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Sensors and Materials, Vol. 36, No. 2 (2024) 451–456
MYU Tokyo
S & M 3531

Luminescence Properties of La2Be2O5 Single Crystal Scintillator
Kensei Ichiba,* Takumi Kato, Daisuke Nakauchi,
Noriaki Kawaguchi, and Takayuki Yanagida
Division of Materials Science, Nara Institute of Science and Technology (NAIST),
8916-5 Takayama, Ikoma, Nara 630-0192, Japan(Received October 30, 2023; accepted January 9, 2024)

Keywords: radiation detector, scintillator, La2Be2O5, single crystal

We synthesized an undoped La2Be2O5 single crystal by the floating zone method andinvestigated its scintillation properties. The synthesized sample was confirmed to have a singlephase structure of La2Be2O5 as shown by the powder X-ray diffraction pattern. The diffuse
transmittance spectrum of the synthesized sample had transmittance at 80% in the range of
250–850 nm. The scintillation spectrum had a broad emission band from 280 to 600 nm, which
was derived from unidentified lattice defects. In the scintillation decay profile, the obtained
decay time constants were 1.1, 6.2, and 386 ns. From pulse height spectra, the light yield of the
La2Be2O5 single crystal was 1000 photons/MeV. In afterglow profiles, the afterglow level was
42.9 ppm.

1.

Introduction

A scintillator is a type of optical material that can immediately transform ionizing radiation
with high photon energy into ultraviolet-visible light with low photon energy. Its primary
application is a radiation detector, which is composed of a scintillator and a photodetector. A
scintillator has been widely used in various fields including nuclear medicine,(1,2) security,(3,4)
astrophysics,(5) and resource exploration.(6) The required properties of a scintillator depend on
its applications. However, high chemical stability, large effective atomic number (Zeff), highdensity (ρ), fast decay time constant, high light yield (LY), and low afterglow level (AL) are
generally required for a scintillator. Since a scintillator with all these required properties has notbeen found yet, various new scintillators have been developed.(7–13) Such development of new
scintillators has been conducted in various forms: glass,(14–17) transparent ceramics,(18–20) single
crystals,(21–25) organic–inorganic composites,(26,27) and plastics.(28,29)
In this study, we focused on the La2Be2O5 single crystal. La2Be2O5 has been studied as thehost material of a scintillator in the field of radiation measurement since it has a high chemical/
radiation resistance, a relatively large Zeff (52.9), and a high ρ (6.1).(30,31) For example, it has beenreported that the Ce-doped La2Be2O5 single crystal has a broad emission band centered at 450
nm(30) and that the LY of the Ce-doped La2Be2O5 is 58% compared with that of Bi4Ge3O12.(32)*Corresponding author: e-mail: ichiba.kensei.if7@ms.naist.jp

https://doi.org/10.18494/SAM4752

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

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

On the other hand, the scintillation properties of the undoped La2Be2O5 single crystal have notbeen studied. In a previous study of the photoluminescence (PL) properties of undoped
La 2Be2O5, the emission wavelength was observed in the range of 280–600 nm due tounidentified lattice defects.(33) This emission wavelength is compatible with the wavelength
sensitivity of the general photodetector. Therefore, undoped La2Be2O5 could also be a candidate
for a scintillator. In addition, it is important to understand the scintillation properties of an
undoped material. One reason for this is that the energy migration efficiency and interaction
probability between ionizing radiation and materials in the scintillation process basically dependon the host material. For the above reason, we synthesized the La2Be2O5 single crystal by thefloating zone (FZ) method and investigated its scintillation properties.

2.

Experimental Method

The La2Be2O5 single crystal was synthesized by the FZ method. La2O3 (99.99%) and BeO(99.99%), which were raw materials, were mixed to form into a cylindrical rod by applyinghydrostatic pressure. The formed rod was sintered at 1200 ℃ for 12 h in air and then crystal
growth was conducted in an FZ furnace (Canon, Machinery FZD0192). The growth parameters
used were a pull-down speed of 3 mm/h and a rotation speed of 30 rpm. The obtained crystalline
rod was cut to obtain the evaluation sample, and a polishing machine (Buehler, MetaServe 250)
was used for polishing the surface. To confirm the crystal phase, the powder X-ray diffraction(PXRD) pattern was obtained with a diffractometer (MiniFlex600, Rigaku). The diffuse
transmittance spectrum was evaluated using a spectrophotometer (SolidSpec-3700, Shimadzu).
The PL excitation and emission spectra were evaluated using a spectrofluorometer (JASCO, FP8600). The resolutions of the spectrofluorometer on the sides of excitation and emission spectra
were 10 and 20 nm, respectively.
Scintillation spectra were evaluated using our original setup.(34) As the radiation source, an
X-ray generator (Spellman, XRB80N100/CB) with an X-ray tube was used, and the bias voltage
and tube current were 80 kV and 1.2 mA, respectively. Scintillation decay time and afterglow
profiles were evaluated using an afterglow characterization system,(35) and pulse height spectra
under γ-rays from 137Cs were evaluated with our previous setup.(27,36)

3.

Results and Discussion

Figure 1(a) shows the picture and PXRD patterns of the La2Be2O5 single crystal. The
synthesized sample appeared colorless and transparent, and the sample size was approximately
4 × 4 × 1.0 mm3. The PXRD pattern of the synthesized sample corresponded to the reference
pattern (La2Be2O5: International Centre for Diffraction Data No. 76–1652). Therefore, it wasconsidered that the synthesized sample had a single-phase structure of La2Be2O5. Figure 1(b)shows the diffuse transmittance spectrum and PL excitation and emission spectra of the
La2Be2O5 single crystal. The diffuse transmittance of the synthesized sample was approximately
80% in the range of 250–800 nm. In addition, there was clear absorption in the range of 220–250
nm, which would be due to the optical band gap of La2Be2O5.(37) In the PL excitation spectrum,Sensors and Materials, Vol. 36, No. 2 (2024)

Intensity [arb. unit]

Synthesized Sample

20

Reference

30

40

50
60
22θq [deg.]
(a)

70

80

100

Diffuse Transmittance
Diffuse Transmittance
[%]
[%]

4 mm

453

80
100
60
80
40
60
20
40
0
200
20

300

400 500 600 700
Wavelength [nm]

800

0
200

300

400 (b)500 600 700
Wavelength [nm]800Fig. 1. (Color online) (a) Picture and PXRD patterns of La2Be2O5 single crystal. The reference pattern isInternational Centre for Diffraction Data No. 76-1652. (b) Diffuse transmittance spectrum and PL excitation andemission spectra of La2Be2O5 single crystal.

the excitation band was observed at approximately 230 nm, which corresponded to theabsorption wavelength observed in the diffuse transmittance spectrum. Under excitation at 230
nm, a broad emission band was observed in the range of 300–500 nm. The broad emission band
was reported in previous research to be attributed to unidentified lattice defects.(38)Figure 2(a) shows the scintillation spectrum of the La2Be2O5 single crystal. The synthesized
sample had a broad emission band located in the range of 300–600 nm. The broad emission band
would be derived from unidentified lattice defects.(33,38) In addition, at least two different defects
would contribute to the emission since the scintillation spectrum had two emission peaks at 350
and 390 nm. Figure 2(b) shows the scintillation decay time profile of the La2Be2O5 single
crystal. The scintillation decay curve was well approximated by a sum of three exponentialfunctions, and the obtained decay time constants were 11, 62, and 386 ns. The fastest component
came from an instrumental response function (IRF). In addition, the others would be assumed tobe the value related to the emission band, which was observed in the scintillation spectrum. Notethat the intensities were 1793 for the first component, 578 for the second component, and 82 forthe third component.
Figure 3(a) shows the pulse height spectra of the La2Be2O5 single crystal under γ-rays from
137Cs (662 keV). In the measurement, the sharping time was set to 2 µs. As a reference,(Lu,Y)2SiO5 (LYSO) with an LY of 22000 photons/MeV was used. The synthesized sample had aclear photoabsorption peak at 364 ch, and its LY was calculated to be 1000 photons/MeV after
considering a peak position with LYSO and the quantum efficiency of the photomultiplier tube.
Figure 3(b) shows the afterglow profile of the La2Be2O5 single crystal. AL was defined by thesignal intensity at 20 ms after X-ray irradiation with the pulse width of 2 ms. The AL of thesynthesized sample was 42.9 ppm, which was lower than that of the Tl-doped CsI (~300 ppm)
evaluated using the same measurement system.(39)Sensors and Materials, Vol. 36, No. 2 (2024)

200

300

400

500

600

200

300

400

500

600

Fig. 2.

Wavelength [nm](a)Wavelength
[nm]

(b)

(Color online) (a) Scintillation spectrum and (b) scintillation decay time profile of La2Be2O5 single crystal.

Gain Gain×25 ×25

100

Intensity [arb. unit]

Normalized
Normalized
Intensity
Intensity[arb. unit][arb. unit]

454

10-1
10-2
10-3
10-4
10-5
0

(a)

10
20
Time [ms]

30

40

(b)

Fig. 3. (Color online) (a) Pulse height spectra of LYSO and La2Be2O5 single crystal under γ-rays from 137Cs. (b)Afterglow profile of La2Be2O5 single crystal.

4.

Conclusion

In this study, we synthesized a La2Be2O5 single crystal to evaluate its scintillation properties.
From the PXRD pattern, the synthesized sample had a single-phase structure of La2Be2O5. In
the scintillation spectrum, the La2Be2O5 single crystal had a broad emission band in the range of
280–600 nm, which was due to unidentified lattice defects. Pulse height spectra revealed that
the LY of the La2Be2O5 single crystal was 1000 photons/MeV. The AL was 42.9 ppm, which was
lower than that of Tl-doped CsI. Judging from these scintillation properties, the La2Be2O5 single
crystal would hold potential as a new scintillator for X-ray and γ-ray detection. However, the LY

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

455

of the La2Be2O5 single crystal is clearly lower than that of commercial scintillators such as
Bi4Ge3O12, LYSO, and CdWO4. Therefore, it is essential to improve LY, and a solution may be tocontrol the lattice defect derived from the emission center by doping impurity ions such as alkali
metal or alkali earth metal.

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), Challenging Exploratory Research (22K18997), and JSPS Fellows (23KJ1591) fromthe Japan Society for the Promotion of Science. The Cooperative Research Project of the
Research Center for Biomedical Engineering, A-STEP (JPMJTM22DM) from JST, Konica
Minolta Science and Technology Foundation, Nakatani Foundation, Kazuchika Okura Memorial
Foundation, Nippon Sheet Glass Foundation, Terumo Life Science Foundation, KRF Foundation,
Tokuyama Science Foundation, Iketani Science and Technology Foundation, and Foundation for
Nara Institute of Science and Technology are also acknowledged.

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