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Micro-ion beam-induced luminescence spectroscopy for evaluating SiAlON scintillators
Nuclear Inst. and Methods in Physics Research B xxx (xxxx) xxx–xxx
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Micro-ion beam-induced luminescence spectroscopy for evaluating SiAlON scintillators W. Kadaa, , T. Satohb, S. Yamadac, M. Kokad, N. Yamadab, K. Miuraa, O. Hanaizumia ⁎
a
Faculty of Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan Takasaki Advanced Radiation Research Institute (TARRI), Quantum Beam Science Research Directorate, National Institutes for Quantum and Radiological Science and Technology (QST), 1233 Watanuki-machi, Takasaki, Gunma 370-1292, Japan c Research & Development Department, Denka Co. Ltd., 2-1-1 Nihonbashi-Muromachi, Chuo-ku, Tokyo 103-8338, Japan d Beam Operation Co., Ltd, 1233, Watanukicho, Takasaki, Gunma 370-1292, Japan b
ARTICLE INFO
ABSTRACT
Keywords: IBIL SiAlON Single grain Spectroscopy Imaging
Ion beam-induced luminescence (IBIL) spectroscopy was used for luminescent material characterization and analysis for the investigation of new scintillator families. Under continuous 3 MeV proton microbeam irradiation, the crystal structures of α-SiAlON:Eu, β-SiAlON:Eu, and CaAlSiN3 (CASN) emitted bright luminescence at peak wavelengths of 605, 540, and 670 nm, respectively. As the irradiation progressed, the IBIL intensity of the conventional ZnS:Ag scintillator decreased sharply, whereas that of the SiAlONs and CASN remained within the detection limit. IBIL spectroscopy was performed on individual grains of the SiAlON scintillators. IBIL imaging and spectroscopy of two grains of β-SiAlON:Eu showed that the main peak in the IBIL spectrum of β-SiAlON:Eu obtained from a large-area beam scan consisted of several small peaks, which were observed in spectra from individual grains. Our experimental results suggest that microscopic spectroscopy of IBIL is an effective tool for microscopic material characterization of luminescent targets.
1. Introduction Scintillators are commonly used in various types of ion-beam experiments, especially where convenient beam profiling or charged particle detection is required [1–5]. The experiments may require the scintillators to be exposed continuously to intense radiation or high temperatures, and most conventional scintillators are susceptible to radiation damage and are degraded by intense or focused irradiation [6,7]. Therefore, scintillators that are more resistant to radiation are particularly desirable. There is growing interest in SiAlON scintillators, originally designed as substrates for white light-emitting diodes [8], that have excellent luminescence properties and outstanding thermal and chemical stabilities [9]. Because SiAlONs have a hexagonal crystal structure equivalent to that of phosphors substituted with silicon nitride (Si3N4) ceramics [10], they are similarly robust. In addition, low thermal quenching is achieved by doping SiAlONs with Eu, and the luminescence of the phosphor is maintained even up to 423 K [11]. Therefore, SiAlONs are also expected to be robust to radiation damage caused by charged particle impacts. Ion beam analysis is used to characterize SiAlON ceramic phosphors [12] and the scintillation properties of α-SiAlON (YL600A) and β-
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SiAlON (GR-200 and MW540H) powders have been evaluated by in situ luminescence measurements by ion beam-induced luminescence (IBIL) analysis under focused proton microbeam irradiation [13,14]. IBIL is utilized for its sensitivity to the chemical composition of inorganic targets [15–17] and radiation damage [18,19]. In our previous studies, SiAlONs showed excellent luminescent yields, comparable with the conventional scintillator, ZnS:Ag [13]. Single broadband emission spectrum which can be described as 4f → 5d transition of Eu2+ sustains its original IBIL intensity during irradiation because of robust crystal structure of SiAlONs. In addition, the advantages of SiAlONs at high temperatures have also been evaluated by temperature-controlled IBIL [14]. IBIL at high temperatures revealed that SiAlONs are candidate scintillators for charged-particle detection at temperatures above 373 K, at which conventional luminescent scintillators (e.g., ZnS:Ag) cannot operate. Hence, SiAlONs are considered as promising scintillators for ionizing radiation monitoring. However, in some occasion, small shifts in peak wavelength of IBIL were observed from different portion of SiAlONs scintillators individually prepared for irradiation. Irradiation might cause such changes but such energy shift may also be generated from minor differences in crystal structures at microscopic level. By comparing peak wavelength of IBIL spectrum with that of
Corresponding author. E-mail address: [email protected] (W. Kada).
https://doi.org/10.1016/j.nimb.2019.10.001 Received 7 May 2019; Received in revised form 1 October 2019; Accepted 2 October 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: W. Kada, et al., Nuclear Inst. and Methods in Physics Research B, https://doi.org/10.1016/j.nimb.2019.10.001
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photoluminescence or cathodoluminescence, similar excitation and radiative recombination occurs by ultraviolet lights, electrons, or ions [20–22]. Taking into account these electron transition processes, beam current would affect IBIL spectra by radiation damage but not the shift in the spectrum. To investigate the origin of such shift, more precise luminescent properties of the scintillators similar to SiAlON phosphor should be characterized by IBIL microscopic spectroscopy using a proton microprobe. In this study, we evaluated additional scintillator candidate of CaAlSiN3 phoshpor as well as previously evaluated scintillators of αSiAlON:Eu, β-SiAlON:Eu by IBIL imaging and spectroscopy with a 3 MeV proton microprobe. CaAlSiN3 (CASN) phosphor has similarity with both SiAlONs in its base crystal structure, activator, adequate thermal stability, and high light emission yield [23]. Since differences in composition might bring interesting information related to the spectral shifts and changes in IBIL, CASN and SiAlON scintillator powders were characterized by combined analysis and imaging with particle-induced X-ray emission (PIXE) and IBIL using a focused proton microbeam. We recorded the IBIL spectra continuously and determined the peak wavelength ranging from 540 to 670 nm, and decay in intensity. In addition, microscopic IBIL spectroscopy was performed in the area visualized by PIXE and IBIL imaging. The experimental results suggested that IBIL imaging and spectroscopy is an effective tool for detailed characterization of the microscopic luminescent properties of scintillators.
cm3 for ZnS:Ag inputted into SRIM2013 software, 3 MeV protons is roughly estimated to penetrate into each target approximately 62.7 and 66.5 μm, respectively. Effective IBIL intensity may be largely affected by actual grain sizes and their aggregate surface coverage homogeneity. Some differences can also be expected due to minor variations in grains chemical composition. All in-depth scintillators proton excitation is guaranteed due to the nominal 30 µm thick grain layer. The samples were sandwiched between two polyimide films of the same thickness. For single-particle observations in a vacuum chamber, carbon tape was used to fix the scintillators. Clusters to single agglomerates of α-SiAlON:Eu, β-SiAlON:Eu, or CaAlSiN3 powder was spread over the adhesive tape by electrostatically collecting and fixing the powder to the tape manually. An aluminum plate was used to fix the tape bearing the scintillator powders to a three-dimensional movable stage in a vacuum chamber. A focused proton microprobe with a typical diameter of 1 μm was used for both the external and vacuum analytical conditions. Beam size was evaluated prior to irradiation using a copper mesh and a secondary electron imaging system [31]. A Faraday cup was placed behind the sample to monitor the beam current. Under vacuum analytical conditions, the beam current passing through the sample holder was also monitored during irradiation. A beam current of less than 100 pA was used for IBIL spectroscopy in conjunction with the micro-PIXE analysis.
2. Materials and methods
3.1. Comparison of IBIL spectra for CASN and SiAlON scintillators
2.1. Experimental setup
The IBIL spectrum of CaAlSiN3 scintillator was compared with that of α-SiAlON:Eu, β-SiAlON:Eu as shown in Fig. 2. The typical exposure time for IBIL spectroscopy was approximately 1000 ms under 3 MeV focused proton microbeam irradiation with a beam current of approximately 10 pA. CaAlSiN3 emitted intense IBIL similar to that from both α-SiAlON:Eu and β-SiAlON:Eu scintillators, which was quite similar to the value previously evaluated elsewhere [14]. All three scintillators gave excellent photon emission yields which is comparable with that of the ZnS:Ag scintillator [32]. The peak wavelengths for IBIL of α-SiAlON:Eu, β-SiAlON:Eu, and CaAlSiN3 were 605, 540, and 670 nm, respectively. Spectrum resolution was also obtained from the full width half maximum (FWHM) of 105.15 nm for CaAlSiN3, compared with 68.25 and 40.62 nm for α-SiAlON:Eu and β-SiAlON:Eu, respectively. The controllability of the peak wavelength is one of the advantages of these scintillators which may be suitable for various radioluminescence applications while their peak wavelengths match the maximum quantum efficiency region of commonly used visible photon sensors and detectors [33]. The differences in crystal structure of SiAlONs and CaAlSiN3 seems to be the origin of the peak shifts. The decay occurred during continuous irradiation of focused beam was investigated by continuous IBIL spectroscopy. During the irradiation, a steep decay in intensity was observed for conventional ZnS:Ag, whereas a slow decrease was observed for α-SiAlON:Eu, β-SiAlON:Eu, and CaAlSiN3 with little fluctuation. The decay in intensity with respect to beam fluence F (ions/cm2) and normalized IBIL intensity IBIL(F) (arb. units) was determined for all scintillators (Fig. 3). IBIL intensity was delivered by integrating for each single broadband envelop appeared in each IBIL spectrum. Then each IBIL intensity IBIL(F) obtained at beam fluence F (ions/cm2) was normalized the value of virgin IBIL (0). The parameters were fitted with a degradation model [14,34] with two decay factors, F1 and F2, as expressed by
3. Results and discussion
IBIL imaging and spectroscopy of SiAlON scintillators was performed with a microbeam setup of the single-ended accelerator at TARRI/QST [24] using custom-designed confocal micro-optics with an anti-reflection coating and a charge-coupled device (CCD) spectrometer (Solid Lambda CCD, Spectra Co-op) [25,26]. A schematic of the experimental setup is shown in Fig. 1. The system can switch between microbeam irradiation in a vacuum or in air. The effective area of IBIL imaging and spectroscopy was approximately 800 μm in diameter, which was determined by the diameter of the fiber and the numerical aperture of the micro-optics. The system shares the focal point of the optics with that of microbeam [27]. Our custom-made data acquisition system designed for a micro-PIXE analysis system [28] was used for simultaneous PIXE imaging by inputting both X-ray-induced pulse and photon-counting pulse signals into the data acquisition system. Continuous IBIL spectroscopy was performed by acquiring the spectra repeatedly with a minimum measurement time of 19 ms using the CCD spectrometer with an effective wavelength range of 300–900 nm, which was determined by the optical throughput of the micro-optics [29]. 2.2. Sample preparation and irradiation conditions α-SiAlON:Eu (YL600A, Denka Co., Ltd.), β-SiAlON:Eu (MW540H, Denka Co., Ltd.), and CaAlSiN3 (RE-650XMD, Denka Co., Ltd.) scintillator powders were used for the irradiation. For comparison, the conventional ZnS:Ag scintillator was also analyzed with the same setup. A polyimide film with an approximate thickness of 6 μm was attached to a plastic sample holder [30] for external microbeam irradiation during continuous IBIL spectroscopy. Although polyimide film is not transparent, it was used because it is robust to prolonged irradiation. The scintillator powders were mixed with water, spread evenly on the substrate film, and the water was removed by drying at room temperature for 10 min. SiAlON and CASN samples were prepared with a similar thickness to that of the commercially available ZnS:Ag scintillator. The thickness of the ZnS:Ag sheet was estimated as approximately 30 μm by micrometer measurement with a precision of 1 μm. By considering material density assuming 3.22 g/cm3 for SiAlON and 4.09 g/
IBIL (F ) = I1 × e F1 and F1 = 0.90 ± F1 = 0.83 ± F1 = 2.32 ± 2
(F ) F1
+ I2 × e
(F ) F2
(1)
F2 were determined as follows: ZnS:Ag, 0.14 × 1014 and F2 = 8.36 ± 0.12 × 1014; α-SiAlON:Eu, 0.15 × 1015 and F2 = 1.43 ± 0.08 × 1016; β-SiAlON:Eu, 0.29 × 1015 and F2 = 1.54 ± 0.14 × 1016; and
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Fig. 1. Schematic of measurement apparatus for ion beam-induced luminescence (IBIL) spectroscopy and imaging. The analytical target was set at the focal point of the confocal optics designed for IBIL spectroscopy and continuous measurements. A scanning proton microbeam was used for microscopic IBIL spectroscopy and imaging.
Fig. 2. IBIL spectra of CaAlSiN3 scintillator compared with that of α-SiAlON:Eu, β-SiAlON:Eu scintillators, as well as a commercially available ZnS:Ag scintillator excited by a focused 3 MeV proton microbeam. While IBIL spectra of αSiAlON:Eu, β-SiAlON:Eu, and ZnS:Ag scintillators showed same peak wavelength obtained in previous work [14], CaAlSiN3 scintillator exhibits broadband envelope spectrum with center wavelength approximately around 670 nm.
Fig. 3. Decay in IBIL spectrum of CaAlSiN3 scintillator with that of αSiAlON:Eu, β-SiAlON:Eu, and ZnS:Ag scintillators during continuous IBIL with a 3 MeV proton microbeam. While decay scheme of α-SiAlON:Eu, β-SiAlON:Eu scintillators showed almost same decay constant obtained in previous work [14]. CaAlSiN3 scintillator exhibits similar decay component comparable with α-SiAlON:Eu, β-SiAlON:Eu.
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Fig. 4. Micro-PIXE identification of α-SiAlON:Eu particles through Si elemental distribution mapping. (a) Micro-PIXE image of whole area with a large scan of 800 × 800 μm2. Dashed red rectangles indicates region of interest ‘A’ and ‘B’. (b) Micro-PIXE image obtained from region of interest A with scanning areas of 25 × 25 μm2. (c) Micro-PIXE image obtained from region of interest ‘B’ with same scanning areas of 25 × 25 μm2.
CaAlSiN3, F1 = 3.95 ± 0.61 × 1015 and F2 = 2.84 ± 1.42 × 1016. CaAlSiN3 had higher decay factors and larger fluctuations than α-SiAlON:Eu and β-SiAlON:Eu. Although we could not visualize large differences in the IBIL decay components for the CASN and SiAlONs, the differences in the crystal structures of these scintillators also have minor effects on the stability of their scintillation schemes.
generates minor shift in energy state of Eu2+. Moreover, some regions showed a small shift in peak wavelength in the IBIL spectrum, and this behavior was clear in similar single-grain analysis of β-SiAlON:Eu scintillator clusters. Fig. 6 shows microscopic PIXE imaging of silicon Kα X-ray obtained from two β-SiAlON:Eu scintillator clusters within an area of 100 × 100 μm2. The IBIL peaks were clearly shifted and separated, as visualized in the comparison of the IBIL spectra obtained from regions 1 and 2 in the IBIL image (Fig. 7). Large area scan over 100 × 100 μm2 including the two grains was also performed and the IBIL spectrum compared with previous two IBIL from specific regions 1 and 2 (Fig. 8). The IBIL spectrum obtained over the area of 100 × 100 μm2 including both two grains were similar to that obtained from a wider area scan with uniform sample distribution shown in Fig. 2. Two IBIL spectra obtained from region 1 and 2 (as shown in Fig. 7) felled within the envelop of IBIL spectrum. The peak wavelengths were 529 and 553 nm for region 1 and 582 nm for region 2. The peak fitting algorithm showed there were three peaks in the whole-area IBIL spectrum at 529, 552, and 589 nm, which corresponded well with those for regions 1 and 2. These experimental results indicated that the conventional IBIL spectrum of SiAlON scintillators can be subdivided into further detailed components by single-grain microscopic IBIL spectroscopy. Those might have origin in differences in crystal structure or chemical compositions by each grain. In previous section we examined the IBIL peak wavelengths of CaAlSiN3 scintillator in comparison of that of α-SiAlON:Eu, β-SiAlON:Eu scintillators. While all scintillators have similar single broadband envelops with common
3.2. Microscopic IBIL spectroscopy IBIL spectroscopy of SiAlON showed minor differences in the structures of the luminescence spectrum envelop compared with cathode luminescence and photoluminescence spectroscopy [14,21]. Thus, more detailed IBIL spectroscopy was performed for particular areas of interest. First, we performed IBIL spectroscopy and imaging in a region of interest covered with the α-SiAlON:Eu scintillator powder by scanning an area from 25 × 25 μm2 to 800 × 800 μm2 with a 3 MeV proton microprobe and a beam current of approximately 50 pA. Fig. 4 show microscopic PIXE images obtained from different areas. IBIL spectra from corresponding three different scanning areas were recorded as shown in Fig. 5. Differences in intensity in PIXE images correspond to different concentrations of the α-SiAlON:Eu scintillator. The IBIL intensity decreased as the density decreased, from almost uniform distribution, as shown in Fig. 5(a), to single grains, as shown in Fig. 5(b). However, the signal-to-noise ratios remained the same and the IBIL peaks could still be distinguished from the background. It was also found that IBIL spectra had different envelop obtained at region A and B. It might be due to the changes in crystal structure which 4
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Fig. 5. Micro-IBIL spectra obtained from areas of interest in the α-SiAlONs targets. (a) The whole-area spectrum corresponding to the signals from the large scan area of 800 × 800 μm2 (b) Micro-IBIL spectra obtained from scanning areas of 25 × 25 μm2 for the region of interest 'A' and region of interest 'B' showing different IBIL intensities and spectrum envelopes.
Fig. 8. Comparison of IBIL spectra from the whole-area scan and from independent regions 1 and 2. The conventional whole-area IBIL spectrum peak encompasses the two peaks from regions 1 and 2.
Fig. 6. MicroPIXE image of two independent β-SiAlON clusters visualized within an area of 100 × 100 μm2. Independent regions 1 and 2 were selected for further IBIL spectroscopy.
activator transition of 4f → 5d for Eu2+, differences in crystal structure and chemical composition shifted IBIL peak wavelength. This could be more specific in microscopic level, where each grain could have varieties in minor differences in crystal structure and chemical compositions. Even thinnest envelop of β-SiAlON:Eu scintillator was able to be deconvoluted into two peaks. These results suggested that microscopic IBIL spectroscopy could be utilized to investigate minor variations in crystal structure or chemical compositions in microscopic level through energy shift in IBIL spectrum. This might be beneficial for ceramic engineering of such phosphors or related optical materials. 4. Conclusions We characterized CaAlSiN3 scintillator with α-SiAlON:Eu, βSiAlON:Eu scintillators by IBIL spectroscopy with a focused proton microbeam probe to investigate the influence of the differences in chemical composition and crystal structure. IBIL spectra were continuously obtained and the peak wavelength and decay in intensity were analyzed. Based on the following findings, IBIL spectroscopy using a proton microprobe is an effective tool for the microscopic characterization of luminescent targets, including SiAlON scintillators.
Fig. 7. Micro-IBIL spectra obtained from independent regions 1 and 2 identified in β-SiAlONs particulate clusters. A clear peak shift is observed in the IBIL spectra between the two regions of interest.
(1) We examined the IBIL peak wavelengths of CaAlSiN3 scintillator in comparison of that of α-SiAlON:Eu, β-SiAlON:Eu scintillators, 5
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which exhibits similar optical properties obtained in previous works, and found that intense IBIL peaks were observed at 605, 540, and 670 nm, respectively. Peaks are differed by its crystal structure of SiAlON and CASN while single broadband envelops were commonly caused by 4f → 5d transition of Eu2+. The typical IBIL peak wavelengths for SiAlON and CASN scintillators are quite similar with the peak wavelength of those in photoluminescence or cathodoluminescence found in literature. Therefore, there are variety and controllability in IBIL peak wavelength with different type of SiAlON phosphor families which may be of interest for various radioluminescence applications. (2) The stability toward ion exposure of CaAlSiN3 and α-SiAlON:Eu, βSiAlON:Eu scintillators were investigated by continuous IBIL spectroscopy. Although we did not visualize large differences in the IBIL decay components of the CASN and SiAlONs, the differences in the crystal structures had minor effects on the stability of the scintillation schemes. (3) Microscopic IBIL spectroscopy was performed for α- and βSiAlON:Eu. Peaks obtained from microscopic IBIL analysis of singlegrain targets showed energy shifts in spectra, it is the subdivision of the spectroscopic components of the conventional IBIL spectra obtained by macroscopic IBIL analysis. Those might have origin in differences in crystal structure or chemical compositions by each grain. Our experimental results suggest that microscopic IBIL spectroscopy is an effective tool for microscopic material characterization of luminescent targets, where minor differences in crystal structure or chemical composition might have influences.
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[18]
Declaration of Competing Interest
[19]
S.Y. is employee of Denka Co., Ltd. The research was partially funded by Denka Co., Ltd.
[20]
Acknowledgment
[21]
This research was partially supported by a MEXT/JSPS Grant-in-Aid for Scientific Research (No. JP26706025).
[22]
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Micro-ion beam-induced luminescence spectroscopy for evaluating SiAlON scintillators W. Kadaa, , T. Satohb, S. Yamadac, M. Kokad, N. Yamadab, K. Miuraa, O. Hanaizumia ⁎
a
Faculty of Science and Technology, Gunma University, 1-5-1 Tenjin-cho, Kiryu, Gunma 376-8515, Japan Takasaki Advanced Radiation Research Institute (TARRI), Quantum Beam Science Research Directorate, National Institutes for Quantum and Radiological Science and Technology (QST), 1233 Watanuki-machi, Takasaki, Gunma 370-1292, Japan c Research & Development Department, Denka Co. Ltd., 2-1-1 Nihonbashi-Muromachi, Chuo-ku, Tokyo 103-8338, Japan d Beam Operation Co., Ltd, 1233, Watanukicho, Takasaki, Gunma 370-1292, Japan b
ARTICLE INFO
ABSTRACT
Keywords: IBIL SiAlON Single grain Spectroscopy Imaging
Ion beam-induced luminescence (IBIL) spectroscopy was used for luminescent material characterization and analysis for the investigation of new scintillator families. Under continuous 3 MeV proton microbeam irradiation, the crystal structures of α-SiAlON:Eu, β-SiAlON:Eu, and CaAlSiN3 (CASN) emitted bright luminescence at peak wavelengths of 605, 540, and 670 nm, respectively. As the irradiation progressed, the IBIL intensity of the conventional ZnS:Ag scintillator decreased sharply, whereas that of the SiAlONs and CASN remained within the detection limit. IBIL spectroscopy was performed on individual grains of the SiAlON scintillators. IBIL imaging and spectroscopy of two grains of β-SiAlON:Eu showed that the main peak in the IBIL spectrum of β-SiAlON:Eu obtained from a large-area beam scan consisted of several small peaks, which were observed in spectra from individual grains. Our experimental results suggest that microscopic spectroscopy of IBIL is an effective tool for microscopic material characterization of luminescent targets.
1. Introduction Scintillators are commonly used in various types of ion-beam experiments, especially where convenient beam profiling or charged particle detection is required [1–5]. The experiments may require the scintillators to be exposed continuously to intense radiation or high temperatures, and most conventional scintillators are susceptible to radiation damage and are degraded by intense or focused irradiation [6,7]. Therefore, scintillators that are more resistant to radiation are particularly desirable. There is growing interest in SiAlON scintillators, originally designed as substrates for white light-emitting diodes [8], that have excellent luminescence properties and outstanding thermal and chemical stabilities [9]. Because SiAlONs have a hexagonal crystal structure equivalent to that of phosphors substituted with silicon nitride (Si3N4) ceramics [10], they are similarly robust. In addition, low thermal quenching is achieved by doping SiAlONs with Eu, and the luminescence of the phosphor is maintained even up to 423 K [11]. Therefore, SiAlONs are also expected to be robust to radiation damage caused by charged particle impacts. Ion beam analysis is used to characterize SiAlON ceramic phosphors [12] and the scintillation properties of α-SiAlON (YL600A) and β-
⁎
SiAlON (GR-200 and MW540H) powders have been evaluated by in situ luminescence measurements by ion beam-induced luminescence (IBIL) analysis under focused proton microbeam irradiation [13,14]. IBIL is utilized for its sensitivity to the chemical composition of inorganic targets [15–17] and radiation damage [18,19]. In our previous studies, SiAlONs showed excellent luminescent yields, comparable with the conventional scintillator, ZnS:Ag [13]. Single broadband emission spectrum which can be described as 4f → 5d transition of Eu2+ sustains its original IBIL intensity during irradiation because of robust crystal structure of SiAlONs. In addition, the advantages of SiAlONs at high temperatures have also been evaluated by temperature-controlled IBIL [14]. IBIL at high temperatures revealed that SiAlONs are candidate scintillators for charged-particle detection at temperatures above 373 K, at which conventional luminescent scintillators (e.g., ZnS:Ag) cannot operate. Hence, SiAlONs are considered as promising scintillators for ionizing radiation monitoring. However, in some occasion, small shifts in peak wavelength of IBIL were observed from different portion of SiAlONs scintillators individually prepared for irradiation. Irradiation might cause such changes but such energy shift may also be generated from minor differences in crystal structures at microscopic level. By comparing peak wavelength of IBIL spectrum with that of
Corresponding author. E-mail address: [email protected] (W. Kada).
https://doi.org/10.1016/j.nimb.2019.10.001 Received 7 May 2019; Received in revised form 1 October 2019; Accepted 2 October 2019 0168-583X/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: W. Kada, et al., Nuclear Inst. and Methods in Physics Research B, https://doi.org/10.1016/j.nimb.2019.10.001
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photoluminescence or cathodoluminescence, similar excitation and radiative recombination occurs by ultraviolet lights, electrons, or ions [20–22]. Taking into account these electron transition processes, beam current would affect IBIL spectra by radiation damage but not the shift in the spectrum. To investigate the origin of such shift, more precise luminescent properties of the scintillators similar to SiAlON phosphor should be characterized by IBIL microscopic spectroscopy using a proton microprobe. In this study, we evaluated additional scintillator candidate of CaAlSiN3 phoshpor as well as previously evaluated scintillators of αSiAlON:Eu, β-SiAlON:Eu by IBIL imaging and spectroscopy with a 3 MeV proton microprobe. CaAlSiN3 (CASN) phosphor has similarity with both SiAlONs in its base crystal structure, activator, adequate thermal stability, and high light emission yield [23]. Since differences in composition might bring interesting information related to the spectral shifts and changes in IBIL, CASN and SiAlON scintillator powders were characterized by combined analysis and imaging with particle-induced X-ray emission (PIXE) and IBIL using a focused proton microbeam. We recorded the IBIL spectra continuously and determined the peak wavelength ranging from 540 to 670 nm, and decay in intensity. In addition, microscopic IBIL spectroscopy was performed in the area visualized by PIXE and IBIL imaging. The experimental results suggested that IBIL imaging and spectroscopy is an effective tool for detailed characterization of the microscopic luminescent properties of scintillators.
cm3 for ZnS:Ag inputted into SRIM2013 software, 3 MeV protons is roughly estimated to penetrate into each target approximately 62.7 and 66.5 μm, respectively. Effective IBIL intensity may be largely affected by actual grain sizes and their aggregate surface coverage homogeneity. Some differences can also be expected due to minor variations in grains chemical composition. All in-depth scintillators proton excitation is guaranteed due to the nominal 30 µm thick grain layer. The samples were sandwiched between two polyimide films of the same thickness. For single-particle observations in a vacuum chamber, carbon tape was used to fix the scintillators. Clusters to single agglomerates of α-SiAlON:Eu, β-SiAlON:Eu, or CaAlSiN3 powder was spread over the adhesive tape by electrostatically collecting and fixing the powder to the tape manually. An aluminum plate was used to fix the tape bearing the scintillator powders to a three-dimensional movable stage in a vacuum chamber. A focused proton microprobe with a typical diameter of 1 μm was used for both the external and vacuum analytical conditions. Beam size was evaluated prior to irradiation using a copper mesh and a secondary electron imaging system [31]. A Faraday cup was placed behind the sample to monitor the beam current. Under vacuum analytical conditions, the beam current passing through the sample holder was also monitored during irradiation. A beam current of less than 100 pA was used for IBIL spectroscopy in conjunction with the micro-PIXE analysis.
2. Materials and methods
3.1. Comparison of IBIL spectra for CASN and SiAlON scintillators
2.1. Experimental setup
The IBIL spectrum of CaAlSiN3 scintillator was compared with that of α-SiAlON:Eu, β-SiAlON:Eu as shown in Fig. 2. The typical exposure time for IBIL spectroscopy was approximately 1000 ms under 3 MeV focused proton microbeam irradiation with a beam current of approximately 10 pA. CaAlSiN3 emitted intense IBIL similar to that from both α-SiAlON:Eu and β-SiAlON:Eu scintillators, which was quite similar to the value previously evaluated elsewhere [14]. All three scintillators gave excellent photon emission yields which is comparable with that of the ZnS:Ag scintillator [32]. The peak wavelengths for IBIL of α-SiAlON:Eu, β-SiAlON:Eu, and CaAlSiN3 were 605, 540, and 670 nm, respectively. Spectrum resolution was also obtained from the full width half maximum (FWHM) of 105.15 nm for CaAlSiN3, compared with 68.25 and 40.62 nm for α-SiAlON:Eu and β-SiAlON:Eu, respectively. The controllability of the peak wavelength is one of the advantages of these scintillators which may be suitable for various radioluminescence applications while their peak wavelengths match the maximum quantum efficiency region of commonly used visible photon sensors and detectors [33]. The differences in crystal structure of SiAlONs and CaAlSiN3 seems to be the origin of the peak shifts. The decay occurred during continuous irradiation of focused beam was investigated by continuous IBIL spectroscopy. During the irradiation, a steep decay in intensity was observed for conventional ZnS:Ag, whereas a slow decrease was observed for α-SiAlON:Eu, β-SiAlON:Eu, and CaAlSiN3 with little fluctuation. The decay in intensity with respect to beam fluence F (ions/cm2) and normalized IBIL intensity IBIL(F) (arb. units) was determined for all scintillators (Fig. 3). IBIL intensity was delivered by integrating for each single broadband envelop appeared in each IBIL spectrum. Then each IBIL intensity IBIL(F) obtained at beam fluence F (ions/cm2) was normalized the value of virgin IBIL (0). The parameters were fitted with a degradation model [14,34] with two decay factors, F1 and F2, as expressed by
3. Results and discussion
IBIL imaging and spectroscopy of SiAlON scintillators was performed with a microbeam setup of the single-ended accelerator at TARRI/QST [24] using custom-designed confocal micro-optics with an anti-reflection coating and a charge-coupled device (CCD) spectrometer (Solid Lambda CCD, Spectra Co-op) [25,26]. A schematic of the experimental setup is shown in Fig. 1. The system can switch between microbeam irradiation in a vacuum or in air. The effective area of IBIL imaging and spectroscopy was approximately 800 μm in diameter, which was determined by the diameter of the fiber and the numerical aperture of the micro-optics. The system shares the focal point of the optics with that of microbeam [27]. Our custom-made data acquisition system designed for a micro-PIXE analysis system [28] was used for simultaneous PIXE imaging by inputting both X-ray-induced pulse and photon-counting pulse signals into the data acquisition system. Continuous IBIL spectroscopy was performed by acquiring the spectra repeatedly with a minimum measurement time of 19 ms using the CCD spectrometer with an effective wavelength range of 300–900 nm, which was determined by the optical throughput of the micro-optics [29]. 2.2. Sample preparation and irradiation conditions α-SiAlON:Eu (YL600A, Denka Co., Ltd.), β-SiAlON:Eu (MW540H, Denka Co., Ltd.), and CaAlSiN3 (RE-650XMD, Denka Co., Ltd.) scintillator powders were used for the irradiation. For comparison, the conventional ZnS:Ag scintillator was also analyzed with the same setup. A polyimide film with an approximate thickness of 6 μm was attached to a plastic sample holder [30] for external microbeam irradiation during continuous IBIL spectroscopy. Although polyimide film is not transparent, it was used because it is robust to prolonged irradiation. The scintillator powders were mixed with water, spread evenly on the substrate film, and the water was removed by drying at room temperature for 10 min. SiAlON and CASN samples were prepared with a similar thickness to that of the commercially available ZnS:Ag scintillator. The thickness of the ZnS:Ag sheet was estimated as approximately 30 μm by micrometer measurement with a precision of 1 μm. By considering material density assuming 3.22 g/cm3 for SiAlON and 4.09 g/
IBIL (F ) = I1 × e F1 and F1 = 0.90 ± F1 = 0.83 ± F1 = 2.32 ± 2
(F ) F1
+ I2 × e
(F ) F2
(1)
F2 were determined as follows: ZnS:Ag, 0.14 × 1014 and F2 = 8.36 ± 0.12 × 1014; α-SiAlON:Eu, 0.15 × 1015 and F2 = 1.43 ± 0.08 × 1016; β-SiAlON:Eu, 0.29 × 1015 and F2 = 1.54 ± 0.14 × 1016; and
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Fig. 1. Schematic of measurement apparatus for ion beam-induced luminescence (IBIL) spectroscopy and imaging. The analytical target was set at the focal point of the confocal optics designed for IBIL spectroscopy and continuous measurements. A scanning proton microbeam was used for microscopic IBIL spectroscopy and imaging.
Fig. 2. IBIL spectra of CaAlSiN3 scintillator compared with that of α-SiAlON:Eu, β-SiAlON:Eu scintillators, as well as a commercially available ZnS:Ag scintillator excited by a focused 3 MeV proton microbeam. While IBIL spectra of αSiAlON:Eu, β-SiAlON:Eu, and ZnS:Ag scintillators showed same peak wavelength obtained in previous work [14], CaAlSiN3 scintillator exhibits broadband envelope spectrum with center wavelength approximately around 670 nm.
Fig. 3. Decay in IBIL spectrum of CaAlSiN3 scintillator with that of αSiAlON:Eu, β-SiAlON:Eu, and ZnS:Ag scintillators during continuous IBIL with a 3 MeV proton microbeam. While decay scheme of α-SiAlON:Eu, β-SiAlON:Eu scintillators showed almost same decay constant obtained in previous work [14]. CaAlSiN3 scintillator exhibits similar decay component comparable with α-SiAlON:Eu, β-SiAlON:Eu.
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Fig. 4. Micro-PIXE identification of α-SiAlON:Eu particles through Si elemental distribution mapping. (a) Micro-PIXE image of whole area with a large scan of 800 × 800 μm2. Dashed red rectangles indicates region of interest ‘A’ and ‘B’. (b) Micro-PIXE image obtained from region of interest A with scanning areas of 25 × 25 μm2. (c) Micro-PIXE image obtained from region of interest ‘B’ with same scanning areas of 25 × 25 μm2.
CaAlSiN3, F1 = 3.95 ± 0.61 × 1015 and F2 = 2.84 ± 1.42 × 1016. CaAlSiN3 had higher decay factors and larger fluctuations than α-SiAlON:Eu and β-SiAlON:Eu. Although we could not visualize large differences in the IBIL decay components for the CASN and SiAlONs, the differences in the crystal structures of these scintillators also have minor effects on the stability of their scintillation schemes.
generates minor shift in energy state of Eu2+. Moreover, some regions showed a small shift in peak wavelength in the IBIL spectrum, and this behavior was clear in similar single-grain analysis of β-SiAlON:Eu scintillator clusters. Fig. 6 shows microscopic PIXE imaging of silicon Kα X-ray obtained from two β-SiAlON:Eu scintillator clusters within an area of 100 × 100 μm2. The IBIL peaks were clearly shifted and separated, as visualized in the comparison of the IBIL spectra obtained from regions 1 and 2 in the IBIL image (Fig. 7). Large area scan over 100 × 100 μm2 including the two grains was also performed and the IBIL spectrum compared with previous two IBIL from specific regions 1 and 2 (Fig. 8). The IBIL spectrum obtained over the area of 100 × 100 μm2 including both two grains were similar to that obtained from a wider area scan with uniform sample distribution shown in Fig. 2. Two IBIL spectra obtained from region 1 and 2 (as shown in Fig. 7) felled within the envelop of IBIL spectrum. The peak wavelengths were 529 and 553 nm for region 1 and 582 nm for region 2. The peak fitting algorithm showed there were three peaks in the whole-area IBIL spectrum at 529, 552, and 589 nm, which corresponded well with those for regions 1 and 2. These experimental results indicated that the conventional IBIL spectrum of SiAlON scintillators can be subdivided into further detailed components by single-grain microscopic IBIL spectroscopy. Those might have origin in differences in crystal structure or chemical compositions by each grain. In previous section we examined the IBIL peak wavelengths of CaAlSiN3 scintillator in comparison of that of α-SiAlON:Eu, β-SiAlON:Eu scintillators. While all scintillators have similar single broadband envelops with common
3.2. Microscopic IBIL spectroscopy IBIL spectroscopy of SiAlON showed minor differences in the structures of the luminescence spectrum envelop compared with cathode luminescence and photoluminescence spectroscopy [14,21]. Thus, more detailed IBIL spectroscopy was performed for particular areas of interest. First, we performed IBIL spectroscopy and imaging in a region of interest covered with the α-SiAlON:Eu scintillator powder by scanning an area from 25 × 25 μm2 to 800 × 800 μm2 with a 3 MeV proton microprobe and a beam current of approximately 50 pA. Fig. 4 show microscopic PIXE images obtained from different areas. IBIL spectra from corresponding three different scanning areas were recorded as shown in Fig. 5. Differences in intensity in PIXE images correspond to different concentrations of the α-SiAlON:Eu scintillator. The IBIL intensity decreased as the density decreased, from almost uniform distribution, as shown in Fig. 5(a), to single grains, as shown in Fig. 5(b). However, the signal-to-noise ratios remained the same and the IBIL peaks could still be distinguished from the background. It was also found that IBIL spectra had different envelop obtained at region A and B. It might be due to the changes in crystal structure which 4
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Fig. 5. Micro-IBIL spectra obtained from areas of interest in the α-SiAlONs targets. (a) The whole-area spectrum corresponding to the signals from the large scan area of 800 × 800 μm2 (b) Micro-IBIL spectra obtained from scanning areas of 25 × 25 μm2 for the region of interest 'A' and region of interest 'B' showing different IBIL intensities and spectrum envelopes.
Fig. 8. Comparison of IBIL spectra from the whole-area scan and from independent regions 1 and 2. The conventional whole-area IBIL spectrum peak encompasses the two peaks from regions 1 and 2.
Fig. 6. MicroPIXE image of two independent β-SiAlON clusters visualized within an area of 100 × 100 μm2. Independent regions 1 and 2 were selected for further IBIL spectroscopy.
activator transition of 4f → 5d for Eu2+, differences in crystal structure and chemical composition shifted IBIL peak wavelength. This could be more specific in microscopic level, where each grain could have varieties in minor differences in crystal structure and chemical compositions. Even thinnest envelop of β-SiAlON:Eu scintillator was able to be deconvoluted into two peaks. These results suggested that microscopic IBIL spectroscopy could be utilized to investigate minor variations in crystal structure or chemical compositions in microscopic level through energy shift in IBIL spectrum. This might be beneficial for ceramic engineering of such phosphors or related optical materials. 4. Conclusions We characterized CaAlSiN3 scintillator with α-SiAlON:Eu, βSiAlON:Eu scintillators by IBIL spectroscopy with a focused proton microbeam probe to investigate the influence of the differences in chemical composition and crystal structure. IBIL spectra were continuously obtained and the peak wavelength and decay in intensity were analyzed. Based on the following findings, IBIL spectroscopy using a proton microprobe is an effective tool for the microscopic characterization of luminescent targets, including SiAlON scintillators.
Fig. 7. Micro-IBIL spectra obtained from independent regions 1 and 2 identified in β-SiAlONs particulate clusters. A clear peak shift is observed in the IBIL spectra between the two regions of interest.
(1) We examined the IBIL peak wavelengths of CaAlSiN3 scintillator in comparison of that of α-SiAlON:Eu, β-SiAlON:Eu scintillators, 5
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which exhibits similar optical properties obtained in previous works, and found that intense IBIL peaks were observed at 605, 540, and 670 nm, respectively. Peaks are differed by its crystal structure of SiAlON and CASN while single broadband envelops were commonly caused by 4f → 5d transition of Eu2+. The typical IBIL peak wavelengths for SiAlON and CASN scintillators are quite similar with the peak wavelength of those in photoluminescence or cathodoluminescence found in literature. Therefore, there are variety and controllability in IBIL peak wavelength with different type of SiAlON phosphor families which may be of interest for various radioluminescence applications. (2) The stability toward ion exposure of CaAlSiN3 and α-SiAlON:Eu, βSiAlON:Eu scintillators were investigated by continuous IBIL spectroscopy. Although we did not visualize large differences in the IBIL decay components of the CASN and SiAlONs, the differences in the crystal structures had minor effects on the stability of the scintillation schemes. (3) Microscopic IBIL spectroscopy was performed for α- and βSiAlON:Eu. Peaks obtained from microscopic IBIL analysis of singlegrain targets showed energy shifts in spectra, it is the subdivision of the spectroscopic components of the conventional IBIL spectra obtained by macroscopic IBIL analysis. Those might have origin in differences in crystal structure or chemical compositions by each grain. Our experimental results suggest that microscopic IBIL spectroscopy is an effective tool for microscopic material characterization of luminescent targets, where minor differences in crystal structure or chemical composition might have influences.
[9] [10] [11]
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[14]
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[18]
Declaration of Competing Interest
[19]
S.Y. is employee of Denka Co., Ltd. The research was partially funded by Denka Co., Ltd.
[20]
Acknowledgment
[21]
This research was partially supported by a MEXT/JSPS Grant-in-Aid for Scientific Research (No. JP26706025).
[22]
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