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Thermoluminescence properties of borosilicate glass doped with ZnO
Journal of Luminescence 186 (2017) 164–169
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Thermoluminescence properties of borosilicate glass doped with ZnO E. Salama a,c,n, H.A. Soliman b, G.M. Youssef a, Sara Hamad a a b c
Physics Department, Faculty of Science, Ain Shams University, Cairo, Egypt National Institute for Standards, Ionizing Radiation Metrology Laboratory, Egypt Basic Science Department, Faculty of Engineering, British University in Egypt (BUE)
art ic l e i nf o
a b s t r a c t
Article history: Received 17 October 2016 Received in revised form 19 January 2017 Accepted 3 February 2017 Available online 4 February 2017
Thermoluminescence (TL) properties of new synthetic borosilicate glass (SiO2-H3BO3-Na2CO3) doped with various concentrations of ZnO are studied. The glasses were prepared by the melt quenching method and irradiated with gamma doses in the range of 0.5–500 Gy. Some properties such as, dose response, minimum detectable dose, reproducibility of the response, thermal and optical fading and IR absorption were studied. Optimum glass composition with a good linear TL gamma dose response over a wide dose range, as well as low fading and excellent reproducibility is obtained. These appealing features indicated that; this new synthetic TL glass material is useful in many radiation detection applications. & 2017 Elsevier B.V. All rights reserved.
Keywords: Thermoluminescence Borosilicate Zinc Glass Ionizing radiation
1. Introduction Accurate determination of radiation doses is a priority issue in most dosimetry applications. Such applications use dosimeters having specific features; namely: a favorable effective atomic number (Zeff), reasonable sensitivity, wide linear range, energy independent dose response, low fading rate and its relatively easy preparation and inexpensive shaping. Doped borate glasses have been extensively studied as dosimetric materials because they have several dosimetry properties compared with other dosimeters. Their Zeff, are very close to those of human biological tissues (Zeff ¼ 7.4) which makes them ideal materials for the environmental and medical dosimetry [1,2]. Pure borate glasses are typically known for the disadvantages of high hygroscopicity and the weak TL glow peak at relatively low temperatures [3]. Borate glasses are considered to be a good host for a wide varieties of dopants, like rare earths and transition metals. Most of the earlier studies on borate glass have been focusing on either the incorporation of different types of metals (alkali/alkaline) as modifiers or transition metals and/or rare earths as dopants and/or co-dopants to improve the stability as well as to enhance the thermoluminescence sensitivity of the dosimeter [4–7]. Alkali n Corresponding author at: Physics Department, Faculty of Science, Ain Shams University, Cairo, Egypt. E-mail addresses: [email protected], [email protected] (E. Salama).
http://dx.doi.org/10.1016/j.jlumin.2017.02.008 0022-2313/& 2017 Elsevier B.V. All rights reserved.
metals doped borate glasses are also better hosts for transition metal ions than the pure borates [4,8,9]. Lithium potassium borate glass co-doped with TiO2 and MgO thermoluminescent has a good linearity, low fading, excellent reproducibility with promising effective atomic number (Zeff ¼ 8.89), which is very close to human biological tissues (Zeff ¼ 7.4) [10]. Other dopants such as samarium were used to produce a high sensitive thermoluminescence borate glass [11]. Copper doped zinc lithium borate glass has good sensitivity, less fading characteristics and linear dose response in the radiotherapy dose range of 0.5–4 Gy [12]. Similar features were obtained with the prepared 0.1 mol% Cu-doped lithium potassium borate glass with Zeff ¼ 8.39 [13]. Borosilicate glass on the other hand is characterized by its extensive imperviousness to sudden changes in temperature, low coefficient of linear expansion and high chemical resistance [14]. The effect of ionizing radiation on a borate glass containing small amounts of impurities, normally found in most samples, involves the electron–hole pair formation and charge carrier trapping at specific sites of the glass; called defect centers. Previously, many authors have been discussing the luminescence mechanisms in borate glasses [15–17]. When irradiated, borosilicate glasses develop radiation-induced defects resulting in the formation of the non-bridging oxygen hole centers (NBOHCs) which are relevant to the coloration in the visible region. The electrons released from the non-bridging oxygen centers by irradiation are trapped at various defect centers. Some of these defect centers already exist in the glasses and can trap electrons while others are due to the ions added to the glass material such as Fe þ 2,
E. Salama et al. / Journal of Luminescence 186 (2017) 164–169
Fe þ 3 and Zn þ 2 [15]. The main objectives of this work were to synthesize and study the thermoluminescence properties of the new inexpensive vitreous system of borosilicate glass doped with ZnO for the purpose of finding conceivable applications in the field of ionizing radiation.
165
irradiated at room temperature at a dose rate of about 1.131 Gy/ min in sample position.
3. Results and discussions 3.1. Powder X-ray diffraction analysis (XRD)
2. Experimental work 2.1. Sample preparation and measurements New vitreous system SiO2-H3BO3-Na2CO3 with different concentrations of ZnO was synthesized by the melt quenching method. The preparation was done by mixing stoichiometric amounts of boric acid with silicon dioxide and sodium carbonates. Zinc oxide was added to the blend as a dopant. The chosen reagents noted for their high purity. To acquire high homogeneity, the composites were blended mechanically for 40 min and afterward put in a ceramic crucible. The blend was melted in a conventional electrical furnace at a temperature of around 1100 °C for a duration of 90 min. The used furnace was Ney-2-525 series II box oven. For the sake of complete homogeneity, the mixture was frequently stirred during the melting process. The molten glass was then poured onto a steel plate, which got subsequently annealed in another furnace at a temperature of 400 °C for 2 h. To release the thermal strain, the heat of furnace was slowly lost to room temperature. The prepared samples had a nominal composition of 20Na2CO3:20SiO2:(60 x)H3BO3:xZnO, 0˂x˂6. The compositions of the prepared samples were illustrated in Table 1. The prepared samples in solid form were polished and formed in the shape of 4 mm x 4 mm transparent square chips with 1 mm thickness. All samples are kept in dark containers to counteract the photoluminescence effect of background light. The TL reader model 3500 (Harshaw USA) available at the national institute for standards (NIS), Giza, Egypt was used in the TL measurements. The X-ray powder diffraction (XRD) was recorded for the representative samples of the prepared glass, to investigate its amorphous state. The XRD analysis was performed with Philips X`pert Pro X-ray powder diffractometer using Cu Kα radiation (1.5418 A) at a scanning speed of 0.3 s. The structural changes which occurred inside the prepared glass network with the variation in the concentration of ZnO can be identified by the infrared (IR) absorption measurements. The FTIR absorption spectra of the glasses were measured using a JASCOFT-IR6200 spectrometer with the KBr pellet technique in the spectral range 400–4000 cm 1.
Fig. 1 shows the XRD patterns of the samples of undoped and 2 mol% ZnO doped borosilicate glass samples. The broadening of the obtained pattern along with the disappear of the ZnO peaks (ICDD: 00–001-1136) indicate that, the samples were pure amorphous. 3.2. Fourier transformation infrared (FTIR) spectra As shown in Fig. 2, the (FTIR) absorption spectra of the studied glasses show major absorption bands around 468.63, 528.40, 717.40, 991.25, and 1382.73 cm 1 with some minor changes in band intensities and peak positions with respect to each other. These bands with their assignments are summarized in Table 2. The glasses doped with ZnO reveal FTIR spectra which are similar to the same vibrational bands identified in the undoped glass, without any changes in the number and position of the IR vibrational bonds. 3.3. TL glow curves Fig. 3 shows the glow curve of undoped and 1–6%ZnO-doped borosilicate samples irradiated by 20 Gy gamma dose. The glow curve, of both undoped and doped samples at 4 °C/s heating rate, exhibit one strong TL peak approximately at 138–206 °C. The maximum TL intensity is obtained at 2 mol % ZnO doped sample as shown in Fig. 4. The role of ZnO in the TL emission in the prepared borosilicate glass can be discussed based on the formation of both electron and hole trapping centers, as follow: Non-bridging oxygen hole centers are formed by irradiation through the following reaction in multicomponent glasses with non-bridging oxygen [15–17]
≡ Si−O−→ ≡ Si−O.+e− where, ≡ Si − O− and ≡ Si − O. represent the non-bridging oxygen (NBO) and non-bridging oxygen hole center (NBOHC) respectively. Adding divalent state zinc ions will increase both the electron and hole - trapping sites as follow: 100
2.2. Radiation source
Intensity (a.u)
Theratron Co-60 gamma source model 780E, manufactured by Theratronics Canada was used for irradiation. Samples were Table 1 The raw materials and composition of prepared glasses. Glass
Sample Sample Sample Sample Sample Sample Sample
Batch composition (mol%)
0 1 2 3 4 5 6
(S0) (S1) (S2) (S3) (S4) (S5) (S6)
SiO2
H3BO3
Na2CO3
ZnO
20 20 20 20 20 20 20
60 59 58 57 56 55 54
20 20 20 20 20 20 20
0 1 2 3 4 5 6
50
(b) (a)
0 20
30
40
50
60
70
2θ (degrees) Fig. 1. XRD patterns of (a) undoped and (b) 2 mol% ZnO -doped borosilicate.
E. Salama et al. / Journal of Luminescence 186 (2017) 164–169
991.25
166
0% ZnO 2% ZnO
15000
TLD Intensity (a.u)
0.2
528.40 468.63
717.40
Absorption (a.u.)
1382.73
0.4
10000
5000
0
2
0.0
40 0
60 0
80 0
10 00
12 00
14 00
16 00
18 00
20 00
Fig. 2. FTIR spectra of some investigated samples. Table 2 Assignment of infrared bands in the spectra of the prepared glass samples. Peak position (cm 1)
Assignment
Reference range
1382.73
Asymmetric stretching relaxation of B–O bonds of trigonal BO3 units B–O-stretching vibrations of BO3 units Stretching vibrations of B–O–Si linkage
̴ 1400 [18,19]
991.25 717.40
Band due to B–O–B linkage in borate network (two silicate chains and borate phases)
528.40 468.63
Vibration of the metal cation Na þ , Zn2 þ
3x10
2x10
1x10
950–1050 [20– 22] ̴ 700 [23,24]
o 600 [25–27]
0 Zn 1 Zn 2 Zn 3 Zn 4 Zn 5 Zn 6 Zn
7
7
7
7
ZnO, a maximum at this concentration and a decline of the efficiency at higher concentrations. Decreasing of the TL sensitivity of the prepared glass at higher concentrations of ZnO above 2 mol % may be attributed to the well-known concentration quenching effect. This effect has been first described by Johnson and Williams [28,29]; and recently developed and presented by several models [30,31]. 3.4. Annealing procedure The best pre-irradiation annealing condition should fulfill the following criteria: completely remove the previous signals, accomplish higher sensitivities, reproduce the readings for many irradiation cycles and minimizing standard deviation for the dosimeter readings [6]. The optimum pre-irradiation annealing condition is investigated by subjecting two groups each of 15 samples to different annealing conditions (temperatures and time) and gamma dose of 10 Gy. The samples of 1st group are annealed at different temperature from 100 °C to 400 °C in step of 100 °C with fixed annealing time (60 min). The samples of the 2nd group are annealed at different annealing time from 10 min to 60 min in step of 10 min and at the optimum temperature obtained from the 1st group. The annealing procedures were performed in the aformentioned conventional electrical oven. Fig. 5 shows the variation of TL emission and its associated standard deviation with different annealing temperatures at fixed annealing time (60 min); while Fig. 6 shows the variation with
7 4
20
3.0x10
4
0
-o-
2.8x10
100
150
200
250
Temperature (
o
300
350
−
≡ Si−O−+Zn2 + → ≡ Si−O.+( Zn2 + ) and
+ Zn2 +
)
4
16
4
14
4
12
4
10
4
8
4
6
4
4
4
2
2.6x10
C)
Fig. 3. The glow curves of undoped and 1–6 mol%ZnO-doped borosilicate samples irradiated by 20 Gy gamma dose.
≡ Si−O.+Zn2 + → ≡ Si−O−+(
2.4x10 2.2x10 2.0x10 1.8x10 1.6x10 1.4x10
)− and ( Zn2+)+ represent the hole and electron trap-
1.2x10
ping center respectively due to the additive ion Zn2 +. The dependence of the thermoluminescence efficiency on the concentration of ZnO in borosilicate glass is shown in Fig. 4. These results showed an increase with the concentration up to 2 mol% of
1.0x10
(
where, Zn2 +
18
400
TL Intensity (a.u)
50
TL response STD
4
100
200
300
400
o
Temperture ( C) Fig. 5. The effect of annealing temperature on TL intensity.
0 500
STD %
TL Intensity (a.u)
4x10
6
Fig. 4. TL intensity of irradiated borosilicate silica glass at different mol% ZnO concentrations.
-1
Wavenumber (cm )
5x10
4
ZnO concentrations (mol %)
E. Salama et al. / Journal of Luminescence 186 (2017) 164–169
4
4
100
200
90
o
2.6x10
210
110
TL -o- STD %
2.2x10
4
70 60
4
50 2.0x10
40
4
STD (%)
TL intensity (a.u)
80 2.4x10
30 1.8x10
20
4
10 1.6x10
4
Glow Peak Temperature ( C)
2.8x10
190
180
170
160
150
140
0 10
20
30
40
50
0
60
2
4
6
8
10
12
14
16
18
20
o
Time (min)
Heating Rate ( C/s)
Fig. 8. The effect of different heating rates on the glow peak temperature.
Fig. 6. Variation of TL intensity with different annealing time.
time. The highest TL mission with low standard deviation is obtained at the annealing condition of 400 °C for 40 min. Therefore, the best annealing condition for the new prepared TLD is evidently 400 °C for 40 min.
6
10
The effect of heating rate on the glow curve of the new glass phosphor and the glow peak temperature are investigated. For this respect, the TL glow curve of an irradiated samples of the prepared dosimeter were measured at different heating rates in the range of 1–20 °C/s. As shown in Fig. 7, the highest TL intensity was observed at heating rate (4 °C/s). The position of TL peaks was shifted from left to right with the increase in heating rate as shown in Fig. 8. This behavior can be explained in terms of the amount of electrons released per thermal stimulation, and the required time for this process. At low heating rates, there will be enough time to evacuate all electrons in the traps and form the glow curve at low temperature. At relatively high heating rates (5–20 °C/s), electrons are to be released from all traps faster, and therefore, it will take less time to complete glow curve shape, and shift glow curves peaks to high temperature will be observed [10,12]. 3.6. TL dose responses The results of the dose response, of the prepared borosilicate
TL Intensity (a.u)
5
3.5. Heating rate
10
4
10
3
10
1
10
100
Dose (Gy) Fig. 9. Linearity of dose response.
silica glass doped with 2 mol% ZnO within the dose range of 0.5– 500 Gy, are shown in Fig. 9. Linear response within this range was obtained with 0.9954 correlation coefficient. Such high correlation coefficient confirmed the linearity of the dose response within this range and indicated the good distribution of deep electron traps within the different levels of the host material. 3.7. Thermal fading
1.3
1.2
TL intensity (Normalized)
167
1.1
1.0
0.9
0.8
0.7
0.6 0
2
4
6
8
10
12
14
16
18
20
o
Heating Rate ( C/s) Fig. 7. Variation of the total glow curve intensity (normalized to the heating rate of 1 °C/s) with different heating rates.
Investigation of the possible fading effects of the new glass phosphor is an important parameter that needed to be determined. Thermal and optical fading can be evaluated as a result of the effect of the variable environmental conditions. The thermal fading of 10 Gy irradiated samples of the synthetic ZnO 2 mol% borosilicate glass was investigated over the period of 30 days. All irradiated samples are stored in an opaque container at room temperature (25–30 °C) and humidity condition between 40 and 50%. The thermal fading characteristics of sampled dosimeters over the period of 30 days were reported as shown in Fig. 10. To subtract the effect of background, annealed (un-irradiated) samples were stored with the irradiated samples and measured at each measurement step. The obtained results showed 11% reduction after 24 h from irradiation and 21.4% after 7 days. No extra thermal fading was obtained in a time window further than one week during the remaining investigation time of one month, as shown in Fig. 11. This result will assist in estimating the correct absorbed doses in case the readout could not take place in time.
168
E. Salama et al. / Journal of Luminescence 186 (2017) 164–169
6
110
4x10
Zero day 1 day 2 days 3 days
6
90 80
Residual Signal (%)
TL Intensity (a.u)
3x10
100
6
2x10
6
1x10
70 60 50 40 30 20 10 0
0 50
100
150
200
250
300
350
0
400
10
20
30
40
50
60
70
80
90
100
Time (Min)
o
Temperture ( C) Fig. 10. Glow curves of 10 Gy irradiated 2 mol% ZnO doped borosilicate glass during its thermal Fading at room temperature.
Fig. 12. Optical Fading of 2 mol% ZnO doped borosilicate glass irradiated at 10 Gy.
1.2
100 1.0
TL intensity (normalized)
Residual signal (%)
80
60
40
20
0.8
0.6
0.4
0.2
0.0
0 0
5
10
15
20
25
30
0
3.8. Optical fading (Sensitivity to light) Optical fading parameter was also determined during this study to quantify the sensitivity of the new synthetic borosilicate glass to sunlight. Direct exposure of irradiated dosimeters (10 Gy absorbed dose) to sunlight for 0.5 h led to a loss of 72% of the stored signals. This loss happened to exceed 82% after 1.5 h of direct exposure as shown in Fig. 12. It is to be mentioned that these numbers may vary according to the light intensity. In the present work we were considering the general effect of light exposure. We wanted to prove that, light exposure has effect on the stored signals of an irradiated samples, no matter what the intensity is. Therefore, it is recommended to use an opaque cover in environmental applications and during dosimeters transfer to avoid the light effect. 3.9. Reproducibility Reproducibility of measured dose by the proposed TLD was determined by using five glass samples. These samples were repeatedly irradiated at 10 Gy after the proper preliminary annealing. As shown in Fig. 13, the prepared dosimeter can repeatedly measure the irradiated dose with less than 3% difference, based on a standard deviation of five consecutive measurements. The small
4
6
8
10
Number of measurements
Storage time (days) Fig. 11. Thermal Fading of 2 mol% ZnO doped borosilicate glass irradiated at 10 Gy.
2
Fig. 13. Reproducibility of 2 mol% ZnO doped borosilicate glass.
differences in the average TL intensity throughout nine cycles readout shows that this glass is reusable in the dose assessment by TL method.
4. Conclusion In this work, new synthetic borosilicate glass (SiO2-H3 BO3-Na2CO3) doped with various concentrations of ZnO was prepared by the melt quenching method. The best concentration of ZnO was found to be 2 mol%. With regard to its utilization as a radiation dosimeter, several dosimetric properties of the new prepared glass have been studied. The best heating rate and annealing procedure was found to be 4 °C/s and 400 °C at 40 min respectively. The obtained glow curves are considerably simple curves. The dose response curve is linear over the dose range 0.5– 500 Gy, with a single prominent peak at about 178 °C. Relatively low rate of fading about 21% over 1 week of storage and good reproducibility of dosimeter response are provided. These characteristics make the new synthetic borosilicate glass very suitable for radiation detection applications.
E. Salama et al. / Journal of Luminescence 186 (2017) 164–169
References [1] M.M. Elkholy, J. Lumin 130 (2010) 1880. [2] M.I. Teixeira, Z.M. Costa, C.R. Costa, W.M. Pontuschka, L.V.E. Caldas, Radiat. Meas. 43 (2008) 480. [3] W.M. Pontuschka, L.S. Kanashiro, L.C. Courrol, Glass Phys. Chem. 27 (2001) 37. [4] S.S. Rojas, K. Yukimitu, A.S.S. de Camargo, L.A.O. Nunes, A.C. Hernandes, J. NonCryst. Solids 352 (2006) 3608. [5] S. Hashim, Y.S.M. Alajerami, A.T. Ramli, S.K. Ghoshal, M.A. Saleh, A.B. Abdul Kadir, M.I. Saripan, K. Alzimami, D.A. Bradley, M.H.A. Mhareb, Appl. Radiat. Isot. 91 (2014) 126. [6] Y. Alajerami, S. Hashim, A.T. Ramli, M. Saleh, T. Kadni, Radiat. Prot. Dosim. 155 (2013) 1. [7] W.E.F. Ayta, V.A. Silva, N.O. Dantas, J. Lumin. 130 (2010) 1032. [8] H. Aboud, H. Wagiran, I. Hossain, R. Hussin, Int. J. Phys. Sci. 7 (2012) 226. [9] Y. Alajerami, S. Hashim, W. Saridan, A.T. Ramli, Physica B 407 (2012) 2390. [10] Y. Alajerami, S. Hashim, S. Ghoshal, M. Saleh, T. Kadni, M. Saripan, K. Alzimami, Z. Ibrahim, D. Bradley, J. Non-Cryst. Solids 74 (2013) 1816. [11] S. Krause, C. Pfau, M. Dyrba, P.-T. Miclea, S. Schweizer, J. Lumin. 151 (2014) 29. [12] A. Saidu, H. Wagiran, M.A. Saeed, Y. Alajerami, J. Radioanal. Nucl. Chem. 304 (2015) 627. [13] H. Aboud, H. Wagiran, R. Hussin, H. Ali, Y. Alajerami, M.A. Saeed, Appl. Radiat. Isot. 90 (2014) 35.
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[14] T. Kloss, G. Lautenschläger, K. Schneider, Glass Technol. 41 (2000) 177. [15] K. Kadono, N. Itakura, T. Akai, M. Yamashita, T. Yazawa, Nucl. Instrum. Methods Phys. Res. B 267 (2009) 2411. [16] S.M. Del Nery, W.M. Pontuschka, C.G. Rouse, Phys. Rev. B 49 (1994) 3760. [17] W.M. Pontuschka, L.S. Kanashiro, L.C. Courrol, Glass Phys. Chem. 27 (1) (2001) 37. [18] E.I. Kamitsos, M.A. Karakassides, G.D. Chryssikos, J. Phys. Chem. 91 (1987) 1073. [19] I. Ardelean, et al., Mod. Phys. Lett. B 14 (17–18) (2000) 653. [20] K. El-Egili, A.H. Oraby, J. Phys. Condens. Matter 8 (46) (1996) 8959. [21] R.C. lucacel, I. Ardelean, J. Optoelectron. Adv. Mater. 8 (2006) 1124. [22] K. El-Egili, H. Doweidar, Y.M. Moustafa, I. Abbas, Physica B 339 (4) (2003) 237. [23] I. Ardelean, P. Pascuta, Mater. Lett. 58 (2004) 3499. [24] I. Ardelean, P. Pascuta, Mod. Phys. Lett. B 16 (14) (2002) 539. [25] H. Aboud, H. Wagiran, I. Hossain, R. Hussin, Int. J. Phys. Sci. 7 (2012) 922. [26] H. Doweidar, M.A.A. Zeid, G.M. El-Damrawy, J. Phys. D 24 (12) (1991) 2222. [27] S.P. Yawale, S.V. Pakade, C.S. Adgaonkar, Indian J. Pure Appl. Phys. 33 (1995) 34. [28] P.D. Johnson, F.E. Williams, J. Chem. Phys. 18 (1950) 322. [29] P.D. Johnson, F.E. Williams, J. Chem. Phys. 18 (1950) 1477. [30] R. Mostefa, A. Kadari, S. Hiadsi, D. Kadri, Optik 127 (2016) 368. [31] R. Chen, J.L. Lawless, V. Pagonis, Radiat. Meas. 46 (2011) 1380.
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Thermoluminescence properties of borosilicate glass doped with ZnO E. Salama a,c,n, H.A. Soliman b, G.M. Youssef a, Sara Hamad a a b c
Physics Department, Faculty of Science, Ain Shams University, Cairo, Egypt National Institute for Standards, Ionizing Radiation Metrology Laboratory, Egypt Basic Science Department, Faculty of Engineering, British University in Egypt (BUE)
art ic l e i nf o
a b s t r a c t
Article history: Received 17 October 2016 Received in revised form 19 January 2017 Accepted 3 February 2017 Available online 4 February 2017
Thermoluminescence (TL) properties of new synthetic borosilicate glass (SiO2-H3BO3-Na2CO3) doped with various concentrations of ZnO are studied. The glasses were prepared by the melt quenching method and irradiated with gamma doses in the range of 0.5–500 Gy. Some properties such as, dose response, minimum detectable dose, reproducibility of the response, thermal and optical fading and IR absorption were studied. Optimum glass composition with a good linear TL gamma dose response over a wide dose range, as well as low fading and excellent reproducibility is obtained. These appealing features indicated that; this new synthetic TL glass material is useful in many radiation detection applications. & 2017 Elsevier B.V. All rights reserved.
Keywords: Thermoluminescence Borosilicate Zinc Glass Ionizing radiation
1. Introduction Accurate determination of radiation doses is a priority issue in most dosimetry applications. Such applications use dosimeters having specific features; namely: a favorable effective atomic number (Zeff), reasonable sensitivity, wide linear range, energy independent dose response, low fading rate and its relatively easy preparation and inexpensive shaping. Doped borate glasses have been extensively studied as dosimetric materials because they have several dosimetry properties compared with other dosimeters. Their Zeff, are very close to those of human biological tissues (Zeff ¼ 7.4) which makes them ideal materials for the environmental and medical dosimetry [1,2]. Pure borate glasses are typically known for the disadvantages of high hygroscopicity and the weak TL glow peak at relatively low temperatures [3]. Borate glasses are considered to be a good host for a wide varieties of dopants, like rare earths and transition metals. Most of the earlier studies on borate glass have been focusing on either the incorporation of different types of metals (alkali/alkaline) as modifiers or transition metals and/or rare earths as dopants and/or co-dopants to improve the stability as well as to enhance the thermoluminescence sensitivity of the dosimeter [4–7]. Alkali n Corresponding author at: Physics Department, Faculty of Science, Ain Shams University, Cairo, Egypt. E-mail addresses: [email protected], [email protected] (E. Salama).
http://dx.doi.org/10.1016/j.jlumin.2017.02.008 0022-2313/& 2017 Elsevier B.V. All rights reserved.
metals doped borate glasses are also better hosts for transition metal ions than the pure borates [4,8,9]. Lithium potassium borate glass co-doped with TiO2 and MgO thermoluminescent has a good linearity, low fading, excellent reproducibility with promising effective atomic number (Zeff ¼ 8.89), which is very close to human biological tissues (Zeff ¼ 7.4) [10]. Other dopants such as samarium were used to produce a high sensitive thermoluminescence borate glass [11]. Copper doped zinc lithium borate glass has good sensitivity, less fading characteristics and linear dose response in the radiotherapy dose range of 0.5–4 Gy [12]. Similar features were obtained with the prepared 0.1 mol% Cu-doped lithium potassium borate glass with Zeff ¼ 8.39 [13]. Borosilicate glass on the other hand is characterized by its extensive imperviousness to sudden changes in temperature, low coefficient of linear expansion and high chemical resistance [14]. The effect of ionizing radiation on a borate glass containing small amounts of impurities, normally found in most samples, involves the electron–hole pair formation and charge carrier trapping at specific sites of the glass; called defect centers. Previously, many authors have been discussing the luminescence mechanisms in borate glasses [15–17]. When irradiated, borosilicate glasses develop radiation-induced defects resulting in the formation of the non-bridging oxygen hole centers (NBOHCs) which are relevant to the coloration in the visible region. The electrons released from the non-bridging oxygen centers by irradiation are trapped at various defect centers. Some of these defect centers already exist in the glasses and can trap electrons while others are due to the ions added to the glass material such as Fe þ 2,
E. Salama et al. / Journal of Luminescence 186 (2017) 164–169
Fe þ 3 and Zn þ 2 [15]. The main objectives of this work were to synthesize and study the thermoluminescence properties of the new inexpensive vitreous system of borosilicate glass doped with ZnO for the purpose of finding conceivable applications in the field of ionizing radiation.
165
irradiated at room temperature at a dose rate of about 1.131 Gy/ min in sample position.
3. Results and discussions 3.1. Powder X-ray diffraction analysis (XRD)
2. Experimental work 2.1. Sample preparation and measurements New vitreous system SiO2-H3BO3-Na2CO3 with different concentrations of ZnO was synthesized by the melt quenching method. The preparation was done by mixing stoichiometric amounts of boric acid with silicon dioxide and sodium carbonates. Zinc oxide was added to the blend as a dopant. The chosen reagents noted for their high purity. To acquire high homogeneity, the composites were blended mechanically for 40 min and afterward put in a ceramic crucible. The blend was melted in a conventional electrical furnace at a temperature of around 1100 °C for a duration of 90 min. The used furnace was Ney-2-525 series II box oven. For the sake of complete homogeneity, the mixture was frequently stirred during the melting process. The molten glass was then poured onto a steel plate, which got subsequently annealed in another furnace at a temperature of 400 °C for 2 h. To release the thermal strain, the heat of furnace was slowly lost to room temperature. The prepared samples had a nominal composition of 20Na2CO3:20SiO2:(60 x)H3BO3:xZnO, 0˂x˂6. The compositions of the prepared samples were illustrated in Table 1. The prepared samples in solid form were polished and formed in the shape of 4 mm x 4 mm transparent square chips with 1 mm thickness. All samples are kept in dark containers to counteract the photoluminescence effect of background light. The TL reader model 3500 (Harshaw USA) available at the national institute for standards (NIS), Giza, Egypt was used in the TL measurements. The X-ray powder diffraction (XRD) was recorded for the representative samples of the prepared glass, to investigate its amorphous state. The XRD analysis was performed with Philips X`pert Pro X-ray powder diffractometer using Cu Kα radiation (1.5418 A) at a scanning speed of 0.3 s. The structural changes which occurred inside the prepared glass network with the variation in the concentration of ZnO can be identified by the infrared (IR) absorption measurements. The FTIR absorption spectra of the glasses were measured using a JASCOFT-IR6200 spectrometer with the KBr pellet technique in the spectral range 400–4000 cm 1.
Fig. 1 shows the XRD patterns of the samples of undoped and 2 mol% ZnO doped borosilicate glass samples. The broadening of the obtained pattern along with the disappear of the ZnO peaks (ICDD: 00–001-1136) indicate that, the samples were pure amorphous. 3.2. Fourier transformation infrared (FTIR) spectra As shown in Fig. 2, the (FTIR) absorption spectra of the studied glasses show major absorption bands around 468.63, 528.40, 717.40, 991.25, and 1382.73 cm 1 with some minor changes in band intensities and peak positions with respect to each other. These bands with their assignments are summarized in Table 2. The glasses doped with ZnO reveal FTIR spectra which are similar to the same vibrational bands identified in the undoped glass, without any changes in the number and position of the IR vibrational bonds. 3.3. TL glow curves Fig. 3 shows the glow curve of undoped and 1–6%ZnO-doped borosilicate samples irradiated by 20 Gy gamma dose. The glow curve, of both undoped and doped samples at 4 °C/s heating rate, exhibit one strong TL peak approximately at 138–206 °C. The maximum TL intensity is obtained at 2 mol % ZnO doped sample as shown in Fig. 4. The role of ZnO in the TL emission in the prepared borosilicate glass can be discussed based on the formation of both electron and hole trapping centers, as follow: Non-bridging oxygen hole centers are formed by irradiation through the following reaction in multicomponent glasses with non-bridging oxygen [15–17]
≡ Si−O−→ ≡ Si−O.+e− where, ≡ Si − O− and ≡ Si − O. represent the non-bridging oxygen (NBO) and non-bridging oxygen hole center (NBOHC) respectively. Adding divalent state zinc ions will increase both the electron and hole - trapping sites as follow: 100
2.2. Radiation source
Intensity (a.u)
Theratron Co-60 gamma source model 780E, manufactured by Theratronics Canada was used for irradiation. Samples were Table 1 The raw materials and composition of prepared glasses. Glass
Sample Sample Sample Sample Sample Sample Sample
Batch composition (mol%)
0 1 2 3 4 5 6
(S0) (S1) (S2) (S3) (S4) (S5) (S6)
SiO2
H3BO3
Na2CO3
ZnO
20 20 20 20 20 20 20
60 59 58 57 56 55 54
20 20 20 20 20 20 20
0 1 2 3 4 5 6
50
(b) (a)
0 20
30
40
50
60
70
2θ (degrees) Fig. 1. XRD patterns of (a) undoped and (b) 2 mol% ZnO -doped borosilicate.
E. Salama et al. / Journal of Luminescence 186 (2017) 164–169
991.25
166
0% ZnO 2% ZnO
15000
TLD Intensity (a.u)
0.2
528.40 468.63
717.40
Absorption (a.u.)
1382.73
0.4
10000
5000
0
2
0.0
40 0
60 0
80 0
10 00
12 00
14 00
16 00
18 00
20 00
Fig. 2. FTIR spectra of some investigated samples. Table 2 Assignment of infrared bands in the spectra of the prepared glass samples. Peak position (cm 1)
Assignment
Reference range
1382.73
Asymmetric stretching relaxation of B–O bonds of trigonal BO3 units B–O-stretching vibrations of BO3 units Stretching vibrations of B–O–Si linkage
̴ 1400 [18,19]
991.25 717.40
Band due to B–O–B linkage in borate network (two silicate chains and borate phases)
528.40 468.63
Vibration of the metal cation Na þ , Zn2 þ
3x10
2x10
1x10
950–1050 [20– 22] ̴ 700 [23,24]
o 600 [25–27]
0 Zn 1 Zn 2 Zn 3 Zn 4 Zn 5 Zn 6 Zn
7
7
7
7
ZnO, a maximum at this concentration and a decline of the efficiency at higher concentrations. Decreasing of the TL sensitivity of the prepared glass at higher concentrations of ZnO above 2 mol % may be attributed to the well-known concentration quenching effect. This effect has been first described by Johnson and Williams [28,29]; and recently developed and presented by several models [30,31]. 3.4. Annealing procedure The best pre-irradiation annealing condition should fulfill the following criteria: completely remove the previous signals, accomplish higher sensitivities, reproduce the readings for many irradiation cycles and minimizing standard deviation for the dosimeter readings [6]. The optimum pre-irradiation annealing condition is investigated by subjecting two groups each of 15 samples to different annealing conditions (temperatures and time) and gamma dose of 10 Gy. The samples of 1st group are annealed at different temperature from 100 °C to 400 °C in step of 100 °C with fixed annealing time (60 min). The samples of the 2nd group are annealed at different annealing time from 10 min to 60 min in step of 10 min and at the optimum temperature obtained from the 1st group. The annealing procedures were performed in the aformentioned conventional electrical oven. Fig. 5 shows the variation of TL emission and its associated standard deviation with different annealing temperatures at fixed annealing time (60 min); while Fig. 6 shows the variation with
7 4
20
3.0x10
4
0
-o-
2.8x10
100
150
200
250
Temperature (
o
300
350
−
≡ Si−O−+Zn2 + → ≡ Si−O.+( Zn2 + ) and
+ Zn2 +
)
4
16
4
14
4
12
4
10
4
8
4
6
4
4
4
2
2.6x10
C)
Fig. 3. The glow curves of undoped and 1–6 mol%ZnO-doped borosilicate samples irradiated by 20 Gy gamma dose.
≡ Si−O.+Zn2 + → ≡ Si−O−+(
2.4x10 2.2x10 2.0x10 1.8x10 1.6x10 1.4x10
)− and ( Zn2+)+ represent the hole and electron trap-
1.2x10
ping center respectively due to the additive ion Zn2 +. The dependence of the thermoluminescence efficiency on the concentration of ZnO in borosilicate glass is shown in Fig. 4. These results showed an increase with the concentration up to 2 mol% of
1.0x10
(
where, Zn2 +
18
400
TL Intensity (a.u)
50
TL response STD
4
100
200
300
400
o
Temperture ( C) Fig. 5. The effect of annealing temperature on TL intensity.
0 500
STD %
TL Intensity (a.u)
4x10
6
Fig. 4. TL intensity of irradiated borosilicate silica glass at different mol% ZnO concentrations.
-1
Wavenumber (cm )
5x10
4
ZnO concentrations (mol %)
E. Salama et al. / Journal of Luminescence 186 (2017) 164–169
4
4
100
200
90
o
2.6x10
210
110
TL -o- STD %
2.2x10
4
70 60
4
50 2.0x10
40
4
STD (%)
TL intensity (a.u)
80 2.4x10
30 1.8x10
20
4
10 1.6x10
4
Glow Peak Temperature ( C)
2.8x10
190
180
170
160
150
140
0 10
20
30
40
50
0
60
2
4
6
8
10
12
14
16
18
20
o
Time (min)
Heating Rate ( C/s)
Fig. 8. The effect of different heating rates on the glow peak temperature.
Fig. 6. Variation of TL intensity with different annealing time.
time. The highest TL mission with low standard deviation is obtained at the annealing condition of 400 °C for 40 min. Therefore, the best annealing condition for the new prepared TLD is evidently 400 °C for 40 min.
6
10
The effect of heating rate on the glow curve of the new glass phosphor and the glow peak temperature are investigated. For this respect, the TL glow curve of an irradiated samples of the prepared dosimeter were measured at different heating rates in the range of 1–20 °C/s. As shown in Fig. 7, the highest TL intensity was observed at heating rate (4 °C/s). The position of TL peaks was shifted from left to right with the increase in heating rate as shown in Fig. 8. This behavior can be explained in terms of the amount of electrons released per thermal stimulation, and the required time for this process. At low heating rates, there will be enough time to evacuate all electrons in the traps and form the glow curve at low temperature. At relatively high heating rates (5–20 °C/s), electrons are to be released from all traps faster, and therefore, it will take less time to complete glow curve shape, and shift glow curves peaks to high temperature will be observed [10,12]. 3.6. TL dose responses The results of the dose response, of the prepared borosilicate
TL Intensity (a.u)
5
3.5. Heating rate
10
4
10
3
10
1
10
100
Dose (Gy) Fig. 9. Linearity of dose response.
silica glass doped with 2 mol% ZnO within the dose range of 0.5– 500 Gy, are shown in Fig. 9. Linear response within this range was obtained with 0.9954 correlation coefficient. Such high correlation coefficient confirmed the linearity of the dose response within this range and indicated the good distribution of deep electron traps within the different levels of the host material. 3.7. Thermal fading
1.3
1.2
TL intensity (Normalized)
167
1.1
1.0
0.9
0.8
0.7
0.6 0
2
4
6
8
10
12
14
16
18
20
o
Heating Rate ( C/s) Fig. 7. Variation of the total glow curve intensity (normalized to the heating rate of 1 °C/s) with different heating rates.
Investigation of the possible fading effects of the new glass phosphor is an important parameter that needed to be determined. Thermal and optical fading can be evaluated as a result of the effect of the variable environmental conditions. The thermal fading of 10 Gy irradiated samples of the synthetic ZnO 2 mol% borosilicate glass was investigated over the period of 30 days. All irradiated samples are stored in an opaque container at room temperature (25–30 °C) and humidity condition between 40 and 50%. The thermal fading characteristics of sampled dosimeters over the period of 30 days were reported as shown in Fig. 10. To subtract the effect of background, annealed (un-irradiated) samples were stored with the irradiated samples and measured at each measurement step. The obtained results showed 11% reduction after 24 h from irradiation and 21.4% after 7 days. No extra thermal fading was obtained in a time window further than one week during the remaining investigation time of one month, as shown in Fig. 11. This result will assist in estimating the correct absorbed doses in case the readout could not take place in time.
168
E. Salama et al. / Journal of Luminescence 186 (2017) 164–169
6
110
4x10
Zero day 1 day 2 days 3 days
6
90 80
Residual Signal (%)
TL Intensity (a.u)
3x10
100
6
2x10
6
1x10
70 60 50 40 30 20 10 0
0 50
100
150
200
250
300
350
0
400
10
20
30
40
50
60
70
80
90
100
Time (Min)
o
Temperture ( C) Fig. 10. Glow curves of 10 Gy irradiated 2 mol% ZnO doped borosilicate glass during its thermal Fading at room temperature.
Fig. 12. Optical Fading of 2 mol% ZnO doped borosilicate glass irradiated at 10 Gy.
1.2
100 1.0
TL intensity (normalized)
Residual signal (%)
80
60
40
20
0.8
0.6
0.4
0.2
0.0
0 0
5
10
15
20
25
30
0
3.8. Optical fading (Sensitivity to light) Optical fading parameter was also determined during this study to quantify the sensitivity of the new synthetic borosilicate glass to sunlight. Direct exposure of irradiated dosimeters (10 Gy absorbed dose) to sunlight for 0.5 h led to a loss of 72% of the stored signals. This loss happened to exceed 82% after 1.5 h of direct exposure as shown in Fig. 12. It is to be mentioned that these numbers may vary according to the light intensity. In the present work we were considering the general effect of light exposure. We wanted to prove that, light exposure has effect on the stored signals of an irradiated samples, no matter what the intensity is. Therefore, it is recommended to use an opaque cover in environmental applications and during dosimeters transfer to avoid the light effect. 3.9. Reproducibility Reproducibility of measured dose by the proposed TLD was determined by using five glass samples. These samples were repeatedly irradiated at 10 Gy after the proper preliminary annealing. As shown in Fig. 13, the prepared dosimeter can repeatedly measure the irradiated dose with less than 3% difference, based on a standard deviation of five consecutive measurements. The small
4
6
8
10
Number of measurements
Storage time (days) Fig. 11. Thermal Fading of 2 mol% ZnO doped borosilicate glass irradiated at 10 Gy.
2
Fig. 13. Reproducibility of 2 mol% ZnO doped borosilicate glass.
differences in the average TL intensity throughout nine cycles readout shows that this glass is reusable in the dose assessment by TL method.
4. Conclusion In this work, new synthetic borosilicate glass (SiO2-H3 BO3-Na2CO3) doped with various concentrations of ZnO was prepared by the melt quenching method. The best concentration of ZnO was found to be 2 mol%. With regard to its utilization as a radiation dosimeter, several dosimetric properties of the new prepared glass have been studied. The best heating rate and annealing procedure was found to be 4 °C/s and 400 °C at 40 min respectively. The obtained glow curves are considerably simple curves. The dose response curve is linear over the dose range 0.5– 500 Gy, with a single prominent peak at about 178 °C. Relatively low rate of fading about 21% over 1 week of storage and good reproducibility of dosimeter response are provided. These characteristics make the new synthetic borosilicate glass very suitable for radiation detection applications.
E. Salama et al. / Journal of Luminescence 186 (2017) 164–169
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