Anomalous fading of the TL, Blue-SL and IR-SL signals of fluorapatite

Radiation Measurements 41 2006) 954 960 www.elsevier.com/locate/radmeas Anomalous fading of the TL, Blue-SL and IR-SL signals of fluorapatite N. Tsirliganis a,, G. Polymeris a,b, Z. Loukou a, G. Kitis b a Cultural and Educational Technology Institute, Archaeometry Laboratory, Tsimiski 58, 67100 Xanthi, Greece b Aristotle University of Thessaloniki, Nuclear Physics Laboratory, 54124 Thessaloniki, Greece Received 22 August 2005; received in revised form 27 March 2006; accepted 17 April 2006 Abstract The thermoluminescence TL), blue stimulated luminescence BSL) and infrared stimulated luminescence IRSL) signals of fluorapatite originating from Durango, Mexico) were measured and found to exhibit strong anomalous fading AF). The experimentally obtained OSL and IRSL decay curves were transformed into pseudo-linear modulated pseudo-lm) decay curves. The resulting glow-curve shaped pseudo-lm decay curves were analyzed using a deconvolution analysis, similar to the one used for the glow-curve deconvolution GCD) of TL glow-curves. It was found that the pseudo-lm OSL and IRSL decay curves consist of two components named fast and slow, respectively, and their individual contribution was estimated. The AF of the remnant TL, BSL and IRSL as a function of the storage time was fitted using the tunneling model equations and the fading rate g, in terms of percentage per decade was evaluated. According to the obtained g values, the AF of the BSL and IRSL is stronger than that of the TL. The AF of the fast component of BSL and IRSL is almost the same. The AF of the slow component of IRSL is, by approximately a factor of 2, stronger than that of the BSL. 2006 Elsevier Ltd. All rights reserved. Keywords: TL; OSL; Anomalous fading; Tunneling model; Fluorapatite 1. Introduction Anomalous or abnormal fading AF) is the term adopted for the rapid decay at room temperature of the high temperature thermoluminescence TL) glow-peak signal, contrary to the expected stability predicted by the basic TL kinetic models. The AF of the TL signal was observed in early 1950 according to Chen and McKeever 1997), however, Wintle first discussed its implications in dating archaeological samples Wintle, 1973, 1977). Several models were proposed to explain the effect Chen and McKeever, 1997; BZtter-Jensen et al., 2003). In these cases where the AF is temperature independent the quantum mechanical tunneling Visocekas et al., 1976; Visocekas, 1985) seems to be the most probable mechanism. For the cases where the AF is temperature dependent the suggested models are: the thermally assisted tunneling Visocekas, 1985), the localized transitions Templer, 1986), and a model which considers AF as normal fading in disguise Chen and Hag-Yahya, 1997). Corresponding author. Tel.: +30 2541078787; fax: +30 2541063656. E-mail address: tnestor@ceti.gr N. Tsirliganis). The TL of Durango apatite exhibits AF Vaz, 1982; Kitis et al., 1991). Kitis et al. 1991) have studied the AF rate as a function of grain size, annealing temperature, pre-dose and irradiation temperature and concluded that tunneling is the probable mechanism responsible for the AF in this material. The aim of the present work was: i) to investigate if the AF exhibited by the TL signal of Durango apatite appears also in the Blue-OSL BSL) and Infrared-OSL IRSL) signals, ii) to examine if tunneling is the responsible mechanism, and iii) to compare and search for possible relations between the AF of the TL, BSL, and IRSL signals. 2. Experimental procedure The study was performed on Durango apatite which is a nearly pure fluorapatite, having the formula Ca 5 PO 4 ) 3 F with a maximum of 0.45 wt% Cl, known to exhibit strong AF of its TL signal Vaz, 1982; Kitis et al., 1991). The sample was initially crushed, and grains with dimensions 2 10 μm were selected and deposited on aluminum disks of 1 cm 2 area using the Zimmerman method Zimmerman, 1971). The grains were annealed at 500 C for 1 h in a furnace Lindberg/Blue 1350-4487/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2006.04.024

N. Tsirliganis et al. / Radiation Measurements 41 2006) 954 960 955 M BF51800 series) with a maximum temperature of 1100 C and electronically controlled heating rate. The annealed samples were allowed to cool to room temperature by placing them on copper blocks. The measurements were performed using the RISO TL/OSL reader model TL/OSL-DA-15) equipped with a high-power blue LED light source, an infrared solid state laser and a 0.085 Gy/s 90 Sr/ 90 Y β-ray source. The TL measurements were performed using a combination of a Pilkington HA-3 heat-absorbing filter and a Corning 7-59 blue filter, using a heating rate of 5 C/s and a maximum temperature of 500 C. The OSL measurements BSL and IRSL) were performed using a Hoya U-340 filter at room temperature. The power level was software controlled and set at 90% for BSL measurements and 45% for ISRL measurements. The stimulation time was 100 and 300 s for BSL and IRSL, respectively. 3. Results and discussion 3.1. Anomalous fading of TL Glow-curve shapes for Durango apatite, annealed at 500 C for 1 h, are shown in Fig. 1. Curve a) was obtained immediately after the end of an irradiation with an administered dose of 15.3 Gy, while curve b) was obtained 10 days after the end of the irradiation. The glow-curve shape remains the same and is not influenced by the strong fading. The glow-curve has three distinct parts. Two groups of peaks centered approximately at 200 and 320 C, respectively, and a high temperature segment above 400 C which is the residual of glow-peaks centered above 500 C that are not erased by the annealing at 500 C for 1 h. Performing a series of successive irradiation-readout cycles, it was found that the sensitivity of the main peak at 320 C remains stable, over 12 cycles, whereas the high temperature integral decreases as a function of irradiation-readout cycle. According to Bowman 1988) the ratio of glow-curve b) Fig. 1. TL of Durango apatite: a) immediately after the end of the irradiation, and b) 10 days after the irradiation. Right axis: fading shown as a function of glow-curve temperature ratio of glow-curve b) over glow-curve a)). over the glow-curve a) should give the AF factor as a function of the glow-curve temperature. This ratio is represented in Fig. 1 by curve c). As can be seen from curve c) in the glowcurve region up to 200 C, the AF rate is higher in the low temperature part. This is consistent with the localized transition of thermal fading Templer, 1986; Chen and McKeever, 1997). The same is observed in the temperature region between 200 and 300 C although the AF rate is much lower. However, a definite argument is not possible since this temperature region is between two successive glow-peak groups. On the other hand, it is apparent that above 300 C 20 C before the main peak s maximum) the AF rate is higher as the temperature of the glow-peak increases. This is characteristic of the tunneling model. Therefore, at least the AF of the main peak is rather due to tunneling. Above 350 C, noconclusions can be derived safely, due to the very intense high temperature residual. Preliminary studies on samples annealed at 800 C and readout up to 600 C show that between 400 and 550 C exists a complex group of peaks, which also exhibits strong AF. The TL or OSL signal monitored in the present work, is the remnant TL or OSL, i.e. the TL or OSL remaining after the storage at room temperature for various times from the end of irradiation. The remnant TL or OSL is defined by the parameter r, which is the ratio of the TL or OSL signal at time t to the initial TL or OSL signal at time t 0. In the case of the tunneling model and for short irradiation times the intensity follows a t 1 low, i.e. I = K t, 1) where K is approximately constant. Then the remnant TL, OSL is given by the equation Chen and McKeever, 1997): ) t t m r = T Lt) T Lt 0 ) = tm dt t t tm dt t0 ln = t ln tm t0 ), 2) where t m is the maximum time for which the tunneling mechanism holds and t 0 is a short time between the end of irradiation and the measurement. Usually instead of Eq. 2), an equation of the following form is used Visocekas et al., 1976; Visocekas, 1985; Auclair et al., 2003): ) t r = A K ln, 3) t 0 where A=1, however, for fitting purposes, it can be considered as a free parameter varying around 1. The AF of the TL signal of the main peak is shown in Fig. 2, where the open circles correspond to the experimental data and the solid lines to their fitting to Eqs. 2) and 3). The resulting fitted lines almost coincide. The parameters obtained by the fitting are listed in Table 1. Combining Eqs. 2) and 3) one can get: K = 1 ln tm t0 ). 4)

956 N. Tsirliganis et al. / Radiation Measurements 41 2006) 954 960 Fig. 2. Anomalous fading of the main TL peak of Durango apatite. Open circles represent the experimental data and the solid line is the fitting of remnant TL versus lnt/t 0 ) to a tunneling model equation. Using the values of t 0 and t m listed in Table 1, it is found that K = 0.089, i.e. in very good agreement with K = 0.0884 found directly by Eq. 3). The fading rate in terms of percentage per decade g Aitken, 1985) is related to K Lamothe et al., 2003; BZtter-Jensen et al., 2003) as g = 230.2 K. 5) Using Eq. 5) a value of g = 20.34% is found for the TL signal of Durango apatite, see Table 1. 3.2. Anomalous fading of blue and IR stimulated luminescence BSL and IRSL decay curves of Durango apatite are shown in upper and lower Fig. 3. The test doses used were 15 Gy for BSl and 30 Gy for IRSL. In both figures, curves a) were obtained immediately after the end of the irradiation, and curves b) 240 and 140 min after the end of the irradiation for BSL and IRSL, respectively. Curve c) right Y-axis) is the ratio of the decay curve b) over curve a) in correspondence with the similar curve for the TL see Fig. 1). Curve c) represents the AF factor as a function of the stimulation time. This ratio clearly demonstrates that the AF rate varies strongly in the fast decay region of the OS decay curve, whereas the variation becomes very slow in the tail of the OS decay curve. The OSL both BSL and IRSL) decay curves a) and b) of Fig. 3, obtained using the continuous wave CW) stimulation, show a rather featureless decay function, similar to the phosphorescence intensity versus time or prompt isothermal decay of TL at a high decay temperature). Efforts to overcome this problem are reported by various authors Randall and Wilkins, 1945; Visocekas et al., 1976; Chen and Kristianpoller, 1986; Kirsh and Chen, 1991) and it was suggested that a plot of t It)as a function of lnt) gives a peak-shaped curve, which resembles the shape of the TL glow-peak with the same order of kinetics. The transformation of the decay curve to a peak-shaped one can be achieved in two ways: i) by extending the TL-like presentation method to OSL decay curves Kitis et al., 2002), and ii) by transforming the CW-OSL curves into pseudo-linear modulated curves pseudo-lm) Bulur, 2000). Using the nomenclature of Bulur 2000) for the transformation of the CW-OSL curves into pseudo-lm OSL curves, the first-order pseudo-lm OSL decay curve is given by I u) = n 0 b P u exp b 2 P u2 ), 6) where u = 2 t P, P is the total illumination time, n 0 is the number of trapped electrons, and b is the decay constant. For the case of general-order kinetics the pseudo-lm OSL decay curve is described by the equation: I u) = n 0 b [1 P u + β 1) b ] β/1 β u2, 7) 2 P where β is the kinetic order. Eqs. 6) and 7) can be further transformed using the relation for u m and I m given by Bulur 2000): P u m β = 1) = b, 8) I m β = 1) = n 0 u m exp 1 2 ), 9) 2 u m β) = β + 1 P ) N β 1) b, 10) n 0 I m β) = 2 n ) 0 β + 1 1 2 β β/1 β. 11) u m β + 1 Table 1 Values of fitting parameters K, t m and t 0 according to Eqs. 2) and 3) Effect Component K t m t 0 K = 1/ lnt m /t 0 ) g = 230.2K TL Main peak 0.0884 ± 0.003 2845.6 0.038 0.08909 20.34 Blue-SL Fast 0.1232 ± 0.004 887.44 0.183 0.1178 28.36 Blue-SL Slow 0.1047 ± 0.002 12008 1.2993 0.1095 24.1 IRSL Fast 0.1246 ± 0.006 62.887 0.023 0.1264 29.2 IRSL Slow 0.1839 ± 0.003 98.29 0.367 0.1789 41.178 The time t is hours for TL and minutes for OSL, IRSL.

N. Tsirliganis et al. / Radiation Measurements 41 2006) 954 960 957 Fig. 3. Upper figure: i) left Y-axis. CW Blue-SL decay curve of Durango apatite, obtained: a) after zero time after the end of irradiation, and b) obtained 240 min after the end of irradiation. ii) Right Y-axis. Ratio of the Blue-SL decay curve b) over curve a). Lower figure: i) left Y-axis. CW IRSL decay curve of Durango apatite, obtained: a) after zero time after the end of irradiation, and b) obtained 140 min after the end of irradiation. ii) Right Y-axis. Ratio of the IRSL decay curve b) over curve a). The equations derived and used in the present work are for first-order kinetics: ) I u) = 1.6488 Im u exp u2 u m 2 u 2 12) m and for general-order kinetics: I u) = I m β 1 u u m 2 β u2 u 2 + β + 1 ) β/1 β. 13) m 2 β The pseudo-lm OSL Eqs. 12) and 13) were used for the deconvolution of all experimental data. The curve fitting was performed using the MINUIT computer program James and Roos, 1981), while the goodness of fit was tested using the figure of FOM) merit of Balian and Eddy 1977) given by FOM = i Y Exper Y Fit, 14) A where Y Exper is the experimental glow-curve, Y Fit is the fitted glow-curve and A is the area of the fitted glow-curve. The background was simulated by an equation of the form: bg = z d t P, 15) Fig. 4. Upper figure: blue stimulated CW-OSL decay curve transformed into a pseudo-lm curve and deconvolution to its individual components FOM value 0.52%). Open circles represent experimental data while continuous lines the fitted lines for: a) the fast component, b) the slow component, and bg) the background. Lower figure: same as in upper figure but for IRSL FOM value 2.2%). where z d is the zero dose OSL signal after blue and IR stimulation and it is evaluated experimentally. The best fitting results were obtained for second-order kinetics Eq. 13)). Examples of the deconvolution are given in upper Fig. 4 for the blue light stimulation and in the lower Fig. 4 for the IR stimulation. In the case of the blue stimulation, the FOM values obtained were in all cases between 0.5% and 0.7%, while in the case of IR stimulation the FOM values were between 2% and 3% except in a few cases where the IRSL signal was drastically faded out and the statistics were bad. Both blue and IR stimulated decay curves consist of two components termed fast and slow. A first test concerns the stability of the OSL signal over many irradiation and readout cycles. The behavior as a function of successive irradiation optical simulation TL readout cycles is shown in upper Fig. 5. It was found that in the case of blue light stimulation the fast component is stable, whereas the slow component is sensitized approximately by a factor of 1.8 between the first and the last cycle. Blair et al. 2005) reported similar results for the case of three feldspars, adopting the explanation of Duller 1991), who attributed the sensitivity changes of feldspars by a

958 N. Tsirliganis et al. / Radiation Measurements 41 2006) 954 960 Fig. 6. Anomalous fading of the Blue-OSL signal. Open symbols represent experimental data, the solid line through curve b) is an eye guide, and solid straight lines are the fitting of remnant OSL, versus lnt/t 0 ) to a tunneling model equation for a) the fast component and b) the slow component. Fig. 5. Upper figure: sensitivity of the Blue-OSL signal as a function of successive irradiation OSL TL readout cycles: a) fast component, and b) slow component. Lower figure: the parameter u m2 of the slow component as a function of successive irradiation OSL TL readout cycles. progressive build-up of trapped charge, which changed the rate of trap filling during subsequent irradiations. The free parameter u m1 of the fast component was stable for all cycles and equal to 10.445 ± 0.007 s. On the other hand, as is shown in the lower Fig. 5, the parameter, u m2 of the slow component increases by a factor of approximately 20% between the first and the last cycle. The parameter u m is related to the stimulation rate b through Eq. 11), which for β = 2 becomes 2 u m = 3 P b N. 16) n 0 The stability of the fast OSL component and of the TL signal measured after each OSL readout, over all the irradiationreadout cycles, indicates that the variation of n 0 is the less probable case. Moreover, if one assumes that the sensitization is due to an increase of n 0, then the parameter u m should decrease contrary to the present observations. The other parameter that can vary is the stimulation rate b, which is equal to Φ σ, where Φ is the incident photon flux, which is constant and σ is the photoionization cross-section. The final suggestion for explaining the results of the upper Fig. 5 is to assume that the successive irradiation OSL TL readout cycles alter the defect state responsible for the slow component thus decreasing its photoionization cross-section. This assumption is also compatible with the analysis of Huntley et al. 1996) who determined σ from the ratio of the OSL intensity to the slope of the OSL decay curve BZtter-Jensen et al., 2003). In the case of the IRSL signal, both components were found to be stable over the 10 cycles tested. The values of the free parameters u m were u m1 =38.326±0.677 s and u m2 =175.396± 2.03 s for the fast and slow components, respectively. The AF of the OSL signal after blue light simulation of both components is shown in Fig. 6. The open symbols are the experimental data. The AF rate of the fast component is much stronger than that of the slow component. The solid lines are the fitted lines to Eqs. 2) and 3). The fitting parameters are shown in Table 1. Therefore, the BSL is removed by AF due to a tunneling effect, as is the case of the TL signal. The AF of the IRSL signal for both components is shown in Fig. 7. The open symbols are the experimental data. The solid lines are the fitted lines to Eqs. 2) and 3). It seems that the tunneling model fails to fit the last experimental points which correspond to the longer storage times. It must be noticed, however, that at these times the fading of the signal has exceeded 90% of that at zero time and as a result, the tunneling effect has been ended. This is verified by the t m tunneling duration) values obtained by the fitting to Eq. 2), which are shorter than the maximum storage times used. The parameters u m resulting from the deconvolution procedure are found to vary as the AF proceeds. The results are shown in Fig. 8. The general behavior is the same for both components and for both stimulation wavelengths. A possible explanation of this behavior is consistent with a decrease of n 0. This should be the case, since as the AF proceeds, the value of

N. Tsirliganis et al. / Radiation Measurements 41 2006) 954 960 959 A final point of interest is the possible alteration of the properties of Durango apatite, with the partial formation of the very sensitive phosphor CaF 2, at temperatures above 450 500 C. The simple readout up to 500 C does not seem to have any influence on either TL sensitivity or AF. The sensitivity was found to be extremely stable over the successive irradiation readout cycles. This property was the same for samples annealed at 500 C for 1 h and samples that were not annealed. Similarly, AF is not changed. However, as it was found by Kitis et al. 1991) the sensitivity of the main peak is increased by a factor of 3 when the sample is heated to a temperature between 500 and 700 C and its position shifts to lower temperatures by about 200 C, whereas AF decreases very slightly. These results could be attributed to the CaF 2 formation within the fluorapatite structure. It seems that heat treatments below 500 C, as is the case in the present work, are not able to form CaF 2 in sufficient quantities that could influence either the sensitivity or the AF. Fig. 7. Anomalous fading of the IRSL signal. Open symbols represent experimental data, the solid line through curve b) is an eye guide, and solid straight lines are the fitting of remnant IRSL, r versus lnt/t 0 ) to a tunneling model equation for: a) the fast component and b) the slow component. 4. Conclusions The present work aimed at the investigation of the responsible mechanism and the comparison of the AF of the TL, BSL and IRSL signals of Durango apatite. The AF of the remnant TL, BSL and IRSL as a function of the storage time was recorded and fitted using the tunneling model equations and the fading rate g, in terms of percentage per decade was evaluated. The results indicate that the AF exhibited by the TL signal appears also in the BSL and IRSL signal. The behavior of AF of the main TL peak follows the tunneling model. In the fast decay region of the OSL decay curve, the AF rate varies strongly, whereas the variation becomes very slow in the tail of the curve. BSL is anomalously fading following the tunneling model, as is the case of the TL signal, while the model fails to describe the last experimental points which correspond to the longer storage times. Also the AF of the BSL and IRSL is stronger than that of the TL. The AF of the fast component of BSL and IRSL is almost the same. The AF of the slow component of IRSL is, by approximately a factor of 2, stronger than that of the BSL. References Fig. 8. Upper figure: the parameters u m1 and u m2 for the fast and slow component, respectively, obtained from the analysis of the results shown in Fig. 6. Lower figure: the parameters u m1 and u m2 of the fast and slow components, respectively, obtained from the analysis of the results shown in Fig. 7. n 0 decreases. Therefore, according to Eq. 16) the increase of the u m values of all components is due to the decrease of n 0 caused by the AF. Aitken, M.J., 1985. Thermoluminescence Dating. Academic Press, London, Appendix F. Auclair, M., Lamothe, M., Huot, S., 2003. Measurement of anomalous fading for feldspar IRSL using SAR. Radiat. Meas. 37, 487 492. Balian, H.G., Eddy, N.W., 1977. Figure-Of-Merit FOM). An improved criterion over the normalized Chi-squared test for assessing goodness-of-fit of gamma-ray spectral peaks. Nucl. Instrum. Methods 145, 389 395. Blair, M.W., Yukihara, E.G., McKeever, S.W.S., 2005. Experiences with single-aliquot OSL procedures using coarse-grain feldspars. Radiat. Meas. 39, 361 374. BZtter-Jensen, L., SMcKeever, S.W., Wintle, A.G., 2003. Optically Stimulated Luminescence Dosimetry. Elsevier, Amsterdam. Bowman, S.G.E., 1988. Observations of anomalous fading in maiolica. Nucl. Tracks Radiat. Meas. 14, 131 137. Bulur, E., 2000. A simple transformation for converting CW-OSL curves to LM-OSL curves. Radiat. Meas. 32, 141 145. Chen, R., Hag-Yahya, A., 1997. A new possible interpretation of the anomalous fading in thermoluminescent materials as normal fading in disguise. Radiat. Meas. 27, 205 210.

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