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Comparison of SAAD and SAR procedures for equivalent dose determination using quartz
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Radiation Measurements 37 (2003) 417 – 424 www.elsevier.com/locate/radmeas
Comparison of SAAD and SAR procedures for equivalent dose determination using quartz Hui Zhaoa , Sheng-Hua Lia;∗ , A.S. Murrayb b The
a Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China Nordic Laboratory for Luminescence Dating, Department of Earth Sciences, Aarhus University, Ris$ National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark
Received 26 August 2002; received in revised form 11 December 2002; accepted 15 December 2002
Abstract Comparisons between the single-aliquot additive-dose (SAAD) and single-aliquot regenerative-dose (SAR) procedures are made using 150 –212 m quartz grains separated from three sediment samples. Growth curves and equivalent doses from the two procedures are compared directly using single aliquots of quartz. With appropriate corrections for the loss of signals in our SAAD protocol, the SAAD and SAR growth curves coincide well. It is con4rmed that the luminescence correction in the SAAD protocol can only be applied to samples for which the natural signal is in the linear part of the growth curve. A new correction method is proposed. c 2003 Elsevier Ltd. All rights reserved. Keywords: Quartz; Equivalent dose; SAAD and SAR
1. Introduction Two main groups of equivalent dose (De ) determination methods, additive-dose and regenerative-dose, have been applied in luminescence dating (Wintle, 1997). A single-aliquot approach to the measurement of De using K-feldspar separation was 4rst developed by Duller (1991), who recommended the use of a single-aliquot additive-dose protocol (SAAD) because of possible sensitivity changes in regeneration procedures. A SAAD protocol for quartz was developed later by Murray et al. (1997). A single-aliquot regenerative-dose (SAR) protocol for the OSL dating of quartz was then proposed, which used the response of the 110◦ C TL peak to correct for sensitivity changes in the OSL signal (Murray and Roberts, 1998). This SAR protocol has since been modi4ed to use the OSL signal from a test dose for sensitivity correction (Murray and Wintle, 2000), and this is now considered the standard regenerative protocol. These two single-aliquot protocols, SAAD and SAR, are now widely employed in optical dating. ∗
Corresponding author. Fax: +852-2517-6912. E-mail address: [email protected] (S.-H. Li).
The SAR protocol tends to be favored over SAAD protocol because sensitivity changes corrected for; De can be determined by interpolation to get more precise and accurate results, and these results have little dependence on the 4tting equations used for the growth curve. However, it is not clear whether the dose responses are the same in both growing cases where OSL grows from emptied OSL traps for SAR procedures and grows from partially 4lled traps for SAAD procedures, and whether they can be expressed in a simple mathematical expression. Although OSL sensitivity change can be corrected by the OSL signals created from a test dose, the 110◦ C TL from the same test dose can be a surrogate to the OSL only when both the signals are linearly correlated. Moreover, other interesting questions concerning SAAD procedures remained: (a) beside the problem of extrapolation, are there other diBculties in the application of the SAAD protocol? (b) Can the ’Australian slide’ method (Prescott et al., 1993) be applied to the data from a single aliquot? In this study, comparisons between the SAAD the SAR procedures are made. Growth curves and equivalent doses from the two procedures are compared using single aliquots of quartz. A new correction method that makes the SAAD
c 2003 Elsevier Ltd. All rights reserved. 1350-4487/03/$ - see front matter doi:10.1016/S1350-4487(03)00019-2
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Table 1 Measurement protocol used in this study Step
Treatment
Observed
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Give dose Di1 (i = 0; 1; 2; 3 : : :; D01 = 0) Preheat (260◦ C, 10 s) OSL (0:2 s at 125◦ C) Test dose Dt TL (up to 160◦ C, heat rate 3◦ C=s) If Di1 = Dmax , return to step 1, else go to step 7 Repeat step 2–5 4ve times Give dose Di2 (i = 1; 2; 3 : : :) Preheat (260◦ C, 10 s) OSL (0:2 s at 125◦ C) OSL (200 s at 125◦ C) Test dose Dt TL (up to 160◦ C, heat rate 3◦ C=s) OSL (200 s at 125◦ C) If Di2 = Dmax , return to step 8, else go to step 16 Repeat steps 9 –13 once more
— — Additive Ladi — Tadi used to correct Ladi sensitivity change — Ladi decay factors — — Regeneration Lrei Empty the OSL trap — Trei used to correct Lrei sensitivity change Empty the OSL trap — Lrei for Di2 = 0
growth curve indistinguishable from that obtained using the SAR protocol is proposed.
2. Samples and measurement procedures 2.1. Samples and facilities Three quartz samples were used in this study. One is a young sample (NP) for which the natural signal is in the linear part of the growth curve; the second (sample WG3) has an intermediate value of De , and the natural signal is just out of the linear region; the third (DaLi) has a very large De value, such that the natural signal is almost in saturation. Sample NP is a carbonate-rich eolianite from the “Tel Aviv” bed, on the Mediterranean beach about 30 km north of Tel Aviv, Israel (Porat and Wintle, 1995). The De obtained on quartz extracted from this sample is 5:31 ± 0:12 Gy, obtained using the SAR protocol. Sample WG3 is a eolian sand from the Hulun Buir sandy land, north China, with an SAR De of 47 ± 2 Gy (Li et al., 2002). Sample DaLi is a sample older than 200 ka from an archaeological site in the northwest China (Xiao et al., 2002). The De is more than 500 Gy. All samples were 4rst treated with HCl and H2 O2 to remove carbonate and organic matter. The samples were then dried and sieved to select the grain size in the 150 –212 m interval. The selected grain fractions were further separated by heavy liquid to separate the quartz and K-feldspar grains. After drying, the quartz grains were treated with 40% HF for 40 min to remove the outer layer irradiated by alpha particles and any remaining feldspar contamination. The grains were then treated with 1 M HCl for 10 min to remove Puorites created by the HF etching. Before measurement of the OSL signals from the quartz aliquots, all aliquots underwent IRSL scanning to check for feldspar contamination
(Aitken, 1998). Aliquots giving a signi4cant IRSL signal were rejected. All luminescence signals were measured using an automated RisH TL/OSL reader system (TL-DA-15; Botter-Jensen et al., 2000). An EMI9635Q photomultiplier tube was used to detect the luminescence signal. For OSL measurements, the stimulating light was 420 –550 nm from a halogen lamp. For detecting the OSL from quartz, one 2 mm thick Schott BG-39 4lter and two 3 mm thick U-340 4lters were used. A 90 Sr= 90 Y beta source delivering 0:0921 Gy=s to the sample was also mounted on the TL/OSL reader for irradiation. 2.2. Measurements procedures 2.2.1. The growth curve from multiple aliquots We 4rst measured a regeneration growth curve using 24 bleached aliquots of sample NP. The aliquots were bleached under sunlight for 5 –6 h. Regenerative doses of 0, 27.6, 55.3, 92.1, 138.2, 184.2, 368.4 and 555:6 Gy were given to separate group of three aliquots. After preheating for 10 s at 260◦ C, the OSL signal of 4rst 0:2 s was measured from every aliquot at 125◦ C, and then the OSL trap was emptied by measuring the OSL for 200 s. Each aliquot was then given a 0:46 Gy test dose and the TL signal measured up to 160◦ C and the OSL signal for 200 s. The sensitivity change of the OSL was corrected by dividing the intensity of the subsequent 110◦ C TL from the test dose. This correction also normalized the OSL signal from diTerent aliquots. 2.2.2. Growth curves from SAAD and SAR with single aliquot In order to compare the SAAD and SAR protocols for quartz, all measurements were conducted on one aliquot. It
H. Zhao et al. / Radiation Measurements 37 (2003) 417 – 424
3. Results 3.1. Multiple-aliquot regenerative-dose and SAR growth curves The multiple-aliquot regenerative-dose growth curve for sample NP is shown in Fig. 1. The average value and the standard error of every group of three aliquots are calculated. The growth curve of multiple aliquots is compared with the SAR growth curve for one aliquot. All OSL signals were corrected and normalized using the subsequent 110◦ C TL response to the test dose. The multiple-aliquot growth curve coincides very well with that from SAR method. This agreement gives us con4dence in the reliability of the growth curves from both procedures.
1.4 corrected OSL (OSL/TL)
has been reported that the 110◦ C TL sensitivity change in some samples is proportional to the OSL sensitivity change (Chen et al., 2000; Murray and Wintle, 2000). The 110◦ C TL peak signal was used to correct the OSL sensitivity change in both the SAAD and SAR protocols. The measurement sequence is outlined in Table 1. Steps 1–7 are used to evaluate the data using the SAAD approach. The aliquot is preheated at 260◦ C for 10 s prior to each short OSL stimulation measurement of 0:2 s. The measured OSL signal is Ladi (step 3). After observing the natural signal (Lad0 ), an additive dose (Di1 ) is given and the aliquot is preheated and measured again. After the last additive dose (Dmax ), the aliquot is preheated and measured 4ve times (step 7) without irradiation, in order to determine the decay of the OSL signal due to preheating and measurement only. The luminescence signal lost in the preheating and stimulation is corrected using such decay factors. In contrast to the SAAD protocol proposed by Murray et al. (1997), a test dose Dt (step 4) has been included in the measurement cycle; the 110◦ C TL peak (Tadi ) induced by Dt is used to correct any OSL sensitivity changes. Growth curves are constructed using the OSL signals after correction for both sensitivity change and decay (see later). The De of the aliquot is then given by extrapolating the growth curve to the dose axis. Steps 8–16 are an implementation of the SAR procedure using the 110◦ C TL peak for sensitivity correction. The same aliquot used in steps 1–7 is used. The aliquot is preheated in step 9 (equals to step 2). The initial 0:2 s OSL signal (Lrei , step 10) is taken as the regenerative-dose signal. A test dose Dt (step 12) is given, as in the SAAD protocol above, and the aliquot heated to 160◦ C to measure the 110◦ C peak (step 13) to allow correction of the OSL signal for sensitivity change. The corrected Ladi and Lrei are then plotted against dose. The corrected Lad0 is the natural signal used with both the additive-dose growth curve and the regenerative-dose growth curve to calculate the De , by extrapolation using the SAAD data and by interpolation using the SAR data.
419
1.2 1 0.8 0.6 0.4
multiple
0.2
SAR
0 0
100
200
300 Dose (Gy)
400
500
600
Fig. 1. The multiple-aliquot regenerative-dose growth curve compared with the SAR growth curve for sample NP. All OSL signals were corrected by dividing by the 110◦ C TL response to the test dose. Every point on the multiple-aliquot growth curve is the mean value of 3 aliquots, and their standard errors are also shown.
3.2. Comparison of luminescence correction of SAAD with SAR The growth curves of the three samples obtained using the luminescence corrected SAAD results and the SAR data are shown in Fig. 2 (the procedure outlined in Section 2.2.2 was used to derive these data). Aliquots of NP and WG3 were divided into two groups. One group was given small doses up to 46 Gy (NP) and 138 Gy (WG3) to obtain the growth curves around the De value (Fig. 2a and c). Another group of aliquots was given larger doses up to 553 Gy (NP) and 1382 Gy (WG3) to study the growth of OSL in non-linear range (Fig. 2b and d). Quartz from DaLi was just given large doses up to 1382 Gy because the natural signal is close to saturation (Fig. 2e). Sample NP has a small regeneration De value of about 5:1 Gy. The additive growth curve is almost parallel to the regenerative growth curve for small doses (Fig. 2a). The De from the SAAD data is 5:4 Gy, similar to the De from the SAR results. However, with larger doses, the behavior of SAAD growth curve is diTerent from that of SAR growth curve (Fig. 2b). The SAR growth curve reaches saturation at about 200 Gy, but the SAAD growth curve still increases with dose. A similar picture is observed using samples WG3 and DaLi. WG3 has a De value of about 50 Gy. The De from SAAD is ∼ 83 Gy, signi4cantly different from the SAR results of ∼ 53 Gy (Fig. 2c). The additive growth curve also saturates more slowly than the regenerative growth curve (Fig. 2d). Sample DaLi has a De of about 210 Gy. The De from SAAD is almost 2 times the De from SAR, and the additive growth curve shape is obviously diTerent from that of the regenerative growth curve (Fig. 2e). The results of sample DaLi are particularly interesting (Fig. 2e). Since the natural signal is nearly in saturation, the SAAD data set (Ladi =Tadi ) without luminescence correction almost coincide with the SAR data set, except for
H. Zhao et al. / Radiation Measurements 37 (2003) 417 – 424 0.6
Ladi/Tadi
0.5
luminescence corrected Ladi/Tadi SAR, Lrei/Trei
0.4
2.5 2
0.3 NP additive De =5.43Gy regenerative De=5.07Gy
0.2 0.1
corrected OSL
corrected OSL
420
0 -5
10
0 -50
40
6
1
5
0.8 0.6 WG3 additive De=83.58Gy regenerative De=51.54Gy
0 35
85 dose (Gy)
135
350
550
dose (Gy) 7
-15
150
(b)
1.2
0.4
NP
0.5
1.4
0.2
(c)
1
corrected OSL
corrected OSL
(a)
25 dose (Gy)
1.5
4 3 2
WG3
1
(d)
0 -50
450
950
1450
dose (Gy)
12
corrected OSL
10 8 6 4
DaLi additive De=375.68Gy regenerative De=249.50Gy
2 0 -50
450
(e)
950
1450
dose (Gy)
Fig. 2. The growth curves of the three samples from the SAAD and SAR procedures are plotted using the 110◦ C TL corrected OSL against irradiation dose. Additive dose growth curves are corrected for sensitivity changes using the 110◦ C TL from a constant test dose, and the luminescence correction method (Duller, 1994; Murray et al., 1997). In this approach, the sensitivity-corrected luminescence lost during each OSL measurement is estimated from the rate of decay of luminescence during repeated measurements, but without further added dose (step 7 in Table 1), and this ‘lost OSL’ is added to the observed data at each dose point.
the 4rst two points. However, the growth curve from the luminescence-corrected SAAD data set increases signi4cantly with dose and is obviously diTerent from the SAR growth curve. This diTerence between SAAD and SAR growth curves clearly con4rms that the luminescence correction cannot be applied to samples for which the natural signals lie in the non-linear region of the growth curves, as was 4rst pointed out by Duller (1994). This comes from the basic assumption of luminescence correction that the amount of trapped charge created by unit dose is independent of dose. This is not the case when the equivalent dose is not in the linear part of the growth curve. Another correction method, such as the dose correction method (Duller, 1994) is required.
4. Growth function correction (GFC) and the results Here we introduce a new correction process for SAAD data, called the growth function corrections (GFC). 4.1. GFC In this approach, every data point is assumed to be on a growth curve of luminescence with ‘eTective’ added dose. It is generally represented by a growth curve function L = L(D):
(1)
In the additive dose data set, after a given additive dose (Di ) in measurement cycle i (i ¿ 1), it is assumed that the luminescence Li is composed of two parts: (a) the
H. Zhao et al. / Radiation Measurements 37 (2003) 417 – 424
luminescence Lri remaining from the previous measurement cycle; and (b) the luminescence created by the new laboratory additive dose Di (Fig. 3). The Lri signal can be thought of as that induced by an eTective dose Dri . Hence Lri = L(Dri )
or
Dri = D(Lri ):
(2)
where Lmax is the saturation signal, f is a constant such that a larger f means lower saturation dose. DeTect is the eTective dose that gives rise to the luminescence signal L. As Lri responds to eTective dose Dri , and Li responds to eTective dose (Dri + Di ) Lri = Lmax (1 − exp(−fDri ));
(2 )
(3)
Li = Lmax [1 − exp(−f(Dri + Di ))]:
(3 )
Thus Li = L(Dri + Di ): Rearranging (2) and (3)
Rearranging these two equations
Li = L(D(Lri ) + Di );
(4)
where Di ; Li and Lri are obtained from the experiments. Estimation of factors in function L(D) can be achieved by 4tting to all measured data points. As an example, we assume function (1) as a single saturation exponential function: (1 )
L = Lmax (1 − exp(−fDeTect ));
corrected Luminescence (Ladi/Tadi)
corrected luminescence
-10
5
421
Li = Lmax − (Lmax − Lri ) exp(−fDi ): Thus the relationship between f and Lmax is Lmax − Li 1 : f = − ln Di Lmax − Lri
(4 )
Here, Di ; Li and Lri are obtained from the measurements during the SAAD procedure. The factors f and Lmax can be derived by choosing the best 4t of Eq. (4 ) to all measurement data points. A growth curve can then be derived using Eq. (1 ).
luminescence correction
4.2. The
growth function correction
4
corrected luminescence remaining after previous measurement cycle
3 2 1 0 0
10
20
30
Dose
Fig. 3. Illustration of growth function correction of SAAD data and comparison with the results of luminescence correction. The translation of the luminescence points to give the eTective dose is indicated by horizontal arrows. (The luminescence correction process is shown by vertical arrows.)
Factors Lmax and f were obtained from an iterative routine to obtain the best 4t. The 4tting process involves large groups of assumed Lmax and f values. The best 4t Lmax and f values were chosen when the relative standard deviation (RSD) of the data set is at a minimum. The algorithm is shown in Table 2. The 4tting program is based on Eq. (4) (and is illustrated using Eq. (4 )). Firstly, an arbitrary value was given to Lmax ; this is normally the largest Li in the SAAD data set (step 1). A group of fi is then calculated using the known Di ; Li and Lri values of the data set (step 2). The relative standard deviation RSD1 of this group of fi values is then calculated. Secondly, a revised value of (Lmax + pace) is assigned to Lmax . The value of pace decides the amount and the precision of the calculation. Another group of fi
Table 2 The computer algorithm used for 4tting the SAAD data set to obtain the f and Lmax for GFC Step
Operation
Note
1 2 3 4
Let Lmax ⇐ arbitrary value Calculate fi of every data point Li Calculate relative standard deviation RSDi of the fi Let Lmax ⇐ Lmax + pace
Normally use the biggest Li (i = 1; 2; 3 : : :) Using Eq. (4) (e.g. Eq. (4 ))
5 6 7 8
Calculate fi+1 for every Li Calculate RSDi+1 of fi+1 If RSDi+1 ¡ RSDi , then RSDi ⇐ RSDi+1 Repeat 4 –7, else next Calculate the average value of fi ⇒ f
9
End
The value of pace determines the amount and the precision of the 4tting process Using Eq. (4) (e.g. Eq. (4 )) Finding the minimum RSD f and Lmax giving the minimum RSD are the best 4tting result Lmax and f are now the best 4t values
422
H. Zhao et al. / Radiation Measurements 37 (2003) 417 – 424
Table 3 The data set for one aliquot of sample DaLi used for the 4tting shown in Fig. 4 Measurement cycle
Added dose Di (i = 0; 1; 2 : : :) (Gy)
Li (i = 0; 1; 2 : : :) (osl/tl)
0 1 2 3 4 5 6
0 46.05 46.05 92.1 184.2 368.4 644.7
3.322 2.830 2.443 2.718 3.666 4.427 5.030
Fig. 4. The RSD of every group of fi against the Lmax of the DaLi aliquot of Table 3. From the derived values, when the Lmax is 5.31, the corresponding RSD reaches its minimum value of 2.56% and, the mean value of fi (0.00287) is the best 4tting result of f.
corresponding to (Lmax + pace) is then calculable from the data set, and the RSD2 of this group of fi can then be calculated. This process is repeated until the minimum RSD is obtained (steps 4 –7). Lmax and the mean value of the group of fi giving this minimum RSD are the best 4t results for the data set. As an example, the calculation process for sample DaLi is shown in Table 3 and Figure 4. Here the Li and Lri are the luminescence signals, sensitivity corrected using the 110◦ C TL. L6 was set as the starting Lmax and the pace was 0.001. For every Lmax value, there were six f values for six additive doses points, from which the mean value and the RSD of f were calculated. We plot the RSD against the corresponding Lmax in Fig. 4. When the RSD reaches its lowest value, the Lmax and the average value of f are accepted as the best 4tting result. The growth curves after correction are compared with those derived from the SAR data set in Fig. 5. (The data sets used are the same as shown in Fig. 2.) 5. Discussion 5.1. SAAD and SAR protocols The multiple-aliquot regenerative growth curve coincides very well with the growth curve from the SAR data set
Lri (i = 1; 2; 3 : : :)
2.485 2.117 1.828 2.033 2.742 3.311
Natural = L0 = 3:322; Lmax start from L6 = 5:030; Step = 0:001.
(Fig. 1). After ‘sliding’ the appropriate De value on the dose axis, the growth curve from the luminescence-corrected SAAD data only coincides with that from SAR in the linear part of the growth curve (Fig. 2). However, after GFC, the coincidence between growth curves of SAAD data sets and the SAR has been signi4cantly improved (Fig. 5). It appears that the SAAD data do describe the relationship between dose and luminescence (as do the SAR results), but the SAAD measurements need to be appropriately corrected for the loss of OSL induced in measurement process. In the SAR procedure, only one correction is required, i.e. correction for sensitivity changes during the measurement process. For SAAD data, as well as correcting for sensitivity change, the loss of luminescence signal must also be corrected for using the concept of ‘eTective dose’. Since an aliquot does not lose all charge from traps during SAAD measurements, any such remaining charge will inPuence subsequent trap 4lling. After luminescence correction (such as the ones used by Duller, 1994; Murray et al., 1997), the SAAD growth curves only coincide with the SAR growth curve in the linear region (Fig. 1). We deduce (as did Duller, 1994) that the luminescence correction can only be used for aliquots for which the natural signal is in the linear part of the growth curve. For ‘old’ samples, luminescence correction of SAAD data sets is not suitable, and a new correction method must be used. 5.2. GFC For samples NP and DaLi, the SAAD growth curves, after GFC correction and oTsetting by De , are in good agreement with the SAR growth curves (Fig. 5a, b, e). For sample WG3, the GFC-corrected SAAD growth curve do not agree well with that from SAR (Fig. 5c, d). The saturation values of GFC, SAAD and SAR do not coincide. This diTerence may come from two sources. One is from the SAAD measurements: from Fig. 2c, the SAAD data points (Ladi =Tadi ) before GFC correction only increase by about 20% with the additive dose. After GFC correction, the data points only make up a small portion of the growth curve (Fig. 5c). This introduces a signi4cant 4tting error when modeling the whole growth curve using these limited data
H. Zhao et al. / Radiation Measurements 37 (2003) 417 – 424 0.5
GFC, Ladi/Tadi
0.4
SAR, Lrei/Trei
corrected OSL
corrected OSL
'Australia slide' GFC
0.3 0.2 NP
0.1 0
-10
10
(a)
30
1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0
NP
-20
50
180
(b)
dose (Gy)
423
380
580
dose (Gy) 6
1
corrected OSL
corrected OSL
5
0.8 0.6 0.4 WG3
0.2
(c)
-30
3 2
WG3
1
0 -80
4
20
70
120
170
0 -150
350
(d)
dose (Gy)
850
1350
dose (Gy)
6
corrected OSL
5 4 3 2 DaLi
1 0
-500
(e)
0
500
1000
1500
dose (Gy)
Fig. 5. The growth curves of the additive dose data sets corrected using GFC compared with those from the SAR procedure. The data sets are same as those used in Fig. 1. The GFC additive growth curves are also shown corrected for the De oTset (‘Australia slide’), to illustrate the similarity to the SAR growth curves.
points. The other source of diTerence is from the growth function used: the growth function for some samples may not be well represented by a single saturating exponential function (Li, 1991). This would result in the GFC-corrected SAAD growth curves being diTerent from the SAR growth curves. The advantage of the GFC method is that it is not limited to only one function; any function that is known to provide a good 4t to the quartz growth curve of the material under study may be used. Although for some samples there are diTerences between GFC-corrected SAAD growth curves and the SAR growth curves, they are unlikely to be signi4cant to the estimates of equivalent doses. 5.3. Duller’s dose correction of SAAD data A dose correction process has been described earlier for SAAD data collected in the non-linear region of the growth
curve (Duller, 1994). In this approach, the decay factor calculated for each of the luminescence measurement runs was assumed to be relevant to the loss of ‘eTective dose’ in the run, and used to correct the ‘dose’ of each data point (rather than the luminescence). As the loss of dose may not have a linear relationship with the luminescence signal, the ‘lost’ luminescence signal (i.e. the amount by which the luminescence must be corrected) must be obtained from the growth function and the corresponding loss of ‘eTective dose’. Hence, the correction of the ‘eTective dose’ must be made using the appropriate growth curve function. The SAAD data sets were also corrected by using Duller’s dose correction method (Duller, 1994). The resulting growth curves are compared with the results of GFC SAAD data sets (Fig. 6). For samples NP and WG3, after allowing for the oTset due to De , the growth curves from dose correction coincide with the beginning of the GFC corrected growth
424
H. Zhao et al. / Radiation Measurements 37 (2003) 417 – 424
agreement with the multiple and SAR growth curves. The agreement of the SAAD and SAR methods helps to increase our con4dence in the validity of our single-aliquot quartz growth curves.
1.6 1.4 corrected OSL
1.2 1 0.8
Acknowledgements
0.6
Dose correction, Ladi/Tadi
0.4
'Australia slide' dose correction
0.2 0 -50
'Australia slide' GFC Ladi/Tadi
150
corrected OSL
(a)
4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 -150
(b)
350
350 dose (Gy)
850
This work was supported by grants to S.-H. Li from the Research Grant Council of HKSAR, China (project No. 7112/98P), and from ERCG of HKU.
550
References
1350
dose (Gy)
Fig. 6. Comparison of the dose corrected SAAD data with the GFC method.
curves. For sample DaLi, the 4rst two luminescence signals in the SAAD data (Ladi =Tadi ) set correspond to additive doses that are apparently smaller than the natural point (Fig. 1e). For this kind of data set, the dose correction cannot be applied, although the GFC approach handles this data set well (Fig. 5e). 6. Conclusion From directly comparing the growth curves from SAAD procedures with those from SAR procedures, it is con4rmed that SAAD procedures can work as good as SAR procedures with appropriate corrections. The luminescence correction of SAAD data can only be applied to aliquots for which the natural signals are in linear part of the growth curve. For those in non-linear range of dose responses, the SAAD data must be corrected for sensitivity changes and using a GFC. This SAAD method produces additive growth curves in good
Aitken, M.J., 1998. An Introduction to Optical Dating: the Dating of Quaternary Sediments by the Use of Photon-Stimulated Luminescence. Oxford University Press, Oxford. Botter-Jensen, L., Bulur, E., Duller, G.A.T., Murray, A.S., 2000. Advances in luminescence instrument systems. Radiat. Meas. 32, 523–528. Chen, G., Li, S.-H., Murray, A.S., 2000. Study of the 110◦ C TL peak sensitivity in optical dating of quartz. Radiat. Meas. 32, 641–645. Duller, G.A.T., 1991. Equivalent dose determination using single aliquots. Nuclear Tracks Radiat. Meas. 18, 257–264. Duller, G.A.T., 1994. Luminescence dating of sediments using single aliquots: new procedures. Quat. Sci. Rev. 13, 149–156. Li, S.-H., 1991. A cautionary note: apparent sensitivity changes resulting from curve 4tting. Ancient TL 9, 12–13. Li, S.-H., Sun, J.-M., Zhao, H., 2002. Optical dating of dune sands in the northeastern deserts of China. Palaeogeogr. Palaeoclimatol. Palaeoecol. 181, 419–429. Murray, A.S., Roberts, R.G., 1998. Measurement of the equivalent dose in quartz using a regenerative-dose single-aliquot protocol. Radiat. Meas. 29, 501–511. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiat. Meas. 32, 57–73. Murray, A.S., Roberts, R.G., Wintle, A.G., 1997. Equivalent dose measurement using a single aliquot of quartz. Radiat. Meas. 27, 171–184. Porat, N., Wintle, A.G., 1995. IRSL dating of eolianites from the late Pleistocene coastal kurkar ridge, Israel. XIV International Congress International Union for Quaternary Research, February 1995, Terra Nostre, Berlin, p. 221 (Abstract). Prescott, J.R., Huntley, D.J., Robertson, G.B., 1993. Estimation of equivalent dose in thermoluminescence dating the Australian lide method. Ancient TL 11, 1–5. Wintle, A.G., 1997. Luminescence dating: laboratory procedures and protocols. Radiat. Meas. 27, 769–817. Xiao, J.-L., Jin, Ch.-Zh., Zhu, Y.-Zh., 2002. Age of the fossil Dali Man in north-central China deduced from chronostratigraphy of the loess-paleosol sequence. Quat. Sci. Rev. 21, 2191–2198.
Radiation Measurements 37 (2003) 417 – 424 www.elsevier.com/locate/radmeas
Comparison of SAAD and SAR procedures for equivalent dose determination using quartz Hui Zhaoa , Sheng-Hua Lia;∗ , A.S. Murrayb b The
a Department of Earth Sciences, The University of Hong Kong, Pokfulam Road, Hong Kong, China Nordic Laboratory for Luminescence Dating, Department of Earth Sciences, Aarhus University, Ris$ National Laboratory, P.O. Box 49, DK-4000 Roskilde, Denmark
Received 26 August 2002; received in revised form 11 December 2002; accepted 15 December 2002
Abstract Comparisons between the single-aliquot additive-dose (SAAD) and single-aliquot regenerative-dose (SAR) procedures are made using 150 –212 m quartz grains separated from three sediment samples. Growth curves and equivalent doses from the two procedures are compared directly using single aliquots of quartz. With appropriate corrections for the loss of signals in our SAAD protocol, the SAAD and SAR growth curves coincide well. It is con4rmed that the luminescence correction in the SAAD protocol can only be applied to samples for which the natural signal is in the linear part of the growth curve. A new correction method is proposed. c 2003 Elsevier Ltd. All rights reserved. Keywords: Quartz; Equivalent dose; SAAD and SAR
1. Introduction Two main groups of equivalent dose (De ) determination methods, additive-dose and regenerative-dose, have been applied in luminescence dating (Wintle, 1997). A single-aliquot approach to the measurement of De using K-feldspar separation was 4rst developed by Duller (1991), who recommended the use of a single-aliquot additive-dose protocol (SAAD) because of possible sensitivity changes in regeneration procedures. A SAAD protocol for quartz was developed later by Murray et al. (1997). A single-aliquot regenerative-dose (SAR) protocol for the OSL dating of quartz was then proposed, which used the response of the 110◦ C TL peak to correct for sensitivity changes in the OSL signal (Murray and Roberts, 1998). This SAR protocol has since been modi4ed to use the OSL signal from a test dose for sensitivity correction (Murray and Wintle, 2000), and this is now considered the standard regenerative protocol. These two single-aliquot protocols, SAAD and SAR, are now widely employed in optical dating. ∗
Corresponding author. Fax: +852-2517-6912. E-mail address: [email protected] (S.-H. Li).
The SAR protocol tends to be favored over SAAD protocol because sensitivity changes corrected for; De can be determined by interpolation to get more precise and accurate results, and these results have little dependence on the 4tting equations used for the growth curve. However, it is not clear whether the dose responses are the same in both growing cases where OSL grows from emptied OSL traps for SAR procedures and grows from partially 4lled traps for SAAD procedures, and whether they can be expressed in a simple mathematical expression. Although OSL sensitivity change can be corrected by the OSL signals created from a test dose, the 110◦ C TL from the same test dose can be a surrogate to the OSL only when both the signals are linearly correlated. Moreover, other interesting questions concerning SAAD procedures remained: (a) beside the problem of extrapolation, are there other diBculties in the application of the SAAD protocol? (b) Can the ’Australian slide’ method (Prescott et al., 1993) be applied to the data from a single aliquot? In this study, comparisons between the SAAD the SAR procedures are made. Growth curves and equivalent doses from the two procedures are compared using single aliquots of quartz. A new correction method that makes the SAAD
c 2003 Elsevier Ltd. All rights reserved. 1350-4487/03/$ - see front matter doi:10.1016/S1350-4487(03)00019-2
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H. Zhao et al. / Radiation Measurements 37 (2003) 417 – 424
Table 1 Measurement protocol used in this study Step
Treatment
Observed
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
Give dose Di1 (i = 0; 1; 2; 3 : : :; D01 = 0) Preheat (260◦ C, 10 s) OSL (0:2 s at 125◦ C) Test dose Dt TL (up to 160◦ C, heat rate 3◦ C=s) If Di1 = Dmax , return to step 1, else go to step 7 Repeat step 2–5 4ve times Give dose Di2 (i = 1; 2; 3 : : :) Preheat (260◦ C, 10 s) OSL (0:2 s at 125◦ C) OSL (200 s at 125◦ C) Test dose Dt TL (up to 160◦ C, heat rate 3◦ C=s) OSL (200 s at 125◦ C) If Di2 = Dmax , return to step 8, else go to step 16 Repeat steps 9 –13 once more
— — Additive Ladi — Tadi used to correct Ladi sensitivity change — Ladi decay factors — — Regeneration Lrei Empty the OSL trap — Trei used to correct Lrei sensitivity change Empty the OSL trap — Lrei for Di2 = 0
growth curve indistinguishable from that obtained using the SAR protocol is proposed.
2. Samples and measurement procedures 2.1. Samples and facilities Three quartz samples were used in this study. One is a young sample (NP) for which the natural signal is in the linear part of the growth curve; the second (sample WG3) has an intermediate value of De , and the natural signal is just out of the linear region; the third (DaLi) has a very large De value, such that the natural signal is almost in saturation. Sample NP is a carbonate-rich eolianite from the “Tel Aviv” bed, on the Mediterranean beach about 30 km north of Tel Aviv, Israel (Porat and Wintle, 1995). The De obtained on quartz extracted from this sample is 5:31 ± 0:12 Gy, obtained using the SAR protocol. Sample WG3 is a eolian sand from the Hulun Buir sandy land, north China, with an SAR De of 47 ± 2 Gy (Li et al., 2002). Sample DaLi is a sample older than 200 ka from an archaeological site in the northwest China (Xiao et al., 2002). The De is more than 500 Gy. All samples were 4rst treated with HCl and H2 O2 to remove carbonate and organic matter. The samples were then dried and sieved to select the grain size in the 150 –212 m interval. The selected grain fractions were further separated by heavy liquid to separate the quartz and K-feldspar grains. After drying, the quartz grains were treated with 40% HF for 40 min to remove the outer layer irradiated by alpha particles and any remaining feldspar contamination. The grains were then treated with 1 M HCl for 10 min to remove Puorites created by the HF etching. Before measurement of the OSL signals from the quartz aliquots, all aliquots underwent IRSL scanning to check for feldspar contamination
(Aitken, 1998). Aliquots giving a signi4cant IRSL signal were rejected. All luminescence signals were measured using an automated RisH TL/OSL reader system (TL-DA-15; Botter-Jensen et al., 2000). An EMI9635Q photomultiplier tube was used to detect the luminescence signal. For OSL measurements, the stimulating light was 420 –550 nm from a halogen lamp. For detecting the OSL from quartz, one 2 mm thick Schott BG-39 4lter and two 3 mm thick U-340 4lters were used. A 90 Sr= 90 Y beta source delivering 0:0921 Gy=s to the sample was also mounted on the TL/OSL reader for irradiation. 2.2. Measurements procedures 2.2.1. The growth curve from multiple aliquots We 4rst measured a regeneration growth curve using 24 bleached aliquots of sample NP. The aliquots were bleached under sunlight for 5 –6 h. Regenerative doses of 0, 27.6, 55.3, 92.1, 138.2, 184.2, 368.4 and 555:6 Gy were given to separate group of three aliquots. After preheating for 10 s at 260◦ C, the OSL signal of 4rst 0:2 s was measured from every aliquot at 125◦ C, and then the OSL trap was emptied by measuring the OSL for 200 s. Each aliquot was then given a 0:46 Gy test dose and the TL signal measured up to 160◦ C and the OSL signal for 200 s. The sensitivity change of the OSL was corrected by dividing the intensity of the subsequent 110◦ C TL from the test dose. This correction also normalized the OSL signal from diTerent aliquots. 2.2.2. Growth curves from SAAD and SAR with single aliquot In order to compare the SAAD and SAR protocols for quartz, all measurements were conducted on one aliquot. It
H. Zhao et al. / Radiation Measurements 37 (2003) 417 – 424
3. Results 3.1. Multiple-aliquot regenerative-dose and SAR growth curves The multiple-aliquot regenerative-dose growth curve for sample NP is shown in Fig. 1. The average value and the standard error of every group of three aliquots are calculated. The growth curve of multiple aliquots is compared with the SAR growth curve for one aliquot. All OSL signals were corrected and normalized using the subsequent 110◦ C TL response to the test dose. The multiple-aliquot growth curve coincides very well with that from SAR method. This agreement gives us con4dence in the reliability of the growth curves from both procedures.
1.4 corrected OSL (OSL/TL)
has been reported that the 110◦ C TL sensitivity change in some samples is proportional to the OSL sensitivity change (Chen et al., 2000; Murray and Wintle, 2000). The 110◦ C TL peak signal was used to correct the OSL sensitivity change in both the SAAD and SAR protocols. The measurement sequence is outlined in Table 1. Steps 1–7 are used to evaluate the data using the SAAD approach. The aliquot is preheated at 260◦ C for 10 s prior to each short OSL stimulation measurement of 0:2 s. The measured OSL signal is Ladi (step 3). After observing the natural signal (Lad0 ), an additive dose (Di1 ) is given and the aliquot is preheated and measured again. After the last additive dose (Dmax ), the aliquot is preheated and measured 4ve times (step 7) without irradiation, in order to determine the decay of the OSL signal due to preheating and measurement only. The luminescence signal lost in the preheating and stimulation is corrected using such decay factors. In contrast to the SAAD protocol proposed by Murray et al. (1997), a test dose Dt (step 4) has been included in the measurement cycle; the 110◦ C TL peak (Tadi ) induced by Dt is used to correct any OSL sensitivity changes. Growth curves are constructed using the OSL signals after correction for both sensitivity change and decay (see later). The De of the aliquot is then given by extrapolating the growth curve to the dose axis. Steps 8–16 are an implementation of the SAR procedure using the 110◦ C TL peak for sensitivity correction. The same aliquot used in steps 1–7 is used. The aliquot is preheated in step 9 (equals to step 2). The initial 0:2 s OSL signal (Lrei , step 10) is taken as the regenerative-dose signal. A test dose Dt (step 12) is given, as in the SAAD protocol above, and the aliquot heated to 160◦ C to measure the 110◦ C peak (step 13) to allow correction of the OSL signal for sensitivity change. The corrected Ladi and Lrei are then plotted against dose. The corrected Lad0 is the natural signal used with both the additive-dose growth curve and the regenerative-dose growth curve to calculate the De , by extrapolation using the SAAD data and by interpolation using the SAR data.
419
1.2 1 0.8 0.6 0.4
multiple
0.2
SAR
0 0
100
200
300 Dose (Gy)
400
500
600
Fig. 1. The multiple-aliquot regenerative-dose growth curve compared with the SAR growth curve for sample NP. All OSL signals were corrected by dividing by the 110◦ C TL response to the test dose. Every point on the multiple-aliquot growth curve is the mean value of 3 aliquots, and their standard errors are also shown.
3.2. Comparison of luminescence correction of SAAD with SAR The growth curves of the three samples obtained using the luminescence corrected SAAD results and the SAR data are shown in Fig. 2 (the procedure outlined in Section 2.2.2 was used to derive these data). Aliquots of NP and WG3 were divided into two groups. One group was given small doses up to 46 Gy (NP) and 138 Gy (WG3) to obtain the growth curves around the De value (Fig. 2a and c). Another group of aliquots was given larger doses up to 553 Gy (NP) and 1382 Gy (WG3) to study the growth of OSL in non-linear range (Fig. 2b and d). Quartz from DaLi was just given large doses up to 1382 Gy because the natural signal is close to saturation (Fig. 2e). Sample NP has a small regeneration De value of about 5:1 Gy. The additive growth curve is almost parallel to the regenerative growth curve for small doses (Fig. 2a). The De from the SAAD data is 5:4 Gy, similar to the De from the SAR results. However, with larger doses, the behavior of SAAD growth curve is diTerent from that of SAR growth curve (Fig. 2b). The SAR growth curve reaches saturation at about 200 Gy, but the SAAD growth curve still increases with dose. A similar picture is observed using samples WG3 and DaLi. WG3 has a De value of about 50 Gy. The De from SAAD is ∼ 83 Gy, signi4cantly different from the SAR results of ∼ 53 Gy (Fig. 2c). The additive growth curve also saturates more slowly than the regenerative growth curve (Fig. 2d). Sample DaLi has a De of about 210 Gy. The De from SAAD is almost 2 times the De from SAR, and the additive growth curve shape is obviously diTerent from that of the regenerative growth curve (Fig. 2e). The results of sample DaLi are particularly interesting (Fig. 2e). Since the natural signal is nearly in saturation, the SAAD data set (Ladi =Tadi ) without luminescence correction almost coincide with the SAR data set, except for
H. Zhao et al. / Radiation Measurements 37 (2003) 417 – 424 0.6
Ladi/Tadi
0.5
luminescence corrected Ladi/Tadi SAR, Lrei/Trei
0.4
2.5 2
0.3 NP additive De =5.43Gy regenerative De=5.07Gy
0.2 0.1
corrected OSL
corrected OSL
420
0 -5
10
0 -50
40
6
1
5
0.8 0.6 WG3 additive De=83.58Gy regenerative De=51.54Gy
0 35
85 dose (Gy)
135
350
550
dose (Gy) 7
-15
150
(b)
1.2
0.4
NP
0.5
1.4
0.2
(c)
1
corrected OSL
corrected OSL
(a)
25 dose (Gy)
1.5
4 3 2
WG3
1
(d)
0 -50
450
950
1450
dose (Gy)
12
corrected OSL
10 8 6 4
DaLi additive De=375.68Gy regenerative De=249.50Gy
2 0 -50
450
(e)
950
1450
dose (Gy)
Fig. 2. The growth curves of the three samples from the SAAD and SAR procedures are plotted using the 110◦ C TL corrected OSL against irradiation dose. Additive dose growth curves are corrected for sensitivity changes using the 110◦ C TL from a constant test dose, and the luminescence correction method (Duller, 1994; Murray et al., 1997). In this approach, the sensitivity-corrected luminescence lost during each OSL measurement is estimated from the rate of decay of luminescence during repeated measurements, but without further added dose (step 7 in Table 1), and this ‘lost OSL’ is added to the observed data at each dose point.
the 4rst two points. However, the growth curve from the luminescence-corrected SAAD data set increases signi4cantly with dose and is obviously diTerent from the SAR growth curve. This diTerence between SAAD and SAR growth curves clearly con4rms that the luminescence correction cannot be applied to samples for which the natural signals lie in the non-linear region of the growth curves, as was 4rst pointed out by Duller (1994). This comes from the basic assumption of luminescence correction that the amount of trapped charge created by unit dose is independent of dose. This is not the case when the equivalent dose is not in the linear part of the growth curve. Another correction method, such as the dose correction method (Duller, 1994) is required.
4. Growth function correction (GFC) and the results Here we introduce a new correction process for SAAD data, called the growth function corrections (GFC). 4.1. GFC In this approach, every data point is assumed to be on a growth curve of luminescence with ‘eTective’ added dose. It is generally represented by a growth curve function L = L(D):
(1)
In the additive dose data set, after a given additive dose (Di ) in measurement cycle i (i ¿ 1), it is assumed that the luminescence Li is composed of two parts: (a) the
H. Zhao et al. / Radiation Measurements 37 (2003) 417 – 424
luminescence Lri remaining from the previous measurement cycle; and (b) the luminescence created by the new laboratory additive dose Di (Fig. 3). The Lri signal can be thought of as that induced by an eTective dose Dri . Hence Lri = L(Dri )
or
Dri = D(Lri ):
(2)
where Lmax is the saturation signal, f is a constant such that a larger f means lower saturation dose. DeTect is the eTective dose that gives rise to the luminescence signal L. As Lri responds to eTective dose Dri , and Li responds to eTective dose (Dri + Di ) Lri = Lmax (1 − exp(−fDri ));
(2 )
(3)
Li = Lmax [1 − exp(−f(Dri + Di ))]:
(3 )
Thus Li = L(Dri + Di ): Rearranging (2) and (3)
Rearranging these two equations
Li = L(D(Lri ) + Di );
(4)
where Di ; Li and Lri are obtained from the experiments. Estimation of factors in function L(D) can be achieved by 4tting to all measured data points. As an example, we assume function (1) as a single saturation exponential function: (1 )
L = Lmax (1 − exp(−fDeTect ));
corrected Luminescence (Ladi/Tadi)
corrected luminescence
-10
5
421
Li = Lmax − (Lmax − Lri ) exp(−fDi ): Thus the relationship between f and Lmax is Lmax − Li 1 : f = − ln Di Lmax − Lri
(4 )
Here, Di ; Li and Lri are obtained from the measurements during the SAAD procedure. The factors f and Lmax can be derived by choosing the best 4t of Eq. (4 ) to all measurement data points. A growth curve can then be derived using Eq. (1 ).
luminescence correction
4.2. The
growth function correction
4
corrected luminescence remaining after previous measurement cycle
3 2 1 0 0
10
20
30
Dose
Fig. 3. Illustration of growth function correction of SAAD data and comparison with the results of luminescence correction. The translation of the luminescence points to give the eTective dose is indicated by horizontal arrows. (The luminescence correction process is shown by vertical arrows.)
Factors Lmax and f were obtained from an iterative routine to obtain the best 4t. The 4tting process involves large groups of assumed Lmax and f values. The best 4t Lmax and f values were chosen when the relative standard deviation (RSD) of the data set is at a minimum. The algorithm is shown in Table 2. The 4tting program is based on Eq. (4) (and is illustrated using Eq. (4 )). Firstly, an arbitrary value was given to Lmax ; this is normally the largest Li in the SAAD data set (step 1). A group of fi is then calculated using the known Di ; Li and Lri values of the data set (step 2). The relative standard deviation RSD1 of this group of fi values is then calculated. Secondly, a revised value of (Lmax + pace) is assigned to Lmax . The value of pace decides the amount and the precision of the calculation. Another group of fi
Table 2 The computer algorithm used for 4tting the SAAD data set to obtain the f and Lmax for GFC Step
Operation
Note
1 2 3 4
Let Lmax ⇐ arbitrary value Calculate fi of every data point Li Calculate relative standard deviation RSDi of the fi Let Lmax ⇐ Lmax + pace
Normally use the biggest Li (i = 1; 2; 3 : : :) Using Eq. (4) (e.g. Eq. (4 ))
5 6 7 8
Calculate fi+1 for every Li Calculate RSDi+1 of fi+1 If RSDi+1 ¡ RSDi , then RSDi ⇐ RSDi+1 Repeat 4 –7, else next Calculate the average value of fi ⇒ f
9
End
The value of pace determines the amount and the precision of the 4tting process Using Eq. (4) (e.g. Eq. (4 )) Finding the minimum RSD f and Lmax giving the minimum RSD are the best 4tting result Lmax and f are now the best 4t values
422
H. Zhao et al. / Radiation Measurements 37 (2003) 417 – 424
Table 3 The data set for one aliquot of sample DaLi used for the 4tting shown in Fig. 4 Measurement cycle
Added dose Di (i = 0; 1; 2 : : :) (Gy)
Li (i = 0; 1; 2 : : :) (osl/tl)
0 1 2 3 4 5 6
0 46.05 46.05 92.1 184.2 368.4 644.7
3.322 2.830 2.443 2.718 3.666 4.427 5.030
Fig. 4. The RSD of every group of fi against the Lmax of the DaLi aliquot of Table 3. From the derived values, when the Lmax is 5.31, the corresponding RSD reaches its minimum value of 2.56% and, the mean value of fi (0.00287) is the best 4tting result of f.
corresponding to (Lmax + pace) is then calculable from the data set, and the RSD2 of this group of fi can then be calculated. This process is repeated until the minimum RSD is obtained (steps 4 –7). Lmax and the mean value of the group of fi giving this minimum RSD are the best 4t results for the data set. As an example, the calculation process for sample DaLi is shown in Table 3 and Figure 4. Here the Li and Lri are the luminescence signals, sensitivity corrected using the 110◦ C TL. L6 was set as the starting Lmax and the pace was 0.001. For every Lmax value, there were six f values for six additive doses points, from which the mean value and the RSD of f were calculated. We plot the RSD against the corresponding Lmax in Fig. 4. When the RSD reaches its lowest value, the Lmax and the average value of f are accepted as the best 4tting result. The growth curves after correction are compared with those derived from the SAR data set in Fig. 5. (The data sets used are the same as shown in Fig. 2.) 5. Discussion 5.1. SAAD and SAR protocols The multiple-aliquot regenerative growth curve coincides very well with the growth curve from the SAR data set
Lri (i = 1; 2; 3 : : :)
2.485 2.117 1.828 2.033 2.742 3.311
Natural = L0 = 3:322; Lmax start from L6 = 5:030; Step = 0:001.
(Fig. 1). After ‘sliding’ the appropriate De value on the dose axis, the growth curve from the luminescence-corrected SAAD data only coincides with that from SAR in the linear part of the growth curve (Fig. 2). However, after GFC, the coincidence between growth curves of SAAD data sets and the SAR has been signi4cantly improved (Fig. 5). It appears that the SAAD data do describe the relationship between dose and luminescence (as do the SAR results), but the SAAD measurements need to be appropriately corrected for the loss of OSL induced in measurement process. In the SAR procedure, only one correction is required, i.e. correction for sensitivity changes during the measurement process. For SAAD data, as well as correcting for sensitivity change, the loss of luminescence signal must also be corrected for using the concept of ‘eTective dose’. Since an aliquot does not lose all charge from traps during SAAD measurements, any such remaining charge will inPuence subsequent trap 4lling. After luminescence correction (such as the ones used by Duller, 1994; Murray et al., 1997), the SAAD growth curves only coincide with the SAR growth curve in the linear region (Fig. 1). We deduce (as did Duller, 1994) that the luminescence correction can only be used for aliquots for which the natural signal is in the linear part of the growth curve. For ‘old’ samples, luminescence correction of SAAD data sets is not suitable, and a new correction method must be used. 5.2. GFC For samples NP and DaLi, the SAAD growth curves, after GFC correction and oTsetting by De , are in good agreement with the SAR growth curves (Fig. 5a, b, e). For sample WG3, the GFC-corrected SAAD growth curve do not agree well with that from SAR (Fig. 5c, d). The saturation values of GFC, SAAD and SAR do not coincide. This diTerence may come from two sources. One is from the SAAD measurements: from Fig. 2c, the SAAD data points (Ladi =Tadi ) before GFC correction only increase by about 20% with the additive dose. After GFC correction, the data points only make up a small portion of the growth curve (Fig. 5c). This introduces a signi4cant 4tting error when modeling the whole growth curve using these limited data
H. Zhao et al. / Radiation Measurements 37 (2003) 417 – 424 0.5
GFC, Ladi/Tadi
0.4
SAR, Lrei/Trei
corrected OSL
corrected OSL
'Australia slide' GFC
0.3 0.2 NP
0.1 0
-10
10
(a)
30
1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0
NP
-20
50
180
(b)
dose (Gy)
423
380
580
dose (Gy) 6
1
corrected OSL
corrected OSL
5
0.8 0.6 0.4 WG3
0.2
(c)
-30
3 2
WG3
1
0 -80
4
20
70
120
170
0 -150
350
(d)
dose (Gy)
850
1350
dose (Gy)
6
corrected OSL
5 4 3 2 DaLi
1 0
-500
(e)
0
500
1000
1500
dose (Gy)
Fig. 5. The growth curves of the additive dose data sets corrected using GFC compared with those from the SAR procedure. The data sets are same as those used in Fig. 1. The GFC additive growth curves are also shown corrected for the De oTset (‘Australia slide’), to illustrate the similarity to the SAR growth curves.
points. The other source of diTerence is from the growth function used: the growth function for some samples may not be well represented by a single saturating exponential function (Li, 1991). This would result in the GFC-corrected SAAD growth curves being diTerent from the SAR growth curves. The advantage of the GFC method is that it is not limited to only one function; any function that is known to provide a good 4t to the quartz growth curve of the material under study may be used. Although for some samples there are diTerences between GFC-corrected SAAD growth curves and the SAR growth curves, they are unlikely to be signi4cant to the estimates of equivalent doses. 5.3. Duller’s dose correction of SAAD data A dose correction process has been described earlier for SAAD data collected in the non-linear region of the growth
curve (Duller, 1994). In this approach, the decay factor calculated for each of the luminescence measurement runs was assumed to be relevant to the loss of ‘eTective dose’ in the run, and used to correct the ‘dose’ of each data point (rather than the luminescence). As the loss of dose may not have a linear relationship with the luminescence signal, the ‘lost’ luminescence signal (i.e. the amount by which the luminescence must be corrected) must be obtained from the growth function and the corresponding loss of ‘eTective dose’. Hence, the correction of the ‘eTective dose’ must be made using the appropriate growth curve function. The SAAD data sets were also corrected by using Duller’s dose correction method (Duller, 1994). The resulting growth curves are compared with the results of GFC SAAD data sets (Fig. 6). For samples NP and WG3, after allowing for the oTset due to De , the growth curves from dose correction coincide with the beginning of the GFC corrected growth
424
H. Zhao et al. / Radiation Measurements 37 (2003) 417 – 424
agreement with the multiple and SAR growth curves. The agreement of the SAAD and SAR methods helps to increase our con4dence in the validity of our single-aliquot quartz growth curves.
1.6 1.4 corrected OSL
1.2 1 0.8
Acknowledgements
0.6
Dose correction, Ladi/Tadi
0.4
'Australia slide' dose correction
0.2 0 -50
'Australia slide' GFC Ladi/Tadi
150
corrected OSL
(a)
4.5 4 3.5 3 2.5 2 1.5 1 0.5 0 -150
(b)
350
350 dose (Gy)
850
This work was supported by grants to S.-H. Li from the Research Grant Council of HKSAR, China (project No. 7112/98P), and from ERCG of HKU.
550
References
1350
dose (Gy)
Fig. 6. Comparison of the dose corrected SAAD data with the GFC method.
curves. For sample DaLi, the 4rst two luminescence signals in the SAAD data (Ladi =Tadi ) set correspond to additive doses that are apparently smaller than the natural point (Fig. 1e). For this kind of data set, the dose correction cannot be applied, although the GFC approach handles this data set well (Fig. 5e). 6. Conclusion From directly comparing the growth curves from SAAD procedures with those from SAR procedures, it is con4rmed that SAAD procedures can work as good as SAR procedures with appropriate corrections. The luminescence correction of SAAD data can only be applied to aliquots for which the natural signals are in linear part of the growth curve. For those in non-linear range of dose responses, the SAAD data must be corrected for sensitivity changes and using a GFC. This SAAD method produces additive growth curves in good
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