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MYU Tokyo
S & M 3548

Synthesis and Radiation Response Properties
of Tm-activated Na2O–ZnO–TeO2–B2O3 Glasses
Kai Okazaki,* Daisuke Nakauchi, Akihiro Nishikawa, Takumi Kato,
Noriaki Kawaguchi, and Takayuki Yanagida
Division of Materials Science, Nara Institute of Science and Technology (NAIST),
8916-5 Takayama-Cho Ikoma, Nara 630-0192, Japan(Received October 30, 2023; accepted January 18, 2024)

Keywords: phosphor, radiation measurement, scintillation, borate glass

Na 2O–ZnO–TeO2 –B2O3 (NZTB) glasses doped with Tm (0.1, 0.5, 1, and 2%) weresynthesized by a conventional melt quenching technique, and their photoluminescence andscintillation properties were investigated. Some emission peaks due to the electronic transitionsof Tm3+ appeared upon ultraviolet light and X-ray irradiation. Decay times were valid as the 4f–
4f transitions of Tm3+. A 1% Tm-doped NZTB glass showed a full absorption peak under 241Am
α-ray irradiation, and the light yield was estimated to be 51 photons/5.5 MeV-α when the peakchannel was compared with a photoabsorption peak of 137Cs γ-rays measured with a Ce-doped
Gd2SiO5 sample.

1.

Introduction

Scintillators are luminescence materials that promptly convert high-energy ionizing radiation
into low-energy photons after absorbing the energy.(1–3) They have been used in many fields, forexample, medical imaging,(4) security inspection,(5) environmental measurement,(6) and resource
exploration.(7) The following properties are generally important for scintillators: high light yield(LY), fast decay, low afterglow, and mechanical and chemical stabilities. There are no scintillators
satisfying all the above properties. However, the required properties vary in applications;
therefore, suitable scintillators are chosen on the basis of their physical and chemical properties.
From the perspective of material form, many types of material have been applied, for instance,
crystals,(8–11) ceramics,(12–14) liquids,(15–17) and glasses.(18–22) Glasses are an attractive form
owing to their low cost, ease of formability, high mechanical strength, and high freedom incomposition selection. In particular, Ce-doped lithium silicate glasses have been intensively
studied for thermal neutron detection,(19,23–25) and GS20, which shows the LY of 6000 photons/
neutron(26) with no hygroscopicity and low density, is a commercial glass scintillator. For
thermal neutron detection, a 3He gas counter is mainly applied by using 3He(n, p)3H reactions;(27)however, alternative detectors are extensively studied because of the limitation of 3He
resources.(28)*Corresponding author: e-mail: okazaki.kai.of0@ms.naist.jp

https://doi.org/10.18494/SAM4753

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

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Borate glasses to be used for thermal neutron detection in addition to Li-containing glasses
have also received attention because 10B has a larger thermal neutron capture cross section (3840barn) than 6Li (940 barn).(29) Some compositions with borate have shown response to neutrons or
α-rays;(30–33) however, there are no commercial ones. In this study, Na2O–ZnO–TeO2–B2O3(NZTB) glasses were developed. The prepared glasses have almost the same effective atomic
number (29) as GS20 (26),(34) and the relatively low value is an advantage for distinguishing
signals of neutrons from those of γ-rays. TeO2 improves thermal strength, chemical stability, andlight output when combined with B2O3.(35,36) Borate glasses generally show high hygroscopicity;
therefore, Na2O was added as a modifier.(37) Moreover, alkali-metal-oxide-containing ZnO–
TeO2–B2O3 glasses were discovered to show good transparency in visible ranges.(38,39) This
optical characteristic is suitable for the host material for scintillators with luminescence centers.
Some rare-earth ions act as luminescence centers.(40–42) Tm exhibits sharp emissions in visible–
infrared regions due to electric dipole-forbidden transitions between 4f orbitals.(43–45) The main
peaks appear in the 300–500 nm range,(46–49) which matches the high wavelength sensitivity
regions of conventional photodetectors such as photomultiplier tubes and Si photodiodes. Here,
we fabricated NZTB glasses doped with different Tm concentrations, and investigated their
optical and scintillation properties.

2.

Materials and Methods

25Na2O–20ZnO–5TeO2–50B2O3 glasses doped with Tm (0, 0.1, 0.5, 1, and 2 mol%) weresynthesized by the melt quenching method. First, Tm2O3 (4N), Na2CO3 (4N), ZnO (4N), TeO2
(4N), and B2O3 (5N) powders were homogeneously mixed with an agate mortar. Then, they weretransferred to an alumina crucible and melted at 900 ℃ for 1 h. After that, the melt was flowed
onto a preheated stainless-steel plate to quench. The obtained samples were annealed at 300 ℃
for 1 h to remove thermal and mechanical strains. The annealing temperature was determined bymeasuring the glass transition temperature (Tg) of the undoped sample with a TG-DTA system(Hitachi High-Tech Corporation, STA7200).
The densities of the prepared samples were measured using an analytical balance (A&D
Company, GR-120). Ultrapure water (Fujifilm Wako) was used in the measurement. Powder
X-ray diffraction (XRD) patterns were determined using a diffractometer (Rigaku,
MiniFlex600). Diffuse transmission spectra were measured with a spectrometer (Shimazu,
SolidSpec-3700). The photoluminescence (PL) excitation and emission spectra, PL quantum
yields (QYs), and PL decay curves were calculated using a Quantaurus-QY (Hamamatsu,
C11347) and a Quantaurus-τ (Hamamatsu, C11367). As radiation response properties, X-rayinduced scintillation spectra, X-ray-induced scintillation decay curves, and pulse height spectraof 241Am α-rays (5.5 MeV) were tested with our original setups.(50,51)

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3.

589

Results and Discussion

Figure 1 shows the photograph and densities of the prepared samples. The surfaces were
polished for the following PL and scintillation measurements. All the samples appearedtransparent and colorless under room light. The Tg of the undoped sample was 500 ℃. The
densities were changed in the range of 2.4–2.7 g/cm3 with respect to the Tm concentration. As
the dopant concentration increased, the densities increased owing to the molecular mass ofTm2O3 being larger than that of the compounds composing the host. The values were comparable
to that of GS20 (2.5 g/cm3).(26) Figure 2 shows the XRD patterns of the samples. Some parts ofthe glasses not used in the PL and scintillation measurements were crushed into powders, and
XRD measurements were conducted. All the samples showed only halo peaks; hence, thesamples had no periodical structures and formed glass phases.
Figure 3 shows the diffuse transmission spectra of NZTB glasses. The transmittance was
80–95% in the 350–850 nm range. Absorption peaks due to the electronic transitions ofTm3+(52,53) were clearly observed at 350, 470, 680, and 790 nm in the 1 and 2% Tm-doped
samples. Absorption edges were confirmed at 260–280 nm. They were slightly shifted to lowenergy regions as the dopant concentration increased. As the Tm concentration increased, thepeaks were shifted to low-energy regions. Figure 4 shows the PL excitation and emission spectraof the undoped and 0.5% Tm-doped NZTB glasses. Although the undoped sample did not show
any emissions, the Tm-doped sample showed several emissions at 450, 480, 650, and 750 nm
under excitation at 360 nm. These emissions were respectively considered to be derived from the1D –3F , 1G –3H , 1G –3F , and 1D –3F transitions of Tm3+.(54,55) The QYs of the 0.1, 0.5, 1, and
2
4
4
6
4
4
2
3
2% Tm-doped samples were respectively 4.3, 8.6, 7.2, and 3.5% when monitored at 400–800 nm
upon excitation at 360 nm. Figure 5 shows the PL decay curves of the Tm-doped samples. The

Fig. 1. Photograph (inset) and densities of NZTB
glasses.

Fig. 2. (Color online) XRD patterns of NZTB
glasses.

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Fig. 3.

(Color online) Diffuse transmission spectra of NZTB glasses.

Fig. 4. (Color online) PL excitation and emission
spectra of undoped and 0.5% Tm-doped NZTB
glasses. Color scales indicate the intensities, where
white and black indicate high and low intensities,
respectively.

Fig. 5. (Color online) PL decay (solid) and fitting(dashed) cur ves of Tm-doped NZTB glasses.
Excitation and monitored wavelengths were 340–390
nm and 455 nm, respectively.

obtained curves matched with an approximation by a single exponential decay function. The
decay times of the 0.1, 0.5, 1, and 2% Tm-doped samples were obtained to be 14.8, 13.7, 12.6, and10.0 μs, respectively. From the above decay times and QYs, radiative (kf) and nonradiative (k nr)
transition rates were estimated and are shown in Table 1. In the estimation, the following
equations were applied: kf = QY/τ and k nr = (1 − QY)/τ. Here, the PL decay time was denoted as τ.
kf decreased as the Tm concentration increased from 0.5 to 1%. The tendency can be describedas concentration quenching that occurred at 1% Tm doping.
Figure 6 shows the X-ray-induced scintillation spectra of the NZTB glasses. Sharp emission
peaks were observed at 350, 360, 450, and 480 nm. Similar scintillation peaks were observed inother Tm-doped materials;(56,57) hence, they were considered to be attributed to the 4f–4ftransitions of Tm3+. Figure 7 shows the X-ray-induced scintillation decay curves of the Tmdoped samples. All the curves were approximated by a single exponential function when the
instrumental response function (IRF) was deconvoluted. The decay times were obtained to beSensors and Materials, Vol. 36, No. 2 (2024)

591

Table 1
Summary of PL characteristics of Tm-doped NZTB glasses.
SamplesQYs (%)
Decay times (μs)
0.1% Tm
4.3
14.8
0.5% Tm
8.6
13.7
1% Tm
7.2
12.6
2% Tm
3.5
10.0

Fig. 6. (Color online) X-ray-induced scintillation
spectra of NZTB glasses.

k f (s−1)
2.9 × 103
6.3 × 103
5.8 × 103
3.5 × 103

k nr (s−1)
6.5 × 104
6.7 × 104
7.4 × 104
9.7 × 104

Fig. 7. (Color online) X-ray-induced scintillation
decay curves of Tm-doped NZTB glasses.

30–50 μs, which were close to the reported scintillation decay times of other Tm-doped
materials.(48,57–59) Therefore, they originated from the 4f–4f transitions of Tm3+. When comparedwith the PL decay times, they became much longer because of the conversion and transportation
processes of scintillation in addition to the direct excitation and emission processes.(60)Figure 8 shows the pulse height spectrum of 241Am α-rays (5.5 MeV) measured using the 1%
Tm-doped NZTB glass. The spectrum of 137Cs γ-rays (0.662 MeV) measured using a Ce-doped
Gd2SiO5 sample (GSO, 8000 photons/MeV) was also displayed as a reference for calculating the
LY of a prepared sample. A full absorption peak appeared only in the 1% Tm-doped sample,
whereas this peak was unclear in the spectra. The rest of the prepared samples did not show fullabsorption peaks owing to their low LYs. From Robbins’ model,(61) LY is considered directly
proportional to QY, but inversely proportion to bandgap energy (Eg). The QY of the 0.5% Tmdoped sample was higher than that of the 1% Tm-doped sample; however, the 0.5% sample did
not show a full absorption peak. This would be due to the difference in Eg: a high concentrationof Tm doping would broaden the impurity bands and tails would reduce Eg.(62) This tendency
was confirmed in the diffuse transmission spectra shown in Fig. 3. Therefore, the LY of the 1%
Tm-doped sample would become higher than that of the 0.5% Tm-doped sample and the fullabsorption peak would appear. By comparing the channel of a full absorption peak (230 ch) witha photoabsorption peak channel of the reference (23750 ch), which was taking the difference ingain into account, the LY of the 1% Tm-doped glass was estimated to be 51 photons/5.5 MeV-α.

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Fig. 8. (Color online) Pulse height spectra of 241Am α-rays (5.5 MeV) measured with 1% Tm-doped NZTB glassand 137Cs γ-rays (0.662 MeV) measured using GSO. Background response was measured without samples.

4.

Conclusions

NZTB glasses doped with Tm (0.1, 0.5, 1, and 2%) were fabricated by a conventional melt
quenching method, and their physical, PL, and scintillation properties were studied. The
prepared samples had halo peaks and no sharp diffraction patterns. Their densities were 2.4–2.7
g/cm3, which were comparable to that of GS20. Under the irradiation of both ultraviolet light and
X-rays, some emission peaks appeared in visible regions, and they originated from the 4f–4ftransitions of Tm3+. Although the prepared samples could not detect thermal neutrons, they
showed signals under 241Am α-ray irradiation (5.5 MeV). The LY of the 1% Tm-doped sample
was estimated to be 51 photons/5.5 MeV-α. Therefore, the result revealed that the glass with acomposition of NZTB had a potential for thermal neutron detection because neutrons weredetected through the observation of the α-rays generated by neutron capture reactions. There is
still huge room for research on NZTB glass scintillators because the molar ratio of hostcompositions, the optimum dopants, and their concentrations have not been investigated. By
considering them, the QY, LY, and capability of neutron detection would be improved.

Acknowledgments
This work was supported by Grants-in-Aid for Scientific Research A (22H00309), Scientific
Research B (21H03733, 21H03736, and 22H03872), Exploratory Research (22K18997), and JSPS
Fellows for Young Scientists (23KJ1592) from the Japan Society for the Promotion of Science.
JST A-STEP, Foundation from Cooperative Research Project of the Research Center for
Biomedical Engineering, Kazuchika Okura Memorial Foundation, Asahi Glass Foundation,
Konica Minolta Science and Technology Foundation, and Nakatani Foundation are also
acknowledged.

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