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Scintillation properties of Ce-doped Tb3Al5O12
Nuclear Instruments and Methods in Physics Research A 866 (2017) 134–139
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Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Scintillation properties of Ce-doped Tb3Al5O12 Tomohisa Oya n, Daisuke Nakauchi, Go Okada, Noriaki Kawaguchi, Takayuki Yanagida Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 13 December 2016 Received in revised form 16 April 2017 Accepted 17 April 2017 Available online 18 April 2017
Tb3Al5O12 (TbAG) crystalline materials doped with different concentrations of Ce were prepared by the floating zone (FZ) method to evaluate their basic optical and scintillation properties. In photoluminescence (PL), a broad emission due to the 5d-4f f transition of Ce3 þ appeared around 550 nm, and the decay time was approximately 40 ns. Under X-ray irradiation, the scintillation due to the 5d-4f transition of Ce3 þ was also observed around 550 nm, but the decay time appeared to be much longer 376–599 ns than that of PL and it depended on the concentration of Ce. The pulse height spectroscopy analyses revealed that 3.0% Ce-doped TbAG showed the highest light yield of 30,100 ph/MeV under 137Cs γ-ray irradiation among the samples investigated in this work. & 2017 Elsevier B.V. All rights reserved.
Keywords: Scintillator Tb3Al5O12 Crystalline materials
1. Introduction Fluorescent substances which have a function to immediately convert the absorbed energy of ionizing radiation (e.g., X-rays) with typically keV-GeV energy to numerous low energy (1–6 eV) photons are called scintillators. The scintillators are used in many fields such as high energy physics [1], nuclear medicine [2], and security inspection at airports [3]. A scintillator material can typically be an inorganic or organic solid. In general, the inorganic scintillators show higher light yield, and the organic scintillator is characterized by fast decay time (faster than 10 ns scale). Important properties of scintillators are light yield, density (ρ), effective atomic number (Zeff) and response speed. In particular, ρ and Zeff are important factors to achieve high detection efficiency against high energy photons since the probability of the photoabsorption events is proportional to ρZeff4. In reality, there is no ideal scintillator that fullfills all the properties required for all the applications. Consequently, users select one from existing scintillators suitable for their purposes while a progressive search for new scintillation materials is of great importance. Among many inorganic scintillators, rare-earth doped garnet (Y, Gd, Lu)3Al5O12 [(Y, Gd, Lu)AG] scintillators show a high light yield and fast response [4–10]. So these garnet scintillators were studied intensively [5,6]. Especially, Ce is the most common rareearth dopant for scintillators, and there are many studies of Cedoped YAG (Ce:YAG) and LuAG (Ce:LuAG) [4,7,11,12]. In recent years, Ce-doped Gd3(Al, Ga)5O12 (Ce:GAGG) crystal scintillator has been developed by our group, and this crystal has become a n
Corresponding author. E-mail address: [email protected] (T. Oya).
http://dx.doi.org/10.1016/j.nima.2017.04.027 0168-9002/& 2017 Elsevier B.V. All rights reserved.
commercial product of Furukawa Co.,Ltd. Single crystal of Ce: GAGG is characterized by the high scintillation light yield (46000 ph/MeV) with fast scintillation decay time ( 140 ns) and longer emission wavelength than the other common commercial materials [8,9]. In addition to these common garnet hosts such as (Y, Gd, Lu)AG, other host consisting of other rare earth elements are of great interest for scintillator. Tb3Al5O12 (TbAG) is also one of the garnet species, and mainly studied for Faraday magneto-optical devices using the polycrystalline form [13,14]. However, it has not been studied for scintillator applications intensively, and as far as we are aware there are only a few literature reporting scintillation properties of TbAG but in a thin-film form [15–17]. Therefore, it is worthful to study scintillation properties of TbAG in a bulk crystalline form. In this study, we synthesized TbAG samples with different concentrations of Ce by the Floating Zone (FZ) method. The synthesized samples were characterized by the scintillation spectra, scintillation decay time profiles, afterglow, pulse height spectroscopy and the thermally-stimulated luminescence (TSL). In addition, we investigated the basic photoluminescence (PL) properties such as emission and excitation spectra and decay time.
2. Experimental Ce-doped TbAG samples were grown by the FZ method. We used Tb4O7 (4 N), Al2O3 (4 N) and CeO2 (4 N) powders as starting materials. First, these compounds with molar ratio of Tb4O7:Al2O3: CeO2 ¼ (75-3x):250:3x were homogeneously mixed, and the mixture powder was formed into a cylinder rod by applying hydrostatic pressure. The obtained rod was sintered at 1600 °C for 9 h in air to obtain a solid ceramic rod, which was used for crystal
T. Oya et al. / Nuclear Instruments and Methods in Physics Research A 866 (2017) 134–139
growth by the FZ method. The crystal growth was performed by using an FZ furnace (FZD0192, Canon Machinery Inc.) in air. During the synthesis, the ceramic rod was rotated at a rate of 20 rpm, and it was vertically translated at a rate of 3–8 mm/h. Powder X-ray diffraction (XRD) pattern was measured by a diffractometer (MiniFlex 600, RIGAKU) using Cu (Kα) X-ray beam with the tube voltage and current of 40 kV and 15 mA, respectively. The PL excitation (PLE) and emission spectra were observed by using a spectrofluorometer (FP8600, JASCO). By using QuantaurusQY (Hamamatsu), PL quantum yield was evaluated. The absolute quantum yield (QY) was calculated via QY ¼ Nemit/Nabsorb where Nemit and Nabsorb were the numbers of emitted and absorbed photons, respectively. PL decay time profiles were evaluated by Quantaurus-τ (Hamamatsu). The radioluminescence (RL) spectra were evaluated at room temperature under X-ray irradiation by using our original setup [18]. The excitation source was a conventional X-ray tube (XRB80P&N200 4550, Spellman) supplied with 40 kV bias voltage and 5.2 mA tube current. The emission spectrum was measured using a spectrometer (Andor DU920-BU2NC CCD) over 180– 700 nm. This detector was cooled down to 188 K by a Peltier device to reduce the thermal noise. Here, the scintillation photons were collected and guided into spectrometer through a 2 m optical fiber. The scintillation decay time and afterglow profiles were evaluated by using our original setup [19,20]. To characterize the absolute scintillation light yield induced by γ-rays, the pulse height spectroscopy measurement was performed. The sample was firmly placed on a photomultiplier tube (PMT; R7600, Hamamatsu) window using optical grease (OKEN6262A) and covered by layers of Teflon tape. The bias voltage of 600 V was applied to the PMT by a DC power supply (ORTEC556). The electrical signal output from the anode of the PMT was amplified by a pre-amplifier (ORTEC113). Then, to obtain the light output upon a single γ-ray event, the amplified signal was processed by a shaping amplifier (ORTEC 572) with 10 ms shaping time. The signal output per event was statically collected and accumulated by a multichannel analyzer (Pocket MCA 8000 A, Amptek). TSL glow curve was measured by a TSL reader (TL-2000, Nanogray). The sample was irradiated by X-rays of 1 Gy prior to the measurement. The heating rate was 1 °C/s over the temperature range from 50 to 490 °C [21].
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Fig. 1. Photograph of undoped and Ce-doped TbAG under room light (top) and UV (302 nm) light (bottom).
3. Results and discussion Fig. 2. XRD patterns of undoped and Ce-doped TbAG samples.
We have successfully obtained non-doped and Ce-doped TbAG samples. The typical size of as-grown sample rod was approximately 4 mm in diameter with 20 mm in length. We cut the sample rod to the thickness of approximately 1 mm for characterizations. Fig. 1 shows all the samples used for evaluations. Due to a strong absorption of 4f-5d transition of Ce3 þ in the UVblue range, Ce-doped samples looked yellow, and the coloration seems to become stronger with increasing the concentration of Ce. In addition, large fraction of cracks were evident. The inclusion of cracks was due to fast cooling rate during the FZ synthesis. Under UV light (302 nm), undoped TbAG showed green emission while the Ce-doped TbAG showed strong yellow emission. Fig. 2 shows the XRD patterns of undoped and Ce-doped TbAG. TbAG (JCPDS card No.76–1111) is also shown as a reference. All samples have the garnet phase structure. In 3.0% Ce-doped sample, TbO2 phase was detected. Therefore, the highly opaque appearance of the 3.0% Ce-doped sample is blamed for the TbO2 phase acting as scattering center to some extent. PLE and PL spectra of undoped and Ce-doped TbAG are shown in Fig. 3. In the PLE spectra of undoped TbAG, strong excitation bands were observed in the range of 200 250 nm and
250 400 nm and the spectral range agree with those of absorption bands due to the 4f-5f and 4f-4f transitions of Tb3 þ , respectively. The PL spectrum of undoped TbAG consisted of several emission lines at 490 nm, 540 nm, 590 nm and 620 nm, and these emission were due to the 5D4-7FJ (J ¼ 6-3) transitions of Tb3 þ [22–24]. The 5D4-7F5 transition line at 540 nm showed the strongest intensity. In the Ce-doped TbAG, PLE spectrum showed strong absorptions due to 4f-5d transition of Ce3 þ in the range of 400 – 500 nm in addition to that of Tb3 þ as observed in the nondoped sample. The PL spectrum of Ce-doped TbAG showed a broad emission due to the 5d-4f transitions of Ce3 þ in the wavelength range from 500 nm to 680 nm [25,26]. The emission peak slightly redshifted in relative to that of Ce:YAG due to modification of local crystal field [27]. Furthermore, emission lines due to Tb3 þ were negligible in the Ce-doped TbAG (even when directly exciting the Tb bands). Under excitation around 470 nm, the QY values of the 0.1%, 0.3%, 1.0% and 3.0% Ce-doped samples are 66%, 59%, 59% and 60%, respectively. These values are smaller than that of Ce:YAG but equivalent to those of Ce:LuAG [28,29]. Fig. 4 represents the PL decay curves of undoped and Ce-doped
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Fig. 3. PL excitation and emission spectra of undoped TbAG (top) and Ce-doped TbAG (bottom).
TbAG, respectively. The decay time of undoped TbAG was derived as 66 μs when monitoring the 5D4-7F5 emission of Tb3 þ (550 nm). It should be pointed out that the decay time of 4f4f transitions of Tb3 þ is often observed to be a few ms [30]. Much faster decay time observed in our sample suggests that concentration quenching is involved and it is reasonable because Tb is one of the constitution elements of the host material. This phenomena has been reported in TbAG [31,32]. In Ce-doped TbAG, the decay time of the 5d-4f transition of Ce3 þ (550 nm) was approximately 40 ns and consistent regardless of the dopant concentration. This latter decay time was typical for the 5d-4f transition of Ce3 þ in the garnet host [18,33,34]. X-ray induced RL spectra of undoped and Ce-doped TbAG are represented in Fig. 5. As for PL spectra, undoped TbAG showed emission lines at 490 nm, 540 nm, 590 nm and 620 nm due to the 5 D4-7FJ (J ¼ 6-3) transitions of Tb3 þ [15,35]. In Ce-doped TbAG, broad emission bands apparently due to the 5d-4f transition of Ce3 þ was dominant but a small contribution of Tb3 þ was also observed especially for those with smaller concentrations of Ce3 þ . As mentioned above, Tb3 þ emission was not observed even when excited at the Tb3 þ excitation bands, and this indicates that energy transfer from the Tb3 þ to Ce3 þ is very efficient. In scintillation, both Tb3 þ and Ce3 þ are excited by energetic secondary electrons generated in the host by X-rays, but since the energy transfer from the Tb3 þ to Ce3 þ so efficient, emission of Tb3 þ appeared to be much smaller than that of Ce3 þ . As in the PL spectrum, the emission peak position was slightly redshifted compared with that of Ce:YAG [36]. Fig. 6 represents X-ray induced scintillation decay time profiles of undoped and Ce-doped TbAG. In undoped TbAG, the measured decay time was 35 μs due to the Tb3 þ emission, and the observed decay time was faster than PL decay time. The observation of faster scintillation decay than that of PL is generally understood to be due to quenching of excited state by much higher excitation density during scintillation process. In Ce-doped TbAG, this fast
Fig. 4. PL decay curves of undoped (top) and Ce-doped (bottom) TbAG.
component was too fast to be discriminated from the instrumental response. We also observed slower response with the decay time of 376–599 ns, and these values were similar to that observed in Ce-doped TbAG films under α-ray excitation [15]. Unlike the Tb3 þ , a slower decay time of scintillation than PL was observed, and this behavior is more typical because scintillation process involves energy transportation process in addition to the luminescence process at the local center. Fig. 7 represent the afterglow curves of undoped and Ce-doped TbAG measured with 2 ms X-ray pulse irradiation. The afterglow levels at 20 ms of undoped and 0.1%, 0.3%, 1.0% and 3.0% Ce-doped TbAG were 0.736%, 0.112%, 0.120%, 0.114% and 0.172%, respectively. The 0.1% Ce-doped TbAG shows the lowest afterglow level. The afterglow levels (A) are calculated in the following expressions as A( %) = 100 ×( I2 − I0)/(I1 − I0) where I0, I1 and I2 were the average intensity before X-ray irradiation, average intensity during X-ray irradiation and intensity at 20 ms after X-ray irradiation, respectively. This definition of afterglow followed the protocol used by NIHON KESSHO KOGAKU CO., Ltd. which has a large market share of scintillation detectors for security and medical applications. Compared with the afterglow levels of Bi4Ge3O12 and CdWO4 ( 10 ppm) which are well-known to show very low afterglow [19], the afterglow levels of Ce-doped TbAG were significantly higher. Fig. 8 represents the pulse height spectra of Ce-doped TbAG measured under 137Cs γ-ray irradiation. The photoabsorption peak
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Fig. 5. X-ray induced RL spectra of undoped (top) and Ce-doped (bottom) TbAG.
channels and the absolute light yield are summarized in Table 1. The absolute light yield of the 0.1%, 0.3%, 1.0% and 3.0% Ce-doped TbAG were deduced to be 9,600, 16,800, 28,600 and 30,100 ph/ MeV with a typical error of 10%, respectively. Among the samples tested, the 3.0% Ce-doped TbAG showed the highest light yield. The energy resolution at 662 keV of the 0.1%, 0.3%, 1.0% and 3.0% Ce-doped TbAG were 19%, 45%, 22%, and 30%. In this analysis, we could not observe any significant signal from the nondoped sample. The absolute light yield of Ce-doped TbAG was comparable to other Ce-doped garnet scintillators such as Ce:YAG and Ce:LuAG [4,37]. However, it is instructive to mention here that since the past studies of Ce-doped TbAG [12] was performed only with the thin film form, it would not have been possible to determine scintillation light yield. In order to calculate the light yield, we used the 0.5% Ce:YAG transparent ceramic scintillator as a reference because the emission wavelength is similar to Ce-doped TbAG [4]. An absolute light yield of reference sample was 20,000 ph/MeV calibrated by using Si-APD and 55Fe source. Fig. 9 exhibits TSL glow curves of undoped and Ce-doped TbAG measured after 1 Gy X-ray irradiation. The starting temperature of 50 °C was the default value of the TSL reader designed for dosimetry purposes. Therefore, it neglected the glow peak around room temperature, and we could not measure the entire glow peak in the lower temperature range. All the samples showed a main glow peak around 150–250 °C. In addition, all the samples showed significantly large signal around 50 °C which is the origin of large afterglow. The undoped TbAG which had the lowest scintillation intensity (Fig. 6 and no detection in Fig. 9) showed the strongest TSL intensity. It suggests that many carriers were trapped instead of being transferred to luminescent center during scintillation. The trapped carries are so large that it showed strong
Fig. 6. X-ray induced scintillation decay time profiles of undoped (top) and Cedoped (bottom) TbAG.
Fig. 7. Afterglow curves of undoped and Ce-doped TbAG.
TSL signal. Recently, it was reported that the scintillation and TSL show a complemental relationship in some materials [38,39], and we think the same interpretation can be applied in this material system. Although this relation on TSL and scintillation was
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form of thin-film. In this study, we reported scintillation properties of TbAG in a bulk crystalline form, and we believe that our research result is very useful for general scintillator applications since scintillators are typically used in a bulk form. The Ce-doped TbAG showed a strong and broad scintillation emission peaking around 550 nm due to the 5d-4f transitions of Ce3 þ under X-irradiation. By pulse height spectroscopy analyses, absolute light yield was estimated, and the 3.0% Ce-doped TbAG showed the highest light yield of 30,100 ph/MeV under 137Cs γ-ray irradiation. The 0.1% Ce-doped TbAG exhibited the lowest afterglow level of 0.112%.
Acknowledgement
Fig. 8. Pulse height spectra of Ce-doped YAG and Ce-doped TbAG under exposure.
137
Cs
Table 1 The photoabsorption peaks and the absolute light yield from the pulse height spectra. Sample
Photoabsorption peak [channel]
Emission peak [nm] (QE of PMT)
Light yield [ph/MeV]
0.1% Ce: TbAG 0.3% Ce: TbAG 1.0% Ce: TbAG 3.0% Ce: TbAG 0.5% Ce: YAG
195
550 (9%)
9600 7 10%
340
550 (9%)
16,800 7 10%
580
550 (9%)
28,600 710%
610
550 (9%)
30,100 7 10%
720
520 (16%)
20,000 7 10%
Fig. 9. TSL glow curves of undoped and Ce-doped TbAG after 1 Gy X-ray irradiation.
observed in some materials [40–44], such a general relationship between scintillation and storage luminescence based on the simple energy conservation was not clearly proven until recently.
4. Conclusion We synthesized Ce-doped TbAG samples by the FZ method and evaluated their PL and scintillation properties. TbAG has not been studied for scintillator applications intensively, and as far as we are aware scintillation properties of TbAG have been only studied in a
This work was supported by a Grant in Aid for Scientific Research (A) 26249147 from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese government (MEXT) and partially by JST A-step. The Cooperative Research Project of Research Institute of Electronics, Shizuoka University, KRF foundation, Hitachi Metals Materials Science foundation, and Inamori foundation are also acknowledged.
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Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Scintillation properties of Ce-doped Tb3Al5O12 Tomohisa Oya n, Daisuke Nakauchi, Go Okada, Noriaki Kawaguchi, Takayuki Yanagida Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), Japan
art ic l e i nf o
a b s t r a c t
Article history: Received 13 December 2016 Received in revised form 16 April 2017 Accepted 17 April 2017 Available online 18 April 2017
Tb3Al5O12 (TbAG) crystalline materials doped with different concentrations of Ce were prepared by the floating zone (FZ) method to evaluate their basic optical and scintillation properties. In photoluminescence (PL), a broad emission due to the 5d-4f f transition of Ce3 þ appeared around 550 nm, and the decay time was approximately 40 ns. Under X-ray irradiation, the scintillation due to the 5d-4f transition of Ce3 þ was also observed around 550 nm, but the decay time appeared to be much longer 376–599 ns than that of PL and it depended on the concentration of Ce. The pulse height spectroscopy analyses revealed that 3.0% Ce-doped TbAG showed the highest light yield of 30,100 ph/MeV under 137Cs γ-ray irradiation among the samples investigated in this work. & 2017 Elsevier B.V. All rights reserved.
Keywords: Scintillator Tb3Al5O12 Crystalline materials
1. Introduction Fluorescent substances which have a function to immediately convert the absorbed energy of ionizing radiation (e.g., X-rays) with typically keV-GeV energy to numerous low energy (1–6 eV) photons are called scintillators. The scintillators are used in many fields such as high energy physics [1], nuclear medicine [2], and security inspection at airports [3]. A scintillator material can typically be an inorganic or organic solid. In general, the inorganic scintillators show higher light yield, and the organic scintillator is characterized by fast decay time (faster than 10 ns scale). Important properties of scintillators are light yield, density (ρ), effective atomic number (Zeff) and response speed. In particular, ρ and Zeff are important factors to achieve high detection efficiency against high energy photons since the probability of the photoabsorption events is proportional to ρZeff4. In reality, there is no ideal scintillator that fullfills all the properties required for all the applications. Consequently, users select one from existing scintillators suitable for their purposes while a progressive search for new scintillation materials is of great importance. Among many inorganic scintillators, rare-earth doped garnet (Y, Gd, Lu)3Al5O12 [(Y, Gd, Lu)AG] scintillators show a high light yield and fast response [4–10]. So these garnet scintillators were studied intensively [5,6]. Especially, Ce is the most common rareearth dopant for scintillators, and there are many studies of Cedoped YAG (Ce:YAG) and LuAG (Ce:LuAG) [4,7,11,12]. In recent years, Ce-doped Gd3(Al, Ga)5O12 (Ce:GAGG) crystal scintillator has been developed by our group, and this crystal has become a n
Corresponding author. E-mail address: [email protected] (T. Oya).
http://dx.doi.org/10.1016/j.nima.2017.04.027 0168-9002/& 2017 Elsevier B.V. All rights reserved.
commercial product of Furukawa Co.,Ltd. Single crystal of Ce: GAGG is characterized by the high scintillation light yield (46000 ph/MeV) with fast scintillation decay time ( 140 ns) and longer emission wavelength than the other common commercial materials [8,9]. In addition to these common garnet hosts such as (Y, Gd, Lu)AG, other host consisting of other rare earth elements are of great interest for scintillator. Tb3Al5O12 (TbAG) is also one of the garnet species, and mainly studied for Faraday magneto-optical devices using the polycrystalline form [13,14]. However, it has not been studied for scintillator applications intensively, and as far as we are aware there are only a few literature reporting scintillation properties of TbAG but in a thin-film form [15–17]. Therefore, it is worthful to study scintillation properties of TbAG in a bulk crystalline form. In this study, we synthesized TbAG samples with different concentrations of Ce by the Floating Zone (FZ) method. The synthesized samples were characterized by the scintillation spectra, scintillation decay time profiles, afterglow, pulse height spectroscopy and the thermally-stimulated luminescence (TSL). In addition, we investigated the basic photoluminescence (PL) properties such as emission and excitation spectra and decay time.
2. Experimental Ce-doped TbAG samples were grown by the FZ method. We used Tb4O7 (4 N), Al2O3 (4 N) and CeO2 (4 N) powders as starting materials. First, these compounds with molar ratio of Tb4O7:Al2O3: CeO2 ¼ (75-3x):250:3x were homogeneously mixed, and the mixture powder was formed into a cylinder rod by applying hydrostatic pressure. The obtained rod was sintered at 1600 °C for 9 h in air to obtain a solid ceramic rod, which was used for crystal
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growth by the FZ method. The crystal growth was performed by using an FZ furnace (FZD0192, Canon Machinery Inc.) in air. During the synthesis, the ceramic rod was rotated at a rate of 20 rpm, and it was vertically translated at a rate of 3–8 mm/h. Powder X-ray diffraction (XRD) pattern was measured by a diffractometer (MiniFlex 600, RIGAKU) using Cu (Kα) X-ray beam with the tube voltage and current of 40 kV and 15 mA, respectively. The PL excitation (PLE) and emission spectra were observed by using a spectrofluorometer (FP8600, JASCO). By using QuantaurusQY (Hamamatsu), PL quantum yield was evaluated. The absolute quantum yield (QY) was calculated via QY ¼ Nemit/Nabsorb where Nemit and Nabsorb were the numbers of emitted and absorbed photons, respectively. PL decay time profiles were evaluated by Quantaurus-τ (Hamamatsu). The radioluminescence (RL) spectra were evaluated at room temperature under X-ray irradiation by using our original setup [18]. The excitation source was a conventional X-ray tube (XRB80P&N200 4550, Spellman) supplied with 40 kV bias voltage and 5.2 mA tube current. The emission spectrum was measured using a spectrometer (Andor DU920-BU2NC CCD) over 180– 700 nm. This detector was cooled down to 188 K by a Peltier device to reduce the thermal noise. Here, the scintillation photons were collected and guided into spectrometer through a 2 m optical fiber. The scintillation decay time and afterglow profiles were evaluated by using our original setup [19,20]. To characterize the absolute scintillation light yield induced by γ-rays, the pulse height spectroscopy measurement was performed. The sample was firmly placed on a photomultiplier tube (PMT; R7600, Hamamatsu) window using optical grease (OKEN6262A) and covered by layers of Teflon tape. The bias voltage of 600 V was applied to the PMT by a DC power supply (ORTEC556). The electrical signal output from the anode of the PMT was amplified by a pre-amplifier (ORTEC113). Then, to obtain the light output upon a single γ-ray event, the amplified signal was processed by a shaping amplifier (ORTEC 572) with 10 ms shaping time. The signal output per event was statically collected and accumulated by a multichannel analyzer (Pocket MCA 8000 A, Amptek). TSL glow curve was measured by a TSL reader (TL-2000, Nanogray). The sample was irradiated by X-rays of 1 Gy prior to the measurement. The heating rate was 1 °C/s over the temperature range from 50 to 490 °C [21].
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Fig. 1. Photograph of undoped and Ce-doped TbAG under room light (top) and UV (302 nm) light (bottom).
3. Results and discussion Fig. 2. XRD patterns of undoped and Ce-doped TbAG samples.
We have successfully obtained non-doped and Ce-doped TbAG samples. The typical size of as-grown sample rod was approximately 4 mm in diameter with 20 mm in length. We cut the sample rod to the thickness of approximately 1 mm for characterizations. Fig. 1 shows all the samples used for evaluations. Due to a strong absorption of 4f-5d transition of Ce3 þ in the UVblue range, Ce-doped samples looked yellow, and the coloration seems to become stronger with increasing the concentration of Ce. In addition, large fraction of cracks were evident. The inclusion of cracks was due to fast cooling rate during the FZ synthesis. Under UV light (302 nm), undoped TbAG showed green emission while the Ce-doped TbAG showed strong yellow emission. Fig. 2 shows the XRD patterns of undoped and Ce-doped TbAG. TbAG (JCPDS card No.76–1111) is also shown as a reference. All samples have the garnet phase structure. In 3.0% Ce-doped sample, TbO2 phase was detected. Therefore, the highly opaque appearance of the 3.0% Ce-doped sample is blamed for the TbO2 phase acting as scattering center to some extent. PLE and PL spectra of undoped and Ce-doped TbAG are shown in Fig. 3. In the PLE spectra of undoped TbAG, strong excitation bands were observed in the range of 200 250 nm and
250 400 nm and the spectral range agree with those of absorption bands due to the 4f-5f and 4f-4f transitions of Tb3 þ , respectively. The PL spectrum of undoped TbAG consisted of several emission lines at 490 nm, 540 nm, 590 nm and 620 nm, and these emission were due to the 5D4-7FJ (J ¼ 6-3) transitions of Tb3 þ [22–24]. The 5D4-7F5 transition line at 540 nm showed the strongest intensity. In the Ce-doped TbAG, PLE spectrum showed strong absorptions due to 4f-5d transition of Ce3 þ in the range of 400 – 500 nm in addition to that of Tb3 þ as observed in the nondoped sample. The PL spectrum of Ce-doped TbAG showed a broad emission due to the 5d-4f transitions of Ce3 þ in the wavelength range from 500 nm to 680 nm [25,26]. The emission peak slightly redshifted in relative to that of Ce:YAG due to modification of local crystal field [27]. Furthermore, emission lines due to Tb3 þ were negligible in the Ce-doped TbAG (even when directly exciting the Tb bands). Under excitation around 470 nm, the QY values of the 0.1%, 0.3%, 1.0% and 3.0% Ce-doped samples are 66%, 59%, 59% and 60%, respectively. These values are smaller than that of Ce:YAG but equivalent to those of Ce:LuAG [28,29]. Fig. 4 represents the PL decay curves of undoped and Ce-doped
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Fig. 3. PL excitation and emission spectra of undoped TbAG (top) and Ce-doped TbAG (bottom).
TbAG, respectively. The decay time of undoped TbAG was derived as 66 μs when monitoring the 5D4-7F5 emission of Tb3 þ (550 nm). It should be pointed out that the decay time of 4f4f transitions of Tb3 þ is often observed to be a few ms [30]. Much faster decay time observed in our sample suggests that concentration quenching is involved and it is reasonable because Tb is one of the constitution elements of the host material. This phenomena has been reported in TbAG [31,32]. In Ce-doped TbAG, the decay time of the 5d-4f transition of Ce3 þ (550 nm) was approximately 40 ns and consistent regardless of the dopant concentration. This latter decay time was typical for the 5d-4f transition of Ce3 þ in the garnet host [18,33,34]. X-ray induced RL spectra of undoped and Ce-doped TbAG are represented in Fig. 5. As for PL spectra, undoped TbAG showed emission lines at 490 nm, 540 nm, 590 nm and 620 nm due to the 5 D4-7FJ (J ¼ 6-3) transitions of Tb3 þ [15,35]. In Ce-doped TbAG, broad emission bands apparently due to the 5d-4f transition of Ce3 þ was dominant but a small contribution of Tb3 þ was also observed especially for those with smaller concentrations of Ce3 þ . As mentioned above, Tb3 þ emission was not observed even when excited at the Tb3 þ excitation bands, and this indicates that energy transfer from the Tb3 þ to Ce3 þ is very efficient. In scintillation, both Tb3 þ and Ce3 þ are excited by energetic secondary electrons generated in the host by X-rays, but since the energy transfer from the Tb3 þ to Ce3 þ so efficient, emission of Tb3 þ appeared to be much smaller than that of Ce3 þ . As in the PL spectrum, the emission peak position was slightly redshifted compared with that of Ce:YAG [36]. Fig. 6 represents X-ray induced scintillation decay time profiles of undoped and Ce-doped TbAG. In undoped TbAG, the measured decay time was 35 μs due to the Tb3 þ emission, and the observed decay time was faster than PL decay time. The observation of faster scintillation decay than that of PL is generally understood to be due to quenching of excited state by much higher excitation density during scintillation process. In Ce-doped TbAG, this fast
Fig. 4. PL decay curves of undoped (top) and Ce-doped (bottom) TbAG.
component was too fast to be discriminated from the instrumental response. We also observed slower response with the decay time of 376–599 ns, and these values were similar to that observed in Ce-doped TbAG films under α-ray excitation [15]. Unlike the Tb3 þ , a slower decay time of scintillation than PL was observed, and this behavior is more typical because scintillation process involves energy transportation process in addition to the luminescence process at the local center. Fig. 7 represent the afterglow curves of undoped and Ce-doped TbAG measured with 2 ms X-ray pulse irradiation. The afterglow levels at 20 ms of undoped and 0.1%, 0.3%, 1.0% and 3.0% Ce-doped TbAG were 0.736%, 0.112%, 0.120%, 0.114% and 0.172%, respectively. The 0.1% Ce-doped TbAG shows the lowest afterglow level. The afterglow levels (A) are calculated in the following expressions as A( %) = 100 ×( I2 − I0)/(I1 − I0) where I0, I1 and I2 were the average intensity before X-ray irradiation, average intensity during X-ray irradiation and intensity at 20 ms after X-ray irradiation, respectively. This definition of afterglow followed the protocol used by NIHON KESSHO KOGAKU CO., Ltd. which has a large market share of scintillation detectors for security and medical applications. Compared with the afterglow levels of Bi4Ge3O12 and CdWO4 ( 10 ppm) which are well-known to show very low afterglow [19], the afterglow levels of Ce-doped TbAG were significantly higher. Fig. 8 represents the pulse height spectra of Ce-doped TbAG measured under 137Cs γ-ray irradiation. The photoabsorption peak
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Fig. 5. X-ray induced RL spectra of undoped (top) and Ce-doped (bottom) TbAG.
channels and the absolute light yield are summarized in Table 1. The absolute light yield of the 0.1%, 0.3%, 1.0% and 3.0% Ce-doped TbAG were deduced to be 9,600, 16,800, 28,600 and 30,100 ph/ MeV with a typical error of 10%, respectively. Among the samples tested, the 3.0% Ce-doped TbAG showed the highest light yield. The energy resolution at 662 keV of the 0.1%, 0.3%, 1.0% and 3.0% Ce-doped TbAG were 19%, 45%, 22%, and 30%. In this analysis, we could not observe any significant signal from the nondoped sample. The absolute light yield of Ce-doped TbAG was comparable to other Ce-doped garnet scintillators such as Ce:YAG and Ce:LuAG [4,37]. However, it is instructive to mention here that since the past studies of Ce-doped TbAG [12] was performed only with the thin film form, it would not have been possible to determine scintillation light yield. In order to calculate the light yield, we used the 0.5% Ce:YAG transparent ceramic scintillator as a reference because the emission wavelength is similar to Ce-doped TbAG [4]. An absolute light yield of reference sample was 20,000 ph/MeV calibrated by using Si-APD and 55Fe source. Fig. 9 exhibits TSL glow curves of undoped and Ce-doped TbAG measured after 1 Gy X-ray irradiation. The starting temperature of 50 °C was the default value of the TSL reader designed for dosimetry purposes. Therefore, it neglected the glow peak around room temperature, and we could not measure the entire glow peak in the lower temperature range. All the samples showed a main glow peak around 150–250 °C. In addition, all the samples showed significantly large signal around 50 °C which is the origin of large afterglow. The undoped TbAG which had the lowest scintillation intensity (Fig. 6 and no detection in Fig. 9) showed the strongest TSL intensity. It suggests that many carriers were trapped instead of being transferred to luminescent center during scintillation. The trapped carries are so large that it showed strong
Fig. 6. X-ray induced scintillation decay time profiles of undoped (top) and Cedoped (bottom) TbAG.
Fig. 7. Afterglow curves of undoped and Ce-doped TbAG.
TSL signal. Recently, it was reported that the scintillation and TSL show a complemental relationship in some materials [38,39], and we think the same interpretation can be applied in this material system. Although this relation on TSL and scintillation was
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form of thin-film. In this study, we reported scintillation properties of TbAG in a bulk crystalline form, and we believe that our research result is very useful for general scintillator applications since scintillators are typically used in a bulk form. The Ce-doped TbAG showed a strong and broad scintillation emission peaking around 550 nm due to the 5d-4f transitions of Ce3 þ under X-irradiation. By pulse height spectroscopy analyses, absolute light yield was estimated, and the 3.0% Ce-doped TbAG showed the highest light yield of 30,100 ph/MeV under 137Cs γ-ray irradiation. The 0.1% Ce-doped TbAG exhibited the lowest afterglow level of 0.112%.
Acknowledgement
Fig. 8. Pulse height spectra of Ce-doped YAG and Ce-doped TbAG under exposure.
137
Cs
Table 1 The photoabsorption peaks and the absolute light yield from the pulse height spectra. Sample
Photoabsorption peak [channel]
Emission peak [nm] (QE of PMT)
Light yield [ph/MeV]
0.1% Ce: TbAG 0.3% Ce: TbAG 1.0% Ce: TbAG 3.0% Ce: TbAG 0.5% Ce: YAG
195
550 (9%)
9600 7 10%
340
550 (9%)
16,800 7 10%
580
550 (9%)
28,600 710%
610
550 (9%)
30,100 7 10%
720
520 (16%)
20,000 7 10%
Fig. 9. TSL glow curves of undoped and Ce-doped TbAG after 1 Gy X-ray irradiation.
observed in some materials [40–44], such a general relationship between scintillation and storage luminescence based on the simple energy conservation was not clearly proven until recently.
4. Conclusion We synthesized Ce-doped TbAG samples by the FZ method and evaluated their PL and scintillation properties. TbAG has not been studied for scintillator applications intensively, and as far as we are aware scintillation properties of TbAG have been only studied in a
This work was supported by a Grant in Aid for Scientific Research (A) 26249147 from the Ministry of Education, Culture, Sports, Science and Technology of the Japanese government (MEXT) and partially by JST A-step. The Cooperative Research Project of Research Institute of Electronics, Shizuoka University, KRF foundation, Hitachi Metals Materials Science foundation, and Inamori foundation are also acknowledged.
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