, Jakub Ruzicka Carla Cannas and Takayuki Yanagida Institute of Physics, Academy of Sciences of the Czech Republic Faculty of Science, Charles University of Prague, Department of Inorganic Chemistry University of Cagliari, Chemistry department New Industry Creation Hatchery Center, Tohoku University Czech Republic Italy Japan 1. Introduction Scintillation materials are empl oyed to detect X-ray and gamma-ray photons, neutrons or accelerated particles. Usually the wide band-gap insulator materials of a high degree of structural perfection are used in the form of artificially made single crystals. They accomplish fast and efficient tr ansformation of incoming high energy photon/particle into a number of electron-hole pairs collected in th e conduction and valence bands, respectively, and their radiative recombination at suitable luminescence centres in the material. omposite Technology 202 single crystal preparation. In less demanding applications the suitably doped glasses are also used. However, their efficiency and stab ility in the radiation environment is usually much inferior to the single crystal systems. Modern technologies bring new material conceptions, e.g. the optical ceramics which found its use in the Computed Tomography medical imaging (Greskovich & Du clos, 1997; van Eijk, 2002). Most recently, an innovative approach was reported consisting in preparation of composite materials composed of an inorganic scintillating phase embedded in an inert optical organic or inorganic material. However, the refracti While semiconductor nanocrystals have been a subject of study for more than thirty years, investigations of wide band-gap insulating nanocrystals begun only recently (Tissue, omposite Scintillators 203 was found to crystallize in the monoclinic P2 omposite Technology 204 Fig. 1. Phase diagram of Y system has one stable crystallographic phase at RT representing the thorveitite structure omposite Scintillators 205 Fig. 2. Schematic diagram for the monoclinic structure of Lu crystal omposite Technology 206 transparent with yellowish colour due to presen ce of Ce doping cations. When the silica matrix had crystallized, samples became milky and non-transparent. after heat-treatment at 1300°C (Fig. 3) omposite Scintillators 207 structure of LPS crystal can be seen. In Fig. 8c , the selected area electron diffraction (SAED) pattern is shown. Fig. 3. X-ray diffraction pattern of the sample Y , annealed at 900, 1100 and 1300 Fig. 4. X-ray diffraction pattern of the sample Lu , annealed at 1100 °C omposite Technology 208 Fig. 5. SEM (A) and TEM (B) of YPS:Ce annealed at 900 °C. Fig. 6. SEM (A) and TEM (B) of YPS annealed at 1100 °C heat treated sample. Left image (SEM) shows the glassy aspect of sample while right image (TEM) shows the formation of Fig. 7. SEM (A) and TEM (B) of 1300 °C heat treated YPS:Ce omposite Scintillators 209 A B C Fig. 8. (a, b, c) HRTEM measurement of Lu 3.1.3 Luminescent properties Radioluminescence spectra of the YPS:Ce sample s were measured under X-ray excitation. The spectra are dominated by the Ce 5d -4f emission peaks at about 440 nm, Fig. 9 The intensity increased strongly with increasing temperature of the heat treatment. The maximum of the spectrum is noticeably long-wavelength shifted wi th respect to that of YPS:Ce single crystals Fig. 9. Radioluminescence spectra dependence on the heat treatment temperature of the Ce- doped YPS (excitation by an X-ray tube, 40 kV ) Spectra can be mutually compared in an absolute scale. Small dips around 510 nm and 565 nm are experimental artifacts omposite Technology 210 Fig. 10. Normalized radioluminescence spectra of YPS:Ce and their dependence on annealing temperature. Small dips around 5 10 nm and 565 nm are experimental artifacts. Photoluminescence decays in Fig. 11 were meas ured for all the YPS:Ce samples. The decays Fig. 11. Normalized photoluminescence deca ys of YPS:Ce heat-treated at different temperatures marked in the legend. Single exponential fit with decay time of 32 ns by a solid line is shown for 1300 C treated samples to evaluate the decay time. exc = 345 nm, em = 440 nm. omposite Scintillators 211 Fig. 12. Normalized PL excitation (em=420 nm ) and RL (X-ray, 40 kV) spectra of Ce-doped LPS annealed at 1100 PL decay shows nanosecond decay time of 34 ns (Fig. 13), which is due to the allowed 5d-4f transition of Ce and that is in very good agreement wi th that measured in LPS:Ce single omposite Technology 212 decay curve might be due to an energy transfer from Ce centres to some defects e.g. at grain surface layer. Fig. 13. Photoluminescence decays of the Ce -doped LPS (exc= 300nm, em=400 nm) annealed C. Decay time of 34 ns was evaluated in the tail of the decay. 3.2 Nanocomposite materials :Ce (RE = Y, Lu) nanocomposite samples were prepared using the same sol-gel method. The overstoichiometric ratio of RE/Si (1/10) was used for the material The nanocomposites are transparent when silic a matrix is amorphous, see Fig. 14, but during the heat treatment at the temperature of more than 1250 °C the crystallisation of amorphous silica matrix takes place and cristo balite structure is formed. Consequently, the Fig. 14. Photo of the transparent SiO / Lu :Ce nanocomposite heat-treated at 1000 °C. omposite Scintillators 213 3.2.1 XRD measurements system was prepared and heat-treated at different temperatures. The samples annealed at 1000°C are amorphous and tran sparent. Further increase of annealing temperature to 1100-1200 C leads to crystallisation of both silica matrix (into crystobalite) and of Y as shown in Fig. 15. Fig. 16 shows XRD spectra of the samples denoted as TK10 which are transparent. Annealing for 2h at 1000°C and 1100°C was carried out on the TK10 samples first and then a rapid thermal treatment (RTT) procedure was applied at 1250°C and 1300°C, consisting of increase of temperature from 1000°C or 1100 C to 1250°C - 1300°C in 2 minutes, 5 minutes at this temperat ure, cooling down in 5 minutes. The RTT temperature of 1300°C keeps sample transparen minescent intensity improves. It is interesting to note that in case of non-transparent crystallized sample -YPS phase is observed in XRD spectra, while in transparent ones rather -YPS phase is found or sample remains purely amorphous. Fig. 15. XRD patterns of SiO :Ce nanocomposites (denoted as A10) heat-treated at different temperatures. omposite Technology 214 Fig. 16. TK samples annealed at 1000 °C, 1100 for 2 hours, RTT procedure then applied at 1250 °C and 1300 °C as described in the text before. Fig. 17. XRD spectra of SiO composite after annealing at different, gradually increasing temperatures. omposite Scintillators 215 a) b) c) Fig. 18. SEM and HR TEM images of TK10 SiO nanocomposite samples: a) corresponds to SEM of natural sample surfac e, annealed at 1000°C, b) HRTEM of sample annealed at 1000°C, c) HR TEM of sample annealed at 1100°C. a) b) c) Fig. 19. HRTEM of SiO nanocomposite heat-treated at 1100 °C (a), 1200 °C (b) and 1300 °C (c). The average particle size of the 1100 °C heat -treated sample is about 30 nm, the 1200 °C heat-treated sample reaches the mean particle size of about 50 nm and the 1300 °C heat- treated nanocomposite shows the particle size of about 100 nm. omposite Technology 216 Fig. 20. Size distribution of LPS nanosphe res for the sample heat-treated at 1100 °C evaluated from HRTEM images. 3.2.3 Thermal analysis Thermal behaviour of the both Yttrium and lu omposite Scintillators 217 nanocomposite YPS:Ce/SiO system heat-treated at 1300 °C . In case of samples which were prepared in powder form from the very be ginning the RL intensity shows even higher values compared to BGO, Fig. 23 Fig. 21. TG and DTA analysis of SiO /LPS nanocomposite Fig. 22. RL spectra of the bulk TK 10 YPS:Ce/SiO nanocomposite sample heat-treated at 1100°C and 1300 °C and the spectrum of BGO standard scintillator sample. omposite Technology 218 Fig. 23. RL spectra of A10 nanocomposite YPS:Ce/SiO samples prepared in powder form. Annealing temperatures are shown in the figure , comparison with BGO standard scintillator sample is provided as well. Fig. 24 shows the PL and PLE spectra of A 10 powder nanocomposite sample. Very good correspondence of both emission and excitati on subbands is found compared to the bands ones, see Fig. 25. Normalized spectr a show high energy shift and smaller FWHM with increasing annealing temperature, see Fig. 26. Position of excitation and emission peaks for T =1250-1300 C match reasonably well those of LPS:Ce single crystal omposite Scintillators 219 Fig. 24. PL (ex=247 nm) and PLE (em=390nm) spectra of YPS:Ce/SiO A10 sample at RT. Fig. 25. RL spectra of LPS:Ce/SiO nanocomposite sample annealed at different temperatures given in the figure. Excitation X-ray, 40 kV. omposite Technology 220 Fig. 26. Normalized RL spectra of LPS:Ce/SiO nanocomposite samples from Fig. 25. Excitation X-ray, 40 kV. PLE spectrum for T =1300 °C and em=410 nm is shown as well Fig. 27. Radioluminescence spectra (excitatio n X-ray, 40 kV) of LPS:Ce single crystal, LPS:Ce/SiO nanocomposite, undoped LPS powder and SiO :Ce glass. The latter three samples were made by an analogous sol-gel route, single crystal was grown by Czochralski omposite Scintillators 221 Fig. 28. PL decay of LPS:Ce/SiO nanocomposite sample. Ex= 300 nm, em=410 nm. Solid line is the fit I(t) given in the figure. Fig. 29. Scintillation decay of LPS:Ce/SiO nanocomposite sample. Excitation by picosecond omposite Technology 222 RTT strongly influences the sh ape, position and intensity of RL spectrum (Fig. 30). In Fig. 30. RL spectra of LPS:Ce/SiO nanocomposite sample after application of RTT procedure, the temperature of which is given in the legend 4. Conclusions Sol-gel method has been successfully used fo r preparation of Ce-dop ed yttrium (YPS) and omposite Scintillators 223 the grain surface and structural flaws. LPS:Ce emission characteristics showed to be closer to the bulk single crystal. Photo- and radioluminescence spectra of YPS:Ce and LPS:Ce crystallized nanophase in SiO host appeared very similar to their bulk single crystal analogs. Slightly shortened Ce decay times in REPS:Ce/SiO can indicate the effect of small size of REPS:Ce nanospheres and different refractive index of surrounding medium. RTT leaves the pyrosilicate phase amorphous although the luminescence efficien cy increases noticeably, small low energy shift of Ce spectrum is consistent with the am orphous character of REPS host. RTT procedure indicates the possibility to obtain transparent bulk op tical elements with scintillating REPS:Ce nanophase, but more expe rimental work has to be done to achieve d crack-free samples. 5. Acknowledgement The support of Grant Agency of AS CR, pr oject KAN300100802 is grat efully acknowledged. Thanks are due to A. Beitlerova, R. Kucerkov a and V. Jary for lumi nescence measurements. V.Tyrpekl and I.Jakubec are acknowledged for SEM, TE CRYTUR Ltd. and P. Horodysky are acknowledged for technical help in the experiments. 6. References Becquerel, H. (1896). Sur les Radiations Invisi bles Emises par les Corps Phosphorescents. Comptes-Rendus Hebdomadaires des Séances de l'Académie des Sciences, Vol. 122, pp. Bihari, B.; Eilers, H. & Tissue, B. M. ( 1997). 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One- and Two-Photon Spectroscopy Ions in LaF Mixed Crystals. Journal of Physics: Condensed Matter, No. 24, (15 June 1992), pp. 5461-5470, ISSN 0953-8984 Pidol, L.; Viana, B., Bessière, A., Galtayries , A., Dorenbos, P. & Ferrand, B. (2007). High