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Effects of various dopants on NaCl and KCl glow curves
Nuclear Instruments and Methods in Physics Research B 218 (2004) 249–254 www.elsevier.com/locate/nimb
Effects of various dopants on NaCl and KCl glow curves A.T. Davidson
a,*
, A.G. Kozakiewicz a, T.E. Derry b, J.D. Comins b, M. Suszynska c
a
c
Physics Department, University of Zululand, Private Bag X1001, Kwadlangezwa 3886, South Africa b Physics Department, University of the Witwatersrand, Johannesburg 2050, South Africa Institute of Low Temperatures and Structure Research, Polish Academy of Sciences, 50-950 Wroclaw, Poland
Abstract We have measured the thermoluminescence of a number of NaCl and KCl crystals following irradiation at ambient temperature with the same dose (10 kGy) of Co-60 c rays. We compare the TL of pure samples and of samples doped with europium and calcium ions. In the case of NaCl, additional impurities (Ni, Pb, Sr and Cr) have been investigated. The effects of irradiation are determined using optical absorption and thermoluminescence. Factors investigated include the effects of different dopants on TL glow curves and the effects of thermal annealing samples at 400 °C before the irradiation. Changes in TL glow curves relating to changes in the state of aggregation of the impurities produced by preirradiation annealing are reported in this paper. Perhaps the most significant effect is a temperature shift of the main glow peak in pre-annealed compared to not pre-annealed samples in the case of Eu doped NaCl. The magnitude of the shift depends on the concentration of the Eu dopant. Shifts are also observed for Ni and Sr impurities in NaCl, but not for Ca and Cr impurities in NaCl. In the case of KCl, glow peaks generally occur at similar temperatures in doped samples and do not shift when doped samples are pre-annealed. Here the main effect of different impurities is to influence the size of the emission and not the structure of the glow curve. Results are discussed in terms of current theories of thermoluminescence. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Thermoluminescence; Pure and doped crystals; Pre-annealing; NaCl; KCl
1. Introduction The controlled doping of crystals is a major research strategy in materials science often leading to spectacular advances in understanding and to useful applications. In the case of ionic crystals, the introduction of impurities is important for activation of phosphors and for the optimization *
Corresponding author. Tel.: +27-35-902-6558; fax: +27-35902-6750. E-mail address: [email protected] (A.T. Davidson).
of radiation dosimeters. The present investigation concerns the effect of different impurities on the thermoluminescence (TL) of NaCl and KCl crystals. In a previous paper [1], we reported the TL of Eu doped crystals and we now report results for nominally pure crystals and for crystals doped with Ca as well as Eu ions. In the case of NaCl, additional impurities such as Ni, Pb, Sr and Cr have been investigated. The effect of incorporating impurity ions into insulating host crystals during crystal growth involves several considerations which include
0168-583X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.01.010
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whether interstitial or substitutional sites are occupied, the clustering of the impurities, possible precipitation of impurities to form new phases and the question of charge compensation of the impurity ions. Some of these aspects have been considered in detail for NaCl and KCl by previous workers [2–4]. Effects produced in TL glow curves by rare earth impurities in a range of host crystals have been discussed by Townsend and coworkers [5,6]. Amongst several interesting effects are a sensitivity of the glow peaks to thermal history and a shift of TL peak temperatures dependant on the size of the impurity ions. In the latter case, typical Tmax shifts of about 20 °C were observed in for example LaF3 doped with a variety of rare earth (RE) ions [7]. For this to occur it is thought that the emitting RE ion should be closely associated with trapping sites for electrons and holes produced during irradiation. The distortions created by RE ions of different size influence the stability of traps resulting in glow peaks which differ in temperature in a systematic way. The present paper provides information about possible impurity size effects on TL in the NaCl/ KCl system. Here defect production proceeds by the efficient excitonic mechanism leading to trapped electrons in the form of F and F aggregate centres and complementary interstitial anions known as H centres. In NaCl/KCl, these radiation induced centres are not particularly stable at ambient temperature or when exposed to background illumination. However, after exposure to ultraviolet illumination, doped crystals of NaCl/ KCl exhibit thermoluminescence, an effect that is not observed in pure crystals [1,8,9]. This has encouraged us to investigate the effects of various impurities on thermoluminescence in NaCl and KCl crystals.
2. Experimental details The crystals under investigation were provided by Prof. M. Suszynska. Pure and doped samples were exposed at ambient temperature to 10 kGy of c rays using a Co-60 source at the Schonland Centre, University of the Witwatersrand. Doping
levels were generally in the range 100–750 ppm. Pre-annealed crystals were heated in air for 1 h at 400 °C immediately prior to irradiation. The optical absorption of samples was measured immediately after irradiation and also prior to the measurement of TL which was done about one month after irradiation. Glow curves were measured at a heating rate of 15 °C min 1 using apparatus described previously [10]. The mass of samples was normalized to allow relative TL intensities to be compared.
3. Results and discussion 3.1. Pure and europium doped crystals Glow curves for pure crystals of KCl and NaCl are given in Fig. 1. We compare results for crystals pre-annealed (pa) before c irradiation and for crystals which were not pre-annealed (npa). Pure KCl is very susceptible to the pre-annealing treatment, which eliminates a peak near 230 °C in the untreated sample (npa). Some variability is apparent as can be seen by the different glow curves obtained for two pre-annealed samples (pa). In NaCl, the main glow peak lies in the range 250–280 °C for both pa and npa samples. A low temperature peak is present near 80 °C in NaCl. Glow curves for crystals containing different concentrations of Eu as an impurity are given in Figs. 2 and 3. In the case of NaCl(Eu) shown in Fig. 2, a prominent glow peak occurs near 230 °C, which shifts to lower temperatures in pa samples. The magnitude of the shift increases with increasing Eu concentration. For KCl(Eu) shown in Fig. 3, two prominent peaks are present, near 130 and 200 °C, in npa samples. The intensity of the low temperature peak is enhanced in pa samples and is seen to consist of two neighboring glow peaks. The high temperature peak occurs at similar temperatures in both pa and npa samples of KCl(Eu). The present crystals have been exposed to the same dose of radiation so it is of interest to compare the results from this point of view. Doping with Eu has enhanced the TL emission of both NaCl and KCl. Other dopants investigated here do not enhance the TL emission to the same extent.
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Temperature (°C) Fig. 2. Glow curves for Eu doped NaCl crystals after irradiation with c rays, 10 kGy, at ambient temperature. Doping levels in ppm are indicated on the diagrams. Open circles indicate pre-annealed crystals (pa). Full circles indicate not pre-annealed crystals (npa).
Our results show clearly that pre-annealing crystals before irradiation often produces significant changes to the glow curves. The effects are different for the Eu doped halides shown in Figs. 2 and 3. Optical absorption (OA) measurements can pro-
vide relevant information in this case. OA spectra of our NaCl(Eu) samples show significant distortions in shape and spectral location of the Eu absorption bands. See for example our earlier paper [1] and papers by other workers [2,11] for
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Temperature (°C) Fig. 3. Glow curves for Eu doped KCl crystals after irradiation with c rays, 10 kGy, at ambient temperature. Doping levels in ppm are indicated on the diagrams. Open circles indicate pre-annealed crystals (pa). Full circles indicate not pre-annealed crystals (npa).
the type of effects observed. Annealing at 400 °C changes the shape of these bands. It has been suggested that these effects seen in OA spectra relate to the state of aggregation of the impurities in which case the glow curves of pa and npa NaCl(Eu) should be considered in this light. This is discussed further in Section 3.3. Our OA spectra for KCl(Eu) indicate that the Eu impurity bands are not changed significantly when samples are annealed suggesting perhaps that impurity aggregation is not a significant factor in this halide.
3.2. NaCl crystals doped with Cr, Sr and Ni In Fig. 4, we show glow curves for NaCl crystals containing impurities with very different chemical characteristics. In the case of Cr (100 ppm), an existing glow peak near 260 °C is enhanced in the pa sample. This behaviour is similar in some respects to pure NaCl shown in Fig. 1. In the case of Sr (100 ppm), a glow peak near 230 °C shifts to 200 °C in the pa sample. This is similar to europium doped samples shown in Fig. 2. In the case of Ni (240 ppm), a glow peak near 220 °C shifts to 185 °C. However, the overall TL emission decreases in the pa sample. We also find that the peak shift is independent of impurity concentra-
tion in this material. So in several respects NaCl(Ni) differs from NaCl(Eu). We note that Ni is the smallest ion of those under investigation. The Ni doped sample is the only one of those shown in Fig. 4 to have impurity absorption bands in the uv-visible region.
3.3. Other impurities and discussion Ca doped samples (about 200 ppm) of both halides give results similar to Eu doped samples reported in Figs. 2 and 3. However, pre-annealing samples before irradiation did not change the glow peak temperatures in any significant way. In the case of Pb doped (100 ppm) NaCl, TL emission is drastically reduced in both pa and npa samples and is the smallest of all the crystals investigated. The present data indicates that dopants in NaCl/KCl affect the size of the TL response. Also important is the state of aggregation of the dopant as evidenced by the effect of pre-annealing samples before irradiation. This enhances the TL response in many cases and can produce temperature shifts of a glow peak near 200 °C particularly in NaCl samples. If we assume that glow peaks are due to the recombination of chlorine interstitials with
A.T. Davidson et al. / Nucl. Instr. and Meth. in Phys. Res. B 218 (2004) 249–254
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Temperature (°C) Fig. 4. Glow curves for NaCl crystals doped with Cr, Sr and Ni ions as indicated. Crystals were irradiated with c rays, 10 kGy, at ambient temperature. Open circles indicate pre-annealed crystals (pa). Full circles indicate not pre-annealed crystals (npa).
aggregated F centres [12], we may envisage the following scenario. In doped samples, annealing will disperse the impurities. After irradiation, crystals will contain interstitial aggregates of small size trapped at the impurities. The TL response is determined by the availability of interstitials to recombine with F centres, if some interstitials become unavailable for recombination by bonding with impurities for example, then a smaller TL signal results. In untreated samples, interstitial aggregates are likely to be larger and interstitials evaporate at higher temperatures as evidenced by a glow peak at higher temperatures. Recent models emphasize the localization of trapped electrons and holes in complexes containing the impurity. During TL readout, recombination energy is transferred to the impurity which can participate in the emission if it has a short enough relaxation time. In this scenario, the TL response is affected by defect-induced strain in the lattice which can cause a peak shift, and also by the relaxation properties of the impurity ion which will influence the size of the signal. Additional experiments are needed to further characterize the role of dopant ions in TL. These include spectral analysis of the emitted light and more information about the state of aggregation of dopants in pre-annealed crystals.
4. Conclusions We have presented TL glow curves for NaCl and KCl crystals, both pure crystals and crystals incorporating various impurities are considered. Impurities have a stabilizing effect on glow curves and affect the size of the emission. The basic pattern of the glow curve is similar in doped crystals and does not depend on the type of impurity. Changes are produced as a result of pre-annealing samples at 400 °C before irradiation. In doped crystals this often includes a shift in the temperature of a glow peak near 200 °C and changes in the overall emission intensity. Some reasons for this are suggested. Further work is needed before we can properly evaluate recently predicted effects of impurities such as the shift of glow peak temperatures with impurity ion size. Good candidates for investigation appear to be NaCl doped with europium or strontium ions.
Acknowledgements We thank Dr. T. Nam for doing the c irradiations. This work was supported by the National Research Foundation and by the Research Committee of UZ.
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References [1] A.T. Davidson, A.G. Kozakiewicz, T.E. Derry, J.D. Comins, M. Suszynska, Radiat. Eff. Def. Solid 157 (2002) 629. [2] F.J. L opez, H.S. Murrieta, J.A. Hernandez, J.O. Rubio, Phys. Rev. B 22 (1980) 6428. [3] J.O. Rubio, H.S. Murrieta, R.C. Powell, W.F. Sibley, Phys. Rev. B 31 (1985) 59. [4] F.M. Matinaga, L.A.O. Nunes, S.C. Zilio, J.C. Castro, Phys. Rev. B 37 (1988) 993. [5] B. Yang, P.D. Townsend, J. Appl. Phys. 88 (2000) 6395. [6] P.D. Townsend, A.K. Jazmati, T. Karali, M. Maghrabi, S.G. Raymond, B. Yang, J. Phys.: Condens. Matter 13 (2001) 2211.
[7] B. Yang, P.D. Townsend, A.P. Rowlands, Phys. Rev. B 57 (1998) 178. [8] I. Aguirre de Carcer, A.P. Rowlands, F. Jaque, P.D. Townsend, Radiat. Meas. 29 (1998) 203. [9] M. Pedroza-Montero, B. Castenada, R. Melendrez, V. Chernov, M. Barboza-Flores, Radiat. Eff. Def. Solid 154 (2001) 319. [10] A.T. Davidson, A.G. Kozakiewicz, D.J. Wilkinson, J.D. Comins, T.E. Derry, Nucl. Instr. and Meth. B 141 (1998) 523. [11] B. Macalik, T. Morawska-Kowal, M. Suszynska, M. Szmida, Radiat. Eff. Def. Solid 150 (1999) 355. [12] G. Baldacchini, A.T. Davidson, V.S. Kalinov, A.G. Kozakiewicz, R.M. Montereali, A.P. Voitovich, J. Lumin. 102–103 (2003) 77.
Effects of various dopants on NaCl and KCl glow curves A.T. Davidson
a,*
, A.G. Kozakiewicz a, T.E. Derry b, J.D. Comins b, M. Suszynska c
a
c
Physics Department, University of Zululand, Private Bag X1001, Kwadlangezwa 3886, South Africa b Physics Department, University of the Witwatersrand, Johannesburg 2050, South Africa Institute of Low Temperatures and Structure Research, Polish Academy of Sciences, 50-950 Wroclaw, Poland
Abstract We have measured the thermoluminescence of a number of NaCl and KCl crystals following irradiation at ambient temperature with the same dose (10 kGy) of Co-60 c rays. We compare the TL of pure samples and of samples doped with europium and calcium ions. In the case of NaCl, additional impurities (Ni, Pb, Sr and Cr) have been investigated. The effects of irradiation are determined using optical absorption and thermoluminescence. Factors investigated include the effects of different dopants on TL glow curves and the effects of thermal annealing samples at 400 °C before the irradiation. Changes in TL glow curves relating to changes in the state of aggregation of the impurities produced by preirradiation annealing are reported in this paper. Perhaps the most significant effect is a temperature shift of the main glow peak in pre-annealed compared to not pre-annealed samples in the case of Eu doped NaCl. The magnitude of the shift depends on the concentration of the Eu dopant. Shifts are also observed for Ni and Sr impurities in NaCl, but not for Ca and Cr impurities in NaCl. In the case of KCl, glow peaks generally occur at similar temperatures in doped samples and do not shift when doped samples are pre-annealed. Here the main effect of different impurities is to influence the size of the emission and not the structure of the glow curve. Results are discussed in terms of current theories of thermoluminescence. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Thermoluminescence; Pure and doped crystals; Pre-annealing; NaCl; KCl
1. Introduction The controlled doping of crystals is a major research strategy in materials science often leading to spectacular advances in understanding and to useful applications. In the case of ionic crystals, the introduction of impurities is important for activation of phosphors and for the optimization *
Corresponding author. Tel.: +27-35-902-6558; fax: +27-35902-6750. E-mail address: [email protected] (A.T. Davidson).
of radiation dosimeters. The present investigation concerns the effect of different impurities on the thermoluminescence (TL) of NaCl and KCl crystals. In a previous paper [1], we reported the TL of Eu doped crystals and we now report results for nominally pure crystals and for crystals doped with Ca as well as Eu ions. In the case of NaCl, additional impurities such as Ni, Pb, Sr and Cr have been investigated. The effect of incorporating impurity ions into insulating host crystals during crystal growth involves several considerations which include
0168-583X/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.01.010
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A.T. Davidson et al. / Nucl. Instr. and Meth. in Phys. Res. B 218 (2004) 249–254
whether interstitial or substitutional sites are occupied, the clustering of the impurities, possible precipitation of impurities to form new phases and the question of charge compensation of the impurity ions. Some of these aspects have been considered in detail for NaCl and KCl by previous workers [2–4]. Effects produced in TL glow curves by rare earth impurities in a range of host crystals have been discussed by Townsend and coworkers [5,6]. Amongst several interesting effects are a sensitivity of the glow peaks to thermal history and a shift of TL peak temperatures dependant on the size of the impurity ions. In the latter case, typical Tmax shifts of about 20 °C were observed in for example LaF3 doped with a variety of rare earth (RE) ions [7]. For this to occur it is thought that the emitting RE ion should be closely associated with trapping sites for electrons and holes produced during irradiation. The distortions created by RE ions of different size influence the stability of traps resulting in glow peaks which differ in temperature in a systematic way. The present paper provides information about possible impurity size effects on TL in the NaCl/ KCl system. Here defect production proceeds by the efficient excitonic mechanism leading to trapped electrons in the form of F and F aggregate centres and complementary interstitial anions known as H centres. In NaCl/KCl, these radiation induced centres are not particularly stable at ambient temperature or when exposed to background illumination. However, after exposure to ultraviolet illumination, doped crystals of NaCl/ KCl exhibit thermoluminescence, an effect that is not observed in pure crystals [1,8,9]. This has encouraged us to investigate the effects of various impurities on thermoluminescence in NaCl and KCl crystals.
2. Experimental details The crystals under investigation were provided by Prof. M. Suszynska. Pure and doped samples were exposed at ambient temperature to 10 kGy of c rays using a Co-60 source at the Schonland Centre, University of the Witwatersrand. Doping
levels were generally in the range 100–750 ppm. Pre-annealed crystals were heated in air for 1 h at 400 °C immediately prior to irradiation. The optical absorption of samples was measured immediately after irradiation and also prior to the measurement of TL which was done about one month after irradiation. Glow curves were measured at a heating rate of 15 °C min 1 using apparatus described previously [10]. The mass of samples was normalized to allow relative TL intensities to be compared.
3. Results and discussion 3.1. Pure and europium doped crystals Glow curves for pure crystals of KCl and NaCl are given in Fig. 1. We compare results for crystals pre-annealed (pa) before c irradiation and for crystals which were not pre-annealed (npa). Pure KCl is very susceptible to the pre-annealing treatment, which eliminates a peak near 230 °C in the untreated sample (npa). Some variability is apparent as can be seen by the different glow curves obtained for two pre-annealed samples (pa). In NaCl, the main glow peak lies in the range 250–280 °C for both pa and npa samples. A low temperature peak is present near 80 °C in NaCl. Glow curves for crystals containing different concentrations of Eu as an impurity are given in Figs. 2 and 3. In the case of NaCl(Eu) shown in Fig. 2, a prominent glow peak occurs near 230 °C, which shifts to lower temperatures in pa samples. The magnitude of the shift increases with increasing Eu concentration. For KCl(Eu) shown in Fig. 3, two prominent peaks are present, near 130 and 200 °C, in npa samples. The intensity of the low temperature peak is enhanced in pa samples and is seen to consist of two neighboring glow peaks. The high temperature peak occurs at similar temperatures in both pa and npa samples of KCl(Eu). The present crystals have been exposed to the same dose of radiation so it is of interest to compare the results from this point of view. Doping with Eu has enhanced the TL emission of both NaCl and KCl. Other dopants investigated here do not enhance the TL emission to the same extent.
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Our results show clearly that pre-annealing crystals before irradiation often produces significant changes to the glow curves. The effects are different for the Eu doped halides shown in Figs. 2 and 3. Optical absorption (OA) measurements can pro-
vide relevant information in this case. OA spectra of our NaCl(Eu) samples show significant distortions in shape and spectral location of the Eu absorption bands. See for example our earlier paper [1] and papers by other workers [2,11] for
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the type of effects observed. Annealing at 400 °C changes the shape of these bands. It has been suggested that these effects seen in OA spectra relate to the state of aggregation of the impurities in which case the glow curves of pa and npa NaCl(Eu) should be considered in this light. This is discussed further in Section 3.3. Our OA spectra for KCl(Eu) indicate that the Eu impurity bands are not changed significantly when samples are annealed suggesting perhaps that impurity aggregation is not a significant factor in this halide.
3.2. NaCl crystals doped with Cr, Sr and Ni In Fig. 4, we show glow curves for NaCl crystals containing impurities with very different chemical characteristics. In the case of Cr (100 ppm), an existing glow peak near 260 °C is enhanced in the pa sample. This behaviour is similar in some respects to pure NaCl shown in Fig. 1. In the case of Sr (100 ppm), a glow peak near 230 °C shifts to 200 °C in the pa sample. This is similar to europium doped samples shown in Fig. 2. In the case of Ni (240 ppm), a glow peak near 220 °C shifts to 185 °C. However, the overall TL emission decreases in the pa sample. We also find that the peak shift is independent of impurity concentra-
tion in this material. So in several respects NaCl(Ni) differs from NaCl(Eu). We note that Ni is the smallest ion of those under investigation. The Ni doped sample is the only one of those shown in Fig. 4 to have impurity absorption bands in the uv-visible region.
3.3. Other impurities and discussion Ca doped samples (about 200 ppm) of both halides give results similar to Eu doped samples reported in Figs. 2 and 3. However, pre-annealing samples before irradiation did not change the glow peak temperatures in any significant way. In the case of Pb doped (100 ppm) NaCl, TL emission is drastically reduced in both pa and npa samples and is the smallest of all the crystals investigated. The present data indicates that dopants in NaCl/KCl affect the size of the TL response. Also important is the state of aggregation of the dopant as evidenced by the effect of pre-annealing samples before irradiation. This enhances the TL response in many cases and can produce temperature shifts of a glow peak near 200 °C particularly in NaCl samples. If we assume that glow peaks are due to the recombination of chlorine interstitials with
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aggregated F centres [12], we may envisage the following scenario. In doped samples, annealing will disperse the impurities. After irradiation, crystals will contain interstitial aggregates of small size trapped at the impurities. The TL response is determined by the availability of interstitials to recombine with F centres, if some interstitials become unavailable for recombination by bonding with impurities for example, then a smaller TL signal results. In untreated samples, interstitial aggregates are likely to be larger and interstitials evaporate at higher temperatures as evidenced by a glow peak at higher temperatures. Recent models emphasize the localization of trapped electrons and holes in complexes containing the impurity. During TL readout, recombination energy is transferred to the impurity which can participate in the emission if it has a short enough relaxation time. In this scenario, the TL response is affected by defect-induced strain in the lattice which can cause a peak shift, and also by the relaxation properties of the impurity ion which will influence the size of the signal. Additional experiments are needed to further characterize the role of dopant ions in TL. These include spectral analysis of the emitted light and more information about the state of aggregation of dopants in pre-annealed crystals.
4. Conclusions We have presented TL glow curves for NaCl and KCl crystals, both pure crystals and crystals incorporating various impurities are considered. Impurities have a stabilizing effect on glow curves and affect the size of the emission. The basic pattern of the glow curve is similar in doped crystals and does not depend on the type of impurity. Changes are produced as a result of pre-annealing samples at 400 °C before irradiation. In doped crystals this often includes a shift in the temperature of a glow peak near 200 °C and changes in the overall emission intensity. Some reasons for this are suggested. Further work is needed before we can properly evaluate recently predicted effects of impurities such as the shift of glow peak temperatures with impurity ion size. Good candidates for investigation appear to be NaCl doped with europium or strontium ions.
Acknowledgements We thank Dr. T. Nam for doing the c irradiations. This work was supported by the National Research Foundation and by the Research Committee of UZ.
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A.T. Davidson et al. / Nucl. Instr. and Meth. in Phys. Res. B 218 (2004) 249–254
References [1] A.T. Davidson, A.G. Kozakiewicz, T.E. Derry, J.D. Comins, M. Suszynska, Radiat. Eff. Def. Solid 157 (2002) 629. [2] F.J. L opez, H.S. Murrieta, J.A. Hernandez, J.O. Rubio, Phys. Rev. B 22 (1980) 6428. [3] J.O. Rubio, H.S. Murrieta, R.C. Powell, W.F. Sibley, Phys. Rev. B 31 (1985) 59. [4] F.M. Matinaga, L.A.O. Nunes, S.C. Zilio, J.C. Castro, Phys. Rev. B 37 (1988) 993. [5] B. Yang, P.D. Townsend, J. Appl. Phys. 88 (2000) 6395. [6] P.D. Townsend, A.K. Jazmati, T. Karali, M. Maghrabi, S.G. Raymond, B. Yang, J. Phys.: Condens. Matter 13 (2001) 2211.
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