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Photoluminescence and scintillation properties of Ce–doped Ca(Gd,Y)Al3O7 single crystals
Optical Materials xxx (xxxx) xxx
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Photoluminescence and scintillation properties of Ce–doped Ca(Gd,Y)Al3O7 single crystals Kenta Igashira a, *, Daisuke Nakauchi a, Yutaka Fujimoto b, Takumi Kato a, Noriaki Kawaguchi a, Takayuki Yanagida a a b
Division of Materials Science, Nara Institute of Science and Technology (NAIST), 8916–5 Takayama, Ikoma, Nara, 630–0192, Japan Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 6–6 Aramaki, Aoba, Sendai, Miyagi, 980–8579, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Scintillator Single crystal Melilite Cerium Photoluminescence Afterglow
Ce–doped Ca(Gd,Y)Al3O7 single crystals were synthesized by the optical floating zone method, and the photo luminescence and scintillation properties were evaluated. Photoluminescence and scintillation with a broad band around 420 nm are observed in all the samples. The decay curves are well–approximated by a sum of two exponential functions, which are due to the 5d–4f transition of Ce3þ and intrinsic luminescence in the host material. As a result of the pulse height spectra under γ–rays irradiation from 241Am, the CaYAl3O7 sample shows the highest scintillation light yield of 6500 ph/MeV among the present samples.
1. Introduction Scintillators immediately convert ionizing radiations to numerous photons with a few eV and have been applied to various fields such as medical imaging [1,2], security [3] and geophysical logging [4]. Since the interaction between materials and ionizing radiations depends on a type of ionizing radiations and a chemical composition of materials, scintillation properties of various materials have been investigated for scintillator applications. In general, we require scintillators to have a short decay time, low afterglow, high scintillation light yield (LY), high energy resolution, high density, non–deliquescence and so on. A short decay time is one of the important scintillation properties to avoid pile up. Since the lumi nescence caused by the 5d–4f transitions of Ce3þ, Pr3þ and Eu2þ ions exhibits a relatively short decay time in a nanosecond order, such ions have been often used as a luminescent center [5,6]. Low afterglow is required particularly for application of integration–type detectors. For example, X–ray computed tomography collects projection data by moving X–ray tubes. Since signal intensities are integrated in about milliseconds, a detector cannot read out appropriate emission intensities owing to adding up background noise from afterglow signals. Then, redundant projection data causes blurred images with low spatial res olution [7]. High LY is also important to detect ionizing radiations with low–energy and low dose because it improves signal–to–noise ratio and
image resolution of imagers. For a charged particle detection like α–ray, it is necessary not to be sensitive to background such as X–ray, γ–ray and neutron so that 6Li, 10B and heavy elements are not suitable. As a result, substances with middle atomic number are often used for that applica tion. In addition, some scintillators with middle effective atomic number such as Ce–doped YAG or YSO is used for commercial small handy–type survey meter for X– and γ–rays, and if the performance is enough, they can be used for practical application. In general, most scintillators have been used in a single crystalline form by following reasons. Since single crystalline insulators show high transmittance in UV–visible regions, emitted photons can be efficiently delivered to photodetectors. In addition, single crystals are more ho mogeneous than polycrystals such as powders and ceramics, resulting the uniformity of scintillation output. This paper focuses on melilite compounds, generally expressed as X2ZT2O7 (X ¼ Ca, Sr and Na, Z ¼ Mg, Zn and Al and T ¼ Al, Fe and Si) [8]. Rare–earth–doped melilite compounds have been reported as phosphors for LEDs [9–12] and long–lasting luminescence materials [13–16]. In our previous work, Ce–doped melilite single crystals were evaluated for scintillator applications [17–19], and Ce:Ca2Al2SiO7 exhibited the LY of 16,000 ph/MeV [17]; therefore, the melilite com pounds are one of the promising candidates for scintillators. In this research, 0.3 at.% Ce–doped Ca(Gd1–xYx)Al3O7 (x ¼ 0, 0.25, 0.5, 0.75 and 1) (Ce:CGYAM) single crystals were grown by the optical floating
* Corresponding author. E-mail address: [email protected] (K. Igashira). https://doi.org/10.1016/j.optmat.2019.109497 Received 9 August 2019; Received in revised form 3 October 2019; Accepted 29 October 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Kenta Igashira, Optical Materials, https://doi.org/10.1016/j.optmat.2019.109497
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Fig. 2. Powder XRD patterns of Ce:CGYAM single crystals (left) and the enlarged figure at around 2θ ¼ 30–33� (right).
Fig. 1. Pictures of synthesized samples (x ¼ 1, 0.75, 0.5, 0.25, 0 samples from the left).
zone (FZ) method, and the photoluminescence (PL) and scintillation properties were evaluated. By the change of Gd and Y ratio, we can expect some modifications in band gap energy and crystal field around emission centers, and such modifications will affect scintillation properties. 2. Experimental 0.3 at.% Ce:CGYAM single crystals were synthesized by the optical FZ method [20]. First, raw powders of CaO (99.99%, Furuuchi Chemi cal), Al2O3 (99.99%, Taimei Chemicals), Gd2O3 (99.99%, Furuuchi Chemical), Y2O3 (99.99%, Furuuchi Chemical) and CeO2 (99.99%, Furuuchi Chemical) were homogeneously mixed with molar ratios of CaO:Al2O3:Gd2O3:Y2O3:CeO2 ¼ 2:3:1–x:x:0.006. Following the mixture process, the mixture was molded into a cylinder rod under hydrostatic pressure. Then, the obtained rods were sintered at 1250 � C for 12 h in air. The single crystals were grown in air by the optical FZ method (FZD0192, Canon Machinery). The pulling–down and the rotation speed were 5 mm/h and 20 rpm, respectively. These crystals show congruent melting, as shown in the previous report [21]. After synthesis, the transparent parts of obtained samples were cut into pieces with about 1 mm thickness to evaluate the PL and scintillation properties. Powder X–ray diffraction (XRD) patterns were measured using the diffractometer (MiniFlex600, Rigaku) over a 2θ range of 5–90� to identify the crystalline phase. The X–ray source is a micro–focus X–ray tube with CuKα target. The bias voltage and tube current were 40 kV and 15 mA, respectively. The PL excitation (PLE), PL spectra and quantum yield (QY) were evaluated using Quantaurus–QY (C11347–01, Hama matsu Photonics). The PL decay curves were measured using Quantaurus–τ (C11367, Hamamatsu Photonics). The emission signals were monitored at 420 nm under excitation at 280 nm. Scintillation spectra under X–ray irradiation were measured our original setup [22]. As an excitation source, the X–ray generator (XRB80N100/CB, Spellman) equips with an ordinary X–ray tube having a tungsten anode target and a beryllium window. Regarding the X–ray tube, the bias voltage and tube current were 40 kV and 1.2 mA, respectively. The scintillation photons emitted from the samples were led to the spectrometer (Andor DU–420–BU2 CCD with Shamrock SR163 monochromator) through an optical fiber to measure the spectrum. The detector was cooled down to 188 K with a Peltier module for reduction of thermal noise. Scintillation decay curves and X–ray induced afterglow curves were measured by an afterglow characterization system, which allowed the measurements by time–correlated single photon counting technique [23]. A photomultiplier tube (PMT) was used in this mea surement covered the spectral range of 160–650 nm. In terms of both PL decay curves and scintillation decay curves, the decay time constants were calculated by the least–squares fitting with a sum of a few expo nential decay functions. For evaluation of pulse height spectra under irradiation of γ–rays from 241Am, a sample covered with a few layers of Teflon reflectors was placed on the window of PMT (R7600U–200, Hamamatsu Photonics). Then, silicon grease (6262A, OKEN) was applied between bottom of the sample and the window to decrease a loss
Fig. 3. PLE/PL spectra of Ce:CGYAM single crystals.
of the scintillation photons. The scintillation light was converted into electrical signals using a preamplifier (113, ORTEC), a shaping–ampli fier (570, ORTEC) and a multichannel analyzer (Pocket MCA 8000A, Amptek). The shaping time for all the samples and a reference sample, Ce:Gd2SiO5 (Ce:GSO) [24], was 0.5 μs 3. Results and discussion The pictures and XRD patterns of the samples are shown in Figs. 1 and 2, respectively. The x ¼ 1 sample looks white translucent, the x ¼ 0 sample looks transparent but partly brown, and the others look colorless and transparent under room light. A purple emission is observed from all the samples under 254 nm UV light excitation. The crystal phase is identified by measuring XRD patterns; according to reference data of CaGdAl3O7 and CaYAl3O7 [25], all the samples belong to melilite sin gle–phase, space group of P421m [8]. As shown in Fig. 2 (right), a value of 2θ becomes bigger by the addition of Y. From this result, it is considered that an average ionic radius becomes smaller when the Y–ratio increases, and it is reasonable because the ionic radius of Y3þ is smaller than that of Gd3þ. The PLE/PL spectra of all the samples are illustrated in Fig. 3. Under the excitation at around 360 nm, a broad emission at around 420 nm is observed. The emission is due to the 5d–4f transitions of Ce3þ [26]. The PLE/PL contour plot of the x ¼ 1 sample presents the longest emission wavelength among the samples. As the Gd–ratio increases, the emission wavelength becomes shorter. From this result, we confirm the effect of the crystal field to emission centers weakens when the amount of Gd increases. Furthermore, the broader emission peak is observed as the amount of Y increases. It would be due to the presence of two different Ce3þ sites such as substitution to Y3þ and Gd3þ sites. PL emission spectra of Ce:CGYAM are consistent with those of the other Ce–doped melilites [17]. PL decay profiles were measured and the obtained PL decay time constants are shown in Table 1. The decay curves are approximated by a 2
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Table 1 PL and scintillation decay time constants of Ce:CGYAM single crystals. Sample
x¼1 x ¼ 0.75 x ¼ 0.5 x ¼ 0.25 x¼0
PL decay time [ns]
PL decay time [ns]
Scintillation decay time [ns]
Scintillation decay time [ns]
Short component
Long component
Short component
Long component
39.8 (99.6%) 39.4 (98.5%) 39.1 (97.5%) 38.4 (96.2%) 35.6 (94.2%)
266 312 274 368 133
41.5 (87.5%) 31.2 (84.4%) 34.3 (79.9%) 35.7 (82.0%) 32.1 (76.2%)
216 266 243 241 244
(0.4%) (1.5%) (2.5%) (3.8%) (5.8%)
(12.5%) (15.6%) (20.1%) (18.0%) (23.8%)
Fig. 6. Scintillation decay curves of Ce:CGYAM single crystals.
Fig. 4. QY values of Ce:CGYAM single crystals as a function of Y–ratio. Fig. 7. Pulse height spectra of γ–rays from single crystals.
241
Am measured using Ce:CGYAM
Table 2 LY, QY, energy migration efficiency from the host to luminescent centers (f (S→A)) and afterglow levels of Ce:CGYAM single crystals. f(S→A) is assumed that β is 2.5 and band gap energy is 6.8 eV.
Fig. 5. Scintillation spectra of Ce:CGYAM single crystals.
Sample
LY [ph/MeV]
QY [%]
f(S→A)
Afterglow levels [ppm]
x¼1 x ¼ 0.75 x ¼ 0.5 x ¼ 0.25 x¼0
6500 1500 1600 2100 690
46.4 59.6 59.3 37.4 16.0
0.24 0.04 0.05 0.10 0.07
1200 1500 2200 1600 700
the PLE/PL contour plots, and it would be due to the presence of two different Ce3þ sites such as substitution to Y3þ and Gd3þ sites. Fig. 6 shows scintillation decay curves, and the obtained scintillation decay time constants and percentage are shown in Table 1. The scintillation decay curves are approximated by a sum of two exponential functions with the decay time constants of 30–40 ns and 200–300 ns, which are slightly shorter than the PL decay time constants. One possible inter pretation for this is that the excitation density is far larger for scintil lation than for common PL; thus, interactions between the multiple excitation states are involved, which would reduce the luminescence decay time. Similar behaviors have been observed in some material systems [29,30]. They are ascribed to the 5d–4f transitions of Ce3þ and intrinsic luminescence, respectively. These values are slightly longer than those of the other Ce–doped melilite groups [17]. In both PL and scintillation, the x ¼ 1 sample shows longer decay time than the x ¼ 0 sample. One possible reason is emission peak shifts because the decay time shows inverse proportional relationship to the emission wave length [31]. The x ¼ 1 sample has longer emission component, as shown in Fig. 5, and the center of the emission wavelength of the x ¼ 1 sample is slightly longer. The other possible reason is that QY increases with the addition of Y, and the increase of QY suppresses quenching. Pulse height spectra under irradiation of γ–rays from 241Am are represented in Fig. 7, and LY, QY and energy migration efficiency from the host to luminescent centers (f(S→A)) of Ce:CGYAM are summarized
sum of two exponential functions, and the decay time constants of the primary components are approximately 40 ns while the secondary ones are 100–300 ns. They are due to the 5d–4f transitions of Ce3þ and intrinsic luminescence, respectively. The decay time constants due to the 5d–4f transitions of Ce3þ are slightly longer than those of other Ce–doped melilite species [17] but typical as Ce–doped phosphors [27, 28]. By the addition of Gd, primary decay time component becomes shorter owing to the shorter emission wavelength of Gd–containing samples. QY of the samples are plotted as a function of Y–ratio in Fig. 4. Among the samples, the x ¼ 0.5 and x ¼ 0.75 samples exhibit relatively high QY of 59.3% and 59.6%, respectively. They are higher than those of the other Ce–doped melilite scintillators such as Ca2Al2SiO7 (~20%) [17] and CaGdAl3O7 (~40%) [26]. As the Gd–ratio becomes higher, QY becomes smaller. It would also affect shortness of the primary decay time component of Gd–containing samples. Scintillation spectra upon X–ray excitation are demonstrated in Fig. 5. A broad emission around 420 nm is observed in all the samples, which is attributable to the 5d–4f transitions of Ce3þ. In addition, a sharp peak at around 310 nm is observed from the x ¼ 0.25, 0.5, 0.75 samples because of the 4f–4f transitions of Gd3þ (6P7/2–8S7/2). In fully Gd–substituted sample (x ¼ 1), the emission due to 4f–4f transition of Gd3þ disappears due to the concentration quenching of Gd. By the addition of Y, the width of the emission peak becomes broader same as 3
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irradiation of γ–rays from 241Am, the CaYAl3O7 sample has the highest LY of 6500 ph/MeV among the present samples. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by Grant–in–Aid for Scientific Research A (17H01375), Scientific Research B (18H03468) and JSPS Research Fellow (17J09488) from JSPS. The Cooperative Research Project of Research Center for Biomedical Engineering, NSG Foundation, Iketani Science and Technology Foundation, Murata Foundation and NAIST Foundation are also acknowledged.
Fig. 8. X–ray induced afterglow curves of Ce:CGYAM single crystals.
in Table 2. Although we generally use 137Cs for this evaluation, we could not observe a photoabsorption peak in light materials (e.g., x ¼ 0). Thus, instead of 137Cs (662 keV), we used 241Am (60 keV) for measurements. Commercial Ce:GSO scintillator (Oxide corp.) was also measured as a reference sample because it had a similar emission wavelength with our samples. The x ¼ 1 sample has the highest LY of 6500 ph/MeV. Although the x ¼ 0.5 and the 0.75 samples show relatively high QY, those LY are relatively low. It would be considered that the x ¼ 0.5 and 0.75 samples have lower f(S→A) than those of the others. LY is written by equation (1), where qA is QY at the luminescent centers, β is the constant parameter and Eg is band gap energy (βEg is the average energy to form a pair of electron and hole). Based on equation (1), LY depends on not only qA but also f(S→A). f(S→A) of all the samples were estimated as β and Eg are supposed 2.5 and 6.8 eV [32], respectively. Although Eg of each compound are different, and samples with smaller x will have smaller Eg, rough analysis will help us to grasp the tendency. Except for the x ¼ 1 sample, all the samples exhibit similar f(S→A), and relatively low f (S→A) would be the reason for low LY of x ¼ 0.5 and 0.75 samples. On the other hand, f(S→A) of the x ¼ 1 sample is the highest among the present samples as well as LY. It may be due to easy energy migration owing to shorter distance between Ce and Y than that between Ce and Gd because lattice constant becomes smaller by the increase of Y–ratio from the XRD patterns of Ce:CGYAM single crystals. Since the estimation of f(S→A) is not common in relating fields, further estimations for many kinds of materials will be required to obtain clear understanding. � LY ½ph = MeV� ¼ 106 ⋅f ðS→AÞ⋅qA βE : g
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(1)
Fig. 8 demonstrates the afterglow curves after X–ray exposure of 2 ms, and afterglow levels of all the samples are summarized in Table 2. The afterglow level (A) is defined as equation (2). Here, I1 is the average signal intensity during X–ray irradiation, I2 is the signal intensity at 20 ms after X–ray irradiation is cut off and IBG is the average background signal. A ½ppm� ¼ 106 ⋅ðI2
� IBG Þ I 1
IBG :
(2)
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Optical Materials journal homepage: http://www.elsevier.com/locate/optmat
Photoluminescence and scintillation properties of Ce–doped Ca(Gd,Y)Al3O7 single crystals Kenta Igashira a, *, Daisuke Nakauchi a, Yutaka Fujimoto b, Takumi Kato a, Noriaki Kawaguchi a, Takayuki Yanagida a a b
Division of Materials Science, Nara Institute of Science and Technology (NAIST), 8916–5 Takayama, Ikoma, Nara, 630–0192, Japan Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 6–6 Aramaki, Aoba, Sendai, Miyagi, 980–8579, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Scintillator Single crystal Melilite Cerium Photoluminescence Afterglow
Ce–doped Ca(Gd,Y)Al3O7 single crystals were synthesized by the optical floating zone method, and the photo luminescence and scintillation properties were evaluated. Photoluminescence and scintillation with a broad band around 420 nm are observed in all the samples. The decay curves are well–approximated by a sum of two exponential functions, which are due to the 5d–4f transition of Ce3þ and intrinsic luminescence in the host material. As a result of the pulse height spectra under γ–rays irradiation from 241Am, the CaYAl3O7 sample shows the highest scintillation light yield of 6500 ph/MeV among the present samples.
1. Introduction Scintillators immediately convert ionizing radiations to numerous photons with a few eV and have been applied to various fields such as medical imaging [1,2], security [3] and geophysical logging [4]. Since the interaction between materials and ionizing radiations depends on a type of ionizing radiations and a chemical composition of materials, scintillation properties of various materials have been investigated for scintillator applications. In general, we require scintillators to have a short decay time, low afterglow, high scintillation light yield (LY), high energy resolution, high density, non–deliquescence and so on. A short decay time is one of the important scintillation properties to avoid pile up. Since the lumi nescence caused by the 5d–4f transitions of Ce3þ, Pr3þ and Eu2þ ions exhibits a relatively short decay time in a nanosecond order, such ions have been often used as a luminescent center [5,6]. Low afterglow is required particularly for application of integration–type detectors. For example, X–ray computed tomography collects projection data by moving X–ray tubes. Since signal intensities are integrated in about milliseconds, a detector cannot read out appropriate emission intensities owing to adding up background noise from afterglow signals. Then, redundant projection data causes blurred images with low spatial res olution [7]. High LY is also important to detect ionizing radiations with low–energy and low dose because it improves signal–to–noise ratio and
image resolution of imagers. For a charged particle detection like α–ray, it is necessary not to be sensitive to background such as X–ray, γ–ray and neutron so that 6Li, 10B and heavy elements are not suitable. As a result, substances with middle atomic number are often used for that applica tion. In addition, some scintillators with middle effective atomic number such as Ce–doped YAG or YSO is used for commercial small handy–type survey meter for X– and γ–rays, and if the performance is enough, they can be used for practical application. In general, most scintillators have been used in a single crystalline form by following reasons. Since single crystalline insulators show high transmittance in UV–visible regions, emitted photons can be efficiently delivered to photodetectors. In addition, single crystals are more ho mogeneous than polycrystals such as powders and ceramics, resulting the uniformity of scintillation output. This paper focuses on melilite compounds, generally expressed as X2ZT2O7 (X ¼ Ca, Sr and Na, Z ¼ Mg, Zn and Al and T ¼ Al, Fe and Si) [8]. Rare–earth–doped melilite compounds have been reported as phosphors for LEDs [9–12] and long–lasting luminescence materials [13–16]. In our previous work, Ce–doped melilite single crystals were evaluated for scintillator applications [17–19], and Ce:Ca2Al2SiO7 exhibited the LY of 16,000 ph/MeV [17]; therefore, the melilite com pounds are one of the promising candidates for scintillators. In this research, 0.3 at.% Ce–doped Ca(Gd1–xYx)Al3O7 (x ¼ 0, 0.25, 0.5, 0.75 and 1) (Ce:CGYAM) single crystals were grown by the optical floating
* Corresponding author. E-mail address: [email protected] (K. Igashira). https://doi.org/10.1016/j.optmat.2019.109497 Received 9 August 2019; Received in revised form 3 October 2019; Accepted 29 October 2019 0925-3467/© 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Kenta Igashira, Optical Materials, https://doi.org/10.1016/j.optmat.2019.109497
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Fig. 2. Powder XRD patterns of Ce:CGYAM single crystals (left) and the enlarged figure at around 2θ ¼ 30–33� (right).
Fig. 1. Pictures of synthesized samples (x ¼ 1, 0.75, 0.5, 0.25, 0 samples from the left).
zone (FZ) method, and the photoluminescence (PL) and scintillation properties were evaluated. By the change of Gd and Y ratio, we can expect some modifications in band gap energy and crystal field around emission centers, and such modifications will affect scintillation properties. 2. Experimental 0.3 at.% Ce:CGYAM single crystals were synthesized by the optical FZ method [20]. First, raw powders of CaO (99.99%, Furuuchi Chemi cal), Al2O3 (99.99%, Taimei Chemicals), Gd2O3 (99.99%, Furuuchi Chemical), Y2O3 (99.99%, Furuuchi Chemical) and CeO2 (99.99%, Furuuchi Chemical) were homogeneously mixed with molar ratios of CaO:Al2O3:Gd2O3:Y2O3:CeO2 ¼ 2:3:1–x:x:0.006. Following the mixture process, the mixture was molded into a cylinder rod under hydrostatic pressure. Then, the obtained rods were sintered at 1250 � C for 12 h in air. The single crystals were grown in air by the optical FZ method (FZD0192, Canon Machinery). The pulling–down and the rotation speed were 5 mm/h and 20 rpm, respectively. These crystals show congruent melting, as shown in the previous report [21]. After synthesis, the transparent parts of obtained samples were cut into pieces with about 1 mm thickness to evaluate the PL and scintillation properties. Powder X–ray diffraction (XRD) patterns were measured using the diffractometer (MiniFlex600, Rigaku) over a 2θ range of 5–90� to identify the crystalline phase. The X–ray source is a micro–focus X–ray tube with CuKα target. The bias voltage and tube current were 40 kV and 15 mA, respectively. The PL excitation (PLE), PL spectra and quantum yield (QY) were evaluated using Quantaurus–QY (C11347–01, Hama matsu Photonics). The PL decay curves were measured using Quantaurus–τ (C11367, Hamamatsu Photonics). The emission signals were monitored at 420 nm under excitation at 280 nm. Scintillation spectra under X–ray irradiation were measured our original setup [22]. As an excitation source, the X–ray generator (XRB80N100/CB, Spellman) equips with an ordinary X–ray tube having a tungsten anode target and a beryllium window. Regarding the X–ray tube, the bias voltage and tube current were 40 kV and 1.2 mA, respectively. The scintillation photons emitted from the samples were led to the spectrometer (Andor DU–420–BU2 CCD with Shamrock SR163 monochromator) through an optical fiber to measure the spectrum. The detector was cooled down to 188 K with a Peltier module for reduction of thermal noise. Scintillation decay curves and X–ray induced afterglow curves were measured by an afterglow characterization system, which allowed the measurements by time–correlated single photon counting technique [23]. A photomultiplier tube (PMT) was used in this mea surement covered the spectral range of 160–650 nm. In terms of both PL decay curves and scintillation decay curves, the decay time constants were calculated by the least–squares fitting with a sum of a few expo nential decay functions. For evaluation of pulse height spectra under irradiation of γ–rays from 241Am, a sample covered with a few layers of Teflon reflectors was placed on the window of PMT (R7600U–200, Hamamatsu Photonics). Then, silicon grease (6262A, OKEN) was applied between bottom of the sample and the window to decrease a loss
Fig. 3. PLE/PL spectra of Ce:CGYAM single crystals.
of the scintillation photons. The scintillation light was converted into electrical signals using a preamplifier (113, ORTEC), a shaping–ampli fier (570, ORTEC) and a multichannel analyzer (Pocket MCA 8000A, Amptek). The shaping time for all the samples and a reference sample, Ce:Gd2SiO5 (Ce:GSO) [24], was 0.5 μs 3. Results and discussion The pictures and XRD patterns of the samples are shown in Figs. 1 and 2, respectively. The x ¼ 1 sample looks white translucent, the x ¼ 0 sample looks transparent but partly brown, and the others look colorless and transparent under room light. A purple emission is observed from all the samples under 254 nm UV light excitation. The crystal phase is identified by measuring XRD patterns; according to reference data of CaGdAl3O7 and CaYAl3O7 [25], all the samples belong to melilite sin gle–phase, space group of P421m [8]. As shown in Fig. 2 (right), a value of 2θ becomes bigger by the addition of Y. From this result, it is considered that an average ionic radius becomes smaller when the Y–ratio increases, and it is reasonable because the ionic radius of Y3þ is smaller than that of Gd3þ. The PLE/PL spectra of all the samples are illustrated in Fig. 3. Under the excitation at around 360 nm, a broad emission at around 420 nm is observed. The emission is due to the 5d–4f transitions of Ce3þ [26]. The PLE/PL contour plot of the x ¼ 1 sample presents the longest emission wavelength among the samples. As the Gd–ratio increases, the emission wavelength becomes shorter. From this result, we confirm the effect of the crystal field to emission centers weakens when the amount of Gd increases. Furthermore, the broader emission peak is observed as the amount of Y increases. It would be due to the presence of two different Ce3þ sites such as substitution to Y3þ and Gd3þ sites. PL emission spectra of Ce:CGYAM are consistent with those of the other Ce–doped melilites [17]. PL decay profiles were measured and the obtained PL decay time constants are shown in Table 1. The decay curves are approximated by a 2
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Table 1 PL and scintillation decay time constants of Ce:CGYAM single crystals. Sample
x¼1 x ¼ 0.75 x ¼ 0.5 x ¼ 0.25 x¼0
PL decay time [ns]
PL decay time [ns]
Scintillation decay time [ns]
Scintillation decay time [ns]
Short component
Long component
Short component
Long component
39.8 (99.6%) 39.4 (98.5%) 39.1 (97.5%) 38.4 (96.2%) 35.6 (94.2%)
266 312 274 368 133
41.5 (87.5%) 31.2 (84.4%) 34.3 (79.9%) 35.7 (82.0%) 32.1 (76.2%)
216 266 243 241 244
(0.4%) (1.5%) (2.5%) (3.8%) (5.8%)
(12.5%) (15.6%) (20.1%) (18.0%) (23.8%)
Fig. 6. Scintillation decay curves of Ce:CGYAM single crystals.
Fig. 4. QY values of Ce:CGYAM single crystals as a function of Y–ratio. Fig. 7. Pulse height spectra of γ–rays from single crystals.
241
Am measured using Ce:CGYAM
Table 2 LY, QY, energy migration efficiency from the host to luminescent centers (f (S→A)) and afterglow levels of Ce:CGYAM single crystals. f(S→A) is assumed that β is 2.5 and band gap energy is 6.8 eV.
Fig. 5. Scintillation spectra of Ce:CGYAM single crystals.
Sample
LY [ph/MeV]
QY [%]
f(S→A)
Afterglow levels [ppm]
x¼1 x ¼ 0.75 x ¼ 0.5 x ¼ 0.25 x¼0
6500 1500 1600 2100 690
46.4 59.6 59.3 37.4 16.0
0.24 0.04 0.05 0.10 0.07
1200 1500 2200 1600 700
the PLE/PL contour plots, and it would be due to the presence of two different Ce3þ sites such as substitution to Y3þ and Gd3þ sites. Fig. 6 shows scintillation decay curves, and the obtained scintillation decay time constants and percentage are shown in Table 1. The scintillation decay curves are approximated by a sum of two exponential functions with the decay time constants of 30–40 ns and 200–300 ns, which are slightly shorter than the PL decay time constants. One possible inter pretation for this is that the excitation density is far larger for scintil lation than for common PL; thus, interactions between the multiple excitation states are involved, which would reduce the luminescence decay time. Similar behaviors have been observed in some material systems [29,30]. They are ascribed to the 5d–4f transitions of Ce3þ and intrinsic luminescence, respectively. These values are slightly longer than those of the other Ce–doped melilite groups [17]. In both PL and scintillation, the x ¼ 1 sample shows longer decay time than the x ¼ 0 sample. One possible reason is emission peak shifts because the decay time shows inverse proportional relationship to the emission wave length [31]. The x ¼ 1 sample has longer emission component, as shown in Fig. 5, and the center of the emission wavelength of the x ¼ 1 sample is slightly longer. The other possible reason is that QY increases with the addition of Y, and the increase of QY suppresses quenching. Pulse height spectra under irradiation of γ–rays from 241Am are represented in Fig. 7, and LY, QY and energy migration efficiency from the host to luminescent centers (f(S→A)) of Ce:CGYAM are summarized
sum of two exponential functions, and the decay time constants of the primary components are approximately 40 ns while the secondary ones are 100–300 ns. They are due to the 5d–4f transitions of Ce3þ and intrinsic luminescence, respectively. The decay time constants due to the 5d–4f transitions of Ce3þ are slightly longer than those of other Ce–doped melilite species [17] but typical as Ce–doped phosphors [27, 28]. By the addition of Gd, primary decay time component becomes shorter owing to the shorter emission wavelength of Gd–containing samples. QY of the samples are plotted as a function of Y–ratio in Fig. 4. Among the samples, the x ¼ 0.5 and x ¼ 0.75 samples exhibit relatively high QY of 59.3% and 59.6%, respectively. They are higher than those of the other Ce–doped melilite scintillators such as Ca2Al2SiO7 (~20%) [17] and CaGdAl3O7 (~40%) [26]. As the Gd–ratio becomes higher, QY becomes smaller. It would also affect shortness of the primary decay time component of Gd–containing samples. Scintillation spectra upon X–ray excitation are demonstrated in Fig. 5. A broad emission around 420 nm is observed in all the samples, which is attributable to the 5d–4f transitions of Ce3þ. In addition, a sharp peak at around 310 nm is observed from the x ¼ 0.25, 0.5, 0.75 samples because of the 4f–4f transitions of Gd3þ (6P7/2–8S7/2). In fully Gd–substituted sample (x ¼ 1), the emission due to 4f–4f transition of Gd3þ disappears due to the concentration quenching of Gd. By the addition of Y, the width of the emission peak becomes broader same as 3
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irradiation of γ–rays from 241Am, the CaYAl3O7 sample has the highest LY of 6500 ph/MeV among the present samples. Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by Grant–in–Aid for Scientific Research A (17H01375), Scientific Research B (18H03468) and JSPS Research Fellow (17J09488) from JSPS. The Cooperative Research Project of Research Center for Biomedical Engineering, NSG Foundation, Iketani Science and Technology Foundation, Murata Foundation and NAIST Foundation are also acknowledged.
Fig. 8. X–ray induced afterglow curves of Ce:CGYAM single crystals.
in Table 2. Although we generally use 137Cs for this evaluation, we could not observe a photoabsorption peak in light materials (e.g., x ¼ 0). Thus, instead of 137Cs (662 keV), we used 241Am (60 keV) for measurements. Commercial Ce:GSO scintillator (Oxide corp.) was also measured as a reference sample because it had a similar emission wavelength with our samples. The x ¼ 1 sample has the highest LY of 6500 ph/MeV. Although the x ¼ 0.5 and the 0.75 samples show relatively high QY, those LY are relatively low. It would be considered that the x ¼ 0.5 and 0.75 samples have lower f(S→A) than those of the others. LY is written by equation (1), where qA is QY at the luminescent centers, β is the constant parameter and Eg is band gap energy (βEg is the average energy to form a pair of electron and hole). Based on equation (1), LY depends on not only qA but also f(S→A). f(S→A) of all the samples were estimated as β and Eg are supposed 2.5 and 6.8 eV [32], respectively. Although Eg of each compound are different, and samples with smaller x will have smaller Eg, rough analysis will help us to grasp the tendency. Except for the x ¼ 1 sample, all the samples exhibit similar f(S→A), and relatively low f (S→A) would be the reason for low LY of x ¼ 0.5 and 0.75 samples. On the other hand, f(S→A) of the x ¼ 1 sample is the highest among the present samples as well as LY. It may be due to easy energy migration owing to shorter distance between Ce and Y than that between Ce and Gd because lattice constant becomes smaller by the increase of Y–ratio from the XRD patterns of Ce:CGYAM single crystals. Since the estimation of f(S→A) is not common in relating fields, further estimations for many kinds of materials will be required to obtain clear understanding. � LY ½ph = MeV� ¼ 106 ⋅f ðS→AÞ⋅qA βE : g
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(1)
Fig. 8 demonstrates the afterglow curves after X–ray exposure of 2 ms, and afterglow levels of all the samples are summarized in Table 2. The afterglow level (A) is defined as equation (2). Here, I1 is the average signal intensity during X–ray irradiation, I2 is the signal intensity at 20 ms after X–ray irradiation is cut off and IBG is the average background signal. A ½ppm� ¼ 106 ⋅ðI2
� IBG Þ I 1
IBG :
(2)
The afterglow level of the x ¼ 0 sample is the lowest among the samples. However, it is still higher than those of typical scintillators like Bi4Ge3O12 and CdWO4 (~10 ppm) [23]. The higher afterglow levels of Gd–containing samples may be affected by the emission from 4f–4f transition of Gd3þ which shows typically ms order decay time. 4. Conclusions Ce–doped Ca(Gd,Y)Al3O7 single crystals were grown by the optical FZ method and PL and scintillation properties were evaluated. The samples show PL and scintillation dominantly with a broad emission peaking at around 420 nm due to the 5d–4f transitions of Ce3þ. The PL and scintillation decay time constants are 30–40 ns and 100–300 ns, and are attributable to the 5d–4f transitions of Ce3þ and intrinsic lumines cence, respectively. Suggested from the pulse height spectra under 4
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