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A portable luminescence dating instrument
Nuclear Instruments and Methods in Physics Research B 269 (2011) 1370–1378
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Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
A portable luminescence dating instrument M.H. Kook a,b,d, A.S. Murray c, T. Lapp d, P.H. Denby d,e, C. Ankjærgaard d,f, K. Thomsen d, M. Jain d, J.H. Choi a,⇑, G.H. Kim a a
Korea Basic Science Institute, Daejeon 305-333, South Korea Department of Nano-Mechatronics, University of Science and Technology, Daejeon 305-333, South Korea c Nordic Laboratory for Luminescence Dating, Department of Earth Sciences, University of Aarhus, Risø DTU, DK-4000 Roskilde, Denmark d Radiation Research Division, Risø DTU, DK-4000 Roskilde, Denmark e Department of Physics and Technology, University of Bergen, Norway f Netherlands Centre for Luminescence Dating, Faculty of Applied Sciences, Delft University of Technology, NL-2629JB Delft, The Netherlands b
a r t i c l e
i n f o
Article history: Received 24 November 2010 Received in revised form 16 March 2011 Available online 24 March 2011
a b s t r a c t We describe a portable luminescence reader suitable for use in remote localities in the field. The instrument weighs about 8 kg and is based around a 30 mm bialkali photomultiplier detecting signals through a glass filter centered on 340 nm. Stimulation is by 470 nm blue LEDs (24 W in total) operating in both continuous wave and pulsed mode; photon counting can be gated such that it is active only during the pulse off-period. There are also two bleaching light sources (470 nm, 5 W and 940 nm, 3 W), and the luminescence signals can be regenerated using a cold-cathode 30 kV X-ray tube, delivering 0.06 Gy.s 1. The three position sampling device has a heating element under each sampling position, able to heat the sample at 3 °C.s 1 up to at least 250 °C. The sampler can be inserted into unconsolidated sediments, and is designed to prevent exposure of the mineral grains to ambient light during sampling. The performance of the instrument in terms of sensitivity and reproducibility is comparable to that of the standard bench-top laboratory TL/OSL Risø reader. We show that the portable luminescence reader is able to measure accurately an 20 Gy quartz burial dose in a natural (unpretreated, no mineral separation) sandy sediment. We also show that, because of the configuration of the measurement head, the portable reader can be used to measure radioluminescence at elevated temperature in the presence of stimulation light; this facility is not available on conventional bench-top instruments. It is concluded that the portable luminescence reader can be used to accurately determine the quartz burial dose in loose sandy sediments in the field, without sample preparation or darkroom facilities. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Luminescence is now widely used to date quaternary sediments from all over the world, including those from remote and inaccessible locations that cannot readily be revisited. In the absence of any prior chronostratigraphic data, it is often difficult to design an appropriate sampling strategy in the field, and it is not uncommon that inadvertent errors in sampling, e.g. of the wrong geological period, are only discovered when the samples are returned for measurement in the laboratory. Any method of obtaining preliminary ages in the field would help to increase the relevance of the samples, and reduce wasted resources. As a result there is a demand for a portable instrument that will provide preliminary estimates of the equivalent dose in samples in the field, and thus preliminary ages to guide the sampling strategy.
⇑ Corresponding author. E-mail address: [email protected] (J.H. Choi). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.03.014
Various authors have attempted to construct such an instrument, usually based on optically stimulation using infrared (IR) or visible wavelengths. Some include an ionising radiation source (usually an X-ray source). Poolton et al. [1] were probably the first to construct a practical instrument intended to determine the dose in natural sediments. Their system was built around a 12 position sample holder which could slide laterally from under a UV lamp (to regenerate the luminescence signals) to the measurement position, where IR diodes (880 nm) stimulated each sample position while photons were detected through glass filters (Schott BG39) by a 30 mm diameter photomultiplier tube (EMI, bialkali photocathode, type 9924B). By sliding the sample holder to the other side of the measurement position, it could be illuminated by a halogen lamp bleaching source. More recently, Takeuchi et al. [2] constructed a compact TL/OSL measurement system similar in concept to the standard Risø TL/ OSL reader [3]. It is equipped with blue (470 nm) and IR (890 nm) LEDs for stimulation, a ceramic heater, a metal-packaged PMT (Hamamatsu, H7421), and a miniature X-ray generator
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(Oxford, Eclipse II). Although it could be considered a portable instrument (weight 15 kg) it must be loaded under darkroom conditions, and samples are expected to have been chemically preprocessed. Sanderson et al. [4] have field tested a simple portable OSL instrument (<5 kg) to assist with sampling decisions. It does not have any heating or irradiation facility but it is able to measure OSL using stimulating light from blue (470 nm) or IR (880 nm) LEDs and a 30 mm photodetector module. The stimulation light can be pulsed, and the photodetector gated so that counts are accumulated only during the off-period of the diode pulse. This instrument must also be loaded in a darkroom, although it has been used with untreated sediment (i.e. minerals not chemically cleaned and separated). In this paper, we outline the design of a portable instrument intended for use in the field, and designed to accept untreated sediment without the need for darkroom facilities. We verify the performance by comparison with a standard laboratory Risø OSL/TL reader and finally test the ability to measure the quartz burial dose recorded by an untreated natural sample. We also demonstrate the potential of the instrument to measure radioluminescence while the sample is held at elevated temperature during simultaneous X-irradiation and blue-light stimulation. 2. Challenges specific to a field portable instrument When estimating age (dose) in the field, several problems arise specific to field sampling and measurement, in addition to the need for the instrumentation to be portable and self-contained. These include: 2.1. Sampling The sediment may be consolidated (i.e. hard and impenetrable) or loose and unconsolidated; it may also be wet. Of course, if optically stimulated luminescence (OSL) is to be measured, it must not be exposed to daylight during sampling. Thus we must be able to take a sample and insert it into the portable instrument without exposure to daylight, and the sampling process and the instrument must be able to cope with significant excess moisture, perhaps even free water. 2.2. Sample pretreatment In laboratory sample preparation, the sediment is normally wet-sieved to recover a particular grains size, and then cleaned using various acid treatments to give a pure quartz extract; this is clearly impractical in the field. This has implications not only for the source of signals (see Section 2.3) but also for signal strength. Iron oxide or clay minerals are common coatings in natural sediments; these will attenuate both the luminescence leaving the grains, and also the stimulating light entering the grains. A portable instrument must be as sensitive as possible, and use intense light sources to minimize problems arising from such coatings.
field) or there must be some instrumental method of separating signals from specific minerals. 2.4. Source of ionizing radiation To calibrate luminescence in terms of dose, the instrument must include some source of ionizing radiation such as the 90 Sr/90Y beta source normally used in the laboratory. Although a beta source has the advantage of being light and not requiring a power source, heavy shielding is needed to protect the operator, and often complex bureaucratic procedures must be complied with if the source is to be transported (especially across borders and/or by air). An alternative radiation source that is inactive when not energised (such as an X-ray tube) is clearly desirable. 3. Design principles Based on our research experience with the Risø TL/OSL reader, a laboratory instrument weighing 80 kg (e.g. [3]) we decided to retain all the facilities already present in this bench instrument in a portable instrument. These are shown diagrammatically in Fig. 1. All the widely-used methods for retrospectively measurement of dose by means of the optically stimulated luminescence (OSL) of natural minerals [5] involve the use of optical stimulation sources, a wavelength-selective light detection system, a sample heater and a source of ionizing radiation. Optical filters (Filter A in Fig. 1) are usually required in front of the stimulation sources to ensure that no light from the short wavelength tail of the stimulation source passes through the detection filter (Filter B). Fig. 2 illustrates the relevant wavelength-dependent characteristics of the optical components selected for use in the instrument described later in this paper. These include: a stimulation LED (peak emission 470 nm, FWHM 25 nm, 4.76 W Luxeon V star); a 420 nm long-pass Schott GG420 filter (3 mm thick, Filter A in Fig. 1); a UV selective Hoya U-340 filter (3 mm thick, peak transmission at 340 nm, FWHM 80 nm, Filter B in Fig. 1); and the photocathode response of a 9125B photomultiplier tube (25 mm diameter photocathode, ET Enterprises Limited). The short wavelength tail of the 470 nm blue diodes extends under the U-340 detection window; this cannot be seen in the diagram above because this tail is many orders of magnitude less intense than the peak emission, but nevertheless it results in a large count rate in the photodetector (many 1000s of count per second). To reduce this breakthrough signal a blocking GG420 filter is placed in front of the diodes. Because of the bialkali cathode, the PM tube has a maximum in quantum efficiency very close to the U-340 pass band.
2.3. Mineral separation The environmental dose rate to the two most common luminescence minerals, quartz and feldspar, can be significantly different because of the 40K content of K-rich feldspars (K-rich feldspar contains up to 14% K). This dose rate difference is typically 30–50% of the total, and results in a different total absorbed dose for the two minerals. Thus for accurate age estimates, either a specific mineral must be physically separated before measurement (difficult in the
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Fig. 1. Outline of facilities required.
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Fig. 4. Photograph showing the portable OSL reader system: (1) portable reader, (2) sampler, (3) X-ray controller, (4) standard laptop.
wavelength (nm) Fig. 2. Comparison of LED emission, GG420, U-340 and PM tube response (LED: www.luxeon.com, GG420: www.schott.com, U-340: www.hoyaoptics.com, PMT: www.electrontubes.com). All curves should be read on the left hand axis except that of the photomultiplier tube, which is read on the quantum efficiency axis.
45 V DC or 220 V AC. The laptop runs a simplified version of the standard Risø Sequence Editor software for control and data storage. 4.1. Measurement head and main body of the Portable OSL reader
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It is well known that the luminescence lifetimes for feldspars are much shorter than those of quartz. Fig. 3 shows time-resolved measurements for the electronic driving signal, and for the photons detected from quartz and feldspar samples; these data were collected using the portable instrument described here and a photon timer attachment [6]. These differences in decay time of the luminescence signals offer an instrumental method of mineral separation; this approach has recently been implemented and tested on a laboratory Risø TL/OSL reader [6–8]. With these various requirements and observations in mind, we have developed a compact portable luminescence dating instrument that includes: (i) a novel 3 position sampler with built-in heaters designed to collect samples without light exposure, (ii) pulsed blue light stimulation for the separation of quartz signals from those of feldspar, and (iii) a 30 kV mini X-ray generator.
The portable reader consists of 3 main parts: the measurement head, measurement table and main body (Fig. 5). The measurement head contains the PM tube, blue stimulation LEDs and the X-ray tube. Luminescence is detected by a P30CWAD5 photodetector module (ET Enterprises Limited) including a 30 mm diameter end window photomultiplier tube with blue-sensitive bialkali photocathode (9125B, ET Enterprises Limited) and an internal negative high-voltage supply and high speed amplifier-discriminator. The PM tube is inclined at 40° to the plane of the reader body. A biconvex lens between the sample and PM cathode improves the light collection efficiency by a factor of 2, and there is 3 mm of U340 glass filter (Hoya) mounted immediately in front of the PM tube. Five Luxeon V star (470 nm, 23.8 W in total) LEDs are used for stimulation. The LEDs can be operated in both continuous wave (CW) and pulsed (POSL) mode, with the pulse parameters preset by the controlling software (Sequence Editor), to give on-times between 1 and 65,535 ls, and off-times between 1 and 65,535 ls. Photon counting can be set to accumulate continuously, or only during the off-time. The cold cathode X-ray tube (Oxford Instruments Eclipse II, 30 kV, 0.1 mA, 3 W, described in detail by Andersen et al. [9] and Thomsen et al. [10]) is mounted vertically to minimize dose rate variations across the sample, and has an anode to sample spacing of 23 mm. The bottom cover of the reader and the locking ring holding the sampler are interlocked to disable the X-ray generator if the irradiated volume is not fully closed. The acceleration voltage and anode current is preset manually using the controller.
4. Instrument description Fig. 4 shows the prototype Portable OSL reader system consisting of the reader itself (1), the sampler (2), X-ray controller (3) and a standard laptop (4). The instrument weighs 8 kg and requires an external power supply delivering up to 4 A at between 9 and
Fig. 5. Sectional drawing of the Portable OSL reader.
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The measurement head and table can be moved horizontally under software control to measure each sample. For instance TL, OSL and radioluminescence (RL) signals from the sample in the centre position sample can be measured as shown in the configuration in Fig. 5. In the same configuration, the left sample and right sample can be illuminated respectively by a 5 W blue (470 nm) LED tile (Lamina, 7-cavity array) and/or a 3.2 W IR (940 nm) LED tile (Lamina, 7-cavity array) mounted on the sliding table. The IR LED source can be used to selectively reduce the luminescence signal from any feldspar grains present in the sample before OSL measurement (see below).
quired in a specified period during the off-time of the diode pulses. We therefore assigned these functions to a second independent microcontroller. Timing parameters of the blue stimulation pulse and the counting window are set using a simplified version of the standard Risø Sequence Editor. These include a stimulation period and pulse on-time, and a start time and stop time for the counting interval, all on a ls-scale. The counting interval is delayed, so that it starts a few ls after the end of the drive pulse to ensure that the LEDs are fully turned off.
4.2. Sampler
Various measurements characterizing the performance of the instrument have been undertaken, and, where relevant, compared with those obtained from a standard laboratory Risø OSL/TL reader (Model DA-15).
The sample holder has three functions: (i) collecting the sample without light exposure, (ii) presenting the sample in the correct position to the measurement head, and (iii) heating samples. The three-position sampler (Fig. 6) can be directly inserted up to 30 cm into unconsolidated sediments (e.g. loose sand). There are three 1 mm deep and 10 mm diameter indentations in the sampler, covered by a rotating shutter. By opening the shutter, the three separate recesses fill with sediment. When the shutter is closed, the samples are sealed from possible light exposure, and the sampler can be withdrawn from the sediment, wiped and inserted directly into the reader; finally the shutter is opened to uncover the 3 sub-samples for measurement. Heating elements are built into the sampling tool under each sample position; each sub-sample can be heated to a maximum temperature of 250 °C, at up to 3 °C.s 1. The heating coil (ThermoCoax, 5 X) and temperature measurement sensor (RTD, Pt100) are held in an aluminum can with high-temperature thermal cement, and the maximum power consumption of one heater is around 30 W at 12 V. An optical window can be mounted in the fixed table (see Fig. 5) to separate the samples from the measurement head. This prevents moisture released during preheating (or during any thermal treatment at 100 °C to dry out samples prior to any measurements) from entering the measurement cavity. 4.3. Electronic block diagram An electronic block diagram of the portable luminescence instrument is shown in Fig. 7. Two 8-bit microcontrollers (Atmel, AT90USB1286 and ATMEGA128) are used on the motherboard: the first (i) controls the reader according to a stored sequence of commands transferred before measurement from the user computer via USB; (ii) positions the measurement head above the appropriate sample position prior to any measurement; (iii) controls the 3 sample heaters; (iv) switches the X-ray generator; (v) monitors safety interlocks. The most time-critical functions are control of the stimulation LEDs and photon counting. In particular, when pulsed OSL is measured, the LED pulse and the PM tube counting window need to be synchronized precisely to ensure that photons counts are only ac-
5. Performance and testing of portable reader
5.1. Heating characteristics Figs. 8a and b show the performance of one of the three sample heaters. The deviations from the real temperature over the entire range of room temperature to 200 °C are all within 4 °C. 5.2. Optical detection sensitivity The relative optical detection sensitivity of the portable system was measured using a comparison of three configurations: (i) a Risø reader with the 50 mm bialkali PM tube (9235B) used normally by this laboratory instrument, (ii) the same Risø reader fitted with the 30 mm bialkali PM tube (9125B) used in the portable reader (same sample-photocathode distance as in (i)), and (iii) the portable reader with the 30 mm bialkali PM tube (9125B). TL and OSL measurements were carried out on the same samples, and all were measured after the same dose was given by the 90 Sr/90Y beta source mounted in the laboratory reader (for measurement in the portable, after irradiation the sample was transferred carefully to the sampler in such a manner as to avoid losing grains). Fig. 9a shows the results of this comparison of detection sensitivity for the 110 °C quartz TL peak. The smaller 30 mm PM tube detects only 32% of the photons collected by the 50 mm PM tube when mounted in the same geometry on the laboratory Risø reader (comparable with the photocathode area reduction of 36%). But when installed in the portable reader, the 30 mm PM tube recorded 12% more photons than the larger PM tube in the standard geometry, reflecting the improved collection angle resulting from the collection lens only 34 mm from the sample. When OSL was measured, the 30 mm PM tube mounted on the portable reader detected about 0.89 of the initial intensity of the OSL signal observed by the 50 mm PM tube on the standard Risø reader (OSL initial intensity is a function of optical sensitivity and diode power). We conclude that the portable reader has a very similar sensitivity to that of the standard Risø reader during both TL and OSL. 5.3. X-ray irradiation and calibration
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Fig. 6. Three heating elements installed in the sampler: (1) aluminum can, (2) heating coil, (3) temperature measurement sensor.
The use of an X-ray generator as an alternative to the radioisotope irradiator for the irradiation of natural minerals has been investigated in detail by both Andersen et al. [9] and Thomsen et al. [10,11]. We have selected the Oxford Instruments Eclipse II (30 kV, 0.1 mA) described by Thomsen et al. [10,11]; these authors showed that if the softer X-rays are not filtered sufficiently, the effective dose rate (compared to that from a standard beta source) is sample dependent. Fig. 10 shows the results of a dose rate calibration test using OSL signals from quartz samples. Increasing the
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Fig. 7. Block diagram of portable luminescence instrument.
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Fig. 8. (a) Set temperature and observed heater temperature as a function of time for a linear ramp to 200 °C at 3 °C.s Deviation between measured (by RTD in heater can) and set temperatures as a function of time.
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tural potassium, and so 40K, in some feldspars) and so record different doses. In addition, they have different luminescence properties, and so some method of separation of the signals from these two mineral is required for accurate age determination. We have chosen to use the post-IR pulsed-blue stimulation method described by Thomsen et al. [7]. This approach makes use of two characteristics of feldspar to differentiate between the feldspar and quartz signals, and so preferentially record only the quartz OSL: (i) at room temperature feldspar is sensitive to IR light, but quartz is not; both are sensitive to blue light. In Thomsen et al.’s approach, a large part of the blue-sensitive OSL signal from feldspar is first removed by stimulating with IR. (ii) The different response of the two minerals to pulsed stimulation (Fig. 3) is then used to separate any residual feldspar signal from quartz.
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Al filtration (µm) Fig. 10. Dose rate as a function of filtration thickness for 30 kV, 0.1 mA. A typical SAR dose response curve ([16]) for the calibration quartz sample is shown inset, with the corrected natural OSL interpolated onto the growth curve to give the seconds of X-ray irradiation required to match the known gamma dose of 4.81 Gy.
filter thickness from 0 to 300 lm Al reduces the apparent dose rate (30 kV/0.1 mA) from 0.28 to 0.04 Gy/s. Thomsen et al. [10,11] suggest 200 lm Al filter is necessary to remove sample dependency; such a filter thickness gives an effective dose rate to quartz of 0.06 Gy/s. 5.4. Reproducibility To determine the overall reproducibility of the portable luminescence instrument, a sample was repeatedly dosed, heated, measured using TL or OSL, and finally bleached; the resulting TL and OSL signals are presented in Fig. 11. The observed relative standard deviation of the results around the smooth trend lines (resulting from systematic sample sensitivity change) is 1.1% and 1.3% for TL and OSL respectively. The standard Risø reader has a corresponding value for OSL of 1.5%. We conclude that the reproducibility of the portable luminescence instrument is comparable with the standard laboratory instrument. 6. Measurement of dose Natural sediments usually contain a mixture of quartz and feldspar. As discussed above, these minerals are exposed to different to different radiation fields (mainly because of the presence of struc-
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6.2. Untreated natural sample We next used a SAR protocol [12] with the post-IR pulsed-blue stimulation method on sample 054620 (Poland). Part of the sample was chemically processed in the usual manner using wet sieving, acid treatment and density separation. The apparent equivalent dose, De, from the resulting clean quartz extract, and from an un-
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To test the ability of our system to separate a quartz signal from a feldspar/quartz mixture, measurements were first undertaken using a quartz extract (180–250 lm), a density-separated potassium-rich feldspar extract (also 180–250 lm), and a 50:50 mixture of the two. The feldspar grains were first bleached under a solar simulator (Hönle) and the mixture prepared. All samples were then bleached further under blue diodes for 60 s, given a dose of 9 Gy and CW-OSL, pulsed OSL and post-IR pulsed OSL signals were measured. The stimulation pulses used for pulsed OSL were on for the first 50 ls of each 100 ls pulse cycle, and off for the remaining 50 ls [8]; signals were recorded from 53 to 99 ls. Fig. 12a and b compare the results from CW-, pulsed- and post-IR pulsed-OSL from quartz and feldspar. The initial luminescence signals decrease to 33% and 2.2% of their values under CW stimulation, respectively, and the ratio of the quartz to feldspar signal increases by a factor of 15. Fig. 12c and d show the corresponding results for the 50:50 quartz/feldspar mixture, on logarithmic and linear vertical scales, respectively. The quartz OSL is much less sensitive than the feldspar, but despite this the rapidly decaying quartz OSL signal (the ‘fast component’) is readily detectable in Fig. 12d, and the improvement in the resolution of this component after prior-IR stimulation is clearly visible.
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treated ‘natural’ portion of the same sample was then measured, using a preheat at 200 °C for 10 s, IR bleaching at 125 °C for 100 s, pulsed-blue stimulation at 125 °C for 80 s, 50 ls on and off time with PM tube gated to count from 53 to 99 ls in each pulse cycle, and a test-dose ‘cutheat’ to 200 °C. All dose response curves consist of three regeneration doses, a recuperation point (0 Gy) and a repeat dose (recycling point). Fig. 13 shows the resulting growth curves and decay curves from one aliquot of quartz and one of the natural sample. In Fig. 13b the initial decay from the untreated sample is somewhat slower than that from the clean quartz sample. This is in contrast to the results from Fig. 12d, where the initial decay rates of the clean quartz and the quartz/feldspar mixture are
very similar. We attribute this difference to grain coatings; in Fig. 12, the quartz-feldspar mixture and the quartz separate had both been acid treated, so that any surface coatings on the quartz grains in both samples had been removed. In Fig. 13, only the quartz separate had received such treatment; quartz in the untreated sample would still have any natural surface coatings present. Such coatings would attenuate the stimulation light, and so reduce the rate of stimulation. Loose sands may also contain significant carbonate, as shell hash or as coatings. Although we have not explicitly tested the instrument with carbonate-rich samples, there is no reason to expect that carbonate will behave otherwise than as an inert diluent
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and attenuator of the OSL signals. At the temperatures involved here, there should be no significant carbonate breakdown, and carbonate does not have a significant OSL sensitivity when compared to quartz and feldspar. The quartz extract was also measured on a conventional Risø reader (using a beta source and CW stimulation) and gave a De of 20.7 ± 0.3 Gy. The average De from the quartz extract measured on the portable reader (using the X-ray source) was 19.7 ± 1.0 Gy (n = 6), and from the natural sample 18 ± 2 Gy (n = 6); all three of these results are indistinguishable. We conclude that by using post-IR pulsed blue stimulation, we are able to separate instrumentally the quartz signal from an untreated natural sample sufficiently to give an accurate estimate of the natural dose in quartz.
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Fig. 15 shows the normalized radioluminescence intensity as a function of irradiation temperature for both the quartz and feldspar extracts described in section 5. The measurements were carried out with the X-ray source set at 30 kV and 0.01 mA. The luminescence process in quartz is known to be less efficient at higher temperatures (due to thermal quenching of the luminescence centres; [15,16]). The solid line represents the expected effect of this thermal quenching based on the model and parameters given by Murray and Wintle [16], and it can be seen that the observed data follow a similar trend. In contrast, feldspar
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a few seconds; thereafter the quartz RL becomes constant, although the feldspar RL signal continues to increase slowly. When the blue light stimulation is switched on, both the quartz and feldspar signals immediately increase because of the additional OSL signal; the quartz luminescence rapidly decreases back to the level of the underlying RL, whereas that from feldspar remains at an elevated level. When the stimulation light is switched off, the luminescence decreases to below the level of the RL, and then rapidly increases again; this effect is most pronounced in the feldspar data. The doses involved in this experiment are small (<1 Gy) and so the build up in signal is unlikely to reflect trap saturation; we presume the luminescence efficiency changes as the luminescent to nonluminescent hole population re-equilibrates after the stimulation light is switched off.
The same aliquots of quartz and K-feldspar extracted from sample 054620 used above were employed here (i.e. after optical resetting in the experiments described in Section 5). In the two measurements described below, the aliquot was held at room 125 °C, and the luminescence signal was observed from the time the X-ray source was activated. In the first, only RL was observed. In the second, the aliquot was first bleached again using OSL at 125 °C for 40 s, and then the X-ray source was activated. After 6.4 s of X-irradiation the blue light source was switched on, and 33.6 s after the beginning of the experiment the blue light source was switched off. The luminescence observed from the quartz and feldspar aliquots is shown in Fig. 14. For comparison, an OSL stimulation curve (observed without the X-ray irradiation) is shown inset in both cases. In both cases the RL increases from close to zero at the beginning of the measurement. The rapid increase in RL only lasts for
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Because the X-ray tube, stimulation source and the PM tube are all situated immediately above the same sample position it is possible to measure luminescence at the same time as the X-ray source is energized (radioluminescence, RL) and the stimulation light is present, with the sample at temperatures up to 250 °C. Although RL of natural minerals (without simultaneous OSL) has been described before (e.g. [13,14]), this combination of irradiation, stimulation light and heat is not available on standard laboratory luminescence instrumentation; it offers new measurement possibilities and new insights into the charge storage and recombination process in natural minerals. These new measurement possibilities are illustrated below.
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IRSL is known to increases with temperature [7]. From the fact that the RL feldspar signal decreases with temperature (rather than increases) it appears that charge in the conduction band (created during X-ray irradiation) follows a different recombination route from infrared stimulated charge (which is known not to go through the conduction band, but probably recombines through a tunneling mechanism; [14]). Assuming both processes use the same recombination centres, then it may be that recombination from the conduction band goes through an excited state in the centre (and thus thermal quenching is possible), whereas tunneling delivers electrons below this excited state. 8. Conclusion We have developed a portable luminescence dating instrument for in situ dose determinations in the field. To overcome the difficulties of sampling and transfer to the instrument without light exposure, we developed a novel 3 position sampler with inbuilt heaters. Infra-red bleaching and subsequent pulsed blue light stimulation is used to measure relatively clean quartz signals using untreated natural sand containing feldspar. A mini X-ray generator (dose rate 0.06 Gy/s) is used to measure the dose response of these signals, both for transportability and safety. Various measurements have been carried out to characterize the performance of the instrument such as heating, optical detection sensitivity, X-ray calibration and reproducibility. We have shown that the optical detection sensitivity of the portable reader is very similar to the standard Risø reader and signals can be measured with a reproducibility of 1.1% and 1.3% for TL and OSL respectively. We then confirmed our ability to reject the feldspar signal from an artificial known mixture of quartz and feldspar using post-IR pulsed-blue stimulation, and conclude that our portable instrument can be used to determine an accurate quartz equivalent dose from untreated natural samples. Finally we have demonstrated the new measurement possibilities of the instrument, and shown how new insights into the charge storage and recombination process in natural minerals can be obtained by
making radioluminescence measurements in conjunction with stimulation light and heating. In summary, our new instrument offers a new approach to sample selection in the field, and promises at least preliminary ages while field work is in progress. It also offers the possibility of new measurements not at present possible with conventional instrumentation. Acknowledgements Initial development of this instrument was made possible by funding through the Nordic Centre of Excellence programme (2003-2007), and partly supported by KBSI grant (Project No. F31604). References [1] N.R.J. Poolton, L. Bøtter-Jensen, A.G. Wintle, J. Jakobsen, F. Jørgensen, K.L. Knudsen, Radiat. Meas. 23 (1994) 529. [2] T. Takeuchi, T. Shibutani, T. Hashimoto, Geochronometria 30 (2008) 17. [3] L. Bøtter-Jensen, C.E. Andersen, G.A.T. Duller, A.S. Murray, Radiat. Meas. 37 (2003) 535. [4] D.C.W. Sanderson, S. Murphy, Quat. Geochronol. 5 (2010) 299. [5] M.J. Aitken, An Introduction to Optical Dating, Oxford University Press, Oxford, 1998. [6] T. Lapp, M. Jain, C. Ankjærgaard, L. Pirtzel, Radiat. Meas. 44 (2009) 571. [7] K.J. Thomsen, M. Jain, A.S. Murray, P.M. Denby, N. Roy, L. Bøtter-Jensen, Radiat. Meas. 43 (2008) 752. [8] C. Ankjærgaard, M. Jain, K.J. Thomsen, A.S. Murray, Radiat. Meas. 45 (2010) 778. [9] C.E. Andersen, L. Bøtter-Jensen, A.S. Murray, Radiat. Meas. 37 (2003) 557. [10] K.J. Thomsen, L. Bøtter-Jensen, P.M. Denby, A.S. Murray, Nucl. Instrum. Meth. B 252 (2006) 267. [11] K.J. Thomsen, L. Bøtter-Jensen, P.M. Denby, P. Moska, A.S. Murray, Radiat. Meas. 41 (2006) 768. [12] A.S. Murray, A.G. Wintle, Radiat. Meas. 32 (2000) 57. [13] N.R.J. Poolton, E. Bulur, J. Wallinga, L. Bøtter-Jensen, A.S. Murray, F. Willumsen, Nucl. Instrum. Meth. B 179 (2001) 575. [14] T. Trautman, M.R. Krbetschek, A. Dietrich, W. Stolz, Radiat. Meas. 29 (1998) 421. [15] N. Spooner, Radiat. Meas. 23 (1994) 593. [16] A.S. Murray, A.G. Wintle, Radiat. Meas. 29 (1998) 65.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb
A portable luminescence dating instrument M.H. Kook a,b,d, A.S. Murray c, T. Lapp d, P.H. Denby d,e, C. Ankjærgaard d,f, K. Thomsen d, M. Jain d, J.H. Choi a,⇑, G.H. Kim a a
Korea Basic Science Institute, Daejeon 305-333, South Korea Department of Nano-Mechatronics, University of Science and Technology, Daejeon 305-333, South Korea c Nordic Laboratory for Luminescence Dating, Department of Earth Sciences, University of Aarhus, Risø DTU, DK-4000 Roskilde, Denmark d Radiation Research Division, Risø DTU, DK-4000 Roskilde, Denmark e Department of Physics and Technology, University of Bergen, Norway f Netherlands Centre for Luminescence Dating, Faculty of Applied Sciences, Delft University of Technology, NL-2629JB Delft, The Netherlands b
a r t i c l e
i n f o
Article history: Received 24 November 2010 Received in revised form 16 March 2011 Available online 24 March 2011
a b s t r a c t We describe a portable luminescence reader suitable for use in remote localities in the field. The instrument weighs about 8 kg and is based around a 30 mm bialkali photomultiplier detecting signals through a glass filter centered on 340 nm. Stimulation is by 470 nm blue LEDs (24 W in total) operating in both continuous wave and pulsed mode; photon counting can be gated such that it is active only during the pulse off-period. There are also two bleaching light sources (470 nm, 5 W and 940 nm, 3 W), and the luminescence signals can be regenerated using a cold-cathode 30 kV X-ray tube, delivering 0.06 Gy.s 1. The three position sampling device has a heating element under each sampling position, able to heat the sample at 3 °C.s 1 up to at least 250 °C. The sampler can be inserted into unconsolidated sediments, and is designed to prevent exposure of the mineral grains to ambient light during sampling. The performance of the instrument in terms of sensitivity and reproducibility is comparable to that of the standard bench-top laboratory TL/OSL Risø reader. We show that the portable luminescence reader is able to measure accurately an 20 Gy quartz burial dose in a natural (unpretreated, no mineral separation) sandy sediment. We also show that, because of the configuration of the measurement head, the portable reader can be used to measure radioluminescence at elevated temperature in the presence of stimulation light; this facility is not available on conventional bench-top instruments. It is concluded that the portable luminescence reader can be used to accurately determine the quartz burial dose in loose sandy sediments in the field, without sample preparation or darkroom facilities. Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction Luminescence is now widely used to date quaternary sediments from all over the world, including those from remote and inaccessible locations that cannot readily be revisited. In the absence of any prior chronostratigraphic data, it is often difficult to design an appropriate sampling strategy in the field, and it is not uncommon that inadvertent errors in sampling, e.g. of the wrong geological period, are only discovered when the samples are returned for measurement in the laboratory. Any method of obtaining preliminary ages in the field would help to increase the relevance of the samples, and reduce wasted resources. As a result there is a demand for a portable instrument that will provide preliminary estimates of the equivalent dose in samples in the field, and thus preliminary ages to guide the sampling strategy.
⇑ Corresponding author. E-mail address: [email protected] (J.H. Choi). 0168-583X/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2011.03.014
Various authors have attempted to construct such an instrument, usually based on optically stimulation using infrared (IR) or visible wavelengths. Some include an ionising radiation source (usually an X-ray source). Poolton et al. [1] were probably the first to construct a practical instrument intended to determine the dose in natural sediments. Their system was built around a 12 position sample holder which could slide laterally from under a UV lamp (to regenerate the luminescence signals) to the measurement position, where IR diodes (880 nm) stimulated each sample position while photons were detected through glass filters (Schott BG39) by a 30 mm diameter photomultiplier tube (EMI, bialkali photocathode, type 9924B). By sliding the sample holder to the other side of the measurement position, it could be illuminated by a halogen lamp bleaching source. More recently, Takeuchi et al. [2] constructed a compact TL/OSL measurement system similar in concept to the standard Risø TL/ OSL reader [3]. It is equipped with blue (470 nm) and IR (890 nm) LEDs for stimulation, a ceramic heater, a metal-packaged PMT (Hamamatsu, H7421), and a miniature X-ray generator
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(Oxford, Eclipse II). Although it could be considered a portable instrument (weight 15 kg) it must be loaded under darkroom conditions, and samples are expected to have been chemically preprocessed. Sanderson et al. [4] have field tested a simple portable OSL instrument (<5 kg) to assist with sampling decisions. It does not have any heating or irradiation facility but it is able to measure OSL using stimulating light from blue (470 nm) or IR (880 nm) LEDs and a 30 mm photodetector module. The stimulation light can be pulsed, and the photodetector gated so that counts are accumulated only during the off-period of the diode pulse. This instrument must also be loaded in a darkroom, although it has been used with untreated sediment (i.e. minerals not chemically cleaned and separated). In this paper, we outline the design of a portable instrument intended for use in the field, and designed to accept untreated sediment without the need for darkroom facilities. We verify the performance by comparison with a standard laboratory Risø OSL/TL reader and finally test the ability to measure the quartz burial dose recorded by an untreated natural sample. We also demonstrate the potential of the instrument to measure radioluminescence while the sample is held at elevated temperature during simultaneous X-irradiation and blue-light stimulation. 2. Challenges specific to a field portable instrument When estimating age (dose) in the field, several problems arise specific to field sampling and measurement, in addition to the need for the instrumentation to be portable and self-contained. These include: 2.1. Sampling The sediment may be consolidated (i.e. hard and impenetrable) or loose and unconsolidated; it may also be wet. Of course, if optically stimulated luminescence (OSL) is to be measured, it must not be exposed to daylight during sampling. Thus we must be able to take a sample and insert it into the portable instrument without exposure to daylight, and the sampling process and the instrument must be able to cope with significant excess moisture, perhaps even free water. 2.2. Sample pretreatment In laboratory sample preparation, the sediment is normally wet-sieved to recover a particular grains size, and then cleaned using various acid treatments to give a pure quartz extract; this is clearly impractical in the field. This has implications not only for the source of signals (see Section 2.3) but also for signal strength. Iron oxide or clay minerals are common coatings in natural sediments; these will attenuate both the luminescence leaving the grains, and also the stimulating light entering the grains. A portable instrument must be as sensitive as possible, and use intense light sources to minimize problems arising from such coatings.
field) or there must be some instrumental method of separating signals from specific minerals. 2.4. Source of ionizing radiation To calibrate luminescence in terms of dose, the instrument must include some source of ionizing radiation such as the 90 Sr/90Y beta source normally used in the laboratory. Although a beta source has the advantage of being light and not requiring a power source, heavy shielding is needed to protect the operator, and often complex bureaucratic procedures must be complied with if the source is to be transported (especially across borders and/or by air). An alternative radiation source that is inactive when not energised (such as an X-ray tube) is clearly desirable. 3. Design principles Based on our research experience with the Risø TL/OSL reader, a laboratory instrument weighing 80 kg (e.g. [3]) we decided to retain all the facilities already present in this bench instrument in a portable instrument. These are shown diagrammatically in Fig. 1. All the widely-used methods for retrospectively measurement of dose by means of the optically stimulated luminescence (OSL) of natural minerals [5] involve the use of optical stimulation sources, a wavelength-selective light detection system, a sample heater and a source of ionizing radiation. Optical filters (Filter A in Fig. 1) are usually required in front of the stimulation sources to ensure that no light from the short wavelength tail of the stimulation source passes through the detection filter (Filter B). Fig. 2 illustrates the relevant wavelength-dependent characteristics of the optical components selected for use in the instrument described later in this paper. These include: a stimulation LED (peak emission 470 nm, FWHM 25 nm, 4.76 W Luxeon V star); a 420 nm long-pass Schott GG420 filter (3 mm thick, Filter A in Fig. 1); a UV selective Hoya U-340 filter (3 mm thick, peak transmission at 340 nm, FWHM 80 nm, Filter B in Fig. 1); and the photocathode response of a 9125B photomultiplier tube (25 mm diameter photocathode, ET Enterprises Limited). The short wavelength tail of the 470 nm blue diodes extends under the U-340 detection window; this cannot be seen in the diagram above because this tail is many orders of magnitude less intense than the peak emission, but nevertheless it results in a large count rate in the photodetector (many 1000s of count per second). To reduce this breakthrough signal a blocking GG420 filter is placed in front of the diodes. Because of the bialkali cathode, the PM tube has a maximum in quantum efficiency very close to the U-340 pass band.
2.3. Mineral separation The environmental dose rate to the two most common luminescence minerals, quartz and feldspar, can be significantly different because of the 40K content of K-rich feldspars (K-rich feldspar contains up to 14% K). This dose rate difference is typically 30–50% of the total, and results in a different total absorbed dose for the two minerals. Thus for accurate age estimates, either a specific mineral must be physically separated before measurement (difficult in the
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Fig. 1. Outline of facilities required.
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Fig. 4. Photograph showing the portable OSL reader system: (1) portable reader, (2) sampler, (3) X-ray controller, (4) standard laptop.
wavelength (nm) Fig. 2. Comparison of LED emission, GG420, U-340 and PM tube response (LED: www.luxeon.com, GG420: www.schott.com, U-340: www.hoyaoptics.com, PMT: www.electrontubes.com). All curves should be read on the left hand axis except that of the photomultiplier tube, which is read on the quantum efficiency axis.
45 V DC or 220 V AC. The laptop runs a simplified version of the standard Risø Sequence Editor software for control and data storage. 4.1. Measurement head and main body of the Portable OSL reader
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It is well known that the luminescence lifetimes for feldspars are much shorter than those of quartz. Fig. 3 shows time-resolved measurements for the electronic driving signal, and for the photons detected from quartz and feldspar samples; these data were collected using the portable instrument described here and a photon timer attachment [6]. These differences in decay time of the luminescence signals offer an instrumental method of mineral separation; this approach has recently been implemented and tested on a laboratory Risø TL/OSL reader [6–8]. With these various requirements and observations in mind, we have developed a compact portable luminescence dating instrument that includes: (i) a novel 3 position sampler with built-in heaters designed to collect samples without light exposure, (ii) pulsed blue light stimulation for the separation of quartz signals from those of feldspar, and (iii) a 30 kV mini X-ray generator.
The portable reader consists of 3 main parts: the measurement head, measurement table and main body (Fig. 5). The measurement head contains the PM tube, blue stimulation LEDs and the X-ray tube. Luminescence is detected by a P30CWAD5 photodetector module (ET Enterprises Limited) including a 30 mm diameter end window photomultiplier tube with blue-sensitive bialkali photocathode (9125B, ET Enterprises Limited) and an internal negative high-voltage supply and high speed amplifier-discriminator. The PM tube is inclined at 40° to the plane of the reader body. A biconvex lens between the sample and PM cathode improves the light collection efficiency by a factor of 2, and there is 3 mm of U340 glass filter (Hoya) mounted immediately in front of the PM tube. Five Luxeon V star (470 nm, 23.8 W in total) LEDs are used for stimulation. The LEDs can be operated in both continuous wave (CW) and pulsed (POSL) mode, with the pulse parameters preset by the controlling software (Sequence Editor), to give on-times between 1 and 65,535 ls, and off-times between 1 and 65,535 ls. Photon counting can be set to accumulate continuously, or only during the off-time. The cold cathode X-ray tube (Oxford Instruments Eclipse II, 30 kV, 0.1 mA, 3 W, described in detail by Andersen et al. [9] and Thomsen et al. [10]) is mounted vertically to minimize dose rate variations across the sample, and has an anode to sample spacing of 23 mm. The bottom cover of the reader and the locking ring holding the sampler are interlocked to disable the X-ray generator if the irradiated volume is not fully closed. The acceleration voltage and anode current is preset manually using the controller.
4. Instrument description Fig. 4 shows the prototype Portable OSL reader system consisting of the reader itself (1), the sampler (2), X-ray controller (3) and a standard laptop (4). The instrument weighs 8 kg and requires an external power supply delivering up to 4 A at between 9 and
Fig. 5. Sectional drawing of the Portable OSL reader.
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The measurement head and table can be moved horizontally under software control to measure each sample. For instance TL, OSL and radioluminescence (RL) signals from the sample in the centre position sample can be measured as shown in the configuration in Fig. 5. In the same configuration, the left sample and right sample can be illuminated respectively by a 5 W blue (470 nm) LED tile (Lamina, 7-cavity array) and/or a 3.2 W IR (940 nm) LED tile (Lamina, 7-cavity array) mounted on the sliding table. The IR LED source can be used to selectively reduce the luminescence signal from any feldspar grains present in the sample before OSL measurement (see below).
quired in a specified period during the off-time of the diode pulses. We therefore assigned these functions to a second independent microcontroller. Timing parameters of the blue stimulation pulse and the counting window are set using a simplified version of the standard Risø Sequence Editor. These include a stimulation period and pulse on-time, and a start time and stop time for the counting interval, all on a ls-scale. The counting interval is delayed, so that it starts a few ls after the end of the drive pulse to ensure that the LEDs are fully turned off.
4.2. Sampler
Various measurements characterizing the performance of the instrument have been undertaken, and, where relevant, compared with those obtained from a standard laboratory Risø OSL/TL reader (Model DA-15).
The sample holder has three functions: (i) collecting the sample without light exposure, (ii) presenting the sample in the correct position to the measurement head, and (iii) heating samples. The three-position sampler (Fig. 6) can be directly inserted up to 30 cm into unconsolidated sediments (e.g. loose sand). There are three 1 mm deep and 10 mm diameter indentations in the sampler, covered by a rotating shutter. By opening the shutter, the three separate recesses fill with sediment. When the shutter is closed, the samples are sealed from possible light exposure, and the sampler can be withdrawn from the sediment, wiped and inserted directly into the reader; finally the shutter is opened to uncover the 3 sub-samples for measurement. Heating elements are built into the sampling tool under each sample position; each sub-sample can be heated to a maximum temperature of 250 °C, at up to 3 °C.s 1. The heating coil (ThermoCoax, 5 X) and temperature measurement sensor (RTD, Pt100) are held in an aluminum can with high-temperature thermal cement, and the maximum power consumption of one heater is around 30 W at 12 V. An optical window can be mounted in the fixed table (see Fig. 5) to separate the samples from the measurement head. This prevents moisture released during preheating (or during any thermal treatment at 100 °C to dry out samples prior to any measurements) from entering the measurement cavity. 4.3. Electronic block diagram An electronic block diagram of the portable luminescence instrument is shown in Fig. 7. Two 8-bit microcontrollers (Atmel, AT90USB1286 and ATMEGA128) are used on the motherboard: the first (i) controls the reader according to a stored sequence of commands transferred before measurement from the user computer via USB; (ii) positions the measurement head above the appropriate sample position prior to any measurement; (iii) controls the 3 sample heaters; (iv) switches the X-ray generator; (v) monitors safety interlocks. The most time-critical functions are control of the stimulation LEDs and photon counting. In particular, when pulsed OSL is measured, the LED pulse and the PM tube counting window need to be synchronized precisely to ensure that photons counts are only ac-
5. Performance and testing of portable reader
5.1. Heating characteristics Figs. 8a and b show the performance of one of the three sample heaters. The deviations from the real temperature over the entire range of room temperature to 200 °C are all within 4 °C. 5.2. Optical detection sensitivity The relative optical detection sensitivity of the portable system was measured using a comparison of three configurations: (i) a Risø reader with the 50 mm bialkali PM tube (9235B) used normally by this laboratory instrument, (ii) the same Risø reader fitted with the 30 mm bialkali PM tube (9125B) used in the portable reader (same sample-photocathode distance as in (i)), and (iii) the portable reader with the 30 mm bialkali PM tube (9125B). TL and OSL measurements were carried out on the same samples, and all were measured after the same dose was given by the 90 Sr/90Y beta source mounted in the laboratory reader (for measurement in the portable, after irradiation the sample was transferred carefully to the sampler in such a manner as to avoid losing grains). Fig. 9a shows the results of this comparison of detection sensitivity for the 110 °C quartz TL peak. The smaller 30 mm PM tube detects only 32% of the photons collected by the 50 mm PM tube when mounted in the same geometry on the laboratory Risø reader (comparable with the photocathode area reduction of 36%). But when installed in the portable reader, the 30 mm PM tube recorded 12% more photons than the larger PM tube in the standard geometry, reflecting the improved collection angle resulting from the collection lens only 34 mm from the sample. When OSL was measured, the 30 mm PM tube mounted on the portable reader detected about 0.89 of the initial intensity of the OSL signal observed by the 50 mm PM tube on the standard Risø reader (OSL initial intensity is a function of optical sensitivity and diode power). We conclude that the portable reader has a very similar sensitivity to that of the standard Risø reader during both TL and OSL. 5.3. X-ray irradiation and calibration
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Fig. 6. Three heating elements installed in the sampler: (1) aluminum can, (2) heating coil, (3) temperature measurement sensor.
The use of an X-ray generator as an alternative to the radioisotope irradiator for the irradiation of natural minerals has been investigated in detail by both Andersen et al. [9] and Thomsen et al. [10,11]. We have selected the Oxford Instruments Eclipse II (30 kV, 0.1 mA) described by Thomsen et al. [10,11]; these authors showed that if the softer X-rays are not filtered sufficiently, the effective dose rate (compared to that from a standard beta source) is sample dependent. Fig. 10 shows the results of a dose rate calibration test using OSL signals from quartz samples. Increasing the
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Main body X-ray AT90USB1287 X-ray control unit
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Fig. 7. Block diagram of portable luminescence instrument.
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tural potassium, and so 40K, in some feldspars) and so record different doses. In addition, they have different luminescence properties, and so some method of separation of the signals from these two mineral is required for accurate age determination. We have chosen to use the post-IR pulsed-blue stimulation method described by Thomsen et al. [7]. This approach makes use of two characteristics of feldspar to differentiate between the feldspar and quartz signals, and so preferentially record only the quartz OSL: (i) at room temperature feldspar is sensitive to IR light, but quartz is not; both are sensitive to blue light. In Thomsen et al.’s approach, a large part of the blue-sensitive OSL signal from feldspar is first removed by stimulating with IR. (ii) The different response of the two minerals to pulsed stimulation (Fig. 3) is then used to separate any residual feldspar signal from quartz.
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6.1. Discriminating against the feldspar signal
Al filtration (µm) Fig. 10. Dose rate as a function of filtration thickness for 30 kV, 0.1 mA. A typical SAR dose response curve ([16]) for the calibration quartz sample is shown inset, with the corrected natural OSL interpolated onto the growth curve to give the seconds of X-ray irradiation required to match the known gamma dose of 4.81 Gy.
filter thickness from 0 to 300 lm Al reduces the apparent dose rate (30 kV/0.1 mA) from 0.28 to 0.04 Gy/s. Thomsen et al. [10,11] suggest 200 lm Al filter is necessary to remove sample dependency; such a filter thickness gives an effective dose rate to quartz of 0.06 Gy/s. 5.4. Reproducibility To determine the overall reproducibility of the portable luminescence instrument, a sample was repeatedly dosed, heated, measured using TL or OSL, and finally bleached; the resulting TL and OSL signals are presented in Fig. 11. The observed relative standard deviation of the results around the smooth trend lines (resulting from systematic sample sensitivity change) is 1.1% and 1.3% for TL and OSL respectively. The standard Risø reader has a corresponding value for OSL of 1.5%. We conclude that the reproducibility of the portable luminescence instrument is comparable with the standard laboratory instrument. 6. Measurement of dose Natural sediments usually contain a mixture of quartz and feldspar. As discussed above, these minerals are exposed to different to different radiation fields (mainly because of the presence of struc-
a
6.2. Untreated natural sample We next used a SAR protocol [12] with the post-IR pulsed-blue stimulation method on sample 054620 (Poland). Part of the sample was chemically processed in the usual manner using wet sieving, acid treatment and density separation. The apparent equivalent dose, De, from the resulting clean quartz extract, and from an un-
TL counts Fit line
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To test the ability of our system to separate a quartz signal from a feldspar/quartz mixture, measurements were first undertaken using a quartz extract (180–250 lm), a density-separated potassium-rich feldspar extract (also 180–250 lm), and a 50:50 mixture of the two. The feldspar grains were first bleached under a solar simulator (Hönle) and the mixture prepared. All samples were then bleached further under blue diodes for 60 s, given a dose of 9 Gy and CW-OSL, pulsed OSL and post-IR pulsed OSL signals were measured. The stimulation pulses used for pulsed OSL were on for the first 50 ls of each 100 ls pulse cycle, and off for the remaining 50 ls [8]; signals were recorded from 53 to 99 ls. Fig. 12a and b compare the results from CW-, pulsed- and post-IR pulsed-OSL from quartz and feldspar. The initial luminescence signals decrease to 33% and 2.2% of their values under CW stimulation, respectively, and the ratio of the quartz to feldspar signal increases by a factor of 15. Fig. 12c and d show the corresponding results for the 50:50 quartz/feldspar mixture, on logarithmic and linear vertical scales, respectively. The quartz OSL is much less sensitive than the feldspar, but despite this the rapidly decaying quartz OSL signal (the ‘fast component’) is readily detectable in Fig. 12d, and the improvement in the resolution of this component after prior-IR stimulation is clearly visible.
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Fig. 11. Reproducibility of the portable reader for (a) TL and (b) OSL.
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Fig. 12. Comparison of luminescence counts for three samples using various stimulation modes. (a) Quartz, (b) feldspar (c) artificial mixture (d) artificial mixture and absolute time. The legend in (a) applies to (b) and (c). Other details are given in the text.
treated ‘natural’ portion of the same sample was then measured, using a preheat at 200 °C for 10 s, IR bleaching at 125 °C for 100 s, pulsed-blue stimulation at 125 °C for 80 s, 50 ls on and off time with PM tube gated to count from 53 to 99 ls in each pulse cycle, and a test-dose ‘cutheat’ to 200 °C. All dose response curves consist of three regeneration doses, a recuperation point (0 Gy) and a repeat dose (recycling point). Fig. 13 shows the resulting growth curves and decay curves from one aliquot of quartz and one of the natural sample. In Fig. 13b the initial decay from the untreated sample is somewhat slower than that from the clean quartz sample. This is in contrast to the results from Fig. 12d, where the initial decay rates of the clean quartz and the quartz/feldspar mixture are
very similar. We attribute this difference to grain coatings; in Fig. 12, the quartz-feldspar mixture and the quartz separate had both been acid treated, so that any surface coatings on the quartz grains in both samples had been removed. In Fig. 13, only the quartz separate had received such treatment; quartz in the untreated sample would still have any natural surface coatings present. Such coatings would attenuate the stimulation light, and so reduce the rate of stimulation. Loose sands may also contain significant carbonate, as shell hash or as coatings. Although we have not explicitly tested the instrument with carbonate-rich samples, there is no reason to expect that carbonate will behave otherwise than as an inert diluent
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Fig. 13. (a) SAR growth curve and (b) the natural signal from aliquots of the quartz extract and the untreated material from sample 054620.
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and attenuator of the OSL signals. At the temperatures involved here, there should be no significant carbonate breakdown, and carbonate does not have a significant OSL sensitivity when compared to quartz and feldspar. The quartz extract was also measured on a conventional Risø reader (using a beta source and CW stimulation) and gave a De of 20.7 ± 0.3 Gy. The average De from the quartz extract measured on the portable reader (using the X-ray source) was 19.7 ± 1.0 Gy (n = 6), and from the natural sample 18 ± 2 Gy (n = 6); all three of these results are indistinguishable. We conclude that by using post-IR pulsed blue stimulation, we are able to separate instrumentally the quartz signal from an untreated natural sample sufficiently to give an accurate estimate of the natural dose in quartz.
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Fig. 15 shows the normalized radioluminescence intensity as a function of irradiation temperature for both the quartz and feldspar extracts described in section 5. The measurements were carried out with the X-ray source set at 30 kV and 0.01 mA. The luminescence process in quartz is known to be less efficient at higher temperatures (due to thermal quenching of the luminescence centres; [15,16]). The solid line represents the expected effect of this thermal quenching based on the model and parameters given by Murray and Wintle [16], and it can be seen that the observed data follow a similar trend. In contrast, feldspar
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a few seconds; thereafter the quartz RL becomes constant, although the feldspar RL signal continues to increase slowly. When the blue light stimulation is switched on, both the quartz and feldspar signals immediately increase because of the additional OSL signal; the quartz luminescence rapidly decreases back to the level of the underlying RL, whereas that from feldspar remains at an elevated level. When the stimulation light is switched off, the luminescence decreases to below the level of the RL, and then rapidly increases again; this effect is most pronounced in the feldspar data. The doses involved in this experiment are small (<1 Gy) and so the build up in signal is unlikely to reflect trap saturation; we presume the luminescence efficiency changes as the luminescent to nonluminescent hole population re-equilibrates after the stimulation light is switched off.
The same aliquots of quartz and K-feldspar extracted from sample 054620 used above were employed here (i.e. after optical resetting in the experiments described in Section 5). In the two measurements described below, the aliquot was held at room 125 °C, and the luminescence signal was observed from the time the X-ray source was activated. In the first, only RL was observed. In the second, the aliquot was first bleached again using OSL at 125 °C for 40 s, and then the X-ray source was activated. After 6.4 s of X-irradiation the blue light source was switched on, and 33.6 s after the beginning of the experiment the blue light source was switched off. The luminescence observed from the quartz and feldspar aliquots is shown in Fig. 14. For comparison, an OSL stimulation curve (observed without the X-ray irradiation) is shown inset in both cases. In both cases the RL increases from close to zero at the beginning of the measurement. The rapid increase in RL only lasts for
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Fig. 15. Radioluminescence as a function of irradiation temperature for quartz and feldspar.
7.1. Radioluminescence with simultaneous stimulation light
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Because the X-ray tube, stimulation source and the PM tube are all situated immediately above the same sample position it is possible to measure luminescence at the same time as the X-ray source is energized (radioluminescence, RL) and the stimulation light is present, with the sample at temperatures up to 250 °C. Although RL of natural minerals (without simultaneous OSL) has been described before (e.g. [13,14]), this combination of irradiation, stimulation light and heat is not available on standard laboratory luminescence instrumentation; it offers new measurement possibilities and new insights into the charge storage and recombination process in natural minerals. These new measurement possibilities are illustrated below.
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Fig. 14. Simultaneous radioluminescence and OSL from (a) quartz and (b) feldspar.
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IRSL is known to increases with temperature [7]. From the fact that the RL feldspar signal decreases with temperature (rather than increases) it appears that charge in the conduction band (created during X-ray irradiation) follows a different recombination route from infrared stimulated charge (which is known not to go through the conduction band, but probably recombines through a tunneling mechanism; [14]). Assuming both processes use the same recombination centres, then it may be that recombination from the conduction band goes through an excited state in the centre (and thus thermal quenching is possible), whereas tunneling delivers electrons below this excited state. 8. Conclusion We have developed a portable luminescence dating instrument for in situ dose determinations in the field. To overcome the difficulties of sampling and transfer to the instrument without light exposure, we developed a novel 3 position sampler with inbuilt heaters. Infra-red bleaching and subsequent pulsed blue light stimulation is used to measure relatively clean quartz signals using untreated natural sand containing feldspar. A mini X-ray generator (dose rate 0.06 Gy/s) is used to measure the dose response of these signals, both for transportability and safety. Various measurements have been carried out to characterize the performance of the instrument such as heating, optical detection sensitivity, X-ray calibration and reproducibility. We have shown that the optical detection sensitivity of the portable reader is very similar to the standard Risø reader and signals can be measured with a reproducibility of 1.1% and 1.3% for TL and OSL respectively. We then confirmed our ability to reject the feldspar signal from an artificial known mixture of quartz and feldspar using post-IR pulsed-blue stimulation, and conclude that our portable instrument can be used to determine an accurate quartz equivalent dose from untreated natural samples. Finally we have demonstrated the new measurement possibilities of the instrument, and shown how new insights into the charge storage and recombination process in natural minerals can be obtained by
making radioluminescence measurements in conjunction with stimulation light and heating. In summary, our new instrument offers a new approach to sample selection in the field, and promises at least preliminary ages while field work is in progress. It also offers the possibility of new measurements not at present possible with conventional instrumentation. Acknowledgements Initial development of this instrument was made possible by funding through the Nordic Centre of Excellence programme (2003-2007), and partly supported by KBSI grant (Project No. F31604). References [1] N.R.J. Poolton, L. Bøtter-Jensen, A.G. Wintle, J. Jakobsen, F. Jørgensen, K.L. Knudsen, Radiat. Meas. 23 (1994) 529. [2] T. Takeuchi, T. Shibutani, T. Hashimoto, Geochronometria 30 (2008) 17. [3] L. Bøtter-Jensen, C.E. Andersen, G.A.T. Duller, A.S. Murray, Radiat. Meas. 37 (2003) 535. [4] D.C.W. Sanderson, S. Murphy, Quat. Geochronol. 5 (2010) 299. [5] M.J. Aitken, An Introduction to Optical Dating, Oxford University Press, Oxford, 1998. [6] T. Lapp, M. Jain, C. Ankjærgaard, L. Pirtzel, Radiat. Meas. 44 (2009) 571. [7] K.J. Thomsen, M. Jain, A.S. Murray, P.M. Denby, N. Roy, L. Bøtter-Jensen, Radiat. Meas. 43 (2008) 752. [8] C. Ankjærgaard, M. Jain, K.J. Thomsen, A.S. Murray, Radiat. Meas. 45 (2010) 778. [9] C.E. Andersen, L. Bøtter-Jensen, A.S. Murray, Radiat. Meas. 37 (2003) 557. [10] K.J. Thomsen, L. Bøtter-Jensen, P.M. Denby, A.S. Murray, Nucl. Instrum. Meth. B 252 (2006) 267. [11] K.J. Thomsen, L. Bøtter-Jensen, P.M. Denby, P. Moska, A.S. Murray, Radiat. Meas. 41 (2006) 768. [12] A.S. Murray, A.G. Wintle, Radiat. Meas. 32 (2000) 57. [13] N.R.J. Poolton, E. Bulur, J. Wallinga, L. Bøtter-Jensen, A.S. Murray, F. Willumsen, Nucl. Instrum. Meth. B 179 (2001) 575. [14] T. Trautman, M.R. Krbetschek, A. Dietrich, W. Stolz, Radiat. Meas. 29 (1998) 421. [15] N. Spooner, Radiat. Meas. 23 (1994) 593. [16] A.S. Murray, A.G. Wintle, Radiat. Meas. 29 (1998) 65.