Optically stimulated luminescence from quartz measured using the linear modulation technique

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1 Radiation Measurements 32 (2000) 407±411 Optically stimulated luminescence from quartz measured using the linear modulation technique E. Bulur a, L. Bùtter-Jensen a, *, A.S. Murray b a Risù National Laboratory, P.O. Box 49, DK 4000, Roskilde, Denmark b Nordic Laboratory for Luminescence Dating, Department of Earth Sciences, Aarhus University, Risù National Laboratory, DK 4000, Roskilde, Denmark Received 19 October 1999; received in revised form 9 March 2000; accepted 1 May 2000 Abstract The optically stimulated luminescence (OSL) from heated natural quartz has been investigated using the linear modulation technique (LMT), in which the excitation light intensity is increased linearly during stimulation. In contrast to conventional stimulation, which usually produces a monotonically decreasing signal, linearly increasing the stimulation power gives peaks in the signal as a function of time. In cases where the OSL signal contains more than one component, the linear increase in power of the stimulation light may result in a curve containing overlapping peaks, where the most easily stimulated component occurs at a shorter time. This allows the separation of the overlapping OSL components, which are assumed to originate from di erent traps. The LM-OSL curve from quartz shows an initial peak followed by a broad one. Deconvolution using curve tting has shown that the composite OSL curve from quartz can be approximated well by using a linear combination of rst-order peaks. In addition to the three known components, i.e. fast, medium and slow components from continuous-wave-osl studies, an additional slow component is also identi ed for the rst time. The dose responses and thermal stabilities of the various components are also studied Elsevier Science Ltd. All rights reserved. Keywords: Optically stimulated luminescence; OSC; Luminescence dating; Quartz; LM-OSL 1. Introduction The conventional technique for measuring optically stimulated luminescence (OSL) uses continuous-wave (cw) stimulation where the prompt luminescence emission is recorded during constant power excitation. A decay of luminescence with time is usually observed; the intensity is a function of the ionising radiation * Corresponding author. Tel.: ; fax: address: lars.boetter@risoe.dk (L. Bùtter-Jensen). energy imparted to the material, i.e. the radiation dose. Recently, a new technique for measuring OSL has been introduced (Bulur, 1996). This is based on the linear increase of the stimulation light intensity from zero to a maximum value during readout (the Linear Modulation Ð LM) technique. Unlike the decay curves obtained with the conventional OSL, the linear modulation gives luminescence curves containing peaks. Assuming a simple trap model which considers a single trap, a single recombination centre, and quasiequilibrium conditions (e.g. Chen and McKeever, 1997), one may obtain analytical expressions for the expected LM-OSL curve. The parameters of the lumi /00/$ - see front matter Elsevier Science Ltd. All rights reserved. PII: S (00)

2 408 E. Bulur et al. / Radiation Measurements 32 (2000) 407±411 nescence curve (the peak height L max and the peak position t max ) are related to the physical parameters of the trap involved in stimulated luminescence production, i.e. the number of trapped electrons (n 0 ) and the photoionisation cross-section a of the electrons. The technique was originally tested on geological sediments containing feldspars and later applied to ZnS and SrS based IR-stimulable storage phosphors (Bulur and GoÈ ksu, 1997). More recently, it has been used to investigate the dose and temperature dependent behaviour of IR-stimulated luminescence from K and Na feldspars (Bulur and GoÈ ksu, 1999). The introduction of blue LED stimulation light sources (with the possibility of ramping the stimulation light intensity) has opened the possibility of applying linear modulation techniques to other materials, such as quartz, which can only be e ectively stimulated by shorter wavelengths (Bùtter-Jensen et al., 1999a). In cases where more than one trap is involved in OSL production, as is the case for many materials, the conventional decay curve measured using cw stimulation will result in a decay curve that contains several components usually poorly resolved. In contrast, linearly increasing the stimulation power during stimulation tends to separate the individual OSL components from di erent traps as a function of time, with those most easily stimulated being measured rst. This leads to the recording of the signals as overlapping peaks. In other words, the LM-OSL gives a luminescence curve, which shows a spectrum of the net detrapping rates; these are proportional to the photoionisation cross-sections. This property can give new insights into OSL production mechanisms by visually separating the individual components making up the measured OSL curve. Quartz is one of the most useful materials for luminescence dating and retrospective dosimetry. It shows a rather complicated OSL decay curve where di erent components have been identi ed as easy-to-bleach and hard-to-bleach (Aitken, 1998). Bailey et al. (1997) have suggested that the OSL decay curve from quartz can be approximated using three rst-order decay components, and recently, a non- rst-order behaviour of the slow component was reported (Bailey, 1998). These components were referred to as the ``fast'', ``medium'' and ``slow'' components according to their respective detrapping rates. In a more recent study, Bùtter-Jensen et al. (1999a) reported that the LM-OSL signal from quartz has a very sharp peak occurring at the initial stages of the LM-OSL curve which is attributed to the easy-to-bleach component in cw-osl decay curve. The slowly decaying component has also been identi ed as an increasing tail in the LM-OSL curve. Bùtter-Jensen et al. (1999a) were also unambiguously able to identify the role played by the 1108C TL trap in the OSL production. Recently, LM-OSL from quartz was investigated to study partial bleaching (Agersnap Larsen et al., 2000), and the behaviour of the characteristics of the LM-OSL signal from quartz samples of di erent origin were investigated by Kuhns et al. (2000). In the present work, we investigate further the applicability of the linearly modulated stimulation of quartz. The various components making up the LM- OSL signal are examined as a function of dose and prior thermal treatments. 2. Experimental details Luminescence measurements were carried out using a Risù TLDA 15 reader equipped with an excitation unit based on blue light emitting diodes (Bùtter-Jensen et al., 1999a, 2000) delivering 025 mw/cm 2 to the sample, with the possibility of linearly ramping the stimulation light intensity. Luminescence emission was measured using two U-340 lters (each 2.5 mm thick) Fig. 1. (a) LM-OSL curves from quartz immediately after irradiation are measured at 25 and 1258C. (b) Repeated readings of the LM-OSL. OSL was measured at 1608C after preheating at 2808C for 10 s. No heating between the measurements.

3 E. Bulur et al. / Radiation Measurements 32 (2000) 407± in front of the PM tube, one being coated with metal oxide to suppress the transmission above 720 nm (Bùtter-Jensen et al., 1999b). Irradiations were performed at room temperature using a 90 Sr= 90 Y source (dose rate 25 mgy/s). Ten milligrams of heated quartz grains (90±140 mm) was placed in nickel cups for measurements. Samples were sensitised by irradiating (to 25 Gy), and heating to high temperature (5508C) several times to obtain a stable and reproducible OSL signal. The sensitivity changes have been monitored by reading the LM-OSL signal for 3600 s, to ensure that the whole luminescence curve has been stabilised. 3. Results and discussions 3.1. The LM-OSL signal from quartz Typical LM-OSL curves from this sample measured at 25 and 1258C are shown in Fig. 1a. The sample was irradiated to 25 Gy and OSL measured by increasing the stimulation light power from zero to the maximum power (25 mw/cm 2 ) over a period of 3600 s. The LM- OSL curve contains a sharp peak in the initial stages, which is followed by an intermediate broad component (at 258C) and then a broad signal occurring at longer times. The sharp peak in the initial stages is identi ed as the easy-to-bleach or fast decaying component of the quartz cw-osl curve. The intermediate broad component was originally reported by Bùtter-Jensen et al. (1999a). Measurements done at various readout temperatures indicated that this component is related to the shallow trap(s) contributing to 1108C peak in the quartz TL glow curve. Comparison of the LM- OSL curves measured at 25 and 1258C (Fig. 1a) support this conclusion. The most slowly decaying part of the LM-OSL curve appears here as a broad peak. This component is usually seen as an almost constant background in cw-osl when recorded for the short times employed to monitor the fast and medium components. This signal was observed by Bùtter-Jensen et al. (1999a) as an increasing background since both the observation time (800 s) and the maximum stimulation power (15 mw/cm 2 ) were not su cient to completely empty the associated traps. The enhancement of the blue LED unit to reach higher stimulation powers (25 mw/cm 2 ) and extending the measurement period has allowed the observation of the maximum in the slowly decaying component for the rst time. The above LM-OSL experiments suggest that it is possible to stimulate completely the slowly decaying part of the OSL signal using blue light. In order to investigate this further, the optical bleaching of the slowly decaying component has been examined by repeatedly measuring the LM-OSL from an irradiated aliquot. The sample was preannealed after the 25 Gy irradiation by heating to 2808C for 10 s. LM-OSL curves were recorded at 1608C by ramping the stimulation light power from 0 to 25 mw/cm 2 over 3600 s. In Fig. 1b, LM-OSL curves obtained over seven successive measurements are shown. The rst measurement was su cient to completely deplete the easy-tobleach initial part of the LM-OSL curve. However, the broad, slowly decaying signal located at longer times in the LM-OSL curve was only partially emptied. Successive measurements show a broad, slowly decaying peak which is progressively eroded. The broad peak has two local maxima (marked with arrows) implying that the slowly decaying component is not a simple trap, but a composite one, which will be discussed later Components underlying the LM-OSL curve Smith and Rhodes (1994) and Bailey et al. (1997) have shown that the cw-osl curve can be approximated using a linear combination of three exponential decay functions. One of the disadvantages of deconvolution of the cw-osl data is the weighting of the data when tting; the initial part of the decay curve may be several orders of magnitudes larger than the slow tail. LM-OSL does not seem to su er from such a problem. Curve tting studies have shown that the LM-OSL curve from quartz can be approximated using a linear combination of four rst-order components of the form (Bulur, 1996) L t ˆ Ab t P exp bt 2 1 2P Fig. 2. LM-OSL curve from quartz. The various components are obtained by curve tting. OSL was measured at 1608C for 7200 s after preheating at 2808C for 10 s.

4 410 E. Bulur et al. / Radiation Measurements 32 (2000) 407±411 where A is the amplitude which is proportional to the trap population n 0, b is proportional to the photoionisation cross-section a and the maximum stimulation light intensity I 0 b ˆ ai 0, and P is the total observation time. In Fig. 2, a typical LM-OSL curve and its components are shown. The LM-OSL curve has been approximated successfully using Eq. (1), with the residuals well within an error band of 25% (calculated using 100 I exp I fit =I exp ). The rst two components lie under the initial peak and correspond to the socalled ``fast and medium components'' (Bailey et al., 1997; Bailey, 1998); they are labelled as peak A and B in Fig. 2. At higher stimulation powers, two slow components are observed (peaks C and D). The relative contributions of the individual components to the total luminescence intensity, as deduced by comparing the integrated intensities of the tted peaks, are 7.2, 1.3, 7.2 and 84% for peaks A, B, C and D, respectively. The radiation dose response and the thermal stability of a signal are important for a signal to be useful in radiation dosimetry and dating. These characteristics are discussed in the following two sections Dose response The radiation dose responses of each component, determined by curve tting, was measured in the range 1.25±640 Gy using a single aliquot of quartz. LM-OSL was measured at 1608C for 3600 s after preheating at 2808C for 10 s following the irradiation. After each OSL measurement, the sample was thermally drained by reading the TL (5508C). The cumulative sensitivity changes of the components Ð as determined using curve tting based on Eq. (1) Ð of the LM-OSL curve was monitored by repeating one of the low dose points and determined to be negligible. In Fig. 3, the dose responses of the amplitudes of components A±D are plotted against the radiation dose. The fast and medium components saturate at lower doses than the slower components; in these cases, the growth curves are approximated using a saturating exponential plus a linear function. The higher dose response of the slow components o ers the possible use of the slowly decaying part of the OSL curve from quartz for dating that may extend the upper limit of luminescence dating with quartz. In the inset to Fig. 3, some examples of LM-OSL curves obtained at various radiation doses are also shown. The peaks of LM-OSL curves measured at various doses show no detectable shift, and similar detrapping parameters (b ) were obtained from curve tting indicating the validity of the assumption of rst-order kinetics. The component C is initially masked by the tails of the fast components and the slow component D; as the radiation dose increases, it can be recognised more easily Thermal stability of the components The thermal stabilities of the four components have been studied by preheating an irradiated aliquot (25 Gy) to a temperature T followed by an LM-OSL measurement. A single aliquot has been used for all the measurements and the LM-OSL was measured at 1608C. The sample was annealed by reading the TL (5508C) following each OSL measurement. In Fig. 4, the amplitudes of the four components obtained by Fig. 3. Dose responses of the amplitudes of the components of the LM-OSL from quartz. LM-OSL was measured at 1608C after preheating at 2808C for 10 s. Inset: LM-OSL curves obtained at various radiation doses: 1.25,10, 80 and 640 Gy from bottom to top. Fig. 4. Pulse annealing of the LM-OSL curve. The amplitudes of the four components are determined by curve tting based on Eq. (1).

5 E. Bulur et al. / Radiation Measurements 32 (2000) 407± curve tting (using Eq. (1)) are plotted as a function of preannealing temperature. The rapidly bleaching component in the OSL curve of quartz has been the subject of many studies which have shown that it is related to the 3258C TL peak in quartz TL glow curve (see Wintle and Murray, 2000 for a recent summary). The sharp decay of the component A in the temperature region between 300 and 3408C con rms that this component is related to the 3258C TL peak. The medium component (component B) starts decaying at 03008C, which continues slowly at 04008C. The pulse-annealing curve of component C shows a structured, rather than a simple, decay where two shoulders (one near 3508C and the other near 4008C) can easily be identi ed. Component D started decaying near 2708C and can be removed completely after preheating at 4808C with a structure showing two broad shoulders at 0300 and 04008C, respectively. Preheating to temperatures >4808C completely removes the OSL signal. This observation disagrees with the measurements made by Bailey et al. (1997), who found that the slow component is stable up to 6508C. The di erences in dose response and thermal stabilities of the four components imply that di erent traps are involved in the OSL process; this is the subject of further study. 4. Summary and conclusions Linearly increasing the stimulation light intensity during measurement has provided a technique, which readily allows discrimination between the various components of the OSL signal. We have shown that the LM-OSL signal from our quartz sample contains a minimum of four components each with di erent thermal stability and bleaching rate. The correct identi cation and characterisation of these components need a more detailed investigation, which should include spectrometric studies. The slow components (C and D) have a number of properties, such as high saturation dose and relatively high thermal stability, which make them possible candidates for use in age determination. As Bailey et al. (1997) have suggested, this may extend the usable range of quartz for luminescence beyond the limits of the rapidly bleaching component. The slowest component (D) makes the greatest contribution (84%) to the total luminescence output, the components A and C have equal contributions of about 7%, and the medium component (B) contributes only about 1%. The large relative importance of component D in the total luminescence output also warrants further investigation. By linearly modulating the stimulation intensity during a luminescence measurement, it has proved to be possible to recognise four components in the OSL signal. Linear modulation provides an important new approach in the analysis of OSL signals, particularly for the study of the characteristics of the minor components in a stimulation curve. References Agersnap Larsen, N., Bulur, E., McKeever, S.W.S., Use of LM-OSL for the detection of partial bleaching in quartz and feldspar. Radiat. Meas., 32, 419±425. Aitken, M.J., An Introduction to Optical Dating: The Dating of Quaternary Sediments by the Use of Photon- Stimulated Luminescence. Oxford Science Publications, Oxford. Bailey, R.M., The form of the optically stimulated luminescence signal of quartz: implications for dating. Unpublished Ph.D. Thesis, University of London. Bailey, R.M., Smith, B.W., Rhodes, E.J., Partial bleaching and decay form characteristics of quartz OSL. Radiat. Meas. 27, 123±136. Bùtter-Jensen, L., Bulur, E., Duller, G.A.T., Murray, A.S., Advances in luminescence measurement systems. Radiat. Meas., 32, 523±528. Bùtter-Jensen, L., Duller, G.A.T., Murray, A.S., Banerjee, D., 1999a. Blue light emitting diodes for optical stimulation of quartz in retrospective dosimetry and dating. Radiat. Prot. Dosim. 84, 335±340. Bùtter-Jensen, L., Murray, A.S., Banerjee, D., 1999b. A new light on OSL. Quaternary Geochronology 18, 303±309. Bulur, E., An alternative technique for optically stimulated luminescence (OSL) experiment. Radiat. Meas. 26, 701±709. Bulur, E., GoÈ ksu, H.Y., IR stimulated luminescence from ZnS and SrS based storage phosphors: a re-examination using the linear modulation technique. Phys. Stat. Sol. A161, R9±R10. Bulur, E., GoÈ ksu, H.Y., IR-stimulated luminescence from feldspars with linearly increasing excitation light intensity. Radiat. Meas. 30, 505±512. Chen, R., McKeever, S.W.S., Theory of Thermoluminescence and Related Phenomena. World Scienti c, Singapore. Kuhns, C.K., Agersnap Larsen, N., McKeever, S.W.S., Characteristics of LM-OSL from several di erent types of quartz. Radiat. Meas., 32, 413±418. Smith, B.W., Rhodes, E.J., Charge movements in quartz and their relevance to optical dating. Radiat. Meas. 23, 329±333. Wintle, A., Murray, S.A., Quartz OSL: e ect of thermal treatments and their relavance to laboratory dating procedures. Radiat. Meas., 32, 387±400.

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