Ce:LuAG - IEEE Xplore

IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 61, NO. 1, FEBRUARY 2014

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Scintillation and Luminescent Properties of Cerium Doped Lutetium Aluminum Garnet (Ce:LuAG) Powders and Transparent Ceramics Jian Xu, Lingcong Fan, Ying Shi, Junlang Li, Jianjun Xie, and Fang Lei

Abstract—Transparent cerium doped lutetium aluminum garnet (Ce:LuAG) ceramic scintillantors with different Ce concentrations were densified successfully from synthetical nanosized Ce:LuAG powders using co-precipitation method. It was found that the 0.5 mol% Ce: LuAG ceramic exhibited the highest luminescent emission intensity under synchrotron radiation excitation which could be divided into two components centered at 505 and 540 nm under low temperature (50–150 K) conditions originated from the d excited level ( D) to the 4f ground state of Ce ( F F ). The quantum efficiency (QE) of the optimized Ce:LuAG ceramic could reach 66.3% under UV excitation of 345 nm. Typical absorption at 170 nm related to possible Lu antisite defect (AD) was observed both in Ce doped and undoped LuAG ceramics which formed electron traps disturbing the energy transfer from host lattice to Ce centers, leading to a slow decay component of 109.4 ns in the 0.5 mol% Ce:LuAG Cs (662 keV) irradiation. transparent ceramic under Index Terms—Cerium doped lutetium aluminum garnet antisite defect, luminescence, transparent (Ce:LuAG), Lu ceramic, vacuum ultraviolet (VUV).

I. INTRODUCTION

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ERIUM doped lutetium based polycrystalline ceramics, such as Ce:LuAlO (Ce:LuAP) [1], [2], Ce:Lu Al O (Ce:LuAG) [3], [4], and Ce:Lu SiO (Ce:LSO) [5], [6] are considered to be potential candidates for high performance inorganic scintillators which are widely used in sophisticated medical imaging systems and radiation detection technology like positron emission tomography (PET), single photon emission computed tomography (SPECT), and high energy physics (HEP) due to their fast timing, high light yield, and good energy resolution [7]–[9]. Compared with single crystals, ceramic scintillators also exhibit other advantages such as better chemical homogeneity, lower fabrication temperatures, higher doping concentrations, and feasibility for larger size specimen [10]–[12]; therefore, they attract much attention in recent years [13], [14]. However, the optical birefringence caused by the anisotropic structure of LuAP (orthorhombic) and LSO (monoclinic) makes Manuscript received May 22, 2013; revised August 14, 2013 and September 06, 2013; accepted September 09, 2013. Date of publication November 01, 2013; date of current version February 06, 2014. This work is supported by the National Science Foundation of China (No. 11079026/A0804). The authors are with Department of Electronics and Information Materials, School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China (e-mail: [email protected]) Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNS.2013.2281520

them difficult to obtain corresponding polycrystalline ceramics with highly optical transparency [2], [15]–[17]. On the contrary, LuAG, owing to its isotropic cubic structure, is more convenient to act as a host matrix for fabrication of fast scintillating transparent ceramics by doping with 5d-4f emitting rare earth ions (especially Ce ) [3], [4], [18]–[20]. So various methods, such as co-precipitation method [4], [18], sol–gel reaction [19], [21], and solvothermal method [22], [23] were employed to synthesize Ce:LuAG nanosized powders with sufficient phase purity and well dispersion. Then the green bodies shaped by these powders were further densified into transparent ceramics after being sintered in vacuum [3], [4], [19] or a H atmosphere [24]. Besides, it is also confirmed that as an alternative, polycrystalline Ce:LuAG ceramics can even possess higher light yield (LY) [25] and shorter decay time [26] than Ce:LuAG single crystals once satisfactory optical transparency is obtained by advanced ceramic processing techniques. In the previous work [27], we fabricated transparent Ce:LuAG ceramics with 74.1% in-line optical transmittance at 550 nm by co-precipitation method without any sintering aids. To our knowledge, little attention was paid to the spectroscopy of Ce:LuAG polycrystalline powders and transparent ceramics under synchrotron radiation (SR) excitation. Here, we aimed to illuminate the Ce doping concentration and temperature effects on luminescent properties of Ce:LuAG powders and ceramics using SR facility. Moreover, scintillation decay time, photoluminescence (PL) emission spectra and quantum efficiency (QE) of the Ce:LuAG ceramics were also investigated in this paper. II. EXPERIMENTAL PROCEDURE We used commercial Lu O (purity 99.99%, Xiyuan International, Shanghai, China), Ce(NO ) 6H O and Al(NO ) 9H O (analytical grade) as starting materials. NH HCO of analytical grade was used as precipitant. Aqueous nitrate solution of Lu was prepared by dissolving corresponding metal oxide in HNO solution under stirring and heating. Al(NO ) and Ce(NO ) solutions were obtained by dissolving Al(NO ) 9H O and Ce(NO ) . 6H O in deionized water, respectively. Ce:LuAG amorphous precursors were synthesized by reverse-strike co-precipitation method (adding the cationic salt solution into the precipitant solution). The mixed nitrate solution containing Lu , Al , and Ce according to (Ce Lu ) Al O 0 0.3% 0.5% 0.8% 1.0% was added at a speed of 2 ml/min into 2 M NH HCO solution

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under severely stirring at room temperature. A 0.8 wt% of HPC ( 99% Aldrich) was added as surfactant in order to obtain the powders with a narrow particle size distribution and highly spherical morphology [28]. After being aged for 24 h, the precipitated slurry was filtered by suction filtration, followed by washing with deionized water and anhydrous alcohol (analytical grade) for several times and drying at 80 C for 36 h. The dried cake was crushed with an alumina pestle, sieved by a griddle of 200 mesh and calcined at 1000 C for 2 h in air. The calcined powders were ball milled by highly pure ZrO ceramic balls with anhydrous alcohol for 8 h, then compacted to form a ceramic green body ( 20 mm (3–4) mm) under uniaxial press of 5 MPa and cold isostatic press (CIP) of 200 MPa. The green body was sintered at 1750 C for 10 h in vacuum atmosphere ( pa) then annealed in air at 1450 C for 20 h to compensate the oxygen vacancies introduced during the vacuum sintering. The phase formation of the obtained powders and ceramics was analyzed by the X-ray diffraction (XRD, D/max 2550 V, Rigaku, Japan), utilizing nickel filtered CuK radiation (1.5406 in the range of 10 80 . The morphologies of the as-prepared powders and the microstructure of the as-sintered ceramics were examined by scanning electron microscopy (SEM, SU-1500, HITACHI, Japan) and high-resolution transmission electron microscopy (HRTEM, JEM-2010F, JEOL, Japan). The PL spectra were recorded on a Perkin-Elmer LS-50B luminescence spectrometer with Monk-Gillieson type monochromators and a 150 W Xe lamp was used as excitation source. QE was measured by an Absolute Photoluminescence Quantum Yield Measurement System (C9920-02, Hamamatsu). The decay time of the Ce:LuAG ceramic under -ray excitation of Cs (662 keV) radioactive source sandwiched between the start scintillator, and the ceramic specimen was measured based on the time dependence of scintillation intensity by a delayed-coincidence method. The VUV excitation spectra and excited emission spectra were measured at U24 VUV spectroscopy beam-line station of National Synchrotron Radiation Laboratory (NSRL), Hefei, China. The pressure in the vacuum chamber during the measurement was pa. The excitation and emission spectra were detected by a Hamamatsu H5920-01 photomultiplier. The bandwidth of 8 nm, integration time of 1 s, and wavelength step of 1 nm were applied for all of the VUV spectra measurements. The excitation spectra were corrected for the photo flux of the excitation beam using the excitation spectra of sodium salicylate as standard. The emission spectra were not corrected for the spectral response. The wavelength ranges of the excitation and emission spectra were limited to be 125–375 and 400–650 nm, respectively. All measurements were carried out at room temperature except for the temperature depending test at VUV beam-line station using liquid nitrogen apparatus. III. RESULTS AND DISCUSSION A. Composition and Microstructure Fig. 1 shows X-ray diffraction (XRD) patterns of the 0.5 mol% Ce:LuAG powders calcined at 1000 C for 2 h in air and (0.3, 0.5, 0.8, 1.0) mol% Ce:LuAG ceramics sintered

Fig. 1. X-ray diffraction patterns of the as-prepared Ce:LuAG powders calcined at 1000 CC for 2 h and Ce:LuAG ceramics sintered at 1750 C for 10 h with different Ce concentrations.

at 1750 C for 10 h in vacuum atmosphere. It reveals that the obtained powders and ceramics are single phase LuAG compounds which match well with the standard card of JCPDS 73-1368, and no other crystalline phases are detected when the concentration of Ce is increased from 0.3 to 1.0 mol%, leading to a sufficient sintering and good solubility of Ce ions in the LuAG host lattice. The average size of the powders is estimated from the full-width at half-maximum (FWHM) of the XRD diffraction peak by the Scherrer equation, as follows: (1) ” is the FWHM of the pure diffraction profile where “ in radians, “ ” is 0.89, “ ” is the wavelength of the CuK (1.5406 ), “ ” is the diffraction angle, and “ ” is the average diameter of the powders. According to the above equation, the average particle size of the Ce:LuAG nanosized powders is 18 nm. High density and satisfactory optical transparency are the two keys for Ce:LuAG optical ceramics to meet the needs of radiation detection and medical imaging [7]–[9]. To achieve this goal, the preparation of fine dispersive powders with little agglomeration is necessary. Fig. 2(a) shows the SEM micrograph of the as-prepared 0.5 mol% Ce:LuAG powders exhibiting a monodisperse morphology with a spherical particle shape and average particle size of 20 nm, which is in accordance with the result calculated by Scherrer equation and beneficial to promote the further densification of Ce:LuAG transparent ceramics. The SEM micrograph of the fractured surface of the 0.5 mol% Ce:LuAG transparent ceramic sintered from the Ce:LuAG powders mentioned above is given in Fig. 2(b); it reveals that the fracture mode of the ceramic is mainly intercrystalline, and almost no micro-pores or secondary phases are observed either in the surface and within grains or at grain boundaries. Moreover, the grain boundaries with the thickness of about 0.5 nm can be easily distinguished in the typical HRTEM micrograph shown in Fig. 2(c). Ikesue et al. [29], [30] pointed out that “clean”

XU et al.: SCINTILLATION AND LUMINESCENT PROPERTIES OF Ce:LuAG POWDERS AND TRANSPARENT CERAMICS

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Fig. 2. SEM micrographs of (a) the 0.5 mol% Ce:LuAG powders calcined at 1000 C for 2 h and (b) the fractured surface of the 0.5 mol% Ce:LuAG transparent ceramic. (c) HRTEM image of the triple junction. (d) Photograph of the 0.5 mol% Ce:LuAG transparent ceramic (double polished 14 mm 0.7 mm).

boundary without micro-pores and secondary phases could decrease the scattering coefficient and improve the optical quality of transparent ceramics. So this “fine” microstructure of our ceramic (double polished mm 0.7 mm) leads to its high transparency and uniform optical quality as shown in Fig. 2(d). B. Scintillation and Luminescent Properties Fig. 3 displays the excitation ( 522 nm) and emission ( 358 nm) spectra of the 0.5 mol% Ce:LuAG powders and ceramic under SR excitation. The ceramic exhibits much higher luminescent intensity than the powders with the same Ce concentration, which is also demonstrated by the photograph inserted [the Ce:LuAG powders were shaped into the word “LAG” in Fig. 3(b)] of the two specimens excited by UV illumination of 350 nm. The luminescence enhancement of the ceramic can be ascribed to the complete crystallization by sufficient solid state reaction during the sintering process compared with plenty of defects existing both on the particle surface and in the lattice of the powders, which results in a smaller particle size and their relatively poorer crystalline. Defects and traps in the powders could block the energy transfer from electrons to Ce centers, resulting in an energy loss during the light absorption and emission process, which finally decreases the luminescent property of the Ce:LuAG powders [31]. In Fig. 3(a), the peak at 145 nm is ascribed to host absorption according to the band gap of about 8.3 eV ( 150 nm) for the LuAG host [32]; the stronger host absorption in powders reflects that there is a larger energy loss in powders than in ceramics when the host is excited and the defects serve as electron traps resulting in nonradiative transition from host lattice to Ce luminescent centers. The peak at 230 nm is ascribed to the electron transition from ground state to the 5d sub-band energy level. The excitation band at 358 nm is ascribed to transition of Ce centers from the ground state to the excited d levels , which is consistent with the results of Ce:LuAG single crystals [33], [34], powders [18], [21], and transparent ceramics [3], [25]. The little aberrance of the excitation curve around 358 nm for the Ce:LuAG ceramic is mainly caused by the spectra correction for the spectral response under SR excitation using the excitation spectra of sodium salicylate as standard. It is also noticed that the emission spectrum of the Ce:LuAG ceramic displays an apparent broadband covering from 1.91 ev (650 nm) to 2.64 ev (470 nm), as shown in

Fig. 3. (a) Excitation ( 522 nm) and (b) emission ( 358 nm) spectra of the 0.5 mol% Ce:LuAG ceramic and powders. (Insert photograph reveals the image of the two Ce:LuAG specimens under UV illumination of 350 nm.)

Fig. 3(b). According to Gaussian fitted curves (green lines), the spectrum can be divided into two components centered at 2.28 ev (544 nm) and 2.42 ev (512 nm), respectively, which is the characteristic double peak emission of Ce ions. Obviously, the emissions are ascribed to the electron transitions from the to lowest crystal-splitting component of d excited level the 4f ground state of Ce ( F ; F ) [3], [4], [33], [34], respectively, which is consistent with that of the Ce:LuAG materials reported before. Scintillation decay time of the 0.5 mol% Ce:LuAG powders and ceramic was presented in Fig. 4. It is approximated by the sum of two exponential terms according to (2). The relative intensity of all components is also reported according to (3), here , as follows: (2) (3) Typical values of the decay time for the Ce:LuAG powders and ceramic of about 40–50 and 100–150 ns are evaluated in the faster and slower component, respectively. The faster component is due to prompt electron–hole (e-h) recombination at the

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TABLE I DECAY TIME

AND ITS RELATIVE INTENSITY OF THE FAST AND SLOW COMPONENT OF THE 0.5 mol% Ce:LuAG POWDERS AND CERAMIC UNDER Cs (662 keV) IRRADIATION COMPARED WITH THAT OF THE Ce:LuAG SINGLE CRYSTAL (SC) [36] UNDER Na (511 keV) IRRADIATION

Fig. 4. Decay curve of the 0.5 mol% Ce:LuAG powders and ceramic under Cs (662 keV) radioactive source irradiation.

Ce centers, while the slower one is related to a delayed recombination during the transport stage. In LuAG garnet single crystals, such kind of slow component is ascribed to shallow electron traps due to Lu antisite defects (AD) [33]–[35]. The temporary localization of the migrating charge carriers at the AD, which act as trapping centers during the energy transfer from the garnet host to Ce emitting centers leads to a decrease of scintillating efficiency [35]. In common viewpoint, ceramics would avoid such kind of defects, considering its comparably lower fabrication temperature than single crystals [36]. However, Shi et al. [37] and Wang et al. [38] also observed the existence of this possible AD under VUV synchrotron excitation in rare earth doped LuAG ceramics. Besides, in polycrystalline ceramics, the presence of traps at grain boundaries also can slow the decay time because grain boundaries with possibly segregated dopants or sintering aid constituents can act as the regions with intrinsical structural disorder where the traps and defects prefer to locate in [39]. Compared with Ce:LuAG single crystals, the decay time of the 0.5 mol% Ce:LuAG powders and ceramic is much shorter, especially for the slow component, while the fast one is not so discrepant (see Table I). We assume that it is quite related to AD in Ce:LuAG single crystals, which could reach the concentration of 0.5 mol% [40], whereas, in Ce:LuAG powders and ceramics the concentration of AD is nearly negligible. On the other hand, owing to the “clean” and thin (about 0.5 nm) grain boundaries of the ceramic specimen without any secondary phases or sintering aid constituents [see Fig. 2(c)], the relative intensity of the slow component (14%) in ceramic specimen is quite lower than that in the powder (69%) specimen, which leads to more efficient energy transfer from the host lattice to the Ce ions in the fully densified Ce:LuAG transparent ceramics (99.8% of the theoretical value) [27] than the dispersed nanosized powders. The excitation spectra of the Ce:LuAG ceramics with different Ce concentrations are shown in Fig. 5(a). The spectra are measured under the same conditions; therefore, they can be compared quantitatively. It reveals that these ceramic specimens

exhibit the same absorption behavior at 145 nm, which is attributed to the host absorption for LuAG, and no typical 5d-4f transition of Ce ions centered at 230 and 358 nm exists in the undoped LuAG specimen. It is also worth noting that the host absorption peaks of all the specimens are overlapped by a 170 nm band, which accords well with the typical position of Lu AD in LuAG single crystals [40]–[42]. It seems that there is no such kind of absorption band existing in the powder specimen [see Fig. 3(a)], and this 170 nm absorption band in ceramic specimens somehow improves with the increase of the Ce concentration. Fig. 5(b) shows the effect of doping concentration on the luminescence of Ce:LuAG ceramics. It suggests that the 0.5 mol% Ce doped ceramic specimen exhibits the best spectral character than other specimens, which is also proved by the photograph of the four unpolished specimens under 350 nm UV illumination. The PL emission intensity of the corresponding specimens under 345 nm UV excitation is shown in Fig. 6. This result is mainly ascribed to the concentration quenching effect. At high concentrations, the crystallization or clustering of Ce ions will occur, which may change a fraction of the Ce ions into PL quenching by transferring their energy to the other nearby Ce ions [43]. Therefore, the concentration will induce the luminescence quenching effect, leading to a decrease in the PL intensity as the Ce concentration reaches to 1.0 mol%, which is also consistent with the result under the X-ray irradiation condition [21]. Thus, it is confirmed that the Ce concentration of Ce:LuAG ceramics is more denser than that of the single crystalline counterpart in single crystal growth, where Ce concentration in LuAG host is limited to be 0.2–0.3 mol% due to the segregation phenomenon [34]. Such low Ce concentration prevents us from getting Ce:LuAG single crystals with high light yield, whereas the ceramic processing techniques can easily overcome this problem [3], [4], [19]. The variation trend of QE (see inserted curve in Fig. 6) of the different doping ceramics is also in accordance with the PL intensity due to concentration quenching, and the top QE of the 0.5 mol% ceramic specimen is 66.3 % under UV excitation of 345 nm at room temperature, which is comparable to the reported commercial Ce:YAG (bulk OSRAM) single crystal scintillator [44]. In Fig. 7(a), analogous Ce -related green (covering from 470 to 650 nm) and ultraviolet (covering from 360 to 410 nm) broadband emissions are observed in the 0.5 mol% Ce:LuAG ceramic specimen which is originated from the electronic transitions from the lowest 5d excited state d and the second lowest 5d excited state d , respectively, to the spin-orbit-split 4f ground state of Ce ions. It is also worth noting that the intensity ratio of the two emissions 1.8


Fig. 5. (a) Excitation ( 522 nm) and (b) emission ( 358 nm) spectra of Ce:LuAG ceramics with different Ce concentrations. (Insert photograph reveals the image of the four unpolished Ce:LuAG specimens under UV illumination of 350 nm.)

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re-trapping of created free charge carriers at unknown defects and traps or some local structure distortions at grain boundaries. Considering the typical Lu AD emission at 325 nm, which is near the 5d -4f emission [36] and the suspicious excitation peak at 170 nm in both Ce doped and undoped LuAG ceramics mentioned, we assume that AD possibly exists in the Ce:LuAG transparent ceramic and the appearance of both the intrinsic and the Ce -related emission under SR excitation in the exciton absorption region point to the fact that energy transfer from the host lattice occurs not only to Ce ions, but also to the AD responsible for the intrinsic emission. The competition between Ce ions and these defects takes place leading to a decrease of 5d -4 f emission intensity, red shift of the 5d -4 f emission from 522 to 526 nm. These phenomena are mainly related to the change of the excitation wavelength from 348 to 145 nm and the slow component of the decay time in the ceramic [shown in Fig. 4(b)]. It is also concluded that the efficiency of the energy transfer from the host lattice to Ce ions can be improved by removing the defects responsible for the intrinsic emission of Ce:LuAG polycrystalline ceramics. However, the existence of the Lu AD in LuAG ceramics is still confused, and the absorption position around 170 nm is also close to the region of excitons (160–180 nm) [32], [33] and absorption position (162–170 nm) arose from self-trapped excitons (STE) [33], [34], [41], [42] or other unknown defects and traps in polycrystalline ceramics located around grain boundaries or micro-pores, which are absent in corresponding single crystals. Therefore, further investigation is needed to clarify this problem using thermal stimulated luminescence (TSL) or other measurements. Temperature effect on the emission of the 0.5 mol% Ce:LuAG ceramic is given in Fig. 7(b). It reveals that under low temperature conditions (especially 50–150 K), the characteristic double peak emission spectra of Ce ions centered at about 505 and 540 nm could be clearly distinguished. As the temperature increased from 50 to 298 K, the intensity of the emission spectra increased as well, and the two independent characteristic peaks became overlapped to be a broadband at room temperature (300 K), no thermal quenching occurred in the Ce:LuAG ceramic. The spectral integral intensity of Ce typical 5d-4f radiative transition ( d F F ) shown inserted in Fig. 7(b) reveals that the sum of the two emissions monotonically increased with the temperature growth, which was mainly ascribed to the thermal enhancement for the free carriers transport at the rising temperature. IV. CONCLUSION

Fig. 6. PL emission spectra and quantum efficiency (QE) of the Ce:LuAG ceramics with different Ce concentrations under UV excitation of 345 nm.

under 145 nm excitation over the band-to-band ( 8.3 eV) is much smaller than that in Ce:LuAG single crystals where the value of 145 is reported [33]. It suggests that the strong enhancement of the 5d -4f emission is mainly caused by the

In summary, high quality optical Ce:LuAG ceramics with different Ce concentrations were sintered in vacuum atmosphere from monodisperse and nanosized Ce:LuAG powders synthesized by co-precipitation method. Under synchrotron VUV excitation, we found that the 0.5 mol% Ce doped LuAG ceramic exhibited the best spectrum character and quantum efficiency than other ceramics. The main emission spectrum of the ceramic could be divided into two components centered at 505 and 540 nm at low temperature (50–150 K), which was the typical 5d-4f radiative transition of Ce ions. Compared with Ce:LuAG single crystal, the decay time of the optimized

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and Prof. T. Tsuboi from Kyoto Sangyo University for the PL emission and QE measurements. REFERENCES

Fig. 7. (a) Emission spectra of the 0.5 mol% Ce:LuAG ceramic under different 145, 230, and 348 nm) (the insert shows the VUV and UV excitations ( mechanism of Ce transitions). (b) Emission spectra of the Ce:LuAG ceramic excited by 358 nm at various temperatures (the inset shows the spectral integral intensity with temperature effect to Ce and sum of their emissions).

Ce:LuAG ceramic was even shorter and with higher relative intensity of fast component. However, there was still 14% slow component of 109.4 ns in this ceramic ascribed to the disturbing by the existence of possible Lu antisite defects during the energy transfer from host lattice to Ce centers. Therefore, the defects creation and defect-related luminescence in LuAG garnet ceramics needs additional works and further investigation to be figured out.

ACKNOWLEDGMENT The authors gratefully acknowledge the support by Prof. G. B. Zhang and Prof. J. Y. Shi from U24 VUV beam-line station of NSRL. They also thank Dr. Y. L Chu from instrumental and analysis research center, Shanghai University for SEM measurement, Prof. Z. M. Zhang from Institute of the High Energy Physics, Chinese Academy of Sciences for decay time measurement, Prof. Y. L. Huang from Soochow University

[1] M. Nikl, “Modern scintillation materials. physics, applications and perspectives,” J. Ceram. Process. Res., vol. 5, no. 2, pp. 101–105, 2004. [2] L. Nadaraian, N. Jalabadze, R. Chedia, and L. Khundadze, “Production of nanopowder and bulk aluminate ceramic scintillators,” Ceram. Int., vol. 39, pp. 2207–2214, 2013. [3] H. L. Li, X. J. Liu, and L. P. Huang, “Fabrication of transparent cerium-doped lutetium aluminum garnet (LuAG:Ce) ceramics by a solid-state reaction method,” J. Amer. Ceram. Soc., vol. 88, no. 11, pp. 3226–3228, 2005. [4] H. L. Li, X. J. Liu, and L. P. Huang, “Fabrication of transparent ceriumdoped lutetium aluminum garnet ceramics by co-precipitation routes,” J. Amer. Ceram. Soc., vol. 89, no. 7, pp. 2356–2358, 2006. [5] Y. Wang, E. L. Van, W. H. Rhodes, J. Glodo, C. brecher, L. Nguyen, A. Lempicki, G. Baldoni, W. M. Higgins, and K. S. Shan, “Lu SiO :Ce optical ceramic scintillator for PET,” IEEE Trans. Nucl. Sci., vol. 59, no. 5, pp. 2587–2593, June 2008. [6] G. E. Jellison, E. D. Specht, L. A. Boatner, D. J. Singh, and C. L. Melcher, “Spectroscopic refractive indices of monoclinic single crystal and ceramic lutetium oxyorthosilicate from 200 to 850 nm,” J. Appl. Phys., vol. 112, no. 063524, 2012. [7] C. W. E. van Eijk, J. Andriessen, P. Dorenbos, and R. Visser, “Ce doped inorganic scintillators,” Nucl. Instrum. Methods A, vol. 348, pp. 546–550, 1994. [8] C. W. E. van Eijk, “Inorganic-scintillator development,” Nucl. Instrum. Methods A, vol. 460, pp. 1–14, 2001. [9] C. Greskovich and S. Duclos, “Ceramic scintillators,” Annu. Rev. Mater. Sci., vol. 27, no. 69, pp. 69–88, 1997. [10] A. Ikesue, I. Furusato, and K. Kamata, “Fabrication of polycrystalline, transparent YAG ceramics by a solid state reaction method,” J. Amer. Ceram. Soc., vol. 78, no. 1, pp. 225–228, 1995. [11] A. Ikesue, T. Kinoshita, K. Kamata, and K. Yoshida, “Fabrication and optical properties of high-performance polycrystalline Nd:YAG ceramics for solid-state lasers,” J. Amer. Ceram. Soc., vol. 78, no. 4, pp. 1033–1040, 1995. [12] A. Ikesue, K. Kamata, and K. Yoshida, “Effects of neodymium concentration on optical characteristics of polycrystalline Nd:YAG laser materials,” J. Amer. Ceram. Soc., vol. 79, no. 7, pp. 1921–1926, 1996. [13] M. Nikl, E. Mihokova, J. Pejchal, A. Vedda, M. Fasoli, I. Fontana, V. V. Laguta, V. Babin, K. Nejezchleb, A. Yoshikawa, H. Ogino, and G. Ren, “Scintillator materials-achievements, opportunities and puzzles,” IEEE Trans. Nucl. Sci., vol. 55, no. 3, pp. 1035–1041, Jun. 2008. [14] D. J. Wisniewski, L. A. Boatner, J. S. Neall, G. E. Jellison, J. O. Ramey, A. North, M. Wisniewska, A. E. Payzant, J. Y. Howe, A. Lempicki, C. Brecher, and J. Glodo, “Development of novel polycrystalline ceramic scintillators,” IEEE Trans. Nucl. Sci., vol. 55, no. 3, pp. 1501–1508, 2008. [15] A. Lempicki, C. Brecher, H. Lingertat, S. R. Miller, J. Glodo, and V. K. Sarin, “A ceramic version of the LSO scintillator,” IEEE Trans. Nucl. Sci., vol. 55, no. 3, pp. 1148–1151, Jun. 2008. [16] J. J. Xie, Y. Shi, L. C. Fan, and Z. B. Xu, “Microstructure and luminescent properties of Ce:Lu SiO ceramic scintillator by spark plasma sintering,” Opt. Mater., vol. 35, pp. 744–747, 2013. [17] S. Roy, H. Ligertat, C. Brecher, and V. K. Sarin, “Spectroscopic doped lutetium and transmittance properties of fine grained Ce oxyorthosilicate,” IEEE Trans. Nucl. Sci., vol. 56, no. 3, pp. 887–891, Jun. 2009. [18] H. L. Li, X. J. Liu, and L. P. Huang, “Synthesis of nanocrystalline lutetium aluminum garnet powders by co-precipitation method,” Ceram. Int., vol. 32, pp. 309–312, 2006. [19] H. L. Li, X. J. Liu, R. J. Xie, and L. P. Huang, “Cerium-doped lutetium aluminum garnet optically transparent ceramics fabricated by a sol-gel combustion process,” J. Mater. Res., vol. 21, no. 6, pp. 1519–1525, 2006. [20] T. Yanagida, Y. Fujimoto, K. Kamada, D. Totsuka, H. Yagi, T. Yanagitani, Y. Futami, S. Yanagida, S. Kurosawa, Y. Yokota, A. Yoshikawa, and M. Nikl, “Scintillation properties of transparent ceramic Pr:LuAG for different Pr concentration,” IEEE Trans. Nucl. Sci., vol. 59, no. 5, pp. 2146–2151, Oct. 2012. [21] H. L. Li, X. J. Liu, and L. P. Huang, “Luminescent properties of LuAG:Ce phosphors with different Ce contents prepared by a sol—Gel combustion method,” Opt. Mater., vol. 29, pp. 1138–1142, 2007.


[22] L. Xing, L. M. Peng, M. Gua, and G. D. Tang, “Solvothermal synthesis of lutetium aluminum garnet nanopowders: Determination of the optimum synthesis conditions,” J. Alloys Compound, vol. 491, pp. 599–604, 2010. [23] L. Xing and L. M. Peng, “Comparison of preparation and formation mechanism of LuAG nanopowders using two different methods,” Micro & Nano Lett., vol. 7, pp. 529–532, 2012. [24] Y. K. Liao, D. Y. Jiang, and J. L. Shi, “Transparent lutetium aluminum garnet sintered from carbonate co-precipitated powders,” Mater. Lett., vol. 59, pp. 3724–3727, 2005. [25] T. Yanagida, Y. Fujimoto, Y. Yokota, K. Kamada, S. Yanagida, A. Yoshikawa, H. Yagi, and T. Yanagitani, “Comparative study of transparent ceramic and single crystal Ce doped LuAG scintillators,” Radiat. Meas., vol. 46, pp. 1503–1505, 2011. [26] N. J. Cherepy, J. D. Kuntz, T. M. Tillotson, D. T. Speaks, S. A. Payne, B. H. T. Chai, Y. Porterchapman, and S. E. Derenzo, “Cerium-doped single crystal and transparent ceramic lutetium aluminum garnet scintillators,” Nucl. Instrum. Methods A, vol. 579, pp. 38–41, 2007. [27] J. Xu, Y. Shi, J. J. Xie, and F. Lei, “Fabrication, microstructure, and luminescent properties of Ce -doped Lu Al O (Ce:LuAG) transparent ceramics by low-temperature vacuum sintering,” J. Amer. Ceram. Soc., vol. 96, no. 6, pp. 1930–1936, 2013. [28] J. L. Look and C. F. Zukoshi, “Alkoxide-derived titania particles: Use of electrolytes to control size and agglomeration levels,” J. Amer. Ceram. Soc., vol. 75, no. 6, pp. 1587–1595, 1992. [29] A. Ikesue, K. Yoshida, T. Yamamoto, and I. Yamaga, “Optical scattering centers in polycrystalline Nd:YAG laser,” J. Amer. Ceram. Soc., vol. 80, no. 6, pp. 1517–1522, 1997. [30] A. Ikesue and K. Yoshida, “Scattering in polycrystalline Nd:YAG lasers,” J. Amer. Ceram. Soc., vol. 81, no. 8, pp. 2194–2196, 1998. [31] J. Barta, V. Cuba, M. Pospısil, V. Jary, and M. Nikl, “Radiation-induced preparation of pure and Ce-doped lutetium aluminium garnet and its luminescent properties,” J. Mater. Chem., vol. 22, pp. 16590–16597, 2012. [32] V. Babin, K. Blazek, A. Krasnikov, K. Nejezchleb, M. Nikl, T. Savikhina, and S. Zazubovich, “Luminescence of undoped LuAG and YAG crystals,” Phys. Stat. Sol. (c), vol. 2, no. 1, pp. 97–100, 2005. [33] K. Blazek, A. Krasnikov, K. Nejezchleb, M. Nikl, T. Savikhina, and S. Zazubovich, “Luminescence and defects creation in Ce -Lu Al O crystals,” Phys. Stat. Sol (b), vol. 241, pp. 1134–1140, 2004. [34] M. Nikl, E. Mihokova, J. A. Mares, A. Vedda, M. Martini, K. Nejezchleb, and K. Blazek, “Traps and timing characteristics of scintillator,” Phys. Stat. Sol (a), vol. 181, pp. 10–12, LuAG:Ce 2000.

379

[35] M. Nikl, A. Vedda, M. Fasoli, I. Fontana, V. V. Laguta, E. Mihokova, J. Pejchal, J. Rosa, and K. Nejezchleb, “Shallow traps and radiative recombination processes in single crystal scintillator,” Phys. Rev. B, vol. 76, no. 195121, 2007. [36] M. Nikl, J. A. Mares, N. Solovieva, H. Li, X. Liu, L. Huang, I. Fontana, M. Fasoli, A. Vedda, and C. D’Ambrosio, “Scintillation characteristics of Lu Al O :Ce optical ceramics,” J. Appl. Phys., vol. 101, no. 033515, 2007. [37] Y. Shi, X. Q. Feng, Y. B. Pan, J. Zhou, Y. Huang, and Z. H. Gao, “Fabrication and photoluminescence characteristic of Pr:LuAG scintillator ceramics,” Radiat. Meas., vol. 45, pp. 457–460, 2010. [38] L. X. Wang, H. J. Zhu, L. Y. Wu, K. M. Deng, C. X. Chang, and M. Yin, “Synthesis and luminescent properties of LuAG :Ce transparent ceramics by solvo-thermal method,” Spectrosc. Spect. Anal., vol. 31, no. 2, pp. 308–311, Feb. 2011. [39] Y. Q. Shen, Y. Shi, X. Q. Feng, Y. B. Pan, J. Li, Y. P. Zeng, M. Nikl, A. Krasnikov, A. Vedda, and F. Moretti, “The harmful effects of sintering aids in Pr:LuAG optical ceramic scintillator,” J. Amer. Ceram. Soc., vol. 95, no. 7, pp. 2130–2132, 2012. [40] Y. Zorenko, T. Voznyak, V. Gorbenko, T. Zorenko, A. Voloshinovskii, V. Vistovsky, M. Nikl, K. Nejezchleb, V. Kolobanov, and D. Spasskii, “Luminescence spectroscopy of excitons and antisite defects in Lu Al O single crystals and single-crystal films,” Opt. Spectrosc., vol. 104, no. 1, pp. 75–87, 2008. [41] Y. Zorenko, V. Gorbenko, A. Voloshinovskii, G. Stryganyuk, V. Mikhailin, V. Kolobanov, D. Spassky, M. Nikl, and K. Blazek, “Exciton-related luminescence in LuAG:Ce single crystals and single crystalline films,” Phys. Stat. Sol (a), vol. 202, no. 6, pp. 1113–1119, 2005. [42] Y. Zorenko, A. Voloshinovskii, V. Savchyn, T. Voznyak, M. Nikl, K. Nejezchleb, V. Mikhailin, V. Kolobanov, and D. Spassky, “Exciton and antisite defect-related luminescence in Lu Al O and Y Al O garnets,” Phys. Stat. Sol (b), vol. 244, no. 6, pp. 2180–2189, 2007. [43] W. Y. Cong, W. M. Zheng, S. M. Li, Y. J. Wang, X. Y. Liu, H. B. Huang, X. Y. Meng, and J. B. Zhai, “Effects of Ce concentrations, annealing conditions and fabrication processes on the photoluminescence properties of Ce-doped silica films,” Phys. Stat. Sol (b), vol. 249, no. 8, pp. 1585–1589, 2012. [44] D. Haranath, H. Chander, P. Sharma, and S. Singh, “Enhanced luminescence of Y Al O :Ce nanophosphor for white light emitting diodes,” Appl. Phys. Lett., vol. 89, no. 173118, 2006.