Growth of Tm3+-Doped Y2O3, Sc2O3, and Lu2O3 Crystals by the

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Growth of Tm3þ-Doped Y2O3, Sc2O3, and Lu2O3 Crystals by the Micropulling down Technique and Their Optical and Scintillation Characteristics Akihiro Fukabori,*,† Valery Chani,† Kei Kamada,‡ Federico Moretti,§ and Akira Yoshikawa†,# †

IMRAM, Tohoku University, 2-1-1, Katahira, Aoba-ku, Sendai, Miyagi, 980-0812, Japan Furukawa Co., Ltd., 1-25-13, Kannondai, Tsukuba, Ibaraki, 305-0856, Japan § Dipartimento di Scienza dei Materiali, Universita di Milano-Bicocca, via Cozzi 53, 20125 Milano, Italy # NICHe, Tohoku University, 6-6-10 Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan ‡

ABSTRACT: Single -crystal growth of Tm3þ-doped Y2O3, Sc2O3, and Lu2O3 sesquioxides, with a melting point around 2400 °C, by the micropulling down technique is reported. Crystal quality and composition of the crystals were evaluated by rocking curve and energy-dispersive X-ray (EDX) analyses, respectively. Optical properties including optical transmittance and photoluminescence decay profiles were also examined. Furthermore, scintillation performance of the crystals including pulse height spectra and radioluminescence spectra under R-ray and γ-ray excitation was evaluated.

1. INTRODUCTION Scintillators emit light in visible and ultraviolet regions of electromagnetic spectra by converting ionizing radiation (for example, R-ray, γ-ray, X-ray, or neutrons) into a number of photons. This physical property is widely used in various technical fields including nuclear medicine (positron emission tomography, X-ray computed tomography, and single photon emission computed tomography), high energy physics, geophysical and resource exploration, nuclear energy, security fields, etc. Most of the existing scintillating bulk single crystals are grown from the melt by Czochralski (CZ) and Bridgman methods. Many kinds of scintillating materials including oxides, fluorides, bromides, iodides, and chlorides are widely studied in terms of both practical applications and academic research. The required scintillating properties are mostly high light yield (LY), short decay time, high density, and high effective atomic number (except for neutron detection). The growth of Y2O3, Sc2O3, and Lu2O3 single crystals is typically performed by various methods. These include CZ,1,2 heat exchanger method (HEM),3 Bridgman method,4 laser heated pedestal growth (LHPG),5,6 Verneuil process,7 floating zone (FZ) method,8 and flux method.9 Recently, growth of these crystals with the micropulling-down (μ-PD) method1013 was also reported. In spite of variety of the growth techniques applicable to these crystals, the main problem of further development of these materials is related to their high melting temperatures (2430 °C for Y2O3, 2430 °C for Sc2O3, 2450 °C for Lu2O32,3). Therefore, the scintillation properties of these sesquioxide crystals are not studied in sufficient degree. Considering scintillator applications, there are two general emission mechanisms that determine performance of the materials. One is intrinsic emission derived from properties of the host material itself like self-trapped exciton (STE), and the other one is extrinsic emission derived from presence of the dopants in the host matrix. In this communication, extrinsic emission originated from Tm3þ ion is discussed. r 2011 American Chemical Society

There are three reasons why Tm3þ ion was selected as a dopant in this study. First, as it was mentioned above, one of the required properties is fast decay. Therefore, Ce3þ- and Pr3þdoped crystals were intensively examined as fast response scintillators. These dopants have very fast 5d4f transition within 100 ns. However, Tm3þ ion possess multiple 4f4f spin forbidden transition with slow decay ranged from several microseconds to several hundreds microseconds14,15 at wavelengths of 300900 nm.16 Therefore, reports regarding slow decay emission originated from 4f-4f spin forbidden transition was not sufficient in the past, and this motivated us to study scintillation properties of Tm3þ-doped Y2O3, Sc2O3, and Lu2O3 single crystals in current project. Second, Tm3þ ion may be considered as appropriate dopant for scintillators because of possession of many 4f4f transitions16 in blue wavelength region. This region has a good match with high sensitivity of photo multiplier tube (PMT) in the blue and ultraviolet wavelength ranges (250 nm-550 nm), even if these transitions are spin forbidden 4f4f transitions with slow response. Third reason was based on the suggestion that Tm-doped Y2O3, Sc2O3, and Lu2O3 single crystals should demonstrate better chemical uniformity (more uniform dopant distribution) than same host crystals doped Pr3þ or Ce3þ. In most cases of the doped crystal growth, the segregation coefficient of the dopant is not equal to unity, K 6¼ 1. As a result, (1) nominal dopant concentration (in the melt) is different from actual dopant concentration (in the crystal), and (2) spatial distribution of the dopant in the crystal is not homogeneous. Therefore, substitution of the matrix host cations with a dopant (guest) that have segregation coefficient K≈1 is highly preferable. Received: February 1, 2011 Revised: March 25, 2011 Published: April 01, 2011 2404

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Crystal Growth & Design

Figure 1. Schematic diagram of the micropulling down method.

Generally, K ≈ 1 means when ionic radius of the dopant is similar to ionic radius of the host. Considering the crystals discussed here, the difference between Tm3þ guest cation radius (1.04 Å) and host cation radii of Y3þ (1.06 Å), Sc3þ (0.83 Å), or Lu3þ (0.99 Å) is significantly less than the difference between the same host cations and Pr3þ (1.16 Å) or Ce3þ (1.18 Å) guest cations, which possess 4f5d transition with fast response. Therefore, uniformity of the Tm3þ-doped crystals was expected to be better than uniformity of the same host crystals doped with Pr3þ or Ce3þ. As a result, the scintillation performance of the crystals including energy resolution was supposed to be better. The main objectives of this report were to demonstrate growth of Tm-doped sesquioxide single crystals by μ-PD method using special hot zone setup and to illustrate that currently the μ-PD process is one of the most suitable crystal growth techniques applicable to sesquioxides. Furthermore, scintillation properties of as grown Tm3þ-doped rare-earth sesquioxide crystals are discussed based on experimental characterization. The results on radio-luminescence under R-ray excitation, light yields under R-ray and γ-ray excitation, and photo luminescence decay kinetics are presented in some details.

2. EXPERIMENTAL SECTION 2.1. Crystal Growth. Y2O3, Sc2O3, and Lu2O3 single crystals were grown by the μ-PD method. The growth was performed using rhenium (Re) crucible and ZrO2 ceramic for thermal insulation (Figure 1). The melting point of Re is about 3200 °C, and it demonstrates excellent mechanical properties at the melting temperature of the sesquioxides discussed here. Moreover, Re does not react chemically with these compounds at growth temperature. However, Re is easily oxidized at high temperature. Therefore, the growth was performed in reducing atmosphere (3% H2 in Ar flow) to prevent the crucible from oxidization. In the growth process, the crucible was thermally isolated by two sets of ZrO2 ceramic tubes both covered with ZrO2 lids. This set up was sufficient to keep appropriate thermal insulation of the hot zone and allowed adjustment of the suitable temperature gradients around the crucible and especially around the die. The heating was performed using a radio frequency coil. Y2O3 (99.9999%, Nippon Yttrium Co., Ltd.), Sc2O3 (99.999%, Metal Rare Earth Limited), Lu2O3 (99.995%, Yixing Xinwei Leeshing Rare

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Figure 2. Views of the crystals: (a) Tm (0.5 mol %):Y2O3 (1.41 mm thick), (b) Tm (0.5 mol %):Sc2O3 (1.18 mm), and (c) Tm (0.5 mol %): Lu2O3 (1.10 mm). Right images are as grown crystals and left images are cut and polished specimens. Earth Co., Ltd.), and Tm2O3 (99.99%, Nippon Yttrium Co., Ltd.) were used as starting powders. The rare-earth sesquioxides were sufficiently blended using alumina mortar and pestle. The nominal concentration of Tm3þ doping cation with respect to corresponding host cations of Y3þ, Sc3þ, or Lu3þ was always 0.5%. The Tm3þ-doped crystals were grown using blocks of corresponding undoped sesquioxides as seeds as is described in details in ref 12. The pulling-down speed was nearly 0.1 mm/min. 2.2. Crystal Quality Analysis (EDX, SEM, and XRC). The obtained crystals were subsequently cut perpendicularly to the growth direction and polished mechanically. The diameter of the characterization-ready disk shaped specimens was about 5.0 mm, and the thickness was 1.41, 1.18, and 1.10 mm (for Tm3þ-doped Y2O3, Sc2O3, and Lu2O3, respectively) as it is illustrated in Figure 2, right. The inspection of the crystals regarding their surface and inner observation, and chemical composition was performed with scanning electron microscope (SEM, Hitachi S-3400N) and energy dispersive X-ray analysis (EDX, Horiba EMAX, X-act) using secondary electron mode at high vacuum. Both the surface of the as-grown crystals and inner parts were examined. To evaluate crystal quality, we measured X-ray rocking curves (XRC) using RIGAKU ATX-E. After CuKR X-ray accelerated by 40 kV and 30 mA was irradiated to the specimens, the 222 reflections were measured and then crystal quality was estimated using the full width at half-maximum (fwhm). 2.3. Optical Transmittance and Radio-Luminescence. Optical transmittance was measured at wavelengths ranged from 200 to 900 nm with UV/Visible spectrophotometer (JASCO V550). Radioluminescence (RL) spectra were studied using a FLS920 spectrofluorometer (Edinburgh Instruments) in the 250550 nm wavelength range. The signals were recorded by attaching the investigated specimens to the surface of the radioactive 241Am source (E = 5.5 MeV, radioactivity = 4 MBq). In this measurement, the excitation part of the FLS920 spectrofluorometer including Xe lamp as excitation source was not used, and only detection part with detector and grating was operated. When spectrum correction was performed, noise in shorter and longer wavelength region raised up in RL spectra. In this case, the emission peaks could not be distinguished clearly enough. Therefore, spectral correction was not applied in these measurements.

2.4. Radiation Response and Photoluminescence Decay Time. To estimate light yield, the pulse height spectra under R-ray and

γ-ray excitation were measured with a bias photomultiplier tube (PMT, Hamamatsu photonics R7600) under accelerating voltage of 700 V. The

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Figure 3. EDX data for (a) surface of Tm: Sc2O3 single crystal (inset image is surface) and (b) inner part of Tm: Sc2O3 single crystal (inset image is inner part). samples were optically connected to the PMT window using an optical grease (OKEN6262A). The PMT output signal was transmitted to a preamplifier (ORTEC model 113), a shaping amplifier (ORTEC 572A), a multichannel analyzer (Amptek pocket MCA), and finally to a personal computer. Shaping time of the shaping amplifier was selected to be 10 μs. Photoluminescence (PL) decay profiles were measured by a FLS920 spectrofluorometer (Edinburgh Instruments) using a hydrogen thyratron nanosecond flashlamp as excitation source. According to the manual of this equipment, pulse width of instrumental response function in H2 filler gas is approximately 5 ns. But, this instrumental response function can vary the operating conditions.

3. RESULTS AND DISCUSSION 3.1. Crystal Growth. The as grown crystals are shown in Figure 2, left. All the crystals were over 8 mm long, and the diameter of the crystals was approximately 5 mm. Crystallization ratios were 7098% depending on process. The growth was performed until all melt from the crucible was consumed. Some part of the melt was lost due to evaporation that depended on growth conditions including total length of the process, melt temperature and its overheating, etc. The initial charge of the crucible was 1.52.7 g depending on crystal composition. The shape of the crystals was reasonably well controlled by the shape of the bottom-attached die. The inner parts of the crystals demonstrated high transparency (Figure 2, right). However, the surface of the as grown crystals was dark-gray or black. This

was result of deposition of Re particles originated from the crucible material and oxidized during the growth run. Most probably, the content of hydrogen in the Arþ3%H2 growth atmosphere was not sufficient to prevent Re crucible from oxidation completely. 3% H2 concentration in the growth experiments described here was selected considering the explosion limit of hydrogen in air that is 4%. Several solutions could be proposed to suppress the surface coloration at least partially. One of them is to increase ArþH2 3% gas flow through the growth chamber. This way, Re is forced out of the chamber before depositing on the grown crystals. Another method is to increase pulling down rate to remove the crystal from the vicinity of the crucible as fast as possible before Re attaches the crystal surface. Finally, increasing of H2 content in the growth atmosphere should certainly help, however this decreases safety of environment. The Re oxidation also reduces lifetime of the crucible; within 2 g of Re was lost after every growth that corresponded to approximately 1% weight loss per growth cycle. 3.2. Crystal Contaminations. To confirm whether Re was incorporated into the crystal or not, EDX and SEM measurements were performed. Figure 3 demonstrates EDX data and SEM image for (a) surface and (b) interior part of the Tm:Sc2O3 single crystal. According to comparison of these data, Re was only detected on the surface of the crystals. Detection limit of EDX is 0.31%. Some signal in inner part looks like Re peaks, but most probably this is noise. Of course, some negligible amount of Re was incorporated into the crystal (probably in ppm order of 2406



Figure 4. Rocking curve data for (a) Tm (0.5 mol %):Y2O3, (b) Tm (0.5 mol %):Sc2O3, and (c) Tm (0.5 mol %):Lu2O3. Crystal quality was evaluated using fwhm of 222 reflection peaks.

magnitude), but quantitative determination of Re content in the inner parts of the crystals with available techniques is currently impossible. In order to confirm whether Re was incorporated into the inner parts, the EPMA measurement that have more precise detection limit than that of EDX was performed. As a result, presence of Re was not detected. This indicated that Re did not affect the results of evaluation of optical and scintillation properties discussed below. Furthermore, various elements including Ca, Al, and C (except Re, Sc, and O originated from the

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crucible and melt materials) were deposited on the crystal surface. Most probably, source of these elements is ZrO2 ceramic used for thermal insulation (Figure 1). According to SEM, the surface of the crystals was rough and coarse-grained as a result of deposition of foreign substances. Similarly, no Re was detected in the inner parts of the Tm:Y2O3 and Tm:Lu2O3 crystals. 3.3. Crystal Quality: X-ray Rocking Curve Analysis. The results of XRC characterization according to Section 2.2 are presented in Figure 4. The measured fwhm were 90 arcsec, 100 arcsec, and 217 arcsec for Tm:Y2O3, Tm:Sc2O3, and Tm:Lu2O3 crystals, respectively. Considering the crystal quality of the commercially available crystals grown by the CZ method (approximately 100 arcsec), the quality of the μ-PD crystals discussed here was sufficiently high except Tm:Lu2O3. It is assumed that following improvement of the structural quality can be achieved through reduction of the solidification (pullingdown) rate, even though a rate of oxidation of Re increase during the growth. Also, increasing of the meniscus thickness may result better ordering of the atoms transferred from the melt to the crystal. It is noted that in the experiments discussed here, the meniscus thickness (height) was estimated to be approximately 100200 μm only. In our experiments, the meniscus height was inspected with high resolution CCD camera focused directly to the crystal growth interface and LCD monitor. The accuracy of the measurement was about (50 μm. We note that the meniscus height for the growth of these crystals was not optimized, but it was kept low enough to stay away from overheating of the melt to decrease its evaporation and to avoid melt overflow. 3.4. Industrial Production: μ-PD vs CZ and Other Methods. The relatively high structural perfection of the above rare-earth sesquioxide crystals was sufficient factor allowing comparison of the μ-PD technique with other crystal growth methods from the point of view of commercialization of these materials. As it was mentioned above, the quality of the μ-PD crystals was comparable to those grown by the CZ method except Tm:Lu2O3. Furthermore, the μ-PD hot zone setup illustrated in Figure 1 is relatively simple when compared to that practiced for the CZ method. As a result, even the current status of development of the μ-PD technology allows production of these crystals in semiindustrial scale. According to previous reports,3,4 the growth of these sesquioxide crystals by the CZ technique is difficult due to unstable temperature gradients. Therefore, the dimensions of the as grown crystals were several millimeters only. Furthermore, every CZ growth run takes approximately one week, including time necessary for the removal of residual material from the crucible. By contrast, the time necessary for the growth of 12 mm long crystal using μ-PD method is approximately two hours. As for the crucible cleaning, the amount of residual material in the μ-PD crucible was negligible because of the growth system design, effect of gravity, and wetting properties of the melt. This way, concerning the materials discussed here, the μ-PD method is excellent crystal growth technique in terms of both high speed growth (and total time consumption including crucible cleaning) and size compared to CZ method. Another representative method applicable to the growth of rare-earth sesquioxide crystals is HEM,3 and it allows preparation of relatively large crystals exceeding 30 mm.3 The crystals of similar dimensions (35 mm) can be also produced by the Bridgman method.4 Considering the crystal size, the μ-PD process in its current design is certainly not a winner in competition with other methods. However, compared to HEM 2407



Figure 5. Optical transmittance of Tm (0.5 mol %):Y2O3, Tm (0.5 mol %): Sc2O3, and Tm (0.5 mol %):Lu2O3 single crystals.

Figure 6. Radio-luminescence spectra of Tm (0.5 mol %):Y2O3, Tm (0.5 mol %):Sc2O3, and Tm (0.5 mol %):Lu2O3 recorded under R-ray excitation. All peaks were assigned to be 4f4f spin forbidden transition of Tm3þ ion and * denotes intrinsic emission.

and Bridgman methods, the advantage of the μ-PD method is its ability to produce crystals with well controlled shape. This conclusion was made based on examination of the shapes of the crystals grown by these two alternative techniques and reported in.3,4 Additional shortcoming of the HEM system is related to separation of the as grown crystals from the Re frame. As a result, the HEM crystals had number of cracks3 produced during the crystal separation from the Re flame. Other methods including LHPG,5,6 Verneuil,7 FZ,8 and flux growth9 have limitations considering both the crystals dimensions and the crystals quality. Summarizing above comparisons, it is concluded that μ-PD method have a chance to be first one employed for commercial mass production of rare-earth sesquioxide single crystals. 3. 5. Optical Transmittance and Radio-Luminescence. Figure 5 illustrates the optical transmittance spectra of the crystals. All the specimens demonstrated high transparency around 75% and had Tm3þ absorption peaks at wavelengths of approximately 360 nm, 470 nm, 650 nm, 680 nm, and 770 nm. These transitions are assigned to 3H6f1D2, 3H6f1G4, 3 H6f3F2, 3H6f3F3, and 3H6f3H4 respectively.17

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Figure 7. Pulse height spectra recorded under R-ray (241Am, 5.5 MeV R-ray) excitation. The samples are Tm (0.5 mol %):Y2O3, Tm (0.5 mol %):Sc2O3, Tm (0.5 mol %):Lu2O3, and BGO as a reference.

Figure 8. Pulse height spectra of Tm (0.5 mol %):Y2O3 recorded under γ-ray (152Eu, 41.2 keV) excitation and that of Tm (0.5 mol %):Sc2O3 recorded under γ-ray (241Am, 59.5 keV) excitation.

The radio-luminescence spectra are shown in Figure 6. Several intensity peaks corresponding to presence of Tm3þ ion in the host matrixes were observed. They were assigned to 1I6f3H6 at 290 nm, 1D2f3H6 at 360 nm, 1D2f3F4 at 460 nm, and 1 G4f3H6 at 480 nm in 4f4f transition,1416 and the most intense peak was related to 1D2f3F4 transition at 460 nm. Furthermore, intrinsic emission peaks were also observed in RL spectra. The position of these peaks is indicated with single asterisk. Undoped Y2O3, Sc2O3, and Lu2O3 crystals emit at 350,12,1824 340,12,2224 and 390 nm12,25,26 wavelengths, and these intrinsic emissions are derived from self-trapped excitons (STE) or self-trapped holes (STH).12,1826 In the case of Tm: Y2O3 and Tm:Sc2O3 RL emission spectra, the intrinsic emissions were overlapped with Tm3þ emission peaks positioned in the wavelength range of 300500 nm.12,1824 Tm:Lu2O3 emitted around 390 nm, and this peak can be considered as Lu2O3 intrinsic emission12,25,26 in the paper. In the case of Lu2O3 intrinsic emission, according to refs 25 and 26, origin of the peak at 390 nm was interpreted as different origin. Therefore, we need to clarify origin concerning this emission peak, hereafter. 3.6. Light Yields and Photoluminescence Decay Time. The pulse height spectra recorded under R- and γ-ray excitations are shown in Figures 7 and 8, respectively. All peaks were fitted by a 2408



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Table 1. Light Yields under r- and X-ray Excitation, r/γ Ratio, and Photoluminescence Decay of Tm-Doped Sesqui-oxides photoluminescence material Tm(0.5 mol %):Y2O3

LY under R (241Am) excitation@RT 3000 ( 300phs/5.5 MeVR

R/γ ratio

decay @RT (μs)

950 ( 95phs/MeV (152Eu41.2 keV)

0.58

5.88

3500 ( 350phs/MeV (241Am59.5 keV)

0.23

5.74

LY under X excitation@RT

(550 ( 55phs/MeV, normalized) Tm(0.5 mol %):Sc2O3

4500 ( 450phs/5.5 MeVR (820 ( 82phs/MeV, normalized)

Tm(0.5 mol %):Lu2O3

1100 ( 110phs/5.5 MeVR

5.77

(200 ( 20phs/MeV, normalized)

single Gaussian function, thus obtaining the peaks channel. The reported light yield (LY) of Bi4Ge3O12 (BGO) of 8200 photons/ MeV27 under γ-ray excitation was used as a reference, as it is illustrated in Figure 7. The quantum efficiency (QE) of the PMT (37% at 460 nm for the Tm:Y2O3, Tm:Sc2O3, and Tm:Lu2O3, and 32% at 480 nm for the BGO) was considered to correct LY magnitudes. QE of the wavelength of 460 nm was selected for all Tm-doped sesquioxide crystals because most intense peaks were observed at this wavelength for all these materials. Comparing the peaks obtained under 5.5 MeV R-ray excitation for the Tmdoped rare-earth oxide crystals and that for BGO, the LYs under R-ray excitation of the Tm:Y2O3, Tm:Sc2O3, and Tm:Lu2O3 crystals were estimated to be about 3000 ( 300 ph/5.5 MeVR, 4500 ( 45 ph/5.5 MeVR, and 1100 ( 110 ph/5.5 MeVR, respectively. Detailed calculation using Tm:Y2O3 data was described as follows LY ¼ 8200 photons=MeV 0:662 MeVðenergy@137 CsÞ

0:32ðQE@480 nmÞ 200 ðchannelÞ 0:37ðQE@460 nmÞ 311

¼ 3000 ( 300photons=5:5 MeVR Unfortunately, when using 137Cs 662 keV γ-ray emitter to evaluate γ-ray response, every photoabsorption peaks of investigated specimen cannot be observed after irradiation. But, when low energy γ radiation source (X-ray source) such as 152 [email protected] keV (weighted average between 40.1 keV and 45.4 keV) and [email protected] keV, peaks can be observed. LY of Tm:Y2O3 after γ-ray ([email protected] keV) irradiation were estimated using same calculation as follows: LY ¼ 8200 photons=MeV

0:662 MeVðenergy@137 CsÞ 0:0412 MeVðenergy@152 EuÞ

0:32ðQE@480 nmÞ 2:58 ðchannelÞ 0:37ðQE@460 nmÞ 311

¼ 950 95 photons=MeV Similarly, LY of Tm:Sc2O3 after γ-ray ([email protected] keV) irradiation were calculated to be about 3500 ( 350 photons/MeV. According to our best knowledge, these results on radiation responses were never reported in the past. However, photo absorption peaks of only Tm-doped Lu2O3 single crystal can not be obtained even if a low-energy γ-radiation source were irradiated. Furthermore, LY under R particle excitation normalized to MeV and R/γ ratio were calculated and summarized in Table 1. There is large difference of R/γ ratio between Tm:Y2O3 and Tm:

Sc2O3. Since density for Sc2O3 (3.83 g/cm3) is less than that for Y2O3 (5.04 g/cm3), stopping power against γ-ray (X-ray) for Sc2O3 is lower than that for Y2O3. Meanwhile, all R-ray energy was absorbed to objects, and then was converted to photons. Therefore, R/γ ratio for Sc2O3 was considerably different from that for Y2O3. Generally speaking, the scintillation decay profile should be completely evaluated to make final decision about applicability of any specific material as a scintillator. Unfortunately, scintillation decay could not be measured accurately using equipment available. Instead, PL decay was characterized using 1D2f3F4 transition at 460 nm because emission at this wavelength was most intensive one. The selected excitation wavelength was 360 nm. The results are demonstrated in Figure 9. The PL decay profiles were fitted by single or double exponential function and dominant one was roughly 5.8 μs for every sample. PL decays for Tm: Y2O3 and Tm:Lu2O3 crystals had short decay components of 80 and 10 ns, which did not have physical meanings because these were only used to fit instrumental response like sharp spike. However, 80 ns decay for Tm:Y2O3 is too slow compare to excitation pulse width; therefore, this decay might be possible to originate from intrinsic emission 18. PL 1D2 level of Tm3þ ion decay times that were obtained in current project were consistent with the data reported previously.14,15,17 In practical applications, the scintillator is commonly coupled to PMT. Therefore, the wavelengths of the emission peaks should match to high sensitivity wavelength region of PMT (250550 nm) as much as possible. 1D2f3F4 transition at 460 nm and 1D2f3H6 transition at 360 nm satisfy this condition very well. Additionally, both these transitions originate from the same 1D2 excitation level. Therefore, both decays possess same decay time14,15 within 10 μs. In the radiation response measurements reported here, 10 μs shaping time was selected because peaks were the largest using 10 μs shaping time. Therefore, LY of both 1D2f3F4 transition at 460 nm and 1D2f3H6 transition at 360 nm can be sufficiently measured using 10 μs shaping time of the shaper. Nevertheless, the measured LY was not sufficiently high. Since decay of 1I6f3H6 transition at 290 nm and 1G4f3H6 transition at 480 nm possess approximately 20 and 100 μs,15 LY of these transitions was not sufficiently measured using 10 μs shaping time. Therefore, real LY might be larger than above as mentioned value. It is assumed that further improvement of the scintillation efficiency can be achieved through optimization of the Tm3þ concentration in the sesquioxide crystals and by using thicker specimens than those discussed in current report. One of the aims in this study was to understand practical capabilities of Tm3þ-doped scintillators. Therefore, the pulse height spectra of Tm3þ-doped sesquioxide crystals bombarded by low energy γ radiation source from [email protected] keV and [email protected] keV sources was measured. It is assumed that emission under 137Cs 2409



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4. CONCLUSIONS AND OUTLOOKS μ-PD is excellent technique for the growth of oxide single crystals with high melting point. Considering growth of Tm: Y2O3, Tm:Sc2O3, and Tm:Lu2O3 crystals, the μ-PD has number of advantages when compared with other crystal growth methods. RL spectra of these crystals consisted of a number of peaks that derived from 4f4f transition of Tm ion. The most intense peaks corresponded to 1D2f3F4 transition at 460 nm. The decay time of the most intense peak was estimated to be about 5.8 μs. The LYs were estimated from the pulse height spectra recorded under R-ray excitation, and the LYs values obtained were 3000 ( 300 ph/5.5 MeVR (for Tm:Y2O3), 4500 ( 45 ph/5.5 MeVR (for Tm:Sc2O3), and 1100 ( 110 ph/5.5 MeVR (Tm:Lu2O3), respectively. Similar characterization was performed under γray excitation, and the LYs values measured were about 950 ( 95 ph/MeV (for Tm:Y2O3, [email protected] keV) and 3500 ( 350 photons/MeV (for Tm:Sc2O3, [email protected] keV). Unfortunately, the photoabsorption peaks of all specimens could not be detected under 137Cs 662 keV γ-ray irradiation. It is assumed that this result can be improved when appropriate Tm concentration will be found and thicker samples than those evaluated in current report will be used for characterization. ’ AUTHOR INFORMATION Corresponding Author

*Phone:þ81-22-217-5662. Fax:þ81-22-217-5102. E-mail: [email protected].

’ ACKNOWLEDGMENT This project was supported by GCOE (Global Center of Excellence) program for Materials Integration of the Ministry of Education, Culture, Sports, Science and Technology of the Japanese Government and partially by Japan Science and Technology Agency (JST) with A-step program for their support. F. M. is grateful to CARIPLO Foundation for the financial support in the framework of the project “Energy transfer and trapping processes in nanostructured scintillator materials” (2008-2011). ’ REFERENCES

Figure 9. Photoluminescence decay profiles of (a) Tm (0.5 mol %): Y2O3, (b) Tm (0.5 mol %):Sc2O3, and (c) Tm (0.5 mol %):Lu2O3. Photoluminescence decay at 460 nm emission was recorded after 360 nm excitation. Red lines are the fitting curves, and blue lines correspond to instrumental response.

662 keV γ-ray irradiation is also achievable when appropriate amount of Tm3þ activator is incorporated into the sesquioxide host matrix. It is also possible that emission under 137Cs 662 keV γ-ray irradiation can be realized when Tm3þ activator is incorporated into other nonsesquioxide matrix. Anyway, the results reported here can be considered as a reference regarding capabilities of Tm3þ-based scintillating rare-earth sesquioxides.

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Growth of Tm3+-Doped Y2O3, Sc2O3, and Lu2O3 Crystals by the

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