Author’s Accepted Manuscript Optically stimulated luminescence study in rare earth doped SrBPO5 Sonali Gaikwad, R.R. Patil, M.S. Kulkarni, S.V. Moharil www.elsevier.com/locate/apradiso
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S0969-8043(16)30655-8 http://dx.doi.org/10.1016/j.apradiso.2017.06.007 ARI7911
To appear in: Applied Radiation and Isotopes Received date: 27 November 2016 Revised date: 6 June 2017 Accepted date: 8 June 2017 Cite this article as: Sonali Gaikwad, R.R. Patil, M.S. Kulkarni and S.V. Moharil, Optically stimulated luminescence study in rare earth doped SrBPO5, Applied Radiation and Isotopes, http://dx.doi.org/10.1016/j.apradiso.2017.06.007 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Optically stimulated luminescence study in rare earth doped SrBPO5 Sonali Gaikwad1 , R.R.Patil*2,M.S.Kulkarni3, S.V.Moharil4 1
Institute of Science R.T. Road Civil Lines Nagpur Institute of Forensic Science R.T. Road Civil Lines Nagpur 3 Radiation Safety Systems Division, Bhabha Atomic Research Centre, Mumbai 4 R.T.M Nagpur University Nagpur 2
Abstract Optically stimulated luminescence (OSL) was studied in rare earth doped SrBPO5 for the possible applications in radiation dosimetry using optically stimulated luminescence. The study shows that the sensitivity of the Eu doped SrBPO5 shows good OSL and the sensitivity is comparable to that of Al2O3:C. It is observed that annealing has a profound effect on the OSL sensitivity. Slowly cooled Eu doped sample shows highest sensitivity and is 77% compared to that Al2O3:C whereas lowest sensitivity is observed in the quenched sample. Other properties like good linearity and low fading will make this phosphor suitable for the applications in radiation dosimetry using OSL.
Keywords: Inorganic materials; Thermoluminescence; Optically stimulated luminescence Optical properties; Radiation effects; Sulphate based phosphors *Corresponding Author: Email:[email protected]
Fax: +91-0712-2565581 Cell.No: +91-09890359291
1 Introduction: Optically stimulated luminescence (OSL) is a optical analogue of the thermoluminescence technique in dosimetry of ionizing radiations. After the development of α-Al2O3:C, and its commercialization this technique is now well accepted for personnel and environmental monitoring as well as in medical dosimetry (Akselrod et al 1990,Akselrod et al 1999). More recently, several OSL phosphors have been proposed which include borates, oxides, fluorides, phosphates, etc.( Bos et al 2006, Dhabekar et al 2011, Dotlzer et al 2007, Qiu et al 1997, Justus et al 1997, Sommerfeld et al 2007, Kristianpoller et al 1997, Marcazzo et al 2011, Lee et al 2012, Kulkarni et al 2008, Patil et al 2012, Barve et al 2013). Each of these systems have their own merits and demerits which have been discussed in literature. One thing is common in these materials that these material satisfy the prime requirement for the material to be a good OSL phosphor which is discussed elsewhere (Barve et al 2013). Briefly for the material to be good OSL phosphor the emission should be in between 350 to 425 nm and the defects can be stimulable by blue-green light (450 – 550 nm) or IR radiations (650 - 800 nm). This limit on wavelength is mostly due to constrain on instrumentation part which is discussed in detail elsewhere (Barve et al 2013). Strontium borate phosphate (SrBPO5) having a crystal structure corresponding to mineral stillwellite has been investigated in detail in recent years. The effects of doping Eu2+ and Ce3+ , Tb3+ ions on the luminescent properties are reported in the literature for the possible applications as for X-ray imaging (Karthikeyani et al 2000), neutron imaging (Sakasai et al 2002 ), green phosphor for tricolor lamp (Nehru et al 2001, Chung-Hsin Lu 2005 ). Since this material is already proposed for imaging, OSL in this material is worth
studying as it is expected that this material will be a good OSL phosphor. With this in mind Photoluminescence, Optically Stimulated Luminescence, and Thermoluminescence in Eu and Ce doped SrBPO5 are studied and reported in this paper. 2 Experimental : All the SrBPO5 based borophosphate phosphors were synthesized via the solidstate reaction at high temperature similar to that described in literature (Chung-Hsin Lu 2005 ). For synthesis, analytical grade Sr2CO3 (NH4)H2PO4, and H3BO3 were thoroughly mixed in agate mortar for several hours. The solution of respective dopant in desired amount (Eu-1000ppm, Ce-1000ppm) was sprinkled on these grinded ingredients and the mixture was dried under drying lamp to evaporate the water. The mixture was initially heated at 400°C for 2 hours in air. The mixture was allowed to cool and then grinded thoroughly for 30 min. This regrinded mixture was heated at 1000o C for 4 hours in air and allowed to cool slowly to the room temperature. To study the quenching effect on the properties another sample was prepared in similar way. After keeping the sample at 1000o C for 4 hours, the heated powder was then quenched to room temperature by quickly transferring the material on to a metal block. Part of the slowly cooled sample was again heated in reducing atmosphere at 800° C for an hour to study the effect of reducing atmosphere. Photoluminescence studies were carried on Hitachi F-4000 Spectrofluorometer with the excitation slit 1.5 nm and the emission slit 5 nm. The CW-OSL response and thermoluminescence of the samples is recorded on the assembly described elsewhere( Kulkarni et al 2007). The assembly uses Luxeon blue LEDs (LXHL-LB5C) emitting at 450 nm for stimulation. Two optical filters viz.UG-1 (acrossPMT), to prevent stimulation light
from reaching PMT(9111B,25mm diameter end window PMT) and GG- 435 (across LEDs), to cut-off the stimulation wavelengths below 435 nm, were used in the assembly. To study TL and OSL response, the samples which are in micro crystalline powder form were irradiated using 90Sr/90Y beta source with the dose rate of 20mGy per min. Every time 10mg of powder sample was used and given a test dose of 100mGy. During the OSL measurements the LED power was adjusted to 11mW/cm2 and signal was recorded for 200s with the acquisition time 0.1s. TL-OSL correlation was studied with 48mW/cm2 LED power. All the thermoluminescence measurements were recorded with rate 4°C/s. 3. Result and Discussion: 3.1 XRD Analysis of SrBPO5: Figure 1 shows XRD pattern for slowly cooled (fig.1 a) as well as quenched SrBPO5 sample(fig.1 b). Both the XRD matches with Strontium Borate Phosphate similar to that reported in ICDD database (ICDD file No.18-1270), indicating the formation of the stillwellite phase. The compound is single phase compound as no additional lines of the precursor’s constituents are seen. Thus it can be seen that heating history yields the same phase, and does not alter the crystal structure. 3.2 Photoluminescence Spectra in doped SrBPO5: Figure 2 (curve a) shows photoluminescence emission spectra of doped SrBPO5 quenched samples. In case of SrBPO5: Ce sample double humped emission of Ce3+ is observed at 337nm and 350 nm corresponding to 5D – 2F5/2 and 5D–2F7/2. The excitation to this band consists of main band around 275 nm with four other bands at 244 nm, 256 nm, 295 nm 308 nm (fig.2 curve c). This is typical Ce3+ excitation which is observed in many
lattices. The SrBPO5 : Eu PL emission is single band emission and is observed at 385nm (fig.2 curve b), the corresponding excitation is observed at 297 nm (fig.2 curve d). This is typical Eu2+ emission corresponding to f-d transitions ( Lu et al 2005). Figure 3 shows the emission spectra of Eu doped sample with various heat treatments. The emission and excitation position is same with different annealing treatments however the emission intensity is found to vary significantly with heating treatment. The slowly cooled sample heated in reducing atmosphere shows the highest intensity among all samples (fig.3 curve c). This is followed by slowly heated sample. The quenched sample shows the lowest intensity among the studied samples (fig.3 curve a). The emission intensity of sample heated in reducing atmosphere is almost eight times to that of emission intensity observed in case of quenched sample whereas the emission intensity of slowly cooled sample is four times to that for quench sample. This increase in emission is due to more and more conversion of Eu ions in 2+ form either from Eu3+ state or from non-luminescent aggregates. No Eu
emission is observed in lattice hence it is concluded that most of the
Eu ions enter in lattice as Eu2+. The observed emission and excitation of Ce3+ and Eu2+ is similar to what has been observed earlier ( Lu et al 2005, Liang et al 2002) . 3.3 OSL Response of doped SrBPO5: Figure 4 shows blue stimulated luminescence of various doped SrBPO5 quenched samples. The OSL from SrBPO5 : Ce sample is weak, the decay is slow and the entire signal decays within 80 seconds (fig. 4 a). Intense OSL is observed in SrBPO5 : Eu sample and is almost 40 times compared to Ce doped sample (fig. 4 b). The decay is very fast with signal decay 2 s.
Figure 5 shows the BSL for Eu doped samples which have been given different annealing treatments. Inset shows normalized OSL curves along with normalized decay curve of Al2O3:C (Landuer Inc.). Highest OSL sensitivity is observed for slowly cooled sample (fig. 5 curve b) whereas lowest OSL is observed for the quenched sample (fig.5 curve a).The OSL intensity for sample heated in reducing atmosphere is intermediate (fig.5 curve c). The decay also depends on the annealing treatment. The decay of quenched sample is relatively faster compared to slowly cooled sample. It can be observed that CWOSL decay of Al2O3:C is quite slow compared to various SrBPO5 samples (inset in fig. 5). Since the decays are different the OSL sensitivity cannot be compared directly. Therefore the Yukihara method was employed to compare the sensitivity with Al2O3:C (Yukihara et al 2004). The method suggests to average counts up to initial few seconds (3s in present case) for both samples and the corresponding ratio then gives the sensitivity factor. Using this method the OSL from quenched SrBPO5-Eu (0.1%)sample is 42% whereas sensitivity of slowly cooled SrBPO5:Eu(0.1%) is 77 % compared to that of Al2O3:C. When same slowly cooled sample, heated in reducing atmosphere sensitivity decreases and found to be 62% compared to that of Al2O3:C (Landuer Inc.). The OSL sensitivity data is summarized in table I. 3.4 Thermo luminescence Glow Curves in doped SrBPO5 : The glow curve structure in various samples consists of one low temperature around 125° C and other at 190° C (fig. 6). The relative intensity of the peaks depends upon the annealing history. The TL in SrBPO5 :Ce sample is weak (fig. 6 curve g) with one less intense peak around 125°C and other main peak around 275°C. Intense TL is observed from Eu doped samples compared to the Ce doped sample. The quenched sample show
intense low temperature peak around 120°C along with less intense peak around 190° C (fig. 6 curve a). It is observed that after OSL ( for stimulation time of 200 s) high temperature peak around 190° C depletes completely whereas the low temperature peak depletes partially correlating the observed TL and OSL (fig.6 curve b). In case of slowly cooled sample the intensity of the peak around 120°C is marginally higher compared to the quenched sample (fig.6 curve c). However the peak intensity of higher temperature peak is higher compared to the quenched sample. An additional shoulder around 225° C is also observed. After OSL the peak around 190° C depletes completely with partial depletion of lower temperature peak (fig.6 d). The peak around 240° C does not get depleted after OSL which indicates that this peak is not responsible for the observed OSL. The TL from sample heated in reducing atmosphere is comparatively less than the samples discussed above (fig.6 e). The high temperature peak structure is similar to that what is observed in slowly cooled sample. In this sample also, after OSL, the low temperature peak depletes partially with complete depletion of 190° C peak. No change in peak around 240° C is observed (fig.6 f). The TL data is summarized in table I. From the table it can be seen that in case of slowly cooled sample the integrated TL after OSL is 50% whereas 30% depletion is observed in sample heated in reducing atmosphere. 3.5 De-convolution of CW-OSL decay curves of various SrBPO5 samples: The de-convolution of decay curves gives various parameters which gives vital information about the various physical processes inside the material. The BSL signal of the samples show exponential decay and could be fitted with the equation given by IOSL = A1 exp (-t/τ1) + A2 exp (-t/τ2) ------ (1)
Where A1, A2 are coefficients & τ1, τ2 are decay constants. Figure 7 shows the deconvolution of decay curves from various SrBPO5 sample. The corresponding values of coefficients, decay constants and photo-ionization cross-section for all three samples of SrBPO5:Eu are presented in table II. From the table it can be observed that in case of Eu doped sample decay constant for fast as well as slow components are nearly same irrespective of the annealing treatment which indicates that OSL decay is independent of the annealing treatment. 3.6 Minimum Detectable Dose in SrBPO5 : The Minimum Detectable dose(MDD) for SrBPO5:Eu samples is calculated. The minimum detectable dose in Eu doped quenched sample is 104 μGy whereas the value of MDD for slowly cooled is 15.6 μGy on the described setup of OSL. 3.7 Dose Linearity Response in SrBPO5: Dose response of the SrBPO5 : Eu sample is studied in the dose range of 10 mGy to 1Gy of beta dose. Plot of OSL integral counts as a function of radiation dose is plotted The dose response is found to be linear over the said dose range (fig. 8). 3.8 OSL Fading in SrBPO5: In order to recommend material as OSL phosphor it is essential that material should have low post irradiation fading. For studying post irradiation fading the sample aliquots were irradiated with same dose. The first OSL is recorded after 24 hrs of irradiation. Then the OSL on subsequent days are recorded and is compared with the data recorded after 24 hrs. Figure 9 shows the plot of percentage OSL intensity verses time in days. From the plot it could be seen that 10% signal fades within 24 hrs, later practically no fading is observed.
4. Conclusion : Rare earth doped SrBPO5 was synthesized using solid state reaction method. The slow cooled as well as quenched sample yield the same phase. However OSL sensitivities are very different. The OSL sensitivity of the Eu doped quenched sample is 42 % whereas slowly cooled sample shows 77% sensitivity compared to that of Al2O3:C. Weak OSL is observed in Ce doped sample. The photoluminescence of Eu is also a function of annealing; slowly cooled sample showing better PL intensity compared to that of quenched sample. The SrBPO5 :Eu shows good linear dose response
in the 10mGy to 1Gy dose
range and practically no fading is observed after initial 10% fading in first 24hrs.
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Figure captions Figure 1. XRD of SrBPO5 samples a) slowly cooled b) quenched Figure 2. Photoluminescence spectra of Ce as well as Eu doped SrBPO5 Emission; a) Ce doped SrBPO5 for excitation 275 nm b) Eu doped SrBPO5 for excitation 297 nm Excitation c) Ce doped SrBPO5 for emission 337 nm d) Eu doped SrBPO5 for emission 385 nm Figure 3 Photoluminescence emission spectra of Eu doped SrBPO5 samples with various heat treatment ; excitation wavelength 297 nm a) Heated at 800° C and quenched b) Heated at 800° C and slowly cooled c) Heated in reducing atmosphere Figure 4 Optically stimulated luminescence in various SrBPO5 quenched samples a) Ce doped b) Eu doped Figure 5 Optically stimulated luminescence in Eu doped SrBPO5 samples with various heat treatment a) quenched b) slowly Cooled c) Heated in reducing atmosphere Inset : Normalized OSL curves of various Eu doped SrBPO5 samples along with normalized OSL curve of Al2O3 :C( blue curve) Figure 6 Thermo luminescence in Ce as well as Eu doped SrBPO5 samples before and after OSL readout TL before OSL readout a) SrBPO5:Eu quenched sample c) SrBPO5:Eu slowly cooled sample e) SrBPO5:Eu annealed in reducing atmosphere g) SrBPO5:Ce quenched sample TL after OSL readout for 200 s
b) SrBPO5:Eu quenched sample d) SrBPO5:Eu slowly cooled sample f) SrBPO5:Eu annealed in reducing atmosphere Figure 7 Deconvolution plot of various Eu doped SrBPO5 samples a) quenched b) slowly Cooled c) Heated in reducing atmosphere Figure 8 Dose response of Eu doped SrBPO5 slowly cooled sample Figure 9 Fading response of Eu doped SrBPO5 slowly cooled sample
TABLE I: Sensitivity of various SrBPO5 samples compared with standard Al2O3: C (Landauer) & TL Data Sample CW-OSL counts Sensitivity Integrated Integrated TL integrated over factor as TL area area under initial 1 second compared to under curve curve after Al2O3: C OSL (Landauer) SrBPO5-Eu; Slowly 14007 77% 18457 9646 cooled SrBPO5-Eu;Slowly 11292 62% 10792 7231 cooled and then Heated in reducing atmosphere SrBPO5-Eu;Quenched 8446 46% 10571 7103 Al2O3: C (Landauer) 18108 100 Table II : CW-OSL parameters for various SrBPO5 samples CW OSL component
Decay constant τ(sec)
Photoionization cross section σ(Cm2)
Decay constant τ(sec)
Photoionization cross section σ(Cm2)
SrBPO5:Eu( slowly cooled)
SrBPO5:Eu( heated in reducing atm.) Fast
Highlights OSL in Ce and Eu doped SrBPO5 is studied Annealing treatment has profound effect on OSL sensitivity The Slowly cooled Eu doped SrBPO5 shows good OSL properties The dose response is linear and practically no fading is observed after initial fading of 10% in first 24 hrs