- Email: [email protected]
Correlation of optically and thermally stimulated luminescence of natural fluorite pellets
Radiation Measurements xxx (2014) 1e4
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Correlation of optically and thermally stimulated luminescence of natural fluorite pellets F.A. Ferreira Jr. *, E.M. Yoshimura, N.K. Umisedo, R.P. do Nascimento Instituto de Física, Universidade de São Paulo, P.O. Box 66318, CEP 05314-970 São Paulo, SP, Brazil
h i g h l i g h t s CaF2:NaCl pellets are extremelly useful as dosimetric materials. Only part of the defects responsible for the TL signal from fluorite is optically active. NaCl signals affects the TL and OSL signals from the pellets. NaCl OSL signal can be avoided with a pre-heat or after about 1 h waiting time.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 December 2013 Received in revised form 23 January 2014 Accepted 20 February 2014
Natural fluorite (CaF2), a dosimetric material of large usage, presents Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL). This study examined the behavior of TL and OSL (stimulated with Blue LEDs) signals from the Brazilian natural fluorite pellets with NaCl as binding agent, as well as their correlations, in order to study and optimize the dosimetric process with this material. A series of experiments were conducted, basically with thermal treatments before OSL acquisition, and optical bleaching before TL readout. The role of NaCl in the TL and OSL emission was investigated. It was observed that natural CaF2 TL signal is still ample to be used in dosimetric applications, as dose reassessment in personal dosimetry after an OSL measurement. Also it was verified that the fluorite OSL signal is extinguished by a 350 C heating and that NaCl has no contribution to the stable part of the OSL signal. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Optically stimulated luminescence Thermoluminescence Natural fluorite
1. Introduction Both natural and synthetic calcium fluoride are widely used as dosimetric TL material mainly due to their high sensitivity. In particular, the use of natural fluorite has extended to many places because of its abundance and low cost. In Brazil we have developed a solid dosimeter, based on natural calcium fluoride and sodium chloride powders pressed together (CaF2:NaCl) (Okuno et al., 1977; Trzeniak et al., 1990) for dosimetric applications. These pellets are extremely useful as dosimetric materials because they can provide information on dose and radiation energy with the use of appropriated filters (Guimarães and Okuno, 2003). Recently, it has been observed that both Indian (Sunta, 1970; Chougaonkar and Bhatt, 2004) and Brazilian (Yoshimura and Yukihara, 2006) fluorite also
* Corresponding author. Tel.: þ55 (0)11 30910849. E-mail address: [email protected] (F.A. Ferreira).
emit OSL when blue light is used for stimulation, and that the emitted light is related to the absorbed dose. In this paper we explore the OSL and TL properties of the fluorite pellets in order to verify the possibility of expanding the dosimetric capabilities with the use of combined OSL and TL emissions. Studies of correlation of OSL and TL signals in fluorite are rare (Polymeris et al., 2006; Chougaonkar and Bhatt, 2004; Bakshi et al., 2009). They show that not all the traps related to TL peaks are optically active and that both photo and thermal transfer processes may be present. The addition of NaCl and the production of pressed pellets with CaF2 powder may affect the correlation of OSL and TL signals. This occurs because the NaCl is itself a luminescent material and the pellet thickness (1 mm in average) may prevent uniform illumination or homogeneous deposition of radiation energy in the pellet (Moralles et al., 2005). Thus, this work aims to investigate the influence of TL readouts on the OSL response and the effect of blue illumination in the TL intensity of natural CaF2:NaCl pellet dosimeters. Thus, it is expected that a better performance of this dosimeter will be achieved.
http://dx.doi.org/10.1016/j.radmeas.2014.02.020 1350-4487/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Ferreira, Jr., F.A., et al., Correlation of optically and thermally stimulated luminescence of natural fluorite pellets, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.02.020
2
F.A. Ferreira Jr. et al. / Radiation Measurements xxx (2014) 1e4
2. Materials and methods The materials used in this work are natural fluorite and sodium chloride (99% pure). Natural fluorite acquired as-from-the-mine was transformed into powder as described by Trzeniak et al. (1990). Sodium chloride was used as binding agent. Aliquots of pure NaCl or of a mixture containing 60% CaF2 and 40% NaCl were compressed to make 5 mm diameter 1 mm thickness (w45 mg mass) pellets. The dosimeters are protected from humidity with storage in dry ambient or with plastic enveloping, to avoid influence of this condition in TL or OSL signals. Before the first use and between subsequent uses of the same sample in a experiment, the sample was heated at 5 C/s up to 450 C, as in a TL acquisition, to empty the traps and erase the TL and OSL signal. The variation of NaCl and CaF2:NaCl pellets sensitivity to repeated irradiation-readout (TL/OSL) cycles was tested over 100 cycles and the statistical dispersion is compatible with the uncertainties presented, without trends. Two groups of ten samples each were selected in accordance with the signal intensity after the same irradiation time under the beta source. One group of pellets was made of pure NaCl and the other of CaF2:NaCl (60:40); each group of selected pellets presented 3.0% standard deviation of the mean OSL signal. The experimental data that are shown in this work are the mean value from these group measurements with their respective uncertainties. OSL and TL measurements were carried out using a commercial automated TL/OSL reader produced by Risø National Laboratory (model DA-20). Luminescence was stimulated using blue light emitting diodes (470 nm, FWHM ¼ 20 nm) delivering 80 mW/cm2 at the sample position. The TL/OSL signals were detected with a bialkali photomultiplier tube (PMT) behind an UV transmitting broad-band glass filter (Hoya U-340, 7.5 mm thick 45 mm diameter) to block the stimulation light while transmitting part of the OSL signal from the samples. Irradiations were performed at room temperature using the built-in 90Sr/90Y beta source of the TL/ OSL reader (dose rate of 11 mGy/s). The stimulation time for OSL measurements was 100 s. Integrated OSL signals were calculated as first 50 s counts considering the last 50 s as background. TL glow curves were obtained using a heating rate of 5 C/s. 3. Results and discussions The experiments described below involve a series of irradiationreadout cycles. Some types of NaCl have shown vulnerability to these cycles, as shown by Polymeris et al. (2011). However, the TL glow curves from the NaCl samples used on this work show no peaks at temperatures higher than 300 C, as can be seen in Fig. 1. These TL glow curves are more similar to those presented by Ekendahl and Judas (2011) than by Polymeris et al. (2011) and may explain that no change of sensitivity of NaCl pellets to irradiationreadout cycles was observed in our work. Fig. 1 shows residual TL glow curves for CaF2:NaCl pellets after CW-OSL with different illumination times. The results for a NaCl pellet are in the insert, and we see that the TL signal is restricted to the low temperature region, and is completely extinguished after a light stimulation of just 5 s. Almost all the signal of the fluorite pellet comes from fluorite, and part of the TL is bleached by the optical stimulus. For a more complete analysis, we show in Fig. 2 the difference between the TL glow curve from a non-illuminated sample and TL glow curves after different illumination times. With this data it is possible to conclude that the shallow traps responsible for the low temperature part of the glow curve (below 150 C) are easily bleached out during the OSL readout e less than 5 s of illumination
Fig. 1. TL glow curves of CaF2:NaCl pellets obtained after different optical stimulation times. The lines connecting points are just guides for the eyes. The insert shows the TL signal of a NaCl pellet, freshly irradiated to the same dose, and after blue light stimulation during 5 s.
are necessary to remove these peaks from the glow curve. As far as the fluorite TL dosimetric peak is concerned, the effect of optical stimulation is to bleach the low temperature part of the peak leaving a very stable signal, centered near 350 C. In fact, after 500 s of illumination, a TL peak centered at 310 C, usually a part of the main peak, is completely bleached out. A satellite peak at 350 C survives the bleaching. Polymeris et al. (2006) have seen a similar effect in the study of natural fluorite, and have predicted the existence of such a peak through the deconvolution of the whole TL glow curve. Here we identify its presence by the bleaching out of the TL signal. A better correlation of TL and OSL signals can be seen in Fig. 3 where the relative integrated TL signal of the fluorite pellets, from room temperature to Tstop is compared to the relative residual OSL signal after a preheating till Tstop. Again it is possible to observe that the NaCl contribution to the OSL signal is connected with shallow traps. Also, we see that the fluorite shallow traps concentrate the
Fig. 2. Differences between the TL glow curve obtained without optical bleach and the TL glow curves after different stimulation times. The lines connecting points are just guides for the eyes.
Please cite this article in press as: Ferreira, Jr., F.A., et al., Correlation of optically and thermally stimulated luminescence of natural fluorite pellets, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.02.020
F.A. Ferreira Jr. et al. / Radiation Measurements xxx (2014) 1e4
Fig. 3. Residual OSL (R-OSL) signal after thermal bleaching up to Tstop compared with TL signal integrated from room temperature to Tstop. Values of R-OSL signals are relative to OSL signal at room temperature and values of TL signals are relative to TL acquisition up to 400 C. Lines connecting data points have been included as a guide for the eye.
majority of charge carriers responsible for a large fraction of the OSL signal. The fluorite OSL signal disappears after a TL readout till 350 C, and the OSL intensity decays very slowly with Tstop variations from about 100 to 250 C, showing a progressive releasing of charges from deeper traps, the same that are responsible for the second and third TL fluorite peaks. The absolute values of TL and OSL signals, obtained with the same set of filters and PMT, can be seen in Fig. 4 where OSL signals for different stimulation times (from 0.01 to 500 s) are compared with residual TL signal for CaF2:NaCl pellets. The insert shows the OSL signal as a function of unbleached minus bleached integrated signal. This curve shows a nonlinear correlation between OSL signal detected during bleaching and the amount of decline of the TL signal. The total light intensity available as TL output equals the very initial OSL signal. The total light emitted as OSL is about 5 times higher than the total TL. Part of the light loss could be due to
3
Fig. 5. Comparison between residual OSL signals from NaCl and CaF2:NaCl pellets as a function of time after irradiation. The data are percentages relative to the measurements obtained just after the irradiation (about 30 s after irradiation). Statistical uncertainties are smaller than the symbol sizes. Lines connecting data points have been included as a guide for the eye.
thermal quenching during TL readout, but this hypothesis has been ruled out by Polymeris et al. (2006). Without further analysis we can simply conclude that not all the OSL emitted light comes from the traps responsible for the TL peaks from 50 to 400 C. As this feature was also observed for another natural CaF2 sample (Polymeris et al., 2006), it probably has to due with intrinsic defects of this material, and also with the fact that, as a natural sample, this mineral has been exposed to natural radiation (soil and cosmic) since it was formed giving rise to filled-up deep traps. The fading curves of integrated OSL signals from CaF2:NaCl and NaCl pellets are shown in Fig. 5. The OSL signal from NaCl pellets is very unstable, being completely removed approximately 1 h after the irradiation, with the sample kept in the dark at room temperature. Regarding the OSL signal from the CaF2:NaCl pellets, a fraction of about 25% of the whole signal is still present after 5000 s. This light intensity is practically as large as the total intensity that can be collected with a TL readout till 400 C. 4. Conclusions
Fig. 4. Integrated OSL signal and residual TL (RTL) signal for CaF2:NaCl pellets after increasing optical stimulation times. The insert shows the OSL signal as a function of unbleached minus bleached glow-curves (TL-RTL). Lines joining data points have been included as a guide for the eye.
It was observed that only part of the defects responsible for the TL signal of fluorite is optically active, and that the defects associated to the OSL of this material are completely emptied by heating it to 350 C. This way, it is possible to use the TL of OSL emptied CaF2 pellets to re-assess doses, e.g. in personal dosimetry. It was not possible to verify the contribution of each TL trap individually to the OSL signal. The initial OSL decay of the pellet signals, either in the case of thermal bleaching or fading, is strongly influenced by the presence of NaCl in the sample composition. This was proved by performing the same experiments with pure NaCl pellets. The OSL signal of NaCl has fast fading after irradiation and its TL signal is almost completely erased after 5 s of illumination or after 150 C pre-heat. For this study, the whole glow-curve was included. Even the low temperature and low intensity TL peaks were considered in order to better understand the correlation between OSL and TL signals. However, for practical applications, such as personal dosimetry, it is usual to consider only the high temperature and high intensity TL peaks, due to their stability and sensitivity. In the case of CaF2:NaCl, the complex peak centered at 310e350 C plays this role.
Please cite this article in press as: Ferreira, Jr., F.A., et al., Correlation of optically and thermally stimulated luminescence of natural fluorite pellets, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.02.020
4
F.A. Ferreira Jr. et al. / Radiation Measurements xxx (2014) 1e4
Furthermore, the signal from NaCl has negligible influence on the intensity of this peak, even without any previous treatment, reinforcing the conclusion that the CaF2:NaCl pellet dosimetric capabilities can be expanded with the use of combined OSL and TL emissions. The results presented here characterize CaF2:NaCl as a possible OSL detector for use in personal dosimetry. The knowledge of the dose response curve and of the best optical zeroing procedure are important for this application and are under investigation for the development of technical aspects. Acknowledgements The authors would like to thank the Brazilian agencies São Paulo Research Foundation (FAPESP) (gs1),grants #2010/16437-0 and #2012/13534-0, National Council for Scientific and Technological Development (CNPq) (gs2) and National Institute of Science and Technology - Radiation Metrology in Medicine (INCT) (gs3). References Bakshi, A., Dhabekar, B., Rawat, N., Singh, S., Joshi, V., Kumar, V., 2009. Study on TL and OSL characteristics of indigenously developed CaF2:Mn phosphor. Nucl. Instrum. Meth. Phys. Res. Sect. B: Beam Interact. Mater. Atoms 267 (3), 548e 553.
Chougaonkar, M.P., Bhatt, B.C., 2004. Blue light stimulated luminescence in calcium fluoride, its characteristics and implications in radiation dosimetry. Radiat. Prot. Dosimetry 112 (2), 311e321. Ekendahl, D., Judas, L., 2011. Nacl as a retrospective and accident dosemeter. Radiat. Prot. Dosim. 145 (1), 36e44. Guimarães, C.C., Okuno, E., 2003. Blind performance testing of personal and environmental dosimeters based on TLD-100 and natural CaF2:NaCl. Radiat. Meas. 37 (2), 127e132. Moralles, M., Guimares, C.C., Okuno, E., 2005. Response of thermoluminescent dosimeters to photons simulated with the Monte Carlo method. Nucl. Instrum. Meth. Phys. Res. Sect. Accel. Spectrom. Detect. Assoc. Equip. 545 (12), 261e 268. Okuno, E., Ferreira, M.M., Cruz, M.T., Watanabe, S., 1977. Polycrystalline dosimetric discs of natural CaF2. In: Proceedings of Fifth International Conference on Luminescence Dosimetry, 14e17 February 1977, Giessen. Germany, pp. 189e196. Polymeris, G.S., Kitis, G., Kiyak, N.G., Sfamba, I., Subedi, B., Pagonis, V., 2011. Dissolution and subsequent re-crystallization as zeroing mechanism, thermal properties and component resolved dose response of salt (NaCl) for retrospective dosimetry. Appl. Radiat. Isot. 69 (9), 1255e1262. Polymeris, G.S., Kitis, G., Tsirliganis, N.C., 2006. Correlation between TL and OSL properties of CaF2:N. Nucl. Instrum. Meth. Phys. Res. Sect. B: Beam Interact. Mater. Atoms 251 (1), 133e142. Sunta, C.M., 1970. Thermoluminescence spectrum of gamma-irradiated natural calcium fluoride. J. Phys. C: Solid State Phys. 3 (9), 1978. Trzeniak, P., Yoshimura, E.M., Cruz, M.T., Okuno, E., 1990. Brazilian fluorite-based dosimetric pellets - history and post-use review. Radiat. Prot. Dosim. 34 (1e4), 167e170. Yoshimura, E.M., Yukihara, E., 2006. Optically stimulated luminescence: searching for new dosimetric materials. Nucl. Instrum. Meth. Phys. Res. Sect. B: Beam Interact. Mater. Atoms 250 (12), 337e341. Proceedings of the 13th International Conference on Radiation Effects in Insulators (REI-2005).
Please cite this article in press as: Ferreira, Jr., F.A., et al., Correlation of optically and thermally stimulated luminescence of natural fluorite pellets, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.02.020
Contents lists available at ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Correlation of optically and thermally stimulated luminescence of natural fluorite pellets F.A. Ferreira Jr. *, E.M. Yoshimura, N.K. Umisedo, R.P. do Nascimento Instituto de Física, Universidade de São Paulo, P.O. Box 66318, CEP 05314-970 São Paulo, SP, Brazil
h i g h l i g h t s CaF2:NaCl pellets are extremelly useful as dosimetric materials. Only part of the defects responsible for the TL signal from fluorite is optically active. NaCl signals affects the TL and OSL signals from the pellets. NaCl OSL signal can be avoided with a pre-heat or after about 1 h waiting time.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 December 2013 Received in revised form 23 January 2014 Accepted 20 February 2014
Natural fluorite (CaF2), a dosimetric material of large usage, presents Thermoluminescence (TL) and Optically Stimulated Luminescence (OSL). This study examined the behavior of TL and OSL (stimulated with Blue LEDs) signals from the Brazilian natural fluorite pellets with NaCl as binding agent, as well as their correlations, in order to study and optimize the dosimetric process with this material. A series of experiments were conducted, basically with thermal treatments before OSL acquisition, and optical bleaching before TL readout. The role of NaCl in the TL and OSL emission was investigated. It was observed that natural CaF2 TL signal is still ample to be used in dosimetric applications, as dose reassessment in personal dosimetry after an OSL measurement. Also it was verified that the fluorite OSL signal is extinguished by a 350 C heating and that NaCl has no contribution to the stable part of the OSL signal. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Optically stimulated luminescence Thermoluminescence Natural fluorite
1. Introduction Both natural and synthetic calcium fluoride are widely used as dosimetric TL material mainly due to their high sensitivity. In particular, the use of natural fluorite has extended to many places because of its abundance and low cost. In Brazil we have developed a solid dosimeter, based on natural calcium fluoride and sodium chloride powders pressed together (CaF2:NaCl) (Okuno et al., 1977; Trzeniak et al., 1990) for dosimetric applications. These pellets are extremely useful as dosimetric materials because they can provide information on dose and radiation energy with the use of appropriated filters (Guimarães and Okuno, 2003). Recently, it has been observed that both Indian (Sunta, 1970; Chougaonkar and Bhatt, 2004) and Brazilian (Yoshimura and Yukihara, 2006) fluorite also
* Corresponding author. Tel.: þ55 (0)11 30910849. E-mail address: [email protected] (F.A. Ferreira).
emit OSL when blue light is used for stimulation, and that the emitted light is related to the absorbed dose. In this paper we explore the OSL and TL properties of the fluorite pellets in order to verify the possibility of expanding the dosimetric capabilities with the use of combined OSL and TL emissions. Studies of correlation of OSL and TL signals in fluorite are rare (Polymeris et al., 2006; Chougaonkar and Bhatt, 2004; Bakshi et al., 2009). They show that not all the traps related to TL peaks are optically active and that both photo and thermal transfer processes may be present. The addition of NaCl and the production of pressed pellets with CaF2 powder may affect the correlation of OSL and TL signals. This occurs because the NaCl is itself a luminescent material and the pellet thickness (1 mm in average) may prevent uniform illumination or homogeneous deposition of radiation energy in the pellet (Moralles et al., 2005). Thus, this work aims to investigate the influence of TL readouts on the OSL response and the effect of blue illumination in the TL intensity of natural CaF2:NaCl pellet dosimeters. Thus, it is expected that a better performance of this dosimeter will be achieved.
http://dx.doi.org/10.1016/j.radmeas.2014.02.020 1350-4487/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Ferreira, Jr., F.A., et al., Correlation of optically and thermally stimulated luminescence of natural fluorite pellets, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.02.020
2
F.A. Ferreira Jr. et al. / Radiation Measurements xxx (2014) 1e4
2. Materials and methods The materials used in this work are natural fluorite and sodium chloride (99% pure). Natural fluorite acquired as-from-the-mine was transformed into powder as described by Trzeniak et al. (1990). Sodium chloride was used as binding agent. Aliquots of pure NaCl or of a mixture containing 60% CaF2 and 40% NaCl were compressed to make 5 mm diameter 1 mm thickness (w45 mg mass) pellets. The dosimeters are protected from humidity with storage in dry ambient or with plastic enveloping, to avoid influence of this condition in TL or OSL signals. Before the first use and between subsequent uses of the same sample in a experiment, the sample was heated at 5 C/s up to 450 C, as in a TL acquisition, to empty the traps and erase the TL and OSL signal. The variation of NaCl and CaF2:NaCl pellets sensitivity to repeated irradiation-readout (TL/OSL) cycles was tested over 100 cycles and the statistical dispersion is compatible with the uncertainties presented, without trends. Two groups of ten samples each were selected in accordance with the signal intensity after the same irradiation time under the beta source. One group of pellets was made of pure NaCl and the other of CaF2:NaCl (60:40); each group of selected pellets presented 3.0% standard deviation of the mean OSL signal. The experimental data that are shown in this work are the mean value from these group measurements with their respective uncertainties. OSL and TL measurements were carried out using a commercial automated TL/OSL reader produced by Risø National Laboratory (model DA-20). Luminescence was stimulated using blue light emitting diodes (470 nm, FWHM ¼ 20 nm) delivering 80 mW/cm2 at the sample position. The TL/OSL signals were detected with a bialkali photomultiplier tube (PMT) behind an UV transmitting broad-band glass filter (Hoya U-340, 7.5 mm thick 45 mm diameter) to block the stimulation light while transmitting part of the OSL signal from the samples. Irradiations were performed at room temperature using the built-in 90Sr/90Y beta source of the TL/ OSL reader (dose rate of 11 mGy/s). The stimulation time for OSL measurements was 100 s. Integrated OSL signals were calculated as first 50 s counts considering the last 50 s as background. TL glow curves were obtained using a heating rate of 5 C/s. 3. Results and discussions The experiments described below involve a series of irradiationreadout cycles. Some types of NaCl have shown vulnerability to these cycles, as shown by Polymeris et al. (2011). However, the TL glow curves from the NaCl samples used on this work show no peaks at temperatures higher than 300 C, as can be seen in Fig. 1. These TL glow curves are more similar to those presented by Ekendahl and Judas (2011) than by Polymeris et al. (2011) and may explain that no change of sensitivity of NaCl pellets to irradiationreadout cycles was observed in our work. Fig. 1 shows residual TL glow curves for CaF2:NaCl pellets after CW-OSL with different illumination times. The results for a NaCl pellet are in the insert, and we see that the TL signal is restricted to the low temperature region, and is completely extinguished after a light stimulation of just 5 s. Almost all the signal of the fluorite pellet comes from fluorite, and part of the TL is bleached by the optical stimulus. For a more complete analysis, we show in Fig. 2 the difference between the TL glow curve from a non-illuminated sample and TL glow curves after different illumination times. With this data it is possible to conclude that the shallow traps responsible for the low temperature part of the glow curve (below 150 C) are easily bleached out during the OSL readout e less than 5 s of illumination
Fig. 1. TL glow curves of CaF2:NaCl pellets obtained after different optical stimulation times. The lines connecting points are just guides for the eyes. The insert shows the TL signal of a NaCl pellet, freshly irradiated to the same dose, and after blue light stimulation during 5 s.
are necessary to remove these peaks from the glow curve. As far as the fluorite TL dosimetric peak is concerned, the effect of optical stimulation is to bleach the low temperature part of the peak leaving a very stable signal, centered near 350 C. In fact, after 500 s of illumination, a TL peak centered at 310 C, usually a part of the main peak, is completely bleached out. A satellite peak at 350 C survives the bleaching. Polymeris et al. (2006) have seen a similar effect in the study of natural fluorite, and have predicted the existence of such a peak through the deconvolution of the whole TL glow curve. Here we identify its presence by the bleaching out of the TL signal. A better correlation of TL and OSL signals can be seen in Fig. 3 where the relative integrated TL signal of the fluorite pellets, from room temperature to Tstop is compared to the relative residual OSL signal after a preheating till Tstop. Again it is possible to observe that the NaCl contribution to the OSL signal is connected with shallow traps. Also, we see that the fluorite shallow traps concentrate the
Fig. 2. Differences between the TL glow curve obtained without optical bleach and the TL glow curves after different stimulation times. The lines connecting points are just guides for the eyes.
Please cite this article in press as: Ferreira, Jr., F.A., et al., Correlation of optically and thermally stimulated luminescence of natural fluorite pellets, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.02.020
F.A. Ferreira Jr. et al. / Radiation Measurements xxx (2014) 1e4
Fig. 3. Residual OSL (R-OSL) signal after thermal bleaching up to Tstop compared with TL signal integrated from room temperature to Tstop. Values of R-OSL signals are relative to OSL signal at room temperature and values of TL signals are relative to TL acquisition up to 400 C. Lines connecting data points have been included as a guide for the eye.
majority of charge carriers responsible for a large fraction of the OSL signal. The fluorite OSL signal disappears after a TL readout till 350 C, and the OSL intensity decays very slowly with Tstop variations from about 100 to 250 C, showing a progressive releasing of charges from deeper traps, the same that are responsible for the second and third TL fluorite peaks. The absolute values of TL and OSL signals, obtained with the same set of filters and PMT, can be seen in Fig. 4 where OSL signals for different stimulation times (from 0.01 to 500 s) are compared with residual TL signal for CaF2:NaCl pellets. The insert shows the OSL signal as a function of unbleached minus bleached integrated signal. This curve shows a nonlinear correlation between OSL signal detected during bleaching and the amount of decline of the TL signal. The total light intensity available as TL output equals the very initial OSL signal. The total light emitted as OSL is about 5 times higher than the total TL. Part of the light loss could be due to
3
Fig. 5. Comparison between residual OSL signals from NaCl and CaF2:NaCl pellets as a function of time after irradiation. The data are percentages relative to the measurements obtained just after the irradiation (about 30 s after irradiation). Statistical uncertainties are smaller than the symbol sizes. Lines connecting data points have been included as a guide for the eye.
thermal quenching during TL readout, but this hypothesis has been ruled out by Polymeris et al. (2006). Without further analysis we can simply conclude that not all the OSL emitted light comes from the traps responsible for the TL peaks from 50 to 400 C. As this feature was also observed for another natural CaF2 sample (Polymeris et al., 2006), it probably has to due with intrinsic defects of this material, and also with the fact that, as a natural sample, this mineral has been exposed to natural radiation (soil and cosmic) since it was formed giving rise to filled-up deep traps. The fading curves of integrated OSL signals from CaF2:NaCl and NaCl pellets are shown in Fig. 5. The OSL signal from NaCl pellets is very unstable, being completely removed approximately 1 h after the irradiation, with the sample kept in the dark at room temperature. Regarding the OSL signal from the CaF2:NaCl pellets, a fraction of about 25% of the whole signal is still present after 5000 s. This light intensity is practically as large as the total intensity that can be collected with a TL readout till 400 C. 4. Conclusions
Fig. 4. Integrated OSL signal and residual TL (RTL) signal for CaF2:NaCl pellets after increasing optical stimulation times. The insert shows the OSL signal as a function of unbleached minus bleached glow-curves (TL-RTL). Lines joining data points have been included as a guide for the eye.
It was observed that only part of the defects responsible for the TL signal of fluorite is optically active, and that the defects associated to the OSL of this material are completely emptied by heating it to 350 C. This way, it is possible to use the TL of OSL emptied CaF2 pellets to re-assess doses, e.g. in personal dosimetry. It was not possible to verify the contribution of each TL trap individually to the OSL signal. The initial OSL decay of the pellet signals, either in the case of thermal bleaching or fading, is strongly influenced by the presence of NaCl in the sample composition. This was proved by performing the same experiments with pure NaCl pellets. The OSL signal of NaCl has fast fading after irradiation and its TL signal is almost completely erased after 5 s of illumination or after 150 C pre-heat. For this study, the whole glow-curve was included. Even the low temperature and low intensity TL peaks were considered in order to better understand the correlation between OSL and TL signals. However, for practical applications, such as personal dosimetry, it is usual to consider only the high temperature and high intensity TL peaks, due to their stability and sensitivity. In the case of CaF2:NaCl, the complex peak centered at 310e350 C plays this role.
Please cite this article in press as: Ferreira, Jr., F.A., et al., Correlation of optically and thermally stimulated luminescence of natural fluorite pellets, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.02.020
4
F.A. Ferreira Jr. et al. / Radiation Measurements xxx (2014) 1e4
Furthermore, the signal from NaCl has negligible influence on the intensity of this peak, even without any previous treatment, reinforcing the conclusion that the CaF2:NaCl pellet dosimetric capabilities can be expanded with the use of combined OSL and TL emissions. The results presented here characterize CaF2:NaCl as a possible OSL detector for use in personal dosimetry. The knowledge of the dose response curve and of the best optical zeroing procedure are important for this application and are under investigation for the development of technical aspects. Acknowledgements The authors would like to thank the Brazilian agencies São Paulo Research Foundation (FAPESP) (gs1),grants #2010/16437-0 and #2012/13534-0, National Council for Scientific and Technological Development (CNPq) (gs2) and National Institute of Science and Technology - Radiation Metrology in Medicine (INCT) (gs3). References Bakshi, A., Dhabekar, B., Rawat, N., Singh, S., Joshi, V., Kumar, V., 2009. Study on TL and OSL characteristics of indigenously developed CaF2:Mn phosphor. Nucl. Instrum. Meth. Phys. Res. Sect. B: Beam Interact. Mater. Atoms 267 (3), 548e 553.
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Please cite this article in press as: Ferreira, Jr., F.A., et al., Correlation of optically and thermally stimulated luminescence of natural fluorite pellets, Radiation Measurements (2014), http://dx.doi.org/10.1016/j.radmeas.2014.02.020