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Ce3+ or Tb3+-doped phosphate and silicate scintillating glasses
Journal of Luminescence 87}89 (2000) 673}675
Ce> or Tb>-doped phosphate and silicate scintillating glasses S. Baccaro , R. Dall'Igna, P. Fabeni *, M. Martini, J.A. Mares, F. Meinardi, M. Nikl, K. Nitsch, G.P. Pazzi, P. Polato, C. Susini, A. Vedda, G. Zanella, R. Zannoni ENEA, INN/TEC, via Anguillarese 301, 00060 S. Maria di Galeria, Rome, Italy Stazione Sperimentale del Vetro, via Briati 10, 30141 Murano-Venezia, Italy IROE/CNR, via Panciatichi 64, 50 127 Florence, Italy INFM and Dept. of Material Sci., University of Milano, via Cozzi 53, Milan, Italy Institute of Physics AS CR, Cukrovarnicka 10, 162 53 Prague, Czech Republic INFN and Department of Physics, University of Padova, via Marzolo 8, 35131 Padova, Italy
Abstract Optical characterization of scintillating phosphate and silicates glasses is presented including absorption, emission spectra and decay kinetics. A new concept of energy transfer sensitization is developed, based on nearly resonant energy migration through a rare earth ion subsystem (Gd> ions at su$ciently high concentration) in the glass matrix, followed by a single-step transfer towards the emission centers created by Ce> or Tb> doping. While Ce> doping in phosphates results in emission spectra in the near UV-violet, the Tb> doping in silicates gives a four-band spectrum with a dominating one at 545 nm. Intense UV irradiation produces a reduction of emission intensity in phosphate glasses. 2000 Elsevier Science B.V. All rights reserved. Keywords: Scintillating glasses; Ce> and Tb> energy transfer
1. Introduction So far scintillating glasses have been little investigated in comparison with single crystals. Glasses generally suffer from low transfer e$ciency and thus show a low light yield (LY) due to the presence of traps (possible centers of nonradiative recombination) which cause an ine$cient transfer of the thermalized electrons and holes towards the emission centers. Ce-doped silicate glasses seem to provide the highest LY, about 6 ph/keV [1]. Li-enriched silicate glasses with Tb doping gave similar LY [2]. While Ce-doped silicate glasses are being developed for X-ray detection, the Tb-doped (Li-enriched) ones are interesting for neutron detection. We have developed a new concept of energy transfer sensitization based on nearly resonant energy migration
* Corresponding author. Fax: #39-055-410-893. E-mail address: speclab@iroe.".cnr.it (P. Fabeni)
through a rare earth ion subsystem in the glass matrix followed by a single-step transfer to the Ce> (Tb>) emission centers. In more detail, Gd> ions at su$ciently high concentration enable e!ective energy migration in the phosphate and silicate-based glass matrices followed by a single-step energy transfer towards emission centers created by Ce> or Tb> doping. In this contribution the optical characterization of Ce-doped phosphate and Tb-doped silicate glasses with su$ciently high percentage of Gd> ions (above 20%) in the glassy matrix is reported. 2. Experimental An optical multichannel analyzer (OMA) EG&G PARC 1461 was used for the emission spectra measurements in continuous wave (CW) or time-resolved (TR, time gate of 20 ns) modes. Luminescence characteristics were obtained by Spectro#uorometer 199S (Edinburgh Instrument) equipped with a steady-state and
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 3 5 3 - 1
674
S. Baccaro et al. / Journal of Luminescence 87}89 (2000) 673}675
ns-pulsed hydrogen and ls-pulsed Xenon lamps. Luminescence decays obtained in extended dynamical and time scales were measured using the excitation by a XeCl (308 nm) excimer laser line and detected by a photomultiplier coupled to the digital oscilloscope Tektronix TDS 680B. Slow decays in ms time scale were measured with Xenon #ash lamp and multichannel analyzer in a scanning regime at Spectro#uorometer 199S. All the spectra are corrected for experimental distortions and the decay times are extracted from decay curves using deconvolution procedures, i.e. removing distortions due to extended width and shape of excitation pulse (essential in the ns time scale). Four samples of Ce-doped phosphate glasses were prepared using NaPO , GdPO and CePO as starting materials. They are noted as Na74Gd25Ce1, Na99Ce1, Na75Gd25 and Na52Gd45Ce3 (numbers denote the molar percentage of the starting materials in the melt). The constituents were melted in a quartz ampoule at around 12003C and then poured into a graphite crucible. The Tb-doped silicate glass, Gd-free, previously examined (Tb10Mg4 in Ref. [2]) had the composition (weight% in the melt): SiO (58%), Al O (16%), Tb O (11.5%), Li O (10.5%), MgO (4%). The silicate glasses were pro duced in a controlled reducing atmosphere furnace. Three other samples with di!erent Gd percentage were prepared (Tb0Gd25, Tb3Gd30 and Tb10Gd22), in order to evaluate the Gd e!ect in the energy transfer processes. Annealing processes were applied to both silicate and phosphate glasses in order to remove thermal stresses before the optical machining.
3. Results and discussion The main emission of Ce-doped phosphate glasses is situated at 350 nm, see Fig. 1. It shows rather structured character at low temperatures with a shoulder around 325 nm. Such a structure might be induced by the splitted Ce> ground state (4f , 4f levels) which is observed in many Ce-doped (based) crystal systems. Partly asymmetric and broadened Ce> emission band can be explained by a #uctuating crystal "eld in the glass matrix. Emission structure smoothing at elevated temperatures is similar to the situation observed earlier, e.g. in CeF [3], which is explained by partial thermalization of 4f , 4f levels of Ce> ground state. Under continuous laser illumination (308 nm XeCl) the glass emission is noticeably decreasing: in 1 h it reduces to 60% or 40% with power density of 15 or 30 kW/mm, respectively. This e!ect can, at least partly, explain the observed intensity decrease in Fig. 1 (measured from 19 up to 300 K), which is usually not observed in Ce-doped (based) single-crystal matrices. Such an e!ect can be of similar nature as the observed photo-darkening of CdS/CdSe quantum dot doped glasses [4], recently explained as due to a thermally induced
Fig. 1. Emission spectra of phosphate glass Na52Gd45Ce3 at di!erent temperatures (308 nm laser excitation); in the inset the absorption spectra of Na74Gd25Ce1 and Na75Gd25, at RT.
escape of an electron from the quantum dot excited state. Here, thermally induced electron escape from Ce> excited state is probable, supported also by the existence of slower (delayed recombination) decay components in Ce-doped, Gd-free, phosphate glass. In the inset of Fig. 1 the absorption spectra are reported for two di!erent phosphate glasses at RT and the transitions related to Ce> and Gd> are evidenced. With reference to the classi"cation reported in [5], this is the case of intermediate crystal "eld, in which the lowest 4f}5d transition of Ce> is of about the same energy as the lowest 4f}4f transition round 312 nm belonging to Gd>. In such a situation, a bi-directional energy transfer is expected between Gd> and Ce> ions at higher temperatures. The photoluminescence decay kinetics of Ce> centers is characterized by a leading fast component (of about 20}30 ns) with a slow non-exponential tail, which increases in the amplitude with elevating temperature (clearly noticed for times greater than 10\ s), as reported in Fig. 2. Non-monotonic course of the decay curves is the consequence of the exciting laser pulse shape, shown in Fig. 2 as well. The convolution of the laser pulse (instrumental response) with a sum of three exponentials, I(t)"RA exp [!t/q ], where A , q stand for pre-exG G G G ponential factor and decay time, respectively, enabled a reasonably good "tting of decay data at all the temperatures (included in Fig. 2 for ¹"300 K). In the inset of Fig. 2 the temperature dependence of the relative slowcomponents integral intensity I is also given calculated 1! according to the displayed equation. Furthermore at every temperature the TR emission spectra (0}20 ns) coincide with the steady-state ones, which means that also delayed recombination processes occur at Ce> centers. The enhancement of the slow decay processes with temperature is another evidence of the back energy transfer
S. Baccaro et al. / Journal of Luminescence 87}89 (2000) 673}675
675
Fig. 3. Excitation and emission spectra of two silicate glasses, at RT.
[6] due to good matching of excited state 4f levels of Gd> and Tb ions. Fig. 2. Photoluminescence decay of phosphate glass Na52Gd45Ce3 at di!erent temperatures; in the inset the percentage of the slow components I vs. temperature is given. 1! These I values were obtained using a 3-exponential approxi1! mation of decay curve (I(t)"RA exp [!t/q ], i"1, 2, 3) and G G equation given in the inset. At 300 K the shown "t gives the values: A /q "1/19 ns, A /q "0.011/123 ns, A /q "0.00058/ 3930 ns.
Ce>PGd>, mentioned above. Energy transfer Gd>PCe> is well demonstrated in shortening of Gd> decay in 312 nm emission line, which gets nonexponential and considerably shorter in the sample with Ce> presence (6.5 ms decay time in Na75Gd25 sample, while 0.52 ms mean decay time is obtained in Na74Gd25Ce1 sample). In Fig. 3, the emission and excitation spectra are given for two silicate glasses with and without Tb-doping. The presence of Gd> in the silicate glass matrix gives rise to the same absorption peaks round 310 nm as in the phosphate matrix. Emission spectrum of Tb10Gd22 shows four leading narrow bands at 485, 545, 585 and 620 nm, which are broadened due to the #uctuations of the crystal "eld in the glassy environment. No Gd-related emission is present in Tb10Gd22 sample round 312 nm, while it is clearly visible in Tb0Gd25 one. In the excitation spectra of Tb> emission (leading 545 nm band) enhanced excitation is evident in the position of Gd> excited state round 275 and 312 nm. Such a feature gives a good evidence of the Gd>PTb> energy transfer. Similar shortening of Gd> decay in 312 nm emission line is observed in Tb-doped silicate glasses as in the phosphate system above (2.1 ms decay time in Tb0Gd25 samples, while 0.02 ms mean decay time is obtained in Tb10Gd22 sample). It is worth noting that Gd>PTb> energy transfer is observed in many other crystal matrix systems
4. Conclusions The Gd> Ce> energy transfer in phosphate glasses and Gd>PTb> energy transfer in silicate glasses is evidenced by measurements of decay kinetics, excitation and emission spectra of Gd>, Ce> and Tb> emission centers. The participation of Gd> ions in the energy transfer processes in the Ce-doped phosphate glasses produces slow components in the luminescence decay, which are increasing in amplitude with elevated temperature due to the Ce>PGd> back energy transfer. UV irradiation by intense 308 nm XeCllaser induces degradation of Ce-doped phosphate glasses emission. Acknowledgements This work was supported by the NEWLUMEN project of the Italian INFN. References [1] G. Zanella et al., Nucl. Instr. and Meth. A345 (1994) 198. [2] G. Zanella, R. Zannoni, R. Dall'Igna, P. Polato, M. Bettinelli, Nucl. Instrum. and Meth. A359 (1995) 547. [3] M. Nikl, J.A. Mares, E. Mihokova, A. Beitlerova, K. Blazek, J. Jindra, Sol. Stat. Commum. 87 (1993) 185. [4] F. Hennenberger, J. Puls, in: F. Hennenberger, S. SchmittRink, E. Gobel (Eds.), Optics of Semiconductor Nanostructures, Akademie Verlag, Berlin, 1993, p. 523. [5] P. Dorenbos, J.C. Spijker, C.W.E. van Eijk, Proceedings of SCINT97, in: Y. Zhiwen, X.Q. Feng, L. Peijum, X. Zhilin (Eds.), CAS, Shanghai Branch Press, Shanghai, China, 1997, pp. 307}310. [6] G. Blasse, J. Alloys Compounds 192 (1993) 17.
Ce> or Tb>-doped phosphate and silicate scintillating glasses S. Baccaro , R. Dall'Igna, P. Fabeni *, M. Martini, J.A. Mares, F. Meinardi, M. Nikl, K. Nitsch, G.P. Pazzi, P. Polato, C. Susini, A. Vedda, G. Zanella, R. Zannoni ENEA, INN/TEC, via Anguillarese 301, 00060 S. Maria di Galeria, Rome, Italy Stazione Sperimentale del Vetro, via Briati 10, 30141 Murano-Venezia, Italy IROE/CNR, via Panciatichi 64, 50 127 Florence, Italy INFM and Dept. of Material Sci., University of Milano, via Cozzi 53, Milan, Italy Institute of Physics AS CR, Cukrovarnicka 10, 162 53 Prague, Czech Republic INFN and Department of Physics, University of Padova, via Marzolo 8, 35131 Padova, Italy
Abstract Optical characterization of scintillating phosphate and silicates glasses is presented including absorption, emission spectra and decay kinetics. A new concept of energy transfer sensitization is developed, based on nearly resonant energy migration through a rare earth ion subsystem (Gd> ions at su$ciently high concentration) in the glass matrix, followed by a single-step transfer towards the emission centers created by Ce> or Tb> doping. While Ce> doping in phosphates results in emission spectra in the near UV-violet, the Tb> doping in silicates gives a four-band spectrum with a dominating one at 545 nm. Intense UV irradiation produces a reduction of emission intensity in phosphate glasses. 2000 Elsevier Science B.V. All rights reserved. Keywords: Scintillating glasses; Ce> and Tb> energy transfer
1. Introduction So far scintillating glasses have been little investigated in comparison with single crystals. Glasses generally suffer from low transfer e$ciency and thus show a low light yield (LY) due to the presence of traps (possible centers of nonradiative recombination) which cause an ine$cient transfer of the thermalized electrons and holes towards the emission centers. Ce-doped silicate glasses seem to provide the highest LY, about 6 ph/keV [1]. Li-enriched silicate glasses with Tb doping gave similar LY [2]. While Ce-doped silicate glasses are being developed for X-ray detection, the Tb-doped (Li-enriched) ones are interesting for neutron detection. We have developed a new concept of energy transfer sensitization based on nearly resonant energy migration
* Corresponding author. Fax: #39-055-410-893. E-mail address: speclab@iroe.".cnr.it (P. Fabeni)
through a rare earth ion subsystem in the glass matrix followed by a single-step transfer to the Ce> (Tb>) emission centers. In more detail, Gd> ions at su$ciently high concentration enable e!ective energy migration in the phosphate and silicate-based glass matrices followed by a single-step energy transfer towards emission centers created by Ce> or Tb> doping. In this contribution the optical characterization of Ce-doped phosphate and Tb-doped silicate glasses with su$ciently high percentage of Gd> ions (above 20%) in the glassy matrix is reported. 2. Experimental An optical multichannel analyzer (OMA) EG&G PARC 1461 was used for the emission spectra measurements in continuous wave (CW) or time-resolved (TR, time gate of 20 ns) modes. Luminescence characteristics were obtained by Spectro#uorometer 199S (Edinburgh Instrument) equipped with a steady-state and
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 3 5 3 - 1
674
S. Baccaro et al. / Journal of Luminescence 87}89 (2000) 673}675
ns-pulsed hydrogen and ls-pulsed Xenon lamps. Luminescence decays obtained in extended dynamical and time scales were measured using the excitation by a XeCl (308 nm) excimer laser line and detected by a photomultiplier coupled to the digital oscilloscope Tektronix TDS 680B. Slow decays in ms time scale were measured with Xenon #ash lamp and multichannel analyzer in a scanning regime at Spectro#uorometer 199S. All the spectra are corrected for experimental distortions and the decay times are extracted from decay curves using deconvolution procedures, i.e. removing distortions due to extended width and shape of excitation pulse (essential in the ns time scale). Four samples of Ce-doped phosphate glasses were prepared using NaPO , GdPO and CePO as starting materials. They are noted as Na74Gd25Ce1, Na99Ce1, Na75Gd25 and Na52Gd45Ce3 (numbers denote the molar percentage of the starting materials in the melt). The constituents were melted in a quartz ampoule at around 12003C and then poured into a graphite crucible. The Tb-doped silicate glass, Gd-free, previously examined (Tb10Mg4 in Ref. [2]) had the composition (weight% in the melt): SiO (58%), Al O (16%), Tb O (11.5%), Li O (10.5%), MgO (4%). The silicate glasses were pro duced in a controlled reducing atmosphere furnace. Three other samples with di!erent Gd percentage were prepared (Tb0Gd25, Tb3Gd30 and Tb10Gd22), in order to evaluate the Gd e!ect in the energy transfer processes. Annealing processes were applied to both silicate and phosphate glasses in order to remove thermal stresses before the optical machining.
3. Results and discussion The main emission of Ce-doped phosphate glasses is situated at 350 nm, see Fig. 1. It shows rather structured character at low temperatures with a shoulder around 325 nm. Such a structure might be induced by the splitted Ce> ground state (4f , 4f levels) which is observed in many Ce-doped (based) crystal systems. Partly asymmetric and broadened Ce> emission band can be explained by a #uctuating crystal "eld in the glass matrix. Emission structure smoothing at elevated temperatures is similar to the situation observed earlier, e.g. in CeF [3], which is explained by partial thermalization of 4f , 4f levels of Ce> ground state. Under continuous laser illumination (308 nm XeCl) the glass emission is noticeably decreasing: in 1 h it reduces to 60% or 40% with power density of 15 or 30 kW/mm, respectively. This e!ect can, at least partly, explain the observed intensity decrease in Fig. 1 (measured from 19 up to 300 K), which is usually not observed in Ce-doped (based) single-crystal matrices. Such an e!ect can be of similar nature as the observed photo-darkening of CdS/CdSe quantum dot doped glasses [4], recently explained as due to a thermally induced
Fig. 1. Emission spectra of phosphate glass Na52Gd45Ce3 at di!erent temperatures (308 nm laser excitation); in the inset the absorption spectra of Na74Gd25Ce1 and Na75Gd25, at RT.
escape of an electron from the quantum dot excited state. Here, thermally induced electron escape from Ce> excited state is probable, supported also by the existence of slower (delayed recombination) decay components in Ce-doped, Gd-free, phosphate glass. In the inset of Fig. 1 the absorption spectra are reported for two di!erent phosphate glasses at RT and the transitions related to Ce> and Gd> are evidenced. With reference to the classi"cation reported in [5], this is the case of intermediate crystal "eld, in which the lowest 4f}5d transition of Ce> is of about the same energy as the lowest 4f}4f transition round 312 nm belonging to Gd>. In such a situation, a bi-directional energy transfer is expected between Gd> and Ce> ions at higher temperatures. The photoluminescence decay kinetics of Ce> centers is characterized by a leading fast component (of about 20}30 ns) with a slow non-exponential tail, which increases in the amplitude with elevating temperature (clearly noticed for times greater than 10\ s), as reported in Fig. 2. Non-monotonic course of the decay curves is the consequence of the exciting laser pulse shape, shown in Fig. 2 as well. The convolution of the laser pulse (instrumental response) with a sum of three exponentials, I(t)"RA exp [!t/q ], where A , q stand for pre-exG G G G ponential factor and decay time, respectively, enabled a reasonably good "tting of decay data at all the temperatures (included in Fig. 2 for ¹"300 K). In the inset of Fig. 2 the temperature dependence of the relative slowcomponents integral intensity I is also given calculated 1! according to the displayed equation. Furthermore at every temperature the TR emission spectra (0}20 ns) coincide with the steady-state ones, which means that also delayed recombination processes occur at Ce> centers. The enhancement of the slow decay processes with temperature is another evidence of the back energy transfer
S. Baccaro et al. / Journal of Luminescence 87}89 (2000) 673}675
675
Fig. 3. Excitation and emission spectra of two silicate glasses, at RT.
[6] due to good matching of excited state 4f levels of Gd> and Tb ions. Fig. 2. Photoluminescence decay of phosphate glass Na52Gd45Ce3 at di!erent temperatures; in the inset the percentage of the slow components I vs. temperature is given. 1! These I values were obtained using a 3-exponential approxi1! mation of decay curve (I(t)"RA exp [!t/q ], i"1, 2, 3) and G G equation given in the inset. At 300 K the shown "t gives the values: A /q "1/19 ns, A /q "0.011/123 ns, A /q "0.00058/ 3930 ns.
Ce>PGd>, mentioned above. Energy transfer Gd>PCe> is well demonstrated in shortening of Gd> decay in 312 nm emission line, which gets nonexponential and considerably shorter in the sample with Ce> presence (6.5 ms decay time in Na75Gd25 sample, while 0.52 ms mean decay time is obtained in Na74Gd25Ce1 sample). In Fig. 3, the emission and excitation spectra are given for two silicate glasses with and without Tb-doping. The presence of Gd> in the silicate glass matrix gives rise to the same absorption peaks round 310 nm as in the phosphate matrix. Emission spectrum of Tb10Gd22 shows four leading narrow bands at 485, 545, 585 and 620 nm, which are broadened due to the #uctuations of the crystal "eld in the glassy environment. No Gd-related emission is present in Tb10Gd22 sample round 312 nm, while it is clearly visible in Tb0Gd25 one. In the excitation spectra of Tb> emission (leading 545 nm band) enhanced excitation is evident in the position of Gd> excited state round 275 and 312 nm. Such a feature gives a good evidence of the Gd>PTb> energy transfer. Similar shortening of Gd> decay in 312 nm emission line is observed in Tb-doped silicate glasses as in the phosphate system above (2.1 ms decay time in Tb0Gd25 samples, while 0.02 ms mean decay time is obtained in Tb10Gd22 sample). It is worth noting that Gd>PTb> energy transfer is observed in many other crystal matrix systems
4. Conclusions The Gd> Ce> energy transfer in phosphate glasses and Gd>PTb> energy transfer in silicate glasses is evidenced by measurements of decay kinetics, excitation and emission spectra of Gd>, Ce> and Tb> emission centers. The participation of Gd> ions in the energy transfer processes in the Ce-doped phosphate glasses produces slow components in the luminescence decay, which are increasing in amplitude with elevated temperature due to the Ce>PGd> back energy transfer. UV irradiation by intense 308 nm XeCllaser induces degradation of Ce-doped phosphate glasses emission. Acknowledgements This work was supported by the NEWLUMEN project of the Italian INFN. References [1] G. Zanella et al., Nucl. Instr. and Meth. A345 (1994) 198. [2] G. Zanella, R. Zannoni, R. Dall'Igna, P. Polato, M. Bettinelli, Nucl. Instrum. and Meth. A359 (1995) 547. [3] M. Nikl, J.A. Mares, E. Mihokova, A. Beitlerova, K. Blazek, J. Jindra, Sol. Stat. Commum. 87 (1993) 185. [4] F. Hennenberger, J. Puls, in: F. Hennenberger, S. SchmittRink, E. Gobel (Eds.), Optics of Semiconductor Nanostructures, Akademie Verlag, Berlin, 1993, p. 523. [5] P. Dorenbos, J.C. Spijker, C.W.E. van Eijk, Proceedings of SCINT97, in: Y. Zhiwen, X.Q. Feng, L. Peijum, X. Zhilin (Eds.), CAS, Shanghai Branch Press, Shanghai, China, 1997, pp. 307}310. [6] G. Blasse, J. Alloys Compounds 192 (1993) 17.