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MYU Tokyo
S & M 3538

Scintillation Properties of Tb-doped SrLu2O4 Single Crystals
Hiromi Kimura,1* Hiroyuki Fukushima,2 Kenichi Watanabe,3 Takeshi Fujiwara,1
Hidetoshi Kato,1 Masahito Tanaka,1 Takumi Kato,4Daisuke Nakauchi,4 Noriaki Kawaguchi,4 and Takayuki Yanagida4
1National Metrology Institute of Japan (NMIJ), National Institute ofAdvanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba, Ibaraki 305-8568, Japan
2Department of Electrical and Electronic Engineering, National Institute of Technology, Fukui College,
Geshi-Cho, Sabae, Fukui 916-8507, Japan
3Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University,
744 Motooka, Nishi, Fukuoka 819-0395, Japan
4Division of Materials Science, Nara Institute of Science and Technology (NAIST),
8916-5 Takayama, Ikoma, Nara 630-0192, Japan(Received November 1, 2023; accepted January 17, 2024)

Keywords: radiation-induced luminescence, scintillators, photoluminescence, single crystals

Tb-doped SrLu2O4 single crystals were grown by the floating zone method, and their opticaland scintillation properties were evaluated. Regarding the scintillation properties, all the
prepared crystals showed the sharp emission peaks assigned to Tb3+ 4f–4f transitions in therange of 350–650 nm. The obtained decay times of 1.0–1.6 ms were typical values for the Tb3+
4f–4f transitions. From the results of the pulse area spectra, the 1.0% Tb-doped crystal showed
the highest light yield of 19600 photons/MeV among the prepared crystals.

1.

Introduction

Scintillators are a type of luminescent material, and they immediately convert the absorbed
energy of ionizing radiation into many low-energy photons. The combination of a scintillator
and a photodetector is used for a typical scintillation detector,(1) which has been used in various
applications including medical diagnosis,(2,3) security,(4) industrial inspection,(5) andenvironmental monitoring.(6–8) The performance of scintillation detectors depends primarily on
the scintillation properties, and the required scintillation properties vary depending on theapplications. Therefore, many types of scintillator have been reported in the form of single
crystals,(9–16) ceramics,(17–21) and glasses.(22–31)
Until the 1990s, UV-blue emitting scintillators have been mainly studied because aphotomultiplier tube (PMT), which has a high sensitivity in the UV-blue region, has mainly beenused as a photodetector in scintillation detectors.(32–34) In the 2000s, with the progress of Sibased photodetectors, two-dimensional X-ray detectors including f lat panel detectors(FPDs)(35–37) and X-ray line scan cameras(38,39) have attracted considerable attention. They
consist of scintillators and Si-based photodetectors, and the required properties of scintillators*Corresponding author: e-mail: h.kimura@aist.go.jp

https://doi.org/10.18494/SAM4767

ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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for the above detectors were high effective atomic number (Zeff), high light yield (LY), highdensity, low fading, and green-to-red emission.(40) Tl-doped CsI crystals (Zeff : ~54, LY: ~54000
photons/MeV),(41) CdWO4 crystals (Zeff : ~66, LY: ~20000 photons/MeV),(42,43) and Ce, Pr-doped
GOS ceramics (Zeff : ~60, LY: ~30000 photons/MeV)(44) were utilized as scintillators in the FPD
and X-ray line scan cameras. However, there are no green-to-red emitting scintillators with large
Zeff and high light yield, and thus the development of new scintillators is expected.
Tb-doped SrRE2O4 (RE = rare-earth ions) has a high potential for scintillators because when
RE = Lu, Zeff and density are ~65 and 8.4 g/cm3, respectively.(45–49) In our previous research,
Tb-doped SrY2O4 single crystals showed green emission bands, and the estimated light yields
were 12000 photons/MeV.(50) However, there are no reports on the scintillation properties of Tbdoped SrLu2O4. In this study, we investigated the optical and scintillation properties of Tbdoped SrLu2O4 single crystals.

2.

Experimental Methods

SrLu2O4 single crystals doped with various concentrations of Tb were grown by the floatingzone technique.(46,51) The raw powders of Tb4O7 (99.99%, Furuuchi Chemical), SrCO3 (99.99%,
Wako Pure Chemical), and Lu2O3 (99.999%, Nippon Yttrium) were mixed in a stoichiometricratio. Here, the concentrations of Tb were 0.1, 0.5, 1.0, and 2.0 mol% with respect to Lu. The
mixture was annealed at 1200 °C for 8 h to desorb carbon dioxide, and then the mixture wasformed to a cylindrical shape by cold isostatic pressing at 15 MPa for 15 min. The cylindricalceramic was sintered at 1400 °C for 8 h using an electric furnace, and the crystal growth was
performed using a floating zone (FZ) furnace (Crystal Systems, FZ-T-12000-X-VPO-PC-YH).
As growth conditions, the pulling speed and rotation rate were 10 mm/h and 20 rpm, respectively.
The surfaces of the obtained crystals were mechanically polished to evaluate the optical andscintillation properties.
To identify the structure phase, X-ray diffraction (XRD) patterns were measured using adiffractometer (Rigaku, MiniFlex400). As optical properties, diffuse transmittance spectra,
photoluminescence (PL) excitation/emission maps, and PL decay curves were evaluated using aspectrometer (Shimadzu, SolidSpec-3700), a Quantaurus-QY (Hamamatsu Photonics, C11347),
and a Quantaurus-τ (Hamamatsu Photonics, C11367), respectively.
For the measurement of the scintillation spectra and decay curves, we used our previously
reported original systems.(52,53) To calculate the scintillation light yields, the pulse area spectraof a 137Cs γ-ray source using the prepared crystals were observed using our customized setup,(54)which can be measured using the scintillators with lifetimes in the ms range.

3.

Results and Discussion

Figure 1 shows the XRD patterns of Tb-doped SrLu2O4 single crystals and the standard cardof SrLu2O4 (JCPDS: 00-032-1242). All the diffraction patterns matched the reference data for an
orthorhombic structure of SrLu2O4. Since other diffraction peaks were not confirmed, we havesuccessfully prepared the single phase of SrLu2O4 by the FZ method. Figure 2 shows theSensors and Materials, Vol. 36, No. 2 (2024)

Fig. 1. (Color online) XRD patterns of Tb-doped
SrLu 2 O 4 single crystals and standard cards of
SrLu 2O4 (JCPDS: 00-032-1242).

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Fig. 2. (Color online) Diffuse-transmittance spectraof Tb-doped SrLu 2 O 4 single cr ystals. (Inset)
Photographs of Tb-doped SrLu 2O4 single crystals.

diffuse-transmittance spectra of Tb-doped SrLu 2O4 single crystals. All the crystals have ahigh transmittance of ~80% in the wavelength range from 450 to 850 nm. The decrease intransmittance was confirmed at wavelengths below 450 nm, and absorption bands were
shifted to longer wavelengths with increasing Tb concentration. In previous reports on Tbdoped phosphors, absorption bands appeared at around 290 and 550 nm, which wereassociated with the Tb3+ 4f–5d and charge transfer (CT) transitions between O2− and Tb 4+,
respectively.(47,48,55–58)
PL excitation/emission maps and quantum yield (QY) values of the Tb-doped SrLu 2O4
single crystals are shown in Fig. 3. All the Tb-doped crystals exhibited several sharp
emission peaks in the wavelength range of 350–650 nm under excitation at 290 nm. The
excitation spectral peak around 290 nm corresponds to the absorption band of the Tb3+ 4f–5dtransition (as shown in Fig. 2). Since the same spectral positions of emission peaks wereconfirmed in previous reports on Tb-doped phosphors, the emission peaks were attributed
to Tb3+ 4f–4f transitions.(49,59,60) With increasing Tb concentration, the emission intensity
decreased below 500 nm, which would be due to the effect of the self-absorptions of Tb4+
and Tb3+. The QY of the 0.1% Tb-doped crystal (29.4%) was the highest among the prepared
Tb-doped crystals, and this value was higher than that of Pr-doped SrLu 2O 4 crystals(~18.4%), which were the same host material.(46)
Under an excitation of 265 nm, the PL decay curves of Tb-doped SrLu 2O4 single crystals
monitored at 550 nm are shown in Fig. 4. A single exponential decay function was used toapproximate the decay curves except for the instrumental response functions (IRFs) and therise part of each decay curve. The derived lifetimes were 1.8–2.3 ms for Tb-doped SrLu 2O4
crystals, which were reasonable for Tb3+ 4f–4f transitions.(7,49,59–61) In SrLu 2O4 with low Tb
concentration, the increase in rise time was observed. This trend was also confirmed inprevious reports on Tb-doped SrY2O4 single crystals, which was attributed to the crossrelaxation process between neighboring Tb3+ ions.(50,62–64)

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Fig. 3. (Color online) PL excitation/emission maps
and QY values of Tb-doped SrLu 2O4 single crystals.

Sensors and Materials, Vol. 36, No. 2 (2024)

Fig. 4. (Color online) PL decay curves of Tb-doped
SrLu 2O4 single crystals monitored at 550 nm under an
excitation of 265 nm.

Figure 5 shows the X-ray-induced scintillation properties of Tb-doped SrLu 2O4 single
crystals. All the crystals showed several sharp peaks originating from Tb3+ 4f–4f transitions,and the spectral features were similar to the results in PL. In a previous report, the nondoped
SrLu 2O4 crystal showed a broad emission peak around 300–500 nm due to self-trapped
excitons,(46) whereas this was not observed in Tb-doped crystals. The 0.1% Tb-doped crystal
showed several sharp emission peaks in the wide range of 350–650 nm. In the 0.5–2.0% Tbdoped SrLu 2O4 single crystals, the green emission was mainly observed because of theeffect of the self-absorptions of Tb 4+ and Tb3+, which was suitable for the wavelength
sensitivity of Si-based photodetectors. Figure 6 shows the X-ray-induced scintillation decay
curves of Tb-doped SrLu 2O4 single crystals. The decay curves can be fitted with a single
exponential decay function except for the IRF. The decay time constants were 1.0–1.6 ms,
which were typical values for Tb3+ 4f–4f transitions under X-ray irradiation.(65,66)
The pulse area spectra of Tb-doped SrLu 2O4 single crystals and the BGO scintillator
under γ-ray irradiation from a 137Cs source are shown in Fig. 7. The commercial BGO
scintillator, which has a LY of ~8200 photons/MeV,(67) was used as the reference. For the 0.1,
0.5, 1.0, and 2.0% Tb-doped crystals, a full-energy absorption peak appeared at the channelsof 745, 277, 175, and 108, respectively. Considering a full-absorption peak position of the
BGO (263 ch), the calculated LYs of the 0.1, 0.5, 1.0, and 2.0% Tb-doped crystals were 16100,
19600, 12400, and 7600 photons/MeV, respectively. Here, the quantum efficiencies (QEs) of
the PMT to emission peak wavelengths at 415 nm (QE = 36% for 0.1% Tb-doped crystals),
480 nm (QE = 25% for BGO), and 545 nm (QE = 11% for 0.5, 1.0, and 2.0% Tb-doped
crystals) were taken into consideration. The LY of the 1.0% Tb-doped SrLu 2O4 single crystal
was higher than that of Tb-doped SrY2O4 single crystals (~12000 photons/MeV) in ourprevious report.(50) In contrast, the LY of Tb-doped SrLu2O4 crystals was lower than that of thecommercial Tl-doped CsI scintillators (LY: ~54000 photons/MeV) for the FPD.(41)Sensors and Materials, Vol. 36, No. 2 (2024)

Fig. 5. (Color online) X-ray-induced scintillation
properties of Tb-doped SrLu 2O4 single crystals.

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Fig. 6. (Color online) X-ray-induced scintillation
decay curves of Tb-doped SrLu 2O4 single crystals.

Fig. 7. (Color online) Pulse area spectra of Tb-doped SrLu 2O4 single crystals under γ-ray irradiation from a 137Cs
source. The inset shows the pulse area spectrum of a commercial Bi4Ge3O12 (BGO) scintillator as the reference.

4.

Conclusions

The structure and optical and scintillation properties of Tb-doped SrLu2O4 single crystals
were investigated. From XRD results, all the prepared crystals had the single phase of SrLu2O4.
Under 290 nm excitation and X-ray irradiation, several emission lines appeared in the range of
350–650 nm, and emission origins were Tb3+ 4f–4f transitions. With increasing Tb
concentration, spectral shapes were changed by the self-absorptions of Tb 4+ and Tb3+. Since the0.5, 1.0, and 2.0% Tb-doped SrLu2O4 single crystals exhibited strong green emission peaks
around 540 nm, they were suitable for the wavelength sensitivity of the Si-photodiode. Amongthe prepared crystals, the light yield of the 1.0% Tb-doped SrLu2O4 crystal was the highest(~19600 photons/MeV), which is higher than that of a commercial BGO scintillator (~8000
photons/MeV).

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Acknowledgments
This work was supported by Grants-in-Aid for Scientific Research A (22H00309), Scientific
Research B (21H03733 and 22H02939), and Research Activity Start-up (22K20489). The
Cooperative Research Project of the Research Center for Biomedical Engineering is also
acknowledged.

References
1 T. Yanagida: Proc. Japan Acad. Ser. B 94 (2018) 75.
2 S. Yamamoto, S. Okumura, N. Kato, and J. Y. Yeom: J. Instrum. 10 (2015) T09002.
3 K. Shinsho, R. Oh, M. Tanaka, N. Sugioka, H. Tanaka, G. Wakabayashi, T. Takata, W. Chang, S. Matsumoto,G. Okada, S. Sugawara, E. Sasaki, K. Watanabe, Y. Koba, K. Nagasaka, S. Yoshihashi, A. Uritani, and T.
Negishi: Jpn. J. Appl. Phys. 62 (2023) 010502.
4 J. Glodo, Y. Wang, R. Shawgo, C. Brecher, R. H. Hawrami, J. Tower, and K. S. Shah: Phys. Procedia 90 (2017)285.
5 C. Connolly: Sens. Rev. 28 (2008) 194.
6 K. Watanabe, T. Yanagida, K. Fukuda, A. Koike, T. Aoki, and A. Uritani: Sens. Mater. 27 (2015) 269.
7 P. Kantuptim, T. Kato, D. Nakauchi, N. Kawaguchi, K. Watanabe, and T. Yanagida: Sens. Mater. 35 (2023) 451.
8 K. Watanabe: Jpn. J. Appl. Phys. 62 (2023) 010507.
9 T. Suto, N. Kawano, K. Okazaki, Y. Takebuchi, H. Fukushima, T. Kato, D. Nakauchi, and T. Yanagida: Jpn. J.
Appl. Phys. 62 (2023) 010610.
10 H. Fukushima, D. Nakauchi, T. Kato, N. Kawaguchi, and T. Yanagida: Sens. Mater. 35 (2023) 429.
11 M. Arai, Y. Fujimoto, M. Koshimizu, H. Kimura, T. Yanagida, and K. Asai: Mater. Res. Bull. 120 (2019)110589.
12 D. Shiratori, H. Fukushima, D. Nakauchi, T. Kato, N. Kawaguchi, and T. Yanagida: Sens. Mater. 35 (2023) 439.
13 C. Dujardin, E. Auffray, E. Bourret-Courchesne, P. Dorenbos, P. Lecoq, M. Nikl, A. N. Vasil’ev, A. Yoshikawa,and R.-Y. Zhu: IEEE Trans. Nucl. Sci. 65 (2018) 1977.
14 M. J. Weber: J. Lumin. 100 (2002) 35.
15 T. Yanagida, T. Kato, D. Nakauchi, and N. Kawaguchi: Jpn. J. Appl. Phys. 61 (2022) SB1040.
16 N. Kawaguchi, T. Kato, D. Nakauchi, and T. Yanagida: Jpn. J. Appl. Phys. 62 (2023) 010611.
17 T. Kunikata, T. Kato, D. Shiratori, P. Kantuptim, D. Nakauchi, N. Kawaguchi, and T. Yanagida: Sens. Mater.
35 (2023) 491.
18 N. Kawaguchi, H. Kimura, D. Nakauchi, T. Kato, and T. Yanagida: Jpn. J. Appl. Phys. 61 (2022) SB1004.
19 N. J. Cherepy, Z. M. Seeley, S. A. Payne, E. L. Swanberg, P. R. Beck, D. J. Schneberk, G. Stone, R. Perry, B.
Wihl, S. E. Fisher, S. L. Hunter, P. A. Thelin, R. R. Thompson, N. M. Harvey, T. Stefanik, and J. Kindem:
Proceedings Volume 9593, Hard X-Ray, Gamma-Ray, and Neutron Detector Physics XVII, L. Franks, R. B.
James, M. Fiederle, and A. Burger, Eds. (2015) 95930P.
20 D. Nakauchi, F. Nakamura, T. Kato, N. Kawaguchi, and T. Yanagida: Sens. Mater. 35 (2023) 467.
21 H. Kimura, T. Kato, T. Fujiwara, M. Tanaka, D. Nakauchi, N. Kawaguchi, and T. Yanagida: Jpn. J. Appl. Phys.
62 (2023) 010504.
22 D. Shiratori, H. Fukushima, D. Nakauchi, T. Kato, N. Kawaguchi, and T. Yanagida: Jpn. J. Appl. Phys. 62(2023) 010608.
23 H. Kimura, T. Fujiwara, M. Tanaka, T. Kato, D. Nakauchi, N. Kawaguchi, and T. Yanagida: Sens. Mater. 35(2023) 513.
24 V. Dormenev, A. Amelina, E. Auffray, K.-T. Brinkmann, G. Dosovitskiy, F. Cova, A. Fedorov, S. Gundacker,
D. Kazlou, M. Korjik, N. Kratochwil, V. Ladygin, V. Mechinsky, M. Moritz, S. Nargelas, R. W. Novotny, P.
Orsich, M. Salomoni, Y. Talochka, G. Tamulaitis, A. Vaitkevicius, A. Vedda, and H.-G. Zaunick: Nucl.
Instrum. Methods Phys. Res. Sect. A 1015 (2021) 165762.
25 Y. Takebuchi, D. Shiratori, T. Kato, D. Nakauchi, N. Kawaguchi, and T. Yanagida: Sens. Mater. 35 (2023) 507.
26 L. Q. Nguyen, G. Gabella, B. L. Goldblum, T. A. Laplace, J. S. Carlson, E. Brubaker, and P. L. Feng: Nucl.
Instrum. Methods Phys. Res. Sect. A 988 (2021) 164898.
27 H. Kimura, T. Kato, D. Nakauchi, N. Kawaguchi, and T. Yanagida: Sens. Mater. 34 (2022) 691.
28 N. P. Giha, W. M. Steinberger, L. Q. Nguyen, J. S. Carlson, P. L. Feng, S. D. Clarke, and S. A. Pozzi: Nucl.
Instrum. Methods Phys. Res. Sect. A 1014 (2021) 165676.

Sensors and Materials, Vol. 36, No. 2 (2024)

513

29 H. Masai, Y. Yanagida, H. Kawamoto, Y. Koguchi, M. Koshimizu, and M. Yamawaki: Sens. Mater. 34 (2022)
699.
30 G. Ito, H. Kimura, D. Shiratori, D. Nakauchi, T. Kato, N. Kawaguchi, and T. Yanagida: Sens. Mater. 34 (2022)
685.
31 K. Shinozaki, G. Okada, N. Kawaguchi, and T. Yanagida: Jpn. J. Appl. Phys. 62 (2023) 010603.
32 D. R. Schaart: Phys. Med. Biol. 66 (2021) 09TR01.
33 M. R. Farukhi: IEEE Trans. Nucl. Sci. 29 (1982) 1237.
34 E. Roncali, M. A. Mosleh-Shirazi, and A. Badano: Phys. Med. Biol. 62 (2017) R207.
35 S. Izumi, S. Kamata, K. Satoh, and H. Miyai: IEEE Trans. Nucl. Sci. 40 (1993) 158.
36 I. G. Valais, I. S. Kandarakis, D. N. Nikolopoulos, I. A. Sianoudis, N. Dimitropoulos, D. A. Cavouras, C. D.
Nomicos, and G. S. Panayiotakis: IEEE Trans. Nucl. Sci. 52 (2005) 1830.
37 T. Fujiwara, H. Miyoshi, Y. Mitsuya, N. L. Yamada, Y. Wakabayashi, Y. Otake, M. Hino, K. Kino, M. Tanaka,
N. Oshima, and H. Takahashi: Rev. Sci. Instrum. 93 (2022) 013304.
38 K. Hyun Kim, H. Yeon Yeo, and I. Sub Jun: J. Nucl. Sci. Technol. 45 (2008) 435.
39 L. Nagornaya, V. Ryzhikov, A. Apanasenko, I. Tupitsyna, V. Chernikov, and V. Vostretsov: Nucl. Instrum.
Methods Phys. Res. Sect. A 486 (2002) 268.
40 E. Kotter and M. Langer: Eur. Radiol. 12 (2002) 2562.
41 M. Jeong and M. Hammig: Nucl. Eng. Technol. 52 (2020) 2572.
42 M. Moszynski, M. Balcerzyk, M. Kapusta, A. Syntfeld, D. Wolski, G. Pausch, J. Stein, and P. Schotanus: IEEE
Trans. Nucl. Sci. 52 (2005) 3124.
43 H. Nanto: Sens. Mater. 30 (2018) 327.
44 T. Yanagida: Opt. Mater. 35 (2013) 1987.
45 L. M. Lopato: Ceramurg. Int. 2 (1976) 18.
46 H. Fukushima, D. Nakauchi, T. Kato, M. Koshimizu, N. Kawaguchi, and T. Yanagida: Solid State Sci. 110(2020) 106471.
47 E. Pavitra, G. S. R. Raju, Y. H. Ko, and J. S. Yu: Phys. Chem. Chem. Phys. 14 (2012) 11296.
48 R. K. Tamrakar and K. Upadhyay: J. Electron. Mater. 47 (2018) 651.
49 J. Zhang, Y. Wang, L. Guo, and Y. Huang: J. Am. Ceram. Soc. 95 (2012) 243.
50 H. Kimura, H. Fukushima, K. Watanabe, T. Fujiwara, M. Tanaka, D. Nakauchi, T. Kato, N. Kawaguchi, and T.
Yanagida: Jpn. J. Appl. Phys. 63 (2024) 01SP21.
51 H. Fukushima, D. Nakauchi, T. Kato, N. Kawaguchi, and T. Yanagida: Opt. Mater. 128 (2022) 112385.
52 T. Yanagida, K. Kamada, Y. Fujimoto, H. Yagi, and T. Yanagitani: Opt. Mater. 35 (2013) 2480.
53 T. Yanagida, Y. Fujimoto, T. Ito, K. Uchiyama, and K. Mori: Appl. Phys. Express 7 (2014) 062401.
54 K. Watanabe, T. Yanagida, D. Nakauchi, and N. Kawaguchi: Jpn. J. Appl. Phys. 60 (2021) 106002.
55 Z. Aoki, Y. Takebuchi, D. Nakauchi, T. Kato, N. Kawaguchi, and T. Yanagida: J. Mater. Sci. Mater. Electron.
34 (2023) 2021.
56 J. Zmojda, M. Kochanowicz, P. Miluski, and D. Dorosz: Fibers 2 (2014) 150.
57 A. K. V. Raj, P. Prabhakar Rao, S. Divya, and T. R. Ajuthara: Powder Technol. 311 (2017) 52.
58 R. Oka, T. Nouchi, and T. Masui: Colorants 1 (2022) 347.
59 Z. Aoki, Y. Takebuchi, D. Nakauchi, T. Kato, N. Kawaguchi, and T. Yanagida: Opt. Mater. 134 (2022) 113068.
60 K. Ichiba, Y. Takebuchi, H. Kimura, T. Kato, D. Nakauchi, N. Kawaguchi, and T. Yanagida: Sens. Mater. 35(2023) 475.
61 D. Nakauchi, H. Fukushima, T. Kato, N. Kawaguchi, and T. Yanagida: Sens. Mater. 34 (2022) 611.
62 T. O. Sales, R. J. Amjad, C. Jacinto, and M. R. Dousti: J. Lumin. 205 (2019) 282.
63 Y. Liu, J. Zhang, C. Zhang, J. Jiang, and H. Jiang: J. Phys. Chem. C 120 (2016) 2362.
64 S. Kalusniak, E. Castellano-Hernández, H. Yalçinoğlu, H. Tanaka, and C. Kränkel: Appl. Phys. B 128 (2022)
33.
65 N. Kawano, T. Kato, G. Okada, N. Kawaguchi, and T. Yanagida: Opt. Mater. 73 (2017) 364.
66 N. Kawano, M. Akatsuka, H. Kimura, G. Okada, N. Kawaguchi, and T. Yanagida: Radiat. Meas. 117 (2018) 52.
67 I. Holl, E. Lorenz, and G. Mageras: IEEE Trans. Nucl. Sci. 35 (1988) 105.