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The fractional thermoluminescence of bleached K-feldspars
Journal of Luminescence 92 (2001) 271–276
The fractional thermoluminescence of bleached K-feldspars A. Chrus´ cin´ska* Institute of Physics, TL Dating Laboratory, N. Copernicus University, ul.Grudziadzka 5, 87-100 Torun´, Poland Received 24 May 2000; received in revised form 14 November 2000; accepted 16 November 2000
Abstract The fractional glow technique was applied for the investigation of trap occupation in optically bleached K-feldspars separated from sediments. Various bleaching times and two spectra ranges of sunlight simulator were used. Four trap groups exhibit different sensitivities to the bleaching. The influence of the spectrum range of the stimulation light on the bleaching efficiency is presented. # 2001 Elsevier Science B.V. All rights reserved. PACS: 78.60.K; 93.85 Keywords: Thermoluminescence; Trap spectroscopy; Thermoluminescence dating; K-feldspar
1. Introduction K-feldspars used in thermoluminescence (TL) and optically stimulated luminescence (OSL) dating for years, were intensively investigated by means of TL [1–3] and spectroscopic methods [4]. Recently, because of the OSL dating, the kinetics of infrared stimulated luminescence is the favourite subject of investigations [5,6]. The measurements of IRSL decay [7] and the activation energy of IRSL traps [8] are good steps for better understanding of this process in Kfeldspars. Simultaneously, the natural bleaching of TL is studied [9]. The bleaching of the luminescence of minerals before their deposition is an essential problem in both TL and OSL dating of sediments. This work presents the first, preliminary results of applying the fractional glow technique (FGT) [10,11] for investigation of the influence of light on *Fax: +48-56-62-25-397.
the population of traps in K-feldspars. The energy density spectrum of traps, and actually the spectrum of occupied traps which are active in TL process, were measured after different bleaching procedures.
2. Experimental 2.1. Apparatus The measurements were carried out using the TL equipment [12] with a temperature control system adapted for FGT with heating–cooling rate correction [13,14]. Heating and cooling with an average rate about 0.18C/s were linear in the greater part of each cycle and only this linear part was used for the determination of trap depth. The temperature range of the whole heating procedure is from room temperature to 3308C. It is enough to depopulate the major part of the traps in Kfeldspars normally explored by TL or OSL dating.
0022-2313/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 1 ) 0 0 1 6 9 - 7
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A. Chrus´cin´ska / Journal of Luminescence 92 (2001) 271–276
The Schott BG-2 filter is installed before the EMI 6097 B photomultiplier in the light detection unit, so the waveband 390–440 nm [15] commonly used in dating was investigated. The sensitivity of the optical arrangement allowed the use of the FGT procedure for unbleached samples as well as for samples after 20 h of bleaching. The sunlight simulator [16] enables two options of bleaching} using the wavelength ranges 300–750 nm or 375– 750 nm (with the UV lamps switched off). The feldspar samples (mainly microcline)}in average about 15 mg of grains separated from sediment}were put directly on the heating strip. They were heated up to 5008C before the irradiation. The X-rays source used for TL excitation yield a dose of 60 Gy in 1 h. The 6 h excitation was followed by the optical bleaching. A few days later a preheat to 2008C with the heating rate 1.78C/s was applied. In the FGT measurement, which was carried out a day after preheat, a procedure with heating over 128C and subsequent cooling over 108C in every cycle was employed. Over 90 such cycles contribute effectively to trap spectrum when the sample is preheated. 2.2. FGT The FGT is quite adequate to complex glow curves analysis and its main advantage is independence from the TL kinetic order. It enables also to obtain good results when several competing recombination centres are present [17]. However, it should be stressed that the experimental results need always a careful handling and interpretation [14,18]. One should not forget that the value of activation energy calculated from experimental data in a FGT cycle is an average of the depth of traps emptied in the cycle. As the computer simulations [13] show, the FGT improved by the heating–cooling rate correction applied in the temperature region in question enables to distinguish discrete trap levels the depths of which differ by several dozen of meV. Experimentally, however, one can have to do with a discrete trap spectrum or a continuous distribution and even with a ‘‘very good looking result’’ one cannot definitely decide for one of these possibilities. So the occupation spectra obtained directly from
measurements should be treated rather as the initial data for further analysis. It should be stressed that this work presents the experimental results, which are waiting now for the detailed analysis.
3. Experimental results and discussion The area under the TL curve registered during the heating and cooling of the sample in the FGT cycle is the light-sum. A graph of the light-sum dependence on maximum temperature in the FGT cycle is a kind of TL glow curve (measured with very low heating rate}here about 0.0068C/s). Fig. 1 presents such curves obtained for an unbleached K-feldspar and after 2 and 20 h of bleaching. The shapes of these curves reflect the influence of optical stimulation on the TL signal. The low- and high-temperature edges of the curves are more easily bleached than the central part. The two types of bleaching treatments show different efficiencies. This is also reflected in the trap occupation spectra presented in the next figures. Every trap spectrum presented is an average of three histograms each of which resulting from a single experiment. The ‘‘arbitrary units’’ of the trap occupation are the same on all the diagrams. Fig. 2 shows the trap occupation spectra of a non-bleached feldspar together with the spectra after 2 h of stimulation applied with either spectral ranges. Three trap groups can be distinguished: traps shallower than 1.3 eV, traps with a depth between 1.3 and 1.6 eV and traps deeper then 1.6 eV. The deepest fraction is the most easily depopulated by the light. The shallower traps are a little bit more resistant to the optical stimulation but in the case of traps about 1.5 eV both spectral ranges seem to be still less effective. Figs. 3 and 4 present the changes of trap population with the time of bleaching separately for reduced (without UV part) and full range of the simulator light spectrum. As one can see, the traps with the energy below 1.3 eV are depopulated more effectively by the light with full spectrum range. There is no significant difference in trap occupation in this range after 2, 4 and even 20 h of bleaching (Fig. 4) so, it seems that a residual level
A. Chrus´cin´ska / Journal of Luminescence 92 (2001) 271–276
273
Fig. 1. The FGT light-sum curves: the area under the glow curve for heating and cooling part of a FGT cycle against the maximum temperature of the cycle; for K-feldspar after X-ray excitation and various bleaching procedures. The FGT measurements were carried out after a preheat to 2008C.
Fig. 2. The trap occupation spectra for (a) non-bleached K-feldspar, (b) after 2 h of bleaching by the light without UV part of spectrum, (c) after 2 h of bleaching with full range of simulator light spectrum. Each histogram is an average of three independent FGT experiments with the cycle procedure of 128C of heating and 108C of cooling. The preheat to 2008C was applied. All the results presented are mass normalised.
of the trap population is obtained or that there are present the weakly populated traps which are unbleachable. The light without UV part (Fig. 3) causes steady but slower emptying of these traps. After 20 h (Fig. 3c) the residual level is still not reached.
The traps over 1.6 eV disappear from the trap occupation spectra already after 2 h of stimulation by the light with a full spectrum (Fig. 4), whereas they remain occupied even after 20 h of bleaching by the light with a reduced range (Fig. 3). Their population is divided by ten but it is still detectable.
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A. Chrus´cin´ska / Journal of Luminescence 92 (2001) 271–276
Fig. 3. The trap occupation spectra for K-feldspar after 2 h (a), 4 h (b) and 20 h (c) of bleaching by the light without UV part of the spectrum. Same measurement procedure as for the results presented in previous figure.
Fig. 4. The trap occupation spectra for K-feldspar after 2 h (a), 4 h (b) and 20 h (c) of bleaching with the light of full simulator spectrum range. FGT procedure same as for the results presented in previous figures.
As one could expect the full range of simulator light spectrum should cause the more effective trap depopulation. This is true for the traps from both early mentioned energy ranges (below 1.3 eV and over 1.6 eV) but the third group of traps stands out. These traps are emptied during the FGT measurement at the temperature 200–2508C and correspond to the part of the light-sum curve, which is the most weakly bleached (Fig. 1). The analysis of Figs. 3 and 4 enables to divide these traps into two groups with the depth ranges:
1.3–1.45 eV and 1.45–1.6 eV. In the case of the first fraction, the residual level of occupation is reached already after 4 h of full spectrum stimulation (Fig. 4b). After 20 h of bleaching these traps remain as the highest populated ones. The traps from the range 1.45–1.6 eV are clearly seen in occupation spectra obtained after 4 h of the full spectrum stimulation (Fig. 4b) and after 20 h of reduced spectrum stimulation (Fig. 3c). This trap fraction is depopulated below the detectable level after 20 h of stimulation by the full range light
A. Chrus´cin´ska / Journal of Luminescence 92 (2001) 271–276
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Fig. 5. The dependence of relative trap occupation on bleaching time. The occupation of a trap fraction (the sum of the histogram bars for this fraction) after the bleaching is divided by the occupation of the same fraction determined for the unbleached sample.
but the light without UV acts much less effective and it seems even to yield the residual level after some hours of bleaching (Fig. 3). A longer time of bleaching should be applied in order to check if there really exists a residual level for one kind of stimulation whereas it does not exist (or is much lower) for the other one. The dependence of relative trap occupation (trap occupation after bleaching divided by the occupation for unbleached sample) on the duration of the optical stimulation is shown in Fig. 5 for both spectral ranges. Some remarks should be added in connection with the trap depth values in K-feldspars described in the earlier papers. Apart from the possible differences between K-feldspar samples investigated by different authors, the correlation between the trap occupation spectra for unbleached samples presented here and the trap energy values determined for microcline by Strickertsson [2] and Visocekas et al. [4] is clear. It is interesting that the trap energy spectra after long optical bleaching (Fig. 4c) agree with the values determined for microcline by Ahmed and Gartia [1]. They give the energy value of about 1.4 eV for the deepest traps. Their FGT measurements were carried out after
applying the relatively low-excitation dose which, results in low trap occupation which is also the result of long optical stimulation. It can be discussed if the obtained trap occupation spectra are in fact the same as they were directly after switching off the bleaching light. According to the measurement procedure: excitation–bleaching–preheat, one can suppose that the results correspond to the equilibrium state reached after these three steps. One can imagine that after the depopulation of one trap fraction by the optical stimulation, the carriers from another, less stable traps repopulate this first fraction again and the thermal stability cannot play a role here alone. However, during all the experiments now presented one has not observed any change of trap occupation spectrum with the time of storage. The trap occupation change after switching off the optical stimulation could eventually be caused by the preheat to 2008C. The comparison of the results of some FGT measurements, which were carried out from the room temperature to 3308C, with those presented here does not show that preheating procedure applied causes the increase of population of traps which are active beyond the 2008C. Summing up, the measured trap
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A. Chrus´cin´ska / Journal of Luminescence 92 (2001) 271–276
occupation results from the bleaching applied and, eventually, a rapid process which occurs straight after the bleaching and does not derive from the temperature stimulation.
4. Conclusion As one can see from the preliminary results, the FGT with the heating-cooling rate correction can be a useful tool to investigate the bleaching processes. The changes of trap occupation after bleaching and the influence of the spectrum range of the stimulation light on the occupation spectra are clearly demonstrated. The traps in the energy range 1.3–1.6 eV seem to be responsible for the residual TL which can be registered even after dozens of hours of bleaching and exists also in the nature in sediments after the sun bleaching. If so, they are the source of the problems with dating of not completely bleached sediments. The unambiguous recognition of trap kinds as discrete or distributed over a determined energy range will be possible after a detailed analysis. Acknowledgements This work was partially supported by the UMK Grant 464-F.
References [1] A.B. Ahmed, R.K. Gartia, Phys. Stat. Sol. A 94 (1985) 645. [2] K. Strickertsson, Nucl. Tracks Radiat. Meas. 10 (1985) 613. [3] Y. Kirsh, S. Shoval, P.D. Townsend, Phys. Stat. Sol. A 101 (1987) 253. [4] R. Visocekas, V. Tale, A. Zink, N.A. Spooner, I. Tale, Rad. Prot. Dosim. 66 (1996) 391. [5] G.A. Duller, A.G. Wintle, Nucl. Tracks Radiat. Meas. 18 (1991) 384. [6] U. Rieser, G. Hutt, M.R. Krbetschek, W. Stolz, Radiat. Meas. 27 (1997) 273. [7] R.J. Clark, I.K. Bailiff, M.J. Tooley, Radiat. Meas. 27 (1997) 211. [8] T. Trautmann, U. Rieser, W. Stolz, Radiat. Meas. 27 (1997) 193. [9] S.W.S. McKeever, Radiat. Meas. 23 (1994) 267. [10] H. Gobrecht, D. Hofmann, J. Phys. Chem. Solids 27 (1966) 509. [11] I.A. Tale, Phys. Stat. Sol. A 66 (1981) 65. [12] A. Pietkun, L. Polakiewicz, H.L. Oczkowski, Acta Phys. Polon. A 81 (1992) 687. [13] A. Chrus´ cin´ska, J. Lumin. 62 (1994) 115. [14] A. Chrus´ cin´ska, H.L. Oczkowski, K.R. Przegietka, Acta Phys. Polon. A 89 (1996) 555. [15] M.R. Krbetschek, J. Gotze, A. Dietrich, T. Trautmann, Radiat. Meas. 27 (1997) 695. [16] H.L. Oczkowski, K.R. Przegietka, Radiat. Meas. 23 (1998) 435. [17] R. Chen, A. Hag-Yahya, Rad. Prot. Dosim. 65 (1996) 17. [18] J.A. Kierstead, P.W. Levy, Nucl. Tracks Radiat. Meas. 18 (1991) 19.
The fractional thermoluminescence of bleached K-feldspars A. Chrus´ cin´ska* Institute of Physics, TL Dating Laboratory, N. Copernicus University, ul.Grudziadzka 5, 87-100 Torun´, Poland Received 24 May 2000; received in revised form 14 November 2000; accepted 16 November 2000
Abstract The fractional glow technique was applied for the investigation of trap occupation in optically bleached K-feldspars separated from sediments. Various bleaching times and two spectra ranges of sunlight simulator were used. Four trap groups exhibit different sensitivities to the bleaching. The influence of the spectrum range of the stimulation light on the bleaching efficiency is presented. # 2001 Elsevier Science B.V. All rights reserved. PACS: 78.60.K; 93.85 Keywords: Thermoluminescence; Trap spectroscopy; Thermoluminescence dating; K-feldspar
1. Introduction K-feldspars used in thermoluminescence (TL) and optically stimulated luminescence (OSL) dating for years, were intensively investigated by means of TL [1–3] and spectroscopic methods [4]. Recently, because of the OSL dating, the kinetics of infrared stimulated luminescence is the favourite subject of investigations [5,6]. The measurements of IRSL decay [7] and the activation energy of IRSL traps [8] are good steps for better understanding of this process in Kfeldspars. Simultaneously, the natural bleaching of TL is studied [9]. The bleaching of the luminescence of minerals before their deposition is an essential problem in both TL and OSL dating of sediments. This work presents the first, preliminary results of applying the fractional glow technique (FGT) [10,11] for investigation of the influence of light on *Fax: +48-56-62-25-397.
the population of traps in K-feldspars. The energy density spectrum of traps, and actually the spectrum of occupied traps which are active in TL process, were measured after different bleaching procedures.
2. Experimental 2.1. Apparatus The measurements were carried out using the TL equipment [12] with a temperature control system adapted for FGT with heating–cooling rate correction [13,14]. Heating and cooling with an average rate about 0.18C/s were linear in the greater part of each cycle and only this linear part was used for the determination of trap depth. The temperature range of the whole heating procedure is from room temperature to 3308C. It is enough to depopulate the major part of the traps in Kfeldspars normally explored by TL or OSL dating.
0022-2313/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 0 1 ) 0 0 1 6 9 - 7
272
A. Chrus´cin´ska / Journal of Luminescence 92 (2001) 271–276
The Schott BG-2 filter is installed before the EMI 6097 B photomultiplier in the light detection unit, so the waveband 390–440 nm [15] commonly used in dating was investigated. The sensitivity of the optical arrangement allowed the use of the FGT procedure for unbleached samples as well as for samples after 20 h of bleaching. The sunlight simulator [16] enables two options of bleaching} using the wavelength ranges 300–750 nm or 375– 750 nm (with the UV lamps switched off). The feldspar samples (mainly microcline)}in average about 15 mg of grains separated from sediment}were put directly on the heating strip. They were heated up to 5008C before the irradiation. The X-rays source used for TL excitation yield a dose of 60 Gy in 1 h. The 6 h excitation was followed by the optical bleaching. A few days later a preheat to 2008C with the heating rate 1.78C/s was applied. In the FGT measurement, which was carried out a day after preheat, a procedure with heating over 128C and subsequent cooling over 108C in every cycle was employed. Over 90 such cycles contribute effectively to trap spectrum when the sample is preheated. 2.2. FGT The FGT is quite adequate to complex glow curves analysis and its main advantage is independence from the TL kinetic order. It enables also to obtain good results when several competing recombination centres are present [17]. However, it should be stressed that the experimental results need always a careful handling and interpretation [14,18]. One should not forget that the value of activation energy calculated from experimental data in a FGT cycle is an average of the depth of traps emptied in the cycle. As the computer simulations [13] show, the FGT improved by the heating–cooling rate correction applied in the temperature region in question enables to distinguish discrete trap levels the depths of which differ by several dozen of meV. Experimentally, however, one can have to do with a discrete trap spectrum or a continuous distribution and even with a ‘‘very good looking result’’ one cannot definitely decide for one of these possibilities. So the occupation spectra obtained directly from
measurements should be treated rather as the initial data for further analysis. It should be stressed that this work presents the experimental results, which are waiting now for the detailed analysis.
3. Experimental results and discussion The area under the TL curve registered during the heating and cooling of the sample in the FGT cycle is the light-sum. A graph of the light-sum dependence on maximum temperature in the FGT cycle is a kind of TL glow curve (measured with very low heating rate}here about 0.0068C/s). Fig. 1 presents such curves obtained for an unbleached K-feldspar and after 2 and 20 h of bleaching. The shapes of these curves reflect the influence of optical stimulation on the TL signal. The low- and high-temperature edges of the curves are more easily bleached than the central part. The two types of bleaching treatments show different efficiencies. This is also reflected in the trap occupation spectra presented in the next figures. Every trap spectrum presented is an average of three histograms each of which resulting from a single experiment. The ‘‘arbitrary units’’ of the trap occupation are the same on all the diagrams. Fig. 2 shows the trap occupation spectra of a non-bleached feldspar together with the spectra after 2 h of stimulation applied with either spectral ranges. Three trap groups can be distinguished: traps shallower than 1.3 eV, traps with a depth between 1.3 and 1.6 eV and traps deeper then 1.6 eV. The deepest fraction is the most easily depopulated by the light. The shallower traps are a little bit more resistant to the optical stimulation but in the case of traps about 1.5 eV both spectral ranges seem to be still less effective. Figs. 3 and 4 present the changes of trap population with the time of bleaching separately for reduced (without UV part) and full range of the simulator light spectrum. As one can see, the traps with the energy below 1.3 eV are depopulated more effectively by the light with full spectrum range. There is no significant difference in trap occupation in this range after 2, 4 and even 20 h of bleaching (Fig. 4) so, it seems that a residual level
A. Chrus´cin´ska / Journal of Luminescence 92 (2001) 271–276
273
Fig. 1. The FGT light-sum curves: the area under the glow curve for heating and cooling part of a FGT cycle against the maximum temperature of the cycle; for K-feldspar after X-ray excitation and various bleaching procedures. The FGT measurements were carried out after a preheat to 2008C.
Fig. 2. The trap occupation spectra for (a) non-bleached K-feldspar, (b) after 2 h of bleaching by the light without UV part of spectrum, (c) after 2 h of bleaching with full range of simulator light spectrum. Each histogram is an average of three independent FGT experiments with the cycle procedure of 128C of heating and 108C of cooling. The preheat to 2008C was applied. All the results presented are mass normalised.
of the trap population is obtained or that there are present the weakly populated traps which are unbleachable. The light without UV part (Fig. 3) causes steady but slower emptying of these traps. After 20 h (Fig. 3c) the residual level is still not reached.
The traps over 1.6 eV disappear from the trap occupation spectra already after 2 h of stimulation by the light with a full spectrum (Fig. 4), whereas they remain occupied even after 20 h of bleaching by the light with a reduced range (Fig. 3). Their population is divided by ten but it is still detectable.
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A. Chrus´cin´ska / Journal of Luminescence 92 (2001) 271–276
Fig. 3. The trap occupation spectra for K-feldspar after 2 h (a), 4 h (b) and 20 h (c) of bleaching by the light without UV part of the spectrum. Same measurement procedure as for the results presented in previous figure.
Fig. 4. The trap occupation spectra for K-feldspar after 2 h (a), 4 h (b) and 20 h (c) of bleaching with the light of full simulator spectrum range. FGT procedure same as for the results presented in previous figures.
As one could expect the full range of simulator light spectrum should cause the more effective trap depopulation. This is true for the traps from both early mentioned energy ranges (below 1.3 eV and over 1.6 eV) but the third group of traps stands out. These traps are emptied during the FGT measurement at the temperature 200–2508C and correspond to the part of the light-sum curve, which is the most weakly bleached (Fig. 1). The analysis of Figs. 3 and 4 enables to divide these traps into two groups with the depth ranges:
1.3–1.45 eV and 1.45–1.6 eV. In the case of the first fraction, the residual level of occupation is reached already after 4 h of full spectrum stimulation (Fig. 4b). After 20 h of bleaching these traps remain as the highest populated ones. The traps from the range 1.45–1.6 eV are clearly seen in occupation spectra obtained after 4 h of the full spectrum stimulation (Fig. 4b) and after 20 h of reduced spectrum stimulation (Fig. 3c). This trap fraction is depopulated below the detectable level after 20 h of stimulation by the full range light
A. Chrus´cin´ska / Journal of Luminescence 92 (2001) 271–276
275
Fig. 5. The dependence of relative trap occupation on bleaching time. The occupation of a trap fraction (the sum of the histogram bars for this fraction) after the bleaching is divided by the occupation of the same fraction determined for the unbleached sample.
but the light without UV acts much less effective and it seems even to yield the residual level after some hours of bleaching (Fig. 3). A longer time of bleaching should be applied in order to check if there really exists a residual level for one kind of stimulation whereas it does not exist (or is much lower) for the other one. The dependence of relative trap occupation (trap occupation after bleaching divided by the occupation for unbleached sample) on the duration of the optical stimulation is shown in Fig. 5 for both spectral ranges. Some remarks should be added in connection with the trap depth values in K-feldspars described in the earlier papers. Apart from the possible differences between K-feldspar samples investigated by different authors, the correlation between the trap occupation spectra for unbleached samples presented here and the trap energy values determined for microcline by Strickertsson [2] and Visocekas et al. [4] is clear. It is interesting that the trap energy spectra after long optical bleaching (Fig. 4c) agree with the values determined for microcline by Ahmed and Gartia [1]. They give the energy value of about 1.4 eV for the deepest traps. Their FGT measurements were carried out after
applying the relatively low-excitation dose which, results in low trap occupation which is also the result of long optical stimulation. It can be discussed if the obtained trap occupation spectra are in fact the same as they were directly after switching off the bleaching light. According to the measurement procedure: excitation–bleaching–preheat, one can suppose that the results correspond to the equilibrium state reached after these three steps. One can imagine that after the depopulation of one trap fraction by the optical stimulation, the carriers from another, less stable traps repopulate this first fraction again and the thermal stability cannot play a role here alone. However, during all the experiments now presented one has not observed any change of trap occupation spectrum with the time of storage. The trap occupation change after switching off the optical stimulation could eventually be caused by the preheat to 2008C. The comparison of the results of some FGT measurements, which were carried out from the room temperature to 3308C, with those presented here does not show that preheating procedure applied causes the increase of population of traps which are active beyond the 2008C. Summing up, the measured trap
276
A. Chrus´cin´ska / Journal of Luminescence 92 (2001) 271–276
occupation results from the bleaching applied and, eventually, a rapid process which occurs straight after the bleaching and does not derive from the temperature stimulation.
4. Conclusion As one can see from the preliminary results, the FGT with the heating-cooling rate correction can be a useful tool to investigate the bleaching processes. The changes of trap occupation after bleaching and the influence of the spectrum range of the stimulation light on the occupation spectra are clearly demonstrated. The traps in the energy range 1.3–1.6 eV seem to be responsible for the residual TL which can be registered even after dozens of hours of bleaching and exists also in the nature in sediments after the sun bleaching. If so, they are the source of the problems with dating of not completely bleached sediments. The unambiguous recognition of trap kinds as discrete or distributed over a determined energy range will be possible after a detailed analysis. Acknowledgements This work was partially supported by the UMK Grant 464-F.
References [1] A.B. Ahmed, R.K. Gartia, Phys. Stat. Sol. A 94 (1985) 645. [2] K. Strickertsson, Nucl. Tracks Radiat. Meas. 10 (1985) 613. [3] Y. Kirsh, S. Shoval, P.D. Townsend, Phys. Stat. Sol. A 101 (1987) 253. [4] R. Visocekas, V. Tale, A. Zink, N.A. Spooner, I. Tale, Rad. Prot. Dosim. 66 (1996) 391. [5] G.A. Duller, A.G. Wintle, Nucl. Tracks Radiat. Meas. 18 (1991) 384. [6] U. Rieser, G. Hutt, M.R. Krbetschek, W. Stolz, Radiat. Meas. 27 (1997) 273. [7] R.J. Clark, I.K. Bailiff, M.J. Tooley, Radiat. Meas. 27 (1997) 211. [8] T. Trautmann, U. Rieser, W. Stolz, Radiat. Meas. 27 (1997) 193. [9] S.W.S. McKeever, Radiat. Meas. 23 (1994) 267. [10] H. Gobrecht, D. Hofmann, J. Phys. Chem. Solids 27 (1966) 509. [11] I.A. Tale, Phys. Stat. Sol. A 66 (1981) 65. [12] A. Pietkun, L. Polakiewicz, H.L. Oczkowski, Acta Phys. Polon. A 81 (1992) 687. [13] A. Chrus´ cin´ska, J. Lumin. 62 (1994) 115. [14] A. Chrus´ cin´ska, H.L. Oczkowski, K.R. Przegietka, Acta Phys. Polon. A 89 (1996) 555. [15] M.R. Krbetschek, J. Gotze, A. Dietrich, T. Trautmann, Radiat. Meas. 27 (1997) 695. [16] H.L. Oczkowski, K.R. Przegietka, Radiat. Meas. 23 (1998) 435. [17] R. Chen, A. Hag-Yahya, Rad. Prot. Dosim. 65 (1996) 17. [18] J.A. Kierstead, P.W. Levy, Nucl. Tracks Radiat. Meas. 18 (1991) 19.