IRSL dating of K-feldspar at elevated temperatures and infrared bleaching of TL

Radialion

Measuremenrs,

Vol.

23. Nos

213.

pp.

387-391,

1994

Copyright 0 1994ElsevierScience Ltd Printed in Great Britain. All rights rcscrwd I35044tI7/94s7.00 + .oo

1350-4487(93)E8035-K

IRSL DATING OF K-FELDSPAR AT ELEVATED TEMPERATURES AND INFRARED BLEACHING OF TL H. WIGGENHORN Heidelberger Akademie der Wissenschaften,

Forschungsstelle

Archiiometrie, am MPI fur Kemphysik,

Saupfercheckweg 1, D-691 17 Heidelberg, F.R.G. stimulated luminescence (IRSL) is mostly measured at room temperature, but IRSL intensity can be increased while measured at elevated temperatures. Here, IRSL obtained at temperatures

Abstract-Infrared

ranging from 30 to 200°C is tested, and the effect on the resulting equivalent doses (ED) is described. A change in ED is found for temperatures above lOO”C, though no obvious dependence on temperature can be seen. A second experiment investigates the effect of infrared stimulation at varying temperatures on TL from K-feldspar. It shows that only a small, fixed amount of TL is bleachable by infrared, independent of bleaching temperature or time. At the same time, elevated temperatures increase the total amount of IRSL from a sample.

1. INTRODUCTION INFRARED stimulated luminescence (IRSL) has been shown to be a possible method for dating K-feldspars from sediments (Hiitt ef al., 1988; Hiitt and Jaek, 1989; Duller, 1991). IRSL dating is closely related to thermoluminescence (TL) dating. In both methods, energy of natural, ambient ionizing radiation is accumulated in the crystal lattice by trapped charges. The trapped charges occupy energy levels between valence and conduction bands. From these levels, the charges can be released by heating the crystal or by exposing it to light, so that the date of sedimentation can be determined. Whereas for TL the charges are thermally evicted from their traps, for IRSL the release has been proposed to be a thermo-optical process (Hiitt et al., 1988; Hiitt and Jaek, 1989). In this process, the charge is evicted from a deep trap to an excited state or intermediate trap by infrared stimulation and then thermally released to the conduction band. The behaviour of IRSL is therefore strongly dependent on the temperature during measurement. A strong increase in IRSL intensity with temperatures up to 200°C has been reported (Duller and Wintle, 1991). From infrared stimulated shine-down curves (Fig. l(a)), it is not possible to distinguish how many or what kind of traps are involved. It is not yet clear which preheat treatment needs to be applied in order to remove all charges from those traps exhibiting long-term instability. A rather complicated plateau test to check long-term stability, comparable to that used in TL, has been proposed by Huntley et al. (1985) for optical dating. If trapped

RM2312.3-j

charges responsible for IRSL and for TL were correlated, simpler checks could possibly be applied. On the other hand, if there is no relation, IRSL could provide additional and independent age information. In this paper, the influence of temperature during IRSL measurements on the equivalent dose (ED) will be investigated, as well as the bleachability of TL by infrared radiation. The latter will explicitly reveal a possible existence of correlation between trapped charges responsible for IRSL and TL.

2. EXPERIMENTAL

AND RESULTS

Two samples, WALL3 and GSU, were selected from two Late Glacial/Lower Holocene dunes of the Upper Rhinegraben, Germany (locations Walldorf and Gewannsee, near Sandhausen). K-feldspars (density of 2.53-2.58 gem-‘) were separated and sieved to the 90-2OOpm coarse grain fraction. Sample GSU has a K-content of 11.7 + 0.3% and sample WALL3 a K-content of 8.4 + 0.6%. TL and IRSL were measured with a Riso TL/IRSL reader. The infrared LEDs have a power of ca 40mW cm-* at a maximum wavelength of 880 nm (Batter-Jensen et al., 1991). All luminescence was detected with a 402 nm interference filter (HW = 19 nm), where IRSL short-shine measurements (i.e. 0.1 s of IR stimulation) of both samples show maximum intensity. Special care was taken to expose samples to as little light (diffused orange or red light) as possible during preparation. 387

H. WIGGENHORN

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‘I 40

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SO

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FIG. 1. Example of determination of ED from IRSL measurements of sample WALW, preheated at 160°C for 4 h. Temperature during infrared stimulation is 30°C. (a) Shine-down curves of natural and irradiated aliquots (IO, 20,30,40 Gy). Each curve shows the mean of 8-9 aliquots. Of every curve, average intensity of the last 6s was subtracted as background. (b) ED with shine-time. (c) Growth curve calculated from integral (5-35 s) of shine-down curves. A linear fit, proportional to variance, was applied.

the scattered light from the IR LEDs is subtracted, but also dose-dependent contributions from harder to bleach traps. The value that is subtracted then also increases with temperature applied during IRSL measurements. The area of 5-35 s was integrated for ED calculation, using a linear fit proportional to variance (Fig. 1). All 19 subsets measured at varying temperatures showed a good plateau between 5 and 35 s. Figure 2 shows the resulting EDs measured at the temperatures indicated on the x-axis for the two different preheats. For lower temperatures, the ED seems to be reproducible. Calculated from the EDs obtained at the three lowest temperatures of each preheat group (i.e. 50, 90, 110°C and 30, 90, lOO”C, respectively), the ED for the subsets preheated at 220°C is 29.55 + 0.21 Gy, while for the 160°C subsets it is 27.64 + 0.55 Gy (la errors). TL of a subset preheated at 160°C was measured with an ED of 28.9 f 1.9 Gy (290-330°C integration area). So far the dose-rate measurements of the natural radioactivity are not complete. With the ED of 29.55 Gy, a preliminary age calculation, based on in situ y -spectrometry, gives an age of 12,000 + 2000 years, in agreement with the expected geological age and preliminary IRSL ages of the same section (Lang, 1994). A “C dating of underlying peat is in progress.

Temperature

2.1. ED-determinations 20

Twenty

subsets

of sample WALL3 were prepared,

consisting of 44 discs each. For every subset, some discs were irradiated with additional doses (from 10 to 4OGy, 6Gy min-‘) and some left unirradiated. One group of subsets was then preheated at 220°C for 200 s, the other at 160°C for 4 h. Afterwards, all subsets were kept at 70°C for a week and at room temperature for an additional week to remove charges susceptible to anomalous fading. The preheat temperatures mentioned have been proposed by Li (1991) and Aitken (1992) for use with IRSL. Aliquots of the natural sample were irradiated with 4OGy and their IRSL, measured at 30°C compared to that from unirradiated samples after preheating at 220°C for 60-320 s or at 160°C for l-6 h. The magnitude of the ratio of (N + fi)/B showed no significant difference (lo error of 7%) due to preheating time for the two temperature groups. Therefore, the preheat times which gave the best plateau of (N + /?)I/3 with shine-time were chosen. IRSL of 19 subsets was then measured while the sample discs were kept at a constant temperature, which varied from 30 up to 200°C for different subsets. Of the 60s IRSL shine-down, the average intensity of the last 6 s was subtracted as background (Aitken and Xie, 1992). With this procedure, not only

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f°C1 140

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preheat: 200s at 220%

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FIG. 2. EDs of WALL3 measured at temperatures indicated on the x-axis during infrared stimulation. (a) Subsets preheated at 220°C. (b) Subsets preheated at 160°C. Shaded area shows 95% confidence interval calculated from EDs obtained at the three lowest temperatures, i.e. 50.90, 110°C in (a) and 30.90, 100°C in (b). Error bars indicate lo errors. TL data were contained from a subset preheated at 160°C integrated over a plateau area from 290 to 330°C.

IRSL DATING TL measurements show that a larger part of trapped charges is removed with the 220°C preheat. The lower ED value, obtained from the subsets preheated at 16o”C, might therefore be due to insufficient removal of charges from unstable traps, but the influence of preheat temperature on IRSL has not yet been fully investigated. In the temperature region of ll&140°C the EDs of the subsets preheated at 160°C are not reproducible and slightly increased, compared to those at lower temperatures, whereas for the 220°C subsets, the EDs decrease with rising temperatures. Taking into account 2a errors, the one outlier at 115°C (Fig. 2(a), la error bars) is not significant, though it might also co&m irreproducibility of EDs at elevated temperatures. The dependence on temperature of the corresponding growth-curve parameters, i.e. y-axis intercept and slope, is shown in Fig. 3. Units for the intercept, which is equal to the fitted natural intensity of IRSL, are chosen, so that for the lowest temperature of each preheat group, slope and intercept overlap. Then, the EDs obtained at higher temperatures are equal to the one obtained at the lowest temperature if again slope and intercept coincide. As expected, the intercept increases strongly and slightly supralinear with temperature for both preheats. For subsets preheated at 220°C (Fig. 3(a)) slope and intercept mostly overlap and diverge only at high temperatures, resulting in

3

(a) t

60

100 Tomp.mtun

160

389

lower EDs (see also Fig. 2(a)). In the other group of subsets (Fig. 3(b)), the slope seems to stop its increase at temperatures around 100°C while the intercept does not. This leads to higher EDs (Fig. 2(b)). From the above results, it is not obvious which temperature to choose for IRSL measurements. EDs obtained at lower temperatures, i.e. below llO”C, seem to be the most reliable, since their reproducibility is very good. Low temperatures also have the advantage that they do not affect stable TL traps or traps filled by phototransfer. From Figs 2 and 3 it appears that two effects dominate the ED behaviour: the ED seems to decrease at higher temperatures, and the ED is not reproducible for temperatures above 110°C. The latter effect is much more prominent in the sample sets preheated at 160°C. A thermal depletion of charges, thermal quenching at increased temperatures, or a wavelength shift of luminescence at higher temperatures can be responsible for changes in IRSL intensity. However, at high temperatures (200°C), IRSL intensity is still increasing strongly and no equilibrium between IRSL efficiency and thermal depletion is reached (at which point, intensity would start to decrease). Therefore, a lower ED at those temperatures might partly be due to an effect that does not affect the natural IRSL intensity, but is supralinear in dose (i.e. slope increasing stronger than intercept, see Fig. 3(a)). It is not yet clear in what way elevated temperatures during infrared stimulation affect the charge populations. Maybe different trap types are sampled or just additional charges from similar trap types (Duller and Wintle, 1991). The increased temperature will certainly change the transition rates from intermediate traps to the conduction band. Any of the effects of temperature on IRSL mentioned above could cause a change in ED. It is more complicated to find an explanation for the irreproducibility of the ED between 110 and 140°C. The subsets preheated at 160°C produce EDs that vary between the ED obtained at lower temperatures and an ED approximately 20% higher. This behaviour could be caused by a variation of ED with different grains, which is supported by a poor reproducibility of TL between single discs. However, this does not explain why only EDs measured at higher temperatures are affected, or why the preheating procedure should be important. Not enough data are available to explain this effect satisfactorily.

11 1

0

OF K-FELDSPAR

200

ITI

2.2. TL-bleaching FIG. 3. Growth-curve parameters, slope and y-axis intercept, calculated from linear fit (see Fig. l(c)). Units of intercept were chosen so that symbols for intercent and slope overlap at 50°C for 220”6 preheat and at 3&C for 160°C preheat. If symbols coincide at higher tcmoeraturcs. the corksponding &set has the same Eb as m&red ai SOor 30°C. If slope is higher than intercept, the resulting ED will be lower. EDs can be calculated by dividing intercept by slope and multiplying the result with the value of ED obtained at 50°C in (a) or at 30°C in (b).

Of sample GSU, natural TL bleached by infrared was compared to unbleached TL (NTL). Examples are given in Fig. 4. Several temperatures during the bleach and several bleaching times were investigated and the missing TL (dTL) recorded. Results are shown in Fig. 5(a). The corresponding IRSL, monitored during the bleach is shown in Fig. 5(b). No significant effect of temperature or bleaching time on

H. WIGGENHORN

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FIG. 4. Examples of infrared bleaching of natural TL (sample GSU). Boxes show the applied bleaching times and the temperatures during bleach.

the bleached part of TL could be determined.

At the

from a sample increased with temperature by a factor of three. After 2 min, already 95% of total IRSL was released, and after 1 h at the latest, IRSL dropped down to a background level. The measurements show that for this sample, only a certain part (13 %) of TL is infrared bleachable, same time, the total amount

1

lR6L 3

2

of IRSL obtainable

20

50.150%

60

120

3600

min

,.,TL

3

d

g 2t P lE 10’ IR-bleaching-time

10’ [mini

FIG. 5. Bleaching of natural TL. (a) Infrared-bleached part of TL with bleaching time (log scale). (b) Corresponding IRSL of bleached sample. IRSL was integrated over bleaching time. After 2 min co 95% of IRSL has been stimulated after 20min no significant IRSL is left. Symbols indicate the different temperatures during bleaching. lo errors are shown.

which also has been reported by Duller and Wintle (1991) for other K-feldspars. The same behaviour is found in sample WALW, though the bleachable part of TL is around 20%. Li and Aitken (1989) on the other hand reported a decrease in TL of K-feldspar by several percent with increasing infrared bleaching times (1000 and 2000 s). All TL curves bleached at low temperatures show a strong phototransfer of charges to traps of TL peaks at IOO-200°C due to infrared bleaching (see Fig. 4). Low infrared bleachability of TL of K-feldspars is quite unexpected, because it leaves the charges responsible for TL and for IRSL mostly uncorrelated. It suggests that for most TL traps, no suitable intermediate traps for infrared stimulation are available, as proposed by Duller and Wintle (1991). The total number of photons, evicted by infrared and thermal stimulation of sample GSU, are of the same order of magnitude. However, as IRSL is increased at elevated temperatures, no additional effect on the infrared bleachability of TL can be seen. Nevertheless, all IRSL is removed by heating the sample up to 500°C. Most charges responsible for IRSL must therefore recombine non-radiatively or at different wavelengths (interference filter used has LX = 402nm, HW = 19 nm). On the other hand, both IRSL and TL of K-feldspars mostly emit in the blue region (Bailiff and Poolton, 1991; Dalal et al., 1988; Huntley et al., 1991; Prescott et al., 1990; Rieser et al., 1994). The recombination process involved in TL and IRSL therefore presents a rather complicated system.

3. CONCLUSIONS As shown above, special care has to be taken regarding any thermal treatment of samples used for IRSL. So far, it is not clear or easily understandable in which way trapped charges are affected by elevated temperatures during IRSL measurements. The reproducibility of the ED obtained at lower temperatures and their accordance with TL data is encouraging, SO that the increase of IRSL intensity with temperature might to some extent be usable for dating. So far, it has not been investigated, if the IRSL measured at elevated temperatures can easily be bleached at room temperature. Infrared bleaching experiments suggest, that at least for some K-feldspars, there is not much correlation between charge traps responsible for TL and for IRSL. A lot more spectral data are needed to propose specific recombination processes of trapped charges with luminescence centres. Because of the rather unrelated sources for TL and IRSL, it is not possible to apply standard TL preheat procedures to IRSL. Thermal stability of traps will have to be checked using IRSL itself. Acknowledgements-This work is supported by the Bundesministerium fiir Forschung und Technologie and is part

IRSL DATING of the project “Optically stimulated luminescence-a new dating-technology~in amhaeometry”. I wish to thank A. Lana. Prof. G. A. Wanner and Dr L. ZGlier for their useful disc&ions and valuable help. REFERENCES Aitken M. J. (1992) Optical dating. Quot. Sci. Rpv. 11, 127-131. Aitken M. J. and Xie J. (1992) Optical dating using infrared diodes: young samples. Quot. Sci. Rev. 11, 147-152. Bailiff I. K. and Poolton N. R. J. (1991) Studies of charge transfer mechanisms in feldspars. Nucl. Trucks Radiur. Meus. 18, 111-118.

Batter-Jensen L., Ditlefsen C. and Mejdahl V. (1991) Combined OSL (infrared) and TL studies of feldspars. Nucl. Tracks Radial. Meas. 18, 257-263. Dalal M. L., Kirsh Y., Rendell H. M. and Townsend P. D. (1988) TL emission spectra of natural feldspar. Nucl. Tracks Radial. Me&

14, 57-62.

Duller G. A. T. (1991) Comnarison of eauivalent doses determined by thermohtminescence ’ and infrared stimulated luminescence for dune sands in New Zealand. Quar. Sci. Rev. 11, 39-43. Duller G. A. T. and Wintle A. G. (1991) On infrared stimulated luminescence at elevated temperatures. Nucl. Tracks Radiar. Meas. 18. 379-384. Huntley D. J., Godfrey-Smith D. L’and Haskell E. H. H. (1991) Light-induced emission spectra from some quartz and feldspars. Nucl. Tracks Radial. Meas. 18, 127-131.

OF K-FELDSPAR Huntley D. J., Godfrey-Smith D. I. and Thewalt M. L. W. (1985) Optical dating of sediments. Nurure 313, 105-107. Hiitt G. and Jaek I. (1989) Infrared photoluminescence (PL) dating of sediments: modification of physical model, equipment and some dating results. In Long and Shorr Range Limirs in Luminescence Daring. Research Laboratory for Archaeology and the History of Art, Oxford, Occasional Publication No. 9. H&t G., Jaek 1. and Tchonka J. (1988) Optical dating: K-feldspars optical response stimulation spectra. Qucrr. Sci. Rev. I, 381-385. Lang A. (1994) Luminescence dating of holocene reworked silty sediments. Quar. Sci. Reu. (in suppl. Quar. Geochron.) (in press). Li S. H. (1991) Removal of the thermally unstable signal in optical dating of K-feldspar. Anci&r TL 9, 2&29. Li S. H. and Aitken M. J. (1989) How far back can we PO with the optical dating of’K-feldspar? In bng a;;d Shorr Range Limits in Luminescence Daring. Research Laboratory for Archaeology and the History of Art, Oxford, Occasional Publication No. 9. Prescott J. R., Akbcr R. A. and Gartia R. K. (1990) Three-dimensional thermoluminescence spectroscopy of minerals. In Spectroscopic Characrerizarion of Minerals and rheir surfaces (Edited by Coyne L. MI, McKeever S. W. S. and Blake D. F.).II DD. . . 180-189. American Chemical Society, Washington, DC. Rieser U., Krbetschek M. and Stolz W. (1994) CCDcamera-based high sensitivity TL/OSL-spectrometer. Radial. Meas. 23, 523-528.