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Thermoluminescence analysis of beta irradiated ZnB2O4: Pr3+ phosphors synthesized by a wet-chemical method
Radiation Physics and Chemistry 160 (2019) 105–111
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Thermoluminescence analysis of beta irradiated ZnB2O4: Pr3+ phosphors synthesized by a wet-chemical method
T
S. Akcaa, M. Oglakcia, Z.G. Portakala, N. Kucukb, M. Bakrc, M. Topaksua, N. Canc,d,∗ a
Physics Department, Arts-Sciences Faculty, Cukurova University, 01330, Adana, Turkey Department of Physics, Faculty of Arts and Sciences, Bursa Uludag University, Gorukle Campus, 16059, Bursa, Turkey c Physics Department, Jazan University, P.O. Box 114, 45142, Jazan, Saudi Arabia d Department of Physics, Faculty of Arts and Sciences, Manisa Celal Bayar University, Muradiye-Manisa, Turkey b
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnB2O4 Rare earths Thermoluminescence Glow curve Trap parameter
In this paper we describe the thermoluminescence (TL) characteristics of ZnB2O4:Pr3+ phosphors with Pr concentrations varying between 0.2 and 1 mol% prepared by a wet-chemical method. The TL glow curves of the phosphor sample consisted of three peaks located at 71 °C (P1), 124 °C (P2) and 233 °C (P3). The TL intensity increased with the beta dose ranging from ∼0.1 to ∼20 Gy. Dose response, reproducibility and trapping parameters of TL glow curves were evaluated to clearly reveal thermoluminescence features. We observed that TL intensity of P2 and P3 peaks decreases as the heating rate increases. Trap parameters were estimated via the Hoogenstraaten, Booth-Bohun-Parfianovitch, the initial rise methods combined with Tm − Tstop experiment and TLanal CGCD program. Heating rates were varied to use in the Hoogenstraaten analyses. The Tm − Tstop investigations on regenerated TL signals for P1 and P2 peaks indicated that ZnB2O4:Pr3+ phosphor has four electron trap levels with energy values in the range of 0.5–1.5 eV These four traps have first order kinetic and are formed at high temperature region. Resulting values are utilized as a reference for the CGCD procedure and the trapping parameters from the TL glow curves are calculated. The figure of merit (FOM) of the TL glow curve during curve fitting procedures is found to be 2.019%. The intensities of the main dosimetric peaks appeared at 124 °C and 233 °C exhibits good linear dose response up to 20 Gy. These results provide valuable knowledge for use of the characteristics of Pr doped ZnB2O4 in dosimetry research, just need to eliminate low temperature TL peak.
1. Introduction
etc. to improve their TL and optically stimulated luminescent (OSL) performances because adding of dopants in host material changes the number and position of electron traps and/or trap centers. Besides, to develop the TL and OSL properties of borates further, several material preparation methods like solid state reaction method, the melting method, combustion method, sol-gel method and nitric acid method etc. have been used and also new ones have been studied for the years. Recently, lanthanide-doped ZnB2O4 has been drawn attention for phosphor luminescence applications. Li et al. investigated some luminescence properties and dosimetric characteristic of Tb3+ doped and Dy3+ doped ZnB2O4 and, the experimental results showed that the mentioned phosphors could be used as potential materials for gammaray TL dosimeter (TLD) for clinical dosimetry (Li et al., 2007, 2008). A number of studies have examined Tb doped borates samples. G. Cedillo Del Rosario et al. reported TL characteristics of ZnB2O4 host incorporated with different concentrations of terbium (Cedillo Del Rosario et al., 2017). They investigated the potentialities of the
Borate compounds like lithium tetra-borate (Li2B4O7), magnesium borate (MgB4O7), barium borate (BaB3O4), zinc borate (ZnB2O4), cadmium tetraborate (CdB4O7) have found a place in many areas (i.e nonlinear optics and radiation dosimeters) due to their wide electronic band gap, thermal durability, chemical stability, good conductivity, easy preparation and relatively low cost (Yu et al., 2002; Isao and Akihiro, 2013; Annalakshmi et al., 2014; Saidu et al., 2015). High TL intensity, chemical stability, soft tissue equivalent (Zeff = 7.4) and its good performance of borate materials for various ionizing radiations makes tetraborates an interesting phosphor to be considered in the development of a Thermoluminescence (TL) dosimeter. An impressive study to enhance the sensitivity using DyO2 with a borate dosimeter was first reported by (Kazanskaye et al., 1974). Borate compounds used for TL dosimetry are generally doped with various divalent or trivalent lanthanides such as Nd3+, Sm3+, Eu2+,3+, Gd3+, Tb3+, Dy3+, Tm3+ ∗
Corresponding author. Department of Physics, Faculty of Arts and Sciences, Manisa Celal Bayar University, Muradiye-Manisa, Turkey. E-mail address: [email protected] (N. Can).
https://doi.org/10.1016/j.radphyschem.2019.03.033 Received 12 January 2019; Received in revised form 22 March 2019; Accepted 24 March 2019 Available online 27 March 2019 0969-806X/ © 2019 Elsevier Ltd. All rights reserved.
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ZnB2O4:Tb3+ for the application in radiation dosimetry at low doses. In 2017, Cruz-Zaragoza et al. investigated TL and OSL properties of the traps in the Tb activated phosphor sample. They suggested that the TL traps are in relation to those of OSL property of this sample. Zheng et al. studied the luminescence properties of Eu3+ doped ZnB2O4 nanoparticles and it was observed that the dopant Eu3+ increased the luminescence intensity when it was added to the system of zinc borate (Zheng et al., 2009). Mu et al. expressed that Gd3+ doped ZnB2O4 phosphors synthesized by solid state reaction method could be a candidate material for luminescence display applications (Mu et al., 2014). Kucuk et al. studied on luminescence properties of ZnB2O4:Ce and La under x-ray and electron excitation and it was seen that Ce doped ZnB2O4 has higher luminescence intensity rather than La doped ZnB2O4 (Küçük et al., 2017). The dosimetric character of a phosphor is basically determined by its energy response, dose-sensitivity, reusability, fading and kinetic parameters including order of kinetics b , activation energy E and frequency factor s . Various methods are used to extract the kinetic parameters from the TL glow curve (Chen and McKeever, 1997; Topaksu and Yazici, 2007; Gökçe et al., 2009; Chen and Pagonis, 2011). One of these methods is various heating rates method suggested and developed by many researchers. One knows that the increasing heating rate β increases the maximum peak temperature Tm , which is also seen in the expression over the maximum of TL glow peak of Randall and Wilkins (1945), namely the peak maximum shifts to higher temperature with the increasing heating rate. Booth (1954), Bohun (1954) and Parfianovitch (1954) independently put forward to obtain the trapping parameters E and s by taking advantage of the shift. Using two different heating rates β 1 and β 2 versus two maximum temperatures Tm1 and Tm2 , respectively, the expression of Randall and Wilkins was re-written for both heating rates and divided one by the other. Thus, the value E obtained from the dividing procedure provides to get the value s. Then, Hoogenstraaten (1958) also suggested the use of various heating rates to attain the values of E and s . Using a plot of ln ( Tm 2/ β ) as a function of 1/ kTm , which gives a straight line, the value E from slope of the straight line and the value s from intercept equalling to the value of ln (E / sk ) are obtained by variable heating rate method. The other methods used for determination of kinetic parameters can be listed as Initial rise (IR), Peak shape (PS), Computerized glow curve deconvolution (CGCD) etc. methods. IR method can be carried out only in the initial region of the TL signal up to approximately %15 of its peak maximum (Im) (Furetta and Weng, 1998). PS method based on measurements of a number of points on the glow peak is a convenient method for calculating the trapping parameters of distinct energy levels (Chen, 1969). CGCD method/software works for the deconvolution of the TL glow curves (Chung et al., 2007). This work has motivation in the potentialities of the borate phosphors for investigating some TL characteristics ZnB2O4 incorporated with Pr3+ was synthesized via a wet chemical method. The TL glow curves of the prepared phosphor were collected after beta exposure. The TL glow curves with the first-order kinetics of the phosphors were recorded using different Pr3+ concentrations, doses, and heating rates. The kinetic parameters of the experimental peaks in the TL glow curves were analysed via various TL analysis methods for the optimized glow curves as well as deconvoluted glow curve via TLAnal CGCD program.
weighed pure powder substances were put into a glass beaker of 400 ml volume and mixed while heating at 80 °C, in 80 ml HNO3 standard solution using a magnetic stirrer. Mixing process was continued until a dry substance was obtained. This dried substance was made into a fine powder using an agate mortar and pestle. The powder substance was then placed in an alumina crucible. The alumina crucible was transferred to a tube oven preheated to 450 °C, and the powder substance was calcined at this temperature for 5 h. The powder substance was once again ground with an agate mortar and pelleted under a pressure of 3 tons/cm2. The pellets were then transferred to a tube oven in an alumina crucible and annealed at 900 °C for 2 h, and the pellets were cooled to room temperature (RT). Finally, the cooled pellets were ground in an agate mortar and placed in Eppendorf tubes before being analysed and measured. TL experiments were performed with an automated Lexsyg Smart TL/OSL reader system under nitrogen flow at Çukurova University in Adana (Bulcar et al., 2018). Irradiation of the system was carried out with an internal 90Sr/90Y beta source with a dose rate of 0.11 Gys−1. For the TL measurements, thin pellet samples of 25 mg with the chemical composition formula ZnB2O4:xPr3+ (x = 0.2%, 0.5%, 0.8%, 1%) were used and the sample doped with 0.2% concentration of Pr was preferred for all TL measurements due to the highest TL intensity. The pellets which have height of 0.70 mm and diameter of 6.00 mm were pressed under 2 tons/cm2 for 20 min. Prior to each TL experiment, a thermal cleaning of the traps was conducted by heating from RT to 450 °C. TL glow curves were collected using a constant heating rate of 2 °Cs−1 from RT to 450 °C except heating rate experiment and a 5 Gy beta dose except dose-response experiment. TL-response with different filters, TL-response against different dopant amounts, TL-response against various heating rates ranging between 0.5 and 5 °Cs−1, TL-response and linearity against doses ranging between 0.1 and 20 Gy, reusability properties and kinetic parameters were investigated to determine characteristics of TL of aforementioned ZnB2O4:Pr3+ samples. 3. Results and discussion 3.1. XRD analysis Zn2B2O4 samples are prepared using a wet chemical route and Fig. 1 displays the XRD patterns of Zn2B2O4:Pr with different doping concentration. The results indicate that all diffraction pattern is good agreement with body centred cubic structure phase of pure Zn2B2O4 (JCPDS 039-1126) which indicate the samples have been successfully synthesized using a wet chemical route. The strongest diffraction peak at 29.24° can be the best probe. Peak intensities of some diffraction peaks slightly vary with doping concentration. This indicates that ZnB2O4 phosphors with different composition exhibit different crystallinity. This finding is in agreement with Cedillo Del Rosario's (Cedillo Del Rosario et al., 2017). On the other hand, the activator (Pr3+) does not affect the main phase structure of ZnB2O4. Therefore XRD patterns shown in Fig. 1 reveal that all prepared ZnB2O4:Pr phosphors were almost pure phase. 3.2. Thermoluminescence Pr incorporated ZnB2O4 phosphors were exposed to different beta doses and the TL glow curves of irradiated phosphors were also collected at different heating rates in order to determine the trap levels. Thus, the effects of heating rate and beta dose on TL glow curves of the phosphor samples as well as on kinetic parameters were studied in detail.
2. Experimental procedures Powder samples of ZnB2O4:Pr3+ were synthesized by the wet chemical method (Kucuk et al., 2018; Portakal et al., 2017; Dogan et al., 2017; Bulcar et al., 2018). The pure substances used in the synthesis are: zinc oxide (ZnO, 99.99% purity, Alfa Aesar), boric acid (H3BO3, 99.99% purity, Alfa Aesar), and praseodymium oxide (Pr2O3, 99.9% purity, Alfa Aesar). These pure substances were weighed out in stoi+ chiometric amounts according to the composition of Zn 0.998B2 O4 : Pr 30.002 , 3+ 3+ 3+ Zn 0.995B2 O4 : Pr 0.005 , Zn 0.992 B2 O4 : Pr 0.008 and Zn 0.990 B2 O4 : Pr 0.010 . The
3.2.1. Glow curves characteristics Knowledge of the emission spectra of the materials is crucial both to explain fundamental TL mechanisms and, from a practical viewpoint, to select the optical filters used to collect the TL curves. The best band pass 106
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with respect to Pr3+ concentration with a fixed heating rate of 2 °Cs−1 after beta exposure of 5 Gy. Strong TL signal located 124 °C has been observed. We did not take into consideration centred at 81 °C because it is low temperature glow peak. Additionally, there is another peak located at 233 °C but its intensity is smaller than P2 and therefore this glow curve has been displayed in inset figure in order to allow a clearer comparison of the glow curve features. As evident in Fig. 2, more Pr3+ ions are incorporated in ZnB2O4 host lattice with increasing dopant concentration of Pr3+ ions, and among all concentrations doped in ZnB2O4:Pr3+ phosphor the maximum intensity of TL glow curve was observed for 0.2 mol%. The TL is extremely a sensitive phenomenon associated with lattice defects which fully rely on the number of present defect states in the gap between the valence and conduction band. Therefore, the incorporating of rare earth ions in host matrix results in the creation of defect energy levels within the band gap and irradiation will also lead to an increase in the number of defects owing to energy source. The number of different energy level traps (defects) within the sample's energy band gap increases with the increasing dopant concentration (Gupta et al., 2016).
3.2.2. Dose response TL curves obtained by setting the linear heating rate at 2°Cs−1 of the TL reader as a function of β doses ranging from 0.1 to 20 Gy are shown in Fig. 3. It is apparent that the TL intensity tends to be increased with excitation dose rate. In all cases, the entire shape of the glow curves remains unchanged with an increasing dose. As it is known, the dependence of the TL responses on radiation dose gives valuable information regarding the trapping process of trapped electrons and holes. Experimental results revealed that the peak positions are not affected with the increasing of the dose amount. A first order kinetic is dominant in nature using a generalized description of thermally stimulated processes (Bos, 2017). The behavior of the first order glow curve is different than that of a second order glow curve. In latter case, the peak positions slightly shift toward lower temperatures (Chen and McKeever, 1997). We mainly focused on P2 and P3 because P1 is low temperature glow peak and can be eliminated somehow but we did not concern that peak in this study. The dose dependence of the TL glow curves (i.e P2 and P3) of Pr doped of ZnB2O4 are shown in Fig. 4. As presented in Fig. 4, the evolutions of the whole areas of induced two TL glow curves exhibit good linear dose response (r = 0.999) and experimental points can be fitted to an equation of the sort y = intercept + slope × x , where y is peak area under the glow curves and x is the given dose. Note that for these selected doses no saturation
Fig. 1. XRD patterns for undoped and Pr3+ doped ZnB2O4.
filter for using in future dosimetric applications of the ZnB2O4 host can be selected by testing different band pass filters (i.e. 365 nm, 410 nm, 565 nm and wideband blue) inserted between the TL sample and the photomultiplier tube and then spectra were collected for each band pass filter (Oglakci et al., 2019). Although not displayed here, the shapes of TL glow curves monitored at 410 nm and 565 nm are similar. However, when examined the intensity of peak maxima located at 81 and 141 °C, TL intensities obtained using the filter of wideband are higher than the others. Therefore, all TL measurements were performed using the filter of IRSL-TL wideband during this study. Fig. 2 compares the TL glow curves of ZnB2O4:Pr3+ synthesized
Fig. 2. TL glow curves of beta irradiated ZnB2O4:Pr3+ phosphor subjected to 5 Gy of beta particle irradiation at various doping concentrations with constant heating rate 2 °Cs−1 (inset: the glow peak located at 233 °C is magnified).
Fig. 3. TL glow curves of beta irradiated ZnB2O4:Pr3+ (0.2%) samples for different dose. Heating rate is 2 °Cs−1. 107
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recombination (Bos, 2017) (b) An appreciable reduction of the intensity can be observed as the heating rate increases. The reduction can be assigned to a thermal quenching effect which is well known phenomenon. Therefore, the probability of non-radiative transitions was increased. Similarly, there is no direct recombination taken place from thermally released trapped charge carries or luminescence centers (i.e. mostly host defects or co-dopants) to recombination center and energy transfer occurs by means of non-radiative transitions (Wintle, 1975). (c) The integrated peak areas of the glow curves were affected from heating rate. (d) Full width at half maximum (FWHM) mostly increased. 3.2.4. Reproducibility Reproducibility is another desirable key factor which points out the practical application of samples in dose measurement (Karabulut et al., 2016; Afouxenidis et al., 2012). A phosphor is being considered to be a good reproducibility and repeatability if its response to ionizing radiation remains unaffected despite several repetitions of exposures and readouts. Also, the ZnB2O4: Pr3+ (0.2%) phosphor sample was examined for its reusability. The TL glow curves of the sample were recorded from RT to 450 °C followed by exposing to β-ray of 5 Gy at RT. The sample is allowed to be cooled much faster to RT and exposed to β radiation again for accumulating the same dose. It is crucial that same cooling procedure was followed. The glow curve was recorded again and, several such exposure readout cycles and glow curve recordings were carried out. In repeated measurements, the TL response to the same dose exposure is expected to be approximately the same. The standard deviation of the experimental results was pretty good and there is no significant variation of TL responses for 10 sequential measurements (not shown here). This result reveals that the TL sensitivities are identical within less than 2% and this phosphor is reusable in radiation dose assessment.
Fig. 4. Dose dependence of the TL glow curves of ZnB2O4:Pr3+ (0.2%) in the range of ∼0.1–∼20 Gy for 2nd and 3rd peak. Heating rate is 2 °Cs−1.
was taken place.
3.2.3. Heating rate effect It is generally accepted that the position of TL glow peak and TL intensity depend on the heating. In other words, the changes in the peak shape and the shifts in the peak position are functions of the heating rate. The immediate initial effects of the heating rate ranging from 1 °Cs−1 to 5 °Cs−1 of 0.2 mol% Pr3+ doped ZnB2O4 phosphor sample exposed to 5 Gy beta dose are seen in Fig. 5 for TL measurements. Above 5 °Cs−1 temperature lag effects are very possible, which will give rise to very lower values of activation energy. We believe that for heating rates 1-5 °Cs−1 are enough otherwise we need take into consideration the temperature lag effects. The effects of several heating rate were observed as follows: (a) the intriguing feature of these analyses was that the temperature at maximum TL intensity at which the TL glow peak occurred shifted to higher temperatures as the heating rate increased. This phenomenon can be stated as follows: Two assumptions need to be made for forming a TL glow peak: (i) an increase in the probability of escape and so a faster recombination rate which causes an increase in the number of photons emitted and (ii) a decrease in number of charge pairs released from a storage trap available for
3.3. Methods for extracting trap parameters Many expressions were derived to evaluate the activation energy from the shape of the TL peak. Some important methods taking into consideration the change in the position of the glow peak with the changing heating rates for obtaining the activation energy E (eV) belong to Booth (1954), Bohun (1954) and Parfianovitch (1954) and Hoogenstraaten (Hoogenstraaten, 1958). 3.3.1. Tm − Tstop experiment and initial rise (IR) method Broadly speaking, the nature of experimentally observed glow peaks may be assigned to several overlapping glow peaks having a trap distribution depth close to each other. Computerized glow curve deconvolution (CGCD) method based upon a number of different models existing in the theory of TL processes are most widely utilized in order to resolve the glow peaks. Estimating the order of kinetics for the various peaks in the TL glow curve can be executed via the Tm − Tstop experiment. The Tm − Tstop procedure verified that the position of the glow peaks shifts toward higher temperatures as the Tstop increases. These results obviously revealed that the glow peaks of the phosphor sample are a superposition of a number of first order glow peaks. The procedure also allows us to separate overlapping of the peaks and calculate the value Tm for each of them. The flat regions in the Tm − Tstop plot show the peak positions in the complex glow curve. Pr activated ZnB2O4 phosphor irradiated with a beta dose of 5 Gy is heated up to a Tstop at a linear heating rate. The phosphor sample is rapidly cooled down to RT and then the aliquot is re-heated all the way to a high temperature of 450 °C with a heating rate of 2 °Cs−1 and the position of the glow peak temperature Tm is recorded. This process is repeated several times on the same irradiated aliquot at a slightly higher temperature Tstop each time in the steps of 10 °C for the complete interval Tstop = 50–280 °C. The glow curves after the Tm − Tstop procedure are illustrated in Fig. 6. A plot of Tm as a function of Tstop displays a stepwise curve after completing Tm − Tstop procedure. This process may help us
Fig. 5. TL intensity versus temperature for ZnB2O4:Pr3+ (0.2%) recorded with various heating rates ranging between 1 and 5 °Cs−1 following constant irradiation dose of 5 Gy (β). 108
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3.3.2. Computerized glow curve deconvolution (CGCD) method CGCD is very popular method for studying the complex TL mechanism which includes multi-peak glow curves. Deconvolution also ensures the advantage in providing a simultaneous evaluation of peak parameters without the need of any further thermal annealing. Additionally, the optimization of some fundamental characteristics of a TL system can be done using this method. It is worth noting that various models, approximation and minimization procedures are taken into consideration for the glow curves analysis via the CGCD programs. The following equation of the first order kinetics of the TL developed at Reactor Institute at Delf (Gartia, 1989) were used to evaluate the glow curves
E s kT 2 E I (T ) = n 0 s exp ⎛− a ⎞ exp ⎡ exp ⎛− a ⎞ ∗ ⎛0.9920 ⎢ β E kT ⎝ kT ⎠ ⎝ ⎠ a ⎝ ⎣ ⎜
− 1.6220
kT ⎞ ⎤ Ea ⎠ ⎥ ⎦ ⎟
where n0 is the concentration of trapped electrons at t = 0, s is the frequency factor, E the trap depth, T absolute temperature, k Boltzmann's constant, β heating rate and b the kinetic order. Under these conditions, there is a doubt as to acceptability of kinetic parameters established via CGCD program. Figure of merit (FOM) or a simple ‘’chi-square’’ parameter that indicates the goodness of the fitting should be determined. FOM has become acceptable by the community of CGCD workers. The function FOM is calculated by
Fig. 6. TL glow curves recorded for Tm-Tstop analysis of ZnB2O4:Pr3+ (0.2%). Preheat temperatures are applied ranging from 50 °C to 280 °C with 5 °C increase at each step, making total 47 TL glow curves. Results are recorded for constant dose of 5 Gy (β).
FOM =
∑ (TLexp − TLfit )/∑ TLfit
× 100
where TLexp is the measured values of intensity at various T in experimental data and TLfit is the corresponding best fitted values. Quality of deconvolution and choice of an appropriate number of peaks are characterized by repeating the procedure by several times to obtain an optimal fit result (i.e minimum FOM) with minimum number of possible peaks. In other words, we can reach much better fit as the number of peaks is increased but the acceptability of such extra peak/peaks will be definitely doubtful and will not reflect any physical meaning. In the present case, we have tried to establish the minimum number of peaks needed to get an acceptable fit which is determined by the FOM. Fig. 8 shows typical set of TL glow curve analysis of ZnB2O4:0.02Pr3+ recorded with a linear heating rate (β = 2 Cs−1) using TLAnal CGCD program. Assuming the presence of six peaks, a much better accordance fitted theoretically with experimental glow curves were obtained. The
Fig. 7. Activation energy versus preheat temperature graph of ZnB2O4:Pr3+ (0.2%) obtained through Tm-Tstop analysis. Note the six peaks, each corresponding to a different class of trap.
to prognosticate the number of the peaks and each plateau region shown in Fig. 7 represents the approximate position of an individual peak. We suggested that five single first order components have a contribution in the entire measured TL glow curve. There are two main parameters (i.e. trap depths and frequency factor) in a TL glow curve. The first is clearly critical as an exponential term, but frequency factor is also another subtle dependence that varies between each class of trap. The initial rise method was carried out and a possible range of the activation energy (E) and frequency factors (s) for each preheat temperature were evaluated for analysed TL glow curves. Application of this method depicted the presence of a continuous distribution of trap depths for the sample preheated up to 280 °C, with energies between 0.37 ± 0.07 eV and 1.54 ± 0.02 eV. Whilst a trap depth identifies a TL feature, it does not provide any details of the structure of the TL site. Therefore, we suggest a site modelling below although it is very speculative. Frequency factor also changes from 1.97 × 10−10 (s−1) to 7.58 × 10−19 (s−1).
Fig. 8. An exemplarily result of deconvolution performed for ZnB2O4:Pr3+ (0.2%) recorded after 5 Gy (β) irradiation using TLAnal CGCD program. P1, P2 and P3 peaks correspond to measured data (the circle line). Note that the sample has been confirmed to behave as a first order trap. 109
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where TM1 is glow peak temperature, TM2 is the temperature of the lower side of the glow curve for β1 and β2 , respectively. The values of the temperatures for each pairs of various heating rates are evaluated from the shape of the glow curves in ZnB2O4:0.2Pr3+ shown in Fig. 5. It was seen that average trap depths of P2 and P3 changes between 0.38 ± 0.02 eV - 0.81 ± 0.02 eV and 0.55 ± 0.03 eV–1.15 ± 0.03 eV respectively once the above equation is solved. The final average trap depths from each average trap depth is found to be 0.55 + 0.02 eV and 0.88 + 0.02 eV.
Table 1 Calculated activation energy (Ea) and frequency factor (s) of 2nd and 3rd peaks of ZnB2O4:Pr (0.2%) with Hoogenstraaten's Equation from Fig. 8.
2nd Peak 3rd Peak
T (°C)
Ea (eV)
s (s−1)
ln(s)
124 233
0.52 ± 0.02 0.84 ± 0.03
3.9243E+05 1.6300E+07
12.88 16.61
obtained best-fit parameters based on the value of the FOM were satisfactory in this study. The FOM value for 5 Gy β irradiated ZnB2O4:Pr3+ phosphor was found to be 2.019%. The FOM values must be less than 5% according to consensus regarding acceptable fit. It is worth noting that the deconvolution of the glow curves shows six peaks that are not necessarily corresponding to those finding by Tm − Tstop (see Fig. 7) using this partial bleaching method due to their overlapped glow peaks. Finally, in this case, the Tm value on the stepwise (see Fig. 7) is only an average value of the maximum temperature of these peaks and not the precise value of the peak obtained by deconvolution method. In this study, a visual model has been also suggested to elucidate the TL response of beta irradiated ZnB2O4 sample. 4 (four) TL glow peaks obtained applying the deconvolution technique to the glow curve with the aid of an efficient computer program TLanal can be interpreted suggesting a model. It is worth noting that there are many factors influencing the interpretation and analysis of experimentally measured the TL glow curves and therefore it is not straight forward. The traps of phosphors are formed by chemical impurities or crystal lattice defects in phosphor compound. The substitution of the Pr3+ ions as dopants into ZnB2O4 host material introduces a charge imbalance, leading to charge compensation. A forbidden band composed of valance band and the conduction band involves two types of traps in this proposed model; namely shallow and deep traps. 4 (four) electron traps assigned to P2 and P3 in Fig. 8 are presumably created by the presence of the structural defects present in the ZnB2O4 compound. The release of electrons liberated from the traps after irradiation may recombine with the holes at the luminescence center causing TL. The existence of competition among these trap levels might exhibit different probability of either being releasing or retrapping probabilities.
3.4. Conclusions In summary, ZnB2O4 phosphors doped with 0.2–1.0 %Pr were successfully prepared by a wet chemical reaction process. The X-ray patterns of the undoped and Pr doped phosphors exhibited the cubic (bcc) phase in this material. The Tm − Tstop analysis of collected data was utilized to estimate the number of peaks that make up the whole TL curve and characteristic trapping centers related with these peaks. The glow curves for P2 and P3 were well deconvolved by four peaks. Expected range of activation energies of distributed four trap centers were determined between 0.37 ± 0.07 eV and 1.54 ± 0.02 eV as a function of Tstop in range of 50–280 °C. Using the deconvolution method, it was revealed that the TL response of ZnB2O4:Pr3+ phosphor sample does follow first order kinetics. All the applied methods for evaluation of the activation energy are comparable. Trap depths for P2 and P3 via Hoogenstraaten's Equation are found to be 0.52 ± 0.03 eV and 0.84 ± 0.03 eV, respectively. It is worth noting that the relatively small variations in E are inevitably reflected to a huge difference of s due to the exponential function. We evaluated reproducibility of the sample via 10 replicate TL measurements and suggested that the prepared samples have good reproducibility for beta exposure. In high dopant situations lattice strains and charge compensation features will cause lattice distortions that favor pairing or clustering of the dopants and associated intrinsic defects. Therefore, trapping and recombination sites may be in intimate proximity. Hence there is no unique activation energy for the TL process. Acknowledgments This work was supported by the Research Fund of the Çukurova University, Turkey (Project Number: FAY-2015-4735).
3.3.3. Methods of Hoogenstraaten and Booth-Bohun-Parfianovitch In principle, Hoogenstraaten's method was only developed for TL glow curves having first-order kinetics but some researchers suggested that the method was also applicable for non -first order cases and provides a good approximation for evaluating activation energy E (eV ) (McKeever, 1985; Furetta, 2003). The following equation at several heating rate was used in order to get a linear relation between ln (Tm2 / β ) and (1/ Tm) :
References Afouxenidis, D., Polymeris, G.S., Tsirliganis, N.C., Kitis, G., 2012. Computerized curve deconvolution of TL/OSL curves using a popular spreadsheet program. Radiat. Prof. Dosim. 149, 363–370. Annalakshmi, O., Jose, M.T., Madhusoodanan, U., Subramanian, J., Venkatraman, B., Amarendra, G., Mandal, A.B., 2014. Thermoluminescence dosimetric characteristics of thulium dopedZnB2O4 phosphor. J. Lumin. 146, 295–301. Bohun, A., 1954. Thermoemission und photoemission von natriumchlorid, Czech. J. Phys. 4, 91–93. Booth, A.H., 1954. Calculation of electron trap depths from thermoluminescence maxima. Can. J. Chem. 32, 214–215. Bos, Adrie J.J., 2017. Thermoluminescence as a research tool to investigate luminescence mechanisms. Materials 10, 1–22. Bulcar, K., Dogan, T., Akca, S., Yüksel, M., Ayvacikli, M., Karabulut, Y., Kucuk, N., Canimoglu, A., Can, N., Topaksu, M., 2018. Thermoluminescence behavior of Sm3+ activated ZnB2O4 phosphors synthesized using low temperature chemical synthesis method. Nucl. Instrum. Methods Phys. Res., Sect. B 428, 65–71. Cedillo Del Rosario, G., Cruz-Zaragoza, E., García Hipólito, M., Marcazzó, J., Hernández, J.M.A., Murrieta, H.S., 2017. Synthesis and stimulated luminescence properties of Zn (BO2)2:Tb3+. Appl. Radiat. Isot. 127, 103–108. Chen, R., 1969. On the calculation of activation energies and frequency factors from glow curves. J. Appl. Phys. 40, 570–585. Chen, R.S., McKeever, W.S., 1997. Theory of Thermoluminescence and Related Phenomena. World Scientific, Singapore City. Chen, R., Pagonis, V., 2011. Thermally and Optically Stimulated Luminescence. John Wiley & Sons Ltd. Chung, K.S., Choe, H.S., Lee, J.I., Kim, J.L., 2007. A new method for the numerical analysis of thermoluminescence glow curve. Radiat. Meas. 42, 731–734. Cruz-Zaragoza, E., Cedillo del Rosario, G., García Hipólito, M., Marcazzó, J., Hernández,
T2 E E 1 ln ⎛⎜ m ⎞⎟ = ⎛ ⎞ ⎛ ⎞ + ln ⎛ ⎞ β k T ks ⎠ ⎝ ⎠ ⎝ m ⎝ ⎠ ⎝ ⎠ ⎜
⎟
where Tm (K ) is the temperature corresponding to the maximum intensity Im of the glow peak, s (s−1) frequency factor, E (eV ) activation energy and β (Ks−1) the linear heating rate. The relation between ln (Tm2 / β ) versus (1/ Tm) are plotted by taking into consideration three TL glow curves for constant dose radiation (5 Gy) to which the phosphor material is exposed. The slope of the plot of ln (Tm2 / β ) versus (1/ Tm) , namely Hoogenstraaten plot, should yield the value E / k (Hoogenstraaten, 1958). Activation energy and frequency factor values for each TL glow curve were calculated using these values and are listed in Table 1. Booth-Bohun-Parfianovitch method is based on first-order kinetics and activation energy is calculated using the following equation (Booth, 1954; Bohun, 1954; Parfianovitch, 1954): 2
E=k
TM1 TM2 ⎡ β TM ⎤ ln 1 ⎜⎛ 1 ⎟⎞ TM1 − TM2 ⎢ β2 ⎝ TM2 ⎠ ⎥ ⎦ ⎣ 110
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excitation. Appl. Radiat. Isot. 127, 35–40. Li, J., Zhang, C.X., Tang, Q., Zhang, Y.L., Hao, J.Q., Su, Q., Wang, S.B., 2007. Synthesis, photoluminescence, thermoluminescence and dosimetry properties of novel phosphor Zn(BO2)2:Tb. J. Phys. Chem. Solids 68, 143–147. Li, J., Zhang, C.X., Tang, Q., Hao, J.Q., Zhang, Y.L., Su, Q., Wang, S.B., 2008. Photoluminescence and thermoluminescence properties of dysprosium doped zinc metaborate phosphors. J. Rare Earths 26, 203–206. McKeever, S.W.S., 1985. Thermoluminescence of Solids. Cambridge University Press. Mu, Z., Hu, Y., Chen, L., Wang, X., Chen, R., Wang, T., Fu, Y., Xu, J., 2014. Synthesis of Bi3+ and Gd3+ doped ZnB2O4 for evaluation as potential materials in luminescent display applications. Displays 35, 147–151. Oglakci, M., Akça, S., Halefoglu, Y.Z., Dogan, T., Ayvacikli, M., Karabulut, Y., Topaksu, M., Can, N., 2019. Characterization and thermoluminescence behavior of beta irradiated NaBaBO3 phosphor synthesized by combustion method. Ceram. Int. 45, 7011–7017. Parfianovitch, I.A., 1954. The determination of the depth of electron traps in crystal phosphors. J. Exp. Theor. Phys. SSSR 26, 696. Portakal, Z.G., Dogan, T., Balci Yegen, S., Küçük, N., Ayvacikli, M., Garcia Guinea, J., Canimoglu, A., Karabulut, Y., Topaksu, M., Can, N., 2017. Luminescence characteristics of Dy3+ incorporated zinc borate powders. J. Lumin. 188, 409–417. Randall, J.T., Wilkins, M.H.F., 1945. Phosphorescence and electron traps. I. The study of trap distributions. Proc. Roy. Soc. Lond. 184 347–365, 366–389, 390–407. Saidu, A., Wagiran, H., Saeed, M.A., Alajerami, Y.S.M., 2015. Thermoluminescence characteristics of zinc lithium borate glass activated with Cu+ (ZnO–Li2O–B2O3:Cu+) for radiation Dosimetry. J. Radioanal. Nucl. Chem. 304, 627–632. Topaksu, M., Yazici, A.N., 2007. The thermoluminescence properties of natural CaF2 after β-irradiation Nucl. Instrum. Methods Phys. Res., Sect. B 264, 293–301. Wintle, A.G., 1975. Thermoluminescence quenching of thermoluminescence in quartz. Geophys. J. R. Astron. Soc. 41, 107. Yu, Z.T., Shi, Z., Chen, W., Jiang, Y.S., Yuan, H.M., Chen, J.S., 2002. Synthesis and X-ray crystal structures of two new alkaline-earth metal borates: SrBO2(OH) and Ba3B6O9(OH)6. J. Chem. Soc., Dalton 9, 2031–2035. Zheng, Y.H., Qu, Y.N., Tian, Y.M., Rong, C.G., Wang, Z.C., Li, S.L., Chen, X., Ma, Y.J., 2009. Effect of Eu3+-doped on the luminescence properties of zinc borate nanoparticles. Colloids Surf., A 349, 19–22.
J.M., Camarillo, E., Murrieta, H., 2017. Radio-Optically- and thermally stimulated luminescence of Zn(BO2)2:Tb3+ exposed to ionizing radiation. J. Nucl. Phys. Mater. Sci. Radiat. Appl. 5 (1), 169–178. Dogan, T., Yüksel, M., Akça, S., Portakal, Z.G., Balci-Yegen, S., Kucuk, N., Topaksu, M., 2017. Normal and anomalous heating rate effects on thermoluminescence of Cedoped ZnB2O4. Appl. Radiat. Isot. 128, 256–262. Furetta, C., 2003. Handbook of Thermoluminescence. World Scientific Publishing Co. Furetta, C., Weng, P.S., 1998. Operational Thermoluminescence Dosimetry. World Scientific, Singapore. Gartia, R.K., Singh, S.J., Mazumdar, P.S., 1989. Determination of the activation energy of thermally stimulated luminescence peaks obeying general-order kinetics. Phys. Status Solidi 114, 407. Gökçe, M., Oğuz, K.F., Karalı, T., Prokic, M., 2009. Influence of heating rate on thermoluminescence of Mg2SiO4 : Tb dosimeter. J. Phys. D Appl. Phys. 42, 105412. Gupta, Kumar, Karan, Kadam, R.M., Dhoble, N.S., Lochab, S.P., Singh, Vijay, Dhoble, S.J., 2016. Photoluminescence, thermoluminescence and evaluation of some parameters of Dy3+ activated Sr5(PO4)3F phosphor synthesized by sol-gel method. J. Alloy. Comp. 688, 982–993. Hoogenstraaten, W., 1958. Electron traps in zinc-sulphide phosphors. Philips Res. Rep. 13, 515–593. Isao, T., Akihiro, K., 2013. Synthesis, characterization and charge–discharge properties of layer-structure lithium zinc borate, LiZnBO3. Mater. Sci. Appl. 4, 246–249. Karabulut, Y., Canimoglu, A., Ekdal, E., Ayvacikli, M., Can, N., Karali, T., 2016. Thermoluminescence studies of Nd doped Bi4Ge3O12 crystals irradiated by UV and beta sources. Appl. Radiat. Isot. 113, 18–21. Kazanskaya, V.A., Kuzmin, V.V., Minaeva, E.E., Sokolov, A.D., 1974. Magnesium borate radiothermoluminescent detectors. In: Proc. 4th Int. Conf. Luminescence Dosimetry, 581-592, Krakow, Poland. Kucuk, N., Bulcar, K., Dogan, T., Garcia Guinea, J., Portakal, Z.G., Karabulut, Y., Ayvacikli, M., Canimoglu, A., Topaksu, M., Can, N., 2018. Doping Sm3+ into ZnB2O4 phosphors and their structural and cathodoluminescence properties. J. Alloy. Comp. 748, 245–251. Küçük, N., Ayvacikli, M., Akça, S., Yüksel, M., Garcia Guinea, J., Karabulut, Y., Canimoglu, A., Topaksu, M., Can, N., 2017. Luminescence studies of zinc borates activated with different concentrations of Ce and La under x-ray and electron
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Thermoluminescence analysis of beta irradiated ZnB2O4: Pr3+ phosphors synthesized by a wet-chemical method
T
S. Akcaa, M. Oglakcia, Z.G. Portakala, N. Kucukb, M. Bakrc, M. Topaksua, N. Canc,d,∗ a
Physics Department, Arts-Sciences Faculty, Cukurova University, 01330, Adana, Turkey Department of Physics, Faculty of Arts and Sciences, Bursa Uludag University, Gorukle Campus, 16059, Bursa, Turkey c Physics Department, Jazan University, P.O. Box 114, 45142, Jazan, Saudi Arabia d Department of Physics, Faculty of Arts and Sciences, Manisa Celal Bayar University, Muradiye-Manisa, Turkey b
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnB2O4 Rare earths Thermoluminescence Glow curve Trap parameter
In this paper we describe the thermoluminescence (TL) characteristics of ZnB2O4:Pr3+ phosphors with Pr concentrations varying between 0.2 and 1 mol% prepared by a wet-chemical method. The TL glow curves of the phosphor sample consisted of three peaks located at 71 °C (P1), 124 °C (P2) and 233 °C (P3). The TL intensity increased with the beta dose ranging from ∼0.1 to ∼20 Gy. Dose response, reproducibility and trapping parameters of TL glow curves were evaluated to clearly reveal thermoluminescence features. We observed that TL intensity of P2 and P3 peaks decreases as the heating rate increases. Trap parameters were estimated via the Hoogenstraaten, Booth-Bohun-Parfianovitch, the initial rise methods combined with Tm − Tstop experiment and TLanal CGCD program. Heating rates were varied to use in the Hoogenstraaten analyses. The Tm − Tstop investigations on regenerated TL signals for P1 and P2 peaks indicated that ZnB2O4:Pr3+ phosphor has four electron trap levels with energy values in the range of 0.5–1.5 eV These four traps have first order kinetic and are formed at high temperature region. Resulting values are utilized as a reference for the CGCD procedure and the trapping parameters from the TL glow curves are calculated. The figure of merit (FOM) of the TL glow curve during curve fitting procedures is found to be 2.019%. The intensities of the main dosimetric peaks appeared at 124 °C and 233 °C exhibits good linear dose response up to 20 Gy. These results provide valuable knowledge for use of the characteristics of Pr doped ZnB2O4 in dosimetry research, just need to eliminate low temperature TL peak.
1. Introduction
etc. to improve their TL and optically stimulated luminescent (OSL) performances because adding of dopants in host material changes the number and position of electron traps and/or trap centers. Besides, to develop the TL and OSL properties of borates further, several material preparation methods like solid state reaction method, the melting method, combustion method, sol-gel method and nitric acid method etc. have been used and also new ones have been studied for the years. Recently, lanthanide-doped ZnB2O4 has been drawn attention for phosphor luminescence applications. Li et al. investigated some luminescence properties and dosimetric characteristic of Tb3+ doped and Dy3+ doped ZnB2O4 and, the experimental results showed that the mentioned phosphors could be used as potential materials for gammaray TL dosimeter (TLD) for clinical dosimetry (Li et al., 2007, 2008). A number of studies have examined Tb doped borates samples. G. Cedillo Del Rosario et al. reported TL characteristics of ZnB2O4 host incorporated with different concentrations of terbium (Cedillo Del Rosario et al., 2017). They investigated the potentialities of the
Borate compounds like lithium tetra-borate (Li2B4O7), magnesium borate (MgB4O7), barium borate (BaB3O4), zinc borate (ZnB2O4), cadmium tetraborate (CdB4O7) have found a place in many areas (i.e nonlinear optics and radiation dosimeters) due to their wide electronic band gap, thermal durability, chemical stability, good conductivity, easy preparation and relatively low cost (Yu et al., 2002; Isao and Akihiro, 2013; Annalakshmi et al., 2014; Saidu et al., 2015). High TL intensity, chemical stability, soft tissue equivalent (Zeff = 7.4) and its good performance of borate materials for various ionizing radiations makes tetraborates an interesting phosphor to be considered in the development of a Thermoluminescence (TL) dosimeter. An impressive study to enhance the sensitivity using DyO2 with a borate dosimeter was first reported by (Kazanskaye et al., 1974). Borate compounds used for TL dosimetry are generally doped with various divalent or trivalent lanthanides such as Nd3+, Sm3+, Eu2+,3+, Gd3+, Tb3+, Dy3+, Tm3+ ∗
Corresponding author. Department of Physics, Faculty of Arts and Sciences, Manisa Celal Bayar University, Muradiye-Manisa, Turkey. E-mail address: [email protected] (N. Can).
https://doi.org/10.1016/j.radphyschem.2019.03.033 Received 12 January 2019; Received in revised form 22 March 2019; Accepted 24 March 2019 Available online 27 March 2019 0969-806X/ © 2019 Elsevier Ltd. All rights reserved.
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ZnB2O4:Tb3+ for the application in radiation dosimetry at low doses. In 2017, Cruz-Zaragoza et al. investigated TL and OSL properties of the traps in the Tb activated phosphor sample. They suggested that the TL traps are in relation to those of OSL property of this sample. Zheng et al. studied the luminescence properties of Eu3+ doped ZnB2O4 nanoparticles and it was observed that the dopant Eu3+ increased the luminescence intensity when it was added to the system of zinc borate (Zheng et al., 2009). Mu et al. expressed that Gd3+ doped ZnB2O4 phosphors synthesized by solid state reaction method could be a candidate material for luminescence display applications (Mu et al., 2014). Kucuk et al. studied on luminescence properties of ZnB2O4:Ce and La under x-ray and electron excitation and it was seen that Ce doped ZnB2O4 has higher luminescence intensity rather than La doped ZnB2O4 (Küçük et al., 2017). The dosimetric character of a phosphor is basically determined by its energy response, dose-sensitivity, reusability, fading and kinetic parameters including order of kinetics b , activation energy E and frequency factor s . Various methods are used to extract the kinetic parameters from the TL glow curve (Chen and McKeever, 1997; Topaksu and Yazici, 2007; Gökçe et al., 2009; Chen and Pagonis, 2011). One of these methods is various heating rates method suggested and developed by many researchers. One knows that the increasing heating rate β increases the maximum peak temperature Tm , which is also seen in the expression over the maximum of TL glow peak of Randall and Wilkins (1945), namely the peak maximum shifts to higher temperature with the increasing heating rate. Booth (1954), Bohun (1954) and Parfianovitch (1954) independently put forward to obtain the trapping parameters E and s by taking advantage of the shift. Using two different heating rates β 1 and β 2 versus two maximum temperatures Tm1 and Tm2 , respectively, the expression of Randall and Wilkins was re-written for both heating rates and divided one by the other. Thus, the value E obtained from the dividing procedure provides to get the value s. Then, Hoogenstraaten (1958) also suggested the use of various heating rates to attain the values of E and s . Using a plot of ln ( Tm 2/ β ) as a function of 1/ kTm , which gives a straight line, the value E from slope of the straight line and the value s from intercept equalling to the value of ln (E / sk ) are obtained by variable heating rate method. The other methods used for determination of kinetic parameters can be listed as Initial rise (IR), Peak shape (PS), Computerized glow curve deconvolution (CGCD) etc. methods. IR method can be carried out only in the initial region of the TL signal up to approximately %15 of its peak maximum (Im) (Furetta and Weng, 1998). PS method based on measurements of a number of points on the glow peak is a convenient method for calculating the trapping parameters of distinct energy levels (Chen, 1969). CGCD method/software works for the deconvolution of the TL glow curves (Chung et al., 2007). This work has motivation in the potentialities of the borate phosphors for investigating some TL characteristics ZnB2O4 incorporated with Pr3+ was synthesized via a wet chemical method. The TL glow curves of the prepared phosphor were collected after beta exposure. The TL glow curves with the first-order kinetics of the phosphors were recorded using different Pr3+ concentrations, doses, and heating rates. The kinetic parameters of the experimental peaks in the TL glow curves were analysed via various TL analysis methods for the optimized glow curves as well as deconvoluted glow curve via TLAnal CGCD program.
weighed pure powder substances were put into a glass beaker of 400 ml volume and mixed while heating at 80 °C, in 80 ml HNO3 standard solution using a magnetic stirrer. Mixing process was continued until a dry substance was obtained. This dried substance was made into a fine powder using an agate mortar and pestle. The powder substance was then placed in an alumina crucible. The alumina crucible was transferred to a tube oven preheated to 450 °C, and the powder substance was calcined at this temperature for 5 h. The powder substance was once again ground with an agate mortar and pelleted under a pressure of 3 tons/cm2. The pellets were then transferred to a tube oven in an alumina crucible and annealed at 900 °C for 2 h, and the pellets were cooled to room temperature (RT). Finally, the cooled pellets were ground in an agate mortar and placed in Eppendorf tubes before being analysed and measured. TL experiments were performed with an automated Lexsyg Smart TL/OSL reader system under nitrogen flow at Çukurova University in Adana (Bulcar et al., 2018). Irradiation of the system was carried out with an internal 90Sr/90Y beta source with a dose rate of 0.11 Gys−1. For the TL measurements, thin pellet samples of 25 mg with the chemical composition formula ZnB2O4:xPr3+ (x = 0.2%, 0.5%, 0.8%, 1%) were used and the sample doped with 0.2% concentration of Pr was preferred for all TL measurements due to the highest TL intensity. The pellets which have height of 0.70 mm and diameter of 6.00 mm were pressed under 2 tons/cm2 for 20 min. Prior to each TL experiment, a thermal cleaning of the traps was conducted by heating from RT to 450 °C. TL glow curves were collected using a constant heating rate of 2 °Cs−1 from RT to 450 °C except heating rate experiment and a 5 Gy beta dose except dose-response experiment. TL-response with different filters, TL-response against different dopant amounts, TL-response against various heating rates ranging between 0.5 and 5 °Cs−1, TL-response and linearity against doses ranging between 0.1 and 20 Gy, reusability properties and kinetic parameters were investigated to determine characteristics of TL of aforementioned ZnB2O4:Pr3+ samples. 3. Results and discussion 3.1. XRD analysis Zn2B2O4 samples are prepared using a wet chemical route and Fig. 1 displays the XRD patterns of Zn2B2O4:Pr with different doping concentration. The results indicate that all diffraction pattern is good agreement with body centred cubic structure phase of pure Zn2B2O4 (JCPDS 039-1126) which indicate the samples have been successfully synthesized using a wet chemical route. The strongest diffraction peak at 29.24° can be the best probe. Peak intensities of some diffraction peaks slightly vary with doping concentration. This indicates that ZnB2O4 phosphors with different composition exhibit different crystallinity. This finding is in agreement with Cedillo Del Rosario's (Cedillo Del Rosario et al., 2017). On the other hand, the activator (Pr3+) does not affect the main phase structure of ZnB2O4. Therefore XRD patterns shown in Fig. 1 reveal that all prepared ZnB2O4:Pr phosphors were almost pure phase. 3.2. Thermoluminescence Pr incorporated ZnB2O4 phosphors were exposed to different beta doses and the TL glow curves of irradiated phosphors were also collected at different heating rates in order to determine the trap levels. Thus, the effects of heating rate and beta dose on TL glow curves of the phosphor samples as well as on kinetic parameters were studied in detail.
2. Experimental procedures Powder samples of ZnB2O4:Pr3+ were synthesized by the wet chemical method (Kucuk et al., 2018; Portakal et al., 2017; Dogan et al., 2017; Bulcar et al., 2018). The pure substances used in the synthesis are: zinc oxide (ZnO, 99.99% purity, Alfa Aesar), boric acid (H3BO3, 99.99% purity, Alfa Aesar), and praseodymium oxide (Pr2O3, 99.9% purity, Alfa Aesar). These pure substances were weighed out in stoi+ chiometric amounts according to the composition of Zn 0.998B2 O4 : Pr 30.002 , 3+ 3+ 3+ Zn 0.995B2 O4 : Pr 0.005 , Zn 0.992 B2 O4 : Pr 0.008 and Zn 0.990 B2 O4 : Pr 0.010 . The
3.2.1. Glow curves characteristics Knowledge of the emission spectra of the materials is crucial both to explain fundamental TL mechanisms and, from a practical viewpoint, to select the optical filters used to collect the TL curves. The best band pass 106
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with respect to Pr3+ concentration with a fixed heating rate of 2 °Cs−1 after beta exposure of 5 Gy. Strong TL signal located 124 °C has been observed. We did not take into consideration centred at 81 °C because it is low temperature glow peak. Additionally, there is another peak located at 233 °C but its intensity is smaller than P2 and therefore this glow curve has been displayed in inset figure in order to allow a clearer comparison of the glow curve features. As evident in Fig. 2, more Pr3+ ions are incorporated in ZnB2O4 host lattice with increasing dopant concentration of Pr3+ ions, and among all concentrations doped in ZnB2O4:Pr3+ phosphor the maximum intensity of TL glow curve was observed for 0.2 mol%. The TL is extremely a sensitive phenomenon associated with lattice defects which fully rely on the number of present defect states in the gap between the valence and conduction band. Therefore, the incorporating of rare earth ions in host matrix results in the creation of defect energy levels within the band gap and irradiation will also lead to an increase in the number of defects owing to energy source. The number of different energy level traps (defects) within the sample's energy band gap increases with the increasing dopant concentration (Gupta et al., 2016).
3.2.2. Dose response TL curves obtained by setting the linear heating rate at 2°Cs−1 of the TL reader as a function of β doses ranging from 0.1 to 20 Gy are shown in Fig. 3. It is apparent that the TL intensity tends to be increased with excitation dose rate. In all cases, the entire shape of the glow curves remains unchanged with an increasing dose. As it is known, the dependence of the TL responses on radiation dose gives valuable information regarding the trapping process of trapped electrons and holes. Experimental results revealed that the peak positions are not affected with the increasing of the dose amount. A first order kinetic is dominant in nature using a generalized description of thermally stimulated processes (Bos, 2017). The behavior of the first order glow curve is different than that of a second order glow curve. In latter case, the peak positions slightly shift toward lower temperatures (Chen and McKeever, 1997). We mainly focused on P2 and P3 because P1 is low temperature glow peak and can be eliminated somehow but we did not concern that peak in this study. The dose dependence of the TL glow curves (i.e P2 and P3) of Pr doped of ZnB2O4 are shown in Fig. 4. As presented in Fig. 4, the evolutions of the whole areas of induced two TL glow curves exhibit good linear dose response (r = 0.999) and experimental points can be fitted to an equation of the sort y = intercept + slope × x , where y is peak area under the glow curves and x is the given dose. Note that for these selected doses no saturation
Fig. 1. XRD patterns for undoped and Pr3+ doped ZnB2O4.
filter for using in future dosimetric applications of the ZnB2O4 host can be selected by testing different band pass filters (i.e. 365 nm, 410 nm, 565 nm and wideband blue) inserted between the TL sample and the photomultiplier tube and then spectra were collected for each band pass filter (Oglakci et al., 2019). Although not displayed here, the shapes of TL glow curves monitored at 410 nm and 565 nm are similar. However, when examined the intensity of peak maxima located at 81 and 141 °C, TL intensities obtained using the filter of wideband are higher than the others. Therefore, all TL measurements were performed using the filter of IRSL-TL wideband during this study. Fig. 2 compares the TL glow curves of ZnB2O4:Pr3+ synthesized
Fig. 2. TL glow curves of beta irradiated ZnB2O4:Pr3+ phosphor subjected to 5 Gy of beta particle irradiation at various doping concentrations with constant heating rate 2 °Cs−1 (inset: the glow peak located at 233 °C is magnified).
Fig. 3. TL glow curves of beta irradiated ZnB2O4:Pr3+ (0.2%) samples for different dose. Heating rate is 2 °Cs−1. 107
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recombination (Bos, 2017) (b) An appreciable reduction of the intensity can be observed as the heating rate increases. The reduction can be assigned to a thermal quenching effect which is well known phenomenon. Therefore, the probability of non-radiative transitions was increased. Similarly, there is no direct recombination taken place from thermally released trapped charge carries or luminescence centers (i.e. mostly host defects or co-dopants) to recombination center and energy transfer occurs by means of non-radiative transitions (Wintle, 1975). (c) The integrated peak areas of the glow curves were affected from heating rate. (d) Full width at half maximum (FWHM) mostly increased. 3.2.4. Reproducibility Reproducibility is another desirable key factor which points out the practical application of samples in dose measurement (Karabulut et al., 2016; Afouxenidis et al., 2012). A phosphor is being considered to be a good reproducibility and repeatability if its response to ionizing radiation remains unaffected despite several repetitions of exposures and readouts. Also, the ZnB2O4: Pr3+ (0.2%) phosphor sample was examined for its reusability. The TL glow curves of the sample were recorded from RT to 450 °C followed by exposing to β-ray of 5 Gy at RT. The sample is allowed to be cooled much faster to RT and exposed to β radiation again for accumulating the same dose. It is crucial that same cooling procedure was followed. The glow curve was recorded again and, several such exposure readout cycles and glow curve recordings were carried out. In repeated measurements, the TL response to the same dose exposure is expected to be approximately the same. The standard deviation of the experimental results was pretty good and there is no significant variation of TL responses for 10 sequential measurements (not shown here). This result reveals that the TL sensitivities are identical within less than 2% and this phosphor is reusable in radiation dose assessment.
Fig. 4. Dose dependence of the TL glow curves of ZnB2O4:Pr3+ (0.2%) in the range of ∼0.1–∼20 Gy for 2nd and 3rd peak. Heating rate is 2 °Cs−1.
was taken place.
3.2.3. Heating rate effect It is generally accepted that the position of TL glow peak and TL intensity depend on the heating. In other words, the changes in the peak shape and the shifts in the peak position are functions of the heating rate. The immediate initial effects of the heating rate ranging from 1 °Cs−1 to 5 °Cs−1 of 0.2 mol% Pr3+ doped ZnB2O4 phosphor sample exposed to 5 Gy beta dose are seen in Fig. 5 for TL measurements. Above 5 °Cs−1 temperature lag effects are very possible, which will give rise to very lower values of activation energy. We believe that for heating rates 1-5 °Cs−1 are enough otherwise we need take into consideration the temperature lag effects. The effects of several heating rate were observed as follows: (a) the intriguing feature of these analyses was that the temperature at maximum TL intensity at which the TL glow peak occurred shifted to higher temperatures as the heating rate increased. This phenomenon can be stated as follows: Two assumptions need to be made for forming a TL glow peak: (i) an increase in the probability of escape and so a faster recombination rate which causes an increase in the number of photons emitted and (ii) a decrease in number of charge pairs released from a storage trap available for
3.3. Methods for extracting trap parameters Many expressions were derived to evaluate the activation energy from the shape of the TL peak. Some important methods taking into consideration the change in the position of the glow peak with the changing heating rates for obtaining the activation energy E (eV) belong to Booth (1954), Bohun (1954) and Parfianovitch (1954) and Hoogenstraaten (Hoogenstraaten, 1958). 3.3.1. Tm − Tstop experiment and initial rise (IR) method Broadly speaking, the nature of experimentally observed glow peaks may be assigned to several overlapping glow peaks having a trap distribution depth close to each other. Computerized glow curve deconvolution (CGCD) method based upon a number of different models existing in the theory of TL processes are most widely utilized in order to resolve the glow peaks. Estimating the order of kinetics for the various peaks in the TL glow curve can be executed via the Tm − Tstop experiment. The Tm − Tstop procedure verified that the position of the glow peaks shifts toward higher temperatures as the Tstop increases. These results obviously revealed that the glow peaks of the phosphor sample are a superposition of a number of first order glow peaks. The procedure also allows us to separate overlapping of the peaks and calculate the value Tm for each of them. The flat regions in the Tm − Tstop plot show the peak positions in the complex glow curve. Pr activated ZnB2O4 phosphor irradiated with a beta dose of 5 Gy is heated up to a Tstop at a linear heating rate. The phosphor sample is rapidly cooled down to RT and then the aliquot is re-heated all the way to a high temperature of 450 °C with a heating rate of 2 °Cs−1 and the position of the glow peak temperature Tm is recorded. This process is repeated several times on the same irradiated aliquot at a slightly higher temperature Tstop each time in the steps of 10 °C for the complete interval Tstop = 50–280 °C. The glow curves after the Tm − Tstop procedure are illustrated in Fig. 6. A plot of Tm as a function of Tstop displays a stepwise curve after completing Tm − Tstop procedure. This process may help us
Fig. 5. TL intensity versus temperature for ZnB2O4:Pr3+ (0.2%) recorded with various heating rates ranging between 1 and 5 °Cs−1 following constant irradiation dose of 5 Gy (β). 108
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3.3.2. Computerized glow curve deconvolution (CGCD) method CGCD is very popular method for studying the complex TL mechanism which includes multi-peak glow curves. Deconvolution also ensures the advantage in providing a simultaneous evaluation of peak parameters without the need of any further thermal annealing. Additionally, the optimization of some fundamental characteristics of a TL system can be done using this method. It is worth noting that various models, approximation and minimization procedures are taken into consideration for the glow curves analysis via the CGCD programs. The following equation of the first order kinetics of the TL developed at Reactor Institute at Delf (Gartia, 1989) were used to evaluate the glow curves
E s kT 2 E I (T ) = n 0 s exp ⎛− a ⎞ exp ⎡ exp ⎛− a ⎞ ∗ ⎛0.9920 ⎢ β E kT ⎝ kT ⎠ ⎝ ⎠ a ⎝ ⎣ ⎜
− 1.6220
kT ⎞ ⎤ Ea ⎠ ⎥ ⎦ ⎟
where n0 is the concentration of trapped electrons at t = 0, s is the frequency factor, E the trap depth, T absolute temperature, k Boltzmann's constant, β heating rate and b the kinetic order. Under these conditions, there is a doubt as to acceptability of kinetic parameters established via CGCD program. Figure of merit (FOM) or a simple ‘’chi-square’’ parameter that indicates the goodness of the fitting should be determined. FOM has become acceptable by the community of CGCD workers. The function FOM is calculated by
Fig. 6. TL glow curves recorded for Tm-Tstop analysis of ZnB2O4:Pr3+ (0.2%). Preheat temperatures are applied ranging from 50 °C to 280 °C with 5 °C increase at each step, making total 47 TL glow curves. Results are recorded for constant dose of 5 Gy (β).
FOM =
∑ (TLexp − TLfit )/∑ TLfit
× 100
where TLexp is the measured values of intensity at various T in experimental data and TLfit is the corresponding best fitted values. Quality of deconvolution and choice of an appropriate number of peaks are characterized by repeating the procedure by several times to obtain an optimal fit result (i.e minimum FOM) with minimum number of possible peaks. In other words, we can reach much better fit as the number of peaks is increased but the acceptability of such extra peak/peaks will be definitely doubtful and will not reflect any physical meaning. In the present case, we have tried to establish the minimum number of peaks needed to get an acceptable fit which is determined by the FOM. Fig. 8 shows typical set of TL glow curve analysis of ZnB2O4:0.02Pr3+ recorded with a linear heating rate (β = 2 Cs−1) using TLAnal CGCD program. Assuming the presence of six peaks, a much better accordance fitted theoretically with experimental glow curves were obtained. The
Fig. 7. Activation energy versus preheat temperature graph of ZnB2O4:Pr3+ (0.2%) obtained through Tm-Tstop analysis. Note the six peaks, each corresponding to a different class of trap.
to prognosticate the number of the peaks and each plateau region shown in Fig. 7 represents the approximate position of an individual peak. We suggested that five single first order components have a contribution in the entire measured TL glow curve. There are two main parameters (i.e. trap depths and frequency factor) in a TL glow curve. The first is clearly critical as an exponential term, but frequency factor is also another subtle dependence that varies between each class of trap. The initial rise method was carried out and a possible range of the activation energy (E) and frequency factors (s) for each preheat temperature were evaluated for analysed TL glow curves. Application of this method depicted the presence of a continuous distribution of trap depths for the sample preheated up to 280 °C, with energies between 0.37 ± 0.07 eV and 1.54 ± 0.02 eV. Whilst a trap depth identifies a TL feature, it does not provide any details of the structure of the TL site. Therefore, we suggest a site modelling below although it is very speculative. Frequency factor also changes from 1.97 × 10−10 (s−1) to 7.58 × 10−19 (s−1).
Fig. 8. An exemplarily result of deconvolution performed for ZnB2O4:Pr3+ (0.2%) recorded after 5 Gy (β) irradiation using TLAnal CGCD program. P1, P2 and P3 peaks correspond to measured data (the circle line). Note that the sample has been confirmed to behave as a first order trap. 109
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where TM1 is glow peak temperature, TM2 is the temperature of the lower side of the glow curve for β1 and β2 , respectively. The values of the temperatures for each pairs of various heating rates are evaluated from the shape of the glow curves in ZnB2O4:0.2Pr3+ shown in Fig. 5. It was seen that average trap depths of P2 and P3 changes between 0.38 ± 0.02 eV - 0.81 ± 0.02 eV and 0.55 ± 0.03 eV–1.15 ± 0.03 eV respectively once the above equation is solved. The final average trap depths from each average trap depth is found to be 0.55 + 0.02 eV and 0.88 + 0.02 eV.
Table 1 Calculated activation energy (Ea) and frequency factor (s) of 2nd and 3rd peaks of ZnB2O4:Pr (0.2%) with Hoogenstraaten's Equation from Fig. 8.
2nd Peak 3rd Peak
T (°C)
Ea (eV)
s (s−1)
ln(s)
124 233
0.52 ± 0.02 0.84 ± 0.03
3.9243E+05 1.6300E+07
12.88 16.61
obtained best-fit parameters based on the value of the FOM were satisfactory in this study. The FOM value for 5 Gy β irradiated ZnB2O4:Pr3+ phosphor was found to be 2.019%. The FOM values must be less than 5% according to consensus regarding acceptable fit. It is worth noting that the deconvolution of the glow curves shows six peaks that are not necessarily corresponding to those finding by Tm − Tstop (see Fig. 7) using this partial bleaching method due to their overlapped glow peaks. Finally, in this case, the Tm value on the stepwise (see Fig. 7) is only an average value of the maximum temperature of these peaks and not the precise value of the peak obtained by deconvolution method. In this study, a visual model has been also suggested to elucidate the TL response of beta irradiated ZnB2O4 sample. 4 (four) TL glow peaks obtained applying the deconvolution technique to the glow curve with the aid of an efficient computer program TLanal can be interpreted suggesting a model. It is worth noting that there are many factors influencing the interpretation and analysis of experimentally measured the TL glow curves and therefore it is not straight forward. The traps of phosphors are formed by chemical impurities or crystal lattice defects in phosphor compound. The substitution of the Pr3+ ions as dopants into ZnB2O4 host material introduces a charge imbalance, leading to charge compensation. A forbidden band composed of valance band and the conduction band involves two types of traps in this proposed model; namely shallow and deep traps. 4 (four) electron traps assigned to P2 and P3 in Fig. 8 are presumably created by the presence of the structural defects present in the ZnB2O4 compound. The release of electrons liberated from the traps after irradiation may recombine with the holes at the luminescence center causing TL. The existence of competition among these trap levels might exhibit different probability of either being releasing or retrapping probabilities.
3.4. Conclusions In summary, ZnB2O4 phosphors doped with 0.2–1.0 %Pr were successfully prepared by a wet chemical reaction process. The X-ray patterns of the undoped and Pr doped phosphors exhibited the cubic (bcc) phase in this material. The Tm − Tstop analysis of collected data was utilized to estimate the number of peaks that make up the whole TL curve and characteristic trapping centers related with these peaks. The glow curves for P2 and P3 were well deconvolved by four peaks. Expected range of activation energies of distributed four trap centers were determined between 0.37 ± 0.07 eV and 1.54 ± 0.02 eV as a function of Tstop in range of 50–280 °C. Using the deconvolution method, it was revealed that the TL response of ZnB2O4:Pr3+ phosphor sample does follow first order kinetics. All the applied methods for evaluation of the activation energy are comparable. Trap depths for P2 and P3 via Hoogenstraaten's Equation are found to be 0.52 ± 0.03 eV and 0.84 ± 0.03 eV, respectively. It is worth noting that the relatively small variations in E are inevitably reflected to a huge difference of s due to the exponential function. We evaluated reproducibility of the sample via 10 replicate TL measurements and suggested that the prepared samples have good reproducibility for beta exposure. In high dopant situations lattice strains and charge compensation features will cause lattice distortions that favor pairing or clustering of the dopants and associated intrinsic defects. Therefore, trapping and recombination sites may be in intimate proximity. Hence there is no unique activation energy for the TL process. Acknowledgments This work was supported by the Research Fund of the Çukurova University, Turkey (Project Number: FAY-2015-4735).
3.3.3. Methods of Hoogenstraaten and Booth-Bohun-Parfianovitch In principle, Hoogenstraaten's method was only developed for TL glow curves having first-order kinetics but some researchers suggested that the method was also applicable for non -first order cases and provides a good approximation for evaluating activation energy E (eV ) (McKeever, 1985; Furetta, 2003). The following equation at several heating rate was used in order to get a linear relation between ln (Tm2 / β ) and (1/ Tm) :
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T2 E E 1 ln ⎛⎜ m ⎞⎟ = ⎛ ⎞ ⎛ ⎞ + ln ⎛ ⎞ β k T ks ⎠ ⎝ ⎠ ⎝ m ⎝ ⎠ ⎝ ⎠ ⎜
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E=k
TM1 TM2 ⎡ β TM ⎤ ln 1 ⎜⎛ 1 ⎟⎞ TM1 − TM2 ⎢ β2 ⎝ TM2 ⎠ ⎥ ⎦ ⎣ 110
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