The thermoluminescence properties and determination of trapping parameters of soda lime glass doped with erbium oxide

Journal of Luminescence 197 (2018) 304–309

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The thermoluminescence properties and determination of trapping parameters of soda lime glass doped with erbium oxide R. Laopaiboon, T. Thumsa-ard, C. Bootjomchai

T

⁎

Glass Technology Excellent Center (GTEC), Department of Physics, Faculty of Science, Ubon Ratchathani University, Ubon Ratchathani 34190, Thailand

A R T I C L E I N F O

A B S T R A C T

Keywords: Soda lime glass Thermoluminescence Physical properties Dosimeters Trapping parameters

The thermoluminescence properties and trapping parameters of soda lime glass doped with erbium oxide (Er2O3) were investigated. The glow curve and linearity evaluations of the glass samples were carried out by irradiation with 100 kV of X-ray tube photon energy at a dose of 14 mGy. The results for the glow curve and linearity show that the S-2 glass sample is more suitable for using as a dosimeter. The lower detection limit (DLDL) of the S-2 glass samples was studied. Reproducibility of the S-2 glass samples was investigated by calculation of the detector variability index (DVI). Trapping parameters (activation energy and frequency factor) supported the results for the glow curve and fading of the glass samples. Physical properties of the glass samples were calculated. The results could explain the behavior of the modifier atoms when these were added into the network structures.

1. Introduction

oxides of some rare earth metals were expected to improve the stability of trapping electrons and the thermoluminescence behavior of materials [8–13]. Trivalent rare earth ions (RE3+); such as, Dy3+, Nd3+, Sm3+, etc., played a direct role in trapping electrons and caused an increase in the TL signal [14–17]. Moreover, the trivalent rare earth ions had a stability for storing electrons in the trap resulting in low fading of the TL signals [15,18]. The Er3+ ion is one of the rare earths, which can act as a very important dopant due to its optical transition in the range of being near-infrared. In addition, the trivalent oxide of erbium has highly suitable active ions for several hosts which apply f-f transition states [19]. Moreover, the structure of the host glass doped with Er3+ was investigated. The results revealed that the effect of Er3+ on the structural properties decreased the connectivity of the network glass for low Er2O3 concentrations while an increase in the Er2O3 concentration slightly increased the elastic moduli [20,21]. These findings indicate that the structure of the host glass is highly suitable with increased concentrations of Er2O3. For this research study, the soda lime glasses from recycled window glass doped with Er2O3 was prepared. The new glass materials were investigated to assess their thermoluminescence properties. The statistical analysis to represent the reproducibility was carried out by using the coefficients of variation analysis method. Trapping parameters and physical properties of the glass samples were studied and discussed to better understand the interior of the glass structures.

Nuclear energy and radioactive sources are used widely for various applications; such as, agriculture, food preservation, radiotherapy, and many other industries. Therefore, the management of radiation protection is extremely important. Personnel monitoring is also important for workers associated with radiation. Thermoluminescence dosimeters (TLD) are one of the most highly efficient means of radiation dosimetry. New materials are now being implemented which allow more sensitivity and linearity in the radiation response of the TL signal output over a wide range of radiation doses. In the case of radiation accidents, retrospective dosimeters play a key role in measuring the amount of radiation at the beginning of the accident. The materials used for retrospective dosimeters should be common materials and personally portable. Watch glass and tooth enamel have successfully been used to assess the doses absorbed by people involved in major radiation accidents, such as the Chernobyl disaster [1–4]. Glass materials have been engaged due to valuable properties; such as, easy handling, chemical inertness, and excellent transparency. Moreover, a dosimeter made from glass possesses several advantages over other dosimeters, not least of which is its effective atomic number (Zeff), which is comparable to that of human biological tissue [5–7]. Recently, commercial window glass was investigated to assess its potential for use as a radiation dosimeter. This is interesting since window glass is a common material. The results found that it has the potential to be developed for dosimetry [7]. However, its capacity in terms of signal storage and reproducibility required improvement. The

⁎

Correspondence to: Glass Technology Excellent Center (GTEC), Department of Physics, Ubon Ratchathani University, Ubon Ratchathani, Thailand. E-mail addresses: [email protected], [email protected] (C. Bootjomchai).

https://doi.org/10.1016/j.jlumin.2018.01.039 Received 8 March 2017; Received in revised form 15 January 2018; Accepted 19 January 2018 Available online 31 January 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.

Journal of Luminescence 197 (2018) 304–309

R. Laopaiboon et al.

W1 ⎞ ρL ρ=⎛ ⎝ W1 − W2 ⎠

[90RWG − 10 Na O] + xEr O

0.001 mol.%

0.100 mol.%

0.010 mol.%

⎜

1.000 mol.%

(g⋅cm−3)

⎟

(1)

where W1 and W2 are the weights of the glass samples in air and in immersion liquid respectively, and ρL is the density of the immersion liquid. The immersion liquid in this study was n-hexane. All of the samples were measured three times at room temperature. The estimated error in these measurements was ± 0.002 g cm−3 . Molar volume (Va ) was evaluated using the equation:

Va =

2. Experimental details The (90)RWG – (10)Na2O – (x)Er2O3 glass system was prepared using a melt quenching technique, where the RWG is recycled window glass (soda lime glass) and x is 0.001, 0.01, 0.1 and 1 mol%. The RWG was prepared by cleaning and grinding into a powder. The analytical reagent grades of Na2O and Er2O3 were used in this research. The chemical oxides (Na2O and Er2O3) and RWG powder were weighed using an electric balance with accuracy in the order of 0.1 mg. The appropriate amounts of chemical were mixed thoroughly in ceramic crucibles. The mixtures were melted in an electric furnace at 1250 °C for about four hours to ensure homogeneity. The homogenous melted glasses were poured into preheated stainless steel molds and then immediately inserted into separate electric furnaces for annealing at 500 °C for two hours before being allowed to cool naturally to room temperature. The obtained glass samples were cut and polished in order to maximize flatness. The nominal compositions and physical observations of the prepared glass samples are given in Fig. 1 and Table 1. For the thermoluminescence measurements, each sample was annealed using a dual step technique (400 °C for one hour and then 100 °C for two hours) before being irradiated with 100 kV of X-ray tube photon energy with a dose range of 0 – 14 mGy to carry out the thermoluminescence response and glow curve assessment. The X-ray machine band KELEX was used with the X-ray tube model MD1100. The X-ray tube had an inherent filtration by oil-insulation with a vacuum system and added filtration by lead-shielded housing to reduce to low photon energy. TL light emitted from the glass samples was detected by a TLD Reader of the brand Harshaw/Bicron Model 3500 Manual. The glow curves were recorded from 60 °C up to a maximum temperature of 300 °C with a heating rate of 10 °C/s. The region of interest facility available in the TLD reader was used to evaluate the responses of different glow peaks resulting from the Computerized Glow Curve Deconvolution (CGCD) procedure. The CGCD data were gathered by taking notes from the MS-DOS files to the Excel program. Each datum point was obtained from an average of five measurements. Archimedes’ principle was used to determine the variation of the density of the glass samples. The density could be calculated using the equation:

N=

RWG

Na2O

Er2O3

S−1 S−2 S−3

90 90 90

10 10 10

0.001 0.01 0.1

S−4

90

10

1

(cm3 mol−1)

(2)

(mol% of RE doped)(ρ)(NA) Mglass

(ions cm−3)

(3)

where NA is Avogadro's number. After the ion concentration was determined, three other related physical properties were evaluated using the equations [22,23]:

Polaron radius:rP =

1 π 1/3 ⎛ ⎞ 2 ⎝ 6N ⎠

(Å)

1 1/3 Interatomic distance:ri = ⎛ ⎞ ⎝N⎠ z Field strength:F = ⎜⎛ 2 ⎟⎞ ⎝ rP ⎠

(4)

(Å)

(5)

(cm−2)

(6)

where z is the valence number of the rare earth ion. 3. Results and discussions The glass samples were doped with Er2O3 from 0.001 mol% to 1 mol % along with physical observations as shown in Fig. 1 and Table 1. The morphology of all glass samples showed good homogeneity. The color of the glass samples was colorless doped with Er2O3 at 0.001–0.01 mol % and became pink when increasing the concentration of Er2O3 from 0.1 to 1 mol%. The glass samples were irradiated with 100 kV of X-ray tube photon energy at doses of 14 mGy. The glow curves of the measurements are shown in Fig. 2. The glass samples showed similar main glow curves for all concentrations at around 180 °C and 270 °C, respectively. From Fig. 2, it is clear that the Er2O3 caused significant changes in the TL intensity by the electron trap created by the Er3+ ion. The important role played by the Er3+ ion in the TL emission is in the trap filling process that may arise through the direct transfer of 0.40

Relative light intensity (AU)

0.35

Table 1 Chemical compositions and physical observations of the glass samples. Chemical compositions (mol%)

ρ

where Mglass is the molecular weight of the glass samples. The error in the molar volume was determined by repeating the density measurements three times and was equal to ± 0.102 cm3 mol−1. The physical properties of the glass samples were investigated. The ion concentration (N) could be calculated using the equation [22]:

Fig. 1. Morphology of the glass samples with different concentrations of Er2O3.

Glass samples

Mglass

Physical observations

S-2

0.30 0.25 S-1

0.20 0.15

S-3

0.10

S-4

0.05

Colorless and homogeneous Colorless and homogeneous Light pink and homogeneous Pink and homogeneous

0.00

50

100

150 200 Temperature (°C)

250

300

Fig. 2. Glow curve of the glass samples irradiated with X-ray tube photon energy of 100 kV at a dose of 14 mGy.

305

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R. Laopaiboon et al. 28

25

24

y = 0.9871 (x) (S-1) R² = 0.9102

15

10

20

y = 0.6207 (x) (S-3) R² = 0.8577

16 12 8

5 y = 0.5004 (x) (S-4) R² = 0.6826

0 0

2

4

6

8 Dose(mGy)

10

4

12

14

16

0

4th Reading

5th Reading

20.58 21.14 20.32 20.54 20.62 20.95 21.25 21.14 20.65 20.57

21.65 21.58 20.32 22.35 20.15 20.69 21.47 20.33 22.14 21.36

21.05 20.54 21.34 20.15 22.31 22.58 21.36 21.89 20.96 20.67

20.49 20.57 22.34 21.55 23.02 20.95 21.54 20.68 22.58 22.74

20.17 20.58 21.52 22.68 20.54 20.94 21.54 20.63 21.45 20.44

99.33

99

98.04

98 97 96

95

average

1

3

2

5

4

1

2

3

4

5

6

The system variability index (SVI): or %CV =

SD × 100 X

(7)

The reader variability index (RVI): or %CV =

SD × 100 mean

(8)

The detector variability index (DVI): DVI =

(SVI)2 − (RVI)2

(9)

The SVI is the mean value of the percentage standard deviation of each TL detector; this amount gives a measure of the reproducibility of the whole system. From Table 3, the variation of SVI is 3.5%. The RVI is the percentage standard deviation of the mean values of each cycle of readings. This quantity gives a measure of the long-term reader reproducibility and exact values are shown in Table 4. From Table 4, the variation of RVI is 1.5%. Finally, the DVI is the quantity which gives a measure of the reproducibility of the TL detectors (S-2 glass sample). From Eq. (9), the DVI was calculated and the obtained value was less than 3.2%. The value of the DVI indicated the high reproducibility of Table 3 Calculation of system variability index (SVI). The average values of the five reading calculated from Table 2.

Reproducibility 3rd Reading

100.06 100.44

100

the average values are represented in Fig. 4. From the values shown in Fig. 4 this indicated that the S-2 glass sample showed a high performance of reproducibility for use in the X-ray dosimeters. The TL signal was normalized to the average value (100%) for each cycle of measurements. The maximum variation range of the TL signal (y-range) was less than 2.14% from the average value. However, statistical analysis was required to confirm the efficiency of the variability of the dosimeters. The coefficient of variation analysis method was used. Covariance (CV) is a measure of the extent to which two random variables change together, and can be defined as follows [28]:

Chip No.

2nd Reading

101

Fig. 4. Reproducibility of the S − 2 glass samples with 14 mGy for five cycles (dash line is the average of the initial read out). Error bars correspond to one standard deviation and subscript with the normalized TL signal to the average value (100%) for each cycle of measurements.

Table 2 Data for reproducibility of TLD measurement of the S − 2 glass samples.

1st Reading

102.14

102

Cycle of measurements

electrons from the excited state of Er3+ to the trap centers [24–26]. However, the greater concentration of Er2O3 resulted in the breakdown of the network structures and led to a decrease in the TL intensity. The total integrated area under the glow peaks showed that the highest TL intensity was at 0.01 mol% of Er2O3 as shown in Fig. 2. The radiation dose response of the glass samples was important for investigation. The radiation dose ranges 0–14 mGy were utilized to study the linearity of the glass samples response to the radiation dose. For the application of the dosimeters, linearity is a very important property since this defines a linear relationship between the TL signal and the dose of radiation. The general behavior of the TL emission is linear at low irradiation dose values but then becomes supralinear and finally saturated at high values of irradiation doses [27]. The obtained results for linearity are shown in Fig. 3. For the corresponding calibration line of the best fit along with the expression of the linear function, the correlation coefficients (R2) were calculated to confirm the linearity of the response in the investigated dose range. For all measures, the background signals specific for each kind of doped samples were subtracted to make all TL signals intercept at the origin point. The obtained results indicated that the glass samples doped with Er2O3 at 0.01 mol% (S-2 glass sample) give the best linearity. The S-2 glass sample was selected to measure a lower detection limit (DLDL ) by using the values of the standard deviations of ten zero doses of the sample. The DLDL could be carried out by using the equation: DLDL = (3σBKG) ΦC , where σBKG was the standard deviation (S.D.) of the unirradiated samples. The value ΦC was the calibration factor for the TL reader and was given by the ratio of the calibration dose divided by the average value of the net TL reading. The value of DLDL obtained was 0.62 mGy. In the investigation of the reproducibility of the S-2 glass sample, ten samples were selected for annealing by using a dual step technique, and then irradiated with X-ray photon energy at a dosage of 14 mGy, and the TL signal was read out. This experiment was repeated for five cycles. The obtained results of these processes are shown in Table 2 and

Chip No.

103

0

Fig. 3. Linear relationships between the TL response and irradiation dose of the glass samples. Error bars correspond to one standard deviation.

1 2 3 4 5 6 7 8 9 10

Normalized to the average value

y = 1.4325 (x) (S-2) R² = 0.9729

TL reading (AU)

Integral TL signal(AU)

20

1 2 3 4 5 6 7 8 9 10 CV SVI = CV × 100

306

Average values of the five reading: X

Standard deviation: SD

20.788 20.882 21.168 21.454 21.328 21.222 21.432 20.934 21.556 21.156

0.576 0.463 0.861 1.101 1.259 0.767 0.126 0.608 0.803 0.954

Covariance:

CV = 0.028 0.022 0.041 0.051 0.059 0.036 0.006 0.029 0.037 0.045 0.035 3.5%

SD X

Journal of Luminescence 197 (2018) 304–309

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Table 4 Calculation of reader variability index (RVI). The values of each reading with the average of the TL signal of all Chip No. in Table 2. 1st Reading

2nd Reading

3rd Reading

4th Reading

5th Reading

Mean

SD

RVI =

20.776

21.204

21.285

21.646

21.049

21.192

0.320

1.5%

SD Mean

× 100

120

Percentage TL intensity (%)

100 80 60 40 20 0 0

100

200

300

400

500

600

700

800

Time (hours)

Fig. 5. Fading of the irradiated S − 2 glass samples at room temperature. Error bars correspond to one standard deviation.

Fig. 6. Comparison mass energy-absorption coefficients between the S − 2 glass samples and soft tissue (ICRU-44).

the S-2 glass sample. The fading or trapped charge effects and their evaluation could be assessed through the loss of the stored TL signal after irradiation, which was the most important problem in the application of the TL techniques; such as, personal pocket meters and environmental and clinical dosimetry [29]. Fig. 5 shows the average integrated TL intensity after a storage period of 720 h at a dose of 14 mGy. The rapid reduction of the TL signal was about 15% in the range of 0 – 5 h for the S-2 glass samples. The TL signal then faded more slowly as time passed and took around 168–720 h to lose 45%. This suggests that the low loss of the TL signals of the S-2 glass samples results from the higher stabilization of the trapped charge effects [18], and this would help in estimating the doses in case the sample could not be read on time for any particular reason [30]. The interaction of radiation with matter was also considered to compare the possibility of using biological tissue and inorganic materials in radiation dose measurements and in particular in accident dosimetry. To verify that the S-2 glass sample was used as an agent of tissue equivalence, the mass energy absorption coefficients were calculated by comparing the S-2 glass samples and soft tissue. The mass attenuation coefficient of the S-2 glass samples was obtained by calculation using the WinXCom program. The mass energy absorption coefficients of the soft tissue were obtained from ICRU-44 [31]. The obtained results showed a fair agreement of the trend lines of the mass energy absorption coefficients between the S-2 samples and soft tissue as illustrated in Fig. 6. However, Fig. 6 shows that the S-2 glass sample is not tissue-equivalent because the mass energy absorption coefficients are significantly larger in the energy range considered (X-ray tube photons produced at 100 kV). The trapping parameters of the glass samples were investigated and are shown in Table 5. Chen [32] reported the general expressions for appraising activation energy and trap depth. Chen's method does not require knowledge of the order of kinetics, which is found by using the symmetry factor (μ = T2 − TM/T2 − T1) from the peak shape. The equations can be summarized as follows:

KT2 Eα = cα ⎜⎛ M ⎟⎞ − bα (2KTM) ⎝ α ⎠

The values of cα and bα are summarized in the equations [32]:

c τ = 1.510 + 3.0(μ − 0.42) and b τ = 1.58 + 4.2(μ − 0.42)

(11)

cδ = 0.976 + 7.3(μ − 0.42) and bδ = 0

(12)

The pre-exponential frequency factor or attempt-to-escape frequency is a constant characteristic of the electron trap. This variation is well known as the frequency factor (S). The frequency factor is proportional to the regularity of the collisions of the electron with the lattice phonons and can be calculated using the expression:

βE E ⎞⎡ 2K T = (S) exp ⎛− 1 + (b − 1) ⎛ B M ⎞ ⎤ KBT2M ⎝ E ⎠⎦ ⎝ KBTM ⎠ ⎣ ⎜

⎟

(13)

where β is the heating rate of the read out. The activation energy directly depends on the ability of trapping electrons and/or the release of modifier ions added into the glass structures [18]. From Table 5, the activation energies (E τ and Eδ ) and frequency factor (S) showed a maximum value (peak 2) in the S-2 glass samples. These results support the glow curve of the glass samples as shown in Fig. 2, as the dominant peak 2. The highest of the variations of the frequency factor supported the highest TL signal of the S-2 glass sample due to the high number of electrons trapped and released. The highest of the variations of the activation energies supported the low fading TL signal of the S-2 glass sample due to higher activation energy leading to a lower loss of electrons from the trap by thermal energy at room temperature. The physical properties included the density, molar volume, ion concentration, Polaron radius, interatomic distance, and field strength of the glass samples. These items were calculated and shown in Table 6. The density of the glass samples increased from 2.580 g cm−3 to 2.875 g cm−3 with an increase in the concentration of Er2O3. This was due to the fact that adding Er2O3 with a high molecular mass (382.717 g mol−1) into the network structure would lead to increases of density in the glass system. The molar volume of the glass samples directly decreased with an increase in the concentrations of Er2O3. These results can be explained by the function of the ionic radius of the network structures and modifiers. The tetrahedral structures of SiO2 acted as the network structures. The effective ionic radii of Si4 + and O2 − were 0.40 and 1.40 Å, respectively. The effective ionic radius of Er 3 + was 89 Å. The ionic radius of the modifier could be inserted into the

(10)

where K is the Boltzmann constant, TM is the glow peak temperature, α is τ or δ (where τ = TM − T1 and δ = T2 − TM ), T1 (rising end) and T2 (falling end) are the temperatures at the half-widths of the glow peak. 307

Journal of Luminescence 197 (2018) 304–309

R. Laopaiboon et al.

Table 5 Data on various trap depth parameters of the glass samples. Glass samples

S S S S

− − − −

1 2 3 4

τ (°C )

Tm (°C )

μ

δ (°C )

S × 108 (s−1)

Eδ (eV )

Peak 1

Peak 2

Peak 1

Peak 2

Peak 1

Peak 2

Peak 1

Peak 2

Peak 1

Peak 2

Peak 1

Peak 2

Peak 1

Peak 2

180 178 184 172

266 262 272 282

42 42 48 36

40 52 64 44

40 48 46 44

68 68 60 42

0.488 0.533 0.489 0.550

0.630 0.567 0.484 0.488

0.560 0.595 0.572 0.591

0.903 0.917 0.840 0.881

0.548 0.666 0.562 0.692

0.998 1.064 0.778 0.825

0.130 1.700 0.162 4.211

135.864 664.607 3.399 6.066

Table 6 Physical properties of the glass samples. Measurements

–

+

–

+

–

+

–

+

+

–

+

–

+

–

+

–

–

+

–

+

–

Glass samples

S−1

S−2

S−3

S−4

Density, ρ (g cm−3)

2.580

2.624

2.690

2.875

Molar volume, Va (cm3 mol−1)

23.243

22.861

22.433

22.180

0.026

0.263

2.684

27.140

13.618

6.286

2.900

1.341

33.799

15.602

7.196

3.328

0.032

0.152

0.714

3.338

Ion concentration, NRE

(×1021

o

Polaron radius, rp (A) o

Interatomic distance, ri (A) Field strength, F

Eτ (eV )

(×1016

cm−2)

Ion cm−3)

–

+

+

–

+

–

–

+

+

–

+

e–

+

+

–

–

+

–

–

+

–

+

–

+

–

+

–

+

Fig. 7. The possible reaction of occurring the formation of NBOs.

interstices of the structures leading to a decrease in the molar volume with the addition of Er2O3. These results indicated that the glass structure was more packed (increase of the number of atoms per unit volume) with the increased concentrations of Er2O3. The ion concentrations of the glass samples showed a clear rise with the increasing content of Er2O3. The obtained results revealed that the oxide of the dopants added into the network structures was in the form of an ion (Er 3 +), or it could be said that the dopants added into the network structures were in the form of non-bridging oxygens (NBOs). Thus, it was possible that the coordination of the former network changing. When the Er2O3 was added into the network structures, the oxygen in Er2O3 converted a tetrahedral bond into a trigonal bond, and the Er atom compensated the charge (Er3+). The oxygen from Er2O3 broke up the silica network leaving oxygen atoms with an unshared electron and released O2 gas molecule in the air between the processes of melting. The possible reaction of this process is shown in Fig. 7. Moreover, the rise of the ion concentration showed that the Er 3 + ions were uniformly spread throughout the network structures [33]. The Polaron radius and interatomic distance showed decreases with the increasing content of Er2O3. This was due to the fact that the increasing of the ion concentration led to the decrease of the distance between the positive and negative ion cores. The decrease of the interatomic distance supported the results of the molar volume of the glass samples. In addition, the Polaron radius can be described as the distance of the interaction of the electron within the ion cores of the materials. The electrons move (conducting electron) in the lattice structures where the atoms (ion cores) move from their equilibrium positions due to the electrons repelling the negative ions (such as O2– ions) and attract the positive ions (such as Si4+, Na+ or Er3+ ions) to decrease the electron mobility (as shown in Fig. 8). A self-induced potential arises, which would act back on the electron (trapping electron) and modify its physical properties. The Polaron size should decrease with the number of atoms being increased. Therefore, the decrease in the Polaron radius would indicate that the stronger force field interaction between the electron and ion

–

+

+

–

+

+

+

–

+

–

+

–

+

–

–

Fig. 8. Artist views of the interaction distance between a moving electron and ion cores of the atoms or Polaron radius.

cores of the solids was due to increasing the numbers of ion concentrations resulting in decreasing the path length that the electron travels and leading to trapping the electron. These results could be confirmed with the value of the field strength. From Table 6, the field strength showed a rise with the increasing Er2O3 content. These results indicate that the R–O bond strength increased producing stronger field strength around the modifier ions. The outcomes strongly support the discussion of the results for the Polaron radius.

4. Conclusions The new glass system was fabricated from recycled window glass as the starting material doped with Er2O3. The glass samples were initially colorless with minimal dopant content and became pink with higher dopant levels. The glow curve and linearity show suitable values for the S-2 glass samples. The reproducibility of the S-2 glass samples was evaluated using calculations of the DVI value. The maximum variation range of the TL signal from the average value was less than 2.14%. The DVI value was 3.2%, which indicated that the S-2 glass samples had high reproducibility. The fading of the S-2 glass samples showed a high remaining TL signal of about 55% after 168 h. The activation energy and frequency factor supported the results for the glow curve and fading of the glass samples. The physical properties of the glass samples showed that the addition of a modifier into the network structures led to the increase of the density while the molar volume decreased. These results indicated that adding Er2O3 into the glass network raised the compactness of the structures. The modifier atom was in the form of 308

Journal of Luminescence 197 (2018) 304–309

R. Laopaiboon et al.

ions when added into the network structures, and resulted in an increase in the field strength. [15]

Acknowledgments The authors gratefully acknowledge the financial support of the Thailand Research Fund (TRF), Office of the Higher Education Commission (CHE) and Ubon Ratchathani University (UBU) (Grant number: MRG6080060).

[16]

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