Optical and scintillation properties of transparent ceramic Yb:Lu2O3 with different Yb concentrations

Optical Materials 36 (2014) 1044–1048

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Optical and scintillation properties of transparent ceramic Yb:Lu2O3 with different Yb concentrations Takayuki Yanagida a,⇑,1, Yutaka Fujimoto a, Hideki Yagi b, Takagimi Yanagitani b a b

Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japan Konoshima Chemical Co., Ltd., 80 Kouda, Takuma, Mitoyo, Kagawa 769-1103, Japan

a r t i c l e

i n f o

Article history: Received 3 October 2013 Received in revised form 17 January 2014 Accepted 20 January 2014 Available online 11 February 2014 Keywords: Transparent ceramic Scintillation detector Yb3+ Scintillator

a b s t r a c t Yb 0.1–100% doped Lu2O3 transparent ceramic scintillators were prepared by Konoshima Chemical. They had 60–80% transparency at wavelength longer than 240 nm and absorption bands around 970 nm due to 4f–4f transition of Yb3+ were observed. In photoluminescence and X-ray induced radioluminescence, Yb3+ charge transfer luminescence appeared at 330 and 490 nm. Photoluminescence and scintillation decay times of the charge transfer luminescence resulted 0.5–1.5 ns. 137Cs excited pulse height spectrum was evaluated to determine the light yield of the fast component and Yb 0.3% doped sample exhibited the highest light yield of 500 ph/MeV. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Scintillators are luminescent materials which convert the energy of a high energy ionizing particle or high-energy photon into a number of visible/UV photons [1]. They are widely used in radiation detection applications in medical imaging [2], security [3], astrophysics [4], particle physics [5], and well-logging [6]. In these practical applications, scintillation c-ray detectors have attracted much attention especially for medical and security applications. Among medical applications, positron emission tomography (PET) [7] is the most famous application and now PET is used to discover the very early stage of cancers all over the world. At the first stage of PET (1990s), Bi4Ge3O12 (BGO) was used as a scintillator and then Y admixed Ce doped Lu2SiO5 (Ce:LYSO) became an alternative to BGO in these years. In order to develop the next generation PET, time-of-flight (TOF) function is required. The TOF information has been investigated in PET applications since 1980s [8–9] using core-valence luminescence materials such as BaF2 [10] and CsF [11]. Due to a low density, a low light yield, a short emission wavelength ranging at VUV–UV range of BaF2, and huge hygroscopicity of CsF, the TOF-PET technique has not been further developed [12] using this principle. In recent years, TOF technique was re-defined and exhibted practical value [13] by extracting the TOF information from the rising edge of bright Ce:⇑ Corresponding author. Tel./fax: +81 93 695 6049. E-mail address: [email protected] (T. Yanagida). Present address: Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japan. 1

http://dx.doi.org/10.1016/j.optmat.2014.01.022 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.

LYSO scintillator decay, which was applied in the latest generation of PET devices[14]. In this work, we investigated Yb-doped Lu2O3 (Yb:Lu2O3) transparent ceramic scintillators with different Yb concentrations because Yb-doped sesquioxides (Sc2O3, Y2O3, and Lu2O3) showed some favorable characteristics for TOF-PET [15]. Though we introduced some ceramic scintillators (e.g., [16]), among them, Yb-doped sesquioxides scintillators had a big merit in the ceramic form. Fabrication of sesquioxide materials in a single crystal form by the conventional melt growth needs enormous costs due to a high melting point around 2400 °C requiring special crucibles and causes a problematic quality due to their high melting point (e.g., numerous defects). However, transparent ceramics sintered at lower temperatures will reduce the fabrication costs although their quality and scintillation performance is reasonable [15]. In this work, Yb 0.1%, 0.3%, 1%, 3%, 9%, and 100% (= Yb2O3) doped samples were prepared by Konoshima Chemical by the conventional vacuum sintering. Basic optical properties including transmittance and photoluminescence (PL) were evaluated systematically. After optical properties evaluation, scintillation light yield and decay time were investigated.

2. Experimental 2.1. Sample preparation In the sample preparation, Lu2O3 transparent ceramic scintillators doped with different concentrations of Yb3+ were prepared by

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the conventional vacuum sintering method using 99.999% purified Lu2O3 and Yb2O3 powders in Konoshima Chemical. Firstly, aqueous solutions of LuCl3 and YbCl3 were mixed. These solutions were heated and then deposition of the colloidal suspension with typically 100 nm particle size was performed. Steps of filtration and washing with water were repeated several times and the resulting material was then dried for 2 days at 120 °C. The precursor obtained in these processes is calcined at 1000 °C in order to produce a raw oxide powder Yb doped sesquioxide. After 24 h ball-milling, the milled slurry was placed in a gypsum mold and dried into the desired form. Finally, after removing the organic components by calcination, the remaining material was vacuum sintered at 1700 °C for 5 h and highly transparent Yb:Lu2O3 and Yb2O3 ceramics were obtained. Yb concentrations were 0.1%, 0.3%, 1%, 3%, 9%, and 100%. The detailed description of the preparation process was described in the past [17]. 2.2. Optical characterization In-line transmittances of all samples were evaluated by using JASCO V670 spectrometer from 190 to 2700 nm with 1 nm step. PL spectra were corrected by Hamamatsu Quantaurus-s instrument under 280 nm excitation which was the same wavelength with the PL decay curve observation. Though this instrument did not have high wavelength resolution, spectral sensitivity was corrected by a manufacturer. The excitation wavelength (280 nm) was the strongest and the shortest wavelength of this instrument. Since the intensity of the excitation source was enough strong and wavelength resolution was rough, Yb3+ centers could be excited if the excitation wavelength did not match exactly. The monitoring wavelength was 330 nm which was the strongest emission band described later. The timing resolution of Quantaurus-s was 60 ps. In decay time analysis, double exponential function was assumed and instrumental response was deconvoluted. All experiments were carried out at room temperature. 2.3. Evaluation of scintillation properties The radioluminescence spectra were recorded at room temperature under X-ray irradiation. The excitation source was our original instrument fabricated by OURSTEX Corporation. The X-ray tube (W target) was supplied with 70 kV bias voltage and 1 mA tube current. The emission spectra were measured using Andor DU-420-BU2 CCD spectrometer in 180–700 nm and DU-492A in 650–1650 nm wavelength ranges. These CCD-based detectors (cooled down to 188 K by a Peltier module) were coupled with a monochromator SR163 (Andor, 1200 grooves/mm, 300 nm blaze wavelength). The scintillation light was fed into spectrometer through a 2 m optical fiber to avoid direct X-ray hit of CCD. In order to cut background radiation, the detector was surrounded by 5 cm thick Pb blocks. The geometry of the setup can be seen in our previous work [18]. In 137Cs pulse height measurements, the samples were wrapped with several layers of Teflon tape to collect scintillation photons and were coupled to the photomultiplier tube (PMT) R7600-200 (Hamamatsu) with an optical grease (OKEN 6262A). The anode signal of PMT was fed into preamplifier (ORTEC 113), shaping amplifier (CP4467) with 20 ns shaping time, and multichannel analyzer (Amptek Pocket MCA). The scintillation decay times were evaluated by our original setup, pulse X-ray streak camera system [19–20] that enabled us to observe time- and wavelength- resolved scintillation phenomenon with 80 ps timing resolution. The mean energy of X-ray quanta was 30 keV and the endpoint energy of the bremsstrahlung spectrum was 40 keV. Monitoring wavelength of ceramic Yb:Lu2O3 samples were same with that of PL decay time evaluations

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(330 ± 15 nm) based on the radioluminescence spectra discussed later. The analysis was carried out with the same manner in PL decay time and all experiments were done at room temperature. 3. Results and discussion 3.1. Sample Fig. 1 represents cut and polished Yb 0.1%, 0.3%, 1%, 3%, 9%, and 100% (= Yb2O3) doped Lu2O3 transparent ceramic. The sizes of the ceramic and single crystal were 5–10 mm /  2 mmt. They were clearly transparent. Though sample sizes were not the same, slight difference did not affect any results discussed on this work because we did not compare the PL and radioluminescence intensities quantitatively that were largely affected by the sample size. The scintillation light yields were evaluated by the pulse height under 137 Cs excitation and this evaluation was not affected by the sample size. 3.2. Optical properties Fig. 2 represents in-line transmittance spectra of Yb:Lu2O3 ceramics. The overall features were almost same in all samples. The absorption features around 970 nm due to Yb3+ 4f–4f transition [21] were proportional to Yb concentrations. Except Yb3+ 4f–4f absorption bands, no particular absorption features were observed and all samples had 60–80% transparency from UV to NIR wavelengths. The transmittance became approximately 0% around 240 nm. It would correspond to 2F7/2 – charge transfer (CT) state transition of Yb3+ or band-to-band transition of sesquioxide materials. PL spectra under 280 nm excitation are shown in Fig. 3. Emission bands appeared at 330, 420, 490, and 550 nm. The most intense peak at 330 nm was ascribed to Yb3+ luminescence from CT state to 2F7/2 level and it resembled to previously reported other Yb3+-doped materials, such as transparent ceramic Yb-doped YAG [22], crystal YbAlO3 [23], and other hosts [24]. The 490 nm peak was attributed to the transition from the CT state to 2F5/2 level since the energies difference of 2F7/2 and 2F5/2 levels were roughly 10,000 cm 1. The energy difference between 330 and 490 nm well coincided to 10,000 cm 1. 550 nm line would be due to an unexpected contamination of other rare earth elements such as Nd3+, Tb3+, Er3+, and Tm3+ because PL decay time monitored at 550 nm was very slow and their ionic radii were close to Lu3+ so that the elimination of rare earth impurities in chemical processes was quite difficult. In spite of the low contamination level not detected in transmittance spectra, they exhibited intense PL signal. In other words, Lu2O3 host was an attractive matrix for 4f–4f rare earth

Fig. 1. Photograph of Yb:Lu2O3 transparent ceramic.

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Fig. 4. PL decay time profiles of Yb:Lu2O3 transparent ceramic under 280 nm excitation. Dotted line represents a fitting function deconvoluted with the instrumental response.

dopant because Yb-doped YAG and LuAG synthesized by using same raw materials did not show such unexpected lines. The origin of the 420 nm peak was not fully understood. At least, we could not find any differences in PL decay time because these emissions (330 and 420 nm) had similar decay times (1 ns). PL decay time profiles are shown in Fig. 4. All decay time profiles were quite similar, characterized by 1 ns fast decay. Yb2O3 sample showed the fastest PL decay due to the concentration quenching. The significant difference was not observed in present samples and they were compared with X-ray induced scintillation decay times later. As the shortest excitation wavelength of our instrument was 280 nm, we could not examine 240 nm excitation.

human body is transparent to NIR (700–1200 nm) wavelength and Lu2O3 is a good host for NIR emitting scintillators from the present work. 137 Cs irradiated pulse height spectra of Yb 0.3%, 1%, and 3% Lu2O3 are shown in Fig. 6. Although the energy resolution was not so high, 137Cs photoabsorption peak was observed in very fast shaping time (20 ns). Other samples (Yb 0.1%, 9%, and 100%) showed detectable signal while photoabsorption peak could not be distinguished. Scintillation light yield of Yb 0.3% doped sample resulted 500 ± 50 ph/MeV taking into account the quantum efficiency around 330 nm of PMT and by using the light yield calibrated BGO standard measured with 2 ls shaping time [29]. Due to the wavelength sensitivity of PMT (280–600 nm) and short shaping time (20 ns), emission from unexpected rare earth contaminations were not accumulated. Although previous studies on scintillation of CT luminescence investigated highly (several tens%) Yb-doped materials [30–35] mainly, the present work demonstrated that high Yb-doping concentration was not adequate for Lu2O3 host because scintillation light yield decreased in higher Yb-doped samples. Fig. 7 demonstrates X-ray induced scintillation decay time profiles of Yb:Lu2O3. Though most samples showed similar time profiles, only Yb2O3 sample was quite fast due to the concentration quenching. Then, Fig. 8 summarizes PL and scintillation decay times of CT luminescence at 330 nm as a function of Yb concentrations. Typical scintillation and PL decay times were 1.3 and 0.9 ns, respectively. Scintillation decay times were few hundreds of picoseconds slower than the photoluminescence ones due to an energy migration from the host to emission centers. In Yb2O3, scintillation

3.3. Scintillation properties Fig. 5 demonstrates X-ray induced radioluminescence spectra from UV to NIR wavelengths. The main emission peak due to Yb3+ CT luminescence appeared around 330 nm while many unexpected contamination lines were detected. 550, 580, and 620 lines were simply ascribed to transitions of Tb3+, 5D4–7F6, 5D4–7F5, 5 D4–7F4, and 5D4–7F3, respectively. The sharp 400 nm line not observed in PL spectra would be caused by Nd3+, Er3+ or Tm3+ since we observed strong scintillation from these dopants via X-ray excitation in garnet host around 400 nm [25–27]. In NIR wavelength, emission around 970–1030 nm was ascribed to Yb3+ 2F5/2–2F7/2 transition [28]. The sharp line around 1064 nm would be blamed for unexpected Nd contamination because the emission from other rare earth ions did not appear at this wavelength. The NIR scintillation is quite interesting for medical applications because the

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long emission wavelength (UV–visible) and a fast response, Yb:Lu2O3 is a promising candidate for high counting rate applications coupled with the photodetectors which have UV–visible wavelength sensitivity. Actually, recent test of positron annihilation measurement proved that the timing resolution of Yb:Lu2O3 was superior to that of conventional BaF2 [39]. In order to achieve higher scintillation performance, purchasing more purified raw materials is required.

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and PL decay times were same since the host and emission center were same in this material. Typical scintillation decay times of Ce3+ at visible wavelength, Pr3+ at UV wavelength, and Nd3+, Ho3+, Er3+, and Tm3+ at vacuum ultra violet wavelength are 20–60 ns (e.g., [36]), 20 ns (e.g., [37]), and 4–8 ns (e.g., [38]), respectively. Compared with above widely used luminescence centers, CT luminescence of Yb3+ showed a unique characteristic with 1 ns decay at UV–visible wavelengths. Timing resolution of scintillation detectors depends on scintillation rise/decay times and photoelectron yield that is a product of the light yield of a scintillator and the quantum efficiency of a photodetector. Concerning a relatively

Optical and scintillation properties of Yb 0.1%, 0.3%, 1%, 3%, and 9% doped Lu2O3 and Yb2O3 transparent ceramic materials fabricated by Konoshima Chemical were investigated. They exhibited 60–80% high transparency from UV to NIR wavelengths except some absorption bands due to Yb3+ 4f–4f transition around 970 nm. CT luminescence bands were detected around 330 nm (CT->2F7/2) and 490 nm (CT->2F5/2) in PL and X-ray induced radioluminescence spectra. The difference of wavelengths of these two bands was consistent with the difference in energies between 2 F7/2 and 2F5/2 levels (10,000 cm 1). Scintillation light yield of Yb 0.3% doped sample was the highest among samples investigated in this work and the light yield was 500 ph/MeV. PL and scintillation decay times of CT luminescence were quite fast, sub-ns and 1.3 ns, respectively. Acknowledgments This work was mainly supported by J.S.T. Sentan, A-step and partially by a Grant in Aid for Young Scientists (A)-23686135, and Challenging Exploratory Research-23656584 from the Ministry of

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Education, Culture, Sports, Science and Technology of the Japanese government (MEXT). Partial assistance from the Yazaki Memorial Foundation for Science and Technology, Shimazu Sci. Foundation, Kato Foundation for Promotion of Science, and Nippon Sheet Glass Foundation for Materials Science and Engineering, Tokuyama Science foundation, Iketani Science and Technology Foundation, and The Asahi Glass Foundation, are also gratefully acknowledged. References [1] T. Yanagida, Opt. Mat. 35 (2013) 1987. [2] T. Yanagida, A. Yoshikawa, Y. Yokota, K. Kamada, Y. Usuki, S. Yamamoto, M. Miyake, M. Baba, K. Sasaki, M. Ito, IEEE. Nucl. Trans. Sci. 57 (2010) 1492. [3] D. Totsuka, T. Yanagida, K. Fukuda, N. Kawaguchi, Y. Fujimoto, Y. Yokota, A. Yoshikawa, Nucl. Instrum. Methods A 659 (2011) 399. [4] K. Yamaoka, M. Ohno, Y. Terada, S. Hong, J. Kotoku, Y. Okada, A. Tsutsui, Y. Endo, K. Abe, Y. Fukazawa, S. Hirakuri, T. Hiruta, K. Itoh, T. Itoh, T. Kamae, M. Kawaharada, N. Kawano, K. Kawashima, T. Kishishita, T. Kitaguchi, M. Kokubun, G.M. Madejski, K. Makishima, T. Mitani, R. Miyawaki, T. Murakami, M.M. Murashima, K. Nakazawa, H. Niko, M. Nomachi, K. Oonuki, G. Sato, M. Suzuki, H. Takahashi, I. Takahashi, T. Takahashi, S. Takeda, K. Tamura, T. Tanaka, M. Tashiro, S. Watanabe, T. Yanagida, D. Yonetoku, IEEE. Trans. Nucl. Sci 52 (2005) 2765. [5] T. Ito, M. Kokubun, T. Takashima, T. Yanagida, S. Hirakuri, R. Miyawaki, H. Takahashi, K. Makishima, T. Tanaka, K. Nakazawa, T. Takahashi, T. Honda, IEEE Trans. Nucl. Sci. 53 (2006) 2983. [6] T. Yanagida, Y. Fujimoto, S. Kurosawa, K. Kamada, H. Takahashi, Y. Fukazawa, M. Nikl, V. Chani, Jpn. J. Appl. Phys. 52 (2013) 076401. [7] W.W. Moses, Nucl. Instrum. Methods A 471 (2001) 209. [8] M.E. Phelps, E.J. Hoffman, N.A. Mullani, M. Ter-Pogossian, J. Nucl. Med. 16 (1975) 210. [9] N.A. Mullani, W. Wong, R. Hartz, et al., IEEE Trans. Nucl. Sci. 30 (1983) 739. [10] C. van Eijk, J. Andriessen, P. Dorenbos, J. Jansons, N. Khaidukov, Z. Rachko, J. Valbis, Heavy Scintillators for Scientific and Industrial Applications, Editions Frontieres (Crystal 2000), 1993, pp. 161–166. ISBN 2-86332-128-5. [11] M. Moszynski, R. Allemand, M.L.R. Odru, J. Vacher, Nucl. Instrum. Methods 205 (1983) 239. [12] M.E. Casey, R. Nutt, IEEE Trans. Nucl. Sci. NS-33 (1986) 463. [13] M. Conti, Eur. J. Nucl. Med. Mol. Imaging 38 (2011) 1147. [14] W.W. Moses, Nucl. Instrum. Methods A 580 (2007) 919. [15] T. Yanagida, Y. Fujimoto, S. Kurosawa, K. Watanabe, H. Yagi, T. Yanagitani, V. Jary, Y. Futami, Y. Yokota, A. Yoshikawa, A. Uritani, T. Iguchi, M. Nikl, Appl. Phys. Express 4 (2011) 126402. [16] H. Takahashi, T. Yanagida, D. Kasama, T. Ito, M. Kokubun, K. Makishima, T. Yanagitani, H. Yagi, T. Shigeta, T. Ito, IEEE Trans. Nucl. Sci. 53 (2006) 2404.

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