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Thermoluminescence response of dysprosium doped strontium tetraborate glasses subjected to electron irradiations
Applied Radiation and Isotopes 102 (2015) 10–14
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Thermoluminescence response of dysprosium doped strontium tetraborate glasses subjected to electron irradiations Tou Ying Lim a,n, H. Wagiran a, R. Hussin a, S. Hashim a a
Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM, Johor Bahru, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 29 September 2014 Received in revised form 28 March 2015 Accepted 13 April 2015 Available online 14 April 2015
The paper presents the thermoluminescence (TL) response of strontium tetraborate glass subjected to electron irradiations at various Dy2O3 concentrations ranging from 0.00 to 1.00 mol%. All glass samples exhibited single broad peak with maximum peak temperature positioned at 170–215 °C. The optimum TL response was found at Dy2O3 concentration 0.75 mol%. This glass showed good linearity and higher sensitivity for 7 MeV compared to 6 MeV electrons. Analysis of kinetic parameters showed that the glasses demonstrate second order kinetic. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Thermoluminescence Electron Glow curve Strontium tetraborate Glass
1. Introduction Thermoluminescence (TL) dosimetry is well known technique for measuring the radiation absorbed dose, particularly for personal, medical and environmental purpose. It has been reported that the TL efficiency of a given dosimeter varied and strongly depends to the type of TL settings implemented during annealing and readout processes, type of impurities used, type of readers, sample storage condition and etc. LiF: Mg, Ti under the trade name of TLD-100 has been recognized as a standard dosimeter in twentieth century. This is because LiF: Mg, Ti has been characterized to have properties close to ideal dosimeter and almost tissue equivalence (Zeff ¼ 8.04). However, the drawback of this dosimeter was apparent especially at low energies since an overresponse of the relative TL efficiency with approximately 10% was detected and thus limits the useful range of this dosimeter (Horowitz, 2006). In addition, its complex glow curve and annealing procedure necessitate the careful analyses and interpretation of dose information. Up to now, many investigations have been conducted to improve the performance of the current dosimeter and explore the new TL materials. Among them, strontium tetraborate is one of the attractive materials for radiation dosimetry since it is not hygroscopic as normally exhibited by other borate compounds (Santiago et al., 2001). In their work, they reported polycrystalline of n
Corresponding author. Fax: þ 6075566162. E-mail address: [email protected] (T.Y. Lim).
http://dx.doi.org/10.1016/j.apradiso.2015.04.005 0969-8043/& 2015 Elsevier Ltd. All rights reserved.
dysprosium doped strontium tetraborate has sensitivity five times higher compared to 7LiF: Mg,Ti. The high mechanical strength of strontium tetraborate also made it as a potential material to be used as radiation dosimetry (Lavat et al., 2004). McKeever et al. (1995) also emphasized the need of deeper research on the control of microstructure of the existing dosimetric materials. In this study, strontium tetraborate in glassy form is proposed and investigated. Dysprosium (Dy) was chosen since it was reported to induce high TL efficiency (Prokic, 1982; 2000). The study aims to measure the TL response of dysprosium doped strontium tetraborate glasses exposed to electron irradiations at room temperature. The kinetic parameters using peak shape method was also presented.
2. Materials and methods 2.1. Sample preparation The glass samples investigated in this work was Dy2O3 doped strontium tetraborate at various Dy2O3 concentrations ranging from 0 to 1.00 mol%. Table 1 shows the nominal compositions of the prepared samples. The binary glasses of B2O3–SrO were prepared by conventional melt quenching method. The ratio of 2:1 were weighed and mixed thoroughly using the analytical grade boric oxide (B2O3) and strontium carbonate (SrCO3) respectively. Small amount of dysprosium oxide (Dy2O3) was added to B2O3– SrO glass to confirm the role of dopant during TL measurements.
T.Y. Lim et al. / Applied Radiation and Isotopes 102 (2015) 10–14
11
Table 1 Nominal composition of Dy2O3 doped strontium tetraborate glasses. Glass notation
S0 S1 S2 S3 S4 S5
Concentration (mol%) B2O3
SrO
Dy2O3
66.67 66.59 66.54 66.42 66.29 66.17
33.33 33.26 33.21 33.08 32.96 32.83
0.00 0.15 0.25 0.50 0.75 1.00
The mixtures were melted at 1100 °C at electric furnace for 15– 30 min depending on their compositions. The melts were quenched on stainless steel metal plate and were pressed immediately. The amorphous nature of these glasses were examined by X-ray diffraction analysis at room temperature. All the glass samples demostrate amorphous nature since no sharp bands were observed and the spectra kept similar but with varying peak intensity.
Fig. 1. Glow curve of various Dy2O3 doped strontium tetraborate glasses at various Dy2O3 concentrations subjected to 6 MeV electron at a dose of 3.5 Gy.
2.2. Electron irradiation The glass samples were cut into small pieces with approximately same thickness. They were weighted using electronic balance with an accuracy of 4 decimal places. They were first annealed in an oven at 350 °C for 15 min prior to electron irradiation to ensure the background reading was standardised. Each piece of the glasses was placed in an perspec holder. The samples were irradiated subjected to 6 MeV and 7 MeV electron energies using linear accelerator Varian 2100C and Siemens Primus M3388 available at University Malaya Medical Centre (UMMC) and Johor Specialist Hospital, respectively. Both linear accelerators are calibrated to delivered the prescribed absorbed dose in such that 1 Monitor Unit (MU) gives an absorbed dose of 1 cGy for a 10 10 cm2 field at 100 cm source- to- surface distance (SSD). The energy chosen is the typical energy used for cancer patients treatment. The TL readouts were performed using TL reader Harshaw model 4500. The linear heating rate of 5 °C s 1 was used after few trials of TL readouts using various heating rates which gives the best resolution of the glow curve. The samples were kept in dark atmosphere for 24 h prior to readouts to eliminate any unstable low temperature readings. The TL readings were then normalized to their mass.
Fig. 2. Glow curve of various Dy2O3 doped strontium tetraborate glasses at various Dy2O3 concentrations subjected to 7 MeV electron at a dose of 3.5 Gy.
Fig. 2. The observed broad peak could be due to overlapping of several glow peaks and this phenomena confims the amorphous nature of glass samples. The Tm of the glow curves were estimated to be centered between 170 and 215 °C. 3.2. Optimum Dy2O3 concentration Despite the type of dopants, the concentrations of dopant material are also have direct influence on TL performance. Fig. 3 shows the plot of TL intensity as a function of Dy2O3 concentration of Dy2O3 doped strontium tetraborate glasses subjected to 6 MeV
3. Results and discussion 3.1. Glow curve Glow curve is one of the important feature for better understanding of TL phenomena. They are usually plotted as light intensity against the heating temperature. The shape and the position of the glow curve can reveal the type of trapping states and their fading characteristics of the corresponding material. It is well known that the ideal glow curve should has single sharp peak positioned between 180 and 250 °C. Fig. 1 shows the glow curves of Dy2O3 doped strontium tetraborate glasses at various Dy2O3 concentrations subjected to 6 MeV electron at a dose of 3.5 Gy. A single broad peak of TL glow curve was observed for all samples with different concentrations of Dy2O3. Most of the maximum peak temperature, Tm of the glow curves was found to shift to higher temperature as Dy2O3 concentration is increased. The same pattern was also observed for the glass samples exposed to 7 MeV electron energy as shown in
Fig. 3. TL intensity as a function of Dy2O3 concentration of Dy2O3 doped strontium tetraborate glasses subjected to 6 MeV electron at a dose of 3.5 Gy.
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T.Y. Lim et al. / Applied Radiation and Isotopes 102 (2015) 10–14
Table 2 Sensitivity and relative sensitivity of S4 glass sample to S0 (undoped) glass sample. Electron energies (MeV)
6 7
Sensitivity(nC mg 1 Gy 1) S4
S0
22.92 25.60
0.39 0.57
Relative sensitivity of S4–S0 glass
58.8 44.9
Table 3 : Calibration factor and minimum detectable dose of Dy2O3 doped strontium tetraborate glass at different electron energies.
Fig. 4. Graph of TL response against dose for S0 and S4 glass samples subjected to 6 MeV electrons irradiation.
Electron energies (MeV)
Calibration factor (Gy nC 1 mg)
Minimum detectable dose (Gy)
6 7
0.0436 0.0391
0.0051 0.0046
Fig. 7. The expressions of τ, δ and ω of the glow curve. Im is the maximum peak intensity of the glow curve.
Fig. 5. Graph of TL response against dose for S0 and S4 glass samples subjected to 7 MeV electrons irradiation.
However, a slight reduction of TL signal was observed for glass sample with 1.00 mol% Dy2O3 (S5). This phenomena is called as concentration quenching. A huge localized energy levels between the conduction band and the valence band are produced which result the enhancement of non–radiative relaxation of charge carriers (Sharma et al., 2008). A similar result was also reported for tellurite glass by other workers (El-Mallawany and Diab, 2012). 3.3. TL response
Fig. 6. Graph of TL response against dose of S4 glass sample subjected to 6 MeV and 7 MeV.
electron at a dose of 3.5 Gy. As expected, the undoped strontium tetraborate glass sample (S0) has the least TL response compared to other Dy2O3 doped samples. This means that dysprosium ion had induced new defect states to glass host which lead to a good improvement in TL performance. Meanwhile, the highest TL response was observed for glass sample with a Dy2O3 concentration of 0.75 mol% (S4). At this dose level, the S4 glass was found to has TL response 58 times higher compared to S0 glass sample.
In order to determine the TL dose dependent on different electron energies, the undoped (S0 glass) and 0.75 mol% Dy2O3 (S4 glass) were used for comparison. Fig. 4 shows the graph of TL response against 6 MeV electrons for doses ranging from 0.50 to 4.00 Gy. It can be seen that the TL response of both glass samples increase as a function of electron doses. Similar pattern was also observed for glasses exposed to 7 MeV electrons at the same dose range as shown in Fig. 5. In order to compare the energy dependence of TL response on S4 glass sample, the TL response of the glass subjected to 6 MeV and 7 MeV electrons was plotted together as shown in Fig. 6. It can be seen that the glass sample exposed to 7 MeV has higher TL response compared to the glass sample irradiated at 6 MeV electrons. The result opposed with the result published by Ribeiro et al. (2008) where they found that the TL sensitivity of pure LiF was inversely proportional to the electron beam energies (i.e. 8, 10 and 14 MeV) in the dose range from 0.10 to 2.00 Gy. However, more radiation energies are needed to confirm the trend of the TL sensitivity of current glass sample. 3.4. TL sensitivity A high sensitivity TL dosimeter is known to be able to measure
T.Y. Lim et al. / Applied Radiation and Isotopes 102 (2015) 10–14
13
Table 4 Kinetic parameters (Eav and s) of the samples subjected to 6 MeV calculated using peak shape method. Sample
Tm (K)
μg ± Δμg
Eτ ± ΔEτ (eV)
Eσ ± ΔEσ (eV)
Eω ± ΔEω (eV)
Eav ± ΔEav (eV)
s ± Δs (s 1)
S0 S1 S2 S3 S4 S5
454 462 489 440 470 462
0.57 70.02 0.56 70.02 0.55 70.01 0.58 70.02 0.57 70.01 0.57 70.02
1.077 0.09 0.81 7 0.05 0.777 0.05 0.977 0.07 0.83 7 0.05 0.88 7 0.06
0.99 7 0.08 0.80 7 0.05 0.78 7 0.04 0.89 7 0.06 0.80 7 0.05 0.84 7 0.05
1.03 7 0.06 0.80 7 0.04 0.78 7 0.03 0.93 7 0.05 0.81 7 0.04 0.86 7 0.04
1.03 70.13 0.80 70.08 0.78 70.07 0.93 70.10 0.81 70.08 0.86 70.09
(7.517 0.94) 1010 (1.247 0.13) 108 (1.737 0.16) 107 (1.23 7 0.14) 1010 (1.117 0.11) 108 (5.84 7 0.61) 108
Table 5 Lifetimes of the undoped and various Dy doped strontium tetraborate glass samples subjected to 6 MeV. Sample
τav (s)
S0 S1 S2 S3 S4 S5
2.90 106 (∼33.6 days) 2.36 105 (∼2.7 days) 7.79 105 (∼9.0 days) 3.67 105 (∼4.3 days) 3.89 105 (∼4.5 days) 5.13 105 (∼5.9 days)
(2)
where μg is dependent to the order of kinetic. The TL material is said to obey the first order kinetics if μg is close to 0.42. While, the TL material is said to obey the second order kinetics provided μg is close to 0.52. Chen (1969) also formulated the expressions for determination of activation energy and frequency factor as below:
⎛ kT2 ⎞ Eα = cα⎜⎜ m ⎟⎟ − bα (2kTm) ⎝ α ⎠
a very low dose in medical, personnel and environmental dosimetry. Bos (2001) defined the sensitivity using a ratio of TL intensity per unit absorbed dose and per unit mass. For useful comparison, the relative sensitivity is used instead of the sensitivity for a given material. The relative sensitivity is defined in the ratio of the sensitivity of that material per unit of the sensitivity of a reference material. Table 2 shows the values of the sensitivity of S4 glass sample and the relative sensitivity of S4–S0 glass samples prepared in this work. The gradients of the graph in Fig. 6 were used to represent the sensitivity of S4 glass sample for 6 MeV and 7 MeV electrons over a dose range of 0.50–4.00 Gy. 3.5. Minimum detectable dose Furetta et al. (2000) defined an expression for minimum detectable dose as:
D0 = (B + 2σB)⋅F
T2 − Tm T2 − T1
μg =
(1)
where B is the average of TL background of annealed but not irradiated samples and sB is the standard deviation of the average TL background. F is the TL calibration factor with a unit of Gy nC 1 mg 1 and can be determined by the slope of the graph of Fig. 6. Table 3 shows the values of calibration factor and minimum detectable dose for 6 and 7 MeV electron energies. The minimum detectable dose was found to vary slightly with electron energies. The glass sample was found to exhibit low minimum detectable dose for 7 MeV compared to 6 MeV electrons. 3.6. Kinetic parameters (Peak shape Method) Studies of kinetic parameters such as activation energy, frequency factor and order of kinetics give valuable information for better understanding of TL phenomenon. There are many calculation methods developed in computing the kinetic parameters. Among them, Chen's peak shape method is the popular method, which applies the values using the width of the glow curve and the temperatures corresponding to the half of peak intensity on the rising and falling sides of glow curve. In this method, a symmetry factor (μg) is defined as follow (Chen and Winer, 1970):
s=
βHR E 2 kTm
Δm =
(3)
⎛ E ⎞ exp⎜ ⎟[1 + (b − 1)Δm ]−1 ⎝ kTm ⎠
(4)
2kTm E
(5)
(
)
(
cτ = 1.51 + 3.0 μg − 0.42 bτ = 1.58 + 4.2 μg − 0.42
(
)
(
)
cδ = 0.976 + 7.3 μg − 0.42 bδ = 0 c w = 2.52 + 10.2 μg − 0.42 bω = 1
)
(6) (7) (8)
where μg is the symmetric factor, Tm is the maximum peak temperature, T1 is the temperature at half intensity on the rising side of the glow curve, T2 is the temperature at half intensity on the falling side of the glow curve, βHR is the heating rate, k is the Boltzmann's constant, α is corresponding to τ, δ and ω. τ is the first half width on the rising side of the glow curve and is equal to the difference between Tm and T1; δ is the second half width on the falling side of the glow curve and is equal to the difference between T2 and Tm; and ω is the total width of the glow curve and is equal to the difference between T2 and T1. Fig. 7 shows the τ, δ and ω of the glow curve. Table 4 shows the calculation of activation energy and the frequency factor for undoped and various doped strontium tetraborate glass samples. It should be noted that Eav is the average activation energy estimated from Eτ, Eδ and Eω (i.e. see Eqs. (3)– (8)). Eτ means the activation energy based on the first half width on the rising side of the glow curve; Eδ means the activation energy based on the second half width on the falling side of the glow curve; Eω means the activation energy based on the total width of the glow curve. From the table, it can be observed that all the glass samples possess second kinetic order. The evaluated symmetry factor for S4 glass is close to 0.52, which is similar as been reported by Li et al. (2005) and Liu et al. (2006) for polycrystalline Sr0.93Dy0.07B4O7 and SrB6O10: Tb3 þ , respectively. The calculated values of Eav and s vary with increasing Dy2O3 concentration. This means that the first half, the second half and the total width of the glow curve vary from sample to sample. In addition, the value of Tm stated in the
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T.Y. Lim et al. / Applied Radiation and Isotopes 102 (2015) 10–14
table have different values in the range between 446 and 490 K; this indicates that the traps has continuous distribution of energies. The values of Eav and s calculated in this work are in the range 0.78–1.03 eV and (0.017–75.10) 109 s 1, respectively. In addition, the decay times or the lifetimes of these glasses at room temperature was found as furnished in Table 5 using the equation
τav = s−1exp
( ) Eav kT
(Chen and Hag-Yahya, 1997). It can be observed
that the undoped glass (∼33.6 days) has larger lifetimes compare to other Dy doped glasses (i.e. in the range between 2.7 and 9.0 days). Thus, it would be best to measure the TL response of these glasses before their lifetimes were reached to achieve high accuracy and reliability of TL readings. Based on this consideration, this glass has a capability to be used for short term radiation dose monitoring.
4. Conclusion The basic TL features of undoped and various Dy doped strontium tetraborate glass system has been investigated subjected to 6 and 7 MeV electron energies. The variation of glow curve, the dose response and the minimum detectable dose have been identified for all glass samples. The glasses were found to have broad single glow peak and linear TL response for 6 and 7 MeV electron irradiations over a dose range from 0.5 to 4.0 Gy. The minimum detectable dose was calculated to be 5.1 and 4.6 mGy for 6 and 7 MeV electron energies, respectively. The optimum concentration of Dy confirmed the importance role of dopant in TL dosimetry. The calculation of kinetic parameters showed that the glass samples illustrate the second order kinetic.
Acknowledgment The authors would like to convey thanks to the Ministry of Higher Education (MOHE) and Universiti Teknologi Malaysia for providing the project grant (Q.J130000.2526.03H65) and laboratories facilities. The authors also would like to take this opportunity to thank the University Malaya Medical Centre and Johor
Specialist Hospital for providing irradiation facilities.
Reference Bos, A.J.J., 2001. High sensitivity thermoluminescence dosimetry. Nucl. Instrum. Methods Phys. Res. B 184, 3–28. Chen, R., Winer, S.A.A., 1970. Effects of various heating rates on glow curves. J. Appl. Phys. 41 (13), 5227–5232. Chen, R., 1969. Glow curves with general order kinetics. J. Electrochem. Soc. 116 (9), 1254–1257. Chen, R., Hag-Yahya, A., 1997. A new possible interpretation of the anomalous fading in thermoluminescent materials as normal fading in disguise. Radiat. Meas. 27 (2), 205–210. El–Mallawany, R., Diab, H.M., 2012. Improving dosimetric properties of tellurite glasses. Physica B. 407, 3580–3585. Furetta, C., Prokić, M., Salamon, R., Kitis, G., 2000. Dosimetric characterization of a new production of MgB4O7: Dy, Na thermoluminescent material. Appl. Radiat. Isot. 52, 243–250. Horowitz, Y., 2006. Microdosimetric response of physical and biological systems to low- and high–LET radiations: theory and applications to dosimetry, A Unified and Comprehensive Theory of the TL Dose Response of Thermoluminescent Systems Applied to LiF:Mg,Ti. Elsevier, The Netherlands, Chapter 2. Lavat, A., Graselli, C., Santiago, M., Pomarico, J., Caselli, E., 2004. Influence of the preparation route on the optical properties of dosimetric phosphors based on rare-earth doped polycrystalline strontium borates. Cryst. Res. Technol. 39 (10), 840–848. Li, J., Hao, J.Q., Li, C.Y., Zhang, C.X., Tang, Q., Zhang, Y.L., Su, Q., Wang, S.B., 2005. Thermally stimulated luminescence studies for dyprsoium doped strontium tetraborate. Radiat. Meas. 39, 229–233. Liu, L.Y., Hao, J.Q., Li, C.Y., Tang, Q., Zhang, C.X., Su, Q., Wang, S.B., 2006. Thermoluminescence characteristics of SrB6O10: Tb. J. Rare Earths 24, 276–280. McKeever, S.W.S., Moskovitch, M., Townsend, P.D., 1995. Thermoluminescence Dosimetry Materials: Properties and Uses. Nuclear Technology Publishing, Ashford. Prokic, M., 2000. Effect of lithium co-dopant on the thermoluminescence response of some phosphors. Appl. Radiat. Isot. 52, 97–103. Prokic, M., 1982. New sintered thermoluminescent dosemeter for personnel and environmental dosimetry. Health Phys. 42 (6), 849. Ribeiro, D.R.S., Souza, D.N., Maia, A.F., Baldochi, S.L., Caldas, L.V.E., 2008. Applicability of pure LiF in dosimetry. Radiat. Meas. 43, 1132–1134. Santiago, M., Graseli, C., Caselli, E., Lester, M., Lavat, A., Spano, F., 2001. Thermoluminescence of SrB4O7: Dy. Phys. Status Solidi A 185 (2), 285–289. Sharma, S.K., Pitale, S.S., Malik, M.M., Dubey, R.N., Qureshi, M.S., 2008. Synthesis and detailed kinetic analysis using computerized glow curve deconvolution technique of nanocrystalline Sr3Al2O6:Pr3 þ – a new phosphor for UV applications. Phys. Status Solidi A 205 (11), 2695–2703.
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Thermoluminescence response of dysprosium doped strontium tetraborate glasses subjected to electron irradiations Tou Ying Lim a,n, H. Wagiran a, R. Hussin a, S. Hashim a a
Department of Physics, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM, Johor Bahru, Malaysia
art ic l e i nf o
a b s t r a c t
Article history: Received 29 September 2014 Received in revised form 28 March 2015 Accepted 13 April 2015 Available online 14 April 2015
The paper presents the thermoluminescence (TL) response of strontium tetraborate glass subjected to electron irradiations at various Dy2O3 concentrations ranging from 0.00 to 1.00 mol%. All glass samples exhibited single broad peak with maximum peak temperature positioned at 170–215 °C. The optimum TL response was found at Dy2O3 concentration 0.75 mol%. This glass showed good linearity and higher sensitivity for 7 MeV compared to 6 MeV electrons. Analysis of kinetic parameters showed that the glasses demonstrate second order kinetic. & 2015 Elsevier Ltd. All rights reserved.
Keywords: Thermoluminescence Electron Glow curve Strontium tetraborate Glass
1. Introduction Thermoluminescence (TL) dosimetry is well known technique for measuring the radiation absorbed dose, particularly for personal, medical and environmental purpose. It has been reported that the TL efficiency of a given dosimeter varied and strongly depends to the type of TL settings implemented during annealing and readout processes, type of impurities used, type of readers, sample storage condition and etc. LiF: Mg, Ti under the trade name of TLD-100 has been recognized as a standard dosimeter in twentieth century. This is because LiF: Mg, Ti has been characterized to have properties close to ideal dosimeter and almost tissue equivalence (Zeff ¼ 8.04). However, the drawback of this dosimeter was apparent especially at low energies since an overresponse of the relative TL efficiency with approximately 10% was detected and thus limits the useful range of this dosimeter (Horowitz, 2006). In addition, its complex glow curve and annealing procedure necessitate the careful analyses and interpretation of dose information. Up to now, many investigations have been conducted to improve the performance of the current dosimeter and explore the new TL materials. Among them, strontium tetraborate is one of the attractive materials for radiation dosimetry since it is not hygroscopic as normally exhibited by other borate compounds (Santiago et al., 2001). In their work, they reported polycrystalline of n
Corresponding author. Fax: þ 6075566162. E-mail address: [email protected] (T.Y. Lim).
http://dx.doi.org/10.1016/j.apradiso.2015.04.005 0969-8043/& 2015 Elsevier Ltd. All rights reserved.
dysprosium doped strontium tetraborate has sensitivity five times higher compared to 7LiF: Mg,Ti. The high mechanical strength of strontium tetraborate also made it as a potential material to be used as radiation dosimetry (Lavat et al., 2004). McKeever et al. (1995) also emphasized the need of deeper research on the control of microstructure of the existing dosimetric materials. In this study, strontium tetraborate in glassy form is proposed and investigated. Dysprosium (Dy) was chosen since it was reported to induce high TL efficiency (Prokic, 1982; 2000). The study aims to measure the TL response of dysprosium doped strontium tetraborate glasses exposed to electron irradiations at room temperature. The kinetic parameters using peak shape method was also presented.
2. Materials and methods 2.1. Sample preparation The glass samples investigated in this work was Dy2O3 doped strontium tetraborate at various Dy2O3 concentrations ranging from 0 to 1.00 mol%. Table 1 shows the nominal compositions of the prepared samples. The binary glasses of B2O3–SrO were prepared by conventional melt quenching method. The ratio of 2:1 were weighed and mixed thoroughly using the analytical grade boric oxide (B2O3) and strontium carbonate (SrCO3) respectively. Small amount of dysprosium oxide (Dy2O3) was added to B2O3– SrO glass to confirm the role of dopant during TL measurements.
T.Y. Lim et al. / Applied Radiation and Isotopes 102 (2015) 10–14
11
Table 1 Nominal composition of Dy2O3 doped strontium tetraborate glasses. Glass notation
S0 S1 S2 S3 S4 S5
Concentration (mol%) B2O3
SrO
Dy2O3
66.67 66.59 66.54 66.42 66.29 66.17
33.33 33.26 33.21 33.08 32.96 32.83
0.00 0.15 0.25 0.50 0.75 1.00
The mixtures were melted at 1100 °C at electric furnace for 15– 30 min depending on their compositions. The melts were quenched on stainless steel metal plate and were pressed immediately. The amorphous nature of these glasses were examined by X-ray diffraction analysis at room temperature. All the glass samples demostrate amorphous nature since no sharp bands were observed and the spectra kept similar but with varying peak intensity.
Fig. 1. Glow curve of various Dy2O3 doped strontium tetraborate glasses at various Dy2O3 concentrations subjected to 6 MeV electron at a dose of 3.5 Gy.
2.2. Electron irradiation The glass samples were cut into small pieces with approximately same thickness. They were weighted using electronic balance with an accuracy of 4 decimal places. They were first annealed in an oven at 350 °C for 15 min prior to electron irradiation to ensure the background reading was standardised. Each piece of the glasses was placed in an perspec holder. The samples were irradiated subjected to 6 MeV and 7 MeV electron energies using linear accelerator Varian 2100C and Siemens Primus M3388 available at University Malaya Medical Centre (UMMC) and Johor Specialist Hospital, respectively. Both linear accelerators are calibrated to delivered the prescribed absorbed dose in such that 1 Monitor Unit (MU) gives an absorbed dose of 1 cGy for a 10 10 cm2 field at 100 cm source- to- surface distance (SSD). The energy chosen is the typical energy used for cancer patients treatment. The TL readouts were performed using TL reader Harshaw model 4500. The linear heating rate of 5 °C s 1 was used after few trials of TL readouts using various heating rates which gives the best resolution of the glow curve. The samples were kept in dark atmosphere for 24 h prior to readouts to eliminate any unstable low temperature readings. The TL readings were then normalized to their mass.
Fig. 2. Glow curve of various Dy2O3 doped strontium tetraborate glasses at various Dy2O3 concentrations subjected to 7 MeV electron at a dose of 3.5 Gy.
Fig. 2. The observed broad peak could be due to overlapping of several glow peaks and this phenomena confims the amorphous nature of glass samples. The Tm of the glow curves were estimated to be centered between 170 and 215 °C. 3.2. Optimum Dy2O3 concentration Despite the type of dopants, the concentrations of dopant material are also have direct influence on TL performance. Fig. 3 shows the plot of TL intensity as a function of Dy2O3 concentration of Dy2O3 doped strontium tetraborate glasses subjected to 6 MeV
3. Results and discussion 3.1. Glow curve Glow curve is one of the important feature for better understanding of TL phenomena. They are usually plotted as light intensity against the heating temperature. The shape and the position of the glow curve can reveal the type of trapping states and their fading characteristics of the corresponding material. It is well known that the ideal glow curve should has single sharp peak positioned between 180 and 250 °C. Fig. 1 shows the glow curves of Dy2O3 doped strontium tetraborate glasses at various Dy2O3 concentrations subjected to 6 MeV electron at a dose of 3.5 Gy. A single broad peak of TL glow curve was observed for all samples with different concentrations of Dy2O3. Most of the maximum peak temperature, Tm of the glow curves was found to shift to higher temperature as Dy2O3 concentration is increased. The same pattern was also observed for the glass samples exposed to 7 MeV electron energy as shown in
Fig. 3. TL intensity as a function of Dy2O3 concentration of Dy2O3 doped strontium tetraborate glasses subjected to 6 MeV electron at a dose of 3.5 Gy.
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T.Y. Lim et al. / Applied Radiation and Isotopes 102 (2015) 10–14
Table 2 Sensitivity and relative sensitivity of S4 glass sample to S0 (undoped) glass sample. Electron energies (MeV)
6 7
Sensitivity(nC mg 1 Gy 1) S4
S0
22.92 25.60
0.39 0.57
Relative sensitivity of S4–S0 glass
58.8 44.9
Table 3 : Calibration factor and minimum detectable dose of Dy2O3 doped strontium tetraborate glass at different electron energies.
Fig. 4. Graph of TL response against dose for S0 and S4 glass samples subjected to 6 MeV electrons irradiation.
Electron energies (MeV)
Calibration factor (Gy nC 1 mg)
Minimum detectable dose (Gy)
6 7
0.0436 0.0391
0.0051 0.0046
Fig. 7. The expressions of τ, δ and ω of the glow curve. Im is the maximum peak intensity of the glow curve.
Fig. 5. Graph of TL response against dose for S0 and S4 glass samples subjected to 7 MeV electrons irradiation.
However, a slight reduction of TL signal was observed for glass sample with 1.00 mol% Dy2O3 (S5). This phenomena is called as concentration quenching. A huge localized energy levels between the conduction band and the valence band are produced which result the enhancement of non–radiative relaxation of charge carriers (Sharma et al., 2008). A similar result was also reported for tellurite glass by other workers (El-Mallawany and Diab, 2012). 3.3. TL response
Fig. 6. Graph of TL response against dose of S4 glass sample subjected to 6 MeV and 7 MeV.
electron at a dose of 3.5 Gy. As expected, the undoped strontium tetraborate glass sample (S0) has the least TL response compared to other Dy2O3 doped samples. This means that dysprosium ion had induced new defect states to glass host which lead to a good improvement in TL performance. Meanwhile, the highest TL response was observed for glass sample with a Dy2O3 concentration of 0.75 mol% (S4). At this dose level, the S4 glass was found to has TL response 58 times higher compared to S0 glass sample.
In order to determine the TL dose dependent on different electron energies, the undoped (S0 glass) and 0.75 mol% Dy2O3 (S4 glass) were used for comparison. Fig. 4 shows the graph of TL response against 6 MeV electrons for doses ranging from 0.50 to 4.00 Gy. It can be seen that the TL response of both glass samples increase as a function of electron doses. Similar pattern was also observed for glasses exposed to 7 MeV electrons at the same dose range as shown in Fig. 5. In order to compare the energy dependence of TL response on S4 glass sample, the TL response of the glass subjected to 6 MeV and 7 MeV electrons was plotted together as shown in Fig. 6. It can be seen that the glass sample exposed to 7 MeV has higher TL response compared to the glass sample irradiated at 6 MeV electrons. The result opposed with the result published by Ribeiro et al. (2008) where they found that the TL sensitivity of pure LiF was inversely proportional to the electron beam energies (i.e. 8, 10 and 14 MeV) in the dose range from 0.10 to 2.00 Gy. However, more radiation energies are needed to confirm the trend of the TL sensitivity of current glass sample. 3.4. TL sensitivity A high sensitivity TL dosimeter is known to be able to measure
T.Y. Lim et al. / Applied Radiation and Isotopes 102 (2015) 10–14
13
Table 4 Kinetic parameters (Eav and s) of the samples subjected to 6 MeV calculated using peak shape method. Sample
Tm (K)
μg ± Δμg
Eτ ± ΔEτ (eV)
Eσ ± ΔEσ (eV)
Eω ± ΔEω (eV)
Eav ± ΔEav (eV)
s ± Δs (s 1)
S0 S1 S2 S3 S4 S5
454 462 489 440 470 462
0.57 70.02 0.56 70.02 0.55 70.01 0.58 70.02 0.57 70.01 0.57 70.02
1.077 0.09 0.81 7 0.05 0.777 0.05 0.977 0.07 0.83 7 0.05 0.88 7 0.06
0.99 7 0.08 0.80 7 0.05 0.78 7 0.04 0.89 7 0.06 0.80 7 0.05 0.84 7 0.05
1.03 7 0.06 0.80 7 0.04 0.78 7 0.03 0.93 7 0.05 0.81 7 0.04 0.86 7 0.04
1.03 70.13 0.80 70.08 0.78 70.07 0.93 70.10 0.81 70.08 0.86 70.09
(7.517 0.94) 1010 (1.247 0.13) 108 (1.737 0.16) 107 (1.23 7 0.14) 1010 (1.117 0.11) 108 (5.84 7 0.61) 108
Table 5 Lifetimes of the undoped and various Dy doped strontium tetraborate glass samples subjected to 6 MeV. Sample
τav (s)
S0 S1 S2 S3 S4 S5
2.90 106 (∼33.6 days) 2.36 105 (∼2.7 days) 7.79 105 (∼9.0 days) 3.67 105 (∼4.3 days) 3.89 105 (∼4.5 days) 5.13 105 (∼5.9 days)
(2)
where μg is dependent to the order of kinetic. The TL material is said to obey the first order kinetics if μg is close to 0.42. While, the TL material is said to obey the second order kinetics provided μg is close to 0.52. Chen (1969) also formulated the expressions for determination of activation energy and frequency factor as below:
⎛ kT2 ⎞ Eα = cα⎜⎜ m ⎟⎟ − bα (2kTm) ⎝ α ⎠
a very low dose in medical, personnel and environmental dosimetry. Bos (2001) defined the sensitivity using a ratio of TL intensity per unit absorbed dose and per unit mass. For useful comparison, the relative sensitivity is used instead of the sensitivity for a given material. The relative sensitivity is defined in the ratio of the sensitivity of that material per unit of the sensitivity of a reference material. Table 2 shows the values of the sensitivity of S4 glass sample and the relative sensitivity of S4–S0 glass samples prepared in this work. The gradients of the graph in Fig. 6 were used to represent the sensitivity of S4 glass sample for 6 MeV and 7 MeV electrons over a dose range of 0.50–4.00 Gy. 3.5. Minimum detectable dose Furetta et al. (2000) defined an expression for minimum detectable dose as:
D0 = (B + 2σB)⋅F
T2 − Tm T2 − T1
μg =
(1)
where B is the average of TL background of annealed but not irradiated samples and sB is the standard deviation of the average TL background. F is the TL calibration factor with a unit of Gy nC 1 mg 1 and can be determined by the slope of the graph of Fig. 6. Table 3 shows the values of calibration factor and minimum detectable dose for 6 and 7 MeV electron energies. The minimum detectable dose was found to vary slightly with electron energies. The glass sample was found to exhibit low minimum detectable dose for 7 MeV compared to 6 MeV electrons. 3.6. Kinetic parameters (Peak shape Method) Studies of kinetic parameters such as activation energy, frequency factor and order of kinetics give valuable information for better understanding of TL phenomenon. There are many calculation methods developed in computing the kinetic parameters. Among them, Chen's peak shape method is the popular method, which applies the values using the width of the glow curve and the temperatures corresponding to the half of peak intensity on the rising and falling sides of glow curve. In this method, a symmetry factor (μg) is defined as follow (Chen and Winer, 1970):
s=
βHR E 2 kTm
Δm =
(3)
⎛ E ⎞ exp⎜ ⎟[1 + (b − 1)Δm ]−1 ⎝ kTm ⎠
(4)
2kTm E
(5)
(
)
(
cτ = 1.51 + 3.0 μg − 0.42 bτ = 1.58 + 4.2 μg − 0.42
(
)
(
)
cδ = 0.976 + 7.3 μg − 0.42 bδ = 0 c w = 2.52 + 10.2 μg − 0.42 bω = 1
)
(6) (7) (8)
where μg is the symmetric factor, Tm is the maximum peak temperature, T1 is the temperature at half intensity on the rising side of the glow curve, T2 is the temperature at half intensity on the falling side of the glow curve, βHR is the heating rate, k is the Boltzmann's constant, α is corresponding to τ, δ and ω. τ is the first half width on the rising side of the glow curve and is equal to the difference between Tm and T1; δ is the second half width on the falling side of the glow curve and is equal to the difference between T2 and Tm; and ω is the total width of the glow curve and is equal to the difference between T2 and T1. Fig. 7 shows the τ, δ and ω of the glow curve. Table 4 shows the calculation of activation energy and the frequency factor for undoped and various doped strontium tetraborate glass samples. It should be noted that Eav is the average activation energy estimated from Eτ, Eδ and Eω (i.e. see Eqs. (3)– (8)). Eτ means the activation energy based on the first half width on the rising side of the glow curve; Eδ means the activation energy based on the second half width on the falling side of the glow curve; Eω means the activation energy based on the total width of the glow curve. From the table, it can be observed that all the glass samples possess second kinetic order. The evaluated symmetry factor for S4 glass is close to 0.52, which is similar as been reported by Li et al. (2005) and Liu et al. (2006) for polycrystalline Sr0.93Dy0.07B4O7 and SrB6O10: Tb3 þ , respectively. The calculated values of Eav and s vary with increasing Dy2O3 concentration. This means that the first half, the second half and the total width of the glow curve vary from sample to sample. In addition, the value of Tm stated in the
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table have different values in the range between 446 and 490 K; this indicates that the traps has continuous distribution of energies. The values of Eav and s calculated in this work are in the range 0.78–1.03 eV and (0.017–75.10) 109 s 1, respectively. In addition, the decay times or the lifetimes of these glasses at room temperature was found as furnished in Table 5 using the equation
τav = s−1exp
( ) Eav kT
(Chen and Hag-Yahya, 1997). It can be observed
that the undoped glass (∼33.6 days) has larger lifetimes compare to other Dy doped glasses (i.e. in the range between 2.7 and 9.0 days). Thus, it would be best to measure the TL response of these glasses before their lifetimes were reached to achieve high accuracy and reliability of TL readings. Based on this consideration, this glass has a capability to be used for short term radiation dose monitoring.
4. Conclusion The basic TL features of undoped and various Dy doped strontium tetraborate glass system has been investigated subjected to 6 and 7 MeV electron energies. The variation of glow curve, the dose response and the minimum detectable dose have been identified for all glass samples. The glasses were found to have broad single glow peak and linear TL response for 6 and 7 MeV electron irradiations over a dose range from 0.5 to 4.0 Gy. The minimum detectable dose was calculated to be 5.1 and 4.6 mGy for 6 and 7 MeV electron energies, respectively. The optimum concentration of Dy confirmed the importance role of dopant in TL dosimetry. The calculation of kinetic parameters showed that the glass samples illustrate the second order kinetic.
Acknowledgment The authors would like to convey thanks to the Ministry of Higher Education (MOHE) and Universiti Teknologi Malaysia for providing the project grant (Q.J130000.2526.03H65) and laboratories facilities. The authors also would like to take this opportunity to thank the University Malaya Medical Centre and Johor
Specialist Hospital for providing irradiation facilities.
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