Journal of the Korean Physical Society, Vol. 62, No. 8, April 2013, pp. 1102∼1107
Synthesis and Characterization of Ce3+ -, Pr3+ -, Tm3+ -doped Li6 Lu(BO3 )3
Phosphors for X-ray and Neutron Imaging
U. Fawad, Myeongjin Oh, H. Park and H. J. Kim∗
Department of Physics, Kyungpook National University, Daegu 702-701, Korea
Sunghwan Kim
Department of Radiological Science, Cheongju University, Cheongju 360-764, Korea
(Received 16 January 2013)
Rare-earth-doped phosphors are drawing attention due to their applications in various fields
including medical imaging. Lithium lutetium borate has very good absorption efficiency in com-
parison to other phosphors. The presence of Lu with its high effective atomic number (Zef f = 52)
and high molecular weight (M.Wt = 393 g/mol) make lithium lutetium borate a potential detector
for X-ray imaging. On the other hand, the presence of 6 Li (lithium) and 10 B (boron) may make
Li6 Lu(BO3 )3 as a promising neutron detector. For this work Li6 Lu(BO3 )3 :Ce3+ , Li6 Lu(BO3 )3 :Pr3+
and Li6 Lu(BO3 )3 :Tm3+ were synthesized by using a solid-state reaction method. The crystallinity
and the particle morphology of the synthesized phosphors were characterized by XRD (X-ray diffrac-
tion) and FE-SEM (field-emission scanning electron microscopy). Luminescence properties such as
X-ray-induced luminescence, the proton-induced luminescence and the UV photoluminescence were
measured for the synthesized phosphors.
PACS numbers: 29.40.Mc, 29.27.Fh
Keywords: Li6 Lu(BO3 )3 , Phosphor, X-ray imaging, Neutron detector
DOI: 10.3938/jkps.62.1102
I. INTRODUCTION (Li6 Y(BO3 )3 :Eu3+ ) has already been studied before, and
the result of the powder XRD has proven to be isotypic
to the monoclinic crystal system, Li6 RE(BO3 )3 , with RE
The inorganic borate compounds have been the fo- denoting rare-earths, with P 21 /c as the space group [9–
cus of researchers due to their various kinds of struc- 11]. This proves that Li6 Lu(BO3 )3 possesses the same
tures, transparency to a wide range of radiations and monoclinic structure with P 21 /c as the space group. Lu-
high optical quality. Borates intrinsically possess benefi- minescent studies for Li6 Lu(BO3 )3 in its crystalline form
cial optical properties, which include a wide transparency are repeated in Ref. 12 but no study has been conducted
range, good thermal and chemical stability, low synthe- on its phosphor form. Li6 Lu(BO3 )3 could be a good can-
sis temperature, radiation hardness, non-linearity and didate for a thermal neutron detector due to the presence
their ability to play the role of good host materials [1–3]. of 6 Li and 10 B [11,13,14]. The luminescent properties of
Rare-earth-doped borates have been reported in applica- rare-earth-doped phosphors strongly depend on the posi-
tions of luminescence materials due to their low sinter- tions of the excited levels, as well as the electronic levels
ing temperature and high physical and chemical stabil- of the host material, concentration quenching has been
ity, which make them potential hosts for investigations shown to be weak because of the anisotropic nature of
of novel phosphors with good properties [4,5]. the system [15,16].
Rare-earth-doped borate phosphors are considered as The Ce3+ -doped Li6 Lu(BO3 )3 crystal has been stud-
prospective materials for X-ray and gamma-ray imag- ied before for its luminescent properties [9]. The Ce3+
ing. The presence of lutetium in a phosphor increases ion is used for its potential to yield fast scintillation in the
the material’s density. Hence, it gives a high light yield 300 to 500 nm wavelength range due to electric-dipole-
in comparison to other rare-earth elements [6]. Rare- allowed 5d → 4f transitions. Nowadays, the Ce3+ ion is
earth dopants considerably influence the luminescence drawing more attention for its application in high-energy
properties of phosphors and the same is the case for physics due to its fast and efficient luminescence in the
Li6 Lu(BO3 )3 phosphor [7, 8]. A similar kind of system UV and blue spectral regions with the radiative lifetimes
of several tens of nanoseconds [15,17]. In this study, how-
∗ E-mail: ever, we have particularly tried to optimize the synthesis
-1102-
, Synthesis and Characterization of Ce3+ -, Pr3+ -, Tm3+ -doped · · · – U. Fawad et al. -1103-
conditions and compare Ce3+ with the (praseodymium a suitable lifetime and havw various applications in the
ion) Pr3+ and the (thulium ion) Tm3+ -doped phosphors. fields of LEDs (light emitting diodes) and PDPs (plasma
Pr3+ in solid phosphors presents an intricate energy display panels) [19]. Nowadays neutron detector has be-
level diagram for its transitions. A parity allowed tran- come the center of focus for its applications in various in-
sition, 5d-4f of Pr3+ has been shown in Ref. 18 and is dustrial fields. Li6 Lu(BO3 )3 phosphor could be a strong
confirmed in our work. These days thulium-doped phos- candidate for neutron detection due to its 10 B and 6 Li
phors have drawn attention for their blue emission with constituents [13,14,20]:
n +10 B →7 Li∗ +4 He (1.47 MeV) + 0.48 MeV(γ) + (Q value = 2.31 MeV, excited state)
→7 Li +4 He (1.78 MeV) + (Q value = 2.79 MeV, ground state, σ = 3840 barn, 96%) (1)
6
Li + n →3 H +4 He (2.05 MeV) + (Q value = 4.78 MeV, σ = 940 barn, 4%) (2)
The aim of our research was to synthesize a novel phos- the crystallinity of the prepared materials. For XRD
phor that could be applied in the field of X-ray and measurements, the scan range was adjusted to 10 ∼ 60◦
neutron imaging. In order to achieve the best lumines- (2θ) with a scan speed of 0.015◦ /s and a step size of
cence properties, we optimized the sintering conditions 0.03◦ . The X-ray source was Cu (Kα ), the accelerat-
in terms of temperature, duration of sintering and con- ing voltage was set at 40 kV and the tube current was
centration of the rare-earth activator to achieve the best set at 30 mA. The results of the XRD analysis were
luminescence properties. compared with reference Powder Diffraction File (PDF)
data. High-resolution field-emission scanning electron
microscopy (HR FE-SEM S-4800, Hitachi, Japan) was
II. EXPERIMENTS AND DISCUSSION used to observe the morphology and grain size of the
sintered phosphor. A Fluorolog-3 spectro-fluorometer
(light source-450 W Xenon lamp) and an Ocean Op-
1. Sample Preparation tics QE 65000 spectrometer (65 kV, 1 mA) were used
to get the UV-excited and X-ray luminescence spectra
Firstly, the three reactants, lithium carbonate at room temperature, respectively. Proton beam irra-
(Li2 CO3 , 99.998%), lutetium oxide (Lu2 O3 , 99.99%) diation of the phosphor was carried out at the Korea
and boric acid (H3 BO3 , 99.99%), were weighed with Institute of Radiological and Medical Sciences, Korea.
appropriate ratios according to the balanced chemi- A proton beam line was used for pilot studies of the
cal reaction in order to get the required host ma- Proton Engineering Frontier Project (PEFP), especially
terial Li6 Lu(BO3 )3 . Secondly, the synthesized host for the studies using low-flux proton beams, 104 ∼ 1010
material Li6 Lu(BO3 )3 was doped with different con- proton/cm2 -s. We used the 45-MeV proton beam which
centrations of three activators, cerium nitrate hexahy- had a 2-nA current, passed through a 0.2-mm-thick alu-
drate (Ce(NO3 )3 ·6H2 O, 99.99%), praseodymium oxide minum window capping a beam pipe with 5 cm of air,
(Pr2 O3 , 99.9%) and thulium oxide (Tm2 O3 , 99.9%). The and losing energy down to 39 MeV [21,22].
synthesis parameters included the ball milling time, the
temperature for sintering, the duration for sintering and
the heating and cooling rates for sintering. The powders
were weighed and then ball milled for 6 hours. The well- 3. Results and Discussion
mixed powders were sintered in an electric furnace at 720
◦
C for 10 hours with slow heating and cooling of about 6
hours each in an air environment. Finally, we were able Figure 1 shows XRD plots for the Li6 Lu(BO3 )3 :Ce3+ ,
to obtain Ce3+ -, Pr3+ - and Tm3+ -doped Li6 Lu(BO3 )3 Pr3+ , Tm3+ phosphors. In this figure, the XRD plots for
phosphors. the Li6 Lu(BO3 )3 :Ce3+ , Pr3+ , Tm3+ phosphors are com-
pared with the standard reference data of Li6 Gd(BO3 )3
(PDF No. 00-054-1119). The comparison proves that the
peaks of the synthesized phosphors exactly match those
2. Characterization of the reference and that no peaks of the phosphor’s com-
ponents were found, confirming that the required phases
Several complementary methods were used to charac- had been achieved for the phosphors. It further verified
terize the properties of the prepared phosphors. X-ray that the sintered phosphors obtained, were monoclinic in
diffraction (XRD) was used to measure the phases and structure having the P 21 /c space group [10].
Synthesis and Characterization of Ce3+ -, Pr3+ -, Tm3+ -doped Li6 Lu(BO3 )3
Phosphors for X-ray and Neutron Imaging
U. Fawad, Myeongjin Oh, H. Park and H. J. Kim∗
Department of Physics, Kyungpook National University, Daegu 702-701, Korea
Sunghwan Kim
Department of Radiological Science, Cheongju University, Cheongju 360-764, Korea
(Received 16 January 2013)
Rare-earth-doped phosphors are drawing attention due to their applications in various fields
including medical imaging. Lithium lutetium borate has very good absorption efficiency in com-
parison to other phosphors. The presence of Lu with its high effective atomic number (Zef f = 52)
and high molecular weight (M.Wt = 393 g/mol) make lithium lutetium borate a potential detector
for X-ray imaging. On the other hand, the presence of 6 Li (lithium) and 10 B (boron) may make
Li6 Lu(BO3 )3 as a promising neutron detector. For this work Li6 Lu(BO3 )3 :Ce3+ , Li6 Lu(BO3 )3 :Pr3+
and Li6 Lu(BO3 )3 :Tm3+ were synthesized by using a solid-state reaction method. The crystallinity
and the particle morphology of the synthesized phosphors were characterized by XRD (X-ray diffrac-
tion) and FE-SEM (field-emission scanning electron microscopy). Luminescence properties such as
X-ray-induced luminescence, the proton-induced luminescence and the UV photoluminescence were
measured for the synthesized phosphors.
PACS numbers: 29.40.Mc, 29.27.Fh
Keywords: Li6 Lu(BO3 )3 , Phosphor, X-ray imaging, Neutron detector
DOI: 10.3938/jkps.62.1102
I. INTRODUCTION (Li6 Y(BO3 )3 :Eu3+ ) has already been studied before, and
the result of the powder XRD has proven to be isotypic
to the monoclinic crystal system, Li6 RE(BO3 )3 , with RE
The inorganic borate compounds have been the fo- denoting rare-earths, with P 21 /c as the space group [9–
cus of researchers due to their various kinds of struc- 11]. This proves that Li6 Lu(BO3 )3 possesses the same
tures, transparency to a wide range of radiations and monoclinic structure with P 21 /c as the space group. Lu-
high optical quality. Borates intrinsically possess benefi- minescent studies for Li6 Lu(BO3 )3 in its crystalline form
cial optical properties, which include a wide transparency are repeated in Ref. 12 but no study has been conducted
range, good thermal and chemical stability, low synthe- on its phosphor form. Li6 Lu(BO3 )3 could be a good can-
sis temperature, radiation hardness, non-linearity and didate for a thermal neutron detector due to the presence
their ability to play the role of good host materials [1–3]. of 6 Li and 10 B [11,13,14]. The luminescent properties of
Rare-earth-doped borates have been reported in applica- rare-earth-doped phosphors strongly depend on the posi-
tions of luminescence materials due to their low sinter- tions of the excited levels, as well as the electronic levels
ing temperature and high physical and chemical stabil- of the host material, concentration quenching has been
ity, which make them potential hosts for investigations shown to be weak because of the anisotropic nature of
of novel phosphors with good properties [4,5]. the system [15,16].
Rare-earth-doped borate phosphors are considered as The Ce3+ -doped Li6 Lu(BO3 )3 crystal has been stud-
prospective materials for X-ray and gamma-ray imag- ied before for its luminescent properties [9]. The Ce3+
ing. The presence of lutetium in a phosphor increases ion is used for its potential to yield fast scintillation in the
the material’s density. Hence, it gives a high light yield 300 to 500 nm wavelength range due to electric-dipole-
in comparison to other rare-earth elements [6]. Rare- allowed 5d → 4f transitions. Nowadays, the Ce3+ ion is
earth dopants considerably influence the luminescence drawing more attention for its application in high-energy
properties of phosphors and the same is the case for physics due to its fast and efficient luminescence in the
Li6 Lu(BO3 )3 phosphor [7, 8]. A similar kind of system UV and blue spectral regions with the radiative lifetimes
of several tens of nanoseconds [15,17]. In this study, how-
∗ E-mail: ever, we have particularly tried to optimize the synthesis
-1102-
, Synthesis and Characterization of Ce3+ -, Pr3+ -, Tm3+ -doped · · · – U. Fawad et al. -1103-
conditions and compare Ce3+ with the (praseodymium a suitable lifetime and havw various applications in the
ion) Pr3+ and the (thulium ion) Tm3+ -doped phosphors. fields of LEDs (light emitting diodes) and PDPs (plasma
Pr3+ in solid phosphors presents an intricate energy display panels) [19]. Nowadays neutron detector has be-
level diagram for its transitions. A parity allowed tran- come the center of focus for its applications in various in-
sition, 5d-4f of Pr3+ has been shown in Ref. 18 and is dustrial fields. Li6 Lu(BO3 )3 phosphor could be a strong
confirmed in our work. These days thulium-doped phos- candidate for neutron detection due to its 10 B and 6 Li
phors have drawn attention for their blue emission with constituents [13,14,20]:
n +10 B →7 Li∗ +4 He (1.47 MeV) + 0.48 MeV(γ) + (Q value = 2.31 MeV, excited state)
→7 Li +4 He (1.78 MeV) + (Q value = 2.79 MeV, ground state, σ = 3840 barn, 96%) (1)
6
Li + n →3 H +4 He (2.05 MeV) + (Q value = 4.78 MeV, σ = 940 barn, 4%) (2)
The aim of our research was to synthesize a novel phos- the crystallinity of the prepared materials. For XRD
phor that could be applied in the field of X-ray and measurements, the scan range was adjusted to 10 ∼ 60◦
neutron imaging. In order to achieve the best lumines- (2θ) with a scan speed of 0.015◦ /s and a step size of
cence properties, we optimized the sintering conditions 0.03◦ . The X-ray source was Cu (Kα ), the accelerat-
in terms of temperature, duration of sintering and con- ing voltage was set at 40 kV and the tube current was
centration of the rare-earth activator to achieve the best set at 30 mA. The results of the XRD analysis were
luminescence properties. compared with reference Powder Diffraction File (PDF)
data. High-resolution field-emission scanning electron
microscopy (HR FE-SEM S-4800, Hitachi, Japan) was
II. EXPERIMENTS AND DISCUSSION used to observe the morphology and grain size of the
sintered phosphor. A Fluorolog-3 spectro-fluorometer
(light source-450 W Xenon lamp) and an Ocean Op-
1. Sample Preparation tics QE 65000 spectrometer (65 kV, 1 mA) were used
to get the UV-excited and X-ray luminescence spectra
Firstly, the three reactants, lithium carbonate at room temperature, respectively. Proton beam irra-
(Li2 CO3 , 99.998%), lutetium oxide (Lu2 O3 , 99.99%) diation of the phosphor was carried out at the Korea
and boric acid (H3 BO3 , 99.99%), were weighed with Institute of Radiological and Medical Sciences, Korea.
appropriate ratios according to the balanced chemi- A proton beam line was used for pilot studies of the
cal reaction in order to get the required host ma- Proton Engineering Frontier Project (PEFP), especially
terial Li6 Lu(BO3 )3 . Secondly, the synthesized host for the studies using low-flux proton beams, 104 ∼ 1010
material Li6 Lu(BO3 )3 was doped with different con- proton/cm2 -s. We used the 45-MeV proton beam which
centrations of three activators, cerium nitrate hexahy- had a 2-nA current, passed through a 0.2-mm-thick alu-
drate (Ce(NO3 )3 ·6H2 O, 99.99%), praseodymium oxide minum window capping a beam pipe with 5 cm of air,
(Pr2 O3 , 99.9%) and thulium oxide (Tm2 O3 , 99.9%). The and losing energy down to 39 MeV [21,22].
synthesis parameters included the ball milling time, the
temperature for sintering, the duration for sintering and
the heating and cooling rates for sintering. The powders
were weighed and then ball milled for 6 hours. The well- 3. Results and Discussion
mixed powders were sintered in an electric furnace at 720
◦
C for 10 hours with slow heating and cooling of about 6
hours each in an air environment. Finally, we were able Figure 1 shows XRD plots for the Li6 Lu(BO3 )3 :Ce3+ ,
to obtain Ce3+ -, Pr3+ - and Tm3+ -doped Li6 Lu(BO3 )3 Pr3+ , Tm3+ phosphors. In this figure, the XRD plots for
phosphors. the Li6 Lu(BO3 )3 :Ce3+ , Pr3+ , Tm3+ phosphors are com-
pared with the standard reference data of Li6 Gd(BO3 )3
(PDF No. 00-054-1119). The comparison proves that the
peaks of the synthesized phosphors exactly match those
2. Characterization of the reference and that no peaks of the phosphor’s com-
ponents were found, confirming that the required phases
Several complementary methods were used to charac- had been achieved for the phosphors. It further verified
terize the properties of the prepared phosphors. X-ray that the sintered phosphors obtained, were monoclinic in
diffraction (XRD) was used to measure the phases and structure having the P 21 /c space group [10].
