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Comparative study of intrinsic luminescence in undoped transparent ceramic and single crystal garnet scintillators
Optical Materials xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Comparative study of intrinsic luminescence in undoped transparent ceramic and single crystal garnet scintillators Yutaka Fujimoto a,⇑, Takayuki Yanagida a, Hideki Yagi b, Takagimi Yanagidani b, Valery Chani c a
Kyushu Institute of Technology, 2-4, Hibikino, Wakamatsu-ku, Kitakyushu, Japan Takuma Works, Konoshima Chemical Co., Ltd., 80 Koda, Takuma, Mitoyo, Kagawa, Japan c Tohoku University, 6-6-04, Aramaki Aza Aoba Aoba-ku, Sendai, Miyagi 980-8579, Japan b
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Scintillator Transparent ceramic Garnet
a b s t r a c t Scintillation properties associated with intrinsic lattice defects of undoped Y3A5O12 (YAG) and Lu3A5O12 (LuAG) transparent ceramics and single crystals are compared. The ceramics excited with X-ray demonstrated relatively low emission intensity when compared with that of the single crystals. Decay times of the ceramics and the single crystals were similar. These parameters were approximately 430 ns (YAG ceramic), 460 ns (YAG single crystal), 30 ns and 1090 ns (LuAG ceramic), and 25 ns and 970 ns (LuAG single crystal). According to the pulse height spectra recorded under 137Cs gamma-ray irradiation, the scintillation light yield of the both ceramics were about 2950 ± 290 ph/MeV. However, the single crystals had greater kight yield of about about 14,300 ± 1430 ph/MeV for YAG and 8350 ± 830 ph/MeV for LuAG. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Various types of translucent and transparent oxide ceramics including alumina [1–3], rare-earth sesquioxides [4,5], spinels [6,7], garnets [8–13] and other compounds [14–18] have been developed for optical device applications. As an example, Nd:Y3Al5O12 laser transparent ceramics prepared by vacuum sintering technique demonstrated several advantages including relatively simplicity of their fabrication at comparatively low sintering temperature and possibility to produce large size specimens. In addition, compositional uniformity and higher achievable doping concentration of the active ions for higher power laser output makes the ceramics more attractive materials than single crystals [13]. Recently, interest has focused on the development of transparent ceramics for both military and civil applications. Also, the development of innovative optoelectronic devices requires development of novel optical materials. Such devices and corresponding applications include but not limited to temperature (IR) sensors, optical fiber communications, and laser interferometers. Moreover, many types of scintillating transparent ceramics have been systematically studied for their applications as radiation detectors for security and baggage inspection, X-ray detectors in computed
⇑ Corresponding author. Tel.: +81 93 695 6049.
tomography (CT scan), block detectors in positron emission tomography (PET) and high-energy physics [19–28]. The transparent ceramics with garnet structure are well developed. These materials are widely studied for X-ray and gamma-ray detection [23–28]. However, the garnet type crystals of Y3Al5O12 (YAG) and Lu3Al5O12 (LuAG) contain antisite defects (Y or Lu cations substitute for Al in the octahedral sites of the garnet structure) and color centers originated from the oxygen vacancies formed in the crystal lattice. These defects have an effect on scintillation performance of these materials that is additional problem to solve [29–33]. Current report is concentrated on development of Ce3+:LuAG and Pr3+:LuAG transparent ceramic scintillators that demonstrate high light yield and undetectable slow decay components comparing to the single crystals studied in the past [25,26]. Probably, this illustrates improvement of the scintillation efficiency. However the details are not clearly understood. That is why, the aim of this paper was to study scintillation properties related to intrinsic defects of undoped YAG and LuAG transparent ceramics in comparison with those of single crystals. At first, optical transparency of the materials was inspected. Thereafter evaluation of the radioluminescence spectra recorded under X-ray excitation and determination of the decay times using the streak camera system equipped with pulsed X-ray source [34,35] was performed. Finally, the relative light yield was estimated from the pulse height spectra measured under 137Cs gamma-ray irradiation.
E-mail address: [email protected] (Y. Fujimoto). http://dx.doi.org/10.1016/j.optmat.2014.06.019 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Fujimoto et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.06.019
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2. Experimental procedures and results 2.1. Preparation of the samples and optical transmittance spectra Transparent ceramics of undoped YAG and LuAG were produced by Konoshima Chemical Company. Aqueous solutions of yttrium, aluminum, and ytterbium chlorides were used as starting materials that were mixed together. Similar preparation process of Nd:YAG laser ceramics is reported in Refs. [8,9]. The obtained powders were sintered under vacuum at 1700 °C for 20 h. As a result, highly transparent ceramic samples were obtained. Single crystals of YAG and LuAG used as reference materials were grown by Czochralski (CZ) method with radio frequency (RF) heating system. Stoichiometric mixtures of 4N Y2O3, Lu2O3, and 5N a-Al2O3 powders (High Purity Chemicals) were used as raw materials for the CZ growths. These mixtures were sintered at 1400 °C for 30 h prior to melting. The sintered powders were loaded into Ir crucible, and then it was heated to a melting point of YAG and LuAG (1950– 2000 °C). The growths were performed at the seed rotation rate of 10–20 rpm, and the pulling rate was in the range of 1–2 mm/ h. An automatic diameter control system based on monitoring of crystal weight was applied to stabilize the crystal diameter. The crystals were grown under N2 atmosphere to prevent oxidization of the crucible material (Ir). The seeding performed on h1 1 1i oriented undoped LuAG crystals. For evaluation of optical and scintillation properties, the ceramics and the single crystals were cut to plate samples and polished. Fig. 1 illustrates view of the specimens. All samples were colorless, transparent and had no any visible inclusions. The transmission spectra of the samples were recorded using JASCO V-670 spectrometer in wavelength range from 190 to 2700 nm. The spectrometer was equipped with photomultiplier tube (PMT) to detect signals in UV–VIS wavelength region and a Peltier-cooled PbS detector is used in the NIR region. The light sources for both regions were D2 and halogen lamp. Fig. 2 illustrates the results. Both ceramic and crystal samples demonstrate transparency exceeding 70%, and no absorption band associated with some impurities in the 300–2700 nm wavelength range were detected. On the other hand, some absorption bands were observed in the UV region for the ceramic samples. These bands are probably originated from presence of F-type centers formed by oxygen vacancies [36]. Intensity of formation of such centers depends on parameters of the synthesis process of the ceramics including sintering temperature, atmosphere, annealing routine and others.
2.2. Radioluminescence spectra and scintillation decay time profile under X-ray excitation X-ray excited radioluminescence spectra were recorded with a SR163i-UV spectrometer (Andor) combined with a DU920P CCD detector (Andor). An original X-ray generator (XRB80P Monoblock,
Spellman) that was equipped with tungsten target was used as excitation source. The generator was supplied with voltage of 80 kV and current of 2.5 mA. The scintillation light emitted from the sample was sent to the spectrometer using optical fiber to avoid direct irradiation of X-ray. The spectra are shown in Fig. 3. The ceramics and the crystals excited with X-ray demonstrated broad emission bands in VIS wavelength regions. The emission bands were identified according to four different origins as follows: (1) self-trapped excitons (STE) in the bands peaked at 260 nm, (2) exitons localized around antisite defects in the bands peaked at 300–320 nm, (3) antisite defects in the bands peaked at 340 nm, and (4) F+ centers (oxygen vacancy with one trapped electron) in the bands peaked at 380–400 nm [31–33]. According to the spectra (Fig. 3), the emission intensities of the ceramics were considerably lower than those of the single crystals that are particularly result of formation of the exitons localized around antisite defects. Specifically, the emission intensities of the YAG ceramic are very low. Therefore it was suggested that concentration of the antisite defects in the ceramics discussed here was relatively low comparatively to the single crystals. Scintillation decay time profiles were recorded using a Picosecond X-ray Induced Fluorescence Spectroscopy System (Hamamatsu) [34,35]. The main modules of the system are a streak camera (C10627, Hamamatsu), a spectrometer chamber, and X-ray tube (N5084, Hamamatsu) controlled by a picosecond laser diode (PLP10-063, Hamamatsu). This system allowed measurement of time- and wavelength-resolved scintillation events in a wavelength range of 110–900 nm. The instrumental response function was about 80 ps at a maximum. The decay times were evaluated by fitting with single and double exponential equation. The profiles are presented in Fig. 4. Following the result of calculations, the decay times were about 430 ns for YAG ceramic, 460 ns for YAG single crystal, 30 and 1090 ns for YAG ceramic, and 25 and 970 ns for LuAG crystal, respectively (Table 1). Thus it is inferred that intrinsic emission band of the single crystals and ceramics
(b) Single crystals
(a) Ceramics YAG
Fig. 2. Transmittance spectra of the transparent ceramics and the single crystals in the wavelength region 190–2700 nm.
LuAG
YAG
LuAG
Fig. 1. View of the YAG and LuAG transparent ceramics (a) and the single crystals (b).
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exitons + antisite defects
exitons + antisite defects F+ centers STE
STE F+ centers
Fig. 3. X-ray excited radioluminescence spectra of the transparent ceramics and the single crystals.
Fig. 4. Pulsed X-ray exited scintillation decay time profiles of the transparent ceramics and the single crystals.
Table 1 Characteristic components of the scintillation decay curves of the transparent ceramics and the single crystals. Sample
Decay time (ns)
YAG ceramic YAG crystal LuAG ceramic LuAG crystal
430 460 30, 1290 25, 970
are due to same origin as evidenced by the similar emission band and decay component. 2.3. Pulse height spectra under
137
Cs gamma-ray irradiation
Also, evaluation of the concentrations of intrinsic defects related to scintillation properties was required. Therefore, comparison of relative scintillation light yield of the ceramics and the single crystals was performed with help of the irradiated pulse height spectra measured under 137Cs gamma-ray irradiation. Bi4Ge3O12 (BGO) commercial scintillator was used as a standard sample because the light yield of BGO under gamma-ray irradiation is well known (8600 ph/MeV [37]). For these measurements, the samples were attached to the window of PMT (R7600U, Hamamatsu) with optical silicon grease (6262A, OKEN). The pulse signals from the PMT were processed using a pre-amplifier (ORTEC 113), a shaping amplifier (ORTEC 572), a multichannel analyzer (Amptec Pocket MCA 8000A), and finally fed into a personal computer for analysis. The shaping time was 2 ls for all the measurements as an optical value. The 137Cs 662 keV photo-absorption peak channels were determined by fitting with a single Gaussian function. The light yields were estimated in accordance with the peak channels and quantum efficiency (QE) of PMT. The QE at emission
wavelength of the single crystals and the ceramics were about 38% (300 nm), and that of BGO is about 30% (500 nm). The spectra are presented in Fig. 5. In the spectra, each 137Cs 662 keV photoabsorption peak was determined at 100 ch for YAG and LuAG ceramics, 340 ch for LuAG single crystal, and 585 ch for YAG single crystal, respectively. That of BGO was at 310 ch. Taking into account the peak channel number and QE of PMT, relative scintillation light yield of the YAG and LuAG ceramics were found to be about 2950 ± 290 ph/MeV, while those of the single crystals were 14,300 ± 1430 ph/MeV (YAG) and 8350 ± 830 ph/MeV (LuAG). Based on the pulse height and radioluminescence spectra, the concentration of intrinsic defects in host lattice associated with scintillation process was expected to be deferent between single crystals and ceramics. Thus, it was suggested that the concentration of the antisite defects in the ceramics was relatively low as compared to that of the single crystals because of the low scintillation light yield.
Fig. 5. 137Cs gamma-ray irradiated pulse height spectra of the transparent ceramics and the single crystals together with that of BGO.
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3. Summary The intrinsic luminescence properties caused by lattice defects of undoped YAG and LuAG transparent ceramic scintillators and the single crystals were compared. Both the single crystals and the ceramics demonstrated broad emission bands at UV–VIS regions when excited with X-rays. These bands originated from STE, exitons localized around antisite defects, antisite defects, and F+ centers. According to pulse height spectra evaluated for 137 Cs gamma-ray irradiation, relative scintillation light yield of the ceramics were estimated to be about 2950 ± 290 ph/MeV. The light yields of the single crystals were 14,300 ± 1430 ph/MeV for YAG and 8350 ± 830 ph/MeV for LuAG and were greater than those of the ceramics and BGO scintillator. In conclusion, the present study has demonstrated that concentration of the lattice defects associated with scintillation in YAG and LuAG transparent ceramics studied here is very low compared to that of the single crystals. Thus, in the case of scintillators activated with rare-earth ions such as Ce3+:LuAG and Pr3+:LuAG, the energy transfer efficiency from host matrix to luminescence centers is most probably governed by the defect concentration in the host lattice. Future research plans includes (1) study thermoluminescence and photo-stimulated luminescence properties of the ceramics in order to estimate shallow and deep electron trap levels in band gap and (2) inspection of other transparent ceramics with cubic crystalline structure including sesquioxides, pyrochlores and spinels.
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Contents lists available at ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Comparative study of intrinsic luminescence in undoped transparent ceramic and single crystal garnet scintillators Yutaka Fujimoto a,⇑, Takayuki Yanagida a, Hideki Yagi b, Takagimi Yanagidani b, Valery Chani c a
Kyushu Institute of Technology, 2-4, Hibikino, Wakamatsu-ku, Kitakyushu, Japan Takuma Works, Konoshima Chemical Co., Ltd., 80 Koda, Takuma, Mitoyo, Kagawa, Japan c Tohoku University, 6-6-04, Aramaki Aza Aoba Aoba-ku, Sendai, Miyagi 980-8579, Japan b
a r t i c l e
i n f o
Article history: Available online xxxx Keywords: Scintillator Transparent ceramic Garnet
a b s t r a c t Scintillation properties associated with intrinsic lattice defects of undoped Y3A5O12 (YAG) and Lu3A5O12 (LuAG) transparent ceramics and single crystals are compared. The ceramics excited with X-ray demonstrated relatively low emission intensity when compared with that of the single crystals. Decay times of the ceramics and the single crystals were similar. These parameters were approximately 430 ns (YAG ceramic), 460 ns (YAG single crystal), 30 ns and 1090 ns (LuAG ceramic), and 25 ns and 970 ns (LuAG single crystal). According to the pulse height spectra recorded under 137Cs gamma-ray irradiation, the scintillation light yield of the both ceramics were about 2950 ± 290 ph/MeV. However, the single crystals had greater kight yield of about about 14,300 ± 1430 ph/MeV for YAG and 8350 ± 830 ph/MeV for LuAG. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Various types of translucent and transparent oxide ceramics including alumina [1–3], rare-earth sesquioxides [4,5], spinels [6,7], garnets [8–13] and other compounds [14–18] have been developed for optical device applications. As an example, Nd:Y3Al5O12 laser transparent ceramics prepared by vacuum sintering technique demonstrated several advantages including relatively simplicity of their fabrication at comparatively low sintering temperature and possibility to produce large size specimens. In addition, compositional uniformity and higher achievable doping concentration of the active ions for higher power laser output makes the ceramics more attractive materials than single crystals [13]. Recently, interest has focused on the development of transparent ceramics for both military and civil applications. Also, the development of innovative optoelectronic devices requires development of novel optical materials. Such devices and corresponding applications include but not limited to temperature (IR) sensors, optical fiber communications, and laser interferometers. Moreover, many types of scintillating transparent ceramics have been systematically studied for their applications as radiation detectors for security and baggage inspection, X-ray detectors in computed
⇑ Corresponding author. Tel.: +81 93 695 6049.
tomography (CT scan), block detectors in positron emission tomography (PET) and high-energy physics [19–28]. The transparent ceramics with garnet structure are well developed. These materials are widely studied for X-ray and gamma-ray detection [23–28]. However, the garnet type crystals of Y3Al5O12 (YAG) and Lu3Al5O12 (LuAG) contain antisite defects (Y or Lu cations substitute for Al in the octahedral sites of the garnet structure) and color centers originated from the oxygen vacancies formed in the crystal lattice. These defects have an effect on scintillation performance of these materials that is additional problem to solve [29–33]. Current report is concentrated on development of Ce3+:LuAG and Pr3+:LuAG transparent ceramic scintillators that demonstrate high light yield and undetectable slow decay components comparing to the single crystals studied in the past [25,26]. Probably, this illustrates improvement of the scintillation efficiency. However the details are not clearly understood. That is why, the aim of this paper was to study scintillation properties related to intrinsic defects of undoped YAG and LuAG transparent ceramics in comparison with those of single crystals. At first, optical transparency of the materials was inspected. Thereafter evaluation of the radioluminescence spectra recorded under X-ray excitation and determination of the decay times using the streak camera system equipped with pulsed X-ray source [34,35] was performed. Finally, the relative light yield was estimated from the pulse height spectra measured under 137Cs gamma-ray irradiation.
E-mail address: [email protected] (Y. Fujimoto). http://dx.doi.org/10.1016/j.optmat.2014.06.019 0925-3467/Ó 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: Y. Fujimoto et al., Opt. Mater. (2014), http://dx.doi.org/10.1016/j.optmat.2014.06.019
2
Y. Fujimoto et al. / Optical Materials xxx (2014) xxx–xxx
2. Experimental procedures and results 2.1. Preparation of the samples and optical transmittance spectra Transparent ceramics of undoped YAG and LuAG were produced by Konoshima Chemical Company. Aqueous solutions of yttrium, aluminum, and ytterbium chlorides were used as starting materials that were mixed together. Similar preparation process of Nd:YAG laser ceramics is reported in Refs. [8,9]. The obtained powders were sintered under vacuum at 1700 °C for 20 h. As a result, highly transparent ceramic samples were obtained. Single crystals of YAG and LuAG used as reference materials were grown by Czochralski (CZ) method with radio frequency (RF) heating system. Stoichiometric mixtures of 4N Y2O3, Lu2O3, and 5N a-Al2O3 powders (High Purity Chemicals) were used as raw materials for the CZ growths. These mixtures were sintered at 1400 °C for 30 h prior to melting. The sintered powders were loaded into Ir crucible, and then it was heated to a melting point of YAG and LuAG (1950– 2000 °C). The growths were performed at the seed rotation rate of 10–20 rpm, and the pulling rate was in the range of 1–2 mm/ h. An automatic diameter control system based on monitoring of crystal weight was applied to stabilize the crystal diameter. The crystals were grown under N2 atmosphere to prevent oxidization of the crucible material (Ir). The seeding performed on h1 1 1i oriented undoped LuAG crystals. For evaluation of optical and scintillation properties, the ceramics and the single crystals were cut to plate samples and polished. Fig. 1 illustrates view of the specimens. All samples were colorless, transparent and had no any visible inclusions. The transmission spectra of the samples were recorded using JASCO V-670 spectrometer in wavelength range from 190 to 2700 nm. The spectrometer was equipped with photomultiplier tube (PMT) to detect signals in UV–VIS wavelength region and a Peltier-cooled PbS detector is used in the NIR region. The light sources for both regions were D2 and halogen lamp. Fig. 2 illustrates the results. Both ceramic and crystal samples demonstrate transparency exceeding 70%, and no absorption band associated with some impurities in the 300–2700 nm wavelength range were detected. On the other hand, some absorption bands were observed in the UV region for the ceramic samples. These bands are probably originated from presence of F-type centers formed by oxygen vacancies [36]. Intensity of formation of such centers depends on parameters of the synthesis process of the ceramics including sintering temperature, atmosphere, annealing routine and others.
2.2. Radioluminescence spectra and scintillation decay time profile under X-ray excitation X-ray excited radioluminescence spectra were recorded with a SR163i-UV spectrometer (Andor) combined with a DU920P CCD detector (Andor). An original X-ray generator (XRB80P Monoblock,
Spellman) that was equipped with tungsten target was used as excitation source. The generator was supplied with voltage of 80 kV and current of 2.5 mA. The scintillation light emitted from the sample was sent to the spectrometer using optical fiber to avoid direct irradiation of X-ray. The spectra are shown in Fig. 3. The ceramics and the crystals excited with X-ray demonstrated broad emission bands in VIS wavelength regions. The emission bands were identified according to four different origins as follows: (1) self-trapped excitons (STE) in the bands peaked at 260 nm, (2) exitons localized around antisite defects in the bands peaked at 300–320 nm, (3) antisite defects in the bands peaked at 340 nm, and (4) F+ centers (oxygen vacancy with one trapped electron) in the bands peaked at 380–400 nm [31–33]. According to the spectra (Fig. 3), the emission intensities of the ceramics were considerably lower than those of the single crystals that are particularly result of formation of the exitons localized around antisite defects. Specifically, the emission intensities of the YAG ceramic are very low. Therefore it was suggested that concentration of the antisite defects in the ceramics discussed here was relatively low comparatively to the single crystals. Scintillation decay time profiles were recorded using a Picosecond X-ray Induced Fluorescence Spectroscopy System (Hamamatsu) [34,35]. The main modules of the system are a streak camera (C10627, Hamamatsu), a spectrometer chamber, and X-ray tube (N5084, Hamamatsu) controlled by a picosecond laser diode (PLP10-063, Hamamatsu). This system allowed measurement of time- and wavelength-resolved scintillation events in a wavelength range of 110–900 nm. The instrumental response function was about 80 ps at a maximum. The decay times were evaluated by fitting with single and double exponential equation. The profiles are presented in Fig. 4. Following the result of calculations, the decay times were about 430 ns for YAG ceramic, 460 ns for YAG single crystal, 30 and 1090 ns for YAG ceramic, and 25 and 970 ns for LuAG crystal, respectively (Table 1). Thus it is inferred that intrinsic emission band of the single crystals and ceramics
(b) Single crystals
(a) Ceramics YAG
Fig. 2. Transmittance spectra of the transparent ceramics and the single crystals in the wavelength region 190–2700 nm.
LuAG
YAG
LuAG
Fig. 1. View of the YAG and LuAG transparent ceramics (a) and the single crystals (b).
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exitons + antisite defects
exitons + antisite defects F+ centers STE
STE F+ centers
Fig. 3. X-ray excited radioluminescence spectra of the transparent ceramics and the single crystals.
Fig. 4. Pulsed X-ray exited scintillation decay time profiles of the transparent ceramics and the single crystals.
Table 1 Characteristic components of the scintillation decay curves of the transparent ceramics and the single crystals. Sample
Decay time (ns)
YAG ceramic YAG crystal LuAG ceramic LuAG crystal
430 460 30, 1290 25, 970
are due to same origin as evidenced by the similar emission band and decay component. 2.3. Pulse height spectra under
137
Cs gamma-ray irradiation
Also, evaluation of the concentrations of intrinsic defects related to scintillation properties was required. Therefore, comparison of relative scintillation light yield of the ceramics and the single crystals was performed with help of the irradiated pulse height spectra measured under 137Cs gamma-ray irradiation. Bi4Ge3O12 (BGO) commercial scintillator was used as a standard sample because the light yield of BGO under gamma-ray irradiation is well known (8600 ph/MeV [37]). For these measurements, the samples were attached to the window of PMT (R7600U, Hamamatsu) with optical silicon grease (6262A, OKEN). The pulse signals from the PMT were processed using a pre-amplifier (ORTEC 113), a shaping amplifier (ORTEC 572), a multichannel analyzer (Amptec Pocket MCA 8000A), and finally fed into a personal computer for analysis. The shaping time was 2 ls for all the measurements as an optical value. The 137Cs 662 keV photo-absorption peak channels were determined by fitting with a single Gaussian function. The light yields were estimated in accordance with the peak channels and quantum efficiency (QE) of PMT. The QE at emission
wavelength of the single crystals and the ceramics were about 38% (300 nm), and that of BGO is about 30% (500 nm). The spectra are presented in Fig. 5. In the spectra, each 137Cs 662 keV photoabsorption peak was determined at 100 ch for YAG and LuAG ceramics, 340 ch for LuAG single crystal, and 585 ch for YAG single crystal, respectively. That of BGO was at 310 ch. Taking into account the peak channel number and QE of PMT, relative scintillation light yield of the YAG and LuAG ceramics were found to be about 2950 ± 290 ph/MeV, while those of the single crystals were 14,300 ± 1430 ph/MeV (YAG) and 8350 ± 830 ph/MeV (LuAG). Based on the pulse height and radioluminescence spectra, the concentration of intrinsic defects in host lattice associated with scintillation process was expected to be deferent between single crystals and ceramics. Thus, it was suggested that the concentration of the antisite defects in the ceramics was relatively low as compared to that of the single crystals because of the low scintillation light yield.
Fig. 5. 137Cs gamma-ray irradiated pulse height spectra of the transparent ceramics and the single crystals together with that of BGO.
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3. Summary The intrinsic luminescence properties caused by lattice defects of undoped YAG and LuAG transparent ceramic scintillators and the single crystals were compared. Both the single crystals and the ceramics demonstrated broad emission bands at UV–VIS regions when excited with X-rays. These bands originated from STE, exitons localized around antisite defects, antisite defects, and F+ centers. According to pulse height spectra evaluated for 137 Cs gamma-ray irradiation, relative scintillation light yield of the ceramics were estimated to be about 2950 ± 290 ph/MeV. The light yields of the single crystals were 14,300 ± 1430 ph/MeV for YAG and 8350 ± 830 ph/MeV for LuAG and were greater than those of the ceramics and BGO scintillator. In conclusion, the present study has demonstrated that concentration of the lattice defects associated with scintillation in YAG and LuAG transparent ceramics studied here is very low compared to that of the single crystals. Thus, in the case of scintillators activated with rare-earth ions such as Ce3+:LuAG and Pr3+:LuAG, the energy transfer efficiency from host matrix to luminescence centers is most probably governed by the defect concentration in the host lattice. Future research plans includes (1) study thermoluminescence and photo-stimulated luminescence properties of the ceramics in order to estimate shallow and deep electron trap levels in band gap and (2) inspection of other transparent ceramics with cubic crystalline structure including sesquioxides, pyrochlores and spinels.
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