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Sensors and Materials, Vol. 36, No. 2 (2024) 597–605
MYU Tokyo
S & M 3549

Radio-photoluminescence Properties
of Sm-doped HfO2–Al2O3–SiO2 Glass Ceramics
Akihiro Nishikawa,1* Daiki Shiratori,2 Takumi Kato,1Daisuke Nakauchi,1 Noriaki Kawaguchi,1 and Takayuki Yanagida1
1Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan
2Tokyo University of Science, 6-3-1 Niijuku, Katsushika-ku, Tokyo 125-8585, Japan(Received October 19, 2023; accepted January 11, 2024)

Keywords: glass ceramics, X-ray imaging, radio-photoluminescence, Sm

In this study, Sm-doped HfO2–Al2O3–SiO2 glass ceramics with different Sm concentrations(0.1, 0.3, 1.0, and 3.0%) were synthesized by the melt quenching method using a floating zone
(FZ) furnace. Their photoluminescence properties, including radio-photoluminescence andspatial resolution, were investigated. The 0.1 and 0.3% Sm-doped samples indicated theluminescence of Sm3+, and the 1.0% Sm-doped sample showed the luminescence of Sm2+ andSm3+. Alternatively, the 3.0% Sm-doped sample mainly showed the luminescence of Sm2+. The
radio-photoluminescence phenomenon (Sm3+ → Sm2+) was observed in the 0.1–1.0% Sm-doped
samples. In addition, the 1.0% Sm-doped sample had a spatial resolution of 10 LP/mm.

1.

Introduction

Various inorganic phosphors have been investigated for radiation measurements,(1,2) andradio-photoluminescence (RPL) materials are one of such phosphors used for dosimetry.(3–7)
RPL is photoluminescence (PL) that occurs through luminescence centers generated by X-ray
irradiation. The amount of luminescence centers is proportional to the irradiation dose; thus,
measuring the luminescence intensity can estimate the irradiation dose. The Ag-doped
phosphate glass is a common RPL material.(8) The RPL center of this material is Ag2+. Before
X-ray irradiation to the Ag-doped phosphate glass, most Ag exists as Ag+ in the glass matrix.
Then, Ag+ changes to Ag2+ with X-ray irradiation.(9) In this case, Ag2+ is stable at room
temperature in the glass matrix, so the luminescence intensity of Ag2+ can be observed
repeatedly. In other words, once dose information is stored by the Ag-doped phosphate glass, we
can read out the dose information repeatedly without losing it.
RPL materials are used primarily for personal dosimetry, but an X-ray imaging plate (IP)using RPL has been considered.(10,11) Currently, optically stimulated luminescence (OSL)
materials are typically used as phosphors for IP;(12–15) however, the stored OSL signal is reducedby the laser during readout. When OSL materials are read out with strong excitation light, thereadout and unintended points are excited simultaneously, resulting in a decrease in OSL*Corresponding author: e-mail: nishikawa.akihiro.nc6@ms.naist.jp

https://doi.org/10.18494/SAM4755

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

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intensity and resolution. On the other hand, RPL has the potential to solve the problem and evenachieve a high resolution by combining confocal laser scanning microscopy. Thus, we focusedon developing new RPL materials to replace the OSL materials in IP applications.
In this study, Sm-doped HfO2–Al2O3–SiO2 glass ceramics were prepared by the melt
quenching method using an optical floating zone (FZ) furnace, and their PL and RPL properties
and spatial resolutions were investigated. The commercial RPL material used for personal
dosimetry has as low effective atomic number (Zeff) as human tissues. However, we aim todevelop RPL materials for IP, so materials with high Zeff are preferred to increase the interaction
probability of X-rays. Thus, HfO2 was chosen to increase the Zeff of the host material. In
addition, Sm was selected as an RPL center. Sm was reported to indicate the RPL phenomenon,
in which Sm3+ changes to Sm2+ through ionizing radiation.(16–19)

2.

Materials and Methods

2.1

Sample preparation

Sm-doped 10HfO2-10Al2O3-80SiO2 glass ceramics were synthesized by the melt quenching
method using an optical FZ furnace. HfO2 has a high melting point; thus, we applied the optical
FZ furnace, not an electric furnace, to prepare the glasses. The starting materials of the hostglass, namely, HfO2 (4N), Al2O3 (4N), and SiO2 (4N) powders, were uniformly mixed at a molar
ratio of 10:10:80, respectively. This host glass has a Zeff of 45. Sm2O3 (4N) was added to the hostglass with concentrations of 0.1, 0.3, 1.0, and 3.0 mol.%. Four grams in total of these powders
was mixed in an agate mortar. The mixed powders were formed into a cylinder rod usinghydrostatic pressure, and this rod was sintered at 1200 ℃ for 8 h in air. The optical FZ furnace(FZ-T-12000-X-VPO-PC-Y, Crystal Systems), which can achieve a temperature of approximately
3000 ℃, melted the obtained ceramic rod with four Xe lamps. In addition, the molten part was
dropped into the water and rapidly quenched. The glass sample obtained by this method has aspherical morphology with a diameter of less than 10 mm. The obtained glass samples weremechanically polished.
2.2

Measurement method

X-ray diffraction (XRD) measurements were conducted to confirm the amorphous phase of
the samples. XRD patterns were measured using MiniFlex 600 (RIGAKU). Diffuse
transmittance spectra were measured with a spectrophotometer (SolidSpec-3700, Shimadzu).
The absorption spectra in the infrared region were also measured using a spectrophotometer(V670, JASCO), and the measurement was performed across the spectral range from 1.7 to 3.0μm.
For the evaluation of RPL properties, the excitation and emission spectra obtained before andafter X-ray irradiation were measured using a spectrofluorometer (FP-8600, JASCO). The
measurement interval was 1 nm. Moreover, the PL lifetimes of the samples observed before andafter X-ray irradiation were measured using Quantaurus-Tau (C11367, Hamamatsu Photonics).

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To expose X-rays to the samples, an X-ray generator (XRBOP&N200X4550, Spellman) wasused. The samples were irradiated with 10 Gy of X-rays.
In terms of the imaging device capability, test charts (Pro-Res RF BarType7, PROPROJECT)
were used to evaluate the spatial resolution. The samples were irradiated with X-rays through atest chart. Subsequently, the samples were excited with 440 nm light, and the emission throughan optical filter (LV0670, Asahi Spectra) was received by a CCD camera (BU-54UV, Bitran) toobtain X-ray images.

3.

Results and Discussion

Figure 1(a) shows the appearance of all samples. All samples were approximately 6 mm indiameter, 1.1 mm thick, and transparent, but the 3.0% Sm-doped sample was reddish. Figure 1(b)shows the XRD patterns of all the samples, and broad and sharp peaks were observed. The
broad peak at 25 degrees was regarded as the halo peak, which was generally observed inamorphous materials, while the sharp peaks at 30, 35, 50, and 60 degrees were suitable for cubicHfO2 (c-HfO2) (space group: Fm-3m, PDF: 00-053-0550) crystal peaks. These peaks were also
observed in the undoped 10HfO2–10Al2O3–80SiO2 glass.(20) It was found that all the samples
were glass ceramics, including the c-HfO2 phase without lost transparency.
Figure 2(a) shows the diffuse transmittance spectra of all the samples. The 0.1, 0.3, and 1.0%
Sm-doped samples indicated an absorption band at around 200–300 nm. This absorption band
was attributed to the absorption of undoped HfO2–Al2O3–SiO2 glass ceramics.(20) In addition, a
sharp absorption at around 400 nm in the 0.1–1.0% Sm-doped samples and an absorption band ataround 300–450 nm in the 3.0% Sm-doped sample were observed. The absorption peaks ataround 400 nm were attributed to the 4f–4f transition of Sm3+.(21–23) The absorption band ataround 300–450 nm in the 3.0% Sm-doped sample would be related to the 4f–5d transition of
Sm2+. Figure 2(b) shows the absorption spectrum of the 1.0% Sm-doped sample in the infrared
region. This absorption spectrum was measured to confirm the presence or absence of the OH
group in the sample because of the use of water in the synthesis process. Since the sameabsorption band was obtained for all the samples, the absorption spectrum of the 1.0% Smdoped sample is shown as a representative. Absorption bands for OH groups are generally
observed at 1.91, 2.22, and 2.52 μm in silicate glass.(24) Such absorption bands were observed in
the 1.0% Sm-doped sample. The presence of OH groups in the samples might be due to the effectof using water for quenching.
Figure 3(a) shows the PL excitation and emission spectra of the 3.0% Sm-doped sample.
Excitation peaks at around 340, 360, and 400 nm under monitoring at 600 nm were attributed totransitions of 6H5/2 → 4H9/2, 6P7/2, and 4F7/2.(25) The excitation peaks monitored at 690 nm
emission were detected at around 350 and 440 nm, which were attributed to the 4f 6 –4f 5–5d1
transition.(26) There were emission peaks at around 560, 600, 650, 690, 710, and 730 nm under
400 nm excitation. On the other hand, the emission peaks were mainly observed at around 690
and 730 nm under 440 nm excitation. Figure 3(b) shows the PL decay curves of all the samples
when the emission wavelength is monitored at 600 nm. The decay curve of the 0.1% Sm-doped
sample was approximated by two exponential decay components, and the decay curves of the600

Sensors and Materials, Vol. 36, No. 2 (2024)(a)Fig. 1.

(b)(Color online) (a) Appearance and (b) XRD patterns of all samples.

(a)

(b)

Fig. 2. (Color online) (a) Diffuse transmittance spectra of all samples and (b) absorption spectrum of 1.0% Smdoped sample in infrared region.

(a)

(b)

Fig. 3. (Color online) (a) PL excitation and emission spectra of 3.0% Sm-doped sample; dashed lines show
excitation spectra and solid lines show emission spectra. (b) PL decay curves of all samples. Instrument Response
Function (IRF) is excitation light leakage.

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other samples were approximated by three ones. The calculated lifetimes are shown in Table 1.
The lifetimes of the 0.1% Sm-doped sample were 1.29 and 2.89 ms, which would be attributed to
Sm3+.(27) In the other samples, PL lifetimes of 169–283 μs and 1.01–2.77 ms were observed. The
first component was assigned to Sm2+,(28) and the second and third ones would be attributed to
Sm3+ as well as the 0.1% Sm-doped sample. On the basis of these emission wavelengths andlifetimes, the emission peaks at around 560, 600, 650, and 710 nm were found to originate fromthe 4G5/2 → 6H5/2, 6H7/2, 6H9/2, and 6H11/2 transitions of Sm3+, and the emission peaks at around690 and 730 nm originated from the 5D0 → 7F0 and 7F2 transitions of Sm2+, respectively.(29–31)
Figure 4 shows the PL emission spectra of all the samples obtained before and after X-ray
irradiation. Before X-ray irradiation, the 0.1 and 0.3% Sm-doped samples showed theluminescence of Sm3+ (560, 600, 650, and 710 nm), and the 1.0% Sm-doped sample indicated theluminescence of Sm2+ (690 and 730 nm) and Sm3+. On the other hand, the 3.0% Sm-doped
sample before X-ray irradiation mainly exhibited the luminescence of Sm2+. After X-ray
irradiation, the decrease in Sm3+ emission intensity and the increase in Sm2+ emission intensity
were observed in the 0.1–1.0% Sm-doped samples, and the 3.0% Sm-doped sample did not show
significant changes. These results are based on the valence change (Sm3+ → Sm2+) caused by
X-ray irradiation.(18) In other words, the 0.1–1.0% Sm-doped samples indicated the RPL
phenomenon.
Figure 5 shows the dose response function of the 1.0% Sm-doped sample. The 1.0% Smdoped sample exhibited the dose response function with a good linear relationship in the range
of 1–30 Gy, and this result supported the fact that Sm2+ was formed by irradiation.
Figure 6(a) shows the repeated measurement of the RPL intensity of the 1.0% Sm-doped
sample. The plots indicated the difference in RPL intensity at 690 nm before and after X-ray
irradiation with normalization by the first measurement intensity. The reset was conducted byannealing at 500 ℃ for 1 h. No significant change in RPL intensity with the number ofmeasurements was observed, suggesting the reusability of the samples. Figure 6(b) shows thechange in RPL intensity with elapsed time. Here, the intensity was normalized at the first day.
The RPL intensity increased one day after X-ray irradiation, suggesting that a buildup
phenomenon had occurred, and it monotonically decreased after the second day.
The X-ray imaging test was conducted using the 1.0% Sm-doped sample because of the highemission intensity of Sm2+ after X-ray irradiation. Figure 7 shows the image obtained by the
X-ray imaging test. The spatial resolution was evaluated using the reciprocal of the width of aline pair that could be considered resolved on the image. As a result, the spatial resolution was 10
LP/mm.

Table 1
PL lifetimes of all samples.
Sample
0.1
0.3
1.0
3.0

1st
1.29
0.283
0.221
0.169

Lifetime (ms)
2nd
2.89
1.29
1.03
1.01

3rd
N/A
2.77
2.60
2.62

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Fig. 4. (Color online) (Clockwise from the top left) PL emission spectra of 0.1, 0.3, 1.0, and 3.0% Sm-doped
samples obtained before and after X-ray irradiation. The irradiation dose is 10 Gy.

Fig. 5.

Dose response function of 1.0% Sm-doped sample.

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(a)

603

(b)

Fig. 6. (a) Repeated measurement of RPL intensity of 1.0% Sm-doped sample and (b) change in RPL intensitywith elapsed time.

Fig. 7.

4.

Image of 1.0% Sm-doped sample obtained by an X-ray imaging test and its luminance intensity.

Conclusion

Sm-doped HfO2–Al2O3 –SiO2 glass ceramics were prepared successfully; however, theglasses included the crystalline phase of c-HfO2. The glasses were transparent despite theinclusion of the crystalline phase. Their PL properties were evaluated using a 3.0% Sm-doped
sample. PL emission peaks were detected at around 560, 600, 650, 690, 710, and 730 nm under
400 and 440 nm excitations. The peaks at around 560, 600, 650, and 710 nm were due to the 4f–
4f transition of Sm3+. On the other hand, the peaks at around 690 and 730 nm were attributed to
the 4f–4f transition of Sm2+. Before X-ray irradiation, the 0.1 and 0.3% Sm-doped samples
indicated only the Sm3+ luminescence, and the 1.0 and 3.0% Sm-doped samples indicated theSm2+ and Sm3+ luminescence. The emission intensity of the 0.1–1.0% Sm-doped samples waschanged by X-ray irradiation, in which the Sm2+ emission intensity increased with decreasing

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Sm3+ emission intensity. This confirmed the existence of the RPL phenomenon. The X-ray
imaging test was conducted using the 1.0% Sm-doped sample. The result was that it had a spatial
resolution of 10 LP/mm.

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), and Challenging Exploratory Research (22K18997) from the 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 and Technology Foundation, andFoundation for Nara Institute of Science and Technology are also acknowledged.

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