Characterization and some fundamental features of Optically Stimulated Luminescence measurements of silver activated lithium tetraborate

Journal of Luminescence 202 (2018) 136–146

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Characterization and some fundamental features of Optically Stimulated Luminescence measurements of silver activated lithium tetraborate A. Ozdemira, V. Altunala, V. Guckana, N. Canb,c, K. Kurtd, I. Yegingile, Z. Yegingila,

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a

Çukurova University, Art Sciences Faculty, Physics Department, Balcalı, 01330 Sarıçam, Adana, Turkey Celal Bayar University, Faculty of Arts and Sciences, Department of Physics, Manisa, Turkey c Physics Department, Jazan University, P.O. Box 114, 45142 Jazan, Saudi Arabia d Mersin University, Art Sciences Faculty, Physics Department, Mersin, Turkey e Hasan Kalyoncu University, Faculty of Engineering, Department of Electrical and Electronics Engineering, Gaziantep, Turkey b

A B S T R A C T

A new lithium tetraborate (Li2B4O7 or abbreviated as LTB) material was produced by adding various concentrations of Ag impurities to allow better luminescent properties using the solution combustion synthesis (SCS) method. The formation of single phase LTB was confirmed using X-ray Diffraction (XRD) data and Scanning Electron Microscopy (SEM) analysis indicated the existence of a tetragonal crystalline domain. Two broad band emissions located at ∼ 272 nm (near UV region) and 526 nm (green region) were observed from room temperature photoluminescence (PL) under 205 nm excitation The synthesized material consisted of polycrystalline LTB with 1 wt% Ag (abbreviated herein as LTB:Ag) exhibits considerable thermoluminescence (TL) and Optically Stimulated Luminescence (OSL) which is several times more sensitive to beta radiation than the other concentrations attempted. It was determined that the OSL signal has been a collection of three component signals. A step-preheating procedure to investigate the depth of the trapping centers associated with the OSL signal was carried out. We suggest that the TL peak at 200 °C mainly contributes to the OSL signal. It was observed that the total OSL area shows a linear dose response for beta doses ranging from 1 to 100 Gy. The minimum detectable dose (MDD) value was found to be around 3 mGy using the total OSL area. Under optimum conditions (irradiation with beta-rays), the reproducibility of total OSL area was determined with a −3% deviation at the end of the 9th irradiation-blue light stimulation-readout cycle. The dark storage stability of the total OSL signals was investigated and fading of the total OSL area was found to be approximately 25% after one week. The trap depth corresponding to the OSL signal was found to be 0.99 eV and 0.94 eV using various heating rate and isothermal annealing methods, respectively. Finally, silver doped lithium tetraborate is shown to have promise as an optically stimulated luminescent dosimeter, particularly in medical and personal applications.

1. Introduction OSL is the emission of energy as photons of light (luminescence) from an insulator or semiconductor previously exposed to irradiation and subsequently stimulated by appropriate optical-frequency radiation. Much of the recent interest in dosimetry has been focused on the precise dose assessments for personal monitoring and on patients during diagnostic or therapeutic procedures using the OSL technique [1–7]. Of the many possibilities for OSL dosimetry a few materials e.g. Al2O3:C (Landauer, Inc.) and Thermalox 995 OSL-Material (Materion Ceramics) offer sufficient stability and linear response to be used in OSL dosimetry applications [8,9]. Nevertheless, they present limitations in dose assessments, in particular in clinical applications as radiological

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Corresponding author. E-mail address: [email protected] (Z. Yegingil).

https://doi.org/10.1016/j.jlumin.2018.05.054 Received 31 October 2017; Received in revised form 13 March 2018; Accepted 21 May 2018 Available online 23 May 2018 0022-2313/ © 2018 Elsevier B.V. All rights reserved.

examinations, 2D dosimetry or neutron dosimetry, motivating the search for new OSL dosimetric materials [10]. LTB phosphors prepared by a /dopant/co-dopant combination with various transition metal ions and lanthanides have a long history in terms of ionizing radiation dose assessment studies. LTB was primarily studied for its TL properties. There is a considerable number of reports regarding its TL properties with different dopants and co-dopants exposed to ionizing radiation but OSL properties have not been investigated in details [11–22]. The lithium borate compounds have effective atomic number 7.37 which is very close to that of the biological tissue (Zeff = 7.4) [23]. When the incident photon energy is in the energy range 20–100 keV, the interaction between material and radiation is most likely photoelectric interaction and depends on the third

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OSL technique when it was exposed to beta dose. The sensitivity, decay curve characteristics, reusability and dark storage stability of OSL signals were performed. The kinetic parameters, trap depths and frequency factors (Ea and s) for both the TL and OSL traps were also estimated.

power of the atomic number. Therefore, when the phosphors are tissue equivalent, their response is comparable to that of the soft tissue. LTB crystal with a large band gap (E ~ 7.5 eV) provides a wide transparent energy window range in the implementation of transition metal ion and lanthanide doping [24]. The appropriate doping of the material in LTB promotes the formation of the deep electron and hole traps, and ultraviolet (UV) luminescence centers providing high storage and luminescence efficiency. Recently, researchers drew their attention to the implementation of light emission from LTB using OSL. OSL characteristics and optical analysis of this material have been studied by different groups [25–31]. OSL measurements are highly dependent on the defect structure and impurities providing charge trapping levels in the gap. The existence of these imperfections plays an important role when the crystal is exposed to ionizing radiation. To date, the literature surveys present that studies on Dy, Ce, Mn, C, Ag, Ni, Cr dopants and (Ag, Cu) co-dopants of LTB using OSL have made up the majority of scientific publications [24]. One of the first implementations of these phosphors using OSL, mainly Mn2+ doped LTB (TLD-800) was rather inspirational. Danilkin et al. studied optical stimulation possibilities of LTB:Mn in the UV band and found that the spectral efficiency of luminescence stimulation has a dominating UV band [25]. LTB single crystals doped with Cu and Ag, grown using the Czochralski method, were reported by Rawat et al. [26]. In that study, CW-OSL measurements on the LTB:Cu,Ag single crystals using blue light stimulation were carried out. The traps responsible for the three TL peaks observed in LTB:Cu,Ag were found to be optically sensitive. Cu doped LTB was prepared using a sintering method by Aydin et al. [27]. The powder LTB:Cu sample was doped with various concentrations of Cu. The OSL dose response curve was observed as linear up to a dose range of 12 Gy. The fading properties was found to be quite stable over long time durations. The reproducibility of the OSL measurements was approximately 5%. Kananen et al. studied Ag+ doped LTB single crystal and observed a large OSL sensitivity to ionizing radiation from a Ag-doped LTB crystal [28]. The Ag+ ions enter the lattice and occupy two distinctly different sites. They replace Li+ ions and they also become interstitials. It is suggested that Ag-doped LTB will be useful as an OSL material because of this unique combination of defect properties. Patra et al. reported OSL measurements on Ag doped LTB single crystal after exposure to various nuclear radiations [29]. Optical absorption and photoluminescence spectral characterization showed metal centered transitions 4d95s ↔ 4d10 of a single type Ag+ ions. OSL intensity was found to be linear in the range from 0.1 Gy to 500 Gy and its sensitivity to neutron detection was determined. Fading of the OSL signal was found to be around 36% in 48 h. Hemam et al. studied LTB nanoparticles doped with different concentrations of Cu, Ag and co-doped with Cu, Ag using solid state sintering technique [30]. It was investigated that the CW-OSL decay curves fitted with third order exponential decay curves and photoionization cross sections of each component were evaluated. The characterization studies mentioned above showed that Ag as a co-dopant plays a role of increasing the sensitivity of the host material when doped with Cu and gives rise to increase the overall emission. The MDD of the synthesized samples were calculated and found to be 15 μGy. The next contribution to be taken into account for the LTB based OSL dosimetry is the study on LTB:Ag,Cu phosphor synthesized using solid state reaction method by Palan et al. [31]. LTB:Ag,Cu phosphors were found 1.3 times more sensitive than that of commercially available Al2O3:C OSL dosimeters. According to the study above mentioned, the dose response was determined as linear up to 1 Gy using the TL/OSL measurements on Ag and Cu doped LTB. In the present study, the LTB:Ag samples were obtained using solution combustion technique. The samples were prepared using different doping concentrations of Ag ions in order to enhance the material's OSL sensitivity. Doping with 1 wt% of Ag was found to be the highest OSL sensitivity to ionizing radiation. We reported OSL, TL and PL properties of Ag doped LTB and its dosimetric characterization using

2. Materials and Methods Polycrystalline LTB:Ag samples were synthesized by using the solution combustion method. In this process, lithium nitrate (SigmaAldrich, LiNO3 BioUltra, ≥ 99.0%) was used as oxidizer and urea (Sigma-Aldrich, CH4N2O, powder, bioReagent) was exploited as fuel due to its high exothermicity. Boric acid (Sigma-Aldrich, H3BO3, 99.999% trace metals basis) was used as the source of boron. The oxidizers are also the oxygen and cation sources for the reaction and the product, respectively. The fuel also serves as the carbon and hydrogen source generating carbon dioxide and water and enables increased mixture homogeneity as complexes tend to form with the metal ions in solution. The precursors lithium nitrate, boric acid and urea were mixed in stoichiometric ratio and dissolved in double distilled water with stirring continuously for 2 h at 300 °C. Then the temperature of the hot plate was increased to 500 °C. The resultant solution was allowed to powder form by keeping the solution for 30 min at 500 °C. Doped LTB samples were prepared by adding silver nitrate (Sigma-Aldrich, AgNO3, 99.9999% trace metals basis) to the master solution. The polycrystal powder sample was obtained by sintering at retention temperature of 800 °C for 2 h for the crystalline form of the samples in an ash furnace (Nabertherm Model P330) in air and afterwards left in the furnace for cooling down to room temperature. Finally, it was grounded. The structures of silver doped LTB microcrystallites were characterized by X-Ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) methods. XRD measurements were performed using a PANalytical EMPYREAN XRD with a copper (Cu) and cobalt (Co) Xray tube operated at 45 kV and 40 mA and using Cu Kα radiation of wavelength λ = 0.1541 nm. The surface morphology of the LTB:Ag phosphor was investigated using FE-SEM with EDX, Zeiss, Supra55 with a spectral slit width of 1.5 nm at room temperature. The emission spectrum of the material was recorded using a spectrofluorometer (Perkin Elmer Fluorescence Spectrometer; model LS55) at room temperature. It has a xenon flash lamp whose output is a continuum; pulsed at 50 Hz with < 10 μs pulse width value at half peak height; power equivalent to 20 kW at continuous operation. TL and OSL measurements were carried out using a combined TL/ OSL reader, model Riso TL/OSL-DA-20. Sample irradiation was obtained using a Sr90/Y90 beta irradiator which emits beta particles with a maximum energy of 2.27 MeV. The dose rate in quartz at the sample position is ~ 110 mGy/s. OSL measurements were carried out using the blue light stimulation source in continuous wave mode (CW-OSL). The TL/OSL reader is equipped with 28 blue LEDs emitting at 470 nm (FWHM = 20 nm). The total power from these 28 LEDs is ~ 80 mW/ cm2 at the sample position. A green long-pass filter (GG-420) is incorporated in front of each blue LED cluster to minimize the directly scattered blue light from reaching the detector system. The photomultiplier tube (PMT) used in the reader is a bialkali EMI 9235 QA with maximum detection efficiency between 300 and 400 nm and is detecting the emission between 160 and 630 nm. In this work, to prevent scattered stimulation light from reaching the PMT, Hoya U-340 (340 ± 40 nm) was placed in front of the PMT. Thermal stimulation is achieved using a heating element inserted close to PMT and the heating strip is cooled down by a nitrogen flow. The samples were heated up to 450 °C at a linear heating rate of 5 °C/s and/or 1 °C/s. The counting time for each data point was 200 ms and the overall stimulation time for each OSL curve was 100 and/or 200 s. The initial intensity of OSL signal (dosimetric signal) was defined by integrating under the OSL curve over the first 1 s interval. We used ~ 10 mg of LTB:Ag powder samples spread out uniformly 137

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tetragonal structure and the sintering temperature and time were enough for crystallization as shown in Fig. 2. According to the SEM images, the LTB:Ag micro-particles have the size distribution in the range of ~ 0.5–1.2 µm. 3.2. Information obtainable from luminescence The PL spectra for the undoped host material LTB and all samples prepared in different Ag concentration were recorded at room temperature (RT) under the identical experimental conditions, to achieve some idea regarding the effect of Ag ions on the LTB. No sign of a perceptible broad emission band in the recorded PL data is observed for un-doped LTB samples upon the excitation of 205 nm as shown in Fig. 3. In contrast, the PL spectra of LTB:Ag with different Ag concentration at room temperature revealed two intense broad luminescence bands, one in the near UV region (272 nm) and one in the green region (526 nm) (see Fig. 3) Thus, PL spectrum contains contributions from both emission bands. The emission band centered at 272 nm was contributed to previously reported spectral characterization of the host phosphor material to metal-centered transition of isolated ion, namely Ag+ (4d10 ← 4d95s) [18,32]. It was found that there is a good agreement between wavelength of earlier reported TL and our PL emission band. We suggest that the Ag dopant is directly involved as isolated ions in TL process and it may act as traps or recombination centers. It is seen that the PL intensity of Ag doped LTB can be enhanced by changing doping concentration of Ag and it is most obvious when the concentration of Ag is 0.3 wt%. Clearly, when more silver is doped in, the concentration quenching occurs which is observed in 0.5 wt%. When the concentration of Ag ions increases, the distance between the Ag ions decreases. This promotes non-radiative energy transfers between Ag ions. Additionally, emission bands at 272 nm and 526 nm are not shifted for a new site with the increase of Ag concentration or under different excitation wavelengths. This suggests that only one local Ag environment exists, because different centers will have different emission spectra. To the best knowledge of authors’, the broad band observed at 526 is a new PL peak. Such a peak has not been addressed previously. Only Brant et al., reported that a broad PL band peaking at 502 nm in LTB crystals doped with Ag ions pre-irradiated at room temperature using X-rays was observed [33]. They did not observe this PL emission band before the X-ray irradiation at room temperature. To date, luminescence mechanism of the Ag2+ ions has not yet been understood. We speculate a mechanism for a charge transfer involving the which causes this broad emission band. During the excitation, an electron moves from an adjacent O2- ion to a Ag2+ ion and accordingly the lattice will relax to a new equilibrium position. This gives rise to an excited state attributed to a Ag+-O- close pair. However, this is a speculative interpretation only and we need further exploration.

Fig. 1. XRD patterns of un-doped Li2B4O7 (LTB) and LTB doped with various Ag concentrations compared with the standard pattern of the International Center for Diffraction Data (ICDD) PDF 98-013-0018 (Li2B4O7). No impurity phases were observed.

on thin planchets to carry out TL and OSL measurements. All the powder samples used were initially annealed in an ash oven at 350 °C for 25 min to eliminate the luminescence effects of any prior excitation. TLD-500 discs, the commercial dosimeters (Al2O3:C, Landauer Inc., Stillwater, OK, USA), were used for comparison of the absorbed dose values using OSL signals. The TL glow curve of the material was compared with the TL glow curve of the commercially available TLD-100 (LiF:Mg,Ti, Thermo Fisher Scientific, PA, USA) dosimeter which is in powder form.

3. Results and Discussion

3.3. The effect of impurity concentration on OSL and TL

3.1. Crystalline structure and morphological analysis

In the present study, we confine our attention to the outputs of two stimulation types: thermally and optically stimulated luminescence of the studied phosphor. OSL decay curves of the samples synthesized by the solution combustion method with different concentration of Ag ions were recorded after 1 Gy beta dose. The OSL decay curve of Ag doped lithium tetraborate (LTB:Ag) showed that OSL sensitivity was low for the un-doped material and increased as the amount of silver impurity increases. Fig. 4 shows the effect of impurity concentration on the maximum intensity of OSL signal. As can be seen from Fig. 4, maximum OSL intensity grows progressively with the increasing silver ion concentration. Doping the material with more Ag concentration does not exhibit quenching process. Hence, LTB:Ag (1 wt%) phosphor was chosen as a sample to be scrutinized in our study. Fig. 5 shows the TL curves of LTB prepared with different concentrations of Ag dopant (0.05, 0.1, 0.3, 0.5 and 1.0 wt%) together with un-doped LTB after 0.5 Gy beta dose exposure. TL elementary peaks are

The samples sintered at 800 °C for 2 h were analyzed using XRD to confirm the crystalline phase. It is revealed that XRD data shows a well crystalline structure and all of the observed peaks satisfy the reflection condition. All of the peaks are found to be well matched with ICDD No 98-013-0018 as seen in Fig. 1. This is direct experimental evidence of the fact that the crystal can be assigned to the structural nature of the LTB phase and an indication that the Ag+ ions were satisfactorily substituted in the lattice without significant changes in the tetragonal phase. The analysis of selected point locations at the surface (or near the surface) of the sample were performed using electrons via a SEM to examine the morphology and homogeneity of the samples. Fig. 2 shows SEM micrographs of LTB:Ag synthesized using SCS method and sintered at 800 °C for 2 h. The LTB crystal crystallizes in a highly homogeneous 138

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Fig. 2. SEM of lithium tetraborate material synthesized using solution combustion method and doped with 1 wt% Ag. Grounded and sieved samples were sintered at 800 °C for 2 h.

Fig. 5. Experimental TL glow curves of all synthesized LTB:Ag phosphors irradiated by a Sr90/Y90 beta source with a dose of 0.5 Gy. Heating rate is 5 °C/s.

Fig. 3. Photoluminescence (PL) spectra of un-doped LTB and LTB with dopants of various Ag concentrations measured at room temperature. Emission spectrum monitoring with excitation wavelength at ʎex = 205 nm.

relative amplitudes of the TL peaks appear to increase with increasing Ag doping concentration. The TL response of the ~ 200 °C peak of LTB:Ag (1 wt%) is approximately 30 times more sensitive than that of un-doped LTB. In a previous work, the similar peaks at ~ 95, 150 and 240 °C were reported for the TL glow curve of LTB:Ag crystal after beta dose of 1.1 Gy [29]. 3.4. Optically Stimulated Luminescence measurements CW-OSL curve measured from the material at 25 °C is shown in Fig. 6. The irradiation was performed with 0.2 Gy beta dose and OSL was recorded over a period of 200 s using blue light stimulation after preheating the samples at 110 °C for 10 s to remove shallow traps. The OSL decay curve was compared with that of commercial Al2O3:C chip (TLD-500) irradiated with the same dose and illuminated with the same light. Fig. 6a presents a typical OSL signal for the material, showing that the intensity of the total area of LTB:Ag OSL signal is approximately 7 times lower than that of OSL signal from Al2O3:C (TLD-500) when using Hoya U-340 filters. This data represents only an order of magnitude comparison between two detector types, one is in powder form and the other is in chip; having the same masses not as absolute comparison between the material's OSL sensitivity. There are several arguments in support of the idea; (i) The samples have different optical transmission properties and Al2O3:C chips are more transparent than LTB:Ag samples (ii) The Al2O3:C content of the TLD-500 type chips is relatively small. The chip detectors used in this study have a mass of typically ~ 10 mg, but a part of this mass is made out of plastic (iii) The average dose delivered to the detectors by the beta source will be slightly different due to different sample thickness (lower for the thicker samples) (iv)

Fig. 4. Effect of Ag impurity concentration on OSL signals. Samples were exposed to 1 Gy of beta irradiation and stimulated with blue light for 200 s.

located at 95, ~ 200 and ~ 300 °C, and the relative peak amplitudes give large differences (heating rate: 5 °C/s). The low temperature peak occurs slightly above ambient temperature. At that temperature the peak decay takes place in a few hours. The second TL peak (main peak) appears at the ~ 200 °C region. Its fading at room temperature can be suggested as sufficiently slight that it might be used for dosimetry. The third peak is the most stable one being located around 300 °C. The 139

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based on various trials and errors, with the expression

t t y = background + S1 + S2 + S3 = bkg + A1 exp ⎛− ⎞ + A2 exp ⎛− ⎞ ⎝ τ1 ⎠ ⎝ τ2 ⎠ t + A3 exp ⎛− ⎞ (1) ⎝ τ3 ⎠ ⎜

⎜

⎟

⎜

⎟

⎟

where S1, S2 , and S3 are the first-order decay functions resembling the fast decay component (thermally unstable), medium decay component and the slow decay component in the tail of the OSL signal, respectively.

y : OSL intensity for stimulation time t background : background value of OSL signal t : stimulation time A1,2,3 : the initial intensities of the components of OSL signal τ1,2,3 : OSL decay life times of each component As pointed out above, the data of OSL decay curve was described successfully by the Eq. (1) using curve fitting. The S1,S2 and S3 components were determined by using the curve fitting based on Eq. (1). The lifetimes τ1, τ2 and τ3 were determined as 1.44 s, 16.02 s and 81.73 s, respectively for the OSL signal exposed to 1 Gy beta dose (see Fig. 7). It should be noted that this model employed to describe the luminescence phenomena are just oversimplifications of the real situations. The curve fitting using three exponential decays is an approximation showing mathematical consistency rather than a proof of the existence of separate physical mechanism (traps). It is evident that more detailed studies on this are necessary. 3.5. The effect of OSL measurement on TL glow curve The OSL measurement has affected the TL glow curve of the material excited with 0.5 Gy beta dose. As is seen from Fig. 8, in the TL glow curve measured after OSL, the 100 °C peak occurred with a very small amplitude; the main peak (peak1) shifted to the right and appeared as a smaller peak, and ~ 300 °C peak (peak2 + peak3; shoulderlike) showed little variation of the response to stimulation. This suggests that there are some shallow OSL traps around 100 °C traps of the TL glow curve. Since the peak1 around 200 °C seemed mostly affected the source of the OSL signal can be correlated to around the 200 °C TL peak. Since the intensity of the TL peak was removed partly around 300 °C (peak2 + peak3), this TL peak (shoulder-like) may have small amount of the light sensitive trapping centers. The bleached TL glow

Fig. 6. (a) Typical OSL decay curve of LTB:Ag compared with Al2O3:C (TLD500), both exposed to 0.2 Gy beta dose. The samples of LTB:Ag (~ 10 mg, in powder form) were annealed at 350 °C for 25 min and Al2O3:C (TLD-500) samples were annealed at 900 °C for 20 min. Both of the samples were stimulated with blue light for 100 s at half an hour after the irradiation. (b) TL comparison of the material with TLD-100 powder sample. TLD-100 powder was annealed at 400 °C for 1 h.

The OSL reader used here is not optimized for either sample in terms of wavelength used for stimulation and wavelength detected for measuring the OSL signal [34]. Fig. 6b presents the comparison between the material and commercially available TLD-100 dosimeter. Note that both samples are in powder form and have the same masses. As can be seen in Fig. 6b, the main peak amplitude of LTB:Ag at ~ 200 °C presents 20% more TL intensity than that of the powder TLD-100 at ~ 235 °C when using Hoya U-340 filters. OSL decay curve in Fig. 6a is an exponential curve. Smith and Rhodes [35] and Bailey et al. [36] reported that the trapping sites as origins of releasing charges under optical stimulation have more than one mean life. They showed that the CW-OSL curve can be approximated using a linear combination of three exponential decay functions. Chen and Lockwood presented that a possible decay function of OSL during as well as following the exposure to stimulating light does not behave according to a simple exponential function and might be closely approximated by the quite ubiquitous stretched-exponential function [37]. OSL components are usually fitted using a first order kinetics model [38,39] and so the traps responsible for them could be clearly characterized. In this study, to find the decay components of the OSL signal, just for simplicity, we assumed that the OSL luminescence processes involved are obeying the first-order kinetics. The OSL signal can be approximated to fit to three exponentially decaying components

Fig. 7. OSL signal observed in LTB:Ag. OSL curve was recorded during a blue light stimulation for 200 s at 25 °C after an exposure of 1 Gy. OSL signal record was started with a preheating process at 110 °C for 10 s just before the stimulation. 140

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Fig. 8. TL glow curve and TL glow curve after OSL measurement of LTB:Ag (dose: 0.5 Gy; heating rate: 5 °C/s; 200 s blue light stimulation; filter: HoyaU340). The inset figure shows the bleached TL curve.

Fig. 10. Increase of the intensity of OSL signal (total OSL area and initial intensity of OSL signal) as the dose amount increases. Three powder samples, each 10 mg, were employed for the measurements. Apertures were used for the high doses. The beta-ray irradiation was delivered with the dose amounts ranging from 0.1 Gy to 100 Gy.

LTB:Ag material. LTB:Ag samples were examined in the dose range between 0.1 Gy and 100 Gy of beta irradiation. After pre-heating at 110 °C for 10 s, the OSL signals were recorded from the phosphor for dose values of 0.1, 0.2, 0.5, 1, 2, 5, 10, 20, 50 and 100 Gy during blue light stimulation for 200 s at room temperature. Fig. 10 shows the changes in total OSL area and initial OSL signal as a function of delivery doses. The OSL signal has a quite linear dose response between 0.1 Gy and 1 Gy with g (D) ≤ 0.9926 values at < 1 Gy [42]. For the dose values from 1 to 100 Gy, g (D) values were changed between 0.9926 ≤ g (D) ≤ 0.9999. So, the behavior of dose response curve tends to linearity as the dose value increases from 1 Gy to 100 Gy. When the OSL signal intensity was calculated according to maximum initial OSL intensity the dose response curve appeared as a sublinear behavior for the dose values from 0.1 Gy to 7.4 Gy with 0.5745 ≤ g (D) ≤ 0.9989 and then a linear behavior from 7.4 Gy to 100 Gy with 0.9990 ≤ g (D) ≤ 0.9999. The following equation has been used to calculate the minimum detectable dose of the material [43]

Fig. 9. Pre-heat plateau of OSL signal for LTB:Ag as a function of maximum preheat temperature; compared to TL glow curve of the material as a function of readout temperature. The pre-heating and TL readout were carried out with 5 °C/s after 0.5 Gy beta dose exposure and optical filter was employed as Hoya U-340.

MDD = [3 . s (BG )/ a]

(2)

where s(BG) is the standard deviation of the background OSL signals of three non-irradiated samples and a is the mean OSL intensity (maximum intensity of OSL signal and/or total OSL counts for the applied stimulation time) of three samples irradiated with 0.1 Gy beta dose. The MDD values were estimated as ~ 3 mGy and ~ 0.1 mGy according to the data for total counts of OSL signal and for initial OSL counts for the first 1 s time period of the stimulation, respectively. The similar measurements of LTB:Ag in single crystal form were carried out by Patra et al. and the MDD value was calculated as ~ 1 mGy [29].

curve is given in the inset of Fig. 8. This has been confirmed by the results from a step-annealing experiment, in which the sample is repeatedly irradiated and heated to increasingly higher temperatures before each the OSL measurement [40]. Fig. 9 gives the same contributions with Fig. 8 that the shallow traps somewhat contribute to the OSL signal since there exists a slight decrease in the OSL signal observed for pre-heating temperatures up to ~ 120 °C. We observe that measuring the OSL after annealing around 120 °C gives a rapidly decaying signal and then decay of OSL signal is completed at ~ 240 °C. The trapping centers related to the main TL glow peak at ~ 200 °C are emptied during optical stimulation and contribute to OSL signal. These results are plotted in Fig. 9. The optically evicted charges from the trap corresponding to the 300 °C TL peak (see Fig. 8) make some non-radiative transitions or the luminescence emission is beyond the detection bandpass (340 ± 40 nm). The same non-radiative transitions were observed in Thermolax995 BeO dosimeter by Bulur and Goksu [41].

3.7. Reproducibility of response after each use Following the identical exposures under carefully controlled laboratory conditions, the reproducibility of the successive OSL readings was estimated to be ~ −3% and −1% for the total OSL area and the initial OSL intensity, respectively. To determine the reproducibility of the OSL intensity after each cycle, the OSL signals from the three samples were recorded following 0.5 Gy beta dose exposure and preheating at 110 °C for 10 s during 200 s blue light stimulation. The OSL sensitivity of the material was reduced by repeated exposure and reprocessing. Nine consecutive measurements on beta-ray for the same batch of LTB:Ag over a period of one week showed a deviation of −3%

3.6. Dose response of OSL signal with beta-ray exposure The study of linearity of OSL dose response was performed for 141

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Fig. 11. OSL response of LTB:Ag as a function of repeated readings. The samples were annealed at 350 °C for 25 min before each measurement. Each data is the average of three samples. A reduction of about −3% in the integrated total OSL area was found for the last 9th cycle. For the initial intensity of OSL signal the reproducibility was determined to be ~ −1% based on the 9 repeated cycles.

and −1% at the end of the 9th cycle. Normalized integrated OSL signals versus number of the experimental cycles are shown in Fig. 11. 3.8. Short term decay of stored energy For studying the fading, the samples were irradiated with 0.5 Gy beta dose and kept in dark at laboratory conditions. To provide the constancy of radiation sensitivity between the samples from the same batch we previously found the calibration factors of each sample before delivering the dose to the samples. Each OSL decay curve was obtained by simultaneous readout of three samples during the blue light stimulation for 200 s following the exposure at the end of the storage series. The results of this OSL signal-storage (total OSL area and initial OSL intensity) stability study are shown in Fig. 12. The error brackets in the inset figure of Fig. 12 show the standard deviation of the mean derived from the spread of different readings of the OSL signal of three samples. The total OSL area of the material revealed a loss of 25% of its signal in 168 h. It is likely due to the shallow trapping centers associated with the

Fig. 13. (a) Different OSL curves after storage times of 0.5 h, 1 h, 6 h and 7 d for LTB:Ag sample. The OSL curves and the exponential decay components for the OSL signal are represented by solid lines and dashed lines, respectively. (b) A comparison of amplitudes of the exponential components of individual OSL signal measured after each storage time as a function of storage time.

TL peak at ~ 100 °C (Fig. 5). It was observed that readout of total OSL area at 0.5 h after exposure (the first readout) was some 20% higher than a readout at 48 h. The initial intensity of OSL signal exhibited somewhat a stronger fading than the total OSL area. It indicates that the shallow traps mostly associate with fast OSL components (high photoionization cross-section) which dominate the initial part of the OSL decay curve [10]. In the present study we confined our attention also to fitting simultaneously the OSL decay curves recorded after each short-term storage (0.5, 1, 2, 3, 6, 12, 24, 48, 168 h). Each OSL curve has fast, medium and slow exponential decay components and a constant background. The decay constants for all the fitted curves have been found the same and determined as 0.63 ± 0.04 s, 3.21 ± 0.09 s and 20.83 ± 0.61 s (experimental standard deviations) for fast, medium and slow components, respectively. As can be seen in Fig. 13a, the OSL decay curves after four storage times (0.5 h, 1 h, 6 h, 7 d) were fitted well with each other and each has individually three decay components and a constant background. The decay components and the background of OSL signal (1 h storage time) and shared decay constants are shown in Fig. 13a. In Fig. 13b, the amplitude of each component is plotted as a function of short storage times. The resultant curves show that all the components (fast, medium and slow) decay exponentially as a function of time, as expected. This compares favorably with the results reported by Patra et al.,

Fig. 12. Short time fading of the OSL signal of LTB:Ag following 0.5 Gy beta dose exposure during dark storage. The OSL signals were recorded with blue light stimulation for 100 s at room temperature after the preheating of 110 °C for 10 s. The OSL counts were taken as total OSL area for each data. Inset: The same record with three samples include error bar at logarithmic scale. 142

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which indicated 36% loss of signal in 48 h for LTB:Ag in single crystal form [29]. Fading measurement in this study is higher than the acceptable amount of fading for the commercially available dosimeters [44]. This may be caused by the influence of all the shallow trap centers contributing to the OSL signal in this material [45]. Studies based on the Ag+ trap centers are necessary to understand more about the decaying of the traps involved in the storage period. The effects of the possible additional doping and improving the synthesis method for decreasing the fading will be studied in the future. 3.9. Kinetic parameters In this work, various heating rate (VHR), isothermal annealing (ITA) and computerized glow curve deconvolution (CGCD) methods were performed to evaluate the trapping parameters, activation energy (Ea) and frequency factor (s) of thermally stimulated luminescence curve and OSL signal in LTB:Ag phosphor. The TL glow curve was analyzed using various heating rate method [46]. This method based on the shifting of maximum peak temperature (Tm) to higher temperature when the occurrence of variable heating rate (β). To determine the relation between Tm and β after 1 Gy beta irradiation, heating rate values of 0.5, 1, 2, 3 and 4 °C s−1 were performed. Maximum peak temperatures of peak1 and peak2 (see CGCD curve in Fig. 17) in the TL glow curve, shifted gradually from 156 to 191 °C and from 259 to 292 °C, respectively when the β was increased from 0.5 to 4 °C/s with the increasing steps of heating rate given above. The shift of the Tm of the peak1 in the bleached TL glow curve (difference between TL glow curve and TL glow curve after OSL) (see inset Fig. 8) was appeared from 151 to 184 °C for peak1 as β was increased. Besides, it was experimentally verified that increase in β gives rise to decrease the TL sensitivity. The relation between Tm and β can be expressed as

T2 Ea Ea ln ⎛⎜ m ⎞⎟ = + ln ( ) kTm sk ⎝ β ⎠

(3)

for the first order kinetics where s is the frequency factor; ln (Ea/ sk ) is the intercept. The plot of ln (Tm2 / β ) against 1/ kTm is expected to be linear with slope Ea and an intercept ln (Ea/ sk ) [46]. Activation energies and frequency factors for peak1 and peak2 were calculated from Fig. 14a as 0.93 eV and 3.04 × 1010 s−1, and 1.53 eV and 1.79 × 1013 s−1, respectively. The activation energy and frequency factor for peak1 of the bleached TL glow curve from Fig. 14b were calculated as 0.99 eV and 5.46 × 1011 s−1, respectively. The thermal stability of radiation induced OSL signal from LTB:Ag samples was studied by isothermal annealing method. In this experiment, the three LTB:Ag samples irradiated with 1 Gy beta dose were kept at various annealing temperatures 110, 120, 130, 140, 150 and 160 °C (heating rate: 1 °C/s) for various duration of times in the range of 0–100 s with the time interval of 20 s. The OSL signals were measured after cooling the samples to the room temperature. The samples were irradiated each time with the same dose after the residual signals were removed with TL reading (up to 450 °C; heating rate: 2 °C/s). The isothermal decay curves for OSL are exponential function of time. When the natural logarithms of the intensity of OSL curves (max. OSL intensity) were plotted as the function of the annealing times, the isothermal decay curves appeared to slope downward as it is indicated in Fig. 15. The slopes of the isothermal decay curves equaling the life times of the signals (τ) are used to determine the trap parameters of the LTB:Ag samples. The plots of the lnτ versus reciprocal of the k times annealing temperature T (1/ kT ) is a straight line with the slope = − Ea and the intercept = lns (see Fig. 16). In this study, the thermal activation energy (Ea) and frequency factor (s) for the traps responsible from the OSL signal were obtained to be 0.94 eV and 1.27 × 109 s−1. Finally, the measurements were made on the estimation of the trap parameters using CGCD method. CGCD analysis has been extensively

Fig. 14. The plot of ln(Tm2/β) vs 1/kTm for (a) TL glow curve (b) bleached TL curve. The slope of the plot gives activation energy E and intercept gives frequency factor s for the TL traps in (a) and for the TL trap responsible from OSL in the material in (b).

Fig. 15. Isothermal annealing of the OSL signal from the samples after a dose delivery of 1 Gy. OSL signals were measured during blue light stimulation for 200 s. The total OSL area intensities were taken for evaluations. Each data is the average of three samples.

used for the determination of the characteristic parameters of experimentally obtained TL glow curves. In this technique, some TL glow curves have been simulated using the general order equation 143

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Fig. 16. Linear relation of ln(τ) versus 1/(kT). Fig. 17. CGCD analysis of TL glow curve of LTB:Ag recorded in the RT – 430 °C region with 1 °C/s heating rate after 1 Gy beta irradiation.

introduced by Kitis et al. [47],

E T − Tm ⎞ ⎡ I (T ) = Im bb /(b −1) exp ⎛ (b−1)(1 ⎝ kT Tm ⎠ ⎢ ⎣ ⎜

−Δ)

T2 Tm2

⎟

− b

E T − Tm ⎞ + Zm ⎤ exp ⎛ ⎥ ⎝ kT Tm ⎠ ⎦ ⎜

was synthesized using solution combustion method and dosimetric properties were examined to clarify whether the luminescent phosphor can be applied successfully for the optically stimulated luminescence dosimetry. The PL emission spectra of LTB:Ag with various concentrations of the impurity took place very nearly at the same wavelength (~ 270 nm) with the same peak shapes but the widths and intensities of the peaks are different. The nature of the luminescence in this region is due to 4d95s → 4d10 transitions of the Ag+ ion substituted for Li+ and agrees well with earlier reported data [26]. The emission band centered at 526 nm is attributed to Ag2+ ions which has no nearby perturbation. The enhancement in PL emission intensity is found to be maximum for LTB containing 0.3 wt% of Ag impurity. We have not found any previous report regarding the emission band located at 526 nm and its origin remains unclear. Therefore, this indicates the need for further study. The material was developed with different concentrations of dopant Ag to achieve the most sensitive OSL response. The luminescence intensity of the sample increased with increasing Ag concentration. The Ag doped LTB exhibits remarkably high TL and OSL sensitivity compared to undoped LTB. In order to understand the luminescence characteristics of LTB:Ag phosphor, TL and OSL sensitivity, dose response, reproducibility, energy storage and thermal stability of the TL and OSL readouts are studied. The OSL decay signal was compared to TLD-500 and was found 6 times less sensitive from the mentioned commercial dosimeter. Good reproducibility, approximately −3% at the end of the 9th cycle, has been obtained on repeated determinations with the total OSL intensity of the samples. The total OSL intensity has a linear dose response over the dose range between ~ 1 Gy and 100 Gy. Experimental data was shown that the use of powder LTB:Ag with optical stimulation is possible above ~ 3 mGy. Determination of the trapping parameters associated with the TL peaks and OSL decay curve is one of the most important aspects in the field of luminescence and is reported here for LTB:Ag. The activation energy for the main glow peak located around 200 °C of the sample was found as 0.93 eV using VHR method that is very close to the activation energy value 0.91 eV obtained with CGCD method. The kinetic parameters Ea and s of only the peak1 of the bleached TL glow curve which is associated with the OSL traps were evaluated using the isothermal annealing method and determined as 0.94 eV and 1.27 × 109 s−1, respectively. Study of LTB:Ag appears to have opened up a feasibility of using the optically stimulated luminescence of this phosphor for dosimetry purposes in the dose range ~ 1–100 Gy for medical applications. It can also

⁄ (b −1)

⎟

(4)

2kT (b−1) Em

where I (T ) is TL intensity; E (eV) with Δ = 2kT / E , Zm = 1 + is the activation energy; Im is the peak maximum intensity; Tm (K) is the peak maximum temperature; b is the order of kinetics; k (eV K−1) is the Boltzmann's constant and T (K) is the absolute temperature. The goodness of the fit is tested by the Figure of Merit (FOM) [48] which is expressed as

FOM =

∑ YExpt − YFit ∑ YFit

x100

(5)

where YExpt is the experimental glow curve and YFit is the fitted glow curve. In the CGCD calculations, the FOM value was found as 2.34. The frequency factor s (s−1) was calculated by Eq. (6) which was derived using a general order approximation [49],

s=

βE kT2m (1+2kTm (b − 1)/E

exp (

E ) kTm

(6)

where β (°C/s) is the heating rate. The TL measurement of a LTB:Ag sample with 1 °C/s heating rate after 1 Gy beta dose exposure was conducted for the CGCD calculations. The trials of the CGCD simulations exhibited totally four peaks under the glow curve together with a slight peak at around ambient temperature (see Fig. 17). All the TL peaks in LTB:Ag follow the non-first order kinetics. The order of kinetics (b) had been determined after the shifting of the maximum peak temperatures (Tm) of the peak1 and peak2 following the increasing dose exposures. The activation energy value of 0.91 eV and the frequency factor of 2.10 × 109 s−1 were estimated for the main peak (peak1) appeared at ~ 150 °C for this sample. The obtained activation energy values and frequency factors of peak2 and peak3 were determined as 1.42 eV and 1.05 × 1012 s−1, and 1.04 eV and 3.55 × 109 s−1, respectively (see Table 1). The b values were determined as 1.87, 1.51 and 1.97 for peak1, peak2, and peak3, respectively and indicate the non-first-order kinetics. As seen from the Table 1, the activation energy values and frequency factors for peak1 and peak2 by CGCD calculations are quite consistent with the results obtained using VHR method. 4. Conclusion In this work, silver activated lithium tetraborate, LTB:Ag (1 wt%) 144

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Table 1 Kinetic analysis of TL glow curve and the bleached TL glow curve (the difference between TL and TL after OSL glow curves). Kinetic order values b of the TL peaks, Peak1, Peak2 and Peak3, as they were determined by CGCD method with 1 °C/s heating rate, fit well to the non-first order kinetics. Method

TL peaks

TL peaks responsible for the observed OSL components

Peak1 Ea (eV) VHRa ITAb CGCDc

a b c

Peak2 s (s−1)

0.93 3.04 × 1010 N/A 0.91 2.10 × 109 Kinetic order value (b) 1.87

Peak3

Ea (eV)

s (s−1)

Ea (eV)

1.53 N/A 1.42

1.79 × 1013

N/A N/A 1.04

1.51

1.05 × 1012

Peak1

1.97

s (s−1)

Ea (eV)

s (s−1)

3.55 × 109

0.99 0.94 N/A

5.46 × 1011 1.27 × 109 N/A

N/A

N/A

Various heating rate method. Isothermal annealing method. Computerized glow curve deconvolution method.

be used in personal dosimetry. The signal stability of LTB:Ag will be studied in detail in the future using different co-dopants with different concentrations and synthesis methods in order to be used for longer storage times. Besides this, neutron dosimetry applications based on OSL properties of this material and imaging applications in neutron radiography can be taken into consideration for future studies.

[16]

[17] [18]

Acknowledgement [19]

This research is sponsored by the Cukurova University Rectorate through the Projects FDK-2017-6833, FUA-2015-4300, FBA-2016-6000, FYL-2016-6065 and FYL-2015-3944. We gratefully acknowledge the financial support given by Cukurova University. We very much appreciate Prof. Dr. Gulfeza Kardas and Fatih Tezcan for analyzing spectroscopic measurements.

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