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Charge movements in quartz and their relevance to optical dating
RadiationMeasuremcn~s, Vol. 23, Nos 213, pp. 329-333, 1994 Caavridtt ch 1994 Ekevier S&me Ltd All rights mcrwd 1350-4487/94 s7.00 + .a0 --r.
US
Printed in C&Britain.
1350-4@7(93)EOO21-D
CHARGE MOVEMENTS IN QUARTZ AND THEIR RELEVANCE TO OPTICAL DATING B. W. SMtrr-t*tand E. J.
RHODES~
*Research Laboratory for Archaeology and the History of Art, Oxford University, 6 Keble Rd. Oxford OX1 345, U.K.; IDepartment of Geography, Royal Holloway, University of London, Egham, Surrey TW20 OEX, U.K.
Abstract
1. INTRODUCTION study investigates the movement of charge in the quartz lattice which accompanies the emission of luminescence. It is based largely on an unpublished work which was circulated within the dating community several years ago. The evidence presented clearly suggests that, during the optical stimulation of quartz by 514nm light, charge is evicted from traps in the crystal lattice associated with the 325°C thermoluminescence (IL) emission and elevated to the conduction band. Part of this charge recombines with luminescence centres to produce the observed optically stimulated luminescence (OSL), and part is re-trapped at the same or different localities in the lattice. Smith et al. (1986) first demonstrated that the initial OSL emission was related to the 325°C TL peak in quartz, and also that the OSL had a tendency to “recuperate” after light exposure. The latter phenomenon was discussed in more detail by Aitken and Smith (1988), who gave an account of the mechanism involved. Since then, a number of papers has discussed the use of OSL to date quartz, but none has given an in-depth account of the charge movements associated with the dating procedure. In the present study, a quartz sample from Chaperon Rouge, Morocco was selected for detailed examination. It was collected from an Upper Aterian dune sand (lab. ref. Ox-724g2) and has been dated by Rhodes (Texier et al., 1988) at 24.0 + 3.1/-4.8 ka.
THE PRESENT
The natural OSL emission is relatively large compared with many European quartz samples. TL measurements have been used to distinguish the traps in which charge is held. For natural Chaperon Rouge quartz, these can be broadly divided into “shallow” traps (with TL emission around 1OO’C) for which the initial charge population is zero, and “deep” traps (280-400°C) which are thermally stable at ambient temperatures but can lose charge when stimulated by light. The TL measured after progressively longer exposures to green 514 run (Ar laser) light is illustrated in Fig. 1. Charge is removed (“bleached”) from the high temperature region, while TL in the 100°C region grows rapidly from zero (by phototransfer) and then eventually decreases. After a 1 ks exposure, the high temperature TL shows no further decrease within experimental errors for the data shown in Fig. 1, although other measurements have suggested that it is still decreasing very slowly. This paper investigates the movement of charge within the trapping sites associated with the TL peaks in Fig. 1. 2. OSL EMISSION Exposure of Chaperon Rouge quartz to 514nm light (during optical dating) results in the prompt emission of OSL, which decreases during the exposure as the source charge is depleted. The decrease in OSL emission with laser exposure does not follow a simple exponential as would be expected
tPresent address: 81 Labs, GWD, DSTO, PO Box 1500, Salisbury 5108, Australia. 329
B. W. SMITH and E. J. RHODES
330
0
100
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Temperature ( o C) FIG. 1. The TL of natural Chaperon Rouge quartz measured after exposure to the laser (under the standard conditions given in Section 3). The TL measurement began 40s after completion of the laser exposure, using a heating rate of 5°C s-‘. The curves are an average of several measurements, with the error in the TL typically about 10%. from first-order detrapping from a single trapping site. This is not surprising as it is clear from Fig. 1 that there are at least two sources of charge when OSL measurements are made at 17°C. First charge is removed from deep traps (corresponding to TL at 280400°C) and then later also from the traps associated with the 100°C TL phototransfer peak. To remove complications introduced by this phototransfer, some OSL measurements have been made on samples held at elevated temperatures. At temperatures above lOO”C, any charge in traps associated with the 100°C TL phototransfer peak will immediately be raised to the conduction band. As the temperature is increased, the OSL is observed to decrease at a faster rate. This clearly suggests that the release of charge from traps proceeds via some thermally assisted mechanism. Figure 2 shows the observed OSL decrease for natural Chaperon Rouge quartz held at 220°C. Note that the decay is still not a simple exponential. This could be due to phototransfer to a peak stable at 220°C but as no such peak is measured by TL, this possibility is discounted. Without phototransfer effects, the source charge could only originate from a single trapping site (with a single mean life or “bleachability”) if significant second-order effects are present due to the retrapping of charge. Retrapping is clearly indicated by the presence of OSL recuperation (Aitken and Smith, 1988). However, calculations (based on quantitative measurements of the amount of recuperated charge relative to luminescence emission) suggest that this retrapping produces only a minor modification to an exponential OSL decrease so long as the traps are not near saturation. Hence, it appears that the released charge originates from trapping sites with more than one mean life under optical stimulation.
The OSL decrease can be broken into three exponential components (the dashed lines in Fig. 2). The long-term component which is evident after an exposure of one hundred seconds (at 220°C) contributes very few counts to the OSL measured when the sample is held at 17°C and can be considered as a small constant addition to the background for normal optical dating measurements. It can be reduced to near-zero by the use of efficient red-rejection filters. When the long-term component is subtracted from the OSL decrease shown in Fig. 2, the remaining data can be fitted well by two exponential components with mean lives which differ by a factor of four (designated as fast- and medium-term components). This was found to be the case whenever the samples were held at temperatures above 150°C; below this temperature, the decay was more complex (presumably because of the interference of phototransferred charge). Caution is required when fitting multiple exponential functions, and although the fast and medium components fit the observed decrease, the existence of a range of mean lives (as may be expected from a spread of trap depths and excited levels) cannot be ruled out. It should also be noted that the exponential decay model may be incorrect. The fast and medium components show severe thermal quenching of the luminescence efficiency which follows a form similar to that derived by Wintle (1975) for the 325°C TL peak in quartz. It was measured by integrating the total OSL emission from these components while holding the sample at a range of temperatures between 17 and 250°C. Luminescence efficiency decreases from 1.OOat room temperature to 0.24 at 170°C and is only 0.03 at 250°C. 3. CHARGE REMOVED FROM DEEP TRAPS The movement of charge was monitored by making TL measurements immediately after exposure of
OSL (kcps)
Exposure (s) FIG. 2. The OSL emission from natural Chaperon
Rouge quartz when it is held at 220°C. The sample was exposed to a 7 mW cm-’ beam of 514.5 nm from an Ar ion laser, and the OSL measured through a 0.5 mm Schott BG39 and four Corning 7-59 filters. The dashed lines indicate the three exponential components into which the OSL decrease can be divided.
CHARGE
MOVEMENTS
the samples to the laser. In all of the following experiments, standardized conditions were used so that the results could be intercompared. Identical samples were prepared, each consisting of a monolayer of 100 pm quartz grains on stainless Steel discs. Measurements were made using 514.5 nm at 2.1 mW cm-’ for OSL and bleaching studies (with the sample held at 17 f O.YC), and a heating rate of 5°C s-l for the TL measurements. The TL and OSL measurements were made in the same chamber, using the same electronics, photomultiplier (EMI 9635Q) and filters (four Corning 7-59 and one 0.5 mm Schott BG39 which limit the detection region to around 380 nm). The Schott BG39 filter is necessary because of the unexpected red sensitivity of the photomultiplier, which needs to be suppressed. The amount of bleachable TL in the high temperature region is illustrated in Fig. 3. The value plotted is the difference between the TL of the natural sample and the residual TL after a 2 ks exposure to the laser. In order to convert the measured TL to units of charge, it is necessary to divide by the luminescence efficiency at the temperature concerned, giving the values plotted in Fig. 3 in units of photoelectrons counted by the photomultiplier. These have been normalized to a typical sample disc of x2 mg which gives an initial OSL count rate of 104 counts s-‘. It can be seen that most of the bleachable charge comes from the 280~340°C region. The exact amount from higher temperatures is uncertain as it is not known how much of the emission is being thermally quenched (luminescence centres normally associated with the 375°C TL peak do not show thermal quenching). The form of decrease in TL with laser exposure was investigated using 20 similar discs of natural Chaperon Rouge quartz The discs were normalized by the TL from a standard dose (delivered after the bleaching studies). The TL decrease is analogous to the OSL decrease shown in Fig. 2, but the data were not accurate enough to allow a detailed analysis. It was expected that the form of decrease would vary with temperature, but in fact all 20°C TL intervals from 240 to 400°C showed similar decay of the bleachable component. When integrating the 260-360°C TL interval to minimize errors, the decrease over 1 ks could be adequately matched by two exponentials with decay constants of 1 = 0.016~~’ (for 0.53 of the initial charge) and .i = 0.0012 s-’ (for 0.47 of the initial charge). Note that these measurements of TL decrease show the net loss of charge after retrapping from shallow traps (which occurred during the heating accompanying TL measurement). 4. TRAPPING
IN SHALLOW
TRAPS
Phototransfer of charge to the 1OOC TL region is shown in Fig. 1. There is also a small phototransfer TL peak at 160°C which is roughly proportional to the 100°C peak. Beta-irradiation of the natural
331
IN QUARTZ
200
240
230
320
Tomprraturo
360
400
(“C)
FIG. 3. The TL removed by a 2 ks laser exposure under standard conditions is shown by the solid curve (left-hand scale). When corrected for thermal quenching effects the dashed curve is obtained, which is plotted in units of photoelectron counts per 20°C interval (right-hand scale). The la error limits are given.
sample shows about the same relative peak heights for 100°C and 160°C as does phototransfer, and also a very small peak at 210°C (which would not be detectable at the lower phototransfer intensities). The 160°C peak contains ~2% of the TL of the 100°C peak, and it is safe to conclude that both the 160°C and 210°C peaks play only a minor role in the movement of charge. In order to calculate the relative probability of charge being transferred to the 100°C TL region, the luminescence emitted during the phototransfer (i.e. the OSL) was compared with the resultant 100°C TL (measured 1 min after completion of the laser exposure). As the laser exposure increased, the ratio of the TL counts to integrated OSL fell from an initial value of 0.087 to 0.042 at 2 ks (using standard conditions). One reason for this could be saturation of the 100°C TL peak, but the population of the peak does not increase beyond 10% of the measured saturation level. A ratio of 0.087 is again obtained from a repeat short exposure following a long exposure (and TL measurement), which suggests that the decrease is due to charge lost from the shallow traps by thermal and/or optical eviction. The initial ratio of 0.087 indicates the relative efficiency of shallow trapping to luminescence emission (i.e. recombination), but note that upon TL measurement, only some of the trapped charge was measured as luminescence. The remainder was transferred to deep traps or underwent non-radiative recombination. 5. RETRAPPING
IN DEEP TRAPS
Retrapping in deep traps is directly shown by the recuperation of OSL during a delay (or heating) after an extended laser exposure (Smith et ol., 1986). Recuperation of natural Chaperon Rouge OSL is illustrated in Fig. I of Aitken and Smith (1988). After 10 days at 17”C, the OSL had reached a stable level of 4.5% of the initial natural OSL and remained
B. W. SMITH and E. J. RHODES
332
constant for at least 1 year. This is within experimental error of the 4.7% level reached immediately after heating a similar sample to 220°C for 5 min to remove the low temperature TL. The increase in recuperation OSL with time can instead be interpreted as a decrease of charge contained in the source traps. This is illustrated in Fig. 4, which shows the “latent recuperation” of OSL (i.e. the recuperation to the 4.5% level which has yet to occur, which is related to the source trap population). Also shown is the decrease in phosphorescence measured over the same period, which can be related to the 100°C TL peak area. The similarity of the two decay forms strongly points to the source of recuperation OSL being charge in shallow traps (giving TL at around 100°C). Other evidence is also available (Aitken and Smith, 1988). The amount of retrapping from shallow to deep traps has been measured by two techniques. First, the phosphorescence shown in Fig. 4 was integrated and compared with the total increase in OSL emission after recuperation. In the second method, the TL emission during a 5 min heating at 220°C (which removed the shallow trapped charge resulting from an extended laser exposure) was compared with the resulting increase in OSL. Both methods gave a surprisingly high ratio of deep trapping to luminescence emission of about 0.7. 6. IMPLICATIONS FOR OPTICAL DATING The results described here apply only to the one quartz sample, but less detailed measurements on other samples show the same general characteristics. The measurements firmly establish the relationship
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Delay (mlnutes) FIG. 4. The form of decay of the “latent” recuperation (circles) after completion of an extended exposure to the laser. The decay IS very similar to that of the phosphorescence (crosses) measured over the same period. The “latent” recuperation (counts per 1mJ crne2 exposure) and phosphorescence (counts s-l) have both been normalized to 100 counts at a time of I min. In each case, the decay can be approximated by two exponential components (dashed lines).
between OSL emission and the removal of charge from high temperature TL traps (principally at 325°C). They also give indications of the possible mechanisms of charge removal under optical stimulation. The similarity of trapping probabilities during irradiation and phototransfer (see Section 4) suggests that, whatever process is at work, the charge ultimately moves via the conduction band. A straight transition from the 325°C TL trap to the conduction band seems unlikely from the known kinetic parameters and the optical energy available. An alternative explanation is that there is an initial sub-conduction band process such as an optically stimulated localized transition (Templer, 1986). The transition would need to take the charge from the high temperature traps to an unstable site with a very short mean life (<< 1 s)---such as one of the phototransfer traps giving rise to TL below 17°C. Such a model would explain why a proportion of deeptrapped charge is stable under optical stimulation, as only those traps with access to a localized transition could be bleached. In many quartz samples, a distinct peak in the 325°C region is still present after long laser exposures, indicating that one portion of this peak is unbleachable by 514 nm light. The rate of retrapping (as derived from quantitative recuperation measurements) is not sufficient to account for this residual peak. It is possible to use a simple model to calculate the charge movement when Chaperon Rouge quartz is exposed to 514 nm light, using some of the parameters reported here. The model assumes that charge moves through the conduction band and has fixed probabilities of capture at each vacant luminescence centre, shallow trap or deep trap. The deep traps are divided into two types (with decay constants and initial populations as given at the end of Section 3). The TL and phosphorescence measurements outlined in this paper give the initial trappedcharge population. the total number of trapping sites and the decay rates (under 514 nm light exposure) for charge in each type of trap. When these parameters are used to predict the form of OSL decrease, they match the observed OSL emission over the first 100 s of laser exposure (about 200 mJ cm-‘). Many of the effects described above have methodological implications for optical dating. After irradiation (to measure the OSL response), there is an excess of charge in shallow traps. It is obvious from the above measurements that this needs to be redistributed in a way which mimics the charge transfer which occurred at ambient temperatures during environmental dosing of the sediment. The movement of charge from shallow traps has been measured to be largely temperature-independent (Rhodes, 1988). This suggests that the charge redistribution can be adequately achieved by heating. In some quartz samples, significant charge is also transferred from the 200-300°C region (Aitken and Smith, 1988) and this necessitates heating for several minutes
CHARGE
MOVEMENTS
above 200°C. The natural sample also requires replicate heating to ensure full transfer and match any induced sensitivity change. Another consequence of this charge transfer is that if shallow-trapped charge is still present after the resetting event, some of it will find its way to the deep traps and cause an over estimation of age. This recuperation has been discussed in detail previously (Aitken and Smith, 1988). The kinetics raise questions about the interpretation of “shine plateaux”, which are plots of the equivalent dose (the dose that the natural sample is determined to have received) as a function of the laser exposure time. For non-first-order kinetics, the form of OSL decrease upon laser exposure will vary with charge population, and care must be taken when comparing OSL emissions at individual exposure times. The safest approach is to integrate the full OSL emission over the longest exposure possible. However, so long as the OSL is not approaching saturation, the shine plateau can give valuable information on other aspects. For example, following the observation that the OSL decrease comprises more than one component, i’t should be possible to detect inadvertent bleaching during sample collection or preparation. Similarly the lack of a plateau may indicate incomplete bleaching at the time of the event
333
IN QUARTZ
being dated, as originally suggested by Huntley et al. (1985). Acknowledgements-This study has been aided by the keen interest shown by Martin A&ken, Nigel Spooncr, Scott Wheeler and Stephen Stokes. The research was supported by the Science-based Archaeology Committee of the U.K. Science and Engineering Research Council.
REFERENCES Aitken M. J. and Smith B. W. (1988) Optical dating: recuperation after bleaching. Quot. Sci. Reu. 7, 381-393. Huntley D. J., Godfrey-Smith D. I. and Thewalt M. L. W. (1985) Optical dating of sediments. Nature 313, 105-107. Rhodes E. J. (1988) Methodological considerations in the optical dating of quartz. Quot. Sci. Reu. 7, 395-400. Smith B. W., Aitken M. J., Rhodes E. J., Robinson P. D. and Geldard D. M. (1986) Optical datina: methodological aspects. Radici. Prbteit. Dosim. 15, 229-233. Temuler R. H. (1986) The localized transition model *of anomalous fading. Radiat. Protect. Dosim. 17, 493-497. Texier J.-P., Huxtable J., Rhodes E., Miallier D. and Ousmoi M. (1988) Nouvelles don&s sur la situation chronologique de.l’Attrien du Maroc et leurs implications. C. R. Acad. Sci. Paris 307. 827-832. Wintle A. G. (1975) Thermal quenching of thermoluminescence in quartz. Geophys. J. Roy. Astron. Sot. 41, 107-l 13.
US
Printed in C&Britain.
1350-4@7(93)EOO21-D
CHARGE MOVEMENTS IN QUARTZ AND THEIR RELEVANCE TO OPTICAL DATING B. W. SMtrr-t*tand E. J.
RHODES~
*Research Laboratory for Archaeology and the History of Art, Oxford University, 6 Keble Rd. Oxford OX1 345, U.K.; IDepartment of Geography, Royal Holloway, University of London, Egham, Surrey TW20 OEX, U.K.
Abstract
1. INTRODUCTION study investigates the movement of charge in the quartz lattice which accompanies the emission of luminescence. It is based largely on an unpublished work which was circulated within the dating community several years ago. The evidence presented clearly suggests that, during the optical stimulation of quartz by 514nm light, charge is evicted from traps in the crystal lattice associated with the 325°C thermoluminescence (IL) emission and elevated to the conduction band. Part of this charge recombines with luminescence centres to produce the observed optically stimulated luminescence (OSL), and part is re-trapped at the same or different localities in the lattice. Smith et al. (1986) first demonstrated that the initial OSL emission was related to the 325°C TL peak in quartz, and also that the OSL had a tendency to “recuperate” after light exposure. The latter phenomenon was discussed in more detail by Aitken and Smith (1988), who gave an account of the mechanism involved. Since then, a number of papers has discussed the use of OSL to date quartz, but none has given an in-depth account of the charge movements associated with the dating procedure. In the present study, a quartz sample from Chaperon Rouge, Morocco was selected for detailed examination. It was collected from an Upper Aterian dune sand (lab. ref. Ox-724g2) and has been dated by Rhodes (Texier et al., 1988) at 24.0 + 3.1/-4.8 ka.
THE PRESENT
The natural OSL emission is relatively large compared with many European quartz samples. TL measurements have been used to distinguish the traps in which charge is held. For natural Chaperon Rouge quartz, these can be broadly divided into “shallow” traps (with TL emission around 1OO’C) for which the initial charge population is zero, and “deep” traps (280-400°C) which are thermally stable at ambient temperatures but can lose charge when stimulated by light. The TL measured after progressively longer exposures to green 514 run (Ar laser) light is illustrated in Fig. 1. Charge is removed (“bleached”) from the high temperature region, while TL in the 100°C region grows rapidly from zero (by phototransfer) and then eventually decreases. After a 1 ks exposure, the high temperature TL shows no further decrease within experimental errors for the data shown in Fig. 1, although other measurements have suggested that it is still decreasing very slowly. This paper investigates the movement of charge within the trapping sites associated with the TL peaks in Fig. 1. 2. OSL EMISSION Exposure of Chaperon Rouge quartz to 514nm light (during optical dating) results in the prompt emission of OSL, which decreases during the exposure as the source charge is depleted. The decrease in OSL emission with laser exposure does not follow a simple exponential as would be expected
tPresent address: 81 Labs, GWD, DSTO, PO Box 1500, Salisbury 5108, Australia. 329
B. W. SMITH and E. J. RHODES
330
0
100
200
300
400
500
Temperature ( o C) FIG. 1. The TL of natural Chaperon Rouge quartz measured after exposure to the laser (under the standard conditions given in Section 3). The TL measurement began 40s after completion of the laser exposure, using a heating rate of 5°C s-‘. The curves are an average of several measurements, with the error in the TL typically about 10%. from first-order detrapping from a single trapping site. This is not surprising as it is clear from Fig. 1 that there are at least two sources of charge when OSL measurements are made at 17°C. First charge is removed from deep traps (corresponding to TL at 280400°C) and then later also from the traps associated with the 100°C TL phototransfer peak. To remove complications introduced by this phototransfer, some OSL measurements have been made on samples held at elevated temperatures. At temperatures above lOO”C, any charge in traps associated with the 100°C TL phototransfer peak will immediately be raised to the conduction band. As the temperature is increased, the OSL is observed to decrease at a faster rate. This clearly suggests that the release of charge from traps proceeds via some thermally assisted mechanism. Figure 2 shows the observed OSL decrease for natural Chaperon Rouge quartz held at 220°C. Note that the decay is still not a simple exponential. This could be due to phototransfer to a peak stable at 220°C but as no such peak is measured by TL, this possibility is discounted. Without phototransfer effects, the source charge could only originate from a single trapping site (with a single mean life or “bleachability”) if significant second-order effects are present due to the retrapping of charge. Retrapping is clearly indicated by the presence of OSL recuperation (Aitken and Smith, 1988). However, calculations (based on quantitative measurements of the amount of recuperated charge relative to luminescence emission) suggest that this retrapping produces only a minor modification to an exponential OSL decrease so long as the traps are not near saturation. Hence, it appears that the released charge originates from trapping sites with more than one mean life under optical stimulation.
The OSL decrease can be broken into three exponential components (the dashed lines in Fig. 2). The long-term component which is evident after an exposure of one hundred seconds (at 220°C) contributes very few counts to the OSL measured when the sample is held at 17°C and can be considered as a small constant addition to the background for normal optical dating measurements. It can be reduced to near-zero by the use of efficient red-rejection filters. When the long-term component is subtracted from the OSL decrease shown in Fig. 2, the remaining data can be fitted well by two exponential components with mean lives which differ by a factor of four (designated as fast- and medium-term components). This was found to be the case whenever the samples were held at temperatures above 150°C; below this temperature, the decay was more complex (presumably because of the interference of phototransferred charge). Caution is required when fitting multiple exponential functions, and although the fast and medium components fit the observed decrease, the existence of a range of mean lives (as may be expected from a spread of trap depths and excited levels) cannot be ruled out. It should also be noted that the exponential decay model may be incorrect. The fast and medium components show severe thermal quenching of the luminescence efficiency which follows a form similar to that derived by Wintle (1975) for the 325°C TL peak in quartz. It was measured by integrating the total OSL emission from these components while holding the sample at a range of temperatures between 17 and 250°C. Luminescence efficiency decreases from 1.OOat room temperature to 0.24 at 170°C and is only 0.03 at 250°C. 3. CHARGE REMOVED FROM DEEP TRAPS The movement of charge was monitored by making TL measurements immediately after exposure of
OSL (kcps)
Exposure (s) FIG. 2. The OSL emission from natural Chaperon
Rouge quartz when it is held at 220°C. The sample was exposed to a 7 mW cm-’ beam of 514.5 nm from an Ar ion laser, and the OSL measured through a 0.5 mm Schott BG39 and four Corning 7-59 filters. The dashed lines indicate the three exponential components into which the OSL decrease can be divided.
CHARGE
MOVEMENTS
the samples to the laser. In all of the following experiments, standardized conditions were used so that the results could be intercompared. Identical samples were prepared, each consisting of a monolayer of 100 pm quartz grains on stainless Steel discs. Measurements were made using 514.5 nm at 2.1 mW cm-’ for OSL and bleaching studies (with the sample held at 17 f O.YC), and a heating rate of 5°C s-l for the TL measurements. The TL and OSL measurements were made in the same chamber, using the same electronics, photomultiplier (EMI 9635Q) and filters (four Corning 7-59 and one 0.5 mm Schott BG39 which limit the detection region to around 380 nm). The Schott BG39 filter is necessary because of the unexpected red sensitivity of the photomultiplier, which needs to be suppressed. The amount of bleachable TL in the high temperature region is illustrated in Fig. 3. The value plotted is the difference between the TL of the natural sample and the residual TL after a 2 ks exposure to the laser. In order to convert the measured TL to units of charge, it is necessary to divide by the luminescence efficiency at the temperature concerned, giving the values plotted in Fig. 3 in units of photoelectrons counted by the photomultiplier. These have been normalized to a typical sample disc of x2 mg which gives an initial OSL count rate of 104 counts s-‘. It can be seen that most of the bleachable charge comes from the 280~340°C region. The exact amount from higher temperatures is uncertain as it is not known how much of the emission is being thermally quenched (luminescence centres normally associated with the 375°C TL peak do not show thermal quenching). The form of decrease in TL with laser exposure was investigated using 20 similar discs of natural Chaperon Rouge quartz The discs were normalized by the TL from a standard dose (delivered after the bleaching studies). The TL decrease is analogous to the OSL decrease shown in Fig. 2, but the data were not accurate enough to allow a detailed analysis. It was expected that the form of decrease would vary with temperature, but in fact all 20°C TL intervals from 240 to 400°C showed similar decay of the bleachable component. When integrating the 260-360°C TL interval to minimize errors, the decrease over 1 ks could be adequately matched by two exponentials with decay constants of 1 = 0.016~~’ (for 0.53 of the initial charge) and .i = 0.0012 s-’ (for 0.47 of the initial charge). Note that these measurements of TL decrease show the net loss of charge after retrapping from shallow traps (which occurred during the heating accompanying TL measurement). 4. TRAPPING
IN SHALLOW
TRAPS
Phototransfer of charge to the 1OOC TL region is shown in Fig. 1. There is also a small phototransfer TL peak at 160°C which is roughly proportional to the 100°C peak. Beta-irradiation of the natural
331
IN QUARTZ
200
240
230
320
Tomprraturo
360
400
(“C)
FIG. 3. The TL removed by a 2 ks laser exposure under standard conditions is shown by the solid curve (left-hand scale). When corrected for thermal quenching effects the dashed curve is obtained, which is plotted in units of photoelectron counts per 20°C interval (right-hand scale). The la error limits are given.
sample shows about the same relative peak heights for 100°C and 160°C as does phototransfer, and also a very small peak at 210°C (which would not be detectable at the lower phototransfer intensities). The 160°C peak contains ~2% of the TL of the 100°C peak, and it is safe to conclude that both the 160°C and 210°C peaks play only a minor role in the movement of charge. In order to calculate the relative probability of charge being transferred to the 100°C TL region, the luminescence emitted during the phototransfer (i.e. the OSL) was compared with the resultant 100°C TL (measured 1 min after completion of the laser exposure). As the laser exposure increased, the ratio of the TL counts to integrated OSL fell from an initial value of 0.087 to 0.042 at 2 ks (using standard conditions). One reason for this could be saturation of the 100°C TL peak, but the population of the peak does not increase beyond 10% of the measured saturation level. A ratio of 0.087 is again obtained from a repeat short exposure following a long exposure (and TL measurement), which suggests that the decrease is due to charge lost from the shallow traps by thermal and/or optical eviction. The initial ratio of 0.087 indicates the relative efficiency of shallow trapping to luminescence emission (i.e. recombination), but note that upon TL measurement, only some of the trapped charge was measured as luminescence. The remainder was transferred to deep traps or underwent non-radiative recombination. 5. RETRAPPING
IN DEEP TRAPS
Retrapping in deep traps is directly shown by the recuperation of OSL during a delay (or heating) after an extended laser exposure (Smith et ol., 1986). Recuperation of natural Chaperon Rouge OSL is illustrated in Fig. I of Aitken and Smith (1988). After 10 days at 17”C, the OSL had reached a stable level of 4.5% of the initial natural OSL and remained
B. W. SMITH and E. J. RHODES
332
constant for at least 1 year. This is within experimental error of the 4.7% level reached immediately after heating a similar sample to 220°C for 5 min to remove the low temperature TL. The increase in recuperation OSL with time can instead be interpreted as a decrease of charge contained in the source traps. This is illustrated in Fig. 4, which shows the “latent recuperation” of OSL (i.e. the recuperation to the 4.5% level which has yet to occur, which is related to the source trap population). Also shown is the decrease in phosphorescence measured over the same period, which can be related to the 100°C TL peak area. The similarity of the two decay forms strongly points to the source of recuperation OSL being charge in shallow traps (giving TL at around 100°C). Other evidence is also available (Aitken and Smith, 1988). The amount of retrapping from shallow to deep traps has been measured by two techniques. First, the phosphorescence shown in Fig. 4 was integrated and compared with the total increase in OSL emission after recuperation. In the second method, the TL emission during a 5 min heating at 220°C (which removed the shallow trapped charge resulting from an extended laser exposure) was compared with the resulting increase in OSL. Both methods gave a surprisingly high ratio of deep trapping to luminescence emission of about 0.7. 6. IMPLICATIONS FOR OPTICAL DATING The results described here apply only to the one quartz sample, but less detailed measurements on other samples show the same general characteristics. The measurements firmly establish the relationship
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f 60
120
160
Delay (mlnutes) FIG. 4. The form of decay of the “latent” recuperation (circles) after completion of an extended exposure to the laser. The decay IS very similar to that of the phosphorescence (crosses) measured over the same period. The “latent” recuperation (counts per 1mJ crne2 exposure) and phosphorescence (counts s-l) have both been normalized to 100 counts at a time of I min. In each case, the decay can be approximated by two exponential components (dashed lines).
between OSL emission and the removal of charge from high temperature TL traps (principally at 325°C). They also give indications of the possible mechanisms of charge removal under optical stimulation. The similarity of trapping probabilities during irradiation and phototransfer (see Section 4) suggests that, whatever process is at work, the charge ultimately moves via the conduction band. A straight transition from the 325°C TL trap to the conduction band seems unlikely from the known kinetic parameters and the optical energy available. An alternative explanation is that there is an initial sub-conduction band process such as an optically stimulated localized transition (Templer, 1986). The transition would need to take the charge from the high temperature traps to an unstable site with a very short mean life (<< 1 s)---such as one of the phototransfer traps giving rise to TL below 17°C. Such a model would explain why a proportion of deeptrapped charge is stable under optical stimulation, as only those traps with access to a localized transition could be bleached. In many quartz samples, a distinct peak in the 325°C region is still present after long laser exposures, indicating that one portion of this peak is unbleachable by 514 nm light. The rate of retrapping (as derived from quantitative recuperation measurements) is not sufficient to account for this residual peak. It is possible to use a simple model to calculate the charge movement when Chaperon Rouge quartz is exposed to 514 nm light, using some of the parameters reported here. The model assumes that charge moves through the conduction band and has fixed probabilities of capture at each vacant luminescence centre, shallow trap or deep trap. The deep traps are divided into two types (with decay constants and initial populations as given at the end of Section 3). The TL and phosphorescence measurements outlined in this paper give the initial trappedcharge population. the total number of trapping sites and the decay rates (under 514 nm light exposure) for charge in each type of trap. When these parameters are used to predict the form of OSL decrease, they match the observed OSL emission over the first 100 s of laser exposure (about 200 mJ cm-‘). Many of the effects described above have methodological implications for optical dating. After irradiation (to measure the OSL response), there is an excess of charge in shallow traps. It is obvious from the above measurements that this needs to be redistributed in a way which mimics the charge transfer which occurred at ambient temperatures during environmental dosing of the sediment. The movement of charge from shallow traps has been measured to be largely temperature-independent (Rhodes, 1988). This suggests that the charge redistribution can be adequately achieved by heating. In some quartz samples, significant charge is also transferred from the 200-300°C region (Aitken and Smith, 1988) and this necessitates heating for several minutes
CHARGE
MOVEMENTS
above 200°C. The natural sample also requires replicate heating to ensure full transfer and match any induced sensitivity change. Another consequence of this charge transfer is that if shallow-trapped charge is still present after the resetting event, some of it will find its way to the deep traps and cause an over estimation of age. This recuperation has been discussed in detail previously (Aitken and Smith, 1988). The kinetics raise questions about the interpretation of “shine plateaux”, which are plots of the equivalent dose (the dose that the natural sample is determined to have received) as a function of the laser exposure time. For non-first-order kinetics, the form of OSL decrease upon laser exposure will vary with charge population, and care must be taken when comparing OSL emissions at individual exposure times. The safest approach is to integrate the full OSL emission over the longest exposure possible. However, so long as the OSL is not approaching saturation, the shine plateau can give valuable information on other aspects. For example, following the observation that the OSL decrease comprises more than one component, i’t should be possible to detect inadvertent bleaching during sample collection or preparation. Similarly the lack of a plateau may indicate incomplete bleaching at the time of the event
333
IN QUARTZ
being dated, as originally suggested by Huntley et al. (1985). Acknowledgements-This study has been aided by the keen interest shown by Martin A&ken, Nigel Spooncr, Scott Wheeler and Stephen Stokes. The research was supported by the Science-based Archaeology Committee of the U.K. Science and Engineering Research Council.
REFERENCES Aitken M. J. and Smith B. W. (1988) Optical dating: recuperation after bleaching. Quot. Sci. Reu. 7, 381-393. Huntley D. J., Godfrey-Smith D. I. and Thewalt M. L. W. (1985) Optical dating of sediments. Nature 313, 105-107. Rhodes E. J. (1988) Methodological considerations in the optical dating of quartz. Quot. Sci. Reu. 7, 395-400. Smith B. W., Aitken M. J., Rhodes E. J., Robinson P. D. and Geldard D. M. (1986) Optical datina: methodological aspects. Radici. Prbteit. Dosim. 15, 229-233. Temuler R. H. (1986) The localized transition model *of anomalous fading. Radiat. Protect. Dosim. 17, 493-497. Texier J.-P., Huxtable J., Rhodes E., Miallier D. and Ousmoi M. (1988) Nouvelles don&s sur la situation chronologique de.l’Attrien du Maroc et leurs implications. C. R. Acad. Sci. Paris 307. 827-832. Wintle A. G. (1975) Thermal quenching of thermoluminescence in quartz. Geophys. J. Roy. Astron. Sot. 41, 107-l 13.