|Year : 2018 | Volume
| Issue : 2 | Page : 61-65
Study of optically stimulated luminescence in K2SO4:Ce
Yogesh K More1, Minakshi S Nikam2, Rajesh R Patil3, Sangeeta P Wankhede4, Mukund S Kulkarni5, Sanjeev V Moharil2
1 Department of Applied Physics, S B Jain Institute of Technology, Management and Research, Nagpur, Maharashtra, India
2 Department of Physics, Nagpur University, Nagpur, Maharashtra, India
3 Department of Physics, Institute of Science, Nagpur, Maharashtra, India
4 Department of Physics, KDK College of Engineering, Nagpur, Maharashtra, India
5 Division of Radiation Safety Systems, Bhabha Atomic Research Centre, Mumbai, Maharashtra, India
|Date of Submission||13-Feb-2018|
|Date of Decision||26-Feb-2018|
|Date of Acceptance||28-Feb-2018|
|Date of Web Publication||24-Aug-2018|
Dr. Yogesh K More
Department of Applied Physics, S B Jain Institute of Technology, Management and Research, Nagpur-441 501, Maharashtra
Source of Support: None, Conflict of Interest: None
Highly sensitive cerium-doped K2SO4phosphor was synthesized for optically stimulated luminescence (OSL) properties. The present study mainly investigated various kinetic parameters and correlation between the thermoluminescence and OSL (TL-OSL) processes. Continuous wave-OSL sensitivity of the phosphor is four times more than that of Al2O3:C (Landauer). The phosphor exhibits good TL-OSL correlation. Most of the charge carriers in K2SO4:Ce get depleted using optical stimulation which confirms their participation in TL as well as OSL processes. Optical bleaching also affects the order of kinetics for TL. The glow curve of K2SO4:Ce without illumination exactly fits with two components, whereas it is fitted with three components when optically bleached for 60 s. Bleaching also shifted the glow peaks toward higher temperature side indicating non- first-order kinetics.
Keywords: Dosimetry, optically stimulated luminescence, phosphor synthesis, potassium sulfate
|How to cite this article:|
More YK, Nikam MS, Patil RR, Wankhede SP, Kulkarni MS, Moharil SV. Study of optically stimulated luminescence in K2SO4:Ce. Radiat Prot Environ 2018;41:61-5
|How to cite this URL:|
More YK, Nikam MS, Patil RR, Wankhede SP, Kulkarni MS, Moharil SV. Study of optically stimulated luminescence in K2SO4:Ce. Radiat Prot Environ [serial online] 2018 [cited 2018 Dec 11];41:61-5. Available from: http://www.rpe.org.in/text.asp?2018/41/2/61/239681
| Introduction|| |
Optically stimulated luminescence (OSL) has emerged out as a powerful tool in the field of dosimetry. It leads to quick, accurate, and more precise calculation of the doses absorbed by the materials either intentionally or accidentally. In this technique, irradiated materials are stimulated using blue or green light. Consequently, they emit the radiation in ultraviolet (UV), visible, or infrared (IR) region, intensity of which is the function of dose absorbed. This technique has proved as a better alternative to the well-known thermoluminescence (TL) technique.
Sulfates are known to be good TL materials. Rare earth (RE)-doped CaSO4 phosphors have been studied for quite some time. Dy  and Tm are known to be efficient activators for TL in a CaSO4 host. CaSO4:Eu and CaSO4:Sm were found to exhibit interesting photoluminescence (PL) properties, and the possibility of using these phosphors in radio-PL (RPL) dosimetry  and UV dosimetry using TL  was indicated. TL in other alkaline earth sulfates was also studied subsequently. In recent years, Dhoble et al. have reported several sulfate-based phosphors which possess good luminescence properties and are useful for measuring ionizing radiations.,,,, They also studied the TL in Li2 SO4:P, Dy and Li2 SO4:P, Eu based phosphors.
The mirabilite → thenardite phase transition using an original self-made thermal control during the X-ray diffraction sequential profiles was studied by Correcher et al. They also studied characterization of the IR-stimulated luminescence, radioluminescence, and TL spectra in the range of 200–800 nm and the influence of thermal phase transition on the emission spectra of Na2 SO4. Alkali sulfates were found to be interesting host for the luminescence of Eu as they were found to incorporate Eu in large concentration and also there exist intermediate compounds in alkali sulfate-RE sulfate systems. Europium enters Li2 SO4 and Na2 SO4 lattices in trivalent form, which is stable for exposures to gamma rays, and hence could not be used for RPL dosimetry. There is also thermal and UV-induced Eu 3+ to Eu 2+ conversion and no complete back conversion to Eu 3+ after the thermal treatment. We have already studied and reported the general OSL properties of Eu-doped K2 SO4. However, the continuous wave-OSL (CW-OSL) sensitivity in this case was only 39% that of the commercial phosphor Al2O3:C (Landauer). We have studied the effect of optical bleaching on kinetics of TL and OSL for K2 SO4:Eu 2+.
The present study mainly investigated various kinetic parameters and correlation between the TL and OSL processes in K2 SO4:Ce.
For the preparation of K2 SO4:Ce, analytical grade K2 SO4 was first dissolved in double distilled water. 1000 ppm cerium nitrate was added to this aqueous solution of K2 SO4. The solution was then allowed to evaporate slowly in air at 60°C on a hot plate. The dried powder was melted in crucible in a preheated furnace at 1200°C and quenched to room temperature. The polycrystalline material thus obtained was crushed in a mortar pestle and sieved to grain size ranging from 72 to 210 μ. The powder samples of material were used for the measurements. The optimized cerium concentration was found to be 0.1% mol.
All TL and OSL readouts were carried out on Riso TL/OSL reader equipped with blue (470 nm) LEDs for optical stimulation, a 40 mCi 90 Sr/90 Y beta-ray source having a dose rate of 1.22 Gy/min at sample position. The reader is fitted with an EMI 9235QA photomultiplier tube.
| Results and Discussion|| |
The phase pure material was obtained using the described synthesis which was confirmed by the X-ray diffraction pattern. [Figure 1] shows the PL spectra of the material taken on Hitachi F-7000 Fluorescence Spectrophotometer. The sample shows broad PL emission ranging from 310 nm to 400 nm. The emission peak was observed at the wavelength 356 nm with a shoulder at 335 nm. The sample shows intense excitation peak at 284 nm for 356 nm emission.
The CW-OSL curve for K2 SO4:Ce was recorded for a test dose of 100 mGy with the stimulating power of 22 mW/cm 2. The CW-OSL sensitivity of K2 SO4:Ce was compared with that of the commercial phosphor Al2O3:C (Landauer) recorded under identical conditions [Figure 2]. The sensitivities were compared using both area integral method and averaging out the OSL counts for first 5 s. The phosphor is four times more sensitive than Al2O3:C by considering integral area under CW-OSL curves, whereas it is five times more sensitive by averaging out the OSL counts for first 5 s. Although phosphor shows high CW-OSL sensitivity, the charge carriers depleted much rapidly than that in Al2O3:C. The decay profile of CW-OSL curves for both these phosphors is shown in inset of [Figure 2]. The CW-OSL decay curve of K2 SO4:Ce was found to be four times faster than in case of Al2O3:C for the same dose. The phosphor shows linear dose response over the measured dose range for few mGy to 5 Gy.
|Figure 2: Continuous wave-optically stimulated luminescence curve for K2SO4:Ce compared with Al2O3:C (Landauer). Data points for K2SO4:Ce are divided with factor 10 to be fitted in scale. Inset shows the normalized continuous wave optically stimulated luminescence decay curve|
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[Figure 3] shows the linearly modulated-OSL (LM-OSL) response of K2 SO4:Ce compared with Al2O3:C. LM-OSL curves were recorded for a test dose of 100 mGy. K2 SO4:Ce shows intense LM-OSL, but the rate of depletion of charge carriers was much higher as compared to Al2O3:C. The integral area under the curve for the phosphor was about 20 times more than that for Al2O3:C.
|Figure 3: Linearly modulated-optically stimulated luminescence response of K2SO4:Ce phosphor compared with commercial phosphor Al2O3:C (Landauer)|
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The shape/geometrical factor μg is defined by the ratio of where , and tm is the time corresponding to maximum intensity of LM-OSL peak (Im) and t1 and t2 are the values of the time for the rising and falling portions of LM-OSL curve at Im/2. In fact, represents full width at the half maximum where . In addition, other shape parameters ω/tm, δ/tm, and τ/tm may also be evaluated and are very helpful in kinetic analysis of LM-OSL curves.
The LM-OSL parameters for K2 SO4:Ce are given in [Table 1]. The value of shape parameter μg for LM-OSL curve was ~0.65 which means that phosphor obeys nearly second-order kinetics or have order of kinetics between one and two.
|Table 1: Linearly modulated optically stimulated luminescence parameters of K2SO4:Ce|
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The TL glow curve for K2 SO4:Ce was recorded for the test dose of 100 mGy with a heating rate of 5°C/s. Phosphor shows intense TL glow peak around 155°C, with a shoulder at 123°C. The TL glow curve for K2 SO4:Ce can be exactly fitted with sum of two peaks with adjusted R2 value tending to one as shown in [Figure 4].
|Figure 4: Thermoluminescence glow curves K2SO4:Ce along with the fitted components|
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The low-temperature component (peak) of the curve was observed at 138°C, whereas second component (peak) was observed at 155°C. Both these components participate in OSL processes, and phosphor shows good TL-OSL correlation which was observed in the depletion of TL peaks on illumination with blue light while performing optical bleaching studies. TL curve for the phosphor post-OSL was recorded as shown in [Figure 5]. From [Figure 5], it is clear that the area under post-CW-OSL TL curve was depleted to 9% of the original value. Blue stimulation also changes the glow peak structure of TL curve, and the TL peak at 155°C was found to be shifted slightly toward the higher temperature side at 165°C. The post-CW-OSL TL curve can be fitted with sum of three peaks at about 117°C, 165°C, and 198°C. It confirms that optical stimulation affects the order of kinetics of TL; however, bleaching does not change the nature of CW-OSL profile.
|Figure 5: Postcontinuous wave optically stimulated luminescence thermoluminescence glow curves K2SO4:Ce along with the fitted components|
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The CW-OSL curve for the phosphor can be exactly fitted with the sum of two exponentials as given in equation 1.
Where IOSL is the initial OSL intensity and τ1 and τ2 are the decay constants of the respective OSL traps. A1 and A2 are the coefficients for respective OSL traps and are measure of the number of charge carriers depleted on stimulation.
[Figure 6] shows the CW-OSL curve for K2 SO4:Ce along with the fitted components. The CW-OSL parameters are given in [Table 2]. From [Table 2], it is observed that ratio of fast to slow coefficient is 8.
|Figure 6: Continuous wave optically stimulated luminescence curves for K2SO4:Ce with fitted components|
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|Table 2: Continuous wave optically stimulated luminescence parameters for K2SO4:Ce|
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The minimum detectable dose (MDD) for K2 SO4:Ce was found to be ~1 μGy. Background counts using bleached samples were recorded for six times to calculate the mean deviation in counts. The phosphor was then exposed to 20 mGy of beta and the CW-OSL response was recorded.
The reusability of OSL signal was measured for 10 cycles of exposure/readout by exposing the phosphor to 100 mGy and recording OSL for 60 s so that phosphor was bleached completely. [Figure 7] shows the plot of relative integral area under CW-OSL curve versus number of readouts. The phosphor shows ± 5% deviation during first 10 cycles which means phosphor can be reused at least for 10 cycles without any change in OSL output.
The stability of beta-induced TL and OSL in K2 SO4:Ce was studied for 3 and 7 days of postirradiation storage, respectively. In this case, the samples were subjected to 100 mGy of beta dose and stored in a lead box at room temperature. The OSL and TL readouts were subsequently taken on each day after irradiation. The relative TL and OSL intensities with respect to the intensity of freshly irradiated samples are plotted against fading time as shown in [Figure 8]. About 45% of TL and 51% of OSL signal fading were observed within 1st day, whereas the fading of both TL and OSL signals stabilized after a day. On comparing the TL and OSL intensities with the respective intensity of sample recorded 1 day after irradiation, it was seen that only 9% of TL signal and 7% of OSL signal fade after 1 day. It means both TL and OSL signals fade more or less equally with time. This confirms that both TL and OSL signals originate from the same trap.
|Figure 8: Thermoluminescence and optically stimulated luminescence fading in K2SO4:Ce|
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| Conclusions|| |
Cerium-doped K2 SO4 phosphor was synthesized. The samples were irradiated with beta radiations to investigate TL-OSL properties. This material exhibits four times more CW-OSL sensitivity than that for commercial Al2O3:C. It also exhibits good TL-OSL correlation which ensures the optical erasing of accumulated dose. Most of the charge carriers in K2 SO4:Ce get depleted using optical stimulation, which confirms their participation in both TL as well as OSL processes. Optical bleaching affects the order of kinetics for TL and shifts the glow peaks toward higher temperature side. The MDD for the phosphor was found to be nearly 1 μGy. The phosphor shows good reusability for minimum 10 cycles of CW-OSL measurement with ± 5% deviation which is within statistical limit. TL signal fading was observed ~45% within 3 days. Similar fading trend was observed in CW-OSL signals also which confirms the uniqueness of TL and OSL traps created within lattice.
Financial support and sponsorship
We are grateful to BRNS for financial support to this work.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]
[Table 1], [Table 2]