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Sensors and Materials, Vol. 30, No. 7 (2018) 1555–1563
MYU Tokyo
S & M 1611

Scintillation and Dosimetric Properties
of Ce:CsCl Transparent Ceramics
Hiromi Kimura,* Fumiya Nakamura, Takumi Kato, Daisuke Nakauchi,
Go Okada, Noriaki Kawaguchi, and Takayuki Yanagida
Nara Institute of Science and Technology, 8916-5 Takayama-cho, Ikoma-shi, Nara 630-0192, Japan(Received January 19, 2018; accepted March 2, 2018)

Keywords: transparent ceramic, CsCl, Ce, scintillation, TSL, OSL

Ce:CsCl transparent ceramics doped with different concentrations of Ce (0.01, 0.1, and1 mol.%) were synthesized by spark plasma sintering, and their optical, scintillation anddosimetric properties were investigated. Regarding the scintillation properties, X-ray-induced
scintillation spectra of the samples consisted of three emission peaks at around 280, 380, and
400 nm, which were due to the Auger-free luminescence and 5d→4f (2F5/2 and 2F7/2) transitionsof Ce3+, respectively. In addition, the samples showed thermally stimulated luminescence (TSL)
and optically stimulated luminescence (OSL) with emission peaks centering at around 400
and 480 nm, in which the origins were attributed to the 5d–4f transitions of Ce3+ and the host,respectively. In particular, the TSL was sensitive to X-rays, and the TSL signals as low as 0.1
mGy were measurable, while a linear response was confirmed up to 100 mGy.

1.

Introduction

Phosphor materials are often used in radiation detectors, and such phosphors are classified
into two types: scintillator and storage phosphors. Scintillators can immediately convert
energy of the absorbed ionizing radiation into a large number of low-energy photons, and they
have been widely used in fields requiring radiation detection, such as medicine,(1) security,(2)environmental monitoring,(3) and high-energy physics.(4) In general, the required propertiesof scintillators include high light yield, short decay time, and low afterglow. In contrast,
storage phosphors store the absorbed energy of ionizing radiation. The energy is stored in theform of carriers trapped at localized centers. The trapped electrons and holes can be releasedby heat or light stimulation to emit photons. The resultant luminescence is called thermallystimulated luminescence (TSL) and optically stimulated luminescence (OSL), respectively.
The luminescence intensity is proportional to the radiation dose; therefore, such storage
phosphors are utilized in dosimetry applications. For this reason, they are often referred to as
dosimeters. The materials showing TSL and OSL have been mainly used in individual radiation
monitoring(5–7) and imaging plates (IPs).(8,9)*

Corresponding author: e-mail: kimura.hiromi.kf1@ms.naist.jp
http://dx.doi.org/10.18494/SAM.2018.1923
ISSN 0914-4935 © MYU K.K.
http://myukk.org/

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The recent trend of development of scintillators is based on a combination of inorganic
materials as the host and rare-earth ions as emission centers, e.g., Ce:Tb3Al5O12(10) andCe:Gd2SiO5.(11) The host has the role of absorbing ionizing radiation effectively, while theemission center emits photons. Among such rare-earth ions, Ce3+ is often used in scintillators
because of the fast emission due to the 5d→4f (2F5/2 and 2F7/2) transitions of Ce3+. Some Cedoped chlorides are known to show high light yields, such as Ce:LaCl3 (61000 ph/MeV)(12) andCe:Cs2LiYCl6 (70000 ph/MeV).(13) CsCl is one of the chlorides, and it is potentially applicableto radiation detectors. CsCl has a high effective atomic number (Zeff = 51.9) close to those of
conventional scintillators such as CsI (Zeff = 54.1) and NaI (Zeff = 50.8), so CsCl can be appliedin X- and gamma-ray detectors as the dosimeter material of scintillator and IPs. Ce:CsCl
is known to show emission peaks at around 250, 360, and 380 nm, which are due to Augerfree luminescence (AFL) and 5d→4f (2F5/2 and 2F7/2) transitions of Ce3+,(14) respectively. The
decay time constants of the AFL at 8 K and the 5d–4f transitions of Ce3+ at room temperature
are known to be ~2 and ~20 ns,(14,15) respectively. Up to now, some scintillation properties of
Ce:CsCl have been reported in the literature; however, most studies were carried out on bulk
single crystals, and no reports can be found for transparent ceramics.
Conventionally, most scintillator and dosimeter materials have been used mainly in theform of bulk single crystals because of their high optical qualities, even though single crystals
have some industrial disadvantages such as low mechanical strength, high cost, and difficultyof upsizing. As an alternative, the use of transparent ceramics has recently attracted attention
in the field of ionizing radiation detectors in order to overcome the above disadvantages. In
addition, some transparent ceramics such as Ce:Gd3(Al, Ga)5O12(16) and Ce:Y3Al5O12(17) are
known to show higher scintillation light yields than the single crystals.
In this study, we developed Ce:CsCl in the form of a transparent ceramic by spark plasma
sintering (SPS) and then evaluated its optical, scintillation, and dosimeter properties. In recent
years, it has been clarified that the scintillation and dosimetric properties of some materials
are complementarily related.(18) Therefore, evaluations of both scintillation and dosimetricproperties are important in order to comprehensively understand the luminescence phenomena
induced by ionizing radiations.

2.

Experiment

Ce:CsCl transparent ceramics doped with different concentrations of Ce (0.01, 0.1, and 1
mol.%) were synthesized by SPS using Sinter Land LabX-100 in vacuum. Raw powders of
CsCl (>99.999%, Mitsuwa Chemical) and CeCl3·xH2O (>99.9%, Kojundo Chemical Laboratory)
were homogeneously mixed using a mortar and pestle. Then, the mixture (total mass: 0.5 g)
was loaded in a cylindrical graphite die (inner diameter: 10.4 mm), in which the mixture washeld between two graphite punches inserted in the cylinder. SPS was performed by applying
uniaxial pressure along the axis of graphite punches while applying a high-current DC pulse.
The sintering temperature was increased from 20 to 580 ℃ at a rate of 56 ℃/min and then heldfor 20 min while applying a pressure of 25 MPa. The wide surfaces of the obtained ceramic
samples were mechanically polished by hand using sandpaper (2000 grits).

Sensors and Materials, Vol. 30, No. 7 (2018)1557

To investigate the optical properties, the in-line transmittance spectra were measured using
a spectrophotometer (V670, JASCO) across the spectral range from 200 to 2700 nm with 1 nm
intervals. The photoluminescence (PL) quantum yields and PL excitation/emission spectra wereobtained using Quantaurus-QY (C11347, Hamamatsu Photonics). The PL decay curves were
evaluated using Quantaurus-τ (C11367, Hamamatsu Photonics).
As a scintillation property, X-ray-induced scintillation spectra were measured using
our original setup.(16) As the radiation source, an X-ray generator (XRB80P&N200X4550,
Spellman) equipped with a conventional X-ray tube was used. The X-ray tube had a W anodetarget and was operated with a tube voltage and current of 40 kV and 1.2 mA, respectively.
During irradiation, the scintillation photons were guided into a spectrometer (Andor DU-420BU2 CCD and Shamrock 163 monochromator) through an optical fiber. X-ray-induced scintillation
decay curves and afterglow profiles were evaluated using an afterglow characterization systemequipped with a pulse X-ray tube. The details of the setup are described elsewhere.(19)As dosimetric properties, TSL and OSL were characterized. TSL glow curves were
measured using a TSL reader (TL-2000, Nanogray Inc.)(20) after irradiation with X-rays(100 mGy). Here, the heating rate was 1 ℃/s, and the measurement temperature range wasfrom 50 to 350 ℃. Just before the measurement, the sample was kept at 50 ℃ for 5 min.
Moreover, to obtain a dose response function, a series of TSL glow curves were measured
with varying irradiation doses from 0.1 to 100 mGy. To obtain TSL spectra, the samples after
X-ray irradiation (~10 Gy) were heated on a ceramic heater system (SCR-SHQ-A, Sakaguchi)
increasing the temperature at a rate of 1 ℃/s, and then the emission spectrum at 90 ℃ wasrecorded using a spectrometer (QEPro, Ocean Optics). OSL spectra were measured using
Quantaurus-τ (C11367, Hamamatsu Photonics) after irradiating the samples with X-rays of 10
Gy. The stimulation wavelength was 630 nm.

3.

Results and Discussion

3.1

SampleFigure 1 shows photographs of the CsCl transparent ceramic samples doped with different
concentrations of Ce (0.01, 0.1, and 1%) under (a) room light and (b) UV light (302 nm). Under
UV irradiation, we observed a strong blue emission in all the samples. In addition, it was

(a)

(b)

Fig. 1.

(Color online) Synthesized Ce:CsCl transparent ceramics under (a) room light and (b) UV light (302 nm).

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Sensors and Materials, Vol. 30, No. 7 (2018)confirmed that transparency was such that the stripe patterns under the samples could be seen.
The thickness of all the samples was fixed at approximately 1.3 mm. In-line transmittancespectra are illustrated in Fig. 2. The transmittance of the sample increased with increasing
Ce concentration. A possible reason may be that the CeCl3·xH2O was activated as a sinteringacid and the densification was improved. The absorption due to 4f–5d transitions of Ce3+ was
confirmed at around 340 nm.
3.2

PL properties

Figure 3 shows a PL excitation/emission contour graph of the Ce(0.1%):CsCl transparent
ceramic sample as a representative example. The emission seems to consist of two broad band
peaks at around 380 and 410 nm under an excitation of around 340 nm. The origins of these are
attributed to the 5d→4f (2F5/2 and 2F7/2) transitions of Ce3+, respectively. In a previous study,
two emission bands at 360 and 380 nm due to the 5d→4f (2F5/2 and 2F7/2) transitions of Ce3+
were reported.(14) Our result is not highly consistent with that in the previous study. Typically,
the difference in the energy levels between 2F5/2 and 2F7/2 is ~2000 cm−1. In fact, our result
matched this typical energy difference. The peak position shifted to a longer wavelength thanthat reported previously,(14) as a result of the self-absorption of Ce3+. In the previous report,
the sample was synthesized by the Czochralski method, and the Ce concentration was expected
to be low. Furthermore, the optimum excitation wavelength agreed well with the absorption
band observed in Fig. 2. The PL quantum yields of the samples with 0.01, 0.1, and 1% Ce were
10.7, 44.6, and 50.0%, respectively. The low PL QY of the Ce 0.01% sample was attributed
to the small number of emission centers, and even in the 1% Ce-doped sample, concentrationquenching did not occur.
PL decay profiles are presented in Fig. 4. Here, the excitation wavelength was 340 nm
while the monitored emission wavelength was 400 nm. All the decay curves of the samples
were well-approximated by a single exponential decay function. The decay time constants ofFig. 2. (Color online) In-line transmittance spectraof Ce:CsCl transparent ceramics.

Fig. 3. (Color onli ne) PL excit ation /emission
contour graph of 0.1% Ce-doped CsCl transparent
ceramics. The vertical and horizontal axes represent
excitation and emission wavelengths, respectively.

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Fig. 4. (Color online) PL decay profiles of Ce:CsCl transparent ceramics. The monitoring wavelength was 400
nm while the excitation wavelength was 340 nm.

the Ce:CsCl samples doped with 0.01, 0.1, and 1% Ce were 27.8, 30.5, and 31.6 ns, respectively.
Those decay constants were of a typical order for the 5d–4f transitions of Ce3+.(14) The decay
time decreased as the Ce concentration increased.
3.3

Scintillation properties

Figure 5 shows X-ray-induced scintillation spectra of the Ce:CsCl samples. It was suggested
that the emission consists of three bands centered at around 280, 380, and 410 nm. The originsof these emissions were attributed to the AFL and 5d→4f (2F5/2 and 2F7/2) transitions of Ce3+,
respectively, according to the previous studies of single crystals.(14) The spectral featuresof Ce3+ were consistent with PL. Among the present samples, Ce(0.1%):CsCl showed thehighest intensity. The emission wavelength is favorable for a typical photomultiplier tube. The
emission intensity of the X-ray-induced scintillation spectrum is qualitative since the radiation
stopping power cannot be accurately corrected; however, compared with Tl- and In-doped
CsCl,(21) the emission intensity of our samples was weaker. In addition, a companion with thepast result of Ce:CsCl(12) was not possible because the light yield under 45 MeV proton beam
excitation, which is unavailable to us, was evaluated previously.
X-ray-induced scintillation decay time profiles of the Ce:CsCl samples are presented in Fig.
6. Each scintillation decay curve was well approximated by the sum of two exponential decayfunctions. The faster decay components of the samples with 0.01, 0.1, and 1% Ce were 1.6, 1.8,
and 1.8 ns, respectively, and these faster decay components were attributed to the mixture of the
AFL and instrumental response. The slower decay time components of the samples with 0.01, 0.1,
and 1% Ce were 29.3, 32.2, and 32.3 ns, respectively. These values were of a typical order for
5d–4f transitions of Ce3+. The decay time increased with the Ce concentration.

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Fig. 5. (Color online) X-ray-induced scintillation
spectra of Ce:CsCl transparent ceramics.

Fig. 6. (Color online) Scintillation decay profiles of
Ce:CsCl transparent ceramics.

The afterglow curves of the Ce:CsCl samples are shown in Fig. 7. The afterglow level (A)is defined as A (%) = 100 × (I2 – IBG)/(I1 – IBG), where I1 is the signal intensity during X-ray
irradiation, I2 is that at 2 ms after the termination of irradiation, and IBG is the background. The
derived afterglow levels of the samples with 0.01, 0.1, and 1% Ce were 3.45, 4.43, and 1.14%,
respectively. The afterglow levels of the current samples were higher than those of common
scintillators such as Tl:CsI crystal.(22)
3.4

Dosimetric properties

Figure 8 shows the TSL glow curves of Ce:CsCl transparent ceramic samples measured after
X-ray irradiation (~100 mGy). All the samples showed a major glow peak at around 100 ℃.
The origin of the glow peak is unknown, and we found no literature on the origin of the glow
peak of Ce:CsCl. In previous works, more complicated features at a similar peak temperature
were observed in Ni-doped and Eu-doped CsCl.(23,24) Therefore, the observed glow peak ispossibly related to the CsCl host since these materials have only the CsCl host in common.
TSL dose response functions of the samples are shown in the inset of Fig. 8. The TSL intensity
represents the integrated signals between 50 and 190 ℃ of the measured glow curve. The
samples showed good linearity from 0.1 to 100 mGy. The detection limit of 0.1 mGy wascomparable to those of some commercial dosimeters.
The TSL spectra of Ce:CsCl transparent ceramic samples are presented in Fig. 9. The
samples were irradiated with X-rays (~10 Gy) before the measurement, and the spectra were
measured at 90 ℃. Each TSL spectrum consists of two emission band peaks at around 400 and
480 nm. A previous study demonstrated that the TSL emission of nondoped CsCl appeared
from 400 to 470 nm. Therefore, we consider that the origin of the TSL emission is both the
CsCl host and 5d–4f transitions of Ce3+.(25)Sensors and Materials, Vol. 30, No. 7 (2018)

Fig. 7. (Color online) Afterglow profiles of Ce:CsCltransparent ceramics. The inset shows the afterglow
level of all the samples.

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Fig. 8. (Color online) TSL glow curves of Ce:CsCltransparent ceramic samples measured after 100 mGy
X-ray irradiation. The inset shows the TSL dose
response curves of the samples from 0.1 to 100 mGy.

Fig. 9. (Color online) TSL spectra of Ce:CsCl transparent ceramic samples measured at 90 ℃ after X-ray
irradiation (~10 Gy).

Figure 10 shows OSL spectra of Ce:CsCl transparent ceramic samples. The samples were
irradiated with X-rays (~10 Gy) before the measurements. The optical stimulation wavelength
was 630 nm. The OSL spectral features of the samples were in approximate agreement withthose of PL. Therefore, the origins of OSL spectra would be the 5d→4f (2F5/2 and 2F7/2)
transitions of Ce3+.
OSL decay profiles of Ce:CsCl transparent ceramic samples are shown in Fig. 11. Before themeasurements, the samples were irradiated with X-rays (~10 Gy). The monitoring wavelength
was 400 nm while the stimulation wavelength was 630 nm. The OSL intensity exponentiallydecreased with time during the stimulation, which was a typical feature of OSL (if the observed
signal is due to up-conversion, the signal intensity must not decrease with time). Each decay
curve was best approximated by a sum of four exponential decay functions. This finding
suggests that at least four different detrapping processes were involved. To the best of ourknowledge, this is the first report of OSL in Ce:CsCl.

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Fig. 10. (Color online) OSL spectra of Ce:CsCltransparent ceramic samples measured under 630 nm
stimulation after X-ray irradiation (~10 Gy).

4.

Fig. 11. (Color online) OSL decay profiles of Ce:CsCltransparent ceramic samples after X-ray irradiation (~10Gy). The inset shows the OSL decay time constantsof all the samples.

Conclusions

We synthesized Ce:CsCl transparent ceramics doped with different concentrations of Ce(0.01, 0.1, and 1%) by SPS and then investigated their optical, scintillation, and dosimeter
properties. The PL spectra of all the samples showed two broad emission band peaks at around
380 and 410 nm under excitation of around 340 nm. The decay time constants of the samples
doped with 0.01, 0.1, and 1% Ce were 27.7, 30.5, and 31.6 ns, respectively, which were of atypical order for the 5d–4f transitions of Ce3+. The X-ray-induced scintillation spectra of all thesamples contained three emission band peaks at around 280, 380, and 410 nm. The emission ataround 280 nm was due to AFL, whereas the emission peaks at around 380 and 410 nm were
attributed to the 5d→4f (2F5/2 and 2F7/2) transitions of Ce3+, respectively, as in PL. Concerning
the dosimetric properties, the samples showed TSL and OSL. The main emission of both typesof luminescence was observed at around 400 nm owing to the 5d→4f transitions of Ce3. In
particular, the TSL was very sensitive to the X-ray dose, and the TSL signal was measurable at
as low as 0.1 mGy.

Acknowledgments
This work was supported by Grant-in-Aid for Scientific Research (A) (17H01375), Grantin-Aid for Young Scientists (B) (17K14911), and Grant-in-Aid for Research Activity Start-up(16H06983) from the Ministry of Education, Culture, Sports, Science and Technology of theJapanese government (MEXT) as well as A-STEP from the Japan Science and Technology
Agency (JST). The Cooperative Research Project of the Research Institute of Electronics,
Shizuoka University, Mazda Foundation, Taisei Foundation, Terumo Foundation for Life
Sciences and Arts, Izumi Science and Technology Foundation, SEI Group CSR Foundation,
Iwatani Naoji Foundation, Konica Minolta Science and Technology Foundation, NAISTFoundation, and TEPCO Memorial Foundation are also acknowledged.

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