- Email: [email protected]
Scintillation efficiency and X-ray imaging with the RE-Doped LuAG thin films grown by liquid phase epitaxy
Radiation Measurements 47 (2012) 311e314
Contents lists available at SciVerse ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Technical report
Scintillation efficiency and X-ray imaging with the RE-Doped LuAG thin films grown by liquid phase epitaxy Jan Tous a, *, Karel Blazek a, Miroslav Kucera b, Martin Nikl c, Jiri A. Mares c a
CRYTUR, Ltd., Palackeho 175, 51119 Turnov, Czech Republic Charles University, Faculty of Mathematics and Physics, Ke Karlovu 5, Prague 2, Czech Republic c Institute of Physics, AS CR, Cukrovarnicka 10, 16253 Prague, Czech Republic b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 November 2011 Received in revised form 23 January 2012 Accepted 30 January 2012
Very thin scintillator imaging plates have recently become of great interest. In high resolution X-ray radiography, very thin scintillator layers of about 5e20 mm are used to achieve 2D-spatial resolutions below 1 mm. Thin screens can be prepared by mechanical polishing from single crystals or by epitaxial growth on single-crystal substrates using the Liquid Phase Epitaxy technique (LPE). Other types of screens (e.g. deposited powder) do no reach required spatial resolutions. This work compares LPE-grown YAG and LuAG scintillator films doped with different rare earth ions (Cerium, Terbium and Europium). Two different fluxes were used in the LPE growth procedure. These LPE films are compared to YAG:Ce and LuAG:Ce screens made from bulk single crystals. Relative light yield was detected by a highly sensitive CCD camera. Scintillator screens were excited by a micro-focus X-ray source and the generated light was gathered by the CCD camera’s optical system. Scintillator 2D-homogeneity is examined in an Xray imaging setup also using the CCD camera. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Single crystal Scintillator LuAG X-ray radiography LPE growth
1. Introduction In X-ray micro-radiography and high resolution non-destructive evaluation (NDE) there is a need for thin (few mm e tens of mm) and large area (up to 50 mm) single crystal plates or films to be used with CCD-based position-sensitive photodetectors to enable imaging of tiny objects down to mm scale. Traditionally, CsI:Tlbased plates and more recently columnary-grown CsI:Tl large area panels became extensively used in medical radiography (Moy, 2000). In the latter case, however, the spatial 2D-resolution is given by the column cross section which is of the order of 10 mm2. Another well-known phosphor used in the powder-based flat displayes in X-ray radiography is Gd2O2S:Pr codoped further with (Ce,F) to reduce afterglow (Yamada et al., 1989). However, due to grain size, the spatial 2D-resolution is also of the order of several mm in the best case. Garnet type single crystals Y3Al5O12 (YAG), and more recently also Lu3Al5O12 (LuAG), doped with various rare earth ions are known to be effective scintillators. LuAG has higher density
* Corresponding author. Tel.: þ420 481 319 511; fax: þ420 322323. E-mail address: [email protected] (J. Tous). 1350-4487/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2012.01.015
(r ¼ 6.73 g/cm3) and significantly larger effective atomic number Zeff ¼ 62.9 in comparison to YAG (r ¼ 4.57, Zeff ¼ 32.0) (Nikl, 2006). YAG:Ce LPE films were reported first for 2D-microimaging purposes (Koch et al., 1998), but soon LuAG-based thin plates or films grown by liquid phase epitaxy (LPE) were used for this purpose due to larger Zeff (Tous et al., 2011; Zorenko et al., 2002). Most recently silicate-based thin LPE films became studied for this purpose (Martin et al., 2009; Zorenko et al., 2011). The aim of this paper is to compare the characteristics of thin imaging screens prepared by mechanical polishing from LuAG:Ce single crystals with those prepared by epitaxial growth on singlecrystal substrates using the LPE technique. Rare earth (RE) Ce3þ, Tb3þ and Eu3þ dopants were used for the LPE-grown LuAG-based films. We use trivalent Ce, Tb and Eu emission centers due to their high internal quantum efficiency in the green and red spectral region, respectively, which is perfectly matched with CCD detector sensitivity range. Other rare earth centers as trivalent Nd, Ho, Er and Tm will emit part of the absorbed energy in infrared region beyond the sensitivity range of CCD detector which will decrease detection efficiency of the whole system. On the contrary, Pr3þ will emit substantial part of absorbed energy in 5d-4f transition in UV region where sensitivity of CCD detector is also substantially diminished, i.e. the same deterioration of system sensitivity would occur.
312
J. Tous et al. / Radiation Measurements 47 (2012) 311e314
Fig. 1. LPE Sample 11LuC3 (Left) and Czochralski grown LuAG:Ce glued on Quartz substrate e see Table 1.
2. Experimental details 2.1. Samples
Fig. 2. Scheme of the experiment.
The garnet films were grown at Charles University by isothermal dipping liquid phase epitaxy onto Czochralski grown YAG and LuAG substrates of (111) orientation, see Fig. 1. The films were grown from PbOeB2O3 and from BaOeB2O3eBaF2 fluxes (referred to herein as PbO flux and BaO flux) at constant supercooling. The flux dissolves the binary Y(Lu) and Al oxides and enables film growth at much lower temperatures (around 1000 C) compared to Czochralski grown crystals. Details of the LPE growth technique have been reported elsewhere (Kucera et al., 2010). Films prepared from the PbO flux had excellent crystallographic properties and a high quality smooth surface. The content of rare earth ions in films was tailored by their concentration in the flux, growth temperature, and supercooling. However, the presence of impurity Pb and Pt ions in the LPE films is a serious disadvantage of the PbO flux resulting in lower scintillation efficiency (Babin et al., 2007; Kucera et al., 2008). LPE films with excellent luminescent and scintillation properties were obtained from the BaO flux (Kucera et al., 2008, 2010). No increase of undesirable impurities was observed for this flux. However, the high viscosity and high surface tension of the BaO flux resulted in more complicated film growth and in less ideal surface morphology in films. Although the surface defects do not influence the emission and scintillation properties of the films, they have to be eliminated in applications requiring high spatial resolution (scintillation screens). Single crystal thin plates (SCP) glued on a BK7 glass support (see Fig. 1) were prepared at CRYTUR Ltd., Turnov, Czech Republic, from
Table 1 Survey of characteristics of the RE-Doped samples. Sample no. LuAG20 YAG20 12LBC2 11LuC1 11LuC3
Thickness [mm] 20 20 3.8 18.4 12.4
Doping Ce Ce Ce Ce Ce
Ce [relat] 1 1 1.3 1.32 1.23
Flux SCP SCP BaO PbO PbO
Sample no. 1LE4 1LE52 1LE53 1LE54
Thickness [mm] 11.1 7.4 11.4 7
Doping Eu Eu Eu Eu
Eu/(Eu þ Lu) 0.02 0.038 0.038 0.038
Flux PbO PbO PbO PbO
Sample no. 1YTb2 1YTb3 1LBT4 1LBT5 2LBT5 2LBT6
Thickness [mm] 12.7 20.7 17.7 17.7 12.4 12.7
Doping Tb Tb Tb Tb Tb/Sc Tb/Sc
Tb/(Tb þ Lu)/Sc/(Al þ Sc) 0.010 0.101 0.024 0.099 0.099/0.085 0.099/0.115
Flux PbO PbO BaO BaO BaO BaO
bulk crystals grown by the Czochralski method in molybdenum crucibles under reducing atmosphere. Table 1 presents the survey of the Cerium, Europium and Terbium doped LPE films, respectively, used in this study. The thickness of epitaxial films was determined by weighing. Composition of samples was determined by EPMA (electron probe microanalysis) and low dopant concentration by GDMS (glow discharge mass spectrometry). The observed halfwidths of 444 diffraction peaks of the best epitaxial films were w12 ang. sec. This value is comparable to that of substrates (10 ang. sec) showing on high crystallographic quality of the films. 2.2. Measurements Radioluminescence spectra were measured with a custommade spectrofluorometer 5000M, Horiba Jobin Yvon (Zorenko et al., 2011) using a low voltage with an X-ray tube (10 kV), which ensures that most of the X-ray energy is absorbed in the scintillator film of sufficient thickness (20 mm). X-ray radiography measurements were performed using an X-Ray WorX micro-focus X-rays source, see scheme in Fig. 2. The transmission type X-ray source has a spot size of about 1 mm at the Copper anode. It was operated at 40 kV and target power of 2 W. A scintillator sample was placed on the holder with the scintillator layer oriented towards the X-ray source. The camera objective was focused into the scintillator plane. A mirror is placed between the objective and scintillator to avoid direct exposure of the CCD chip by X-rays. The CCD camera uses an 11 MPix sensor
Table 2 ISDE measurement results. Sample
ISDE [ADU]
RLE [%]
LuAG20 YAG20 12LBC2 11LuC1 11LuC3 1LE4 1LE52 1LE53 1LE54 1YTb2 1YTb3 1LBT4 1LBT5 2LBT5 2LBT6
27784 13591 32789 9717 37947 5892 6975 5583 12448 1529 9821 40169 31979 29632 31805
100 48.9 118.0 35.0 136.6 21.2 25.1 20.1 44.8 5.5 35.4 144.6 115.1 106.7 114.5
J. Tous et al. / Radiation Measurements 47 (2012) 311e314
313
Fig. 5. Absorption in 20 mm thick YAG and LuAG.
3. Results and discussion Fig. 3. RL spectra concentration dependence of Tb-doped LuAG LPE Films. Excitation by X-Ray, 10 kV, 50 mA. Inset e RL spectra integral. Concentration of Tb in samples: 1LBT2 e 0.12%, 1LBT3 e 0.5%, 1LBT4 e 2.4%, 1LBT5 e 10%.
with 9 mm square pixels. The image size is 36.45 mm 23.67 mm. The spectral response of the camera CCD matches a wide range of 450 nme700 nm with QE w 15%e25%. The camera is equipped with a macro optical system with 1:1 magnification. During X-ray exposure, the luminescence image from the scintillator was collected by the CCD camera in order to compare the scintillation efficiency and film/plate homogeneity of the samples. The acquisition parameters were fixed for all samples. The acquisition time was 60 s. The light intensity (mean value) of the images was measured in the images, see Table 2. The ADU values present the image light intensity collected by the CCD sensor and are given by a 16-bit number from CCD camera analogue-to-digital converter. The light yield thus obtained is a steady-state scintillation and optical system efficiency called the imaging system detection efficiency (ISDE). To compare the imaging capability of the LPE film and bulk plate samples, a golden 8 mm wire grid was placed between the X-ray source and the sample. Due to X-ray absorption in the grid, a dark image of the grid is obtained at the sample surface, which is monitored by the same CCD camera and objective.
Fig. 4. RL spectrum of the Eu-Doped LuAG. Excitation X-ray, 10 kV.
X-ray excited emission of the Ce-doped LuAG LPE films shows the well-known broad emission band at about 510 nm due to the 5d1-4f transition of Ce3þ centre (Kucera et al., 2010). Examples of radioluminescence spectra of the Tb and Eu doped LPE films are shown in Figs. 3 and 4 respectively. In the case of Tb3þ, the emission from the 5D3 level (370e480 nm) is dominant at lower Tb concentrations while that from the 5D4 level (above 480 nm) becomes dominant at higher ones due to the well-known crossrelaxation process. We note that maximum RL intensity (spectrum integral) is obtained for an intermediate Tb concentration with still an intense emission from the 5D3 level (see inset). In the case of Eu dopant, the characteristic emission from the 5D0 level to the ground state multiplet 7Fx is observed with an intense line at 710 nm belonging to a transition ending at the 7F4 level. In the measurement of imaging system detection efficiency, the samples were exposed to a conical X-ray beam. The CCD camera exposure time was 60 s integrating the optical signal from the scintillator. About 17.7% of the X-ray energy (for U ¼ 40 kV, Cu anode) was deposited in the 20 mm thick layer of LuAG, and about 12.3% in YAG. LuAG has higher absorption as illustrated in Fig. 5. Sample images obtained with the LPE film and SCP plate are shown in Fig. 6. As the detector is rather insensitive in the region of the YAG intrinsic emission (300e350 nm), a rather small distortion of measured values due to substrate emission can occur, and almost no signal was registered from the BK7 substrate glass in the case of the bulk plate sample as well. Since the thicknesses of the LPE films are different, the ISDE results have been corrected by a calculated absorption of X-rays generated by a Cu anode in the range of 1e40 kV. The data are not corrected for different spectral QE of the CCD for different emission spectra of the samples. The results are
Fig. 6. Camera image of LPE 11LuC3 (Left) and SCP samples in X-rays.
314
J. Tous et al. / Radiation Measurements 47 (2012) 311e314
bulk analogues, as far as their scintillation efficiency and quality of 2D imaging down to a few micron resolution is considered. Scintillation efficiency of the Eu3þ-doped LuAG is comparatively lower. Scintillation efficiency of LPE-grown films depends on the flux used: the BaO flux is clearly superior even if the surface morphology of films becomes worse due to its high viscosity. LPE film-based screens might be advantageous especially for larger diameters (YAG crystals up to 4 inch in diameter can be grown at CRYTUR nowadays) where mechanical processing of thin plates becomes very difficult. Fig. 7. Radiography of the grid using the LPE (Left) and SCP samples.
given in Table 2. The estimated experimental error is below 5%. The ISDE values are in ADU units. ADU is a 16-bit number after analogue-to-digital conversion of the CCD signal. RLE means relative luminescence efficiency while the results are compared to Czochralski-prepared bulk scintillator LuAG:Ce. Fig. 7 presents an absorption image (X-ray radiography) of the test grid using the LPE film and SCP plate. The 2D-spatial resolution of the scintillator element is comparable for both samples. The white spots in the LPE film are morphological flaws and surface defects. In the case of the PbO-based flux, Pb contamination of 10e50 wt% ppm was found in the films by GDMS analysis. Pb2þ ions are wellknown luminescence centres and their emission characteristics have been described recently for LPE-grown LuAG:Ce thin films (Babin et al., 2007). Pb2þ centres capture the energy deposited in the LuAG host lattice under high energy excitation. As their emission is severely quenched at room temperature, they can effectively lower the scintillation efficiency of the material due to nonradiative energy losses, where for they are responsible. Moreover, their presence in the lattice seems to induce the localization (trapping) of exciton nearby the Pb2þ ion (Babin et al., 2007), which is also an unwanted phenomenon as the resulting emission occurs in the UV region. 4. Conclusions In conclusion, the LPE-grown thin films of the Ce3þ or Tb3þ doped LuAG appear comparable to or even competitive with their
Acknowledgements Financial support of Czech GA AV project KAN300100802 is gratefully acknowledged. References Babin, V., Gorbenko, V., Makhov, A., Mares, J.A., Nikl, M., Zazubovich, S., Zorenko, Yu, 2007. Luminescence characteristics of Pb2þ centres in undoped and Ce3þ-doped Lu3Al5O12 single-crystalline films and Pb2þ/Ce3þ energy transfer processes. J. Lumin. 127, 384e390. Koch, A., Raven, C., Spanne, P., Snigirev, A., 1998. X-ray imaging with submicrometer resolution employing transparent luminescent screens. J. Opt. Soc. Am. A 15, 1940e1951. Kucera, M., Nitsch, K., Kubova, M., Solovieva, N., Nikl, M., Mares, J.A., 2008. Ce-doped YAG and LuAG epitaxial films for scintillation detectors. IEEE Trans. Nucl. Sci. 55, 1201e1205. Kucera, M., Nitsch, K., Nikl, M., Hanus, M., Danis, S., 2010. Growth and characterization of YAG and LuAG epitaxial films for scintillation applications. J. Cryst. Growth 312, 1538e1545. Martin, T., Douissard, P.A., Couchaud, M., Cecilia, A., Baumbach, T., Dupré, K., Rack, A., 2009. LSO-based single crystal film scintillator for synchrotron-based hard x-ray micro-imaging. IEEE Trans. Nucl. Sci. 56, 1412. Moy, J.-P., 2000. Recent developments in X-ray imaging detectors. Nucl. Instrum. Methods Phys. Res. A 442, 26e37. Nikl, M., 2006. Scintillation detectors for X-rays. Meas. Sci. Technol. 17, R37eR54. Tous, J., Horodysky, P., Blazek, K., Nikl, M., Mares, J.A., 2011. High resolution low energy X-ray microradiography using a CCD camera. J. Inst. 6 (C01048), 1e5. Yamada, H., Suzuki, A., Uchida, Y., Yoshida, M., Yamamoto, H., 1989. A scintillator Gd2O2S: Pr, Ce, F for x-ray computed tomography. J. Electrochem. Soc. 136, 2713e2716. Zorenko, Yu., Gorbenko, V., Konstankevych, I., Grinev, B., Globus, M., 2002. Scintillation properties of Lu3Al5O12:Ce single crystalline films. NIM A486, 309e314. Zorenko, Yu., Nikl, M., Gorbenko, V., Savchyn, V., Voznyak, T., Kucerkova, R., Sidletskiy, O., Grynyov, B., Fedorov, A., 2011. Growth and luminescent properties of Lu2SiO5 and Lu2SiO5:Ce single crystalline films. Opt. Mat 33, 846e852l.
Contents lists available at SciVerse ScienceDirect
Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas
Technical report
Scintillation efficiency and X-ray imaging with the RE-Doped LuAG thin films grown by liquid phase epitaxy Jan Tous a, *, Karel Blazek a, Miroslav Kucera b, Martin Nikl c, Jiri A. Mares c a
CRYTUR, Ltd., Palackeho 175, 51119 Turnov, Czech Republic Charles University, Faculty of Mathematics and Physics, Ke Karlovu 5, Prague 2, Czech Republic c Institute of Physics, AS CR, Cukrovarnicka 10, 16253 Prague, Czech Republic b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 November 2011 Received in revised form 23 January 2012 Accepted 30 January 2012
Very thin scintillator imaging plates have recently become of great interest. In high resolution X-ray radiography, very thin scintillator layers of about 5e20 mm are used to achieve 2D-spatial resolutions below 1 mm. Thin screens can be prepared by mechanical polishing from single crystals or by epitaxial growth on single-crystal substrates using the Liquid Phase Epitaxy technique (LPE). Other types of screens (e.g. deposited powder) do no reach required spatial resolutions. This work compares LPE-grown YAG and LuAG scintillator films doped with different rare earth ions (Cerium, Terbium and Europium). Two different fluxes were used in the LPE growth procedure. These LPE films are compared to YAG:Ce and LuAG:Ce screens made from bulk single crystals. Relative light yield was detected by a highly sensitive CCD camera. Scintillator screens were excited by a micro-focus X-ray source and the generated light was gathered by the CCD camera’s optical system. Scintillator 2D-homogeneity is examined in an Xray imaging setup also using the CCD camera. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Single crystal Scintillator LuAG X-ray radiography LPE growth
1. Introduction In X-ray micro-radiography and high resolution non-destructive evaluation (NDE) there is a need for thin (few mm e tens of mm) and large area (up to 50 mm) single crystal plates or films to be used with CCD-based position-sensitive photodetectors to enable imaging of tiny objects down to mm scale. Traditionally, CsI:Tlbased plates and more recently columnary-grown CsI:Tl large area panels became extensively used in medical radiography (Moy, 2000). In the latter case, however, the spatial 2D-resolution is given by the column cross section which is of the order of 10 mm2. Another well-known phosphor used in the powder-based flat displayes in X-ray radiography is Gd2O2S:Pr codoped further with (Ce,F) to reduce afterglow (Yamada et al., 1989). However, due to grain size, the spatial 2D-resolution is also of the order of several mm in the best case. Garnet type single crystals Y3Al5O12 (YAG), and more recently also Lu3Al5O12 (LuAG), doped with various rare earth ions are known to be effective scintillators. LuAG has higher density
* Corresponding author. Tel.: þ420 481 319 511; fax: þ420 322323. E-mail address: [email protected] (J. Tous). 1350-4487/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2012.01.015
(r ¼ 6.73 g/cm3) and significantly larger effective atomic number Zeff ¼ 62.9 in comparison to YAG (r ¼ 4.57, Zeff ¼ 32.0) (Nikl, 2006). YAG:Ce LPE films were reported first for 2D-microimaging purposes (Koch et al., 1998), but soon LuAG-based thin plates or films grown by liquid phase epitaxy (LPE) were used for this purpose due to larger Zeff (Tous et al., 2011; Zorenko et al., 2002). Most recently silicate-based thin LPE films became studied for this purpose (Martin et al., 2009; Zorenko et al., 2011). The aim of this paper is to compare the characteristics of thin imaging screens prepared by mechanical polishing from LuAG:Ce single crystals with those prepared by epitaxial growth on singlecrystal substrates using the LPE technique. Rare earth (RE) Ce3þ, Tb3þ and Eu3þ dopants were used for the LPE-grown LuAG-based films. We use trivalent Ce, Tb and Eu emission centers due to their high internal quantum efficiency in the green and red spectral region, respectively, which is perfectly matched with CCD detector sensitivity range. Other rare earth centers as trivalent Nd, Ho, Er and Tm will emit part of the absorbed energy in infrared region beyond the sensitivity range of CCD detector which will decrease detection efficiency of the whole system. On the contrary, Pr3þ will emit substantial part of absorbed energy in 5d-4f transition in UV region where sensitivity of CCD detector is also substantially diminished, i.e. the same deterioration of system sensitivity would occur.
312
J. Tous et al. / Radiation Measurements 47 (2012) 311e314
Fig. 1. LPE Sample 11LuC3 (Left) and Czochralski grown LuAG:Ce glued on Quartz substrate e see Table 1.
2. Experimental details 2.1. Samples
Fig. 2. Scheme of the experiment.
The garnet films were grown at Charles University by isothermal dipping liquid phase epitaxy onto Czochralski grown YAG and LuAG substrates of (111) orientation, see Fig. 1. The films were grown from PbOeB2O3 and from BaOeB2O3eBaF2 fluxes (referred to herein as PbO flux and BaO flux) at constant supercooling. The flux dissolves the binary Y(Lu) and Al oxides and enables film growth at much lower temperatures (around 1000 C) compared to Czochralski grown crystals. Details of the LPE growth technique have been reported elsewhere (Kucera et al., 2010). Films prepared from the PbO flux had excellent crystallographic properties and a high quality smooth surface. The content of rare earth ions in films was tailored by their concentration in the flux, growth temperature, and supercooling. However, the presence of impurity Pb and Pt ions in the LPE films is a serious disadvantage of the PbO flux resulting in lower scintillation efficiency (Babin et al., 2007; Kucera et al., 2008). LPE films with excellent luminescent and scintillation properties were obtained from the BaO flux (Kucera et al., 2008, 2010). No increase of undesirable impurities was observed for this flux. However, the high viscosity and high surface tension of the BaO flux resulted in more complicated film growth and in less ideal surface morphology in films. Although the surface defects do not influence the emission and scintillation properties of the films, they have to be eliminated in applications requiring high spatial resolution (scintillation screens). Single crystal thin plates (SCP) glued on a BK7 glass support (see Fig. 1) were prepared at CRYTUR Ltd., Turnov, Czech Republic, from
Table 1 Survey of characteristics of the RE-Doped samples. Sample no. LuAG20 YAG20 12LBC2 11LuC1 11LuC3
Thickness [mm] 20 20 3.8 18.4 12.4
Doping Ce Ce Ce Ce Ce
Ce [relat] 1 1 1.3 1.32 1.23
Flux SCP SCP BaO PbO PbO
Sample no. 1LE4 1LE52 1LE53 1LE54
Thickness [mm] 11.1 7.4 11.4 7
Doping Eu Eu Eu Eu
Eu/(Eu þ Lu) 0.02 0.038 0.038 0.038
Flux PbO PbO PbO PbO
Sample no. 1YTb2 1YTb3 1LBT4 1LBT5 2LBT5 2LBT6
Thickness [mm] 12.7 20.7 17.7 17.7 12.4 12.7
Doping Tb Tb Tb Tb Tb/Sc Tb/Sc
Tb/(Tb þ Lu)/Sc/(Al þ Sc) 0.010 0.101 0.024 0.099 0.099/0.085 0.099/0.115
Flux PbO PbO BaO BaO BaO BaO
bulk crystals grown by the Czochralski method in molybdenum crucibles under reducing atmosphere. Table 1 presents the survey of the Cerium, Europium and Terbium doped LPE films, respectively, used in this study. The thickness of epitaxial films was determined by weighing. Composition of samples was determined by EPMA (electron probe microanalysis) and low dopant concentration by GDMS (glow discharge mass spectrometry). The observed halfwidths of 444 diffraction peaks of the best epitaxial films were w12 ang. sec. This value is comparable to that of substrates (10 ang. sec) showing on high crystallographic quality of the films. 2.2. Measurements Radioluminescence spectra were measured with a custommade spectrofluorometer 5000M, Horiba Jobin Yvon (Zorenko et al., 2011) using a low voltage with an X-ray tube (10 kV), which ensures that most of the X-ray energy is absorbed in the scintillator film of sufficient thickness (20 mm). X-ray radiography measurements were performed using an X-Ray WorX micro-focus X-rays source, see scheme in Fig. 2. The transmission type X-ray source has a spot size of about 1 mm at the Copper anode. It was operated at 40 kV and target power of 2 W. A scintillator sample was placed on the holder with the scintillator layer oriented towards the X-ray source. The camera objective was focused into the scintillator plane. A mirror is placed between the objective and scintillator to avoid direct exposure of the CCD chip by X-rays. The CCD camera uses an 11 MPix sensor
Table 2 ISDE measurement results. Sample
ISDE [ADU]
RLE [%]
LuAG20 YAG20 12LBC2 11LuC1 11LuC3 1LE4 1LE52 1LE53 1LE54 1YTb2 1YTb3 1LBT4 1LBT5 2LBT5 2LBT6
27784 13591 32789 9717 37947 5892 6975 5583 12448 1529 9821 40169 31979 29632 31805
100 48.9 118.0 35.0 136.6 21.2 25.1 20.1 44.8 5.5 35.4 144.6 115.1 106.7 114.5
J. Tous et al. / Radiation Measurements 47 (2012) 311e314
313
Fig. 5. Absorption in 20 mm thick YAG and LuAG.
3. Results and discussion Fig. 3. RL spectra concentration dependence of Tb-doped LuAG LPE Films. Excitation by X-Ray, 10 kV, 50 mA. Inset e RL spectra integral. Concentration of Tb in samples: 1LBT2 e 0.12%, 1LBT3 e 0.5%, 1LBT4 e 2.4%, 1LBT5 e 10%.
with 9 mm square pixels. The image size is 36.45 mm 23.67 mm. The spectral response of the camera CCD matches a wide range of 450 nme700 nm with QE w 15%e25%. The camera is equipped with a macro optical system with 1:1 magnification. During X-ray exposure, the luminescence image from the scintillator was collected by the CCD camera in order to compare the scintillation efficiency and film/plate homogeneity of the samples. The acquisition parameters were fixed for all samples. The acquisition time was 60 s. The light intensity (mean value) of the images was measured in the images, see Table 2. The ADU values present the image light intensity collected by the CCD sensor and are given by a 16-bit number from CCD camera analogue-to-digital converter. The light yield thus obtained is a steady-state scintillation and optical system efficiency called the imaging system detection efficiency (ISDE). To compare the imaging capability of the LPE film and bulk plate samples, a golden 8 mm wire grid was placed between the X-ray source and the sample. Due to X-ray absorption in the grid, a dark image of the grid is obtained at the sample surface, which is monitored by the same CCD camera and objective.
Fig. 4. RL spectrum of the Eu-Doped LuAG. Excitation X-ray, 10 kV.
X-ray excited emission of the Ce-doped LuAG LPE films shows the well-known broad emission band at about 510 nm due to the 5d1-4f transition of Ce3þ centre (Kucera et al., 2010). Examples of radioluminescence spectra of the Tb and Eu doped LPE films are shown in Figs. 3 and 4 respectively. In the case of Tb3þ, the emission from the 5D3 level (370e480 nm) is dominant at lower Tb concentrations while that from the 5D4 level (above 480 nm) becomes dominant at higher ones due to the well-known crossrelaxation process. We note that maximum RL intensity (spectrum integral) is obtained for an intermediate Tb concentration with still an intense emission from the 5D3 level (see inset). In the case of Eu dopant, the characteristic emission from the 5D0 level to the ground state multiplet 7Fx is observed with an intense line at 710 nm belonging to a transition ending at the 7F4 level. In the measurement of imaging system detection efficiency, the samples were exposed to a conical X-ray beam. The CCD camera exposure time was 60 s integrating the optical signal from the scintillator. About 17.7% of the X-ray energy (for U ¼ 40 kV, Cu anode) was deposited in the 20 mm thick layer of LuAG, and about 12.3% in YAG. LuAG has higher absorption as illustrated in Fig. 5. Sample images obtained with the LPE film and SCP plate are shown in Fig. 6. As the detector is rather insensitive in the region of the YAG intrinsic emission (300e350 nm), a rather small distortion of measured values due to substrate emission can occur, and almost no signal was registered from the BK7 substrate glass in the case of the bulk plate sample as well. Since the thicknesses of the LPE films are different, the ISDE results have been corrected by a calculated absorption of X-rays generated by a Cu anode in the range of 1e40 kV. The data are not corrected for different spectral QE of the CCD for different emission spectra of the samples. The results are
Fig. 6. Camera image of LPE 11LuC3 (Left) and SCP samples in X-rays.
314
J. Tous et al. / Radiation Measurements 47 (2012) 311e314
bulk analogues, as far as their scintillation efficiency and quality of 2D imaging down to a few micron resolution is considered. Scintillation efficiency of the Eu3þ-doped LuAG is comparatively lower. Scintillation efficiency of LPE-grown films depends on the flux used: the BaO flux is clearly superior even if the surface morphology of films becomes worse due to its high viscosity. LPE film-based screens might be advantageous especially for larger diameters (YAG crystals up to 4 inch in diameter can be grown at CRYTUR nowadays) where mechanical processing of thin plates becomes very difficult. Fig. 7. Radiography of the grid using the LPE (Left) and SCP samples.
given in Table 2. The estimated experimental error is below 5%. The ISDE values are in ADU units. ADU is a 16-bit number after analogue-to-digital conversion of the CCD signal. RLE means relative luminescence efficiency while the results are compared to Czochralski-prepared bulk scintillator LuAG:Ce. Fig. 7 presents an absorption image (X-ray radiography) of the test grid using the LPE film and SCP plate. The 2D-spatial resolution of the scintillator element is comparable for both samples. The white spots in the LPE film are morphological flaws and surface defects. In the case of the PbO-based flux, Pb contamination of 10e50 wt% ppm was found in the films by GDMS analysis. Pb2þ ions are wellknown luminescence centres and their emission characteristics have been described recently for LPE-grown LuAG:Ce thin films (Babin et al., 2007). Pb2þ centres capture the energy deposited in the LuAG host lattice under high energy excitation. As their emission is severely quenched at room temperature, they can effectively lower the scintillation efficiency of the material due to nonradiative energy losses, where for they are responsible. Moreover, their presence in the lattice seems to induce the localization (trapping) of exciton nearby the Pb2þ ion (Babin et al., 2007), which is also an unwanted phenomenon as the resulting emission occurs in the UV region. 4. Conclusions In conclusion, the LPE-grown thin films of the Ce3þ or Tb3þ doped LuAG appear comparable to or even competitive with their
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