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Energy transfer from Gd3+ to Sm3+ and luminescence characteristics of CaO–Gd2O3–SiO2–B2O3 scintillating glasses
Journal of Luminescence 181 (2017) 382–386
Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Energy transfer from Gd3 þ to Sm3 þ and luminescence characteristics of CaO–Gd2O3–SiO2–B2O3 scintillating glasses N. Wantana a,b, S. Kaewjaeng c, S. Kothan c, H.J. Kim d, J. Kaewkhao a,b,n a
Center of Excellence in Glass Technology and Materials Science (CEGM), Nakhon Pathom Rajabhat University, Nakhon Pathom 73000, Thailand Physics Program, Faculty of Science and Technology, Nakhon Pathom Rajabhat University, Nakhon Pathom 73000, Thailand c Department of Radiologic Technology, Faculty of Associated Medical Sciences, Chiang Mai University, Chiang Mai 50200, Thailand d Department of Physics, Kyungpook National University, Daegu 702-701, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 17 May 2016 Received in revised form 12 September 2016 Accepted 22 September 2016
In this work, density, optical, photoluminescence and x-ray scintillation properties of calcium gadolinium silicoborate glass (CaO:Gd2O3:SiO2:B2O3) have been investigated. Glasses were prepared by the conventional melt quenching technique. The results show that the density of glass increased with increasing of Sm2O3. The optical spectra of glass shows eleven discrete absorption bands at 360, 373, 403, 475, 944, 1077, 1225, 1368, 1468, 1520 and 1586 nm which are due to the transitions of 6H5/2 to 4D3/2, 6P7/2, 6P3/2, 4 I11/2, 6F11/2, 6F9/2, 6F7/2, 6F5/2, 6F3/2, 6H15/2 and 6F1/2, respectively. The emission spectra were observed and assigned to 312, 563, 600, 646 and 703 nm by excitation at 275 nm. The emission intensity of Sm3 þ increased with increasing of Sm2O3 concentration until 0.25 mol%, while decay time decrease with increasing of Sm2O3 content. For results of the radioluminescence (RL), they perform four emission peaks with the strongest emission at 598 nm. Glass doped with 0.35 mol% Sm2O3 show the highest RL emission. The integral scintillation efficiency of 0.35 mol% Sm2O3 doped glass was determined as 25% of commercial BGO scintillator crystal. & 2016 Elsevier B.V. All rights reserved.
Keyword: Glass Luminescence Scintillator BGO
1. Introduction Glass scintillator can be used for detection of X-rays and neutrons. Glass material performs the variety of advantage such as easy synthesis, low-price, high optical homogeneity, large volume production and various shaping [1,2]. Borate glasses are the technologically important glass formers and play a significant role in various applications. Furthermore, increase of B2O3 content from 10 mol% to 30 mol% resulted in the emergence of [BO3] units and Non-Bridging Oxygen (NBOs) growth [3–5]. In part of the silicate glasses, they were used as host material for luminescence study of rare-earth and transition metal ions, due to they have good optical and mechanical properties as well as good chemical durability [6].Cooperative of these two glass formers, borosilicate glasses perform low melting temperature, low coefficients of thermal expansion and can be used in relatively high temperatures [7]. The effect of adding calcium, it can increase intensity of optical absorption and luminescence emission in glass [8]. For rare earth n Correspond author at: Center of Excellence in Glass Technology and Materials Science (CEGM), Nakhon Pathom Rajabhat University, Nakhon Pathom 73000, Thailand.
http://dx.doi.org/10.1016/j.jlumin.2016.09.050 0022-2313/& 2016 Elsevier B.V. All rights reserved.
oxides, intensive Gd2O3 are popular material due to the efficient energy transfer from Gd3 þ ions to the luminescence activators, high thermal neutron capture cross-section and increase the light yield of emission [9,10]. In recent years, glass scintillators containing high Gd2O3 contents have been succeed in various germinate, phosphate, silicate and borosilicate glasses with fast decay time [1,11]. Glasses containing Sm3 þ ions have the extensive interest due to their potential application for high-density optical storage, under sea communication and color displays [12]. The optical properties of Sm3 þ doped glasses have attracted much attention because of their technological applications. Additionally, the Sm3 þ ions exhibit broad emission bands due to 4G5/2-6HJ (J ¼5/2, 7/2, 9/2, 11/2) transitions in any host matrix [13]. It is also well known that the intensities of emission bands of Sm3 þ ion in glasses depend on its concentration and glass composition [14]. Several literatures were published luminescence properties of Sm3 þ in different glass host such as tellurite, phosphate, silicate and borate glasses [13,15–20]. In this work, a new series of CaO:Gd2O3:SiO2:B2O3 glass have been prepared by melted-quenching technique. Sm2O3 was doped in CaO:Gd2O3:SiO2:B2O3 glasses with different concentrations for study in physical, optical, luminescence properties and scintillation efficiency. This glass has never been studied in any detail and
N. Wantana et al. / Journal of Luminescence 181 (2017) 382–386
this work is the first time for understanding the effects of glass composition and Sm2O3 concentration on properties in CaO: Gd2O3:SiO2:B2O3 glass system, developed for using to glass scintillator.
383
of glasses were measured with a Cu target X-ray generator (Inel, XRG3D-E), whose X-ray source was operated at 50 kV and 30 mA, and the spectrometer (QE65 Pro, Ocean Optics) with an optical fiber to detect the emission spectra, as shown in Fig. 1.
2. Experiment 3. Results Glasses compositions of 10CaO:25Gd2O3:10SiO2:(55-x)B2O3: xSm2O3 (where x are 0.05, 0.15, 0.25, 0.35, 0.45, 0.50 and 1.00 mol%) were prepared by melt quenching technique. The high purity chemicals, CaO, Gd2O3, SiO2, H3BO3 and Sm2O3 were mixed thoroughly in an alumina crucible. Each batch of formulas was weighted to 30 g melted in an electric furnace at a temperature of 1400 °C for 3 hours. The melted material was quenched in preheated stainless-steel molds, which would yielded several batches of glass samples. The glass samples were further annealed at 550 °C for 3 hours in order to remove thermal strains. Glasses were cut and polished to 1.0 1.5 0.3 cm3 for property investigations. The density measurement was applied by Archimedes's principle, glasses were weighted in air and water as an immersion liquid using a 4-digit sensitive microbalance (AND, HR-200). The optical spectra of glass samples were measured with a UV–vis–NIR spectrophotometer (Shimadzu UV-3600) in the wavelength range 350–1800 nm. The excitation, emission spectra and decay curves were recorded by using a spectrofluorophotometer (Cary-Eclipse) with xenon lamp as a light source. The non-exponential decay curves were fitted, to evaluate energy transfer parameter (Q), with the Inokuti–Hirayama (IH) model ( 3=s ) t t I ðt Þ ¼ I 0 exp Q
τ0
All the Sm3 þ doped CaO:Gd2O3:SiO2:B2O3 glasses exhibit soft yellow color and high transparency, as shown in Fig. 2.
4. Density Glass density tend to increase with increasing of Sm2O3 concentration, as shown in Fig. 3. It means that there is a change in the structural arrangement of the atoms with Sm2O3 addition in CaO:Gd2O3:SiO2:B2O3 network which shows that borate is replaced by samarium oxide since the density of Sm2O3 is 8.347 g/cm3 while that of B2O3 is 2.460 g/cm3. The increased density of the samples is due to higher molecular weight of samarium than any other component of the given glass system.
τ0
where I(t) is fluorescence intensity at time after excitation (t), I0 is fluorescence intensity at initiate time, and τ0 is the intrinsic decay time of the donors in the absence of acceptors. The value of S (6, 8 or 10) depends on whether the dominant mechanism of the interaction is dipole–dipole, dipole–quadrupole or quadrupole– quadrupole, respectively [21,22]. The X-ray luminescence spectra
Fig. 2. The Sm3 þ doped CaO:Gd2O3:SiO2:B2O3 glasses. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 1. (a) Experimental setup and (b) schematic diagram of the X-ray induced optical luminescence spectrometer.
Fig. 3. The density of the Sm2O3 doped CaO:Gd2O3:SiO2:B2O3 glasses.
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N. Wantana et al. / Journal of Luminescence 181 (2017) 382–386
Fig. 4. The absorption spectra of the Sm2O3 doped CaO:Gd2O3:SiO2:B2O3 glasses.
Fig. 6. The emission spectra of the Sm2O3 doped CaO:Gd2O3:SiO2:B2O3 glasses.
Fig. 5. The excitation spectrum 0.05 mol% Sm2O3 doped CaO:Gd2O3:SiO2:B2O3 glass.
5. Optical property The absorption spectra for the Sm2O3 doped CaO:Gd2O3:SiO2: B2O3 glasses as shown in Fig. 4. The optical spectra of glass shows eleven discrete absorption bands at 360, 373, 403, 475, 944, 1077, 1225, 1368, 1468, 1520 and 1586 nm which are due to the transitions of 6H5/2 to 4D3/2, 6P7/2, 6P3/2, 4I11/2, 6F11/2, 6F9/2, 6F7/2, 6F5/2, 6F3/ 6 6 2, H15/2 and F1/2, respectively [12–23]. Furthermore, absorption bands show more sharpness with increasing of Sm2O3 concentration. This represent higher absorption ability of glass sample with higher amount of Sm3 þ in glass.
6. Photoluminescence Fig. 5 shows the typical excitation spectrum at 0.05 mol% of Sm3 þ doped CaO:Gd2O3:SiO2:B2O3 glass, monitoring emission at 600 nm. Gd3 þ has exhibited four major bands centered at 246 nm (8S7/2-6D7/2), 253 nm (8S7/2-6D9/2), 275 nm (8S7/2-6I9/2) and 312 nm (8S7/2-6P7/2) [11,24], other peak excitation of Sm3 þ such as 345 nm (6H5/2-4D7/2), 361 nm (6H5/2-4D3/2), 375 nm (6H5/2-6P7/ 403 nm (6H5/2-6P3/2), 415 nm (6H5/2-6P5/2), 439 nm 2),
Fig. 7. Energy level diagram of Gd3 þ and Sm3 þ ions in the Gd2O3:CaO:SiO2:B2O3 glasses.
(6H5/2-4G9/2), 462 nm (6H5/2-4I13/2), 475 nm (6H5/2-4M15/2) and 500 nm (6H5/2-4G7/2) [18–22]. From the excitation spectrum, glass show the strongest peak at 275 nm then glasses were excited by this wavelength to investigate the emission spectra as shown in Fig. 6. These spectra show five emission bands at 563, 600, 646 and 703 nm representing the energy transitions of the Sm3 þ from 4G5/2 ground state to 6H5/2, 6H7/2, 6H9/2 and 6H11/2, respectively [12–23]. These emission cause by the energy transfer from Gd3 þ to Sm3 þ as shown in energy level diagram in Fig. 7. The emission intensity of Gd3 þ at 312 nm decrease with increasing of Sm2O3 concentration, while all emission intensity of Sm3 þ increase until
N. Wantana et al. / Journal of Luminescence 181 (2017) 382–386
385
Table 1 Experimental lifetimes (τexp, ms) of the 4G5/2 level of Sm3 þ and the energy transfer parameter (Q) of the Sm3 þ doped CaO:Gd2O3:SiO2:B2O3 glasses. Sm3 þ ions concentration, mol%
τexp (ms)
Q
0.05 0.15 0.25 0.35 0.45 0.50 1.00
2.320 2.090 1.885 1.685 1.585 1.501 1.075
– 0.143 0.296 0.451 0.512 0.576 1.003
Fig. 8. The emission spectra of 0.25 mol% Sm2O3 doped glasses with different Gd2O3 concentration.
Fig. 10. The X-rays induced optical luminescence spectra of the Sm3 þ doped CaO: Gd2O3:SiO2:B2O3 glasses.
Fig. 9. Decay curves of the Sm3 þ doped CaO:Gd2O3:SiO2:B2O3 glasses.
0.25 mol% concentration of Sm2O3. Reducing of Gd3 þ emission simultaneously with increasing of Sm3 þ emission is a quite evidence of the energy transfer phenomena from Gd3 þ to Sm3 þ . The Gd3 þ -Sm3 þ energy transfer still appear even in glass that dope Sm2O3 higher than 0.25 mol% but in this condition, it was influenced with concentration quenching effect that make Sm3 þ emission decrease. To study the optimal concentration of Gd2O3 in glasses, Glass samples with 10CaO:yGd2O3:10SiO2:(79.75-y)B2O3:0.25Sm2O3 (where y are 15, 20, 25, 30 and 35 mol%) system have been fabricated in the similar preparing condition. The emission spectra are shown in Fig. 8. These spectra exhibit emission bands at 563, 600, 646 and 703 nm representing the energy transitions of the Sm3 þ from 4G5/2 ground state to 6H5/2, 6H7/2, 6H9/2 and 6H11/2, respectively [12–23]. The emission intensity of Sm3 þ increase with increasing Gd2O3 concentration until 25 mol%. Therefore the optimum concentrations of Gd3 þ ion is 25 mol%. The decay curve for different concentrations of Sm3 þ doped CaO:Gd2O3:SiO2:B2O3 glasses are obtained using excitation wavelength 275 nm by monitoring the emission wavelength at 600 nm. These decay curves are shown in Fig. 9, they decrease with increasing of Sm2O3 concentration. Similar decay time behavior
has been observed in cased borate glass system such as Sm3 þ in Li2O–Gd2O3–(BaO/Bi2O3)–B2O3 [25], Sm3 þ in Li2O–Y2O3–B2O3 [26], Eu3 þ in ZnO–Bi2O3–B2O3 glass [27], Dy3 þ in Gd2O3–CaO– SiO2–B2O3 glass [8] and Er3 þ in Gd2O3–CaO–SiO2–B2O3 glass [28]. For Sm3 þ doping at low concentration, interaction between 3þ Gd and Sm3 þ ion is insignificant, decay curve therefore can be fitted exponentially. When Sm2O3 concentration increases, it cause interaction between Gd3 þ and Sm3 þ ion becomes so major. The energy transfer between an excited Gd3 þ ion (donor) and a nonexcited Sm3 þ ions (acceptor) occur with higher probability, leading to a non-exponential shape of the decay curves. The nonexponential nature of the decay curves is fitted in frame of the Inokuti–Hirayama (IH) model, glasses are fitted IH model this case of dipole-dipole interactions (S ¼6). Lifetime and the energy transfer parameter (Q) are shown in Fig. 9 and Table 1. The acquired Q value, which indicates quantity of Gd3 þ - Sm3 þ energy transfer, increases with increasing of Sm2O3 concentration.
7. X-ray luminescence The X-ray induced luminescence spectra of the Sm3 þ doped CaO:Gd2O3:SiO2:B2O3 glasses are shown in Fig. 10. The X-ray of 50 kV and 30 mA were irradiated to glass samples. Sm3 þ doped glasses have four peaks emission at 564, 602, 648 and 708 nm, which are assigned to 4G5/2-6H5/2, 4G5/2-6H7/2, 4G5/2-6H9/2 and 4 G5/2-6H11/2 transitions [12–23]. The strongest peak is centered at 598 nm wavelength. Furthermore, it was found that intensity of glass increased with increasing of Sm2O3 concentration until 0.35 mol% then it decreased, caused by concentration quenching effect. The optimal concentrations of Sm2O3 for the PL and RL are
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N. Wantana et al. / Journal of Luminescence 181 (2017) 382–386
Fig. 11. BGO and 0.35 mol% Sm2O3 doped CaO:Gd2O3:SiO2:B2O3 glass for compared RL measurement.
ground state to higher state. Absorption of photon with 275 nm results to the strongest excitation for Gd3 þ which then generates the emission of Sm3 þ via energy transfer. The emission spectra with 275 nm excitation wavelength performs the strongest peak at 600 nm, with the highest intensity obtained from 0.25 mol% Sm2O3 doped glass. Decay time decrease with increasing of Sm2O3 concentration, it cause by the energy transfer from Gd3 þ to Sm3 þ ion with dipole-dipole interaction, proved by IH model. The X-ray luminescence spectra show similar pattern with PL emission spectra, but instead by the highest intensity from 0.35 mol% Sm2O3 doped glass. For comparison of scintillation between glass sample and BGO crystal, it was found that the integral scintillation efficiency of glass was 25% of BGO. Since this glasses have long decay time as a few millisecond that it can be used for integration mode of scintillation such as in medical and industrial X-ray imaging inspection systems as well as portal imaging system at MeV energies.
Acknowledgments The authors wish to thanks Nakhon Pathom Rajabhat University (NPRU) and National Research Council of Thailand (NRCT) to support this research.
References
Fig. 12. The X-rays induced optical luminescence spectra of the 0.35 mol% Sm2O3 doped CaO:Gd2O3:SiO2:B2O3 glasses, compared with BGO scintillator.
different as 0.25 mol% and 0.35 mol%, respectively. It can be explain that host material and activator ion have different interaction mechanism with UV and X-ray excitation. UV can directly excite electrons of Ln ions, while X-ray excite electrons of both Ln ions and host. X-ray excitation in host glass results to hole electron interaction, then the large number of secondary electrons are produced. Electrons finally indirectly and directly excite the Ln ions in glass [8,29]. Glass doped with Sm2O3 0.35 mol% was then cut and polished to be same size and shape with commercial BGO crystal as shown in Fig. 11. Comparison of X-ray luminescence spectrum between such glass and BGO crystal have been shown in Fig. 12. From total area under peaks of spectrum, the integral scintillation efficiency of 0.35 mol% Sm2O3 doped glass was determined, and found to be 25% of BGO scintillator crystal. Since this glasses have long decay time as a few millisecond that it can be used for integration mode of scintillation such as in medical and industrial X-ray imaging inspection systems as well as portal imaging system at MeV energies [30].
8. Conclusion The Sm3 þ doped CaO:Gd2O3:SiO2:B2O3 glasses were prepared by melt quenching technique, developed glass was investigated in density, optical, photoluminescence, and scintillation properties. From the results, glass densities show increase trend with increasing of Sm2O3 concentrations. Glasses absorb photon in visible light and near infrared regions, assigned to the transitions from the 6H5/2
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Contents lists available at ScienceDirect
Journal of Luminescence journal homepage: www.elsevier.com/locate/jlumin
Full Length Article
Energy transfer from Gd3 þ to Sm3 þ and luminescence characteristics of CaO–Gd2O3–SiO2–B2O3 scintillating glasses N. Wantana a,b, S. Kaewjaeng c, S. Kothan c, H.J. Kim d, J. Kaewkhao a,b,n a
Center of Excellence in Glass Technology and Materials Science (CEGM), Nakhon Pathom Rajabhat University, Nakhon Pathom 73000, Thailand Physics Program, Faculty of Science and Technology, Nakhon Pathom Rajabhat University, Nakhon Pathom 73000, Thailand c Department of Radiologic Technology, Faculty of Associated Medical Sciences, Chiang Mai University, Chiang Mai 50200, Thailand d Department of Physics, Kyungpook National University, Daegu 702-701, Republic of Korea b
art ic l e i nf o
a b s t r a c t
Article history: Received 17 May 2016 Received in revised form 12 September 2016 Accepted 22 September 2016
In this work, density, optical, photoluminescence and x-ray scintillation properties of calcium gadolinium silicoborate glass (CaO:Gd2O3:SiO2:B2O3) have been investigated. Glasses were prepared by the conventional melt quenching technique. The results show that the density of glass increased with increasing of Sm2O3. The optical spectra of glass shows eleven discrete absorption bands at 360, 373, 403, 475, 944, 1077, 1225, 1368, 1468, 1520 and 1586 nm which are due to the transitions of 6H5/2 to 4D3/2, 6P7/2, 6P3/2, 4 I11/2, 6F11/2, 6F9/2, 6F7/2, 6F5/2, 6F3/2, 6H15/2 and 6F1/2, respectively. The emission spectra were observed and assigned to 312, 563, 600, 646 and 703 nm by excitation at 275 nm. The emission intensity of Sm3 þ increased with increasing of Sm2O3 concentration until 0.25 mol%, while decay time decrease with increasing of Sm2O3 content. For results of the radioluminescence (RL), they perform four emission peaks with the strongest emission at 598 nm. Glass doped with 0.35 mol% Sm2O3 show the highest RL emission. The integral scintillation efficiency of 0.35 mol% Sm2O3 doped glass was determined as 25% of commercial BGO scintillator crystal. & 2016 Elsevier B.V. All rights reserved.
Keyword: Glass Luminescence Scintillator BGO
1. Introduction Glass scintillator can be used for detection of X-rays and neutrons. Glass material performs the variety of advantage such as easy synthesis, low-price, high optical homogeneity, large volume production and various shaping [1,2]. Borate glasses are the technologically important glass formers and play a significant role in various applications. Furthermore, increase of B2O3 content from 10 mol% to 30 mol% resulted in the emergence of [BO3] units and Non-Bridging Oxygen (NBOs) growth [3–5]. In part of the silicate glasses, they were used as host material for luminescence study of rare-earth and transition metal ions, due to they have good optical and mechanical properties as well as good chemical durability [6].Cooperative of these two glass formers, borosilicate glasses perform low melting temperature, low coefficients of thermal expansion and can be used in relatively high temperatures [7]. The effect of adding calcium, it can increase intensity of optical absorption and luminescence emission in glass [8]. For rare earth n Correspond author at: Center of Excellence in Glass Technology and Materials Science (CEGM), Nakhon Pathom Rajabhat University, Nakhon Pathom 73000, Thailand.
http://dx.doi.org/10.1016/j.jlumin.2016.09.050 0022-2313/& 2016 Elsevier B.V. All rights reserved.
oxides, intensive Gd2O3 are popular material due to the efficient energy transfer from Gd3 þ ions to the luminescence activators, high thermal neutron capture cross-section and increase the light yield of emission [9,10]. In recent years, glass scintillators containing high Gd2O3 contents have been succeed in various germinate, phosphate, silicate and borosilicate glasses with fast decay time [1,11]. Glasses containing Sm3 þ ions have the extensive interest due to their potential application for high-density optical storage, under sea communication and color displays [12]. The optical properties of Sm3 þ doped glasses have attracted much attention because of their technological applications. Additionally, the Sm3 þ ions exhibit broad emission bands due to 4G5/2-6HJ (J ¼5/2, 7/2, 9/2, 11/2) transitions in any host matrix [13]. It is also well known that the intensities of emission bands of Sm3 þ ion in glasses depend on its concentration and glass composition [14]. Several literatures were published luminescence properties of Sm3 þ in different glass host such as tellurite, phosphate, silicate and borate glasses [13,15–20]. In this work, a new series of CaO:Gd2O3:SiO2:B2O3 glass have been prepared by melted-quenching technique. Sm2O3 was doped in CaO:Gd2O3:SiO2:B2O3 glasses with different concentrations for study in physical, optical, luminescence properties and scintillation efficiency. This glass has never been studied in any detail and
N. Wantana et al. / Journal of Luminescence 181 (2017) 382–386
this work is the first time for understanding the effects of glass composition and Sm2O3 concentration on properties in CaO: Gd2O3:SiO2:B2O3 glass system, developed for using to glass scintillator.
383
of glasses were measured with a Cu target X-ray generator (Inel, XRG3D-E), whose X-ray source was operated at 50 kV and 30 mA, and the spectrometer (QE65 Pro, Ocean Optics) with an optical fiber to detect the emission spectra, as shown in Fig. 1.
2. Experiment 3. Results Glasses compositions of 10CaO:25Gd2O3:10SiO2:(55-x)B2O3: xSm2O3 (where x are 0.05, 0.15, 0.25, 0.35, 0.45, 0.50 and 1.00 mol%) were prepared by melt quenching technique. The high purity chemicals, CaO, Gd2O3, SiO2, H3BO3 and Sm2O3 were mixed thoroughly in an alumina crucible. Each batch of formulas was weighted to 30 g melted in an electric furnace at a temperature of 1400 °C for 3 hours. The melted material was quenched in preheated stainless-steel molds, which would yielded several batches of glass samples. The glass samples were further annealed at 550 °C for 3 hours in order to remove thermal strains. Glasses were cut and polished to 1.0 1.5 0.3 cm3 for property investigations. The density measurement was applied by Archimedes's principle, glasses were weighted in air and water as an immersion liquid using a 4-digit sensitive microbalance (AND, HR-200). The optical spectra of glass samples were measured with a UV–vis–NIR spectrophotometer (Shimadzu UV-3600) in the wavelength range 350–1800 nm. The excitation, emission spectra and decay curves were recorded by using a spectrofluorophotometer (Cary-Eclipse) with xenon lamp as a light source. The non-exponential decay curves were fitted, to evaluate energy transfer parameter (Q), with the Inokuti–Hirayama (IH) model ( 3=s ) t t I ðt Þ ¼ I 0 exp Q
τ0
All the Sm3 þ doped CaO:Gd2O3:SiO2:B2O3 glasses exhibit soft yellow color and high transparency, as shown in Fig. 2.
4. Density Glass density tend to increase with increasing of Sm2O3 concentration, as shown in Fig. 3. It means that there is a change in the structural arrangement of the atoms with Sm2O3 addition in CaO:Gd2O3:SiO2:B2O3 network which shows that borate is replaced by samarium oxide since the density of Sm2O3 is 8.347 g/cm3 while that of B2O3 is 2.460 g/cm3. The increased density of the samples is due to higher molecular weight of samarium than any other component of the given glass system.
τ0
where I(t) is fluorescence intensity at time after excitation (t), I0 is fluorescence intensity at initiate time, and τ0 is the intrinsic decay time of the donors in the absence of acceptors. The value of S (6, 8 or 10) depends on whether the dominant mechanism of the interaction is dipole–dipole, dipole–quadrupole or quadrupole– quadrupole, respectively [21,22]. The X-ray luminescence spectra
Fig. 2. The Sm3 þ doped CaO:Gd2O3:SiO2:B2O3 glasses. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
Fig. 1. (a) Experimental setup and (b) schematic diagram of the X-ray induced optical luminescence spectrometer.
Fig. 3. The density of the Sm2O3 doped CaO:Gd2O3:SiO2:B2O3 glasses.
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Fig. 4. The absorption spectra of the Sm2O3 doped CaO:Gd2O3:SiO2:B2O3 glasses.
Fig. 6. The emission spectra of the Sm2O3 doped CaO:Gd2O3:SiO2:B2O3 glasses.
Fig. 5. The excitation spectrum 0.05 mol% Sm2O3 doped CaO:Gd2O3:SiO2:B2O3 glass.
5. Optical property The absorption spectra for the Sm2O3 doped CaO:Gd2O3:SiO2: B2O3 glasses as shown in Fig. 4. The optical spectra of glass shows eleven discrete absorption bands at 360, 373, 403, 475, 944, 1077, 1225, 1368, 1468, 1520 and 1586 nm which are due to the transitions of 6H5/2 to 4D3/2, 6P7/2, 6P3/2, 4I11/2, 6F11/2, 6F9/2, 6F7/2, 6F5/2, 6F3/ 6 6 2, H15/2 and F1/2, respectively [12–23]. Furthermore, absorption bands show more sharpness with increasing of Sm2O3 concentration. This represent higher absorption ability of glass sample with higher amount of Sm3 þ in glass.
6. Photoluminescence Fig. 5 shows the typical excitation spectrum at 0.05 mol% of Sm3 þ doped CaO:Gd2O3:SiO2:B2O3 glass, monitoring emission at 600 nm. Gd3 þ has exhibited four major bands centered at 246 nm (8S7/2-6D7/2), 253 nm (8S7/2-6D9/2), 275 nm (8S7/2-6I9/2) and 312 nm (8S7/2-6P7/2) [11,24], other peak excitation of Sm3 þ such as 345 nm (6H5/2-4D7/2), 361 nm (6H5/2-4D3/2), 375 nm (6H5/2-6P7/ 403 nm (6H5/2-6P3/2), 415 nm (6H5/2-6P5/2), 439 nm 2),
Fig. 7. Energy level diagram of Gd3 þ and Sm3 þ ions in the Gd2O3:CaO:SiO2:B2O3 glasses.
(6H5/2-4G9/2), 462 nm (6H5/2-4I13/2), 475 nm (6H5/2-4M15/2) and 500 nm (6H5/2-4G7/2) [18–22]. From the excitation spectrum, glass show the strongest peak at 275 nm then glasses were excited by this wavelength to investigate the emission spectra as shown in Fig. 6. These spectra show five emission bands at 563, 600, 646 and 703 nm representing the energy transitions of the Sm3 þ from 4G5/2 ground state to 6H5/2, 6H7/2, 6H9/2 and 6H11/2, respectively [12–23]. These emission cause by the energy transfer from Gd3 þ to Sm3 þ as shown in energy level diagram in Fig. 7. The emission intensity of Gd3 þ at 312 nm decrease with increasing of Sm2O3 concentration, while all emission intensity of Sm3 þ increase until
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Table 1 Experimental lifetimes (τexp, ms) of the 4G5/2 level of Sm3 þ and the energy transfer parameter (Q) of the Sm3 þ doped CaO:Gd2O3:SiO2:B2O3 glasses. Sm3 þ ions concentration, mol%
τexp (ms)
Q
0.05 0.15 0.25 0.35 0.45 0.50 1.00
2.320 2.090 1.885 1.685 1.585 1.501 1.075
– 0.143 0.296 0.451 0.512 0.576 1.003
Fig. 8. The emission spectra of 0.25 mol% Sm2O3 doped glasses with different Gd2O3 concentration.
Fig. 10. The X-rays induced optical luminescence spectra of the Sm3 þ doped CaO: Gd2O3:SiO2:B2O3 glasses.
Fig. 9. Decay curves of the Sm3 þ doped CaO:Gd2O3:SiO2:B2O3 glasses.
0.25 mol% concentration of Sm2O3. Reducing of Gd3 þ emission simultaneously with increasing of Sm3 þ emission is a quite evidence of the energy transfer phenomena from Gd3 þ to Sm3 þ . The Gd3 þ -Sm3 þ energy transfer still appear even in glass that dope Sm2O3 higher than 0.25 mol% but in this condition, it was influenced with concentration quenching effect that make Sm3 þ emission decrease. To study the optimal concentration of Gd2O3 in glasses, Glass samples with 10CaO:yGd2O3:10SiO2:(79.75-y)B2O3:0.25Sm2O3 (where y are 15, 20, 25, 30 and 35 mol%) system have been fabricated in the similar preparing condition. The emission spectra are shown in Fig. 8. These spectra exhibit emission bands at 563, 600, 646 and 703 nm representing the energy transitions of the Sm3 þ from 4G5/2 ground state to 6H5/2, 6H7/2, 6H9/2 and 6H11/2, respectively [12–23]. The emission intensity of Sm3 þ increase with increasing Gd2O3 concentration until 25 mol%. Therefore the optimum concentrations of Gd3 þ ion is 25 mol%. The decay curve for different concentrations of Sm3 þ doped CaO:Gd2O3:SiO2:B2O3 glasses are obtained using excitation wavelength 275 nm by monitoring the emission wavelength at 600 nm. These decay curves are shown in Fig. 9, they decrease with increasing of Sm2O3 concentration. Similar decay time behavior
has been observed in cased borate glass system such as Sm3 þ in Li2O–Gd2O3–(BaO/Bi2O3)–B2O3 [25], Sm3 þ in Li2O–Y2O3–B2O3 [26], Eu3 þ in ZnO–Bi2O3–B2O3 glass [27], Dy3 þ in Gd2O3–CaO– SiO2–B2O3 glass [8] and Er3 þ in Gd2O3–CaO–SiO2–B2O3 glass [28]. For Sm3 þ doping at low concentration, interaction between 3þ Gd and Sm3 þ ion is insignificant, decay curve therefore can be fitted exponentially. When Sm2O3 concentration increases, it cause interaction between Gd3 þ and Sm3 þ ion becomes so major. The energy transfer between an excited Gd3 þ ion (donor) and a nonexcited Sm3 þ ions (acceptor) occur with higher probability, leading to a non-exponential shape of the decay curves. The nonexponential nature of the decay curves is fitted in frame of the Inokuti–Hirayama (IH) model, glasses are fitted IH model this case of dipole-dipole interactions (S ¼6). Lifetime and the energy transfer parameter (Q) are shown in Fig. 9 and Table 1. The acquired Q value, which indicates quantity of Gd3 þ - Sm3 þ energy transfer, increases with increasing of Sm2O3 concentration.
7. X-ray luminescence The X-ray induced luminescence spectra of the Sm3 þ doped CaO:Gd2O3:SiO2:B2O3 glasses are shown in Fig. 10. The X-ray of 50 kV and 30 mA were irradiated to glass samples. Sm3 þ doped glasses have four peaks emission at 564, 602, 648 and 708 nm, which are assigned to 4G5/2-6H5/2, 4G5/2-6H7/2, 4G5/2-6H9/2 and 4 G5/2-6H11/2 transitions [12–23]. The strongest peak is centered at 598 nm wavelength. Furthermore, it was found that intensity of glass increased with increasing of Sm2O3 concentration until 0.35 mol% then it decreased, caused by concentration quenching effect. The optimal concentrations of Sm2O3 for the PL and RL are
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Fig. 11. BGO and 0.35 mol% Sm2O3 doped CaO:Gd2O3:SiO2:B2O3 glass for compared RL measurement.
ground state to higher state. Absorption of photon with 275 nm results to the strongest excitation for Gd3 þ which then generates the emission of Sm3 þ via energy transfer. The emission spectra with 275 nm excitation wavelength performs the strongest peak at 600 nm, with the highest intensity obtained from 0.25 mol% Sm2O3 doped glass. Decay time decrease with increasing of Sm2O3 concentration, it cause by the energy transfer from Gd3 þ to Sm3 þ ion with dipole-dipole interaction, proved by IH model. The X-ray luminescence spectra show similar pattern with PL emission spectra, but instead by the highest intensity from 0.35 mol% Sm2O3 doped glass. For comparison of scintillation between glass sample and BGO crystal, it was found that the integral scintillation efficiency of glass was 25% of BGO. Since this glasses have long decay time as a few millisecond that it can be used for integration mode of scintillation such as in medical and industrial X-ray imaging inspection systems as well as portal imaging system at MeV energies.
Acknowledgments The authors wish to thanks Nakhon Pathom Rajabhat University (NPRU) and National Research Council of Thailand (NRCT) to support this research.
References
Fig. 12. The X-rays induced optical luminescence spectra of the 0.35 mol% Sm2O3 doped CaO:Gd2O3:SiO2:B2O3 glasses, compared with BGO scintillator.
different as 0.25 mol% and 0.35 mol%, respectively. It can be explain that host material and activator ion have different interaction mechanism with UV and X-ray excitation. UV can directly excite electrons of Ln ions, while X-ray excite electrons of both Ln ions and host. X-ray excitation in host glass results to hole electron interaction, then the large number of secondary electrons are produced. Electrons finally indirectly and directly excite the Ln ions in glass [8,29]. Glass doped with Sm2O3 0.35 mol% was then cut and polished to be same size and shape with commercial BGO crystal as shown in Fig. 11. Comparison of X-ray luminescence spectrum between such glass and BGO crystal have been shown in Fig. 12. From total area under peaks of spectrum, the integral scintillation efficiency of 0.35 mol% Sm2O3 doped glass was determined, and found to be 25% of BGO scintillator crystal. Since this glasses have long decay time as a few millisecond that it can be used for integration mode of scintillation such as in medical and industrial X-ray imaging inspection systems as well as portal imaging system at MeV energies [30].
8. Conclusion The Sm3 þ doped CaO:Gd2O3:SiO2:B2O3 glasses were prepared by melt quenching technique, developed glass was investigated in density, optical, photoluminescence, and scintillation properties. From the results, glass densities show increase trend with increasing of Sm2O3 concentrations. Glasses absorb photon in visible light and near infrared regions, assigned to the transitions from the 6H5/2
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