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MYU Tokyo
S & M 3542

Comparative Study on Optically Stimulated Luminescence
Properties of BaCl2:Ce and BaCl2:Eu Transparent Ceramics
Shota Otake,* Hirotaka Sakaguchi, Yuta Yoshikawa, Takumi Kato,
Daisuke Nakauchi, Noriaki Kawaguchi, and Takayuki Yanagida
Division of Materials Science, Nara Institute of Science and Technology (NAIST),
8916-5 Takayama-cho, Ikoma-shi, Nara 630-0192, Japan(Received October 26, 2023; accepted January 23, 2024)

Keywords: photoluminescence, optically stimulated luminescence, radiation measurement, transparent
ceramics, phosphor

0.1% BaCl2:Ce and BaCl2:Eu transparent ceramics were synthesized by the spark plasma
sintering method, and the optical and dosimetric properties of BaCl2:Ce and BaCl2:Eu werecompared. BaCl2:Ce and BaCl2:Eu transparent ceramics showed photoluminescence (PL) andoptically stimulated luminescence (OSL) peaks at around 380 nm due to the 5d–4f transition of
Ce3+ and Eu2+. The PL quantum yields of BaCl2:Ce and BaCl2:Eu transparent ceramics were 3.8
and 25.6%, respectively. The PL decay times of BaCl2:Ce and BaCl2:Eu were 21 and 400 ns,
respectively. In OSL dose responses, the BaCl2:Eu sample showed better OSL intensity than
BaCl2:Ce, and the OSL signal was detectable from 0.01 mGy. In terms of detection sensitivity,
this value was comparable to those of some commercially available materials.

1.

Introduction

Phosphor-based radiation detectors are classified into two categories, namely, scintillation
detectors(1–3) and dosimeters with storage phosphors.(4–6) They are widely applied to many fields
such as medicine,(7–9) environmental monitoring,(10–12) security,(13–15) and astrophysics.(16–18)
Dosimeters with storage phosphors have the function of storing the energy of ionizing radiation
in a form of trapped electrons and holes at localized sites. The trapped carriers can be releasedby external stimulation, which eventually causes the recombination of electrons and holes withthe emission of photons. If the stimulation is optical, the process is called optically stimulated
luminescence (OSL).(19–25) OSL materials are mainly used in radiation measurements such as
personal radiation dosimeters(26–28) and imaging plates (IPs).(29–33) In particular, the required
properties of OSL materials for IPs include high intensity, wide dynamic range, low fading, andhigh effective atomic number.
Conventionally, storage phosphors such as BaFBr:Eu and CsBr:Eu have been developed andused for commercially available IPs.(34–37) These phosphors show a high OSL intensity and awide dynamic range. Moreover, the OSL wavelengths of BaFBr:Eu and CsBr:Eu are 390 and 440*Corresponding author: e-mail: otake.shota.os4@ms.naist.jp

https://doi.org/10.18494/SAM4759

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

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nm, respectively, which can be effectively detected by conventional photodetectors.(38,39)
Recently, our group has developed transparent ceramics of BaFBr:Eu and CsBr:Eu and has
evaluated their OSL properties.(40,41) Transparent ceramics have attracted attention as new
optical materials because of their superior heat resistance and mechanical strength, as well as
their advantages of shape flexibility and mass production compared with single crystals.(42–47)
In this study, we focused on BaCl2 as a host material of transparent ceramics. BaCl2 has ahigh effective atomic number (46.8) as do BaFBr (47.8) and CsBr (51.9). In addition to thischaracteristic, BaCl2 has almost no hygroscopicity, which is also an advantage in applying BaCl2
to OSL materials. Up to now, single crystals of BaCl2:Ce(48) and BaCl2:Eu(49) have been reported
to have OSL properties; however, the potential of BaCl2:Ce and BaCl2:Eu transparent ceramics
for use in OSL dosimeters remains unclear. Therefore, the OSL properties of BaCl2:Ce and
BaCl 2:Eu transparent ceramics were investigated and compared. In addition, theirphotoluminescence (PL) properties were also evaluated to specify the difference in OSL
properties between BaCl2:Ce and BaCl2:Eu.

2.

Experimental Methods

BaCl2:Ce and BaCl2:Eu transparent ceramics were synthesized in a vacuum by spark plasma
sintering (SPS) (LABOX-300, Sinterland). Raw powders of BaCl2 (4N, Rare Metals) andCeCl 3  xH 2O (3N, Kojundo Chemical) or EuCl 3  6H 2O (3N, Furuuchi Chemical) were
homogeneously mixed in a molar ratio of 99.9:0.1 respectively. The mixture of BaCl2 andCeCl3xH2O was dried at 270 °C for 4 h using a small electric furnace under vacuum. On theother hand, the mixture of BaCl2 and EuCl36H2O was dried at 400 °C for 2 h using the same
electric furnace under vacuum. Subsequently, the mixtures were enclosed with a cylindricaltungsten carbide die and a pair of tungsten carbide punches. The sintering temperature of
BaCl2:Ce was raised from 20 to 300 °C over 20 min, while the pressure was raised from 0 to 300
MPa for 20 min. In contrast, BaCl2:Eu was sintered at a temperature of 350 °C. Thesetemperatures and pressure (350 or 300 °C and 300 MPa) were held for 10 min, and the sintering
was completed. The surfaces of the fabricated samples were polished with sandpaper.
Diffuse transmittance spectra, PL quantum yields (QYs), and PL decay curves were measured
using a spectrophotometer (SolidSpec-3700, Shimadzu), Quantaurus-QY (C11347, Hamamatsu
Photonics), and Quantaurus-τ (C11367, Hamamatsu Photonics), respectively. OSL spectra were
measured using a spectrofluorometer (FP-8600, JASCO). The sample was irradiated with X-rays
with a dose in the range from 0.01 to 100 mGy, and the dose response curves were obtained. An
X-ray generator (XRBOP&N200X4550, Spellman) was used for X-ray irradiation.

3.

Results and Discussion

Figure 1(a) shows the appearance of BaCl2:Ce and BaCl2:Eu transparent ceramics. The
samples appeared transparent and showed blue luminescence under UV light. Figure 1(b)indicates the diffuse transmittance spectra of BaCl2:Ce and BaCl2:Eu transparent ceramics. An
absorption band was detected at 250–400 nm for both samples. This absorption band is due toSensors and Materials, Vol. 36, No. 2 (2024)

541

Fig. 1. (Color online) (a) Synthesized BaCl2:Ce and BaCl2:Eu transparent ceramics under room light and UV (302nm) light. (b) Diffuse transmittance spectra of BaCl2:Ce and BaCl2:Eu transparent ceramics.

the 4f–5d transitions of Ce3+(50) and Eu2+.(51,52) The diffuse transmittance of BaFBr:Eu andCsBr:Eu transparent ceramics fabricated by our group is ~50% in the visible range,(40,41) and thesamples fabricated in this study showed comparable transmittance.
Figure 2 shows the PL excitation and emission maps of BaCl2:Ce and BaCl2:Eu transparent
ceramics. An emission peak was observed at 380 nm under 330 nm excitation in BaCl2:Ce and at
400 nm under 325 nm excitation in BaCl2:Eu. The excitation and emission wavelengths are wellmatched to previously reported ones of BaCl2:Ce and BaCl2:Eu;(48,51) thus, the luminescenceorigin is the 5d–4f transitions of Ce3+ and Eu2+. The PL QYs of BaCl2:Ce and BaCl2:Eu
transparent ceramics were 3.8 and 25.6%, respectively, and the PL QY of BaCl2:Eu was higher
than that of BaCl2:Ce. The inset of Fig. 2 shows their PL decay curves. The emission wavelengths
were monitored at 380 nm (BaCl2:Ce) and 400 nm (BaCl2:Eu) under 340 nm excitation, whichwere chosen from data of Fig. 2. The decay curves of both samples were approximated by asingle exponential decay function. The PL decay times obtained by fitting analysis were 21 and
400 ns for BaCl2:Ce and BaCl2:Eu, respectively. Compared with the previous reports,(51,53,54)
these decay time constants were reasonable values for 5d–4f transitions of Ce3+ and Eu2+ in
BaCl2.
Figure 3 shows the OSL spectra of BaCl2:Ce and BaCl2:Eu transparent ceramics. Under
stimulation of 520 nm, BaCl2:Ce and BaCl2:Eu showed an OSL emission peak at around 380 nm.
The spectral features are consistent with those of PL; thus, the OSL peak is due to the 5d–4ftransition of Ce3+ and Eu2+. The stimulation spectra had a broad tail feature, and the intensity
increased with a decrease in stimulation wavelength. No stimulation peak was observed since
the peak position seems to be close to the emission peak (or maybe at even shorter wavelengths
than that of the emission). According to previous reports,(48,55–57) the trapping centers in thiscase are F centers.
Figure 4 shows the OSL decay curves of BaCl2:Ce and BaCl2:Eu transparent ceramics during
a constant stimulation at 520 nm after X-ray irradiation of 10 mGy. The monitored wavelengths

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(a)

(b)

Fig. 2. (Color online) PL excitation and emission maps of (a) BaCl2:Ce and (b) BaCl2:Eu transparent ceramics. The
inset shows PL decay curves of (a) BaCl2:Ce and (b) BaCl2:Eu transparent ceramics, where the black solid and red
dashed lines show the decay curves and fitted curves, respectively.

(a)Fig. 3.

(b)

OSL spectra of (a) BaCl2:Ce and (b) BaCl2:Eu transparent ceramics after 1000 mGy X-ray exposure.

(a)

(b)

Fig. 4. (Color online) OSL decay curves of (a) BaCl2:Ce and (b) BaCl2:Eu transparent ceramics. Solid and dashedlines are raw data and fitted curves, respectively.

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Fig. 5.

543(Color online) OSL dose response functions of BaCl2:Ce and BaCl2:Eu transparent ceramics.

of BaCl2:Ce and BaCl2:Eu were 380 nm, whereas the stimulation wavelengths were 520 nm. The
emission intensities of both samples decreased with continuous stimulation; therefore, theemission is entirely due to the OSL phenomenon. The decay curves were best approximated by asum of three exponential decay functions. Thus, there must be three different paths for charges
photoionized and transferred to recombination centers. Comparison between BaCl2:Ce and
BaCl2:Eu shows that their OSL decay time constants were almost the same. Since the number ofcomponents and decay times were close regardless of dopant, the host BaCl2 was thought to playa major role in the OSL phenomenon.
Figure 5 shows the OSL dose response of BaCl2:Ce and BaCl2:Eu transparent ceramics. Here,the OSL intensity represents an integrated value of the OSL curves (shown in Fig. 4) at each
X-ray dose. The irradiation dose range was from 0.01 to 1000 mGy. The dynamic ranges of
BaCl2:Ce and BaCl2:Eu were 0.1–1000 and 0.01–100 mGy, respectively. Note that the upperdetection limit of BaCl2:Eu reached the detection limit of the instrument, and the OSL signal of
BaCl2:Eu could not be read out correctly at 1000 mGy. When the lower detection limits of
BaCl2:Ce and BaCl2:Eu were compared, BaCl2:Eu showed higher sensitivity than BaCl2:Ce.
Furthermore, the BaCl 2:Eu sample could detect a signal of lower dose than could thecommercially available IPs (Fujifilm, HR-BD).(40)4.

Conclusions

The optical and OSL properties of 0.1 mol% BaCl2:Ce and BaCl2:Eu transparent ceramics
synthesized by SPS were investigated. The PL excitation and emission maps showed theemission peaks at 380 (BaCl2:Ce) and 400 (BaCl2:Eu) nm. The PL decay time constants of
BaCl2:Ce and BaCl2:Eu were 21 and 400 ns, respectively. Furthermore, the OSL spectra of thesamples correspond to the results of the PL spectra. The PL QY of BaCl2:Eu was higher than thatof BaCl2:Ce, which caused the superior lower detection limit of BaCl2:Eu.

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Acknowledgments
This work was supported by Grants-in-Aid for Scientific Research A (22H00309), Scientific
Research B (22H03872, 22H02939, 21H03733, and 21H03736), 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 GlassFoundation, Terumo Life Science Foundation, KRF Foundation, Tokuyama Science Foundation,
Iketani Science and Technology Foundation, and Foundation for Nara Institute of Science andTechnology are also acknowledged.

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