PHASE DEPENDENT OPTICALLY STIMULATED LUMINESCENCE IN CU-DOPED Sr4Si3O8Cl4

PHASE DEPENDENT OPTICALLY STIMULATED LUMINESCENCE IN CU-DOPED Sr4Si3O8Cl4 Abstract Phase dependent optically stimulated luminescence (OSL) is studied in Cu-doped Sr4Si3O8Cl4. The Study shows that samples in which amount of contributing strontium metasilicate phase in Cu-doped Sr4Si3O8Cl4 is less, show intense OSL while those samples in which strontium metasilicate phase is more show weak OSL. The observed Cu luminescence is also found to be phase dependent. Sample in which Sr4Si3O8Cl4 phase is dominant, the observed Cu luminescence is around 350 nm whereas an additional longer wavelength band around 450 nm is observed when the strontium metasilicate phase is present in significant amount. The relatively phase pure, Cu-doped Sr4Si3O8Cl4 shows good OSL properties. The sensitivity of such material is 3.89 times more compared to commercial Al2O3:C (Landauer Inc.). High sensitivity, good linearity and reusability, along with low fading make this material as good OSL phosphor and may find applications in OSL based radiation dosimetry. INTRODUCTION Developing optically stimulated luminescence (OSL) phosphors for radiation dosimetric applications is of interest in recent past. Several efforts have been made by our group to study OSL in various lattices for the development of OSL phosphors(1–8). One of the areas under interest has been the phase dependent OSL properties of the material. The phase of the material is known to play an important role in determining several properties of the material. In particular, abrupt changes in several luminescence parameters have been observed during phase transitions(9). In the recent past significant changes in the intensity of 110°C TL peak are reported in bioglass(10). In our earlier work on Cu-doped Na2SO4, we have observed changes in luminescence spectra of Cu+ as well as OSL with respect to various phases(11, 12). Recently, it has been observed that the presence of microphases of Li4P2O7 and Li2Cu2P6O18 in Cu-doped Li3PO4 affect the OSL properties significantly(13). Rare earth-doped Sr4Si3O8Cl4 is of interest for several applications(14, 15). Several synthesis methods such as solid state synthesis as well as gel combustion have been reported. It has been reported that the phase purity of this material depends upon the synthesis conditions. The most common parasitic phase in this compound is strontium metasilcate, SrSiO3. The amount of this phase is known to vary depending on various synthesis parameters(16, 17). Therefore, this material is ideally suited for studying phase dependent properties. In this article we report the OSL in Cu-doped Sr4Si3O8Cl4 and its correlation to the SrSiO3 phase which is formed during the annealing of the material. The sensitivity of various Cu-doped Sr4Si3O8Cl4 is compared with commercial Al2O3:C (Landauer Inc). EXPERIMENTAL Synthesis of Cu-doped Sr4Si3O8Cl4 was carried out by the modified combustion synthesis method. Sr(NO3)2, silicic acid, SrCl2 (all AR grade from Loba chemie) were weighed in stoichoimetric proportion and mixed intimately with appropriate amount of urea and ammonium nitrate to get a paste. Cu in desired amount (0.1 mol%) in the form of CuCl2 solution was added to the paste and transferred to a china dish for heating in a furnace maintained at 500°C. After some time the paste swells due to evolution of gases and catches fire. The flame extinguishes within few seconds and a foamy white material is formed. This foamy material was then crushed, sieved and divided into three batches labeled as B2 to B4. All the batches were heated in reactive atmosphere of ammonium chloride at 500°C prior to heating at elevated temperature. This treatment is necessary to incorporate Cu in monovalent form. The details of this method are described elsewhere(18). The Sr4Si3O8Cl4 was also prepared using solid state synthesis method reported earlier(16). This batch is labeled as B1. The Cu-doped SrSiO3 is also prepared using conventional solid state synthesis method. In this method, AR grade SrCO3 and silicilic acid in stoichiometric proportion were intimately mixed for 1 h and then heated at 400°C for 4 h. After the heat treatment the mixture is again grinded for 30 min and heated at 800°C for 8 h to get the SrSiO3. This is labeled as batch B5. Each batch is given different heat treatment as summarized in Table 1. The powder material is sieved in 90–210 μm particle size. For all measurements powder samples having particle size in the range of 90–210 μm are used. Table 1. Various batches of Sr4Si3O8Cl4:Cu. S/N Batch name Material Heat treatment 1 B1 Sr4Si3O8Cl4:Cu Heated at 800°C 14 h in air solid state method 2 B2 Sr4Si3O8Cl4:Cu Heated at 900°C 2 h in air 3 B3 Sr4Si3O8Cl4:Cu Heated at 900°C 2 h in reducing atm. 4 B4 Sr4Si3O8Cl4:Cu Heated at 800°C 2 h in reducing atm 5 B5 SrSiO3:Cu Heated at 800°C 2 h in air S/N Batch name Material Heat treatment 1 B1 Sr4Si3O8Cl4:Cu Heated at 800°C 14 h in air solid state method 2 B2 Sr4Si3O8Cl4:Cu Heated at 900°C 2 h in air 3 B3 Sr4Si3O8Cl4:Cu Heated at 900°C 2 h in reducing atm. 4 B4 Sr4Si3O8Cl4:Cu Heated at 800°C 2 h in reducing atm 5 B5 SrSiO3:Cu Heated at 800°C 2 h in air Table 1. Various batches of Sr4Si3O8Cl4:Cu. S/N Batch name Material Heat treatment 1 B1 Sr4Si3O8Cl4:Cu Heated at 800°C 14 h in air solid state method 2 B2 Sr4Si3O8Cl4:Cu Heated at 900°C 2 h in air 3 B3 Sr4Si3O8Cl4:Cu Heated at 900°C 2 h in reducing atm. 4 B4 Sr4Si3O8Cl4:Cu Heated at 800°C 2 h in reducing atm 5 B5 SrSiO3:Cu Heated at 800°C 2 h in air S/N Batch name Material Heat treatment 1 B1 Sr4Si3O8Cl4:Cu Heated at 800°C 14 h in air solid state method 2 B2 Sr4Si3O8Cl4:Cu Heated at 900°C 2 h in air 3 B3 Sr4Si3O8Cl4:Cu Heated at 900°C 2 h in reducing atm. 4 B4 Sr4Si3O8Cl4:Cu Heated at 800°C 2 h in reducing atm 5 B5 SrSiO3:Cu Heated at 800°C 2 h in air All the thermoluminescence (TL) and OSL measurements were carried out under identical experimental conditions on a Risø TL/OSL-DA-20 (Risø National Laboratory, Denmark) reader system. The reader system is provided with a bi-alkali photomultiplier tube for light detection (ET 9235QB15) and a set of optical filters like Hoya 340, Schott BG-39 and BG-3 for TL/OSL measurements. The continuous wave (CW) OSL readouts were taken using blue (470 ± 20 nm) LED based light stimulation available in the reader system. The LED power was kept ~50 mW/cm2 and CW-OSL signal was recorded for 60 s with the acquisition time of 0.1 s. The OSL from all the samples is compared with commercial Al2O3:C(Landauer Inc.) OSL phosphor. The TL measurements were carried out at a heating rate of 4°C/s. The samples were given a test dose of 100 mGy using the built-in 90Sr/90Y beta source. The photoluminescence (PL) studies were carried on Hitachi-7000 spectrofluorometer with excitation and emission band pass 2.5 nm. RESULTS AND DISCUSSION Figure 1 shows the XRD pattern of various batches of Cu-doped Sr4Si3O8Cl4 samples. In all the batches, lines corresponding to SrSiO3 and Sr4Si3O8Cl4 are seen. In case of B1, the dominant phase is Sr4Si3O8Cl4 with significant amount of SrSiO3 (Figure 1a). Of the two hexagonal and monoclinic phases of SrSiO3, lines corresponding to monoclinic phase are observed (ICDD file No. 36-0018). The Sr4Si3O8Cl4 crystallizes in orthorhombic phase (ICDD file No. 40-0036). In case of batches B2 to B4, similar phases are observed. The relative contribution of phases depends on the annealing treatment. In case of batch B2 dominant phase is strontium metasilicate (Figure 1b), whereas in case of batch B3 the dominant phase is Sr4Si3O8Cl4 (Figure 1c). The Sr4Si3O8Cl4 phase in this batch is more predominant compared to batch B1 as observed from the relative intensities of XRD lines. In case of batch B4 which is heated at 800°C under reducing atmosphere, the dominant phase is again SrSiO3. This shows that temperature as well as annealing atmosphere plays an important role in deciding the relative amount of phases in the material. Figure 1. View largeDownload slide XRD pattern of various batches of Sr4Si3O8Cl4 samples. (a) Batch B1, (b) batch B2, (c) batch B3 and (d) batch B4. Figure 1. View largeDownload slide XRD pattern of various batches of Sr4Si3O8Cl4 samples. (a) Batch B1, (b) batch B2, (c) batch B3 and (d) batch B4. Figure 2 shows the PL spectra of Cu in various batches. The emission in batch B5 (SrSiO3 sample) consists of a broad band around 430 nm (Figure 2a) with the double humped excitation around 254 and 360 nm (Figure 2b). This emission could be attributed to parity forbidden 3d94s1←→3d10 transitions of Cu+ ions. The emission in Sr4Si3O8Cl4 is different. In case of batch B1, an emission band is observed around 360 nm with a less intense shoulder around 430 nm (Figure 2c). The excitation is observed around 240 nm (Figure 2d). The PL emission in batch B2 is similar to batch B1. However, two distinct emission and excitation bands are observed. The peak emission intensity is nearly three times more compared to batch B1. The emission bands are observed around 360 and 430 nm (Figure 2e and f). The 360 nm band could be excited by 240 nm band, whereas the 430 nm band excitation is around 254 nm. The batch B3 also shows similar bands (Figure 3a and c). The intensity of emission is better as compared to batch B1 as well as B2. This is due to reduction of more Cu2+ ions into Cu+ ions as the material is annealed under the reducing atmosphere. The Cu emission in case of batch B4 consists of a single band around 425 nm (Figure 3e) with the excitation around 254 nm (Figure 3f). Figure 2. View largeDownload slide Photoluminescence spectra of various Cu-doped Sr4Si3O8Cl4 samples. Emission: (a) batch B5, (c) batch B1, (e) batch B2 with excitation 240 nm, (f) batch B2 with excitation 254 nm. Excitation: (b) batch B5, (d) batch B1. Figure 2. View largeDownload slide Photoluminescence spectra of various Cu-doped Sr4Si3O8Cl4 samples. Emission: (a) batch B5, (c) batch B1, (e) batch B2 with excitation 240 nm, (f) batch B2 with excitation 254 nm. Excitation: (b) batch B5, (d) batch B1. Figure 3. View largeDownload slide Photoluminescence spectra of various Cu-doped Sr4Si3O8Cl4 samples. Emission: (a) batch B3 with excitation 240 nm, (c) batch B3 with excitation 254 nm, (e) batch B4 with excitation 254 nm. Excitation: (b) batch B3 excitation for 360 nm emission, (d) batch B3 excitation for 430 nm emission. Figure 3. View largeDownload slide Photoluminescence spectra of various Cu-doped Sr4Si3O8Cl4 samples. Emission: (a) batch B3 with excitation 240 nm, (c) batch B3 with excitation 254 nm, (e) batch B4 with excitation 254 nm. Excitation: (b) batch B3 excitation for 360 nm emission, (d) batch B3 excitation for 430 nm emission. In all the samples two emission bands and corresponding excitations could be attributed to Cu ions in SrSiO3 and Sr4Si3O8Cl4 phases. The batches in which the SrSiO3 phase is dominant (batches B2, B4), longer wavelength band around 425 nm is prominent. In batches B1 and B3 the shorter wavelength band around 360 nm is prominent which can be attributed to the dominant Sr4Si3O8Cl4 phase in these batches. Figure 4 shows blue stimulated luminescence (BSL) in B1 to B5. The plotted OSL is weight normalized so that the sensitivities can be directly compared from the plot. The inset figure shows normalized curves. The OSL in batch B5 is weak (Figure 4a, curve i). Substantial OSL is observed in the batches B2 and B4. The OSL decay in case of batch B2 is slower and the whole signal decays within 15 s (Figure 4a, curve ii) as compared to 5 s for batch B4 (Figure 4a, curve iii). The OSL sensitivity is compared with commercial α-Al2O3:C (Landauer Inc., USA) using area integration method. The data are summarized in Table 2. From this table it can be seen that the sensitivities of batches B2 and B4 are 42 and 17%, respectively compared to that of commercial Al2O3:C under identical conditions. Figure 4. View largeDownload slide Blue stimulated luminescence in various Cu-doped Sr4Si3O8Cl4 samples (inset show normalized OSL curves).(a) (i) Batch B5, (ii) batch B2, (iii) batch B4, (iv) Al2O3:C (Landaur Inc.). (b) (i) Batch B1, (ii) batch B3. Figure 4. View largeDownload slide Blue stimulated luminescence in various Cu-doped Sr4Si3O8Cl4 samples (inset show normalized OSL curves).(a) (i) Batch B5, (ii) batch B2, (iii) batch B4, (iv) Al2O3:C (Landaur Inc.). (b) (i) Batch B1, (ii) batch B3. Table 2. OSL comparison of various batches of Cu-doped Sr4Si3O8Cl4 with Al2O3:C. S/N Batch Integral counts Percent OSL intensity in comparison with Al2O3:C (Landuer Inc.) Percent OSL intensity in comparison with B3 1 B1 34 448 73 18.77 2 B2 8391 18 4.57 3 B3 183 545 389 100 4 B3(heated at 400°C for 10 min prior to irradiation) 168 325 357 91.70 5 B4 19 872 42 10.83 6 B5 559 1 0.30 7 Al2O3:C (Landauer Inc.) 47 166 100.0 25.70 S/N Batch Integral counts Percent OSL intensity in comparison with Al2O3:C (Landuer Inc.) Percent OSL intensity in comparison with B3 1 B1 34 448 73 18.77 2 B2 8391 18 4.57 3 B3 183 545 389 100 4 B3(heated at 400°C for 10 min prior to irradiation) 168 325 357 91.70 5 B4 19 872 42 10.83 6 B5 559 1 0.30 7 Al2O3:C (Landauer Inc.) 47 166 100.0 25.70 View Large Table 2. OSL comparison of various batches of Cu-doped Sr4Si3O8Cl4 with Al2O3:C. S/N Batch Integral counts Percent OSL intensity in comparison with Al2O3:C (Landuer Inc.) Percent OSL intensity in comparison with B3 1 B1 34 448 73 18.77 2 B2 8391 18 4.57 3 B3 183 545 389 100 4 B3(heated at 400°C for 10 min prior to irradiation) 168 325 357 91.70 5 B4 19 872 42 10.83 6 B5 559 1 0.30 7 Al2O3:C (Landauer Inc.) 47 166 100.0 25.70 S/N Batch Integral counts Percent OSL intensity in comparison with Al2O3:C (Landuer Inc.) Percent OSL intensity in comparison with B3 1 B1 34 448 73 18.77 2 B2 8391 18 4.57 3 B3 183 545 389 100 4 B3(heated at 400°C for 10 min prior to irradiation) 168 325 357 91.70 5 B4 19 872 42 10.83 6 B5 559 1 0.30 7 Al2O3:C (Landauer Inc.) 47 166 100.0 25.70 View Large Intense OSL is obtained in batches B1 and B3 (Figure 4b, curves i and ii). The OSL decay in case of batch B3 is the slowest among all the batches and is observed to be 20 s (Figure 4b, curve ii). The OSL sensitivities of batches B1 and B3 are 73 and 389%, respectively compared to that of Al2O3:C. The main emission band from Al2O3:C is at 420 nm and from the samples in this work is around 360 nm. Therefore, the sensitivity figures are only indicative and give idea that the sensitivity of the samples is comparable to that of Al2O3:C. The exact comparison is not possible due to different response of filter to the emission of the samples and Al2O3:C. Since the sensitivity is comparable with Al2O3:C, the phosphor can be useful for low dose measurements and hence the minimum detectable dose (MDD) will give better idea about the utility of the phosphor at low dose levels. On the described set-up, the MDD (equivalent to 3σ of the background counts of the annealed and unexposed sample) is found to be 1.4 μGy. Figure 5 shows the TL glow curves of various Cu-doped Sr4Si3O8Cl4 samples as well as TL taken after BSL in all batches. Weak TL is observed in case of batches B5 and B1 (Figure 5a and b). After OSL the TL depletes significantly and reduces to background level (Figure 5a and b; curve ii). In case of batch B2, broad TL peak is observed around 150°C with a shoulder around 225°C (Figure 5c, curve i). After OSL readout, the entire TL peak gets depleted which indicates that the traps associated with this peak are responsible for the observed OSL. Intense TL is observed in case of batch B3 (Figure 5d, curve i). The main TL peak is observed around 300°C with a shoulder around 250°C. In this case too, after recording OSL, TL peak reduces to background, indicating that OSL and TL traps are the same in this batch (Figure 5d, curve ii). Similar but less intense TL is observed in batch B4 (Figure 5e, curve i). After OSL, substantial reduction in TL peak is observed, but this reduction is less as compared to batch B3, which indicates the presence of composite TL/OSL traps as well as pure TL traps (Figure 5e, curve ii). Figure 5. View largeDownload slide TL in various Cu-doped Sr4Si3O8Cl4 samples: (a) Batch B5: (i) TL immediately after irradiation and (ii) TL after taking BSL. (b) Batch B1: (i) TL immediately after irradiation and (ii) TL after taking BSL. (c) Batch B2: (i) TL immediately after irradiation and (ii) TL after taking BSL. (d) Batch B3: (i) TL immediately after irradiation and (ii) TL after taking BSL. (e) Batch B4: (i) TL immediately after irradiation and (ii) TL after taking BSL. Figure 5. View largeDownload slide TL in various Cu-doped Sr4Si3O8Cl4 samples: (a) Batch B5: (i) TL immediately after irradiation and (ii) TL after taking BSL. (b) Batch B1: (i) TL immediately after irradiation and (ii) TL after taking BSL. (c) Batch B2: (i) TL immediately after irradiation and (ii) TL after taking BSL. (d) Batch B3: (i) TL immediately after irradiation and (ii) TL after taking BSL. (e) Batch B4: (i) TL immediately after irradiation and (ii) TL after taking BSL. The above observations indicate that in batch B3, TL-OSL traps are same and the TL peak is at 350°C. This makes it different from the phosphors studied in our earlier work(1–8) where in most phosphors multiple TL peaks are observed. The lower temperature TL peak only showed light sensitivity which get depleted after OSL readout. Moreover if the light sensitive lower temperature peak is depleted thermally then also the sample shows OSL due to deep OSL traps. Thus in all the samples composite TL/OSL traps, pure OSL traps and pure TL traps are observed. The peak at lower temperature causes loss in the signal with time. Also when the TL and OSL traps are different, loss in OSL signal and built up in TL signal with time is observed due to tunneling of charge carriers from OSL traps to TL traps. This causes fading in the OSL signal with time which was indeed observed. The TL peak in sample B3 which is also responsible for the OSL is around 350°C. Therefore, the OSL signal will be stable and negligible fading is expected similar to that observed in Al2O3:C Since the batch B3 showed good OSL response, it was studied for dose response, linearity, reusability, MDD, etc. To study the dose response, eight aliquots were prepared and exposed to a test dose in the range 20 mGy–2.3 Gy. The OSL readout of each aliquot was taken for the 60 s. The OSL curves were normalized with respect to weight. The integral OSL counts vs. dose given were plotted. A linear relationship between the dose given and the integral counts up to 2.3 Gy is observed (Figure 6). Figure 6. View largeDownload slide Dose response of Cu-doped Sr4Si3O8Cl4 sample, batch B3. Figure 6. View largeDownload slide Dose response of Cu-doped Sr4Si3O8Cl4 sample, batch B3. For carrying out reusability study, a single aliquot is given a test dose of 20 mGy and the OSL was recorded. After recording OSL, TL readout was taken up to 450°C to erase the residual TL. The aliquot was again delivered a test dose and the same process was repeated up to 10 cycles. Figure 7 shows the plot of OSL response and the cycle number. From the figure, around 8% decrease in integral OSL intensity is observed up to third cycle, after that OSL is stabilized. Therefore, a heat treatment of 400°C for 10 min is recommended to nullify the decrease in OSL during first three cycles. When the OSL measurement of the sample heated at 400°C for 10 min before the irradiation is carried out then it was observed that there is 8% decrease in sensitivity compared to that of unheated sample. However, on subsequent readouts no change in sensitivity is observed. Figure 7. View largeDownload slide Reusability plot of Cu-doped Sr4Si3O8Cl4 sample, batch B3. Figure 7. View largeDownload slide Reusability plot of Cu-doped Sr4Si3O8Cl4 sample, batch B3. CONCLUSION The OSL sensitivity in Cu-doped Sr4Si3O8Cl4 depends on the relative contribution of strontium metasilicate phase which is formed during the synthesis of Sr4Si3O8Cl4 phase. Samples in which contributing strontium metasilicate phase is less, show intense OSL compared to that of Al2O3:C, whereas sample in which strontium metasilicate phase is larger show relatively weak OSL. The observed Cu luminescence is also found to be phase dependent. For sample having Sr4Si3O8Cl4 as a dominant phase, the observed Cu luminescence is around 350 nm whereas an additional longer wavelength band around 450 nm is observed when the strontium metasilicate phase is present in significant amount. The relatively phase pure Cu-doped Sr4Si3O8Cl4 shows good OSL sensitivity which is 3.89 times more compared to commercial Al2O3:C. It has MDD ~ 1.4 μGy on the described set up. The sample shows good linear response up to 2.3 Gy and is reusable up to at least seven cycles. All these properties along with the ease in preparation technique will make this phosphor suitable for radiation dosimetry using OSL. REFERENCES 1 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 . Phys. B 407 ( 4 ), 629 – 634 ( 2012 ). Google Scholar Crossref Search ADS 2 Barve , R.A. , Patil , R. R. , Gaikwad , N. P. , Kulkarni , M. S. , Mishra , D. R. , Soni , A. , Bhatt , B. C. and Moharil , S. V. Optically stimulated luminescence and thermoluminescence in some Cu+ doped alkali fluoro-silicates . Radiat. Meas. 59 , 73 – 80 ( 2013 ). Google Scholar Crossref Search ADS 3 Mandlik , N. , Sahare , P. D. , Kulkarni , M. S. , Bhatt , B. C. , Bhoraskar , V. N. and Dhole , S. D. Study of TL and optically stimulated luminescence of K2Ca2(SO4)3:Cu nanophospher for radiation dosimetry . J. Lumin. 146 , 128 – 132 ( 2014 ). 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PHASE DEPENDENT OPTICALLY STIMULATED LUMINESCENCE IN CU-DOPED Sr4Si3O8Cl4

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Abstract

Abstract Phase dependent optically stimulated luminescence (OSL) is studied in Cu-doped Sr4Si3O8Cl4. The Study shows that samples in which amount of contributing strontium metasilicate phase in Cu-doped Sr4Si3O8Cl4 is less, show intense OSL while those samples in which strontium metasilicate phase is more show weak OSL. The observed Cu luminescence is also found to be phase dependent. Sample in which Sr4Si3O8Cl4 phase is dominant, the observed Cu luminescence is around 350 nm whereas an additional longer wavelength band around 450 nm is observed when the strontium metasilicate phase is present in significant amount. The relatively phase pure, Cu-doped Sr4Si3O8Cl4 shows good OSL properties. The sensitivity of such material is 3.89 times more compared to commercial Al2O3:C (Landauer Inc.). High sensitivity, good linearity and reusability, along with low fading make this material as good OSL phosphor and may find applications in OSL based radiation dosimetry. INTRODUCTION Developing optically stimulated luminescence (OSL) phosphors for radiation dosimetric applications is of interest in recent past. Several efforts have been made by our group to study OSL in various lattices for the development of OSL phosphors(1–8). One of the areas under interest has been the phase dependent OSL properties of the material. The phase of the material is known to play an important role in determining several properties of the material. In particular, abrupt changes in several luminescence parameters have been observed during phase transitions(9). In the recent past significant changes in the intensity of 110°C TL peak are reported in bioglass(10). In our earlier work on Cu-doped Na2SO4, we have observed changes in luminescence spectra of Cu+ as well as OSL with respect to various phases(11, 12). Recently, it has been observed that the presence of microphases of Li4P2O7 and Li2Cu2P6O18 in Cu-doped Li3PO4 affect the OSL properties significantly(13). Rare earth-doped Sr4Si3O8Cl4 is of interest for several applications(14, 15). Several synthesis methods such as solid state synthesis as well as gel combustion have been reported. It has been reported that the phase purity of this material depends upon the synthesis conditions. The most common parasitic phase in this compound is strontium metasilcate, SrSiO3. The amount of this phase is known to vary depending on various synthesis parameters(16, 17). Therefore, this material is ideally suited for studying phase dependent properties. In this article we report the OSL in Cu-doped Sr4Si3O8Cl4 and its correlation to the SrSiO3 phase which is formed during the annealing of the material. The sensitivity of various Cu-doped Sr4Si3O8Cl4 is compared with commercial Al2O3:C (Landauer Inc). EXPERIMENTAL Synthesis of Cu-doped Sr4Si3O8Cl4 was carried out by the modified combustion synthesis method. Sr(NO3)2, silicic acid, SrCl2 (all AR grade from Loba chemie) were weighed in stoichoimetric proportion and mixed intimately with appropriate amount of urea and ammonium nitrate to get a paste. Cu in desired amount (0.1 mol%) in the form of CuCl2 solution was added to the paste and transferred to a china dish for heating in a furnace maintained at 500°C. After some time the paste swells due to evolution of gases and catches fire. The flame extinguishes within few seconds and a foamy white material is formed. This foamy material was then crushed, sieved and divided into three batches labeled as B2 to B4. All the batches were heated in reactive atmosphere of ammonium chloride at 500°C prior to heating at elevated temperature. This treatment is necessary to incorporate Cu in monovalent form. The details of this method are described elsewhere(18). The Sr4Si3O8Cl4 was also prepared using solid state synthesis method reported earlier(16). This batch is labeled as B1. The Cu-doped SrSiO3 is also prepared using conventional solid state synthesis method. In this method, AR grade SrCO3 and silicilic acid in stoichiometric proportion were intimately mixed for 1 h and then heated at 400°C for 4 h. After the heat treatment the mixture is again grinded for 30 min and heated at 800°C for 8 h to get the SrSiO3. This is labeled as batch B5. Each batch is given different heat treatment as summarized in Table 1. The powder material is sieved in 90–210 μm particle size. For all measurements powder samples having particle size in the range of 90–210 μm are used. Table 1. Various batches of Sr4Si3O8Cl4:Cu. S/N Batch name Material Heat treatment 1 B1 Sr4Si3O8Cl4:Cu Heated at 800°C 14 h in air solid state method 2 B2 Sr4Si3O8Cl4:Cu Heated at 900°C 2 h in air 3 B3 Sr4Si3O8Cl4:Cu Heated at 900°C 2 h in reducing atm. 4 B4 Sr4Si3O8Cl4:Cu Heated at 800°C 2 h in reducing atm 5 B5 SrSiO3:Cu Heated at 800°C 2 h in air S/N Batch name Material Heat treatment 1 B1 Sr4Si3O8Cl4:Cu Heated at 800°C 14 h in air solid state method 2 B2 Sr4Si3O8Cl4:Cu Heated at 900°C 2 h in air 3 B3 Sr4Si3O8Cl4:Cu Heated at 900°C 2 h in reducing atm. 4 B4 Sr4Si3O8Cl4:Cu Heated at 800°C 2 h in reducing atm 5 B5 SrSiO3:Cu Heated at 800°C 2 h in air Table 1. Various batches of Sr4Si3O8Cl4:Cu. S/N Batch name Material Heat treatment 1 B1 Sr4Si3O8Cl4:Cu Heated at 800°C 14 h in air solid state method 2 B2 Sr4Si3O8Cl4:Cu Heated at 900°C 2 h in air 3 B3 Sr4Si3O8Cl4:Cu Heated at 900°C 2 h in reducing atm. 4 B4 Sr4Si3O8Cl4:Cu Heated at 800°C 2 h in reducing atm 5 B5 SrSiO3:Cu Heated at 800°C 2 h in air S/N Batch name Material Heat treatment 1 B1 Sr4Si3O8Cl4:Cu Heated at 800°C 14 h in air solid state method 2 B2 Sr4Si3O8Cl4:Cu Heated at 900°C 2 h in air 3 B3 Sr4Si3O8Cl4:Cu Heated at 900°C 2 h in reducing atm. 4 B4 Sr4Si3O8Cl4:Cu Heated at 800°C 2 h in reducing atm 5 B5 SrSiO3:Cu Heated at 800°C 2 h in air All the thermoluminescence (TL) and OSL measurements were carried out under identical experimental conditions on a Risø TL/OSL-DA-20 (Risø National Laboratory, Denmark) reader system. The reader system is provided with a bi-alkali photomultiplier tube for light detection (ET 9235QB15) and a set of optical filters like Hoya 340, Schott BG-39 and BG-3 for TL/OSL measurements. The continuous wave (CW) OSL readouts were taken using blue (470 ± 20 nm) LED based light stimulation available in the reader system. The LED power was kept ~50 mW/cm2 and CW-OSL signal was recorded for 60 s with the acquisition time of 0.1 s. The OSL from all the samples is compared with commercial Al2O3:C(Landauer Inc.) OSL phosphor. The TL measurements were carried out at a heating rate of 4°C/s. The samples were given a test dose of 100 mGy using the built-in 90Sr/90Y beta source. The photoluminescence (PL) studies were carried on Hitachi-7000 spectrofluorometer with excitation and emission band pass 2.5 nm. RESULTS AND DISCUSSION Figure 1 shows the XRD pattern of various batches of Cu-doped Sr4Si3O8Cl4 samples. In all the batches, lines corresponding to SrSiO3 and Sr4Si3O8Cl4 are seen. In case of B1, the dominant phase is Sr4Si3O8Cl4 with significant amount of SrSiO3 (Figure 1a). Of the two hexagonal and monoclinic phases of SrSiO3, lines corresponding to monoclinic phase are observed (ICDD file No. 36-0018). The Sr4Si3O8Cl4 crystallizes in orthorhombic phase (ICDD file No. 40-0036). In case of batches B2 to B4, similar phases are observed. The relative contribution of phases depends on the annealing treatment. In case of batch B2 dominant phase is strontium metasilicate (Figure 1b), whereas in case of batch B3 the dominant phase is Sr4Si3O8Cl4 (Figure 1c). The Sr4Si3O8Cl4 phase in this batch is more predominant compared to batch B1 as observed from the relative intensities of XRD lines. In case of batch B4 which is heated at 800°C under reducing atmosphere, the dominant phase is again SrSiO3. This shows that temperature as well as annealing atmosphere plays an important role in deciding the relative amount of phases in the material. Figure 1. View largeDownload slide XRD pattern of various batches of Sr4Si3O8Cl4 samples. (a) Batch B1, (b) batch B2, (c) batch B3 and (d) batch B4. Figure 1. View largeDownload slide XRD pattern of various batches of Sr4Si3O8Cl4 samples. (a) Batch B1, (b) batch B2, (c) batch B3 and (d) batch B4. Figure 2 shows the PL spectra of Cu in various batches. The emission in batch B5 (SrSiO3 sample) consists of a broad band around 430 nm (Figure 2a) with the double humped excitation around 254 and 360 nm (Figure 2b). This emission could be attributed to parity forbidden 3d94s1←→3d10 transitions of Cu+ ions. The emission in Sr4Si3O8Cl4 is different. In case of batch B1, an emission band is observed around 360 nm with a less intense shoulder around 430 nm (Figure 2c). The excitation is observed around 240 nm (Figure 2d). The PL emission in batch B2 is similar to batch B1. However, two distinct emission and excitation bands are observed. The peak emission intensity is nearly three times more compared to batch B1. The emission bands are observed around 360 and 430 nm (Figure 2e and f). The 360 nm band could be excited by 240 nm band, whereas the 430 nm band excitation is around 254 nm. The batch B3 also shows similar bands (Figure 3a and c). The intensity of emission is better as compared to batch B1 as well as B2. This is due to reduction of more Cu2+ ions into Cu+ ions as the material is annealed under the reducing atmosphere. The Cu emission in case of batch B4 consists of a single band around 425 nm (Figure 3e) with the excitation around 254 nm (Figure 3f). Figure 2. View largeDownload slide Photoluminescence spectra of various Cu-doped Sr4Si3O8Cl4 samples. Emission: (a) batch B5, (c) batch B1, (e) batch B2 with excitation 240 nm, (f) batch B2 with excitation 254 nm. Excitation: (b) batch B5, (d) batch B1. Figure 2. View largeDownload slide Photoluminescence spectra of various Cu-doped Sr4Si3O8Cl4 samples. Emission: (a) batch B5, (c) batch B1, (e) batch B2 with excitation 240 nm, (f) batch B2 with excitation 254 nm. Excitation: (b) batch B5, (d) batch B1. Figure 3. View largeDownload slide Photoluminescence spectra of various Cu-doped Sr4Si3O8Cl4 samples. Emission: (a) batch B3 with excitation 240 nm, (c) batch B3 with excitation 254 nm, (e) batch B4 with excitation 254 nm. Excitation: (b) batch B3 excitation for 360 nm emission, (d) batch B3 excitation for 430 nm emission. Figure 3. View largeDownload slide Photoluminescence spectra of various Cu-doped Sr4Si3O8Cl4 samples. Emission: (a) batch B3 with excitation 240 nm, (c) batch B3 with excitation 254 nm, (e) batch B4 with excitation 254 nm. Excitation: (b) batch B3 excitation for 360 nm emission, (d) batch B3 excitation for 430 nm emission. In all the samples two emission bands and corresponding excitations could be attributed to Cu ions in SrSiO3 and Sr4Si3O8Cl4 phases. The batches in which the SrSiO3 phase is dominant (batches B2, B4), longer wavelength band around 425 nm is prominent. In batches B1 and B3 the shorter wavelength band around 360 nm is prominent which can be attributed to the dominant Sr4Si3O8Cl4 phase in these batches. Figure 4 shows blue stimulated luminescence (BSL) in B1 to B5. The plotted OSL is weight normalized so that the sensitivities can be directly compared from the plot. The inset figure shows normalized curves. The OSL in batch B5 is weak (Figure 4a, curve i). Substantial OSL is observed in the batches B2 and B4. The OSL decay in case of batch B2 is slower and the whole signal decays within 15 s (Figure 4a, curve ii) as compared to 5 s for batch B4 (Figure 4a, curve iii). The OSL sensitivity is compared with commercial α-Al2O3:C (Landauer Inc., USA) using area integration method. The data are summarized in Table 2. From this table it can be seen that the sensitivities of batches B2 and B4 are 42 and 17%, respectively compared to that of commercial Al2O3:C under identical conditions. Figure 4. View largeDownload slide Blue stimulated luminescence in various Cu-doped Sr4Si3O8Cl4 samples (inset show normalized OSL curves).(a) (i) Batch B5, (ii) batch B2, (iii) batch B4, (iv) Al2O3:C (Landaur Inc.). (b) (i) Batch B1, (ii) batch B3. Figure 4. View largeDownload slide Blue stimulated luminescence in various Cu-doped Sr4Si3O8Cl4 samples (inset show normalized OSL curves).(a) (i) Batch B5, (ii) batch B2, (iii) batch B4, (iv) Al2O3:C (Landaur Inc.). (b) (i) Batch B1, (ii) batch B3. Table 2. OSL comparison of various batches of Cu-doped Sr4Si3O8Cl4 with Al2O3:C. S/N Batch Integral counts Percent OSL intensity in comparison with Al2O3:C (Landuer Inc.) Percent OSL intensity in comparison with B3 1 B1 34 448 73 18.77 2 B2 8391 18 4.57 3 B3 183 545 389 100 4 B3(heated at 400°C for 10 min prior to irradiation) 168 325 357 91.70 5 B4 19 872 42 10.83 6 B5 559 1 0.30 7 Al2O3:C (Landauer Inc.) 47 166 100.0 25.70 S/N Batch Integral counts Percent OSL intensity in comparison with Al2O3:C (Landuer Inc.) Percent OSL intensity in comparison with B3 1 B1 34 448 73 18.77 2 B2 8391 18 4.57 3 B3 183 545 389 100 4 B3(heated at 400°C for 10 min prior to irradiation) 168 325 357 91.70 5 B4 19 872 42 10.83 6 B5 559 1 0.30 7 Al2O3:C (Landauer Inc.) 47 166 100.0 25.70 View Large Table 2. OSL comparison of various batches of Cu-doped Sr4Si3O8Cl4 with Al2O3:C. S/N Batch Integral counts Percent OSL intensity in comparison with Al2O3:C (Landuer Inc.) Percent OSL intensity in comparison with B3 1 B1 34 448 73 18.77 2 B2 8391 18 4.57 3 B3 183 545 389 100 4 B3(heated at 400°C for 10 min prior to irradiation) 168 325 357 91.70 5 B4 19 872 42 10.83 6 B5 559 1 0.30 7 Al2O3:C (Landauer Inc.) 47 166 100.0 25.70 S/N Batch Integral counts Percent OSL intensity in comparison with Al2O3:C (Landuer Inc.) Percent OSL intensity in comparison with B3 1 B1 34 448 73 18.77 2 B2 8391 18 4.57 3 B3 183 545 389 100 4 B3(heated at 400°C for 10 min prior to irradiation) 168 325 357 91.70 5 B4 19 872 42 10.83 6 B5 559 1 0.30 7 Al2O3:C (Landauer Inc.) 47 166 100.0 25.70 View Large Intense OSL is obtained in batches B1 and B3 (Figure 4b, curves i and ii). The OSL decay in case of batch B3 is the slowest among all the batches and is observed to be 20 s (Figure 4b, curve ii). The OSL sensitivities of batches B1 and B3 are 73 and 389%, respectively compared to that of Al2O3:C. The main emission band from Al2O3:C is at 420 nm and from the samples in this work is around 360 nm. Therefore, the sensitivity figures are only indicative and give idea that the sensitivity of the samples is comparable to that of Al2O3:C. The exact comparison is not possible due to different response of filter to the emission of the samples and Al2O3:C. Since the sensitivity is comparable with Al2O3:C, the phosphor can be useful for low dose measurements and hence the minimum detectable dose (MDD) will give better idea about the utility of the phosphor at low dose levels. On the described set-up, the MDD (equivalent to 3σ of the background counts of the annealed and unexposed sample) is found to be 1.4 μGy. Figure 5 shows the TL glow curves of various Cu-doped Sr4Si3O8Cl4 samples as well as TL taken after BSL in all batches. Weak TL is observed in case of batches B5 and B1 (Figure 5a and b). After OSL the TL depletes significantly and reduces to background level (Figure 5a and b; curve ii). In case of batch B2, broad TL peak is observed around 150°C with a shoulder around 225°C (Figure 5c, curve i). After OSL readout, the entire TL peak gets depleted which indicates that the traps associated with this peak are responsible for the observed OSL. Intense TL is observed in case of batch B3 (Figure 5d, curve i). The main TL peak is observed around 300°C with a shoulder around 250°C. In this case too, after recording OSL, TL peak reduces to background, indicating that OSL and TL traps are the same in this batch (Figure 5d, curve ii). Similar but less intense TL is observed in batch B4 (Figure 5e, curve i). After OSL, substantial reduction in TL peak is observed, but this reduction is less as compared to batch B3, which indicates the presence of composite TL/OSL traps as well as pure TL traps (Figure 5e, curve ii). Figure 5. View largeDownload slide TL in various Cu-doped Sr4Si3O8Cl4 samples: (a) Batch B5: (i) TL immediately after irradiation and (ii) TL after taking BSL. (b) Batch B1: (i) TL immediately after irradiation and (ii) TL after taking BSL. (c) Batch B2: (i) TL immediately after irradiation and (ii) TL after taking BSL. (d) Batch B3: (i) TL immediately after irradiation and (ii) TL after taking BSL. (e) Batch B4: (i) TL immediately after irradiation and (ii) TL after taking BSL. Figure 5. View largeDownload slide TL in various Cu-doped Sr4Si3O8Cl4 samples: (a) Batch B5: (i) TL immediately after irradiation and (ii) TL after taking BSL. (b) Batch B1: (i) TL immediately after irradiation and (ii) TL after taking BSL. (c) Batch B2: (i) TL immediately after irradiation and (ii) TL after taking BSL. (d) Batch B3: (i) TL immediately after irradiation and (ii) TL after taking BSL. (e) Batch B4: (i) TL immediately after irradiation and (ii) TL after taking BSL. The above observations indicate that in batch B3, TL-OSL traps are same and the TL peak is at 350°C. This makes it different from the phosphors studied in our earlier work(1–8) where in most phosphors multiple TL peaks are observed. The lower temperature TL peak only showed light sensitivity which get depleted after OSL readout. Moreover if the light sensitive lower temperature peak is depleted thermally then also the sample shows OSL due to deep OSL traps. Thus in all the samples composite TL/OSL traps, pure OSL traps and pure TL traps are observed. The peak at lower temperature causes loss in the signal with time. Also when the TL and OSL traps are different, loss in OSL signal and built up in TL signal with time is observed due to tunneling of charge carriers from OSL traps to TL traps. This causes fading in the OSL signal with time which was indeed observed. The TL peak in sample B3 which is also responsible for the OSL is around 350°C. Therefore, the OSL signal will be stable and negligible fading is expected similar to that observed in Al2O3:C Since the batch B3 showed good OSL response, it was studied for dose response, linearity, reusability, MDD, etc. To study the dose response, eight aliquots were prepared and exposed to a test dose in the range 20 mGy–2.3 Gy. The OSL readout of each aliquot was taken for the 60 s. The OSL curves were normalized with respect to weight. The integral OSL counts vs. dose given were plotted. A linear relationship between the dose given and the integral counts up to 2.3 Gy is observed (Figure 6). Figure 6. View largeDownload slide Dose response of Cu-doped Sr4Si3O8Cl4 sample, batch B3. Figure 6. View largeDownload slide Dose response of Cu-doped Sr4Si3O8Cl4 sample, batch B3. For carrying out reusability study, a single aliquot is given a test dose of 20 mGy and the OSL was recorded. After recording OSL, TL readout was taken up to 450°C to erase the residual TL. The aliquot was again delivered a test dose and the same process was repeated up to 10 cycles. Figure 7 shows the plot of OSL response and the cycle number. From the figure, around 8% decrease in integral OSL intensity is observed up to third cycle, after that OSL is stabilized. Therefore, a heat treatment of 400°C for 10 min is recommended to nullify the decrease in OSL during first three cycles. When the OSL measurement of the sample heated at 400°C for 10 min before the irradiation is carried out then it was observed that there is 8% decrease in sensitivity compared to that of unheated sample. However, on subsequent readouts no change in sensitivity is observed. Figure 7. View largeDownload slide Reusability plot of Cu-doped Sr4Si3O8Cl4 sample, batch B3. Figure 7. View largeDownload slide Reusability plot of Cu-doped Sr4Si3O8Cl4 sample, batch B3. CONCLUSION The OSL sensitivity in Cu-doped Sr4Si3O8Cl4 depends on the relative contribution of strontium metasilicate phase which is formed during the synthesis of Sr4Si3O8Cl4 phase. Samples in which contributing strontium metasilicate phase is less, show intense OSL compared to that of Al2O3:C, whereas sample in which strontium metasilicate phase is larger show relatively weak OSL. The observed Cu luminescence is also found to be phase dependent. For sample having Sr4Si3O8Cl4 as a dominant phase, the observed Cu luminescence is around 350 nm whereas an additional longer wavelength band around 450 nm is observed when the strontium metasilicate phase is present in significant amount. The relatively phase pure Cu-doped Sr4Si3O8Cl4 shows good OSL sensitivity which is 3.89 times more compared to commercial Al2O3:C. It has MDD ~ 1.4 μGy on the described set up. The sample shows good linear response up to 2.3 Gy and is reusable up to at least seven cycles. All these properties along with the ease in preparation technique will make this phosphor suitable for radiation dosimetry using OSL. REFERENCES 1 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 . Phys. B 407 ( 4 ), 629 – 634 ( 2012 ). Google Scholar Crossref Search ADS 2 Barve , R.A. , Patil , R. R. , Gaikwad , N. P. , Kulkarni , M. S. , Mishra , D. R. , Soni , A. , Bhatt , B. C. and Moharil , S. V. Optically stimulated luminescence and thermoluminescence in some Cu+ doped alkali fluoro-silicates . Radiat. Meas. 59 , 73 – 80 ( 2013 ). Google Scholar Crossref Search ADS 3 Mandlik , N. , Sahare , P. D. , Kulkarni , M. S. , Bhatt , B. C. , Bhoraskar , V. N. and Dhole , S. D. Study of TL and optically stimulated luminescence of K2Ca2(SO4)3:Cu nanophospher for radiation dosimetry . J. Lumin. 146 , 128 – 132 ( 2014 ). 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For Permissions, please email: journals.permissions@oup.com This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model)

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Radiation Protection DosimetryOxford University Press

Published: Oct 1, 2018

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