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MYU Tokyo
S & M 3532

Tm Concentration Dependence on Scintillation Propertiesof Y3Al5O12 Garnet Single Crystals
Toshiaki Kunikata,1* Prom Kantuptim,2 Daiki Shiratori,3 Takumi Kato,1Daisuke Nakauchi,1 Noriaki Kawaguchi,1 and Takayuki Yanagida1
1Nara Institute of Science and Technology, 8916-5 Takayama, Ikoma, Nara 630-0192, Japan

2Department of Physics, Faculty of Science, King Mongkut’s University of Technology Thonburi,

Bangkok 10140, Thailand

3Tokyo University of Science (TUS) 6-3-1 Niijuku, Katsushika-ku, Tokyo 125-8585, Japan(Received October 27, 2023; accepted January 16, 2024)

Keywords: Tm-doped YAG, scintillation, single crystals, radiation detector

Tm-doped Y3Al5O12 (YAG) single crystals with different Tm concentrations (0.1, 0.5, 1.0, and2.0%) were grown using the floating zone method. X-ray-induced scintillation spectra,
scintillation decay curves, and γ-ray-induced pulse height spectra of the obtained Tm-doped
YAG single crystals were investigated. All the samples showed emission peaks due to the 4f–4ftransitions of Tm3+ ions, and the decay times were around 40–20 μs under X-ray excitation. The
0.5% Tm-doped YAG sample showed the highest light yield under 662 keV γ-ray excitation
among the samples. The optimal dopant concentration of Tm-doped YAG single crystals was
confirmed to be 0.5%.

1.

Introduction

Radiation detectors play an important role in various applications, such as medical
diagnoses,(1,2) security screenings,(3,4) and industrial inspections.(5) A high proportion ofradiation detectors are equipped with scintillators that convert ionizing radiation into a lot of
low-energy photons. Scintillators are coupled with photodetectors including a photomultipliertube (PMT) and a Si photodiode. Therefore, the performance of scintillators strongly affects thatof radiation detectors. Various scintillators, such as films,(6) ceramics,(7–12) glasses,(13–22) single
crystals,(23–32) and nanoparticles,(33) have been investigated to date. Single crystal materials tend
to show excellent properties and are widely used as scintillators.
Yttrium aluminum garnet (YAG, Y3Al5O12) single crystals are one of the good candidates for
scintillators because they have high radiation resistance and acceptable density (4.57 g/cm3) for
X-ray detection, and can form a substitutional solid solution with rare-earth ions. In particular,
there are many reports on the scintillation properties of Ce-doped YAG single crystals with fast
decay time and high light yield.(34–37) Although other rare-earth ions can also show attractive
luminescence properties, there were relatively few studies on those of other rare-earth-iondoped YAG single crystals.
*Corresponding author: e-mail: kunikata.toshiaki.kt1@ms.naist.jp

https://doi.org/10.18494/SAM4754

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

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In this study, the scintillation properties of Tm-doped YAG single crystals were investigated.
Tm3+ ions can show bright blue-green luminescence due to 4f-4f transitions, and emission
wavelengths match a wavelength range at which typical PMTs show high sensitivities. In aprevious study, the scintillation properties of 0.5% Tm-doped YAG and 0.5% Tm-doped
Lu3Al5O12 (LuAG) single crystals, which were grown by the micro-pulling down method, were
evaluated, and photoabsorption peaks were observed under γ-ray excitation.(38) However, the Tm
concentration dependence on the scintillation properties of Tm-doped YAG single crystals has
not yet been investigated. In this paper, we show the Tm concentration dependence on the
scintillation properties of Tm-doped YAG single crystals.

2.

Materials and Methods

Tm-doped (0.1, 0.5, 1.0, and 2.0%) YAG single crystals were grown by the floating zone (FZ)
method. The chemical composition is represented by (Tm x,Y1−x)3Al5O12 (x = 0.001, 0.005, 0.01,
and 0.02); 0.1, 0.5, 1.0, and 2.0% correspond to x = 0.001, 0.005, 0.01, and 0.02, respectively. The
raw powders of Y2O3 (99.99%; Furuuchi Chemical), Al 2O3 (99.99%; Kojundo Chemical
Laboratory), and Tm2O3 (99.99%; Furuuchi Chemical) were mixed and ground using an agate
mortar and pestle. The mixed powders became rod-shaped by applying hydrostatic pressure andwere sintered at 1400 ℃ for 10 h using an electric furnace. The sintered rods were partiallymelted and gradually solidified using an optical FZ furnace (Canon Machinery, FZD0192). The
pull-down rate was set to 3 mm/h during crystal growth. The obtained Tm-doped YAG single
crystals were cut and polished using a polishing machine (Buehler, MetaServ 250).
Photoluminescence quantum yields (PL QYs) were evaluated using Quantaurus-QY (C11347,
Hamamatsu Photonics). The X-ray diffraction (XRD) patterns and scintillation properties of theobtained samples were investigated. The XRD patterns of the samples were measured in therange of 10–70° with an X-ray diffractometer (Rigaku, MiniFlex 600). The X-ray-induced
scintillation spectra of the samples were evaluated using our original setup equipped with an
X-ray generator (Spellman, XRB80N100/CB).(39) In this measurement, the tube voltage andoperation current of the X-ray generator were set to 40 kV and 1.2 mA, respectively. The X-rayinduced scintillation decay curves of the samples were measured using our other original
setup.(40) Pulse height spectra were measured under γ-ray irradiation from a 137Cs sealed source(662 keV). The samples were optically coupled with a PMT (Hamamatsu Photonics,
R7600U-200) whose applied voltage is −700 V. The shaping time was set to 10 µs. Signals fromthe PMT were analyzed using a preamplifier, a shaping amplifier, and an analog-to-digitalconverter, which are shown in a previous report.(41) The light yield of the 0.5% Tm-doped YAG
sample was estimated by comparison with that of the commercially available Ce-doped Gd2SiO5
single crystal (7000 photons/MeV).

3.

Results and Discussion

Figure 1 shows the Tm-doped YAG single crystals obtained in this study. The thicknessesof the samples were approximately 1.0 mm. All the samples were transparent and the 1.0 andSensors and Materials, Vol. 36, No. 2 (2024)

Fig. 1.

459(Color online) Appearance of Tm-doped YAG samples.

2.0% Tm-doped YAG samples showed brown color. In general, the colors of single crystals
can be changed by the presense of oxygen vacancies. There is a possibility that the browncolors of the 1.0 and 2.0% Tm-doped YAG samples were caused by the effect of oxygen
vacancies. Figure 2 shows the XRD patterns of the Tm-doped YAG samples and thereference data of YAG (Crystallography Open Database, No. 2003066). All the XRD peaksof the Tm-doped YAG samples were ascribed to that of the reference data of YAG. No
undefined phases were observed in this measurement.
The PL QYs of the 0.1, 0.5, 1.0, and 2.0% Tm-doped YAG samples were 17, 31, 9, and 7%,
respectively. In the 1.0–2.0% Tm-doped samples, the PL QY decreased with increasing
dopant concentration owing to the concentration quenching phenomenon.
Figure 3 shows the X-ray-induced scintillation spectra of the Tm-doped YAG samples.
The multiple emission peaks due to the 4f-4f transitions of Tm 3+ ions were observed ataround 250–550 nm. The observed emission peaks are consistent with those of Tm-doped
materials in the previous reports.(38,42,43) The peaks at around 280, 350, 460, and 520 nm are
ascribed to 1I6 -3H6, 1D2→3H6, 1G4→3H6, and 1D2→3H5 transitions, respectively. The 0.1 and
0.5% Tm-doped samples showed the broad emission in the wavelength range from 300 to
450 nm. This can be ascribed to the intrinsic luminescence of YAG, which is explained byemissions due to defects(38) or self-trapped excitons.(44)Figure 4 shows the X-ray induced scintillation decay curves of the Tm-doped YAG
samples and instrument response function (IRF). The scintillation decay curves were fitted tosingle exponential functions in the time ranges that did not overlap with that of the IRF
component. The estimated scintillation decay times of the 0.1, 0.5, 1.0, and 2.0% Tm-doped
YAG samples were 43, 44, 34, and 23 µs, respectively. The scintillation decay times are roughly
consistent with that of Tm-doped YAG shown in a previous report.(38) When the Tm
concentration was 0.5–2.0%, the scintillation decay time decreased with increasing dopant
concentration. This seems to be due to the concentration quenching phenomenon, similar to theresults of PL QY.
Figure 5 shows the pulse height spectra of the Tm-doped YAG samples under γ-ray
irradiation from the 137Cs sealed source. The photoabsorption peaks of the 0.1, 0.5, 1.0, and
2.0% Tm-doped YAG samples were observed at 585, 705, 410 and 240 channels, respectively.
The light yield of the 0.5% Tm-doped YAG sample is the highest among the samples
according to the peak positions. The estimated light yield of the 0.5% Tm-doped YAG
sample was 3500 photons/MeV. This value was lower than those of Tm-doped YAG (11000

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Fig. 2. (Color online) XRD patterns of Tm-doped
YAG samples and reference data of YAG.

Fig. 3. (Color online) X-ray-induced scintillation
spectra of Tm-doped YAG samples.

Fig. 4. (Color online) X-ray-induced scintillation
decay curves of Tm-doped YAG samples and IRF of
the equipment.

Fig. 5. (Color online) Pulse height spectra of Tmdoped YAG samples under γ-ray irradiation.

photons/MeV) and Tm-doped LuAG (6000 photons/MeV).(38) There are two possible reasonsfor this result. One is the difference in shaping time. The shaping time in our measurement
was 10 µs (the maximum value of our setup), whereas that time in the previous study was 30µs.(38) The obtained light yields could be underestimated. The other possible reason is thedifference in the single crystal growth method used. The FZ method was used in this study,
whereas the micro-pulling down method was used in the previous study.(38) In general, thelight yields of single crystals were strongly affected by defects. Defect formation during
single crystal growth depends on the growth method used. Figure 6 shows the normalized
light yields of the Tm-doped YAG samples under γ-ray irradiation. In the 1.0–2.0% Tmdoped YAG samples, the light yield tended to decrease with increasing dopant concentration,
similar to the results of PL QY. This result can be explained by the concentration quenching
phenomenon.

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Light yield

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

Fig. 6.

4.

Normalized light yields of Tm-doped YAG samples.

Conclusions

We grew Tm-doped YAG single crystals with different Tm concentrations (0.1, 0.5, 1.0, and2.0%) by the FZ method and evaluated their XRD patterns and scintillation properties. The
XRD patterns of the Tm-doped YAG samples were ascribed to the reference data of YAG. The
obtained scintillation spectra and scintillation decay times were consistent with those in aprevious report(38) on Tm-doped YAG. In the pulse height spectra under γ-ray irradiation, the
0.5% Tm-doped YAG sample showed the highest light yield. The scintillation decay times andlight yields of the 1.0 and 2.0% Tm-doped YAG samples were lower than those of the 0.5 % Tmdoped YAG sample, which can be explained by the concentration quenching phenomenon.

Acknowledgments
This work was supported by Grants-in-Aid for Scientific Research A (22H00309), Scientific
Research B (21H03736), and Challenging Exploratory Research (22K18997) from the Japan
Society for the Promotion of Science.

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