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Thermoluminescence of B2O3–Li2O glass system doped with MgO
ARTICLE IN PRESS Journal of Luminescence 130 (2010) 1880–1892
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Thermoluminescence of B2O3–Li2O glass system doped with MgO M.M. Elkholy Physics Department, Faculty of Science, Menoufia University, Shibin El-Koom 32511, Egypt
a r t i c l e in f o
a b s t r a c t
Article history: Received 6 October 2009 Received in revised form 28 April 2010 Accepted 3 May 2010 Available online 7 May 2010
Lithium borate (LiB) glasses in the system (100 x)B2O3–xLi2O with x¼ 20, 30, 40, 50, 60 and 70 mol% were prepared. The glasses were doped with different concentrations of the order of 10 1, 10 2, 10 3, 10 4 and 10 5 of MgO and their thermoluminescent (TL) response was investigated. The irradiations were performed using g rays from a 60Co source in the dose range from 0.1 to 25 kGy. The material displayed good sensitivity for g-rays and intensity of TL signals is dependent on g-ray dose and Li2O content. For each dose level and investigated temperature range (50–350 1C), exactly single isolated glow peak appears in the temperature range of 165–205 1C depending on both Li2O concentrations and time of exposure. The shape of the glow peak has altered significantly with increase in the gamma ray dose or Li2O concentrations. The glass composition with x ¼50 mol% doped with 10 3 mol% of MgO presented the best TL response. The results of the present study indicated that the recorded single and isolated high temperature peak is a good candidate for TL dosimetric investigations. This indicates that 50 B2O3–50Li2O-doped with 10 3 mol% of MgO is possibly used as materials for radiation dosimetry in the dose range of 0.1–20 kGy. & 2010 Elsevier B.V. All rights reserved.
Keywords: Thermoluminescence Lithium borate Glasses Dosimetry
1. Introduction The knowledge on radiation-induced defect centers in glasses has been a motivating focus of investigation in recent years, since such studies help in probing the stability of the glasses for radiation dosimetry applications. In radiotherapy and radiodiagnosis, in order to map doses in tissues, there exists a need for highly sensitive in vivo dosimetry systems of high spatial resolution, commercially available thermoluminescence dosimeters (TLDs) being limited to a capability of few millimeters. A number of investigators have examined the photon response of optical fibers at kilovoltage energies and below [1–5]. Recently to date, investigation has been made on commercially available germanium-doped optical fibers in terms of their response to photons in the therapeutic energy and dose range [6,7]. Encouraging results from such studies have paved the way for development of fibre radiation dosimeters specifically tailored to the task of dosimetry in the therapeutic and diagnostics dose regimes. Study on radiation induced defect centers in glasses has been an interesting subject of investigation in recent years, since such studies help in examining the suitability of the glasses for radiation dosimetry applications. Extensive studies on the activating or killing effect of luminescence due to some transition metal ions like Fe2 + , Cu2 + , Ti4 + , etc., in amorphous materials are
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available in the literature [8–12]. In recent years, many authors have done a commendable work on luminescence mechanisms in borate glasses [13–16]. Thermoluminescence studies of borate glasses are of great interest because of their near tissue equivalent absorption coefficient, very low cost and easy handling process although their sensitivity and thermal stability vary widely and depend strongly on the preparation method. The hygroscopic nature and low temperature glow peak are the disadvantages of pure borate glasses, which may limit its applications. Alkali oxy borate glasses are considered as good materials for dosimetry applications since they are relatively moisture resistant when compared with the pure borate glasses. Addition of CaF2 into the glass matrix lowers the viscosity and decreases the liquidus temperature to a substantial extent and further it acts as an effective mineralizer, giving scope for the formation of a large concentration of color centers when the glasses are exposed to ionizing radiation. F ions in CaF2 act as co-activators and facilitate the substitution of activators into the lattice. Manganese ion is interesting because it exists in different valence states in different glass matrices, for example, as Mn3 + in borate glass with octahedral coordination whereas in silicate and germinate glasses it exists in Mn2 + state with both octahedral and tetrahedral coordination [17]. Further, among different manganese ions, Mn2 + and Mn4 + ions are identified as luminescence activators [18]. The content of manganese in different forms in different valence states that exist in the glass depends on the quantitative properties of modifiers and glass formers, the size of the ions in glass structure, their field strength, the mobility of the modifier
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cation, etc. Hence, the relation between the state, the position of the manganese ion and the luminescence properties of the glass is expected to be very interesting. Lithium borate (Li2B4O7) glasses are of great interest because of their good ionic conductance properties [19]. Since the addition of Li2O to the borate glass adds extra oxygen atoms, which gets accommodated in the network, a transfer of some boron atoms from triangle BO3 to tetrahedral BO4 occurs [20]. The introduction of transition metals ions (TMI) into an oxide glass change the oxide network. TMI were used as a constituent in several borate glasses in order to achieve useful physical properties. Though MgO is not a glass-forming oxide by itself, it can incorporate in substantial quantities into these glassforming oxide systems. Borate glasses are easily melted and are good hosts for transition metals ions and from a number of spectroscopic
Table 1 Starting and analyzed compositions of (100–x)B2O3–xLi2O glass system Glass notation
80Li2O–20B2O3 70Li2O–30B2O3 60Li2O–40B2O3 50Li2O–50B2O3 40Li2O–60B2O3 30Li2O–70B2O3
LiB-8020 LiB-7030 LiB-6040 LiB-5050 LiB-4060 LiB-3070
Starting composition
Analyzed composition
Li2O
B2O3
Li2O
B2O3
80 70 60 50 40 30
20 30 40 50 60 70
81.6 71.4 61.2 52.5 43.8 35.6
18.4 28.6 38.8 47.5 56.2 64.4
20
18
14000 xLi2O-(1-x)B2O3:0.001MgO Exposed to 10 kGy and heating rate 10° C/s
x=20 x=30 x=40 x=50 x=60 x=70
12000
10000
TL intensity (µC)
techniques, these properties of glasses have been established in terms of the stereochemical environment they provide and the oxidation number that they favour for the transition metal ion. The second advantage of borate glasses is that boron has the smallest mass compared to other network forming elements [21]. In lithium borate, nLi2O–B2O3 glass system, these physical properties as the function of the glass composition were extensively studied [22–26]. This glass system extends over a wide range of the glass composition; therefore, it is appropriate to study the validity and range of adaptation of each model in this glass system. Despite the doped lithium tetraborate studied in recent years, there is limited knowledge about the energy storage and luminescence mechanisms in these materials. In this study, thermoluminescence characteristics of lithium–borate glasses and their dependence on several MgO concentrations aimed at a deeper understanding of the thermoluminescence processes in doped glass. The present study was directed to study the usefulness of using the prepared glass as gamma ray dosimeter. For this purpose, the prepared glass samples are doped with low
Glow peak height (arb. units)
Glass formula
1881
8000
16
14
12
10
6000
8 0
10
20
30
40
50
60
70
80
Li2O content (mol.%) 4000
Fig. 2. The height of the main glow curve as a function of Li2O content.
2000
Table 2 Concentrations of activator and co-activator added to 50LiO2–50B2O5 glass sample.
0 0
50
100
150
200
250
300
350
400
Temperature (°C) Fig. 1. TL intensity as a function of temperature for (1–x)B2O3–xLi2O2 doped with 10 3 wt% of MgO and exposed to 10 kGy.
Glass formula
Glass notation
LiO2
B2O3
Mg
50Li2O–50 B2O3 50Li2O–50 B2O3 50Li2O–50 B2O3 50LiO–50 B2O3 50Li2O–50 B2O3
LiB-5050(1) LiB-5050(2) LiB-5050(3) LiB-5050(4) LiB-5050(5)
50 50 50 50 50
50 50 50 50 50
10 1 10 2 10 3 10 4 10 5
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concentrations of some MgO, which are very suitable centers for TLD purposes, to detect gamma rays of wide dose range.
thermoluminescence (TL) of dosed glass sample. It is one of a series of thermoluminescence analyzers developed by this company for personal, environmental, diagnostic and therapeutic radiation dosimetry as well as for research applications.
2. Experimental 2.1. Glass preparation
3. Results
For the present study, binary (100 x)B2O3–xLi2O glass system with different ratios of lithium oxide was prepared. For each glass, the thermal history (i.e. melting temperature, melting time, annealing temperature and time) and all preparation conditions are as similar as possible. The binary (100x)B2O3–xLi2O glasses have been synthesized using high purity analar grade chemicals: lithium carbonate (Li2CO3), manganese dioxide (MgO) and orthoboric acid (H3BO3). Mixtures of these materials in appropriate proportions are thoroughly mixed and melted in porcelain crucibles at about 1200 1C. The glasses suitable for thermoluminescence measurements are prepared by the melt quenching method. All the samples annealed below their transition temperature. Traces of transition metals responsible for thermoluminescence traps of the order of 10 1, 10 2, 10 3, 10 4 and 10 5 of MgO were added as 0.001 wt% for all samples as activator and co-activator. 2.2. Measurement of thermoluminescence A thermoluminescence analyzer supplied by Harshaw Chemical Company (USA), Harshaw TLD model 4500, measured
3.1. Determination of the final composition of (100 x)B2O3-xLi2O For the present study, binary (100 x)B2O3–xLi2O glasses with different concentrations of Li2O, where x¼20, 30, 40, 50, 60 and 70, were prepared. The resultant glass samples are cylindrical in form, transparent, and its transparency changes from sample to sample depending on Li2O concentration. All the prepared glass samples and their notations are tabulated in Table 1. During the glass preparation, we noted that at temperatures lower than the melting temperature of pure Li2O (Tm 1000 1C) some vapors were shown in the muffle zone. These vapors are essentially arising from B2O3 (Tm 580 1C). Therefore, substantial loss in B2O3 content has occured during the melting process; therefore, the composition of the final product may slightly differ from the starting composition. Because of these, the prepared glasses were chemically analyzed to determine the exact content of the glass components. The chemically analyzed composition data have shown that, a substantial loss of B2O3 content may have occurred during the melting processes (see Table 1). Several authors [27–29] have investigated phosphate glass using
16000
14000
50Li2O-50B2O3:MgO Exposed to 10 kGy
12000
0.50 kGy 0.75 kGy 1 kGy 2.5 kGy 5 kGy 7.5 kGy 10 kGy 15 kGy 20 kGy
50Li2O-50B2O3:MgO
10-3 wt.% MgO 10-2 wt.% MgO 10-4 wt.% MgO 10-1 wt.% MgO 10-5 wt.% MgO
14000
12000 10000
TL Intensity (µC)
TL intensity (µC)
10000 8000
6000
8000
6000
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4000
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2000
0 0
50
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350
400
Temperature (°C) Fig. 3. The glow curves of samples: LiB5050(1), LiB5050(2), LiB5050(3), LiB5050(4) and LiB5050(5) exposed to 10 kGy of gamma irradiation.
0 0.000001
0.00001
0.0001
0.001
0.01
0.1
1
MgO wt.% Fig. 4. TL response as a function of MgO concentrations for glass sample LiB5050(3) after exposed to 10 kGy of gamma irradiation.
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16000
12000 0.50 kGy 0.75 kGy 1 kGy 2.5 kGy 5 kGy 7.5 kGy 10 kGy 15 kGy 20 kGy
10000
0.50 kGy 0.75 kGy 1 kGy 2.5 kGy 5 kGy 7.5 kGy 10 kGy 15 kGy 20 kGy
50Li2O-50B2O3:0.00001MgO
14000
12000
50Li2O-50B2O3:0.001MgO
8000
TL intensity (µ µC)
TL Intensity (µC)
10000
6000
8000
6000 4000
4000 2000
2000
0 0
50
100
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350
400
Temperature (° C)
0
12000
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50
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Temperature (°C)
Fig. 5. Glow curves recorded at different doses of 0.5, 0.75, 1.0, 2.5, 5.0, 7.5, 10, 15 and 20 kGy for the 50B2O5–50 Li2O samples doped with 10 5 MgO.
0.50 kGy 0.75 kGy 1 kGy 2.5 kGy 5 kGy 7.5 kGy 10 kGy 15 kGy 20 kGy
0
50Li2O-50B2O3:0.0001MgO
Fig. 7. Glow curves recorded at different doses of 0.5, 0.75, 1.0, 2.5, 5.0, 7.5, 10, 15 and 20 kGy for the 50B2O5–50 Li2O samples doped with 10 3 MgO.
chemical analysis. It is found that, like phosphate glasses, when borate glasses were prepared by melting the constituents in air, loss in B2O3 oxygen and boron occurred. From chemical analysis results, as expected, loss of borate contents was observed (see Table, 1). In addition, the data of Table 1 have established that the range of glass formation of binary (100 x) B2O3-xLi2O glass system is continuous from x¼30 to 80, with no region of stable immiscibility existing.
TL Intensity (µ µC)
8000
3.2. Thermoluminescence for (1 x)B2O3–xLi2O 6000
4000
2000
0 0
50
100
150
200
250
300
350
400
Temperature (°C) Fig. 6. Glow curves recorded at different doses of 0.5, 0.75, 1.0, 2.5, 5.0, 7.5, 10, 15 and 20 kGy for the 50B2O5–50 Li2O samples doped with 10 4 MgO.
To measure the response to gamma radiation, we used TL technique. The TL analysis was performed using a Harshaw 4500 analyzer in a nitrogen atmosphere, with a linear heating rate of 10 1C/s from room temperature up to 350 1C. Traces of MgO with the order of 10 3 wt% were added to (1 x)B2O3–xLi2O to determine the sensitive sample to the present study. Calibration was carried out by irradiating standard dosimeters (such as commercial lithium fluoride: LiF,Mg, Ti (TL-100)), in the same conditions and geometry. Each experimental data point represents the mean of at least two measurements. Fig. 1 shows the TL intensity as a function of temperature for (1 x)B2O3–xLi2O doped with 10 3 wt% of MgO and exposed to 10 kGy. This figure shows two glow peaks: one of them is weak and positioned around 100 1C and the other main one is highly intense and lay around 225 1C, which is very suitable for dosimetry purposes. Also Fig. 1 shows that the intensity and the height of the main glow curve markedly increase with increase in Li2O content up to 50 mol% followed by a dramatic decrease with further increase in Li2O content (see Fig. 2).
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3.3. Determination of the optimum concentration of transition metal suitable as TL centers The glass sample LiB5050 of composition 50 mole% Li2O–50 mole% B2O3 was used as a base glass due to its good stability and its good TL response as indicated in Figs. 1 and 2. Different concentrations, 10 1, 10 2, 10 3, 10 4 and 10 5 wt% of MgO, were added to the above glass sample, LiB5050, to obtain the samples marked as LiB5050(1), LiB5050(2), LiB5050(3), LiB5050(4) and LiB5050(5), respectively (see Table 2). The prepared samples were exposed to intermediate dose 10 kGy of cobalt-60 gamma rays at room temperature. Fig. 3 shows the glow curves of samples LiB5050(1), LiB5050(2), LiB5050(3), LiB5050(4) and LiB5050(5) exposed to 10 kGy of gamma irradiation. This figure shows single isolated glow curves for all doped samples with different Mg concentrations. For LiB5050(2) and LiB5050(3), additional small glow peak around 100 1C was also detected. The positions of the recorded glow curves seem unchanged and independent of MgO concentrations. The total TL intensity that reflected from either the total integrated area under glow peaks or that recorded in microCoulombs in the reading system increases with increase in MgO concentrations up to 10 3 wt% followed by a sharp decrease with further increase in MgO concentrations up to 10 5 wt%. Therefore, we concluded that among all the prepared samples, the sample doped with MgO concentration of 10 3 wt% is the most thermoluminescence efficient one. Fig. 4 shows TL response as a function of MgO concentrations for glass sample LiB5050(3) after being exposed to 10 kGy of gamma irradiation. From this figure we noted that, the total thermoluminescence response hardly depends on MgO
concentrations. Also, from the figure, it can be easily seen that the TL response is increased with increase in MgO concentration up to 10 3 wt% in glass system followed by a strong decrease in the total TL response with MgO concentration up to 10 5 wt% in this glass system. So, the optimum concentration of Mg responsible for TL in this glass system is 10 3 wt%. Therefore, the concentration of 10 3 wt% MgO was used as an optimum concentration of Mg, as an activator for the rest of the prepared glass samples. 3.4. Thermoluminescence and thermoluminescence dosimetry for 50B2O5–50Li2O Figs. 5–9 show typical glow curves recorded at different doses of 0.5, 0.75, 1.0, 2.5, 5.0, 7.5, 10, 15 and 20 kGy for the prepared 50B2O5–50Li2O samples with different MgO concentrations (see Table 1). The curves were recorded using a heating rate of 10 1C/s. For each dose level and investigated temperature range (100–350 1C), exactly single isolated glow peak appears in the temperature range of 215–225 1C. The shape of the glow peak has not altered significantly with increase in the gamma ray dose. Two significant effects were observed, one of which is a very slight shift of the peaks towards higher temperatures at higher dose levels namely 15 and 20 kGy. The second one is a marked increase in the intensity of the peak height and area under glow curves with g-dose due to the total charge in microCoulombs (mC), collected and displayed on the TLD analysis. This charge represents the total integrated area under the full glow curves of the irradiated thermoluminescent sample. When this charge is plotted as a function of the gamma ray dose, the TL-response curves for the measured samples are given. From Figs. 5–9, the thermoluminescent 9000
14000
12000
10000
50Li2O-50B2O3:0.1MgO
0.50 kGy 0.75 kGy 1 kGy 2.5 kGy 5 kGy 7.5 kGy 10 kGy 15 kGy 20 kGy
50Li2O-50B2O3:0.01MgO
0.50 kGy 0.75 kGy 1 kGy 2.5 kGy 5 kGy 7.5 kGy 10 kGy 15 kGy 20 kGy
8000
7000
TL Intensity (mC)
TL Intensity (µC)
6000
8000
6000
5000
4000
3000 4000
2000
2000
1000
0
0 0
50
100
150
200
250
300
350
400
Gamma Dose (kGy) Fig. 8. Glow curves recorded at different doses of 0.5, 0.75, 1.0, 2.5, 5.0, 7.5, 10, 15 and 20 kGy for the 50B2O5–50 Li2O samples doped with 10 2 MgO.
0
50
100
150
200
250
300
350
400
Temperature (°C) Fig. 9. Glow curves recorded at different doses of 0.5, 0.75, 1.0, 2.5, 5.0, 7.5, 10, 15 and 20 kGy for the 50B2O5–50Li2O samples doped with 10 1 MgO.
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glow peaks are detected around 215 1C for binary B2O5–Li2O glasses with different concentrations of MgO. Figs. 5–9 show also that the glow peak intensities are hardly dependent on the MgO concentration. The above observation coincides with the idea that the trapping is intrinsic to the host lattice, i.e. there is a cross link between the impurity and the trapping centers. From the recorded glow curves for the same glass sample, it is very clear from each figure that a slight shift is observed in the maximum temperature of the glow curve to higher temperature at higher g-radiation dose level. A similar behavior observed in a number of glow curves studied on CaSO4: Dy [30]. Mangia et al. [30] concluded that this behavior could be explained by assuming either a multilevel or a continuous distribution of trap depths, which was associated with the main thermoluminescent glow peak, with a single associated frequency factor. This picture of a Gaussian distribution of trap depths (observed in the present work) coincided with Spurny and Novothy’s [31] conclusion that the most probable environment for a trapped electron or hole is nonspherical symmetric cavity, which contains a distribution of activation energies.
1885
The total charge which is plotted as a function of the gamma ray dose, at a running end temperature in Figs. 10–14, shows that for all samples, the profile of the dose response curves does not show significant changes. In general, the curves reveal two characteristic ranges. At low gamma ray dose, below 10 kGy, the TL intensity increases linearly with increase in gamma dose. In the second range, the total TL response shows a supralinear relation with gamma dose. Above this range, the TL response tends to saturate. Also it is clear that the sample containing 10 3 wt% MgO is the most efficient thermoluminescent sample. This is clearer from the glow curves and from the TL response dose curves where the intensity of the TL response of this samples is much higher than that of other samples. Results of the attempts made to investigate the changes in the TL response and the height of the measured glow curve with irradiation dose shown in Figs. 7 and 12 for the sample containing 10 3 wt% MgO indicate that this sample is the most sensitive for irradiation. Apparently, the TL response and the peak height increase linearly with increase in the irradiation dose up to 10 kGy.
Fig. 10. TL response as a function of gamma dose for glass sample LiB5050(5) at different temperatures.
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Fig. 11. TL response as a function of gamma dose for glass sample LiB5050(4) at different temperatures.
Above this dose level, supralinearity exists and all curves with different doses show the same trend observed in the dose-response curves. Supralinearity is attributed to the following factors [32]: 1. the creation of additional trapping sites; 2. the creation of new recombination centers; and 3. an increase in thermoluminescent efficiency of the used material. The observed effects of gamma radiation in the present doped glasses can be discussed in terms of two main phenomena which operate in the same time in the material during irradiation. Gamma ray creates trapping centers, the number of which increases with increase in gamma doses. In addition, they also act as an exciting source for the material raising electrons from the valence band of the material, which later are trapped in the states created by defects produced either by radiation or by the impurities. When the trapped electrons are released by heating, it will recombine with holes left in the filled band to give the thermal glow curves.
The observed increase in the glow intensity with increase in the radiation dose up to approximately 10 kGy results from the increased number of excited electrons, leading to an increase in the recombination probability. The saturation of all glow peaks (noted at higher gamma dose) is caused by a new group of traps created by radiation at such dose levels. These new traps participate in the trapping process reducing the number of electrons originally available for other trap levels.
3.5. Effect of heating rate on the thermoluminescence response in 50B2O5–50Li2O:10 3MgO glass system The dependence of dosimeter sensitivity upon the heating rate used during its readout is a very important effect. The influence of the heating rates on experimental glow curve of the present 50B2O5–50Li2O:10 3MgO at a fixed dose of 10 kGy is shown in Fig. 15. The shape of the glow curve does not change as a function of the heating rate. From these figures, it can be easily seen that, the thermoluminescence intensity and glow peak position are
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Fig. 12. TL response as a function of gamma dose for glass sample LiB5050(3) at different temperatures.
hardly dependent on heating rate. The glow peak position and shape dramatically and linearly shifted to higher temperature with increase in heating rate (see Fig. 16). It is clear that the heating rate of 10 1C/s is the most suitable rate for obtaining high TL response and the highest glow peak besides the most suitable glow peak temperature, which is very good for dosimetry purposes. Fig. 17 shows a comparison between the TL intensity and some slandered TLD phosphors and the TL intensity of the present glass samples. From this figure, it can be noticed that the prepared new glass samples are good and promising radiation dosimetry samples. 3.6. Calculation of trap depth from glow curves 3.6.1. The methods used for determining trap depth from glow curves The most important parameter for TL to be determined is E the trap depth, which is the thermal energy required to liberate a trapped electron.
Several methods based on the analysis of the glow curves have been proposed and used for determining E. The full details of these methods are investigated in my previous work [33]. We can classify these methods into three main types: (1) Methods based on Tm, where Tm is the peak temperature at the maximum intensity. (2) Methods based on the shape of the glow curve. (3) Methods based on both Tm and the shape. 3.6.2. Numerical calculation of trap depth from glow curves The measured glow curves are analyzed to obtain the activation energy, E, for the trapped electrons present in lithium borate glass doped with 10 3 wt% of Mg. The resulting data for E calculated by these methods are tabulated in Table 3. The used methods were concerned with first and second kinetic orders. In the present work, we have used most of the above methods to estimate the activation energy values from the resulting glow peaks. Apparently, the methods used gave different values of
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Fig. 13. TL response as a function of gamma dose for glass sample LiB5050(2) at different temperatures.
activation energy, which show some scatter due to different basic techniques on which these methods were established.
4. Discussion The diborate composition was chosen since it contains the largest fraction of BO4 units in Li borate glasses, with Li ions occurring in a charge compensating position [40]. In Li-bearing crystalline phases, the mean Li–O distance in tetrahedral site is 1.98 A˚ [40]. In crystalline Li-diborate, Li occurs in a distorted fourfold site, with a mean Li–O distance of 2.05 A˚ [41]. Therefore, the mean Li–O distance of 1.99 A˚ is consistent with an average tetrahedral site in Li-diborate glass. According to the very large ˚ average Debye–Waller factor (0.115 A), this site is highly ˚ distorted with two broad components at 1.97 and 2.03 A. Similarly, Li atoms occur in distorted tetrahedral sites in Lialuminosilicate [42] and Li-disilicate glasses [43], with mean Li–O ˚ respectively. The Li environment distances of 2.10 and 2.02 A,
obtained by molecular dynamics (MD) simulations on Li borate glasses consists of various distorted sites with 4–8 oxygen neighbors [44]. The charge compensating positions were identified with the largest coordinated sites [42,43] having a mean Li–O ˚ However, these data suggest that the Li sites distance of 2.12 A. related to modifying and charge-compensating positions are not so well distinct than indicated by MD, and Li may occur in a broad site distribution rather than in two types of sites in borate glasses. In the present glass samples, the Li ions have a closed structure, do not have energy levels within 10 eV of the ground state and hence these ions do not participate directly in the luminescence but may act as activator ions. In the activation process, the doping of glass by doubly charged alkaline earth Mg2 + causes a charge imbalance. Since the alkaline earth ions are doubly charged, the energy levels of the surrounding oxygen ions will be slightly lifted up in comparison with the normal ions and give rise to occupied energy levels close to the top of the valence band and these levels form the ground state of the luminescence. The lifting of these energy levels occurs to a lesser extent in
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Fig. 14. TL response as a function of gamma dose for glass sample LiB5050(1) at different temperatures.
˚ glasses containing Mg ions, since the ionic radius of Mg2 + (0.74 A) ˚ Sr2 + is small when compared to the ionic radii of Ca2 + (1.04 A), ˚ and Ba2 + (1.38 A) ˚ ions. This indicates the presence of (1.20 A) deeper traps and color centers in the case of MgO, which give rise to larger TL light output in the high temperature region as observed. Thermoluminescence is a consequence of radiative recombination between the electrons (released by heating from the electron center) and an antibonding molecular orbital of the nearest of the oxygen hole centers. However, the TL emission due to such recombination process is generally possible only at low temperatures in borate glasses [45]. Alternatively, the TL emission in these glasses may be explained as follows. During the heating process, the electrons captured by metal ions are liberated and later trapped by holes in the recombination centers giving out TL light output. The first small glow peak may be attributed to be due to such a process. The action of g-ray irradiation on glasses is to produce secondary electrons from the sites where they are in a stable state and have an excess energy. Such electrons may
traverse in the glass network depending upon their energy and the compositions of the glasses finally trapped, thus forming color centers. The trapping sites may be the metal cations that constitute the glass structure, ions of admixtures to the main composition and/or the structural defects due to impurities in the glass. Thus this process leads to the formation of (1) boron electron centers, (2) non-bridging oxygen hole centers and (3) boron oxygen hole centers [14,45]. In the absence of Mg ion in the glass network, each electron released by heating from electron centers would be trapped by an antibonding molecular orbital of the nearest of the oxygen hole center giving out maximum TL light output. During the heating process, the electrons from such centers are liberated and later trapped by holes in the recombination centers giving out TL light output. The response of the glasses to gamma rays irradiation is related to the rate of formation and accumulation of induced defects during progressive irradiation and hence the production of characteristic color center. According to Friebele [2], the color
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50Li2O-50B2O3:0.001MgO Exposed to 10 kGy
7.5 oC/s
14000
10 oC/s 12.5 oC/s 15 oC/s 17.5 oC/s
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20 oC/s
TL intensity (µ µC)
10000
8000
6000
4000
2000
0 0
100
200
300
400
Temperature (°C)
Fig. 16. The influence of the heating rates on the glow peak maximum temperature for 50B2O3–50Li2O:10 3MgO at a fixed dose of 10 kGy.
Fig. 15. The influence of the heating rates on glow curve of 50B2O3–50Li2O:10 3 MgO at a fixed dose of 10 kGy.
8 TLD-100 * LBO:Cu,Ag,P * present paper 50Li2O-50B2O3:0.001MgO
7
6
TL intensity (arb. units)
centers may include boron E center, nonbridging oxygen holes centers, this can be defined as HC, hole trapped on a single nonbridging oxygen,HC2 hole trapped on two nonbridging oxygen hole centers BOHC1 and BOHC2 and alkaline earth defect center (i.e. Mg in this work). There are few reports in the literature with TL emission in borate glasses. Pontuschka and co-workers [14,45,46] had shown with EPR results that in barium or calcium aluminoborate glasses, intrinsic electron trap centers connected to boron (BEC) act only at temperatures below 300 K, but a large concentration of hole centers, due to boron and oxygen (BOHC), is available to recombine with free electrons at higher temperatures. Also, Pascoal et al. [46] verified that in the calcium aluminoborate glasses, the increase of Ca content above 20 mol% of CaO decreases the TL emission at 470 K, because the presence of this divalent ion decreases the BOHC concentration, due to the increase of NBO (non-bridging oxygen) in the matrix. Rao et al. [47] reports also the TL response of borate glasses (mixed with CaF2 and alkali metal oxides): glow curves of undoped samples show complex peaks in the range of 180–205 1C, with activation energies in the range 0.29–0.39 eV. Addition of nickel shifts the peaks to higher temperatures, with higher activation energies (0.31–0.53 eV depending on the metal oxide). The authors attribute to Ca ions the role of electron trapping centers in those borate glasses, showing also that other impurities as Fe, Mn and Ni kill the TL emission [46,47]. In fact, due to the presence of F ions and their associated charge compensation in the glass, the calcium ions occupy the alkali ion positions in the matrix, diversely of what happens in the samples used in Pascoal’s paper [46]. According to these arguments, we can suggest that, in the
5
4
3
2
1
0 0
50
100
150
200
250
300
350
Temperature (°C) * Data taken from: B.T. Huy , V.X. Quang, H.T.B. Chau, Journal of Luminescence 128 (2008) 1601–1605 Fig. 17. Comparison between the TL intensity and some slandered TLD phosphors and the TL intensity of the present glass samples.
ARTICLE IN PRESS
2kTM
1.06 0.6 0.56
1.02 0.77 0.72
1 0.77 0.72
1 0.62 0.58
1.01 0.59 0.55
0.68
0.56 1.13 0.55 0.62 0.88
0.96
0.47 0.95 0.46 0.65 0.91
0.81
0.45 0.9 0.44 0.63 0.88
0.77
0.5 0.99 0.49 0.6 0.84
0.85
0.48 0.97 0.47 0.58 0.82
0.83
0.54 0.76
0.62
0.35
0.73
0.36
o
Lushchik’s 1st order method [37]
2 kTM
ðEc Þo ¼ 3:54
d
Chen’s 1st order method [36]
0.56 0.62 0.54 0.54
0.54
0.71 0.78 0.45 0.59
0.72
0.71 0.77 0.43 0.57
0.71
0.58 0.64 0.48 0.53
0.57
0.56 0.61 0.47 0.51
0.54
0.67 0.74 0.34 0.5
0.68
Halperin and Braner’s 2nd order method [38] Christodoulides method, second half width (I/Im ¼1/2) [39]
Christodoulides method, first half width (I/Im ¼1/2) [39]
1891
calcium matrix the intrinsic hole centers (BOHC) are less abundant giving rise to a less intense TL signal, as observed also by Pascoal et al. [46]. Also in matrices studied here, Mg probably acts as an electron-trapping center, and the intrinsic defects (BOHC) are the recombination centers for the TL emission. Lithium ions and magnesium, acting as co-dopants, definitely enhance the thermoluminescence of all the investigated TL materials, when compared with that of the related phosphors without lithium co-dopant. Lithium borate glasses doped with magnesium is given a preirradiation anneal at 350 1C (standard phosphor anneal of 1 h) and cooled quickly to normal ambient temperature; the resulting glow curve after irradiation contains single isolated glow peak between normal ambient temperatures and 350 1C. By convention, this is named the main characteristic peak. The precise readout temperature and the resolution of the peak depend on the heating rate employed. This peak is the normally used one for practical dosimetry. The magnesium ions presumed to form electron traps in combination with certain defect centers in the lattice. The influence of magnesium in the trapping process is unclear and its role is thought to be primarily in the formation of luminescence recombination centers. The divalent magnesium ion (Mg2 + ) is introduced into a lattice consisting of an array of monovalent lithium (Li) and boron (B) ions. The substitution of Li ion by Mg2 + ion results in an excess positive charge at the lattice site. Columbic attraction results in the formation of nearest-neighbor pairs (dipoles) consisting of a substitutional Mg ion in combination with a Li ion vacancy. Under certain thermal conditions, the dipoles are believed to aggregate forming dimers, trimers and higher-order complexes. There is evidence that the complex dipoles arrangement is associated with the electron trapping centers responsible for the high-temperature glow peaks. The aggregation of simple dipoles, which may be present to form complexes, is critically dependent on temperature, heating and cooling rates. Dipoles are electrically neutral in the lattice and the actual electron trapping centers may be similar to the modified F centers, such as a dipole in proximity to a fluorine ion vacancy [48]. This model for trapping is used to explain the changes in the relative heights of the glow peaks.
5. Conclusion
Christodoulides method, total width (I/Im ¼1/2) [39]
Table 3 Activation energy calculated by different methods (eV).
Halperin and Braner’s 1st. order method [38]
Chen’s 2nd order method [36]
Chen modified Lushchik’s 2nd order method [36]
Chen modified Lushchik’s 1st order method [36]
Lushchik’s 2nd order method [37]
ðEc Þd ¼ 0:706
2 kTM
Chen’s modified method [36]
0.73
Grossweiner’s method [35]
0.96
Urbach’s empirical method [34]
Sample
80Li2O– 20B2O3 70Li2O– 30 B2O3 60Li2O– 40B2O3 50Li2O– 50 B2O3 40Li2O– 60 B2O3 30Li2O– 70B2O3
M.M. Elkholy / Journal of Luminescence 130 (2010) 1880–1892
We study the optical and spectroscopic properties of Li2O–B2O3 glass system in a separate work [49]. TL glow curves of Li2O–B2O3 glass specimens doped with different concentrations of magnesium oxide are studied. The studied glasses exhibited almost single TL peak approximately at 165–205 1C. The TL intensity and the glow shape and heights are depending on Li2O content and MgO doping concentrations. Drastic reduction in the TL light output with increase in MgO concentrations above 10 3 wt% was observed. The intensity and the height of the main glow curve markedly increase with increase in Li2O content up to 50 mol% followed by a dramatic decrease with further increase of Li2O content. Therefore, the sample containing 50Li2O–50B2O3 has the highest TL response. Among all the prepared samples, the sample doped with MgO concentration of 10 3 wt% is the most thermoluminescent efficient one. Therefore, the concentration of 10 3 wt% MgO was used as an optimum concentration of Mg, as an activator to the prepared glass samples for dosimetry purposes. The glow peak position and shape dramatically and linearly shifted to higher temperature with increase in heating rate. The heating rate of 10 1C/s is the most suitable rate for obtaining high
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TL response and the highest glow peak besides the most suitable glow peak temperature, which is very good for dosimetry purposes. References [1] E.J. Friebele, P.C. Schattz, M.E. Gingerich, Appl. Opt. 19 (1980) 2910. [2] E.J. Friebele, in: Optical Properties of Glass, The American Ceramic Society, Weserville, Ohaio, 1991. [3] Y. Kirsh, J.E. Townsend, P.D. Townsend, Phys. Status Solidi 114 (1989) 739. [4] A.E. Ellis, P.D. Moskowitz, J.E. Townsend, P.D. Townsend, J. Phys. D 22 (1989) 1758. [5] M.R. Khanlary, P.D. Townsend, J. Phys. D 26 (1993) 379. [6] Y.A. Abdulla, Y.M. Amin, D.A. Bradley, Radiat. Phys. Chem. 61 (2001) 409. [7] Y.A. Abdulla, Y.M. Amin, D.A. Bradley, J. Fiz. Malaysia 22 (3 & 4) (2001) 49. [8] M. Rami Reddy, P. Yadagiri Reddy, N. Veeraiah, Ind. J. Phys. 74A (2000) 601. [9] M.V. Ramana, G.S. Sastry, Phys. Status Solidi A 116 (1989) K205. [10] N. Rabie, A.F. Abbas, A.A. Ali, Ind. J. Pure Appl. Phys. 37 (1999) 914. [11] Y. Fukuda, T. Okuno, N. Takeuchi, Rad. Prot. Dosimetry 6 (1984) 309. [12] G. Srinivasarao, N. Veeraiah, J. Alloys Compd. 327 (2001) 52. [13] W.M. Pontusuchka, L.S. Kanashiro, L.C. Courrol, Fiz. Khim. Stekla 27 (2001) 54. [14] S.M. DelNery, W.M. Pontuschka, C.G. Rouse, Phys. Rev. B 49 (1994) 3760. [15] W.W. Furtado, T. Tome, S. Isotani, A.R. Antonini, A.R. Blak, W.M. Pontuschka, S.R. Rabbani, Anais da Academia Brasileira de Ciencias 61 (1989) 397. [16] W.M. Pontusuchka, L.S. Kanashiro, L.C. Courrol, Fiz. Khim. Stekla 27 (2001) 54. [17] A. Van Die, A.C.H.I. Leenaers, W.F.V. Der Weg, J. Non-Cryst. Solids 99 (1988) 32. [18] A. Van Der Ziel, in: Solid State Physics Electronics, Prentice–Hall of India, New Delhi, 1971. [19] W. Soppe, C. Van Der Marel, H.W. Den Hartog, J. Non-Cryst. Solids 101 (1988) 101. [20] L. Cervinka, F. Rocca, P. Fornasini, G. Dalba, J. Non-Cryst. Solids 150 (1992) 140. [21] F.H. El Batal, A.A. El Kheshen, M.A. Azooz, S.M. Abo-Naf, Opt. Mater. 30 (2008) 881. [22] H.P. Lim, A. Karki, S. Feller, J.E. Kasper, G. Sumcad, J. Non- Cryst. Solids 91 (1987) 324. [23] S.A. Feller, W.J. Dell, P.J. Bray, J. Non-Cryst. Solids 51 (1982) 21. [24] M. Affatigato, S. Feller, E.J. Khaw, D. Feil, B. Teohand, O. Mathews, Phys. Chem. Glass. 31 (1990) 19.
[25] B.C.L. Chong, S.H. Choo, S. Feller, B. Teoh, O. Mathews, E.J. Khaw, D. Feil, K.H. Chong, M. Affatigato, D. Bain, K. Hazen, K. Farooqui, J. Non-Cryst. Solids 109 (1989) 105. [26] P.J. Bray, S.A. Feller, G.E. Jellison Jr.,, Y.H. Yun, J. Non-Cryst. Solids 38 (39) (1980) 93. [27] Bh.V. Janakivam-Rao, J. Am. Ceram. Soc. 49 (1966) 605. [28] F.R. Landsbergerad, P.J. Brag, J. Chem. Phys. 53 (1970) 2757. [29] A.A. Higazy, B. Bridge, J. Mater. Sci. 20 (1985) 2345. [30] M. Mangia, Oliveri, E., and Fiorella, O., Proceedings of the 5th International Conference on Luminescent Dosimetry, Sao Paulo, Brazil, Physikalisches Institut. Glessen, 1977, p.29. [31] Z. Spurny, J. Novotny, in: Proceedings of the 3rd International Conference ‘‘Luminescent dosimetry’’, Danish Atomic Energy Commission, Riso, Denmark, 1971 p.132. [32] J.R. Cameron, N. Suntharlingam, N.G. Kenney, in: Thermoluminescence Dosimetry, The University of Winconson Press, London, 1968. [33] M.M. Elkholy, Mater. Chem. Phys. 77 (2002) 321. [34] Urbach, Wiener Ber. IIa 139 (1953) 363. [35] L.I. Grossweiner, J. Appl. Phys. 24 (1953) 1306. [36] R. Chen, J. Electrochem. Soc. 116 (1969) 1254. [37] C.B. Lushchik, Sov. Phys. JETP 3 (1956) 390. [38] A. Halperin, A.A. Braner, Phys. Rev. 117 (1960) 408. [39] C. Christodoulides, J. Appl. Phys. D 18 (1985) 1501. [40] O. Majerus, L. Cormier, G. Calas, B. Beuneu, Phys. Rev. B 67 (2003) 024210. [41] J. Krogh-Moe, Acta Cryst. (1968) 179. [42] L. Cormier, P.H. Gaskell, G. Calas, J. Zhao, A.K. Soper, Phys. Rev. B 57 (1998) R8067. [43] J. Zhao, P.H. Gaskell, M.M. Cluckie, A.K. Soper, J. Non-Cryst. Solids 232–234 (1998) 721. [44] C.P.E. Varsamis, A. Vegiri, E.I. Kamitsos, Phys. Rev. B 65 (2002) 104203. [45] W.M. Pontuschka, L.S. Kanashiro, L.C. Courrol, Phys. Chem. Glass. 27 (2001) 37. [46] H.B. Pascoal, W.M. Pontuschka, H.R. Rechenberg, J. Non-Cryst. Solids 258 (1999) 92. [47] G.V. Rao, N. Veeraiah, P.Y. Reddy, Opt. Mater. 22 (2003) 295. [48] M.R. Mayhugh, Christy R.W. and Johnson N.M., in: J.A. Auxier et al. (Ed.), Proceedings of the 2nd International Conference in Luminescence Dosimetry, Gaimberg, USAEC and Oak Ridge National Laboratory, Tennessee, Conf.680920, 1968, p.294. [49] L.M. Sharaf El-Deen and M.M. Elkholy, Egypt. J. Solids (in press).
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Thermoluminescence of B2O3–Li2O glass system doped with MgO M.M. Elkholy Physics Department, Faculty of Science, Menoufia University, Shibin El-Koom 32511, Egypt
a r t i c l e in f o
a b s t r a c t
Article history: Received 6 October 2009 Received in revised form 28 April 2010 Accepted 3 May 2010 Available online 7 May 2010
Lithium borate (LiB) glasses in the system (100 x)B2O3–xLi2O with x¼ 20, 30, 40, 50, 60 and 70 mol% were prepared. The glasses were doped with different concentrations of the order of 10 1, 10 2, 10 3, 10 4 and 10 5 of MgO and their thermoluminescent (TL) response was investigated. The irradiations were performed using g rays from a 60Co source in the dose range from 0.1 to 25 kGy. The material displayed good sensitivity for g-rays and intensity of TL signals is dependent on g-ray dose and Li2O content. For each dose level and investigated temperature range (50–350 1C), exactly single isolated glow peak appears in the temperature range of 165–205 1C depending on both Li2O concentrations and time of exposure. The shape of the glow peak has altered significantly with increase in the gamma ray dose or Li2O concentrations. The glass composition with x ¼50 mol% doped with 10 3 mol% of MgO presented the best TL response. The results of the present study indicated that the recorded single and isolated high temperature peak is a good candidate for TL dosimetric investigations. This indicates that 50 B2O3–50Li2O-doped with 10 3 mol% of MgO is possibly used as materials for radiation dosimetry in the dose range of 0.1–20 kGy. & 2010 Elsevier B.V. All rights reserved.
Keywords: Thermoluminescence Lithium borate Glasses Dosimetry
1. Introduction The knowledge on radiation-induced defect centers in glasses has been a motivating focus of investigation in recent years, since such studies help in probing the stability of the glasses for radiation dosimetry applications. In radiotherapy and radiodiagnosis, in order to map doses in tissues, there exists a need for highly sensitive in vivo dosimetry systems of high spatial resolution, commercially available thermoluminescence dosimeters (TLDs) being limited to a capability of few millimeters. A number of investigators have examined the photon response of optical fibers at kilovoltage energies and below [1–5]. Recently to date, investigation has been made on commercially available germanium-doped optical fibers in terms of their response to photons in the therapeutic energy and dose range [6,7]. Encouraging results from such studies have paved the way for development of fibre radiation dosimeters specifically tailored to the task of dosimetry in the therapeutic and diagnostics dose regimes. Study on radiation induced defect centers in glasses has been an interesting subject of investigation in recent years, since such studies help in examining the suitability of the glasses for radiation dosimetry applications. Extensive studies on the activating or killing effect of luminescence due to some transition metal ions like Fe2 + , Cu2 + , Ti4 + , etc., in amorphous materials are
E-mail address: [email protected] 0022-2313/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2010.05.002
available in the literature [8–12]. In recent years, many authors have done a commendable work on luminescence mechanisms in borate glasses [13–16]. Thermoluminescence studies of borate glasses are of great interest because of their near tissue equivalent absorption coefficient, very low cost and easy handling process although their sensitivity and thermal stability vary widely and depend strongly on the preparation method. The hygroscopic nature and low temperature glow peak are the disadvantages of pure borate glasses, which may limit its applications. Alkali oxy borate glasses are considered as good materials for dosimetry applications since they are relatively moisture resistant when compared with the pure borate glasses. Addition of CaF2 into the glass matrix lowers the viscosity and decreases the liquidus temperature to a substantial extent and further it acts as an effective mineralizer, giving scope for the formation of a large concentration of color centers when the glasses are exposed to ionizing radiation. F ions in CaF2 act as co-activators and facilitate the substitution of activators into the lattice. Manganese ion is interesting because it exists in different valence states in different glass matrices, for example, as Mn3 + in borate glass with octahedral coordination whereas in silicate and germinate glasses it exists in Mn2 + state with both octahedral and tetrahedral coordination [17]. Further, among different manganese ions, Mn2 + and Mn4 + ions are identified as luminescence activators [18]. The content of manganese in different forms in different valence states that exist in the glass depends on the quantitative properties of modifiers and glass formers, the size of the ions in glass structure, their field strength, the mobility of the modifier
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cation, etc. Hence, the relation between the state, the position of the manganese ion and the luminescence properties of the glass is expected to be very interesting. Lithium borate (Li2B4O7) glasses are of great interest because of their good ionic conductance properties [19]. Since the addition of Li2O to the borate glass adds extra oxygen atoms, which gets accommodated in the network, a transfer of some boron atoms from triangle BO3 to tetrahedral BO4 occurs [20]. The introduction of transition metals ions (TMI) into an oxide glass change the oxide network. TMI were used as a constituent in several borate glasses in order to achieve useful physical properties. Though MgO is not a glass-forming oxide by itself, it can incorporate in substantial quantities into these glassforming oxide systems. Borate glasses are easily melted and are good hosts for transition metals ions and from a number of spectroscopic
Table 1 Starting and analyzed compositions of (100–x)B2O3–xLi2O glass system Glass notation
80Li2O–20B2O3 70Li2O–30B2O3 60Li2O–40B2O3 50Li2O–50B2O3 40Li2O–60B2O3 30Li2O–70B2O3
LiB-8020 LiB-7030 LiB-6040 LiB-5050 LiB-4060 LiB-3070
Starting composition
Analyzed composition
Li2O
B2O3
Li2O
B2O3
80 70 60 50 40 30
20 30 40 50 60 70
81.6 71.4 61.2 52.5 43.8 35.6
18.4 28.6 38.8 47.5 56.2 64.4
20
18
14000 xLi2O-(1-x)B2O3:0.001MgO Exposed to 10 kGy and heating rate 10° C/s
x=20 x=30 x=40 x=50 x=60 x=70
12000
10000
TL intensity (µC)
techniques, these properties of glasses have been established in terms of the stereochemical environment they provide and the oxidation number that they favour for the transition metal ion. The second advantage of borate glasses is that boron has the smallest mass compared to other network forming elements [21]. In lithium borate, nLi2O–B2O3 glass system, these physical properties as the function of the glass composition were extensively studied [22–26]. This glass system extends over a wide range of the glass composition; therefore, it is appropriate to study the validity and range of adaptation of each model in this glass system. Despite the doped lithium tetraborate studied in recent years, there is limited knowledge about the energy storage and luminescence mechanisms in these materials. In this study, thermoluminescence characteristics of lithium–borate glasses and their dependence on several MgO concentrations aimed at a deeper understanding of the thermoluminescence processes in doped glass. The present study was directed to study the usefulness of using the prepared glass as gamma ray dosimeter. For this purpose, the prepared glass samples are doped with low
Glow peak height (arb. units)
Glass formula
1881
8000
16
14
12
10
6000
8 0
10
20
30
40
50
60
70
80
Li2O content (mol.%) 4000
Fig. 2. The height of the main glow curve as a function of Li2O content.
2000
Table 2 Concentrations of activator and co-activator added to 50LiO2–50B2O5 glass sample.
0 0
50
100
150
200
250
300
350
400
Temperature (°C) Fig. 1. TL intensity as a function of temperature for (1–x)B2O3–xLi2O2 doped with 10 3 wt% of MgO and exposed to 10 kGy.
Glass formula
Glass notation
LiO2
B2O3
Mg
50Li2O–50 B2O3 50Li2O–50 B2O3 50Li2O–50 B2O3 50LiO–50 B2O3 50Li2O–50 B2O3
LiB-5050(1) LiB-5050(2) LiB-5050(3) LiB-5050(4) LiB-5050(5)
50 50 50 50 50
50 50 50 50 50
10 1 10 2 10 3 10 4 10 5
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concentrations of some MgO, which are very suitable centers for TLD purposes, to detect gamma rays of wide dose range.
thermoluminescence (TL) of dosed glass sample. It is one of a series of thermoluminescence analyzers developed by this company for personal, environmental, diagnostic and therapeutic radiation dosimetry as well as for research applications.
2. Experimental 2.1. Glass preparation
3. Results
For the present study, binary (100 x)B2O3–xLi2O glass system with different ratios of lithium oxide was prepared. For each glass, the thermal history (i.e. melting temperature, melting time, annealing temperature and time) and all preparation conditions are as similar as possible. The binary (100x)B2O3–xLi2O glasses have been synthesized using high purity analar grade chemicals: lithium carbonate (Li2CO3), manganese dioxide (MgO) and orthoboric acid (H3BO3). Mixtures of these materials in appropriate proportions are thoroughly mixed and melted in porcelain crucibles at about 1200 1C. The glasses suitable for thermoluminescence measurements are prepared by the melt quenching method. All the samples annealed below their transition temperature. Traces of transition metals responsible for thermoluminescence traps of the order of 10 1, 10 2, 10 3, 10 4 and 10 5 of MgO were added as 0.001 wt% for all samples as activator and co-activator. 2.2. Measurement of thermoluminescence A thermoluminescence analyzer supplied by Harshaw Chemical Company (USA), Harshaw TLD model 4500, measured
3.1. Determination of the final composition of (100 x)B2O3-xLi2O For the present study, binary (100 x)B2O3–xLi2O glasses with different concentrations of Li2O, where x¼20, 30, 40, 50, 60 and 70, were prepared. The resultant glass samples are cylindrical in form, transparent, and its transparency changes from sample to sample depending on Li2O concentration. All the prepared glass samples and their notations are tabulated in Table 1. During the glass preparation, we noted that at temperatures lower than the melting temperature of pure Li2O (Tm 1000 1C) some vapors were shown in the muffle zone. These vapors are essentially arising from B2O3 (Tm 580 1C). Therefore, substantial loss in B2O3 content has occured during the melting process; therefore, the composition of the final product may slightly differ from the starting composition. Because of these, the prepared glasses were chemically analyzed to determine the exact content of the glass components. The chemically analyzed composition data have shown that, a substantial loss of B2O3 content may have occurred during the melting processes (see Table 1). Several authors [27–29] have investigated phosphate glass using
16000
14000
50Li2O-50B2O3:MgO Exposed to 10 kGy
12000
0.50 kGy 0.75 kGy 1 kGy 2.5 kGy 5 kGy 7.5 kGy 10 kGy 15 kGy 20 kGy
50Li2O-50B2O3:MgO
10-3 wt.% MgO 10-2 wt.% MgO 10-4 wt.% MgO 10-1 wt.% MgO 10-5 wt.% MgO
14000
12000 10000
TL Intensity (µC)
TL intensity (µC)
10000 8000
6000
8000
6000
4000
4000
2000
2000
0 0
50
100
150
200
250
300
350
400
Temperature (°C) Fig. 3. The glow curves of samples: LiB5050(1), LiB5050(2), LiB5050(3), LiB5050(4) and LiB5050(5) exposed to 10 kGy of gamma irradiation.
0 0.000001
0.00001
0.0001
0.001
0.01
0.1
1
MgO wt.% Fig. 4. TL response as a function of MgO concentrations for glass sample LiB5050(3) after exposed to 10 kGy of gamma irradiation.
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1883
16000
12000 0.50 kGy 0.75 kGy 1 kGy 2.5 kGy 5 kGy 7.5 kGy 10 kGy 15 kGy 20 kGy
10000
0.50 kGy 0.75 kGy 1 kGy 2.5 kGy 5 kGy 7.5 kGy 10 kGy 15 kGy 20 kGy
50Li2O-50B2O3:0.00001MgO
14000
12000
50Li2O-50B2O3:0.001MgO
8000
TL intensity (µ µC)
TL Intensity (µC)
10000
6000
8000
6000 4000
4000 2000
2000
0 0
50
100
150
200
250
300
350
400
Temperature (° C)
0
12000
10000
50
100
150
200
250
300
350
400
Temperature (°C)
Fig. 5. Glow curves recorded at different doses of 0.5, 0.75, 1.0, 2.5, 5.0, 7.5, 10, 15 and 20 kGy for the 50B2O5–50 Li2O samples doped with 10 5 MgO.
0.50 kGy 0.75 kGy 1 kGy 2.5 kGy 5 kGy 7.5 kGy 10 kGy 15 kGy 20 kGy
0
50Li2O-50B2O3:0.0001MgO
Fig. 7. Glow curves recorded at different doses of 0.5, 0.75, 1.0, 2.5, 5.0, 7.5, 10, 15 and 20 kGy for the 50B2O5–50 Li2O samples doped with 10 3 MgO.
chemical analysis. It is found that, like phosphate glasses, when borate glasses were prepared by melting the constituents in air, loss in B2O3 oxygen and boron occurred. From chemical analysis results, as expected, loss of borate contents was observed (see Table, 1). In addition, the data of Table 1 have established that the range of glass formation of binary (100 x) B2O3-xLi2O glass system is continuous from x¼30 to 80, with no region of stable immiscibility existing.
TL Intensity (µ µC)
8000
3.2. Thermoluminescence for (1 x)B2O3–xLi2O 6000
4000
2000
0 0
50
100
150
200
250
300
350
400
Temperature (°C) Fig. 6. Glow curves recorded at different doses of 0.5, 0.75, 1.0, 2.5, 5.0, 7.5, 10, 15 and 20 kGy for the 50B2O5–50 Li2O samples doped with 10 4 MgO.
To measure the response to gamma radiation, we used TL technique. The TL analysis was performed using a Harshaw 4500 analyzer in a nitrogen atmosphere, with a linear heating rate of 10 1C/s from room temperature up to 350 1C. Traces of MgO with the order of 10 3 wt% were added to (1 x)B2O3–xLi2O to determine the sensitive sample to the present study. Calibration was carried out by irradiating standard dosimeters (such as commercial lithium fluoride: LiF,Mg, Ti (TL-100)), in the same conditions and geometry. Each experimental data point represents the mean of at least two measurements. Fig. 1 shows the TL intensity as a function of temperature for (1 x)B2O3–xLi2O doped with 10 3 wt% of MgO and exposed to 10 kGy. This figure shows two glow peaks: one of them is weak and positioned around 100 1C and the other main one is highly intense and lay around 225 1C, which is very suitable for dosimetry purposes. Also Fig. 1 shows that the intensity and the height of the main glow curve markedly increase with increase in Li2O content up to 50 mol% followed by a dramatic decrease with further increase in Li2O content (see Fig. 2).
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3.3. Determination of the optimum concentration of transition metal suitable as TL centers The glass sample LiB5050 of composition 50 mole% Li2O–50 mole% B2O3 was used as a base glass due to its good stability and its good TL response as indicated in Figs. 1 and 2. Different concentrations, 10 1, 10 2, 10 3, 10 4 and 10 5 wt% of MgO, were added to the above glass sample, LiB5050, to obtain the samples marked as LiB5050(1), LiB5050(2), LiB5050(3), LiB5050(4) and LiB5050(5), respectively (see Table 2). The prepared samples were exposed to intermediate dose 10 kGy of cobalt-60 gamma rays at room temperature. Fig. 3 shows the glow curves of samples LiB5050(1), LiB5050(2), LiB5050(3), LiB5050(4) and LiB5050(5) exposed to 10 kGy of gamma irradiation. This figure shows single isolated glow curves for all doped samples with different Mg concentrations. For LiB5050(2) and LiB5050(3), additional small glow peak around 100 1C was also detected. The positions of the recorded glow curves seem unchanged and independent of MgO concentrations. The total TL intensity that reflected from either the total integrated area under glow peaks or that recorded in microCoulombs in the reading system increases with increase in MgO concentrations up to 10 3 wt% followed by a sharp decrease with further increase in MgO concentrations up to 10 5 wt%. Therefore, we concluded that among all the prepared samples, the sample doped with MgO concentration of 10 3 wt% is the most thermoluminescence efficient one. Fig. 4 shows TL response as a function of MgO concentrations for glass sample LiB5050(3) after being exposed to 10 kGy of gamma irradiation. From this figure we noted that, the total thermoluminescence response hardly depends on MgO
concentrations. Also, from the figure, it can be easily seen that the TL response is increased with increase in MgO concentration up to 10 3 wt% in glass system followed by a strong decrease in the total TL response with MgO concentration up to 10 5 wt% in this glass system. So, the optimum concentration of Mg responsible for TL in this glass system is 10 3 wt%. Therefore, the concentration of 10 3 wt% MgO was used as an optimum concentration of Mg, as an activator for the rest of the prepared glass samples. 3.4. Thermoluminescence and thermoluminescence dosimetry for 50B2O5–50Li2O Figs. 5–9 show typical glow curves recorded at different doses of 0.5, 0.75, 1.0, 2.5, 5.0, 7.5, 10, 15 and 20 kGy for the prepared 50B2O5–50Li2O samples with different MgO concentrations (see Table 1). The curves were recorded using a heating rate of 10 1C/s. For each dose level and investigated temperature range (100–350 1C), exactly single isolated glow peak appears in the temperature range of 215–225 1C. The shape of the glow peak has not altered significantly with increase in the gamma ray dose. Two significant effects were observed, one of which is a very slight shift of the peaks towards higher temperatures at higher dose levels namely 15 and 20 kGy. The second one is a marked increase in the intensity of the peak height and area under glow curves with g-dose due to the total charge in microCoulombs (mC), collected and displayed on the TLD analysis. This charge represents the total integrated area under the full glow curves of the irradiated thermoluminescent sample. When this charge is plotted as a function of the gamma ray dose, the TL-response curves for the measured samples are given. From Figs. 5–9, the thermoluminescent 9000
14000
12000
10000
50Li2O-50B2O3:0.1MgO
0.50 kGy 0.75 kGy 1 kGy 2.5 kGy 5 kGy 7.5 kGy 10 kGy 15 kGy 20 kGy
50Li2O-50B2O3:0.01MgO
0.50 kGy 0.75 kGy 1 kGy 2.5 kGy 5 kGy 7.5 kGy 10 kGy 15 kGy 20 kGy
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7000
TL Intensity (mC)
TL Intensity (µC)
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0
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Gamma Dose (kGy) Fig. 8. Glow curves recorded at different doses of 0.5, 0.75, 1.0, 2.5, 5.0, 7.5, 10, 15 and 20 kGy for the 50B2O5–50 Li2O samples doped with 10 2 MgO.
0
50
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300
350
400
Temperature (°C) Fig. 9. Glow curves recorded at different doses of 0.5, 0.75, 1.0, 2.5, 5.0, 7.5, 10, 15 and 20 kGy for the 50B2O5–50Li2O samples doped with 10 1 MgO.
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glow peaks are detected around 215 1C for binary B2O5–Li2O glasses with different concentrations of MgO. Figs. 5–9 show also that the glow peak intensities are hardly dependent on the MgO concentration. The above observation coincides with the idea that the trapping is intrinsic to the host lattice, i.e. there is a cross link between the impurity and the trapping centers. From the recorded glow curves for the same glass sample, it is very clear from each figure that a slight shift is observed in the maximum temperature of the glow curve to higher temperature at higher g-radiation dose level. A similar behavior observed in a number of glow curves studied on CaSO4: Dy [30]. Mangia et al. [30] concluded that this behavior could be explained by assuming either a multilevel or a continuous distribution of trap depths, which was associated with the main thermoluminescent glow peak, with a single associated frequency factor. This picture of a Gaussian distribution of trap depths (observed in the present work) coincided with Spurny and Novothy’s [31] conclusion that the most probable environment for a trapped electron or hole is nonspherical symmetric cavity, which contains a distribution of activation energies.
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The total charge which is plotted as a function of the gamma ray dose, at a running end temperature in Figs. 10–14, shows that for all samples, the profile of the dose response curves does not show significant changes. In general, the curves reveal two characteristic ranges. At low gamma ray dose, below 10 kGy, the TL intensity increases linearly with increase in gamma dose. In the second range, the total TL response shows a supralinear relation with gamma dose. Above this range, the TL response tends to saturate. Also it is clear that the sample containing 10 3 wt% MgO is the most efficient thermoluminescent sample. This is clearer from the glow curves and from the TL response dose curves where the intensity of the TL response of this samples is much higher than that of other samples. Results of the attempts made to investigate the changes in the TL response and the height of the measured glow curve with irradiation dose shown in Figs. 7 and 12 for the sample containing 10 3 wt% MgO indicate that this sample is the most sensitive for irradiation. Apparently, the TL response and the peak height increase linearly with increase in the irradiation dose up to 10 kGy.
Fig. 10. TL response as a function of gamma dose for glass sample LiB5050(5) at different temperatures.
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Fig. 11. TL response as a function of gamma dose for glass sample LiB5050(4) at different temperatures.
Above this dose level, supralinearity exists and all curves with different doses show the same trend observed in the dose-response curves. Supralinearity is attributed to the following factors [32]: 1. the creation of additional trapping sites; 2. the creation of new recombination centers; and 3. an increase in thermoluminescent efficiency of the used material. The observed effects of gamma radiation in the present doped glasses can be discussed in terms of two main phenomena which operate in the same time in the material during irradiation. Gamma ray creates trapping centers, the number of which increases with increase in gamma doses. In addition, they also act as an exciting source for the material raising electrons from the valence band of the material, which later are trapped in the states created by defects produced either by radiation or by the impurities. When the trapped electrons are released by heating, it will recombine with holes left in the filled band to give the thermal glow curves.
The observed increase in the glow intensity with increase in the radiation dose up to approximately 10 kGy results from the increased number of excited electrons, leading to an increase in the recombination probability. The saturation of all glow peaks (noted at higher gamma dose) is caused by a new group of traps created by radiation at such dose levels. These new traps participate in the trapping process reducing the number of electrons originally available for other trap levels.
3.5. Effect of heating rate on the thermoluminescence response in 50B2O5–50Li2O:10 3MgO glass system The dependence of dosimeter sensitivity upon the heating rate used during its readout is a very important effect. The influence of the heating rates on experimental glow curve of the present 50B2O5–50Li2O:10 3MgO at a fixed dose of 10 kGy is shown in Fig. 15. The shape of the glow curve does not change as a function of the heating rate. From these figures, it can be easily seen that, the thermoluminescence intensity and glow peak position are
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Fig. 12. TL response as a function of gamma dose for glass sample LiB5050(3) at different temperatures.
hardly dependent on heating rate. The glow peak position and shape dramatically and linearly shifted to higher temperature with increase in heating rate (see Fig. 16). It is clear that the heating rate of 10 1C/s is the most suitable rate for obtaining high TL response and the highest glow peak besides the most suitable glow peak temperature, which is very good for dosimetry purposes. Fig. 17 shows a comparison between the TL intensity and some slandered TLD phosphors and the TL intensity of the present glass samples. From this figure, it can be noticed that the prepared new glass samples are good and promising radiation dosimetry samples. 3.6. Calculation of trap depth from glow curves 3.6.1. The methods used for determining trap depth from glow curves The most important parameter for TL to be determined is E the trap depth, which is the thermal energy required to liberate a trapped electron.
Several methods based on the analysis of the glow curves have been proposed and used for determining E. The full details of these methods are investigated in my previous work [33]. We can classify these methods into three main types: (1) Methods based on Tm, where Tm is the peak temperature at the maximum intensity. (2) Methods based on the shape of the glow curve. (3) Methods based on both Tm and the shape. 3.6.2. Numerical calculation of trap depth from glow curves The measured glow curves are analyzed to obtain the activation energy, E, for the trapped electrons present in lithium borate glass doped with 10 3 wt% of Mg. The resulting data for E calculated by these methods are tabulated in Table 3. The used methods were concerned with first and second kinetic orders. In the present work, we have used most of the above methods to estimate the activation energy values from the resulting glow peaks. Apparently, the methods used gave different values of
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Fig. 13. TL response as a function of gamma dose for glass sample LiB5050(2) at different temperatures.
activation energy, which show some scatter due to different basic techniques on which these methods were established.
4. Discussion The diborate composition was chosen since it contains the largest fraction of BO4 units in Li borate glasses, with Li ions occurring in a charge compensating position [40]. In Li-bearing crystalline phases, the mean Li–O distance in tetrahedral site is 1.98 A˚ [40]. In crystalline Li-diborate, Li occurs in a distorted fourfold site, with a mean Li–O distance of 2.05 A˚ [41]. Therefore, the mean Li–O distance of 1.99 A˚ is consistent with an average tetrahedral site in Li-diborate glass. According to the very large ˚ average Debye–Waller factor (0.115 A), this site is highly ˚ distorted with two broad components at 1.97 and 2.03 A. Similarly, Li atoms occur in distorted tetrahedral sites in Lialuminosilicate [42] and Li-disilicate glasses [43], with mean Li–O ˚ respectively. The Li environment distances of 2.10 and 2.02 A,
obtained by molecular dynamics (MD) simulations on Li borate glasses consists of various distorted sites with 4–8 oxygen neighbors [44]. The charge compensating positions were identified with the largest coordinated sites [42,43] having a mean Li–O ˚ However, these data suggest that the Li sites distance of 2.12 A. related to modifying and charge-compensating positions are not so well distinct than indicated by MD, and Li may occur in a broad site distribution rather than in two types of sites in borate glasses. In the present glass samples, the Li ions have a closed structure, do not have energy levels within 10 eV of the ground state and hence these ions do not participate directly in the luminescence but may act as activator ions. In the activation process, the doping of glass by doubly charged alkaline earth Mg2 + causes a charge imbalance. Since the alkaline earth ions are doubly charged, the energy levels of the surrounding oxygen ions will be slightly lifted up in comparison with the normal ions and give rise to occupied energy levels close to the top of the valence band and these levels form the ground state of the luminescence. The lifting of these energy levels occurs to a lesser extent in
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Fig. 14. TL response as a function of gamma dose for glass sample LiB5050(1) at different temperatures.
˚ glasses containing Mg ions, since the ionic radius of Mg2 + (0.74 A) ˚ Sr2 + is small when compared to the ionic radii of Ca2 + (1.04 A), ˚ and Ba2 + (1.38 A) ˚ ions. This indicates the presence of (1.20 A) deeper traps and color centers in the case of MgO, which give rise to larger TL light output in the high temperature region as observed. Thermoluminescence is a consequence of radiative recombination between the electrons (released by heating from the electron center) and an antibonding molecular orbital of the nearest of the oxygen hole centers. However, the TL emission due to such recombination process is generally possible only at low temperatures in borate glasses [45]. Alternatively, the TL emission in these glasses may be explained as follows. During the heating process, the electrons captured by metal ions are liberated and later trapped by holes in the recombination centers giving out TL light output. The first small glow peak may be attributed to be due to such a process. The action of g-ray irradiation on glasses is to produce secondary electrons from the sites where they are in a stable state and have an excess energy. Such electrons may
traverse in the glass network depending upon their energy and the compositions of the glasses finally trapped, thus forming color centers. The trapping sites may be the metal cations that constitute the glass structure, ions of admixtures to the main composition and/or the structural defects due to impurities in the glass. Thus this process leads to the formation of (1) boron electron centers, (2) non-bridging oxygen hole centers and (3) boron oxygen hole centers [14,45]. In the absence of Mg ion in the glass network, each electron released by heating from electron centers would be trapped by an antibonding molecular orbital of the nearest of the oxygen hole center giving out maximum TL light output. During the heating process, the electrons from such centers are liberated and later trapped by holes in the recombination centers giving out TL light output. The response of the glasses to gamma rays irradiation is related to the rate of formation and accumulation of induced defects during progressive irradiation and hence the production of characteristic color center. According to Friebele [2], the color
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16000 1 oC/s 2.5 oC/s 5 oC/s
50Li2O-50B2O3:0.001MgO Exposed to 10 kGy
7.5 oC/s
14000
10 oC/s 12.5 oC/s 15 oC/s 17.5 oC/s
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20 oC/s
TL intensity (µ µC)
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Temperature (°C)
Fig. 16. The influence of the heating rates on the glow peak maximum temperature for 50B2O3–50Li2O:10 3MgO at a fixed dose of 10 kGy.
Fig. 15. The influence of the heating rates on glow curve of 50B2O3–50Li2O:10 3 MgO at a fixed dose of 10 kGy.
8 TLD-100 * LBO:Cu,Ag,P * present paper 50Li2O-50B2O3:0.001MgO
7
6
TL intensity (arb. units)
centers may include boron E center, nonbridging oxygen holes centers, this can be defined as HC, hole trapped on a single nonbridging oxygen,HC2 hole trapped on two nonbridging oxygen hole centers BOHC1 and BOHC2 and alkaline earth defect center (i.e. Mg in this work). There are few reports in the literature with TL emission in borate glasses. Pontuschka and co-workers [14,45,46] had shown with EPR results that in barium or calcium aluminoborate glasses, intrinsic electron trap centers connected to boron (BEC) act only at temperatures below 300 K, but a large concentration of hole centers, due to boron and oxygen (BOHC), is available to recombine with free electrons at higher temperatures. Also, Pascoal et al. [46] verified that in the calcium aluminoborate glasses, the increase of Ca content above 20 mol% of CaO decreases the TL emission at 470 K, because the presence of this divalent ion decreases the BOHC concentration, due to the increase of NBO (non-bridging oxygen) in the matrix. Rao et al. [47] reports also the TL response of borate glasses (mixed with CaF2 and alkali metal oxides): glow curves of undoped samples show complex peaks in the range of 180–205 1C, with activation energies in the range 0.29–0.39 eV. Addition of nickel shifts the peaks to higher temperatures, with higher activation energies (0.31–0.53 eV depending on the metal oxide). The authors attribute to Ca ions the role of electron trapping centers in those borate glasses, showing also that other impurities as Fe, Mn and Ni kill the TL emission [46,47]. In fact, due to the presence of F ions and their associated charge compensation in the glass, the calcium ions occupy the alkali ion positions in the matrix, diversely of what happens in the samples used in Pascoal’s paper [46]. According to these arguments, we can suggest that, in the
5
4
3
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1
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350
Temperature (°C) * Data taken from: B.T. Huy , V.X. Quang, H.T.B. Chau, Journal of Luminescence 128 (2008) 1601–1605 Fig. 17. Comparison between the TL intensity and some slandered TLD phosphors and the TL intensity of the present glass samples.
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2kTM
1.06 0.6 0.56
1.02 0.77 0.72
1 0.77 0.72
1 0.62 0.58
1.01 0.59 0.55
0.68
0.56 1.13 0.55 0.62 0.88
0.96
0.47 0.95 0.46 0.65 0.91
0.81
0.45 0.9 0.44 0.63 0.88
0.77
0.5 0.99 0.49 0.6 0.84
0.85
0.48 0.97 0.47 0.58 0.82
0.83
0.54 0.76
0.62
0.35
0.73
0.36
o
Lushchik’s 1st order method [37]
2 kTM
ðEc Þo ¼ 3:54
d
Chen’s 1st order method [36]
0.56 0.62 0.54 0.54
0.54
0.71 0.78 0.45 0.59
0.72
0.71 0.77 0.43 0.57
0.71
0.58 0.64 0.48 0.53
0.57
0.56 0.61 0.47 0.51
0.54
0.67 0.74 0.34 0.5
0.68
Halperin and Braner’s 2nd order method [38] Christodoulides method, second half width (I/Im ¼1/2) [39]
Christodoulides method, first half width (I/Im ¼1/2) [39]
1891
calcium matrix the intrinsic hole centers (BOHC) are less abundant giving rise to a less intense TL signal, as observed also by Pascoal et al. [46]. Also in matrices studied here, Mg probably acts as an electron-trapping center, and the intrinsic defects (BOHC) are the recombination centers for the TL emission. Lithium ions and magnesium, acting as co-dopants, definitely enhance the thermoluminescence of all the investigated TL materials, when compared with that of the related phosphors without lithium co-dopant. Lithium borate glasses doped with magnesium is given a preirradiation anneal at 350 1C (standard phosphor anneal of 1 h) and cooled quickly to normal ambient temperature; the resulting glow curve after irradiation contains single isolated glow peak between normal ambient temperatures and 350 1C. By convention, this is named the main characteristic peak. The precise readout temperature and the resolution of the peak depend on the heating rate employed. This peak is the normally used one for practical dosimetry. The magnesium ions presumed to form electron traps in combination with certain defect centers in the lattice. The influence of magnesium in the trapping process is unclear and its role is thought to be primarily in the formation of luminescence recombination centers. The divalent magnesium ion (Mg2 + ) is introduced into a lattice consisting of an array of monovalent lithium (Li) and boron (B) ions. The substitution of Li ion by Mg2 + ion results in an excess positive charge at the lattice site. Columbic attraction results in the formation of nearest-neighbor pairs (dipoles) consisting of a substitutional Mg ion in combination with a Li ion vacancy. Under certain thermal conditions, the dipoles are believed to aggregate forming dimers, trimers and higher-order complexes. There is evidence that the complex dipoles arrangement is associated with the electron trapping centers responsible for the high-temperature glow peaks. The aggregation of simple dipoles, which may be present to form complexes, is critically dependent on temperature, heating and cooling rates. Dipoles are electrically neutral in the lattice and the actual electron trapping centers may be similar to the modified F centers, such as a dipole in proximity to a fluorine ion vacancy [48]. This model for trapping is used to explain the changes in the relative heights of the glow peaks.
5. Conclusion
Christodoulides method, total width (I/Im ¼1/2) [39]
Table 3 Activation energy calculated by different methods (eV).
Halperin and Braner’s 1st. order method [38]
Chen’s 2nd order method [36]
Chen modified Lushchik’s 2nd order method [36]
Chen modified Lushchik’s 1st order method [36]
Lushchik’s 2nd order method [37]
ðEc Þd ¼ 0:706
2 kTM
Chen’s modified method [36]
0.73
Grossweiner’s method [35]
0.96
Urbach’s empirical method [34]
Sample
80Li2O– 20B2O3 70Li2O– 30 B2O3 60Li2O– 40B2O3 50Li2O– 50 B2O3 40Li2O– 60 B2O3 30Li2O– 70B2O3
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We study the optical and spectroscopic properties of Li2O–B2O3 glass system in a separate work [49]. TL glow curves of Li2O–B2O3 glass specimens doped with different concentrations of magnesium oxide are studied. The studied glasses exhibited almost single TL peak approximately at 165–205 1C. The TL intensity and the glow shape and heights are depending on Li2O content and MgO doping concentrations. Drastic reduction in the TL light output with increase in MgO concentrations above 10 3 wt% was observed. The intensity and the height of the main glow curve markedly increase with increase in Li2O content up to 50 mol% followed by a dramatic decrease with further increase of Li2O content. Therefore, the sample containing 50Li2O–50B2O3 has the highest TL response. Among all the prepared samples, the sample doped with MgO concentration of 10 3 wt% is the most thermoluminescent efficient one. Therefore, the concentration of 10 3 wt% MgO was used as an optimum concentration of Mg, as an activator to the prepared glass samples for dosimetry purposes. The glow peak position and shape dramatically and linearly shifted to higher temperature with increase in heating rate. The heating rate of 10 1C/s is the most suitable rate for obtaining high
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