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Sensitivity change of thermoluminescence signals after laboratory optical bleaching: experiments with loess fine grains
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0277-379 I(94)E0039-D
SENSITIVITY CHANGE OF T H E R M O L U M I N E S C E N C E SIGNALS AFTER LABORATORY OPTICAL BLEACHING: EXPERIMENTS WITH LOESS FINE GRAINS L.P. Zhou*, and A.G. Wintle-: * 77w Godwin Lahoratorv .lor Quatermto' Research, University of Camhridge, Free School Lane, Cambridge CB2 3RS, U.K. :i:lnstmae o t" Earth Studies, UniveJwitv College of Wales, A hervstwvth, D3J'ed S Y23 3DB, U.K.
Experiments were designed to stud}' the effect of laboratory optical bleaching on thermoluminescence (TL) sensitivity ch~,nge using line grains extracted from Chinese loess. There is a tendency for an increase in TL sensitivity after laboratory bleaching treatment for "artificially aged" samples, i.e. subsamples of a yot, ng sample irradiated with progressively increased laboratory doses. The sensitivity increase tends to become signiticant at a threshold dose and seems to approach an upper limit of 20-30% increase compared with the sensitiviw of the unbleached young sample. The implications of the results for dating relatively old loess samples are discussed and ways of correcting and circumventing the sensitivity change are explored.
INTRODUCTION In thermoluminesccncc (TL) dating, the TL sensitivity of the sample, defined as the luminescence intensity per unit radiation dose for a given weight, has to be determined. If this quantity remains the same under natural and laboratory conditions, its use in the equivalent dose (ED) determination (e.g. Aitken, 1985) is justified and straightforward. However. laboratory treatments which emplo.~ different light sources, a single type of radiation and proceed at a time-scale different from that in nature (the dose rates in nature and in the laborator\ differ bx about Ill,") may result in a different sample response, i.e. a change in TL sensitivity (Rendell el a/.. 1983). This could lead to either underestimation or overestimation of the TL age of the sample. Although our understanding of the mechanisms of TL sensitivity, c h a n o c e h a s b e e n ~ r c a t h . i n l p r o \ e d - for example, through intcnsixc studies of the l lll(" signal from quartz (of. McKcc~cr. 1 9 t i l l - - problenls in the regeneration method of El) determination for fine-grained mixed minerals remain, particuhlrl.x for loess samples which are dominated b \ tcldspar T[. signals (Debenham. 1985: \Vinllc. 1985:.1). Models have been put forward to a c c o u n t f o f t h e ;.ipllal-Clll p o s l bleach sensitivity changes obscrxcd in line grains from European and other loess (\Vintlc. 1985b: Rcndcll and Townsend, 1988). In this paper, x~c present the results of a series of experiments designed to investigate the effects of laboratory illunlination on the TL sensitivil~ of fine grains extracted from loess samples from China.
.Present address: The M c l ) o n : d d ln,,lilute lor ..\rchaeological Research, Universitx el ('ambridgc. l)ox~nmg Street. ('ambridgc CB2 3ER, U.K.
In particular, we examine the relationship between the sensitivity change and previous radiation dose history of a sample. The implications of these results for ED determinations and possible approaches of correcting and circumventing the effects of the sensitivity change are discussed.
NATURE OF LABORATORY OPTICAL BLEACHING One widely noted feature in optical bleaching is the build-up of TL in the low temperature part of the glow curve while the higher temperature signals are reduced (Wintle and Huntley. 1980). An experiment was performed to determine whether the traps giving rise to the transferred TL are related to a TL signal from a narrow glow curve temperature region, or from a range of TL signals at higher temperatures. Seven of eight groups of fine grain discs (natural samples of O T L I I T J from Xifeng, central Loess Plateau) were first glowed from room temperature to 1511, 200, 2511, 3110. 350,401t and 450°C, respectively. These partially annealed discs, together with the unheated natural TL (NTL) discs, were then subjected to 180 min of illumination with a Honlc SOL2 solar simulator. Measurements were made immediately after the bleach and the results for the ultraviolet (UV) emission measured using a Schott U G I I and Chance Pilkington HA-3 filter combination are given in Fig. I. It can be seen that the TL intensity of the 170°C peak decreases with the progressively increased partial annealing temperature, with the discs annealed at 450°C yielding the lowest signal for this peak (Fig. la). The gradual decrease of the 170°C peak with increased partial annealing temperature suggests that the deep traps responsible for the induced 170°C peak arc related to a range of
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TL signals corresponding to a glow curve t e m p e r a t u r e from 200 to about 400°C, rather than from a signal confined to certain t e m p e r a t u r e (Fig. lb). A similar dependence on the partial annealing t e m p e r a t u r e was found for TL signals measured in the blue and green spectra using filter combinations listed later.
To test whether the difference in the spectral distribution of the light sources resulted in different effects on the sensitivity change, a very old sample from Lanzhou. Gansu (QTL99S8B, expected age >500 ka, J. Wang, pers. cornmun., 1988) was studied. The three bleaching light sources were blue sky sunlight (Cambridge, U.K.), the SOL2 solar simulator and the unfiltered 300 W mercury sunlamp. The illumination time was 600 minutes. A test beta dose of about 20 Gy was added 48 hours after the optical bleaching, The TL m e a s u r e m e n t was made with the U G 11 and HA-3 filter combination. The relationship between TL sensitivity and residual TL level (Io) is similar to the results for the sample above. As is shown in Fig. 3, a lower I o corresponds to a lower TL sensitivity.
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Illumination Time Dependence Polymineral fine grains from sample Q T L I I 6 Z Z . which was collected below a palaeosol believed to be of the last interglacial, hence with an expected age of 130 ka, at Lishi, Shanxi, were used to study the effect of bleaching time on T L sensitivity. Laboratory bleaching was p e r f o r m e d with either the SOL2 solar simulator or a glass-filtered 300 W mercury sunlamp. Illumination times were 30, 90, 300 and 600 min for the SOL2 and 16 hr for the mercury sunlamp. A test beta dose of about 20 Gy was added 48 hr after the optical bleaching. The T L m e a s u r e m e n t was made with the U G 11 and HA-3 filter combination. The TL sensitivity was found to decrease with increasing illumination time for the SOL2 (Fig. 2). The aliquot bleached with the mercury sunlamp for 960 min yielded an intermediate sensitivity which was very close to that for 90 rain SOL2 exposure.
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Fl(i. 3. Post-hlcach TL sensitivity variation with different light ,,C,tll-CCSlor UV emission (QTL99SSB). Illumination time 66(l inin. The SOL2 solar simuhttor has a similar spectral distribution to daylight and produces a constant intensity up to 6.5 times that of natural sunlight, whereas the unfiltered mercury sunlamp is expected to have a much stronger UV component. The apparent linear relationship between I o and the T L sensitivity seems to suggest that the differences in T L sensitivity may also be related to the different intensities of the light sources.
Dose Dependence In a previous T L dating study of Chinese loess, wc found that sensitivity changes tend to occur for the
TL Scnsitivilx ('han,,c ot L.cs~
relatively older samples (Zhou and Wintle, 1989). As shown in Fig. 4, for fine grains of young samples, the additive and regenerated curves fall on top of each other when one is shifted along the dose axis, suggesting that there is a negligible change in sensitivity resulting from the laboratory optical bleach for the regenerated curve• In contrast, for the older samples the two curves cannot bc shifted in such a way as to overlay one another• This results in a significantly higher ED for the total bleach method•
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As a result of the complex emission of the constituent feldspar and quartz in the fine grains, it has been difficult to assess the effect of the sensitivity change observed in the loess samples. We carried out an experiment to simulate specifically the situation for the old samples with laboratory irradiation. The design of the experiments was based on the postbleach dose-dependent sensitization model proposed by Wintle (1985b). Similar experiments have been reported by Frechen (1991) and for optically stimulated luminescence (Stoneham and Stokes, 1991) and infrared stimulated luminescence Li and Wintle, 1992).
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The experiment scheme Is shown in the inset of Fig. 5. Fine grains of a young sample (QTL117A, dated to 12 ka) from a loess section at Xifeng were used. All discs were prepared at the same time using standard procedures (Wintle and Huntlcv, 1980). They were divided into eight groups, six of which were "artificially aged" by given beta doses from 70 to 623 Gv. They were then bleached with the SO[,2 solar simulator for 300 min, as has been used in dating by the regeneration method. A set of unirradiatcd natural discs was also bleached. A test beta dose (ca. 35 Gy) was then
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L.P. Zhou and A.G. Wintlc
46(1
added to these bleached discs. Thermoluminescence m e a s u r e m e n t s (heating rate 5°C/sec) were made on a D a y b r e a k T L reader after a 16 hr preheat at 140°C as routinely used in dating. The U G l l and HA-3 filter combination was used. A period of 48 hr was introduced between each laboratory treatment described above. The natural TL sensitivity is defined as Sn, the slope of a straight line linking the N T L and N + 13 point, and the regeneration TL sensitivity as Sr, the slope of a straight line connecting the bleached N + 13 aliquot (I.) and the I. + [3 point. The TL sensitivities of the "artificially aged" samples, both relatively "young" and "old', are c o m p a r e d with that of a natural TL sample. Similar experiments were performed on the same young sample (QTL117A) using another filter combination, an Oriel green dichroic filter and an HA-3 filter (referred to subsequently as the green filter set), and on a young sample from Baoji (QTL120A, dated to 16 ka using a blue filter combination), southern Loess Plateau using a Corning 5-58 and an HA-3 filter (which passes blue-dominant TL).
a function of laboratory added dose obtained with the green filter combination. At the glow curve t e m p e r a t u r e of 290°C, an 8 + 3 % increase in TL sensitivity is observed when the laboratory added doses are relatively low. The Sr is increased for the higher added doses up to 612 Gv, giving a mean of 27_+6% increase in T L sensitivity.
Blue dominant TL Plots of Sr versus laboratory added dose up to 612 Gv for QTL120A arc shown in Fig. 7 for the blue emission. There is a difference in the TL sensitivity between the two ranges with low ( < 7 0 Gy) and high (140-623 Gy) added beta dose. The regeneration TL sensitivity Sr for the latter range have an average value of 10.2+0.2 at the 320°C coordinate, which is about 26% higher than that for the lower added dose range. At the 360°C t e m p e r a t u r e coordinate, a similar difference is observed for the two dose ranges, with average Sr values being 7.4+(I.1 and 9.3+(I.5, respectively (Fig. 7).
Results
DISCUSSION
UV dominant TL The Sr values as a function of the added dose arc shown for TL at the 30(1°C glow curve temperature (Fig. 5). At the lower dose range there is a relatively small change in the Sr value, which is very similar to the S;; value, plotted against the E D of the young sample used. There then appears to be a rising trend in the Sr with increasing irradiation dose, with a noticeable TL sensitivity increase being seen when the added dose reaches around 14() Gv. The increase then appears to slow down but, at the 623 Gv dose level, the sensitivity decreases sharply. Green dominant TL Figure 6 shows the natural TL sensitivity, 5;n, and variation in the regeneration TL sensitivitx, 3r, as
The trend of decreasing Sr with smaller 1o in polymincralic grains (Figs 2 and 3) implies that extended optical bleaching, which removes more natu-
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461
TL Sensitivity Change ol Loess
ral TL signal, could give rise to a higher ED. For sediments such as loess, which are expected to have been exposed thoroughly to sunlight before deposition, this effect should be small, and the preferred laboratory treatment should be a long bleach. The aliquots with laboratory added beta doses may be taken to represent progressively older samples from the sarnc section as the young sample. The Sr values observed are then considered as TL sensitivity, which is related to previous radiation dose, i.e. the 'age" of the sample. Although the variation of Sr values is fairly largc and the differences in TL sensitivity between the unbleached young sample and the artificially aged subsamples are not more than 3 0 % , the overall trend of these results indicates that there is a sensitivity increase with increasing laboratory added radiation dose, which may be attributed to the laboratory optical bleaching before the test beta dose. This is consistent with the prediction of the model proposed by Wintle (1985b). As a sensitivity increase will lead to an ED underestimation, the experimental results above may provide one, but not necessarily the only, explanation for those regeneration ages which are found to be younger than expected (Wintle, 1990). Two points may be noted from the modelling experiments presented here. First, there appears to be a threshold dose level above which sensitivity enhancement becomes significant. This dose value is found around 20(I Gy (corresponding to an age of about 45 ka for Chinese loess) in these experiments, but it may vary for samples from different sites. Second, there seems to be an upper limit of post-bleach sensitivity enhancement of around 20-30% for samples with an ED of the order of a few hundred grays.
sensitivity change was attributed to defect clustering, which favours the formation of larger, more stable defect complexes during storage or preheating of the bleached samples (Bowall et al., 1987). To test the relevance of these findings to Chinese loess, several storage experiments were carried out and the EDs thus obtained were compared with those obtained when one day was allowed between bleaching and beta irradiation (Table 1). For a loess sample from the Xifeng section Q T L I 1 7 Q (underneath the last interglacial palaeosol, thus expected to be ca. 130 ka), a 58 day room temperature storage was allowed for bleached discs before adding the regeneration beta doses. No E D plateau emerged until a glow temperature of 340°C was reached, when a slightly higher mean E D of 451 _+13 Gy was obtained, compared with 414_+ 12 Gy without storage. If the recovery from sensitization is a thermal process, the restoration of the original sensitivity should be accelerated by holding the bleached discs at elevated temperatures for a certain period of time. This hypothesis led to another experiment with the same sample. Bleached discs were held at 140°C for 16 hr before beta doses were added to regenerate the TL signal. Measurements were made with the UG11 and HA-3 filters, as for the above measurements, and E D values were 423_+ 11 Gv around 300°C and 452_+15 Gy around 350°C. Compared with the 414_+12 Gy results, the increases in ED arc hardly significant.
T A B L E 1. Effects of laboratory treatments on equivalent doses (ED) Sample QTL117Q
CIRCUMVENTING THE SENSITIVITY CHANGE Q'FLgt.~ZII0
In the preceding sections, we have shown that the sensitivity change may occur for the relatively old samples when the regeneration method is used to determine the EDs. Although more effort will be directed to understanding the mechanism of the sensitivity change and to developing more accurate correction procedures, it may be more desirable to explore ways of circumventing the sensitivity change. In this section, we examine the effects of laboratory treatments on the sensitivity change and discuss the applicability of a recent adaptation of the regeneration method which requires no optical bleaching (Smith, 1983).
Effects of
Laboratory Treatment
Bowall et a/. (1987) and Rendell et al. (1988) suggested that sensitivity change is a time-dependent effect for loess samples from Pakistan. The original TL sensitivity appeared to be restored if a delay was introduced between bleaching and laboratory irradiation and particularly between laboratory irradiation and readout. Such a transitory behaviour of
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Rendell et al. (1988) showed that the time-dependent effect was most pronounced in the green emission for Pakistan loess. Bleached discs of sample QTL99Z00 from Lanzhou were stored at room temperature for over 11 months before beta irradiation was administered to regenerate the TL signal. The green filter combination was used for TL measurement and thc E D obtained was the same as that without the post-bleach delay within the error limits, i.e. 243+15 compared with 257_+22 Gy (Table 1). The apparent lack of transient effect of sensitivity change in these samples indicates some differences in the T L behaviour between Chinese loess and Pakistan loess. For the samples examined here the relaxation time may be longer than months and the restoration of the original sensitivity does not seem to be particularly sensitive to the temperature used in thc experiment.
462
L.P. Zhou and A.G. Wintlc
ED Determination by Regeneration without Bleaching In dating coarse grain quartz from sands in Australia, Smith (1983) introduced a regeneration method which requires no optical bleaching for most samples. In this method (hereafter called Smith's method), the EDs for the older samples from a section are obtained by comparing their natural TL levels with the N + 13 growth curve constructed with the youngest sample and by adding the ED of the youngest sample (Fig. 8). Ideally, this method may be used to date samples which may show post-bleach TL sensitivity change. Two assumptions are made for samples from different levels of the same section. First, the residual TL signals reached after solar bleaching in antiquity are similarly low. Second, the TL sensitivities of the samples are largely the same. Figure 9 shows the EDs obtained by Smith's method in the UV filter pack and compares them with those derived from the additive and regeneration methods for the same samples from the Xifeng section. The
correspondence of the two ED sets is reasonably good for the three youngest samples, but becomes rather poor for the rest. The EDs obtained using Smith's method appear to be closer to those of the conventional regeneration method and lower than those of the total bleach method. The most likely cause of the poor correlation may be related to the assumption that the sensitivity is the same for all the samples. The glow curves for samples from different levels are not identical and normalization for the natural discs of the progressively old samples was required. Second glow normalization was used, but this procedure may have caused an additional dose-dependent sensitivity change which would result in the EDs being lower than the true values for these old samples. Better controlled sample preparation will be required to establish whether the differences in TL sensitivity between the samples are intrinsic to the samples or are artifacts of sample preparation.
CONCLUSIONS
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The preliminary experimental results presented in this study indicate an optically induced sensitivity change in fine grain loess samples from Chinese Loess Plateau. The modelling experiments suggest that the sensitivity change may be related to the previous radiation dose, i.e. the age of the sample. The sensitivity increase occurs at a threshold dose level around 200 Gy (corresponding to an age of about 45 ka for Chinese loess) and tends to approach an upper limit of around 20-30% enhancement. Similar dosedependent post-bleach sensitivity change is observed using three emission regions, in the UV, blue and green. Further work is needed to identify the association of the TL sensitivity change with particular mineral fractions in loess fine grains. The original sensitivity cannot be restored by the laboratory treatments used in this study. When the regeneration method is used for the ED determination for older samples, a sensitivity correction is likely to be required. A rough estimation of the dcgrcc of the sensitivity change in older samples may bc made bv modelling experiments using the youngest sample from the studied section. The application of Smith's method of determining EDs using the response of the youngest sample needs further testing for loess sections, as its fundamental assumption that samples from the same section have identical TL properties did not hold up at the test site of Xifeng.
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ACKNOWLEDGEMENTS
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FIG. 9. Comparison of EDs (300-320°C integrals. SD mostly around 10%) obtained by Smith's method with EDs b~ the Iota] bleach method (EDadd) and the regeneration method" (EDrcg) for UV emission.
LPZ thanks St John's College, Cambridge and the U.K. Government (Overseas Research Studentship Awards) for financial support. We thank Dr H.M. Rendell for constructive comments. This is publication number 359 of the Institute of Earth Studies, UCW, Aberystwyth.
TL Sensitivity Change of Loess
REFERENCES Aitken, M.J. (1985). Thermoluminescence Dating. Academic Press, London, 359 pp. Bowall, L., Frean, R., McKeogh, K.J., Rendell. H.M. and Townsend, P.D. (1987). Sensitivity changes in the TL of quartz and feldspar after bleaching. Crystal Lattice Defects and Amorphous Materials, 16, 37--43. Debenham, N.C. (1985). Use of UV emission in TL dating of sediments. Nuclear Tracks and Radiation Measurements, 10, 717-724. Frechen, M. (1991). Thermolumineszenz-Datierungen an L6ssen des Mittelrheingebiets. Ph.D. Thesis, Geologisches Institut, Universitfil K61n, zu SonderverOffentlichungen, 79, 137 pp. Li, S.H. and Wintle, A.G. (1992) Luminescence sensitivity change due to bleaching of sediments. Nuclear Tracks and Radiation Measurements, 20,567-573. McKeever, S.W.S. (1991). Mechanisms of thermoluminescence production: some problems and a few answers? Nuclear Tracks and Radiation Measurements, 18, 5-12. Rendell, H.M. and Townsend, P.D. (1988). Thermoluminescence dating of a 1(I m loess profile in Pakistan. Quaternary Science Reviews, 7, 251-255. Rendell, H.M., Gamble, I.J.A. and Townsend, P.D. (1983). Thermoluminescence dating of loess from the Potwar Plateau, northern Pakistan. PACT, 9, 555-562.
463
Rendell, H.M., Mann, S.J. and Townsend, P.D. (1988). Spectral measurements of loess TL. Nuclear Tracks atul Radiation Measurements, 14, 63-72. Smith, B.W. (1983). New applications of thermoluminescence dating and comparisons with other methods. Ph.D. Thesis, University of Adelaide. Stoneham, D. and Stokes, S. (199t). An investigation of the relationship between the 110°C TL peak and optically stimulated luminescence in sedimentary quartz. Nuclear Tracks and Radiation Measurentents. 18, 119-123. Wintle, A.G. (1985a). Stability of TL signal in fine grains from loess. Nuclear Tracks and Radiation Measurements, 10,725-730. Wintle, A.G. (1985b). Sensitization of TL signal by exposure to light. Ancient TL, 3. 16--21. Wintle, A.G. (1990). A review of current research on TL dating of loess. Quaterna O' Science Review, 9. 385-397. Wintle, A.G. and Huntley, D.J. (1980). Thermoluminescence dating of ocean sediments. Canadian Journal of Earth Sciences, 17,348-360. Zhou, L.P. and Wintle, A.G. (1989). Underestimation of regeneration ED encountered in Chinese loess. In: Aitken, M.J. (ed.), Long and Short Range Limits in Luminescence Dating. Occasional Paper No. 9 pp. 139-144. Research Laboratory for Archaeology and the History of Art, Oxford University.
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('op?.ughl *E Itlq4 [!lsc~icr Socncc Ltd l'rintcd in (;real Britain All right,, resorted [I,-7-U'tll ,q4 $201111
Pergamon
0277-379 I(94)E0039-D
SENSITIVITY CHANGE OF T H E R M O L U M I N E S C E N C E SIGNALS AFTER LABORATORY OPTICAL BLEACHING: EXPERIMENTS WITH LOESS FINE GRAINS L.P. Zhou*, and A.G. Wintle-: * 77w Godwin Lahoratorv .lor Quatermto' Research, University of Camhridge, Free School Lane, Cambridge CB2 3RS, U.K. :i:lnstmae o t" Earth Studies, UniveJwitv College of Wales, A hervstwvth, D3J'ed S Y23 3DB, U.K.
Experiments were designed to stud}' the effect of laboratory optical bleaching on thermoluminescence (TL) sensitivity ch~,nge using line grains extracted from Chinese loess. There is a tendency for an increase in TL sensitivity after laboratory bleaching treatment for "artificially aged" samples, i.e. subsamples of a yot, ng sample irradiated with progressively increased laboratory doses. The sensitivity increase tends to become signiticant at a threshold dose and seems to approach an upper limit of 20-30% increase compared with the sensitiviw of the unbleached young sample. The implications of the results for dating relatively old loess samples are discussed and ways of correcting and circumventing the sensitivity change are explored.
INTRODUCTION In thermoluminesccncc (TL) dating, the TL sensitivity of the sample, defined as the luminescence intensity per unit radiation dose for a given weight, has to be determined. If this quantity remains the same under natural and laboratory conditions, its use in the equivalent dose (ED) determination (e.g. Aitken, 1985) is justified and straightforward. However. laboratory treatments which emplo.~ different light sources, a single type of radiation and proceed at a time-scale different from that in nature (the dose rates in nature and in the laborator\ differ bx about Ill,") may result in a different sample response, i.e. a change in TL sensitivity (Rendell el a/.. 1983). This could lead to either underestimation or overestimation of the TL age of the sample. Although our understanding of the mechanisms of TL sensitivity, c h a n o c e h a s b e e n ~ r c a t h . i n l p r o \ e d - for example, through intcnsixc studies of the l lll(" signal from quartz (of. McKcc~cr. 1 9 t i l l - - problenls in the regeneration method of El) determination for fine-grained mixed minerals remain, particuhlrl.x for loess samples which are dominated b \ tcldspar T[. signals (Debenham. 1985: \Vinllc. 1985:.1). Models have been put forward to a c c o u n t f o f t h e ;.ipllal-Clll p o s l bleach sensitivity changes obscrxcd in line grains from European and other loess (\Vintlc. 1985b: Rcndcll and Townsend, 1988). In this paper, x~c present the results of a series of experiments designed to investigate the effects of laboratory illunlination on the TL sensitivil~ of fine grains extracted from loess samples from China.
.Present address: The M c l ) o n : d d ln,,lilute lor ..\rchaeological Research, Universitx el ('ambridgc. l)ox~nmg Street. ('ambridgc CB2 3ER, U.K.
In particular, we examine the relationship between the sensitivity change and previous radiation dose history of a sample. The implications of these results for ED determinations and possible approaches of correcting and circumventing the effects of the sensitivity change are discussed.
NATURE OF LABORATORY OPTICAL BLEACHING One widely noted feature in optical bleaching is the build-up of TL in the low temperature part of the glow curve while the higher temperature signals are reduced (Wintle and Huntley. 1980). An experiment was performed to determine whether the traps giving rise to the transferred TL are related to a TL signal from a narrow glow curve temperature region, or from a range of TL signals at higher temperatures. Seven of eight groups of fine grain discs (natural samples of O T L I I T J from Xifeng, central Loess Plateau) were first glowed from room temperature to 1511, 200, 2511, 3110. 350,401t and 450°C, respectively. These partially annealed discs, together with the unheated natural TL (NTL) discs, were then subjected to 180 min of illumination with a Honlc SOL2 solar simulator. Measurements were made immediately after the bleach and the results for the ultraviolet (UV) emission measured using a Schott U G I I and Chance Pilkington HA-3 filter combination are given in Fig. I. It can be seen that the TL intensity of the 170°C peak decreases with the progressively increased partial annealing temperature, with the discs annealed at 450°C yielding the lowest signal for this peak (Fig. la). The gradual decrease of the 170°C peak with increased partial annealing temperature suggests that the deep traps responsible for the induced 170°C peak arc related to a range of
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TL signals corresponding to a glow curve t e m p e r a t u r e from 200 to about 400°C, rather than from a signal confined to certain t e m p e r a t u r e (Fig. lb). A similar dependence on the partial annealing t e m p e r a t u r e was found for TL signals measured in the blue and green spectra using filter combinations listed later.
To test whether the difference in the spectral distribution of the light sources resulted in different effects on the sensitivity change, a very old sample from Lanzhou. Gansu (QTL99S8B, expected age >500 ka, J. Wang, pers. cornmun., 1988) was studied. The three bleaching light sources were blue sky sunlight (Cambridge, U.K.), the SOL2 solar simulator and the unfiltered 300 W mercury sunlamp. The illumination time was 600 minutes. A test beta dose of about 20 Gy was added 48 hours after the optical bleaching, The TL m e a s u r e m e n t was made with the U G 11 and HA-3 filter combination. The relationship between TL sensitivity and residual TL level (Io) is similar to the results for the sample above. As is shown in Fig. 3, a lower I o corresponds to a lower TL sensitivity.
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Illumination Time Dependence Polymineral fine grains from sample Q T L I I 6 Z Z . which was collected below a palaeosol believed to be of the last interglacial, hence with an expected age of 130 ka, at Lishi, Shanxi, were used to study the effect of bleaching time on T L sensitivity. Laboratory bleaching was p e r f o r m e d with either the SOL2 solar simulator or a glass-filtered 300 W mercury sunlamp. Illumination times were 30, 90, 300 and 600 min for the SOL2 and 16 hr for the mercury sunlamp. A test beta dose of about 20 Gy was added 48 hr after the optical bleaching. The T L m e a s u r e m e n t was made with the U G 11 and HA-3 filter combination. The TL sensitivity was found to decrease with increasing illumination time for the SOL2 (Fig. 2). The aliquot bleached with the mercury sunlamp for 960 min yielded an intermediate sensitivity which was very close to that for 90 rain SOL2 exposure.
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Fl(i. 3. Post-hlcach TL sensitivity variation with different light ,,C,tll-CCSlor UV emission (QTL99SSB). Illumination time 66(l inin. The SOL2 solar simuhttor has a similar spectral distribution to daylight and produces a constant intensity up to 6.5 times that of natural sunlight, whereas the unfiltered mercury sunlamp is expected to have a much stronger UV component. The apparent linear relationship between I o and the T L sensitivity seems to suggest that the differences in T L sensitivity may also be related to the different intensities of the light sources.
Dose Dependence In a previous T L dating study of Chinese loess, wc found that sensitivity changes tend to occur for the
TL Scnsitivilx ('han,,c ot L.cs~
relatively older samples (Zhou and Wintle, 1989). As shown in Fig. 4, for fine grains of young samples, the additive and regenerated curves fall on top of each other when one is shifted along the dose axis, suggesting that there is a negligible change in sensitivity resulting from the laboratory optical bleach for the regenerated curve• In contrast, for the older samples the two curves cannot bc shifted in such a way as to overlay one another• This results in a significantly higher ED for the total bleach method•
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As a result of the complex emission of the constituent feldspar and quartz in the fine grains, it has been difficult to assess the effect of the sensitivity change observed in the loess samples. We carried out an experiment to simulate specifically the situation for the old samples with laboratory irradiation. The design of the experiments was based on the postbleach dose-dependent sensitization model proposed by Wintle (1985b). Similar experiments have been reported by Frechen (1991) and for optically stimulated luminescence (Stoneham and Stokes, 1991) and infrared stimulated luminescence Li and Wintle, 1992).
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The experiment scheme Is shown in the inset of Fig. 5. Fine grains of a young sample (QTL117A, dated to 12 ka) from a loess section at Xifeng were used. All discs were prepared at the same time using standard procedures (Wintle and Huntlcv, 1980). They were divided into eight groups, six of which were "artificially aged" by given beta doses from 70 to 623 Gv. They were then bleached with the SO[,2 solar simulator for 300 min, as has been used in dating by the regeneration method. A set of unirradiatcd natural discs was also bleached. A test beta dose (ca. 35 Gy) was then
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L.P. Zhou and A.G. Wintlc
46(1
added to these bleached discs. Thermoluminescence m e a s u r e m e n t s (heating rate 5°C/sec) were made on a D a y b r e a k T L reader after a 16 hr preheat at 140°C as routinely used in dating. The U G l l and HA-3 filter combination was used. A period of 48 hr was introduced between each laboratory treatment described above. The natural TL sensitivity is defined as Sn, the slope of a straight line linking the N T L and N + 13 point, and the regeneration TL sensitivity as Sr, the slope of a straight line connecting the bleached N + 13 aliquot (I.) and the I. + [3 point. The TL sensitivities of the "artificially aged" samples, both relatively "young" and "old', are c o m p a r e d with that of a natural TL sample. Similar experiments were performed on the same young sample (QTL117A) using another filter combination, an Oriel green dichroic filter and an HA-3 filter (referred to subsequently as the green filter set), and on a young sample from Baoji (QTL120A, dated to 16 ka using a blue filter combination), southern Loess Plateau using a Corning 5-58 and an HA-3 filter (which passes blue-dominant TL).
a function of laboratory added dose obtained with the green filter combination. At the glow curve t e m p e r a t u r e of 290°C, an 8 + 3 % increase in TL sensitivity is observed when the laboratory added doses are relatively low. The Sr is increased for the higher added doses up to 612 Gv, giving a mean of 27_+6% increase in T L sensitivity.
Blue dominant TL Plots of Sr versus laboratory added dose up to 612 Gv for QTL120A arc shown in Fig. 7 for the blue emission. There is a difference in the TL sensitivity between the two ranges with low ( < 7 0 Gy) and high (140-623 Gy) added beta dose. The regeneration TL sensitivity Sr for the latter range have an average value of 10.2+0.2 at the 320°C coordinate, which is about 26% higher than that for the lower added dose range. At the 360°C t e m p e r a t u r e coordinate, a similar difference is observed for the two dose ranges, with average Sr values being 7.4+(I.1 and 9.3+(I.5, respectively (Fig. 7).
Results
DISCUSSION
UV dominant TL The Sr values as a function of the added dose arc shown for TL at the 30(1°C glow curve temperature (Fig. 5). At the lower dose range there is a relatively small change in the Sr value, which is very similar to the S;; value, plotted against the E D of the young sample used. There then appears to be a rising trend in the Sr with increasing irradiation dose, with a noticeable TL sensitivity increase being seen when the added dose reaches around 14() Gv. The increase then appears to slow down but, at the 623 Gv dose level, the sensitivity decreases sharply. Green dominant TL Figure 6 shows the natural TL sensitivity, 5;n, and variation in the regeneration TL sensitivitx, 3r, as
The trend of decreasing Sr with smaller 1o in polymincralic grains (Figs 2 and 3) implies that extended optical bleaching, which removes more natu-
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461
TL Sensitivity Change ol Loess
ral TL signal, could give rise to a higher ED. For sediments such as loess, which are expected to have been exposed thoroughly to sunlight before deposition, this effect should be small, and the preferred laboratory treatment should be a long bleach. The aliquots with laboratory added beta doses may be taken to represent progressively older samples from the sarnc section as the young sample. The Sr values observed are then considered as TL sensitivity, which is related to previous radiation dose, i.e. the 'age" of the sample. Although the variation of Sr values is fairly largc and the differences in TL sensitivity between the unbleached young sample and the artificially aged subsamples are not more than 3 0 % , the overall trend of these results indicates that there is a sensitivity increase with increasing laboratory added radiation dose, which may be attributed to the laboratory optical bleaching before the test beta dose. This is consistent with the prediction of the model proposed by Wintle (1985b). As a sensitivity increase will lead to an ED underestimation, the experimental results above may provide one, but not necessarily the only, explanation for those regeneration ages which are found to be younger than expected (Wintle, 1990). Two points may be noted from the modelling experiments presented here. First, there appears to be a threshold dose level above which sensitivity enhancement becomes significant. This dose value is found around 20(I Gy (corresponding to an age of about 45 ka for Chinese loess) in these experiments, but it may vary for samples from different sites. Second, there seems to be an upper limit of post-bleach sensitivity enhancement of around 20-30% for samples with an ED of the order of a few hundred grays.
sensitivity change was attributed to defect clustering, which favours the formation of larger, more stable defect complexes during storage or preheating of the bleached samples (Bowall et al., 1987). To test the relevance of these findings to Chinese loess, several storage experiments were carried out and the EDs thus obtained were compared with those obtained when one day was allowed between bleaching and beta irradiation (Table 1). For a loess sample from the Xifeng section Q T L I 1 7 Q (underneath the last interglacial palaeosol, thus expected to be ca. 130 ka), a 58 day room temperature storage was allowed for bleached discs before adding the regeneration beta doses. No E D plateau emerged until a glow temperature of 340°C was reached, when a slightly higher mean E D of 451 _+13 Gy was obtained, compared with 414_+ 12 Gy without storage. If the recovery from sensitization is a thermal process, the restoration of the original sensitivity should be accelerated by holding the bleached discs at elevated temperatures for a certain period of time. This hypothesis led to another experiment with the same sample. Bleached discs were held at 140°C for 16 hr before beta doses were added to regenerate the TL signal. Measurements were made with the UG11 and HA-3 filters, as for the above measurements, and E D values were 423_+ 11 Gv around 300°C and 452_+15 Gy around 350°C. Compared with the 414_+12 Gy results, the increases in ED arc hardly significant.
T A B L E 1. Effects of laboratory treatments on equivalent doses (ED) Sample QTL117Q
CIRCUMVENTING THE SENSITIVITY CHANGE Q'FLgt.~ZII0
In the preceding sections, we have shown that the sensitivity change may occur for the relatively old samples when the regeneration method is used to determine the EDs. Although more effort will be directed to understanding the mechanism of the sensitivity change and to developing more accurate correction procedures, it may be more desirable to explore ways of circumventing the sensitivity change. In this section, we examine the effects of laboratory treatments on the sensitivity change and discuss the applicability of a recent adaptation of the regeneration method which requires no optical bleaching (Smith, 1983).
Effects of
Laboratory Treatment
Bowall et a/. (1987) and Rendell et al. (1988) suggested that sensitivity change is a time-dependent effect for loess samples from Pakistan. The original TL sensitivity appeared to be restored if a delay was introduced between bleaching and laboratory irradiation and particularly between laboratory irradiation and readout. Such a transitory behaviour of
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Rendell et al. (1988) showed that the time-dependent effect was most pronounced in the green emission for Pakistan loess. Bleached discs of sample QTL99Z00 from Lanzhou were stored at room temperature for over 11 months before beta irradiation was administered to regenerate the TL signal. The green filter combination was used for TL measurement and thc E D obtained was the same as that without the post-bleach delay within the error limits, i.e. 243+15 compared with 257_+22 Gy (Table 1). The apparent lack of transient effect of sensitivity change in these samples indicates some differences in the T L behaviour between Chinese loess and Pakistan loess. For the samples examined here the relaxation time may be longer than months and the restoration of the original sensitivity does not seem to be particularly sensitive to the temperature used in thc experiment.
462
L.P. Zhou and A.G. Wintlc
ED Determination by Regeneration without Bleaching In dating coarse grain quartz from sands in Australia, Smith (1983) introduced a regeneration method which requires no optical bleaching for most samples. In this method (hereafter called Smith's method), the EDs for the older samples from a section are obtained by comparing their natural TL levels with the N + 13 growth curve constructed with the youngest sample and by adding the ED of the youngest sample (Fig. 8). Ideally, this method may be used to date samples which may show post-bleach TL sensitivity change. Two assumptions are made for samples from different levels of the same section. First, the residual TL signals reached after solar bleaching in antiquity are similarly low. Second, the TL sensitivities of the samples are largely the same. Figure 9 shows the EDs obtained by Smith's method in the UV filter pack and compares them with those derived from the additive and regeneration methods for the same samples from the Xifeng section. The
correspondence of the two ED sets is reasonably good for the three youngest samples, but becomes rather poor for the rest. The EDs obtained using Smith's method appear to be closer to those of the conventional regeneration method and lower than those of the total bleach method. The most likely cause of the poor correlation may be related to the assumption that the sensitivity is the same for all the samples. The glow curves for samples from different levels are not identical and normalization for the natural discs of the progressively old samples was required. Second glow normalization was used, but this procedure may have caused an additional dose-dependent sensitivity change which would result in the EDs being lower than the true values for these old samples. Better controlled sample preparation will be required to establish whether the differences in TL sensitivity between the samples are intrinsic to the samples or are artifacts of sample preparation.
CONCLUSIONS
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The preliminary experimental results presented in this study indicate an optically induced sensitivity change in fine grain loess samples from Chinese Loess Plateau. The modelling experiments suggest that the sensitivity change may be related to the previous radiation dose, i.e. the age of the sample. The sensitivity increase occurs at a threshold dose level around 200 Gy (corresponding to an age of about 45 ka for Chinese loess) and tends to approach an upper limit of around 20-30% enhancement. Similar dosedependent post-bleach sensitivity change is observed using three emission regions, in the UV, blue and green. Further work is needed to identify the association of the TL sensitivity change with particular mineral fractions in loess fine grains. The original sensitivity cannot be restored by the laboratory treatments used in this study. When the regeneration method is used for the ED determination for older samples, a sensitivity correction is likely to be required. A rough estimation of the dcgrcc of the sensitivity change in older samples may bc made bv modelling experiments using the youngest sample from the studied section. The application of Smith's method of determining EDs using the response of the youngest sample needs further testing for loess sections, as its fundamental assumption that samples from the same section have identical TL properties did not hold up at the test site of Xifeng.
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ACKNOWLEDGEMENTS
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FIG. 9. Comparison of EDs (300-320°C integrals. SD mostly around 10%) obtained by Smith's method with EDs b~ the Iota] bleach method (EDadd) and the regeneration method" (EDrcg) for UV emission.
LPZ thanks St John's College, Cambridge and the U.K. Government (Overseas Research Studentship Awards) for financial support. We thank Dr H.M. Rendell for constructive comments. This is publication number 359 of the Institute of Earth Studies, UCW, Aberystwyth.
TL Sensitivity Change of Loess
REFERENCES Aitken, M.J. (1985). Thermoluminescence Dating. Academic Press, London, 359 pp. Bowall, L., Frean, R., McKeogh, K.J., Rendell. H.M. and Townsend, P.D. (1987). Sensitivity changes in the TL of quartz and feldspar after bleaching. Crystal Lattice Defects and Amorphous Materials, 16, 37--43. Debenham, N.C. (1985). Use of UV emission in TL dating of sediments. Nuclear Tracks and Radiation Measurements, 10, 717-724. Frechen, M. (1991). Thermolumineszenz-Datierungen an L6ssen des Mittelrheingebiets. Ph.D. Thesis, Geologisches Institut, Universitfil K61n, zu SonderverOffentlichungen, 79, 137 pp. Li, S.H. and Wintle, A.G. (1992) Luminescence sensitivity change due to bleaching of sediments. Nuclear Tracks and Radiation Measurements, 20,567-573. McKeever, S.W.S. (1991). Mechanisms of thermoluminescence production: some problems and a few answers? Nuclear Tracks and Radiation Measurements, 18, 5-12. Rendell, H.M. and Townsend, P.D. (1988). Thermoluminescence dating of a 1(I m loess profile in Pakistan. Quaternary Science Reviews, 7, 251-255. Rendell, H.M., Gamble, I.J.A. and Townsend, P.D. (1983). Thermoluminescence dating of loess from the Potwar Plateau, northern Pakistan. PACT, 9, 555-562.
463
Rendell, H.M., Mann, S.J. and Townsend, P.D. (1988). Spectral measurements of loess TL. Nuclear Tracks atul Radiation Measurements, 14, 63-72. Smith, B.W. (1983). New applications of thermoluminescence dating and comparisons with other methods. Ph.D. Thesis, University of Adelaide. Stoneham, D. and Stokes, S. (199t). An investigation of the relationship between the 110°C TL peak and optically stimulated luminescence in sedimentary quartz. Nuclear Tracks and Radiation Measurentents. 18, 119-123. Wintle, A.G. (1985a). Stability of TL signal in fine grains from loess. Nuclear Tracks and Radiation Measurements, 10,725-730. Wintle, A.G. (1985b). Sensitization of TL signal by exposure to light. Ancient TL, 3. 16--21. Wintle, A.G. (1990). A review of current research on TL dating of loess. Quaterna O' Science Review, 9. 385-397. Wintle, A.G. and Huntley, D.J. (1980). Thermoluminescence dating of ocean sediments. Canadian Journal of Earth Sciences, 17,348-360. Zhou, L.P. and Wintle, A.G. (1989). Underestimation of regeneration ED encountered in Chinese loess. In: Aitken, M.J. (ed.), Long and Short Range Limits in Luminescence Dating. Occasional Paper No. 9 pp. 139-144. Research Laboratory for Archaeology and the History of Art, Oxford University.