Thermoluminescence Characteristics of Terbium Doped Zinc Borates

In this work, structural and thermoluminescence (TL) characteristics for ZnB2O4:xTb (x = 0.01, 0.02, 0.03, 0.04, 0.05, and 0.10 mole) phosphors were investigated. The phosphors were prepared via synthesis of nitric acid. The X-ray diffraction (XRD) studies show that the synthesized samples can be indexed to nearly single-phase cubic ZnB2O4. The TL characteristics following 90Sr beta irradiation (40 mCi) were studied. TL intensity is found to depend on Tb concentration. The optimal concentration of the doped Tb3+ is 0.03 mol in TL measurements. TL dose responses of the phosphors to beta doses of 0.143, 0.715, 1.43, 15, 30 and 60 Gy showed fairly linear behavior. The minimum detectable dose (MDD) value for ZnB2O4:0.04Tb was found to be 87 mGy. The kinetic parameters of the ZnB2O4:0.03Tb sample were estimated by the glow curve deconvolution, the initial rise, the curve fitting, and the peak shape methods. The results indicate that these phosphors are thought to be promising candidates as TL materials. The results provide valuable knowledge of the characteristics of Tb-doped ZnB2O4 for use in dosimetry research.


Results and Discussion
Obtained powder samples were subject to X-ray powder diffraction (XRD) in order to check their phase purity. Furthermore, the standard XRD pattern of ZnB 2 O 4 (PDF Card No. 39-1126) was selected for comparison. Powder XRD patterns of ZnB 2 O 4 doped with various concentrations of Tb 3+ ions are illustrated in Figure 1a. Powder XRD patterns showed intense and sharp peaks at almost the same angle position. This is a strong indication that the samples have high crystallinity. These diffraction patterns are in good agreement with the standard JCPDS card No. 39-1126, which represents the pure phase of ZnB 2 O 4 . No additional signal was observed in the XRD patterns of other phosphors, except for the ZnB 2 O 4 :0.10Tb 3+ phosphor. The decrease in crystallization of Tb-doped ZnB 2 O 4 with increased quantities of Tb-dopant was seen in the XRD patterns of ZnB 2 O 4 :0.10Tb 3+ phosphor. This can be attributed to the change in dopant loads leading to defect formation in the cage. In general, it can be concluded from these data that the structure of the samples does not change after doping with Tb 3+ . The effective ionic radii of dopant and host ions are given as follows: Zn 2+ = 0.60 Å (Coordination IV), B 3+ = 0.21 Å (Coordination IV), Tb 3+ = 0.92 Å (Coordination VI) [17]. Tb 3+ doping ions cannot replace the B 3+ ions site in the ZnB 2 O 4 host lattice due to their different ionic radii. This can be linked to the smaller ionic radius of B 3+ compared to that of Zn 2+ . For this reason, it is more likely for Tb 3+ ions to replace Zn 2+ ions in the host lattice. The concentration of the impurity phases is so small that even though the concentration of Tb 3+ doping is as high as 10 mol%, Tb 3+ has no effect on the crystal structure of ZnB 2 O 4 . It should be remembered that replacing Zn 2+ ions with Tb 3+ causes not only a charge imbalance but also a volume imbalance. This leads to crystal lattice expansion because the Tb 3+ radius is greater than that of the altered Zn 2+ . However, the recorded XRD results are consistent with the literature reports [1,10,11].
Crystals 2019, 9, x FOR PEER REVIEW 3 of 10 (Coordination VI) [17]. Tb 3+ doping ions cannot replace the B 3+ ions site in the ZnB2O4 host lattice due to their different ionic radii. This can be linked to the smaller ionic radius of B 3+ compared to that of Zn 2+ . For this reason, it is more likely for Tb 3+ ions to replace Zn 2+ ions in the host lattice. The concentration of the impurity phases is so small that even though the concentration of Tb 3+ doping is as high as 10 mol%, Tb 3+ has no effect on the crystal structure of ZnB2O4. It should be remembered that replacing Z (a) (b) After refining XRD patterns using FullProf software, lattice parameters were calculated. The refined XRD of ZnB2O4:0.03Tb 3+ sample is seen in Figure 1(b). In this figure, the observed pattern, the calculated pattern and the difference between the observed-calculated patterns are represented by red circles, a thick black line and a thin black line, respectively. ZnB2O4:Tb 3+ samples exhibit a cubic structure with the space group I-43m. It can be said that the crystal structure remains the same with the addition of Tb and that the lattice parameters undergo small changes. The maximum increase in lattice parameter "a" is 0.028% for 3% Tb. When the amount of Tb doping increases by 3%, the lattice parameter "a" and the unit cell volume slowly increase as the atomic radius of Tb is greater than that of Zn. When the doping amount is above 3%, the lattice parameter "a" and unit cell volume are reduced due to lattice distortion or migration of the doping ions to the interstitial positions [18]. In addition, the diffraction pattern of the ZnB2O4:0.10Tb 3+ sample indicates the presence of small impurity phases, which are H3BO3 and B2O3. The parameters obtained for ZnB2O4:Tb 3+ samples are given in Table 1. Figure 2 shows an SEM image of the ZnB2O4:0.03Tb 3+ sample. It is observed that the particles in the zinc borate are slightly agglomerated and thus result in an inhomogeneous structure. SEM-EDX analysis was performed to determine the chemical composition of the sample in an area of 10 × 10 μm 2 . Table 2 shows the atomic ratios of Zn/O, Tb/Zn and Tb/O. The results show that the nominal ratios of zinc borate are consistent with the EDX results.  After refining XRD patterns using FullProf software, lattice parameters were calculated. The refined XRD of ZnB 2 O 4 :0.03Tb 3+ sample is seen in Figure 1b. In this figure, the observed pattern, the calculated pattern and the difference between the observed-calculated patterns are represented by red circles, a thick black line and a thin black line, respectively. ZnB 2 O 4 :Tb 3+ samples exhibit a cubic structure with the space group I-43m. It can be said that the crystal structure remains the same with the addition of Tb and that the lattice parameters undergo small changes. The maximum increase in lattice parameter "a" is 0.028% for 3% Tb. When the amount of Tb doping increases by 3%, the lattice parameter "a" and the unit cell volume slowly increase as the atomic radius of Tb is greater than that of Zn. When the doping amount is above 3%, the lattice parameter "a" and unit cell volume are reduced due to lattice distortion or migration of the doping ions to the interstitial positions [18]. In addition, the diffraction pattern of the ZnB 2 O 4 :0.10Tb 3+ sample indicates the presence of small impurity phases, which are H 3 BO 3 and B 2 O 3 . The parameters obtained for ZnB 2 O 4 :Tb 3+ samples are given in Table 1. Figure 2 shows an SEM image of the ZnB 2 O 4 :0.03Tb 3+ sample. It is observed that the particles in the zinc borate are slightly agglomerated and thus result in an inhomogeneous structure. SEM-EDX analysis was performed to determine the chemical composition of the sample in an area of 10 × 10 µm 2 . Table 2 shows the atomic ratios of Zn/O, Tb/Zn and Tb/O. The results show that the nominal ratios of zinc borate are consistent with the EDX results.    Figure 3 shows the TL glow curves for the selected ZnB2O4:xTb 3+ (x = 0.01, 0.03, and 0.05 mol) phosphors. As seen from Figure 3, the TL glow curves showed a dosimetric peak at about 200 °C for ZnB2O4:0.01Tb 3+ and about 210 °C for ZnB2O4:0.03Tb 3+ and ZnB2O4:0.05Tb 3+ . In addition, as the beta dose irradiation increases, a low-severity shoulder is seen in the temperature range of 260-300 °C. No significant shifts were observed in the maximum peak temperature of the main glow peak with increasing beta dose irradiation for all the phosphors produced. Additionally, the increase in the beta dose irradiation caused an increase in the intensity of the TL. It is known that the dependence of the TL responses on the beta dose irradiation provides important information about the trapping of trapped electrons and holes. Thus, it can be assumed that more charge carriers are trapped as the beta dose irradiation increases. Subsequent heating releases some of the trapped electrons with an associated emission of light in the form of glow curves [19]. This result shows that there are strong interactions between internal defects and dopant. For the 30 Gy beta dose, the glow peak intensities of ZnB2O4:0.01Tb 3+ , ZnB2O4:0.03Tb 3+ and ZnB2O4:0.05Tb 3+ phosphors increased by about 19, 46 and 21 times compared to the 1.43 Gy beta dose.
(a1) (a2)   Figure 3 shows the TL glow curves for the selected ZnB 2 O 4 :xTb 3+ (x = 0.01, 0.03, and 0.05 mol) phosphors. As seen from Figure 3, the TL glow curves showed a dosimetric peak at about 200 • C for ZnB 2 O 4 :0.01Tb 3+ and about 210 • C for ZnB 2 O 4 :0.03Tb 3+ and ZnB 2 O 4 :0.05Tb 3+ . In addition, as the beta dose irradiation increases, a low-severity shoulder is seen in the temperature range of 260-300 • C. No significant shifts were observed in the maximum peak temperature of the main glow peak with increasing beta dose irradiation for all the phosphors produced. Additionally, the increase in the beta dose irradiation caused an increase in the intensity of the TL. It is known that the dependence of the TL responses on the beta dose irradiation provides important information about the trapping of trapped electrons and holes. Thus, it can be assumed that more charge carriers are trapped as the beta dose irradiation increases. Subsequent heating releases some of the trapped electrons with an associated emission of light in the form of glow curves [19]. This result shows that there are strong interactions between internal defects and dopant. For the 30 Gy beta dose, the glow peak intensities of   Figure 4 shows the TL glow curves of all the phosphors exposed to the 30 Gy beta dose. According to this figure, it has been observed that all the phosphors have different TL intensities and there are very few differences in the peak structures. The low-intensity shoulder observed at 260-290 °C can be said to vary with beta dose irradiation. It is clear that the main TL peak intensity of ZnB2O4:0.03Tb 3+ phosphor is much more severe than that of the other phosphors. It is also found that max peak intensity and max peak temperatures of the glow curve are dependent on the amount of Tb concentration. Due to the Tb-concentration quenching effect, the TL intensity initially increases with increasing Tb concentration, and then reaches the maximum value at the optimum concentration level [5]. The Tb 3+ ions surrounded by the host lattice form the luminescence centers in the sample. At a high Tb 3+ concentration, stability of luminescence centers and lattice structure can be disrupting and this will lead to an increased interaction between the periodicity of the lattice and luminescence center. This will cause a decrease of the luminescence intensity [20]. Furthermore, the peak temperature of the main glow peak appears to shift slightly as the Tb-concentration level increases. Thus, it can be deduced that the distributions of the traps produced by beta dose irradiation can be changed by changing the concentrations of Tb 3+ ion added to zinc borate.  Figure 4 shows the TL glow curves of all the phosphors exposed to the 30 Gy beta dose. According to this figure, it has been observed that all the phosphors have different TL intensities and there are very few differences in the peak structures. The low-intensity shoulder observed at 260-290 • C can be said to vary with beta dose irradiation. It is clear that the main TL peak intensity of ZnB 2 O 4 :0.03Tb 3+ phosphor is much more severe than that of the other phosphors. It is also found that max peak intensity and max peak temperatures of the glow curve are dependent on the amount of Tb concentration. Due to the Tb-concentration quenching effect, the TL intensity initially increases with increasing Tb concentration, and then reaches the maximum value at the optimum concentration level [5]. The Tb 3+ ions surrounded by the host lattice form the luminescence centers in the sample. At a high Tb 3+ concentration, stability of luminescence centers and lattice structure can be disrupting and this will lead to an increased interaction between the periodicity of the lattice and luminescence center. This will cause a decrease of the luminescence intensity [20]. Furthermore, the peak temperature of the main glow peak appears to shift slightly as the Tb-concentration level increases. Thus, it can be deduced that the distributions of the traps produced by beta dose irradiation can be changed by changing the concentrations of Tb 3+ ion added to zinc borate.  Figure 5 shows TL dose-response curves of the selected ZnB2O4:0.02Tb 3+ and ZnB2O4:0.05Tb 3+ phosphors. TL = a(D) k is the best fit equation for the line passing through the data points [14,21]. TL dose-response curves were found to be linear in the 0.143-60 Gy beta dose range. The k values of phosphors are in the range of 0.864 to 1.050. MDD values for ZnB2O4:Tb 3+ phosphors were determined as three-times the background count [22,23]. The best MDD value for ZnB2O4:0.04Tb 3+ was found to be 0.087 Gy. MDD values for ZnB2O4:0.01Tb 3+ , ZnB2O4:0.02Tb 3+ , ZnB2O4:0.03Tb 3+ , ZnB2O4:0.05Tb 3+ and ZnB2O4:0.10Tb 3+ were reported as 0.189, 0.135, 0.101, 0.140 and 0.105 Gy, respectively. For ZnB2O4:Tb 3+ phosphors, maximum peak temperature (Tm), background counts, values of k with standard error (Δk) and MDD values are given in Table 3.  0.864 ± 0.096 0.105 ± 0.010 Kinetic parameters provide important information regarding the TL mechanism of the material. Well-known kinetic parameters increase the reliability of dosimetric studies. Therefore, the kinetic parameters of ZnB2O4:0.03Tb 3+ were determined using the glow curve deconvolution, the   Table 3.  Figure 5 shows TL dose-response curves of the selected ZnB2O4:0.02Tb 3+ and ZnB2O4:0.05Tb 3+ phosphors. TL = a(D) k is the best fit equation for the line passing through the data points [14,21]. TL dose-response curves were found to be linear in the 0.143-60 Gy beta dose range. The k values of phosphors are in the range of 0.864 to 1.050. MDD values for ZnB2O4:Tb 3+ phosphors were determined as three-times the background count [22,23]. The best MDD value for ZnB2O4:0.04Tb 3+ was found to be 0.087 Gy. MDD values for ZnB2O4:0.01Tb 3+ , ZnB2O4:0.02Tb 3+ , ZnB2O4:0.03Tb 3+ , ZnB2O4:0.05Tb 3+ and ZnB2O4:0.10Tb 3+ were reported as 0.189, 0.135, 0.101, 0.140 and 0.105 Gy, respectively. For ZnB2O4:Tb 3+ phosphors, maximum peak temperature (Tm), background counts, values of k with standard error (Δk) and MDD values are given in Table 3.  0.864 ± 0.096 0.105 ± 0.010 Kinetic parameters provide important information regarding the TL mechanism of the material. Well-known kinetic parameters increase the reliability of dosimetric studies. Therefore, the kinetic parameters of ZnB2O4:0.03Tb 3+ were determined using the glow curve deconvolution, the  Kinetic parameters provide important information regarding the TL mechanism of the material. Well-known kinetic parameters increase the reliability of dosimetric studies. Therefore, the kinetic parameters of ZnB 2 O 4 :0.03Tb 3+ were determined using the glow curve deconvolution, the initial rise, the curve fitting, and the peak shape methods. The glow curve deconvolution method has great advantages over experimental methods [24]. It can be used to simultaneously determine the kinetic parameters of TL peaks without additional thermal treatments and experimental repetitions. In addition, this method uses all the data points in the glow curve during curve fitting procedures [19]. Maximum peak temperature for a constant heating rate should not be influenced by other experimental parameters. It was noted that the peak maximum positions did not change with dose increase for first-order kinetics. However, the peak shifts to the lower-temperature zone as the dose increases for general-order kinetics (1 < b < 2). As shown in Figure 3, the peak temperature of the luminance peak at about 200 • C does not shift with dose increase, and this point indicates that this peak has first-order kinetics [21][22][23]. In this study, decomposition was made with GlowFit software on the TL glow curve of the ZnB 2 O 4 :0.03Tb 3+ sample selected for a 30 Gy beta dose. Figure 6a shows the analyzed glow curve of the ZnB 2 O 4 :0.03Tb 3+ sample. As can be seen, this TL glow curve consists of five separated peaks. The TL glow curves of the ZnB 2 O 4 :0.03Tb 3+ sample exhibit one prominent glow peak at around 215 • C (peak 2), two-less dominant glow peaks at around 201 • C (peak 1) and around 235 • C (peak 3), one low-intensity peak at around 265 • C, and one shoulder peak at around 358 • C (peak 5). The accuracy of these values is supported by the FOM (figure of merit) value. A parameter set is acceptable if the FOM is less than 5% [12]. The FOM value of the ZnB 2 O 4 :0.03Tb 3+ sample was found to be 2.32%.
Crystals 2019, 9, x FOR PEER REVIEW 7 of 10 initial rise, the curve fitting, and the peak shape methods. The glow curve deconvolution method has great advantages over experimental methods [24]. It can be used to simultaneously determine the kinetic parameters of TL peaks without additional thermal treatments and experimental repetitions. In addition, this method uses all the data points in the glow curve during curve fitting procedures [19]. Maximum peak temperature for a constant heating rate should not be influenced by other experimental parameters. It was noted that the peak maximum positions did not change with dose increase for first-order kinetics. However, the peak shifts to the lower-temperature zone as the dose increases for general-order kinetics (1 < b < 2). As shown in Figure 3, the peak temperature of the luminance peak at about 200 °C does not shift with dose increase, and this point indicates that this peak has first-order kinetics [21][22][23]. In this study, decomposition was made with GlowFit software on the TL glow curve of the ZnB2O4:0.03Tb 3+ sample selected for a 30 Gy beta dose. Figure 6a shows the analyzed glow curve of the ZnB2O4:0.03Tb 3+ sample. As can be seen, this TL glow curve consists of five separated peaks. The TL glow curves of the ZnB2O4:0.03Tb 3+ sample exhibit one prominent glow peak at around 215 °C (peak 2), two-less dominant glow peaks at around 201 °C (peak 1) and around 235 °C (peak 3), one low-intensity peak at around 265 °C, and one shoulder peak at around 358 °C (peak 5). The accuracy of these values is supported by the FOM (figure of merit) value. A parameter set is acceptable if the FOM is less than 5% [12]. The FOM value of the ZnB2O4:0.03Tb 3+ sample was found to be 2.32%. The kinetic parameters obtained by the glow curve deconvolution method are given in Table 4, together with Tm values. Ea is the activation energy and s is the frequency factor. The activation energies and the frequency factors of the ZnB2O4:0.03Tb 3+ sample were determined as 1.49 eV and 2.79 × 10 15 s −1 for the first peak, 1.27 eV and 3.88 × 10 12 s −1 for the second peak, 1.20 eV and 2.20 × 10 11 s −1 for the third peak, 0.71 eV and 6.20 × 10 5 s −1 for the fourth peak, and 0.50 eV and 7.24 × 10 2 s −1 for the fifth peak, respectively.
The initial rise method-a powerful technique independent from the order of kinetics in the TL processes-was also used to calculate the activation energy. The TL intensity in the initial portion (∼10% of its maximum intensity) of the glow curve is proportional to exp(-Ea/kT) [25]. This relation gives an opportunity to evaluate the Ea value from the slope of the ln(ITL) vs. 1/T graph. Figure 6b presents the corresponding plots (open shapes) and their linear fits (solid lines). The values of Ea obtained by this method for the ZnB2O4:0.03Tb 3+ sample are as obtained 1.17 eV ( Table 4).
The curve fitting method is based on the fitting of the observed glow curve to the theoretical expressions. The details of this method were reported in reference [26]. The fitting process has been carried using the equations given in Reference [26] for different values of the parameter, b. The best fitting was achieved for b = 1.1, which states the presence of first order kinetics. The activation energy obtained from the fitted curve was 1.20 eV and is given in Table 4. The kinetic parameters obtained by the glow curve deconvolution method are given in Table 4, together with T m values. E a is the activation energy and s is the frequency factor. The activation energies and the frequency factors of the ZnB 2 O 4 :0.03Tb 3+ sample were determined as 1.49 eV and 2.79 × 10 15 s −1 for the first peak, 1.27 eV and 3.88 × 10 12 s −1 for the second peak, 1.20 eV and 2.20 × 10 11 s −1 for the third peak, 0.71 eV and 6.20 × 10 5 s −1 for the fourth peak, and 0.50 eV and 7.24 × 10 2 s −1 for the fifth peak, respectively.
The initial rise method-a powerful technique independent from the order of kinetics in the TL processes-was also used to calculate the activation energy. The TL intensity in the initial portion (~10% of its maximum intensity) of the glow curve is proportional to exp(-E a /kT) [25]. This relation gives an opportunity to evaluate the E a value from the slope of the ln(I TL ) vs. 1/T graph. Figure 6b presents the corresponding plots (open shapes) and their linear fits (solid lines). The values of E a obtained by this method for the ZnB 2 O 4 :0.03Tb 3+ sample are as obtained 1.17 eV ( Table 4).
The curve fitting method is based on the fitting of the observed glow curve to the theoretical expressions. The details of this method were reported in reference [26]. The fitting process has been carried using the equations given in Reference [26] for different values of the parameter, b. The best fitting was achieved for b = 1.1, which states the presence of first order kinetics. The activation energy obtained from the fitted curve was 1.20 eV and is given in Table 4.
The peak shape method is generally called Chen's method, which is used to determine the kinetic parameters of the main glow peak of the TL materials [27]. This method is mainly based on the temperatures T m , T 1 and T 2 , which are the peak temperatures, the temperatures at half of the maximum intensity, on the ascending and descending parts of the peak, respectively. Geometrical shape parameters (i.e., T m , T 1 , T 2 , τ, δ and ω) were initially determined with the help of the TL glow curves. Where ω = T 2 − T 1 is the total peak width at half of the maximum intensity, τ = T m − T 1 is the lower-temperature half-width, δ = T 2 − T m is the upper-temperature half-width, and µ g = δ/ω is the symmetry factor. Then, the peak shape method was applied to ZnB 2 O 4 :0.03Tb 3+ phosphor for 30 Gy. The mean values of E a and s obtained by this method for the ZnB 2 O 4 :0.03Tb 3+ sample were determined as 1.25 eV and 8.32 × 10 15 s −1 for the first peak, 1.29 eV and 1.33 × 10 12 s −1 for the second peak, 1.21 eV and 2.19 × 10 11 s −1 for the third peak, 0.73 eV and 2.13 × 10 5 s −1 for the fourth peak, and 0.51 eV and 1.02 × 10 2 s −1 for the fifth peak, respectively ( Table 4). As seen from Table 4, there is a good agreement between all the results.

Conclusions
ZnB 2 O 4 :Tb 3+ phosphors were successfully prepared using a nitric acid synthesis method. The results of the XRD analysis showed that the synthesized phosphors could be indexed to a pure, cubic phase of ZnB 2 O 4 . The TL properties and dependence on the dose of beta irradiation of the phosphors have been investigated by the TL technique. The TL peaks of the samples are present at around 200-210 • C. It has been found that the TL intensity of the ZnB 2 O 4 :0.03Tb 3+ sample is much stronger than that of the other samples. The dose responses to beta irradiation of all the powder samples showed good linearity in the dose range between 0.143-60 Gy. The MDD value for ZnB 2 O 4 :0.04Tb 3+ sample was found to be 87 mGy. MDD values for ZnB 2 O 4 :0.01Tb 3+ , ZnB 2 O 4 :0.02Tb 3+ , ZnB 2 O 4 :0.03Tb 3+ , ZnB 2 O 4 :0.05Tb 3+ and ZnB 2 O 4 :0.10Tb 3+ samples were reported as 0.189, 0.135, 0.101, 0.140 and 0.105 Gy, respectively. The kinetic parameters of the ZnB 2 O 4 :0.03Tb 3+ sample were calculated by the glow curve deconvolution, the initial rise, the curve fitting, and the peak shape methods. In conclusion, ZnB 2 O 4 :0.03Tb 3+ phosphor could be a promising material for dosimetric applications.