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MYU Tokyo
S & M 1607

Scintillation Properties of (Lu,Y)AlO3 Doped with Nd
Masaki Akatsuka,* Yuki Usui, Daisuke Nakauchi,
Go Okada, Noriaki Kawaguchi, and Takayuki Yanagida
Graduate School of Materials Science, Nara Institute of Science and Technology,
8916-5 Takayama, Ikoma, Nara 630-0192, Japan(Received January 19, 2018; accepted March 2, 2018)

Keywords: scintillation, near infrared, Nd, ceramic(Lu,Y)AlO3 crystals doped with different concentrations of Nd were synthesized by thefloating-zone (FZ) method to evaluate their scintillation properties, particularly in the nearinfrared (NIR) range. Under X-ray irradiation, scintillation due to the 4F3/2→4I11/2 transition of
Nd3+ was observed at 1064 nm. The scintillation decay time profile was approximated by an
exponential decay function, and the obtained value varied from 2–3 to 150 μs depending on theconcentration of Nd3+.

1.

Introduction

Scintillators have a function to immediately convert the absorbed energy of ionizing
radiation (typically of keV to GeV) to thousands of low-energy photons.(1) The application fields
of scintillators are very broad, and they include medical imaging,(2) security,(3) environmental
monitoring,(4) and high-energy physics.(5) The scintillators are in various kinds of material
forms, such as inorganic/organic solid, liquid, and gas, which affect the scintillation properties.
Among them, inorganic solid-state scintillators are the most common. Scintillators are often
considered in terms of the density (ρ), effective atomic number (Zeff), scintillation decay time,afterglow level, light yield, and radiation tolerance. In particular, ρ and Zeff are important
for achieving high detection efficiency against high-energy photons since the probability ofphotoelectric interaction events with the material is proportional to ρZeff4. In reality, there is
no ideal scintillator to fulfill all the properties required for all the applications; therefore, users
must select suitable scintillators for their purposes. Among many inorganic scintillators, garnet
materials have, in particular, superior fluorescence properties, and many studies have been
performed to investigate their scintillation properties.(6,7)
Recently, scintillators emitting near-infrared (NIR) photons have attracted much attention
because a human body can transmit NIR photons.(8–12) Since there is an optical window from
700–1200 nm in the human body, the scintillator materials emitting NIR photons can be used
for bioimaging. For example, in radiation therapy, a small NIR scintillator is embedded in theaffected part of the human body so that it emits NIR light when radiation is delivered to the*Corresponding author: e-mail: akatsuka.masaki.ad5@ms.naist.jp
http://dx.doi.org/10.18494/SAM.2018.1922
ISSN 0914-4935 © MYU K.K.
http://myukk.org/

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appropriate part. The NIR scintillation can be transmitted through the optical window of thepatient՚s body; therefore, the scintillation signal can be detected by a photodetector in real timein order to ensure the appropriate radiation delivery. In addition, the photon energies of NIR
quanta are approximately 1 eV lower than those of UV or VIS light; therefore, NIR-emitting
scintillators are considered to have a higher scintillation light yield, which is the number ofscintillation photons per absorbed energy of ionizing radiation. Furthermore, in high-dose
environments (e.g., nuclear reactors), we expect such NIR-emitting scintillators to be effective
tools for monitoring radiation dose. In general, scintillation detectors for monitoring highradiation doses are a combination of a scintillator and optical fiber. However, the optical fiber
suffers from radiation damage and the UV–VIS light transmittance is degraded,(13) strongly
weakening the scintillation emission if the scintillator emission is in the UV–VIS range. On
the other hand, NIR-emitting scintillators are useful in such a measurement configuration
because the radiation damage has no effect in the NIR range. Despite such usefulness, there
have been only a few reports on NIR-emitting scintillators, and there remains much room forstudying this topic. One reason is that the conventional Si-based photodetectors do not havesufficient sensitivity of emission with wavelengths longer than 800 nm, so in previous studies,
only emissions up to 800 nm could be characterized.(14,15) Recently, on the other hand, we havedramatically expanded the spectral range of measurement by using an InGaAs-based detector
to study NIR-emitting scintillators for bioimaging and high-dose-monitoring applications. Up
to now, we have reported NIR scintillations (600–1650 nm) in oxide garnets (Nd3+-doped),sesquioxides (Yb3+-doped Lu2O3), and fluorides (Nd3+-doped YLiF4).(16–20)
Most inorganic crystalline scintillators of oxides and fluorides have superior scintillation
characteristics. In order to extend the investigation on NIR-emitting scintillators, in this study,
we focused on Nd-doped (Lu,Y)AlO3 [grown by the floating-zone (FZ) method] since mostgarnet materials have been intensively studied, while perovskite materials are also suitable for
scintillator uses. As the perovskite host, we selected (Lu,Y)AlO3 since ρ and Zeff of common
YAlO3 [yttrium aluminum perovskite (YAP)] are not sufficiently high for scintillators. The
Nd was added as an emission center for NIR emission since Nd3+ is known to be a luminous
emission center, especially in laser applications. The obtained crystals were evaluated in termsof photoluminescence (PL), scintillation, and thermoluminescence (TL) properties.

2.

Experimental Procedure

Nd-doped (Lu,Y)AlO3 (Nd = 0.1, 0.3, 1.0, 3.0, 10 mol.%) samples were synthesized by the FZ
method. Here, the ratio of Lu to Y was fixed to Lu:Y = 1:1, and Nd was added as a substitutefor lutetium. In addition, we prepared Nd-free (Lu,Y)AlO3 for comparison. The raw material
powders used were Lu2O3 (5N), Al2O3 (4N), Y2O3 (4N), and Nd2O3 (4N). They were mixed tothe compositions as above. After mixing, the powders were formed into a cylindrical rod byapplying hydrostatic pressure. After the shaping process, the cylinders of all the compositions
were sintered at 1100 ℃ for 8 h in air to obtain ceramic rods. Finally, we conducted crystal
growth by melting the ceramic rod via the FZ method in air. The FZ furnace used was Canon
Machinery FZD0192. Here, the rotation rate was 20 rpm, and the pull-down rate was 2.5–5 mm/h.

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The PL excitation/emission contour spectrum (or PL map) and PL quantum yield (QY) were
measured using Quantaurus-QY (C11347, Hamamatsu). The excitation and emission wavelength
ranges for the PL map were 250–800 and 300–950 nm, respectively. The measurement interval
for excitation wavelength was 10 nm. The absolute QY was calculated as QY = Nemit/Nabsorb,
where Nemit and Nabsorb are the numbers of emitted and absorbed photons, respectively. PL
decay time profiles were evaluated using Quantaurus-τ (C11367, Hamamatsu), and the excitationand monitoring wavelengths were selected on the basis of the obtained PL map.
The scintillation spectra were evaluated at room temperature under X-ray irradiation in
our original setup.(21) The excitation source was an X-ray generator (XRB80N100, Spellman)equipped with a conventional X-ray tube, supplied with 40 kV bias voltage and 1.2 mA tube
current. The emission spectra were measured using two different spectrometers to cover awide spectral range from UV to NIR: Andor Newton 920 for 180–700 nm and Andor iDUS
for 650–1650 nm. The CCDs of the Andor spectrometers were cooled to 193 K using a Peltier
device to reduce thermal noise. In order to avoid the CCD being exposed directly to X-rays, thespectrometers were placed off the irradiation axis, and the scintillation light was fed into thespectrometers through 2 m of optical fiber. The scintillation decay time profiles were also evaluatedin our original setup.(22) In addition, a previous report indicated that YAP showed very strong
TL;(23) therefore, we also evaluated the TL glow curve by heating a sample at a constant rate (1℃/s) over 50 to 490 ℃, using a TL reader (TL-2000, Nanogray).(24)3.

Results and Discussion

3.1

Sample appearance

As-synthesized rods were typically 4 mm in diameter and 15–20 mm in length. These rods
were cut into pieces for characterization. Figure 1 shows the samples used for characterization.
The Nd-free and 0.1–3.0% Nd-doped samples looked opaque, indicating that these samples arein a ceramic (or polycrystalline) form. However, the 10% Nd-doped sample looked transparent,
so we conclude that this was grown in a single crystal.

Fig. 1.

(Color online) Photograph of (Lu,Y)AlO3 samples doped with different concentrations of Nd.

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Figure 2 shows the powder X-ray diffraction (XRD) patterns of prepared samples. All thediffraction patterns measured corresponded well with the standard crystallographic data (JCPDS
Nos. 871290, 240690, 330844, and 731368). Moreover, LuAlO3 and YAlO3 have the same
crystal structure and very similar XRD patterns; therefore, we used (Lu,Y)AlO3. The 10% Nddoped sample seems to have a perovskite single-crystal layer. On the other hand, the Nd-free
and 0.1–3.0% Nd-doped samples seem to have not only the perovskite structure but also garnet
and monoclinic structures. In this result, we considered that, in general, the mixed crystal of
Lu4Al2O9 and Lu3Al5O12 is easier to grow than the (Lu,Y)AlO3 single crystal. However, the (f)sample was doped with a large amount of Nd compared with other samples and we consideredthat the large amount of Nd facilitated the growth of the perovskite structure. Thus, the (f)sample was grown as a (Lu,Y)AlO3 single crystal. From the viewpoint of the diffraction angle,
the diffraction angles of 0.1–3.0% Nd-doped samples are the same as that of the Nd-free sample.
Thus, it is expected that Nd cations will mainly exist in the interface of crystallites.
3.2

PL properties

Figure 3 shows the PL emission map of 1.0% Nd-doped (Lu,Y)AlO3 as a representative
example. It shows strong emission at around 900 nm owing to the 4f–4f transitions of Nd3+.(25)Here, the integrated emission range was from 800 to 950 nm for all samples. As a result, QY
values of Nd-free, 0.1, 0.3, 1.0, 3.0, and 10.0% samples were 0, 26.9, 28.2, 22.1, 9.3, and 0.3%,
respectively. Among all the samples, the 0.3% Nd-doped sample showed the highest QY value,
and the 10% Nd-doped sample showed very low QY, although it had a high transparency. We
consider that the 10% Nd-doped sample suffered from concentration quenching. The wellknown intense emission of Nd3+ at 1064 nm is not included because our apparatus cannot
measure photons of wavelengths larger than 960 nm.
The PL decay curves of the Nd-doped samples are illustrated in Fig. 4. Here, the monitoring
wavelength was around 900 nm. The excitation wavelength was selected to be the one at which
the QY is the highest. All the decay curves followed a simple exponential decay function. The
Nd-free sample did not show any signal. For the 0.1–3.0% Nd-doped samples, the PL decay

Fig. 2. (Color online) Powder XRD patterns of (Lu,Y)
AlO3 doped with different concentrations of Nd.

Fig. 3. (Color online) PL map of 0.3% Nd-doped(Lu,Y)AlO3. The horizontal and vertical axes show
emission and excitation wavelengths, respectively.

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Fig. 4. (Color online) PL decay curves of (Lu,Y)AlO3 doped with different concentrations of Nd. The monitoring
wavelength was around 900 nm.

time of 4f–4f transitions of Nd3+ was 220–550 μs. These decay times were typical for the 4f–4ftransitions of Nd3+ and agreed with the values reported in previous studies.(25–27) The decay
time of the 10% Nd-doped sample was very short. This finding supports the idea that the 10%
Nd-doped sample suffered from concentration quenching. Furthermore, the decay time of the 3.0%
Nd-doped sample was shorter than those of the 0.1–1.0% Nd-doped samples. Therefore, the 3.0%
Nd-doped sample also possibly suffered from concentration quenching.
3.3

Scintillation properties

Figure 5 shows X-ray-induced scintillation spectra measured in the (a) UV–VIS and (b)NIR ranges. The Nd-free sample showed emission at around 300 nm. In previous studies,
nondoped YAlO3 and Y3Al5O12 showed emission at around 300 and 355 nm owing to latticedefects, respectively.(28) For the Nd-doped samples, the 4f–4f transitions of Nd3+ were observedin the UV–VIS range, which were not observed in PL. The emission features were typicaland can be identified as the electronic transitions of 2F5/2→4F5/2 (400 nm), 2F5/2→4F9/2 (450nm), 2F5/2→4G5/2 (480 nm), 2F5/2→4G7/2 (540 nm), and 2F5/2→4G9/2 (550 nm).(16,19,29,30) In
the NIR range, all the Nd-doped samples showed emissions due to the electronic transitionsof 4F3/2→4I9/2 (910 nm), 4F3/2→4I11/2 (1064 nm), and 4F3/2→4I13/2 (1320 nm). The emission of1064 nm is well known for laser applications,(31) and the emission intensities of 0.1–3.0% Nddoped samples were high. Although the intensity of the integrated scintillation dealt with hereis qualitative (unlike in the case of the pulse-counting technique), the 0.3% Nd-doped sample
showed the strongest NIR emission intensity. The emission of the 10% Nd-doped sample wasvery weak, which was consistent with the results of PL.
Figure 6 shows X-ray-induced scintillation decay time profiles of Nd-doped (Lu,Y)AlO3.
The decay curves of the 0.1–3.0% Nd-doped samples were approximated as the sum of two
exponential decay functions, and the faster decay component was considered to be a tail of the
instrumental response (~10 µs) while the longer component was considered to be the signal fromthe sample. In contrast, the 10% Nd-doped sample was approximated by a single exponential
decay function. These components were due to the 4f–4f transitions of Nd3+. These decay
times, except for the 10% Nd-doped sample, were typical values of the 4f–4f transitions of1530

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(a)

(b)

Fig. 5. (Color online) X-ray-induced scintillation spectra of (Lu,Y)AlO3 doped with different concentrations of Nd
in the (a) UV–VIS and (b) NIR ranges.

Fig. 6. (Color online) X-ray-induced scintillation decay time profiles of (Lu,Y)AlO3 doped with different
concentrations of Nd. The monitoring wavelength was 400–900 nm.

Nd3+ reported from previous studies.(17–19) In general, the scintillation phenomenon consists ofthree different processes: generation of electrons and holes, energy transport, and emission atluminescence centers. Therefore, the scintillation decay time is generally slower than that of
PL. However, the scintillation decay times of all the samples were faster than those of PL in ourmaterials. A possible reason is some kind of quenching among secondary electrons generatedby X-ray irradiation. The same behavior was observed in different phosphors reported
elsewhere.(32,33)
3.4

TL properties

Figure 7 shows TL glow curves of (Lu,Y)AlO3 doped with different concentrations of Nd.
The glow curves were measured after irradiating the samples with X-rays of 0.1 Gy. The Ndfree and 0.1% Nd-doped samples demonstrate glow peaks at around 150 and 300 ℃. However,
the glow peaks of the 0.1% Nd-doped sample are weaker than those of the Nd-free sample, andSensors and Materials, Vol. 30, No. 7 (2018)

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Fig. 7. (Color online) TL glow curves of (Lu,Y)AP doped with different concentrations of Nd. The samples were
irradiated with X-rays of 0.1 Gy before measurement. The monitored wavelength was 250–550 nm.

the samples doped with 0.3% or more Nd showed small glow peaks. This tendency was thesame as those in Eu-doped YAlO3.(23) It is suggested that, in the Nd-doped samples, most of theenergy of secondary electrons excited by X-ray irradiation was transferred to the scintillation.
In addition, the 10% Nd-doped sample shows neither scintillation nor TL. In both scintillation
and TL, the final luminescence process depends on QY. Thus, the 10% Nd-doped sample,
which showed a very low QY, should not show intense emission.

4.

Conclusions

We synthesized Nd-doped (Lu,Y)AlO3 by the FZ method to evaluate their PL, scintillation,
and TL properties. For scintillation, the 0.1–3.0% Nd-doped samples demonstrated a strong
emission peak at 1064 nm owing to 4F3/2→4I11/2 transitions of Nd3+. The latter decay time
was of a typical order; however, that of PL was longer than that of scintillation. This finding
suggests that some sort of quenching took place during the scintillation process. For TL
properties, the Nd-free sample exhibited the strongest glow peak among all the samples, andthe origin of this emission would be due to the host since a similar glow curve was observed in
YAlO3.

Acknowledgments
This work was supported by Grant-in-Aid for Scientific Research (A) (17H01375) and
Grant-in-Aid for Young Scientists (B) (17K14911) from the Ministry of Education, Culture,
Sports, Science and Technology of the Japanese government (MEXT) as well as A-STEP fromthe Japan Science and Technology Agency (JST). The Cooperative Research Project of the
Research Institute of Electronics, Shizuoka University, Mazda Foundation, Konica Minolta
Science and Technology Foundation, Taisei Foundation, SEI Group CSR Foundation, NAISTFoundation, and TEPCO Memorial Foundation are also acknowledged.

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