Towards the development of a preheat procedure for OSL dating of quartz

PII:

Radiation Measurements Vol. 29, No. 1, pp. 81±94, 1998 # 1998 Elsevier Science Ltd. All rights reserved Printed in Great Britain 1350-4487/98 $19.00 + 0.00 S1350-4487(97)00228-X

TOWARDS THE DEVELOPMENT OF A PREHEAT PROCEDURE FOR OSL DATING OF QUARTZ A. G. WINTLEa* and A. S. MURRAYb Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB, U.K. and b The Nordic Laboratory for Luminescence Dating, Risù National Laboratory, DK-4000, Roskilde, Denmark

a

(Received 28 May 1997; accepted 21 September 1997) AbstractÐIsothermal decay data for the natural optically stimulated luminescence (OSL) of a quartz sample from the Kimberley region of Western Australia are presented. These show that >99% of the initial OSL signal measured using a broad-band blue/green-light stimulation source is derived from a single trap. A lifetime at 208C of about 850 Ma is predicted for this OSL trap. Since these measurements may be a€ected by charge transfer or sensitivity changes, both of which are thought to occur as a result of heating, the e€ects of prior heating (preheating) on the shape of the quartz OSL decay curve have been investigated. The relationship between the initial OSL (®rst 0.4 s), the OSL at the end of a 100 s stimulation period, and the integrated OSL (0±100 s), is presented at a constant stimulation temperature of 1258C, but following preheats at temperatures between 1108C and 3808C. It is deduced that both the natural and regenerated OSL are dominated by a single trap/ luminescence centre combination for preheats up to above 3008C. There is probably a small contribution [about 1% of the natural OSL integral (0±100 s) without preheating] from traps which are not emptied by heating to 3408C, and which have a longer lifetime. Based on the observation that the initial OSL signal from 0.1 s stimulation correlates well with the net integral OSL (0±100 s), the e€ect of preheat on the natural and regenerated OSL is examined again using single aliquots. These curves are then corrected for changes in luminescence eciency with preheating obtained using the 1108C thermoluminescence from a 0.1 Gy test dose, before ®tting with a simple two trap model. We draw conclusions regarding the implications of this study for dating. # 1998 Elsevier Science Ltd. All rights reserved

decay of the OSL due to storage at temperatures from 1608C to 2568C was observed using the initial OSL signal. The exponential decay obtained led them to conclude that they had isolated a single OSL signal with a life time at 208C of 06  108 years. On the other hand, studies of the thermal decay of the natural OSL of an old Australian quartz (De0350 Gy) on storage at temperatures ranging from 143 to 2238C gave very di€erent results (Huntley et al., 1996). No single exponential decay was obtained, but the data could be explained by ®tting the sum of four exponential terms. They concluded that the OSL in this 800,000 year old sample (SESA-63) was derived from four electron traps; these contributed 20, 20, 49 and 1% of the natural OSL signal and had half-lives at 208C of 14 ka, 2 Ma, 28 Ma and 4700 Ma, respectively. In this paper, we present isothermal decay studies on the natural OSL of a younger Australian quartz, in order to identify components which might require speci®c preheat treatments. We also present the results of experiments to explore the e€ects of

1. INTRODUCTION On exposure to visible wavelength light, previously irradiated quartz emits light (Huntley et al., 1985). This optically stimulated luminescence (OSL) decreases as optical stimulation continues, resulting in a decay curve. The signal may be observed at room temperature, or above, until a temperature is reached such that the electron traps responsible for the signal are thermally emptied. The synchronous behaviour of the integrated OSL signal and the 3258C TL peak (Smith et al., 1986; Spooner, 1994; Wintle and Murray, 1997) indicates that the majority of the OSL signal is due to the optical release of charge from the trap responsible for the 3258C peak. Having established such an empirical relationship, Smith et al. (1986) developed a preheat procedure (5 min at 2208C) which speci®cally emptied the thermally unstable TL traps below 3258C, but removed less than 10% of the 3258C TL trap. Isothermal experiments were carried out by Smith et al. (1990) using a laboratory irradiated sample which had been given this preheat. The isothermal

*The Nordic Laboratory for Luminescence Dating is a section of The Department of Earth Sciences, Aarhus University, C.F. Mùllers AlleÂ, DK-8000 AÊrhus C, Denmark. 81

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A. G. WINTLE and A. S. MURRAY

preheating on OSL signals from this sample, in particular the e€ects on charge transfer and sensitivity changes. 2. LABORATORY PREHEATING 2.1. The need for preheating in dating studies Since the size of the OSL signal relates to radiation exposure, the signal can be used for dating. It is of particular interest for dating sediments (Wintle, 1993), because the trapped charge which could give rise to an OSL signal is likely to be removed by the exposure to sunlight during erosion, transport and deposition. Hence the OSL signal from natural sedimentary quartz which has remained shielded from light since deposition, should provide information on the age of the sediment when combined with separate measurements of the annual dose-rate (Aitken, 1985). However, the natural signal needs to be compared to the signal from laboratory irradiation, to allow the determination of the equivalent dose (De). The conditions of irradiation are di€erent in the laboratory, compared with in nature. For example, the 1108C TL trap remains empty during exposure to environmental radiation because of thermal decay; for laboratory irradiations at room temperature (which are usually on a time scale of min to h), this trap will hold charge which would otherwise be trapped at the deeper OSL traps. To some degree this e€ect can be compensated for by heating prior to OSL measurement (preheating), i.e. by heating the sample, after irradiation, to some temperature which empties shallow traps (and thus transfers some of their contents to deeper traps), but does not signi®cantly thermally erode the deep OSL trap. The need for a preheat procedure in optical dating was ®rst suggested by Huntley et al. (1985) in the ®rst paper on optical dating; this requirement was later demonstrated by Godfrey-Smith (1994), who observed the decay of OSL signals from unheated samples with storage after laboratory irradiation. It has been suggested that a preheat of both the natural and laboratory irradiated samples is required to ensure the equal redistribution of charge, in a way which is equivalent to the e€ects of environmental temperature over thousands of years (Rhodes, 1988). Observations of the phototransferred thermoluminescence (PTTL) at 1108C, which can be seen after room temperature OSL measurement, indicate that the electron trap which gives rise to the thermoluminescence (TL) at 1108C plays a role in charge transfer under optical stimulation (Wintle and Murray, 1997). Traps other than the 1108C trap may also a€ect the OSL signal, e.g. those at 1608C and 2608C. PTTL is seen at 1608C when optical stimulation is carried out after heating to 2808C to remove any TL peaks (Wintle and

Murray, 1997) and this peak was shown to be slightly light sensitive. Other traps which are only weakly light sensitive, e.g. at 2808C, also compete for charge, although PTTL has not been observed at this glow curve temperature. The above discussion is relevant to dating procedures which involve a number of di€erent sample aliquots, each of which have received a di€erent radiation dose prior to measurement, i.e. natural, and di€erent laboratory irradiations. However, there is also a need for preheating during single aliquot dating procedures (Duller, 1995; Murray et al., 1997); in these cases the sample may be dosed and preheated many times, and it becomes imperative to understand the e€ect of repeated preheats on the accumulating OSL signal.

2.2. Summary of preheats used in routine dating applications Various preheats have been proposed, e.g. 1 min at 2408C, which would remove the 2808C peak with negligible e€ect on the 3258C peak (Franklin et al., 1995); 5 min at 2208C on the basis of a N/(N + b) ``plateau'' test using 5 min preheats from 1608C to 2808C in 308C intervals (Rhodes, 1988); 16 h at 1608C (Stokes, 1992); and 48 h at 1508C (Wolfe et al., 1994). However, preheat procedures are controversial (Roberts et al., 1994) and di€erent preheats have not always been found to give the same values of De, even when they are thought to be equivalent on the basis of laboratory experiments. Roberts et al. (1993, Fig. 6) found that the De obtained for measurements of the integrated OSL signal after a preheat of 2208C (for 5 min) was double that for a preheat of 1608C (for 16 h). Further intercomparisons using the same two preheats for 45 samples with ages ranging from a few hundred to about 125,000 yr showed negligible di€erences between the two values of De (Stokes, 1996). For younger samples (<2000 yr), Murray (1996) chose a preheat of 1908C for 10 s, based on De ``plateau'' tests, using 10 s preheats from 1608C to 2608C in 208C steps. Murray et al. (1997) show 5 preheat plateau tests (using 10 s preheats from 1608C to 3008C) for samples of various ages (<5 yr to 050 kyr) obtained using a single-aliquot additive-dose protocol. They show that the older the sample, the higher is the minimum preheat temperature required. Part of the contention over preheats relates to the part of the OSL curve employed, e.g. the total OSL (integrated until the instantaneous signal has decreased to some very small level, such as 1% of the initial signal) or the initial signal, perhaps due to a stimulation period of a fraction of a second. The di€erent behaviour of these signals has been investigated for the sample used in this study (Wintle and Murray, 1997; Murray and Wintle, 1998). These studies have shown that several dis-

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83

tinct temperature dependent processes may occur under di€erent measurement conditions. These are further investigated in this paper, with particular reference as to how they a€ect the OSL signal measured after preheating. 2.3. Possible e€ects of preheating Two e€ects of preheating a sample prior to OSL measurement have been reported: (a) the transfer of charge from light-insensitive traps that are thermally stable to the OSL trap(s) (Rhodes, 1988; Godfrey-Smith and Haskell, 1993). This is a particular problem for young sediments which at deposition have their light sensitive traps empty, but other traps may remain ®lled (cf. ceramics which have all traps emptied by heating). An increase in OSL of ten times was reported by Godfrey-Smith et al. (1988) when a zero age sample, in which the OSL traps were empty, was heated in steps to temperatures from 1258C to 4258C. (b) sensitivity changes. Besides OSL signals increasing because of charge transfer to the OSL trap, increased signals can arise from increased luminescence eciency, i.e. from an increase in the number of detectable photons emitted per number of electrons recombining. This phenomenon was ®rst reported by Aitken and Smith (1988), who studied a wind-blown sand from France. Using a 0.2 Gy test dose, they monitored the OSL sensitivity change after heating to various temperatures up to 6008C. Sensitivity changes with temperature had been found previously for ®red quartz from pottery; these changes have been explained by the thermal transfer of holes from non-radiative centres to luminescence centres (Zimmerman, 1971).

3. SAMPLE CHARACTERISATION All experimental work was carried out on 90± 125 mm quartz extracted from a sample of aeolian sand from Widgingarri in the Kimberley region of the northern part of Western Australia (WIDG8). The sample was expected to have been well bleached at deposition, over 30,000 yr ago (Veth, 1995), and has an equivalent dose of 582 6 Gy as measured by an additive-dose multiple-aliquot OSL procedure (Murray et al., 1997). The natural TL is given in Fig. 1(a) and shows a prominent peak at 03108C when a heating rate of 58C/s is used (this is referred to in the literature as the 3258C TL peak, or the ``rapidly bleaching peak''). Experimental details of the optical detection system are given by Wintle and Murray (1997), who compared the e€ects of blue/green light stimu-

Fig. 1. (a) Natural TL of WIDG8 showing peak at 03108C for heating rate of 58C/s observed with an HA3 and two 3 mm thick U-340 ®lters. (b) TL glow curves obtained from natural quartz which had been given a 43 Gy dose, and a 10 s preheat at 2808C. One disc was exposed to blue/green light for 100 s at 1258C prior to measurement. The di€erence in TL (right hand axis) shows TL lost at 3108C and gained at 1608C. (c) E€ect of blue/green light on regenerated TL. Two aliquots were exposed to blue/green light (100 s at 258C), given a 43 Gy dose, and preheated at 1108C for 10 s. One aliquot was then exposed to blue/green light for 2 s, and the di€erence between the two subsequent TL glow curves is shown. Note that the increase in TL below 1258C has been multiplied by 0.4 for display.

lation on various parts of the TL curve, including this peak. A further two natural aliquots were given an additional 43 Gy followed by a 10 s preheat at 2808C, and one of them was then exposed to 100 s of blue/ green light. The signal that was lost, when compared to the unexposed sample, only had a peak at 03108C [Fig. 1(b)Ðtaken from Fig. 2(a) of Wintle and Murray, 1997]. Wintle and Murray showed that this ``lost TL'' signal varies in proportion to the integrated OSL signal as the blue/ green light exposure time was varied from 0.1 to 100 s. The blue/green light stimulation from a Risù TL/OSL reader covers the wavelength region from 420 to 550 nm (Bùtter-Jensen and Duller, 1992), but has been shown to behave as a stimulation source with an average wavelength of about 470 nm (Murray and Wintle, 1998). For aliquots which had been optically bleached with the blue/green light, irradiated (43 Gy) and then given a 10 s preheat at 1108C, the e€ect of a 2 s blue/green light exposure was observed as a loss in TL signal [Wintle and Murray, Fig. 1(b)]; this

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A. G. WINTLE and A. S. MURRAY

Fig. 2. (a) and (b) Isothermal decay data over 30 h and 1.5 h, respectively. Initial OSL (0.1 s stimulation) obtained during light exposure of one natural aliquot at 1258C after preheating for various times at 1608C (uppermost solid circles). Optical decay is estimated at <0.2% per stimulation. The experiment was then repeated on other aliquots, each heated to a temperature between 1808C and 2808C for varying lengths of time. Data are normalised to the ®rst measurement. The solid lines are best ®ts of the sum of two exponential decay functions. (c) Mean lifetimes, 1/l, derived from the data of Fig. 2(a) and (b), plotted against 1/kBT, as described in the text for the two exponential decay functions with component (a) being the major component with the shorter lifetime. The solid lines are best ®ts of l = S exp(ÿE/kBT), where E and S are the trap depth and frequency factor, respectively.

signal is reproduced in Fig. 1(c). This lost TL signal is broader, with a substantial shoulder just below 3008C, as well as the main peak at 03108C. There is also an optically sensitive peak at 01508C. 4. ISOTHERMAL DECAY CURVES Huntley et al. (1996) found that the isothermal decay of the natural signal in their old sample (De0350 Gy) required at least four traps to satisfactorily explain the data. This was despite being their being able to represent the e€ects of preheating at

di€erent temperatures with a ®rst order model. However, Murray et al. (1997) have shown that the decay of more than 90% of the initial OSL signal from WIDG8 can be adequately represented by a single exponential, when using repeated pulse stimulation with heating to 2808C for 10 s between each 0.1 s measurement on a single sample. Although only about 70% of their decay at each step was attributed to thermal excitation (the remainder was due to optical excitation because the stimulation source was used with the aperture fully open), this suggests that most of the OSL signal originates from a single trap. Because of this apparent disagreement with Huntley et al. (1996), we undertook experiments similar to theirs, but using our sample. Isothermal decay measurements were carried out on the natural OSL signal from WIDG8. One aliquot was stimulated for 0.1 s at 1258C using a reduced illumination aperture to minimise the optical decay of the OSL signal (<0.2% decay per 0.1 s stimulation). The sample was then heated at 2808C for a minimum period of 10 s, before cooling rapidly, and remeasuring the OSL. The heating to 2808C was repeated for various increasing times, with two OSL measurements between each heating cycle. It is assumed that no further changes in sensitivity (beyond that which might have arisen from the ®rst 10 s of heating) occurred when the aliquot was held at temperature for longer times. This experiment was repeated on di€erent aliquots, each heated to a di€erent temperature between 1608C and 2808C, in 208C steps. The 1608C data set was carried out to a cumulative time of 48 h, all others stop at or before 24 h. An instrument background was determined by repeated exposure of a previously annealed (5008C for 1 h) sample, and this background was subtracted from all data. The resulting OSL signals are shown in Fig. 2(a) and (b) normalised to the initial OSL signal. We have ®tted the data in two ways, each using the sum of two exponentials. First, we allowed the relative proportions of the two exponential components to vary from one temperature data set to another. Figure 2(c) shows the values of 1/l (lifetime) for the two components (i.e. two traps) obtained from this method, plotted on a logarithmic scale against 1/kBT, where kB is Boltzmann's constant and T is absolute temperature. The lifetime of the minor component (b) is about an order of magnitude more than that of the major component (a) at each temperature. The solid straight lines are best ®ts of l = S exp(ÿE/kBT), where E is the trap depth and S is a constant, sometimes called the frequency factor (McKeever, 1985). All points shown in Fig. 2(c) were used in the ®tting procedure. [Uncertainties in l for the minor component became very large for temperatures below 2208C, and these data are not included in Fig. 2(c).] Second, we constrained the initial ratio of the two exponentials (at t = 0) to be constant with

OSL DATING OF QUARTZ

85

Table 1. Parameters values obtained from isothermal decay data for natural OSL only

Unconstrained Constrained

log10(Sa) (S in sÿ1)

Ea eV

log10(Sb) (S in sÿ1)

Eb eV

b/a

15.920.3 19.0020.06

1.882 0.03 1.8342 0.006

162 3 17.92 0.2

2.0 20.3 1.81 20.02

0.0056 0.0089

Subscripts a and b refer to the two exponential components ®tted to the data. The ®nal column gives the relative contribution of the two components to the initial OSL signal.

temperature, by simultaneously ®tting all data. This is what would be expected if any sensitivity change with temperature was the same for both traps, and is an assumption. The smooth lines shown in Fig. 2(a) and (b) are the result of the second approach; there was no signi®cant improvement in the quality of the ®t using the less constrained ®rst method. The parameter values obtained are shown in Table 1. The frequency factor S is expected to be less than the lattice vibration frequency, typically thought to be 1012 to 1014 sÿ1 (McKeever, 1985). For this reason, the parameters from the unconstrained data ®tting procedure are preferred. The ratio of the two components (b/a) is given in the ®nal column of Table 1; <1% of the initial signal is derived from trap (b). We conclude that there is only evidence for two traps containing the natural electron population in our sample. The main trap (a), which accounts for

>99% of the initial OSL signal, has E = 1.8820.03 eV and log10S = 15.9 20.3, indistinguishable from those of the trap identi®ed by Smith et al. (1990), who assigned values of E = 1.8420.07 eV and log10S = 15.3 based on their isothermal decay data. This is in contrast to the sample described by Huntley et al. (1996), which appeared to require four traps to explain the observations. Our results for trap (a) suggest a lifetime of about 850 Ma at 208C, compared with the 600 Ma predicted by Smith et al. (1990).

5. EFFECT OF PRIOR HEATING ON SHAPE OF OSL DECAY CURVES OBTAINED AT 1258C In the following section the behaviour of the initial and integrated (0±100 s) OSL signal after pre-

Fig. 3. (a) OSL stimulation curves for natural OSL obtained at 1258C following a 10 s preheat at temperatures from 1108C to 3608C. Only the ®rst 60 s of the 100 s of stimulation is shown. Samples were normalised using a 0.1 s OSL measurement at 258C prior to the preheat. (b) First 0.4 s OSL plotted against integrated (0±100 s) OSL signal less ®rst 0.4 s OSL, all obtained at 1258C after 10 s preheating at a given temperature. The labels on data points give the preheat temperature in 8C. (c) Last 0.4 s OSL (99.6±100 s) signal plotted against ®rst 0.4 s OSL signal, all obtained at 1258C after 10 s preheat at the given temperature. The labels on data points give the preheat temperature in 8C. (d) Variation in integrated (0±100 s, circles), ®rst (0±0.4 s, squares) and last (99.6±100 s, triangles) OSL signals obtained at 1258C after 10 s preheat, as a function of stimulation temperature (logarithmic plot).

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A. G. WINTLE and A. S. MURRAY

heats for 10 s at various temperatures is presented. This provides an understanding of the charge transfer brought about by preheating and its e€ects on the OSL signal. In the ®rst two experiments, data are obtained under continuous optical stimulation at 1258C, ®rstly for the natural signal and secondly for samples which have been optically bleached and given a laboratory dose. Comparison of these two data sets is relevant to measurements made in multiple aliquot dating studies.

5.1. Continuous stimulation of natural OSL Using natural samples of WIDG8, we have studied the e€ect of 10 s preheats at temperatures from 1108C to 3908C on the shape of the decay curves obtained subsequently. OSL measurements were made at 1258C to minimise any e€ects of the 1108C TL peak. Selected decay curves (normalised using the OSL from a 0.1 s stimulation before preheating) are shown in Fig. 3(a). There is no systematic e€ect on OSL curve shapes until a preheat temperature of >2808C is used. A plot of the ®rst 0.4 s OSL vs the integrated OSL signal (0±100 s, less the ®rst 0.4 s to avoid auto-correlation) shows a linear relationship

for preheats up to 3408C [Fig. 3(b)], with a small intercept (01% of the initial integral) presumably due to the inclusion of a signal coming from peaks above 3258C. The slope of 32.521.1% up to 3408C is similar to the slope of 31% seen in the relationship between the initial and integrated OSL for stimulation at elevated temperatures (Murray and Wintle, 1998). This suggests that a single trap/luminescence centre combination is responsible for the majority of this OSL, up to about 3408C. In Fig. 3(c), the OSL signals from the last 0.4 s are compared with the ®rst 0.4 s signal. Within the reproducibility of the last 0.4 s measurements (<150 counts), the relationship is linear up to a preheat temperature of 3208C. The intercept of 602 8 on the vertical axis is made up of instrument background (5 counts/0.4 s) and a contribution from the traps above 3258C, which include the dicult-to-bleach 3758C trap. This intercept amounts to about 40% of the last channel signal after a low temperature preheat and 100 s of stimulation. The e€ect of the preheat on the various parts of the OSL signal as a function of preheat temperature, and relative to the 1108C preheat data, can be seen in Fig. 3(d). There is no signi®cant change in

Fig. 4. (a) OSL stimulation curves for samples given a 43 Gy dose after blue/green-light bleaching for 200 s at 1258C to erase the natural OSL. Measured OSL was stimulated for 100 s at 1258C following a 10 s preheat at temperatures from 1108C to 3608C. (b) First 0.4 s OSL plotted against integrated (0± 100 s) OSL signal less ®rst 0.4 s OSL, all obtained at 1258C after 10 s preheat at a given temperature. A dose of 43, Gy was given as described in caption to Fig. 4(a). The labels on data points give the preheat temperature in 8C. (c) Last 0.4 s OSL (99.6±100 s) signal plotted against ®rst 0.4 s signal, all obtained at 1258C after 10 s preheat at the given temperature. A dose of 43 Gy was given as described in caption to Fig. 4(a). The labels on data points give the preheat temperature in 8C. (d) Variation in integrated (0±100 s), ®rst (0±0.4 s) and last 0.4 s (99.6±100 s) OSL signals obtained at 1258C after 10 s preheat, as a function of preheat temperature [logarithmic plot, symbols as in Fig. 3(d)]. A dose of 43 Gy was given as described in caption to Fig. 4(a).

OSL DATING OF QUARTZ the 100 s integral and the initial OSL until a preheat of 2708C is exceeded. This appears to con®rm the report by Franklin et al. (1995) that the natural OSL integral is una€ected by preheating up to 2408C. Murray et al. (1997) have also observed in their single-aliquot study that the natural OSL signal is una€ected by preheats <2608C (of 10 s duration). However, this does not necessarily mean that there is no e€ect on the main OSL trap. Using a fully bleached sample of WIDG8, Wintle and Murray (1997) have shown that there is a 25% increase in the OSL luminescence sensitivity (stimulated at room temperature) and in the 1108C TL luminescence sensitivity between about 2008C and 2708C; we show later that this result also applies qualitatively to stimulation at 1258C. The OSL trap must lose a corresponding fraction of the trapped electron population by 2708C for the overall signal to remain approximately constant. It should also be stressed that the responses at elevated temperature (Murray and Wintle, 1998) cannot be compared directly with the current data set, since for each measurement made at elevated temperature, the absolute values will be lower because of thermal quenching. 5.2. Continuous stimulation of regenerated OSL A similar experiment was carried out in which aliquots were ®rst bleached with blue/green light for 200 s at 1258C to erase the natural OSL signal, and then were each given a dose of 43 Gy to regenerate the OSL. Preheats for 10 s were carried out at several temperatures from 1108C to 3808C and the OSL decay curves were measured at 1258C [Fig. 4(a)]. The results for the ®rst 0.4 s and the last 0.4 s are compared with the integrated values for 0 to 100 s in Fig. 4(d). The slope of the correlation between the ®rst 0.4 s and the integral (less ®rst 0.4 s) is 37.6 21.7%, and it extends from 1108C to 3208C [Fig. 4(b)]. In this experiment all shallow traps had been ®lled by the regeneration dose. Nevertheless, the correlation between the ®rst 0.4 s and the 100 s integral is very similar to that of the natural sample [Fig. 3(b)] and to those data sets collected at elevated temperatures (Wintle and Murray, 1997). This suggests that the ®xed trap/luminescence centre combination is common to all data sets, whether or not the shallow traps above 1258C (e.g. at 1608C and 2608C) were previously ®lled. The 1108C TL trap does not play a role, since stimulation in Figs 3 and 4 was at 1258C. 5.3. Comparison of natural and regenerated OSL signals Comparison of Fig. 3(b) and 4(b) suggests that the direct contribution to the majority of the OSL

87

signal from charge trapped in shallow traps prior to initial illumination is not signi®cantÐthe regenerated data set shows a similar linear relationship between the initial and integrated OSL signals to that of the natural data, from 1108C to 3208C. Compared with the data in Fig. 3(d) for the natural signal, which show little variation in OSL up to preheat temperatures of 3008C, Fig. 4(d) shows a signi®cant rise (050%) in OSL with preheating up to 2808C in both the ®rst channel and the integral. Wintle and Murray (1997) argue that this increase is likely to be the result of trapped charge from the shallower traps being redistributed to the 3258C peak during the preheat, and we shall reconsider this argument in Section 7. In any case, direct comparison of the natural and regenerated OSL will result in a De which decreases with increasing preheat temperature. This behaviour was shown by Murray et al. (1997) for single-aliquot De measurements. Figure 4(c) shows that the OSL in the last 0.4 s of stimulation decreases rapidly with increasing preheat temperature from 1108C to 2808C, although that in the ®rst 0.4 s increases. Above 2808C the two signals decrease together with a slope of (0.021 20.002)%; this is similar to the slope of Fig. 3(c), and again suggests a common trap/luminescence centre combination. It is deduced that when shallow traps below 2808C contain a regeneration dose, they contribute about 50% to the OSL observed at 1258C after 100 s of illumination. This may be by transfer of charge to the main OSL traps, followed by restimulation and recombination, or by direct stimulation/recombination of electrons from the shallow traps. Such charge transfer would not be detected by the experiments described in Fig. 2, because they used natural aliquots; the data of Fig. 3 suggest that shallow traps do not contribute to the natural OSL, and so we conclude they must be e€ectively empty.

6. EFFECT OF PREHEAT ON LUMINESCENCE EFFICIENCY Aitken and Smith (1988) ®rst proposed the mechanism of thermal activation of the luminescence centres used by optically evicted electrons in quartz; they invoked this to explain an increase in sensitivity in their recuperation OSL as they heated quartz from a dune sand to progressively higher temperatures. The main change in sensitivity occurred between 2208C and 2808C, and the enhancement continued up to 6008C; these changes were con®rmed by monitoring the sensitivity changes using the OSL obtained from a small test dose. They commented that the increase in sensitivity of the 1108C TL peak followed that of the OSL signal closely, which they interpreted as suggesting a common mechanism. It has since been

88

A. G. WINTLE and A. S. MURRAY

shown that the 1108C TL peak and the OSL trap probably use the same luminescence recombination centre (Franklin et al., 1995). The relationship between the 1108C TL peak and the OSL signal has also been explored by Stoneham and Stokes (1991); they suggested that it might be possible to use the 1108C TL peak to monitor, and thus correct for, sensitivity changes which (a) result from preheating, and (b) are dose dependent. In an extension of this study, Stokes (1994) showed that in their sample the sensitivity change of the 1108C peak occurred after dosing plus preheating, whereas the sensitivity change of the OSL signal was seen only when optical bleaching preceded dosing plus preheating. He concluded that it may be possible to use the 1108C TL peak to correct for sensitivity changes in a single-aliquot regeneration protocol.

Wintle and Murray (1997) have also reported OSL sensitivity changes after preheating in WIDG8, when the OSL was measured using room temperature stimulation. Here, we examine changes in sensitivity in more detail using stimulation at 1258C. We ®rst bleached an aliquot with a 200 s blue/green light exposure at 1258C, preheated it at 1608C for 10 s, allowed the sample to cool for 5 min and gave it a test dose of 0.2 Gy. The sample was then heated to 1608C to measure the sensitivity of the 1108C TL peak, and optically stimulated at 1258C for 100 s with blue/green light to measure the OSL sensitivity. The process was repeated on the same aliquot using preheats which were increased by 108C each cycle, up to 5008C. Although bleaching for 200 s at 1258C e€ectively removes all the quickly decaying OSL, it is well known that a slower component will be left only

Fig. 5. (a) Variations in the sensitivity of: (i) the integrated OSL signal (0±100 s); (ii) the initial (®rst 0.4 s) OSL; and (iii) the ®nal OSL (last 0.4 s of stimulation, calculated as a tenth of the last 4 s) with preceding preheat temperature, in 108C steps. [Symbols are as used in Fig. 3(d).] A single sample was bleached for 200 s at 1258C. The 1108C TL and OSL sensitivities were then measured using a 0.2 Gy test dose; the sample was heated to 1608C to read out the 1108C TL peak, and was then held at the preheat temperature for 10 s. The OSL was measured subsequently at 1258C for 100 s, before the next test dose was administered, and the cycle repeated with the preheat temperature increased by 108C. An estimate of the underlying remnant signal from the natural dose was estimated by treating a second aliquot identically, except with the omission of the test doses. These background signals were subtracted to give the data shown. (b) First 0.4 s OSL plotted against the net OSL (0±100 s integral less a background estimated from the last 4 s of stimulation), all obtained at 1258C after 10 s preheat at a given temperature. The net 0.4 s signal has been subtracted from the net signal to avoid autocorrelation. A test dose of 0.2 Gy was given as described in caption to Fig. 5(a). The labels on data points give the preheat temperature in 8C. (c) Last 0.4 s OSL (99.6±100 s, calculated as a tenth of the last 4 s) signal plotted against ®rst 0.4 s signal, all obtained at 1258C after 10 s preheat at the given temperature. A test dose of 0.2 Gy was given as described in caption to Fig. 5(a). The labels on data points give the preheat temperature in 8C. (d) 1108C TL peak area (measured during preheat) plotted against ®rst 0.4 s OSL signal obtained at 1258C after 10 s preheat at the given temperature. A test dose of 0.2 Gy was given as described in caption to Fig. 5(a). The labels on data points give the preheat temperature in 8C. The straight line is ®tted to the 1608C to 3108C data.

OSL DATING OF QUARTZ partially bleached (e.g. Murray, 1996) Because it is derived from a large dose (the natural, 058 Gy), this slow component can contribute signi®cantly to the OSL signal derived from the subsequent 0.2 Gy test dose. To allow for this, a further sample was treated identically to the ®rst, except that the test dose irradiation was omitted. The two data sets were normalised using the natural signal from the initial stimulation, and then the appropriate components (see below) of the second data set were subtracted from the corresponding values in the data obtained using the test dose. The resulting signals from various parts of the OSL decay curve are shown against previous preheat temperature in Fig. 5(a): the full OSL signal (integrated from 0 to 100 s); the initial (®rst 0.4 s) OSL; and the ®nal OSL (last 0.4 s of stimulation, calculated as a tenth of the last 4 s). The ®rst 0.4 s and the integrated signal show an initial slight decrease, before a signi®cant increase commencing at about 2508C. There is then again a small decrease in both signals commencing at about 3308C. Only the integral signal then increases again, towards 5008C. The initial OSL is compared with the net OSL (i.e. the 0±100 s OSL less a background calculated from the last 4 s of stimulation) in Fig. 5(b). The net content of the ®rst 0.4 s have also been subtracted from the net signal, to avoid auto-correlation. The two signals behave very similarly, although there may be an intercept on the vertical axis. The sensitivity changes of the ®rst 0.4 s OSL thus vary in the same way with temperature as those of the integrated signal. However, the sensitivity of the slow component (i.e. that found in the last 0.4 s of stimulation) does not change in the same manner as the initial 0.4 s OSL (or the net OSL signal)Ðsee Fig. 5(a) and (c). It decreases slowly to about 3108C, and then increases by more than an order of magnitude with increasing preheat temperature. Some part of these changes in sensitivity could result from changes, with increasing preheat temperature, in the relative trapping cross-sections of the OSL trap and competing traps (such as the 1108C TL trap), during irradiation with the test dose. Figure 5(d) shows the 1108C TL peak area plotted against the ®rst 0.4 s OSL signal. There is a good linear relationship between the TL peak area and the initial OSL signal, for preheats up to and including 3108C. A 10 s preheat at this temperature is sucient to thermally empty the 3258C region of the TL glow curve. This sample had been optically bleached before being heated to this temperature. However, Wintle and Murray (1997) have shown that about 50% of the natural TL at 3258C remains after the optical signal has been fully bleached. It may be that the thermally stimulated recombination of these electrons contributes to the decrease in sensitivity (McKeever, 1994) observed for preheats above

89

3208C. Whatever the reason, we conclude that there has been no signi®cant change in the relative trapping cross-sections of the 1108C TL trap and the OSL trap for preheats up to 3108C, and only a small change for preheats between 3308C and 5008C. Since the slope of the 1108C TL against ®rst 0.4 s OSL data above 3308C is parallel to that up to 3108C, we further conclude that this small change occurs at about 3208C, and above this temperature remains constant.

7. COMBINED MEASUREMENT OF SENSITIVITY CHANGE AND THERMAL EROSION OF OSL TRAP Wintle and Murray (1997) used sensitivity change data obtained using the integrated signal to correct the integrated OSL signal from both the natural and regenerated samples shown in Fig. 3(d) and 4(d); however, the data (reproduced on a linear scale in Fig. 8 of Wintle and Murray, 1997) were relatively imprecise because they were obtained from a suite of aliquots. From Fig. 5(b), we note that the initial OSL signal correlates well with the net signal. Hence, by using only a very short illumination, which does not signi®cantly deplete the total OSL signal, all measurements of thermal erosion/transfer can be made on a single aliquot, with corresponding improvements in precision. These measurements were made using a single natural aliquot, which was given a short illumination (0.1 s with reduced aperture) at 1258C and preheated to 1608C for 10 s. The illumination is estimated to reduce the OSL signal by <0.2%, and so corrections for optically stimulated decay are negligible. The measurement sequence was then repeated, with the preheat temperature increasing in 108C steps until 5008C. Another aliquot was given an initial short illumination at 1258C (for normalisation), and then illuminated for 200 s at 1258C, to empty the natural dose in the OSL trap. After giving a laboratory dose of 56 Gy, the measurement cycle used on the natural aliquot was repeated. These observed 0.1 s OSL signals were normalised using the initial 0.1 s OSL obtained prior to any treatment, and are presented in Fig. 6(a). The data show a broadly similar pattern to the integral data of Fig. 3(d) and 4(d) (also reproduced in Wintle and Murray, 1997, Fig. 8, on a linear scale), but with less scatter. The initial (0.4 s) sensitivity change data for stimulation at 1258C [Fig. 5(a)] was then used to correct these initial OSL signals. The sensitivity change data are reproduced on a linear scale in Fig. 6(b), and the corrected data are given in Fig. 6(c). The shape of the resulting curves remains largely unchanged compared with those in Fig. 6(a), although the rapid decrease above 2508C becomes slightly less steep after correction.

90

A. G. WINTLE and A. S. MURRAY was given an initial short illumination (for normalisation), and then illuminated for 200 s at 1258C, to empty the natural dose in the OSL trap. A laboratory dose of 56 Gy was then administered, and the full measurement cycle (up to 5008C) used on the natural aliquot was repeated. The normalised OSL signals observed during the 0.1 s illumination are shown in Fig. 7(a). These are essentially indistinguishable from those of Fig. 6(a), although even with normalisation, the relative intensities are slightly di€erent. The 1108C TL peak areas, scaled to unity for the natural sensitivity at 5008C, are shown in Fig. 7(b). The shape of the sensitivity change curve observed during the natural

Fig. 6. (a) Initial OSL (0.1 s stimulation) obtained during light exposure of one aliquot at 1258C after preheating for 10 s at temperatures from 1608C to 5008C. Optical decay is estimated at <0.2% per stimulation. Open circles, natural luminescence. Solid circles, the aliquot was optically bleached at 1258C for 200 s and then given a 56 Gy dose to regenerate the OSL signal before measurement. The two curves have been normalised using the OSL from a 0.1 s stimulation of the natural signal of each aliquot prior to any treatment. The solid lines represent the best ®ts of [3], as described in the text. (b) The initial (0.4 s stimulation) OSL signal as a function of the previous 10 s preheat temperature taken from Fig. 5(a) but plotted on a linear scale. This data set provides a monitor of the change in sensitivity of the luminescence centres as a result of the previous preheat. (c) OSL data from Fig. 6(a) corrected for thermal activation of luminescence centres using data from Fig. 6(b). Symbols as in Fig. 6(a). The solid lines represent the best ®ts of [3], as described in the text.

The observation that the change in 1108C TL sensitivity correlates well with the change in OSL sensitivity over the temperature range of interest [Fig. 5(d)] o€ers a more direct method of measuring the OSL sensitivity change during preheat. A single natural aliquot was given a short illumination (0.1 s with reduced aperture) at 1258C. The aliquot was then given a test dose of 0.1 Gy (<0.2% of the natural dose), and preheated to 1608C for 10 s. The 1108C TL peak area was measured during the preheat. The measurement sequence was then repeated, with the preheat temperature increasing in 108C steps until 5008C. The cumulative test dose was 3.9 Gy (07% of the natural dose). A further aliquot

Fig. 7. (a) Initial OSL (0.1 s stimulation) obtained during light exposure of one aliquot at 1258C after preheating for 10 s at temperatures from 1608C to 5008C. Optical decay is estimated at <0.2% per stimulation. A 0.1 Gy test dose was given after each OSL measurement, and the sample heated to 1608C to measure the 1108C TL peak. Open circles, natural luminescence. Solid circles, the aliquot was optically bleached at 1258C for 200 s and then given a 56 Gy dose to regenerate the OSL signal before measurement. The two curves have been normalised using the OSL from a 0.1 s stimulation of each aliquot prior to any treatment. The solid lines represent the best ®ts of [3], as described in the text. (b) Area of 1108C TL peak measured following 0.1 Gy test dose given as described in (a). Symbols as in (a). The areas have been normalised to unity for the natural result following a 5008C preheat. (c) OSL data from (a) corrected for thermal activation o¯uminescence centres using data from Fig. 7(b). Symbols as in (a). The solid lines represent the best ®ts of [3], as described in the text.

OSL DATING OF QUARTZ aliquot measurement is similar to that of Fig. 6(b) (observed on a fully bleached sample) although the amplitude is smaller. However, the shape of the sensitivity change observed during measurement of the regenerated data is markedly di€erent at low temperatures; the overall sensitivity increases by about three times from 1608C to 3008C. Above 3508C, the shape is similar to that of the natural data, and at 5008C, the sensitivity change for the regenerated aliquot is only 10% higher than that of the natural aliquot. The data of Fig. 7(a) were then corrected for the apparent sensitivity changes described in Fig. 7(b) to give the data of Fig. 7(c). Because our correction is intended to minimise any e€ects of changes in luminescence sensitivity with temperature, the corrected data are presumed to re¯ect only the movement of charge between traps. The shapes are signi®cantly di€erent from those of Fig. 6(a), (c) and 7(a), and there is now some suggestion of an increase in the natural signal, at about 2408C, which could re¯ect charge transfer from a shallow trap to the deep OSL trap. However, there is no such evidence in the regenerated data set; the corrected OSL signal decreases slightly at about 2008C, and then more rapidly above about 2508C. This is in contrast to the data of Fig. 6(c), and Fig. 8(c) of Wintle and Murray (1997), which appeared to show evidence for a signi®cant charge transfer around 2508C. It is dicult to identify any TL trap responsible for the initial drop in OSL below 2008C. There is a signi®cant light sensitive TL peak at 1508C [Fig. 1(c)], but this is at too low a temperature to be a likely candidate. The insets in Fig. 7(a) and (c) show the ratio of the natural to regenerated OSL signals as a function of preheat temperature (from 1608C to 3008C) for the uncorrected and corrected data, respectively. Comparison of these ratios as a function of temperature shows that (a) the ratio of the corrected data set is closer to unity, as expected for a regeneration dose of 56 Gy (De058 Gy), and (b) the variation with temperature is less for the corrected data set.

8. FURTHER ESTIMATES OF OSL TRAP DEPTH Huntley et al. (1996) used data similar to those of Fig. 6(a) and 7(a) to estimate trap depth, assuming a single trap. We have undertaken similar calculation on both the uncorrected [Figs 6 and 7(a)] and corrected [Fig. 6(c) and 7(c)] data sets. We begin by assuming that there are two electron traps, a shallow trap which does not give OSL, and a deeper OSL trap; the latter is identi®ed with the dominant trap (a) in Section 4, which is responsible for >99% of the initial OSL signal. We assume that, with time at a particular temperature, the shallow

91

trap empties, and that some of the electrons are retrapped by the OSL trap. Then the rate of change with time, t, in the number of trapped electrons, Ns, in the shallow trap can be described by dNs ˆ ÿls Ns : dt

…1†

The decay constant, ls, is assumed to be given by Ss exp(ÿEs/kBT), where kB is Boltzmann's constant, Es is the trap depth, and Ss is the frequency factor; the subscript s refers to the shallow trap. The rate of change in the number of trapped electrons, Nd, in the deeper OSL trap is then given by dNd ˆ ÿld Nd ‡ f ls Ns dt

…2†

where f is the fraction of electrons released from the shallow trap which are retrapped by the OSL trap. The decay constant ld is assumed to be given by Sd exp(ÿEd/kBT); the meanings are as above, except that the subscript d refers to the OSL trap. If we assume that the OSL signal, L, observed during a short illumination is proportional to Nd, the solution to [2] is L ˆ CL0d

l`s …eÿl`s t ÿ eÿld t † ‡ L0d eÿld t ld ÿ l`s

…3†

where L0d is the initial OSL signal (assumed ®nite), C is a constant, and l's=flS. At all elevated temperatures t = 10 s, the preheat time. (We assume here that the previous preheat history, at temperatures at least 108C lower, does not give rise to any e€ect additional to that of the current preheat.) This equation has been ®tted to both the corrected and uncorrected data sets of Figs 6 and 7, and the ®ts are shown there as solid lines. Note that to ®t the natural data in Fig. 6(a), (c) and 7(a), C was set to zero, as there was insucient evidence for two traps from the computer analysis. It is clear from examining the ®ts in Figs 6 and 7 that [3] provides an excellent description of all the data sets. The ®tted parameters for the corrected data of Fig. 6(c) and 7(c) are summarised in Table 2. Although the data are well described by [3], the parameter values are very sensitive to small features of the data sets, which in turn are very dependent on the sensitivity corrections. It would thus be unwise to place too much credence on the parameter values and uncertainties. The values obtained for the OSL trap may be compared with those obtained using isothermal decay by Smith et al. (1990), of log10S = 15.3, and E = 1.84 20.07 eV, and those derived in Section 4 of this paper, log10S = 15.9, and E = 1.88 20.03 eV.

9. DISCUSSION The values of E and S for the dominant OSL trap obtained from the isothermal decay exper-

A. G. WINTLE and A. S. MURRAY

92

Table 2. Results of ®tting [3] to thermal decay data of Fig. 6(c) and 7(c)

Corrected natural [Fig. 6(c)] Corrected regenerated data Corrected natural [Fig. 7(c)] Corrected regenerated data

Es, eV

log10(fSs) (fS in sÿ1)

Ed, eV

log10(Sd) (S in sÿ1)

C

Ð

Ð

1.562 0.04

13.1 20.4

Ð

0.8020.08

6.52 0.8

1.52 0.2

13 22

2.8 20.5

1.220.5

1225

1.542 0.03

12.9 20.3

0.08 20.02

1.6820.13

14.121.1

1.702 0.14

17.1 21.6

500 2300

Subscripts d and s refer to the deep OSL trap and a putative shallow trap, respectively.

iments agree with those obtained by Smith et al. (1990); this leads to a similar estimate of the lifetime at 208C. The two data sets suggest a lifetime in excess of 500 Ma, which should enable dating to be reliable up to at least 5 Ma, provided that trap saturation is not important. However, saturation may be the limiting factor if the behaviour reported by Huntley et al. (1996) for the OSL signal from a 122 ka aeolian quartz is typical. In this example, the natural OSL was close to 50% of the apparent saturation light level. When deriving our decay constants from the isothermal decay data of Fig. 2, we assumed that any sensitivity changes that may be taking place were complete within the ®rst 10 s of heating at a particular temperature. This assumption is common to all isothermal decay measurements reported for the OSL of quartz. A lifetime >500 Ma is considerably higher than values of 0.014, 2.0 and 28 Ma that were calculated by Huntley et al. (1996) for the three dominant components in their older sample, which they estimated to have received a dose of 350 Gy. However, all isothermal decay analyses may be a€ected by continuing luminescence sensitivity changes, which would be expected to be more severe for a sample at, or close to, saturation. Based on our data of Fig. 5(d), it may be possible to monitor such changes using the 1108C TL peak derived from a test dose administered after each period at elevated temperature. The two data sets showing the dependence of absolute natural and regenerated OSL on preheat temperature (Figs 3 and 4) suggest that a ®xed trap/ luminescence centre combination is responsible for the majority of OSL signal. Furthermore, the trap responsible is not thermally emptied by 10 s preheating at temperatures below 3008C. However, there is some evidence for a small contribution from shallower traps to the long term OSL signal [observed in Fig. 4(c) as the last 0.4 s OSL in a 100 s stimulation curve]. This is the only direct evidence we have for a contribution to OSL from a shallow trap (apart from the 1108C peak; our OSL measurements were made at 1258C). In Fig. 4(d), the regenerated OSL signal is seen to increase with preheat temperature, in contrast to the natural OSL data of Fig. 3(d). At ®rst glance,

these data may be explained by thermal transfer of charge from shallow electron traps to the deeper OSL trap. The data of Fig. 5(d) justi®es monitoring the preheat-dependent luminescence sensitivity changes of a fully bleached aliquot with the 1108C TL peak. We assume that this approach can be extended to natural and regenerated aliquots [Fig. 7(b)]. After correction for this sensitivity change, the low temperature di€erence in the natural and regenerated OSL curves [Fig. 7(a)] can be accounted for almost entirely [Fig. 7(c)]. At this point it is appropriate to consider why any preheat should be applied to quartz in dating protocols, since it appears that one of the main arguments, viz. the need for thermal transfer of charge from shallow traps to the deeper OSL, has been shown to be unimportant, at least in our sample. Godfrey-Smith (1994) showed that the OSL signal obtained after laboratory irradiation decreases with time, suggesting that there may be a component derived from shallow traps. It is possible that this component could be removed by storage at room temperature for about a week. However, the di€erence between the sensitivity of the natural and regenerated aliquots after a 1608C preheat [Fig. 7(b)] suggests that there is a fundamental di€erence in sensitivity of close to a factor of 2. This might be related to partial or complete activation of the sensitisation process occurring in nature. This could account for the pronounced di€erence between the two aliquots for preheat temperatures from 1508C to about 2508C [Fig. 7(b)]. If this is the case, then preheating is required to equally sensitise the natural and laboratory-induced signals. This cannot be achieved by any practical storage at room temperature. 10. CONCLUSIONS The experiments reported in this paper are concerned with the e€ect of preheating on both the trapped charge population and the luminescence eciency using a single Australian quartz. It is shown that these two characteristics are not independent. For this sample (with a De=5826 Gy) isothermal decay curves obtained at temperatures from

OSL DATING OF QUARTZ 1608C to 2808C show that more than 99% of the initial natural OSL signal is derived from a single trap. The trap depth and frequency factor, obtained by plotting the mean lifetimes against reciprocal temperature, are similar to those obtained by Smith et al. (1990) and predict a lifetime at 208C of around 850 million yr. The OSL decay curves for a natural sample were obtained at 1258C, to keep the 1108C trap empty. Using 10 s preheats, the natural signal showed a linear relationship between the initial (0.4 s) and integrated OSL signals for preheats up to 3408C, suggesting that a single trap/luminescence centre combination is responsible for 99% of the OSL signal. For 10 s preheats up to 2708C no signi®cant changes in the natural OSL signal occur, but between 2708C and 3408C the signal decreases rapidly. When the same decay curves were obtained for a regenerated (bleach plus dose equal to De) OSL signal, a similar relationship between the initial and integrated OSL signal was found. However both signals increased by about 50% when the preheat temperature was increased from 1608C to 2808C. Comparison of the natural and regenerated OSL signals as a function of preheat temperature gave rise to values of De which decrease with increasing temperature, with more credible values of De being found for higher temperature preheats. When using small test doses (0.2 Gy) on a bleached sample, both the initial and integrated OSL signals showed sensitivity increases with increasing preheat temperature from 2308C to 3108C, and they were linearly related to the 1108C TL peak from the 0.2 Gy test dose. We therefore assume that the 1108C TL peak from a small test dose can be used to monitor luminescence sensitivity changes in a single aliquot, containing either a natural or a regenerative dose. For this sample, the observed sensitivity changes for the natural and regenerated aliquots were quite di€erent. particularly from 1608C to 2808C. In this temperature region the regenerated OSL showed a rise of more than twice that of the natural. Application of the individual 1108C TL measurements to the OSL data set resulted in much less variation in the value of De with preheat temperature, and a value of De which is much closer to that given by multiple aliquot measurements. From the above, we suggest that it is possible to use the response of the 1108C TL peak to monitor the sensitivity change as a function of preheat temperature. This can be applied to two discs, one natural and one regenerated, to obtain De as a function of temperature. This method of monitoring the sensitivity change could also be incorporated into a simple regenerated-dose single-aliquot procedure, in which the natural OSL is compared with the regenerated OSL from a known dose.

93

AcknowledgementsÐThe pre-treated sample of WIDG8 was generously provided by R. G. Roberts. Fundamental luminescence research in Aberystwyth is currently supported by NERC grant GST/02/0762 and this is LOIS publication number 344.

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Wintle, A. G. (1975) Thermal quenching of thermoluminescence in quartz. Geophysical Journal of the Royal Astronomical Society 41, 107±113. Wintle, A. G. (1993) Luminescence dating of sands: an overview. In The Dynamics and Environmental Context of Aeolian Sedimentary Systems. Geological Society Special Publication, ed. K. Pye, 72, pp. 49± 58. Wintle, A. G. and Murray, A. S. (1997) The relationship between quartz thermoluminescence, phototransferred-thermoluminescence and optically stimulated luminescence. Radiation Measurement 27, 611±624. Wolfe, A. S., Huntley, D. J. and Ollerhead, J. (1994) Recent and late Holocene sand dune activity in southwestern Saskatchewan. In Current Research of the Geological Society of Canada B, 131±140. Zimmerman, J. (1971) The radiation-induced increase of thermoluminescence sensitivity of ®red quartz. Journal of Physics C; Solid State Physics 4, 3277± 3291.