JOURNAL OF RARE EARTHS, Vol. 34, No. 8, Aug. 2016, P. 782

Luminescence properties of Gd2Si2O7:Ce and Gd2Si2O7:La,Ce undervacuum ultraviolet irradiation
M. Koshimizu1,*, T. Yanagida2, Y. Fujimoto1, K. Asai1(1. Department of Applied Chemistry, Graduate School of Engineering, Tohoku University, 6-6-07 Aoba, Aramaki, Aoba-ku, Sendai 980-8579, Japan; 2.
Graduate School of Materials Science, Nara Institute of Science and Technology, 8916-5 Takayama-Cho, Ikoma, Nara 630-0192, Japan)
Received 21 December 2015; revised 25 February 2016

Abstract: The luminescence properties of Ce- or Ce and La-doped gadolinium pyrosilicate (Gd2Si2O7, GPS) were characterized usingvacuum ultraviolet (VUV) excitation light. A prominent emission band was observed in the luminescence spectra with excitation at
60 nm and ascribed to 5d-4f transition of Ce3+. Because the excitation wavelength of 60 nm corresponded to the excitation in the hostmatrix, this result indicated that the excitation energy transfer occurred from the host matrix to Ce3+ ions. On the basis of the rise inthe luminescence time profiles with excitation at 60 nm, the energy transfer occurred within 2 ns, which was much shorter than that ofCe-doped Gd2SiO5. For Ce-doped GPS, the decay rate was slower for the host excitation than that for direct excitation of Ce3+. In
contrast, for Ce and La-doped GPS, no significant difference was observed for the host excitation and direct excitation of Ce3+. This
result indicated that the energy transfer from the host to Ce3+ ions led to a different radiative decay process, and that La doping had aneffect on the energy transfer and decay process.
Keywords: energy transfer; scintillation; vacuum ultraviolet; rare earths

Gadolinium pyrosilicate (Gd2Si2O7, abbreviated asGPS hereafter) doped with Ce has been attracting
considerable attention owing to its excellent scintillation
properties. At early stage, excellent scintillation
properties were reported by Kawamura et al.[1–3]. They
reported structural, optical, and scintillation properties indetail. According to their papers, Ce-doped GPS crystals
exhibit efficient scintillation owing to 5d-4f transition of
Ce3+, and the scintillation light yield is significantly larger than that of Ce-doped Gd2SiO5 (GSO). In addition,
the scintillation decay is slightly faster than that ofCe-doped GSO. Subsequently, several groups reported
the scintillation properties of Ce-doped GPS fabricatedby various synthesis techniques and their applications for
gamma- ray, α-ray, or thermal neutron detection[4–7].
Recently, Kurosawa et al. reported the stabilization of thepyrosilicate phase by La doping and the excellent
scintillation properties of Ce and La-doped GPS[8,9].
The aim of this study is the analysis of the scintillation
processes in Ce-doped and Ce and La-doped GPS. The
scintillation processes can be divided into three stages[10]:Stage 1: generation of excited states in the host matrix byenergy deposition of ionizing radiation, Stage 2: excitation energy transfer from the host matrix to luminescencecenters (dopants such as Ce3+), and Stage 3: photon
emission by radiative deexcitation of the luminescencecenters. Among them, the energy transfer process is important; however, it has not been thoroughly analyzed.

As for the energy transfer process in Ce and La-doped
GPS, a detailed analysis has been reported in a previous
paper[11]. They proposed an energy level diagram anddiscussed the photoluminescence properties based on thediagram.
In this study, we analyzed the energy transfer processusing vacuum ultraviolet (VUV) spectroscopy. In orderto analyze the energy transfer process, it is necessary togenerate excited state in the host matrix. In a previousstudy[11], the band-gap energy of La-admixed GPS isestimated to be 6.6–6.8 eV. VUV photons having anenergy larger than the band-gap energy, as well as
ionizing radiation, generate excited states in the host matrix. Thus far, Feng et al. have reported the excitation
spectra of Ce-doped GPS in VUV region and discussed
the contribution of higher 5d levels within theband-gap[12]. In this study, we used VUV excitation
photons with energies higher than the band-gap energy,
and analyzed the energy transfer process. In addition tothe measurements of luminescence spectra with VUVexcitations, luminescence decay behavior wascharacterized using a single-photon counting method.

1 Experimental
The samples were GPS crystals doped with Ce at 1mol.% or 10 mol.%, and GPS crystals doped with Ce at 1mol.% and La at 10 mol.%. These crystals were grown* Corresponding author: M. Koshimizu (E-mail: koshi@qpc.che.tohoku.ac.jp; Tel.: +81227157219)
DOI: 10.1016/S1002-0721(16)60094-7

M. Koshimizu et al., Luminescence properties of Gd2Si2O7:Ce and Gd2Si2O7:La,Ce under vacuum ultraviolet irradiation 783
with floating zone method. Raw material powders wereLu2O3 (99.999%), SiO2 (99.999%), La2O3 (99.99%) andCeO2 (99.99%) and they were mixed as the above molar
ratios. After the mixing, powders were shaped to thecylinder by the hydrostatic pressure. The cylinders weresintered at 1600 °C for 6 h under an air atmosphere toobtain GPS ceramic. In the crystal growth, the rotation
rate was 20 r/min and we pulled down the sintering rod
to obtain a crystal with a rate of 3–8 mm/h. Hereafter thesamples are referred to with their dopant concentrationsin the melt. The X-ray-irradiated radioluminescencespectra were obtained using an X-ray generator. The applied voltage and the tube current was 80 kV and 2.5 mA,respectively. The spectra were recorded with a CCD detector (Andor, DU-420-OE) equipped with a grating
monochromator (Oriel Instruments)[13]. The characterization of the luminescence properties was performed at
BL-7B of UVSOR, Institute for Molecular Science, Japan. The samples were placed in a vacuum chamber andexcited with VUV or UV photons. The luminescence
spectra were recorded with a CCD detector (Roper Scientific) equipped with a monochromator (Acton, 300i).
The luminescence decay behavior was observed using
time-correlated single photon counting technique. Single
bunch operation of the UVSOR was used, and the time
period between the successive excitation pulses was 176
ns. Emitted photons were monochromatized with themonochromator and detected with a multichannel-platemounted photomultiplier tube (Hamamatsu, R3809U-50).
The time difference of the luminescence photon detection signal and the excitation pulse were accumulated
into a histogram as luminescence time profile.

Fig. 2 Luminescence spectra of the Ce-doped and Ce andLa-doped GPS (The excitation wavelength was 60 nm)

photon energy much higher than the band-gap energy,
and excited states are generated in the host matrix. In
both spectra, a prominent band is observed at approximately 390 nm and ascribed to 5d-4f transition of Ce3+.
Thus, Ce3+ emission is observed in the case of photoexcitation of the host matrix, as well as in the case of scintillation. The result in Fig. 2 indicates that the emission
occurs after the energy transfer from the host matrix toCe3+ ions.
Figs. 3–5 show the luminescence time profiles ofCe-doped GPS at 1 mol.%, Ce-doped GPS at 10 mol.%,and Ce and La-doped GPS crystals, respectively. The excitation wavelengths were 60 and 300 nm, which correspond to excitation in the host matrix and of Ce3+ ions,respectively. The excitation wavelength of 60 nm corresponds to the photon energy of about three times the2 Results and discussion
Fig. 1 shows the X-ray-irradiated radioluminescence
spectra of the Ce-doped and Ce and La-doped GPS crystals. Fig. 2 shows the luminescence spectra of theCe-doped and Ce and La-doped GPS crystals. The excitation wavelength was 60 nm. Judged from the host excitation band peaking at 126 nm in a previous paper[12],the excitation wavelength of 60 nm corresponds to a

Fig. 1 X-ray-irradiated radioluminescence spectra of the Cedoped and Ce and La-doped GPS

Fig. 3 Luminescence time profiles of GPS doped with Ce at 1 mol.%(a) In linear scale; (b) In semi-log scale

784

Fig. 4 Luminescence time profiles of GPS doped with Ce at 10mol.%(a) In linear scale; (b) In semi-log scale

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

The emission wavelength was 380 nm. For comparison,the luminescence time profile of GSO doped with Ce
at 1 mol.% is shown in Fig. 6. The emission wavelength
was 440 nm. As mentioned in the above paragraph, theexcitation wavelength of 60 nm corresponds to the excitation in the host matrix. Hence, the time profiles reflect
the energy transfer process in addition to the radiativetransition of Ce3+ ions. In contrast, the excitation wavelength of 300 nm corresponds to the direct excitation ofCe3+ ions, and thus the time profiles reflect only the radiative transition of Ce3+ ions.
At first, a clear difference is seen at the rise of the time
profiles of GPS and GSO. The rise of the time profile ofGSO with excitation at 60 nm is much slower than thatwith excitation at 300 nm. This result is in line with thatof a previous paper[15]. The slow rise indicates the slow
energy transfer process, i.e., the typical time of the energy transfer corresponds to the rise time of ~20 ns. On
the other hand, in the time profiles of the GPS crystals,
no significant difference in the rise was observed. This
result indicates that the energy transfer in GPS crystals ismuch faster than that in GSO, although the chemicalcompositions of the crystals are similar. On the basis of
the rise in the time profiles, the energy transfer in GPS
crystals occurs in less than 2 ns with excitation at 60 nm.
In addition, the decay behavior was different for GPS
crystals with and without La. Table 1 shows the decay
time constants for GPS samples. For Ce-doped GPS, thedecay is slower with excitation at 60 nm than that withexcitation at 300 nm. In contrast, in the case of Ce andLa-doped GPS, the decay behavior was similar for bothcases. As clearly shown in Figs. 3–5 with a vertical axis
in logarithmic scale, the difference in the decay behavior
cannot be explained with appearance or disappearance ofanother component. The decay rate was different at least
for 100 ns with different excitation wavelengths for
Ce-doped GPS. If a slow energy transfer from the hostmatrix to Ce3+ ions contributes to the time profile, a slowrise would be observed similarly to the case of Ce-doped
GSO. The difference lies in the decay part, hence, the radiative decay process is different. The results in Figs. 3and 4 indicate that the energy transfer from the host ma-Fig. 5 Luminescence time profiles of GPS doped with Ce at 1mol.% and La at 10 mol.%(a) In linear scale; (b) In semi-log scale

band-gap energy. For such photon energy, thermalizationand diffusion of electron-hole pairs in the host matrix
play an important role in the luminescence process. Suchprocess has been analyzed in a previous paper in detail[14].

Fig. 6 Luminescence time profiles of GSO doped with Ce at
1 mol.%

M. Koshimizu et al., Luminescence properties of Gd2Si2O7:Ce and Gd2Si2O7:La,Ce under vacuum ultraviolet irradiation 785
Table 1 Decay time constants obtained by fitting the decay
part of the luminescence time profiles of the GPSsamples
Samples

Excitation at

Excitation at

300 nm/ns

60 nm/ns

GPS doped with Ce at 1 mol.%4156

GPS doped with Ce at 10 mol.%

41535260

GPS doped with Ce at 1 mol.% and La at
10 mol.%trix to Ce3+ ions leads to a decay process significantly
different from that in the case of direct excitation of Ce3+.
In addition, the results of Figs. 3–5 indicate that the La
doping influences the energy transfer and decay process.
The 5d levels of La is considered to form the bottom of
the conduction band in GPS[11]. According to a previous
paper on scintillation properties of Ce and La-dopedGPS[11], the La doping increased scintillation efficiency,
which was attributed to the local fluctuation in theband-gap energy leading to suppressed diffusion ofcharge carriers and enhanced radiative recombination
probability. Taking this discussion into consideration, theeffect of La doping in this study may be ascribed to thechange in the energy transfer process owing to thechange in the conduction band structure.

3 Conclusions
The luminescence properties of Ce- and Ce and Ladoped GPS crystals were characterized using VUV excitation light. With excitation at 60 nm, which corresponded
to the excitation in the host matrix, a prominent emission
band owing to 5d-4f transition of Ce3+ was observed. Therise in the luminescence time profiles indicated that theenergy transfer from the host matrix to Ce3+ ions in theGPS crystals occurred in less than 2 ns, which was much
faster than that of Ce-doped GSO. In addition, the decay
was slower for the host excitation than that for direct excitation of Ce3+ in Ce-doped GPS. This difference indicatedthe different radiative decay processes for Ce3+ direct excitation or via energy transfer from the host matrix. For theLa-doped crystal, this difference was not observed. Theseresults indicated that the La doping had an effect on theenergy transfer and radiative decay process in the photoluminescence process, similarly to the case of scintillation.
Acknowledgements: Part of this work was carried out at theUVSOR facility and it was supported by the Joint Studies Program (2014, No. 26–546) of the Institute for Molecular Science.
This work was partially supported by the SPRITS program,
Kyoto University and the Cooperative Research Project of Research Institute of Electronics, Shizuoka University.

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