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MYU Tokyo
S & M 3539

Scintillation Properties of Eu-doped KI Single Crystals
Keiichiro Miyazaki,* Daisuke Nakauchi, Takumi Kato,
Noriaki Kawaguchi, and Takayuki Yanagida
Division of Materials Science, Nara Institute of Science and Technology,
8916-5 Takayama-cho, Ikoma-shi, Nara 630-0192, Japan(Received October 20, 2023; accepted January 18, 2024)

Keywords: scintillator, photoluminescence, alkali halide, Bridgman–Stockbarger method, light yield

The photoluminescence and scintillation properties of KI:Eu synthesized by the vertical
Bridgman–Stockbarger method were investigated. All the samples showed a broad peak at 430–
450 nm under UV and X-ray irradiations, and the origin of emission was ascribed to the 4f65d1–
4f 7 transition of Eu2+. Furthermore, a tail emission at 500 nm under X-ray irradiation wasobserved, the origin of which could be ascribed to color centers associated with oxygen
impurities. According to the pulse height spectra, the highest light yield (LY) under 241Am γ-ray(60 keV) irradiation was 3400 photons/MeV among the prepared samples. In terms of LY andafterglow, the optimal concentration of Eu was found to be 0.3%.

1.

Introduction

Scintillators are phosphors that convert ionizing radiation to numerous low-energy photons
in the range of ultraviolet (UV) to near-infrared (NIR) light. By combining scintillators andphotodetectors such as photomultiplier tubes (PMTs) and photodiodes (PDs), ionizing radiation
can be detected as an electrical signal.(1) Therefore, scintillators have been used in radiation
measurement fields such as medicine,(2) environmental measurement,(3,4) well logging,(5) andsecurity.(6) The following properties of X- and γ-ray scintillators are required: high light yield(LY), high energy resolution, large effective atomic number (Zeff ), short decay time, low
afterglow, and no hygroscopicity. There are no scintillators that meet all the required properties,
and material exploration continues at present. So far, material forms such as glass,(7–16)
ceramics,(17–26) and single crystals(27–36) have been developed. Among the material forms, single
crystals have mainly been studied because of their high luminescence intensity.
While most halide materials are hygroscopic,(37) some of them have been used in practiceowing to their high LY.(38) In addition, the manufacturing cost of these materials is low because
the melting point is relatively low. The typical halide scintillators, NaI:Tl and CsI:Tl, show high
LY and energy resolution, and the emission wavelengths of NaI:Tl and CsI:Tl are suitable for
PMTs and PDs, respectively.(39,40) Since dopant-activated NaI and CsI have been widely used fora long time, other alkali iodides are also high-potential scintillator hosts. In our previous study,*Corresponding author: e-mail: miyazaki.keiichiro.mg5@ms.naist.jp

https://doi.org/10.18494/SAM4756

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

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the scintillation properties of KI:Tl and RbI:Tl were investigated because they have lesshygroscopicity than NaI,(41,42) and KI:Tl showed a moderate LY of 7300 photons/MeV. In addition
to Tl-doped ones, Eu-doped alkali iodides such as LiI:Eu, NaI:Eu, and CsI:Eu have also beenreported to show a high LY of 15000–20000 photons/MeV;(43–45) therefore, KI:Eu would be
expected to show a high LY. So far, there have been a few reports on the photoluminescence (PL)of KI:Eu.(46) However, no reports on the scintillation properties of KI:Eu exist. In this study,
KI:Eu single crystals were grown by the vertical Bridgman–Stockbarger method, and theconcentration dependences of Eu on photoluminescence and scintillation properties were
evaluated.

2.

Experimental Methods

Undoped KI and KI:Eu single crystals were synthesized by the vertical Bridgman–
Stockbarger method, and the concentrations of Eu were 0.03, 0.1, 0.3, and 1%. KI (High Purity
Chemicals, 2N) and EuI2 (Sigma Aldrich, 5N) were mixed in stoichiometric ratio in a glove box(AZGBV065, Glovebox Japan inc.). These powders were put into a quartz tube and dried byheating at 200 oC for 2 h in vacuum. Then, the quartz tube was sealed using a gas burner(47,48)
and set in a Bridgman furnace (VFK-1800, Crystal Systems). The temperature of the furnace
was 730 oC, which was ~50 oC above the melting point of KI,(49) and the pull-down speed was 5mm/h. After growth, the quartz tubes were crushed to obtain samples with the size of ~5 mmφ ×
10–20 mm. The two large surfaces were polished using sandpaper. In the case of PL properties,
PL contour plots and quantum yields (QYs) were obtained using Quantaurus-QY (C11347,
Hamamatsu Photonics). PL decay curves were measured using Quantaurus-τ (C11367,
Hamamatsu Photonics). X-ray-induced scintillation spectra were measured using our original
setup.(50) The applied voltage and current of the X-ray tube were 60 kV and 1.2 mA, respectively.
X-ray-induced scintillation decay curves and afterglow profiles were obtained using an X-rayinduced afterglow characterization system.(51) To estimate LY, pulse height spectra were
measured using PMT (R7600U-200, Hamamatsu Photonics) and a 241Am γ-ray source.

3.

Results and Discussion

Pictures of the samples under room light and UV light irradiation are shown in Fig. 1. The
thickness of all the samples was unified as 1–2 mm. All the samples were colorless andtransparent under room light, and all the samples showed blue emission under 365 nm UV
irradiation. The PL contour plots of all the samples are illustrated in Fig. 2. All the samples
showed an emission peak at 430–450 nm under excitation at 300–400 nm. The emission
wavelength was shifted toward long wavelengths with increasing Eu concentration, and thevalues of the Stokes shift in the 0.03, 0.1, 0.3, and 1% Eu-doped samples under 390 nm irradiation
were 45.1, 48.1, 51.9, and 52.6 nm, respectively. The QYs of the 0.03, 0.1, 0.3, and 1% Eu-doped
samples under excitation at 360 nm were >99, >99, 97, and 96%, respectively. The PL decay time
curves monitored at 440 nm under excitation at 365 nm are shown in Fig. 3. All the decay curves
were composed of one exponential function, and the decay time constant (τ) was 1.18–1.49 ms.

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Fig. 1. (Color online) Pictures of crystalline KI doped with various concentrations of Eu (0.03, 0.1, 0.3, and 1%)
under room light (top) and UV light at 365 nm (bottom).

Fig. 2. (Color online) PL contour plots of KI:Eu.
The vertical and horizontal axes are emission andexcitation wavelengths, respectively.

Fig. 3. (Color online) PL decay curves monitored at
440 nm under 365 nm excitation. Dashed lines are
fitting curves.

This emission would be ascribed to the 4f65d1–4f 7 transition of Eu2+ because the emission
wavelength and PL decay time constant were close to those of KI:Eu.(46,52)
The X-ray-induced scintillation spectra of KI:Eu and an undoped KI are presented in Fig. 4.
A broad peak at 430–450 nm was observed, and this peak would be ascribed to the 4f65d1–4f 7transition of Eu2+ since the emission wavelength was close to PL (Fig. 2). The peak shift wasobserved as well as PL in Fig. 2. In addition, a tail emission was observed at ~500 nm. In the
undoped KI, the emission signals were observed at around 300, 400, and 500 nm. According toprevious reports,(53,54) the emissions at 300 and 400 nm were on- and off-center configurationsof self-trapped excitons, respectively, and the luminescence at 500 nm was ascribed to color
centers associated with oxygen impurities.(55) Scintillation decay curves under X-ray irradiationin KI:Eu are indicated in Fig. 5. All the decay profiles consisted of a sum of two exponentialfunctions. Here, the fast and slow components were indicated as τ1 and τ 2, respectively. Because
τ1 was close to the PL decay time constant, τ1 would be attributed to the 4f65d1–4f 7 transition of
Eu2+. The τ 2 would be color centers associated with oxygen impurities. In some materials, thehigh excitation density of X-ray irradiation generates many secondary electrons, and theinteraction between many localized secondary electrons causes quenching.(56–60) Suchquenching makes the scintillation decay time constants shorter than those of PL decays.

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Fig. 4. (Color online) X-ray-induced scintillation
spectra of KI:Eu and undoped KI (inset).

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

Fig. 5. (Color online) X-ray-induced scintillation
decay profiles of KI:Eu. Dashed lines are fitting
curves.

Afterglow profiles after X-ray exposure for 2 ms in KI:Eu are shown in Fig. 6. Here, theafterglow levels at 20 ms after the X-ray was turned off were defined using the following
equation: Afterglow level [ppm] = 106 × (I2 − IBG)/(I1 − IBG),(61,62) where I1 is the signal intensity
during X-ray irradiation, I2 is the signal intensity at 20 ms after the X-ray is turned off, and IBG
is the background signal intensity. The afterglow levels of the 0.03, 0.1, 0.3, and 1% Eu-doped
samples were 1600, 1200, 700, and 9500 ppm, respectively. The 0.3% Eu-doped sample showed
the lowest afterglow level among all the samples. In the 0.03, 0.1, and 0.3% Eu-doped samples,Eu2+ would be substituted at the K+ site. However, excess Eu2+ might distort the crystal structurein the 1% Eu-doped sample. Such a lattice distortion can lead to the generation of defects, andthe afterglow level of the 1% Eu-doped sample can become high.
Pulse height spectra measured using KI:Eu and reference Gd2SiO5:Ce (GSO:Ce) under 241Am
γ-ray (60 keV) irradiation are presented in Fig. 7. The shaping times of KI:Eu and GSO:Ce were
3 and 0.5 ms, respectively. The LY of GSO:Ce was reported to be 8000 photons/MeV,(63–65) andthe peak channel was directly compared with the calculated LY since the wavelengths of KI:Euand GSO:Ce were almost the same. Concerning the signals due to the Compton edge, thephotoabsorption peaks of low-energy γ-rays and 60 keV were overlapped in the 0.03 and 0.1%
Eu-doped samples, and a photoabsorption peak was observed at ~200 ch in the 0.3 and 1% Eudoped samples. The LYs of the 0.3 and 1% Eu-doped samples were 3400 and 3300 photons/MeV,
respectively. Since the measured value contained a 10% error, the LYs of the 0.3 and 1% Eudoped samples were not significantly different. The scintillation LY can be expressed using thefollowing equation: LY ∝ S × QY, where S is the energy transportation efficiency from a host to aluminescence center.(66) The PL QY of all the samples was >96%. Therefore, energy
transportation would be inefficient. Scintillation LYs have been reported to be inversely
correlated with storage-type luminescence, such as thermoluminescence and opticallystimulated luminescence.(67) In this study, the afterglow did not correlate with the scintillation
LY probably because the afterglow did not include storage luminescence but only
thermoluminescence at room temperature.

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Fig. 6. (Color online) Afterglow profiles after X-ray
exposure for 2 ms.

4.

Fig. 7. (Color online) Pulse height spectra of KI:Euand Gd 2SiO5:Ce (GSO:Ce) under 241Am γ-ray (60keV) irradiation.

Conclusions

The photoluminescence and scintillation properties of KI:Eu synthesized by the vertical
Bridgman–Stockbarger method were investigated. A broad peak due to the 4f65d1–4f 7 transitionof Eu2+ was observed at 430–450 nm under UV and X-ray irradiations. In addition, a tailemission ascribed to color centers associated with oxygen impurities was observed at 500 nm
under X-ray irradiation. According to the pulse height spectra under 241Am γ-ray (60 keV)
irradiation, the highest LY was 3400 photons/MeV among the prepared samples. In terms of LY
and afterglow, the optimal concentration of Eu was 0.3% in KI. However, KI:Eu showed a lower
LY than commercial scintillators, and the energy transportation would be inefficient. Therefore,
the LY can be improved if the synthesis conditions are optimized to decrease the number ofdefects.

Acknowledgments
This work was supported by Grants-in-Aid for Scientific Research A (22H00309), Scientific
Research B (21H03733, 21H03736, 22H02939, and 22H03872), and Exploratory Research(22K18997) from the Japan Society for the Promotion of Science. The Foundation fromCooperative Research Project of the Research Center for Biomedical Engineering, A-STEP(JPMJTM22DN) from JST, Kazuchika Okura Memorial Foundation, Asahi Glass Foundation,
Nakatani Foundation, and Konica Minolta Science and Technology Foundation are also
acknowledged.

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