Incomplete stimulation of luminescence in young quartz sediments and its effect on the regenerated signal

Radiation Measurements. Vol. 26, No. 2. pp. 221-231, 1996

Pergamon 1350-4487(95)00298-7

Published by ElsevierScienceLid Printed in Great Britain 1350-4487/96 $15.00+ 0.00

INCOMPLETE STIMULATION OF LUMINESCENCE IN YOUNG QUARTZ SEDIMENTS AND ITS EFFECT ON THE REGENERATED SIGNAL A. S. MURRAY CSIRO Division of Water Resources, P.O. Box 1666, Canberra, ACT 2601, Australia (Received 11 January 1995; revised 20 June 1995; in final form 2 August 1995)

Abstract--The accuracy of measurements of the equivalent dose in young fluvial quartz when using regeneration of optically stimulated luminescence (OSL), such as is involved in some single aliquot techniques, can be compromised by the presence of a residual light signal. This residual underlies the regenerated signal and is proportional to the initial additive doses rather than regeneration doses. For matched initial and regeneration doses, this residual may, depending on measurement parameters, amount to 20% of the regenerated light integral. Some characteristics of this signal in a sequence of samples 0-2000 years old are described. An analytical method that estimates the effective dose at the time of deposition is proposed, based on the separation of the OSL signal into components which are easy to bleach, difficult to bleach, and unbleachable on a laboratory time-scale. For these samples, this dose is at most equivalent to 35 years burial. It is shown that the residual signal depends on dose cycle and pre-heat temperature but is essentially independent of dose. An equivalent-dose measurement protocol is proposed which avoids the problems caused by the residual signal--in particular, by its dependence on dose cycle.

INTRODUCTION The dating of young water-lain or aeolian sediments using optically stimulated luminescence (OSL) is receiving increasing attention. Reported precisions on dates of up to a few hundred years are comparable to, or better than, those obtained by radiocarbon for samples of similar age (Stokes and Rhodes, 1989; Stokes, 1992; Ollerhead et al., 1994; Murray, 1996). However, systematic uncertainties over these time-scales have received little attention. This paper examines one of the sources of such uncertainty, which is of potential significance in analytical protocols that involve regeneration of the quartz OSL signal after initial laboratory optical stimulation. When quartz is optically stimulated in the laboratory, even prolonged illumination fails to completely reduce the luminescence to background levels. This is usually attributed to the existence of a suite of traps, only some of which bleach easily (Aitken and Smith, 1988; Aitken, 1992; Smith et al., 1990). Aitken and Smith suggested that the existence of diffficult-to-bleach traps may result in a significant OSL signal being measured in a recently deposited sample; they maintain this could result in a nonzero equivalent dose (DE) which would limit the usefulness of the technique for young samples ( < 1000 years old). The effect of diffficult-to-bleach traps will be more profound if the analytical protocol involves regeneration of the OSL signal, such as in single aliquot

and related techniques (Duller, 1991; Mejdahl and Botter-Jensen, 1994). The existence of a significant dose remaining to be measured as an OSL signal after the first and subsequent optical stimulations will induce systematic errors. This signal is likely to be of particular importance if, in order to avoid the use of sunlight or other short wavelength sources, a green light is used for laboratory stimulation and/or bleaching of quartz. Green light sources are sometimes used, not just for stimulation, but also for bleaching OSL signals, because sunlight and other sources contain short wavelengths which are likely to provide undesirable stimulation and retrapping of electrons from, for example, the incompletely bleached 375°C thermoluminescence (TL) trap. This TL peak bleaches relatively slowly (Spooner, 1994) and will be incompletely bleached in fluvial sediments; electrons may be subsequently transferred (via low temperature but optically insensitive traps) into traps related to the OSL signal. Large residual OSL signals have been reported for quartz from recent (< 200 y) glacio-fluvial sediments from the upper Ganges River, north India; the quartz grains were exposed to bright daylight for 8 h and given a pre-heat of 220°C for 5 min before measurement (Rhodes and Pownall, 1994). On the other hand, similar "recuperation" signals were not observed for quartz from a fluvial sediment with an age of over 100 kyears (Perkins and Rhodes, 1994). However, it is clear that it is important to understand the behaviour of the residual OSL signal, 221

222

A.S. M U R R A Y

particularly for quartz from young fluvial sediments (which form the experimental samples used in this paper). The behaviour of residual signals is also of importance for De measurement protocols based on OSL signals which are regenerated after a laboratory bleach to reduce the initial natural OSL. This may be a single aliquot regeneration protocol, of the type originally proposed by Duller (1991) for IRSL dating of feldspars, or a modification of this approach proposed by Mejdahl and Botter-Jensen (1994), known as 'SARA'--Single Aliquot/ Regeneration and Added dose. This procedure is based on a minimum of 2 aliquots. It was developed to allow for the sensitivity changes that often occur after erasure of the natural signal by either heating (as in the case of TL) or bleaching (as in the case of OSL) of quartz and feldspars. They prepare for each quartz aliquot a limited growth curve based on three points (lst, 2nd and 3rd regenerations, see Fig. 1) selected so that the OSL signal (derived from the natural dose received during burial and integrated from 0-50 s) falls within the range of the regenerated data. After each measurement (during which only heating completely erases the luminescence signal), the aliquot is irradiated and pre-heated. The laboratory dose which would give regenerated light equivalent to the natural OSL integral is then determined by interpolation. Different aliquots are given different additive doses (0, fib f12) prior to these measurements, enabling the interpolated dose (CN, CN+/~I, CN+~Z) to be plotted against the additive dose. The true dose (De), equivalent to the natural dose, is then given by the negative intercept on the additive-dose axis. This is shown schematically in Fig. 1. Mejdahl and Botter-Jensen (1994) applied this protocol to heated materials (e.g. bricks and stones) for which the residual luminescence signal at deposition is zero. From their study, they suggest that the 'SARA' approach, besides having many of the advantages of the single aliquot technique (Duller, 1991), can also cope with significant sensitivity changes. (Observation of such changes in feldspars caused Duller to reject the regenerated approach to single aliquots.) In particular, it has the advantage of normalizing the light output from each aliquot using regenerated signals from the same aliquot, thus eliminating one of the principle sources of scatter in conventional growth-curve analysis. Implicit in their protocol are the following assumptions: (a) there is no significant dose-dependent sensitivity change between the additive and any regenerated data, although changes independent of dose are allowed. (b) non-dose-dependent sensitivity changes from one regeneration cycle to another may also occur, but they must be constant (i.e. independent of dose cycle).

Their TL and OSL test program on eight archaeological samples heated in antiquity, each with an independent age estimate, was very convincing. The aim of this paper is to determine whether the behaviour of unheated quartz from very young fluvial sediments meets the assumptions required for the 'SARA' protocol. For unheated samples the natural OSL signal may contain a residual signal (as discussed above), but this should not be important for De determination using 'SARA'. However, if after completion of the OSL measurement there is a residual signal which relates to the dose received since deposition (either in nature or in the laboratory), a systematic error may be introduced into the De values obtained. This paper describes some characteristics of this residual dose-dependent luminescence in young quartz samples (0-2000 years old) and proposes an analytical method to estimate the dose at the time of deposition, or at least a signal proportional to it. The dependence of the residual luminescence on regeneration cycle, dose, and pre-heat temperature is also examined. The implications of these measurements for the application of the 'SARA' protocol are discussed, and a modification to the protocol is proposed.

SAMPLING PROCEDURES, ANALYTICAL FACILITIES AND SAMPLE INFORMATION Sediment samples were collected from an exposed face in a 6 m thick accumulation of flood deposits at the junction of the Murrumbidgee River with a minor tributary, Tuggeranong Creek, near Canberra, ACT, Australia. The catchment upstream of the sampling site covers about 5000 km 2. PVC tubing (50 mm diameter) was forced into a freshly cleaned sediment face, withdrawn, and the ends capped. Quartz grains (106-212 #m diameter) were later extracted in the laboratory under subdued yellow light by sieving and heavy-liquid separation (sodium polytungstate solution, density 2.68 and 2.62 g/cm3). They were then etched in 40% hydrofluoric acid to remove the outer 10 /tm layer (Aitken, 1985), and the purity of the quartz extract checked using infrared stimulation (Spooner and Questiaux, 1989). All OSL analyses were undertaken using 10 mg samples placed loose in the cups of a Riso automated TL/OSL reader, which was fitted with an EMI 9635QA photomultiplier tube and two U-340 transmission filters. The reader was also equipped with a green light source (pass-band 420-575 nm), giving an illumination intensity of about 16 mW/cm 2 (Botter-Jensen and Duller, 1992; B~tter-Jensen et al., 1993), and a 9°Sr/9°Y beta source delivering 0.036 Gy/s to quartz. All samples were heated between irradiation and optical stimulation. Various 'pre-heat' regimes for quartz have been described in the literature (Smith et al., 1986; Rhodes, 1988; Stokes, 1992; Roberts

INCOMPLETE S T I M U L A T I O N O F LUMINESCENCE et al., 1993); all have been intended for application to old samples for which the low temperature traps (below 325°C) are likely to be thermally unstable compared with the age range of interest. The work presented in this paper concentrates on young samples, to which this assumption does not apply• It has been shown (Murray, 1996) that for the oldest sample considered here (2000 years old, see below) a plateau of equivalent dose exists for preheat temperatures between 180 and 260°C (heated at 5°C/s and held at temperature for 10 s). For younger samples, doses derived using a pre-heat to 160°C were consistent with those obtained using higher temperatures. In this paper pre-heat temperatures of between 160 and 260°C are considered; the pre-heat temperature used is stated as each data set is discussed• Light outputs were collected in intervals of 0.4 s, and so multiplication by 2.5 of the arbitrary units shown in the figures converts the scale to pulses/s detected by the photomultiplier. Carbon-14 dates on associated charcoal, and direct observation by R. J. Wasson (private communication, 1993), indicate that the uppermost sample (20-70 mm below surface) was deposited between 5 and 300 + 80 years BP, and the bottom layer (5.3 m) at 2010 + 210 BP (14C laboratory numbers CS834 and NZA1163, respectively). Based on high-resolution gamma spectrometry of the sediments (Murray et al., 1987; Murray and Aitken, 1988) and an estimate of the cosmic-ray dose rate (Prescott and Hutton. 1988), the top layer has received a dose of between 0.02 and 1.0 Gy, and the bottom layer about 6.8 Gy during burial. These values are consistent with the observed equivalent doses of 0.40 + 0.04 Gy and 6.6 +__0.2 Gy, obtained using the ' S A R A ' procedure, modified as suggested in this paper and described in detail elsewhere (Murray, 1996). It was noted above that the ' S A R A ' protocol assumes the absence of dose-dependent sensitivity changes• Although not the subject of this paper, this author has also investigated the dose dependence of any OSL sensitivity change between the additive and 1st (and subsequent) regeneration cycles in these samples (Murray, 1996). It has been shown that there is no detectable dose dependence (to < 3 % ) between the first three regeneration cycles, or (to < 5%) between the additive and first regeneration cycles. The difference in uncertainty reflects, for the two procedures employed, the different ability to detect change in the dependence of sensitivity on dose.

GREEN LIGHT S T I M U L A T I O N OF OSL Figure 2(a) presents the first 300 s of the OSL curve for an aliquot of the oldest sample (laboratory code 938006, burial dose 6.8 Gy) to which had been added a laboratory dose of 11 Gy.

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Fig. 1. Schematic representation of SARA protocol: (a) natural luminescence (LN) compared with that induced by dose given after zeroing (by heating or bleaching) for three different doses DI, /)2 and D3; (b) luminescence (LN + 8) for a second aliquot given an additional dose fl compared with that induced by doses given after zeroing for three different doses, where DI, D2 and D3 are different from those in (a); (c) values of interpolated dose (CN and C~v+ ¢~ from (a) and (b)) are plotted against the additional dose fl and extrapolated to give De. Experimental details are given in the figure caption. The curve includes any background arising from scattered stimulation light. The familiar rapid decrease in luminescence output can be seen up to about 20 s. This is followed by a much slower decrease as the charge transferred to (or already present in) the more difficult-to-bleach traps is partially redistributed into the easy-to-bleach 325°C TL trap before radiative recombination (McKeever, 1994). Even after 300 s of net illumination, the OSL is still more than a factor of 20 above the level obtained from an illuminated empty cup (light level of about 4 counts/0.4 s in Fig. 2). Some recuperation (note the logarithmic y-axis) of the

224

A.S. MURRAY

easy-to-bleach component (type A, Aitken, 1992) is also observed on both the 2nd and 3rd OSL curves; this is very small compared with the original signal. Note that in the pause at the end of the 2nd 100 s of the additive-dose stimulation curve the sample was heated again to observe the behaviour of the l l0°C TL peak. (These data are reported in Murray, 1996.) The effect of this additional heating was to increase slightly the size of the easy-tobleach recuperation signal, and reduce slightly the size of the subsequent residual OSL. Figure 2(b) shows corresponding data for an additive dose of 1.8 Gy given to a younger sample (laboratory code 938001, estimated burial dose about 1.1 Gy). These data were obtained using the same pre-heat and room temperature storage procedures as for sample 938006. The curves are broadly the same as in Fig. 2(a), except that the decrease of the signal with time in the 2nd and 3rd I00 s stimulation periods is much less marked, and recuperation of the easy-to-bleach component is undetectable by the 3rd 100 s stimulation period. At the end of 300 s of net stimulation the residual is still more than 15 times the measured empty-cup signal (and at least 4 times the calculated contribution from any non-dose-dependent light--see next section). The residual OSL at the end of the first 100 s is 0.5% (Fig. 2(a)) and 1.8% (Fig. 2(b)) of the instantaneous initial output (measured over first 0.4 s). It may appear appropriate, therefore, to restrict the measured signal to the first few seconds of stimulation, to improve the signal to residual ratio. However, several authors (Smith et al., 1990; Smith and Rhodes, 1994; Roberts et al,, 1994) have pointed out that because of the non-first-order kinetics in quartz, the light output at any time during the stimulation period will not necessarily be proportional to the light sum. This relationship may change with dose, and from additive-dose to regeneration cycle. Smith and Rhodes (1994) conclude that, for the determination of De, "the safest approach is to integrate the full OSL emission over the longest exposure possible"--this suggestion is adopted here. Such integration increases the relative contribution to the total OSL signal of the difficultto-bleach component which, by its nature, is likely to accumulate from one dose cycle to the next. In order to investigate the progressive accumulation of the difficult-to-bleach residual OSL signal with regeneration cycle, the 100-200 s intervals from an N and an N + fl stimulation curve were compared with the 100-200 s intervals from two subsequent regeneration measurements on the same aliquots. Figure 2(c) shows this section of three stimulation curves for each of two aliquots of sample 938006. The lower set of three curves are from an aliquot initially containing only the natural dose of 6.8 Gy, followed by regeneration doses of 6.1 and 6.4 Gy. The upper set are from a further

aliquot initially containing a total additive dose (N + fl) of 17.5 Gy, and regeneration doses of 17.5 and 16 Gy. Other experimental details are given in the figure caption. Figure 2(d) shows similar data for two aliquots of sample 938001; the initial additive doses were 1.1 and 2.9 Gy, and the regenerative doses were 0.9 and 1.2 Gy, and 3.2 and 2.5 Gy, respectively. In each of the 4 cases, the additive-dose residual OSL curve can be regarded as the baseline on top of which can be seen the increased residual level after the 1st regeneration dose, and again after the 2nd regeneration dose. No similar increases are seen from 1st to 2nd to 3rd stimulation cycle in Figs 2(a) or (b). The only difference between these procedures and those of Figs 2(c) and (d) is the addition of laboratory doses, and so it must be concluded that this OSL is dose dependent. Thus, if a dose was given for regeneration similar to the expected sum of the burial and additive dose (N + fl), as is required by the ' S A R A ' protocol, a residual light integral would be measured in addition to the regeneration signal. This residual would be a constant fraction of the OSL derived from the N or N + fl dose, provided that the shape of the stimulation curve is independent of dose. Since the regeneration dose is chosen to be close to the N + fl dose, then the residual integral will also be a constant fraction of the OSL derived from the regeneration dose. Since the additive doses (and thus the regenerative doses) vary from one aliquot to another, the behaviour shown in Figs 2(c) and (d) would lead to an apparent sensitivity change, even in the absence of any true change. Had the sample in Fig. 2(a) been stimulated for 100 s, dosed, and stimulated again for 100 s, 10% of the 100 s integral of the initial OSL output would have been present under the integral of the regenerated curve. In the younger sample the corresponding level would have been 19%. Thus, the apparent sensitivity change could be as much as 10-20%. This change would continue to occur each regeneration cycle. Clearly an understanding of the behaviour of the residual OSL signal is required before this effect can be allowed for in any regeneration-based analytical protocol.

PREDICTING THE BEHAVIOUR OF THE RESIDUAL LIGHT INTEGRAL The behaviour of the residual luminescence, Lr, with respect to the additive-dose OSL curve can be predicted if it is assumed that the luminescence integral, tt, is made up of two parts: a background integral, B, which is unbleachable on a laboratory time-scale and includes any scattered light from the stimulation source, and a bleachable integral, Lo. Then, Lr = foLo + B, where fo is a fraction between 0 and 1, and Lr is integrated over the same time period as Lt. The bleachable signal, Lo, is con-

INCOMPLETE STIMULATION OF LUMINESCENCE

(a) Additive done for sample 938006 k" m J\i) Total Dose = 18Gy ~ '0°°° t ~ i) 0-100 s

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Fig. 2. Additive-dose OSL curves for (a) 938006 (2000 years old, burial dose 6.8 Gy) and (b) 938001 (300 years old, burial dose 1.1 Gy) showing the difference between the two in the rate of decrease in OSL after the first 20 s. Additive and regenerated residual OSL curves for 10(O200 s stimulation periods for two aliquots each of (e) 938006 and (d) 938001, showing the accumulation of the residual signal for aliquots containing both N and N +/~ initial doses. Note the logarithmic vertical axes. Doses received during each treatment: (c) aliquot i, i = N (6.8 Gy); ii = 1st regen. (6.1 Gy); iii = 2nd regen. (6.4 Gy); aliquot 2, iv = N + fl (17.5 Gy); v = 1st regen. (17.5 Gy); vi = 2nd regen. (16.0 Gy): (d) aliquot 1, i = N (1.1 Gy); ii = 1st regen. (0.9 Gy); iii = 2nd regen. (1.2 Gy); aliquot 2, iv = N + fl (2.9 Gy); v = 1st regen. (3.2 Gy); vi = 2nd regen. (2.5 Gy). (Experimental details: Sample 938006---After dosing, the sample was pre-heated at 5°C/s to 220°C, held at that temperature for 10 s, and then held at room temperature for about 35 min before initial optical stimulation. Between both the 1st and 2nd (Figs 2(a)-(d)), and the 2nd and 3rd 100 s (Figs 2(a) and (b) only) stimulation period there was a 20 min break at room temperature, further heating at 5°C/s to 220°C with a 10 s hold, and another 35 rain delay at room temperature. Sample 938001 was treated identically except that the pre-heat temperature was 160°C.)

sidered to be made up of an easy-to-bleach component, ( 1 - f o ) L o , and a component more difficult to bleach, foLo, as is shown in Fig. 2. These components are operationally defined, since the value of fo is likely to depend, inter alia, on pre-heat temperature and stimulation time. Since Lt = Zo + B Lr = f o L , + (1 - fo)B,

(1)

i.e. there should be a linear relationship between a total stimulation integral, L , and a residual integral, Lr, if they are both dose dependent. For a plot of L, against Lt, the slope fo is a measure of the fraction of the light output that is difficult to bleach, and thus would be present under a subsequent regenerated luminescence curve. (This neglects recuperation effects, and assumes the change in light output rate with time during the stimulation of Lr is small, These phenomena will tend to cancel.) T h e intercept, ( l - f o ) B , gives a

direct measure of any light which cannot be bleached on a laboratory time-scale. In general, this will not just be the scattered light derived from the stimulation source; any very diffficult-to-bleach light remaining from a previous dose which does not vary from one aliquot to another will also appear as a constant. For very young samples, where it is always safer to assume some degree of incomplete bleaching prior to deposition, B includes residual light arising from any partially bleached dose present at deposition. This conclusion must be applied with caution, however. Recuperation effects imply that B may underestimate this partially bleached signal because of charge redistribution into easy-tobleach traps during burial. Nevertheless, this method of determining B is to be preferred over the alternative using a prolonged stimulation, because here the risk of over-bleaching the residual light

226

A.S. MURRAY

plied by 10, to make them equivalent to the integration time (0-100 s) of the additive-dose OSL, Lt.) As predicted by equation (1), well-defined linear correlations are observed, and the constants fo and .~.~ 90-100 s B are summarized in Table l (sample 938006). The 1x105 ] ¢oO." existence of these correlations confirms that Lr ,o 190-200 s remains dose dependent up to at least 300 s of stimulation, and it is clear that it would take pro-.,i .~..-. longed illumination to completely bleach the dose¢0 S ¢D ..4-dependent OSL. Even at the end of 300 s, about rv 0.'- - . . . . . . . 4% of the initial 100 s integral derived from the additive-dose sequence would be present under a 2x 105 (b) Ist Regeneration ~/ subsequent regenerated dose. tIt should also be noted that the scatter about the ~, "9o-I oo s O (J line in Fig. 3(a) is, in part, a reflection of the vari¢, "E ation from one aliquot to another of the ratio .,o 190-200s between the residual light at the end of a stimu~o lx105 lation period and the light integral over the first 100 s. This scatter supports the suggestion made above no that it would be unwise to use only the first few sec.== • =' °~" IiF i. ~ , ~ =., ' " , ~1'~, . , . , . ,,, onds of the OSL curve: over these time-scales there n, 0 appears to be variation in the proportion of the light sum emitted during various time periods in the +,= 2x105. OSL curve. Observation of the OSL stimulation (c) 2nd Reoenerotion ," ¢curves of individual aliquots with the same initial ,~90-100 s 0 0 dose suggested that the shape of individual curves o~ can be different. This was confirmed by regressing ,d~ 190- 200 s 95-100 s Lr integrals against the Lt values from 0,5 1x105 s. The correlation coefficient (R 2) was only 0.52, o compared with a value of 0.85 for Fig. 3(a) (90,100 ,=o . .... 9o-3oo, s); the data are twice as scattered about the re'10 gression line. It is concluded that longer integration rv 0 6 1o5 8 1o5 lO6 times are required for these samples to produce 0 " 2 1o reproducible results. OSL Integral (100 s), counts Figures 3(b) and (c) show similar linear correFig. 3. (a) Dependence of residual OSL on the additive- lations in the 1st and 2nd regeneration data, dose OSL integral for sample 938006. The additive doses obtained using the same 24 aliquots used in ranged between the natural dose and the natural dose plus 7.5 Gy. (b) Dependence of residual signal on the 1st regen- Fig. 3(a). There is less scatter in these data than in eration OSL integral. Regeneration doses varied between 4 Fig. 3(a), indicating that variability in the proportion of the light-sum emitted at the end of the and 11 Gy. (c) Dependence of residual signal on the 2nd regeneration OSL integral. Regeneration doses varied stimulation period referred to above is smaller in between 4 and 11 Gy. (Experimental details: These doses the regeneration data than in the additive-dose were pre-heated to 160°C for 10 s before OSL measurement. There was a delay of about 45 rain between the in- data. The regeneration data were produced by itial (0-100 s) and 2nd (100-200 s) stimulations, and a net 'matching' light levels, i.e. by giving a dose to an indelay of 55 min and a pre-heat to 160°C for 10 s, between dividual aliquot so that the 1st regenerated output, the 2nd and 3rd (200-300 s) stimulations.) L t t = L1 + Lr, approximately matches the light output of the additive-dose luminescence, Lt (for these data, to about 18%). In these circumstances, the arising from the partially bleached pre-deposition new residual light signal, Lrl, is made up of three components: the constant (i,e. independent dose is negligible. The appropriateness of equation (1) is demon- of regeneration cycle or post-depositional dose) strated in Fig. 3 for the older sample (938006, light level, B, discussed above; a light level, foLo, proportional to the additive-dose dependent De = 6.8 Gy). Twenty-four aliquots were given light output; and a light level, fiLl, proportional additive doses from 0 to 7.5 Gy. The OSL was then to the regenerated light output. The subscript measured for a total of 300 s, with pre-heat and '1' refers to the 1st regeneration. Thus, delays as given in the figure caption. Figure 3(a) Lrl = f i L l + foLo + B =flL~ + Lr. This derivation shows Lr plotted against Lt, for three different inteneglects recuperation effects, and assumes that the gration periods for Lr, from 90-100, 190-200 and continued decrease in Lr with stimulation time is 290-300 s. (The residual integrals were then multi- negligible, and that there is no significant change in

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INCOMPLETE STIMULATION OF LUMINESCENCE luminescence efficiency between cycles. Remembering that L,I ~ Lt, substituting into equation (1) gives the 1st regeneration OSL residual

Lrj = Lt,(f~ + fo - f ~ f o ) + B(I - f ,

- f o + f l fo). (2)

Similarly, for the 2nd regeneration residual Lr2 = Lt2(f2 + f l + fo - f 2 f l - f 2 f o - f i f o

+ f2flfo)

+ e ( l - f2 - ft - fo + f2fl + f i f o + f i f o - f f f l f o ) .

(3) The derived values o f f and B for Figs 3(b) and (c), analysed using equations (2) and (3), are also given in Table 1, along with the summary of a similar analysis for the younger sample of Fig. 2. As expected, values for f decrease rapidly for the older sample as stimulation time is extended (from 0.25 to 0.04 in the additive-dose data). In contrast, the values for the younger sample show a less marked drop with stimulation time (from 0.16 to 0.09 in the additive-dose data). There is a significant change in almost all the values of f from the additive-dose cycle to the 1st and 2nd regeneration cycle (e.g. from 0.10 to 0.03 for the 190--200 s residual integral in sample 938001). For sample 938006 all values of B are statistically consistent with the weighted mean o f 4600 +__ 1200. After subtraction of the empty cup signal, this background light level is only 1.8 + 0.6% of the average natural light level for this sample integrated over the first 100 s. The analysis of the younger sample (938001) gives a mean background level of

227

10 + 4% o f the natural light level, corresponding to a dose of about 0.11 Gy (assuming linearity). Assuming the degree of bleaching of the sediment at the time o f deposition was similar in the case of the older sample, this dose would provide about 1.7% of the burial-dose-derived light, consistent with the observed value of 1.8 + 0.6%. In terms of age, this background dose is equivalent to only about 35 years burial dose, and must be considered an upper limit because it includes the additional scatter of stimulation light that presumably occurs from the grain surfaces. This argument has also ignored the possibility of the redistribution during burial of any residual dose which might have been present in low temperature traps at the time of deposition. This would tend to make the time dependence of the stimulation of the residual dose more similar to that of any burial dose and additive laboratory doses. This similarity arises if the residual dose is stored in more difficult-to-bleach but thermally unstable traps. These will then be partially redistributed into the easy-to-bleach 325'C trap. The good agreement between the J4C and OSL data reported in Murray (1996) and summarized earlier in this paper puts an upper limit to this phenomenon of a dose equivalent to about 50--100 years.

DETERMINING f USING REGENERATION DATA One disadvantage of the use of equations (I)-(3) to determine values for the residual fraction, ./~ is that errors accumulate from additive-dose cycle

Table 1. Correlation data for the regression of residual light against the total OSL integral over the first 100 s Residual time B Average B2 Dose cycle integral, s arbitrary units arbitrary units Sample 938006

Additive-dose

1st Regeneration

2nd Regeneration

Sample 935001

Additive-dose

90-100 190-200 290-300

fo 0.248 4- 0.011 0.062 4- 0.004 0.039 4- 0.003 f, 0.267 ± 0.009 0.049 4- 0.004 0.022 4- 0.012 f2 0.261 4- 0.010 0.039 4- 0.005 0.011 ± 0.005

90-100 190-200 290-300

fo 0.164 + 0.014 0.103 4- 0.011 0.091 + 0.010

90-100 190-200 290-300 90-100 190-200 290-300

1st Regeneration

90-100 190-200

2nd Regeneration

90-100 190-200

f, 0.173 + 0.016 0.092 4- 0.015 f2 0.125 + 0.028 0.034 4- 0.027

--13000 4- 13000 17004- 3500 2800 ± 2700

2000±2100

780 4- 7670 5970 4- 2220 5420 ± 2070

5500 ± 2000

1490 4- 8490 6920 4- 2460 6340 4- 2560 Weighted average

64004- 2200 46004- 1200

4340 4- 2360 3260 4- 1770 2960 4- 1600

3470 4- 1600

3140 4- 2060 2950 4- 1630

3020 _+ 1630

4780 4- 3700 3710 4- 2430 Weighted average

4700 ___3100 3400 4- 1100

Notes: I. Uncertainties are 1 standard error. 2. Over the same 100 s integration period, the illuminated empty-cup output was 1090 counts. 3. Pre-heat temperature was 160°C for 10 s in both cases. I~ 26:2-F

A. S. M U R R A Y

228

Table 2 Correlation data for the regression of residual light against total OSL for 5 stratigraphically related samples Sample

Pre-heat temp., °C

938007

190

938001

Method (a)

Method (b)

fo .fl fz

0,168 ± 0,028 0,206 ± 0.037 0.142 + 0.046

-0.160 +_ 0.013 0.166 + 0.010

160

Jo fl .f2

0.164 + 0.014 0.173 ± 0.016 0.125 -4-_0.026

-0.157 _ 0.010 0.140 + 0.009

938002

190

fo fl f2

0.183 _+ 0.017 0.181 +_ 0.020 0.095 ± 0.021

-0.131 ± 0.008 0.123 + 0.006

938004

190

fo .fl f:

0.185 + 0.015 0.160 ± 0.012 0.126 ± 0,013

-0.141 + 0.011 0.137 + 0.009

938006

180

fo .fl f2

0.193 ± 0~017 0.191 ± 0,010 0.154 ± 0.012

-0.186+0.009 0.166 + 0,005

-0.220 ± 0.008 0.153 + 0.011 0.132 __. 0.015

-0.216 5:0.008 0.155 ±0.006 0.138 ± 0.006

Repeat of youngest sample under more controlled conditions: 938007 160 fo fl fz f3

Notes: 1. Method (a) uses equation (1), (2), or (3) as appropriate; method (b) uses equation (4). 2. Uncertainties are l standard error. 3. Residual values obtained by integrating between 90 and 100 s of stimulation ( x 10), total OSL integrals between 0 and 100 s.

through the regeneration cycles. For instance, the value of fo (derived from equation (1)) substituted into equation (2) might be in error because the additive-dose residual continued to decrease significantly during stimulation of the 1st regeneration OSL. This would produce a systematic error in the value offl derived from equation (2). There is another more direct method for determining f from regenerated data, because an estimate of the residual light level prior to regeneration is available explicitly for each cup. (It is therefore not applicable to additive-dose data.) Using the same terminology as before, the regeneration residual Lrt =fiLl + Lr, and Ltl = L1 + Lr. Lr is taken as the residual immediately prior to regeneration, which will be a good approximation if subsequent recuperation and OSL time dependence are small Then (Lr~ -

Ld

= A(L,:

-

Lr),

(4)

i.e. if the residual level immediately prior to regeneration is subtracted from both the 0-100 s regenerated OSL integral and the regenerated residual, the fraction fl may be determined directly. A similar equation applies to the 2nd regeneration data. Table 2 summarizes the results of calculating fo, f] and f2 by equations (1)-(4) for five samples from the sequence of flood deposits. Laboratory codes 938007 and 938006 are the youngest and oldest, respectively, the others lie between these in stratigraphic sequence in the order shown in the table. Equations (1)-(3) are referred to as method (a), equation (4) as method (b). In all samples examined, the intercept when regressing ( L r l - L r ) against ( L t I - - L r ) was consistent with zero, as expected

from equation (4), confirming that recuperation/ stimulation time dependency was small. The agreement between the two methods for measuring f~ and f2 is satisfactory, but some systematic effects do exist. For instance, ft by method (a) is always greater than fl by method (b), but the reverse is true for f2. This is probably in part because these data were obtained from a routine dating protocol, in which additive-dose and regeneration light levels were not precisely matched, but it may also be caused by the cumulative effect referred to above. Nevertheless, the mean values for each sample (i.e. average of methods (a) and (b)) for each of fo, fl and f2 are probably not significantly different from each other; the weighted average values are 0.179 _+ 0.008, 0.168 _ 0.007 and 0.140 _+ 0.010, respectively. Method (a) is the only approach available for determining fo on additive-dose data, because no independent estimate of the prior residual is possible. However, accurate estimates of the behaviour of fo with respect to dose and stimulation time are required. It is thus important to confirm that the two methods indeed give the same result under more controlled conditions, and in particular to confirm that equation (1) is reliable. To test this, 24 cups of 938007, the youngest sample, were stimulated for two 100 s periods. These cups were then dosed in the usual way to produce a 1st regeneration OSL data set for which the residual prior to stimulation was known and the same for all aliquots (relative to subsequent OSL data). This enabled equations (1) and (4) to be applied directly to the same data set. Care was also taken to match the subsequent regenerated light levels to within

INCOMPLETE STIMULATION OF LUMINESCENCE 10%. The resulting f values are summarized at the end of Table 2. The agreement is excellent, confirming that the two methods agree well, and that any systematic effects in the earlier data set probably resulted from inadequate matching of light levels. It is worth noting that all values of B for the samples listed in Table 2 are consistent with a weighted mean of 5300 + 1200 (53 + 12 cps); the two values from Table 1 are also consistent with this mean, which is no more than five times the background from an empty cup. It is concluded that any signals left from the pre-deposition dose which are unbleachable on a laboratory time-scale either are not significant, or do not vary detectably from sample to sample over a 2000 y period. It has been shown above that equations (1)-(4) provide accurate methods for calculating the residual fraction, f, and that this fraction depends on dose cycle. These methods can now be used to investigate the behaviour of the residual light fraction with respect to dose and pre-heat temperature.

229

This is shown in Fig. 4(a) for the oldest (938006) sample and in Fig. 4(b) for the youngest (938007). Any dose dependence is undetectable in Fig. 4(a), which covers the limited dose range equal to the additive doses used as part of a dating protocol; this is typical of the range used to produce the data used in this paper. There may be a detectable dose dependence in Fig. 4(b) in the 90--100 s derived data in the 2nd regeneration (which covers a dose range of two orders of magnitude--note the logarithmic x-axis). Given the uncertainties, however, this dependence has not been convincingly established. In any case, it would appear that, for practical purposes, dose dependence o f f in regeneration data can be neglected over the low-dose range present in these young materials. The linearity of the correlations shown in Fig. 3(a), and of those found in the additive-dose data in all other samples, strongly suggests that this conclusion also applies to additive-dose data.

O F f O N PRE-HEAT TEMPERATURE

DEPENDENCE DOSE DEPENDENCE

OF f

The dose dependence o f f can be explicitly examined in regeneration data, using equation (4). The ratio of ( L r ~ - L 0 to ( L , t - L O against regeneration dose should be constant if f is dose independent.

Since the choice of pre-heat procedure has been shown to be important for OSL dating applications (using any multiple aliquot protocol, including 'SARA'; Roberts et al., 1993; Aitken, 1992; Smith et al., 1990; Murray, 1996) the effect of pre-heat (a) Sample g38006 temperature on f was investigated using the oldest v sample, 938006. 'Pre-heating' involved heating at "~ 0.2 5°C/s to the required temperature, holding at that ¢90--100 s ~ • • - -O- -Q ..... CT - - 0 - - 0 - - 0 - - - 0 temperature for 10 s, and then cooling before E stimulation. Figure 5 presents the results for the _O additive-dose and 1st and 2rid regeneration-dose -o0.1 data sets. Values of f were determined using both o methods (a) and (b) where possible; the agreement 190-200 s 6 • • • • o • fl (1st regeneration dose) was good, and mean values, calculated for both the o Of 2 (2nd regeneration dose) o 90-100 and 190-200 s residual level, are plotted in 0.0 2'0 Figs 5(b) and (c). There is a clear dependence o f f 1'o 0 on pre-heat temperature, especially for the 90-100 s t-. 0.3, (b) Sample 938007 J~ data; the relative importance of the difficult-tobleach signal is reduced compared to that of the "1o O easy-to-bleach signal as the pre-heat temperature -; .... "~ 0.2 increases. 10b'; . . . . . o ~ E L T T 0 This suggests that pre-heating excites stored n charge from the more difficult-to-bleach traps, perno 0.1 haps into those more easy to bleach. This is consistent with the model suggested by Aitken and Smith u •Of l (let reqeneratlon T dose) (1988) to explain recuperation in samples previously o u Of 2 (2nd regeneration dole) bleached in the laboratory (which is also observed -- 0.¢ 1:o 16.0 0.1 in these samples, see Fig. 2). They proposed that Regeneration Dose, Gy trapped charge is redistributed during storage, or at Fig. 4. Dose dependence of residual fractions fl and J~ for elevated temperature, back into the easy-to-bleach the oldest sample, 938006, and the youngest, 938007 (pre- 325°C TL peak from the difficult-to-bleach but therheat 180 and 160°C, respectively). Residual integrations mally unstable low-temperature peaks (e.g. at 160, are for 90-100 and 190-200 s. Points represent the mean 190, and 230°C). In this case, the 325°C peak is not and standard error based on three measurements; the weighted average values are shown as solid and dashed empty, and it can only be presumed that it traps some of the thermally excited charge from the lowlines. t.-

.

.

.

.

.

.

.

.

.

.

230

A.S. M U R R A Y

as discussed earlier. In principle, this does not preclude the use of 'SARA'. However, although a *4-,,,.,, ~ ~ - 9 0 - 1 0 0 s residual knowledge of the size of the sensitivity change (and ._. --~ [ o 1 9 0 - 2 0 0 s residual thus the residual fraction, f) is not required, it is im*d oO.2t " ~ ~ portant that it should remain constant from the 1st oo ) to the 3rd regeneration. The results presented here ~ 0 demonstrate that this is not true for these samples. °~,, 1 o Table 2 shows that values o f f vary by as much as 50% from the 1st to the 2nd regeneration, with ab-o- -o solute values (for 100 s stimulation) around 0.1 to O.Ol 0.2. For the 50 s stimulation used by Mejdahl and ~'0.3. (b) 1st Regeneration Dose Botter-Jensen (1994), these would be significantly ~l-- "O tlarger (see Fig. 2). Thus, in its present form, the • 9 0 - 1 0 0 s residual t- O o 190-200 s residual 'SARA' protocol would appear to be unsuitable for application to these samples. Subtraction of the residual from the total OSL integral does not resolve this problem, because the net integral remaining is -~ ~0.1. then a fraction of the dose-dependent integral. This "N o fraction varies with regeneration cycle. 0 -- "o~ -=~--O--O._ O. _ 19 However the value o f f is effectively independent EO.O of dose. Thus, a simple comparison of additive and 1st regeneration light integrals (assuming close -~ 0.3. (c) 2nd Regeneration Dose matching) would circumvent the problems of vary• •9O-lOO s residual ing f (and apparent sensitivity) with regeneration o 190-200 s residual cycle. Since, in practice, such matching is difficult, ~0.2an alternative is to apply two (or more) different 1st regeneration doses to aliquots having the same additive-dose treatment (e.g. natural dose or natural plus beta dose) in such a way as to provide one aliquot where the regenerated-light integral slightly E o.o exceeds the additive integral, and one where it is 260 2~i0 --" 15o slightly lower. Then the matching dose can be dePre-heot Temperature, °C rived by interpolation between the two observed Fig. 5. Dependence of residual fraction, /, on pre-heat doses. This modification to the 'SARA' protocol temperature for sample 938006. Residual integrations are has been used with success on the samples discussed for 90--100 and 190-200 s. Data at any one temperature in in this paper, and is presented in detail in Murray (a)-(c) are derived from the same set of 24 aliquots, with a different set being used for each pre-heat temperature. (1996). Note that although smooth curves have been drawn through the data, it is possible that discontinuities exist between 160 and 180, and 210 and 220°C, which may be CONCLUSIONS associated with the emptying of low temperature TL traps.

0.31 (o) Additive

Dose

1

temperature peaks. However, the reduction in the difficult-to-bleach light relative to the easy-to-bleach component is entirely consistent with a low-temperature trap association. At least from the point of view of minimizing the residual fraction, the higher pre-heat temperatures are to be preferred.

DISCUSSION The 'SARA' protocol, as originally proposed, makes the implicit assumption that the regenerated light integrals, whether TL or OSL, start from a residual of zero, although a sensitivity change from additive to regenerated data is allowed. Given that the regenerated doses are selected to bracket the additive-dose light integrals, a residual fraction of the additive dose will appear as a sensitivity change,

In this study of young fluvial quartz, it has been shown that the essentially unbleachable light integral remaining from the pre-depositional dose is small---equivalent to a maximum of about 35 years of burial dose. However, there is a more significant difficult-to-bleach residual-light integral arising from routine (100 s) laboratory stimulations, which can amount to 10 to 20% of the measured integral. This continues to be read out under any regenerated signal, and will give rise to an apparent sensitivity change if not properly accounted for. At least for these young sediments, the residual fraction does not vary significantly with applied dose, but decreases with increasing pre-heat temperature. It also tends to decrease with successive regeneration cycles. The latter conclusion prevents the direct application of the 'SARA' protocol to these sediments. However, by modifying the protocol to match light

INCOMPLETE STIMULATION OF LUMINESCENCE levels using only one regeneration cycle, rather than three as proposed by Mejdahl and Botter-Jensen (1994), any need to explicitly determine the residual fraction is avoided, and the ' S A R A ' protocol can then be employed successfully. AcknowledgementsIThis work was made possible by the

generous hospitality of the Nordic Laboratory for Luminescence Dating, and the willing and friendly assistance of both the Laboratory staff and the staff of the Riso National Laboratory. Drs Mejdahl and Botter-Jensen provided constant and enthusiastic encouragement, and Dr R. J. Wasson guided the collection of the samples. Dr R. G. Roberts provided valuable comment, and the manuscript was much improved by Dr A. G. Wintle's careful and detailed criticism. A travel grant from the CSIRO Division of Water Resources is also acknowledged.

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