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Sensors and Materials, Vol. 36, No. 2 (2024) 481–488
MYU Tokyo
S & M 3535

Photoluminescence, Scintillation, and Thermally Stimulated
Luminescence Properties of Eu-doped Al2O3 Single Crystals
Ren Tsubouchi,1 Hiroyuki Fukushima,1 Takumi Kato,2Daisuke Nakauchi,2 Satoshi Saijo,1 Toru Matsuura,1Noriaki Kawaguchi,2 Tomoaki Yoneda,1 and Takayuki Yanagida2*
1Department of Electrical and Electronic Engineering, National Institute of Technology, Fukui College, Geshi,

Sabae, Fukui 916-8507, Japan

2Division of Materials Science, Nara Institute of Science and Technology,

8916-5 Takayama-cho, Ikoma, Nara 630-0192, Japan(Received October 31, 2023; accepted January 22, 2024)

Keywords: photoluminescence, scintillation, TSL, single crystal, floating zone

The photoluminescence (PL), scintillation, and thermally stimulated luminescence (TSL)
properties of Eu-doped (0.01, 0.1, and 1%) Al2O3 single crystals grown by the floating zone
method were investigated systematically. The PL of the samples showed several sharp emission
peaks across 550–750 nm originating from the 4f–4f transitions of Eu3+ and the 3d–3d transitionsof Cr3+ impurity ions. The scintillation spectrum of the samples showed a broad emission peak at
320 nm due to the F+ center and an emission peak at 700 nm due to the 3d–3d transitions of Cr3+
impurity ions. All the samples showed TSL glow peaks at around 200, 320, and 375 °C. The TSL
intensity of the 1% Eu-doped sample was the highest among the present three samples, and the
TSL response was proportional to the irradiated X-ray dose in the range from 10 to 1000 mGy.
The Eu-doped Al2O3 single crystal could be a novel candidate for personal dose monitoring
applications.

1.

Introduction

Radiation-induced phosphors can be classified into two types: scintillators, in which carriers
are excited by ionizing radiation irradiation at the luminescent center and immediately emit
light, and phosphor-based dosimeters, in which captured carriers are re-excited by external
stimulation such as heat or light, leading to emission.(1,2) Scintillators are applied in medical
imaging equipment such as X-ray computed tomography (CT) and positron emission tomography(PET), security equipment such as baggage screening devices, and environmental
monitoring.(1,3,4) In contrast, phosphor-based dosimeters are mainly applied in personal dosemonitoring.(5–7) The luminescence types of phosphor-based dosimeters are classified as follows:
thermally stimulated luminescence (TSL), in which carriers are re-excited by thermal
stimulation; optically stimulated luminescence (OSL), in which carriers are re-excited by light;*Corresponding author: e-mail: t-yanagida@ms.naist.jp

https://doi.org/10.18494/SAM4763

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

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and radio-photoluminescence (RPL), in which the interaction with ionizing radiation creates
new emission centers in the material.(8–12)
So far, LiF single crystals doped with Ti and Mg(13) and Cu-doped LiB4O7 ceramics(14) for
TSL, BeO ceramics(15) for OSL, and Ag-doped phosphate glass(16,17) for RPL have been studied.
In particular, personal dosimeters should have an effective atomic number (Zeff) close to that ofhuman soft tissue (Zeff = 7.51) from the viewpoint of bioequivalence, in addition to highsensitivity dose–response characteristics and low fading.(18) To develop novel phosphor-based
dosimeters, the TSL, OSL, and RPL properties of many new materials have been
investigated.(19–29) In particular, Al2O3 has high thermal stability, chemical stability, and low
Zeff ; thus, C-doped Al2O3 is commercially developed as TSL and OSL personal dosimeters.(7,30)
In recent years, transparent ceramics, which have more defects than single crystals, havebeen attracting attention in phosphor-based dosimeter applications.(31–34) Moreover, the TSL and
OSL properties of Al2O3 transparent ceramics doped with C, Mg, Ti, Ce, and Eu have also beenstudied.(35–38) In contrast, there are few reports on Al2O3 single crystals doped with impurities
other than C, leaving room for further research.
Single crystals are expected to have a higher detection efficiency than opaque ceramics
because of their high transmittance and the ease of extracting internally generated light to theoutside. In addition, the mass production of large-area transparent ceramics is difficult withcurrent technology. In contrast, single crystals are suitable for mass production because largearea single crystals can be fabricated by the pulling method such as the Czochralski method.
Therefore, if dosimeters that emit light with high efficiency in single-crystal form are discovered,they could be one of the best candidates for new dosimeter materials compared with opaque andtransparent ceramics. Recently, MgAl2O4 and Mg2SiO4 single crystals doped with some dopants
have been studied as candidates for new dosimeter materials, realizing defects produced insingle-crystal form with high transmittance and high TSL and OSL intensities.(39–44) In thisstudy, Eu3+ is selected as a luminescence center. Eu3+ ions are often used as luminescencecenters because they have intense red emission due to the 4f–4f transition.(45) To date, the PL,
scintillation, and TSL properties of Eu-doped Al 2O3 transparent ceramics have beenreported.(38,46) In Al2O3, it is considered necessary to evaluate dosimetric properties not only inthe transparent ceramic form but also in the single-crystal form. For the above reasons, Eudoped Al2O3 single crystals exhibit intense luminescence and good TSL properties.
In this study, Eu-doped Al2O3 single crystals were synthesized by the floating zone (FZ)
method, and in addition to their PL and scintillation properties, their TSL response properties as
a dosimeter were also investigated.

2.

Materials and Methods

Eu-doped Al2O3 single crystals were synthesized using an FZ furnace equipped with fourxenon lamps. Al2O3 (99.99%) and Eu2O3 (99.99%) were used as starting materials and weremixed uniformly using a mortar and pestle. The nominal doped concentrations of Eu were 0.01,
0.1, and 1%. The X-ray powder diffraction (XRD) patterns were measured in the range of 20–
80º (Rigaku, UltimaⅣ). The PL emission was measured using a CCD-based spectrometerSensors and Materials, Vol. 36, No. 2 (2024)

483(Otsuka Electronics, MCPD-9800-2285C). The excitation source was a xenon lamp with amonochromator, and the excitation light was led to the sample through an optical fiber. The
X-ray-induced scintillation spectra were measured using laboratory-made setups.(47) The applied
voltage and tube current during the measurement of scintillation spectra were 40 kV and 1.2
mA, respectively. The TSL glow curves were measured using a TSL reader (TL-2000, Nanogray)after X-ray irradiation. No background processing was conducted for the blackbody radiation
signal.

3.

Results and Discussion

Figure 1 shows a photograph of the Eu-doped Al2O3 single crystals. The samples wereapproximately 1 mm thick and colorless. The following measurements were conducted using
these samples. The actual Eu concentration inside the crystal was considered to be lower than
the nominal concentration. The weights of the 0.01, 0.1, and 1% Eu-doped Al2O3 single crystals
were 0.0592, 0.0211, and 0.0204 g, respectively.
Figure 2 shows the XRD patterns of the Eu-doped Al2O3 single crystals. The remaining
crushed samples were used for XRD measurements. The XRD patterns of the samples matched
the reference pattern for Al2O3 (ICSD. No. 31545). Therefore, the samples are considered to be
single-phase Al2O3. The coincidence of the valence and coordination numbers of Al3+ and Eu3+
suggested that Eu3+ ions were substituted at the Al3+ site.
Figure 3 shows the PL excitation and emission spectra of the Eu-doped Al2O3 single crystals.
The Eu-doped samples showed emission peaks at 587, 616, 678, and 714 nm. These emission
peaks are consistent with those attributed to the 4f–4f transition of Eu3+.(38,48,49) The sharp
emission peak at 693 nm is considered to be due to the 3d–3d transition of Cr3+.(37,46) The
intensity of the emission peak due to the 4f–4f transition of Eu3+ weakened as the Eu doping
concentration increased. This is the same trend reported previously for Eu-doped Al 2O3

Intensity [arb. unit.]

1% Eu
0.1% Eu
0.01% Eu
Al2O3 ICSD 31545

20

Fig. 1. (Color online) Photograph of Eu-doped
Al2O3 single crystals.

30

40

50
2θ [deg]

60

70

80

Fig. 2. (Color online) XRD patterns of Eu-doped
Al2O3 single crystals.

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Sensors and Materials, Vol. 36, No. 2 (2024)

Fig. 3.

(Color online) PL emission and excitation spectra of Eu-doped Al2O3 single crystals.

transparent ceramics. Since the samples in this study were synthesized in air ambient, emission
peaks originating from the 4f–4f transition of Eu3+ were observed while no emission bands
originating from the 5d–4f transition of Eu2+ were observed. Excitation spectra showed sharp
peaks at 365, 380, 395, and 415 nm. These peaks are attributed to the 4f–4f transition of
Eu3+.(38,48,49) The broad peak at about 300 nm is attributed to charge transfer transitions between
Eu3+ and O2−.
Figure 4 shows X-ray-induced scintillation spectra of Eu-doped Al2O3 single crystals. A
broad scintillation peak was observed at ~320 nm. On the basis of previous studies, thescintillation peak at 320 nm is attributed to the F+ center.(35,46,50) As with PL, the emission near
700 nm is considered to be a peak due to the 3d–3d transition of Cr3+. No emission due to the
4f–4f transition of Eu3+, which was observed in the PL spectra, was observed. PL directly excites
carriers to the luminescent center, and recombination leads to luminescence. In scintillation,
carriers are ionized, transferred to the luminescent center via energy transport, and recombined
to emit light. The luminescence mechanism differs between PL and scintillation, and in
scintillation, the carriers do not always recombine at the desired emission center. Therefore, in
Eu-doped Al2O3 single crystals, the luminescence was attributed to Cr3+ rather than Eu3+ ions.
Figure 5 shows the TSL glow curves of Eu-doped Al2O3 single crystals after 1000 mGy
X-ray irradiation. The TSL intensity was normalized by sample weight. All the samples showed
TSL glow peaks at ~200, ~320, and ~375 ℃. The 1% Eu-doped sample showed the highest TSL
intensity among the present samples. Since the ionic radius of Eu3+ (0.947 Å, 6-coordination) islarger than that of Al3+ (0.535 Å, 6-coordination), strain is generated, and defects are formed
when Eu3+ ions are substituted at the Al3+ sites. Therefore, 1% Eu-doped Al2O3 single crystals
have the strongest TSL intensity because the generated number of defects is greater than those of
the other samples. Figure 6 shows the fitted curves of the TSL glow curve of the 1% Eu-doped
Al2O3 single crystal. The TSL glow curve was fitted with a general order kinetics in the formula
shown below.(51) The obtained parameters are listed in Table 1. The 1% Eu-doped sample showed
peaks at 150, 187, 221, 320, and 380 ℃. The shapes of the TSL glow curves between Eu-doped
Al2O3 single crystals and those in transparent ceramics are different. In a previous study, Eu-Sensors and Materials, Vol. 36, No. 2 (2024)

485

TSL Intensity [arb. unit.]

0.01% Eu
0.1% Eu
1% Eu
1000mGy

50

Fig. 4. (Color online) X-ray-induced scintillation
spectra of Eu-doped Al2O3 single crystals.

Fig. 5. (Color online) TSL glow curves of Eu-doped
Al2O3 single crystals.

TSL Intensity [arb. unit.]

Raw data
Peak 1
Peak 2
Peak 3
Peak 4
Peak 5
Total fit

50

Fig. 6.

100 150 200 250 300 350 400 450
Temperature [°C]

100 150 200 250 300 350 400 450
Temperature [°C]

(Color online) Fitted curves of TSL glow curve of 1% Eu-doped Al2O3 single crystal.

Table 1
Parameters of fitted glow curves using 1% Eu-doped Al2O3 single crystal.
Peak intensity Peak temperature Trap depth Frequency factor(arb. unit)
(℃)
(eV)
(s−1)
Peak 133
150
1.02
1.48 × 1011
Peak 218
187
1.11
1.38 × 1011
7
221
1.14
3.66 × 1010
1% Eu Peak 3
Peak 4201
320
1.21
1.21 × 109
Peak 5182
380
1.51
2.93 × 1010

doped Al2O3 transparent ceramics showed the TSL glow peak at ~100 °C.(46) In contrast, thepresent Eu-doped Al2O3 single crystals showed no TSL glow peak at ~100 °C. The defects
generated depend on the fabrication method. Typically, single crystals have fewer defects thantransparent ceramics.

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Fig. 7.

I (T ) I M=(Color online) TSL dose response functions of Eu-doped Al2O3 single crystals.

−b2
 E T − TM    b −1 (1)T − TM  2kTM
 2kT   T+ ( b − 1) 1 − 1 + ( b − 1)
    2 exp EE   TM kT TM  
 kT TM   b Ebb −1 exp

Figure 7 shows the TSL dose response functions of Eu-doped Al2O3 single crystals. The TSL
intensity at each dose represents the maximum intensity at 300 ℃. The TSL intensity increased
linearly with irradiation dose in the range of 10–1000 mGy for the 1% Eu-doped sample and in
the range of 100–1000 mGy for the 0.01 and 0.1% Eu-doped samples. In the previous study of
Eu-doped Al2O3 transparent ceramics, the TSL intensity decreased with increasing Eu doping
concentration.(46) In contrast, Eu-doped Al2O3 single crystals showed a trend toward higher TSL
intensity with increasing Eu doping concentration. The number of defects may increase withincreasing Eu doping concentration, and the TSL intensity depends on the number of defects.
Thus, the highly Eu-doped Al2O3 single crystals exhibited strong TSL intensity. The lower
sensitivity limit of the Eu-doped Al2O3 single crystal was 10 mGy, which is higher than thedetection limit of 10–100 μGy in the personal dose monitoring application. In future works, wewould like to improve the TSL intensity by growing Al2O3 doped with different luminescentcenters and annealing in a reducing atmosphere.

4.

Conclusions

Eu-doped (0.01, 0.1, and 1%) Al2O3 single crystals were successfully grown by the FZ
method. In terms of PL, the samples showed several sharp emission peaks across 550–750 nm
originating from the 4f–4f transitions of Eu3+. In terms of scintillation, the samples showed abroad emission peak at 320 nm due to the F+ center. Furthermore, the Eu-doped Al2O3 single
crystals showed PL and scintillation due to not only Eu3+ but also Cr3+ impurity ions. In terms of
TSL, all the samples showed TSL glow peaks at ~200, 320, and 375 °C, and the TSL intensity of
the 1% Eu-doped sample was the highest among the present samples. The absence of the TSL
peak at ~100 °C is suggested to be advantageous for applications such as personal dosemonitoring. The TSL response was conﬁrmed to be linearly related to the irradiated X-ray doseSensors and Materials, Vol. 36, No. 2 (2024)

487

in the range from 10 to 1000 mGy. Therefore, the Eu-doped Al2O3 single crystal can be a novel
candidate for personal dose monitoring applications.

Acknowledgments
This work was supported by Grants-in-Aid for Scientific Research A (22H00309), Scientific
Research B (22H03872, 22H02939, 21H03733, and 21H03736), Early-Career Scientists(23K13689), and Challenging Exploratory Research (22K18997) from the Japan Society for the
Promotion of Science. The Cooperative Research Project of the Research Center for Biomedical
Engineering, A-STEP (JPMJTM22DN) from JST, Kazuchika Okura Memorial Foundation,
Asahi Glass Foundation, Nakatani Foundation, and Konica Minolta Science and Technology
Foundation are also acknowledged.

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