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A fast lutetium aluminum garnet scintillation ceramic with Ce3+ and Ca2+ co-dopants
Journal of Luminescence 216 (2019) 116728
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
A fast lutetium aluminum garnet scintillation ceramic with Ce3+ and Ca2+ co-dopants
T
Wanqiu Maa,b, Benxue Jianga,∗∗, Shuilin Chena,b, Xiqi Fengd, Qiangqiang Zhua, Long Zhanga,c,∗, Krittiya Sreebunpenge a
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, Shanghai, 201800, China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China c Collaborative Innovation Center of IFSA (CICIFSA), Shanghai Jiao Tong University, Shanghai 200240, China d Key Laboratory of Transparent and Opto-functional Advanced Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China e Faculty of Science, Chandrakasem Rajabhat University, Bangkok 10900, Thailand b
A R T I C LE I N FO
A B S T R A C T
Keywords: Ca2+ co-doping Scintillation ceramic Fast component
A Ca2+ ions codoped LuAG:Ce scintillation ceramic was fabricated and oxygen annealing treatment was then applied to eliminate the oxygen vacancies. The Ca2+ codoped ceramic exhibits a faster decay time (42 ns) than the uncodoped one. And the light yield at 1μs is about 24,100 ph/MeV and reaches to 28,200 ph/MeV when using a shaping time of 12 μs. The slow scintillation component is also suppressed. The fast-to-slow ratio calculated as LY(1μs)/LY(12 μs) is approximately 85.5%. With Ca2+ ions codopant, the intensity of thermally stimulated luminescence was greatly reduced and Ce3+ ions was partially oxidized to Ce4+ ions. The significant improvement of the scintillation performance indicate that the Ca2+ ions codopoing is a good method to modify the performance of LuAG:Ce ceramic scintillator.
1. Introduction
scintillation performance [5–7]. Further, as reported by Chen Hu et al., the light yield and transmittance of LuAG:Ce ceramic remain almost unchanged after 220 Mrad gamma-ray radiation [8]. Commonly used scintillators include crystals and ceramics. Among LuAG crystals, owing to the high melting point and long growth time, they are prone to antisite defects (AD), which is one of the reasons for the slow component in scintillators [9–11]. Consequently, the light yield is reportedly 18,000 ph/MeV, which is far below the theoretical value [10]. In the contrast, a lower preparation temperature would greatly reduce antisite defects in ceramics. Besides, the preparation cost of ceramic materials is lower than crystal materials, making ceramic scintillators a competitive material [11]. Recent studies of Ce-doped LuAG crystals and ceramics have shown that Me+/Me2+ ion co-doping can improve the scintillation performance, mainly because Ce3+ is partially converted to Ce4+. Compared to Ce3+, Ce4+ has a faster luminescence process, which could reduce the slow component. MgO is typically used as a sintering aid, and a Mg2+ and Ce3+ co-doped LuAG ceramic was found to show a light yield of 21,000 ph/MeV due to less shallow traps and valence change of some
Electromagnetic calorimeter is an indispensable part of high-energy physics (HEP) experiments. As the core material of electromagnetic calorimeters, the performance of scintillators, like light yield, decay time and radiation hardness, will greatly affect the detection efficiency. For HEP experiments, scintillators should have high density, fast decay time, good radiation damage resistance, and small slow component contamination [1]. Since the existence of the Higgs boson has been validated, faster and radiation-harder scintillators have become necessary to meet the demanding challenges of future HEP experiments. Among many scintillators, LYSO:Ce crystals are considered to be the most promising candidate material for updating the CMS at CERN because of its excellent scintillation performance [2,3]. However, after γray irradiation with a dose of 106 rad, the transmittance of a LYSO crystal decreased by approximately 8%, and its light yield decreased by approximately 10–15% [4]. Therefore, materials with better radiation damage resistance are in need. Lu3Al5O12:Ce (LuAG:Ce) is a very competitive scintillator with good
∗
Corresponding author. Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, Shanghai, 201800, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (B. Jiang), [email protected] (L. Zhang). https://doi.org/10.1016/j.jlumin.2019.116728 Received 22 April 2019; Received in revised form 12 July 2019; Accepted 1 September 2019 Available online 03 September 2019 0022-2313/ © 2019 Published by Elsevier B.V.
Journal of Luminescence 216 (2019) 116728
W. Ma, et al.
Ce3+ ions [12]. However, Mg2+ ions would not only replace Lu3+, but also easily enter interstitial sites and form nonradiative centers. LuAG:Ce ceramics co-doped with Li+ have also reportedly shown a higher light yield and faster decay time than the undoped ceramic [13]. However, like Mg2+ ions, Li+ ions have a smaller radius, which makes it difficult for them to enter the lattice and causes defects that degrade the scintillation properties. The fast-to-slow ratio calculated as LY(1 μs)/LY(10 μs) is only 61%. And the decay time needs further improvement as well. In 2013, Blahuta et al. found that a LYSO:Ce crystal co-doped with Ca2+ ions had more Ce4+ ions and fewer defects than the Mg2+ co-doped crystal, which makes it have higher light yield and better energy resolution [14]. Further, it was reported that the decay time of GGAG:Ce crystals became faster with Ca2+ co-dopant, as did the rise time [15]. Thus, it is expected that Ca2+ ions can also be co-doped into LuAG:Ce ceramics to realize better performance. In this study, a Ca2+ ions co-doped LuAG:Ce scintillation ceramic was fabricated. Oxygen-annealing treatment was then applied to optimize the performance of the ceramic samples. The phase composition, microstructure, and optical absorption spectrum were studied in detail. The radioluminescence spectra, decay time and light yield were also analyzed to evaluate the applicability of the obtained Ca2+ ions codoped ceramics. In order to have a sight of the defects effects, the thermally stimulated luminescence curves were carefully recorded.
Fig. 1. XRD patterns of the fabricated LuAG:Ce and LuAG:Ce, Ca ceramics.
(4f–5d2) and 445 (4f–5d1) nm [17–19]. It should be noted that for the Ca2+ ions codoped ceramic sample, the two absorption peaks become weaker, whereas the absorption below 320 nm is clearly enhanced as shown in the insert. This enhancement is related to the charge-transfer transition of Ce4+ [14,18]. In analogy to the Mg2+ codoped LuAG:Ce ceramic, substitution of Ca2+ ions for Lu3+ ions will change the valence state of some Ce3+ ions and introduce point defects like oxygen vacancies and O− centers in order to maintain the charge balance. The processes of Ca2+ ion getting into the LuAG lattice and charge compensation can be described as follows:
2. Experiment procedure A traditional solid-state reaction method was used to fabricate LuAG:Ce, Ca (doped with 0.3 at% Ce and 0.2 at% Ca) ceramic. And ceramic without Ca2+ ions was also made for comparison. High purity commercial powders including Lu2O3, CaF2, Al2O3, and CeO2 were used. After being ball-milled, dried, sieved and pre-sintered, the powder was dry-pressed and cold isostatic pressed into green body. The prepared green bodies were then sintered in vacuum atmosphere at temperature higher than 1800 °C for 12–18 h. Afterwards, a high temperature oxygen-annealing treatment was performed to the ceramics. Then, the prepared ceramics were polished to 1 mm. The X-ray diffraction (XRD) measurements were performed between 10 and 90° (Rigaku Co.). The absorption spectra were measured by JASCOV-570 UV/Vis/IR spectrophotometer at 200–700 nm. A fieldemission scanning electron microscopy was used to have a look in the microstructure of prepared ceramics. In order to analyze the distribution of ceramic grain size, a software Nano Measurer 1.2 was used. Besides, an X-ray excited luminescence spectrometer was used to obtain the radioluminescence spectra. For the thermally stimulated luminescence (TSL) curve measurement, the ceramics were heated to 500 K after X-ray irradiation at 78 K for 15 min and the glow curves were recorded. The measurement details of decay time and light yield were described in Ref. [16].
× ′ + VO⋅⋅ + F2 ↑ CaF2 + LuLu → CaLu × ′ + OO⋅ + F2 ↑ CaF2 + LuLu + OO× → CaLu ⋅ ′ + CeLu CaF2 + Lu (Ce )×Lu (Ce) → CaLu + F2 ↑ × ⋅ VO⋅⋅ + CeLu + O2 → CeLu + OO×
It can also be seen from Fig. 3 that after annealed in oxygen atmosphere, the absorption intensity of the UV region is further enhanced. This indicates that another portion of Ce3+ ions is oxidized to the tetravalent state. The radiolumiscence spectra of the LuAG:Ce and LuAG:Ce, Ca ceramics before (denoted by S) and after (denoted by O) oxygen-annealing are presented in Fig. 4. For comparison, a BGO single crystal was also analyzed. The typical Ce3+ ions emission band ranges from 480 to 700 nm and the peak located at 524 nm is owing to the 5d1 – 4f transition, which is in accordance with previous reports [7,20–23]. After annealed in oxygen atmosphere, the XEL intensity of the ceramics both increases due to elimination of oxygen vacancies. It is a notable fact that the ceramic codoped with Ca2+ ions has a lower XEL intensity than the uncodoped ceramic, regardless of whether the ceramic is annealed or not. This is because the incorporation of Ca2+ ions will introduce some point defects, like oxygen vacancies and O− centers, which will block the transport process of carriers. Besides, the emission spectrum of ceramic matches with the sensitive detection wavelength region of Si-based photodiodes, which is helpful to improve the detection efficiency. Although the XEL intensity of the annealed Ca2+ codoped ceramic is lower than the uncodoped one, its light yield is more higher. The values of scintillation LY of the oxygen-annealed LuAG:Ce and LuAG:Ce, Ca ceramics are presented in Table 1. At a shaping time of 1 μs, the LY of LuAG:Ce, Ca is as high as 24,100 ph/MeV, while the LY value of the uncodoped ceramic is 13,100 ph/MeV. Fig. 5 shows the relative light yield of the two annealed samples as a function of shaping time, from
3. Results and discussion To identify the phase composition of fabricated ceramics, XRD measurements were conducted and the patterns are presented in Fig. 1. The patterns matched PDF card No. 73–1368 well. No impurity peaks were observed, demonstrating that Ca2+ ions codoping does not hinder the realization of pure LuAG:Ce ceramics. Fig. 2 exhibits the microstructure of etched (a) and fracture (b) surface of as-prepared LuAG:Ce, Ca ceramic, from which the grain boundaries are very clean. And the sample has a dense structure with no pores observed, as can be seen from Fig. 2(b). In addition, the distribution of grain size was analyzed by software Nano Measure 1.2; about 150 counts were made to ensure accuracy. As shown in the insert of Fig. 2(a), the grain size ranges from 0.6 to 6 μm, and most of the grains have a size of about 2.1 μm. The absorption spectra of the prepared LuAG:Ce and LuAG:Ce, Ca ceramics are shown in Fig. 3. For samples without oxygen-annealing treatment, the absorption peaks are centered at approximately 345 2
Journal of Luminescence 216 (2019) 116728
W. Ma, et al.
Fig. 2. SEM micrographs of etched (a) and fracture (b) surface of the fabricated LuAG:Ce, Ca ceramic.
decay time curves were recorded, as presented in Fig. 6. Because of the instantaneous and delayed radiative combination at the luminescence centers, the scintillation decay exhibits two components of fast and slow. The obtained decay curves can be fitted with the function described blew:
I (t ) = A1 exp (−t / τ1) + A2 exp (−t / τ2) + B among which τ1 and τ2 are the fast and slow component of the decay time, respectively [13,24]. In addition, the relative intensity of each part can be calculated as
Ii = Ai τi /(A1 τ1 + A2 τ2), i = 1,2 Table 2 present the values of decay time and the relatively intensity of each part. The Ca2+ ions codoped ceramic has a fast decay of about 42 ns, which is faster than that of the uncodoped ceramic. In addition, the relative intensity of fast decay in the LuAG:Ce ceramic is approximately 41%, and it increases to about 68% when Ca2+ ions are present. Thermally stimulated luminescence measurements were performed to investigate the effect of Ca2+ ions codoping on defects. Fig. 7 shows the TSL curves of the LuAG:Ce and LuAG:Ce, Ca ceramics in 80–500 K. The TSL peaks match well with those reported earlier in LuAG:Ce single crystal [25]. Obviously, the TSL peak intensity of Ca2+ ions codopoed ceramic was significantly reduced. And the annealing treatment could further reduce the TSL intensity, as can be seen from Fig. 7, which means the negative effect of defects in the annealed Ca2+ codoped LuAG:Ce ceramic was greatly reduced [26–29]. As mentioned above, for the purpose of maintaining the charge balance, Ca2+ ions entered into the LuAG lattice would introduce point defects like oxygen vacancies and O− centers in the vicinity. Moreover, ⋅ Ce3+ ions would be partially oxidized to Ce4+ (CeLu ) ions as well. The subsequent annealing treatment would eliminate oxygen vacancies and further oxidize Ce3+ ions to Ce4+ ions. During luminescence process, the Ce4+ centers would complete to capture electrons with electron defects from the conduction band, producing excited (Ce3+)* centers. Then, the excited (Ce3+)* centers would release photons without capturing holes from the valence band, which makes the luminescence process faster than the Ce3+ ions. To return to the initial form of Ce4+ ions, the de-excited Ce3+ centers would capture holes from the valence band or hole traps. Further, the O− centers located nearby the de-excited Ce3+ centers can act as hole sources [9,30,31]. In addition, in Me2+ ions codoped LuAG:Ce scintillators, O− will reportedly be affected by the nearby Me2+ ions and form a perturbed center (O−–Me2+) [32]. This disturbed center makes the cyclic scintillation of Ce4+ center efficiency, thus making the scintillation LY higher than the uncodoped one. And the hole transition from O− to the de-excited Ce3+ center is non-radiative, which will do no harm to the scintillation process. Besides, in the LuAG host, Me2+ ions would not only replace Lu3+ (0.98 Å, dodecahedron) [33], but also easily enter the interstitial sites
Fig. 3. Absorption spectra of the prepared LuAG:Ce and LuAG:Ce, Ca ceramics before (denoted by S) and after (denoted by O) annealing. Inset is the difference absorption spectrum of the unannealed LuAG:Ce and LuAG:Ce, Ca ceramics.
Fig. 4. Radiolumiscence spectra of LuAG:Ce and LuAG:Ce, Ca ceramics before (denoted by S) and after (denoted by O) oxygen-annealing.
which the value of LY gradually increases with the shaping time increases. Moreover, the LY (1 μs)/LY (12 μs) ratio of the LuAG:Ce, Ca ceramic is approximately 85.5%, indicating a relatively small proportion of slow component contamination in the scintillation response. To find out the impact of Ca2+ ions on timing characteristic, the 3
Journal of Luminescence 216 (2019) 116728
W. Ma, et al.
Table 1 Light yield value of oxygen-annealed LuAG:Ce and LuAG:Ce, Ca ceramics. Shaping time (μs)
1
2
4
8
12
LY (1 μs)/LY(12 μs) (%)
LY of LuAG:Ce O (ph/Mev) LY of LuAG:Ce, Ca O (ph/MeV)
13,100 24,100
14,100 24,300
15,500 24,600
17,300 26,200
18,900 28,200
69.3% 85.5%
Fig. 5. Relative light yield of the annealed LuAG:Ce and LuAG:Ce, Ca ceramics at different shaping time from 1 μs to 12 μs. The curves are normalized by the LY for 1 μs shaping time.
Fig. 7. TSL curves of LuAG:Ce and LuAG:Ce, Ca ceramics before (denoted by S) and after (denoted by O) oxygen-annealing.
(1.12 Å, dodecahedron) are more likely to substitute Lu3+ ions. And the relatively low codoping concentration (0.2 at%) would reduce the content of Ca2+ ions at interstitial sites so that the effect of nonradiative recombination centers was also reduced. Indeed, the TSL peak intensity of the annealed LuAG:Ce, Ca ceramic was greatly reduced, which enables energy to be transferred quickly and efficiently to the luminescence centers and results in a faster scintillation decay time and a higher light yield value.
4. Conclusions In this paper, a Ca2+ ions codoped LuAG:Ce ceramic was fabricated. With Ca2+ ions codoping, the optical absorption at 200–320 nm was clearly enhanced, indicating an increase of the Ce4+ ions content. And the XEL intensity of LuAG:Ce, Ca ceramic is lower than that of the uncodoped one whether annealing treatment was performed or not. In addition, the oxygen-annealed Ca2+ ions codoped LuAG:Ce ceramic exhibited a high light yield of 24,100 ph/MeV at 1μs and 28,200 ph/ MeV at 12 μs; the fast decay time was reduced to 42 ns, and slow component was greatly reduced. In the LuAG:Ce, Ca ceramic, Ce3+ were partially oxided to Ce4+, which can effectively compete with electron traps to capture electrons. Moreover, due to the Ca2+ co-dopant and annealing treatment, the intensity of TSL peaks were also suppressed, leading to the improvement of decay time and light yield. The good scintillation performance of the fabricated Ca2+ ions codoped LuAG:Ce ceramic indicates its great promise for applications in HEP experiments.
Fig. 6. Decay curves of oxygen-annealed LuAG:Ce and LuAG:Ce, Ca ceramics. Table 2 Scintillation decay time of the oxygen-annealed LuAG:Ce and LuAG:Ce, Ca ceramics. And the calculated relative intensity of each part is also presented. Sample
τ1 (I1) ns (%)
τ2 (I2) ns (%)
LuAG:Ce LuAG:Ce, Ca
55 (41) 42(68)
517 (59) 241 (32)
and form nonradiative recombination centers, which degrades the scintillation performance. Such as the Mg2+ (0.89 Å, dodecahedron) codoped LuAG:Ce ceramic, it have been reported that the light yield decreases gradually if the concentration of Mg2+ ions was higher than 0.3 at%. It was believed that this decrease was caused by the negative effect of the interstitial sites atoms and phase separation [34]. Note that because the solid solution energy of Ca2+ ions in the LuAG lattice is the lowest among Me2+ codopants (except for Cd2+) [35], Ca2+ ions
Acknowledgment This work was financially supported by the National Key Research & Development Program of China (2017YFB0310503), National Nature Science Funds of China (Nos U1830125 and 11535010), Youth Innovation Promotion Association of CAS, and General Financial Grant from the China Postdoctoral Science Foundation (No. 2016M601654). 4
Journal of Luminescence 216 (2019) 116728
W. Ma, et al.
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
A fast lutetium aluminum garnet scintillation ceramic with Ce3+ and Ca2+ co-dopants
T
Wanqiu Maa,b, Benxue Jianga,∗∗, Shuilin Chena,b, Xiqi Fengd, Qiangqiang Zhua, Long Zhanga,c,∗, Krittiya Sreebunpenge a
Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, Shanghai, 201800, China Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China c Collaborative Innovation Center of IFSA (CICIFSA), Shanghai Jiao Tong University, Shanghai 200240, China d Key Laboratory of Transparent and Opto-functional Advanced Inorganic Materials, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China e Faculty of Science, Chandrakasem Rajabhat University, Bangkok 10900, Thailand b
A R T I C LE I N FO
A B S T R A C T
Keywords: Ca2+ co-doping Scintillation ceramic Fast component
A Ca2+ ions codoped LuAG:Ce scintillation ceramic was fabricated and oxygen annealing treatment was then applied to eliminate the oxygen vacancies. The Ca2+ codoped ceramic exhibits a faster decay time (42 ns) than the uncodoped one. And the light yield at 1μs is about 24,100 ph/MeV and reaches to 28,200 ph/MeV when using a shaping time of 12 μs. The slow scintillation component is also suppressed. The fast-to-slow ratio calculated as LY(1μs)/LY(12 μs) is approximately 85.5%. With Ca2+ ions codopant, the intensity of thermally stimulated luminescence was greatly reduced and Ce3+ ions was partially oxidized to Ce4+ ions. The significant improvement of the scintillation performance indicate that the Ca2+ ions codopoing is a good method to modify the performance of LuAG:Ce ceramic scintillator.
1. Introduction
scintillation performance [5–7]. Further, as reported by Chen Hu et al., the light yield and transmittance of LuAG:Ce ceramic remain almost unchanged after 220 Mrad gamma-ray radiation [8]. Commonly used scintillators include crystals and ceramics. Among LuAG crystals, owing to the high melting point and long growth time, they are prone to antisite defects (AD), which is one of the reasons for the slow component in scintillators [9–11]. Consequently, the light yield is reportedly 18,000 ph/MeV, which is far below the theoretical value [10]. In the contrast, a lower preparation temperature would greatly reduce antisite defects in ceramics. Besides, the preparation cost of ceramic materials is lower than crystal materials, making ceramic scintillators a competitive material [11]. Recent studies of Ce-doped LuAG crystals and ceramics have shown that Me+/Me2+ ion co-doping can improve the scintillation performance, mainly because Ce3+ is partially converted to Ce4+. Compared to Ce3+, Ce4+ has a faster luminescence process, which could reduce the slow component. MgO is typically used as a sintering aid, and a Mg2+ and Ce3+ co-doped LuAG ceramic was found to show a light yield of 21,000 ph/MeV due to less shallow traps and valence change of some
Electromagnetic calorimeter is an indispensable part of high-energy physics (HEP) experiments. As the core material of electromagnetic calorimeters, the performance of scintillators, like light yield, decay time and radiation hardness, will greatly affect the detection efficiency. For HEP experiments, scintillators should have high density, fast decay time, good radiation damage resistance, and small slow component contamination [1]. Since the existence of the Higgs boson has been validated, faster and radiation-harder scintillators have become necessary to meet the demanding challenges of future HEP experiments. Among many scintillators, LYSO:Ce crystals are considered to be the most promising candidate material for updating the CMS at CERN because of its excellent scintillation performance [2,3]. However, after γray irradiation with a dose of 106 rad, the transmittance of a LYSO crystal decreased by approximately 8%, and its light yield decreased by approximately 10–15% [4]. Therefore, materials with better radiation damage resistance are in need. Lu3Al5O12:Ce (LuAG:Ce) is a very competitive scintillator with good
∗
Corresponding author. Key Laboratory of Materials for High Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Science, Shanghai, 201800, China. ∗∗ Corresponding author. E-mail addresses: [email protected] (B. Jiang), [email protected] (L. Zhang). https://doi.org/10.1016/j.jlumin.2019.116728 Received 22 April 2019; Received in revised form 12 July 2019; Accepted 1 September 2019 Available online 03 September 2019 0022-2313/ © 2019 Published by Elsevier B.V.
Journal of Luminescence 216 (2019) 116728
W. Ma, et al.
Ce3+ ions [12]. However, Mg2+ ions would not only replace Lu3+, but also easily enter interstitial sites and form nonradiative centers. LuAG:Ce ceramics co-doped with Li+ have also reportedly shown a higher light yield and faster decay time than the undoped ceramic [13]. However, like Mg2+ ions, Li+ ions have a smaller radius, which makes it difficult for them to enter the lattice and causes defects that degrade the scintillation properties. The fast-to-slow ratio calculated as LY(1 μs)/LY(10 μs) is only 61%. And the decay time needs further improvement as well. In 2013, Blahuta et al. found that a LYSO:Ce crystal co-doped with Ca2+ ions had more Ce4+ ions and fewer defects than the Mg2+ co-doped crystal, which makes it have higher light yield and better energy resolution [14]. Further, it was reported that the decay time of GGAG:Ce crystals became faster with Ca2+ co-dopant, as did the rise time [15]. Thus, it is expected that Ca2+ ions can also be co-doped into LuAG:Ce ceramics to realize better performance. In this study, a Ca2+ ions co-doped LuAG:Ce scintillation ceramic was fabricated. Oxygen-annealing treatment was then applied to optimize the performance of the ceramic samples. The phase composition, microstructure, and optical absorption spectrum were studied in detail. The radioluminescence spectra, decay time and light yield were also analyzed to evaluate the applicability of the obtained Ca2+ ions codoped ceramics. In order to have a sight of the defects effects, the thermally stimulated luminescence curves were carefully recorded.
Fig. 1. XRD patterns of the fabricated LuAG:Ce and LuAG:Ce, Ca ceramics.
(4f–5d2) and 445 (4f–5d1) nm [17–19]. It should be noted that for the Ca2+ ions codoped ceramic sample, the two absorption peaks become weaker, whereas the absorption below 320 nm is clearly enhanced as shown in the insert. This enhancement is related to the charge-transfer transition of Ce4+ [14,18]. In analogy to the Mg2+ codoped LuAG:Ce ceramic, substitution of Ca2+ ions for Lu3+ ions will change the valence state of some Ce3+ ions and introduce point defects like oxygen vacancies and O− centers in order to maintain the charge balance. The processes of Ca2+ ion getting into the LuAG lattice and charge compensation can be described as follows:
2. Experiment procedure A traditional solid-state reaction method was used to fabricate LuAG:Ce, Ca (doped with 0.3 at% Ce and 0.2 at% Ca) ceramic. And ceramic without Ca2+ ions was also made for comparison. High purity commercial powders including Lu2O3, CaF2, Al2O3, and CeO2 were used. After being ball-milled, dried, sieved and pre-sintered, the powder was dry-pressed and cold isostatic pressed into green body. The prepared green bodies were then sintered in vacuum atmosphere at temperature higher than 1800 °C for 12–18 h. Afterwards, a high temperature oxygen-annealing treatment was performed to the ceramics. Then, the prepared ceramics were polished to 1 mm. The X-ray diffraction (XRD) measurements were performed between 10 and 90° (Rigaku Co.). The absorption spectra were measured by JASCOV-570 UV/Vis/IR spectrophotometer at 200–700 nm. A fieldemission scanning electron microscopy was used to have a look in the microstructure of prepared ceramics. In order to analyze the distribution of ceramic grain size, a software Nano Measurer 1.2 was used. Besides, an X-ray excited luminescence spectrometer was used to obtain the radioluminescence spectra. For the thermally stimulated luminescence (TSL) curve measurement, the ceramics were heated to 500 K after X-ray irradiation at 78 K for 15 min and the glow curves were recorded. The measurement details of decay time and light yield were described in Ref. [16].
× ′ + VO⋅⋅ + F2 ↑ CaF2 + LuLu → CaLu × ′ + OO⋅ + F2 ↑ CaF2 + LuLu + OO× → CaLu ⋅ ′ + CeLu CaF2 + Lu (Ce )×Lu (Ce) → CaLu + F2 ↑ × ⋅ VO⋅⋅ + CeLu + O2 → CeLu + OO×
It can also be seen from Fig. 3 that after annealed in oxygen atmosphere, the absorption intensity of the UV region is further enhanced. This indicates that another portion of Ce3+ ions is oxidized to the tetravalent state. The radiolumiscence spectra of the LuAG:Ce and LuAG:Ce, Ca ceramics before (denoted by S) and after (denoted by O) oxygen-annealing are presented in Fig. 4. For comparison, a BGO single crystal was also analyzed. The typical Ce3+ ions emission band ranges from 480 to 700 nm and the peak located at 524 nm is owing to the 5d1 – 4f transition, which is in accordance with previous reports [7,20–23]. After annealed in oxygen atmosphere, the XEL intensity of the ceramics both increases due to elimination of oxygen vacancies. It is a notable fact that the ceramic codoped with Ca2+ ions has a lower XEL intensity than the uncodoped ceramic, regardless of whether the ceramic is annealed or not. This is because the incorporation of Ca2+ ions will introduce some point defects, like oxygen vacancies and O− centers, which will block the transport process of carriers. Besides, the emission spectrum of ceramic matches with the sensitive detection wavelength region of Si-based photodiodes, which is helpful to improve the detection efficiency. Although the XEL intensity of the annealed Ca2+ codoped ceramic is lower than the uncodoped one, its light yield is more higher. The values of scintillation LY of the oxygen-annealed LuAG:Ce and LuAG:Ce, Ca ceramics are presented in Table 1. At a shaping time of 1 μs, the LY of LuAG:Ce, Ca is as high as 24,100 ph/MeV, while the LY value of the uncodoped ceramic is 13,100 ph/MeV. Fig. 5 shows the relative light yield of the two annealed samples as a function of shaping time, from
3. Results and discussion To identify the phase composition of fabricated ceramics, XRD measurements were conducted and the patterns are presented in Fig. 1. The patterns matched PDF card No. 73–1368 well. No impurity peaks were observed, demonstrating that Ca2+ ions codoping does not hinder the realization of pure LuAG:Ce ceramics. Fig. 2 exhibits the microstructure of etched (a) and fracture (b) surface of as-prepared LuAG:Ce, Ca ceramic, from which the grain boundaries are very clean. And the sample has a dense structure with no pores observed, as can be seen from Fig. 2(b). In addition, the distribution of grain size was analyzed by software Nano Measure 1.2; about 150 counts were made to ensure accuracy. As shown in the insert of Fig. 2(a), the grain size ranges from 0.6 to 6 μm, and most of the grains have a size of about 2.1 μm. The absorption spectra of the prepared LuAG:Ce and LuAG:Ce, Ca ceramics are shown in Fig. 3. For samples without oxygen-annealing treatment, the absorption peaks are centered at approximately 345 2
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Fig. 2. SEM micrographs of etched (a) and fracture (b) surface of the fabricated LuAG:Ce, Ca ceramic.
decay time curves were recorded, as presented in Fig. 6. Because of the instantaneous and delayed radiative combination at the luminescence centers, the scintillation decay exhibits two components of fast and slow. The obtained decay curves can be fitted with the function described blew:
I (t ) = A1 exp (−t / τ1) + A2 exp (−t / τ2) + B among which τ1 and τ2 are the fast and slow component of the decay time, respectively [13,24]. In addition, the relative intensity of each part can be calculated as
Ii = Ai τi /(A1 τ1 + A2 τ2), i = 1,2 Table 2 present the values of decay time and the relatively intensity of each part. The Ca2+ ions codoped ceramic has a fast decay of about 42 ns, which is faster than that of the uncodoped ceramic. In addition, the relative intensity of fast decay in the LuAG:Ce ceramic is approximately 41%, and it increases to about 68% when Ca2+ ions are present. Thermally stimulated luminescence measurements were performed to investigate the effect of Ca2+ ions codoping on defects. Fig. 7 shows the TSL curves of the LuAG:Ce and LuAG:Ce, Ca ceramics in 80–500 K. The TSL peaks match well with those reported earlier in LuAG:Ce single crystal [25]. Obviously, the TSL peak intensity of Ca2+ ions codopoed ceramic was significantly reduced. And the annealing treatment could further reduce the TSL intensity, as can be seen from Fig. 7, which means the negative effect of defects in the annealed Ca2+ codoped LuAG:Ce ceramic was greatly reduced [26–29]. As mentioned above, for the purpose of maintaining the charge balance, Ca2+ ions entered into the LuAG lattice would introduce point defects like oxygen vacancies and O− centers in the vicinity. Moreover, ⋅ Ce3+ ions would be partially oxidized to Ce4+ (CeLu ) ions as well. The subsequent annealing treatment would eliminate oxygen vacancies and further oxidize Ce3+ ions to Ce4+ ions. During luminescence process, the Ce4+ centers would complete to capture electrons with electron defects from the conduction band, producing excited (Ce3+)* centers. Then, the excited (Ce3+)* centers would release photons without capturing holes from the valence band, which makes the luminescence process faster than the Ce3+ ions. To return to the initial form of Ce4+ ions, the de-excited Ce3+ centers would capture holes from the valence band or hole traps. Further, the O− centers located nearby the de-excited Ce3+ centers can act as hole sources [9,30,31]. In addition, in Me2+ ions codoped LuAG:Ce scintillators, O− will reportedly be affected by the nearby Me2+ ions and form a perturbed center (O−–Me2+) [32]. This disturbed center makes the cyclic scintillation of Ce4+ center efficiency, thus making the scintillation LY higher than the uncodoped one. And the hole transition from O− to the de-excited Ce3+ center is non-radiative, which will do no harm to the scintillation process. Besides, in the LuAG host, Me2+ ions would not only replace Lu3+ (0.98 Å, dodecahedron) [33], but also easily enter the interstitial sites
Fig. 3. Absorption spectra of the prepared LuAG:Ce and LuAG:Ce, Ca ceramics before (denoted by S) and after (denoted by O) annealing. Inset is the difference absorption spectrum of the unannealed LuAG:Ce and LuAG:Ce, Ca ceramics.
Fig. 4. Radiolumiscence spectra of LuAG:Ce and LuAG:Ce, Ca ceramics before (denoted by S) and after (denoted by O) oxygen-annealing.
which the value of LY gradually increases with the shaping time increases. Moreover, the LY (1 μs)/LY (12 μs) ratio of the LuAG:Ce, Ca ceramic is approximately 85.5%, indicating a relatively small proportion of slow component contamination in the scintillation response. To find out the impact of Ca2+ ions on timing characteristic, the 3
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Table 1 Light yield value of oxygen-annealed LuAG:Ce and LuAG:Ce, Ca ceramics. Shaping time (μs)
1
2
4
8
12
LY (1 μs)/LY(12 μs) (%)
LY of LuAG:Ce O (ph/Mev) LY of LuAG:Ce, Ca O (ph/MeV)
13,100 24,100
14,100 24,300
15,500 24,600
17,300 26,200
18,900 28,200
69.3% 85.5%
Fig. 5. Relative light yield of the annealed LuAG:Ce and LuAG:Ce, Ca ceramics at different shaping time from 1 μs to 12 μs. The curves are normalized by the LY for 1 μs shaping time.
Fig. 7. TSL curves of LuAG:Ce and LuAG:Ce, Ca ceramics before (denoted by S) and after (denoted by O) oxygen-annealing.
(1.12 Å, dodecahedron) are more likely to substitute Lu3+ ions. And the relatively low codoping concentration (0.2 at%) would reduce the content of Ca2+ ions at interstitial sites so that the effect of nonradiative recombination centers was also reduced. Indeed, the TSL peak intensity of the annealed LuAG:Ce, Ca ceramic was greatly reduced, which enables energy to be transferred quickly and efficiently to the luminescence centers and results in a faster scintillation decay time and a higher light yield value.
4. Conclusions In this paper, a Ca2+ ions codoped LuAG:Ce ceramic was fabricated. With Ca2+ ions codoping, the optical absorption at 200–320 nm was clearly enhanced, indicating an increase of the Ce4+ ions content. And the XEL intensity of LuAG:Ce, Ca ceramic is lower than that of the uncodoped one whether annealing treatment was performed or not. In addition, the oxygen-annealed Ca2+ ions codoped LuAG:Ce ceramic exhibited a high light yield of 24,100 ph/MeV at 1μs and 28,200 ph/ MeV at 12 μs; the fast decay time was reduced to 42 ns, and slow component was greatly reduced. In the LuAG:Ce, Ca ceramic, Ce3+ were partially oxided to Ce4+, which can effectively compete with electron traps to capture electrons. Moreover, due to the Ca2+ co-dopant and annealing treatment, the intensity of TSL peaks were also suppressed, leading to the improvement of decay time and light yield. The good scintillation performance of the fabricated Ca2+ ions codoped LuAG:Ce ceramic indicates its great promise for applications in HEP experiments.
Fig. 6. Decay curves of oxygen-annealed LuAG:Ce and LuAG:Ce, Ca ceramics. Table 2 Scintillation decay time of the oxygen-annealed LuAG:Ce and LuAG:Ce, Ca ceramics. And the calculated relative intensity of each part is also presented. Sample
τ1 (I1) ns (%)
τ2 (I2) ns (%)
LuAG:Ce LuAG:Ce, Ca
55 (41) 42(68)
517 (59) 241 (32)
and form nonradiative recombination centers, which degrades the scintillation performance. Such as the Mg2+ (0.89 Å, dodecahedron) codoped LuAG:Ce ceramic, it have been reported that the light yield decreases gradually if the concentration of Mg2+ ions was higher than 0.3 at%. It was believed that this decrease was caused by the negative effect of the interstitial sites atoms and phase separation [34]. Note that because the solid solution energy of Ca2+ ions in the LuAG lattice is the lowest among Me2+ codopants (except for Cd2+) [35], Ca2+ ions
Acknowledgment This work was financially supported by the National Key Research & Development Program of China (2017YFB0310503), National Nature Science Funds of China (Nos U1830125 and 11535010), Youth Innovation Promotion Association of CAS, and General Financial Grant from the China Postdoctoral Science Foundation (No. 2016M601654). 4
Journal of Luminescence 216 (2019) 116728
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