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Kinetic analysis of the 300 °C TL peak in Solonópole natural quartz sensitized by heat and gamma radiation
Radiation Measurements 46 (2011) 1421e1425
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Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Kinetic analysis of the 300 C TL peak in Solonópole natural quartz sensitized by heat and gamma radiation P.L. Guzzo a, *, L.B.F. Souza b, H.J. Khoury b a b
Department of Mining Engineering, Federal University of Pernambuco, Avenida Acadêmico Hélio Ramos, s/n, 50740-530 CDU Recife, PE, Brazil Department of Nuclear Energy, Federal University of Pernambuco, 50740-540 CDU Recife, PE, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 November 2010 Received in revised form 4 February 2011 Accepted 27 February 2011
Earlier, the sensitization of a thermoluminescence (TL) peak near 300 C in Solonópole quartz was investigated taking into account the combined effect of heat-treatments and irradiation with high doses of g rays. At that time, it was shown that the sequence: heat at 500 C, irradiation at 25 kGy and annealing at 400 C, was a suitable procedure to measure doses as low as 0.1 mGy using this quartz. However, sensitization with doses higher than 50 kGy and the kinetic analysis of the 300 C peak were not considered. The aim of the present study is to measure the TL sensitivity and the main kinetic parameters of this peak as a function of various sensitization conditions. Five lots of samples were sensitized at different heating temperatures and incremental doses up to 200 kGy. The activation energy was calculated by the peak-shape and the repeated initial rise methods. As a result, the decay noticed in TL sensitivity above 100 kGy was less intense for the lots that were heated before irradiation. The geometrical factor analysis was important to clarify the kinetic-order and the probability of retrapping for each condition of sensitization. The activation energies measured by each method are in fair agreement with each other and were mainly affected by the heat-treatment performed at 1000 C before irradiation. Activation energies averaging (1.51 0.06) eV were calculated for untreated and heat-treated lots at 500 and 800 C. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Single crystal Thermoluminescence Sensitivity Activation energy Geometrical factor
1. Introduction The possibility to sensitize the intensity of a TL peak near 300 C in quartz single crystals was originally reported by Khoury et al. (2007). Combining g doses like 25 kGy and post heat-treatments at 400 C, the TL sensitivity of quartz near the 280e320 C region increased more than 1000 times. As a result, it has been possible to assess doses as low as 104 Gy with quartz single crystals taken from the Solonópole district located in the Ceará State, Brazil (Khoury et al., 2008). It was also shown that the sensitized peak shifts toward higher temperatures when the Solonópole quartz is heated beyond the aeb phase transition before being irradiated at high doses. Later, when TL data were related to the impurity-content of quartz crystals extracted from different sites, it was shown that the sensitization depends, among other things, on the impurity-content of Li/Al and Li/OH ratios; the impurities are known to be associated with point-defects into the quartz lattice (Guzzo et al., 2009). In a more detailed study, Souza et al. (2010) observed that the TL intensity of the sensitized peak near 300 C saturates at g doses * Corresponding author. Tel.: þ55 81 2126 8244; fax: þ55 81 2126 8249. E-mail address: [email protected] (P.L. Guzzo). 1350-4487/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2011.02.024
ranging from 15 to 50 kGy. This behavior was explained by the saturation in the number of electron traps because the number of recombination centers was still increasing with the increase of g dose. However, the parameters (activation energy and frequency factor) of the electron traps related to the sensitized peak have not yet been characterized. Thus, a kinetic analysis is essential now to make clear if the above mentioned peak corresponds to one of the well documented peaks observed in quartz glow curves at 325 and 375 C or not. Such kind of study is important because natural quartz is extensively used in luminescence dating and retrospective dosimetry and also because trap depth parameters are very sensitive to the environmental conditions under which quartz was grown (Wintle, 2008; Preusser et al., 2009). In addition, the models that have been proposed to describe the sensitization of TL peaks in quartz are based on the former Zimmermann’s model which is restricted to explain the sensitization of the 110 C glow peak (Bailey, 2001) and, as known, the kinetic analysis is an essential tool to determine trap depth parameters useful to modeling the TL output of luminescent minerals such as quartz. Thus, the aim of this study is to investigate the TL sensitivity and trap depth parameters of the peak at 300 C in Solonópole quartz as a function of different conditions of sensitization up to accumulated doses of 200 kGy.
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P.L. Guzzo et al. / Radiation Measurements 46 (2011) 1421e1425
2. Experimental The quartz samples used in this study (30 discs 6 mm in diameter and 1 mm thick) are those previously used by Khoury et al. (2008). The discs were divided into 5 lots. Lot 1 was irradiated with 60Co g rays in a g cell irradiator with a dose rate of 10.04 kGy h1 in March 2007. The doses were delivered in steps varying from 5 to 50 kGy, until a cumulative dose of 200 kGy. After each irradiation increment, the samples were annealed as follows: heating up to 400 C; annealing for 1 h at 400 C; cooling; annealing for 2 h at 100 C; and cooling again. This thermal cycle was adopted as the standard annealing procedure throughout this work. The procedure was repeated at least 3 times after each irradiation increment with high dose. Lots 2, 3 and 4 were heattreated during 2 h at 500, 800 and 1000 C, respectively. The treatments were performed in a muffle furnace at atmospheric pressure with a heating rate of 5 C min1 and slowly cooled (w1 C min1). Then, these lots were irradiated with dose increments of 25 and 50 kGy. For comparison with the heat-treated lots, an untreated lot (lot 5) was also irradiated with 25 and 50 kGy. After each step of sensitization, the glow curves and the TL responses of each lot were measured as follows. For glow curve analysis, the samples were irradiated in air with g rays from a 137Cs source (43 mGy h1) with test-dose of 30 mGy. The TL glow curves were recorded using a Harshaw 3500 reader with a heating rate (b) equal to 2 C s1. The TL intensity was normalized to sample weight and test-dose. A typical glow curve is shown in Fig. 1. Besides the peak at 90 C, which corresponds to the so-called 110 C peak, the glow curves show a weak peak near 180 C and a strong peak centered at 285 C. For TL intensity vs. dose response, the TL signals were recorded with a 2800 M Victoreen reader by using the stepheating mode with doses in the range of 0.1e30 mGy. The intensity of the TL emission was evaluated by integrating the area under the peak appearing from 160 to 350 C and was given as the TL signal per unit of mass (nC mg1). The data points associated with each condition of sensitization were fitted linearly taking into account the standard deviation related to the average value of 5 measurements for each test-dose. A typical TL response curve is shown in the inset plot of Fig. 1. All TL response curves showed a linear fitting factor (R2) better than 0.996. The shape of the glow peak near 300 C was analyzed after each step of sensitization using the glow curves obtained with the Harshaw instrument. Besides the temperature at maximum TL intensity (Tm), the low- and high-temperature half-heights (T1 and T2) were determined. The kinetic order (b) of the glow peak was
Fig. 1. Typical TL glow curve (b ¼ 2 C s1) and TL response curve of Solonópole quartz sensitized with g radiation (25 kGy).
estimated after the determination of the geometrical factor (mg) as described by Chen and McKeever (1997). For a general order kinetics, the activation energy (Ea) is given by:
Ea ¼
2:52 þ 10:2 mg 0:42
kT 2 m
u
!
a 1 þ ð2kTm Þ 2
(1)
where u is the full width of the peak at its half-maximum intensity, k is the Boltzmann’s constant and a is the temperature dependent pre-exponential factor ðIðTÞaT a Þ. As kT was much smaller than Ea, we assumed that a ¼ 0. The second method used to determine Ea was the repeated initial rise (RIR) method (Chen and McKeever, 1997). Two samples of each lot were irradiated with a test-dose of 20 mGy. Initially, the samples were used to get the complete TL signal of the glow peak. From this curve, the temperature at the TL maximum (Imax) and the corresponding temperature to 0.1Imax were determined. After a new irradiation with 20 mGy, the samples were heated to a temperature just beyond the temperature related to 0.1(Imax) called Tstop. Then, the samples were quickly cooled and the TL signals were collected again at a temperature Tstop slightly higher than the previous one. This procedure was repeated with Tstop values ranging from 200 to 350 C in steps of 10 C. As a result, a total of 15 partial glow curves were obtained for each sample. Ea was obtained from a plot of ln(I) vs. 1/T, the slope being Ea/k. Then, using Ea determined by the RIR method, the pre-exponential frequency factor (s) was calculated using the condition of maximum TL intensity to peak of general kinetic order, as follows:
"kT 2 exp Ea #1 m 2km ðb 1Þ kTm 1þ s ¼ bEa Ea
(2)
3. Results and discussion The sensitivity of the TL response curves as a function of accumulated increments of g doses is shown in Fig. 2. As shown in Fig. 2(a), TL sensitivity of the unheated lots has a notable increase up to 15 kGy. From 15 to 50 kGy, the TL sensitivity remains almost constant indicating saturation in the sensitization process. Above 50 kGy, TL sensitivity decreases almost linearly suggesting that high g doses desensitize the 300 C peak. In the past, Sawakuchi and Okuno (2004) reported that the TL intensity glow peaks near 135 and 185 C decreased and those at the 250 and 325 C peaks saturated when test-doses higher than 20 kGy were delivered to quartz particles. These authors explained this behavior using a phenomenological model in which radiation annihilates traps related to each individual peak. In the present study, high g doses have been used to sensitize the quartz and not as a test-dose as was used by Sawakuchi and Okuno (2004). Thus, it assumed that the quartz desensitization corresponding to the 300 C peak of the glow curve is associated with the saturation of electron traps when accumulated doses exceed 50 kGy, probably because above this point, the annealing treatments at 400 C are not enough to empty all the traps related to this peak. Due to the annealing procedure carried out after each test-dose, the heating time accumulated by the lot sensitized with small increments (lot 1) is much higher than with the lot irradiated with large increments (lot 5) of g doses. By observing Fig. 2(a), we can see that the additional annealing time at 400 C may only be affecting the saturation level because the TL sensitivity of lot 5 is slightly higher than that of lot 1. Up to 50 kGy, the accumulated time of annealing for lots 1 and 5 was 126 and 36 h, respectively. As suggested before, the successive treatments at 400 C gradually release electrons from deep traps that are originally populated
P.L. Guzzo et al. / Radiation Measurements 46 (2011) 1421e1425
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Fig. 2. TL sensitivity of Solonópole quartz sensitized with high g doses (a) and heat-treatments plus high g doses (b).
during the first irradiations with high g doses. Later, during the irradiation with test-doses, the deep traps may be competing with the shallow ones which are responsible for the TL peak at 285 C (Souza et al., 2010). Once saturation is attained, the TL sensitivity is no more affected by the cumulative time of annealing because an almost linear decrease in TL sensitivity was also noticed for the lot irradiated with large dose increments. After 200 kGy, the accumulated annealing time for lots 1 and 5 was 193 and 74 h, respectively. The effect of accumulated doses in samples that were heattreated before irradiation is shown in Fig. 2(b). Compared to Fig. 2(a), a decrease in TL sensitivity is noticed from 25 to 50 kGy, but desensitization occurs less rapidly than in untreated samples for doses higher than 100 kGy. Up to this dose, it is observed that the TL sensitivity of the lot treated at 1000 C is slightly higher than the sensitivity of the other lots. In addition, the decay of the TL sensitivity noticed for the lot treated at 500 C is slightly different than that observed in lots treated at 800 and 1000 C. Fig. 3 shows the effect of accumulated doses on the geometrical factor (mg) calculated from the glow curves obtained at different sensitization conditions. For untreated samples, Fig. 3(a) shows that mg varies from values around (0.51 0.02) to (0.46 0.01) when the accumulated dose goes from 25 to 100 kGy. For heattreated samples, Fig. 3(b) shows that mg randomly varies around (0.47 0.02) independently of the accumulated dose. Comparing both plots, it may be inferred that heat-treatment carried out before irradiation has more influence on peak morphology than the accumulated time of annealing performed after irradiation. The different values of mg shown in Fig. 3(a) suggest that the kinetic-order of the 300 C peak varies with the increase of the
accumulated dose. Thus, the kinetic-order parameter (b) was calculated separately, as follows: (i) from the beginning of sensitization up to 50 kGy; (ii) from 100 up to 200 kGy. As a result, it was found that b passes from (1.76 0.24) to (1.29 0.13) for untreated samples and from (1.33 0.25) to (1.37 0.19) for heat-treated samples. In case of untreated samples, as b approaches first-order kinetics, it is concluded that the probability of retrapping during the TL output of the 300 C peak decreases with the increase of accumulated dose. Compared to the untreated samples sensitized with 25 kGy, one observes that the heat-treatments performed before irradiation also reduce the probability of retrapping during the TL output of the sensitized peak, but the successive doses administered up to 200 kGy did not change the kinetic order of the 300 C. Activation energies measured by the peak-shape method are shown in Fig. 4. In case of untreated samples, it is seen in Fig. 4(a) that Ea increases from dose of 5e50 kGy and then remains almost constant. For those samples that were heat-treated before irradiation, Fig. 4(b) shows that Ea randomly varies all over the range of doses investigated here. Even so, it is shown that the lot heat-treated at 1000 C has higher Ea than those lots treated at 500 and 800 C. This fact is in agreement with our previous observation that the glow peak of the lot heat-treated at 1000 C occurs beyond 300 C (Khoury et al., 2008). Measuring Tm throughout the accumulated doses, it is found that the Tm values for untreated and heat-treated lots at 500 C are similar to each other and their average value is (283.4 11.0) C. For the lots treated at 800 and 1000 C, the average value for Tm is (293.0 4.9) and (317.1 6.7) C, respectively. The activation energy measured by the RIR method was carried out after accumulated doses of 25 and 200 kGy. For all conditions, the mean values measured by this method are in good agreement
Fig. 3. Geometrical factor (mg) for Solonópole quartz sensitized with high g doses (a) and heat-treatments plus high g doses (b).
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P.L. Guzzo et al. / Radiation Measurements 46 (2011) 1421e1425
Fig. 4. Activation energy (Ea) for Solonópole quartz sensitized with high g doses (a) and heat-treatments plus high g doses (b) measured by the peak-shape method.
with the values measured by the peak-shape method. The plots of Ea against Tstop showed the presence of single plateau regions in the temperature range of 250e350 C. This implies that the sensitized peak is indeed corresponding to a single trap depth. Table 1 shows the values of trapping parameters Ea and s measured after 25 and 200 kGy. It is observed that Ea for the untreated lot measured after 25 kGy is fairly lower than the values measured in the other sensitization conditions. On the other hand, Ea measured in the lot treated at 1000 C shows higher values. Removing these outliers and comparing the mean of Ea (1.51 0.06 eV) with previous data reported in the literature, one observes that the trap depth of this peak is lower than that measured at the 325 C (1.65 0.06 eV) and 375 C (1.88 0.03 eV) quartz peaks (Spooner and Franklin, 2002). In addition, the mean Ea and s values resulting from this study are in satisfactory agreement with trap depth parameters measured by the RIR method for a TL peak at 280 C of quartz grains extracted from sediments (Spooner and Questiaux, 2000). The effect of heat-treatments on trapping parameters can be observed when the untreated lot is compared to heat-treated lots after irradiation with 25 kGy. It is noticed that Ea and s yield higher values for the lot heat-treated at 1000 C. This effect is in agreement with the shift of the glow peak toward higher temperatures. As far as we know, the changes in trap parameters reported in this study cannot be accounted for quartz phase transitions taking place at 573 C (a- to b-quartz) and 870 C (b-quartz to tridymite) due to the following reasons: (i) as no phase transition occurs up to 500 C, the changes observed between untreated and 500 C heat-treated lots can be merely related to the heat-treatment itself; (ii) differences in trap parameters of the untreated lot was observed after 200 kGy where the cumulative time of annealing at 400 C also increased; (iii) trap parameters for heat-treated lots at 500 and 800 C are quite similar although one of the lots was treated in the b-phase. Based on these observations, it is suggested that as temperature (or heating time) increases, the rearrangement of existing point defects, acting as electron traps and recombination centers, also increases by enhancement of thermal energy and by diffusion mechanisms into the quartz lattice. This rearrangement can be Table 1 Trapping parameters of the 300 C peak measured by the RIR method after sensitization with 25 and 200 kGy. Heat-treatment
25 kGy
200 kGy s (s1)
Ea (eV) No 500 C 800 C 1000 C
1.36 1.49 1.50 1.73
0.09 0.05 0.02 0.05
6.3 1.2 2.5 1.8
s (s1)
Ea (eV) 11
10 1013 1012 1014
1.52 1.51 1.51 1.62
0.06 0.07 0.06 0.06
6.4 8.5 4.0 1.2
1012 1012 1012 1013
more favorable at 1000 C because the tridymite phase has a more open framework than a- and b-quartz (Heaney, 1994). 4. Conclusion The use of peak-shape and repeated initial rise methods revealed the presence of a single trap in the 280e320 C region of the glow curve of Solonópole quartz after being sensitized with g doses of 25 and 50 kGy. The analysis of peak symmetry showed that both accumulated radiation doses and heat-treatments reduce the probability of retrapping during the TL output at 300 C. Mean activation energy of (1.51 0.06) eV calculated for untreated and heat-treated lots at 500 and 800 C suggests that the trap depth related to this peak is not the same as that related to the 325 and 375 C quartz glow peaks. The increase observed in the activation energy and pre-exponential frequency factor caused by heat-treatment at 1000 C is more likely to be associated with small rearrangements of existing electron and hole centers than with the direct effect of the quartz-tridymite phase transition. Moreover, it was shown that the TL intensity of the sensitized peak near 300 C starts to decrease when accumulated doses exceed 50 kGy probably by the saturation of the existing electron traps. The onset and rate of desensitization are affected by the heat-treatments performed before irradiation and are not affected by the size of incremental doses. The similarities between the lots sensitized with small and large increments suggest that the 300 C peak of Solonópole quartz has a good thermal stability up to an annealing temperature around 400 C. Acknowledgments This work was supported by CNPq, FACEPE and LMRI/UFPE. References Bailey, R.M., 2001. Towards a general kinetic model for optically and thermally stimulated luminescence of quartz. Radiat. Meas. 33, 17e45. Chen, R., McKeever, S.W.S., 1997. Theory of Thermoluminescence and Related Phenomena. World Scientific. Guzzo, P.L., Khoury, H.J., Miranda, M.R., Barreto, S.B., Shinohara, A.H., 2009. Point defects and pre-dose requirements for sensitization of the 300 C TL peak in natural quartz. Phys. Chem. Miner. 36, 75e85. Heaney, P.J., 1994. Structure and chemistry of the low-pressure silica polymorphs. Rev. Mineral. 29, 1e40. Khoury, H.J., Guzzo, P.L., Brito, S.B., Hazin, C.A., 2007. Effect of high gamma doses on the sensitization of natural quartz used for thermoluminescence dosimetry. Radiat. Eff. Defects Solids 162, 101e107. Khoury, H.J., Guzzo, P.L., Souza, L.B.F., Farias, T.M.B., Watanabe, S., 2008. TL dosimetry of natural quartz sensitized by heat-treatment and high dose irradiation. Radiat. Meas. 43, 487e491. Preusser, F., Chithambo, M.L., Götte, T., Martini, M., Ramseyer, K., Sendezera, E.J., Susino, G.J., Wintle, A.G., 2009. Quartz as a natural luminescent dosimeter. Earth-Sci. Rev. 97, 196e226.
P.L. Guzzo et al. / Radiation Measurements 46 (2011) 1421e1425 Sawakuchi, G.O., Okuno, E., 2004. Effects of high gamma ray doses in quartz. Nucl. Instrum. Methods Phys. Res. B 218, 217e221. Souza, L.B.F., Guzzo, P.L., Khoury, H.J., 2010. Correlating the TL response of g-irradiated natural quartz to aluminum and hydroxyl point defects. J. Lumin. 130, 1551e1556.
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Spooner, N.A., Questiaux, D.G., 2000. Kinetics of red, blue and UV thermoluminescence and optically-stimulated luminescence from quartz. Radiat. Meas. 32, 659e666. Spooner, N.A., Franklin, A.D., 2002. Effect of the heating rate on the red TL of quartz. Radiat. Meas. 35, 59e66. Wintle, A.G., 2008. Fifty years of luminescence dating. Archaeometry 50, 276e312.
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Kinetic analysis of the 300 C TL peak in Solonópole natural quartz sensitized by heat and gamma radiation P.L. Guzzo a, *, L.B.F. Souza b, H.J. Khoury b a b
Department of Mining Engineering, Federal University of Pernambuco, Avenida Acadêmico Hélio Ramos, s/n, 50740-530 CDU Recife, PE, Brazil Department of Nuclear Energy, Federal University of Pernambuco, 50740-540 CDU Recife, PE, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 November 2010 Received in revised form 4 February 2011 Accepted 27 February 2011
Earlier, the sensitization of a thermoluminescence (TL) peak near 300 C in Solonópole quartz was investigated taking into account the combined effect of heat-treatments and irradiation with high doses of g rays. At that time, it was shown that the sequence: heat at 500 C, irradiation at 25 kGy and annealing at 400 C, was a suitable procedure to measure doses as low as 0.1 mGy using this quartz. However, sensitization with doses higher than 50 kGy and the kinetic analysis of the 300 C peak were not considered. The aim of the present study is to measure the TL sensitivity and the main kinetic parameters of this peak as a function of various sensitization conditions. Five lots of samples were sensitized at different heating temperatures and incremental doses up to 200 kGy. The activation energy was calculated by the peak-shape and the repeated initial rise methods. As a result, the decay noticed in TL sensitivity above 100 kGy was less intense for the lots that were heated before irradiation. The geometrical factor analysis was important to clarify the kinetic-order and the probability of retrapping for each condition of sensitization. The activation energies measured by each method are in fair agreement with each other and were mainly affected by the heat-treatment performed at 1000 C before irradiation. Activation energies averaging (1.51 0.06) eV were calculated for untreated and heat-treated lots at 500 and 800 C. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Single crystal Thermoluminescence Sensitivity Activation energy Geometrical factor
1. Introduction The possibility to sensitize the intensity of a TL peak near 300 C in quartz single crystals was originally reported by Khoury et al. (2007). Combining g doses like 25 kGy and post heat-treatments at 400 C, the TL sensitivity of quartz near the 280e320 C region increased more than 1000 times. As a result, it has been possible to assess doses as low as 104 Gy with quartz single crystals taken from the Solonópole district located in the Ceará State, Brazil (Khoury et al., 2008). It was also shown that the sensitized peak shifts toward higher temperatures when the Solonópole quartz is heated beyond the aeb phase transition before being irradiated at high doses. Later, when TL data were related to the impurity-content of quartz crystals extracted from different sites, it was shown that the sensitization depends, among other things, on the impurity-content of Li/Al and Li/OH ratios; the impurities are known to be associated with point-defects into the quartz lattice (Guzzo et al., 2009). In a more detailed study, Souza et al. (2010) observed that the TL intensity of the sensitized peak near 300 C saturates at g doses * Corresponding author. Tel.: þ55 81 2126 8244; fax: þ55 81 2126 8249. E-mail address: [email protected] (P.L. Guzzo). 1350-4487/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2011.02.024
ranging from 15 to 50 kGy. This behavior was explained by the saturation in the number of electron traps because the number of recombination centers was still increasing with the increase of g dose. However, the parameters (activation energy and frequency factor) of the electron traps related to the sensitized peak have not yet been characterized. Thus, a kinetic analysis is essential now to make clear if the above mentioned peak corresponds to one of the well documented peaks observed in quartz glow curves at 325 and 375 C or not. Such kind of study is important because natural quartz is extensively used in luminescence dating and retrospective dosimetry and also because trap depth parameters are very sensitive to the environmental conditions under which quartz was grown (Wintle, 2008; Preusser et al., 2009). In addition, the models that have been proposed to describe the sensitization of TL peaks in quartz are based on the former Zimmermann’s model which is restricted to explain the sensitization of the 110 C glow peak (Bailey, 2001) and, as known, the kinetic analysis is an essential tool to determine trap depth parameters useful to modeling the TL output of luminescent minerals such as quartz. Thus, the aim of this study is to investigate the TL sensitivity and trap depth parameters of the peak at 300 C in Solonópole quartz as a function of different conditions of sensitization up to accumulated doses of 200 kGy.
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P.L. Guzzo et al. / Radiation Measurements 46 (2011) 1421e1425
2. Experimental The quartz samples used in this study (30 discs 6 mm in diameter and 1 mm thick) are those previously used by Khoury et al. (2008). The discs were divided into 5 lots. Lot 1 was irradiated with 60Co g rays in a g cell irradiator with a dose rate of 10.04 kGy h1 in March 2007. The doses were delivered in steps varying from 5 to 50 kGy, until a cumulative dose of 200 kGy. After each irradiation increment, the samples were annealed as follows: heating up to 400 C; annealing for 1 h at 400 C; cooling; annealing for 2 h at 100 C; and cooling again. This thermal cycle was adopted as the standard annealing procedure throughout this work. The procedure was repeated at least 3 times after each irradiation increment with high dose. Lots 2, 3 and 4 were heattreated during 2 h at 500, 800 and 1000 C, respectively. The treatments were performed in a muffle furnace at atmospheric pressure with a heating rate of 5 C min1 and slowly cooled (w1 C min1). Then, these lots were irradiated with dose increments of 25 and 50 kGy. For comparison with the heat-treated lots, an untreated lot (lot 5) was also irradiated with 25 and 50 kGy. After each step of sensitization, the glow curves and the TL responses of each lot were measured as follows. For glow curve analysis, the samples were irradiated in air with g rays from a 137Cs source (43 mGy h1) with test-dose of 30 mGy. The TL glow curves were recorded using a Harshaw 3500 reader with a heating rate (b) equal to 2 C s1. The TL intensity was normalized to sample weight and test-dose. A typical glow curve is shown in Fig. 1. Besides the peak at 90 C, which corresponds to the so-called 110 C peak, the glow curves show a weak peak near 180 C and a strong peak centered at 285 C. For TL intensity vs. dose response, the TL signals were recorded with a 2800 M Victoreen reader by using the stepheating mode with doses in the range of 0.1e30 mGy. The intensity of the TL emission was evaluated by integrating the area under the peak appearing from 160 to 350 C and was given as the TL signal per unit of mass (nC mg1). The data points associated with each condition of sensitization were fitted linearly taking into account the standard deviation related to the average value of 5 measurements for each test-dose. A typical TL response curve is shown in the inset plot of Fig. 1. All TL response curves showed a linear fitting factor (R2) better than 0.996. The shape of the glow peak near 300 C was analyzed after each step of sensitization using the glow curves obtained with the Harshaw instrument. Besides the temperature at maximum TL intensity (Tm), the low- and high-temperature half-heights (T1 and T2) were determined. The kinetic order (b) of the glow peak was
Fig. 1. Typical TL glow curve (b ¼ 2 C s1) and TL response curve of Solonópole quartz sensitized with g radiation (25 kGy).
estimated after the determination of the geometrical factor (mg) as described by Chen and McKeever (1997). For a general order kinetics, the activation energy (Ea) is given by:
Ea ¼
2:52 þ 10:2 mg 0:42
kT 2 m
u
!
a 1 þ ð2kTm Þ 2
(1)
where u is the full width of the peak at its half-maximum intensity, k is the Boltzmann’s constant and a is the temperature dependent pre-exponential factor ðIðTÞaT a Þ. As kT was much smaller than Ea, we assumed that a ¼ 0. The second method used to determine Ea was the repeated initial rise (RIR) method (Chen and McKeever, 1997). Two samples of each lot were irradiated with a test-dose of 20 mGy. Initially, the samples were used to get the complete TL signal of the glow peak. From this curve, the temperature at the TL maximum (Imax) and the corresponding temperature to 0.1Imax were determined. After a new irradiation with 20 mGy, the samples were heated to a temperature just beyond the temperature related to 0.1(Imax) called Tstop. Then, the samples were quickly cooled and the TL signals were collected again at a temperature Tstop slightly higher than the previous one. This procedure was repeated with Tstop values ranging from 200 to 350 C in steps of 10 C. As a result, a total of 15 partial glow curves were obtained for each sample. Ea was obtained from a plot of ln(I) vs. 1/T, the slope being Ea/k. Then, using Ea determined by the RIR method, the pre-exponential frequency factor (s) was calculated using the condition of maximum TL intensity to peak of general kinetic order, as follows:
"kT 2 exp Ea #1 m 2km ðb 1Þ kTm 1þ s ¼ bEa Ea
(2)
3. Results and discussion The sensitivity of the TL response curves as a function of accumulated increments of g doses is shown in Fig. 2. As shown in Fig. 2(a), TL sensitivity of the unheated lots has a notable increase up to 15 kGy. From 15 to 50 kGy, the TL sensitivity remains almost constant indicating saturation in the sensitization process. Above 50 kGy, TL sensitivity decreases almost linearly suggesting that high g doses desensitize the 300 C peak. In the past, Sawakuchi and Okuno (2004) reported that the TL intensity glow peaks near 135 and 185 C decreased and those at the 250 and 325 C peaks saturated when test-doses higher than 20 kGy were delivered to quartz particles. These authors explained this behavior using a phenomenological model in which radiation annihilates traps related to each individual peak. In the present study, high g doses have been used to sensitize the quartz and not as a test-dose as was used by Sawakuchi and Okuno (2004). Thus, it assumed that the quartz desensitization corresponding to the 300 C peak of the glow curve is associated with the saturation of electron traps when accumulated doses exceed 50 kGy, probably because above this point, the annealing treatments at 400 C are not enough to empty all the traps related to this peak. Due to the annealing procedure carried out after each test-dose, the heating time accumulated by the lot sensitized with small increments (lot 1) is much higher than with the lot irradiated with large increments (lot 5) of g doses. By observing Fig. 2(a), we can see that the additional annealing time at 400 C may only be affecting the saturation level because the TL sensitivity of lot 5 is slightly higher than that of lot 1. Up to 50 kGy, the accumulated time of annealing for lots 1 and 5 was 126 and 36 h, respectively. As suggested before, the successive treatments at 400 C gradually release electrons from deep traps that are originally populated
P.L. Guzzo et al. / Radiation Measurements 46 (2011) 1421e1425
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Fig. 2. TL sensitivity of Solonópole quartz sensitized with high g doses (a) and heat-treatments plus high g doses (b).
during the first irradiations with high g doses. Later, during the irradiation with test-doses, the deep traps may be competing with the shallow ones which are responsible for the TL peak at 285 C (Souza et al., 2010). Once saturation is attained, the TL sensitivity is no more affected by the cumulative time of annealing because an almost linear decrease in TL sensitivity was also noticed for the lot irradiated with large dose increments. After 200 kGy, the accumulated annealing time for lots 1 and 5 was 193 and 74 h, respectively. The effect of accumulated doses in samples that were heattreated before irradiation is shown in Fig. 2(b). Compared to Fig. 2(a), a decrease in TL sensitivity is noticed from 25 to 50 kGy, but desensitization occurs less rapidly than in untreated samples for doses higher than 100 kGy. Up to this dose, it is observed that the TL sensitivity of the lot treated at 1000 C is slightly higher than the sensitivity of the other lots. In addition, the decay of the TL sensitivity noticed for the lot treated at 500 C is slightly different than that observed in lots treated at 800 and 1000 C. Fig. 3 shows the effect of accumulated doses on the geometrical factor (mg) calculated from the glow curves obtained at different sensitization conditions. For untreated samples, Fig. 3(a) shows that mg varies from values around (0.51 0.02) to (0.46 0.01) when the accumulated dose goes from 25 to 100 kGy. For heattreated samples, Fig. 3(b) shows that mg randomly varies around (0.47 0.02) independently of the accumulated dose. Comparing both plots, it may be inferred that heat-treatment carried out before irradiation has more influence on peak morphology than the accumulated time of annealing performed after irradiation. The different values of mg shown in Fig. 3(a) suggest that the kinetic-order of the 300 C peak varies with the increase of the
accumulated dose. Thus, the kinetic-order parameter (b) was calculated separately, as follows: (i) from the beginning of sensitization up to 50 kGy; (ii) from 100 up to 200 kGy. As a result, it was found that b passes from (1.76 0.24) to (1.29 0.13) for untreated samples and from (1.33 0.25) to (1.37 0.19) for heat-treated samples. In case of untreated samples, as b approaches first-order kinetics, it is concluded that the probability of retrapping during the TL output of the 300 C peak decreases with the increase of accumulated dose. Compared to the untreated samples sensitized with 25 kGy, one observes that the heat-treatments performed before irradiation also reduce the probability of retrapping during the TL output of the sensitized peak, but the successive doses administered up to 200 kGy did not change the kinetic order of the 300 C. Activation energies measured by the peak-shape method are shown in Fig. 4. In case of untreated samples, it is seen in Fig. 4(a) that Ea increases from dose of 5e50 kGy and then remains almost constant. For those samples that were heat-treated before irradiation, Fig. 4(b) shows that Ea randomly varies all over the range of doses investigated here. Even so, it is shown that the lot heat-treated at 1000 C has higher Ea than those lots treated at 500 and 800 C. This fact is in agreement with our previous observation that the glow peak of the lot heat-treated at 1000 C occurs beyond 300 C (Khoury et al., 2008). Measuring Tm throughout the accumulated doses, it is found that the Tm values for untreated and heat-treated lots at 500 C are similar to each other and their average value is (283.4 11.0) C. For the lots treated at 800 and 1000 C, the average value for Tm is (293.0 4.9) and (317.1 6.7) C, respectively. The activation energy measured by the RIR method was carried out after accumulated doses of 25 and 200 kGy. For all conditions, the mean values measured by this method are in good agreement
Fig. 3. Geometrical factor (mg) for Solonópole quartz sensitized with high g doses (a) and heat-treatments plus high g doses (b).
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P.L. Guzzo et al. / Radiation Measurements 46 (2011) 1421e1425
Fig. 4. Activation energy (Ea) for Solonópole quartz sensitized with high g doses (a) and heat-treatments plus high g doses (b) measured by the peak-shape method.
with the values measured by the peak-shape method. The plots of Ea against Tstop showed the presence of single plateau regions in the temperature range of 250e350 C. This implies that the sensitized peak is indeed corresponding to a single trap depth. Table 1 shows the values of trapping parameters Ea and s measured after 25 and 200 kGy. It is observed that Ea for the untreated lot measured after 25 kGy is fairly lower than the values measured in the other sensitization conditions. On the other hand, Ea measured in the lot treated at 1000 C shows higher values. Removing these outliers and comparing the mean of Ea (1.51 0.06 eV) with previous data reported in the literature, one observes that the trap depth of this peak is lower than that measured at the 325 C (1.65 0.06 eV) and 375 C (1.88 0.03 eV) quartz peaks (Spooner and Franklin, 2002). In addition, the mean Ea and s values resulting from this study are in satisfactory agreement with trap depth parameters measured by the RIR method for a TL peak at 280 C of quartz grains extracted from sediments (Spooner and Questiaux, 2000). The effect of heat-treatments on trapping parameters can be observed when the untreated lot is compared to heat-treated lots after irradiation with 25 kGy. It is noticed that Ea and s yield higher values for the lot heat-treated at 1000 C. This effect is in agreement with the shift of the glow peak toward higher temperatures. As far as we know, the changes in trap parameters reported in this study cannot be accounted for quartz phase transitions taking place at 573 C (a- to b-quartz) and 870 C (b-quartz to tridymite) due to the following reasons: (i) as no phase transition occurs up to 500 C, the changes observed between untreated and 500 C heat-treated lots can be merely related to the heat-treatment itself; (ii) differences in trap parameters of the untreated lot was observed after 200 kGy where the cumulative time of annealing at 400 C also increased; (iii) trap parameters for heat-treated lots at 500 and 800 C are quite similar although one of the lots was treated in the b-phase. Based on these observations, it is suggested that as temperature (or heating time) increases, the rearrangement of existing point defects, acting as electron traps and recombination centers, also increases by enhancement of thermal energy and by diffusion mechanisms into the quartz lattice. This rearrangement can be Table 1 Trapping parameters of the 300 C peak measured by the RIR method after sensitization with 25 and 200 kGy. Heat-treatment
25 kGy
200 kGy s (s1)
Ea (eV) No 500 C 800 C 1000 C
1.36 1.49 1.50 1.73
0.09 0.05 0.02 0.05
6.3 1.2 2.5 1.8
s (s1)
Ea (eV) 11
10 1013 1012 1014
1.52 1.51 1.51 1.62
0.06 0.07 0.06 0.06
6.4 8.5 4.0 1.2
1012 1012 1012 1013
more favorable at 1000 C because the tridymite phase has a more open framework than a- and b-quartz (Heaney, 1994). 4. Conclusion The use of peak-shape and repeated initial rise methods revealed the presence of a single trap in the 280e320 C region of the glow curve of Solonópole quartz after being sensitized with g doses of 25 and 50 kGy. The analysis of peak symmetry showed that both accumulated radiation doses and heat-treatments reduce the probability of retrapping during the TL output at 300 C. Mean activation energy of (1.51 0.06) eV calculated for untreated and heat-treated lots at 500 and 800 C suggests that the trap depth related to this peak is not the same as that related to the 325 and 375 C quartz glow peaks. The increase observed in the activation energy and pre-exponential frequency factor caused by heat-treatment at 1000 C is more likely to be associated with small rearrangements of existing electron and hole centers than with the direct effect of the quartz-tridymite phase transition. Moreover, it was shown that the TL intensity of the sensitized peak near 300 C starts to decrease when accumulated doses exceed 50 kGy probably by the saturation of the existing electron traps. The onset and rate of desensitization are affected by the heat-treatments performed before irradiation and are not affected by the size of incremental doses. The similarities between the lots sensitized with small and large increments suggest that the 300 C peak of Solonópole quartz has a good thermal stability up to an annealing temperature around 400 C. Acknowledgments This work was supported by CNPq, FACEPE and LMRI/UFPE. References Bailey, R.M., 2001. Towards a general kinetic model for optically and thermally stimulated luminescence of quartz. Radiat. Meas. 33, 17e45. Chen, R., McKeever, S.W.S., 1997. Theory of Thermoluminescence and Related Phenomena. World Scientific. Guzzo, P.L., Khoury, H.J., Miranda, M.R., Barreto, S.B., Shinohara, A.H., 2009. Point defects and pre-dose requirements for sensitization of the 300 C TL peak in natural quartz. Phys. Chem. Miner. 36, 75e85. Heaney, P.J., 1994. Structure and chemistry of the low-pressure silica polymorphs. Rev. Mineral. 29, 1e40. Khoury, H.J., Guzzo, P.L., Brito, S.B., Hazin, C.A., 2007. Effect of high gamma doses on the sensitization of natural quartz used for thermoluminescence dosimetry. Radiat. Eff. Defects Solids 162, 101e107. Khoury, H.J., Guzzo, P.L., Souza, L.B.F., Farias, T.M.B., Watanabe, S., 2008. TL dosimetry of natural quartz sensitized by heat-treatment and high dose irradiation. Radiat. Meas. 43, 487e491. Preusser, F., Chithambo, M.L., Götte, T., Martini, M., Ramseyer, K., Sendezera, E.J., Susino, G.J., Wintle, A.G., 2009. Quartz as a natural luminescent dosimeter. Earth-Sci. Rev. 97, 196e226.
P.L. Guzzo et al. / Radiation Measurements 46 (2011) 1421e1425 Sawakuchi, G.O., Okuno, E., 2004. Effects of high gamma ray doses in quartz. Nucl. Instrum. Methods Phys. Res. B 218, 217e221. Souza, L.B.F., Guzzo, P.L., Khoury, H.J., 2010. Correlating the TL response of g-irradiated natural quartz to aluminum and hydroxyl point defects. J. Lumin. 130, 1551e1556.
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Spooner, N.A., Questiaux, D.G., 2000. Kinetics of red, blue and UV thermoluminescence and optically-stimulated luminescence from quartz. Radiat. Meas. 32, 659e666. Spooner, N.A., Franklin, A.D., 2002. Effect of the heating rate on the red TL of quartz. Radiat. Meas. 35, 59e66. Wintle, A.G., 2008. Fifty years of luminescence dating. Archaeometry 50, 276e312.