- Email: [email protected]
Optically stimulated luminescence in Cu+ doped lithium orthophosphate
Physica B 458 (2015) 117–123
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Optically stimulated luminescence in Cu þ doped lithium orthophosphate R.A. Barve a,n,1, R.R. Patil a, S.V. Moharil b, B.C. Bhatt c, M.S. Kulkarni d a
Government Institute of Science, R.T. Road, Civil Lines, Nagpur, India R.T.M Nagpur University, Nagpur, India c C/o RP&AD, BARC, Mumbai, India d RP&AD, Bhabha Atomic Research Centre, Mumbai, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 22 July 2014 Received in revised form 13 November 2014 Accepted 17 November 2014 Available online 20 November 2014
Optically stimulated luminescence (OSL) in Cu þ doped Li3PO4 synthesized by co-precipitation technique using different phosphorus precursors was studied. Changes in the luminescent properties were observed with change in the phosphorus precursors. All the synthesized phosphors showed intense fading but the OSL sensitivity was comparable to that of the commercially available Al2O3:C (Landauer Inc.). In general, BSL (blue stimulated luminescence) decay was very fast but the GSL (green stimulated luminescence) decay was comparable to that of Al2O3:C phosphor. Phosphors with fast decay, good sensitivity and intense fading are suitable for real-time dosimetry. Therefore, Cu-doped Li3PO4 could be developed for real-time dosimetry using a fiber optic based OSL reader system. & 2014 Elsevier B.V. All rights reserved.
Keywords: Inorganic materials Chemical synthesis Luminescence Transition metal ions
1. Introduction In the recent years, optical stimulated luminescence (OSL) technique is being widely used in almost all the dosimetric applications. In OSL the localized defects act as traps and capture electrons or holes generated by the ionizing radiation. These traps are stimulated by the light in the visible/IR region, which results in release of charge carriers of one sign, which then are able to recombine with charge carriers of the opposite sign, leading to the emission of light. OSL was first used in archeological dating [1] and later, with the development of Al2O3:C, proposed for personnel and environmental monitoring [2]. The general requirement for a material to be a good OSL phosphor is that the emission should be between 350 and 425 nm and the defects should have a high photoionization cross-section in blue-green region (450–550 nm) or IR region (650–800 nm). This limit on the wavelength is due to the availability of suitable filters, stimulation sources as well as sensitive PM tubes in this range, and most importantly the requirement of separation of stimulating wavelength from the emission wavelength which ensures better signal-to-noise ratio. Over nearly 20 years, Al2O3:C has been used for various dosimetric applications, using OSL technique, due to its excellent dosimetric properties. Several efforts have been made towards n
Corresponding author. E-mail address: [email protected] (R.A. Barve). 1 Present address: Radiological Safety Division, IGCAR, Kalpakkam, Tamil Nadu.
http://dx.doi.org/10.1016/j.physb.2014.11.024 0921-4526/& 2014 Elsevier B.V. All rights reserved.
development of different phosphors for dosimetric applications but it is mainly centered on natural/synthetic quartz, feldspars, oxides, fluorides, chlorides, etc. [3–7]. More recently, OSL of cerium, europium, samarium and copper ions doped into borate and silicate glasses has been reported [8–12]. Attempts were also made to develop/characterize materials like MgO:Tb [13], NaMgF3:Eu [14], BeO [15], LiMgPO4:Tb,B [16], Cu-doped alkali fluorosilicates [17], etc. But except for BeO, they still remain in development stage as far as their use in routine radiation dosimetry is concerned. Cudoped Li3PO4 could be another candidate as an OSL phosphor. Cu þ emission in Li3PO4 has been reported as early as 2001 [18]. In Li3PO4:Cu the emission is observed around 367 nm and thus it satisfies one of the important criteria for a material to be a good OSL phosphor. Earlier, thermoluminescence (TL) in Li3PO4 was studied by Naranje et al. [19] with the intention to the study role of phosphorus in LiF:Mg, Cu, P. This study led to the development of new Li3PO4 based phosphor for TL dosimetry. Detailed characterization of this phosphor was later done by Bhatt et al. [20]. Aghalte et al. [21] reported OSL in Li3PO4:Cu but the detailed study of this phosphor is not yet done. In the recent past we have studied the effect of micro phases of lithium phosphate on the OSL sensitivity in Cu-doped Li3PO4 [22]. In this paper we report the OSL properties of this phosphor synthesized using different precursors and their effect on TL/OSL properties. This study may lead to the development of highly sensitive Cu-doped Li3PO4 OSL phosphor for radiation dosimetry.
118
R.A. Barve et al. / Physica B 458 (2015) 117–123
2. Experimental Cu-doped Li3PO4 was synthesized using co-precipitation method by reacting an analytical grade lithium chloride and the phosphorus precursor. CuCl2 solution in desired proportion (500 ppm) was added in lithium chloride solution before the reaction. The obtained precipitate was filtered, washed several times by double distilled water and then dried under a drying lamp to get the dried powder of doped Li3PO4. Some lithium and phosphorus precursor combinations did not yield precipitate. In such cases, the solution was evaporated to dryness to get the white powder. This powder was then washed several times to remove the unwanted byproducts/un-reacted salts and yield Li3PO4. All the powders were heated in air in graphite crucible for 25 min at 600 °C. Various synthesized samples were labeled as Sample A: from the reaction 3LiCl þ Na2HPO4-Li3PO4 þ2NaClþ HCl Sample B: from the reaction 3LiCl þ H3PO4-Li3PO4 þ 3HCl Sample C: from the reaction 3LiCl þ NaH2PO4-Li3PO4 þNaClþ2HCl For studying the TL/OSL response, all the samples were exposed to a test dose of 100 mGy using 90Sr/90Y beta source with the dose rate of 20 mGy/min. All the samples were normalized with respect to weight and dose and the measurements were carried out under identical conditions. Photoluminescence studies were carried on Hitachi-4000 Spectrofluorometer. OSL and TL measurements were taken on the locally fabricated set-up [23]. The assembly consists of array of blue/green LEDs as a stimulation source with power adjustable from 11 to 48 mW/cm2. Two optical filters viz. UG-1 (across PMT), to prevent stimulation signal from reaching PMT (9111B, 25 mm diameter end window PMT) and GG-435 (across LEDs), to cut-off the stimulation wavelengths below 435 nm, were used in the assembly. All the operations in the assembly are controlled by the suitable software. During all the OSL measurements the LED power was kept at 11 mW/cm2 and signal was recorded for 200 s with the acquisition time 0.1 s. All the TL measurements were taken at the heating rate 4 °C/s. The TL was recorded in air atmosphere with HA-3 filter used on PMT side.
Fig. 1. X-ray diffraction patterns of various Li3PO4 samples heated at 600 °C. (a) Sample A, (b) sample C and (c) sample B. Table 1 Relative intensity of 100% line of Li4P2O7 and Li2Cu2P6O18 phases with the most intense diffraction line of Li3PO4 in the three synthesized samples. Li3PO4 sample
Li4P2O7
Li2Cu2P6O18
Sample A (%) Sample C (%) Sample B (%)
20 12 4
2.2 3.5 5.7
3. Results and discussion 3.1. X-ray diffraction analysis Fig. 1 shows the X-ray diffraction patterns of various Li3PO4 samples. Fig. 1a shows the XRD pattern of sample A heated at 600 °C. In this pattern along with major phase of Li3PO4, minor phase of Li4P2O7 (JCPDS no.: 77-1045) along with Li2Cu2P6O18 (JCPDS no.: 27-1232) is detected. The relative percentage of these phases can be found out by comparing the relative ratio of the most intense diffraction line of Li3PO4 to that of the 100% intense lines of these phases. When the relative ratios were compared, percentages of Li4P2O7 and Li2Cu2P6O18 in this sample were found to be 20% and 2.2%, respectively. Similar features were observed for samples B (Fig. 1c) and C (Fig. 1b) with different relative percentages of the minor phases and are summarized in Table 1. From the table it could be seen that the relative percentage of Li4P2O7 is highest in sample A, whereas relatively high percentage of Li2Cu2P6O18 was observed in sample B.
Fig. 2. Photoluminescence spectra of Cu-doped phosphor synthesized using different phosphorus precursors and heated at 600 °C. (a) Excitation curve of sample A, (b) excitation curve of sample C, (c) emission curve of sample A and (d) emission curve of sample C.
3.2. Photoluminescence properties Fig. 2 shows the photoluminescence spectra of Cu-doped phosphor synthesized using different phosphorus precursors. As-
prepared and un-doped samples do not exhibit any Cu fluorescence. Cu-doped sample A heated at 600 °C shows emission around 365 nm (Fig. 2c) with excitation at 254 nm (Fig. 2a). Cu-
R.A. Barve et al. / Physica B 458 (2015) 117–123
119
Fig. 3. Blue stimulated response (BSL) of various Cu-doped Li3PO4 samples heated in air: (a) sample A, (b) sample B, (c) sample C and (d) Al2O3:C.
doped sample C heated at 600 °C shows similar emission around 365 nm (Fig. 2d) having excitation at 257 nm (Fig. 2b). However, the PL intensity is found to be reduced and is nearly 80% as compared to that of sample A. Sample B heated at 600 °C does not yield Cu fluorescence. The observed Cu þ emission could be attributed to 3d94s1-3d10 transitions of Cu ion and the decrease in the PL intensity could be correlated to the increase of Li2Cu2P6O18 phase. In this phase the valency of Cu is 2 þ and hence substantial Cu þ ions get converted into Cu2 þ ions reducing the Cu þ photoluminescence. In sample B, the relative percentage of this phase is highest and hence no Cu photoluminescence is observed. Thus it can be concluded that the formation of
Li2Cu2P6O18 phase is responsible for quenching of Cu luminescence in Li3PO4. 3.3. Optically stimulated luminescence properties Fig. 3 shows the BSL (blue stimulated luminescence) response of various Cu-doped Li3PO4 samples heated in air. The BSL of sample A decays within 12 s (Fig. 3a) whereas the BSL signals of samples B and C decay relatively slowly. The complete signal decays within 16 s (Fig. 3b and c). The decay of green stimulated luminescence (GSL) (Fig. 4) is slower as compared to the BSL. The GSL signals of samples A and C
Fig. 4. Green stimulated response (GSL) of various Cu-doped Li3PO4 samples heated in air: (a) sample A, (b) sample B, (c) sample C and (d) Al2O3:C.
120
R.A. Barve et al. / Physica B 458 (2015) 117–123
Table 2 Comparison of CW-OSL sensitivities of various Li3PO4 samples with Al2O3:C (Landauer Inc.). BSL sensitivity as compared to Al2O3:C Sample A 39.6 Sample C 17.4 Sample B 0.66
GSL sensitivity as compared to Al2O3:C 7.2 3.0 0.10
decay in 120 s (Fig. 4a and c). However, the GSL decay of sample B is relatively fast as compared to that of sample A. The complete signal decays within 100 s (Fig. 4b). Of all the samples, sample A was observed to be highly sensitive. The BSL sensitivity of the samples B and C was found to be 2% and 43%, respectively, as compared to that of sample A, using area integration method. The GSL sensitivities of the samples B and C were found to be 1% and 49%, respectively, as compared to that of sample A. The BSL and GSL sensitivities of various Li3PO4 samples were also compared with the BSL and GSL sensitivities of Al2O3:C (Landauer Inc.). Since the BSL signals of Li3PO4 and Al2O3:C decay at different rates, area integration method may not be suitable to compare the sensitivities of the phosphors. Hence, the method suggested by Yukihara et al. [24] was employed. However, the GSL decay of both the phosphors is comparable. Hence the sensitivity comparison for GSL is done on the basis of area integration method-OSL integrated over the time period of 200 s. The corresponding factors are presented in Table 2. From the table it can be seen that the sample A shows the highest OSL sensitivity whereas the sample B shows the lowest. Observed intense OSL in sample A could be attributed to the presence of minor phase of Li4P2O7 which leads to production of large number of defects and relatively higher density of filled traps during irradiation with ionizing radiation. Such observations were also made earlier in LiF:Mg, Cu, P TLD phosphor in which the TL sensitivity as well as shape of the TL glow curve was found to be dependent on minor phases of Li4P2O7 and Li2Cu2P6O18 [25]. Thus, the observed decrease in the OSL sensitivity in samples B and C may be due to decrease in the contribution of Li4P2O7 phase which reduces the number of defects responsible for TL and OSL. Fig. 5 shows the BSL decay curve of Cu-doped sample A along with the fitted components. The decay curve can be fitted with the equation:
IOSL = A1 exp (−t /τ1) + A2 exp (−t /τ2 ) + A 3 exp (−t /τ3 )
Fig. 5. Fitted BSL decay curve of Cu-doped sample A.
(1)
where IOSL is the initial OSL intensity, A1, A2, A3 are constant coefficients and τ1, τ2, τ3 are the decay constants of the respective OSL traps. The BSL decay curves of Cu-doped samples B and C are also similar to that represented by the Eq. (1). This indicates the presence of wide distribution of traps having different optical trap depths and photoionization cross-sections. The constant coefficients, photoionization cross-sections and decay constants for the three samples are given in Table 3. Fig. 6 shows the GSL decay curve of Cu-doped Li3PO4 along with the fitted components. The decay curve can be exactly fitted with the equation:
IOSL = A1 exp ( − t /τ1) + A2 exp ( − t /τ2 )
((2)
where IOSL is the initial OSL intensity, A1 and A2 are constant coefficients and τ1 and τ2 are the decay constants of the respective OSL traps. The GSL decay curve of Cu-doped sample B can also be fitted with sum of two exponentials similar to that represented by Eq. (2). However, the GSL decay curve of Cu-doped sample C can be fitted with the sum of three exponentials similar to that for BSL (i.e. Eq. (1)). The coefficients, photoionization cross-sections and decay constant values are given in Table 3. If the photoionization cross-sections of the various components of BSL response of various Li3PO4 samples are compared to that of the GSL response then it can be seen that the photoionization cross-section for BSL is far greater than that for GSL. This indicates the better and faster readout possibility from the sample when stimulated with the blue light. 3.4. Thermoluminescence properties Fig. 7 shows the TL response of Cu-doped samples A, B and C heated in air at 600 °C. The undoped and as-prepared samples do not exhibit any TL. Sample A shows an intense TL peak at 120 °C (Fig. 7a). The TL recorded after taking OSL shows depletion of the low temperature peak with a build-up of small peaks around 185 °C and broad peak around 350 °C, correlating the low temperature peak to the observed OSL (Fig. 7b). Fig. 8 shows CW-OSL of sample A recorded after thermally removing the low temperature peak. The integral OSL after depleting low temperature peak is found to be 6% of the original signal (Fig. 8a and b) thereby indicating that 94% of the total signal is attributable to the traps up to 120 °C glow peak. Since 94% OSL is from the shallow traps, 94% post-irradiation fading could be expected. This is indeed observed when the BSL was recorded after 192 h from the time of irradiation as shown in the inset of Fig. 8 (curves c and d). Similar behavior is observed in case of samples B and C. The fading in these samples is 98% after 192 h from the time of irradiation. The BSL decay curve of sample A recorded after heating up to 120 °C obeys exponential decay and could be fitted using Eq. (2). The corresponding values of the decay constants and photoionization cross-sections are presented in Table 3. From Table 3 it can be observed that the medium component in the BSL decay curve for an OSL readout taken after heating the sample up to 120 °C vanishes. This indicates that removal of the low temperature peak depletes the medium component traps along with the partial depletion of fast component traps. Fig. 7 also shows the TL glow curve for sample A obtained after thermally removing the low temperature peak followed by its BSL readout (Fig. 7c). Two peaks are observed; one peak around 185 °C and another broad peak around 320 °C similar to the curve b. But, the intensity is marginally less than that observed in the curve b. The TL glow curve for the sample B is similar to that of the sample A. Two peaks, one around 115 °C and another broad peak
R.A. Barve et al. / Physica B 458 (2015) 117–123
121
Table 3 CW-OSL parameters of various Cu-doped Li3PO4 samples. CW-OSL component
Coefficient
Decay constant τ (s)
Photoionization cross-section s (cm2)
Coefficient
Decay constant τ (s)
Photoionization cross-section s (cm2)
BSL response of sample A Fast 9 105 (A1) 0.7 Medium 3 105 (A2) 2.4 Slow 2 104 (A3) 9.9
0.6 10 16 0.2 10 16 0.4 10 17
BSL response of sample B 9 103 (A1) 0.6 7 103 (A2) 1.7 7 102 (A3) 7.7
0.7 10 16 0.2 10 16 0.5 10 17
BSL of sample A recorded after heating up to 120 °C Fast 9 103 (A1) 0.8 Medium – – Slow 6 103 (A3) 6.4
0.5 10 16 – 0.7 10 17
GSL response of sample B 8 102 (A1) 7 – – 3 102 (A3) 25.3
0.5 10 17 – 0.1 10 17
GSL response of sample A Fast 9 104 (A1) 5.2 Medium – – Slow 4 104 (A3) 23.8
0.7 10 17 – 0.2 10 17
BSL response of sample C 3 105 (A1) 0.6 2 105 (A2) 1.9 2 104 (A3) 7.6
0.7 10 16 0.2 10 16 0.6 10 17
GSL response of sample C Fast 4 104 (A1) 0.98 Medium 2 104 (A2) 7.5 Slow 104 (A3) 30.8
0.4 10 16 0.6 10 17 0.1 10 17
Fig. 6. Fitted GSL decay curve of Cu-doped sample A.
around 340 °C, are observed (Fig. 7d). However, the TL sensitivity is found to be 16% of the sample A. TL recorded after taking OSL shows depletion of low temperature peak with the appearance of broad peak around 350 °C; thus correlating the low temperature peak to the observed OSL (Fig. 7e). Similar to the sample B, the sample C shows intense TL peak at 110 °C having a shoulder peak at 160 °C (Fig. 7f), but the TL sensitivity is found to be 52% of that of the sample A. No TL is observed in the sample after recording OSL; thus correlating both the peaks to the observed OSL. The observed decrease in the TL sensitivity of the samples B and C could be attributed to decrease in the minor phase Li4P2O7. Decrease in the contribution of this phase reduces the number of defects responsible for TL resulting in decrease in the TL sensitivity. The enhancement of high temperature peak in the samples A and C suggests that carriers generated during OSL readout get re-trapped at the optically insensitive TL traps responsible for the TL peaks around 200–400 °C. This indicates that two types of traps are present viz., (i) traps amenable to TL as well as OSL and (ii) traps amenable to TL but do not yield OSL signal during optical stimulation.
Fig. 7. Thermoluminescence (TL) response of various Cu-doped Li3PO4 samples heated in air: (a) TL of sample A recorded immediately after irradiation, (b) TL of sample A recorded after taking its BSL readout, (c) TL of sample A recorded after depleting a low temperature peak followed by BSL, (d) TL of sample B recorded immediately after irradiation, (e) TL of sample B recorded after taking its BSL readout, and (f) TL of sample C recorded immediately after irradiation.
3.5. Dose response The dose response of the OSL signal of various samples is studied in the dose range of 10 mGy to 1 Gy using 90Sr/90Y beta-ray source. For each dose the OSL measurement is taken at room temperature for 200 s. The dose response plots of Cu-doped samples A and B are shown in Fig. 9a and b, respectively. The OSL output from a sample is usually directly related to the radiation dose absorbed by the sample. General equation for dose vs OSL response of an OSL phosphor can be represented as
R = kDm
(3)
where R is OSL output (weight normalized) from the sample, k is a proportionality constant having units counts. Gy m (generally, represented as counts/Gym), D is absorbed dose in gray (Gy) and m is a dimensionless parameter which measures the proportionality between absorbed dose (D) and R. Eq. (3) is a heuristic
122
R.A. Barve et al. / Physica B 458 (2015) 117–123
0.92 and 0.95, respectively. The deviation from m ¼1 (for linear response) indicates some sub-linearity in the dose response curve. 3.6. Minimum detectable dose On the described set-up, the minimum detectable dose (equivalent to 3s of the background counts of the annealed and unexposed sample) is found to be 0.18 mGy. For the calculation of MDD the standard deviation (s) is calculated by recording OSL from an unexposed sample (10 measurements) and comparing it with the OSL of the irradiated sample (20 mGy).
4. Conclusion
Fig. 8. Blue stimulated response (BSL) of Cu-doped sample A heated in air: (a) BSL recorded after depleting a low temperature peak, (b) BSL recorded immediately after irradiation, (c) (inset) BSL recorded immediately after irradiation, and (d) (inset) BSL recorded after 192 h from the time of irradiation.
The study shows that Cu-doped lithium phosphate could be synthesized by a simple co-precipitation method. All the samples synthesized using different phosphorus precursors yields Li3PO4 in same phase along with minor phases of Li4P2O7 and Li2Cu2P6O18. The relative percentage of these phases was found to be dependent on phosphorus precursor; these phases have profound effect on the luminescent properties of Li3PO4. Amongst the studied phosphors, intense OSL is observed in the Cu-doped sample A and its sensitivity is much higher as compared to that observed for commercial Al2O3:C. The BSL decay of sample A is very fast as compared to that of commercial Al2O3:C. However, the GSL decay is slow, similar to that of the commercial Al2O3:C. The observed decrease in PL, TL and OSL sensitivity of Cu-doped samples B and C could be attributed to the decrease in the contribution of the Li4P2O7 phase and increase in contribution of the Li2Cu2P6O18 phase. Even though good sensitivity is observed in most of the samples, they exhibit intense post-irradiation fading in 192 h. Though a fast fading OSL phosphor is not suitable for application in personnel and environmental dosimetry, it may find application for real-time dosimetry in a readout system using fiber optics, such as medical dosimetry.
Acknowledgment The authors are grateful to B.R.N.S. (Sanction no. 2008/37/20) for funding this work.
References
Fig. 9. OSL dose response of Cu-doped samples A and B. (a) Dose response of Cudoped sample A and (b) dose response of Cu-doped sample B.
presentation that usually holds up to a certain dose where saturation effects are usually observed. Corresponding logarithmic equation will be
log R = log k + m log D
(4)
If m ¼0, it implies that the response is independent from the dose and when m ¼ 1, dependence is said to be linear. However, in the present case, the values of m for Fig. 9a and b are found to be
[1] D.J. Huntley, D.I. Godfrey-Smith, M.L.W. Thewatt, Nature 313 (1985) 105. [2] M.S. Akselrod, V.S. Kortov, D.J. Kravetsky, V.I. Gotlib, Radiat. Prot. Dosim. 32 (1990) 15. [3] N. Kristianpoller, M. Abu-Rayya, R. Chen, J. Lumin. 60&61 (1994) 540. [4] L. Bøtter-Jensen, S.W.S. McKeever, Radiat. Prot. Dosim. 65 (1996) 273. [5] L. Bøtter-Jensen, H. Jungner, V. Mejdahl, Radiat. Prot. Dosim. 47 (1993) 643. [6] I. Jaek, G. Hutt, I. Vassiltchenko, Radiat. Meas. 27 (1997) 473. [7] A.S. Pradhan, J.I. Lee, J.L. Kim, J. Med. Phys. 33 (3) (2008) 85. [8] J. Qiu, Y. Shimizugawa, Y. Iwabuchi, K. Hirao, Appl. Phys. Lett. 71 (1997) 43. [9] J. Qiu, N. Sugimoto, Y. Iwabuchi, K. Hirao, J. Non-Cryst. Solids 209 (1997) 200. [10] J. Qiu, Y. Shimizugawa, Y. Iwabuchi, K. Hirao, Appl. Phys. Lett. 71 (1997) 759. [11] J. Qiu, Y. Shimizugawa, N. Sugimoto, K. Hirao, J. Non-Cryst. Solids 222 (1997) 290. [12] B.L. Justus, S. Rychnovsky, M.A. Miller, K.J. Pawlovich, A.L. Huston, Radiat. Prot. Dosim. 74 (1997) 151. [13] A.J.J. Bos, M. Prokic, J.C. Brouwer, Radiat. Prot. Dosim. 119 (2006) 130. [14] C. Dotzler, G.V.M. Williams, U. Reiser, A. Edgar, Appl. Phys. Lett. 91 (2007) 121910–121911. [15] M. Sommer, R. Fraudenberg, J. Henniger, Radiat. Meas. 42 (2007) 617. [16] B.S. Dhabekar, S.N. Menon, A. Raja, A.K. Bakshi, A.K. Singh, M.P. Chougaonkar, Y.S. Mayya, Nucl. Instrum. Methods B 269 (2011) 1844. [17] R. Barve, R.R. Patil, N.P. Gaikwad, M.S. Kulkarni, D.R. Mishra, A. Soni, B.C. Bhatt, S.V. Moharil, Radiat. Meas. 59 (2013) 73. [18] R.R. Patil, S.V. Moharil, Phys. Status Solidi A 187 (2001) 557. [19] S.M. Naranje, S.V. Moharil, Phys. Status Solidi A 165 (1998) 489. [20] B.C. Bhatt, B.S. Dhabekar, S.S. Sanaye, S.S. Shinde, S.V. Moharil, T.K. Gundu Rao,
R.A. Barve et al. / Physica B 458 (2015) 117–123
Radiat. Prot. Dosim. 100 (1–4) (2002) 251. [21] G.A. Aghalte, S.K. Omanwar and S.V. Moharil, 2009, Proceedings of the National Conference on Luminescence and its Application (NCLA-2009) at IACS, CGCRI, Kolkata, Vol. XIX 71, pp. 272. [22] R. Barve, R.R. Patil, M.S. Kulkarni, B.C. Bhatt, S.V. Moharil, J. Lumin. 156 (2014) 25. [23] M.S. Kulkarni, D.R. Mishra, D.N. Sharma, Nucl. Instrum. Methods B 262 (2007) 348.
123
[24] E.G. Yukihara, V.H. Whitley, S.W.S. McKeever, A.E. Akselrod, M.S. Akselrod, Radiat. Meas. 38 (2004) 317. [25] F. Sun, L. Jiao, J. Wu, Z. Yang, S. Yuan, G. Dai, Radiat. Prot. Dosim. 51 (3) (1994) 183.
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Optically stimulated luminescence in Cu þ doped lithium orthophosphate R.A. Barve a,n,1, R.R. Patil a, S.V. Moharil b, B.C. Bhatt c, M.S. Kulkarni d a
Government Institute of Science, R.T. Road, Civil Lines, Nagpur, India R.T.M Nagpur University, Nagpur, India c C/o RP&AD, BARC, Mumbai, India d RP&AD, Bhabha Atomic Research Centre, Mumbai, India b
art ic l e i nf o
a b s t r a c t
Article history: Received 22 July 2014 Received in revised form 13 November 2014 Accepted 17 November 2014 Available online 20 November 2014
Optically stimulated luminescence (OSL) in Cu þ doped Li3PO4 synthesized by co-precipitation technique using different phosphorus precursors was studied. Changes in the luminescent properties were observed with change in the phosphorus precursors. All the synthesized phosphors showed intense fading but the OSL sensitivity was comparable to that of the commercially available Al2O3:C (Landauer Inc.). In general, BSL (blue stimulated luminescence) decay was very fast but the GSL (green stimulated luminescence) decay was comparable to that of Al2O3:C phosphor. Phosphors with fast decay, good sensitivity and intense fading are suitable for real-time dosimetry. Therefore, Cu-doped Li3PO4 could be developed for real-time dosimetry using a fiber optic based OSL reader system. & 2014 Elsevier B.V. All rights reserved.
Keywords: Inorganic materials Chemical synthesis Luminescence Transition metal ions
1. Introduction In the recent years, optical stimulated luminescence (OSL) technique is being widely used in almost all the dosimetric applications. In OSL the localized defects act as traps and capture electrons or holes generated by the ionizing radiation. These traps are stimulated by the light in the visible/IR region, which results in release of charge carriers of one sign, which then are able to recombine with charge carriers of the opposite sign, leading to the emission of light. OSL was first used in archeological dating [1] and later, with the development of Al2O3:C, proposed for personnel and environmental monitoring [2]. The general requirement for a material to be a good OSL phosphor is that the emission should be between 350 and 425 nm and the defects should have a high photoionization cross-section in blue-green region (450–550 nm) or IR region (650–800 nm). This limit on the wavelength is due to the availability of suitable filters, stimulation sources as well as sensitive PM tubes in this range, and most importantly the requirement of separation of stimulating wavelength from the emission wavelength which ensures better signal-to-noise ratio. Over nearly 20 years, Al2O3:C has been used for various dosimetric applications, using OSL technique, due to its excellent dosimetric properties. Several efforts have been made towards n
Corresponding author. E-mail address: [email protected] (R.A. Barve). 1 Present address: Radiological Safety Division, IGCAR, Kalpakkam, Tamil Nadu.
http://dx.doi.org/10.1016/j.physb.2014.11.024 0921-4526/& 2014 Elsevier B.V. All rights reserved.
development of different phosphors for dosimetric applications but it is mainly centered on natural/synthetic quartz, feldspars, oxides, fluorides, chlorides, etc. [3–7]. More recently, OSL of cerium, europium, samarium and copper ions doped into borate and silicate glasses has been reported [8–12]. Attempts were also made to develop/characterize materials like MgO:Tb [13], NaMgF3:Eu [14], BeO [15], LiMgPO4:Tb,B [16], Cu-doped alkali fluorosilicates [17], etc. But except for BeO, they still remain in development stage as far as their use in routine radiation dosimetry is concerned. Cudoped Li3PO4 could be another candidate as an OSL phosphor. Cu þ emission in Li3PO4 has been reported as early as 2001 [18]. In Li3PO4:Cu the emission is observed around 367 nm and thus it satisfies one of the important criteria for a material to be a good OSL phosphor. Earlier, thermoluminescence (TL) in Li3PO4 was studied by Naranje et al. [19] with the intention to the study role of phosphorus in LiF:Mg, Cu, P. This study led to the development of new Li3PO4 based phosphor for TL dosimetry. Detailed characterization of this phosphor was later done by Bhatt et al. [20]. Aghalte et al. [21] reported OSL in Li3PO4:Cu but the detailed study of this phosphor is not yet done. In the recent past we have studied the effect of micro phases of lithium phosphate on the OSL sensitivity in Cu-doped Li3PO4 [22]. In this paper we report the OSL properties of this phosphor synthesized using different precursors and their effect on TL/OSL properties. This study may lead to the development of highly sensitive Cu-doped Li3PO4 OSL phosphor for radiation dosimetry.
118
R.A. Barve et al. / Physica B 458 (2015) 117–123
2. Experimental Cu-doped Li3PO4 was synthesized using co-precipitation method by reacting an analytical grade lithium chloride and the phosphorus precursor. CuCl2 solution in desired proportion (500 ppm) was added in lithium chloride solution before the reaction. The obtained precipitate was filtered, washed several times by double distilled water and then dried under a drying lamp to get the dried powder of doped Li3PO4. Some lithium and phosphorus precursor combinations did not yield precipitate. In such cases, the solution was evaporated to dryness to get the white powder. This powder was then washed several times to remove the unwanted byproducts/un-reacted salts and yield Li3PO4. All the powders were heated in air in graphite crucible for 25 min at 600 °C. Various synthesized samples were labeled as Sample A: from the reaction 3LiCl þ Na2HPO4-Li3PO4 þ2NaClþ HCl Sample B: from the reaction 3LiCl þ H3PO4-Li3PO4 þ 3HCl Sample C: from the reaction 3LiCl þ NaH2PO4-Li3PO4 þNaClþ2HCl For studying the TL/OSL response, all the samples were exposed to a test dose of 100 mGy using 90Sr/90Y beta source with the dose rate of 20 mGy/min. All the samples were normalized with respect to weight and dose and the measurements were carried out under identical conditions. Photoluminescence studies were carried on Hitachi-4000 Spectrofluorometer. OSL and TL measurements were taken on the locally fabricated set-up [23]. The assembly consists of array of blue/green LEDs as a stimulation source with power adjustable from 11 to 48 mW/cm2. Two optical filters viz. UG-1 (across PMT), to prevent stimulation signal from reaching PMT (9111B, 25 mm diameter end window PMT) and GG-435 (across LEDs), to cut-off the stimulation wavelengths below 435 nm, were used in the assembly. All the operations in the assembly are controlled by the suitable software. During all the OSL measurements the LED power was kept at 11 mW/cm2 and signal was recorded for 200 s with the acquisition time 0.1 s. All the TL measurements were taken at the heating rate 4 °C/s. The TL was recorded in air atmosphere with HA-3 filter used on PMT side.
Fig. 1. X-ray diffraction patterns of various Li3PO4 samples heated at 600 °C. (a) Sample A, (b) sample C and (c) sample B. Table 1 Relative intensity of 100% line of Li4P2O7 and Li2Cu2P6O18 phases with the most intense diffraction line of Li3PO4 in the three synthesized samples. Li3PO4 sample
Li4P2O7
Li2Cu2P6O18
Sample A (%) Sample C (%) Sample B (%)
20 12 4
2.2 3.5 5.7
3. Results and discussion 3.1. X-ray diffraction analysis Fig. 1 shows the X-ray diffraction patterns of various Li3PO4 samples. Fig. 1a shows the XRD pattern of sample A heated at 600 °C. In this pattern along with major phase of Li3PO4, minor phase of Li4P2O7 (JCPDS no.: 77-1045) along with Li2Cu2P6O18 (JCPDS no.: 27-1232) is detected. The relative percentage of these phases can be found out by comparing the relative ratio of the most intense diffraction line of Li3PO4 to that of the 100% intense lines of these phases. When the relative ratios were compared, percentages of Li4P2O7 and Li2Cu2P6O18 in this sample were found to be 20% and 2.2%, respectively. Similar features were observed for samples B (Fig. 1c) and C (Fig. 1b) with different relative percentages of the minor phases and are summarized in Table 1. From the table it could be seen that the relative percentage of Li4P2O7 is highest in sample A, whereas relatively high percentage of Li2Cu2P6O18 was observed in sample B.
Fig. 2. Photoluminescence spectra of Cu-doped phosphor synthesized using different phosphorus precursors and heated at 600 °C. (a) Excitation curve of sample A, (b) excitation curve of sample C, (c) emission curve of sample A and (d) emission curve of sample C.
3.2. Photoluminescence properties Fig. 2 shows the photoluminescence spectra of Cu-doped phosphor synthesized using different phosphorus precursors. As-
prepared and un-doped samples do not exhibit any Cu fluorescence. Cu-doped sample A heated at 600 °C shows emission around 365 nm (Fig. 2c) with excitation at 254 nm (Fig. 2a). Cu-
R.A. Barve et al. / Physica B 458 (2015) 117–123
119
Fig. 3. Blue stimulated response (BSL) of various Cu-doped Li3PO4 samples heated in air: (a) sample A, (b) sample B, (c) sample C and (d) Al2O3:C.
doped sample C heated at 600 °C shows similar emission around 365 nm (Fig. 2d) having excitation at 257 nm (Fig. 2b). However, the PL intensity is found to be reduced and is nearly 80% as compared to that of sample A. Sample B heated at 600 °C does not yield Cu fluorescence. The observed Cu þ emission could be attributed to 3d94s1-3d10 transitions of Cu ion and the decrease in the PL intensity could be correlated to the increase of Li2Cu2P6O18 phase. In this phase the valency of Cu is 2 þ and hence substantial Cu þ ions get converted into Cu2 þ ions reducing the Cu þ photoluminescence. In sample B, the relative percentage of this phase is highest and hence no Cu photoluminescence is observed. Thus it can be concluded that the formation of
Li2Cu2P6O18 phase is responsible for quenching of Cu luminescence in Li3PO4. 3.3. Optically stimulated luminescence properties Fig. 3 shows the BSL (blue stimulated luminescence) response of various Cu-doped Li3PO4 samples heated in air. The BSL of sample A decays within 12 s (Fig. 3a) whereas the BSL signals of samples B and C decay relatively slowly. The complete signal decays within 16 s (Fig. 3b and c). The decay of green stimulated luminescence (GSL) (Fig. 4) is slower as compared to the BSL. The GSL signals of samples A and C
Fig. 4. Green stimulated response (GSL) of various Cu-doped Li3PO4 samples heated in air: (a) sample A, (b) sample B, (c) sample C and (d) Al2O3:C.
120
R.A. Barve et al. / Physica B 458 (2015) 117–123
Table 2 Comparison of CW-OSL sensitivities of various Li3PO4 samples with Al2O3:C (Landauer Inc.). BSL sensitivity as compared to Al2O3:C Sample A 39.6 Sample C 17.4 Sample B 0.66
GSL sensitivity as compared to Al2O3:C 7.2 3.0 0.10
decay in 120 s (Fig. 4a and c). However, the GSL decay of sample B is relatively fast as compared to that of sample A. The complete signal decays within 100 s (Fig. 4b). Of all the samples, sample A was observed to be highly sensitive. The BSL sensitivity of the samples B and C was found to be 2% and 43%, respectively, as compared to that of sample A, using area integration method. The GSL sensitivities of the samples B and C were found to be 1% and 49%, respectively, as compared to that of sample A. The BSL and GSL sensitivities of various Li3PO4 samples were also compared with the BSL and GSL sensitivities of Al2O3:C (Landauer Inc.). Since the BSL signals of Li3PO4 and Al2O3:C decay at different rates, area integration method may not be suitable to compare the sensitivities of the phosphors. Hence, the method suggested by Yukihara et al. [24] was employed. However, the GSL decay of both the phosphors is comparable. Hence the sensitivity comparison for GSL is done on the basis of area integration method-OSL integrated over the time period of 200 s. The corresponding factors are presented in Table 2. From the table it can be seen that the sample A shows the highest OSL sensitivity whereas the sample B shows the lowest. Observed intense OSL in sample A could be attributed to the presence of minor phase of Li4P2O7 which leads to production of large number of defects and relatively higher density of filled traps during irradiation with ionizing radiation. Such observations were also made earlier in LiF:Mg, Cu, P TLD phosphor in which the TL sensitivity as well as shape of the TL glow curve was found to be dependent on minor phases of Li4P2O7 and Li2Cu2P6O18 [25]. Thus, the observed decrease in the OSL sensitivity in samples B and C may be due to decrease in the contribution of Li4P2O7 phase which reduces the number of defects responsible for TL and OSL. Fig. 5 shows the BSL decay curve of Cu-doped sample A along with the fitted components. The decay curve can be fitted with the equation:
IOSL = A1 exp (−t /τ1) + A2 exp (−t /τ2 ) + A 3 exp (−t /τ3 )
Fig. 5. Fitted BSL decay curve of Cu-doped sample A.
(1)
where IOSL is the initial OSL intensity, A1, A2, A3 are constant coefficients and τ1, τ2, τ3 are the decay constants of the respective OSL traps. The BSL decay curves of Cu-doped samples B and C are also similar to that represented by the Eq. (1). This indicates the presence of wide distribution of traps having different optical trap depths and photoionization cross-sections. The constant coefficients, photoionization cross-sections and decay constants for the three samples are given in Table 3. Fig. 6 shows the GSL decay curve of Cu-doped Li3PO4 along with the fitted components. The decay curve can be exactly fitted with the equation:
IOSL = A1 exp ( − t /τ1) + A2 exp ( − t /τ2 )
((2)
where IOSL is the initial OSL intensity, A1 and A2 are constant coefficients and τ1 and τ2 are the decay constants of the respective OSL traps. The GSL decay curve of Cu-doped sample B can also be fitted with sum of two exponentials similar to that represented by Eq. (2). However, the GSL decay curve of Cu-doped sample C can be fitted with the sum of three exponentials similar to that for BSL (i.e. Eq. (1)). The coefficients, photoionization cross-sections and decay constant values are given in Table 3. If the photoionization cross-sections of the various components of BSL response of various Li3PO4 samples are compared to that of the GSL response then it can be seen that the photoionization cross-section for BSL is far greater than that for GSL. This indicates the better and faster readout possibility from the sample when stimulated with the blue light. 3.4. Thermoluminescence properties Fig. 7 shows the TL response of Cu-doped samples A, B and C heated in air at 600 °C. The undoped and as-prepared samples do not exhibit any TL. Sample A shows an intense TL peak at 120 °C (Fig. 7a). The TL recorded after taking OSL shows depletion of the low temperature peak with a build-up of small peaks around 185 °C and broad peak around 350 °C, correlating the low temperature peak to the observed OSL (Fig. 7b). Fig. 8 shows CW-OSL of sample A recorded after thermally removing the low temperature peak. The integral OSL after depleting low temperature peak is found to be 6% of the original signal (Fig. 8a and b) thereby indicating that 94% of the total signal is attributable to the traps up to 120 °C glow peak. Since 94% OSL is from the shallow traps, 94% post-irradiation fading could be expected. This is indeed observed when the BSL was recorded after 192 h from the time of irradiation as shown in the inset of Fig. 8 (curves c and d). Similar behavior is observed in case of samples B and C. The fading in these samples is 98% after 192 h from the time of irradiation. The BSL decay curve of sample A recorded after heating up to 120 °C obeys exponential decay and could be fitted using Eq. (2). The corresponding values of the decay constants and photoionization cross-sections are presented in Table 3. From Table 3 it can be observed that the medium component in the BSL decay curve for an OSL readout taken after heating the sample up to 120 °C vanishes. This indicates that removal of the low temperature peak depletes the medium component traps along with the partial depletion of fast component traps. Fig. 7 also shows the TL glow curve for sample A obtained after thermally removing the low temperature peak followed by its BSL readout (Fig. 7c). Two peaks are observed; one peak around 185 °C and another broad peak around 320 °C similar to the curve b. But, the intensity is marginally less than that observed in the curve b. The TL glow curve for the sample B is similar to that of the sample A. Two peaks, one around 115 °C and another broad peak
R.A. Barve et al. / Physica B 458 (2015) 117–123
121
Table 3 CW-OSL parameters of various Cu-doped Li3PO4 samples. CW-OSL component
Coefficient
Decay constant τ (s)
Photoionization cross-section s (cm2)
Coefficient
Decay constant τ (s)
Photoionization cross-section s (cm2)
BSL response of sample A Fast 9 105 (A1) 0.7 Medium 3 105 (A2) 2.4 Slow 2 104 (A3) 9.9
0.6 10 16 0.2 10 16 0.4 10 17
BSL response of sample B 9 103 (A1) 0.6 7 103 (A2) 1.7 7 102 (A3) 7.7
0.7 10 16 0.2 10 16 0.5 10 17
BSL of sample A recorded after heating up to 120 °C Fast 9 103 (A1) 0.8 Medium – – Slow 6 103 (A3) 6.4
0.5 10 16 – 0.7 10 17
GSL response of sample B 8 102 (A1) 7 – – 3 102 (A3) 25.3
0.5 10 17 – 0.1 10 17
GSL response of sample A Fast 9 104 (A1) 5.2 Medium – – Slow 4 104 (A3) 23.8
0.7 10 17 – 0.2 10 17
BSL response of sample C 3 105 (A1) 0.6 2 105 (A2) 1.9 2 104 (A3) 7.6
0.7 10 16 0.2 10 16 0.6 10 17
GSL response of sample C Fast 4 104 (A1) 0.98 Medium 2 104 (A2) 7.5 Slow 104 (A3) 30.8
0.4 10 16 0.6 10 17 0.1 10 17
Fig. 6. Fitted GSL decay curve of Cu-doped sample A.
around 340 °C, are observed (Fig. 7d). However, the TL sensitivity is found to be 16% of the sample A. TL recorded after taking OSL shows depletion of low temperature peak with the appearance of broad peak around 350 °C; thus correlating the low temperature peak to the observed OSL (Fig. 7e). Similar to the sample B, the sample C shows intense TL peak at 110 °C having a shoulder peak at 160 °C (Fig. 7f), but the TL sensitivity is found to be 52% of that of the sample A. No TL is observed in the sample after recording OSL; thus correlating both the peaks to the observed OSL. The observed decrease in the TL sensitivity of the samples B and C could be attributed to decrease in the minor phase Li4P2O7. Decrease in the contribution of this phase reduces the number of defects responsible for TL resulting in decrease in the TL sensitivity. The enhancement of high temperature peak in the samples A and C suggests that carriers generated during OSL readout get re-trapped at the optically insensitive TL traps responsible for the TL peaks around 200–400 °C. This indicates that two types of traps are present viz., (i) traps amenable to TL as well as OSL and (ii) traps amenable to TL but do not yield OSL signal during optical stimulation.
Fig. 7. Thermoluminescence (TL) response of various Cu-doped Li3PO4 samples heated in air: (a) TL of sample A recorded immediately after irradiation, (b) TL of sample A recorded after taking its BSL readout, (c) TL of sample A recorded after depleting a low temperature peak followed by BSL, (d) TL of sample B recorded immediately after irradiation, (e) TL of sample B recorded after taking its BSL readout, and (f) TL of sample C recorded immediately after irradiation.
3.5. Dose response The dose response of the OSL signal of various samples is studied in the dose range of 10 mGy to 1 Gy using 90Sr/90Y beta-ray source. For each dose the OSL measurement is taken at room temperature for 200 s. The dose response plots of Cu-doped samples A and B are shown in Fig. 9a and b, respectively. The OSL output from a sample is usually directly related to the radiation dose absorbed by the sample. General equation for dose vs OSL response of an OSL phosphor can be represented as
R = kDm
(3)
where R is OSL output (weight normalized) from the sample, k is a proportionality constant having units counts. Gy m (generally, represented as counts/Gym), D is absorbed dose in gray (Gy) and m is a dimensionless parameter which measures the proportionality between absorbed dose (D) and R. Eq. (3) is a heuristic
122
R.A. Barve et al. / Physica B 458 (2015) 117–123
0.92 and 0.95, respectively. The deviation from m ¼1 (for linear response) indicates some sub-linearity in the dose response curve. 3.6. Minimum detectable dose On the described set-up, the minimum detectable dose (equivalent to 3s of the background counts of the annealed and unexposed sample) is found to be 0.18 mGy. For the calculation of MDD the standard deviation (s) is calculated by recording OSL from an unexposed sample (10 measurements) and comparing it with the OSL of the irradiated sample (20 mGy).
4. Conclusion
Fig. 8. Blue stimulated response (BSL) of Cu-doped sample A heated in air: (a) BSL recorded after depleting a low temperature peak, (b) BSL recorded immediately after irradiation, (c) (inset) BSL recorded immediately after irradiation, and (d) (inset) BSL recorded after 192 h from the time of irradiation.
The study shows that Cu-doped lithium phosphate could be synthesized by a simple co-precipitation method. All the samples synthesized using different phosphorus precursors yields Li3PO4 in same phase along with minor phases of Li4P2O7 and Li2Cu2P6O18. The relative percentage of these phases was found to be dependent on phosphorus precursor; these phases have profound effect on the luminescent properties of Li3PO4. Amongst the studied phosphors, intense OSL is observed in the Cu-doped sample A and its sensitivity is much higher as compared to that observed for commercial Al2O3:C. The BSL decay of sample A is very fast as compared to that of commercial Al2O3:C. However, the GSL decay is slow, similar to that of the commercial Al2O3:C. The observed decrease in PL, TL and OSL sensitivity of Cu-doped samples B and C could be attributed to the decrease in the contribution of the Li4P2O7 phase and increase in contribution of the Li2Cu2P6O18 phase. Even though good sensitivity is observed in most of the samples, they exhibit intense post-irradiation fading in 192 h. Though a fast fading OSL phosphor is not suitable for application in personnel and environmental dosimetry, it may find application for real-time dosimetry in a readout system using fiber optics, such as medical dosimetry.
Acknowledgment The authors are grateful to B.R.N.S. (Sanction no. 2008/37/20) for funding this work.
References
Fig. 9. OSL dose response of Cu-doped samples A and B. (a) Dose response of Cudoped sample A and (b) dose response of Cu-doped sample B.
presentation that usually holds up to a certain dose where saturation effects are usually observed. Corresponding logarithmic equation will be
log R = log k + m log D
(4)
If m ¼0, it implies that the response is independent from the dose and when m ¼ 1, dependence is said to be linear. However, in the present case, the values of m for Fig. 9a and b are found to be
[1] D.J. Huntley, D.I. Godfrey-Smith, M.L.W. Thewatt, Nature 313 (1985) 105. [2] M.S. Akselrod, V.S. Kortov, D.J. Kravetsky, V.I. Gotlib, Radiat. Prot. Dosim. 32 (1990) 15. [3] N. Kristianpoller, M. Abu-Rayya, R. Chen, J. Lumin. 60&61 (1994) 540. [4] L. Bøtter-Jensen, S.W.S. McKeever, Radiat. Prot. Dosim. 65 (1996) 273. [5] L. Bøtter-Jensen, H. Jungner, V. Mejdahl, Radiat. Prot. Dosim. 47 (1993) 643. [6] I. Jaek, G. Hutt, I. Vassiltchenko, Radiat. Meas. 27 (1997) 473. [7] A.S. Pradhan, J.I. Lee, J.L. Kim, J. Med. Phys. 33 (3) (2008) 85. [8] J. Qiu, Y. Shimizugawa, Y. Iwabuchi, K. Hirao, Appl. Phys. Lett. 71 (1997) 43. [9] J. Qiu, N. Sugimoto, Y. Iwabuchi, K. Hirao, J. Non-Cryst. Solids 209 (1997) 200. [10] J. Qiu, Y. Shimizugawa, Y. Iwabuchi, K. Hirao, Appl. Phys. Lett. 71 (1997) 759. [11] J. Qiu, Y. Shimizugawa, N. Sugimoto, K. Hirao, J. Non-Cryst. Solids 222 (1997) 290. [12] B.L. Justus, S. Rychnovsky, M.A. Miller, K.J. Pawlovich, A.L. Huston, Radiat. Prot. Dosim. 74 (1997) 151. [13] A.J.J. Bos, M. Prokic, J.C. Brouwer, Radiat. Prot. Dosim. 119 (2006) 130. [14] C. Dotzler, G.V.M. Williams, U. Reiser, A. Edgar, Appl. Phys. Lett. 91 (2007) 121910–121911. [15] M. Sommer, R. Fraudenberg, J. Henniger, Radiat. Meas. 42 (2007) 617. [16] B.S. Dhabekar, S.N. Menon, A. Raja, A.K. Bakshi, A.K. Singh, M.P. Chougaonkar, Y.S. Mayya, Nucl. Instrum. Methods B 269 (2011) 1844. [17] R. Barve, R.R. Patil, N.P. Gaikwad, M.S. Kulkarni, D.R. Mishra, A. Soni, B.C. Bhatt, S.V. Moharil, Radiat. Meas. 59 (2013) 73. [18] R.R. Patil, S.V. Moharil, Phys. Status Solidi A 187 (2001) 557. [19] S.M. Naranje, S.V. Moharil, Phys. Status Solidi A 165 (1998) 489. [20] B.C. Bhatt, B.S. Dhabekar, S.S. Sanaye, S.S. Shinde, S.V. Moharil, T.K. Gundu Rao,
R.A. Barve et al. / Physica B 458 (2015) 117–123
Radiat. Prot. Dosim. 100 (1–4) (2002) 251. [21] G.A. Aghalte, S.K. Omanwar and S.V. Moharil, 2009, Proceedings of the National Conference on Luminescence and its Application (NCLA-2009) at IACS, CGCRI, Kolkata, Vol. XIX 71, pp. 272. [22] R. Barve, R.R. Patil, M.S. Kulkarni, B.C. Bhatt, S.V. Moharil, J. Lumin. 156 (2014) 25. [23] M.S. Kulkarni, D.R. Mishra, D.N. Sharma, Nucl. Instrum. Methods B 262 (2007) 348.
123
[24] E.G. Yukihara, V.H. Whitley, S.W.S. McKeever, A.E. Akselrod, M.S. Akselrod, Radiat. Meas. 38 (2004) 317. [25] F. Sun, L. Jiao, J. Wu, Z. Yang, S. Yuan, G. Dai, Radiat. Prot. Dosim. 51 (3) (1994) 183.