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Sensors and Materials, Vol. 36, No. 2 (2024) 579–585
MYU Tokyo
S & M 3547

Scintillation Properties of Tb4O7–Al2O3 Glasses
Yuma Takebuchi,1* Atsunobu Masuno,2 Daiki Shiratori,3Kensei Ichiba,1 Akihiro Nishikawa,1 Takumi Kato,1Daisuke Nakauchi,1 Noriaki Kawaguchi,1 and Takayuki Yanagida1
1Division of Materials Science, Nara Institute of Science and Technology (NAIST), Ikoma, Nara 630-0192, Japan
2Graduate School of Engineering, Kyoto University, Nishikyo-ku, Kyoto 615-8510, Japan
3Department of Electrical Engineering, Tokyo University of Science,Niijuku, Katsushika-ku, Tokyo 125-8585, Japan(Received October 30, 2023; accepted January 12, 2024)

Keywords: scintillator, glass, containerless method, radiation measurement

Undoped and Ce-doped Tb4O7–Al2O3 glasses without glass network-former oxides weresynthesized by the containerless method. The X-ray diffraction measurement indicated that thesynthesized glass samples were amorphous. In diffuse transmission spectra, the undoped sample
showed absorption due to Tb4+, and the reduction of Tb4+ to Tb3+ was observed in Ce-doped
samples. In X-ray-induced scintillation spectra, the Ce-doped samples showed luminescence dueto the 4f-4f transitions of Tb3+. This result indicated that CeCl3 acted as a reducer and enhanced
the luminescence due to Tb3+. In addition, the Ce-doped samples suppressed the thermallystimulated luminescence glow peaks at the low-temperature region.

1.

Introduction

A scintillator, a type of phosphor for ionizing radiation detection, has a function to convert
high-energy ionizing radiation into ultraviolet–visible–near-infrared photons. When combinedwith a photodetector, scintillators are used in many fields such as nuclear medicine,(1) security,(2)and high-energy physics.(3) In general, scintillators require high light yield, high chemical
stability, low afterglow level, short decay time, and high energy resolution. For X- or γ-ray
detection, a large effective atomic number (Zeff ) is also required. Because there are no
scintillators with all of the above requirements, many researchers are still studying novel
scintillators.(4–14) Until now, single crystalline scintillators are mainly used owing to their opticalquality. Glass scintillators also have high optical quality and several industrial advantages overcrystalline scintillators, such as low production cost, upsizing, and workability. On the other
hand, glass scintillators tend to show a lower light yield than crystalline scintillators. Even a Liglass scintillator, the only commercially available glass scintillator, has a light yield of about 1/10
those of crystalline scintillators.(15) Moreover, glass materials with a chemical composition
resulting in a large Zeff have been difficult to form. Because conventional melting methods
generally need network-former oxides such as SiO2, P2O5, and B2O3, the Zeff of glasses becomes*Corresponding author: e-mail: takebuchi.yuma.ty1@ms.naist.jp

https://doi.org/10.18494/SAM4751

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

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small. Therefore, novel glass scintillators with high light yield and large Zeff should be developed.
Actually, some rapid quenching methods have been used to synthesize glasses withouta network former.(16,17) However, the obtained glasses have a f lake-like form; they areinappropriate for practical use as well as for measurements of fundamental physicalproperties. The containerless method prevents heterogeneous nucleation from occurringon the container walls and promotes the deep undercooling of the melt. As a result, thecontainerless method has been used to synthesize some glasses in bulk form with lowglass-forming-ability materials such as the R 2O3 –Al 2O3 (R = rare-earth or Y) binary
system without network-former oxides.(18) In the case of crystals, the R 2O3 –Al 2O3 binary
system allows the use of RAlO3 perovskite and R 3Al 5O12 garnet. Since the crystals haveshown good scintillation properties, the glasses using the R 2O3 –Al 2O3 binary system arealso expected to show good scintillation properties.(19,20)
In this study, we synthesized Ce-doped Tb4O7–Al2O3 glasses by the containerlessmethod and evaluated their radiation-induced luminescence properties. The Tb4O7–Al2O3
glasses have a large Z eff (>50) owing to Tb. This Zeff value is much larger than that of a Liglass scintillator (22.8) and comparable to those of conventional crystalline scintillatorssuch as Tl-doped NaI (50.7) and Tl-doped CsI (54.0). In addition, the Ce-doped Tb3Al 5O12 ,one of the Tb4O7–Al2O3 crystals, has shown a high light yield of 57,000 ph/MeV.(21)Therefore, the Tb4O7–Al2O3 glass is one of the candidates for a novel glass scintillator
with large Z eff and high light yield.

2.

Materials and Methods

Tb4O7 (4N) and Al2O3 (4N) were mixed homogeneously. As a dopant, CeCl3 (4N) powder
was chosen to obtain Ce3+. After mixing, the powders were pressed into tablets and sintered at
1400 °C for 8 h in air. Then, the tablets were crushed and the fragments were obtained. The
containerless method was used to fabricate the glasses using the fragments.(22) The fragments
were melted using a CO2 laser (λ = 10.6 μm) during floating by O2 gas flow. The melt was cooled
to room temperature by turning off the laser. Ten pieces were prepared for each composition.
Some of the pieces were crushed into powder to conduct the X-ray diffraction (XRD)
measurement. A diffractometer (Rigaku, MiniFlex600) was used for the XRD measurement.
The glass transition temperature (Tg) was investigated using a TG-DTA system (Hitachi,
STA7200). A spectrophotometer (Shimadzu, SolidSpec-3700) was used to obtain diffuse
transmission spectra. An original setup was used to investigate X-ray-induced scintillation
spectra.(23) The bias voltage and tube current of an X-ray generator (Spellman, XRB80N100/CB)
were set to 80 kV and 1.2 mA, respectively. Thermally stimulated luminescence (TSL) glow
curves were examined using a TSL reader (NanoGray Inc., TL-2000).(24) The heating rate was 1°C/s and the heating range was 50–490 °C. Before the TSL measurement, MiniFlex 600
irradiated X-rays to the samples.

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3.

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Results and Discussion

By the containerless method, the samples with four different Tb contents were prepared. The
Tb/(Tb + Al) = 30, 32, 37.5, and 40 samples were labeled as 30Tb, 32Tb, 37.5Tb, and 40Tb,
respectively. The Zeff values of the samples were 54.6, 54.7, 55.9, and 56.4, respectively. The Ceconcentration was fixed at 1%. A Ce-free Tb/(Tb + Al) = 37.5 sample, with the same Tb/Al ratio
as that of Tb3Al5O12, was also prepared as a reference and labeled as undoped. Figure 1 shows aphotograph of the prepared glasses. Spherical samples of 2 mm diameter were obtained. Fromthe photograph, the color of the undoped sample was observed to be black, which changed toyellow after Ce doping. The Ce-doped samples with low Tb content showed a visibly light color.
Figure 2 shows the results of the XRD measurement of the samples. All the samples showeda halo peak at around 30° and no sharp peaks due to some crystalline phases were detected.
Therefore, the synthesized samples were amorphous. The Tg values obtained from the DTA
curves for the undoped, 30Tb, 32Tb, 37.5Tb, and 40Tb samples were 867, 864, 858, 855, and 859°C, respectively.
Figure 3 shows the diffuse transmission spectra of the samples. The undoped sample showed
an absorption band at a wavelength shorter than 570 nm. Because the Tb4O7 raw powder has the
one-to-one mixture of Tb3+ and Tb4+, the origin of the absorption was considered to be
Tb4+.(25–27) In the Ce-doped samples, the absorption shifted toward the short-wavelength region.
Moreover, the samples with low Tb content exhibited the absorption at a short wavelength. From
the relationship of complementary colors, the change in absorption wavelength corresponded to
the visual color shown in Fig. 1. The results implied the reduction of Tb4+ to Tb3+. Comparedwith the undoped and 37.5Tb samples, the only difference in the synthesis was the presence of
CeCl3. During melting, therefore, Cl2 gas derived from CeCl3 might have acted as a reducer. Thechange in the absorption wavelength among the Ce-doped samples was due to the difference in2θ
Fig. 1. (Color online) Photograph of synthesized
samples.

Fig. 2.

(Color online) XRD patterns of samples.

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Fig. 3.

(Color online) Diffuse transmission spectra of undoped and Ce-doped samples.

the amount of Tb4+. The 4f–5d transitions of Ce3+ possibly contributed to the absorption at 400–
500 nm.(21,28) A small absorption peak at 480 nm was also observed in the Ce-doped samples.
The origin of the peak was the 7F6 –5D4 transitions of Tb3+.(25)
Figure 4 shows the X-ray-induced scintillation spectra of the samples. Note that no valencechanges such as the reduction of Tb4+ to Tb3+ or the oxidation of Tb3+ to Tb4+ induced byionizing radiation were observed. In the undoped sample, no significant signals were detected.
On the other hand, the Ce-doped samples showed an emission peak at 540 nm. Additionally, thesamples with low Tb content also showed emission peaks at 490, 590, and 620 nm. From theemission wavelengths, the emission peaks at 490, 540, 590, and 620 nm were attributed to the
4f-4f transitions of Tb3+ (5D4 –7F6, 5D4 –7F5, 5D4 –7F4, and 5D4 –7F3, respectively).(29–35) The
relative emission intensity of the samples monitored at 540 nm is shown in Fig. 5. Corresponding
to the result of diffuse transmittance, the samples doped with Ce had low Tb content and highluminescence intensity. One of the reasons for the difference in emission intensity was selfabsorption. Although Ce3+ had no contribution to the luminescence, CeCl3 decreased the amountof Tb4+ and increased the amount of Tb3+. In this study, the samples were synthesized using
Tb4O7 powder and O2 gas; under this condition, Tb3+ oxidized easily to Tb4+. The light yield of
the glasses was unclear because of the low luminescence intensity, which might be caused by thepresence of Tb4+. The oxidation of Tb3+ to Tb4+ decreased the amount of the luminescencecenter and increased the effect of self-absorption. According to a previous study on the TbAlO3
crystal, the crystal growth under a reducing (a mixture of nitrogen and hydrogen gases)
atmosphere prevented the generation of Tb 4+, and the crystal showed strong Tb3+
luminescence.(25) Thus, the glass samples synthesized using a reducing gas flow are expected toshow strong luminescence.
Figure 6 shows the TSL glow curves of the samples after X-ray irradiation of 100 Gy. The
intensities were normalized at 400 °C. In the undoped sample, three glow peaks were observedat around 130, 200, and 400 °C, and the peak at 130 °C was dominant. In contrast, the peaks atSensors and Materials, Vol. 36, No. 2 (2024)

Fig. 4. (Color online) X-ray-induced scintillation
spectra of samples.

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Fig. 5. (Color online) Comparison of luminescenceintensity at 540 nm.

Fig. 6. (Color online) TSL glow curves of samples after X-ray irradiation of 100 Gy. The intensities werenormalized at 400 °C.

130 and 200 °C were suppressed, and the peak at 400 °C was dominant in the Ce-doped samples.
Note that the signal intensities included dark signals and that the signal at low temperature waspredominantly a dark signal. In particular, the 37.5Tb sample indicated no significant signals,
and only the signal due to black body radiation was observed. Although the luminescence origin
in the TSL process was unclear, Tb3+ possibly acted as a luminescence center as with
scintillation. The TSL glow peak means the presence of defects. Thus, the result indicated that
the CeCl3 doping into the glass suppressed the formation of defects in the undoped sample. The
low temperature peaks are affected by room temperature and cause afterglow, although themeasurement of the afterglow level of the present glasses was difficult because of the low signal
intensity. Therefore, the Ce-doped samples are better than the undoped samples for scintillatorsfrom the viewpoint of afterglow.

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Conclusions

Undoped and Ce-doped Tb4O7–Al2O3 glasses were fabricated by the containerlessmethod. The glass samples without network-former oxides were successfully synthesizedin bulk form. The Ce-doped samples showed luminescence due to the 4f–4f transitions of
Tb3+. Although Ce3+ has not affected the luminescence directly, doping CeCl 3 decreased
the amount of Tb4+ and increased the amount of Tb3+. As a result, the luminescence intensity
increased owing to the reducing self-absorption. In addition, the Ce-doped samples suppressed
the glow peaks at the low-temperature region and were expected to show low afterglow levels. In
future work, investigating the effect of reducing gas flow during glass synthesis and otheractivators would be interesting to improve the scintillation properties.

Acknowledgments
This work was supported by Grants-in-Aid for Scientific Research A (22H00309), Scientific
Research B (22H03872, 22H02939, 21H03733, and 21H03736), Early-Career Scientists(23K13689), Challenging Exploratory Research (22K18997), and JSPS Fellow (21J22668) fromthe Japan Society for the Promotion of Science. The Cooperative Research Project of the
Research Center for Biomedical Engineering, Nippon Sheet Glass Foundation, Terumo Life
Science Foundation, KRF Foundation, Tokuyama Science Foundation, Iketani Science andTechnology Foundation, and Foundation for Nara Institute of Science and Technology are also
acknowledged.

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