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The influence of optical bleaching on lifetimes and luminescence intensity in the slow component of optically stimulated luminescence of natural quartz from Nigeria
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Journal of Luminescence 128 (2008) 1561–1569 www.elsevier.com/locate/jlumin
The influence of optical bleaching on lifetimes and luminescence intensity in the slow component of optically stimulated luminescence of natural quartz from Nigeria F.O. Ogundarea,, M.L. Chithambob a
Department of Physics, University of Ibadan, Ibadan, Oyo State, Ibadan, Nigeria Department of Physics and Electronics, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
b
Received 21 August 2007; received in revised form 23 February 2008; accepted 3 March 2008 Available online 12 March 2008
Abstract The intensity of optically stimulated luminescence may be decreased to a slow or medium component of its decay curve by optical bleaching, that is, by prolonged exposure of the luminescent sample to stimulating light. In this paper, we report on the influence of irradiation and measurement temperature on luminescence lifetimes as well as on the effect of measurement temperature on luminescence intensity in annealed natural quartz from Nigeria. Measurements were carried out in the slow component region using time-resolved optical stimulation at 470 nm on samples annealed at 500 and 600 1C. Luminescence lifetimes were determined from the resultant timeresolved luminescence spectra by analysing the portion of each spectrum after the stimulating light pulse of duration 11 ms. In preparatory tests, the influence of the duration of optical bleaching on lifetimes was investigated. It was found that lifetimes in samples annealed at 500 1C are independent of the duration of optical bleaching, whereas lifetimes in quartz annealed at 600 1C are affected, decreasing towards a constant value with duration of bleaching. Concerning measurements in the slow-component region, lifetimes were found to decrease with irradiation dose for samples annealed at either 500 or 600 1C. The temperature dependence of lifetimes in both sets of quartz is similar with lifetimes constant at about 36 ms between 20 and 120 1C, but decreasing consistently from then on to about 5 ms at 200 1C, the maximum measurement temperature used in experiments. The luminescence intensity was observed to typically go through a peak as the stimulation temperature was increased from 20 to 200 1C, following a brief initial decrease, a change better exemplified in the quartz annealed at 600 1C. The initial decrease in luminescence intensity is attributed to the dominance of optical stimulation over thermal stimulation. On the other hand, the subsequent change of luminescence intensity with temperature is discussed as evidence of thermal assistance to optical stimulation, initially with activation energy of 0.2770.07 eV and of thermal quenching subsequently with activation energy equal to 0.9370.23 eV for samples annealed at either 500 or 600 1C. The temperature dependence of lifetimes is explained as showing increased thermal effect on lifetimes with activation energy values within 0.8370.01 eV. On the other hand, the influence of irradiation on lifetimes is accounted for in terms of an energy band model for quartz consisting of three luminescence centres and one non-radiative recombination centre. r 2008 Elsevier B.V. All rights reserved. PACS: 39.90; 78.47; 91.60.M; 78.60 Keywords: Time-resolved luminescence; Quartz; Lifetimes; Bleaching; Slow component
1. Introduction Quartz is a natural mineral that is widely used for the determination of archaeological and geological age using Corresponding author. Tel.: +234 8050397616.
E-mail address: [email protected] (F.O. Ogundare). 0022-2313/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.03.001
luminescence [1–3]. The common techniques used for dating are thermoluminescence (TL) and optically stimulated luminescence (OSL). Thermoluminescence involves stimulation of luminescence from a previously irradiated sample by controlled heating, whereas OSL is obtained when the sample is exposed to light. Once the dose rate, a measure of the rate at which energy is absorbed by quartz
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grains from environmental radiation, and the equivalent dose, the total dose from the environmental radiation following deposition of the quartz grains are known, the age can be calculated as age=(equivalent dose)/dose rate. There are two possible methods for determining the equivalent dose using OSL. One is the regeneration method and the other is the additive dose method [3]. Both alternatives involve several laboratory procedures including irradiation, optical bleaching and preheating of a sample [4]. Any of these procedures may affect the OSL and in turn, the equivalent dose and hence the age determined. Previous measurements from sedimentary quartz of continuous-wave optically stimulated luminescence (CW-OSL) in which the luminescence is recorded simultaneously with stimulating light to produce a decay curve, showed the presence of at least three distinct components in the decay curve [6–8]. For convenience, the components were referred to as fast, medium or slow [8], and are identified as separate exponential terms in the timedependent change of the OSL intensity, otherwise called the decay curve. On the other hand, if the luminescence is measured using linear modulation where the intensity of stimulating light is increased linearly during measurement [9], the various components appear as separate peaks in a plot of luminescence intensity against time. As discussed by Chithambo and Galloway [10], each of the components in a CW-OSL decay curve may not necessarily be associated with a distinct physical mechanism, with the method of multi-exponential fitting only providing a means to conveniently represent the shape of a decay curve. In contrast, linearly modulated luminescence (LM-OSL) enables donor defects with different photoionization cross-section to be reliably and unambiguously identified as separate peaks [11]. The accuracy of the equivalent dose and hence the age depends on the proportion of each of the components in the portion of the decay curve used in the calculation. For example, if the fast component was zeroed by sunlight, but the slow component was not, the resulting equivalent dose may be inaccurate [12,13] because then the contribution from components other than the fast one will be significant. It is therefore necessary to understand the general characteristics of different OSL components in quartz and on this aspect, Jain et al. [11] used LM-OSL to investigate the influence on OSL components of (a) sensitization (b) thermal stability (c) infrared sensitivity and (d) recuperation. Based on their results, Jain and coworkers [11] concluded that the various OSL components do not undergo identical sensitivity changes and that this characteristic could therefore lead to inaccurate estimates of the equivalent dose used in dating calculations. Chithambo and Galloway [10] using time-resolved optically stimulated luminescence (TR-OSL) measurements, in which the stimulation and emission of luminescence is separated in time, calculated activation energies associated with thermal assistance and thermal quenching in the slowcomponent region of CW-OSL from quartz and demonstrated that the two effects are separable. This work was
extended to include effects of measurement temperature and irradiation dose on lifetimes and luminescence intensity in the various components of the CW-OSL decay curve in quartz [14,15]. Time-resolved luminescence spectra to be described in this paper should not be confused with decay curves obtained in continuous optical stimulation. A decay curve shows the time dependence of luminescence intensity, whereas a time-resolved luminescence spectrum displays the delay between stimulation and emission of luminescence. Besides the studies by Chithambo and Galloway [10] and Chithambo [14,15], the component-resolved study of lifetimes and luminescence intensity using time-resolved luminescence has otherwise remained largely unexplored. Indeed, results of Chithambo [15] showed that lifetimes decrease slightly with the duration of pre-measurement optical bleaching, suggested a need to further investigate and understand the effect. This work therefore reports detailed studies concerned with the dependence of lifetimes (the delay between stimulation and emission of luminescence) as well as the luminescence intensity on measurement temperature and irradiation dose in the slow component region of optically stimulated luminescence from annealed natural quartz from Nigeria. This is preceded by a report on preparatory investigations concerned with the influence on lifetimes of optical bleaching used to reduce the luminescence intensity to the slowcomponent region. The overall aim of these studies is to better understand the physical processes of luminescence in quartz. 2. Experimental details The material used in all measurements was sedimentary quartz, grain size 90–180 mm, collected from Oro, Kwara state, in the west central region of Nigeria. The sample was analyzed for feldspar, which usually occurs concurrently with natural quartz, using a JEOL superprobe 733 electron microprobe (JEOL company, Japan) as in a previous study using the same material [16]. The analysis determined the percentage of oxides of Si, Al, Ca, Na and K in the sample with a lower limit of detection (LLD) in each case of 0.017, 0.013, 0.012, 0.018 and 0.013, respectively. In this method, any oxide with detection value less than its LLD is not detected by the microprobe. The analysis showed that oxides of Al, Ca, Na and K were not detected which indicated that the feldspar content of the sample was negligible. The quartz was thus used without any chemical treatment required for removal of feldspar as is usual in OSL experiments [3,17]. Prior to use, samples were annealed at 500 and 600 1C for 10 min in an oven (Gallenkamp Muffle furnace), and then immediately cooled in air thereafter. The samples were annealed to remove any residual natural OSL signal and to enhance the sensitivity of the quartz to optical stimulation [3,10,14,15], as well as to provide annealed samples for the study.
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Measurements of time-resolved luminescence spectra for this paper were made using multichannel scaling, an established technique for time-correlated photon counting experiments reviewed in many texts example.g. Ref. [18–20]. In the method, a multichannel scaler is used to acquire data corresponding to the number of events that occur during a time interval ti to ti+Dt as a function of time where the time-gap Dt is known as the dwell time. In the context of this work, an event would be the detection of a luminescence photon. The total measurement time t otherwise called the dynamic range is subdivided into an integral number of channels nDt, where n is the number of specific channels. When a measurement is started, data are acquired sequentially, channel to channel from the first channel until all selected channels have been addressed, that is, until the dynamic range has been covered or until the scaler has scanned or swept through the preselected number n of channels. For purposes of experimental expediency, for example, to reduce scatter in data points, it is usual to obtain data over multiple sweeps. The timeresolved luminescence spectrum generated in this way is a plot of cumulative photon counting rates against time. Detailed features of such instrumentation for measurement of time-resolved luminescence from quartz have been presented by Chithambo and Galloway [21–23]. Time-resolved luminescence spectra for this paper were recorded on an LED pulsing system modified from that described by Chithambo and Galloway [21]. In its present configuration, the system uses a multichannel scaler (ORTEC MCS-plusTM) to simultaneously initiate a sweep and to trigger a set of LEDs via a multivibrator, an integrated circuit 2N74221. Luminescence is stimulated at pulse-widths set by the timing components on the multivibrator. In this study, each time-resolved luminescence spectrum was measured over a dynamic range of 600 ms at a dwell time of 2 ms and 200,000 sweeps. Luminescence was stimulated by a set of four 470 nm blue LEDs (Nichia NSPB-500) each operated at a pulse width of 11 ms. The mean light intensity at the sample was 0.60 mW/cm2 as determined by a PIN diode (Ealing Electro-optics). An EMI 9635QA photomultiplier (Electron tubes Ltd, UK) with a combination of BG-39 and UG-11 filters in front was used for the detection of luminescence. Each LED has a 2 mm thick GG420 filter in front to attenuate the shorter wavelength end of the LED spectrum. The measurement procedure is such that the luminescence increases progressively over photomultiplier noise and constant scatter from luminescence stimulating light during stimulation and then decreases in time over photomultiplier noise only after the stimulating light pulse as has been illustrated on a number of occasions for natural quartz [14,21,24,25] and for synthetic quartz [26]. Luminescence lifetimes t were extracted from timeresolved luminescence spectra by fitting exponential functions of the form LðtÞ ¼ B þ A expðt=tÞ
(1)
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to the portion of each spectrum after the stimulation light pulse where L(t) is the luminescence counting rate, t is time, A is a scaling factor and B is a constant added to account for the background. The justification for the method is that if luminescence is pulse stimulated from a charge trap with an initial electron population n0, with a probability for stimulation per unit time p, and if l is the probability that a stimulated electron will produce luminescence, then the rate of emission of luminescence l(t) after the light pulse is given by lðtÞ ¼ pno ½expðlt1 Þ 1 expðltÞ
(2)
where t1 is the pulse width and t ¼ 1/l [27]. It should be noted that although Eqs. (1) and (2) describe the evaluation of lifetimes from the portion of a time-resolved luminescence spectrum following the light pulse, the lifetimes may also be found from the portion during the light pulse [27]. The relevant formulae for the latter case as well as general theoretical models for the analysis of time-resolved luminescence spectra including computer-simulated and experimental aspects have been reported recently by Chithambo [24,27]. The methods used for evaluating lifetimes and various kinetic parameters from quartz experimental data [10,14–16,22] thus discussed, are based on a simple model consisting of one electron trap and one kind of recombination centre with emission involving electron transition through the conduction band just like in discussions of optically stimulated luminescence by McKeever and Chen [28] and Chen and Leung [29]. However, in typical timeresolved optical stimulation measurements, the stimulating light pulse is brief (of the order of nanoseconds to several microseconds) such that the reduction in trapped charge due to optical stimulation in that time is negligible and as a result retrapping is also assumed to be negligible [27]. Analytical formulae developed on this basis as well as computer-simulated data on the same assumptions are consistent with experimental results where available, for example data drawn from pulsed luminescence from quartz and a-Al2O3:C [27]. The influence of retrapping on pulsed luminescence from quartz was considered in a preliminary study by Chithambo and Galloway [30], who suggested that a decrease in lifetimes as the temperature of measurement was increased was due to a reduction in retrapping from shallow traps. In a subsequent investigation [10], the concomitant temperature dependence of lifetimes evaluated from time-resolved luminescence spectra and the intensity of luminescence integrated from the same spectra showed that the temperature dependence of lifetimes and luminescence intensity could be properly explained on the basis of the Mott–Seitz model of thermal quenching and assumption of negligible retrapping. The influence of retrapping on pulsed luminescence in general was also considered by Chen and Leung [29] whose theoretical model on luminescence intensity predicted that if retrapping were dominant, the growth curve, that is, the dependence of the luminescence intensity
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on dose should change from being linear initially to subsequently showing nonlinear behaviour such as quadratic dose dependence in some cases. However, experimental cases of obvious nonnegligible retrapping on time-resolved luminescence from quartz remain to be examined. In view of the discussions above, all calculations in this paper are based on the assumption of negligible retrapping as discussed previously [10,24,27]. Fig. 1 shows an example of a time-resolved luminescence spectrum measured from a sample of quartz dosed up to 257 Gy. The solid line through data points is the best fit of Eq. (3) which in this example gave a lifetime value of 41.8570.40 ms. The first set of measurements concerned the dependence of luminescence lifetimes on the duration of optical bleaching. For this experiment, thirty aliquots were prepared and then separated into 10 groups each containing three aliquots. Each sample, consisting of a few milligrams of quartz on a 10 mm aluminium sample disc, was irradiated to a dose of 343 Gy using a 90Sr/90Y beta source. Each irradiated sample in every group was then bleached using a set of 16 continuously operated 470 nm blue LEDs. A different duration of bleaching was used for each group giving in effect 10 different bleaching duration tests. Measurements of time-resolved luminescence spectra corresponding in each case to a given duration of bleaching were then carried out on each of the three aliquots in each group. The second set of measurements was on the dependence of luminescence lifetimes and luminescence intensity on irradiation dose and measurement temperature in the slow component region of luminescence, optically stimulated from quartz annealed at 500 and 600 1C. Unless otherwise specified, samples were irradiated to 343 Gy and then exposed to stimulating light for 2500 s at 25 1C thereby
reducing the intensity to about 5% of the peak intensity, that is, to the slow-component region. Time-resolved luminescence measurements were made without preheating, a usual procedure in CW-OSL experiments [17]. As in previous studies [16,25,26,31], the preheating could be omitted because the spontaneously emitted phosphorescence that follows irradiation is not correlated in time with stimulation light pulses, but simply adds a level background to the time-resolved luminescence spectrum and so does not influence the value of the lifetime [22]. 3. Results and discussion 3.1. Dependence of luminescence lifetimes on the duration of pre-measurement optical bleaching Fig. 2 shows the change of luminescence lifetimes with the duration of optical bleaching in quartz annealed at 500 and 600 1C. The lifetimes presented in Fig. 2 are each the average from three measurements. The lifetime of the sample annealed at 500 1C is constant with an average value of 46.870.6 ms throughout the measurement period. In contrast, the lifetimes determined from the sample annealed at 600 1C decrease from 41.470.2 to 38.570.3 ms when the duration of optical bleaching is increased from 250 to 1500 s. Thereafter the lifetimes are constant at an average value of 38.570.3 ms. The results in Fig. 2 for the sample annealed at 500 1C can be compared with similar measurements carried out on unannealed sedimentary quartz from the UK (hereafter referred to as BDH quartz) as well as subsamples from the same BDH quartz annealed at 500 1C where the lifetimes were also independent of the duration of optical bleaching [32]. In a latter study (also on BDH quartz) concerned with the link between thermal influence on lifetimes and
5500 48
5000
46 500°C
4000 Lifetime (µs)
Intensity (a.u.)
4500
3500 3000
600°C
44
42
2500 40 2000 38
1500 0
100
200
400 300 Time (µs)
500
600
Fig. 1. A time-resolved luminescence spectrum measured from a quartz sample annealed at 500 1C and given a dose of 257 Gy prior to measurement at room temperature. The data after the stimulation pulse are shown fitted by Eq. (1).
0
500
1000 1500 2000 Pre-measurement bleaching time (s)
2500
Fig. 2. Dependence of lifetimes on the duration of pre-measurement optical bleaching. The samples were irradiated to a dose of 343 Gy before the optical bleaching.
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bleaching of luminescence, Chithambo [15] showed that lifetimes were independent of duration of pre-measurement of optical bleaching for sedimentary quartz annealed at 600 and 800 1C, but that the lifetimes decreased with the duration of bleaching in samples annealed at 1000 1C. The general feature inferred from results of Fig. 2 and those on BDH quartz [10,15] is that the dependence of lifetimes on the duration of optical bleaching differs on either side of the appropriate phase transition temperature in quartz. In the quartz used in this work, the dependence of lifetimes on the duration of optical bleaching is different, on either side of 573 1C, the first phase transition temperature, whereas in the sedimentary quartz of Chithambo et al. [10,15] the same can be said for the second phase transition temperature at 873 1C. A preliminary explanation of this effect would be to assume that the structural change that occurs during phase transition may affect the spatial link between traps and luminescence centres as was pointed out in a similar case by Raymond et al. [33] in their work on the thermoluminescence spectra of doped Bi4Ge3O12. In order to explain the dependence of luminescence lifetimes on the duration of optical bleaching, we use the band model for quartz [34,35] as adapted for time-resolved optically stimulated luminescence by Galloway [22]. One of the main features of this model is that annealing causes a transfer of holes from a non-radiative recombination centre to luminescence centres as well as between the luminescence centres. An energy-band schematic diagram of the model developed by Galloway [22] is shown in Fig. 3 and consists of three luminescence centres (labelled LH, LL and LS), one non-radiative recombination centre (LN) and three electron traps (1,2,3). The luminescence centres LH, LL and LS are associated with lifetimes tH, tL and tS, respectively, where tH4tL4tS. Conduction band e e
1
e
2
3
LS LL
LH
LN
Valence band Fig. 3. The energy band model used in the text to explain the dependence of luminescence lifetimes on the duration of optical bleaching and irradiation in quartz. The diagram shows three luminescence centres, LH, LL and LS with which lifetimes tH, tL and tS are associated. LN is a nonradiative recombination centre. Electron traps are denoted 1, 2 and 3 and stand for shallow traps, optically sensitive traps and optically inaccessible deep traps, respectively.
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With reference to Fig. 3, we assume that annealing at 500 1C causes a transfer of holes from centre LH to LL and LS centres, but that much of the transfer is to the LL centre. This is a similar assumption as that made by Bøtter-Jensen et al. [35] where the preferential capture of holes by nonradiative recombination centres rather than radiative ones was required to sufficiently account for sensitivity changes caused by annealing in quartz. The result of the inter-centre hole transfer due to annealing at 500 1C is that the luminescence from the LL centre dominates the measured signal in samples annealed at this temperature (500 1C) such that the lifetime associated with the LL centre becomes the characteristic lifetime of the sample annealed at 500 1C. The lifetime associated with the quartz annealed at 500 1C which is independent of the duration of bleaching as seen in Fig. 2 may therefore be denoted tL. Therefore a numerical value associated with tL, taken as the mean of the ten lifetimes plotted in Fig. 2 for the sample annealed at 500 1C is 46.870.6 ms. The dependence of lifetimes on the duration of bleaching in the sample annealed at 600 1C can be understood on the basis of two assumptions, which also involve preferential capture of charge. The aim is to first explain the reason lifetimes in the quartz annealed at 500 1C are less than those in the quartz annealed at 600 1C and then secondly to account for the dependence of lifetimes on the duration of optical bleaching. On the first point, concerning the difference in lifetime values we assume that in this case the net number of holes transferred to luminescence centre LS from other centres is comparatively greater than that transferred to LL centres from other centers. Now, for quartz annealed at this temperature, contributions to the measured luminescence from both LL and LS centres are significant. In turn, lifetimes consist of contributions from both tL and tS where tS is less than tL. Thus overall the effective lifetime measured from the quartz annealed at 600 1C should be less than that measured from the quartz annealed at 500 1C in which the lifetime tL is dominant. Concerning the dependence of lifetimes on the duration of optical bleaching, we assume that the recombination probability of electrons with holes in centre LL is greater than for recombination in the LS centre. This means that during optical bleaching the percentage of holes lost to electron recombination to produce luminescence is higher for the LL centre than for the LS centre. Therefore the luminescence contribution from LS centres relative to that from LL centres increases with optical stimulation time. The increase in signal from LS centres is thus manifested as the decrease of measured lifetime between 250 and 1500 s as seen in Fig. 2. Beyond 1500 s of optical bleaching, lifetimes become constant because the luminescence from centres LL becomes negligible. Thus the measured mean lifetime approaches tS with a mean value of 38.570.4 ms. Note that it is not luminescence from LS that becomes negligible because if this were the case, the measured mean lifetime would have tended to increase towards the larger value of tL.
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3.2. The slow component
3.2.2. Dependence of lifetimes on radiation dose for luminescence in the slow-component region Fig. 5 shows the dependence of lifetimes on beta dose for quartz annealed at 500 1C and in the inset at 600 1C. In
Percentage intensity (%)
80 100 80
60 60 40
40
20
20
0 0
20 40 60 80 100 120 140 160 180 200
0 0
500
1000 1500 2000 Stimulation time (s)
2500
3000
Fig. 4. The relative importance of components of a decay curve as a function of stimulation time for quartz sample irradiated to a dose of 172 Gy. The inset compares the relative values of the fast, medium and slow components for the first 200 s. 40
38
48 46
36
44 Lifetime (µs)
3.2.1. Determination of the slow-component region The slow component was identified by fitting a sum of three exponential functions each of form exp(lit) to a decay curve and noting the component with the slowest decay parameter l Such a fit for a decay curve of a sample irradiated to 172 Gy gave l1, l2 and l3 as 0.1441, 0.0024 and 0.0006 s1 respectively. In this case, the slow compo, nent is the one with the decay parameter of 0.0006 s1. A one-to-one comparison of these decay parameters with published data may not be that illustrative since the luminescence decay rate is sensitive not only to the wavelength of stimulation, but also to a number of other factors including stimulation power at sample, temperature of measurement and charge concentration [3,36–38]. However, as a general indicator of decay rate values only, we note that the decay-rate 0.1441 s1 for the fast component in this work is comparable to the value of 0.1401 s1 for a CW-OSL fast component stimulated from quartz sediments using green light [39]. On the other hand, the values for decay rates obtained in this work for the medium and slow components differ from the corresponding values, that is, 0.0488 and 0.0020 s1 from the slow and medium components, respectively, of the latter material [39]. The difference may be attributed to some of the factors alluded to earlier. The relative importance of the fast, medium or slow component in a decay curve as a function of measurement time was determined as a fraction of the total measured signal as done previously by Chithambo and Galloway [10]. Fig. 4 shows the time-dependent contribution of each of the components to the luminescence intensity. The inset to Fig. 4 shows, for better clarity, the relative features for the first 200 s. Fig. 4 shows that in the initial transient (that is in the first 1 s) nearly 89% of the measured luminescence is due to the fast component with the slow and medium components contributing about 4% and 7%, respectively. As stimulation continues, the fast component decreases to a negligible value whereas the medium component increases in importance to about 57% in the first 50 s, but progressively decreases thereafter. On the other hand, the contribution from the slow component increases from about 4% at the start of the stimulation to be predominant by 2400 s. The measurements reported in the following sections were carried out in the slow-component region of luminescence from annealed natural quartz stimulated using pulsed 470 nm blue light. The results may be of interest to others interested in the application of luminescence from the slow-component region using optically stimulated luminescence methods.
fast medium slow
100
34
42 40
0
100
200
300
400
500
600
38 36 34 0
100
200
300 Dose (Gy)
400
500
600
Fig. 5. Dose dependence of luminescence lifetimes for quartz annealed at 500 1C and in the inset for the sample annealed at 600 1C and bleached to slow component region after irradiation. The luminescence lifetimes in the main graph as well as in the inset, each correspond to the same dose values.
both cases the lifetimes decreased with dose. In the sample annealed at 500 1C, lifetimes decreased from 47.070.8 to 36.670.5 ms when the beta dose was increased from 14 to 515 Gy. In the case of quartz annealed at 600 1C (inset), lifetimes change from 38.770.5 to 35.170.4 ms for beta doses up to about 200 Gy and then become constant thereafter. In comparison, lifetimes calculated from the same sample when not subjected to any optical bleaching were constant up to about 100 Gy from which dose the lifetime then decreased [16]. The dependence of lifetimes on beta dose shown in Fig. 5 can also be explained using the energy band model of
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3.2.3. Effect of measurement temperature on luminescence lifetimes in the slow-component region The influence of the temperature of the quartz during stimulation on the luminescence lifetime was investigated for quartz annealed at 500 and 600 1C. Samples were irradiated to a beta dose of 343 Gy before measurement. Time-resolved luminescence spectra were recorded at various temperatures between 20 and 200 1C on the same sample. The lifetimes presented in this section are averages of values from three separate measurements. The dependence of lifetimes on measurement temperature for samples annealed at 500 and 600 1C is shown in Fig. 6. It can be seen that for the sample annealed at 500 1C values of the lifetime between 20 and 120 1C are essentially constant at about 35.572.7 ms. From then on the lifetimes decrease to a minimum of 4.771.5 ms at 200 1C. The change of lifetimes with measurement temperature for the sample annealed at 600 1C is similar. In this case, the lifetimes are constant within 35.974.2 ms between 20 and 120 1C and decrease thereafter to 4.670.7 ms at 200 1C. The dependence of lifetimes on measurement temperature as shown in Fig. 6 is the evidence of thermal quenching effect on lifetimes t and is described on the basis of the Mott–Seitz configurational coordinate model [40] as a
55 50
500°C 600°C
45 40 Lifetime (µs)
Fig. 3. Firstly, the number of electron-hole pairs created by irradiation increases with the beta dose. In turn, the number of holes trapped by each of the three luminescence centres as well as the non-radiative recombination centre also increases. Again, as in the Bøtter-Jensen et al. [35] description of sensitization by preferential capture of holes by non-radiative recombination centres rather than radiative ones, we assume that during irradiation the LS centre has the highest hole capture probability followed by that of the LL and then the LH centre. Therefore increasing irradiation dose increases the number of holes in LS centres compared to those in LL centres in samples annealed at 500 1C as in quartz annealed at 600 1C. Now with regards to the measurements on the unbleached samples where lifetimes were constant with dose [16], it should be understood that the lifetimes were constant because of the dominance of luminescence from LL centres over that from LS centres. In contrast, the bleaching done to reduce the luminescence to the slow component in the present study significantly decreases the number of holes trapped in LL centres compared to those trapped in LS centres since the recombination, probability of electrons with holes for the LL centre is greater than that for the LS centre. This effect therefore means that the amount of luminescence from LS centers is greater than that from LL centers in the time-resolved luminescence spectrum used to determine the mean lifetime. This is seen as a decrease with the dose of mean lifetimes as shown in Fig. 5 since tS (where tSotL) corresponding to Ls luminescence increases in importance compared to tL associated with the LL centre.
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35 30 25 20 15 10 5 0 0
20
40
60 80 100 120 140 160 Measurement temperature (°C)
180
200
Fig. 6. The dependence of luminescence lifetimes on measurement temperature for samples irradiated to a dose of 343 Gy and then bleached to the slow component region before the measurement of time-resolved luminescence. The dotted line is the best fit of Eq. (3) to the data for the sample annealed at 500 1C (open cycles) and the solid line for the one annealed at 600 1C (open squares).
dependence on absolute temperature T as t¼
t0 1 þ CeDE=kT
(3)
where to is the lifetime at 0 K and DE is the activation energy for thermal quenching and C is a constant [10,22,24,41]. The same equation describes the temperature dependence of F centre luminescence from some materials for example, KCl [42] and a-Al2O3:C [43,44]. As reviewed elsewhere [14,19,22,24,43,44], from the Mott–Seitz model, the temperature-dependent distribution of lifetimes t is found to comprise radiative, phonon-assisted, and nonradiative contributions, that is, 1 1 _o DE ¼ þ g coth þ n exp (4) t t0 kT kT where to is the radiative lifetime at absolute zero of temperature, g is a constant, o is the phonon vibration frequency,_ is Planck’s constant, k is Boltzmann’s constant and DE and n are the activation energy and frequency factor for the non-radiative process. It should be noted that parameters to and DE have the same meaning as defined for Eq. (3). If the phonon-assisted process is negligible, the temperature-dependent distribution of the measured mean lifetimes as expressed as in Eq. (3) then emerges. The theoretical and experimental aspects of the analysis of time-resolved optically stimulated luminescence from quartz using Eq. (3) and related formulae has recently been discussed by Chithambo [24,27]. The data in Fig. 6 are shown fitted by Eq. (3). The values of DE determined from this fit are 0.8370.01 and 0.9370.23 eV for samples annealed at 500 and 600 1C, respectively. These values are comparable to 0.8370.01
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Table 1 A comparison of thermal quenching (DE) and thermal assistance (Ea) activation energies in various samples of quartz. Component of OSL used
Parameter analysed
Stimulation wavelength (nm)
Annealing temperature (1C)
Dose (Gy)
DE (eV)
Ea(eV)
Referencesa
Slow
Lifetime
470 470 470 420–560 470
500 600 600 No anneal 500
343 343 343 Natural 343
0.8370.01 0.9370.23 0.9370.27 0.7970.02 0.8370.01
–
Fig. 5 Fig. 5 Fig. 6(b) [30] [16]
470
600
343
0.8270.01
470
500
343
0.9870.05
0.2570.01
[16]
470
600
343
0.8270.01
0.2370.02
[16]
Lifetime
470
500
150
0.6670.04
–
[10]
Intensity
470
500
150
0.6870.11
0.0870.01
[10]
Intensity Lifetime
525 525 525
500 500 800
150 25 25
0.5370.15 0.7770.06 0.7970.15
0.1470.02 – –
[14] [23] [23]
Slow Integrated OSL
Intensity Intensity Lifetime
Intensity
Slow
Slow Slow
0.2770.07 0.2370.03 –
[16]
a The sample used in Ref. [16] is the same as the one as used in this work. In Refs. [10], [14] and [23], the sample used was ‘‘acid washed’’ quartz from BDH Ltd, UK. Quartz from a Tunisian interglacial raised beach deposit was used in Ref. [30].
100000 80000 Intensity (AU)
and 0.8270.01 eV reported by Ogundare and Chithambo [16] for samples annealed at 500 and 600 1C, respectively for unbleached samples, that is, ones with all luminescence components present. A more expansive list of activation energy values is shown for comparison in Table 1 in which it is evident that the values from component resolved work in this paper are consistent with those from a series of such tests on sedimentary quartz for studies in various regions of the decay curve.
60000 40000 20000
3.2.4. Effect of measurement temperature on luminescence intensity The influence of measurement temperature on luminescence intensity was also investigated in quartz annealed at 500 and 600 1C. The intensity at each measurement temperature was obtained by integrating the portion of each time-resolved luminescence spectrum after the stimulating light pulse between 12 and 600 ms. The dependence of the luminescence intensity on temperature during stimulation is shown in Fig. 7 for the sample annealed at 600 1C. As shown in Fig. 7, the luminescence intensity initially decreases from 20 to 60 1C and increases thereafter passing through a maximum at 140 1C. The same behaviour of the luminescence going through a peak as a function of measurement temperature was also apparent in the sample annealed at 500 1C, but is better exemplified in the sample annealed at 600 1C as shown in Fig. 7. The fall in luminescence intensity between 20 and 60 1C in Fig. 7 is typical of optical stimulation at a constant temperature as electron traps are successively depleted. The initial drop in intensity also shows that the change in the sample temperature during stimulation had
0 20
40
60
80
100 120 140 160 180 200 220 Temperature (°C)
Fig. 7. The dependence of luminescence intensity on measurement temperature for samples annealed at 600 1C, irradiated to a dose of 343 Gy and then bleached to the slow component region before the measurement of time-resolved luminescence. The solid line is the fit of Eq. (5) to the portion of the data where the influence of measurement temperature is appreciable. The plotted data are averages of values from the three measurements.
little influence on the luminescence intensity [10,22,45]. The effect of sample temperature on luminescence intensity then becomes significant for measurement temperatures greater than 60 1C compared with the effect on luminescence intensity due to optical stimulation alone. The initial increase in luminescence intensity is due to the additional thermal assistance to optical stimulation of luminescence whereas the subsequent decrease is attributed to decreased luminescence emission efficiency otherwise referred to as thermal quenching [10,22,38,45]. Overall the dependence of
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luminescence intensity I(T) on temperature is I 0 eE a =kT (5) 1 þ CeDE=kT where Io is the initial value of luminescence intensity, Ea and DE are, respectively, the activation energy values for thermal assistance and thermal quenching, and C is a constant. The values of DE and Ea evaluated from Fig. 7 for the sample annealed at 600 1C are 0.9370.27 and 0.2770.07 eV, respectively. In contrast, the values of DE and Ea could not be reliably determined from the comparatively and poorly defined intensity–temperature data from the sample annealed at 500 1C. The value of DE from Fig. 7 is in agreement with values obtained from the dependence of lifetime on measurement temperature for corresponding time-resolved luminescence spectra discussed earlier in Fig. 6. The values of DE and Ea obtained from this analysis are also consistent with the published values as shown in Table 1. IðTÞ ¼
4. Conclusion The dependence of luminescence lifetimes on the duration of optical bleaching has been studied in quartz from Nigeria, as has the dependence of luminescence lifetimes and luminescence intensity on irradiation dose and sample temperature during stimulation in the slow-component region of CW-OSL from the same quartz. Luminescence lifetimes in samples annealed at 500 1C were found to be independent of the duration of optical bleaching, whereas lifetimes in quartz annealed at 600 1C were affected, decreasing towards a constant value with the duration of bleaching. Concerning dependence on the irradiation dose, luminescence lifetimes were found to decrease with irradiation dose for samples annealed at either 500 or 600 1C. The activation energy of thermal assistance and thermal quenching estimated from the temperature dependence of luminescence intensity are 0.277 0.07 and 0.9370.23 eV, respectively, while from the temperature dependence of lifetimes the values of the activation energy for thermal quenching are within 0.8370.01 eV. These results have been explained using a model consisting of three electron traps, three luminescence centres and one nonradiative recombination centre. References [1] M.J. Aitken, An Introduction to Optical Dating, Oxford University Press, Oxford, 1998. [2] A.G. Wintle, A.S. Murray, Radiat. Meas. 41 (2006) 369. [3] L. Bøtter-Jensen, S.W.S. McKeever, A.G. Wintle, Optically Stimulated Luminescence Dosimetry, Elsevier, Amsterdam, 2003.
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[4] G. Chen, S.H. Li, J. Phys. D: Appl. Phys. 33 (2000) 437. [6] B.W. Smith, E.J. Rhodes, Radiat. Meas. 23 (1994) 329. [7] S.W.S. McKeever, L. Bøtter-Jensen, N. Larsen, G.A.T. Duller, Radiat. Meas. 27 (1997) 161. [8] R.M. Bailey, B.W. Smith, E.J. Rhodes, Radiat. Meas. 27 (1997) 123. [9] E. Bulur, Radiat. Meas. 26 (1996) 701. [10] M.L. Chithambo, R.B. Galloway, Nucl. Instrum. Methods B 183 (2001) 358. [11] M. Jain, A.S. Murray, L. Bøtter-Jensen, Radiat. Meas. 37 (2003) 441. [12] T. Watanuki, Chronological study of loess palaeosol by improved method of luminescence dating and application to reconstruct past environmental changes, Unpublished Ph.D. Thesis, Tokyo Metropolitan University, 2002. [13] J.H. Choi, A.S. Murray, M. Jain, C.S. Cheong, H.W. Chang, Quat. Sci. Rev. 22 (2003) 407. [14] M.L. Chithambo, Radiat. Meas. 37 (2003) 167. [15] M.L. Chithambo, Radiat. Meas. 37 (2003) 467. [16] F.O. Ogundare, M.L. Chithambo, Opt. Mater. 29 (2007) 1844. [17] A.G. Wintle, Radiat. Meas. 27 (1997) 769. [18] G. Sole´, L.E. Bausa´, D. Jaque, Optical Spectroscopy of Inorganic Solids, Wiley, Chichester, 2005, pp. 25–28. [19] F. Agullo-Lopez F, C.R.A. Catlow, P.D. Townsend, Point Defects in Materials, Academic Press, London, 1988, p. 265. [20] J.R. Lakowicz, Principles of Fluorescence Spectroscopy, second ed., Kluwer Academic, New York, 1999, pp. 95–118. [21] M.L. Chithambo, R.B. Galloway, Meas. Sci. Technol. 11 (2000) 418. [22] R.B. Galloway, Radiat. Meas. 35 (2002) 67. [23] M.L. Chithambo, Ancient TL 23 (2005) 39. [24] M.L. Chithambo, J. Phys. D: Appl. Phys. 40 (2007) 1880. [25] M.L. Chithambo, F.O. Ogundare, J. Feathers, D.G. Hong, Phys. Stat. Sol. (c) 5 (2008) 630. [26] M.L. Chithambo, Radiat. Meas. 38 (2004) 553. [27] M.L. Chithambo, J. Phys. D: Appl. Phys. 40 (2007) 1874. [28] S.W.S. McKeever, R. Chen, Radiat. Meas. 27 (1997) 625. [29] R. Chen, P.L. Leung, Radiat. Meas. 37 (2002) 519. [30] M.L. Chithambo, R.B. Galloway, Radiat. Meas. 32 (2000) 627. [31] M.L. Chithambo, F. Preusser, K. Ramseyer, F.O. Ogundare, Radiat. Meas. 42 (2007) 205. [32] M.L. Chithambo, R.B. Galloway, Radiat. Eff. Def. Solids 154 (2001) 361. [33] S.G. Raymond, B.J. Luff, P.D. Townsend, X.Q. Feng, G.Q. Hu, Radiat. Meas. 23 (1994) 195. [34] J. Zimmerman, J. Phys. C 4 (1971) 3265. [35] L. Bøtter-Jensen, N. Agersnap Larsen, V. Mejdahl, N.R.J. Poolton, M.F. Morris, S.W.S. McKeever Radiat, Radiat. Meas. 24 (1995) 535. [36] J.S. Singarayer, R.M. Bailey, Radiat. Meas. 37 (2003) 451. [37] R.M. Bailey, Radiat. Meas. 32 (2000) 233. [38] A.S. Murray, A.G. Wintle, Radiat. Meas. 29 (1998) 65. [39] C.K. Kuhns, N. Agersnap Larsen, S.W.S. McKeever, Radiat. Meas. 32 (2000) 413. [40] S.W.S. McKeever, Thermoluminescence of Solids Cambridge, University Press, Cambridge, 1985, p. 84. [41] I.K. Bailiff, Radiat. Meas. 32 (2000) 401. [42] R.K. Swank, F.C. Brown, Phys. Rev. 130 (1963) 34. [43] M.S. Akselrod, N. Agersnap-Larsen, V. Whitley, S.W.S. McKeever, J. Appl. Phys. 84 (1998) 3364. [44] M.S. Akselrod, N. Agersnap-Larsen, V. Whitley, S.W.S. McKeever, Radiat. Prot. Dosimetry 84 (1999) 39. [45] N.A. Spooner, Radiat. Meas. 23 (1994) 593.
Journal of Luminescence 128 (2008) 1561–1569 www.elsevier.com/locate/jlumin
The influence of optical bleaching on lifetimes and luminescence intensity in the slow component of optically stimulated luminescence of natural quartz from Nigeria F.O. Ogundarea,, M.L. Chithambob a
Department of Physics, University of Ibadan, Ibadan, Oyo State, Ibadan, Nigeria Department of Physics and Electronics, Rhodes University, PO Box 94, Grahamstown 6140, South Africa
b
Received 21 August 2007; received in revised form 23 February 2008; accepted 3 March 2008 Available online 12 March 2008
Abstract The intensity of optically stimulated luminescence may be decreased to a slow or medium component of its decay curve by optical bleaching, that is, by prolonged exposure of the luminescent sample to stimulating light. In this paper, we report on the influence of irradiation and measurement temperature on luminescence lifetimes as well as on the effect of measurement temperature on luminescence intensity in annealed natural quartz from Nigeria. Measurements were carried out in the slow component region using time-resolved optical stimulation at 470 nm on samples annealed at 500 and 600 1C. Luminescence lifetimes were determined from the resultant timeresolved luminescence spectra by analysing the portion of each spectrum after the stimulating light pulse of duration 11 ms. In preparatory tests, the influence of the duration of optical bleaching on lifetimes was investigated. It was found that lifetimes in samples annealed at 500 1C are independent of the duration of optical bleaching, whereas lifetimes in quartz annealed at 600 1C are affected, decreasing towards a constant value with duration of bleaching. Concerning measurements in the slow-component region, lifetimes were found to decrease with irradiation dose for samples annealed at either 500 or 600 1C. The temperature dependence of lifetimes in both sets of quartz is similar with lifetimes constant at about 36 ms between 20 and 120 1C, but decreasing consistently from then on to about 5 ms at 200 1C, the maximum measurement temperature used in experiments. The luminescence intensity was observed to typically go through a peak as the stimulation temperature was increased from 20 to 200 1C, following a brief initial decrease, a change better exemplified in the quartz annealed at 600 1C. The initial decrease in luminescence intensity is attributed to the dominance of optical stimulation over thermal stimulation. On the other hand, the subsequent change of luminescence intensity with temperature is discussed as evidence of thermal assistance to optical stimulation, initially with activation energy of 0.2770.07 eV and of thermal quenching subsequently with activation energy equal to 0.9370.23 eV for samples annealed at either 500 or 600 1C. The temperature dependence of lifetimes is explained as showing increased thermal effect on lifetimes with activation energy values within 0.8370.01 eV. On the other hand, the influence of irradiation on lifetimes is accounted for in terms of an energy band model for quartz consisting of three luminescence centres and one non-radiative recombination centre. r 2008 Elsevier B.V. All rights reserved. PACS: 39.90; 78.47; 91.60.M; 78.60 Keywords: Time-resolved luminescence; Quartz; Lifetimes; Bleaching; Slow component
1. Introduction Quartz is a natural mineral that is widely used for the determination of archaeological and geological age using Corresponding author. Tel.: +234 8050397616.
E-mail address: [email protected] (F.O. Ogundare). 0022-2313/$ - see front matter r 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2008.03.001
luminescence [1–3]. The common techniques used for dating are thermoluminescence (TL) and optically stimulated luminescence (OSL). Thermoluminescence involves stimulation of luminescence from a previously irradiated sample by controlled heating, whereas OSL is obtained when the sample is exposed to light. Once the dose rate, a measure of the rate at which energy is absorbed by quartz
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F.O. Ogundare, M.L. Chithambo / Journal of Luminescence 128 (2008) 1561–1569
grains from environmental radiation, and the equivalent dose, the total dose from the environmental radiation following deposition of the quartz grains are known, the age can be calculated as age=(equivalent dose)/dose rate. There are two possible methods for determining the equivalent dose using OSL. One is the regeneration method and the other is the additive dose method [3]. Both alternatives involve several laboratory procedures including irradiation, optical bleaching and preheating of a sample [4]. Any of these procedures may affect the OSL and in turn, the equivalent dose and hence the age determined. Previous measurements from sedimentary quartz of continuous-wave optically stimulated luminescence (CW-OSL) in which the luminescence is recorded simultaneously with stimulating light to produce a decay curve, showed the presence of at least three distinct components in the decay curve [6–8]. For convenience, the components were referred to as fast, medium or slow [8], and are identified as separate exponential terms in the timedependent change of the OSL intensity, otherwise called the decay curve. On the other hand, if the luminescence is measured using linear modulation where the intensity of stimulating light is increased linearly during measurement [9], the various components appear as separate peaks in a plot of luminescence intensity against time. As discussed by Chithambo and Galloway [10], each of the components in a CW-OSL decay curve may not necessarily be associated with a distinct physical mechanism, with the method of multi-exponential fitting only providing a means to conveniently represent the shape of a decay curve. In contrast, linearly modulated luminescence (LM-OSL) enables donor defects with different photoionization cross-section to be reliably and unambiguously identified as separate peaks [11]. The accuracy of the equivalent dose and hence the age depends on the proportion of each of the components in the portion of the decay curve used in the calculation. For example, if the fast component was zeroed by sunlight, but the slow component was not, the resulting equivalent dose may be inaccurate [12,13] because then the contribution from components other than the fast one will be significant. It is therefore necessary to understand the general characteristics of different OSL components in quartz and on this aspect, Jain et al. [11] used LM-OSL to investigate the influence on OSL components of (a) sensitization (b) thermal stability (c) infrared sensitivity and (d) recuperation. Based on their results, Jain and coworkers [11] concluded that the various OSL components do not undergo identical sensitivity changes and that this characteristic could therefore lead to inaccurate estimates of the equivalent dose used in dating calculations. Chithambo and Galloway [10] using time-resolved optically stimulated luminescence (TR-OSL) measurements, in which the stimulation and emission of luminescence is separated in time, calculated activation energies associated with thermal assistance and thermal quenching in the slowcomponent region of CW-OSL from quartz and demonstrated that the two effects are separable. This work was
extended to include effects of measurement temperature and irradiation dose on lifetimes and luminescence intensity in the various components of the CW-OSL decay curve in quartz [14,15]. Time-resolved luminescence spectra to be described in this paper should not be confused with decay curves obtained in continuous optical stimulation. A decay curve shows the time dependence of luminescence intensity, whereas a time-resolved luminescence spectrum displays the delay between stimulation and emission of luminescence. Besides the studies by Chithambo and Galloway [10] and Chithambo [14,15], the component-resolved study of lifetimes and luminescence intensity using time-resolved luminescence has otherwise remained largely unexplored. Indeed, results of Chithambo [15] showed that lifetimes decrease slightly with the duration of pre-measurement optical bleaching, suggested a need to further investigate and understand the effect. This work therefore reports detailed studies concerned with the dependence of lifetimes (the delay between stimulation and emission of luminescence) as well as the luminescence intensity on measurement temperature and irradiation dose in the slow component region of optically stimulated luminescence from annealed natural quartz from Nigeria. This is preceded by a report on preparatory investigations concerned with the influence on lifetimes of optical bleaching used to reduce the luminescence intensity to the slowcomponent region. The overall aim of these studies is to better understand the physical processes of luminescence in quartz. 2. Experimental details The material used in all measurements was sedimentary quartz, grain size 90–180 mm, collected from Oro, Kwara state, in the west central region of Nigeria. The sample was analyzed for feldspar, which usually occurs concurrently with natural quartz, using a JEOL superprobe 733 electron microprobe (JEOL company, Japan) as in a previous study using the same material [16]. The analysis determined the percentage of oxides of Si, Al, Ca, Na and K in the sample with a lower limit of detection (LLD) in each case of 0.017, 0.013, 0.012, 0.018 and 0.013, respectively. In this method, any oxide with detection value less than its LLD is not detected by the microprobe. The analysis showed that oxides of Al, Ca, Na and K were not detected which indicated that the feldspar content of the sample was negligible. The quartz was thus used without any chemical treatment required for removal of feldspar as is usual in OSL experiments [3,17]. Prior to use, samples were annealed at 500 and 600 1C for 10 min in an oven (Gallenkamp Muffle furnace), and then immediately cooled in air thereafter. The samples were annealed to remove any residual natural OSL signal and to enhance the sensitivity of the quartz to optical stimulation [3,10,14,15], as well as to provide annealed samples for the study.
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Measurements of time-resolved luminescence spectra for this paper were made using multichannel scaling, an established technique for time-correlated photon counting experiments reviewed in many texts example.g. Ref. [18–20]. In the method, a multichannel scaler is used to acquire data corresponding to the number of events that occur during a time interval ti to ti+Dt as a function of time where the time-gap Dt is known as the dwell time. In the context of this work, an event would be the detection of a luminescence photon. The total measurement time t otherwise called the dynamic range is subdivided into an integral number of channels nDt, where n is the number of specific channels. When a measurement is started, data are acquired sequentially, channel to channel from the first channel until all selected channels have been addressed, that is, until the dynamic range has been covered or until the scaler has scanned or swept through the preselected number n of channels. For purposes of experimental expediency, for example, to reduce scatter in data points, it is usual to obtain data over multiple sweeps. The timeresolved luminescence spectrum generated in this way is a plot of cumulative photon counting rates against time. Detailed features of such instrumentation for measurement of time-resolved luminescence from quartz have been presented by Chithambo and Galloway [21–23]. Time-resolved luminescence spectra for this paper were recorded on an LED pulsing system modified from that described by Chithambo and Galloway [21]. In its present configuration, the system uses a multichannel scaler (ORTEC MCS-plusTM) to simultaneously initiate a sweep and to trigger a set of LEDs via a multivibrator, an integrated circuit 2N74221. Luminescence is stimulated at pulse-widths set by the timing components on the multivibrator. In this study, each time-resolved luminescence spectrum was measured over a dynamic range of 600 ms at a dwell time of 2 ms and 200,000 sweeps. Luminescence was stimulated by a set of four 470 nm blue LEDs (Nichia NSPB-500) each operated at a pulse width of 11 ms. The mean light intensity at the sample was 0.60 mW/cm2 as determined by a PIN diode (Ealing Electro-optics). An EMI 9635QA photomultiplier (Electron tubes Ltd, UK) with a combination of BG-39 and UG-11 filters in front was used for the detection of luminescence. Each LED has a 2 mm thick GG420 filter in front to attenuate the shorter wavelength end of the LED spectrum. The measurement procedure is such that the luminescence increases progressively over photomultiplier noise and constant scatter from luminescence stimulating light during stimulation and then decreases in time over photomultiplier noise only after the stimulating light pulse as has been illustrated on a number of occasions for natural quartz [14,21,24,25] and for synthetic quartz [26]. Luminescence lifetimes t were extracted from timeresolved luminescence spectra by fitting exponential functions of the form LðtÞ ¼ B þ A expðt=tÞ
(1)
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to the portion of each spectrum after the stimulation light pulse where L(t) is the luminescence counting rate, t is time, A is a scaling factor and B is a constant added to account for the background. The justification for the method is that if luminescence is pulse stimulated from a charge trap with an initial electron population n0, with a probability for stimulation per unit time p, and if l is the probability that a stimulated electron will produce luminescence, then the rate of emission of luminescence l(t) after the light pulse is given by lðtÞ ¼ pno ½expðlt1 Þ 1 expðltÞ
(2)
where t1 is the pulse width and t ¼ 1/l [27]. It should be noted that although Eqs. (1) and (2) describe the evaluation of lifetimes from the portion of a time-resolved luminescence spectrum following the light pulse, the lifetimes may also be found from the portion during the light pulse [27]. The relevant formulae for the latter case as well as general theoretical models for the analysis of time-resolved luminescence spectra including computer-simulated and experimental aspects have been reported recently by Chithambo [24,27]. The methods used for evaluating lifetimes and various kinetic parameters from quartz experimental data [10,14–16,22] thus discussed, are based on a simple model consisting of one electron trap and one kind of recombination centre with emission involving electron transition through the conduction band just like in discussions of optically stimulated luminescence by McKeever and Chen [28] and Chen and Leung [29]. However, in typical timeresolved optical stimulation measurements, the stimulating light pulse is brief (of the order of nanoseconds to several microseconds) such that the reduction in trapped charge due to optical stimulation in that time is negligible and as a result retrapping is also assumed to be negligible [27]. Analytical formulae developed on this basis as well as computer-simulated data on the same assumptions are consistent with experimental results where available, for example data drawn from pulsed luminescence from quartz and a-Al2O3:C [27]. The influence of retrapping on pulsed luminescence from quartz was considered in a preliminary study by Chithambo and Galloway [30], who suggested that a decrease in lifetimes as the temperature of measurement was increased was due to a reduction in retrapping from shallow traps. In a subsequent investigation [10], the concomitant temperature dependence of lifetimes evaluated from time-resolved luminescence spectra and the intensity of luminescence integrated from the same spectra showed that the temperature dependence of lifetimes and luminescence intensity could be properly explained on the basis of the Mott–Seitz model of thermal quenching and assumption of negligible retrapping. The influence of retrapping on pulsed luminescence in general was also considered by Chen and Leung [29] whose theoretical model on luminescence intensity predicted that if retrapping were dominant, the growth curve, that is, the dependence of the luminescence intensity
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on dose should change from being linear initially to subsequently showing nonlinear behaviour such as quadratic dose dependence in some cases. However, experimental cases of obvious nonnegligible retrapping on time-resolved luminescence from quartz remain to be examined. In view of the discussions above, all calculations in this paper are based on the assumption of negligible retrapping as discussed previously [10,24,27]. Fig. 1 shows an example of a time-resolved luminescence spectrum measured from a sample of quartz dosed up to 257 Gy. The solid line through data points is the best fit of Eq. (3) which in this example gave a lifetime value of 41.8570.40 ms. The first set of measurements concerned the dependence of luminescence lifetimes on the duration of optical bleaching. For this experiment, thirty aliquots were prepared and then separated into 10 groups each containing three aliquots. Each sample, consisting of a few milligrams of quartz on a 10 mm aluminium sample disc, was irradiated to a dose of 343 Gy using a 90Sr/90Y beta source. Each irradiated sample in every group was then bleached using a set of 16 continuously operated 470 nm blue LEDs. A different duration of bleaching was used for each group giving in effect 10 different bleaching duration tests. Measurements of time-resolved luminescence spectra corresponding in each case to a given duration of bleaching were then carried out on each of the three aliquots in each group. The second set of measurements was on the dependence of luminescence lifetimes and luminescence intensity on irradiation dose and measurement temperature in the slow component region of luminescence, optically stimulated from quartz annealed at 500 and 600 1C. Unless otherwise specified, samples were irradiated to 343 Gy and then exposed to stimulating light for 2500 s at 25 1C thereby
reducing the intensity to about 5% of the peak intensity, that is, to the slow-component region. Time-resolved luminescence measurements were made without preheating, a usual procedure in CW-OSL experiments [17]. As in previous studies [16,25,26,31], the preheating could be omitted because the spontaneously emitted phosphorescence that follows irradiation is not correlated in time with stimulation light pulses, but simply adds a level background to the time-resolved luminescence spectrum and so does not influence the value of the lifetime [22]. 3. Results and discussion 3.1. Dependence of luminescence lifetimes on the duration of pre-measurement optical bleaching Fig. 2 shows the change of luminescence lifetimes with the duration of optical bleaching in quartz annealed at 500 and 600 1C. The lifetimes presented in Fig. 2 are each the average from three measurements. The lifetime of the sample annealed at 500 1C is constant with an average value of 46.870.6 ms throughout the measurement period. In contrast, the lifetimes determined from the sample annealed at 600 1C decrease from 41.470.2 to 38.570.3 ms when the duration of optical bleaching is increased from 250 to 1500 s. Thereafter the lifetimes are constant at an average value of 38.570.3 ms. The results in Fig. 2 for the sample annealed at 500 1C can be compared with similar measurements carried out on unannealed sedimentary quartz from the UK (hereafter referred to as BDH quartz) as well as subsamples from the same BDH quartz annealed at 500 1C where the lifetimes were also independent of the duration of optical bleaching [32]. In a latter study (also on BDH quartz) concerned with the link between thermal influence on lifetimes and
5500 48
5000
46 500°C
4000 Lifetime (µs)
Intensity (a.u.)
4500
3500 3000
600°C
44
42
2500 40 2000 38
1500 0
100
200
400 300 Time (µs)
500
600
Fig. 1. A time-resolved luminescence spectrum measured from a quartz sample annealed at 500 1C and given a dose of 257 Gy prior to measurement at room temperature. The data after the stimulation pulse are shown fitted by Eq. (1).
0
500
1000 1500 2000 Pre-measurement bleaching time (s)
2500
Fig. 2. Dependence of lifetimes on the duration of pre-measurement optical bleaching. The samples were irradiated to a dose of 343 Gy before the optical bleaching.
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bleaching of luminescence, Chithambo [15] showed that lifetimes were independent of duration of pre-measurement of optical bleaching for sedimentary quartz annealed at 600 and 800 1C, but that the lifetimes decreased with the duration of bleaching in samples annealed at 1000 1C. The general feature inferred from results of Fig. 2 and those on BDH quartz [10,15] is that the dependence of lifetimes on the duration of optical bleaching differs on either side of the appropriate phase transition temperature in quartz. In the quartz used in this work, the dependence of lifetimes on the duration of optical bleaching is different, on either side of 573 1C, the first phase transition temperature, whereas in the sedimentary quartz of Chithambo et al. [10,15] the same can be said for the second phase transition temperature at 873 1C. A preliminary explanation of this effect would be to assume that the structural change that occurs during phase transition may affect the spatial link between traps and luminescence centres as was pointed out in a similar case by Raymond et al. [33] in their work on the thermoluminescence spectra of doped Bi4Ge3O12. In order to explain the dependence of luminescence lifetimes on the duration of optical bleaching, we use the band model for quartz [34,35] as adapted for time-resolved optically stimulated luminescence by Galloway [22]. One of the main features of this model is that annealing causes a transfer of holes from a non-radiative recombination centre to luminescence centres as well as between the luminescence centres. An energy-band schematic diagram of the model developed by Galloway [22] is shown in Fig. 3 and consists of three luminescence centres (labelled LH, LL and LS), one non-radiative recombination centre (LN) and three electron traps (1,2,3). The luminescence centres LH, LL and LS are associated with lifetimes tH, tL and tS, respectively, where tH4tL4tS. Conduction band e e
1
e
2
3
LS LL
LH
LN
Valence band Fig. 3. The energy band model used in the text to explain the dependence of luminescence lifetimes on the duration of optical bleaching and irradiation in quartz. The diagram shows three luminescence centres, LH, LL and LS with which lifetimes tH, tL and tS are associated. LN is a nonradiative recombination centre. Electron traps are denoted 1, 2 and 3 and stand for shallow traps, optically sensitive traps and optically inaccessible deep traps, respectively.
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With reference to Fig. 3, we assume that annealing at 500 1C causes a transfer of holes from centre LH to LL and LS centres, but that much of the transfer is to the LL centre. This is a similar assumption as that made by Bøtter-Jensen et al. [35] where the preferential capture of holes by nonradiative recombination centres rather than radiative ones was required to sufficiently account for sensitivity changes caused by annealing in quartz. The result of the inter-centre hole transfer due to annealing at 500 1C is that the luminescence from the LL centre dominates the measured signal in samples annealed at this temperature (500 1C) such that the lifetime associated with the LL centre becomes the characteristic lifetime of the sample annealed at 500 1C. The lifetime associated with the quartz annealed at 500 1C which is independent of the duration of bleaching as seen in Fig. 2 may therefore be denoted tL. Therefore a numerical value associated with tL, taken as the mean of the ten lifetimes plotted in Fig. 2 for the sample annealed at 500 1C is 46.870.6 ms. The dependence of lifetimes on the duration of bleaching in the sample annealed at 600 1C can be understood on the basis of two assumptions, which also involve preferential capture of charge. The aim is to first explain the reason lifetimes in the quartz annealed at 500 1C are less than those in the quartz annealed at 600 1C and then secondly to account for the dependence of lifetimes on the duration of optical bleaching. On the first point, concerning the difference in lifetime values we assume that in this case the net number of holes transferred to luminescence centre LS from other centres is comparatively greater than that transferred to LL centres from other centers. Now, for quartz annealed at this temperature, contributions to the measured luminescence from both LL and LS centres are significant. In turn, lifetimes consist of contributions from both tL and tS where tS is less than tL. Thus overall the effective lifetime measured from the quartz annealed at 600 1C should be less than that measured from the quartz annealed at 500 1C in which the lifetime tL is dominant. Concerning the dependence of lifetimes on the duration of optical bleaching, we assume that the recombination probability of electrons with holes in centre LL is greater than for recombination in the LS centre. This means that during optical bleaching the percentage of holes lost to electron recombination to produce luminescence is higher for the LL centre than for the LS centre. Therefore the luminescence contribution from LS centres relative to that from LL centres increases with optical stimulation time. The increase in signal from LS centres is thus manifested as the decrease of measured lifetime between 250 and 1500 s as seen in Fig. 2. Beyond 1500 s of optical bleaching, lifetimes become constant because the luminescence from centres LL becomes negligible. Thus the measured mean lifetime approaches tS with a mean value of 38.570.4 ms. Note that it is not luminescence from LS that becomes negligible because if this were the case, the measured mean lifetime would have tended to increase towards the larger value of tL.
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3.2. The slow component
3.2.2. Dependence of lifetimes on radiation dose for luminescence in the slow-component region Fig. 5 shows the dependence of lifetimes on beta dose for quartz annealed at 500 1C and in the inset at 600 1C. In
Percentage intensity (%)
80 100 80
60 60 40
40
20
20
0 0
20 40 60 80 100 120 140 160 180 200
0 0
500
1000 1500 2000 Stimulation time (s)
2500
3000
Fig. 4. The relative importance of components of a decay curve as a function of stimulation time for quartz sample irradiated to a dose of 172 Gy. The inset compares the relative values of the fast, medium and slow components for the first 200 s. 40
38
48 46
36
44 Lifetime (µs)
3.2.1. Determination of the slow-component region The slow component was identified by fitting a sum of three exponential functions each of form exp(lit) to a decay curve and noting the component with the slowest decay parameter l Such a fit for a decay curve of a sample irradiated to 172 Gy gave l1, l2 and l3 as 0.1441, 0.0024 and 0.0006 s1 respectively. In this case, the slow compo, nent is the one with the decay parameter of 0.0006 s1. A one-to-one comparison of these decay parameters with published data may not be that illustrative since the luminescence decay rate is sensitive not only to the wavelength of stimulation, but also to a number of other factors including stimulation power at sample, temperature of measurement and charge concentration [3,36–38]. However, as a general indicator of decay rate values only, we note that the decay-rate 0.1441 s1 for the fast component in this work is comparable to the value of 0.1401 s1 for a CW-OSL fast component stimulated from quartz sediments using green light [39]. On the other hand, the values for decay rates obtained in this work for the medium and slow components differ from the corresponding values, that is, 0.0488 and 0.0020 s1 from the slow and medium components, respectively, of the latter material [39]. The difference may be attributed to some of the factors alluded to earlier. The relative importance of the fast, medium or slow component in a decay curve as a function of measurement time was determined as a fraction of the total measured signal as done previously by Chithambo and Galloway [10]. Fig. 4 shows the time-dependent contribution of each of the components to the luminescence intensity. The inset to Fig. 4 shows, for better clarity, the relative features for the first 200 s. Fig. 4 shows that in the initial transient (that is in the first 1 s) nearly 89% of the measured luminescence is due to the fast component with the slow and medium components contributing about 4% and 7%, respectively. As stimulation continues, the fast component decreases to a negligible value whereas the medium component increases in importance to about 57% in the first 50 s, but progressively decreases thereafter. On the other hand, the contribution from the slow component increases from about 4% at the start of the stimulation to be predominant by 2400 s. The measurements reported in the following sections were carried out in the slow-component region of luminescence from annealed natural quartz stimulated using pulsed 470 nm blue light. The results may be of interest to others interested in the application of luminescence from the slow-component region using optically stimulated luminescence methods.
fast medium slow
100
34
42 40
0
100
200
300
400
500
600
38 36 34 0
100
200
300 Dose (Gy)
400
500
600
Fig. 5. Dose dependence of luminescence lifetimes for quartz annealed at 500 1C and in the inset for the sample annealed at 600 1C and bleached to slow component region after irradiation. The luminescence lifetimes in the main graph as well as in the inset, each correspond to the same dose values.
both cases the lifetimes decreased with dose. In the sample annealed at 500 1C, lifetimes decreased from 47.070.8 to 36.670.5 ms when the beta dose was increased from 14 to 515 Gy. In the case of quartz annealed at 600 1C (inset), lifetimes change from 38.770.5 to 35.170.4 ms for beta doses up to about 200 Gy and then become constant thereafter. In comparison, lifetimes calculated from the same sample when not subjected to any optical bleaching were constant up to about 100 Gy from which dose the lifetime then decreased [16]. The dependence of lifetimes on beta dose shown in Fig. 5 can also be explained using the energy band model of
ARTICLE IN PRESS F.O. Ogundare, M.L. Chithambo / Journal of Luminescence 128 (2008) 1561–1569
3.2.3. Effect of measurement temperature on luminescence lifetimes in the slow-component region The influence of the temperature of the quartz during stimulation on the luminescence lifetime was investigated for quartz annealed at 500 and 600 1C. Samples were irradiated to a beta dose of 343 Gy before measurement. Time-resolved luminescence spectra were recorded at various temperatures between 20 and 200 1C on the same sample. The lifetimes presented in this section are averages of values from three separate measurements. The dependence of lifetimes on measurement temperature for samples annealed at 500 and 600 1C is shown in Fig. 6. It can be seen that for the sample annealed at 500 1C values of the lifetime between 20 and 120 1C are essentially constant at about 35.572.7 ms. From then on the lifetimes decrease to a minimum of 4.771.5 ms at 200 1C. The change of lifetimes with measurement temperature for the sample annealed at 600 1C is similar. In this case, the lifetimes are constant within 35.974.2 ms between 20 and 120 1C and decrease thereafter to 4.670.7 ms at 200 1C. The dependence of lifetimes on measurement temperature as shown in Fig. 6 is the evidence of thermal quenching effect on lifetimes t and is described on the basis of the Mott–Seitz configurational coordinate model [40] as a
55 50
500°C 600°C
45 40 Lifetime (µs)
Fig. 3. Firstly, the number of electron-hole pairs created by irradiation increases with the beta dose. In turn, the number of holes trapped by each of the three luminescence centres as well as the non-radiative recombination centre also increases. Again, as in the Bøtter-Jensen et al. [35] description of sensitization by preferential capture of holes by non-radiative recombination centres rather than radiative ones, we assume that during irradiation the LS centre has the highest hole capture probability followed by that of the LL and then the LH centre. Therefore increasing irradiation dose increases the number of holes in LS centres compared to those in LL centres in samples annealed at 500 1C as in quartz annealed at 600 1C. Now with regards to the measurements on the unbleached samples where lifetimes were constant with dose [16], it should be understood that the lifetimes were constant because of the dominance of luminescence from LL centres over that from LS centres. In contrast, the bleaching done to reduce the luminescence to the slow component in the present study significantly decreases the number of holes trapped in LL centres compared to those trapped in LS centres since the recombination, probability of electrons with holes for the LL centre is greater than that for the LS centre. This effect therefore means that the amount of luminescence from LS centers is greater than that from LL centers in the time-resolved luminescence spectrum used to determine the mean lifetime. This is seen as a decrease with the dose of mean lifetimes as shown in Fig. 5 since tS (where tSotL) corresponding to Ls luminescence increases in importance compared to tL associated with the LL centre.
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35 30 25 20 15 10 5 0 0
20
40
60 80 100 120 140 160 Measurement temperature (°C)
180
200
Fig. 6. The dependence of luminescence lifetimes on measurement temperature for samples irradiated to a dose of 343 Gy and then bleached to the slow component region before the measurement of time-resolved luminescence. The dotted line is the best fit of Eq. (3) to the data for the sample annealed at 500 1C (open cycles) and the solid line for the one annealed at 600 1C (open squares).
dependence on absolute temperature T as t¼
t0 1 þ CeDE=kT
(3)
where to is the lifetime at 0 K and DE is the activation energy for thermal quenching and C is a constant [10,22,24,41]. The same equation describes the temperature dependence of F centre luminescence from some materials for example, KCl [42] and a-Al2O3:C [43,44]. As reviewed elsewhere [14,19,22,24,43,44], from the Mott–Seitz model, the temperature-dependent distribution of lifetimes t is found to comprise radiative, phonon-assisted, and nonradiative contributions, that is, 1 1 _o DE ¼ þ g coth þ n exp (4) t t0 kT kT where to is the radiative lifetime at absolute zero of temperature, g is a constant, o is the phonon vibration frequency,_ is Planck’s constant, k is Boltzmann’s constant and DE and n are the activation energy and frequency factor for the non-radiative process. It should be noted that parameters to and DE have the same meaning as defined for Eq. (3). If the phonon-assisted process is negligible, the temperature-dependent distribution of the measured mean lifetimes as expressed as in Eq. (3) then emerges. The theoretical and experimental aspects of the analysis of time-resolved optically stimulated luminescence from quartz using Eq. (3) and related formulae has recently been discussed by Chithambo [24,27]. The data in Fig. 6 are shown fitted by Eq. (3). The values of DE determined from this fit are 0.8370.01 and 0.9370.23 eV for samples annealed at 500 and 600 1C, respectively. These values are comparable to 0.8370.01
ARTICLE IN PRESS F.O. Ogundare, M.L. Chithambo / Journal of Luminescence 128 (2008) 1561–1569
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Table 1 A comparison of thermal quenching (DE) and thermal assistance (Ea) activation energies in various samples of quartz. Component of OSL used
Parameter analysed
Stimulation wavelength (nm)
Annealing temperature (1C)
Dose (Gy)
DE (eV)
Ea(eV)
Referencesa
Slow
Lifetime
470 470 470 420–560 470
500 600 600 No anneal 500
343 343 343 Natural 343
0.8370.01 0.9370.23 0.9370.27 0.7970.02 0.8370.01
–
Fig. 5 Fig. 5 Fig. 6(b) [30] [16]
470
600
343
0.8270.01
470
500
343
0.9870.05
0.2570.01
[16]
470
600
343
0.8270.01
0.2370.02
[16]
Lifetime
470
500
150
0.6670.04
–
[10]
Intensity
470
500
150
0.6870.11
0.0870.01
[10]
Intensity Lifetime
525 525 525
500 500 800
150 25 25
0.5370.15 0.7770.06 0.7970.15
0.1470.02 – –
[14] [23] [23]
Slow Integrated OSL
Intensity Intensity Lifetime
Intensity
Slow
Slow Slow
0.2770.07 0.2370.03 –
[16]
a The sample used in Ref. [16] is the same as the one as used in this work. In Refs. [10], [14] and [23], the sample used was ‘‘acid washed’’ quartz from BDH Ltd, UK. Quartz from a Tunisian interglacial raised beach deposit was used in Ref. [30].
100000 80000 Intensity (AU)
and 0.8270.01 eV reported by Ogundare and Chithambo [16] for samples annealed at 500 and 600 1C, respectively for unbleached samples, that is, ones with all luminescence components present. A more expansive list of activation energy values is shown for comparison in Table 1 in which it is evident that the values from component resolved work in this paper are consistent with those from a series of such tests on sedimentary quartz for studies in various regions of the decay curve.
60000 40000 20000
3.2.4. Effect of measurement temperature on luminescence intensity The influence of measurement temperature on luminescence intensity was also investigated in quartz annealed at 500 and 600 1C. The intensity at each measurement temperature was obtained by integrating the portion of each time-resolved luminescence spectrum after the stimulating light pulse between 12 and 600 ms. The dependence of the luminescence intensity on temperature during stimulation is shown in Fig. 7 for the sample annealed at 600 1C. As shown in Fig. 7, the luminescence intensity initially decreases from 20 to 60 1C and increases thereafter passing through a maximum at 140 1C. The same behaviour of the luminescence going through a peak as a function of measurement temperature was also apparent in the sample annealed at 500 1C, but is better exemplified in the sample annealed at 600 1C as shown in Fig. 7. The fall in luminescence intensity between 20 and 60 1C in Fig. 7 is typical of optical stimulation at a constant temperature as electron traps are successively depleted. The initial drop in intensity also shows that the change in the sample temperature during stimulation had
0 20
40
60
80
100 120 140 160 180 200 220 Temperature (°C)
Fig. 7. The dependence of luminescence intensity on measurement temperature for samples annealed at 600 1C, irradiated to a dose of 343 Gy and then bleached to the slow component region before the measurement of time-resolved luminescence. The solid line is the fit of Eq. (5) to the portion of the data where the influence of measurement temperature is appreciable. The plotted data are averages of values from the three measurements.
little influence on the luminescence intensity [10,22,45]. The effect of sample temperature on luminescence intensity then becomes significant for measurement temperatures greater than 60 1C compared with the effect on luminescence intensity due to optical stimulation alone. The initial increase in luminescence intensity is due to the additional thermal assistance to optical stimulation of luminescence whereas the subsequent decrease is attributed to decreased luminescence emission efficiency otherwise referred to as thermal quenching [10,22,38,45]. Overall the dependence of
ARTICLE IN PRESS F.O. Ogundare, M.L. Chithambo / Journal of Luminescence 128 (2008) 1561–1569
luminescence intensity I(T) on temperature is I 0 eE a =kT (5) 1 þ CeDE=kT where Io is the initial value of luminescence intensity, Ea and DE are, respectively, the activation energy values for thermal assistance and thermal quenching, and C is a constant. The values of DE and Ea evaluated from Fig. 7 for the sample annealed at 600 1C are 0.9370.27 and 0.2770.07 eV, respectively. In contrast, the values of DE and Ea could not be reliably determined from the comparatively and poorly defined intensity–temperature data from the sample annealed at 500 1C. The value of DE from Fig. 7 is in agreement with values obtained from the dependence of lifetime on measurement temperature for corresponding time-resolved luminescence spectra discussed earlier in Fig. 6. The values of DE and Ea obtained from this analysis are also consistent with the published values as shown in Table 1. IðTÞ ¼
4. Conclusion The dependence of luminescence lifetimes on the duration of optical bleaching has been studied in quartz from Nigeria, as has the dependence of luminescence lifetimes and luminescence intensity on irradiation dose and sample temperature during stimulation in the slow-component region of CW-OSL from the same quartz. Luminescence lifetimes in samples annealed at 500 1C were found to be independent of the duration of optical bleaching, whereas lifetimes in quartz annealed at 600 1C were affected, decreasing towards a constant value with the duration of bleaching. Concerning dependence on the irradiation dose, luminescence lifetimes were found to decrease with irradiation dose for samples annealed at either 500 or 600 1C. The activation energy of thermal assistance and thermal quenching estimated from the temperature dependence of luminescence intensity are 0.277 0.07 and 0.9370.23 eV, respectively, while from the temperature dependence of lifetimes the values of the activation energy for thermal quenching are within 0.8370.01 eV. These results have been explained using a model consisting of three electron traps, three luminescence centres and one nonradiative recombination centre. References [1] M.J. Aitken, An Introduction to Optical Dating, Oxford University Press, Oxford, 1998. [2] A.G. Wintle, A.S. Murray, Radiat. Meas. 41 (2006) 369. [3] L. Bøtter-Jensen, S.W.S. McKeever, A.G. Wintle, Optically Stimulated Luminescence Dosimetry, Elsevier, Amsterdam, 2003.
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