JOURNAL OF RARE EARTHS, Vol. 34, No. 8, Aug. 2016, P. 757

Optical and scintillation properties of Nd-doped SrAl2O4 crystals
Daisuke Nakauchi1,*, Go Okada1, Masanori Koshimizu2, Takayuki Yanagida1(1. Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST) 8916-5 Takayama-cho, Ikoma-shi, Nara 630-0192, Japan; 2. Department of Applied Chemistry, Graduate School of Engineering, Tohoku University 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan)
Received 15 December 2015; revised 11 March 2015

Abstract: Nd-doped SrAl2O4 (Nd:SrAl2O4) crystals were prepared by a floating zone (FZ) method with different dopant concentrations. The photoluminescence (PL) measurements under visible light excitations confirmed strong near infrared (NIR) emissions
around 890 nm, which is due to the 4f-4f transitions of Nd3+, and the decay time constants were 300–400 μs. Furthermore, VUV excitations revealed two broad emissions around 250–450 and 500–700 nm, which were attributed to self-trapped excitons (STEs) perturbed by Nd3+. Moreover, the X-ray induced scintillation spectra showed some small peaks at around 380 nm in addition to NIR
emissions at 1064 nm similarly seen in PL. The former scintillation decay time constants were 30–40 μs. In the undoped sample,
these emissions mentioned above were not present but a weak broad emission around 450 nm appeared. In thermally stimulated luminescence (TSL) after X-ray irradiation, strong TSL glow peaks were observed in the Nd-doped samples at around 100 and 250 ºC.
Keywords: scintillation; photoluminescence; storage luminescence; single crystal; rare earths

Inorganic scintillators convert ionizing radiation to
thousands of low energy photons (1–6 eV) immediately,
so they have been playing an important role in various
fields of radiation detections including medical imaging[1], security[2], astrophysics[3], and geophysical and resources exploration, e.g. oil-dwelling[4]. In these applications, market volumes of medical and security fields areespecially large, so scintillators for particular X-ray andγ-ray detectors have been intensively studied. A conventional scintillation detector consists of a scintillator,
which emits scintillation photons, and a photodetector,
which converts the incident photons to electrons. Generally, visible light emitting scintillators are preferable because the most conventional photodetector, PMT, has ahigh sensitivity around the blue wavelength. In addition
to such a conventional visible emitting scintillator, vacuum ultra violet (VUV), UV and near infrared (NIR)
scintillators are also attractive for various applications[5,6].
To achieve such a wavelength functionality, rare-earth
ions are generally doped as an emission center in bulk
insulator or semiconductor materials.
In this study, we investigated the scintillation properties of Nd-doped SrAl2O4 (Nd:SrAl2O4) crystals. In addition, we evaluated the dosimetric properties in order tocomprehensively understand the material system.
Rare-earth-doped SrAl2O4 have been widely studied in
the fields of phosphorescent phosphors[7], and scintilla-tion from Eu and Dy doped SrAl2O4 in the nano powder
or sintered ceramic forms has also been reported[8–12].
Although most scintillation detectors utilize single crystals mainly due to their high detection efficiency andhigh optical quality, most reported studies of SrAl2O4
materials are in the powder form. For scintillator uses, abulk form is particularly important since the interaction
probability of ionizing radiations with materials simply
depends on the detector volume, so except for some specific detectors, most practical detectors use bulk crystalline scintillators. While Nd-doped materials have been
regarded as one of the most popular NIR phosphors inlaser systems[13], it is also of our great interest as a newgeneration NIR emitting scintillator. Nd is known toshow NIR emission due to the 4f-4f transitions, which
has great transparency to biological tissue[14], so it has aconsiderable potential for bio-imaging applications[6]. It
is worth mentioning here that scintillation detectors areusually classified into photon-counting and integration
types as commonly utilized in positron emission tomography (PET) and X-ray computed tomography (X-ray
CT), respectively. Unlike photon-counting based detection, the integration type does not strictly require a fast
scintillator and lifetime of such typical scintillators is on
an order of micro-seconds. Up to now, our group also
investigated some Nd-doped materials[15,16], and most ofthem showed detectable scintillation and some materials

Foundation item: Project supported by the Aid for Scientific Research ((A)-26249147) from the Ministry of Education, Culture, Sports, Science andTechnology of the Japanese government (MEXT), JST A-step, Green Photonics Research from MEXT, the Adaptable and Seamless
Technology Transfer Program (A-STEP) by the Japan Science and Technology (JST) Agency, the Murata Science Foundation, and acooperative research project of the Research Institute of Electronics, Shizuoka University* Corresponding author: Daisuke Nakauchi (E-mail: nakauchi.daisuke.mv7@ms.naist.jp; Tel.: +810743726144)
DOI: 10.1016/S1002-0721(16)60090-X

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were found to be usable even in the photon-counting
mode. As far as we are aware, no studies have been reported for the scintillation properties of Nd-doped
SrAl2O4.

with Shamrock SR163 monochromator) was cooled
down to 188 K by a Peltier module to reduce the thermal
noise. Furthermore, we have measured the scintillation
decay time and afterglow profiles using a pulsed X-ray
source equipped afterglow characterization system[18]. In
both PL and scintillation decay time profiles, we fitted
the data only in the tail part assuming a single exponential function because we cannot evaluate an instrumental
response. Here, the afterglow level (A) is represented by
A (%)=100×(I2–I0)/(I1–I0), where I0, I1 and I2 denote theaveraged signal intensity before the X-ray irradiation, theaveraged signal intensity during X-ray irradiation andsignal intensity at t=20 ms after the X-ray cut off, respectively. Although the evaluation manners of the afterglow
differ in each company, we referred the formula from themanner of NIHON KESSHO KOGAKU CO., LTD.
which is one of the famous companies for integrated-type
scintillation detectors.
In order to characterize relatively shallow trapping
centers, the thermally stimulated luminescence (TSL)
glow curves were measured by TL-2000 (Nanogray, Japan) with the heating rate of 1 ºC/s over the temperature
range from 50 to 490 ºC[19]. Except for the TSL, the sample for all the other measurements mentioned above wasat room temperature.

1 Experimental
Nd-doped SrAl2O4 crystals were synthesized by afloating zone (FZ) method using an FZ furnace(FZD0192, Canon Machinery Inc.). We used SrCO3(99.99%, 1.0 equiv.), Al2O3 (99.999%, 1.0 equiv.) andNd2O3 (99.99%, x/2 mol.%, x=0, 0.5, 1.0 and 2.0) powders as raw materials. Hereafter, the molar ratio of Ndions with respect to the SrAl2O4 compound are used to
identify the samples. Here, the Nd ion is expected to substitute the Sr site. These starting compounds were mixed
first, and the mixture was shaped to the cylinder by thehydrostatic pressure. Then the cylinder was sintered at
1500 °C for 6 h. The sintered rod was put into the FZ
furnace and the crystal growth was conducted with thepulling rate of 5 mm/h and the rotation rate of 20 r/min.
The heat radiation source of the FZ furnace is halogen
lamp. The synthesized sample was characterized by themeasurements below.
To identify the obtained crystalline phase, powder
X-ray diffraction (XRD) pattern was measured with adiffractometer (MiniFlex600, Rigaku) over the 2θ rangeof 5º–70º. The X-ray source is a conventional X-ray tube
with a CuKα target operated at 40 kV and 15 mA. The
photoluminescence (PL) and PL excitiation (PLE) spectra were measured by using a spectrofluorometer(FP8600, JASCO), and PL quantum yield (QY) were
evaluated by using Quantaurus-QY (Hamamatsu
Photonics). The PL decay time profile monitoring at 890
nm under 575–625 nm excitation was evaluated by using
Quantaurus-τ (Hamamatsu Photonics). The excitation
source was a white light source, and the band pass filter(11K0350, OptoSigma) was used to select excitation
photons. The decay time was deduced by the single exponential fitting. In addition, the emission spectra under
VUV photon excitations were measured at the synchrotron facility (UVSOR). The tested excitation wavelength
was from 50 to 200 nm.
As a scintillation property, X-ray induced radioluminescence (RL) spectra were measured by utilizing ouroriginal setup[17]. The irradiation source was X-ray generator equipped with a tungsten anode target(XRB80P&N200X4550, Spellman). During the measurements, the X-ray generator was supplied with thevoltage of 80 kV and tube current of 2.5 mA. While thesample was irradiated by X-rays, the scintillation emissions from the sample were fed into the spectrometer
through a 2 m optical fiber to measure the scintillation
spectrum. The spectrometer (Andor DU-420-BU2 CCD
for visible and Andor DU492A-1.7 for NIR wavelengths

2 Results and discussion
The synthesized undoped and Nd:SrAl2O4 crystals areshown in Fig. 1. The samples doped with 0–1.0 at.% Nd
look white and translucent, and the 2.0%Nd-doped sample looks colorless and transparent. Since the 2.0%Nddoped sample has the strongest NIR absorption due tothe 4f-4f transitions of Nd3+ where the halogen lamp
equipped with the FZ furnace has the peak emission
wavelength, the crystal growth was stably performed
without much creation of cracks and defects. We broke

Fig. 1 Appearances of the synthesized Nd:SrAl2O4 crystals

Daisuke Nakauchi et al., Optical and scintillation properties of Nd-doped SrAl2O4 crystals

these crystalline rods to get a few cubic millimeter samples for characterizations.
Fig. 2 shows XRD patterns of the Nd:SrAl2O4 crystals
and the comparison with Crystallography Open Database(COD) information card for 2002284. The XRD patterns
are similar for all the samples, and the patterns demonstrate diffraction peaks corresponding to the expected
SrAl2O4 single phase. These materials belong to P21
space group of the monoclinic crystal system. No impurity phases are observed in whole samples.
The PL and PLE spectra of the 0.5%Nd:SrAl2O4 crystal are represented in Fig. 3. Some emissions are observed in the 850–950 nm range upon excitations by several different wavelengths. These emissions are due tothe 4f-4f transition of Nd3+ (4F3/2→4I9/2)[20]. Under excitation at 585 nm, the QY of the 0.5%, 1.0% and 2.0%
Nd:SrAl2O4 crystals are 29%, 19% and 8.5%, respectively. In the undoped sample, no emissions can be detected. The detection limit of our PL instrument was1000 nm so we could not observe the most famous Nd3+
emission at around 1064 nm.
The PL decay time profiles of the Nd:SrAl2O4 crystals
are represented in Fig. 4. Under an excitation at 575–625 nm

759

Fig. 4 PL decay time profiles of the Nd:SrAl2O4 crystals

Fig. 2 XRD patterns of the Nd:SrAl2O4 crystals and the comparison with COD information card for 2002284

and monitoring for 4F3/2→4I9/2 (around 900 nm) transitionof Nd3+, the PL decay curves are composed of two components. The faster component around the rise part is ascribed to the instrumental response, and the slower component is due to Nd3+ emission. The time constants of the
0.5%, 1.0% and 2.0%Nd:SrAl2O4 crystals are 390, 389
and 372 μs, respectively. These are almost the same as a
typical decay time constant of Nd3+ [21]. By dense
Nd-doping, PL decay time became faster and it was a
typical behavior of rare-earth-doped phosphors.
The PL emission spectra of the undoped and 0.5%Nddoped SrAl2O4 under VUV photon excitations at
UVSOR are represented in Fig. 5. When the Nd-doped
sample was excited by 160–180 nm photons, two broad
peaks at around 250–450 and 500–700 nm were observed. These peaks cannot be seen in the undoped sample. The band-gap energy of the SrAl2O4 is reported to be
6.5 eV[22], which corresponds to the energy of a photonwith the wavelength of ~190 nm. The excitation wavelength region at which these bands were observed corresponds to the interband excitation. In addition, thesebands were not observed with photo-excitation within the
band-gap. Hence, electronic transitions within the
band-gap at defects or impurities are excluded as the origin. A plausible origin of these bands is self-trapped excitons (STEs) perturbed by Nd3+ ions.

Fig. 3 PL and PLE spectra of the 0.5%Nd:SrAl2O4 crystal

Fig. 5 PL spectra of the undoped and 0.5%Nd-doped SrAl2O4
crystals under VUV photon excitations

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Fig. 6 X-ray induced scintillation spectra of the Nd:SrAl2O4 crystals in UV-Vis (a) and NIR (b) wavelengths

The X-ray induced scintillation spectra are shown in
Fig. 6. In UV-Vis range, a sharp emission peak at 380 nm
and several emission bands around 450 nm due to 4f-4ftransitions of Nd3+ (2F(2)5/2→4F5/2, 2F(2)5/2→4F9/2,
2
F(2)5/2→4G5/2, 2F(2)5/2→4G7/2 and 2F(2)5/2→4G9/2)[23] areobserved in Nd-doped samples. Unlike PL, Nd3+ emissions in UV-Vis range are detected clearly. In the NIR
wavelength, emission peaks are observed at 900, 1064
and 1320 nm. These peaks are attributed to the typicalemissions by Nd3+ due to the 4f-4f transitions (4F3/2→4I9/2,
4
F3/2→4I11/2 and 4F3/2→4I13/2)[15]. In the undoped sample,
broad emission bands from 400 to 600 nm is observed.
These emission bands were not observed in PL. According to the previous literature[24], the emission around 450
nm was observed only in Eu-doped SrAl2O4 and the origin was interpreted as the charge transfer from the oxygento residual Eu3+. This scenario is not likely for our sample
because our sample does not contain Eu. To consider theorigin of these emissions, we also investigated the PL
excitation spectrum by using FP8600 (JASCO). The excitation bands monitoring at 450 nm emission appearing
was 320 nm. The emission origin was not exciton-related
because the excitation energy was smaller than the
bandgap, and the origin would be ascribed to some kindsof defects (e.g., oxygen vacancy).
X-ray induced scintillation decay time profiles of the
Nd:SrAl2O4 crystals are represented in Fig. 7. The decay

Fig. 7 X-ray induced scintillation decay time profiles of the
Nd:SrAl2O4 crystals

curves consist of one exponential functions, and the time
constants of the 0.5%, 1.0% and 2.0%Nd-doped samples
are 39.0, 36.4 and 34.8 μs, respectively. As the Nd concentration increase, the decay time become faster. The
scintillation decay time is shorter than those in PL due tothe difference of the instruments and the evaluation condition. The PMT used in the scintillation evaluation has awavelength sensitivity from VUV to Vis wavelengths
and the sensitivity in NIR wavelength is very low.
Therefore main emissions in scintillation decay time profiles are 380–500 nm emission bands, and decay timeobserved is typical with Nd3+ emission in these wavelength ranges[15].
We investigated the afterglow properties after an
X-ray irradiation with a pulse duration of 2 ms. The results are shown in Fig. 8. The afterglow levels of the the0, 0.5%, 1.0% and 2.0% Nd-doped samples are 0.09%,
0.48%, 0.15%, 0.29%, and the 1.0% Nd-doped sample
shows the lowest afterglow level among the Nd-doped
samples. However, it is much bigger than commercial
scintillators such as CdWO4 and BGO (~10 ppm)[18], andthese Nd-doped SrAl2O4 are not preferable to highcounting rate applications.
The TSL glow curves are shown in Fig. 9. The
1.0%Nd-doped sample shows a TSL glow peak only at
lower temperature around 110 ºC, and the 0.5%Nd-

Fig. 8 Afterglow curves of the Nd:SrAl2O4 crystals after the
X-ray irradiation was cut off

Daisuke Nakauchi et al., Optical and scintillation properties of Nd-doped SrAl2O4 crystals

Fig. 9 TSL glow curves of the Nd:SrAl2O4 crystals

doped sample shows a relatively high temperature peakat 250 ºC as well. In the undoped and 2.0%Nd-doped
samples, no significant TSL signal is observed. These
results are consistent with the afterglow levels since theafterglow is TSL around the room temperature. It is wellknown that scintillation and PL intensities have a dopant
concentration dependence and the optimum doping concentration. If we think analogically with them, TSL may
also have an optimum doping concentration, and in thiscase it was around 1.0%.

3 Conclusions
We investigated the optical, scintillation and dosimetric properties of the Nd:SrAl2O4 crystals synthesized by
the FZ method. The Nd:SrAl2O4 crystals showed photoand radio-luminescence peaks due to the 4f-4f transitionsof Nd3+ and the decay time constants were almost thesame as the typical emissions from Nd3+-doped phosphors. The QY of the 0.5%Nd:SrAl2O4 crystals was thehighest among the present samples. Under VUV photon
excitations, two broad peaks were observed, and the origins were attributed to STEs perturbed by Nd3+ ions. In
the afterglow, the undoped sample represented the lowest
afterglow level. TSL results suggested that the 0.5%–
1.0%Nd-doping concentration was optimum when they
worked as the TSL dosimeter.

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