(Lu2SiO5:Ce, namely LSO:Ce) became of interest in early nineties of last century [1], [2] and is being extensively studied as a promising scintillator in the field of ...
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SCINT2009 Jeju, Korea Thermoluminescence (TL) measurements after RT x-ray irradiation were performed from RT to 500 oC with a linear heating rate of 1 oC /s by using a ROSB TL/OSL 3DS wavelength-resolved TL spectrometer. The LSO:Ce crystal samples were irradiated by incandescent light with a power of 25W for 30 minutes before the afterglow measurements; after that the samples were coupled to XP2262B PMT, which was then loaded by a voltage of -1600V, and the afterglow signal (shown as voltage) was detected by TDS-220 oscillograph . LSO:Ce samples with dimensions of 3×3×1 mm3 were used for TL measurements, and dimensions of 10×10×2 mm3 were used for all the other measurements. B. Sample Preparation for XANES Measurement In order to obtain high-quality XANES spectra, the content of probed ions in the samples, must be at least few percent. The concentration of cerium ions in LSO:Ce single crystal samples grown from the melt can hardly exceed some 0.1wt%. However, LSO:Ce polycrystalline samples with much higher content of cerium can be prepared, in fact the LSO:Ce polycrystalline samples with high content of cerium ((Lu0.95Ce0.05)2SiO5, the content of cerium ions in it is 3.08wt%) were prepared and used in this study. We sintered these samples in the conditions similar to those of crystal preparation and post-processing. By analyzing the valence state of cerium ion in such polycrystalline samples, we can conclude about the valence state changes of cerium ion in LSO:Ce crystal samples.
T11-2 < hours respectively to form polycrystalline (Lu0.95Ce0.05)2SiO5.
Fig. 2. X-ray excited emission spectra of LSO:Ce single crystal before (as indicated by thick black solid lines) and after (as indicated by thick red dashed lines) annealing in air atmosphere at 1400 oC for 10 hours. The thin black solid lines correspond to the fitting bands before annealing, the thin red dashed lines correspond to the fitting bands after annealing, and the thin red solid lines are the fitting curves. The small figure plotted in the upper right corner of the figure presents a comparison of the integral intensity of emission spectra before and after annealing, showing that the total luminescence intensity increases after annealing. TABLE I THE INTEGRAL INTENSITY AND INTENSITY RATIOS OF FITTING BANDS OF UV AND X-RAY EXCITED EMISSION SPECTRA OF LSO:CE SINGLE CRYSTAL O BEFORE AND AFTER ANNEALING IN AIR ATMOSPHERE AT 1400 C FOR 10 HOURS
Excit ation
Anneal ing
Before UV After
Before Fig. 1. UV excitation (λem=404nm) and emission (λex=359nm) spectra of LSO:Ce single crystal before (as indicated by thick black solid lines) and after (as indicated by thick red dashed lines) annealing in air atmosphere at 1400 oC for 10 hours. The thin black solid lines correspond to the fitting bands before annealing, the thin red dashed lines correspond to the fitting bands after annealing, and the thin green solid lines are the fitting curves. The small figure plotted in the upper left corner of the figure presents a comparison of the intensity normalized UV excited emission spectra before and after annealing.
The polycrystalline samples of (Lu0.95Ce0.05)2SiO5 were prepared for XANES measurements according to the following procedure. Their starting materials were Lu2O3, SiO2 and CeO2 powders with purity of 99.99%; they were weighed precisely, mixed fully and then compressed to tablets under a pressure of 20Mpa. Half of these tablets were sintered in air atmosphere at 1600 oC for 48 hours and in argon atmosphere at 1700 oC for 10
Xray After
Fitting emission bands Peaks
R
S
T
U
Position (nm) Integral intensity Intensity ratio Position (nm) Integral intensity Intensity ratio Position (nm) Integral intensity Intensity ratio Position (nm) Integral intensity Intensity ratio
393
412
8988
14508
1 395
1.61 412
10139
16998
1 391
1.68 415
434 2040 1 2,27 436 3468 3 3.42 435
476
854
981
1496
840
1 391
1.15 416
1.75 439
0.98 479
570
661
1424
2418
1
1.16
2.50
4.24
X-ray diffraction (XRD) analysis results show that the polycrystals are pure LSO phase. The LⅢ edge XANES spectra of cerium were obtained at the XAFS station of BSRF (Beijing Synchrotron Radiation Facility) under normal storage ring conditions (2.2GeV and 150-250mA). Spectra were recorded around Ce LⅢ edge (5.723keV) using a Si (111) double-crystal monochromator, in fluorescence mode for LSO:Ce polycrystalline samples, and in transmission mode for Ce4+ and Ce3+ standard samples (CeO2 and CeF3). All of the samples were measured at RT, and the background of their spectra was
> SCINT2009 Jeju, Korea removed with a standard procedure. III. RESULTS AND DISCUSSION A. Annealing Effects on Luminous Efficiency The LSO:Ce single crystal sample with dimensions of 10×10×2 mm3 was annealed in air atmosphere at 1400 oC for 10 hours. By comparing the spectra before and after the annealing treatments, we found that both UV excited emission spectra and XEL spectra show an intensity increase. The peak profile of luminescence changes—the portion of luminescent intensity at the long wavelength side in the total luminescent intensity increases. As shown in Fig.1 and Fig.2, the emission spectra can be fitted with three or four Gaussian bands with peak positions at 393, 412, 434 and 476nm (marked with R, S, T and U respectively). Such a fit is made solely to compare quantitatively the changes in emission spectra induced by the annealing. As shown in table I, by comparing the area (integral intensity of luminescence) under the fitting bands, we found that the percentages of areas with bands peaking at T and U in the whole luminescence increase after annealing. Besides, we found that the area ratio of the bands S and R remains almost the same before and after annealing, whether the spectra were excited by UV (the ratio being 1.61 vs. 1.68) or X-ray (the ratio being 1.15 vs. 1.16). This means the luminescence with band peak positions at R and S belongs to the luminescence of cerium at the same crystallographical site. These two emission subbands are due to the transition of 5d1→4f level of trivalent cerium. The peak positions of fitting bands are in agreement with previous work [10]. Bands peaking at R and S correspond to the luminescence of Ce1 (7-oxygen-coordinated) [11], while bands peaking at T and U can be attributed to the luminescence of Ce2 (6-oxygen-coordinated) [11].
T11-2 < The pulse height spectra excited by gamma ray (as shown in Fig. 3) show that the light output (indicated by channel number, shortened as C.N.) increases after annealing (it is about 28% higher than that of the sample before annealing), while energy resolution (shortened as E.R.) gets worse from 8.4% to 10.8%. The increase of light output is in agreement with the previous analysis. The reason why E.R. gets worse is not clear but may be related to the increase of luminescence intensity of Ce2 (6-oxygen-coordinated) and its increased involvement into scintillation mechanism.
Fig. 4. The wavelength-resolved TL spectra of LSO:Ce single crystal before annealing (indicated in (a1)) and after annealing (indicated in (b1)) in air atmosphere at 1400 oC for 10 hours. The contour line diagrams before and after annealing are shown in (a2) and (b2) respectively.
Fig. 3. Gamma-ray spectra of 662KeV line of 137Cs measured with LSO:Ce sample before and after annealing in air atmosphere at 1400 oC for 10 hours. The peak position indicates position of 662KeV line.
Fig. 5. The TL curves of the LSO:Ce crystal before and after annealing in air atmosphere at 1400 oC for 10 hours.
The integral intensity of UV excited luminescence of the LSO:Ce single crystal sample is about 40% higher than that of the sample before annealing, while the XEL intensity increases only 20% after annealing. Possible causes for this phenomenon may consist in different excitation energy transfer when using the UV or X-ray excitation.
B. Annealing Effects on Electron Traps As shown in fig. 4, the wavelength-resolved TL spectra present the feature of the luminescence of LSO:Ce single crystal (with peak position around 400nm) before and after annealing. The peak positions in TL curves, being 89 and 161
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SCINT2009 Jeju, Korea Charles L. Melcher failed to validate the existence of Ce4+ in LSO:Ce single crystal [15], XANES was used in this paper to
T11-2 < the sample synthesized in argon atmosphere, the Ce3+ absorption feature is obtained, which agrees with what was reported in [15]. The XANES spectra of cerium in the sample synthesized in air atmosphere apparently possess the Ce4+ absorption feature (located at 5.732keV). These results show that high temperature combined with air atmosphere may cause oxidization of Ce3+ in LSO:Ce into Ce4+ (a nonradiative center). As shown in fig.9, the intensity of XEL of (Lu0.95Ce0.05)2SiO5 sintered in air atmosphere is much lower than that of (Lu0.95Ce0.05)2SiO5 sintered in Ar atmosphere. Besides, it was reported in [16] that the luminescence intensity of LPS:Ce began to decrease when the annealing time exceeded about 5 hours (annealing in air atmosphere at 1400 oC with the sample dimensions of 7×7×7 mm3). The main reason for these phenomena may consist in the oxidization of Ce3+ to Ce4+. IV. CONCLUSION
Fig. 8. The upper part of the figure is the X-ray absorption near edge spectra (XANES) of Ce4+ and Ce3+ guide samples recorded near cerium’s LⅢ-edge (5.723keV), and the lower part of the figure is XANES spectra (near LⅢ-edge) of cerium in (Lu0.95Ce0.05)2SiO5 samples, which were sintered in Argon atmosphere at 1700 oC for 10 hours and in air atmosphere at 1600 oC for 48 hours respectively.
Concluding, the annealing of LSO:Ce single crystal in air atmosphere helps to reduce the content of deep electron traps, which possibly include oxygen vacancy, lowers the intensity of TL and afterglow, shortens the decay time of afterglow and increases the luminescence intensity of Ce2. However, such annealing may also cause cerium ion in the crystal to be oxidized to +4 charge state, which may introduce an additional nonradiative center, thereby weakening luminescence. To optimize the scintillation figure-of-merit of LSO:Ce crystal, it is necessary to establish a proper annealing conditions according to the atmosphere in which LSO:Ce crystals are grown and the size of the sample. Besides, the intrinsic mechanism of the oxidization of cerium ion needs to be further explored. ACKNOWLEDGMENT
Fig. 9. The XEL spectra of LSO:Ce polycrystalline samples sintered in Ar atmosphere at 1700 oC for 10 hours and in air atmosphere at 1600 oC for 48 hours respectively.
study the valence state of cerium ion in the orthosilicate system. To enhance the signal for easy detection, we doped LSO with a high content of cerium (5at.%), and synthesized polycrystalline materials, see Section II. B.. The synthesized polycrystalline powder samples were analyzed by XRD method, and both of them present a pure LSO structure. We chose CeO2 (Ce4+), CeF3 (Ce3+) and fully mixed mixture with the mol ratio of n(CeO2):n(CeF3)=1 (Ce4+ and Ce3+) as the standard sample for cerium ion valence state characterization. As shown in fig.8, the XANES spectra of these standard samples differ sharply from each other, and those of the above-mentioned synthesized LSO:Ce powder samples do show the valence of cerium ion: in
We are grateful to Prof. Hu TianDou, Xie YaNing, Kui ReXi and Ph. D candidate Yang FeiFei in Beijing Synchrotron Radiation Facility for the XAFS, photoelectron spectroscopy beam time (Project ID: vr-08012) and XANES data acquisition. Our thanks also go to Prof. Xu ZiZong in University of Science and Technology of China for the help in afterglow measurements, as well as Prof. Tang Qiang in Zhongshan University in China for the help in TL spectra data acquisition. REFERENCES [1] [2]
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