Radiation Protection Dosimetry (2015), Vol. 163, No. 4, pp. 439 –445 Advance Access publication 1 August 2014
doi:10.1093/rpd/ncu255
Na2SiF6:Cu,P: A NEW OSL PHOSPHOR FOR THE RADIATION DOSIMETRIC APPLICATIONS R. A. Barve1,5, *, R. R. Patil1, S. V. Moharil2, N. P. Gaikwad3, B. C. Bhatt4, Ratna Pradeep3, D. R. Mishra3 and M. S. Kulkarni3 1 Government Institute of Science, Nagpur, India 2 RTM Nagpur University, Nagpur, India 3 Radiological Protection and Advisory Division, BARC, Mumbai, India 4 DST Fellow, C/o RPAD., BARC, Mumbai, India 5 Present Address: Radiological Safety Division, IGCAR, Kalpakkam, Tamil Nadu, India *Corresponding author: [email protected] Received 19 May 2014; revised 29 June 2014; accepted 2 July 2014 A new Cu,P-doped, sodium fluorosilicate-based optically stimulated luminescence (OSL) phosphor is developed. This phosphor shows good OSL properties, and the sensitivity is comparable with that of the commercial Al2O3:C (Landauer, Inc.) phosphor. For the luminescence averaged over initial 1 s, blue-stimulated luminescence and green-stimulated luminescence sensitivities were found to be 0.76 and 3.8 times, respectively, of Al2O3:C (Landauer, Inc.) with 28 % of post-irradiation fading in 3 days and nil thereafter. The simple preparation procedure, fast decay, very good sensitivity and moderate fading will make this phosphor suitable for radiation dosimetry, using OSL.
INTRODUCTION Optically stimulated luminescence (OSL) is a technique in dosimetry of ionising radiations. In OSL, the localised defects act as traps and capture electrons or holes generated by the ionising radiations. These traps are stimulated by the light in the visible (blue – green region) or IR region, which results in release of either electron or hole from these localised traps. The subsequent annihilation at the recombination centre leads to the emission of light, which is at the shorter wavelength compared with the wavelength of the stimulating light. The general requirement for the 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 photo-ionisation 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. OSL was first used in archaeological dating(1) and later proposed for personnel monitoring and environmental monitoring of radiation with the development of Al2O3:C(2). More recently, OSL of cerium, europium, samarium and copper ions doped into borate and silicate glasses has been reported(3 – 7). Attempts are also made to develop materials like BeO(8), NaMgF3(9), and YAG: MgO:Tb(10), LiMgPO4:Tb,B(11), LiAlO(12) 2 C(13), but, except for BeO, they still remain in development stage as far as their use in routine radiation
dosimetry is concerned. Fluorosilicates, particularly alkali fluorosilicates, could be other candidates as OSL materials. Fluorosilicates are the class of materials that are termed as complex fluorides. The general formula is M2SiF6 and NSiF6 where M and N are any of the alkali or alkaline earth ions, respectively. All the alkali, alkaline earth silicofluorides lose SiF4 on decomposition and follow the reactions: M2SiF6 ! 2MF þ SiF4; NSiF6 ! NF þ SiF4 (M—alkali ion; N—alkaline earth ion). The temperature range for decomposition is in between 300 and 7008C and is well reported in literature. As these materials decompose on heating, these could not be used as TL phosphors but could be good OSL phosphors as the readout does not involve heating. In the authors’ earlier work, the authors successfully doped Cuþ in alkali fluorosilicates such as Na2SiF(14) 6 . This phosphor shows Cuþ luminescence of 358 nm with the excitation at 254 nm. This phosphor also exhibits very good OSL sensitivity, but intense post-irradiation fading is observed making this material unsuitable for dosimetric applications. Doping P along with Cu reduces post-irradiation fading significantly without affecting other dosimetric properties. In this study, the OSL properties of Cu,P-doped Na2SiF6 are presented in detail. EXPERIMENTAL Analytical-grade Na2SiF6, procured from Loba Chemie, was used to synthesise various Na2SiF6 phosphors. Synthesis of Cu-doped Na2SiF6 is
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
R. A. BARVE ET AL.
reported in the authors’ earlier work(14). For preparing Cu,P-doped phosphor, phosphorus was first incorporated in Na2SiF6 lattice. For this, Na2SiF6 in desired amount was dissolved in the appropriate amount of double-distilled water at 1008C and phosphorous in the form of NH4H2PO4 was added to it. The solution was then evaporated at 808C for several hours to remove the excess water to get dried powder. Then, CuCl2 (0.01 mol %) solution was sprinkled on this dried powder. This wet powder was dried under the IR lamp and was then heated under reactive atmosphere of NH4Cl for 3 h at 3508C. For diffusion of Cu in lattice, this temperature is chosen as no decomposition is reported in the literature around this temperature(15, 16). Several samples with varying concentration of P in the range 0.1– 2 mol % were prepared. Out of the several samples, best properties were obtained for P ¼ 1 mol %, and hence, the properties discussed are for this concentration of P. For studying the TL and OSL response, all the samples were irradiated using 90Sr/90Y beta source with the dose rate of 20 mGy min21. The samples were given a test dose of 100 mGy. The CW-OSL response and thermoluminescence of the samples were recorded on the TL-OSL reader assembly described elsewhere(17). The assembly consists of an array of blue/green LEDs as a stimulation source with power adjustable from 11 to 48 mW cm22. 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 cm22 and signal was recorded for 200 s with the acquisition time of 0.1 s. All the TL measurements were taken at the heating rate of 48C s21. Photoluminescence (PL) studies were carried on Hitachi F-4000 Spectrofluorometer. Xenon lamp was used as an excitation source for the PL studies.
Na2SiF6 (JCPDS File no.78-0393). No peaks of decomposition products such as NaF or SiO2 are observed. This indicates that no decomposition has occurred during heating the sample at 3508C for the diffusion of Cu impurity, and thus, the observed properties are of Na2SiF6:Cu,P. Cu emission in Na2SiF6, which could be attributed to 3d 94s1 !3d10 transitions, is discussed in the authors’ earlier work(14). The PL spectra for various concentration of P were recorded (not plotted). It is observed that the PL intensity decreases with the concentration of P, which indicates that P acts as a quenching ion to the Cuþ luminescence in Na2SiF6. For the P concentration equal to 1 mol %, the emission is observed at 358 nm (Figure 1d). The excitation to this Cuþ band is observed around 254 nm (Figure 1b). This emission is with lesser intensity compared with the Cuþ excitation and emission in sample doped with Cuþ alone. (Figure 1a and c). Intense OSL is observed in Cu- and Cu,P-doped Na2SiF6. Figure 2 shows the CW-OSL response of Cu- and Cu,P-doped Na2SiF6. The inset in Figure 2a shows the blue-stimulated luminescence (BSL), and Figure 2b shows the green-stimulated luminescence (GSL) response of Al2O3:C (Landauer, Inc.). The BSL decay in Na2SiF6:Cu-doped sample is very fast and whole signal decays within 1.5 s (Figure 2a, curve i) whereas the GSL signal decays in 5 s (Figure 2b, curve i). Incorporation of P along with Cu affects the CW-OSL decay and makes the decay slower. The BSL (Figure 2a, curve ii) and GSL signal of Cu,P (Figure 2b, curve ii)-doped sample decays in 3 and 8 s, respectively. Doping of P also influences the OSL sensitivity. The BSL and GSL sensitivity of Cu,Pdoped sample using area integration method is found to be 56 and 30 %, respectively, compared with that of the Cu-doped sample. From the results presented earlier, it is clear that Cu plays an important role in imparting OSL sensitivity. Sample with intense
RESULTS AND DISCUSSION Most alkali fluorosilicates decompose on heating, and the decomposition profile of various alkali fluorosilicates is different and is well reported in literature(18). Therefore, incorporation of impurities in fluorosilicate lattices must be carried out well below the decomposition temperature. In the present case, though the diffusion is carried out well below the decomposition temperature to ensure that no decomposition of Na2SiF6 has taken place during heating, the X-ray diffraction of sample heated at 3508C was taken, which was similar to that observed in the authors’ earlier work(14). The diffraction pattern corresponds to the well-defined, intense diffraction peaks of
Figure 1. Photoluminescence spectra of Cu- and Cu,Pdoped Na2SiF6. (a) Excitation of Cu-doped Na2SiF6. (b) Excitation of Cu,P-doped Na2SiF6. (c) Emission of Cu-doped Na2SiF6. (d) Emission of Cu,P-doped Na2SiF6.
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Figure 3. Reproducibility of CW-OSL signal within aliquots of same batch of Na2SiF6:Cu,P. Figure 2. (a) BSL response of Cu- and Cu,P-doped Na2SiF6 [inset shows BSL response of Al2O3:C (Landauer, Inc.)]. (i) BSL response of Cu-doped Na2SiF6 recorded immediately after irradiation. (ii) BSL response of Cu,P-doped Na2SiF6 recorded immediately after irradiation. (iii) BSL response of Cu,P-doped Na2SiF6 recorded after heating up to 1208C. (b) GSL response of Cu- and Cu,P-doped Na2SiF6 [inset shows GSL response of Al2O3:C (Landauer, Inc.)]. (i) GSL response of Cu-doped Na2SiF6 recorded immediately after irradiation. (ii) GSL response of Cu,P-doped Na2SiF6 recorded immediately after irradiation.
fluorescence of Cuþ shows better sensitivity than the sample with less intense fluorescence. As very weak OSL is observed in un-doped sample, it can be deduced that the doping introduces several metastable traps that are responsible for imparting high sensitivity to the material. The BSL and GSL sensitivity of Cu,P-doped samples is also compared with the BSL and GSL sensitivity of commercial Al2O3:C (Landauer, Inc.). All the measurements were carried out under identical conditions and are normalised with respect to weight and dose. Before exposing to radiation, both the samples were bleached in blue light (470 nm) with an intensity of 50 mw cm22 for 15 min to make the base level zero. Since the decay of CW-OSL signals of doped Na2SiF6 and Al2O3:C are very different, the integral area method may not be a suitable one for
comparing the two phosphors. Therefore, the method suggested by Yukihara et al.(19) is employed. In this method, the two phosphors are compared on the basis of initial OSL intensity (luminescence averaged over first 1 s) measurements. The BSL and GSL sensitivity is found to be 0.76 and 3.8 times the BSL and GSL sensitivity of commercial alumina. Though this method gives an idea about the relative sensitivity, the calculation of minimum detectable dose gives better idea about the absolute sensitivity of the phosphors. On the described set-up, the minimum detectable dose [MDD (equivalent to 3s of the background counts of the annealed and unexposed sample)] is found to be 10.9 mR. For the calculation of MDD, the standard deviation (s) is calculated by recording OSL from unexposed sample and compared with the OSL of irradiated sample (20 mGy); 10 measurements for each. However, the actual lower limit has to be established in the optimised readout conditions. Reproducibility of the signal within a batch and between different batches is one of the important criteria for suitability of the phosphor for dosimetric applications. To test the reproducibility within the same batch, five identical aliquots were prepared and were given a test dose of 100 mGy. It is observed that the integral OSL signal is reproducible and reproducibility is within +3 % (Figure 3). To study the reproducibility between different batches, integral OSL signal from three batches were tested. Four aliquots,
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Figure 5. Thermoluminescence response of Cu- and Cu,Pdoped Na2SiF6. (a) TL response of Cu-doped Na2SiF6 recorded immediately after irradiation. (b) TL response of Cu-doped Na2SiF6 recorded after recording BSL. (c) TL response of Cu,P-doped Na2SiF6 recorded immediately after irradiation. (d) TL response of Cu,P-doped Na2SiF6 recorded after recording BSL.
Figure 4. Reproducibility of CW-OSL signal within different batches of Na2SiF6:Cu,P.
one from each batch, were given a test dose of 100 mGy and the OSL was recorded. Amongst all batches, the signal was also found to be reproducible within +2 % (Figure 4). Figure 5 shows the thermoluminescence glow curves of Cu-doped Na2SiF6. Two peaks, one around 1008C and another around 3258C, are observed in Cu-doped sample (Figure 5a). The TL recorded after taking OSL in Na2SiF6:Cu shows depletion of lowtemperature peak whereas 3258C peak increases with a shoulder appearing around 2408C, correlating the low-temperature peak to the observed OSL (Figure 5b). The changes in the structure of high-temperature peak suggest that carriers generated during optical stimulation get re-trapped in the traps responsible for TL peaks around 240 –3508C. In a P-codoped sample, the relative ratio of TL peaks is found to be reversed compared with the Cu-doped sample (Figure 5c). The high-temperature peak, which is observed around 3208C, is more pronounced compared with the low-temperature peak around 908C. Thus, the incorporation of P reduces traps responsible for the low-temperature peak whereas that increases
the traps responsible for 3208C peak. However, no significant change in the TL sensitivity is observed as integral TL area is nearly same for the two samples (Cu doped and Cu,P doped). The TL glow curve recorded after recording OSL shows complete depletion of low-temperature peak with partial depletion of high-temperature peak (Figure 5d). The observed OSL after heating the sample up to 1208C indicates that some deep traps are also making significant contribution to the total OSL in the sample (Figure 2a curve iii). The integral OSL after heating up to 1208C was found to be 46 % of integrated OSL of the commercial Al2O3:C and 85 % of the original signal. This observation suggests that at least 15 % fading is expected in the sample. Figure 6 shows the fading of Na2SiF6:Cu,P sample. During first 7 d, 28 % fading is observed and then signal is found to be stable. Thus, the observed fading is 13 % for the sample given 1208C post-irradiation thermal treatment. Fading of OSL signal is not desirable for the phosphor if it is to be used for dosimetric applications, particularly for personnel monitoring. In personnel monitoring, the doses are to be evaluated after a month or a quarter. Therefore, fading in personnel monitoring can be taken care of by delaying the OSLD read out by 3 d since the OSL signal stabilises in 3 d. However, the checking of long-term stability (over several months) is necessary before advocating the phosphor for practical applications. Figure 7 shows the dose response of Cu,P-doped Na2SiF6 studied in the range from 10 mGy to 1 Gy. It is found to be linear over the entire range and could be fitted with the linear equation of the form: Y ¼ A þ B X, where A is the intercept on the Y-axis having
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Figure 6. Fading response of Cu,P-doped Na2SiF6.
Figure 8. Curve fitting of BSL decay curve of Cu,P-doped Na2SiF6 recorded immediately after irradiation.
Figure 7. Dose response of Cu,P-doped Na2SiF6.
Figure 9. Curve fitting of GSL decay curve of Cu,P-doped Na2SiF6 recorded immediately after irradiation.
value 5.05 with unit integral counts. The constant B in the equation is the slope having value 1 with unit integral counts/dose.
The BSL decay curve recorded after thermally removing the low-temperature peak can be represented by the following equation:
De-convolution of CW-OSL decay curves Figure 8 shows the BSL decay curve of Cu,P-doped Na2SiF6 along with the fitted components. The sum of three components of CW-OSL decay curve is shown by the fitted curve. The decay curve can be represented by the following equation: t t t þA2 exp þA3 exp ð1Þ IOSL ¼A1 exp t1 t2 t3 where IOSL is the initial OSL intensity and t1, t2 and t3 are the decay constants of the respective OSL traps.
IOSL ¼A1 exp
t t þA2 exp t1 t2
ð2Þ
where IOSL is the initial OSL intensity and t1 and t2 are the decay constants of the respective OSL traps. The GSL decay curve of Cu,P-doped Na2SiF6 (Figure 9) can also be represented by Equation (2). The corresponding values of decay constants and photo-ionisation cross section are given in Table 1. The photo-ionisation cross section is calculated using
443
0.9` 10217 4.1
4.2` 10216
0.3` 10216
0.1
1.35
17 479 (A1) — 682 (A3)
0.6
6` 10217
the following formula: 1 s¼ ðwtÞ where w is the incident photon flux ( photons incident per cm2 per second) and t is the decay constant (in seconds). The figure of merit (FOM) analysis with these parameters shows that FOM for BSL curve [Equation (1)] comes out to be 2 % whereas for GSL curve [Equation (2)], it is found to be 4 %. If the photo-ionisation cross section of the various components of BSL response is compared with that of GSL response (Table 1), then it can be seen that the photo-ionisation cross section for BSL is greater than that for GSL. This indicates the better and faster OSL readout possibility from the sample when stimulated with the blue light. Also, in case of BSL recorded after depleting the low-temperature peak, the medium component is found to be absent. The ratio of coefficients between fast and slow component is found to be 122. This factor is far less compared with the ratio for BSL recorded without depleting the low-temperature peak, which is 198. Thus, the depletion of low-temperature peak removes the fast and slow component partially. However, the medium component is found to be depleted completely.
193 617 (A1) 9940 (A2) 975 (A3)
0.07 0.37 2.4
5.2` 10216 0.98` 10216 0.2` 10216
110 502 (A1) — 908 (A3)
CONCLUSION The new Cu,P-doped, sodium fluorosilicate-based phosphor shows good OSL properties, and the BSL and GSL sensitivity is comparable with that of the commercial Al2O3:C (Landauer, Inc.) phosphor. The OSL decay is very fast as compared with commercial alumina. The synthesis method is very simple, and phosphor can be synthesised with commonly available chemicals. Twenty-eight per cent of post-irradiation fading is observed in Na2SiF6:Cu,P phosphor, which is mostly due to low-temperature peak of 908C. In personnel monitoring, where the doses are to be evaluated after a month or a quarter, fading can be taken care of by delaying the OSLD read out by 3 days since the OSL signal stabilises after 3 days. Thus, Na2SiF6:Cu,P is a suitable phosphor for the dosimetric applications.
FUNDING The authors are grateful to B.R.N.S. (Sanction No. 2008/37/20) for funding this work. REFERENCES
Fast Medium Slow
Decay Photo-ionisation constant t (s) cross section s (cm2) Decay Photo-ionisation constant t (s) cross section s (cm2) Coefficient Decay Photo-ionisation constant t (s) cross section s (cm2) Coefficient CW-OSL component
BSL recorded after heating up to 1208C BSL response
Table 1. CW-OSL decay curve parameters for Cu,P-doped Na2SiF6.
Coefficient
GSL response
R. A. BARVE ET AL.
1. Huntley, D. J., Godfrey-Smith, D. I. and Thewatt, M. L. W. Optical dating of sediments. Nature 313, 105– 107 (1985).
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PHOTOLUMINESCENCE, OSL, FLUOROSILICATES, DOSIMETRY 2. Akselrod, M. S., Kortov, V. S., Kravetsky, D. J. and Gotlib, V. I. Highly sensitive thermoluminescent aniondefective alpha-Al203: C single crystal detectors. Radiat. Prot. Dosim. 32, 15–20 (1990). 3. Qiu, J., Shimizugawa, Y., Iwabuchi, Y. and Hirao, K. Photostimulated luminescence of Ce3þ-doped alkali borate glasses. Appl. Phys. Lett. 71, 43–45 (1997). 4. Qiu, J., Sugimoto, N., Iwabuchi, Y. and Hirao, K. Photostimulted luminescence in Ce3þdoped silicate glasses. J. Non-Crystal. Solids 209, 200– 203 (1997). 5. Qiu, J., Shimizugawa, Y., Iwabuchi, Y. and Hirao, K. Photostimulated luminescence in Eu2þdoped fluoroaluminate glasses. Appl. Phys. Lett. 71, 759–761 (1997). 6. Qiu, J., Shimizugawa, Y., Sugimoto, N. and Hirao, K. Photostimulated luminescence in borate glasses doped with Eu2þ and Sm3þ ions. J. Non-Crystal. Solids 222, 290 –295 (1997). 7. Justus, B. L., Rychnovsky, S., Miller, M. A., Pawlovich, K. J. and Huston, A. L. Optically stimulated luminescence radiation dosimetry using doped silica glass. Radiat. Prot. Dosim. 74, 151–154 (1997). 8. Sommer, M., Fraudenberg, R. and Henniger, J. New aspects of a BeO-based optically stimulated luminescence dosimeter. Radiat. Meas. 42, 617 (2007). 9. Dotzler, C., Williams, G. V. M., Reiser, U. and Edgar, A. Optically stimulated luminescence in NaMgF3:Eu2þ. Appl. Phys. Lett. 91, 121910-1-3 (2007). 10. Bos, A. J., Prokic, M. and Brouwer, J. C. Optically and thermally stimulated luminescence characteristics of MgO: Tb3þ. Radiat. Prot. Dosim. 119, 130–133 (2006). 11. Dhabekar, B., Menon, S. N., Alagu Raja, E., Bakshi, A. K., Singh, A. K., Chougaonkar, M. P. and Mayya, Y. S. LiMgPO4:Tb,B—a new sensitive OSL phosphor for
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dosimetry. Nucl. Instr. Meth. Phys. Res. B 269, 1844–1848 (2011). Lee, J. I., Pradhan, A. S., Kim, J. L., Chang, I., Kim, B. H. and Chung, K. S. Preliminary study on development and characterization of high sensitivity LiAlO2 optically stimulated luminescence material. Radiat. Meas. 47, 837–840 (2012). Kulkarni, M. S., Muthe, K. P., Rawat, N. S., Mishra, D. R., Kakde, M. B. and Ramanathan, S. Carbon doped yttrium aluminum garnet (YAG:C)—A new phosphor for radiation dosimetry. Radiat. Meas. 43, 492–496 (2008). Patil, R. R., Barve, R., Kulkarni, M. S., Bhatt, B. C. and Moharil, S. V. Synthesis and luminescence in some fluoro-silicates for the possible applications in OSL dosimetry. Physica. B. 407, 629–634 (2012). Leal-Cruz, A. L. and Pech-Canul, M. I. Thermal and microstructural characterization of Na2SiF6 decomposition-Si3N4 formation by HYSYCVD in the Na2SiF6-N2. Adv. Tech. Mater. Mater. Proc. J. 10, 33–40 (2008). Stodolski, R. and Kolditz, L. Some aspects of real structure and thermal decomposition of K2SiF6. J. Fluorine Chem. 29, 73 (1985). Kulkarni, M. S., Mishra, D. R. and Sharma, D. N. A versatile integrated system for thermoluminescence and optically stimulated luminescence measurements. Nucl. Instr. Meth. Phys. Res. B 262, 348 –356 (2007). Zachara, J. and Wigniewski, W. Electronegativity force of cations and thermal decomposition of complex fluorides. J. Therm. Anal. 44, 363–373 (1995). Yukihara, E. G., Whitley, V. H., McKeever, S. W. S., Akselrod, A. E. and Akselrod, M. S. Effect of high-dose irradiation on the optically stimulated luminescence of Al2O3.C. Radiat. Meas. 38, 317–330 (2004).
doi:10.1093/rpd/ncu255
Na2SiF6:Cu,P: A NEW OSL PHOSPHOR FOR THE RADIATION DOSIMETRIC APPLICATIONS R. A. Barve1,5, *, R. R. Patil1, S. V. Moharil2, N. P. Gaikwad3, B. C. Bhatt4, Ratna Pradeep3, D. R. Mishra3 and M. S. Kulkarni3 1 Government Institute of Science, Nagpur, India 2 RTM Nagpur University, Nagpur, India 3 Radiological Protection and Advisory Division, BARC, Mumbai, India 4 DST Fellow, C/o RPAD., BARC, Mumbai, India 5 Present Address: Radiological Safety Division, IGCAR, Kalpakkam, Tamil Nadu, India *Corresponding author: [email protected] Received 19 May 2014; revised 29 June 2014; accepted 2 July 2014 A new Cu,P-doped, sodium fluorosilicate-based optically stimulated luminescence (OSL) phosphor is developed. This phosphor shows good OSL properties, and the sensitivity is comparable with that of the commercial Al2O3:C (Landauer, Inc.) phosphor. For the luminescence averaged over initial 1 s, blue-stimulated luminescence and green-stimulated luminescence sensitivities were found to be 0.76 and 3.8 times, respectively, of Al2O3:C (Landauer, Inc.) with 28 % of post-irradiation fading in 3 days and nil thereafter. The simple preparation procedure, fast decay, very good sensitivity and moderate fading will make this phosphor suitable for radiation dosimetry, using OSL.
INTRODUCTION Optically stimulated luminescence (OSL) is a technique in dosimetry of ionising radiations. In OSL, the localised defects act as traps and capture electrons or holes generated by the ionising radiations. These traps are stimulated by the light in the visible (blue – green region) or IR region, which results in release of either electron or hole from these localised traps. The subsequent annihilation at the recombination centre leads to the emission of light, which is at the shorter wavelength compared with the wavelength of the stimulating light. The general requirement for the 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 photo-ionisation 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. OSL was first used in archaeological dating(1) and later proposed for personnel monitoring and environmental monitoring of radiation with the development of Al2O3:C(2). More recently, OSL of cerium, europium, samarium and copper ions doped into borate and silicate glasses has been reported(3 – 7). Attempts are also made to develop materials like BeO(8), NaMgF3(9), and YAG: MgO:Tb(10), LiMgPO4:Tb,B(11), LiAlO(12) 2 C(13), but, except for BeO, they still remain in development stage as far as their use in routine radiation
dosimetry is concerned. Fluorosilicates, particularly alkali fluorosilicates, could be other candidates as OSL materials. Fluorosilicates are the class of materials that are termed as complex fluorides. The general formula is M2SiF6 and NSiF6 where M and N are any of the alkali or alkaline earth ions, respectively. All the alkali, alkaline earth silicofluorides lose SiF4 on decomposition and follow the reactions: M2SiF6 ! 2MF þ SiF4; NSiF6 ! NF þ SiF4 (M—alkali ion; N—alkaline earth ion). The temperature range for decomposition is in between 300 and 7008C and is well reported in literature. As these materials decompose on heating, these could not be used as TL phosphors but could be good OSL phosphors as the readout does not involve heating. In the authors’ earlier work, the authors successfully doped Cuþ in alkali fluorosilicates such as Na2SiF(14) 6 . This phosphor shows Cuþ luminescence of 358 nm with the excitation at 254 nm. This phosphor also exhibits very good OSL sensitivity, but intense post-irradiation fading is observed making this material unsuitable for dosimetric applications. Doping P along with Cu reduces post-irradiation fading significantly without affecting other dosimetric properties. In this study, the OSL properties of Cu,P-doped Na2SiF6 are presented in detail. EXPERIMENTAL Analytical-grade Na2SiF6, procured from Loba Chemie, was used to synthesise various Na2SiF6 phosphors. Synthesis of Cu-doped Na2SiF6 is
# The Author 2014. Published by Oxford University Press. All rights reserved. For Permissions, please email: [email protected]
R. A. BARVE ET AL.
reported in the authors’ earlier work(14). For preparing Cu,P-doped phosphor, phosphorus was first incorporated in Na2SiF6 lattice. For this, Na2SiF6 in desired amount was dissolved in the appropriate amount of double-distilled water at 1008C and phosphorous in the form of NH4H2PO4 was added to it. The solution was then evaporated at 808C for several hours to remove the excess water to get dried powder. Then, CuCl2 (0.01 mol %) solution was sprinkled on this dried powder. This wet powder was dried under the IR lamp and was then heated under reactive atmosphere of NH4Cl for 3 h at 3508C. For diffusion of Cu in lattice, this temperature is chosen as no decomposition is reported in the literature around this temperature(15, 16). Several samples with varying concentration of P in the range 0.1– 2 mol % were prepared. Out of the several samples, best properties were obtained for P ¼ 1 mol %, and hence, the properties discussed are for this concentration of P. For studying the TL and OSL response, all the samples were irradiated using 90Sr/90Y beta source with the dose rate of 20 mGy min21. The samples were given a test dose of 100 mGy. The CW-OSL response and thermoluminescence of the samples were recorded on the TL-OSL reader assembly described elsewhere(17). The assembly consists of an array of blue/green LEDs as a stimulation source with power adjustable from 11 to 48 mW cm22. 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 cm22 and signal was recorded for 200 s with the acquisition time of 0.1 s. All the TL measurements were taken at the heating rate of 48C s21. Photoluminescence (PL) studies were carried on Hitachi F-4000 Spectrofluorometer. Xenon lamp was used as an excitation source for the PL studies.
Na2SiF6 (JCPDS File no.78-0393). No peaks of decomposition products such as NaF or SiO2 are observed. This indicates that no decomposition has occurred during heating the sample at 3508C for the diffusion of Cu impurity, and thus, the observed properties are of Na2SiF6:Cu,P. Cu emission in Na2SiF6, which could be attributed to 3d 94s1 !3d10 transitions, is discussed in the authors’ earlier work(14). The PL spectra for various concentration of P were recorded (not plotted). It is observed that the PL intensity decreases with the concentration of P, which indicates that P acts as a quenching ion to the Cuþ luminescence in Na2SiF6. For the P concentration equal to 1 mol %, the emission is observed at 358 nm (Figure 1d). The excitation to this Cuþ band is observed around 254 nm (Figure 1b). This emission is with lesser intensity compared with the Cuþ excitation and emission in sample doped with Cuþ alone. (Figure 1a and c). Intense OSL is observed in Cu- and Cu,P-doped Na2SiF6. Figure 2 shows the CW-OSL response of Cu- and Cu,P-doped Na2SiF6. The inset in Figure 2a shows the blue-stimulated luminescence (BSL), and Figure 2b shows the green-stimulated luminescence (GSL) response of Al2O3:C (Landauer, Inc.). The BSL decay in Na2SiF6:Cu-doped sample is very fast and whole signal decays within 1.5 s (Figure 2a, curve i) whereas the GSL signal decays in 5 s (Figure 2b, curve i). Incorporation of P along with Cu affects the CW-OSL decay and makes the decay slower. The BSL (Figure 2a, curve ii) and GSL signal of Cu,P (Figure 2b, curve ii)-doped sample decays in 3 and 8 s, respectively. Doping of P also influences the OSL sensitivity. The BSL and GSL sensitivity of Cu,Pdoped sample using area integration method is found to be 56 and 30 %, respectively, compared with that of the Cu-doped sample. From the results presented earlier, it is clear that Cu plays an important role in imparting OSL sensitivity. Sample with intense
RESULTS AND DISCUSSION Most alkali fluorosilicates decompose on heating, and the decomposition profile of various alkali fluorosilicates is different and is well reported in literature(18). Therefore, incorporation of impurities in fluorosilicate lattices must be carried out well below the decomposition temperature. In the present case, though the diffusion is carried out well below the decomposition temperature to ensure that no decomposition of Na2SiF6 has taken place during heating, the X-ray diffraction of sample heated at 3508C was taken, which was similar to that observed in the authors’ earlier work(14). The diffraction pattern corresponds to the well-defined, intense diffraction peaks of
Figure 1. Photoluminescence spectra of Cu- and Cu,Pdoped Na2SiF6. (a) Excitation of Cu-doped Na2SiF6. (b) Excitation of Cu,P-doped Na2SiF6. (c) Emission of Cu-doped Na2SiF6. (d) Emission of Cu,P-doped Na2SiF6.
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PHOTOLUMINESCENCE, OSL, FLUOROSILICATES, DOSIMETRY
Figure 3. Reproducibility of CW-OSL signal within aliquots of same batch of Na2SiF6:Cu,P. Figure 2. (a) BSL response of Cu- and Cu,P-doped Na2SiF6 [inset shows BSL response of Al2O3:C (Landauer, Inc.)]. (i) BSL response of Cu-doped Na2SiF6 recorded immediately after irradiation. (ii) BSL response of Cu,P-doped Na2SiF6 recorded immediately after irradiation. (iii) BSL response of Cu,P-doped Na2SiF6 recorded after heating up to 1208C. (b) GSL response of Cu- and Cu,P-doped Na2SiF6 [inset shows GSL response of Al2O3:C (Landauer, Inc.)]. (i) GSL response of Cu-doped Na2SiF6 recorded immediately after irradiation. (ii) GSL response of Cu,P-doped Na2SiF6 recorded immediately after irradiation.
fluorescence of Cuþ shows better sensitivity than the sample with less intense fluorescence. As very weak OSL is observed in un-doped sample, it can be deduced that the doping introduces several metastable traps that are responsible for imparting high sensitivity to the material. The BSL and GSL sensitivity of Cu,P-doped samples is also compared with the BSL and GSL sensitivity of commercial Al2O3:C (Landauer, Inc.). All the measurements were carried out under identical conditions and are normalised with respect to weight and dose. Before exposing to radiation, both the samples were bleached in blue light (470 nm) with an intensity of 50 mw cm22 for 15 min to make the base level zero. Since the decay of CW-OSL signals of doped Na2SiF6 and Al2O3:C are very different, the integral area method may not be a suitable one for
comparing the two phosphors. Therefore, the method suggested by Yukihara et al.(19) is employed. In this method, the two phosphors are compared on the basis of initial OSL intensity (luminescence averaged over first 1 s) measurements. The BSL and GSL sensitivity is found to be 0.76 and 3.8 times the BSL and GSL sensitivity of commercial alumina. Though this method gives an idea about the relative sensitivity, the calculation of minimum detectable dose gives better idea about the absolute sensitivity of the phosphors. On the described set-up, the minimum detectable dose [MDD (equivalent to 3s of the background counts of the annealed and unexposed sample)] is found to be 10.9 mR. For the calculation of MDD, the standard deviation (s) is calculated by recording OSL from unexposed sample and compared with the OSL of irradiated sample (20 mGy); 10 measurements for each. However, the actual lower limit has to be established in the optimised readout conditions. Reproducibility of the signal within a batch and between different batches is one of the important criteria for suitability of the phosphor for dosimetric applications. To test the reproducibility within the same batch, five identical aliquots were prepared and were given a test dose of 100 mGy. It is observed that the integral OSL signal is reproducible and reproducibility is within +3 % (Figure 3). To study the reproducibility between different batches, integral OSL signal from three batches were tested. Four aliquots,
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Figure 5. Thermoluminescence response of Cu- and Cu,Pdoped Na2SiF6. (a) TL response of Cu-doped Na2SiF6 recorded immediately after irradiation. (b) TL response of Cu-doped Na2SiF6 recorded after recording BSL. (c) TL response of Cu,P-doped Na2SiF6 recorded immediately after irradiation. (d) TL response of Cu,P-doped Na2SiF6 recorded after recording BSL.
Figure 4. Reproducibility of CW-OSL signal within different batches of Na2SiF6:Cu,P.
one from each batch, were given a test dose of 100 mGy and the OSL was recorded. Amongst all batches, the signal was also found to be reproducible within +2 % (Figure 4). Figure 5 shows the thermoluminescence glow curves of Cu-doped Na2SiF6. Two peaks, one around 1008C and another around 3258C, are observed in Cu-doped sample (Figure 5a). The TL recorded after taking OSL in Na2SiF6:Cu shows depletion of lowtemperature peak whereas 3258C peak increases with a shoulder appearing around 2408C, correlating the low-temperature peak to the observed OSL (Figure 5b). The changes in the structure of high-temperature peak suggest that carriers generated during optical stimulation get re-trapped in the traps responsible for TL peaks around 240 –3508C. In a P-codoped sample, the relative ratio of TL peaks is found to be reversed compared with the Cu-doped sample (Figure 5c). The high-temperature peak, which is observed around 3208C, is more pronounced compared with the low-temperature peak around 908C. Thus, the incorporation of P reduces traps responsible for the low-temperature peak whereas that increases
the traps responsible for 3208C peak. However, no significant change in the TL sensitivity is observed as integral TL area is nearly same for the two samples (Cu doped and Cu,P doped). The TL glow curve recorded after recording OSL shows complete depletion of low-temperature peak with partial depletion of high-temperature peak (Figure 5d). The observed OSL after heating the sample up to 1208C indicates that some deep traps are also making significant contribution to the total OSL in the sample (Figure 2a curve iii). The integral OSL after heating up to 1208C was found to be 46 % of integrated OSL of the commercial Al2O3:C and 85 % of the original signal. This observation suggests that at least 15 % fading is expected in the sample. Figure 6 shows the fading of Na2SiF6:Cu,P sample. During first 7 d, 28 % fading is observed and then signal is found to be stable. Thus, the observed fading is 13 % for the sample given 1208C post-irradiation thermal treatment. Fading of OSL signal is not desirable for the phosphor if it is to be used for dosimetric applications, particularly for personnel monitoring. In personnel monitoring, the doses are to be evaluated after a month or a quarter. Therefore, fading in personnel monitoring can be taken care of by delaying the OSLD read out by 3 d since the OSL signal stabilises in 3 d. However, the checking of long-term stability (over several months) is necessary before advocating the phosphor for practical applications. Figure 7 shows the dose response of Cu,P-doped Na2SiF6 studied in the range from 10 mGy to 1 Gy. It is found to be linear over the entire range and could be fitted with the linear equation of the form: Y ¼ A þ B X, where A is the intercept on the Y-axis having
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PHOTOLUMINESCENCE, OSL, FLUOROSILICATES, DOSIMETRY
Figure 6. Fading response of Cu,P-doped Na2SiF6.
Figure 8. Curve fitting of BSL decay curve of Cu,P-doped Na2SiF6 recorded immediately after irradiation.
Figure 7. Dose response of Cu,P-doped Na2SiF6.
Figure 9. Curve fitting of GSL decay curve of Cu,P-doped Na2SiF6 recorded immediately after irradiation.
value 5.05 with unit integral counts. The constant B in the equation is the slope having value 1 with unit integral counts/dose.
The BSL decay curve recorded after thermally removing the low-temperature peak can be represented by the following equation:
De-convolution of CW-OSL decay curves Figure 8 shows the BSL decay curve of Cu,P-doped Na2SiF6 along with the fitted components. The sum of three components of CW-OSL decay curve is shown by the fitted curve. The decay curve can be represented by the following equation: t t t þA2 exp þA3 exp ð1Þ IOSL ¼A1 exp t1 t2 t3 where IOSL is the initial OSL intensity and t1, t2 and t3 are the decay constants of the respective OSL traps.
IOSL ¼A1 exp
t t þA2 exp t1 t2
ð2Þ
where IOSL is the initial OSL intensity and t1 and t2 are the decay constants of the respective OSL traps. The GSL decay curve of Cu,P-doped Na2SiF6 (Figure 9) can also be represented by Equation (2). The corresponding values of decay constants and photo-ionisation cross section are given in Table 1. The photo-ionisation cross section is calculated using
443
0.9` 10217 4.1
4.2` 10216
0.3` 10216
0.1
1.35
17 479 (A1) — 682 (A3)
0.6
6` 10217
the following formula: 1 s¼ ðwtÞ where w is the incident photon flux ( photons incident per cm2 per second) and t is the decay constant (in seconds). The figure of merit (FOM) analysis with these parameters shows that FOM for BSL curve [Equation (1)] comes out to be 2 % whereas for GSL curve [Equation (2)], it is found to be 4 %. If the photo-ionisation cross section of the various components of BSL response is compared with that of GSL response (Table 1), then it can be seen that the photo-ionisation cross section for BSL is greater than that for GSL. This indicates the better and faster OSL readout possibility from the sample when stimulated with the blue light. Also, in case of BSL recorded after depleting the low-temperature peak, the medium component is found to be absent. The ratio of coefficients between fast and slow component is found to be 122. This factor is far less compared with the ratio for BSL recorded without depleting the low-temperature peak, which is 198. Thus, the depletion of low-temperature peak removes the fast and slow component partially. However, the medium component is found to be depleted completely.
193 617 (A1) 9940 (A2) 975 (A3)
0.07 0.37 2.4
5.2` 10216 0.98` 10216 0.2` 10216
110 502 (A1) — 908 (A3)
CONCLUSION The new Cu,P-doped, sodium fluorosilicate-based phosphor shows good OSL properties, and the BSL and GSL sensitivity is comparable with that of the commercial Al2O3:C (Landauer, Inc.) phosphor. The OSL decay is very fast as compared with commercial alumina. The synthesis method is very simple, and phosphor can be synthesised with commonly available chemicals. Twenty-eight per cent of post-irradiation fading is observed in Na2SiF6:Cu,P phosphor, which is mostly due to low-temperature peak of 908C. In personnel monitoring, where the doses are to be evaluated after a month or a quarter, fading can be taken care of by delaying the OSLD read out by 3 days since the OSL signal stabilises after 3 days. Thus, Na2SiF6:Cu,P is a suitable phosphor for the dosimetric applications.
FUNDING The authors are grateful to B.R.N.S. (Sanction No. 2008/37/20) for funding this work. REFERENCES
Fast Medium Slow
Decay Photo-ionisation constant t (s) cross section s (cm2) Decay Photo-ionisation constant t (s) cross section s (cm2) Coefficient Decay Photo-ionisation constant t (s) cross section s (cm2) Coefficient CW-OSL component
BSL recorded after heating up to 1208C BSL response
Table 1. CW-OSL decay curve parameters for Cu,P-doped Na2SiF6.
Coefficient
GSL response
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