Nuclear Instruments and Methods in Physics Research B 377 (2016) 89–93

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Nuclear Instruments and Methods in Physics Research B
journal homepage: www.elsevier.com/locate/nimb

Evaluation of Ce:SrAl2O4 crystalline scintillators
Daisuke Nakauchi a,⇑, Go Okada a, Masanori Koshimizu b, Takayuki Yanagida aab

Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0192, Japan
Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba, Sendai 980-8579, Japana r t i c l e

i n f oArticle history:
Received 10 February 2016
Received in revised form 23 March 2016
Accepted 9 April 2016

Keywords:
Scintillation
Photoluminescence
Thermally stimulated luminescence
Optically stimulated luminescence
Strontium aluminate
CeriumSingle crystal

a b s t r a c t
In this study, Ce-doped SrAl2O4 (Ce:SrAl2O4) crystals were prepared by a floating zone (FZ) method andexperimental evaluations were performed for scintillator applications. In photoluminescence (PL) with an
excitation around 320 nm, an intense broad emission peak appears around 380 nm due to the 5d-4f transitions. The PL decay time constants are approximately 30 ns. The X-ray induced scintillation spectrum
also shows an intense broad emission peak around 380 nm. Among the samples evaluated with different
concentrations of Ce, the absolute light yield under 137Cs is considerably high (3900 ph/MeV) when theconcentration of Ce is 1.0%. In thermally stimulated luminescence (TSL) after X-ray irradiation, strong TSL
glow peaks are observed at 110, 210 and 330 °C. The samples also shows optically stimulated luminescence (OSL) after X-ray irradiation, in which the emission appears around 380 nm due to the 5d-4f emission bands of Ce3+ while the samples is exposed to the stimulation light at 655 nm.
Ó 2016 Published by Elsevier B.V.

1. Introduction
Scintillating materials convert incident ionizing radiation suchas high energy photons and particles to a large number of ultraviolet–visible photons immediately. Such scintillators play an important role for radiation detection applied in various fields, forexample medical imaging [1], security [2], astrophysics [3] andgeophysical and resources exploration, e.g. oil-dwelling [4]. Amongthese applications, especially scintillators for X-ray and c-ray
detectors have attracted considerable attentions since the market
size of medical and security fields are large. A conventional radiation detector using a scintillator consists of a scintillator (to convert radiation to light) and photodetectors (to convert light toelectronic signal). Such detection technique can be further classified into two types. One is based on a photon-counting (pulse
height) technique and the other is based on signal integration. Asa photodetector, photomultiplier tube (PMT) is a strong tool andoften used since it has a very fast response and high quantum efficiency in the UV-blue region. Therefore, a scintillator to be usedwith a PMT is preferred to show UV-blue light emission.
The recent trend of developing new scintillator is based on a
combination of insulator as a host and rare earth ion as emission
center. Among such phosphors, Ce3+-doped materials are one of
the most common scintillator due to the strong optical absorption⇑ Corresponding author.
E-mail address: nakauchi.daisuke.mv7@ms.naist.jp (D. Nakauchi).
http://dx.doi.org/10.1016/j.nimb.2016.04.017
0168-583X/Ó 2016 Published by Elsevier B.V.

and emission in the near ultraviolet–visible region due to the parity and spin allowed electron transitions. The emission due to the
5d-4f transitions of Ce3+ is characterized by a broad emission bandwith relatively a longer wavelength than 5d-4f transitions of otherrare earth elements (e.g., vacuum ultra violet (VUV) emission from
Nd3+ [5]) and fast decay time of several tens of nanoseconds. Such
fast scintillation is advantageous especially in positron emission
tomography (PET), which requires radiation detection with highcounting rate. Up to now, the most common Ce-doped scintillators
are rare earth silicate materials [6–9], and many efforts have been
paid to develop new Ce-doped scintillators.
To date, rare-earth doped SrAl2O4 have been widely studied in
the fields of long persistent phosphors [10–15], and scintillationfrom SrAl2O4 co-doped with Eu and Dy in the nano powder or sintered ceramic forms has been also reported [16–20]. However, no
studies have been reported for the scintillation properties of Cedoped SrAl2O4 as far as we know. In addition, for typical scintillator
uses, a bulk form is particularly important since the interaction
probability of ionizing radiations with materials simply depends
on the detector volume, so bulk crystalline scintillators are practically used except for some special applications.
In this study, we investigated the scintillation properties of
undoped and Ce-doped SrAl2O4 (Ce:SrAl2O4) crystals with different
Ce concentrations. In addition, we also study the dosimetric properties such as thermally stimulated luminescence (TSL) and optically stimulated luminescence (OSL) to comprehensively evaluate
radiation induced luminescence properties since scintillation and90

D. Nakauchi et al. / Nuclear Instruments and Methods in Physics Research B 377 (2016) 89–93

dosimeter properties are complementary related [21]. Also, some
defect centers are expected to be created when the Ce3+ ion substitutes the Sr2+ site as these valence states are different, and thesecenters may act charge trapping centers.

2. Experimental
Undoped and Ce-doped SrAl2O4 crystals were synthesized by afloating zone (FZ) method using the FZ furnace (FZD0192, Canon
Machinery Inc.). We used SrCO3 (4N), Al2O3 (4N), and CeO2 (4N)
powders as raw materials. The concentration of Ce varies as 0%,0.5%, 1.0% and 2.0%, and the Ce dopant is externally introduced
to the chemical composition of SrAl2O4. The starting compounds
were mixed first, and the mixture was shaped in a cylinder formby the hydrostatic pressure. Then, the cylinder was sintered at
1500 °C for 6 h to obtain a SrAl2O4 ceramic rod. The sintered rod
was loaded into the FZ furnace and the crystal growth was conducted with the translation rate of 5 mm/h and the rotation rateof 20 rpm. The heat source of the FZ furnace is halogen lamp. The
synthesized sample was characterized by the measurements
below.
The photoluminescence (PL) emission map and quantum yield(QY) were evaluated by using Quantaurus-QY (Hamamatsu Photonics). The PL decay time profile monitoring at 380 nm under
280 nm excitation was evaluated by using Quantaurus-s (Hamamatsu Photonics). The decay time was deduced by the least square
fitting of single exponential decay function. In addition, the excitation spectrum in near ultraviolet region was observed by using
FP8600 spectrofluorometer (JASCO) while the excitation spectrum
in the VUV range was measured at a synchrotron facility (UVSOR).
The range of measured excitation wavelength in the VUV region
was from 50 to 190 nm.
As a scintillation property, X-ray induced radioluminescence(RL) spectrum was measured by utilizing our original setup [22].
The irradiation source is an X-ray generator equipped with a tungsten anode target (XRB80P&N200X4550, Spellman). During themeasurements, the X-ray generator was supplied with the bias
voltage of 80 kV and tube current of 2.5 mA. While the sample
was irradiated by X-rays, the RL emission from the sample was
fed into the spectrometer through a 2 m optical fiber to measure
the spectrum. The spectrometer (Andor DU-420-BU2 CCD with
Shamrock SR163 monochromator) was cooled down to 188 K by
a Peltier module to reduce the thermal noise. Further, we havemeasured the scintillation decay time and the afterglow profiles
using a pulsed X-ray source equipped afterglow characterization
system [23]. In both PL and scintillation decay time profiles, we fitted the data only in the tail part assuming a single exponentialfunction. In the pulse height measurement, we mounted each crystal on the PMT (R877-100, Hamamatsu) with a silicon grease(6262A, OKEN), and the sample was covered by several layers of
Teflon reflectors to guide scintillation photons to the PMT. Here,the scintillation light is converted to electronical signal by the
PMT, which is then processed by the preamplifier (113, ORTEC),
shaping-amplifier (572, ORTEC) with 0.5 ls shaping time and multichannel analyzer (AMPTEK Pocket MCA). The pulse height spectrum is eventually stored on a computer.
In order to characterize relatively shallow trapping centers, thethermally stimulated luminescence (TSL) glow curve was measured by TL-2000 (Nanogray, Japan) with the heating rate of 1 °C/
s over the temperature range from 50 to 490 °C [24]. On the other
hand, for deeper trapping centers, optically stimulated luminescence (OSL) was also studied under 655 nm stimulation by using
FP8600 spectrofluorometer (JASCO). Except for the TSL, the sample
for all the other measurements mentioned above was at room
temperature.

3. Results and discussion
The synthesized undoped and Ce:SrAl2O4 crystals are shown in
Fig. 1. Although no reports can be found for bulk shape, we could
successfully obtain bulk crystalline samples with a typical sizesof 4–5 mm /  20–30 mm in length. Visibly they are white andtranslucent. Under UV lamp irradiation (254 nm), a bright blue
luminescence due to the 5d-4f transition of Ce3+ can be seen by
naked eye. We cut these crystalline rods to extract a relativelytransparent part with a typical size of 4–5 mm /  2 mm forcharacterizations.
The PL emission and excitation map of 1.0% Ce:SrAl2O4 crystal isrepresented in Fig. 2. Under excitation around 320 nm, an intense
broad emission around 380 nm is observed. This emission origin
is attributed to the 5d-4f transition of Ce3+ from the past study
in powder forms [13–15]. The QY of the 0.5%, 1.0% and 2.0% Ce:
SrAl2O4 crystals upon excitation at 330 nm are 13%, 39% and 14%,respectively. In the undoped sample, no emissions can be detected.
The 1.0% Ce-doped sample shows the best PL QY among the present
samples.
The PL decay time profiles of the Ce:SrAl2O4 crystals are shownin Fig. 3. The decay time constants of the samples monitoring at
380 nm emission band upon 280 nm excitation are approximately
30 ns. These are almost the same as the typical time constants of
5d-4f transition of Ce3+ [15], and no slow component is detectedin PL. In PL decay time profiles, no significant difference is observedin different Ce concentrations.
In order to investigate 5d1-5d5 levels of Ce3+, PL excitation spectra of the 1.0% Ce-doped sample monitoring at the 5d-4f emissionof Ce3+ are displayed in Fig. 4. The monitoring wavelength at
UVSOR was 400–410 nm which was the tail part of the emission(see Fig. 2) to avoid any influences of the diffraction light. Six excitation bands are observed, and the bands around 255, 280 and
325 nm are observed by former works [11,12]. These bands are
ascribed to the 4f-5d1-3 levels transitions of Ce3+. In the VUV range,
the excitation band around 180 nm would correspond to the transitions from between the top of valence band and bottom of conduction band since the band gap energy of SrAl2O4 is reported tobe 6.5 eV (190 nm) [12]. Therefore, the remaining two bands, 120
and 150 nm will be assigned to 4f-5d4-5 transitions. Therefore,
the excitation bands around 120, 150, 255, 280 and 325 nm would
be attributed to the 4f-5d1-5 transitions of Ce3+ (2F5/2 ? 2D).
The X-ray induced RL spectra are shown in Fig. 5. An intense
broad emission peak around 380 nm is observed as the same forPL, so the emission origin is ascribed to the 5d-4f transition of
Ce3+. Compared with other famous Ce-doped scintillators, emission
wavelength is similar to the scintillation from Ce-doped rare earth
silicate materials [6,7], and shorter than Ce-doped garnet scintillators [22,25]. In the undoped sample, broad emission bands around
500 nm appear. The emission origin of the undoped sample is discussed in elsewhere based on luminescence properties under VUVphoton excitations at Synchrotron facility, and we concluded theorigin as the perturbed self-trapped exciton (STE) [26]. Although
RL are qualitative evaluation since we cannot correct the stopping
power of each scintillator material, the scintillation intensity of theundoped sample was significantly weaker than Ce-doped samples.
By the Ce-doping, this STE based host emission is suppressed.
X-ray induced RL decay time profiles of the Ce:SrAl2O4 crystals
are represented in Fig. 6. The decay curves consist of one exponential function, and the decay time constants of the 0.5%, 1.0% and2.0% Ce-doped samples are 47.3, 48.0 and 51.7 ns, respectively.
These are slightly slower than those in PL due to the differenceof the energy migration processes of the PL and scintillation. In
PL, we observe the direct excitation and de-excitation of energy
levels of Ce3+ while the additional transportation process fromthe host to emission centers occurs in scintillation. In addition,

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91

Fig. 1. Appearances of partly cut Ce:SrAl2O4 crystals synthesized by the FZ method.

Fig. 2. PL emission and excitation map of the 1.0% Ce:SrAl2O4 crystal as arepresentative. The horizontal axis shows the emission and the vertical axis showsthe excitation wavelengths, respectively.

Fig. 4. PL excitation spectra of the 1.0% Ce:SrAl2O4 crystal monitoring at the 5d-4f
emission of Ce3+. The spectra in the VUV (dashed line) and near ultraviolet (solidline) were measured by Synchrotron facility (UVSOR) and common spectrofluorometer (FP8600, JASCO), respectively.

Fig. 5. X-ray induced RL spectra of the Ce:SrAl2O4 crystals.
Fig. 3. PL decay time profiles of the Ce:SrAl2O4 crystals.

some other mechanisms such as exciton [27] or a consecutive capture of electron-hole pairs [28] may affect the deceleration of thescintillation. As the Ce concentration increase, the decay time constants become faster, and it is a common tendency of the rare earth
doped scintillators. Ce:SrAl2O4 has almost the same decay time
constant compared to Ce-doped silicate materials [6,7], and the
decay time is fast enough for scintillator uses. Due to the low emis-

sivity, we cannot observe the decay time profile of the undoped
sample.
Pulse height spectra of the Ce-doped samples measured under137
Cs c-ray exposure are shown in Fig. 7. The 0.5% Ce:YAG transparent ceramic scintillator [25] was used as a reference which
had an absolute light yield of 20,000 ph/MeV calibrated by using
Si-APD and 55Fe source. Only the 1.0% Ce:SrAl2O4 crystal shows asignificantly large signal. Taking into account the quantum

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D. Nakauchi et al. / Nuclear Instruments and Methods in Physics Research B 377 (2016) 89–93

Fig. 6. X-ray induced RL decay time profiles of the Ce:SrAl2O4 crystals.

Fig. 7. Pulse height spectra of the Ce:SrAl2O4 crystals under 137Cs exposure. The
inset shows a reduced plot of the 1.0% Ce-doped sample.

efficiency of the PMT and the light yield of Ce:YAG scintillator as areference, the absolute light yield of the 1.0% Ce-doped SrAl2O4 isdeduced to be 3900 ph/MeV with a typical error of 10% if we
assume 200 ch corresponds to the full energy deposition. The
2.0% Ce-doped sample shows a smaller light yield, and the 0.5%
Ce-doped sample shows only a slightly higher signal than the background. The poor energy resolution would be due to the imperfection of the crystals containing significant amount of cracks. The
latter may be due to the charge imbalance of Ce3+ and Sr2+ in crystals. Compared with the Eu-doped SrAl2O4 crystals which exhibit
46,000 ph/MeV [29], scintillation light yield of Ce:SrAl2O4 islower. To our experience, we can detect 137Cs signal in the pulse
height from only a few% of newly developed crystals, and thisresult suggests that the matching of energy levels of Ce3+ emission
center and SrAl2O4 host is promising. In this point of view, studyinga Ba-substituted crystal, BaAl2O4, as a scintillator is an interesting
future assignment since typically alkaline earth aluminate materials show similar luminescence properties.
The afterglow curves of the Ce:SrAl2O4 crystals by an X-ray irradiation are represented in Fig. 8. Here, the X-ray pulse duration is
2 ms. The afterglow levels of the 0%, 0.5%, 1.0% and 2.0% Ce:SrAl2O4
crystals are 0.090%, 0.082%, 0.146% and 0.110%, respectively. The
0.5% Ce:SrAl2O4 crystal shows relatively low afterglow level among
the present samples. In these evaluations, after glow levels (A) aredefined as A (%) = 100  (I2 I0)/(I1 I0) where I0, I1 and I2 denotethe averaged signal intensity before the X-ray irradiation, the averaged signal intensity during X-ray irradiation and signal intensity
at t = 20 ms after the X-ray cut off, respectively. Although the evaluation manners of the afterglow differ by each manufacture, theFig. 8. Afterglow curves of the Ce:SrAl2O4 crystals.

latter method is the way used by NIHON KESSHO KOGAKU CO.,Ltd. which is one of the well-known manufactures of scintillation
detectors for medical and security applications. Compared withthe afterglow levels of other scintillators used in practice such as
CdWO4 and Bi4Ge3O12 [23], the afterglow levels of our samples
are higher.
Fig. 9 shows TSL glow curves of the samples after X-ray exposure of 10 Gy. In the 1.0% Ce-doped sample, TSL glow peaks areobserved around 110 and 330 °C. In addition to these peaks, TSL
glow peaks around 210 °C appears in the 2.0% Ce-doped sample.
These peaks were reported in past studies measured with a powder
form of SrAl2O4 co-doped with Eu-Ce [14] and Ce-Mn [30].
Although the emission origins were not clear in the past studies
since they only presented the data of co-doped materials [14,30],
the present work clarifies that the Ce-doping plays an importantrole to generate these glow peaks. In the undoped and 0.5% Cedoped samples, there are barely measurable emissions. It is suggested that the doping of Ce makes more defects in the SrAl2O4
possibly due to the charge imbalance of Sr2+ and Ce3+. These results
are roughly consistent with the afterglow levels (shown in Fig. 8)
since afterglow is a type of TSL at room temperature.
The OSL spectra of the Ce:SrAl2O4 crystals are illustrated inFig. 10. Here, the samples were irradiated with X-rays of 1.0 Gy.
While exposing the samples to the stimulation light at 655 nm,
emission bands by Ce3+ are observed at the wavelength around380 nm. The 1.0% Ce-doped sample shows the highest intensityof OSL among the samples, and the undoped sample does not show
the OSL signal. Throughout these studies, it is confirmed that Ce:
SrAl2O4 not only works as a scintillator but also works as TSL and

Fig. 9. TSL glow curves of the Ce:SrAl2O4 crystals after 10 Gy X-ray exposure.

D. Nakauchi et al. / Nuclear Instruments and Methods in Physics Research B 377 (2016) 89–93

Fig. 10. OSL spectra of the Ce:SrAl2O4 crystals under 655 nm stimulation.

OSL dosimeters. Therefore, this material system is an interesting
candidate to be used for radiation detector applications. It should
be pointed out here that for the purpose of dosimeter applications,the effective atomic number (Zeff) of detector materials should beclose to that of human issue (Zeff = 7.13). Therefore, Sr substituted
compositions such as MgAl2O4 and BeAl2O4 would be interesting toinvestigate in the future.
4. Conclusions
We investigated the optical, scintillation and dosimetric properties of the Ce:SrAl2O4 crystals prepared by the FZ method. The Ce:
SrAl2O4 crystals show PL and RL peaks due to the 5d-4f transitionof Ce3+, and the decay time constants are almost the same as the
typical emissions from Ce3+ doped phosphors. The QY of the 1.0%
Ce-doped sample is the highest among the present samples. Frompulse height spectra, the light yield of the 1.0% Ce-doped sample is3900 ph/MeV. Furthermore, Ce3+:Sr2Al2O4 also shows dosimeter
properties such as TSL and OSL. The presence of dosimeter properties indicates that the material has a potential to further improveas a scintillator as scintillation and dosimeter properties are understood to be complementally related.
Acknowledgements
This work was supported by a Grant in Aid for Scientific
Research (A)-26249147 from the Ministry of Education, Culture,
Sports, Science and Technology of the Japanese government(MEXT) and partially by JST A-step. Green Photonics Research from
MEXT is also acknowledged. It is also partially supported by the
Murata Science Foundation, KRF foundation and a cooperative
research project of the Research Institute of Electronics, Shizuoka
University.
References[1] T. Yanagida, A. Yoshikawa, Y. Yokota, K. Kamada, Y. Usuki, S. Yamamoto, M.
Miyake, M. Baba, K. Sasaki, M. Ito, Development of Pr: LuAG scintillator array
and assembly for positron emission mammography, IEEE Trans. Nucl. Sci. 57(2010) 1492–1495.
[2] D. Totsuka, T. Yanagida, K. Fukuda, N. Kawaguchi, Y. Fujimoto, Y. Yokota, A.
Yoshikawa, Performance test of Si PIN photodiode line scanner for thermal
neutron detection, Nucl. Instrum. Methods A 659 (2011) 399–402.

93[3] T. Ito, M. Kokubun, T. Takashima, T. Yanagida, S. Hirakuri, R. Miyawaki, H.
Takahashi, K. Makishima, T. Tanaka, K. Nakazawa, T. Takahashi, T. Honda,
Developments of a new 1-dimensional c-ray position sensor using scintillators
coupled to a Si strip detector, IEEE Trans. Nucl. Sci. 53 (2006) 2983–2990.
[4] C.L. Melcher, Scintillators for well logging applications, Nucl. Instrum. Methods
B 40–41 (1989) 1214–1218.
[5] T. Yanagida, K. Fukuda, N. Kawaguchi, S. Kurosawa, Y. Fujimoto, Y. Futami, Y.
Yokota, K. Taniue, H. Sekiya, H. Kubo, A. Yoshikawa, T. Tanimori, Scintillation
properties of Nd3+, Tm3+, and Er3+ doped LuF3 scintillators in the vacuum ultra
violet region, Nucl. Instrum. Methods A 659 (2011) 258–261.
[6] C.L. Melcher, J.S. Schweitzer, Cerium-doped lutetium oxyorthosilicate: a fast,
efficient new scintillator, IEEE Trans. Nucl. Sci. 39 (1992) 502–505.
[7] L. Pidol, A. Kahn-Harari, B. Viana, E. Virey, B. Ferrand, P. Dorenbos, J.T.M. deHaas, C.W.E. van Eijk, High efficiency of lutetium silicate scintillators, Cedoped LPS, and LYSO crystals, IEEE Trans. Nucl. Sci. 51 (2004) 1084–1087.
[8] M.A. Spurrier, P. Szupryczynski, K. Yang, A.A. Carey, C.L. Melcher, Effects of Ca2+
Co-doping on the scintillation properties of LSO:Ce, IEEE Trans. Nucl. Sci. 55(2008) 1178–1182.
[9] K. Takagi, T. Fukazawa, Cerium-activated Gd2SiO5 single crystal scintillator,
Appl. Phys. Lett. 42 (1983) 43–45.
[10] T. Matsuzawa, Y. Aoki, N. Takeuchi, Y. Murayama, A new long phosphorescentphosphor with high brightness, SrAl2O4: Eu2+, Dy3+, J. Electrochem. Soc. 143(1996) 2670–2673.
[11] Z. Fu, S. Zhou, S. Zhang, Study on optical properties of rare-earth ions in
nanocrystalline monoclinic SrAl2O4: Ln (Ln = Ce3+, Pr3+, Tb3+), J. Phys. Chem. B
109 (2005) 14396–14400.
[12] D. Jia, X.-J. Wang, W. Jiad, W.M. Yen, Trapping processes of 5d electrons in Ce3+
doped SrAl2O4, J. Lumin. 122–123 (2007) 311–314.
[13] B.Y. Geng, J.Z. Ma, F.M. Zhan, A solution chemistry approach for onedimensional needle-like SrAl2O4 nanostructures with Ln (Ce3+, Eu2+ andTb3+) as activator/dopant, J. Alloys Compd. 473 (2009) 530–533.
[14] X. Xu, Y. Wang, X. Yu, Y. Li, Y. Gong, Investigation of Ce–Mn energy transfer in
SrAl2O4:Ce3+, Mn2+, J. Am. Ceram. Soc. 94 (2011) 160–163.
[15] R. Zheng, L. Xu, W. Qin, J. Chen, B. Dong, L. Zhang, H. Song, Electrospinning
preparation and photoluminescence properties of SrAl2O4:Ce3+ nanowires, J.
Mater. Sci. 46 (2011) 7517–7524.
[16] P.J.R. Montes, M.E.G. Valerio, G.M. Azevedo, Radioluminescence and X-ray
excited optical luminescence of SrAl2O4: Eu nanopowders, Nucl. Instrum.
Methods B 266 (2008) 2923–2927.
[17] R. Meléndrez, O. Arellano-Tánori, M. Pedroza-Montero, W.M. Yen, M. BarbozaFlores, Temperature dependence of persistent luminescence in b-irradiated
SrAl2O4:Eu2+, Dy3+ phosphor, J. Lumin. 129 (2009) 679–685.
[18] P.J.R. Montes, M.E.G. Valerio, Radioluminescence properties of rare earths
doped SrAl2O4 nanopowders, J. Lumin. 130 (2010) 1525–1530.
[19] K. Korthout, K. Van den Eeckhout, J. Botterman, S. Nikitenko, D. Poelman, P.F.
Smet, Luminescence and x-ray absorption measurements of persistent
SrAl2O4:Eu, Dy powders: evidence for valence state changes, Phys. Rev. B 84
(2011) 085140.
[20] M. Ayvacıklı, A. Ege, N. Can, Radioluminescence of SrAl2O4:Ln3+ (Ln = Eu, Sm,Dy) phosphor ceramic, Opt. Mater. 34 (2011) 138–142.
[21] T. Yanagida, Y. Fujimoto, K. Watanabe, K. Fukuda, N. Kawaguchi, Y. Miyamoto,
H. Nanto, Scintillation and optical stimulated luminescence of Ce-doped CaF2,
Radiat. Meas. 71 (2014) 162–165.
[22] T. Yanagida, K. Kamada, Y. Fujimoto, H. Yagi, T. Yanagitani, Comparative studyof ceramic and single crystal Ce:GAGG scintillator, Opt. Mater. 35 (2013)
2480–2485.
[23] T. Yanagida, Y. Fujimoto, T. Ito, K. Uchiyama, K. Mori, Development of X-rayinduced afterglow characterization system, Appl. Phys. Exp. 7 (2014) 062401.
[24] T. Yanagida, Y. Fujimoto, N. Kawaguchi, S. Yanagida, Dosimeter properties of
AlN, J. Ceram. Soc. Jpn. 121 (2013) 988–991.
[25] T. Yanagida, H. Takahashi, T. Ito, D. Kasama, M. Kokubun, K. Makishima, T.
Yanagitani, H. Yagi, T. Shigeta, T. ITO, Evaluation of properties of YAG (Ce)ceramic scintillators, IEEE Trans. Nucl. Sci. 52 (2005) 1836–1841.
[26] D. Nakauchi, G. Okada, M. Koshimizu, T. Yanagida, Optical and scintillation
properties of Nd-doped SrAl2O4 crystals, J. Rare Earths (2016). in press.
[27] R. Visser, P. Dorenbos, C.W.E. van Eijk, A. Meijerink, G. Blasse, H.W. den Hartog,
Energy transfer processes involving different luminescence centres in BaF2:Ce,J. Phys. Condens. Mater. 5 (1993) 1659–1680.
[28] A.J. Wojtowicz, P. Szupryczynski, D. Wisniewski, J. Glodo, W. Drozdowski,
Electron traps and scintillation mechanism in LuAlO3:Ce, J. Phys. Condens.
Mater. 13 (2001) 9599.
[29] D. Nakauchi, G. Okada, M. Koshimizu, T. Yanagida, Storage luminescence andscintillation properties of Eu-doped SrAl2O4 crystals, J. Lumin. (2016), http://dx.doi.org/10.1016/j.jlumin.2016.04.008. in press.
[30] T. Katsumata, S. Toyomane, R. Sakai, S. Komuro, T. Morikawa, Trap levels in Eudoped SrAl2O4 phosphor crystals co-doped with rare-earth elements, J. Am.
Ceram. Soc. 89 (2006) 932–936.