Experimental features of natural thermally assisted OSL (NTA-OSL) signal in various quartz samples; preliminary results

Nuclear Instruments and Methods in Physics Research B 349 (2015) 24–30

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Experimental features of natural thermally assisted OSL (NTA-OSL) signal in various quartz samples; preliminary results George S. Polymeris a,⇑, Eren Sß ahiner a, Niyazi Meriç a, George Kitis b a b

Institute of Nuclear Sciences, Ankara University, Besßevler, 06100 Ankara, Turkey Laboratory of Nuclear Physics and Elementary Particles, Physics Department, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece

a r t i c l e

i n f o

Article history: Received 23 May 2014 Received in revised form 30 October 2014 Accepted 21 January 2015

Keywords: Very deep traps (VDT) Natural thermally assisted OSL (NTA-OSL) Thermal activation Quartz

a b s t r a c t The access to the OSL signals from very deep traps is achieved by an alternative experimental method which comprises combined action of thermal and optical stimulation, termed as thermally assisted OSL (TA-OSL). This experimental technique was suggested in order to not only measure the signal of the deep traps without heating the sample to temperatures greater than 500 °C, but also use the former for dosimetry purposes as well, due to exhibiting a number of interesting properties which could be effectively used towards dosimetry purposes, especially for large accumulated artificial doses. The present study provides for the first time in the literature with preliminary results towards the feasibility study of the naturally occurring TA-OSL signal in coarse grains of natural quartz towards its effective application to geological dating. The samples subjected to the present study were collected from fault lines in Kütahya-Simav, Western Anatolia Region, Turkey; independent luminescence approaches yielded an equivalent dose larger than 100 Gy. Several experimental luminescence features were studied, such as sensitivity, reproducibility, TA-OSL curve shape as well as the correlation between NTA-OSL and NTL/ NOSL. Nevertheless, special emphasis was addressed towards optimizing the measuring conditions of the TA-OSL signal. The high intensity of the OSL signal confirms the existence of a transfer phenomenon from deep electron traps. The increase of the integrated TA-OSL signal as a function of temperature is monitored for temperatures up to 180 °C, indicating the later as the most effective stimulation temperature. At all temperatures of the studied temperature range between 75 and 260 °C, the shape of the signal resembles much the shape of a typical CW-OSL curve. However, a long-lived, residual NTA-OSL component was monitored after the primary, initial NTA-OSL measured at 180 °C; the intensity of this component increases with increasing stimulation temperature. The prevalence of these luminescent features was investigated, while the implications on dating applications of these features were also discussed. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Luminescence stands among the basic research tools in the fields of (a) ionizing radiation dosimetry, (b) archeological dating, geochronology and retrospective dosimetry and (c) authenticity testing of archeological artifacts [1,2]. Both thermoluminescence (TL) and optically stimulated luminescence (OSL) are passive dosimetric methods in the sense that the energy of ionizing radiation is stored in form of electron (and holes) trapped at electron (and hole) trapping levels having long lifetimes. Under thermal and optical stimulation the electrons are released from their traps ⇑ Corresponding author. E-mail addresses: [email protected] (G.S. Polymeris), sahiner@ankara. edu.tr (E. S ß ahiner), [email protected] (N. Meriç), [email protected] (G. Kitis).

http://dx.doi.org/10.1016/j.nimb.2015.01.079 0168-583X/Ó 2015 Elsevier B.V. All rights reserved.

and recombine with holes at luminescence centers giving rise to TL and OSL signals. In general luminescence dating and retrospective dosimetry is based on the fact that naturally-occurring minerals like quartz and feldspars act as natural dosimeters and preserve a record of irradiation dose, i.e., energy per unit mass, received through time mainly from the decay of natural radionuclides, i.e., 232Th, 40K, 87 Rb and natural U, along with cosmic rays [3,4]. The luminescence dating technique was firmly established in the 1970s and has undergone rapid development and enhancement in the 1980s, 1990s and 2000s, while the ability to image individual grains in conjunction with developments in measurement procedures have made important contributions to the field. Measurement procedures have been developed in which the equivalent dose is obtained on single aliquots for quartz, feldspars as well as polymineral samples [5,6]. In single-aliquot regenerative-dose

G.S. Polymeris et al. / Nuclear Instruments and Methods in Physics Research B 349 (2015) 24–30

procedures, the natural OSL signal is compared with the OSL signals resulting from doses being given to the same aliquot. In particular, a single-aliquot regenerative-dose (SAR) protocol was developed [7] in which correction for sensitivity change during the measurement sequence takes place. Currently, the electron trapping levels used for the applications of TL and OSL are exclusively the levels that can be thermally excited at temperatures well less than 500 °C. By stimulating these trapping levels, either by TL or OSL, the upper ages estimated by TL are limited to 50–100 kyr whereas the age limits by OSL are less than 1 Myr. However, there is a high need, especially in the geochronology community, for further improving the above age limits for covering at least the last four million years. Most of TL and OSL phosphors, as wide band gap materials, also hold some deep energy level defects. These are called as very deep traps (VDT) hereafter. As VDT are considered those traps, which correspond to TL glow peak having their peak maximum temperature, Tmax beyond the 500 °C. These deep electron traps have, at least, all the benefits of the shallower traps, along with an additional un-comparable advantage, being the very long lifetimes expected for these trapping levels. This latter, basic characteristic of any trapping level stands among the cornerstones for all TL and OSL application. However, its value is even more important in the cases of archeological and geological dating, because presets the age evaluation limits. Studying the luminescence resulting from charge release from these deep traps is difficult because of both thermal quenching, in conjunction to instrumental limitations [8]. In this case, new techniques are required in order to access the signal from VDT, which so far in the literature, was mostly indirectly monitored. The access to the luminescence signal from VDT is achieved by either photo-transferred TL [9,10] or indirectly [11,8 and references there in]. Therefore, alternative experimental methods, including a combined action of thermal and optical stimulation such as the thermally assisted OSL (TAOSL) [12,13] were suggested in order to not only measure the signal of the deep traps without heating the sample to temperatures greater than 500 °C, but also use the former for dosimetry purposes as well. The presence of VDT has been experimentally verified in many cases of luminescent materials, such as CaF2:N [16], Al2O3:C [8,12– 15], sedimentary quartz [17] and very recently apatites [18,19] as well as to some materials consisting the ground layers of wood and canvas paintings, such as yellow ochre, BaSO4, gypsum and chalk [20]. Among all the aforementioned citations: (a) TA-OSL signals from CaF2:N, quartz and Al2O3:C crystals exhibit a number of interesting properties which could be effectively used towards dosimetry purposes, especially for large accumulated doses. Among these properties, the most notable are the straightforward relation observed between the TA-OSL integrated intensity and the dose, along with the simple TA-OSL curve shape. From a theoretical point of view the very long lifetimes expected for these traps provide one of the main pre-requirements towards the extension of the age limits. Therefore, the role of the VDT in dating very old samples could be very important as well as significant. Nevertheless, especially in the case of previously heated quartz, it was recently argued [17] that the lower detectable dose limit of the VDT is of the order of 1 Gy but it could be further improved. (b) While apatites, and especially Durango apatite [18,22,23] are natural materials which are known to exhibit strong anomalous fading effects in the corresponding TL and conventional OSL signals, recent works indicated that the TA-OSL signal after artificial irradiation is much more stable compared to the other two aforementioned luminescence entities [18,19].

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(c) VDT are populated by a combined action of annealing at temperatures of the order of 500–1185 °C [8,12–20] in order to empty all traps from residual electrons, followed by a (post annealing) irradiation with a large dose. In all the aforementioned studies, the occupancy of very deep traps was controlled by selectively either filling or emptying them after using different types of artificial irradiations and/or different irradiation temperatures. Nevertheless, studying the naturally occurring signal arising from VDT has not been previously reported in the literature. The present work provides for the first time in the literature with preliminary results towards the feasibility study of the naturally occurring TA-OSL (or NTA-OSL) signal in coarse grains of natural quartz towards its effective application to geological dating. The motivation for this study arises from the cases of calcium carbonate as well as obsidian samples [21], for which the lack of natural, fast decaying OSL signal excludes the application of typical SAR OSL protocols, despite the intense OSL signal after artificial irradiation. This study will mostly stress on investigating the dosimetric potentiality of the naturally occurring luminescence signal arising from VDT; however, specific properties such as sensitization and repeatability were studied to both naturally occurring as well as artificial signals. Towards this primary aim, secondary aims of the present work include (a) establishing a robust protocol towards both measuring as well as taking full advantage of this specific signal and (b) studying the prevalence of specific properties of this specific signal such as activation energy of the thermally assisting phenomenon. 2. Materials and method 2.1. Origin of the quartz samples The samples subjected to the present study were sedimentary, geological quartzes collected from several fault lines in Kütahya-Simav, the Aegean Anatolia region, Turkey. The term ‘‘geological’’ is used in order to indicate quartz samples with large equivalent dose accumulated, resulting in very much intense naturally occurring luminescence signal. Five different quartz samples, each one obtained from a different fault line, were studied. For these cases the zeroing mechanism is not the light bleaching but the heat induced because of friction. In an independent dating approach for each fault, the equivalent dose for each sample was estimated in the range between 175 and 250 Gy. The laboratory code names as well as the independently estimated equivalent dose for each one are presented in Table 1. Aliquots-discs made out of aluminum substrate 0.5 mm thick and 9 mm in diameter, were prepared. The preparation of samples was formed under dim red light conditions. After sieving, grains of dimensions 90–180 lm were obtained. These grains were treated with HCI (10%), H2O2 (10%), HF (40%) and a final treatment with HCI (10%) in order to obtain a clean quartz extract. Aliquots with mass of 5 mg each were prepared by mounting the material on stainless-steel disks. Mass reproducibility was checked to be within ±5%. All aliquots were checked with infrared (IR) stimulation (880 nm) at ambient temperature to ensure the absence of feldspars. 2.2. Apparatus and measurement conditions All luminescence measurements were carried out using a Risø TL/OSL reader (model TL/OSL-DA-20), equipped with a 90Sr/90Y beta particle source, delivering a nominal dose rate of 0.130 ± 0.004 Gy/s. A 9635QA photomultiplier tube was used for

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Table 1 A summary on the luminescent data of the studied quartz samples.

*

Sample code name

ED (Gy)

Optimum NTA-OSL measurement T (°C)

Eact1 (eV)*

Eact2 (eV)*

Sensitization

YKT2A-02

239 ± 6

180

233 ± 5

180

YKT2A-08

313 ± 15

180

YKT1A-02

164 ± 7

180–200

YKT1A-03

100 ± 2

180

0.95 (0.07) 0.97 (0.06) 1.03 (0.07) 0.98 (0.06) 1.05 (0.07)

No

YKT2A-03

0.47 (0.06) 0.45 (0.05) 0.53 (0.07) 0.51 (0.06) 0.48 (0.06)

Yes No No Yes

Corrected for thermal quenching.

light detection. The stimulation wavelength is 470 (±20) nm for the case of blue stimulation, delivering at the sample position a maximum power of 40 mW cm2 [24]. The detection optics consisted of a 7.5 mm Hoya U-340 filter (kp 340 nm, FWHM 80 nm). All heatings and TL measurements were performed in a nitrogen atmosphere with a low constant heating rate of 1 °C/s, in order to avoid significant temperature lag; for the case of TL the samples were heated up to the maximum temperature of 500 °C. All TA-OSL measurements were performed at elevated temperatures for 500 s. Unless otherwise stated, all conventional blue OSL measurements were performed at 110 °C in the continuous wave mode (CWOSL). In all cases of OSL measurements, 1 data point was received for each second of stimulation.

temperature will be further decreased; (b) to check whether the PM tube suffers from overflowing. The quartz samples of the present study yield very large equivalent doses. Thus the corresponding NTL signal of step 1 is very intense, as Fig. 1 reveals. This very intense signal could result in enhanced dark counts background signal due to overflowing PM Tube. The inset of Fig. 1 presents a typical measurement according to step 2. Fortunately, besides the extremely intense NTL signal, no overflowing problems were monitored. Towards the selection of the optimum stimulation temperature for the TA-OSL, the Ti ranged from room temperature (RT throughout) to 280 °C. After establishing the optimum temperature for stimulation, all following measurements were performed at this temperature. In some cases, prior to the NTL measurement of step 1, a NOSL measurement was performed at 110 °C subsequent preheat at 260 °C for 10 s, in the continuous wave mode (CW-OSL). These OSL measurements were performed in order to exploit the correlation between the TA-OSL and the conventional OSL signal, if any.

3. Results and discussion 3.1. Selection of the optimum stimulation temperature for the NTAOSL signal in quartz

The general experimental protocol that was applied in the framework of the present study aimed at the identification of the optimum stimulation temperature, including the following steps: Step 1: NTL measurement. Step 2: Isothermal TL (ITL) at room temperature for 60 s. Step 3: TA-OSL at varying Ti (°C) for 500 s. Step 4: Residual TL (RTL). The aim of step 2 is twofold: (a) to ensure that the temperature of the hot plate is decreased to room temperature. The Risø reader is configured so that the lift will not rise until the hotplate temperature is 60 °C or less, so for temperatures below 60 °C, the temperature control is lost, as the system cannot cool directly the hotplate. Therefore, extra time is required so that the

OSL measurements performed at room temperature could not stimulate VDT. Nevertheless, this measurement could be used as dark count background signal. In order to define the optimum stimulation temperature for NTA-OSL, the optical stimulation was performed at increasing OSL measuring temperatures in order to study the thermal behavior of the source traps. The experimental protocol applied, includes the four steps described at Section 2.3, for varying temperatures ranging between RT and 280 °C. The upper stimulation temperature was selected to be slightly lower than the Tmax of the 325 °C glow peak of quartz. Each cycle of steps 1–4 was performed for each different TA-OSL measurement temperature to two different fresh aliquots of quartz crystal. TA-OSL curves for the natural signal, measured at various temperatures ranging between 75 and 260 °C are presented in Fig. 2. This specific figure includes the plots corresponding to one, unique quartz sample with code name YKT02–02; however the results were similar to all other cases of quartz samples. The signal is of extremely low intensity in the case of low stimulation temperatures of RT and 50 °C; this is the reason why these OSL curves are not plotted in Fig. 2. However, as the temperature is slightly

Fig. 1. Natural TL (NTL) signal measured at step 1 of the applied protocol. The signal is extremely intense. Inset: The level of the dark counts-background signal measured during step 2, immediately after the NTL measurement.

Fig. 2. Natural TA-OSL curves received at various temperatures ranging between 75 °C up to 260 °C.

2.3. Experimental protocols

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increasing, measureable signal could be detected, even at low stimulating temperatures as 75 °C. Another interesting feature arises from the strong increase of the integrated TA-OSL intensity, as a function of the stimulation temperature, up to stimulation temperatures of 180–200 °C. For higher stimulation temperatures, a decrease of the TA-OSL intensity is monitored. Eventually, the most intense TA-OSL signal, in both terms of initial as well as integrated intensity, is monitored for the stimulation temperature of 180 °C. For stimulation temperatures within the range between 50 °C, which is the first temperature indicating measurable natural TA-OSL signal, and 180 °C, the initial intensity is increased by at least one order of magnitude. Similar results were yielded for all quartz samples subjected to the present study, establishing thus the temperature of 180 °C as the optimum measurement temperature for natural TA-OSL, based on its enhanced sensitivity. It should be emphasized that RTL signal resembled much the typical background signal level for all of the stimulation temperatures within the aforementioned range. Hereafter, unless otherwise stated, all other TA-OSL measurements were performed at 180 °C.

(A)

(B)

3.2. Shape of natural TA-OSL curves The natural TA-OSL signal in quartz yields a typical decaying OSL curve shape, similar with that reported for the cases of artificially irradiated quartz [17] as well as apatites [18,19], being dependent on the temperature of the measurement. On the contrary, this shape is different compared to the corresponding shape of the (artificially irradiated) TA-OSL signal reported for the cases of either CaF2:N [16] or Al2O3:C [8,12], which resembles much the peak shaped linearly modulated OSL (LM-OSL). After 300 s of stimulation at the optimum temperature of 180 °C, as Fig. 3A reveals, the signal becomes flat with intensity almost one order of magnitude higher than ordinary OSL dark count background level of 60 counts/s. In order to study further the origin of this specific flat signal, after the typical TA-OSL measurement at 180 °C, sequential NTA-OSL measurements were performed to the same aliquot, at increasing temperatures. These curves were obtained and are presented in Fig. 3A, for the temperature range between 200 and 280 °C. All curves present similar features, such as the absence of any initial and quickly decaying part; instead, a very slowly decaying signal, whose shape is extremely flat with very large intensity. It is worth emphasizing that with increasing stimulation temperature the intensity of this flat NTA-OSL component is also increased, reaching the level of 2000 counts/s for stimulation at 280 °C, further supporting thus the fact that this signal corresponds to a slowly decaying TA-OSL component. 3.3. Quantifying NTA-OSL signal – correlation to conventional luminescence signals Fig. 3B presents 10 curves of natural TA-OSL obtained at 180 °C from 10 different aliquots of the same mass for the sample with code name YKT2A-02, in order to check the reproducibility of the signal. As it becomes apparent, the naturally occurring TA-OSL signal is not reproducible in terms of intensity. This lack of reproducibility could be possibly attributed to the wide range of the grain sizes of the samples subjected to the present study. At the same time, irreproducibility is also monitored in the case of NTL signals among the different aliquots, strongly supporting thus the sensitive relation of both NTL as well as NTA-OSL intensities on the grain size. Similar lack of reproducibility was also monitored after artificial irradiation as well. Due to this aforementioned lack of reproducibility, the appropriate quantification of the natural TA-OSL signal becomes extremely important. Towards the selection of the signal integration limits, two different approaches were adopted. In the

Fig. 3. NTA-OSL signal measured at 180 °C, along with sequential TA-OSL measurements which were performed to the same aliquot, at increasing temperatures within the range between 200 and 280 °C in step of 20 °C (plot A). Note the absence of any initial and quickly decaying part along with the presence of a very slowly decaying signal, whose shape is extremely flat with very large intensity. Plot B presents natural TA-OSL curves received at 180 °C from 10 different aliquots of the same sample with laboratory code name YKT2A-02. The obvious lack of reproducibility becomes apparent.

framework of each approach, a correlation was exploited with the integrated NTL signal; once for the case of the integrated natural TA-OSL throughout the entire stimulation duration (500 s), as well as once for the case of the initial signal, corresponding to the initial first second of stimulation. For this correlation, a total number of 25 aliquots were used, including five aliquots from each sample. Fig. 4 presents the results for both cases. As this figure is going to further reveal, the relation of the integrated NTL and NTA-OSL signals is not plausible. In Fig. 4A, the straight line corresponds to an approach to correlate these two luminescence entities linearly; nevertheless, both Fig. 4A as well as the linearity coefficient of 0.673, both indicate the failure of such a correlation. Once again, this lack of correlation could be attributed to the fact that the NTA-OSL signal comprises of at least two different contributing components. On the contrary, Fig. 4B presents the initial NTA-OSL intensity versus the integrated NTA-OSL signal. Linearity coefficient is 0.956, indicating a sufficient linear relation between the two signals. According to these results, hereafter quantification of the NTA-OSL signal takes place through the initial second of stimulation. Having found the appropriate way to quantify the NTA-OSL signal without applying the de-convolution procedure, a series of measurements were performed, in which prior to the NTL

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first-order kinetics assumption for the case of quartz. The first-order kinetics assumption is based on a direct extrapolation from the experience gained from the TL studies of the glow-curve of quartz, where no higher order kinetics glow-peaks exist. Fig. 5B presents the initial NTA-OSL intensity versus the contribution of the medium component to the Blue OSL signal, in terms of integrated intensity over the entire duration of stimulation. There is a clear indication towards a straightforward relation between the initial NTA-OSL intensity and the latter contribution, providing thus a strong indication regarding the possibility of using the same luminescence centre for both luminescence entities.

(B)

(A)

3.4. Thermal assistance for NTA-OSL – Arrhenius plots

Fig. 4. Integrated NTA-OSL signal (plot A) as well as initial NTA-OSL signal (plot B) measured at 180 °C plotted versus integrated NTL signal. In the latter case, a linear relation between the two signal entities is monitored.

measurement of step 1, a natural OSL (NOSL) measurement was performed at 110 °C in the continuous wave mode (CW-OSL), subsequent preheat at 260 °C for 10 s. For this experiment, a total number of 20 aliquots were used, including four aliquots from each sample. The insertion of the NOSL measurement to the protocol does not change the shape of the TA-OSL. However, the following, very interesting results were yielded: in the case where the NOSL signal is very fast, as in the case of the Fig. 5A, the intensity of the NTA-OSL is very low. Moreover, all NOSL curves were de-convolved by using the general order kinetics (GOK hereafter) expression for OSL theory [23,25,26]:

  1 t b1 I ¼ I0  1 þ ðb  1Þ  ; b–1

s

ð1Þ

where I(t) is the intensity of the luminescence signal as a function of time, s = 1/k (s) is the lifetime and b is the order of kinetics. For the case of general order kinetics the value of kinetic order b was left to vary freely in the range between 1.00001 and 2. In practise, a sum of three components was applied in all cases, with C1 being the fast component, C2 being the medium component and C3 being the slow OSL component, while in all cases, the b parameter for the order of kinetic yielded values between 1.03 and 1.07, verifying the

Fig. 6A shows the behavior of the initial naturally occurring TAOSL as a function of the stimulation temperature for the sample with code name YKT2A-02. It is rather prominent that the NTAOSL signals in Fig. 6A increase continuously up to the stimulation temperature of 180 °C, while for higher stimulation temperatures a decrease is monitored for the signal intensity. Similar behaviors were also yielded for all other four quartz samples. The natural logarithm of the NTA-OSL values is drawn against 1/kT in Fig. 6C for the same quartz sample, where T represents the (absolute) stimulation temperature, and the slope of this graph represents the thermal activation energy E. Put simply, Fig. 6C presents the respective Arrhenius plot, which is linear with a slope corresponding to activation energy of 0.28 (±0.03) eV. However, (a) only four points are involved in the fitting procedure of Fig. 6C, while (b) the above analysis does not take into account the possible presence of thermal quenching in the quartz samples. It is well known that luminescence signals from quartz exhibit a reduced efficiency as the stimulation temperature is getting increased, due to the phenomenon of thermal quenching. Even though the presence of thermal quenching effects is commonly assumed during TL/OSL studies, the prevalence of this phenomenon for all types of quartz samples has recently been discussed by Subedi et al. [27]. Nevertheless, the influence of the thermal quenching effect on the (artificially irradiated) TA-OSL signal in quartz was indicated by Kitis et al. [17]. Therefore, in the present analysis of the data collected for all five samples, the TA-OSL signals were corrected for thermal quenching. The correction was attempted by using the typical values of the thermal quenching parameters W = 0.67 eV and C = (3.7)107 which were suggested by [27] and stand in very good agreement with those given by Wintle [28]. Initially, the values of the thermal quenching efficiency g(T) are evaluated for each one of the stimulation temperatures T using the well known expression for the luminescence efficiency [29,30]:

gðTÞ ¼

Fig. 5. NOSL decay curve (right-hand-side plot), de-convolved into 3 individual components. Inset presents the NTA-OSL signal corresponding to a quartz sample with NOSL dominated by the fast component. Left-hand-side plot presents the NTAOSL initial intensity versus the contribution of the fast OSL component C1 to the entire NOSL signal in terms of integrated signal.

1 ; 1 þ C expð W=kTÞ

where k is the Boltzmann constant and T is the stimulation temperature. Next the NTA-OSL intensity at each temperature T is corrected for the effect of thermal quenching by dividing by the corresponding value of g(T). The corrected initial intensity values as well as the efficiency g(T) are plotted versus stimulation temperature in Fig. 6B. After correction, the intensity is monotonically increasing with stimulation temperature throughout the entire temperature region applied. The exponential behavior of the intensity in this figure indicates the effectiveness of the correction procedure, while at the same time allows the evaluation of the un-biased activation energy E of this process for the samples studied. Finally Arrhenius plots of the logarithm of the corrected NTA-OSL values against 1/kT are drawn, with the slopes of these graphs representing the thermal activation energy E for this process. The new results are shown in Fig. 6D for the same, typical quartz sample, while the new

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(A)

(B)

(C)

(D)

Fig. 6. The dependence of the initial NTA-OSL intensity on the stimulation temperature without correction for thermal quenching (plot A) as well as after correcting for thermal quenching (plot B). In the latter plot the thermal quenching efficiency g(T) is also presented. Plots C and D present the corresponding Arrhenius plots before and after correction for thermal quenching respectively. Corresponding slopes indicate the thermal activation energies for the thermally assisted phenomena.

value for activation energy assuming thermal quenching effects was found to be E = 0.47 (±0.06) eV. Table 1 presents in a tabular form the activation energies estimated with correction for thermal quenching, indicating values within the range of 0.45 and 0.53 eV. Values in parentheses indicate the corresponding error values. A similar analysis was also performed for the case of the flat, slowly decaying NTA-OSL signal component presented in Fig. 3A. The temperature range was within 180–280 °C while the integration limits this time were restricted to the final 50 s of stimulation, in order to also include the flat component signal measured at 180 °C. The signals were also corrected for thermal quenching, according to the above described procedure. Fig. 7A presents the corrected luminescence intensity values as well as the efficiency g(T) are plotted versus stimulation temperature, while Fig. 7B draws the corresponding Arrhenius plots of the logarithm of the corrected NTA-OSL values against 1/kT. A perfect linearity is yielded for this specific plot, indicating a slope of 0.95 (±0.07) eV. Similar results were indicated for the cases of all four other quartz samples, indicating values for the activation energies ranging between 0.95 and 1.05 eV. This latter value stands in very good agreement with the corresponding values of 0.93 and 1.02 eV reported by [17]. Once again, Table 1 presents in a tabular form the activation energies estimated after correction for thermal quenching, with values in parentheses indicating the corresponding error values. In summary, if the data are analyzed neglecting the effect of thermal quenching, the NTA-OSL signals from VDT can be described by a thermally assisted process with a mean activation energy of 0.3 (±0.04) eV. However, if the data are corrected for thermal quenching effect, the same signals are described by a thermally assisted process with a higher activation energy within the range between 0.45 and 0.53 eV, indicating a mean value of 0.49 (±0.03) eV. Moreover, a second thermally assisted NTA-OSL component was yielded, indicating NTA-OSL signals assisted by an activation process with energies much larger, being of the order of 1 (±0.04) eV. The presence of this, second NTA-OSL component, indicates the high need for a de-convolution analysis on the NTAOSL signal in quartz.

(A)

(B)

Fig. 7. Similar to Fig. 6 but for the flat, very slowly decaying NTA-OSL signal.

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3.5. Sensitization Even though the current study is mostly stressing on investigating the naturally occurring luminescence signal arising from VDT, however, sensitization was studied to artificially irradiated signals. Three different test doses were applied, namely 45, 90 and 135 Gy. Monitoring sensitivity changes of the NTA-OSL signal is a very useful test, especially when a single aliquot dating protocol is to be established. The NTA-OSL sensitivity was studied for ten successive irradiation–TL–NTA-OSL readout cycles for all quartz samples of the present study. It was found that the initial sensitivity remains stable within less than 1.5% for the three quartz samples. This result indicates the remarkable stability of the NTA-OSL signal from quartz, which was also monitored for the flat, slowly decaying signal component for prolonged stimulation duration. Unfortunately, this stability is not of prevalent nature, because it is only monitored for three samples. For the other two, intense sensitization was monitored for the initial NTA-OSL signal. For these two latter samples however, no sensitization is monitored for the flat, slowly decaying signal component for prolonged stimulation duration. The results were yielded regardless of the test dose applied. 4. Conclusions In the present study 5 different natural quartz samples quartz samples were studied in order to investigate both the existence of naturally occurring TA-OSL signal as well as the prevalence of specific properties of this specific signal such as activation energy of the thermally assisting phenomenon. The conclusions were as follows: (a) Intense NTA-OSL signal is monitored for all five different quartz samples subjected to the present study. The shape of this signal resembles much to a typical CW-OSL decay curve, with its intensity being dependent on the stimulation temperature. At prolonged stimulation durations, the signal becomes flat with intensity almost one order of magnitude higher than ordinary OSL dark count background level. (b) For the NTA-OSL signals of all five different quartz samples, the optimum measuring temperature was indicated to be at 180 °C. (c) In all five cases, the NTA-OSL signal comprises of at least two different contributing components, with one corresponding to the intensity during the initial one second of stimulation, while the other corresponds to the flat, slowly decaying signal after 300 s of stimulation. This result is strongly supported by the two individual values of the thermal activation energies yielded by the corresponding Arrhenius plots. (d) The values yielded for each one between the two different thermal activation energies are of prevalent nature for the five different quartz samples subjected to the present study. (e) In terms of intensity, the lack of reproducibility for the naturally occurring TA-OSL signal could be possibly attributed to the wide range of the grain sizes of the samples subjected to the present study. (f) The initial intensity during the first second of stimulation could be directly correlated to both the NTL integrated signal and the contribution of the medium OSL components to the entire NOSL signals. (g) Sensitization of the TA-OSL signal after artificial irradiation does not show prevalent nature. Further work is required in order to study the bleaching and zeroing properties of this signal. Nevertheless, based on the results of the present study, establishing a robust protocol towards both

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