crystals Article
LPE Growth of Single Crystalline Film Scintillators Based on Ce3+ Doped Tb3−xGdxAl5−yGayO12 Mixed Garnets Vitalii Gorbenko 1, *, Tetiana Zorenko 1 , Sandra Witkiewicz 1 , Kazimierz Paprocki 1 , Oleg Sidletskiy 2 , Alexander Fedorov 3 , Paweł Bilski 4 ID , Anna Twardak 4 and Yuriy Zorenko 1, * 1 2 3 4
*
ID
Institute of Physics, Kazimierz Wielki University in Bydgoszcz, 85090 Bydgoszcz, Poland;
[email protected] (T.Z.);
[email protected] (S.W.);
[email protected] (K.P.) Institute for Scintillation Materials, National Academy of Sciences of Ukraine, 61001 Kharkiv, Ukraine;
[email protected] SSI Institute for Single Crystals, National Academy of Sciences of Ukraine, 61178 Kharkiv, Ukraine;
[email protected] Institute of Nuclear Physics, Polish Academy of Sciences, 31342 Krakow, Poland;
[email protected] (P.B.);
[email protected] (A.T.) Correspondence:
[email protected] (V.G.);
[email protected] (Y.Z.)
Received: 24 July 2017; Accepted: 25 August 2017; Published: 30 August 2017
Abstract: The growth of single crystalline films (SCFs) with excellent scintillation properties based on the Tb1.5 Gd1.5 Al5−y Gay O12 :Ce mixed garnet at y = 2–3.85 by Liquid Phase Epitaxy (LPE) method onto Gd3 Al2.5 Ga2.5 O12 (GAGG) substrates from BaO based flux is reported in this work. We have found that the best scintillation properties are shown by Tb1.5 Gd1.5 Al3 Ga2 O12 :Ce SCFs. These SCFs possess the highest light yield (LY) ever obtained in our group for LPE grown garnet SCF scintillators exceeding by at least 10% the LY of previously reported Lu1.5 Gd1.5 Al2.75 Ga2.25 O12 :Ce and Gd3 Al2–2.75 Ga3–2.25 O12 :Ce SCF scintillators, grown from BaO based flux. Under α-particles excitation, the Tb1.5 Gd1.5 Al3 Ga2 O12 :Ce SCF show LY comparable with that of high-quality Gd3 Al2.5 Ga2.5 O12 :Ce single crystal (SC) scintillator with the LY above 10,000 photons/MeV but faster (at least by 2 times) scintillation decay times t1/e and t1/20 of 230 and 730 ns, respectively. The LY of Tb1.5 Gd1.5 Al2.5 Ga2.5 O12 :Ce SCFs, grown from PbO flux, is comparable with the LY of their counterparts grown from BaO flux, but these SCFs possess slightly slower scintillation response with decay times t1/e and t1/20 of 330 and 990 ns, respectively. Taking into account that the SCFs of the Tb1.5 Gd1.5 Al3–2.25 Ga2–2.75 O12 :Ce garnet can also be grown onto Ce3+ doped GAGG substrates, the LPE method can also be used for the creation of the hybrid film-substrate scintillators for simultaneous registration of the different components of ionization fluxes. Keywords: liquid phase epitaxy; single crystalline films; scintillators; mixed garnets; Tb3+ cations
1. Introduction The development of detectors for 2D/3D microimaging using X-ray sources and synchrotron radiation demands the creation of thin (from a few microns thick up to 20 microns) single crystalline film (SCF) scintillating screens with an extremely high ability for X-ray absorption and a micron-submicron spatial resolution [1–4]. More recently, for this purpose, the visible emitting scintillating screens based on the SCF of Ce doped Y3 Al5 O12 (YAG) and Lu3 Al5 O12 (LuAG) garnets grown by the Liquid Phase Epitaxy (LPE) method have been used and the spatial resolution of the detector in the micron range has been achieved using synchrotron radiation with energy in the 8–20 keV range [1,2]. After that, the SCFs of Eu3+ , Tb3+ doped Gd3 Al5 O12 (GGG) and Sc3+ doped LuAG garnets [3,4], Tb3+ and double Tb3+ , Ce3+ doped Lu2 SiO5 (LSO) orthosilicates [5–14], Ce3+ , Tb3+ and Crystals 2017, 7, 262; doi:10.3390/cryst7090262
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Eu3+ doped LuAlO3 (LuAP) and (Gd,Lu)AlO3 (GLAP) perovskites [15–19] and recently Ce3+ doped Tb3 Al5 O12 (TbAG) garnets [20,21], have also been successfully developed in the last decade for microimaging detectors using the LPE method. The fabrication of screens with higher spatial resolution of X-ray images in the submicron range demands the creation of new scintillating film screens with extremely high absorption ability for X-rays—which is proportional to $Zeff 4 , where $ is the density and Zeff is the effective atomic number of scintillators [2,3]—as well as the development of novel concepts for microimaging. During the last years, two novel concepts for the creation of a detector for microtomography have been proposed [15,19,21]. The first concept is related to the engineering of K-edge of X-ray absorption multilayer-film scintillators using the solid solution of oxide compounds containing the Lu, Gd and Tb ions [15,19,21]. Indeed, the absorption ability of the film scintillator can be significantly improved in the 20–65 keV range due to the significant broadening of the K-edge of X-ray absorption in such mixed materials [15,19,21]. The second concept is based on using the complex multilayer-film scintillator with a separate pathway for registration of the optical signal from each layer and final overlapping of the images coming from the different parts of the complex scintillator [15,21]. By using such multilayer-film scintillators one can significantly improve the contrast and resolution of images even in the submicron range. Two such novel concepts also demand the fabrication of different sets of heavy and efficient SCF scintillators which can be deposited onto the same substrates. The Ce3+ doped Lu3−x Gdx Al5−y Gay O12 and Gd3 Al5−y Gay O12 mixed garnets are related to the efficient and heavy scintillators with very high (up to 50,000 photons/MeV) light yield (LY) under γ quanta excitation [22–25]. For this reason, these compounds are also used for the fabrication of the scintillation screens with high absorption ability for X-rays [26–33]. With the aim of increasing the energy transfer efficiency from the host of mixed garnets to the Ce3+ ions, the Tb3 Al5 O12 , Lu3−x Tbx Al5 O12 and Gd3−x Tbx Al5 O12 SCFs were also crystallized by the LPE method and their luminescent and scintillation properties were investigated [20,21,34,35]. The Ga co-doped analogues of these garnets can also be considered as very interesting matrixes for this purpose and their luminescent and scintillation properties were briefly reported by us as well [36]. At the same time, the possibility of the creation of the efficient SCF scintillators on the basis of the mentioned Tb containing garnets by the LPE method still needs the following technological and experimental evidence. First of all, estimation of the real potential of different garnet compositions for producing the scintillation screens strongly requires the LPE crystallization of these compounds in the SCF form from the different types of fluxes due to very large influence of flux related dopants on their scintillation characteristics [31,33,37,38] as well as the crystal analogs of these garnets using MPD [39] or Czochralski methods. In this paper, we present the new results of the research on the creation of the advanced SCF scintillation screens based on Ce doped Tb1.5 Gd1.5 Al5−y Gay O12 mixed garnets at y = 2–3.85, grown by the LPE method from the novel lead free BaO based flux (later called Tb1.5 Gd1.5 Al5−y Gay O12 :Ce (BaO) SCFs) and compare their properties with those of Tb3−x Gdx Al5−y Gay O12 SCFs at x = 0–2.1 and y = 0–2.75, grown from the traditional PbO based flux [21,37] (later called Tb3−x Gdx Al5−y Gay O12 :Ce (PbO) SCFs). For engineering the scintillator composition we apply the combination of Ce3+ 5d-level positioning [20] and band-gap engineering [40] in the Ce doped Tb3−x Gdx Al5−y Gay O12 mixed garnet using the substitution by Gd3+ cations of the dodecahedral sites of Tb3 Al5 O15 garnet lattice with the concentration x = 1.5 and the substitution by Ga3+ ions of the Al3+ cations in both the tetrahedral and octahedral positions of the garnet host at concentration y = 2–3.85. Additionally, we can expect an increase in the energy transfer efficiency from the host of the Tb3−x Gdx Al5−y Gay O12 garnet to the Ce3+ ions using the sublattices of Tb3+ and Gd3+ cations. 2. Growth of Tb3−x Gdx Al5−y Gay O12 :Ce Single Crystalline Films The SCFs of Tb3−x Gdx Al5−y Gay O12 :Ce garnets were grown by the LPE method onto Gd3 Al2.5 Ga2.5 O12 (GAGG) substrates with a relatively high lattice constant of 12.228 Å in comparison with
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the traditional Y3 Al5 O12 (YAG) substrates with a lattice constant of 12.003 Å from supercooled melt solutions using both BaO-B2 O3 -BaF2 and PbO-B2 O3 fluxes. Firstly, the sets of optical quality perfect Crystals 2017, 7, 262x values in the 0–2.1 range, y values changing in the 0–2.85 range and 3 of thickness 14 SCF samples—with in the 16–38 µm range—were crystallized onto GAGG substrates with a square of 1 × 1 cm2 with solutions using both BaO-B2O3-BaF2 and PbO-B2O3 fluxes. Firstly, the sets of optical quality perfect the (100) orientation (Figure 1, left) using conventional PbO flux for more detail of SCF samples—with x values in the 0–2.1the range, y values changing in the 0–2.85 range anddetermination thickness the optimal ofrange—were Gd and Gacrystallized concentrations x andsubstrates y, respectively, in comparison work [36]. in theranges 16–38 μm onto GAGG with a square of 1 × 1 cm2 with with the (100)other orientation 1, left) using the conventional PbO for more detail determination of the After that, sets of(Figure optically good quality Tb1.5 Gd1.5 Al5flux Ga O :Ce SCF samples—with y values y 12 −y optimal ranges of Gd and Ga concentrations x and y, respectively, in comparison with work [36]. changing in the 2–3.8 range and thickness in the 10.5–22 µm range—were also successfully crystallized After that, other sets of optically good quality Tb1.5Gd1.5Al5−yGayO12:Ce SCF samples—with y values onto GAGG substrates from novel lead-free BaO based flux (see Figure 1, middle and right figures, changing in the 2–3.8 range and thickness in the 10.5–22 μm range—were also successfully and Table 1). The components of this flux have significantly smaller influence on their scintillation crystallized onto GAGG substrates from novel lead-free BaO based flux (see Figure 1, middle and properties than in the of1). PbO grown SCF scintillators [31,33,37,38]. At influence the sameon time, right figures, andcase Table Theflux components of this flux have significantly smaller theirthe high viscosity of this BaO based than fluxinleads to formation of different structural macro-defects scintillation properties the case of PbO flux grown SCF scintillators [31,33,37,38]. At theand samestrongly time, theuniformity high viscosity this BaO based[31,33,37]. flux leads toSuch formation of different structural macro-defects decreases the ofofSCF surface unwanted effects are also observed in the and strongly decreases the uniformity of SCF surface [31,33,37]. Such unwanted effects are alsousing the case of growing the Tb1.5 Gd1.5 Al5−y Gay O12 :Ce SCFs from BaO based flux. For this reason, observed in the case of growing the Tb1.5Gd1.5Al5−yGayO12:Ce SCFs from BaO based flux. For this PbO-B2 O3 solvent—due to its low viscosity and good kinematic properties—is preferable for producing high reason, using the PbO-B2O3 solvent—due to its low viscosity and good kinematic properties—is preferable for quality SCF scintillators for ensuring the best structural of screens forscreens high-resolution producing high quality SCF scintillators for ensuring theand best surface structuralquality and surface quality of for X-ray imaging [6,7,18,19]. high-resolution X-ray imaging [6,7,18,19]. GAGG substrate TGAGG (PbO) SCF TGAGG (BaO) SCF
Figure 1. Images of undoped Gd3AlGa 2.5Ga2.5O12 (GAGG) substrate (left), Tb3Al2.5Ga2.5O12:Ce (PbO) Figure 1. Images of undoped Gd3 Al 2.5 2.5 O12 (GAGG) substrate (left), Tb3 Al2.5 Ga2.5 O12 :Ce (PbO) (middle) and Tb1.5Gd1.5Al3Ga2O12:Ce (BaO) (right) SCF scintillators, grown by the LPE method onto (middle) and Tb1.5 Gd1.5 Al3 Ga2 O12 :Ce (BaO) (right) SCF scintillators, grown by the LPE method onto GAGG substrates. GAGG substrates. Table 1. Growth conditions, luminescent and scintillation properties of Tb3−xGdxAl5−yGayO12:Ce SCFs. of CL spectra, t1/e and t1/20 scintillation 1/ey O12 :Ce misfit, luminescent λmax—maximum Growth conditions, and scintillation properties of Tb3−decay Al5 −to Table 1.M—SCF/substrate y Ga x Gdxtimes 239Pu (5.15 and 1/20 levels, respectively; LY—photoelectron light yield under α-particle excitation by SCFs. M—SCF/substrate misfit, λmax —maximum of CL spectra, t1/e and t1/20 scintillation decay times MeV) source with respect to the standard YAG:Ce SCF with a photoelectron LY of 360 phels/MeV (light 239 to 1/e and 1/20 levels, respectively; LY—photoelectron light yield under α-particle excitation by Pu yield of 2650 photons/MeV) [38] and reference Gd3Al2–2.5Ga3–2.5O12:Ce bulk crystals with a photoelectron (5.15 MeV) source with respect to the standard YAG:Ce SCF with a photoelectron LY of 360 phels/MeV LY of 1300–1370 phels/MeV (light yield of about 10,100 photons/MeV). (light yield of 2650 photons/MeV) [38] and reference Gd3 Al2–2.5 Ga3–2.5 O12 :Ce bulk crystals with a Content of SCF Samples Flux Substrate m, % λmax, nm t1/e/t1/20, ns LY, % Reference photoelectron LY of 1300–1370 phels/MeV (light yield of about 10,100 photons/MeV).
YAG:Ce LuAG:Ce Content of Tb SCF Samples Flux 1.5Gd1.5Al3Ga2O12:Ce 1.5 Gd 1.5 Al 2.5 Ga 2.5 O 12 :Ce Tb YAG:Ce PbO :Ce Tb1.5Gd1.5Al1.5Ga3.5O12PbO LuAG:Ce Gd1.5O Al1.15Ga3.85O12BaO :Ce Tb1.5 Gd1.5Tb Al1.53 Ga 2 12 :Ce 5O12:Ce Tb3Al Tb1.5 Gd1.5 Al2.5 Ga O :Ce BaO 2.5 12 3Ga2O12:Ce Tb1.5 Gd1.5 Al1.5Tb Ga3Al BaO 3.5 O12 :Ce 3Al2.5Ga2.5O12:Ce Tb Tb1.5 Gd1.5 Al1.15 Ga3.85 O12 :Ce BaO 2GdAl2.5Ga2.5O12:Ce Tb3 Al5Tb O12 :Ce PbO 1.5Gd1.5Al2.5Ga2.5O12:Ce Tb Tb3 Al3 Ga2 O12 :Ce PbO 2Al2.5Ga2.5O12:Ce TbGd Tb3 Al2.5 Ga PbO 2.5 O12 :Ce 2.5Ga2.5O12:Ce SC Tb2 GdAl2.5Gd Ga3Al PbO 2.5 O12 :Ce 3Al2Ga3O12:Ce SC Tb Gd AlGdGa O :Ce PbO
PbO YAG PbO YAG Substrate BaO GAGG BaO YAG GAGG BaO YAG GAGG BaOGAGG GAGG PbO GAGG GAGG PbO GAGG GAGG PbO GAGG GAGG PbO GAGG GAGG PbO GAGG GAGG PbO GAGG GAGG - GAGG - GAGG -
−0.82 m, −1.30 % −0.79 +0.05 −0.82 +0.5 −1.30 −1.29 −0.79 −0.49 +0.05 −0.37 +0.5 −0.20 −1.29 −0.12 −0.49 −0.04 −0.37 −0.20−0.12-
535 67.3 100 509 52.8 205 λ553 t1/e /t1/20380 , ns max , nm 228/728 543 201/893 270 535 67.3 543 183/728 52.8160 509 543 103/868 50 553 228/728 560 195 543 306/1795 201/893 543 200 543 435/1340 183/728 235 543 456/1368 543 103/868 543 291/883 254 560 306/1795 380 543 333/990 543 435/1340 543 299/88 160 543 456/1368 381 547 543 441/1536 291/883 549 240/876 365 543 333/990
[21,31] [21,31]
LY, %
Reference
100 [21,31] 205 [21,31] 380 [21] 270 [36] 160 [36] 50 [36] 195 [21] [36] 200 [36] [36] 235 [36] [21,31] 254 [36] [21,31] 380 [36] 1.5 1.5 2.5 2.5 12 TbGd2 Al2.5 Ga2.5 O12 :Ce PbO GAGG −0.04 543 299/88 160 [36] The concentration of -CeO2 activating oxide- was 10 and with respect to 381 the garnetGd3 Al2.5 Ga 5475 mole% 441/1536 [21,31] 2.5 O12 :Ce SC components in the- cases of SCF fluxes, respectively. Gd3forming Al2 Ga3 O - growth -using PbO and 549 BaO based 240/876 365 [21,31] 12 :Ce SC
The concentration of CeO2 activating oxide was 10 and 5 mole% with respect to the garnet-forming components in the cases of SCF growth using PbO and BaO based fluxes, respectively.
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The composition of SCF samples was determined using a JEOL JSM-820 electronic microscope, equipped by an EDX microanalyzer with IXRF 500 and LN2 Eumex detectors. From the microanalysis of the content of the SCF samples we have also found that the segregation coefficient of Ga3+ ions inCrystals Tb3 Al Ga O :Ce and Tb3−x Gdx Al5−y Gay O12 :Ce SCFs, grown from PbO 4based flux, 5−y 2017, 7, 262 y 12 of 14 was equal to 0.59–0.65 and 0.735–0.82, respectively (Table 2). The segregation coefficient of Gd3+ The composition SCF samples was determined usingto a JEOL JSM-820The electronic microscope, ions in Tb3− (PbO) SCFs was equal 0.95–1.05. segregation coefficient x Al5−y Gaof yO x Gd 12 :Ce equipped by an EDX microanalyzer with IXRF 500 and LN2 Eumex detectors. From the microanalysis 3+ of Ce ions was equal to about 0.004–0.005 and 0.0095–0.02 in Tb3 Al5−y Gay O12 :Ce SCF and of the content of the SCF samples we have also found that the segregation coefficient of Ga3+ ions in Tb3−x Gdx Al5−y Gay O12 :Ce SCFs, in the case of using PbO based flux for their growth (Table 2). Tb3Al5−yGayO12:Ce and Tb3−xGdxAl5−yGayO12:Ce SCFs, grown from PbO based flux, was equal to 0.59– 3+ We0.65have foundrespectively that the(Table segregation coefficientscoefficient of Ga3+of and and also 0.735–0.82, 2). The segregation Gd3+ Ce ions inions in Tb3−x Gd Al SCFs SCFs in the using BaO fluxcoefficient were significantly y OyO Tbx3−x Gd 5−yGa :Ce (PbO) wascase equalof to 0.95–1.05. The based segregation of Ce3+ ions larger. 5−xAl y Ga 1212:Ce 3+ was equal about 0.004–0.005 and 0.0095–0.02 Tb3Al5−y yO12:Ce SCF and Tb3−xGd xAl5−yBaO GayO12based :Ce Specifically, the tosegregation coefficient of Ga in ions inGa these SCFs, grown from flux, SCFs, the caseand of using basedlarger flux forthan theirthe growth (Table 2).values in the SCFs grown from PbO was equal toin1.0–1.1 was PbO notably respective We have also found that the segregation coefficients of Ga3+ and Ce3+ ions in based flux (Table 2). The segregation coefficient of Ce3+ ions in the Tb3−x Gdx Al5−y Gay O12 :Ce SCFs was Tb3−xGdxAl5−yGayO12:Ce SCFs in the case of using BaO based flux were significantly larger. also significantly larger in the case of using BaO based flux and was equal to 0.012–0.14 in comparison Specifically, the segregation coefficient of Ga3+ ions in these SCFs, grown from BaO based flux, was with theequal 0.004–0.005 innotably Tb3 Allarger :Ce and 0.0095–0.02 inSCFs Tb3−grown Al5−PbO y O12 y O12 :Ce SCF 5−y Ga x Gdxfrom y Gabased to 1.0–1.1 value and was than the respective values in the 3+ counterparts grown from PbO based flux (Table Atinthe time, coefficient of flux (Table 2). The segregation coefficient of Ce 2). ions thesame Tb3−xGd xAl5−ythe GayOsegregation 12:Ce SCFs was also significantly larger to in the case of BaO based andx Al was 0.012–0.14 in comparison Gd3+ ions, being equal 1.0–1.1 inusing the case of Tbflux O12 :Ce SCFs, grown from BaO 3−x Gd 5−equal y Gayto with was the 0.004–0.005 value in Tb 3Al5−yGayO12:Ce and 0.0095–0.02 in Tb3−xGdxAl5−yGayO12:Ce SCF based flux, only slightly larger than that in the case of using PbO based flux (Table 2). counterparts grown from PbO based flux (Table 2). At the same time, the segregation coefficient of The 3+XRD measurements (spectrometer DRON 4, CuKα X-ray source) were used for Gd ions, being equal to 1.0–1.1 in the case of Tb3−xGdxAl5−yGayO12:Ce SCFs, grown from BaO based characterization of the structural quality ofthe Tbcase Al5− :Ce (Table SCFs,2). grown from BaO based y O12flux 1.5 Gd y Ga flux, was only slightly larger than that in of1.5 using PbO based flux (FigureThe 2). XRD Frommeasurements the respective XRD patterns of these at source) y valuewere in the 2–3.85 (spectrometer DRON 4, CuKαSCFs X-ray used for range of the structural quality of Tb 1.5Gd1.5Al5−y Gathe yO12:Ce SCFs, grown from BaO based fluxand the (Figure characterization 2), we can also estimate the lattice constants of different garnet compositions (Figure 2).the From the respective XRD of these SCFssubstrate at y value in (Figure misfit between lattice constants ofpatterns SCFs and GAGG ∆athe= 2–3.85 (aSCF range − asub )/asub2),× 100% we can also estimate the lattice constants of the different garnet compositions and the misfit between (Table 1). Namely, the lattice constant of Tb1.5 Gd1.5 Al5−y Gay O12 :Ce (BaO) SCFs at y = 2–3.85 changed the lattice constants of SCFs and GAGG substrate Δa = (aSCF − asub)/asub × 100% (Table 1). Namely, the from 12.069 Å for Tb1.5 Gd1.5 Al3 Ga2 O12 :Ce SCFs to 12.2913 Å for Tb1.5 Gd1.5 Al1.2 Ga3.8 O12 :Ce SCFs lattice constant of Tb1.5Gd1.5Al5−yGayO12:Ce (BaO) SCFs at y = 2–3.85 changed from 12.069 Å for (Figure Tb 2)1.5and value of misfit m changed from −1.3% to +0.52% for these SCF samples (Table 1). Gd1.5the Al3Ga 2O12:Ce SCFs to 12.2913 Å for Tb1.5Gd1.5Al1.2Ga3.8O12:Ce SCFs (Figure 2) and the value of misfit m changed from −1.3% to +0.52% for these SCF samples (Table 1).
Table 2. Segregation coefficients of different ions in Tb3−x Gdx Al5−y Gay O12 :Ce SCFs grown onto 2 Segregation coefficients of different ions in Tb3−xGdxAl5−yGayO12:Ce SCFs grown onto GAGG GAGG Table substrates. substrates. Type of Flux
Garnet Content Garnet Content
Type of Flux
Tb O1212:Ce :Ce Tb33Al Al5–2.9 5–2.9Ga0–2.1 0–2.1O Tb Gd1–2.1 Al2.25–2.4 Ga 2.25–2.4 2.75–2.6 12 :Ce Tb2–0.9 2–0.9Gd 1–2.1Al Ga 2.75–2.6 O12O:Ce Tb1.5 Gd1.5 Al3–1.15 Ga2–3.85 O12 :Ce
PbO PbO PbO PbO BaO BaO
Tb1.5Gd1.5Al3–1.15Ga2–3.85O12:Ce Kα1
1,0
1- Tb1.5Gd1.5Al3Ga2O12:Ce (BaO) m = - 1.30 %; a=12.0687 ? 2- Tb1.5Gd1.5Al2.5Ga2.5O12:Ce (BaO)
0,8
Counts
SegregationCoefficient Coefficient Segregation 3+ 3+ 3+ 3+ 3+3+ Ce GdGd GaGa Ce 0.59–0.65 0.004–0.005 0.004–0.005 0.59–0.65 1.0–1.1 0.735–0.82 0.735–0.820.0095–0.02 0.0095–0.02 1.0–1.1 1.0–1.05 1.0–1.1 0.012–0.14 1.0–1.05 1.0–1.1 0.012–0.14
m = - 0.79 %; a=12.1312 ? 3- Tb1.5Gd1.5Al1.5Ga3.5O12:Ce (BaO)
3
0,6
4
0,4
Kα2
2
1
m = + 0.05 %; a=12.2337 ? 4- Tb1.5Gd1.5Al1.15Ga3.85O12:Ce (BaO) m = + 0.52 %; a=12.2913 ?
(1200) plane
GAGG sub
0,2
0,0
97
98
99
100
2θ (degrees)
101
Figure 2. patterns XRD patterns (1200)planes planes ofofTbTb 1.5Gd1.5 Al5−yGayO12:Ce (BaO) SCFs at y = 2 (1); 2.5 (2); 3.5 (3) and Figure 2. XRD of of (1200) 1.5 Gd1.5 Al5 −y Gay O12 :Ce (BaO) SCFs at y = 2 (1); 2.5 (2); 3.85 (4). The film/substrate lattice misfit m lies in the −1.3% < m < +0.5% range. 3.5 (3) and 3.85 (4). The film/substrate lattice misfit m lies in the −1.3% < m < +0.5% range.
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The measurements of rocking curves in the ω and 2θ-ω scanning modes were applied for The measurements of rocking curves theGd ω1.5and scanning modes were applied for characterization of the structural quality of in Tb1.5 Al5−2θ-ω y Gay O12 :Ce (BaO) SCFs at different Ga characterization of the structural quality of Tb 1.5 Gd 1.5 Al 5−y Ga y O 12 :Ce (BaO) different Ga figures, content content y in the 2.0–3.85 range (Figure 3a,b, respectively). As can beSCFs seenatfrom these y in the 2.0–3.85 range (Figure 3a,b, respectively). As can be seen from these figures, the quality of the the quality of the SCFs, which is proportional to FWHM of rocking curves, significantly increases at SCFs, which is proportional to FWHM rocking curves, significantly increases at lower SCF-substrate lower SCF-substrate misfit values m inofTb Gd Al Ga O :Ce (BaO) SCFs. Namely, the smallest y 12 1.5 1.5 5−y misfit values mof in0.0182 Tb1.5Gd 1.5Al5−yGayO12:Ce (BaO) SCFs. Namely, the smallest FWHM values of 0.0182 FWHM values and 0.0121 degrees are observed for Tb1.5 Gd1.5 Al1.5 Ga3.5 O12 :Ce (BaO) SCFs 1.5Gd1.5Al1.5Ga3.5O12:Ce (BaO) SCFs (Figure 3a,b, respectively) and 0.0121 are observed for Tb (Figure 3a,b,degrees respectively) grown onto GAGG substrates with the lowest SCF/substrate misfit value grown onto GAGG substrates with the lowest SCF/substrate misfit value m = +0.05% (Table 1). m = +0.05% (Table 1). 1,0
1 - Tb1.5Gd1.5Al3Ga2O12:Ce FWHM=0.037? 2 - Tb1.5Gd1.5Al2.5Ga2.5O12:Ce
0,8
FWHM=0.047? 3 - Tb1.5Gd1.5Al1.5Ga3.5O12:Ce
Counts
(a)
FWHM=0.025? 4 - Tb1.5Gd1.5Al1.15Ga3.85O12:Ce
0,6
1,0
0,8
2 - Tb1.5Gd1.5Al2.5Ga2.5O12:Ce FWHM=0.0383? 3 - Tb1.5Gd1.5Al1.5Ga3.5O12:Ce FWHM=0.0246? 4- Tb1.5Gd1.5Al1.15Ga3.85O12:Ce
(b)
FWHM=0.0218?
0,6
SCF rocking curves (2θ-ω scan) hkl=(800)
FWHM=0.0182?
1
SCF rocking curves (ω-scan) hkl=(800)
0,4
0,4
3 4 2
2 0,2
0,2
3 0,0 -0,15
-0,10
-0,05
4
0,00
ω, degrees
0,05
0,10
0,15
0,0 -0,10
-0,05
0,00
0,05
θ, degrees
Figure Gd1.5 Al5−y O:Ce (BaO) SCFsgrown grownonto ontoGGAG GGAG substrates substrates at 1.5Al 5−Ga y Ga 12 :Ce Figure 3. 3. Rocking Rocking curves curves of of Tb Tb1.5 1.5Gd yOy12 (BaO) SCFs at different y values: y = 2 (1); 2.5 (2); 3.5 (3) and 3.85 (4) recorded in ω (a) and 2θ-ω (b) scanning modes. different y values: y = 2 (1); 2.5 (2); 3.5 (3) and 3.85 (4) recorded in ω (a) and 2θ-ω (b) scanning modes.
3. Scintillation Properties Propertiesof ofTb Tb3−x O12Single :Ce Single 3. Luminescent and Scintillation Gd xAl 5−yGa O12y:Ce Crystalline Films x Al 3− x Gd 5−yyGa Crystalline Films For characterization of the optical properties of Ce3+ doped Tb3−xGdxAl5−yGayO12:Ce SCFs, the 3+ doped Tb For characterization the optical properties of Cedecay O12well :Ce as SCFs, x Gdx Al5−y Gayas cathodoluminescence (CL)ofspectra, LY and scintillation kinetics3−measurements the the cathodoluminescence (CL) spectra, LY and scintillation decay kinetics measurements as well as the thermostimulated luminescence (TSL) glow curves under excitation by α-particles were performed. thermostimulated luminescence (TSL) curves under excitation by α-particles performed. The CL spectra were measured at glow the room temperature (RT) using an electronwere microscope SEM CL spectra were measured at with the room temperature (RT) using an electron microscope SEM JEOLThe JSM-820, additionally equipped a spectrometer Stellar Net with TE-cooled CCD detector JEOL JSM-820, equipped withscintillation a spectrometer Stellar with time TE-cooled detector working in theadditionally 200–1200 nm range. The LY with a Net shaping of 14 CCD μs and decay working the 200–1200 nm range. The scintillation LY setup with a based shapingon time of 14 µs and decay kinetics in measurements were performed using the a Hamamatsu H6521kinetics PMT, measurements were performed using the setup based on a Hamamatsu H6521 PMT, multichannel multichannel analyzer and digital TDS3052 oscilloscope under excitation by α-particles of Pu239 (5.15 239 (5.15 MeV) source. analyzer and digital TDS3052 oscilloscope under byisα-particles MeV) source. The energy resolution (ER) of SCF excitation scintillators calculatedof asPu a ratio of the FWHM of The energy resolution of SCFcentroid scintillators is calculated as a ratio of the FWHM of the full energy the full energy peak to(ER) the peak’s position: E = FWHM/centroid [%]. The TSL measurements peak to the peak’s centroid position: E = FWHM/centroid [%]. The TSL measurements were performed were performed in the 300–800 K temperature range using a commercial Risoe DA-20 TL/OSL reader 241 in the 300–800 K temperature range using a commercial Risoe DA-20 TL/OSL reader (Denmark) after (Denmark) after α-particle excitation by the Am source which is built into the DA-20 reader. The TL 241 −1. The The α-particle excitation by the Am into theof DA-20 TL glow curves glow curves were registered from source 50 °C which to 450 is °Cbuilt at the rate 5 °C·sreader. measurements were ◦ C at the rate of 5 ◦ C·s−1 . The measurements were conducted with were registered 50 ◦BG C to39450 conducted withfrom a Shott green filter, with transmission from 350 to 700 nm. This filter is well 3+ luminescence aadapted Shott BG green filter, with transmission from in 350 to SCF 700 nm. Thisunder filter isstudy. well adapted for the for39the registration of Ce the samples Meanwhile, the registration of Ce3+ luminescence the SCFxAl samples under Meanwhile, spectrally resolved spectrally resolved TSL spectra ofinTb 3−xGd 5−yGayO12 SCF study. with different Gd3+the and Tb3+ content (not 3+ and Tb3+ content (not present in the 3+ luminescence TSL spectra of Tb O12 SCF with different x Al5− present in the paper) show only in theGd green-yellow ranges and absence of the 3−x Gd y GayCe 3+ 3+ or paper) show only Ce in the green-yellow ranges and absence of the emission of Tb3+ emission of Tb Gd3+luminescence cations. or Gd3+ cations. 3.1. Cathodoluminescence Spectra 3.1. Cathodoluminescence Spectra The RT CL spectra of Tb3Al5−yGayO12:Ce and Tb3−xGdxAl5−yGayO12:Ce SCFs, grown from PbO The RTwith CL spectra Tb3 Alin andand Tb3− Gay O12in:Ce grownare from PbO y Ga2–2.5 y O12 :Ce 5 −the x Gdx Al5x −yvalues based flux, close yofvalues range different theSCFs, 0–2 range, shown based flux, with close y values in the 2–2.5 range and different x values in the 0–2 range, are shown in Figure 4a,b, respectively, in comparison with the spectrum of TbAG:Ce SCF (Figure 4a, curve 1). in Figure 4a,b, respectively, in 1.5 comparison the spectrum of TbAG:Ce SCFflux, (Figure curve 1). The RT CL spectra of Tb1.5Gd Al5−yGayO12with :Ce SCFs, grown from BaO based with4a, different y The RT CL spectra of Tb Gd Al Ga O :Ce SCFs, grown from BaO based flux, with different y y 1.5 1.5 5 − y 12 values in the 2–3.85 range, are presented in Figure 4c, curves 1–4 in comparison with the CL spectra of standard Gd3Al2.5Ga2.5O12:Ce bulk SC (Figure 4b, curve 5).
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values in the 2–3.85 range, are presented in Figure 4c, curves 1–4 in comparison with the CL spectra of standard Gd3 Al2.5 Ga2.5 O12 :Ce bulk SC (Figure 4b, curve 5). The CL spectra of all the SCFs under study show only the dominant luminescence of Ce3+ or Tb3+ Crystals 2017, 7, 262 6 of 14 ions in the visible range without any bands in the UV range related to the luminescence of antisite defects [41–44], which typically observed in the bulk analogues of these of garnets The CL spectra of all the are SCFs under study show onlycrystal the dominant luminescence Ce3+ or [45,46]. Tb3+ 3+ 3+ The presented Figure 4a–c, indicate that the related complicated Gd →Tb of→antisite Ce3+ →Tb3+ ionsresults, in the visible range in without any bands in the UV range to the luminescence cascade energy transfer is observed in observed the Tb3 −in Gay Oanalogues garnet, with[45,46]. large content defects [41–44], which typically are thex Al bulk crystal of these garnets x Gd 5 −y 12 :Ce mixed 3+→Tb3+→Ce3+→Tb3+ 3+ 3+ 3+ 3+ The results, presented in Figure 4a–c, indicate that the complicated Gd of Gd and Ga cations due to overlapping of the Gd and Tb emission bands and the absorption 3+ ions transfer is observed the Tb3−xGd xAl5−yGayO12:Ce mixed garnet, with large content of 4f-5d bandscascade of Tb3+energy and Ce [47–51].inNamely, the change of the positions of Ce3+ and Tb3+ Gd3+ and Ga3+ cations due to overlapping of the Gd3+ and Tb3+ emission bands and the3+ absorption 3+ 3+ absorption and emission bands at different concentrations of Tb , Gd and3+ Ga cations leads bands of Tb3+ and Ce3+ ions [47–51]. Namely, the change of3+the positions of Ce and Tb3+ 4f-5d 3+ 3+ 3+ to a strong variation of the efficiency of Gd →Tb →Ce → Tb energy transfer processes in absorption and emission bands at different concentrations of Tb3+, Gd3+ and Ga3+ cations leads to a Tb3−x Gd Al Ga O :Ce SCFs and results in the respective changes of their CL spectra (Figure x y 3+ 3+ 3+ 3+ y 12 of the efficiency of Gd →Tb →Ce →Tb strong 5− variation energy transfer processes in 4a–c). 3+ and Ga3+ cations in Tb The estimation of the optimal content of Gd Gd Al Ga x y O12 :Ce 3 −x (Figure 5 −y4a–c). Tb3−xGdxAl5−yGayO12:Ce SCFs and results in the respective changes of their CL spectra 3+ 3+ SCFs in theThe x =estimation 1–1.5 andofythe = 2–3 ranges wasof firstly performed in [36]. will explain such a choice in optimal content Gd and Ga cations in TbWe 3−xGd xAl5−y GayO12:Ce SCFs in the xin= more 1–1.5 and y =based 2–3 ranges was firstlyof performed [36]. We will explain a choice in 3+ thisions in this work detail on the results their CL in spectra (Figure 4a,b). such Alloying of Ga 3+ ions in in more the results of range their CL spectra Alloying of Tb3 Al5work O12 :Ce (PbO)detail SCF based in theon concentration above x =(Figure 2 leads4a,b). to a decrease ofGa the garnet band Tb3Al5O12:Ce (PbO) SCF in the concentration range above x = 2 leads to a decrease of the garnet band gap value [34] and increasing the centroid shift of O-Ga bonding in comparison with O-Al bonding. gap value [34] and increasing the centroid shift of O-Ga bonding in comparison with O-Al bonding. That results in the subsequent blue shift of the Ce3+3+ emission spectra in Tb3 Al5−y Gay O12 :Ce (PbO) That results in the subsequent blue shift of the Ce emission spectra in Tb3Al5−yGayO12:Ce (PbO) SCFs SCFs (Figure 4a,curves curves 2 and 3) and Gd1.5 :Ce (BaO) (Figure curves −:Ce y Ga y O12SCFs 512 (Figure 4a, 2 and 3) and Tb1.5Tb Gd1.5 1.5Al5−y GaAl yO (BaO) (FigureSCFs 4c, curves 1–4).4c, It is most 1–4). 3+ alloying in these SCFs above y = 2–2.5 It is most important here that increasing the level of Ga 3+ important here that increasing the level of Ga alloying in these SCFs above y = 2–2.5 also leads to a 3+ emission contribution to the total spectrum of the CL also leads a strong of the Ce strongtodecrease of decrease the Ce3+ emission contribution to the total spectrum of the CL luminescence of these SCFofsamples (Figure 4a,c). (Figure 4a,c). luminescence these SCF samples 1,0
544 559
Ga content
Intensity (arb. units)
0,8
2- Tb3Al3Ga2O12:Ce (PbO) 3- Tb3Al2.5Ga2.5O12:Ce (PbO)
2
0,6
2
0.6
(a)
(b)
1 - Tb2GdGa2.5Al2,5O12:Ce (PbO) 2 - Tb2GdGa2.5Al2,5O12:Ce (PbO) 3 - Tb1.5Gd1.5Al2,5Ga2.5O12:Ce (PbO)
3
4 - Tb1Gd2Al2,5Ga2,5O12:Ce (PbO)
4
0.4
0,2
0,0
Gd content
0.8
3
0,4
544 550
1.0
1
1- Tb3Al5O12:Ce (PbO)
0.2
300
400
500
600
Wavelength (nm)
700
800
0.0
300
400
500
600
700
800
Wavelength (nm)
Ga content
553
1,0
1- Tb1.5Gd1.5Al3Ga2O12:Ce (BaO)
2- Tb1.55Gd1.45Al2.5Ga2.5O12:Ce (BaO)
5
0,8
3- Tb1,6Gd1.4Al2,15Ga2,85O12:Ce (BaO)
2 3
0,6
4- Tb1.5Gd1.5Al1.15Ga3.85O12:Ce (BaO) 5- Gd3Al2.5Ga2.5O12:Ce SC
(c) 0,4
1
4
0,2
0,0
300
400
500
600
Wavelength (nm)
700
800
4. Normalized CL spectra at K300 Tb 3Al 5−yGa yO :Ce (PbO) (PbO) (a), (a), Tb Tb33−x xAl 12:Ce FigureFigure 4. Normalized CL spectra at 300 of K Tbof AlGa yO x 5−y y O12 :Ce 3 Al 5− y Ga 1212:Ce −Gd x Gd 5−yyOGa (PbO) (b) and Tb 3−x Gd x Al 5−y Ga y O 12 :Ce (BaO) (c) SCFs with different x and y values (see legend of the (PbO) (b) and Tb3−x Gdx Al5−y Gay O12 :Ce (BaO) (c) SCFs with different x and y values (see legend of figure) comparison with with CL 3Al5O12:Ce (PbO) SCF (1a) and Gd3Al2.5Ga2.5O12:Ce bulk the figure) inincomparison CLspectra spectraofofTbTb 3 Al5 O12 :Ce (PbO) SCF (1a) and Gd3 Al2.5 Ga2.5 O12 :Ce SC (5b). bulk SC (5b).
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3+ Alloying GayyOO1212 :Ce(PbO) (PbO)SCFs SCFsin in the the concentration concentration range −yGa 55−y AlloyingofofGd Gd3+ions ionsininTb Tb3 Al 3Al :Ce range xx == 0–1.5 0–1.5has has 3+ 3+ 3+ an opposite effect than the Ga alloying and leads to the notable red shift of their CL spectra. The an opposite effect than the Ga alloying and leads to the notable red shift of their CL spectra.Gd The alloying in concentrations up to x up = 1 to increases the Ce3+ the luminescence contribution (Figure 4b, curves4b, 2) Gd3+ alloying in concentrations x = 1 increases Ce3+ luminescence contribution (Figure 3+ in comparison with the CL spectra of Gd free SCF samples (Figure 4a, curve 1). Meanwhile, the Gd curves 2) in comparison with the CL spectra of Gd free SCF samples (Figure 4a, curve 1). Meanwhile, alloying the concentration range above x = 1.0 in Tb GaxyAl O12 also results x Gd the Gd3+inalloying in the concentration range above x 3=−1.0 inx Al Tb5 − 3−xyGd 5−y:Ce Gay(PbO) O12:CeSCFs (PbO) SCFs also 3+ in the notable the Ce ofemission to the total spectrum of the SCF luminescence results in thedecrease notable of decrease the Ce3+contribution emission contribution to the total spectrum of the SCF and respectiveand increase of theincrease Tb3+ luminescence contributioncontribution (Figure 4a, (Figure curves 4a, 3–5). This 3–5). can luminescence respective of the Tb3+ luminescence curves 3+ 3+ 3+ 3+ 3+→Ce 3+ energy be caused by caused variation the efficiency Gd →Tbof Gd and3+→Tb Tb 3+ →Ce energy transfer processes This can be byofvariation of theofefficiency and Tb transfer 3+ and3+Tb3+ in Tb Gd Al Ga O :Ce SCFs due to the change of the respective positions of Ce − x x − y y 3 5 12 processes in Tb3−xGdxAl5−yGayO12:Ce SCFs due to the change of the respective positions of Ce and 3+ cations. 3+ cations. absorption and emission bandsbands at different concentration of Tb3+ Tb3+ absorption and emission at different concentration of and Tb3+Gd and Gd Based Basedon onthe theresults resultspresented presentedin inFigure Figure4a,b, 4a,b,we wecan canconfirm confirmhere herethat thatthe theoptimal optimalvalues valuesof ofthe the 3+ 3+ 3+ 3+ Gd and Ga concentrations in Tb Gd Al Ga O :Ce SCFs are x = 1–1.5 and y = 2–2.5 ranges, x y xGdxAl5−y 5−Ga y yO12:Ce 12 SCFs are x = 1–1.5 and y = 2–2.5 ranges, Gd and Ga concentrations in Tb3− 3−x respectively. thethe respective CLCL spectra of Tb Ga 1.5 Al 3–2.5 2–2.5 respectively.At Atthese theseconcentrations concentrations respective spectra of1.5 TbGd 1.5Gd 1.5Al 3–2.5 Ga 2–2.5O O1212:Ce :Ce (PbO) (PbO) 3+ 3+ and and(BaO) (BaO)SCFs SCFsshow showthe thedominant dominantCe Ce3+ emission emissionband bandwith withrelatively relativelysmall smallcontribution contributionofofthe theTb Tb3+ luminescence luminescence(Figure (Figure4b, 4b,curves curves22and and33and andFigure Figure4c, 4c,curves curves11and and2). 2).
3.2. 3.2.Scintillation ScintillationDecay DecayKinetics Kinetics The decay kinetics of Tb GaGa :Ce TbTb AlAl :Ce SCFs, x Al −y5−y yO x Gd 1212 1.5 Gd 1.51.5 5−5−y y Ga Thescintillation scintillation decay kinetics of3−Tb 3−xGdx5Al yO :Ceand and 1.5Gd GayyO O12 12:Ce SCFs, grown both from PbO (a) and BaO (b) fluxes with different x and y values in the 0–2 and 2–2.5 grown both from PbO (a) and BaO (b) fluxes with different x and y values in the 0–2 and 2–2.5 ranges, ranges, respectively, are shown correspondingly in Figure 5a,b. Generally, the Tb Gd Al Ga O :Ce x y 3 − x 5 − y 12 respectively, are shown correspondingly in Figure 5a,b. Generally, the Tb3−xGdxAl5−yGayO12:Ce SCF SCF scintillators scintillatorsdemonstrate demonstratethe thenotably notablyslower slowernon-exponential non-exponential kinetics kinetics similarly similarly to to their theirYAG:Ce YAG:Ceand and 3+ luminescence is typical LuAG:Ce SCF counterparts [21,36]. Such slower decay kinetics of the Ce 3+ LuAG:Ce SCF counterparts [21,36]. Such slower decay kinetics of the Ce luminescence is typical for 3+ and Gd3+ based SCF scintillators, where the cascade energy transfer via both sublattices of for Tband Tb3+ Gd3+ based SCF scintillators, where the cascade energy transfer via both sublattices of Gd3+ 3+ 3+ cations is more complicated [36] in comparison with their YAG:Ce and LuAG:Ce SCF Gd and and Tb3+ Tb cations is more complicated [36] in comparison with their YAG:Ce and LuAG:Ce SCF 3+ ions dominates [21]. analogues, where the toto CeCe 3+ ions dominates [21]. analogues, where thedirect directenergy energytransfer transferfrom fromthe thegarnet garnethost host 1
Intensity (arb. units)
1
1 - Tb1,5Gd1,5Al3Ga2O12:Ce (BaO)
1 - Tb3Al5O12:Ce (PbO);
2 - Tb1,5Gd1,5Al2,5Ga2,5O12:Ce (BaO)
2 - Tb1,5Gd1,5Al2,5Ga2,5O12:Ce (PbO);
3 - Gd3Al2,5Ga2,5O12:Ce SC
3 - Tb1Gd2Al2,5Ga2,5O12:Ce (PbO);
(b)
(a)
0,1
0,1
1 0
500
1
2
3
1000
Time (ns)
1500
0
500
3 2 1000
Time (ns)
1500
Figure5.5.(a) (a)The Thenormalized normalized scintillation decay kinetics of TbGd 3−xGdxAl2.5Ga2.5O12:Ce (PbO) SCFs Figure scintillation decay kinetics of Tb x Al2.5 Ga2.5 O12 :Ce (PbO) SCFs 3−x (curves 2, 3) with different Gd and Ga contents in comparison with the decay decay kinetics kinetics of of the the (curves 2, 3) with different Gd and Ga contents in comparison with the Tb 3Al5O12:Ce (PbO) SCF counterpart (curve 1); (b) the normalized scintillation decay kinetics of Tb3 Al5 O12 :Ce (PbO) SCF counterpart (curve 1); (b) the normalized scintillation decay kinetics of Tb1.5Gd Gd1.5Al 5−yGayO12:Ce (BaO) SCFs (curves 1, 2) with different Ga content in comparison with the Tb 1.5 1.5 Al5−y Gay O12 :Ce (BaO) SCFs (curves 1, 2) with different Ga content in comparison with the Al2.5Ga2.5O12:Ce bulk SC (curve 3). decay kinetics ofGd Gd3Al decay kinetics of 3 2.5 Ga2.5 O12 :Ce bulk SC (curve 3).
The influence of Ga3+ and Gd3+ alloying on the scintillation decay of Tb3−xGdxAl5−yGayO12:Ce (PbO) The influence of Ga3+ and Gd3+ alloying on the scintillation decay of Tb3+3−x Gdx Al −y Gay O12 :Ce SCFs was firstly considered in [36]. We will explain the choice of optimal Gd and Ga3+ 5concentrations (PbO) SCFs was firstly considered in [36]. We will explain the choice of optimal Gd3+ and Ga3+ in these SCFs in more detail based on the results of their scintillation decay kinetics (Figure 5a). The concentrations in these SCFs in more detail based on the results of their scintillation decay kinetics Ga3+ alloying with the concentration up to y = 2.5 in Tb3Al5−yGayO12:Ce (PbO) SCFs leads to a significant (Figure 5a). The Ga3+ alloying with the 3+ concentration up to y = 2.5 in Tb3 Al5 −y Gay O12 :Ce (PbO) slowdown of the decay kinetics of the Ce emission in comparison with the TbAG:Ce SCF due to the SCFs leads to a significant slowdown of the decay kinetics of the Ce3+ emission in comparison with lowering of the bottom of the conductive band and the arising electron transitions from the excited the TbAG:Ce3+SCF due to the lowering of the bottom of the conductive band and the arising electron levels of Ce to the conductive band. This results in the strong elongation of the scintillation decay of Tb3Al5−yGayO12:Ce (PbO) SCFs (not present in Figure 5a). Contrary to the influence of Ga3+ cations, the Gd3+ alloying in the concentration range x = 1.5–2 strongly accelerates the scintillation decay of
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transitions from the excited levels of Ce3+ to the conductive band. This results in the strong elongation of the scintillation decay of Tb3 Al5−y Gay O12 :Ce (PbO) SCFs (not present in Figure 5a). Contrary to the influence of Ga3+ cations, the Gd3+ alloying in the concentration range x = 1.5–2 strongly accelerates the scintillation decay of Tb3−x Gdx Al2.5 Ga2.5 O12 :Ce (PbO) SCFs (Figure 5a, curves 2 and 3). It is also important to note here that the scintillation decay times t1/e and t1/20 in Tb3−x Gdx Al5−y Gay O12 (PbO) SCFs at high Gd and Ga concentrations (x = 1.5–2; y = 2.5) (Figure 5a, curves 2 and 3) are close or even faster with respect to the corresponding values for Tb3 Al5 O12 :Ce SCF (see also Table 1). The Tb1.5 Gd1.5 Al5−y Gay O12 :Ce SCF scintillators, grown from the BaO based flux (Figure 5b), demonstrate significantly more exponential kinetics than that in their SCF counterparts, grown from the PbO based flux (Figure 5a). Such a difference can be caused by eliminating the delay of energy transfer from the garnet hosts to Ce3+ ions caused by the defect centers related to the Pb2+ ions in Tb3–x Gdx Al5–y Gay O12 :Ce (PbO) SCF scintillators. The concentration of these defect centers is significantly smaller in their analogues, prepared from the BaO based flux due to the very low contamination with Ba2+ ions. It is necessary also to note here that the scintillation decay of Tb1.5 Gd1.5 Al3–2.5 Ga2–2.5 O12 :Ce (BaO) SCFs (Figure 5b, curves 1 and 2) is also notably faster than that in Gd3 Al2.5 Ga2.5 O12 :Ce SC counterparts (Figure 5b, curve 3) which can also be used as a substrate for producing SCF scintillators. In such a way these SCFs and the high-quality Gd3 Al2.5 Ga2.5 O12 :Ce SCs can also be used for the creation of advanced hybrid film-substrate scintillators using the LPE method for simultaneous registration of the different components of ionization fluxes [52]. In such scintillators, the separation of the signal coming from the film and crystal parts of the hybrid scintillator can be performed using the differences in their scintillation decay kinetics (Figure 5b, curves 1 and 3, respectively). 3.3. TSL Properties The results of the TSL investigations after irradiation by alpha particles of Tb3−x Gdx Al5−y Gay O12 :Ce SCFs with different x and y values, grown from both PbO (a) and BaO (b) based fluxes, in the above RT range are shown in Figure 6. The TSL in these SCFs arises at the thermal liberation of electrons from traps and their recombination with the holes localized around Ce3+ ions [21,28,33,42,43]. Taking into account the low temperature of SCF preparation in oxygen containing atmosphere (air), the formation of these traps can be caused mainly by the presence of Pb2+ and Ba2+ (from flux) and Pt4+ (from crucible) contaminations in SCF samples. This leads to the creation of different locally non-compensated lattice defects, such as the oxygen or cation vacancies, around the mentioned impurities, which act as trapping centers [21,31,36,53,54]. In Tb3 Al5 O12 :Ce (PbO) SCFs, the position of the main TSL peak is located at 440 K (Figure 6a, curve 1). Ga3+ alloying of Tb3 Al5−x Gax O12 :Ce SCFs in the concentration range up to y = 2 leads to the shift of the TSL peaks to 407 K and substantially decreases the TSL signal in the 350–600 K range (Figure 6a, curve 2) (see also [31,36]). Gd3+ alloying additionally shifts the TSL peak to 390 K and slightly decreases the TSL signal of Tb1.5 Gd1.3 Al2.5 Ga2.5 O12 :Ce (PbO) SCFs in the 450–650 K high-temperature range in comparison with Gd free SCF samples (Figure 6a, curve 3). These results are in good correlation with the significant increase of the LY in Tb1.5 Gd1.5 Al3–2.5 Ga2–2.5 O12 :Ce (PbO) SCFs (Table 1), most probably due to elimination of the participation of high-temperature trap-related centers in the scintillation processes in the SCF samples with the mentioned optimal content. Using the lead free BaO based flux for the growth of Tb3−x Gdx Al5−y Gay O12 :Ce SCFs leads also to the low intensity of the TSL peaks in the above RT range in SCF scintillators (Figure 6b). Namely, the TSL intensity of Tb1.5 Gd1.5 Al3–2.5 Ga2–2.5 O12 :Ce (BaO) SCFs in the 450–600 K range is also negligible (Figure 6b). This result is in good correlation with the high LY in Tb1.5 Gd1.5 Al3–2.5 Ga2–2.5 O12 :Ce (BaO) SCFs (Table 1), due to the low Ba2+ contamination and related with them ptrapping centers as well as to the additional elimination of the trap-related phenomena in the scintillation processes in these SCFs by Ga3+ and Gd3+ alloying.
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2 - Tb 3Al3Ga 2O 12:Ce (PbO)
Gd alloying TL signal (a.u.)
1000
1 - Tb 3Al5O 12:Ce (PbO)
1000
1 - Tb1.5Gd1.5Al3Ga2O12:Ce (BaO)
3 - Tb 1.5Gd 1.5Al2.5Ga2.5O 12:Ce (PbO)
2 - Tb1.5Gd1.5Al2.5Ga2.5O12:Ce (BaO)
440
1 100
390
Alpha irradiation 241 by Am source
100
407
1 Ga alloying
(a)
2 10
3 300
350
400
450
(b)
2 10
Alpha irradiation 241 by Am source 500
550
Temperature (K)
600
650
700
300
350
400
450
500
550
Temperature (K)
600
650
700
Figure 6.6. TSL TSL(in (inthe thelog logscale) scale)ofof GdxxAl Al55−y :Ce content of ofGd Gd3+3+ Figure TbTb GayyOO1212 :Ce(PbO) (PbO)(a) (a)with with aa different different content 3−3−x x Gd −yGa 3+ 3+ and Ga Ga cations cations (see (seelegend legendofofthe thefigure) figure) and TbGd 1.5Gd1.5 Al 3–2.5Ga Ga2–2.5 2–2.5O after and and Tb1.5 O1212:Ce :Ce(BaO) (BaO) (b) (b) SCFs after 1.5 Al 3–2.5 241 α-particles and registration of the Ce3+ 3+luminescence. irradiation by by Am Am241 irradiation luminescence.
Thus, the Ga3+ alloying in the concentration range y = 2–2.5 positively affects not only the Thus, the Ga3+ alloying in the concentration range y = 2–2.5 positively affects not only the scintillation properties of melt-grown mixed garnet crystals with a large concentration of antisite scintillation properties of melt-grown mixed garnet crystals with a large concentration of antisite defects and oxygen vacancies [22–24] but antisite free SCF counterparts as well [31–33] due to burying defects and oxygen vacancies [22–24] but antisite free SCF counterparts as well [31–33] due to burying the trap levels by the bottom of the conductive band in the Ga-containing garnets. The additional the trap levels by the bottom of the conductive band in the Ga-containing garnets. The additional positive effect on the elimination of the trap centers in scintillation phenomena, which is observed in positive effect on the elimination of the trap centers in scintillation phenomena, which is observed in Tb3−xGdxAl5−xGaxO12:Ce SCF samples, is most probably caused by burying the deeper trap levels by Tb3−x Gdx Al5−x Gax O12 :Ce SCF samples, is most probably caused by burying the deeper trap levels by high-energy states of Tb3+ and Gd3+ cations in such Tb,Gd-rich garnets [36]. high-energy states of Tb3+ and Gd3+ cations in such Tb,Gd-rich garnets [36]. 3.4. Photoelectron Photoelectron Light 3.4. Light Yield Yield Measurements Measurements The scintillation scintillationLY LYofofTbTb3−xGd GdxxAl Al5−yGa yO12:Ce SCFs, grown from PbO and BaO based fluxes at The 3−x 5−y Gay O12 :Ce SCFs, grown from PbO and BaO based fluxes at 3+ and Ga3+ 3+ cations, measured with a shaping time of 14 μs under excitation by 3+ different content of Gd different content of Gd and Ga cations, measured with a shaping time of 14 µs under excitation by 239 (5.15 MeV) source, is shown in Table 1 and Figure 7. α-particles of of Pu Pu239 α-particles (5.15 MeV) source, is shown in Table 1 and Figure 7. 3+ 3+ 3+ doping In principle, the Ga3+ doping of TbAG:Ce SCFsSCFs can also In principle, thesimultaneous simultaneousinfluence influenceofofGd Gdand and Ga of TbAG:Ce canresult also in a strong increase in the LY of Tb 3−xGdxAl5−yGayO12:Ce SCF scintillators at x = 0–1.5 and y = 2–3 result in a strong increase in the LY of Tb3−x Gdx Al5−y Gay O12 :Ce SCF scintillators at x = 0–1.5 and 3+ and 3+ 3+ cations Thus, the determination of the optimal contentcontent and ratio the Gdthe y[21,36]. = 2–3 [21,36]. Thus, the determination of the optimal andbetween ratio between GdGaand Ga3+ in the TbAG:Ce garnet host is the most important task for the optimization of the properties of cations in the TbAG:Ce garnet host is the most important task for the optimization of the properties of Tb 3−xGdxAl5−yGayO12:Ce SCF scintillators in the case of growth from both PbO and especially lead free Tb3−x Gdx Al5−y Gay O12 :Ce SCF scintillators in the case of growth from both PbO and especially lead BaOBaO based flux.flux. free based 3+3+doping We have observedthat thatGa Ga dopingininthe the concentration range = 2–2.5 leads to the increase of We have observed concentration range y =y2–2.5 leads to the increase of the 3+ 3+ doping theof LYTbofAl Tb3Al5−yGayO12:Ce (PbO) SCF scintillators. Indeed, with the Ga dopingat at the the concentration concentration LY 3 5−y Gay O12 :Ce (PbO) SCF scintillators. Indeed, with the Ga y = 2.5, the LY of these scintillators notably (up to 20%) overcomes the LY of the Tb 3Al5O12:Ce SCF y = 2.5, the LY of these scintillators notably (up to 20%) overcomes the LY of the Tb3 Al5 O12 :Ce sample (Table 1). 1). ThisThis is caused mainly bybythe phenomena in in SCF sample (Table is caused mainly theelimination elimination of of trap-related trap-related phenomena Tb3Al Al5−yGa yO12:Ce SCF scintillators due to the decrease of the Tb3Al5O12 band gap in the case of Ga Tb 3 5−y Gay O12 :Ce SCF scintillators due to the decrease of the Tb3 Al5 O12 band gap in the case alloying in thein mentioned concentration rangerange (see Part for3.3 details). of Ga alloying the mentioned concentration (see3.3 Part for details). In addition to the positive trend caused by the doping with Ga3+3+ ions, ions, the the significant significant increase increase of of In addition to the positive trend caused by the doping with Ga 3+ 3+ doping the LY LYisisobserved observedin inTb Tb3−xGd xAl5−yGayO12:Ce (PbO) SCFs due to Gd doping at the concentration x= the at the concentration 3−x Gdx Al5−y Gay O12 :Ce (PbO) SCFs due to Gd 3+ emitting 3+ 1–1.5 (Table 1). This effect can be caused by the deepest localization of the Ce levels inside x = 1–1.5 (Table 1). This effect can be caused by the deepest localization of the Ce emitting levels the band and better of themof with respect the levels of levels conductive band. Namely, inside thegap band gap andseparation better separation them with to respect to the of conductive band. the Tb 1.5Gd1.5Al2.5Ga2.5O12:Ce (PbO) SCF possess excellent scintillation properties (Table 1 and Figure Namely, the Tb1.5 Gd1.5 Al2.5 Ga2.5 O12 :Ce (PbO) SCF possess excellent scintillation properties (Table 1 7). The photoelectron LY of this SCF sample significantly overcomes overcomes the LY of the samples of and Figure 7). The photoelectron LY of this SCF sample significantly thebest LY of the best LuAG:CeofSCF [53], LuSCF 1.5Gd1.5Al2.75Ga2.25O12:Ce SCF [31] and Tb3Al5O12:Ce SCF [21], grown from PbO samples LuAG:Ce [53], Lu1.5 Gd1.5 Al2.75 Ga2.25 O12 :Ce SCF [31] and Tb3 Al5 O12 :Ce SCF [21], based flux, up to 1.85, and times,2.62 respectively (see Table 1). This LY is Table the highest oneLY ever grown from PbO based2.62 flux, up1.95 to 1.85, and 1.95 times, respectively (see 1). This is obtained in our group for the garnet SCF scintillators grown by the LPE from traditional PbO-B2O3 based flux [21,31,36,38,53].
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the highest one ever obtained in our group for the garnet SCF scintillators grown by the LPE from Crystals 2017,PbO-B 7, 262 O based flux [21,31,36,38,53]. 10 of 14 traditional 2 3 500
1 - YAG:Ce (PbO) SCF 2 - Tb1.5Gd1.5Al2.5Ga2.5O12:Ce (PbO) SCF
400
3 - Tb1.5Gd1.5Al3Ga2O12:Ce (BaO) SCF 4 - Gd3Al2.5Ga2.5O12:Ce SC
4
300
6.9 % 200 12.1 % 12.4 %
100 0
0
20%
1
1000
2000
3
3000
2 4000
Number of Chanel Figure Gd 1.5Al Al2.5 Ga2.52.5 (PbO) (LY = 1368 phels/MeV (380%); 2.5Ga 12 :Ce Figure7.7.Pulse Pulseheight heightspectra spectraofofTb Tb1.5 1.5Gd1.5 OO 12:Ce (PbO) (2)(2) (LY = 1368 phels/MeV (380%); E= E20%) = 20%) and Tb Gd Al Ga O :Ce (BaO) (3) (LY = 1370 phels/MeV (380%); E = 12.1%) in 1.51.5Al1.5 3 12:Ce 2 (BaO) 12 and Tb1.5Gd 3Ga2O (3) (LY = 1370 phels/MeV (380%); E = 12.1%) in comparison comparison with YAG:Ce SCFs (1) phels/MeV (LY = 360 phels/MeV = 12.4%) Gd3 Al O12(LY :Ce= 2.5 Ga with YAG:Ce SCFs (1) (LY = 360 (100%), E =(100%), 12.4%)Eand Gd3Aland 2.5Ga2.5O 12:Ce SC2.5(4) 239 Pu (5.15 MeV) source and SC (4) (LY = 1375 phels/MeV (381%); E = 6.9%) excited by α-particles of 239 1375 phels/MeV (381%); E = 6.9%) excited by α-particles of Pu (5.15 MeV) source and registered registered with atime shaping with a shaping of 14time μs. of 14 µs. 2+ contamination Theelimination elimination of of Pb Pb2+ based flux in principle can The contaminationby byusing usingthe thelead-free lead-freeBaO BaO based flux in principle result in the of theofLY and resolution of Tb3−xof GdTb xAl 5−y GayxO SCF can result in improvement the improvement the LYenergy and energy resolution Al125:Ce y Oscintillators 3− x Gd −y Ga 12 :Ce SCF (Table 1 and(Table Figure 7). Indeed, 1.5Gd1.5Al 12 :Ce (BaO) SCF sample also possess excellent scintillators 1 and Figure the 7). Tb Indeed, the3Ga Tb2O Gd Al Ga O :Ce (BaO) SCF sample also 1.5 1.5 3 2 12 scintillation properties. Probably due to the elimination of the quenching and trap related phenomena possess excellent scintillation properties. Probably due to the elimination of the quenching and trap causedphenomena by lead ions, the by best LYions, and the energy resolution in these SCF in is these observed lower Ga related caused lead best LY and energy resolution SCF at is observed y = 2 in the Tb 1.5Gd Al3Ga O12 :Ce (BaO) SCF sample than in the case of using thecase PbO atconcentration lower Ga concentration y= 2 in1.5the Tb21.5 Gd Al Ga O :Ce (BaO) SCF sample than in the 1.5 3 2 12 1). The of these SCFs notably (up to notably 10%) exceeds LYexceeds of the the best ofbased usingflux the (Table PbO based fluxLY (Table 1). The LYalso of these SCFs also (up to the 10%) Luof 3−xthe Gdxbest Al5−yLu Ga3y− Ox12Gd :Cex Al (BaO) and Gd 3 Al 5−y Ga x O 12 :Ce (BaO) SCF scintillators [31] (Table 1). Without LY Ga O :Ce (BaO) and Gd Al Ga O :Ce (BaO) SCF scintillators [31] y x 5−y 12 3 5−y 12 doubt,1). thisWithout is the highest garnet SCFLY scintillators, grown by the LPEgrown method BaOmethod based flux (Table doubt, LY thisfor is the highest for garnet SCF scintillators, byfrom the LPE fromin our group [31].inHoverer, the[31]. increase of thethe LYincrease of Tb3−xGd xAl5−y GaofyOTb 12:Ce to the elimination BaO based flux our group Hoverer, of the LY Gdx Aldue :Ce SCFs y O12 3−x SCFs 5−y Ga 2+ flux related dopants relatively andisisrelatively not so significant asisinnot the of negative influence of Pb due to the elimination negative influence of Pb2+isflux relatedsmall dopants small and case of producing Lu 1−x Gd x Al 5−y Ga y O 12 :Ce and Gd 3 Al 5−y Ga y O 12 :Ce SCFs scintillators from PbO and so significant as in the case of producing Lu1−x Gdx Al5−y Gay O12 :Ce and Gd3 Al5−y Gay O12 :Ce SCFs BaO basedfrom fluxes [31]. be fluxes caused[31]. by This a more situation in terms ofsituation the energy scintillators PbO andThis BaOcan based canfavourable be caused by a more favourable in transfer phenomena via the phenomena sublattice of via Tb3+the cations to Ce3+ofions the casetoofCe Tb3+3−xions GdxAl Gacase yO12:Ce terms of the energy transfer sublattice Tb3+incations in5−y the of (PbO) SCFs enables the production of these scintillators with high LY and excellentwith structural Tb SCFs which enables the production of these scintillators high x Al5which 3−x Gd −y Gay O 12 :Ce (PbO) quality from thestructural traditional PbO based flux.traditional PbO based flux. LY and excellent quality from the Thus,the themain mainreason reasonforfor such increase of the LYTb of3−Tb 3−xGd Ga yO 12:Ce :Ce SCFs SCFs isisthe the Thus, such anan increase of the LY of Ga x Alx5Al yO x Gd −y5−y 12 optimizedcation cation content content with with respect to the optimized the crystal crystal field fieldstrength strengthand andenergy energytransfer transferefficiency efficiencyto 3+ and 3+ cations. 3+ 3+ ions directly from thethe mixed garnet host andand via via the the sublattice of Tb and Gd3+Gd cations. Most toCeCe ions directly from mixed garnet host sublattice of3+Tb 3+ 3+ 3+ 3+ 3+ and Gd 3+ cations is probably, the relative position of Ce levels with respect to the levels of Tb Most probably, the relative position of Ce levels with respect to the of Tb and Gd cations is optimalininTbTb 1.4–1.5Gd Gd1.6–1.5 1.6–1.5Al Ga 2–2.5 O12O:Ce garnet hosts efficiencyof ofthe the optimal Al3–2.5 Ga garnet hostsfrom fromthe thepoint pointof of view view of efficiency 1.4–1.5 3–2.5 2–2.5 12 :Ce 3+3+→ 3+→Tb3+ complicatedGd Gd →Tb →Ce details of of such suchaatransfer transfer complicated Tb3+3+→ Ce3+ →Tb3+energy energytransfer transferin inthese these matrixes. matrixes. The details arepresented presentedininthe theseparate separatepapers papers[34,35,49–51]. [34,35,49–51]. are alsoimportant important note LYTbof1.5Tb 2.5Ga 2.512 O:Ce 12:Ce (PbO) and TbGd 1.5Gd ItItisisalso to to note thatthat the the LY of Gd1.51.5Gd Al1.52.5Al Ga (PbO) and Tb1.5 AlAl 2.5 O 1.51.5 33 2O 12 :Ce (BaO) SCFs is comparable with that in the high-quality Gd 3 Al 2.5–2 Ga 2.5–3 O 12 :Ce SC samples Ga2 O Ga :Ce (BaO) SCFs is comparable with that in the high-quality Gd Al Ga O :Ce SC samples 12 3 2.5–2 2.5–3 12 (Table1 1and andFigure Figure7). 7).This Thisalso alsoenables enablesproducing producingthe thehybrid hybridfilm-substrate film-substratedetectors detectorsusing usingthe theLPE LPE (Table methodwith withhigh highLY LYboth bothininfilm filmand andsubstrate substratescintillators scintillatorswith withoptimized optimizedcontents, contents,taking takinginto into method account the requirement for the decay times of the signals coming from the film and substrate components of the hybrid scintillator differs by a factor of at least two [52]. Such a demand is fully realized in the case of Tb1.5Gd1.5Al3Ga2O12:Ce (BaO) SCF/Gd3Al2.5Ga2.5O12:Ce SC hybrid scintillators (Figure 4b). These scintillators can be proposed for different types of application, especially for
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account the requirement for the decay times of the signals coming from the film and substrate components of the hybrid scintillator differs by a factor of at least two [52]. Such a demand is fully realized in the case of Tb1.5 Gd1.5 Al3 Ga2 O12 :Ce (BaO) SCF/Gd3 Al2.5 Ga2.5 O12 :Ce SC hybrid scintillators (Figure 4b). These scintillators can be proposed for different types of application, especially for registration of the components of mixed ionizing flux [52] and multi-layer screens for visualization of X-ray images [21]. 4. Conclusions In this work, we report the creation of advanced single crystalline film (SCF) screens with excellent scintillation properties based on the Tb1.5 Gd1.5 Al3–2.5 Ga2–2.5 O12 :Ce mixed garnet compounds grown by the LPE method from both novel lead free BaO and traditional PbO based fluxes onto Gd3 Al2.5 Ga2.5 O12 (GAGG) substrates. The optimization of Gd3+ and Ga3+ content in the Tb3−x Gdx Al5−y Gay O12 :Ce garnet at x = 1.5 and y = 2–2.5 results in the strong improvement of the energy transfer efficiency from the Tb3+ -Gd3+ based matrix to Ce3+ ions due to the modification of the band gap value and Ce3+ energy structure, as well as the elimination of the TSL peaks above room temperature. Namely, the Tb1.5 Gd1.5 Al3 Ga2 O12 :Ce SCFs grown from BaO based flux under α-particle excitation possess the highest LY values among all the LPE grown garnet SCF scintillators obtained in our group, which exceeds by at least 10% the LY of the best samples of the recently developed Lu1.5 Gd1.5 Al2.75 Ga2.25 O12 :Ce and Gd3 Al2.75 Ga2.25 O12 :Ce SCF scintillators grown from BaO based flux [31]. The photoelectron LY of these SCF scintillators under excitation by 239 Pu (5.15 MeV) source is comparable with that in high-quality Gd3 Al2.5−3 Ga2.5−3 O12 :Ce reference bulk crystal analogue with a photoelectron LY of 1370 phels/MeV (light yield of about 10,000 photons/MeV). Tb1.5 Gd1.5 Al3 Ga2 O12 :Ce SCFs also have a relatively fast scintillation response in the hundred ns range under α-particle excitation with decay times t1/e and t1/20 of 230 and 730 ns, respectively. Meanwhile, the structural uniformity and optical quality of these SCF scintillators are strongly influenced by the high-viscosity of BaO based melt. The SCFs of Tb1.5 Gd1.5 Al2.5 Ga2.5 O12 :Ce garnets, grown from PbO based flux onto GAGG substrates, possess very high structural quality and excellent scintillation properties. Under α-particle excitation, the LY of Tb1.5 Gd1.5 Al2.5 Ga2.5 O12 :Ce (PbO) SCFs is comparable with that of their analogues grown from BaO flux and these SCFs possess only slightly slower scintillation response with decay times t1/e and t1/20 of 330 and 990 ns, respectively. It is important to note that the negative quenching influence of the Pb2+ flux related dopants is not so significant during manufacturing the Tb1.5 Gd1.5 Al2.5 Ga2.5 O12 :Ce SCF scintillators as in the case of Lu1−x Gdx Al5−y Gay O12 :Ce and Gd3 Al5−y Gay O12 :Ce SCFs analogues grown from PbO based fluxes [31]. This enables the production of Tb3−x Gdx Al5−y Gay O12 :Ce SCF scintillators with high LY and excellent structural quality from the traditional PbO based flux. Most probably, this positive trend is observed only in the Tb containing scintillators, where very complicated but efficient energy transfer from Tb3+ and Gd3+ cation sub-lattices to Ce3+ ions can be realized in comparison with Lu- and Gd-containing scintillators where such transfer is absent. We have also found that the scintillation decay of Tb1.5 Gd1.5 Al2.5 Ga2.5 O12 :Ce (PbO) and especially Tb1.5 Gd1.5 Al3 Ga2 O12 :Ce (BaO) SCFs in the 0–2 µs range is notably faster (at least by 2 times) than that in Gd3 Al2.5 Ga2.5 O12 :Ce SC counterparts which can also be used as a substrate for producing SCF scintillators. In such a way these SCFs and the high-quality Gd3 Al2.5 Ga2.5 O12 :Ce crystals can be used for the creation of hybrid film-substrate scintillators using the LPE method for simultaneous registration of the different components of ionization fluxes. In such types of hybrid scintillators, the separation of the signal coming from the film and crystal parts can be performed using the differences in the scintillation decay kinetics. Acknowledgments: The work was performed in the framework of NCN No 2016/21/B/ST8/03200 and NCBR NANOLUX2014 No 286 projects and also partly supported by the Ministry of Education and Science of Ukraine in the framework of SF-20 F projects.
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Author Contributions: Vitalii Gorbenko performed the SCF growth experiments and wrote growth part of paper, Tetiana Zorenko performed the scintillation LY and decay kinetic measurements, Kazimierz Paprocki performed the CL spectra measurements; Alexander Fedorov performed the XRD investigations and analysis of SCF structural quality, Oleg Sidletskiy performed the GAGG substrates preparation; Sandra Witkiewicz collected and analyzed the SCF optical properties; Paweł Bilski and Anna Twardak performed the TSL measurements of the SCF samples and Yuriy Zorenko analyzed experimental materials and wrote the Introduction, Third part and Conclusion of the paper. Conflicts of Interest: The authors declare no conflict of interest.
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