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Abstract— single crystals were grown by the -PD method with RF heating system. In these crystals,. 4f-5d emission is observed within 500-530 nm wavelength.
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Crystal Growth of Ce Doped Single Crystal by the Micro-Puling-Down Method and Their Scintillation Properties Kei Kamada, Takayuki Yanagida, Jan Pejchal, Martin Nikl, Takanori Endo, Kousuke Tsutumi, Yutaka Fujimoto, Akihiro Fukabori, and Akira Yoshikawa

Abstract— single crystals were grown by the -PD method with RF heating system. In 4f-5d emission is observed within 500-530 these crystals, nm wavelength. Emission peak shifts to shorter wavelength and the decay accelerates with increasing Ga concentration. In the case of series,the Ce0.2% crystal showed the highest emission intensity. In order to determine light yield, the energy spectra were measured under 662 keV ã-ray excitation ( source) and detection by an APD S8664-55(Hamamatsu). The light yield was direct irradiation peak to APD. The light calibrated from yield of Ce0.2%: sample was of about 30,000 photon/MeV. Dominant scintillation decay time was of about 50 ns. Index Terms—Crystals, luminescence, solid scintillation detectors.

I. INTRODUCTION

S

CINTILLATOR materials combined with photodetectors are used to detect high energy photons and accelerated particles in medical imaging techniques, high energy and nuclear physics detectors, high-tech industrial applications and most recently also in the advanced homeland security related techniques. [1] In the last two decades, great R&D effort brought several new material systems, namely the Ce-doped orthosilicates as (GSO), (LSO), (LYSO), pyrosilicates based on and most recently single crystal hosts Manuscript received November 04, 2011; revised January 05, 2012; accepted January 09, 2012. This work was supported in part by JST Sentan, in part by a Grant in Aid for Young Scientists (B)-15686001, (A)-23686135, and Challenging Exploratory Research-23656584 from the Ministry of Education, Culture, Sports, Science, and Technology of the Japanese Government (MEXT), and in part by Czech AV M100100910 and GACR 202/08/0893 Projects. K. Kamada, T. Endo, and K. Tsutsumi are with the Materials Research Laboratory, Furukawa Co., Ltd., Ibaraki 305-0856, Japan (e-mail: [email protected]; [email protected]; [email protected]). T. Yanagida, Y. Fujimoto, A. Fukabori, and A. Yoshikawa are with the IMR, Tohoku University, Miyagi 980-8577, Japan (e-mail: yanagida@imr. tohoku.ac.jp; [email protected]; [email protected]; [email protected]). J. Pejchal and M. Nikl are with the Institute of Physics, AS CR, Prague 162 53, Czech Republic (e-mail: [email protected]; [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.2012.2184141

[1]–[11] . The best combination of stopping power, decay time, and light yield is currently achieved by Ce-activated materials. Oxide materials based on garnet structure single crystals are promising candidates for scintillator applications because of well mastered technology developed for laser hosts and other applications, optical transparency and easy doping by rare-earth elements. As an analog to the classical scintillator [12] the Ce-doped (Ce:LuAG) single crystals became the subject of systematic studies [13] due to its considerably higher density of 6.7 g/cm3 and fast scintillation response of about 60–80 ns due to the 5d1-4f radiative transition of providing the emission around 500–520 nm which is suitable also for semiconductor-based photodetectors. The reported light yield was gradually increasing from about 12,000 [14] up to recently reported 25,000 photon/MeV. [15] It has been also reported that Ga- admixture in LuAG host makes the energy transfer to or emission centers faster and more efficient which accelerate the scintillation response of such a material. [16]. Slightly increased light yield was obtained for Ga concentration up to 20 at% in Lu3(Al-Ga)5O12:Ce. [17] Recently, our group found that single crystal could be grown from melt by substituting Al site with Ga. Ce:GAGG single crystal grown by micro pulling down method showed the high light yield of 42000 photon/MeV. [18]. In this report, the (LYGAG) single crystals were grown by the micro-pulling down ( -PD) method. Luminescence and scintillation properties were measured. The substitution effects of the sites with in has been studied. II. MATERIALS NAD METHODS A. Crystal Growth , , , A stoichiometric mixture of and powders (High Purity Chemicals and Co.) was used as starting material. More specifically, according to the formula sites were substituted by . Single crystals of of Ce:LYGAG were grown by the -PD method with an RF heating system. The z was 0.01 and additionally 2 mol% of was added to compensate the ignition loss. A schematic layout of the -PD growth apparatus is given in [19],

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[20]. Typical pulling rates were 0.05–0.07 mm/min and the diameter was around 3 mm. Crystals were grown from an Ir crucible under atmosphere with , 2% of added to prevent evaporation of gallium oxide. The seed crystals were oriented undope LuAG crystals. Plates of were cut and polished for the purposes of the luminescence spectra and gamma-ray response measurements. Quantitative chemical analysis of the crystals for the Al, Ga, Ce, Y and Lu content along the growth direction were performed by electron probe microanalysis (EPMA; JXA-8621MX, JEOL). So called ZAF correction was used, where Z stands for atomic number, A for absorption correction factor and F for fluorescence correction factor, respectively. 1) Luminescence and Gamma-Ray Response Measurement Procedure: Radioluminescence spectra were measured at custom made model 5000M Horiba Jobin Yvon spectrometer using an X-ray tube (operated at 35 kV and 16 mA, Mo cathode) for the excitation. Light yield measurements were performed by using an avalanche photodiode (APD) (Hamamatsu, S8664-55). Sample pieces with dimensions of were cut from the grown single crystals, surfaces were mechanically polished. The samples were coupled by the APD (Hamamatsu, S8664-55) using silicone grease (OKEN, 6262A). The measured sample was covered by Teflon tape. The signal was fed into a pre-amplifier (CP580K), shaping amplifier (CP 4417), pocket MCA (Amptec 8000A) and finally to a personal computer. The bias for the APD was supplied by a CP 6641 module. Since the APD was highly sensitive to the ambient temperature, a heat bath was used to control the temperature at , within . For the decay time measurement the same setup with a photomultiplier tube (PMT Hamamatsu H6521) and tal oscilloscope TD5032B were used. III. GROWTH OF CE:LYGAG SINGLE CRYSTALS (x = 0, 1, 2, 3, 4 and 5), , and crystals were grown by the -PD method. Example photos are shown in Fig. 1. The grown crystals were transparent with yellow color, 2–3 mm in diameter and 10–30 mm in length. Some of them looked slightly cloudy because of the rough surface caused by gallium oxide evaporation or thermal etching. However, the inner part of all the crystals was perfectly transparent. The composition distribution in grown crystals was monitored by the EPMA. The composition distribution along the growth direction of is shown in Fig. 2. The effective segregation coefficients of Y, Ga and Ce ions showed , 0.97 and 0.62 values, respectively. IV. SCINTILLATION PROPERTIES A. Luminescence Properties Radioluminescence spectra were measured using an X-ray tube (operated at 35 kV and 16 mA, Mo cathode). Fig. 3 shows radioluminescence spectra of series. The 5d-4f emission of at 520-530 nm was observed. The

Fig. 1. Example photos of grown Ce:LYGAG crystals.

Fig.

2. The

composition distribution crystal.

along

growth

direction

of

sample shows the highest intensity of 4f-5d emission. The emission peaks shifted to shorter wavelength with increasing Ga concentration. It is fully consistent with characteristics of previous studies of YAG-based materials [21], [22]. 1) Gamma-Ray Response: The typical energy spectra of excited by at room temperature and measured using the APD are shown in Fig. 4. The light yield of the sample was calibrated from the direct irradiation peak to the APD. Such direct irradiation generates electron-hole pairs [23]. Based on this value, LY of is without correcting quantum efficiency (QE) of the APD. After correcting the QE, which is at 80% at 520 nm, the total LY becomes , which is around 80% of LY of a reference Ce:LYSO scintillator (35,000 photon/MeV, QE corrected) measured at the same experiment arrangement. Energy resolution of the sample was 14.2%@662 keV. Scintillation decay curves were measured by using the PMT and digital oscilloscope TD5032B under excitation by radioisotope. Scintillation decay curves of

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SINGLE CRYSTAL

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Fig. 6. Scintillation decay curve of Ce0.2, 1 and 3% crystals using the PMT and digital irradiated by . Fig. 3. Radioluminescence spectra of grown crystals. LIGHT YIELD

Fig. 4. Energy spectra of Ce0.2% excited by a 662 keV gamma-ray using the APD at

AND

TABLE I SCINTILLATION DECAY TIME .

OF

and Ce:LYSO standard .

Table I shows light yield and scintillation decay time samples. The sample shows the highest light yield of 30000 phot/MeV. Theoretically, density increases with increasing Ga concentration in chemical formula. has theoretical density which is comparable to the density of LuAG . Higher Ga concentration crystals such as and show no luminescence. 5d-4f luminescence is quenched in these crystalss because of positioning 5d states of in the host conduction band which is well-known problem in other garnet materials as well [17], [18], [20], [24]. of

Fig. 5. Scintillation decay curve of the PMT and digital irradiated by

crystals using .

(X = 0, 2 and 3) crystals were shown in Fig. 5. The scintillation decays were accelerated with increasing Ga concentration. The scintillation decay times were 77.4 ns, 56.8 ns and 50.0 ns in (x = 0, 2 and 3) samples, respectively. Scintillation decay curves of Ce0.2, 1 and 3% crystals were shown in Fig. 6. The scintillation decay times became shorter with increasing Ce concentration. The scintillation decay times were 52.4 ns, 50.0 ns and 43.6 ns in Ce0.2, 1 and 3% samples, respectively.

V. CONCLUSION single crystals were grown by the -PD method. Luminescence and scintillation properties were measured. The substitution phenomenon in the sites with and sites with in garnet structure has been studied. 5d-4f emission within 520–530 nm was observed in the Ga 0-80 at.% substituted samples. In the point view of light yield, decay time and density, Ce0.2%: is the most advantageous composition among the samples. The Ce0.2%: sample

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shows the highest light yield of 30000 photon/MeV. The energy resolution was 14.2%@662 keV. The scintillation decays were accelerated with increasing Ce and Ga concentration in . The scintillation decay time of the Ce0.2%: sample was 52.4 ns. Theoretical density of is . Furthermore, by improving the crystal quality of crystals by using method, scintillation properties are expected to be improved in the future. REFERENCES [1] M. Nikl, “Scintillation detectors for X-rays,” Meas. Sci. Technol., vol. 17, pp. R37–R54, Feb. 2006. [2] C. L. Melcher and J. S. Schweitzer, “Cerium-doped lutetium orthosilicate: A fast, efficient new scintillator,” IEEE Trans. Nucl. Sci., vol. 39, no. 4, pp. 502–505, Aug. 1992. [3] M. Kapusta, P. Szupryczynski, C. L. Melcher, M. Moszynski, M. Balcerzyk, A. A. Carey, W. Czarnacki, M. A. Spurrier, and A. Syntfeld, “Nonproportionality and thermoluminescence of LSO:Ce,” IEEE Trans. Nucl. Sci., vol. 52, no. 4, pp. 1098–1104, Aug. 2005. [4] M. A. Spurrier, P. Szupryczynski, A. A. Carey, and C. L. Melcher, co-doping on the scintillation properties of LSO:Ce,” “Effects of IEEE Trans. Nucl. Sci., vol. NS-55, no. 3, pp. 1178–1182, Jun. 2008. [5] P. Lecoq and M. Korzhik, “New inorganic scintillation materials development for medical imaging,” IEEE Trans. Nucl. Sci., vol. 49, no. 4, pp. 1651–1654, Aug. 2002. [6] M. Moszynski, D. Wolski, T. Ludziejewski, M. Kapusta, A. Lempicki, C. Brecher, D. Wiśniewski, and A. J. Wojtowiczc, “Properties of the new LuAP:Ce scintillator,” Nucl. Instr. Meth. A, vol. 385, pp. 123–131, Jan. 1997. [7] S. Weber, D. Christ, M. Kurzeja, R. Engels, G. Kemmerling, and H. Halling, “Comparison of LuYAP, LSO, BGO as scintillators for high resolution pet detectors,” IEEE Trans. Nucl. Sci., vol. 50, no. 5, pp. 1370–1372, Oct. 2003. [8] M. Korzhik et al., “Development of scintillator materials for PET scanners,” Nucl. Instr. Meth. A, vol. 571, pp. 122–125, Jan. 2007. [9] K. S. Shah, J. Glodo, M. Klugerman, W. W. Moses, S. E. Derenzo, and M. J. Weber, “LaBr3:Ce scintillators for gamma-ray spectroscopy,” IEEE Trans. Nucl. Sci., vol. 50, no. 6, pp. 2410–2413, Dec. 2003. [10] J. Glodo, W. W. Moses, W. M. Higgins, E. V. D. van Loef, P. Wong, S. E. Derenzo, M. J. Weber, and K. S. Shah, “Effects of Ce concentration on scintillation properties of LaBr3:Ce,” IEEE Trans. Nucl. Sci., vol. 52, no. 5, pp. 1805–1808, Oct. 2005. [11] Saint-Gobain Crystals, “Brillance 380 Data Sheet,” 2007 [Online]. Available: http://www.detectors.saint-gobain.com/

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