Abstract Lithium tetraborate (Li2B4O7) nanoparticles (NPs) doped with manganese (Mn) were prepared for the first time by the solid-state sintering method. NPs were characterized using X-ray diffraction, scanning electron microscopy, photoluminescence and thermoluminescence (TL) techniques. The synthesized NPs exhibited highest TL response at 0.3 wt% of Mn dopant under gamma irradiation. TL dose response is linear for the absorbed dose from 1 Gy to 20 kGy and beyond this range behaves sub-linear. Such feature makes the synthesized nanophosphor as a promising material for high-dose dosimetry applications. Low fading and good reusability were obtained for the synthesized NPs. Tm−Tstop and computerized glow curve deconvolution procedures were utilized to identify the component TL glow peaks and kinetic parameters of the produced phosphor. Other TL dosimetry features of the prepared NPs are also presented and discussed. INTRODUCTION Lithium tetraborate (LTB) is amongst the most advantageous materials for medical, clinical and personnel dosimetry applications due to its excellent human tissue equivalence. Its effective atomic number of 7.3 is very close to that of soft tissue, so it is excellent in personnel monitoring(1, 2). This material can be used as TL dosimeter for measuring absorbed dose in different ionizing radiations such as X-ray, gamma and neutrons. The elements of both Li and B have large absorption cross-section for neutrons(3). Li2B4O7 doped with rare earth and transition elements has also attracted attention as tissue equivalent TL dosimeter and efficient scintillator(4). Mn-doped LTB was the first commercially available material reported for TL dosimetry applications known as TLD-800. The first study on Li2B4O7:Mn phosphor as a TL material was performed by Schulman et al.(5) They showed that Li2B4O7 doped with 0.1 wt% of Mn has a main dosimetry peak at ~200°C and its TL response is 10 times lower than that of LiF:Mg, Ti (TLD-100). There are numerous publications about TL of single crystalline, polycrystalline and glass LTB:Mn samples(6–11). Recently, Annalakshmi et al. prepared LTB:Mn by solid-state sintering. Their study showed that TL glow curve has a dosimetry peak around 280°C and its sensitivity is less than TLD-100 by a factor of 0.9(12). Ozdemir et al. worked on preparation of powder and pellet lithium tetra borates doped with Mn by solution combustion technique. They observed that dose response is linear for both samples in the range of 0.1–10 Gy using beta rays and the TL glow curve is composed of two glow peaks located at ~100 and 260°C(13). Over the past few years, attempts have been focused on TL properties of nanostructured materials because of their exceptional TL properties compared with their bulk counterparts. The major problem with the microscale TL materials is saturation of trapping states at high levels of absorbed dose which causes the TL response become sub-linear in high radiation exposure. A general feature of nanophosphors is that they exhibit linear dose response at higher absorbed doses, where the conventional micro-scaled TL phosphors saturate(14–19). Up to the present, only a few number of reports have been addressed to study LTB nanophosphor. In this regard, Singh et al.(19) prepared Li2B4O7 nanophosphor doped with Cu by combustion technique. They found that nanoparticles (NPs) have a linear dose response in the range of 10–450 Gy of absorbed dose. Similarly, TL characteristics of Li2B4O7 NPs doped with Cu and co-doping of Cu, Ag have been studied in detail(20, 21). However, the TL response of LTB NPs doped with Mn has not been reported until now. The aim of this research is to present the preparation method of Li2B4O7:Mn NPs by solid-state sintering method and investigate its dosimetry properties including TL response, annealing conditions, fading, reusability and other TL dosimetry characteristics. EXPERIMENTAL Mn-doped LTB nanophosphor was prepared by solid-state sintering technique, where stoichiometry amounts of lithium carbonate (Li2CO3, Merck), boric acid (H3BO3, Merck) and manganese chloride (MnCl2, Merck) as the dopant source of Mn ion were mixed together homogeneously. After that, the mixture was heated in two stages. At the initial stage the mixed material was heated at 350°C for 2 h in the muffle furnace and then the furnace temperature was raised to 700°C and kept at this temperature for 2 h. After cooling the furnace gradually, Li2B4O7:Mn white powder was obtained. CHARACTERIZATION The sample formation and crystalline phases were identified using an X-ray diffractometer (Bruker D8 Advance) with Cu-Kα radiation (λ= 1.54 A°) under the conditions of 40 kV and 30 mA, at a step size of 2θ = 0.02°. SEM images were obtained using a Philips model XL-30 ESEM, equipped with an energy dispersive spectrometer (EDS). Photoluminescence (PL) spectrum was measured over the wavelength range 200–800 nm using a Perkin-Elmer spectrometer-model LS55 with a photo multiplier tube and a Xenon lamp operating at room temperature. All irradiations were performed using a 60Co gamma source. TL measurements were obtained in a Harshaw model 4500 computer-based TL reader using a heater belt (planchet) as contact heating, where the temperature of planchet is recorded as indicator of temperature of the sample with an accuracy of 1°C. All TL readouts (except for readouts in studying the fading characteristic) were carried out 3 h after irradiation to allow tunneling and transitions between trapping states. The heating rate for readout was 2°C/s (with preheat of 50°C) to a maximum temperature of 350°C. The samples were annealed at 400°C for 15 min using a programmable oven with accuracy of 1°C and then were cooled rapidly to room temperature (75°C/min). The TL responses of the samples of different batches under the same irradiation conditions were the same through an uncertainty of 5%. Since the TL response depends on the mass of the sample; it was kept constant at 0.003 g using Sartorius Research R 160 P weigher (with a mass accuracy of ± 0.01 mg). RESULTS AND DISCUSSION XRD and SEM study Figure 1 shows the X-ray diffraction (XRDthe Bragg angle. Using) pattern. This pattern exhibits a tetragonal structure that is in correspondence with ICDD collection code no. 22-1140. The average particle size may also be determined from the widths of the XRD peaks using Scherrer’s formula: D=0.9λβcosθ (1)where D is the average crystalline size, λ is the wavelength of Cu-Kα line (1.540562 A°), β is the full width at half-maximum (in radians) and θ is the Bragg angle. Using the XRD pattern and the dominant peak, the average crystalline size was calculated to be ~28 nm. Figure 1. View largeDownload slide X-ray diffraction pattern of the synthesized Li2B4O7:Mn NPs. Figure 1. View largeDownload slide X-ray diffraction pattern of the synthesized Li2B4O7:Mn NPs. Scanning electron microscopy (SEM) was done to study the size and shape of Li2B4O7:Mn NPs. According to SEM images of Figure 2, particles are uniform in size and their shape is spherical. A particle size distribution between 30 and 60 nm was identified which is in accordance with that calculated by XRD pattern. Figure 2. View largeDownload slide SEM image of synthesized nanocrystalline LTB:Mn. Figure 2. View largeDownload slide SEM image of synthesized nanocrystalline LTB:Mn. Photoluminescence study PL emission of the synthesized NPs is presented in Figure 3. The excitation wavelength of 336 nm was used for recording the PL spectrum. There are various reports on PL emission of Li2B4O7 doped with manganese in the red-orange region. It has been shown that Mn2+ ions in natural minerals in the same Td coordination exhibit green PL near 500 nm if emitting level is 4T2g (4G), while orange PL (near 600 nm) was observed when 4T1g (4G) level is involved(6, 22). Figure 3. View largeDownload slide PL emission spectra of Mn-doped lithium tetraborate nanoparticles. Figure 3. View largeDownload slide PL emission spectra of Mn-doped lithium tetraborate nanoparticles. It is worth noting that PL emission spectra of Li2B4O7 NPs doped with manganese has broad emission peaked at 481 nm; whereas this excitation wavelength produces broad PL emission peaked at 527 nm in bulk sample(6). The blue shifting is a result of quantum size effect leading to increase in the band gap and reduced quantum allowed states. Thermoluminescence study Glow curve Figure 4 shows the TL glow curve of the synthesized Li2B4O7:Mn NPs exposed to 1 kGy gamma-ray radiation by 60Co source. TL glow curve was recorded while heating the sample from 50 to 350°C with heating rate of 2°C/s. The shape of glow curve exhibits a main stable TL peak centered at 254°C and a small low temperature peak located around 122°C. Figure 4. View largeDownload slide TL glow curve of nanocrystalline Li2B4O7:Mn. Figure 4. View largeDownload slide TL glow curve of nanocrystalline Li2B4O7:Mn. The influence of dopant concentration Adding Mn impurity to Li2B4O7 host material significantly affects the accessible TL intensity. Figure 5 shows the variation of TL response with concentration of Mn dopant. It is observed that the optimum Mn concentration which yields the highest TL response is around 0.3 wt%. These samples were used for all subsequent studies. The optimum concentration of 0.1 wt% of Mn dopant for bulk Li2B4O7:Mn phosphor reported by Schulman et al.(5) gives very poor sensitivity in equivalent nanoscaled phosphor. Figure 5. View largeDownload slide Variation of TL response with manganese concentration for the absorbed dose of 1 kGy. Figure 5. View largeDownload slide Variation of TL response with manganese concentration for the absorbed dose of 1 kGy. Annealing Suitable annealing regime is a main parameter which should be identified for a TL dosimeter. An accepted annealing procedure not only eliminates all of the stored energy trapped in the crystalline lattice during the exposures, but restores the original sensitivity(23). In this work, firstly all of specimens were subjected to an optimized firing temperature at 750°C for 30 min and then annealing was carried out in furnace at various temperatures of 300, 400 and 500°C for time intervals of 15, 30 and 60 min in air. As can be seen in Figure 6 and in the inset of the figure, the highest TL response was achieved at annealing temperature of 400°C and duration of 15 min. Optimization of time and temperature of annealing was performed at a fixed absorbed dose of 1 kGy. Figure 6. View largeDownload slide TL glow curves of Li2B4O7:Mn NPs for different annealing temperatures and duration of 15 min for absorbed dose of 1 kGy. The inset of the figure confirms that optimum annealing regime is 400°C for 15 min. Figure 6. View largeDownload slide TL glow curves of Li2B4O7:Mn NPs for different annealing temperatures and duration of 15 min for absorbed dose of 1 kGy. The inset of the figure confirms that optimum annealing regime is 400°C for 15 min. Dose response Dose responses of LTB:Mn nanophosphor was measured by irradiating the samples of the same mass to different gamma doses in the range of 0.1–50 kGy. As can be seen in Figure 7, TL dose response is poor and non-linear in absorbed dose region <1 Gy which indicates that the synthesized nanophosphor cannot be used for low dose measurements. At dose levels lower than 0.1 Gy, the uncertainties raised due to thermal noise and readout of different samples become comparable with the average count of the TLD reader. Therefore, the absorbed dose of 0.1 Gy is the minimum measureable dose by LBO NPs. Figure 7. View largeDownload slide TL dose response for absorbed dose ranging from 0.1 Gy to 50 kGy in log–log scale. Figure 7. View largeDownload slide TL dose response for absorbed dose ranging from 0.1 Gy to 50 kGy in log–log scale. A linear dose response is observed in the range 1 Gy–20 kGy with a slope close to 1 in a log–log scale. The outstanding feature of Li2B4O7 NPs is its linear response at high doses of gamma radiation. By further increasing the absorbed dose, the response becomes sub-linear. In a recent work, Annalakshmi et al.(6) found that bulk Li2B4O7 doped with Mn has linear dose response up to 10 Gy. Also TLD-800 (commercialized by Harshaw) shows linear response in the range 10−4–3 Gy(23). Therefore, the synthesized LTB:Mn nanopowder while exhibit a linear dose response in a wide range of absorbed dose compared to its bulk equivalent, its TL sensitivity is lower than that of corresponding microcrystalline. A comparison of TL response of the synthesized LTB:Mn nanophosphor and TLD-100 is shown in Figure 8, which reveals that TL sensitivity of nanophosphor is ~0.04 times that of TLD-100. Figure 8. View largeDownload slide TL glow curves of synthesized Mn-doped Li2B4O7 and TLD-100 irradiated with 1 Gy gamma-radiation. Figure 8. View largeDownload slide TL glow curves of synthesized Mn-doped Li2B4O7 and TLD-100 irradiated with 1 Gy gamma-radiation. Fading Fading characteristics of LTB:Mn nanophosphor is shown in Figure 9. Thermal fading is described as loss of the TL signal with storage time at room temperature(23). In order to determine the stability of TL glow curve, the samples were initially annealed and irradiated to 1 kGy dose from 60Co gamma source and were stored in the dark place at room temperature. Figure 9. View largeDownload slide TL glow curve areas of LTB:Mn NPs recorded after different storage times. All samples were first irradiated to 1 kGy by a 60Co source and then stored in the dark at room temperature. Figure 9. View largeDownload slide TL glow curve areas of LTB:Mn NPs recorded after different storage times. All samples were first irradiated to 1 kGy by a 60Co source and then stored in the dark at room temperature. TL glow curves are shown in Figure 9 for different storage times up to 30 days. The inset of the figure is the variation of total glow curve area with the storage time. The low‐temperature peak disappeared rapidly after 7 days but the intensity of the main peak decreased slightly after this period of time. The total fading recorded was around 12% in 30 days whereas TLD-800 (Li2B4O7:Mn) has fading of 10% in 3 months (5% in first month). Reusability From dosimetry application point of view, an appropriate TL dosimeter should have a glow curve with no considerable change in the shape and total glow curve area after several cycles of annealing–irradiation–TL readout. In order to examine the reusability of the synthesized LTB:Mn nanophosphor, the samples were annealed at 400°C for 15 min, irradiated with gamma ray (1 kGy) and readout. This process was repeated for six times. The TL response and the standard deviations for six above cycles for the synthesized LTB:Mn nanophosphor is observed in Figure 10. It is observed that the TL response remains approximately stable after six cycles of annealing, irradiation and readout. Figure 10. View largeDownload slide Reusability test plot including TL responses for six repeated cycles of annealing, irradiation to 1 kGy gamma ray and readout of LTB:Mn nanophoshor. Figure 10. View largeDownload slide Reusability test plot including TL responses for six repeated cycles of annealing, irradiation to 1 kGy gamma ray and readout of LTB:Mn nanophoshor. Tm – Tstop method and Kinetic parameters Tm − Tstop method was used to find of the number of glow peaks contained in the TL glow curve. Using this method, the samples were firstly exposed to gamma irradiation, followed by annealing with heating rate of 5°C/s to the temperature Tstop, then cooled quickly down to room temperature and without emerging the sample from the reader, it reheated with rate of 2°C/s for recording the remaining glow curve. This procedure was repeated several times at different Tstop values between 50 and 350°C with the step size of 10°C and the corresponding maximum temperatures Tm were obtained. The plot of Tm versus Tstop is shown in Figure 11. As is evident; by increasing the Tstop, Tm moves towards higher temperatures but this variation is not regular and several sudden increases are observed in Tm values. Each plateau region in Tm−Tstop plot corresponds to a single glow peak. The plateaus with lower slopes (flat regions) indicate that the corresponding glow peak obeys near first-order kinetics and higher slopes correspond to higher values for the order of kinetics(23). Figure 11. View largeDownload slide Variation of peak maximum temperature (Tm) against Tstop. The plateau region at low temperature is attributed to the first glow peak. Figure 11. View largeDownload slide Variation of peak maximum temperature (Tm) against Tstop. The plateau region at low temperature is attributed to the first glow peak. As can be seen in Figure 11 two plateaus and one separate jump are observed in Tm − Tstop pattern of Li2B4O7:Mn nanocrystalline. In the first plateau, the Tm is nearly constant; an indication of nearly first order TL glow peak. This region is located between 90 and 120°C and activation energy extracted from computerized glow curve deconvolution (CGCD) is around 0.91 eV. The second plateau consists of two regions; the first is flat appearing between 120 and 180°C with activation energy of around 1.04 eV and in the second region, Tm increases gradually with Tstop. This means that two overlapped peaks are within this plateau. In the former region, the peak is first order and in the latter, it appears as near second order peak. The CGCD used to curve fitting and identify the kinetic parameters of the isolated glow peaks has been written in our laboratory using Levenberg–Marquart algorithm based on non-linear least square method. In this study, general order of kinetics was used to obtain the parameters of the deconvoluted glow peaks(15–17, 24). The accuracy of fitting is checked by the figure of merit (FOM): FOM=∑jijf100|yi−y(xi)|A (2)where ji and jf are the numbers of the initial and final temperature interval ΔT used for curve fitting, yi is the TL intensity in the ith interval obtained from experiment and y(xi) the intensity expected from equation of general-order glow curve deconvolution function and A, the total area underneath the fitted glow peak between ji and jf. The FOM values lower than 2.5% shows that there is good agreement between experimental and theoretical results(25). Considering three component glow peaks (found from Tm−Tstop analysis) as input parameters in CGCD for Li2B4O7:Mn NPs, three constituent glow peaks at 122, 222 and 260°C were obtained. Excellent fitness of experimental and theoretical glow peaks with FOM value of 0.89% is observed in Figure 12. Figure 12. View largeDownload slide Experimental TL glow curve (hollow triangles), fitted glow curve (solid curve) along with component glow peaks (dashed curves) for Mn-doped lithium tetraborate NPs for absorbed dose of 1 kGy. Figure 12. View largeDownload slide Experimental TL glow curve (hollow triangles), fitted glow curve (solid curve) along with component glow peaks (dashed curves) for Mn-doped lithium tetraborate NPs for absorbed dose of 1 kGy. Kinetic parameters of component glow peaks including activation energy, kinetic order and maximum temperature are shown in Table 1. Table 1. TL kinetic parameters of the component glow peaks of the synthesized LTB:Mn NPs obtained by CGCD method for the absorbed dose of 1 kGy. Peak b E (eV) Tm (K) Im (a.u.) 1 1.17 0.91 395 493 2 1.68 1.04 495 9159 3 1.90 1.48 533 15 618 Peak b E (eV) Tm (K) Im (a.u.) 1 1.17 0.91 395 493 2 1.68 1.04 495 9159 3 1.90 1.48 533 15 618 Table 1. TL kinetic parameters of the component glow peaks of the synthesized LTB:Mn NPs obtained by CGCD method for the absorbed dose of 1 kGy. Peak b E (eV) Tm (K) Im (a.u.) 1 1.17 0.91 395 493 2 1.68 1.04 495 9159 3 1.90 1.48 533 15 618 Peak b E (eV) Tm (K) Im (a.u.) 1 1.17 0.91 395 493 2 1.68 1.04 495 9159 3 1.90 1.48 533 15 618 CONCLUSIONS LTB:Mn NPs as nanostructured counterpart of the known TLD-800 TL dosimeter was synthesized by solid-state sintering method. PL spectrum confirms that Mn2+ ion is incorporated in the nanocrystalline lattice. LTB:Mn NPs are found to have linear dose–response in the range from 1 Gy to 20 kGy and relatively low fading of the main glow peak. The reproducibility, stable TL response after multiple cycles of usage and other proper dosimetry features of the synthesized phosphor designate that Li2B4O7 NPs doped with manganese is capable of being used as a TL dosimeter for ionizing radiation dosimetry, particularly for estimating high levels of radiation dose. 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Published: Mar 1, 2018
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