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Bridgman growth of Bi4Si3O12 scintillation crystals and doped effects on radiation resistance
Progress in Crystal Growth and Characterization PERGAMON
Progress in Crystal Growth and Characterization of Materials of Materials (2000) 189-194 http://www.elsevier.eom/locate/perysgrow
Bridgman Growth of BiaSi3012 Scintillation Crystals and Doped Effects on Radiation Resistance Fei Y i t i n g ", F a n Shiji ~, Sun R e n y i n g ", X u Jiayue ~, M . Ishii b aShanghai Institute of Ceramics, ChineseAcademy of Sciences, Shanghai 200050, P.R.China bShonan Institute of Technology. Fujisawa251, Japan
Abstract Ce, Nd and Eu doped BSO crystals 20x20xl00mm3 in size have been grown by vertical Bridgman method, and the doped effects on radiation resistanceof BSO have also been studied for the first time. Nd and Eu dopings were found to improve the radiationresistanceof BSO. However, Ce and Nd dopings degrade the light output of BSO except that Eu doping has almost no effect on it. Therefore,Eu may be the most promisingdopantcandidatefor improvingthe scintillationpropertiesof BSO crystal. PACS: 42.70; 81.10; 78.70 Key words: DopedBSO crystals; Bridgmangrowth; Light output; Radiationresistance
1. Introduction Bismuth germanate (Bi4Ge3012, BGO) crystal has attracted much interest [1] over the years as an excellent scintillator for applications in medicine, geological exploration, nuclear physics and high energy physics. However, the radiation damage suffered by BGO upon irradiation has limited its use for high precision calorimeters. Recently, bismuth silicate (Bi4Si3012, BSO) crystal has attracted ever-increasing interest and was once studied [2-4] as one of the possible candidates for an alternative to BGO for high energy physics experiments. BSO has the same crystalline structure as BGO and its scintillation comes from the excitation of bismuth similarly as in BGO. Except that BSO has higher radiation resistance, faster decay time and smaller light output than BGO, it resembles BGO in many respects including physical, optical and scintillation characteristics. The material cost of the scintillator may be significantly reduced if GeO2 can be replaced by Si02. Single crystals of BSO were grown by Philipsborn [5] in 1971 using Czochralski method and by Fan [6] and Ishii [7] using Bridgman method. In the present paper, we report the growth of large size and high optical quality doped BSO single crystals by the vertical Bridgman method, and then evaluate their radiation resistance characteristics. We studied the improvement in the scintillation characteristics of BSO through doping investigation. 2. Crystal growth Undoped and doped BSO single crystals were grown in a vertical Bridgman furnace, in which separated molybdenum disilicide bars were used as the heating elements and the vertical temperature gradient at the growth interface was 20-30°C/em. The crucible was fitted with two Pt-10%Rh/Pt thermoeouples, an upper one and a lower one, and the temperature of the furnace was controlled by a DWT-702 fine temperature controller with an accuracy of _+0.5°C. Fig. 1 shows a schematic of the Bridgman furnace used for growing BSO crystals, and its axial temperature profile is shown in Fig.2, in which curves A and B were obtained by the upper and lower thermocouples, respectively. BSO melts congruently with a melting point of about 1030°C. The feed material was prepared by weighing and thoroughly mixing together stoichiometric (2:3) amounts ofBi203 (5N) and SiO2 (4N) as well as a small amount of doped oxides as seen in Table 1, then charged in a platinum crucible of 20×20 mm 2 in square section and 200 mm in length with a BSO seed at its bottom. The crucible was near-sealed in order to control effectively the volatilization of BSO melt during growth, and then placed in a refractory tube filled with alumina powder to steady the crucible as well as to isolate it from external temperature fluctuations. After the crucible was heated to about 1100°C and held at that temperature for several hours to ensure complete melting and mixing of the charge, seeding was performed by adjusting the crucible position and melt temperature so that only the top of the seed melted. Growth was then driven by lowering the crucible at 0960-8974/00/$ - see front matter © 2000 Published by Elsevier Science Ltd. PII: S0960-8974(00)00004-8
190
Fei Yiting et al. / Prog. Crystal Growth and Charact. 40 (2000) 189-194
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Fei Yiting et aL /Prog. Crystal Growth and Charact. 40 (2000) 189-194
191
a rate of 0.2-0.5mm/h. To prevent cracking resulting from thermal stress in BSO crystals, the as-grown boules were heat-treated before cooling to room temperature. The sizes of undoped and doped BSO boules grown under above-mentioned growth conditions were all 20×20×100mm 3. Specimens used for measurements were cut from the as-grown crystals and all the specimen surfaces were lapped and polished. The scintillation properties of doped BSO crystals were measured at Shonan Institute of Technology (SIT) in Japan.
Table 1 BSO samples for measurements Sample Size (ram3) Dopant Concentration (mol%) BSO 17.0x16.0x14.7 None 0 Ce:BSO 19.1×14.1×10.9 CeO2 0.25 Nd:BSO 13.5x10.9×9.7 Nd203 0.2 Eu:BSO 10.7x7.6x6.4 Eu20~ 0.2
Table 2 Light outputsand FWHMenergyresolutions of BSO samples Sample Light output FWHM(%) BGO 100 10 BSO 23 18 Ce:BSO 10 26 Nd:BSO 19 21 Eu:BSO 24 17
3. Results
3.1 Scintillation light output The experimental set-up for measuring light output is shown in Fig. 3. The scintillation light output of BSO samples irradiated with a 137Cs ]/-ray source (0.662MeV) in units of photoelectron numbers per MeV of energy deposition was measured by using a Hamamatsu photomultiplier tube (PMT) R878 at room temperature. The crystals were wrapped with teflon sheet and air-coupled on one end to the PMT with bialkali photocathode. The output of the photomultiplier was shaped by a preamplifier and analyzed by a pulse height analyzer (PHA) operated in the peak voltage mode. To compare the light output of doped BSO with that of B G O , a standard BGO sample 20×20x20 mm 3 in size was also measured under the same experimental conditions. The light outputs (relative to BGO) of undoped and doped BSO crystals and the FWHM energy resolutions are given in Table 2. The results suggest that the FWHM energy resolution of BSO is 18% at 662 KeV and its light output per fixed energy deposit is about 23% of BGO, which is a little larger than the result (~20%) by Kobayashi et al. [2,3]. However, Ce and Nd dopings degrade the light output of BSO to 10% and 19%, respectively, and Eu doping has almost no effect on the light output of BSO. The doping effects on the FWHM energy resolution are similar as that on the light output. 3.2 Transmission spectra and radiation damage Radiation damage was measured using a spectrophotometer (Hitachi U-3210) by comparing the optical transmittance of BSO samples before and after irradiation performed with 6°Co 7-ray source. Six cycles of irradiation followed by transmission measurement were carried out to cover the accumulated dose from 103 to l0 s rad by increasing the accumulated dose by a factor of 10 per cycle. The transmission spectra of BSO samples with different dopants before and after irradiation with 6°Co ]/-rays are shown in Fig. 4 and the corresponding degradation in transmittance at the emission peak (480rim) after irradiation is listed in Table 3. From the results it is evident that Nd and Eu dopings improve the radiation resistance of BSO greatly except for the poor radiation resistance of Ce:BSO. 3.3 Radiation-induced absorption spectra The degradation in transmittance and increasing in absorption caused by irradiation can be quantitatively analyzed in terms of induced absorption coefficient/xi~, which defined as = (I/d) In (To/T),
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Wavelength (am) Fig.4 Transmission spectra of BSO samples before and after irradiation with 6°Co v-rays of different doses, in which the transmission with (ram) of BSO, Ce:BSO, Nd:BSO and Eu:BSO is 14.7, 10.9, 10.9 and 6.4, respectively.
Table 3 Degradation in transmittance ofBSO atthe emission peak (480nm) after irradiation Dose (tad) 103 104 105 106 107 108
BSO (%) -5.3 -12.1 -12.3 -15.0 -12.1 -14,5
Ce:BSO (%) 0.9 -7.6 -43.9 -60.3 -59.9 -61.8
Nd:BSO (%) 5.6 -0.8 0.6 -2.6 -3.5 -18.1
Eu:BSO (%) -2.1 -3.5 -4.9 -5.6 -6.0 -12,9
Table 4 Induced absorption coefficient ~t~for BSO at the emission peak (480nm) after irradiation Dose (rad) 103 104
105 106 l0 T l0 s
BSO (m-z) 3.7 8.7 8.9 11.0 8.7 10.6
Ce:BSO (ma ) -0.8 7.2 52.9 84.6 83,7 88.2
Nd:BSO fin-z) -5.0 0.7 -0.6 2.4 3.3 18,4
Eu:BSO (m"z) 3.2 5.5 7.8 8.9 9.5 21.4
where d is the thickness across which the transmittance To (before irradiation) and T (after irradiation) were measured. Table 4 presents induced absorption coefficient ~t~ for BSO samples at the peak emission
193
Fei }Tting et aL / Prog. Crystal Growth and Charact. 40 (2000) 189-194
wavelength 480nm after irradiation. It can be seen that Ixi~in Nd:BSO increase slowly to 3.3 m ~ at 107 rad and in Eu:BSO increase smoothly from 3.2 m -~ at 10~ tad to 9.5 m q at 107 rad. All of these two dopings decrease the induced absorption in BSO at 480nm except larger/a~ at the accumulated dose 10~ rad. Fig. 5 shows the induced absorption spectra of BSO samples after irradiation with 6°Co 3,-rays of different doses, in which some characteristic absorption peaks corresponding to transitions of 4f electrons of Nd 3+ appear in induced absorption spectrum of Nd:BSO and no significant peaks are detected in the spectrum of Eu:BSO due to the weak absorption of Eu 3+. The results also indicate that the induced absorption in Nd and Eu dopings is much less than that of undoped BSO under the same dosage, and therefore these two dopings improve the radiation hardness of BSO.
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4. Discussions and conclusions BSO crystal belongs to cubic system, in which Bi 3+ and Si4+ are sited in the distorted [BiO6] octahedron and [SiO4] tetrahedron, respectively. The doped ions substitute regularly for the eigenelements and occupy Bi > or Si4+sites while doping. Kobayashi et al. [4] have pointed out that the dominant colour centers in undoped BSO might be due to intrinsic defects such as F-centers rather than to any specific impurity ions. According to the research done by Wei et al. [8] on doped BGO, an isomorph of BSO, we can also conclude that oxygen vacancies present in BSO crystals would be the origin of the radiation induced colour center, and electrons excited by irradiation are trapped in oxygen vacancies and form the colour centers. In Ce doped BSO crystal, Ce 4+ ions can compete for electrons in the lattice and transform to Ce ~÷ through Ce 4+ + e --+ Ce 3÷ so that two valence states, Ce 4÷ and Ce 3+, coexist in Ce doped BSO crystal. However, 4f electrons of Ce3~ can be easily excited to 5d levels whose distribution is broadened by the crystal field
194
Fei Yiting et al. / Prog. Crystal Growth and Charact. 40 (2000) 189-194
because the low 5d levels and high 4f levels overlap partly. The transition 4f ---> 5d of Ce3+ corresponds to the broad band-shaped absorption which shifts the absorption edge of the transmission spectrum to long wave and lowers the light output significantly. The characteristic peaks in the transmission or emission spectra of Nd and Eu doped BSO crystals are due to the light absorption or emission of certain wavelengths resulting from allowed electric-dipole transitions between 4f energy levels of Nd 3+ and Eu 3+. For the strong light absorption, the excited luminescence of Nd 3+ weakens that of Bi3+ so as to lower the light output. However, Eu3+ has little effect on the light output because of its weak light absorption. In addition, Eu 3+ in BSO crystal compete with the oxygen vacancies, and trap most of the electrons which are free in the lattice when the crystal is irradiated. Therefore, the Eu doped BSO has a lower colour center density under the same irradiation condition, which contributes to improve its radiation resistance. In conclusion, the vertical Bridgman growth and scintillation characteristics of Ce, Nd and Eu doped BSO crystals with the sizes 20×20xl00mm 3 have been studied for the first time. Nd and Eu dopings can improve the radiation resistance of BSO. However, Ce and Nd dopings degrade the light output of BSO except that Eu doping has almost no effect on it. For these reasons, we can expect that Eu is the most promising dopant candidate for improving the scintillation properties of BSO crystal.
Acknowledgements The authors would like to express their thanks to the supports by National Natural Science Foundation of China (No. 59472001) and National Laboratory of Crystal Materials at Shandong University.
References [1] MJ. Webber and R.R. Monchamp, J. Appl. Phys. 44 (1973) 5496. [2] M. Kobayashi, M. Ishii, K. Harada, K. and I. Yamaga, Nucl. Instr. Moth. Phys. Res. A372 (1996) 45. [3] M. Kobayashi, K. Morimoto, H. Yoshida, S. Sugimoto, S. Kobayashi, M. Chiba, M. Ishii, S, Akiyama and H. Ishibashi0 Nucl. Instr. Moth. 205 (1983) 133. [4] M. Kobayashi, K. Flamda, Y. Hirose, M. Ishii and I. Yamaga, Nucl. Instr. Meth. Phys. Res. A400 (1997) 392. [5] H.V. Philipsbom, J. Crystal Growth 11 (1971) 348. [6] S.J. Fan, R.Y. Sun, J.D. Wu and Y.F. Ya, The Eleventh International Conference on Crystal Growth, Advance Program (1995) p.30. [7] M. Ishii, M. Kobayashi and I. Yamaga, Heavy Scintill. Sci. Ind. Appl., Proc. "Cryst. 2000" Int. Workshop (1992) p427. [8] Z.Y. Wei, R.Y. Zhu, H. Newman and Z.W. Yin, Nucl. Instr. Meth. Phys. Res. A297 (1990) 163.
Progress in Crystal Growth and Characterization of Materials of Materials (2000) 189-194 http://www.elsevier.eom/locate/perysgrow
Bridgman Growth of BiaSi3012 Scintillation Crystals and Doped Effects on Radiation Resistance Fei Y i t i n g ", F a n Shiji ~, Sun R e n y i n g ", X u Jiayue ~, M . Ishii b aShanghai Institute of Ceramics, ChineseAcademy of Sciences, Shanghai 200050, P.R.China bShonan Institute of Technology. Fujisawa251, Japan
Abstract Ce, Nd and Eu doped BSO crystals 20x20xl00mm3 in size have been grown by vertical Bridgman method, and the doped effects on radiation resistanceof BSO have also been studied for the first time. Nd and Eu dopings were found to improve the radiationresistanceof BSO. However, Ce and Nd dopings degrade the light output of BSO except that Eu doping has almost no effect on it. Therefore,Eu may be the most promisingdopantcandidatefor improvingthe scintillationpropertiesof BSO crystal. PACS: 42.70; 81.10; 78.70 Key words: DopedBSO crystals; Bridgmangrowth; Light output; Radiationresistance
1. Introduction Bismuth germanate (Bi4Ge3012, BGO) crystal has attracted much interest [1] over the years as an excellent scintillator for applications in medicine, geological exploration, nuclear physics and high energy physics. However, the radiation damage suffered by BGO upon irradiation has limited its use for high precision calorimeters. Recently, bismuth silicate (Bi4Si3012, BSO) crystal has attracted ever-increasing interest and was once studied [2-4] as one of the possible candidates for an alternative to BGO for high energy physics experiments. BSO has the same crystalline structure as BGO and its scintillation comes from the excitation of bismuth similarly as in BGO. Except that BSO has higher radiation resistance, faster decay time and smaller light output than BGO, it resembles BGO in many respects including physical, optical and scintillation characteristics. The material cost of the scintillator may be significantly reduced if GeO2 can be replaced by Si02. Single crystals of BSO were grown by Philipsborn [5] in 1971 using Czochralski method and by Fan [6] and Ishii [7] using Bridgman method. In the present paper, we report the growth of large size and high optical quality doped BSO single crystals by the vertical Bridgman method, and then evaluate their radiation resistance characteristics. We studied the improvement in the scintillation characteristics of BSO through doping investigation. 2. Crystal growth Undoped and doped BSO single crystals were grown in a vertical Bridgman furnace, in which separated molybdenum disilicide bars were used as the heating elements and the vertical temperature gradient at the growth interface was 20-30°C/em. The crucible was fitted with two Pt-10%Rh/Pt thermoeouples, an upper one and a lower one, and the temperature of the furnace was controlled by a DWT-702 fine temperature controller with an accuracy of _+0.5°C. Fig. 1 shows a schematic of the Bridgman furnace used for growing BSO crystals, and its axial temperature profile is shown in Fig.2, in which curves A and B were obtained by the upper and lower thermocouples, respectively. BSO melts congruently with a melting point of about 1030°C. The feed material was prepared by weighing and thoroughly mixing together stoichiometric (2:3) amounts ofBi203 (5N) and SiO2 (4N) as well as a small amount of doped oxides as seen in Table 1, then charged in a platinum crucible of 20×20 mm 2 in square section and 200 mm in length with a BSO seed at its bottom. The crucible was near-sealed in order to control effectively the volatilization of BSO melt during growth, and then placed in a refractory tube filled with alumina powder to steady the crucible as well as to isolate it from external temperature fluctuations. After the crucible was heated to about 1100°C and held at that temperature for several hours to ensure complete melting and mixing of the charge, seeding was performed by adjusting the crucible position and melt temperature so that only the top of the seed melted. Growth was then driven by lowering the crucible at 0960-8974/00/$ - see front matter © 2000 Published by Elsevier Science Ltd. PII: S0960-8974(00)00004-8
190
Fei Yiting et al. / Prog. Crystal Growth and Charact. 40 (2000) 189-194
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Fei Yiting et aL /Prog. Crystal Growth and Charact. 40 (2000) 189-194
191
a rate of 0.2-0.5mm/h. To prevent cracking resulting from thermal stress in BSO crystals, the as-grown boules were heat-treated before cooling to room temperature. The sizes of undoped and doped BSO boules grown under above-mentioned growth conditions were all 20×20×100mm 3. Specimens used for measurements were cut from the as-grown crystals and all the specimen surfaces were lapped and polished. The scintillation properties of doped BSO crystals were measured at Shonan Institute of Technology (SIT) in Japan.
Table 1 BSO samples for measurements Sample Size (ram3) Dopant Concentration (mol%) BSO 17.0x16.0x14.7 None 0 Ce:BSO 19.1×14.1×10.9 CeO2 0.25 Nd:BSO 13.5x10.9×9.7 Nd203 0.2 Eu:BSO 10.7x7.6x6.4 Eu20~ 0.2
Table 2 Light outputsand FWHMenergyresolutions of BSO samples Sample Light output FWHM(%) BGO 100 10 BSO 23 18 Ce:BSO 10 26 Nd:BSO 19 21 Eu:BSO 24 17
3. Results
3.1 Scintillation light output The experimental set-up for measuring light output is shown in Fig. 3. The scintillation light output of BSO samples irradiated with a 137Cs ]/-ray source (0.662MeV) in units of photoelectron numbers per MeV of energy deposition was measured by using a Hamamatsu photomultiplier tube (PMT) R878 at room temperature. The crystals were wrapped with teflon sheet and air-coupled on one end to the PMT with bialkali photocathode. The output of the photomultiplier was shaped by a preamplifier and analyzed by a pulse height analyzer (PHA) operated in the peak voltage mode. To compare the light output of doped BSO with that of B G O , a standard BGO sample 20×20x20 mm 3 in size was also measured under the same experimental conditions. The light outputs (relative to BGO) of undoped and doped BSO crystals and the FWHM energy resolutions are given in Table 2. The results suggest that the FWHM energy resolution of BSO is 18% at 662 KeV and its light output per fixed energy deposit is about 23% of BGO, which is a little larger than the result (~20%) by Kobayashi et al. [2,3]. However, Ce and Nd dopings degrade the light output of BSO to 10% and 19%, respectively, and Eu doping has almost no effect on the light output of BSO. The doping effects on the FWHM energy resolution are similar as that on the light output. 3.2 Transmission spectra and radiation damage Radiation damage was measured using a spectrophotometer (Hitachi U-3210) by comparing the optical transmittance of BSO samples before and after irradiation performed with 6°Co 7-ray source. Six cycles of irradiation followed by transmission measurement were carried out to cover the accumulated dose from 103 to l0 s rad by increasing the accumulated dose by a factor of 10 per cycle. The transmission spectra of BSO samples with different dopants before and after irradiation with 6°Co ]/-rays are shown in Fig. 4 and the corresponding degradation in transmittance at the emission peak (480rim) after irradiation is listed in Table 3. From the results it is evident that Nd and Eu dopings improve the radiation resistance of BSO greatly except for the poor radiation resistance of Ce:BSO. 3.3 Radiation-induced absorption spectra The degradation in transmittance and increasing in absorption caused by irradiation can be quantitatively analyzed in terms of induced absorption coefficient/xi~, which defined as = (I/d) In (To/T),
Fei Yiting et al. / Prog. Crystal Growth and Charact. 40 (2000) 189-194
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Wavelength (am) Fig.4 Transmission spectra of BSO samples before and after irradiation with 6°Co v-rays of different doses, in which the transmission with (ram) of BSO, Ce:BSO, Nd:BSO and Eu:BSO is 14.7, 10.9, 10.9 and 6.4, respectively.
Table 3 Degradation in transmittance ofBSO atthe emission peak (480nm) after irradiation Dose (tad) 103 104 105 106 107 108
BSO (%) -5.3 -12.1 -12.3 -15.0 -12.1 -14,5
Ce:BSO (%) 0.9 -7.6 -43.9 -60.3 -59.9 -61.8
Nd:BSO (%) 5.6 -0.8 0.6 -2.6 -3.5 -18.1
Eu:BSO (%) -2.1 -3.5 -4.9 -5.6 -6.0 -12,9
Table 4 Induced absorption coefficient ~t~for BSO at the emission peak (480nm) after irradiation Dose (rad) 103 104
105 106 l0 T l0 s
BSO (m-z) 3.7 8.7 8.9 11.0 8.7 10.6
Ce:BSO (ma ) -0.8 7.2 52.9 84.6 83,7 88.2
Nd:BSO fin-z) -5.0 0.7 -0.6 2.4 3.3 18,4
Eu:BSO (m"z) 3.2 5.5 7.8 8.9 9.5 21.4
where d is the thickness across which the transmittance To (before irradiation) and T (after irradiation) were measured. Table 4 presents induced absorption coefficient ~t~ for BSO samples at the peak emission
193
Fei }Tting et aL / Prog. Crystal Growth and Charact. 40 (2000) 189-194
wavelength 480nm after irradiation. It can be seen that Ixi~in Nd:BSO increase slowly to 3.3 m ~ at 107 rad and in Eu:BSO increase smoothly from 3.2 m -~ at 10~ tad to 9.5 m q at 107 rad. All of these two dopings decrease the induced absorption in BSO at 480nm except larger/a~ at the accumulated dose 10~ rad. Fig. 5 shows the induced absorption spectra of BSO samples after irradiation with 6°Co 3,-rays of different doses, in which some characteristic absorption peaks corresponding to transitions of 4f electrons of Nd 3+ appear in induced absorption spectrum of Nd:BSO and no significant peaks are detected in the spectrum of Eu:BSO due to the weak absorption of Eu 3+. The results also indicate that the induced absorption in Nd and Eu dopings is much less than that of undoped BSO under the same dosage, and therefore these two dopings improve the radiation hardness of BSO.
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550
600
4. Discussions and conclusions BSO crystal belongs to cubic system, in which Bi 3+ and Si4+ are sited in the distorted [BiO6] octahedron and [SiO4] tetrahedron, respectively. The doped ions substitute regularly for the eigenelements and occupy Bi > or Si4+sites while doping. Kobayashi et al. [4] have pointed out that the dominant colour centers in undoped BSO might be due to intrinsic defects such as F-centers rather than to any specific impurity ions. According to the research done by Wei et al. [8] on doped BGO, an isomorph of BSO, we can also conclude that oxygen vacancies present in BSO crystals would be the origin of the radiation induced colour center, and electrons excited by irradiation are trapped in oxygen vacancies and form the colour centers. In Ce doped BSO crystal, Ce 4+ ions can compete for electrons in the lattice and transform to Ce ~÷ through Ce 4+ + e --+ Ce 3÷ so that two valence states, Ce 4÷ and Ce 3+, coexist in Ce doped BSO crystal. However, 4f electrons of Ce3~ can be easily excited to 5d levels whose distribution is broadened by the crystal field
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because the low 5d levels and high 4f levels overlap partly. The transition 4f ---> 5d of Ce3+ corresponds to the broad band-shaped absorption which shifts the absorption edge of the transmission spectrum to long wave and lowers the light output significantly. The characteristic peaks in the transmission or emission spectra of Nd and Eu doped BSO crystals are due to the light absorption or emission of certain wavelengths resulting from allowed electric-dipole transitions between 4f energy levels of Nd 3+ and Eu 3+. For the strong light absorption, the excited luminescence of Nd 3+ weakens that of Bi3+ so as to lower the light output. However, Eu3+ has little effect on the light output because of its weak light absorption. In addition, Eu 3+ in BSO crystal compete with the oxygen vacancies, and trap most of the electrons which are free in the lattice when the crystal is irradiated. Therefore, the Eu doped BSO has a lower colour center density under the same irradiation condition, which contributes to improve its radiation resistance. In conclusion, the vertical Bridgman growth and scintillation characteristics of Ce, Nd and Eu doped BSO crystals with the sizes 20×20xl00mm 3 have been studied for the first time. Nd and Eu dopings can improve the radiation resistance of BSO. However, Ce and Nd dopings degrade the light output of BSO except that Eu doping has almost no effect on it. For these reasons, we can expect that Eu is the most promising dopant candidate for improving the scintillation properties of BSO crystal.
Acknowledgements The authors would like to express their thanks to the supports by National Natural Science Foundation of China (No. 59472001) and National Laboratory of Crystal Materials at Shandong University.
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