetting'' event are included in an attempt to account for the fact that in nature the irradiations are delivered at low, natural dose rates, and thus, traps of low ther-
N. Agersnap Larsen et al. / Radiation Measurements 32 (2000) 419±425
mal stability will not ®ll. The intent of the pre-heat phases, therefore, is to empty the shallow traps in order to mimic as closely as possible the natural situations. The simulated dating procedure described above generates two OSL signals: a signal corresponding to irradiations of the sample before and during burial (termed the ``natural'' signal) and a ``regenerated'' signal which follows from the administration of the laboratory regeneration dose. Fig. 2(a) shows LM-OSL curves following a variety of bleaching periods Dt during resetting of the pre-depositional signal, and an LM-OSL curve following the regeneration dose. (The shape of the latter curve did not change signi®cantly as a function of the number of dierent bleaching cycles.) The dierent peaks in the LM-OSL curves are clearly seen to bleach at dierent rates and to approach a level corresponding to complete resetting as the bleaching duration increases. In the case of complete resetting of the pre-depositional signal, the ratio between
Fig. 3. Schematic of the model used to simulate the OSL experiments. The model consists of three optically sensitive traps; an optically disconnected deep trap and a recombination center. The photoionization cross-sections used in the calculation are experimentally estimated from the quartz aliquot Q914805.The following parameters were used in the calculation: photoionization cross-sections s1 9 10 ÿ17 cmÿ2, s2 6 10 ÿ19 cmÿ2 and s3 4:2 10 ÿ20 cmÿ2; trap concentrations N1 14:4 1010 cmÿ3, N2 10 1010 cmÿ3, cmÿ3, Nd 1:0 1013 cmÿ3 and N3 95 1010 H 1:0 1013 cmÿ3; transition probabilities a1 a2 a3 ad ar a^ 1:0 10 ÿ7 cmÿ3 sÿ1. The ionizing radiation and light exposure were controlled through the generation rate of electron-hole pairs, R (01010 cmÿ3 sÿ1), and photon ¯ux, F (<2 1016 cmÿ2 sÿ1), respectively. nc and hv are the concentrations of free electrons and holes in the delocalized bands, respectively; ni is the concentrations of occupied electron traps, and h is the concentration of occupied hole traps.
423
the natural and dose-regenerated signals is expected to be 1 (since the same doses were chosen in this study for all irradiation phases), whereas incomplete resetting results in the ratio being greater than 1. Thus, plateaux of values >1 are expected due to variations in the ease-of-bleaching of the dierent traps. This is con®rmed in Fig. 2(b), where the most bleached case (1000 s) gives a ratio close to 1 for times less than 100 s, while the less-bleached cases yield plateaux of values >1. A second observation is that the easy-to-bleach component is well-bleached for bleaching periods even as short as 10 s, while the harder-to-bleach components
Fig. 4. Computer-simulated (a) LM-OSL curves and (b) ratio plots for various bleaching durations using the model sketched in Fig. 3. (c) Model prediction of the ratio between the natural and dose-regenerated LM-OSL curves using dierent bleaching durations. Notice the good agreement with the experimental results in Figs. 1 and 2.
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need more than 1000 s of bleaching time. The decrease in the ratio at the end of the curve is probably due to a residual contribution to the dose-regenerated LMOSL signal from the natural signal in the harder-tobleach traps.
4. Model The properties of bleaching of this particular quartz aliquot have been simulated using a model consisting of three optically sensitive traps having photoionization cross-sections equal to those values estimated from the experimental results. The model includes a luminescence center and a thermally and optically disconnected trap (to ensure properties such as linear dose response). To simplify the simulation, thermal assistance and shallow traps have been excluded. Fig. 3 contains a sketch of the model, and the de®nitions of the terms and the parameter values used are given in the ®gure caption. The set of equations describing the trac of charge between the traps and center are: dnc dh dhv X dni R ÿ dt dt dt dt i dni ÿfsi ni ai nc Ni ÿ ni dt
statement can be applied to a real dating scenario will depend on several factors, including the natural dose, the dose response, the dose response for dierent OSL signals and the signal level and counting statistics. However, the key is that the LM-OSL method is able to separate the various components through dierences in the photoionization cross-section of the traps. This separation is more easily achieved with LM-OSL than with conventional CW-OSL, no matter what the sample is. By using the ratio of the natural and the dose-regenerated LM-OSL signals, the partial bleaching/incomplete resetting of the various traps can be easily recognized. As a consequence, the signal from the easy-to-bleach traps can be quickly separated from that from the harder-to-bleach traps. In conventional CW-OSL curves there is often a considerable overlap among the various components and their separation is less easily achieved. As demonstrated by Kuhns et al. (2000), separating the various OSL components by curve ®tting is easier for the LM-OSL signal than for the CW-OSL signal. To demonstrate the potential further, however, additional studies performed on natural sedimentary quartz are necessary. In this way, the utility of the method to characterize the optical resetting event in luminescence dating will be further evaluated.
i 2 f1, 2, 3g
dnd ad nc Nd ÿ nd dt dh ^ v H ÿ h ÿcnc h ah dt dhv ^ v H ÿ h R ÿ ah dt The results of the simulation of the partial bleaching and the dating scenario are shown in Fig. 4. A direct comparison of the simulation results and experimental data (Figs. 1 and 2) immediately shows an excellent qualitative agreement. In particular, the dierent traps are illustrated by the appearance of three plateaux in the ratio plots.
5. Conclusion Through experiments under controlled laboratory conditions, and the use of computer simulations, we have demonstrated that it is possible to recognize incomplete bleaching of dierent OSL components by using the LM-OSL technique. The extent to which this
References Bailey, R.M., Smith, B.W., Rhodes, E.J., 1997. Partial bleaching and the decay from characteristics of quartz OSL. Radiat. Meas. 27, 123±136. Bùtter-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., 1996. An alternative technique for optically stimulated luminescence (OSL) experiments. Radiat. Meas. 26, 701±709. Bulur, E., Bùtter-Jensen, L., Murray, A.S., 2000. Optically stimulated luminescence from quartz measured using the linear modulation technique. Radiat. Meas. 32, 407±411. Huntley, D.J., Berger, G.W., 1995. Scatter in luminescence data for optical dating some models. Ancient TL 13, 5±9. Huntley, D.J., Godfrey-Smith, D.I., Thewalt, M.L.W., 1985. Optical dating of sediments. Nature 313, 105±107. Kuhns, C.K., Agersnap Larsen, N., McKeever, S.W.S., 2000. Characteristics of LM-OSL from several dierent types of quartz. Radiat. Meas. 32, 413±418. Murray, A.S., Roberts, R.G., 1998. Measurement of the equivalent dose in quartz using a regeneration-dose singlealiquot protocol. Radiat. Meas. 29, 503±515. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiat. Meas, 32, 57±73. Olley, J.M., Caitcheon, G., Murray, A.S., 1998. The distribution of apparent dose as determined by optically stimu-
N. Agersnap Larsen et al. / Radiation Measurements 32 (2000) 419±425 lated luminescence in small aliquots of ¯uvial quartz: implications for dating young sediments. Quat. Sci. Rev. (Quat. Geochronol.) 17, 1033±1040.
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Smith, B.W., Rhodes, E.J., 1994. Charge movements in quartz and their relevance to optical dating. Radiat. Meas. 23, 329±334.
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Use of the LM-OSL technique for the detection of partial bleaching in quartz N. Agersnap Larsen a, E. Bulur b, L. Bùtter-Jensen b, S.W.S. McKeever a,* a
Department of Physics, Oklahoma State University, Stillwater, OK 74078-3072, USA b Risù National Laboratory, DK-4000 Roskilde, Denmark Received 30 October 1999; accepted 1 March 2000
Abstract We present a study of the sensitivity to light (ease-of-bleaching) of the trapped charge in sedimentary quartz grains using an optically stimulated luminescence (OSL) technique in which the intensity of the stimulation light is linearly increased during the measurement period. The technique is known as linear modulation OSL (LM-OSL). In controlled laboratory conditions, this technique has been employed to study the ease-of-bleaching of the trapped charge in quartz by comparing the OSL curves of quartz aliquots which have been either: (1) fully bleached, followed by a laboratory dose of b-irradiation, or (2) partially bleached, followed by the laboratory b-dose. The ratio of the OSL signals due to the b-dose from the partly and fully bleached aliquots is illustrated to be a potential indicator of the degree of optical resetting of the OSL signal in dating material. The key parameter governing the ease-of-bleaching is the photoionization cross-section of the trap involved. The concept is also demonstrated in a model study from which very good agreement with the experimental observations has been found. Potential applications of the technique to dating are discussed. 7 2000 Elsevier Science Ltd. All rights reserved.
1. Introduction One of the necessary criteria for successful dating using optically stimulated luminescence (OSL) is a complete resetting to zero, or to a known level, of the latent OSL signal at the time of deposition through exposure of the sediment to sunlight. Incomplete resetting (partial bleaching) at deposition time leaves the material with an unknown residual signal leading to an apparent higher accumulated paleodose during the burial period and this in turn results in an overestimation of the age. This is particularly a danger for ¯uvial
* Corresponding author. Tel.: +1-405-744-5802; fax: +1405-744-6811. E-mail address: [email protected] (S.W.S. McKeever).
and colluvial sediments since these materials may have been transported under conditions of short transport distances and reduced light intensities owing to the sediment load in the water during transport. A multigrain aliquot of these sediments may therefore consist of grains that have received dierent light exposure energies, leaving grains with dierent residuals at deposition time. This consequently causes a grain-to-grain variation of the apparent paleodose, or age. These circumstances cause not only the OSL dating technique to overestimate the age, but also to be sensitive to the number of grains used in an aliquot, which in turn leads to poor reproducibility (Huntley and Berger, 1995). The demand for a tool to recognize and characterize incomplete resetting at the time of deposition is therefore high in OSL dating of sediments. Sev-
1350-4487/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 1 3 5 0 - 4 4 8 7 ( 0 0 ) 0 0 0 7 1 - 8
420
N. Agersnap Larsen et al. / Radiation Measurements 32 (2000) 419±425
eral methods have been suggested in the literature to extract information on incomplete resetting. Olley et al. (1998) suggested an approach utilizing the grain-to-grain variation of the paleodose in quartz sediments. They represented the paleodose variation in a histogram constructed from, typically, 100 small aliquots of 60±100 grains and concluded that the lowest value observed gave the optimum estimate of the true paleodose. Huntley et al. (1985), in their pioneering work on OSL dating, proposed a test for incomplete resetting in quartz by plotting paleodose against OSL illumination time. A constant illumination intensity was employed to stimulate the OSL signal (using a 514 nm Ar2+ laser) and the technique is hereafter termed continuous wave OSL (CW-OSL). In the case of partial bleaching, the paleodose is expected to increase with longer illumination time since the OSL signal at longer times originates from traps which are more dicult to empty optically. In contrast, in a sample which has been fully bleached, the paleodose is constant with stimulation time. (This procedure is sometimes called the ``shine-plateau'' test.) A more detailed study of CW-OSL curves from quartz stimulated at the same wavelength as above was reported by Smith and Rhodes (1994), wherein they concluded that the luminescence was associated with three traps characterized by having dierent decay constants Ð the so-called ``fast'', ``medium'' and ``slow'' components. Bailey et al. (1997) studied CW-OSL curves from quartz when stimulated with a broadband light source (420±550 nm) and reached the same conclusions as those of previous authors. The presence of dierential light sensitivity of the traps led the latter authors to propose a method of detecting partial bleaching in an aliquot by comparing certain parts of the natural CW-OSL decay curve to that from a matching regeneration dose delivered in the laboratory. Recently, Bulur (1996) introduced an OSL technique in which the intensity of the stimulation light is linearly increased during the measurement period. The technique is known as the linear modulation OSL (LM-OSL). The temporal order for which the peaks in an LM-OSL curve occur is inversely proportional to the photoionization cross-section of the involved traps. Consequently, the ®rst LM-OSL peak corresponds to the easy-to-bleach traps, i.e. having a high cross-section. Correspondingly, the signals for the harder-tobleach traps, having lower cross-sections, follow later. This distinct property of the LM-OSL technique has been applied in this work to study partial bleaching in quartz. Following Bulur (1996), the time dependence of LM-OSL from a single, ®rst-order, optically sensitive trap is expressed by:
1 I t I0 sgt exp ÿ sgt 2 2
1
where s is the photoionization cross-section of the trap. Thermal assistance from the trap has been speci®cally excluded. The ¯ux of the stimulation light F is linearly ramped in according to F t gt, where g is the ramp rate. I0 is the area under the curve and t is the time elapsed since the start of the OSL measurement. For further details of the expected shape of LMOSL curves and their relationship to conventional CW-OSL decay curves, see the accompanying papers in these proceedings by Kuhns et al. (2000) and Bulur et al. (2000). The purpose of this paper is to present an investigation of partial bleaching in quartz and to develop a method to characterize incomplete resetting of the preburial OSL signal using the LM-OSL technique. The work describes bleaching experiments of a previously heated quartz aliquot and its comparison with model simulations based on parameters extracted from the obtained data. The implications for detecting incomplete resetting using this technique are discussed.
2. Experimental details OSL measurements were performed using an array of blue light emitting diodes of type Nichia NSPB500S using a Risù automated TL/OSL apparatus (Bùtter-Jensen et al., 1999). The diodes had an emission wavelength at 470 D 20 nm and delivered an optical power output of about 20 mW cmÿ2 at the sample position. The luminescence was recorded by a 9168AQ PMT ®tted with two 3 mm Hoya U340 ®lters. Schott GG420 long-pass ®lters placed in front of each blue diode reduced the detection of the most energetic light from these diodes. The sample was exposed to a 90 Sr/90Y b-source delivering a dose rate of 25 mGy ÿ1 s . All the experiments were carried out on the same material, a glacio¯uvial quartz sediment from a marine core in Jutland, Denmark (Risù laboratory code Q914805) which was kindly provided by A.S. Murray at The Nordic Laboratory for Luminescence Dating, Risù National Laboratory, Denmark. The work was performed with 10 mg of approximately 125 mm grains, extracted from the sediments. Prior to the study, the aliquot was irradiated, illuminated and heated to 5508C repeatedly in the laboratory until the OSL sensitivity and the shape of the OSL curve were unchanged. This procedure inevitably results in some dierences between the behavior of these samples and those of as-found sedimentary materials, especially in terms of the sensitivity. Nevertheless, our concern in this work was to enable comparison between the dier-
N. Agersnap Larsen et al. / Radiation Measurements 32 (2000) 419±425
ent measurements conducted with the same aliquot in order to establish the principle of the method. Additional studies will be required to determine the utility of the procedures to natural sedimentary quartz. The blue light emitting diodes were used to both bleach and readout the OSL signal. 3. Results 3.1. Partial bleaching and ratio plots The eect of partial bleaching of the OSL signal in one aliquot of quartz was studied by exposing an irradiated aliquot to light using dierent illumination durations prior to the OSL measurements. The details of the measuring sequence are: Laboratory dose
partial bleaching
z}|{ z}|{ annealed 4 irradiation 4 preheat 4 bleaching 25 Gy at RT 5508C=10 s 2208C=10 s Dt at 1608C OSL curve
z}|{ 4 preheat 4 OSL readout 4000s at 1608C 2208C=10 s
421
The preheat and the elevated temperatures during the bleaching and OSL measurement were applied to eliminate accumulation of charge in the shallow traps in order to prevent thermal release of charge from these traps during the OSL readout. The illumination duration of the partial bleach Dt was varied from 0 to 10,000 s (at maximum light power), and the LM-OSL signal was stimulated by ramping the light power linearly from 0 to 100% of maximum light power in 4000 s at the elevated temperature of 1608C. Fig. 1(a) shows the LM-OSL curves measured after these various partial bleaches. The LM-OSL curves contain up to three separate peaks. The position (in time) of each peak is seen to be the same in each of the OSL curves. By assuming that the peaks are caused by the optically stimulated release of charge from three dierent traps obeying ®rst-order kinetics, the LMOSL curve for no bleaching Dt 0 s) was ®tted to the sum of three ®rst-order curves given by Eq. (1). The traps were found to be characterized by photoionization cross-sections of s1 9:0 10 ÿ17 cmÿ2 (easy-tobleach), s2 6:0 10 ÿ19 cmÿ2 (medium bleachability) and s3 4:2 10 ÿ20 cmÿ2 (hard-to-bleach). (These values should be compared with similar estimates by Kuhns et al. (2000) for LM-OSL emission from quartz from Oklahoma eolian and ¯uvial sediments using
Fig. 1. Partially bleached LM-OSL curves (a), and CW-OSL curves (c) recorded at an elevated temperature of 1608C. The ratio between the partially bleached and the non-bleached LM-OSL curves (b) and CW-OSL curves (d) for various bleaching durations.
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longer wavelength, green light stimulation.) Consistency between the photoionization cross-section and the degree of partial bleaching is clearly observed as the easy-to-bleach peak reduces faster than the harder-tobleach peaks. To describe the bleaching eect more qualitatively, the ratio between the partially bleached and the unbleached OSL curves has been plotted in Fig. 1(b). These ratio plots show distinct plateaux, of dierent durations, apparently corresponding to the presence of each of the three traps. In addition, the plot contains direct information of the degree of depletion of each of the dierent traps. Also plotted in Fig. 1 are CW-OSL curves obtained for the same sample after similar irradiation and partial bleaching sequences (Fig. 1(c)), and the corresponding ratio plots of the OSL from the partially bleached samples to that from the unbleached sample (Fig. 1(d)). The regions corresponding to the dierent
traps are much less distinct. No true plateaux are obtained but instead upwardly sloping curves are observed over narrow time regions. This arises because of the greater overlap of the easy-to-bleach and hardto-bleach signals in the CW-OSL curves compared to the LM-OSL curves. 3.2. The dating scenario Sediments examined in dating studies are expected to have undergone several irradiation and bleaching stages corresponding to pre-depositional (i.e. pre-burial) irradiation, solar resetting (partial or complete), and depositional irradiation (i.e. during burial) before the dating procedure is applied in the laboratory. The dating technique adopted in this work is the single aliquot regeneration (SAR) procedure described initially by Murray and Roberts (1998) and recently improved by Murray and Wintle (2000). The following measuring sequence has been conducted to imitate the dating scenario: pre-deposition dose z}|{ annealed 4 irradiation 4 preheat 25Gy at RT 5508C=10 s 2208C=10 s resetting event
deposition dose
z}|{ z}|{ 4 bleaching 4 preheat 4 irradiation 25 Gy at RT Dt at1608C 2208C=10 s natural OSL signal
z}|{ 4 4 preheat 4 OSL readout 4000 s at 1608C 2208C=10 s regeneration dose
z}|{ 4 4 bleaching 4 irradiation 25 Gy at RT 1000 s at 1608C dose-regenerated OSL signal z}|{ 4 4 preheat 4 OSL readout 4000 s at 1608C 2208C=10 s
Fig. 2. (a) Natural and dose-regenerated LM-OSL curves from a single aliquot of quartz which we have subjected to the imitated dating scenario, as described in the text. (b) Ratio between the natural and dose-regenerated LM-OSL curves for various bleaching durations.
The stages included consist of a ``pre-depositional'' irradiation, an optical resetting event using various illumination durations Dt from 0 s to 10000 s, a ``depositional'' irradiation followed by a single regenerative-dose step to imitate part of the SAR procedure. To simplify the study, the pre-depositional, depositional and the regeneration doses were selected to be identical (25 Gy). The preheat stages (2208C for 10 s) introduced after the ``pre-depositional'' dose and the ``resetting'' event are included in an attempt to account for the fact that in nature the irradiations are delivered at low, natural dose rates, and thus, traps of low ther-
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mal stability will not ®ll. The intent of the pre-heat phases, therefore, is to empty the shallow traps in order to mimic as closely as possible the natural situations. The simulated dating procedure described above generates two OSL signals: a signal corresponding to irradiations of the sample before and during burial (termed the ``natural'' signal) and a ``regenerated'' signal which follows from the administration of the laboratory regeneration dose. Fig. 2(a) shows LM-OSL curves following a variety of bleaching periods Dt during resetting of the pre-depositional signal, and an LM-OSL curve following the regeneration dose. (The shape of the latter curve did not change signi®cantly as a function of the number of dierent bleaching cycles.) The dierent peaks in the LM-OSL curves are clearly seen to bleach at dierent rates and to approach a level corresponding to complete resetting as the bleaching duration increases. In the case of complete resetting of the pre-depositional signal, the ratio between
Fig. 3. Schematic of the model used to simulate the OSL experiments. The model consists of three optically sensitive traps; an optically disconnected deep trap and a recombination center. The photoionization cross-sections used in the calculation are experimentally estimated from the quartz aliquot Q914805.The following parameters were used in the calculation: photoionization cross-sections s1 9 10 ÿ17 cmÿ2, s2 6 10 ÿ19 cmÿ2 and s3 4:2 10 ÿ20 cmÿ2; trap concentrations N1 14:4 1010 cmÿ3, N2 10 1010 cmÿ3, cmÿ3, Nd 1:0 1013 cmÿ3 and N3 95 1010 H 1:0 1013 cmÿ3; transition probabilities a1 a2 a3 ad ar a^ 1:0 10 ÿ7 cmÿ3 sÿ1. The ionizing radiation and light exposure were controlled through the generation rate of electron-hole pairs, R (01010 cmÿ3 sÿ1), and photon ¯ux, F (<2 1016 cmÿ2 sÿ1), respectively. nc and hv are the concentrations of free electrons and holes in the delocalized bands, respectively; ni is the concentrations of occupied electron traps, and h is the concentration of occupied hole traps.
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the natural and dose-regenerated signals is expected to be 1 (since the same doses were chosen in this study for all irradiation phases), whereas incomplete resetting results in the ratio being greater than 1. Thus, plateaux of values >1 are expected due to variations in the ease-of-bleaching of the dierent traps. This is con®rmed in Fig. 2(b), where the most bleached case (1000 s) gives a ratio close to 1 for times less than 100 s, while the less-bleached cases yield plateaux of values >1. A second observation is that the easy-to-bleach component is well-bleached for bleaching periods even as short as 10 s, while the harder-to-bleach components
Fig. 4. Computer-simulated (a) LM-OSL curves and (b) ratio plots for various bleaching durations using the model sketched in Fig. 3. (c) Model prediction of the ratio between the natural and dose-regenerated LM-OSL curves using dierent bleaching durations. Notice the good agreement with the experimental results in Figs. 1 and 2.
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need more than 1000 s of bleaching time. The decrease in the ratio at the end of the curve is probably due to a residual contribution to the dose-regenerated LMOSL signal from the natural signal in the harder-tobleach traps.
4. Model The properties of bleaching of this particular quartz aliquot have been simulated using a model consisting of three optically sensitive traps having photoionization cross-sections equal to those values estimated from the experimental results. The model includes a luminescence center and a thermally and optically disconnected trap (to ensure properties such as linear dose response). To simplify the simulation, thermal assistance and shallow traps have been excluded. Fig. 3 contains a sketch of the model, and the de®nitions of the terms and the parameter values used are given in the ®gure caption. The set of equations describing the trac of charge between the traps and center are: dnc dh dhv X dni R ÿ dt dt dt dt i dni ÿfsi ni ai nc Ni ÿ ni dt
statement can be applied to a real dating scenario will depend on several factors, including the natural dose, the dose response, the dose response for dierent OSL signals and the signal level and counting statistics. However, the key is that the LM-OSL method is able to separate the various components through dierences in the photoionization cross-section of the traps. This separation is more easily achieved with LM-OSL than with conventional CW-OSL, no matter what the sample is. By using the ratio of the natural and the dose-regenerated LM-OSL signals, the partial bleaching/incomplete resetting of the various traps can be easily recognized. As a consequence, the signal from the easy-to-bleach traps can be quickly separated from that from the harder-to-bleach traps. In conventional CW-OSL curves there is often a considerable overlap among the various components and their separation is less easily achieved. As demonstrated by Kuhns et al. (2000), separating the various OSL components by curve ®tting is easier for the LM-OSL signal than for the CW-OSL signal. To demonstrate the potential further, however, additional studies performed on natural sedimentary quartz are necessary. In this way, the utility of the method to characterize the optical resetting event in luminescence dating will be further evaluated.
i 2 f1, 2, 3g
dnd ad nc Nd ÿ nd dt dh ^ v H ÿ h ÿcnc h ah dt dhv ^ v H ÿ h R ÿ ah dt The results of the simulation of the partial bleaching and the dating scenario are shown in Fig. 4. A direct comparison of the simulation results and experimental data (Figs. 1 and 2) immediately shows an excellent qualitative agreement. In particular, the dierent traps are illustrated by the appearance of three plateaux in the ratio plots.
5. Conclusion Through experiments under controlled laboratory conditions, and the use of computer simulations, we have demonstrated that it is possible to recognize incomplete bleaching of dierent OSL components by using the LM-OSL technique. The extent to which this
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