Luminescence sensitivity change due to bleaching of sediments

0735-245X/92 $5.00+ .00 Pergamon Press Ltd

Nucl. Tracks Radiat. Meas., Vol. 20, No. 4, pp. 567-573, 1992 Int. J. Radiat. AppL Instrum., Part D

Printed in Great Britain

LUMINESCENCE SENSITIVITY CHANGE DUE TO BLEACHING OF SEDIMENTS SHENG-HuALi and A. G. WIWrLE Institute of Earth Studies, University College of Wales, Aberystwyth SY23 3DB, U.K. (Received 20 June 1991;/n revised form 1 April 1992)

Abstract--Different sensitivity changes of luminescence signals were observed in colluvial and aeolian sediments. It is suggested that not only does the sensitivity change relate to the laboratory bleaching and the age of the sample, but it also relates to the exposure prior to deposition. This sensitivity change is explained by a model, which involves the competition of hard and easy-to-bleach trapped charges. The implications for optical dating of sediments are discussed.

2. EXPERIMENTAL P R O C E D U R E S

1. I N T R O D U C T I O N SENSITIVITYchanges after laboratory bleaching have been reported in several luminescence dating studies. For aeolian sediments, e.g. loess and sand dune samples, the sensitivity of the luminescence signal (thermoluminescence (TL) and optically stimulated luminescence (OSL)) has been found to increase after laboratory bleaching (Rendell et al., 1983; Smith et al., 1990). For TL measurements this sensitivity increase was significant for old samples but negligible for young samples (Rendell and Townsend, 1988; Zhou and Wintle, 1989). A simple model which hypothesized that the sensitivity increase was dosedependent was put forward by Wintle (1985); but Bowall et al. (1987) reported a sensitivity change which depended upon the bleaching time. In a more recent study, infrared stimulated luminescence (IRSL) signals from colluvial sediments (colluvial sediments contain material that has been transported across, and deposited on slopes as a result of wash and mass movement processes) from South Africa showed a sensitivity decrease, rather than increase, after laboratory bleaching with a solar simulator (Li and Wintle, 1991). Not only did this sensitivity change occur for old samples (36 ka), but it also occurred for a young sample (5.1 ka). Other experiments showed that no change was found after prolonged exposure to infrared (IR) radiation. TL studies on these samples indicated that the TL signal was not well bleached at deposition. The differences in bleaching response led us to consider the effect of the main difference between aeolian and colluvial sediments, i.e. the degree of exposure to sunlight prior to deposition. Aeolian sediments have been exposed to sunlight for much longer than colluvial sediments. In this paper the effects of this difference are explored.

2.1. Design o f experiment

Two sets of samples were prepared: one set was well-exposed to light from a solar simulator (SOL 2) to emulate an aeolian deposit; and the other was given a much shorter exposure to the light source to emulate colluvium at deposition. Measurements of their IRSL and TL response to subsequent irradiation and bleaching treatments were then made. Altogether eight subgroups were taken from the 192 discs prepared as described in the following section. Three subgroups (Fig. 1) were exposed to the solar simulator for a relatively short time (0.5 h) and these form the "artificial colluvial sediment". One of these subgroups was given five additional irradiation doses to construct an "additive growth curve" which thus simulated the increase in luminescence signal with age. The other two subgroups of "artificial coiluvial sediment" were given a single radiation dose of 120min to simulate a colluvium of a particular age. These two groups were then given different light exposures with the solar simulator (0.5 and 48 h) as used in typical dating procedures. The discs were then given the same five irradiation doses to produce two "regeneration growth curves", after short and long bleaching, respectively. The remaining five subgroups (Fig. 2) were exposed to the solar simulator for 48 h and form the "artificial aeolian sediment". One subgroup was given five additional doses to construct an "additive growth curve" and thus simulated the increase in luminescence signal with age. The remaining four subgroups were divided into two groups which were given radiation doses of 20 and 120 rain and thus simulated aeolian sediment of two different ages ("young" and 567

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SHENG-HUA LI and A. G. WINTLE THREE GROUPS OF DISCS

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FIG. 1. Expefimen~l ~quence of the "a~ifi~al colluvial sediment". "old", respectively). For each of those groups, one subgroup was given a short light exposure (0.5 h) and the other a long light exposure (48 h) before a "regeneration growth curve" was constructed for each, as already described for the "artificial colluvial sediment". 2.2. Experimental conditions The sample used in this experiment was a 700-kaold loess from Luochuan, Shaanxi, China. The luminescence signal of this sample had reached an equilibrium level, and hence it could be treated as typical source material. Fine grains (4-11/am), extracted by our routine method, were deposited onto the 10 mm diameter aluminium discs. For all natural discs, the IRSL from a 0.1 s infrared exposure was used for normalization. The reduction of either the IRSL or the TL as a result of this 0.1 s exposure is negligible. A solar simulator (SOL 2 from Dr Hohnle, Martinsried, F.R.G.) was used for the bleaching

experiments. According to the manufacturer, this solar simulator has a similar spectral distribution to sunlight, and produces a constant intensity up to 6.5 times that of natural sunlight. To ensure the bleaching was the same for all the groups, discs were placed on an aluminium plate and bleached together for each of the long and short bleaching times. The growth curves were constructed with the same applied doses: 0; 20; 40; 80; 120 and 200 rain of beta irradiation. A Daybreak 9°Sr-9°Y beta source was used for the irradiation. The dose rate was 3.79 Gy min- t. In order to remove the thermally unstable IRSL components, discs were preheated together in an oven for 16 h at 140°C. In IRSL measurements the stimulating infrared was 880 nm (A80 nm) from a diode array whose power at the sample disc was about 20 m W c m -2. The IRSL was detected by an EMI 9635QB photomultiplier with one 2 mm thick Schott BG39 glass filter in front of it. An infrared exposure of 1 s was used for the IRSL measurements, which resulted in a 4.5% reduction of the IRSL signal and

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L U M I N E S C E N C E SENSITIVITY C H A N G E a negligible reduction of the TL signal. Therefore, the discs used in the IRSL measurement were also used in the TL measurement, allowing the TL and IRSL results to be compared directly. The TL measurements were performed in a Riso automatic TL reader. The optical filter pack, containing a Coming 7-59 and a Chance Pilkington HA3, was placed in front of an EMI 9635QB photomultiplier. The heating rate was 5°C s-t up to 450°C. For the dose range used the IRSL and TL signals showed non-linear growth with dose and a third order polynomial fitting was used. The sensitivity of the luminescence signals after bleaching was given by the initial slope of the growth curve. To minimize the effect due to different experimental conditions, discs were treated together at the same time for bleaching and preheating. The same irradiation doses were employed for all the growth curves. At least 24 h delay was introduced between each experimental procedure

3. RESULTS The results of the IRSL and TL measurements carried out on the same discs are summarized in Figs 3 and 4, and Table I. Each data point is the mean of the measurements for four discs. In Fig. 4(a)-(c) and in Table 1, the TL signal is taken as the integral of the TL signal in the region between 280 and 380°C. To ensure more direct comparison of the data sets, the additive dose curve has been shifted along the dose axis by the appropriate dose (20 or 120rain at 3.79Gymin-~). Sensitivity changes greater than 5% are taken to be statistically significant.

3.1. Infrared stimulated luminescence (IRSL ) 3.1.1. Colluvial sediment. Compared with the "additive dose" growth curve, the "regeneration" growth curve for the "artificial colluvial sediment" showed a significant sensitivity decrease ( - 2 7 % ) after the long bleach (Fig. 3(a), Table 1), but a negligible sensitivity change ( - 2 . 1 % ) for the short bleach. 3.1.2. Aeolian sediments. "Young sample". No significant sensitivity changes were found in the young sample after either the long or short laboratory bleaches (Fig. 3(b), Table !). "Old sample". A significant sensitivity increase ( + 1 8 . 5 % ) was found for the aeolian sediments after the short laboratory bleach, but no significant change ( - 4 . 6 % ) after the long laboratory bleaching (Fig. 3(c), Table 1). It should be noted that the sensitivity changes for the colluvial and the aeolian sediments are not due to the residual IRSL signal. Even after the short bleach, the IRSL was close to the background level, although consistently higher than the level after the long N T ~0/4---D

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bleach. The background measured on a blank disc was 335 cps.

3.2. Thermoluminescence ( TL ) The TL results are more complicated. As expected, significantly different residual TL signals were found for the short and long bleaches (Fig. 4(a)--(c)). This clearly demonstrates that knowledge of the residual TL is critical when the equivalent dose (ED) is determined by the regeneration method. Because there is a finite residual signal, the sensitivity of the TL signal cannot be defined.

4. DISCUSSION For the "artificial colluvial sediment", the sensitivity of the IRSL signal is decreased after the long laboratory bleach, but not after the short bleach. For the "old .... artificial aeolian sediment", the sensitivity increases after a short bleach but no significant change occurs after a long bleach. The "young" "artificial aeolian sediment" did not show any significant sensitivity change. The important point to notice is that the sensitivity change after the short bleach was substantially different for the two "artificial aeolian sediments". This implies that sensitivity changes would lead to an erroneous ED determination for the regeneration method using this short bleach. We can also compare the behaviour of the "old" "'artificial aeolian sediment" and the "artificial colluvial sediment", since both were given the same initial dose (120min). Two sensitivity changes were ob° served: for the "colluvial sediment", there was a sensitivity decrease after the long bleach, whereas for the "'aeolian sediment" there was a sensitivity increase after the short bleach. These different responses must be the result of the amounts of light received prior to the 120 min dose, i.e. equivalent to the light exposure at deposition. This suggests that not only is the sensitivity change relative to the laboratory bleaching and age of the sample, but also the last exposure to light. These results for "artificial sediments" mirror sensitivity changes reported in the literature (Li and Wintle, 1991). The TL results further confirm the importance of being able to estimate the residual signal (Aitken, 1985). Unfortunately the size of the residual signals in the experiments precludes firm conclusions from being drawn concerning TL sensitivity changes particularly after the short bleach--the slope of the growth curve above the residual TL level is dependent upon the relative magnitude of the residual level, particularly for the "'colluvium". However, comparing the shapes of the shifted "additive growth curve" and the "regeneration curve" in Fig. 4(a), it may be concluded that there has been a negligible change in sensitivity as a result of the 0.5 h bleach.

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L U M I N E S C E N C E SENSITIVITY C H A N G E 5. A QUALITATIVE M O D E L In a recent paper, McKeever (1991) has attempted to model the effects of optical bleaching of quartz. Despite the lack of experimental information concerning the physical nature of the traps and recombination centres, McKeever developed a model which simulated the response of the TL signals as a function of bleaching exposure. He concluded that the bleaching process alters the distribution of trapped charge and that because several variables are involved (past irradiation, duration of bleaching, and the wavelength of illumination) it is not possible to predict whether the sensitivity will increase or decrease after bleaching.

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In this paper we suggest that the behaviour of the IRSL signal is simpler and can be explained by an empirical model. In IRSL measurement, and in the bleaching of the IRSL signal by IR exposure, the IR causes electrons to be excited from a deep trapping level to an immediate level from which they are thermally released at room temperature into the conduction band (Hftt et al., 1988). Activation energies of 0.1-0.2 eV have been obtained for this thermal release for a variety of feldspars (Bailiff" and Poolton, 1991; Duller and Wintle, 1991). We put forward a model based on the IR stimulated response alone. We hypothesize that there are two types of electron traps--those that contribute to the luminescence signal and those which do not.

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Table 1. Sensitivity change and equivalent doses Groups Expected ED (rain) Bleach IRSL sensitivity change ED (IRSL) (rain) ED (TL) (rain)

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The former bleach rapidly on exposure to light, whereas the latter do not. F o r convenience we will refer to them as E-type (easy to bleach) and H-type (hard to bleach) analogous to the nomenclature of McKeever, based on that of Wintle and Huntley (1980). These traps compete for electrons during irradiation. If colluvial sediments receive only a short sunlight exposure, the E-type traps would have been emptied, but the H-type traps remain relatively full. During irradiation after deposition the electrons are preferentially trapped at the E-type traps. If in the subsequent dating procedure a bleach is applied which results in the emptying of not just the E-type traps but also the H-type, then a decreased IRSL response to dose will be observed. This would result in an overestimate of the ED. It would not matter whether a sunlamp or sunlight was employed. However, if bleaching is carried out with an IR source which has the same spectrum as that used for the measurement, the H-type traps will remain full. This can be seen from the TL signal which remains after the IRSL is reduced to < 1 % by 1000s o f l R exposure (Li and Wintle, 1992). In this case the E-type traps will fill under the same competing regime as in nature, and the sensitivity of the IRSL response to dose when regenerated will be the same. Hence the ED will be a correct evaluation of the past dose. This implies that the ED obtained by a single disc regeneration method will be correct provided that bleaching with IR is carried out between each dose step. However, aeolian sediments will behave differently if we hypothesize that the H-type traps are emptied at deposition. During irradiation after deposition, electrons are able to be trapped at either H- or E-type traps resulting in a lower natural IRSL sensitivity (compared with colluvial sediment). For extended spectrum bleaching (e.g. with SOL 2) the E-type traps empty very fast, but the H-type traps will alter with the period of laboratory bleaching. It is even more likely to be affected by the extent of filling of the H-type traps that occurred during the natural irradiation, i.e. upon the age of the sediment. An IRSL sensitivity increase would be expected after a short bleaching because a relatively large number of H-type traps remained filled. This was observed experimentally. For a sample of the same age, the sensitivity change will decrease with increased light exposure.

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6. C O N C L U S I O N Either increases or decreases in IRSL sensitivity can occur after laboratory bleaching and irradiation. These sensitivity changes depend upon the degree o f sunlight exposure of the sediment prior to deposition and the amount of radiation received by the sample since deposition. Only the latter effect has been reported for TL measurements. Although the effect of limited sunlight exposure has long been recognized as a problem in TL dating of water-lain sediments (see Berger, 1988, for review), it might have been thought that, provided exposure was long enough to zero the IRSL signal, IRSL dating methods would yield the correct age. This seems to be unlikely if the regeneration method is applied without careful choice of the most appropriate laboratory bleaching procedure. Change in sensitivity of the IRSL signal brought about by laboratory bleaching might be explained in terms of a competition model involving two trapped charge populations. For many samples the degree of sunlight exposure is likely to lie between those used in the experiment. The luminescence sensitivity can therefore increase or decrease depending upon the age of the sample, laboratory bleaching and light exposure prior to deposition. If the sensitivity is dependent on the H-type charges as mentioned in the model, it may be predicted that: (1) the sensitivity of a particular sample will decrease as the laboratory bleaching time is extended; (2) under similar laboratory bleaching conditions, the sensitivity change will reflect the degree of sunlight exposure prior to deposition for two different sediment types of the same age; and (3) if the bleaching light source is the same as the stimulating light used for dating, the sensitivity will increase for most samples to an extent which is dependent on the age of the sample, and the extent of light exposure. Acknowledgements--Useful discussions with Mr G. A. T. Duller and Mrs F. M. Musson are acknowledged. This is publication No. 237 of the Institute of Earth Studies, UCW, Aberystwyth. REFERENCES

Aitken M. J. (1985) Thermoluminescence Dating. Academic Press, London.

LUMINESCENCE SENSITIVITY CHANGE Bailiff I. K. and Poolton N. R. J. (1991) Studies of charge transfer mechanisms in feldspars. Nucl. Tracks Radiat. Meas. 18, 111-118. lkrger G. W. (1988) Dating Quaternary deposits by luminescence. In Dating Quaternary Sediments (Edited by Easterbrook D. J.). Geological Society of America, Special paper 227, pp. 13-50. Bowall L., Frean R., McKeogh K. J., Rendell H. M. and Townsend P. D. (1987) Sensitivity change in the TL of quartz and feldspar after bleaching. Cryst. Latt. Defect Amorph. Mater. 16, 37-43. Duller G. A. T. and Wintle A. G. (1991) On infrared stimulated luminescence at elevated temperatures. Nucl. Tracks Radiat. Meas. 18, 379-384. Hiitt G., Jack I. and Tchonka J. (1988) Optical dating: K-feldspars optical response stimulation spectra. Quat. Sci. Rev. 7, 381-385. Li S.-H. and Wintle A. G. (1991) Sensitivity changes of luminescence signals from colluvial sediments after different bleaching procedures. Ancient TL 9, 50-54. Li S.-H. and Wintle A. G. (1992) A global view of the stability of himinescence signals from loess. Quat. Sci. Rev. 11, 133-137.

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McKcever S. W. S. (1991) Mechanisms of thermoluminescence production: some problems and a few answers? Nucl. Tracks Radiat. Meas. 18, 5-12. Rendell H. M., Gamble I. J. A. and Townsend P. D. (1983) Thermoluminescence dating of loess from the Potwar Plateau, northern Pakistan. P A C T 9, 555-562. Rcndell H. M. and Townsend P. D. (1988) Thermoluminescence dating of a 10 m loess profile in Pakistan. Quat. Sci. Rev. 7, 251-255. Smith B. W., Rhodes E. J., Stokes S., Spooner N. A. and Aitkcn M. J. (1990) Optical dating of sediments: initial quartz results from Oxford. Archaeometry 32, 19-31. Wintle A. G. (1985) Sensitization of TL signal by exposure to light. Ancient TL 3, 16-21. Wintl¢ A. G. and Huntley D. J. (1980) Thermoluminescence dating of ocean sediments. Can. J. Earth Sci. 17, 348-360. Zhou L.-P. and Wintle A. G. (1989) Underestimation of regeneration ED encountered in Chinese loess. In synopsis from a workshop on long and short range limits in luminescence dating, Oxford, April 1989, Res. Lab. Archaeo. Hist. Art, Oxford University, occasional publication, Vol. 9.