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Sensors and Materials, Vol. 30, No. 7 (2018) 1547–1554
MYU Tokyo
S & M 1610

Characterizations of Ce-doped Y4Al2O9 Crystals
for Scintillator Applications
Go Okada,1* Masaki Akatsuka,1 Hiromi Kimura,1 Masaki Mori,2†
Naoki Kawano,1 Noriaki Kawaguchi,1 and Takayuki Yanagida1
1

Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST),
8916-5 Takayama, Ikoma, Nara 630-0192, Japan
2
Yamazaki Manufacturing Department, Hitachi Metals Ltd.
15-17 2 Chome, Egawa, Shimamoto Mishima, Osaka 618-0013, Japan(Received December 19, 2017; accepted April 11, 2018)

Keywords: scintillator, Y4Al2O9, YAM, Ce3+

Ce-doped Y4Al2O9 (YAM) crystals were grown by the floating zone method, and several
different characterizations were conducted for scintillator applications. Both photoluminescence(PL) and scintillation show emissions predominantly due to the 5d–4f transitions of Ce3+ over
400–600 nm. The luminescence lifetime is approximately 10 ns for PL while the scintillation
decay curve is best fitted by a second-order exponential decay function with the lifetimes ofapproximately 30 and 300 ns. These lifetimes are faster than that for Ce:Y3Al5O12 (YAG) (~120ns) and close to that of Ce:YAlO3 (YAP) (~30 ns). The Ce-doped YAM shows a relatively highafterglow signal induced by X-rays (2 ms pulse) possibly due to the inclusion of a large numberof defect centers. Pulse height spectroscopy was demonstrated under alpha-ray irradiation, anda successful sample showed a full-energy peak.

1.

Introduction

Inorganic phosphors are often a tool for radiation measurement applications.(1) Such devices
are commonly referred to as scintillators and storage phosphors depending on the phenomenon
involved. The scintillators convert the incident radiation energy to light immediately so that one
can detect radiation signals using conventional photodetectors on-line. The storage phosphors,
on the other hand, store and accumulate the incident radiation energy for a certain period oftime until read-out. The stored signal is then read out as light emission stimulated by heat orlight, in which the light emission intensity is proportional to the stored energy, hence the stored
radiation dose. Both scintillators and storage phosphors involve different mechanisms; however,they are considered to undergo some common processes. First, upon interactions of incident
radiation with matter, in both scintillators and dosimeters, electrons and holes are generated.
In scintillators, next, the generated charges transfer to luminescent centers and recombine toemit light, while in storage phosphors, the charges are captured by trapping centers and stored.
*

Corresponding author: e-mail: go-okada@ms.naist.jp
Former student of NAIST and currently at Hitachi Metals Ltd.
http://dx.doi.org/10.18494/SAM.2018.1919†

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

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By applying external heat or light, the captured charges are ionized and then recombine atluminescent centers to emit light, as in scintillators. When the stimulant is heat, the resultant
luminescence is so-called thermally stimulated luminescence (TSL),(2) while the luminescencegenerated by light stimulation is the so-called optically stimulated luminescence (OSL).(3,4)
With knowledge of the luminescence processes above, it is clear that the luminescence
efficiencies of scintillators and storage phosphors conflict.(5–8) Those phosphor materials
are used in a wide range of application fields such as medicine,(9) security,(10) high-energy
physics,(11) and geophysical and resource explorations.(12) The required properties depend on
the application so there are many different scintillator materials available, but no scintillators
satisfy the requirements of all the applications. In addition, the application technologies
continue to advance and require sensing materials with better properties. Therefore, a search
for new materials is continuously demanded.
Yttrium aluminum monoclinic (YAM) crystals are our current interest. The YAM is
one in the Y2O3–Al2O3 compound group after yttrium aluminum garnet (YAG) and yttrium
aluminum perovskite (YAP). It can be expressed by the formula Y4Al2O9 and has the spacegroup P21/c with a monoclinic crystal structure.(13,14) The yttrium site can be substitutedby, for example, a series of rare-earth ions to act as a luminescence center while the Al site
can be substituted by Ga and some transitional metals. These features are similar to those of
the garnet and perovskite groups. The luminescence properties, including scintillation andstorage-luminescence properties, of garnets and perovskites have been intensively studied withmany different combinations of these constituent elements.(15–20) Despite the similarities to
the garnets and perovskites, fewer studies have been reported on the luminescence propertiesof the monoclinic crystals. As far as we are aware, the scintillation properties of Ce-doped
YAM (Ce:YAM) have not been reported, and only a few groups have investigated some opticalproperties including photoluminescence.(21,22)
This contribution is a comprehensive report on the optical properties of Ce:YAM crystals
for scintillator applications. We have synthesized the Ce:YAM crystals with different
concentrations of Ce3+ from 0.3 to 1 mol% by the floating zone (FZ) method. Furthermore, wehave investigated the photoluminescence (PL) spectrum, PL quantum yield (QY), PL lifetime,
scintillation spectrum/lifetime, afterglow, and pulse height spectroscopic properties.

2.

Materials and Methods

Ce:YAM single crystals with different concentrations of Ce from 0.3 to 1 mol.% weresynthesized by the FZ method. The raw powders of Y2O3, Al2O3, and CeO2 of reagent grade(4N purity) were homogeneously mixed in stoichiometric compositions. Here, the Ce ion was
introduced to the host constitution as (Y1−xCex)4Al2O9 (x = 0.003, 0.005, 0.008, and 0.010). The
mixed powder of each composition was formed into a cylindrical rod (⌀4 mm × 50 mm) byapplying hydrostatic pressure inside a pencil balloon and then sintered at 1500 °C for 6 h in air
to obtain a solid ceramic rod. Crystal growth was accomplished by the FZ method using an FZ
furnace (FZD0192, Canon Machinery) in air. A pair of ceramic rods was placed at the centerof the mirror furnace equipped with two halogen lamps. The infrared light from the lamps

Sensors and Materials, Vol. 30, No. 7 (2018)1549

was focused on adjacent ends of the ceramic rods to cause melting, and the rods were rotatedin alternating directions to each other at a rate of 20 rpm. The set of ceramic rods was slowly
translated vertically at 5–6 mm/h to slowly cool the melt and obtain a crystal.
PL emission and excitation spectra were measured using a spectrofluorometer (FP8600,
JASCO). The PL decay curve was measured using Quantaurus-τ (C11367, Hamamatsu
Photonics) to obtain the lifetime. The lifetime was extracted from the measured decay curve byleast-squares fitting with a sum of two or three exponential decay functions. The system offersa time resolution of 60 ps.
Scintillation spectra were measured using a laboratory-constructed setup. Detailed
descriptions can be found elsewhere;(19) but briefly, a CCD-based spectrometer (DU920-BU2NC,
ANDOR) was used to measure a scintillation spectrum while the sample was irradiated by
X-rays generated by a conventional X-ray tube (40 kV and 5.2 mA; W anode; Be window). The
scintillation photons were guided to the spectrometer through an optical fiber.
The X-ray-induced scintillation decay time and afterglow profiles were measured using acharacterization system equipped with a pulse X-ray source. The details of system descriptions
can be found elsewhere.(23) The system is available as a custom-ordered product from
Hamamatsu Photonics. The system offers a time resolution of a few ns, and a decay curve
profile was collected on the basis of a time-correlated single photon counting (TCSPC). The
voltage applied to the X-ray tube was 30 kV.
Pulse height spectroscopy measurements were performed in order to demonstrate thecapability of being used in a scintillation counter and to test the light yield, which is the numberof emission photons per incident radiation quantum. A sample was placed on the window of
the photomultiplier tube (PMT; R7600, Hamamatsu Photonics) with optical coupling grease(OKEN6262A), and the sample was covered by several layers of Teflon reflectors. A power
supply (ORTEC556) was used to apply a bias voltage of −700 V to the PMT. The output
electrical signal from the PMT was sent to a preamplifier (ORTEC 113) and then a shaping
amplifier (ORTEC 572) with a 0.5 µs shaping time. Furthermore, the signal was processed bya multichannel analyzer (Pocket MCA 8000A, Amptek) to convert analog signals to digital, andthe pulse height spectrum was constructed on the computer. We used sealed radioactive 241Am
α-ray (5.5 MeV) and 137Cs γ-ray (662 keV) sources in the measurements.

3.

Results and Discussion

Figure 1 illustrates the synthesized YAM crystals with different concentrations of Ce. The
typical size of the obtained crystal rods was ~4 mm in diameter and ~50 mm in length. The
obtained crystal rods were cut into small pieces for characterization. The obtained crystals
appear milky, the degree of which seems to increase with increasing dopant concentration.
The cause can be inclusions of cracks and possibly secondary phases [although no secondary
phase was detected by X-ray diffraction (XRD)]. Figure 2 shows the XRD patterns of the
YAM crystals synthesized. All the samples showed the monoclinic structure with the patternconsistent with that in an earlier study.(24) The dominant crystal phase is monoclinic.

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Fig. 1. (Color online) Synthesized YAM crystals
doped with various concentrations of Ce.

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

Fig. 2. (Color online) Powder XRD patterns ofobtained crystals with different concentrations of Ce.
A reference pattern of Y4Al2O9 crystal is also given at
the bottom.(24)The PL excitation and emission spectra are illustrated in Fig. 3. When excited at 280 nm, a
broad emission was observed from 400 to 600 nm. The latter emission has a weak excitation
band around 360 nm and a strong band below 300 nm. The origins of the emission andexcitation bands were ascribed to the 5d1 → 4f transitions (emission) and 4f → 5d1 transitions(excitation) of the Ce3+ ion. A strong absorption band below 300 nm is most likely due to the
4f → 5d2 transitions of Ce3+ as well as absorption by the Ce4+ ion. The PL quantum yields were
too low to measure, i.e., less than ~3%.
Figure 4 shows the PL decay profiles. The excitation and emission wavelengths were fixedat 365 and 480 nm for all samples. The obtained curves were best-fitted with a sum of two
exponential decay functions to determine the lifetimes. The lifetimes (and relative intensities)
obtained are: 2.4 ns (90%) and 10 ns (10%) for 0.3%, 2.1 ns (85%) and 11 ns (15%) for 0.5%,
2.2 ns (89%) and 9.6 ns (11%) for 0.8%, and 2.0 ns (83%) and 9.9 (17%) ns for 1% Ce-doped
samples. The shorter values for each set of lifetimes are considered to be the instrumental
response, and the longer values of approximately 10 ns are the PL lifetimes due to the 5d–4ftransitions of Ce3+. These values are the fastest among the Ce-doped Y2O3–Al2O3 compounds(~30 ns for Ce:YAlO3(25) and ~120 ns for Ce:Y3Al5O12(26)). The correlation with the dopant
concentration is not significant, but the lifetime seems to decrease with the concentration,
possibly due to increasing nonradiative relaxation processes upon concentration quenching. It
is worth mentioning that an earlier study(21) reported that the lifetime of PL is on the order of afew ns. Some possible explanations of the difference from the results of our study are that (1)the reported emission component is hidden in the instrumental response in our measurements,and (2) the slower signal was not detected in the earlier report because the measurement time
range was short (~10 ns).
The X-ray-induced scintillation spectra are demonstrated in Fig. 5. As observed for PL (Fig. 3),
the scintillation spectrum consists of a broad band over 400–600 nm. For the similarities in the
spectral features with those of PL, it is straightforward that the emission origin can be attributed
to the 5d–4f transitions of Ce3+. As indicated in the figure, the peak position seems to red-Sensors and Materials, Vol. 30, No. 7 (2018)

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Fig. 3. (Color online) PL excitation and emission
spectra of YAM samples doped with Ce.

Fig. 4. (Color online) PL decay profiles of Ce:YAM.
The excitation wavelength is 365 nm while theemission wavelength monitored is 480 nm.

Fig. 5. (Color online) X-ray-induced scintillation
spectra of Ce:YAM crystals. The X-ray irradiation
was performed by using a conventional X-ray tube (80keV, W anode, Be window).

Fig. 6. (Color online) X-ray-induced scintillation
decay cur ves of Ce:YAM cr ystals. The X-ray
irradiation was performed by using a pulse X-ray tube
(30 keV, W anode, Be window).

shift with increasing concentration of Ce. This is typically seen in many Ce-doped materials
because of self-absorption in which a fraction of emission is absorbed by the material itself as
it travels in the matrix. It should be noted that a clear redshift was not apparent in PL owing to
the difference in the measurement geometry. The PL was measured on the same sample surface
of excitation while the scintillation photon was collected to be measured on the opposite sidefrom the irradiation source so the self-absorption is more strongly affected. Despite the effectof self-absorption, the emission wavelength seems to be shorter than that of YAG:Ce and longer
than that of YAP:Ce where the peak positions are 540 and 360 nm, respectively.
Figure 6 shows scintillation decay curves of Ce:YAM crystal samples induced by pulse
X-rays. All the decay curves are best approximated by a third-order exponential decayfunction. The deducted lifetimes (and relative intensities) are: 1.9 ns (80%), 37 ns (14%), and
400 ns (6%) for 0.3%, 6.4 ns (90%), 38 ns (4%), and 240 ns (6%) for 0.5%, 5.6 ns (90%), 28 ns(6%), and 250 ns (4%) for 0.8%, and 1.6 ns (80%), 25 ns (15%), and 320 ns (5%) for 1% Ce-1552

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

doped samples. The shortest lifetimes detected for all the samples are due to the instrumental
response. The intermediate values are of a similar order to those observed in PL for the 5d–4ftransitions of Ce3+, so we attribute the same origins. The origins of the longest contributions are
unknown and further investigations are needed to identify them; however, a possibility is thatsome types of defects act as luminescent centers. No luminescence features are observed in the
scintillation spectrum other than that for Ce3+ (Fig. 5); thus, most likely, the luminescence of
this slowest component is located in the same spectral range and overlapped with the emissionof Ce3+. These lifetimes are found to be longer than that of PL. An explanation for this is thatthe processes of excitation and radiative relaxation at only the localized luminescent center are
observed by PL whereas the energy transfer process from the host matrix to the luminescentcenter must also be taken into account. Thus, it appears to decay slower in scintillation.
Furthermore, compared with Ce-doped YAG (Ce:YAG) and Ce-doped YAP (Ce:YAP) crystals,
the intermediate luminescence due to Ce3+ in Ce:YAM is considerably shorter than that of
Ce:YAG (~120 ns) and equivalent to that of Ce:YAP (~30 ns).
The X-ray-induced afterglow profiles of Ce:YAM crystal samples are shown in Fig. 7. Here,the afterglow level is defined as the percentage of intensity at 20 ms after a pulse irradiation
relative to the intensity during irradiation. The afterglow levels of the 0.3, 0.5, 0.8, and 1.0%
Ce-doped samples are 0.05, 0.07, 0.03, and 0.05%, respectively. Compared with common X-ray
scintillators (e.g., Bi4Ge3O12, CdWO4, and Tb and Pr co-doped Gd2O2S),(23) those afterglow
levels are still considerably higher as common scintillators have afterglows of ~10 ppm atseveral milliseconds after irradiation.
Pulse height spectroscopy was demonstrated using the Ce:YAM crystal samples with 5.5
MeV α-ray irradiation of 241Am, and the obtained spectra are illustrated in Fig. 8. Among thesamples tested, only the 0.3% Ce-doped sample showed a full-energy peak. We think this isbecause, on the basis of the visual appearance, the crystal quality of this sample is the best, andscintillation light is effectively detected by the PMT without scattering loss. Therefore, better
performance is expected for the other samples by improving the crystal quality. In addition, theFig. 7. (Color online) X-ray-induced afterglow
profiles. The X-ray source was a pulse X-ray tube (30keV, W anode, Be window), and the irradiation pulse
width was 2 ms.

Fig. 8. (Color online) Pulse height spectra of 5.5
MeV α-rays of 241Am measured using the Ce:YAM
crystal samples.

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

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YAM structure has four different Y sites in a unit cell (substituted by Ce3+ ions), which could
cause concentration quenching of luminescence by the Ce3+ ion.(27) We also examined pulse
height spectroscopy under 137Cs γ-ray irradiation, but the light yield was so small that a fullenergy peak was not observed even with the 0.3% Ce-doped sample.

4.

Conclusions

Ce:YAM crystals were synthesized by the FZ technique and the optical properties werestudied for scintillator applications. For both PL and scintillation, a broad emission band
is observed over 400–600 nm owing to the 5d–4f transitions of the Ce3+ ion. The spectral
position is, in fact, shorter than that of Ce:YAG (540 nm) and longer than that of Ce:YAP (360nm). The peak position seems to slightly redshift with increasing concentration of Ce3+ owing
to self-absorption. Despite the similarity in the spectral features between PL and scintillation,
the decay time constants are notably different. The PL decay constants are ~10 nm and almost
independent of the concentration of Ce, whereas the scintillation decay times are much longer (~30
and 200–300 ns). In addition, the scintillation decay time slightly decreased with increasing
concentration of Ce, most likely due to the concentration quenching effect. Compared with
Ce:YAG (~120 ns), the scintillation decay time is much shorter but equivalent to that of Ce:YAP(~30 ns). The X-ray-induced afterglow level is on the order of 10−2, which is considerablylarger than those of commercial scintillators (ppm). A pulse height spectrum of α-rays (241Am)
using Ce:YAM shows a clear full-energy peak, but the light yield and crystalline quality are
insufficient for a clear peak to be observed under γ-rays. Further efforts must be made toenhance the scintillation properties before Ce:YAM crystals can be considered for a practicalscintillator.

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, Konica Minolta Science and Technology Foundation,
NAIST Foundation, and TEPCO Memorial Foundation are also acknowledged.

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