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Single grain laser luminescence (SGLL) measurements using a novel automated reader
Nuclear Instruments and Methods in Physics Research B 155 (1999) 506±514
www.elsevier.nl/locate/nimb
Single grain laser luminescence (SGLL) measurements using a novel automated reader G.A.T. Duller
a,*
, L. Bùtter-Jensen b, A.S. Murray c, A.J. Truscott
a
a
c
Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB, UK b Risù National Laboratory, DK-4000 Roskilde, Denmark Nordic Laboratory for Thermoluminescence Dating 1, Risù National Laboratory, DK-4000 Roskilde, Denmark Received 4 March 1999; received in revised form 27 May 1999
Abstract Optically stimulated luminescence (OSL) is used widely for reconstructing past radiation exposure, either in connection with accidental release of radionuclides into the environment, or for dating the time since geological materials were deposited. Measurements of the optically stimulated luminescence properties of crystals are conventionally undertaken on groups of many hundred to many thousand sand-sized (90±300 lm) grains. However, it has long been known that dierent grains may have dierent luminescence properties (e.g., sensitivity to dose) and that more information could be gained if single grains could be measured separately, and thus avoid the eect of averaging. Here we describe an automated system that makes the routine measurement of OSL of a large number of single grains feasible for the ®rst time. The concepts underlying the design are described, and initial measurements demonstrate that a reproducibility of 3% can be achieved in repeated OSL measurements of a single grain of Al2 O3 :C. Measurements on a geological quartz sample demonstrate that the system can also analyse natural samples. Ó 1999 Elsevier Science B.V. All rights reserved. Keywords: Luminescence; Quartz; Dose reconstruction; Optically stimulated luminescence
1. Introduction Luminescence is used in many applications where it is desired to reconstruct the radiation dose
*
Corresponding author. Tel.: +44-1970-622611; fax:+441970-622658; e-mail: geo[email protected] 1 The Nordic Laboratory for Luminescence Dating is a section of the Department of Earth Sciences, Aarhus University, C.F. Mùllers Alle, DK-8000 Aarhus, Denmark.
to which a sample has been exposed. Two of the most common uses are in the reconstruction of radiation doses following the accidental release of radioactive isotopes [1], and in dating geological and archaeological materials [2,3]. Measurements are normally made on groups of grains extracted from the sample that is being studied. These grains are normally separated on the basis of grain diameter and bulk mineralogy, quartz and feldspar being the most commonly used. Typically between 1000 and 7000 grains in the size range from 90 to
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 4 8 8 - 7
G.A.T. Duller et al. / Nucl. Instr. and Meth. in Phys. Res. B 155 (1999) 506±514
300 lm are mounted on a single 9.7 mm diameter metal plate and the luminescence emitted from all of these grains is measured simultaneously. Gr un et al. [4] showed that dierent grains may have very dierent luminescence properties. This is not surprising given the variability in chemical composition and mineralogical structure that is likely in natural materials. Therefore there is considerable potential advantage in being able to make measurements of the luminescence properties of single grains. Until now this has only been possible by two methods, each with severe disadvantages, and the eect has been that this avenue of research has not progressed rapidly. The ®rst approach is to pick individual grains by hand and to mount each grain on a separate sample disc [5± 7]. These can then be analysed using conventional luminescence instrumentation. However, this procedure is very time-consuming, both in sample preparation and in instrument time. A second approach has been to use an ultra-sensitive imaging system such as an imaging photon detector (IPD [8,9]) or a charge coupled device (CCD [10]). These both have the ability to spatially resolve the luminescence signal from a sample. Often the aim of luminescence measurements is to assess the radiation dose to which the sample has been exposed. In recent years a number of methods have been developed that allow dose assessment to be performed by measuring only a single aliquot of a sample [11]. However, these procedures require that repeat measurements be made on each aliquot. In the case of imaging systems, such as the CCD and IPD systems mentioned above, this requires that the luminescence signal coming from the same area of the sample over several measurement cycles can be resolved, despite physical movement of the sample or detector. This is dicult at the scale of tens of microns (as required for routine single grain measurements) and McFee [12] has estimated that the reproducibility with which his IPD system could remeasure a grain was 25%. The ®rst prototype of a system which enabled the routine measurement of the optically stimulated luminescence signal from single grains of quartz or feldspar in the size range from 100 to 300 lm was developed at Risù [13]. This system has
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been improved since then and the next section describes the modi®ed single grain OSL system. We have speci®cally addressed the need to make repeat measurements of the same grain. This is the ®rst instrument that allows individual measurements of many grains to be made in a routine manner without individually manipulating each grain and mounting it on a separate sample holder. 2. Existing system description The design of a single grain OSL unit to measure sand-sized grains was constrained by the need to incorporate it into an already well-established automated luminescence reader. This system has been described in detail elsewhere [14,15]. It allows up to 48 samples, presented on 9.7 mm diameter discs (usually aluminium or stainless steel), to be heated to any temperature from room temperature to 700°C in a controlled manner, to be exposed to a beta irradiation source, and to have their luminescence signal measured using a photomultiplier tube. This automated system was speci®cally designed to enable measurement of the total luminescence signal from many grains of sample (typically 1000±7000) mounted on each sample disc. Hand picking individual grains for analysis on this standard system is inecient for several reasons. Firstly, an essential part of the protocols routinely used for analysing the luminescence behaviour of samples is to expose them to a calibrated beta radiation source built into the instrument (e.g., [11]). This system has been designed to irradiate the whole area of a sample disc (9.7 mm diameter) evenly, and so can irradiate the many thousands of grains mounted on a single disc in the same time that it would take to irradiate a single grain mounted on a disc. Given that these irradiations may constitute in excess of 75% of the time required for an analysis, the optimisation of this is essential. Secondly, another part of the analytical procedure is to hold the sample at moderate temperatures (200±300°C) for a period of time in order to complete any thermally unstable charge movement. As with irradiations, this procedure will take the same length of time whether a
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sample disc contains one grain or many thousands. Thirdly, the automated system is able to excite optically stimulated luminescence (OSL) from each sample using a range of optical sources (e.g., [16±18]). These have all been optimised so that they evenly illuminate the whole area of the 9.7 mm diameter sample disc. If only a single grain is mounted then less than 0.1% of the available optical energy is made use of. Finally, a maximum of 48 sample discs can be mounted within the reader at a time, and hence 48 single grains. Mounting a single grain on a disc is clearly inecient in the use of the beta irradiator and the oven, and it limits the number of analyses that can be undertaken automatically at one time to 48 grains. 3. Single grain laser luminescence (SGLL) system We have designed a new optical stimulation unit to ®t onto the existing automated reader in order to improve its use for single grain measurements. The underlying design requirement was that each 9.7 mm sample disc should be able to hold as many grains as possible so that one gains from the ability to simultaneously irradiate and heat them, but that the OSL signal from each grain can be measured individually. The system employs a 10 mW Nd:YVO4 solidstate diode-pumped laser emitting at 532 nm producing a spot approximately 120 lm in diameter at the surface of the sample disc. This provides an estimated power density at the sample of about 50 W cmÿ2 . An electronic shutter controls the entry of the beam into the measurement chamber. The individual grains are located in 300 lm wide and 300 lm deep holes drilled into the surface of a 9.7 mm diameter, 0.5 mm thick aluminium disc. Once in the hole, the laser beam is re¯ected o the sides of the hole so that the grain is illuminated relatively evenly. The holes are located precisely in an eight by eight grid pattern giving a total of 64 grains on each sample disc. The location of the laser spot on the sample disc is controlled by two mirrors which are mounted orthogonally, making it possible to illuminate only a single grain and thus measure the OSL signal it produces.
3.1. Sample disc manufacture An essential prerequisite of this approach is that the grains placed on each sample disc are located at precisely de®ned locations relative to one another. This has the obvious advantage that it enables the laser beam to be moved so that it is centred on only one grain. An alternative approach is to scan the laser beam across the entire surface of the sample disc and hence remove the need to de®ne grain locations [19]. However, this is inecient when the number of grains on a sample disc is low since most of the time the laser beam is not hitting a sample grain. This has the serious consequence that many grains are inadvertently partially illuminated by scattered light before the laser spot is focussed directly on them. The grid of 64 sample holes were drilled using a CAD/CAM controlled computerised numerical control system type DMU 50V from DMG, Germany. It works with an accuracy of better than 1±2 lm. The holes (300 lm deep) were drilled automatically step by step using specially hardened drills (300 lm diameter). At the same time three further holes were drilled at the periphery of the disc. These were 400 lm in diameter and pass completely through the disc. The position of the grid of 64 holes and the three peripheral holes relative to the circumference of the disc is not precisely de®ned. However, the locations of all the holes relative to one another are well-de®ned and form the basis for the location of the grains as described in Section 3.3. 3.2. Laser beam movement The laser beam is focussed using a simple set of three lenses to give an approximately gaussian beam with 90% of the power contained within a spot 120 lm in diameter at the sample. The movement of the spot in two dimensions across the surface of the disc is achieved by movement of the two mirrors. Each mirror is set at 45° to the line of the beam and can be moved so that it causes the position of the spot on the sample disc to move along one axis. The mirrors are driven using two stepper motors attached to gear trains so that a single 1.8° step of the motor results in a 2 lm
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movement of the mirror. For practical reasons the system is con®gured so that the minimum movement is 5 steps, or 10 lm. The maximum scan speed of the two mirrors is 2000 lm/s, and the maximum travel distance is 20 000 lm. The single grain OSL attachment, consisting of the laser, the two mirrors and two stepper motors, is mounted in a plane at 45° to the sample disc (Fig. 1). This is a compromise between the need to direct the beam onto the sample at as high an angle as possible in order to reduce the possibility of scattered light aecting adjacent samples, and the need to position the photocathode of the photomultiplier tube as close to the sample as possible for optimal detection eciency. 3.3. Locating individual grains The automatic sample changer of the existing TL/OSL reader consists of a carousel on which the sample discs are placed, and a heater strip that can be raised up through holes in the carousel in order to lift the sample disc above the plane of the carousel into the measurement position (Fig. 1). This introduces some uncertainty in the position of the sample disc relative to the laser system. Even when great care is taken in loading the carousel, it is not possible to locate the samples precisely on the scale of a single grain (100±300 lm), and a small rotation of the aluminium sample disc may occur with repeated carousel and sample movement. Thus, a third component of the system is some means by which the relative location of the laser system and
Fig. 1. Schematic diagram of the new single grain unit mounted on the automated Risù reader.
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the sample disc can be automatically determined before each luminescence measurement. Our approach is to attenuate the laser beam using a neutral density ®lter placed into the beam line and to use this attenuated beam to locate the positions of the two holes drilled into the periphery of the sample disc on opposite sides (Fig. 2(a)). A phototransistor is built into the measurement chamber, mounted so that it will be struck by the laser beam if it is re¯ected o the surface of the sample disc. The variation in the intensity of the re¯ected beam can be measured while scanning the laser beam across the sample disc (Fig. 2(b)) and the results de®ne the edges and centre point of the hole in the sample disc. The third hole on the periphery is useful to help orientate the disc and during initial system set up. However, it is not used in routine measurements. This procedure is repeated on both the X and Y axes in order to de®ne the centre positions of both locating holes. In this way the position of the location holes in the sample disc are de®ned precisely in terms of the co-ordinate system of the laser and its associated optics. From this information the control software calculates the centre of the sample disc, and its angle of rotation relative to the laser system. By combining this information with a detailed description of the positions of the various holes drilled into the sample disc, the laser spot can be moved to any of the holes containing sample grains. During the location measurements, the laser beam is attenuated by a factor of 30 by the neutral density ®lter. This reduces possible damage to adjacent sample grains due to inadvertent exposure to scattered laser light. This initial search routine is the starting point for each measurement of a sample disc since it de®nes the position and angle of rotation of the disc. Previously [13] this information was used to calculate each grain position and no further position checking was undertaken during routine measurement. As a diagnostic tool one can use the same technique of looking for changes in re¯ectivity around a location hole to check the location of each of the 64 grain positions. Even though these holes are smaller than the locating holes on the periphery of the disc, do not pass completely
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Fig. 2. (a) A scanning electron microscope image of a sample disc showing the eight by eight grid of 300 lm diameter holes used to hold the grains, and the two 400 lm holes on the periphery, drilled completely through the disc, which are used to locate the disc. (b) The change in re¯ectivity as the attenuated laser beam is scanned across one of the two locating holes. The position of the 400 lm wide hole is clearly de®ned by the low re¯ectivity values in the middle of the scan.
through the disc, and have a grain in them they still produce a clear change in re¯ectivity that can be used to de®ne the hole position. Fig. 3(a) shows the results from one such set of measurements using the system described by Duller et al. [13]. The standard deviations of the distributions are 32 lm on the X-axis and 41 lm on the Y-axis; although the distributions are poorly resolved, in all cases the beam would have fallen within the grain hole. However, this performance was observed to be quite variable, with some sets of measurements giving lower standard deviations and others higher. The major source of uncertainty appears to be in the stepper motors which occasionally ÔstallÕ and hence lose position. This inaccuracy is cumulative
over the 64 measurements. To correct for this a new measurement procedure was developed. The new procedure begins with using the laser operating at the reduced power level to ®nd the two peripheral locating holes in order to de®ne the disc position. The laser is then moved to the calculated position of grain number one and an OSL measurement made using full laser power. Once this measurement is complete the attenuator for the laser beam is inserted again and the position of the ®rst sample grain hole is checked using the pattern of re¯ectivity. This new estimate for the position of grain number one is then used to update the calculation for the position of grain number two. Grain two is then measured at full
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Fig. 3. The accuracy with which the single grain reader is able to locate the holes on a disc into which the grains are placed. (a) For the earlier prototype the errors are quite large and irregularly distributed. (b) Using the new algorithm, in which the positioning of the laser is constantly updated, the errors are smaller and more normally distributed.
laser power, its position checked, and the new estimate used to calculate the position of grain number three. This process continues across the whole disc, with the laser positioning constantly being updated based upon the position of the last grain that was measured. In this way, any positioning errors are constantly corrected. Fig. 3(b) shows the results from measurement of 6400 grains. The errors on the positioning are 25 lm on the X-axis and 30 lm on the Y-axis, about 10 lm better that those obtained using the earlier method. These errors are approaching the minimum step size for the movement of the laser, which is 10 lm. 4. System reproducibility As discussed earlier, the most important requirement of a single grain system is that it should be able to make accurate repeat measurements on a single grain so that single aliquot dose evaluation procedures [11,20] can be applied. To test this a disc was prepared that contained a single 90±105
lm grain of Al2 O3 :C ®xed in each sample hole. Al2 O3 :C was chosen because of its high luminescence sensitivity [21] so that any measurements of the reproducibility of the instrument would not be limited by poor counting statistics. The Al2 O3 :C grains were ®xed in each hole using a resin adhesive, to prevent contamination of the measurement chamber. This disc was given a beta dose in the instrument of approximately 8 Gy, and then the OSL signal from each grain was measured for 20 s. All measurements were made at room temperature with a constant time interval between irradiation and measurement. No preheating procedure was applied since the resin could not withstand high temperatures. This irradiation and OSL measurement cycle was repeated 11 times. For each grain, the OSL signal was integrated over the 20 s of the measurement. Most grains showed some systematic variation in OSL response as the sequence of measurements progressed. This was foreseen since no thermal annealing was undertaken between each measurement cycle [21]. In order to calculate the reproducibility of the SGLL reader, the set of
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11 repeat OSL measurements for each grain were plotted as a function of measurement number, ®tted by linear regression, and the standard deviation of the residuals from this regression analysis was expressed as a percentage of the average OSL signal from the grain. This procedure was designed to remove the eect of systematic changes in the behaviour of the Al2 O3 :C, and thus to isolate the uncertainty due to the measurement alone. This analysis gave an average reproducibility of 3% [22], an order of magnitude better than that reported for the IPD and CCD systems described earlier. As an additional test, the same disc containing 64 grains of Al2 O3 :C was given a beta dose of 100 s (1 Gy) and the disc was then analysed using a modi®ed form of the single-aliquot regenerativedose protocol [20] to see whether this dose could be accurately recovered. The mean of the resulting dose estimates was 100 s with a standard deviation of 6.7 s, and a standard error of 0.9 s (Fig. 4). The standard deviation is two times larger than that calculated above for a single OSL measurement. Some of this decreased precision is because, in this protocol, each dose estimate requires several measurements of the OSL signal from an individual grain. The uncertainty on each reconstructed dose is then controlled by the combination of uncertainties on at least four independent OSL measurements ± two to de®ne the natural and two
Al2 O3 :C was chosen to test the performance of the system because of its high luminescence sensitivity per unit dose. However, in most applications natural materials such as quartz and feldspars are used as the dosimeters. Therefore, the performance of the single grain laser luminescence (SGLL) system was further tested using a set of 320 quartz grains mounted on ®ve sample discs. These grains were extracted from a modern dune sand from the Queensland Coast, Australia, and have been shown in other measurements to have been given a dose of approximately 0.05 Gy in nature [23]. The ®ve discs were exposed to the beta source on the automated reader for 2000 s, giving them a known dose of 17.6 Gy. The single aliquot regeneration (SAR) procedure was then used in order to attempt to measure this known dose. The SAR procedure that was used required 14 repeat measurements of each grain. Fig. 5 shows how the normalised luminescence signal from a single grain varies as a function of the
Fig. 4. The distribution of apparent dose in 56 grains of Al2 O3 :C that had previously been given a beta dose of 100 s using the beta source in the automated reader. The mean of the distribution is 100.0 s, with a standard deviation of 6.7 s.
Fig. 5. The response of the luminescence signal from a single grain of quartz to laboratory radiation. The inset shows the luminescence signal for a single grain during 2 s of laser stimulation.
to de®ne the regenerated signal. Taking an uncertainty of 3% on each of the four measurements gives an expected error of 6%, close to that actually observed. 5. Application to quartz grains
G.A.T. Duller et al. / Nucl. Instr. and Meth. in Phys. Res. B 155 (1999) 506±514
radiation dose to which it is exposed (see [20] for a detailed description of the normalisation process). The signal from the 17.6 Gy dose given to the sample prior to this test is plotted on the left most axis as a ®lled square. Subsequent measurements were made of the luminescence signal after exposure to the irradiation source for dierent periods of time, and these were used to construct the dose response for this grain. It is then possible to calculate the initial dose by interpolation, as shown by the dotted line. For this grain the value determined is very close to the known dose of 17.6 Gy. Of the 320 grains which were analysed, only 80 gave sucient luminescence signal to allow analysis. This proportion of grains giving light (25%) is higher than has typically been found in previous measurements of single grains (e.g., [6]), and this is probably due to the nature of the sample and the intense optical stimulation that is used. Fig. 6 shows the distribution of equivalent dose for the 80 grains from which signals were obtained. The distribution is normal with a mean value of 17.8 Gy and a standard deviation of 2.2 Gy. The mean value of the absorbed dose measured for this sample using the single grain attachment is consistent with the known dose that was administered and demonstrates that the system can be used to recover a dose from a natural quartz sample. The uncertainty in the calculated dose value is 12%, approximately double that observed for the Al2 O3 :C grains. This increased uncertainty
513
is thought to relate primarily to counting statistics, but this is currently the subject of further research. 6. Conclusions We have constructed a system that makes it feasible to measure the luminescence behaviour of a large number of grains for the ®rst time. The initial results described here show that it is capable of locating each grain accurately, and that it is possible to make repeat measurements on a grain of Al2 O3 :C with a precision of 3%. When used for reconstructing an absorbed dose in the same material using a single aliquot regeneration procedure these uncertainties increase because a minimum of four OSL measurements are required for each dose estimate. The system is also suciently sensitive to measure the OSL signal from single grains of quartz derived from a geological sample. The single grain system was able to recover a known dose from quartz accurately, though the precision was reduced to approximately 12%. It is thought that the lower signal level from the quartz is the primary cause of the lower precision obtained for the quartz compared with the Al2 O3 :C. Future work will be required to test the system on further geological and archaeological samples, and to attempt to reduce the uncertainties involved in calculating the absorbed radiation dose. Acknowledgements This work was funded by Danish Natural Science Research Council grant 9701837, the EU project ÔDose EstimationÕ and NERC grants GR3/ E0087 and GT04/98/290/ES. GATD was also funded by an award from the British Council. References
Fig. 6. The distribution of apparent dose in 80 grains given a test dose of 17.6 Gy. The mean value of the distribution is 17.8 Gy, with a standard deviation of 2.2 Gy, and a standard error of 0.25 Gy.
[1] I.K. Baili, Radiat. Protection Dosimetry 84 (1999) 411. [2] M.J. Aitken, An Introduction to Optical Dating: The Dating of Quaternary Sediments by the Use of PhotonStimulated Luminescence, Oxford University Press, Oxford, 1998.
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[3] R.G. Roberts, Radiat. Measurements 27 (1997) 819. [4] R. Gr un, S.C. Packman, K. Pye, in: Long and Short Range Limits in Luminescence Dating, Research Laboratory for Archaeology and the History of Art, vol. 9, Occasional Publication, Oxford, 1989, p. 13. [5] M. Lamothe, S. Balescu, M. Auclair, Radiat. Measurements 23 (1994) 555. [6] A.S. Murray, R.G. Roberts, Earth and Planetary Sci. Lett. 152 (1997) 163. [7] R. Roberts, G. Walsh, A.S. Murray, J. Olley, R. Jones, M. Morwood, C. Tuniz, E. Lawson, M. Macphail, D. Bowdery, I. Naumann, Nature 387 (1997) 696. [8] B.W. Smith, G.C.W.S. Wheeler, E.J. Rhodes, N.A. Spooner, Nucl. Tracks and Radiat. Measurements 18 (1991) 273. [9] C.J. McFee, M.S. Tite, Archaeometry 40 (1998) 153. [10] G.A.T. Duller, L. Bùtter-Jensen, B.G. Markey, Radiat. Measurements 27 (1997) 91. [11] G.A.T. Duller, Radiat. Measurements 24 (1995) 217. [12] C.J. McFee, Ancient TL 16 (1998) 11. [13] G.A.T. Duller, L. Bùtter-Jensen, P. Kohsiek, A.S. Murray, Radiat. Protection Dosimetry 84 (1999) 325.
[14] L. Bùtter-Jensen, Nucl. Tracks and Radiat. Measurements 14 (1988) 177. [15] B.G. Markey, L. Bùtter-Jensen, G.A.T. Duller, Radiat. Measurements 27 (1997) 83. [16] L. Bùtter-Jensen, C. Ditlefsen, V. Mejdahl, Nucl. Tracks and Radiat. Measurements 18 (1991) 257. [17] L. Bùtter-Jensen, G.A.T. Duller, Nucl. Tracks and Radiat. Measurements 20 (1992) 549. [18] L. Bùtter-Jensen, D. Banerjee, G.A.T. Duller, A.S. Murray, Radiat. Protection Dosimetry 84 (1999) 335. [19] I.K. Baili, R.J. Clark, S.M. Barnett, A high resolution scanning system for measuring spatially resolved optically stimulated luminescence, Abstracts, Eighth International LED Conference, Canberra, 1996. [20] A.S. Murray, A.G. Wintle, Radiat. Measurements, in press. [21] L. Bùtter-Jensen, N.A. Larsen, B.G. Markey, S.W.S. McKeever, Radiat. Measurements 27 (1997) 295. [22] A.J. Truscott, L. Bùtter-Jensen, G.A.T. Duller, A.S. Murray, in preparation. [23] A.S. Murray, R.G. Roberts, A.G. Wintle, Radiat. Measurements 27 (1997) 171.
www.elsevier.nl/locate/nimb
Single grain laser luminescence (SGLL) measurements using a novel automated reader G.A.T. Duller
a,*
, L. Bùtter-Jensen b, A.S. Murray c, A.J. Truscott
a
a
c
Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB, UK b Risù National Laboratory, DK-4000 Roskilde, Denmark Nordic Laboratory for Thermoluminescence Dating 1, Risù National Laboratory, DK-4000 Roskilde, Denmark Received 4 March 1999; received in revised form 27 May 1999
Abstract Optically stimulated luminescence (OSL) is used widely for reconstructing past radiation exposure, either in connection with accidental release of radionuclides into the environment, or for dating the time since geological materials were deposited. Measurements of the optically stimulated luminescence properties of crystals are conventionally undertaken on groups of many hundred to many thousand sand-sized (90±300 lm) grains. However, it has long been known that dierent grains may have dierent luminescence properties (e.g., sensitivity to dose) and that more information could be gained if single grains could be measured separately, and thus avoid the eect of averaging. Here we describe an automated system that makes the routine measurement of OSL of a large number of single grains feasible for the ®rst time. The concepts underlying the design are described, and initial measurements demonstrate that a reproducibility of 3% can be achieved in repeated OSL measurements of a single grain of Al2 O3 :C. Measurements on a geological quartz sample demonstrate that the system can also analyse natural samples. Ó 1999 Elsevier Science B.V. All rights reserved. Keywords: Luminescence; Quartz; Dose reconstruction; Optically stimulated luminescence
1. Introduction Luminescence is used in many applications where it is desired to reconstruct the radiation dose
*
Corresponding author. Tel.: +44-1970-622611; fax:+441970-622658; e-mail: geo[email protected] 1 The Nordic Laboratory for Luminescence Dating is a section of the Department of Earth Sciences, Aarhus University, C.F. Mùllers Alle, DK-8000 Aarhus, Denmark.
to which a sample has been exposed. Two of the most common uses are in the reconstruction of radiation doses following the accidental release of radioactive isotopes [1], and in dating geological and archaeological materials [2,3]. Measurements are normally made on groups of grains extracted from the sample that is being studied. These grains are normally separated on the basis of grain diameter and bulk mineralogy, quartz and feldspar being the most commonly used. Typically between 1000 and 7000 grains in the size range from 90 to
0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 4 8 8 - 7
G.A.T. Duller et al. / Nucl. Instr. and Meth. in Phys. Res. B 155 (1999) 506±514
300 lm are mounted on a single 9.7 mm diameter metal plate and the luminescence emitted from all of these grains is measured simultaneously. Gr un et al. [4] showed that dierent grains may have very dierent luminescence properties. This is not surprising given the variability in chemical composition and mineralogical structure that is likely in natural materials. Therefore there is considerable potential advantage in being able to make measurements of the luminescence properties of single grains. Until now this has only been possible by two methods, each with severe disadvantages, and the eect has been that this avenue of research has not progressed rapidly. The ®rst approach is to pick individual grains by hand and to mount each grain on a separate sample disc [5± 7]. These can then be analysed using conventional luminescence instrumentation. However, this procedure is very time-consuming, both in sample preparation and in instrument time. A second approach has been to use an ultra-sensitive imaging system such as an imaging photon detector (IPD [8,9]) or a charge coupled device (CCD [10]). These both have the ability to spatially resolve the luminescence signal from a sample. Often the aim of luminescence measurements is to assess the radiation dose to which the sample has been exposed. In recent years a number of methods have been developed that allow dose assessment to be performed by measuring only a single aliquot of a sample [11]. However, these procedures require that repeat measurements be made on each aliquot. In the case of imaging systems, such as the CCD and IPD systems mentioned above, this requires that the luminescence signal coming from the same area of the sample over several measurement cycles can be resolved, despite physical movement of the sample or detector. This is dicult at the scale of tens of microns (as required for routine single grain measurements) and McFee [12] has estimated that the reproducibility with which his IPD system could remeasure a grain was 25%. The ®rst prototype of a system which enabled the routine measurement of the optically stimulated luminescence signal from single grains of quartz or feldspar in the size range from 100 to 300 lm was developed at Risù [13]. This system has
507
been improved since then and the next section describes the modi®ed single grain OSL system. We have speci®cally addressed the need to make repeat measurements of the same grain. This is the ®rst instrument that allows individual measurements of many grains to be made in a routine manner without individually manipulating each grain and mounting it on a separate sample holder. 2. Existing system description The design of a single grain OSL unit to measure sand-sized grains was constrained by the need to incorporate it into an already well-established automated luminescence reader. This system has been described in detail elsewhere [14,15]. It allows up to 48 samples, presented on 9.7 mm diameter discs (usually aluminium or stainless steel), to be heated to any temperature from room temperature to 700°C in a controlled manner, to be exposed to a beta irradiation source, and to have their luminescence signal measured using a photomultiplier tube. This automated system was speci®cally designed to enable measurement of the total luminescence signal from many grains of sample (typically 1000±7000) mounted on each sample disc. Hand picking individual grains for analysis on this standard system is inecient for several reasons. Firstly, an essential part of the protocols routinely used for analysing the luminescence behaviour of samples is to expose them to a calibrated beta radiation source built into the instrument (e.g., [11]). This system has been designed to irradiate the whole area of a sample disc (9.7 mm diameter) evenly, and so can irradiate the many thousands of grains mounted on a single disc in the same time that it would take to irradiate a single grain mounted on a disc. Given that these irradiations may constitute in excess of 75% of the time required for an analysis, the optimisation of this is essential. Secondly, another part of the analytical procedure is to hold the sample at moderate temperatures (200±300°C) for a period of time in order to complete any thermally unstable charge movement. As with irradiations, this procedure will take the same length of time whether a
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sample disc contains one grain or many thousands. Thirdly, the automated system is able to excite optically stimulated luminescence (OSL) from each sample using a range of optical sources (e.g., [16±18]). These have all been optimised so that they evenly illuminate the whole area of the 9.7 mm diameter sample disc. If only a single grain is mounted then less than 0.1% of the available optical energy is made use of. Finally, a maximum of 48 sample discs can be mounted within the reader at a time, and hence 48 single grains. Mounting a single grain on a disc is clearly inecient in the use of the beta irradiator and the oven, and it limits the number of analyses that can be undertaken automatically at one time to 48 grains. 3. Single grain laser luminescence (SGLL) system We have designed a new optical stimulation unit to ®t onto the existing automated reader in order to improve its use for single grain measurements. The underlying design requirement was that each 9.7 mm sample disc should be able to hold as many grains as possible so that one gains from the ability to simultaneously irradiate and heat them, but that the OSL signal from each grain can be measured individually. The system employs a 10 mW Nd:YVO4 solidstate diode-pumped laser emitting at 532 nm producing a spot approximately 120 lm in diameter at the surface of the sample disc. This provides an estimated power density at the sample of about 50 W cmÿ2 . An electronic shutter controls the entry of the beam into the measurement chamber. The individual grains are located in 300 lm wide and 300 lm deep holes drilled into the surface of a 9.7 mm diameter, 0.5 mm thick aluminium disc. Once in the hole, the laser beam is re¯ected o the sides of the hole so that the grain is illuminated relatively evenly. The holes are located precisely in an eight by eight grid pattern giving a total of 64 grains on each sample disc. The location of the laser spot on the sample disc is controlled by two mirrors which are mounted orthogonally, making it possible to illuminate only a single grain and thus measure the OSL signal it produces.
3.1. Sample disc manufacture An essential prerequisite of this approach is that the grains placed on each sample disc are located at precisely de®ned locations relative to one another. This has the obvious advantage that it enables the laser beam to be moved so that it is centred on only one grain. An alternative approach is to scan the laser beam across the entire surface of the sample disc and hence remove the need to de®ne grain locations [19]. However, this is inecient when the number of grains on a sample disc is low since most of the time the laser beam is not hitting a sample grain. This has the serious consequence that many grains are inadvertently partially illuminated by scattered light before the laser spot is focussed directly on them. The grid of 64 sample holes were drilled using a CAD/CAM controlled computerised numerical control system type DMU 50V from DMG, Germany. It works with an accuracy of better than 1±2 lm. The holes (300 lm deep) were drilled automatically step by step using specially hardened drills (300 lm diameter). At the same time three further holes were drilled at the periphery of the disc. These were 400 lm in diameter and pass completely through the disc. The position of the grid of 64 holes and the three peripheral holes relative to the circumference of the disc is not precisely de®ned. However, the locations of all the holes relative to one another are well-de®ned and form the basis for the location of the grains as described in Section 3.3. 3.2. Laser beam movement The laser beam is focussed using a simple set of three lenses to give an approximately gaussian beam with 90% of the power contained within a spot 120 lm in diameter at the sample. The movement of the spot in two dimensions across the surface of the disc is achieved by movement of the two mirrors. Each mirror is set at 45° to the line of the beam and can be moved so that it causes the position of the spot on the sample disc to move along one axis. The mirrors are driven using two stepper motors attached to gear trains so that a single 1.8° step of the motor results in a 2 lm
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movement of the mirror. For practical reasons the system is con®gured so that the minimum movement is 5 steps, or 10 lm. The maximum scan speed of the two mirrors is 2000 lm/s, and the maximum travel distance is 20 000 lm. The single grain OSL attachment, consisting of the laser, the two mirrors and two stepper motors, is mounted in a plane at 45° to the sample disc (Fig. 1). This is a compromise between the need to direct the beam onto the sample at as high an angle as possible in order to reduce the possibility of scattered light aecting adjacent samples, and the need to position the photocathode of the photomultiplier tube as close to the sample as possible for optimal detection eciency. 3.3. Locating individual grains The automatic sample changer of the existing TL/OSL reader consists of a carousel on which the sample discs are placed, and a heater strip that can be raised up through holes in the carousel in order to lift the sample disc above the plane of the carousel into the measurement position (Fig. 1). This introduces some uncertainty in the position of the sample disc relative to the laser system. Even when great care is taken in loading the carousel, it is not possible to locate the samples precisely on the scale of a single grain (100±300 lm), and a small rotation of the aluminium sample disc may occur with repeated carousel and sample movement. Thus, a third component of the system is some means by which the relative location of the laser system and
Fig. 1. Schematic diagram of the new single grain unit mounted on the automated Risù reader.
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the sample disc can be automatically determined before each luminescence measurement. Our approach is to attenuate the laser beam using a neutral density ®lter placed into the beam line and to use this attenuated beam to locate the positions of the two holes drilled into the periphery of the sample disc on opposite sides (Fig. 2(a)). A phototransistor is built into the measurement chamber, mounted so that it will be struck by the laser beam if it is re¯ected o the surface of the sample disc. The variation in the intensity of the re¯ected beam can be measured while scanning the laser beam across the sample disc (Fig. 2(b)) and the results de®ne the edges and centre point of the hole in the sample disc. The third hole on the periphery is useful to help orientate the disc and during initial system set up. However, it is not used in routine measurements. This procedure is repeated on both the X and Y axes in order to de®ne the centre positions of both locating holes. In this way the position of the location holes in the sample disc are de®ned precisely in terms of the co-ordinate system of the laser and its associated optics. From this information the control software calculates the centre of the sample disc, and its angle of rotation relative to the laser system. By combining this information with a detailed description of the positions of the various holes drilled into the sample disc, the laser spot can be moved to any of the holes containing sample grains. During the location measurements, the laser beam is attenuated by a factor of 30 by the neutral density ®lter. This reduces possible damage to adjacent sample grains due to inadvertent exposure to scattered laser light. This initial search routine is the starting point for each measurement of a sample disc since it de®nes the position and angle of rotation of the disc. Previously [13] this information was used to calculate each grain position and no further position checking was undertaken during routine measurement. As a diagnostic tool one can use the same technique of looking for changes in re¯ectivity around a location hole to check the location of each of the 64 grain positions. Even though these holes are smaller than the locating holes on the periphery of the disc, do not pass completely
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Fig. 2. (a) A scanning electron microscope image of a sample disc showing the eight by eight grid of 300 lm diameter holes used to hold the grains, and the two 400 lm holes on the periphery, drilled completely through the disc, which are used to locate the disc. (b) The change in re¯ectivity as the attenuated laser beam is scanned across one of the two locating holes. The position of the 400 lm wide hole is clearly de®ned by the low re¯ectivity values in the middle of the scan.
through the disc, and have a grain in them they still produce a clear change in re¯ectivity that can be used to de®ne the hole position. Fig. 3(a) shows the results from one such set of measurements using the system described by Duller et al. [13]. The standard deviations of the distributions are 32 lm on the X-axis and 41 lm on the Y-axis; although the distributions are poorly resolved, in all cases the beam would have fallen within the grain hole. However, this performance was observed to be quite variable, with some sets of measurements giving lower standard deviations and others higher. The major source of uncertainty appears to be in the stepper motors which occasionally ÔstallÕ and hence lose position. This inaccuracy is cumulative
over the 64 measurements. To correct for this a new measurement procedure was developed. The new procedure begins with using the laser operating at the reduced power level to ®nd the two peripheral locating holes in order to de®ne the disc position. The laser is then moved to the calculated position of grain number one and an OSL measurement made using full laser power. Once this measurement is complete the attenuator for the laser beam is inserted again and the position of the ®rst sample grain hole is checked using the pattern of re¯ectivity. This new estimate for the position of grain number one is then used to update the calculation for the position of grain number two. Grain two is then measured at full
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Fig. 3. The accuracy with which the single grain reader is able to locate the holes on a disc into which the grains are placed. (a) For the earlier prototype the errors are quite large and irregularly distributed. (b) Using the new algorithm, in which the positioning of the laser is constantly updated, the errors are smaller and more normally distributed.
laser power, its position checked, and the new estimate used to calculate the position of grain number three. This process continues across the whole disc, with the laser positioning constantly being updated based upon the position of the last grain that was measured. In this way, any positioning errors are constantly corrected. Fig. 3(b) shows the results from measurement of 6400 grains. The errors on the positioning are 25 lm on the X-axis and 30 lm on the Y-axis, about 10 lm better that those obtained using the earlier method. These errors are approaching the minimum step size for the movement of the laser, which is 10 lm. 4. System reproducibility As discussed earlier, the most important requirement of a single grain system is that it should be able to make accurate repeat measurements on a single grain so that single aliquot dose evaluation procedures [11,20] can be applied. To test this a disc was prepared that contained a single 90±105
lm grain of Al2 O3 :C ®xed in each sample hole. Al2 O3 :C was chosen because of its high luminescence sensitivity [21] so that any measurements of the reproducibility of the instrument would not be limited by poor counting statistics. The Al2 O3 :C grains were ®xed in each hole using a resin adhesive, to prevent contamination of the measurement chamber. This disc was given a beta dose in the instrument of approximately 8 Gy, and then the OSL signal from each grain was measured for 20 s. All measurements were made at room temperature with a constant time interval between irradiation and measurement. No preheating procedure was applied since the resin could not withstand high temperatures. This irradiation and OSL measurement cycle was repeated 11 times. For each grain, the OSL signal was integrated over the 20 s of the measurement. Most grains showed some systematic variation in OSL response as the sequence of measurements progressed. This was foreseen since no thermal annealing was undertaken between each measurement cycle [21]. In order to calculate the reproducibility of the SGLL reader, the set of
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11 repeat OSL measurements for each grain were plotted as a function of measurement number, ®tted by linear regression, and the standard deviation of the residuals from this regression analysis was expressed as a percentage of the average OSL signal from the grain. This procedure was designed to remove the eect of systematic changes in the behaviour of the Al2 O3 :C, and thus to isolate the uncertainty due to the measurement alone. This analysis gave an average reproducibility of 3% [22], an order of magnitude better than that reported for the IPD and CCD systems described earlier. As an additional test, the same disc containing 64 grains of Al2 O3 :C was given a beta dose of 100 s (1 Gy) and the disc was then analysed using a modi®ed form of the single-aliquot regenerativedose protocol [20] to see whether this dose could be accurately recovered. The mean of the resulting dose estimates was 100 s with a standard deviation of 6.7 s, and a standard error of 0.9 s (Fig. 4). The standard deviation is two times larger than that calculated above for a single OSL measurement. Some of this decreased precision is because, in this protocol, each dose estimate requires several measurements of the OSL signal from an individual grain. The uncertainty on each reconstructed dose is then controlled by the combination of uncertainties on at least four independent OSL measurements ± two to de®ne the natural and two
Al2 O3 :C was chosen to test the performance of the system because of its high luminescence sensitivity per unit dose. However, in most applications natural materials such as quartz and feldspars are used as the dosimeters. Therefore, the performance of the single grain laser luminescence (SGLL) system was further tested using a set of 320 quartz grains mounted on ®ve sample discs. These grains were extracted from a modern dune sand from the Queensland Coast, Australia, and have been shown in other measurements to have been given a dose of approximately 0.05 Gy in nature [23]. The ®ve discs were exposed to the beta source on the automated reader for 2000 s, giving them a known dose of 17.6 Gy. The single aliquot regeneration (SAR) procedure was then used in order to attempt to measure this known dose. The SAR procedure that was used required 14 repeat measurements of each grain. Fig. 5 shows how the normalised luminescence signal from a single grain varies as a function of the
Fig. 4. The distribution of apparent dose in 56 grains of Al2 O3 :C that had previously been given a beta dose of 100 s using the beta source in the automated reader. The mean of the distribution is 100.0 s, with a standard deviation of 6.7 s.
Fig. 5. The response of the luminescence signal from a single grain of quartz to laboratory radiation. The inset shows the luminescence signal for a single grain during 2 s of laser stimulation.
to de®ne the regenerated signal. Taking an uncertainty of 3% on each of the four measurements gives an expected error of 6%, close to that actually observed. 5. Application to quartz grains
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radiation dose to which it is exposed (see [20] for a detailed description of the normalisation process). The signal from the 17.6 Gy dose given to the sample prior to this test is plotted on the left most axis as a ®lled square. Subsequent measurements were made of the luminescence signal after exposure to the irradiation source for dierent periods of time, and these were used to construct the dose response for this grain. It is then possible to calculate the initial dose by interpolation, as shown by the dotted line. For this grain the value determined is very close to the known dose of 17.6 Gy. Of the 320 grains which were analysed, only 80 gave sucient luminescence signal to allow analysis. This proportion of grains giving light (25%) is higher than has typically been found in previous measurements of single grains (e.g., [6]), and this is probably due to the nature of the sample and the intense optical stimulation that is used. Fig. 6 shows the distribution of equivalent dose for the 80 grains from which signals were obtained. The distribution is normal with a mean value of 17.8 Gy and a standard deviation of 2.2 Gy. The mean value of the absorbed dose measured for this sample using the single grain attachment is consistent with the known dose that was administered and demonstrates that the system can be used to recover a dose from a natural quartz sample. The uncertainty in the calculated dose value is 12%, approximately double that observed for the Al2 O3 :C grains. This increased uncertainty
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is thought to relate primarily to counting statistics, but this is currently the subject of further research. 6. Conclusions We have constructed a system that makes it feasible to measure the luminescence behaviour of a large number of grains for the ®rst time. The initial results described here show that it is capable of locating each grain accurately, and that it is possible to make repeat measurements on a grain of Al2 O3 :C with a precision of 3%. When used for reconstructing an absorbed dose in the same material using a single aliquot regeneration procedure these uncertainties increase because a minimum of four OSL measurements are required for each dose estimate. The system is also suciently sensitive to measure the OSL signal from single grains of quartz derived from a geological sample. The single grain system was able to recover a known dose from quartz accurately, though the precision was reduced to approximately 12%. It is thought that the lower signal level from the quartz is the primary cause of the lower precision obtained for the quartz compared with the Al2 O3 :C. Future work will be required to test the system on further geological and archaeological samples, and to attempt to reduce the uncertainties involved in calculating the absorbed radiation dose. Acknowledgements This work was funded by Danish Natural Science Research Council grant 9701837, the EU project ÔDose EstimationÕ and NERC grants GR3/ E0087 and GT04/98/290/ES. GATD was also funded by an award from the British Council. References
Fig. 6. The distribution of apparent dose in 80 grains given a test dose of 17.6 Gy. The mean value of the distribution is 17.8 Gy, with a standard deviation of 2.2 Gy, and a standard error of 0.25 Gy.
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[14] L. Bùtter-Jensen, Nucl. Tracks and Radiat. Measurements 14 (1988) 177. [15] B.G. Markey, L. Bùtter-Jensen, G.A.T. Duller, Radiat. Measurements 27 (1997) 83. [16] L. Bùtter-Jensen, C. Ditlefsen, V. Mejdahl, Nucl. Tracks and Radiat. Measurements 18 (1991) 257. [17] L. Bùtter-Jensen, G.A.T. Duller, Nucl. Tracks and Radiat. Measurements 20 (1992) 549. [18] L. Bùtter-Jensen, D. Banerjee, G.A.T. Duller, A.S. Murray, Radiat. Protection Dosimetry 84 (1999) 335. [19] I.K. Baili, R.J. Clark, S.M. Barnett, A high resolution scanning system for measuring spatially resolved optically stimulated luminescence, Abstracts, Eighth International LED Conference, Canberra, 1996. [20] A.S. Murray, A.G. Wintle, Radiat. Measurements, in press. [21] L. Bùtter-Jensen, N.A. Larsen, B.G. Markey, S.W.S. McKeever, Radiat. Measurements 27 (1997) 295. [22] A.J. Truscott, L. Bùtter-Jensen, G.A.T. Duller, A.S. Murray, in preparation. [23] A.S. Murray, R.G. Roberts, A.G. Wintle, Radiat. Measurements 27 (1997) 171.