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Thermally modulated optically stimulated luminescence (TM-OSL) of quartz
Journal of Luminescence 195 (2018) 435–440
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Thermally modulated optically stimulated luminescence (TM-OSL) of quartz ⁎
T
A. Chruścińska , A. Szramowski Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5/7, 87-100 Torun, Poland
A B S T R A C T It has recently been shown in computer simulations of the OSL process that increasing the temperature during optical stimulation allows the probability of electron excitation from traps to the conduction band to be modulated. This, in turn, facilitates the efficient separation of individual OSL components. A result of the thermally modulated OSL (TM-OSL) measurement method is a peak shaped curve. In addition, the position of the TM-OSL peak on the temperature axis can be controlled by the stimulation energy, heating rate and photon flux used for optical stimulation. Here, the results of a TM-OSL study for natural quartz samples are presented. The basic features of the TM-OSL curves of quartz are shown for different parameters that control their shape. The investigation is focused on traps that are most sensitive to light and which are the source of the OSL signal that is the most useful in retrospective dosimetry. A method by which it can be measured separately from the slower components is presented. The very first attempt at directly estimating the optical depth and the parameters determining the strength of the electron-phonon coupling for the traps responsible for the fast OSL component in quartz has been made.
1. Introduction In the investigation of the optically stimulated luminescence (OSL) of solids, the main aim is usually to determine the parameters of traps responsible for the individual components of the total OSL signal. Knowing these parameters enables the changes in luminescence with the temperature and the type of light used for stimulation to be predicted. In the most basic description of the phenomenon that takes into account the crystal lattice vibrations, there are three uniquely determined parameters defining a trap: the optical trap depth (E), the energy of lattice vibration mode (ħω) and the Huang-Rhys factor (S). They are related to the thermal trap depth ET by the formula [1,2]:
E = E T + Sħω.
(1)
In the results obtained by the most popular stimulation techniques, however, these parameters are hidden in a single physical quantity called the optical cross-section (OCS) that determines the OSL decay rate. It has recently been shown that, when the optical cross-section is modulated during the OSL measurements, a more advantageous kind of OSL curve can be acquired for a single trap kind [2–5]. The curve has the form of a peak whose position and shape are uniquely determined by all three trap parameters. Thus, they can be found by using curve shape analysis, e.g. by fitting the theoretical curves. As can be seen from the OCS dependence on the stimulation energy presented in Supplementary material, the OCS also depends on temperature.
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Therefore, one of the possibilities of OCS modulation is increasing the temperature during optical stimulation. A similar kind of stimulation was previously theoretically investigated by Chen and Pagonis [6] for a specific case when a trap has an excited electronic state below the conduction band and the trap emptying is a two-stage process. Such a process is reported in the case of K-feldspars and can be recognized by a characteristic peak in the infrared part of its OSL stimulation spectra [7]. Recent simulations of the one-stage OSL process, using a model that takes into account the optical cross-section dependence on temperature, showed that the socalled thermally modulated OSL method not only allows the estimation of trap parameters but also facilitates the efficient separation of individual OSL components [4]. The position of the TM-OSL peak on the temperature axis is very strongly dependent on the trap parameters and can be easily controlled by the stimulation energy, heating rate and photon flux used for optical stimulation. Here, the results of recent computer modeling are used for planning and interpreting the results of the TM-OSL measurements performed for natural quartz samples. The model assuming the one-stage emptying of traps seems to be suitable for quartz, for which no evidence of the more complex trap ionization is observed in the OSL stimulation spectra [8]. Quartz was chosen as the first material whose TM-OSL was intensively investigated for two reasons. Firstly, it has a rich spectrum of deep traps and, secondly, it is widely used in luminescence dating and retrospective dosimetry. The OSL of quartz has been intensively
Corresponding author. E-mail address: alicja@fizyka.umk.pl (A. Chruścińska).
https://doi.org/10.1016/j.jlumin.2017.12.004 Received 1 June 2017; Received in revised form 11 October 2017; Accepted 2 December 2017 Available online 05 December 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
Journal of Luminescence 195 (2018) 435–440
A. Chruścińska, A. Szramowski
exploited for estimating the age of quaternary geological sediments for more than 30 years. OSL dating is a well-established method for sediments that were exposed to sun light during their creation process. The nature of the OSL signal of quartz is complex. Three to six components can be observed depending on the sample [9–14]. The fastest decaying component is the most appropriate for age estimation because of the requirement of OSL signal zeroing prior to the sediment layer creation. In the most widely used OSL dating protocols, the very first part of the OSL decay curve is used for age estimation. It is assumed that it corresponds mainly to the fastest OSL component. Such a solution can be accepted for samples whose fast component dominates the total OSL signal. This fact, however, is hard to verify by standard measurements during the dating procedure. The OSL measurement techniques applied up to now do not allow the selective detection of the OSL components. Using the analytical techniques of the OSL curve decomposition into individual components before the age determination is as dubious as it is unrealistic in dating practice. There is a need to find a way of effectively separating the fast OSL component during the measurement stage. It will be shown that the application of thermally modulated optical stimulation provides such a possibility. The basic features of the TM-OSL curves of quartz are shown for different parameters controlling their shape. The first preliminary attempt at directly determining the optical trap depth and the parameters determining the strength of the electron-phonon coupling is demonstrated. Estimation of these parameters allows the direct correlation of the traps active in the OSL and TL processes to be performed.
Fig. 1. TL curves of quartz samples used in the study. The heating rate used in the measurement is 1 Ks−1 and the excitation dose about 10 Gy.
of stimulated luminescence for the samples was erased by fast heating to 500 °C. 3. Results 3.1. TM-OSL measurements for different experimental conditions The TL curves of all three investigated samples are shown in Fig. 1. As can be seen, the peak called the 100 °C peak (here for the heating rate of 1 K s−1 below 80 °C) dominates in all samples, but the intensities of the subsequent peaks at higher temperatures differ significantly from sample to sample. As will be reported below, the presence of the traps shallower than these that are the main object of the investigation can complicate the results of TM-OSL. On the one hand, it can make the analysis of experimental curves harder but, on the other, the explicit manifestation of the activity of shallow traps in the TM-OSL measurements reveals the mechanism of luminescence. This is, for example, much harder to recognize in the CW-OSL experiments. For describing the potential of a measurement method, however, it is reasonable to use rather simpler cases of experimental results. Therefore, most of the data is presented here for the FQ sample, which has the smallest concentration of traps responsible for peaks below 200 °C. The depth of traps which can be investigated by the TM-OSL method is limited by the temperature region RT − 500 °C available when using the Risø system. From previous TM-OSL simulations, it is known that it is useful to establish the experimental conditions in such a way that the investigated traps are emptied mainly by light. This means that the TMOSL peak should be observed below the temperature region of the effective thermal emptying of the appropriate trap. Because of that, the measurement of the explicit TM-OSL peaks for traps that in quartz contribute to a TL signal below 200 °C was not possible when using the equipment described above. However, the traps that are important for dating applications, because they are able to hold electrons for long enough, are thermally emptied above 250 °C. From room temperature to 250 °C should be a broad enough temperature range to detect a purely optical TM-OSL signal. The TL peaks below 250 °C originating from the shallower traps were erased by the adequate preheat. Fig. 2 presents the TL curve obtained with the heating rate of 0.2 K s−1 for the FQ sample after a preheat at 275 °C. It also presents the TM-OSL curves obtained after the analogical preheat, with the same heating rate, for the wavelength of the stimulation light equal to 620 nm and two different values of the photon flux. The shift of the TMOSL peak toward lower temperatures can be observed for the increasing photon flux, just as was seen earlier by simulations ([4], Figure 6). The opposite direction of the peak shift is caused by applying higher heating rates. This is illustrated by Fig. 3 for the WP9 sample. It is interesting to notice that the TM-OSL peak position stays fixed
2. Experimental The main part of the measurement equipment is the Risø TL/OSL System TL-DA-12. The stimulation was carried out using two kinds of stimulation units. The first one enables the application of one of three LED stimulation modules, with the maxima of their spectral bands at: 660, 620 and 530 nm. Each module contains a 10 × 10 LED matrix (produced by R2T Industry(HK) Ltd.) and is connected to the Risø System by means of a fiber optic bundle with a 10 mm diameter and made of 1.0 mm high-performance plastic fibers (Mitsubishi Rayon Co. LTD.). The maximal photon flux achievable on the sample position for all the LED modules is over 2 × 1017 cm−2 s−1. The second stimulation system uses as a light source a 1000 W xenon lamp (Kiloarc by Optical Building Blocks Corp.), as described in detail earlier [15]. Luminescence detection is carried out by an EMI 9235QA photomultiplier with a 2 mm UG11-IRB filter that is an improved version of a UG11 filter (the main transmission band 300–380 nm), which more efficiently blocks light with wavelengths above 700 nm. Data saving is realized through the standard program for measurement control of the Risø reader. A 90 Sr/90Y β source giving a dose rate of about 30 mGys−1 has been used for irradiation. The main presented results were performed for quartz grains of 150–250 mm extracted from Fontainebleau sand (palaeo-coastal dune system Fontainebleau, Seine-et-Marne, Ile-de-France, France), hereinafter referred to as FQ. The thermoluminescence (TL) characteristic of this sample is very beneficial while presenting the results of TM-OSL experiments because of the relatively low participation of the peaks about 165 °C and 200 °C in its TL signal. As will be shown, the strong retrapping of electrons by traps responsible for these peaks during optical stimulation can significantly deform the shape of the TM-OSL curve. Two other kinds of quartz were used: one extracted from sediments – WP9, and natural quartz provided by Merck KGaA – marked as MQ. Quartz grains of the WP9 sample with the diameter of about 100–200 µm were extracted from sediment by the standard procedure, including separation in heavy liquids and etching in 40% HF. The Merck quartz was sieved and the fraction used in the experiments is 100–125 µm. Aliquots of 4 mm diameter were prepared on steel discs using Silkospray silicone oil for each kind of sample. The natural signal 436
Journal of Luminescence 195 (2018) 435–440
A. Chruścińska, A. Szramowski
Fig. 5. The TM-OSL curves obtained by computer simulations for a trap with the optical depth of 2 eV, the Huang-Rhys factor S1 = 5 and vibrational mode energy ħω1 = 0.01 eV. The wavelength of the stimulation light was 660 nm. The curves were simulated for different combinations of heating rate and photon flux values selected in such a manner that the ratio of these quantities was constant. The simulations were performed for the model that consists of two traps and one luminescence center (Supplementary material, equations 1–5) for the following parameters: N1 = 1012 cm−3, N2 = 1013 cm−3, M = 2 × 1013 cm−3, A1 = A2 = 10−11 cm3 s−1, Am = 4×10−11 cm3 s−1, β = 10−8 cm3 s−1, E2 = 3 eV, S2 = 5, ħω2 = 0.01 eV, s1 = s2 = 1013 s−1.
Fig. 2. TM-OSL curves for the FQ sample obtained with the stimulation light wavelength 620 nm, the heating rate 0.2 Ks−1 and two different photon flux values 1016 cm−2 s−1 (gray line) and 3×1016 cm−2 s−1 (black line). The TL curve obtained with the same heating rate is shown for comparison (light gray line). All curves were measured after an irradiation dose of about 10 Gy, 25 s of preheat at 275 °C subsequent to heating with the rate 1 Ks−1.
for a constant ratio of photon flux to heating rate. A convincing illustration of this effect is given in Fig. 4, where the TM-OSL curves of the FQ sample are shown for the heating rates of 1, 2 and 4 Ks−1 and the photon flux values of 9.5 × 1015, 1.9 × 1016 and 3.8 × 1016 cm−2s−1 respectively. This is a simple consequence of the fact that the number of photons reaching the sample during its heating by one degree is the same in all cases. Analogous results can be obtained by computer simulations of the TM-OSL process. A few examples are presented in Fig. 5. From both the experimental and simulation results, it is clear that higher heating rates and photon flux values are more advantageous for the TM-OSL measurements. There are two benefits. The first is the better separation of the TM-OSL peak from the TL peak region, which is always at higher temperatures for the higher heating rates. The second is the higher intensity of the measured TM-OSL signal. The OCS depends on the stimulation energy, especially dynamic for the stimulation energies much below the threshold, hence the TM-OSL peak position can also be controlled by this experimental parameter. The TM-OSL measurements have been conducted for different wavelengths of the stimulation light. The TM-OSL peak below 200 °C could be observed for wavelengths as high as 700 nm. Fig. 6 presents an example of TM-OSL curves realized for three different stimulation energies for the FQ sample. As can be seen, the TM-OSL peak shifts into
Fig. 3. TM-OSL curves for the WP9 sample were obtained with the photon flux 3.2 × 1016 cm−2 s−1 and two different heating rates 2 Ks−1 (black line) and 0.2 Ks−1 (gray line). The measurements were performed using the 660 nm LED module after irradiation with the dose of 20 Gy and preheat to 300 °C.
Fig. 4. TM-OSL curves for the FQ sample obtained with three different heating rates and photon flux values: 1 Ks−1 and 9.5 × 1015 cm−2 s−1 (solid line), 2 Ks−1 and 1.9 × 1016 cm−2 s−1 (dashed line), 4 Ks−1 and 3.8 × 1016 cm−2 s−1 (dotted line). The ratio of the photon flux value to the heating rates is the same for all curves. The stimulation light wavelength was 620 nm and the excitation dose about 10 Gy. Fig. 6. TM-OSL curves for the FQ sample measured with the heating rate of 0.2 Ks−1 and the photon flux of about 3.8 × 1016 cm−2s−1 after a preheat to 230 °C for three different stimulation wavelengths.
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Fig. 8. CW-OSL curves of the FQ sample obtained with the stimulation wavelength of 530 nm after a preheat to 230 °C (black line) and after the same preheat and the TM-OSL with the stimulation wavelength of 620 nm, heating rate of 4 Ks−1, and photon flux of 1.3×1017 cm−2s−1 (gray dashed line). The inset presents the TM-OSL curves of the same sample measured without prior optical stimulation (gray line) and after 15 s (light-gray line) and 200 s (black line) of CW-OSL stimulation (530 nm).
Fig. 7. Three curves measured after a preheat to 270 °C with the heating rate of 0.2 Ks−1 are compared: the TM-OSL (gray line) measured to 230 °C with the stimulation wavelength of 620 nm, the TL curve obtained after the TM-OSL measurement (light gray line) and the TL curve measured directly after the preheat (black line). The irradiation dose was about 10 Gy. Traps responsible for the TM-OSL peak observed below 200 °C are also the source of the TL peak at 260 °C (325 °C when the heating rate 5 Ks−1 is used).
higher temperatures during stimulation with lower energy when values of the photon flux and the heating rate remain unchanged.
3.2. Selecting the fast OSL component by the TM-OSL method The comparison of the TM-OSL and TL curves measured with the same heating rate and after a preheat to the same temperature can directly reveal the origin of the TM-OSL signal. In Fig. 2, some gap in the TM-OSL signal can clearly be seen above 250 °C. The TL peak observed in this temperature region during heating with the rate of 0.2 Ks−1 is detected at 325 °C when TL is measured with the much faster heating (5 Ks−1). The TL peak at 325 °C in quartz is known as being related to traps which are the source of the fast OSL component [9,16–18]. The difference between the TL curves obtained after the TMOSL measurement carried out to 230 °C and directly after the preheat to 270 °C is explicitly shown in Fig. 7. The TM-OSL curve is also shown in order to demonstrate the difference between the intensities of both the TM-OSL and TL signals. Thermal quenching causes the decrease in TL intensity at higher temperatures [19–23]. The maximum intensity of the TM-OSL peak is over 13 times higher than the intensity of the related TL peak. A more direct confirmation of the relationship of the TM-OSL peak obtained using the 620 nm stimulation with the fast OSL component observed in conventional OSL measurements is the comparison of CWOSL curves measured before and after TM-OSL measurement. Both curves are shown in Fig. 8 for the FQ sample. The first CW-OSL curve is measured directly after the preheat to 230 °C, and the second one after the same preheat and subsequent TM-OSL measurement with the wavelength of 620 nm continued to the temperature of the preheat. CWOSL was measured at 125 °C with the stimulation wavelength of 530 nm. As can be observed, the fast component of the CW-OSL signal is almost erased by the TM-OSL stimulation. The CW-OSL curve for the FQ sample can be fitted by a sum of two first-order curves (exponential decays). The intensity of the faster component (lifetime of about 2 s) decreases by about 94% after the TM-OSL measurement that was stopped at 230 °C. As can be seen, this temperature does not ensure the total emptying of the related traps. The second component is not noticeably reduced. The impact of CW-OSL stimulation on the TM-OSL signal measured with the wavelength of 620 nm is shown in the inset of Fig. 8. There is not much left from the TM-OSL signal measured with 620 nm after the first 5 s of CW-OSL stimulation with 530 nm. After measuring the TM-OSL signal with the stimulation wavelength 620 nm and emptying solely the traps responsible for the fast OSL
Fig. 9. TM-OSL curves of the FQ sample obtained using the stimulation wavelength of 560 and 575 nm, the heating rate of 0.2 Ks−1 and the photon flux of 4.8 × 1016 and 4.6 × 1016 cm−2s−1, respectively. Before the TM-OSL measurement realized with the higher stimulation energy, the traps related to the fast OSL component were depopulated. The inset shows the gradual reset of the fast OSL component by 3-fold TM-OSL measurement with 620 nm stimulation carried out to 230 °C with the heating rate of 4 Ks−1 and the TL curve (4 Ks−1) obtained after these TM-OSL measurements.
component in quartz, the slower component can be measured by the TM-OSL method using a higher stimulation energy. The effects of such an experiment can be seen in Fig. 9 for two different wavelengths of the stimulation light. 4. Discussion It should be noted that the TM-OSL corresponding to the fast OSL component in quartz can not always be observed as such a clear peak as in the FQ or WP9 samples. The FQ sample, for example, as was mentioned earlier, is rather a special, advantageous case of natural quartz. In its simple CW-OSL curve, consisting only of two components, the fast component dominates. Its initial intensity is over 5 times higher than the initial intensity of the second component. In light of the latest findings concerning the nature of the medium component in quartz [24], the simplicity of the CW-OSL curve for the FQ sample can be linked to the very low concentrations of traps responsible for the peaks at about 145 and 195 °C occurring in this sample (see Fig. 1). The sample of Merck quartz is quite another case. Here, one can observe the large activity of shallow traps and the CW-OSL curve for the same stimulation wavelength (530 nm) can be decomposed into three firstorder curves. The relation between their intensities is 85:84:62. The fast 438
Journal of Luminescence 195 (2018) 435–440
A. Chruścińska, A. Szramowski
analysis easier. It is generally accepted that the thermal depth of the trap responsible for the 325 °C TL peak in quartz is 1.7 eV and its frequency factor is about 5 × 1013 s−1 [20,25], so these values were used for the first attempt at fitting a theoretical TM-OSL curve to the experimental TM-OSL curve obtained for quartz. For this purpose, a series of simulations was performed for a model consisting of one OSL trap, one deep, thermally disconnected trap and one luminescence center. The parameters of this model are presented in detail in Supplementary Material. The values of the thermal trap depth and frequency factor were fixed and shapes of the TM-OSL curve were initially tested for the S and ħω values from wide ranges: 2 − 100 and 0.005 − 0.1 eV, respectively. It should be stressed that curves with the shape of a peak below 200 °C could be obtained only for photon flux values much lower than the one used in the experiments, but none of the obtained curves resembled the experimental curves. A few exemplary results of these tests are shown in Fig. S1-S3 in Supplementary Material. It is worth noting here that it is very hard to estimate the thermal trap depth and frequency factor for a high temperature TL peak that is not isolated from other peaks. This is because the methods allowing these quantities to be estimated were established for well isolated TL peaks and assume the independence of different traps [26]. The TL peak in quartz at 325 °C, as can be observed in Fig. 1, is neither isolated nor can the corresponding trap be considered as independent of others. Moreover, the temperature range above 300 °C is very inconvenient for TL parameter investigation because of the possible thermal lag between the heating element and the sample. Additionally, the peak position at 325 °C can be obtained for different combinations of the ET and s parameters that were also used in the simulations aimed at reproducing the TM-OSL curve measured for quartz. The shapes of simulated TMOSL curves similar to the shapes of experimental curves were obtained for significantly larger values of the thermal trap depth than 1.7 eV. The reason for differences between the shapes of the TM-OSL curves obtained for different thermal trap depths can be explained by the dynamics of the OCS increase with temperature. Two examples of this dependency for different trap depths are presented in Fig. S4 in Supplementary material. As a rule, the lower the stimulation energy below the threshold energy (and simultaneously higher values of photon flux density) used for stimulation, the narrower the TM-OSL peaks. This effect is also illustrated in Supplementary material in Fig.
Fig. 10. TM-OSL curves for all three samples measured after a preheat to 275 °C, for the heating rate of 4 Ks−1, the photon flux density of 1.3 × 1017 cm−2s−1 and the stimulation wavelength equal 620 nm. The inset presents TM-OSL curves for the Merck sample for two different heating rates.
component does not prevail and it is hard to maintain, as is usually assumed in OSL dating, that the beginning of the CW-OSL curve corresponds solely to this component. In this application, the samples whose CW-OSL are more complex and the fast component intensity is comparable with the intensities of slower components are always more problematic. In the case of the Merck sample, after the TM-OSL measurement with the stimulation wavelength of 620 nm, both the fast and the medium components decrease by 70% and 14%, respectively, but simultaneously the values of their lifetimes increase noticeably. This suggests that the optical emptying of traps during the CW-OSL measurement may proceed with efficient competition from the shallow traps, and that the analysis of the CW-OSL curve applying the first-order kinetic assumption is not quite an appropriate method here. The strong activity of the shallow traps in the case of the Merck sample can be observed directly during the TM-OSL measurements that are presented in Fig. 10. The TM-OSL peak is clearly deformed by a contribution from TL peaks related to the shallow traps. The latter, although empty after the preheat, are filled during the optical stimulation as a result of the phototransfer phenomenon. The inset shows the TM-OSL peaks for two different heating rates and the photon flux values appropriately selected in order to fix the TM-OSL peak position. The shape of the curve changes significantly because the TL peak position changes with the heating rate. In Fig. 10 the TM-OSL curves for other samples are shown for comparison. In these cases, it is hard to observe any deformation, however, since the weak phototransfered 100 °C TL peak can be measured after the CW-OSL stimulation at room temperature (see Fig. S6 in Supplementary material), it can be supposed that there may be a slight impact of this peak on the shape of the TMOSL curves. Three facts concerning the Merck sample can be linked: the exceptionally high concentration of the TL peaks below 200 °C, the presence of phototransfered TL peaks in its TM-OSL results, and the abovementioned partial decay of the medium component of the CW-OSL curve after the TM-OSL measurement that removes solely the fast component in the case of other samples. All these effects confirm that the medium component in quartz may not be related to the optical depopulation of a particular deep trap, but rather is a result of retrapping the electrons in the traps responsible for TL peaks below 200 °C during optical stimulation [24]. The TM-OSL peak obtained during the stimulation with 620 nm can be used for the estimation of parameters that uniquely determine the related trap. There are five such parameters: ET, s (the frequency factor), E, S and ħω, but when one assumes that the thermal depth of the trap, ET which is related to the optical trap depth by relation (1), and the frequency factor are known from earlier works, then only two parameters, S and ħω, are independent. This make the curve shape
Fig. 11. Theoretical TM-OSL curves obtained as a result of simulations using the model described in Supplementary material. The thermal depth of trap ET1 used in the simulations is 1.9 eV and frequency factor s is 2×1015 s−1. Two examples that reproduce the position of the TM-OSL peak properly are shown for extreme values of S and ħω. All other curves that have the maximum at the same temperature are between these two extreme curves. The stimulation parameters both for the experiment and simulations are 620 nm and 4 Ks−1. The photon flux density is assumed to be the less accurately estimated quantity, so its different values were tested. They are given in the legend for the shown cases.
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sediments. Future works should include the investigation of the dose dependency of the component separated by the TM-OSL method, working out the dating protocol exploiting this method and its tests for samples of known age. Acknowledgements This work has been financed by the grant of the National Science Centre, Poland, No. 2016/21/B/ST10/01867. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jlumin.2017.12.004. References Fig. 12. Results of fitting the theoretical TM-OSL curve to the experimental one obtained by taking into account the effect of thermal quenching (details are given in the text). The parameters in the simulation that determine the strength of electron-phonon coupling are S = 27 and ħω = 20 meV, the stimulation wavelength 620 nm, heating rate 4 Ks−1 and the photon flux density 5×1016 cm−2s−1.
[1] J.M. Noras, Photoionisation and phonon coupling, J. Phys. C: Solid State Phys. 13 (1980) 4779–4789. [2] A. Chruścińska, On some fundamental features of optically stimulated luminescence measurements, Radiat. Meas. 45 (2010) 991–999. [3] A. Chruścińska, Estimating the parameters of traps in quartz by the variable energy of stimulation optically stimulated luminescence (VES-OSL) method, Radiat. Meas. 81 (2015) 205–211. [4] A. Chruścińska, N. Kijek, Thermally modulated optically stimulated luminescence (TM-OSL) as a tool of trap parameter analysis, J. Lumin. 174 (2016) 42–48. [5] A. Chruścińska, N. Kijek, S. Topolewski, Recent development in the optical stimulation of luminescence, Radiat. Meas. (2017). [6] R. Chen, V. Pagonis, Modeling TL-like thermally assisted optically stimulated luminescence (TA-OSL), Radiat. Meas. 56 (2013) 6–12. [7] S.M. Barnett, I.K. Bailiff, Infrared stimulation spectra of sediments containing feldspars, Radiat. Meas. 27 (1997) 237–242. [8] G. Hütt, I. Jaek, V. Vasilchenko, Photoionization of radiation-induced traps in quartz and alkali feldspars, Appl. Radiat. Isot. 54 (2001) 175–182. [9] M. Jain, M., A.S. Murray, L. Bøtter-Jensen, Characterisation of blue-light stimulated luminescence components in different quartz samples: implications for dose measurement, Radiation Measurements 37 (2003), pp. 441–449. [10] G. Kitis, G.S. Polymeris, N.G. Kiyak, Component-resolved thermal stability and recuperation study of the LM-OSL curves of four sedimentary quartz samples, Radiat. Meas. 42 (2007) 1273–1279. [11] G. Kitis, V. Pagonis, Computerized curve deconvolution analysis for LMOSL, Radiat. Meas. 43 (2008) 737–741. [12] G. Kitis, N.G. Kiyak, G.S. Polymeris, V. Pagonis, Investigation of OSL signals from very deep traps in unfired and fired quartz samples, Nucl. Instrum. Methods Phys. Res. B 268 (2010) 592–598. [13] N. Kijek, A. Chruścińska, K.R. Przegiętka, The dependence of quartz OSL on stimulation energy, Radiat. Meas. 71 (2014) 108–112. [14] A.G. Wintle, G. Adamiec, Optically stimulated luminescence signals from quartz: a review, Radiat. Meas. 98 (2017) 10–33. [15] A. Chruścińska, Experimental demonstration of the variable energy of stimulation optically stimulated luminescence (VES-OSL) method, Radiat. Meas. 71 (2014) 247–250. [16] B.W. Smith, M.J. Aitken, E.J. Rhodes, E.J. Robinson, D.M. Geldard, Optical dating: methodological aspects, Radiat. Prot. Dosim. 17 (1986) 229–233. [17] N.A. Spooner, On the optical dating signal from quartz, Radiat. Meas. 23 (1994) 593–600. [18] A.S. Murray, A.S. Wintle, Factors controlling the shape of the OSL decay curve in quartz, Radiat. Meas. 29 (1998) 65–79. [19] A.G. Wintle, Thermal quenching of thermoluminescence in quartz, Geophys. J. R. Astron. Soc. 41 (1975) 107–113. [20] R.M. Bailey, Towards a general kinetic model for optically and thermally stimulated luminescence of quartz, Radiat. Meas. 32 (2001) 17–45. [21] R. Nanjundaswamy, K. Lepper, S.W.S. McKeever, Thermal quenching of thermoluminescence in natural quartz, Radiat. Prot. Dosim. 100 (2002) 305–308. [22] J.S. Singarayer, R.M. Bailey, Further investigations of the quartz optically stimulated luminescence components using linear modulation, Radiat. Meas. 37 (2003) 451–458. [23] V. Pagonis, C. Ankjærgaard, A.S. Murray, M. Jain, R. Chen, J. Lawless, S. Greilich, Modelling the thermal quenching mechanism in quartz based on time resolved optically stimulated luminescence, J. Lumin. 130 (2010) 902–909. [24] X.L. Wang, J.H. Du, G. Adamiec, A.G. Wintle, The origin of the medium OSL component in West Australian quartz, J. Lumin. 159 (2015) 147–157. [25] L. Bøtter-Jensen, S.W.S. McKeever, A.G. Wintle, Optically Stimulated Luminescence Dosimetry, Elsevier, Amsterdam, 2003. [26] R. Chen, S.W.S. McKeever, Theory of Thermoluminescence and Related Phenomena, World Scientific, London, 1997.
S5. Fig. 11 shows a comparison of the experimental TM-OSL curve with two cases of the simulation outcomes that in the best way imitate the position of experimental results for the extreme S and ħω values. They were acquired using the values 1.9 eV and 2 × 1015 s−1 for ET and s, respectively. As can be seen, the simulated TM-OSL curves are much wider than the measured ones, but if one takes into account the effect of thermal quenching then the TM-OSL peak is narrower. An exemplary adequate calculation was performed using the thermal quenching parameters given by Wintle [19]:
Wq Icorr (T ) = I (T )/ ⎡1 + K exp ⎛− ⎞ ⎤ ⎢ kT ⎠ ⎥ ⎝ ⎦ ⎣ ⎜
⎟
(2) 7
where the coefficients K and Wq are 2.8 × 10 and 0.64 eV, respectively. As can be seen in Fig. 12, this rough operation leads to quite reasonable results. The optical depth of the 1st trap is estimated to be about 2.4 eV (1.9 eV + 27 × 0.02 eV). This value should be confirmed by more extensive investigation focused on TM-OSL curve fitting for several different experimental parameters, e.g. different heating rates or stimulation wavelengths, and also compared with the optical trap depth estimated for the traps responsible for the slower components of OSL in quartz. However, the results presented above of a simple approach show that TM-OSL measurements are very helpful in determining the parameters that uniquely determine a trap in the OSL process. 5. Conclusions The experimental results confirmed the recent simulation outcomes that the dependency of the OCS on temperature can be used to effectively separate the different OSL components. The proper selection of the stimulation energy, heating rate and photon flux density leads to the TM-OSL peak in the temperature range below the thermal activity of a trap. The subsequent repetition of the TM-OSL measurements with higher stimulation energies allows the TM-OSL peaks to be obtained repeatedly for deeper traps. The shape of TM-OSL peaks is determined unambiguously by trap parameters, so the latter can be estimated by fitting theoretical curves to the experimental ones. The results presented for natural quartz samples shows the possibility of selectively measuring the fast OSL component, which is the most favorable for age estimation in the luminescence dating of
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Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Thermally modulated optically stimulated luminescence (TM-OSL) of quartz ⁎
T
A. Chruścińska , A. Szramowski Institute of Physics, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5/7, 87-100 Torun, Poland
A B S T R A C T It has recently been shown in computer simulations of the OSL process that increasing the temperature during optical stimulation allows the probability of electron excitation from traps to the conduction band to be modulated. This, in turn, facilitates the efficient separation of individual OSL components. A result of the thermally modulated OSL (TM-OSL) measurement method is a peak shaped curve. In addition, the position of the TM-OSL peak on the temperature axis can be controlled by the stimulation energy, heating rate and photon flux used for optical stimulation. Here, the results of a TM-OSL study for natural quartz samples are presented. The basic features of the TM-OSL curves of quartz are shown for different parameters that control their shape. The investigation is focused on traps that are most sensitive to light and which are the source of the OSL signal that is the most useful in retrospective dosimetry. A method by which it can be measured separately from the slower components is presented. The very first attempt at directly estimating the optical depth and the parameters determining the strength of the electron-phonon coupling for the traps responsible for the fast OSL component in quartz has been made.
1. Introduction In the investigation of the optically stimulated luminescence (OSL) of solids, the main aim is usually to determine the parameters of traps responsible for the individual components of the total OSL signal. Knowing these parameters enables the changes in luminescence with the temperature and the type of light used for stimulation to be predicted. In the most basic description of the phenomenon that takes into account the crystal lattice vibrations, there are three uniquely determined parameters defining a trap: the optical trap depth (E), the energy of lattice vibration mode (ħω) and the Huang-Rhys factor (S). They are related to the thermal trap depth ET by the formula [1,2]:
E = E T + Sħω.
(1)
In the results obtained by the most popular stimulation techniques, however, these parameters are hidden in a single physical quantity called the optical cross-section (OCS) that determines the OSL decay rate. It has recently been shown that, when the optical cross-section is modulated during the OSL measurements, a more advantageous kind of OSL curve can be acquired for a single trap kind [2–5]. The curve has the form of a peak whose position and shape are uniquely determined by all three trap parameters. Thus, they can be found by using curve shape analysis, e.g. by fitting the theoretical curves. As can be seen from the OCS dependence on the stimulation energy presented in Supplementary material, the OCS also depends on temperature.
⁎
Therefore, one of the possibilities of OCS modulation is increasing the temperature during optical stimulation. A similar kind of stimulation was previously theoretically investigated by Chen and Pagonis [6] for a specific case when a trap has an excited electronic state below the conduction band and the trap emptying is a two-stage process. Such a process is reported in the case of K-feldspars and can be recognized by a characteristic peak in the infrared part of its OSL stimulation spectra [7]. Recent simulations of the one-stage OSL process, using a model that takes into account the optical cross-section dependence on temperature, showed that the socalled thermally modulated OSL method not only allows the estimation of trap parameters but also facilitates the efficient separation of individual OSL components [4]. The position of the TM-OSL peak on the temperature axis is very strongly dependent on the trap parameters and can be easily controlled by the stimulation energy, heating rate and photon flux used for optical stimulation. Here, the results of recent computer modeling are used for planning and interpreting the results of the TM-OSL measurements performed for natural quartz samples. The model assuming the one-stage emptying of traps seems to be suitable for quartz, for which no evidence of the more complex trap ionization is observed in the OSL stimulation spectra [8]. Quartz was chosen as the first material whose TM-OSL was intensively investigated for two reasons. Firstly, it has a rich spectrum of deep traps and, secondly, it is widely used in luminescence dating and retrospective dosimetry. The OSL of quartz has been intensively
Corresponding author. E-mail address: alicja@fizyka.umk.pl (A. Chruścińska).
https://doi.org/10.1016/j.jlumin.2017.12.004 Received 1 June 2017; Received in revised form 11 October 2017; Accepted 2 December 2017 Available online 05 December 2017 0022-2313/ © 2017 Elsevier B.V. All rights reserved.
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exploited for estimating the age of quaternary geological sediments for more than 30 years. OSL dating is a well-established method for sediments that were exposed to sun light during their creation process. The nature of the OSL signal of quartz is complex. Three to six components can be observed depending on the sample [9–14]. The fastest decaying component is the most appropriate for age estimation because of the requirement of OSL signal zeroing prior to the sediment layer creation. In the most widely used OSL dating protocols, the very first part of the OSL decay curve is used for age estimation. It is assumed that it corresponds mainly to the fastest OSL component. Such a solution can be accepted for samples whose fast component dominates the total OSL signal. This fact, however, is hard to verify by standard measurements during the dating procedure. The OSL measurement techniques applied up to now do not allow the selective detection of the OSL components. Using the analytical techniques of the OSL curve decomposition into individual components before the age determination is as dubious as it is unrealistic in dating practice. There is a need to find a way of effectively separating the fast OSL component during the measurement stage. It will be shown that the application of thermally modulated optical stimulation provides such a possibility. The basic features of the TM-OSL curves of quartz are shown for different parameters controlling their shape. The first preliminary attempt at directly determining the optical trap depth and the parameters determining the strength of the electron-phonon coupling is demonstrated. Estimation of these parameters allows the direct correlation of the traps active in the OSL and TL processes to be performed.
Fig. 1. TL curves of quartz samples used in the study. The heating rate used in the measurement is 1 Ks−1 and the excitation dose about 10 Gy.
of stimulated luminescence for the samples was erased by fast heating to 500 °C. 3. Results 3.1. TM-OSL measurements for different experimental conditions The TL curves of all three investigated samples are shown in Fig. 1. As can be seen, the peak called the 100 °C peak (here for the heating rate of 1 K s−1 below 80 °C) dominates in all samples, but the intensities of the subsequent peaks at higher temperatures differ significantly from sample to sample. As will be reported below, the presence of the traps shallower than these that are the main object of the investigation can complicate the results of TM-OSL. On the one hand, it can make the analysis of experimental curves harder but, on the other, the explicit manifestation of the activity of shallow traps in the TM-OSL measurements reveals the mechanism of luminescence. This is, for example, much harder to recognize in the CW-OSL experiments. For describing the potential of a measurement method, however, it is reasonable to use rather simpler cases of experimental results. Therefore, most of the data is presented here for the FQ sample, which has the smallest concentration of traps responsible for peaks below 200 °C. The depth of traps which can be investigated by the TM-OSL method is limited by the temperature region RT − 500 °C available when using the Risø system. From previous TM-OSL simulations, it is known that it is useful to establish the experimental conditions in such a way that the investigated traps are emptied mainly by light. This means that the TMOSL peak should be observed below the temperature region of the effective thermal emptying of the appropriate trap. Because of that, the measurement of the explicit TM-OSL peaks for traps that in quartz contribute to a TL signal below 200 °C was not possible when using the equipment described above. However, the traps that are important for dating applications, because they are able to hold electrons for long enough, are thermally emptied above 250 °C. From room temperature to 250 °C should be a broad enough temperature range to detect a purely optical TM-OSL signal. The TL peaks below 250 °C originating from the shallower traps were erased by the adequate preheat. Fig. 2 presents the TL curve obtained with the heating rate of 0.2 K s−1 for the FQ sample after a preheat at 275 °C. It also presents the TM-OSL curves obtained after the analogical preheat, with the same heating rate, for the wavelength of the stimulation light equal to 620 nm and two different values of the photon flux. The shift of the TMOSL peak toward lower temperatures can be observed for the increasing photon flux, just as was seen earlier by simulations ([4], Figure 6). The opposite direction of the peak shift is caused by applying higher heating rates. This is illustrated by Fig. 3 for the WP9 sample. It is interesting to notice that the TM-OSL peak position stays fixed
2. Experimental The main part of the measurement equipment is the Risø TL/OSL System TL-DA-12. The stimulation was carried out using two kinds of stimulation units. The first one enables the application of one of three LED stimulation modules, with the maxima of their spectral bands at: 660, 620 and 530 nm. Each module contains a 10 × 10 LED matrix (produced by R2T Industry(HK) Ltd.) and is connected to the Risø System by means of a fiber optic bundle with a 10 mm diameter and made of 1.0 mm high-performance plastic fibers (Mitsubishi Rayon Co. LTD.). The maximal photon flux achievable on the sample position for all the LED modules is over 2 × 1017 cm−2 s−1. The second stimulation system uses as a light source a 1000 W xenon lamp (Kiloarc by Optical Building Blocks Corp.), as described in detail earlier [15]. Luminescence detection is carried out by an EMI 9235QA photomultiplier with a 2 mm UG11-IRB filter that is an improved version of a UG11 filter (the main transmission band 300–380 nm), which more efficiently blocks light with wavelengths above 700 nm. Data saving is realized through the standard program for measurement control of the Risø reader. A 90 Sr/90Y β source giving a dose rate of about 30 mGys−1 has been used for irradiation. The main presented results were performed for quartz grains of 150–250 mm extracted from Fontainebleau sand (palaeo-coastal dune system Fontainebleau, Seine-et-Marne, Ile-de-France, France), hereinafter referred to as FQ. The thermoluminescence (TL) characteristic of this sample is very beneficial while presenting the results of TM-OSL experiments because of the relatively low participation of the peaks about 165 °C and 200 °C in its TL signal. As will be shown, the strong retrapping of electrons by traps responsible for these peaks during optical stimulation can significantly deform the shape of the TM-OSL curve. Two other kinds of quartz were used: one extracted from sediments – WP9, and natural quartz provided by Merck KGaA – marked as MQ. Quartz grains of the WP9 sample with the diameter of about 100–200 µm were extracted from sediment by the standard procedure, including separation in heavy liquids and etching in 40% HF. The Merck quartz was sieved and the fraction used in the experiments is 100–125 µm. Aliquots of 4 mm diameter were prepared on steel discs using Silkospray silicone oil for each kind of sample. The natural signal 436
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Fig. 5. The TM-OSL curves obtained by computer simulations for a trap with the optical depth of 2 eV, the Huang-Rhys factor S1 = 5 and vibrational mode energy ħω1 = 0.01 eV. The wavelength of the stimulation light was 660 nm. The curves were simulated for different combinations of heating rate and photon flux values selected in such a manner that the ratio of these quantities was constant. The simulations were performed for the model that consists of two traps and one luminescence center (Supplementary material, equations 1–5) for the following parameters: N1 = 1012 cm−3, N2 = 1013 cm−3, M = 2 × 1013 cm−3, A1 = A2 = 10−11 cm3 s−1, Am = 4×10−11 cm3 s−1, β = 10−8 cm3 s−1, E2 = 3 eV, S2 = 5, ħω2 = 0.01 eV, s1 = s2 = 1013 s−1.
Fig. 2. TM-OSL curves for the FQ sample obtained with the stimulation light wavelength 620 nm, the heating rate 0.2 Ks−1 and two different photon flux values 1016 cm−2 s−1 (gray line) and 3×1016 cm−2 s−1 (black line). The TL curve obtained with the same heating rate is shown for comparison (light gray line). All curves were measured after an irradiation dose of about 10 Gy, 25 s of preheat at 275 °C subsequent to heating with the rate 1 Ks−1.
for a constant ratio of photon flux to heating rate. A convincing illustration of this effect is given in Fig. 4, where the TM-OSL curves of the FQ sample are shown for the heating rates of 1, 2 and 4 Ks−1 and the photon flux values of 9.5 × 1015, 1.9 × 1016 and 3.8 × 1016 cm−2s−1 respectively. This is a simple consequence of the fact that the number of photons reaching the sample during its heating by one degree is the same in all cases. Analogous results can be obtained by computer simulations of the TM-OSL process. A few examples are presented in Fig. 5. From both the experimental and simulation results, it is clear that higher heating rates and photon flux values are more advantageous for the TM-OSL measurements. There are two benefits. The first is the better separation of the TM-OSL peak from the TL peak region, which is always at higher temperatures for the higher heating rates. The second is the higher intensity of the measured TM-OSL signal. The OCS depends on the stimulation energy, especially dynamic for the stimulation energies much below the threshold, hence the TM-OSL peak position can also be controlled by this experimental parameter. The TM-OSL measurements have been conducted for different wavelengths of the stimulation light. The TM-OSL peak below 200 °C could be observed for wavelengths as high as 700 nm. Fig. 6 presents an example of TM-OSL curves realized for three different stimulation energies for the FQ sample. As can be seen, the TM-OSL peak shifts into
Fig. 3. TM-OSL curves for the WP9 sample were obtained with the photon flux 3.2 × 1016 cm−2 s−1 and two different heating rates 2 Ks−1 (black line) and 0.2 Ks−1 (gray line). The measurements were performed using the 660 nm LED module after irradiation with the dose of 20 Gy and preheat to 300 °C.
Fig. 4. TM-OSL curves for the FQ sample obtained with three different heating rates and photon flux values: 1 Ks−1 and 9.5 × 1015 cm−2 s−1 (solid line), 2 Ks−1 and 1.9 × 1016 cm−2 s−1 (dashed line), 4 Ks−1 and 3.8 × 1016 cm−2 s−1 (dotted line). The ratio of the photon flux value to the heating rates is the same for all curves. The stimulation light wavelength was 620 nm and the excitation dose about 10 Gy. Fig. 6. TM-OSL curves for the FQ sample measured with the heating rate of 0.2 Ks−1 and the photon flux of about 3.8 × 1016 cm−2s−1 after a preheat to 230 °C for three different stimulation wavelengths.
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Fig. 8. CW-OSL curves of the FQ sample obtained with the stimulation wavelength of 530 nm after a preheat to 230 °C (black line) and after the same preheat and the TM-OSL with the stimulation wavelength of 620 nm, heating rate of 4 Ks−1, and photon flux of 1.3×1017 cm−2s−1 (gray dashed line). The inset presents the TM-OSL curves of the same sample measured without prior optical stimulation (gray line) and after 15 s (light-gray line) and 200 s (black line) of CW-OSL stimulation (530 nm).
Fig. 7. Three curves measured after a preheat to 270 °C with the heating rate of 0.2 Ks−1 are compared: the TM-OSL (gray line) measured to 230 °C with the stimulation wavelength of 620 nm, the TL curve obtained after the TM-OSL measurement (light gray line) and the TL curve measured directly after the preheat (black line). The irradiation dose was about 10 Gy. Traps responsible for the TM-OSL peak observed below 200 °C are also the source of the TL peak at 260 °C (325 °C when the heating rate 5 Ks−1 is used).
higher temperatures during stimulation with lower energy when values of the photon flux and the heating rate remain unchanged.
3.2. Selecting the fast OSL component by the TM-OSL method The comparison of the TM-OSL and TL curves measured with the same heating rate and after a preheat to the same temperature can directly reveal the origin of the TM-OSL signal. In Fig. 2, some gap in the TM-OSL signal can clearly be seen above 250 °C. The TL peak observed in this temperature region during heating with the rate of 0.2 Ks−1 is detected at 325 °C when TL is measured with the much faster heating (5 Ks−1). The TL peak at 325 °C in quartz is known as being related to traps which are the source of the fast OSL component [9,16–18]. The difference between the TL curves obtained after the TMOSL measurement carried out to 230 °C and directly after the preheat to 270 °C is explicitly shown in Fig. 7. The TM-OSL curve is also shown in order to demonstrate the difference between the intensities of both the TM-OSL and TL signals. Thermal quenching causes the decrease in TL intensity at higher temperatures [19–23]. The maximum intensity of the TM-OSL peak is over 13 times higher than the intensity of the related TL peak. A more direct confirmation of the relationship of the TM-OSL peak obtained using the 620 nm stimulation with the fast OSL component observed in conventional OSL measurements is the comparison of CWOSL curves measured before and after TM-OSL measurement. Both curves are shown in Fig. 8 for the FQ sample. The first CW-OSL curve is measured directly after the preheat to 230 °C, and the second one after the same preheat and subsequent TM-OSL measurement with the wavelength of 620 nm continued to the temperature of the preheat. CWOSL was measured at 125 °C with the stimulation wavelength of 530 nm. As can be observed, the fast component of the CW-OSL signal is almost erased by the TM-OSL stimulation. The CW-OSL curve for the FQ sample can be fitted by a sum of two first-order curves (exponential decays). The intensity of the faster component (lifetime of about 2 s) decreases by about 94% after the TM-OSL measurement that was stopped at 230 °C. As can be seen, this temperature does not ensure the total emptying of the related traps. The second component is not noticeably reduced. The impact of CW-OSL stimulation on the TM-OSL signal measured with the wavelength of 620 nm is shown in the inset of Fig. 8. There is not much left from the TM-OSL signal measured with 620 nm after the first 5 s of CW-OSL stimulation with 530 nm. After measuring the TM-OSL signal with the stimulation wavelength 620 nm and emptying solely the traps responsible for the fast OSL
Fig. 9. TM-OSL curves of the FQ sample obtained using the stimulation wavelength of 560 and 575 nm, the heating rate of 0.2 Ks−1 and the photon flux of 4.8 × 1016 and 4.6 × 1016 cm−2s−1, respectively. Before the TM-OSL measurement realized with the higher stimulation energy, the traps related to the fast OSL component were depopulated. The inset shows the gradual reset of the fast OSL component by 3-fold TM-OSL measurement with 620 nm stimulation carried out to 230 °C with the heating rate of 4 Ks−1 and the TL curve (4 Ks−1) obtained after these TM-OSL measurements.
component in quartz, the slower component can be measured by the TM-OSL method using a higher stimulation energy. The effects of such an experiment can be seen in Fig. 9 for two different wavelengths of the stimulation light. 4. Discussion It should be noted that the TM-OSL corresponding to the fast OSL component in quartz can not always be observed as such a clear peak as in the FQ or WP9 samples. The FQ sample, for example, as was mentioned earlier, is rather a special, advantageous case of natural quartz. In its simple CW-OSL curve, consisting only of two components, the fast component dominates. Its initial intensity is over 5 times higher than the initial intensity of the second component. In light of the latest findings concerning the nature of the medium component in quartz [24], the simplicity of the CW-OSL curve for the FQ sample can be linked to the very low concentrations of traps responsible for the peaks at about 145 and 195 °C occurring in this sample (see Fig. 1). The sample of Merck quartz is quite another case. Here, one can observe the large activity of shallow traps and the CW-OSL curve for the same stimulation wavelength (530 nm) can be decomposed into three firstorder curves. The relation between their intensities is 85:84:62. The fast 438
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analysis easier. It is generally accepted that the thermal depth of the trap responsible for the 325 °C TL peak in quartz is 1.7 eV and its frequency factor is about 5 × 1013 s−1 [20,25], so these values were used for the first attempt at fitting a theoretical TM-OSL curve to the experimental TM-OSL curve obtained for quartz. For this purpose, a series of simulations was performed for a model consisting of one OSL trap, one deep, thermally disconnected trap and one luminescence center. The parameters of this model are presented in detail in Supplementary Material. The values of the thermal trap depth and frequency factor were fixed and shapes of the TM-OSL curve were initially tested for the S and ħω values from wide ranges: 2 − 100 and 0.005 − 0.1 eV, respectively. It should be stressed that curves with the shape of a peak below 200 °C could be obtained only for photon flux values much lower than the one used in the experiments, but none of the obtained curves resembled the experimental curves. A few exemplary results of these tests are shown in Fig. S1-S3 in Supplementary Material. It is worth noting here that it is very hard to estimate the thermal trap depth and frequency factor for a high temperature TL peak that is not isolated from other peaks. This is because the methods allowing these quantities to be estimated were established for well isolated TL peaks and assume the independence of different traps [26]. The TL peak in quartz at 325 °C, as can be observed in Fig. 1, is neither isolated nor can the corresponding trap be considered as independent of others. Moreover, the temperature range above 300 °C is very inconvenient for TL parameter investigation because of the possible thermal lag between the heating element and the sample. Additionally, the peak position at 325 °C can be obtained for different combinations of the ET and s parameters that were also used in the simulations aimed at reproducing the TM-OSL curve measured for quartz. The shapes of simulated TMOSL curves similar to the shapes of experimental curves were obtained for significantly larger values of the thermal trap depth than 1.7 eV. The reason for differences between the shapes of the TM-OSL curves obtained for different thermal trap depths can be explained by the dynamics of the OCS increase with temperature. Two examples of this dependency for different trap depths are presented in Fig. S4 in Supplementary material. As a rule, the lower the stimulation energy below the threshold energy (and simultaneously higher values of photon flux density) used for stimulation, the narrower the TM-OSL peaks. This effect is also illustrated in Supplementary material in Fig.
Fig. 10. TM-OSL curves for all three samples measured after a preheat to 275 °C, for the heating rate of 4 Ks−1, the photon flux density of 1.3 × 1017 cm−2s−1 and the stimulation wavelength equal 620 nm. The inset presents TM-OSL curves for the Merck sample for two different heating rates.
component does not prevail and it is hard to maintain, as is usually assumed in OSL dating, that the beginning of the CW-OSL curve corresponds solely to this component. In this application, the samples whose CW-OSL are more complex and the fast component intensity is comparable with the intensities of slower components are always more problematic. In the case of the Merck sample, after the TM-OSL measurement with the stimulation wavelength of 620 nm, both the fast and the medium components decrease by 70% and 14%, respectively, but simultaneously the values of their lifetimes increase noticeably. This suggests that the optical emptying of traps during the CW-OSL measurement may proceed with efficient competition from the shallow traps, and that the analysis of the CW-OSL curve applying the first-order kinetic assumption is not quite an appropriate method here. The strong activity of the shallow traps in the case of the Merck sample can be observed directly during the TM-OSL measurements that are presented in Fig. 10. The TM-OSL peak is clearly deformed by a contribution from TL peaks related to the shallow traps. The latter, although empty after the preheat, are filled during the optical stimulation as a result of the phototransfer phenomenon. The inset shows the TM-OSL peaks for two different heating rates and the photon flux values appropriately selected in order to fix the TM-OSL peak position. The shape of the curve changes significantly because the TL peak position changes with the heating rate. In Fig. 10 the TM-OSL curves for other samples are shown for comparison. In these cases, it is hard to observe any deformation, however, since the weak phototransfered 100 °C TL peak can be measured after the CW-OSL stimulation at room temperature (see Fig. S6 in Supplementary material), it can be supposed that there may be a slight impact of this peak on the shape of the TMOSL curves. Three facts concerning the Merck sample can be linked: the exceptionally high concentration of the TL peaks below 200 °C, the presence of phototransfered TL peaks in its TM-OSL results, and the abovementioned partial decay of the medium component of the CW-OSL curve after the TM-OSL measurement that removes solely the fast component in the case of other samples. All these effects confirm that the medium component in quartz may not be related to the optical depopulation of a particular deep trap, but rather is a result of retrapping the electrons in the traps responsible for TL peaks below 200 °C during optical stimulation [24]. The TM-OSL peak obtained during the stimulation with 620 nm can be used for the estimation of parameters that uniquely determine the related trap. There are five such parameters: ET, s (the frequency factor), E, S and ħω, but when one assumes that the thermal depth of the trap, ET which is related to the optical trap depth by relation (1), and the frequency factor are known from earlier works, then only two parameters, S and ħω, are independent. This make the curve shape
Fig. 11. Theoretical TM-OSL curves obtained as a result of simulations using the model described in Supplementary material. The thermal depth of trap ET1 used in the simulations is 1.9 eV and frequency factor s is 2×1015 s−1. Two examples that reproduce the position of the TM-OSL peak properly are shown for extreme values of S and ħω. All other curves that have the maximum at the same temperature are between these two extreme curves. The stimulation parameters both for the experiment and simulations are 620 nm and 4 Ks−1. The photon flux density is assumed to be the less accurately estimated quantity, so its different values were tested. They are given in the legend for the shown cases.
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sediments. Future works should include the investigation of the dose dependency of the component separated by the TM-OSL method, working out the dating protocol exploiting this method and its tests for samples of known age. Acknowledgements This work has been financed by the grant of the National Science Centre, Poland, No. 2016/21/B/ST10/01867. Appendix A. Supporting information Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jlumin.2017.12.004. References Fig. 12. Results of fitting the theoretical TM-OSL curve to the experimental one obtained by taking into account the effect of thermal quenching (details are given in the text). The parameters in the simulation that determine the strength of electron-phonon coupling are S = 27 and ħω = 20 meV, the stimulation wavelength 620 nm, heating rate 4 Ks−1 and the photon flux density 5×1016 cm−2s−1.
[1] J.M. Noras, Photoionisation and phonon coupling, J. Phys. C: Solid State Phys. 13 (1980) 4779–4789. [2] A. Chruścińska, On some fundamental features of optically stimulated luminescence measurements, Radiat. Meas. 45 (2010) 991–999. [3] A. Chruścińska, Estimating the parameters of traps in quartz by the variable energy of stimulation optically stimulated luminescence (VES-OSL) method, Radiat. Meas. 81 (2015) 205–211. [4] A. Chruścińska, N. Kijek, Thermally modulated optically stimulated luminescence (TM-OSL) as a tool of trap parameter analysis, J. Lumin. 174 (2016) 42–48. [5] A. Chruścińska, N. Kijek, S. Topolewski, Recent development in the optical stimulation of luminescence, Radiat. Meas. (2017). [6] R. Chen, V. Pagonis, Modeling TL-like thermally assisted optically stimulated luminescence (TA-OSL), Radiat. Meas. 56 (2013) 6–12. [7] S.M. Barnett, I.K. Bailiff, Infrared stimulation spectra of sediments containing feldspars, Radiat. Meas. 27 (1997) 237–242. [8] G. Hütt, I. Jaek, V. Vasilchenko, Photoionization of radiation-induced traps in quartz and alkali feldspars, Appl. Radiat. Isot. 54 (2001) 175–182. [9] M. Jain, M., A.S. Murray, L. Bøtter-Jensen, Characterisation of blue-light stimulated luminescence components in different quartz samples: implications for dose measurement, Radiation Measurements 37 (2003), pp. 441–449. [10] G. Kitis, G.S. Polymeris, N.G. Kiyak, Component-resolved thermal stability and recuperation study of the LM-OSL curves of four sedimentary quartz samples, Radiat. Meas. 42 (2007) 1273–1279. [11] G. Kitis, V. Pagonis, Computerized curve deconvolution analysis for LMOSL, Radiat. Meas. 43 (2008) 737–741. [12] G. Kitis, N.G. Kiyak, G.S. Polymeris, V. Pagonis, Investigation of OSL signals from very deep traps in unfired and fired quartz samples, Nucl. Instrum. Methods Phys. Res. B 268 (2010) 592–598. [13] N. Kijek, A. Chruścińska, K.R. Przegiętka, The dependence of quartz OSL on stimulation energy, Radiat. Meas. 71 (2014) 108–112. [14] A.G. Wintle, G. Adamiec, Optically stimulated luminescence signals from quartz: a review, Radiat. Meas. 98 (2017) 10–33. [15] A. Chruścińska, Experimental demonstration of the variable energy of stimulation optically stimulated luminescence (VES-OSL) method, Radiat. Meas. 71 (2014) 247–250. [16] B.W. Smith, M.J. Aitken, E.J. Rhodes, E.J. Robinson, D.M. Geldard, Optical dating: methodological aspects, Radiat. Prot. Dosim. 17 (1986) 229–233. [17] N.A. Spooner, On the optical dating signal from quartz, Radiat. Meas. 23 (1994) 593–600. [18] A.S. Murray, A.S. Wintle, Factors controlling the shape of the OSL decay curve in quartz, Radiat. Meas. 29 (1998) 65–79. [19] A.G. Wintle, Thermal quenching of thermoluminescence in quartz, Geophys. J. R. Astron. Soc. 41 (1975) 107–113. [20] R.M. Bailey, Towards a general kinetic model for optically and thermally stimulated luminescence of quartz, Radiat. Meas. 32 (2001) 17–45. [21] R. Nanjundaswamy, K. Lepper, S.W.S. McKeever, Thermal quenching of thermoluminescence in natural quartz, Radiat. Prot. Dosim. 100 (2002) 305–308. [22] J.S. Singarayer, R.M. Bailey, Further investigations of the quartz optically stimulated luminescence components using linear modulation, Radiat. Meas. 37 (2003) 451–458. [23] V. Pagonis, C. Ankjærgaard, A.S. Murray, M. Jain, R. Chen, J. Lawless, S. Greilich, Modelling the thermal quenching mechanism in quartz based on time resolved optically stimulated luminescence, J. Lumin. 130 (2010) 902–909. [24] X.L. Wang, J.H. Du, G. Adamiec, A.G. Wintle, The origin of the medium OSL component in West Australian quartz, J. Lumin. 159 (2015) 147–157. [25] L. Bøtter-Jensen, S.W.S. McKeever, A.G. Wintle, Optically Stimulated Luminescence Dosimetry, Elsevier, Amsterdam, 2003. [26] R. Chen, S.W.S. McKeever, Theory of Thermoluminescence and Related Phenomena, World Scientific, London, 1997.
S5. Fig. 11 shows a comparison of the experimental TM-OSL curve with two cases of the simulation outcomes that in the best way imitate the position of experimental results for the extreme S and ħω values. They were acquired using the values 1.9 eV and 2 × 1015 s−1 for ET and s, respectively. As can be seen, the simulated TM-OSL curves are much wider than the measured ones, but if one takes into account the effect of thermal quenching then the TM-OSL peak is narrower. An exemplary adequate calculation was performed using the thermal quenching parameters given by Wintle [19]:
Wq Icorr (T ) = I (T )/ ⎡1 + K exp ⎛− ⎞ ⎤ ⎢ kT ⎠ ⎥ ⎝ ⎦ ⎣ ⎜
⎟
(2) 7
where the coefficients K and Wq are 2.8 × 10 and 0.64 eV, respectively. As can be seen in Fig. 12, this rough operation leads to quite reasonable results. The optical depth of the 1st trap is estimated to be about 2.4 eV (1.9 eV + 27 × 0.02 eV). This value should be confirmed by more extensive investigation focused on TM-OSL curve fitting for several different experimental parameters, e.g. different heating rates or stimulation wavelengths, and also compared with the optical trap depth estimated for the traps responsible for the slower components of OSL in quartz. However, the results presented above of a simple approach show that TM-OSL measurements are very helpful in determining the parameters that uniquely determine a trap in the OSL process. 5. Conclusions The experimental results confirmed the recent simulation outcomes that the dependency of the OCS on temperature can be used to effectively separate the different OSL components. The proper selection of the stimulation energy, heating rate and photon flux density leads to the TM-OSL peak in the temperature range below the thermal activity of a trap. The subsequent repetition of the TM-OSL measurements with higher stimulation energies allows the TM-OSL peaks to be obtained repeatedly for deeper traps. The shape of TM-OSL peaks is determined unambiguously by trap parameters, so the latter can be estimated by fitting theoretical curves to the experimental ones. The results presented for natural quartz samples shows the possibility of selectively measuring the fast OSL component, which is the most favorable for age estimation in the luminescence dating of
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