Comparative study of ceramic and single crystal Ce:GAGG scintillator

Optical Materials 35 (2013) 2480–2485

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Comparative study of ceramic and single crystal Ce:GAGG scintillator Takayuki Yanagida a,⇑, Kei Kamada b, Yutaka Fujimoto a, Hideki Yagi c, Takagimi Yanagitani c a

Kyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japan Furukawa 1-25-13, Kannondai, Tsukuba, Ibaraki 305-0856, Japan c Konoshima Chemical Co., Ltd., 80 Kouda, Takuma, Mitoyo, Kagawa 769-1103, Japan b

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Article history: Received 7 March 2013 Received in revised form 16 June 2013 Accepted 1 July 2013 Available online 20 July 2013 Keywords: Single crystal Ceramic Scintillation detector Ce3+ Scintillator

a b s t r a c t Recent study revealed that single crystal Ce:Gd3(Al,Ga)5O12 (Ce:GAGG) showed good scintillation response under c-ray exposure. We discover here that ceramic Ce:GAGG scintillator exhibited better performance than the single crystal counterpart. We developed Ce 1% doped ceramic and single crystal GAGG scintillators with 1 mm thick and compared their properties. In radioluminescence spectra, they showed intense emission peaking at 530 nm due to Ce3+ 5d–4f transition. The 137Cs c-ray induced light yields of ceramic and single crystal resulted 70 000 ph/MeV and 46 000 ph/MeV with primary decay times of 165 and 143 ns, respectively. At present, the observed light yield was the brightest in oxide scintillators. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Inorganic scintillators which convert high energy ionizing radiation to thousands of visible photons, have been playing a major role in many fields of radiation detection, including medical imaging, security, astrophysics [1], and geophysical and resources exploration, e.g. oil-dwelling [2]. In these applications, scintillators for c-ray detectors have attracted much attention especially for medical and security applications. Up to now, most scintillation detectors have consisted of single crystals mainly due to their high optical quality. However, thanks to developments of the manufacturing technique of sintering crystalline nano-micrograins into a bulk optical ceramic with a high optical quality, we have a new way to develop ceramic inorganic scintillators. In 1990s, (Y,Gd)2O3:Eu,Pr and Gd2O2S:Pr,Ce,F appeared competitive with single crystal systems in X-ray CTs where scintillation response below a few milliseconds was acceptable for the image reconstruction techniques [3,4]. Around the same time, the fast, Ce3+ activated transparent ceramic Y3Al5O12 (Ce:YAG) scintillator was introduced [5,6]. Its light yield (LY), however, was inferior to single crystals due to trapping phenomena at grain surfaces and interfaces [7]. Following these studies, we succeeded to develop ceramic Ce:YAG scintillator which showed higher LY than that of single crystalline counterpart [8,9]. In Ce:YAG ceramic, the superior scintillation performance would ⇑ Corresponding author. Tel./fax: +81 93 695 6049. E-mail address: [email protected] (T. Yanagida). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.07.002

be owed to the suppression of anti-site defects consisting in retrapping of electron on the anti-site levels in the forbidden gap [7]. In single crystals, anti-site defects caused a slow luminescence and it was not accumulated in pulse height measurements. Then, we improved its chemical composition to Ce:(Gd–Y)3Al5O12 for higher c-ray stopping power [10]. Few years ago, simultaneously Gd and Ga admixed YAG:Ce scintillator ceramics (Ce:GYGAG) has been reported with the excellent scintillation performance [11]. Very recently, fully Gd-substituted Gd3(Al,Ga)5O12:Ce (Ce:GAGG) crystal scintillator was developed by Furukawa [12–14] and Ce:GAGG single crystal exhibited the brightest light yields of 46 000 ph/MeV [14] among oxide crystalline scintillators. In this work, we developed ceramic Ce:GAGG scintillator because sometimes ceramic scintillators exceeded single crystal counterparts in the light yield. Up to now, we introduced some ceramic scintillators [8,9,15–22] and among them, Ce:YAG [9], Ce:LuAG [21] and Pr:LuAG [22] exhibited higher scintillation performance than single crystal counterparts. Although Ce:GAGG ceramic has been evaluated mainly for structure analysis with different chemical compositions [23], optical properties and scintillation responses based on pulse height analysis (photon counting type) has not been reported yet. Therefore, photoluminescence (PL), PL decay time, X-ray induced radioluminescence, pulse height analysis, and X-ray induced wavelength resolved scintillation decay time were systematically studied and compared with the single crystal Ce:GAGG through this work.

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2. Experimental 2.1. Sample preparation The single crystal Ce:GAGG was fabricated by the conventional Czochralski (CZ) method in Furukawa [13]. On the other hand, the ceramic counterpart was fabricated by the vacuum sintering with high pressure treatment in Konoshima Chemical. The fabrication method for GAGG ceramic was basically same with that of Nd:YAG ceramic [24]. The only difference between the fabrication ways of ceramic YAG and GAGG was a sintering temperature. The sintering temperature of GAGG was lower (1500 °C) than that of YAG (1700 °C). In the ceramic sample, X-ray diffraction (XRD) analysis was examined for phase analysis by using RINT-2000 (Rigaku corporation). 2.2. Optical characterization PL emission map was evaluated by using Quantaurus-QY (Hamamatsu). This instrument enables us to evaluate absolute PL quantum yield (e.g., [25]). The monitoring excitation and emission wavelength ranges in the emission map were from 250 to 500 and from 200 to 950 nm, respectively. The wavelength step was 10 nm. The absolute quantum yield (QY) was calculated by the following equation, QY = Nemit/Nabsorb where Nemit and Nabsorb were number of emission and absorption photons, respectively. We used Quantaurus-s that had a timing resolution of 100 ps for PL decay time investigation (e.g., [26]). Based on PL and radioluminescence results described later, emission wavelengths at 380, 415, and 525 nm under 280 nm excitation were monitored. 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 and 1 mA. The emission spectra were measured using Andor DU-420-BU2 CCD spectrometer. Its CCD-based detector (cooled down to 188 K by a Peltier module) was 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 exposure of CCD. In order to cut background radiation, the detector was surrounded by 5 cm thick Pb blocks. A schematic drawing of the experimental setup is shown in Fig. 1(a). In 137Cs pulse height measurements, the samples were wrapped with several layers of Teflon tape and were coupled to the UV-enhanced APD (Hamamatsu) with optical grease (OKEN 6262A). The APD and the sample were mounted in the 5 mm thick Al case

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equipped with BNC connector. Basic properties of Si-APD itself were investigated in past reports [27,28]. The Al case was connected with preamplifier (CP581K), shaping amplifier (CP4417) with 2 ls shaping time, and multichannel analyzer (Amptek Pocket MCA), in order. To evaluate the absolute light yield, 5.9 keV X-rays from 55Fe radioisotope were directly irradiated to Si-APD. When 5.9 keV X-rays were detected by APD, its photoabsorption peak corresponded to 1640 e–h pairs and we could calibrate MCA channel number to electron numbers. Because APD multiplication gain is sensitive to the environmental temperature, the temperature was controlled to 20 °C by the constant temperature reservoir (Espec, SH661) with 5–8 cm thick wall. Though 55Fe was put into the Al case, the 137Cs radiation was coming from outside of the reservoir so that the photofraction was not so high compared with ideal setups where the scintillator and the radioisotope were close. Schematic drawing of the experimental set up for pulse height measurement is presented in Fig. 1(b). The scintillation decay times were measured by our original setup, pulse X-ray streak camera system [29–32] 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 wavelengths of ceramic GAGG were same with PL decay time, 380 ± 15, 415 ± 15, and 525 ± 15 nm based on radioluminescence spectrum discussed later.

3. Results and discussion 3.1. Sample Fig. 2 represents cut and polished ceramic and single crystal Ce 1% doped GAGG scintillators. The sizes of the ceramic and single crystal were 5 mmu  2 mmt and 5  5  2 mm3, respectively. The color of them was yellowish as same as Ce:YAG. Although the ceramic sample was opaque, diffusive transmittance using an integrated sphere was around 50% and it was similar to that of visibly opaque ceramic Gd2O2S:Pr,Ce,F. On the other hand, straight line transmittance of the single crystal was around 80% at wavelength longer than 500 nm. In XRD analysis, the diffraction pattern of the ceramic GAGG showed both garnet and perovskite phases. When we compared XRD intensities of primary peaks of these two phases, the ratio of garnet and perovskite phases was roughly

(a)

(b)

Fig. 1. Experimental setups for X-ray induced radioluminescence (a) and pulse height analysis (b).

Fig. 2. Appearances of single crystal (top left) and ceramic (top right) Ce 1% doped GAGG scintillators. The bottom panel shows their emission under UV (365 nm) excitation.

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9 to 1. Although XRD intensity was not a quantitative value, the existence of the perovskite phase with significant amount was confirmed. 3.2. Optical properties Fig. 3 represents PL emission maps of ceramic and crystal Ce:GAGG. Their main emission appeared at 525 nm due to Ce3+ 5d–4f transitions and the observed peak well coincided with previous results [11–14]. Excitation bands for this 525 nm emission were 450, 340, and 250 nm corresponding to 5d1–3 levels. These excitation bands were also typical for Ce3+ doped garnet materials. However in ceramic sample, relatively strong emission at 380 nm under 300 nm excitation and weak emission at 415 nm under 280 nm excitation were also detected. QY values of ceramic and crystal Ce:GAGG were similar levels, 89.3% and 89.1%, respectively. PL decay time profiles are shown in Fig. 4. In the 525 nm emission, both ceramic and crystal Ce:GAGG exhibited typical decay curves of Ce-doped garnet materials, fast 90 ns and slow 194 ns decay times. 380 and 415 nm luminescence of the ceramic sample resulted a very fast few ns and relatively slow 120 ns decay times. In this PL-based observation, the origin of these two emission bands will be considered similar. 3.3. Scintillation properties Fig. 5 exhibits X-ray induced radioluminescence spectra of both samples. Ce3+ 5d–4f transition luminescence appeared from 500 to 700 nm. The shape of emission spectra quite resembled to those of other Ce-doped garnet material. In this primary emission, differences between the ceramic and the crystal were not so high. However in the ceramic, weak emission peaks clearly appeared at 380 and 415 nm as same as the PL emission map. Unlike other garnet scintillators, antisite defect emission around 260–300 nm was not observed. Fig. 6 demonstrates pulse height spectra of ceramic and single crystal Ce:GAGG under 137Cs exposure. Clear photoabsorption peaks were detected in both samples. Based on 55Fe 5.9 keV

Fig. 3. PL emission maps of ceramic (top) and crystal (bottom) Ce:GAGG. The horizontal axis and the vertical axis are emission and excitation wavelengths, respectively.

Fig. 4. From top to bottom, PL decay time profiles of single crystal (em = 525 nm), ceramic (em = 525 nm), ceramic (em = 415 nm), and ceramic (em = 380 nm) under 280 nm excitation. Dotted line is an instrumental response (blue) and a fitting curve (red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Wavelength (nm) Fig. 5. X-ray induced radioluminescence spectra of ceramic and single crystal Ce:GAGG scintillators.

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enhancement. Though we did not evaluate thicker ceramic sample due to a technical difficulty in the fabrication, crystal with different thickness (5 mm) prepared by Furukawa was also evaluated. The observed light yield of the 1 mm thick one was 10% higher than that of the 5 mm thick sample and the sample thickness also affected the scintillation light yield. The energy migration relating to the impurity phase will be discussed later based on scintillation decay times. This high light yield is a big advantage for imaging scintillation detectors because most of them apply the charge center of gravity method to obtain two dimensional images. It is well known that the spatial resolution of this method is highly affected by the light yield of scintillators. In scintillatiors, there is an empirical relation between the number of generated electron–hole pairs in scintillation ne–h and the band gap Eg as ne–h = Ec/bEg, where Eg means the energy of c-quanta [33]. This relation was proposed in 1980–1990s [34–36] and well established in 2000s based on experimental results of many materials [33]. Thus the light yield (LY) is simply expressed as LY = 106SQ/ bEg [ph/MeV] where S is the energy transport efficiency from the host to the emission center, Q is the luminescence quantum efficiency at the emission center, and empirically b is a constant close to 2.5 [33] in famous (bright) scintillators. In previous reports, the theoretical limit assuming no loss of optical phonons was reported

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MCA channel Fig. 6. 137Cs induced pulse height spectra of ceramic and single crystal Ce:GAGG scintillators. 55Fe 5.9 keV X-ray peak is also plotted.

X-ray peak plotted in the same figure and taking into consideration the quantum efficiency of the used APD of 80% at 525 nm, absolute light yields of ceramic and single crystal Ce:GAGG were determined as 70 000 ph/MeV and 46 000 ph/MeV, respectively. The light yield of the single crystal was same with the previous report [12] and the ceramic counterpart highly exceeded the single crystal. In previously introduced ceramic Ce:GYGAG, the light yield was around 50 000 ph/MeV [13] and the present ceramic Ce:GAGG was the brightest oxide scintillator by far. The enhancement of the light yield will be ascribed to some complicated reasons as follows, (i) difference of the actual Ce concentration, (ii) light scattering near the detection layers of APD, and (iii) an energy migration due to the impurity phase discussed later. Based on the previous work by Furukawa [12], actual Ce concentration of nominally Ce 1% doped GAGG single crystal was around 0.3–0.4%. On the other hand, Ce concentration of the ceramic was close to 1% because it was fabricated by the solid state reaction. Therefore higher Ce concentration will also affect the light yield enhancement. In addition, the light scattering on ceramic grains and boundaries could cause the observed light yield enhancement that the light is effectively collected only from a layer near the PD, which is much thinner than the sample itself. Generally, a scintillator thickness affects the scintillation light yield; the thinner the sample, the higher the light yield. Connection with the light scattering above mentioned, the thickness could affect the light yield

Fig. 7. From top to bottom, scintillation decay time profiles of single crystal (wavelength was 525 ± 15 nm), ceramic (wavelength was 525 ± 15 nm), ceramic (wavelength was 415 ± 15 nm), and ceramic (wavelength was 380 ± 15 nm). Dotted lines are fitting functions.

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2.3 [34,35]. Through this work, we have already known LY and Q so that we could calculate the energy transportation efficiency S. The bandgap of GAGG was not experimentally determined and we assumed a typical bandgap of Ga-substituted garnet materials 6 eV. Thus, S of the ceramic and crystal Ce:GAGG were deduced to 1.18 and 0.78, respectively. If we assume no loss of optical phonon (b = 2.3), S were 1.08 and 0.72. Although the simple assumption of above formula predicts S < 1, we must note that the calculated value of Scera = 1.18 (or 1.08) still has large uncertainties in Eg, and LY due to few% systematic error in optical attachment and readout of the quantum efficiency of the photodetector. Otherwise it may simply suggest that the limit of b is higher. The important thing in this calculation is that the energy transportation efficiency of ceramic Ce:GAGG highly exceeds the single crystal. The wavelength resolved decay time profiles of single crystal and ceramic are demonstrated in Fig. 7. Decay times due to Ce3+ 5d–4f transition at 525 nm were similar profiles in both scintillators. The primary decay times of single crystal and ceramic Ce:GAGG were 143 and 165 ns, respectively. Though slower component was also observed, physically significant values were not obtained due to a large uncertainty. Then in ceramic, weak emission peaks at 380 and 415 nm were investigated. They both consisted of two decay time components: one was fast and the other was slow. The fast component at 380 and 415 nm turned out to be 3.4 and 3.5 ns, respectively. In the present evaluation, the slower component of 415 nm emission could not be determined by a large uncertainty due to low emissivity and that of 380 nm one was similar with PL,120 ns. Based on XRD, PL emission maps, PL decay times, and scintillation decay times, the origin of the weak emission in the ceramic is attributed to the perovskite phase because sometimes the perovskite phase appears in garnet materials. In previous study, optical and scintillation properties of Ce-doped GdAlO3 (Ce:GAP) were reported [37–39]. In those report, the emission peak appeared around 350 nm with wide peak width from 300 to 400 nm [37– 39] and decay time components of Ce:GAP were fast 5.75 ns and moderate several tens ns [37]. The evaluations for decay times were also reported as 30 and 180 ns in the other work [38]. In addition similar decay times with present results, absorption bands around 280–300 nm of the ceramic Ce:GAGG well coincided with the excitation bands of Ce:GAP [39]. The most recent study about Ce-doped (Gdx,Y1 x)AP (0 < x < 1) crystals revealed the emission peak in relatively longer wavelength (350–400 nm) [40] than previous reports [37–39]. This variability will be blamed for the difficulty in the fabrication of GAP. The observed emission spectra can be interpreted as following. There are some absorption bands of Ce:GAGG at 340 and 450 nm and most emission from the perovskite phase is self-absorbed. Thus the rest (non-absorbed) parts of the emission from the perovskite phase appeared at 380 and 415 nm. This overlapping of the emission from the perovskite and the absorption of the garnet will cause an energy migration and contribute to high light yield in Ce:GAGG ceramic. The slight delay of the scintillation decay in the ceramic with respect to the single crystal will be the evidence of energy migration from the perovskite to the garnet generally because higher Ce concentration makes scintillation decay fast. In addition, the large transportation efficiency (S) of the ceramic may be also affected by the impurity phase because above mentioned formula assumed the energy migration from one type host to one type emission center. Although the PL QYs of both samples were same, the scintillation efficiency differed largely possibly due to the existence of the impurity phase as well as higher Ce concentration than the crystal, the scattering effect, and the sample thickness mentioned above. The present result gives us a new knowledge that the existence of impurity phases is not necessarily negative in scintillator materials.

4. Conclusion Concluding ceramic and single crystal Ce 1% doped GAGG scintillators were prepared by the vacuum sintering and the Cz method, respectively. The PL QYs of them were same values of 89% with typical PL decay times of 90 ns and 194 ns. In 1 mm thick samples, they exhibited high scintillation light yields of 46 000 (single crystal) and 70 000 ph/MeV (ceramic) with main decay times of 165 and 143 ns, respectively. The existence of the perovskite phase contributes to the enhancement of the light yield in the ceramic sample as well as higher Ce concentration than the crystal, the scattering effect, and the sample thickness. At present, ceramic Ce:GAGG is the top brightest oxide scintillator. Acknowledgments This work was mainly supported by JST Sentan, A-step and partially by a Grant in Aid for Young Scientists (A)-23686135, and Challenging Exploratory Research-23656584 from the Ministry of 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] 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. [2] C.L. Melcher, Nucl. Instrum. Methods B (40/41) (1989) 1214. [3] C. Greskovich, S. Duclos, Annu. Rev. Mater. Sci. 27 (1997) 69. [4] S.J. Duclos, C.D. Greskovich, R.J. Lyons, J.S. Vartuli, D.M. Hoffman, R.J. Riedner, M.J. Lynch, Nucl. Instrum. Methods Phys. Res. A 505 (2003) 68. [5] E. Zych, C. Brecher, A.J. Wojtowicz, H. Lingertat, J. Lumin. 75 (1997) 193. [6] E. Zych, C. Brecher, J. Glodo, J. Phys. Condens. Matter 12 (2000) 1947. [7] E. Mihóková, M. Nikl, J.A. Mareš, A. Beitlerová, A. Vedda, K. Nejezchleb, K. Blazˇek, C. D’Ambrosio, J. Lumin. 126 (2007) 77. [8] T. Yanagida, H. Takahashi, T. Ito, D. Kasama, M. Kokubun, K. Makishima, T. Yanagitani, H. Yagi, T. Shigeta, T. ITO, in: IEEE. Nucl. Trans. Sci. 51 (2005) 1836– 1841. [9] 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– 2408. [10] T. Yanagida, T. Ito, H. Takahashi, M. Sato, T. Enoto, M. Kokubun, K. Makishima, T. Yanagitani, H. Yagi, T. Shigeta, T. ITO, in: Nucl. Instrum. Meth. A 579 (2007) 23. [11] N.J. Cherepy, S.A. Payne, B.W. Sturm, J.D. Kuntz, Z.M. Seeley, B.L. Rupert, R.D. Sanner, O.B. Drury, T.A. Hurst, S.E. Fisher, M. Groza, L. Matei, A. Burger, R. Hawrami, K.S. Shah, L.A. Boatner, IEEE Nuc. Sci. Symp. Conf. Record (2010) 1288–1291. [12] K. Kamada, T. Yanagida, J. Pejchal, M. Nikl, T. Endo, K. Tsutsumi, Y. Fujimoto, A. Fukabori, A. Yoshikawa, IEEE Trans. Nucl. Sci. 59 (2012) 2112–2115. [13] K. Kamada, T. Yanagida, T. Endo, K. Tsutumi, Y. Usuki, M. Nikl, Y. Fujimoto, A. Fukabori, A. Yoshikawa, J. Cryst. Growth 352 (2012) 88–90. [14] K. Kamada, K. Tsutsumi, T. Endo, T. Yanagida, Y. Fujimoto, A. Fukabori, A. Yoshikawa, J. Pejchal, M. Nikl, Cryst. Growth Des. 11 (2011) 4484–4490. [15] Y. Fujimoto, T. Yanagida, S. Wakahara, H. Yagi, T. Yanagidani, S. Kurosawa, A. Yoshikawa, Opt. Mater. 35 (2013) 778–781. [16] T. Yanagida, Y. Fujimoto, H. Yagi, T. Yanagitani, M. Sugiyama, A. Yamaji, M. Nikl, Opt. Mater. 35 (2013) 788–792. [17] T. Yanagida, K. Kamada, Y. Fujimoto, Y. Yokota, A. Yoshikawa, H. Yagi, T. Yanagitani, Nucl. Instrum. Methods A 631 (2011) 54–57. [18] T. Yanagida, Y. Fujimoto, Y. Yokota, A. Yoshikawa, S. Kuretake, Y. Kintaka, N. Tanaka, K. Kageyama, V. Chani, Opt. Mater. 34 (2011) 414–418. [19] Y. Fujimoto, T. Yanagida, Y. Yokota, A. Ikesue, A. Yoshikawa, Opt. Mater. 34 (2011) 448–451. [20] 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.

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