Radiation Protection Dosimetry (2015), Vol. 163, No. 4, pp. 430 – 438 Advance Access publication 10 July 2014
doi:10.1093/rpd/ncu226
DOSIMETRIC INVESTIGATIONS OF Tb31-DOPED STRONTIUM SILICATE PHOSPHOR R. A. Barve1, N. Suriyamurthy1,*, B. S. Panigrahi2 and B. Venkatraman1 1 Radiological Safety Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India 2 Technical Services Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India *Corresponding author: [email protected] Received 29 January 2014; revised 23 May 2014; accepted 14 June 2014
INTRODUCTION Silicates are efficient luminescent hosts, mainly because of their rigid and very stable crystal structures. Several silicate materials doped with rare earth ions, either divalent or trivalent, have been proposed for or used as commercial phosphors in tricolour fluorescent lamps, scintillators, etc.(1) The luminescent materials based on silicates as host material exhibit strong chemical stability, high-energy ion irradiation resistance, high UV and visible light transmittance. Variety of silicates such as M2SiO4, M3SiO5 and M3MgSi2O8 (M ¼ Ca, Sr, Ba) have been investigated as the host materials for phosphors doped with trivalent rare earth ions or transition metal ions(2 – 7). The silicate hosts exhibit shift in emission band of cerium and europium ions when cation of silicates changed from calcium to strontium. Also amongst several rare earths, Tb3þ is an intense green emitter having narrow line emission due to f –f electronic transition peaking at 544 nm(8). Besides their photonic applications, silicates were investigated for development of possible dosimetric materials. The first TL material based on magnesium orthosilicate was introduced in radiation dosimetry in 1971(9, 10). Tb3þ-doped Mg2SiO4 is known to be a promising TLD material for the dosimetry due to its high TL sensitivity(11) that can also be used as a fibre optic dosemeter to access the real-time dose(12). There exist several methods inter alia solid-state reactions method, hydrothermal method, sol gel, which are being used for the synthesis of various Ba-, Ca- or Sr-based silicates and require sophisticated instruments and high calcination temperature(6, 7, 13, 14). However, the co-precipitation technique is one of the facile methods to prepare the silicate materials that introduce significant amount of defects leading to improved thermoluminescence (TL) in polycrystalline materials.
This paper discusses about the cross-relaxation phenomenon and thermoluminescence dosimetric properties of SrSiO3:Tb3þ with special emphasis on the determination of kinetic parameters using different methods. EXPERIMENTAL AR grade Sr(NO3)2 and Na2SiO3 taken in stoichiometric ratio were used as precursors along with Tb3þ as dopant for synthesising SrSiO3:Tb3þ by co-precipitation technique. The obtained precipitate was then washed several times with water to remove the unreacted compounds and allowed to dry for several hours under drying lamp to remove excess amount of water. This powder was annealed in air atmosphere at 7008C for 3 h and allowed to cool slowly to room temperature. The PL spectrum was recorded using Shimadzu Spectrofluorometer equipped with 150 W of Xenon lamp, and TL was recorded using TL/OSL-DA-20 Risoe reader with various heating rates from 0.1 to 6 K s21 depending on the purpose of investigation. All the samples were normalised with respect to weight and dose. The samples are irradiated with in-built 90 Sr beta source having dose rate of 0.1 Gy s21. RESULTS AND DISCUSSION X-ray diffraction analysis Figure 1 shows the XRD profile of SrSiO3:Tb3þ. All the important diffraction profiles match with SrSiO3 (monoclinic) phase (JCPDS No. 87-0474). Sharp X-ray diffraction peaks indicate the formation of highly crystalline phase after annealing at 7008C. Besides, no impurity peaks and no signatures from dopant are noticed.
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Tb31-doped SrSiO3 phosphor synthesised by co-precipitation technique exhibits intense green emission due to cross-relaxation phenomena between Tb31 ions. Dosimetric properties of this phosphor have been investigated using thermoluminescence (TL) technique. A dosimetrically useful glow peak observed was at 581 K along with a linear dose response over the wide dose range (100 mGy– 4 Gy). TL parameters such as trap depth (E), frequency factor (s) and the order of kinetics (b) are determined by different methods such as Chen’s peak shape, initial rise, isothermal decay and variable heating rate methods. Results of these methods are compared and reported in this study.
THERMOLUMINESCENCE STUDIES OF SrSiO3:Tb3þ
diagrams of the RE3þ series. On the other hand, the gap below the 5D3 level is 5700 cm21, and hence, cross-relaxation from this energy level is insignificant (16). The intense green emission from this host prompted the authors to study its TL response too. Figure 1. X-ray diffraction pattern of SrSiO3:Tb3þ heated at 7008C for 3 h in air.
Cross-relaxation of terbium luminescence Figure 2 shows the photoluminescence spectra of Tb3þ-doped SrSiO3. The sample exhibits excitation bands at 237, 352 and 379 nm (Figure 2a). The band at 237 nm is charge transfer band between O22 and Tb3þ, and this is completely allowed. On the other hand, peaks at 352 and 379 nm are essentially due to forbidden absorption transitions. Upon exciting at 379 nm, the authors could observe well-defined narrow emission lines due to f –f transitions of 490 and 550 nm (Figure 2b) corresponding to 5D4 – 7F6 and 5D4 – 7F5 transitions, respectively. Cross-relaxation is a process wherein the excitation energy from terbium ion is transferred partially to neighbouring luminescent terbium ion resulting in quenching of blue emission and enhancement of green emission(15). Exploiting this property, one can selectively tune the luminescent material for enhanced green emission. In this host, terbium undergoes cross-relaxation and hence emission intensity of peaks at 437, 460 and 488 nm originating from 5D3 is less than the intensity of 544-nm emission from 5D4. This phenomenon can be manipulated by readjusting the concentration of terbium ions. Cross-relaxation does not occur from all energy level. The probability of non-radiative relaxation from the 5D4 level is very low, because the band gap to the next available level is very large 14 600 cm21, one of the largest gaps in the energy-level
Thermoluminescence Thermoluminescence phosphors generally exhibit glow curves with one or more peaks when the charge carriers are released. The glow curve is the characteristic of different trap levels that lie in the band gap of materials. The traps are specified by the some parameters including trap depth (E) and frequency factor (s). Figure 3 shows the thermo-stimulated luminescence response of Tb3þ-activated SrSiO3. For the absorbed dose of 1 Gy delivered by beta-radiation, the sample exhibits well-defined glow peak at 593 K. TL glow peak position and peak width are the finger prints of TL dosimetry. The observed glow peak is broad and hence de-convoluted using Origin 6.1. After the deconvolution, two glow peaks having maxima at 535 and 595 K have been obtained. The peak at 593 K is dosimetrically useful peak and hence investigated extensively for its possible TL applications. The figure of merit was found to be 0.5 %, which reflects that the fit is very good. Effect of heating rate on peak maxima (Tm) and TL sensitivity Studies on the role of heating rate indicated the change in glow peak maxima that was shifting towards high temperature while keeping the dose constant. Here, the heating rate was varied from 0.5 to 6 K s21. At lower heating rate, the time spent by charge carriers is long enough that the thermal release of charge carriers from traps depends on the half-life whereas for higher heating rate, the thermal release of
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Figure 2. Photoluminescence spectra of SrSiO3:Tb3þ. (a) Excitation spectrum (lemi-544 nm), (b) emission spectrum (lexi-379 nm).
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carriers decreases. Therefore, at this heating rate, higher temperature is required to ensure effective detrapping of carriers and hence is the observed shifting of glow peak maxima (Figure 4). Though peak maxima have shifted, there has been no change in shape of glow curve with heating rate. On other hand, as the heating rate is increased, the TL sensitivity of this phosphor decreases (Figure 5). This is probably due to fact that, at faster heating rate, the shallow traps are getting emptied faster than resulting in reduced integrated area under the glow curve(17). The TL sensitivity being proportional to area under the curve, the sensitivity consequently decreased. Another factor that influences the luminescence efficiency is thermal quenching, which is manifested when the temperature of phosphor material increases. Therefore, increasing the heating rate also contributes to increasing the temperature of phosphor and thereby decreasing TL sensitivity. Having recorded the TL response for different heating rate, the authors have also studied the dose response of this phosphor with respect to beta irradiation (90Sr). The dose response varied linearly over the wide dose range from 100 mGy to 4 Gy (Figure 6). Linearity of dose response makes calibration easy and straightforward. A linear TL signal– dose relationship starting from dose zero is preferred for many applications. Reproducibility of the TL response between different batches is one of the important criteria for suitability of the phosphor for dosimetric applications. To study the reproducibility between different batches, TL response from five batches were tested. Five aliquots one from each batch were given a dose of 1 Gy, and the TL glow curve was recorded. Amongst in all batches, the TL was found to be reproducible within +10 %. Determination of kinetic parameters The shape of a TL glow peak plays an important role in basic research and in TL applications. In the case
Figure 4. Shifting of glow peak maxima with respect to heating rate.
Figure 5. Variation of TL sensitivity with respect to heating rate.
Figure 6. Dose linearity response of SrSiO3:Tb3þ phosphor.
of basic TL research, it is the basis of important and convenient methods for calculating the trapping parameters of distinct energy levels within the crystal. These methods are based on measurements of a few
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Figure 3. TL glow curve of SrSiO3:Tb3þ recorded at heating rate of 4 K s21.
THERMOLUMINESCENCE STUDIES OF SrSiO3:Tb3þ
points on the glow-peak. The complete description of the TL characteristics of TL material requires the knowledge of these parameters. The TL glow curves are characterised by determining various factors like the order of kinetics (b), activation energy (E) and frequency factor (s). These parameters determine the stability of the traps. In general, for a better stability of the traps, the activation energy (E) should be high, which results in the higher glow peak temperature and hence less fading. Such kinetic parameters of the TL glow peaks can be evaluated by using various methods viz. Chen’s peak shape method(18), initial rise (IR) method(19), whole glow peak method and isothermal decay method(20).
The peak shape methods are based on certain characteristics of a single glow peak, namely the peak maximum temperature Tm, and the temperatures at half-maximum TL intensity T1 and T2 at the lowand high-temperature side of the glow peak, respectively. These quantities are used to define further the peak widths v ¼ T2 2 T1, d ¼ T2 2 Tm and t ¼ Tm 2 T1 as well as the symmetry factor of the glow peak mg ¼ d/ v(21). The equation deduced by Chen is [Ea ¼ 2 /a) 2 (ba(2kTm)], where Ea represents activa(cakTm tion energy, k is Boltzmann’s constant and a can be either w ¼ T2 2 T1 (where T1 and T2 are low- and high-maximum intensity temperature), t ¼ T1 2 Tm or d ¼ T2 2 Tm (where Tm is the glow peak maximum). The values of c and b are constants given by Chen(19) and are summarised as follows: ct ¼ 1:510 þ 3:0ðm 0:42Þ bt ¼ 1:58 þ 4:2ðm 0:42Þ cd ¼ 0:976 þ 7:3ðm 0:42Þ bd ¼ 0 cv ¼ 2:52 þ 10:2ðm 0:42Þ bv ¼ 1
Variable heating rate method The symmetry factor mg is given by mg ¼ d/v, which is used to determine the order of the kinetics
This method essentially makes use of change in glow peak temperature Tm with respect to heating rate.
Table 1. Kinetic parameters calculated for the TL glow curve using the peak shape method at various heating rates.
b (K s21)
0.5 1 2 3 4 6 Average
Tm (K)
572 579 584 593 595 602
2 Tm /t
6961 9061 7414 7033 7861 6969
2 Tm /d
7436 7450 6687 7645 8851 6969
2 Tm /v
3595 3810 3516 3663 4165 3775
mg ¼ d/v
0.48 0.51 0.53 0.48 0.47 0.46 0.49
Frequency factor (s21)
Activation energy E (eV) Et
Ed
Ew
St
Sd
Sw
0.889 1.214 0.955 0.893 1.022 0.887 0.977
1.096 1.098 0.985 1.126 1.304 1.214 1.137
0.998 1.062 0.972 1.015 1.168 1.048 1.044
9.7` 105 1.43` 109 1` 107 3.1` 106 5.5` 107 4.1` 106 2.5` 108
8.1` 107 1.3` 108 1.9` 107 3.8` 108 1.8` 1010 3.1` 109 3.6` 109
1` 107 5.9` 107 1.5` 107 3.9` 107 1.1` 109 1.1` 108 2.2` 108
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Chen’s peak shape method
of the glow peak. The standard value of mg for the first-order kinetics is 0.42, and for second-order kinetics, it is 0.52(22). The calculated value of the symmetry factor is found to be 0.5. The glow peaks are de-convoluted using the software ‘Origin Pro-6.1’ from which the values of T1, T2 and Tm are determined. The calculated trap parameters are given in Table 1. 2 /t From Table 1, it is observed that the ratios of Tm 2 and Tm /d were found to scatter strongly as compared 2 /v. However, it is expected that these with that of Tm ratios should be constant. Previous studies indicate that the presence of temperature lag between the TL sample and heating element due to improper thermal contact (23) could be one of the reasons for such variations. Such changes were also reflected in the corresponding values of activation energies and frequency 2 /v ratios are factors. However, since the values of Tm relatively constant, no significant variations were observed in the corresponding values of activation energies and frequency factors. It is also evident from Table 1 that, as the heating rate increased, the glow peak maxima (Tm) has shifted to high temperature. Since this shifting is marginal, no significant variation in trap depth has been noticed. However, frequency factor increased with respect to heating rate. In the present study, all the measurements were carried out with powdered sample, and hence, the effect of temperature lag can be considered as negligible for reasons that (a) powder samples have a better contact with heating element (24 – 26), (b) due to their very low thermal mass, they do not have appreciable thermal gradient within the sample(27) and (c) the powder is spread over a small area of the planchet in order to avoid the possible temperature gradient along the heating element (27).
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From the Booth– Bohun method(28, 29), for different sets of heating rates and their corresponding Tm, the activation energy (E) and frequency factor (s) can be calculated by the following expressions: " 2 !# kTm1 Tm2 b1 Tm2 In ð1Þ E¼ Tm1 Tm2 b2 Tm1 2 3 2 Tm 2 !ðT 1 T 2 m mÞ E b1 6 b1 Tm2 7 s¼ 5 24 b 1 kðTm1 Þ 2 Tm
ð2Þ
Initial rise method This method(19) is based on the assumption that the amount of trapped electrons in the low-temperature tail of a TL glow peak is approximately constant, since the dependence of n(T ) on temperature T is negligible in that temperature region. This remains true for temperatures up to a cut-off temperature Tc, corresponding to TL intensity ‘Ic’ smaller than 15 % of the maximum TL intensity ‘Im’. However, the IR technique can be used only when the glow peak is well defined and is clearly separated from the other peaks. To calculate the trap depth and the frequency factor of both Peaks 1 and 2, the graph is plotted as [1/kT] versus ln (I) (Figures 7a and b). The plot of ln (I) versus S[1/kT] gave straight line. The slope of the graph gives the trap depth, and the frequency factor can be calculated using the formula: s ¼ b exp (intercept). The calculated values are mentioned in Table 2. The value of the activation energy calculated for Peak 2 was found to differ significantly from the values obtained by Chen’s peak method. Thus, to minimise the error in E, a Tangent method proposed by Ilich(30) is used. Taking a point on initial rising part of a TL Peak 2 (Figure 7c), a tangent is drawn at the point X (Tc, Ic) and slope is calculated. The activation energy E is calculated by using the formula: E ¼ kTc2/(Tc 2 T0). The corresponding value is found to be 0.988 eV, which correlates well with that obtained by Chen’s peak method. The corresponding value of frequency factor is found to be 1.3` 107. Whole glow peak method It is also known as ‘area method’(20) and is based on the measurement of the integral area under a glow peak. This method can be applied when a glow peak is well defined and isolated. The value of the integral
Figure 7. Initial rise plot of beta-irradiated SrSiO3:Tb3þ for Peak 1 (a), Peak 2 (b) and (c) tangent method for correcting activation energy.
n(T ) of the TL intensity over a certain temperature region can be estimated by the area under the glow curve from a given temperature T0 in the IR region, up to the final temperature Tf at the end of the glow peak by using the following equation: 1 n¼ b
T ðf
I dt To
Figure 8 shows the plot of ln (TL/areab) versus 1/kT for Peak 1 and Peak 2. The TL is recorded at the heating rate of 4 K s21. For several values of order namely b ¼ 1.6, 1.8, 2.0, and 2.2, the plot clearly shows the straight lines without any deviation. The straight line fitted with highest value of R 2 is selected, and the activation energy and frequency factors are determined from the slope and intercept of the graph. In the present case, the highest value for R 2 in case of Peak 1 is 0. 99985 (Figure 8a), and for Peak 2,
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1 2 From Table 1, the values of Tm and Tm are 584 and 593 K and the values of b1 and b2 are 2 and 3 K s21, respectively. The values of ‘E’ and ‘s’ calculated by using Equations (1) and (2) are found to be 1.24 eV and 4.5` 109 s21, respectively.
THERMOLUMINESCENCE STUDIES OF SrSiO3:Tb3þ Table 2. Kinetic parameters calculated by various methods. Methods
Peak 1 (Tm ¼ 535 K)
Peak 2 (Tm ¼ 595 K)
Trap depth (eV)
Frequency factor (s21)
Trap depth (eV)
Frequency factor (s21)
Chen’s d method Initial rise method
0.65+0.009 0.6 + 0.005
1.5` 105 4.9` 108
Whole glow peak method
0.6 + 0.002
1.2` 102
1.3 + 0.021 2.2 + 0.008 0.988 + 0.02a 2.3 + 0.008 1.37 + 0.02a
2` 1010 1.4` 1023 1.3` 107a 4.2` 1016 5.2` 107a
a
Corrected E-values by using methods proposed by Ilich(30) and Halperin and Braner(31).
Isothermal decay method
Figure 8. Plot of ln (TL/areab) versus 1/kT for (a) Peak 1 and (b) Peak 2 for several values of the kinetic order b ¼ 1.6, 1.8, 2.0 and 2.2.
the value of R 2 is 0.99940 (Figure 8b) when b ¼ 2.2. The corresponding values of activation energy and frequency factor are presented in Table 2. Similar to IR method, the value of E calculated for Peak 2 using this method is also found to vary significantly. Hence, the method suggested by Halperin and Braner(31) is used. In this method, the value of n is calculated by the area of the glow peak (Figure 7c) from a given point to the end of the peak. The value of activation energy thus obtained is found to be 1.37 eV,
The isothermal decay experiment (20) consists of quickly heating the sample after irradiation to a particular temperature (close to the peak temperature) and keeping the sample at this temperature for a certain time interval. During this time interval, the light output (also termed phosphorescence decay) is measured as a function of time, and so, it is possible to evaluate the decay rate of trapped electrons. This procedure is repeated for various temperatures, namely for 548, 553, 558, 563 and 568 K with the heating rate of 4 K s21. Figure 9a presents the plot of TL intensity versus time at constant temperature (558 K). They are called isothermal decay curves. The value of b corresponding to the straight line fitted with highest R 2 is selected, and the graph for various temperatures for selected b value is plotted between TL intensity and time (Figure 9b). The straight line is obtained from which the slope of the line corresponding to different temperature can be determined. To calculate the activation energy and frequency factor, the graph between ln (jslopej) versus 1/kT (Figure 10) is plotted, which is again a straight line with slope ¼ 2E and a Y-intercept equal to ln (s). Thus, the activation energy and the frequency factor are found to be 1.2 eV and 5.1` 108 s21, respectively. However, during the thermal annealing process because of closely overlapping peaks, there is always an uncertainty that the preceding peak has not been completely removed. Also, the activation energy ‘E’ is known to vary depending upon the degree of overlapping and the temperature to which the sample is heated. This may lead to inaccuracy in obtaining ‘E’ values. Since frequency factor‘s’ is found to be dependent on ‘E’, it varies by several order of magnitude even for a small change in ‘E’. Thus, there is a possible uncertainty in calculating the values of trapping parameters by this method especially in case of complex glow curves.
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which also correlates well with that obtained by Chen’s peak method. The corresponding value of frequency factor is found to be 5.2` 107.
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CONCLUSION
Figure 10. Plot of ln (jslopej) versus 1/kT for b ¼ 2.
Trap depths and frequency factors have been determined using three different methods. Table 2 compares the trap depth and frequency factor estimated using IR, Chen’s and whole glow peak methods.
Tb3þ-doped SrSiO3 synthesised by simple co-precipitation technique exhibits intense green emission at 543 nm due to cross-relaxation. High-temperature TL peak appearing at 593 K is found to a dosimetrically useful glow peak. Due to the presence of high-temperature glow peak, this phosphor can be explored further as a TL phosphor, which may find application in radiation dosimetry. Linear dose response is observed over a wide range of radiation dose. The kinetic parameters are calculated by different methods found to correlate well, and the order of kinetics was found to be 2. Amongst several methods discussed earlier, in case of glow peak method, the whole glow curve is utilised in the analysis rather than just few data points on the glow curve as in Chen’s peak method. Thus, the probability for accurate
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Figure 9. Isothermal decay curves of beta-irradiated SrSiO3:Tb3þ. (a) Isothermal decay curves at constant temperature (558 K) for various values of kinetic order b. (b) Isothermal decay curves for kinetic order b ¼ 2 at temperatures (548, 553 and 558 K).
According to Mott and Gurney(32), frequency factor ‘s’ should be of the order of magnitude of Debye frequency (1010 2 1013 s21). The order of magnitude of ‘s’ gives the conclusion about the nature of recombination mechanism involved in the TL process. The values of ‘s’ observed in case of Chen0 s peak and whole glow peak methods for Peak 1 are ,105 s21. This indicates that the recombination process is occurred due to delocalised transitions. However, in case of Peak 2, it is greater indicating the recombination process of carriers occurs as a thermal release of carriers from a trap and diffusion of electrons or holes to a recombination centre via the conduction or the valence band(33). Traps of similar depth have been identified in IR and whole glow peak method and Chen0 s method (0.6 eV) for Peak-1. Also, the values of frequency factors by several methods correlate well for Peak 1. However, marginal variation is observed in IR method in case of Peak 2. The value obtained by Chen’s peak method and whole glow peak method correlates well. These variations could be attributed to (a) the presence of thermal quenching effects and (b) the formation of trap clusters instead of randomly distributed defects, which causes the variations in the kinetics of trapping and recombination process and thus influencing the TL properties of a phosphor(34). The cluster formation has also been reported earlier for host materials containing rare-earth elements as impurities(35, 36). Similar effects have also been reported by M. Geokce et al.(37) in case of Tb-doped Mg2SiO4. This effect is more serious in the ‘residual’ isothermal decay technique, where the samples are held at the decay temperatures for long time and thermal quenching effect with heating rate. Hence, the observed variation in the value of activation energy calculated by isothermal decay method could be attributed to defect clustering and impurity precipitation occurring at the isothermal decay temperatures.
THERMOLUMINESCENCE STUDIES OF SrSiO3:Tb3þ
determination of trapping parameters is high. Thus, the values of ‘E’ and ‘s’ obtained by this method can be considered as more consistent as compared with several other methods.
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15. Benz, F., Strunk, H. P., Schaab, J., Kunecke, U. and Wellmann, P. Tuning the emission colour by manipulating terbium-terbium interactions: terbium doped aluminum nitride as an example system. J. Appl. Phys. 114, 0735181-6 (2013). 16. Som, S. and Sharma, S. K. Eu3þ/Tb3þ-codoped Y2O3 nanophosphors: Rietveld refinement, bandgap and photoluminescence optimization. J. Phys. D: Appl. Phys. 45, 415102-1-12 (2012). 17. Ege, A., Ekdal, E., Karali, T., Can, N. and Prokic, M. Effect of heating rate on kinetic parameters of b-irradiated Li2B4O7:Cu, Ag, P in TSL measurements. Meas. Sci. Technol. 18, 889– 892 (2007). 18. Chen, R. On the calculation of activation energies and frequency factors from glow curves. J. Appl. Phys. 40, 570–584 (1969). 19. Garlick, G. F. and Gibson, A. F. The electron trap Mechanism of luminescence in sulphide and silicate phosphors. Proc. Phys. Soc. 60, 574– 590 (1948). 20. Chen, R. and McKeever, S. W. S. Theory of thermoluminescence and related phenomena (World Scientific Publishing) (1997). 21. Kitis, G., Chen, R. and Pagonis, V. Thermoluminescence glow-peak shape methods based on mixed order kinetics. Phys. Stat. Sol. 205, 1181–1189 (2008). 22. Horowitz, Y. S., Avila, O. and Rodriguez-Villafuerte, M. Theory of heavy charged particle response (efficiency and supralinearity) in TL materials. Nucl. Instrum. Meth. Phys. Res. B 184, 85–112 (2001). 23. Montalvo, T., Furetta, C., Kitis, G., Azorı´n, J. and Vite, R. M. Influence of heating rate on thermoluminescence of zirconium oxide UV irradiated. Radiat. Eff. Defects solids. 159, 217– 222 (2004). 24. Taylor, G. C. and Lilley, E. Rapid readout rate studies of thermoluminescence in LiF (TLD-100) crystals: III. J. Phys. D: Appl. Phys. 15, 2053– 2065 (1982). 25. Betts, D. S., Couturier, L., Khayrat, A. H., Luff, B. J. and Townsend, P. D. Temperature distribution in thermoluminescence experiments I: experimental results. J. Phys. D: Appl. Phys. 26, 843–848 (1993). 26. Facey, R. A. Heating-rate effects in glow peak measurements for thermoluminescent dosimetry. Health Phys. 12, 717–720 (1966). 27. Furetta, C., Kitis, G., Kuo, J. H., Vismara, L. and Weng, P. S. Impact of non-ideal heat transfer on the determination of thermoluminescent kinetic parameters. J. Lumin. 75, 341–351 (1997). 28. Booth, A. H. Calculation of electron trap depths from thermoluminescence maxima. Can. J. Chem. 32, 214–215 (1954). 29. Bohun, A. Thermoemission und Photoemission von Natriumchlorid. Czech J. Phys. 4, 91–93 (1954). 30. Ilich, B. M. Method of determination of trap depth. Sov. Phys. Solid State. 21, 1880– 1882 (1979). 31. Halperin, A. and Braner, A. A. Evaluation of thermal activation energies from glow curves. Phys. Rev. 117, 408–412 (1960). 32. Mott, N. F. and Gurney, R. W. Electronic Processes in Ionic Crystals (Oxford Univ. Press) (1948). 33. Kui, H. W., Lo, D., Tsang, Y. C., Khaidukov, N. M. and Makarov, V. N. Thermoluminescence properties of double potassium yttrium fluorides singly doped with Ce3þ, Tb3þ, Dy3þ and Tm3þ in response to a and b irradiation J. Lumin. 117, 29–38 (2006).
R. A. BARVE ET AL. 34. Ogundare, F. O., Balogun, F. A. and Hussain, L. A. Heating rate effects on the thermoluminescence of fluorite. Radiat. Meas. 40, 60–64 (2005). 35. Karali, T., Rowlands, A. P., Townsend, P. D., Prokic, M. and Olivares, J. Spectral comparison of Dy, Tm and Dy/Tm in CaSO4 thermoluminescent dosimeters. J. Phys. D: Appl. Phys. 31, 754– 765 (1998).
36. Holsa, J., Aitasalo, T., Jungner, H., Lastusaari, M., Niittykoski, J. and Spano, G. Role of defect states in persistent luminescence materials. J. Alloy Compd. 374, 56– 59 (2004). 37. Goekce, M., Oguz, K. F., Karali, T. and Prokic, M. Influence of heating rate on thermoluminescence of Mg2SiO4 : Tb dosimeter. J. Phys. D: Appl. Phys. 42, 105412-1-5 (2009).
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DOSIMETRIC INVESTIGATIONS OF Tb31-DOPED STRONTIUM SILICATE PHOSPHOR R. A. Barve1, N. Suriyamurthy1,*, B. S. Panigrahi2 and B. Venkatraman1 1 Radiological Safety Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India 2 Technical Services Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603102, India *Corresponding author: [email protected] Received 29 January 2014; revised 23 May 2014; accepted 14 June 2014
INTRODUCTION Silicates are efficient luminescent hosts, mainly because of their rigid and very stable crystal structures. Several silicate materials doped with rare earth ions, either divalent or trivalent, have been proposed for or used as commercial phosphors in tricolour fluorescent lamps, scintillators, etc.(1) The luminescent materials based on silicates as host material exhibit strong chemical stability, high-energy ion irradiation resistance, high UV and visible light transmittance. Variety of silicates such as M2SiO4, M3SiO5 and M3MgSi2O8 (M ¼ Ca, Sr, Ba) have been investigated as the host materials for phosphors doped with trivalent rare earth ions or transition metal ions(2 – 7). The silicate hosts exhibit shift in emission band of cerium and europium ions when cation of silicates changed from calcium to strontium. Also amongst several rare earths, Tb3þ is an intense green emitter having narrow line emission due to f –f electronic transition peaking at 544 nm(8). Besides their photonic applications, silicates were investigated for development of possible dosimetric materials. The first TL material based on magnesium orthosilicate was introduced in radiation dosimetry in 1971(9, 10). Tb3þ-doped Mg2SiO4 is known to be a promising TLD material for the dosimetry due to its high TL sensitivity(11) that can also be used as a fibre optic dosemeter to access the real-time dose(12). There exist several methods inter alia solid-state reactions method, hydrothermal method, sol gel, which are being used for the synthesis of various Ba-, Ca- or Sr-based silicates and require sophisticated instruments and high calcination temperature(6, 7, 13, 14). However, the co-precipitation technique is one of the facile methods to prepare the silicate materials that introduce significant amount of defects leading to improved thermoluminescence (TL) in polycrystalline materials.
This paper discusses about the cross-relaxation phenomenon and thermoluminescence dosimetric properties of SrSiO3:Tb3þ with special emphasis on the determination of kinetic parameters using different methods. EXPERIMENTAL AR grade Sr(NO3)2 and Na2SiO3 taken in stoichiometric ratio were used as precursors along with Tb3þ as dopant for synthesising SrSiO3:Tb3þ by co-precipitation technique. The obtained precipitate was then washed several times with water to remove the unreacted compounds and allowed to dry for several hours under drying lamp to remove excess amount of water. This powder was annealed in air atmosphere at 7008C for 3 h and allowed to cool slowly to room temperature. The PL spectrum was recorded using Shimadzu Spectrofluorometer equipped with 150 W of Xenon lamp, and TL was recorded using TL/OSL-DA-20 Risoe reader with various heating rates from 0.1 to 6 K s21 depending on the purpose of investigation. All the samples were normalised with respect to weight and dose. The samples are irradiated with in-built 90 Sr beta source having dose rate of 0.1 Gy s21. RESULTS AND DISCUSSION X-ray diffraction analysis Figure 1 shows the XRD profile of SrSiO3:Tb3þ. All the important diffraction profiles match with SrSiO3 (monoclinic) phase (JCPDS No. 87-0474). Sharp X-ray diffraction peaks indicate the formation of highly crystalline phase after annealing at 7008C. Besides, no impurity peaks and no signatures from dopant are noticed.
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
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Tb31-doped SrSiO3 phosphor synthesised by co-precipitation technique exhibits intense green emission due to cross-relaxation phenomena between Tb31 ions. Dosimetric properties of this phosphor have been investigated using thermoluminescence (TL) technique. A dosimetrically useful glow peak observed was at 581 K along with a linear dose response over the wide dose range (100 mGy– 4 Gy). TL parameters such as trap depth (E), frequency factor (s) and the order of kinetics (b) are determined by different methods such as Chen’s peak shape, initial rise, isothermal decay and variable heating rate methods. Results of these methods are compared and reported in this study.
THERMOLUMINESCENCE STUDIES OF SrSiO3:Tb3þ
diagrams of the RE3þ series. On the other hand, the gap below the 5D3 level is 5700 cm21, and hence, cross-relaxation from this energy level is insignificant (16). The intense green emission from this host prompted the authors to study its TL response too. Figure 1. X-ray diffraction pattern of SrSiO3:Tb3þ heated at 7008C for 3 h in air.
Cross-relaxation of terbium luminescence Figure 2 shows the photoluminescence spectra of Tb3þ-doped SrSiO3. The sample exhibits excitation bands at 237, 352 and 379 nm (Figure 2a). The band at 237 nm is charge transfer band between O22 and Tb3þ, and this is completely allowed. On the other hand, peaks at 352 and 379 nm are essentially due to forbidden absorption transitions. Upon exciting at 379 nm, the authors could observe well-defined narrow emission lines due to f –f transitions of 490 and 550 nm (Figure 2b) corresponding to 5D4 – 7F6 and 5D4 – 7F5 transitions, respectively. Cross-relaxation is a process wherein the excitation energy from terbium ion is transferred partially to neighbouring luminescent terbium ion resulting in quenching of blue emission and enhancement of green emission(15). Exploiting this property, one can selectively tune the luminescent material for enhanced green emission. In this host, terbium undergoes cross-relaxation and hence emission intensity of peaks at 437, 460 and 488 nm originating from 5D3 is less than the intensity of 544-nm emission from 5D4. This phenomenon can be manipulated by readjusting the concentration of terbium ions. Cross-relaxation does not occur from all energy level. The probability of non-radiative relaxation from the 5D4 level is very low, because the band gap to the next available level is very large 14 600 cm21, one of the largest gaps in the energy-level
Thermoluminescence Thermoluminescence phosphors generally exhibit glow curves with one or more peaks when the charge carriers are released. The glow curve is the characteristic of different trap levels that lie in the band gap of materials. The traps are specified by the some parameters including trap depth (E) and frequency factor (s). Figure 3 shows the thermo-stimulated luminescence response of Tb3þ-activated SrSiO3. For the absorbed dose of 1 Gy delivered by beta-radiation, the sample exhibits well-defined glow peak at 593 K. TL glow peak position and peak width are the finger prints of TL dosimetry. The observed glow peak is broad and hence de-convoluted using Origin 6.1. After the deconvolution, two glow peaks having maxima at 535 and 595 K have been obtained. The peak at 593 K is dosimetrically useful peak and hence investigated extensively for its possible TL applications. The figure of merit was found to be 0.5 %, which reflects that the fit is very good. Effect of heating rate on peak maxima (Tm) and TL sensitivity Studies on the role of heating rate indicated the change in glow peak maxima that was shifting towards high temperature while keeping the dose constant. Here, the heating rate was varied from 0.5 to 6 K s21. At lower heating rate, the time spent by charge carriers is long enough that the thermal release of charge carriers from traps depends on the half-life whereas for higher heating rate, the thermal release of
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Figure 2. Photoluminescence spectra of SrSiO3:Tb3þ. (a) Excitation spectrum (lemi-544 nm), (b) emission spectrum (lexi-379 nm).
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carriers decreases. Therefore, at this heating rate, higher temperature is required to ensure effective detrapping of carriers and hence is the observed shifting of glow peak maxima (Figure 4). Though peak maxima have shifted, there has been no change in shape of glow curve with heating rate. On other hand, as the heating rate is increased, the TL sensitivity of this phosphor decreases (Figure 5). This is probably due to fact that, at faster heating rate, the shallow traps are getting emptied faster than resulting in reduced integrated area under the glow curve(17). The TL sensitivity being proportional to area under the curve, the sensitivity consequently decreased. Another factor that influences the luminescence efficiency is thermal quenching, which is manifested when the temperature of phosphor material increases. Therefore, increasing the heating rate also contributes to increasing the temperature of phosphor and thereby decreasing TL sensitivity. Having recorded the TL response for different heating rate, the authors have also studied the dose response of this phosphor with respect to beta irradiation (90Sr). The dose response varied linearly over the wide dose range from 100 mGy to 4 Gy (Figure 6). Linearity of dose response makes calibration easy and straightforward. A linear TL signal– dose relationship starting from dose zero is preferred for many applications. Reproducibility of the TL response between different batches is one of the important criteria for suitability of the phosphor for dosimetric applications. To study the reproducibility between different batches, TL response from five batches were tested. Five aliquots one from each batch were given a dose of 1 Gy, and the TL glow curve was recorded. Amongst in all batches, the TL was found to be reproducible within +10 %. Determination of kinetic parameters The shape of a TL glow peak plays an important role in basic research and in TL applications. In the case
Figure 4. Shifting of glow peak maxima with respect to heating rate.
Figure 5. Variation of TL sensitivity with respect to heating rate.
Figure 6. Dose linearity response of SrSiO3:Tb3þ phosphor.
of basic TL research, it is the basis of important and convenient methods for calculating the trapping parameters of distinct energy levels within the crystal. These methods are based on measurements of a few
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Figure 3. TL glow curve of SrSiO3:Tb3þ recorded at heating rate of 4 K s21.
THERMOLUMINESCENCE STUDIES OF SrSiO3:Tb3þ
points on the glow-peak. The complete description of the TL characteristics of TL material requires the knowledge of these parameters. The TL glow curves are characterised by determining various factors like the order of kinetics (b), activation energy (E) and frequency factor (s). These parameters determine the stability of the traps. In general, for a better stability of the traps, the activation energy (E) should be high, which results in the higher glow peak temperature and hence less fading. Such kinetic parameters of the TL glow peaks can be evaluated by using various methods viz. Chen’s peak shape method(18), initial rise (IR) method(19), whole glow peak method and isothermal decay method(20).
The peak shape methods are based on certain characteristics of a single glow peak, namely the peak maximum temperature Tm, and the temperatures at half-maximum TL intensity T1 and T2 at the lowand high-temperature side of the glow peak, respectively. These quantities are used to define further the peak widths v ¼ T2 2 T1, d ¼ T2 2 Tm and t ¼ Tm 2 T1 as well as the symmetry factor of the glow peak mg ¼ d/ v(21). The equation deduced by Chen is [Ea ¼ 2 /a) 2 (ba(2kTm)], where Ea represents activa(cakTm tion energy, k is Boltzmann’s constant and a can be either w ¼ T2 2 T1 (where T1 and T2 are low- and high-maximum intensity temperature), t ¼ T1 2 Tm or d ¼ T2 2 Tm (where Tm is the glow peak maximum). The values of c and b are constants given by Chen(19) and are summarised as follows: ct ¼ 1:510 þ 3:0ðm 0:42Þ bt ¼ 1:58 þ 4:2ðm 0:42Þ cd ¼ 0:976 þ 7:3ðm 0:42Þ bd ¼ 0 cv ¼ 2:52 þ 10:2ðm 0:42Þ bv ¼ 1
Variable heating rate method The symmetry factor mg is given by mg ¼ d/v, which is used to determine the order of the kinetics
This method essentially makes use of change in glow peak temperature Tm with respect to heating rate.
Table 1. Kinetic parameters calculated for the TL glow curve using the peak shape method at various heating rates.
b (K s21)
0.5 1 2 3 4 6 Average
Tm (K)
572 579 584 593 595 602
2 Tm /t
6961 9061 7414 7033 7861 6969
2 Tm /d
7436 7450 6687 7645 8851 6969
2 Tm /v
3595 3810 3516 3663 4165 3775
mg ¼ d/v
0.48 0.51 0.53 0.48 0.47 0.46 0.49
Frequency factor (s21)
Activation energy E (eV) Et
Ed
Ew
St
Sd
Sw
0.889 1.214 0.955 0.893 1.022 0.887 0.977
1.096 1.098 0.985 1.126 1.304 1.214 1.137
0.998 1.062 0.972 1.015 1.168 1.048 1.044
9.7` 105 1.43` 109 1` 107 3.1` 106 5.5` 107 4.1` 106 2.5` 108
8.1` 107 1.3` 108 1.9` 107 3.8` 108 1.8` 1010 3.1` 109 3.6` 109
1` 107 5.9` 107 1.5` 107 3.9` 107 1.1` 109 1.1` 108 2.2` 108
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Chen’s peak shape method
of the glow peak. The standard value of mg for the first-order kinetics is 0.42, and for second-order kinetics, it is 0.52(22). The calculated value of the symmetry factor is found to be 0.5. The glow peaks are de-convoluted using the software ‘Origin Pro-6.1’ from which the values of T1, T2 and Tm are determined. The calculated trap parameters are given in Table 1. 2 /t From Table 1, it is observed that the ratios of Tm 2 and Tm /d were found to scatter strongly as compared 2 /v. However, it is expected that these with that of Tm ratios should be constant. Previous studies indicate that the presence of temperature lag between the TL sample and heating element due to improper thermal contact (23) could be one of the reasons for such variations. Such changes were also reflected in the corresponding values of activation energies and frequency 2 /v ratios are factors. However, since the values of Tm relatively constant, no significant variations were observed in the corresponding values of activation energies and frequency factors. It is also evident from Table 1 that, as the heating rate increased, the glow peak maxima (Tm) has shifted to high temperature. Since this shifting is marginal, no significant variation in trap depth has been noticed. However, frequency factor increased with respect to heating rate. In the present study, all the measurements were carried out with powdered sample, and hence, the effect of temperature lag can be considered as negligible for reasons that (a) powder samples have a better contact with heating element (24 – 26), (b) due to their very low thermal mass, they do not have appreciable thermal gradient within the sample(27) and (c) the powder is spread over a small area of the planchet in order to avoid the possible temperature gradient along the heating element (27).
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From the Booth– Bohun method(28, 29), for different sets of heating rates and their corresponding Tm, the activation energy (E) and frequency factor (s) can be calculated by the following expressions: " 2 !# kTm1 Tm2 b1 Tm2 In ð1Þ E¼ Tm1 Tm2 b2 Tm1 2 3 2 Tm 2 !ðT 1 T 2 m mÞ E b1 6 b1 Tm2 7 s¼ 5 24 b 1 kðTm1 Þ 2 Tm
ð2Þ
Initial rise method This method(19) is based on the assumption that the amount of trapped electrons in the low-temperature tail of a TL glow peak is approximately constant, since the dependence of n(T ) on temperature T is negligible in that temperature region. This remains true for temperatures up to a cut-off temperature Tc, corresponding to TL intensity ‘Ic’ smaller than 15 % of the maximum TL intensity ‘Im’. However, the IR technique can be used only when the glow peak is well defined and is clearly separated from the other peaks. To calculate the trap depth and the frequency factor of both Peaks 1 and 2, the graph is plotted as [1/kT] versus ln (I) (Figures 7a and b). The plot of ln (I) versus S[1/kT] gave straight line. The slope of the graph gives the trap depth, and the frequency factor can be calculated using the formula: s ¼ b exp (intercept). The calculated values are mentioned in Table 2. The value of the activation energy calculated for Peak 2 was found to differ significantly from the values obtained by Chen’s peak method. Thus, to minimise the error in E, a Tangent method proposed by Ilich(30) is used. Taking a point on initial rising part of a TL Peak 2 (Figure 7c), a tangent is drawn at the point X (Tc, Ic) and slope is calculated. The activation energy E is calculated by using the formula: E ¼ kTc2/(Tc 2 T0). The corresponding value is found to be 0.988 eV, which correlates well with that obtained by Chen’s peak method. The corresponding value of frequency factor is found to be 1.3` 107. Whole glow peak method It is also known as ‘area method’(20) and is based on the measurement of the integral area under a glow peak. This method can be applied when a glow peak is well defined and isolated. The value of the integral
Figure 7. Initial rise plot of beta-irradiated SrSiO3:Tb3þ for Peak 1 (a), Peak 2 (b) and (c) tangent method for correcting activation energy.
n(T ) of the TL intensity over a certain temperature region can be estimated by the area under the glow curve from a given temperature T0 in the IR region, up to the final temperature Tf at the end of the glow peak by using the following equation: 1 n¼ b
T ðf
I dt To
Figure 8 shows the plot of ln (TL/areab) versus 1/kT for Peak 1 and Peak 2. The TL is recorded at the heating rate of 4 K s21. For several values of order namely b ¼ 1.6, 1.8, 2.0, and 2.2, the plot clearly shows the straight lines without any deviation. The straight line fitted with highest value of R 2 is selected, and the activation energy and frequency factors are determined from the slope and intercept of the graph. In the present case, the highest value for R 2 in case of Peak 1 is 0. 99985 (Figure 8a), and for Peak 2,
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1 2 From Table 1, the values of Tm and Tm are 584 and 593 K and the values of b1 and b2 are 2 and 3 K s21, respectively. The values of ‘E’ and ‘s’ calculated by using Equations (1) and (2) are found to be 1.24 eV and 4.5` 109 s21, respectively.
THERMOLUMINESCENCE STUDIES OF SrSiO3:Tb3þ Table 2. Kinetic parameters calculated by various methods. Methods
Peak 1 (Tm ¼ 535 K)
Peak 2 (Tm ¼ 595 K)
Trap depth (eV)
Frequency factor (s21)
Trap depth (eV)
Frequency factor (s21)
Chen’s d method Initial rise method
0.65+0.009 0.6 + 0.005
1.5` 105 4.9` 108
Whole glow peak method
0.6 + 0.002
1.2` 102
1.3 + 0.021 2.2 + 0.008 0.988 + 0.02a 2.3 + 0.008 1.37 + 0.02a
2` 1010 1.4` 1023 1.3` 107a 4.2` 1016 5.2` 107a
a
Corrected E-values by using methods proposed by Ilich(30) and Halperin and Braner(31).
Isothermal decay method
Figure 8. Plot of ln (TL/areab) versus 1/kT for (a) Peak 1 and (b) Peak 2 for several values of the kinetic order b ¼ 1.6, 1.8, 2.0 and 2.2.
the value of R 2 is 0.99940 (Figure 8b) when b ¼ 2.2. The corresponding values of activation energy and frequency factor are presented in Table 2. Similar to IR method, the value of E calculated for Peak 2 using this method is also found to vary significantly. Hence, the method suggested by Halperin and Braner(31) is used. In this method, the value of n is calculated by the area of the glow peak (Figure 7c) from a given point to the end of the peak. The value of activation energy thus obtained is found to be 1.37 eV,
The isothermal decay experiment (20) consists of quickly heating the sample after irradiation to a particular temperature (close to the peak temperature) and keeping the sample at this temperature for a certain time interval. During this time interval, the light output (also termed phosphorescence decay) is measured as a function of time, and so, it is possible to evaluate the decay rate of trapped electrons. This procedure is repeated for various temperatures, namely for 548, 553, 558, 563 and 568 K with the heating rate of 4 K s21. Figure 9a presents the plot of TL intensity versus time at constant temperature (558 K). They are called isothermal decay curves. The value of b corresponding to the straight line fitted with highest R 2 is selected, and the graph for various temperatures for selected b value is plotted between TL intensity and time (Figure 9b). The straight line is obtained from which the slope of the line corresponding to different temperature can be determined. To calculate the activation energy and frequency factor, the graph between ln (jslopej) versus 1/kT (Figure 10) is plotted, which is again a straight line with slope ¼ 2E and a Y-intercept equal to ln (s). Thus, the activation energy and the frequency factor are found to be 1.2 eV and 5.1` 108 s21, respectively. However, during the thermal annealing process because of closely overlapping peaks, there is always an uncertainty that the preceding peak has not been completely removed. Also, the activation energy ‘E’ is known to vary depending upon the degree of overlapping and the temperature to which the sample is heated. This may lead to inaccuracy in obtaining ‘E’ values. Since frequency factor‘s’ is found to be dependent on ‘E’, it varies by several order of magnitude even for a small change in ‘E’. Thus, there is a possible uncertainty in calculating the values of trapping parameters by this method especially in case of complex glow curves.
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which also correlates well with that obtained by Chen’s peak method. The corresponding value of frequency factor is found to be 5.2` 107.
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CONCLUSION
Figure 10. Plot of ln (jslopej) versus 1/kT for b ¼ 2.
Trap depths and frequency factors have been determined using three different methods. Table 2 compares the trap depth and frequency factor estimated using IR, Chen’s and whole glow peak methods.
Tb3þ-doped SrSiO3 synthesised by simple co-precipitation technique exhibits intense green emission at 543 nm due to cross-relaxation. High-temperature TL peak appearing at 593 K is found to a dosimetrically useful glow peak. Due to the presence of high-temperature glow peak, this phosphor can be explored further as a TL phosphor, which may find application in radiation dosimetry. Linear dose response is observed over a wide range of radiation dose. The kinetic parameters are calculated by different methods found to correlate well, and the order of kinetics was found to be 2. Amongst several methods discussed earlier, in case of glow peak method, the whole glow curve is utilised in the analysis rather than just few data points on the glow curve as in Chen’s peak method. Thus, the probability for accurate
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Figure 9. Isothermal decay curves of beta-irradiated SrSiO3:Tb3þ. (a) Isothermal decay curves at constant temperature (558 K) for various values of kinetic order b. (b) Isothermal decay curves for kinetic order b ¼ 2 at temperatures (548, 553 and 558 K).
According to Mott and Gurney(32), frequency factor ‘s’ should be of the order of magnitude of Debye frequency (1010 2 1013 s21). The order of magnitude of ‘s’ gives the conclusion about the nature of recombination mechanism involved in the TL process. The values of ‘s’ observed in case of Chen0 s peak and whole glow peak methods for Peak 1 are ,105 s21. This indicates that the recombination process is occurred due to delocalised transitions. However, in case of Peak 2, it is greater indicating the recombination process of carriers occurs as a thermal release of carriers from a trap and diffusion of electrons or holes to a recombination centre via the conduction or the valence band(33). Traps of similar depth have been identified in IR and whole glow peak method and Chen0 s method (0.6 eV) for Peak-1. Also, the values of frequency factors by several methods correlate well for Peak 1. However, marginal variation is observed in IR method in case of Peak 2. The value obtained by Chen’s peak method and whole glow peak method correlates well. These variations could be attributed to (a) the presence of thermal quenching effects and (b) the formation of trap clusters instead of randomly distributed defects, which causes the variations in the kinetics of trapping and recombination process and thus influencing the TL properties of a phosphor(34). The cluster formation has also been reported earlier for host materials containing rare-earth elements as impurities(35, 36). Similar effects have also been reported by M. Geokce et al.(37) in case of Tb-doped Mg2SiO4. This effect is more serious in the ‘residual’ isothermal decay technique, where the samples are held at the decay temperatures for long time and thermal quenching effect with heating rate. Hence, the observed variation in the value of activation energy calculated by isothermal decay method could be attributed to defect clustering and impurity precipitation occurring at the isothermal decay temperatures.
THERMOLUMINESCENCE STUDIES OF SrSiO3:Tb3þ
determination of trapping parameters is high. Thus, the values of ‘E’ and ‘s’ obtained by this method can be considered as more consistent as compared with several other methods.
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