- Email: [email protected]
Laser induced effects in the optical properties of Tb3+-doped phosphate scintillating glasses
Radiation Measurements 33 (2001) 721–723
www.elsevier.com/locate/radmeas
Laser induced e ects in the optical properties of Tb3+-doped phosphate scintillating glasses P. Fabenia , G.P. Pazzia , M. Martinib; ∗ , A. Veddab , M. Niklc , K. Nitschc , S. Baccarod b INFM
a IROE del CNR, Via Panciatichi 64, 50127 Firenze, Italy & Dipartimento di Scienza dei Materiali dell’ Universit)a di Milano-Bicocca, Via Cozzi 53, 20125 Milano, Italy c Institute of Physics, AS CR, Cukrovarnick1 a 10, 162 53 Prague, Czech Republic d ENEA, INN=TEC, Casaccia, S. Maria di Galeria, 00060 Roma, Italy
Received 20 August 2000; received in revised form 2 February 2001; accepted 15 February 2001
Abstract Optical absorption, photo- and thermo-luminescence measurements were performed on (NaPO3 )x (GdPO4 )y (TbPO4 )z (x = 60–77 mol%; y = 20–30 mol%; z = 3–10 mol%) glasses. Pronounced changes of photoluminescence intensity of the leading 542 nm emission line of Tb3+ were observed during prolonged excitation (irradiation) by 308 nm XeCl excimer laser line into the Gd 3+ 6 P5=2 excited state. Laser irradiation also gave rise also to optical absorption that peaks at around 5:4 MeV and to a ◦ thermoluminescence glow peak at 195 C. Similar optical absorption and thermoluminescence features were found by ionising (X-ray) irradiation. The results are explained by taking into account energy exchange between the Gd-sub-lattice and deep c 2001 Elsevier Science Ltd. All rights reserved. traps in the glass matrix. Keywords: Phosphate glasses; Thermoluminescence; Point defects; Luminescence
1. Introduction Scintillators based on glass matrices are potentially interesting materials because of their low production cost and their capability of incorporation of di erent activator ions in high concentrations. The main drawback of such systems is their lower light output with respect to scintillating crystals, mainly related to the presence of a relatively high concentration of point defects; these can act as trapping sites and give rise to nonradiative recombination paths of the carriers, thus lowering the energy transfer e@ciency of the material. Recently, new glass scintillators with an enhanced energy transfer e@ciency have been developed, namely Na–Gd phosphate glasses doped with Ce3+ or Tb3+ activator ions providing emission centres in the near UV and green spectral regions, respectively (Baccaro et al., 2000a, b). The presence of Gd 3+ ions in the phosphate glass matrix at concentrations ∗
Corresponding author. Fax: +39-0264485400. E-mail address: [email protected] (M. Martini).
around 30 mol% gives rise to “energy guiding sub-lattice”, which e@ciently delivers the energy (free charge carriers) obtained in the process of conversion of X-ray photons into electron–hole pairs towards the emission centres themselves. In this way, transfer losses (unavoidable in glass matrices) can be noticeably lowered. The Gd 3+ → Ce3+ (Tb3+ ) energy transfer has been described previously for micro-crystalline phosphor materials (Blasse, 1982, 1993) and is of importance for modern single crystal scintillator materials as well (Dorenbos et al., 1997). Such concept can in principle work e@ciently in a glass matrix too, under the assumption of a negligible energy transfer from the Gd 3+ sub-system into matrix defect states (deep mid-gap traps), which can compete for energy capture with the emission centres. This contribution will show that, in the phosphate glasses considered here, such parasitic energy transfer processes do exist at a level, which can noticeably a ect the overall e@ciency of the material. Particularly, the results put in evidence the time and temperature modiHcations of the luminescence intensity under excimer laser (308 nm XeCl line)
c 2001 Elsevier Science Ltd. All rights reserved. 1350-4487/01/$ - see front matter PII: S 1 3 5 0 - 4 4 8 7 ( 0 1 ) 0 0 0 9 0 - 7
722
P. Fabeni et al. / Radiation Measurements 33 (2001) 721–723
irradiation, which can be at least partly related to the population of several trapping states as demonstrated by parallel thermally stimulated luminescence (TSL) and optical absorption measurements. 2. Experimental conditions Samples of Tb-doped phosphate glass were prepared using NaPO3 ; GdPO4 and TbPO4 starting materials of 99.9% purity. In the following, the samples are labelled as Nax Gd y Tbz , where x; y; z denote the molar percentage of the corresponding starting materials in the melt. Mixed con◦ stituents were melted in a quartz ampoule at around 1200 C and after homogenisation poured into a graphite crucible. As a post-growth procedure an appropriate annealing had to be applied to remove thermal stresses before the cutting and polishing procedures. For laser excited luminescence, the samples were placed in a closed cycle cryogenerator (operating in the temperature range 20 –330 K) and excited by a laser with a Xe–Cl mixture (frep = 10 Hz, Epulse (max) = 8 mJ, em = 308 nm, FWHM = 10 ns). The laser beam was focused on the sample surface in a pencil-shape beam and the emission was observed at right angle through a monochromator whose output was sent to an optical multichannel analyser or to a photomultiplier connected to a digital oscilloscope. TSL measurements were performed after X-ray (Machlett OEG 50 X-ray tube operated at 30 kV) or laser excitation, ◦ from RT up to 300 C. Two di erent apparatus were used: in the Hrst one the total emitted light was recorded in photon counting mode by means of a photomultiplier (EMI 9635 QB). The second one was a home made TSL spectrometer measuring the TSL emission both as a function of the temperature and wavelength; the detector was a double-stage micro-channel plate followed by a 512 diode array; the dispersive element was a 140 lines=mm holographic grating, the detection range being 200 –800 nm. In both kinds of ◦ measurements the heating rate was 1 C=s. Optical absorption measurements in the range 200 –800 nm were performed at RT by a Varian Cary 50 spectrophotometer. 3. Experimental results and discussion A 308 nm excitation performed on Na77 Gd 20 Tb3 glass, well matching the lowest absorption transition of Gd 3+ , gives rise to a typical, inhomogeneously broadened luminescence line spectrum dominated by four lines at 482, 542, 585 and 621 nm and belonging to 5 D4 → 7 Fx transitions (x = 6; 5; 4; 3, respectively) (inset of Fig. 1). Instabilities of the luminescence intensity were observed: at RT, a noticeable luminescence increase as a function of time during prolonged laser irradiation was observed (monitored in the most intense Tb3+ 542 nm line), reaching a saturation level
Fig. 1. Dependence of the 542 nm emission line of Tb3+ on irradiation time (UV excitation 308 nm, XeCl excimer laser line, pulse power of 130 kW=mm2 with a spot of about 10 × 0:4 mm2 ) at 295 K (curve a) and 25 K (curve b) for the Na77 Gd 20 Tb3 glass. In the inset the emission spectrum under UV (308 nm) excitation is displayed.
after approximately 200 min of irradiation (Fig. 1, curve a). It is worth remarking that the amplitude of the fast initial intensity rise, its overall rise and the time needed to reach the saturation level depend on the speciHc position of the laser spot on the sample plate, and that the intensity increases by a factor 1.3 to 9. After switching o the laser irradiation, a slight increase of the luminescence intensity is observed for a few minutes, followed by a luminescence decay which can be reasonably approximated by two exponential components. Preliminary Ht evaluations gave decay time constants of about 1 and 190 h, respectively. On the other hand, if the same sample is cooled down below 100 K, an opposite behaviour of the luminescence intensity by increasing the irradiation time is observed (Fig. 1, curve b). Such luminescence instabilities suggest the existence of recombination processes competing with prompt Tb3+ luminescence, possibly governed in a complex way by several point defects with di erent temperature dependencies. Similar results were obtained also for glasses with di erent compositions, as Na65 Gd 30 Tb5 and Na60 Gd 30 Tb10 . Point defects were monitored by TSL and optical absorption measurements after laser irradiation. The TSL glow curve of Na77 Gd 20 Tb3 obtained after laser irradiation is shown in Fig. 2, curve (a): it features a broad and proba◦ bly composite peak centred at about 220 C. The glow curve obtained after X-irradiation is shown in curve (b) of Fig. 2 ◦ for comparison: in this case, two peaks at around 70 C and ◦ 195 C were detected. The emission spectrum of the X-ray induced TSL was also performed, and evidenced the Tb3+ emission lines. A correspondence between the two high temperature structures can be argued; the absence of the lower temperature peak in the glow curve obtained after laser irradiation could be explained by taking into account the fact that, while the X-ray induced TSL glow curve was recorded just after irradiation, the laser induced TSL could be monitored only several days after irradiation. In such a case the
P. Fabeni et al. / Radiation Measurements 33 (2001) 721–723
Fig. 2. TSL glow curves of Na77 Gd 20 Tb3 after X-ray irradiation (103 Gy, curve a) and 308 nm laser line irradiation (curve b) at RT.
Fig. 3. RT optical absorption of Na77 Gd 20 Tb3 : (a) as received; (b) after 308 nm laser line irradiation; (c) after subsequent TSL run. In the inset, the di erence between curves (b) and (a) is reported (laser induced absorption). ◦
traps related to 70 C glow peak became empty due to their estimated lifetime of a few hours at room temperature. The di erent experimental procedures could also explain the observed higher temperature of the broad structure in the glow curve after laser irradiation, as due to partial emptying of the responsible trap states. Optical absorption measurements performed on laser irradiated samples displayed a wide absorption extending from 2.5 up to 5:5 eV, (Fig. 3). By performing the di erence between the spectrum obtained after laser irradiation and that of the unirradiated sample, a laser induced absorption centred at around 5:4 eV was observed (inset of Fig. 3). This laser induced absorption was mostly erased by a TSL run up ◦ to 300 C (Fig. 3, curve c), indicating that carriers freed during TSL partly recombine at defects responsible for optical absorption bands. It is worth remarking that similar optical absorption results were also obtained after X-ray irradiation. The observed dependence of the Tb3+ emission intensity on the 308 nm laser line irradiation (i.e. under selective
723
excitation into the Gd 3+ subsystem) evidences complex energy transfer processes from the Gd 3+ energy guiding sub-lattice, probably involving deep trapping states which compete for the energy capture with the Tb3+ emission centres. The strong temperature dependence of this process (Fig. 1) exempliHes the complicated character of these interactions. One might, for example, suggest that the increase of prompt luminescence observed at RT can be related to a higher e@ciency of radiative recombination after Hlling of the competing traps (part of which are monitored by TSL), up to saturation. We also remark that the two-component time decay of the luminescence after switching o the laser light might be related to the presence of two (groups of) traps monitored by TSL. On the other hand, the opposite time dependence observed at 100 K could suggest that at low T laser irradiation leads to the population of other defect centres (unstable at RT), which lowers the radiative recombination. The complex character of the phenomenology surely does not allow to reach a precise picture: however, it is important to emphasise that all the experimental Hndings suggest energy transfer phenomena from the Gd sub-lattice to defect states, so that sub-band-gap laser excitation results in the population of defect levels similar to ionising X-irradiation. Moreover, as the intensity variations at RT are strongly position dependent, the homogeneity of the glass matrix is in question. From the present data, it can be concluded that the understanding and control of point defects is an important step in the optimisation of the luminescence properties of glasses, even in the presence of a Gd 3+ energy guiding sub-lattice. In this respect, future studies should focus on the possible role of impurities and of the glass structure.
References Baccaro, S., Dall’Igna, R., Fabeni, P., Martini, M., Mares, J.A., Meinardi, F., Mihokova, E., Nikl, M., Nitsch, K., Pazzi, G.P., Polato, P., Vedda, A., Zanella, G., Zannoni, R., 2000a. High light yield Ce3+ (Tb3+ )-doped phosphate and oxide scintillation glasses. In: Mikhailin, V. (Ed.), Proceedings of SCINT99. August 1999, Moscow, Russia, Lomonosov Moscow State University, ISBN 5-8279-0007-9, pp. 106 –112. Baccaro, S., Dall’Igna, R., Fabeni, P., Martini, M., Mares, J.A., Meinardi, F., Nikl, M., Nitsch, K., Pazzi, G.P., Polato, P., Vedda, A., Zanella, G., Zannoni, R., 2000b. Ce3+ or Tb3+ -doped phosphate and silicate scintillating glasses. J. Luminescence 87–89, 673–675. Blasse, G., 1982. Energy transfer phenomena in the system (Y,Ce,Gd,Tb)F3 . Phys. Stat. Sol. (a) 73, 205–208. Blasse, G., 1993. Luminescence of rare-earth ions at the end of the century. J. Alloys Compounds 192, 17–21. Dorenbos, P., Spijker, J.C., van Eijk, C.W.E., 1997. Luminescence and scintillation mechanisms in Ce3+ activated Gd-compounds. In: Yin Zhiwen, Feng Xiqi, Li Peijun, Xue Zhilin (Eds.), Proceedings of SCINT97. Shanghai, China, September 22–25, CAS Shangai Branch Press, pp. 307–310.
www.elsevier.com/locate/radmeas
Laser induced e ects in the optical properties of Tb3+-doped phosphate scintillating glasses P. Fabenia , G.P. Pazzia , M. Martinib; ∗ , A. Veddab , M. Niklc , K. Nitschc , S. Baccarod b INFM
a IROE del CNR, Via Panciatichi 64, 50127 Firenze, Italy & Dipartimento di Scienza dei Materiali dell’ Universit)a di Milano-Bicocca, Via Cozzi 53, 20125 Milano, Italy c Institute of Physics, AS CR, Cukrovarnick1 a 10, 162 53 Prague, Czech Republic d ENEA, INN=TEC, Casaccia, S. Maria di Galeria, 00060 Roma, Italy
Received 20 August 2000; received in revised form 2 February 2001; accepted 15 February 2001
Abstract Optical absorption, photo- and thermo-luminescence measurements were performed on (NaPO3 )x (GdPO4 )y (TbPO4 )z (x = 60–77 mol%; y = 20–30 mol%; z = 3–10 mol%) glasses. Pronounced changes of photoluminescence intensity of the leading 542 nm emission line of Tb3+ were observed during prolonged excitation (irradiation) by 308 nm XeCl excimer laser line into the Gd 3+ 6 P5=2 excited state. Laser irradiation also gave rise also to optical absorption that peaks at around 5:4 MeV and to a ◦ thermoluminescence glow peak at 195 C. Similar optical absorption and thermoluminescence features were found by ionising (X-ray) irradiation. The results are explained by taking into account energy exchange between the Gd-sub-lattice and deep c 2001 Elsevier Science Ltd. All rights reserved. traps in the glass matrix. Keywords: Phosphate glasses; Thermoluminescence; Point defects; Luminescence
1. Introduction Scintillators based on glass matrices are potentially interesting materials because of their low production cost and their capability of incorporation of di erent activator ions in high concentrations. The main drawback of such systems is their lower light output with respect to scintillating crystals, mainly related to the presence of a relatively high concentration of point defects; these can act as trapping sites and give rise to nonradiative recombination paths of the carriers, thus lowering the energy transfer e@ciency of the material. Recently, new glass scintillators with an enhanced energy transfer e@ciency have been developed, namely Na–Gd phosphate glasses doped with Ce3+ or Tb3+ activator ions providing emission centres in the near UV and green spectral regions, respectively (Baccaro et al., 2000a, b). The presence of Gd 3+ ions in the phosphate glass matrix at concentrations ∗
Corresponding author. Fax: +39-0264485400. E-mail address: [email protected] (M. Martini).
around 30 mol% gives rise to “energy guiding sub-lattice”, which e@ciently delivers the energy (free charge carriers) obtained in the process of conversion of X-ray photons into electron–hole pairs towards the emission centres themselves. In this way, transfer losses (unavoidable in glass matrices) can be noticeably lowered. The Gd 3+ → Ce3+ (Tb3+ ) energy transfer has been described previously for micro-crystalline phosphor materials (Blasse, 1982, 1993) and is of importance for modern single crystal scintillator materials as well (Dorenbos et al., 1997). Such concept can in principle work e@ciently in a glass matrix too, under the assumption of a negligible energy transfer from the Gd 3+ sub-system into matrix defect states (deep mid-gap traps), which can compete for energy capture with the emission centres. This contribution will show that, in the phosphate glasses considered here, such parasitic energy transfer processes do exist at a level, which can noticeably a ect the overall e@ciency of the material. Particularly, the results put in evidence the time and temperature modiHcations of the luminescence intensity under excimer laser (308 nm XeCl line)
c 2001 Elsevier Science Ltd. All rights reserved. 1350-4487/01/$ - see front matter PII: S 1 3 5 0 - 4 4 8 7 ( 0 1 ) 0 0 0 9 0 - 7
722
P. Fabeni et al. / Radiation Measurements 33 (2001) 721–723
irradiation, which can be at least partly related to the population of several trapping states as demonstrated by parallel thermally stimulated luminescence (TSL) and optical absorption measurements. 2. Experimental conditions Samples of Tb-doped phosphate glass were prepared using NaPO3 ; GdPO4 and TbPO4 starting materials of 99.9% purity. In the following, the samples are labelled as Nax Gd y Tbz , where x; y; z denote the molar percentage of the corresponding starting materials in the melt. Mixed con◦ stituents were melted in a quartz ampoule at around 1200 C and after homogenisation poured into a graphite crucible. As a post-growth procedure an appropriate annealing had to be applied to remove thermal stresses before the cutting and polishing procedures. For laser excited luminescence, the samples were placed in a closed cycle cryogenerator (operating in the temperature range 20 –330 K) and excited by a laser with a Xe–Cl mixture (frep = 10 Hz, Epulse (max) = 8 mJ, em = 308 nm, FWHM = 10 ns). The laser beam was focused on the sample surface in a pencil-shape beam and the emission was observed at right angle through a monochromator whose output was sent to an optical multichannel analyser or to a photomultiplier connected to a digital oscilloscope. TSL measurements were performed after X-ray (Machlett OEG 50 X-ray tube operated at 30 kV) or laser excitation, ◦ from RT up to 300 C. Two di erent apparatus were used: in the Hrst one the total emitted light was recorded in photon counting mode by means of a photomultiplier (EMI 9635 QB). The second one was a home made TSL spectrometer measuring the TSL emission both as a function of the temperature and wavelength; the detector was a double-stage micro-channel plate followed by a 512 diode array; the dispersive element was a 140 lines=mm holographic grating, the detection range being 200 –800 nm. In both kinds of ◦ measurements the heating rate was 1 C=s. Optical absorption measurements in the range 200 –800 nm were performed at RT by a Varian Cary 50 spectrophotometer. 3. Experimental results and discussion A 308 nm excitation performed on Na77 Gd 20 Tb3 glass, well matching the lowest absorption transition of Gd 3+ , gives rise to a typical, inhomogeneously broadened luminescence line spectrum dominated by four lines at 482, 542, 585 and 621 nm and belonging to 5 D4 → 7 Fx transitions (x = 6; 5; 4; 3, respectively) (inset of Fig. 1). Instabilities of the luminescence intensity were observed: at RT, a noticeable luminescence increase as a function of time during prolonged laser irradiation was observed (monitored in the most intense Tb3+ 542 nm line), reaching a saturation level
Fig. 1. Dependence of the 542 nm emission line of Tb3+ on irradiation time (UV excitation 308 nm, XeCl excimer laser line, pulse power of 130 kW=mm2 with a spot of about 10 × 0:4 mm2 ) at 295 K (curve a) and 25 K (curve b) for the Na77 Gd 20 Tb3 glass. In the inset the emission spectrum under UV (308 nm) excitation is displayed.
after approximately 200 min of irradiation (Fig. 1, curve a). It is worth remarking that the amplitude of the fast initial intensity rise, its overall rise and the time needed to reach the saturation level depend on the speciHc position of the laser spot on the sample plate, and that the intensity increases by a factor 1.3 to 9. After switching o the laser irradiation, a slight increase of the luminescence intensity is observed for a few minutes, followed by a luminescence decay which can be reasonably approximated by two exponential components. Preliminary Ht evaluations gave decay time constants of about 1 and 190 h, respectively. On the other hand, if the same sample is cooled down below 100 K, an opposite behaviour of the luminescence intensity by increasing the irradiation time is observed (Fig. 1, curve b). Such luminescence instabilities suggest the existence of recombination processes competing with prompt Tb3+ luminescence, possibly governed in a complex way by several point defects with di erent temperature dependencies. Similar results were obtained also for glasses with di erent compositions, as Na65 Gd 30 Tb5 and Na60 Gd 30 Tb10 . Point defects were monitored by TSL and optical absorption measurements after laser irradiation. The TSL glow curve of Na77 Gd 20 Tb3 obtained after laser irradiation is shown in Fig. 2, curve (a): it features a broad and proba◦ bly composite peak centred at about 220 C. The glow curve obtained after X-irradiation is shown in curve (b) of Fig. 2 ◦ for comparison: in this case, two peaks at around 70 C and ◦ 195 C were detected. The emission spectrum of the X-ray induced TSL was also performed, and evidenced the Tb3+ emission lines. A correspondence between the two high temperature structures can be argued; the absence of the lower temperature peak in the glow curve obtained after laser irradiation could be explained by taking into account the fact that, while the X-ray induced TSL glow curve was recorded just after irradiation, the laser induced TSL could be monitored only several days after irradiation. In such a case the
P. Fabeni et al. / Radiation Measurements 33 (2001) 721–723
Fig. 2. TSL glow curves of Na77 Gd 20 Tb3 after X-ray irradiation (103 Gy, curve a) and 308 nm laser line irradiation (curve b) at RT.
Fig. 3. RT optical absorption of Na77 Gd 20 Tb3 : (a) as received; (b) after 308 nm laser line irradiation; (c) after subsequent TSL run. In the inset, the di erence between curves (b) and (a) is reported (laser induced absorption). ◦
traps related to 70 C glow peak became empty due to their estimated lifetime of a few hours at room temperature. The di erent experimental procedures could also explain the observed higher temperature of the broad structure in the glow curve after laser irradiation, as due to partial emptying of the responsible trap states. Optical absorption measurements performed on laser irradiated samples displayed a wide absorption extending from 2.5 up to 5:5 eV, (Fig. 3). By performing the di erence between the spectrum obtained after laser irradiation and that of the unirradiated sample, a laser induced absorption centred at around 5:4 eV was observed (inset of Fig. 3). This laser induced absorption was mostly erased by a TSL run up ◦ to 300 C (Fig. 3, curve c), indicating that carriers freed during TSL partly recombine at defects responsible for optical absorption bands. It is worth remarking that similar optical absorption results were also obtained after X-ray irradiation. The observed dependence of the Tb3+ emission intensity on the 308 nm laser line irradiation (i.e. under selective
723
excitation into the Gd 3+ subsystem) evidences complex energy transfer processes from the Gd 3+ energy guiding sub-lattice, probably involving deep trapping states which compete for the energy capture with the Tb3+ emission centres. The strong temperature dependence of this process (Fig. 1) exempliHes the complicated character of these interactions. One might, for example, suggest that the increase of prompt luminescence observed at RT can be related to a higher e@ciency of radiative recombination after Hlling of the competing traps (part of which are monitored by TSL), up to saturation. We also remark that the two-component time decay of the luminescence after switching o the laser light might be related to the presence of two (groups of) traps monitored by TSL. On the other hand, the opposite time dependence observed at 100 K could suggest that at low T laser irradiation leads to the population of other defect centres (unstable at RT), which lowers the radiative recombination. The complex character of the phenomenology surely does not allow to reach a precise picture: however, it is important to emphasise that all the experimental Hndings suggest energy transfer phenomena from the Gd sub-lattice to defect states, so that sub-band-gap laser excitation results in the population of defect levels similar to ionising X-irradiation. Moreover, as the intensity variations at RT are strongly position dependent, the homogeneity of the glass matrix is in question. From the present data, it can be concluded that the understanding and control of point defects is an important step in the optimisation of the luminescence properties of glasses, even in the presence of a Gd 3+ energy guiding sub-lattice. In this respect, future studies should focus on the possible role of impurities and of the glass structure.
References Baccaro, S., Dall’Igna, R., Fabeni, P., Martini, M., Mares, J.A., Meinardi, F., Mihokova, E., Nikl, M., Nitsch, K., Pazzi, G.P., Polato, P., Vedda, A., Zanella, G., Zannoni, R., 2000a. High light yield Ce3+ (Tb3+ )-doped phosphate and oxide scintillation glasses. In: Mikhailin, V. (Ed.), Proceedings of SCINT99. August 1999, Moscow, Russia, Lomonosov Moscow State University, ISBN 5-8279-0007-9, pp. 106 –112. Baccaro, S., Dall’Igna, R., Fabeni, P., Martini, M., Mares, J.A., Meinardi, F., Nikl, M., Nitsch, K., Pazzi, G.P., Polato, P., Vedda, A., Zanella, G., Zannoni, R., 2000b. Ce3+ or Tb3+ -doped phosphate and silicate scintillating glasses. J. Luminescence 87–89, 673–675. Blasse, G., 1982. Energy transfer phenomena in the system (Y,Ce,Gd,Tb)F3 . Phys. Stat. Sol. (a) 73, 205–208. Blasse, G., 1993. Luminescence of rare-earth ions at the end of the century. J. Alloys Compounds 192, 17–21. Dorenbos, P., Spijker, J.C., van Eijk, C.W.E., 1997. Luminescence and scintillation mechanisms in Ce3+ activated Gd-compounds. In: Yin Zhiwen, Feng Xiqi, Li Peijun, Xue Zhilin (Eds.), Proceedings of SCINT97. Shanghai, China, September 22–25, CAS Shangai Branch Press, pp. 307–310.