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MYU Tokyo
S & M 3541

Scintillation Properties of Eu-doped SrF2 Transparent Ceramics
Takumi Kato,* Daisuke Nakauchi, Noriaki Kawaguchi, and Takayuki Yanagida
Nara Institute of Science and Technology (NAIST), 8916-5, Takayama-cho, Ikoma-shi, Nara 630-0192, Japan(Received October 30, 2023; accepted January 9, 2024)

Keywords: transparent ceramics, SrF2, Eu, scintillator

Eu-doped SrF2 transparent ceramics were synthesized by spark plasma sintering, and their
optical and scintillation properties were studied. The Eu-doped SrF2 transparent ceramics
showed an emission peak at 410 nm with excitations of 250 and 350 nm, and the PL quantum
yield of the 0.1% Eu-doped SrF2 transparent ceramic was 8.1%. Moreover, the Eu-doped SrF2
transparent ceramics showed scintillation with emission peaks at 300 and 410 nm under X-ray
irradiation. Based on the measured scintillation decay times, the origins of the emissions at 300
and 410 nm were self-trapped excitation and 5d–4f transitions of Eu2+ ions, respectively. The
scintillation light yield of the 0.1% Eu-doped SrF2 transparent ceramic was evaluated to be about
6100 photons/MeV under 137Cs γ-rays.

1.

Introduction

Scintillators are a class of phosphors that absorb and immediately convert the energy ofincident ionizing radiation to thousands of ultraviolet/visible photons. Attention to new
scintillator materials is promoted by an increasing number of new applications such as medical(X-ray CT and PET),(1) security,(2)particularly the presence of hidden special nuclear materials(SNM oil logging),(3) environmental monitoring,(4) astronomy,(5) and particle physics.(6) The
most common scintillators are bromides and iodides doped with Tl+, Eu2+, or Ce3+ ions such as
cesium iodide doped with Tl (CsI:Tl), strontium iodide doped with Eu (SrI2:Eu), and lanthanum
bromide doped with Ce (LaBr3:Ce).(7,8) These crystals have high light yield (100000 photons/
MeV for SrI2:Eu), good energy resolution, and short decay time.(9) In general, bromides andiodides have high hygroscopicity and deliquescence, which are disadvantages. In addition, it wasreported that SrI2:Eu showed the temperature-related instability of the light yield.(10) On theother hand, alkaline earth fluorides, which have been applied as scintillators,(11,12) have low
hygroscopicity, low phonon energy, and high radiation resistance. For instance, barium fluoride(BaF2) single crystals are known as scintillator materials. It has been reported that the
scintillation of BaF2 crystals is due to Auger-free luminescence and self-trapped excitation(STE).(13–17) In particular, BaF2 has a relatively large effective atomic number, so it has a great
advantage for use in γ-ray detections. Calcium fluoride (CaF2) single crystals are known to show
*Corresponding author: e-mail: kato.takumi.ki5@ms.naist.jp

https://doi.org/10.18494/SAM4749

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

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scintillation due to STE at 270 nm.(18–20) Moreover, it has large band gap energy and relatively
high light yield (13000 photons/MeV),(21) and CaF2 has been investigated for potential as the
scintillator in astrophysics.(22)
Nowadays, almost all the scintillators used in practice are bulk single crystals because they
have excellent properties such as high transparency, high light yield, and high detection
efficiency due to the volume of bulk single crystals.(23–25) On the other hand, recent studies haveopened up a new possibility of using transparent ceramics as scintillator materials owing to theachievement of improved synthesis technologies developed with the advancement of the laser
field.(26–29) In comparison with single crystals, ceramics have many advantages such as costeffectiveness, flexible geometric shape, and mechanical strength. Furthermore, they have also
been found to show an equivalent or even superior performance as a scintillator compared withthe single crystal form.(26–28) However, these studies have only dealt with oxide materials;
therefore, we have started to investigate the scintillation properties of halogen compound
ceramics with transparent or translucent forms. Thus far, our groups have developed andevaluated CaF2, BaF2, CsCl, and CsBr transparent ceramics for scintillator applications.(30–33)
In this paper, to extend our previous investigations, Eu-doped SrF2 transparent ceramics weredeveloped by spark plasma sintering (SPS), and their photoluminescence (PL) and scintillation
properties were characterized. The fabrication method and optical properties of undoped andYb-doped SrF2 transparent ceramics have already been reported.(34,35) In addition, we havecompared the scintillation properties of transparent ceramics and single crystals of undoped
SrF2;(36) however, no report on the scintillation properties of Eu-doped SrF2 transparent
ceramics can be found.

2.

Materials and Methods

Eu-doped SrF2 transparent ceramic samples were synthesized by SPS using Sinter Land
LabX-100. Reagent-grade EuF3 and SrF2 powders were homogeneously mixed using a mortar,and Eu concentrations were 0.01, 0.1, and 1.0 mol% with respect to SrF2. The obtained powder
was loaded in a graphite die and sealed with two graphite punches from up and down directions.
The graphite was covered with a carbon felt sheet to keep it warm and placed in a furnace tosinter. The sintering conditions used were as follows: the temperature was increased from 20 to890 °C at a rate of 10 °C/min and held for 10 min while applying a pressure of 10 MPa, and then
the temperature was increased to 1080 °C at a rate of 10 °C/min and held for 20 min while
applying a pressure of 100 MPa in a vacuum. The surfaces of the sintered samples were opticallypolished with a polishing machine (MetaServ 250, Buehler).
To investigate the optical properties, transmittance spectra were measured using aspectrophotometer (SolidSpec-3700, Shimadzu) in the spectral range from 200 to 800 nm with
0.5 nm intervals. PL and excitation spectra were measured using a spectrofluorometer (FP8600,
JASCO). PL quantum yields (QYs) were estimated using Quantaurus-QY (C11347, Hamamatsu
Photonics). To calculate PL QY, the numbers of absorbed photons (Nabsob, 340 nm) and emitted
photons (Nemit, from 390 to 600 nm) were determined on the basis of PL intensities with andwithout the sample, and PL QY was calculated as QY = Nemit/Nabsob. PL decay curves monitored

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at 410 nm under 340 nm excitation were measured using Quantaurus-τ (C11367, Hamamatsu
Photonics). Then, PL decay constants were evaluated by the least-square fitting of decay curves
using an exponential decay function.
Radiation-induced luminescence properties were characterized as follows. X-ray-induced
scintillation spectra from 250 to 700 nm were measured using a setup.(28) A monochromatorequipped with a CCD-based detector (Shamrock 163 monochromator and DU-420-BU2 CCD,
Andor) was used to measure the scintillation spectra. The sample was attached to an opticalfiber, which guided the scintillation light to the detector. The X-ray source used here was an
X-ray tube (W anode/Be window) equipped with a generator unit (XRB80N100, Spellman). The
supplied tube voltage and current were 80 kV and 1.2 mA, respectively. To measure X-rayinduced scintillation decay curves, we used an afterglow characterization system equipped witha pulse X-ray tube. The obtained information can be found elsewhere.(37) A pulse heightspectroscopy measurement was conducted to estimate scintillation light yields induced by
γ-rays. In the measurement, a sample was firmly placed on a window of the photomultiplier tube
(PMT; R877-100, Hamamatsu) using an optical grease (6262 A, OKEN) and covered by several
layers of Teflon sheet to guide the scintillation photons to the PMT effectively. A pre-amplifier(113, ORTEC) was subsequently used to amplify PMT output signals, and the signals were
processed by a shaping amplifier (572, ORTEC) with a 10 μs shaping time. The signal output
was integrated and registered using a multichannel analyzer (Pocket MCA 8000 A, Amptek),
and then a pulse height spectrum was constructed on a personal computer. In this measurement,a sealed radioactive 137Cs γ-ray was used as the radiation source.

3.

Results and Discussion

Transmittance spectra are shown in Fig. 1. The thicknesses of the 0.01, 0.1, and 1.0% Eudoped samples were 0.56, 0.55, and 0.56 mm, respectively. All the samples in the inset werevisually transparent and indicated a transmittance of around 60% over a wavelength range of

Fig. 1. (Color online) Transmittance spectra from 200 to 800 nm. The inset shows the appearance of the obtained
samples.

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400–800 nm. Because the band gap of SrF2 is 11.3 eV (110 nm),(38,39) no optical absorption edge
was observed in the measurement range. On the other hand, absorption bands were observed ataround 230 and 350 nm for all the samples, and the absorptions clearly increased with anincrease in dopant concentration. It was considered that these absorptions were due to the 4f–5dtransitions of Eu2+ ions.
PL excitation and emission spectra are presented in Fig. 2. A broad emission band was
detected at 410 nm under excitations at 250 and 350 nm. Excitation wavelengths agreed with theabsorption wavelengths shown in the transmittance spectra. Similar spectral shapes andpositions were reported in earlier works on Eu-doped SrF2.(40) The QY values of the 0.01, 0.1, and
1.0% Eu-doped samples were 5.1, 8.1, and 2.5%, respectively, and the QY value of the 0.1% Eudoped sample was maximum among the present samples. The reason for the low QYs is that the
samples prepared by SPS often include lattice defects and are contaminated by carbon during
sintering. Under 390 nm excitation, PL emission peaks were observed at 590 and 615 nm,
originating from the 4f–4f transitions of Eu3+.(41–43) Figure 3 shows PL decay time profiles whena monitored wavelength was fixed to 410 nm under 340 nm excitation. The decay curves werefitted with an exponential decay function. The delivered decay time constants from theapproximation were 380–435 ns, which are consistent with those of Eu2+ ions.(44,45) By theseevaluations, we assigned the emission origin of the 410 nm peak to the 5d–4f transitions of Eu 2+
ions.
X-ray-induced scintillation spectra are presented in Fig. 4. Under X-ray irradiation, all thesamples showed an emission band at 410 nm due to the 5d–4f transition of Eu2+ ions as observedin the PL spectra (Fig. 3). In addition to this, all the samples showed an emission band at 300 nm
attributed to STE. The scintillation intensity at 300 nm decreased with an increase in Eu
concentration. The possible reason is that the local structure of the host lattice was affected by
the increase in Eu concentration, so the generation of STE was suppressed.(46–48) Moreover, only
the 1.0% Eu-doped sample exhibited some remarkable peaks between 450 and 700 nm, which
are due to the 4f–4f transitions of Eu3+ ions.(41–43) These characteristics coincided with thespectra of single crystals with the same chemical composition.(49)

Fig. 2. (Color online) PL excitation and emission
spectra. The inset shows emission spectra under 390
nm excitation.

Fig. 3. (Color online) PL decay time profiles under
340 nm excitation.

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These assignments were confirmed on the basis of scintillation decay time constants. Figure
5 shows the X-ray-induced scintillation decay time profiles. The scintillation decay curves of all
the samples were approximated by a sum of three exponential functions. The obtained decay
time constants were 154, 439, and 1107 ns for the 0.1% Eu-doped sample. The first and third
decay time constants were consistent with the values due to STE in the previous report.(50) The
obtained second decay time constants agreed with the PL decay time constants measured in thisstudy and that of 5d–4f transitions of Eu2+ ions.(51)Figure 6 shows pulse height spectra under 137Cs γ-ray irradiation. The photoabsorption peaksof the 0.01, 0.1, and 1.0% Eu-doped samples appeared at 305, 315, and 110 ch, respectively. The
light yield was estimated by comparing the Eu-doped samples with Ce-doped (Lu,Y)2SiO5(LYSO), showing a light yield of 22000 photons/MeV. The light yields of the 0.01, 0.1, and 1.0%
Eu-doped samples were 5900, 6100, and 2100 photons/MeV, respectively. The light yield of the
1.0% Eu-doped sample was smaller than those of the 0.01 and 0.1% Eu-doped samples. Thisdecrease is associated with the decrease in PL QY calculated under 340 nm excitation and theexistence of Eu3+ ions observed in scintillation spectra. In general, the light yield is proportional

Fig. 4. (Color online) X-ray-induced scintillation
spectra.

Fig. 6.

Fig. 5. (Color online) X-ray-induced scintillation
decay time profiles.

(Color online) Pulse height spectra under 137Cs γ-ray irradiation.

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to PL QY, and the scintillation from Eu3+ ions cannot effectively contribute to the light yieldowing to its relatively long decay time. Compared with that of Eu-doped CaF2 transparent
ceramics (17700 photons/MeV), the light yield of Eu-doped SrF2 transparent ceramics waslow.(52)

4.

Conclusions

We synthesized Eu-doped SrF2 transparent ceramics by SPS and then investigated their
optical and scintillation properties. From experimental results of PL and scintillation, it was
confirmed that the Eu-doped SrF2 transparent ceramics showed the emission peak at 410 nm
due to the 5d–4f transitions of Eu 2+ ions. In addition, the 1.0% Eu-doped SrF2 exhibitedsome emission peaks caused by the 4f–4f transitions of Eu 3+ ions. The highest light yield
was approximately 6100 photons/MeV among the present samples.

Acknowledgments
This work was supported by Grants-in-Aid for Scientific Research A (22H00309), Grants-inAid for 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,
and Foundation for Nara Institute of Science and Technology are also acknowledged.

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