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TL dosimetry of natural quartz sensitized by heat-treatment and high dose irradiation
Radiation Measurements 43 (2008) 487 – 491 www.elsevier.com/locate/radmeas
TL dosimetry of natural quartz sensitized by heat-treatment and high dose irradiation H.J. Khoury a,∗ , P.L. Guzzo b , L.B.F. Souza a , T.M.B. Farias c , S. Watanabe c a Department of Nuclear Energy, Federal University of Pernambuco, 50740-540 CDU Recife, PE, Brazil b Department of Mining Engineering, Federal University of Pernambuco, 50740-530 CDU Recife, PE, Brazil c Institute of Physics, University of São Paulo, 05509-001 Cidade Universitária, São Paulo, SP, Brazil
Abstract The aim of this paper is to report the sensitization of the TL peak appearing at 270 ◦ C in the glow curve of natural quartz by using the combined effect of heat-treatments and irradiation with high doses. For this, thirty discs with 6 × 1 mm2 were prepared from plates parallell to a rhombohedral crystal face. The specimens were separated into four lots according to its TL read out between 160 and 320 ◦ C. One lot was submitted to doses of 60 Co radiation starting at 2 kGy and going up until a cumulative dose of 25 kGy. The other three lots were initially heat-treated at 500, 800 and 1000 ◦ C and then irradiated with a single dose of 25 kGy. The TL response of each lot was determined as a function of test-doses ranging from 0.1 to 30 mGy. As a result, it was observed that heat-treatments themselves did not produce the strong peak at 270 ◦ C that was observed after the administration of high doses. This peak is associated with the optical absorption band appearing at 470 nm which is due to the formation of [AlO4 ]◦ acting as electron–hole recombination centers. The formation of the 270 ◦ C peak was preliminary analyzed in relation to aluminum- and oxygen-vacancy-related centers found in crystalline quartz. © 2008 Elsevier Ltd. All rights reserved. Keywords: Quartz single crystal; Thermoluminescence; -rays; Pre-dose; Heat-treatment; Al-hole center
1. Introduction Sensitization is the ability to increase the sensitivity of a thermoluminescent (TL) phosphor using pre-doses and/or heattreatments. Zimmerman (1971) was one of the first to report that the TL sensitivity of the 110 ◦ C peak of quartz appears after the absorption of ionizing radiation at room temperature (pre-dose), followed by heating at approximately 500 ◦ C. Later, sensitization of 110, 220 and 325 ◦ C TL peaks were observed in quartz prepared from single crystals, ancient ceramics and geological sediments using or pre-doses in the range of 50 Gy to 4 kGy followed by heat-treatments from 200 up to 1200 ◦ C (Bailiff and Haskell, 1983; Chen et al., 1988; Yang and McKeever, 1990; Benny and Bhatt, 1997; Kristianpoller et al., 1997; Petrov and Bailiff, 1997; Wintle, 1997; Benny et al., 2000). In these studies, the test-dose used to determine the sensitization factor in relation to the TL response of the ∗ Corresponding author. Tel.: +55 81 2126 8708; fax: +55 81 2126 7988.
E-mail address: [email protected] (H.J. Khoury). 1350-4487/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2008.01.028
normal sample ranged from 0.1 to 50 Gy. Pre-dose and optical sensitization of 110 and 325 ◦ C peaks was reproduced with a satisfactory level of accuracy by the kinetic models describing the luminescent behavior of quartz grains (Bailey, 2001; Itoh, 2002). According to these studies, the sensitivity of both 110 and 325 ◦ C TL peaks are affected by the redistribution of charges that occurs between optically stimulated traps during illumination at room temperature and subsequent heating. It was also established that the maximum in TL emission of both peaks occurs at 380 nm suggesting that the same recombination centers are taking part in the TL process of these peaks. Yang and McKeever (1990) attributed the 380 nm emission to recombination of electrons at [H3 O4 ]◦ centers. According to previous investigations, the peaks appearing above 200 ◦ C seem to be associated to electron–hole recombination at [AlO4 ]◦ centers (Halliburton, 1985). Additional TL peaks occurring at 140, 160, 245, 280, 375 and 480 ◦ C have been observed in glow curves of quartz (Ichikawa, 1968; Jani et al., 1983; Wintle, 1997; Roque et al., 2004). However, much less is known about the pre-dose sensitization
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of these peaks, especially for pre-doses in the range of kGy. Recently we have reported the sensitization of the peak between 200 and 300 ◦ C after the administration of high doses (25–175 kGy) followed by heat-treatment at 400 ◦ C (Guzzo et al., 2006; Khoury et al., 2007). After the sensitization it was possible to observe this peak with doses in range of 10–20 mGy. Compared to previous studies, the main differences of our sensitization process can be summarized as follows: (i) the sensitized peak is not observed in as-received samples; (ii) the pre-dose is much higher than those usually reported; (iii) considering the background signal as a reference, a sensitization factor of 5000 is obtained using a test-dose of 20 mGy; (iv) the sample is a single crystal prepared from a natural quartz with high Li/Al and Li/OH impurity content ratios. Based on these reports, the aim of this paper is to investigate the combined effect of heat-treatment and high doses of irradiation for the sensitization of the TL peak appearing between 200 and 300 ◦ C in the glow curve of natural quartz. 2. Experimental A single crystal with 500 g taken from the Solonópole deposit located in the Northeast region of Brazil was cut into plates with 2 mm thick parallel to one of its rhombohedral face. The plates were lapped with Al2 O3 with decreasing grit sizes to obtain samples with 1 mm thick. Using a stationary ultrasonic machining apparatus, 30 discs with diameter of 6 mm were extracted from crystal plates with a cutting tool made by stainless steel and SiC grits with 38 m. The samples were cleaned with acetone and their weights were determined with an analytical balance (0.0001 g). Afterwards, they were annealed in a muffle furnace as follows: continuous heating up to 400 ◦ C, annealing for one hour at 400 ◦ C, cooling, annealing for two hours at 100 ◦ C, and cooling. This thermal cycle was adopted as the standard annealing procedure throughout this work. In order to evaluate the TL response of Solonópole quartz prior to any sensitization process (we call here “as-received” condition) all samples were irradiated with 60 Co radiation with a dose of 10 Gy. The sequence for irradiation-TL reading-annealing was repeated three times. After that, the samples were separated into four lots according to its TL output between 160 and 320 ◦ C. The lot 1 was irradiated with -rays of 60 Co in a cell irradiator with a dose rate of 10 kGy h−1 . The initial pre-doses were 2 and 3 kGy. Then, the pre-doses were administrated in steps of 5 kGy up to a cumulative dose of 25 kGy. After each step, three annealing cycles were successively performed in order to guarantee the release of charge carriers from the trap levels. The lots 2, 3 and 4 were heat treated during two hours at 500, 800 and 1000 ◦ C, respectively. The heat-treatments were performed in a muffle furnace at atmospheric pressure with a heating rate of 5 ◦ C min−1 . Later on, these lots were submitted to a single pre-dose of 25 kGy. After each sensitization step, the four lots were irradiated in air with -rays of a 137 Cs source (43 mGy h−1 ) with doses in the range of 0.1–30 mGy. In order to guarantee the electronic equilibrium was attained, the samples were fixed behind a 5 mm thick acrylic plate and placed in the center of the radiation beam.
The TL response was carried out with a 2800 M Victoreen reader by using the step-heating mode. The step parameters were set at 10 s from room temperature to 160 ◦ C and at 20 s from 160 to 320 ◦ C. The intensity of the TL emission was evaluated by integrating the area under the peak appearing in the second region and the sensitivity was defined as the TL intensity per unit of mass (nC mg−1 ). Typical TL glow curves were recorded using a Harshaw 3500 reader with a heating rate equal to 2 ◦ C s−1 in order to determine the position of the TL peaks. The formation of [AlO4 ]◦ centers in each quartz lot was evaluated by optical absorption spectroscopy. For this, specimens with 2 mm thick were prepared from the same crystal and submitted to the same sensitization procedures. After each step of sensitization, ultraviolet–visible (UV–VIS) spectra were carried out using a Lambda-6 Perkin Elmer spectrometer. The absorption band appearing near 470 nm, associated with the EPR (electron paramagnetic resonance) signal of [AlO4 ]◦ center, was evaluated by the Beer–Lambert equation (Koumvakalis, 1980; Guzzo et al., 1997). 3. Results Typical glow curves are shown in Fig. 1 prior and after the sensitization with heat-treatment and high doses. The glow curve in Fig. 1(a) corresponds to TL signal of Solonópole quartz in its as-received condition, i.e., just after performing annealing and irradiation with 10 Gy. An intense TL peak is observed at 90 ◦ C which corresponds to the so-called 110 ◦ C peak of quartz. A weak increment on TL signal is noticed at around 130 and 325 ◦ C, as can be observed in the insert plot of Fig. 1(a). The TL signal integrated from 160 to 320 ◦ C is negligible compared to the output related to the first peak. The effect of the heat-treatment on TL glow curves is shown in Fig. 1(b). The first peak is observed near 120 ◦ C and its intensity is higher for samples heat-treated with 1000 ◦ C. This peak matches the contribution from those signals observed at 90 and 130 ◦ C in the as-received condition. The ground level for the TL intensity above 200 ◦ C is higher for the sample heattreated at 1000 ◦ C and there is evidence for a glow peak near 215 ◦ C in the inset plot. Fig. 1(c) shows the glow curve after the cumulative dose of 25 kGy. The glow region up to 160 ◦ C is similar to that observed in the as-received condition with the first peak appearing near 90 ◦ C. In the second region, a strong TL peak appears centered at 270 ◦ C, which is responsible to the TL output from 160 to 320 ◦ C. The combined effect of heat-treatment and high dose on the TL response of Solonópole quartz is shown in Fig. 1(d). The first peak appears in the temperature range of 90 ◦ C but the position of the second peak is dependent on the temperature in which the heat-treatment was performed. Such peak is centered at 270, 295 and 305 ◦ C for samples heated at 500, 800 and 1000 ◦ C, respectively. Fig. 2 shows the TL response for sample’s lots sensitized in different conditions. Here, the TL intensity corresponds to the TL signal integrated from 160 to 320 ◦ C. Fig. 2(a) shows that
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Fig. 1. TL glow curves of natural quartz from Solonópole recorded before and after the sensitization with heat-treatments and high doses: (a) as-received, (b) heat-treated, (c) 25 kGy, and (d) heat-treated and 25 kGy (heating rate : 2 ◦ C s−1 ).
the TL intensities increase monotonically with the increase in the test-doses. Although the standard deviations associated to TL values measured with test-doses higher than 10 mGy are not negligible, the factors (r 2 ) associated to the linear fitting are better than 0.995. Similar TL response vs. test-dose dependencies are shown in Fig. 2(b) for those lots submitted to heat-treatment and irradiation with 25 kGy. The slope of the linear fitting is slightly higher for samples heat-treated with 500 and 800 ◦ C when compared with the slopes shown in Fig. 2(a). The reason for the lower TL responses recorded for the lot heat-treated with 1000 ◦ C is an artifact caused by the shift of the second peak to higher temperatures. As can be observed in Fig. 1(d), the tail of this peak dropped out the region set for the TL readout. Comparing the standard deviations associated to the mean values shown in Fig. 2, one observes that they are lower for the lots 2, 3 and 4 which were heated prior to the administration of the pre-dose. UV–VIS absorption spectra of as-received and sensitized samples with high doses are shown in Fig. 3. The band centered at 470 nm increases with the increasing of high dose.
The UV–VIS spectra of the heat-treated samples are similar as that shown in Fig. 3 for the as-received condition. After irradiating with a single dose of 25 kGy, the sample heat-treated at 1000 ◦ C showed a higher absorption near 470 nm compared to untreated and heat-treated samples at 500 and 800 ◦ C. Fig. 4 summarizes the effect of high doses on the absorption coefficient at 470 nm (470 ) and on the slope of TL responses shown in Fig. 2(a). It is observed that 470 increase always with high doses suggesting a continuous formation of [AlO4 ]◦ centers. On the other hand, the slope of the TL response increases up to 15 kGy remaining unchanged for additional doses. These observations suggest that the saturation in TL response above 15 kGy is probably associated with a limit in the concentration of electron traps created by high doses in this crystal. 4. Discussion Yang and McKeever (1990) furnished convincing evidences that the TL peak at 110 ◦ C is produced by the recombination of electrons with holes trapped at [H3 O4 ]◦ and [AlO4 ]◦ centers.
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The thermal anneal undergone by the EPR signal of [AlO4 ]◦ centers in the region 200–300 ◦ C reported by Jani et al. (1983) and Halliburton (1985) showed that TL signals recorded in this range of temperature are associated with the electron–hole recombination at [AlO4 ]◦ . The increase in 470 with high doses shown in Fig. 3 lend support to the assumption that [AlO4 ]◦ center is concerned with the strong TL emission observed at 270 ◦ C. However, one of the difficulties in interpreting the TL response of natural quartz in a consistent manner is the fact that the identity of the electron traps for both 110 ◦ C and 200–300 ◦ C peaks remain unclear. In the next paragraph an attempt to explain the mechanism involved with the creation of the 270 ◦ C peak is proposed. According to the results shown in Fig. 4, it is assumed that pre-doses with -rays produced remarkable changes in the point defect structure of quartz. The saturation observed in TL response for doses higher than 15 kGy is probably associated
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with saturation in electron trap creation because the number of [AlO4 ]◦ was still increasing. In the as-received condition, Al-centers in natural quartz are compensated by H+ or alkalis giving rise to [AlO4 /H+ ]◦ or [AlO4 /M+ ]◦ . Based on the relationship between impurity content and the darkening grade produced by irradiation, it was shown that [AlO4 /M+ ]◦ is mainly compensated by Li+ in natural quartz (Guzzo et al., 1997). It was also shown that [AlO4 /M+ ]◦ is easily changed to [AlO4 ]◦ when both natural or synthetic quartz is irradiated at room temperature (Halliburton, 1985; Guzzo et al., 1997). Besides aluminum centers, oxygen-vacancy-associated centers form an important class of point defects in quartz usually referred to as E centers (Halliburton, 1985). As it was firstly reported by Weeks and Nelson (1960), if the crystal is heated to 300 ◦ C after the room temperature irradiation, more than one order of magnitude enhancement in the concentration of
H.J. Khoury et al. / Radiation Measurements 43 (2008) 487 – 491
E centers is observed. Taking into account these observations, it is suggested that the pre-dose with -rays is able to create a number of acceptor sites to host the Li+ ions that were released from the [AlO4 /Li+ ]◦ centers. The annealing at 400 ◦ C shall be responsible for the creation of E centers that act as stable electron traps during the irradiation with the test-dose. At the same time, the annealing shall restore the [AlO4 /Li+ ]◦ centers which are the precursors for [AlO4 ]◦ . 5. Concluding remarks The TL responses recorded in Solonópole quartz made clear that the creation of the TL peak near 270 ◦ C is, amongst other things, controlled by the administration of pre-doses of -rays like 15 kGy followed by annealing at 400 ◦ C. The heattreatments performed at 500 ◦ C prior to high dose did not change the TL intensity of this peak but contributed to decrease the standard deviation around the mean values of TL. Further attempts are required to explain the shift noticed in this peak when quartz was heated at 800 and 1000 ◦ C. The saturation in TL response for cumulative doses higher than 15 kGy is probably connected with a maximum concentration of point defects, created by pre-dose plus annealing, acting as electron traps in the TL emission. From the results shown in this work, it is concluded that heat-treatment at 500 ◦ C followed by irradiation with 25 kGy and annealing at 400 ◦ C shall be a suitable procedure for future use of Solonópole quartz in TL dosimetry of doses as lower as 0.1 mGy. Acknowledgments The authors wish to thank Dra. Maria Betânia P. Pinheiro and Mr. Ricardo Sena from DNPM (10th District) for their support during the visit of Solonópole (CE). The authors are also grateful to Dr. Manfred Schwartz (Central Analítica, UFPE) to get them the permission to use the UV–VIS spectrometer. This work was supported in part by CNPq and LMRI/UFPE. References Bailey, R.M., 2001. Towards a general kinetic model for optically and thermally stimulated luminescence of quartz. Radiat. Meas. 33, 17–45. Bailiff, I.K., Haskell, E.H., 1983. The use of the pre-dose technique for environmental dosimetry. Radiat. Prot. Dosim. 6, 245–248.
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Benny, P.G., Bhatt, B.C., 1997. Sensitization of 220 ◦ C TL peak in quartz separated from sand. Radiat. Meas. 27, 67–69. Benny, P.G., Sanjeev, N., Gundu Rao, T.K., Bhatt, B.C., 2000. Gamma ray induced sensitization of 110 ◦ C TL peak in quartz separated from sand. Radiat. Meas. 32, 247–252. Chen, R., Yang, X.H., McKeever, S.W.S., 1988. The strong superlinear dose dependence of thermoluminescence in synthetic quartz. J. Phys. D 21, 1452–1457. Guzzo, P.L., Iwasaki, I., Iwasaki, H., 1997. Al-related centers in relation to -irradiation response in natural quartz. Phys. Chem. Miner. 27, 254–263. Guzzo, P.L., Khoury, H.J., Souza, C.P., Sóuza Jr., A.M., Schwartz, M.O.E., Azevedo, W.A., 2006. Defect analysis in natural quartz from Brazilian sites for ionizing radiation dosimetry. Radiat. Prot. Dosim. 119, 168–171. Halliburton, L.E., 1985. Defects models and radiation damage mechanisms in alpha-quartz. Cryst. Latt. Defects Amorph. Mater 12, 163–190. Ichikawa, Y., 1968. Thermoluminescence of natural quartz irradiated by gamma rays. Jpn. J. Appl. Phys. 7, 220–226. Itoh, N., 2002. Ionic and electronic processses in quartz: mechanisms of thermoluminescence and optically stimulated luminescence. J. Appl. Phys. 92, 5036–5044. Jani, M.G., Halliburton, L.E., Kohnke, E.E., 1983. Point defects in SiO2 : thermally stimulated luminescence above room temperature. J. Appl. Phys. 54, 6321–6328. 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, 101–107. Koumvakalis, N., 1980. Defects in crystalline SiO2 : optical absorption of aluminum-associated hole center. J. Appl. Phys. 51, 5528–5582. Kristianpoller, N., Abu-Rayya, M., Shmilevich, A., Chen, R., 1997. Effects of pre-dose treatment on the thermoluminescence in synthetic quartz. J. Lumin. 72–74, 687–688. Petrov, S.A., Bailiff, I.K., 1997. Determination of trap depths associated with TL peaks in synthetic quartz (350–550 K). Radiat. Meas. 27, 185–191. Roque, C., Guibert, P., Duttine, M., Vartanian, E., Chapoulie, R., Bechtel, F., 2004. Dependence of luminescence characteristics of irradiated quartz with the thermal treatment and consequences for TL dating. Geochronometria 23, 1–8. Weeks, R.A., Nelson, C.M., 1960. Tapped electrons in irradiated quartz and silica: II. Electron spin resonance. J. Am. Ceram. Soc. 43, 399–404. Wintle, A.G., 1997. Luminescence dating: laboratory procedures and protocols. Radiat. Meas. 27, 769–817. Yang, X.H., McKeever, S.W.S., 1990. The pre-dose effect in crystalline quartz. J. Phys. D 23, 237–244. Zimmerman, J., 1971. The radiation-induced increase of the 100 ◦ C thermoluminescence sensitivity of fired quartz. J. Phys. C 4, 3265–3276.
TL dosimetry of natural quartz sensitized by heat-treatment and high dose irradiation H.J. Khoury a,∗ , P.L. Guzzo b , L.B.F. Souza a , T.M.B. Farias c , S. Watanabe c a Department of Nuclear Energy, Federal University of Pernambuco, 50740-540 CDU Recife, PE, Brazil b Department of Mining Engineering, Federal University of Pernambuco, 50740-530 CDU Recife, PE, Brazil c Institute of Physics, University of São Paulo, 05509-001 Cidade Universitária, São Paulo, SP, Brazil
Abstract The aim of this paper is to report the sensitization of the TL peak appearing at 270 ◦ C in the glow curve of natural quartz by using the combined effect of heat-treatments and irradiation with high doses. For this, thirty discs with 6 × 1 mm2 were prepared from plates parallell to a rhombohedral crystal face. The specimens were separated into four lots according to its TL read out between 160 and 320 ◦ C. One lot was submitted to doses of 60 Co radiation starting at 2 kGy and going up until a cumulative dose of 25 kGy. The other three lots were initially heat-treated at 500, 800 and 1000 ◦ C and then irradiated with a single dose of 25 kGy. The TL response of each lot was determined as a function of test-doses ranging from 0.1 to 30 mGy. As a result, it was observed that heat-treatments themselves did not produce the strong peak at 270 ◦ C that was observed after the administration of high doses. This peak is associated with the optical absorption band appearing at 470 nm which is due to the formation of [AlO4 ]◦ acting as electron–hole recombination centers. The formation of the 270 ◦ C peak was preliminary analyzed in relation to aluminum- and oxygen-vacancy-related centers found in crystalline quartz. © 2008 Elsevier Ltd. All rights reserved. Keywords: Quartz single crystal; Thermoluminescence; -rays; Pre-dose; Heat-treatment; Al-hole center
1. Introduction Sensitization is the ability to increase the sensitivity of a thermoluminescent (TL) phosphor using pre-doses and/or heattreatments. Zimmerman (1971) was one of the first to report that the TL sensitivity of the 110 ◦ C peak of quartz appears after the absorption of ionizing radiation at room temperature (pre-dose), followed by heating at approximately 500 ◦ C. Later, sensitization of 110, 220 and 325 ◦ C TL peaks were observed in quartz prepared from single crystals, ancient ceramics and geological sediments using or pre-doses in the range of 50 Gy to 4 kGy followed by heat-treatments from 200 up to 1200 ◦ C (Bailiff and Haskell, 1983; Chen et al., 1988; Yang and McKeever, 1990; Benny and Bhatt, 1997; Kristianpoller et al., 1997; Petrov and Bailiff, 1997; Wintle, 1997; Benny et al., 2000). In these studies, the test-dose used to determine the sensitization factor in relation to the TL response of the ∗ Corresponding author. Tel.: +55 81 2126 8708; fax: +55 81 2126 7988.
E-mail address: [email protected] (H.J. Khoury). 1350-4487/$ - see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2008.01.028
normal sample ranged from 0.1 to 50 Gy. Pre-dose and optical sensitization of 110 and 325 ◦ C peaks was reproduced with a satisfactory level of accuracy by the kinetic models describing the luminescent behavior of quartz grains (Bailey, 2001; Itoh, 2002). According to these studies, the sensitivity of both 110 and 325 ◦ C TL peaks are affected by the redistribution of charges that occurs between optically stimulated traps during illumination at room temperature and subsequent heating. It was also established that the maximum in TL emission of both peaks occurs at 380 nm suggesting that the same recombination centers are taking part in the TL process of these peaks. Yang and McKeever (1990) attributed the 380 nm emission to recombination of electrons at [H3 O4 ]◦ centers. According to previous investigations, the peaks appearing above 200 ◦ C seem to be associated to electron–hole recombination at [AlO4 ]◦ centers (Halliburton, 1985). Additional TL peaks occurring at 140, 160, 245, 280, 375 and 480 ◦ C have been observed in glow curves of quartz (Ichikawa, 1968; Jani et al., 1983; Wintle, 1997; Roque et al., 2004). However, much less is known about the pre-dose sensitization
488
H.J. Khoury et al. / Radiation Measurements 43 (2008) 487 – 491
of these peaks, especially for pre-doses in the range of kGy. Recently we have reported the sensitization of the peak between 200 and 300 ◦ C after the administration of high doses (25–175 kGy) followed by heat-treatment at 400 ◦ C (Guzzo et al., 2006; Khoury et al., 2007). After the sensitization it was possible to observe this peak with doses in range of 10–20 mGy. Compared to previous studies, the main differences of our sensitization process can be summarized as follows: (i) the sensitized peak is not observed in as-received samples; (ii) the pre-dose is much higher than those usually reported; (iii) considering the background signal as a reference, a sensitization factor of 5000 is obtained using a test-dose of 20 mGy; (iv) the sample is a single crystal prepared from a natural quartz with high Li/Al and Li/OH impurity content ratios. Based on these reports, the aim of this paper is to investigate the combined effect of heat-treatment and high doses of irradiation for the sensitization of the TL peak appearing between 200 and 300 ◦ C in the glow curve of natural quartz. 2. Experimental A single crystal with 500 g taken from the Solonópole deposit located in the Northeast region of Brazil was cut into plates with 2 mm thick parallel to one of its rhombohedral face. The plates were lapped with Al2 O3 with decreasing grit sizes to obtain samples with 1 mm thick. Using a stationary ultrasonic machining apparatus, 30 discs with diameter of 6 mm were extracted from crystal plates with a cutting tool made by stainless steel and SiC grits with 38 m. The samples were cleaned with acetone and their weights were determined with an analytical balance (0.0001 g). Afterwards, they were annealed in a muffle furnace as follows: continuous heating up to 400 ◦ C, annealing for one hour at 400 ◦ C, cooling, annealing for two hours at 100 ◦ C, and cooling. This thermal cycle was adopted as the standard annealing procedure throughout this work. In order to evaluate the TL response of Solonópole quartz prior to any sensitization process (we call here “as-received” condition) all samples were irradiated with 60 Co radiation with a dose of 10 Gy. The sequence for irradiation-TL reading-annealing was repeated three times. After that, the samples were separated into four lots according to its TL output between 160 and 320 ◦ C. The lot 1 was irradiated with -rays of 60 Co in a cell irradiator with a dose rate of 10 kGy h−1 . The initial pre-doses were 2 and 3 kGy. Then, the pre-doses were administrated in steps of 5 kGy up to a cumulative dose of 25 kGy. After each step, three annealing cycles were successively performed in order to guarantee the release of charge carriers from the trap levels. The lots 2, 3 and 4 were heat treated during two hours at 500, 800 and 1000 ◦ C, respectively. The heat-treatments were performed in a muffle furnace at atmospheric pressure with a heating rate of 5 ◦ C min−1 . Later on, these lots were submitted to a single pre-dose of 25 kGy. After each sensitization step, the four lots were irradiated in air with -rays of a 137 Cs source (43 mGy h−1 ) with doses in the range of 0.1–30 mGy. In order to guarantee the electronic equilibrium was attained, the samples were fixed behind a 5 mm thick acrylic plate and placed in the center of the radiation beam.
The TL response was carried out with a 2800 M Victoreen reader by using the step-heating mode. The step parameters were set at 10 s from room temperature to 160 ◦ C and at 20 s from 160 to 320 ◦ C. The intensity of the TL emission was evaluated by integrating the area under the peak appearing in the second region and the sensitivity was defined as the TL intensity per unit of mass (nC mg−1 ). Typical TL glow curves were recorded using a Harshaw 3500 reader with a heating rate equal to 2 ◦ C s−1 in order to determine the position of the TL peaks. The formation of [AlO4 ]◦ centers in each quartz lot was evaluated by optical absorption spectroscopy. For this, specimens with 2 mm thick were prepared from the same crystal and submitted to the same sensitization procedures. After each step of sensitization, ultraviolet–visible (UV–VIS) spectra were carried out using a Lambda-6 Perkin Elmer spectrometer. The absorption band appearing near 470 nm, associated with the EPR (electron paramagnetic resonance) signal of [AlO4 ]◦ center, was evaluated by the Beer–Lambert equation (Koumvakalis, 1980; Guzzo et al., 1997). 3. Results Typical glow curves are shown in Fig. 1 prior and after the sensitization with heat-treatment and high doses. The glow curve in Fig. 1(a) corresponds to TL signal of Solonópole quartz in its as-received condition, i.e., just after performing annealing and irradiation with 10 Gy. An intense TL peak is observed at 90 ◦ C which corresponds to the so-called 110 ◦ C peak of quartz. A weak increment on TL signal is noticed at around 130 and 325 ◦ C, as can be observed in the insert plot of Fig. 1(a). The TL signal integrated from 160 to 320 ◦ C is negligible compared to the output related to the first peak. The effect of the heat-treatment on TL glow curves is shown in Fig. 1(b). The first peak is observed near 120 ◦ C and its intensity is higher for samples heat-treated with 1000 ◦ C. This peak matches the contribution from those signals observed at 90 and 130 ◦ C in the as-received condition. The ground level for the TL intensity above 200 ◦ C is higher for the sample heattreated at 1000 ◦ C and there is evidence for a glow peak near 215 ◦ C in the inset plot. Fig. 1(c) shows the glow curve after the cumulative dose of 25 kGy. The glow region up to 160 ◦ C is similar to that observed in the as-received condition with the first peak appearing near 90 ◦ C. In the second region, a strong TL peak appears centered at 270 ◦ C, which is responsible to the TL output from 160 to 320 ◦ C. The combined effect of heat-treatment and high dose on the TL response of Solonópole quartz is shown in Fig. 1(d). The first peak appears in the temperature range of 90 ◦ C but the position of the second peak is dependent on the temperature in which the heat-treatment was performed. Such peak is centered at 270, 295 and 305 ◦ C for samples heated at 500, 800 and 1000 ◦ C, respectively. Fig. 2 shows the TL response for sample’s lots sensitized in different conditions. Here, the TL intensity corresponds to the TL signal integrated from 160 to 320 ◦ C. Fig. 2(a) shows that
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Fig. 1. TL glow curves of natural quartz from Solonópole recorded before and after the sensitization with heat-treatments and high doses: (a) as-received, (b) heat-treated, (c) 25 kGy, and (d) heat-treated and 25 kGy (heating rate : 2 ◦ C s−1 ).
the TL intensities increase monotonically with the increase in the test-doses. Although the standard deviations associated to TL values measured with test-doses higher than 10 mGy are not negligible, the factors (r 2 ) associated to the linear fitting are better than 0.995. Similar TL response vs. test-dose dependencies are shown in Fig. 2(b) for those lots submitted to heat-treatment and irradiation with 25 kGy. The slope of the linear fitting is slightly higher for samples heat-treated with 500 and 800 ◦ C when compared with the slopes shown in Fig. 2(a). The reason for the lower TL responses recorded for the lot heat-treated with 1000 ◦ C is an artifact caused by the shift of the second peak to higher temperatures. As can be observed in Fig. 1(d), the tail of this peak dropped out the region set for the TL readout. Comparing the standard deviations associated to the mean values shown in Fig. 2, one observes that they are lower for the lots 2, 3 and 4 which were heated prior to the administration of the pre-dose. UV–VIS absorption spectra of as-received and sensitized samples with high doses are shown in Fig. 3. The band centered at 470 nm increases with the increasing of high dose.
The UV–VIS spectra of the heat-treated samples are similar as that shown in Fig. 3 for the as-received condition. After irradiating with a single dose of 25 kGy, the sample heat-treated at 1000 ◦ C showed a higher absorption near 470 nm compared to untreated and heat-treated samples at 500 and 800 ◦ C. Fig. 4 summarizes the effect of high doses on the absorption coefficient at 470 nm (470 ) and on the slope of TL responses shown in Fig. 2(a). It is observed that 470 increase always with high doses suggesting a continuous formation of [AlO4 ]◦ centers. On the other hand, the slope of the TL response increases up to 15 kGy remaining unchanged for additional doses. These observations suggest that the saturation in TL response above 15 kGy is probably associated with a limit in the concentration of electron traps created by high doses in this crystal. 4. Discussion Yang and McKeever (1990) furnished convincing evidences that the TL peak at 110 ◦ C is produced by the recombination of electrons with holes trapped at [H3 O4 ]◦ and [AlO4 ]◦ centers.
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The thermal anneal undergone by the EPR signal of [AlO4 ]◦ centers in the region 200–300 ◦ C reported by Jani et al. (1983) and Halliburton (1985) showed that TL signals recorded in this range of temperature are associated with the electron–hole recombination at [AlO4 ]◦ . The increase in 470 with high doses shown in Fig. 3 lend support to the assumption that [AlO4 ]◦ center is concerned with the strong TL emission observed at 270 ◦ C. However, one of the difficulties in interpreting the TL response of natural quartz in a consistent manner is the fact that the identity of the electron traps for both 110 ◦ C and 200–300 ◦ C peaks remain unclear. In the next paragraph an attempt to explain the mechanism involved with the creation of the 270 ◦ C peak is proposed. According to the results shown in Fig. 4, it is assumed that pre-doses with -rays produced remarkable changes in the point defect structure of quartz. The saturation observed in TL response for doses higher than 15 kGy is probably associated
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with saturation in electron trap creation because the number of [AlO4 ]◦ was still increasing. In the as-received condition, Al-centers in natural quartz are compensated by H+ or alkalis giving rise to [AlO4 /H+ ]◦ or [AlO4 /M+ ]◦ . Based on the relationship between impurity content and the darkening grade produced by irradiation, it was shown that [AlO4 /M+ ]◦ is mainly compensated by Li+ in natural quartz (Guzzo et al., 1997). It was also shown that [AlO4 /M+ ]◦ is easily changed to [AlO4 ]◦ when both natural or synthetic quartz is irradiated at room temperature (Halliburton, 1985; Guzzo et al., 1997). Besides aluminum centers, oxygen-vacancy-associated centers form an important class of point defects in quartz usually referred to as E centers (Halliburton, 1985). As it was firstly reported by Weeks and Nelson (1960), if the crystal is heated to 300 ◦ C after the room temperature irradiation, more than one order of magnitude enhancement in the concentration of
H.J. Khoury et al. / Radiation Measurements 43 (2008) 487 – 491
E centers is observed. Taking into account these observations, it is suggested that the pre-dose with -rays is able to create a number of acceptor sites to host the Li+ ions that were released from the [AlO4 /Li+ ]◦ centers. The annealing at 400 ◦ C shall be responsible for the creation of E centers that act as stable electron traps during the irradiation with the test-dose. At the same time, the annealing shall restore the [AlO4 /Li+ ]◦ centers which are the precursors for [AlO4 ]◦ . 5. Concluding remarks The TL responses recorded in Solonópole quartz made clear that the creation of the TL peak near 270 ◦ C is, amongst other things, controlled by the administration of pre-doses of -rays like 15 kGy followed by annealing at 400 ◦ C. The heattreatments performed at 500 ◦ C prior to high dose did not change the TL intensity of this peak but contributed to decrease the standard deviation around the mean values of TL. Further attempts are required to explain the shift noticed in this peak when quartz was heated at 800 and 1000 ◦ C. The saturation in TL response for cumulative doses higher than 15 kGy is probably connected with a maximum concentration of point defects, created by pre-dose plus annealing, acting as electron traps in the TL emission. From the results shown in this work, it is concluded that heat-treatment at 500 ◦ C followed by irradiation with 25 kGy and annealing at 400 ◦ C shall be a suitable procedure for future use of Solonópole quartz in TL dosimetry of doses as lower as 0.1 mGy. Acknowledgments The authors wish to thank Dra. Maria Betânia P. Pinheiro and Mr. Ricardo Sena from DNPM (10th District) for their support during the visit of Solonópole (CE). The authors are also grateful to Dr. Manfred Schwartz (Central Analítica, UFPE) to get them the permission to use the UV–VIS spectrometer. This work was supported in part by CNPq and LMRI/UFPE. References Bailey, R.M., 2001. Towards a general kinetic model for optically and thermally stimulated luminescence of quartz. Radiat. Meas. 33, 17–45. Bailiff, I.K., Haskell, E.H., 1983. The use of the pre-dose technique for environmental dosimetry. Radiat. Prot. Dosim. 6, 245–248.
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Benny, P.G., Bhatt, B.C., 1997. Sensitization of 220 ◦ C TL peak in quartz separated from sand. Radiat. Meas. 27, 67–69. Benny, P.G., Sanjeev, N., Gundu Rao, T.K., Bhatt, B.C., 2000. Gamma ray induced sensitization of 110 ◦ C TL peak in quartz separated from sand. Radiat. Meas. 32, 247–252. Chen, R., Yang, X.H., McKeever, S.W.S., 1988. The strong superlinear dose dependence of thermoluminescence in synthetic quartz. J. Phys. D 21, 1452–1457. Guzzo, P.L., Iwasaki, I., Iwasaki, H., 1997. Al-related centers in relation to -irradiation response in natural quartz. Phys. Chem. Miner. 27, 254–263. Guzzo, P.L., Khoury, H.J., Souza, C.P., Sóuza Jr., A.M., Schwartz, M.O.E., Azevedo, W.A., 2006. Defect analysis in natural quartz from Brazilian sites for ionizing radiation dosimetry. Radiat. Prot. Dosim. 119, 168–171. Halliburton, L.E., 1985. Defects models and radiation damage mechanisms in alpha-quartz. Cryst. Latt. Defects Amorph. Mater 12, 163–190. Ichikawa, Y., 1968. Thermoluminescence of natural quartz irradiated by gamma rays. Jpn. J. Appl. Phys. 7, 220–226. Itoh, N., 2002. Ionic and electronic processses in quartz: mechanisms of thermoluminescence and optically stimulated luminescence. J. Appl. Phys. 92, 5036–5044. Jani, M.G., Halliburton, L.E., Kohnke, E.E., 1983. Point defects in SiO2 : thermally stimulated luminescence above room temperature. J. Appl. Phys. 54, 6321–6328. 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, 101–107. Koumvakalis, N., 1980. Defects in crystalline SiO2 : optical absorption of aluminum-associated hole center. J. Appl. Phys. 51, 5528–5582. Kristianpoller, N., Abu-Rayya, M., Shmilevich, A., Chen, R., 1997. Effects of pre-dose treatment on the thermoluminescence in synthetic quartz. J. Lumin. 72–74, 687–688. Petrov, S.A., Bailiff, I.K., 1997. Determination of trap depths associated with TL peaks in synthetic quartz (350–550 K). Radiat. Meas. 27, 185–191. Roque, C., Guibert, P., Duttine, M., Vartanian, E., Chapoulie, R., Bechtel, F., 2004. Dependence of luminescence characteristics of irradiated quartz with the thermal treatment and consequences for TL dating. Geochronometria 23, 1–8. Weeks, R.A., Nelson, C.M., 1960. Tapped electrons in irradiated quartz and silica: II. Electron spin resonance. J. Am. Ceram. Soc. 43, 399–404. Wintle, A.G., 1997. Luminescence dating: laboratory procedures and protocols. Radiat. Meas. 27, 769–817. Yang, X.H., McKeever, S.W.S., 1990. The pre-dose effect in crystalline quartz. J. Phys. D 23, 237–244. Zimmerman, J., 1971. The radiation-induced increase of the 100 ◦ C thermoluminescence sensitivity of fired quartz. J. Phys. C 4, 3265–3276.