- Email: [email protected]
OSL DA-15 reader
Radiation Measurements 35 (2002) 275 – 280
Technical note
www.elsevier.com/locate/radmeas
Quanti cation of cross-irradiation and cross-illumination using a RisH TL=OSL DA-15 reader H.E. Bray ∗ , R.M. Bailey, S. Stokes School of Geography and the Environment, University of Oxford, Manseld Road, Oxford OX1 3TB, UK Received 21 April 2001; accepted 26 November 2001
Abstract The automated RisH TL=OSL DA-15 reader is widely used for luminescence dating applications. The nature of the device is such that the irradiation and illumination of a sample during the OSL procedures may a5ect discs in adjacent positions. It is critical to con rm that these factors do not introduce signi cant systematic errors. The e5ects of such cross-talk (cross-irradiation and cross-bleaching) are examined and quanti ed, both for speci c and general cases. Depending upon the protocol used, cross-irradiation and cross-bleaching could accumulate to have an e5ect on the De estimate, and for the latter this could be c 2002 Elsevier Science Ltd. All rights reserved. signi cant (i.e. 10%).
1. Introduction Computerised and automated (dose-heat-illuminate) measurement equipment has rapidly become the norm in luminescence dating applications (Aitken, 1998). It is essential to ensure that these devices do not complicate or corrupt critical dating measurements. Of the two commercially available devices the RisH automated reader (BHtter-Jensen et al., 1999a, 2000) would appear to be gaining majority market share, and is the focus of this investigation. The design of the RisH TL=OSL DA-15 reader optimises the spatial distribution of measurement discs by packing them closely. The disc carousel has 24, 36 or 48 positions, we focus on the 48-seater arrangement. The centres of adjacent positions on the 48-seater carousel are 17 mm apart. The RisH TL=OSL reader is provided with a combined blue=IR illumination unit and 90 Sr= 90 Y beta irradiation unit. The blue diodes employed for the stimulation ( = 470 nm; Pmax ∼18 mW cm−2 ) are focused on the sample from a distance of about 25 mm and the beta source (40 mCi) is 5 mm from the sample during irradiation
∗
Corresponding author. E-mail address: [email protected] (H.E. Bray).
(BHtter-Jensen et al., 2000). 1 A consequence of this close packing is that irradiation and illumination of one disc may a5ect neighbouring discs. Such ‘cross-talk’ of either stimulation photons or beta-radiation is a means by which systematic errors can be introduced into luminescence measurements and data. Documentation provided by RisH (Markey et al., 1997) alludes only to the extent of cross-talk due to irradiation, measured for both the nearest and second nearest neighbouring samples on a 48-position carousel. The nearest neighbouring sample is reported to receive 60 Gy Gy−1 , or 0.006%, of the dose received by the irradiated sample. The second nearest neighbouring disc receives 6 Gy Gy−1 (0.0006%). The same e5ect on the adjacent disc is measured by BHtter-Jensen et al. (2000) as 0:1735 ± 0:0004%, signi cantly greater than the earlier gure. Optical cross-talk (cross-bleaching) is not reported by Markey et al. (1997). It is, however, measured as 0:00599 ± 0:00007% to the neighbouring sample due to stimulation at full (35 mW cm−2 ; = 470 nm) power by BHtter-Jensen et al. (2000). 1 As our applications are currently on quartz, we have not tested the cross-illumination of feldspathic or polymineral samples using the IR source.
c 2002 Elsevier Science Ltd. All rights reserved. 1350-4487/02/$ - see front matter PII: S 1 3 5 0 - 4 4 8 7 ( 0 2 ) 0 0 0 4 5 - 8
276
H.E. Bray et al. / Radiation Measurements 35 (2002) 275 – 280
This study was undertaken to provide an independent test for cross-talk within an automated RISI TL=OSL DA-15 reader tted with a blue diode array by examining the e5ects of optical exposure and irradiation on a series of discs of annealed quartz from modern beach sand (Eastbourne, UK) with grain size 90 –125 m (Bailey, 2000).
Power/dose Cross-talk Discs Adjacent Measurement Adjacent (Adj) (M) (Adj) Fig. 1. Diagram showing the positioning of the adjacent and measurement discs described in the text. Measurements of cross-talk on both adjacent discs were made, and the results averaged.
2. Experimental design The luminescence signal emitted from the samples was measured with a photomultiplier (type 9235QA) ltered with 2 Corning U-340 glass lters. In the following description, we refer to the “adjacent” and “measurement” discs, as de ned in Fig. 1. The results presented are the averages of those from both adjacent discs.
further details of the procedure. Adjacent discs either side of the measurement disc were given a beta dose of 13 Gy (step ◦ a1) and a pre-heat of 200 C for 10 s (step a2). The size of the remaining signal was subsequently monitored using a 0:1 s OSL measurement (step a3) [0:1 s at 5% total diode power, 0:95 mW cm−2 ]. This was followed by a 1000 s optical stimulation on the measurement disc (step a4). ◦ In step a5, the adjacent disc was heated to 200 C. This heating was included for two reasons. First of all, it allows the extent of PTTL induced during cross-bleaching to be established. Secondly, it causes “recuperation” of the OSL signal, whereby some of the source charge of
2.1. Tests of optical cross-talk To test for the extent of cross-bleaching to neighbouring discs we looked for (i) evidence of phototransferred thermoluminescence (PTTL); and (ii) loss of OSL signal, following illumination of adjacent disc positions. Fig. 2a provides
Irradiation (M) (1000 Gy) (nx=0)
a1
a2
a3
Dose (Adj) (c. 13 Gy)
b1
Irradiation (Adj) (nx Gy) (nx=0,2,4,6,8 Gy)
b2
TL (Adj) (200°C, 10s)
Pre-heat (Adj) (200°C, 10s)
Short-shine OSL (0.1s, 5% power, 130°C) (Adj)
OSL (Adj) 50s,
b3 90% power,130°C a4
a5
a6
1000s OSL (M)
TL (Adj) (200°C)
b4
Test Dose (Adj) (c. 1.5 Gy)
b5
Pre-heat (Adj) (200°C, 10s)
Repeat for all x
×4
Short-shine OSL (0.1s, 5% power, 130°C) (Adj)
OSL (Adj) 50s,
b6 90% power,130°C (a)
(b)
Fig. 2. Flow chart showing the procedure followed for measuring cross-talk to neighbouring discs (Adj) due to (a) cross-bleaching; (b) cross-irradiation, where n1 = natural (i.e. dose received as cross-talk from measurement disc (M)), n2 –n5 are regeneration doses to the adjacent discs. Measurements were made on both adjacent discs and the results averaged.
H.E. Bray et al. / Radiation Measurements 35 (2002) 275 – 280 ◦
5000 TL (counts/s)
the PTTL signal, which temporarily resides in the 110 C trap, is thermally transferred back in to the traps responsible for OSL (Aitken and Smith, 1988). Thermal stimulation thus allows measurement of the true signal loss in the adjacent disc, which would otherwise be overestimated. In step a6, the OSL remaining in the adjacent disc, following cross-bleaching, is measured using another 0:1 s (5% blue-diode power) OSL measurement. Steps a4 –a6 were then repeated 4 times. The depletion of the OSL signal of the adjacent disc due to the measurement cycle alone was measured using a control disc, which was given the same dose and pre-heat and series of short-shine illuminations. To further quantify the degree of signal loss due to cross-bleaching a similar sequence of measurements was used on a separate aliquot. Instead of being bleached through cross-talk from a 1000 s shine down to a neighbouring disc, this disc was itself given a 0:2 s short-shine (at 95% power—18 mW cm−2 ). The depletion caused by this was measured with a 0:1 s short-shine, as for step a3, described above. Comparison of the half-life of signal loss due to cross-talk and to direct short-shine bleaching allowed us to quantify the cross-talk in terms of illumination power.
4000 3000 2000 1000 0 0
3. Results
50
100 150 Temp (C)
200
Fig. 3. Example of phototransferred thermoluminescence induced ◦ at 110 C in the adjacent disc due to 1000 s illumination to the measurement disc. The initial dose to the adjacent disc was 13 Gy. Table 1 Results from cross-bleaching experiments (a). Figures are the average from the two discs either side of the measurement disc (M—measurement disc, Adj—adjacent disc). (b) shows the set of measurements made using direct optical stimulation on an aliquot in order to calibrate the optical cross-talk measurements
2.2. Tests of cross-talk during irradiation To quantify the e5ects of cross-talk to adjacent discs during irradiation a set of aliquots were rst “reset” using a ◦ ◦ ramp heat to 400 C (5 C s−1 ) and a subsequent 50 s illumination (470 nm, 18 mW cm−2 ). Fig. 2b details the further stages of the procedure. A beta dose of 7500 s (ca.1000 Gy) was given to the measurement disc. The dose received by the adjacent discs due to cross-talk from this irradiation was measured using the SAR protocol (steps b1–b6) (following Murray and Wintle, 2000).
277
(a) Cumulative 0 1000 2000 3000 4000
OSL signal from Adj (c=s)
Cumulative signal loss from Adj (%)
exposure time to M 8320 ± 91 6467 ± 80 5019 ± 71 4132 ± 64 3403 ± 58
— 22:3 ± 1:3 39:7 ± 1:1 50:3 ± 0:9 59:1 ± 0:83
(b) Cumulative 0:2 s short-shine to Adj 0 5426 ± 74 0.2 3830 ± 62 0.4 2692 ± 52 0.6 1960 ± 44 0.8 1465 ± 38
— 29:4 ± 1:5 50:4 ± 1:2 63:9 ± 1:0 73:0 ± 0:8
3.1. Extent of optical cross-talk The existence of optical cross-talk can be con rmed by the presence of a PTTL signal from an irradiated (and preheated) aliquot due to illumination of the adjacent disc. This phototransfer is caused by the transfer of electrons to ◦ PTTL traps (in this case at 110 C) during illumination and is clearly visible in the samples adjacent to the measurement disc (Fig. 3). As there is no reason to believe that the power required to halve the signal from the disc receiving cycles of 0:2 s illumination at 95% diode power (18 mW cm−2 ) is di5erent to the power required to halve the signal due to cross-bleaching (i.e. there is a linear relationship between power and signal depletion rate: see Spooner, 1994; Bailey, 2001), we “calibrated” the depletion due to cross-bleaching by comparing it to direct bleaching (as we know the power density of
direct bleaching). The degree of cross-bleaching can then be expressed in terms of e5ective power or energy deposition. The control discs showed no signi cant depletion of the signal due to the 0:1 s (5% illumination power) short-shines over the ve cycles measured (see Fig. 2a, steps a4 –a6). We can therefore conclude that all signal loss observed in the adjacent disc (see Table 1) is due to cross-talk. The half-life of the depletion of the signal due to cross-bleaching was found to be 3100 s (of illumination to the measurement disc) (Fig. 4a). The equivalent gure on the disc receiving 0:2 s short-shines is 0:422 s illumination at 18 mW cm−2 (Table 1, Fig. 4b) and thus 7:60 mJ cm−2 cross-talk to the adjacent disc occurred during bleaching. Over the 3100 s this is 2:45 W cm−2 cross-talk, which is 0.014% of the light to the measurement disc
278
H.E. Bray et al. / Radiation Measurements 35 (2002) 275 – 280
1.2
1
Normalised OSL
Normalised OSL
1.2
0.8 0.6 0.4 0.2
-0.22x
y = 0.98e
1 0.8 0.6 0.4
0
y = 0.98e
0 0
(a)
-0.33x
0.2
1 2 3 4 Measurement cycle (1000s to M)
0
(b)
1 2 3 4 Measurement cycle (0.2s shine)
Fig. 4. Signal loss, (a) in adjacent disc (Adj) due to 1000 s illumination to measurement disc (M); (b) repeated 0:2 s OSL measurements of the adjacent aliquot (Adj). Table 2 Results from cross-irradiation experiments (ss=stainless steel, Al= aluminium). Discs 3 and 10 were irradiated for 7500 s (c.1000 Gy)
1.4 1.2
Norm OSL
1 0.8 0.1
0.6
Disc
Disc substrate
Equivalent dose (s)
2 4 9 11
ss ss Al Al
0:51 ± 0:01 0:17 ± 0:005 0:39 ± 0:001 0:58 ± 0:043
0.4 0
0.2 0
0
0
2
4 6 Dose (s)
0.5
8
Markey et al. (1997) and is 2 orders of magnitude less than that of BHtter-Jensen et al. (2000).
1
10
Fig. 5. Example regeneration curve of the “natural” (crossirradiation) signal to a disc adjacent to one receiving a 1000 Gy dose; inset shows curve enlarged to show natural signal. Error bars t within symbols.
(18 mW cm−2 ). This gure is signi cantly greater (i.e. more than double) than the ∼ 0:006% cross-talk measured by BHtter-Jensen et al. (2000), but is still small in percentage terms. 3.2. Extent of cross-talk during irradiation The regeneration curves showed slight supra-linearity, so the De was interpolated from a linear t of the rst two points to maximise accuracy (Fig. 5). Our gure for cross-irradiation is thus likely to be an underestimate of the true value. Following an irradiation of 7500 s duration, cross-irradiation is shown to be equivalent to a dose of 0:41 ± 0:09 s. This is 0:0055 ± 0:012%; or 55 Gy Gy−1 of the dose given to the measurement disc. These values represent the average of measurements from a sample on either side of the irradiated sample (Table 2). The magnitude of cross-irradiation measured here agrees with the gure of
4. Discussion 4.1. Cross-bleaching In order to determine whether these e5ects are negligible and can be ignored it is useful to put them in the context of a standard OSL protocol. Using the SAR procedure, for example, a standard illumination duration of 50 or 100 s may be adopted (Murray and Wintle, 2000). The calculations presented below are for the worst case scenario where all the OSL and irradiation procedures are carried out on disc 1 before any measurements are started on disc 2. For the general case it is possible to calculate the magnitude of the cross-bleaching e5ect using a simple integration. If there is 0.014% cross-talk, 1 s of OSL measurement will give power equivalent to 0:00014 s illumination to the adjacent disc. Thus Tp , the e5ective illumination time to the adjacent disc, is given by Tp = 0:00014Tn;
(1)
where the product Tn is the total illumination duration of the measurement disc (T being the duration of each individual illumination and n the number of illumination periods, including test dose illuminations).
H.E. Bray et al. / Radiation Measurements 35 (2002) 275 – 280
279
Table 3 The magnitude of cross-talk during irradiation for some standard OSL sequences assuming a cross-talk value of 55 Gy Gy−1 Expected De (Gy) Regeneration doses (Gy) Standard test dose (Gy) Total dose (Gy) Cross-irradiation (mGy) Proportion of De (%)
10 6,8,10,12,6,0 2 56 3.08 0.038
50 10,20,50,80,10,0 5 205 11.28 0.023
Typically, the rst few seconds of quartz OSL decay can be approximated by a single exponential decay, written OSL(t) = I0 e−t ;
(2)
where I0 is the initial signal intensity, is the decay constant (typically of the order of 1 s−1 for 90% diode power, with ◦ a sample temperature of 130 C) and t is illumination time. The amount of signal lost due to cross-bleaching can be estimated by integrating this decaying signal (Eq. (2)) between t = 0 and Tp . Generally, the integral of only the rst few seconds (a duration de ned here as Tc seconds) of the OSL signal is used to calculate the De . The proportion of this part of the signal remaining following cross-bleaching, of duration Tp , (approximating the OSL decay with a single exponential) can be calculated as T c I0 Tc I˜0 e−t dt {( ) − [( I0 )1 − e−Tp ]}e−t dt = 0 P = 0Tc Tc I0 e−t dt I0 e−t dt 0 0 = e−Tp ;
(3)
where I˜0 is the initial intensity following modi cation (depletion) due to cross-bleaching (of duration Tp ) and I0 is for Tp = 0 (no prior cross-bleaching). As expected, Eq. (3) simpli es to an exponential dependence of P on Tp . We have used Eqs. (1) – (3) to simulate the expected depletion of OSL due to a full SAR procedure on an adjacent position. Here we de ne our “full” SAR procedure as being a total of 14 OSL measurements (comprising measurement of the natural signal, four regeneration points, a repeat point and a zero-dose point, each with associated sensitivity measurements), each of 100 s duration (again, we set =1). For this case, we have Tp = 0:196 (Eq. (1)) and the resulting depletion of the OSL signal of the adjacent disc is 17.8%. Note that reducing the illumination time to 50 s reduces the depletion to 9.3%. In order to check the accuracy of our calculations, we performed an experiment in which two natural quartz aliquots (RAK=00=A1, United Arab Emirates) were rst bleached ◦ (100 s blue LED illumination at 130 C) and subsequently given a beta dose of 10 Gy. A SAR procedure was performed in order to recover this dose on the rst aliquot. The resultant De was 9:75 ± 0:30 Gy (i.e. within errors of 10 Gy). Before measurement of the second aliquot, a blank disc to the left of the second aliquot was illuminated for 2000 s. The prediction from Eq. (3) (again, using = 1) is that 24.4%
100 30,60,100,120,30,0 5 375 20.63 0.021
of the signal would be removed, leaving an estimated De of ∼7:56 Gy (Tp = 0:28). The measured De for the second aliquot was 7:44 ± 0:24 Gy, and in agreement with the predicted value. Based on these results, we conclude that Eqs. (1) – (3) o5er a suSciently accurate means of assessing the e5ect of cross-illumination. 4.2. Cross-irradiation The extra dose to the adjacent disc of 55 Gy Gy−1 agrees with that measured by Markey et al. (1997). It would be received at least 5 times during a typical SAR measurement sequence. The test dose is generally small, and thus unlikely to have signi cant cross-irradiation e5ects. Depending upon the age of the sample and thus the regeneration doses being used, irradiation cross-talk could be, at most, of the order of a few mGy (see Table 3). Again, we can formulate an equation to describe the amount of cross-irradiation received in the general case. The summed value of the regeneration doses and the standard test doses to a sample for a given expected equivalent dose (De ) can be multiplied by the expected proportion of cross-irradiation (C ), measured here to be 55 × 10−6 or by Markey et al. (1997) as 60×10−6 and by BHtter-Jensen et al. (2000) as 170 × 10−6 , to give the expected cross-irradiation dose. The general equation is thus as follows: i=n Ctot = C Ri + (n + 1)S Gy; (4) i=0
where n is the number of regeneration cycles (the index i running from 0, the natural signal, to n), R is the regeneration dose and S is the standard test dose. This can then be divided by the expected De to give the cross-irradiation as a proportion of the expected De (Table 3). These e5ects will be seen only on discs in carousel position 2 and above in sequences run one at a time, but may be important for ultra high precision De determinations such as those employed in radiation microdosimetry (e.g. Banerjee et al., 1999; BHtter-Jensen et al., 1999b). 5. Conclusions 1. We con rm the occurrence of cross-talk between adjacent discs on the 48-position carousel for the RISI TL=OSL DA-15 reader.
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H.E. Bray et al. / Radiation Measurements 35 (2002) 275 – 280
2. Depending upon the protocol and sequence order used, cross-bleaching and cross-irradiation could accumulate to have an e5ect on the De estimate. The cross-illumination e5ect may be severe (i.e. up to 20% signal reduction on adjacent aliquots) while the cross-irradiation is of a far lower magnitude. 3. It is important to con rm and quantify the occurrence of all systematic errors in luminescence analyses. There may be merit in more widespread application of robust analyses such as those described above for other aspects of the optical dating procedure. 4. It would be pertinent to conduct similar tests on other automated TL=OSL readers. References 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. Aitken, M.J., Smith, B.W., 1988. Optical dating: recuperation after bleaching. Quat. Sci. Rev. 7, 387–393. Bailey, R.M., 2000. The interpretation of quartz optically stimulated luminescence equivalent dose versus time plots. Radiat. Meas. 32, 129–140.
Bailey, R.M., 2001. Towards a general kinetic model for optically and thermally stimulated luminescence of quartz. Radiat. Meas. 33 (1), 17–45. Banerjee, D., BHtter-Jensen, L., Murray, A.S., 1999. Retrospective dosimetry: preliminary use of the single aliquot regeneration (SAR) protocol for the measurement of quartz dose in young house bricks. Radiat. Prot. Dosim. Part 1 84 (1– 4), 412–426. BHtter-Jensen, L., Mejdahl, V., Murray, A.S., 1999a. New light on OSL. Quat. Sci. Rev. 18 (2), 303–309. BHtter-Jensen, L., Banerjee, D., Jungner, H., Murray, A.S., 1999b. Retrospective assessment of environmental dose rates using optically stimulated luminescence from Al(2)O(3):C and quartz. Radiat. Prot. Dosim. Part 1 84 (1– 4), 537–542. BHtter-Jensen, L., Bulur, E., Duller, G.A.T., Murray, A.S., 2000. Advances in luminescence instrument systems. Radiat. Meas. 32, 523–528. Markey, B.G., BHtter-Jensen, L., Duller, G.A.T., 1997. A new, Vexible system for measuring thermally and optically stimulated luminescence. Radiat. Meas. 27, 83–89. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiat. Meas. 32, 57–73. Spooner, N.A., 1994. On the optical dating signal from quartz. Radiat. Meas. 23 (2–3), 593–600.
Technical note
www.elsevier.com/locate/radmeas
Quanti cation of cross-irradiation and cross-illumination using a RisH TL=OSL DA-15 reader H.E. Bray ∗ , R.M. Bailey, S. Stokes School of Geography and the Environment, University of Oxford, Manseld Road, Oxford OX1 3TB, UK Received 21 April 2001; accepted 26 November 2001
Abstract The automated RisH TL=OSL DA-15 reader is widely used for luminescence dating applications. The nature of the device is such that the irradiation and illumination of a sample during the OSL procedures may a5ect discs in adjacent positions. It is critical to con rm that these factors do not introduce signi cant systematic errors. The e5ects of such cross-talk (cross-irradiation and cross-bleaching) are examined and quanti ed, both for speci c and general cases. Depending upon the protocol used, cross-irradiation and cross-bleaching could accumulate to have an e5ect on the De estimate, and for the latter this could be c 2002 Elsevier Science Ltd. All rights reserved. signi cant (i.e. 10%).
1. Introduction Computerised and automated (dose-heat-illuminate) measurement equipment has rapidly become the norm in luminescence dating applications (Aitken, 1998). It is essential to ensure that these devices do not complicate or corrupt critical dating measurements. Of the two commercially available devices the RisH automated reader (BHtter-Jensen et al., 1999a, 2000) would appear to be gaining majority market share, and is the focus of this investigation. The design of the RisH TL=OSL DA-15 reader optimises the spatial distribution of measurement discs by packing them closely. The disc carousel has 24, 36 or 48 positions, we focus on the 48-seater arrangement. The centres of adjacent positions on the 48-seater carousel are 17 mm apart. The RisH TL=OSL reader is provided with a combined blue=IR illumination unit and 90 Sr= 90 Y beta irradiation unit. The blue diodes employed for the stimulation ( = 470 nm; Pmax ∼18 mW cm−2 ) are focused on the sample from a distance of about 25 mm and the beta source (40 mCi) is 5 mm from the sample during irradiation
∗
Corresponding author. E-mail address: [email protected] (H.E. Bray).
(BHtter-Jensen et al., 2000). 1 A consequence of this close packing is that irradiation and illumination of one disc may a5ect neighbouring discs. Such ‘cross-talk’ of either stimulation photons or beta-radiation is a means by which systematic errors can be introduced into luminescence measurements and data. Documentation provided by RisH (Markey et al., 1997) alludes only to the extent of cross-talk due to irradiation, measured for both the nearest and second nearest neighbouring samples on a 48-position carousel. The nearest neighbouring sample is reported to receive 60 Gy Gy−1 , or 0.006%, of the dose received by the irradiated sample. The second nearest neighbouring disc receives 6 Gy Gy−1 (0.0006%). The same e5ect on the adjacent disc is measured by BHtter-Jensen et al. (2000) as 0:1735 ± 0:0004%, signi cantly greater than the earlier gure. Optical cross-talk (cross-bleaching) is not reported by Markey et al. (1997). It is, however, measured as 0:00599 ± 0:00007% to the neighbouring sample due to stimulation at full (35 mW cm−2 ; = 470 nm) power by BHtter-Jensen et al. (2000). 1 As our applications are currently on quartz, we have not tested the cross-illumination of feldspathic or polymineral samples using the IR source.
c 2002 Elsevier Science Ltd. All rights reserved. 1350-4487/02/$ - see front matter PII: S 1 3 5 0 - 4 4 8 7 ( 0 2 ) 0 0 0 4 5 - 8
276
H.E. Bray et al. / Radiation Measurements 35 (2002) 275 – 280
This study was undertaken to provide an independent test for cross-talk within an automated RISI TL=OSL DA-15 reader tted with a blue diode array by examining the e5ects of optical exposure and irradiation on a series of discs of annealed quartz from modern beach sand (Eastbourne, UK) with grain size 90 –125 m (Bailey, 2000).
Power/dose Cross-talk Discs Adjacent Measurement Adjacent (Adj) (M) (Adj) Fig. 1. Diagram showing the positioning of the adjacent and measurement discs described in the text. Measurements of cross-talk on both adjacent discs were made, and the results averaged.
2. Experimental design The luminescence signal emitted from the samples was measured with a photomultiplier (type 9235QA) ltered with 2 Corning U-340 glass lters. In the following description, we refer to the “adjacent” and “measurement” discs, as de ned in Fig. 1. The results presented are the averages of those from both adjacent discs.
further details of the procedure. Adjacent discs either side of the measurement disc were given a beta dose of 13 Gy (step ◦ a1) and a pre-heat of 200 C for 10 s (step a2). The size of the remaining signal was subsequently monitored using a 0:1 s OSL measurement (step a3) [0:1 s at 5% total diode power, 0:95 mW cm−2 ]. This was followed by a 1000 s optical stimulation on the measurement disc (step a4). ◦ In step a5, the adjacent disc was heated to 200 C. This heating was included for two reasons. First of all, it allows the extent of PTTL induced during cross-bleaching to be established. Secondly, it causes “recuperation” of the OSL signal, whereby some of the source charge of
2.1. Tests of optical cross-talk To test for the extent of cross-bleaching to neighbouring discs we looked for (i) evidence of phototransferred thermoluminescence (PTTL); and (ii) loss of OSL signal, following illumination of adjacent disc positions. Fig. 2a provides
Irradiation (M) (1000 Gy) (nx=0)
a1
a2
a3
Dose (Adj) (c. 13 Gy)
b1
Irradiation (Adj) (nx Gy) (nx=0,2,4,6,8 Gy)
b2
TL (Adj) (200°C, 10s)
Pre-heat (Adj) (200°C, 10s)
Short-shine OSL (0.1s, 5% power, 130°C) (Adj)
OSL (Adj) 50s,
b3 90% power,130°C a4
a5
a6
1000s OSL (M)
TL (Adj) (200°C)
b4
Test Dose (Adj) (c. 1.5 Gy)
b5
Pre-heat (Adj) (200°C, 10s)
Repeat for all x
×4
Short-shine OSL (0.1s, 5% power, 130°C) (Adj)
OSL (Adj) 50s,
b6 90% power,130°C (a)
(b)
Fig. 2. Flow chart showing the procedure followed for measuring cross-talk to neighbouring discs (Adj) due to (a) cross-bleaching; (b) cross-irradiation, where n1 = natural (i.e. dose received as cross-talk from measurement disc (M)), n2 –n5 are regeneration doses to the adjacent discs. Measurements were made on both adjacent discs and the results averaged.
H.E. Bray et al. / Radiation Measurements 35 (2002) 275 – 280 ◦
5000 TL (counts/s)
the PTTL signal, which temporarily resides in the 110 C trap, is thermally transferred back in to the traps responsible for OSL (Aitken and Smith, 1988). Thermal stimulation thus allows measurement of the true signal loss in the adjacent disc, which would otherwise be overestimated. In step a6, the OSL remaining in the adjacent disc, following cross-bleaching, is measured using another 0:1 s (5% blue-diode power) OSL measurement. Steps a4 –a6 were then repeated 4 times. The depletion of the OSL signal of the adjacent disc due to the measurement cycle alone was measured using a control disc, which was given the same dose and pre-heat and series of short-shine illuminations. To further quantify the degree of signal loss due to cross-bleaching a similar sequence of measurements was used on a separate aliquot. Instead of being bleached through cross-talk from a 1000 s shine down to a neighbouring disc, this disc was itself given a 0:2 s short-shine (at 95% power—18 mW cm−2 ). The depletion caused by this was measured with a 0:1 s short-shine, as for step a3, described above. Comparison of the half-life of signal loss due to cross-talk and to direct short-shine bleaching allowed us to quantify the cross-talk in terms of illumination power.
4000 3000 2000 1000 0 0
3. Results
50
100 150 Temp (C)
200
Fig. 3. Example of phototransferred thermoluminescence induced ◦ at 110 C in the adjacent disc due to 1000 s illumination to the measurement disc. The initial dose to the adjacent disc was 13 Gy. Table 1 Results from cross-bleaching experiments (a). Figures are the average from the two discs either side of the measurement disc (M—measurement disc, Adj—adjacent disc). (b) shows the set of measurements made using direct optical stimulation on an aliquot in order to calibrate the optical cross-talk measurements
2.2. Tests of cross-talk during irradiation To quantify the e5ects of cross-talk to adjacent discs during irradiation a set of aliquots were rst “reset” using a ◦ ◦ ramp heat to 400 C (5 C s−1 ) and a subsequent 50 s illumination (470 nm, 18 mW cm−2 ). Fig. 2b details the further stages of the procedure. A beta dose of 7500 s (ca.1000 Gy) was given to the measurement disc. The dose received by the adjacent discs due to cross-talk from this irradiation was measured using the SAR protocol (steps b1–b6) (following Murray and Wintle, 2000).
277
(a) Cumulative 0 1000 2000 3000 4000
OSL signal from Adj (c=s)
Cumulative signal loss from Adj (%)
exposure time to M 8320 ± 91 6467 ± 80 5019 ± 71 4132 ± 64 3403 ± 58
— 22:3 ± 1:3 39:7 ± 1:1 50:3 ± 0:9 59:1 ± 0:83
(b) Cumulative 0:2 s short-shine to Adj 0 5426 ± 74 0.2 3830 ± 62 0.4 2692 ± 52 0.6 1960 ± 44 0.8 1465 ± 38
— 29:4 ± 1:5 50:4 ± 1:2 63:9 ± 1:0 73:0 ± 0:8
3.1. Extent of optical cross-talk The existence of optical cross-talk can be con rmed by the presence of a PTTL signal from an irradiated (and preheated) aliquot due to illumination of the adjacent disc. This phototransfer is caused by the transfer of electrons to ◦ PTTL traps (in this case at 110 C) during illumination and is clearly visible in the samples adjacent to the measurement disc (Fig. 3). As there is no reason to believe that the power required to halve the signal from the disc receiving cycles of 0:2 s illumination at 95% diode power (18 mW cm−2 ) is di5erent to the power required to halve the signal due to cross-bleaching (i.e. there is a linear relationship between power and signal depletion rate: see Spooner, 1994; Bailey, 2001), we “calibrated” the depletion due to cross-bleaching by comparing it to direct bleaching (as we know the power density of
direct bleaching). The degree of cross-bleaching can then be expressed in terms of e5ective power or energy deposition. The control discs showed no signi cant depletion of the signal due to the 0:1 s (5% illumination power) short-shines over the ve cycles measured (see Fig. 2a, steps a4 –a6). We can therefore conclude that all signal loss observed in the adjacent disc (see Table 1) is due to cross-talk. The half-life of the depletion of the signal due to cross-bleaching was found to be 3100 s (of illumination to the measurement disc) (Fig. 4a). The equivalent gure on the disc receiving 0:2 s short-shines is 0:422 s illumination at 18 mW cm−2 (Table 1, Fig. 4b) and thus 7:60 mJ cm−2 cross-talk to the adjacent disc occurred during bleaching. Over the 3100 s this is 2:45 W cm−2 cross-talk, which is 0.014% of the light to the measurement disc
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1.2
1
Normalised OSL
Normalised OSL
1.2
0.8 0.6 0.4 0.2
-0.22x
y = 0.98e
1 0.8 0.6 0.4
0
y = 0.98e
0 0
(a)
-0.33x
0.2
1 2 3 4 Measurement cycle (1000s to M)
0
(b)
1 2 3 4 Measurement cycle (0.2s shine)
Fig. 4. Signal loss, (a) in adjacent disc (Adj) due to 1000 s illumination to measurement disc (M); (b) repeated 0:2 s OSL measurements of the adjacent aliquot (Adj). Table 2 Results from cross-irradiation experiments (ss=stainless steel, Al= aluminium). Discs 3 and 10 were irradiated for 7500 s (c.1000 Gy)
1.4 1.2
Norm OSL
1 0.8 0.1
0.6
Disc
Disc substrate
Equivalent dose (s)
2 4 9 11
ss ss Al Al
0:51 ± 0:01 0:17 ± 0:005 0:39 ± 0:001 0:58 ± 0:043
0.4 0
0.2 0
0
0
2
4 6 Dose (s)
0.5
8
Markey et al. (1997) and is 2 orders of magnitude less than that of BHtter-Jensen et al. (2000).
1
10
Fig. 5. Example regeneration curve of the “natural” (crossirradiation) signal to a disc adjacent to one receiving a 1000 Gy dose; inset shows curve enlarged to show natural signal. Error bars t within symbols.
(18 mW cm−2 ). This gure is signi cantly greater (i.e. more than double) than the ∼ 0:006% cross-talk measured by BHtter-Jensen et al. (2000), but is still small in percentage terms. 3.2. Extent of cross-talk during irradiation The regeneration curves showed slight supra-linearity, so the De was interpolated from a linear t of the rst two points to maximise accuracy (Fig. 5). Our gure for cross-irradiation is thus likely to be an underestimate of the true value. Following an irradiation of 7500 s duration, cross-irradiation is shown to be equivalent to a dose of 0:41 ± 0:09 s. This is 0:0055 ± 0:012%; or 55 Gy Gy−1 of the dose given to the measurement disc. These values represent the average of measurements from a sample on either side of the irradiated sample (Table 2). The magnitude of cross-irradiation measured here agrees with the gure of
4. Discussion 4.1. Cross-bleaching In order to determine whether these e5ects are negligible and can be ignored it is useful to put them in the context of a standard OSL protocol. Using the SAR procedure, for example, a standard illumination duration of 50 or 100 s may be adopted (Murray and Wintle, 2000). The calculations presented below are for the worst case scenario where all the OSL and irradiation procedures are carried out on disc 1 before any measurements are started on disc 2. For the general case it is possible to calculate the magnitude of the cross-bleaching e5ect using a simple integration. If there is 0.014% cross-talk, 1 s of OSL measurement will give power equivalent to 0:00014 s illumination to the adjacent disc. Thus Tp , the e5ective illumination time to the adjacent disc, is given by Tp = 0:00014Tn;
(1)
where the product Tn is the total illumination duration of the measurement disc (T being the duration of each individual illumination and n the number of illumination periods, including test dose illuminations).
H.E. Bray et al. / Radiation Measurements 35 (2002) 275 – 280
279
Table 3 The magnitude of cross-talk during irradiation for some standard OSL sequences assuming a cross-talk value of 55 Gy Gy−1 Expected De (Gy) Regeneration doses (Gy) Standard test dose (Gy) Total dose (Gy) Cross-irradiation (mGy) Proportion of De (%)
10 6,8,10,12,6,0 2 56 3.08 0.038
50 10,20,50,80,10,0 5 205 11.28 0.023
Typically, the rst few seconds of quartz OSL decay can be approximated by a single exponential decay, written OSL(t) = I0 e−t ;
(2)
where I0 is the initial signal intensity, is the decay constant (typically of the order of 1 s−1 for 90% diode power, with ◦ a sample temperature of 130 C) and t is illumination time. The amount of signal lost due to cross-bleaching can be estimated by integrating this decaying signal (Eq. (2)) between t = 0 and Tp . Generally, the integral of only the rst few seconds (a duration de ned here as Tc seconds) of the OSL signal is used to calculate the De . The proportion of this part of the signal remaining following cross-bleaching, of duration Tp , (approximating the OSL decay with a single exponential) can be calculated as T c I0 Tc I˜0 e−t dt {( ) − [( I0 )1 − e−Tp ]}e−t dt = 0 P = 0Tc Tc I0 e−t dt I0 e−t dt 0 0 = e−Tp ;
(3)
where I˜0 is the initial intensity following modi cation (depletion) due to cross-bleaching (of duration Tp ) and I0 is for Tp = 0 (no prior cross-bleaching). As expected, Eq. (3) simpli es to an exponential dependence of P on Tp . We have used Eqs. (1) – (3) to simulate the expected depletion of OSL due to a full SAR procedure on an adjacent position. Here we de ne our “full” SAR procedure as being a total of 14 OSL measurements (comprising measurement of the natural signal, four regeneration points, a repeat point and a zero-dose point, each with associated sensitivity measurements), each of 100 s duration (again, we set =1). For this case, we have Tp = 0:196 (Eq. (1)) and the resulting depletion of the OSL signal of the adjacent disc is 17.8%. Note that reducing the illumination time to 50 s reduces the depletion to 9.3%. In order to check the accuracy of our calculations, we performed an experiment in which two natural quartz aliquots (RAK=00=A1, United Arab Emirates) were rst bleached ◦ (100 s blue LED illumination at 130 C) and subsequently given a beta dose of 10 Gy. A SAR procedure was performed in order to recover this dose on the rst aliquot. The resultant De was 9:75 ± 0:30 Gy (i.e. within errors of 10 Gy). Before measurement of the second aliquot, a blank disc to the left of the second aliquot was illuminated for 2000 s. The prediction from Eq. (3) (again, using = 1) is that 24.4%
100 30,60,100,120,30,0 5 375 20.63 0.021
of the signal would be removed, leaving an estimated De of ∼7:56 Gy (Tp = 0:28). The measured De for the second aliquot was 7:44 ± 0:24 Gy, and in agreement with the predicted value. Based on these results, we conclude that Eqs. (1) – (3) o5er a suSciently accurate means of assessing the e5ect of cross-illumination. 4.2. Cross-irradiation The extra dose to the adjacent disc of 55 Gy Gy−1 agrees with that measured by Markey et al. (1997). It would be received at least 5 times during a typical SAR measurement sequence. The test dose is generally small, and thus unlikely to have signi cant cross-irradiation e5ects. Depending upon the age of the sample and thus the regeneration doses being used, irradiation cross-talk could be, at most, of the order of a few mGy (see Table 3). Again, we can formulate an equation to describe the amount of cross-irradiation received in the general case. The summed value of the regeneration doses and the standard test doses to a sample for a given expected equivalent dose (De ) can be multiplied by the expected proportion of cross-irradiation (C ), measured here to be 55 × 10−6 or by Markey et al. (1997) as 60×10−6 and by BHtter-Jensen et al. (2000) as 170 × 10−6 , to give the expected cross-irradiation dose. The general equation is thus as follows: i=n Ctot = C Ri + (n + 1)S Gy; (4) i=0
where n is the number of regeneration cycles (the index i running from 0, the natural signal, to n), R is the regeneration dose and S is the standard test dose. This can then be divided by the expected De to give the cross-irradiation as a proportion of the expected De (Table 3). These e5ects will be seen only on discs in carousel position 2 and above in sequences run one at a time, but may be important for ultra high precision De determinations such as those employed in radiation microdosimetry (e.g. Banerjee et al., 1999; BHtter-Jensen et al., 1999b). 5. Conclusions 1. We con rm the occurrence of cross-talk between adjacent discs on the 48-position carousel for the RISI TL=OSL DA-15 reader.
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2. Depending upon the protocol and sequence order used, cross-bleaching and cross-irradiation could accumulate to have an e5ect on the De estimate. The cross-illumination e5ect may be severe (i.e. up to 20% signal reduction on adjacent aliquots) while the cross-irradiation is of a far lower magnitude. 3. It is important to con rm and quantify the occurrence of all systematic errors in luminescence analyses. There may be merit in more widespread application of robust analyses such as those described above for other aspects of the optical dating procedure. 4. It would be pertinent to conduct similar tests on other automated TL=OSL readers. References 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. Aitken, M.J., Smith, B.W., 1988. Optical dating: recuperation after bleaching. Quat. Sci. Rev. 7, 387–393. Bailey, R.M., 2000. The interpretation of quartz optically stimulated luminescence equivalent dose versus time plots. Radiat. Meas. 32, 129–140.
Bailey, R.M., 2001. Towards a general kinetic model for optically and thermally stimulated luminescence of quartz. Radiat. Meas. 33 (1), 17–45. Banerjee, D., BHtter-Jensen, L., Murray, A.S., 1999. Retrospective dosimetry: preliminary use of the single aliquot regeneration (SAR) protocol for the measurement of quartz dose in young house bricks. Radiat. Prot. Dosim. Part 1 84 (1– 4), 412–426. BHtter-Jensen, L., Mejdahl, V., Murray, A.S., 1999a. New light on OSL. Quat. Sci. Rev. 18 (2), 303–309. BHtter-Jensen, L., Banerjee, D., Jungner, H., Murray, A.S., 1999b. Retrospective assessment of environmental dose rates using optically stimulated luminescence from Al(2)O(3):C and quartz. Radiat. Prot. Dosim. Part 1 84 (1– 4), 537–542. BHtter-Jensen, L., Bulur, E., Duller, G.A.T., Murray, A.S., 2000. Advances in luminescence instrument systems. Radiat. Meas. 32, 523–528. Markey, B.G., BHtter-Jensen, L., Duller, G.A.T., 1997. A new, Vexible system for measuring thermally and optically stimulated luminescence. Radiat. Meas. 27, 83–89. Murray, A.S., Wintle, A.G., 2000. Luminescence dating of quartz using an improved single-aliquot regenerative-dose protocol. Radiat. Meas. 32, 57–73. Spooner, N.A., 1994. On the optical dating signal from quartz. Radiat. Meas. 23 (2–3), 593–600.