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Determination of thermoluminescence kinetic parameters of Li2B4O7:Cu,Ag,P
Radiation Measurements 42 (2007) 1280 – 1284 www.elsevier.com/locate/radmeas
Determination of thermoluminescence kinetic parameters of Li2 B4 O7: Cu, Ag, P A. (Türkler) Ege a , E. Ekdal a , T. Karali a,∗ , N. Can b a Institute of Nuclear Sciences, Ege University, 35100 Bornova- Izmir, ˙ Turkey b Department of Physics, Celal Bayar University, Faculty of Arts and Sciences, 45140 Manisa, Turkey
Received 24 February 2006; received in revised form 20 April 2007; accepted 19 June 2007
Abstract The kinetic parameters of a newly prepared tissue-equivalent, highly sensitive thermoluminescent detector, Li2 B4 O7 : Cu, Ag, P of sintered pellets have been calculated. Thermoluminescence (TL) glow curves of Li2 B4 O7 : Cu, Ag, P samples after -irradiation showed peaks at about 384 and 446 K with a heating rate of 1 K s−1 . The kinetic parameters, namely activation energy (E) and frequency factor (s) associated with the main glow peak (446 K) of Li2 B4 O7 : Cu, Ag, P have been determined using isothermal decay (ID) and peak shape (PS) methods. The activation energies obtained by ID and PS methods are calculated to be 1.12 and 1.13 eV (mean), respectively. The frequency factors obtained by both methods are 7.61 × 1011 and 3.53 × 1011 s−1 (mean), respectively. Results obtained using both methods are compared and discussed. © 2007 Elsevier Ltd. All rights reserved. Keywords: Li2 B4 O7 : Cu, Ag, P; Kinetic parameters; Thermoluminescence; Dosimeter
1. Introduction The measurement of absorbed dose in soft biological tissue exposed to ionizing radiation such as X- and - rays requires the dosimeter material to have a similar atomic composition as that of human tissue, i.e. to be tissue equivalent (Zeff =7.42). There are very few tissue equivalent thermoluminescent (TL) materials used in radiation dosimetry applications. Consequently lithium borate-based dosimeters (TLDs) appeared to be one of the most attractive materials in personal dosimetry due to its effective atomic number of 7.3. TLDs are effective tools to assess personal radiation exposure from substances or equipment that emits radiation. To achieve accurate measurement of the absorbed dose, the dosimeter material should have a similar response as the medium being irradiated. The performance of TLD is evaluated by taking into account properties such as linearity, dose range, energy response, reproducibility, stability of stored information and isotropy (McKeever, 1985). ∗ Corresponding author. Tel./fax: +90 232 3886466.
E-mail address: [email protected] (T. Karali). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.06.008
Either naturally occurring or artificially created point defects that exist in the structure of insulators create electronic states in the forbidden band. Understanding of the TL phenomenon depends on characterization of these defect structures. TL can be defined as an emission of light from a material when it is heated where charge carrier traps induced by impurities called dopants are responsible for the TL process. TL materials exhibit a glow curve with one or more peaks when the trapped charge carriers are released (Azorín, 1986). The shape and size of the glow peak are defined by the kinetic parameters E (activation energy of trap) and s (frequency factor), and also by the number of trapped electrons at the start of heating (Kiyak and Bulu¸s, 2001). The characteristics of dosimeters used in TL process are related to kinetic parameters, and these parameters quantitatively describe the trapping–emitting centres. Detailed studies of the kinetic parameters provide valuable information regarding the TL mechanism responsible for the dosimetric applications (Kitis et al., 2000). A reliable dosimetric study of a TL material should be based on a good knowledge of the kinetics parameters. For example, the loss of the dosimetric information stored in the material after irradiation is strongly dependent on the position of the trapping levels within the forbidden gap.
A. (Türkler) Ege et al. / Radiation Measurements 42 (2007) 1280 – 1284
400
TL Intensity (a.u.)
In personal dosimetry, it is highly desirable for the dosimetric material to be tissue equivalent. The absorbed dose in soft biological tissue exposed to ionizing radiation can be determined more accurately, if the material is tissue equivalent, i.e. Zeff =7.42 (Kitis et al., 2000; Furetta et al., 2001; Proki´c, 2002). In the present study, Li2 B4 O7 : Cu, Ag, P dosimeters, having tissue like characteristics and high sensitivity were used. The aim of this paper is to present the kinetic parameters of the main glow peak (446 K) of Li2 B4 O7 : Cu, Ag, P, which have important aspects in the general description of physical characteristics of TL materials, using isothermal decay (ID) and peak shape (PS) methods, namely the activation energy, E (in eV), the frequency factor, s (in s −1 ). We believe this to be the first of such a work using Li2 B4 O7 : Cu, Ag, P.
300
200
100
0
350
2. Material and methods Copper is known to be one of the most effective activators. The use of copper as an activator in Li2 B4 O7 material may overcome the drawback of poor TL sensitivity and move the red TL emission spectra to an emission spectrum of copper activator, at about 370 nm (Takenaga et al., 1980). Many experimental results of the luminescence in some inorganic systems show that Cu and Ag as activator and co-activator are very efficient luminescent centres. Li2 B4 O7 : Cu, Ag, P TL material were prepared by sintering by Dr. M. Proki´c at the Institute of Nuclear Sciences-Vinca, Belgrade, Serbia and Montenegro (Proki´c, 2002). Li2 B4 O7 samples were prepared by wet reaction between stoichiometric amounts of Li2 CO3 and H3 BO3 with the addition of SiO2 to prevent adverse effects of humidity. The optimal stoichiometric lithium–borate ratio eliminated the effect of moisture on these TLDs. The addition of SiO2 and the proper choice of the chemical form of activators and co-activators resulted in a good chemical and moisture stability of prepared TLDs. The pellets (4.5 mm in diameter and 0.95 mm in thickness) were produced by a cold pressing method of polycrystalline power, having grain sizes between 75 and 200 m. The pellets were then sintered at a temperature over 1123 K in an air atmosphere (Proki´c, 2002). During routine applications of Li2 B4 O7 : Cu, Ag, P dosimeters, a readout anneal showed to be a very convenient procedure. Irradiation is performed by a 90 Sr–90 Y beta source (650 MBq) delivering an equivalent dose rate of 20 cGy m−1 . Since Li2 B4 O7 : Cu, Ag, P is sensitive to light, it was protected from direct light during handling, irradiation and readout process. Experiments were performed immediately after irradiation in order to overcome any fading effect which may influence the trap parameters. The TL glow curves were recorded by TLD reader (Harshaw Model 3500) with a linear heating rate of 1 K s−1 . A heat absorbent filter (Shott KG-1) has been placed over PM tube in order to prevent the blackbody radiation reaching the PM tube. 3. Results and discussion TL glow curve of Li2 B4 O7 : Cu, Ag, P sample was recorded after irradiation with a 90 Sr– 90 Y beta source for 1 min (Fig. 1).
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400 450 Temperature (K)
500
Fig. 1. TL glow curve of Li2 B4 O7 : Cu, Ag, P dosimeter irradiated with 90 Sr– 90 Y beta source for 1 min.
The TL glow curve of Li2 B4 O7 : Cu, Ag, P TL material is composed of two well-separated TL peaks with the main one located at about 446 K, and the low temperature peak at about 384 K. Trapping parameters such as order of kinetics, trap-depth and the frequency factor have appreciable influence on the TL properties of a phosphor. Hence, knowledge of them is of paramount importance for understanding the TL phenomenon in the phosphor, and there have been many approaches in determining these parameters experimentally (Bindi et al., 1994). There are various methods of analysis developed for obtaining TL parameters such as heating rate, area measurement under the curve, glow curve shape etc. The methods used in the current study are briefly summarized and the results obtained by each method are given in the following sections. 3.1. ID method ID is a general technique for determining E and s and does not employ any particular heating cycle. The pre-excited TL material is rapidly heated to a particular constant temperature and the light emission (i.e. phosphorescence), which decays exponentially as a function of time, is monitored. The kinetic parameters of a TL glow curve can be determined using the procedure described by Manam and Sharma. In this method, three or more temperatures were chosen below the maximum temperature of the glow peak. After irradiation, the sample is heated to the selected temperature and then maintained for recording the variation of the TL emission with time at a constant temperature. This procedure is repeated for other chosen temperatures for the material (McKeever, 1985; Manam and Sharma, 2005). ID measurements have been used to determine the kinetic order (b). In the case of first-order kinetics the plots of Ln [I] versus time (t) and for the second-order kinetics (I0 /I )1/2 − 1 versus time should yield a straight line, where I0 is the intensity at t = 0, I is the intensity at any instant t. If it is not obtained in either case, then the experimental data are to be fitted with a
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(1/T)x103 (K-1)
T=438 K
-2.6
T=433 K
6
T=428 K
-3.0
T=423 K
-3.2
2
Ln [m(T)]
4
0
2.28
2.30
2.32
2.34
2.36
2.38
-2.8
-3.4 -3.6 -3.8
0
20
40
60 80 Time (sec)
100
120
140
Fig. 2. Isothermal decay curves of Li2 B4 O7 : Cu, Ag, P dosimeter for 446 K glow peak.
linear function of (I0 /I )(b−1)/b −1 versus time. Different values of b (1 < b < 2) are tested and the value of b that gives the best linear fit would be the order of kinetics. The slopes of these lines for each chosen temperature [m(T )] are calculated and logarithmic values of these versus 1/T are plotted. Activation energy is calculated from the slope of this line and the frequency factor derived from the intersection of it by using the following equation: Ln[m(T )] = −
E + Ln[s(b − 1)], kT
(1)
where k is the Boltzman constant. To determine the kinetic parameters of the main peak (446 K) of Li2 B4 O7 : Cu, Ag, P dosimeter using this method, four temperatures (423, 428, 433 and 438 K) were chosen below the maximum temperature of the main glow peak (446 K). The order of kinetic is found to be 1.6, applying the different b values to the plot of [(I0 /I )(b−1)/b − 1] versus time where a straight line is obtained as seen in Fig. 2. The slopes of lines in Fig. 2 for each temperature are calculated and logarithmic values of these Ln [m(T )] versus 1/T are plotted as shown in Fig. 3. Activation energy was calculated from the slope of this line and found to be 1.12 ± 0.01 eV, and the frequency factor was derived from the intersection of it and calculated to be (7.61 ± 0.08) × 1011 s−1 by using Eq. (1). In this method, the positions of TL peaks are observed as Tstop is increased. This method may also estimate the number and positions of the component peaks within a complex glow curve. An initially irradiated material is heated at a linear rate to a temperature Tstop corresponding to a position on the low temperature tail of the glow peak. The sample is then cooled rapidly to room temperature and reheated at the same linear rate in order to record the entire remaining glow curve. The temperature of the maximum peak intensity (Tm ) in the glow curve is noted. The procedure is repeated on a freshly irradiated sample, slightly increasing the Tstop with small steps of 3 K. The Tm position shifts to high temperatures after each repeated cycle. The temperatures corresponding to each flat region on the plot
-4.0 -4.2 Fig. 3. Plot of Ln [m(T )] versus 1/T for the 446 K glow peak of Li2 B4 O7 : Cu, Ag, P.
500
480
Tm (K)
[(Io/I)3/8−1]
8
460
440
420
375
400
425 Tstop (K)
450
475
Fig. 4. Tm –Tstop plot of the main peak of Li2 B4 O7 : Cu, Ag, P dosimeter with a heating rate of 1 K s−1 . Tstop temperature is increased in 3 K steps for each experiment.
reveal the presence of an individual peak at these temperatures (McKeever, 1980). For complex TL glow curves, this method is used to determine the approximate number of peaks in the TL glow curve. After irradiation, Tm –Tstop analysis was applied to the main peak of Li2 B4 O7 : Cu, Ag, P dosimeter. During analysis, the Tstop value was first set to 370 K and then raised to 470 K in 3 K steps. The recorded glow curves were used to plot the Tm of the first peak as a function of Tstop (Fig. 4). The Tm –Tstop plot shows that Tm changes more rapidly above 435 K. The figure also indicates that there are noticeable plateaus at about 447 and 461 K. 3.2. PS method The PS method commonly known as Chen’s method is used to determine the kinetic parameters of the main glow peak (Chen, 1969). This method is based on the measurement of Tm ,
A. (Türkler) Ege et al. / Radiation Measurements 42 (2007) 1280 – 1284
where stands for , , and ; in which the low temperature half width = Tm − T1 , the high temperature half width = T2 − Tm and the total half intensity width = T2 − T1 . When the ascending and descending parts of the peak are used the quantities become c = 1.51 + 3.0( g − 0.42),
c = 0.976 + 7.3( g − 0.42), c = 2.52 + 10.2( g − 0.42),
900
TL Intensity (a.u.)
T1 and T2 , which are the peak temperatures, the temperatures at half of the maximum intensity, on the ascending and descending parts of the peak, respectively. The expression deduced by Chen, valid for any kinetics is 2 kT m − b (2kT m ), (2) E = c
700 500 300 100
b = 1.58 + 4.2( g − 0.42),
350
400
b = 0, b = 1,
(3)
where g = / is the symmetry factor that determines the order of the kinetics (if g is 0.42 or 0.52 we have first or second-order kinetics, respectively). The frequency factor can be evaluated using E E s= [1 + (b − 1) m ]−1 , exp (4) kT m kT 2m where is heating rate and m =2kTm /E (Chen, 1969; Halperin and Braner, 1960). The PS method does not give correct values in materials having complex glow curves (Kitis et al., 2000). For this reason before applying the method, firstly the Tm –Tstop method was used to determine the number and position of glow peaks of the material and then pre-annealing was conducted at 390 K to eliminate possible contribution of low temperature peak to the main peak. Then glow curve deconvolution (GCD) analysis is used to separate each peak from this resulting cleaned peak. First-order TL peaks were represented successfully using the Weibull distribution function and second-order and generalorder TL peaks were represented accurately by using the logistic asymmetric functions with varying symmetry parameters (Pagonis and Kitis, 2002). GCD analysis was performed on the cleaned main peak using obtained Tm –Tstop results, and the real and isolated peaks shown in Fig. 5 were obtained. Then PS method was applied to the cleaned main peak determined by means of GCD analysis and trap energy (E), frequency factor (s) and symmetry factor were calculated using Eqs. (2)–(4). Symmetry factor ( g ), average trap energy and frequency factor were found to be 0.46, 1.13 ± 0.003 eV and (3.53 ± 0.43) × 1011 s−1 , respectively. The GCD and PS methods depend on the same properties of the peak, i.e. shape, FWHM, etc; as a consequence, the PS method should give results similar to those obtained by GCD. All trap energy and frequency factor results obtained by the two methods are given in Table 1. As seen from the table, there is a good agreement between the results obtained by PS and ID methods. Please note that the E values calculated by the PS method are not affected by any temperature gradient between the heater strip and the emissive surface of the sample since the
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450 Temperature (K)
500
550
Fig. 5. GCD analysis of glow curve of Li2 B4 O7 : Cu, Ag, P dosimeter. Table 1 Kinetic parameters of the main peak of Li2 B4 O7 : Cu, Ag, P dosimeter obtained by two methods
PS method
ID method
Activation energy (E)
(eV)
Frequency factor (s)
(s−1 )
E E E
1.13 1.13 1.14
s s s
3.42 × 1011 3.17 × 1011 4.01 × 1011
E
1.12 ± 0.01
s
(7.61±0.08)×1011
term Tm2 / Eq. (2) does not vary as a function of heating rate. Moreover, the effect of thermal gradient on Tm is cancelled by the same effect on (Furetta et al., 1997). 4. Conclusion We have clearly demonstrated that there is a good agreement in evaluated kinetic parameters obtained by PS and ID methods. Therefore it seems reasonable that both methods, PS and ID and the measured parameters are reliable and accurate. As far as we are concerned there is no such work in literature in calculating TL kinetic parameters of newly prepared Li2 B4 O7 : Cu, Ag, P TLDs using any of the methods of analysis aforementioned. References Azorín, J., 1986. Determination of thermoluminescence parameters from glow curves I. A Review. Int. J. Radiat. Appl. Instrum. Part D Nucl. Tracks 11 (3), 159–166. Bindi, R., Lapraz, D., Iacconi, P., Boutayeb, S., 1994. Theoretical analysis of the simultaneous detection method of thermally stimulated conductivity (TSC) and luminescence (TSL); application to an alphaAl2 O3 monocrystal. J. Phys. D 27, 2395. Chen, R., 1969. Glow curves with general order kinetics. J. Electrochem. Soc. 116, 1254–1257. Furetta, C., Kitis, G., Kuo, J.H., Vismara, L., Weng, P.S., 1997. Impact of non-ideal heat transfer on the determination of thermoluminescent kinetics parameters. J. Lumin. 75, 341. Furetta, C., Proki´c, M., Salamon, R., Proki´c, V., Kitis, G., 2001. Dosimetric characteristics of tissue equivalent thermoluminescence solid TL detectors
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based on lithium borate. Nucl. Instrum. Methods Phys. Res. A 456, 411–417. Halperin, A., Braner, A.A., 1960. Evaluation of thermal activation energies from glow curves. Phys. Rev. 117, 408–415. Kitis, G., Furetta, C., Proki´c, M., Proki´c, V., 2000. Kinetic parameters of some tissue equivalent thermoluminescence materials. J. Phys. D 33, 1252–1262. Kiyak, N.G., Bulu¸s, E., 2001. Effect of annealing temperature on determining trap depths of quartz by various heating rates method. Radiat. Meas. 33, 879–882. Manam, J., Sharma, S.K., 2005. Evaluation of trapping parameters of thermally stimulated luminescence glow curves in Cu-doped Li2 B4 O7 Phosphor. Radiat. Phys. Chem. 72 (4), 423–427.
McKeever, S.W.S., 1980. On the analysis of complex thermoluminescence glow curves: resolution into individual peaks. Phys. Status Solidi A. 62, 331. McKeever, S.W.S., 1985. Thermoluminescence of Solids. Cambridge University Press, New York. Pagonis, V., Kitis, G., 2002. On the possibility of using commercial software packages for thermoluminescence glow curve deconvolution analysis. Radiat. Prot. Dosim. 101 (1–4), 93–98. Proki´c, M., 2002. Dosimetric characteristics of Li2 B4 O7 : Cu, Ag, P solid TL detectors. Radiat. Prot. Dosim. 100, 265–268. Takenaga, M., Yamamoto, O., Yamashita, T., 1980. Preparation and characteristics of Li2 B4 O7 : Cu phosphor. Nucl. Instrum. Methods 175, 77.
Determination of thermoluminescence kinetic parameters of Li2 B4 O7: Cu, Ag, P A. (Türkler) Ege a , E. Ekdal a , T. Karali a,∗ , N. Can b a Institute of Nuclear Sciences, Ege University, 35100 Bornova- Izmir, ˙ Turkey b Department of Physics, Celal Bayar University, Faculty of Arts and Sciences, 45140 Manisa, Turkey
Received 24 February 2006; received in revised form 20 April 2007; accepted 19 June 2007
Abstract The kinetic parameters of a newly prepared tissue-equivalent, highly sensitive thermoluminescent detector, Li2 B4 O7 : Cu, Ag, P of sintered pellets have been calculated. Thermoluminescence (TL) glow curves of Li2 B4 O7 : Cu, Ag, P samples after -irradiation showed peaks at about 384 and 446 K with a heating rate of 1 K s−1 . The kinetic parameters, namely activation energy (E) and frequency factor (s) associated with the main glow peak (446 K) of Li2 B4 O7 : Cu, Ag, P have been determined using isothermal decay (ID) and peak shape (PS) methods. The activation energies obtained by ID and PS methods are calculated to be 1.12 and 1.13 eV (mean), respectively. The frequency factors obtained by both methods are 7.61 × 1011 and 3.53 × 1011 s−1 (mean), respectively. Results obtained using both methods are compared and discussed. © 2007 Elsevier Ltd. All rights reserved. Keywords: Li2 B4 O7 : Cu, Ag, P; Kinetic parameters; Thermoluminescence; Dosimeter
1. Introduction The measurement of absorbed dose in soft biological tissue exposed to ionizing radiation such as X- and - rays requires the dosimeter material to have a similar atomic composition as that of human tissue, i.e. to be tissue equivalent (Zeff =7.42). There are very few tissue equivalent thermoluminescent (TL) materials used in radiation dosimetry applications. Consequently lithium borate-based dosimeters (TLDs) appeared to be one of the most attractive materials in personal dosimetry due to its effective atomic number of 7.3. TLDs are effective tools to assess personal radiation exposure from substances or equipment that emits radiation. To achieve accurate measurement of the absorbed dose, the dosimeter material should have a similar response as the medium being irradiated. The performance of TLD is evaluated by taking into account properties such as linearity, dose range, energy response, reproducibility, stability of stored information and isotropy (McKeever, 1985). ∗ Corresponding author. Tel./fax: +90 232 3886466.
E-mail address: [email protected] (T. Karali). 1350-4487/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2007.06.008
Either naturally occurring or artificially created point defects that exist in the structure of insulators create electronic states in the forbidden band. Understanding of the TL phenomenon depends on characterization of these defect structures. TL can be defined as an emission of light from a material when it is heated where charge carrier traps induced by impurities called dopants are responsible for the TL process. TL materials exhibit a glow curve with one or more peaks when the trapped charge carriers are released (Azorín, 1986). The shape and size of the glow peak are defined by the kinetic parameters E (activation energy of trap) and s (frequency factor), and also by the number of trapped electrons at the start of heating (Kiyak and Bulu¸s, 2001). The characteristics of dosimeters used in TL process are related to kinetic parameters, and these parameters quantitatively describe the trapping–emitting centres. Detailed studies of the kinetic parameters provide valuable information regarding the TL mechanism responsible for the dosimetric applications (Kitis et al., 2000). A reliable dosimetric study of a TL material should be based on a good knowledge of the kinetics parameters. For example, the loss of the dosimetric information stored in the material after irradiation is strongly dependent on the position of the trapping levels within the forbidden gap.
A. (Türkler) Ege et al. / Radiation Measurements 42 (2007) 1280 – 1284
400
TL Intensity (a.u.)
In personal dosimetry, it is highly desirable for the dosimetric material to be tissue equivalent. The absorbed dose in soft biological tissue exposed to ionizing radiation can be determined more accurately, if the material is tissue equivalent, i.e. Zeff =7.42 (Kitis et al., 2000; Furetta et al., 2001; Proki´c, 2002). In the present study, Li2 B4 O7 : Cu, Ag, P dosimeters, having tissue like characteristics and high sensitivity were used. The aim of this paper is to present the kinetic parameters of the main glow peak (446 K) of Li2 B4 O7 : Cu, Ag, P, which have important aspects in the general description of physical characteristics of TL materials, using isothermal decay (ID) and peak shape (PS) methods, namely the activation energy, E (in eV), the frequency factor, s (in s −1 ). We believe this to be the first of such a work using Li2 B4 O7 : Cu, Ag, P.
300
200
100
0
350
2. Material and methods Copper is known to be one of the most effective activators. The use of copper as an activator in Li2 B4 O7 material may overcome the drawback of poor TL sensitivity and move the red TL emission spectra to an emission spectrum of copper activator, at about 370 nm (Takenaga et al., 1980). Many experimental results of the luminescence in some inorganic systems show that Cu and Ag as activator and co-activator are very efficient luminescent centres. Li2 B4 O7 : Cu, Ag, P TL material were prepared by sintering by Dr. M. Proki´c at the Institute of Nuclear Sciences-Vinca, Belgrade, Serbia and Montenegro (Proki´c, 2002). Li2 B4 O7 samples were prepared by wet reaction between stoichiometric amounts of Li2 CO3 and H3 BO3 with the addition of SiO2 to prevent adverse effects of humidity. The optimal stoichiometric lithium–borate ratio eliminated the effect of moisture on these TLDs. The addition of SiO2 and the proper choice of the chemical form of activators and co-activators resulted in a good chemical and moisture stability of prepared TLDs. The pellets (4.5 mm in diameter and 0.95 mm in thickness) were produced by a cold pressing method of polycrystalline power, having grain sizes between 75 and 200 m. The pellets were then sintered at a temperature over 1123 K in an air atmosphere (Proki´c, 2002). During routine applications of Li2 B4 O7 : Cu, Ag, P dosimeters, a readout anneal showed to be a very convenient procedure. Irradiation is performed by a 90 Sr–90 Y beta source (650 MBq) delivering an equivalent dose rate of 20 cGy m−1 . Since Li2 B4 O7 : Cu, Ag, P is sensitive to light, it was protected from direct light during handling, irradiation and readout process. Experiments were performed immediately after irradiation in order to overcome any fading effect which may influence the trap parameters. The TL glow curves were recorded by TLD reader (Harshaw Model 3500) with a linear heating rate of 1 K s−1 . A heat absorbent filter (Shott KG-1) has been placed over PM tube in order to prevent the blackbody radiation reaching the PM tube. 3. Results and discussion TL glow curve of Li2 B4 O7 : Cu, Ag, P sample was recorded after irradiation with a 90 Sr– 90 Y beta source for 1 min (Fig. 1).
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400 450 Temperature (K)
500
Fig. 1. TL glow curve of Li2 B4 O7 : Cu, Ag, P dosimeter irradiated with 90 Sr– 90 Y beta source for 1 min.
The TL glow curve of Li2 B4 O7 : Cu, Ag, P TL material is composed of two well-separated TL peaks with the main one located at about 446 K, and the low temperature peak at about 384 K. Trapping parameters such as order of kinetics, trap-depth and the frequency factor have appreciable influence on the TL properties of a phosphor. Hence, knowledge of them is of paramount importance for understanding the TL phenomenon in the phosphor, and there have been many approaches in determining these parameters experimentally (Bindi et al., 1994). There are various methods of analysis developed for obtaining TL parameters such as heating rate, area measurement under the curve, glow curve shape etc. The methods used in the current study are briefly summarized and the results obtained by each method are given in the following sections. 3.1. ID method ID is a general technique for determining E and s and does not employ any particular heating cycle. The pre-excited TL material is rapidly heated to a particular constant temperature and the light emission (i.e. phosphorescence), which decays exponentially as a function of time, is monitored. The kinetic parameters of a TL glow curve can be determined using the procedure described by Manam and Sharma. In this method, three or more temperatures were chosen below the maximum temperature of the glow peak. After irradiation, the sample is heated to the selected temperature and then maintained for recording the variation of the TL emission with time at a constant temperature. This procedure is repeated for other chosen temperatures for the material (McKeever, 1985; Manam and Sharma, 2005). ID measurements have been used to determine the kinetic order (b). In the case of first-order kinetics the plots of Ln [I] versus time (t) and for the second-order kinetics (I0 /I )1/2 − 1 versus time should yield a straight line, where I0 is the intensity at t = 0, I is the intensity at any instant t. If it is not obtained in either case, then the experimental data are to be fitted with a
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A. (Türkler) Ege et al. / Radiation Measurements 42 (2007) 1280 – 1284
(1/T)x103 (K-1)
T=438 K
-2.6
T=433 K
6
T=428 K
-3.0
T=423 K
-3.2
2
Ln [m(T)]
4
0
2.28
2.30
2.32
2.34
2.36
2.38
-2.8
-3.4 -3.6 -3.8
0
20
40
60 80 Time (sec)
100
120
140
Fig. 2. Isothermal decay curves of Li2 B4 O7 : Cu, Ag, P dosimeter for 446 K glow peak.
linear function of (I0 /I )(b−1)/b −1 versus time. Different values of b (1 < b < 2) are tested and the value of b that gives the best linear fit would be the order of kinetics. The slopes of these lines for each chosen temperature [m(T )] are calculated and logarithmic values of these versus 1/T are plotted. Activation energy is calculated from the slope of this line and the frequency factor derived from the intersection of it by using the following equation: Ln[m(T )] = −
E + Ln[s(b − 1)], kT
(1)
where k is the Boltzman constant. To determine the kinetic parameters of the main peak (446 K) of Li2 B4 O7 : Cu, Ag, P dosimeter using this method, four temperatures (423, 428, 433 and 438 K) were chosen below the maximum temperature of the main glow peak (446 K). The order of kinetic is found to be 1.6, applying the different b values to the plot of [(I0 /I )(b−1)/b − 1] versus time where a straight line is obtained as seen in Fig. 2. The slopes of lines in Fig. 2 for each temperature are calculated and logarithmic values of these Ln [m(T )] versus 1/T are plotted as shown in Fig. 3. Activation energy was calculated from the slope of this line and found to be 1.12 ± 0.01 eV, and the frequency factor was derived from the intersection of it and calculated to be (7.61 ± 0.08) × 1011 s−1 by using Eq. (1). In this method, the positions of TL peaks are observed as Tstop is increased. This method may also estimate the number and positions of the component peaks within a complex glow curve. An initially irradiated material is heated at a linear rate to a temperature Tstop corresponding to a position on the low temperature tail of the glow peak. The sample is then cooled rapidly to room temperature and reheated at the same linear rate in order to record the entire remaining glow curve. The temperature of the maximum peak intensity (Tm ) in the glow curve is noted. The procedure is repeated on a freshly irradiated sample, slightly increasing the Tstop with small steps of 3 K. The Tm position shifts to high temperatures after each repeated cycle. The temperatures corresponding to each flat region on the plot
-4.0 -4.2 Fig. 3. Plot of Ln [m(T )] versus 1/T for the 446 K glow peak of Li2 B4 O7 : Cu, Ag, P.
500
480
Tm (K)
[(Io/I)3/8−1]
8
460
440
420
375
400
425 Tstop (K)
450
475
Fig. 4. Tm –Tstop plot of the main peak of Li2 B4 O7 : Cu, Ag, P dosimeter with a heating rate of 1 K s−1 . Tstop temperature is increased in 3 K steps for each experiment.
reveal the presence of an individual peak at these temperatures (McKeever, 1980). For complex TL glow curves, this method is used to determine the approximate number of peaks in the TL glow curve. After irradiation, Tm –Tstop analysis was applied to the main peak of Li2 B4 O7 : Cu, Ag, P dosimeter. During analysis, the Tstop value was first set to 370 K and then raised to 470 K in 3 K steps. The recorded glow curves were used to plot the Tm of the first peak as a function of Tstop (Fig. 4). The Tm –Tstop plot shows that Tm changes more rapidly above 435 K. The figure also indicates that there are noticeable plateaus at about 447 and 461 K. 3.2. PS method The PS method commonly known as Chen’s method is used to determine the kinetic parameters of the main glow peak (Chen, 1969). This method is based on the measurement of Tm ,
A. (Türkler) Ege et al. / Radiation Measurements 42 (2007) 1280 – 1284
where stands for , , and ; in which the low temperature half width = Tm − T1 , the high temperature half width = T2 − Tm and the total half intensity width = T2 − T1 . When the ascending and descending parts of the peak are used the quantities become c = 1.51 + 3.0( g − 0.42),
c = 0.976 + 7.3( g − 0.42), c = 2.52 + 10.2( g − 0.42),
900
TL Intensity (a.u.)
T1 and T2 , which are the peak temperatures, the temperatures at half of the maximum intensity, on the ascending and descending parts of the peak, respectively. The expression deduced by Chen, valid for any kinetics is 2 kT m − b (2kT m ), (2) E = c
700 500 300 100
b = 1.58 + 4.2( g − 0.42),
350
400
b = 0, b = 1,
(3)
where g = / is the symmetry factor that determines the order of the kinetics (if g is 0.42 or 0.52 we have first or second-order kinetics, respectively). The frequency factor can be evaluated using E E s= [1 + (b − 1) m ]−1 , exp (4) kT m kT 2m where is heating rate and m =2kTm /E (Chen, 1969; Halperin and Braner, 1960). The PS method does not give correct values in materials having complex glow curves (Kitis et al., 2000). For this reason before applying the method, firstly the Tm –Tstop method was used to determine the number and position of glow peaks of the material and then pre-annealing was conducted at 390 K to eliminate possible contribution of low temperature peak to the main peak. Then glow curve deconvolution (GCD) analysis is used to separate each peak from this resulting cleaned peak. First-order TL peaks were represented successfully using the Weibull distribution function and second-order and generalorder TL peaks were represented accurately by using the logistic asymmetric functions with varying symmetry parameters (Pagonis and Kitis, 2002). GCD analysis was performed on the cleaned main peak using obtained Tm –Tstop results, and the real and isolated peaks shown in Fig. 5 were obtained. Then PS method was applied to the cleaned main peak determined by means of GCD analysis and trap energy (E), frequency factor (s) and symmetry factor were calculated using Eqs. (2)–(4). Symmetry factor ( g ), average trap energy and frequency factor were found to be 0.46, 1.13 ± 0.003 eV and (3.53 ± 0.43) × 1011 s−1 , respectively. The GCD and PS methods depend on the same properties of the peak, i.e. shape, FWHM, etc; as a consequence, the PS method should give results similar to those obtained by GCD. All trap energy and frequency factor results obtained by the two methods are given in Table 1. As seen from the table, there is a good agreement between the results obtained by PS and ID methods. Please note that the E values calculated by the PS method are not affected by any temperature gradient between the heater strip and the emissive surface of the sample since the
1283
450 Temperature (K)
500
550
Fig. 5. GCD analysis of glow curve of Li2 B4 O7 : Cu, Ag, P dosimeter. Table 1 Kinetic parameters of the main peak of Li2 B4 O7 : Cu, Ag, P dosimeter obtained by two methods
PS method
ID method
Activation energy (E)
(eV)
Frequency factor (s)
(s−1 )
E E E
1.13 1.13 1.14
s s s
3.42 × 1011 3.17 × 1011 4.01 × 1011
E
1.12 ± 0.01
s
(7.61±0.08)×1011
term Tm2 / Eq. (2) does not vary as a function of heating rate. Moreover, the effect of thermal gradient on Tm is cancelled by the same effect on (Furetta et al., 1997). 4. Conclusion We have clearly demonstrated that there is a good agreement in evaluated kinetic parameters obtained by PS and ID methods. Therefore it seems reasonable that both methods, PS and ID and the measured parameters are reliable and accurate. As far as we are concerned there is no such work in literature in calculating TL kinetic parameters of newly prepared Li2 B4 O7 : Cu, Ag, P TLDs using any of the methods of analysis aforementioned. References Azorín, J., 1986. Determination of thermoluminescence parameters from glow curves I. A Review. Int. J. Radiat. Appl. Instrum. Part D Nucl. Tracks 11 (3), 159–166. Bindi, R., Lapraz, D., Iacconi, P., Boutayeb, S., 1994. Theoretical analysis of the simultaneous detection method of thermally stimulated conductivity (TSC) and luminescence (TSL); application to an alphaAl2 O3 monocrystal. J. Phys. D 27, 2395. Chen, R., 1969. Glow curves with general order kinetics. J. Electrochem. Soc. 116, 1254–1257. Furetta, C., Kitis, G., Kuo, J.H., Vismara, L., Weng, P.S., 1997. Impact of non-ideal heat transfer on the determination of thermoluminescent kinetics parameters. J. Lumin. 75, 341. Furetta, C., Proki´c, M., Salamon, R., Proki´c, V., Kitis, G., 2001. Dosimetric characteristics of tissue equivalent thermoluminescence solid TL detectors
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based on lithium borate. Nucl. Instrum. Methods Phys. Res. A 456, 411–417. Halperin, A., Braner, A.A., 1960. Evaluation of thermal activation energies from glow curves. Phys. Rev. 117, 408–415. Kitis, G., Furetta, C., Proki´c, M., Proki´c, V., 2000. Kinetic parameters of some tissue equivalent thermoluminescence materials. J. Phys. D 33, 1252–1262. Kiyak, N.G., Bulu¸s, E., 2001. Effect of annealing temperature on determining trap depths of quartz by various heating rates method. Radiat. Meas. 33, 879–882. Manam, J., Sharma, S.K., 2005. Evaluation of trapping parameters of thermally stimulated luminescence glow curves in Cu-doped Li2 B4 O7 Phosphor. Radiat. Phys. Chem. 72 (4), 423–427.
McKeever, S.W.S., 1980. On the analysis of complex thermoluminescence glow curves: resolution into individual peaks. Phys. Status Solidi A. 62, 331. McKeever, S.W.S., 1985. Thermoluminescence of Solids. Cambridge University Press, New York. Pagonis, V., Kitis, G., 2002. On the possibility of using commercial software packages for thermoluminescence glow curve deconvolution analysis. Radiat. Prot. Dosim. 101 (1–4), 93–98. Proki´c, M., 2002. Dosimetric characteristics of Li2 B4 O7 : Cu, Ag, P solid TL detectors. Radiat. Prot. Dosim. 100, 265–268. Takenaga, M., Yamamoto, O., Yamashita, T., 1980. Preparation and characteristics of Li2 B4 O7 : Cu phosphor. Nucl. Instrum. Methods 175, 77.