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

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Nuclear Instruments and Methods in Physics Research B

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

http://dx.doi.org/10.1016/j.nimb.2015.01.0790168-583X/� 2015 Elsevier B.V. All rights reserved.

⇑ Corresponding author.E-mail addresses: gspolymeris@ankara.edu.tr (G.S. Polymeris), sahiner@ankara.

edu.tr (E. S�ahiner), meric@ankara.edu.tr (N. Meriç), gkitis@physics.auth.gr (G. Kitis).

George S. Polymeris a,⇑, Eren S�ahiner a, Niyazi Meriç a, George Kitis b

a Institute of Nuclear Sciences, Ankara University, Bes�evler, 06100 Ankara, Turkeyb 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 a b s t r a c t

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

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

The access to the OSL signals from very deep traps is achieved by an alternative experimental methodwhich comprises combined action of thermal and optical stimulation, termed as thermally assistedOSL (TA-OSL). This experimental technique was suggested in order to not only measure the signal ofthe deep traps without heating the sample to temperatures greater than 500 �C, but also use the formerfor dosimetry purposes as well, due to exhibiting a number of interesting properties which could be effec-tively used towards dosimetry purposes, especially for large accumulated artificial doses. The presentstudy provides for the first time in the literature with preliminary results towards the feasibility studyof the naturally occurring TA-OSL signal in coarse grains of natural quartz towards its effective applica-tion to geological dating. The samples subjected to the present study were collected from fault lines inKütahya-Simav, Western Anatolia Region, Turkey; independent luminescence approaches yielded anequivalent dose larger than 100 Gy. Several experimental luminescence features were studied, such assensitivity, 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 ofthe TA-OSL signal. The high intensity of the OSL signal confirms the existence of a transfer phenomenonfrom deep electron traps. The increase of the integrated TA-OSL signal as a function of temperature ismonitored for temperatures up to 180 �C, indicating the later as the most effective stimulationtemperature. At all temperatures of the studied temperature range between 75 and 260 �C, the shapeof the signal resembles much the shape of a typical CW-OSL curve. However, a long-lived, residualNTA-OSL component was monitored after the primary, initial NTA-OSL measured at 180 �C; the intensityof this component increases with increasing stimulation temperature. The prevalence of these lumines-cent features was investigated, while the implications on dating applications of these features were alsodiscussed.

� 2015 Elsevier B.V. All rights reserved.

1. Introduction

Luminescence stands among the basic research tools in thefields of (a) ionizing radiation dosimetry, (b) archeological dating,geochronology and retrospective dosimetry and (c) authenticitytesting of archeological artifacts [1,2]. Both thermoluminescence(TL) and optically stimulated luminescence (OSL) are passive dosi-metric methods in the sense that the energy of ionizing radiation isstored in form of electron (and holes) trapped at electron (andhole) trapping levels having long lifetimes. Under thermal andoptical stimulation the electrons are released from their traps

and recombine with holes at luminescence centers giving rise toTL and OSL signals.

In general luminescence dating and retrospective dosimetry isbased on the fact that naturally-occurring minerals like quartzand feldspars act as natural dosimeters and preserve a record ofirradiation dose, i.e., energy per unit mass, received through timemainly from the decay of natural radionuclides, i.e., 232Th, 40K,87Rb and natural U, along with cosmic rays [3,4]. The luminescencedating technique was firmly established in the 1970s and hasundergone rapid development and enhancement in the 1980s,1990s and 2000s, while the ability to image individual grains inconjunction with developments in measurement procedures havemade important contributions to the field. Measurementprocedures have been developed in which the equivalent dose isobtained on single aliquots for quartz, feldspars as well aspolymineral 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 25

procedures, the natural OSL signal is compared with the OSLsignals resulting from doses being given to the same aliquot. Inparticular, a single-aliquot regenerative-dose (SAR) protocol wasdeveloped [7] in which correction for sensitivity change duringthe measurement sequence takes place.

Currently, the electron trapping levels used for the applicationsof TL and OSL are exclusively the levels that can be thermally excit-ed at temperatures well less than 500 �C. By stimulating thesetrapping levels, either by TL or OSL, the upper ages estimated byTL are limited to 50–100 kyr whereas the age limits by OSL are lessthan 1 Myr. However, there is a high need, especially in the geo-chronology community, for further improving the above age limitsfor covering at least the last four million years.

Most of TL and OSL phosphors, as wide band gap materials, alsohold some deep energy level defects. These are called as very deeptraps (VDT) hereafter. As VDT are considered those traps, whichcorrespond to TL glow peak having their peak maximum tem-perature, Tmax beyond the 500 �C. These deep electron traps have,at least, all the benefits of the shallower traps, along with an addi-tional un-comparable advantage, being the very long lifetimesexpected for these trapping levels. This latter, basic characteristicof any trapping level stands among the cornerstones for all TLand OSL application. However, its value is even more importantin the cases of archeological and geological dating, because presetsthe age evaluation limits. Studying the luminescence resultingfrom charge release from these deep traps is difficult because ofboth thermal quenching, in conjunction to instrumental limita-tions [8]. In this case, new techniques are required in order toaccess the signal from VDT, which so far in the literature, wasmostly indirectly monitored. The access to the luminescence signalfrom VDT is achieved by either photo-transferred TL [9,10] or indi-rectly [11,8 and references there in]. Therefore, alternativeexperimental methods, including a combined action of thermaland optical stimulation such as the thermally assisted OSL (TA-OSL) [12,13] were suggested in order to not only measure the sig-nal of the deep traps without heating the sample to temperaturesgreater than 500 �C, but also use the former for dosimetry purposesas well.

The presence of VDT has been experimentally verified in manycases of luminescent materials, such as CaF2:N [16], Al2O3:C [8,12–15], sedimentary quartz [17] and very recently apatites [18,19] aswell as to some materials consisting the ground layers of wood andcanvas 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 crystalsexhibit a number of interesting properties which could beeffectively used towards dosimetry purposes, especially forlarge accumulated doses. Among these properties, the mostnotable are the straightforward relation observed betweenthe TA-OSL integrated intensity and the dose, along withthe simple TA-OSL curve shape. From a theoretical point ofview the very long lifetimes expected for these traps provideone of the main pre-requirements towards the extension ofthe age limits. Therefore, the role of the VDT in dating veryold samples could be very important as well as significant.Nevertheless, especially in the case of previously heatedquartz, it was recently argued [17] that the lower detectabledose limit of the VDT is of the order of 1 Gy but it could befurther improved.

(b) While apatites, and especially Durango apatite [18,22,23] arenatural materials which are known to exhibit strong anoma-lous fading effects in the corresponding TL and conventionalOSL signals, recent works indicated that the TA-OSL signalafter artificial irradiation is much more stable compared tothe other two aforementioned luminescence entities [18,19].

(c) VDT are populated by a combined action of annealing attemperatures of the order of 500–1185 �C [8,12–20] in orderto empty all traps from residual electrons, followed by a(post annealing) irradiation with a large dose. In all theaforementioned studies, the occupancy of very deep trapswas controlled by selectively either filling or emptying themafter using different types of artificial irradiations and/or dif-ferent irradiation temperatures. Nevertheless, studying thenaturally occurring signal arising from VDT has not beenpreviously reported in the literature.

The present work provides for the first time in the literaturewith preliminary results towards the feasibility study of thenaturally occurring TA-OSL (or NTA-OSL) signal in coarse grainsof natural quartz towards its effective application to geological dat-ing. The motivation for this study arises from the cases of calciumcarbonate as well as obsidian samples [21], for which the lack ofnatural, fast decaying OSL signal excludes the application of typicalSAR OSL protocols, despite the intense OSL signal after artificialirradiation. This study will mostly stress on investigating the dosi-metric potentiality of the naturally occurring luminescence signalarising from VDT; however, specific properties such as sensitiza-tion and repeatability were studied to both naturally occurring aswell as artificial signals. Towards this primary aim, secondary aimsof the present work include (a) establishing a robust protocoltowards both measuring as well as taking full advantage of thisspecific signal and (b) studying the prevalence of specific proper-ties of this specific signal such as activation energy of the thermallyassisting 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-Si-mav, the Aegean Anatolia region, Turkey. The term ‘‘geological’’ isused in order to indicate quartz samples with large equivalent doseaccumulated, resulting in very much intense naturally occurringluminescence signal. Five different quartz samples, each oneobtained from a different fault line, were studied. For these casesthe zeroing mechanism is not the light bleaching but the heatinduced because of friction. In an independent dating approachfor each fault, the equivalent dose for each sample was estimatedin the range between 175 and 250 Gy. The laboratory code namesas well as the independently estimated equivalent dose for eachone are presented in Table 1.

Aliquots-discs made out of aluminum substrate 0.5 mm thickand 9 mm in diameter, were prepared. The preparation of sampleswas formed under dim red light conditions. After sieving, grains ofdimensions 90–180 lm were obtained. These grains were treatedwith HCI (10%), H2O2 (10%), HF (40%) and a final treatment withHCI (10%) in order to obtain a clean quartz extract. Aliquots withmass of 5 mg each were prepared by mounting the material onstainless-steel disks. Mass reproducibility was checked to be with-in ±5%. All aliquots were checked with infrared (IR) stimulation(880 nm) at ambient temperature to ensure the absence offeldspars.

2.2. Apparatus and measurement conditions

All luminescence measurements were carried out using a RisøTL/OSL reader (model TL/OSL-DA-20), equipped with a 90Sr/90Ybeta particle source, delivering a nominal dose rate of0.130 ± 0.004 Gy/s. A 9635QA photomultiplier tube was used for

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

Samplecodename

ED (Gy) Optimum NTA-OSLmeasurementT (�C)

Eact1

(eV)*

Eact2

(eV)*

Sensitization

YKT2A-02 239 ± 6 180 0.47(0.06)

0.95(0.07)

No

YKT2A-03 233 ± 5 180 0.45(0.05)

0.97(0.06)

Yes

YKT2A-08 313 ± 15 180 0.53(0.07)

1.03(0.07)

No

YKT1A-02 164 ± 7 180–200 0.51(0.06)

0.98(0.06)

No

YKT1A-03 100 ± 2 180 0.48(0.06)

1.05(0.07)

Yes

* Corrected for thermal quenching.

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

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

2.3. Experimental protocols

The general experimental protocol that was applied in theframework of the present study aimed at the identification of theoptimum 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 readeris configured so that the lift will not rise until the hotplatetemperature is 60 �C or less, so for temperatures below 60 �C, thetemperature control is lost, as the system cannot cool directlythe hotplate. Therefore, extra time is required so that the

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

temperature will be further decreased; (b) to check whether thePM tube suffers from overflowing. The quartz samples of thepresent study yield very large equivalent doses. Thus the corre-sponding NTL signal of step 1 is very intense, as Fig. 1 reveals. Thisvery intense signal could result in enhanced dark countsbackground signal due to overflowing PM Tube. The inset ofFig. 1 presents a typical measurement according to step 2. Fortu-nately, besides the extremely intense NTL signal, no overflowingproblems were monitored.

Towards the selection of the optimum stimulation temperaturefor the TA-OSL, the Ti ranged from room temperature (RT through-out) to 280 �C. After establishing the optimum temperature forstimulation, all following measurements were performed at thistemperature. In some cases, prior to the NTL measurement of step1, a NOSL measurement was performed at 110 �C subsequent pre-heat at 260 �C for 10 s, in the continuous wave mode (CW-OSL).These OSL measurements were performed in order to exploit thecorrelation 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 NTA-OSL signal in quartz

OSL measurements performed at room temperature could notstimulate VDT. Nevertheless, this measurement could be used asdark count background signal. In order to define the optimumstimulation temperature for NTA-OSL, the optical stimulationwas performed at increasing OSL measuring temperatures in orderto study the thermal behavior of the source traps. The experimen-tal protocol applied, includes the four steps described at Section 2.3,for varying temperatures ranging between RT and 280 �C. Theupper stimulation temperature was selected to be slightly lowerthan the Tmax of the 325 �C glow peak of quartz. Each cycle of steps1–4 was performed for each different TA-OSL measurement tem-perature to two different fresh aliquots of quartz crystal.

TA-OSL curves for the natural signal, measured at various tem-peratures ranging between 75 and 260 �C are presented in Fig. 2.This specific figure includes the plots corresponding to one, uniquequartz sample with code name YKT02–02; however the resultswere similar to all other cases of quartz samples. The signal is ofextremely low intensity in the case of low stimulation tem-peratures of RT and 50 �C; this is the reason why these OSL curvesare not plotted in Fig. 2. However, as the temperature is slightly

Fig. 2. Natural TA-OSL curves received at various temperatures ranging between75 �C up to 260 �C.

(A)

(B)

Fig. 3. NTA-OSL signal measured at 180 �C, along with sequential TA-OSL mea-surements which were performed to the same aliquot, at increasing temperatureswithin the range between 200 and 280 �C in step of 20 �C (plot A). Note the absenceof any initial and quickly decaying part along with the presence of a very slowlydecaying signal, whose shape is extremely flat with very large intensity. Plot Bpresents natural TA-OSL curves received at 180 �C from 10 different aliquots of thesame sample with laboratory code name YKT2A-02. The obvious lack of repro-ducibility becomes apparent.

G.S. Polymeris et al. / Nuclear Instruments and Methods in Physics Research B 349 (2015) 24–30 27

increasing, measureable signal could be detected, even at lowstimulating temperatures as 75 �C. Another interesting featurearises from the strong increase of the integrated TA-OSL intensity,as a function of the stimulation temperature, up to stimulationtemperatures of 180–200 �C. For higher stimulation temperatures,a decrease of the TA-OSL intensity is monitored. Eventually, themost intense TA-OSL signal, in both terms of initial as well as inte-grated intensity, is monitored for the stimulation temperature of180 �C. For stimulation temperatures within the range between50 �C, which is the first temperature indicating measurable naturalTA-OSL signal, and 180 �C, the initial intensity is increased by atleast one order of magnitude. Similar results were yielded for allquartz samples subjected to the present study, establishing thusthe temperature of 180 �C as the optimum measurement tem-perature for natural TA-OSL, based on its enhanced sensitivity. Itshould be emphasized that RTL signal resembled much the typicalbackground signal level for all of the stimulation temperatureswithin the aforementioned range. Hereafter, unless otherwise stat-ed, all other TA-OSL measurements were performed at 180 �C.

3.2. Shape of natural TA-OSL curves

The natural TA-OSL signal in quartz yields a typical decayingOSL curve shape, similar with that reported for the cases of artifi-cially irradiated quartz [17] as well as apatites [18,19], beingdependent on the temperature of the measurement. On the con-trary, this shape is different compared to the corresponding shapeof the (artificially irradiated) TA-OSL signal reported for the casesof either CaF2:N [16] or Al2O3:C [8,12], which resembles muchthe peak shaped linearly modulated OSL (LM-OSL). After 300 s ofstimulation at the optimum temperature of 180 �C, as Fig. 3Areveals, the signal becomes flat with intensity almost one orderof magnitude higher than ordinary OSL dark count backgroundlevel of �60 counts/s. In order to study further the origin of thisspecific flat signal, after the typical TA-OSL measurement at180 �C, sequential NTA-OSL measurements were performed tothe same aliquot, at increasing temperatures. These curves wereobtained and are presented in Fig. 3A, for the temperature rangebetween 200 and 280 �C. All curves present similar features, suchas the absence of any initial and quickly decaying part; instead, avery slowly decaying signal, whose shape is extremely flat withvery large intensity. It is worth emphasizing that with increasingstimulation temperature the intensity of this flat NTA-OSL compo-nent is also increased, reaching the level of 2000 counts/s forstimulation at 280 �C, further supporting thus the fact that this sig-nal corresponds to a slowly decaying TA-OSL component.

3.3. Quantifying NTA-OSL signal – correlation to conventionalluminescence signals

Fig. 3B presents 10 curves of natural TA-OSL obtained at 180 �Cfrom 10 different aliquots of the same mass for the sample withcode name YKT2A-02, in order to check the reproducibility of thesignal. As it becomes apparent, the naturally occurring TA-OSLsignal is not reproducible in terms of intensity. This lack of repro-ducibility could be possibly attributed to the wide range of thegrain sizes of the samples subjected to the present study. At thesame time, irreproducibility is also monitored in the case of NTLsignals among the different aliquots, strongly supporting thus thesensitive relation of both NTL as well as NTA-OSL intensities onthe grain size. Similar lack of reproducibility was also monitoredafter artificial irradiation as well.

Due to this aforementioned lack of reproducibility, theappropriate quantification of the natural TA-OSL signal becomesextremely important. Towards the selection of the signalintegration limits, two different approaches were adopted. In the

framework of each approach, a correlation was exploited withthe integrated NTL signal; once for the case of the integrated nat-ural TA-OSL throughout the entire stimulation duration (500 s),as well as once for the case of the initial signal, corresponding tothe initial first second of stimulation. For this correlation, a totalnumber of 25 aliquots were used, including five aliquots from eachsample. Fig. 4 presents the results for both cases. As this figure isgoing to further reveal, the relation of the integrated NTL andNTA-OSL signals is not plausible. In Fig. 4A, the straight line corre-sponds to an approach to correlate these two luminescence entitieslinearly; nevertheless, both Fig. 4A as well as the linearity coeffi-cient of 0.673, both indicate the failure of such a correlation. Onceagain, this lack of correlation could be attributed to the fact thatthe NTA-OSL signal comprises of at least two different contributingcomponents. On the contrary, Fig. 4B presents the initial NTA-OSLintensity versus the integrated NTA-OSL signal. Linearity coeffi-cient is 0.956, indicating a sufficient linear relation between thetwo signals. According to these results, hereafter quantification ofthe NTA-OSL signal takes place through the initial second ofstimulation.

Having found the appropriate way to quantify the NTA-OSLsignal without applying the de-convolution procedure, a series ofmeasurements were performed, in which prior to the NTL

(A) (B)

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 linearrelation between the two signal entities is monitored.

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

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

I ¼ I0 � 1þ ðb� 1Þ � ts

� �� 1b�1

; b–1 ð1Þ

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

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

first-order kinetics assumption for the case of quartz. The first-orderkinetics assumption is based on a direct extrapolation from theexperience gained from the TL studies of the glow-curve of quartz,where no higher order kinetics glow-peaks exist. Fig. 5B presentsthe initial NTA-OSL intensity versus the contribution of the mediumcomponent to the Blue OSL signal, in terms of integrated intensityover the entire duration of stimulation. There is a clear indicationtowards a straightforward relation between the initial NTA-OSLintensity and the latter contribution, providing thus a strongindication regarding the possibility of using the same luminescencecentre for both luminescence entities.

3.4. Thermal assistance for NTA-OSL – Arrhenius plots

Fig. 6A shows the behavior of the initial naturally occurring TA-OSL as a function of the stimulation temperature for the samplewith code name YKT2A-02. It is rather prominent that the NTA-OSL signals in Fig. 6A increase continuously up to the stimulationtemperature of 180 �C, while for higher stimulation temperaturesa decrease is monitored for the signal intensity. Similar behaviorswere also yielded for all other four quartz samples. The naturallogarithm of the NTA-OSL values is drawn against 1/kT in Fig. 6Cfor the same quartz sample, where T represents the (absolute)stimulation temperature, and the slope of this graph representsthe thermal activation energy E. Put simply, Fig. 6C presents therespective Arrhenius plot, which is linear with a slope correspond-ing to activation energy of 0.28 (±0.03) eV. However, (a) only fourpoints are involved in the fitting procedure of Fig. 6C, while (b) theabove analysis does not take into account the possible presence ofthermal quenching in the quartz samples. It is well known thatluminescence signals from quartz exhibit a reduced efficiency asthe stimulation temperature is getting increased, due to the phe-nomenon of thermal quenching. Even though the presence of ther-mal quenching effects is commonly assumed during TL/OSLstudies, the prevalence of this phenomenon for all types of quartzsamples has recently been discussed by Subedi et al. [27].Nevertheless, the influence of the thermal quenching effect onthe (artificially irradiated) TA-OSL signal in quartz was indicatedby Kitis et al. [17]. Therefore, in the present analysis of the data col-lected for all five samples, the TA-OSL signals were corrected forthermal quenching. The correction was attempted by using thetypical values of the thermal quenching parameters W = 0.67 eVand C = (3.7)�107 which were suggested by [27] and stand in verygood agreement with those given by Wintle [28]. Initially, the val-ues of the thermal quenching efficiency g(T) are evaluated for eachone of the stimulation temperatures T using the well knownexpression for the luminescence efficiency [29,30]:

gðTÞ ¼ 11þ C expð�W=kTÞ ;

where k is the Boltzmann constant and T is the stimulationtemperature. Next the NTA-OSL intensity at each temperature T iscorrected for the effect of thermal quenching by dividing by the cor-responding value of g(T). The corrected initial intensity values aswell as the efficiency g(T) are plotted versus stimulation tem-perature in Fig. 6B. After correction, the intensity is monotonicallyincreasing with stimulation temperature throughout the entiretemperature region applied. The exponential behavior of the inten-sity in this figure indicates the effectiveness of the correction proce-dure, while at the same time allows the evaluation of the un-biasedactivation energy E of this process for the samples studied. FinallyArrhenius plots of the logarithm of the corrected NTA-OSL valuesagainst 1/kT are drawn, with the slopes of these graphs representingthe thermal activation energy E for this process. The new results areshown in Fig. 6D for the same, typical quartz sample, while the new

(A)

(B)

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

(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 forthermal 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 aftercorrection for thermal quenching respectively. Corresponding slopes indicate the thermal activation energies for the thermally assisted phenomena.

G.S. Polymeris et al. / Nuclear Instruments and Methods in Physics Research B 349 (2015) 24–30 29

value for activation energy assuming thermal quenching effects wasfound to be E = 0.47 (±0.06) eV. Table 1 presents in a tabular formthe activation energies estimated with correction for thermalquenching, 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 integra-tion limits this time were restricted to the final 50 s of stimulation,in order to also include the flat component signal measured at180 �C. The signals were also corrected for thermal quenching,according to the above described procedure. Fig. 7A presents thecorrected luminescence intensity values as well as the efficiencyg(T) are plotted versus stimulation temperature, while Fig. 7Bdraws the corresponding Arrhenius plots of the logarithm of thecorrected NTA-OSL values against 1/kT. A perfect linearity is yield-ed for this specific plot, indicating a slope of 0.95 (±0.07) eV. Simi-lar results were indicated for the cases of all four other quartzsamples, indicating values for the activation energies rangingbetween 0.95 and 1.05 eV. This latter value stands in very goodagreement with the corresponding values of 0.93 and 1.02 eVreported by [17]. Once again, Table 1 presents in a tabular formthe activation energies estimated after correction for thermalquenching, with values in parentheses indicating the correspond-ing error values.

In summary, if the data are analyzed neglecting the effect ofthermal quenching, the NTA-OSL signals from VDT can bedescribed by a thermally assisted process with a mean activationenergy of 0.3 (±0.04) eV. However, if the data are corrected forthermal quenching effect, the same signals are described by a ther-mally assisted process with a higher activation energy within therange between 0.45 and 0.53 eV, indicating a mean value of 0.49(±0.03) eV. Moreover, a second thermally assisted NTA-OSLcomponent was yielded, indicating NTA-OSL signals assisted byan activation process with energies much larger, being of the orderof 1 (±0.04) eV. The presence of this, second NTA-OSL component,indicates the high need for a de-convolution analysis on the NTA-OSL signal in quartz.

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

3.5. Sensitization

Even though the current study is mostly stressing on investigat-ing 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 use-ful test, especially when a single aliquot dating protocol is to beestablished. The NTA-OSL sensitivity was studied for ten successiveirradiation–TL–NTA-OSL readout cycles for all quartz samples ofthe present study. It was found that the initial sensitivity remainsstable within less than 1.5% for the three quartz samples. Thisresult indicates the remarkable stability of the NTA-OSL signalfrom quartz, which was also monitored for the flat, slowly decay-ing signal component for prolonged stimulation duration. Unfortu-nately, this stability is not of prevalent nature, because it is onlymonitored for three samples. For the other two, intense sensitiza-tion was monitored for the initial NTA-OSL signal. For these twolatter samples however, no sensitization is monitored for the flat,slowly decaying signal component for prolonged stimulation dura-tion. The results were yielded regardless of the test dose applied.

4. Conclusions

In the present study 5 different natural quartz samples quartzsamples were studied in order to investigate both the existenceof naturally occurring TA-OSL signal as well as the prevalence ofspecific properties of this specific signal such as activation energyof the thermally assisting phenomenon. The conclusions were asfollows:

(a) Intense NTA-OSL signal is monitored for all five differentquartz samples subjected to the present study. The shapeof this signal resembles much to a typical CW-OSL decaycurve, with its intensity being dependent on the stimulationtemperature. At prolonged stimulation durations, the signalbecomes flat with intensity almost one order of magnitudehigher 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 at180 �C.

(c) In all five cases, the NTA-OSL signal comprises of at least twodifferent contributing components, with one correspondingto the intensity during the initial one second of stimulation,while the other corresponds to the flat, slowly decayingsignal after 300 s of stimulation. This result is strongly sup-ported by the two individual values of the thermal activationenergies yielded by the corresponding Arrhenius plots.

(d) The values yielded for each one between the two differentthermal activation energies are of prevalent nature for thefive different quartz samples subjected to the present study.

(e) In terms of intensity, the lack of reproducibility for thenaturally occurring TA-OSL signal could be possiblyattributed to the wide range of the grain sizes of the samplessubjected to the present study.

(f) The initial intensity during the first second of stimulationcould be directly correlated to both the NTL integrated signaland the contribution of the medium OSL components to theentire NOSL signals.

(g) Sensitization of the TA-OSL signal after artificial irradiationdoes not show prevalent nature.

Further work is required in order to study the bleaching andzeroing properties of this signal. Nevertheless, based on the resultsof the present study, establishing a robust protocol towards both

measuring as well as taking full advantage of this specific signalfor geological dating purposes could be feasible, based on thedirect correlation of the initial NTA-OSL signal with both fast NOSLcomponent and NTL signal. The effective use of this specific lumi-nescence signals in some cases could be extremely innovative.

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