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Sensors and Materials, Vol. 36, No. 2 (2024) 559–564
MYU Tokyo
S & M 3544

Radio-photoluminescence Properties of Eu-doped CaBPO5Hirozumi Ito,1 Go Okada,1* Yasuhiro Koguchi,2,1Wataru Kada,3 Kenichi Watanabe,4 and Hidehito Nanto1,2
1Co-creative Research Center of Industrial Science and Technology, Kanazawa Institute of Technology,

3-1 Yatsukaho, Hakusan, Ishikawa 924-0838, Japan

2Oarai Research Center, Chiyoda Technol Corporation,3681 Narita-cho, Oarai-machi, Higashi-ibaraki-gun, Ibaraki 311-1313, Japan

3Department of Quantum Science and Energy Engineering, Tohoku University,

6-6-01-2 Aramaki-Aza-Aoba, Aoba, Sendai, Miyagi 980-8579, Japan

4Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University,

744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan(Received October 31, 2023; accepted January 16, 2023)

Keywords: radio-photoluminescence, RPL, Europium, CaBPO5

Eu-doped CaBPO5 was synthesized by the solid-state reaction route, and then its radiophotoluminescence properties were studied. On the basis of steady-state and time-resolved
photoluminescence (PL) studies, it was confirmed that Eu2+ is formed by X-ray irradiation due
to the valence change of the Eu ion (Eu3+ → Eu2+). In addition, for dosimetric applications, it
was demonstrated that the PL intensity of Eu2+ is proportional to the accumulated radiationdose, is stable after irradiation, and can be reversed by heat treatment at 500 ℃ for 100 s.

1.

Introduction

Radio-photoluminescence (RPL) has attracted considerable attention in luminescence
dosimetry applications.(1–3) RPL is a phenomenon whereby a luminescence center is formed viainteractions with ionizing radiation. The luminescence center formed can be detected by aconventional photoluminescence (PL) technique, and the luminescence intensity is used as aprobe of radiation dose since the number of luminescence centers generated (i.e., luminescenceintensity) is proportional to the accumulated radiation dose. Despite the usefulness of RPL,
conventional materials considered for radiation dosimetry are limited to Ag-doped phosphateglass,(4–6) LiF,(7,8) and Al2O3:C,Mg.(9,10) In contrast, recent studies of exploring new RPL
materials revealed additional choices such as Sm-doped,(11–18) Eu-doped,(19–22) and Yb-doped(23)compounds, as well as undoped compounds.(24–27)
In this study, as part of the quest for new RPL materials, we have synthesized Eudoped CaBPO5 by the solid-state reaction route, and then we found that it exhibits RPL.
Furthermore, the origin of RPL as well as the properties for dosimetric applications werediscussed in detail.
*Corresponding author: e-mail: go.okada@neptune.kanazawa-it.ac.jp

https://doi.org/10.18494/SAM4765

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

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Materials and Methods

Eu-doped CaBPO5 was synthesized by the solid-state reaction route. CaCO3 (99.99%),
NH4H 2PO 4 (99.999%), H 3BO3 (99.99%), and Eu 2O3 (99.9%) were first weighed to thestoichiometric ratio and then homogeneously mixed by mortar and pestle for 20 min together
with 10 mol.% Na2CO3 (99.9%) as a sintering aid. Next, the mixture was loaded to an alumina
crucible and then sintered in air at 200 ℃ for 2 h, followed by 950 ℃ for 4 h using an electric
furnace (FT-101FMW, Full-Tech). The PL excitation spectrum of the obtained sample wascharacterized using a spectrofluorometer (F-4500, Hitachi). The time-resolved PL spectrum andPL decay curve were obtained using a 349 nm pulse laser (Explore One 349-120, Spectra
Physics) as an excitation source, a multichannel spectrometer (PMA-12, Hamamatsu) to obtain aspectrum, and a delay generator (DG535, Stanford Research Systems). The details of theinstrumental setup can be found elsewhere.(28) Furthermore, to characterize RPL properties,
X-ray irradiation, PL spectrum measurement, and thermal treatment were performed using theTSL/OSL/RPL Automated and Integrated Measurement System (TORAIMS).(29)3.

Results and Discussion

The obtained material was found to be a single phase of CaBPO5, and no impurity phaseassociated with Na was found by X-ray diffraction analysis. The PL properties of the Eu-doped
CaBPO5 are summarized in Fig. 1. Figure 1(a) shows the PL emission and excitation spectra ofas-prepared (0 Gy) and X-ray-irradiated (5 Gy) samples. The PL spectrum of the as-preparedsample consists of a strong emission band peaking at 400 nm as well as multiple sharp emission
lines across 600–700 nm. The former emission is accompanied by a broad excitation featureacross 200–350 nm, while the excitation spectrum of the latter emission feature consists of abroad band across 200–300 nm and sharp peaks at 318, 365, and 394 nm. The comparison of theemission intensities between the as-prepared and X-ray-irradiated samples shows that the 400nm band is stronger, whereas the 600–700 nm peaks are slightly smaller after X-ray irradiation(5 Gy). To study the above luminescence origins, time-resolved PL spectra and PL decay curves
are characterized. Figure 1(b) shows the time-resolved PL spectra of the nanosecond andmillisecond ranges. From the spectra of the nanosecond range, it is shown that the 400 nm
emission band decays very rapidly, whereas the 600–700 nm emission band remains unchanged.
In the millisecond range, however, a decay of the 600–700 nm emission band is clearlydemonstrated. In Fig. 1(c), the intensities of these two sets of emissions are plotted as a functionof time after pulse excitation. The square-root curve fitting with a single exponential decayfunction reveals that the 400 nm emission band has a lifetime of 118 ns, whereas the latteremission lines across 600–700 nm have a lifetime of 2.6 ms. It is commonly known that Eu cantake its valence state to be either divalent or trivalent. On the basis of the above observations ofthe spectrum features and lifetime, it is reasonable to assign the luminescence origin of the600–700 nm emission to the 4f-4f transitions of Eu3+. In contrast, the origin of the 400 nmemission band is considered to be the 5d→4f transitions of Eu2+ based on the spectrum features,
whereas the lifetime is smaller than the typical value by approximately one order of magnitude.

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(a)

(b)

(c)
Fig. 1. (Color online) PL properties of Eu-doped CaBPO5. (a) PL excitation and emission spectra measured before
and after X-ray irradiation (5 Gy), (b) time-resolved PL spectra, and (c) PL decay curves.

The lifetime appeared to be smaller because the probability of nonradiative transition is high inthis system. Note that earlier studies on Eu-doped CaBPO5 reported the same emission at 400nm, whose origin is assigned to Eu2+.(30)
It is considered that the emission intensity of Eu2+ increases, whereas that of Eu3+ decreases
after X-ray irradiation [see also Fig. 2(a)]. This indicates that the number of Eu2+ ions increases,
whereas that of Eu3+ ions decreases following X-ray irradiation. Since Eu2+ as a luminescencecenter is formed by X-rays, it is considered that the Eu-doped CaBPO5 exhibits RPL.

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The RPL of Eu-doped CaBPO5 has been considered for dosimetric applications, andrepresentative properties are illustrated in Fig. 2. In particular, Fig. 2(a) demonstrates a dose
response curve. Here, the doping concentration of Eu is 0.1%. It shows that the response (whichis the integrated PL intensity between 350 and 450 nm induced by X-ray irradiation)
monotonically increases with the radiation dose. The intersection of the dose response curvewith the 3σ line is about 4 Gy, which is the lowest detection limit (LDL) with the presentenvironment. Note that LDL varies with the type of reader system used. The present reader used
is TORAIMS, in which the LDL of a commercially available Ag-doped phosphate glass is about

(a)

(b)

(c)
Fig. 2. (Color online) Dosimetric properties of RPL of 0.1% Eu-doped CaBPO5. (a) Dose response curve, (b)stability after X-ray irradiation, and (c) reproducibility.

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5 mGy, whereas the LDL obtained by using the optimized commercially available reader isabout 10 μGy.
Figure 2(b) shows the stability of RPL, which is equivalent to the Eu2+ center formed by theX-ray irradiation of 10 Gy. It is seen that the response value decreases by about 20% of its
original value after 20 min, whereas the majority of the signal remains after the dose. Therefore,
it can be considered that the RPL signal is reasonably stable, and no sign of buildup is observed.
Thus, the RPL of Eu-doped CaBPO5 can be considered for real-time dose monitoring.(31,32)
Figure 2(c) demonstrates the reproducibility of RPL. For the characterization, the X-rayirradiation of 5 Gy was performed first to record the response value, then the sample was heattreated at 500 ℃ for 100 s to reverse the response signal. These treatments were repeated 10times to study the reproducibility of RPL. As illustrated in Fig. 2(c), in the first cycle, the signal
intensity increases after irradiation. After heat treatment, it considerably increases, so the initialvalue of the second cycle is about three times larger than that of the first cycle after irradiation.
After the irradiation of the second cycle, the intensity increases further; then, the following heat
treatment effectively decreases the signal, unlike the first behavior. Despite the fluctuation of
the signal by heat treatment, the response value, which is an increased signal after X-ray
irradiation, is almost consistent regardless of the treatment cycle. Therefore, the RPL itself isreasonably reproducible.

4.

Conclusions

Eu-doped CaBPO5 was synthesized by the solid-state reaction route, and then it wasconfirmed that it exhibits RPL due to the valence change of the Eu ion (Eu3+ → Eu2+) inducedby radiation. In addition, the dosimetric properties of RPL were studied: the PL intensity of Eu2+
increases as a function of dose and is stable over 20 min, RPL can be reversed by heat treatment(500 ℃ for 100 s), and RPL is reproducible multiple times.

Acknowledgments
This work was supported by a Grant-in-Aid for Scientific Research (B) (22H02009) from the
Ministry of Education, Culture, Sports, Science and Technology of the Japanese government(MEXT) and the Research Center for Biomedical Engineering.

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