Optical Materials 36 (2014) 2016–2019

Contents lists available at ScienceDirect

Optical Materials
journal homepage: www.elsevier.com/locate/optmat

Positive hysteresis of Ce-doped GAGG scintillator
Takayuki Yanagida a,⇑, Yutaka Fujimoto a, Masanori Koshimizu b, Kenichi Watanabe c, Hiroki Sato d,
Hideki Yagi e, Takagimi Yanagitani eaKyushu Institute of Technology, 2-4 Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japan
Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japanc
Nagoya University, Furocho, Chikusa, Nagoya 464-8603, Japand
Furukawa 1-25-13, Kannondai, Tsukuba, Ibaraki 305-0856, Japane
Konoshima Chemical Co., Ltd., 80 Kouda, Takuma, Mitoyo, Kagawa 769-1103, Japanba r t i c l e

i n f oArticle history:
Available online 17 January 2014
Keywords:
Single crystal
Ceramic
Scintillation detector
Ce3+
Pr3+
Scintillatora b s t r a c t
Positive hysteresis and radiation tolerance to high-dose radiation exposure were investigated for Ce 1%
and 3% doped Gd3(Al, Ga)5O12 (Ce:GAGG) crystal scintillator on comparison with other garnet scintillators such Ce:YAG, Ce:LuAG, Pr:LuAG, and ceramic Ce:GAGG. When they were irradiated by several Gy
60
Co c-rays, Ce 1% doped GAGG crystal exhibited 20% light yield enhancement (positive hysteresis). Thisis the ﬁrst time to observe positive hysteresis in Ce doped GAGG. On the other hand, other garnet materials did not show the positive hysteresis and their light yields were stable after 800 Gy irradiation except
Pr:LuAG. The light yield of Pr:LuAG decreased largely. When irradiated Ce:GAGG which showed positive
hysteresis was evaluated in Synchrotron facility (UVSOR), new excitation band was created around
60 nm.
Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction
Inorganic scintillators which convert high energy ionizing radiation to thousands of UV–visible photons [1], have been playing amajor role in many ﬁelds of radiation detection, including medical
imaging [2], security [3], astrophysics [4], particle physics [5], andwell-logging [6]. In these applications, scintillators for c-ray detectors have attracted much attention because of the large volume ofmarket especially in medical and security applications. Radiation
tolerance is one of the important properties for scintillators andmany materials were examined in this property. In radiation tolerance experiments, one interesting phenomenon, positive hysteresis was discovered. Positive hysteresis is a phenomenon in which
the light yield of the scintillator is increased after exposure to ionizing radiation [7]. At present, Ce-doped Gd2SiO5 (Ce:GSO) [8] andTl-doped CsI (Tl:CsI) [9] showed positive hysteresis. The origin of
positive hysteresis is still under discussion, but there are someknown experimental results that positive hysteresis is a temporary
phenomenon and continues few days depending on the irradiateddose.
The aim of this work is to examine radiation tolerance andpositive hysteresis of famous garnet scintillators, including crystal
Ce 1% and 3% doped Gd3(Al, Ga)5O12 (Ce:GAGG) [10], Ce 0.5% doped⇑ Corresponding author. Present address: Kyushu Institute of Technology, 2-4
Hibikino, Wakamatsu-ku, Kitakyushu 808-0196, Japan. Tel./fax: +81 93 695 6049.
E-mail address: yanagida@lsse.kyutech.ac.jp (T. Yanagida).
0925-3467/$ - see front matter Ó 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.optmat.2013.12.043

Y3Al5O12 ceramic (Ce:YAG) [11], Ce 0.5% doped Lu3Al5O12(Ce:LuAG) ceramic [12], Pr 0.25% doped LuAG (Pr:LuAG) [13], andceramic Ce 1% doped GAGG [14]. Crystalline scintillators were
fabricated by the conventional Czochralski method in Furukawa
and ceramic ones were prepared by the vacuum sintering in
Konoshima Chemical, respectively. Among above luminescentmaterials, crystal Ce:GAGG and Pr:LuAG by Furukawa arecommercially available for scintillator uses, and ceramic Ce:YAG
by Konoshima is a product for general phosphor applications.
Fig. 1 shows appearances of samples for present study. Most samples had thin plate shape while Pr:LuAG had rectangular shapebecause it was fabricated for Positron Emission Mammography [2].

2. Experimental proceduresFirst, optical in-line transmittance was evaluated by using V670
spectrometer (JASCO) before 60Co c-ray evaluation since it was ageneral way to evaluate radiation tolerance on a degradation oftransmittance. Photoluminescence (PL) quantum yield (QY) was
also evaluated using a Hamamatsu Quantaurus-QY. The absolute
PL QY was calculated by the following equation, QY = Nemit/Nabsorb,
where Nemit and Nabsorb are the numbers of emission and absorption photons, respectively [15]. These properties were also evaluated after 800 Gy 60Co c-ray exposure. Because our laboratory
and the irradiation facility were not so close, these basic opticalproperties were evaluated 6, 24, 48, and 72 h after the irradiation.

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T. Yanagida et al. / Optical Materials 36 (2014) 2016–2019

3. Results and discussion
3.1. Basic scintillation characteristics before the irradiation
Fig. 2 represents normalized X-ray excited radioluminescence(scintillation) spectra of present samples. Around 520 nm emissionin Ce-doped materials was ascribed to Ce3+ 5d-4f transition and
300 nm emission in Pr:LuAG was due to Pr3+ 5d–4f transition,
respectively. In ceramic Ce:GAGG emission around 380 nm was arized from impurity (perovskite) phase as discussed previously [14].
Since crystal ﬁeld affects 5d levels of materials, emission peaks in
Ce-doped materials differed. Then, scintillation decay time proﬁlesof garnet scintilaltors are demonstrated in Fig. 3. All Ce-doped scintillators exhibited 100 ns primary decay time while that of
Pr:LuAG was 20 ns. These values were typical for Ce3+ and Pr3+
5d–4f transition. Throughout these evaluations, our samples were
not speciﬁc materials in scintillation characteristics.

Ce 1% GAGG (Furukawa)
Ce 3% GAGG (Furukawa)
Ce 1% GAGG (Konoshima)

Ce 0.5% YAG (Konoshima)
Ce 0.5% LuAG (Konoshima)

Pr 0.25% LuAG (Furukawa)

Fig. 1. Appearance of sample scintillators.

Pr:LuAG was studied and transmittance of Pr:LuAG degraded largely in high dose irradiation [19]. In several hundreds Gy exposure
by the present work, no transmittance degradation occurred. Compared with Ce:GSO studied recently which showed transmittancedegradation [8], garnet scintillators had high resistivity to radiation exposure in optical transmittance.
Fig. 4 demonstrates pulse height spectra of Ce 1% doped GAGG
from 0 to 20 Gy exposure and Pr:LuAG from 0 to 600 Gy. After
800 Gy irradiation, we could not detect signal from Pr:LuAG due
to a degradation of light yield and a huge phosphorescence in lower MCA channels. As described later, other samples were stable inpulse height spectra. Ce 1% doped GAGG showed positive hysteresis and the light yield was enhanced 20%. This tendency did notchange to 800 Gy irradiation in this sample. We investigated 1–3
and 20 Gy exposure so that the threshold of positive hysteresis in
Ce:GAGG was around 10 Gy. It was the ﬁrst time to observe positive hysteresis in Ce:GAGG. On the other hand, Pr:LuAG exhibiteda huge degradation of light yield and it was consistent with recent
result that Pr:LuAG was not so resistive to high dose exposure [19].
Though it was not a main object of this project, the reason of worse
energy resolution of Pr:LuAG before the irradiation was blamed forthe optical attachment. To measure all samples quickly, we attached the wide surface of the sample to PMT and light collection
efﬁciency was not so good in this experiment. Fig. 5 summarizes
normalized scintillation light yield in pulse height after irradiation.
In this ﬁgure, light yield (662 keV photoabsorption peak channel)
before the irradiation was deﬁned to 1. Except Ce 1% doped GAGG
and Pr:LuAG, scintillation light yield was stable. In Ce-doped GAGG
samples, low Ce concentration sample exhibited positive hysteresis and the tendency was same with the case of Ce:GSO [8]. Taking

1.2
Ce1% GAGG
Ce3% GAGG
Ceramic GAGG
Ce:YAG
Ce:LuAG
Pr:LuAG1

Intensity (a. u.)Then, basic scintillation properties such as X-ray-induced radioluminescence spectra and scintillation decay time proﬁles were
evaluated at the Kyushu Institute of Technology to conﬁrm that
our materials were not speciﬁc ones. X-ray-induced emission spectra were measured using a DU420A-BU2 Charge Coupled Device(CCD) camera equipped with an SR163 monochromator, made byAndor Technology. The CCD was cooled to 188 K using a Peltier
module. The excitation source was an X-ray generator suppliedwith 80 kV and 1 mA. The scintillation emission was fed to themonochromator through a 2 m optical ﬁber to avoid X-rays hitting
the CCD directly. A detailed explanation of the radioluminescence
setup was previously published [16]. Scintillation decay time proﬁles were evaluated using our original setup, a pulse X-ray streak
camera system also described previously [17,18].
In the irradiation facility of Nagoya Univ., 800 km distant fromthe Kyushu Institute of Technology, 60Co was used to irradiate
samples in steps of 200 Gy from 0 to 800 Gy. At each step, we evaluated the pulse height spectrum using a 137Cs source. As describedlater, Ce 1% doped GAGG crystal exhibited the positive hysteresis,
and we irradiated smaller dose (1–20 Gy) to this sample to determine the threshold of the phenomenon. In pulse height measurements, we mounted each crystal on the PMT (R7600-200,
Hamamatsu) with silicon grease (OKEN 6262A). When c-rays from137
Cs were detected, the signals were fed into a preampliﬁer(ORTEC 113), a shaping-ampliﬁer (ORTEC 572) with 2 ls shaping
time, a multichannel analyzer (hereafter MCA, Kromek K102),
and ﬁnally to a personal computer. The time interval between
the 60Co exposure and pulse height spectrum measurement was
typically a few minutes, the time required to walk from the irradiation room to the measurement room in the same building. All
experiments were carried out at room temperature.
Finally, excitation bands in higher energy range (50–200 nm) of
Ce 1% doped GAGG was evaluated in Synchrotron facility (UVSOR).
In this experiment, we prepared two samples and one of them wasirradiated with 2000 Gy at Nagoya Univ. After one of our colleagues took the sample from Nagoya Univ. to UVSOR, we compared excitation bands of irradiated and non-irradiated Ce 1%
doped GAGG samples. The time interval of transportation from Nagoya Univ. to UVSOR was around 1.5 h. In UVSOR, we used BL-7B
beam line and the measurements were carried out at room
temperature.

0.8
0.6
0.4
0.2

3.2. Radiation tolerance
0First, transmittance spectra of samples in this work were investigated. Consequently, no signiﬁcant differences were observed
after the 60Co c-ray irradiation. Recently, radiation damage of

200

300

400

500

600

700

Wavelength (nm)Fig. 2. X-ray induced radioluminescence spectra of sample scintillaors.

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T. Yanagida et al. / Optical Materials 36 (2014) 2016–2019

1.4
5

Normarized pulse height

10

4

Intensity (a. u.)

10

Pr:LuAG

1000

Ce:LuAG

Ce:YAG

100

Ce3% GAGG

10increase

1
0.8

0 Gy
200 Gy400 Gy600 Gy800 Gy

0.6
0.4
0.2

Ceramic GAGG

decrease

Ce1:GAGG ceramic GAGG

0

1
0.1

1.2

Ce3:GAGG

Ce1% GAGG

Ce:LuAG

Ce:YAG

Pr:LuAG

Fig. 5. Normalized scintillation light yield after 0–800 Gy X-ray exposure.

0

0.2

0.4

0.6

0.8

1

Time (µ s)100into account the segregation coefﬁcient of crystal Ce:GAGG [10],actual doping concentrations of Ce 1% and 3% doped crystals were0.45% and 1.2%, respectively. As the ceramic is free from the segregation in macroscopic scale, actual Ce concentrations were Ce 1%
crystal < Ce 1% ceramic < Ce 3% crystal. It must be noted that
ceramic Ce:GAGG had higher Ce concentration than Ce 1% doped
crystal because it was fabricated by the solid state reaction andfree from the segregation which was not avoidable in conventional
melt growth.
In scintillation, luminescence efﬁciency were simply divided to
two kinds; one is a transfer from host to emission centers (mainly
outside of the band gap) and efﬁciency at luminescence center(inside the band gap). Simple formula, ne–h = BEc/bEg, was raised[20–23]. In this formula, Ec is the energy deposited by c-quanta,
The factor B relates the phonon energy loss during the ionizationand is expressed as B = 1/(1 + 0.65K), where K is deﬁned as the ratioof energy lost by optical phonons to the energy lost for an ionization event [24]. Thus, the light yield (LY) is simply expressed as
LY = 106SQB/bEg [ph/MeV], where S is the energy transport
efﬁciency from the host to the emission center, Q is the luminescence quantum efﬁciency at the emission center which equals to
QY in this work, and b is an empirical constant close to 2.5 infamous scintillators and is theoretically 2.3. Thus to understandthe phenomenon, separation of B, S, and Q is important. Fig. 6
shows QY of present samples before (0 h) and after (6, 24, 48,
72 h) irradiation. Ce:LuAG exhibited an interesting change thatthe degradation and recovery. In Pr:LuAG, QY decreased from 92%
to 40% and it did not recover at least within 72 h. Therefore,
90% light yield degradation of Pr:LuAG was ascribed to the degradation of both QY and BS. On the other hand, other Ce-doped garnet
scintillators did not show signiﬁcant changes and it meant that the

PL Quantum yield (%)

Fig. 3. Scintillation time proﬁles of garnet scintillators under pulse X-ray excitation.

70
before irradiation
6h
24h
48h
72h

60
50

30
Fig. 6. PL QY after 800 Gy X-ray exposure. Measurements were done 6, 24, 48, 72 hlater ﬁnishing the irradiation.

positive hysteresis of Ce 1% doped GAGG was mainly caused by thechange of BS.
Fig. 7 represents excitation spectra monitoring at 500–650 nm
integrated counts of irradiated and non-irradiated Ce 1% doped
GAGG. Around 200 nm, 160, and 80 nm peaks were detected inboth samples. The 200 nm band corresponded to band gap of
GAGG host and 160 nm 80 nm bands would be due to 5d4 and5d5 levels of Ce3+ because 5d1–3 bands appeared around 460, 340,
and 230 nm in Ce-doped garnet materials. In the irradiated sample,
new excitation band around 60 nm was created. 60 nm is close to
2.5  Eg (see above formula) which corresponded to typical average energy to create one electron–hole pair in materials under highenergy ionizing radiation exposure and such energy would havesome physical meanings.
Though we did not conclude this newly created band fully or
partially affected positive hysteresis by presented data sets, itwould relate the phenomenon. Based on the meta-stable
1x104

0 Gy
1 Gy
2 Gy
3 Gy
20 Gy

100

increase

10

0

100

0 Gy
200 Gy400 Gy600 Gy1000

Counts/channel

Counts/channel

80

40

1000

1

90

200

300

MCA channel

400

500

decrease

100

10

1

0

50

100

150

200

MCA channel

Fig. 4. 137Cs pulse height spectra of Ce 1% doped GAGG crystal (left) and Pr:LuAG (right) after several dose irradiation.

T. Yanagida et al. / Optical Materials 36 (2014) 2016–2019

doped GAGG crystal exhibited positive hysteresis and the lightyield was enhanced 20%. This is the ﬁrst time to observe positive
hysteresis in Ce doped GAGG. The radiation resistivity of Pr:LuAG
was not so high compared with other oxide scintillators. Other
materials, Ce 3% doped GAGG, Ce:YAG, Ce:LuAG, and ceramic
Ce:GAGG was highly resistive to ionizing radiation bombardment.

15

Intensity (a. u.)

IrradiatedNon-irradiated10

Acknowledgments

5

0
50

2019

100

150

200

Wavelength (nm)Fig. 7. Excitation spectra of irradiated (dotted line) and non-irradiated (solid line)
Ce 1% doped GAGG crystal. The monitoring wavelength was integrated from 500 to
650 nm.

characteristics of the positive hysteresis, it is reasonable to consider that irradiation-induced meta-stable centers are involved in
the scintillation process after irradiation. In the previous paragraph, we mentioned that the positive hysteresis is due to thechange in BS. In the following, we propose two possible mechanisms for the change in B or S and the appearance of the excitation
band at 60 nm.
The change in B would be caused by the change in the electronic
band structure due to the creation of the meta-stable centers. Thechange in the band structure may lead to the change in the fractionof the energy loss via phonon emission, because this fraction is related to the electronic energy structure [25]. In this case, the positive hysteresis is explained in terms of the decrease in the energy
loss fraction. In this scenario, the appearance of the 60 nm excitation band is explained as follows: Because the photon energy at
60 nm is close to the average energy for electron–hole pair creation, the number of electron–hole pairs created by a single VUVphoton is close to unity down to 60 nm, and electron–hole pairmultiplication sets in for shorter wavelength. If we assume the decrease in the energy loss fraction, it is reasonable to consider thatthe probability of the multiple electron–hole pairs would be enhanced at 60 nm. Thus, the positive hysteresis and the appearanceof the excitation band at 60 nm can be explained in a consistentmanner.
On the other hand, a scenario for the change in S is the change inthe energy transfer process. In this scenario, the meta-stable centers are assumed to promote the energy transfer to Ce3+. Considering the appearance of the excitation band at 60 nm, this promotion
is effective particularly for the electron–hole pairs having a relatively higher energy, because the photon energy at 60 nm is significantly higher than the bandgap energy of GAGG. One possible
explanation in this line is that the meta-stable centers change
the relaxation path of such high-energy electron–hole pairs, whichleads to some relaxed states preferable to the energy transfer to
Ce3+.
Most probably oxygen vacancy was created by c-ray irradiationand such vacancies temporary created meta-stable level beyond
the band gap of the material. If this high energy excitation bandis profoundly related to the origin of the positive hysteresis, it
has a big merit for understanding and engineering scintillation
light yield of materials through elaboration of the high-energy
electronic band structure.
4. Conclusion
Crystal Ce 1% and 3% doped GAGG, Pr:LuAG, ceramic Ce:YAG,
Ce:LuAG, and Ce:GAGG were investigated on their basic scintillation characteristics and radiation tolerance. Among them, Ce 1%

This work was mainly supported by JST Sentan, A-step and partially by a Grant in Aid for Young Scientists (A)-23686135, andChallenging Exploratory Research-23656584 from the Ministry of
Education, Culture, Sports, Science and Technology of the Japanese
government (MEXT). Partial assistance from the Yazaki Memorial
Foundation for Science and Technology, Shimazu Sci. Foundation,
Kato Foundation for Promotion of Science, and Nippon Sheet GlassFoundation for Materials Science and Engineering, Tokuyama
Science foundation, Iketani Science and Technology Foundation,
Hitachi Metals Materials Science Foundation, Mazda Foundation,
JFE 21st century Foundation, and The Asahi Glass Foundation, arealso gratefully acknowledged.
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