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On the slow component of luminescence stimulated from quartz by pulsed blue light-emitting diodes
Nuclear Instruments and Methods in Physics Research B 183 (2001) 358±368
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On the slow component of luminescence stimulated from quartz by pulsed blue light-emitting diodes M.L. Chithambo, R.B. Galloway
*
Department of Physics and Astronomy, The University of Edinburgh, James Clerk Maxwell Building, The Kings Buildings, May®eld Road, Edinburgh EH9 3JZ, UK Received 6 June 2000; received in revised form 19 April 2001
Abstract Characteristics of luminescence lifetimes and luminescence intensity from quartz have been studied as a function of temperature at long stimulation times (the `slow component' region). In this region, luminescence lifetime values are strongly aected by temperature especially from 373 to 473 K. Luminescence intensity values increase from 293 to 398 K and then decrease until 473 K, the maximum temperature investigated. These changes are independent of the order in which the measurements are made and have been discussed in terms of thermal assistance and thermal quenching of the luminescence. Activation energies for thermal assistance and thermal quenching evaluated from temperature-dependent changes of both luminescence intensity and luminescence lifetimes are given. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 78.47.+p Keywords: Luminescence lifetimes; Quartz; Pulsed LEDs; Thermal assistance; Thermal quenching
1. Introduction The fall of optically stimulated luminescence (OSL) with time that results from continuous excitation of luminescence from quartz can usually be described by a sum of three exponentials. Examples of this were reported by Smith and Rhodes [1] for luminescence stimulated from a naturally bleached Morrocan quartz sample using a 514 nm *
Corresponding author. Tel.: +44-131-650-1000; fax: +44131-662-7174. E-mail address: [email protected] (R.B. Galloway).
laser, by Bailey et al. [2] on an aliquot from the same material with stimulation by a ®ltered (420± 560 nm) halogen lamp and also noted by Murray and Wintle [3], and McKeever et al. [4]. In cases where multi-exponential regression was used to describe the time dependence of the OSL [1,2], the third exponential term was required to properly account for luminescence emission after extended stimulation. The presence of luminescence above background level at long stimulation times has been discussed as being indicative of either marked non-®rst-order eects such as electron re-trapping or the release of charge from multiple traps each
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 6 9 4 - 2
M.L. Chithambo, R.B. Galloway / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 358±368
with a distinct probability of stimulation [2±4]. It is interesting to note that measurements made at elevated temperatures do not get rid of the slow component altogether (for example, [2,5]). This work investigated the temperature and dose-dependent properties of luminescence lifetimes (s), and the in¯uence of measurement temperature on luminescence intensity in the slow component region of quartz OSL. Luminescence was stimulated by pulsed 470 nm blue light-emitting diodes (LEDs). The preliminary conference reports on the in¯uence of temperature on luminescence lifetimes from quartz [6,7] did not speci®cally concern the slow component, did not have any model based ®tting to the measured data and used green stimulating light which is not so ecient in the stimulation of luminescence from quartz as the blue light used in the present work. The aim of the conference reports as of this work is to use time-resolved luminescence techniques to investigate the physical processes of luminescence recombination in quartz. The measured lifetimes are closely linked with the emitted luminescence intensity which in turn is aected by probabilities of radiative and non-radiative transitions [8]. If the probability of radiative recombination is assumed to be independent of temperature, and that of non-radiative recombination is related to temperature by the factor exp DE=kT ) [8±11], the change of luminescence lifetime as a function of temperature may be expressed as s0 s ; 1 1 C e DE=kT where s0 is the radiative lifetime at absolute zero of temperature, DE is the thermal activation energy of quenching, k is Boltzmann's constant, T is absolute temperature and C is a constant related to the eective density of states [12]. Using similar assumptions, the change of luminescence intensity as a function of temperature can be appropriately described by thermal quenching of the luminescence intensity [3,11±13], I0 I T ; 2 1 C e DE=kT where I0 denotes the initial value of luminescence intensity and the other parameters are as described
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previously. These equations also describe the behaviour of the F centre involved in luminescence emission from alkali halides [8] and from a-Al2 O3 [14,15]. In contrast such thermal quenching is not observed from feldspars [16,17]. 2. Experimental methods Quartz samples were prepared from commercially available (BDH Ltd.) `acid washed' sand, grain size 90±500 lm. Prior to use, the samples were heated at 773 K for 2 min to remove any remaining OSL signal and to increase the sensitivity of samples to luminescence stimulation. Tests on the purity of the quartz by infrared excitation of dosed samples indicated a negligible feldspar content and samples were used without additional chemical treatment. The same quartz has been used in previous studies by Galloway et al. [18]. Owing to the reported thermal stability of the slow component [2,5], further tests were performed to assess the amount of slow component signal present in the samples after annealing at 773 K as described above. A comparison of the slow component OSL from undosed samples, some of which had been preheated, with that from a dosed and preheated sample, indicated that the counting rate from the undosed quartz was 3% of that in the slow component OSL of the dosed sample. Thus the quartz has a negligible remanent slow component OSL signal compared to the slow component from the dosed samples to be discussed here. The test dose used was 4.5 Gy, the smallest of the doses used in the present study. Each sample was given a beta dose and preheated at 493 K for 5 min before measurement of time-resolved luminescence spectra. Beta doses used were 4.5, 15 and 150 Gy. A set of four 470 nm blue LEDs (Nichia NSPB-500) were used for pulsed stimulation of luminescence. The LEDs were pulsed at a pulse width and duty cycle of 11 ls and 12.4%, respectively, and operated with a pulsed current of about 70 mA per LED, as described by Chithambo and Galloway [19]. Using all four light emitting diodes each operated as described above, the mean light intensity at the
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sample was measured as 0:60 mW cm 2 using a calibrated PIN diode (Ealing Electro-Optics). All times quoted for experiments performed in this work have been corrected for the duty cycle, that is, they are the total time of exposure to the luminescence stimulating light. An EMI 9635QA photomultiplier with a DUG-11 ®lter in front was used for detection of luminescence and a HA3 heat absorbing ®lter was also used to protect the photomultiplier and DUG-11 ®lter at high temperatures. A 2 mm thick GG420 ®lter was put in front of each LED in order to attenuate the low wavelength end of the LED spectrum. Time-resolved luminescence spectra were recorded at 20 K intervals between 293 and 473 K using a combination of an ORTEC model 467 time-to-pulse-height convertor and a 256-channel analog-to-digital convertor (Laben 8213) with each spectrum covering a 64 ls dynamic range [19]. Luminescence lifetimes were calculated from time-resolved luminescence spectra after the end of the stimulating light pulse on the assumption that if N t is the number of stimulated electrons at time t, and j is the probability of decay of a stimulated electron, the luminescence, L after the end of a stimulating pulse of width t1 falls away exponentially as L jN t1 e
t t1 =s
;
with decay parameters kA ; kB ; kC , where kC < kB < kA . The component associated with the slowest decay parameter, kC is variously referred to as the long tail, long-term or slow component [1,2,4]. Although each of the three exponential components may not necessarily be associated with a separable physical mechanism, the method of multi-exponential ®tting provides a convenient representation of the shape of the decay curve. The relative intensities of exponential components extracted from a decay curve of a sample used in this study are shown in Fig. 1 where the ratio of each component to the sum of the three components is plotted against the time of optical stimulation. As Fig. 1 shows, in the initial transient, nearly 75% of the measured luminescence is due to the fast component with the slow component contributing just under 10%. As stimulation progresses, the fast component declines to an insigni®cant percentage of the total while the medium component increases in importance from about 20% to 40% of the sum in the ®rst 30 s and progressively declines thereafter. In contrast, the contribution from the slow component evolves from a minimum of about 10±50% of the total in
3
where s is the lifetime of the decay. The data showed no indication of more than one exponential decay component. Time-resolved luminescence spectra used in these calculations corresponded to a counting rate of less than one luminescence photon for each luminescence stimulating pulse. All the calculations of thermal quenching and thermal assistance assume that the emission spectrum has not changed as a function of temperature. However, this may be a potential source of systematic error in OSL as in TL [20].
3. The slow component Luminescence decay curves from quartz samples can be ®tted by a sum of three exponentials
Fig. 1. The relative importance of the components Cn of a decay curve as a function of stimulation time, showing the fall of the fast component (C1 ), the rise and fall of the medium component (C2 ) and the eventual dominance of the slow component (C3 ). The sample was given a beta dose of 150 Gy and preheated at 473K for 5 min prior to measurement of the luminescence.
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the ®rst 50 s and by 150 s the measured luminescence consists predominantly of the slow component. Although in Fig. 1 the abscissa is presented in hundreds of seconds, the slow component can still be measured after tens of thousands of seconds as some of our preparatory measurements showed repeatedly. This work was therefore concerned with investigating properties of luminescence lifetimes and luminescence intensity for samples of quartz whose luminescence had been bleached to the slow component region. 4. Results and discussion 4.1. Eect of temperature on luminescence intensity In the ®rst test to observe the relationship between temperature and luminescence intensity, a series of 1 s measurements were made from 293 K, in 20 K intervals on a sample which still retained its fast component, immediately after administering a dose of 4.5 Gy and preheating. In this test, the luminescence intensity from the sample held at successively higher temperatures was compared with that from a second sample held at room temperature. It was observed that under continual optical stimulation the luminescence decreased in both the sample at constant temperature (as expected) as well as in the sample subjected to heating. From 293 K to about 373 K the decrease from the two samples was indistinguishable. Above 373 K, the luminescence from the sample being heated decreased more rapidly. This behaviour can be further clari®ed in a plot of the ratio of intensity values from the heated (Ih ) to the unheated sample (Iuh ) against the temperature of the heated sample as shown in Fig. 2. In this case, the ratio of intensities remains constant for the ®rst 5 data points (293±373 K for the sample subject to heating) showing that the rate of decay of luminescence from the two samples is equivalent. Beyond 373 K, the faster fall in intensity from the sample being heated causes the monotonic decrease in the ratio of intensities. The change in luminescence intensity as a function of temperature as shown in Fig. 2 can be
Fig. 2. Ratio of intensities of luminescence stimulated from a sample under simultaneous heating and optical stimulation (Ih ) to that under optical stimulation only (Iuh ). The ratios are plotted against the temperature of the heated sample.
described by the thermal quenching relationship of Eq. (2). The solid line through the data points in Fig. 2 is the best ®t of Eq. (2). The values of DE and C evaluated from the best ®t to the experimental data are 0:63 0:07 eV and 2 107 , respectively. The temperature-dependent variations of luminescence intensity in the slow component region for a sample of quartz dosed to 150 Gy before preheating and measurement of luminescence are shown in Fig. 3. Fig. 3(a), where the measurements
Fig. 3. The relationship between luminescence intensity and temperature for the slow component of quartz luminescence for measurements made from 293 to 473 K (a) and from 473 to 293 K (b).
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were made from 293 to 473 K, shows that the increase in temperature is accompanied by an increase in luminescence intensity, the maximum of which occurs at about 398 K. From 293 to 398 K, the luminescence intensity has approximately doubled and between 398 and 473 K, the luminescence intensity decreases continuously. When the sequence of measurements is reversed, i.e. 473± 293 K as shown in Fig 3(b), the intensity again goes through a peak as a function of temperature with the maximum at about 398 K. The dierence in intensity between corresponding data points in Figs. 3(a) and (b) represents the luminescence lost due to the sequence of measurements at elevated temperature. In these graphs, the change in luminescence intensity with temperature is easy to observe since in the slow component region the luminescence due to repeated stimulations at constant temperature changes only slowly over the measurement period. In the slow component region, an additional thermal assistance term exponentially related to temperature [13] was required to fully account for the temperature related changes in luminescence intensity as follows: I T
I0 e Ea =kT ; 1 Ce DE=kT
4
where Ea is the activation energy of thermal assistance and the other parameters are as previously de®ned. The solid curves in Fig. 3 are the best ®ts of Eq. (4) to the measurements made with temperature increasing (a) and with temperature decreasing (b). The ®ts were made on the basis that the initial increase in luminescence intensity with temperature was a result of thermal assistance to optical stimulation and that the subsequent decrease of luminescence intensity was due to thermal quenching of the luminescence, and that the two processes act independently. The luminescence suers from thermal quenching as electrons in the excited state of the luminescence centre can absorb a thermal energy DE and make a transition to the ground state without the emission of luminescence [9,11]. The resulting value of thermal quenching and thermal assistance activation energies, DE and
Ea , respectively, and the constant C are as follows: DE 0:68 0:11 eV, Ea 0:09 0:01 eV, C 7 107 (Fig. 3(a)) and DE 0:57 0:08 eV, Ea 0:08 0:01 eV, C 2 107 (Fig. 3(b)). The two values of the activation energy for thermal quenching in the slow component region are thus similar to the corresponding value (0:63 0:07 eV) evaluated from intensity changes as function of temperature in the fast component region of the OSL as are the values of C 2 107 . Eq. (4) was used by Murray and Wintle [3] to describe an analogous change of OSL with temperature in quartz and to calculate values of both DE, and Ea . The loss of signal between the two runs in Fig. 3 although expected is signi®cant. Thus there may be a need for correcting for loss of signal. As is unclear what such correction might involve, values of DE, and Ea from Fig. 3 should be treated with caution. Qualitatively, the loss of signal in the sequence of measurements with temperature increasing should cause the luminescence intensity to be too low at higher temperatures whereas in the sequence of measurements with temperature decreasing the luminescence intensity should be too low at the lower temperatures. Thus curves (a) and (b) in Fig. 3 should bracket the true behaviour and the parameters deduced should serve as upper and lower limits on the parameter values. It may be noted from Table 1 that the dierence between the parameters deduced from curves (a) and (b) in Fig. 3 is not statistically signi®cant. 4.2. Eect of temperature on luminescence lifetimes The eect of temperature on luminescence lifetimes was investigated in order to understand further the physical mechanisms associated with the emission of luminescence in the slow component region of the luminescence from quartz. Fig. 4 is an example of a time-resolved luminescence spectrum recorded from a sample dosed up to 4.5 Gy and preheated at 493 K prior to measurement of luminescence. During the stimulating pulse, the luminescence signal builds up over a background consisting of photomultiplier noise and scattered stimulating light from the LEDs. After the pulse, the measured luminescence, over a
Slow component
Intensity
Integrated OSL
293±473 293±473 293±473 473±293 473±293 293±473 473±293 293±473 298±498 323±673 473±553 123±473 293±653c 293±493
Measurement temperatures (°C)
454
470 470 470 470 470 470 470 470 420±550a 420±560a 420±560a 481
Stimulation wavelength (nm)
b
350
b
d
b
4.5 15 150 15 150 150 150 15 58 31 0.64
0:64 0:03 0:69 0:07 0:66 0:04 0:67 0:05 0:58 0:06 0:68 0:11 0:57 0:08 0:63 0:07 0:636 0:013 0.60 0:79 0:02
Radiation dose DE (eV) (Gy)
b
The eective wavelength of the broad band source (a ®ltered halogen lamp) was deduced to be 468 nm [3]. Measurements refer to natural luminescence. c Deduced from Fig. 3 and Fig. 4 of Wintle [21]. d The dose rate of the 90 Sr=90 Y beta source was 1:7 mGy s 1 .
a
Fast component Integrated OSL Integrated OSL Slow component Integrated OSL
Slow component
Lifetimes
Intensity Intensity Intensity Intensity Radioluminescence Intensity
Luminescence intensity region
Luminescence parameter analysed
Table 1 A comparison of thermal assistance and thermal quenching parameters in quartz
0.05
0.07
0.044
0:09 0:01 0:08 0:01
Ea (eV)
2:8 107
Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. [3] [4] [5] [13] [21] [22]
3 107 8 107 4 107 5 107 5 106 7 107 2 106 2 107 3:4 107 7:9 106 5:75 108
6(a) 6(b) 6(c) 6(d) 6(e) 3(a) 3(b) 2(b)
Reference
C
M.L. Chithambo, R.B. Galloway / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 358±368 363
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M.L. Chithambo, R.B. Galloway / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 358±368
Fig. 4. (a) Time-resolved luminescence from quartz stimulated at 470 nm, and measured over a dynamic range of 64 ls following a beta dose of 4.5 Gy and preheating at 493 K for 5 min. The measurements were made with a Schott DUG-11 (transmission band 300±380 nm FWHM) ®lter in front of the photomultiplier, an EMI 9635QA. (b) Background recorded from an undosed sample. The higher counting rate from the undosed sample from 0 to 4 ls is due to scattered light during the last 4 ls of the stimulating pulse.
background of photomultiplier noise only, was described by the single exponential function of Eq. (3) from which the lifetime was evaluated. Fig. 5 shows the change of lifetimes with temperature at which time-resolved luminescence is recorded for a sample dosed up to 15 Gy. Measurements were made from 293 to 473 K for cycle 1 (Fig. 5(a)) and repeated vice-versa for cycle 2 (Fig. 5(b)). In Fig. 5(a), values of lifetime between 293 and 373 K are essentially constant at about 36 2 ls. From then on the lifetimes decrease to a minimum of 7:8 0:2 ls, a decrease of about 75% from 373 to 473 K. The repeat measurements from 473 to 293 K shown in Fig. 5(b) follow a similar pattern. From a minimum value of 8:0 0:2 ls, the lifetime values increase as the temperature is reduced down to 373 K from where the values are constant within 36 2 ls, in agreement with values in cycle 1 (Fig. 5(a)). Experimental data in both cases are well ®tted by Eq. (1). In analogous measurements made using pulsed green LEDs [6] on a sample of quartz dosed to 150 Gy, luminescence lifetimes also decreased monotonically with temperature and at an even greater rate above 398 K. The values of luminescence
Fig. 5. The relationship between luminescence lifetime and temperature in the slow component region of quartz luminescence for measurements made from 293 to 473 K (a) from 473 to 293 K (b). The inset shows examples of time-resolved spectra measured at 293 K (c) and 473 K (d). Lifetimes were determined only from the part of the time-resolved luminescence spectrum following the end of the light pulse. For the inset, zero time is after the end of the stimulating light pulse (that is, at 4:5 ls in Fig. 4).
lifetimes were measured as 29:0 0:5 ls at 293 K, 24:2 0:3 ls at 398 K, and 7:4 0:1 ls at 473 K. These values represent a decrease of approximately 17% and 70% in the intervals 293±398 K and 398±473 K, respectively, which can be compared to the case of blue light stimulation described above. 4.3. Dependence of lifetimes on radiation dose This experiment was conducted to investigate whether the size of the initial dose (i.e. the initial charge concentration) in¯uenced the lifetimes in the slow component region of quartz OSL. Three separate samples of quartz were given beta doses of 4.5, 15 and 150 Gy and preheated at 493 K for 5 min. The luminescence from each sample was then bleached down to the slow component region, taken as a small fraction (1±1.5%) of the initial intensity. As a means of con®rming that the luminescence intensity had reached the slow component region of the OSL, the bleaching was done sequentially in stages by exposure to the blue LEDs for a ®xed number of seconds, and the luminescence intensity monitored at the end of each run.
M.L. Chithambo, R.B. Galloway / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 358±368
The variation of lifetime with temperature for measurements from 293 to 473 K (cycle 1) are shown in Figs. 6(a)±(c) and for measurements from 473 to 293 K (cycle 2) in Figs. 6(d) and (e). It is apparent that for all measurements in cycle 1 (Figs. 6(a)±(c)), the lifetime values between 293 and 373 K are eectively constant within 36 2 ls despite the variation in radiation dose. Once the measurement temperature is increased above 373 K, the lifetime values decrease rapidly. For example, the lifetime values for the sample irradiated to 4.5 Gy (Fig. 6(a)) changes by about 76% between 373 and 473 K, i.e. from about 34 ls at 373 K to about 8 ls at 473 K. In general, however, there is similarity in lifetime values associated with dierent doses over the whole temperature range (293±473 K). It is interesting to note that
Fig. 6. Graphs showing changes in luminescence lifetime when measurements are made from 293 to 473 K in the slow component region for samples given dierent initial doses (a) 4.5 Gy, (b) 15 Gy, (c) 150 Gy and from 473 to 293 K (d) 15 Gy, (e) 150 Gy.
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even in measurements concerning the time dependence of luminescence lifetimes at constant temperature for heated quartz stimulated by pulsed green light [7], the change of lifetimes with stimulation time was independent of radiation dose over the range 15±150 Gy. The repeat measurements (473±293 K) shown in Figs. 6(d) and (e) evidently show good agreement with results shown in set Figs. 6(a)±(c) in terms of the change of lifetime with temperature, and in the values of lifetime at speci®c temperatures. The values of lifetime are similar regardless of the immediate thermal history of the sample, at least for the two heating cycles used. The results of Fig. 6 show that lifetime values in the slow component region for quartz under blue light stimulation decrease with temperature above 373 K and that this change is independent of the size of the initial dose. The temperature-dependent change of lifetimes in the slow component can be ascribed to increased thermal quenching of luminescence according to Eq. (1). Values of DE, the activation energy of thermal quenching and the parameter C evaluated from the best ®t of Eq. (1) to the data of lifetime variations with temperature are summarised in Table 1 and compared with values determined from the temperature dependence of luminescence intensity in the slow component region (Figs. 3(a) and (b)) and in the fast component region (Fig. 2). The full range of values of activation energy for thermal quenching DE, thermal assistance Ea , and the constant C are compared in Table 1 with corresponding values evaluated from temperature related changes in luminescence intensity published elsewhere [3±5,13,21,22]. All data in Table 1 are for luminescence detected in the region 300±450 nm as de®ned by the transmission ®lters used. It is evident from Table 1 that values of DE and C determined from either lifetime or intensity analysis in this work are consistent. Values of DE and C are not only independent of the order of change in measurement temperature, but are also not signi®cantly in¯uenced by the size of beta dose or whether the measurement is made in the fast or the slow component region of the OSL. It is encouraging to note that values of DE; Ea and C
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M.L. Chithambo, R.B. Galloway / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 358±368
evaluated from analysis of both luminescence intensity and lifetimes in this work are consistent with values worked out from the temperature dependence of luminescence intensity as reported elsewhere (Table 1). 4.4. Thermal quenching of luminescence lifetimes Interpretation of thermal quenching of luminescence lifetimes in quartz in terms of a speci®c physical mechanism is complicated by the fact that the luminescent centre in quartz is ambiguous [23] and the mechanisms of charge eviction from shallow traps are by no means clear. Luminescence lifetimes may be associated with three possible physical mechanisms, namely (a) the time to evict an electron from a trap, se ; (b) time for an electron to travel from the trap through the lattice to the recombination centre sl and (c) lifetime of the excited state at a recombination centre sf [9,17]. In photon-induced charge eviction from a trap, estimates of the lifetime in the trap se are usually short and of the order of 1 ns or less [17]. The time of transport from trap to recombination centre, sl , is less easy to estimate as it can be aected by retrapping of electrons prior to recombination as well as by lattice vibrations. The in¯uence of shallow traps on lifetimes was demonstrated by Akselrod et al. [14,15] who reported that shallow traps in a-Al2 O3 caused an increase in the measured luminescence lifetime near room temperature due to re-trapping processes. In most cases, however, the lifetime is dominated by intraluminescence centre relaxation time sf which can usually be associated with a given transition at the luminescence centre [24±26]. If the lifetime values in this ®nal possibility have contributions from thermal excitation and radiative decay, then overall the lifetime will depend on temperature as shown earlier in Eq. (1). Thermal quenching of lifetimes de®ned in this sense is not uncommon and has been observed on other materials such as KCl [8] and a-Al2 O3 [14,15]. In these materials, the fall in lifetimes with temperature was ascribed to thermal quenching at the excited F centres. It should be emphasised however that the precise factors leading to this
quenching were not described explicitly, although the model fully accounted for the decrease in luminescence intensity. In other materials, thermal quenching of luminescence is not always accompanied by quenching of lifetimes. For instance, Barnett and Baili [16] observed that lattice vibrations were associated with thermal quenching of luminescence in potassium feldspars. In studies of lifetimes in similar materials by Clark et al. [17], there was no evidence of consistent variation with temperature that could be described in terms of thermal quenching. Thus it appears that the eect of temperature on the recombination centres and temperature related changes in the lattice may be relevant in a discussion of the fall of luminescence lifetimes with temperature. The increase in temperature may alter the spatial separation of traps and recombination centres thus aecting the associated con®gurational coordinate. The distortions in the con®gurational coordinate that occur may cause near contact between the ground state and the relevant excited state [24,27,28], a proximity leading to non-radiative transitions and thus a fall in luminescence intensity as a function of temperature. If, in addition, the density of recombination centres increased with temperature, the probability of recombination at higher temperature would increase. A direct consequence of this would be that recombinations would occur faster after electron release from a trap, and thus the measured lifetimes would be shorter than at room temperature. However, since it was observed that increase in temperature is also accompanied by a decrease in luminescence intensity, it must be that more of the recombinations at higher temperatures are nonradiative. Although this work has not addressed the nature of recombination centres, some experimental work, e.g. [23], suggests that an increase in the number of recombination centres as a function of temperature is a possibility. In answer to Bailey [5], ``the question of whether the slow component exhibits thermal quenching remains largely unresolved'', we have found both thermal quenching and thermal stimulation in the slow component of quartz OSL.
M.L. Chithambo, R.B. Galloway / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 358±368
5. Conclusions Characteristics of luminescence lifetimes and luminescence intensity from quartz under pulsed blue LED stimulation have been studied at long stimulation times. It was shown that the slow component increases in importance from just under 10% in the initial transient to about 50% after 50 s, and from 150 s onwards the measured luminescence consists of the slow component. The change of luminescence intensity as a function of temperature was studied in the slow component region where it was observed that increase in temperature produces a peak at 398 K in the intensity of luminescence measured as a function of temperature between 293 and 473 K. In the fast component region, increase in temperature above 383 K leads to a much more rapid reduction in intensity of luminescence with duration of stimulation than that observed at room temperature. Dynamic temperature investigations of lifetimes in the slow component region revealed that lifetimes are strongly aected by temperature of measurement. Luminescence lifetimes measured between 293 and 398 K are constant within 36:0 2:0 ls. From 373 K lifetimes decrease to about 8 ls at 473 K, the maximum temperature investigated. The temperature-dependent changes in both luminescence intensity and mean lifetimes are independent of the order in which the measurements are made. The increase of luminescence intensity with temperature was ascribed to thermal assistance to optical stimulation whereas the decrease of both luminescence intensity and lifetimes with temperature could be adequately described by thermal quenching functions. Processes of thermal assistance and thermal quenching were assumed to act independently hence cases where the luminescence intensity went through a peak as a function of temperature could be adequately analysed by combination of the thermal assistance and thermal quenching functions. Values of activation energy for thermal assistance and thermal quenching abstracted from analysis of luminescence intensity in this work are consistent with corresponding values evaluated using similar techniques reported else-
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where [3±5,13,21,22]. Values of the activation energy for thermal quenching derived from temperature-related changes in luminescence lifetimes are consistent with values determined from temperature-dependent changes in the luminescence intensity. Acknowledgements We wish to thank Mr. Harry Napier and Mr. Paul Harris for technical assistance. MC thanks the Commonwealth Scholarship Commission for ®nancial support. References [1] B.W. Smith, E.J. Rhodes, Radiat. Meas. 23 (1994) 329. [2] R.M. Bailey, B.W. Smith, E.J. Rhodes, Radiat. Meas. 27 (1997) 123. [3] A.S. Murray, A.G. Wintle, Radiat. Meas. 29 (1998) 65. [4] S.W.S. McKeever, L. Bùtter-Jensen, N. Agersnap Larsen, G.A.T. Duller, Radiat. Meas. 27 (1997) 161. [5] R.M. Bailey, Radiat. Meas. 32 (2000) 233. [6] M.L. Chithambo, R.B. Galloway, Radiat.Meas. 32 (2000) 627. [7] M.L. Chithambo, R.B. Galloway, Radiat. Meas. 32 (2000) 621. [8] R.K. Swank, F.C. Brown, Phys. Rev. 130 (1963) 34. [9] S.W.S. McKeever, Thermoluminescence of Solids, Cambridge University Press, Cambridge, 1985. [10] D. Curie, Luminescence in Crystals, Wiley, New York, 1963. [11] N.F. Mott, Proc. Roy. Soc. London A. 167 (1938) 384. [12] J.P. Liu, M.Y. Kong, J.J. Si, D.D. Huang, J.P. Liu, D.Z. Sun, J. Phys. D 31 (1988) 85. [13] N.A. Spooner, Radiat. Meas. 27 (1994) 593. [14] M.S. Akselrod, N. Agersnap Larsen, V. Whitley, S.W.S. McKeever, J. Appl. Phys. 84 (1998) 3364. [15] M.S. Akselrod, N. Agersnap Larsen, V. Whitley, S.W.S. McKeever, Radiat. Prot. Dosim. 84 (1999) 39. [16] S.M. Barnett, I.K. Baili, J. Phys. D 30 (1997) 683. [17] R.J. Clark, I.K. Baili, M.J. Tooley, Radiat. Meas. 27 (1997) 211. [18] R.B. Galloway, D.G. Hong, H. Napier, Meas. Sci. Technol. 8 (1997) 267. [19] M.L. Chithambo, R.B. Galloway, Meas. Sci. Technol. 11 (2000) 418. [20] A.D. Franklin, J.R. Prescott, R.B. Schole®eld, J. Lumin. 63 (1995) 417. [21] A.G. Wintle, Geophys. J. R. Astron. Soc. 41 (1975) 107. [22] D.J. Huntley, M.A. Short, K. Dunphy, Can. J. Phys. 74 (1996) 81.
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[23] N.R.J. Poolton, G.M. Smith, P.C. Riedi, E. Bulur, L. Bùtter-Jensen, A.S. Murray, M. Adrian, J. Phys. D 33 (2000) 1007. [24] A.S. Marfunin, Spectroscopy, Luminescence and Radiation Centres in Minerals, Springer, Berlin, 1979. [25] F. Agullo-Lopez, C.R.A. Catlow, P.D. Townsend, Point Defects in Materials, Academic Press, New York, 1988.
[26] S.R. Elliot, Physics of Amorphous Materials, Longman, New York, 1990. [27] F.C. Brown, The Physics of Solids, Benjamin, New York, 1967. [28] A.J. Dekker, Solid State Physics, Macmillan, New York, 1958.
www.elsevier.com/locate/nimb
On the slow component of luminescence stimulated from quartz by pulsed blue light-emitting diodes M.L. Chithambo, R.B. Galloway
*
Department of Physics and Astronomy, The University of Edinburgh, James Clerk Maxwell Building, The Kings Buildings, May®eld Road, Edinburgh EH9 3JZ, UK Received 6 June 2000; received in revised form 19 April 2001
Abstract Characteristics of luminescence lifetimes and luminescence intensity from quartz have been studied as a function of temperature at long stimulation times (the `slow component' region). In this region, luminescence lifetime values are strongly aected by temperature especially from 373 to 473 K. Luminescence intensity values increase from 293 to 398 K and then decrease until 473 K, the maximum temperature investigated. These changes are independent of the order in which the measurements are made and have been discussed in terms of thermal assistance and thermal quenching of the luminescence. Activation energies for thermal assistance and thermal quenching evaluated from temperature-dependent changes of both luminescence intensity and luminescence lifetimes are given. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 78.47.+p Keywords: Luminescence lifetimes; Quartz; Pulsed LEDs; Thermal assistance; Thermal quenching
1. Introduction The fall of optically stimulated luminescence (OSL) with time that results from continuous excitation of luminescence from quartz can usually be described by a sum of three exponentials. Examples of this were reported by Smith and Rhodes [1] for luminescence stimulated from a naturally bleached Morrocan quartz sample using a 514 nm *
Corresponding author. Tel.: +44-131-650-1000; fax: +44131-662-7174. E-mail address: [email protected] (R.B. Galloway).
laser, by Bailey et al. [2] on an aliquot from the same material with stimulation by a ®ltered (420± 560 nm) halogen lamp and also noted by Murray and Wintle [3], and McKeever et al. [4]. In cases where multi-exponential regression was used to describe the time dependence of the OSL [1,2], the third exponential term was required to properly account for luminescence emission after extended stimulation. The presence of luminescence above background level at long stimulation times has been discussed as being indicative of either marked non-®rst-order eects such as electron re-trapping or the release of charge from multiple traps each
0168-583X/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 1 ) 0 0 6 9 4 - 2
M.L. Chithambo, R.B. Galloway / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 358±368
with a distinct probability of stimulation [2±4]. It is interesting to note that measurements made at elevated temperatures do not get rid of the slow component altogether (for example, [2,5]). This work investigated the temperature and dose-dependent properties of luminescence lifetimes (s), and the in¯uence of measurement temperature on luminescence intensity in the slow component region of quartz OSL. Luminescence was stimulated by pulsed 470 nm blue light-emitting diodes (LEDs). The preliminary conference reports on the in¯uence of temperature on luminescence lifetimes from quartz [6,7] did not speci®cally concern the slow component, did not have any model based ®tting to the measured data and used green stimulating light which is not so ecient in the stimulation of luminescence from quartz as the blue light used in the present work. The aim of the conference reports as of this work is to use time-resolved luminescence techniques to investigate the physical processes of luminescence recombination in quartz. The measured lifetimes are closely linked with the emitted luminescence intensity which in turn is aected by probabilities of radiative and non-radiative transitions [8]. If the probability of radiative recombination is assumed to be independent of temperature, and that of non-radiative recombination is related to temperature by the factor exp DE=kT ) [8±11], the change of luminescence lifetime as a function of temperature may be expressed as s0 s ; 1 1 C e DE=kT where s0 is the radiative lifetime at absolute zero of temperature, DE is the thermal activation energy of quenching, k is Boltzmann's constant, T is absolute temperature and C is a constant related to the eective density of states [12]. Using similar assumptions, the change of luminescence intensity as a function of temperature can be appropriately described by thermal quenching of the luminescence intensity [3,11±13], I0 I T ; 2 1 C e DE=kT where I0 denotes the initial value of luminescence intensity and the other parameters are as described
359
previously. These equations also describe the behaviour of the F centre involved in luminescence emission from alkali halides [8] and from a-Al2 O3 [14,15]. In contrast such thermal quenching is not observed from feldspars [16,17]. 2. Experimental methods Quartz samples were prepared from commercially available (BDH Ltd.) `acid washed' sand, grain size 90±500 lm. Prior to use, the samples were heated at 773 K for 2 min to remove any remaining OSL signal and to increase the sensitivity of samples to luminescence stimulation. Tests on the purity of the quartz by infrared excitation of dosed samples indicated a negligible feldspar content and samples were used without additional chemical treatment. The same quartz has been used in previous studies by Galloway et al. [18]. Owing to the reported thermal stability of the slow component [2,5], further tests were performed to assess the amount of slow component signal present in the samples after annealing at 773 K as described above. A comparison of the slow component OSL from undosed samples, some of which had been preheated, with that from a dosed and preheated sample, indicated that the counting rate from the undosed quartz was 3% of that in the slow component OSL of the dosed sample. Thus the quartz has a negligible remanent slow component OSL signal compared to the slow component from the dosed samples to be discussed here. The test dose used was 4.5 Gy, the smallest of the doses used in the present study. Each sample was given a beta dose and preheated at 493 K for 5 min before measurement of time-resolved luminescence spectra. Beta doses used were 4.5, 15 and 150 Gy. A set of four 470 nm blue LEDs (Nichia NSPB-500) were used for pulsed stimulation of luminescence. The LEDs were pulsed at a pulse width and duty cycle of 11 ls and 12.4%, respectively, and operated with a pulsed current of about 70 mA per LED, as described by Chithambo and Galloway [19]. Using all four light emitting diodes each operated as described above, the mean light intensity at the
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sample was measured as 0:60 mW cm 2 using a calibrated PIN diode (Ealing Electro-Optics). All times quoted for experiments performed in this work have been corrected for the duty cycle, that is, they are the total time of exposure to the luminescence stimulating light. An EMI 9635QA photomultiplier with a DUG-11 ®lter in front was used for detection of luminescence and a HA3 heat absorbing ®lter was also used to protect the photomultiplier and DUG-11 ®lter at high temperatures. A 2 mm thick GG420 ®lter was put in front of each LED in order to attenuate the low wavelength end of the LED spectrum. Time-resolved luminescence spectra were recorded at 20 K intervals between 293 and 473 K using a combination of an ORTEC model 467 time-to-pulse-height convertor and a 256-channel analog-to-digital convertor (Laben 8213) with each spectrum covering a 64 ls dynamic range [19]. Luminescence lifetimes were calculated from time-resolved luminescence spectra after the end of the stimulating light pulse on the assumption that if N t is the number of stimulated electrons at time t, and j is the probability of decay of a stimulated electron, the luminescence, L after the end of a stimulating pulse of width t1 falls away exponentially as L jN t1 e
t t1 =s
;
with decay parameters kA ; kB ; kC , where kC < kB < kA . The component associated with the slowest decay parameter, kC is variously referred to as the long tail, long-term or slow component [1,2,4]. Although each of the three exponential components may not necessarily be associated with a separable physical mechanism, the method of multi-exponential ®tting provides a convenient representation of the shape of the decay curve. The relative intensities of exponential components extracted from a decay curve of a sample used in this study are shown in Fig. 1 where the ratio of each component to the sum of the three components is plotted against the time of optical stimulation. As Fig. 1 shows, in the initial transient, nearly 75% of the measured luminescence is due to the fast component with the slow component contributing just under 10%. As stimulation progresses, the fast component declines to an insigni®cant percentage of the total while the medium component increases in importance from about 20% to 40% of the sum in the ®rst 30 s and progressively declines thereafter. In contrast, the contribution from the slow component evolves from a minimum of about 10±50% of the total in
3
where s is the lifetime of the decay. The data showed no indication of more than one exponential decay component. Time-resolved luminescence spectra used in these calculations corresponded to a counting rate of less than one luminescence photon for each luminescence stimulating pulse. All the calculations of thermal quenching and thermal assistance assume that the emission spectrum has not changed as a function of temperature. However, this may be a potential source of systematic error in OSL as in TL [20].
3. The slow component Luminescence decay curves from quartz samples can be ®tted by a sum of three exponentials
Fig. 1. The relative importance of the components Cn of a decay curve as a function of stimulation time, showing the fall of the fast component (C1 ), the rise and fall of the medium component (C2 ) and the eventual dominance of the slow component (C3 ). The sample was given a beta dose of 150 Gy and preheated at 473K for 5 min prior to measurement of the luminescence.
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the ®rst 50 s and by 150 s the measured luminescence consists predominantly of the slow component. Although in Fig. 1 the abscissa is presented in hundreds of seconds, the slow component can still be measured after tens of thousands of seconds as some of our preparatory measurements showed repeatedly. This work was therefore concerned with investigating properties of luminescence lifetimes and luminescence intensity for samples of quartz whose luminescence had been bleached to the slow component region. 4. Results and discussion 4.1. Eect of temperature on luminescence intensity In the ®rst test to observe the relationship between temperature and luminescence intensity, a series of 1 s measurements were made from 293 K, in 20 K intervals on a sample which still retained its fast component, immediately after administering a dose of 4.5 Gy and preheating. In this test, the luminescence intensity from the sample held at successively higher temperatures was compared with that from a second sample held at room temperature. It was observed that under continual optical stimulation the luminescence decreased in both the sample at constant temperature (as expected) as well as in the sample subjected to heating. From 293 K to about 373 K the decrease from the two samples was indistinguishable. Above 373 K, the luminescence from the sample being heated decreased more rapidly. This behaviour can be further clari®ed in a plot of the ratio of intensity values from the heated (Ih ) to the unheated sample (Iuh ) against the temperature of the heated sample as shown in Fig. 2. In this case, the ratio of intensities remains constant for the ®rst 5 data points (293±373 K for the sample subject to heating) showing that the rate of decay of luminescence from the two samples is equivalent. Beyond 373 K, the faster fall in intensity from the sample being heated causes the monotonic decrease in the ratio of intensities. The change in luminescence intensity as a function of temperature as shown in Fig. 2 can be
Fig. 2. Ratio of intensities of luminescence stimulated from a sample under simultaneous heating and optical stimulation (Ih ) to that under optical stimulation only (Iuh ). The ratios are plotted against the temperature of the heated sample.
described by the thermal quenching relationship of Eq. (2). The solid line through the data points in Fig. 2 is the best ®t of Eq. (2). The values of DE and C evaluated from the best ®t to the experimental data are 0:63 0:07 eV and 2 107 , respectively. The temperature-dependent variations of luminescence intensity in the slow component region for a sample of quartz dosed to 150 Gy before preheating and measurement of luminescence are shown in Fig. 3. Fig. 3(a), where the measurements
Fig. 3. The relationship between luminescence intensity and temperature for the slow component of quartz luminescence for measurements made from 293 to 473 K (a) and from 473 to 293 K (b).
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were made from 293 to 473 K, shows that the increase in temperature is accompanied by an increase in luminescence intensity, the maximum of which occurs at about 398 K. From 293 to 398 K, the luminescence intensity has approximately doubled and between 398 and 473 K, the luminescence intensity decreases continuously. When the sequence of measurements is reversed, i.e. 473± 293 K as shown in Fig 3(b), the intensity again goes through a peak as a function of temperature with the maximum at about 398 K. The dierence in intensity between corresponding data points in Figs. 3(a) and (b) represents the luminescence lost due to the sequence of measurements at elevated temperature. In these graphs, the change in luminescence intensity with temperature is easy to observe since in the slow component region the luminescence due to repeated stimulations at constant temperature changes only slowly over the measurement period. In the slow component region, an additional thermal assistance term exponentially related to temperature [13] was required to fully account for the temperature related changes in luminescence intensity as follows: I T
I0 e Ea =kT ; 1 Ce DE=kT
4
where Ea is the activation energy of thermal assistance and the other parameters are as previously de®ned. The solid curves in Fig. 3 are the best ®ts of Eq. (4) to the measurements made with temperature increasing (a) and with temperature decreasing (b). The ®ts were made on the basis that the initial increase in luminescence intensity with temperature was a result of thermal assistance to optical stimulation and that the subsequent decrease of luminescence intensity was due to thermal quenching of the luminescence, and that the two processes act independently. The luminescence suers from thermal quenching as electrons in the excited state of the luminescence centre can absorb a thermal energy DE and make a transition to the ground state without the emission of luminescence [9,11]. The resulting value of thermal quenching and thermal assistance activation energies, DE and
Ea , respectively, and the constant C are as follows: DE 0:68 0:11 eV, Ea 0:09 0:01 eV, C 7 107 (Fig. 3(a)) and DE 0:57 0:08 eV, Ea 0:08 0:01 eV, C 2 107 (Fig. 3(b)). The two values of the activation energy for thermal quenching in the slow component region are thus similar to the corresponding value (0:63 0:07 eV) evaluated from intensity changes as function of temperature in the fast component region of the OSL as are the values of C 2 107 . Eq. (4) was used by Murray and Wintle [3] to describe an analogous change of OSL with temperature in quartz and to calculate values of both DE, and Ea . The loss of signal between the two runs in Fig. 3 although expected is signi®cant. Thus there may be a need for correcting for loss of signal. As is unclear what such correction might involve, values of DE, and Ea from Fig. 3 should be treated with caution. Qualitatively, the loss of signal in the sequence of measurements with temperature increasing should cause the luminescence intensity to be too low at higher temperatures whereas in the sequence of measurements with temperature decreasing the luminescence intensity should be too low at the lower temperatures. Thus curves (a) and (b) in Fig. 3 should bracket the true behaviour and the parameters deduced should serve as upper and lower limits on the parameter values. It may be noted from Table 1 that the dierence between the parameters deduced from curves (a) and (b) in Fig. 3 is not statistically signi®cant. 4.2. Eect of temperature on luminescence lifetimes The eect of temperature on luminescence lifetimes was investigated in order to understand further the physical mechanisms associated with the emission of luminescence in the slow component region of the luminescence from quartz. Fig. 4 is an example of a time-resolved luminescence spectrum recorded from a sample dosed up to 4.5 Gy and preheated at 493 K prior to measurement of luminescence. During the stimulating pulse, the luminescence signal builds up over a background consisting of photomultiplier noise and scattered stimulating light from the LEDs. After the pulse, the measured luminescence, over a
Slow component
Intensity
Integrated OSL
293±473 293±473 293±473 473±293 473±293 293±473 473±293 293±473 298±498 323±673 473±553 123±473 293±653c 293±493
Measurement temperatures (°C)
454
470 470 470 470 470 470 470 470 420±550a 420±560a 420±560a 481
Stimulation wavelength (nm)
b
350
b
d
b
4.5 15 150 15 150 150 150 15 58 31 0.64
0:64 0:03 0:69 0:07 0:66 0:04 0:67 0:05 0:58 0:06 0:68 0:11 0:57 0:08 0:63 0:07 0:636 0:013 0.60 0:79 0:02
Radiation dose DE (eV) (Gy)
b
The eective wavelength of the broad band source (a ®ltered halogen lamp) was deduced to be 468 nm [3]. Measurements refer to natural luminescence. c Deduced from Fig. 3 and Fig. 4 of Wintle [21]. d The dose rate of the 90 Sr=90 Y beta source was 1:7 mGy s 1 .
a
Fast component Integrated OSL Integrated OSL Slow component Integrated OSL
Slow component
Lifetimes
Intensity Intensity Intensity Intensity Radioluminescence Intensity
Luminescence intensity region
Luminescence parameter analysed
Table 1 A comparison of thermal assistance and thermal quenching parameters in quartz
0.05
0.07
0.044
0:09 0:01 0:08 0:01
Ea (eV)
2:8 107
Fig. Fig. Fig. Fig. Fig. Fig. Fig. Fig. [3] [4] [5] [13] [21] [22]
3 107 8 107 4 107 5 107 5 106 7 107 2 106 2 107 3:4 107 7:9 106 5:75 108
6(a) 6(b) 6(c) 6(d) 6(e) 3(a) 3(b) 2(b)
Reference
C
M.L. Chithambo, R.B. Galloway / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 358±368 363
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Fig. 4. (a) Time-resolved luminescence from quartz stimulated at 470 nm, and measured over a dynamic range of 64 ls following a beta dose of 4.5 Gy and preheating at 493 K for 5 min. The measurements were made with a Schott DUG-11 (transmission band 300±380 nm FWHM) ®lter in front of the photomultiplier, an EMI 9635QA. (b) Background recorded from an undosed sample. The higher counting rate from the undosed sample from 0 to 4 ls is due to scattered light during the last 4 ls of the stimulating pulse.
background of photomultiplier noise only, was described by the single exponential function of Eq. (3) from which the lifetime was evaluated. Fig. 5 shows the change of lifetimes with temperature at which time-resolved luminescence is recorded for a sample dosed up to 15 Gy. Measurements were made from 293 to 473 K for cycle 1 (Fig. 5(a)) and repeated vice-versa for cycle 2 (Fig. 5(b)). In Fig. 5(a), values of lifetime between 293 and 373 K are essentially constant at about 36 2 ls. From then on the lifetimes decrease to a minimum of 7:8 0:2 ls, a decrease of about 75% from 373 to 473 K. The repeat measurements from 473 to 293 K shown in Fig. 5(b) follow a similar pattern. From a minimum value of 8:0 0:2 ls, the lifetime values increase as the temperature is reduced down to 373 K from where the values are constant within 36 2 ls, in agreement with values in cycle 1 (Fig. 5(a)). Experimental data in both cases are well ®tted by Eq. (1). In analogous measurements made using pulsed green LEDs [6] on a sample of quartz dosed to 150 Gy, luminescence lifetimes also decreased monotonically with temperature and at an even greater rate above 398 K. The values of luminescence
Fig. 5. The relationship between luminescence lifetime and temperature in the slow component region of quartz luminescence for measurements made from 293 to 473 K (a) from 473 to 293 K (b). The inset shows examples of time-resolved spectra measured at 293 K (c) and 473 K (d). Lifetimes were determined only from the part of the time-resolved luminescence spectrum following the end of the light pulse. For the inset, zero time is after the end of the stimulating light pulse (that is, at 4:5 ls in Fig. 4).
lifetimes were measured as 29:0 0:5 ls at 293 K, 24:2 0:3 ls at 398 K, and 7:4 0:1 ls at 473 K. These values represent a decrease of approximately 17% and 70% in the intervals 293±398 K and 398±473 K, respectively, which can be compared to the case of blue light stimulation described above. 4.3. Dependence of lifetimes on radiation dose This experiment was conducted to investigate whether the size of the initial dose (i.e. the initial charge concentration) in¯uenced the lifetimes in the slow component region of quartz OSL. Three separate samples of quartz were given beta doses of 4.5, 15 and 150 Gy and preheated at 493 K for 5 min. The luminescence from each sample was then bleached down to the slow component region, taken as a small fraction (1±1.5%) of the initial intensity. As a means of con®rming that the luminescence intensity had reached the slow component region of the OSL, the bleaching was done sequentially in stages by exposure to the blue LEDs for a ®xed number of seconds, and the luminescence intensity monitored at the end of each run.
M.L. Chithambo, R.B. Galloway / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 358±368
The variation of lifetime with temperature for measurements from 293 to 473 K (cycle 1) are shown in Figs. 6(a)±(c) and for measurements from 473 to 293 K (cycle 2) in Figs. 6(d) and (e). It is apparent that for all measurements in cycle 1 (Figs. 6(a)±(c)), the lifetime values between 293 and 373 K are eectively constant within 36 2 ls despite the variation in radiation dose. Once the measurement temperature is increased above 373 K, the lifetime values decrease rapidly. For example, the lifetime values for the sample irradiated to 4.5 Gy (Fig. 6(a)) changes by about 76% between 373 and 473 K, i.e. from about 34 ls at 373 K to about 8 ls at 473 K. In general, however, there is similarity in lifetime values associated with dierent doses over the whole temperature range (293±473 K). It is interesting to note that
Fig. 6. Graphs showing changes in luminescence lifetime when measurements are made from 293 to 473 K in the slow component region for samples given dierent initial doses (a) 4.5 Gy, (b) 15 Gy, (c) 150 Gy and from 473 to 293 K (d) 15 Gy, (e) 150 Gy.
365
even in measurements concerning the time dependence of luminescence lifetimes at constant temperature for heated quartz stimulated by pulsed green light [7], the change of lifetimes with stimulation time was independent of radiation dose over the range 15±150 Gy. The repeat measurements (473±293 K) shown in Figs. 6(d) and (e) evidently show good agreement with results shown in set Figs. 6(a)±(c) in terms of the change of lifetime with temperature, and in the values of lifetime at speci®c temperatures. The values of lifetime are similar regardless of the immediate thermal history of the sample, at least for the two heating cycles used. The results of Fig. 6 show that lifetime values in the slow component region for quartz under blue light stimulation decrease with temperature above 373 K and that this change is independent of the size of the initial dose. The temperature-dependent change of lifetimes in the slow component can be ascribed to increased thermal quenching of luminescence according to Eq. (1). Values of DE, the activation energy of thermal quenching and the parameter C evaluated from the best ®t of Eq. (1) to the data of lifetime variations with temperature are summarised in Table 1 and compared with values determined from the temperature dependence of luminescence intensity in the slow component region (Figs. 3(a) and (b)) and in the fast component region (Fig. 2). The full range of values of activation energy for thermal quenching DE, thermal assistance Ea , and the constant C are compared in Table 1 with corresponding values evaluated from temperature related changes in luminescence intensity published elsewhere [3±5,13,21,22]. All data in Table 1 are for luminescence detected in the region 300±450 nm as de®ned by the transmission ®lters used. It is evident from Table 1 that values of DE and C determined from either lifetime or intensity analysis in this work are consistent. Values of DE and C are not only independent of the order of change in measurement temperature, but are also not signi®cantly in¯uenced by the size of beta dose or whether the measurement is made in the fast or the slow component region of the OSL. It is encouraging to note that values of DE; Ea and C
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M.L. Chithambo, R.B. Galloway / Nucl. Instr. and Meth. in Phys. Res. B 183 (2001) 358±368
evaluated from analysis of both luminescence intensity and lifetimes in this work are consistent with values worked out from the temperature dependence of luminescence intensity as reported elsewhere (Table 1). 4.4. Thermal quenching of luminescence lifetimes Interpretation of thermal quenching of luminescence lifetimes in quartz in terms of a speci®c physical mechanism is complicated by the fact that the luminescent centre in quartz is ambiguous [23] and the mechanisms of charge eviction from shallow traps are by no means clear. Luminescence lifetimes may be associated with three possible physical mechanisms, namely (a) the time to evict an electron from a trap, se ; (b) time for an electron to travel from the trap through the lattice to the recombination centre sl and (c) lifetime of the excited state at a recombination centre sf [9,17]. In photon-induced charge eviction from a trap, estimates of the lifetime in the trap se are usually short and of the order of 1 ns or less [17]. The time of transport from trap to recombination centre, sl , is less easy to estimate as it can be aected by retrapping of electrons prior to recombination as well as by lattice vibrations. The in¯uence of shallow traps on lifetimes was demonstrated by Akselrod et al. [14,15] who reported that shallow traps in a-Al2 O3 caused an increase in the measured luminescence lifetime near room temperature due to re-trapping processes. In most cases, however, the lifetime is dominated by intraluminescence centre relaxation time sf which can usually be associated with a given transition at the luminescence centre [24±26]. If the lifetime values in this ®nal possibility have contributions from thermal excitation and radiative decay, then overall the lifetime will depend on temperature as shown earlier in Eq. (1). Thermal quenching of lifetimes de®ned in this sense is not uncommon and has been observed on other materials such as KCl [8] and a-Al2 O3 [14,15]. In these materials, the fall in lifetimes with temperature was ascribed to thermal quenching at the excited F centres. It should be emphasised however that the precise factors leading to this
quenching were not described explicitly, although the model fully accounted for the decrease in luminescence intensity. In other materials, thermal quenching of luminescence is not always accompanied by quenching of lifetimes. For instance, Barnett and Baili [16] observed that lattice vibrations were associated with thermal quenching of luminescence in potassium feldspars. In studies of lifetimes in similar materials by Clark et al. [17], there was no evidence of consistent variation with temperature that could be described in terms of thermal quenching. Thus it appears that the eect of temperature on the recombination centres and temperature related changes in the lattice may be relevant in a discussion of the fall of luminescence lifetimes with temperature. The increase in temperature may alter the spatial separation of traps and recombination centres thus aecting the associated con®gurational coordinate. The distortions in the con®gurational coordinate that occur may cause near contact between the ground state and the relevant excited state [24,27,28], a proximity leading to non-radiative transitions and thus a fall in luminescence intensity as a function of temperature. If, in addition, the density of recombination centres increased with temperature, the probability of recombination at higher temperature would increase. A direct consequence of this would be that recombinations would occur faster after electron release from a trap, and thus the measured lifetimes would be shorter than at room temperature. However, since it was observed that increase in temperature is also accompanied by a decrease in luminescence intensity, it must be that more of the recombinations at higher temperatures are nonradiative. Although this work has not addressed the nature of recombination centres, some experimental work, e.g. [23], suggests that an increase in the number of recombination centres as a function of temperature is a possibility. In answer to Bailey [5], ``the question of whether the slow component exhibits thermal quenching remains largely unresolved'', we have found both thermal quenching and thermal stimulation in the slow component of quartz OSL.
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5. Conclusions Characteristics of luminescence lifetimes and luminescence intensity from quartz under pulsed blue LED stimulation have been studied at long stimulation times. It was shown that the slow component increases in importance from just under 10% in the initial transient to about 50% after 50 s, and from 150 s onwards the measured luminescence consists of the slow component. The change of luminescence intensity as a function of temperature was studied in the slow component region where it was observed that increase in temperature produces a peak at 398 K in the intensity of luminescence measured as a function of temperature between 293 and 473 K. In the fast component region, increase in temperature above 383 K leads to a much more rapid reduction in intensity of luminescence with duration of stimulation than that observed at room temperature. Dynamic temperature investigations of lifetimes in the slow component region revealed that lifetimes are strongly aected by temperature of measurement. Luminescence lifetimes measured between 293 and 398 K are constant within 36:0 2:0 ls. From 373 K lifetimes decrease to about 8 ls at 473 K, the maximum temperature investigated. The temperature-dependent changes in both luminescence intensity and mean lifetimes are independent of the order in which the measurements are made. The increase of luminescence intensity with temperature was ascribed to thermal assistance to optical stimulation whereas the decrease of both luminescence intensity and lifetimes with temperature could be adequately described by thermal quenching functions. Processes of thermal assistance and thermal quenching were assumed to act independently hence cases where the luminescence intensity went through a peak as a function of temperature could be adequately analysed by combination of the thermal assistance and thermal quenching functions. Values of activation energy for thermal assistance and thermal quenching abstracted from analysis of luminescence intensity in this work are consistent with corresponding values evaluated using similar techniques reported else-
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where [3±5,13,21,22]. Values of the activation energy for thermal quenching derived from temperature-related changes in luminescence lifetimes are consistent with values determined from temperature-dependent changes in the luminescence intensity. Acknowledgements We wish to thank Mr. Harry Napier and Mr. Paul Harris for technical assistance. MC thanks the Commonwealth Scholarship Commission for ®nancial support. References [1] B.W. Smith, E.J. Rhodes, Radiat. Meas. 23 (1994) 329. [2] R.M. Bailey, B.W. Smith, E.J. Rhodes, Radiat. Meas. 27 (1997) 123. [3] A.S. Murray, A.G. Wintle, Radiat. Meas. 29 (1998) 65. [4] S.W.S. McKeever, L. Bùtter-Jensen, N. Agersnap Larsen, G.A.T. Duller, Radiat. Meas. 27 (1997) 161. [5] R.M. Bailey, Radiat. Meas. 32 (2000) 233. [6] M.L. Chithambo, R.B. Galloway, Radiat.Meas. 32 (2000) 627. [7] M.L. Chithambo, R.B. Galloway, Radiat. Meas. 32 (2000) 621. [8] R.K. Swank, F.C. Brown, Phys. Rev. 130 (1963) 34. [9] S.W.S. McKeever, Thermoluminescence of Solids, Cambridge University Press, Cambridge, 1985. [10] D. Curie, Luminescence in Crystals, Wiley, New York, 1963. [11] N.F. Mott, Proc. Roy. Soc. London A. 167 (1938) 384. [12] J.P. Liu, M.Y. Kong, J.J. Si, D.D. Huang, J.P. Liu, D.Z. Sun, J. Phys. D 31 (1988) 85. [13] N.A. Spooner, Radiat. Meas. 27 (1994) 593. [14] M.S. Akselrod, N. Agersnap Larsen, V. Whitley, S.W.S. McKeever, J. Appl. Phys. 84 (1998) 3364. [15] M.S. Akselrod, N. Agersnap Larsen, V. Whitley, S.W.S. McKeever, Radiat. Prot. Dosim. 84 (1999) 39. [16] S.M. Barnett, I.K. Baili, J. Phys. D 30 (1997) 683. [17] R.J. Clark, I.K. Baili, M.J. Tooley, Radiat. Meas. 27 (1997) 211. [18] R.B. Galloway, D.G. Hong, H. Napier, Meas. Sci. Technol. 8 (1997) 267. [19] M.L. Chithambo, R.B. Galloway, Meas. Sci. Technol. 11 (2000) 418. [20] A.D. Franklin, J.R. Prescott, R.B. Schole®eld, J. Lumin. 63 (1995) 417. [21] A.G. Wintle, Geophys. J. R. Astron. Soc. 41 (1975) 107. [22] D.J. Huntley, M.A. Short, K. Dunphy, Can. J. Phys. 74 (1996) 81.
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[26] S.R. Elliot, Physics of Amorphous Materials, Longman, New York, 1990. [27] F.C. Brown, The Physics of Solids, Benjamin, New York, 1967. [28] A.J. Dekker, Solid State Physics, Macmillan, New York, 1958.