Tb-activated scintillating glasses with high Ln2O3 (Ln=Gd, Y, Lu) concentration up to 40mol% have been prepared. The effects of Ln3+ ions on the density, thermal properties, transmission and luminescence properties under both UV and X-ray excitation have been investigated. The glasses containing Gd2O3 or Lu2O3 exhibit a high density of more than 6.0g/cm3. Energy transfer from Gd3+ to Tb3+ takes place in Gd-containing glass and as a result the Gd-containing glass shows a light yield 2.5 times higher than the Y-or Lu-containing glass. The Effect of the substitution of fluorine for oxygen on the optical properties was also investigated

Bross

Ebendorff-Heidepriem

Greskovich

Hobson

Jie Fu

John M. Parker

Masaaki Kobayashi

Melcher

Nakamura

Placious

Takagi

English

Fu, J.

Kobayashi, M.

Parker, J.M.

White Rose Research Online

Terbium-activated heavy scintillating glasses

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promoting access to White Rose research papers    White Rose Research Online   Universities of Leeds, Sheffield and York http://eprints.whiterose.ac.uk/    This is an author produced version of a paper due to be published in Journal of Luminescence.   White Rose Research Online URL for this paper: http://eprints.whiterose.ac.uk/3527/    Published paper Fu, J., Kobayashi, M. and Parker, J.M. (2008) Terbium-activated heavy scintillating glasses, Journal of Luminescence 128 (1), 99 –104.    eprints@whiterose.ac.uk  Terbium-activated heavy scintillating glasses Jie Fu R & D Department, Ohara Inc.,  1-15-30 Oyama, Sagamihara-shi, Kanagawa, 229-1186, Japan  Masaaki Kobayashi High Energy Accelerator Research Organization (KEK) 1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan  J.M. Parker Department of Engineering materials, The University of Sheffield, Mappin Street, Sheffield S1 3JD, UK  Abstract Tb-activated scintillating glasses with high Ln2O3 (Ln=Gd, Y, Lu) concentration up to 40mol% have been prepared. The effects of Ln3+ ions on the density, thermal properties, transmission and luminescence properties under both UV and X-ray excitation have been investigated. The glasses containing Gd2O3 or Lu2O3 exhibit a high density of more than 6.0g/cm3. Energy transfer from Gd3+ to Tb3+ takes place in Gd-containing glass and as a result the Gd-containing glass shows a light yield 2.5 times higher than the Y-or Lu-containing glass. The Effect of the substitution of fluorine for oxygen on the optical properties was also investigated.   1. Introduction Glasses actived with rare earth ions are promising alternatives to single crystals and ceramic scintillators for applications in high energy physics, in X-CT for industrial and medical areas. The main advantages of the glasses scintillators lie in the possibility of being produced with low cost and being easily manufactured to desired size and shape, such as fibers. Among the properties required for most of the applications, for an example, for X-CT application, the density of the scintillators is very important because the high density could result in an increased X-ray absorption across section, thus significantly increasing the image signal-to-noise. However, the density of the existing glass scintillators is low below 4 g/cm3 [1, 2]. Although the density of the glasses can be easily increased to more than 6.0 g/cm3, which is desirable for most applications, by the introduction of some heavy components such as PbO, Bi2O3, this is often accompanied by a dramatic decrease in the luminescence response of rare earth ions.  Introduction of heavy metal oxides Gd2O3, Y2O3, Lu2O3 can be expected to increase the density of the glasses. Moreover these oxides are main host components of some commercial scintillators such as Gd2SiO5:Ce, Lu2SiO5:Ce singles and (Y, Gd) 2O3: Eu, Gd2O2S:Tb ceramics [3-6]. However, to the present authors’ knowledge, effects of these oxides at high concentration of more than 30mol% in glasses on the scintillating properties have not been reported. In this work, we report on terbium-actived dense scintillating glasses which contain high concentrations of Ln2O3 (Ln=Gd, Y, Lu) up to 40mol%. Effects of Ln3+ ions on the scintillating properties under X-ray excitation have been investigated. Furthermore, fluorine is introduced to the glasses and the effect of fluorine on the scintillating properties has been studied.  2. Experimental Glasses with mole compositions 15SiO2-25B2O3-5P2O5-15Ga2O3-38Ln2O3-2Tb2O3 (Ln=Gd, Y, Lu) and 15SiO2-25B2O3-5P2O5-15Ga2O3-28Ln2O3-10GdF3-2Tb2O3 were prepared using the following procedure. Reagent-grade SiO2, H3BO3, NH4H2PO4, Ga2O3, Gd2O3, Y2O3, Lu2O3 and GdF3 were used as starting materials. 50g batches were placed in platinum crucibles and melted under an air atmosphere at 1520 - 1550ºC for 3 h. The melts were poured into preheated stainless-steel plate and pressed into plates 4mm thickness. The glasses were cut and polished to 2mm thickness for various spectroscopic measurements. Density was measured by Archimedes’ method using distilled water as an immersion liquid. Thermal properties were measured by differential thermal analysis (DTA) at a heating rate of 10ºC/min. Ultraviolet/visible transmission spectra were recorded on a spectrometer. Luminescence under UV excitation using Xe lamp as excitation source was recorded by a JASCO LP-750 spectrometer. Luminescence under X-ray excitation was measured using an X-ray tube with Cu anode operated at 50kV and 30mA. Emission in the range of 300-700nm was collected and spectral sensitivity of the detection system was corrected. Light yield of the samples was estimated relative to that of a Bi4G3O12 crystal.  All the above measurements were performed at room temperature.  3. Results 3.1. Effects of Ln3+ ions on the physical properties  Density and refractive index of the 15SiO2-25B2O3-5P2O5-15Ga2O3-38Ln2O3-2Tb2O3 (Ln=Gd, Y, Lu) glasses are listed in Table 1. As expected the density increases in the order of Y2O3 <Gd2O3 <Lu2O3. It is worth noting that the glass containing Gd2O3 or Lu2O3 exhibits a density of more than 6.0 g/cm3, which meets basically the requirement for the practical applications in high energy physics and medical field. The refractive index (nd) of these glasses is very high, up to 1.83 for Gd-containing glass.  The thermal properties of the above glasses based on DTA results are shown in Fig. 1. Tg decreases from 780℃ to 760℃ and (Tx-Tg) decrease from 195℃ to 140℃ with the increase of ionic radius of Ln3+ ions. The difference of (Tx-Tg) are usually used to judge the stability of a glass against to crystallization, the higher the difference, more stable the glass. Therefore, the glass containing Lu exhibits the highest stability.  3.2. Effects of Ln3+ ions on the optical properties (Ln=Gd, Y, Lu) Figure 2 shows the transmission spectra of 15SiO2-25B2O3-5P2O5-15Ga2O3-38Ln2O3-2Tb2O3 (Ln=Gd, Y, Lu) glasses together with those of Tb-free glasses for comparison. There exist two weak absorption peaks at 375nm and 480nm in all Tb-doped glasses and an extra peak at 313nm only in Gd-containing glasses. The former two peaks are due to f-f transition of Tb3+ and latter one to that of Gd3+. The Tb-doped glasses show the almost same transmission regardless of the kind of Ln3+ ions, but they obviously have a broad absorption band ranging from the absorption edges to 580nm in comparison with Tb-free glasses, indicating the broad absorption originates from terbium ions. Figure 3 shows the luminescence spectra of 15SiO2-25B2O3-5P2O5-15Ga2O3-38Ln2O3-2Tb2O3 (Ln=Gd, Y, Lu) glasses when excited with 276nm light. All observed emission peaks are attributable to the 5D4-7FJ transition of Tb3+. The Gd-containing glass has a luminescent response at least a factor of 2 higher than the Y or Lu-containing one. Almost the same results have been observed under X-ray excitation as shown in Fig. 4. From the emission peaks, the light yield was estimated relative to BGO single crystal and the results were shown in Fig. 5. The Gd-containing glass exhibits higher light yield 2.5 times that of Y- or Lu-containing one. From the decay curves obtained by monitoring the intensity of the strongest emission peak at 545nm, the decay time for all the glasses investigated above was estimated. It was shown that the decay time did not change too much with the kind of Ln3+ and had a value around 1.8ms. Dependence of the light yield on terbium concentration for the Gd-containing glasses is shown in Fig. 6. It can be seen that the light yield increases slightly with the increase in the terbium concentration. Figure 7 shows the excitation spectra of 15SiO2-25B2O3-5P2O5-15Ga2O3-38Ln2O3-2Tb2O3 (Ln=Gd, Y, Lu) glasses for the Tb3+ emission at 545nm. Same excitation bands are observed for the Y- and Lu-containing glasses. The broad band peaking at 260nm is due to the 4f8-4f75d1 transition of Tb3+ and the remaining bands at longer wavelength to the 4f-4f transitions of Tb3+. For the Gd-containing glass, however, there exist extra two sharp and strong excitation bands at 276nm and 313nm. These two bands can be attributed to Gd3+ ion as they are identical with the 8S2/7 → 6IJ and 8S7/2 → 6P7/2 transitions of Gd3+ ion, respectively.  3.3. Effect of fluorine on the optical properties  To investigate the effect of fluorine on the properties, 10mol% of Gd2O3 in the 15SiO2-25B2O3-5P2O5-15Ga2O3-38Gd2O3-2Tb2O3 glass was replaced with GdF3. Emission spectra under UV excitation indicated that the introduction of fluorine resulted in an increase in emission intensity, and similar result is also obtained under x-ray excitation as shown in Fig. 8. Transmission curves before and after the introduction of fluorine is shown in Fig. 9. It is obvious that the broad absorption becomes weaker significantly after the introduction of fluorine.  4. Discussion Terbium in a glass exists mainly as Tb3+, usually being accompanied by a small amount of Tb4+. The ratio of these two ions depends on the melting atmosphere and the compositions of the host. Tb3+ ion emits fluorescence, but Tb4+ does not. The Tb3+ gives several sharp absorption peaks due to f-f transition in the visible region while Tb4+ has a broad absorption from ultraviolet to visible region peaking at 320-370nm due to the charge transfer transition [7]. It is therefore reasonable to assign the observed broad absorption in the present Tb-doped glasses (Fig. 3) to Tb4+ ions. Almost the same transmission for Gd-, Y- and Lu-containing glasses implies that the ratio of Tb3+/Tb4+ is almost equal in these glasses, i.e. the same Tb3+ concentration.  Improvement of the transmission after the introduction of fluorine indicates the fluorine-containing glass has a lower concentration of Tb4+ and hence a higher concentration of Tb3+. This is consistent with fact that the fluorine favors the lower oxidation states of ions. Appearance of excitation bands of Gd3+ in the excitation spectra of Tb3+ indicates that energy transfer from Gd3+ to Tb3+ takes place in Gd2O3-containing glass. This must be responsible for the much stronger luminescence response under either UV or X-ray excitation in Gd-containing glass than that in Y- or Lu-containing glass. To investigate the optimum concentration of Gd3+ ion for energy transfer to Tb3+, the substitution of Lu2O3 for Gd2O3 was made in the 15SiO2-25B2O3-5P2O5-15Ga2O3-(38-x)Lu2O3-xGd2O3-2Tb2O3 glasses and emission spectra under the X-ray excitation were shown in Fig. 10. In addition to the emission peaks at longer wavelength beyond 350nm due to Tb3+ ions, emission peak due to the  transmission of 6P7/2 level → 8S7/2 level of Gd3+ ions is also observed. The intensity of Tb3+ emission increases with the increase of Gd3+ concentration, but the intensity of Gd3+ emission decreases significantly and even disappears at high concentration more than 16mol%. The enhanced emission of Tb3+ ions with the increase of Gd3+ concentration could be explained by the following energy transfer process from Gd3+ to Tb3+. At low Gd3+ concentration, excited Gd3+ ions return to ground state by two routs; one is by transferring its energy to Tb3+ and the other by radiative decay from 6P7/2 level to 8S7/2 level. At higher Gd3+ concentration, however, besides the energy transfer from Gd3+ to Tb3+, the energy transfer between Gd3+ ions becomes more probable than the radiative decay of the Gd3+ ions. Energy migrates among Gd3+ ions and finally is trapped by Tb3+, hence resulting in a decrease in Gd3+ emission and an increase in Tb3+ emission.  Figure 8 reveals that fluorine is effective in improving the light yield. This is most likely due to the higher Tb3+ concentration in the fluorine-containing glass as discussed above. The dependence of light yield on the terbium concentration (Fig.6) supports this speculation.  5. Conclusions Tb-activated scintillating glasses containing high Ln2O3 concentration (Ln=Gd, Y, Lu) have been studied. The glasses are very heavy, having density up to 6.0g/cm3 for Gd2O3-containing glass and 6.6g/cm3 for Lu2O3-containing glass. Due to the energy transfer from Gd3+ to Tb3+, the Gd-containing glass shows a light yield 2.5 times higher than the Y or Lu-containing glass. Part substitution of fluorine for oxygen is effective in improving the light yield.  References [1] A. D. Bross, Nuclear Instruments and Methods in Physics Research, A247(1986)319. [2] R.C. Placious, D. Polansky, H. Berger, C. Bueno, C.L. Vosberg, R.A. Betz and D.J. Rogerson, Materials Evaluation/November, 1991, P.1419. [3] K. Takagi, T. Fukazawa, Appl. Phys. Lett., 43(1983)43-45 [4] C.L. Melcher, J.S. Schweitzer, Nucl. Instr. Meth., A314(1992)212-214. [5] Chariles D. Greskovich, D. Cusano, D. Hoffman and Robert J. Riedner, American Ceramic Society Bulletin, 71(1992)1120-1130. [6] R. Nakamura, J. Am. Ceram. Soc., 82(1999)9 [7] H. Ebendorff-Heidepriem, D. Ehrt, Optical Meterials, 15(2000)7. Figure Captions  Fig. 1. Tg and (Tx-Tg) of 15SiO2-25B2O3-5P2O5-15Ga2O3-38Ln2O3-2Tb2O3  (Ln=Gd, Y, Lu) glasses.  Fig. 2. Transmission spectra of 15SiO2-25B2O3-5P2O5-15Ga2O3-38Ln2O3-2Tb2O3  (Ln=Gd, Y, Lu) glasses, (a) 15SiO2-25B2O3-5P2O5-15Ga2O3-40Gd2O3 glass  and (b) 15SiO2-25B2O3-5P2O5-15Ga2O3-40Lu2O3 glass  Fig. 3. Emission spectra of 15SiO2-25B2O3-5P2O5-15Ga2O3-38Ln2O3-2Tb2O3  (Ln=Gd, Y, Lu) glasses when excited with 276nm.  Fig. 4. Emission spectra of 15SiO2-25B2O3-5P2O5-15Ga2O3-38Ln2O3-2Tb2O3  (Ln=Gd, Y, Lu) glasses under X-ray excitation.  Fig. 5. Light yield of 15SiO2-25B2O3-5P2O5-15Ga2O3-38Ln2O3-2Tb2O3  (Ln=Gd, Y, Lu) glasses under X-ray excitation.  Fig. 6. Dependence of the light yield on the terbium concentration  for 15SiO2-25B2O3-5P2O5-15Ga2O3-(40-x)Gd2O3-xTb2O3 glasses  Fig. 7. Excitation spectra of 15SiO2-25B2O3-5P2O5-15Ga2O3-38Ln2O3-2Tb2O3  (Ln=Gd, Y, Lu) glasses for the Tb3+ emission at 545nm.  Fig. 8. Emission spectra of (a) 15SiO2-25B2O3-5P2O5-15Ga2O3-38Gd2O3-2Tb2O3 glass and (b) 15SiO2-25B2O3-5P2O5-15Ga2O3-28Gd2O3-10GdF3-2Tb2O3 glass  under x-ray excitation.  Fig. 9. Transmission spectra of (a) 15SiO2-25BB2O3-5P2O5-15Ga2O3-38Gd2O3-2Tb2O3 glass and (b) 15SiO2-25B2O3-5P2O5-15Ga2O3-28Gd2O3-10GdF3-2Tb2O3 glass.  Fig. 10. Emission spectra of 15SiO2-25B2O3-5P2O5-15Ga2O3- (38-x) Lu2O3-xGd2O3-2Tb2O3 glasses under x-ray excitation.   Ln Density (g/cm3) Refractive index (nd)Y 4.52 1.793 Gd 5.96 1.832 Lu 6.56 1.796  Table 1. Density and refractive index of  15SiO2-25B2O3-5P2O5-15Ga2O3-38Ln2O3-2Tb2O3 glasses (Ln=Gd, Y, Lu).  7557657757850.82 0.86 0.9 0.94Ionic radius (Å)Tg (℃)120140160180200Tx-Tg (℃)GYLu020406080100200 300 400 500 600 700Wavelength (nm)Transmission (%)GdYLu(a): Tb-free(b): Tb-free0200400600450 500 550 600 650Wavelength (nm )Intensity (a. u. )GdYLu01000200030004000450 500 550 600 650Wavelength (nm)Intensity (a. u. )GdYLu051015202530Gd Y LuRelative light yield (%)0102030400 2 4 6x (mol%)Relative light yield (%)0200400600200 250 300 350 400 450Wavelength (nm)Intensity (a. u. )GdYLu010002000300040005000450 550 650Wavelength (nm)Intensity (a. ui.)(a) F-free glass(b) F-containing glass020406080100200 300 400 500 600 700Wavelength (nm)Intensity (a. u. )(a) F-free glass(b) F-containing glass01000200030004000300 350 400 450 500 550 600 650Wavelength (nm)Intensity(au)x=4x=8x=16x=38 

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Terbium-activated heavy scintillating glasses

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