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Component resolved bleaching study in natural calcium fluoride using CW-OSL, LM-OSL and residual TL glow curves after bleaching
Author’s Accepted Manuscript Component resolved bleaching study in natural calcium fluoride using CW-OSL, LM-OSL and residual TL glow curves after bleaching Vasiliki Angeli, George S. Polymeris, Ioanna K. Sfampa, Nestor C. Tsirliganis, George Kitis www.elsevier.com/locate/apradiso
PII: DOI: Reference:
S0969-8043(16)30230-5 http://dx.doi.org/10.1016/j.apradiso.2017.01.014 ARI7736
To appear in: Applied Radiation and Isotopes Received date: 7 June 2016 Revised date: 5 January 2017 Accepted date: 17 January 2017 Cite this article as: Vasiliki Angeli, George S. Polymeris, Ioanna K. Sfampa, Nestor C. Tsirliganis and George Kitis, Component resolved bleaching study in natural calcium fluoride using CW-OSL, LM-OSL and residual TL glow curves after bleaching, Applied Radiation and Isotopes, http://dx.doi.org/10.1016/j.apradiso.2017.01.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Component resolved bleaching study in natural calcium fluoride using CW-OSL, LM-OSL and residual TL glow curves after bleaching Vasiliki Angelia, George S. Polymerisb*, Ioanna K. Sfampaa, Nestor C. Tsirliganisc, George Kitisa a
Aristotle University of Thessaloniki, Nuclear Physics Laboratory, 54124-Thessaloniki, Greece.
b
Insitute of Nuclear Sciences, Ankara University, TR-06100, Beşevler-Ankara, Turkey.
c
Laboratory of Radiation Applications and Archaeological Dating, Department of Archaeometry and Physicochemical Measurements, 'Athena' - Research and Innovation Center in Information, Communication and Knowledge Technologies, Kimmeria University Campus, 67100, Xanthi, Greece. *
Corresponding author. [email protected]
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[email protected],
Abstract Natural calcium fluoride has been commonly used as thermoluminescence (TL) dosimeter due to its high luminescence intensity. The aim of this work is to attempt a correlation between specific TL glow curves after bleaching and components of linearly modulated optically stimulated luminescence (LM-OSL) as well as continuous wave OSL (CW-OSL). A component resolved analysis was applied to the integrated intensity of the RTL glow curves and the OSL decay curves, by using a Computerized Glow-Curve De-convolution (CGCD) procedure. All the CW-OSL and LM-OSL components are correlated with the decay components of the integrated RTL signal, apart from two RTL components which cannot be directly correlated with either LM-OSL or CW-OSL component. The unique, stringent criterion for this correlation deals with the value of the decay constant λ of each bleaching component. There is only one, unique bleaching component present in all three luminescence entities which were the subject of the present study, indicating that each TL trap yields at least three different bleaching components, while different TL traps can indicate bleaching components with similar values. According to the data of this work each RTL bleaching component receives electrons from at least two peaks. The results of the present study suggest that the traps that contribute to TL and OSL are the same.
1. Introduction Calcium Fluoride, either natural (CaF2:N), or in synthetic form, doped with other rare earth elements such as Dy, Eu and Tm, is commonly used as TL dosimeter (Kharita et al., 1995; Polymeris et al., 2006; Topaksu et al., 2007; Tugay et al., 2009). Besides the 1
plethora of its dosimetric advantages, such as its high mechanical strength, its high luminescence intensity and its low cost, it is well known that the complex TL glow curve (Ogundare et al., 2004; Polymeris et al., 2006; Topaksu and Yazici, 2007), as well as its lack of tissue equivalence (Becker, 1973; McKeever et al., 1995) stand as its major shortcomings. Because of its high sensitivity to light (Furetta et al., 1985; Kitis et al., 1993; Chougaonkar et al., 2004; Polymeris et al., 2006;), CaF2:N could be also used effectively in optically stimulated luminescence dosimetry (OSLD). Bernhardt and Herforth have already suggested its use as an OSL dosimeter as early as 1974. Nevertheless, according to the authors’ best knowledge, bleaching has not been used to erase neither the natural signal nor the remnant signal after irradiation. During bleaching, an important proportion of the trapped electrons abandon the traps when the dosimeter is exposed to artificial or natural light. Bleaching can take place naturally or under strict experimental conditions. In the latter case, measuring the number of trapped electrons that are emitted following stimulation results in optically stimulated luminescence (OSL) curves. Consequently, it is assumed that bleaching and OSL have the same outcome to the trapped charge population. The only difference is that during the OSL, stimulation takes place using monochromatic light, while in bleaching the entire optical spectrum is effective. The bleaching properties of TL traps in various materials have been an attractive topic of research nowadays. Using OSL and residual TL glow curves, Spooner suggested that the optical dating signal in the case of quartz is related with an electron trap which is responsible for the TL glow peak at 325 °C (Spooner, 1994). Nevertheless, (Kitis et al., 2010) have reached the conclusion that this TL glow-curve is related to more than one electron traps. A basic result of the latter citation was that the LM-OSL signal for a stimulation time of 50s is a composite signal consisting of three components (fast, medium and slow) with different component lifetime values. By considering this, there is no specific relation of the fast component with a specific electron trap responsible for this TL glow curve. A similar approach has been also adopted by both (Jain et al., 2003) as well as (Polymeris et al., 2008) in their bleaching study of the TL signal in quartz using Infrared Stimulated luminescence (IRSL) at elevated temperatures. Similar conclusions were also reported by Dallas et al. (2010) for the case of KMgF3:Ce3+. These authors have extensively studied the bleaching of each, individual TL peak of the perovskite material, while arrived to the following conclusions: i) none of the individual glow-peak can be related to a single OSL component and ii) overlapping of the bleaching decay constants, having similar values although originating from different TL peaks. This experimental feature was anticipated for the case that the decay constants of two OSL components differ less than 15-20% (Kitis and Pagonis, 2008).
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The present study follows on directly from the aforementioned citation of Dallas et al. (2010), which ends by questioning whether similar bleaching analysis could be also performed to other bleachable TL materials. In the framework of this aim we attempt to continue Dallas’s work as far as the correlation between specific RTL glow curves and LM-OSL components in natural CaF2:N. This specific material has been selected due to its high sensitivity to light (Furetta et al., 1985; Kitis et al., 1993; Chougaonkar et al., 2004; Polymeris et al., 2006) and because of its complex TL glow curve (Balogun et al., 1999; Polymeris et al., 2006;). According to the authors’ best knowledge, still not thorough similar study has been attempted for CaF2:N. Moreover, this study will not limit just to LM-OSL and RTL curves, but will also include the component resolved analysis of the CW-OSL. The main outcome of this componentresolved analysis deals with the comparison of the decay constant λ values obtained from the analysis of integrated RTL, LM-OSL as well as CW-OSL curves. 2. Experimental a) Sample and equipment The study was performed on natural calcium fluoride, CaF2:N, which was obtained from a fluorite mine of Nigeria, Africa; it is the same sample that has been previously studied by Polymeris et al., 2006. The sample was gently crushed using an agate mortar and grains of dimensions between 80μm and 140μm were obtained after sieving. Aliquots with mass of 3mg each were prepared by mounting the material on stainless-steel disks of 1cm2 area (Polymeris et al., 2006). Mass reproducibility was checked to be within ±5%. All measurements were performed using one, unique aliquot (single aliquot measurements) containing material that had been freshly annealed at 500 °C; this was feasible due to the lack of sensitization (Polymeris et al., 2006). All luminescent measurements were performed using a RISØ TL/OSL reader (model TL/OSL-DA-15) equipped with a high-power blue LED light source, an infrared solid state laser and a 0.073Gy/s 90Sr/90Y β-ray source. The reader is fitted with an EMI 9635QA PM Tube. The detection optics consisted of a 7.5 mm Hoya U-340 filter (λ ~ 340 nm, FWHM ~ 80 nm). 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. All heatings and TL measurements were performed in a nitrogen atmosphere with a low constant heating rate of 1 oC/s, in order to avoid significant temperature lag (Kitis et al., 2015), up to the temperature of 500 oC. OSL measurements were performed in both continuous wave as well as linear mode configurations (CW-OSL and LM-OSL respectively). In the former case, the power level was software controlled and set at 90% of the maximum stimulation intensity for blue LEDs. For LM-OSL curves (Bulur, 1996), the ramping rate was 0.04 mW/cm2/s, increasing the stimulation light power from zero up to the maximum power (40 mW/cm2) over a period of 1000 s. Unless otherwise stated, all OSL measurements 3
were performed at room temperature, namely 25 oC. The test dose attributed was 1 Gy. b) Experimental Protocol The experimental protocol used to obtain the OSL signals and the residual TL after bleaching in different bleaching/stimulation durations consists of the following steps: Step 0: Test Dose and TL measurement. The purpose of this step is to obtain the unbleached, reference TL signal of natural calcium fluoride, which corresponds to bleaching/stimulation time ti=0s. This step has been measured only once. Step 1: Test Dose and CW-OSL measurement for stimulation time ti. The purpose of this step is to monitor the CW-OSL signal corresponding to time ti. Step 2: TL measurement. The purpose of this step is to monitor the residual TL (hereafter RTL) after bleaching/stimulation for ti. Steps 1-2 are performed using different stimulation times ti=2, 4, 8, 16, 32, 50, 75, 100, 200, 400, 600, 800, 1000, 1500 and 2000s. Step 3: Blue LM-OSL for 1000s to receive the LM-OSL signal. This step has been performed only once, after steps 1-2 corresponding to the longest bleaching/stimulation duration, being 2000s. 3. Results a) The shape of the RTL glow curves The residual TL glow curves of CaF2:N received after the CW-OSL for various stimulation times are shown in Fig. 1 for a selection of bleaching/stimulation durations. The difference between the TL glow curve of the non bleached sample and the RTL glow curves after bleaching for various times becomes very obvious. According to the data of this plot, it is possible to conclude that the shallow trap responsible for the low temperature TL peak (below 150 oC) is very easily and fast bleached within stimulation time intervals between 2s and 10s; similar results were also reported by Ferreira et al., 2014. On the other hand, the peak located around 300 o C survives for small stimulation/bleaching times. Nevertheless, for stimulation durations longer than 100s, this complex TL peak is also substantially bleached. Moreover, it is worth mentioning the shift of the peak maximum temperature of the main dosimetric complex peak, towards higher temperatures as the stimulation/bleaching time increases. This shift is attributed to the complex nature of the prominent main dosimetric peak of CaF2:N (Polymeris et al., 2006; Topaksu and Yazici, 2007). Finally, as this latter Fig. 1 is going to further reveal, there is no
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bleaching of the RTL glow curve part corresponding to temperatures higher than 350 o C. b) Method of analysis In a previous similar work of Dallas et al. (2010), each one of the RTL glow curves was de-convolved into its individual glow-peaks using a Computerized Glow-Curve De-convolution (CGCD) procedure (Kitis et al., 2000). In the framework of this analysis approach, a good resolution is required for all RTL glow curves; therefore, while measuring the TL glow curves in the case of Dallas et al. (2010), one data point was received for each temperature degree. Nevertheless, due to the complexity of the TL glow curve shape and structure of CaF2:N (Ogundare et al., 2004; Polymeris et al., 2006; Topaksu and Yazici, 2007), the de-convolution approach might be complex, time consuming and some erroneous. Therefore, in the framework of the present study, an alternative approach has been adopted. For each RTL glow curve, measured up to 500 oC, only 100 data points were deliberately obtained. Thus, each experimental data point of any RTL curve, corresponds to an intensity integration over a step temperature interval of ΔΤ=5 oC. By plotting this integrated intensity for each temperature interval Τi+ΔΤ= Τi+5 oC as a function of the stimulation/bleaching time, we get a total of 100 plots, a selection of which is presented in Fig. 2. Each one of these aforementioned 100 plots of integrated RTL intensities versus stimulation time, was analyzed into its individual components by using a Computerized GlowCurve De-convolution (CGCD) procedure (Kitis et all., 1999b) and using general order kinetics equation (Bøtter-Jensen et al., 2003; Kitis and Pagonis, 2008): ( )
(
[
)
]
(1)
where I(t) is the intensity of luminescence signal as a function of time, I0 is the initial luminescence intensity, τ =1/λ is the component lifetime and λ (1/s) stands as the respective decay constant and b is the order of kinetics which varies between the values 1.001 and 2.05. The same aforementioned equation was also applied for the deconvolution of the CW-OSL decay curves. As far as LM-OSL curves are concerned, these were also de-convolved into individual components according to (Kitis and Pagonis, 2008) and (Polymeris et al., 2006): ()
(
[
)
(
)
]
(2)
where Im and tm are the values of OSL intensity and time respectively and β stands for the parameter of the kinetic order. The background was simulated by an equation of the form (Polymeris et al., 2009):
5
(
) (3)
where zd is the zero dose OSL signal after blue stimulation and P the total stimulation time. The parameter zd is evaluated experimentally, by measuring the background OSL signal at non-irradiated samples. c is a constant with value which is expected to be very close to unity (Polymeris et al., 2009). Finally, the λ value of each component was estimated according to the equations suggested by Bulur (1996). All curve fittings were performed using the software package Microsoft Excel, with the Solver utility (Afouxenidis et al., 2012), while the goodness of fit was tested using the Figure Of Merit (FOM) of Balian and Eddy, (1977) given by: ∑[
] (4)
where Yexper is the experimental data, Yfit is the fitted data and A is the area of the fitted curve. The aim of the present exercise includes comparing and discussing the results concerning the λ values obtained by de-convolving these three luminescence entities, namely integrated RTL in steps of 5 °C, LM-OSL and CW-OSL. c) De-convolution results Fig. 2 presents the integrated RTL signal over a step temperature interval of ΔΤ=5 °C, for a selection of four different temperatures Τi+ΔΤ, namely 95-100 °C, 185-190 °C, 245-250 °C and 295-300 °C plotted as a function of the bleaching/stimulation time. In all cases, the plots were de-convolved using three different components. As it can be seen from Fig. 2, it was impossible to achieve good fit by using less than three components of general order kinetics. Following the terminology of Dallas et al. (2010), these components can be termed hereafter as fast (or component CRTL1), medium (or component CRTL2) and slow (or component CRTL3). The main outcome of this de-convolution analysis deals with the comparison of the decay constant λ values for each component obtained from the analysis of integrated RTL throughout the temperature region, LM-OSL as well as CW-OSL curves. The reason for selecting these four aforementioned temperatures for Fig. 2 is mainly twofold: (a) according to the de-convolution of the TL glow curve of CaF2:N (Polymeris et al., 2006; Topaksu and Yazici, 2007) five different TL glow peaks can be observed within the temperature range between room temperature (RT) and 350 oC of each RTL glow curve. The main glow-peak appears at high temperature, near 275 oC, with prominent peaks also at around 90 oC and 183 oC. Both the main glow-peak as well as the one at 90 oC, have one satellite peak each; nevertheless, the satellite peak of the 90 oC TL peak is considered as negligible, while the satellite of the main peak has a delocalising temperature of around 300 oC. All aforementioned temperatures refer to heating rate of 1 oC/s. Therefore, the aforementioned temperatures of 95 oC, 185 oC, 245 oC and 295 oC correspond to four main de-convolved TL glow peaks, (b) The analysis of all RTL intervals indicated a grouping on the values of the λ parameter 6
within three main different temperature ranges, which are located within 60 oC - 120 o C, 160 oC - 190 oC and 230 oC - 320 oC, namely in the same temperature area of the first, second and third plus fourth TL peaks respectively. It is worth emphasizing that there is no discrimination for the decay constants corresponding to the third and fourth TL peaks. Following the discussion above, all the bleaching components λ, originated from the de-convolution of the integrated RTL signal versus the bleaching/stimulation times are represented in Fig. 3, as a function of temperature. For each interval within the ΔΤi=5 °C, three different decay curves were obtained; these are presented as experimental data points with corresponding ±1σ error. As it becomes obvious by this latter plot, six out of the total of nine components λ are well correlated to each other, with the overlapping of bleaching components among the three temperature peak regions being obvious, at least for three cases, namely the fast components (CRTL1) of both first and second TL peaks, the medium components (CRTL2) of both second and third TL peaks as well as slow components (CRTL3) of first and second TL peaks. According to the discussion above, the average value for each decay constant λ1, λ2 and λ3 is presented in Table 1 for the case of components CRTL1 (fast), CRTL2 (medium) and CRTL3 (slow) respectively, as these were yielded by the de-convolution of the corresponding temperature ranges of the RTL glow curves. A basic observation deals with the overlapping monitored, as the decay constants of the fast components derived for the first two peaks will be seen as one component. The same also holds for the slow bleaching decay constants of the first and second TL peaks as well as for the decay constants of the fast component of the third peak with the medium component of the second peak. According to Kitis and Pagonis, (2008), such overlapping is anticipated for all cases where two different OSL/bleaching components indicate decay constants which differ by less than 15-20%. Nevertheless, the bleaching ability of the glow curves of any material is reflected to the rate at which the TL glow curve intensity is decreased after exposure to light. According to equation (1), the parameter b, defining the order of kinetics, stands as a fitting parameter. Knowledge of this parameter could be very helpful in understanding the bleaching mechanism. Fig. 4 presents the order of kinetic parameter b as a function of the temperature of the glow curve for all three components, namely fast, medium and slow. It becomes clear from this latter feature that throughout the entire glow curve temperature range, the fast bleaching component (CRTL1) is described by second order of kinetics, implying re-trapping. On the other hand, the slow bleaching component (CRTL3) is a fist-order-kinetic component, excluding thus the possibility of re-trapping. Finally it is worth mentioning that the medium bleaching component (CRTL2) yields intermediate order of kinetics, with b values ranging between 1.4 and 1.55. According to the data of the present study, each RTL bleaching component receives electrons from at least two peaks. Therefore it seems that for the fast
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component, there is re-trapping for the electrons of the two different traps contributing, while partly this is the case also for the medium component. Moreover, due to this overlapping monitored, one can safely conclude that the LMOSL curve will not be consisting of 9-12 components, but of much less. Indeed, a typical example of the LM-OSL curve shape is shown in Fig. 5. The LM-OSL signal for a stimulation time of 1000s is a composite signal consisting of four components as obtained from the computerized de-convolution analysis. As it can be seen from Fig. 5 the fourth component has not totally decayed for the stimulation time of 1000s (Kitis et al., 2010). This latter component could be possibly comprising of the sum of more, long living components. The decay constants of each LM-OSL component, hereafter termed as CLM-OSL1, CLM-OSL2, CLM-OSL3 and CLM-OSL4 are presented in Table 2. Nevertheless, the results of this component-resolved analysis of the LM-OSL curve stand in good agreement with all previous results concerning the de-convolution of the LM-OSL curves reported previously (Chougaonkar and Bhatt, 2004; Polymeris et al., 2006), in both terms of number of components as well as the values of decay constants. The de-convolution results of the LM-OSL curve yielded another interesting feature, regarding the order of kinetics value for the components; the fast component CLM-OSL1 is described by second order of kinetics, the component CLM-OSL2 by general order kinetics with b parameter around 1.46, while components CLM-OSL3 and CLM-OSL4 are described by first order of kinetics. This experimental result stands in good agreement with the corresponding results concerning the order of kinetics obtained from the analysis of the decay of the integrated RTL signal. According to the applied protocol, which has been presented previously, CW-OSL signals were also measured for various stimulation durations. In fact, four CW-OSL decay curves were analyzed to the corresponding individual components. Four different CW-OSL decay curves, corresponding to 800, 1000, 1500 and 2000 s of stimulation were selected, since these are the four highest stimulation durations applied in the framework of the present study. In all cases, three non-first order OSL components are required for best fitting. The corresponding decay constants are also presented in Table 2, while a CW-OSL de-convolution example for the case of 1000 s of stimulation is presented in Fig. 6. According to the standard deviation values presented in Table 2, reproducibility on the values of the decay constants for each of the three CW-OSL components through different stimulation time is excellent. It is worth mentioning that in all cases of the de-convolution results of the CW-OSL curves, the fast and medium components CCW-OSL1 and CCW-OSL2 are both described by second order of kinetics, while the slow component CCW-OSL3 is described by first order of kinetics. As far as the order-of-kinetics is concerned, these aforementioned results stand in good agreement with the results presented in Fig. 4 for fast and slow components of the integrated RTL signal, CRTL1 and CRTL3 respectively. For the case of the medium components CRTL2 and CCW-OSL2, re-trapping was indicated for both, despite the fact that different b value was yielded by the de-convolution analysis. 8
According to the discussion above the LM-OSL signal is represented by four components, while the CW-OSL signal by three components, hereafter termed as CCW-OSL1, CCW-OSL2, and CCW-OSL3. Table 3 shows the integral of each one of these components along with the percentage contribution of each component to the total corresponding signal. The contribution of components CCW-OSL3 and CLM-OSL3 is the same, being 16%. The same holds for component CCW-OSL2 and the CLM-OSL2, since the contribution of each one of them is 62% and 56% respectively. A difference appears for the components CLM-OSL1 and the CCW-OSL1. The contribution of the CCW-OSL1 to the total CW-OSL signal is 11%, while the contribution of the CLM-OSL1 has almost the double value, being around 28%. 4. Discussion Bleaching of the TL traps in various phosphors has been reported to be a controversial topic. According to the pioneer studies of Bailey et al. (1997), Bailey (2001) and Adamiec et al. (2006) and the corresponding models, TL and OSL traps in quartz are different. These models were reported to be able to reliably reproduce a wide variety of quartz luminescence phenomena. Nevertheless, subsequent studies of Dallas et al. (2008) in KMgFe3:Ce3+ and Kitis et al. (2010) in quartz, verified that both OSL and TL simulate the same traps. In the framework of the present study, the authors try to support the latter assumption of Dallas et al. (2008) and Kitis et al. (2010). Based on our results, similarly to the case of KMgFe3:Ce3+ (Dallas et al., 2010), the bleaching of each individual glow-peak takes place in three different rates; namely in a fast, medium and slow rate. Initially, each bleaching rate is expected to be related to a single OSL component with a specific decay constant in an OSL curve. Four different TL glow peaks could be easily identified in the glow curve of CaF2:N. Therefore, assuming the contribution of three components for the bleaching/OSL decay of each one of them, one can safely assume that a total of twelve components are required in order to describe the OSL signal measured at room temperature, since at room temperature all TL glow curves contribute to the OSL signal (Polymeris et al., 2006). However, since there is no discrimination for the decay constants corresponding to the third and fourth TL peaks, this number of twelve components is reduced to nine. Nevertheless, even these values for the decay constants are degenerated, due to the overlapping of many values that has been previously described. As a result, these nine different bleaching components are forming groups and overlap, so that five, distinguishable decay constant values are required in order to describe efficiently the decay of the integrated RTL signal of CaF2:N received after bleaching for various times. This number of components required is very close to the four components required to achieve good fitting for the case of the LM-OSL curve as well as to the 3 components required for the best fitting of the CW-OSL decay curve. By comparing the characteristic parameters of each component presented in Tables 1 and 2 it is safe to conclude the following: 9
There is only one, unique bleaching component present in all three luminescence entities of the present study. Specifically, components CLM-OSL1, CCW-OSL2, as well as medium component CRTL2 of TL peak at 90oC, indicate decay constant values very similar, namely , and s-1 respectively. There are various bleaching components present to at least two luminescence entities, such as (i) fast components CRTL1 of both first and second TL peaks along with component CCW-OSL1, (ii) medium components CRTL2 of both second and third (main) TL peaks along with component CLM-OSL2, (iii) component CCW-OSL3 along with component CLM-OSL3 and finally (iv) slow components CRTL3 of both second and third (main) TL peaks along with component CLM-OSL4. There are two RTL bleaching components which cannot be directly correlated to either LM-OSL or CW-OSL component, being components CRTL1 and CRTL3 of the main TL peak.
According to Kitis and Pagonis (2008), the fitting resolution for the case of LM-OSL is 15-20%; in a nutshell, it is difficult to resolve through computerized glow curve deconvolution LM-OSL components with decay constants which differ by less than 1520%. Based on this, these components will be seen as one component in an LM-OSL curve. According to the results of the present study, the fitting resolution for the case of CW-OSL is much worse. In fact, it becomes obvious that for CW-OSL curves, it is difficult to resolve through computerized glow curve de-convolution bleaching components with decay constants which differ by less than 50%. This really bad resolution could be attributed to the featureless of the decay curve shape of CW-OSL. Moreover, unlike the TL process, the traps are not emptied serially due to the optical stimulation. The same stands for the case of the integrated RTL signal versus bleaching, since both CW-OSL as well as integrated RTL signal versus bleaching\stimulation time are described by the same equation. Nevertheless, in the case of the latter plots versus bleaching time, another important drawback exists, considering the low number of data points used for all plots of Fig. 2. Even though for the cases of both LM-OSL and CW-OSL, the number of data points could be selected as an experimental parameter, for the case of the RTL curves after bleaching, this resolution totally depends on the number of the stimulation durations applied within the experiment. Better resolution could possibly result by selecting many stimulation durations. Nevertheless, even after applying the maximum possible stimulation times, there is poorer resolution than in the case of CW-OSL, where each signal can be described within at least 1000 data points. According to the results of the present study, regarding the origin of the components CRTLi, each RTL decay component receives electrons from at least two peaks. More specifically, the electrons originating from the traps of first and second peak have similar decay constants, so it is difficult to separate them. Taking this into account, both fast components of first and second peaks yield the same decay constant, indicating that these two components utilize the electrons arising from the same traps. 10
The same stands for medium components CRTL2 with electrons originating from the second and the third-forth peak, as well as for slow components CRTL3 which outcome from the first and second peak. Also, an important notation that is observed from Fig. 3 deals with the overlapping yielded between the component C1 of LM-OSL signal, the component C2 of CW-OSL signal, as well as component C2 of the first RTL peak (at 90oC) as already shown by the Τables 1 and 2. Finally, it is worth emphasizing in another feature which could be yielded by Fig. 3. Firstly, it becomes obvious that the continuous wave configuration can easily deplete the electrons of the first two peaks, resulting to the fast and medium RTL decay components of these respective peaks. Unfortunately, due to the values of decay constant obtained in the present study, CW-OSL hardly achieves, even partially, depletion neither of the slow components of the RTL decay nor for the third TL glow peak. In order to achieve this specific bleaching the application of LM-OSL is highly required.
5. Conclusions A component resolved bleaching study was attempted for CaF2:N . The luminescence signals subjected to de-convolution were the LM-OSL, CW-OSL and RTL decay curves after bleaching. Thus, each experimental data point of any RTL curve, corresponds to an intensity integration. The integrated intensity over a step temperature interval of ΔΤ=5 oC, namely for each temperature interval Τi+ΔΤ= Τi+5 o C of the RTL decay curves, when plotted as a function of the stimulation/bleaching time, indicated the presence of nine different groups, based on the criterion of the bleaching decay constant λ. Nevertheless, the degenerated decay constant values monitored resulted in overlapping for the values of these groups. Finally six, different components, each one yielding distinguishable decay constant value, are sufficient in order to describe efficiently the decay of the RTL glow curves of CaF2:N received after bleaching for various times. Similarly, four different LM-OSL components are required for achieving best fit for LM-OSL curves while only three, degenerated components can fit the CW-OSL signal. There is only one, unique bleaching component present in all three luminescence entities of the present study, while there are various bleaching components present to at least two of the luminescence entities which were the subject of the present study. The results of the present study indicate that each TL trap yields at least three different bleaching components, while different TL traps can indicate bleaching components with similar decay constant. According to the data of the present study, each RTL bleaching component receives electrons from at least two peaks. In this case, these components will be seen as one component and the same holds for the cases of CW-OSL signal, where the resolution is not so good, while it must be noted that CW-OSL can distinguish two or more components if the corresponding decay constants differ by at least 50%. CW-OSL removes 11
substantially the fast and medium components of the first two peaks while very smoothly either the slow components or the third and forth peak. In order to achieve depletion of the high temperature TL peak, the application of LM-OSL configuration is required. The results of the present study suggest that the traps that contribute to TL and OSL are the same. References Adamiec, G., Bluszcz, A., Bailey, R.M., Garcia-Talavera, M., 2006. Finding model parameters: Genetic algorithms and the numerical modelling of quartz luminescence. Radiation Measurements 41 (2006) 897 – 902. Afouxenidis, D., Polymeris, G.S., Tsirliganis, N.C., Kitis, G., 2012. Computerised curve de-convolution of TL/OSL curves using a popular spreadsheet program. Radiat.Prot.Dosim. 149(4), 363–370. 1.1.Balogun. F. A., Ojo. J. O., Ogundare. F. O., Fasasi. M. K., Hussein. L. A., 1999. TL response of a natural fluorite. Radiation Measurements 47 (2012) 182-184. 1.2.Bailey, R.M., 2001. Towards a general kinetic model for optically and thermally stimulated luminescence of quartz. Radiation Measurements 33 1745. 1.3.Bailey, R.M., Smith, B.W., Rhodes, E.J., 1997. Partial bleaching and the decay form characteristics of quartz OSL. Radiation Measurements 27 (2), 123-136. Becker, K., 1973. Solid State Dosimetry, CRC Press. Bernhardt, R., Herforth, L., 1974. Proc. 4th Int. Conf. Lumin. Dosimetry, Krakow, Poland, p. 1091.
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1.4.
Bulur. E., 1996. An alternative technique for optically stimulated luminescence (OSL) experiment. Radiation Measurements 26 (1996) 701
Chougaonkar. M. P., Bhatt. B. C., 2004. Blue light stimulated luminescence incalcium flouride, its characteristics and implications in radiation dosimetry. Radiation Protection Docimetry 112 (2004) 311-321 1.5.Dallas. G. I., Polymeris. G. S., Afouxenidis. D., Tsirliganis. N. C., Tsagas. N. F., Kitis. G., 2010. Correlation between TL and OSL signals in KMgF3:Ce3+: Bleaching study of individual glow-peaks. Radiation Measurements 45 (2010) 537-539 1.6.Ferreira Jr. F. A., Yoshimura. E. M., Umisedo. N. K., do Nascimento. R. P., 2014. Correlation of optically and thermally stimulated luminescence of natural fluorite pellets. Radiation Measurements 71 (2014) 254-257 1.7.Furetta. C., Tuyn. J. W. N., 1985. Annealing cycle effects on the TL sensitivity of CaF2:Tm (TLD-300). The International Journal of Applied Radiation and Isotopes 36 (1985) 1000-1002 Jain, M, Murray, A.S., Bøtter-Jensen, L., 2003. Characterisation of blue light stimulated luminescence components in different quartz samples: implications for dose measurement. Radiation Measurements 37(4-5): 441–449. Kharita. M. H., Stokes. R., Durrani. S. A., 1995.
TL and PTTL in natural
fluorite previously irradiated with gamma rays and heavy ions. Radiation Measurements 24 (1995) 469-472 Kitis, G., Kiyak, N.G., Polymeris, G.S., 2015. Temperature lags of luminescence measurements in a commercial luminescence reader. Nuclear Instruments and Methods in Physics Research B 359 (2015) 60–63. 1.8.Kitis. G., Kiyak. N., Polymeris. G. S., Tsirliganis. N. G., 2010. The correlation of fast OSL component with the TL peak at 325oC in quartz of various origins. Journal of Luminescence 130 (2010) 298-303
1.9.
Kitis. G., Pagonis. V., 2008. Computerized curve deconvolution analysis for LM-OSL. Radiation Measurements 43 (2008) 737-741
1.10. Kitis. G., Ros-Gomez, 1999. Thermoluminescence glow-curve deconvolution functions for mixed order of kinetics and continuous trap distribution. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 31 (1999) 2636-2641 McKeever, S.W.S., Moscovitch, M., Townsend, P.D., 1995. Thermoluminescence Dosimetry Materials: Properties and Uses, Nuclear Technology Publishing. 13
Polymeris, G.S., Afouxenidis, D., Tsirliganis., N.C., Kitis, G., 2009. The TL and room temperature OSL properties of the glow peak at 110 oC in natural milky quartz: A case study. Radiation Measurements 44 (2009) 23–31. Polymeris, Kiyak, N.G., G.S., Kitis, G., 2008. Component resolved IR bleaching study of the blue LM-OSL signal of various quartz samples. Geochronometria 32, 79– 85. Polymeris. G. S., Kitis. G., Tsirliganis. N. C., 2006. Correlation between TL and OSL properties of CaF2:N. Nuclear Instruments And Methods in Physics:Research B 251 (2006) 133-142
1.11. Spooner. N. A, 1994. On the optical dating signal from quartz. Radiation Measurements 23 (1994) 593-600 Topaksu. M., Yazici. A. N., 2007. The thermoluminescence properties of natural CaF2. Nuclear Instruments and Methods in Physics: Research B 264 (2007) 293-301 Tugay. H., Yegingil. Z., Dogan. T., Nur. N., Yazici. N., 2009. The thermoluminescence properties of natural calcium fluoride for radiation dosimetry. Nuclear Instruments and Methods In Physics: Research B 267 (2009) 3640-3651 Table 1. Average λ parameters (in units of s-1) of bleaching decay curve components, as these were yielded from the de-convolution of the integrated signals of the residual TL glow curves. Error values (presented in parenthesis) are estimated to be within 4-7%, since this was the margin of the FOM value obtained through the de-convolution procedure. 0
λ of CRTL1 (fast)
λ of CRTL2 (medium)
λ of CRTL3 (slow)
Peak1 (60° C-120° C )
0.594660 (0.031621)
0.047751 (0.002967)
0.000257 (0.000011)
Peak2 (160 ° C-190° C)
0.420346 (0.034638)
0.021321 (0.001786)
0.000103
Temperature
C
(0.000013) Peak3 (230 ° C-320° C)
0.109844 (0.007986)
0.018624 (0.001536)
0.000011 (7.8·10-7)
14
Table 2. Decay constant values (in units of s-1) for LM-OSL as well as CW-OSL components based on a component resolved analysis. Error values are estimated to be within 2%, since this was the highest value for the FOM value. For the case of CW-OSL curves, the values presented are the mean values over 4 independent measurements of different duration, with reproducibility of the order of 0.2%.
λ of CLM-OSL1
λ of CLM-OSL2
λ of CLM-OSL3
λ of CLM-OSL4
0.0526137
0.0174463
0.0050832
0.000145
λ of CCW-OSL1
λ of CCW-OSL2
λ of CCW-OSL3
λ of CCW-OSL4
0.604588
0.0491235
0.007486
-
Table 3. Integral of the four LM-OSL components and of the three CW-OSL components. The third column shows the % participation of each component to the total LM-OSL and CW-OSL signal respectively.
LM-OSL Ci
Integral (counts)
%
CW-OSL Ci
Integral (counts)
%
C LM-OSL1
42319
11
C CW-OSL1
82442
28
C LM-OSL2
246093
62
C CW-OSL2
167878
56
C LM-OSL3
61805
16
C CW-OSL3
48809
16
C LM-OSL4
44722
11
C CW-OSL4
0
0
Fig. 1. Residual TL (RTL) glow curves of CaF2:N for various stimulation times. Traps responsible for all TL peaks are shown to contribute to the OSL signal. Fig. 2. Integrated RTL intensity over intervals of ΔT=5 °C, as a function of CW-OSL stimulation times. Four different temperature intervals, namely 95-100 °C, 185-190 °C, 245-250 °C and 295-300 °C, are presented clockwise. Experimental data are presented as rhombus, while the three components (fast, medium and slow) obtained from the curve fitting as well as the corresponding sum, are presented as continuous lines.
15
Fig. 3. Experimental points indicate the values of decay constants of components CRTLi with the corresponding errors of ±1σ. The values of the errors were estimated by using the corresponding F.O.M values (ranging within 4-7% for the λ value of each CRTLi) multiplied by the values of the components. Overlapping of bleaching components among the three temperature peak regions is obvious, at least for three cases, namely the fast components (CRTL1) of both first and second TL peaks, the medium components (CRTL2) of both second and third TL peaks as well as slow components (CRTL3) of first and second TL peaks. Fig. 4. The dependence of the order of kinetic parameter b versus temperature for all bleaching components CRTLi (fast, medium and slow) of the integrated RTL intensity over the temperature integrals betweenΤi and Τi+ΔΤ. Fig. 5. LM-OSL curve de-convolution analysis. Experimental data are presented as filled dots, while components CLM-OSL1, CLM-OSL2, CLM-OSL3 and CLM-OSL4 obtained from the curve fitting as well as the sum of all theoretical curves are presented as continuous lines. Fig. 6. CW-OSL curve de-convolution analysis. Experimental data are presented as filled dots, while components CCW-OSL1, CCW-OSL2, and CCW-OSL3 obtained from the curve fitting as well as the sum of all theoretical curves are presented as continuous lines.
0s - unbleached sample
Residual TL (a.u)
10000
1000
2000s
100 100
200
300
400
500
o Temperature ( C)
o T=185 C
1000
RTL Intensity (a.u)
RTL Intensity (a.u)
T = 95°C C1 C2
100
100
C3 1
C1 1000
10
100
StimulationTime (s)
1000
C2
C3
1
10
100
1000
StimulationTime (s)
16
10000
C1
10000
RTL Intensity (a.u)
RTL Intensity (a.u)
T = 295oC
C2
C1
1000
C3
1000 1
C2
C3
100
10
100
1000
1
Stimulation Time (s)
10
100
1000
StimulationTime (s)
fast - CRTL1
0.1 0.01
medium - CRTL2
-1
(s )
0
T= 250 C
1E-3 1E-4 slow - CRTL3
1E-5 50
100
150 200 250 Temperature (°C)
300
350
17
2,2
RTL decay order of kinetic
2,0
Fast
1,8
1,6
Medium
1,4
1,2
1,0
Slow
50
100
150
200
250
300
350
o Temperature ( C)
1600
LM-OSL Intensity (a.u)
1400 1200
CLM-OSL1
1000 800 600
CLM-OSL2
400 200
CLM-OSL3 0
200
CLM-OSL4 400
600
800
1000
CW-OSL Intensity (a.u)
Stimulation Time (s)
CCW-OSL1
10000
CCW-OSL2 1000
CCW-OSL3
1
100 0.1
1
10
100
1000
StimulationTime (s)
18
Highlights
A component resolved bleaching study was attempted to CaF2:N in terms of CW-OSL, LM-OSL and RTL. Bleaching decay constants originating from different TL peaks yield overlapping values. Three to five individual components were used in order to describe the bleaching behavior in all luminescence entities. There is only one, unique bleaching component present in all three luminescence entities.
19
PII: DOI: Reference:
S0969-8043(16)30230-5 http://dx.doi.org/10.1016/j.apradiso.2017.01.014 ARI7736
To appear in: Applied Radiation and Isotopes Received date: 7 June 2016 Revised date: 5 January 2017 Accepted date: 17 January 2017 Cite this article as: Vasiliki Angeli, George S. Polymeris, Ioanna K. Sfampa, Nestor C. Tsirliganis and George Kitis, Component resolved bleaching study in natural calcium fluoride using CW-OSL, LM-OSL and residual TL glow curves after bleaching, Applied Radiation and Isotopes, http://dx.doi.org/10.1016/j.apradiso.2017.01.014 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Component resolved bleaching study in natural calcium fluoride using CW-OSL, LM-OSL and residual TL glow curves after bleaching Vasiliki Angelia, George S. Polymerisb*, Ioanna K. Sfampaa, Nestor C. Tsirliganisc, George Kitisa a
Aristotle University of Thessaloniki, Nuclear Physics Laboratory, 54124-Thessaloniki, Greece.
b
Insitute of Nuclear Sciences, Ankara University, TR-06100, Beşevler-Ankara, Turkey.
c
Laboratory of Radiation Applications and Archaeological Dating, Department of Archaeometry and Physicochemical Measurements, 'Athena' - Research and Innovation Center in Information, Communication and Knowledge Technologies, Kimmeria University Campus, 67100, Xanthi, Greece. *
Corresponding author. [email protected]
Tel.:
[email protected],
Abstract Natural calcium fluoride has been commonly used as thermoluminescence (TL) dosimeter due to its high luminescence intensity. The aim of this work is to attempt a correlation between specific TL glow curves after bleaching and components of linearly modulated optically stimulated luminescence (LM-OSL) as well as continuous wave OSL (CW-OSL). A component resolved analysis was applied to the integrated intensity of the RTL glow curves and the OSL decay curves, by using a Computerized Glow-Curve De-convolution (CGCD) procedure. All the CW-OSL and LM-OSL components are correlated with the decay components of the integrated RTL signal, apart from two RTL components which cannot be directly correlated with either LM-OSL or CW-OSL component. The unique, stringent criterion for this correlation deals with the value of the decay constant λ of each bleaching component. There is only one, unique bleaching component present in all three luminescence entities which were the subject of the present study, indicating that each TL trap yields at least three different bleaching components, while different TL traps can indicate bleaching components with similar values. According to the data of this work each RTL bleaching component receives electrons from at least two peaks. The results of the present study suggest that the traps that contribute to TL and OSL are the same.
1. Introduction Calcium Fluoride, either natural (CaF2:N), or in synthetic form, doped with other rare earth elements such as Dy, Eu and Tm, is commonly used as TL dosimeter (Kharita et al., 1995; Polymeris et al., 2006; Topaksu et al., 2007; Tugay et al., 2009). Besides the 1
plethora of its dosimetric advantages, such as its high mechanical strength, its high luminescence intensity and its low cost, it is well known that the complex TL glow curve (Ogundare et al., 2004; Polymeris et al., 2006; Topaksu and Yazici, 2007), as well as its lack of tissue equivalence (Becker, 1973; McKeever et al., 1995) stand as its major shortcomings. Because of its high sensitivity to light (Furetta et al., 1985; Kitis et al., 1993; Chougaonkar et al., 2004; Polymeris et al., 2006;), CaF2:N could be also used effectively in optically stimulated luminescence dosimetry (OSLD). Bernhardt and Herforth have already suggested its use as an OSL dosimeter as early as 1974. Nevertheless, according to the authors’ best knowledge, bleaching has not been used to erase neither the natural signal nor the remnant signal after irradiation. During bleaching, an important proportion of the trapped electrons abandon the traps when the dosimeter is exposed to artificial or natural light. Bleaching can take place naturally or under strict experimental conditions. In the latter case, measuring the number of trapped electrons that are emitted following stimulation results in optically stimulated luminescence (OSL) curves. Consequently, it is assumed that bleaching and OSL have the same outcome to the trapped charge population. The only difference is that during the OSL, stimulation takes place using monochromatic light, while in bleaching the entire optical spectrum is effective. The bleaching properties of TL traps in various materials have been an attractive topic of research nowadays. Using OSL and residual TL glow curves, Spooner suggested that the optical dating signal in the case of quartz is related with an electron trap which is responsible for the TL glow peak at 325 °C (Spooner, 1994). Nevertheless, (Kitis et al., 2010) have reached the conclusion that this TL glow-curve is related to more than one electron traps. A basic result of the latter citation was that the LM-OSL signal for a stimulation time of 50s is a composite signal consisting of three components (fast, medium and slow) with different component lifetime values. By considering this, there is no specific relation of the fast component with a specific electron trap responsible for this TL glow curve. A similar approach has been also adopted by both (Jain et al., 2003) as well as (Polymeris et al., 2008) in their bleaching study of the TL signal in quartz using Infrared Stimulated luminescence (IRSL) at elevated temperatures. Similar conclusions were also reported by Dallas et al. (2010) for the case of KMgF3:Ce3+. These authors have extensively studied the bleaching of each, individual TL peak of the perovskite material, while arrived to the following conclusions: i) none of the individual glow-peak can be related to a single OSL component and ii) overlapping of the bleaching decay constants, having similar values although originating from different TL peaks. This experimental feature was anticipated for the case that the decay constants of two OSL components differ less than 15-20% (Kitis and Pagonis, 2008).
2
The present study follows on directly from the aforementioned citation of Dallas et al. (2010), which ends by questioning whether similar bleaching analysis could be also performed to other bleachable TL materials. In the framework of this aim we attempt to continue Dallas’s work as far as the correlation between specific RTL glow curves and LM-OSL components in natural CaF2:N. This specific material has been selected due to its high sensitivity to light (Furetta et al., 1985; Kitis et al., 1993; Chougaonkar et al., 2004; Polymeris et al., 2006) and because of its complex TL glow curve (Balogun et al., 1999; Polymeris et al., 2006;). According to the authors’ best knowledge, still not thorough similar study has been attempted for CaF2:N. Moreover, this study will not limit just to LM-OSL and RTL curves, but will also include the component resolved analysis of the CW-OSL. The main outcome of this componentresolved analysis deals with the comparison of the decay constant λ values obtained from the analysis of integrated RTL, LM-OSL as well as CW-OSL curves. 2. Experimental a) Sample and equipment The study was performed on natural calcium fluoride, CaF2:N, which was obtained from a fluorite mine of Nigeria, Africa; it is the same sample that has been previously studied by Polymeris et al., 2006. The sample was gently crushed using an agate mortar and grains of dimensions between 80μm and 140μm were obtained after sieving. Aliquots with mass of 3mg each were prepared by mounting the material on stainless-steel disks of 1cm2 area (Polymeris et al., 2006). Mass reproducibility was checked to be within ±5%. All measurements were performed using one, unique aliquot (single aliquot measurements) containing material that had been freshly annealed at 500 °C; this was feasible due to the lack of sensitization (Polymeris et al., 2006). All luminescent measurements were performed using a RISØ TL/OSL reader (model TL/OSL-DA-15) equipped with a high-power blue LED light source, an infrared solid state laser and a 0.073Gy/s 90Sr/90Y β-ray source. The reader is fitted with an EMI 9635QA PM Tube. The detection optics consisted of a 7.5 mm Hoya U-340 filter (λ ~ 340 nm, FWHM ~ 80 nm). 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. All heatings and TL measurements were performed in a nitrogen atmosphere with a low constant heating rate of 1 oC/s, in order to avoid significant temperature lag (Kitis et al., 2015), up to the temperature of 500 oC. OSL measurements were performed in both continuous wave as well as linear mode configurations (CW-OSL and LM-OSL respectively). In the former case, the power level was software controlled and set at 90% of the maximum stimulation intensity for blue LEDs. For LM-OSL curves (Bulur, 1996), the ramping rate was 0.04 mW/cm2/s, increasing the stimulation light power from zero up to the maximum power (40 mW/cm2) over a period of 1000 s. Unless otherwise stated, all OSL measurements 3
were performed at room temperature, namely 25 oC. The test dose attributed was 1 Gy. b) Experimental Protocol The experimental protocol used to obtain the OSL signals and the residual TL after bleaching in different bleaching/stimulation durations consists of the following steps: Step 0: Test Dose and TL measurement. The purpose of this step is to obtain the unbleached, reference TL signal of natural calcium fluoride, which corresponds to bleaching/stimulation time ti=0s. This step has been measured only once. Step 1: Test Dose and CW-OSL measurement for stimulation time ti. The purpose of this step is to monitor the CW-OSL signal corresponding to time ti. Step 2: TL measurement. The purpose of this step is to monitor the residual TL (hereafter RTL) after bleaching/stimulation for ti. Steps 1-2 are performed using different stimulation times ti=2, 4, 8, 16, 32, 50, 75, 100, 200, 400, 600, 800, 1000, 1500 and 2000s. Step 3: Blue LM-OSL for 1000s to receive the LM-OSL signal. This step has been performed only once, after steps 1-2 corresponding to the longest bleaching/stimulation duration, being 2000s. 3. Results a) The shape of the RTL glow curves The residual TL glow curves of CaF2:N received after the CW-OSL for various stimulation times are shown in Fig. 1 for a selection of bleaching/stimulation durations. The difference between the TL glow curve of the non bleached sample and the RTL glow curves after bleaching for various times becomes very obvious. According to the data of this plot, it is possible to conclude that the shallow trap responsible for the low temperature TL peak (below 150 oC) is very easily and fast bleached within stimulation time intervals between 2s and 10s; similar results were also reported by Ferreira et al., 2014. On the other hand, the peak located around 300 o C survives for small stimulation/bleaching times. Nevertheless, for stimulation durations longer than 100s, this complex TL peak is also substantially bleached. Moreover, it is worth mentioning the shift of the peak maximum temperature of the main dosimetric complex peak, towards higher temperatures as the stimulation/bleaching time increases. This shift is attributed to the complex nature of the prominent main dosimetric peak of CaF2:N (Polymeris et al., 2006; Topaksu and Yazici, 2007). Finally, as this latter Fig. 1 is going to further reveal, there is no
4
bleaching of the RTL glow curve part corresponding to temperatures higher than 350 o C. b) Method of analysis In a previous similar work of Dallas et al. (2010), each one of the RTL glow curves was de-convolved into its individual glow-peaks using a Computerized Glow-Curve De-convolution (CGCD) procedure (Kitis et al., 2000). In the framework of this analysis approach, a good resolution is required for all RTL glow curves; therefore, while measuring the TL glow curves in the case of Dallas et al. (2010), one data point was received for each temperature degree. Nevertheless, due to the complexity of the TL glow curve shape and structure of CaF2:N (Ogundare et al., 2004; Polymeris et al., 2006; Topaksu and Yazici, 2007), the de-convolution approach might be complex, time consuming and some erroneous. Therefore, in the framework of the present study, an alternative approach has been adopted. For each RTL glow curve, measured up to 500 oC, only 100 data points were deliberately obtained. Thus, each experimental data point of any RTL curve, corresponds to an intensity integration over a step temperature interval of ΔΤ=5 oC. By plotting this integrated intensity for each temperature interval Τi+ΔΤ= Τi+5 oC as a function of the stimulation/bleaching time, we get a total of 100 plots, a selection of which is presented in Fig. 2. Each one of these aforementioned 100 plots of integrated RTL intensities versus stimulation time, was analyzed into its individual components by using a Computerized GlowCurve De-convolution (CGCD) procedure (Kitis et all., 1999b) and using general order kinetics equation (Bøtter-Jensen et al., 2003; Kitis and Pagonis, 2008): ( )
(
[
)
]
(1)
where I(t) is the intensity of luminescence signal as a function of time, I0 is the initial luminescence intensity, τ =1/λ is the component lifetime and λ (1/s) stands as the respective decay constant and b is the order of kinetics which varies between the values 1.001 and 2.05. The same aforementioned equation was also applied for the deconvolution of the CW-OSL decay curves. As far as LM-OSL curves are concerned, these were also de-convolved into individual components according to (Kitis and Pagonis, 2008) and (Polymeris et al., 2006): ()
(
[
)
(
)
]
(2)
where Im and tm are the values of OSL intensity and time respectively and β stands for the parameter of the kinetic order. The background was simulated by an equation of the form (Polymeris et al., 2009):
5
(
) (3)
where zd is the zero dose OSL signal after blue stimulation and P the total stimulation time. The parameter zd is evaluated experimentally, by measuring the background OSL signal at non-irradiated samples. c is a constant with value which is expected to be very close to unity (Polymeris et al., 2009). Finally, the λ value of each component was estimated according to the equations suggested by Bulur (1996). All curve fittings were performed using the software package Microsoft Excel, with the Solver utility (Afouxenidis et al., 2012), while the goodness of fit was tested using the Figure Of Merit (FOM) of Balian and Eddy, (1977) given by: ∑[
] (4)
where Yexper is the experimental data, Yfit is the fitted data and A is the area of the fitted curve. The aim of the present exercise includes comparing and discussing the results concerning the λ values obtained by de-convolving these three luminescence entities, namely integrated RTL in steps of 5 °C, LM-OSL and CW-OSL. c) De-convolution results Fig. 2 presents the integrated RTL signal over a step temperature interval of ΔΤ=5 °C, for a selection of four different temperatures Τi+ΔΤ, namely 95-100 °C, 185-190 °C, 245-250 °C and 295-300 °C plotted as a function of the bleaching/stimulation time. In all cases, the plots were de-convolved using three different components. As it can be seen from Fig. 2, it was impossible to achieve good fit by using less than three components of general order kinetics. Following the terminology of Dallas et al. (2010), these components can be termed hereafter as fast (or component CRTL1), medium (or component CRTL2) and slow (or component CRTL3). The main outcome of this de-convolution analysis deals with the comparison of the decay constant λ values for each component obtained from the analysis of integrated RTL throughout the temperature region, LM-OSL as well as CW-OSL curves. The reason for selecting these four aforementioned temperatures for Fig. 2 is mainly twofold: (a) according to the de-convolution of the TL glow curve of CaF2:N (Polymeris et al., 2006; Topaksu and Yazici, 2007) five different TL glow peaks can be observed within the temperature range between room temperature (RT) and 350 oC of each RTL glow curve. The main glow-peak appears at high temperature, near 275 oC, with prominent peaks also at around 90 oC and 183 oC. Both the main glow-peak as well as the one at 90 oC, have one satellite peak each; nevertheless, the satellite peak of the 90 oC TL peak is considered as negligible, while the satellite of the main peak has a delocalising temperature of around 300 oC. All aforementioned temperatures refer to heating rate of 1 oC/s. Therefore, the aforementioned temperatures of 95 oC, 185 oC, 245 oC and 295 oC correspond to four main de-convolved TL glow peaks, (b) The analysis of all RTL intervals indicated a grouping on the values of the λ parameter 6
within three main different temperature ranges, which are located within 60 oC - 120 o C, 160 oC - 190 oC and 230 oC - 320 oC, namely in the same temperature area of the first, second and third plus fourth TL peaks respectively. It is worth emphasizing that there is no discrimination for the decay constants corresponding to the third and fourth TL peaks. Following the discussion above, all the bleaching components λ, originated from the de-convolution of the integrated RTL signal versus the bleaching/stimulation times are represented in Fig. 3, as a function of temperature. For each interval within the ΔΤi=5 °C, three different decay curves were obtained; these are presented as experimental data points with corresponding ±1σ error. As it becomes obvious by this latter plot, six out of the total of nine components λ are well correlated to each other, with the overlapping of bleaching components among the three temperature peak regions being obvious, at least for three cases, namely the fast components (CRTL1) of both first and second TL peaks, the medium components (CRTL2) of both second and third TL peaks as well as slow components (CRTL3) of first and second TL peaks. According to the discussion above, the average value for each decay constant λ1, λ2 and λ3 is presented in Table 1 for the case of components CRTL1 (fast), CRTL2 (medium) and CRTL3 (slow) respectively, as these were yielded by the de-convolution of the corresponding temperature ranges of the RTL glow curves. A basic observation deals with the overlapping monitored, as the decay constants of the fast components derived for the first two peaks will be seen as one component. The same also holds for the slow bleaching decay constants of the first and second TL peaks as well as for the decay constants of the fast component of the third peak with the medium component of the second peak. According to Kitis and Pagonis, (2008), such overlapping is anticipated for all cases where two different OSL/bleaching components indicate decay constants which differ by less than 15-20%. Nevertheless, the bleaching ability of the glow curves of any material is reflected to the rate at which the TL glow curve intensity is decreased after exposure to light. According to equation (1), the parameter b, defining the order of kinetics, stands as a fitting parameter. Knowledge of this parameter could be very helpful in understanding the bleaching mechanism. Fig. 4 presents the order of kinetic parameter b as a function of the temperature of the glow curve for all three components, namely fast, medium and slow. It becomes clear from this latter feature that throughout the entire glow curve temperature range, the fast bleaching component (CRTL1) is described by second order of kinetics, implying re-trapping. On the other hand, the slow bleaching component (CRTL3) is a fist-order-kinetic component, excluding thus the possibility of re-trapping. Finally it is worth mentioning that the medium bleaching component (CRTL2) yields intermediate order of kinetics, with b values ranging between 1.4 and 1.55. According to the data of the present study, each RTL bleaching component receives electrons from at least two peaks. Therefore it seems that for the fast
7
component, there is re-trapping for the electrons of the two different traps contributing, while partly this is the case also for the medium component. Moreover, due to this overlapping monitored, one can safely conclude that the LMOSL curve will not be consisting of 9-12 components, but of much less. Indeed, a typical example of the LM-OSL curve shape is shown in Fig. 5. The LM-OSL signal for a stimulation time of 1000s is a composite signal consisting of four components as obtained from the computerized de-convolution analysis. As it can be seen from Fig. 5 the fourth component has not totally decayed for the stimulation time of 1000s (Kitis et al., 2010). This latter component could be possibly comprising of the sum of more, long living components. The decay constants of each LM-OSL component, hereafter termed as CLM-OSL1, CLM-OSL2, CLM-OSL3 and CLM-OSL4 are presented in Table 2. Nevertheless, the results of this component-resolved analysis of the LM-OSL curve stand in good agreement with all previous results concerning the de-convolution of the LM-OSL curves reported previously (Chougaonkar and Bhatt, 2004; Polymeris et al., 2006), in both terms of number of components as well as the values of decay constants. The de-convolution results of the LM-OSL curve yielded another interesting feature, regarding the order of kinetics value for the components; the fast component CLM-OSL1 is described by second order of kinetics, the component CLM-OSL2 by general order kinetics with b parameter around 1.46, while components CLM-OSL3 and CLM-OSL4 are described by first order of kinetics. This experimental result stands in good agreement with the corresponding results concerning the order of kinetics obtained from the analysis of the decay of the integrated RTL signal. According to the applied protocol, which has been presented previously, CW-OSL signals were also measured for various stimulation durations. In fact, four CW-OSL decay curves were analyzed to the corresponding individual components. Four different CW-OSL decay curves, corresponding to 800, 1000, 1500 and 2000 s of stimulation were selected, since these are the four highest stimulation durations applied in the framework of the present study. In all cases, three non-first order OSL components are required for best fitting. The corresponding decay constants are also presented in Table 2, while a CW-OSL de-convolution example for the case of 1000 s of stimulation is presented in Fig. 6. According to the standard deviation values presented in Table 2, reproducibility on the values of the decay constants for each of the three CW-OSL components through different stimulation time is excellent. It is worth mentioning that in all cases of the de-convolution results of the CW-OSL curves, the fast and medium components CCW-OSL1 and CCW-OSL2 are both described by second order of kinetics, while the slow component CCW-OSL3 is described by first order of kinetics. As far as the order-of-kinetics is concerned, these aforementioned results stand in good agreement with the results presented in Fig. 4 for fast and slow components of the integrated RTL signal, CRTL1 and CRTL3 respectively. For the case of the medium components CRTL2 and CCW-OSL2, re-trapping was indicated for both, despite the fact that different b value was yielded by the de-convolution analysis. 8
According to the discussion above the LM-OSL signal is represented by four components, while the CW-OSL signal by three components, hereafter termed as CCW-OSL1, CCW-OSL2, and CCW-OSL3. Table 3 shows the integral of each one of these components along with the percentage contribution of each component to the total corresponding signal. The contribution of components CCW-OSL3 and CLM-OSL3 is the same, being 16%. The same holds for component CCW-OSL2 and the CLM-OSL2, since the contribution of each one of them is 62% and 56% respectively. A difference appears for the components CLM-OSL1 and the CCW-OSL1. The contribution of the CCW-OSL1 to the total CW-OSL signal is 11%, while the contribution of the CLM-OSL1 has almost the double value, being around 28%. 4. Discussion Bleaching of the TL traps in various phosphors has been reported to be a controversial topic. According to the pioneer studies of Bailey et al. (1997), Bailey (2001) and Adamiec et al. (2006) and the corresponding models, TL and OSL traps in quartz are different. These models were reported to be able to reliably reproduce a wide variety of quartz luminescence phenomena. Nevertheless, subsequent studies of Dallas et al. (2008) in KMgFe3:Ce3+ and Kitis et al. (2010) in quartz, verified that both OSL and TL simulate the same traps. In the framework of the present study, the authors try to support the latter assumption of Dallas et al. (2008) and Kitis et al. (2010). Based on our results, similarly to the case of KMgFe3:Ce3+ (Dallas et al., 2010), the bleaching of each individual glow-peak takes place in three different rates; namely in a fast, medium and slow rate. Initially, each bleaching rate is expected to be related to a single OSL component with a specific decay constant in an OSL curve. Four different TL glow peaks could be easily identified in the glow curve of CaF2:N. Therefore, assuming the contribution of three components for the bleaching/OSL decay of each one of them, one can safely assume that a total of twelve components are required in order to describe the OSL signal measured at room temperature, since at room temperature all TL glow curves contribute to the OSL signal (Polymeris et al., 2006). However, since there is no discrimination for the decay constants corresponding to the third and fourth TL peaks, this number of twelve components is reduced to nine. Nevertheless, even these values for the decay constants are degenerated, due to the overlapping of many values that has been previously described. As a result, these nine different bleaching components are forming groups and overlap, so that five, distinguishable decay constant values are required in order to describe efficiently the decay of the integrated RTL signal of CaF2:N received after bleaching for various times. This number of components required is very close to the four components required to achieve good fitting for the case of the LM-OSL curve as well as to the 3 components required for the best fitting of the CW-OSL decay curve. By comparing the characteristic parameters of each component presented in Tables 1 and 2 it is safe to conclude the following: 9
There is only one, unique bleaching component present in all three luminescence entities of the present study. Specifically, components CLM-OSL1, CCW-OSL2, as well as medium component CRTL2 of TL peak at 90oC, indicate decay constant values very similar, namely , and s-1 respectively. There are various bleaching components present to at least two luminescence entities, such as (i) fast components CRTL1 of both first and second TL peaks along with component CCW-OSL1, (ii) medium components CRTL2 of both second and third (main) TL peaks along with component CLM-OSL2, (iii) component CCW-OSL3 along with component CLM-OSL3 and finally (iv) slow components CRTL3 of both second and third (main) TL peaks along with component CLM-OSL4. There are two RTL bleaching components which cannot be directly correlated to either LM-OSL or CW-OSL component, being components CRTL1 and CRTL3 of the main TL peak.
According to Kitis and Pagonis (2008), the fitting resolution for the case of LM-OSL is 15-20%; in a nutshell, it is difficult to resolve through computerized glow curve deconvolution LM-OSL components with decay constants which differ by less than 1520%. Based on this, these components will be seen as one component in an LM-OSL curve. According to the results of the present study, the fitting resolution for the case of CW-OSL is much worse. In fact, it becomes obvious that for CW-OSL curves, it is difficult to resolve through computerized glow curve de-convolution bleaching components with decay constants which differ by less than 50%. This really bad resolution could be attributed to the featureless of the decay curve shape of CW-OSL. Moreover, unlike the TL process, the traps are not emptied serially due to the optical stimulation. The same stands for the case of the integrated RTL signal versus bleaching, since both CW-OSL as well as integrated RTL signal versus bleaching\stimulation time are described by the same equation. Nevertheless, in the case of the latter plots versus bleaching time, another important drawback exists, considering the low number of data points used for all plots of Fig. 2. Even though for the cases of both LM-OSL and CW-OSL, the number of data points could be selected as an experimental parameter, for the case of the RTL curves after bleaching, this resolution totally depends on the number of the stimulation durations applied within the experiment. Better resolution could possibly result by selecting many stimulation durations. Nevertheless, even after applying the maximum possible stimulation times, there is poorer resolution than in the case of CW-OSL, where each signal can be described within at least 1000 data points. According to the results of the present study, regarding the origin of the components CRTLi, each RTL decay component receives electrons from at least two peaks. More specifically, the electrons originating from the traps of first and second peak have similar decay constants, so it is difficult to separate them. Taking this into account, both fast components of first and second peaks yield the same decay constant, indicating that these two components utilize the electrons arising from the same traps. 10
The same stands for medium components CRTL2 with electrons originating from the second and the third-forth peak, as well as for slow components CRTL3 which outcome from the first and second peak. Also, an important notation that is observed from Fig. 3 deals with the overlapping yielded between the component C1 of LM-OSL signal, the component C2 of CW-OSL signal, as well as component C2 of the first RTL peak (at 90oC) as already shown by the Τables 1 and 2. Finally, it is worth emphasizing in another feature which could be yielded by Fig. 3. Firstly, it becomes obvious that the continuous wave configuration can easily deplete the electrons of the first two peaks, resulting to the fast and medium RTL decay components of these respective peaks. Unfortunately, due to the values of decay constant obtained in the present study, CW-OSL hardly achieves, even partially, depletion neither of the slow components of the RTL decay nor for the third TL glow peak. In order to achieve this specific bleaching the application of LM-OSL is highly required.
5. Conclusions A component resolved bleaching study was attempted for CaF2:N . The luminescence signals subjected to de-convolution were the LM-OSL, CW-OSL and RTL decay curves after bleaching. Thus, each experimental data point of any RTL curve, corresponds to an intensity integration. The integrated intensity over a step temperature interval of ΔΤ=5 oC, namely for each temperature interval Τi+ΔΤ= Τi+5 o C of the RTL decay curves, when plotted as a function of the stimulation/bleaching time, indicated the presence of nine different groups, based on the criterion of the bleaching decay constant λ. Nevertheless, the degenerated decay constant values monitored resulted in overlapping for the values of these groups. Finally six, different components, each one yielding distinguishable decay constant value, are sufficient in order to describe efficiently the decay of the RTL glow curves of CaF2:N received after bleaching for various times. Similarly, four different LM-OSL components are required for achieving best fit for LM-OSL curves while only three, degenerated components can fit the CW-OSL signal. There is only one, unique bleaching component present in all three luminescence entities of the present study, while there are various bleaching components present to at least two of the luminescence entities which were the subject of the present study. The results of the present study indicate that each TL trap yields at least three different bleaching components, while different TL traps can indicate bleaching components with similar decay constant. According to the data of the present study, each RTL bleaching component receives electrons from at least two peaks. In this case, these components will be seen as one component and the same holds for the cases of CW-OSL signal, where the resolution is not so good, while it must be noted that CW-OSL can distinguish two or more components if the corresponding decay constants differ by at least 50%. CW-OSL removes 11
substantially the fast and medium components of the first two peaks while very smoothly either the slow components or the third and forth peak. In order to achieve depletion of the high temperature TL peak, the application of LM-OSL configuration is required. The results of the present study suggest that the traps that contribute to TL and OSL are the same. References Adamiec, G., Bluszcz, A., Bailey, R.M., Garcia-Talavera, M., 2006. Finding model parameters: Genetic algorithms and the numerical modelling of quartz luminescence. Radiation Measurements 41 (2006) 897 – 902. Afouxenidis, D., Polymeris, G.S., Tsirliganis, N.C., Kitis, G., 2012. Computerised curve de-convolution of TL/OSL curves using a popular spreadsheet program. Radiat.Prot.Dosim. 149(4), 363–370. 1.1.Balogun. F. A., Ojo. J. O., Ogundare. F. O., Fasasi. M. K., Hussein. L. A., 1999. TL response of a natural fluorite. Radiation Measurements 47 (2012) 182-184. 1.2.Bailey, R.M., 2001. Towards a general kinetic model for optically and thermally stimulated luminescence of quartz. Radiation Measurements 33 1745. 1.3.Bailey, R.M., Smith, B.W., Rhodes, E.J., 1997. Partial bleaching and the decay form characteristics of quartz OSL. Radiation Measurements 27 (2), 123-136. Becker, K., 1973. Solid State Dosimetry, CRC Press. Bernhardt, R., Herforth, L., 1974. Proc. 4th Int. Conf. Lumin. Dosimetry, Krakow, Poland, p. 1091.
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1.4.
Bulur. E., 1996. An alternative technique for optically stimulated luminescence (OSL) experiment. Radiation Measurements 26 (1996) 701
Chougaonkar. M. P., Bhatt. B. C., 2004. Blue light stimulated luminescence incalcium flouride, its characteristics and implications in radiation dosimetry. Radiation Protection Docimetry 112 (2004) 311-321 1.5.Dallas. G. I., Polymeris. G. S., Afouxenidis. D., Tsirliganis. N. C., Tsagas. N. F., Kitis. G., 2010. Correlation between TL and OSL signals in KMgF3:Ce3+: Bleaching study of individual glow-peaks. Radiation Measurements 45 (2010) 537-539 1.6.Ferreira Jr. F. A., Yoshimura. E. M., Umisedo. N. K., do Nascimento. R. P., 2014. Correlation of optically and thermally stimulated luminescence of natural fluorite pellets. Radiation Measurements 71 (2014) 254-257 1.7.Furetta. C., Tuyn. J. W. N., 1985. Annealing cycle effects on the TL sensitivity of CaF2:Tm (TLD-300). The International Journal of Applied Radiation and Isotopes 36 (1985) 1000-1002 Jain, M, Murray, A.S., Bøtter-Jensen, L., 2003. Characterisation of blue light stimulated luminescence components in different quartz samples: implications for dose measurement. Radiation Measurements 37(4-5): 441–449. Kharita. M. H., Stokes. R., Durrani. S. A., 1995.
TL and PTTL in natural
fluorite previously irradiated with gamma rays and heavy ions. Radiation Measurements 24 (1995) 469-472 Kitis, G., Kiyak, N.G., Polymeris, G.S., 2015. Temperature lags of luminescence measurements in a commercial luminescence reader. Nuclear Instruments and Methods in Physics Research B 359 (2015) 60–63. 1.8.Kitis. G., Kiyak. N., Polymeris. G. S., Tsirliganis. N. G., 2010. The correlation of fast OSL component with the TL peak at 325oC in quartz of various origins. Journal of Luminescence 130 (2010) 298-303
1.9.
Kitis. G., Pagonis. V., 2008. Computerized curve deconvolution analysis for LM-OSL. Radiation Measurements 43 (2008) 737-741
1.10. Kitis. G., Ros-Gomez, 1999. Thermoluminescence glow-curve deconvolution functions for mixed order of kinetics and continuous trap distribution. Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 31 (1999) 2636-2641 McKeever, S.W.S., Moscovitch, M., Townsend, P.D., 1995. Thermoluminescence Dosimetry Materials: Properties and Uses, Nuclear Technology Publishing. 13
Polymeris, G.S., Afouxenidis, D., Tsirliganis., N.C., Kitis, G., 2009. The TL and room temperature OSL properties of the glow peak at 110 oC in natural milky quartz: A case study. Radiation Measurements 44 (2009) 23–31. Polymeris, Kiyak, N.G., G.S., Kitis, G., 2008. Component resolved IR bleaching study of the blue LM-OSL signal of various quartz samples. Geochronometria 32, 79– 85. Polymeris. G. S., Kitis. G., Tsirliganis. N. C., 2006. Correlation between TL and OSL properties of CaF2:N. Nuclear Instruments And Methods in Physics:Research B 251 (2006) 133-142
1.11. Spooner. N. A, 1994. On the optical dating signal from quartz. Radiation Measurements 23 (1994) 593-600 Topaksu. M., Yazici. A. N., 2007. The thermoluminescence properties of natural CaF2. Nuclear Instruments and Methods in Physics: Research B 264 (2007) 293-301 Tugay. H., Yegingil. Z., Dogan. T., Nur. N., Yazici. N., 2009. The thermoluminescence properties of natural calcium fluoride for radiation dosimetry. Nuclear Instruments and Methods In Physics: Research B 267 (2009) 3640-3651 Table 1. Average λ parameters (in units of s-1) of bleaching decay curve components, as these were yielded from the de-convolution of the integrated signals of the residual TL glow curves. Error values (presented in parenthesis) are estimated to be within 4-7%, since this was the margin of the FOM value obtained through the de-convolution procedure. 0
λ of CRTL1 (fast)
λ of CRTL2 (medium)
λ of CRTL3 (slow)
Peak1 (60° C-120° C )
0.594660 (0.031621)
0.047751 (0.002967)
0.000257 (0.000011)
Peak2 (160 ° C-190° C)
0.420346 (0.034638)
0.021321 (0.001786)
0.000103
Temperature
C
(0.000013) Peak3 (230 ° C-320° C)
0.109844 (0.007986)
0.018624 (0.001536)
0.000011 (7.8·10-7)
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Table 2. Decay constant values (in units of s-1) for LM-OSL as well as CW-OSL components based on a component resolved analysis. Error values are estimated to be within 2%, since this was the highest value for the FOM value. For the case of CW-OSL curves, the values presented are the mean values over 4 independent measurements of different duration, with reproducibility of the order of 0.2%.
λ of CLM-OSL1
λ of CLM-OSL2
λ of CLM-OSL3
λ of CLM-OSL4
0.0526137
0.0174463
0.0050832
0.000145
λ of CCW-OSL1
λ of CCW-OSL2
λ of CCW-OSL3
λ of CCW-OSL4
0.604588
0.0491235
0.007486
-
Table 3. Integral of the four LM-OSL components and of the three CW-OSL components. The third column shows the % participation of each component to the total LM-OSL and CW-OSL signal respectively.
LM-OSL Ci
Integral (counts)
%
CW-OSL Ci
Integral (counts)
%
C LM-OSL1
42319
11
C CW-OSL1
82442
28
C LM-OSL2
246093
62
C CW-OSL2
167878
56
C LM-OSL3
61805
16
C CW-OSL3
48809
16
C LM-OSL4
44722
11
C CW-OSL4
0
0
Fig. 1. Residual TL (RTL) glow curves of CaF2:N for various stimulation times. Traps responsible for all TL peaks are shown to contribute to the OSL signal. Fig. 2. Integrated RTL intensity over intervals of ΔT=5 °C, as a function of CW-OSL stimulation times. Four different temperature intervals, namely 95-100 °C, 185-190 °C, 245-250 °C and 295-300 °C, are presented clockwise. Experimental data are presented as rhombus, while the three components (fast, medium and slow) obtained from the curve fitting as well as the corresponding sum, are presented as continuous lines.
15
Fig. 3. Experimental points indicate the values of decay constants of components CRTLi with the corresponding errors of ±1σ. The values of the errors were estimated by using the corresponding F.O.M values (ranging within 4-7% for the λ value of each CRTLi) multiplied by the values of the components. Overlapping of bleaching components among the three temperature peak regions is obvious, at least for three cases, namely the fast components (CRTL1) of both first and second TL peaks, the medium components (CRTL2) of both second and third TL peaks as well as slow components (CRTL3) of first and second TL peaks. Fig. 4. The dependence of the order of kinetic parameter b versus temperature for all bleaching components CRTLi (fast, medium and slow) of the integrated RTL intensity over the temperature integrals betweenΤi and Τi+ΔΤ. Fig. 5. LM-OSL curve de-convolution analysis. Experimental data are presented as filled dots, while components CLM-OSL1, CLM-OSL2, CLM-OSL3 and CLM-OSL4 obtained from the curve fitting as well as the sum of all theoretical curves are presented as continuous lines. Fig. 6. CW-OSL curve de-convolution analysis. Experimental data are presented as filled dots, while components CCW-OSL1, CCW-OSL2, and CCW-OSL3 obtained from the curve fitting as well as the sum of all theoretical curves are presented as continuous lines.
0s - unbleached sample
Residual TL (a.u)
10000
1000
2000s
100 100
200
300
400
500
o Temperature ( C)
o T=185 C
1000
RTL Intensity (a.u)
RTL Intensity (a.u)
T = 95°C C1 C2
100
100
C3 1
C1 1000
10
100
StimulationTime (s)
1000
C2
C3
1
10
100
1000
StimulationTime (s)
16
10000
C1
10000
RTL Intensity (a.u)
RTL Intensity (a.u)
T = 295oC
C2
C1
1000
C3
1000 1
C2
C3
100
10
100
1000
1
Stimulation Time (s)
10
100
1000
StimulationTime (s)
fast - CRTL1
0.1 0.01
medium - CRTL2
-1
(s )
0
T= 250 C
1E-3 1E-4 slow - CRTL3
1E-5 50
100
150 200 250 Temperature (°C)
300
350
17
2,2
RTL decay order of kinetic
2,0
Fast
1,8
1,6
Medium
1,4
1,2
1,0
Slow
50
100
150
200
250
300
350
o Temperature ( C)
1600
LM-OSL Intensity (a.u)
1400 1200
CLM-OSL1
1000 800 600
CLM-OSL2
400 200
CLM-OSL3 0
200
CLM-OSL4 400
600
800
1000
CW-OSL Intensity (a.u)
Stimulation Time (s)
CCW-OSL1
10000
CCW-OSL2 1000
CCW-OSL3
1
100 0.1
1
10
100
1000
StimulationTime (s)
18
Highlights
A component resolved bleaching study was attempted to CaF2:N in terms of CW-OSL, LM-OSL and RTL. Bleaching decay constants originating from different TL peaks yield overlapping values. Three to five individual components were used in order to describe the bleaching behavior in all luminescence entities. There is only one, unique bleaching component present in all three luminescence entities.
19