Partial bleaching and the decay form characteristics of quartz OSL

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Pergamon

Radiation Measurements,Vol. 27, No. 2, pp. 123-136, 1997 © 1997 ElsevierScienceLtd. All rights reserved Printed in Great Britain PII: S1350.-4487(96)00157-6 1350-4487/97 $17.00+ 0.00

PARTIAL BLEACHING AND THE DECAY FORM CHARACTERISTICS OF QUARTZ OSL R. M. BAILEY, B. W SMITH* and E. J. RHODES Department of Geography, Royal Holloway, University of London, Surrey TW20 0EX, U.K. Abstract--Three exponential components have been isolated from observed high temperature OSL decay data of quartz. These components have been found to display differential bleaching and growth characteristics. It is postulated that changes in the ratio of the first two components (the "fast" and the "medium") are responsible for the changes in decay form observed in partially bleached samples. Measurements have shown that a comparison of the observed ratios to the expected ratios (had a sample been fully bleached) may be able to differentiate sediments for which the OSL has only been partially reset prior to deposition. © 1997 Elsevier Science Ltd

1. INTRODUCTION A fundamental requirement of any dating technique is that the time dependent signal measured is set either to zero or to a known value at the time of the event to be dated. In the OSL dating of quartz, the signal is reset during the exposure of the sample to light. Clearly, if the duration of the exposnre is insufficient for full resetting, the measured signal will not be a true reflection of the elapsed time since the event of interest. Events where material is transported in low light conditions and/or over short distances may be difficult to date because of these problems. The ability to recognize samples where the OSL signal was partially reset (or "partially bleached") prior to deposition is thus vital to those using the OSL technique to date materials reset by light. Also it is important that samples inadvertently exposed to light during sample collection or preparation be identified. The purpose of this work was to study the characteristics of the quartz OSL signal and, based on these findings, to assess the possibility of finding a reliable means of identifying sediments that have been only partly reset prior to deposition. Any means of identifying partially bleached sediments should fulfill certain requirements. First of all, there is the need for some property of the signal to have changed following partial bleaching. Secondly, it is necessary that remnants of this change be identifiable after subsequent additional dose during the burial period. In this paper we describe experiments which demonstrate that the OSL decay in quartz comprises three components (as proposed in Smith and Rhodes, 1994) which are each affected to a different degree by

partial bleaching. Laboratory measurements and theoretical calculations suggest that this difference is sufficient to allow the detection of samples which were only partially bleached on deposition. 2. INTERPRETATIONS OF THE OSL DECAY F O R M

2. t. Observations of the OSL decay form All measurements except those noted separately were made on an automated Riso-set (TL-DA-12), using a filtered halogen lamp light source (constant for all measurements), stimulating in a band between 420 and 560 nm (2.9-2.2 eV, nominally 12 mW.cm 2) and filtering the emissions with a 1 mm BG39 and two U340 glass filters. A broad band of stimulating wavelengths is likely to cause a slight spread in the OSL decay rates measured (Section 3). However, a comparison of these results to those obtained using 514.5 nm monochromatic laser light shows the effect to be small. The possible complicating effects on the decay form of grains shading one another from the incident stimulation light, leading to a range of incident powers (and therefore detrapping rates), has been investigated and shown to be insignificant. Other factors which may also contribute to a spreading of incident powers, such as non-uniform illumination and variations in bulb intensity, are not believed to be significant. Figure 1 shows the results of an experiment in which individual aliquots of natural Chaperon Rouge quartz (natural dose of about 12 Gy) were exposed to white light for a range of durations and preheated (at 220°C for 5 rain) prior to OSL measurement at room temperature. There is a trend in the data for the decay

* Present address: WSD, DSTO, PO Box 1500, Salisbury 5108, Australia. RM 27/2

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R . M . BAILEY et al.

124

to become slower (flatter) following lengthier partial bleaches. Similar effects were also observed if filtered green light was used for partial bleaching (supporting the findings of Rhodes, 1990, where 514rim stimulation was used). A second observation is illustrated in Fig. 2(b), which shows the decay of the measured OSL signal from the same sample held at 160°C during measurement. When a sample is held above 100°C, any charge transferred to TL traps in the 100'C region is immediately evicted, simplifying the form of the OSL observed. The chosen temperature for OSL measurement must be sufficiently low that thermal quenching effects are minimal and that the eviction of high temperature TL during measurement is insignificant, whilst being sufficiently high to minimise the effects of retrapping to lower temperature TL traps during OSL measurement (especially TL traps in the 10OC TL region). A temperature of 160C was found to be a good compromise. No detectable signals were observed when samples were held at 160':'Cfor 1000 s either without illumination or whilst illuminated with infra-red light. It can be seen from Fig. 2(a,b) that the OSL decay is not a simple exponential decay but can be described by the sum of three exponential functions (Smith and Rhodes, 1994). In the discussion which follows we call these the "'fast", "medium" and "slow"

components. There are two possible interpretations for the observation that the OSL decay is not a single exponential. In the first interpretation the stimulating light initially releases charge from a single type of trap with a single bleachability or rate of charge release. Non-first order effects (e.g. charge retrapping) must then be deemed responsible for the non-exponential decay of the OSL. The second interpretation is to consider that the effect of the non-first order processes on the observed OSL decay is minor, and that the decay represents the release of charge from traps with different rates of charge loss. If the first interpretation is correct, it seems unlikely that the occurrence of partial bleaching could be detected. However, the second interpretation would suggest that the observed form of OSL decay would change depending on the degree of bleaching, and that partial bleaching should be detectable. The two interpretations are discussed in more detail below. 2.2. Single OSL trap interpretation From Fig. 2(a,b) it is evident that if the OSL originates from charge released from a single trap type at a single rate, there must be significant non-first order effects for the OSL decay to be so far removed from a single exponential. Such effects could

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Fig. 1. Changes in room temperature decay form following partial bleaching. Natural Chaperon Rouge quartz was exposed to natural daylight and then preheated (220°C for 5 rain). The percentage of the natural signal intensity present following preheating, as monitored by 0.1 s room temperature OSL measurements, is shown. Following background subtraction (about 0.4% of the natural initial intensity), the data were normalised to the same initial intensity (I0 = 1) to allow a direct comparison of the decay forms (each curve being the sum of four similar aliquots).

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Fig. 2. (a) The OSL decay measured from a natural Chaperon Rouge quartz sample held at 160°C during OSL measurement can be accurately described by the sum of the three exponential components shown. (b) A superimposition of the summed modelled exponential components [shown in (a)] on to the observed data.

be retrapping in the same traps, in other bleachable traps, or in traps which are thermally unstable at the 160°C measurement temperature. A gradual increase

in the availability of recombination centres through the OSL measurement (i.e. increased OSL sensitivity) could also explain the observed OSL decay form.

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Smith and Rhodes (1994) showed that there was significant retrapping in the traps responsible for the OSL in quartz, but such retrapping will only produce a non-exponential decay if the paths available to the released charge change in their relative proportions during the OSL measurement (see the Appendix). A change in the proportions of charge distribution may occur if any of the traps, recombination centres or killer centres approach saturation. Saturation of the OSL traps or luminescence centres appears unlikely given the measurement of relatively high OSL saturation levels (with respect to the natural OSL) and minimal luminescence sensitivity changes through the OSL decay. Other measurements (Fig. 3) show that transfer rates of charge to traps in the l l 0 C TL region are about the same at both the beginning and end of the OSL decay (Smith and Rhodes, 1994). This suggests that the routes available to detrapped charge do not change proportionally during the OSL measurement. Hence changes in trapping rates are unlikely to be responsible for the non-exponential nature of the OSL decay form shown in Fig. 2(b). The proposed mechanism of a gradual increase in the availability of recombination centres through the OSL measurement is also discounted for the same reason.

The possibility of a constant rate of trapping in other bleachable or thermally unstable traps cannot be completely discounted. The 160c'C measurement temperature removes the influence of the TL traps in the 100+C region, but other traps could be involved. The phototransfer of charge to thermally unstable traps can be discounted as no phosphorescence is observed after the optical stimulation is halted, whatever the length of exposure the sample has been given. It is difficult to fully discount the possibility of phototransfer to traps which are thermally stable, but susceptible to optical stimulation. However, examination of the TL after progressive bleaching periods (e.g. Smith and Rhodes, 1994, Figure 1) shows only a net loss of charge throughout the 200-500°C TL region. The slow component of the OSL emission shown in Fig. 2 represents a considerable amount of charge which would be expected to be evident as phototransferred TL if thermal or optical eviction of charge from unstable traps were the source of the slow component. After considering the evidence above, we believe that a single trap type is unlikely to be responsible for the OSL measured. The results shown in Fig. 1 and in other experiments described later in this paper appear to confirm this conclusion.

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Exposure Time (s) Fig. 3. The ratio of integrated phototransferred TL in the 110C peak to the integrated OSL emission during the laser exposure which produced the TL peak, plotted as a function of exposure time. The OSL measurementswere made using an excitation wavelengthof 514.5 nm at 2. l mW-cm- -'with the Chaperon Rouge quartz sample held at 17°C. The TL measurements began 1 minute after completion of the laser exposure. The decrease in ratio as a function of exposure is thought to be due to the thermal and optical detrapping of phototransferred charge in the 110~CTL trap which occurs during laser exposure. A repeat short laser exposure and measurement of the phototransferred 110C TL gives a ratio equivalent to that initiallyobserved for short exposures, independent of the previous bleach time. Similarcharacteristics have been measured for a second quartz sample (Lake Woods, Australia), except that the ratio for short shine exposures is only 0.045.

CHARACTERISTICS OF Q U A R T Z OSL

127

Table 1. The half lives for the fast, medium and slow components from three different quartz samples each measured at 160°C (natural OSL). These half lives are for the standard Riso set halogen lamp power (nominally 12 mW.cm-:) used throughout this paper Sample Chaperon Rouge Chebba Hergla

Half life (s) under illumination at 160°C Fast

Medium

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2.34 4- 0.28 2.29 ___0.37 2.72 4- 0.74

7.88 4- 1.02 7.65 + 1.66 7.60 4- 1.94

775 4- 46.5 883 4- 42.9 705 + 54.5

2.3. Multiple OSL trap interpretation If non-first order effects are discounted as a major contributor to the form of OSL decay shown in Fig. 2(b), then the decay form most likely results from the release of charge from several trap types which each have different rates of charge loss (bleachabilities). If the decay rates from each of these trap types remain constant through the OSL measurement, the OSL decay should comprise several exponential components. Smith and Rhodes (1994) found that the 514.5 nm stimulated OSL from Chaperon Rouge quartz could be adequately described by three exponential components, and this is also demonstrated in Fig. 2 using the Riso-set system (420-560 nm stimulation). The existence of three exponential components does not rule out non-first order effects in this sample. Indeed, as stated above, Smith and Rhodes 11994) found significant retrapping in the traps responsible for the OSL. These non-first order processes will change the net decay rates, but will still give an exponential decay for each component unless, as discussed above and in the Appendix, the paths available to the released charge change in their relative proportions during the OSL measurement. Note that the interpretation of multiple OSL components does not necessarily imply significantly different TL properties for each component. Smith et al. (1986) found that the OSL in quartz originates principally from the 325°C TL trap. The usual interpretation of OSL emission (as illustrated in Figure 10 of Spooner, 1994), is that the charge in the 325°C TL trap has access to excited levels to which it can be thermally raised. The electrons in excited levels can then be raised to the conduction band (i.e. evicted from the trap) by optical stimulation. We propose that the available excited levels may vary depending on the exact location of the trap within the lattice (i.e. the local electronic environment). The available number of excited levels, or

perhaps their spacing, will affect the probability of optical excitation and hence determine the rate of OSL signal depletion. However, the measured TL properties will be similar (but not identical). Note that we associate only the first two OSL components with the rapidly bleachable region of the TL glowcurve (centred on 325°C) and suggest that there are two different OSL traps, each with different available excited levels. Measurements by Smith and Rhodes (1994) on the bleaching of the 325°C TL peak showed that the TL also contains faster and slower bleaching components, with the faster component bleaching at about the same rate as observed for the fast component of the OSL. Further discussion on the kinetics of the individual components is given in Section 4.

3. THE EXPONENTIAL COMPONENTS

3.1. Isolation of the three components Custom written software was used to perform a stripping procedure in which three individual exponential decays were isolated from the observed data from samples measured at 160°C. Table 1 shows the half lives of the components in each of three natural quartz samples. The Chebba and Hergla samples are from oxygen isotope stage 5e Mediterranean raised beach deposits (Wood, 1994), samples C319 and H335 respectively, courtesy of P. B. Wood. The three exponential components show good consistency in the decay rates between each of the samples. Table 2 lists the contribution of each component to the total (integrated) OSL. The total contribution of the slow component could not be directly measured because of its slow decay rate, but a calculation was made of the total OSL emission assuming exponential decay. Note that the ratio of fast to medium components varies significantly from sample to sample. We interpret this as normal

Table 2. Fractional contributions of each of ~he three exponential components to the total observed signal measured at 160°C Sample Chaperon Rouge Chebba Hergla

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0.75 4- 0.01 0.46 4- 0.07 0.54 4- 0.01

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R . M . BAILEY et al.

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variation between samples, dependent on defect levels. As can be seen in Table 1 and Fig. 2, the bleaching rate of the slow component is considerably less than the other two components. When the OSL is measured at room temperature the decay of the slow component is reduced further. For relatively short OSL measurements at room temperature the decay of the slow component is negligible and it can be considered to contribute a constant level of OSL. Hence the variation in the OSL decay form illustrated in Fig. 1 must originate from changes in OSL from the fast and medium components only. 3.2. The fast and medium components Another experiment measured the relative proportions of the fast and medium OSL components remaining after varied bleaching periods (Fig. 4). It can be seen that the relative proportion of the fast component decreases with increasing bleaching time. It is suggested therefore that the observed changes in OSL decay form measured at room temperature following partial bleaching (Fig. I) are a function of a progressive change in the ratio of signal donated by the fast and medium components. As the duration of the partial bleach increases, a greater proportion of the fast signal is depleted than is the case for the medium component. The decay form will therefore become flatter as the overall (combined) decay is slower. It is shown in Fig. 4 that the fast component still apparently contributes 60% of the fast plus medium

signal following extended bleaching periods at room temperature. This is interpreted to be due to repopulation of the fast component by recuperation during the preheat (Aitken and Smith, 1988). When the sample is bleached at room temperature, a portion of the released charge is retrapped in the 100~C TL region. During the preheat this charge is redistributed and a significant amount is deposited in the traps responsible for OSL. It could be expected that the relative proportions retrapped in the fast and medium components will approximate that observed in the natural sample (i.e. about nine times more to the fast than to the medium component for Chaperon Rouge quartz). For short bleach times the recuperated signal is a relatively small contribution to the measured OSL. Figure 4 indicates that recuperation has a significant effect on the measured ratio of fast to medium components when the bleach time exceeds about 20 s. This is consistent with experiments which show the recuperation OSL of Chaperon Rouge after an extended bleach to be equal to about 4.5% of the natural signal (Smith and Rhodes, 1994), and Fig. 2(a) which indicates that after 20 s the fast plus medium components of the natural OSL have also reduced to about this level. Hence the recuperated component of OSL would be expected to be prominent for exposures over 20 s. If the bleach time is extended beyond the 250 s point shown in Fig. 4, the percentage of fast plus medium component OSL contributed by the fast component again rises towards 90%. This indicates that the OSL decay form of the recuperated OSL

CHARACTERISTICS O F Q U A R T Z OSL closely matches that observed in the natural sample (as demonstrated by Aitken and Smith, 1988, Figure 2, for the same sample). Hence, after a long bleach the measured ratio of fast to medium components would suggest that the sample had been well bleached-as indeed it had just been. As illustrated above, the overall effect of recuperation is to dilute the change in OSL decay form that we are trying to detect. Detection of partial bleaching may therefore be more difficult in samples which show large recuperation effects (as discussed in Section 6.3).

129

BG39 filter) and hence the decay reported is deficient of the slow component. The longer emission wavelength of the slow component suggests a source of charge different from the fast and medium components (as it obviously recombines at different luminescence centres). Other characteristics of the slow component are its thermal stability (to >650°C) and its activation when the sample is heated prior to OSL measurement (Aitken et al., 1989). Figure 5 shows the results of an experiment where a single sample of quartz grains was first given an extended light exposure to remove the fast and medium components, and then successively heated to higher temperatures with the OSL monitored (at 140°C) between each heating. The small increase to 260°C is interpreted as due to recuperation of the OSL by charge thermally released in the 140-260°C region. The fast and medium components are clearly evident in the OSL decay after preheats in this range. Beyond a 320°C preheat the OSL is presumed to originate from the slow component only, and indeed the decay rate of the OSL is the same as the slow component decay rate prior to any preheating. The more significant increase in OSL for preheats above 400°C shows a striking resemblance to the

3.3. The slow component There are several characteristics of the slow component which set it apart from the fast and medium components. The first is that the slow component results from OSL emitted at a longer wavelength (Smith et al., 1990; Smith and Rhodes, 1994). This is clearly demonstrated by comparing the OSL in Figure 2 of Smith and Rhodes (1994) with that for the same sample (and stimulating wavelength) given in Figure 3 of Spooner (1994). Spooner used a more efficient red-rejecting filter combination (a 2 mm Schot BG39 and 5.1 mm Corning 7-51 instead of four Corning 7-59 filters and a 0 5 mm

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Fig. 5. A natural aliquot of Chaperon Rouge quartz grains was initially given an 11 min exposure to 514.5 nm light (Ar ion laser at the Oxford laboratory) at 7 mW.cm -2 while the sample was held at 140°C. The OSL was reduced to 0.2% of the initial level (to 538 cps as shown at the 140°C point). At 30 s after this exposure the sample was heated to 161°C at 5°C-s- ~and immediately cooled back to 140°C at which temperature another OSL measurement (usually 2 min) was made. The procedure was repeated with heating to successively higher temperatures. The OSL plotted is the level initially measured after preheating to the temperature shown. The OSL is influenced by recuperation of the fast and medium components until the preheat rises above 340°C. After heating beyond this temperature the OSL consists purely of the slow component which shows a considerable sensitivity increase with heating. The sample was stored for five days at room temperature after the preheat to 520°C, following which the OSL had decreased. This figure appeared in Aitken et al. (1989).

130

R . M . BAILEY et al.

thermal activation characteristic of the l l0CC TL peak in quartz, and is attributed to a similar sensitisation process. The decay in the signal after a 5 day delay (after the 52OC preheat) is similar to that reported by Godfrey-Smith et al. (1988). The accepted interpretation for the activation of the 100':C TL peak (Zimmerman, 1971) is that the heating transfers holes from reservoir traps to the luminescence centres, thereby activating them and increasing the TL efficiency. Aitken and Smith (1988) found that in Chaperon Rouge quartz the OSL enhancement and the I I0"C TL enhancement followed each other closely. Their measurements used test doses and relate principally to the fast and medium OSL components which emit in the same wavelength region as the l lffC TL trap (at about 370 nm). The slow component emits at a longer wavelength and therefore must be using a different set of luminescence centres. However, using the Zimmerman model, there is no reason why these luminescence centres should not also be activated when holes are released from the reservoir traps. The decrease in the slow component emission after storage suggests that the activated luminescence centres may be unstable at room temperature. Such effects have not been reported for the I I O C TL activation. An alternative explanation for the thermal activation of the slow component is that a small amount of charge reaches the blue luminescence centres which become the principal source of emission after activation. Spectral studies should indicate if this is the case. The rapid decrease in OSL after heating beyond 650C is also similar to the l l0~'C TL activation characteristic and suggests that the luminescence centres are unstable at this temperature (the continued OSL emission, although reduced, suggests that the slow component traps have not themselves been emptied).

4. FURTHER PROPERTIES OF THE COMPONENTS

4.1. Variation in O S L em&sion with measurement temperature

One characteristic of the OSL emission from quartz is that charge is released more rapidly if the sample is held at higher temperatures. This is interpreted to be due to two effects. The first is the thermal population of excited levels, as proposed for the fast and medium components in Section 2.3. The second effect is the increase in lattice vibrations with temperature, which aids detrapping. Urbach's rule (Urbach, 1953; Kurik, 1971), an empirical relationship for absorption, is sometimes used to describe the optical detrapping of charge. The OSL is then assumed to be an exponential function of photon energy at a certain temperature. Huntley et al. (1996) have examined the OSL from quartz and found that Urbach's rule provides an approximate

description of both the energy and temperature dependence of the excitation (by the stimulating light). However, the physical meaning of the parameters derived from Urbach's rule are not fully understood at present. In a series of experiments we held quartz samples at temperatures between 120 and 240°C during OSL measurement. The decay rates of all three components was dependent on temperature. The slope of linear fits to data on Arrhenius plots [of (In 10) vs ( l / k T ) , where I, is intensity at t = 0; k is Boltzmann's constant and T is absolute temperature, see Fig. 6] give values of 0.10 eV for the fast component, 0.14 eV for the medium component and 0.11 eV for the slow component. These activation energies are in very good agreement with the slope on a plot of(In 2) vs ( I / k T ) , where 2 is the decay constant from the exponential function l = l o . e x p ( - 2 . t ) describing signal decay, where I = intensity and t = stimulation time. The latter method is independent of normalization and thermal quenching correction factors, and should also be invariant to shifts in luminescence emission wavelengths with temperature (which have been proposed by Franklin et al., 1995). The medium component displays a non-linear relationship on the Arrhenius plot, which is not understood at present. The lower temperature portion of the plot was used for the above calculations of activation energy. it is satisfying to note the agreement of our results to those of both Spooner (1994) and Huntley et al. (1996). In both of these studies, short-shine OSL measurements were used to determine the activation energies. In Chaperon Rouge quartz the initial OSL intensity (which is sampled by short shines) is overwhelmingly dominated by the fast component (the medium component contributing only about 3% as much as the fast). It would therefore be expected that the results from short shine experiments would equate to our fast component results, as indeed they do, with each study reporting the same 0.10 eV result for similar stimulation energies. The relationship of eviction rate to temperature for each component can be extrapolated back to room temperature. This gives mean lifetimes under illumination at 2OC of 6.2, 25 and 4300 s respectively for the fast, medium and slow components (the medium component lifetime being calculated from the lower temperature portion of the data). These lifetimes have been used to model the changes in decay form as a function of partial bleaching (to be presented elsewhere). Thermal quenching was seen to affect all components to a similar degree (see Fig. 7). Note that the slow component shows similar quenching characteristics even though it is thought to originate from a different luminescence centre (Section 3.3). The thermal quenching is consistent with that measured by Smith and Rhodes (1994) and Spooner (1994).

C H A R A C T E R I S T I C S O F Q U A R T Z OSL

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4.2.

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populations. The high saturation level of the slow component suggests the possibility of its use as a long range chronometer, especially in the light of its considerable thermal stability (Section 3.3). If this dose response is c o m m o n for all samples, then many previously dismissed as undatable due to saturation

Figure 8 shows the regenerated growth curves of each of the components. Table 3 lists the exponential fit parameters. That each component has such distinct growth characteristics suggests strongly that each component represents independent charge

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R . M . BAILEY et al.

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(Gy)

Fig. 8. Regenerated growth curves for each component. Integrated signal intensity for each component is plotted against added beta-dose. Aliquots were bleached for 1 h in natural sunlight prior to dosing and preheating (220°C for 5 min). OSL measurements were then made at 160°C. Each point is the average of three normalized totals, with the standard errors shown. Data were normalized by a 0.1 s room temperature OSL measurement made prior to daylight exposure. (Chaperon Rouge quartz.) (where only the fast and medium components have been considered) may be dateable using the slow component. There are two caveats to this statement. First, the relatively poor bleachability of the slow component means that incomplete bleaching prior to deposition may preclude many samples from being successfully dated this way unless bleaching history could be confidently assumed. Second, the fading of the activation characteristic shown in Fig. 5 (after the 5 day delay) indicates a stability uncertainty, although this is probably related to the luminescence centre rather than the traps. 4.3. Isothermal annealing Preliminary results from ongoing isothermal annealing experiments suggest the fast and medium components have distinct values of E and s (from the thermal lifetime equation z = s-~.exp(E/kT), where s = "frequency factor", E = trap depth, k is Boltzmann's constant and T is absolute temperature). These results have not been included owing to uncertainties concerning the experimental conditions.

Further measurements will be made to verify the initial findings and presented elsewhere. The fast and medium components are completely removed from the OSL after a short preheat to 350°C. As reported in Section 3.3, the slow component is present until the sample is heated beyond 650°C. Huntley et al. (1996) have isolated multiple components in the quartz OSL signal, derived from the shape of the isothermal decay curves rather than by looking at the OSL decay form. They measured only the initial OSL using short shines and thus were measuring the sum of our fast, medium and slow components. Four exponential components were required to fit the experimental data and at this stage it is unclear how these components relate to our three OSL decay components. Some caution should be expressed at the practice of sampling only the initial portion of the OSL in order to obtain kinetic parameters. As the sample temperature rises the processes of increased emission rate and decreased luminescence efficiency (i.e.

Table 3. Summary of growth curve characteristics of each component for Chaperon Rouge quartz (OSL was measured at 160°C). lm~xis the normalised OSL intensity at saturation level; Do is the "characteristic dose" at which point the slope of the growth curve is equal to l/e of the initial value Component Fast Medium Slow

lm,~ (OSL intensity)

Do (Gy)

68.2 + 4.7 33.2 + 6.5 280.0 + 197.6

116.9 _ 17.5 328.0 + 90.9 888.9 _ 716.5

CHARACTERISTICS OF QUARTZ OSL Conduction Band .

.

.

.

.

.

.

.

.

.

0

..~T F

r

?

M

r k

Valence Band

Fig. 9. A schematic diagram of the proposed paths for movement of charge in quartz during optical stimulation. O indicates optical transitions and T thermal transitions. Electron trap P is related to the 110°C TL emission Traps F and M are related to the fast and medium components of OSL, and trap S to the slow component of OSL. The hole traps k indicate killer centres and other undetectable losses as described in the text. Hole traps b indicate the luminescence centres responsible for the fast and medium component OSL (and the 110°C TL). Hole traps r are the slow component OSL luminescence centres, to which the optically stimulated electrons may move through a common excited level. The question marks indicate a second possible route for charge responsible for the slow component of OSL.

thermal quenching) work in opposite directions. Unless the whole of each OSL component is measured it can be difficult to isolate these two processes.

5. A PROPOSED STRUCTURE FOR QUARTZ After considering all of the observations described above, we propose an electronic structure for quartz which is illustrated schematically in Fig. 9. The fast and medium OSL components arise from charge trapped in two slightly different locations in the lattice where the available excited levels differ (as reported in Section 2.3). This charge is also believed responsible for the TL emission centred on 325'C, as originally proposed by Smith et al. (1986). Charge from the fast and medium component traps is believed to be optically stimulated to the conduction band, where it preferentially recombines at luminescence centres which are also responsible for the 110'~C TL emission. An unknown quantity of charge is probably also lost to "killer" centres or other luminescence centres which emit outside the detected wavelength band. A portion of charge released to the conduction band moves to the trap responsible for the 110cC TL emission. For Chaperon Rouge quartz this is. only about 8% of the measurable charge released to the conduction band (Smith and Rhodes, 1994). This charge is immediately released if the sample is held at a temperature above 100°C. At room temperature the charge in this trap is slowly released (Fig. 3) and has

133

a small effect on the observed OSL decay of samples measured below 100°C. The mechanism for detrapping from the trap responsible for the slow OSL component is less clear. The longer wavelength emission (at least before thermal activation) suggests preferential recombination at a different luminescence centre to that used by the fast and medium components. It is uncertain how such thermally stable charge can be detrapped. One possibility is a localised transition to the luminescence centre via a common excited level. At this stage movement of charge to the conduction band cannot be ruled out. Further spectral studies are needed to clarify the relationship between the slow component and the available luminescence centres.

6. IDENTIFYING PARTIALLY BLEACHED SEDIMENTS 6.1. A

proposed method for bleached samples

detecting partially

We propose here an experimental method for detecting samples that were partially bleached on deposition, based on our observations above. Figures 1 and 4 demonstrate that the form of OSL decay is altered by partial bleaching and we believe remnants of this change should still be visible in the OSL decay after a subsequent dose during the sediment burial. In our proposed technique the observed ratio of the integrated signal from the fast and medium components found in the natural sample is compared to that from a similar aliquot which has been bleached and dosed artificially to the same level. The procedure is as follows: 1. Make the OSL measurements for an N +/~ growth curve at 160°C; 2. Make the OSL measurements for a regenerated growth curve at 160'~C; 3. Compare the ratio of fast:medium signal in the natural sample to that measured in the regenerated OSL at the dose level corresponding to the equivalent dose measured for the fast component. Comparison to the fast component equivalent dose is made because it is this component that will have been reset to the greatest degree. If there is a significantly larger medium component measured in the natural OSL, it suggests that the sample was not fully bleached on the last deposition. Of course, this method will only work so long as neither the fast nor medium components is near saturation in the natural sample. 6.2. Partial bleaching prior to deposition: a laboratoo' simulation To test the proposed procedure, a simulation was performed in which samples of Chaperon Rouge

134

R . M . BAILEY et al.

quartz were partially bleached (using green light, at room temperature) and then dosed. Table 4 summarises the measured ratios of the fast and medium OSL components for two different bleaching periods (12 and 25 s). Note that a preheat of 5 min at 220°C was given after bleaching (events 2 and 4) and dosing (events 3 and 5) steps. Comparison of the observed fast to medium ratios after events 3 and 5 reveals that the bleach during event 2 was, in both case (i) and case (ii), insufficient to reset both the fast and medium components fully. Had the sample been fully reset during event 2 and received the same dose as observed in the "natural" sample (following event 3, i.e. ca. 16 Gy in the fast component following the 12 s bleach), the total dose recorded in both the fast and medium components following events 3 and 5 would have been the same, as would the ratio of fast to medium integrated signals. That they are not, and that the ratio is medium component enriched, successfully identifies partial bleaching during event 2. 6.3. Discussion: identifying partially bleached sedimen ts

The ability of the scheme described in Sections 6.1 and 6.2 to identify partially bleached sediments may be reduced by a number of factors. One that has already been mentioned is recuperation. The repopulation of the fast and medium traps following bleaching (as discussed in Section 3.2) is likely to reduce the differences seen in the fast to medium signal ratio of a fully and partially bleached sample. Figure 4 (which includes the effects of recuperation, as all aliquots were preheated following bleaching) shows that there are still marked differences observed in partially bleached samples even when recuperation is included. However, the relative size of the signals following lengthier bleaching times is only very small. This is highlighted in the results of the simulation described in Section 6.2. Following partial bleaching (event 2) the aliquots which were bleached for 25 s had a significantly larger proportion of medium component

than those bleached for 12 s. However, following the 12 Gy additional dose (event 3) the aliquots which were bleached for longer (25 s) had a fast to medium ratio much closer to the 9:1 ratio of the fully bleached sample, suggesting that these aliquots had been more effectively (although not fully) bleached than those bleached for only 12 s. The ratio of fast to medium signal changes as a result of bleaching, but it is the size of these two remnant signals compared to the added signal that determines the detectability of partial bleaching. This means that it is more difficult to identify a partially bleached sediment where the remnant signal is only a small fraction of the total. This could be the case where either the partial bleaching reduces the signal to a very low level, although not completely, or where the additional (in situ) dose is relatively large compared to the remnant dose. In both cases the presence of a remnant signal would only affect the final age determination by a proportionately small amount. Sediments where the remnant dose contributes significantly to the measured equivalent dose should be more clearly discernible. Other thermal transfer effects, such as those described by Rhodes and Bailey (1997) would similarly be expected to reduce the effectiveness of this method. It is possible that the remnant signal itself may be an accumulation of partially bleached signals. Successive partial bleaching would cause the OSL to become progressively richer in medium component signal. Each phase of partial bleaching prior to deposition would therefore serve to make the presence of a remnant signal more conspicuous. The results discussed so far in this paper have been concerned with uniform partial bleaching, i.e. where each grain receives the same light exposure and therefore has its OSL reduced by the same amount. This is unlikely to be the case when mineral grains are transported by natural processes. Sediments are likely to comprise grains that have a range of remnant signals present. The method presented in Sections 6.1 and 6.2 should still differentiate such sediments from those where all grains have

Table 4. Simulation of partial bleaching prior to deposition and subsequent dose for Chaperon Rouge quartz. Figures (i) and (ii) refer to subsamples which received different bleach treatments. The 12 Gy pre-bleach dose was accumulated naturally during burial, and determined using the multiple aliquot additive dose technique The ratio of fast to medium component signals observed following the numbered event

Event (as in natural environment) 1. Pre-bleach dose (12 Gy) 2. Bleaching 3. Additional dose (12 Gy) (Regeneration

9.00 + 0.15 (i) 12 s (ii) 25 s (i)

4.26 ___1.46 2.85 + 0.94 5.47 + 0.09

(ii)

6.69 + 0.44

measurement) 4. Total bleach 5. Added dose (16 Gy)

8.90 + 0.10

CHARACTERISTICS been exposed to light for substantial times. The OSL signal measured from each aliquot will be more rich in m e d i u m c o m p o n e n t OSL t h a n h a d all grains been fully reset. Again, the problem o f partial bleaching should be more easily identified for samples where a residual signal has c o n t r i b u t e d significantly to the natural OSL.

7. C O N C L U S I O N We have d e m o n s t r a t e d that the form o f the OSL signal decay at room t e m p e r a t u r e is sensitive to partial bleaching. We believe that this change is a function of the relative p r o p o r t i o n s of OSL signal derived from three distinct sources. These sources each give rise to OSL that can be well described by a single exponential decay when the sample is held at raised t e m p e r a t u r e d u r i n g OSL measurement. The total observed signal is therefore a sum of these three exponential components. The c o m p o n e n t s display different thermal activation energies a n d also different growth characteristics following artificial dosing. The fast a n d medium c o m p o n e n t s a p p e a r to be associated with the 3 2 Y C TL peak whereas the slow c o m p o n e n t remains stable until at least 65ffC. The substantial thermal stability and high s a t u r a t i o n level o f the slow c o m p o n e n t suggests its potential use as a long range chronometer. L a b o r a t o r y measurements have d e m o n s t r a t e d that alterations to the OSL decay form caused by partial bleaching are still evident after subsequent dosing. Results from simulations suggest t h a t a c o m p a r i s o n of the observed natural signal to that from a sample regenerated to a similar level can identify sediments in which the OSL signal was not fully reset prior to the burial dose. The ability of this technique to detect partial bleaching is closely related to the m a g n i t u d e o f the r e m n a n t signal. Acknowledgements--RMB is financially supported by the NERC (reference GF4/94/363/G). The authors wish to express their gratitude to P. B. Wood for providing two samples and to Prof. M. J. Lea for useful discussien.

REFERENCES

Aitken M. J. and Smith B. W. (1988) Optical dating: recuperation after bleaching. Quat. Sci. Rev. 7, 387-393. Aitken M. J., Smith B. W. and Rhodes E. J. (1989) Optical dating: recapitulation on recuperation. Synopses from a Workshop on Long and Short Range Limits in Luminescence Dating, Oxford. RLAHA, Oxford University, Occasional Publication No. 9, p p . l 6. Franklin A. D., Prescott J. R. and Scholefield R. B. (1995) The mechanism of thermoluminescence in an Australian sedimentary quartz. J. Lumm. 63, 317-326. Godfrey-Smith D. I., Huntley D. J. and Chen W. H. (1988) Optical dating studies of quartz and feldspar sediment extracts. Quat. Sci. Rev. 7, 373-380.

OF QUARTZ

OSL

135

Huntley D., Short M. A. and Dunphy K. (1996) Deep traps in quartz and their use for optical dating. Can. J. Phys. 74, 81-91. Kurik M. V. 0971) Urbach rule. Phys. Stat. Sol. (a) 8, 9~15. Rhodes E. J. (1990) Optical dating of quartz from sediments. Unpublished D. Phil. thesis, University of Oxford. Rhodes E. J. and Bailey R. M. (1997) Thermal transfer effects observed in the luminescence of quartz from recent glaciofluvial sediments. Quaternary Geochronology (Quat. Sci. Rev.) 16, 291-298. Smith B. W., Aitken M. J., Rhodes E. J., Robinson P. D. and Gerald D. M. (1986) Optical dating: methodological aspects. Rad. Prot. Dosim. 17, 229-233. Smith B. W., Rhodes E. J., Stokes S. and Spooner N. A. (1990) The optical dating of sediments using quartz. Rad. Prot. Dosim. 34, 75 78. Smith B. W. and Rhodes E. J. (1994) Charge movements in quartz and their relevance to optical dating. Radiat. Meas. 23, 329 333. Spooner N. A. (1994) On the optical dating signal from quartz. Radiat. Meas. 23, 593-600. Urbach F. (1953) The long-wave edge of photographic sensitivity and of the electrical absorption of solids. Phys. Rev. 92, 1324-1325. Wood P. (1994) Optically stimulated luminescence dating of a late Quaternary shoreline deposit. Quat. Sci. Rev. 13, 513-516. Zimmerman J. (1971 ) The radiation-induced increase of the 100~'C thermoluminescence sensitivity of fired quartz. J. Phys. C: Solid State Phys. 4, 3265 3276.

APPENDIX

The theoretical basis .for expecting exponential O S L decay components In this Appendix we outline the reasons for expecting to observe exponential OSL decay components when quartz is held at around 160°C during optical stimulation, even when there is retrapping of charge into the bleachable traps. The two fundamental assumptions that are made here are discussed in the text. The first is that the paths available to the released charge do not change in their relative proportions during OSL measurement. The second is that there are no traps of intermediate thermal stability at the 160°C measurement temperature: either charge is immediately released due to thermal excitation (e.g. charge in the TL traps in the 100°C region) or it is held in traps which are stable at 160°C. There is sound experimental evidence to support both of these assumptions (see Sections 2 and 3 for discussion). It is important to understand that the equations below describe a highly simplified version of a far more complex system. However, two crucial properties that are believed to have the most significant effect on the observable decay form (namely those described by the two assumptions) have been sustained. Consider first a simplified electronic structure as illustrated in Fig. AI, consisting of two electron traps, with trap concentrations N~ and N,, having trapped electron concentrations n~ and n: respectively. There is one type of recombination centre (M) with a trapped hole concentration m. Population n~ is thermally stable but can be optically excited to the conduction band at a rate B. Population n, is optically and thermally stable. Once charge from population n~ is raised to the conduction band, it can be captured by the N~-type traps, the N_,-type traps, or the recombination centres. The sites can be assigned values A~, A., and A,,, describing the relative probability of electron capture at each of the respective sites

R. M. B A I L E Y

136 Conduction Band

et al.

This gives a solution for no: -

B ' n l + n¢.A,i + nc'A,~_ + nc'Aml = 0

i.e. n~ = B ' n l / ( A , t

+ A,2 + A,,O.

nl

Substitution into equations (5) and (8) gives:

n2

dn~/dt = B.n,.( dm/dt

=

-

Valence Band

Fig. A 1. The simplified electronic structure described in the text of the Appendix. The two electron trapping levels (with trapped electron concentrations n~ and n2) and luminescence centre (concentration m) are as described in the text. The arrows indicate the paths available to charge during optical stimulation.

dnl/dt =

-

C.n~

(11)

dm/dt

-

D.n~

(12)

and

=

-

B ' n t + nc.(N~ -

nO.At

= nc'(N2 - n2)'A2

(1) (2)

- nc'(N2 - n2)'Az - n~.m.Am =

- no're'Am .

(3) (4)

The first assumption, and experimental evidence from quartz, suggests that N,>>n~, Nz>>n: and that m can be approximated as a l~rge constant. Note that this implies that the charge being moved optically is only a small fraction of the total trapped charge in the lattice (n2>>nt), with m >>n, and (N2 - ng>>n,. Hence these equations can be simplified by introducing constants A.t = [(N~--n,).A~], A,2 = [(N2- n2)'A2] and A~ = [m'Am]. This results in the following set of equations: dnt/dt =

-

B . n j + n~'A,t

= B.nL -- n~'A,t -- nc'A~2 -- n~.Am~ dm/dt

=

-

(5) (6)

d n 2 / d t = nc'A,2 dnddt

(7) (8)

nc'Am~ .

Equations (5)-(8) directly illustrate the proportional distribution of charge between the different sites which follows from the first assumption. Making the usual assumption that the population of free carriers, no, is much lower than that in either the traps or the recombination centre dn~/dt + dnddt

-

= nlo'exp( -

C.t)

where n~0 is the initial population of trap type I. Hence from equation (12) the luminescence intensity I, determined from the population m, is given as a function of time by: l(t) =

-- d m / d t

= D'nt(t)

= D'nl0"exp( -

C.t)

= 10-exp( -

d n c / d t = B . n , - n~.(N, - nO'A~

dm/dt

=

where C and D are positive constants. Solving equation (11) for n,, the population of n, as a function of time, t, is given by: n,(t)

N~, N2 and M. If n~ is used to denote the concentration of free electrons in the conduction band, the rate equations for this system can be written as follows:

dnddt

dm/dt

(9) (10)

Again following the first assumption, the parts of equations (9) and (10) that deal with the capture probabilities can be considered to be constant. As B is also a constant, equations (9) and (10) reduce to:

m

dnjdt

1 + A . , / ( A , ~ + A,2 + Am,))

B ' n ~ ' A m , / ( A ° , + A,2 + Amt) .

= O.

C.t)

(13)

where Io is the initial luminescence intensity. The OSL therefore decays exponentially with a time constant of C% Reference to equation (9) confirms that as retrapping in the source trap increases (i.e. A°, increases), the OSL decay slows. This simple model can be extended to include a second bleachable trap. It can then be shown in a similar manner that the OSL will be the summation of two exponential components. The addition of further hole centre types changes only the proportion of charge recombining at the luminescence centre and does not alter the exponential decay of the OSL. If a trap of intermediate thermal stability is added to the model, the loss of charge with time from this trap will destroy the exponential nature of the luminescence decay. This result matches the experimental observation that, when stimulated at temperatures below 100°C, the decay of the quartz OSL signal is more complex than that measured at 160°C and not well described using a sum of three exponential decays (due to charge transfer to the 100°C TL region). The complex electronic structure shown in Fig. 9 has been modelled in more detail than is described here without the assumptions made above. The model yields exponential components corresponding to each bleachable trap, in agreement with the simplified analysis.